Bhattrai, A.; Raikes, A. C.; Brinton, R. D.

INTRODUCTION: Age, Apolipoprotein E4 (APOE4) genotype, and biological sex are major risk factors for late-onset Alzheimer's disease (LOAD). Neuroimaging is central to its characterization, and preclinical mouse models enable controlled investigation of these factors. To date, humanized APOE4 has not recapitulated LOAD-relevant brain phenotypes. Given the central role of amyloid precursor protein (APP) in LOAD pathogenesis, incorporating humanized APP (hAPP) alongside humanized APOE (hAPOE) may therefore improve translational modeling of structural brain changes. METHODS: Aged mice (mean age = 23.25 months) carrying murine (m) or humanized (h) APP and either murine Apoe or hAPOE3/3, hAPOE3/4, or hAPOE4/4 underwent in-skull ex vivo volumetric MRI. Regional volumes were quantified in absolute terms and relative to total brain volume (TBV). Linear models included APP type, APOE genotype, and sex, with FDR correction applied within contrasts. RESULTS: Brain volumes were primarily determined by APP background, with hAPP globally reducing total and regional volumes relative to mAPP mice. Across hAPP models, hAPOE4/4 exhibited the greatest brain-wide reductions, which was mitigated by a single hAPOE3 allele. In contrast, mouse APP exerted modest effect in hAPOE, with hAPOE4 carriers exhibiting greater total volume without regional specificity. After TBV adjustment, hAPP mice exhibited subcortical vulnerability with relative cortical preservation. Females exhibited larger brain volumes than males, independent of APP or APOE genotype. DISCUSSION: These findings demonstrate that APP background is a primary driver of mouse brain volume, with hAPP producing global reductions amplified by the hAPOE4/4 genotype. In contrast, hAPOE4 effects in the mAPP background were modest and nonspecific, consistent with normative aging. Together, these results suggest that hAPP and hAPOE4 act synergistically, and that hAPOE4 alone is insufficient to recapitulate AD-relevant brain changes in mice. The hAPP/hAPOE4/4 model yields a brain-wide phenotype consistent with LOAD-associated volumetric patterns, whereas mAPP/hAPOE models may better reflect non-pathological aging.