Ultrastructural analysis of wildtype and RIM1α knock-out active zones in a large cortical synapse
Rab3A-interacting molecule (RIM) is crucial for fast Ca2+-triggered synaptic vesicle (SV) release in presynaptic active zones (AZ). While loss of RIM1 impairs long-term plasticity at hippocampal giant mossy fiber boutons (MFB), it remains unclear how AZ ultrastructure is altered. We investigated MFB AZ architecture in 3D using electron tomography of rapid cryo-immobilized acute brain slices in RIM1-/- and wild-type mice. In RIM1-/-, AZs are larger with increased synaptic cleft heights and with a three-fold reduced number of tightly docked SVs (0-2nm). The distance of tightly docked SVs to the AZ center is increased from 110 to 195 nm, and the width of their electron dense material between outer SV membrane and AZ membrane is reduced. Furthermore, the SV pool in RIM1-/- is more heterogeneous. Thus, RIM1, beside its role in tight SV docking, is crucial for synaptic architecture and vesicle pool organization in MFBs.