Olfactory dysfunction in the 3xTg-AD model of Alzheimer's disease

Abstract

Alzheimer's disease (AD) is an incurable neurodegenerative disease in which the risk of development increases with age. People with AD are plagued with deficits in their cognition, memory, and basic social skills. Many of these deficits are believed to be caused by the formation of amyloid-β plaques and neurofibrillary tangles in regions of the brain associated with memory, such as the hippocampus. However, one of the early, preclinical symptoms of AD is the loss of olfactory detection and discrimination. To determine if a mouse model of AD expresses the same olfactory dysfunction seen in human AD, 3xTg-AD mice were given a buried food test and, unlike previous studies, compared to their background and parental strains. Results showed that over 52 weeks, the 3xTg-AD mice took significantly longer to find the buried food than the control strains. The olfactory bulbs of the 3xTg-AD mice were removed, sliced, and stained using Congo red for histological analysis. Amyloid deposits were observed predominantly in the granule layer of the olfactory bulb beginning at 13 weeks of age in 3xTg-AD mice, but not in the control strains of mice. Further examination of the buried food test data revealed that 3xTg-AD females had a significantly longer latency to detect the buried food than males beginning at 26 weeks of age. Overall, this study provides further validation of the 3xTg-AD mouse model of AD and supports the idea that simple olfactory testing could be part of the diagnostic process for human AD.

Keywords: 3xTg-AD; Alzheimer’s; Buried food test; Congo red; Olfaction.

Fig. 1

BFT from all animal strains displayed as a clustered boxplot. Within each box on the boxplot, the solid horizontal line towards the middle of the box represents the median of the data set; above the line is the upper or third quartile and below the line is the first or lower quartile of that dataset. The maximum and minimum values (excluding outliers) are represented by the whiskers above and below the boxes. Outliers are represented by the circles and stars above the top whisker, as calculated by SPSS (although they were included in the statistical analysis). At 39 weeks of age, C56BL/6 mice took the shortest time to find the buried food compared to all other mice at that age (#p < 0.01). At 52 weeks of age, 3xTg-AD mice took significantly longer to find the buried food than all other mice at that age (##p < 0.001).

Fig. 2

Comparison of male and female 3xTg-AD mice on the BFT. Displayed is the clustered boxplot of the mice at each time point. At 26, 39, and 52 weeks of age, female 3xTg-AD mice took significantly longer to find the buried food than male 3xTg-AD mice (*p < 0.05). All data was included in the statistical analysis. Circles and gray stars above the whiskers represent outliers.

Fig. 3

Congo red staining in the OBs of 3xTg-AD mice. (A) Shows a 52-week-old female with Congo red and H&E labeling visible, with an arrow pointing to a possible amyloid deposit. Image taken at 4x; scale bar = 200 µm. Images B and C are the same 52-week-old female. When confirming the presence of amyloid, the deposit boxed in (A) displayed birefringence; (B) is the brightfield image, while (C) shows the birefringence under polarized light at 20x magnification. Images D-G are a 52-week-old male (D, E) and a 52-week-old female (F, G). The left column (D, F) show bright-field images, while the right column (E, G) shows birefringence under polarized light. No amyloid was observed in the 129, C57BL/6 nor Hybrid mice. Note in C, E, and G the presence of the amyloid deposits confirmed with the use of a polarizing lens. Images (B-G) taken at 20x; scale bar = 50 µm. Abbreviations: GR = granule layer; EPL = external plexiform layer; GL = glomerular layer.