Citrullination of Amyloid-β Peptides in Alzheimer's Disease

Protein citrullination (deimination of arginine residue) is a well-known biomarker of inflammation. Elevated protein citrullination has been shown to colocalize with extracellular amyloid plaques in postmortem AD patient brains. Amyloid-beta (A beta) peptides which aggregate and accumulate in the plaques of Alzheimer's disease (AD) have sequential N-terminal truncations and multiple post-translational modifications (PTM) such as isomerization, pyroglutamate formation, phosphorylation, nitration, and dityrosine cross-linking. However, no conclusive biochemical evidence exists whether citrullinated A beta is present in AD brains. In this study, using high-resolution mass spectrometry, we have identified citrullination of A beta in sporadic and familial AD brains by characterizing the tandem mass spectra of endogenous N-truncated citrullinated A beta peptides. Our quantitative estimations demonstrate that similar to 35% of pyroglutamate3-A beta pool was citrullinated in plaques in the sporadic AD temporal cortex and similar to 22% in the detergent-insoluble frontal cortex fractions. Similarly, hypercitrullinated pyroglutamate3-A beta (similar to 30%) was observed in both the detergent-soluble as well as insoluble A beta pool in familial AD cases. Our results indicate that a common mechanism for citrullination of A beta exists in both the sporadic and familial AD. We establish that citrullination of A beta is a remarkably common PTM, closely associated with pyroglutamate3-A beta formation and its accumulation in AD. This may have implications for A beta toxicity, autoantigenicity of A beta, and may be relevant for the design of diagnostic assays and therapeutic targeting.