Isomerized Aβ in the brain can distinguish the status of amyloidosis in the Alzheimer's disease spectrum

Abstract

Extracellular amyloid plaques, the pathognomonic hallmark of Alzheimer's disease (AD), are also observed in cognitively unimpaired subjects in the preclinical stages. Progressive accumulation of fibrillar amyloid-β (Aβ) as plaques and perivascular deposits occur two decades before clinical onset, making Aβ a long-lived peptide. To characterize the amyloid plaques biochemically, both the Aβ-load as well the post-translational modifications (PTMs) could serve as markers for distinguishing the pre-clinical stage compared to later prodromal and clinical stages of AD. Recently, we described the presence of extensive isomerization of the Aβ N-terminus in AD post-mortem brains that is significantly increased compared to the age-matched non-AD control brains with Aβ aggregates. In this report, we performed Lys-N enzymatic digestion followed by mass spectrometry-based quantitative analysis of the most common PTMs associated with plaque Aβ. We focused on pyroglutamation (pGlu3), citrullination (cit5), N-terminal truncation (Aβ_4-x), C-terminal isoforms (Aβ_x-42 and Aβ_x-40), and isomerization of aspartic acid residues (Asp-1 and Asp-7) in postmortem human brain tissue from pathologically negative (no Aβ plaques) controls (n = 23), controls with Aβ plaques (n = 35), Parkinson's disease (PD) with (n = 28) and without Aβ accumulation/plaques (n = 30) and symptomatic AD (n = 60). The AD cases contained statistically significant amounts of Asp-1 and Asp-7 isomerized Aβ (~ 90%) compared to controls (preclinical AD) and PD brains with fibrillar Aβ aggregates/deposits. We find that the ratio of isomerized N-terminus Aβ (Aβ_1-x) species in the brain detergent soluble pool differentiates older fibrillar Aβ deposits in symptomatic AD brain compared to Aβ deposits detected in preclinical AD and PD. Citrullinated pGlu3-Aβ was increased only in symptomatic AD, highlighting this Aβ PTM is a unique feature of parenchymal plaques in advanced AD. Our results have implications for early therapeutic targeting of these modified species as well as potential for better biofluid biomarker development for drug efficacy monitoring.

Keywords: Alzheimer’s disease; Amyloid-β; Isomerization; Mass spectrometry; Post-translational modifications (PTMs).