Neurotransmitter release: the last millisecond in the life of a synaptic vesicle

Abstract

During an action potential, Ca(2+) entering a presynaptic terminal triggers synaptic vesicle exocytosis and neurotransmitter release in less than a millisecond. How does Ca(2+) stimulate release so rapidly and precisely? Work over the last decades revealed that Ca(2+) binding to synaptotagmin triggers release by stimulating synaptotagmin binding to a core fusion machinery composed of SNARE and SM proteins that mediates membrane fusion during exocytosis. Complexin adaptor proteins assist synaptotagmin by activating and clamping this core fusion machinery. Synaptic vesicles containing synaptotagmin are positioned at the active zone, the site of vesicle fusion, by a protein complex containing RIM proteins. RIM proteins activate docking and priming of synaptic vesicles and simultaneously recruit Ca(2+) channels to active zones, thereby connecting in a single complex primed synaptic vesicles to Ca(2+) channels. This architecture allows direct flow of Ca(2+) ions from Ca(2+) channels to synaptotagmin, which then triggers fusion, thus mediating tight millisecond coupling of an action potential to neurotransmitter release.

Figure 1. Organization of the presynaptic release machinery

A. Drawing of a synapse with synaptic vesicles (SV), an active zone containing Ca²⁺-channels (blue), and a postsynaptic cluster of receptors (orange). One vesicle in the active zone is depicted in the process of fusing, with red neurotransmitters emitting from the fusion pore. B. Schematic of the molecular machinery mediating Ca²⁺-triggered vesicle fusion. The drawing depicts a segment of a docked synaptic vesicle on the top right, and the presynaptic active zone in the middle. The three functional elements of the neurotransmitter release machinery are depicted from right to left. On the right, the core fusion machine composed of the SNARE/SM protein complex is shown; this machine comprises the SNARE proteins synaptobrevin/VAMP, syntaxin-1, and SNAP-25 and the SM protein Munc18-1. The Ca²⁺-sensor synaptotagmin-1 is depicted in the middle; it is composed of a short intravesicular sequence, a single transmembrane region, and two cytoplasmic C2-domains that bind Ca²⁺, and it functions using complexin (bound to the SNARE complex) as an assistant. The active zone protein complex containing RIM, Munc13, and RIM-BP, and a Ca²⁺-channel in the presynaptic plasma membrane is shown on the left. In this protein complex, RIM binding to specific target proteins coordinates all three functions of the active zone: RIM binding to vesicular rab proteins (Rab3 and Rab27 isoforms) mediates vesicle docking; RIM binding to the central priming factor Munc13 activates vesicle priming; and RIM binding to the Ca²⁺-channel, both directly and indirectly via RIM-BP, recruits the Ca²⁺-channels within 100 nm of the docked vesicles for fast excitation-secretion coupling. The overall design of the neurotransmitter release machinery depicted here enables in a single nanodevice fast and efficient triggering of release in response to an action potential by combining a fusion machine with a Ca²⁺-trigger and an active zone protein complex that positions all elements into appropriate proximity (modified from Kaeser et al., 2011).

Figure 2. Schematic of the SNARE/SM protein cycle mediating fusion, and the role of synaptotagmin and complexin in Ca²⁺-triggering of fusion

SNARE and SM proteins undergo a cycle of assembly and disassembly, such that the vesicular SNARE protein synaptobrevin assembles during priming into a trans-SNARE complex with the plasma membrane SNARE proteins syntaxin-1 and SNAP-25. Prior to SNARE-complex assembly, syntaxin-1 is present in a closed conformation in which its Habc-domain folds back onto its SNARE motif; this conformation precludes SNARE-complex assembly, and syntaxin-1 has to ‘open’ for SNARE-complex assembly to initiate. Prior to SNARE-complex assembly, Munc18-1 is associated with monomeric syntaxin-1 when syntaxin-1 is in a closed conformation; as syntaxin-1 opens during SNARE-complex assembly, Munc18-1 alters the mode of its binding to syntaxin-1 by binding to assembling trans-SNARE complexes via interacting with the syntaxin-1 N-peptide. Once SNARE complexes are partially assembled, complexin binds to further increase their priming. The ‘superprimed’ SNARE/SM protein complexes are then substrate for Ca²⁺-triggered fusion pore opening by Ca²⁺-binding to synaptotagmin, which causes an interaction of synaptotagmin with SNAREs and phospholipids. However, even before Ca²⁺-triggering synaptotagmin likely at least partly interacts with the fusion machinery as evidenced by the unclamping of spontaneous mini release in Syt1 knockout neurons. After fusion-pore opening, the resulting cis-SNARE complexes are disassembled by the NSF/SNAP ATPases, and vesicles are recycled, refilled with neurotransmitters, and reused for release (modified from Südhof, 2013).

Figure 3. Energy landscape of fusion and proposed role of the SM protein Munc18-1 in promoting fusion-pore opening

A. Schematic diagram of the energy level of a vesicle that is docked, primed, and fused. The diagram illustrates that partial SNARE-complex assembly during priming is proposed to provide most of the energy required for fusion, such that Ca²⁺-triggering only adds a small amount of additional energy to induce fusion-pore opening. B. Model of Munc18-1 function in fusion. Prior to priming of docked vesicles, Munc18-1 is bound to the closed conformation of syntaxin-1; this interaction is primarily regulatory to maintain a defined rate of entry into the fusion reaction, and additionally serves for the mutual stabilization of Munc18-1 and syntaxin-1 for each other (Gerber et al., 2008; Zhou et al., 2013a). Partial SNARE-complex assembly during priming (middle) is associated with a dramatic conformational change in syntaxin-1 which has to open, and in Munc18-1 whose binding changes from that to a closed syntaxin-1 conformation to binding to the open syntaxin-1 conformation via the N-peptide of syntaxin-1 (Dulubova et al., 2007). Full SNARE-complex assembly (right) produces fusion-pore opening. Munc18-1 is proposed to ride on the assembly SNARE complexes throughout the fusion reaction, and to couple the approximation of the membranes produced by the energy released by SNARE-complex assembly to fusion, possibly by mediating phospholipid mixing.

Figure 4. Complementary roles of synaptotagmin-1 (Syt1) and -7 (Syt7) Ca²⁺-sensors in triggering synchronous and asynchronous neurotransmitter release

A. Domain structures and localizations of Syt1 as the paradigmatic fast synaptotagmin isoform, and of Syt7 as the slow synaptotagmin isoform. Syt1 and Syt7 have identical domain structures, except that Syt1 contains an N-terminal N-glycosylation site in the vesicle that is lacking from Syt7, and that Syt1 is primarily localized to synaptic vesicles where Syt7 is primarily absent from synaptic vesicles. B. Inhibitory postsynaptic currents (IPSCs) measured in response to isolated action potentials in cultured hippocampal neurons from Syt1 knockout mice that were either infected with a control lentivirus, a lentivirus overexpressing wild-type Syt7, a lentivirus expressing a Syt7 shRNA that blocks most Syt7 expression, a lentivirus co-expressing the Syt7 shRNA together with wild-type Syt7, or a lentivirus co-expressing the Syt7 shRNA with a mutant Syt7 in which the C2A- and C2B-domain Ca²⁺-binding sites were mutated. Representative traces are shown on the left, and summary graphs on the right. Data shown are means ± SEM; numbers in the bars indicate the numbers of neurons/cultures analyzed. Statistical significance was assessed by one-way ANOVA comparing all test conditions to the control (**, p<0.01; ***, p<0.001). C. Same as B, except that IPSCs were measured in response to a 10 Hz 1 sec stimulus train. Data and figure were adapted from Bacaj et al. (2013).

Figure 5. Schematic representation of different synaptic Ca²⁺-controlled neurotransmitter release pathways

After docking and priming of synaptic vesicles mediated by the active zone protein complex containing RIM proteins as central elements, vesicles are further ‘superprimed’ by binding of complexin to the assembling SNARE complexes. The superprimed vesicles are the substrates of fast (Syt1/Syt2/Syt7) and slow (Syt7) synaptotagmins as synaptic Ca²⁺-sensors. Fast and slow synaptotagmins are differentially expressed, compete with each other, and are further regulated by alternative splicing and phosphorylation, such that synapses differ in the amount and short-term plasticity of Ca²⁺-induced release. In the absence of complexin or the fast synaptotagmin Ca²⁺-sensors, an alternative nonphysiological pathway becomes enabled (dotted line) whereby an as yet unidentified Ca²⁺-sensor with a Ca²⁺-cooperativity that differs from that of synaptotagmins mediates spontaneous ‘mini’ release. Key proteins are listed in the drawing at their points of action; SNARE and SM proteins are not shown for clarity sake, but are obviously vital components for the entire fusion reaction.