AKAP-anchored PKA maintains neuronal L-type calcium channel activity and NFAT transcriptional signaling

Abstract

L-type voltage-gated Ca2+ channels (LTCC) couple neuronal excitation to gene transcription. LTCC activity is elevated by the cyclic AMP (cAMP)-dependent protein kinase (PKA) and depressed by the Ca2+-dependent phosphatase calcineurin (CaN), and both enzymes are localized to the channel by A-kinase anchoring protein 79/150 (AKAP79/150). AKAP79/150 anchoring of CaN also promotes LTCC activation of transcription through dephosphorylation of the nuclear factor of activated T cells (NFAT). We report here that the basal activity of AKAP79/150-anchored PKA maintains neuronal LTCC coupling to CaN-NFAT signaling by preserving LTCC phosphorylation in opposition to anchored CaN. Genetic disruption of AKAP-PKA anchoring promoted redistribution of the kinase out of postsynaptic dendritic spines, profound decreases in LTCC phosphorylation and Ca2+ influx, and impaired NFAT movement to the nucleus and activation of transcription. Thus, LTCC-NFAT transcriptional signaling in neurons requires precise organization and balancing of PKA and CaN activities in the channel nanoenvironment, which is only made possible by AKAP79/150 scaffolding.

Figure 1. Characterization of PKA-anchoring deficient AKAP150ΔPKA mice

(A) Diagram depicting the mouse Akap5 gene encoding the AKAP150 WT allele (top), the targeting construct containing the ΔPKA mutation (middle), and the targeted ΔPKA allele (bottom). The single AKAP150 coding exon is represented by the black box. The red rectangle indicates the 30 bp encoding the 10 amino acids of the ΔPKA deletion, yellow rectangle indicates the in-frame insertion of a c-myc epitope tag at the AKAP150 C-terminus, and green triangles indicate loxP sites flanking the neomycin resistance cassette in the 3′ genomic DNA. (B) Diagram of AKAP150 protein primary structure indicating removal of 709-LLIETASSLV-718 to selectively disrupt PKA-RII anchoring. (C) PCR-based genotyping of WT and heterozygous and homozygous AKAP150ΔPKA littermate mice. (D) Detection of AKAP150ΔPKA protein in whole-cell hippocampal extracts from homozygous mice by anti-myc and anti-AKAP150 immunoblotting (IB). (E) The AKAP150ΔPKA mutation or AKAP150 KO (−/−) eliminates anti-AKAP150 co-immunoprecipitation (IP) of PKA-RII and C subunits but not CaNA. Ext = whole-cell hippocampal extract. IgG = IP with non-immune immunoglobulin. (F) HEK293 cells co-transfected with AKAP79 WT or ΔPKA-CFP (magenta) and PKA RII-YFP subunits (green). Co-localization appears white in the merged panel. (G and H) tsA-201 cells co-transfected with C or N-teminally tagged Ca_V1.2-YFP (FRET acceptor, green) and WT or ΔPKA AKAP79-CFP (FRET donor, blue). Corrected FRET (FRETc) shown in pseudocolor gated to CFP. (I and J) Quantification of apparent FRET efficiency measured between Ca_V1.2-YFP and WT or ΔPKA AKAP79-CFP. Data expressed as mean ± SEM. (Not significantly different by t-test; n = 7–17). Scale bars = 10μm.

Figure 2. AKAP79/150 anchoring regulates CaN and PKA localization to dendritic spines

(A) AKAP79 WT, ΔPIX and ΔPKA-mCh localization in maximum intensity projection images of rat hippocampal neurons. (B) Quantification of AKAP79-mCh WT and mutant dendritic spine localization as a spine/dendrite shaft fluorescence intensity ratio. (C) AKAP79ΔPKA causes a loss of PKA-RIIα–CFP from spines relative to 79WT and 79ΔPIX. (D) Quantification of PKA-RIIα-CFP spine/shaft ratios in neurons expressing WT or mutant AKAPs. (E) AKAP79ΔPIX reduces spine localization of CaNA-YFP relative to WT and 79ΔPKA. (F) Quantification of CaNAα-YFP spine/shaft ratios in neurons expressing WT or mutant AKAPs. (G) Merged images of AKAP79-mCh, PKA-RIIα-CFP, and CaNAα-YFP in WT and mutant AKAP expressing neurons. Arrowheads indicate representative spines. Data expressed as mean ± SEM (*p<0.05, **p<0.01 by ANOVA with Dunnett’s post-hoc test; n = 18–23). Scale bars = 10μm

Figure 3. AKAP79/150 anchoring of both CaN and PKA regulates depolarization-triggered NFAT movement to the nucleus in hippocampal neurons

(A) Schematic of the high K⁺ depolarization protocol (left) and quantification of NFATc3-GFP localization in the cytoplasm relative to the nucleus (nucleus/cytoplasm ratio) at the indicated times in control mouse hippocampal neurons (right). (B) Summed intensity projection images of NFATc3-GFP (green) and nuclei (DAPI, blue) in WT mouse hippocampal neurons under non-stimulated (NS) conditions and 10 min after high K⁺ stimulation in DMSO (Vehicle) or nimodipine (Nim). (C) Quantification of NFATc3-GFP translocation to nucleus 10 min after high K⁺ stimulation measured as the fold-change in intrinsic GFP fluorescence nucleus/cytoplasm ratio relative to NS conditions. (D) Images of NFATc3-GFP (anti-GFP; green) and nuclei (DAPI, blue) under NS conditions and 10 min after high K⁺ stimulation in rat neurons transfected with pSilencer empty vector (Control) or AKAP150 RNAi plus mCh alone, AKAP79-mCh WT, ΔPIX, or ΔPKA as indicated (mCh images not shown). (E) Quantification of fold-change in NFATc3-GFP immunostaining nucleus/cytoplasm ratio 10 min after high K⁺ stimulation relative to NS conditions. (F) Hippocampal neurons from WT, AKAP150 KO, ΔPIX, or ΔPKA mice immunostained for NFATc3-GFP (green) and nuclei (DAPI, blue) under NS conditions and 10 min after a high K⁺ stimulation. (G) Quantification of NFATc3-GFP nucleus/cytoplasm ratio for mouse neurons from panel F performed as in panel E. Data expressed as mean ± SEM (**p<0.01, ***p<0.001; ANOVA with Dunnett post-hoc test or t-test; n = 9–21). Scale bars = 10 μm

Figure 4. AKAP79/150 anchoring of both CaN and PKA is required for depolarization-triggered NFAT translocation from dendritic spines

(A and B) Summed intensity projection images of control WT hippocampal neuron dendrites visualized by immunostaining of mCh (white) and overlaid by NFATc3-GFP immunostaining in pseudocolor with a relative scale from blue (low intensity) to red (high intensity). Images represent (A) non-stimulated conditions and (B) 10 min after high K⁺ stimulation. (C) Quantification of mCh and NFATc3-GFP spine/dendrite shaft fluorescence ratios under basal conditions, (D) following high K⁺ stimulation, and (E) change from baseline for conditions where AKAP79/150 expression and anchoring is altered as indicated. Data expressed as mean ± SEM. (*p<0.05, **p<0.01, and ***p<0.001 by paired t-test or ANOVA with Dunnett post-hoc test compared to Control; n = 5–19). Scale bars = 10μm

Figure 5. AKAP79/150 anchoring of both CaN and PKA is necessary for NFAT-dependent transcription

(A) Modified high K⁺ stimulation protocol for stimulation of NFAT-dependent transcription. (B) Diagram of the 3xNFAT/AP1-CFPnls transcriptional reporter construct used for single-cell imaging of NFAT-dependent transcription. (C) Summed intensity projection images of neuronal cell bodies and proximal dendrites in NS conditions and 16 hrs after high K⁺ stimulation (KCl). Neurons were transfected with the 3xNFAT/AP1-CFPnls reporter along with pSilencer empty vector (Control) or 150RNAi plus YFP or the indicated AKAP79-YFP constructs. YFP fluorescence is in white and nuclear-localized CFP fluorescence is in pseudocolor. (D) Quantification of the fold-change in nuclear fluorescence of the 3xNFAT-AP1-CFP-NLS reporter following high K⁺ stimulation for the indicated conditions. (E) Diagram of the pGL3NFAT plasmid that drives NFAT-dependent transcription of firefly luciferase and the internal transfection control plasmid pRLSV40 driving constitutive transcription of Renilla luciferase. (F) Quantification of NFAT-dependent transcription as normalized luciferase activity measured from lysates of WT and AKAP150 mutant mouse neurons. Data expressed as mean ± SEM. (*p<0.05, **^,##p<0.01, and ***p<0.001 by ANOVA with Dunnett post-hoc test; n = 14–49 (rat); n = 6–14 (mouse)). Scale bars = 10μm

Figure 6. Anchoring of both CaN and PKA to AKAP79/150 regulates basal phosphorylation of Ca_V1.2

(A) Representative immunoblots of pS1700, pS1928, and total Ca_V1.2 and from the indicated cultured mouse hippocampal neuron extracts. (B) Quantification of Ca_V1.2 pS1928 over total Ca_V1.2 band intensity and normalized to WT. (C) Quantification of Ca_V1.2 pS1700 over total Ca_V1.2 band intensity and normalized to WT. Data represented as mean ± SEM. (*p<0.05 by ANOVA with Dunnett post-hoc test; n = 4–5).

Figure 7. LTCC Ca²⁺ signals are reduced in the absence of AKAP79/150-PKA anchoring

(A) Diagram of the high K⁺ depolarization protocol to trigger LTCC Ca²⁺ influx. (B) Time-course of RGECO-1 Ca²⁺-indicator fluorescence change, shown in pseudocolor, in response to K⁺ depolarization in control rat neurons. (C) Representative live-cell images of rat hippocampal neurons expressing RGECO-1 (red) overlaid with YFP or AKAP79WT-YFP (green) prior to Ca²⁺ imaging. (D–F) Time-course (left panel) and quantification of area under the curve (right panel) for mean RGECO-1 fluorescence change over time in response to K⁺ depolarization in rat neurons for the indicated conditions (Control = pSilencer empty vector). (G) Representative live-cell images of WT and AKAP150 mutant mouse hippocampal neurons expressing RGECO-1 (red) overlaid with YFP (green) prior to Ca²⁺ imaging. (H–K) Time courses (left panel) and quantification of integrated area under the response curve (right panel) for mean RGECO-1 fluorescence change over time in response to K⁺ stimulation for WT and AKAP150 mutant mouse neurons. Data expressed as mean ± SEM. (**p<0.01, ***p<0.001 by Student’s t-test; n = 11–18 (rat); n = 9–16 (mouse)). Scale bars = 10μm