RIM-BP2 regulates Ca²⁺ channel abundance and neurotransmitter release at hippocampal mossy fiber terminals

Synaptic vesicles dock and fuse at the presynaptic active zone (AZ), the specialized site for transmitter release. AZ proteins play multiple roles such as recruitment of Ca²⁺ channels as well as synaptic vesicle docking, priming, and fusion. However, the precise role of each AZ protein type remains unknown. In order to dissect the role of RIM-BP2 at mammalian cortical synapses having low release probability, we applied direct electrophysiological recording and super-resolution imaging to hippocampal mossy fiber terminals of RIM-BP2 knockout (KO) mice. By using direct presynaptic recording, we found the reduced Ca²⁺ currents. The measurements of excitatory postsynaptic currents (EPSCs) and presynaptic capacitance suggested that the initial release probability was lowered because of the reduced Ca²⁺ influx and impaired fusion competence in RIM-BP2 KO. Nevertheless, larger Ca²⁺ influx restored release partially. Consistent with presynaptic recording, STED microscopy suggested less abundance of P/Q-type Ca²⁺ channels at AZs deficient in RIM-BP2. Our results suggest that the RIM-BP2 regulates both Ca²⁺ channel abundance and transmitter release at mossy fiber synapses.