Potential diagnostic applications of side chain oxysterols analysis in plasma and cerebrospinal fluid

The neurospecific cholesterol 24-hydroxylase converts excess brain cholesterol into 24S-hydroxycholesterol (24OHC) which, via the liver X receptor (LXR), can increase the expression and synthesis of astrocyte ApoE. 24OHC effluxes directly from brain into plasma where it is considered an indicator of brain cholesterol turnover. It is reduced in neurodegenerative disease states proportionally to the severity of disease and the degree of brain atrophy. In the early phases of active disease, a higher rate of turnover may result in transitory increases in plasma 24OHC. Less than 1% of the total brain excretion of 24OHC occurs via the cerebrospinal fluid (CSF) whereas almost all 27-hydroxycholesterol (27OHC) excretion is dependent on the function of the blood-cerebrospinal fluid barrier. Iincreased CSF oxysterols were found in patients with neurodegenerative and neuroinflammatory diseases in the presence of barrier dysfunction. In neurodegeneration, free cholesterol released from dying cells may engulf neurons. Cholesterol also increases Amyloid β (Aβ) deposition and tau pathology. ApoE, 24OHC, tau and soluble APP were correlated in Alzheimer disease (AD) samples. Excess of cholesterol converted into 24OHC may up-regulate ApoE synthesis which is a scavenger for Aβ and Tau. In AD this protective mechanism seems to be inefficient, probably due to the presence of high concentrations of 27OHC, microvascular dysfunction and the decreased efficiency of ApoE4 as lipid transporter and Aβ scavenger. 24OHC itself was cytotoxic. Analysis of side chain oxysterols in the CSF is likely to provided useful information about cholesterol metabolism and ApoE function in the pathogenesis of AD.