APP substrate ectodomain defines Aβ length by restraining γ-secretase processivity and facilitating product release

Abstract

AbstractSequential proteolysis of the amyloid precursor protein (APP) by γ-secretases (GSECs) generates amyloid-β (Aβ) and defines the proportion of short-to-long Aβ peptides, which is tightly connected to Alzheimer’s disease (AD) pathogenesis.Here, we study the mechanism controlling substrate processing by GSECs and defining product length. We found that polar interactions established by the APPC99ectodomain (ECD), involving but not limited to its juxtamembrane region, restrain both the extent and degree of GSEC processive cleavage by destabilizing enzyme-substrate (E-S) interactions. We show that increasing hydrophobicity at APPC99-ECD – due to mutation or ligand binding – attenuates this substrate-driven product release mechanism, and rescues the effects that AD pathogenic variants exert on Aβ profiles. In addition, our study reveals that APPC99-ECD facilitates the paradoxical production of longer Aβs caused by some GSEC inhibitors that act as high-affinity competitors to the substrate.These findings assign a pivotal role to the substrate ECD in the sequential proteolysis by GSEC and suggest it as a sweet spot for the potential design of APP targeting compounds selectively promoting its processing by GSEC.