Involvement of nitric oxide/reactive oxygen species signaling via 8-nitro-cGMP formation in 1-methyl-4-phenylpyridinium ion-induced neurotoxicity in PC12 cells and rat cerebellar granule neurons

To investigate the role of nitric oxide (NO)/reactive oxygen species (ROS) redox signaling in Parkinson's disease-like neurotoxicity, we used 1-methyl-4-phenylpyridinium (MPP⁺) treatment (a model of Parkinson's disease). We show that MPP⁺-induced neurotoxicity was dependent on ROS from neuronal NO synthase (nNOS) in nNOS-expressing PC12 cells (NPC12 cells) and rat cerebellar granule neurons (CGNs). Following MPP⁺ treatment, we found production of 8-nitroguanosine 3',5'-cyclic monophosphate (8-nitro-cGMP), a second messenger in the NO/ROS redox signaling pathway, in NPC12 cells and rat CGNs, that subsequently induced S-guanylation and activation of H-Ras. Additionally, following MPP⁺ treatment, extracellular signal-related kinase (ERK) phosphorylation was enhanced. Treatment with a mitogen-activated protein kinase (MAPK)/ERK kinase (MEK) inhibitor attenuated MPP⁺-induced ERK phosphorylation and neurotoxicity. In conclusion, we demonstrate for the first time that NO/ROS redox signaling via 8-nitro-cGMP is involved in MPP⁺-induced neurotoxicity and that 8-nitro-cGMP activates H-Ras/ERK signaling. Our results indicate a novel mechanism underlying MPP⁺-induced neurotoxicity, and therefore contribute novel insights to the mechanisms underlying Parkinson's disease.

Regulation of nitric oxide/reactive oxygen species redox signaling by nNOS splicing variants

We previously demonstrated different expression patterns of the neuronal nitric oxide synthase (nNOS) splicing variants, nNOS-μ and nNOS-α, in the rat brain; however, their exact functions have not been fully elucidated. In this study, we compared the enzymatic activities of nNOS-μ and nNOS-α and investigated intracellular redox signaling in nNOS-expressing PC12 cells, stimulated with a neurotoxicant, 1-methyl-4-phenylpyridinium ion (MPP⁺), to enhance the nNOS uncoupling reaction. Using in vitro studies, we show that nNOS-μ produced nitric oxide (NO), as did nNOS-α, in the presence of tetrahydrobiopterin (BH₄), an important cofactor for the enzymatic activity. However, nNOS-μ generated more NO and less superoxide than nNOS-α in the absence of BH₄. MPP ⁺ treatment induced more reactive oxygen species (ROS) production in nNOS-α-expressing PC12 cells than in those expressing nNOS-μ, which correlated with the intracellular production of 8-nitroguanosine 3',5'-cyclic monophosphate (8-nitro-cGMP), a downstream messenger of nNOS redox signaling, and apoptosis in these cells. Furthermore, post-treatment with 8-nitro-cGMP aggravated MPP⁺-induced cytotoxicity via activation of the H-Ras/extracellular signal-regulated kinase signaling pathway. In conclusion, our results provide strong evidence that nNOS-μ exhibits distinctive enzymatic properties of NO/ROS production, contributing to the regulation of intracellular redox signaling, including the downstream production of 8-nitro-cGMP.