Protein S-nitrosylation modulates important cellular processes, including neurotransmission, vasodilation, proliferation, and apoptosis in various cell types. We have previously reported that protein disulfide isomerase (PDI) is S-nitrosylated in brains of patients with sporadic neurodegenerative diseases. This modification inhibits PDI enzymatic activity and consequently leads to the accumulation of unfolded/misfolded proteins in the endoplasmic reticulum (ER) lumen. Here, we describe S-nitrosylation of additional ER pathways that affect the unfolded protein response (UPR) in cell-based models of Parkinson’s disease (PD). We demonstrate that nitric oxide (NO) can S-nitrosylate the ER stress sensors IRE1α and PERK. While S-nitrosylation of IRE1α inhibited its ribonuclease activity, S-nitrosylation of PERK activated its kinase activity and downstream phosphorylation/inactivation or eIF2α. Site-directed mutagenesis of IRE1α(Cys931) prevented S-nitrosylation and inhibition of its ribonuclease activity, indicating that Cys931 is the predominant site of S-nitrosylation. Importantly, cells overexpressing mutant IRE1α(C931S) were resistant to NO-induced damage. Our findings show that nitrosative stress leads to dysfunctional ER stress signaling, thus contributing to neuronal cell death.

Nitric oxide (NO) plays a pivotal function in neurotransmission, vasodilation, proliferation, and apoptosis in various types of cells via protein S-nitrosylation. Previously we demonstrated that protein disulfide isomerase (PDI) is S-nitrosylated in brains manifesting sporadic neurodegenerative diseases. This modification results in dysfunction of its enzymatic activity and consequently the accumulation of unfolded/misfolded proteins in the endoplasmic reticulum (ER). The aim of this study was to clarify the detailed function of NO on unfolded protein response (UPR) branches. We here found that the ER stress sensor IRE1α is S-nitrosylated. Interestingly, NO specifically abrogates ribonuclease activity, but not oligomerization or autophosphorylation of IRE1α. Site-directed mutagenesis revealed that Cys 931 and Cys951 in IRE1 are targets for S-nitrosylation. These mutants expressing in IRE1α knockout MEF showed a resistant role to the inhibition of nuclease activity by NO. Thus, we elucidated the effects of S-nitrosylation on ER stress sensors that mediate the UPR, and thus contribute to cell death pathways.