Proteomic alterations in the brain and blood-brain barrier during brain Aβ accumulation in an APP knock-in mouse model of Alzheimer's disease

Background Blood-brain barrier (BBB) dysfunction is supposed to be an early event in the development of Alzheimer's disease (AD). This study aimed to investigate the relationship between BBB alterations and AD progression in terms of amyloid-beta peptide (A beta) accumulation in the brains of humanized amyloid precursor protein knock-in (APP-KI) mice.Methods Brain A beta accumulation was examined using immunohistochemical analysis. Alterations in differentially expressed proteins were determined using sequential window acquisition of all theoretical fragment ion mass spectroscopy (SWATH-MS)-based quantitative proteomics, and Metascape, STRING, Gene Ontology, and KEGG were used for network analyses of altered biological pathways and processes. Statistical significance was determined using the unpaired two-tailed Student's t-test and Welch's t-test for two groups and one-way analysis of variance followed by Tukey's test for more than two groups. Correlations between two groups were determined using Pearson's correlation analysis.Results Brain A beta accumulation in APP-KI mice was detectable at 2 months, increased significantly at 5 months, and remained elevated at 12 months of age. The levels of differentially expressed proteins in isolated brain capillaries were higher in younger mice, whereas those in the brain were higher in older mice. Network analyses indicated changes in basement membrane-associated and ribosomal proteins in the brain capillaries. There were no significant changes in key proteins involved in drug or A beta transport at the BBB. In contrast, solute carrier transporter levels in astrocytes, microglia, and neurons were altered in the brain of older mice. Moreover, the levels of the lipid transporters Apoe and Apoj were upregulated in both the brain and isolated brain capillaries after A beta accumulation.Conclusions Our results suggest that changes in the brain occurred after advanced A beta accumulation, whereas initial A beta accumulation was sufficient to cause alterations in the BBB. These findings may help elucidate the role of BBB alterations in AD progression and predict the distribution of drugs across the BBB in the brain of patients with AD.