Homocysteine potentiates copper- and amyloid beta peptide-mediated toxicity in primary neuronal cultures: possible risk factors in the Alzheimer's-type neurodegenerative pathways

Oxidative stress may have an important role in the progression of neurodege nerative disorders such as Alzheimer's disease (AD) and prion diseases. Oxi dative damage could result from interactions between highly reactive transi tion metals such as copper (Cu) and endogenous reducing and/or oxidizing mo lecules in the brain. One such molecule, homocysteine, a thiol-containing a mino acid, has previously been shown to modulate Cu toxicity in HeLa and en dothelial cells in vitro. Due to a possible link between hyperhomocysteinem ia and AD, we examined whether interaction between homocysteine and Cu coul d potentiate Cu neurotoxicity. Primary mouse neuronal cultures were treated with homocysteine and either Cu (II), Fe (II or III) or Zn (II). Homocyste ine was shown to selectively potentiate toxicity from low micromolar concen trations of Cu. The toxicity of homocysteine/Cu coincubation was dependent on the ability of homocysteine to reduce Cu(II) as reflected by the inhibit ion of toxicity with the Cu(I)-specific chelator, bathocuproine disulphonat e. This was supported by data showing that homocysteine reduced Cu (II) mor e effectively than cysteine or methionine but did not reduce Fe (III) to Fe (II). Homocysteine also generated high levels of hydrogen peroxide in the presence of Cu (II) and promoted A beta /Cu-mediated hydrogen peroxide prod uction and neurotoxicity. The potentiation of metal toxicity did not involv e excitotoxicity as ionotropic glutamate receptor antagonists had no effect on neurotoxicity. Homocysteine alone also had no effect on neuronal glutat hione levels. These studies suggest that increased copper and/or homocystei ne levels in the elderly could promote significant oxidant damage to neuron s and may represent additional risk factor pathways which conspire to produ ce AD or related neurodegenerative conditions.