α-SNAP interferes with the zippering of the SNARE protein membrane fusion machinery

PIP2 Is An Electrostatic Catalyst for Vesicle Fusion by Lowering the Hydration Energy: Arresting Vesicle Fusion by Masking PIP2

Lipids are key factors in regulating membrane fusion. Lipids are not only structural components to form membranes but also active catalysts for vesicle fusion and neurotransmitter release, which are driven by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. SNARE proteins seem to be partially assembled before fusion, but the mechanisms that arrest vesicle fusion before Ca2+ influx are still not clear. Here, we show that phosphatidylinositol 4,5-bisphosphate (PIP2) electrostatically triggers vesicle fusion as an electrostatic catalyst by lowering the hydration energy and that a myristoylated alanine-rich C-kinase substrate (MARCKS), a PIP2-binding protein, arrests vesicle fusion in a vesicle docking state where the SNARE complex is partially assembled. Vesicle-mimicking liposomes fail to reproduce vesicle fusion arrest by masking PIP2, indicating that native vesicles are essential for the reconstitution of physiological vesicle fusion. PIP2 attracts cations to repel water molecules from membranes, thus lowering the hydration energy barrier.

Munc18c accelerates SNARE-dependent membrane fusion in the presence of regulatory proteins α-SNAP and NSF

Intracellular vesicle fusion is driven by the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and their cofactors, including Sec1/Munc18 (SM), α-SNAP, and NSF. α-SNAP and NSF play multiple layers of regulatory roles in the SNARE assembly, disassembling the cis-SNARE complex and the prefusion SNARE complex. How SM proteins coupled with NSF and α-SNAP regulate SNARE-dependent membrane fusion remains incompletely understood. Munc18c, an SM protein involved in the exocytosis of the glucose transporter GLUT4, binds and activates target (t-) SNAREs to accelerate the fusion reaction through a SNARE-like peptide (SLP). Here, using an in vitro reconstituted system, we discovered that α-SNAP blocks the GLUT4 SNAREs-mediated membrane fusion. Munc18c interacts with t-SNAREs to displace α-SNAP, which overcomes the fusion inhibition. Furthermore, Munc18c shields the trans-SNARE complex from NSF/α-SNAP-mediated disassembly and accelerates SNARE-dependent fusion kinetics in the presence of NSF and α-SNAP. The SLP in domain 3a is indispensable in Munc18c-assisted resistance to NSF and α-SNAP. Together, our findings demonstrate that Munc18c protects the prefusion SNARE complex from α-SNAP and NSF, promoting SNARE-dependent membrane fusion through its SLP.

Requirement of Cholesterol for Calcium-Dependent Vesicle Fusion by Strengthening Synaptotagmin-1-Induced Membrane Bending

Cholesterol is essential for neuronal activity and function. Cholesterol depletion in the plasma membrane impairs synaptic transmission. However, the molecular mechanisms by which cholesterol deficiency leads to defects in vesicle fusion remain poorly understood. Here, it is shown that cholesterol is required for Ca²⁺ -dependent native vesicle fusion using the in vitro reconstitution of fusion and amperometry to monitor exocytosis in chromaffin cells. Purified native vesicles are crucial for the reconstitution of physiological Ca²⁺ -dependent fusion, because vesicle-mimicking liposomes fail to reproduce the cholesterol effect. Intriguingly, cholesterol has no effect on the membrane binding of synaptotagmin-1, a Ca²⁺ sensor for ultrafast fusion. Cholesterol strengthens local membrane deformation and bending induced by synaptotagmin-1, thereby lowering the energy barrier for Ca²⁺ -dependent fusion to occur. The data provide evidence that cholesterol depletion abolishes Ca²⁺ -dependent vesicle fusion by disrupting synaptotagmin-1-induced membrane bending, and suggests that cholesterol is an essential lipid regulator for Ca²⁺ -dependent fusion.

Alpha-SNAP (M105I) mutation promotes neuronal differentiation of neural stem/progenitor cells through overactivation of AMPK

Background: The M105I point mutation in α-SNAP (Soluble N-ethylmaleimide-sensitive factor attachment protein-alpha) leads in mice to a complex phenotype known as hyh (hydrocephalus with hop gait), characterized by cortical malformation and hydrocephalus, among other neuropathological features. Studies performed by our laboratory and others support that the hyh phenotype is triggered by a primary alteration in embryonic neural stem/progenitor cells (NSPCs) that leads to a disruption of the ventricular and subventricular zones (VZ/SVZ) during the neurogenic period. Besides the canonical role of α-SNAP in SNARE-mediated intracellular membrane fusion dynamics, it also negatively modulates AMP-activated protein kinase (AMPK) activity. AMPK is a conserved metabolic sensor associated with the proliferation/differentiation balance in NSPCs. Methods: Brain samples from hyh mutant mice (hydrocephalus with hop gait) (B6C3Fe-a/a-Napahyh/J) were analyzed by light microscopy, immunofluorescence, and Western blot at different developmental stages. In addition, NSPCs derived from WT and hyh mutant mice were cultured as neurospheres for in vitro characterization and pharmacological assays. BrdU labeling was used to assess proliferative activity in situ and in vitro. Pharmacological modulation of AMPK was performed using Compound C (AMPK inhibitor) and AICAR (AMPK activator). Results: α-SNAP was preferentially expressed in the brain, showing variations in the levels of α-SNAP protein in different brain regions and developmental stages. NSPCs from hyh mice (hyh-NSPCs) displayed reduced levels of α-SNAP and increased levels of phosphorylated AMPKα (pAMPKα^Thr172), which were associated with a reduction in their proliferative activity and a preferential commitment with the neuronal lineage. Interestingly, pharmacological inhibition of AMPK in hyh-NSPCs increased proliferative activity and completely abolished the increased generation of neurons. Conversely, AICAR-mediated activation of AMPK in WT-NSPCs reduced proliferation and boosted neuronal differentiation. Discussion: Our findings support that α-SNAP regulates AMPK signaling in NSPCs, further modulating their neurogenic capacity. The naturally occurring M105I mutation of α-SNAP provokes an AMPK overactivation in NSPCs, thus connecting the α-SNAP/AMPK axis with the etiopathogenesis and neuropathology of the hyh phenotype.

Electrostatic regulation of the cis- and trans-membrane interactions of synaptotagmin-1

Synaptotagmin-1 is a vesicular protein and Ca²⁺ sensor for Ca²⁺-dependent exocytosis. Ca²⁺ induces synaptotagmin-1 binding to its own vesicle membrane, called the cis-interaction, thus preventing the trans-interaction of synaptotagmin-1 to the plasma membrane. However, the electrostatic regulation of the cis- and trans-membrane interaction of synaptotagmin-1 was poorly understood in different Ca²⁺-buffering conditions. Here we provide an assay to monitor the cis- and trans-membrane interactions of synaptotagmin-1 by using native purified vesicles and the plasma membrane-mimicking liposomes (PM-liposomes). Both ATP and EGTA similarly reverse the cis-membrane interaction of synaptotagmin-1 in free [Ca²⁺] of 10-100 μM. High PIP₂ concentrations in the PM-liposomes reduce the Hill coefficient of vesicle fusion and synaptotagmin-1 membrane binding; this observation suggests that local PIP₂ concentrations control the Ca²⁺-cooperativity of synaptotagmin-1. Our data provide evidence that Ca²⁺ chelators, including EGTA and polyphosphate anions such as ATP, ADP, and AMP, electrostatically reverse the cis-interaction of synaptotagmin-1.

Molecular Mechanisms Underlying Neurotransmitter Release

Major recent advances and previous data have led to a plausible model of how key proteins mediate neurotransmitter release. In this model, the soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptor (SNARE) proteins syntaxin-1, SNAP-25, and synaptobrevin form tight complexes that bring the membranes together and are crucial for membrane fusion. NSF and SNAPs disassemble SNARE complexes and ensure that fusion occurs through an exquisitely regulated pathway that starts with Munc18-1 bound to a closed conformation of syntaxin-1. Munc18-1 also binds to synaptobrevin, forming a template to assemble the SNARE complex when Munc13-1 opens syntaxin-1 while bridging the vesicle and plasma membranes. Synaptotagmin-1 and complexin bind to partially assembled SNARE complexes, likely stabilizing them and preventing fusion until Ca²⁺ binding to synaptotagmin-1 causes dissociation from the SNARE complex and induces interactions with phospholipids that help trigger release. Although fundamental questions remain about the mechanism of membrane fusion, these advances provide a framework to investigate the mechanisms underlying presynaptic plasticity.

Selected tools to visualize membrane interactions

In the past decade, we developed various fluorescence-based methods for monitoring membrane fusion, membrane docking, distances between membranes, and membrane curvature. These tools were mainly developed using liposomes as model systems, which allows for the dissection of specific interactions mediated by, for example, fusion proteins. Here, we provide an overview of these methods, including two-photon fluorescence cross-correlation spectroscopy and intramembrane Förster energy transfer, with asymmetric labelling of inner and outer membrane leaflets and the calibrated use of transmembrane energy transfer to determine membrane distances below 10 nm. We discuss their application range and their limitations using examples from our work on protein-mediated vesicle docking and fusion.

Isolation of large dense-core vesicles from bovine adrenal medulla for functional studies

Large dense-core vesicles (LDCVs) contain a variety of neurotransmitters, proteins, and hormones such as biogenic amines and peptides, together with microRNAs (miRNAs). Isolation of LDCVs is essential for functional studies including vesicle fusion, vesicle acidification, monoamine transport, and the miRNAs stored in LDCVs. Although several methods were reported for purifying LDCVs, the final fractions are significantly contaminated by other organelles, compromising biochemical characterization. Here we isolated LDCVs (chromaffin granules) with high yield and purity from bovine adrenal medulla. The fractionation protocol combines differential and continuous sucrose gradient centrifugation, allowing for reducing major contaminants such as mitochondria. Purified LDCVs show robust acidification by the endogenous V-ATPase and undergo SNARE-mediated fusion with artificial membranes. Interestingly, LDCVs contain specific miRNAs such as miR-375 and miR-375 is stabilized by protein complex against RNase A. This protocol can be useful in research on the biological functions of LDCVs.

Munc18-1 is crucial to overcome the inhibition of synaptic vesicle fusion by αSNAP

Munc18-1 and Munc13-1 orchestrate assembly of the SNARE complex formed by syntaxin-1, SNAP-25 and synaptobrevin, allowing exquisite regulation of neurotransmitter release. Non-regulated neurotransmitter release might be prevented by αSNAP, which inhibits exocytosis and SNARE-dependent liposome fusion. However, distinct mechanisms of inhibition by αSNAP were suggested, and it is unknown how such inhibition is overcome. Using liposome fusion assays, FRET and NMR spectroscopy, here we provide a comprehensive view of the mechanisms underlying the inhibitory functions of αSNAP, showing that αSNAP potently inhibits liposome fusion by: binding to syntaxin-1, hindering Munc18-1 binding; binding to syntaxin-1-SNAP-25 heterodimers, precluding SNARE complex formation; and binding to trans-SNARE complexes, preventing fusion. Importantly, inhibition by αSNAP is avoided only when Munc18-1 binds first to syntaxin-1, leading to Munc18-1-Munc13-1-dependent liposome fusion. We propose that at least some of the inhibitory activities of αSNAP ensure that neurotransmitter release occurs through the highly-regulated Munc18-1-Munc13-1 pathway at the active zone.

Munc18-1 and Munc13-1 are key for the exquisite regulation of neurotransmitter release. Here biophysical experiments show how αSNAP inhibits liposome fusion mediated by the neuronal SNAREs and how Munc18-1 overcomes this inhibition, ensuring that release depends on Munc18-1 and Munc13-1.

Multiple factors maintain assembled trans-SNARE complexes in the presence of NSF and αSNAP

Neurotransmitter release requires formation of trans-SNARE complexes between the synaptic vesicle and plasma membranes, which likely underlies synaptic vesicle priming to a release-ready state. It is unknown whether Munc18-1, Munc13-1, complexin-1 and synaptotagmin-1 are important for priming because they mediate trans-SNARE complex assembly and/or because they prevent trans-SNARE complex disassembly by NSF-αSNAP, which can lead to de-priming. Here we show that trans-SNARE complex formation in the presence of NSF-αSNAP requires both Munc18-1 and Munc13-1, as proposed previously, and is facilitated by synaptotagmin-1. Our data also show that Munc18-1, Munc13-1, complexin-1 and likely synaptotagmin-1 contribute to maintaining assembled trans-SNARE complexes in the presence of NSF-αSNAP. We propose a model whereby Munc18-1 and Munc13-1 are critical not only for mediating vesicle priming but also for precluding de-priming by preventing trans-SNARE complex disassembly; in this model, complexin-1 also impairs de-priming, while synaptotagmin-1 may assist in priming and hinder de-priming.

Single-Molecule Optical Tweezers Study of Regulated SNARE Assembly

Intracellular membrane fusion mediates material and information exchange among different cells or cellular compartments with high accuracy and spatiotemporal resolution. Fusion is driven by ordered folding and assembly of soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptors (SNAREs) and regulated by many other proteins. Understanding regulated SNARE assembly is key to dissecting mechanisms and physiologies of various fusion processes and their associated diseases. Yet, it remains challenging to study regulated SNARE assembly using traditional ensemble-based experimental approaches. Here, we describe our new method to measure the energy and kinetics of neuronal SNARE assembly in the presence of α-SNAP, using a single-molecule manipulation approach based on high-resolution optical tweezers. Detailed experimental protocols and methods of data analysis are shown. This approach can be widely applied to elucidate the effects of regulatory proteins on SNARE assembly and membrane fusion.

Mechanism of neurotransmitter release coming into focus

Research for three decades and major recent advances have provided crucial insights into how neurotransmitters are released by Ca²⁺ -triggered synaptic vesicle exocytosis, leading to reconstitution of basic steps that underlie Ca²⁺ -dependent membrane fusion and yielding a model that assigns defined functions for central components of the release machinery. The soluble N-ethyl maleimide sensitive factor attachment protein receptors (SNAREs) syntaxin-1, SNAP-25, and synaptobrevin-2 form a tight SNARE complex that brings the vesicle and plasma membranes together and is key for membrane fusion. N-ethyl maleimide sensitive factor (NSF) and soluble NSF attachment proteins (SNAPs) disassemble the SNARE complex to recycle the SNAREs for another round of fusion. Munc18-1 and Munc13-1 orchestrate SNARE complex formation in an NSF-SNAP-resistant manner by a mechanism whereby Munc18-1 binds to synaptobrevin and to a self-inhibited "closed" conformation of syntaxin-1, thus forming a template to assemble the SNARE complex, and Munc13-1 facilitates assembly by bridging the vesicle and plasma membranes and catalyzing opening of syntaxin-1. Synaptotagmin-1 functions as the major Ca²⁺ sensor that triggers release by binding to membrane phospholipids and to the SNAREs, in a tight interplay with complexins that accelerates membrane fusion. Many of these proteins act as both inhibitors and activators of exocytosis, which is critical for the exquisite regulation of neurotransmitter release. It is still unclear how the actions of these various proteins and multiple other components that control release are integrated and, in particular, how they induce membrane fusion, but it can be expected that these fundamental questions can be answered in the near future, building on the extensive knowledge already available.

Arrest of trans-SNARE zippering uncovers loosely and tightly docked intermediates in membrane fusion

Soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE) proteins mediate intracellular membrane fusion in the secretory pathway. They contain conserved regions, termed SNARE motifs, that assemble between opposing membranes directionally from their N termini to their membrane-proximal C termini in a highly exergonic reaction. However, how this energy is utilized to overcome the energy barriers along the fusion pathway is still under debate. Here, we have used mutants of the SNARE synaptobrevin to arrest trans-SNARE zippering at defined stages. We have uncovered two distinct vesicle docking intermediates where the membranes are loosely and tightly connected, respectively. The tightly connected state is irreversible and independent of maintaining assembled SNARE complexes. Together, our results shed new light on the intermediate stages along the pathway of membrane fusion.

The heptad repeat domain 1 of Mitofusin has membrane destabilization function in mitochondrial fusion

Mitochondria are double-membrane-bound organelles that constantly change shape through membrane fusion and fission. Outer mitochondrial membrane fusion is controlled by Mitofusin, whose molecular architecture consists of an N-terminal GTPase domain, a first heptad repeat domain (HR1), two transmembrane domains, and a second heptad repeat domain (HR2). The mode of action of Mitofusin and the specific roles played by each of these functional domains in mitochondrial fusion are not fully understood. Here, using a combination of in situ and in vitro fusion assays, we show that HR1 induces membrane fusion and possesses a conserved amphipathic helix that folds upon interaction with the lipid bilayer surface. Our results strongly suggest that HR1 facilitates membrane fusion by destabilizing the lipid bilayer structure, notably in membrane regions presenting lipid packing defects. This mechanism for fusion is thus distinct from that described for the heptad repeat domains of SNARE and viral proteins, which assemble as membrane-bridging complexes, triggering close membrane apposition and fusion, and is more closely related to that of the C-terminal amphipathic tail of the Atlastin protein.

Sec17 (α-SNAP) and an SM-tethering complex regulate the outcome of SNARE zippering in vitro and in vivo

Zippering of SNARE complexes spanning docked membranes is essential for most intracellular fusion events. Here, we explore how SNARE regulators operate on discrete zippering states. The formation of a metastable trans-complex, catalyzed by HOPS and its SM subunit Vps33, is followed by subsequent zippering transitions that increase the probability of fusion. Operating independently of Sec18 (NSF) catalysis, Sec17 (α-SNAP) either inhibits or stimulates SNARE-mediated fusion. If HOPS or Vps33 are absent, Sec17 inhibits fusion at an early stage. Thus, Vps33/HOPS promotes productive SNARE assembly in the presence of otherwise inhibitory Sec17. Once SNAREs are partially zipped, Sec17 promotes fusion in either the presence or absence of HOPS, but with faster kinetics when HOPS is absent, suggesting that ejection of the SM is a rate-limiting step.

Energetics, kinetics, and pathway of SNARE folding and assembly revealed by optical tweezers

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are universal molecular engines that drive membrane fusion. Particularly, synaptic SNAREs mediate fast calcium-triggered fusion of neurotransmitter-containing vesicles with plasma membranes for synaptic transmission, the basis of all thought and action. During membrane fusion, complementary SNAREs located on two apposed membranes (often called t- and v-SNAREs) join together to assemble into a parallel four-helix bundle, releasing the energy to overcome the energy barrier for fusion. A long-standing hypothesis suggests that SNAREs act like a zipper to draw the two membranes into proximity and thereby force them to fuse. However, a quantitative test of this SNARE zippering hypothesis was hindered by difficulties to determine the energetics and kinetics of SNARE assembly and to identify the relevant folding intermediates. Here, we first review different approaches that have been applied to study SNARE assembly and then focus on high-resolution optical tweezers. We summarize the folding energies, kinetics, and pathways of both wild-type and mutant SNARE complexes derived from this new approach. These results show that synaptic SNAREs assemble in four distinct stages with different functions: slow N-terminal domain association initiates SNARE assembly; a middle domain suspends and controls SNARE assembly; and rapid sequential zippering of the C-terminal domain and the linker domain directly drive membrane fusion. In addition, the kinetics and pathway of the stagewise assembly are shared by other SNARE complexes. These measurements prove the SNARE zippering hypothesis and suggest new mechanisms for SNARE assembly regulated by other proteins.

Gasotransmitter Heterocellular Signaling

SIGNIFICANCE

The family of gasotransmitter molecules, nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H₂S), has emerged as an important mediator of numerous cellular signal transduction and pathophysiological responses. As such, these molecules have been reported to influence a diverse array of biochemical, molecular, and cell biology events often impacting one another. Recent Advances: Discrete regulation of gasotransmitter molecule formation, movement, and reaction is critical to their biological function. Due to the chemical nature of these molecules, they can move rapidly throughout cells and tissues acting on targets through reactions with metal groups, reactive chemical species, and protein amino acids.

CRITICAL ISSUES

Given the breadth and complexity of gasotransmitter reactions, this field of research is expanding into exciting, yet sometimes confusing, areas of study with significant promise for understanding health and disease. The precise amounts of tissue and cellular gasotransmitter levels and where they are formed, as well as how they react with molecular targets or themselves, all remain poorly understood.

FUTURE DIRECTIONS

Elucidation of specific molecular targets, characteristics of gasotransmitter molecule heterotypic interactions, and spatiotemporal formation and metabolism are all important to better understand their true pathophysiological importance in various organ systems. Antioxid. Redox Signal. 26, 936-960.

Na+ influx via Orai1 inhibits intracellular ATP-induced mTORC2 signaling to disrupt CD4 T cell gene expression and differentiation

T cell effector functions require sustained calcium influx. However, the signaling and phenotypic consequences of non-specific sodium permeation via calcium channels remain unknown. α-SNAP is a crucial component of Orai1 channels, and its depletion disrupts the functional assembly of Orai1 multimers. Here we show that α-SNAP hypomorph, hydrocephalus with hopping gait, Napa^hyh/hyh mice harbor significant defects in CD4 T cell gene expression and Foxp3 regulatory T cell (Treg) differentiation. Mechanistically, TCR stimulation induced rapid sodium influx in Napa^hyh/hyh CD4 T cells, which reduced intracellular ATP, [ATP]_i. Depletion of [ATP]_i inhibited mTORC2 dependent NFκB activation in Napa^hyh/hyh cells but ablation of Orai1 restored it. Remarkably, TCR stimulation in the presence of monensin phenocopied the defects in Napa^hyh/hyh signaling and Treg differentiation, but not IL-2 expression. Thus, non-specific sodium influx via bonafide calcium channels disrupts unexpected signaling nodes and may provide mechanistic insights into some divergent phenotypes associated with Orai1 function.

DOI:http://dx.doi.org/10.7554/eLife.25155.001

An activated Q-SNARE/SM protein complex as a possible intermediate in SNARE assembly

Assembly of the SNARE proteins syntaxin1, SNAP25, and synaptobrevin into a SNARE complex is essential for exocytosis in neurons. For efficient assembly, SNAREs interact with additional proteins but neither the nature of the intermediates nor the sequence of protein assembly is known. Here, we have characterized a ternary complex between syntaxin1, SNAP25, and the SM protein Munc18-1 as a possible acceptor complex for the R-SNARE synaptobrevin. The ternary complex binds synaptobrevin with fast kinetics, resulting in the rapid formation of a fully zippered SNARE complex to which Munc18-1 remains tethered by the N-terminal domain of syntaxin1. Intriguingly, only one of the synaptobrevin truncation mutants (Syb1-65) was able to bind to the syntaxin1:SNAP25:Munc18-1 complex, suggesting either a cooperative zippering mechanism that proceeds bidirectionally or the progressive R-SNARE binding via an SM template. Moreover, the complex is resistant to disassembly by NSF Based on these findings, we consider the ternary complex as a strong candidate for a physiological intermediate in SNARE assembly.

MicroRNA exocytosis by large dense-core vesicle fusion

Neurotransmitters and peptide hormones are secreted into outside the cell by a vesicle fusion process. Although non-coding RNA (ncRNA) that include microRNA (miRNA) regulates gene expression inside the cell where they are transcribed, extracellular miRNA has been recently discovered outside the cells, proposing that miRNA might be released to participate in cell-to-cell communication. Despite its importance of extracellular miRNA, the molecular mechanisms by which miRNA can be stored in vesicles and released by vesicle fusion remain enigmatic. Using next-generation sequencing, vesicle purification techniques, and synthetic neurotransmission, we observe that large dense-core vesicles (LDCVs) contain a variety of miRNAs including miR-375. Furthermore, miRNA exocytosis is mediated by the SNARE complex and accelerated by Ca²⁺. Our results suggest that miRNA can be a novel neuromodulator that can be stored in vesicles and released by vesicle fusion together with classical neurotransmitters.

Review: Progresses in understanding N-ethylmaleimide sensitive factor (NSF) mediated disassembly of SNARE complexes

N-ethylmaleimide sensitive factor (NSF) is a key protein of intracellular membrane traffic. NSF is a highly conserved protein belonging to the ATPases associated with other activities (AAA+ proteins). AAA+ share common domains and all transduce ATP hydrolysis into major conformational movements that are used to carry out conformational work on client proteins. Together with its cofactor SNAP, NSF is specialized on disassembling highly stable SNARE complexes that form after each membrane fusion event. Although essential for all eukaryotic cells, however, the details of this reaction have long been enigmatic. Recently, major progress has been made in both elucidating the structure of NSF/SNARE complexes and in understanding the reaction mechanism. Advances in both cryo EM and single molecule measurements suggest that NSF, together with its cofactor SNAP, imposes a tight grip on the SNARE complex. After ATP hydrolysis and phosphate release, it then builds up mechanical tension that is ultimately used to rip apart the SNAREs in a single burst. Because the AAA domains are extremely well-conserved, the molecular mechanism elucidated for NSF is presumably shared by many other AAA+ ATPases. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 518-531, 2016.

Chaperoning SNARE assembly and disassembly

Intracellular membrane fusion is mediated in most cases by membrane-bridging complexes of soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). However, the assembly of such complexes in vitro is inefficient, and their uncatalysed disassembly is undetectably slow. Here, we focus on the cellular machinery that orchestrates assembly and disassembly of SNARE complexes, thereby regulating processes ranging from vesicle trafficking to organelle fusion to neurotransmitter release. Rapid progress is being made on many fronts, including the development of more realistic cell-free reconstitutions, the application of single-molecule biophysics, and the elucidation of X-ray and high-resolution electron microscopy structures of the SNARE assembly and disassembly machineries 'in action'.

α-SNAP Enhances SNARE Zippering by Stabilizing the SNARE Four-Helix Bundle

Intracellular membrane fusion is mediated by dynamic assembly and disassembly of soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptors (SNAREs). α-SNAP guides NSF to disassemble SNARE complexes after membrane fusion. Recent experiments showed that α-SNAP also dramatically enhances SNARE assembly and membrane fusion. How α-SNAP is involved in these opposing activities is not known. Here, we examine the effect of α-SNAP on the stepwise assembly of the synaptic SNARE complex using optical tweezers. We found that α-SNAP destabilized the linker domain (LD) of the SNARE complex but stabilized its C-terminal domain (CTD) through a conformational selection mechanism. In contrast, α-SNAP minimally affected assembly of the SNARE N-terminal domain (NTD), indicating that α-SNAP barely bound the partially assembled trans-SNARE complex. Thus, α-SNAP recognizes the folded CTD for SNARE disassembly with NSF and subtly modulates membrane fusion by altering the stabilities of the SNARE CTD and LD.

Reconstituting Intracellular Vesicle Fusion Reactions: The Essential Role of Macromolecular Crowding

Intracellular vesicle fusion is mediated by SNAREs and Sec1/Munc18 (SM) proteins. Despite intensive efforts, the SNARE-SM mediated vesicle fusion reaction has not been faithfully reconstituted in biochemical assays. Here, we present an unexpected discovery that macromolecular crowding is required for reconstituting the vesicle fusion reaction in vitro. Macromolecular crowding is known to profoundly influence the kinetic and thermodynamic behaviors of macromolecules, but its role in membrane transport processes such as vesicle fusion remains unexplored. We introduced macromolecular crowding agents into reconstituted fusion reactions to mimic the crowded cellular environment. In this crowded assay, SNAREs and SM proteins acted in concert to drive efficient membrane fusion. In uncrowded assays, by contrast, SM proteins failed to associate with the SNAREs and the fusion rate decreased more than 30-fold, close to undetectable levels. The activities of SM proteins were strictly specific to their cognate SNARE isoforms and sensitive to biologically relevant mutations, further supporting that the crowded fusion assay accurately recapitulates the vesicle fusion reaction. Using this crowded fusion assay, we also showed that the SNARE-SM mediated fusion reaction can be modulated by two additional factors: NSF and α-SNAP. These findings suggest that the vesicle fusion machinery likely has been evolutionarily selected to function optimally in the crowded milieu of the cell. Accordingly, macromolecular crowding should constitute an integral element of any reconstituted fusion assay.

Synaptotagmin-1 binds to PIP(2)-containing membrane but not to SNAREs at physiological ionic strength

The Ca(2+) sensor synaptotagmin-1 is thought to trigger membrane fusion by binding to acidic membrane lipids and SNARE proteins. Previous work has shown that binding is mediated by electrostatic interactions that are sensitive to the ionic environment. However, the influence of divalent or polyvalent ions, at physiological concentrations, on synaptotagmin's binding to membranes or SNAREs has not been explored. Here we show that binding of rat synaptotagmin-1 to membranes containing phosphatidylinositol 4,5-bisphosphate (PIP2) is regulated by charge shielding caused by the presence of divalent cations. Surprisingly, polyvalent ions such as ATP and Mg(2+) completely abrogate synaptotagmin-1 binding to SNAREs regardless of the presence of Ca(2+). Altogether, our data indicate that at physiological ion concentrations Ca(2+)-dependent synaptotagmin-1 binding is confined to PIP2-containing membrane patches in the plasma membrane, suggesting that membrane interaction of synaptotagmin-1 rather than SNARE binding triggers exocytosis of vesicles.

Differential Effects of Munc18s on Multiple Degranulation-Relevant Trans-SNARE Complexes

Mast cell exocytosis, which includes compound degranulation and vesicle-associated piecemeal degranulation, requires multiple Q- and R- SNAREs. It is not clear how these SNAREs pair to form functional trans-SNARE complexes and how these trans-SNARE complexes are selectively regulated for fusion. Here we undertake a comprehensive examination of the capacity of two Q-SNARE subcomplexes (syntaxin3/SNAP-23 and syntaxin4/SNAP-23) to form fusogenic trans-SNARE complexes with each of the four granule-borne R-SNAREs (VAMP2, 3, 7, 8). We report the identification of at least six distinct trans-SNARE complexes under enhanced tethering conditions: i) VAMP2/syntaxin3/SNAP-23, ii) VAMP2/syntaxin4/SNAP-23, iii) VAMP3/syntaxin3/SNAP-23, iv) VAMP3/syntaxin4/SNAP-23, v) VAMP8/syntaxin3/SNAP-23, and vi) VAMP8/syntaxin4/SNAP-23. We show for the first time that Munc18a operates synergistically with SNAP-23-based non-neuronal SNARE complexes (i to iv) in lipid mixing, in contrast to Munc18b and c, which exhibit no positive effect on any SNARE combination tested. Pre-incubation with Munc18a renders the SNARE-dependent fusion reactions insensitive to the otherwise inhibitory R-SNARE cytoplasmic domains, suggesting a protective role of Munc18a for its cognate SNAREs. Our findings substantiate the recently discovered but unexpected requirement for Munc18a in mast cell exocytosis, and implicate post-translational modifications in Munc18b/c activation.

Cortical Granule Exocytosis Is Mediated by Alpha-SNAP and N-Ethilmaleimide Sensitive Factor in Mouse Oocytes

Cortical granule exocytosis (CGE), also known as cortical reaction, is a calcium- regulated secretion that represents a membrane fusion process during meiotic cell division of oocytes. The molecular mechanism of membrane fusion during CGE is still poorly understood and is thought to be mediated by the SNARE pathway; nevertheless, it is unkown if SNAP (acronym for soluble NSF attachment protein) and NSF (acronym for N-ethilmaleimide sensitive factor), two key proteins in the SNARE pathway, mediate CGE in any oocyte model. In this paper, we documented the gene expression of α-SNAP, γ-SNAP and NSF in mouse oocytes. Western blot analysis showed that the expression of these proteins maintains a similar level during oocyte maturation and early activation. Their localization was mainly observed at the cortical region of metaphase II oocytes, which is enriched in cortical granules. To evaluate the function of these proteins in CGE we set up a functional assay based on the quantification of cortical granules metaphase II oocytes activated parthenogenetically with strontium. Endogenous α-SNAP and NSF proteins were perturbed by microinjection of recombinant proteins or antibodies prior to CGE activation. The microinjection of wild type α-SNAP and the negative mutant of α-SNAP L294A in metaphase II oocytes inhibited CGE stimulated by strontium. NEM, an irreversibly inhibitor of NSF, and the microinjection of the negative mutant NSF D1EQ inhibited cortical reaction. The microinjection of anti-α-SNAP and anti-NSF antibodies was able to abolish CGE in activated metaphase II oocytes. The microinjection of anti-γ SNAP antibody had no effect on CGE. Our findings indicate, for the first time in any oocyte model, that α-SNAP, γ-SNAP, and NSF are expressed in mouse oocytes. We demonstrate that α-SNAP and NSF have an active role in CGE and propose a working model.

Spring-loaded unraveling of a single SNARE complex by NSF in one round of ATP turnover

During intracellular membrane trafficking, N-ethylmaleimide-sensitive factor (NSF) and alpha-soluble NSF attachment protein (α-SNAP) disassemble the soluble NSF attachment protein receptor (SNARE) complex for recycling of the SNARE proteins. The molecular mechanism by which NSF disassembles the SNARE complex is largely unknown. Using single-molecule fluorescence spectroscopy and magnetic tweezers, we found that NSF disassembled a single SNARE complex in only one round of adenosine triphosphate (ATP) turnover. Upon ATP cleavage, the NSF hexamer developed internal tension with dissociation of phosphate ions. After latent time measuring tens of seconds, NSF released the built-up tension in a burst within 20 milliseconds, resulting in disassembly followed by immediate release of the SNARE proteins. Thus, NSF appears to use a "spring-loaded" mechanism to couple ATP hydrolysis and unfolding of substrate proteins.

The proteins of exocytosis: lessons from the sperm model

Exocytosis is a highly regulated process that consists of multiple functionally, kinetically and/or morphologically definable stages such as recruitment, targeting, tethering and docking of secretory vesicles with the plasma membrane, priming of the fusion machinery and calcium-triggered membrane fusion. After fusion, the membrane around the secretory vesicle is incorporated into the plasma membrane and the granule releases its contents. The proteins involved in these processes belong to several highly conserved families: Rab GTPases, SNAREs (soluble NSF-attachment protein receptors), α-SNAP (α-NSF attachment protein), NSF (N-ethylmaleimide-sensitive factor), Munc13 and -18, complexins and synaptotagmins. In the present article, the molecules of exocytosis are reviewed, using human sperm as a model system. Sperm exocytosis is driven by isoforms of the same proteinaceous fusion machinery mentioned above, with their functions orchestrated in a hierarchically organized and unidirectional signalling cascade. In addition to the universal exocytosis regulator calcium, this cascade includes other second messengers such as diacylglycerol, inositol 1,4,5-trisphosphate and cAMP, as well as the enzymes that synthesize them and their target proteins. Of special interest is the cAMP-binding protein Epac (exchange protein directly activated by cAMP) due in part to its enzymatic activity towards Rap. The activation of Epac and Rap leads to a highly localized calcium signal which, together with assembly of the SNARE complex, governs the final stages of exocytosis. The source of this releasable calcium is the secretory granule itself.

To protect or reject

A protein known for its role in dismantling faulty SNARE complexes can also help to maintain complexes that have formed properly during membrane fusion.