Age-Stratified Analysis of Blood–Brain Barrier Proteins Identifies Lower Occludin Abundance in the 3xTg-AD Model

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder in which blood–brain barrier (BBB) dysfunction may arise early. We performed an age-stratified, cross-sectional analysis of pericyte- and endothelial-associated markers and junctional proteins (claudin-1, -3, -5, occludin, and VE-cadherin) in cerebral capillary-enriched fractions from male Non-Tg and 3xTg-AD mice at asymptomatic, early, and advanced stages (3, 6, 12, and 16 months of age). Protein abundance was assessed by Western blot, and BBB permeability to 3–5 kDa FITC-dextran was evaluated at 3 months in an independent cohort. At 3 months, the pSRC/SRC ratio and an MMP-9-immunoreactive band were also evaluated in a separate cohort as candidate signaling markers related to occludin dysregulation. We detected no significant age or genotype effects for the pericyte- and endothelial-associated markers or for most junctional protein outcomes. Occludin was the only protein outcome with a significant overall genotype effect, with the largest mean reductions observed at 3 (~80%) and 6 months (~68%). A small exploratory cohort showed a higher hemisphere-derived permeability index in 3xTg-AD mice at 3 months. Neither pSRC/SRC nor the abundance of the MMP-9-immunoreactive band differed significantly between genotypes. These findings identify lower occludin abundance as the principal BBB-associated protein alteration in the 3xTg-AD model and provide exploratory evidence of increased BBB permeability at 3 months. These independent observations warrant longitudinal and translational evaluation to determine whether occludin abundance and BBB permeability could serve as indicators of early neurovascular dysfunction in AD.