Oxidative inactivation of amyloid beta-degrading proteases by cholesterol-enhanced mitochondrial stress

Familial early-onset forms of Alzheimer's disease (AD) are linked to overproduction of amyloid beta (A beta) peptides, while decreased clearance of A beta is the driving force leading to its toxic accumulation in late-onset (sporadic) AD. Oxidative modifications and defective function have been reported in A beta-degrading proteases such as neprilysin (NEP) and insulin-degrading enzyme (IDE). However, the exact mechanisms that regulate the proteolytic clearance of A beta and its deficits are largely unknown. We have previously showed that cellular cholesterol loading, by depleting the mitochondrial GSH (mGSH) content, stimulates.beta-induced mitochondrial oxidative stress and promotes AD-like pathology in APP-PSEN1-SREBF2 mice. Here, using the same AD mouse model we examined whether cholesterol-enhanced mitochondrial oxidative stress affects NEP and IDE function. We found that brain extracts from APP-PSEN1-SREBF2 mice displayed increased presence of oxidatively modified forms of NEP and IDE, associated with impaired enzymatic activities. Both alterations were substantially recovered after an in vivo treatment with the cholesterol-lowering agent 2-hydroxypropyl-beta-cyclodextrin. The recovery of the proteolytic activity after treatment was accompanied with a significant reduction of A beta levels. Supporting these results, cholesterol-enriched SH-SY5Y cells were more sensitive to A beta-induced impairment of IDE and NEP function in vitro. The rise of cellular cholesterol also stimulated the extracellular release of IDE by an unconventional autophagy-coordinated mechanism. Recovery of depleted pool of mGSH in these cells not only prevented the detrimental effect of A beta on intracellular A beta DPs activities but also had an impact on extracellular IDE levels and function, stimulating the extracellular A beta degrading activity. Therefore, changes in brain cholesterol levels by modifying the mGSH content would play a key role in IDE and NEP-mediated proteolytic elimination of A beta peptides and AD progression.