Towards reconstitution of membrane fusion mediated by SNAREs and other synaptic proteins

Molecular structures and function of the autophagosome-lysosome fusion machinery

Macroautophagy (also known as autophagy) plays a pivotal role in maintaining cellular homeostasis. The terminal step of the multi-step autophagy degradation pathway involves fusion between the cargo-laden, double-membraned autophagosome and the lytic organelle lysosome/vacuole. Over the past decade, various core components of the molecular machinery that execute this critical terminal autophagy event have been identified. This review highlights recent advances in understanding the molecular structures, biochemical functions, and regulatory mechanisms of key components of this highly sophisticated machinery including the SNARE fusogens, tethering factors, Rab GTPases and associated guanine nucleotide exchange factors, and other accessory factors.

The Role of α-Synuclein in SNARE-mediated Synaptic Vesicle Fusion

Neuronal communication depends on exquisitely regulated membrane fusion between synaptic vesicles and presynaptic neurons, which results in neurotransmitter release in precisely timed patterns. Presynaptic dysfunctions are known to occur prior to the onset of neurodegenerative diseases, including Parkinson's disease. Synaptic accumulation of α-synuclein (α-Syn) oligomers has been implicated in the pathway leading to such outcomes. α-Syn oligomers exert aberrant effects on presynaptic fusion machinery through their interactions with synaptic vesicles and proteins. Here, we summarize in vitro bulk and single-vesicle assays for investigating the functions of α-Syn monomers and oligomers in synaptic vesicle fusion and then discuss the current understanding of the roles of α-Syn monomers and oligomers in synaptic vesicle fusion. Finally, we suggest a new therapeutic avenue specifically targeting the mechanisms of α-Syn oligomer toxicity rather than the oligomer itself.

The function of VAMP2 in mediating membrane fusion: An overview

Vesicle-associated membrane protein 2 (VAMP2, also known as synaptobrevin-2), encoded by VAMP2 in humans, is a key component of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex. VAMP2 combined with syntaxin-1A (SYX-1A) and synaptosome-associated protein 25 (SNAP-25) produces a force that induces the formation of fusion pores, thereby mediating the fusion of synaptic vesicles and the release of neurotransmitters. VAMP2 is largely unstructured in the absence of interaction partners. Upon interaction with other SNAREs, the structure of VAMP2 stabilizes, resulting in the formation of four structural domains. In this review, we highlight the current knowledge of the roles of the VAMP2 domains and the interaction between VAMP2 and various fusion-related proteins in the presynaptic cytoplasm during the fusion process. Our summary will contribute to a better understanding of the roles of the VAMP2 protein in membrane fusion.

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.

The Membrane Interaction of Alpha-Synuclein

A presynaptic protein closely related to Parkinson's disease (PD), α-synuclein (α-Syn), has been studied extensively regarding its pathogenic mechanisms. As a physiological protein in presynapses, however, α-Syn's physiological function remains unclear. Its location in nerve terminals and effects on membrane fusion also imply its functional role in synaptic transmission, including its possible interaction with high-curvature membranes via its N-terminus and amorphous C-terminus. PD-related mutants that disrupt the membrane interaction (e.g., A30P and G51D) additionally suggest a relationship between α-Syn's pathogenic mechanisms and physiological roles through the membrane binding. Here, we summarize recent research on how α-Syn and its variants interact with membranes and influence synaptic transmission. We list several membrane-related connections between the protein's physiological function and the pathological mechanisms that stand to expand current understandings of α-Syn.

In vitro single vesicle fusion assays based on pore-spanning membranes: merits and drawbacks

Neuronal fusion mediated by soluble N-ethylmaleimide-sensitive-factor attachment protein receptors (SNAREs) is a fundamental cellular process by which two initially distinct membranes merge resulting in one interconnected structure to release neurotransmitters into the presynaptic cleft. To get access to the different stages of the fusion process, several in vitro assays have been developed. In this review, we provide a short overview of the current in vitro single vesicle fusion assays. Among those assays, we developed a single vesicle assay based on pore-spanning membranes (PSMs) on micrometre-sized pores in silicon, which might overcome some of the drawbacks associated with the other membrane architectures used for investigating fusion processes. Prepared by spreading of giant unilamellar vesicles with reconstituted t-SNAREs, PSMs provide an alternative tool to supported lipid bilayers to measure single vesicle fusion events by means of fluorescence microscopy. Here, we discuss the diffusive behaviour of the reconstituted membrane components as well as that of the fusing synthetic vesicles with reconstituted synaptobrevin 2 (v-SNARE). We compare our results with those obtained if the synthetic vesicles are replaced by natural chromaffin granules under otherwise identical conditions. The fusion efficiency as well as the different fusion states observable in this assay by means of both lipid mixing and content release are illuminated.

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.

Fusion Pore Formation Observed during SNARE-Mediated Vesicle Fusion with Pore-Spanning Membranes

Planar pore-spanning membranes (PSMs) have been shown to be a versatile tool to resolve elementary steps of the neuronal fusion process. However, in previous studies, we monitored only lipid mixing between fusing large unilamellar vesicles and PSMs and did not gather information about the formation of fusion pores. To address this important step of the fusion process, we entrapped sulforhodamine B at self-quenching concentrations into large unilamellar vesicles containing the v-SNARE synaptobrevin 2, which were docked and fused with lipid-labeled PSMs containing the t-SNARE acceptor complex ΔN49 prepared on gold-coated porous silicon substrates. By dual-color spinning disk fluorescence microscopy with a time resolution of ∼20 ms, we could unambiguously distinguish between bursting vesicles, which was only rarely observed (<0.01%), and fusion pore formation. From the time-resolved dual-color fluorescence time traces, we were able to identify different fusion pathways, including remaining three-dimensional postfusion structures with released content and transient openings and closings of the fusion pores. Our results on fusion pore formation and lipid diffusion from the PSM into the fusing vesicle let us conclude that the content release, i.e., fusion pore formation after the merger of the two lipid membranes occurs almost simultaneously.

Fusion of Bipolar Tetraether Lipid Membranes Without Enhanced Leakage of Small Molecules

A major challenge in liposomal research is to minimize the leakage of encapsulated cargo from either uncontrolled passive permeability across the liposomal membrane or upon fusion with other membranes. We previously showed that liposomes made from pure Archaea-inspired bipolar tetraether lipids exhibit exceptionally low permeability of encapsulated small molecules due to their capability to form more tightly packed membranes compared to typical monopolar lipids. Here, we demonstrate that liposomes made of synthetic bipolar tetraether lipids can also undergo membrane fusion, which is commonly accompanied by content leakage of liposomes when using typical bilayer-forming lipids. Importantly, we demonstrate calcium-mediated fusion events between liposome made of glycerolmonoalkyl glycerol tetraether lipids with phosphatidic acid headgroups (GMGTPA) occur without liposome content release, which contrasts with liposomes made of bilayer-forming EggPA lipids that displayed ~80% of content release under the same fusogenic conditions. NMR spectroscopy studies of a deuterated analog of GMGTPA lipids reveal the presence of multiple rigid and dynamic conformations, which provide evidence for the possibility of these lipids to form intermediate states typically associated with membrane fusion events. The results support that biomimetic GMGT lipids possess several attractive properties (e.g., low permeability and non-leaky fusion capability) for further development in liposome-based technologies.

Molecular Mechanisms of Fast Neurotransmitter Release

This review summarizes current knowledge of synaptic proteins that are central to synaptic vesicle fusion in presynaptic active zones, including SNAREs (soluble N-ethylmaleimide sensitive factor attachment protein receptors), synaptotagmin, complexin, Munc18 (mammalian uncoordinated-18), and Munc13 (mammalian uncoordinated-13), and highlights recent insights in the cooperation of these proteins for neurotransmitter release. Structural and functional studies of the synaptic fusion machinery suggest new molecular models of synaptic vesicle priming and Ca²⁺-triggered fusion. These studies will be a stepping-stone toward answering the question of how the synaptic vesicle fusion machinery achieves such high speed and sensitivity.

Single-vesicle measurement of protein-induced membrane tethering

Functions of the proteins involved in membrane tethering, a crucial step in membrane trafficking, remain elusive due to the lack of effective tools to investigate protein-lipid interaction. To address this challenge, we introduce a method to study protein-induced membrane tethering via in vitro reconstitution of lipid vesicles, including detailed steps from the preparation of the PEGylated slides to the imaging of single vesicles. Furthermore, we demonstrate the measurement of protein-vesicle interaction in tethered vesicle pairs using two representative proteins, the cytoplasmic domain of synaptotagmin-1 (C2AB) and α-synuclein. Results from Förster (fluorescence) resonance energy transfer (FRET) reveal that membrane tethering is distinguished from membrane fusion. Single-vesicle measurement also allows for assessment of dose-dependent effects of proteins and ions on membrane tethering. We envision that the continuous development of advanced techniques in the single-vesicle measurement will enable the investigation of complex protein-membrane interactions in live cells or tissues.

Biochemical studies of membrane fusion at the single-particle level

To study membrane fusion mediated by synaptic proteins, proteoliposomes have been widely used for in vitro ensemble measurements with limited insights into the fusion mechanism. Single-particle techniques have proven to be powerful in overcoming the limitations of traditional ensemble methods. Here, we summarize current single-particle methods in biophysical and biochemical studies of fusion mediated by soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and other synaptic proteins, together with their advantages and limitations.

Single-Molecule Fluorescence Measurement of SNARE-Mediated Vesicle Fusion

This chapter expounds the single vesicle-vesicle fluorescence resonance energy transfer (FRET) measurement to study the membrane fusion mediated by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins. The formation of a four-α-helix bundle of SNARE proteins can drive two membranes to a close proximity for fusion. Through single-molecule FRET-based microscopy, the lipid-mixing process at the single-vesicle level can be tracked in real time. This reconstitution system is applicable to study the molecular mechanism of SNAREs during different membrane fusion stages, such as docking, hemifusion, and full fusion. Four main parts are described in this chapter, including SNARE reconstitution, imaging preparation, data collection, and analysis.

A potential mechanism for first-person internal sensation of memory provides evidence for the relationship between learning and LTP induction

Studies conducted to verify learning-induced changes anticipated from Hebb's postulate led to the finding of long-term potentiation (LTP). Even though several correlations have been found between behavioural markers of memory retrieval and LTP, it is not known how memories are retrieved using learning-induced changes. In this context, the following non-correlated findings between learning and LTP induction provide constraints for discovering the mechanism: 1) Requirement of high stimulus intensity for LTP induction in contrast to what is expected for a learning mechanism, 2) Delay of at least 20 to 30 s from stimulation to LTP induction, in contrast to mere milliseconds for associative learning, and 3) A sudden drop in peak-potentiated effect (short-term potentiation) that matches with short-lasting changes expected during working memory and occurs only at the time of delayed LTP induction. When memories are viewed as first-person internal sensations, a newly uncovered mechanism provides explanation for the relationship between memory and LTP. This work interconnects large number of findings from the fields of neuroscience and psychology and provides a further verifiable mechanism of learning.

Ca2+-Triggered Synaptic Vesicle Fusion Initiated by Release of Inhibition

Recent structural and functional studies of the synaptic vesicle fusion machinery suggest an inhibited tripartite complex consisting of neuronal soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNAREs), synaptotagmin, and complexin prior to Ca2+-triggered synaptic vesicle fusion. We speculate that Ca2+-triggered fusion commences with the release of inhibition by Ca2+ binding to synaptotagmin C2 domains. Subsequently, fusion is assisted by SNARE complex zippering and by active membrane remodeling properties of synaptotagmin. This additional, inhibitory role of synaptotagmin may be a general principle since other recent studies suggest that Ca2+ binding to extended synaptotagmin C2 domains enables lipid transport by releasing an inhibited state of the system, and that Munc13 may nominally be in an inhibited state, which is released upon Ca2+ binding to one of its C2 domains.

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.

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.

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.

Regulation of Exocytotic Fusion Pores by SNARE Protein Transmembrane Domains

Calcium-triggered exocytotic release of neurotransmitters and hormones from neurons and neuroendocrine cells underlies neuronal communication, motor activity and endocrine functions. The core of the neuronal exocytotic machinery is composed of soluble N-ethyl maleimide sensitive factor attachment protein receptors (SNAREs). Formation of complexes between vesicle-attached v- and plasma-membrane anchored t-SNAREs in a highly regulated fashion brings the membranes into close apposition. Small, soluble proteins called Complexins (Cpx) and calcium-sensing Synaptotagmins cooperate to block fusion at low resting calcium concentrations, but trigger release upon calcium increase. A growing body of evidence suggests that the transmembrane domains (TMDs) of SNARE proteins play important roles in regulating the processes of fusion and release, but the mechanisms involved are only starting to be uncovered. Here we review recent evidence that SNARE TMDs exert influence by regulating the dynamics of the fusion pore, the initial aqueous connection between the vesicular lumen and the extracellular space. Even after the fusion pore is established, hormone release by neuroendocrine cells is tightly controlled, and the same may be true of neurotransmitter release by neurons. The dynamics of the fusion pore can regulate the kinetics of cargo release and the net amount released, and can determine the mode of vesicle recycling. Manipulations of SNARE TMDs were found to affect fusion pore properties profoundly, both during exocytosis and in biochemical reconstitutions. To explain these effects, TMD flexibility, and interactions among TMDs or between TMDs and lipids have been invoked. Exocytosis has provided the best setting in which to unravel the underlying mechanisms, being unique among membrane fusion reactions in that single fusion pores can be probed using high-resolution methods. An important role will likely be played by methods that can probe single fusion pores in a biochemically defined setting which have recently become available. Finally, computer simulations are valuable mechanistic tools because they have the power to access small length scales and very short times that are experimentally inaccessible.

Exceptionally tight membrane-binding may explain the key role of the synaptotagmin-7 C2A domain in asynchronous neurotransmitter release

Synaptotagmins (Syts) act as Ca²⁺ sensors in neurotransmitter release by virtue of Ca²⁺-binding to their two C₂ domains, but their mechanisms of action remain unclear. Puzzlingly, Ca²⁺-binding to the C₂B domain appears to dominate Syt1 function in synchronous release, whereas Ca²⁺-binding to the C₂A domain mediates Syt7 function in asynchronous release. Here we show that crystal structures of the Syt7 C₂A domain and C₂AB region, and analyses of intrinsic Ca²⁺-binding to the Syt7 C2 domains using isothermal titration calorimetry, did not reveal major differences that could explain functional differentiation between Syt7 and Syt1. However, using liposome titrations under Ca²⁺ saturating conditions, we show that the Syt7 C₂A domain has a very high membrane affinity and dominates phospholipid binding to Syt7 in the presence or absence of l-α-phosphatidylinositol 4,5-diphosphate (PIP₂). For Syt1, the two Ca²⁺-saturated C₂ domains have similar affinities for membranes lacking PIP₂, but the C₂B domain dominates binding to PIP₂-containing membranes. Mutagenesis revealed that the dramatic differences in membrane affinity between the Syt1 and Syt7 C₂A domains arise in part from apparently conservative residue substitutions, showing how striking biochemical and functional differences can result from the cumulative effects of subtle residue substitutions. Viewed together, our results suggest that membrane affinity may be a key determinant of the functions of Syt C₂ domains in neurotransmitter release.

Molecular Mechanisms of Synaptic Vesicle Priming by Munc13 and Munc18

Munc13 catalyzes the transit of syntaxin from a closed complex with Munc18 into the ternary SNARE complex. Here we report a new function of Munc13, independent of Munc18: it promotes the proper syntaxin/synaptobrevin subconfiguration during assembly of the ternary SNARE complex. In cooperation with Munc18, Munc13 additionally ensures the proper syntaxin/SNAP-25 subconfiguration. In a reconstituted fusion assay with SNAREs, complexin, and synaptotagmin, inclusion of both Munc13 and Munc18 quadruples the Ca²⁺-triggered amplitude and achieves Ca²⁺ sensitivity at near-physiological concentrations. In Munc13-1/2 double-knockout neurons, expression of a constitutively open mutant of syntaxin could only minimally restore neurotransmitter release relative to Munc13-1 rescue. Together, the physiological functions of Munc13 may be related to regulation of proper SNARE complex assembly.

Lipid-Coated Gold Nanoparticles and FRET Allow Sensitive Monitoring of Liposome Clustering Mediated by the Synaptotagmin-7 C2A Domain

Synaptotagmin (Syt) family proteins contain tandem C2 domains, C2A and C2B, which insert into anionic membranes in response to increased cytosolic Ca²⁺ concentration and facilitate exocytosis in neuronal and endocrine cells. The C2A domain from Syt7 binds lipid membranes much more tightly than the corresponding domain from Syt1, but the implications of this difference for protein function are not yet clear. In particular, the ability of the isolated Syt7 C2A domain to initiate membrane apposition and/or aggregation has been previously unexplored. Here, we demonstrate that Syt7 C2A induces apposition and aggregation of liposomes using Förster resonance energy transfer (FRET) assays, dynamic light scattering, and spectroscopic techniques involving lipid-coated gold nanoparticles (LCAuNPs). Protein-membrane binding, membrane apposition, and macroscopic aggregation are three separate phenomena with distinct Ca²⁺ requirements: the threshold Ca²⁺ concentration for membrane binding is lowest, followed by apposition and aggregation. However, aggregation is highly sensitive to protein concentration and can occur even at submicromolar Syt7 C2A; thus, highly sensitive assays are needed for measuring apposition without complications arising from aggregation. Notably, the localized surface plasmon resonance of the LCAuNP is sensitive to ≤10 nM Syt7 C2A concentrations. Furthermore, when the LCAuNPs were added into a FRET-based liposome apposition assay, the resultant energy transfer increased; possible explanations are discussed. Overall, LCAuNP-based methods allow for highly sensitive detection of protein-induced membrane apposition under conditions that miminize large-scale aggregation.

Simultaneous lipid and content mixing assays for in vitro reconstitution studies of synaptic vesicle fusion

This protocol describes reconstitution assays to study how the neurotransmitter release machinery triggers Ca²⁺-dependent synaptic vesicle fusion. The assays monitor fusion between proteoliposomes containing the synaptic vesicle SNARE synaptobrevin (with or without the Ca²⁺ sensor synaptotagmin-1) and proteoliposomes initially containing the plasma membrane SNAREs syntaxin-1 and soluble NSF attachment protein (SNAP)-25. Lipid mixing (from fluorescence de-quenching of Marina-Blue-labeled lipids) and content mixing (from development of fluorescence resonance energy transfer (FRET) between phycoerythrin-biotin (PhycoE-Biotin) and Cy5-streptavidin trapped in the two proteoliposome populations) are measured simultaneously to ensure that true, nonleaky membrane fusion is monitored. This protocol is based on a method developed to study yeast vacuolar fusion. In contrast to other protocols used to study the release machinery, this assay incorporates N-ethylmaleimide sensitive factor (NSF) and α-SNAP, which disassemble syntaxin-1 and SNAP-25 heterodimers. As a result, fusion requires Munc18-1, which binds to the released syntaxin-1, and Munc13-1, which, together with Munc18-1, orchestrates SNARE complex assembly. The protocol can be readily adapted to investigation of other types of intracellular membrane fusion by using appropriate alternative proteins. Total time required for one round of the assay is 4 d.

A Membrane-Fusion Model That Exploits a β-to-α Transition in the Hydrophobic Domains of Syntaxin 1A and Synaptobrevin 2

Parallel zippering of the SNARE domains of syntaxin 1A/B, SNAP-25, and VAMP/synaptobrevin 2 is widely regarded as supplying the driving force for exocytotic events at nerve terminals and elsewhere. However, in spite of intensive research, no consensus has been reached concerning the molecular mechanism by which these SNARE proteins catalyze membrane fusion. As an alternative to SNARE-based models, a scenario was developed in which synaptotagmin 1 (or, 2) can serve as a template to guide lipid movements that underlie fast, synchronous exocytosis at nerve terminals. This “dyad model” advanced a novel proposal concerning the membrane disposition of the palmitoylated, cysteine-rich region of these synaptotagmins. Unexpectedly, it now emerges that a similar principle can be exploited to reveal how the hydrophobic, carboxyl-terminal domains of syntaxin 1A and synaptobrevin 2 can perturb membrane structure at the interface between a docked synaptic vesicle and the plasma membrane. These “β-to-α transition” models will be compared and contrasted with other proposals for how macromolecules are thought to intervene to drive membrane fusion.

Membrane Fusion Involved in Neurotransmission: Glimpse from Electron Microscope and Molecular Simulation

Membrane fusion is one of the most fundamental physiological processes in eukaryotes for triggering the fusion of lipid and content, as well as the neurotransmission. However, the architecture features of neurotransmitter release machinery and interdependent mechanism of synaptic membrane fusion have not been extensively studied. This review article expounds the neuronal membrane fusion processes, discusses the fundamental steps in all fusion reactions (membrane aggregation, membrane association, lipid rearrangement and lipid and content mixing) and the probable mechanism coupling to the delivery of neurotransmitters. Subsequently, this work summarizes the research on the fusion process in synaptic transmission, using electron microscopy (EM) and molecular simulation approaches. Finally, we propose the future outlook for more exciting applications of membrane fusion involved in synaptic transmission, with the aid of stochastic optical reconstruction microscopy (STORM), cryo-EM (cryo-EM), and molecular simulations.

Sequence-specific assignment of methyl groups from the neuronal SNARE complex using lanthanide-induced pseudocontact shifts

Neurotransmitter release depends critically on the neuronal SNARE complex formed by syntaxin-1, SNAP-25 and synaptobrevin, as well as on other proteins such as Munc18-1, Munc13-1 and synaptotagmin-1. Although three-dimensional structures are available for these components, it is still unclear how they are assembled between the synaptic vesicle and plasma membranes to trigger fast, Ca²⁺-dependent membrane fusion. Methyl TROSY NMR experiments provide a powerful tool to study complexes between these proteins, but assignment of the methyl groups of the SNARE complex is hindered by its limited solubility. Here we report the assignment of the isoleucine, leucine, methionine and valine methyl groups of the four SNARE motifs of syntaxin-1, SNAP-25 and synaptobrevin within the SNARE complex based solely on measurements of lanthanide-induced pseudocontact shifts. Our results illustrate the power of this approach to assign protein resonances without the need of triple resonance experiments and provide an invaluable tool for future structural studies of how the SNARE complex binds to other components of the release machinery.

In Vitro Reconstitution of Autophagosome-Lysosome Fusion

SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) proteins are a highly regulated class of membrane proteins lying in the center of membrane fusion. In conjunction with accessory proteins, SNAREs drive efficient merger of two distinct lipid bilayers into one interconnected structure. This chapter describes our fluorescence resonance energy transfer (FRET)-based proteoliposome fusion assays for the roles of various SNARE proteins, accessory proteins, and effects of different lipid compositions on membrane fusion involved in autophagy.

A One Donor-Two Acceptor Lipid Bilayer FRET Assay Based on Asymmetrically Labeled Liposomes

The fusion of two opposing membranes is essential in biological functions such as fertilization, viral entry, membrane trafficking and synaptic transmission. Before the membrane bilayers are fully connected, at some stage a hemifusion intermediate-when the outer leaflets are merged but not the inner leaflets-is formed. However, the position of hemifusion in the energy landscape and the duration of it vary and have not been fully mapped out. To date, there has not been a way to differentiate lipid mixing of the two leaflets directly in a single experiment. Herein we demonstrate labeling of the outer and inner leaflets with different fluorophores, which can be distinguished by their fluorescence lifetimes. As a proof of concept, the asymmetrically labeled liposomes were used as acceptor liposomes in a novel one donor-two acceptor Förster resonance energy transfer (FRET) assay to monitor membrane fusion reactions mediated by the synaptic proteins soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) in microfluidic devices. Initial hemifusion was clearly indicated by the acceptor fluorescence lifetime originating solely from FRET acceptors on the outer leaflet (Oregon Green 488; τ_Fl ∼ 4.8 ns). Progression to full fusion was then indicated by the significantly increasing lifetime contribution from acceptors on the inner leaflet (nitrobenzoxadiazole; τ_Fl ∼ 6.7 ns). The new labeling strategy creates many possibilities in the design of bulk and single-molecule experiments.

A modular platform for one-step assembly of multi-component membrane systems by fusion of charged proteoliposomes

An important goal in synthetic biology is the assembly of biomimetic cell-like structures, which combine multiple biological components in synthetic lipid vesicles. A key limiting assembly step is the incorporation of membrane proteins into the lipid bilayer of the vesicles. Here we present a simple method for delivery of membrane proteins into a lipid bilayer within 5 min. Fusogenic proteoliposomes, containing charged lipids and membrane proteins, fuse with oppositely charged bilayers, with no requirement for detergent or fusion-promoting proteins, and deliver large, fragile membrane protein complexes into the target bilayers. We demonstrate the feasibility of our method by assembling a minimal electron transport chain capable of adenosine triphosphate (ATP) synthesis, combining Escherichia coli F1Fo ATP-synthase and the primary proton pump bo3-oxidase, into synthetic lipid vesicles with sizes ranging from 100 nm to ∼10 μm. This provides a platform for the combination of multiple sets of membrane protein complexes into cell-like artificial structures.

N-terminal domain of complexin independently activates calcium-triggered fusion

Complexin activates Ca(2+)-triggered neurotransmitter release and regulates spontaneous release in the presynaptic terminal by cooperating with the neuronal soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and the Ca(2+)-sensor synaptotagmin. The N-terminal domain of complexin is important for activation, but its molecular mechanism is still poorly understood. Here, we observed that a split pair of N-terminal and central domain fragments of complexin is sufficient to activate Ca(2+)-triggered release using a reconstituted single-vesicle fusion assay, suggesting that the N-terminal domain acts as an independent module within the synaptic fusion machinery. The N-terminal domain can also interact independently with membranes, which is enhanced by a cooperative interaction with the neuronal SNARE complex. We show by mutagenesis that membrane binding of the N-terminal domain is essential for activation of Ca(2+)-triggered fusion. Consistent with the membrane-binding property, the N-terminal domain can be substituted by the influenza virus hemagglutinin fusion peptide, and this chimera also activates Ca(2+)-triggered fusion. Membrane binding of the N-terminal domain of complexin therefore cooperates with the other fusogenic elements of the synaptic fusion machinery during Ca(2+)-triggered release.

SNARE-mediated membrane fusion in autophagy

Autophagy, a conserved self-eating process for the bulk degradation of cytoplasmic materials, involves double-membrane autophagosomes formed when an isolation membrane emerges and their direct fusion with lysosomes for degradation. For the early biogenesis of autophagosomes and their later degradation in lysosomes, membrane fusion is necessary, although different sets of genes and autophagy-related proteins involved in distinct fusion steps have been reported. To clarify the molecular mechanism of membrane fusion in autophagy, to not only expand current knowledge of autophagy, but also benefit human health, this review discusses key findings that elucidate the unique membrane dynamics of autophagy.

Versatile Structures of α-Synuclein

α-Synuclein (α-syn) is an intrinsically disordered protein abundantly distributed in presynaptic terminals. Aggregation of α-syn into Lewy bodies (LB) is a molecular hallmark of Parkinson’s disease (PD). α-Syn features an extreme conformational diversity, which adapts to different conditions and fulfills versatile functions. However, the molecular mechanism of α-syn transformation and the relation between different structural species and their functional and pathogenic roles in neuronal activities and PD remain unknown. In this mini-review, we summarize the recent discoveries of α-syn structures in the membrane-bound state, in cytosol, and in the amyloid state under physiological and pathological conditions. From the current knowledge on different structural species of α-syn, we intend to find a clue about its function and toxicity in normal neurons and under disease conditions, which could shed light on the PD pathogenesis.

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'.

Functional synergy between the Munc13 C-terminal C1 and C2 domains

Neurotransmitter release requires SNARE complexes to bring membranes together, NSF-SNAPs to recycle the SNAREs, Munc18-1 and Munc13s to orchestrate SNARE complex assembly, and Synaptotagmin-1 to trigger fast Ca²⁺-dependent membrane fusion. However, it is unclear whether Munc13s function upstream and/or downstream of SNARE complex assembly, and how the actions of their multiple domains are integrated. Reconstitution, liposome-clustering and electrophysiological experiments now reveal a functional synergy between the C₁, C₂B and C₂C domains of Munc13-1, indicating that these domains help bridging the vesicle and plasma membranes to facilitate stimulation of SNARE complex assembly by the Munc13-1 MUN domain. Our reconstitution data also suggest that Munc18-1, Munc13-1, NSF, αSNAP and the SNAREs are critical to form a ‘primed’ state that does not fuse but is ready for fast fusion upon Ca²⁺ influx. Overall, our results support a model whereby the multiple domains of Munc13s cooperate to coordinate synaptic vesicle docking, priming and fusion.

DOI:http://dx.doi.org/10.7554/eLife.13696.001

In the brain, neurons communicate with each other using small molecules called neurotransmitters. Electrical signals in one neuron trigger the release of the neurotransmitters, which then bind to receptor proteins on another neuron nearby. Neurotransmitters are packaged into small compartments called synaptic vesicles and are released from the neuron when these vesicles fuse with the membrane that surrounds the cell. Many proteins are involved in regulating this process to ensure that neurotransmitters are released at the right place and time.

A large protein called Munc13 plays an important role in the release of neurotransmitters. It contains many different regions, including a long domain called MUN and three additional domains called C₁, C₂B and C₂C among others. However, it is not clear how all these domains work together to control neurotransmitter release. Here Liu, Seven et al. address this question using purified proteins inserted into membranes as well as experiments in neurons from mice. The experiments show that the C₁, C₂B and C₂C domains all play key roles in neurotransmitter release. Together with the MUN domain, these three domains help to form bridges between synaptic vesicles and the membrane surrounding the neuron. These bridges could help other proteins involved in neurotransmitter release to form a group that induces vesicle fusion.

Liu, Seven et al.’s findings also suggest that Munc13 proteins cooperate with other proteins to form a 'primed' state in which a synaptic vesicle is ready to rapidly fuse with a neuron’s membrane when triggered to do so by an electrical signal. A future challenge is to find out how the proteins that form this primed state promote vesicle fusion.

DOI:http://dx.doi.org/10.7554/eLife.13696.002

The Secret Life of Tethers: The Role of Tethering Factors in SNARE Complex Regulation

Trafficking in eukaryotic cells is a tightly regulated process to ensure correct cargo delivery to the proper destination organelle or plasma membrane. In this review, we focus on how the vesicle fusion machinery, the SNARE complex, is regulated by the interplay of the multisubunit tethering complexes (MTC) with the SNAREs and Sec1/Munc18 (SM) proteins. Although these factors are used in different stages of membrane trafficking, e.g., Golgi to plasma membrane transport vs. vacuolar fusion, and in a variety of diverse eukaryotic cell types, many commonalities between their functions are being revealed. We explore the various protein-protein interactions and findings from functional reconstitution studies in order to highlight both their common features and the differences in their modes of regulation. These studies serve as a starting point for mechanistic explorations in other systems.

High cholesterol obviates a prolonged hemifusion intermediate in fast SNARE-mediated membrane fusion

Cholesterol is essential for exocytosis in secretory cells, but the exact molecular mechanism by which it facilitates exocytosis is largely unknown. Distinguishing contributions from the lateral organization and dynamics of membrane proteins to vesicle docking and fusion and the promotion of fusion pores by negative intrinsic spontaneous curvature and other mechanical effects of cholesterol have been elusive. To shed more light on this process, we examined the effect of cholesterol on SNARE-mediated membrane fusion in a single-vesicle assay that is capable of resolving docking and elementary steps of fusion with millisecond time resolution. The effect of cholesterol on fusion pore formation between synaptobrevin-2 (VAMP-2)-containing proteoliposomes and acceptor t-SNARE complex-containing planar supported bilayers was examined using both membrane and content fluorescent markers. This approach revealed that increasing cholesterol in either the t-SNARE or the v-SNARE membrane favors a mechanism of direct fusion pore opening, whereas low cholesterol favors a mechanism leading to a long-lived (>5 s) hemifusion state. The amount of cholesterol in the target membrane had no significant effect on docking of synaptobrevin vesicles. Comparative studies with α-tocopherol (vitamin E) show that the negative intrinsic spontaneous curvature of cholesterol and its presumed promotion of a very short-lived (<50 ms) lipid stalk intermediate is the main factor that favors rapid fusion pore opening at high cholesterol. This study also shows that this single-vesicle fusion assay can distinguish between hemifusion and full fusion with only a single lipid dye, thereby freeing up a fluorescence channel for the simultaneous measurement of another parameter in fast time-resolved fusion assays.