Clioquinol Mediates Copper Uptake and Counteracts Copper Efflux Activities of the Amyloid Precursor Protein of Alzheimer's Disease

Abstract

The key protein in Alzheimer's disease, the amyloid precursor protein (APP), is a ubiquitously expressed copper-binding glycoprotein that gives rise to the A␤ amyloid peptide. Whereas overexpression of APP results in significantly reduced brain copper levels in three different lines of transgenic mice, knock-out animals revealed increased copper levels. A provoked rise in peripheral levels of copper reduced concentrations of soluble amyloid peptides and resulted in fewer pathogenic A␤ plaques. Contradictory evidence has been provided by the efficacy of copper chelation treatment with the drug clioquinol. Using a yeast model system, we show that adding clioquinol to the yeast culture medium drastically increased the intracellular copper concentration but there was no significant effect observed on zinc levels. This finding suggests that clioquinol can act therapeutically by changing the distribution of copper or facilitating copper uptake rather than by decreasing copper levels. The overexpression of the human APP or APLP2 extracellular domains but not the extracellular domain of APLP1 decreased intracellular copper levels. The expression of a mutant APP deficient for copper binding increased intracellular copper levels severalfold. These data uncover a novel biological function for APP and APLP2 in copper efflux and provide a new conceptual framework for the formerly diverging theories of copper supplementation and chelation in the treatment of Alzheimer's disease.The amyloid precursor protein (APP) 1 of Alzheimer's disease (AD) is a ubiquitously expressed copper-binding glycoprotein that gives rise to the A␤ amyloid peptide (1). The APP paralogs and orthologs demonstrate a significant evolutionary change in the function of the N-terminal copper-binding domain (CuBD). In higher species, CuBD has a gain-in activity toward promoting Cu(II) reduction (2, 3).Copper deprivation strongly down-regulates APP gene expression and reduces APP levels (4). Animal model systems revealed that APP is actively involved in balancing copper concentrations in vivo. Whereas overexpression of APP results in significantly reduced brain copper levels in three different lines of transgenic mice, APP23, Tg2576, and TgCRND8 (5-7), conversely, APP and APLP2 knock-out mouse revealed increased copper levels in cerebral cortex and liver (8). Whereas the relevance of a disturbed metal ion homeostasis in Alzheimer's disease is presently unclear (9 -11), studies in two different transgenic mouse models convincingly demonstrate that a rise in peripheral copper levels reduced soluble A␤ peptide levels (5) and the number of A␤ plaques (7). In a mammalian cell line, increasing intracellular copper levels attenuated APP processing into A␤ and stimulated secreted APP levels (5, 12). This finding demonstrates that copper-binding to APP alters the turnover of brain APP and concomitantly offers an alternative therapeutic intervention. Based on earlier observations that copper and chelators have an effect on amyloid fibril formation (13), the Cu,Zn chelat...

Toxicity of Alzheimer's disease-associated Aβ peptide is ameliorated in a Drosophila model by tight control of zinc and copper availability

“…A phase III clinical trial termed PBT2 (see http:// www.alzforum.org/drg/drc/detail.asp?ids110) is under way. However, even if clioquinol turns out to counteract AD, it might not act by merely scavenging copper, but rather to function as a copper carrier to effectively increase intracellular copper levels, as was shown in a yeast model (Treiber et al, 2004). Thus, while elevated zinc levels promote Ab aggregation and have adverse effects in vivo, the role of copper remains to be elucidated.…”

Biometals and Their Therapeutic Implications in Alzheimer's Disease

“…Clioquinol has a moderate affinity for iron, copper, and zinc (Kd Cu is 1.2×10 -10 M, Kd Zn is 7×10 -8 M) [197], and for this reason it was explored as a drug for AD. Although it was initially considered a chelator [198][199][200][201], it has more recently been characterized as a copper/zinc ionophore, which functions to redistribute these metals into cells [196,[202][203][204][205][206]. Clioquinol is still considered a moderate iron chelator as it has been shown to lower iron levels in animal models of iron overload [64,148,[207][208][209][210], and has not been shown to redistribute iron into cells using ionophore assays.…”

Oxidative Stress and Neurotoxicity

“…However, studies to ascertain whether chelators might function in this regard revealed a complex interplay of effects on metal ion homeostasis. Thus, in the case of clioquinol, a drug that underwent initial clinical trials for treatment of AD, benefits appear to arise from counteracting the intracellular copper-depleting effects of A PP (157).…”