Bidirectional Control of Synaptic GABAAR Clustering by Glutamate and Calcium

Abstract

GABAergic synaptic transmission regulates brain function by establishing the appropriate excitation-inhibition (E/I) balance in neural circuits. The structure and function of GABAergic synapses are sensitive to destabilization by impinging neurotransmitters. However, signaling mechanisms that promote the restorative homeostatic stabilization of GABAergic synapses remain unknown. Here, by quantum dot single-particle tracking, we characterize a signaling pathway that promotes the stability of GABAA receptor (GABAAR) postsynaptic organization. Slow metabotropic glutamate receptor signaling activates IP3 receptor-dependent calcium release and protein kinase C to promote GABAAR clustering and GABAergic transmission. This GABAAR stabilization pathway counteracts the rapid cluster dispersion caused by glutamate-driven NMDA receptor-dependent calcium influx and calcineurin dephosphorylation, including in conditions of pathological glutamate toxicity. These findings show that glutamate activates distinct receptors and spatiotemporal patterns of calcium signaling for opposing control of GABAergic synapses.

Graphical abstract

Figure 1

Gene KO of IP₃R1 Conducts to the Dispersal of GABA_AR and Gephyrin Clusters (A and B) Hippocampal neurons from 15–18 DIV wild-type (WT) or IP₃R1 knockout (IP₃R1KO) mice stained for GABA_AR γ2 subunit (A) and gephyrin (B). Arrows in upper panels indicate the dendrites enlarged in lower panels. Color code in lower panels is as follows: green, GABA_AR or gephyrin clusters; magenta, synapsin punctae; and white, GABA_AR or gephyrin clusters facing synapsin-labeled boutons. Note the decrease in GABA_AR and gephyrin immunoreactivities in IP₃R1KO neurons as compared to WT neurons. Scale bars, 50 and 10 μm in upper and lower panels, respectively. (C and D) Number of synaptic clusters per dendrite length (left) and fluorescent intensities (right) of synaptic GABA_AR (C) and gephyrin (D) clusters. Values represent mean ± SEM and were normalized to their respective control values (C, n = 36 cells for WT, n = 38 for IP₃R1KO; D, n = 37 for WT, n = 38 for IP₃R1KO; ^∗∗∗p < 0.005, Welch’s t-test).

Figure 2

mGluR/IICR-Signaling Cascade Promotes Clustering of GABA_AR and Gephyrin (A–D) Effect of pharmacological blockade of IICR on GABA_AR (A and B) and gephyrin (C and D) clustering in 21–27 DIV hippocampal neurons. (A and C) Representative examples show GABA_AR (A) or gephyrin (C) immunoreactivity after 100 μM 2APB or 0.1% DMSO treatment for 30, 60, and 90 min. (B and D) The number of cluster per dendrite length (left) and fluorescence intensities (right) of synaptic GABA_AR (B) or gephyrin (D) clusters in cells exposed to DMSO (gray), 2APB (red), U73122 (blue), or MCPG (green) are shown. Plots show mean values ± SEM in function of time. Data were normalized to their respective control values. (E) Time course of DHPG-induced intracellular Ca²⁺ elevation as reported by measurement of Fluo-4 F/F₀ ratio (means ± SEM; n = 41). (F and G) Representative images (F) and quantifications (G) of the fluorescence intensity of synaptic GABA_AR clusters showing that DHPG increases GABA_AR clustering. Values (mean ± SEM) were normalized to their respective control values (n ≥ 30 cells per condition; ^∗∗∗p < 0.005, ^∗p < 0.05, t-test). Scale bars, 10 μm. See also Figures S1–S5.

Figure 3

Lower Efficacy of GABA Synapses after Reduced IICR Activity (A) Examples show GABAergic mIPSC traces recorded in cultured hippocampal neurons in the absence (left) or presence (right) of mGluR inhibitor MCPG. (B) Distribution of mIPSC amplitudes in the absence (white, top and bottom) or presence (gray, middle and bottom) of MCPG. Note that the distribution of mIPSC amplitudes is shifted toward lower values in MCPG-treated neurons. (C and D) Cumulative distributions of time to peak (C) and decay time constant (D) of mIPSCs in the absence (open) or presence of MCPG (close). The first 15 events were collected from seven neurons per condition. (E) Distributions of mIPSC amplitudes recorded in pyramidal neurons from P14–P16 WT (white, top and bottom) or IP₃R1KO mice (gray, middle and bottom) hippocampal slices. The overlay emphasizes the reduction in mIPSC amplitudes in IP₃R1KO. (F and G) Distributions of time to peak (F) and decay time constant (G) of mIPSCs of WT (white circle) and IP₃R1KO (black circle). The first 250 events were collected from four neurons per condition (^∗∗∗p < 0.005; ns, not significant; Mann-Whitney U-test).

Figure 4

mGluR/IICR Contribute to the Positive Control of GABA_AR Clustering through Separate Non-overlapping Mechanisms with Ca²⁺ Influx (A–C) Inhibition of IICR prevents re-clustering of GABA_AR at synapses after NMDA washout. (A) Examples show GABA_AR γ2 immunoreactivities after 3-min exposure to NMDA (top) or after 15 min of NMDA washout in the presence of DMSO (middle) or 2APB (bottom). Scale bar, 10 μm. (B and C) Quantification of the fluorescence intensity of GABA_ARγ2 before (NMDA) and after washout (Recovery), in the presence or absence of 2APB (B) or MCPG (C), is shown. Values (mean ± SEM) were normalized to their respective control values (n = 30 cells/condition; ^∗∗∗p < 0.005, t-test). (D–I) Pre-activation of IICR prevents NMDA-mediated dispersion of GABA_AR. Neurons pre-incubated to NMDA (D–F) or DHPG (G–I) were then exposed to NMDA in the presence of DHPG. Images (E and H) and quantifications of synaptic cluster fluorescence intensities (F and I) reveal that DHPG restored GABA_AR clustering only when neurons were pre-treated with DHPG. Scale bars, 10 μm. Values (mean ± SEM) were normalized to their respective control values (n = 30–60 cells per condition; ^∗∗∗p < 0.005, Tukey’s range test in ANOVA).

Figure 5

Reduced IICR Activity Increases the Lateral Diffusion of GABA_AR γ2 Subunits in the Presence of Calcineurin Activity QD-SPT tracking of GABA_AR γ2 subunits in hippocampal neurons. (A, E, I, M, and Q) Examples show QD trajectories (green), reconstructed from recording sequences (15.2 s for I and 38.4 s for others), overlaid with FM4-64 signals (gray) in order to identify synapses. Scale bars, 5 μm. (B, F, J, N, and R) Quantifications of median diffusion coefficients are shown (median D ± 25%–75% Interquartile Range [IQR]). (C, G, K, O, and S) The mean (±SEM) synaptic dwell time is shown. (D, H, L, P, and T) Size of confinement domains is shown (mean ± SEM). (A–H) Tracking of QD-bound GABA_AR γ2 subunits in neurons exposed to IP₃R blocker 2APB for 0–30 (A–D) or 60–90 (E–H) min. Note that exploration and diffusion coefficient of GABA_AR increased after 2APB application (A, B, E, and F). In contrast, the mean synaptic dwell time (C and G) and size of confinement domains (D and H) decreased and increased, respectively, only after 60 min, but not before 30 min of drug treatment. (I–L) Increased diffusion and reduced synaptic confinement in IP₃R1KO neurons (KO) compared with WT, analyzed at 15–18 DIV. (M–P) The blockade of group I mGluRs also led to an increase in diffusion of GABA_AR. (Q–T) Neurons were exposed to 2APB in the absence or presence of the calcineurin inhibitor Cyclosporin A (CysA) and FK506. Both CysA and FK506 prevented the 2APB-dependent increase in GABA_AR diffusion. NS, p > 0.05; ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.005; Mann-Whitney U-test (B, F, J, N, and R) and Welch’s t-test for others. Numbers of QDs analyzed are shown in Table S1. See also Figures S5 and S6.

Figure 6

IICR Activity Regulates the Expression Level and Clustering of Ca²⁺-Dependent PKC (A) Protein expression levels of α, β2, and γ PKC subtypes and GABA_AR β3 subunit after 60-min exposure of neurons to DMSO or 2APB are shown. (B) Quantification of the PKC/GABA_AR β3 subunit protein level ratio showing the PKCγ/GABA_AR β3 ratio significantly decreased after 2APB exposure for 60 min. Values (mean ± SEM) were normalized to the respective DMSO control condition (n = 6; ^∗∗∗p < 0.005, t-test). (C) Co-staining of GABA_AR γ2 subunit and α, β2, or γ PKC subtypes after 60 min of DMSO or 2APB treatment. Arrowheads in upper panels indicate the dendrites enlarged in lower panels. Color codes in lower panels are as follows: green, GABA_AR; magenta, α, β2, or γ PKC; and white, GABA_AR and PKC colocalized clusters. Scale bars, 10 μm. Note that 2APB decreased the PKC β2 and γ, but not α, isoform immunoreactivities. (D) PKC fluorescence intensity per pixel below GABA_AR punctae. Values (mean ± SEM) were normalized to the respective DMSO control condition (n = 30 cells for PKCα and PKCβ2; n = 40 for PKCγ; ^∗∗∗p < 0.005, t-test).

Figure 7

Constraint of GABA_AR Lateral Diffusion Requires Ca²⁺-Dependent PKC Activity (A) A diagram showing the time course of the experiment. Drugs were applied as indicated by horizontal bars. (B–E) Blockade of Ca²⁺-dependent PKC by Gö6976 (60–90 min) enhances GABA_AR surface exploration (B, green), diffusion coefficients (C, median D values ± IQR), synaptic escape (D, mean dwell time ± SEM), and size of confinement domain (E, mean ± SEM). (F–I) The PKC activator PMA, but not its inactive form (4α-PMA), prevented the 2APB-induced enhancement of GABA_AR mobility (^∗∗∗p < 0.005; NS, p > 0.05; Mann-Whitney U-test (C and G) and Welch’s t-test (D, E, H, and I). Numbers of QDs analyzed are shown in Table S1. Scale bars, 5 μm (A and E). (J) Conceptual diagrams showing summary of our finding. Massive Ca²⁺ influx through NMDAR activates calcineurin (CN), exceeds phosphorylation by PKC, and results in increasing lateral diffusion of GABA_AR on the plasma membrane (PM). In contrast, the mGluR/PLC/IICR pathway constitutively activates PKC. This activation of mGluR/IICR/PKC process constrains lateral diffusion of GABA_AR, counteracting basal activity of CN. See also Figure S7.