Role of the different domains of PSD-95 in basal synaptic transmission

Abstract

Learning and memory has been postulated to rely on changes in synaptic transmission strength. At excitatory synapses in the hippocampus CA1 region, synaptic strength is mainly regulated by AMPA receptor function at the post‐synaptic density. PSD‐95, a member of DLG‐MAGUK family along with PSD‐93, SAP97 and SAP102, is involved in the regulation of AMPAR function. DLG‐MAGUKs share a similar modular domain structure, composed of three copies of PDZ domain, an SH3 domain and a catalytically inactive GK domain. PSD‐95 is the most abundant DLG‐MAGUK at excitatory mature synapses and interacts with va rious cellular proteins. To examine the requirement of specific PSD‐95 domains and their role in the excitatory synaptic transmission regulation, I combined two approaches. On one hand, I made use of the molecular replacement approach by acutely knocking‐down PSD‐95 and expressing simultaneously a mutant form of PSD‐95, in a spatiotemporally specific manner among an intact network of neurons in rat hippocampal slices. On the other hand, I took advantage of the PSD‐95 knockout mouse line, to express mutants of PSD‐95 in a PSD‐95 free background. To assess which domain of PSD‐95 is important for the protein to mediate its effect on basal synaptic transmission, I evaluated if specific form of truncated PSD‐95 could rd escue the eficiency caused bythe absence of endogenous PSD‐95. With this study I could show that PSD‐95 lacking its PDZ3, SH3 and GK domains could not mediate proper basal synaptic strength in the absence of endogenous PSD‐95. Moreover, the PDZ3 or SH3 domain seems dispensable for the protein to be functional. Finally, to be functional PSD‐95 requires its GK domain. However, this seems dependent on SAP102, another DLG‐MAGUK regulating synaptic transmission in immature synapses. The requirement of the GK domain appears dependant on the maturational state of the synapse with a strongest effect on more immature synapses. In conclusion, my data demonstrate the importance of the N‐terminal PDZ12 domains acting in concert with the GK domain and a permissive role of the SH3 and PDZ3 domains in regulating the strength of AMPAR function.