PROCESSING OF THE BETA-AMYLOID PRECURSOR - MULTIPLE PROTEASES GENERATE AND DEGRADE POTENTIALLY AMYLOIDOGENIC FRAGMENTS

Proteolytic processing of the beta-amyloid precursor proteins (APP) is required for release of the beta/A4 protein and its deposition into t he amyloid plaques characteristic of aging and Alzheimer's disease. We have examined the involvement of acidic intracellular compartments in APP processing in cultured human cells. The use of acidotropic agents and inhibitors to a specific class of lysosomal protease, coupled wit h metabolic labeling and immunoprecipitation, revealed that APP is deg raded within an acidic compartment to produce at least 12 COOH-termina l fragments. Nine likely contain the entire beta/A4 domain and, theref ore, are potentially amyloidogenic. Treatment with E64 or Z-Phe-Ala-CH N2 irreversibly blocked activities of the lysosomal cysteine proteases cathepsins B and L but did not inhibit the lysosomal aspartic proteas e cathepsin D and did not alter the production of potentially amyloido genic fragments. Instead, the inhibitors prevented further degradation of the fragments. Thus, large numbers of potentially amyloidogenic fr agments of APP are routinely generated in an acidic compartment by non cysteine proteases and then are eliminated within lysosomes by cystein e proteases. Immunoblot and immunohistochemical analyses confirmed tha t chronic cysteine protease inhibition leads to accumulation of potent ially amyloidogenic APP fragments in lysosomes. The results provide fu rther support for the hypothesis that an acidic compartment may be inv olved in amyloid formation and begin to define the proteolytic events that may be important for amyloidogenesis.