The amyloid precursor protein is concentrated in neuronal lysosomes in normal and Alzheimer disease subjects.

The 4.2-kilodalton (kDa) polypeptide associated with the cerebral amyloid deposits of Alzheimer's disease (AD) derives from a much larger protein that is encoded by a gene on chromosome 21. In the present study, we have used antibodies raised against portions of the amyloid protein precursor (APP) to map its normal distribution and to gain further insights into the events that lead to amyloid deposition. Antibodies raised against several different portions of APP reacted with proteins having apparent molecular sizes of 65, 67, and 132 kDa on Western blots. In sections through the normal human brain, immunocytochemistry revealed punctate concentrations of the protein in pyramidal cells of the neocortex, particularly in associative regions, and intense staining in the CA1 pyramidal cells of the hippocampus. By electron microscopy, this punctate distribution coincided with dense concentrations of the protein in secondary lysosomes. In the hippocampus of several AD cases examined, abnormally dense immunostaining in enlarged intracellular domains accompanied a severe atrophy of the CA1 neurons. These data suggest that accumulations of APP in lysosomes of particular neurons may, in AD, lead to proteolytic events that form the insoluble 4.2-kDa amyloid peptide.