Lu Nicoll 2009

Lu Nicoll • 2009 • Neuron • PDF

Subunit composition of synaptic AMPA receptors revealed by a single-cell genetic approach.

ABSTRACT

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The precise subunit composition of synaptic ionotropic receptors in the brain is poorly understood. This information is of particular importance with regard to AMPA-type glutamate receptors, the multimeric complexes assembled from GluA1-A4 subunits, as the trafficking of these receptors into and out of synapses is proposed to depend upon the subunit composition of the receptor. We report a molecular quantification of synaptic AMPA receptors (AMPARs) by employing a single-cell genetic approach coupled with electrophysiology in hippocampal CA1 pyramidal neurons. In contrast to prevailing views, we find that GluA1A2 heteromers are the dominant AMPARs at CA1 cell synapses (approximately 80%). In cells lacking GluA1, -A2, and -A3, synapses are devoid of AMPARs, yet synaptic NMDA receptors (NMDARs) and dendritic morphology remain unchanged. These data demonstrate a functional dissociation of AMPARs from trafficking of NMDARs and neuronal morphogenesis. This study provides a functional quantification of the subunit composition of AMPARs in the CNS and suggests novel roles for AMPAR subunits in receptor trafficking.

Fig 1

Figure 1. Outside-Out Patch Recordings of AMPAR-Mediated Current from CA1 Pyramidal Neurons in GluA2 KO and WT Mice (A) (Top) Example of the strongly inwardly rectifying I/V curve of glutamate-evoked AMPARmediated current, in the presence of 100 mM cyclothiazide, from acute hippocampal slice from the germline 2- to 3-week-old GluA2-KO mouse, with an RI value of 0.16. (Bottom) In a subsequent glutamate application in the same OOP, held at ~60 mV, ~99% of the current could be blocked by 100 nM PhTx-433. (B) Example of the linear I/V curve of glutamateevoked current in a WT littermate, with an RI value of 0.83. (Bottom) In the same OOP, glutamateevoked current was untouched by 100 nM PhTx- 433. (C) Bar graph showing average RI values for each of the genotypes: GluA2-KO mice, RI = 0.12 ± 0.02 (n = 9); WT mice, RI = 0.82 ± 0.02 (n = 15). (D) Bar graph showing the average percent block (%) of glutamate-evoked currents by 100 nM PhTx-433 in GluA2-KO mice; average percent block is 97.9% ± 0.4% (n = 6) and, in WT mice, 0% (n = 5).

Fig 2

Figure 2. Synaptic Physiology and Morphology of CA1 Pyramidal Neurons without AMPARs (A) Confocal images (left, low magnification; right, high magnification of the boxed area in left) show mosaic expression of Cre-GFP in the CA1 region of a typical hippocampal acute slice made from a triple-GRIAXfl/fl mouse at P25 injected at P0 with rAAV-Cre-GFP. Scale bar, left, 0.2 mm; right, 20 mm. (B) Scatter plots show amplitudes of EPSCs for single pairs (open circles) and mean ± SEM (filled circles), respectively. The scatter plots represented the data recorded from acute slices (P22–P30) infected with rAAV-CRE-GFP at P0. Distributions of EPSC amplitudes show a virtual elimination of AMPAR EPSCs (B1, Control [Cnt], ~127.1 ± 26.6 pA; Cre, ~3.1 ± 1.0 pA; n = 13; *p < 0.001) but no change in NMDAR EPSCs (B2, control, 32.0 ± 5.1 pA; Cre, 34.7 ± 8.0 pA, n = 13; p = 0.73). (Inset in B1) Sample traces are as follows: black, control cell; green, Cre cell. (B3) Bar graph shows average AMPAR (top) and NMDAR (bottom) EPSCs presented in (B1 and B2). (C) Traces of glutamate-evoked currents from OOPs in control (black) and Cre cells (green). Bar graph shows that deletion of GluA1, -A2, and -A3 eliminated the AMPAR-mediated current (Cnt, ~648.7 ± 45.2 pA; n = 23; Cre, ~1.0 ± 0.7 pA; n = 8; *p < 0.001). Scale bar, 200 pA, 1 s. (D) Bar graph shows the decay time constant of NMDAR EPSCs recorded in NBQX at +40 mV (Cnt, 0.24 ± 0.01 s, n = 22; Cre, 0.23 ± 0.01 s, n = 24; p > 0.05). Scale bar, 0.5 s. (E) (E1 and E2) Ifenprodil (3 mM) depressed NMDAR EPSCs recorded at +40 mV in Cnt and Cre cells to a similar extent. (E2) Traces of NMDAR EPSCs from the two groups of cells before and 30 min after ifenprodil application were shown on the right. Bar graph shows the average percentage of NMDAR EPSCs remaining after ifenprodil application (Cnt, 66.8% ± 3.7%, n = 4; Cre, 74% ± 4.8%, n = 5; p > 0.05). Scale bar, 50 pA, 0.1 s. (F) I/Vs of synaptic NMDARs. NMDAR EPSCs were recorded at various holding potentials (~80, ~60, ~40, ~20, 0, +20, and +40 mV) with 4 mM Mg2+. Junction potentials have been corrected. (G) Representative confocal stacks from Cnt and Cre cells. Bar graph in right shows average number of dendritic branchpoints and dendritic length (Cnt, n = 10; Cre, n = 8; p > 0.05). Scale bar, 20 mm. (H) Representative confocal stacks of 20 mm secondary apical dendrites from Cnt and Cre cells. Bar graph in right shows average spine density (Cnt, n = 11; Cre, n = 11; p > 0.05). Scale bar, 2 mm. (A–H) The recordings and anatomy were made from acute slices (P20–P30) from animals injected at P0 with rAAV-Cre-GFP.

Fig 3

Figure 3. Excitatory Synaptic Transmission at CA1 Pyramidal Neurons Is Mediated Primarily by GluA1A2 Heteromers (A) (A1 and A2) The time course for changes in AMPAR EPSCs in hippocampal slice cultures from GRIA1fl/fl mice after transfection of Cre-IRES-GFP. For DGluA1, shown are the ratio of AMPAR-EPSCs (closed circles, 3–5 days, 1.02; 6 days, 0.75; 7–8 days, 0.43; 9–10 days, 0.37; 11–12 days, 0.28; 12–14 days, 0.23; >14 days, 0.21) and ratio of NMDAR-EPSCs (closed diamonds, 3–5 days, 0.98; 6 days, 1.15; 7–8 days, 1.11; 9–10 days, 1.11; 11–12 days, 1.16; 12–14 days, 0.92; >14 days, 1.09) from transfected cells to neighboring control cells, respectively. (A2) Bar graph shows the percentage of AMPAR EPSCs (21.2% ± 3.1%; n = 15; *p < 0.0001) and NMDAR EPSCs (104.8% ± 17.6%; n = 14; p = 0.53) to controls. (B) (B1–B4) Scatter plots (B1 and B2) and bar graphs (B3 and B4) show amplitudes of EPSCs for single pairs (open circles) and mean ± SEM (filled circles) for GRIA1fl/fl (B1, pooled data from acute slices [P19–P24] from animals injected at P0–P2 and from hippocampal slice cultures) and GRIA3fl/fl (B2, data from acute slices [P20-P25] from animals injected at P0–P2), respectively. (Inset in B1 and B2) Sample traces are as follows: black, control; green, Cre. (B3) EPSC amplitudes show a significant reduction in AMPAR EPSCs for the deletion of either subunit (DGluA1, Cnt, ~77.7 ± 12.7 pA; Cre, ~15.1 ± 2.4 pA; n = 31, *p < 0.0001; DGluA3, Cnt, ~56.4 ± 6.0 pA; Cre, ~47.2 ± 5.6 pA; n = 19; *p < 0.05). (B4) There was no change in the NMDAR EPSCs (GluA1, Cnt, 40.0 ± 9.4 pA; Cre, 33.6 ± 6.9 pA, n = 29; p = 0.31; DGluA3, Cnt, 40.4 ± 7.7 pA; Cre, 39.0 ± 7.8 pA, n = 19; p = 0.97). (C and D) Bar graphs show average RI (C) (Cnt, 0.99 ± 0.03, n = 30; DGluA1, 1.02 ± 0.08, n = 14; p = 0.63; DGluA3, 1.06 ± 0.04, n = 15; p = 0.15) and average paired-pulse ratio (PPR, [D]) (Cnt, n = 84; DGluA1, n = 40; DGluA3, n = 9; p > 0.05 for both conditions). Left were sample traces. (E) Sample traces of mEPSCs shown at a low gain and sweep speed (traces on left; scale bar, 10 pA, 500 ms) and averaged mEPSCs at a high gain and sweep speed (traces on right). Control trace (black) has been superimposed on the trace from a Cre cell. Scale bar, 5 pA, 10 ms. mEPSCs were recorded from acute hippocampal slices (P20–P27) from animals injected at P0–P2. (F) (F1) Bar graphs show mEPSC amplitude (Cnt, ~10.5 ± 0.4 pA; DGluA1, ~7.9 ± 0.5 pA; *p < 0.001; DGluA3, ~10.7 ± 0.1 pA; p = 0.77), (F2) frequency (Cnt, 0.28 ± 0.03 Hz; DGluA1, 0.08 ± 0.01 Hz; p* < 0.001; DGluA3, 0.27 ± 0.05 Hz, p = 0.68), and (F3) decay (Cnt, 11.30 ± 0.49 ms; DGluA1, 7.73 ± 1.41 ms; *p < 0.01; DGluA3, 11.60 ± 1.20 ms; p = 0.81). n = 22, 10, and 20 for Cnt, DGluA1, and DGluA3, respectively.

Fig 4

Figure 4. AMPARs Adjust Rapidly to the Deletion of GluA2 (A) (A1 and A2) The time course for the changes in synaptic transmission in hippocampal slice cultures from GRIA2fl/fl mice after transfection of Cre-IRES-GFP. Ratio of RI (open circles, 3–5 days, 0.95; 6 days, 0.99; 7–8 days, 0.71; 9–10 days, 0.60; 11–12 days, 0.34; 12–14 days, 0.16; >14 days, 0.15), ratio of AMPAR EPSCs (closed circle, 3–5 days, 0.96; 6 days, 0.49; 7–8 days, 0.57; 9–10 days, 0.56; 11–12 days, 0.50; 12–14 days, 0.57; >14 days, 0.51), and ratio of NMDAR EPSCs (closed diamonds, 3–5 days, 1.08; 6 days, 1.01; 7–8 days, 1.06; 9–10 days, 1.04; 11–12 days, 0.99; 12–14 days, 1.01; >14 days, 1.10) from transfected cells to neighboring control cells, respectively. Open square shows RI from CA1 pyramidal neurons from germline GluA2 KO mice (0.13 ± 0.02, n = 5). (A2) Graph shows the percentage of the average AMPAR EPSCs (51.7% ± 5.2%; n = 86; *p < 0.0001), NMDAR EPSCs (97.8% ± 13.2%; n = 64; p = 0.81), and RI (15.0% ± 1.8%; n = 19; *p < 0.0001) from transfected cells or GluA2 KO cells (13.3% ± 2.0%; n = 5; *p < 0.01) to control cells. (B) (B1–B3) Scatter plots (B1) and bar graphs (B2 and B3) show amplitudes of EPSCs for single pairs (open circles) and mean ± SEM (filled circles) for GRIA2fl/fl. (Inset in B1) Sample traces are as follows: black, control; green, Cre. (B2) EPSC amplitudes show a significant reduction in the AMPAR EPSCs (Cnt, ~66.2 ± 3.8 pA; Cre, ~34.2 ± 2.5 pA; n = 86; *p < 0.0001). (B3) There was no change in the NMDAR EPSCs (GluA2, Cnt, 40.0 ± 3.7 pA; Cre, 39.1 ± 3.4 pA, n = 64; p = 0.81). The data were pooled from acute hippocampal slices (P13–P17) from animals injected at P0–P2 and from hippocampal slice cultures. (C and D) Bar graphs show average RI (C) (Cnt, 0.99 ± 0.03, n = 30; DGluA2, 0.15 ± 0.02, n = 19; *p < 0.001) and average PPR (D) (Cnt, n = 84; DGluA2, n = 29; p > 0.05). Left were sample traces. (E) Sample traces of mEPSCs shown at a low gain and sweep speed (traces on left; scale bar, 10 pA, 500 ms) and averaged mEPSCs at a high gain and sweep speed (traces on right). Control trace (black) has been superimposed on the trace from a Cre cell. Scale bar, 5 pA, 10 ms. mEPSCs were recorded from acute hippocampal slices (P13–P18) from animals injected at P0–P2. (F) (F1) Bar graphs show mEPSCs amplitude (Cnt, ~10.51 ± 0.37 pA; DGluA2, 11.08 ± 0.65 pA; p = 0.42), (F2) frequency (Cnt, 0.28 ± 0.03 Hz; DGluA2, 0.16 ± 0.03 Hz; *p < 0.001), and (F3) decay (Cnt, 11.30 ± 0.49 ms; DGluA2, 9.75 ± 1.14 ms; p = 0.27). n = 22 and 17 for Cnt and DGluA2, respectively.

Fig 5

Figure 5. Deletion of GluA2A3, GluA1A3, or GluA1A2 in CA1 Pyramidal Cells (A) (A1–A5) Scatter plots (A1–A3) and bar graphs (A4 and A5) show amplitudes of EPSCs for single pairs (open circles) and mean ± SEM (filled circles) for GRIA2A3fl/fl (A1), GRIA1A3fl/fl (A2), and GRIA1A2fl/fl (A3), respectively. (A4) The amplitudes of AMPAR EPSCs were significantly reduced in all three cases (DGluA2A3, Cnt, ~58.1 ± 11.4 pA; Cre, ~24.9 ± 3.3 pA; n = 14; *p < 0.01; DGluA1A3, Cnt, ~128.4 ± 19.7 pA; Cre, ~15.6 ± 3.10 pA; n = 12; *p < 0.001; DGluA1A2, Cnt, ~84.3 ± 10.1 pA; Cre, ~4.9 ± 0.8 pA; n = 24; *p < 0.001). (A5) No change in the size of NMDAR EPSCs was observed (DGluA2A3, Cnt, 40.3 ± 7.4 pA; Cre, 38.0 ± 6.3 pA, n = 12; p = 0.82; DGluA1A3, Cnt, 49.2 ± 11.7 pA; Cre, 49.0 ± 13.7 pA, n = 11; p = 0.99; DGluA1A2; Cnt, 36.3 ± 5.8 pA; Cre, 31.0 ± 4.2 pA, n = 23; p = 0.31). (Inset in A1–A3) Sample traces are as follows: black, control; green, Cre. (B and C) Bar graphs show average RI (B) (Cnt, 0.99 ± 0.03, n = 30; DGluA2A3, 0.14 ± 0.02, n = 13; *p < 0.001; DGluA1A3, 1.06 ± 0.2, n = 5; p = 0.59; DGluA1A2, 0.1 ± 0.02, n = 6; *p < 0.001) and average PPR (C) (Cnt, n = 84; DGluA2A3, n = 14; DGluA1A3, n = 6; DGluA1A2, n = 11; p > 0.05 for each conditions). Left were sample traces. For GRIA1A2fl/fl cells, the stimulus was increased to record measurable EPSCs, and only recordings from the Cre cell were shown. (D) Sample recordings of mEPSCs at low gain and sweep speed (traces on left; scale bar, 10 pA, 500 ms) and averaged mEPSCs at high gain and sweep speed (traces on right). Control trace (black) has been superimposed on the trace from a Cre cell. Scale bar, 5 pA, 10 ms. (E) (E1) Bar graphs show mEPSC amplitude (top, Cnt, ~10.51 ± 0.37 pA; DGluA2A3, ~10.56 ± 0.60 pA; p = 0.93; DGluA1A3, ~7.21 ± 0.36 pA; *p < 0.001; DGluA1A2, ~6.79 ± 0.20 pA; *p < 0.001), (E2) frequency (middle, Cnt, 0.28 ± 0.03 Hz; DGluA2A3, 0.17 ± 0.05 Hz; *p < 0.005; DGluA1A3, 0.03 ± 0.01 Hz; *p < 0.001; DGluA1A2, 0.06 ± 0.01 Hz, *p < 0.001), and (E3) decay (bottom, Cnt, 11.30 ± 0.49 ms; DGluA2A3, 10.18 ± 1.2 ms; p = 0.33; DGluA1A3, 14.70 ± 0.71 ms; *p < 0.01; DGluA1A2, 4.20 ± 0.71 ms; *p < 0.001). n = 22, 14, 7, and 9 for Cnt, DGluA2A3, DGluA1A3, and DGluA1A2, respectively. (A–E) The recordings were made from acute hippocampal slices (P20–P27) from animals injected at P0–P1.

Fig 6

Figure 6. Analysis of Extrasynaptic AMPARs (A) Sample traces of AMPAR currents from OOPs from uninfected control (black) and Cre (green) cells from CA1 pyramidal neurons from various genetic backgrounds. Scale bar, 200 pA, 1 s. The recordings were made from acute hippocampal slices (P13–P17 for DGluA2 and P20– P28 for all other genetic backgrounds) from animals injected at P0–P2. (B) I/V curves of AMPAR currents from OOPs. Control, black; Cre, green. Deletion of the GluA2 subunit, but not other subunits, caused strong inward rectification of the evoked current. Bar graph at the bottom shows the RI for each condition (Cnt, 0.85 ± 0.02, n = 8; DGluA1, 0.81 ± 0.04, n = 5; p = 0.39; DGluA2, 0.09 ± 0.01, n = 6; *p < 0.001; DGluA3, 0.80 ± 0.03, n = 5; p = 0.22; DGluA2A3, 0.10 ± 0.02, n = 6; *p < 0.001; DGluA1A2, 0.15 ± 0.03, n = 5; *p < 0.001). (C) Summary bar graph shows consequences of deletion of respective genes on AMPAR current from OOPs (Cnt, ~648.7 ± 45.2 pA, n = 23; DGluA1, ~35.3 ± 13.1 pA, n = 16, *p < 0.001; DGluA2, ~684.3 ± 92.2 pA, n = 11, p = 0.70; DGluA3, ~674.2 ± 63.5 pA, n = 13, p = 0.74; DGluA2A3, ~656.8 ± 76.3 pA, n = 14, p = 0.92; DGluA1A3, ~2.5 ± 1.0 pA, n = 14, *p < 0.001; DGluA1A2, ~24.1 ± 5.2 pA, n = 25, *p < 0.001; DGluA1A2A3, ~1.01 ± 0.65 pA, n = 8, *p < 0.001). (D) Summary bar graph shows consequences of deletion of respective genes on AMPAR EPSCs (percent control: DGluA1, 19.4 ± 3.1%, n = 31, *p < 0.001; DGluA2, 51.7 ± 3.8%, n = 86, *p < 0.001; DGluA3, 83.8 ± 1.0%, n = 19, *p < 0.05; DGluA2A3, 42.8 ± 5.2%, n = 14, *p < 0.001; DGluA1A3, 12.1 ± 2.4%, n = 12, *p < 0.001; DGluA1A2, 5.7 ± 1.4%, n = 24, *p < 0.001; DGluA1A2A3, 2.4 ± 0.6%, n = 13, *p < 0.001). (E) Models for AMPAR compositions at synaptic and extrasynaptic membranes. At CA1 pyramidal neurons, ~80% synaptic AMPARs are GluA1A2 heteromers, and ~16% synaptic AMPARs are GluA2A3 heteromers. On the other hand, ~95% extrasynaptic AMPARs are GluA1A2 heteromers.