3xTgAD mice exhibit altered behavior and elevated Aβ after chronic mild social stress

Abstract

Chronic stress may be a risk factor for developing Alzheimer's disease (AD), but most studies of the effects of stress in models of AD utilize acute adverse stressors of questionable clinical relevance. The goal of this work was to determine how chronic psychosocial stress affects behavioral and pathological outcomes in an animal model of AD, and to elucidate underlying mechanisms. A triple-transgenic mouse model of AD (3xTgAD mice) and nontransgenic control mice were used to test for an affect of chronic mild social stress on blood glucose, plasma glucocorticoids, plasma insulin, anxiety, and hippocampal amyloid β-particle (Aβ), phosphorylated tau (ptau), and brain-derived neurotrophic factor (BDNF) levels. Despite the fact that both control and 3xTgAD mice experienced rises in corticosterone during episodes of mild social stress, at the end of the 6-week stress period 3xTgAD mice displayed increased anxiety, elevated levels of Aβ oligomers and intraneuronal Aβ, and decreased brain-derived neurotrophic factor levels, whereas control mice did not. Findings suggest 3xTgAD mice are more vulnerable than control mice to chronic psychosocial stress, and that such chronic stress exacerbates Aβ accumulation and impairs neurotrophic signaling.

Figure 1. 3xTgAD mice exhibit elevated basal plasma corticosterone levels and prolonged responses to social stress compared to WT control mice

Blood plasma corticosterone (A), whole-blood glucose (B) and blood plasma insulin (C) levels at 2 hours, 6 hours and the morning after a 6 hour period of social stress in WT and 3xTgAD mice (AD). Measurements taken in the morning were done after an overnight fast. Basal corticosterone was increased in AD mice compared to WT mice (A) and although both strains displayed a rise in corticosterone in stressed mice compared to non-stressed controls, this increase was shorter in WT mice, which displayed normal corticosterone levels by 6 hours after the onset of stress compared to AD mice which still displayed elevated circulating corticosterone at this time point. Interestingly, after an overnight fast, corticosterone levels were dramatically increased in mice that had not previously been exposed to social stress, but not in mice that experienced stress (A). This phenomenon was noted in both strains. Basal glucose levels were lower in AD mice compared to WT mice (B) and at 2 hours after the beginning of stress, both strains experienced a non-significant rise in glucose levels. By 6 hours, both strains displayed a significant (p< 0.001) decrease in glucose levels, but by the morning after a 6 hour stress session, glucose was again elevated in stressed mice compared to non-stressed controls, an increase which was significant for both strains (p< 0.03; B). Similar to glucose levels, insulin levels were also decreased at 6 hours after the beginning of stress, however this was non-significant and, in contrast to glucose patterns, came after a small drop in insulin at the 2 hour time point (C).

Figure 2. 3xTgAD mice exhibit an anxiety phenotype that is exacerbated by mild chronic social stress

The distance traveled in the open field, a test of anxiety. Less distance traveled corresponds to greater anxiety. Results are expressed as distance traveled in zone 1 (the center area), the residual (area nearest the perimeter of the apparatus) and total distance traveled for wild type and AD mice. Basal anxiety was higher in AD mice compared to WT mice. After 6 weeks of social stress, AD mice displayed heightened anxiety compared to non-stressed controls whereas WT mice did not.

Figure 3. Mild social stress enhances intraneuronal Aβ immunoreactivity, but not tau hyperphosphorylation in hippocampal neurons of 3xTgAD mice

Results for Aβ (A–D) and ptau (E–H) immunostaining, densitometry and western blotting. Images show an increase in Aβ in the CA1 region in AD mice after social stress (B) compared to non-stressed controls (A). Insert box shows intraneuronal Aβ reactivity at a higher magnification. Qualitative results are confirmed by densitometry quantifying mean pixel intensity (C) showing a significant (p=0.049) increase in pixel intensity for stressed samples compared to control. Western blot results confirm staining (D) and show no Aβ aggregates in the ~70kDa range for wild-type mice, either control or stressed. Immunohistochemistry for ptau shows no differences in expression between stressed and unstressed AD mice (E&F), which is confirmed by densitometry (G) and western blot (H).

Figure 4. Mild chronic social stress results in a decrease in levels of BDNF in the hippocampus of 3xTgAD mice

Hippocampal BDNF levels as measured by ELISA. AD mice displayed elevated hippocampal BDNF levels compared to WT mice, that were reduced in stressed 3xTgAD mice compared to non-stressed controls.