The galvanization of -amyloid in Alzheimer’s disease

W a decade ago thoughts of metals and Alzheimer’s disease (AD) conjured up thoughts of tossing out your aluminum cookware, more recently, zinc, copper, and iron have been implicated in AD pathology. These metals are not derived from your saucepan or deodorant, but are already resident in the brain. Zinc is not a trace metal in the brain. In fact, zinc, copper, and iron concentrations in gray matter are in the same order of magnitude as magnesium (0.1– 0.5 mM; refs. 1 and 2) and their participation in major neurological diseases is being increasingly appreciated (3). The argument for exploiting the interaction between -amyloid (A ), and cortical zinc and copper, in designing novel therapies for AD has gathered considerable momentum over the last 5 years. This notion was originally prompted by the finding that the precipitation and redox activity of A are modulated by copper, iron, and zinc (4–11). In this issue of PNAS, Lee et al. (12) report the marked decrease in A deposition in the brains of Tg2576 mice lacking the synaptic ZnT3 zinc transporter. These findings provide in vivo evidence that the characteristic amyloid neuropathology of AD is principally caused by zinc released during neurotransmission. These data will likely have a significant impact on the development of drugs aimed at attenuating amyloid pathology underlying AD neurodegeneration (13). The zinc model for AD (Fig. 1) that emerges from these and other findings is more complex than the widely held A autoaggregation model. However, it is also more satisfying because it can explain mysteries such as why A deposits exclusively in the brain, why women more frequently develop Alzheimer’s disease, and why rats and mice do not. AD is the most prevalent of agedependent neurodegenerative disorders, and the most common cause of dementia, affecting about 10% of people over the age of 60, or over 4 million Americans. With the graying of society, it is becoming increasingly more urgent to find a cure. Current therapeutics aim at enhancing neurotransmitter systems, and do not address the underlying etiology, which remains uncertain. Because A is implicated in the pathogenesis of AD, emerging therapeutic approaches have targeted the inhibition of A production [e.g., protease (secretase) inhibitors that inhibit the generation of A from the amyloid precursor protein, APP], or the enhancement of A clearance (e.g., the A ‘‘vaccine’’) (14). These approaches simplistically assume that A precipitation in the brain only requires elevated levels of A . However, neurochemical reactions apart from A production also contribute to amyloid deposition in AD. The zinc model can explain why -amyloid deposits are limited to the neocortex even though A is ubiquitously produced in the brain. At the histological level, the deposits are focal (related to synapses, and the cerebrovascular lamina media), implicating a unique chemical interaction in these microregions that causes A to precipitate. The extracellular concentration of zinc, driven up to 300 M during synaptic transmission, is likely to be far higher in this space than in any other extracellular compartment in the body (15). A biochemical link between the metabolisms of APP and zinc was first recognized when APP copurified with a zincmodulated proteolytic complex in human plasma (16). This then led to the identification of a zinc binding site on the cysteine-rich ectodomain of APP, a feature that is conserved in all members of the APP superfamily (17, 18). Importantly, the highest concentrations of synaptic zinc occur in areas of the neocortex that are most prone to A deposition (19). The seminal discovery in 1992 that A is a soluble component of biological f luids (20) prompted studies aimed at determining whether Zn2 could influence the solubility and metabolism of A . In 1994, it was reported that Zn2 , at physiologically plausible concentrations, rapidly precipitated soluble A 1–40 into proteaseresistant, amyloid-like aggregates in vitro (4, 5). The histidine at residue 13 plays a critical role in Zn2 -mediated aggregation (21). Intermolecular His(N )–Zn2 – His(N ) bridges form (22) as part of a structure that is strikingly similar to superoxide dismutase 1 (23). In contrast, rat mouse A 1–40 [with substitutions of Arg3 Gly, Tyr3 Phe, and His3 Arg at positions 5, 10, and 13, respectively (24), therefore lacking the bridging histidine] is not precipitated by Zn2 at physiological concentrations (5), which could explain why mice and rats do not deposit cerebral A amyloid (25). Although Zn2 is the only physiologically available metal ion to precipitate A at pH 7.4 (5, 8), Cu2 (and Fe3 , which has much lower affinity) can induce limited A aggregation, which is exaggerated by slightly acidic conditions (8). Importantly, A generates cytotoxic H2O2 through the reduction of Cu2 and Fe3 by using O2 as a substrate (9, 10), perhaps explaining the overwhelming H2O2-mediated damage to the neocortex in AD and in Tg2576 mice (11, 26). A can bind Zn2 and Cu2 simultaneously through selective binding sites; however, the affinity for Cu2 is much greater (attomolar for A 1–42) (27). Co-incubation with Zn2 (which is redox inert) partially inhibits Cu2 mediated H2O2 production and A toxicity (11, 28). Therefore, the high concentrations of Zn2 in plaques ( 1 mM; ref. 1) could explain the inverse correlation between oxidation (8-OH guanosine) levels in AD-affected tissue and histological amyloid burden (11). Nevertheless, oxidative adducts are still abnormally elevated even in the AD cases where plaque burden is heaviest (11), indicating that the Zn2 quenching of H2O2 is not sufficient to abolish damage. In this case, neurodegeneration is likely to be mediated by soluble (possibly Cu or Fe bound) forms of A (29–31) dissociating from the amyloid mass (Fig. 1). Unlike the case in AD, plaque formation in Tg2576 mice posi-