Post‐Translational Modifications Distinguish Amyloid‐β Isoforms in Cerebral Amyloid Angiopathy and Alzheimer's Disease

Cerebral amyloid angiopathy (CAA) shares amyloid-β (Aβ) deposition as a pathological hallmark with the extracellular plaques of Alzheimer's disease (AD). While both disease processes involve progressive, decades-long deposition of fibrillar Aβ peptide, they differ in isoform composition. We hypothesized that post-translational modifications (PTMs) on Aβ would also differ between CAA and parenchymal plaques. Using Lys-N enzymatic digestion followed by quantitative mass spectrometry, we profiled Aβ isoforms and N-terminus PTMs (aspartic acid isomerization and pyroglutamate formation) across CAA severity and compared them to parenchymal plaque Aβ in AD. Moderate to severe CAA were dominated by intact N-terminus (Aβ_1-x ~ 95%) with minimal N-truncated species (Aβ_2-x, Aβ_3pGlu-x, and Aβ_4-x), whereas parenchymal plaques displayed diverse N-terminus truncations and PTMs. Increasing CAA severity correlated with a shift from longer, hydrophobic C-terminal isoforms (Aβ₄₁, Aβ₄₂, and Aβ₄₃) to shorter, less hydrophobic C-terminal isoforms (Aβ₃₇, Aβ₃₈, Aβ₃₉, and Aβ₄₀). Importantly, moderate and severe CAA displayed minimal isomerization of Asp1 and Asp7 residues. These patterns suggest distinct Aβ aggregation mechanisms in CAA versus parenchymal plaques. We propose that the intact and unmodified N-terminus found in CAA is due to its inclusion within the protofibril structure making them less disordered and inaccessible to post-translational modifications, in contrast to plaque-associated Aβ. These biochemical differences may reflect underlying structural distinctions in protofibril architectures, with potential implications for biomarker development for early CAA detection and therapeutic targeting of vascular versus parenchymal Aβ.