Extraordinary Diversity of Amyloid-beta Proteoforms in Human Alzheimer's Disease Brain

Amyloid-beta (Aβ) plays a key role in the neuropathogenesis of Alzheimer’s disease, but little is known about the proteoforms (i.e., all protein variants of a single gene including post-translational modifications and sequence variants) present in human AD brain. We used high-resolution mass spectrometry to analyze intact, whole Aβ from soluble and insoluble aggregates in brains of 6 cases with severe dementia and pathologically confirmed Alzheimer’s disease. We found an extraordinary diversity of Aβ peptides totaling 91 unique proteoforms including various N- and C-terminal truncations, 17 different types of post-translational modifications (PTMs), and remarkably 5 amino acid variants in the peptide sequence never before described. Ratios of Aβ proteoforms did not distinguish between soluble and insoluble aggregates, but four individual Aβ proteoforms, three in the insoluble and one in the soluble fraction, were markedly increased when compared across aggregates. Such heterogeneity of the Aβ peptide both deepens our understanding of AD, but perhaps also warrants pause and consideration in our investigation into pathological mechanisms of the disease and therapeutic development.