Pharmacological evidences for DFK167-sensitive presenilin-independent γ-secretase-like activity

Amyloid‐β (Aβ) peptides production is thought to be a key event in the neurodegenerative process ultimately leading to Alzheimer’s disease (AD) pathology. A bulk of studies concur to propose that the C‐terminal moiety of Aβ is released from its precursor β‐amyloid precursor protein by a high molecular weight enzymatic complex referred to as γ‐secretase, that is composed of at least, nicastrin (NCT), Aph‐1, Pen‐2, and presenilins (PS) 1 or 2. They are thought to harbor the γ‐secretase catalytic activity. However, several lines of evidence suggest that additional γ‐secretase‐like activities could potentially contribute to Aβ production. By means of a quenched fluorimetric substrate (JMV2660) mimicking the β‐amyloid precursor protein sequence targeted by γ‐secretase, we first show that as expected, this probe allows monitoring of an activity detectable in several cell systems including the neuronal cell line telencephalon specific murine neurons (TSM1). This activity is reduced by DFK167, N‐N‐(3,5‐difluorophenacetyl)‐L‐alanyl]‐S‐phenylglycine t‐butyl ester (DAPT), and LY68458, three inhibitors known to functionally interact with PS. Interestingly, JMV2660 but not the unrelated peptide JMV2692, inhibits Aβ production in an in vitroγ‐secretase assay as expected from a putative substrate competitor. This activity is enhanced by PS1 and PS2 mutations known to be responsible for familial forms of AD and reduced by aspartyl mutations inactivating PS or in cells devoid of PS or NCT. However, we clearly establish that residual JMV2660‐hydrolysing activity could be recovered in PS‐ and NCT‐deficient fibroblasts and that this activity remained inhibited by DFK167. Overall, our study describes the presence of a proteolytic activity displaying γ‐secretase‐like properties but independent of PS and still blocked by DFK167, suggesting that the PS‐dependent complex could not be the unique γ‐secretase activity responsible for Aβ production and delineates PS‐independent γ‐secretase activity as a potential additional therapeutic target to fight AD pathology.