Upregulation of brain cholesterol levels inhibits mitophagy in Alzheimer disease

Abstract

Mitochondrial dysfunction is behind several neurodegenerative diseases, including Alzheimer disease (AD). Accumulation of damaged mitochondria is already observed at the early stages of AD and has been linked to impaired mitophagy, but the mechanisms underlying this alteration are still not fully known. In our recent study, we show that intracellular cholesterol enrichment can downregulate amyloid beta (Aβ)-induced mitophagy. Mitochondrial glutathione depletion resulting from high cholesterol levels promotes PINK1 (PTEN induced kinase 1)-mediated mitophagosome formation; however, mitophagy flux is ultimately disrupted, most likely due to fusion deficiency of endosomes-lysosomes caused by cholesterol. Meanwhile, in APP-PSEN1-SREBF2 mice, an AD mouse model that overexpresses the cholesterol-related transcription factor SREBF2, cholesterol accumulation prompts an oxidative- and age-dependent cytosolic aggregation of the mitophagy adaptor OPTN (optineurin), which prevents mitophagosome formation despite enhanced PINK1-PRKN/parkin signaling. Hippocampal neurons from postmortem brain of AD individuals reproduce the progressive accumulation of OPTN in aggresome-like structures accompanied by high levels of mitochondrial cholesterol in advanced stages of the disease. Overall, these data provide new insights into the impairment of the PINK1-PRKN mitophagy pathway in AD and suggest the combination of mitophagy inducers with strategies focused on restoring the cholesterol homeostasis and mitochondrial redox balance as a potential disease-modifying therapy for AD.

Keywords: Alzheimer disease; Mitophagy; PINK1; aggresomes; autophagy; cholesterol; optineurin; parkin.

Figure 1.

Schema illustrating our proposed model of differential cholesterol (CHO) regulation of mitophagy in in vitro and in vivo AD models. In cholesterol-enriched SH-SY5Y cells and SREBF2 mouse embryonic neurons, Aβ exposure induces a PINK1-mediated and PRKN-independent mitophagy that cannot be properly resolved due to cholesterol-mediated inhibition of mitophagosome-to-lysosome fusion. In contrast, during AD progression (in old APP-PSEN1-SREBF2 mice and postmortem brain tissue of late-stage AD individuals), sustained Aβ-induced mitochondrial damage, exacerbated by cholesterol loading, prevents mitophagosome formation despite PINK1-PRKN signaling induction; in this scenario, the chronic increase of mitochondrial oxidative stress promotes the cytosolic accumulation of the autophagy receptor OPTN (optineurin) in aggresome-like structures, thereby preventing its recruitment to mitochondria and mitophagy progression