Insulin-degrading enzyme rapidly removes the β-amyloid precursor protein intracellular domain (AICD)

The intramembranous γ-secretase cleavage of the β-amyloid precursor protein (APP) is dependent on biologically active presenilins (PS). Notch also undergoes a similar PS-dependent γ-secretase-like cleavage, resulting in the liberation of the Notch intracellular domain (NICD), which is critically required for developmental signal transduction. γ-Secretase processing of APP results in the production of a similar fragment called AICD (APP intracellular domain), which may function in nuclear signaling as well. AICD, like NICD, is rapidly removed. By using a battery of protease inhibitors we demonstrate that AICD, in contrast to NICD, is degraded by a cytoplasmic metalloprotease. In vitro degradation of AICD can be reconstituted with cytoplasmic fractions obtained from neuronal and non-neuronal cells. Taking into account the inhibition profile and the cytoplasmic localization, we identified three candidate enzymes (neurolysin, thimet oligopeptidase, and insulin-degrading enzyme (IDE), also known as insulysin), which all are involved in the degradation of bioactive peptides in the brain. When insulin, a well characterized substrate of IDE, was added to the in vitro degradation assay, removal of AICD was efficiently blocked. Moreover, overexpression of IDE resulted in enhanced degradation of AICD, whereas overexpression of the inactive IDE E111Q mutant did not affect AICD degradation. Finally, immunodepletion of IDE significantly reduced the AICD degrading activity. Therefore our data demonstrate that IDE, which is one of the proteases implicated in the removal of extracellular Aβ, also removes the cytoplasmic product of γ-secretase cleaved APP.