Active zone scaffolds differentially accumulate Unc13 isoforms to tune Ca2+ channel-vesicle coupling

Böhme, Mathias A. and Beis, Christina and Reddy-Alla, Suneel and Reynolds, Eric and Mampell, Malou M. and Grasskamp, Andreas T. and Lützkendorf, Janine and Bergeron, Dominique Dufour and Driller, Jan H. and Babikir, Husam and Göttfert, Fabian and Robinson, Iain M. and O'Kane, Cahir J. and Hell, Stefan W. and Wahl, Markus C. and Stelzl, Ulrich and Loll, Bernhard and Walter, Alexander M. and Sigrist, Stephan J. (2016) Active zone scaffolds differentially accumulate Unc13 isoforms to tune Ca2+ channel-vesicle coupling. Nature Neuroscience, 19 (10). pp. 1311-1320. ISSN 1097-6256

Abstract

Brain function relies on fast and precisely timed synaptic vesicle (SV) release at active zones (AZs). Efficacy of SV release depends on distance from SV to Ca2+ channel, but molecular mechanisms controlling this are unknown. Here we found that distances can be defined by targeting two unc-13 (Unc13) isoforms to presynaptic AZ subdomains. Super-resolution and intravital imaging of developing Drosophila melanogaster glutamatergic synapses revealed that the Unc13B isoform was recruited to nascent AZs by the scaffolding proteins Syd-1 and Liprin-α, and Unc13A was positioned by Bruchpilot and Rim-binding protein complexes at maturing AZs. Unc13B localized 120 nm away from Ca2+ channels, whereas Unc13A localized only 70 nm away and was responsible for docking SVs at this distance. Unc13A null mutants suffered from inefficient, delayed and EGTA-supersensitive release. Mathematical modeling suggested that synapses normally operate via two independent release pathways differentially positioned by either isoform. We identified isoform-specific Unc13-AZ scaffold interactions regulating SV-Ca2+ -channel topology whose developmental tightening optimizes synaptic transmission.