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

Proteolytic processing of the amyloid precursor protein by beta- and gamma-secretase generates the amyloid-beta (A beta) peptides, which are principal drug targets in Alzheimer disease therapeutics. gamma-Secretase has imprecise cleavage specificity and generates the most abundant A beta 40 and A beta 42 species together with longer and shorter peptides such as A beta 38. Several mechanisms could explain the production of multiple A beta peptides by gamma-secretase, including sequential processing of longer into shorter A beta peptides. A novel class of gamma-secretase modulators (GSMs) that includes some non-steroidal anti-inflammatory drugs has been shown to selectively lower A beta 42 levels with out a change in A beta 40 levels. A signature of GSMs is the concomitant increase in shorter A beta peptides, such as A beta 38, leading to the suggestion that generation of A beta 42 and A beta 38 peptide species by gamma-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 A beta 42 reductions as compared with wild type PS1. We have now used this model system to investigate whether A beta 38 production would be similarly affected indicating coupled generation of A beta 42 and A beta 38 peptides. Surprisingly, treatment with the GSM sulindac sulfide increased A beta 38 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 A beta peptide species. These data provide evidence that A beta 42 and A beta 38 species can be independently generated by gamma-secretase and argue against a precursor-product relationship between these peptides.