S-Glutathionyl quantification in the attomole range using glutaredoxin-3-catalyzed cysteine derivatization and capillary gel electrophoresis with laser-induced fluorescence detection

S-Glutathionylation and S-Nitrosylation in Mitochondria: Focus on Homeostasis and Neurodegenerative Diseases

Redox post-translational modifications are derived from fluctuations in the redox potential and modulate protein function, localization, activity and structure. Amongst the oxidative reversible modifications, the S-glutathionylation of proteins was the first to be characterized as a post-translational modification, which primarily protects proteins from irreversible oxidation. However, a growing body of evidence suggests that S-glutathionylation plays a key role in core cell processes, particularly in mitochondria, which are the main source of reactive oxygen species. S-nitrosylation, another post-translational modification, was identified >150 years ago, but it was re-introduced as a prototype cell-signaling mechanism only recently, one that tightly regulates core processes within the cell’s sub-compartments, especially in mitochondria. S-glutathionylation and S-nitrosylation are modulated by fluctuations in reactive oxygen and nitrogen species and, in turn, orchestrate mitochondrial bioenergetics machinery, morphology, nutrients metabolism and apoptosis. In many neurodegenerative disorders, mitochondria dysfunction and oxidative/nitrosative stresses trigger or exacerbate their pathologies. Despite the substantial amount of research for most of these disorders, there are no successful treatments, while antioxidant supplementation failed in the majority of clinical trials. Herein, we discuss how S-glutathionylation and S-nitrosylation interfere in mitochondrial homeostasis and how the deregulation of these modifications is associated with Alzheimer’s, Parkinson’s, amyotrophic lateral sclerosis and Friedreich’s ataxia.

Dysregulation of the glutaredoxin/S-glutathionylation redox axis in lung diseases

Glutathione is a major redox buffer, reaching millimolar concentrations within cells and high micromolar concentrations in airways. While glutathione has been traditionally known as an antioxidant defense mechanism that protects the lung tissue from oxidative stress, glutathione more recently has become recognized for its ability to become covalently conjugated to reactive cysteines within proteins, a modification known as S-glutathionylation (or S-glutathiolation or protein mixed disulfide). S-glutathionylation has the potential to change the structure and function of the target protein, owing to its size (the addition of three amino acids) and charge (glutamic acid). S-glutathionylation also protects proteins from irreversible oxidation, allowing them to be enzymatically regenerated. Numerous enzymes have been identified to catalyze the glutathionylation/deglutathionylation reactions, including glutathione S-transferases and glutaredoxins. Although protein S-glutathionylation has been implicated in numerous biological processes, S-glutathionylated proteomes have largely remained unknown. In this paper, we focus on the pathways that regulate GSH homeostasis, S-glutathionylated proteins, and glutaredoxins, and we review methods required toward identification of glutathionylated proteomes. Finally, we present the latest findings on the role of glutathionylation/glutaredoxins in various lung diseases: idiopathic pulmonary fibrosis, asthma, and chronic obstructive pulmonary disease.

The roles of S-nitrosylation and S-glutathionylation in Alzheimer's disease

Alzheimer's disease (AD) is a debilitating dementia with complex pathophysiological alterations including modifications to endogenous cysteine. S-nitrosylation (SNO) is a well-studied posttranslational modification (PTM) in the context of AD while S-glutathionylation (PSSG) remains less studied. Excess reactive oxygen and reactive nitrogen species (ROS/RNS) directly or indirectly generate SNO and PSSG. SNO is dysregulated in AD and plays a pervasive role in processes such as protein function, cell signaling, metabolism, and apoptosis. Despite some studies into the role of SNO in AD, multiple identified SNO proteins lack deep investigation and SNO modifications outside of brain tissues are limited, leaving the full role of SNO in AD to be elucidated. PSSG homeostasis is perturbed in AD and may affect a myriad of cellular processes. Here we overview the role of nitric oxide (NO) in AD, discuss proteomic methodologies to investigate SNO and PSSG, and review SNO and PSSG in AD. A more thorough understanding of SNO, PSSG, and other cysteinyl PTMs in AD will be helpful for the development of novel therapeutics against neurodegenerative diseases.

Glutathionylation: a regulatory role of glutathione in physiological processes

Glutathione (γ-glutamyl-cysteinyl-glycine) is an intracellular thiol molecule and a potent antioxidant that participates in the toxic metabolism phase II biotransformation of xenobiotics. It can bind to a variety of proteins in a process known as glutathionylation. Protein glutathionylation is now recognised as one of important posttranslational regulatory mechanisms in cell and tissue physiology. Direct and indirect regulatory roles in physiological processes include glutathionylation of major transcriptional factors, eicosanoids, cytokines, and nitric oxide (NO). This review looks into these regulatory mechanisms through examples of glutathione regulation in apoptosis, vascularisation, metabolic processes, mitochondrial integrity, immune system, and neural physiology. The focus is on the physiological roles of glutathione beyond biotransformational metabolism.

Corticotropin-releasing factor receptor-1 modulates biomarkers of DNA oxidation in Alzheimer's disease mice

Increased production of hydroxyl radical is the main source of oxidative damage in mammalian DNA that accumulates in Alzheimer’s disease (AD). Reactive oxygen species (ROS) react with both nuclear DNA (nDNA) and mitochondrial DNA (mtDNA) to generate 8-hydroxy-2’-deoxyguanosine (8-OHdG), both of which can be measured in the urine. Knowledge of this pathway has positioned measurement of urine 8-OHdG as a reliable index of DNA oxidation and a potential biomarker target for tracking early cellular dysfunction in AD. Furthermore, epigenetic studies demonstrate decreased global DNA methylation levels (e.g. 5-methyl-2’-deoxycytidine, 5-mdC) in AD tissues. Moreover, stress hormones can activate neuronal oxidative stress which will stimulate the release of additional stress hormones and result in damages to hippocampal neurons in the AD brain. Our previous work suggests that treating AD transgenic mice the type-1 corticotropin-releasing factor receptor (CRFR1) antagonist, R121919, to reduce stress signaling, prevented onset of cognitive impairment, synaptic/dendritic loss and Aβ plaque accumulation. Therefore, to investigate whether levels of DNA oxidation can be impacted by the same therapeutic approach, urine levels of hydrogen peroxide, 8-OHdG, 5-mdC and total antioxidant capacity (TAC) were analyzed using an AD Tg mouse model. We found that Tg animals had an 80% increase in hydrogen peroxide levels compared to wild type (Wt) counterparts, an effect that could be dramatically reversed by the chronic administration with R121919. A significant decrease of 8-OHdG levels was observed in Tg mice treated with CRFR1 antagonist. Collectively our data suggest that the beneficial effects of CRFR1 antagonism seen in Tg mice may be mechanistically linked to the modulation of oxidative stress pathways.

Quantitative proteomic characterization of redox-dependent post-translational modifications on protein cysteines

Protein thiols play a crucial role in redox signaling, in the regulation of enzymatic activity and protein function, and in maintaining redox homeostasis in living systems. The unique chemical reactivity of the thiol group makes protein cysteines susceptible to reactions with reactive oxygen and nitrogen species that form various reversible and irreversible post-translational modifications (PTMs). The reversible PTMs in particular are major components of redox signaling and are involved in the regulation of various cellular processes under physiological and pathological conditions. The biological significance of these redox PTMs in both healthy and disease states has been increasingly recognized. Herein, we review recent advances in quantitative proteomic approaches for investigating redox PTMs in complex biological systems, including general considerations of sample processing, chemical or affinity enrichment strategies, and quantitative approaches. We also highlight a number of redox proteomic approaches that enable effective profiling of redox PTMs for specific biological applications. Although technical limitations remain, redox proteomics is paving the way to a better understanding of redox signaling and regulation in both healthy and disease states.

Corticotropin-Releasing Factor Receptor-1 Antagonism Reduces Oxidative Damage in an Alzheimer’s Disease Transgenic Mouse Model

Reports from Alzheimer’s disease (AD) biomarker work have shown a strong link between oxidative stress and AD neuropathology. The nonenzymatic antioxidant, glutathione (GSH), plays a crucial role in defense against reactive oxygen species and maintenance of GSH redox homeostasis. In particular, our previous studies on GSH redox imbalance have implicated oxidative stress induced by excessive reactive oxygen species as a major mediator of AD-like events, with the presence of S- glutathionylated proteins (Pr-SSG) appearing prior to overt AD neuropathology. Furthermore, evidence suggests that oxidative stress may be associated with dysfunction of the hypothalamic-pituitary-adrenal axis, leading to activation of inflammatory pathways and increased production of corticotropin-releasing factor (CRF). Therefore, to investigate whether oxidative insults can be attenuated by reduction of central CRF signaling, we administered the type-1 CRF receptor (CRFR1) selective antagonist, R121919, to AD-transgenic mice beginning in the preclinical/prepathologic period (30-day-old) for 150 days, a time point where behavioral impairments and pathologic progression should be measureable. Our results indicate that R121919 treatment can significantly reduce Pr-SSG levels and increase glutathione peroxide activity, suggesting that interference of CRFR1 signaling may be useful as a preventative therapy for combating oxidative stress in AD.

The Measurement of Reversible Redox Dependent Post-translational Modifications and Their Regulation of Mitochondrial and Skeletal Muscle Function

Mitochondrial oxidative stress is a common feature of skeletal myopathies across multiple conditions; however, the mechanism by which it contributes to skeletal muscle dysfunction remains controversial. Oxidative damage to proteins, lipids, and DNA has received the most attention, yet an important role for reversible redox post-translational modifications (PTMs) in pathophysiology is emerging. The possibility that these PTMs can exert dynamic control of muscle function implicates them as a mechanism contributing to skeletal muscle dysfunction in chronic disease. Herein, we discuss the significance of thiol-based redox dependent modifications to mitochondrial, myofibrillar, and excitation-contraction (EC) coupling proteins with an emphasis on how these changes could alter skeletal muscle performance under chronically stressed conditions. A major barrier to a better mechanistic understanding of the role of reversible redox PTMs in muscle function is the technical challenges associated with accurately measuring the changes of site-specific redox PTMs. Here we will critically review current approaches with an emphasis on sample preparation artifacts, quantitation, and specificity. Despite these challenges, the ability to accurately quantify reversible redox PTMs is critical to understanding the mechanisms by which mitochondrial oxidative stress contributes to skeletal muscle dysfunction in chronic diseases.

Characterization of ATP alternations in an Alzheimer's disease transgenic mouse model

Mitochondrial impairment as evidenced by decline in adenosine 5'-triphosphate (ATP) is associated with oxidative stress in Alzheimer's disease neuropathology and suggests that mitochondria may fail to maintain cellular energy, through reduced ATP production in neurons. To gain insights into the ATP characteristics of Alzheimer's disease transgenic (Tg) mice, we investigated ATP contents in the brain and whole blood of Tg mice at three ages (1-, 5-, and 24-months old). Overall, our results demonstrate that tissue ATP contents in Tg mice are significantly reduced, suggesting a decrease of tissue ATP production and mitochondrial dysfunction.

Comparability of a three-dimensional structure in biopharmaceuticals using spectroscopic methods

Protein structure depends on weak interactions and covalent bonds, like disulfide bridges, established according to the environmental conditions. Here, we present the validation of two spectroscopic methodologies for the measurement of free and unoxidized thiols, as an attribute of structural integrity, using 5,5'-dithionitrobenzoic acid (DTNB) and DyLight Maleimide (DLM) as derivatizing agents. These methods were used to compare Rituximab and Etanercept products from different manufacturers. Physicochemical comparability was demonstrated for Rituximab products as DTNB showed no statistical differences under native, denaturing, and denaturing-reducing conditions, with Student's t-test P values of 0.6233, 0.4022, and 0.1475, respectively. While for Etanercept products no statistical differences were observed under native (P = 0.0758) and denaturing conditions (P = 0.2450), denaturing-reducing conditions revealed cysteine contents of 98% and 101%, towards the theoretical value of 58, for the evaluated products from different Etanercept manufacturers. DLM supported equality between Rituximab products under native (P = 0.7499) and denaturing conditions (P = 0.8027), but showed statistical differences among Etanercept products under native conditions (P < 0.001). DLM suggested that Infinitam has fewer exposed thiols than Enbrel, although DTNB method, circular dichroism (CD), fluorescence (TCSPC), and activity (TNF α neutralization) showed no differences. Overall, this data revealed the capabilities and drawbacks of each thiol quantification technique and their correlation with protein structure.

Proteomic identification and quantification of S-glutathionylation in mouse macrophages using resin-assisted enrichment and isobaric labeling

S-Glutathionylation (SSG) is an important regulatory posttranslational modification on protein cysteine (Cys) thiols, yet the role of specific cysteine residues as targets of modification is poorly understood. We report a novel quantitative mass spectrometry (MS)-based proteomic method for site-specific identification and quantification of S-glutathionylation across different conditions. Briefly, this approach consists of initial blocking of free thiols by alkylation, selective reduction of glutathionylated thiols, and covalent capture of reduced thiols using thiol affinity resins, followed by on-resin tryptic digestion and isobaric labeling with iTRAQ (isobaric tags for relative and absolute quantitation) for MS-based identification and quantification. The overall approach was initially validated by application to RAW 264.7 mouse macrophages treated with different doses of diamide to induce glutathionylation. A total of 1071 Cys sites from 690 proteins were identified in response to diamide treatment, with ~90% of the sites displaying >2-fold increases in SSG modification compared to controls. This approach was extended to identify potential SSG-modified Cys sites in response to H2O2, an endogenous oxidant produced by activated macrophages and many pathophysiological stimuli. The results revealed 364 Cys sites from 265 proteins that were sensitive to S-glutathionylation in response to H2O2 treatment, thus providing a database of proteins and Cys sites susceptible to this modification under oxidative stress. Functional analysis revealed that the most significantly enriched molecular function categories for proteins sensitive to SSG modifications were free radical scavenging and cell death/survival. Overall the results demonstrate that our approach is effective for site-specific identification and quantification of SSG-modified proteins. The analytical strategy also provides a unique approach to determining the major pathways and cellular processes most susceptible to S-glutathionylation under stress conditions.

Long-Term Electrophysiological and Behavioral Analysis on the Improvement of Visual Working Memory Load, Training Gains, and Transfer Benefits

Recent evidence demonstrates that with training, one can enhance visual working memory (VWM) capacity and attention over time in the near transfer tasks. Not only do these studies reveal the characteristics of VWM load and the influences of training, they may also provide insights into developing effective rehabilitation for patients with VWM deficiencies. However, few studies have investigated VWM over extended periods of time and evaluated transfer benefits on non-trained tasks. Here, we combined behavioral and electroencephalographical approaches to investigate VWM load, training gains, and transfer benefits. Our results reveal that VWM capacity is directly correlated to the difference of event-related potential waveforms. In particular, the “magic number 4” can be observed through the contralateral delay amplitude and the average capacity is 3.25-item over 15 participants. Furthermore, our findings indicate that VWM capacity can be improved through training; and after training exercises, participants from the training group are able to dramatically improve their performance. Likewise, the training effects on non-trained tasks can also be observed at the 12th week after training. Therefore, we conclude that participants can benefit from training gains, and augmented VWM capacity sustained over long periods of time on specific variety of tasks.

Resin-assisted enrichment of thiols as a general strategy for proteomic profiling of cysteine-based reversible modifications

Reversible modifications of cysteine thiols have a key role in redox signaling and regulation. A number of reversible redox modifications, including disulfide formation, S-nitrosylation (SNO) and S-glutathionylation (SSG), have been recognized for their significance in various physiological and pathological processes. Here we describe a procedure for the enrichment of peptides containing reversible cysteine modifications. Starting with tissue or cell lysate samples, all of the unmodified free thiols are blocked using N-ethylmaleimide (NEM). This is followed by the selective reduction of those cysteines bearing the reversible modification(s) of interest. The reduction is achieved by using different reducing reagents that react specifically with each type of cysteine modification (e.g., ascorbate for SNO). This protocol serves as a general approach for enrichment of thiol-containing proteins or peptides derived from reversibly modified proteins. The approach uses a commercially available thiol-affinity resin (thiopropyl Sepharose 6B) to directly capture free thiol-containing proteins through a disulfide exchange reaction, followed by on-resin protein digestion and multiplexed isobaric labeling to facilitate liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based quantitative site-specific analysis of cysteine-based reversible modifications. The overall approach requires a simpler workflow with increased specificity compared with the commonly used biotinylation-based assays. The procedure for selective enrichment and analyses of SNO and the level of total reversible cysteine modifications (or total oxidation) is presented to demonstrate the utility of this general strategy. The entire protocol requires ∼3 d for sample processing with an additional day for LC-MS/MS and data analysis.

Recent developments in capillary and microchip electroseparations of peptides (2011-2013)

The review presents a comprehensive survey of recent developments and applications of capillary and microchip electroseparation methods (zone electrophoresis, ITP, IEF, affinity electrophoresis, EKC, and electrochromatography) for analysis, isolation, purification, and physicochemical and biochemical characterization of peptides. Advances in the investigation of electromigration properties of peptides, in the methodology of their analysis, including sample preseparation, preconcentration and derivatization, adsorption suppression and EOF control, as well as in detection of peptides, are presented. New developments in particular CE and CEC modes are reported and several types of their applications to peptide analysis are described: conventional qualitative and quantitative analysis, determination in complex (bio)matrices, monitoring of chemical and enzymatical reactions and physical changes, amino acid, sequence and chiral analysis, and peptide mapping of proteins. Some micropreparative peptide separations are shown and capabilities of CE and CEC techniques to provide relevant physicochemical characteristics of peptides are demonstrated.

Causes and consequences of cysteine S-glutathionylation

Post-translational S-glutathionylation occurs through the reversible addition of a proximal donor of glutathione to thiolate anions of cysteines in target proteins, where the modification alters molecular mass, charge, and structure/function and/or prevents degradation from sulfhydryl overoxidation or proteolysis. Catalysis of both the forward (glutathione S-transferase P) and reverse (glutaredoxin) reactions creates a functional cycle that can also regulate certain protein functional clusters, including those involved in redox-dependent cell signaling events. For translational application, S-glutathionylated serum proteins may be useful as biomarkers in individuals (who may also have polymorphic expression of glutathione S-transferase P) exposed to agents that cause oxidative or nitrosative stress.

Detection and quantification of 8-hydroxy-2'-deoxyguanosine in Alzheimer's transgenic mouse urine using capillary electrophoresis

8-Hydroxy-2'-deoxyguanosine (8-OHdG) is one of the major forms of oxidative DNA damage, and is commonly analyzed as an excellent marker of DNA lesions. The purpose of this study was to develop a sensitive method to accurately and rapidly quantify the 8-OHdG by using CE-LIF detection. The method involved the use of specific antibody to detect the DNA lesion (8-OHdG) and consecutive fluorescence labeling. Next, urinary 8-OHdG fluorescently labeled along with other constituents were resolved by capillary electrophoretic system and the lesion of interest was detected using a fluorescence detector. The limit of detection was 0.18 fmol, which proved sufficient sensitivity for detection and quantification of 8-OHdG in untreated urine samples. The relative standard deviation was found to be 11.32% for migration time and 5.52% for peak area. To demonstrate the utility of this method, the urinary concentration of 8-OHdG in an Alzheimer's transgenic mouse model was determined. Collectively, our results indicate that this methodology offers great advantages, such as high separation efficiency, good selectivity, low limit of detection, simplicity and low cost of analysis.

Prediction of S-glutathionylated proteins progression in Alzheimer's transgenic mouse model using principle component analysis

To date, prediction of Alzheimer's disease (AD) is mainly based on clinical criteria because no well-established biochemical biomarkers for routine clinical diagnosis of AD currently exist. We developed an approach to aid in the early diagnosis of AD by using principal component analysis (PCA)-based spectral analysis of oxidized protein electrophoretic profiling. We found that the combination of capillary electrophoresis and PCA analysis of S-glutathionylation distribution characterization can be used in the sample classification and molecular weight (Mw) prediction. The comparison of leave-one-out AD versus non-AD gives the sensitivity of 100% and 93.33% in brain tissues and blood samples, respectively, while the specificity of 100% in brain and 90.0% in blood samples. Our findings demonstrate that PCA of S-glutathionylation electrophoretic profiling detects AD pathology features, and that the molecular weight based electrophoretic profiling of blood and brain S-glutathionylated proteins are sensitive to change, even at the early stage of the disease. Our results offer a previously unexplored diagnostic approach by using electrophoretic characteristics of oxidized proteins to serve as a predictor of AD progression and early stage screening.

A fluorometric method to quantify protein glutathionylation using glutathione derivatization with 2,3-naphthalenedicarboxaldehyde

This study reports the development of a new assay for the rapid determination of protein glutathionylation in tissues and cell lines using commercially available reagents and standard instrumentation. In this method cells are homogenized in the presence of N-ethylmaleimide to eliminate free thiols and the proteins are precipitated with acetone. Subsequently, the disulfide-bound glutathione is eluted from the protein by the addition of tris(2-carboxyethyl)phosphine and reacted with 2,3-napthalenedicarboxaldehyde to generate a highly fluorescent product. Lymphoblastoid cell lines were found to have glutathionylation levels in the range of 0.3-3 nmol/mg protein, which were significantly elevated after treatment of the cells with S-nitrosoglutathione. Mouse tissues including liver, kidney, lung, heart, brain, spleen, and testes were found to have glutathionylation levels between 1 and 2.5 nmol/mg protein and the levels tended to increase after treatment of mice with doxorubicin. In contrast, mouse skeletal muscle glutathionylation was significantly higher (4.2 ± 0.33 nmol/mg, p < 0.001) than in other tissues in untreated mice and decreased to 1.9 ± 0.15 nmol/mg after doxorubicin treatment. This new method allows rapid measurement of cellular glutathionylation in a high-throughput 96-well plate format.