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A Molecular Machine for Neurotransmitter Release: Synaptotagmin and Beyond

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A Molecular Machine for Neurotransmitter Release: Synaptotagmin and Beyond LASKER MEDICAL BASIC RESEARCH AWARD ESSAY A molecular machine for neurotransmitter release: synaptotagmin and beyond Thomas C Südhof Fifty years ago, Bernard Katz’s seminal work revealed that calcium triggers neurotransmitter Synaptic vesicle membrane release by stimulating ultrafast synaptic vesicle fusion. But how a presynaptic terminal achieves the speed and precision of calcium-triggered fusion remained unknown. My colleagues and I set out to study this fundamental problem more C than two decades ago. Munc18 How do the synaptic vesicle and the plasma membrane fuse during transmitter release? How does calcium trigger synaptic vesicle fusion? Synaptobrevin Synaptotagmin? How is calcium influx localized to release sites N in order to enable the fast coupling of an action α-, β- or γ-SNAP– potential to transmitter release? Together with NSF complex? contributions made by other scientists, most Syntaxin prominently James Rothman, Reinhard Jahn and SNAP-25 (HPC-1) Richard Scheller, and assisted by luck and good fortune, we have addressed these questions over the last decades. Presynaptic plasma membrane © 2013 Nature America, Inc. All rights reserved. America, Inc. © 2013 Nature As I describe below, we now know of a general mechanism of membrane fusion that Figure 1 Description of the SNARE–SM protein complex that mediates synaptic vesicle fusion. The initial operates by the interaction of SNAREs (for sol- study2 identified the SM protein Munc18-1 as a component of the fusion machinery that also contains npg uble N-ethylmaleimide–sensitive factor (NSF)- the SNARE proteins synaptobrevin (also known as VAMP), SNAP-25 and syntaxin (also known as HPC-1), attachment protein receptors) and SM proteins although the detailed protein-protein interactions involved were defined only later (reproduced from ref. 2). (for Sec1/Munc18-like proteins). We also have now a general mechanism of calcium-triggered Starting my lab cells, but the molecular mechanisms involved fusion that operates by calcium binding to syn- When I started my laboratory in 1986 at the in these fusion reactions were unclear. The lack aptotagmins, plus a general mechanism of ves- University of Texas Southwestern in Dallas, of knowledge about how synaptic vesicle fusion icle positioning adjacent to calcium channels, exquisite electrophysiological studies had already happens and how such fusion is controlled by which involves the interaction of the so-called characterized neurotransmitter release in detail. calcium intrigued me and led me to search for RIM proteins with these channels and synap- These studies showed that calcium triggers molecular mechanisms. tic vesicles. Thus, a molecular framework that release within a few hundred microseconds, We chose a simple approach: to isolate and accounts for the astounding speed and preci- exhibits amazing plasticity and displays a non- clone all of the major proteins present in pre- sion of neurotransmitter release has emerged. linear dependence on calcium. However, aside synaptic terminals. Initially, in collaboration with In describing this framework, I have been asked from calcium, not a single molecule important Reinhard Jahn, we focused on synaptic vesicles to describe primarily my own work. I apologize for release had been identified. because they could be isolated at high yield and for the many omissions of citations to work of Genetic screens by Sidney Brenner, Randy purity. Later on, we expanded this approach to others; please consult a recent review for addi- Scheckman and their colleagues had isolated the presynaptic active zone and plasma mem- tional references1. gene mutations that disrupt synaptic transmis- brane. The goal was to achieve a molecular cata- sion in Caenorhabditis elegans or impair the log of protein components of the presynaptic Thomas C. Südhof is at the Department of secretory pathway in yeast, but the function of terminal as a starting point for a functional Molecular and Cellular Physiology and the Howard the corresponding proteins were unknown. In dissection. For over a decade, we purified and Hughes Medical Institute, Stanford University pioneering work, Rothman performed in vitro cloned a series of major synaptic proteins— School of Medicine, Palo Alto, California, USA. membrane fusion assays using non-neuronal among others, synaptophysin, synaptobrevin, NATURE MEDICINE VOLUME 19 | NUMBER 10 | OCTOBER 2013 1227 ESSAY Membranes poised for fusion the action of SM proteins; see below) opens the Presynaptic R-SNARE fusion pore (step 3). Fusion-pore expansion terminal (synaptobrevin/ VAMP) 4. Snare complex disassembly SV 1. SNARE complex assembly transforms the initial trans-SNARE complexes and vesicle recycling (chaperones: CSPα, β, or γ + into cis-SNARE complexes that are then disso- Qa-SNARE SM protein (chaperones: NSF + (syntaxin) Qb- & Qc-SNARE (Munc18) α-, β- or γ-synucleins) ciated by the ATPase NSF, which binds SNARE α-, β- or γ-SNAPs) (SNAP-25) complexes via a-, b- or g-SNAP adaptor proteins, N thereby completing the cycle (step 4). In a physiological context, SNARE complex Synaptic cleft assembly forces opposing membranes into Membranes after fusion Membranes primed for fusion close proximity but is insufficient to mediate cis-SNARE complex– Partial trans-SNARE complex– SM protein assembly SM protein assembly fusion. Our initial proposal that SM proteins SNARE– generally contribute to membrane fusion2 SM protein N N met with skepticism because biochemical N N cycle data seemed to argue against this hypothesis. However, studies over the past decade have revealed that Munc18-1 remains associated Fusion pore opening during fusion with SNARE proteins through- trans-SNARE complex/ SM protein assembly out their assembly-disassembly cycle and that 4 3. Fusion pore 2. Fusion pore this association is essential for fusion . This finding resolved the questions surrounding expansion N N opening Munc18-1 and strongly supported the idea that Munc18-1 and other SM proteins are obligatory Fusion pore components of fusion machines1. Figure 2 Model of the SNARE–SM protein cycle during synaptic vesicle fusion. During step 1, synaptic vesicles are primed for fusion; this step involves opening of the closed conformation of syntaxin, a How calcium controls membrane fusion switch of the Munc18-binding mode of syntaxin from the closed to the open conformation and partial The above discussion describes the major prog- assembly of trans-SNARE complexes. Step 1 is facilitated by recently discovered chaperones (cysteine ress that was made in determining the mecha- string proteins (CSPs) and synucleins) that enhance SNARE complex assembly and whose dysfunction nism of membrane fusion. At the same time, my 3 is related to neurodegeneration . During step 2, the fusion pore opens, with full trans-SNARE complex laboratory was focusing on a question crucial for assembly. During step 3, the fusion pore expands, converting trans-SNARE into cis-SNARE complexes. neuronal function: how is this process triggered In step 4, NSF and SNAPs mediate disassembly of the SNARE complex, leading to vesicle recycling. The cycle shown here for synaptic vesicle fusion is paradigmatic for most cytoplasmic fusion reactions, in microseconds when calcium enters the pre- although the details differ. In knockout experiments, deletion of the SM protein Munc18-1 produces synaptic terminal? the most severe phenotype22, possibly because loss of SNARE components of the fusion machinery is While examining the fusion machinery, we better compensated for than loss of SM proteins. SNAREs are generally classified into four types wondered how it could possibly be controlled so (R, Qa, Qb and Qc) that assemble into SNARE complexes in an obligatory R-Qa-Qb-Qc combination. tightly by calcium. Starting with the description of synaptotagmin-1 (Syt1)5, we worked over two © 2013 Nature America, Inc. All rights reserved. America, Inc. © 2013 Nature synapsins, synaptotagmins, Munc13s, Munc18s, activities of botulinum and tetanus toxins, which decades to show that calcium-dependent exocy- complexins, RIMs, RIM-BPs, neurexins and were known to block neurotransmitter release. tosis is mediated by synaptotagmins as calcium neuroligins. These findings strongly supported the idea that sensors. npg the three SNARE proteins function in synaptic Synaptotagmins are evolutionarily conserved Elucidating the mechanism of fusion vesicle fusion. Thus, at the end of 1993, most transmembrane proteins with two cytoplasmic 5,6 As a molecular catalog of synaptic vesicle pro- components of the synaptic fusion machinery C2 domains (Fig. 3a) . When we cloned Syt1, teins emerged, the question of how synaptic had been identified. But how do SNARE and nothing was known about C2 domains except vesicles may interact with the presynaptic plasma SM proteins mediate fusion? It took two decades that they represented the ‘second constant membrane became approachable. Other labora- until a plausible model emerged1. sequence’ in protein-kinase C isozymes. Because tories cloned SNAP-25 and syntaxin-1, presyn- During fusion, SNARE and SM proteins protein kinase C had been shown to interact with aptic plasma membrane proteins now known as undergo a cycle of association and dissociation, phospholipids by an unknown mechanism, we t-SNAREs. In a crucial study, Rothman showed which is maintained by chaperones that sup- speculated that Syt1 C2 domains may bind phos- that syntaxin and SNAP-25 form a complex port SNARE complex assembly (cysteine string pholipids, which we indeed found to be the case5.
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