The Relationship between Intracellular Free Iron and Cell Injury in Cultured Neurons, Astrocytes, and Oligodendrocytes

Iron is an essential element for cells but may also be an important cytotoxin. However, very little is known about iron transport, redox status, or toxicity specifically inside cells. In this study, we exploited the sensitivity of fura-2 to quenching by ferrous iron (Fe(2+)) to detect intracellular free iron (Fe(2+)(i)) in neurons, astrocytes, and oligodendrocytes in primary culture. All cell types exposed to Fe(2+) in the presence of the ionophore pyrithione rapidly accumulated Fe(2+) to a similar extent. The heavy-metal chelators bipyridyl and N,N,N',N'-tetrakis(2-pyridalmethyl)ethyl-enediamine rapidly reversed the increase in Fe(2+)(i), whereas desferrioxamine had little effect. Interestingly, the Fe(2+)-mediated quenching of fura-2 fluorescence was reversed in a concentration-dependent manner by hydrogen peroxide. This was likely caused by the oxidation of Fe(2+) to Fe(3+) inside the cell. Acute exposure of cells to Fe(2+) was only toxic when the metal was applied together with pyrithione, showing that Fe(2+) is only toxic when elevated inside cells. Interestingly, only neurons and oligodendrocytes were injured by this elevation in Fe(2+)(i), whereas astrocytes were unaffected, although Fe(2+)(i) was elevated to the same degree in each cell type. These studies provide a novel approach for detecting Fe(2+)(i) in a manner sensitive to the redox state of the metal. These studies also provide a model system for the study of the toxic consequences of elevated Fe(2+)(i) in neural cells.