LTP requires a unique postsynaptic SNARE fusion machinery

Abstract

Membrane fusion during exocytosis is mediated by assemblies of SNARE (soluble NSF-attachment protein receptor) and SM (Sec1/Munc18-like) proteins. The SNARE/SM proteins involved in vesicle fusion during neurotransmitter release are well understood, whereas little is known about the protein machinery that mediates activity-dependent AMPA receptor (AMPAR) exocytosis during long-term potentiation (LTP). Using direct measurements of LTP in acute hippocampal slices and an in vitro LTP model of stimulated AMPAR exocytosis, we demonstrate that the Q-SNARE proteins syntaxin-3 and SNAP-47 are required for regulated AMPAR exocytosis during LTP but not for constitutive basal AMPAR exocytosis. In contrast, the R-SNARE protein synaptobrevin-2/VAMP2 contributes to both regulated and constitutive AMPAR exocytosis. Both the central complexin-binding and the N-terminal Munc18-binding sites of syntaxin-3 are essential for its postsynaptic role in LTP. Thus, postsynaptic exocytosis of AMPARs during LTP is mediated by a unique fusion machinery that is distinct from that used during presynaptic neurotransmitter release.

Figure 1. Postsynaptic Syntaxin-3 is Required for Surface AMPAR Delivery During LTP

(A) Schematics of lentivirus vectors. (B) Images of cultured hippocampal neurons (left panels) and summary graph (right panel) showing surface GluA1 staining following glycine treatment. (C) Schematic showing in vivo injection of lentiviruses. DIC (left panels) and fluorescence (right panels) images from acute hippocampal slice showing GFP-expressing CA1 pyramidal cells (upper/lower panel scale bar: 50 μm/10 μm). (D) Schematic of Stx-4 showing its functional domains; (Habc, Habc domain; SNARE, soluble NSF-attachment protein receptor motif; TMR, transmembrane region; red indicates non-complexin binding sequence). (E) Sample images and summary graph (mean ± SEM) of glycine-induced changes in surface GluA1 in control and Stx-4 KD cultured neurons. (F) Sample experiments (top) and summary graphs (bottom) of LTP in control and Stx-4 KD CA1 pyramidal cells, both recorded from slices prepared from animals injected with Stx-4 shRNA lentiviruses. In this and all subsequent figures: arrow indicates tetanic stimulation; sample averaged EPSCs during the baseline (1) and 35 min post-LTP induction (2) are shown next to the LTP graphs. Right panel shows scatter plots of individual experiments with mean ± SEM indicated by symbol with different color. (G) Schematic of Stx-1 structure (complexin binding sequence is indicated in green). (H) Sample images and summary graph of glycine-induced changes in surface GluA1 in control and Stx-1 KD cultured neurons. (I) Sample experiments (top) and summary graphs (bottom) of LTP in control and Stx-1 KD CA1 pyramidal cells. Right panel shows scatter plots of individual experiments with mean ± SEM. (J) Schematic of Stx-3 (complexin binding sequence in green). (K) Sample images and summary graph of glycine-induced changes in surface GluA1 in control and Stx-3 KD cultured neurons. (L) Sample experiments (top) and summary graphs (bottom) of LTP in control and Stx-3 KD CA1 pyramidal cells. Right panel shows scatter plots of individual experiments with mean ± SEM. (M) Schematic illustrating the replacement strategy. (N) Sample images and summary graph showing rescue of glycine-induced increases in surface GluA1 by Stx-3in Stx-3 KD cultured neurons. (O) Sample experiments (top) and summary graphs (bottom) of LTP in control and interleaved Stx-3 Rep CA1 pyramidal cells. Right panel shows scatter plots of individual experiments with mean ± SEM. Scale bar in all panels represents 20 μm unless otherwise indicated. Calibration bars for sample EPSC traces represent 30 ms and 50 pA. In all panels, each bar represents mean ± SEM (*p<0.05). See also Figures S1 and S2.

Figure 2. Postsynaptic Stx-3 KD in vivo Does Not Alter Basal Synaptic Transmission or Constitutive Recycling

(A) Merged images of dendrites stained for endogenous Stx-3 and PSD95. Right panel shows enlargement of single dendrite (box in left panel). See also Movie S1. (B–C) Mean amplitude (B) and frequency (C) of mEPSCs are unchanged by the Stx-3 KD. Cumulative probability graphs of mEPSC amplitudes and frequencies are shown on left and summary graphs (means ± SEM) on the right. (D) The ratio of AMPAR- to NMDAR-mediated EPSCs is unchanged by the Stx-3 KD. Representative EPSCs recorded at −60 mV and + 40 mV are shown with scatter plots of individual recordings. Different colored points indicated mean ± SEM. Scale bar represents 50 ms/25 pA. (E) Voltage-dependence of NMDAR EPSCs is not affected by Stx-3 KD. (F) Representative images and summary graphs showing endosome recycling. Alexa 488-transferrin (Alx-Tf) uptake at time=0 and after 20 min recycling was measured in control and Stx-3 KD cultured neurons. Loss of Alx-Tf reflects recycling. Bar graphs represent means ± SEM. See also Figure S3.

Figure 3. Binding of Complexin to Stx-3 is Required for LTP

(A) Schematic showing swap of non-complexin binding sequence of Stx-4 into Stx-3. (B) Sample images and summary graph of glycine-induced changes in surface GluA1 in control neurons and neurons in which the Stx-3/4 chimera replaced Stx-3. (C) Sample experiments (top) and summary graphs (bottom) of LTP in control and Stx-3/4 CA1 pyramidal cells. Right panel shows scatter plots of individual experiments with mean ± SEM. (D) Schematic showing swap of complexin binding sequence of Stx-3 into Stx-4. (E) Sample images and summary graph of glycine-induced changes in surface GluA1 in control and Stx-4/3 cultured neurons. (F) Sample experiments (top) and summary graphs (bottom) of LTP in control and Stx-4/3 CA1 pyramidal cells. Right panel shows scatter plots of individual experiments with mean ± SEM. (G) Schematic showing Stx-4 replacement. (H) Sample images and summary graph of glycine-induced changes in surface GluA1 in control neurons and neurons in which Stx-4 replaced Stx-3. (I) Sample experiments (top) and summary graphs (bottom) of LTP in control and Stx-4 Rep CA1 pyramidal cells. Right panel shows scatter plots of individual experiments with mean ± SEM. (J) Schematic showing mutant form of Stx-3^10–287, in which the first 10 amino acids of Stx-3 are deleted, as the replacement construct in Stx-3 KD cells. (K) Sample images and summary graph of glycine-induced changes in surface GluA1 in control cultured neurons and neurons in which Stx-3^10–287 replaced Stx-3. (L) Sample experiments (top) and summary graphs (bottom) of LTP in control and Stx-3^10–287 CA1 pyramidal cells. Right panel shows scatter plots of individual experiments with mean ± SEM. Scale bars in all images represent 20 μm. Calibration bars represent 30 ms and 50 pA. Bar graphs represent means ± SEM *p<0.05. See also Figure S4.

Figure 4. Postsynaptic SNAP-25 KD Impairs LTP and NMDAR-Mediated Transmission

(A) Schematic of SNAP-23 showing its functional domains. C indicates palmitoylated cysteine residues. The colored carboxi-terminus represents amino acids cleaved by botulinum toxin E. (B) Sample images and summary graph of glycine-induced changes in surface GluA1 in control and SNAP-23 KD neurons. (C) Sample experiments (top) and summary graphs (bottom) of LTP in control and SNAP-23 KD CA1 pyramidal cells. Right panel shows scatter plots of individual experiments with mean ± SEM. (D) Schematic of SNAP-25. Sequence of amino acids cleaved by botulinum toxin E differs from that in SNAP-23 (in green). (E) Sample images and summary graph of glycine-induced changes in surface GluA1 in control and SNAP-25 KD neurons. (F) Sample experiments (top) and summary graphs (bottom) of LTP in control and SNAP-25 KD CA1 pyramidal cells. Right panel shows scatter plots of individual experiments with mean ± SEM. (G) Sample images of dendritic GluN1 staining. Bar graph shows total GluN1 staining normalized to control cells. Scale bar represents 10 μm. (H) Ratio of AMPAR- to NMDAR-mediated EPSCs is increased in SNAP-25 KD cells. Representative EPSCs recorded at −60 mV and + 40 mV are shown (mean ± SEM is indicated by symbol with different color). Calibration bars represent 50 ms and 25 pA. (I) Paired-pulse ratios of AMPAR EPSCs are unchanged by postsynaptic SNAP-25 KD. Representative EPSCs are shown above summary graph. Calibration bars represent 100 ms/100 pA. Scale bars in B–E represent 20 μm. Calibration bars in C and F represent 30 ms/50 pA. Bar graphs represent means ± SEM. *p<0.05. See also Figure S5.

Figure 5. Postsynaptic SNAP-47 KD Impairs LTP Without Altering Basal Synaptic Transmission or Constitutive Recycling

(A) Schematic of SNAP-47 showing its main functional domains. Note the elongated amino-terminus and linker without palmitoylated cysteine residues. Lack of botulinum toxin E cleavage site is indicated in red. (B) Sample images and summary graph of glycine-induced changes in surface GluA1 in control and SNAP-47 KD neurons. (C) Sample experiments (top) and summary graphs (bottom) of LTP in control and SNAP-47 KD CA1 pyramidal cells. Right panel shows scatter plots of individual experiments with mean ± SEM. (D) Schematic showing molecular replacement strategy for SNAP-47. (E) Sample images and summary graph showing rescue of glycine-induced increases in surface GluA1 by SNAP-47. (F) Sample experiments (top) and summary graphs (bottom) of LTP in control and interleaved SNAP-47 Rep CA1 pyramidal cells. Right panel shows scatter plots of individual experiments with mean ± SEM. (G) Merged images of dendrites stained for endogenous SNAP-47 and PSD95. Right panel shows enlargement of single dendrite (box in left panel). See also Movie S2. (H, I) Mean amplitude (H) and frequency (I) of mEPSCs are unchanged by postsynaptic SNAP-47 KD. Cumulative probability graphs of mEPSC amplitudes and frequencies are shown on left and summary graphs (means ± SEM) on right. (J) Ratio of AMPAR- to NMDAR-mediated EPSCs is unchanged by postsynaptic SNAP-47 KD. Representative EPSCs recorded at −60 mV and + 40 mV (left). Scatter plots of individual experiments with mean ± SEM (righ). Scale bar represents 50 ms/25 pA. (K) Voltage-dependence of NMDAR EPSCs is not affected by postsynaptic SNAP-47 KD. (L) Representative images and summary graphs showing endosome recycling. Uptake and 20 min recycling was measured in control and SNAP-47 KD cells. Loss of Alx-Tf reflects recycling. Calibration bars in C and F represent 30 ms and 50 pA. Bar graphs represent means ± S.EM. *p<0.05. See also Figure S6.

Figure 6. SNARE-Dependent Interaction is Required for SNAP-47’s Role in LTP

(A) Schematic illustrating SNAP-47^ΔCt. (B) Sample images and summary graph of glycine-induced changes in surface GluA1 in control and SNAP-47^ΔCt cultured neurons. (C) Sample experiments (top) and summary graphs (bottom) of LTP in control and SNAP-47^ΔCt CA1 pyramidal cells. Right panel shows scatter plots of individual experiments with mean ± SEM. (D) Schematic showing the SNAP-47 amino-terminus (Nt) swap mutant SNAP-47^25Nt in which the Nt of SNAP-47 is replaced by the Nt of SNAP-25. (E) Sample images and summary graph of glycine-induced changes in surface GluA1 in control and SNAP-47^25Nt neurons. (F) Sample experiments (top) and summary graphs (bottom) of LTP in control and SNAP-47^25Nt CA1 pyramidal cells. Right panel shows scatter plots of individual experiments with mean ± SEM. In B and E, scale bars represent 20 μm. Calibration bars represent 30 ms/50 pA. Bar graphs represent means ± SEM. *p<0.05

Figure 7. Synaptobrevin-2 is Required for AMPAR Surface Delivery

(A) Sample images and summary graph of glycine-induced changes in surface GluA1 in control and cultured neurons prepared from Syb-2 KO mice. Scale bars represent 20 μm. (B–C) Summary graphs of mean mEPSC amplitude and frequency in control and Syb-2 KO cultured neurons before and after glycine application. (D) Sample images and summary graph of glycine-induced changes in surface GluA1 in control cultured neurons and Syb-2 KO cultured neurons infected with lentivirus expressing Syb-2 (Syb-2 Res). Scale bars represent 20 μm. (E–F) Summary graphs of mean mEPSC amplitude and frequency in control and Syb-2 Res cultured neurons before and after glycine application. (G) Images and summary graphs showing endosome recycling. Alx-Tf uptake and 20 min recycling was measured in cultured hippocampal neurons under basal conditions in control and Syb-2 KO cells. (H) The same as G with Syb-2 rescue (Syb-2 Res). Bar graphs represent means ± SEM. *p<0.05.

Figure 8. Model of the SNARE proteins involved in the activity-dependent exocytosis of AMPARs during LTP

Top panel illustrates different components of the critical SNARE complexes including Stx-3, SNAP-47, and Syb-2. SNAP-25 is shown anchored to the plasma membrane where it influences NMDAR trafficking whereas anchored Stx-3 regulates AMPAR trafficking. * indicates a TARP bound to synaptic AMPARs; ** indicates PSD95 or another MAGUK family member. Bottom panel: Upon NMDAR activation, calcium influx promotes SNARE-dependent membrane fusion of AMPAR-containing endosomes. Inset panel: the AMPAR exocytosis is mediated by a SNARE complex containing Stx-3, SNAP-47, Syb-2, complexin, as well as a Munc18-like protein.