Implications for soluble iron accumulation, oxidative stress, and glial glutamate release in motor neuron death associated with sporadic amyotrophic lateral sclerosis.

Oxidative stress in sporadic amyotrophic lateral sclerosis (ALS) has been evidenced by accumulation of oxidatively modified products of nucleic acids, lipids, sugars, and proteins in the motor neuron system of brains and spinal cords obtained at autopsy from the patients. We recently demonstrated soluble iron accumulation in activated microglia of sporadic ALS spinal cords. This finding could indicate that iron-mediated Fenton reaction is most likely to be responsible for oxidative stress associated with this disease. The excitatory amino acid neurotoxicity hypothesis for sporadic ALS has been proposed based on increased glutamate and aspartate concentrations in cerebrospinal fluid from the patients. Initially, the increase in extracellular excitatory amino acid levels was considered to reflect excessive release from the axon terminal of upper motor neurons. However, it is a question of whether the damaged upper motor neurons continue releasing glutamate even in advanced stage of this disease. To address this issue, we hypothesized that glial cells might be a glutamate release source. Our immunohistochemical analysis on autopsied human spinal cords revealed that ferritin, hepcidin, ferroportin, aconitase 1, tumor necrosis factor-α (TNF-α), TNF-α-converting enzyme (TACE), and glutaminase-C (GAC) were expressed mainly in microglia and that cystine/glutamate antiporter (xCT) was expressed mainly in astrocytes. We next performed cell culture experiments. Cultured microglia treated with soluble iron over-released glutamate and TNF-α via aconitase 1 and TACE, respectively. Cultured microglia treated with TNF-α over-released glutamate via GAC. Cultured microglia treated with hepcidin, of which expression is known to be upregulated by TNF-α, showed downregulated expression of ferroportin. Cultured astrocytes treated with hydrogen peroxide over-released glutamate via xCT. These observations provide in vivo and in vitro evidence that microglia and astrocytes are glutamate suppliers in response to soluble iron overload and oxidative stress, respectively, in sporadic ALS.