Regulation of amyloid processing in neurons by astrocyte-derived cholesterol

Alzheimer’s Disease (AD), a common and burdensome neurodegenerative disorder, is characterized by the presence of β-Amyloid (Aβ) plaques, inflammation, and loss of cognitive function. A cholesteroldependent process sorts Aβ-producing enzymes into nanoscale lipid compartments (also called lipid rafts). Genetic variation in a cholesterol transport protein, apolipoprotein E (apoE), is the most common genetic marker for sporadic AD. Evidence suggests apoE links to Aβ production through lipid rafts, but so far there has been little scientific validation of this link in vivo. Here we use super-resolution imaging to show apoE utilizes astrocyte-derived cholesterol to specifically traffic amyloid precursor protein (APP) into lipid rafts where it interacts with β- and γ-secretases to generate Aβ-peptide. We find that targeted deletion of astrocyte cholesterol synthesis robustly reduces amyloid burden in a mouse model of AD. Treatment with cholesterol-free apoE or knockdown of cholesterol synthesis in astrocytes decreases cholesterol levels in cultured neurons and causes APP to traffic out of lipid rafts where it interacts with α-secretase and gives rise to soluble APPα (sAPPα), a neuronal protective product of APP. Changes in cellular cholesterol have no effect on α-, β-, and γ-secretase trafficking, suggesting the ratio of Aβ to sAPPα is regulated by the trafficking of the substrate, not the enzymes. Treatment of astrocytes with inflammatory cytokines IL-1β, IL-6 and TNF-α upregulates the synthesis of cholesterol in the astrocytes. We conclude that cholesterol is a signaling molecule kept low in neurons to inhibit raft function, decrease Aβ formation, and enable astrocyte regulation of APP by cholesterol control of substrate presentation.