The Involvement of zinc in Alzheimer's disease

Abstract

Zinc is an important trace metal in human biology. It plays an active role in enzymic reactions, or simply serves in a structural capacity on a number of cytoplasmic and nuclear proteins. Zinc modulates receptor responses to various excitatory and inhibitory neurotransmitters. It has biphasic effects on several enzymes critical for cell survival and the induction of apoptotic cell death. At a particular concentration threshold, it is cytotoxic both in vitro and in vivo. Recent studies have indicated that zinc may be involved in several neuropathological conditions such as Alzheimer's disease, epilepsy, traumatic head injury and cerebral stroke. This investigation focused on the role of zinc in the context of an Alzheimer's disease paradigm. Rat primary cortical neurons were exposed to freshly-prepared (non-aged) or aggregated (aged) Aβ1-42 protein (20 µM; a highly toxic Aβ species), in the presence or absence of equimolar concentrations of zinc chloride (ZnCl2), or copper chloride (CuCl2).Zinc significantly attenuated, while copper potentiated the neurotoxic effects of non-aged Aβ1-42 after 48 h chronic exposure. Similarly, zinc, but not copper, reduced neuronal death 48 h following exposure to the aged peptide. A metal chelator, DTPA, also showed a protective, but limited effect (only observable at 24 h post-treatment) against the neurotoxicity of aged peptide. At the concentration tested, zinc alone had no effect on neuronal survival, although copper was found to be slightly neurotoxic after 48 h incubation. As hydrogen peroxide (H2O2) production by Aβ has been reported to mediate its cytotoxicity, an in vitro test system was used to identify if zinc affected this process. It was found that co-incubation of zinc (10 µM) with Aβ1-42 peptide alone (10 µM) or with Aβ1-42 and copper (1 µM) showed a significant decrease of Aβ-mediated H2O2 formation using TCEP assay in vitro. This finding suggested that the neuroprotective effect of zinc may not only be due to its capacity to hinder Aβ's redox activity, but also zinc's ability to preclude Aβ-mediated H2O2 generation. A mechanism for these effects of zinc is yet to be determined. As oxidative/nitrosative stress has been widely reported to occur in AD brain, and since zinc metabolism is believed to be dysfunctional in AD brain, the current study also set out to elucidate if cerebral zinc metabolism may be affected by nitrosating agents in vivo. Three unrelated nitric oxide-generating compounds were administered into rat hippocampus. Using Timm's and TSQ stain, a histochemical and a fluorescent staining for zinc, respectively, it was observed that sodium nitroprusside (≥2 nmol) and spermine-nitric oxide complex (≤200 nmol), but not the peroxynitrite-producing agent 3-morpholinosydnonimine (≤200 nmol), caused perikaryal zinc accumulation among certain neurons in the hippocampus. Both membrane impermeable and permeable metal chelators, EDTA and TPEN, respectively, blocked perikaryal zinc staining of hippocampal neurons. Data obtained from EDTA treatment suggested that the source of perikaryal zinc staining was mostly extracellular. Previous reports have shown that metal chelating agents have the capacity to protect neurons and minimize damage from brain insults. The preceding studies showed that chelators minimized perikaryal zinc acccumulation, precluded Aβ's redox activity, and partly reduced Aβ neurotoxicity. Thus, to further assess the possible beneficial effect of these compounds, DTPA, BP, or BC (25 µM) was incubated with a mixture of Aβ1-42 (2 µM) and Aβ1-40 (20 µM). DTPA abolished, while BP delayed the Aβ1-42-mediated Aβ1-40 "seeding" process. This result suggested that contaminating trace metals have an obligatory role in the nucleation-dependent Aβ fibrillogenesis, which is believed to be linked to Aβ neurotoxicity, and AD neuropathology. These results imply that zinc has a biphasic role in AD etiology and disease progression, and that the use of metal chelators to buffer pathologically excessive zinc and other metals, particularly the redox active copper and iron, may have a potential therapeutic value against AD symptomatology.