AMPA receptor incorporation into synapses during LTP: the role of lateral movement and exocytosis

Abstract

The regulated trafficking of AMPA receptors (AMPARs) to synapses is thought to underlie the enhanced transmission in long-term potentiation (LTP), a cellular model of memory. However, there is controversy regarding the nonsynaptic site, either on the surface or intracellularly, from which AMPARs move into synapses during LTP. Using recombinant surface-fluorescent receptors in organotypic rat hippocampal slices, we show that the majority of AMPARs incorporated into synapses during LTP is from lateral diffusion of spine surface receptors containing GluR1, an AMPAR subunit. Following synaptic potentiation, AMPARs in intracellular pools containing GluR1 are driven to the surface primarily on dendrites. These exocytosed receptors likely serve to replenish the local extrasynaptic pool available for subsequent bouts of plasticity. These results clarify the role of intracellular and surface AMPARs during synaptic plasticity.

Figure 1. Subunit-specific AMPAR mobility on spines before and after cLTP

(A) Experimental design. (B) Example of a GluR2-SEP-expressing hippocampal CA1 pyramidal neuron and photobleaching of spine GluR2-SEP. S and B indicate spine and background, respectively. (C) Time-lapse images following photobleaching spine GluR2-SEP. Top and bottom panels show dual-color images and pseudo-color-coded SEP intensity images, respectively. Arrowhead indicates the spine that has undergone photobleaching. (D) Distinct fluorescence recovery on spines between GluR1-SEP (n = 14, 4 cells) and GluR2-SEP (n = 17, 4 cells, **p < 0.01). (E) Example of structural potentiation at 40 min after cLTP. (F) Mean changes in spine volume (n = 21, 8 cells for GluR1-SEP-expressing cell, **p < 0.01; n = 13, 3 cells for GluR2-SEP-expressing cell, *p < 0.05) and SEP-tagged AMPARs on spines (n = 21, 8 cells for GluR1-SEP, *p < 0.05; n = 13, 3 cells for GluR2-SEP, p = 0.19) following cLTP. (G) FRAP of GluR1-SEP obtained 50 min after LTP induction shows a significant synaptic immobile pool at the spine (n = 14, 4 cells for before cLTP; n = 21, 8 cells for after cLTP, **p < 0.01). (H) GluR2-SEP mobility on a spine is not affected by cLTP (n = 17, 4 cells for before cLTP; n = 14, 4 cells for after cLTP, p = 0.51).

Figure 2. Input-specific reduction of GluR1-SEP mobility on spines after two-photon glutamate-uncaging-evoked LTP

(A) Experimental design. (B) Example of glutamate uncaging-evoked LTP (two-photon laser pulse location indicated by yellow filled circle) and FRAP in potentiated (red arrowhead) and nearby non-potentiated (white arrowhead) spines. Baseline image obtained 10 min before FRAP was captured 40 min after LTP induction. (C) Change in volume and GluR1-SEP in the potentiated and nearby non-potentiated spines shown in B (40 min after LTP induction). (D) Fluorescence recovery of GluR1-SEP after photobleaching the potentiated and non-potentiated spines shown in B. (E) Mean changes in spine volume and GluR1-SEP in potentiated (n = 14, 10 cells) and non-potentiated (n = 11, 7 cells, **p < 0.01) spines (40 min after LTP induction). (F) Fluorescence recovery of GluR1-SEP after photobleaching in potentiated (n = 14, 10 cells) and non-potentiated (n = 11, 7 cells, *p < 0.05) spines. (G) Mean changes in spine volume and GluR2-SEP in potentiated (n = 8, 3 cells) and non-potentiated (n = 8, 4 cells, *p < 0.05, p = 0.44, respectively) spines (40 min after LTP induction). (H) Fluorescence recovery of GluR2-SEP after photobleaching in potentiated (n = 8, 3 cells) and non-potentiated (n = 8, 4 cells, p = 0.51) spines.

Figure 3. Subunit-specific AMPAR exocytosis onto dendrites during cLTP

(A) Experimental design. (B) Example of photobleaching dendrite GluR2-SEP in a hippocampal CA1 pyramidal neuron. (C) Examples of subunit-specific and activity-dependent AMPAR exocytosis; images captured every 2 min. Green arrow indicates dendrite photobleaching. (D) Number of exocytotic events within 12 μm from the center of photobleached region (~25 μm) at 8 min after cLTP induction (5 cells for GluR1-SEP basal condition, 4 cells for GluR2-SEP basal condition, 5 cells for GluR1-SEP cLTP and 3 cells for GluR2-SEP cLTP, **p < 0.01, Kruskal-Wallis test). (E) Fluorescence recovery of spine GluR1-SEP with (n = 33, 5 cells) and without cLTP (n = 24, 4 cells, p = 0.56). (F) Fluorescence recovery of spine GluR2-SEP with (n = 31, 3 cells) and without cLTP (n = 43, 4 cells, p = 0.26). Spine integrated green fluorescence was not normalized to spine integrated red fluorescence to avoid underestimation of spine AMPAR-SEP recovery in cLTP conditions. (G) Left: Example of GluR1-SEP exocytotic events without (control) or with coexpression of BoNT/A-LC. Images were acquired at 5 min prior to (baseline), and 8 min after photobleaching and cLTP induction (8 min). Green arrow indicates dendrite photobleaching. Right: Number of GluR1-SEP exocytotic events within 30 μm from the center of photobleached region (~40 μm) without (control, 5 cells) and with co-expression of BoNT/A-LC (7 cells, *p < 0.05).

Figure 4. GluR1 exocytosis occurs primarily on dendrites during two-photon glutamate uncaging-evoked LTP and is compartmentalized to a stretch of dendrite

(A) Experimental design (images acquired at ~2 sec/frame). (B) Example of time-lapse images of highly compartmentalized GluR1 exocytosis along with an increase in spine volume mediated by single-spine LTP. Note that spine enlargement precedes GluR1 exocytosis. (C) Mean changes in spine volume in potentiated (n = 9, 8 cells) and nearby non-potentiated (n = 8, 5 cells, **p < 0.01) spines. (D) Fluorescence recovery of GluR1-SEP in potentiated (n = 9, 8 cells) and nearby non-potentiated (n = 8, 5 cells *p < 0.05) spines after photobleaching dendrites. (E) Mean changes in spine GluR1-SEP/volume in potentiated (n = 9, 8 cells) and non-potentiated (n = 8, 5 cells, p = 0.96) spines. (F) Example of compartmentalized GluR1 exocytosis. Left panel shows the morphology of a hippocampal CA1 pyramidal neuron and right panel shows a pseudo-color-coded rate/intensity map of GluR1 exocytosis (two-photon laser pulse location indicated by yellow filled circle). These images were obtained by integrating over 200 time-lapse images acquired over ~7 min. (G) Frequency of GluR1 exocytosis within 5 min of glutamate uncaging plotted as a function of distance from a potentiated spine (binned by 2 μm, 8 cells). Exponential fit with a length constant λ = 2.70 μm. (H) Time-course of GluR1-SEP fluorescence recovery in dendrites next to the potentiated spine in B. Note the transient increase in the GluR1-SEP signal minutes following glutamate uncaging. (I) Frequency of GluR1 exocytosis after glutamate uncaging plotted as a function of time. The number of events was summed across 8 cells. Note the frequency of GluR1 exocytosis increases considerably after the last laser pulse of glutamate uncaging.

Figure 5. Exocytosis of endogenous AMPARs detected by two-photon glutamate uncaging-evoked responses on dendrites

(A) Example of structural changes following glutamate uncaging-evoked LTP; glutamate uncaging-mediated responses obtained before and after LTP at locations indicated by the yellow filled circles. (B) Example of AMPAR-mediated currents in the spine and dendrite before and after glutamate uncaging-evoked LTP. Right panel shows response traces (average of 3 responses). (C) Mean changes in AMPAR-mediated inward currents in spines (n = 10, 10 cells) and dendrites (n = 17, 17 cells) following glutamate uncaging-evoked LTP. Black points indicate dendrite responses obtained in the absence of the LTP induction protocol (n = 7, 7 cells, *p < 0.05). Each point is an average of 3 responses obtained every 20 sec. (D) Correlation between changes in spine volume and spine responses (n = 9, 9 cells, r = 0.87, **p < 0.01). (E) Significant correlation between changes in spine responses and changes in dendrite responses at 10 min (n = 10, 10 cells, r = 0.69, *p < 0.05) but not at 1 min after LTP induction (n = 10, 10 cells, p = 0.16).

Figure 6. The majority of synaptic AMPARs originate from a spine surface GluR1-containing receptor pool

(A) Example of an increase in spine volume and GluR1-SEP following two-photon glutamate uncaging-evoked LTP (images acquired at ~2 sec/frame). (B) Mean changes in spine volume and GluR1-SEP following glutamate uncaging-evoked LTP (n = 14, 12 cells). (C) Example of increases in spine volume and GluR1-SEP following photobleaching spines and glutamate uncaging-evoked LTP (images acquired at ~2 sec/frame). (D) Mean changes in spine volume and GluR1-SEP following photobleaching spines and glutamate uncaging-evoked LTP (n = 11, 9 cells). (E) Mean changes in spine GluR1-SEP/volume for glutamate uncaging-evoked LTP only (n = 14, 12 cells) and photobleaching plus glutamate uncaging-evoked LTP (n = 11, 9 cells, *p < 0.05).