title: Site-specific characterization of threonine, serine, and tyrosine glycosylations of amyloid precursor protein/amyloid beta-peptides in human cerebrospinal fluid.
abstract: The proteolytic processing of human amyloid precursor protein (APP) into shorter aggregating amyloid β (Aβ)-peptides, e.g., Aβ1-42, is considered a critical step in the pathogenesis of Alzheimer's disease (AD). Although APP is a well-known membrane glycoprotein carrying both N- and O-glycans, nothing is known about the occurrence of released APP/Aβ glycopeptides in cerebrospinal fluid (CSF). We used the 6E10 antibody and immunopurified Aβ peptides and glycopeptides from CSF samples and then liquid chromatography-tandem mass spectrometry for structural analysis using collision-induced dissociation and electron capture dissociation. In addition to 33 unglycosylated APP/Aβ peptides, we identified 37 APP/Aβ glycopeptides with sialylated core 1 like O-glycans attached to Thr(-39, -21, -20, and -13), in a series of APP/AβX-15 glycopeptides, where X was -63, -57, -52, and -45, in relation to Asp1 of the Aβ sequence. Unexpectedly, we also identified a series of 27 glycopeptides, the Aβ1-X series, where X was 20 (DAEFRHDSGYEVHHQKLVFF), 19, 18, 17, 16, and 15, which were all uniquely glycosylated on Tyr10. The Tyr10 linked O-glycans were (Neu5Ac)(1-2)Hex(Neu5Ac)HexNAc-O- structures with the disialylated terminals occasionally O-acetylated or lactonized, indicating a terminal Neu5Acα2,8Neu5Ac linkage. We could not detect any glycosylation of the Aβ1-38/40/42 isoforms. We observed an increase of up to 2.5 times of Tyr10 glycosylated Aβ peptides in CSF in six AD patients compared to seven non-AD patients. APP/Aβ sialylated O-glycans, including that of a Tyr residue, the first in a mammalian protein, may modulate APP processing, inhibiting the amyloidogenic pathway associated with AD.
authors.affiliation: Department of Clinical Chemistry and Transfusion Medicine, Institute of Biomedicine, University of Gothenburg, SE-413 45 Gothenburg, Sweden.
authors.forename: Adnan
authors.initials: A
authors.lastname: Halim
authors.forename: Gunnar
authors.initials: G
authors.lastname: Brinkmalm
authors.forename: Ulla
authors.initials: U
authors.lastname: Rüetschi
authors.forename: Ann
authors.initials: A
authors.lastname: Westman-Brinkmalm
authors.forename: Erik
authors.initials: E
authors.lastname: Portelius
authors.forename: Henrik
authors.initials: H
authors.lastname: Zetterberg
authors.forename: Kaj
authors.initials: K
authors.lastname: Blennow
authors.forename: Göran
authors.initials: G
authors.lastname: Larson
authors.forename: Jonas
authors.initials: J
authors.lastname: Nilsson
mesh_terms: D000544:Alzheimer Disease / Q000378:metabolism / Q000503:physiopathology*; D000595:Amino Acid Sequence; D016229:Amyloid beta-Peptides / Q000235:genetics / Q000378:metabolism*; D016564:Amyloid beta-Protein Precursor / Q000378:metabolism*; D002853:Chromatography, Liquid; D006031:Glycosylation; D006801:Humans; D008969:Molecular Sequence Data; D040901:Proteomics; D012694:Serine / Q000378:metabolism*; D053719:Tandem Mass Spectrometry; D013912:Threonine / Q000378:metabolism*; D014443:Tyrosine / Q000378:metabolism*
chemical_list: D016229:Amyloid beta-Peptides; D016564:Amyloid beta-Protein Precursor; D013912:Threonine; D014443:Tyrosine; D012694:Serine