In order to investigate the influence of cholesterol (Ch) and monosialoganglioside (GM1) on the release and subsequent deposition/aggregation of amyloid ␤ peptide (A␤)-(1-40) and A␤-(1-42), we have examined A␤ peptide model membrane interactions by circular dichroism, turbidity measurements, and transmission electron microscopy (TEM). Model liposomes containing A␤ peptide and a lipid mixture composition similar to that found in the cerebral cortex membranes (CCM-lipid) have been prepared. In all, four A␤-containing liposomes were investigated: CCM-lipid; liposomes with no GM1 (GM1-free lipid); those with no cholesterol (Ch-free lipid); liposomes with neither cholesterol nor GM1 (Ch-GM1-free lipid). In CCM liposomes, A␤ was rapidly released from membranes to form a well defined fibril structure. However, for the GM1-free lipid, A␤ was first released to yield a fibril structure about the membrane surface, then the membrane became disrupted resulting in the formation of small vesicles. In Ch-free lipid, a fibril structure with a phospholipid membrane-like shadow formed, but this differed from the well defined fibril structure seen for CCM-lipid. In Ch-GM1-free lipid, no fibril structure formed, possibly because of membrane solubilization by A␤. The absence of fibril structure was noted at physiological extracellular pH (7.4) and also at liposomal/endosomal pH (5.5). Our results suggest a possible role for both Ch and GM1 in the membrane release of A␤ from brain lipid bilayers. The pathology of Alzheimer's disease (AD)1 includes extracellular amyloid plaques, intraneuronal neurofibrillary tangles, synaptic loss, and neuronal cell death. The major components of amyloid plaques are the amphiphilic 40 and 42 residue peptides, A␤-(1-40) and A␤-(1-42) (1, 2). Amyloid ␤-peptide (A␤) consists of a hydrophilic N-terminal region (residues 1-28) and a hydrophobic C-terminal region (residues 29 -40 or 29 -42). The hydrophobic part of A␤ is originally part of a transmembrane ␣-helix of APP anchored in the membrane of several subcellular compartments, including the ER (3). Proteolysis by the enzyme(s) ␥-secretase leads to the formation of A␤ within the membrane. Thus, the membrane release of A␤ following this enzyme cleavage should play a pivotal role in subsequent amyloid plaque formation.Recent studies have shown that the interaction of A␤ and lipids plays an important role in the pathogenesis of AD. For instance, the fibrillogenic properties of A␤ are in part a consequence of the composition of the membrane in which it resides, its peptide sequence, and its mode of assembly within the membrane (4). In terms of membrane composition, Ch and GM1 in neuronal cell membranes are widely accepted to be modulators of membrane-associated A␤ fibrillogenesis and neurotoxicity (5, 6). The formation of GM1-bound A␤, which is thought to be a seed for the formation of toxic amyloid fiber, depends on the concentration of Ch in model membranes prepared from GM1/Ch/sphingomyelin (SM) (7). Additionally, oligomeric A␤ can promote the release of l...