Paradoxical Condensation of Copper with Elevated β-Amyloid in Lipid Rafts under Cellular Copper Deficiency Conditions

Abstract

Redox-active copper is implicated in the pathogenesis of Alzheimer disease (AD), ␤-amyloid peptide (A␤) aggregation, and amyloid formation. A␤⅐copper complexes have been identified in AD and catalytically oxidize cholesterol and lipid to generate H 2 O 2 and lipid peroxides. The site and mechanism of this abnormality is not known. Growing evidence suggests that amyloidogenic processing of the ␤-amyloid precursor protein (APP) occurs in lipid rafts, membrane microdomains enriched in cholesterol. ␤-and ␥-secretases, and A␤ have been identified in lipid rafts in cultured cells, human and rodent brains, but the role of copper in lipid raft amyloidogenic processing is presently unknown. In this study, we found that copper modulates flotillin-2 association with cholesterol-rich lipid raft domains, and consequently A␤ synthesis is attenuated via copper-mediated inhibition of APP endocytosis. We also found that total cellular copper is associated inversely with lipid raft copper levels, so that under intracellular copper deficiency conditions, A␤⅐copper complexes are more likely to form. This explains the paradoxical hypermetallation of A␤ with copper under tissue copper deficiency conditions in AD.Imbalance of metal ions has been recognized as one of the key factors in the pathogenesis of Alzheimer disease (AD).2 Aberrant interactions between copper or zinc with the ␤-amyloid peptide (A␤) released into the glutamatergic synaptic cleft vicinity could result in the formation of toxic A␤ oligomers and aggregation into plaques characteristic of AD brains (reviewed in Ref. 1). Copper, iron, and zinc are highly concentrated in extracellular plaques (2, 3), and yet brain tissues from AD (4 -6) and human ␤-amyloid precursor protein (APP) transgenic mice (7-10) are paradoxically copper deficient compared with agematched controls. Elevation of intracellular copper levels by genetic, dietary, and pharmacological manipulations in both AD transgenic animal and cell culture models is able to attenuate A␤ production (7,9,(11)(12)(13)(14)(15). However, the underlying mechanism is at present unclear.Abnormal cholesterol metabolism is also a contributing factor in the pathogenesis of AD. Hypercholesterolemia increases the risk of developing AD-like pathology in a transgenic mouse model (16). Epidemiological and animal model studies show that a hypercholesterolemic diet is associated with A␤ accumulation and accelerated cognitive decline, both of which are further aggravated by high dietary copper (17, 18). In contrast, biochemical depletion of cholesterol using statins, inhibitors of 3-hydroxy-3-methyglutaryl coenzyme A reductase, and methyl-␤-cyclodextrin, a cholesterol sequestering agent, inhibit A␤ production in animal and cell culture models (19 -25).Cholesterol is enriched in lipid rafts, membrane microdomains implicated in A␤ generation from APP cleavage by ␤-and ␥-secretases. Recruitment of BACE1 (␤-secretase) into lipid rafts increases the production of sAPP ␤ and A␤ (23, 26). The ␤-secretase-cleaved APP C-terminal fragment (␤-C...

The Amyloid Precursor Protein Copper Binding Domain Histidine Residues 149 and 151 Mediate APP Stability and Metabolism

“…Previous findings showed that copper increases APP cell surface localization (58) and promotes APP trafficking from the Golgi to intracellular compartments and to the cell surface (32). This supports H147N-APP decreased post-Golgi trafficking because His 147 might be necessary for APP copper binding.…”

Copper Promotes the Trafficking of the Amyloid Precursor Protein

“…It has been reported that exposure to copper decreases A␤ levels (31,32,39,71,72). Under conditions used in this study, we did not observe any detectable changes in APP processing.…”

“…Copper-stimulated Relocalization of APP in Cultured Neuronal and Polarized Epithelial Cells-We previously reported that in the human neuroblastoma SH-SY5Y cell line, an increase in copper concentration stimulated an increase in the level of APP at the cell surface (39). In the present study, we further investigated the influence of copper on APP trafficking in both neuronal and polarized epithelial cell models using confocal fluorescence imaging techniques.…”

“…This could be explained by different expression levels of APP in cell models used in different studies. Previous studies used cell models of AD where WT APP or the familial AD mutant, APPsw (APP K670N/M671L), were overexpressed (31,32,39,71,72). Overexpression of APPsw promotes its processing by the amyloidogenic pathway, thus generating more A␤.…”

“…Overexpression of APP in cultured cell and animal models leads to decreased cellular copper levels (35,37,38). We recently reported that copper promotes an in-crease in the level of APP at the cell surface in SH-SY5Y human neuroblastoma cells with a reduction in lipid raft-mediated APP processing (39).…”