Sexual dimorphism, altered hippocampal glutamatergic neurotransmission, and cognitive impairment in APP knock-in mice

Abstract

Background: It is well established that glutamatergic neurotransmission plays an essential role in learning and memory. Previous studies indicate that glutamate dynamics shift with Alzheimer's disease (AD) progression, contributing to negative cognitive outcomes.

Objective: In this study, we characterized hippocampal glutamatergic signaling with age and disease progression in a knock-in mouse model of AD (APP^NL-F/NL--F).

Methods: At 2-4 and 18+ months old, male and female APP^NL/NL, APP^NL-F/NL-F, and C57BL/6 mice underwent cognitive assessment using Morris water maze (MWM) and Novel Object Recognition (NOR). Then, basal and 70 mM KCl stimulus-evoked glutamate release was measured in the dentate gyrus (DG), CA3, and CA1 regions of the hippocampus using a glutamate-selective microelectrode in anesthetized mice.

Results: Glutamate recordings support elevated stimulus-evoked glutamate release in the DG and CA3 of young APP^NL-F/NL-F male mice that declined with age compared to age-matched control mice. Young female APP^NL-F/NL-F mice exhibited increased glutamate clearance in the CA1 that slowed with age compared to age-matched control mice. Male and female APP^NL-F/NL-F mice exhibited decreased CA1 basal glutamate levels, while males also showed depletion in the CA3. Cognitive assessment demonstrated impaired spatial cognition in aged male and female APP^NL-F/NL-F mice, but only aged females displayed recognition memory deficits compared to age-matched control mice.

Conclusions: These findings confirm a sex-dependent hyper-to-hypoactivation glutamatergic paradigm in APP^NL-F/NL-F mice. Further, data illustrate a sexually dimorphic biological aging process resulting in a more severe cognitive phenotype for female APP^NL-F/NL-F mice than their male counterparts. Research outcomes mirror that of human AD pathology and provide further evidence of divergent AD pathogenesis between sexes.

Keywords: Alzheimer's disease; amyloid-β; cognition; glutamate; learning; memory; sex differences.

Figure 1.

Elevated evoked glutamate release in 2–4 months old APP^NL-F/NL-F male mice. (A) Representative traces of 70 mM KCl stimulus-evoked glutamate recordings in the DG, CA3, and CA1 hippocampal subregions. (B, C) Basal and stimulus-evoked glutamate concentration by subregion. (D) Average amount of 70 mM KCl locally applied to evoke glutamate release. (E) Time to 80% of glutamate clearance. One-way ANOVA, Tukey’s post-hoc, *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 2.

Altered glutamate clearance in the CA1 of 2–4 months old female APP^NL-F/NL-F mice. (A) Representative traces of 70 mM KCl stimulus-evoked glutamate recordings in the DG, CA3, and CA1 hippocampal subregions. (B, C) Basal and stimulus-evoked glutamate concentration by subregion. (D) Average amount of 70 mM KCl locally applied to evoke glutamate release. (E) Time to 80% of glutamate clearance. One-way ANOVA, Tukey’s post-hoc, *p < 0.05.

Figure 3.

Sex-dependent cognitive decline in aged AD mice. Cumulative distance to the platform throughout the five trial days for male (A) and female (B) mice. Insets depict area under the curve (AUC) for the five-day learning period. (C-E) Probe challenge measures include: cumulative distance, platform entries per distance swam, and path efficiency to first entry. (F) Representative track plots from the probe challenge for each sex and genotype. (G, H) Distance and average speed during the novel object retention day. (I) Evaluation of retention memory through novelty preference. MWM Trials: Two-way ANOVA, Tukey’s post-hoc; MWM Probe and NOR: One-way ANOVA, Tukey’s post-hoc, *p < 0.05, **p < 0.01.

Figure 4.

Hypoactive glutamate dynamics in the hippocampus of 18 + months old male AD mice. (A) Representative traces from 70 mM KCl stimulus-evoked glutamate recordings in the DG, CA3, and CA1 hippocampal subregions. (B, C) Basal and stimulus-evoked glutamate concentration by subregion. (D) Average amount of 70 mM KCl locally applied to evoke glutamate release. (E) Time to 80% of glutamate clearance. One-way ANOVA, Tukey’s post-hoc, *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 5.

Decreased basal glutamate levels in the CA1 of 18 + months old female APP^NL-F/NL-F mice. (A) Representative traces from the DG, CA3, and CA1. (B, C) Basal and stimulus-evoked glutamate concentration by subregion. (D) Average amount of 70 mM KCl locally applied to evoke glutamate release. (E) Time to 80% of glutamate clearance. One-way ANOVA, Tukey’s post-hoc, *p < 0.05, **p < 0.01.