PT  - JOURNAL ARTICLE
AU  - Tanvi Butola
AU  - Theocharis Alvanos
AU  - Anika Hintze
AU  - Peter Koppensteiner
AU  - David Kleindienst
AU  - Ryuichi Shigemoto
AU  - Carolin Wichmann
AU  - Tobias Moser
TI  - RIM-Binding Protein 2 organizes Ca<sup>2+</sup> channel topography and regulates release probability and vesicle replenishment at a fast central synapse
AID  - 10.1101/2021.03.20.435607
DP  - 2021 Jan 01
TA  - bioRxiv
PG  - 2021.03.20.435607
4099  - http://biorxiv.org/content/early/2021/03/20/2021.03.20.435607.short
4100  - http://biorxiv.org/content/early/2021/03/20/2021.03.20.435607.full
AB  - RIM-Binding Protein 2 (RIM-BP2) is a multi-domain protein of the presynaptic active zone (AZ). By binding to Rab-interacting protein (RIM), bassoon and voltage-gated Ca²⁺ channels (CaV), it is considered to be a central organizer of the topography of CaV and release sites of synaptic vesicles (SVs) at the AZ. Here, we investigated the role of RIM-BP2 at the endbulb of Held synapse of auditory nerve fibers with bushy cells of the cochlear nucleus, a fast relay of the auditory pathway with high release probability. Disruption of RIM-BP2 lowered release probability altering short-term plasticity and reduced evoked excitatory postsynaptic currents (EPSCs). Analysis of SV pool dynamics during high frequency train stimulation indicated a reduction of SVs with high release probability but an overall normal size of the readily releasable SV pool (RRP). The Ca2+-dependent fast component of SV replenishment after RRP depletion was slowed. Ultrastructural analysis by super-resolution light and electron microscopy revealed an impaired topography of presynaptic CaV and a reduction of docked and membrane-proximal SVs at the AZ. We conclude that RIM-BP2 organizes the topography of CaV, and promotes SV tethering and docking. This way RIM-BP2 is critical for establishing a high initial release probability as required to reliably signal sound onset information that we found to be degraded in bushy cells of RIM-BP2-deficient mice in vivo.Significance Statement RIM-binding proteins (RIM-BPs) are key organizers of the active zone (AZ). Using a multidisciplinary approach to the calyceal endbulb of Held synapse that transmit auditory information at rates of up to hundreds of Hertz with sub-millisecond precision we demonstrate a requirement for RIM-BP2 for normal auditory signaling. Endbulb synapses lacking RIM-BP2 show a reduced release probability despite normal whole-terminal Ca2+ influx and abundance of the key priming protein Munc13-1, a reduced rate of SV replenishment, as well as an altered topography of CaV2.1 Ca2+ channels, and fewer docked and membrane proximal synaptic vesicles. This hampers transmission of sound onset information likely affecting downstream neural computations such as of sound localization.Competing Interest StatementThe authors have declared no competing interest.