It is time to move: role of lateral diffusion in AMPA receptor trafficking.

Editor's Note: These short reviews of a recent paper in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to mimic the journal clubs that exist in your own departments or institutions. For more information on the format and purpose of the Journal Club, please see Review of Ashby et al. AMPA receptors (AMPARs) contribute to synaptic plasticity by exocytosis and en-docytosis of receptors between membrane and intracellular pools (Collingridge et al., 2004). Single-molecule fluorescence microscopy also demonstrates that surface AMPARs may diffuse laterally to ex-trasynaptic sites where they cycle with in-tracellular pools (Borgdorff and Choquet, 2002). A recent paper in the Journal of Neuroscience (Ashby et al., 2006) investigated the role of lateral diffusion in AMPAR constitutive trafficking, and demonstrate that spine morphology affects lateral movement. To selectively visualize surface-expressed AMPARs, the investigators tagged glutamate receptor subtype 2 (GluR2) subunits with the pH-sensitive green fluorescent protein [superecliptic pHluorin (SEP)]. SEP loses fluorescence in an acidic environment but is bright at neutral pH. Because vesicular internal or-ganelles containing AMPARs are acidi-fied, bright fluorescence, therefore, should be detected only from surface SEP-GluR2s exposed to the extracellular area fluorescence from the spine head, whereas alkalinization that increased internal pH had no effect on spine fluorescence [Ashby et al. F1)], suggesting that most of the tagged AMPARs in the spine head were on the surface. Using fluorescence recovery after pho-tobleaching (FRAP), the authors noticed that the recovery of fluorescence in a single spine soon reached a plateau, implying con-stitutive cycling of AMPARs on the spine surface [Ashby et al. To exclude the possibility that on– off cycling between the membrane and intracellular pools also contributed to the FRAP, the authors performed antibody cross-linking. This procedure blocked lateral movement of membrane AMPARs and correspondingly the FRAP [Ashby et al. F2)]. Indeed, exocytosis of AMPARs was unable to rescue the blockade of FRAP by antibody cross-linking [Ashby et al. (2006), their Fig. 2 that constitutive AMPAR exchange was driven by lateral diffusion. A comparison of FRAP in spiny and nonspiny regions showed that AMPAR movement was slower in spines [Ashby et al. T1)]. The slow diffusion rate of AMPARs on the spiny membrane could be attributable to special spine structures. To test this hypothesis, the authors focused on the narrow neck region of spines and monitored fluorescence loss in photobleaching from adjacent regions of the spine head. If the neck region acts …