6 caspase activity and thus promoting proapoptotic signaling [85]. 4.2.4. S-Nitrosylation of GAPDH. Solomon Snyder’s group has shown that the important metabolic enzyme glycer- aldehyde-3-phosphate dehydrogenase (GAPDH) can be S- nitrosylated to form SNO-GAPDH. SNO-GAPDH manifests a loss of enzymatic activity [87]. More importantly, S- nitrosylated GAPDH eﬃciently binds Siah1 protein and then translocates to the nucleus. In the nucleus, this protein complex activates ubiquitination and thus degradation of several nuclear proteins, including nuclear receptor corepres- sor (N-COR); this process contributes to cell death [87]. These studies suggest that S-nitrosylation of GAPDH, on one hand, compromises its metabolic enzymatic activity, but, on the other hand, in conjunction with Siah1, forms an important signaling complex to promote cell death and neurodegeneration. 4.2.5. S-Nitrosylation of PDI. As discussed above, pro- tein misfolding and ER stress can be precipitated by S- nitrosylation of PDI, thereby potentially contributing to neuronal injury in AD. Our laboratory has also shown this scenario to be true in PD models. For example, when SH-SY5Y dopaminergic cells were treated with rotenone, a pesticide implicated in the pathogenesis of PD, we observed an increase in SNO-PDI levels concomitant with a decrease in PDI chaperone activity. Additionally, we found dramatically increased levels of SNO-PDI in human postmortem PD brains compared to controls [26]. Since ER stress due to protein misfolding is thought to contribute to the neurodegenerative process in PD [88], our ﬁnding of a substantial degree of SNO-PDI in PD brains has both pathogenic and therapeutic implications. 4.2.6. S-Nitrosylation of DJ-1. Deletions or point mutations in the protein DJ-1 (PARK7) have been shown to be responsible for an early-onset, autosomal-recessive form of PD [89]. Interestingly, DJ-1-mediated signaling pathways have also been implicated in the much more common sporadic form of PD. It has been postulated that the increase in DJ-1 expression observed in cells undergoing nitrosative stress induced by the herbicide, paraquat, represents an attempt to protect the cells [90]. Consistent with this notion, DJ-1 knockdown makes neuronal-like cells more susceptible to peroxide-, MPP+-, and 6-hydroxydopamine-induced cell death [91, 92]. Additionally, DJ-1 deﬁcient ﬂies [93–96] or mice [97] are more vulnerable to environmental neurotoxins associated with dopaminergic degeneration. Sequence anal- ysis of protein DJ-1 reveals three potentially redox-active cysteine residues, two of which (Cys46 and Cys53) appear to be susceptible to S-nitrosylation in vitro and in cell-based systems [98]. Our group has also observed S-nitrosylation of a critical redox-active cysteine in the crystal structure of DJ- 1. These ﬁndings and others suggest that posttranslational modiﬁcations of DJ-1, including protein S-nitrosylation, can disrupt the antioxidant action of DJ-1 in dopaminergic neurons, rendering them more susceptible to damage in International Journal of Cell Biology sporadic PD. However, the elucidation of additional eﬀects of SNO-DJ-1 in PD will require further investigation. In summary, a number of studies suggest that nitrosative stress contributes to PD pathogenesis by altering neuro- protective proteins such as parkin, Prx2, PDI, GAPDH, and XIAP (Figure 2). These ﬁndings indicate that aberrant S-nitrosylation reactions may play an important role in this neurodegenerative disorder, providing additional insight into nitrosative mechanisms of PD pathogenesis as well as potential novel targets for the treatment of PD. 5. Conclusions Nitric oxide signaling can be both beneﬁcial and harmful to the nervous system depending on (i) the concentration of NO and (ii) the cell signaling pathways aﬀected by various levels of NO. Physiological levels of NO activate both cGMP-cGKI and S-nitrosylation pathways responsible for various physiological processes, including those aﬀect- ing synaptic transmission and plasticity. In contrast, high levels of NO compromise cellular functions by a variety of posttranslational modiﬁcations including aberrant S- nitrosylation reactions that would not normally occur in the presence of physiological levels of NO. In this paper, we have discussed data accumulated over the past several years that highlight the importance of protein S-nitrosylation in perturbing vital cell functions, including mitochondrial dynamics, protein folding, ubiquitination, synaptic trans- mission, and signal transduction pathways. Alteration of one or several of these events contributes to neuronal cell death and the development of neurodegenerative disorders (Figure 2). Although we have discussed the role of S- nitrosylation of several proteins here, including Drp1, PDI, GAPDH, ApoE, parkin, XIAP, Prx2, and DJ-1 in AD and PD, this list is by no means complete. Proteome-wide studies have already found hundreds, if not thousands, of proteins that are S-nitrosylated [99]. Future studies will unravel the role of S-nitrosylation of additional proteins in various cellular cascades and its implications for the pathogenesis and treatment of neurodegenerative disorders. Acknowledgments The authors thank Dr. Scott McKercher and Emily Holland for critically reading the paper and for useful suggestions. The work described herein was supported in part by NIH Grants P01 HD29587, P01 ES016738, P30 NS076411, and R01 EY05477 (to S. A. Lipton) and by a postdoctoral fellowship from the Spanish Ministry (to C. Sunico). 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