Apolipoprotein e genotype-dependent paradoxical short-term effects of (56)fe irradiation on the brain

Abstract

Purpose: In humans, apolipoprotein E (apoE) is encoded by three major alleles (ε2, ε3, and ε4) and, compared to apoE3, apoE4 increases the risk of developing Alzheimer disease and cognitive impairments following various environmental challenges. Exposure to irradiation, including that of (56)Fe, during space missions poses a significant risk to the central nervous system, and apoE isoform might modulate this risk.

Methods and materials: We investigated whether apoE isoform modulates hippocampus-dependent cognitive performance starting 2 weeks after (56)Fe irradiation. Changes in reactive oxygen species (ROS) can affect cognition and are induced by irradiation. Therefore, after cognitive testing, we assessed hippocampal ROS levels in ex vivo brain slices, using the ROS-sensitive fluorescent probe, dihydroethidium (DHE). Brain levels of 3-nitrotyrosine (3-NT), CuZn superoxide dismutase (CuZnSOD), extracellular SOD, and apoE were assessed using Western blotting analysis.

Results: In the water maze, spatial memory retention was impaired by irradiation in apoE2 and apoE4 mice but enhanced by irradiation in apoE3 mice. Irradiation reduced DHE-oxidation levels in the enclosed blade of the dentate gyrus and levels of 3-NT and CuZnSOD in apoE2 but not apoE3 or apoE4 mice. Finally, irradiation increased apoE levels in apoE3 but not apoE2 or apoE4 mice.

Conclusions: The short-term effects of (56)Fe irradiation on hippocampal ROS levels and hippocampus-dependent spatial memory retention are apoE isoform-dependent.

Fig. 1

Novel object recognition of sham-irradiated and irradiated mice. The total time (sec) (A) and percentage of time exploring the novel object (B) are shown. n = 8 mice/genotype/treatment. ***, p < 0.001 versus familiar object.

Fig. 2

(A) Contextual and cued fear conditioning of sham-irradiated and irradiated mice. (A) Baseline motion index. (B) Motion index during the shock. (C) Contextual fear conditioning. (D) Cued fear conditioning task. n = 8 mice/genotype/treatment. ***, p < 0.001 compared to apoE3 and apoE4 mice; *, p < 0.05 versus the time prior to the tone (pre-tone) during the training was paired with the shock.

Fig. 3

(A) First (A), second (B), and third (C) water maze probe trials. n = 8 mice/genotype/treatment. *, p < 0.05; **, p < 0.01.

Fig. 4

Levels of brain apoE (A), 3-NT (B), EC-SOD (C), and CuZnSOD (D) in sham-irradiated and irradiated mice. The 125-kDa 3-NT, 75-kDa CuZnSOD, and 19-kDa EC-SOD bands were used for quantification. The gel lanes match the order of the bar graph. β-Actin was the loading control (not shown). *, p < 0.05; **, p = 0.01 compared to apoE2 irradiated mice.

Fig. 5

Representative images of DHE oxidation of sham-irradiated (A, C, and E) and irradiated (B, D, and F) mice 16 min after exposure to DHE.

Fig. 6

(A) Diagram of hippocampal regions used for DHE analysis. (B) ApoE isoform-dependent effects of irradiation on hippocampal superoxide levels assessed by fluorescent imaging of DHE oxidation. *, p < 0.05. n = 3 to 4 slices per genotype per treatment and region.