Rapid Protein Translation Governs Persistent Changes in AMPAR Trafficking in a Form of Long-term Synaptic Plasticity

Activation of group 1 metabotropic glutamate receptors (mGluRs) induces long-term depression of glutamatergic synapses (mGluR-LTD). Postsynaptic endocytosis of ionotropic α-amino-5-hydroxy-3-methyl-4-isoxazole propionic acid receptors (AMPARs) accompanies mGluR-LTD, and long-term decreases in AMPAR surface expression most likely mediate this form of synaptic plasticity. In support of this idea, both mGluR-LTD and decreases in AMPA receptors require rapid protein synthesis in dendrites. To understand how newly synthesized proteins maintain decreases in AMPAR surface expression, we examined how mGluRs persistently alter AMPAR trafficking.

Using biochemical and immunocytochemical methods in dissociated rat hippocampal cultures, we find that brief activation of mGluRs by the group 1 mGluR selective agonist, DHPG, results in a rapid (10 min) increase in AMPAR endocytosis rate that persists for at least one hour after the removal of agonist. This persistent increase in endocytosis rate is blocked by the protein synthesis inhibitor anisomycin, suggesting that components of the endocytosis machinery are synthesized and necessary for mGluR-LTD. In contrast, treatment of cultures with NMDA, which induces NMDA receptor-dependent LTD causes a long-term (60 min) decrease in AMPAR surface expression, but does not persistently increase endocytosis rate.

Recent work has implicated activity-regulated cytoskeletal associated protein (Arc) in the regulation of AMPAR endocytosis through its interactions with endophilin and dynamin, and Arc mRNA is induced in hippocampal CA1 dendrites following behavioral activity. However, little is known about how Arc is locally synthesized at synapses or whether its local synthesis contributes to synaptic plasticity. We find that DHPG induces rapid increases in local and synaptic dendritic Arc protein expression within 10 minutes in hippocampal neurons. Knockdown of Arc by lentiviral delivery of short-hairpin RNA increases basal surface AMPAR expression and synaptic transmission as measured by mEPSC amplitude. Arc knockdown blocks mGluR-induced decreases in surface AMPARs, AMPAR endocytosis as well as mGluR-LTD. Acute inhibition of new Arc translation with antisense nucleotides also blocks mGluR-induced persistent changes in AMPAR trafficking and mGluR-LTD. The involvement of rapid Arc synthesis in mGluR regulation of synaptic function provides a link between behavior-driven neuronal activity and plasticity at the synapse.