Abstract
Spatial relationships between Cav channels and release sensors at active zones (AZs) are a major determinant of synaptic fidelity. They are regulated developmentally, but the underlying molecular mechanisms are largely unclear. Here, we show that Munc13-3 regulates the density of Cav2.1 and Cav2.2 channels, alters the localization of Cav2.1, and is required for the development of tight, nanodomain coupling at parallel-fiber AZs. We combined EGTA application and Ca2+-channel pharmacology in electrophysiological and two-photon Ca2+ imaging experiments with quantitative freeze-fracture immunoelectron microscopy and mathematical modeling. We found that a normally occurring developmental shift from release being dominated by Ca2+ influx through Cav2.1 and Cav2.2 channels with domain overlap and loose coupling (microdomains) to a nanodomain Cav2.1 to sensor coupling is impaired in Munc13-3-deficient synapses. Thus, at AZs lacking Munc13-3, release remained triggered by Cav2.1 and Cav2.2 microdomains, suggesting a critical role of Munc13-3 in the formation of release sites with calcium channel nanodomains.
Highlights
The details of the nanoscopic arrangement between voltageactivated Ca2+ channels (Cavs) and the sensors for transmitter release are crucial for setting fundamental synaptic properties, including release probability and plasticity (Eggermann et al, 2011)
We found that a normally occurring developmental shift from release being dominated by Ca2+ influx through Cav2.1 and Cav2.2 channels with domain overlap and loose coupling to a nanodomain Cav2.1 to sensor coupling is impaired in Munc13-3-deficient synapses
At active zones (AZs) lacking Munc13-3, release remained triggered by Cav2.1 and Cav2.2 microdomains, suggesting a critical role of Munc13-3 in the formation of release sites with calcium channel nanodomains
Summary
The details of the nanoscopic arrangement between voltageactivated Ca2+ channels (Cavs) and the sensors for transmitter release are crucial for setting fundamental synaptic properties, including release probability (pr) and plasticity (Eggermann et al, 2011). At the calyx of Held (Fedchyshyn and Wang, 2005) and the parallel fiber (PF)-to-Purkinje neuron (PN) synapse (Baur et al, 2015), the coupling distance undergoes substantial developmental tightening, associated with changes in pr and short-term plasticity. Septin has been suggested to operate as a developmentally downregulated ‘‘spacer’’ between Cavs and release sensors at the calyx of Held; that is, its downregulation is permissive to the establishment of tight coupling (Yang et al, 2010). No developmental mediators of tight coupling were identified to date (Eggermann et al, 2011)
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