The Dynamics of SAP90/PSD-95 Recruitment to New Synaptic Junctions

Abstract

SAP90/PSD-95 is thought to be a central organizer of the glutamatergic synapse postsynaptic reception apparatus. To assess its potential role during glutamatergic synapse formation, we used GFP-tagged SAP90/PSD-95, time lapse confocal microscopy, and cultured hippocampal neurons to determine its dynamic recruitment into new synaptic junctions. We report that new SAP90/PSD-95 clusters first appeared at new axodendritic contact sites within 20–60 min of contact establishment. SAP90/PSD-95 clustering was rapid, with kinetics that fit a single exponential with a mean time constant of ∼23 min. Most new SAP90/PSD-95 clusters were found juxtaposed to functional presynaptic boutons as determined by labeling with FM 4–64. No evidence was found for the existence of discrete transport particles similar to those previously reported to mediate presynaptic active zone cytoskeleton assembly. Instead, we found that SAP90/PSD-95 is recruited to nascent synapses from a diffuse dendritic cytoplasmic pool. Our findings show that SAP90/PSD-95 is recruited to nascent synaptic junctions early during the assembly process and indicate that its assimilation is fundamentally different from that of presynaptic active zone components.