Membrane-mediated Amyloidogenesis and the Promotion of Oxidative Lipid Damage by Amyloid β Proteins

Abstract

Evidence of oxidative stress and the accumulation of fibrillar amyloid ␤ proteins (A␤) in senile plaques throughout the cerebral cortex are consistent features in the pathology of Alzheimer disease. To define a mechanistic link between these two processes, various aspects of the relationship between oxidative lipid membrane damage and amyloidogenesis were characterized by chemical and physical techniques. Earlier studies of this relationship demonstrated that oxidatively damaged synthetic lipid membranes promoted amyloidogenesis. The studies reported herein specify that 4-hydroxy-2-nonenal (HNE) is produced in both synthetic lipids and human brain lipid extracts by oxidative lipid damage and that it can account for accelerated amyloidogenesis. A␤ promotes the copper-mediated generation of HNE from polyunsaturated lipids, and in turn, HNE covalently modifies the histidine side chains of A␤. HNE-modified A␤ have an increased affinity for lipid membranes and an increased tendency to aggregate into amyloid fibrils. Thus, the prooxidant activity of A␤ leads to its own covalent modification and to accelerated amyloidogenesis. These results illustrate how lipid membranes may be involved in templating the pathological misfolding of A␤, and they suggest a possible chemical mechanism linking oxidative stress with amyloid formation. Alzheimer disease (AD)3 is an age-related neurodegenerative disorder characterized by misfolded and aggregated fibrillar amyloid ␤ proteins (A␤) in the brain. Among the factors associated with the pathogenesis of AD, oxidative stress is one of the most closely scrutinized (1, 2). It has been shown, for example, that the brain in AD has increased susceptibility to oxidative stress (3-5) and that isoprostanes, markers of oxidative stress, are specifically elevated (6 -8). Isoprostanes are chemically stable and nonreactive compounds that arise nonenzymatically from the spontaneous decomposition of lipid hydroperoxides. These hydroperoxides may decompose along other pathways, however, yielding highly reactive short chain alkenals such as 4-oxo-2-nonenal and 4-hydroxy-2-nonenal (HNE) (9 -17). HNE concentrations in human ventricular fluid are 8 -15 M and elevated in AD (3,18,19). HNE has a well known propensity to react with the side chains of various amino acid residues, and HNE-protein adducts have been used as biomarkers of oxidative stress (15).In light of these observations, it is noteworthy that lipid oxidation products such as HNE modify A␤ and increase A␤ misfolding (20 -25). Moreover, the immunoreactivity of antibodies to HNE-modified His residues localizes to amyloid plaques (26,27). This suggests that not only does A␤ promote lipid oxidation but that there may also be a mechanistic link between the lipid oxidation products formed during oxidative stress and A␤ misfolding (21). Conversely, several lines of evidence suggest that A␤ contribute to oxidative stress. For example, the overexpression of A␤ in transgenic mice, in Caenorhabditis elegans, and in cell culture results in an increase in...

Site-specific modification of Alzheimer's peptides by cholesterol oxidation products enhances aggregation energetics and neurotoxicity

“…1A), derived from cholesterol, and the aldol product of the latter (see 2 in Fig. 1 A) (24), covalently modify A␤ and increase its amyloidogenicity (16)(17)(18)(25)(26)(27). Prior data indicated that cholesterol oxidation products 1 and 2, denoted 1(2) hereafter because they interconvert via an aldol/retro-aldol reaction (16,18,24), react with A␤ via Schiff base formation at the N-terminal ␣-amine of Asp-1 (D1) and the side-chain -amines of Lys-16 (K16) and Lys-28 (K28) (18).…”

The covalent modification of spectrin in red cell membranes by the lipid peroxidation product 4-hydroxy-2-nonenal

“…Various diseases involve proteins modified by HNE and protein cross-linking through HNE adducts contributes to disease pathogenecities [14,[16][17][18]. Here, we showed that spectrins are the primary targets of HNE, and that HNE-modified spectrins generate irreversibly cross-linked aggregates in human RBCs.…”

“…HNE reportedly modifies the three His residues of amyloid β protein (Aβ). These HNE-modified Aβ molecules have an increased affinity for membrane lipids and possess a conformation similar to that of mature amyloid fibrils [16,25]. The observed enrichment of HNE-His adducts (Fig.…”

“…Therefore, in the absence or under reduced concentrations of MgATP, greater HNE adduct formation may occur in spectrins that are dissociated from the lipid bilayer and have increased susceptibility to the amphiphilic HNE. Then, local aggregation of HNE-modified spectrins is inevitable, since these spectrins contain Cys-, His-, and Lys-Michael adducts and Schiff bases on the Lys residues, that are required for intra-or intermolecular cross-linking [10,12,13,16,17]. Local spectrin aggregation may free the lipid bilayer from underlying spectrin-actin skeletons, possibly leading to membrane surface area loss by extrusion, which occurs in RBCs under oxidative conditions [8,9] and various RBC membrane disorders [4,7].…”

“…HNE is a highly reactive electrophile that reacts with the side chains of various amino acid residues, including Cys, His, and Lys, to form Michael adducts [10,13]. Protein modification by HNE causes serious detrimental effects in the cell, due to the resulting functional defects and cross-linking of proteins [14][15][16][17][18]. HNE also accumulates in RBCs [19], leading to the covalent modification of membrane proteins [20].…”