Synthetic standard aided quantification and structural characterization of amyloid-beta glycopeptides enriched from cerebrospinal fluid of Alzheimer's disease patients

An early pathological hallmark of Alzheimer's disease (AD) is amyloid-beta (A beta) deposits in the brain, which largely consist of up to 43 amino acids long A beta peptides derived from the amyloid precursor protein (APP). We previously identified a series of sialylated Tyr-10 O-glycosylated A beta peptides, 15-20 residues long, from human cerebrospinal fluid (CSF) and observed a relative increase of those in AD vs non-AD patients. We report here on the synthesis and use of an isotopically double-labeled A beta 1-15 glycopeptide, carrying the core 1Gal beta 3GalNAc alpha 1-O-Tyr-10 structure, to (1) identify by HCD LC-MS/MS the definite glycan core 1 structure of immunopurified and desialylated A beta glycopeptides in human CSF and to (2) establish a LC-MS/MS quantification method for desialylated A beta 1-15 (and A beta-17) glycopeptides and to (3) compare the concentrations of these A beta glycopeptides in CSF from 20 AD patients and 20 healthy controls. Although we unambiguously identified the core 1 structures and Tyr-10 attachment sites of the glycopeptides, we did not observe any quantitative differences, determined through both peptide and oxonium ion fragments, of the desialylated A beta 1-15 or A beta 1-17 glycopeptides between the AD and non-AD group. The new quantitative glycoproteomic approach described, using double-labeled glycopeptide standards, will undoubtedly facilitate future studies of glycopeptides as clinical biomarkers but should also embrace sialylated A beta standards to reveal specific sialylation patterns of individual A beta glycopeptides in AD patients and controls.