Isomerized Aβ in the brain can distinguish the status of amyloidosis in the Alzheimer’s Disease spectrum

Extracellular amyloid plaques, the pathognomonic hallmark of Alzheimers 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 prior to 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 are significantly increased compared to the age-matched non-AD control brains with Aβ aggregates in the frontal cortex. In this report, we used targeted mass spectrometry to conduct a quantitative analysis of the most common PTMs associated with Aβ; pyroglutamation, citrullination, N-terminal truncation (Aβ _4-x ), C-terminal truncation (Aβ ₄₂ and Aβ ₄₀ ), and isomerization of aspartic acid residues (Asp-1 and Asp-7) in postmortem human brain tissue from pathologically negative (no Aβ plaques) controls, controls with Aβ plaques, Parkinson’s disease (PD) with and without Aβ accumulation/plaques and symptomatic AD. The AD cases contained statistically significant amounts of Asp-1and Asp-7 isomerized Aβ _1-15 (∼ 90 %) compared to controls (preclinical AD) and PD brains with fibrillar Aβ aggregates/deposits. We find that ratio of isomerized N-terminus Aβ (Aβ _1-15 ) 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 Aβ _3pglu-15 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 potential for better biofluid biomarker development for drug efficacy monitoring.