RIM-binding protein 2 regulates release probability by fine-tuning calcium channel localization at murine hippocampal synapses

Significance

Highly regulated and precise positioning of Ca ²⁺ channels at the active zone (AZ) controls Ca ²⁺ nanodomains at release sites. Their exact localization affects vesicular release probability (P _VR) and is important for proper synaptic transmission during repetitive stimulation. We provide a detailed analysis of synaptic transmission combined with superresolution imaging of the AZ organization in mouse hippocampal synapses lacking Rab-interacting molecule-binding protein 2 (RIM-BP2). By dual- and triple-channel time-gated stimulated emission depletion (gSTED) microscopy, we directly show that RIM-BP2 fine-tunes voltage-gated Ca ²⁺ channel 2.1 (Ca _V2.1) localization at the AZ. We reveal that RIM-BP2 likely regulates the Ca ²⁺ nanodomain by positioning Ca _V2.1 channels close to synaptic vesicle release sites. Loss of RIM-BP2 reduces P _VR and alters short-term plasticity.

Abstract

The tight spatial coupling of synaptic vesicles and voltage-gated Ca ²⁺ channels (Ca _Vs) ensures efficient action potential-triggered neurotransmitter release from presynaptic active zones (AZs). Rab-interacting molecule-binding proteins (RIM-BPs) interact with Ca ²⁺ channels and via RIM with other components of the release machinery. Although human RIM-BPs have been implicated in autism spectrum disorders, little is known about the role of mammalian RIM-BPs in synaptic transmission. We investigated RIM-BP2–deficient murine hippocampal neurons in cultures and slices. Short-term facilitation is significantly enhanced in both model systems. Detailed analysis in culture revealed a reduction in initial release probability, which presumably underlies the increased short-term facilitation. Superresolution microscopy revealed an impairment in Ca _V2.1 clustering at AZs, which likely alters Ca ²⁺ nanodomains at release sites and thereby affects release probability. Additional deletion of RIM-BP1 does not exacerbate the phenotype, indicating that RIM-BP2 is the dominating RIM-BP isoform at these synapses.