Identification of Novel APP/Aβ Isoforms in Human Cerebrospinal Fluid

Background: Aggregation of beta-amyloid (A beta) into oligomers and plaques is the central pathogenic mechanism in Alzheimer's disease ( AD). A beta is produced from the amyloid precursor protein (APP) by beta- and gamma-secretases, whereas, in the nonamyloidogenic pathway, alpha-secretase cleaves within the A beta sequence, and thus precludes A beta formation. A lot of research has focused on A beta production and the neurotoxic 42-amino-acid form of A beta (A beta 1-42), while less is known about the nonamyloidogenic pathway and how A beta is degraded. Objective: To study the A beta metabolism in man by searching for novel A beta peptides in cerebrospinal fluid (CSF). Methods: Immunoprecipitation, using an anti-A beta antibody, 6E10, was combined with either matrix-assisted laser desorption/ionization time-of-flight mass spectrometry or nanoflow liquid chromatography and tandem mass spectrometry. Results: We identified 12 truncated APP/A beta peptides in the CSF, all of which end at amino acid 15 in the A beta sequence, i.e. 1 amino acid before the proposed alpha-secretase site. Of these 12 APP/A beta peptides, 11 are novel peptides and start N-terminally of the beta-secretase site. The most abundant APP/A beta peptide starts 25 amino acids before the beta-secretase site, APP/A beta (-25 to 15), and had a concentration of approximately 80 pg/ml. The identity of all the APP/A beta peptides was verified in a cohort of AD patients and controls. A first pilot study also showed that the intensity of several APP/A beta peaks in CSF was higher in AD cases than in controls. Conclusion: These data suggest an enzymatic activity that cleaves the precursor protein in a specific manner that may reflect a novel metabolic pathway for APP and A beta. Copyright (C) 2009 S. Karger AG, Basel