An early pathological hallmark of Alzheimer’s disease (AD) is amyloid-β (Aβ) deposits in the brain,
which largely consist of up to 43 amino acids long Aβ peptides derived from the amyloid precursor
protein (APP). We previously identified a series of sialylated Tyr-10 O-glycosylated Aβ 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 Aβ1-15
glycopeptide, carrying the core 1 Galβ3GalNAcα1-O-Tyr-10 structure, to (1) identify by HCD LC-MS/MS
the definite glycan core 1 structure of immunopurified and desialylated Aβ glycopeptides in human
CSF and to (2) establish a LC-MS/MS quantification method for desialylated Aβ1-15 (and Aβ1-17)
glycopeptides and to (3) compare the concentrations of these Aβ 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 Aβ1-15 or Aβ1-17 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 Aβ standards to reveal specific sialylation
patterns of individual Aβ glycopeptides in AD patients and controls. |
