A GluR1-cGKII interaction regulates AMPA receptor trafficking

Abstract

Trafficking of AMPA receptors (AMPARs) is regulated by specific interactions of the subunit intracellular C-terminal domains (CTDs) with other proteins, but the mechanisms involved in this process are still unclear. We have found that the GluR1 CTD binds to cGMP-dependent protein kinase II (cGKII) adjacent to the kinase catalytic site. Binding of GluR1 is increased when cGKII is activated by cGMP. cGKII and GluR1 form a complex in the brain, and cGKII in this complex phosphorylates GluR1 at S845, a site also phosphorylated by PKA. Activation of cGKII by cGMP increases the surface expression of AMPARs at extrasynaptic sites. Inhibition of cGKII activity blocks the surface increase of GluR1 during chemLTP and reduces LTP in the hippocampal slice. This work identifies a pathway, downstream from the NMDA receptor (NMDAR) and nitric oxide (NO), which stimulates GluR1 accumulation in the plasma membrane and plays an important role in synaptic plasticity.

Figure 1. Interaction between cGKII and GluR1

(A) Yeast two-hybrid analysis of the GluR1 CTD and the cGKII catalytic domain interaction. (Ai) Schematic of the GluR1 CTD and the regions required for binding with protein 4.1N and SAP97. (Aii) The GluR1 CTD and deletion mutants were screened for interaction with the aa 400-762 fragment of cGKII (cGKII aa 400–762) by the Sos recruitment system as described in Experimental Procedures. (B) Schematic of cGKII fragments and interaction with the GluR1 CTD in the mammalian recruitment assay (See Figure S1). (C) Interaction of the GluR1 CTD with full-length cGKII, and stimulation by cGMP. Lysates of 293T cells expressing wt cGKII were incubated with GST-GluR1C. 293T cells were either treated with 8-Br-cGMP (50 μM) for 10 min, or left untreated (control), before lysis. The supernatant (SN) and the pellet (P) fractions were fractionated by SDS PAGE. The P fraction contains bound proteins and the SN, unbound proteins. The whole P and approximately 10% of the SN were fractionated in the gel. Proteins were detected by IB. Bottom panel shows the Coomassie blue staining of GST species and their degradation products, which are commonly seen. (D) Specificity of the GluR1 CTD and cGKII interaction. Lysates from 293T cells expressing wt cGKII were incubated with GST, GST-GluR1C and GST-GluR2C. 293T cells were treated with 8-Br-cGMP (50 μM) for 10 min before lysis. Bound proteins were detected by IB. Bottom panel shows the Coomassie blue staining of GST species. (E) GST pulldown experiment. Lysates from 293T cells expressing cGKII were incubated with GST alone or with GST-GluR1C deletion mutants. 293T cells were treated with 8-Br-cGMP (50 μM) for 10 min before lysis. Bound proteins were detected by IB. A 23 aa region between aa 850 and aa 873 in the GluR1 CTD is necessary and sufficient for binding with cGKII.

Figure 2. cGKII interacts with GluR1 in the brain and in cortical cultures, colocalizes with GluR1 in cultured hippocampal neurons, and phosphorylates GluR1 S845 in vitro and in vivo

(A) Subcellular fractionation of adult rat brains. Equal amounts of rat brain fractions were analyzed by IB and probed as indicated. P2, crude membranes; PSD, postsynaptic density. (B) Hippocampal neuron stained for endogenous MAP2 (blue), cGKII (red) and synaptophysin (green). Scale bar, 20 μm. Arrows indicate cGKII and synaptophysin colocalization. (C) Endogenous cGKII (green) and GluR1 (red) staining in hippocampal neurons. Scale bar, 5 μm. Arrows indicate cGKII and GluR1 colocalization. (D) cGKII associates with GluR1 in rat brain tissue. Rat brain synaptosomal lysates were precipitated with a control IgG, an anti-cGKII Ab or an anti-GluR1 Ab. Bound proteins were detected by IB. (E) 8-Br-cGMP increases the interaction of cGKII with GluR1 in cortical cultures. Cells were treated with 8-Br-cGMP, or saline (control). Cellular lysates were precipitated with an anti-cGKII Ab, and bound proteins were detected by IB. (F) CoIP performed as in (E). Glycine and the NO donor NOR-3 increase the interaction of GluR1 with cGKII. (G) In vitro phosphorylation of the GluR1 CTD by cGKII. GST, GST-GluR1C and GST-GluR2C were incubated with the indicated purified kinases and [γ-³²P] ATP, resolved by SDS-PAGE, and analyzed by autoradiography. (i) Phosphorylation of GST-GluR1C by cGKII. The arrows indicate autophosphorylated cGKII. The lower cGKII band is a degradation product of the purified cGKII used in these assays. (ii) Phosphorylation of GST-GluR1C by PKA. (H) Phosphorylated GST-GluR1C pulls down autophosphorylated cGKII. (i) GST-GluR1C was incubated with purified cGKII in the presence of [γ-³²P] ATP. After incubation, protein complexes containing GST-R1C were analyzed by pulldown and autoradiography. The arrows indicate autophosphorylated cGKII. The lower cGKII band is a degradation product of the purified cGKII used in these assays. (ii) Same as in (i), but the reaction was incubated with unlabeled ATP. Proteins were detected by IB. The same two bands seen with labeled ATP in (i) were seen after IB with an anti-cGKII Ab. (I) In vitro phosphorylation of GST-GluR1C at S845 by cGKII and PKA. Reactions were performed as in (A) but with unlabeled ATP. Levels of S845-PO₄ and of total GluR1C were determined by SDS-PAGE and IB using phospho-specific and total GluR1 Abs, respectively. (J) GluR1 phosphorylation at S845 in cortical cultures is increased by stimulation of the NMDA-NO-cGKII pathway. Cortical cultures were treated with glycine, the NO donor NOR-3, 8-Br-cGMP or with saline (control). Inhibitors used as indicated were: AP5, NMDAR antagonist, and KT5823, cGK inhibitor. Cultures were preincubated for 30 min with okadaic acid (1 μM) to inhibit phosphatase activity. Total amount of GluR1 was assayed with an anti-GluR1 Ab. (K) Quantitation of the cGKII phosphorylation of GluR1 S845 from several experiments as the one shown in (D). Phospho-specific signal was normalized to the total amount of GluR1, and final values were normalized to control. *p<0.05, n=4.

Figure 3. Effects of cGKII activation on GluR1 trafficking

(Ai) Hippocampal neurons expressing HAGluR1 and treated for 5 min with saline (control), 8-Br-cGMP, or 8-Br-cGMP following pretreatment with the cGK inhibitor KT5823. Neurons were stained live for surface HA (red), followed by fixation, permeabilization and staining for total HA (green). Scale bar, 20 μm. (Aii) Quantitation of surface/total ratio of HAGluR1 on neurons from experiments as the one shown in (Ai). Bar graph shows mean±SEM; n=35 for control, 30 for 8-Br-cGMP and 20 for 8-Br-cGMP+KT5823; **p<0.01. (Bi) Representative images of hippocampal neurons expressing HAGluR1 or HAGluR1-S845A and treated for 5 min with saline (control) or 8-Br-cGMP. Staining was done as in (Ai). Scale bar, 20 μm. (Bii) Quantitation of surface/total ratio of HAGluR1 and HAGluR1-S845A on neurons from experiments as the one shown in (Bi). Bar graph shows mean±SEM; n=33 for HAGluR1 wt, 20 for HAGluR1-S845A and 20 for HAGluR1-S845A+8-Br-cGMP; NS = non-significant. (C) Quantitation of surface/total ratio of HAGluR1 wt and HAGluR1-S845A on neurons treated for 10 min with saline (control) or NOR-3. Bar graph shows mean±SEM; n=20 for HAGluR1 wt control, 18 for HAGluR1 wt + NOR-3, 22 for HAGluR1-S845A control and 24 for HAGluR1-S845A + NOR-3; **p<0.01. (D) Images of surface GluR1 (red) and SV2 (green) staining in hippocampal neurons treated with saline (control), glycine for chemLTP, 8-Br-cAMP or 8-Br-cGMP. Neurons were stained live for surface GluR1, and after fixation and permeabilization, for SV2. Scale bar, 2 μm. (E) Quantitation of surface levels of GluR1 on neurons from experiments as the one shown in (D). Bar graph shows mean±SEM; n=18 for control, 15 for glycine, 16 for 8-Br-cAMP and 19 for 8-Br-cGMP; ***p<0.001. (F) Quantitation of synaptic GluR1 on neurons from experiments similar to those of (D). Synaptic GluR1 was expressed as the intensity of the GluR1 signal that overlapped with SV2 divided by the total intensity of the GluR1 signal. Bar graph shows mean±SEM; n = same as in (B); ***p<0.001. (G) Examples of mEPSCs before (-5 min), and 5 and 25 min after the application of glycine. (H) Application of glycine (n=6, filled red circles), but neither 8-Br-cGMP (n=6, open green squares), nor 8-Br-cAMP (n=6, open black squares), enhanced mEPSC amplitude in cultured hippocampal neurons. Controls were performed with vehicle (n=5, open red circles). The thick line represents the duration of glycine, 8-Br-cGMP or 8-Br-cAMP application and error bars represent SEM.

Figure 4. Trafficking of GluR1 by cGMP and cAMP

(Ai) Images of surface GluR1 in neurons treated for 5 min with saline (control), 8-Br-cGMP, 8-Br-cGMP following pretreatment with the PKA inhibitor KT5720, or 8-Br-cGMP following pretreatment with the cGK inhibitor KT5823. Neurons were then stained live for surface GluR1. Scale bar, 2 μm. (Aii) Quantitation of surface levels of GluR1 on neurons from experiments as the ones shown in (Ai). Bar graph shows mean±SEM; n=20 for all experimental groups; *p<0.05, ***p<0.001. (Bi) Images of surface GluR1 in neurons treated for 5 min with saline (control), 8-Br-cAMP, 8-Br-cAMP following pretreatment with the cGK inhibitor KT5823, or 8-Br-cAMP following pretreatment with the PKA inhibitor KT5720. Staining was done as in (A). Scale bar, 2 μm. (Bii) Quantitation of surface levels of GluR1 on neurons from experiments as the ones shown in (Bi). Bar graph shows mean±SEM; n=20 for control, 14 for 8-Br-cAMP, and 16 for 8-Br-cAMP+KT2823 and 8-Br-cAMP+KT5720; **p<0.005, ***p<0.001. (Ci) Images of surface GluR1 (red) in neurons treated for 5 min with saline (control), 8-Br-cAMP or 8-Br-cAMP following pretreatment with the cGKII inhibitor Rp-8-pCPT-cGMPS. Neurons were then stained live for surface GluR1, followed by fixation, permeabilization and staining for MAP2 (green). Scale bar, 2 μm. (Cii) Quantitation of surface levels of GluR1 on neurons from experiments as the one shown shown in (Ci). Bar graph shows mean±SEM; n=30 for control, 30 for 8-Br-cAMP and 27 for 8-Br-cAMP+Rp-8-pCPT-cGMPS. ***p<0.001. (Di) Images of surface GluR1 (red) in neurons treated for 5 min with saline (control), 8-Br-cGMP or 8-Br-cGMP following pretreatment with the cGKII inhibitor Rp-8-pCPT-cGMPS. Neurons were then stained live for surface GluR1, followed by fixation, permeabilization and staining for MAP2 (green). Scale bar, 2 μm. (Dii) Quantitation of surface levels of GluR1 on neurons from experiments as the one shown in (Di). Bar graph shows mean±SEM; n=29 for control, 27 for 8-Br-cGMP and 26 for 8-Br-cGMP+Rp-8-pCPT-cGMPS. ***p<0.001.

Figure 5. Different molecular mechanisms mediate phosphorylation and trafficking of GluR1 by PKA and cGKII

(Ai) Lysates from neurons infected with HAGluR1 wt were precipitated with an anti-HA Ab. The IPs were probed with the indicated Abs. NTD = N terminal domain. (Aii) Quantitation of S845 phosphorylation from several experiments as the one shown in (Ai). Phospho-specific signal was normalized to the total amount of GluR1, and final values were normalized to control. ***p<0.001, n=3. (Bi) Lysates from neurons infected with HAGluR1Δ7 were precipitated with an anti-HA Ab. The IPs were probed with the indicated Abs. Note that HAGluR1Δ7 is not recognized by the GluR1 CTD Ab, since this Ab recognizes a sequence present in the C terminus of GluR1, which has been deleted in HAGluR1Δ7 (see text). (Bii) Quantitation of S845 phosphorylation from several experiments as the one shown in (Bi). Phospho-specific signal was normalized to the total amount of GluR1, and final values were normalized to control. ***p<0.001, n=3. (C) Hippocampal neurons expressing HAGluR1Δ7 and treated for 5 min with saline (control), 8-Br-cAMP, or 8-Br-cGMP. Neurons were stained live for surface HA (red), followed by fixation, permeabilization and staining for total HA (green). Scale bar, 20 μm; lower panel, 2 μm. The right lower panel shows the quantitation of surface/total ratio of HAGluR1. Bar graph shows mean±SEM; n=35 for control, 30 for 8-Br-cAMP and 20 for 8-Br-cGMP + KT5823; ***p<0.001. (Di) In vitro phosphorylation of GST-GluR1C wt, GST-GluR1C R837A and GST-GluR1C R837E at S845 by cGKII. Levels of S845-PO₄ and of total GluR1C were determined by SDS-PAGE and IB using phospho-specific and total GluR1 Abs, respectively. (Dii) Quantitation of S845 phosphorylation from several experiments as the one shown in (Di). Phospho-specific signal was normalized to the total amount of GluR1, and final values were normalized to control. *p<0.05, **p<0.005, n=3. (E) Hippocampal neurons expressing HAGluR1 wt, HAGluR1 R837A and HAGluR1 R837E. Neurons were stained live for surface HA (red), followed by fixation, permeabilization and staining for total HA (green). Scale bar, 20 μm; lower panel, 2 μm. (F) Quantitation of surface/total ratio of HAGluR1 on neurons from experiments as the ones shown in (E). Bar graph shows mean±SEM; n=18 for HAGluR1 wt, 20 for HAGluR1 R837A and 16 for HAGluR1 R837E; **p<0.01, ***p<0.001. (G) Quantitation of surface/total ratio of HAGluR1 R837A on neurons treated with saline (control), treated with 8-Br-cGMP or with 8-Br-cAMP. Bar graph shows mean±SEM; n=18 for control, 20 for 8-Br-cGMP and 8-Br-cAMP; ***p<0.001. (H) Quantitation of surface/total ratio of HAGluR1 R837E on neurons treated with saline (control), 8-Br-cGMP or 8-Br-cAMP. Bar graph shows mean±SEM; n=20 for control, 17 for 8-Br-cGMP and 16 for 8-Br-cAMP; ***p<0.001.

Figure 6. Immunocytochemical and electrophysiological experiments on cultures and slices validate a model in which NMDAR transmission activates cGKII via nNOS and sGC activity, to increase AMPARs at the plasma membrane

(A) Quantitation of surface levels of GluR1 in neurons treated with saline (control), glycine, or with glycine following pretreatment for 30 min with the following inhibitors: AP5, nNOS inhibitor I, ODQ, KT5823, and Rp-8-pCPT-cGMPS. Bar graph shows mean±SEM; n=20 for each group; ***p<0.001. (B) Quantitation of surface levels of GluR1 in neurons treated with saline (control), NOR-3, or with NOR-3 following pretreatment for 30 min with the following inhibitors: AP5, nNOS inhibitor I, ODQ, KT5823 and Rp-8-pCPT-cGMPS. Bar graph shows mean±SEM; n=20 for each group; ***p<0.001. (C) and (D) Average changes in frequency (C) and amplitude (D) of mEPSCs following application of vehicle (n=3, open red circles), glycine (n=9, filled red circles), nNOS inhibitor I paired with glycine (n=6, green squares), ODQ paired with glycine (n=5, black squares), KT5823 paired with glycine (n=8, blue squares) and Rp-8-pCPT-cGMPS paired with glycine (n=5, filled blue squares). Application of glycine for 3 min produced a rapid and long-lasting increase in frequency (p<0.001) and amplitude of mEPSCs (p<0.001). Perfusion of nNOS inhibitor I, ODQ, KT5823 and Rp-8-pCPT-cGMPS blocked glycine-induced increase in frequency and amplitude of mEPSCs. The mean frequencies were 3.5±0.8Hz, 8.3±0.9Hz, 3.6±0.9Hz, 4.1±0.7Hz, 4.1±0.5Hz, and 3.7±0.6Hz for vehicle, glycine, nNOS inhibitor I+glycine, ODQ+glycine, KT5823+glycine and Rp-8-pCPT-cGMPS+glycine groups, respectively. The mean amplitudes were 10.5±0.5pA, 14.7±0.8pA, 10.9±0.6pA, 11.1±0.5pA, 11.1±0.7pA, and 10.7±0.6pA for vehicle, glycine, nNOS inhibitor I+glycine, ODQ+glycine, KT5823+glycine, and Rp-8-pCPT-cGMPS+glycine groups, respectively. The thick line represents the duration of glycine application and error bars represent SEM. (E) Cumulative probability distribution of mEPSC inter-event interval and mEPSC amplitude in vehicle (control), glycine, nNOS inhibitor I+glycine, ODQ+glycine, KT5823+glycine and Rp-8-pCPT-cGMPS+glycine groups. (F) LTP is strongly reduced by 20 min perfusion with 10 μM nNOS inhibitor I (F_(1,10)=24.30, p<0.001 compared to tetanized vehicle treated slices). The horizontal bar indicates the period during which the drug was added to the bath solution. In this and in the other figures the arrows indicate the tetanus application. The insets show representative fEPSP before (continuous line) and after (dotted line) tetanus. (G) The sGC inhibitor ODQ (10 μM) blocks LTP (F_(1,11)=28.43, p<0.001 compared to vehicle+tetanus). (H) Treatment with the cGK inhibitor KT5823 (2 μM) for 10 min impaired LTP (F_(1,11)=58.94, p<0.0001, compared to vehicle+tetanus). (I) Treatment with a second specific inhibitor for cGKII, Rp-8-pCPT-cGMPS (10 μM), for 10 min, impaired LTP (F_(1,10)=47.77, p<0.0001 compared to vehicle+tetanus).

Figure 7. GFP-cGKII-i blocks trafficking of GluR1 in hippocampal neurons after application of 8-Br-cGMP but not 8-Br-cAMP

(A) Interaction of infected GFP-cGKII-i with endogenous cGKII in cultured neurons. Cells expressing GFP or GFP-cGKII-i were lysed and precipitated with an anti-GFP Ab. The IPs and inputs were probed with an anti-cGKII Ab. (B) Hippocampal cell in culture expressing GFP-cGKII-i and stained with an Ab that recognizes both cGKII-i and endogenous cGKII. Scale bar, 20 μm; lower panel, 2 μm. (C) Hippocampal neurons expressing control GFP (green) were treated with saline (control), 8-Br-cGMP or 8-Br-cAMP. Neurons were stained live for surface GluR1 (red). Scale bar, 4 μm. (D) Quantitation of surface/total ratio of HAGluR1 on neurons from experiments as the ones shown in (C). Bar graph shows mean±SEM; n=15 for control, 24 for 8-Br-cGMP and 18 for 8-Br-cAMP; ***p <0.001. (E) Hippocampal neurons expressing GFP-cGKII-i (green) were treated with saline (control), 8-Br-cGMP or 8-Br-cAMP. Neurons were stained live for surface GluR1 (red). Scale bar, 4 μm. (F) Quantitation of surface/total ratio of HAGluR1 on neurons from experiments as the ones shown in (E). Bar graph shows mean±SEM; n=15 for control, 24 for 8-Br-cGMP and 18 for 8-Br-cAMP; ***p <0.001.

Figure 8. Expression of GFP-cGKII-i blocks glycine-induced synaptic plasticity in cultured hippocampal neurons and LTP in hippocampal slices

(A) and (B) Average changes in frequency (A) and amplitude (B) of mEPSCs following application of glycine (n=9, filled red circles), GFP expression paired with glycine (n=6, green squares), GFP-cGKII-i expression paired with glycine (n=6, black squares), and vehicle (control) (n=3, open red circles). Glycine application (3 min) produced a rapid and long-lasting increase in frequency (p<0.001) and amplitude of mEPSCs (p<0.001) in neurons expressing GFP. However, expression of GFP-cGKII-i completely blocked glycine-induced increase in frequency and amplitude of mEPSCs. The mean basal frequencies were 3.9±0.5Hz, 3.9±0.6Hz, 4.1±0.6Hz and 3.5±0.4Hz for control, glycine, GFP+glycine, and GFP-cGKII-i+glycine groups, respectively. The mean basal amplitudes were 9.8±0.8pA, 10.2±0.8pA, 10.8±0.9pA and 9.8±0.7pA for control, glycine, GFP+glycine, and GFP-cGKII-i+glycine groups, respectively. The thick line represents the duration of glycine application and error bars represent SEM. (C) Cumulative probability distribution of mEPSC inter-event interval and mEPSC amplitude in vehicle (control), glycine, GFP expression+glycine, GFP-cGKII-i expression+glycine groups. (D) Cannulas were implanted bilaterally into the dorsal hippocampus (schematic representation). (E) Lysates from uninfected hippocampus and hippocampus infected with GFP-cGKII-i were analyzed by IB using an anti-GFP Ab. IB for GFP confirmed that slices infused with Sindbis virus carrying a construct for GFP-cGKII-i were infected. (F) Representative examples of hippocampal slices viewed on a confocal microscope (20X objective) 24 hrs after injection of Sindbis virus expressing GFP-cGKII-i or vehicle. GFP is visible in the CA1 cell body area only in slices infected with GFP-cGKII-i (right panel). (G) Basal synaptic transmission is not impaired in animals injected with GFP-cGKII-i virus compared to slices from animals injected with GFP virus or vehicle (F_(2,14)=0.02, p=0.98) (H) The GFP-cGKII-i virus impaired LTP (F_(1,10)=10.15, p=0.01, compared to vehicle) whereas GFP virus did not affect LTP (F_(1,10)=0.77, p=0.39, compared to vehicle). (Ii) Model for the regulation of GluR1 trafficking by cGKII. Based on the lipid raft location of the kinase and the fact that GluR1 surface levels are increased after S845 phosphorylation, the kinase is depicted at the plasma membrane and the receptor intracellularly, before S845 phosphorylation. However, the subcellular locations of the kinase and the receptor at the time of phosphorylation have not yet been established. See Discussion. (Iii) cGKII autoinhibitory domain homology with GluR1 CTD binding site for cGKII. GluR1 S845 and cGKII S126 (red), and homologous residues (blue), are shown.