The η-secretase-derived APP fragment ηCTF is localized in Golgi, endosomes and extracellular vesicles and contributes to Aβ production

The processing of the amyloid precursor protein (APP) is one of the key events contributing to Alzheimer's disease (AD) etiology. Canonical cleavages by beta- and gamma-secretases lead to A beta production which accumulate in amyloid plaques. Recently, the matrix metalloprotease MT5-MMP, referred to as eta-secretase, has been identified as a novel APP cleaving enzyme producing a transmembrane fragment, eta CTF that undergoes subsequent cleavages by alpha- and beta-secretases yielding the A eta alpha and A eta beta peptides, respectively. The functions and contributions of eta CTF and its related fragments to AD pathology are poorly understood. In this study, we designed a novel immunological probe referred to as eta CTF-NTer antibody that specifically interacts with the N-terminal part of eta CTF targeting eta CTF, A eta alpha, A eta beta but not C99, C83 and A beta. We examined the fate and localization of eta CTF fragment in various cell models and in mice. We found that overexpressed eta CTF undergoes degradation in the proteasomal and autophagic pathways and accumulates mainly in the Golgi and in endosomes. Moreover, we observed the presence of eta CTF in small extracellular vesicles purified from neuroblastoma cells or from mouse brains expressing eta CTF. Importantly, the expression of eta CTF in fibroblasts devoid on APP leads to A beta production demonstrating its contribution to the amyloidogenic pathway. Finally, we observed an eta CTF-like immunoreactivity around amyloid plaques and an age-dependent accumulation of eta CTF in the triple-transgenic mouse AD model. Thus, our study suggests that the eta CTF fragment likely contributes to AD pathology by its exosomal spreading and involvement in A beta production.

Automated summary

This study identifies eta CTF as a novel APP cleaving enzyme and suggests its involvement in the pathogenesis of Alzheimer's disease. The study also reveals the localization of eta CTF in Golgi and endosomes, as well as its presence in small extracellular vesicles. Furthermore, the expression of eta CTF in APP-null fibroblasts leads to A beta production, implicating its role in amyloid plaque formation.