INCREASING AGE ALTERS TRANSBILAYER FLUIDITY AND CHOLESTEROL ASYMMETRYIN SYNAPTIC PLASMA-MEMBRANES OF MICE

Previous studies examining age differences in membrane fluidity and ch olesterol content have reported on the average or total change in memb rane structure, respectively. However, a membrane consists of an exofa cial leaflet and a cytofacial leaflet that differ in fluidity and chol esterol distribution. The purpose of the present experiments was to de termine fluidity and cholesterol distribution of the exofacial and cyt ofacial leaflets of brain synaptic plasma membranes (SPMs) from 3-4-, 14-15-, and 24-25-month-old C57BL/6NNIA mice by using trinitrobenzenes ulfonic acid (TNBS)-quenching techniques and fluorescent probes. The e xofacial 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 grou p. Total SPM cholesterol and the cholesterol-to-phospholipid molar rat io did not differ among the three different age groups of mice. Howeve r, considerable differences were observed in the distribution of chole sterol in the two SPM leaflets, The exofacial leaflet contained substa ntially less cholesterol than did the cytofacial leaflet (13 vs, 87%, respectively) in SPMs of young mice. This asymmetric distribution of c holesterol was significantly modified with increasing age. There was a n approximately twofold increase in exofacial leaflet cholesterol in t he oldest group compared with the youngest age group. Transbilayer flu idity and cholesterol asymmetry were altered in SPMs of older mice. Th is approach is a new and different way of viewing how aging modifies m embrane structure. Age differences in SPM leaflet structure may be an important factor regulating activity of certain membrane proteins.