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, 11 (5), 727-36

Increased Expression of the PI3K Enhancer PIKE Mediates Deficits in Synaptic Plasticity and Behavior in Fragile X Syndrome

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Increased Expression of the PI3K Enhancer PIKE Mediates Deficits in Synaptic Plasticity and Behavior in Fragile X Syndrome

Christina Gross et al. Cell Rep.

Abstract

The PI3K enhancer PIKE links PI3K catalytic subunits to group 1 metabotropic glutamate receptors (mGlu1/5) and activates PI3K signaling. The roles of PIKE in synaptic plasticity and the etiology of mental disorders are unknown. Here, we show that increased PIKE expression is a key mediator of impaired mGlu1/5-dependent neuronal plasticity in mouse and fly models of the inherited intellectual disability fragile X syndrome (FXS). Normalizing elevated PIKE protein levels in FXS mice reversed deficits in molecular and cellular plasticity and improved behavior. Notably, PIKE reduction rescued PI3K-dependent and -independent neuronal defects in FXS. We further show that PI3K signaling is increased in a fly model of FXS and that genetic reduction of the Drosophila ortholog of PIKE, CenG1A rescued excessive PI3K signaling, mushroom body defects, and impaired short-term memory in these flies. Our results demonstrate a crucial role of increased PIKE expression in exaggerated mGlu1/5 signaling causing neuronal defects in FXS.

Figures

Figure 1
Figure 1. Genetic reduction of PIKE expression decreases elevated PI3K activity in the cortex of Fmr1KO mice
(A) Centg1 heterozygosity reduces PIKE-L protein levels in both Fmr1WT and Fmr1KO background (2-way ANOVA, significant effect of Centg1 heterozygosity on PIKE-L protein levels (F(1,37)=29.94, p<0.0001), no effect of Fmr1KO (F(1,37)=3.462, p=0.0708), no interaction (F(1,37)=0.1241, p=0.7266)). Representative western blots are shown at the left. Protein levels were normalized to α-tubulin. Also see Figure S1A for breeding scheme and Figures S1B and C for quantification of PIKE mRNA levels in Centg1 heterozygous mice. (B–D) Increased p110β- and mGlu5-associated PI3K activity is reduced to WT levels in cortical synaptic fractions (p110β) or cortical lysates (mGlu5) from Centg1 heterozygous Fmr1KO mice, whereas IRS2-associated PI3K activity is not affected by Fmr1 or Centg1 genotype. PI3K enzymatic activity of p110β-, mGlu5- or IRS-2-specific immunoprecipitates was measured by ELISA (B, 2-way ANOVA, p(Fmr1)=0.079, F(1,36)=3.275; p(Centg1)=0.053, F(1,36)=4.013; p(interaction)=0.011, F(1,36)=7.255; *p=0.015, #p=0.011; C, 2-way ANOVA, p(Fmr1)=0.08, F(1, 27)=3.320; p(Centg1)=0.018, F(1,27)=6.35; p(interaction)=0.036, F(1,27)=4.892; *p=0.042, #p=0.017; D, 2-way ANOVA, p(Fmr1)=0.115, F(1, 27)=2.659, p(Centg1)=0.228, F(1,27)=1.52; p(interaction)=0.947, F(1,27)=0.005). (E) Elevated PIP3/PIP2 ratios in Fmr1KO hippocampus are significantly decreased by genetic reduction of Centg1 (2-way ANOVA, p(Fmr1)=0.0225, F(1, 25)=5.919; p(Centg1)=0.0398, F(1,25)=4.702; p(interaction)=0.0275, F(1,25)=5.478; *p=0.0179, #p=0.0218). Error bars represent SEM, n represents individual mice from at least 5 litters, n indicated in each figure.
Figure 2
Figure 2. Genetic reduction of Centg1 rescues dysregulated mGlu5-mediated PI3K activity and protein synthesis, increased dendritic spine density and impaired synaptic plasticity in Fmr1KO mice
(A) Genetic reduction of Centg1 restores the mGlu1/5-induced increase in p110β enzymatic activity in cortical synaptic fractions (100 µM DHPG for 10 min; 2-way ANOVA, p(Fmr1)=0.052, F(1, 36)=4.0; p(Centg1)=0.15, F(1,36)=2.2; p(interaction)=0.001, F(1,36)=12.4; *p=0.002, #p=0.006). (B) Increased basal protein synthesis rates in Fmr1KO cortical synaptic fractions, as measured by incorporation of radiolabeled amino acids, were significantly reduced to wild type levels by genetic reduction of Centg1 (2-way ANOVA, p(Fmr1)=0.28, F(1, 36)=1.2; p(Centg1)=0.043, F(1,36)=4.4; p(interaction)=0.011, F(1,36)=7.1; *p=0.053, #p=0.009). (C) Genetic reduction of Centg1 restores the mGlu1/5-induced increase in protein synthesis rates in Fmr1KO cortical synaptic fractions (50 µM DHPG for 20 min; 2-way ANOVA, p(Fmr1)=0.0001, F(1, 36)=18.6; p(Centg1)=0.011, F(1,36)=7.1; p(interaction)<0.0001, F(1,36)=27.1; *p<0.0001, #p<0.0001). Error bars represent SEM, n represents individual mice from at least 5 different litters. (D,E) Genetic reduction of Centg1 normalizes dendritic spine density in CA1 apical dendrites to wild type levels (2-way ANOVA, p(Fmr1)<0.0001, F(1, 99)=32.4; p(Centg1)<0.0001, F(1,99)=27.0; p(interaction)<0.0001, F(1,99)=33.1; *p<0.0001, #p<0.0001). Example images are shown in D, quantification of number of dendritic spines per 10 µm is shown in E. N indicates number of secondary dendrites analyzed (60–100 µm length each, starting from the primary shaft), 3–5 mice/genotype, 4–8 neurons/mouse, 1 dendrite/neuron. See Figure S2 for additional analyses. Scale bar is 3 µm. (F,G) Exaggerated DHPG-induced mGluR-LTD in Fmr1KO hippocampal slices is rescued by Centg1 heterozygosity. (F) Shown are mean field excitatory postsynaptic potentials (fEPSPs) normalized to baseline as a function of time (Fmr1WT: n=7, Fmr1KO: n=8, Fmr1KO/Centg1HET: n=9; repeated measures two-way ANOVA (genotype X time), n(genotype)=3, n(time points)=26, p(genotype) =0.0013, F(2,21)=9.3; p(time)<0.0001, F(15,525)=12.99; p(interaction)<0.0001, F(50,525)=2.3; Tukey’s posthoc tests: *p=0.019, #p=0.001, pns(Fmr1wt/Centg1WTFmr1KO/Centg1HET)=0.595). (G) Average fEPSPs at different time points before and after DHPG treatment shows significantly lower fEPSPs in Fmr1KO 60 and 90 min post DHPG compared to both Fmr1WT and Fmr1KO/Centg1HET, whereas Fmr1WT and Fmr1KO/Centg1HET were not different from each other (1-way ANOVAs; -30 min: p=0.106, F(2,21)=2.5; 21 min: p=0.055, F(2,21)=3.3; 60 min: p=0.0002, F(2,21)=13.5, *p=0.0016, #p=0.0003; 90 min: (n different than previous time points: Fmr1WT: n=4, Fmr1KO: n=4, Fmr1KO/Centg1HET: n=5) p=0.005, F(2,10)=9.4, *p=0.017, #p=0.006). Error bars represent SEM.
Figure 3
Figure 3. Genetic reduction of Centg1 reduces neocortical hyperactivity and repetitive behaviors, and improves nest building in Fmr1KO mice
(A,B) Genetic reduction of Centg1 decreases duration of UP states in acute thalamocortical slices from Fmr1KO mice. Example traces for each genotype are shown in (A), and quantification in (B) (2-way ANOVA, p(Fmr1)<0.001, F(1, 148)=15.4; p(Centg1)=0.002, F(1,148)=9.8; p(interaction)=0.131, F(1,148)=2.3, data square root transformed twice to achieve normal distribution). Also see Figures S3A and B showing that UP states in Fmr1WT and Fmr1KO thalamocortical slices are independent of PI3K signaling. (C) Genetic reduction of Centg1 reduces increased susceptibility to audiogenic seizures in Fmr1KO mice (Fisher’s exact tests, *p=0.0002; #p=0.008; p(Fmr1WT/Centg1WTFmr1KO/Centg1HET)=0.213). (D) Genetic reduction of Centg1 rescues increased marble burying in Fmr1KO mice. Shown are number of marbles buried more than 50% after 15 min (2-way ANOVA, p(Fmr1)=0.037, F(1, 33)=4.7; p(Centg1)=0.106, F(1,33)=2.8; p(interaction)=0.039, F(1,33)=4.6; *p=0.021). (E,F) Impaired nesting behavior is improved in Fmr1WT and Fmr1KO mice by genetic reduction of Centg1. Shown are the nest score (E) and the average amount of unused nestlet after 24 hours (F) (E, 2-way ANOVA, p(Fmr1)=0.003, F(1, 35)=16.1; p(Centg1)=0.003, F(1,35)=10.0; p(interaction)=0.418, F(1,35)=0.7; F, 2-way ANOVA, p(Fmr1)=0.015, F(1, 35)=6.6; p(Centg1)=0.039, F(1,35)=4.6; p(interaction)=0.699, F(1,35)=0.15). Representative pictures of nests and analyses after 72 hours are shown in Figures S3C–E. Error bars represent SEM. N indicates number of slices for B, and individual mice from at least 5 different litters for C–F.
Figure 4
Figure 4. Genetic reduction of CenG1A rescues excessive PI3K activity and neuronal defects in a Drosophila model of FXS
(A) Significantly increased PIP3/PIP2 ratio in dFmr1Δ50 mutant fly heads is decreased by CenG1A EY01217 heterozygosity (2-way ANOVA, p(dFmr1)=0.025, F(1, 15)=6.2; p(CenG1A)=0.001, F(1,15)=16.7; p(interaction)=0.372; F(1,15)=0.8). Also see Figure S4A for confirmation of loss of dFMR1 expression in the dFmr1 mutant flies. (B,C) Western blot analyses suggest that increased S6K1 phosphorylation (B) and increased Akt phosphorylation (C) in dFmr1Δ50 mutant fly heads is reduced by genetic reduction of CenG1A (B, related samples Friedman’s test by ranks, p=0.034; Wilcoxon signed rank posthoc analyses: *p=0.008, p(n.s.)=0.257; C, 2-way ANOVA p(dFmr1)=0.194, F(1, 12)=1.9; p(CenG1A)=0.088, F(1,12)=3.5; p(interaction)=0.061, F(1,12)=4.3). (D) Viability of dFmr1Δ50 mutant flies is increased (to or above Mendelian ratios) by CenG1AEY01217 heterozygosity (n(dFmr1Δ50)=339; n(CenG1AEY01217;dFmr1Δ50)=331, Fisher’s exact test p<0.001). (E) Heterozygosity for the CenG1AEY01217 allele rescues β-lobe fusion in mushroom bodies of dFmr1Δ50 mutant flies. Example images of Fasciclin II-stained mushroom bodies are shown as merged z-stack projections (left) and single optical sections through the transverse midline (right) (Fisher’s exact tests: p(wt-dFmr1 Δ50)<0.001, p(wt-CenG1AEY)=1, p(dFmr1Δ50-CenG1AEY;dFmr1Δ50)=0.006, p(wt-CenG1AEY;dFmr1Δ50)=0.477). Scale bar is 50 µm. (F) Heterozygosity for the CenG1AEY01217 allele rescues loss of courtship short-term memory in dFmr1 mutant flies and impairs courtship memory in wild type background (one-way ANOVA with Sidak’s multiple comparison; p<0.0001, F(7,227)=16.5, *p=0.0097, #p=0.0012. nsp>0.9; n(naïve, trained): wt(48,23);dFmr1 Δ50(30,17); CenG1AEY(33,24); CenG1AEY;dFmr1Δ50(36,24)). Data are presented as memory index, i.e. the relative difference between the mean courtship index (CI) of trained and naïve flies ((CI(naïve)-CI(trained))/CI(naïve)), average CI+/− SEM are shown in Figure S4B. Error bars are SEM, n indicated in the figure. N represents individual experiments pooling 15–30 fly heads per condition in A–C, and individual flies in D–F.
Figure 5
Figure 5. Proposed simplified model of PIKE’s role in dysregulated mGlu5-dependent neuronal functions in FXS
(A) mGlu5 receptors mediate activation of PI3K catalytic subunits (p110) through either Homer-PIKE scaffolds or through heterotrimeric G proteins (GβGγ). FMRP directly regulates expression levels of PIKE, and thus provides crucial control of mGlu5-dependent functions, including PI3K/mTOR-dependent downstream signaling and activity-regulated protein synthesis (1), as well as PI3K- and protein synthesis-independent functions (2). (B) In the absence of FMRP-mediated translational repression, PIKE levels are elevated (1). This contributes to increased mGlu5-mediated activation of PI3K catalytic subunits (2). Increased PIKE levels may also cause receptor-independent, PIKE-mediated PI3K activation (3), which could contribute to the overall increase in PIP3/PIP2 ratios. Together, this contributes to increased mGlu1/5-mediated PI3K/mTOR signaling, causing defects in protein synthesis and synaptic plasticity (4). In addition, increased PIKE contributes to impairments in other mGlu5-dependent, but protein synthesis- and PI3K–independent neuronal functions, such as prolonged neocortical UP states (5). Our study suggests that reduction of PIKE in Fmr1KO animal models limits excessive mGlu1/5-dependent downstream signaling, both PI3K dependent and -independent, leading to normalized signal transduction, protein synthesis, neuronal structure and function.

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