Studying protein-reconstituted proteoliposome fusion with content indicators in vitro

A new method for isolation and purification of fusion-competent inhibitory synaptic vesicles

Synaptic vesicles specific to inhibitory GABA-releasing neurons are critical for regulating neuronal excitability. To study the specific molecular composition, architecture, and function of inhibitory synaptic vesicles, we have developed a new method to isolate and purify GABA synaptic vesicles from mouse brains. GABA synaptic vesicles were immunoisolated from mouse brain tissue using an engineered fragment antigen-binding region (Fab) against the vesicular GABA transporter (vGAT) and purified. Western blot analysis confirmed that the GABA synaptic vesicles were specifically enriched for vGAT and largely depleted of contaminants from other synaptic vesicle types, such as vesicular glutamate transporter (vGLUT1), and other cellular organelles. This degree of purity was achieved despite the relatively low abundance of vGAT vesicles compared to the total synaptic vesicle pool in mammalian brains. Cryo-electron microscopy images of these isolated GABA synaptic vesicles revealed intact morphology with circular shape and protruding proteinaceous densities. The GABA synaptic vesicles are functional, as assessed by a hybrid (ex vivo/in vitro) vesicle fusion assay, and they undergo synchronized fusion with synthetic plasma membrane mimic vesicles in response to Ca2+-triggering, but, as a negative control, not to Mg2+-triggering. Our immunoisolation method could also be applied to other types of vesicles.

Neutral lysophosphatidylcholine mediates α-synuclein-induced synaptic vesicle clustering

α-synuclein (α-Syn) is a presynaptic protein that is involved in Parkinson's and other neurodegenerative diseases and binds to negatively charged phospholipids. Previously, we reported that α-Syn clusters synthetic proteoliposomes that mimic synaptic vesicles. This vesicle-clustering activity depends on a specific interaction of α-Syn with anionic phospholipids. Here, we report that α-Syn surprisingly also interacts with the neutral phospholipid lysophosphatidylcholine (lysoPC). Even in the absence of anionic lipids, lysoPC facilitates α-Syn-induced vesicle clustering but has no effect on Ca2+-triggered fusion in a single vesicle-vesicle fusion assay. The A30P mutant of α-Syn that causes familial Parkinson disease has a reduced affinity to lysoPC and does not induce vesicle clustering. Taken together, the α-Syn-lysoPC interaction may play a role in α-Syn function.

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.

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.

Green fluorescence protein-based content-mixing assay of SNARE-driven membrane fusion

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins mediate intracellular membrane fusion by forming a ternary SNARE complex. A minimalist approach utilizing proteoliposomes with reconstituted SNARE proteins yielded a wealth of information pinpointing the molecular mechanism of SNARE-mediated fusion and its regulation by accessory proteins. Two important attributes of a membrane fusion are lipid-mixing and the formation of an aqueous passage between apposing membranes. These two attributes are typically observed by using various fluorescent dyes. Currently available in vitro assay systems for observing fusion pore opening have several weaknesses such as cargo-bleeding, incomplete removal of unencapsulated dyes, and inadequate information regarding the size of the fusion pore, limiting measurements of the final stage of membrane fusion. In the present study, we used a biotinylated green fluorescence protein and streptavidin conjugated with Dylight 594 (DyStrp) as a Föster resonance energy transfer (FRET) donor and acceptor, respectively. This FRET pair encapsulated in each v-vesicle containing synaptobrevin and t-vesicle containing a binary acceptor complex of syntaxin 1a and synaptosomal-associated protein 25 revealed the opening of a large fusion pore of more than 5 nm, without the unwanted signals from unencapsulated dyes or leakage. This system enabled determination of the stoichiometry of the merging vesicles because the FRET efficiency of the FRET pair depended on the molar ratio between dyes. Here, we report a robust and informative assay for SNARE-mediated fusion pore opening.

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.

Planar Supported Membranes with Mobile SNARE Proteins and Quantitative Fluorescence Microscopy Assays to Study Synaptic Vesicle Fusion

Synaptic vesicle membrane fusion, the process by which neurotransmitter gets released at the presynaptic membrane is mediated by a complex interplay between proteins and lipids. The realization that the lipid bilayer is not just a passive environment where other molecular players like SNARE proteins act, but is itself actively involved in the process, makes the development of biochemical and biophysical assays particularly challenging. We summarize in vitro assays that use planar supported membranes and fluorescence microscopy to address some of the open questions regarding the molecular mechanisms of SNARE-mediated membrane fusion. Most of the assays discussed in this mini-review were developed in our lab over the last 15 years. We emphasize the sample requirements that we found are important for the successful application of these methods.

C-terminal domain of mammalian complexin-1 localizes to highly curved membranes

In presynaptic nerve terminals, complexin regulates spontaneous "mini" neurotransmitter release and activates Ca²⁺-triggered synchronized neurotransmitter release. We studied the role of the C-terminal domain of mammalian complexin in these processes using single-particle optical imaging and electrophysiology. The C-terminal domain is important for regulating spontaneous release in neuronal cultures and suppressing Ca²⁺-independent fusion in vitro, but it is not essential for evoked release in neuronal cultures and in vitro. This domain interacts with membranes in a curvature-dependent fashion similar to a previous study with worm complexin [Snead D, Wragg RT, Dittman JS, Eliezer D (2014) Membrane curvature sensing by the C-terminal domain of complexin. Nat Commun 5:4955]. The curvature-sensing value of the C-terminal domain is comparable to that of α-synuclein. Upon replacement of the C-terminal domain with membrane-localizing elements, preferential localization to the synaptic vesicle membrane, but not to the plasma membrane, results in suppression of spontaneous release in neurons. Membrane localization had no measurable effect on evoked postsynaptic currents of AMPA-type glutamate receptors, but mislocalization to the plasma membrane increases both the variability and the mean of the synchronous decay time constant of NMDA-type glutamate receptor evoked postsynaptic currents.

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.

High Affinity Host-Guest FRET Pair for Single-Vesicle Content-Mixing Assay: Observation of Flickering Fusion Events

Fluorescence-based single-vesicle fusion assays provide a powerful method for studying mechanisms underlying complex biological processes of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor)-mediated vesicle fusion and neurotransmitter release. A crucial element of these assays is the ability of the fluorescent probe(s) to reliably detect key intermediate events of fusion pore opening and content release/mixing. Here, we report a new, reliable, and efficient single-vesicle content-mixing assay using a high affinity, fluorophore tagged host-guest pair, cucurbit[7]uril-Cy3 and adamantane-Cy5 as a fluorescence resonance energy transfer (FRET) pair. The power of these probes is demonstrated by the first successful observation of flickering dynamics of the fusion pore by in vitro assay using neuronal SNARE-reconstituted vesicles.

Lipid molecules influence early stages of yeast SNARE-mediated membrane fusion

Lipid molecules, structural components of biomembranes, have been proposed for an important role in membrane fusion. Through various techniques based on a protein-reconstituted vesicle-vesicle fusion system, we investigated the influence of several lipid molecules on different stages of a yeast soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated membrane fusion process. Lipid compositions played a significant role in the early stages, docking and lipid mixing, while only exhibiting a minor effect on fusion pore formation and dilation phases, indicated by both small and large content mixing.

Reconstituting SNARE-mediated membrane fusion at the single liposome level

Successful reconstitutions of SNARE-mediated intracellular membrane fusion have been achieved in bulk fusion assays since 1998 and in single liposome fusion assays since 2004. Especially in neuronal presynaptic SNARE-mediated exocytosis, fusion is controlled by numerous accessory proteins, of which some functions have also been reconstituted in vitro. The development of and results obtained with two fundamentally different single liposome fusion assays, namely liposome-to-supported membrane and liposome-to-liposome, are reviewed. Both assays distinguish between liposome docking and fusion steps of the overall fusion reaction and both assays are capable of resolving hemi-and full-fusion intermediates and end states. They have opened new windows for elucidating the mechanisms of these fundamentally important cellular reactions with unprecedented time and molecular resolution. Although many of the molecular actors in this process have been discovered, we have only scratched the surface of looking at their fascinating plays, interactions, and choreographies that lead to vesicle traffic as well as neurotransmitter and hormone release in the cell.

Molecular origins of synaptotagmin 1 activities on vesicle docking and fusion pore opening

Synaptotagmin 1 (Syt1), a major Ca²⁺ sensor in neuroexocytosis, utilizes SNARE- and membrane-binding to regulate vesicle fusion, a required process for neurotransmitter release at the synapse. However, the mechanism by which Syt1 orchestrates SNARE- and membrane- binding to control individual vesicle fusion steps is still unclear. In this study, we used a number of single vesicle assays that can differentiate intermediates of neuroexocytosis, to focus on Syt1 mutants that might impair Syt1-SNARE/PIP₂ interaction, Ca²⁺-binding, or membrane penetration. Our results show that, although putative Syt1-SNARE/PIP₂ coupling through the polybasic region of the C2B domain is critical for vesicle docking, its disruption does not affect content release. In contrast, Ca²⁺-binding and membrane-penetration mutants significantly reduce content release. Our results thus delineate multiple functions of Syt1 along the pathway of Ca²⁺-triggered exocytosis in unprecedented detail.

Munc18a does not alter fusion rates mediated by neuronal SNAREs, synaptotagmin, and complexin

Sec1/Munc18 (SM) proteins are essential for membrane trafficking, but their molecular mechanism remains unclear. Using a single vesicle-vesicle content-mixing assay with reconstituted neuronal SNAREs, synaptotagmin-1, and complexin-1, we show that the neuronal SM protein Munc18a/nSec1 has no effect on the intrinsic kinetics of both spontaneous fusion and Ca²⁺-triggered fusion between vesicles that mimic synaptic vesicles and the plasma membrane. However, wild type Munc18a reduced vesicle association ∼50% when the vesicles bearing the t-SNAREs syntaxin-1A and SNAP-25 were preincubated with Munc18 for 30 min. Single molecule experiments with labeled SNAP-25 indicate that the reduction of vesicle association is a consequence of sequestration of syntaxin-1A by Munc18a and subsequent release of SNAP-25 (i.e. Munc18a captures syntaxin-1A via its high affinity interaction). Moreover, a phosphorylation mimic mutant of Munc18a with reduced affinity to syntaxin-1A results in less reduction of vesicle association. In summary, Munc18a does not directly affect fusion, although it has an effect on the t-SNARE complex, depending on the presence of other factors and experimental conditions. Our results suggest that Munc18a primarily acts at the prefusion stage.

Complexin inhibits spontaneous release and synchronizes Ca2+-triggered synaptic vesicle fusion by distinct mechanisms

Previously we showed that fast Ca²⁺-triggered vesicle fusion with reconstituted neuronal SNAREs and synaptotagmin-1 begins from an initial hemifusion-free membrane point contact, rather than a hemifusion diaphragm, using a single vesicle–vesicle lipid/content mixing assay (Diao et al., 2012). When complexin-1 was included, a more pronounced Ca²⁺-triggered fusion burst was observed, effectively synchronizing the process. Here we show that complexin-1 also reduces spontaneous fusion in the same assay. Moreover, distinct effects of several complexin-1 truncation mutants on spontaneous and Ca²⁺-triggered fusion closely mimic those observed in neuronal cultures. The very N-terminal domain is essential for synchronization of Ca²⁺-triggered fusion, but not for suppression of spontaneous fusion, whereas the opposite is true for the C-terminal domain. By systematically varying the complexin-1 concentration, we observed differences in titration behavior for spontaneous and Ca²⁺-triggered fusion. Taken together, complexin-1 utilizes distinct mechanisms for synchronization of Ca²⁺-triggered fusion and inhibition of spontaneous fusion.

DOI:http://dx.doi.org/10.7554/eLife.03756.001

Fast, Ca2+-dependent exocytosis at nerve terminals: shortcomings of SNARE-based models

Investigations over the last two decades have made major inroads in clarifying the cellular and molecular events that underlie the fast, synchronous release of neurotransmitter at nerve endings. Thus, appreciable progress has been made in establishing the structural features and biophysical properties of the calcium (Ca2+) channels that mediate the entry into nerve endings of the Ca2+ ions that trigger neurotransmitter release. It is now clear that presynaptic Ca2+ channels are regulated at many levels and the interplay of these regulatory mechanisms is just beginning to be understood. At the same time, many lines of research have converged on the conclusion that members of the synaptotagmin family serve as the primary Ca2+ sensors for the action potential-dependent release of neurotransmitter. This identification of synaptotagmins as the proteins which bind Ca2+ and initiate the exocytotic fusion of synaptic vesicles with the plasma membrane has spurred widespread efforts to reveal molecular details of synaptotagmin's action. Currently, most models propose that synaptotagmin interfaces directly or indirectly with SNARE (soluble, N-ethylmaleimide sensitive factor attachment receptors) proteins to trigger membrane fusion. However, in spite of intensive efforts, the field has not achieved consensus on the mechanism by which synaptotagmins act. Concurrently, the precise sequence of steps underlying SNARE-dependent membrane fusion remains controversial. This review considers the pros and cons of the different models of SNARE-mediated membrane fusion and concludes by discussing a novel proposal in which synaptotagmins might directly elicit membrane fusion without the intervention of SNARE proteins in this final fusion step.

Nonaggregated α-synuclein influences SNARE-dependent vesicle docking via membrane binding

α-Synuclein (α-Syn), a major component of Lewy body that is considered as the hallmark of Parkinson’s disease (PD), has been implicated in neuroexocytosis. Overexpression of α-Syn decreases the neurotransmitter release. However, the mechanism by which α-Syn buildup inhibits the neurotransmitter release is still unclear. Here, we investigated the effect of nonaggregated α-Syn on SNARE-dependent liposome fusion using fluorescence methods. In ensemble in vitro assays, α-Syn reduces lipid mixing mediated by SNAREs. Furthermore, with the more advanced single-vesicle assay that can distinguish vesicle docking from fusion, we found that α-Syn specifically inhibits vesicle docking, without interfering with the fusion. The inhibition in vesicle docking requires α-Syn binding to acidic lipid containing membranes. Thus, these results imply the existence of at least two mechanisms of inhibition of SNARE-dependent membrane fusion: at high concentrations, nonaggregated α-Syn inhibits docking by binding acidic lipids but not v-SNARE; on the other hand, at much lower concentrations, large α-Syn oligomers inhibit via a mechanism that requires v-SNARE interaction [Choi et al. Proc. Natl. Acad. Sci. U. S. A.2013, 110 (10), 4087−409223431141].

Complexin-1 enhances the on-rate of vesicle docking via simultaneous SNARE and membrane interactions

In synaptic terminals, complexin is thought to have inhibitory and activating roles for spontaneous “mini” release and evoked synchronized neurotransmitter release, respectively. We used single vesicle–vesicle microscopy imaging to study the effect of complexin-1 on the on-rate of docking between vesicles that mimic synaptic vesicles and the plasma membrane. We found that complexin-1 enhances the on-rate of docking of synaptic vesicle mimics containing full-length synaptobrevin-2 and full-length synaptotagmin-1 to plasma membrane-mimicking vesicles containing full-length syntaxin-1A and SNAP-25A. This effect requires the C-terminal domain of complexin-1, which binds to the membrane, the presence of PS in the membrane, and the core region of complexin-1, which binds to the SNARE complex.