ABSTRACT
Based on the evidence that one of the endogenous foci for Aβ aggregation, GM1 ganglioside-bound Aβ, is generated in a cholesterol-dependent manner, we attempted to examine the possibility that concentration and/or distribution of cholesterol in neuronal membranes may be altered by risk factors for the development of AD. We performed a lipid chemical analysis of human apoE knock-in mice (apoE3 and apoE4). In this experiment, we isolated subcellular fractions, including smooth and rough endoplasmic reticulum, and plasma membrane fractions.14 Synaptic plasma membranes (SPM) were also isolated from mouse brains. From these materials, we
determined the levels of lipids, including phospholipids and cholesterol (free and esterified). We also determined the transbilayer distribution of cholesterol in SPM by fluorescence-quenching assay using trinitrobenzene-sulfonic acid (TNBS) and dehydroergosterol (DHE). There were no significant differences in the levels of phospholipids and cholesterol in these fractions, including SPM, between wild-type, apoE3 knock-in, and apoE4 knock-in mice. However, in apoE4 knock-in mice, there was a significant increase and a concomitant decrease in cholesterol level in the exofacial and cytofacial leaflets of SPM, respectively.14 These results suggest that apoE modulates the transbilayer distribution of cholesterol in the SPM in an isoform-dependent manner, leading to an increase in cholesterol level in the exofacial leaflet of SPM. Together with the results of our previous study, the increase in cholesterol level of the neuronal surface is likely to induce the formation of a GM1 ganglioside cluster,13 which may be a receptor for soluble Aβ.
