Effects of gamma-Secretase Inhibition on the Amyloid beta Isoform Pattern in a Mouse Model of Alzheimer's Disease

Background: Accumulation of amyloid beta (A beta) in the brain is believed to represent one of the earliest events in the Alzheimer disease process. A beta is generated from amyloid precursor protein after sequential cleavage by beta- and gamma-secretase. Alternatively, alpha-secretase cleaves within the A beta sequence, thus, precluding the formation of A beta. A lot of research has focused on A beta production, while less is known about the non-amyloidogenic pathway. We have previously shown that A beta is present in human cerebrospinal fluid (CSF) as several shorter C-terminal truncated isoforms (e. g. A beta 1-15 and A beta 1-16), and that the levels of these shorter isoforms are elevated in media from cells that have been treated with gamma-secretase inhibitors. Objective: To explore the effect of N-[N-(3,5-difluorophenacetyl-L-alanyl)]-S-phenylglycine t-butyl ester (DAPT), a gamma-secretase-inhibitor, treatment on the A beta isoform pattern in brain tissue and CSF from Tg2576 mice. Methods: Immunoprecipitation using the anti-A beta antibodies 6E10 and 4G8 was combined with either matrix-assisted laser desorption/ionization time-of-flight mass spectrometry or nanoflow liquid chromatography and tandem mass spectrometry. Results: All fragments longer than and including A beta 1-17 displayed a tendency towards decreased levels upon gamma-secretase inhibition, whereas A beta 1-15 and A beta 1-16 indicated slightly elevated levels during treatment. Conclusion: These data suggest that A beta 1-15 and A beta 1-16 may be generated through a third metabolic pathway independent of gamma-secretase, and that these A beta isoforms may serve as biomarkers for secretase inhibitor treatment. Copyright (C) 2009 S. Karger AG, Basel