SNARE complex oligomerization by synaphin/complexin is essential for synaptic vesicle exocytosis

A Role for the V0 Sector of the V-ATPase in Neuroexocytosis: Exogenous V0d Blocks Complexin and SNARE Interactions with V0c

V-ATPase is an important factor in synaptic vesicle acidification and is implicated in synaptic transmission. Rotation in the extra-membranous V1 sector drives proton transfer through the membrane-embedded multi-subunit V0 sector of the V-ATPase. Intra-vesicular protons are then used to drive neurotransmitter uptake by synaptic vesicles. V0a and V0c, two membrane subunits of the V0 sector, have been shown to interact with SNARE proteins, and their photo-inactivation rapidly impairs synaptic transmission. V0d, a soluble subunit of the V0 sector strongly interacts with its membrane-embedded subunits and is crucial for the canonic proton transfer activity of the V-ATPase. Our investigations show that the loop 1.2 of V0c interacts with complexin, a major partner of the SNARE machinery and that V0d1 binding to V0c inhibits this interaction, as well as V0c association with SNARE complex. The injection of recombinant V0d1 in rat superior cervical ganglion neurons rapidly reduced neurotransmission. In chromaffin cells, V0d1 overexpression and V0c silencing modified in a comparable manner several parameters of unitary exocytotic events. Our data suggest that V0c subunit promotes exocytosis via interactions with complexin and SNAREs and that this activity can be antagonized by exogenous V0d.

Structure and Function of the Mammalian Neuromuscular Junction

The mammalian neuromuscular junction (NMJ) comprises a presynaptic terminal, a postsynaptic receptor region on the muscle fiber (endplate), and the perisynaptic (terminal) Schwann cell. As with any synapse, the purpose of the NMJ is to transmit signals from the nervous system to muscle fibers. This neural control of muscle fibers is organized as motor units, which display distinct structural and functional phenotypes including differences in pre- and postsynaptic elements of NMJs. Motor units vary considerably in the frequency of their activation (both motor neuron discharge rate and duration/duty cycle), force generation, and susceptibility to fatigue. For earlier and more frequently recruited motor units, the structure and function of the activated NMJs must have high fidelity to ensure consistent activation and continued contractile response to sustain vital motor behaviors (e.g., breathing and postural balance). Similarly, for higher force less frequent behaviors (e.g., coughing and jumping), the structure and function of recruited NMJs must ensure short-term reliable activation but not activation sustained for a prolonged period in which fatigue may occur. The NMJ is highly plastic, changing structurally and functionally throughout the life span from embryonic development to old age. The NMJ also changes under pathological conditions including acute and chronic disease. Such neuroplasticity often varies across motor unit types. © 2022 American Physiological Society. Compr Physiol 12:1-36, 2022.

Syntaxin Clustering and Optogenetic Control for Synaptic Membrane Fusion

Membrane fusion during synaptic transmission mediates the trafficking of chemical signals and neuronal communication. The fast kinetics of membrane fusion on the order of millisecond is precisely regulated by the assembly of SNAREs and accessory proteins. It is believed that the formation of the SNARE complex is a key step during membrane fusion. Little is known, however, about the molecular machinery that mediates the formation of a large pre-fusion complex, including multiple SNAREs and accessory proteins. Syntaxin, a transmembrane protein on the plasma membrane, has been observed to undergo oligomerization to form clusters. Whether this clustering plays a critical role in membrane fusion is poorly understood in live cells. Optogenetics is an emerging biotechnology armed with the capacity to precisely modulate protein-protein interaction in time and space. Here, we propose an experimental scheme that combines optogenetics with single-vesicle membrane fusion, aiming to gain a better understanding of the molecular mechanism by which the syntaxin cluster regulates membrane fusion. We envision that newly developed optogenetic tools could facilitate the mechanistic understanding of synaptic transmission in live cells and animals.

8-Nitro-cGMP attenuates context-dependent fear memory in mice

We previously reported that 8-nitroguanosine 3',5'-cyclic monophosphate (8-nitro-cGMP) is endogenously produced via nitric oxide/reactive oxygen species signaling pathways and it reacts with protein thiol residues to add cGMP structure to proteins through S-guanylation. S-Guanylation occurs on synaptosomal-associated protein 25 (SNAP-25), which is a part of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex that regulates exocytosis. However, the biological relevance of 8-nitro-cGMP in the nervous system remains unclear. Here, we investigated the effects of intracerebroventricular (icv) infusion of 8-nitro-cGMP on mouse brain functions. The results of an open-field test and fear-conditioning task revealed that icv infusion of 8-nitro-cGMP decreased the vertical activity and context-dependent fear memory of mice, which are both associated with the hippocampus. Immunohistochemical analysis revealed increased c-Fos-positive cells in the dentate gyrus in 8-nitro-cGMP-infused mice. Further, biochemical analyses showed that icv infusion of 8-nitro-cGMP increased S-guanylated proteins including SNAP-25 and SNARE complex formation as well as decreased complexes containing complexin, which regulates exocytosis by binding to the SNARE complex, in the hippocampus. These findings suggest that accumulation of 8-nitro-cGMP in the hippocampus affects its functions, including memory, via S-guanylation of hippocampal proteins such as SNAP-25.

Search for a minimal machinery for Ca2+-triggered millisecond neuroexocytosis

Neurons have the remarkable ability to release a batch of neurotransmitters into the synapse immediately after an action potential. This signature event is made possible by the simultaneous fusion of a number of synaptic vesicles to the plasma membrane upon Ca²⁺ entry into the active zone. The outcomes of both cellular and in vitro studies suggest that soluble N-ethylmaleimide-sensitive-factor attachment protein receptors (SNAREs) and synaptotagmin 1 (Syt1) constitute the minimal fast exocytosis machinery in the neuron. Syt1 is the major Ca²⁺-sensor and orchestrates the synchronous start of individual vesicle fusion events while SNAREs are the membrane fusion machinery that dictates the kinetics of each single fusion event. The data also suggest that Ca²⁺-bound Syt1 is involved in the upstream docking step which leads to an increase in the number of fusion events or the size of the release, leaving the SNARE complex alone to carry out membrane fusion by themselves.

8-Nitro-cGMP Attenuates the Interaction between SNARE Complex and Complexin through S-Guanylation of SNAP-25

8-Nitroguanosine 3',5'-cyclic monophosphate (8-nitro-cGMP) is the second messenger in nitric oxide/reactive oxygen species redox signaling. This molecule covalently binds to protein thiol groups, called S-guanylation, and exerts various biological functions. Recently, we have identified synaptosomal-associated protein 25 (SNAP-25) as a target of S-guanylation, and demonstrated that S-guanylation of SNAP25 enhanced SNARE complex formation. In this study, we have examined the effects of S-guanylation of SNAP-25 on the interaction between the SNARE complex and complexin (cplx), which binds to the SNARE complex with a high affinity. Pull-down assays and coimmunoprecipitation experiments have revealed that S-guanylation of Cys90 in SNAP-25 attenuates the interaction between the SNARE complex and cplx. In addition, blue native-PAGE followed by Western blot analysis revealed that the amount of cplx detected at a high molecular weight decreased upon 8-nitro-cGMP treatment in SH-SY5Y cells. These results demonstrated for the first time that S-guanylation of SNAP-25 attenuates the interaction between the SNARE complex and cplx.

The Multifaceted Role of SNARE Proteins in Membrane Fusion

Membrane fusion is a key process in all living organisms that contributes to a variety of biological processes including viral infection, cell fertilization, as well as intracellular transport, and neurotransmitter release. In particular, the various membrane-enclosed compartments in eukaryotic cells need to exchange their contents and communicate across membranes. Efficient and controllable fusion of biological membranes is known to be driven by cooperative action of SNARE proteins, which constitute the central components of the eukaryotic fusion machinery responsible for fusion of synaptic vesicles with the plasma membrane. During exocytosis, vesicle-associated v-SNARE (synaptobrevin) and target cell-associated t-SNAREs (syntaxin and SNAP-25) assemble into a core trans-SNARE complex. This complex plays a versatile role at various stages of exocytosis ranging from the priming to fusion pore formation and expansion, finally resulting in the release or exchange of the vesicle content. This review summarizes current knowledge on the intricate molecular mechanisms underlying exocytosis triggered and catalyzed by SNARE proteins. Particular attention is given to the function of the peptidic SNARE membrane anchors and the role of SNARE-lipid interactions in fusion. Moreover, the regulatory mechanisms by synaptic auxiliary proteins in SNARE-driven membrane fusion are briefly outlined.

The role of protein intrinsic disorder in major psychiatric disorders

Although new candidate genes for Autism Spectrum Disorder (ASD), Schizophrenia (SCZ), Attention-Deficit/Hyperactivity Disorder (ADHD), and Bipolar Disorder (BD) emerged from genome-wide association studies (GWAS), their underlying molecular mechanisms remain poorly understood. Evidences of the involvement of intrinsically disordered proteins in diseases have grown in the last decade. These proteins lack tridimensional structure under physiological conditions and are involved in important cellular functions such as signaling, recognition and regulation. The aim of the present study was to identify the role and abundance of intrinsically disordered proteins in a set of psychiatric diseases and to test whether diseases are different regarding protein intrinsic disorder. Our hypothesis is that differences across psychiatric illnesses phenotypes and symptoms may arise from differences in intrinsic protein disorder content and properties of each group. A bioinformatics prediction of intrinsic disorder was performed in proteins retrieved based on top findings from GWAS, Copy Number Variation and candidate gene investigations for each disease. This approach revealed that about 80% of studied proteins presented long stretches of disorder. This amount was significantly higher than that observed in general eukaryotic proteins, and those involved in cardiovascular diseases. These results suggest that proteins with intrinsic disorder are a common feature of neurodevelopment and synaptic transmission processes which are potentially involved in the etiology of psychiatric diseases. Moreover, we identified differences between ADHD and ASD when the binary prediction of structure and putative binding sites were compared. These differences may be related to variation in symptom complexity between both diseases. © 2016 Wiley Periodicals, Inc.

Insulin signaling controls neurotransmission via the 4eBP-dependent modification of the exocytotic machinery

Altered insulin signaling has been linked to widespread nervous system dysfunction including cognitive dysfunction, neuropathy and susceptibility to neurodegenerative disease. However, knowledge of the cellular mechanisms underlying the effects of insulin on neuronal function is incomplete. Here, we show that cell autonomous insulin signaling within the Drosophila CM9 motor neuron regulates the release of neurotransmitter via alteration of the synaptic vesicle fusion machinery. This effect of insulin utilizes the FOXO-dependent regulation of the thor gene, which encodes the Drosophila homologue of the eif-4e binding protein (4eBP). A critical target of this regulatory mechanism is Complexin, a synaptic protein known to regulate synaptic vesicle exocytosis. We find that the amounts of Complexin protein observed at the synapse is regulated by insulin and genetic manipulations of Complexin levels support the model that increased synaptic Complexin reduces neurotransmission in response to insulin signaling.

Exploring the Formation and the Structure of Synaptobrevin Oligomers in a Model Membrane

SNARE complexes have been shown to act cooperatively to enable the synaptic vesicle fusion in neuronal transmission at millisecond timescale. It has previously been suggested that the oligomerization of SNARE complexes required for cooperative action in fusion is mediated by interactions between transmembrane domains (TMDs). We study the oligomerization of synaptobrevin TMD using ensembles of molecular dynamics (MD) simulations at coarse-grained resolution for both the wild-type (WT) and selected mutants. Trimerization and tetramerization of the sybII WT and mutants displayed distinct kinetics depending both on the rate of dimerization and the availability of alternative binding interfaces. Interestingly, the tetramerization kinetics and propensity for the sybII W89A-W90A mutant was significantly increased as compared with the WT; the tryptophans in WT sybII impose sterical restraints on oligomer packing, thereby maintaining an appropriate plasticity and accessibility of sybII to the binding of its cognate SNARE partners during membrane fusion. Higher-order oligomeric models (ranging from pentamer to octamer), built by incremental addition of peptides to smaller oligomers, revealed substantial stability and high compactness. These larger sybII oligomers may induce membrane deformation, thereby possibly facilitating fast fusion exocytosis.

Glutamate Release

Our aim was to review the processes of glutamate release from both biochemical and neurophysiological points of view. A large body of evidence now indicates that glutamate is specifically accumulated into synaptic vesicles, which provides strong support for the concept that glutamate is released from synaptic vesicles and is the major excitatory neurotransmitter. Evidence suggests the notion that synaptic vesicles, in order to sustain the neurotransmitter pool of glutamate, are endowed with an efficient mechanism for vesicular filling of glutamate. Glutamate-loaded vesicles undergo removal of Synapsin I by CaM kinase II-mediated phosphorylation, transforming to the release-ready pool. Vesicle docking to and fusion with the presynaptic plasma membrane are thought to be mediated by the SNARE complex. The Ca(2+)-dependent step in exocytosis is proposed to be mediated by synaptotagmin.

Stabilization of spontaneous neurotransmitter release at ribbon synapses by ribbon-specific subtypes of complexin

Ribbon synapses of tonically releasing sensory neurons must provide a large pool of releasable vesicles for sustained release, while minimizing spontaneous release in the absence of stimulation. Complexins are presynaptic proteins that may accomplish this dual task at conventional synapses by interacting with the molecular machinery of synaptic vesicle fusion at the active zone to retard spontaneous vesicle exocytosis yet facilitate release evoked by depolarization. However, ribbon synapses of photoreceptor cells and bipolar neurons in the retina express distinct complexin subtypes, perhaps reflecting the special requirements of these synapses for tonic release. To investigate the role of ribbon-specific complexins in transmitter release, we combined presynaptic voltage clamp, fluorescence imaging, electron microscopy, and behavioral assays of photoreceptive function in zebrafish. Acute interference with complexin function using a peptide derived from the SNARE-binding domain increased spontaneous synaptic vesicle fusion at ribbon synapses of retinal bipolar neurons without affecting release triggered by depolarization. Knockdown of complexin by injection of an antisense morpholino into zebrafish embryos prevented photoreceptor-driven migration of pigment in skin melanophores and caused the pigment distribution to remain in the dark-adapted state even when embryos were exposed to light. This suggests that loss of complexin function elevated spontaneous release in illuminated photoreceptors sufficiently to mimic the higher release rate normally associated with darkness, thus interfering with visual signaling. We conclude that visual system-specific complexins are required for proper illumination-dependent modulation of the rate of neurotransmitter release at visual system ribbon synapses.

Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis

The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.

Fusion pore regulation in peptidergic vesicles

Regulated exocytosis, which involves fusion of secretory vesicles with the plasma membrane, is an important mode of communication between cells. In this process, signalling molecules that are stored in secretory vesicles are released into the extracellular space. During the initial stage of fusion, the interior of the vesicle is connected to the exterior of the cell with a narrow, channel-like structure: the fusion pore. It was long believed that the fusion pore is a short-lived intermediate state leading irreversibly to fusion pore dilation. However, recent results show that the diameter of the fusion pore can fluctuate, suggesting that the fusion pore is a subject of stabilization. A possible mechanism is addressed in this article, involving the local anisotropicity of membrane constituents that can stabilize the fusion pore. The molecular nature of such a stable fusion pore to predict how interacting molecules (proteins and/or lipids) mediate changes that affect the stability of the fusion pore and exocytosis is also considered. The fusion pore likely attains stability via multiple mechanisms, which include the shape of the lipid and protein membrane constituents and the interactions between them.

SNARE requirements en route to exocytosis: from many to few

Although it has been known for almost two decades that the ternary complex of N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) constitutes the functional unit driving membrane fusion, our knowledge about the dynamical arrangement and organization of SNARE proteins and their complexes before and during vesicle exocytosis is still limited. Here, we review recent progress in this expanding field with emphasis on the question of fusion complex stoichiometry, i.e., how many SNARE proteins and complexes are needed for the fusion of a vesicle with the plasma membrane.

Two synaptobrevin molecules are sufficient for vesicle fusion in central nervous system synapses

Exocytosis of synaptic vesicles (SVs) during fast synaptic transmission is mediated by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex assembly formed by the coil-coiling of three members of this protein family: vesicle SNARE protein, synaptobrevin 2 (syb2), and the presynaptic membrane SNAREs syntaxin-1A and SNAP-25. However, it is controversially debated how many SNARE complexes are minimally needed for SV priming and fusion. To quantify this effective number, we measured the fluorescence responses from single fusing vesicles expressing pHluorin (pHl), a pH-sensitive variant of GFP, fused to the luminal domain of the vesicular SNARE syb2 (spH) in cultured hippocampal neurons lacking endogenous syb2. Fluorescence responses were quantal, with the unitary signals precisely corresponding to single pHluorin molecules. Using this approach we found that two copies of spH per SV fully rescued evoked fusion whereas SVs expressing only one spH were unable to rapidly fuse upon stimulation. Thus, two syb2 molecules and likely two SNARE complexes are necessary and sufficient for SV fusion during fast synaptic transmission.

Expression, localization, and functional role for synaptotagmins in pancreatic acinar cells

Secretagogue-induced changes in intracellular Ca(2+) play a pivotal role in secretion in pancreatic acini yet the molecules that respond to Ca(2+) are uncertain. Zymogen granule (ZG) exocytosis is regulated by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes. In nerve and endocrine cells, Ca(2+)-stimulated exocytosis is regulated by the SNARE-associated family of proteins termed synaptotagmins. This study examined a potential role for synaptotagmins in acinar secretion. RT-PCR revealed that synaptotagmin isoforms 1, 3, 6, and 7 are present in isolated acini. Immunoblotting and immunofluorescence using three different antibodies demonstrated synaptotagmin 1 immunoreactivity in apical cytoplasm and ZG fractions of acini, where it colocalized with vesicle-associated membrane protein 2. Synaptotagmin 3 immunoreactivity was detected in membrane fractions and colocalized with an endolysosomal marker. A potential functional role for synaptotagmin 1 in secretion was indicated by results that introduction of synaptotagmin 1 C2AB domain into permeabilized acini inhibited Ca(2+)-dependent exocytosis by 35%. In contrast, constructs of synaptotagmin 3 had no effect. Confirmation of these findings was achieved by incubating intact acini with an antibody specific to the intraluminal domain of synaptotagmin 1, which is externalized following exocytosis. Externalized synaptotagmin 1 was detected exclusively along the apical membrane. Treatment with CCK-8 (100 pM, 5 min) enhanced immunoreactivity by fourfold, demonstrating that synaptotagmin is inserted into the apical membrane during ZG fusion. Collectively, these data indicate that acini express synaptotagmin 1 and support that it plays a functional role in secretion whereas synaptotagmin 3 has an alternative role in endolysosomal membrane trafficking.

Arg206 of SNAP-25 is essential for neuroexocytosis at the Drosophila melanogaster neuromuscular junction

An analysis of SNAP-25 isoform sequences indicates that there is a highly conserved arginine residue (198 in vertebrates, 206 in the genus Drosophila) within the C-terminal region, which is cleaved by botulinum neurotoxin A, with consequent blockade of neuroexocytosis. The possibility that it may play an important role in the function of the neuroexocytosis machinery was tested at neuromuscular junctions of Drosophila melanogaster larvae expressing SNAP-25 in which Arg206 had been replaced by alanine. Electrophysiological recordings of spontaneous and evoked neurotransmitter release under different conditions as well as testing for the assembly of the SNARE complex indicate that this residue, which is at the P1′ position of the botulinum neurotoxin A cleavage site, plays an essential role in neuroexocytosis. Computer graphic modelling suggests that this arginine residue mediates protein–protein contacts within a rosette of SNARE complexes that assembles to mediate the fusion of synaptic vesicles with the presynaptic plasma membrane.

Fast vesicle fusion in living cells requires at least three SNARE complexes

Exocytosis requires formation of SNARE [soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor] complexes between vesicle and target membranes. Recent assessments in reduced model systems have produced divergent estimates of the number of SNARE complexes needed for fusion. Here, we used a titration approach to answer this question in intact, cultured chromaffin cells. Simultaneous expression of wild-type SNAP-25 and a mutant unable to support exocytosis progressively altered fusion kinetics and fusion-pore opening, indicating that both proteins assemble into heteromeric fusion complexes. Expressing different wild-type:mutant ratios revealed a third-power relation for fast (synchronous) fusion and a near-linear relation for overall release. Thus, fast fusion typically observed in synapses and neurosecretory cells requires at least three functional SNARE complexes, whereas slower release might occur with fewer complexes. Heterogeneity in SNARE-complex number may explain heterogeneity in vesicular release probability.

Complexin 2 modulates vesicle-associated membrane protein (VAMP) 2-regulated zymogen granule exocytosis in pancreatic acini

Complexins are soluble proteins that regulate the activity of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes necessary for vesicle fusion. Neuronal specific complexin 1 has inhibitory and stimulatory effects on exocytosis by clamping trans-SNARE complexes in a prefusion state and promoting conformational changes to facilitate membrane fusion following cell stimulation. Complexins are unable to bind to monomeric SNARE proteins but bind with high affinity to ternary SNARE complexes and with lower affinity to target SNARE complexes. Far less is understood about complexin function outside the nervous system. Pancreatic acini express the complexin 2 isoform by RT-PCR and immunoblotting. Immunofluorescence microscopy revealed complexin 2 localized along the apical plasma membrane consistent with a role in secretion. Accordingly, complexin 2 was found to interact with vesicle-associated membrane protein (VAMP) 2, syntaxins 3 and 4, but not with VAMP 8 or syntaxin 2. Introduction of recombinant complexin 2 into permeabilized acini inhibited Ca(2+)-stimulated secretion in a concentration-dependent manner with a maximal inhibition of nearly 50%. Mutations of the central α-helical domain reduced complexin 2 SNARE binding and concurrently abolished its inhibitory activity. Surprisingly, mutation of arginine 59 to histidine within the central α-helical domain did not alter SNARE binding and moreover, augmented Ca(2+)-stimulated secretion by 130% of control. Consistent with biochemical studies, complexin 2 colocalized with VAMP 2 along the apical plasma membrane following cholecystokinin-8 stimulation. These data demonstrate a functional role for complexin 2 outside the nervous system and indicate that it participates in the Ca(2+)-sensitive regulatory pathway for zymogen granule exocytosis.

Transmembrane-domain determinants for SNARE-mediated membrane fusion

Neurosecretion involves fusion of vesicles with the plasma membrane. Such membrane fusion is mediated by the SNARE complex, which is composed of the vesicle-associated protein synaptobrevin (VAMP2), and the plasma membrane proteins syntaxin-1A and SNAP-25. Although clearly important at the point of membrane fusion, the precise structural and functional requirements for the transmembrane domains (TMDs) of SNAREs in bringing about neurosecretion remain largely unknown. Here, we used a bimolecular fluorescence complementation (BiFC) approach to study SNARE protein interactions involving TMDs in vivo. VAMP2 molecules were found to dimerise through their TMDs in intact cells. Dimerisation was abolished when replacing a glycine residue in the centre of the TMD with residues of increasing molecular volume. However, such mutations still were fully competent in bringing about membrane-fusion events, suggesting that dimerisation of the VAMP2 TMDs does not have an important functional role. By contrast, a series of deletion or insertion mutants in the C-terminal half of the TMD were largely deficient in supporting neurosecretion, whereas mutations in the N-terminal half did not display severe secretory deficits. Thus, structural length requirements, largely confined to the C-terminal half of the VAMP2 TMD, seem to be essential for SNARE-mediated membrane-fusion events in cells.

Exocytosis mechanisms underlying insulin release and glucose uptake: conserved roles for Munc18c and syntaxin 4

Type 2 diabetes has been coined "a two-hit disease," as it involves specific defects of glucose-stimulated insulin secretion from the pancreatic beta cells in addition to defects in peripheral tissue insulin action required for glucose uptake. Both of these processes, insulin secretion and glucose uptake, are mediated by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein core complexes composed of syntaxin, SNAP-23/25, and VAMP proteins. The SNARE core complex is regulated by the Sec1/Munc18 (SM) family of proteins, which selectively bind to their cognate syntaxin isoforms with high affinity. The process of insulin secretion uses multiple Munc18-syntaxin isoform pairs, whereas insulin action in the peripheral tissues appears to use only the Munc18c-syntaxin 4 pair. Importantly, recent reports have linked obesity and Type 2 diabetes in humans with changes in protein levels and single nucleotide polymorphisms (SNPs) of Munc18 and syntaxin isoforms relevant to these exocytotic processes, although the molecular mechanisms underlying the observed phenotypes remain incomplete (5, 104, 144). Given the conservation of these proteins in two seemingly disparate processes and the need to design and implement novel and more effective clinical interventions, it will be vitally important to delineate the mechanisms governing these conserved SNARE-mediated exocytosis events. Thus, we provide here an up-to-date historical review of advancements in defining the roles and molecular mechanisms of Munc18-syntaxin complexes in the pathophysiology of Type 2 diabetes.

The role of lithium in modulation of brain genes: relevance for aetiology and treatment of bipolar disorder

Bipolar disorder is a debilitating disorder of the brain with a lifetime prevalence of 1.0% for bipolar I, 1.1% for bipolar II disorder and 2.4-4.7% for subthreshold bipolar disorder. Medications, including lithium, have demonstrated efficacy in the treatment of bipolar disorder, but their molecular targets and mode of action are largely unknown. A few studies have begun to shed light on potential targets of lithium treatment that may be involved in lithium's therapeutic effect. We have recently conducted a microarray study of rat frontal cortex following chronic treatment (21 days) with lithium. Chronic treatment with lithium led to a significant (at least 1.5-fold) down-regulation of 151 genes and up-regulation of 57 genes. We discuss our results in the context of previous microarray studies involving lithium and gene-association studies to identify key genes associated with chronic lithium treatment. A number of genes associated with bipolar disorder, including Comt (catechol-O-methyltransferase), Vapa (vesicle-associated membrane protein-associated protein A), Dtnb (dystrobrevin beta) and Pkd1 (polycystic kidney disease 1), were significantly altered in our microarray dataset along with genes associated with synaptic transmission, apoptosis and transport among other functions.

Complexin II regulates degranulation in RBL-2H3 cells by interacting with SNARE complex containing syntaxin-3

Recent studies have revealed that SNARE proteins are involved in the exocytotic release (degranulation) in mast cells. However, the roles of SNARE regulatory proteins are poorly understood. Complexin is one such regulatory protein and it plays a crucial role in exocytotic release. In this study, we characterized the interaction between SNARE complex and complexin II in mast cells by GST pull-down assay and in vitro binding assay. We found that the SNARE complex that interacted with complexin II consisted of syntaxin-3, SNAP-23, and VAMP-2 or -8, whereas syntaxin-4 was not detected. Recombinant syntaxin-3 binds to complexin II by itself, but its affinity to complexin II was enhanced upon addition of VAMP-8 and SNAP-23. Furthermore, the region of complexin II responsible for binding to the SNARE complex, was near the central alpha-helix region. These results suggest that complexin II regulates degranulation by interacting with the SNARE complex containing syntaxin-3.

Self-assembly of SNARE fusion proteins into star-shaped oligomers

Three evolutionarily conserved proteins known as SNAREs (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors) mediate exocytosis from single cell eukaryotes to neurons. Among neuronal SNAREs, syntaxin and SNAP-25 (synaptosome-associated protein of 25 kDa) reside on the plasma membrane, whereas synaptobrevin resides on synaptic vesicles prior to fusion. The SNARE motifs of the three proteins form a helical bundle which probably drives membrane fusion. Since studies in vivo suggested an importance for multiple SNARE complexes in the fusion process, and models appeared in the literature with large numbers of SNARE bundles executing the fusion process, we analysed the quaternary structure of the full-length native SNARE complexes in detail. By employing a preparative immunoaffinity procedure we isolated all of the SNARE complexes from brain, and have shown by size-exclusion chromatography and negative stain electron microscopy that they exist as approx. 30 nm particles containing, most frequently, 3 or 4 bundles emanating from their centre. Using highly purified, individual, full-length SNAREs we demonstrated that the oligomerization of SNAREs into star-shaped particles with 3 to 4 bundles is an intrinsic property of these proteins and is not dependent on other proteins, as previously hypothesized. The average number of the SNARE bundles in the isolated fusion particles corresponds well with the co-operativity observed in calcium-triggered neuronal exocytosis.

Single-molecule studies of the neuronal SNARE fusion machinery

SNAREs are essential components of the machinery for Ca(2+)-triggered fusion of synaptic vesicles with the plasma membrane, resulting in neurotransmitter release into the synaptic cleft. Although much is known about their biophysical and structural properties and their interactions with accessory proteins such as the Ca(2+) sensor synaptotagmin, their precise role in membrane fusion remains an enigma. Ensemble studies of liposomes with reconstituted SNAREs have demonstrated that SNAREs and accessory proteins can trigger lipid mixing/fusion, but the inability to study individual fusion events has precluded molecular insights into the fusion process. Thus, this field is ripe for studies with single-molecule methodology. In this review, we discuss applications of single-molecule approaches to observe reconstituted SNAREs, their complexes, associated proteins, and their effect on biological membranes. Some of the findings are provocative, such as the possibility of parallel and antiparallel SNARE complexes or of vesicle docking with only syntaxin and synaptobrevin, but have been confirmed by other experiments.

Differential protein expression profiles in salicylate ototoxicity of the mouse cochlea

The purpose of this study was to investigate protein expression profiles of salicylate ototoxicity using proteomic analysis, and to identify whether salicylates induce apoptosis in organotypic culture of mouse cochlear cells. The adult mice were injected intraperitoneally with 400mg/kg of sodium salicylate. Approximately 30dB threshold shift was observed 3h after the injection, and the hearing threshold returned to normal range within 3 days. Proteomic analysis of mouse cochlea was performed 3h after salicylate injection, because this was the time to show maximal ototoxic effect in salicylate intoxication. Expression pattern of proteomic analysis at 3h was compared with those of normal cochlea and cochlea 3 days after salicylate injection. Sixteen proteins were transiently up-regulated threefolds or more at 3h after the injection compared with normal cochlea, and three proteins were down-regulated at 3h. Similar protein expression profiles were also observed between normal and 3 days group. These up-regulated and down-regulated proteins at 3h were analyzed by MALDI-TOF MS. The mRNA expressions of nine selected genes from 16 up-regulated protein profiles were also investigated by RT-PCR, and their expression levels at 3h were found to be higher than those of normal cochlea. We also confirmed the ototoxicity of salicylate in organotypic culture of cochlear cells using MTT assay, Hoechst staining and DNA laddering assay in vitro, and found that salicylate decreased the viability of cells in a time and dose-dependent manner, and that induced apoptosis in organotypic culture of cochlear cells. This study demonstrated that some proteins can be related to salicylate ototoxicity, and provides basic information about candidate proteins which are related to pathologic changes in salicylate-induced ototoxicity.

The carboxy-terminal domain of complexin I stimulates liposome fusion

Regulated exocytosis requires tight coupling of the membrane fusion machinery to a triggering signal and a fast response time. Complexins are part of this regulation and, together with synaptotagmins, control calcium-dependent exocytosis. Stimulatory and inhibitory functions have been reported for complexins. To test if complexins directly affect membrane fusion, we analyzed the 4 known mammalian complexin isoforms in a reconstituted fusion assay. In contrast to complexin III (CpxIII) and CpxIV, CpxI and CpxII stimulated soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-pin assembly and membrane fusion. This stimulatory effect required a preincubation at low temperature and was specific for neuronal t-SNAREs. Stimulation of membrane fusion was lost when the carboxy-terminal domain of CpxI was deleted or serine 115, a putative phosphorylation site, was mutated. Transfer of the carboxy-terminal domain of CpxI to CpxIII resulted in a stimulatory CpxIII-I chimera. Thus, the carboxy-terminal domains of CpxI and CpxII promote the fusion of high-curvature liposomes.

Identification of a minimal segment of complexin II essential for preferential distribution in axons

Complexin II (CPLX2) is a soluble pre-synaptic protein believed to regulate neurotransmitter release from pre-synaptic terminals. CPLX2 is localized in pre-synaptic terminals in mature brain, but the mechanism of selective localization remains unclear. Here we identified an essential segment of CPLX2 for preferential axonal distribution. Myc-tagged CPLX2 was expressed in cultured rat hippocampal neurons and its distribution between axons and dendrites was compared by immunocytochemistry and image analysis. Fluorescence signals were detected in both axons and dendrites; however, their respective distribution varied significantly. Despite the fact that signal intensity decreased almost linearly from the base to the tip of the dendrite, a substantial level was sustained along the axon, even at a position near the tip. Image analyses using a series of mutants indicated that the deletion of 19 amino acid residues, G71-P89, within the 'central core' for binding to soluble N-ethylmaleimide sensitive factor attachment protein receptor proteins resulted in the loss of preferential axonal distribution. The enhanced green fluorescent protein derivative fused with the G71-P89 fragment exhibited a similar localization to that of wild type CPLX2, indicating that the G71-P89 region of CPLX2 is essential and sufficient for preferential axonal distribution.

Direct interaction of SNARE complex binding protein synaphin/complexin with calcium sensor synaptotagmin 1

Although the binding of synaphin (also called complexin) to the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex is critical for synaptic vesicle exocytosis, the exact role of synaphin remains unclear. Here, we show that synaphin directly binds to synaptotagmin 1, a major Ca(2+) sensor for fast neurotransmitter release, in a 1:1 stoichiometry. Mapping of the synaphin site involved in synaptotagmin 1 binding revealed that the C-terminal region is essential for the interaction between these two proteins. Binding was sensitive to ionic strength, suggesting the involvement of charged residues in the C-terminus region. Mutation of the seven consecutive glutamic acid residues (residues 108-114) at the C-terminal region of synaphin to alanines or glutamines resulted in a dramatic reduction in synaptotagmin 1 binding activity. Furthermore, a peptide from the C-terminus of synaphin (residues 91-124) blocked the binding of synaptotagmin 1 to synaphin, an effect that was abolished by mutating the consecutive glutamic acid residues to alanine. Immunoprecipitation experiments with brain membrane extracts showed the presence of a complex consisting of synaphin, synaptotagmin 1, and SNAREs. We propose that synaphin recruits synaptotagmin 1 to the SNARE-based fusion complex and synergistically functions with synaptotagmin 1 in mediating fast synaptic vesicle exocytosis.

Dual inhibition of SNARE complex formation by tomosyn ensures controlled neurotransmitter release

Neurotransmitter release from presynaptic nerve terminals is regulated by soluble NSF attachment protein receptor (SNARE) complex–mediated synaptic vesicle fusion. Tomosyn inhibits SNARE complex formation and neurotransmitter release by sequestering syntaxin-1 through its C-terminal vesicle-associated membrane protein (VAMP)–like domain (VLD). However, in tomosyn-deficient mice, the SNARE complex formation is unexpectedly decreased. In this study, we demonstrate that the N-terminal WD-40 repeat domain of tomosyn catalyzes the oligomerization of the SNARE complex. Microinjection of the tomosyn N-terminal WD-40 repeat domain into neurons prevented stimulated acetylcholine release. Thus, tomosyn inhibits neurotransmitter release by catalyzing oligomerization of the SNARE complex through the N-terminal WD-40 repeat domain in addition to the inhibitory activity of the C-terminal VLD.

Complexin-I-deficient sperm are subfertile due to a defect in zona pellucida penetration

Upon adhesion to the zona pellucida, sperm undergo regulated exocytosis of the acrosome. Although it is necessary for sperm to penetrate the zona pellucida and fertilize an egg, the acrosomal membrane fusion process is poorly understood. Complexins I and II are small, cytosolic proteins that bind to a complex of proteins termed the solubleN-ethylmaleimide-sensitive factor attachment protein receptor complex to regulate synaptic vesicle exocytosis. Complexin-II-deficient mice are fertile but the fertility of sperm from complexin-I-deficient male mice is unclear because the mice have ataxia and cannot mate. Here, we show that the genes encoding complexins I and II are expressed in primary spermatocytes and spermatids. Complexin proteins were found in/near the developing acrosome in spermatids and in or around the acrosome of mature sperm. Cell fractionation demonstrated that complexins I and II were predominantly found in the cytosolic fraction. Furthermore, sperm from complexin-I-deficient mice had normal morphology, number, and only small differences in motility, as assessed by computer-assisted semen analysis. Complexin-I-deficient sperm capacitated normally and bound to the zona pellucida. But when sperm from complexin-I-deficient mice were inseminated into females, a defect in fertility was observed, in concordance with previous data showing thatin vitrofertilization rate was also reduced. If the zona pellucida was removed prior toin vitrofertilization, fertility was normal, demonstrating that zona pellucida penetration was defective, a step requiring acrosomal exocytosis. Therefore, complexin-I-deficient sperm are subfertile due to faulty zona pellucida penetration.

Membrane fusion

Subcellular compartmentalization, cell growth, hormone secretion and neurotransmission require rapid, targeted, and regulated membrane fusion. Fusion entails extensive lipid rearrangements by two apposed (docked) membrane vesicles, joining their membrane proteins and lipids and mixing their luminal contents without lysis. Fusion of membranes in the secretory pathway involves Rab GTPases; their bound ‘effector’ proteins, which mediate downstream steps; SNARE proteins, which can ‘snare’ each other, in cis (bound to one membrane) or in trans (anchored to apposed membranes); and SNARE-associated proteins (SM proteins; NSF or Sec18p; SNAP or Sec17p; and others) cooperating with specific lipids to catalyze fusion. In contrast, mitochondrial and cell-cell fusion events are regulated by and use distinct catalysts.

Distinct domains of complexins bind SNARE complexes and clamp fusion in vitro

In regulated exocytosis, the core membrane fusion machinery proteins, the SNARE proteins, are assisted by a group of regulatory factors in order to couple membrane fusion to an increase of intracellular calcium ion (Ca(2+)) concentration. Complexin-I and synaptotagmin-I have been shown to be key elements for this tightly regulated process. Many studies suggest that complexin-I can arrest the fusion reaction and that synaptotagmin-I can release the complexin-I blockage in a calcium-dependent manner. Although the actual molecular mechanism by which they exert their function is still unknown, recent in vivo experiments postulate that domains of complexin-I produce different effects on neurotransmitter release. Herein, by using an in vitro flipped SNARE cell fusion assay, we have identified and characterized the minimal functional domains of complexin-I necessary to couple calcium and synaptotagmin-I to membrane fusion. Moreover, we provide evidence that other isoforms of complexin, complexin-II, -III, and -IV, can also be functionally coupled to synaptotagmin-I and calcium. These correspond closely to results from in vivo experiments, providing further validation of the physiological relevance of the flipped SNARE system.

Supramolecular SNARE assembly precedes hemifusion in SNARE-mediated membrane fusion

Formation of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex facilitates intracellular membrane fusion. A single SNARE complex is thought to be insufficient; multiple copies of SNARE complexes must work cooperatively. However, the mechanism by which such a higher-order SNARE protein structure is assembled is unknown. EPR and fluorescence analyses show that at least three copies of target-membrane SNARE proteins self-assemble through the interaction between the transmembrane domains (TMDs), and this multimeric structure serves as scaffolding for trans-SNARE assembly. SNARE core formation in solution induces oligomerization of the TMDs of vesicle-associated SNAREs in the apposing membrane, transiently forming a supramolecular protein structure spanning two membranes. This higher-order protein intermediate evolves, by involving lipid molecules, to the hemifusion state. Hemifusion is subsequently followed by distal leaflet mixing and formation of the cis-SNARE complex.

Regulation of complexin 1 and complexin 2 in the developing human prefrontal cortex

Complexin 1 (CX1) and complexin 2 (CX2) are presynaptic proteins that modulate neurotransmitter release and are used as markers of inhibitory and excitatory synapses, respectively. The aim of this study was to gain insight into the development of inhibitory and excitatory synapses in human prefrontal cortex (PFC) by examining the expression of CX1 and CX2 in postmortem tissues. Relative complexin protein levels were measured by Western blotting in postmortem dorsolateral prefrontal cortex (DLPFC) of 42 subjects without neurological or psychiatric disease ranging in age from 18 gestational weeks to 25 years. Samples were batched a priori into fetal, 0-12 month, 1-5 years, 6-10 years, 11-15 years, 16-20 years, and 21-25 years age groups. CX1 and CX2 expression and CX2/CX1 demonstrated a significant effect of age group by ANOVA. Group CX1 level increased progressively across development and was lowest in the fetal group and highest in the young adult group, whereas group CX2 level increased between the fetal and the 6-10 years groups and then plateaued. Consistent with these divergent patterns, there was a significant effect of age group on CX2/CX1, which was higher in fetal and infant groups than in the young adult group. Furthermore, regression analysis demonstrated linear relationships of CX1 and CX2/CX1 with age, whereas CX2 was better described as having a curvilinear relationship with age. These data indicate that complexin expression increases during synaptic maturation in human DLPFC and that an increase in the influence of inhibitory synapses relative to that of excitatory synapses occurs during development in this cortical region.

A role for soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex dimerization during neurosecretion

The interactions underlying the cooperativity of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes during neurotransmission are not known. Here, we provide a molecular characterization of a dimer formed between the cytoplasmic portions of neuronal SNARE complexes. Dimerization generates a two-winged structure in which the C termini of cytosolic SNARE complexes are in apposition, and it involves residues from the vesicle-associated SNARE synaptobrevin 2 that lie close to the cytosol-membrane interface within the full-length protein. Mutation of these residues reduces stability of dimers formed between SNARE complexes, without affecting the stability of each individual SNARE complex. These mutations also cause a corresponding decrease in the ability of botulinum toxin-resistant synaptobrevin 2 to rescue regulated exocytosis in toxin-treated neuroendocrine cells. Moreover, such synaptobrevin 2 mutants give rise to a dominant-negative inhibition of exocytosis. These data are consistent with an important role for SNARE complex dimers in neurosecretion.

Complexins facilitate neurotransmitter release at excitatory and inhibitory synapses in mammalian central nervous system

Complexins (Cplxs) are key regulators of synaptic exocytosis, but whether they act as facilitators or inhibitors is currently being disputed controversially. We show that genetic deletion of all Cplxs expressed in the mouse brain causes a reduction in Ca(2+)-triggered and spontaneous neurotransmitter release at both excitatory and inhibitory synapses. Our results demonstrate that at mammalian central nervous system synapses, Cplxs facilitate neurotransmitter release and do not simply act as inhibitory clamps of the synaptic vesicle fusion machinery.

Glucose/oxygen deprivation and reperfusion upregulate SNAREs and complexin in organotypic hippocampal slice cultures

Brain ischemia activates Ca(2+)-dependent synaptic vesicle exocytosis. The synaptosomal-associated protein 25 (SNAP-25) and syntaxin proteins, located on presynaptic terminals, are components of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex and play a key role in regulating exocytosis. Changes in the expression of SNAREs could affect SNARE complex formation, fusion of vesicles with the presynaptic membrane, and release of neurotransmitters through exocytosis. To investigate the relationship of glucose/oxygen deprivation (GOD)/reperfusion-induced neuronal damage and alteration of presynaptic function, we examined the expression of SNAREs and complexin during GOD and reperfusion using organotypic hippocampal slice cultures. Microtubule-associated protein 2 (MAP-2) staining and transmission electron microscopy showed that neuronal damage increased in a time-dependent manner and both types of neuronal death can occur at different times during GOD and reperfusion. The immunoreactivity of SNAREs such as SNAP-25, vesicle-associated membrane protein and syntaxin and complexin increased in pyramidal cell bodies in the CA1 and CA3 areas in a time-dependent manner following reperfusion. Our data suggest that alteration of presynaptic function may play a partial role in delayed neuronal death during GOD and reperfusion in organotypic hippocampal slice cultures.

For better or for worse: complexins regulate SNARE function and vesicle fusion

In contrast to constitutive secretion, SNARE-mediated synaptic vesicle fusion is controlled by multiple regulatory proteins, which determine the Ca(2+) sensitivity of the vesicle fusion process and the speed of excitation-secretion coupling. Complexins are among the best characterized SNARE regulators known to date. They operate by binding to trimeric SNARE complexes consisting of the vesicle protein synaptobrevin and the plasma membrane proteins syntaxin and SNAP-25. The question as to whether complexins facilitate or inhibit SNARE-mediated fusion processes is currently a matter of significant controversy. This is mainly because of the fact that biochemical experiments in vitro and studies on vertebrate complexins in vivo have yielded apparently contradictory results. In this review, I provide a summary of available data on the role of complexins in SNARE-mediated vesicle fusion and attempt to define a model of complexin function that incorporates evidence for both facilitatory and inhibitory roles of complexins in SNARE-mediated fusion.

Accessory proteins stabilize the acceptor complex for synaptobrevin, the 1:1 syntaxin/SNAP-25 complex

Syntaxin/SNAP-25 interactions precede assembly of the ternary SNARE complex that is essential for neurotransmitter release. This binary complex has been difficult to characterize by bulk methods because of the prevalence of a 2:1 dead-end species. Here, using single-molecule fluorescence, we find the structure of the 1:1 syntaxin/SNAP-25 binary complex is variable, with states changing on the second timescale. One state corresponds to a parallel three-helix bundle, whereas other states show one of the SNAP-25 SNARE domains dissociated. Adding synaptobrevin suppresses the dissociated helix states. Remarkably, upon addition of complexin, Munc13, Munc18, or synaptotagmin, a similar effect is observed. Thus, the 1:1 binary complex is a dynamic acceptor for synaptobrevin binding, and accessory proteins stabilize this acceptor. In the cellular environment the binary complex is actively maintained in a configuration where it can rapidly interact with synaptobrevin, so formation is not likely a limiting step for neurotransmitter release.

Photolysis of a caged peptide reveals rapid action of N-ethylmaleimide sensitive factor before neurotransmitter release

The time at which the N-ethylmaleimide-sensitive factor (NSF) acts during synaptic vesicle (SV) trafficking was identified by time-controlled perturbation of NSF function with a photoactivatable inhibitory peptide. Photolysis of this caged peptide in the squid giant presynaptic terminal caused an abrupt (0.2 s) slowing of the kinetics of the postsynaptic current (PSC) and a more gradual (2-3 s) reduction in PSC amplitude. Based on the rapid rate of these inhibitory effects relative to the speed of SV recycling, we conclude that NSF functions in reactions that immediately precede neurotransmitter release. Our results indicate the locus of SNARE protein recycling in presynaptic terminals and reveal NSF as a potential target for rapid regulation of transmitter release.

Tag team action at the synapse

Communication between neurons relies on chemical synapses and the release of neurotransmitters into the synaptic cleft. Neurotransmitter release is an exquisitely regulated membrane fusion event that requires the linking of an electrical nerve stimulus to Ca(2+) influx, which leads to the fusion of neurotransmitter-filled vesicles with the cell membrane. The timing of neurotransmitter release is controlled through the regulation of the soluble N-ethylmaleimide sensitive factor attachment receptor (SNARE) proteins-the core of the membrane fusion machinery. Assembly of the fusion-competent SNARE complex is regulated by several neuronal proteins, including complexin and the Ca(2+)-sensor synaptotagmin. Both complexin and synaptotagmin bind directly to SNAREs, but their mechanism of action has so far remained unclear. Recent studies revealed that synaptotagmin-Ca(2+) and complexin collaborate to regulate membrane fusion. These compelling new results provide a molecular mechanistic insight into the functions of both proteins: complexin 'clamps' the SNARE complex in a pre-fusion intermediate, which is then released by the action of Ca(2+)-bound synaptotagmin to trigger rapid fusion.

RNA editing in regulating gene expression in the brain

Adenosine to inosine RNA editing, catalyzed by Adenosine Deaminases Acting on RNA (ADARs), represents an evolutionary conserved post-transcriptional mechanism which harnesses RNA structures to produce proteins that are not literally encoded in the genome. The species-specific alteration of functionally important residues in a multitude of neuronal ion channels and pre-synaptic proteins through RNA editing has been shown to have profound importance for normal nervous system function in a wide range of invertebrate and vertebrate model organisms. ADARs have also been shown to regulate neuronal gene expression through a remarkable variety of disparate processes, including modulation of the RNAi pathway, the creation of alternative splice sites, and the abolition of stop codons. In addition, ADARs have recently been revealed to have a novel role in the primate lineage: the widespread editing of Alu elements, which comprise approximately 10% of the human genome. Thus, as well as enabling the cell-specific regulation of RNAi and selfish genetic elements, the unshackling of the proteome from the constraints of the genome through RNA editing may have been fundamental to the evolution of complex behavior.

Modification of a hydrophobic layer by a point mutation in syntaxin 1A regulates the rate of synaptic vesicle fusion

Both constitutive secretion and Ca²⁺-regulated exocytosis require the assembly of the soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE) complexes. At present, little is known about how the SNARE complexes mediating these two distinct pathways differ in structure. Using the Drosophila neuromuscular synapse as a model, we show that a mutation modifying a hydrophobic layer in syntaxin 1A regulates the rate of vesicle fusion. Syntaxin 1A molecules share a highly conserved threonine in the C-terminal +7 layer near the transmembrane domain. Mutation of this threonine to isoleucine results in a structural change that more closely resembles those found in syntaxins ascribed to the constitutive secretory pathway. Flies carrying the I254 mutant protein have increased levels of SNARE complexes and dramatically enhanced rate of both constitutive and evoked vesicle fusion. In contrast, overexpression of the T254 wild-type protein in neurons reduces vesicle fusion only in the I254 mutant background. These results are consistent with molecular dynamics simulations of the SNARE core complex, suggesting that T254 serves as an internal brake to dampen SNARE zippering and impede vesicle fusion, whereas I254 favors fusion by enhancing intermolecular interaction within the SNARE core complex.

Most living cells constantly renew their membrane compositions and frequently communicate with neighboring cells by delivering cargo molecules from small vesicles. A key step in cargo delivery requires the fusion of the vesicle membrane with the target membrane mediated by SNARE proteins. In most cellular compartments, fusion occurs constitutively, requiring little participation of other molecules. In other cellular compartments, such as synapses in the nervous system, vesicle fusion is predominantly triggered by intracellular calcium ions. At present, constitutive and regulated fusion modes are not well understood.

In this study, we found that a mutant SNARE protein, syntaxin at the synapse, contained a building block commonly conserved for syntaxins functioning along constitutive secretory pathways. Further, our modeling predicted that the mutant syntaxin could form a tightly packed SNARE bundle closely resembling that found in the endosome, but differing from the relatively loosely packed bundle found at the wild-type synapse. Our experimental data support the hypothesis that the mutant syntaxin lowered the energy barrier for vesicle fusion by tightening the SNARE bundle. These findings reveal a novel, intrinsic structural feature of the SNARE complex that regulates vesicle fusion rate at different cellular compartments.

A syntaxin 1A threonine to isoleucine mutation is found to enhance SNARE complex formation and vesicle fusion. This structural change results in a syntaxin that resembles those in constitutive secretory pathways.

Synaptotagmin C2A loop 2 mediates Ca2+-dependent SNARE interactions essential for Ca2+-triggered vesicle exocytosis

Synaptotagmins contain tandem C2 domains and function as Ca(2+) sensors for vesicle exocytosis but the mechanism for coupling Ca(2+) rises to membrane fusion remains undefined. Synaptotagmins bind SNAREs, essential components of the membrane fusion machinery, but the role of these interactions in Ca(2+)-triggered vesicle exocytosis has not been directly assessed. We identified sites on synaptotagmin-1 that mediate Ca(2+)-dependent SNAP25 binding by zero-length cross-linking. Mutation of these sites in C2A and C2B eliminated Ca(2+)-dependent synaptotagmin-1 binding to SNAREs without affecting Ca(2+)-dependent membrane binding. The mutants failed to confer Ca(2+) regulation on SNARE-dependent liposome fusion and failed to restore Ca(2+)-triggered vesicle exocytosis in synaptotagmin-deficient PC12 cells. The results provide direct evidence that Ca(2+)-dependent SNARE binding by synaptotagmin is essential for Ca(2+)-triggered vesicle exocytosis and that Ca(2+)-dependent membrane binding by itself is insufficient to trigger fusion. A structure-based model of the SNARE-binding surface of C2A provided a new view of how Ca(2+)-dependent SNARE and membrane binding occur simultaneously.

A complexin fusion clamp regulates spontaneous neurotransmitter release and synaptic growth

Neuronal signaling occurs through both action potential-triggered synaptic vesicle fusion and spontaneous release, although the fusion clamp machinery that prevents premature exocytosis of synaptic vesicles in the absence of calcium is unknown. Here we demonstrate that spontaneous release at synapses is regulated by complexin, a SNARE complex-binding protein. Analysis of Drosophila melanogaster complexin null mutants showed a marked increase in spontaneous fusion and a profound overgrowth of synapses, suggesting that complexin functions as the fusion clamp in vivo and may modulate structural remodeling of neuronal connections by controlling the rate of spontaneous release.

Distinct domains of complexin I differentially regulate neurotransmitter release

Complexins constitute a family of four synaptic high-affinity SNARE complex binding proteins. They positively regulate a late, post-priming step in Ca²⁺-triggered synchronous neurotransmitter release, but the underlying molecular mechanisms are unclear. We show here that SNARE complex binding of Complexin I via its central α-helix is necessary but unexpectedly not sufficient for its key function in promoting neurotransmitter release. An accessory α-helix N-terminal of the SNARE complex binding region plays an inhibitory role in fast synaptic exocytosis, while its N-terminally adjacent sequences facilitate Ca²⁺-triggered release even in the absence of the Ca²⁺ sensor Synaptotagmin 1. Our results indicate that distinct functional domains of Complexins differentially regulate synaptic exocytosis, and that via the interplay between these domains Complexins play a crucial role in fine-tuning Ca²⁺-triggered fast neurotransmitter release.