The roles of GluR2/3 C-termini and their PDZ domain interacting proteins, GRIP1 and GRIP2, on AMPA receptor trafficking

AMPA receptors conduct the majority of fast excitatory synaptic transmission in the mammalian central nervous system (CNS). Dynamic regulation of AMPA receptor trafficking in and out of synapses modulates the efficacy of synaptic transmission and synaptic plasticity. One of the main mechanisms that govern the trafficking of AMPA receptors is through the intracellular C-termini of AMPA receptor subunits and their dynamic interactions with PDZ domain containing proteins. GRIP1 and GRIP2 are two homologous multi-PDZ domain containing proteins that interact with the C-termini of AMPA receptor GluR2 and GluR3 subunits. Although previous data have suggested that GluR2/3 C-termini and their interactions with GRIP proteins regulate AMPA receptor trafficking and synaptic plasticity, the results are not consistent. In this thesis, we investigated the effects of AMPA receptor GluR2/3 C-termini and GRIP1/2 on AMPA receptor trafficking in vivo by examining several genetic mutant mice. We generated knock-in mice of GluR2 and GluR3, which lack the last seven amino acids of GluR2 or GluR3 (GluR2Delta7 and GluR3Delta7), GRIP1 conditional knockout mice in which GRIP1 would be deleted by introducing Cre recombinase, and GRIP2 knockout mice with GRIP2 gene deleted. We found that AMPA receptor surface expression was severely diminished in GluR2/3Delta7 double mutant neurons. Deletion of GRIP1 and GRIP2 slowed down AMPA receptor recycling induced by NMDA receptor activation, without affecting steady state AMPA receptor trafficking. GRIP1 was found to interact with Sec8 and Mint proteins in vitro and in vivo. When the interaction between GRIP1 and Sec8 was disrupted by a dominant negative construct, AMPA receptor recycling induced by NMDA receptor activation was significantly inhibited, suggesting that GRIP1 regulates AMPA receptor recycling through its interaction with Sec8. These results indicate that C-termini of GluR2/3 subunits are key motifs directing the surface expression of AMPA receptor. Furthermore, the interactions between GluR2/3 C-termini and GRIP proteins are important for activity-regulated AMPA receptor trafficking.