Loss of neprilysin alters protein expression in the brain of Alzheimer's disease model mice

Alzheimer's disease (AD) is a neurodegenerative disease displaying extracellular plaques formed by the neurotoxic amyloid β‐peptide (Aβ), and intracellular neurofibrillary tangles consisting of protein tau. However, how these pathologies relate to the massive neuronal death that occurs in AD brains remain elusive. Neprilysin is the major Aβ‐degrading enzyme and a lack thereof increases Aβ levels in the brain twofold. To identify altered protein expression levels induced by increased Aβ levels, we performed a proteomic analysis of the brain of the AD mouse model APPsw and compared it to that of APPsw mice lacking neprilysin. To this end we established an LC‐MS/MS method to analyze brain homogenate, using an ¹⁸O‐labeled internal standard to accurately quantify the protein levels. To distinguish between alterations in protein levels caused by increased Aβ levels and those induced by neprilysin deficiency independently of Aβ, the brain proteome of neprilysin deficient APPsw mice was also compared to that of neprilysin deficient mice. By this approach we identified approximately 600 proteins and the levels of 300 of these were quantified. Pathway analysis showed that many of the proteins with altered expression were involved in neurological disorders, and that tau, presenilin and APP were key regulators in the identified networks. The data have been deposited to the ProteomeXchange Consortium with identifiers PXD000968 and PXD001786 ( http://proteomecentral.proteomexchange.org/dataset/PXD000968 and ( http://proteomecentral.proteomexchange.org/dataset/PXD001786 ). Interestingly, the levels of several proteins, including some not previously reported to be linked to AD, were associated with increased Aβ levels.