Complex Relationships between Substrate Sequence and Sensitivity to Alterations in γ-Secretase Processivity Induced by γ-Secretase Modulators

gamma-Secretase catalyzes the final cleavage of the amyloid precursor protein (APP), resulting in the production of amyloid-beta (A beta) peptides with different carboxyl termini. Presenilin (PSEN) and amyloid precursor protein (APP) mutations linked to early onset familial Alzheimer's disease modify the profile of A beta isoforms generated, by altering both the initial gamma-secretase cleavage site and subsequent processivity in a manner that leads to increased levels of the more amyloidogenic A beta 42 and in some circumstances A beta 43. Compounds termed gamma-secretase modulators (GSMs) and inverse GSMs (iGSMs) can decrease and increase levels of A beta 42, respectively. As GSMs lower the level of production of pathogenic forms of long A beta isoforms, they are of great interest as potential Alzheimer's disease therapeutics. The factors that regulate GSM modulation are not fully understood; however, there is a growing body of evidence that supports the hypothesis that GSM activity is influenced by the amino acid sequence of the gamma-secretase substrate. We have evaluated whether mutations near the luminal border of the transmembrane domain (TMD) of APP alter the ability of both acidic, nonsteroidal anti-inflammatory drug-derived carboxylate and nonacidic, phenylimidazole-derived classes of GSMs and iGSMs to modulate gamma-secretase cleavage. Our data show that point mutations can dramatically reduce the sensitivity to modulation of cleavage by GSMs but have weaker effects on iGSM activity. These studies support the concept that the effect of GSMs may be substrate selective; for APP, it is dependent on the amino acid sequence of the substrate near the junction of the extracellular domain and luminal segment of the TMD.