The human brain processes information by transmitting signals across neuronal synapses. Underneath the postsynaptic membrane lies a condensed, proteinaceous region termed the postsynaptic density (PSD). Decades of studies on the PSD have revealed its pivotal role in synapse formation, signal transmission and synaptic plasticity. Therefore, an understanding of the structural organization and regulation of the PSD can provide valuable insights into the mechanistic details underlying major synaptic functions.

PSD-95 and SAPAP, two of the most abundant proteins in the PSD, function as a core scaffold that orchestrates PSD formation and plasticity. In the first part of my doctoral study, I have shown that SAPAP GBRs interact with PSD-95 in a phosphorylation dependent manner. This w...[ Read more ]

The human brain processes information by transmitting signals across neuronal synapses. Underneath the postsynaptic membrane lies a condensed, proteinaceous region termed the postsynaptic density (PSD). Decades of studies on the PSD have revealed its pivotal role in synapse formation, signal transmission and synaptic plasticity. Therefore, an understanding of the structural organization and regulation of the PSD can provide valuable insights into the mechanistic details underlying major synaptic functions.

PSD-95 and SAPAP, two of the most abundant proteins in the PSD, function as a core scaffold that orchestrates PSD formation and plasticity. In the first part of my doctoral study, I have shown that SAPAP GBRs interact with PSD-95 in a phosphorylation dependent manner. This was evidenced by the atomic structure of the PSD-95 GK domain in complex with a phosphorylated SAPAP peptide. Later on by expressing dominant negative SAPAP constructs in cultured hippocampus neurons, I further demonstrated that this phosphorylation-dependent interaction is critical for SAPAP synaptic targeting and that disruption of the phosphorylation on SAPAP GBR leads to defects in dendritic spine development.

Recent progress towards the in vitro reconstitution of the PSD has transformed our textbook view of the PSD assembly and introduced a novel mechanism whereby PSD formation is driven by phase separation. Multivalent and specific scaffold-scaffold interactions provide the main driving force for this phase separation while client molecules, such as enzymes, can be recruited into the condensed phase via their interactions with the scaffold proteins. One such example is illustrated by the GIT/PIX pair of GTPase regulatory enzymes. In the second part of my doctoral study, I have shown that the GIT and PIX proteins form a highly specific complex. Interestingly, the association between GIT1 and β-Pix leads to autonomous condensation via phase separation of this enzyme complex both in vitro and inside cells. Importantly, the GIT1/β-Pix condensate functions as a versatile module that can bind to different adaptor proteins through which it can then be targeted to distinct cellular signaling processes. In synapses, this complex is recruited to the PSD through the interaction between β-Pix and Shank, a key PSD scaffold protein.

Altogether, this study illustrates the molecular mechanism underlying the interaction between two major PSD scaffold proteins, SAPAP and PSD-95. Via this core scaffold, the PSD like other membraneless cellular compartments can then specifically recruit and concentrate versatile enzymatic modules such as GIT/PIX in a process mediated by phase separation.