Synapse Transmission

Neurotransmitter release from synaptic vesicle fusion is the fundamental mechanism for neuronal communication at synapses. Since the work of Bernard Katz, it has been known that synaptic vesicle fusion is triggered by Ca2+ influx into the presynaptic terminal. Vesicular trafficking pathways are ubiquitous among eukaryotes, with fusion driven by the SNARE complex. Neurotransmitters can be released during evoked fusion following an action potential, or through spontaneous fusion of vesicles (termed “minis”) in the absence of nerve stimulation. One of the major goals of my research program has been to define how vesicular trafficking mediates synaptic transmission, and how this process can be altered to change neuronal communication. At synapses, SNARE-mediated fusion is uniquely regulated to allow rapid and Ca2+-triggered synaptic vesicle fusion. Several key adaptations to the core fusion machinery require synapse specific SNARE-binding proteins, including Synaptotagmin 1 (Syt 1) and Complexin (Cpx). We have extensively studied how these proteins control synaptic output. Syt 1 is a synaptic vesicle protein that binds to SNARE complexes and membrane phospholipids in a Ca2+-dependent manner. My lab and others have show that Syt 1 functions as the Ca2+ sensor for fast synchronous neurotransmitter release. Cpx is a small cytosolic α-helical protein that binds assembled SNARE complexes. We have shown that association of Cpx with SNAREs allows it to function as a facilitator for synaptic vesicle fusion and as a fusion clamp to prevent premature exocytosis in the absence of Ca2+. A key avenue of our ongoing research program is to understand how these proteins, together with other key regulators like Munc18 and Munc13, regulate cycles of SNARE assembly and disassembly in a precisely coordinated fashion to mediate synaptic transmission. Similarly, we also seek to understand how the core fusion machinery can be rapidly modified, for example through phosphorylation, to alter presynaptic function and synaptic plasticity.

Although we have a research program in place to address how synapses form and function, I will highlight one approach below that provides a framework for how we use the Drosophila NMJ as a model synapse to uncover general principles of synapse biology. We have recently developed a new toolkit to optically visualize single synaptic vesicle fusion events from individual active zones. Evoked release following an action potential has been well characterized for its function in activating the postsynaptic cell, but the significance of spontaneous release is less clear. Whether receptors on the postsynaptic cell have the ability to distinguish between neurotransmitters released through the two independent fusion pathways is still under debate. If both vesicle release mechanisms activate the same set of receptors, crosstalk would occur between the two modes of fusion. However, if spontaneous release occurs at distinct release sites, the postsynaptic cell may be capable of differentiating between the two modes of release, suggesting spontaneous fusion may represent a separate information channel independent of the traditional Ca2+-activated evoked release pathway. The major confound to this question has been the inability to examine vesicle fusion at individual active zones. Classical electrophysiological studies of synaptic transmission measure the postsynaptic effect of neurotransmitter release over a population of release sites, precluding an analysis of how individual active zones participate in and regulate these two modes of vesicle fusion. We have developed transgenically expressed Ca2+ sensors that allow imaging of postsynaptic glutamate receptor activation following vesicle fusion, providing a mechanism to visualize all exocytotic events occurring through both spontaneous and evoked release pathways at Drosophila glutamatergic neuromuscular junctions. This toolkit allows us to generate release probability maps for both spontaneous and evoked fusion for all active zones at a synapse. By imaging a defined neuromuscular connection with ~ 300 active zones between the synaptic partners, we have begun to analyze the relationship between spontaneous and evoked release and determine how release mode and release probability are regulated at individual active zones at this model glutamatergic synapse. Unexpectedly, we have discovered that a subset of active zones are specifically dedicated to spontaneous release, indicating a population of postsynaptic receptors is uniquely activated by this mode of vesicle fusion. We have also found that release probability can vary more than 100-fold between neighboring active zones. This is an exciting finding and provides us a mechanism to understand how single active zone release probability is regulated. The ability to measure individual active zones for their release probability also allows us to determine how active zone release properties can change during synaptic plasticity. We hypothesize that evoked and spontaneous active zone release probability is not a static property of synaptic connections, and can be differentially regulated during distinct modes of synaptic plasticity. Synaptic plasticity mechanisms can be manifested through changes in N (the number of release sites), P (probability of release) or Q (quantal size). Many factors have been suggested to regulate these properties. However, the lack of a tool to examine single active zone release probability has made it difficult to dissect how these changes play out over a population of release sites. We are now in a position to examine how plastic changes in release properties at the NMJ map onto alterations in individual active zone release probability. By mapping single active zone release probability during both acute and homeostatic synaptic plasticity and following the properties, we can gain new insights into presynaptic mechanisms underlying synaptic communication and plasticity at a model glutamatergic synapse.

The Drosophila NMJ contains spatially segregated active zones (termed T-bars) that oppose a distinct population of clustered glutamate receptors in the postsynaptic muscle (Fig. 2). This spatial arrangement of release sites and clustered receptors was used to analyze evoked release probability at individual release sites by measuring Ca2+ influx that occurs through postsynaptic glutamate receptors following vesicle fusion with a transgenic membrane-tethered (myristoylated) variant of the genetically encoded Ca2+-sensitive GFP (GCaMP6) that robustly detects postsynaptic responses to both evoked and spontaneous synaptic vesicle fusion events. We focused our initial analysis on NMJs at larval muscle fiber 4 which contains several hundred individual active zones (Fig. 2). Each synaptic bouton contains between 5 to 30 active zones that are spatially separated and opposed by a discrete cluster of glutamate receptors, providing a one-to-one match between the release machinery and the postsynaptic receptive unit. Transgenic lines expressing postsynaptic myrGCaMP show robust and spatially segregated signals for Ca2+ influx through glutamate receptors that occurs following synaptic vesicle fusion. Subsequent events at the same active zone produce similar Ca2+ profiles, allowing us to follow vesicle fusion at defined regions of interest (ROIs) over continuous imaging sessions for up to 30 minutes. Distinct ROIs defined by spatially segregated Ca2+ signals can be identified over the entire arbor, providing spatial information over time for release at all active zones. We observed that spontaneous Ca2+ signals detected optically are correlated with simultaneous electrophysiological recordings of minis at the NMJ for 99.8% of the events we detect. We find that spontaneous release probability per active zone is not uniform. Indeed, the distribution of spontaneous release probability is highly non-Poisson, with many low probability release sites, and a smaller subset of highly active release sites.