1. The study used high-resolution fluorescence recovery after photobleaching (FRAP) to examine the dynamics and organization of AMPA receptors (AMPARs) within the postsynaptic density (PSD) of single dendritic spines in cultured hippocampal neurons. 2. They found that under basal conditions, AMPARs showed limited lateral diffusion within the PSD, but clustered together in a matrix that continuously reshaped in an actin-dependent manner. 3. Application of glutamate increased the intrasynaptic mobility of AMPARs, suggesting activated synapses promote exchange of receptors among subdomains. This supports the idea that the PSD regulates subsynaptic receptor distribution.

1. The Journal of Neuroscience, May 23, 2012 • 32(21):7103–7105 • 7103
Journal Club
Editor’s Note: These short, critical reviews of recent papers in the Journal, written exclusively by graduate students or postdoctoral
fellows, are intended to summarize the important findings of the paper and provide additional insight and commentary. For more
information on the format and purpose of the Journal Club, please see http://www.jneurosci.org/misc/ifa_features.shtml.
Microscale AMPAR Reorganization and Dynamics of the
Postsynaptic Density
Sandra Jurado1 and Shira Knafo2
1 Nancy Pritzker Laboratory, Department of Psychiatry and Behavioral Sciences, Stanford University School of Medicine, Palo Alto, California, 94304, and
2 Centro de Biología Molecular “Severo Ochoa,” Consejo Superior de Investigaciones Científicas/Universidad Autonoma de Madrid, 28049 Madrid, Spain
´
Review of Kerr and Blanpied
AMPA-type receptors (AMPARs) are tutive trafficking that involves both exocytic ing (FRAP) (Jacobson et al., 1976) to
glutamate-gated channels whose post- delivery from intracellular compartments quantify the dynamics of proteins and lip-
synaptic activation convey the major (Gerges et al., 2006) and fast exchange with ids within a defined subcellular compart-
depolarization in brain excitatory neu- surface extrasynaptic receptors through lat- ment. In this assay, fluorescent molecules
rotransmission. Trafficking of these recep- eral diffusion (Tardin et al., 2003). Still, are irreversibly photobleached in a small
tors to and from synapses is tightly regulated knowledge is lacking regarding the organi- area of the cell by a focused laser beam.
in neurons and underlies long-lasting forms zation and regulation of AMPARs within Subsequent diffusion of surrounding
of synaptic plasticity. For example, export of the postsynaptic density (PSD) and the nonbleached molecules into the bleached
AMPARs from the endoplasmic reticulum events triggering their repositioning. area leads to a total or partial recovery of
to the Golgi (Vandenberghe and Bredt, AMPAR trafficking can be evaluated fluorescence that is proportional to the
2004) is suggested to contribute to the ex- by biochemical, electrophysiological, and mobility of a given molecule under differ-
pression of certain types of synaptic plastic- imaging approaches. Synaptic delivery of ent experimental conditions (Fig. 1).
ity (Broutman and Baudry, 2001). In endogenous AMPARs can be monitored However, with regular FRAP, it is techni-
addition, endocytosis removes AMPARs by measuring levels of specific AMPAR cally challenging to pinpoint subcellular
from synapses during LTD (Beattie et al., subunits in synaptoneurosomes (a frac- AMPAR movement within living syn-
2000) and in response to other stimuli (Man tion of brain extracts enriched in synaptic apses with sufficient temporal and spatial
et al., 2000). Internalized AMPARs can be elements) (Heynen et al., 2000). To track resolution.
degraded in lysosomes or recycled back to specifically the movement of endogenous The study by Kerr and Blanpied (2012)
the surface membrane (Ehlers, 2000; Gru- AMPARs on the cell surface, rapid time- overcame these technical limitations us-
enberg, 2001). This AMPAR sorting is regu- lapse imaging of individual semiconductor ing high-resolution photobleaching of re-
lated by synaptic activity (Ehlers, 2000) and quantum dots coupled to AMPAR antibod- combinant fluorescent receptors on the
provides the local intracellular pool of ies are performed (Dahan et al., 2003). surface of single spines. To view exclu-
AMPARs needed for LTP expression (Park Overexpression of GluAl-GFP or GluA2 sively surface AMPARs (but not the re-
et al., 2004). AMPARs also undergo consti- (R586Q)-GFP AMPAR subunits allows vi- ceptors in intracellular compartments)
sualization of recombinant AMPARs to the authors used primary neurons ex-
detect general distribution and movement. pressing GluA1 and GluA2 AMPAR sub-
Received March 2, 2012; revised April 4, 2012; accepted April 4, 2012.
This work was supported by a grant from the Spanish Ministry of Science This overexpression results in formation units fused to the pH-sensitive GFP
and Innovation (SAF2010-15676 to S.K.). S.K. is the recipient of a “Ramon y
´ primarily of homomeric AMPARs that have [Super Ecliptic pHluorin (SEP)], which
Cajal” contract from the Spanish Ministry of Science and Innovation. We different conductance properties than en- does not fluoresce in the acid environ-
thank Prof. Jose A. Esteban for commenting on the manuscript. dogenous AMPARs (Shi et al., 2001). This ment of intracellular compartments. In
Correspondence should be addressed to either of the following: Dr.
Sandra Jurado, Nancy Pritzker Laboratory, Department of Psychiatry and
“electrophysiological tagging” is a powerful addition, Kerr and Blanpied (2012) took
Behavioral Sciences, Stanford University School of Medicine, Palo Alto, CA tool to detect trafficking of AMPARs into advantage of the fact that in primary hip-
94304, E-mail: sjurado@stanford.edu; or Dr. Shira Knafo, Centro de Bi- synapses, yet it does not identify movements pocampal neurons, GluA1-GFP subunits
ología Molecular “Severo Ochoa,” Consejo Superior de Investigaciones of AMPARs within synapses. spontaneously concentrate at synapses.
Científicas/Universidad Autonoma de Madrid, 28049 Madrid, Spain,
´
E-mail: sknafo@cbm.uam.es.
The past decades have witnessed a re- This is in contrast to neurons in organo-
DOI:10.1523/JNEUROSCI.1048-12.2012 markable increase in the application of typic slice cultures in which GluA1-GFP
Copyright©2012theauthors 0270-6474/12/327103-03$15.00/0 fluorescence recovery after photobleach- subunits distribute diffusely throughout

2. 7104 • J. Neurosci., May 23, 2012 • 32(21):7103–7105 Jurado and Knafo • Journal Club
Figure 1. Principles of FRAP experiment with AMPARs. Left, Scheme illustrating photobleaching and recovery of a whole synapse (top, Full Bleaching) versus half of a synapse (bottom, Partial
Bleaching). Before the bleach event, fluorescent AMPARs can be viewed on the synaptic surface (A, green dots, baseline). Immediately after photobleaching, AMPARs are no longer fluorescent (B,
gray dots, total bleaching) and then fluorescence gradually recovers (C, green and gray dots, recovery) as unbleached AMPARs move into the bleached area. Note that, under basal conditions, full
bleaching and partial bleaching result with the same recovery graph (right, blue line and dashed red line, respectively). When intrasynaptic mobility of AMPARs is increased (e.g., after glutamate
application), there is a stronger increase in recovery following partial bleaching (right, solid red line).
the dendritic tree and require LTP-like PSD-95, GKAP, Shank, and Homer, all of monomer assembly into filaments (with
events to efficiently enter into dendritic which are postsynaptic scaffolding pro- latrunculin) and stabilizing actin polymer-
spines (Shi et al., 1999). Therefore, in pri- teins. A high RF between a scaffold protein ization (with jasplakinolide) transformed
mary neurons, recombinant AMPARs at and AMPARs at individual spines indicated the AMPAR clusters into absolutely rigid
synapses can be viewed without preceding they had similar subsynaptic distribu- structures. This finding suggests that consti-
manipulations. With these methods, the tions. The highest RF was found between tutive reshaping of the synaptic AMPAR
authors demonstrated the use of FRAP AMPARs and PSD-95, although the C ter- clusters requires ongoing actin turnover.
as a practical and reproducible method mini of AMPA receptor subunits do not di- Contrary to some predictions, acute appli-
to study AMPARs repositioning within rectly bind to this scaffolding protein. cation of latrunculin did not increase
the PSD. Nevertheless, this tight colocalization may AMPAR loss from the synapse nor did it af-
Kerr and Blanpied (2012) first aimed account for the crucial role PSD-95 has fect intrasynaptic receptor mobility, as dis-
to elucidate whether, under basal condi- in controlling the number of synaptic covered by subdomain FRAP. These are
tions, AMPARs diffuse laterally within the AMPARs (Schnell et al., 2002). important findings, because they suggest
PSD of single spines. They found that the The immobility of receptors within the that actin treadmilling is not acutely neces-
fluorescence recovery curve in synapses that PSD led Kerr and Blanpied (2012) to ex- sary for AMPAR synaptic retention or mo-
were entirely photobleached (Fig. 1, Full amine whether the overall structure of bility, challenging the notion that actin
Bleaching) was similar to the curve of syn- individual AMPAR clusters is rigid over anchors AMPARs at synapses.
apses in which only a subdomain was time. To this end, the authors per- To test whether AMPAR activation
bleached (Fig. 1, Partial Bleaching), formed extended (1 h) time-lapse imag- promotes internal AMPAR repositioning,
implying that no AMPAR exchange oc- ing of synaptic clusters composed of Kerr and Blanpied (2012) applied gluta-
curred within the PSD. This is in agreement surface AMPARs. As expected from pre- mate to cultured neurons. This manipula-
with previous studies demonstrating re- vious studies showing a substantial PSD tion induced a significant increase in the
stricted diffusion of AMPARs within indi- flexibility (Blanpied et al., 2008), they intrasynaptic mobility of AMPARs that
vidual synapses (Tardin et al., 2003; Makino observed that individual AMPAR clus- became evident when only a subdomain
and Malinow, 2009). Thus, the postsynaptic ters exhibit substantial and continuous of the spine was photobleached (Fig. 1).
scaffolding matrix significantly restricts the changes in their morphology. In contrast This suggests that activated synapses in-
redistribution of AMPARs within the syn- to the continuously dynamic structure of crease their exchange rate of receptors
apse. It is, however, possible that the overex- AMPAR clusters, SEP fluorescence inten- among different subdomains. These re-
pression of AMPAR subunits (also leading sity was extremely stable over time. These sults are consistent with the notion that
to formation of homomeric receptors in results imply that the structural flexibility the PSD acts as a network that regulates
nonphysiological levels, instead of the natu- of AMPAR clusters is not accompanied by subsynaptic receptor distribution so re-
ral heteromeric receptors) physically re- significant changes in the number of sur- ceptors can respond with high efficacy to
stricts their own mobility. face receptors. glutamate release (Elias and Nicoll, 2007).
Kerr and Blanpied (2012) hypothe- Actin, a cytoskeletal protein highly en- Does intrasynaptic receptor mobility in-
sized that AMPAR distribution within the riched in dendritic spine heads, where it is crease during LTP as well? A hint for this
PSD depends on their association with thought to anchor AMPARs, was an obvi- question can be found in a recent study
specific postsynaptic scaffold proteins. ous candidate for the control of the ob- (Makino and Malinow, 2009) using similar
They determined the degree of this asso- served reshaping of AMPAR clusters and approaches (i.e., expression of fluorescent
ciation by calculating the pixel-wise fluo- perhaps for AMPAR retention within the receptors in organotypic hippocampal slices
rescence correlation coefficient (RF) for PSD. Remarkably, both preventing actin combined with FRAP and glutamate un-

3. Jurado and Knafo • Journal Club J. Neurosci., May 23, 2012 • 32(21):7103–7105 • 7105
caging). Makino and Malinow (2009) References fluorescence recovery after photobleaching.
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