Protein S-nitrosylation and denitrosylation in the mouse spinal cord upon injury of the sciatic nerve

Cold avoidance and heat pain hypersensitivity in neuronal nucleoredoxin knockout mice

Nucleoredoxin is a thioredoxin-like oxidoreductase that mainly acts as oxidase and thereby regulates calcium calmodulin kinase Camk2a, an effector of nitric oxide mediated synaptic potentiation and nociceptive sensitization. We asked here if and how NXN affects thermal sensation and nociception in mice using pan-neuronal NXN deletion driven by Nestin-Cre, and sensory neuron specific deletion driven by Advillin-Cre. In a thermal gradient ring, where mice can freely choose the temperature of well-being, Nestin-NXN^-/- mice avoided unpleasant cold temperatures. In neuropathic and inflammatory nociceptive models, Nestin-NXN^-/- and Advillin-NXN^-/- mice displayed subtle phenotypes of heightened heat nociception. Abnormal thermal in vivo responses were associated with heightened calcium influx upon stimulation of transient receptor channels, with heightened oxygen consumption upon disruption of the mitochondrial membrane potential and with higher density of neurite trees of primary sensory neurons of the dorsal root ganglia in cultures. The data suggest that loss of NXN's balancing redox functions leads to maladaptive changes in sensory neurons that manifest in vivo as polyneuropathy-like abnormal cold sensitivity and heat "pain".

Critical roles of protein disulfide isomerases in balancing proteostasis in the nervous system

Protein disulfide isomerases (PDIs) constitute a family of oxidoreductases promoting redox protein folding and quality control in the endoplasmic reticulum. PDIs catalyze disulfide bond formation, isomerization, and reduction, operating in concert with molecular chaperones to fold secretory cargoes in addition to directing misfolded proteins to be refolded or degraded. Importantly, PDIs are emerging as key components of the proteostasis network, integrating protein folding status with central surveillance mechanisms to balance proteome stability according to cellular needs. Recent advances in the field driven by the generation of new mouse models, human genetic studies, and omics methodologies, in addition to interventions using small molecules and gene therapy, have revealed the significance of PDIs to the physiology of the nervous system. PDIs are also implicated in diverse pathologies, ranging from neurodevelopmental conditions to neurodegenerative diseases and traumatic injuries. Here, we review the principles of redox protein folding in the ER with a focus on current evidence linking genetic mutations and biochemical alterations to PDIs in the etiology of neurological conditions.

Protein post-translational modifications after spinal cord injury

Deficits in intrinsic neuronal capacities in the spinal cord, a lack of growth support, and suppression of axonal outgrowth by inhibitory molecules mean that spinal cord injury almost always has devastating consequences. As such, one of the primary targets for the treatment of spinal cord injury is to develop strategies to antagonize extrinsic or intrinsic axonal growth-inhibitory factors or enhance the factors that support axonal growth. Among these factors, a series of individual protein level disorders have been identified during the generation of axons following spinal cord injury. Moreover, an increasing number of studies have indicated that post-translational modifications of these proteins have important implications for axonal growth. Some researchers have discovered a variety of post-translational modifications after spinal cord injury, such as tyrosination, acetylation, and phosphorylation. In this review, we reviewed the post-translational modifications for axonal growth, functional recovery, and neuropathic pain after spinal cord injury, a better understanding of which may elucidate the dynamic change of spinal cord injury-related molecules and facilitate the development of a new therapeutic strategy for spinal cord injury.

Reduced exploratory behavior in neuronal nucleoredoxin knockout mice

Nucleoredoxin is a thioredoxin-like redoxin that has been recognized as redox modulator of WNT signaling. Using a Yeast-2-Hybrid screen, we identified calcium calmodulin kinase 2a, Camk2a, as a prominent prey in a brain library. Camk2a is crucial for nitric oxide dependent processes of neuronal plasticity of learning and memory. Therefore, the present study assessed functions of NXN in neuronal Nestin-NXN^-/- deficient mice. The NXN-Camk2a interaction was confirmed by coimmunoprecipitation, and by colocalization in neuropil and dendritic spines. Functionally, Camk2a activity was reduced in NXN deficient neurons and restored with recombinant NXN. Proteomics revealed reduced oxidation in the hippocampus of Nestin-NXN^-/- deficient mice, including Camk2a, further synaptic and mitochondrial proteins, and was associated with a reduction of mitochondrial respiration. Nestin-NXN^-/- mice were healthy and behaved normally in behavioral tests of anxiety, activity and sociability. They had no cognitive deficits in touchscreen based learning & memory tasks, but omitted more trials showing a lower interest in the reward. They also engaged less in rewarding voluntary wheel running, and in exploratory behavior in IntelliCages. Accuracy was enhanced owing to the loss of exploration. The data suggested that NXN maintained the oxidative state of Camk2a and thereby its activity. In addition, it supported oxidation of other synaptic and mitochondrial proteins, and mitochondrial respiration. The loss of NXN-dependent pro-oxidative functions manifested in a loss of exploratory drive and reduced interest in reward in behaving mice.

TCA cycle metabolic compromise due to an aberrant S-nitrosoproteome in HIV-associated neurocognitive disorder with methamphetamine use

In the brain, both HIV-1 and methamphetamine (meth) use result in increases in oxidative and nitrosative stress. This redox stress is thought to contribute to the pathogenesis of HIV-associated neurocognitive disorder (HAND) and further worsening cognitive activity in the setting of drug abuse. One consequence of such redox stress is aberrant protein S-nitrosylation, derived from nitric oxide, which may disrupt normal protein activity. Here, we report an improved, mass spectrometry-based technique to assess S-nitrosylated protein in human postmortem brains using selective enrichment of S-nitrosocysteine residues with an organomercury resin. The data show increasing S-nitrosylation of tricarboxylic acid (TCA) enzymes in the setting of HAND and HAND/meth use compared with HIV+ control brains without CNS pathology. The consequence is systematic inhibition of multiple TCA cycle enzymes, resulting in energy collapse that can contribute to the neuronal and synaptic damage observed in HAND and meth use.

Gel-based fluorescent proteomic tools for investigating cell redox signaling. A mini-review

The specific chemical reactivity of thiol groups makes protein cysteines susceptible to reactions with reactive oxygen species (ROS) and reactive nitrogen species (RNS) resulting in the formation of various reversible and irreversible oxidative post-translational modifications (oxPTMs). This review highlights a number of gel-based redox proteomic approaches to detect protein oxPTMs, with particular emphasis on S-nitrosylation, which we believe are currently one of the most accurate way to analyze changes in the redox status of proteins. The information collected in this review relates to the recent progress regarding methods for the enrichment and identification of redox-modified proteins, with an emphasis on fluorescent gel proteomics. Gel-based fluorescent proteomic strategies are low-cost and easy-to-use tools for investigating the thiol proteome and can provide substantial information on redox signaling.

Nitric Oxide-Dependent Protein Post-Translational Modifications Impair Mitochondrial Function and Metabolism to Contribute to Neurodegenerative Diseases

Significance: Most brains affected by neurodegenerative diseases manifest mitochondrial dysfunction as well as elevated production of reactive oxygen species and reactive nitrogen species (RNS), contributing to synapse loss and neuronal injury. Recent Advances: Excessive production of RNS triggers nitric oxide (NO)-mediated post-translational modifications of proteins, such as S-nitrosylation of cysteine residues and nitration of tyrosine residues. Proteins thus affected impair mitochondrial metabolism, mitochondrial dynamics, and mitophagy in the nervous system. Critical Issues: Identification and better characterization of underlying molecular mechanisms for NO-mediated mitochondrial dysfunction will provide important insights into the pathogenesis of neurodegenerative disorders. In this review, we highlight recent discoveries concerning S-nitrosylation of the tricarboxylic acid cycle enzymes, mitochondrial fission GTPase dynamin-related protein 1, and mitophagy-related proteins Parkin and phosphatase and tensin homolog-induced putative kinase protein 1. We delineate signaling cascades affected by pathologically S-nitrosylated proteins that diminish mitochondrial function in neurodegenerative diseases. Future Directions: Further elucidation of the pathological events resulting from aberrant S-nitrosothiol or nitrotyrosine formation may lead to new therapeutic approaches to ameliorate neurodegenerative disorders.

Transient receptor potential (TRP) channels: Biosensors for redox environmental stimuli and cellular status

Transient receptor potential (TRP) channels are a family of cation channels that depolarizes the membrane potential and regulates intracellular concentrations of cations such as Ca²⁺. TRP channels are also known to function as "biosensors" to detect changes of the surrounding environment and cellular status. Lines of evidence have unveiled that numerous proteins are subject to redox modification and subsequent signaling. For example, TRPM2, TRPC5, TRPV1, and TRPA1 are known as redox sensors activated by hydrogen peroxide (H₂O₂), nitric oxide (NO), and electrophiles. Thus, these channels facilitate the influx of cations which in turn triggers the appropriate cellular responses against environmental redox stimuli and cellular redox status. In this review, we focus on the recent findings regarding the functions of TRP channels in relation to other ion channels, and other proteins which also go through redox modification of cysteine (Cys) residues. We aim to understand the structural and molecular basis of the redox-sensing mechanisms of TRP channels in exerting various functions under physiological conditions as well as pathological conditions such as cancer malignancy. Their future potential as drug targets will also be discussed.

Mitoproteomics: Tackling Mitochondrial Dysfunction in Human Disease

Mitochondria are highly dynamic and regulated organelles that historically have been defined based on their crucial role in cell metabolism. However, they are implicated in a variety of other important functions, making mitochondrial dysfunction an important axis in several pathological contexts. Despite that conventional biochemical and molecular biology approaches have provided significant insight into mitochondrial functionality, innovative techniques that provide a global view of the mitochondrion are still necessary. Proteomics fulfils this need by enabling accurate, systems-wide quantitative analysis of protein abundance. More importantly, redox proteomics approaches offer unique opportunities to tackle oxidative stress, a phenomenon that is intimately linked to aging, cardiovascular disease, and cancer. In addition, cutting-edge proteomics approaches reveal how proteins exert their functions in complex interaction networks where even subtle alterations stemming from early pathological states can be monitored. Here, we describe the proteomics approaches that will help to deepen the role of mitochondria in health and disease by assessing not only changes to mitochondrial protein composition but also alterations to their redox state and how protein interaction networks regulate mitochondrial function and dynamics. This review is aimed at showing the reader how the application of proteomics approaches during the last 20 years has revealed crucial mitochondrial roles in the context of aging, neurodegenerative disorders, metabolic disease, and cancer.

Nitric oxide mediated redox regulation of protein homeostasis

Nitric oxide is a versatile diffusible signaling molecule, whose biosynthesis by three NO synthases (NOS) is tightly regulated at transcriptional and posttranslational levels, availability of co-factors, and calcium binding. Above normal levels of NO have beneficial protective effects for example in the cardiovascular system, but also contribute to the pathophysiology in the context of inflammatory diseases, and to aging and neurodegeneration in the nervous system. The effect specificity relies on the functional and spatial specificity of the NOS isoenzymes, and on the duality of two major signaling mechanisms (i) activation of soluble guanylycylase (sGC)-dependent cGMP production and (ii) direct S-nitrosylation of redox sensitive cysteines of susceptible proteins. The present review summarizes the functional implications of S-nitrosylation in the context of proteostasis, and focuses on two NO target proteins, heat shock cognate of 70 kDa (Hsc70/HSPA8) and the ubiquitin 2 ligase (UBE2D), because both are modified on functionally critical cysteines and are key regulators of chaperone mediated and assisted autophagy and proteasomal protein degradation. SNO modifications of these candidates are associated with protein accumulations and adoption of a senescent phenotype of neuronal cells suggesting that S-nitrosylations of protein homeostatic machineries contribute to aging phenomena.

Redox-proteomes of human NOS1-transduced versus MOCK SH-SY5Y neuroblastoma cells under full nutrition, serum-free starvation, and rapamycin treatment

Upregulations of neuronal nitric oxide synthase (nNOS/NOS1) in the mouse brain upon aging and stress suggest a role of NO-dependent redox protein modifications for age-associated protein imbalances or dysfunctions. We generated a cell model, in which constitutive expression of nNOS in SH-SY5Y cells at a level comparable with mouse brain replicates the aging phenotype, that is, slowing of cell proliferation, cell enlargement, and expression of senescence markers. nNOS+ and MOCK cells were exposed to proteostasis stress by the treatment with rapamycin or serum-free starvation versus control conditions. To analyze NO-mediated S-nitrosylations (SNO) and other reversible protein modifications including disulfides and sulfoxides, we used complimentary proteomic approaches encompassing 2D-SNO-DIGE (differential gel electrophoresis), SNO-site identification (SNOSID), SNO Super-SILAC, SNO BIAM-Switch, and Redox-BIAM switch. The redox proteomes were analyzed using hybrid liquid chromatography/mass spectrometry (LC/MS). Full scan MS-data were acquired using Xcalibur, and raw mass spectra were analyzed using the proteomics software MaxQuant. The human reference proteome sets from uniprot were used as templates to identify peptides and proteins and quantify protein expression. The DiB data file contains MaxQuant output tables of the redox-modified proteins.The tables include peptide and protein identification, accession numbers, protein, and gene names, sequence coverage and quantification values of each sample. Differences in protein redox modifications in MOCK versus nNOS+ SH-SY5Y cells and interpretation of results are presented in (Valek et al., 2018).

Nitric oxide contributes to protein homeostasis by S-nitrosylations of the chaperone HSPA8 and the ubiquitin ligase UBE2D

Upregulations of neuronal nitric oxide synthase (nNOS) in the rodent brain have been associated with neuronal aging. To address underlying mechanisms we generated SH-SY5Y neuronal cells constitutively expressing nNOS at a level similar to mouse brain (nNOS+ versus MOCK). Initial experiments revealed S-nitrosylations (SNO) of key players of protein homeostasis: heat shock cognate HSC70/HSPA8 within its nucleotide-binding site, and UBE2D ubiquitin conjugating enzymes at the catalytic site cysteine. HSPA8 is involved in protein folding, organelle import/export and chaperone-mediated LAMP2a-dependent autophagy (CMA). A set of deep redox and full proteome analyses, plus analysis of autophagy, CMA and ubiquitination with rapamycin and starvation as stimuli confirmed the initial observations and revealed a substantial increase of SNO modifications in nNOS+ cells, in particular targeting protein networks involved in protein catabolism, ubiquitination, carbohydrate metabolism and cell cycle control. Importantly, NO-independent reversible oxidations similarly occurred in both cell lines. Functionally, nNOS caused an accumulation of proteins, including CMA substrates and loss of LAMP2a. UBE2D activity and proteasome activity were impaired, resulting in dysregulations of cell cycle checkpoint proteins. The observed changes of protein degradation pathways caused an expansion of the cytoplasm, large lysosomes, slowing of the cell cycle and suppression of proliferation suggesting a switch of the phenotype towards aging, supported by downregulations of neuronal progenitor markers but increase of senescence-associated proteins. Hence, upregulation of nNOS in neuronal cells imposes aging by SNOing of key players of ubiquitination, chaperones and of substrate proteins leading to interference with crucial steps of protein homeostasis.

Biotin Switch Assays for Quantitation of Reversible Cysteine Oxidation

Thiol groups in protein cysteine residues can be subjected to different oxidative modifications by reactive oxygen/nitrogen species. Reversible cysteine oxidation, including S-nitrosylation, S-sulfenylation, S-glutathionylation, and disulfide formation, modulate multiple biological functions, such as enzyme catalysis, antioxidant, and other signaling pathways. However, the biological relevance of reversible cysteine oxidation is typically underestimated, in part due to the low abundance and high reactivity of some of these modifications, and the lack of methods to enrich and quantify them. To facilitate future research efforts, this chapter describes detailed procedures to target the different modifications using mass spectrometry-based biotin switch assays. By switching the modification of interest to a biotin moiety, these assays leverage the high affinity between biotin and avidin to enrich the modification. The use of stable isotope labeling and a range of selective reducing agents facilitate the quantitation of individual as well as total reversible cysteine oxidation. The biotin switch assay has been widely applied to the quantitative analysis of S-nitrosylation in different disease models and is now also emerging as a valuable research tool for other oxidative cysteine modifications, highlighting its relevance as a versatile, robust strategy for carrying out in-depth studies in redox proteomics.

Nitroxidative Signaling Mechanisms in Pathological Pain

Tissue injury can initiate bidirectional signaling between neurons, glia, and immune cells that creates and amplifies pain. While the ability for neurotransmitters, neuropeptides, and cytokines to initiate and maintain pain has been extensively studied, recent work has identified a key role for reactive oxygen and nitrogen species (ROS/RNS; nitroxidative species), including superoxide, peroxynitrite, and hydrogen peroxide. In this review we describe how nitroxidative species are generated after tissue injury and the mechanisms by which they enhance neuroexcitability in pain pathways. Finally, we discuss potential therapeutic strategies for normalizing nitroxidative signaling, which may also enhance opioid analgesia, to help to alleviate the enormous burden of pathological pain.

Protein Phosphorylation and Redox Modification in Stomatal Guard Cells

Post-translational modification (PTM) is recognized as a major process accounting for protein structural variation, functional diversity, and the dynamics and complexity of the proteome. Since PTMs can change the structure and function of proteins, they are essential to coordinate signaling networks and to regulate important physiological processes in eukaryotes. Plants are constantly challenged by both biotic and abiotic stresses that reduce productivity, causing economic losses in crops. The plant responses involve complex physiological, cellular, and molecular processes, with stomatal movement as one of the earliest responses. In order to activate such a rapid response, stomatal guard cells employ cellular PTMs of key protein players in the signaling pathways to regulate the opening and closure of the stomatal pores. Here we discuss two major types of PTMs, protein phosphorylation and redox modification that play essential roles in stomatal movement under stress conditions. We present an overview of PTMs that occur in stomatal guard cells, especially the methods and technologies, and their applications in PTM identification and quantification. Our focus is on PTMs that modify molecular components in guard cell signaling at the stages of signal perception, second messenger production, as well as downstream signaling events and output. Improved understanding of guard cell signaling will enable generation of crops with enhanced stress tolerance, and increased yield and bioenergy through biotechnology and molecular breeding.

Redoxins in peripheral neurons after sciatic nerve injury

Peripheral nerve injury causes redox stress in injured neurons by upregulations of pro-oxidative enzymes, but most neurons survive suggesting an activation of endogenous defense against the imbalance. As potential candidates we assessed thioredoxin-fold proteins, called redoxins, which maintain redox homeostasis by reduction of hydrogen peroxide or protein dithiol-disulfide exchange. Using a histologic approach, we show that the peroxiredoxins (Prdx1-6), the glutaredoxins (Glrx1, 2, 3 and 5), thioredoxin (Txn1 and 2) and their reductases (Txnrd1 and 2) are expressed in neurons, glial and/or vascular cells of the dorsal root ganglia (DRGs) and in the spinal cord. They show distinct cellular and subcellular locations in agreement with the GO terms for "cellular component". The expression and localization of Glrx, Txn and Txnrd proteins was not affected by sciatic nerve injury but peroxiredoxins were upregulated in the DRGs, Prdx1 and Prdx6 mainly in non-neuronal cells and Prdx4 and Prdx5 in DRG neurons, the latter associated with an increase of respective mRNAs and protein accumulation in peripheral and/or central fibers. The upregulation of Prdx4 and Prdx5 in DRG neurons was reduced in mice with a cre-loxP mediated deficiency of hypoxia inducible factor 1 alpha (HIF1α) in these neurons. The results identify Prdx4 and Prdx5 as endogenous HIF1α-dependent, transcriptionally regulated defenders of nerve injury evoked redox stress that may be important for neuronal survival and regeneration.

Crosstalk between nitric oxide and glutathione is required for NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1)-dependent defense signaling in Arabidopsis thaliana

Nitric oxide (NO) is a ubiquitous signaling molecule involved in a wide range of physiological and pathophysiological processes in animals and plants. Although its significant influence on plant immunity is well known, information about the exact regulatory mechanisms and signaling pathways involved in the defense response to pathogens is still limited. We used genetic, biochemical, pharmacological approaches in combination with infection experiments to investigate the NO-triggered salicylic acid (SA)-dependent defense response in Arabidopsis thaliana. The NO donor S-nitrosoglutathione (GSNO) promoted the nuclear accumulation of NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1) protein accompanied by an elevated SA concentration and the activation of pathogenesis-related (PR) genes, leading to induced resistance of A. thaliana against Pseudomonas infection. Moreover, NO induced a rapid change in the glutathione status, resulting in increased concentrations of glutathione, which is required for SA accumulation and activation of the NPR1-dependent defense response. Our data imply crosstalk between NO and glutathione, which is integral to the NPR1-dependent defense signaling pathway, and further demonstrate that glutathione is not only an important cellular redox buffer but also a signaling molecule in the plant defense response.

Differential alkylation-based redox proteomics--Lessons learnt

Cysteine is one of the most reactive amino acids. This is due to the electronegativity of sulphur atom in the side chain of thiolate group. It results in cysteine being present in several distinct redox forms inside the cell. Amongst these, reversible oxidations, S-nitrosylation and S-sulfenylation are crucial mediators of intracellular redox signalling, with known associations to health and disease. Study of their functionalities has intensified thanks to the development of various analytical strategies, with particular contribution from differential alkylation-based proteomics methods. Presented here is a critical evaluation of differential alkylation-based strategies for the analysis of S-nitrosylation and S-sulfenylation. The aim is to assess the current status and to provide insights for future directions in the dynamically evolving field of redox proteomics. To achieve that we collected 35 original research articles published since 2010 and analysed them considering the following parameters, (i) resolution of modification site, (ii) quantitative information, including correction of modification levels by protein abundance changes and determination of modification site occupancy, (iii) throughput, including the amount of starting material required for analysis. The results of this meta-analysis are the core of this review, complemented by issues related to biological models and sample preparation in redox proteomics, including conditions for free thiol blocking and labelling of target cysteine oxoforms.

Nitric oxide-mediated pain processing in the spinal cord

A large body of evidence indicates that nitric oxide (NO) plays an important role in the processing of persistent inflammatory and neuropathic pain in the spinal cord. Several animal studies revealed that inhibition or knockout of NO synthesis ameliorates persistent pain. However, spinal delivery of NO donors caused dual pronociceptive and antinociceptive effects, pointing to multiple downstream signaling mechanisms of NO. This review summarizes the localization and function of NO-dependent signaling mechanisms in the spinal cord, taking account of the recent progress made in this field.

Reversible redox modifications in the microglial proteome challenged by beta amyloid

Reversible redox modifications of the microglial proteome contribute to switching of these neuronal sentinel cells toward a neuroinflammatory phenotype.

Hypoxia-inducible factor 1 regulates heat and cold pain sensitivity and persistence

AIMS

The present study assessed the functions of the transcription factor hypoxia-inducible factor (HIF) in sensory neurons in models of acute, inflammatory, ischemic, and neuropathic pain. The alpha subunit, HIF1α, was specifically deleted in neurons of the dorsal root ganglia by mating HIF1α(fl/fl) mice with SNScre mice.

RESULTS

SNS-HIF1α(-/-) mice were more sensitive to noxious heat and cold pain stimulation than were HIF1α(fl/fl) control mice. They also showed heightened first-phase nociceptive responses in the formalin and capsaicin tests with increased numbers of cFos-positive neurons in the dorsal horn, and intensified hyperalgesia in early phases after paw inflammation and hind limb ischemia/reperfusion. The behavioral cold and heat pain hypersensitivity was explained by increased calcium fluxes after transient receptor potential channel activation in primary sensory neurons of SNS-HIF1α(-/-) mice and lowered electrical activation thresholds of sensory fibers. SNS-HIF1α(-/-) mice however, developed less neuropathic pain after sciatic nerve injury, which was associated with an abrogation of HIF1-mediated gene up-regulation.

INNOVATION

The results suggest that HIF1α is protective in terms of acute heat and cold pain but in case of ongoing activation in injured neurons, it may promote the development of neuropathic pain.

CONCLUSION

The duality of HIF1 in pain regulation may have an impact on the side effects of drugs targeting HIF1, which are being developed, for example, as anticancer agents. Specifically, in patients with cancer neuropathy, however, temporary HIF1 inhibition might provide a welcome combination of growth and pain reduction.

Mitochondrial respiratory chain complexes as sources and targets of thiol-based redox-regulation

The respiratory chain of the inner mitochondrial membrane is a unique assembly of protein complexes that transfers the electrons of reducing equivalents extracted from foodstuff to molecular oxygen to generate a proton-motive force as the primary energy source for cellular ATP-synthesis. Recent evidence indicates that redox reactions are also involved in regulating mitochondrial function via redox-modification of specific cysteine-thiol groups in subunits of respiratory chain complexes. Vice versa the generation of reactive oxygen species (ROS) by respiratory chain complexes may have an impact on the mitochondrial redox balance through reversible and irreversible thiol-modification of specific target proteins involved in redox signaling, but also pathophysiological processes. Recent evidence indicates that thiol-based redox regulation of the respiratory chain activity and especially S-nitrosylation of complex I could be a strategy to prevent elevated ROS production, oxidative damage and tissue necrosis during ischemia-reperfusion injury. This review focuses on the thiol-based redox processes involving the respiratory chain as a source as well as a target, including a general overview on mitochondria as highly compartmentalized redox organelles and on methods to investigate the redox state of mitochondrial proteins. This article is part of a Special Issue entitled: Thiol-Based Redox Processes.

Proteomic approaches to evaluate protein S-nitrosylation in disease

Many of nitric oxide (NO) actions are mediated through the coupling of a nitroso moiety to a reactive cysteine leading to the formation of a S-nitrosothiol (SNO), a process known as S-nitrosylation or S-nitrosation. In many cases this reversible post-translational modification is accompanied by altered protein function and aberrant S-nitrosylation of proteins, caused by altered production of NO and/or impaired SNO homeostasis, has been repeatedly reported in a variety of pathophysiological settings. A growing number of studies are directed to the identification and characterization of those proteins that undergo S-nitrosylation and the analysis of S-nitrosoproteomes under pathological conditions is beginning to be reported. The study of these S-nitrosoproteomes has been fueled by advances in proteomic technologies that are providing researchers with improved tools for exploring this post-translational modification. Here we review novel refinements and improvements to these methods, and some recent studies of the S-nitrosoproteome in disease.

Salinity-induced changes in S-nitrosylation of pea mitochondrial proteins

Together with reactive oxygen species, nitric oxide is an essential part of the signal transduction induced by stress conditions. In this work we study the pattern of S-nitrosylated proteins from mitochondria of pea plants subjected to 150mM NaCl for 5 and 14days. A differential pattern of target proteins was found during plant development and salt stress, with a minor number of S-nitrosylated proteins at 14 days specifically some key enzymes related to respiration and photorespiration. At this time of stress, only ATP synthase β subunit, peroxiredoxin and Hsp90 were S-nitrosylated and no changes in protein levels were observed, although the activity of PrxII F may be reduced by S-nitrosylation. The NADH/NAD(+) ratio was also high at 14days but not the NADPH/NADP(+). An enhancement in NO measured by fluorimetry and confocal microscopy was observed in leaves, being part of the NO localized in mitochondria. An increase in mitochondrial GSNOR activity was produced in response to short and long-term NaCl treatment, where a higher number of nitrated proteins were also observed. The results indicated that posttranslational modifications seem to modulate respiratory and photorespiratory pathways, as well as some antioxidant enzymes, through differential S-nitrosylation/denitrosylation in control conditions and under salt stress.