Longitudinal PET Monitoring of Amyloidosis and Microglial Activation in a Second Generation Amyloid-beta Mouse Model

Abstract

Aim: Non-physiological overexpression of β-amyloid (Aβ) precursor protein in common transgenic Aβ mouse models of Alzheimer’s disease (AD) likely hampers their translational potential. The novel AppNL-G-F mouse incorporates a mutated knock-in, potentially presenting an improved model of AD for Aβ-targeting treatment trials. We aimed to establish serial small animal positron-emission-tomography (µPET) of amyloidosis and neuroinflammation in AppNL-G-F mice as a tool for therapy monitoring. Methods: AppNL-G-F mice (homozygous n = 20; heterozygous n = 21) and age-matched wild-type mice (n = 12) were investigated longitudinally from 2.5 to 10 months of age with ¹⁸F-florbetaben Aβ-µPET and ¹⁸F-GE-180 18kDa translocator protein (TSPO)-µPET. Voxel-wise analysis of standardized-uptake-value-ratios (SUVR) images was performed using statistical parametric mapping. All mice underwent a Morris water maze test of spatial learning after their final µPET scan. Quantification of fibrillar Aβ and activated microglia by immunohistochemistry and biochemistry served for validation of µPET results. Results: The periaqueductal gray emerged as a suitable pseudo-reference tissue for both tracers. Homozygous AppNL-G-F mice had rising SUVR in cortex and hippocampus for Aβ- (+9.1%, +3.8%) and TSPO- (+19.8%, +14.2%) µPET from 2.5 to 10 months of age (all P < 0.05), whereas heterozygous AppNL-G-F mice did not show significant changes with age. Significant voxel-wise clusters of Aβ deposition and microglial activation in homozygous mice appeared at five months of age. Immunohistochemical and biochemical findings correlated strongly with µPET data. Water maze escape latency was significantly elevated in homozygous AppNL-G-F mice compared to wild-type at ten months of age and was associated with high TSPO binding. Conclusion: Longitudinal µPET in AppNL-G-F knock-in mice enables monitoring of amyloidogenesis and neuroinflammation in homozygous mice, but is insensitive to minor changes in heterozygous animals. The combination of µPET with behavioral tasks in AppNL-G-F treatment trails is poised to provide important insights in preclinical drug development.