γ-secretase modulators and APH1 isoforms modulate γ-secretase cleavage but not position of ε-cleavage of the amyloid precursor protein (APP)

Abstract

The relative increase in Aβ42 peptides from familial Alzheimer disease (FAD) linked APP and PSEN mutations can be related to changes in both Îμ -cleavage site utilization and subsequent step-wise cleavage. Cleavage at the Îμ -site releases the amyloid precursor protein (APP) intracellular domain (AICD), and perturbations in the position of Îμ -cleavage are closely associated with changes in the profile of amyloid Iβ-protein (AIβ) species that are produced and secreted. The mechanisms by which γ-secretase modulators (GSMs) or FAD mutations affect the various γ-secretase cleavages to alter the generation of AIβ peptides have not been fully elucidated. Recent studies suggested that GSMs do not modulate Îμ - cleavage of APP, but the data were derived principally from recombinant truncated epitope tagged APP substrate. Here, using full length APP from transfected cells, we investigated whether GSMs modify the Îμ -cleavage of APP under more native conditions. Our results confirmed the previous findings that Îμ -cleavage is insensitive to GSMs. In addition, fenofibrate, an inverse GSM (iGSM), did not alter the position or kinetics of Îμ -cleavage position in vitro. APH1A and APH1B, a subunit of the γ-secretase complex, also modulated AIβ42/AIβ40 ratio without any alterations in Îμ -cleavage, a result in contrast to what has been observed with PS1 and APP FAD mutations. Consequently, GSMs and APH1 appear to modulate γ- secretase activity and AIβ42 generation by altering processivity but not Îμ -cleavage site utilization.