Apolipoprotein E4 (APOE ε4) is the most prevalent genetic risk factor for Alzheimer's disease (AD). Targeted replacement mice that express either APOE ε4 or its AD benign isoform, APOE ε3, are used extensively in behavioral, biochemical, and physiological studies directed at assessing the phenotypic effects of APOE ε4 and at unraveling the mechanisms underlying them. Such experiments often involve pursuing biochemical and behavioral measurements on the same cohort of mice. In view of the possible cross-talk interactions between brain parameters and cognitive performance, we presently investigated the extent to which the phenotypic expression of APOE ε4 and APOE ε4 in targeted replacement mice is affected by behavioral testing. This was performed using young, naïve APOE ε4 and APOE ε3 mice in which the levels of distinct brain parameters are affected by the APOE genotype (e.g., elevated levels of amyloid beta [Aβ] and hyperphosphorylated tau and reduced levels of vesicular glutamate transporter (VGLUT) in hippocampal neurons of APOE ε4 mice). These mice were exposed to a fear-conditioning paradigm, and the resulting effects on the brain parameters were examined. The results obtained revealed that the levels of Aβ, hyperphosphorylated tau, VGluT, and doublecortin of the APOE ε4 and APOE ε3 mice were markedly affected following the exposure of APOE ε4 and APOE ε3 mice to the fear-conditioning paradigm such that the isoform-specific effects of APOE ε4 on these parameters were greatly diminished. The finding that behavioral testing affects the APOE ε3 and APOE ε4 phenotypes and masks the differences between them has important theoretical and practical implications and suggests that the assessment of brain and behavioral parameters should be performed using different cohorts.
Keywords: Alzheimer's disease; Apolipoprotein E4 (APOE ε4); Behavior; Fear conditioning; Learning; Synapses; Targeted replacement mice.