RIM-Binding Protein 2 positively regulates the abundance and release site coupling of presynaptic Ca²⁺ channels at a fast central synapse

Abstract

RIM-Binding Protein 2 (RIM-BP2) is a multidomain protein of the presynaptic active zone (AZ). By binding to Rab-interacting protein (RIM), Bassoon and voltage-gated Ca²⁺ channels (CaV), it is considered a to be central organizer of the topography of CaV and release sites of synaptic vesicles (SVs). Here, we investigated the role of RIM-BP2 at the endbulb of Held synapse, a fast relay of the auditory pathway with high release probability. Disruption of RIM-BP2 reduced the amplitude of evoked excitatory postsynaptic currents (EPSCs) and altered short-term plasticity due to reduced vesicular release probability. In addition, SV replenishment to the readily releasable SV pool was slowed. Augmenting Ca²⁺ influx by adding extracellular Ca²⁺ restored normal transmission. Presynaptic CaV channels were reduced and their topography altered. Moreover, there were fewer SVs in a distance of 2-20 nm to the AZ membrane. We conclude that RIM-BP2 positively regulates the clustering and SV coupling of CaV channels at the endbulb of Held.