Subunit-dependent and independent rules of AMPA receptor trafficking during long-term depression in hippocampal neurons

Long-term potentiation (LTP) and depression (LTD) of excitatory neurotransmission are believed to be the neuronal basis of learning and memory. Both processes are primarily mediated by neuronal activity-induced transport of postsynaptic AMPA-type glutamate receptors (AMPARs). While AMPAR subunits and their specific phosphorylation sites mediate differential AMPAR trafficking, LTP and LTD could also occur in a subunit-independent manner. Thus, it remains unclear whether and how, certain AMPAR subunits with phosphorylation sites are preferentially recruited to or removed from synapses during LTP and LTD. Here, we show that phosphorylation of the membrane-proximal region (MPR), which only occurs in GluA1 AMPAR subunits, mediates the subunit-dependent endosomal transport of AMPARs during LTD. AP-2 and AP-3, adaptor protein complexes necessary for clathrin-mediated endocytosis and late endosomal/lysosomal trafficking, respectively, are reported to be recruited to AMPARs by binding to the AMPAR auxiliary subunit, stargazin (STG), in an AMPAR subunit-independent manner. However, the association of AP-3, but not AP-2, with STG was indirectly inhibited by the phosphomimetic mutation in the MPR of GluA1. Thus, although AMPARs containing the phosphomimetic mutation at the MPR of GluA1 were endocytosed by a LTD-inducing stimulus, they were quickly recycled back to the cell surface in hippocampal neurons. These results could explain how the phosphorylation status of GluA1-MPR plays a dominant role in subunit-independent STG-mediated AMPAR trafficking during LTD. * LTP : long term potentiation LTD : long term depression AMPA : α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid MPR : membrane-proximal region STG : stargazing NMDA : N-methyl-D-aspartate C-terminus, CaMKII : calcium/calmodulin-dependent protein kinase II PKA : protein kinase A PKC : protein kinase C TARP : transmembrane AMPAR regulatory protein PSD95 : postsynaptic density 95 HEK293 : human embryonic kidney 293 GST : glutathione S-transferase HA : hemagglutinin MAP2 : Microtubule-associated protein 2 PBS : phosphate-buffered saline BSA : bovine serum albumin