Identification of <scp>ADAMTS5</scp> as <scp>APP</scp> ‐Cleaving Enzyme at the <scp>APP669</scp> Site

Cerebral amyloid-β (Aβ) deposition is a pathological hallmark of the earliest phases of Alzheimer disease (AD). We previously reported APP669-711 as a novel Aβ-related peptide detectable in human plasma and developed a composite biomarker that combines APP669-711/Aβ1-42 and Aβ1-40/Aβ1-42 ratios to serve as a plasma surrogate of cerebral Aβ burden. We also identified ADAMTS4 (a disintegrin and metalloproteinase with thrombospondin type 1 motif 4) as an APP669-cleaving enzyme that catalyzes the rate-limiting step of APP669-711 production. However, ADAMTS4 accounts for approximately 40% of APP669-site cleavage, leaving the enzymes responsible for the remaining 60% unknown. Here, we identify ADAMTS5 (a disintegrin and metalloproteinase with thrombospondin type 1 motif 5) as a protease with stronger APP669-site cleavage activity in vitro. We further show that the difference in APP669-site cleavage activity between ADAMTS4 and ADAMTS5 is explained by the regulation through the spacer (Sp) domain. Nonetheless, in vivo experiments did not confirm a role for ADAMTS5 in plasma APP669-711 production under healthy conditions. Because ADAMTS5 expression increases in certain pathological states, our results suggest that ADAMTS5 may contribute to plasma APP669-711 production in AD patients with comorbid conditions.