SNARE assembly enlightened by cryo-EM structures of a synaptobrevin-Munc18-1-syntaxin-1 complex

Evaluation of synaptotagmin-1 action models by all-atom molecular dynamics simulations

Neurotransmitter release is triggered in microseconds by the two C2 domains of the Ca2+ sensor synaptotagmin-1 and by SNARE complexes, which form four-helix bundles that bridge the vesicle and plasma membranes. The synaptotagmin-1 C2B domain binds to the SNARE complex via a 'primary interface', but the mechanism that couples Ca2+-sensing to membrane fusion is unknown. Widespread models postulate that the synaptotagmin-1 Ca2+-binding loops accelerate membrane fusion by inducing membrane curvature, perturbing lipid bilayers or helping bridge the membranes, but these models do not seem compatible with SNARE binding through the primary interface, which orients the Ca2+-binding loops away from the fusion site. To test these models, we performed molecular dynamics simulations of SNARE complexes bridging a vesicle and a flat bilayer, including the synaptotagmin-1 C2 domains in various configurations. Our data do not support the notion that insertion of the synaptotagmin-1 Ca2+-binding loops causes substantial membrane curvature or major perturbations of the lipid bilayers that could facilitate membrane fusion. We observed membrane bridging by the synaptotagmin-1 C2 domains, but such bridging or the presence of the C2 domains near the site of fusion hindered the action of the SNAREs in bringing the membranes together. These results argue against models predicting that synaptotagmin-1 triggers neurotransmitter release by inducing membrane curvature, perturbing bilayers or bridging membranes. Instead, our data support the hypothesis that binding via the primary interface keeps the synaptotagmin-1 C2 domains away from the site of fusion, orienting them such that they trigger release through a remote action.

SNARE complex assembly and disassembly dynamics in response to Ca2+ current activation in live cells

A SNAP25-based FRET construct named SCORE (SNARE complex reporter) has revealed a transient FRET increase that specifically occurred at fusion sites preceding fusion events by tens of milliseconds and presumably reflects vesicle priming. The FRET increase lasts for a few seconds until it is reversed. In those experiments, the FRET increase was found to be localized to areas <0.5 μm2 at sites of transmitter release as detected amperometrically using electrochemical detector arrays. Due to the localization to such small areas, it was unknown if the reversal of the FRET increase is due to local dispersion of high-FRET SCORE copies leaving the site after fusion and exchange with surrounding low-FRET copies, or if it reflects disassembly of the high-FRET complexes. To resolve this question, we performed whole-cell patch-clamp pulse stimulation experiments, imaging the entire footprint of the cells in total internal reflection fluorescence (TIRF) excitation mode such that diffusional exchange between high-FRET and low-FRET copies does not produce a net FRET change. We show here that pulse stimulation of calcium currents results in FRET ratio transients with a time course very similar to those related to fusion events. By comparing the kinetics of the FRET ratio decay with analytical and numerical diffusion simulation results, we show that the experimentally observed kinetics cannot be explained by diffusional exchange and conclude that the SCORE FRET ratio transients reflect incorporation of SCORE in SNARE complexes followed by SNARE complex disassembly. Experiments using Synaptobrevin 2/Cellubrevin double knockout mouse embryonal chromaffin cells showed no pulse-induced FRET change, indicating that the vSNARE is required for the incorporation of SCORE (or SNAP25 in wild-type cells) in the SNARE complex during priming.

Drosophila and human Headcase define a new family of ribonucleotide granule proteins required for stress response

Cells have means to adapt to environmental stresses such as temperature fluctuations, toxins, or nutrient availability. Stress responses, being dynamic, extend beyond transcriptional control and encompass post-transcriptional mechanisms allowing for rapid changes in protein synthesis. Previous research has established headcase as a fundamental gene for stress responses and survival of the Drosophila adult progenitor cells (APCs). However, the molecular role of Headcase has remained elusive. Here, we identify Headcase as a component of ribonucleoprotein (RNP) granules. We also show that, Headcase is required for proper RNP granule formation and remodeling upon stress and is crucial for translation control. Likewise, the human Headcase homolog (HECA) is identified as a component of RNP granules and has similar roles in translational regulation and stress protection. Thus, Headcase proteins define a new family contributing to specific roles among the RNP heterogeneous network.

The dynamics of SNARE complex assembly and disassembly in response to Ca 2+ current activation in live chromaffin cells

A SNAP25 based FRET construct named SCORE (SNARE COmplex REporter) has revealed a transient FRET increase that specifically occurred at fusion sites preceding fusion events by tens of milliseconds and presumably reflects vesicle priming. The FRET increase lasts for a few seconds until it is reversed. In those experiments, the FRET increase was found to be localized to areas <0.5 µm 2 at sites of transmitter release as detected amperometrically using electrochemical detector arrays. Due to the localization to such small areas, it was unknown if the reversal of the FRET increase is due to local dispersion of high-FRET SCORE copies leaving the site after fusion and exchange with surrounding low-FRET copies, or if it reflects disassembly of the high-FRET complexes. To resolve this question, we performed whole-cell patch clamp pulse stimulation experiments, imaging the entire footprint of the cells in Total Internal Reflection Fluorescence (TIRF) excitation mode such that diffusional exchange between high-FRET and low-FRET copies does not produce a net FRET change. We show here that pulse stimulation of calcium currents results in FRET ratio transients with a time course very similar that related to fusion events. By comparing the kinetics of the FRET ratio decay with analytical and numerical diffusion simulation results, we show that the experimentally observed kinetics cannot be explained by diffusional exchange and conclude that the SCORE FRET ratio transients reflect incorporation of SCORE in SNARE complexes followed by SNARE complex disassembly. Experiments using Synaptobrevin 2/Cellubrevin double knock-out mouse embryonal chromaffin cells showed no pulse induced FRET change, indicating that the vSNARE is required for the incorporation of SCORE (or SNAP25 in wild type cells) in the SNARE complex during priming.

Statement of Significance

In chromaffin cells, SNAP25-based FRET constructs (SCORE) revealed transient FRET increases within <0.5 µm 2 areas, preceding individual fusion events that reversed within seconds. It remained unknown whether this reversal stems from high-FRET complex disassembly or diffusion-mediated exchange with low-FRET complexes. Here, we performed whole-cell patch-clamp pulse stimulation with TIRF microscopy, imaging large ∼30 µm 2 areas of the cell footprint. Calcium currents induced FRET transients with the same decay time constant of ∼1.5 s, significantly shorter than the time scale of diffusion. The SCORE FRET ratio thus reports in real time the dynamics of SNRE complex assembly and disassembly in live cells. Using Synaptobrevin 2/Cellubrevin double knock-out mouse chromaffin cells we show that vSNAREs are required for SNAP25 incorporation into SNARE complexes during priming.

A lever hypothesis for Synaptotagmin-1 action in neurotransmitter release

Neurotransmitter release is triggered in microseconds by Ca2+-binding to the Synaptotagmin-1 C2-domains and by SNARE complexes that form four-helix bundles between synaptic vesicles and plasma membranes, but the coupling mechanism between Ca2+-sensing and membrane fusion is unknown. Release requires extension of SNARE helices into juxtamembrane linkers that precede transmembrane regions (linker zippering) and binding of the Synaptotagmin-1 C2B domain to SNARE complexes through a "primary interface" comprising two regions (I and II). The Synaptotagmin-1 Ca2+-binding loops were believed to accelerate membrane fusion by inducing membrane curvature, perturbing lipid bilayers, or helping bridge the membranes, but SNARE complex binding through the primary interface orients the Ca2+-binding loops away from the fusion site, hindering these putative activities. To clarify this paradox, we have used NMR and fluorescence spectroscopy. NMR experiments reveal that binding of C2B domain arginines to SNARE acidic residues at region II remains after disruption of region I, and that a mutation that impairs spontaneous and Ca2+-triggered neurotransmitter release enhances binding through region I. Moreover, fluorescence assays show that Ca2+ does not induce dissociation of Synaptotagmin-1 from membrane-anchored SNARE complex but causes reorientation of the C2B domain. Based on these results and electrophysiological data described by Toulme et al. (https://doi.org/10.1073/pnas.2409636121), we propose that upon Ca2+ binding the Synaptotagmin-1 C2B domain reorients on the membrane and dissociates from the SNAREs at region I but not region II, acting remotely as a lever that pulls the SNARE complex and facilitates linker zippering or other SNARE structural changes required for fast membrane fusion.

Complexin regulation of synaptic vesicle release: mechanisms in the central nervous system and specialized retinal ribbon synapses

Synaptic ribbons, recognized for their pivotal role in conveying sensory signals in the visual pathway, are intricate assemblages of presynaptic proteins. Complexin (CPX) regulates synaptic vesicle fusion and neurotransmitter release by modulating the assembly of the soluble NSF attachment protein receptor (SNARE) complex, ensuring precise signal transmission in the retina and the broader central nervous system (CNS). While CPX1 or CPX2 isoforms (CPX1/2) play crucial roles in classical CNS synapses, CPX3 or CPX4 isoforms (CPX3/4) specifically regulate retinal ribbon synapses. These isoforms are essential for sustaining synaptic plasticity related to light signaling, adapting to changes in circadian rhythms, and dynamically regulating visual function under varying light conditions. This review explores the regulation of synaptic vesicle release by CPX in both the CNS and retinal ribbon synapses, with a focus on the mechanisms governing CPX3/4 function in the retina. Additionally, by reviewing the role of CPX and ribbon synapse dysfunction in non-retinal diseases, we further hypothesize the potential mechanisms of CPX in retinal diseases and propose therapeutic strategies targeting CPX to address retinal and CNS disorders associated with synaptic dysfunction.

After their membrane assembly, Sec18 (NSF) and Sec17 (SNAP) promote membrane fusion

The energy that drives membrane fusion can come from either complete SNARE zippering, from Sec17 and Sec18, or both. Sec17 and Sec18 initially form a complex which binds membranes. Sec17, Sec18, and the apolarity of a loop on the N-domain of Sec17 are required for their interdependent membrane association. To determine whether Sec18 and the Sec17 loop apolarity are still required for fusion after their membrane arrival, a hydrophobic transmembrane (TM) anchor was affixed to the N-terminus of Sec17, forming TM-Sec17. Fusion without energy from complete SNARE zippering requires Sec18 as well as either Sec17 or TM-Sec17. Even without the need for membrane targeting, the TM-Sec17 apolar loop strongly stimulates Sec17/18-driven fusion. Thus, Sec18 and the Sec17 apolar loop are first required for membrane targeting, and once bound, drive rapid fusion. Each of these variables-the absence or presence of Sec17, its N-loop apolarity, addition or omission of Sec18, and unimpeded or diminished energy from SNARE zippering-has almost no effect on the amount of trans-SNARE complex, but instead regulates the capacity of docked membranes to fuse.

Membrane fusion reactions limited by defective SNARE zippering or stiff lipid fatty acyl composition have distinct requirements for Sec17, Sec18, and adenine nucleotide

Intracellular membrane fusion is catalyzed by SNAREs, Rab GTPases, SM proteins, tethers, Sec18/NSF and Sec17/SNAP. Membrane fusion has been reconstituted with purified vacuolar proteins and lipids to address 3 salient questions: whether ATP hydrolysis by Sec18 affects its promotion of fusion, whether fusion promotion by Sec17 and Sec18 is only seen with mutant SNAREs or can also be seen with wild-type SNAREs, and whether Sec17 and Sec18 only promote fusion when they work together or whether they can each work separately. Fusion is driven by two engines, completion of SNARE zippering (which does not need Sec17/Sec18) and Sec17/Sec18-mediated fusion (needing SNAREs but not the energy from their complete zippering). Sec17 is required to rescue fusion that is blocked by incomplete zippering, though optimal rescue also needs the ATPase Sec18. ATP is an essential Sec18 ligand, but at limiting Sec17 levels Sec18 ATP hydrolysis also drives release of Sec17 without concomitant trans-SNARE complex disassembly. At higher (physiological) Sec17 levels, or without ATP hydrolysis, fusion prevails over Sec17 release. Stiff 16:0, 18:1 fatty acyl chain lipids provide an alternative route to suppressing fusion, with entirely wild-type SNAREs and without impediment to zippering. In this case, Sec17 and Sec18 restore comparable fusion with either ATP or a nonhydrolyzable analog. Fusion blocked by impaired zippering can be restored by concentrated Sec17 alone (but not by Sec18), while fusion inhibited by stiff fatty acyl chains is partially restored by Sec18 alone (but not by Sec17). With distinct fusion impediments, Sec18 and Sec17 have both shared roles and independent roles in promoting fusion.

Neurotransmitter release is triggered by a calcium-induced rearrangement in the Synaptotagmin-1/SNARE complex primary interface

The Ca2+ sensor synaptotagmin-1 (Syt1) triggers neurotransmitter release together with the neuronal sensitive factor attachment protein receptor (SNARE) complex formed by syntaxin-1, SNAP25, and synaptobrevin. Moreover, Syt1 increases synaptic vesicle (SV) priming and impairs spontaneous vesicle release. The Syt1 C2B domain binds to the SNARE complex through a primary interface via two regions (I and II), but how exactly this interface mediates distinct functions of Syt1 and the mechanism underlying Ca2+ triggering of release are unknown. Using mutagenesis and electrophysiological experiments, we show that region II is functionally and spatially subdivided: Binding of C2B domain arginines to SNAP-25 acidic residues at one face of region II is crucial for Ca2+-evoked release but not for vesicle priming or clamping of spontaneous release, whereas other SNAP-25 and syntaxin-1 acidic residues at the other face mediate priming and clamping of spontaneous release but not evoked release. Mutations that disrupt region I impair the priming and clamping functions of Syt1 while, strikingly, mutations that enhance binding through this region increase vesicle priming and clamping of spontaneous release, but strongly inhibit evoked release and vesicle fusogenicity. These results support previous findings that the primary interface mediates the functions of Syt1 in vesicle priming and clamping of spontaneous release and, importantly, show that Ca2+ triggering of release requires a rearrangement of the primary interface involving dissociation of region I, while region II remains bound. Together with biophysical studies presented in [K. Jaczynska et al., bioRxiv [Preprint] (2024). https://doi.org/10.1101/2024.06.17.599417 (Accessed 18 June 2024)], our data suggest a model whereby this rearrangement pulls the SNARE complex to facilitate fast SV fusion.

Pre-fusion AAA+ remodeling of target-SNARE protein complexes enables synaptic transmission

Membrane fusion is driven by SNARE complex formation across cellular contexts, including vesicle fusion during synaptic transmission. Multiple proteins organize trans-SNARE complex assembly and priming, leading to fusion. One target membrane SNARE, syntaxin, forms nanodomains at the active zone, and another, SNAP-25, enters non-fusogenic complexes with it. Here, we show that the AAA+ protein NSF (N-ethylmaleimide sensitive factor) and SNAP (soluble NSF attachment protein) must act prior to fusion. We show that syntaxin clusters are conserved, that NSF colocalizes with them, and characterize SNARE populations within and near these clusters using cryo-EM. Supercomplexes of NSF, α-SNAP, and either a syntaxin tetramer or two binary complexes of syntaxin-SNAP-25 reveal atomic details of SNARE processing and show how sequential ATP hydrolysis drives disassembly. These results suggest a functional role for syntaxin clusters as reservoirs and a corresponding role for NSF in syntaxin liberation and SNARE protein quality control preceding fusion.

Exploring the structural dynamics of the vesicle priming machinery

Various cell types release neurotransmitters, hormones and many other compounds that are stored in secretory vesicles by exocytosis via the formation of a fusion pore traversing the vesicular membrane and the plasma membrane. This process of membrane fusion is mediated by the Soluble N-ethylmaleimide-Sensitive Factor Attachment Proteins REceptor (SNARE) protein complex, which in neurons and neuroendocrine cells is composed of the vesicular SNARE protein Synaptobrevin and the plasma membrane proteins Syntaxin and SNAP25 (Synaptosomal-Associated Protein of 25 kDa). Before a vesicle can undergo fusion and release of its contents, it must dock at the plasma membrane and undergo a process named 'priming', which makes it ready for release. The primed vesicles form the readily releasable pool, from which they can be rapidly released in response to stimulation. The stimulus is an increase in Ca2+ concentration near the fusion site, which is sensed primarily by the vesicular Ca2+ sensor Synaptotagmin. Vesicle priming involves at least the SNARE proteins as well as Synaptotagmin and the accessory proteins Munc18, Munc13, and Complexin but additional proteins may also participate in this process. This review discusses the current views of the interactions and the structural changes that occur among the proteins of the vesicle priming machinery.

Intermediate steps in the formation of neuronal SNARE complexes

Neuronal exocytosis requires the assembly of three SNARE proteins, syntaxin and SNAP25 on the plasma membrane and synaptobrevin on the vesicle membrane. However, the precise steps in this process and the points at which assembly and fusion are controlled by regulatory proteins are unclear. In the present work, we examine the kinetics and intermediate states during SNARE assembly in vitro using a combination of time resolved fluorescence and EPR spectroscopy. We show that syntaxin rapidly forms a dimer prior to forming the kinetically stable 2:1 syntaxin:SNAP25 complex and that the 2:1 complex is not diminished by the presence of excess SNAP25. Moreover, the 2:1 complex is temperature-dependent with a reduced concentration at 37 °C. The two segments of SNAP25 behave differently. The N-terminal SN1 segment of SNAP25 exhibits a pronounced increase in backbone ordering from the N- to the C-terminus that is not seen in the C-terminal SNAP25 segment SN2. Both the SN1 and SN2 segments of SNAP25 will assemble with syntaxin; however, while the association of the SN1 segment with syntaxin produces a stable 2:2 (SN1:syntaxin) complex, the complex formed between SN2 and syntaxin is largely disordered. Synaptobrevin fails to bind syntaxin alone but will associate with syntaxin in the presence of either the SN1 or SN2 segments; however, the synaptobrevin:syntaxin:SN2 complex remains disordered. Taken together, these data suggest that synaptobrevin and syntaxin do not assemble in the absence of SNAP25 and that the SN2 segment of SNAP25 is the last to enter the SNARE complex.

Presynaptic Rac1 in the hippocampus selectively regulates working memory

One of the most extensively studied members of the Ras superfamily of small GTPases, Rac1 is an intracellular signal transducer that remodels actin and phosphorylation signaling networks. Previous studies have shown that Rac1-mediated signaling is associated with hippocampal-dependent working memory and longer-term forms of learning and memory and that Rac1 can modulate forms of both pre- and postsynaptic plasticity. How these different cognitive functions and forms of plasticity mediated by Rac1 are linked, however, is unclear. Here, we show that spatial working memory in mice is selectively impaired following the expression of a genetically encoded Rac1 inhibitor at presynaptic terminals, while longer-term cognitive processes are affected by Rac1 inhibition at postsynaptic sites. To investigate the regulatory mechanisms of this presynaptic process, we leveraged new advances in mass spectrometry to identify the proteomic and post-translational landscape of presynaptic Rac1 signaling. We identified serine/threonine kinases and phosphorylated cytoskeletal signaling and synaptic vesicle proteins enriched with active Rac1. The phosphorylated sites in these proteins are at positions likely to have regulatory effects on synaptic vesicles. Consistent with this, we also report changes in the distribution and morphology of synaptic vesicles and in postsynaptic ultrastructure following presynaptic Rac1 inhibition. Overall, this study reveals a previously unrecognized presynaptic role of Rac1 signaling in cognitive processes and provides insights into its potential regulatory mechanisms.

Neurotransmitter release is triggered by a calcium-induced rearrangement in the Synaptotagmin-1/SNARE complex primary interface

The Ca2+ sensor synaptotagmin-1 triggers neurotransmitter release together with the neuronal SNARE complex formed by syntaxin-1, SNAP25 and synaptobrevin. Moreover, synaptotagmin-1 increases synaptic vesicle priming and impairs spontaneous vesicle release. The synaptotagmin-1 C2B domain binds to the SNARE complex through a primary interface via two regions (I and II), but how exactly this interface mediates distinct functions of synaptotagmin-1, and the mechanism underlying Ca2+-triggering of release is unknown. Using mutagenesis and electrophysiological experiments, we show that region II is functionally and spatially subdivided: binding of C2B domain arginines to SNAP-25 acidic residues at one face of region II is crucial for Ca2+-evoked release but not for vesicle priming or clamping of spontaneous release, whereas other SNAP-25 and syntaxin-1 acidic residues at the other face mediate priming and clamping of spontaneous release but not evoked release. Mutations that disrupt region I impair the priming and clamping functions of synaptotagmin-1 while, strikingly, mutations that enhance binding through this region increase vesicle priming and clamping of spontaneous release, but strongly inhibit evoked release and vesicle fusogenicity. These results support previous findings that the primary interface mediates the functions of synaptotagmin-1 in vesicle priming and clamping of spontaneous release, and, importantly, show that Ca2+-triggering of release requires a rearrangement of the primary interface involving dissociation of region I, while region II remains bound. Together with modeling and biophysical studies presented in the accompanying paper, our data suggest a model whereby this rearrangement pulls the SNARE complex to facilitate fast synaptic vesicle fusion.

A lever hypothesis for Synaptotagmin-1 action in neurotransmitter release

Neurotransmitter release is triggered in microseconds by Ca2+-binding to the Synaptotagmin-1 C2 domains and by SNARE complexes that form four-helix bundles between synaptic vesicles and plasma membranes, but the coupling mechanism between Ca2+-sensing and membrane fusion is unknown. Release requires extension of SNARE helices into juxtamembrane linkers that precede transmembrane regions (linker zippering) and binding of the Synaptotagmin-1 C2B domain to SNARE complexes through a 'primary interface' comprising two regions (I and II). The Synaptotagmin-1 Ca2+-binding loops were believed to accelerate membrane fusion by inducing membrane curvature, perturbing lipid bilayers or helping bridge the membranes, but SNARE complex binding orients the Ca2+-binding loops away from the fusion site, hindering these putative activities. Molecular dynamics simulations now suggest that Synaptotagmin-1 C2 domains near the site of fusion hinder SNARE action, providing an explanation for this paradox and arguing against previous models of Sytnaptotagmin-1 action. NMR experiments reveal that binding of C2B domain arginines to SNARE acidic residues at region II remains after disruption of region I. These results and fluorescence resonance energy transfer assays, together with previous data, suggest that Ca2+ causes reorientation of the C2B domain on the membrane and dissociation from the SNAREs at region I but not region II. Based on these results and molecular modeling, we propose that Synaptotagmin-1 acts as a lever that pulls the SNARE complex when Ca2+ causes reorientation of the C2B domain, facilitating linker zippering and fast membrane fusion. This hypothesis is supported by the electrophysiological data described in the accompanying paper.

SM protein Sly1 and a SNARE Habc domain promote membrane fusion through multiple mechanisms

SM proteins including Sly1 are essential cofactors of SNARE-mediated membrane fusion. Using SNARE and Sly1 mutants and chemically defined in vitro assays, we separate and assess proposed mechanisms through which Sly1 augments fusion: (i) opening the closed conformation of the Qa-SNARE Sed5; (ii) close-range tethering of vesicles to target organelles, mediated by the Sly1-specific regulatory loop; and (iii) nucleation of productive trans-SNARE complexes. We show that all three mechanisms are important and operate in parallel, and that close-range tethering promotes trans-complex assembly when cis-SNARE assembly is a competing process. Further, we demonstrate that the autoinhibitory N-terminal Habc domain of Sed5 has at least two positive activities: it is needed for correct Sed5 localization, and it directly promotes Sly1-dependent fusion. "Split Sed5," with Habc presented solely as a soluble fragment, can function both in vitro and in vivo. Habc appears to facilitate events leading to lipid mixing rather than promoting opening or stability of the fusion pore.

Phosphorylation of Doc2 by EphB2 modulates Munc13-mediated SNARE complex assembly and neurotransmitter release

At the synapse, presynaptic neurotransmitter release is tightly controlled by release machinery, involving the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins and Munc13. The Ca2+ sensor Doc2 cooperates with Munc13 to regulate neurotransmitter release, but the underlying mechanisms remain unclear. In our study, we have characterized the binding mode between Doc2 and Munc13 and found that Doc2 originally occludes Munc13 to inhibit SNARE complex assembly. Moreover, our investigation unveiled that EphB2, a presynaptic adhesion molecule (SAM) with inherent tyrosine kinase functionality, exhibits the capacity to phosphorylate Doc2. This phosphorylation attenuates Doc2 block on Munc13 to promote SNARE complex assembly, which functionally induces spontaneous release and synaptic augmentation. Consistently, application of a Doc2 peptide that interrupts Doc2-Munc13 interplay impairs excitatory synaptic transmission and leads to dysfunction in spatial learning and memory. These data provide evidence that SAMs modulate neurotransmitter release by controlling SNARE complex assembly.

Sec18 binds the tethering/SM complex HOPS to engage the Qc-SNARE for membrane fusion

Membrane fusion is regulated by Rab GTPases, their tethering effectors such as HOPS, SNARE proteins on each fusion partner, SM proteins to catalyze SNARE assembly, Sec17 (SNAP), and Sec18 (NSF). Though concentrated HOPS can support fusion without Sec18, we now report that fusion falls off sharply at lower HOPS levels, where direct Sec18 binding to HOPS restores fusion. This Sec18-dependent fusion needs adenine nucleotide but neither ATP hydrolysis nor Sec17. Sec18 enhances HOPS recognition of the Qc-SNARE. With high levels of HOPS, Qc has a Km for fusion of a few nM. Either lower HOPS levels, or substitution of a synthetic tether for HOPS, strikingly increases the Km for Qc to several hundred nM. With dilute HOPS, Sec18 returns the Km for Qc to low nM. In contrast, HOPS concentration and Sec18 have no effect on Qb-SNARE recognition. Just as Qc is required for fusion but not for the initial assembly of SNAREs in trans, impaired Qc recognition by limiting HOPS without Sec18 still allows substantial trans-SNARE assembly. Thus, in addition to the known Sec18 functions of disassembling SNARE complexes, oligomerizing Sec17 for membrane association, and allowing Sec17 to drive fusion without complete SNARE zippering, we report a fourth Sec18 function, the Sec17-independent binding of Sec18 to HOPS to enhance functional Qc-SNARE engagement.

Molecular mechanism underlying SNARE-mediated membrane fusion enlightened by all-atom molecular dynamics simulations

The SNAP receptor (SNARE) proteins syntaxin-1, SNAP-25, and synaptobrevin mediate neurotransmitter release by forming tight SNARE complexes that fuse synaptic vesicles with the plasma membranes in microseconds. Membrane fusion is generally explained by the action of proteins on macroscopic membrane properties such as curvature, elastic modulus, and tension, and a widespread model envisions that the SNARE motifs, juxtamembrane linkers, and C-terminal transmembrane regions of synaptobrevin and syntaxin-1 form continuous helices that act mechanically as semirigid rods, squeezing the membranes together as they assemble ("zipper") from the N to the C termini. However, the mechanism underlying fast SNARE-induced membrane fusion remains unknown. We have used all-atom molecular dynamics simulations to investigate this mechanism. Our results need to be interpreted with caution because of the limited number and length of the simulations, but they suggest a model of membrane fusion that has a natural physicochemical basis, emphasizes local molecular events over general membrane properties, and explains extensive experimental data. In this model, the central event that initiates fast (microsecond scale) membrane fusion occurs when the SNARE helices zipper into the juxtamembrane linkers which, together with the adjacent transmembrane regions, promote encounters of acyl chains from both bilayers at the polar interface. The resulting hydrophobic nucleus rapidly expands into stalk-like structures that gradually progress to form a fusion pore, aided by the SNARE transmembrane regions and without clearly discernible intermediates. The propensity of polyunsaturated lipids to participate in encounters that initiate fusion suggests that these lipids may be important for the high speed of neurotransmitter release.

Control of Munc13-1 Activity by Autoinhibitory Interactions Involving the Variable N-terminal Region

Regulation of neurotransmitter release during presynaptic plasticity underlies varied forms of information processing in the brain. Munc13s play essential roles in release via their conserved C-terminal region, which contains a MUN domain involved in SNARE complex assembly, and controls multiple presynaptic plasticity processes. Munc13s also have a variable N-terminal region, which in Munc13-1 includes a calmodulin binding (CaMb) domain involved in short-term plasticity and a C2A domain that forms an inhibitory homodimer. The C2A domain is activated by forming a heterodimer with the zinc-finger domain of αRIMs, providing a link to αRIM-dependent short- and long-term plasticity. However, it is unknown how the functions of the N- and C-terminal regions are integrated, in part because of the difficulty of purifying Munc13-1 fragments containing both regions. We describe for the first time the purification of a Munc13-1 fragment spanning its entire sequence except for a flexible region between the C2A and CaMb domains. We show that this fragment is much less active than the Munc13-1 C-terminal region in liposome fusion assays and that its activity is strongly enhanced by the RIM2α zinc-finger domain together with calmodulin. NMR experiments show that the C2A and CaMb domains bind to the MUN domain and that these interactions are relieved by the RIM2α ZF domain and calmodulin, respectively. These results suggest a model whereby Munc13-1 activity in promoting SNARE complex assembly and neurotransmitter release are inhibited by interactions of the C2A and CaMb domains with the MUN domain that are relieved by αRIMs and calmodulin.

Tomosyns attenuate SNARE assembly and synaptic depression by binding to VAMP2-containing template complexes

Tomosyns are widely thought to attenuate membrane fusion by competing with synaptobrevin-2/VAMP2 for SNARE-complex assembly. Here, we present evidence against this scenario. In a novel mouse model, tomosyn-1/2 deficiency lowered the fusion barrier and enhanced the probability that synaptic vesicles fuse, resulting in stronger synapses with faster depression and slower recovery. While wild-type tomosyn-1m rescued these phenotypes, substitution of its SNARE motif with that of synaptobrevin-2/VAMP2 did not. Single-molecule force measurements indeed revealed that tomosyn's SNARE motif cannot substitute synaptobrevin-2/VAMP2 to form template complexes with Munc18-1 and syntaxin-1, an essential intermediate for SNARE assembly. Instead, tomosyns extensively bind synaptobrevin-2/VAMP2-containing template complexes and prevent SNAP-25 association. Structure-function analyses indicate that the C-terminal polybasic region contributes to tomosyn's inhibitory function. These results reveal that tomosyns regulate synaptic transmission by cooperating with synaptobrevin-2/VAMP2 to prevent SNAP-25 binding during SNARE assembly, thereby limiting initial synaptic strength and equalizing it during repetitive stimulation.

The stability of the primed pool of synaptic vesicles and the clamping of spontaneous neurotransmitter release rely on the integrity of the C-terminal half of the SNARE domain of syntaxin-1A

The SNARE proteins are central in membrane fusion and, at the synapse, neurotransmitter release. However, their involvement in the dual regulation of the synchronous release while maintaining a pool of readily releasable vesicles remains unclear. Using a chimeric approach, we performed a systematic analysis of the SNARE domain of STX1A by exchanging the whole SNARE domain or its N- or C-terminus subdomains with those of STX2. We expressed these chimeric constructs in STX1-null hippocampal mouse neurons. Exchanging the C-terminal half of STX1's SNARE domain with that of STX2 resulted in a reduced RRP accompanied by an increased release rate, while inserting the C-terminal half of STX1's SNARE domain into STX2 leads to an enhanced priming and decreased release rate. Additionally, we found that the mechanisms for clamping spontaneous, but not for Ca2+-evoked release, are particularly susceptible to changes in specific residues on the outer surface of the C-terminus of the SNARE domain of STX1A. Particularly, mutations of D231 and R232 affected the fusogenicity of the vesicles. We propose that the C-terminal half of the SNARE domain of STX1A plays a crucial role in the stabilization of the RRP as well as in the clamping of spontaneous synaptic vesicle fusion through the regulation of the energetic landscape for fusion, while it also plays a covert role in the speed and efficacy of Ca2+-evoked release.

Presynaptic Rac1 in the hippocampus selectively regulates working memory

One of the most extensively studied members of the Ras superfamily of small GTPases, Rac1 is an intracellular signal transducer that remodels actin and phosphorylation signaling networks. Previous studies have shown that Rac1-mediated signaling is associated with hippocampal-dependent working memory and longer-term forms of learning and memory and that Rac1 can modulate forms of both pre- and postsynaptic plasticity. How these different cognitive functions and forms of plasticity mediated by Rac1 are linked, however, is unclear. Here, we show that spatial working memory is selectively impaired following the expression of a genetically encoded Rac1-inhibitor at presynaptic terminals, while longer-term cognitive processes are affected by Rac1 inhibition at postsynaptic sites. To investigate the regulatory mechanisms of this presynaptic process, we leveraged new advances in mass spectrometry to identify the proteomic and post-translational landscape of presynaptic Rac1 signaling. We identified serine/threonine kinases and phosphorylated cytoskeletal signaling and synaptic vesicle proteins enriched with active Rac1. The phosphorylated sites in these proteins are at positions likely to have regulatory effects on synaptic vesicles. Consistent with this, we also report changes in the distribution and morphology of synaptic vesicles and in postsynaptic ultrastructure following presynaptic Rac1 inhibition. Overall, this study reveals a previously unrecognized presynaptic role of Rac1 signaling in cognitive processes and provides insights into its potential regulatory mechanisms.

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.

Mechanisms of SNARE proteins in membrane fusion

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are a family of small conserved eukaryotic proteins that mediate membrane fusion between organelles and with the plasma membrane. SNAREs are directly or indirectly anchored to membranes. Prior to fusion, complementary SNAREs assemble between membranes with the aid of accessory proteins that provide a scaffold to initiate SNARE zippering, pulling the membranes together and mediating fusion. Recent advances have enabled the construction of detailed models describing bilayer transitions and energy barriers along the fusion pathway and have elucidated the structures of SNAREs complexed in various states with regulatory proteins. In this Review, we discuss how these advances are yielding an increasingly detailed picture of the SNARE-mediated fusion pathway, leading from first contact between the membranes via metastable non-bilayer intermediates towards the opening and expansion of a fusion pore. We describe how SNARE proteins assemble into complexes, how this assembly is regulated by accessory proteins and how SNARE complexes overcome the free energy barriers that prevent spontaneous membrane fusion.

Control of Munc13-1 Activity by Autoinhibitory Interactions Involving the Variable N-terminal Region

Regulation of neurotransmitter release during presynaptic plasticity underlies varied forms of information processing in the brain. Munc13s play essential roles in release via their conserved C-terminal region, which contains a MUN domain involved SNARE complex assembly, and control multiple presynaptic plasticity processes. Munc13s also have a variable N-terminal region, which in Munc13-1 includes a calmodulin binding (CaMb) domain involved in short-term plasticity and a C2A domain that forms an inhibitory homodimer. The C2A domain is activated by forming a heterodimer with the zinc-finger domain of αRIMs, providing a link to αRIM-dependent short- and long-term plasticity. However, it is unknown how the functions of the N- and C-terminal regions are integrated, in part because of the difficulty of purifying Munc13-1 fragments containing both regions. We describe for the first time the purification of a Munc13-1 fragment spanning its entire sequence except for a flexible region between the C2A and CaMb domains. We show that this fragment is much less active than the Munc13-1 C-terminal region in liposome fusion assays and that its activity is strongly enhanced by the RIM2α zinc-finger domain together with calmodulin. NMR experiments show that the C2A and CaMb domains bind to the MUN domain and that these interactions are relieved by the RIM2α ZF domain and calmodulin, respectively. These results suggest a model whereby Munc13-1 activity in promoting SNARE complex assembly and neurotransmitter release are inhibited by interactions of the C2A and CaMb domains with the MUN domain that are relieved by αRIMs and calmodulin.

Interrogation and validation of the interactome of neuronal Munc18-interacting Mint proteins with AlphaFold2

Munc18-interacting proteins (Mints) are multidomain adaptors that regulate neuronal membrane trafficking, signaling, and neurotransmission. Mint1 and Mint2 are highly expressed in the brain with overlapping roles in the regulation of synaptic vesicle fusion required for neurotransmitter release by interacting with the essential synaptic protein Munc18-1. Here, we have used AlphaFold2 to identify and then validate the mechanisms that underpin both the specific interactions of neuronal Mint proteins with Munc18-1 as well as their wider interactome. We found that a short acidic α-helical motif within Mint1 and Mint2 is necessary and sufficient for specific binding to Munc18-1 and binds a conserved surface on Munc18-1 domain3b. In Munc18-1/2 double knockout neurosecretory cells, mutation of the Mint-binding site reduces the ability of Munc18-1 to rescue exocytosis, and although Munc18-1 can interact with Mint and Sx1a (Syntaxin1a) proteins simultaneously in vitro, we find that they have mutually reduced affinities, suggesting an allosteric coupling between the proteins. Using AlphaFold2 to then examine the entire cellular network of putative Mint interactors provides a structural model for their assembly with a variety of known and novel regulatory and cargo proteins including ADP-ribosylation factor (ARF3/ARF4) small GTPases and the AP3 clathrin adaptor complex. Validation of Mint1 interaction with a new predicted binder TJAP1 (tight junction-associated protein 1) provides experimental support that AlphaFold2 can correctly predict interactions across such large-scale datasets. Overall, our data provide insights into the diversity of interactions mediated by the Mint family and show that Mints may help facilitate a key trigger point in SNARE (soluble N-ethylmaleimide-sensitive factor attachment receptor) complex assembly and vesicle fusion.

Exploring the conformational changes of the Munc18-1/syntaxin 1a complex

Neurotransmitters are released from synaptic vesicles, the membrane of which fuses with the plasma membrane upon calcium influx. This membrane fusion reaction is driven by the formation of a tight complex comprising the plasma membrane N-ethylmaleimide-sensitive factor attachment receptor (SNARE) proteins syntaxin-1a and SNAP-25 with the vesicle SNARE protein synaptobrevin. The neuronal protein Munc18-1 forms a stable complex with syntaxin-1a. Biochemically, syntaxin-1a cannot escape the tight grip of Munc18-1, so formation of the SNARE complex is inhibited. However, Munc18-1 is essential for the release of neurotransmitters in vivo. It has therefore been assumed that Munc18-1 makes the bound syntaxin-1a available for SNARE complex formation. Exactly how this occurs is still unclear, but it is assumed that structural rearrangements occur. Here, we used a series of mutations to specifically weaken the complex at different positions in order to induce these rearrangements biochemically. Our approach was guided through sequence and structural analysis and supported by molecular dynamics simulations. Subsequently, we created a homology model showing the complex in an altered conformation. This conformation presumably represents a more open arrangement of syntaxin-1a that permits the formation of a SNARE complex to be initiated while still bound to Munc18-1. In the future, research should investigate how this central reaction for neuronal communication is controlled by other proteins.

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

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

Sundaram RV, Ramakrishnan S, Pincet F, Rothman JE. Synaptophysin chaperones the assembly of 12 SNAREpins under each ready-release vesicle

The synaptic vesicle protein Synaptophysin (Syp) has long been known to form a complex with the Vesicle associated soluble N-ethylmaleimide sensitive fusion protein attachment receptor (v-SNARE) Vesicle associated membrane protein (VAMP), but a more specific molecular function or mechanism of action in exocytosis has been lacking because gene knockouts have minimal effects. Utilizing fully defined reconstitution and single-molecule measurements, we now report that Syp functions as a chaperone that determines the number of SNAREpins assembling between a ready-release vesicle and its target membrane bilayer. Specifically, Syp directs the assembly of 12 ± 1 SNAREpins under each docked vesicle, even in the face of an excess of SNARE proteins. The SNAREpins assemble in successive waves of 6 ± 1 and 5 ± 2 SNAREpins, respectively, tightly linked to oligomerization of and binding to the vesicle Ca++ sensor Synaptotagmin. Templating of 12 SNAREpins by Syp is likely the direct result of its hexamer structure and its binding of VAMP2 dimers, both of which we demonstrate in detergent extracts and lipid bilayers.

MARCKS Effector Domain, a reversible lipid ligand, illuminates late stages of membrane fusion

Yeast vacuolar HOPS tethers membranes, catalyzes trans-SNARE assembly between R- and Q-SNAREs, and shepherds SNAREs past early inhibition by Sec17. After partial SNARE zippering, fusion is driven slowly by either completion of SNARE zippering or by Sec17/Sec18, but rapid fusion needs zippering and Sec17/Sec18. Using reconstituted-vacuolar fusion, we find that MARCKS Effector Domain (MED) peptide, a lipid ligand, blocks fusion reversibly at a late reaction stage. The MED fusion blockade is overcome by either salt extraction, inactivation with the MED ligand calmodulin, or addition of Sec17/Sec18. During incubation with MED, SNAREs assemble stable complexes in trans and fusion becomes resistant to antibody to the Qa SNARE. When Q-SNAREs are preassembled, a synthetic tether can replace HOPS for fusion. With a synthetic tether, fusion needs both complete SNARE zippering and Sec17/Sec18 to overcome a MED block. In contrast, when SNARE domains are only two-third zippered, only HOPS will support Sec17/Sec18 driven fusion without needing complete zippering. HOPS thus remains engaged with SNAREs during zippering. MED facilitates the study of distinct fusion stages: tethering, initial trans-SNARE assembly and its sensitivity to Sec17, SNARE zippering, Sec17/Sec18 engagement, and lipid and lumenal mixing.

Regulation of Syntaxin3B-Mediated Membrane Fusion by T14, Munc18, and Complexin

Retinal neurons that form ribbon-style synapses operate over a wide dynamic range, continuously relaying visual information to their downstream targets. The remarkable signaling abilities of these neurons are supported by specialized presynaptic machinery, one component of which is syntaxin3B. Syntaxin3B is an essential t-SNARE protein of photoreceptors and bipolar cells that is required for neurotransmitter release. It has a light-regulated phosphorylation site in its N-terminal domain at T14 that has been proposed to modulate membrane fusion. However, a direct test of the latter has been lacking. Using a well-controlled in vitro fusion assay, we found that a phosphomimetic T14 syntaxin3B mutation leads to a small but significant enhancement of SNARE-mediated membrane fusion following the formation of the t-SNARE complex. While the addition of Munc18a had only a minimal effect on membrane fusion mediated by SNARE complexes containing wild-type syntaxin3B, a more significant enhancement was observed in the presence of Munc18a when the SNARE complexes contained a syntaxin3B T14 phosphomimetic mutant. Finally, we showed that the retinal-specific complexins (Cpx III and Cpx IV) inhibited membrane fusion mediated by syntaxin3B-containing SNARE complexes in a dose-dependent manner. Collectively, our results establish that membrane fusion mediated by syntaxin3B-containing SNARE complexes is regulated by the T14 residue of syntaxin3B, Munc18a, and Cpxs III and IV.

Turbocharging synaptic transmission

Evidence from biochemistry, genetics, and electron microscopy strongly supports the idea that a ring of Synaptotagmin is central to the clamping and release of synaptic vesicles (SVs) for synchronous neurotransmission. Recent direct measurements in cell-free systems suggest there are 12 SNAREpins in each ready-release vesicle, consisting of six peripheral and six central SNAREpins. The six central SNAREpins are directly bound to the Synaptotagmin ring, are directly released by Ca++ , and they initially open the fusion pore. The six peripheral SNAREpins are indirectly bound to the ring, each linked to a central SNAREpin by a bridging molecule of Complexin. We suggest that the primary role of peripheral SNAREpins is to provide additional force to 'turbocharge' neurotransmitter release, explaining how it can occur much faster than other forms of membrane fusion. The SV protein Synaptophysin forms hexamers that bear two copies of the v-SNARE VAMP at each vertex, one likely assembling into a peripheral SNAREpin and the other into a central SNAREpin.

Roles for diacylglycerol in synaptic vesicle priming and release revealed by complete reconstitution of core protein machinery

Here, we introduce the full functional reconstitution of genetically validated core protein machinery (SNAREs, Munc13, Munc18, Synaptotagmin, and Complexin) for synaptic vesicle priming and release in a geometry that enables detailed characterization of the fate of docked vesicles both before and after release is triggered with Ca2+. Using this setup, we identify new roles for diacylglycerol (DAG) in regulating vesicle priming and Ca2+-triggered release involving the SNARE assembly chaperone Munc13. We find that low concentrations of DAG profoundly accelerate the rate of Ca2+-dependent release, and high concentrations reduce clamping and permit extensive spontaneous release. As expected, DAG also increases the number of docked, release-ready vesicles. Dynamic single-molecule imaging of Complexin binding to release-ready vesicles directly establishes that DAG accelerates the rate of SNAREpin assembly mediated by chaperones, Munc13 and Munc18. The selective effects of physiologically validated mutations confirmed that the Munc18-Syntaxin-VAMP2 "template" complex is a functional intermediate in the production of primed, release-ready vesicles, which requires the coordinated action of Munc13 and Munc18.

The machinery of vesicle fusion

The compartmentalization of eukaryotic cells is reliant on the fidelity of vesicle-mediated intracellular transport. Vesicles deliver their cargo via membrane fusion, a process requiring membrane tethers, Sec1/Munc18 (SM) proteins, and SNAREs. These components function in concert to ensure that membrane fusion is efficient and accurate, but the mechanisms underlying their cooperative action are still in many respects mysterious. In this brief review, we highlight recent progress toward a more integrative understanding of the vesicle fusion machinery. We focus particular attention on cryo-electron microscopy structures of intact multisubunit tethers in complex with SNAREs or SM proteins, as well as a structure of an SM protein bound to multiple SNAREs. The insights gained from this work emphasize the advantages of studying the fusion machinery intact and in context.

Efficient fusion requires a membrane anchor on the vacuolar Qa-SNARE

As a prelude to fusion, the R-SNARE on one membrane zippers with Qa-, Qb-, and Qc-SNAREs from its apposed fusion partner, forming a four-helical bundle that draws the two membranes together. Because Qa- and Qb-SNAREs are anchored to the same membrane and are adjacent in the 4-SNARE bundle, their two anchors might be redundant. Using the recombinant pure protein catalysts of yeast vacuole fusion, we now report that the specific distribution of transmembrane (TM) anchors on the Q-SNAREs is critical for efficient fusion. A TM anchor on the Qa-SNARE supports rapid fusion even when the other two Q-SNAREs are unanchored, while a TM anchor on the Qb-SNARE is dispensable and is insufficient for rapid fusion as the sole Q-SNARE anchor. This does not depend on which specific TM domain is attached to the Qa-SNARE but rather is due to the Qa-SNARE being anchored per se. The need for Qa-SNARE anchoring is even seen when the homotypic fusion and vacuole protein sorting protein (HOPS), the physiological catalyst of tethering and SNARE assembly, is replaced by an artificial tether. The need for a Qa TM anchor is thus a fundamental property of vacuolar SNARE zippering-induced fusion and may reflect the need for the Qa juxtamembrane (JxQa) region to be anchored between its SNARE and TM domains. This requirement for Qa-SNARE anchoring and correct JxQa position is bypassed by Sec17/Sec18, exploiting a platform of partially zippered SNAREs. Because Qa is the only synaptic Q-SNARE with a TM anchor, the need for Qa-specific anchoring may reflect a general requirement for SNARE-mediated fusion.

A Frame-by-Frame Glance at Membrane Fusion Mechanisms: From Viral Infections to Fertilization

Viral entry and fertilization are distinct biological processes that share a common mechanism: membrane fusion. In viral entry, enveloped viruses attach to the host cell membrane, triggering a series of conformational changes in the viral fusion proteins. This results in the exposure of a hydrophobic fusion peptide, which inserts into the host membrane and brings the viral and host membranes into close proximity. Subsequent structural rearrangements in opposing membranes lead to their fusion. Similarly, membrane fusion occurs when gametes merge during the fertilization process, though the exact mechanism remains unclear. Structural biology has played a pivotal role in elucidating the molecular mechanisms underlying membrane fusion. High-resolution structures of the viral and fertilization fusion-related proteins have provided valuable insights into the conformational changes that occur during this process. Understanding these mechanisms at a molecular level is essential for the development of antiviral therapeutics and tools to influence fertility. In this review, we will highlight the biological importance of membrane fusion and how protein structures have helped visualize both common elements and subtle divergences in the mechanisms behind fusion; in addition, we will examine the new tools that recent advances in structural biology provide researchers interested in a frame-by-frame understanding of membrane fusion.

Identification of residues critical for the extension of Munc18-1 domain 3a

BACKGROUND

Neurotransmitter release depends on the fusion of synaptic vesicles with the presynaptic membrane and is mainly mediated by SNARE complex assembly. During the transition of Munc18-1/Syntaxin-1 to the SNARE complex, the opening of the Syntaxin-1 linker region catalyzed by Munc13-1 leads to the extension of the domain 3a hinge loop, which enables domain 3a to bind SNARE motifs in Synaptobrevin-2 and Syntaxin-1 and template the SNARE complex assembly. However, the exact mechanism of domain 3a extension remains elusive.

RESULTS

Here, we characterized residues on the domain 3a hinge loop that are crucial for the extension of domain 3a by using biophysical and biochemical approaches and electrophysiological recordings. We showed that the mutation of residues T323/M324/R325 disrupted Munc13-1-mediated SNARE complex assembly and membrane fusion starting from Munc18-1/Syntaxin-1 in vitro and caused severe defects in the synaptic exocytosis of mouse cortex neurons in vivo. Moreover, the mutation had no effect on the binding of Synaptobrevin-2 to isolated Munc18-1 or the conformational change of the Syntaxin-1 linker region catalyzed by the Munc13-1 MUN domain. However, the extension of the domain 3a hinge loop in Munc18-1/Syntaxin-1 was completely disrupted by the mutation, leading to the failure of Synaptobrevin-2 binding to Munc18-1/Syntaxin-1.

CONCLUSIONS

Together with previous results, our data further support the model that the template function of Munc18-1 in SNARE complex assembly requires the extension of domain 3a, and particular residues in the domain 3a hinge loop are crucial for the autoinhibitory release of domain 3a after the MUN domain opens the Syntaxin-1 linker region.

Synaptophysin Chaperones the Assembly of 12 SNAREpins under each Ready-Release Vesicle

The synaptic vesicle protein Synaptophysin has long been known to form a complex with the v-SNARE VAMP, but a more specific molecular function or mechanism of action in exocytosis has been lacking because gene knockouts have minimal effects. Utilizing fully-defined reconstitution and single-molecule measurements, we now report that Synaptophysin functions as a chaperone that determines the number of SNAREpins assembling between a ready-release vesicle and its target membrane bilayer. Specifically, Synaptophysin directs the assembly of 12 ± 1 SNAREpins under each docked vesicle, even in the face of an excess of SNARE proteins. The SNAREpins assemble in successive waves of 6 ± 1 and 5 ± 2 SNAREpins, respectively, tightly linked to oligomerization of and binding to the vesicle Ca++ sensor Synaptotagmin. Templating of 12 SNAREpins by Synaptophysin is likely the direct result of its hexamer structure and its binding of VAMP2 dimers, both of which we demonstrate in detergent extracts and lipid bilayers.

Novel Roles for Diacylglycerol in Synaptic Vesicle Priming and Release Revealed by Complete Reconstitution of Core Protein Machinery

Here we introduce the full functional reconstitution of genetically-validated core protein machinery (SNAREs, Munc13, Munc18, Synaptotagmin, Complexin) for synaptic vesicle priming and release in a geometry that enables detailed characterization of the fate of docked vesicles both before and after release is triggered with Ca 2+ . Using this novel setup, we discover new roles for diacylglycerol (DAG) in regulating vesicle priming and Ca 2+- triggered release involving the SNARE assembly chaperone Munc13. We find that low concentrations of DAG profoundly accelerate the rate of Ca 2+ -dependent release, and high concentrations reduce clamping and permit extensive spontaneous release. As expected, DAG also increases the number of ready-release vesicles. Dynamic single-molecule imaging of Complexin binding to ready-release vesicles directly establishes that DAG accelerates the rate of SNAREpin assembly mediated by Munc13 and Munc18 chaperones. The selective effects of physiologically validated mutations confirmed that the Munc18-Syntaxin-VAMP2 'template' complex is a functional intermediate in the production of primed, ready-release vesicles, which requires the coordinated action of Munc13 and Munc18.

SIGNIFICANCE STATEMENT

Munc13 and Munc18 are SNARE-associated chaperones that act as "priming" factors, facilitating the formation of a pool of docked, release-ready vesicles and regulating Ca 2+ -evoked neurotransmitter release. Although important insights into Munc18/Munc13 function have been gained, how they assemble and operate together remains enigmatic. To address this, we developed a novel biochemically-defined fusion assay which enabled us to investigate the cooperative action of Munc13 and Munc18 in molecular terms. We find that Munc18 nucleates the SNARE complex, while Munc13 promotes and accelerates the SNARE assembly in a DAG-dependent manner. The concerted action of Munc13 and Munc18 stages the SNARE assembly process to ensure efficient 'clamping' and formation of stably docked vesicles, which can be triggered to fuse rapidly (∼10 msec) upon Ca 2+ influx.

Forty years of the adrenal chromaffin cell through ISCCB meetings around the world

This historical review focuses on the evolution of the knowledge accumulated during the last two centuries on the biology of the adrenal medulla gland and its chromaffin cells (CCs). The review emerged in the context of a series of meetings that started on the Spanish island of Ibiza in 1982 with the name of the International Symposium on Chromaffin Cell Biology (ISCCB). Hence, the review is divided into two periods namely, before 1982 and from this year to 2022, when the 21st ISCCB meeting was just held in Hamburg, Germany. The first historical period extends back to 1852 when Albert Kölliker first described the fine structure and function of the adrenal medulla. Subsequently, the adrenal staining with chromate salts identified the CCs; this was followed by the establishment of the embryological origin of the adrenal medulla, and the identification of adrenaline-storing vesicles. By the end of the nineteenth century, the basic morphology, histochemistry, and embryology of the adrenal gland were known. The twentieth century began with breakthrough findings namely, the experiment of Elliott suggesting that adrenaline was the sympathetic neurotransmitter, the isolation of pure adrenaline, and the deciphering of its molecular structure and chemical synthesis in the laboratory. In the 1950s, Blaschko isolated the catecholamine-storing vesicles from adrenal medullary extracts. This switched the interest in CCs as models of sympathetic neurons with an explosion of studies concerning their functions, i.e., uptake of catecholamines by chromaffin vesicles through a specific coupled transport system; the identification of several vesicle components in addition to catecholamines including chromogranins, ATP, opioids, and other neuropeptides; the calcium-dependence of the release of catecholamines; the underlying mechanism of exocytosis of this release, as indicated by the co-release of proteins; the cross-talk between the adrenal cortex and the medulla; and the emission of neurite-like processes by CCs in culture, among other numerous findings. The 1980s began with the introduction of new high-resolution techniques such as patch-clamp, calcium probes, marine toxins-targeting ion channels and receptors, confocal microscopy, or amperometry. In this frame of technological advances at the Ibiza ISCCB meeting in 1982, 11 senior researchers in the field predicted a notable increase in our knowledge in the field of CCs and the adrenal medulla; this cumulative knowledge that occurred in the last 40 years of history of the CC is succinctly described in the second part of this historical review. It deals with cell excitability, ion channel currents, the exocytotic fusion pore, the handling of calcium ions by CCs, the kinetics of exocytosis and endocytosis, the exocytotic machinery, and the life cycle of secretory vesicles. These concepts together with studies on the dynamics of membrane fusion with super-resolution imaging techniques at the single-protein level were extensively reviewed by top scientists in the field at the 21st ISCCB meeting in Hamburg in the summer of 2022; this frontier topic is also briefly reviewed here. Many of the concepts arising from those studies contributed to our present understanding of synaptic transmission. This has been studied in physiological or pathophysiological conditions, in CCs from animal disease models. In conclusion, the lessons we have learned from CC biology as a peripheral model for brain and brain disease pertain more than ever to cutting-edge research in neurobiology. In the 22nd ISCCB meeting in Israel in 2024 that Uri Asheri is organizing, we will have the opportunity of seeing the progress of the questions posed in Ibiza, and on other questions that undoubtedly will arise.

Lytic granule exocytosis at immune synapses: lessons from neuronal synapses

Regulated exocytosis is a central mechanism of cellular communication. It is not only the basis for neurotransmission and hormone release, but also plays an important role in the immune system for the release of cytokines and cytotoxic molecules. In cytotoxic T lymphocytes (CTLs), the formation of the immunological synapse is required for the delivery of the cytotoxic substances such as granzymes and perforin, which are stored in lytic granules and released via exocytosis. The molecular mechanisms of their fusion with the plasma membrane are only partially understood. In this review, we discuss the molecular players involved in the regulated exocytosis of CTL, highlighting the parallels and differences to neuronal synaptic transmission. Additionally, we examine the strengths and weaknesses of both systems to study exocytosis.

A non-canonical target-binding site in Munc18-1 domain 3b for assembling the Mint1-Munc18-1-syntaxin-1 complex

As the prototype of Sec1/Munc18 (SM) family proteins, Munc18-1 can manipulate the distinct conformations of syntaxin-1 for controlling intracellular membrane fusion. The Munc18-1-interacting domain of Mint1 (Mint1-MID) binds to Munc18-1 together with syntaxin-1 to form a Mint1-Munc18-1-syntaxin-1 complex, but the mechanism underlying the complex assembly remains unclear. Here, we determine the structure of the Mint1-MID-Munc18-1-syntaxin-1 complex. Unexpectedly, Munc18-1 recognizes Mint1-MID and syntaxin-1 simultaneously via two opposite sites. The canonical central cavity between domains 1 and 3a of Munc18-1 embraces closed syntaxin-1, whereas the non-canonical basic pocket in domain 3b captures the acidic Mint1-MID helix. The domain 3b-mediated recognition of an acidic-helical motif is distinct from other target-recognition modes of Munc18-1. Mutations in the interface between domain 3b and Mint1-MID disrupt the assembly of the Mint1-Munc18-1-syntaxin-1 complex. This work reveals a non-canonical target-binding site in Munc18-1 domain 3b for assembling the Mint1-Munc18-1-syntaxin-1 complex.

SNARE Proteins in Synaptic Vesicle Fusion

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

Analysis of tripartite Synaptotagmin-1-SNARE-complexin-1 complexes in solution

Characterizing interactions of Synaptotagmin-1 with the SNARE complex is crucial to understand the mechanism of neurotransmitter release. X-ray crystallography revealed how the Synaptotagmin-1 C2 B domain binds to the SNARE complex through a so-called primary interface and to a complexin-1-SNARE complex through a so-called tripartite interface. Mutagenesis and electrophysiology supported the functional relevance of both interfaces, and extensive additional data validated the primary interface. However, ITC evidence suggesting that binding via the tripartite interface occurs in solution was called into question by subsequent NMR data. Here, we describe joint efforts to address this apparent contradiction. Using the same ITC approach with the same C2 B domain mutant used previously (C2 BKA-Q ) but including ion exchange chromatography to purify it, which is crucial to remove polyacidic contaminants, we were unable to observe the substantial endothermic ITC signal that was previously attributed to binding of this mutant to the complexin-1-SNARE complex through the tripartite interface. We were also unable to detect substantial populations of the tripartite interface in NMR analyses of the ITC samples or in measurements of paramagnetic relaxation effects, despite the high sensitivity of this method to detect weak protein complexes. However, these experiments do not rule out the possibility of very low affinity (KD  > 1 mm) binding through this interface. These results emphasize the need to develop methods to characterize the structure of synaptotagmin-1-SNARE complexes between two membranes and to perform further structure-function analyses to establish the physiological relevance of the tripartite interface.

Extracellular Vesicles (EVs) in Tumor Diagnosis and Therapy

In recent years, extracellular vesicles (EVs) have gained significant attention due to their tremendous potential for clinical applications. EVs play a crucial role in various aspects, including tumorigenesis, drug resistance, immune escape, and reconstruction of the tumor microenvironment. Despite the growing interest in EVs, many questions still need to be addressed before they can be practically applied in clinical settings. This paper aims to review EVs' isolation methods, structure research, the roles of EVs in tumorigenesis and their mechanisms in multiple types of tumors, their potential application in drug delivery, and the expectations for their future in clinical research.

A dynamic template complex mediates Munc18-chaperoned SNARE assembly

Munc18 chaperones assembly of three membrane-anchored soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) into a four-helix bundle to mediate membrane fusion between vesicles and plasma membranes, leading to neurotransmitter or insulin release, glucose transporter (GLUT4) translocation, or other exocytotic processes. Yet, the molecular mechanism underlying chaperoned SNARE assembly is not well understood. Recent evidence suggests that Munc18-1 and Munc18-3 simultaneously bind their cognate SNAREs to form ternary template complexes - Munc18-1:Syntaxin-1:VAMP2 for synaptic vesicle fusion and Munc18-3:Syntaxin-4:VAMP2 for GLUT4 translocation and insulin release, which facilitate the binding of SNAP-25 or SNAP-23 to conclude SNARE assembly. Here, we further investigate the structure, dynamics, and function of the template complexes using optical tweezers. Our results suggest that the synaptic template complex transitions to an activated state with a rate of 0.054 s-1 for efficient SNAP-25 binding. The transition depends upon the linker region of syntaxin-1 upstream of its helical bundle-forming SNARE motif. In addition, the template complex is stabilized by a poorly characterized disordered loop region in Munc18-1. While the synaptic template complex efficiently binds both SNAP-25 and SNAP-23, the GLUT4 template complex strongly favors SNAP-23 over SNAP-25, despite the similar stabilities of their assembled SNARE bundles. Together, our data demonstrate that a highly dynamic template complex mediates efficient and specific SNARE assembly.

Rabphilin 3A binds the N-peptide of SNAP-25 to promote SNARE complex assembly in exocytosis

Exocytosis of secretory vesicles requires the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins and small GTPase Rabs. As a Rab3/Rab27 effector protein on secretory vesicles, Rabphilin 3A was implicated to interact with SNAP-25 to regulate vesicle exocytosis in neurons and neuroendocrine cells, yet the underlying mechanism remains unclear. In this study, we have characterized the physiologically relevant binding sites between Rabphilin 3A and SNAP-25. We found that an intramolecular interplay between the N-terminal Rab-binding domain and C-terminal C2AB domain enables Rabphilin 3A to strongly bind the SNAP-25 N-peptide region via its C2B bottom α-helix. Disruption of this interaction significantly impaired docking and fusion of vesicles with the plasma membrane in rat PC12 cells. In addition, we found that this interaction allows Rabphilin 3A to accelerate SNARE complex assembly. Furthermore, we revealed that this interaction accelerates SNARE complex assembly via inducing a conformational switch from random coils to α-helical structure in the SNAP-25 SNARE motif. Altogether, our data suggest that the promotion of SNARE complex assembly by binding the C2B bottom α-helix of Rabphilin 3A to the N-peptide of SNAP-25 underlies a pre-fusion function of Rabphilin 3A in vesicle exocytosis.

Assembly-promoting protein Munc18c stimulates SNARE-dependent membrane fusion through its SNARE-like peptide

Intracellular vesicle fusion requires the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and their cognate Sec1/Munc18 (SM) proteins. How SM proteins act in concert with trans-SNARE complexes to promote membrane fusion remains incompletely understood. Munc18c, a broadly distributed SM protein, selectively regulates multiple exocytotic pathways, including GLUT4 exocytosis. Here, using an in vitro reconstituted system, we discovered a SNARE-like peptide (SLP), conserved in Munc18-1 of synaptic exocytosis, is crucial to the stimulatory activity of Munc18c in vesicle fusion. The direct stimulation of the SNARE-mediated fusion reaction by SLP further supported the essential role of this fragment. Interestingly, we found SLP strongly accelerates the membrane fusion rate when anchored to the target membrane but not the vesicle membrane, suggesting it primarily interacts with t-SNAREs in cis to drive fusion. Furthermore, we determined the SLP fragment is competitive with the full-length Munc18c protein and specific to the cognate v-SNARE isoforms, supporting how it could resemble Munc18c’s activity in membrane fusion. Together, our findings demonstrate that Munc18c facilitates SNARE-dependent membrane fusion through SLP, revealing that the t-SNARE-SLP binding mode might be a conserved mechanism for the stimulatory function of SM proteins in vesicle fusion.