Synaptotagmin rings as high-sensitivity regulators of synaptic vesicle docking and fusion

Synchronous release at neuronal synapses is accomplished by a machinery that senses calcium influx and fuses the synaptic vesicle and plasma membranes to release neurotransmitters. Previous studies suggested the calcium sensor synaptotagmin (Syt) is a facilitator of vesicle docking and both a facilitator and inhibitor of fusion. On phospholipid monolayers, the Syt C2AB domain spontaneously oligomerized into rings that are disassembled by Ca2+, suggesting Syt rings may clamp fusion as membrane-separating "washers" until Ca2+-mediated disassembly triggers fusion and release [J. Wang et al., Proc. Natl. Acad. Sci. U.S.A. 111, 13966-13971 (2014)].). Here, we combined mathematical modeling with experiment to measure the mechanical properties of Syt rings and to test this mechanism. Consistent with experimental results, the model quantitatively recapitulates observed Syt ring-induced dome and volcano shapes on phospholipid monolayers and predicts rings are stabilized by anionic phospholipid bilayers or bulk solution with ATP. The selected ring conformation is highly sensitive to membrane composition and bulk ATP levels, a property that may regulate vesicle docking and fusion in ATP-rich synaptic terminals. We find the Syt molecules hosted by a synaptic vesicle oligomerize into a halo, unbound from the vesicle, but in proximity to sufficiently phosphatidylinositol 4,5-bisphosphate (PIP2)-rich plasma membrane (PM) domains, the PM-bound trans Syt ring conformation is preferred. Thus, the Syt halo serves as landing gear for spatially directed docking at PIP2-rich sites that define the active zones of exocytotic release, positioning the Syt ring to clamp fusion and await calcium. Our results suggest the Syt ring is both a Ca2+-sensitive fusion clamp and a high-fidelity sensor for directed docking.

Calcium-dependent docking of synaptic vesicles

The concentration of calcium ions in presynaptic terminals regulates transmitter release, but underlying mechanisms have remained unclear. Here we review recent studies that shed new light on this issue. Fast-freezing electron microscopy and total internal reflection fluorescence microscopy studies reveal complex calcium-dependent vesicle movements including docking on a millisecond time scale. Recordings from so-called 'simple synapses' indicate that calcium not only triggers exocytosis, but also modifies synaptic strength by controlling a final, rapid vesicle maturation step before release. Molecular studies identify several calcium-sensitive domains on Munc13 and on synaptotagmin-1 that are likely involved in bringing the vesicular and plasma membranes closer together in response to calcium elevation. Together, these results suggest that calcium-dependent vesicle docking occurs in a wide range of time domains and plays a crucial role in several phenomena including synaptic facilitation, post-tetanic potentiation, and neuromodulator-induced potentiation.

Myosin V Regulates Spatial Localization of Different Forms of Neurotransmitter Release in Central Synapses

Synaptic active zone (AZ) contains multiple specialized release sites for vesicle fusion. The utilization of release sites is regulated to determine spatiotemporal organization of the two main forms of synchronous release, uni-vesicular (UVR) and multi-vesicular (MVR). We previously found that the vesicle-associated molecular motor myosin V regulates temporal utilization of release sites by controlling vesicle anchoring at release sites in an activity-dependent manner. Here we show that acute inhibition of myosin V shifts preferential location of vesicle docking away from AZ center toward periphery, and results in a corresponding spatial shift in utilization of release sites during UVR. Similarly, inhibition of myosin V also reduces preferential utilization of central release sites during MVR, leading to more spatially distributed and temporally uniform MVR that occurs farther away from the AZ center. Using a modeling approach, we provide a conceptual framework that unites spatial and temporal functions of myosin V in vesicle release by controlling the gradient of release site release probability across the AZ, which in turn determines the spatiotemporal organization of both UVR and MVR. Thus myosin V regulates both temporal and spatial utilization of release sites during two main forms of synchronous release.

Ultrastructural Correlates of Presynaptic Functional Heterogeneity in Hippocampal Synapses

Although similar in molecular composition, synapses can exhibit strikingly distinct functional transmitter release and plasticity characteristics. To determine whether ultrastructural differences co-define this functional heterogeneity, we combine hippocampal organotypic slice cultures, high-pressure freezing, freeze substitution, and 3D-electron tomography to compare two functionally distinct synapses: hippocampal Schaffer collateral and mossy fiber synapses. We find that mossy fiber synapses, which exhibit a lower release probability and stronger short-term facilitation than Schaffer collateral synapses, harbor lower numbers of docked synaptic vesicles at active zones and a second pool of possibly tethered vesicles in their vicinity. Our data indicate that differences in the ratio of docked versus tethered vesicles at active zones contribute to distinct functional characteristics of synapses.

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Electron tomography enables the dissection of vesicle pools at synaptic active zones

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Docked and primed vesicle availability contributes to initial release probability

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The ratio of docked and tethered vesicles may co-determine short-term plasticity

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Hippocampal mossy fibers contain three morphological types of docked vesicles

Active Zone Material-Directed Orientation, Docking, and Fusion of Dense Core Vesicles Alongside Synaptic Vesicles at Neuromuscular Junctions

Active zone material is an organelle that is common to active zones along the presynaptic membrane of chemical synapses. Electron tomography on active zones at frog neuromuscular junctions has provided evidence that active zone material directs the docking of synaptic vesicles (SVs) on the presynaptic membrane at this synapse. Certain active zone material macromolecules connect to stereotypically arranged macromolecules in the membrane of undocked SVs, stably orienting a predetermined fusion domain of the vesicle membrane toward the presynaptic membrane while bringing and holding the two membranes together. Docking of the vesicles is required for the impulse-triggered vesicle membrane-presynaptic membrane fusion that releases the vesicles’ neurotransmitter into the synaptic cleft. As at other synapses, axon terminals at frog neuromuscular junctions contain, in addition to SVs, vesicles that are larger, are much less frequent and, when viewed by electron microscopy, have a distinctive electron dense core. Dense core vesicles at neuromuscular junctions are likely to contain peptides that are released into the synaptic cleft to regulate formation, maintenance and behavior of cellular apparatus essential for synaptic impulse transmission. We show by electron tomography on axon terminals of frog neuromuscular junctions fixed at rest and during repetitive impulse transmission that dense core vesicles selectively dock on and fuse with the presynaptic membrane alongside SVs at active zones, and that active zone material connects to the dense core vesicles undergoing these processes in the same way it connects to SVs. We conclude that undocked dense core vesicles have a predetermined fusion domain, as do undocked SVs, and that active zone material directs oriented docking and fusion of these different vesicle types at active zones of the presynaptic membrane by similar macromolecular interactions.

PtdIns(4,5)P2 is not required for secretory granule docking

Phosphoinositides (PtdIns) play important roles in exocytosis and are thought to regulate secretory granule docking by co-clustering with the SNARE protein syntaxin to form a docking receptor in the plasma membrane. Here we tested this idea by high-resolution total internal reflection imaging of EGFP-labeled PtdIns markers or syntaxin-1 at secretory granule release sites in live insulin-secreting cells. In intact cells, PtdIns markers distributed evenly across the plasma membrane with no preference for granule docking sites. In contrast, syntaxin-1 was found clustered in the plasma membrane, mostly beneath docked granules. We also observed rapid accumulation of syntaxin-1 at sites where granules arrived to dock. Acute depletion of plasma membrane phosphatidylinositol (4,5) bisphosphate (PtdIns(4,5)P₂ ) by recruitment of a 5'-phosphatase strongly inhibited Ca²⁺ -dependent exocytosis, but had no effect on docked granules or the distribution and clustering of syntaxin-1. Cell permeabilization by α-toxin or formaldehyde-fixation caused PtdIns marker to slowly cluster, in part near docked granules. In summary, our data indicate that PtdIns(4,5)P₂ accelerates granule priming, but challenge a role of PtdIns in secretory granule docking or clustering of syntaxin-1 at the release site.

Liprin-α3 controls vesicle docking and exocytosis at the active zone of hippocampal synapses

The presynaptic active zone provides sites for vesicle docking and release at central nervous synapses and is essential for speed and accuracy of synaptic transmission. Liprin-α binds to several active zone proteins, and loss-of-function studies in invertebrates established important roles for Liprin-α in neurodevelopment and active zone assembly. However, Liprin-α localization and functions in vertebrates have remained unclear. We used stimulated emission depletion superresolution microscopy to systematically determine the localization of Liprin-α2 and Liprin-α3, the two predominant Liprin-α proteins in the vertebrate brain, relative to other active-zone proteins. Both proteins were widely distributed in hippocampal nerve terminals, and Liprin-α3, but not Liprin-α2, had a prominent component that colocalized with the active-zone proteins Bassoon, RIM, Munc13, RIM-BP, and ELKS. To assess Liprin-α3 functions, we generated Liprin-α3-KO mice by using CRISPR/Cas9 gene editing. We found reduced synaptic vesicle tethering and docking in hippocampal neurons of Liprin-α3-KO mice, and synaptic vesicle exocytosis was impaired. Liprin-α3 KO also led to mild alterations in active zone structure, accompanied by translocation of Liprin-α2 to active zones. These findings establish important roles for Liprin-α3 in active-zone assembly and function, and suggest that interplay between various Liprin-α proteins controls their active-zone localization.

Hydrodynamics govern the pre-fusion docking time of synaptic vesicles

Synaptic vesicle fusion is a crucial step in the neurotransmission process. Neurotransmitter-filled vesicles are pre-docked at the synapse by the mediation of ribbon structures and SNARE proteins at the ribbon synapses. An electrical impulse triggers the fusion process of pre-docked vesicles, leading to the formation of a fusion pore and subsequently resulting in the release of neurotransmitter into the synaptic cleft. In this study, a continuum model of lipid membrane along with lubrication theory is used to determine the traverse time of the synaptic vesicle under the influence of hydrodynamic forces. We find that the traverse time is strongly dependent on how fast the driving force decays or grows with closure of the gap between the vesicle and the plasma membrane. If the correct behaviour is chosen, the traverse time obtained is of the order of a few hundred milliseconds and lies within the experimentally obtained value of approximately 250 ms (Zenisek D, Steyer JA, Almers W. 2000 Nature406, 849-854 (doi:10.1038/35022500)). We hypothesize that there are two different force behaviours, which complies with the experimental findings of pre-fusion docking of synaptic vesicles at the ribbon synapses. The common theme in the proposed force models is that the driving force has to very rapidly increase or decrease with the amount of clamping.

Traumatic Brain Injury Impairs Soluble N-Ethylmaleimide-Sensitive Factor Attachment Protein Receptor Complex Formation and Alters Synaptic Vesicle Distribution in the Hippocampus

Traumatic brain injury (TBI) impairs neuronal function and can culminate in lasting cognitive impairment. While impaired neurotransmitter release has been well established after experimental TBI, little is understood about the mechanisms underlying this consequence. In the synapse, vesicular docking and neurotransmitter release requires the formation of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex. Impairments in vesicle docking, and alterations in SNARE complex formation are associated with impaired neurotransmitter release. We hypothesized that TBI reduces SNARE complex formation and disrupts synaptic vesicle distribution in the hippocampus. To examine the effect of TBI on the SNARE complex, rats were subjected to controlled cortical impact (CCI) or sham injury, and the brains were assessed at 6 h, 1 d, one week, two weeks, or four weeks post-injury. Immunoblotting of hippocampal homogenates revealed significantly reduced SNARE complex formation at one week and two weeks post-injury. To assess synaptic vesicles distribution, rats received CCI or sham injury and the brains were processed for transmission electron microscopy at one week post-injury. Synapses in the hippocampus were imaged at 100k magnification, and vesicle distribution was assessed in pre-synaptic terminals at the active zone. CCI resulted in a significant reduction in vesicle number within 150 nm of the active zone. These findings provide the first evidence of TBI-induced impairments in synaptic vesicle docking, and suggest that reductions in the pool of readily releasable vesicles and impaired SNARE complex formation are two novel mechanisms contributing to impaired neurotransmission after TBI.

Differential Regulation of Synaptic Vesicle Tethering and Docking by UNC-18 and TOM-1

The assembly of SNARE complexes between syntaxin, SNAP-25 and synaptobrevin is required to prime synaptic vesicles for fusion. Since Munc18 and tomosyn compete for syntaxin interactions, the interplay between these proteins is predicted to be important in regulating synaptic transmission. We explored this possibility, by examining genetic interactions between C. elegans unc-18(Munc18), unc-64(syntaxin) and tom-1(tomosyn). We have previously demonstrated that unc-18 mutants have reduced synaptic transmission, whereas tom-1 mutants exhibit enhanced release. Here we show that the unc-18 mutant release defect is associated with loss of two morphologically distinct vesicle pools; those tethered within 25 nm of the plasma membrane and those docked with the plasma membrane. In contrast, priming defective unc-13 mutants accumulate tethered vesicles, while docked vesicles are greatly reduced, indicating tethering is UNC-18-dependent and occurs in the absence of priming. C. elegans unc-64 mutants phenocopy unc-18 mutants, losing both tethered and docked vesicles, whereas overexpression of open syntaxin preferentially increases vesicle docking, suggesting UNC-18/closed syntaxin interactions are responsible for vesicle tethering. Given the competition between vertebrate tomosyn and Munc18, for syntaxin binding, we hypothesized that C. elegans TOM-1 may inhibit both UNC-18-dependent vesicle targeting steps. Consistent with this hypothesis, tom-1 mutants exhibit enhanced UNC-18 plasma membrane localization and a concomitant increase in both tethered and docked synaptic vesicles. Furthermore, in tom-1;unc-18 double mutants the docked, primed vesicle pool is preferentially rescued relative to unc-18 single mutants. Together these data provide evidence for the differential regulation of two vesicle targeting steps by UNC-18 and TOM-1 through competitive interactions with syntaxin.

Role of cAMP cascade in synaptic stability and plasticity: ultrastructural and physiological analyses of individual synaptic boutons in Drosophila memory mutants

Mutations of the genes rutabaga (rut) and dunce (dnc) affect the synthesis and degradation of cAMP, respectively, and disrupt learning in Drosophila. Combined ultrastructural analysis and focal electrophysiological recording in the larval neuromuscular junction revealed a loss of stability and fine tuning of synaptic structure and function in both mutants. Increased ratios of docked/undocked vesicles and poorly defined synaptic specializations characterized dnc synapses. In contrast, rut boutons possessed fewer, although larger, synapses with lower proportions of docked vesicles. At reduced Ca(2+) levels, decreased quantal content coupled with an increase in failure rate was seen in rut boutons and reduced pair-pulse facilitation were found in both rut and dnc mutants. At physiological Ca(2+) levels, strong enhancement, instead of depression, in evoked release was observed in some dnc and rut boutons during 10 Hz tetanus. Furthermore, increased variability of synaptic transmission, including fluctuation and asynchronicity of evoked release, paralleled an increase in synapse size variation in both dnc and rut boutons, which might impose problems for effective signal processing in the nervous system. Pharmacological and genetic studies indicated broader ranges of physiological alteration by dnc and rut mutations than either the acute effects of cAMP analogs or the available mutations that affect cAMP-dependent protein kinase (PKA) activity. This is consistent with previous reports of more severe learning defects in dnc and rut mutations than these PKA mutants and allows identification of the phenotypes involving long-term developmental regulation and those conferred by PKA.

Impairments in high-frequency transmission, synaptic vesicle docking, and synaptic protein distribution in the hippocampus of BDNF knockout mice

Brain-derived neurotrophic factor (BDNF) promotes long-term potentiation (LTP) at hippocampal CA1 synapses by a presynaptic enhancement of synaptic transmission during high-frequency stimulation (HFS). Here we have investigated the mechanisms of BDNF action using two lines of BDNF knockout mice. Among other presynaptic impairments, the mutant mice exhibited more pronounced synaptic fatigue at CA1 synapses during high-frequency stimulation, compared with wild-type animals. Quantitative analysis of CA1 synapses revealed a significant reduction in the number of vesicles docked at presynaptic active zones in the mutant mice. Synaptosomes prepared from the mutant hippocampus exhibited a marked decrease in the levels of synaptophysin as well as synaptobrevin [vesicle-associated membrane protein (VAMP-2)], a protein known to be involved in vesicle docking and fusion. Treatment of the mutant slices with BDNF reversed the electrophysiological and biochemical deficits in the hippocampal synapses. Taken together, these results suggest a novel role for BDNF in the mobilization and/or docking of synaptic vesicles to presynaptic active zones.

Morphologically docked synaptic vesicles are reduced in synaptotagmin mutants of Drosophila

Nerve terminal specializations include mechanisms for maintaining a subpopulation of vesicles in a docked, fusion-ready state. We have investigated the relationship between synaptotagmin and the number of morphologically docked vesicles by an electron microscopic analysis of Drosophila synaptotagmin (syt) mutants. The overall number of synaptic vesicles in a terminal was reduced, although each active zone continued to have a cluster of vesicles in its vicinity. In addition, there was an increase in the number of large vesicles near synapses. Examining the clusters, we found that the pool of synaptic vesicles immediately adjacent to the presynaptic membrane, the pool that includes the docked population, was reduced to 24 +/- 5% (means +/- SEM) of control in the sytnull mutation. To separate contributions of overall vesicle depletion and increased spontaneous release from direct effects of synaptotagmin on morphological docking, we examined syt mutants in an altered genetic background. Recombining syt alleles onto a second chromosome bearing an as yet uncharacterized mutation resulted in the expected decrease in evoked release but suppressed the increase in spontaneous release frequency. Motor nerve terminals in this genotype contained more synaptic vesicles than control, yet the number of vesicles immediately adjacent to the presynaptic membrane near active zones was still reduced (33 +/- 4% of control). Our findings demonstrate that there is a decrease in the number of morphologically docked vesicles seen in syt mutants. The decreases in docking and evoked release are independent of the increase in spontaneous release. These results support the hypothesis that synaptotagmin stabilizes the docked state.

Disruption of syntaxin-mediated protein interactions blocks neurotransmitter secretion

The membrane protein syntaxin participates in several protein-protein interactions that have been implicated in neurotransmitter release. To probe the physiological importance of these interactions, we microinjected into the squid giant presynaptic terminal botulinum toxin C1, which cleaves syntaxin, and the H3 domain of syntaxin, which mediates binding to other proteins. Both reagents inhibited synaptic transmission yet did not affect the number or distribution of synaptic vesicles at the presynaptic active zone. Recombinant H3 domain inhibited the interactions between syntaxin and SNAP-25 that underlie the formation of stable SNARE complexes in vitro. These data support the notion that syntaxin-mediated SNARE complexes are necessary for docked synaptic vesicles to fuse.