A novel mechanism underlying the susceptibility of neuronal cells to nitric oxide: the occurrence and regulation of protein S-nitrosylation is the checkpoint. 
The susceptibility of neuronal cells to nitric oxide (NO) is a key issue in NO-mediated neurotoxicity.  However, the underlying mechanism remains unclear.  As a cyclic guanosine monophosphate (cGMP)-independent NO signaling pathway, S-nitrosylation (or S-nitrosation) has been suggested to occur as a post-translational modification in parallel with O-phosphorylation.  The underlying mechanism of the involvement of protein S-nitrosylation in the susceptibility of neuronal cells to NO has been little investigated.  In this study, we focused on the role of S-nitrosothiols (RSNO) in the susceptibility of a cerebellar cell line R2 to NO.  Our results showed the following: (i) S-nitrosoglutathione (GSNO) induced a burst of RSNO in GSH-depleted R2 cells, the majority of which were primarily contributed by the S-nitrosylation of proteins (Pro-SNOs), and was followed by severe neuronal necrosis; (ii) the elevation in the level of Pro-SNOs resulted from a dysfunction of S-nitroglutathione reductase (GSNOR) as a result of its substrate, GSNO, being unavailable in GSH-depleted cells.  In the meantime, the suppression of GSNOR increased NO-mediated neurotoxicity in R2 cells, as well as in cerebellar granule neurons; (iii) Our results also demonstrate that the burst of RSNO is the "checkpoint" of cell fate: if RSNO can be reduced to free thiol proteins, cells will survive; if they are further oxidized, cells will die; and (iv) GSH-ethyl ester and Vitamin C protected R2 cells against GSNO neurotoxicity through two distinct mechanisms: by inhibiting the elevation of Pro-SNOs and by reducing Pro-SNOs to free thiol proteins, respectively.  A novel mechanism underlying the susceptibility of neuronal cells to NO is proposed and some potential strategies to prevent the NO-mediated neurotoxicity are discussed.