The Role of PSD-95 and Kinase Interactions in Synaptic Transmission

During development various structural and molecular changes take place in the brain leading to its maturation. These changes occur at multiple levels, including the alterations in protein expression level of single cells and in the synaptic strength of neuronal connections. PSD-95, one of the most important scaffold proteins, in excitatory synapses, was shown to be involved in some of these developmental processes. In this study, by using diverse electrophysiological and biochemical methods in combination with in vivo and in vitro lentiviral injection techniques, I examined the role of PSD-95 and its domains in the regulation of basal synaptic transmission and developmental NMDA receptor subunit switch. NMDA receptor subunit switch is one of the most important events taking place during early postnatal development. The regulation of the GluN2B-containing NMDA receptor surface expression was proposed to involve PSD-95 and Src kinase interaction, and this interaction was shown to depend on the CDK5 phosphorylation state of PSD-95. However, the effect of this phosphorylation on synaptic transmission is unknown. In this study, I showed that a mutant form of PSD-95 mimicking the phosphorylated state enhanced both AMPAR and NMDAR transmission in a Src kinase- and GluN2B subunit-dependent manner. In addition, I could demonstrate that PSD-95 is indeed involved in the developmental NMDAR subunit switch in layer 2/3 neurons of mouse visual cortex and that the expression of the phospho-mimicking mutant of PSD-95 in these neurons prevented the NMDAR subunit switch. Besides the changes in the phosphorylation state, the importance of the SH3 and GK domains of PSD-95 was emphasized in the regulation of basal synaptic transmission. In order to dissect the specific roles of these domains, we generated molecular replacement constructs of PSD-95 lacking one or more of its domains. Expression of these constructs in CA1 region neurons of rat and PSD-95 knockout mouse hippocampus, demonstrated that the SH3 domain is necessary for creating a functional link between PSD-95 and SAP102, whereas GK domain in the absence of SH3 domain funcitons in a SAP102 independent way. Overall, these data suggest a domain- and CDK5 phosphorylation state-specific effect of PSD-95 on the basal synaptic transmission of CA1 region neurons as well as a CDK5 phosphorylation state-specific effect of PSD-95 on the NMDAR subunit switch in layer 2/3 neurons of visual cortex. These findings contribute to a better understanding of synaptic regulation mechanisms, hence of learning and memory