Mitochondria are devoid of amyloid beta-protein (A beta)-producing secretases: Evidence for unlikely occurrence within mitochondria of A beta generation from amyloid precursor protein

摘要

Mitochondrial dysfunction is implicated in the pathological mechanism of Alzheimer's disease (AD). Amyloid beta-protein (A beta), which plays a central role in AD pathogenesis, is reported to accumulate within mitochondria. However, a question remains as to whether A beta is generated locally from amyloid precursor protein (APP) within mitochondria. We investigated this issue by analyzing the expression patterns of APP, APP-processing secretases, and APP metabolites in mitochondria separated from human neuroblastoma SH-SY5Y cells and those expressing Swedish mutant APR APP, BACE1, and PEN-2 protein levels were significantly lower in crude mitochondria than microsome fractions while those of ADAM10 and the other gamma-secretase complex components (presenilin 1, nicastrin, and APH-1) were comparable between fractions. The crude mitochondrial fraction containing substantial levels of cathepsin D, a lysosomal marker, was further separated via iodixanol gradient centrifugation to obtain mitochondria- and lysosome-enriched fractions. Mature APP, BACE1, and all gamma-secretase complex components (in particular, presenilin 1 and PEN -2) were scarcely present in the mitochondria-enriched fraction, compared to the lysosome-enriched fraction. Moreover, expression of the beta-C-terminal fragment (beta-CTF) of APP was markedly low in the mitochondria-enriched fraction. Additionally, immunocytochemical analysis showed very little co-localization between presenilin 1 and Tom20, a marker protein of mitochondria. In view of the particularly low expression levels of BACE1, gamma-secretase complex proteins, and beta-CTF in mitochondria, we propose that it is unlikely that A beta generation from APP occurs locally within this organelle. (C) 2017 Elsevier Inc. All rights reserved.