Apolipoprotein E4 Causes Age-Dependent Disruption of Slow Gamma Oscillations during Hippocampal Sharp-Wave Ripples

Abstract

Apolipoprotein (apo) E4 is the major genetic risk factor for Alzheimer's disease (AD), but the mechanism by which it causes cognitive decline is unclear. In knockin (KI) mice, human apoE4 causes age-dependent learning and memory impairments and degeneration of GABAergic interneurons in the hippocampal dentate gyrus. Here we report two functional apoE4-KI phenotypes involving sharp-wave ripples (SWRs), hippocampal network events critical for memory processes. Aged apoE4-KI mice had fewer SWRs than apoE3-KI mice and significantly reduced slow gamma activity during SWRs. Elimination of apoE4 in GABAergic interneurons, which prevents learning and memory impairments, rescued SWR-associated slow gamma activity but not SWR abundance in aged mice. SWR abundance was reduced similarly in young and aged apoE4-KI mice; however, the full SWR-associated slow gamma deficit emerged only in aged apoE4-KI mice. These results suggest that progressive decline of interneuron-enabled slow gamma activity during SWRs critically contributes to apoE4-mediated learning and memory impairments. VIDEO ABSTRACT.

Figure 1. LFP recording of hippocampal network activity in apoE3-KI and apoE4-KI mice

(A) Schematic representation of probe placement in the dorsal hippocampus, targeting CA1, CA3, and the DG. (B) Representative Nissl staining of brain sections after electrolytic lesioning indicates probe placement, shown in sequential coronal sections. (C) Sample data (800 ms) shown across all recording sites. Arrowheads indicate SWRs in CA1 pyramidal cell layer sites.

Figure 2. SWR abundance is reduced in aged apoE4-KI mice

(A) Representative raw traces and filtered ripple band traces (150–250 Hz) from aged apoE3-KI and apoE4-KI mice. (B) The abundance of SWRs detected with thresholds of 3, 4, 5, and 6 SD above baseline (unpaired t tests; t(25) = 7.302, 6.193, 4.565, and 3.488, respectively). (C) Percent power spectra of aged apoE3-KI and apoE4-KI mice during SWRs. (D) Frequency of peak ripple power in aged apoE3-KI and apoE4-KI mice (unpaired t test; t(25) = 10.34). (E) Distribution of SWR sizes (in units of SD). Inset: Average ripple size (in SD). (F) SWR abundance after exposure to novel environments, 5 SD threshold. Two-way ANOVA shows significant effect of genotype (P < 0.001, F(1,50) = 23.88) and environment (P < 0.01, F(1,50) = 7.421), but no interaction between them. **P < 0.01; ***P < 0.001. n = 14 and 13 for aged apoE3-KI and apoE4-KI groups, respectively. Error bars and shading indicate SEM. See also Figure S1.

Figure 3. Transient slow gamma oscillation during SWRs in CA1, CA3, and DG of the hippocampus in apoE3-KI mice

(A) Representative examples of SWR-triggered spectrograms from CA1 pyramidal cell layer (CA1-pyr), CA1 stratum radiatum (CA1-sr), CA3 pyramidal cell layer (CA3-pyr), and DG hilus (DG-hil) in an apoE3-KI mouse. White dashed line represents threshold crossing for SWR detection. (B) Average increase in slow gamma power from baseline (350 ms before SWR detection) to peak of SWR in DG for all apoE3-KI mice (paired t test; t(13) = 4.348, n = 14). (C) Distribution of instantaneous frequency measurements in all three hippocampal subregions. (D) Schematic representation of site location in an apoE3-KI mouse receiving a probe with 100-μm site spacing. Filled circles indicate sites used for CSD analysis. (E) SWR-triggered CSD profile indicates distinct regions of slow gamma activity in DG region. Color and traces represent averaged (traces) and smoothed (color) CSD values based on LFP signal filtered for slow gamma activity (30–50 Hz), averaged across SWRs. Note phase reversal across granule cell layers, indicating local origin of activity. Sites include CA1 stratum lacunosum-moleculare (CA1-slm), DG molecular layer (DG-mol), DG granule cell layer (DG-gc), and DG hilus (DG-hil). Deep sites are located outside the hippocampal formation. (F) SWR-triggered coherogram between DG hilus and CA1 stratum radiatum. (G) Quantification of coherence between DG and CA1 stratum radiatum at baseline (400–300 ms before SWR detection) and SWR peak (0–100 ms after SWR detection) for all apoE3-KI mice (paired t test; t(13) = 10.86, n = 14). (H) SWR-triggered coherogram between CA3 pyramidal cell layer and CA1 stratum radiatum. (I) Quantification of coherence between CA3 and CA1 stratum radiatum at baseline (400–300 ms before SWR detection) and SWR peak (0–100 ms after SWR detection) for all apoE3-KI mice (paired t test; t(13) = 8.082, n = 14). ***P < 0.001. See also Figure S2

Figure 4. SWR-associated slow gamma power is reduced in aged apoE4-KI mice

(A) Raw and slow gamma filtered traces (30–50 Hz) surrounding a representative SWR (same SWR as in Figure 2) in aged apoE3-KI and apoE4-KI mice. (B) SWR-triggered spectrograms from representative aged apoE3-KI and apoE4-KI mice from CA1 pyramidal cell layer (CA1-pyr), CA1 stratum radiatum (CA1-sr), CA3 pyramidal cell layer (CA3-pyr), and DG hilus (DG-hil). White dashed line represents threshold crossing for SWR detection. (C) Distribution of instantaneous frequency measurements of slow gamma in CA1-sr of aged apoE3-KI and apoE4-KI mice. (D) Quantification of z-scored slow gamma power during SWRs in CA1-sr, CA3, and DG (unpaired t test; t(25) = 5.236 and 4.896 for CA1-sr and DG, respectively; n = 14 and 13 for apoE3-KI and apoE4-KI groups, respectively. t(21) = 4.093 for CA3; n = 12 and 11, for apoE3-KI and apoE4-KI groups, respectively). (E) Distribution of SWR-associated slow gamma power (z-scored) in CA1-sr across all SWRs in aged apoE3-KI and apoE4-KI mice. ***P < 0.001; ****P < 0.0001. Error bars and shading indicate SEM. See also Figure S3 and S4.

Figure 5. Removal of apoE4 from GABAergic interneurons rescues SWR-associated slow gamma power but not SWR abundance

(A) SWR abundance at various thresholds. Significance indicated in respect to apoE4-KI group (one-way ANOVA with corrected post-hoc tests; F(2,32) = 47.2, 32.59, 17.02, 9.265). (B) Representative SWR-triggered spectrograms from CA1 pyramidal cell layer (CA1-pyr), CA1 stratum radiatum (CA1-sr), CA3 pyramidal cell layer (CA3-pyr), and DG hilus (DG-hil) of aged apoE4-KI and apoE4-KI/Dlx-Cre mice. White dashed line represents threshold crossing for SWR detection. (C) Distribution of instantaneous frequency measurements in CA1-sr. (D) Quantification of SWR-associated slow gamma power (z-scored) in hippocampal subregions (for CA1-sr, CA3, and DG, one-way ANOVA with corrected post-hoc tests; F(2,32) = 16.16, 7.701, 13.65 respectively). (E) Distribution of SWR-associated slow gamma power (z-scored) in CA1-sr. *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001. n = 14, 13, and 8 for apoE3-KI, apoE4-KI, and apoE4/Dlx-Cre groups, respectively, except in CA3 subregion, where n = 12, 11, and 8, respectively. Error bars and shading indicate SEM. See also Figure S5.

Figure 6. SWR and SWR-associated slow gamma characteristics in young apoE3-KI and apoE4-KI mice are differentially affected by aging

(A) SWR abundance in 4–5-month-old apoE3-KI and apoE4-KI mice at 3, 4, 5, and 6 SD thresholds. (unpaired t tests; t(14) = 3.129, 3.126, 2.939, and 2.634, respectively). (B) Percent power spectra during SWRs. Frequency of peak ripple power is significantly lower in young apoE4-KI mice than in young apoE3-KI mice; 156.0 ± 1.562 Hz vs 175.1 ± 1.98 Hz, (unpaired t test, P < 0.0001; t(14) = 7.252). (C) Quantification of average SWR length in young apoE3-KI and young apoE4-KI mice (unpaired t-test, t(14) = 3.431) (D) Quantification of SWR-associated slow gamma power (z-scored) in hippocampal subregions (unpaired t test; t(14) = 3.281 for CA1-sr). (E) Distribution of SWR-associated slow gamma power (z-scored) in CA1-sr. Inset: Recalculation of average SWR-associated slow gamma power excluding events with z-scored slow gamma power > 7 (dashed line). (F) Distribution of SWR-associated slow gamma power (z-scored) in CA1-sr for all genotype and age groups. (G) SWR-associated slow gamma power in CA1 stratum radiatum recalculated after exclusion of events with z-scored slow gamma power > 7 (dashed line in panel F). Two-way ANOVA with corrected post-hoc tests shows significant effects of genotype (P < 0.0001, F(1,39) = 23.00) and age (P = 0.0388, F(1,39) =4.571) and significant interaction between the two (P = 0.0263, F(1,39) = 5.332). (H) Fraction of SWRs with suppressed CA1 stratum radiatum slow gamma activity below baseline (z-score power < 0) for all genotype and age groups. Two way ANOVA with corrected post-hoc tests shows significant effects of genotype (P < 0.0001, F(1,39) = 32.84) and age (P = 0.0013, F(1,39) = 11.92) and significant interaction between the two (P = 0.0033, F(1,39) = 9.823). *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001. n = 9 and 7 for young apoE3-KI and apoE4-KI, respectively; n = 14 and 13 for aged apoE3-KI and apoE4-KI, respectively. Error bars and shading indicate SEM. See also Figure S6.

Figure 7. Deficit in DG-enabled slow gamma oscillations during SWRs likely contributes to apoE4-induced learning and memory impairments in aged mice

Compared to aged apoE3-KI mice, aged apoE4-KI mice show significant loss of GABAergic interneurons in the hilus of the DG. This loss is prevented by removing apoE4 from GABAergic interneurons, as seen in aged apoE4-KI/Dlx-Cre mice. Aged ApoE4-KI and apoE4-KI/Dlx-Cre mice have fewer SWRs than aged apoE3-KI mice. Transient slow gamma activity occurs in the CA1, CA3, and DG during SWRs and is attenuated in throughout the hippocampal circuit in aged apoE4-KI mice. Strikingly, in aged apoE4-KI/Dlx-Cre mice, which do not develop learning and memory impairments (Knoferle et al., 2014), SWR-associated slow gamma power is restored to the level as in aged apoE3-KI mice. Together, these findings suggest that progressive dysregulation of SWR-associated slow gamma activity contributes to age-dependent learning and memory impairment in apoE4-KI mice.