Brain microstructure abnormalities in the 3xTg-AD mouse - A diffusion MRI and morphology correlation study

Abstract

The widely studied triple transgenic (3xTg-AD) mouse provides a robust model of Alzheimer's disease (AD) with region dependent patterns of progressive amyloid-β (Aß) and tau pathology. Using diffusion MRI (dMRI), we investigated the sensitivity of dMRI measures in capturing AD pathology associated microstructure alterations in older 3xTg-AD mice, and the degree to which dMRI changes correlate with measurements of Aβ and tau pathology. 3xTg-AD and normal control (NC) mice, 15 to 21 months of age, were used in this study. In vivo dMRI data were acquired for the generation of diffusion tensor (DT) and diffusional kurtosis (DK) measures within the hippocampus and fimbria (Fi). For these same brain regions, Aβ and tau pathology were quantified by morphological analysis of Aß1-42 and AT8 immunoreactivity. Two-tailed, two-sample t-tests were performed to assess group differences in each brain region of interest (ROI), with the Benjamini-Hochberg false discovery rate (FDR) method being applied to adjust for multiple comparisons. Spearman correlation coefficients were calculated to investigate associations between diffusion and morphological measures. Our results revealed, depending on the brain region, DT and DK measures were able to detect group differences. In the dorsal hippocampus (HD), fractional anisotropy (FA) was significantly higher in the 3xTg-AD mice compared with NC mice. In the subiculum (SUB), FA, axial diffusivity (D_||) and radial kurtosis (K_┴) were significantly higher in 3xTg-AD mice compared with NC mice. Morphological quantification of Aß1-42 and AT8 immunoreactivity showed elevated Aß and tau in the Fi, ventral hippocampus (HV) and SUB of 3xTg-AD mice. The presence of Aβ and tau was significantly correlated with several DT and DK measures, particularly in the SUB, where an increase in tau correlated with an increase in mean kurtosis (MK) and K_┴. This work demonstrates significant dMRI differences between older 3xTg-AD and NC mice in the hippocampus and Fi. Significant correlations were found between dMRI and morphological measures of Aβ and tau pathology. These results support the potential of dMRI-derived parameters as biomarkers of AD pathology. Since the imaging methods employed here are easily translatable to clinical MRI, our results are also relevant for human AD patients.

Keywords: 3xTg-AD mouse; Alzheimer's disease; Diffusion MRI; Diffusional kurtosis imaging.

Figure 1.

(a) Slice positioning for the 15 coronal slices. (b) Representative fractional anisotropy maps of a 3xTg-AD mouse illustrating the selected ROIs: Fimbria (FI - yellow), dorsal (HD - orange), ventral (HV - red) and subiculum (SUB - blue) of the hippocampus (c) Representative dMRI parametric maps of all diffusion measures, for a single anatomical slice, from a normal control (NC) and a 3xTg-AD mouse. DT measures are mean diffusivity (MD), axial diffusivity (D_∥), radial diffusivity (D_⊥), and fractional anisotropy (FA). DK measures are mean kurtosis (MK), axial kurtosis (K_∥), radial kurtosis (K_⊥), and kurtosis fractional anisotropy (KFA). Scale bars: 0–1 for FA and KFA; 0–2 μm²/ms for MD, D_∥ and D_⊥; 0–3 for MK, K_∥, and K_⊥.

Figure 2.

Representative of the (a) Aß1-42 and (b) AT8 immunoreactivity (4x magnification) in the fimbria (Fi) and subiculum (SUB) from a 21-month-old normal control (NC) and a 3xTg-AD mouse (TG). Scale bar = 100μm.

Figure 3.

Quantitative analysis of the Aß and tau immunoreactivity; values are expressed as group-averaged means ± standard error of the mean (SE) for each group from all time points pooled together. Optical density (OD) = log (max intensity/mean intensity) for fimbria (Fi), dorsal (DH), ventral (VH) and subiculum (SUB) of the hippocampus. Note that the vertical axis scales are variable across regions and type of staining, and error bars represents standard error of the mean (SE). Statistical significance at the level of p≤ 0.05.

Figure 4.

Spearman’s correlation values (r) and p-values between diffusion MRI (dMRI) metrics and histological quantitative measures for Aß and tau immunoreactivity. Graphs showing the Spearman’s correlation between K_⊥ and optical density (O.D.) of Aß and tau. Statistical significance (at the level of p≤ 0.05), unadjusted p-values in bold. Fimbria (FI), dorsal (HD), ventral (HV) and subiculum (SUB) of the hippocampus. Fraction anisotropy (FA); axial diffusivity (D_∥); axial kurtosis (K_∥); kurtosis fraction anisotropy (KFA); mean kurtosis (MK); radial kurtosis (K_⊥).