Proteomics Time-Course Study of App Knock-In Mice Reveals Novel Presymptomatic Aβ42-Induced Pathways to Alzheimer's Disease Pathology

Background: The 42 amino acids long amyloid-beta peptide, A(beta 42), may initiate a cascade of events leading to the severe neurodegeneration observed in Alzheimer's disease (AD) brain. However, the underlying molecular mechanisms remain to be established. Objective: To find early A(beta 42)-induced AD related mechanisms, we performed a brain proteomics time-course study on a novel App knock-in AD mouse model, App(NL-F), expressing high levels of A(beta 42 )without A beta PP overexpression artifacts. Methods: Hippocampus and cortex were analyzed separately by using O-18-labelling mass spectrometry to reveal alterations in protein levels. Pathway analysis of proteomics data was used to identify altered biological functions Immunohistochemistry was used to further investigate a significant key regulatory protein. Results: Around 100 proteins were differently expressed in App(NL-F) mice at each time point (3, 6, 9, and 18 months of age) as compared to wild type mice. Strikingly, already at 3 months of age-long before Ap plaque development and memory impairment-several pathways, including long-term potentiation and synaptic plasticity, were downregulated, and neuritogenesis was increased. Huntingtin (HTT) was identified as an upstream regulator, i.e., a key protein affecting the levels of several proteins. Increased levels of HTT in hippocampus of App(NL-F) mice was supported by immunofluorescence microscopy. Conclusion: Notably, the proteome was significantly altered already at 3 months of age, 6 months before the development of plaques. Differentially expressed proteins varied over time, indicating that increased A(beta 42 )levels initiate a cascade of events that eventually manifests in amyloid depositions, inflammation, and decline in memory.