The etiology of neurodegenerative orders has been linked to damage caused by oxidative stress. One mechanism by which oxidative stress can be mediated is an excess of reactive iron, which catalyzes the formation of free radicals. The mechanism by which iron is stored in the brain can induce neuronal death has not been clear because cells are normally protected against the deleterious effects of iron. The investigator have purified an activity that induces apoptosis in embryonic day 8 (E8) chick ciliary ganglion neurons. In contrast more mature (>E10) ciliary ganglion neurons were not killed. N-terminal sequencing revealed that this activity was ovotransferrin. Death inducing activity required that iron be bound to the transferrin. The EC50 of diferric recombinant transferrin in inducing apoptosis was 5nM, well within the levels found in embryonic extracellular fluid and blood (30-40 uM). This effect of FeTr was not limited to ciliary ganglion neurons: lumbar sympathetic ganglia contain two populations of neurons, one which survives in nerve growth factor and does not die when exposed to FeTf, and another which survives in clilary neurotrophic factor (CNTF) and is killed by FeTf. The developmental switch in CG neuron sensitivity to FeTf suggests that susceptibility of neurons to transferrin-mediated apoptosis is likely to be a normal developmental event that is regulated by cell-cell interactions. The investigator proposes to study the molecular basis for the differential sensitivity of neurons to killing caused by transferrin-mediated iron transport. These studies are likely to lead to important clues as to how the process may go awry in neurodegenerative disease.
The specific aims are: (1) to test the hypothesis that neurons become sensitive to killing by FeTf when they fail to downregulate transferrin receptor, human transferrin receptor will be overexpressed in insensitive neuronal populations in order to test whether it confers sensitivity to killing by human FeTf; (2) to test the hypothesis that susceptibility to FeTf is mediated by reduced levels of ferritin, the intracellular iron binding protein, ferritin heavy chain will be overexpressed in order test whether neurons are protected from death induced by FeTf; (3) to test the hypothesis that intracellular iron induces apoptosis by accumulating in mitochondria through reduced levels of frataxin, a protein that stimulates iron transport out of mitochondria, frataxin will be overexpressed in neurons in order to determine if they can be protected from killing by FeTf; and (4) to test the hypothesis that intracellular iron kills cells by generating excess free radicals which damage mitochondria, it will be determined if: (1) FeTf generates free radicals; (2) anti-oxidants rescue neurons from FeTf; (3) inhibitors of oxidative phosphorylation induce apoptosis with the same characteristics as FeTf; (4) cytochrome c is released by FeTf treated neurons; (5) overexpression of the mitochondrial anti-apoptotic protein Bcl-2 protects cells from FeTf; and (6) caspase inhibitors rescue neurons from FeTf.
