The extracellular microenvironment of the brain contains numerous
biological redox agents, including ascorbate,
glutathione,
cysteine and
homocysteine. During
ischemia/reperfusion, aging or neurological disease, extracellular levels of
reductants can increase dramatically owing to dysregulated homeostasis. The extracellular concentrations of transition metals such as
copper and
iron are also substantially elevated during aging and in some
neurodegenerative disorders. Increases in the extracellular redox capacity can potentially generate neurotoxic
free radicals from reduction of Cu(II) or Fe(III), resulting in neuronal cell death. To investigate this in vitro, the effects of extracellular
reductants (ascorbate,
glutathione,
cysteine,
homocysteine or
methionine) on primary cortical neurons was examined. All redox agents except
methionine induced widespread neuronal oxidative stress and subsequent cell death at concentrations occurring in normal conditions or during neurological insults. This neurotoxicity was totally dependent on trace Cu (>or=0.4 microM) already present in the culture medium and did not require addition of exogenous Cu. Toxicity involved generation of Cu(I) and H(2)O(2), while other trace metals did not induce toxicity. Surprisingly, administration of Fe(II) or Fe(III) (>or=2.5 microM) completely abrogated
reductant-mediated neurotoxicity. The potent protective activity of Fe correlated with Fe inhibiting
reductant-mediated Cu(I) and H(2)O(2) generation in cell-free assays and reduced cellular Cu uptake by neurons. This demonstrates a novel role for Fe in blocking Cu-mediated neurotoxicity in a high reducing environment. A possible pathogenic consequence for these phenomena was demonstrated by abrogation of Fe neuroprotection after pre-exposure of cultures to the Alzheimer's
amyloid beta peptide (Abeta). The loss of Fe neuroprotection against
reductant toxicity was greater
after treatment with human Abeta1-42 than with human Abeta1-40 or rodent Abeta1-42, consistent with the central role of Abeta1-42 in
Alzheimer's disease. These findings have important implications for trace
biometal interactions and
free radical-mediated damage during neurodegenerative illnesses such as
Alzheimer's disease and old-age
dementia.