Cryo-electron tomography reveals postsynaptic nanoblocks in excitatory synapses

The nanoscale organization of proteins within synapses is critical for maintaining and regulating synaptic transmission and plasticity. Here, we use cryogenic electron tomography to directly visualize the three-dimensional architecture and supramolecular organization of pre-cleft-postsynaptic components in their near-native cellular context in both synaptosomes from rat hippocampi and synapses from rat primary cultured neurons. High-resolution electron microscopy and quantitative analyses revealed that postsynaptic density (PSD) is composed of membrane-associated nanoblocks of various sizes, which are in close relationship with potential presynaptic release sites through adhesion molecules spanning the synaptic cleft, as well as with post-synaptic receptors. Subtomogram averaging from synaptosomes showed two distinct types of postsynaptic membrane proteins at resolutions of 24 Å and 26 Å respectively. Furthermore, our data reveal that majority of potential release sites and ∼50% subtomogram averaged receptor-like particles are located within the boundary of PSD nanoblocks, while PSD nanoblocks might be redundant for neurotransmission. The results of this study provide a more comprehensive understanding of synaptic ultrastructure and suggest that PSD is composed of clustering of various nanoblocks, which likely underlies the dynamic nature of PSD to modulate synaptic strength.