Iron-overload toxicity in central nervous system-associated cells and its attenuation by calcium channel blockers
Gaasch, Julie Ann
Iron is essential to many vital biological pathways within the brain through its participation in oxidation-reduction reactions. Conversely, conditions of iron overload such as hemorrhagic stroke, or iron accumulation as in neurodegenerative disorders, can lead to saturation of iron transport and storage proteins to increase free iron levels within the brain. Free iron is highly reactive and in cases of overload can induce oxidative stress by producing hydroxyl free radicals that cause damage to cellular DNA, membranes, and proteins. Traditionally, iron entry into cells occurs by receptor-mediated endocytosis of transferrin-bound iron. However, studies in heart cells have demonstrated an alternate route of iron entry under conditions of iron overload through the L-type voltage-gated calcium channel. This dissertation investigates two important concepts related to iron-overload toxicity in brain, i.e. 1) the existence of differential responses of select central nervous system cells to iron-induced toxicity, and 2) the existence of an alternative route of entry for iron into brain cells in iron overload conditions via voltage-gated calcium channels. Our studies indicate that iron competes with calcium for entry via voltage-gated calcium channels and that iron uptake is inhibited in a dose-dependent manner by nimodipine, a calcium channel blocker, in neuronal cells. Studies measuring iron toxicity in astrocytes, neurons, and endothelial cells demonstrate that these three cell types respond differentially to iron-induced toxicity when cultured in isolation, and that calcium channel blockers are effective in attenuating the toxicity observed in neuronal and endothelial cells, but not in astrocytic cells. NGP1-01, a polycyclic cage compound, has been shown to be a dual blocker of neuronal voltage gated calcium channels as well as N-methyl-D-aspartate (NMDA) receptors. Our studies investigated the neuroprotective effect of NGP1-01 in iron-loaded brain endothelial cells. Similar to nimodipine, NGP1-01 demonstrated a significant attenuation of iron-induced cellular toxicity. Taken as a whole, these studies suggest that calcium channel blockers may be utilized as neuroprotective agents in conditions of brain iron overload, particularly at the blood brain barrier where brain endothelial cells are intimately affected.