Environmental enrichment mitigates cognitive deficits in a mouse model of Alzheimer's disease

Abstract

Epidemiological studies suggest that individuals with greater education or more cognitively demanding occupations have diminished risk of developing dementia. We wanted to test whether this effect could be recapitulated in rodents using environmental enrichment, a paradigm well documented to attenuate behavioral deficits induced by various pathological insults. Here, we demonstrate that learning and memory deficits observed in a transgenic mouse model of Alzheimer's disease can be ameliorated by enrichment. Female transgenic mice overexpressing amyloid precursor protein and/or presenilin-1 and nontransgenic controls were placed into enriched or standard cages at 2 months of age and tested for cognitive behavior after 6 months of differential housing. Enrichment significantly improved performance of all genotypes in the radial water maze and in the classic and repeated-reversal versions of the Morris water maze. However, enrichment did not benefit all genotypes equally. Mice overproducing amyloid-beta (Abeta), particularly those with amyloid deposits, showed weaker memory for the platform location in the classic Morris water maze and learned new platform positions in the repeated-reversals task less quickly than their nontransgenic cagemates. Nonetheless, enrichment normalized the performance of Abeta-overproducing mice to the level of standard-housed nontransgenic mice. Moreover, this functional preservation occurred despite increased neuritic plaque burden in the hippocampus of double-transgenic animals and elevated steady-state Abeta levels, because both endogenous and transgene-derived Abeta are increased in enriched animals. These results demonstrate that the generation of Abeta in vivo and its impact on the function of the nervous system can be strongly modulated by environmental factors.

Figures

Figure 1.

Enrichment attenuates cognitive deficits in APP and APP/PS1 mice. a-c, Classic MWM. a, Mean distance traveled in consecutive days of training (average of 10 trials per day ± SEM). Genotype-related differences were significant in both the standard-housed [two-way (genotype by day) ANOVA; F(3,37) = 5.40; p < 0.004] and enriched (F(3,27) = 6.78; p < 0.002) groups and decreased as training progressed (genotype-by-day interaction; p < 0.05). b, Average distance to find the platform for 5 d of testing. Enrichment decreased the path length of all genotypes [two-way (enrichment by genotype) ANOVA; enrichment: F(1,64) = 72.89, p < 0.0001; genotype: F(3,64) = 10.17, p < 0.0002]. c, Percentage of time in the correct quadrant during the final probe trial (day 5). Enrichment improved memory in all but APPswe/PS1dE9 mice (enrichment: F(1,64) = 32.28, p < 0.0001; genotype-by-enrichment interaction: F(3,64) = 3.44, p < 0.02). The horizontal line indicates the chance level of performance. d, e, Repeated-reversal training. d, Mean distance swam in consecutive training trials averaged across three sessions. The standard-housed APPswe and APPswe/PS1dE9 groups remained impaired throughout training [two-way (genotype by trial) ANOVA; genotype: F(3,37) = 10.24, p < 0.0001; trial: F(9,333) = 4.87, p < 0.0001]. In contrast, the performance of the enriched APPswe and APPswe/PS1dE9 groups improved with consecutive trials, reaching levels comparable with the NTg and PS1dE9 groups (trial: F(9,243) = 11.42, p < 0.0001; genotype-by-trial interaction: F(24,216) = 2.53, p < 0.02). e, Average distance to find the platform over training trials 2-10 of three reversal sessions. Enrichment improved performance in all genotypes (enrichment: F(1,64) = 55.04, p < 0.0001; genotype: F(3,64) = 11.49, p < 0.0001). Simple main-effect ANOVA confirmed deficits in standard-housed (p < 0.001) but not enriched (p > 0.3) APPswe and APPswe/PS1dE9 mice. f, g, Six-arm radial water maze. f, Number of errors (incorrect arm entries) averaged by trial over 5 d of testing. Performance improved significantly with training for both housing conditions (trial: F = 4.21-5.52; p < 0.002). Standard-housed but not enriched APPswe and APPswe/PS1dE9 groups were significantly impaired compared with the NTg group (F(3,37) = 4.89; p < 0.006). g, Average number of errors during training trials 2-5 over 5 d of testing. Enrichment improved performance in all groups (enrichment: F(1,64) = 39.29, p < 0.0001; genotype: F(3,64) = 6.28, p < 0.02). Simple main-effect ANOVA confirmed impairments in standard-housed (p < 0.006) but not enriched APPswe and APPswe/PS1dE9 mice. Asterisks indicate a significant difference between enriched and standard-housed groups of the same genotype (ANOVA; p < 0.01). Diamonds indicate a significant difference from NTg mice of the same housing condition (Newman-Keuls post hoc test; p < 0.01 applied to significant effect of genotype in the simple main-effect ANOVA). Error bars represent mean ± SEM.

Figure 2.

Hippocampal Aβ levels are increased by enrichment. a-d, Sandwich ELISA was used to measure human Aβ40 and Aβ42 in formic acid extracts from enriched and standard-housed mice. a, b, Enriched APPswe single-transgenic animals display a specific increase in Aβ42 (F(1,14) = 4.57; p = 0.05), with no significant change in Aβ40 (F(1,14) = 1.24; p < 0.28; n = 9 control; n = 8 enriched). One outlier (data not shown), with Aβ levels >3 SDs from the group mean, was removed from the control cohort before statistical analysis. c, d, Enrichment increases the level of both peptides in APPswe/PS1dE9 double-transgenic mice (Aβ42: F(1,16) = 42.70, p < 0.0001; Aβ40: F(1,16) = 55.14, p < 0.0001; n = 11 control; n = 8 enriched). •, Enriched; ○, control. Error bars represent mean ± SEM. *p = 0.05; ***p < 0.0005.

Figure 3.

Enriched mice develop more amyloid than age-matched controls. a-d, Hippocampal sections stained for amyloid with 4G8 (a, b) or Hirano modified silver (c, d). These images were taken from an APPswe/PS1dE9 sibling pair separated after weaning into enriched (b, d) or standard (a, c) housing and killed at 9 months of age. Magnification, 100×. e, Ubiquitin immunostaining, which labels dystrophic neurites surrounding dense-core deposits, was used to quantify amyloid burden by nonbiased stereology. Magnification, 400×. f, Average plaque load (percentage of surface area occupied by ubiquitin-positive neuritic deposits) for each animal (n = 11 control; n = 8 enriched). g, Average plaque load in each cohort. Error bars represent mean ± SEM. ***p < 0.005. h, Plaque load versus age at harvest for each animal. Most enriched mice have higher amyloid burdens than controls of similar age. EE, Enriched.