Complexin cooperates with Bruchpilot to tether synaptic vesicles to the active zone cytomatrix

Abstract

Information processing by the nervous system depends on the release of neurotransmitter from synaptic vesicles (SVs) at the presynaptic active zone. Molecular components of the cytomatrix at the active zone (CAZ) regulate the final stages of the SV cycle preceding exocytosis and thereby shape the efficacy and plasticity of synaptic transmission. Part of this regulation is reflected by a physical association of SVs with filamentous CAZ structures. However, our understanding of the protein interactions underlying SV tethering by the CAZ is far from complete. The very C-terminal region of Bruchpilot (Brp), a key component of the Drosophila CAZ, participates in SV tethering. Yet so far, no vesicular or cytoplasmic molecules have been reported to engage in an interaction with Brps C-terminus. Here, we carried out an in vivo screen for molecules that link the Brp C-terminus to SVs. This strategy identified the conserved SNARE (soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptor) regulator Complexin (Cpx) as a vesicular interaction partner of Brp. We show that Brp and Cpx interact genetically and functionally. Interfering with Cpx targeting to SVs mirrored distinctive features of a C-terminal Brp truncation: impaired SV recruitment to the CAZ and enhanced short-term synaptic depression. Extending the study beyond Drosophila synapses, we interrogated active zones of mouse rod bipolar cells. Here, too, we collected evidence for an evolutionarily conserved role of Cpx upstream of SNARE complex assembly where it participates in SV tethering to the CAZ.