Increasing Age Alters Transbilayer Fluidity and Cholesterol Asymmetry in Synaptic Plasma Membranes of Mice

Previous studies examining age differences in membrane fluidity and cholesterol content have reported on the average or total change in membrane structure, respectively. However, a membrane consists of an exofacial leaflet and a cytofacial leaflet that differ in fluidity and cholesterol distribution. The purpose of the present experiments was to determine fluidity and cholesterol distribution of the exofacial and cytofacial leaflets of brain synaptic plasma membranes (SPMs) from 3-4-, 14-15-, and 24-25-month old C57BL/6NNIA mice by using trinitrobenzenesulfonic acid (TNBS)-quenching techniques and fluorescent probes. The exofacial leaflet of SPMs from young mice was significantly more fluid compared with the cytofacial leaflet. The large difference in fluidity between the two leaflets was abolished in SPMs of the oldest age group. Total SPM cholesterol and the cholesterol-to-phospholipid molar ratio did not differ among the three different age groups of mice. However, considerable differences were observed in the distribution of cholesterol in the two SPM leaflets. The exofacial leaflet contained substantially less cholesterol than did the cytofacial leaflet (13 vs. 87%, respectively) in SPMs of young mice. This asymmetric distribution of cholesterol was significantly modified with increasing age. There was an approximately twofold increase in exofacial leaflet cholesterol in the oldest group compared with the youngest age group. Transbilayer fluidity and cholesterol asymmetry were altered in SPMs of older mice. This approach is a new and different way of viewing how aging modifies membrane structure. Age differences in SPM leaflet structure may be an important factor regulating activity of certain membrane proteins.