Lysosomal enzyme tripeptidyl peptidase 1 destabilizes fibrillar A beta by multiple endoproteolytic cleavages within the beta-sheet domain

Accumulation of amyloid-beta (A beta), which is associated with Alzheimer's disease, can be caused by excess production or insufficient clearance. Because of its beta-sheet structure, fibrillar A beta is resistant to proteolysis, which would contribute to slow degradation of A beta plaques in vivo. Fibrillar A beta can be internalized by microglia, which are the scavenger cells of the brain, but the fibrils are degraded only slowly in microglial lysosomes. Cathepsin B is a lysosomal protease that has been shown to proteolyze fibrillar A beta. Tripeptidyl peptidase 1 (TPP1), a lysosomal serine protease, possesses endopeptidase activity and has been shown to cleave peptides between hydrophobic residues. Herein, we demonstrate that TPP1 is able to proteolyze fibrillar A beta efficiently. Mass spectrometry analysis of peptides released from fibrillar A beta digested with TPP1 reveals several endoproteolytic cleavages including some within beta-sheet regions that are important for fibril formation. Using molecular dynamics simulations, we demonstrate that these cleavages destabilize fibrillar beta-sheet structure. The demonstration that TPP1 can degrade fibrillar forms of A beta provides insight into the turnover of fibrillar A beta and may lead to new therapeutic methods to increase degradation of A beta plaques.