LYSOSOMAL PROCESSING OF AMYLOID PRECURSOR PROTEIN TO A-BETA PEPTIDES - A DISTINCT ROLE FOR CATHEPSIN-S

To investigate the potential contribution of the lysosomal compartment in the processing of amyloid precursor protein (APP) to amyloid beta- peptides (A beta s), we stably overexpressed a series of lysosomal pro teases (the cysteine proteases, cathepsins B, L and S, and the asparti c protease, cathepsin D) in a human kidney epithelial cell line (293) transfected to express high levels of beta APP. preliminary experiment s indicated that 293 cells endogenously synthesize cathepsins B, L and D, but not cathepsin S. A beta secretion was assessed by immunoprecip itation and ELISA and found to be increased similar to 2-fold followin g cathepsin S expression, but to be unchanged (cathepsins B, L) or dec reased (cathepsin D) in the other double transfectants. E-64d, an inhi bitor of lysosomal cysteine proteases, significantly reduced A beta se cretion by the cathepsin S transfectants, but had no effect on cells e xpressing the other proteases. Radiosequencing of AP secreted by cathe psin S-expressing cells revealed that a previously unreported variant beginning at Met -1 (relative to the most common A beta N-terminus, As p -1) accounted for most of the increase in A beta secretion. Immunost aining of human brain sections revealed cathepsin S in cortical neuron s and glia in samples of brain from patients with Alzheimer's disease. These results provide evidence in living cells for a pathway in which cathepsin S generates A beta from amyloidogenic fragments of beta APP in the endosomal/lysosomal compartment. This pathway appears to be in ducible, distinct from a constitutive pathway used by 293 and other ce lls to generate A beta, and may be relevant to the pathogenesis of Alz heimer's disease.