Increased generation of alternatively cleaved beta-amyloid peptides in cells expressing mutants of the amyloid precursor protein defective in endocytosis

The subcellular location of the secretases processing the beta-amyloid precursor protein (APP) is not established yet. We analyzed the generation of the beta-amyloid peptide (Abeta) in human embryonic kidney 293 cell lines stably expressing wild-type and noninternalizing mutants of human APP. APP lacking the entire cytoplasmic domain or with both tyrosine residues of the motif GYENPTY mutated to alanine showed at least fivefold reduced endocytosis. In these cell lines, the production of Abeta1-40 was substantially reduced, but accompanied by the appearance of two prominent alternative Abeta peptides differing at the amino-termini. Based on antibody reactivity and mobility in high- resolution gels in comparison with defined Abeta fragments, these peptides were identified as Abeta3-40 and Abeta5-40. Notably, these alternative Abeta peptides were not generated when the APP mutants were retained in the early secretory pathway by treatment with brefeldin A. These results indicate that the alternative processing is the result of APP accumulation at the plasma membrane and provide evidence of distinct beta-secretase activities. Cleavage amino-terminal to position 1 of Abeta occurs predominantly in endosomes, whereas the processing at positions 3 or 5 takes place at the plasma membrane.