Independent generation of Aβ42 and Aβ38 peptide species by γ-secretase

Abstract

Proteolytic processing of the amyloid precursor protein by β-and γ-secretase generates the amyloid-β (Aβ) peptides, which are principal drug targets in Alzheimer disease therapeutics. γ-Secretase has imprecise cleavage specificity and generates the most abundant Aβ40 and Aβ42 species together with longer and shorter peptides such as Aβ38. Several mechanisms could explain the production of multiple Aβ peptides by γ-secretase, including sequential processing of longer into shorter Aβ peptides. A novel class of γ-secretase modulators (GSMs) that includes some non-steroidal anti-inflammatory drugs has been shown to selectively lower Aβ42 levels without a change in Aβ40 levels. A signature of GSMs is the concomitant increase in shorter Aβ peptides, such as Aβ38, leading to the suggestion that generation of Aβ42 and Aβ38 peptide species by γ-secretase is coordinately regulated. However, no evidence for or against such a precursor-product relationship has been provided. We have previously shown that stable overexpression of aggressive presenilin-1 (PS1) mutations associated with early-onset familial Alzheimer disease attenuated the cellular response to GSMs, resulting in greatly diminished Aβ42 reductions as compared with wild type PS1. We have now used this model system to investigate whether Aβ38 production would be similarly affected indicating coupled generation of Aβ42 and Aβ38 peptides. Surprisingly, treatment with the GSM sulindac sulfide increased Aβ38 production to similar levels in four different PS1 mutant cell lines as compared with wild type PS1 cells. This was confirmed with the structurally divergent GSMs ibuprofen and indomethacin. Mass spectrometry analysis and high resolution urea gel electrophoresis further demonstrated that sulindac sulfide did not induce detectable compensatory changes in levels of other Aβ peptide species. These data provide evidence that Aβ42 and Aβ38 species can be independently generated by γ-secretase and argue against a precursor-product relationship between these peptides. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc.