Impaired dendritic expression and plasticity of h-channels in the fmr1(-/y) mouse model of fragile X syndrome

Abstract

Despite extensive research into both synaptic and morphological changes, surprisingly little is known about dendritic function in fragile X syndrome (FXS). We found that the dendritic input resistance of CA1 neurons was significantly lower in fmr1(-/y) versus wild-type mice. Consistent with elevated dendritic I(h), voltage sag, rebound, and resonance frequency were significantly higher and temporal summation was lower in the dendrites of fmr1(-/y) mice. Dendritic expression of the h-channel subunit HCN1, but not HCN2, was higher in the CA1 region of fmr1(-/y) mice. Interestingly, whereas mGluR-mediated persistent decreases in I(h) occurred in both wildtype and fmr1(-/y) mice, persistent increases in I(h) that occurred after LTP induction in wild-type mice were absent in fmr1(-/y) mice. Thus, chronic upregulation of dendritic I(h) in conjunction with impairment of homeostatic h-channel plasticity represents a dendritic channelopathy in this model of mental retardation and may provide a mechanism for the cognitive impairment associated with FXS.

Figure 1. h-Channel Dependent Dendritic Properties Are Significantly Different between WT and fmr1^−/y Mice

(A) Diagram of recording locations and representative dendritic recordings from WT and fmr1^−/y mice. (B) Group data showing that dendritic R_N was significantly lower in fmr1^−/y mice (n = 6) versus WT (n = 6). (C) Group data showing that application of ZD significantly increased R_N in both WT (n = 4) and fmr1^−/y mice (n = 4). (D) Representative dendritic whole-cell recordings showing that there was more sag (arrow) in the dendrites of fmr1^−/y mice versus WT. (E) Group data showing that dendritic sag was significantly higher in fmr1^−/y mice. (F) Group data showing that application of ZD significantly decreased sag in both WT and fmr1^−/y mice. (G) Representative dendritic whole-cell recordings (left) and plot used to calculate rebound slope (right) from WT and fmr1^−/y mice. (H) Group data showing that dendritic rebound slope was significantly higher in fmr1^−/y mice. (I) Group data showing that application of ZD significantly decreased rebound in both WT and fmr1^−/y mice. (J) Representative voltage traces and ZAP used to determine f_R (dashed line). Note the rightward shift in the fmr1^−/y (red) versus the WT (black). (K) Group data showing that dendritic f_R was significantly higher in fmr1^−/y mice. (L) Group data showing that application of ZD significantly decreases f_R in both WT and fmr1^−/y mice. (M) Representative traces used to measure temporal summation of fmr1^−/y mice versus WT. (N) Group data showing that dendritic temporal summation was significantly less in fmr1^−/y mice. (O) Group data showing that application of ZD significantly increased dendritic summation in both WT and fmr1^−/y mice. Group data are presented as mean ± SEM. *p < 0.05; **p < 0.01; ***p < 0.005 significantly different from WT. †p < 0.05 significantly different from baseline.

Figure 2. Distal Dendritic HCN1 Expression in Area CA1 Is Higher in fmr1^−/y Mice

(A) Representative fluorescent images showing HCN1 staining from a WT (A₁) and an fmr1 ^−/y (A₂) mouse section (Sub, subiculum; hf, hippocampal fissure; DG, dentate gyrus). Fluorescent intensity was measured along the dashed line. (B) Representative fluorescent images showing HCN2 staining from a WT (B₁) and an fmr1^−/y (B₂) mouse section. (C and D) Region indicated by the box in (A₁) and (A₂). sp, stratum pyramidale; sr, stratum radiatum; slm, stratum lacunosummoleculare. (E) A plot of intensity as a function of location showing that distal dendritic HCN1 staining was higher in fmr1^−/y mice versus WT mice. (F) Intensity profile showing that there was no difference in HCN2 staining between fmr1^−/y and WT mice. (G) The total amount of HCN1 protein in the CA1 region of the fmr1^−/y hippocampus was significantly higher than WT. (H) Representative western blot from WT and fmr1^−/y mice for HCN2 and tubulin. There was no significant difference in the total amount of HCN2 protein between fmr1^−/y and WT hippocampus. Group data are presented as mean ± SEM.**p < 0.01.

Figure 3. mGluR-Dependent Plasticity Is Present in Both WT and fmr1^−/y Mice

(A) Time course of membrane potential change following a 10 min DHPG application. (B) Representative traces from WT and fmr1^−/y mice before and after DHPG application showing LTD of EPSPs and increased R_N. (C) Group data showing the normalized decrease in EPSP slope 30 min after DHPG washout for both WT (n = 4) and fmr1^−/y mice (n = 4). Inset shows expanded time scale of traces in (B). (D) Group data showing that R_N significantly increased after DHPG washout in both WT and fmr1^−/y mice. (E) Group data showing that f_R significantly decreased after DHPG washout in both WT and fmr1^−/y mice. (F) Group data showing that rebound slope significantly decreased after DHPG washout in both WT and fmr1^−/y mice. (G) Group data showing that sag significantly decreased after DHPG washout in WT but not fmr1^−/y mice. Group data are presented as mean ± SEM.*p < 0.05; **p < 0.01.

Figure 4. TBP-Dependent I_h Plasticity Is Absent in fmr1^−/y Mice

(A) Representative traces (from time points indicated by a and b in B) from WT (black) and fmr1^−/y (red) mice before and after TBP showing LTP of EPSPs and decreased R_N. (B) The time course of EPSP slope change after TBP for WT (n = 6) and fmr1^−/y mice (n = 4). (C) The time course of RN change after TBP for WT (n = 6) and fmr1^−/y mice (n = 4). (D) Group data showing that R_N significantly decreased after TBP in WT but not fmr1^−/y mice. (E) Group data showing that f_R significantly increased after TBP in WT but not fmr1^−/y mice. (F) Group data showing that rebound slope significantly increased after TBP in WT but not fmr1^−/y mice. (G) Group data showing that sag significantly increased after TBP in WT but not fmr1^−/y mice. Group data are presented as mean ± SEM.*p < 0.05; **p < 0.01; ***p < 0.005.