Multimodal MRI combined with RNA sequencing reveals pathological signatures in the 9-month-old 3×Tg-AD mouse brain

The triple transgenic mouse model of Alzheimer's disease (3×Tg-AD) is a widely used model that exhibits region-dependent patterns of progressive amyloid-β and tau pathology. Although structural brain abnormalities on magnetic resonance imaging have been observed in 3×Tg-AD mice at later disease stages (> 12 months) and as early as 2 months, few studies have investigated changes in these mice during the stage with extensive amyloid-β deposition and onset of tau pathology (around 9 months). This study aimed to assess brain morphometry and microstructure alterations in 9 month-old 3×Tg-AD mice to better understand the neural mechanisms underlying these specific pathological features. Voxel-based analyses were employed on T2-weighted and diffusion tensor imaging to identify differences between 3×Tg-AD and control mice. Compared with controls, 3×Tg-AD mice exhibited lower gray matter volume in several regions including both hippocampal regions, the right thalamus, the left caudoputamen, and the cortex. Reduced white matter volume was observed in fiber tracts including the corpus callosum, internal capsule, stria terminalis, and olfactory tract. Whole-brain diffusion tensor imaging analysis revealed a significant decrease in fractional anisotropy and an increase in both radial and mean diffusivity within the left dentate gyrus of the hippocampal region and right striatumlike amygdala nuclei, with no significant difference in axial diffusivity. Correlation analyses demonstrated significant associations between behavioral performance measures, with both gray and white matter volumes within regions showing significant morphometric differences. Notably, behavioral performance also exhibited significant correlations with diffusion tensor imaging measures particularly within the left dentate gyrus of the hippocampal region and right striatumlike amygdala nuclei. Immunofluorescence analysis confirmed increased amyloid-β plaques and p-Tau protein expression in the hippocampal regions of 3×Tg- AD mice, which corroborated the magnetic resonance imaging findings. Transcriptome analysis in hippocampus tissue identified 1389 differentially expressed genes. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses revealed that numerous differentially expressed genes were enriched in biological processes relevant to synapse structure, cognition, learning, and memory, with particular emphasis on Wnt and mitogen-activated protein kinase signaling pathways. Collectively, these findings suggest that intricate anatomical and microstructural alterations occur in 3×Tg-AD model mice at the onset of pathology around 9 months, potentially driven by gene expression alterations. Moreover, our results support the potential utility of brain volume and diffusion metrics as biomarkers for Alzheimer's disease pathology, which could have significant implications for clinical diagnosis of AD patients.