SciCombinator

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Concept: Synaptotagmin

85

Propofol is the most commonly used general anesthetic in humans. Our understanding of its mechanism of action has focused on its capacity to potentiate inhibitory systems in the brain. However, it is unknown whether other neural mechanisms are involved in general anesthesia. Here, we demonstrate that the synaptic release machinery is also a target. Using single-particle tracking photoactivation localization microscopy, we show that clinically relevant concentrations of propofol and etomidate restrict syntaxin1A mobility on the plasma membrane, whereas non-anesthetic analogs produce the opposite effect and increase syntaxin1A mobility. Removing the interaction with the t-SNARE partner SNAP-25 abolishes propofol-induced syntaxin1A confinement, indicating that syntaxin1A and SNAP-25 together form an emergent drug target. Impaired syntaxin1A mobility and exocytosis under propofol are both rescued by co-expressing a truncated syntaxin1A construct that interacts with SNAP-25. Our results suggest that propofol interferes with a step in SNARE complex formation, resulting in non-functional syntaxin1A nanoclusters.

Concepts: Brain, Cell membrane, Anesthesia, General anaesthesia, Neurophysiology, SNAP-25, Synaptotagmin, SNARE

27

Soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors (SNAREs) mediate vesicle fusion with the plasma membrane on activation by calcium binding to synaptotagmin. In the present study, we used fluorescence resonance energy transfer (FRET) and fluorescence lifetime imaging microscopy between fluorescently labeled SNARE proteins expressed in cultured rat hippocampal neurons to detect resting SNARE complexes, their conformational rearrangement on exocytosis, their disassembly before endocytosis of vesicular proteins, and SNARE assembly at newly docked vesicles. Assembled SNAREs are not only present in docked vesicles; unexpected residual “orphan SNARE complexes” also reside in para-active zone regions. Real-time changes in FRET between N-terminally labeled SNAP-25 and VAMP reported a reorientation of the SNARE motif upon exocytosis, SNARE disassembly in the active zone periphery, and SNARE reassembly in newly docked vesicles. With VAMP labeled C-terminally, decreased fluorescence in C-terminally labeled syntaxin (extracellular) reported trans-cis-conformational changes in SNAREs on vesicle fusion. After fusion SNAP-25 and syntaxin disperse along with VAMP, as well as the FRET signal itself, indicating diffusion of intact SNAREs after vesicle fusion but before their peripheral disassembly. Our measurements of spatiotemporal dynamics of SNARE conformational changes and movements refine models of SNARE function. Technical advances required to detect tiny changes in fluorescence in small fractions of labeled proteins in presynaptic boutons on a time scale of seconds permit the detection of rapid intermolecular interactions between small proportions of protein partners in cellular subcompartments.

Concepts: Protein, Neuron, Cell membrane, Synapse, Förster resonance energy transfer, SNAP-25, Synaptotagmin, SNARE

25

Although largely overlooked relative to the process of phagophore formation, the mechanism through which autophagosomes fuse with lysosomes is a critical aspect of macroautophagy that is not fully understood. In particular, this step must be carefully regulated to prevent premature fusion of an incomplete autophagosome (that is, a phagophore) with a lysosome, because such an event would not allow access of the partially sequestered cargo to the lysosome lumen. The identification of the autophagosome-associated SNARE protein STX17 (syntaxin 17) provided some clue in the understanding of this process. STX17 is recruited specifically to mature autophagosomes, and functions in mediating autophagosome-lysosome fusion by forming a complex with the Qbc SNARE SNAP29 and the lysosomal R-SNARE VAMP8. Additionally, STX17 plays a role in the early events of autophagy by interacting with the phosphatidylinositol 3-kinase complex component ATG14. Upon autophagy induction STX17 is strictly required for ATG14 recruitment to the ER-mitochondria contact sites, a critical step for the assembly of the phagophore and therefore for autophagosome formation. In their recent paper, Diao and collaborators now show that the ATG14-STX17-SNAP29 interaction mediates autophagosome-lysosome tethering and fusion events, thus revealing a novel function of ATG14 in the later steps of the autophagy pathway.

Concepts: Cell, Golgi apparatus, Organelle, Endoplasmic reticulum, Emergence, Lysosome, SNAP-25, Synaptotagmin

3

The fusion pore is the first crucial intermediate formed during exocytosis, yet little is known about the mechanisms that determine the size and kinetic properties of these transient structures. Here, we reduced the number of available SNAREs (proteins that mediate vesicle fusion) in neurons and observed changes in transmitter release that are suggestive of alterations in fusion pores. To investigate these changes, we employed reconstituted fusion assays using nanodiscs to trap pores in their initial open state. Optical measurements revealed that increasing the number of SNARE complexes enhanced the rate of release from single pores and enabled the escape of larger cargoes. To determine whether this effect was due to changes in nascent pore size or to changes in stability, we developed an approach that uses nanodiscs and planar lipid bilayer electrophysiology to afford microsecond resolution at the single event level. Both pore size and stability were affected by SNARE copy number. Increasing the number of vesicle (v)-SNAREs per nanodisc from three to five caused a twofold increase in pore size and decreased the rate of pore closure by more than three orders of magnitude. Moreover, pairing of v-SNAREs and target (t)-SNAREs to form trans-SNARE complexes was highly dynamic: flickering nascent pores closed upon addition of a v-SNARE fragment, revealing that the fully assembled, stable SNARE complex does not form at this stage of exocytosis. Finally, a deletion at the base of the SNARE complex, which mimics the action of botulinum neurotoxin A, markedly reduced fusion pore stability. In summary, trans-SNARE complexes are dynamic, and the number of SNAREs recruited to drive fusion determines fundamental properties of individual pores.

Concepts: Membrane biology, Neurophysiology, SNAP-25, Synaptotagmin, Clostridium botulinum, SNARE, Porosome, Vesicle fusion

3

The membrane fusion necessary for vesicle trafficking is driven by the assembly of heterologous SNARE proteins orchestrated by the binding of Sec1/Munc18 (SM) proteins to specific syntaxin SNARE proteins. However, the precise mode of interaction between SM proteins and SNAREs is debated, as contrasting binding modes have been found for different members of the SM protein family, including the three vertebrate Munc18 isoforms. While different binding modes could be necessary given their roles in different secretory processes in different tissues, the structural similarity of the three isoforms makes this divergence perplexing. Though the neuronal isoform Munc18a is well established to bind tightly to both the closed conformation and the N-peptide of Syntaxin 1a, thereby inhibiting SNARE complex formation, Munc18b and c, which have a more widespread distribution, are reported to mainly interact with the N-peptide of their partnering syntaxins and are thought to instead promote SNARE complex formation. We have re-investigated the interaction between Munc18c and Syntaxin 4 (Syx4). Using isothermal titration calorimetry, we found that Munc18c, like Munc18a, binds to both the closed conformation and the N-peptide of Syx4. Furtermore, using a novel kinetic approach, we found that Munc18c, like Munc18a, slows down SNARE complex formation through high-affinity binding to syntaxin. This strongly suggests that secretory Munc18s in general control the accessibility of the bound syntaxin, probably preparing it for SNARE complex assembly.

Concepts: Proteins, Cell, Molecular biology, Endoplasmic reticulum, Membrane biology, SNAP-25, Synaptotagmin, Syntaxin

2

Gi/o-coupled GPCRs can inhibit neurotransmitter release at synapses via multiple mechanisms. In addition to Gβγ-mediated modulation of voltage-gated calcium channels(VGCC), inhibition can also be mediated through the direct interaction of Gβγ subunits with the soluble N-ethylmaleimide attachment protein receptor (SNARE) complex of the vesicle fusion apparatus. Binding studies with soluble SNARE complexes have shown that Gβγ binds to both ternary SNARE complexes, t-SNARE heterodimers, and monomeric SNAREs, competing with synaptotagmin(syt)1 for binding sites on t-SNARE. However, in secretory cells, Gβγ, SNAREs, and synaptotagmin interact in the lipid environment of a vesicle at the plasma membrane. To approximate this environment, we show that fluorescently-labeled Gβγ interacts specifically with lipid-embedded t-SNAREs consisting of full-length syntaxin 1 and SNAP-25B at the membrane, as measured by fluorescence polarization. Fluorescently-labeled syt 1 undergoes competition with Gβγ for SNARE binding sites in lipid environments. Mutant Gβγ subunits that were previously shown to be more efficacious at inhibiting Ca2+-triggered exocytotic release than wild-type Gβγ were also shown to bind SNAREs at higher affinity than wild-type in a lipid environment. These mutant Gβγ subunits were unable to inhibit VGCC currents. Specific peptides, corresponding to regions on Gβ and Gγ shown to be important for the interaction, disrupt the interaction in a concentration-dependent manner. In in vitro fusion assays utilizing full-length t- and v-SNAREs embedded in liposomes, Gβγ inhibited Ca2+/synaptotagmin-dependent fusion. Together, these studies demonstrate the importance of these regions for the Gβγ-SNARE interaction, and show that the target of Gβγ, downstream of VGCC, is the membrane-embedded SNARE complex.

Concepts: Protein, Cell membrane, Hormone, SNAP-25, Synaptotagmin, SNARE, Porosome, Vesicle fusion

1

Botulinum neurotoxin serotype C (BoNT/C) is a neuroparalytic toxin associated with outbreaks of animal botulism, particularly in birds, and is the only BoNT known to cleave two different SNARE proteins, SNAP-25 and syntaxin. BoNT/C was shown to be a good substitute for BoNT/A1 in human dystonia therapy because of its long lasting effects and absence of neuromuscular damage. Two triple mutants of BoNT/C, namely BoNT/C S51T/R52N/N53P (BoNT/C α-51) and BoNT/C L200W/M221W/I226W (BoNT/C α-3W), were recently reported to selectively cleave syntaxin and have been used here to evaluate the individual contribution of SNAP-25 and syntaxin cleavage to the effect of BoNT/C in vivo. Although BoNT/C α-51 and BoNT/C α-3W toxins cleave syntaxin with similar efficiency, we unexpectedly found also cleavage of SNAP-25, although to a lesser extent than wild type BoNT/C. Interestingly, the BoNT/C mutants exhibit reduced lethality compared to wild type toxin, a result that correlated with their residual activity against SNAP-25. In spite of this, a local injection of BoNT/C α-51 persistently impairs neuromuscular junction activity. This is due to an initial phase in which SNAP-25 cleavage causes a complete blockade of neurotransmission, and to a second phase of incomplete impairment ascribable to syntaxin cleavage. Together, these results indicate that neuroparalysis of BoNT/C at the neuromuscular junction is due to SNAP-25 cleavage, while the proteolysis of syntaxin provides a substantial, but incomplete, neuromuscular impairment. In light of this evidence, we discuss a possible clinical use of BoNT/C α-51 as a botulinum neurotoxin endowed with a wide safety margin and a long lasting effect.

Concepts: Action potential, Neurophysiology, Toxin, SNAP-25, Synaptotagmin, Botulinum toxin, Microbial toxins, Clostridium botulinum

1

Botulinum neurotoxin B is a Food and Drug Administration-approved therapeutic toxin. However, it has lower binding affinity toward the human version of its major receptor, synaptotagmin II (h-Syt II), compared to mouse Syt II, because of a residue difference. Increasing the binding affinity to h-Syt II may improve botulinum neurotoxin B’s therapeutic efficacy and reduce adverse effects. Here we utilized the bacterial adenylate cyclase two-hybrid method and carried out a saturation mutagenesis screen in the Syt II-binding pocket of botulinum neurotoxin B. The screen identifies E1191 as a key residue: replacing it with M/C/V/Q enhances botulinum neurotoxin B binding to human synaptotagmin II. Adding S1199Y/W or W1178Q as a secondary mutation further increases binding affinity. Mutant botulinum neurotoxin B containing E1191M/S1199Y exhibits ~11-fold higher efficacy in blocking neurotransmission than wild-type botulinum neurotoxin B in neurons expressing human synaptotagmin II, demonstrating that enhancing receptor binding increases the overall efficacy at functional levels. The engineered botulinum neurotoxin B provides a platform to develop therapeutic toxins with improved efficacy.Humans are less sensitive to the therapeutic effects of botulinum neurotoxin B (BoNT/B) than the animal models it is tested on due to differences between the human and the mouse receptors. Here, the authors engineer BoNT/B to improve its affinity to human receptors and enhance its therapeutic efficacy.

Concepts: Protein, Receptor, Cell signaling, Toxin, Synaptotagmin, Microbial toxins, Clostridium botulinum, Adenylate cyclase

1

Synaptic vesicle fusion is mediated by SNARE proteins forming in between synaptic vesicle (v-SNARE) and plasma membrane (t-SNARE), one of which is Syntaxin-1A. Although exocytosis mainly occurs at active zones, Syntaxin-1A appears to cover the entire neuronal membrane. By using STED super-resolution light microscopy and image analysis of Drosophila neuro-muscular junctions, we show that Syntaxin-1A clusters are more abundant and have an increased size at active zones. A computational particle-based model of syntaxin cluster formation and dynamics is developed. The model is parametrized to reproduce Syntaxin cluster-size distributions found by STED analysis, and successfully reproduces existing FRAP results. The model shows that the neuronal membrane is adjusted in a way to strike a balance between having most syntaxins stored in large clusters, while still keeping a mobile fraction of syntaxins free or in small clusters that can efficiently search the membrane or be traded between clusters. This balance is subtle and can be shifted toward almost no clustering and almost complete clustering by modifying the syntaxin interaction energy on the order of only 1 kBT. This capability appears to be exploited at active zones. The larger active-zone syntaxin clusters are more stable and provide regions of high docking and fusion capability, whereas the smaller clusters outside may serve as flexible reserve pool or sites of spontaneous ectopic release.

Concepts: Cell membrane, Chemical synapse, Neurophysiology, SNAP-25, Synaptotagmin, SNARE, Porosome, Vesicle fusion

0

SNAP-25 is a protein involved in regulated membrane fusion and part of the SNARE complex. It exists as two splicing variants, SNAP-25a and SNAP-25b, which differ in 9 out of 206 amino acids. SNAP-25 together with Syntaxin 1 and VAMP-2 forms the ternary SNARE complex essential for mediating activity-dependent release of hormones and neurotransmitters. The functional difference between SNAP-25a and SNAP-25b is poorly understood as both can participate in SNARE complexes and mediate membrane fusion. However, we recently demonstrated that SNAP-25b-deficiency results in metabolic disease and increased insulin secretion. Here we investigated if SNAP-25a and SNAP-25b differently affect interactions with other SNAREs and SNARE-interacting proteins in mouse hippocampus. Adult mice almost exclusively express the SNAP-25b protein in hippocampus whereas SNAP-25b-deficient mice only express SNAP-25a. Immunoprecipitation studies showed no significant differences in amount of Syntaxin 1 and VAMP-2 co-precipitated with the different SNAP-25 isoforms. In contrast, Munc18-1, that preferentially interacts with SNAP-25 via Syntaxin 1 and/or the trimeric SNARE complex, demonstrated an increased ability to bind protein-complexes containing SNAP-25b. Moreover, we found that both SNAP-25 isoforms co-precipitated the Gβγ subunits of the heterotrimeric G proteins, an interaction known to play a role in presynaptic inhibition. We have identified Gβ1and Gβ2as the interacting partners of both SNAP-25 isoforms in mouse hippocampus, but Gβ2was less efficiently captured by SNAP-25a. These results implicate that the two SNAP-25 isoforms could differently mediate protein interactions outside the ternary SNARE core complex and thereby contribute to modulate neurotransmission.

Concepts: Protein, Gene, Amino acid, Signal transduction, Metabolism, SNAP-25, Synaptotagmin, Syntaxin