Platelet-activating factor antagonists enhance intracellular degradation of amyloid-β₄₂ in neurons via regulation of cholesterol ester hydrolases

The progressive dementia that is characteristic of Alzheimer’s disease is associated with the accumulation of amyloid-beta (Aβ) peptides in extracellular plaques and within neurons. Aβ peptides are targeted to cholesterol-rich membrane micro-domains called lipid rafts. Observations that many raft proteins undertake recycling pathways that avoid the lysosomes suggest that the accumulation of Aβ in neurons may be related to Aβ targeting lipid rafts. Here we tested the hypothesis that the degradation of Aβ by neurons could be increased by drugs affecting raft formation.

Primary neurons were incubated with soluble Aβ preparations. The amounts of Aβ₄₂ in neurons or specific cellular compartments were measured by enzyme-linked immunosorbent assay. The effects of drugs on the degradation of Aβ₄₂ were studied.

Aβ₄₂ was targeted to detergent-resistant, low-density membranes (lipid rafts), trafficked via a pathway that avoided the lysosomes, and was slowly degraded by neurons (half-life was greater than 5 days). The metabolism of Aβ₄₂ was sensitive to pharmacological manipulation. In neurons treated with the cholesterol synthesis inhibitor squalestatin, less Aβ₄₂ was found within rafts, greater amounts of Aβ₄₂ were found in lysosomes, and the half-life of Aβ₄₂ was reduced to less than 24 hours. Treatment with phospholipase A₂ inhibitors or platelet-activating factor (PAF) antagonists had the same effects on Aβ₄₂ metabolism in neurons as squalestatin. PAF receptors were concentrated in the endoplasmic reticulum (ER) along with enzymes that constitute the cholesterol ester cycle. The addition of PAF to ER membranes triggered activation of cholesterol ester hydrolases and the release of cholesterol from stores of cholesterol esters. An inhibitor of cholesterol ester hydrolases (diethylumbelliferyl phosphate) also increased the degradation of Aβ₄₂ in neurons.

We conclude that the targeting of Aβ₄₂ to rafts in normal cells is a factor that affects its degradation. Critically, pharmacological manipulation of neurons can significantly increase Aβ₄₂ degradation. These results are consistent with the hypothesis that the Aβ-induced production of PAF controls a cholesterol-sensitive pathway that affects the cellular localization and hence the fate of Aβ₄₂ in neurons.