MOLECULAR BIOLOGICAL STUDIES OF APP PROCESSING

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The principal neuropathological feature of Alzheimer's disease is extracellular deposition of about 4 kDa proteinous fragment, designated as amyloid beta peptides (Abeta) derived by proteolytic cleavage from amyloid precursor protein (APP), large cell surface receptor like protein. There have been evidences that APP is proteolytically degraded in the secretory pathway, and endosomal/lysosomal pathway. The pathway is not still identified, in which APP is cleaved to generate Abeta. To clarify the intracellular processing of APP into the generation of Abeta, we detected and characterized potentially amyloidogenic or non-amyloidogenic fragments using newly established monoclonal and polyclonal antibodies, which were raised against eight different synthetic peptides deduced from the sequence of APP770 cDNA,with subcellular fractions obtained from the cultured cells with or without leupeptin, potent lysosomal protease inhibitor of lysosome. APP fragments of 50 and 20 kDa molecular mass containing full-length Abeta were identified in the cultured cells. Immunoblot analysis, biochemical study for specific marker enzyme activity of the fractions obtained from subcellular fractionation, sucrose density gradient centrifugation, together with immunohistochemical observations indicated that the 50 kDa APP fragment was produced in the compartment closely related to endosomal/lysosomal system. Our data suggest that the endosomal/lysosomal pathway is involved in the processing and generation of Abeta.