Mass spectrometry based investigation of Alzheimer’s disease and its biomarkers

Abstract

The overall aim of this work was to find biomarkers for Alzheimer’s disease (AD) in such an easily accessible body fluid as blood and investigate the implications of these biomarkers for the AD mechanism. The central and most studied histopathological features in AD brain are the amyloid plaques. Even at the preclinical stage of AD, amyloid beta (Aβ) peptides already start to form plagues. It still remains unclear whether Aβ oligomers and fibrils initiate the molecular cascade that ultimately leads to AD. So far, all Aβ-centered therapy approaches have had little success. Therefore, a fresh, unorthodox look at the AD initiation and biomarkers of the disease development is required.

Since the primary risk factor for AD is age, biomarkers of protein aging could hypothetically be linked to AD. One of the main protein aging pathways is isoaspartate (isoAsp) accumulation; increased isoAsp levels are found in AD amyloid plaques. Thus we developed a mass spectrometry-based platform and workflow for isoAsp detection and quantification (Paper I), and applied them for the analysis of blood plasma samples from AD patients and healthy controls. We found increased levels of isoAsp in all stages of dementia (Paper II), which supported the hypothetical mechanism in which isoAsp accumulation triggers a sequence of events resulting in AD. To further investigate the impact of isoAsp accumulation in the central nervous system, we analyzed the brains of the transgenic mouse model lacking isoAsp repair via methylation (Paper III), and found that, along with the increased isoAsp levels, glutamate pathway is seriously distorted. IsoAsp can be cleared from the organism, besides repair, by means of proteases. When analyzing protein abundances in blood, we found links between these protein functions and AD development (Paper IV). The changes in the protein abundances were small, but nevertheless predictive of the AD progression. How early can one detect the imminent AD development by protein biomarkers? To address this question in the best possible scenario (a well-controlled system, brain biopsy), we analyzed the brain proteome of young transgenic mice and found statistically significant changes occurring long before the onset of AD-like symptoms (Paper V). Therefore, protein abundances, together with isoAsp levels, are promising biomarker candidates for AD diagnostics and prognostics, and mass spectrometry is an adequate tool for analyzing them.