Identification of ADAMTS5 as APP ‐Cleaving Enzyme at the APP669 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.