Ryanodine receptor-mediated Ca(2+) release underlies iron-induced mitochondrial fission and stimulates mitochondrial Ca(2+) uptake in primary hippocampal neurons

Mounting evidence indicates that iron accumulation impairs brain function. We have reported previously that addition of sub-lethal concentrations of iron to primary hippocampal neurons produces Ca(2)(+) signals and promotes cytoplasmic generation of reactive oxygen species. These Ca(2)(+) signals, which emerge within seconds after iron addition, arise mostly from Ca(2)(+) release through the redox-sensitive ryanodine receptor (RyR) channels present in the endoplasmic reticulum. We have reported also that addition of synaptotoxic amyloid-β oligomers to primary hippocampal neurons stimulates RyR-mediated Ca(2)(+) release, generating long-lasting Ca(2)(+) signals that activate Ca(2)(+)-sensitive cellular effectors and promote the disruption of the mitochondrial network. Here, we describe that 24 h incubation of primary hippocampal neurons with iron enhanced agonist-induced RyR-mediated Ca(2)(+) release and promoted mitochondrial network fragmentation in 43% of neurons, a response significantly prevented by RyR inhibition and by the antioxidant agent N-acetyl-L-cysteine. Stimulation of RyR-mediated Ca(2)(+) release by a RyR agonist promoted mitochondrial Ca(2)(+) uptake in control neurons and in iron-treated neurons that displayed non-fragmented mitochondria, but not in neurons with fragmented mitochondria. Yet, the global cytoplasmic Ca(2)(+) increase induced by the Ca(2)(+) ionophore ionomycin prompted significant mitochondrial Ca(2)(+) uptake in neurons with fragmented mitochondria, indicating that fragmentation did not prevent mitochondrial Ca(2)(+) uptake but presumably decreased the functional coupling between RyR-mediated Ca(2)(+) release and the mitochondrial Ca(2)(+) uniporter. Taken together, our results indicate that stimulation of redox-sensitive RyR-mediated Ca(2)(+) release by iron causes significant neuronal mitochondrial fragmentation, which presumably contributes to the impairment of neuronal function produced by iron accumulation.