Munc13-3 Is Required for the Developmental Localization of Ca2+ Channels to Active Zones and the Nanopositioning of Cav2.1 Near Release Sensors

Kusch V, Bornstein G, Loreth D, Bank J, Jordan J, Baur D, Watanabe M, Kulik A, Heckmann M, Eilers J, Schmidt H (2018)

Cell Reports 22, 1965–73

Spatial relationships between Ca_v 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 Ca_v2.1 and Ca_v2.2 channels, alters the localization of Ca_v2.1, 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 Ca_v2.1 and Ca_v2.2 channels with domain overlap and loose coupling (microdomains) to a nanodomain Ca_v2.1 to sensor coupling is impaired in Munc13-3-deficient synapses. Thus, at AZs lacking Munc13-3, release remained triggered by Ca_v2.1 and Ca_v2.2 microdomains, suggesting a critical role of Munc13-3 in the formation of release sites with calcium channel nanodomains.

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