A Covalent Protein Painting Assay Differentiates Amyloid-β Polymorphs in Alzheimer Disease

The aggregation of amyloid-β (Aβ) into fibrils is a molecular hallmark of Alzheimer disease (AD). Cryo-EM results show that Aβ fibrils are polymorphic, with distinct molecular fibril structures depending on the AD subtype. Here, we used a mass spectrometry-based chemical protein footprinting method that is named covalent protein painting (CPP) to obtain conformational information on filamentous Aβ in tissue samples of aged persons without dementia (5), with AD (5) or cerebral cardiovascular angiopathy (CAA, 1) or AD with CAA (3). We measured and quantified the relative abundances of Aβ40 and Aβ42 polymorphs with a newly developed CPP-Aβ-PRM assay. The CPP-Aβ-PRM assay differentiates four conformational states per Aβ peptide, which allows us to deduce the type of Aβ fibril polymorph in the sample. The polymorph composition differed in the brain and meninges. Aβ42 type I polymorphs correlated with sporadic AD in the absence of amyloid angiopathy, whereas Aβ40 type II polymorphs were enriched in the meninges of patients with CAA. The presence of Aβ40 type I polymorphs in brain parenchyma might indicate a type 1 CAA phenotype in patients with AD. We show that the CPP-Aβ-PRM assay might be a novel method to quantify Aβ polymorph information in AD patient samples.