Choquet

Malinow

Molecular Methods

Quantum Dots

Choquet

AMPAR

Study Timeline - PubMed

2003

Study 1

Tardin, Cognet, Bats, Lounis, Choquet • 2003 • EMBO - PDF

Direct imaging of lateral movements of AMPA receptors inside synapses

In this study they tested the effects of glutamate application and calcium influx on AMPAR diffusion. Being one of their earlier studies aimed at measuring diffusion, they used antibodies instead of Qdots.

Glutamate Effect

• Bath application of 100 uM Glutamate
  • increased the diffusion rate of GluR2-containting AMPAR
  • increased (85%) AMPAR endocytosis within 15 min
  • decreased (22%) total membrane expression
  • increased (55%) AMPAR diffusion within synapses
  • no change of diffusion rate outside of synapses
  • decreased the number of completely immobile AMPARs by 30%

Calcium Influx Effect

• induced calcium influx with biccuculine, strychnine, glycine
• to mimic NMDAR stimulation?
  • decreased number of mobile AMPARs
  • increased (59%) AMPAR membrane expression

Calcium Blocking Effect

• used BAPTA to block calcium influx
  • increased number of mobile AMPARs

Notes

• Glutamate causes endocytosis of AMPARs, and internal AMPARs are immobile. Therefore it seems like glutamate may be causing a general endocytotic episode at non-synaptic AMPARs, perhaps not even at the synapse that received the glutamate application.
• Newly inserted receptors were found to be initially diffusive and then stabilized at synaptic sites.

Summary

• they found that bath application of glutamate induces rapid depletion of AMPARs from PSDs increases synaptic diffusion rate, decreases % of completely immobile receptors, increases proportion of receptors in the area surrounding the synapse (juxtasynaptic region). Activation of NMDARs results in increased surface expression of AMPARs -- in the first few minutes there is mainly a decrease in the proportion of immobile synaptic receptors, but after 40 min, both diffusion rates and percentages of immobile synaptic receptors are back to control values and the proportion of juxtasynaptic receptors is decreased. This observation relates to the fate of newly exocytosed AMPARs: using cleavable extracellular tags, it was observed that at early times after exocytosis, new GluR1 containing AMPARs are diffusively distributed along dendrites. This is followed by their lateral translocation and accumulation into synapses (Passafaro et al., 2001). GluR2 subunits were addressed directly at synapses. In our experiments, we followed the movement of native GluR2 containing AMPARs, where the data suggests that at the level of synapses themselves, newly added receptors are initially diffusive and then stabilize over time.

Methods

• Anti-GluR2 antibodies were labeled with Cy5 or Alexa-647 molecules at low labeling ratio (mean labeling ratio of 0.4 dye per antibody) so that individual antibodies were labeled at most with one fluorophore. A small proportion of surface expressed AMPA receptors containing the GluR2 subunit were selectively labeled in live neurons through short incubations with these antibodies. We could thus image and resolve discrete fluorescence spots with an epifluorescence imaging setup

2004

Groc L, Heine M, Cognet L, Brickley K, Stephenson FA, Lounis B, Choquet D. • 2004 • Nature Neuroscience - - PDF

Differential activity-dependent regulation of the lateral mobilities of AMPA and NMDA receptors

In this study they tried out Qdots for the first time, but didn't use the data. They simply made the claim that the results from Qdot-antibodies gave similar results as the Cy3-antibodies (but only in extrasynaptic space, Qdots were 5x slower in synaptic space). They found that AMPARs diffuse 4x faster than NMDARs when outside the synapse (10.0 vs 2.3 nm/s) and at similar speeds in the synapse (28.0 vs 21.0 nm/s). Note that diffusion was faster outside the synapse than inside the synapse (that doesn't make sense!). KCl was used to stimulate neurons - this caused extrasynaptic AMPARs to diffuse 5x faster than baseline; whereas AMPAR mobility didn't change much in the synapse (but lower N). They didn't report NMDAR changes from KCl. They stimulated PKC activity using TPA, resulting in significant mobility increases for both NMDAR and AMPAR in synaptic and extrasynaptic space.

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Figure 1

Differential lateral diffusion of AMPARs (grey) and NMDARs (black) at the surface of hippocampal neurons. Because AMPARs and NMDARs undergo exo-endocytosis cycling we carried out controls to show that the vast majority of labeled receptors were located at the surface of neurons during recording sessions. (a,b) Histograms of extrasynaptic AMPAR (a) and NMDAR diffusions (b). AMPAR diffusion was approximately four times higher than NMDAR. Insets, examples of GluR2R (a) and NR1R (b) trajectories (bars, 100 nm). Average trajectory length is 455, range 260–9750 ms. (c,d) Diffusion histograms of synaptic AMPARs and NMDARs. (e) Fractions of mobile AMPARs and NMDARs. Note the smaller fraction of mobile receptors in extrasynaptic membranes. (f) Median diffusion of mobile AMPAR and NMDAR

Abstract

• The basis for differences in activity-dependent trafficking of AMPA receptors (AMPARs) and NMDA receptors (NMDARs) remains unclear. Using single-molecule tracking, we found different lateral mobilities for AMPARs and NMDARs: changes in neuronal activity modified AMPAR but not NMDAR mobility, whereas protein kinase C activation modified both. Differences in mobility were mainly detected for extrasynaptic AMPARs, suggesting that receptor diffusion between synaptic and extrasynaptic domains is involved in plasticity processes.

Methods

• Here, we directly compared the lateral mobilities of AMPARs and NMDARs. For this, we measured the diffusion of GluR2 subunit–containing AMPAR and NR1 subunit–containing NMDAR at the surface of cultured hippocampal neurons at 9–11 days in vitro by single-molecule fluorescence microscopy tracking of receptors labeled with appropriate Cy3-coupled antibodies. We compared AMPAR diffusion obtained with Cy3-conjugated antibodies with quantum dot (QD)-coupled antibodies, because QDs are more photostable than organic dyes. In the extrasynaptic membrane, diffusion distributions measured using Cy3-conjugated and QD-coupled antibodies were similar, thus validating the diffusion estimates obtained with Cy3-conjugated antibodies. Distributions of synaptic receptor diffusions were significantly different, however, with QD-coupled receptor diffusion being five times slower. This effect could be due to limitation of receptor diffusion within the glutamatergic synaptic cleft by the bound QD (QD diameter >10–15 nm). Therefore, in the present study, AMPAR and NMDAR diffusions were estimated only from Cy3-conjugated antibodies.

Results

• We treated neurons with either potassium chloride increase neural activity, or tetrodotoxin to decrease neuronal activity. After 2 min of 40 mM KCl treatment, extrasynaptic AMPAR diffusion greatly increased (560% of control), owing to less immobile receptors (from 47% to 32%). Notably, the AMPAR diffusion measured after KCl treatment is comparable to the previously published ‘control’ value (median 0.11 μm2/s, n = 309) obtained after similar KCl treatment needed to stain synapses with FM1-43. When the spontaneous neuronal activity was blocked with TTX for only 10 min, no changes were observed, consistent with the low spontaneous basal activity of our cultured neurons. However, blocking neuronal activity for 48 h greatly decreased extrasynaptic AMPAR diffusion (12% of control), owing to more immobile receptors (from 47% to 67%).
• Activation of protein kinase C (PKC) induces rapid dispersal of NMDARs from a clustered to a uniform membrane distribution as well as endocytosis and redistribution of GluR2-containing AMPARs to synaptic sites. We thus investigated whether NMDAR and AMPAR mobilities were affected by the PKC agonist TPA. After TPA treatment, NMDAR diffusion was increased in both extrasynaptic (12-fold) and synaptic membranes (5-fold). This is consistent with the reported TPA-induced dispersion of NMDARs. Extrasynaptic and synaptic AMPAR diffusions were also significantly affected by TPA treatment (extrasynaptic +34%) (synaptic +333%).

2007

Bats, Groc, Choquet • 2007 • Neuron - PDF

The Interaction between Stargazin and PSD-95 Regulates AMPA Receptor Surface Trafficking

• Quantum Dot
• FRAP
• Live hippocampal neurons
• exchange of AMPAR by lateral diffusion between extrasynaptic and synaptic sites mostly depends on the interaction of Stargazin with PSD-95 and not upon the GluR2 AMPAR subunit C terminus.
• Disruption of interactions between Stargazin and PSD-95 strongly increases AMPAR surface diffusion, preventing AMPAR accumulation at postsynaptic sites.
• AMPARs and Stargazin diffuse as complexes in and out synapses.

2012

Czöndör, Mondin, Garcia, Heine, Frischknecht, Choquet, Sibarita, Thoumine • 2012 • PNAS - PDF

Unified quantitative model of AMPA receptor trafficking at synapses

Trafficking of AMPA receptors (AMPARs) plays a key role in synaptic transmission. However, a general framework integrating the two major mechanisms regulating AMPAR delivery at postsynapses (i.e., surface diffusion and internal recycling) is lacking. To this aim, we built a model based on numerical trajectories of individual AMPARs, including free diffusion in the extrasynaptic space, confinement in the synapse, and trapping at the postsynaptic density (PSD) through reversible interactions with scaffold proteins. The AMPAR/scaffold kinetic rates were adjusted by comparing computer simulations to single-particle tracking and fluorescence recovery after photobleaching experiments in primary neurons, in different conditions of synapse density and maturation. The model predicts that the steady-state AMPAR number at synapses is bidirectionally controlled by AMPAR/scaffold binding affinity and PSD size. To reveal the impact of recycling processes in basal conditions and upon synaptic potentiation or depression, spatially and temporally defined exocytic and endocytic events were introduced. The model predicts that local recycling of AMPARs close to the PSD, coupled to short-range surface diffusion, provides rapid control of AMPAR number at synapses. In contrast, because of long-range diffusion limitations, extrasynaptic recycling is intrinsically slower and less synapse-specific. Thus, by discriminating the relative contributions of AMPAR diffusion, trapping, and recycling events on spatial and temporal bases, this model provides unique insights on the dynamic regulation of synaptic strength.

Choquet Email

Hi Roberto,

I hope you’re doing well, haven’t seen each other in a while. As far as receptor tracking in slices go, we’ve not progressed much. As you’ve done, we routinely use FRAP of phluorin-tagged receptors to evaluate mobility in slices, and this works well, except for the over-expression issue. As for quantum dot tracking in slices, our own trials have been quite unsuccessful, most QDs being generally too sticky and not diffusing well in tissue. Thus, as for tracking endogenous receptors, I think it’s quite hopeless. I do have seen in a few other labs people using GFP tagged proteins and managing to track them with anti-GFP coated QDs, but I have no direct experience with this approach as if I’m to use a tagged receptor, I prefer then to use FRAP in slice as it’s less prone to artifacts I think. Sorry I can’t help more, sure I’d wish we could do that……

All the best and see you in the near future

Best

Daniel