Cholesterol Is an Important Factor Affecting the Membrane Insertion of β-Amyloid Peptide (Aβ1–40), Which May Potentially Inhibit the Fibril Formation

Abstract

␤-Amyloid peptide (A␤), a normal constituent of neuronal and non-neuronal cells, has been proven to be the major component of extracellular plaque of Alzheimer's disease. Interactions between A␤ and neuronal membranes have been postulated to play an important role in the neuropathology of Alzheimer's disease. Here we show that A␤ is able to insert into lipid bilayer. The membrane insertion ability of A␤ is critically controlled by the ratio of cholesterol to phospholipids. In a low concentration of cholesterol A␤ prefers to stay in membrane surface region mainly in a ␤-sheet structure. In contrast, as the ratio of cholesterol to phospholipids rises above 30 mol%, A␤ can insert spontaneously into lipid bilayer by its C terminus. During membrane insertion A␤ generates about 60% ␣-helix and removes almost all ␤-sheet structure. Fibril formation experiments show that such membrane insertion can reduce fibril formation. Our findings reveal a possible pathway by which A␤ prevents itself from aggregation and fibril formation by membrane insertion.The formation of extracellular amyloid plaques is one of the characteristics of Alzheimer's disease. The core component of plaque is A␤, 1 which is the proteolytic product of the larger transmembrane amyloid precursor protein (APP) (1, 2). A␤ contains 39 -42 amino acid residues with a molecular mass of approximate 4 kDa. A␤ is an amphiphilic peptide with a hydrophilic N-terminal domain (residues 1-28) and a hydrophobic C-terminal (residues 29 -40 (42)), the latter corresponding to a part of the transmembrane domain of APP.A␤ is a normal constituent of neuronal and non-neuronal cells (3, 4). It can be detected in cerebrospinal fluid at subnanomolar concentrations in normal individuals. Such a concentration of A␤ has its own physiological functions, for example, increasing tyrosine phosphorylation, increasing the activity of phosphoinositol 3-kinase, and inducing the rapid change of cellular calcium and extracellular protein kinase C (5). It was reported that in cultured hippocampal neurons, A␤ at a low concentration (10 Ϫ11 ϳ10 Ϫ10 M) is neurotrophic to undifferentiated, immature hippocampal neurons (6, 7). As a proteolytic fragment of APP, A␤ can be secreted by membrane-anchored APP or by reinternalized APP (8 -10). Also, A␤ can be degraded either via LRP-mediated endocytosis into primary neurons and astrocytes (11-15) or via scavenger receptor-mediated uptake of aggregates of A␤ into microglial cells (16). The hydrolytic enzymes in lysosomes then can degrade A␤. Thus, whether A␤ is the primary effector of the disease is questioned.Several studies on conformation show that A␤ in the core of amyloid plaques adopts an antiparallel ␤-sheet (17). So A␤ in the form of ␤-sheet may be in favor of aggregating into fibril. In addition, in vitro studies with cell cultures have demonstrated that fibrillar A␤ is toxic to neurons, but monomeric A␤ is not (18 -20). Therefore, the factors inducing A␤ to generate ␤-sheet may contribute to the pathogenesis of Alzheimer's disease.The neurotoxic...

Conformational transition of amyloid β-peptide

“…5b). In fact, the concentration of cholesterol in the cytofacial leaflet of brain synaptic plasma membrane is lower in aged people (60), and A␤ cannot insert into the membrane containing less cholesterol (61). Once A␤ exits the membrane and enters into the extracellular solution, the segment of residues 29-40 has a high tendency to form short ␤-sheets (Figs.…”

The Yeast Prion Ure2p Native-like Assemblies Are Toxic to Mammalian Cells Regardless of Their Aggregation State

“…Indeed, it has been repeatedly reported that anionic phospholipids, such as phosphatidylserine, which becomes exposed in the outer membrane leaflet in apoptotic and tumoral cells, are putative docking sites for amyloid aggregates or can favor aggregate formation, thus initiating membrane impairment (38,50,51). Membrane cholesterol modulates membrane fluidity and may affect the incorporation of toxic aggregates into exposed cells (24,52) as well as membrane-associated Abeta fibrillogenesis and toxicity in neuronal cells (53,54).…”

Membrane Localization of β-Amyloid 1–42 in Lysosomes

“…Experiments were conducted as described in our previous work (37). Lipids were dissolved in a solvent of chloroform/methanol (7:1 v/v, 1.0 mg/ml).…”