Reduction of oxidative stress, amyloid deposition, and memory deficit by manganese superoxide dismutase overexpression in a transgenic mouse model of Alzheimer's disease

Abstract

In Alzheimer's disease (AD), oxidative stress is present early and contributes to disease pathogenesis. We previously reported that in Tg19959 transgenic AD mice, partial deficiency of the mitochondrial antioxidant enzyme manganese superoxide dismutase (MnSOD) exacerbated amyloid pathology. We therefore asked whether MnSOD overexpression would prove beneficial against AD pathogenesis, by studying the offspring of Tg19959 mice crossed with MnSOD-overexpressing mice. At 4 mo of age, there was a 2- to 3-fold increase in MnSOD protein levels in Tg19959-MnSOD mice compared to Tg19959 littermates. Tg19959-MnSOD mice also had a 50% increase in catalase protein levels, a 50% decrease in levels of oxidized protein, and a 33% reduction in cortical plaque burden compared to Tg19959 littermates. Spatial memory was impaired and synaptophysin levels were decreased in Tg19959 mice compared to wild-type littermates, but memory and synaptophysin levels were restored to wild-type levels in Tg19959-MnSOD littermates. These benefits occurred without changes in sodium dodecyl sulfate-soluble or formic acid-soluble Abeta pools or Abeta oligomers in Tg19959-MnSOD mice compared to Tg19959 littermates. These data demonstrate that facilitation of the mitochondrial antioxidant response improves resistance to Abeta, slows plaque formation or increases plaque degradation, and markedly attenuates the phenotype in a transgenic AD mouse model.

Figure 1.

MnSOD overexpression increased levels of MnSOD and catalase and decreased oxidative stress. A, B) Western blots of MnSOD (A) and catalase (B). Densitometric values are normalized to β-actin. MnSOD and catalase protein levels were increased in Tg19959-MnSOD mice compared to Tg19959 littermates. C) Western blots of brain protein carbonyl levels. Densitometric values are normalized to β-actin. MnSOD overexpression decreased oxidative stress in Tg19959 mice. *P < 0.05.

Figure 2.

MnSOD overexpression reduced amyloid plaque burden and microgliosis in Tg19959 mice. A, B) Amyloid plaques per 0.75 mm² (A) and percentage area covered by plaque (B) in Tg19959 mice and Tg19959-MnSOD littermates. At 4 mo of age, MnSOD overexpression reduced amyloid plaques in Tg19959 mouse brains. C) Representative photographs of amyloid plaques detected by Aβ42 antibody AB5078P. D) Percentage area covered by reactive microglia (mean±se). There was a trend toward a decrease of cortical microgliosis in Tg19959-MnSOD mice compared to Tg19959 littermates. *P < 0.05.

Figure 3.

MnSOD overexpression rescued spatial memory retention and restored synaptophysin levels in Tg19959 mice. A, B) Latency (A) and distance traveled (B) during the acquisition period over 5 d. Tg19959 and Tg19959-MnSOD mice were slower to learn than MnSOD and wild-type littermates. C) Percentage time spent in each quadrant during first 15 s of probe trial. Northwest quadrant was the target quadrant. Tg19959-MnSOD mice spent more time in the target quadrant compared to Tg19959 littermates. D) Western blot of synaptophysin, expressed as ratio of protein to α-tubulin. MnSOD overexpression restored synaptophysin levels in Tg19959 mice. Values are means ± se. *P < 0.05.

Figure 4.

MnSOD overexpression did not change APP processing or Aβ levels. A–E) Western blots (A) and analysis of full-length APP (B), α-CTFs (C), β-CTFs (D), and 6% SDS-soluble Aβ monomers (E). Densitometric values are normalized to α-tubulin, in Tg19959 mice and Tg19959-MnSOD littermates. In Tg19959 mice, overexpression of MnSOD did not change APP processing or 6% SDS-soluble Aβ monomer levels. F–I) Quantification of Aβ1-40 and Aβ1-42 levels by ELISA in 6% SDS-soluble and 6% SDS-insoluble 70% FA-soluble pools. In Tg19959 mice, overexpression of MnSOD did not affect 6% SDS-soluble or 6% SDS-insoluble 70% FA-soluble Aβ levels. J, K) A11-immunoreactive patches per 0.75 mm² (J) and percentage area occupied by A11-immunoreactivity (K). No significant differences were found between Tg19959 mice and Tg19959-MnSOD littermates. APP, amyloid precursor protein; CTF, carboxyterminal fragment of APP; SDS, sodium dodecyl sulfate; FA, formic acid. Values are means ± se.