Early Induction of Oxidative Stress in Mouse Model of Alzheimer Disease with Reduced Mitochondrial Superoxide Dismutase Activity

While oxidative stress has been linked to Alzheimer's disease, the underlying pathophysiological relationship is unclear. To examine this relationship, we induced oxidative stress through the genetic ablation of one copy of mitochondrial antioxidant superoxide dismutase 2 (Sod2) allele in mutant human amyloid precursor protein (hAPP) transgenic mice. The brains of young (5–7 months of age) and old (25–30 months of age) mice with the four genotypes, wild-type (Sod2+/+), hemizygous Sod2 (Sod2+/−), hAPP/wild-type (Sod2+/+), and hAPP/hemizygous (Sod2+/−) were examined to assess levels of oxidative stress markers 4-hydroxy-2-nonenal and heme oxygenase-1. Sod2 reduction in young hAPP mice resulted in significantly increased oxidative stress in the pyramidal neurons of the hippocampus. Interestingly, while differences resulting from hAPP expression or Sod2 reduction were not apparent in the neurons in old mice, oxidative stress was increased in astrocytes in old, but not young hAPP mice with either Sod2+/+ or Sod2+/−. Our study shows the specific changes in oxidative stress and the causal relationship with the pathological progression of these mice. These results suggest that the early neuronal susceptibility to oxidative stress in the hAPP/Sod2+/− mice may contribute to the pathological and behavioral changes seen in this animal model

Neuronal HO-1 immunostaining in the pyramidal cells of the hippocampus of young (5–7 month-old) mice (A–D).

<p>Densitometric quantitation demonstrates that HO-1 levels tend to be higher in Sod2^+/− and hAPP/Sod2^+/− mice relative to mice with normal Sod2 levels (Sod^+/+ and hAPP/Sod2^+/+) (E). Bars represent the mean + SEM, n = 5−7 per group.</p

High magnification images of the double immunofluorescence staining in the hippocampus of Sod2^+/−, hAPP/Sod2^+/+, and hAPP/Sod2^+/− mice at 25–30 months of age using antibodies against GFAP (Green fluorescence), HNE (Red fluorescence in A), and HO-1 (Red fluorescence in B).

<p>In Sod2^+/− mice there is a moderate level of astrocytic (GFAP) staining, and moderate levels of HNE (A) or HO-1 (B) localized to glial cells (Merged images). In hAPP/Sod2^+/+ and hAPP/Sod2^+/− mice there is extensive plaque formation (Asterisk), striking increases in levels of glial HNE(A), and HO-1 (B) relative to mice without hAPP/Aβ expression. The magnification is 20×.</p

Sod2 reduction, in synergy with hAPP/Aβ expression, results in increased levels of the lipid peroxidation product HNE, in young (5–7 month-old) mice.

<p>Pyramidal neurons in the CA1 region of the hippocampus exhibit similar levels of HNE immunostaining in the hAPP-/Sod2^+/+ (A), hAPP-/Sod2^+/− (B), and the hAPP+/Sod2^+/+ (C) mice, whereas the hAPP+/Sod2^+/− mice exhibit increased levels of HNE (D). Quantification of the intensity of the neuronal HNE levels using densitometric analysis reveals the hAPP+/Sod2^+/− mice have significantly higher levels of neuronal HNE adducts as compared to hAPP+/Sod2^+/+, as well as the hAPP-/Sod2^+/+, hAPP-/Sod2^+/− mice (E). Bars represent mean + SEM, n = 5−7 per group *p<0.05, **p<0.01 by one-way ANOVA + Tukey Kramer post hoc analysis. ***p<0.05 by a one-tailed student's <i>t</i>-test.</p