Based on evidence that the docked and primed synaptic vesicle state is very dynamic, we propose a three-step process for the buildup of the molecular machinery that mediates synaptic vesicle fusion: ...(1) loose tethering and docking of vesicles to release sites, forming the nucleus of SNARE-complex assembly, (2) tightening of the complex by association of additional proteins, and partial SNARE-complex zippering, and (3) Ca2+-triggered fusion. We argue that the distinction between “phasic synapses” and “tonic synapses” reflects differences in resting occupancy and stability of the loosely and tightly docked states, and we assign corresponding timescales: with high-frequency synaptic activity and concomitantly increased Ca2+-concentrations, step (1) can proceed within 10–50 ms, step (2) within 1–5 ms, and step (3) within 0.2–1 ms.
Synaptic vesicle priming determines synaptic strength and short-term plasticity. Neher and Brose propose a model of loosely and tightly primed vesicle states, which can be interconverted rapidly, and whose occupancy defines the distinction between “phasic” and “tonic” synapses.
In contrast to constitutive secretion, SNARE-mediated synaptic vesicle fusion is controlled by multiple regulatory proteins, which determine the Ca²⁺ sensitivity of the vesicle fusion process and the ...speed of excitation-secretion coupling. Complexins are among the best characterized SNARE regulators known to date. They operate by binding to trimeric SNARE complexes consisting of the vesicle protein synaptobrevin and the plasma membrane proteins syntaxin and SNAP-25. The question as to whether complexins facilitate or inhibit SNARE-mediated fusion processes is currently a matter of significant controversy. This is mainly because of the fact that biochemical experiments in vitro and studies on vertebrate complexins in vivo have yielded apparently contradictory results. In this review, I provide a summary of available data on the role of complexins in SNARE-mediated vesicle fusion and attempt to define a model of complexin function that incorporates evidence for both facilitatory and inhibitory roles of complexins in SNARE-mediated fusion.
Highlights ► Transsynaptic NX–NL complexes control synapse formation, maturation, validation, and function. ► NXs and NLs control synapse maturation by recruiting the presynaptic and postsynaptic ...signaling apparatus. ► Neuronal activity and alternative splicing control the specificity and function of NX–NL interactions. ► Alternative binding partners increase the ‘combinatorial space’ of NX and NL interactions. ► Combinations of NX and NL interaction partners at synapses may determine synapse specificity.
Munc13 catalyzes the transit of syntaxin from a closed complex with Munc18 into the ternary SNARE complex. Here we report a new function of Munc13, independent of Munc18: it promotes the proper ...syntaxin/synaptobrevin subconfiguration during assembly of the ternary SNARE complex. In cooperation with Munc18, Munc13 additionally ensures the proper syntaxin/SNAP-25 subconfiguration. In a reconstituted fusion assay with SNAREs, complexin, and synaptotagmin, inclusion of both Munc13 and Munc18 quadruples the Ca2+-triggered amplitude and achieves Ca2+ sensitivity at near-physiological concentrations. In Munc13-1/2 double-knockout neurons, expression of a constitutively open mutant of syntaxin could only minimally restore neurotransmitter release relative to Munc13-1 rescue. Together, the physiological functions of Munc13 may be related to regulation of proper SNARE complex assembly.
•Munc13-1 has a function independent from Munc18-1•Munc13-1 and Munc18-1 cooperate to promote proper SNARE complex assembly•Proper SNARE complex assembly yields near-physiological Ca2+ sensitivity in vitro•Bypass of Munc13 in neurons with an open mutant of syntaxin is incomplete
Lai et al. discovered that Munc13 promotes proper SNARE complex assembly together with Munc18, increasing evoked release probability. This suggests that the physiological functions of Munc13 in priming and short-term presynaptic plasticity are related to regulation of proper assembly of synaptic complexes.
Synaptic vesicle docking, priming, and fusion at active zones are orchestrated by a complex molecular machinery. We employed hippocampal organotypic slice cultures from mice lacking key presynaptic ...proteins, cryofixation, and three-dimensional electron tomography to study the mechanism of synaptic vesicle docking in the same experimental setting, with high precision, and in a near-native state. We dissected previously indistinguishable, sequential steps in synaptic vesicle active zone recruitment (tethering) and membrane attachment (docking) and found that vesicle docking requires Munc13/CAPS family priming proteins and all three neuronal SNAREs, but not Synaptotagmin-1 or Complexins. Our data indicate that membrane-attached vesicles comprise the readily releasable pool of fusion-competent vesicles and that synaptic vesicle docking, priming, and trans-SNARE complex assembly are the respective morphological, functional, and molecular manifestations of the same process, which operates downstream of vesicle tethering by active zone components.
•Synapses lacking Munc13/CAPS priming proteins have few or no docked synaptic vesicles•Synapses lacking SNAREs have strongly reduced numbers of docked synaptic vesicles•Synaptotagmin-1 and Complexins are dispensable for synaptic vesicle docking•Vesicle docking, priming, and SNARE complex assembly represent the same process
Synaptic vesicle docking and priming have previously been interpreted as independent, sequential steps prior to Ca2+-dependent fusion. Imig et al. now show that docking, priming, and trans-SNARE complex assembly are respective morphological, functional, and molecular manifestations of the same process.
Increased levels of the second messenger lipid diacylglycerol (DAG) induce downstream signaling events including the translocation of C1-domain-containing proteins toward the plasma membrane. Here, ...we introduce three light-sensitive DAGs, termed PhoDAGs, which feature a photoswitchable acyl chain. The PhoDAGs are inactive in the dark and promote the translocation of proteins that feature C1 domains toward the plasma membrane upon a flash of UV-A light. This effect is quickly reversed after the termination of photostimulation or by irradiation with blue light, permitting the generation of oscillation patterns. Both protein kinase C and Munc13 can thus be put under optical control. PhoDAGs control vesicle release in excitable cells, such as mouse pancreatic islets and hippocampal neurons, and modulate synaptic transmission in Caenorhabditis elegans. As such, the PhoDAGs afford an unprecedented degree of spatiotemporal control and are broadly applicable tools to study DAG signaling.
Dendritic spines are the major transmitter reception compartments of glutamatergic synapses in most principal neurons of the mammalian brain and play a key role in the function of nerve cell ...circuits. The formation of functional spine synapses is thought to be critically dependent on presynaptic glutamatergic signaling. By analyzing CA1 pyramidal neurons in mutant hippocampal slice cultures that are essentially devoid of presynaptic transmitter release, we demonstrate that the formation and maintenance of dendrites and functional spines are independent of synaptic glutamate release.
•Elimination of presynaptic transmitter release in hippocampal organotypic slices•Dendrite growth and maintenance are independent of presynaptic glutamate release•Spine formation and maintenance are independent of presynaptic glutamate release•Recruitment of NMDA/AMPA receptors is independent of presynaptic glutamate release
Spines are major neurotransmitter reception compartments of nerve cells. Contrary to the currently held dogma, Sigler et al. show that spine formation in the hippocampus is independent of synaptic neurotransmitter release, indicating that brain circuits are established by activity-independent cellular programs.
Unlike most other secretory processes, neurotransmitter release at chemical synapses is extremely fast, tightly regulated, spatially restricted, and dynamically adjustable at the same time. In this ...review, we focus on recent discoveries of molecular and cell biological processes that determine how fusion competence of vesicles is achieved and controlled in order to suit the specific requirements of synaptic transmitter release with respect to speed and spatial selectivity.
Active zones consist of protein scaffolds that are tightly attached to the presynaptic plasma membrane. They dock and prime synaptic vesicles, couple them to voltage-gated Ca
channels, and direct ...neurotransmitter release toward postsynaptic receptor domains. Simultaneous RIM + ELKS ablation disrupts these scaffolds, abolishes vesicle docking, and removes active zone-targeted Munc13, but some vesicles remain releasable. To assess whether this enduring vesicular fusogenicity is mediated by non-active zone-anchored Munc13 or is Munc13-independent, we ablated Munc13-1 and Munc13-2 in addition to RIM + ELKS in mouse hippocampal neurons. The hextuple knockout synapses lacked docked vesicles, but other ultrastructural features were near-normal despite the strong genetic manipulation. Removing Munc13 in addition to RIM + ELKS impaired action potential-evoked vesicle fusion more strongly than RIM + ELKS knockout by further decreasing the releasable vesicle pool. Hence, Munc13 can support some fusogenicity without RIM and ELKS, and presynaptic recruitment of Munc13, even without active zone anchoring, suffices to generate some fusion-competent vesicles.
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