A Possible Role for Cathepsins D, E, and B in the Processing of β‐amyloid Precursor Protein in Alzheimer's Disease

Abstract

Formation of the 4-kDa peptides, which are essential constituents of the extracellular plaques in Alzheimer's disease, involves the sequential cleavage of the amyloid precursor protein (APP) by p-and y-secretases. The carboxy-terminal 99-amino-acid peptide which is liberated from APP by p-secretase was used as a potential native substrate of the y-secretase(s). With the addition of an initiator Met and a FLAG sequence at the C-terminus (PAPP100-FLAG), it was expressed in Escherichia coli under the control of the T7 promotor. The preferred site(s) of cleavage in the N-terminal 40-amino-acid P-amyloid peptide and PAPPI 00-FLAG by potential y-secretase(s) were rapidly identified using matrix-assisted Iaser-desorptionlionization time-of-flight mass spectroscopy in addition to peptide mapping followed by protein sequence analysis. Since y-secretases seem to be active at acidic pH, three cathepsins (D, E and B) were selected for testing. Studies using different detergents indicated that the cleavage preference of cathepsin D for the PAPP100-FLAG is highly dependent on the surfactant used to solubilize this substrate. All three cathepsins were found to be capable of catabolizing both p-amyloid peptides and the PAPPIOO-FLAG. As cathepsin D was found to cleave the PAPP100-FLAG in the vicinity of the C-terminus of the /l-aniyloid peptides and cathepsin B has a high carboxypeptidase activity at low pH, the possibility cannot be excluded that cathepsins D and B are involved in the amyloidogenic processing of APP.

Protease Inhibitor Coinfusion with Amyloid β-Protein Results in Enhanced Deposition and Toxicity in Rat Brain

“…Leupeptin is well known to inhibit the thiol cathepsins B, H, and L, calpain, and some serine proteases, like trypsin. A likely explanation is the inhibition of cathepsin B, a protease that has been shown to degrade A␤ and that has high carboxypeptidase activity at acidic pH (MacKay et al, 1997). Because the leupeptin effects correlate with the endosome labeling of A␤, we believe the primary effects that were observed are lysosomal.…”

Autophagy, amyloidogenesis and Alzheimer disease

“…The effect requires targeting of overexpressed CD-MPR specifically to Rab5 endosomes, although whether cathepsins contribute indirectly or directly to this APP processing is not known. In this regard, cathepsins D and B do have ␤-and ␥-secretase activity towards APP model peptides in vitro (Mackay et al, 1997;Ladror et al, 1994; Dreyer et al, 1994; Chevallier et at., 1997;Hook et al, 2007), although degradation of A␤ is likely to be their principal function, except possibly in some pathological circumstances.…”

“…The effect requires targeting of overexpressed CD-MPR specifically to Rab5 endosomes, although whether cathepsins contribute indirectly or directly to this APP processing is not known. In this regard, cathepsins D and B do have ␤-and ␥-secretase activity towards APP model peptides in vitro (Mackay et al, 1997;Ladror et al, 1994; Dreyer et al, 1994; Chevallier et at., 1997;Hook et al, 2007), although degradation of A␤ is likely to be their principal function, except possibly in some pathological circumstances.As early endosomes become degradative late endosomes, regions of the surface membrane bud off into the endosome lumen to form a multivesicular body (MVB) (van der Goot and Gruenberg, 2006). This facilitates ubiquitin-dependent sorting of MVB cargo for degradation by lysosomal hydrolases or recycling to other cellular sites (Russell et al, 2006).…”

Therapeutic Potential of Amyloid β-Degrading Enzymes in Alzheimer‘s Disease

“…CatB has been implicated in AD owing to its prominent association with neuritic plaques and enlarged endosomes in AD brains [32][33][34][35][36]. However, it had been postulated that CatB might contribute to Aβ generation through proteolysis of APP.…”