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...

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