Fe65 Stimulates Proteolytic Liberation of the β-Amyloid Precursor Protein Intracellular Domain

The β-amyloid precursor protein (APP)-binding protein Fe65 is involved in APP nuclear signaling and several steps in APP proteolytic processing. In this study, we show that Fe65 stimulates γ-secretase-mediated liberation of the APP intracellular domain (AICD). The mechanism of Fe65-mediated stimulation of AICD formation appears to be through enhanced production of the carboxyl-terminal fragment substrates of γ-secretase and direct stimulation of processing by γ-secretase. The stimulatory capacity of Fe65 is isoform-dependent, as the non-neuronal and a2 isoforms promote APP processing more effectively than the exon 9 inclusive neuronal form of Fe65. Intriguingly, Fe65 stimulation of AICD production appears to be inversely related to pathogenic β-amyloid production as the Fe65 isoforms profoundly stimulate AICD production and simultaneously decrease Aβ42 production. Despite the capacity of Fe65 to stimulate γ-secretase-mediated APP proteolysis, it does not rescue the loss of proteolytic function associated with the presenilin-1 familial Alzheimer disease mutations. These data suggest that Fe65 regulation of APP proteolysis may be integrally associated with its nuclear signaling function, as all antecedent proteolytic steps prior to release of Fe65 from the membrane are fostered by the APP-Fe65 interaction.