Abstract
Amyloid-beta (Aβ) plays a key role in the pathogenesis of Alzheimer’s disease (AD), but little is known about the proteoforms present in AD brain. We used high-resolution mass spectrometry to analyze intact Aβ from soluble aggregates and insoluble material in brains of six cases with severe dementia and pathologically confirmed AD. The soluble aggregates are especially relevant because they are believed to be the most toxic form of Aβ. We found a diversity of Aβ peptides, with 26 unique proteoforms including various N- and C-terminal truncations. N- and C-terminal truncations comprised 73% and 30%, respectively, of the total Aβ proteoforms detected. The Aβ proteoforms segregated between the soluble and more insoluble aggregates with N-terminal truncations predominating in the insoluble material and C- terminal truncations segregating into the soluble aggregates. In contrast, canonical Aβ comprised the minority of the identified proteoforms (15.3%) and did not distinguish between the soluble and more insoluble aggregates. The relative abundance of many truncated Aβ proteoforms did not correlate with post-mortem interval, suggesting they are not artefacts. This heterogeneity of Aβ proteoforms deepens our understanding of AD and offers many new avenues for investigation into pathological mechanisms of the disease, with implications for therapeutic development.
Highlights
Toward this end, we used high-resolution mass spectrometry to obtain highly accurate measurements and peptide-sequencing information of intact, undigested Aβ from soluble and insoluble Aβ aggregates isolated from human Alzheimer’s disease (AD) brain (Table 1)
We analyzed Aβ extracted from brain tissue of six cases of AD each of whom had clinically severe AD dementia at expiration in an untargeted mass spectrometry approach (Supplementary Figs 2–4)
Used Aβ1–40, Aβ1–42, or Aβ1-x enzyme-linked immunosorbent assays (ELISAs) would not detect the majority of these forms, which fits well with our previous report that much of the Aβ measured by ELISA in human brain interstitial fluid is neither Aβ1–40 nor Aβ1–4221
Summary
Focus has shifted in recent years to the most toxic forms of Aβ, soluble aggregates previously termed ‘oligomers’ and other appellations[13], as they demonstrate a strong correlation with dementia[14,15,16,17,18,19]. Toward this end, we used high-resolution mass spectrometry to obtain highly accurate measurements and peptide-sequencing information of intact, undigested Aβ from soluble and insoluble Aβ aggregates isolated from human AD brain (Table 1). The advantages of nLC-MS/MS over other techniques like matrix-assisted laser desorption/ionization (MALDI) include: i) chromatographic separation of various proteoforms before mass
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