Regulation of beta-amyloid production in neurons by astrocyte-derived cholesterol

Significance

The accumulation of amyloid β (Aβ) in the brain appears to be a necessary event in the pathogenesis of Alzheimer’s disease (AD). However, processes linked to the endogenous regulation of Aβ production are still not completely understood. Here, the authors show that Aβ accumulation in neurons is tightly regulated by cholesterol synthesis and apoE transport from astrocytes. The study provides a molecular context for understanding the endogenous regulation of Aβ accumulation and why it correlates with AD. The tight regulation suggests that Aβ may perform an important cellular function. A complete understanding of the mechanism is likely necessary to predict whether the selective removal of Aβ has potential for a therapeutic benefit.

Abstract

Alzheimer’s disease (AD) is characterized by the presence of amyloid β (Aβ) plaques, tau tangles, inflammation, and loss of cognitive function. Genetic variation in a cholesterol transport protein, apolipoprotein E (apoE), is the most common genetic risk factor for sporadic AD. In vitro evidence suggests that apoE links to Aβ production through nanoscale lipid compartments (lipid clusters), but its regulation in vivo is unclear. Here, we use superresolution imaging in the mouse brain to show that apoE utilizes astrocyte-derived cholesterol to specifically traffic neuronal amyloid precursor protein (APP) in and out of lipid clusters, where it interacts with β- and γ-secretases to generate Aβ-peptide. We find that the targeted deletion of astrocyte cholesterol synthesis robustly reduces amyloid and tau 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 clusters, where it interacts with α-secretase and gives rise to soluble APP-α (sAPP-α), a neuronal protective product of APP. Changes in cellular cholesterol have no effect on α-, β-, and γ-secretase trafficking, suggesting that the ratio of Aβ to sAPP-α is regulated by the trafficking of the substrate, not the enzymes. We conclude that cholesterol is kept low in neurons, which inhibits Aβ accumulation and enables the astrocyte regulation of Aβ accumulation by cholesterol signaling.