Cell‐type specific visualization and biochemical isolation of endogenous synaptic proteins in mice

In recent years, the remarkable molecular complexity of synapses has been revealed, with over 1000 proteins identified in the synapse proteome. Although it is known that different receptors and other synaptic proteins are present in different types of neurons, the extent of synapse diversity across the brain is largely unknown. This is mainly due to the limitations of current techniques. Here we report an efficient method for the purification of synaptic protein‐complexes, fusing a high‐affinity tag to endogenous PSD95 in specific cell types.

We also developed a strategy which enables the visualization of endogenous PSD95 with fluorescent‐proteins tag in Cre‐recombinase expressing cells. We demonstrate the feasibility of proteomic analysis of synaptic protein‐complexes and visualization of these in specific cell types. We find that the composition of PSD95‐complexes purified from specific cell types differs from those extracted from tissues with diverse cellular composition. The results suggest that there might be differential interactions in the PSD95‐complexes in different brain regions. We have detected differentially interacting proteins by comparing datasets from the whole hippocampus and the CA3 subfield of the hippocampus. Therefore, these novel conditional PSD95 tagging lines will not only serve as powerful tools for precisely dissecting synapse diversity in specific brain regions and subsets of neuronal cells, but also provide an opportunity to better understand brain region‐ and cell type‐specific alterations associated with various psychiatric/neurological diseases. These newly developed conditional gene‐tagging methods can be applied to many different synaptic proteins and will facilitate research on the molecular complexity of synapses.