Amyloid ß-peptide (Aß) aggregates are one of the key neuropathological characteristics of Alzheimer's disease. Aß belongs to a group of proteins that aggregate and form ß-sheets, and some of these proteins bind cholesterol and other lipids.

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. Key Words: Fluidity-Cholesterol-Brain-Synaptic plasma membrane-Lipid asymmetry-Aging-Mice.

The effects of soluble and aggregated amyloid ␤-peptide (A␤) on cortical synaptic plasma membrane (SPM) structure were examined using small angle x-ray diffraction and fluorescence spectroscopy approaches. Electron density profiles generated from the x-ray diffraction data demonstrated that soluble and aggregated A␤ 1-40 peptides associated with distinct regions of the SPM. The width of the SPM samples, including surface hydration, was 84 Å at 10°C. Following addition of soluble A␤ 1-40 , there was a broad increase in electron density in the SPM hydrocarbon core ؎0 -15 Å from the membrane center, and a reduction in hydrocarbon core width by 6 Å. By contrast, aggregated A␤ 1-40 contributed electron density to the phospholipid headgroup/hydrated surface of the SPM ؎24 -37 Å from the membrane center, concomitant with an increase in molecular volume in the hydrocarbon core. The SPM interactions observed for A␤ 1-40 were reproduced in a brain lipid membrane system. In contrast to A␤ 1-40 , aggregated A␤ 1-42 intercalated into the lipid bilayer hydrocarbon core ؎0 -12 Å from the membrane center. Fluorescence experiments showed that both soluble and aggregated A␤ 1-40 significantly increased SPM bulk and protein annular fluidity. Physico-chemical interactions of A␤ with the neuronal membrane may contribute to mechanisms of neurotoxicity, independent of specific receptor binding. Alzheimer's disease (AD)1 is a progressive neurodegenerative disorder characterized by the accumulation of neuritic plaques composed of amyloid ␤-peptide (A␤) variants, extracellular matrix components, and apolipoproteins (1, 2). A␤ is an amphipathic, 39 -42-residue peptide that is derived by proteolytic cleavage of the transmembrane glycoprotein, the amyloid precursor protein; the A␤ domain is composed of 28 extracellular and 12-14 transmembrane amino acid residues of amyloid precursor protein (3). An increase in the production and abnormal accumulation of A␤ in the brain has been implicated in the etiology of AD. Several studies have shown that A␤ analogs can directly disrupt neuronal function, contributing to cell death associated with the development of AD (4 -8).It has been postulated that the biological activity of A␤ is related to its ability to form insoluble aggregates in solution (9 -11), although the cellular mechanism of action is not well understood. A recent study from Cotman and co-workers (12) showed that A␤ neurotoxicity is independent of stereoisomerspecific ligand-receptor interaction because both all-D-and all-L-stereoisomers of A␤ [25][26][27][28][29][30][31][32][33][34][35] and A␤ 1-40 had similar neurotoxic activity. This finding suggests that A␤ modulates membrane function by a nonreceptor-mediated mechanism, potentially as a result of altering the physico-chemical properties of membrane constituents, including lipids and proteins (13-16). Indeed, previous membrane equilibrium binding experiments have demonstrated that the A␤ 25-35 fragment is highly lipophilic (K P Ͼ 10 2 ); the peptide intercalates deep into the membra...