| Forms of synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD) are widely studied cellular models of learning and memory. Synaptic targeting and clustering of AMPA type glutamate receptors, the major excitatory neurotransmitter receptors in the brain, play crucial roles in synaptic plasticity. The molecular mechanisms that underlie synaptic targeting of AMPA receptors are unclear. Identifying intracellular proteins that interact with AMPA receptors can help us to understand how AMPA receptors target to synapses and how this process is regulated in synaptic plasticity. Using the yeast two-hybrid system, we identified a PDZ domain containing protein, PICK1, that interacts with the C-termini of AMPA receptors in vitro and in vivo. In neurons, PICK1 specifically co-localizes with AMPA receptors at excitatory synapses. PICK1 can multimerize via its coiled-coil domain and can induce clustering of AMPA receptors in heterologous expression systems. Mutations that disrupt the interaction between AMPA receptors and PICK1 also disrupt AMPA receptor clustering. Phosphorylation of the AMPA receptor C-terminal tail, specifically serine-880 of the GluR2 subunit, regulates its binding with PICK1 and induces internalization of GluR2, suggesting PICK1 may decrease synaptic transmission by prompting the internalization of AMPA receptor at synapses. Indeed, treatments that disrupt GluR2-PICK1 interactions in neurons, such as intracellular perfusion of GluR2 C-terminal peptide and PICK1 antibody, block LTD induction in the cerebellum. Furthermore, perfusion of dominant negative mutant fusion proteins of PICK1 into neurons also blocks LTD expression in cerebellum. In conclusion, these findings provide a molecular model for synaptic plasticity. During LTD induction, PKC activation recruits PICK1 to synapses, where it binds to AMPA receptors and promotes AMPA receptor internalization. This decreases the number of AMPA receptors on the surface and leads to decreased synaptic transmission. In addition, the potential roles of small G-proteins in PICK1 function were also examined, since PICK1 has a region homologous to POR1, a small G-protein binding protein. However, no small G-protein was found to interact with PICK1. Finally, PICK1 knockout mice are being generated in order to study the function of PICK1 in intact animals. |
