Modular transport of postsynaptic density-95 clusters and association withstable spine precursors during early development of cortical neurons

The properties of filopodia and spines and their association with the posts ynaptic density (PSD) protein PSD-95 were studied during early development of cultured cortical neurons using time-lapse confocal microscopy. Neurons were transfected with recombinant PSD-95 constructs fused to green fluoresc ent protein (GFP) for, on average, either 8 d in vitro (DIV) or 14 DIV. We find that, during 1 hr of imaging, filopodia and spines bearing PSD-95/GFP clusters are significantly more stable (i.e., do not turnover) than those l acking clusters. When present within a spine precursor, a PSD-95/GFP cluste r appeared to nucleate a relatively stable structure around which filopodiu m-spine membranes can move. Although processes bearing clusters were genera lly stable, in 8 DIV neurons, we observed that a subset (similar to 10%) of PSD-95/GFP clusters underwent rapid modular translocation between filopodi a-spines and dendritic shafts. We conclude that, during early synaptic matu ration, prefabricated PSD-95 clusters are trafficked in a developmentally r egulated process that is associated with filopodial stabilization and synap se formation.