TRANSBILAYER DISTRIBUTION OF CHOLESTEROL IS MODIFIED IN BRAIN SYNAPTIC PLASMA-MEMBRANES OF KNOCKOUT MICE DEFICIENT IN THE LOW-DENSITY-LIPOPROTEIN RECEPTOR, APOLIPOPROTEIN-E, OR BOTH PROTEINS

Both apolipoprotein E (apoE) and the low-density lipoprotein (LDL) rec eptor are present in brain; however, little is known regarding the fun ction of these proteins in brain, in particular with respect to brain cholesterol, The role of apoE and the LDL receptor in modulating the t ransbilayer or asymmetric distribution of cholesterol in the exofacial and cytofacial leaflets of synaptic plasma membranes (SPMs) was exami ned in mutant mice deficient in apoE, the LDL receptor, or both protei ns by using the fluorescent sterol dehydroergosterol and fluorescent q uenching procedures, Fluidity of the exofacial and cytofacial leaflets was also measured. Cholesterol asymmetry of SPMs was altered in the m utant mice, with the largest effect observed in the LDL receptor-defic ient mice. There was an approximately twofold increase in the percent distribution of cholesterol in the exofacial leaflet of the LDL recept or-deficient mice (32%) compared with C57BL/6J mice (15%). Mice defici ent in apoE or both proteins also showed a significantly higher percen t distribution of cholesterol (23 and 26%, respectively) in the exofac ial leaflet compared with the C57BL/6J mice. Although the percent dist ribution of cholesterol was highest in the exofacial leaflet of the LD L receptor-deficient mice, fluidity of the exofacial leaflet of that g roup was significantly lower. However, the cholesterol-to-phospholipid ratio of SPMs of the LDL receptor-deficient mice was significantly lo wer, and this difference was largely the result of a significant incre ase in the total amount of SPM phospholipid. This study demonstrates f or the first time that SPM lipid structure is altered in mice deficien t in apoE or the LDL receptor. Although the mechanism that maintains t he asymmetric distribution of cholesterol in plasma membranes is not w ell understood, data of the present experiments indicate that both apo E and the LDL receptor are involved in maintaining the transbilayer di stribution of cholesterol.