Histological and Behavioral Phenotypes of a Novel Mutated APP Knock-In Mouse

Gene mutations within amyloid precursor protein (APP or AβPP) and/or presenilin 1 (PS1) genes are determinants of familial Alzheimer's disease (fAD) and remain fundamental for experimental models. Here, we generated a neuronal knock-in mouse (PLB2APP) with mutated human APPSwe/Lon and investigated histopathology and behavioral phenotypes. Additionally, PLB2APP mice were cross-bred with a presenilin (PS1A246E) line to assess the impact of this gene combination. Immunohistochemistry determined amyloid-β (Aβ) pathology, astrogliosis (via GFAP labelling), and neuronal densities in hippocampal and cortical brain regions. One-year old PLB2APP mice showed higher levels of intracellular Aβ in CA1, dentate gyrus, and cortical regions compared to PLBWT controls. Co-expression of PS1 reduced hippocampal but elevated cortical Aβ build-up. Amyloid plaques were sparse in aged PLB2APP mice, and co-expression of PS1 promoted plaque formation. Heightened GFAP expression followed the region-specific pattern of Aβ in PLB2APP and PLB2APP/PS1 mice. Behaviorally, habituation to a novel environment was delayed in 6-month-old PLB2APP mice, and overall home-cage activity was reduced in both lines at 6 and 12 months, particularly during the dark phase. Spatial learning in the water maze was impaired in PLB2APP mice independent of PS1 expression and associated with reduced spatial navigation strategies. Memory retrieval was compromised in PLB2APP mice only. Our data demonstrate that low expression of APP is sufficient to drive histopathological and cognitive changes in mice without overexpression or excessive plaque deposition. AD-like phenotypes were altered by co-expression of PS1, including a shift from hippocampal to cortical Aβ pathology, alongside reduced deficits in spatial learning.