Monomers Of Ampa-Type Glutamate Receptor Subunits Diffuse In And Out Of Spines; Unraveling By Single-Molecule Tracking

Memory and learning require structural synaptic plasticity. Among the key players for synaptic plasticity are the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) subtype of ionotropic glutamate receptors (AMPARs), which are concentrated in the synapse to mediate excitatory transmission. AMPARs are composed of four subunits (GluA1-4), and work as ion channels in the forms of homo- and hetero-tetramers. These tetramers are formed at their exit from the endoplasmic reticulum and it was widely believed that these tetramers are very stable entities. However, we considered that the direct evidence for the existence of stable tetramers in the plasma membrane (PM) is lacking, and that if the tetramers were very stable entities, modulating the tetramer compositions in the synapse, which is known to occur rapidly after stimulation, would be difficult. We examined this issue using single-molecule imaging and tracking. We found that, at variance with the general belief, many GluA1 and GluA2 molecules expressed in the PM of HEK293 cells exist as monomers, and form metastable homo- and hetero-tetramers (∼100 and ∼200 ms, respectively), while they instantaneously fall apart into monomers, dimers, or trimers, which again form tetramers readily. In the dendritic-shaft PM of mouse hippocampal neurons, GluA1 and GluA2 also dynamically merged into greater oligomers on similar time scales to those in the HEK293-PM, suggesting that they form only metastable tetramers in neuronal PM. GluA1 and GluA2 monomers and dimers were much more mobile than tetramers in the dendritic-shaft PM and entered the spines quite readily. These results suggest a novel mechanism for synaptic plasticity: During synaptic stimulation, the AMPAR subunit numbers and compositions in the postsynaptic membranes could be regulated readily by recruiting the preferred subunits pooled in dendritic-shaft PM, mostly in the forms of monomers and dimers.