APP intracellular domain formation and unaltered signaling in the presence of familial Alzheimer’s disease mutations

One of the cardinal neuropathological findings in brains from Alzheimer’s disease (AD) patients is the occurrence of amyloid β-peptide (Aβ) deposits. The γ-secretase-mediated intramembrane proteolysis event generating Aβ also results in the release of the APP intracellular domain (AICD), which may mediate nuclear signaling. It was recently shown that AICD starts at a position distal to the site predicted from γ-secretase cleavage within the membrane. This novel site, the ε site, is located close to the inner leaflet of the membrane bilayer. The relationship between proteolysis at the γ and ε sites has not been fully characterized. Here we studied AICD signaling in intact cells using a chimeric C99 molecule and a luciferase reporter system. We show that the release of AICD from the membrane takes place in a compartment downstream of the endoplasmic reticulum, is dependent on presenilin proteins, and can be inhibited by treatment with established γ-secretase inhibitors. Moreover, we find that AICD signaling remains unaltered from C99 derivatives containing mutations associated with increased Aβ42 production and familial AD. These findings indicate that there are very similar routes for Aβ and AICD formation but that FAD-linked mutations in APP primarily affect γ-secretase-mediated Aβ42 formation, and not AICD signaling.