Distribution and Fluidizing Action of Soluble and Aggregated Amyloid β-Peptide in Rat Synaptic Plasma Membranes

Abstract

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...

Cellular Membrane Composition Defines Aβ-Lipid Interactions

“…Previous studies (15,16) have demonstrated accumulation of A␤42 in lysosomal compartments that results in membrane damage as shown by release of lysosomal hydrolases and the lysosomal specific dye acridine orange. Furthermore, A␤-synaptic plasma membrane interactions demonstrate that A␤ has a fluidizing effect on membrane structure as a result of A␤ insertion into the fatty acyl chain region of the bilayer (17). The role of proteins associated with synaptic plasma membranes could not be distinguished from A␤-lipid interactions alone in this study.…”

“…that addition of A␤ to membranes isolated from cerebellum, cortex, hippocampus, and striatum or synthetic lipid vesicles results in a decrease in membrane fluidity. In contrast, Mason et al (17) reported that A␤, both random and aggregated, increased synaptic plasma membrane fluidity by insertion of random A␤ into the fatty acyl chain and the presence of aggregated A␤ at the lipid head groups. Our results are in partial agreement with these results, as we also propose that A␤ exerts its bilayer effects by inserting into the fatty acyl chains, but differ in that our results demonstrate a rigidizing effect.…”

Permeabilization of Lipid Bilayers Is a Common Conformation-dependent Activity of Soluble Amyloid Oligomers in Protein Misfolding Diseases

“…The different effects of low molecular weight A␤ and oligomeric A␤ in increasing membrane conductance is correlated with the different effects on membrane structure that have been reported as determined by small angle x-ray diffraction (17). Monomeric or low molecular weight A␤ produced a broad increase in electron density in the center of the bilayer accompanied by a reduction in the width of the bilayer from 51 to 46 Å.…”

Induction of the Cholesterol Transporter ABCA1 in Central Nervous System Cells by Liver X Receptor Agonists Increases Secreted Aβ Levels

“…Indeed, A␤ cleaved from APP is found in plasma membranes (50). We reasoned that soluble A␤ levels are not only dependent on A␤ production and A␤ degradation but also on the transfer of A␤ between intracellular and extracellular compartments.…”