[Characterization of sialo-apolipoprotein E in human cerebrospinal fluid]

Apolipoprotein E (apoE) in human cerebrospinal fluid (CSF) is extensively sialylated, and the sialylation of certain proteins is known to modulate their biological function. We investigated the effects of a sialic acid moiety of apoE on the apoE-Abeta interaction using the surface plasmon resonance assay. Further, we established a method for the determination of the sialic acid content of CSF apoE, and carried out a more detailed characterization of CSF apoE-containing lipoproteins. The binding avidity of asialo-apoE3 was approximately 50% of that of sialo-apoE3 (p<0.0001). The binding avidity of the apoE-containing lipoproteins for Abeta reduced after neuraminidase treatment. The sialic acid levels in the CSF apoE-containing lipoprotein fractions were 5.3 +/- 1.3% of the total CSF sialic acid, and were correlated with the CSF apoE concentrations. However, the ratios of sialic acid to apoE were inversely proportional to the CSF lipid concentrations. The lipoprotein particle sizes were larger when the ratios of sialic acid to CSF apoE were greater. The sialic acid moiety of the apoE molecules affects the interaction of apoE with Abeta and the formation of apoE-containing lipoprotein particles. The posttranslational modification of apoE, such as the presence of sialic acid moieties, may be involved in the regulation of lipid transport to the brain.