Cerebrospinal fluid CEFA composition is enriched in saturated fatty acids and it is altered in Alzheimer's disease

Cholesterol esterification is a fundamental step in cholesterol metabolism and transport, and in humans, it is operated by three enzymes. Lecithin:cholesterol acyltransferase (LCAT) is responsible of cholesterol esterification in plasma and other biological fluids including cerebrospinal fluid (CSF), where it is mainly activated by apolipoprotein E. Esterification of cholesterol within cells is instead operated by sterol O-1 and O-2 acyltransferases (SOAT1 and SOAT2). SOAT1 is expressed in all cell types, while SOAT2 is expressed in hepatocytes and enterocytes, where it produces cholesteryl esters (CEs) to be assembled within VLDL and chylomicrons. LCAT and SOAT1/2 have different substrate specificity; LCAT has a preference for the unsaturated fatty acids, while the SOAT enzymes prefer the saturated (SFAs) and monounsaturated fatty acids (MUFAs). Here, we show that CSF CEs have a different composition compared to plasma CEs and specifically are more enriched in SFAs and MUFAs, typical substrates of the SOAT2 enzyme, and less frequently used by LCAT. Protein and RNA analysis in astrocytes, the main lipoprotein-producing cells in the central nervous system, excluded the presence of SOAT2, thus suggesting that CSF CEs are products of the LCAT enzyme. In line with this hypothesis, CSF phosphatidylcholine, the substrate of LCAT, is enriched in SFAs and MUFAs and depleted in polyunsaturated fatty acids. Moreover, we show that in Alzheimer's disease patients, CSF CEs are enriched in SFA, thus adding new insights into our recent observation that LCAT-mediated cholesterol esterification is hampered in Alzheimer's disease. In conclusion, the present findings not only clarify the enzymatic origin of CSF CEs but also open avenues for developing enzyme-specific biomarkers and therapeutic strategies aimed at restoring lipid homeostasis in the brain.