1-Methyl-4-phenylpyridinium-induced Apoptosis in Cerebellar Granule Neurons Is Mediated by Transferrin Receptor Iron-dependent Depletion of Tetrahydrobiopterin and Neuronal Nitric-oxide Synthase-derived Superoxide

In this study, we investigated the molecular mechanisms of toxicity of 1-methyl-4-phenylpyridinium (MPP⁺), an ultimate toxic metabolite of a mitochondrial neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, that causes Parkinson-like symptoms in experimental animals and humans. We used rat cerebellar granule neurons as a model cell system for investigating MPP ⁺ toxicity. Results show that MPP⁺ treatment resulted in the generation of reactive oxygen species from inhibition of complex I of the mitochondrial respiratory chain, and inactivation of aconitase. This, in turn, stimulated transferrin receptor (TfR)-dependent iron signaling via activation of the iron-regulatory protein/iron-responsive element interaction. MPP ⁺ caused a time-dependent depletion of tetrahydrobiopterin (BH ₄) that was mediated by H₂O₂ and transferrin iron. Depletion of BH₄ decreased the active, dimeric form of neuronal nitric-oxide synthase (nNOS). MPP⁺-mediated "uncoupling" of nNOS decreased ^.NO and increased superoxide formation. Pretreatment of cells with sepiapterin to promote BH ₄ biosynthesis or cell-permeable iron chelator and TfR antibody to prevent iron-catalyzed BH₄ decomposition inhibited MPP⁺ cytotoxicity. Preincubation of cerebellar granule neurons with nNOS inhibitor exacerbated MPP⁺-induced iron uptake, BH₄ depletion, proteasomal inactivation, and apoptosis. We conclude that MPP ⁺-dependent aconitase inactivation, Tf-iron uptake, and oxidant generation result in the depletion of intracellular BH₄, leading to the uncoupling of nNOS activity. This further exacerbates reactive oxygen species-mediated oxidative damage and apoptosis. Implications of these results in unraveling the molecular mechanisms of neurodegenerative diseases (Parkinson's and Alzheimer's disease) are discussed.