Emerging roles of S-nitrosylation in protein misfolding and neurodegenerative diseases

Exhaled Nitric Oxide as Biomarker of Type 2 Diseases

Nitric oxide (NO) is a short-lived gas molecule which has been studied for its role as a signaling molecule in the vasculature and later, in a broader view, as a cellular messenger in many other biological processes such as immunity and inflammation, cell survival, apoptosis, and aging. Fractional exhaled nitric oxide (FeNO) is a convenient, easy-to-obtain, and non-invasive method for assessing active, mainly Th2-driven, airway inflammation, which is sensitive to treatment with standard anti-inflammatory therapy. Consequently, FeNO serves as a valued tool to aid the diagnosis and monitoring of several asthma phenotypes. More recently, FeNO has been evaluated in several other respiratory and/or immunological conditions, including allergic rhinitis, chronic rhinosinusitis with/without nasal polyps, atopic dermatitis, eosinophilic esophagitis, and food allergy. In this review, we aim to provide an extensive overview of the current state of knowledge about FeNO as a biomarker in type 2 inflammation, outlining past and recent data on the application of its measurement in patients affected by a broad variety of atopic/allergic disorders.

Deciphering the Path of S-nitrosation of Human Thioredoxin: Evidence of an Internal NO Transfer and Implication for the Cellular Responses to NO

Nitric oxide (NO) is a free radical with a signaling capacity. Its cellular functions are achieved mainly through S-nitrosation where thioredoxin (hTrx) is pivotal in the S-transnitrosation to specific cellular targets. In this study, we use NMR spectroscopy and mass spectrometry to follow the mechanism of S-(trans)nitrosation of hTrx. We describe a site-specific path for S-nitrosation by measuring the reactivity of each of the 5 cysteines of hTrx using cysteine mutants. We showed the interdependence of the three cysteines in the nitrosative site. C73 is the most reactive and is responsible for all S-transnitrosation to other cellular targets. We observed NO internal transfers leading to C62 S-nitrosation, which serves as a storage site for NO. C69-SNO only forms under nitrosative stress, leading to hTrx nuclear translocation.

Nitrative Stress and Auditory Dysfunction

Nitrative stress is increasingly recognized as a critical mediator of apoptotic cell death in many pathological conditions. The accumulation of nitric oxide along with superoxide radicals leads to the generation of peroxynitrite that can eventually result in the nitration of susceptible proteins. Nitrotyrosine is widely used as a biomarker of nitrative stress and indicates oxidative damage to proteins. Ototoxic insults, such as exposure to noise and ototoxic drugs, enhance the generation of 3-nitrotyrosine in different cell types in the cochlea. Nitrated proteins can disrupt critical signaling pathways and eventually lead to apoptosis and loss of sensory receptor cells in the cochlea. Accumulating evidence shows that selective targeting of nitrative stress attenuates cellular damage. Anti-nitrative compounds, such as peroxynitrite decomposition catalysts and inducible nitric oxide synthase inhibitors, prevent nitrative stress-mediated auditory damage. However, the role of nitrative stress in acquired hearing loss and its potential significance as a promising interventional target is yet to be fully characterized. This review provides an overview of nitrative stress mechanisms, the induction of nitrative stress in the auditory tissue after ototoxic insults, and the therapeutic value of targeting nitrative stress for mitigating auditory dysfunction.

Intracellular Protein S-Nitrosylation—A Cells Response to Extracellular S100B and RAGE Receptor

Human S100B is a small, multifunctional protein. Its activity, inside and outside cells, contributes to the biology of the brain, muscle, skin, and adipocyte tissues. Overexpression of S100B occurs in Down Syndrome, Alzheimer’s disease, Creutzfeldt–Jakob disease, schizophrenia, multiple sclerosis, brain tumors, epilepsy, melanoma, myocardial infarction, muscle disorders, and sarcopenia. Modulating the activities of S100B, related to human diseases, without disturbing its physiological functions, is vital for drug and therapy design. This work focuses on the extracellular activity of S100B and one of its receptors, the Receptor for Advanced Glycation End products (RAGE). The functional outcome of extracellular S100B, partially, depends on the activation of intracellular signaling pathways. Here, we used Biotin Switch Technique enrichment and mass-spectrometry-based proteomics to show that the appearance of the S100B protein in the extracellular milieu of the mammalian Chinese Hamster Ovary (CHO) cells, and expression of the membrane-bound RAGE receptor, lead to changes in the intracellular S-nitrosylation of, at least, more than a hundred proteins. Treatment of the wild-type CHO cells with nanomolar or micromolar concentrations of extracellular S100B modulates the sets of S-nitrosylation targets inside cells. The cellular S-nitrosome is tuned differently, depending on the presence or absence of stable RAGE receptor expression. The presented results are a proof-of-concept study, suggesting that S-nitrosylation, like other post-translational modifications, should be considered in future research, and in developing tailored therapies for S100B and RAGE receptor-related diseases.

Prevention and reversal of ketamine-induced experimental psychosis in mice by the neuroactive flavonoid, hesperidin: The role of oxidative and cholinergic mechanisms

Currently, prevailing evidence have identified cholinergic and oxidative pathways as important therapeutic targets for abating ketamine-induced schizophrenia-like behavior. Thus, this study evaluated the ability of hesperidin, a naturally occurring antioxidant and neuroprotective flavonoid, to prevent and reverse ketamine-induced schizophrenia-like behaviors and changes in cholinergic, oxidative and nitrergic status in mice. Forty-eight male Swiss mice were allotted into the preventive and reversal studies with 4 groups (n = 6) each. In the preventive study, groups 1 and 2 received vehicle (10 mL/kg/p.o./day), while groups 3 and 4 had hesperidin (100 mg/kg/p.o./day) for 14 days, but ketamine (20 mg/kg/i.p./day) was concurrently given to groups 2 and 4 from days 8-14. In the reversal study, groups 1 and 3 received vehicle, groups 2 and 4 were pretreated with ketamine for 14 days. Nevertheless, groups 3 and 4 additionally received hesperidin from days 8-14. Thereafter, schizophrenia-like behavior from exploratory activity, open-field (positive symptoms), Y-maze (cognitive symptoms) and social interaction (negative symptoms) tests were evaluated. Brain levels of oxidative/nitrergic (glutathione, superoxide-dismutase, malondialdehyde and nitrite levels) and cholinergic (acetylcholinesterase activity) markers were measured in the prefrontal-cortex, striatum and hippocampus. Hesperidin prevents and reverses ketamine-induced hyperactivities, social withdrawal and cognitive impairment. Also, hesperidin prevented and reversed ketamine-induced decrease in glutathione and superoxide-dismutase levels in the prefrontal-cortical, striatal and hippocampal brain regions in mice. Consequently, hesperidin attenuated ketamine-induced increase in malondialdehyde, nitrite levels and acetylcholinesterase activities in the prefrontal-cortex, striatum and hippocampus, respectively. The study showed that hesperidin prevents and reverses ketamine-induced schizophrenia-like behavior through inhibition of oxidative/nitrergic stress and acetylcholinesterase activity in mice brains. Therefore, these findings suggest that hesperidin dietary supplementation could provide natural nutritional intervention to protect against epigenetic-induced mental ill-health like schizophrenia, and thus serve as an important agent for nutritional psychiatry.

S-Denitrosylase-like Activity of Cyclic Diselenides Conjugated with Xaa-His Dipeptide: Role of Proline Spacer as a Key Activity Booster

This study developed dipeptide-conjugated 1,2-diselenan-4-amine (1), i.e., 1-Xaa-His, as a new class of S-denitrosylase mimic. The synthesized compounds, especially 1-Pro-His, remarkably promoted S-denitrosylation of nitrosothiols (RSNO) via a catalytic cycle involving the reversible redox reaction between the diselenide and its corresponding diselenol ([SeH,SeH]) form with coexisting reductant thiols (R'SH), during which the [SeH,SeH] form as a key reactive species reduces RSNO to the corresponding thiol (RSH). Structural analyses of 1-Pro-His suggested that the peptide backbone of [SeH,SeH] is rigidly bent to form a γ-turn, possibly including an NH···Se hydrogen bond between the imidazole ring of His and selenol group, thus stabilizing the [SeH,SeH] form thermodynamically, and dramatically enhancing the catalytic activity. Furthermore, the synthetic compounds were found to prohibit S-nitrosylation-induced protein misfolding in the presence of RSNO, eventually implying their potential as a drug seed for misfolding diseases caused by the dysregulation of the S-denitrosylation system.

Melatonin, Its Metabolites and Their Interference with Reactive Nitrogen Compounds

Melatonin and several of its metabolites are interfering with reactive nitrogen. With the notion of prevailing melatonin formation in tissues that exceeds by far the quantities in blood, metabolites come into focus that are poorly found in the circulation. Apart from their antioxidant actions, both melatonin and N¹-acetyl-5-methoxykynuramine (AMK) downregulate inducible and inhibit neuronal NO synthases, and additionally scavenge NO. However, the NO adduct of melatonin redonates NO, whereas AMK forms with NO a stable product. Many other melatonin metabolites formed in oxidative processes also contain nitrosylatable sites. Moreover, AMK readily scavenges products of the CO₂-adduct of peroxynitrite such as carbonate radicals and NO₂. Protein AMKylation seems to be involved in protective actions.

Inactivation Mechanism of Neuronal Nitric Oxide Synthase by (S)-2-Amino-5-(2-(methylthio)acetimidamido)pentanoic Acid: Chemical Conversion of the Inactivator in the Active Site

Neuronal nitric oxide synthase (nNOS) is one of the three isoforms of nitric oxide synthase (NOS). The other two isoforms include inducible NOS (iNOS) and endothelial NOS (eNOS). These three isoforms of NOS are widely present in both human and other mammals and are responsible for the biosynthesis of NO. As an essential biological molecule, NO plays an essential role in neurotransmission, immune response, and vasodilation; however, the overproduction of NO can cause a series of diseases. Thus, the selective inhibition of three isoforms of NOS has been considered to be important in treating related diseases. The active sites of the three enzymes are highly conserved, causing the selective inhibition of the three enzymes to be a great challenge. (S)-2-Amino-5-(2-(methylthio)acetimidamido)pentanoic acid (1) has been experimentally proved to be a selective and time-dependent irreversible inhibitor of nNOS, and three pathways, including sulfide oxidation, oxidative dethiolation, and oxidative demethylation, have been suggested. In this work, we performed quantum mechanics/molecular mechanics calculations to verify the chemical conversion of inactivator 1. Although we agree with the previously suggested chemical transformation process, our calculations demonstrated that there are lower energy pathways to accomplish both oxidative dethiolation and oxidative demethylation. These three branching reactions are competitive, but only dethiolation and demethylation reactions can generate inhibitory intermediates. As a powerful time-dependent irreversible inhibitor of nNOS, the key sulfur atom and middle imine are all necessary. Our calculation results not only verified the chemical reaction of inhibitor 1 occurring in the enzymatic active site but also explained the inactivation mechanism of inhibitor 1. This is also the first verified example of the heme-enzyme-catalyzed S-demethylation mechanism.

Targeting S100B Protein as a Surrogate Biomarker and its Role in Various Neurological Disorders

Neurological disorders (ND) are the central nervous system (CNS) related complications originated by enhanced oxidative stress, mitochondrial failure and overexpression of proteins like S100B. S100B is a helix-loop-helix protein with the calcium-binding domain associated with various neurological disorders through activation of the MAPK pathway, increased NF-kB expression resulting in cell survival, proliferation and gene up-regulation. S100B protein plays a crucial role in Alzheimer's disease, Parkinson's disease, multiple sclerosis, Schizophrenia and epilepsy because the high expression of this protein directly targets astrocytes and promotes neuroinflammation. Under stressful conditions, S100B produces toxic effects mediated through receptor for advanced glycation end products (AGE) binding. S100B also mediates neuroprotection, minimizes microgliosis and reduces the expression of tumor necrosis factor (TNF-alpha) but that are concentration- dependent mechanisms. Increased level of S100B is useful for assessing the release of inflammatory markers, nitric oxide and excitotoxicity dependent neuronal loss. The present review summarizes the role of S100B in various neurological disorders and potential therapeutic measures to reduce the prevalence of neurological disorders.

Protective effect of cerium oxide nanoparticles on cisplatin and oxaliplatin primary toxicities in male albino rats

Cisplatin and oxaliplatin are widely used anticancer drugs. Their use is restricted by their dose-limiting side effects: nephrotoxicity and neurotoxicity, respectively. Cerium oxide nanoparticles (CONPs) are promising antioxidant and anti-inflammatory agent. To test the possible ameliorative impact of CONPs on the toxic effect of cisplatin and oxaliplatin in male albino rats. Forty eight rats were divided into 6 groups: control group, CONPs group, cisplatin group, cisplatin and CONPs group, oxaliplatin group, and oxaliplatin and CONPs group. After 4 weeks, serum urea and creatinine, renal tissue level of interleukin 10 (IL10), and total antioxidant (TAO) were measured in control, CONPs, and cisplatin groups. The other kidney was used for histopathological and immunohistochemical studies. The right sciatic nerves and the lumbar spinal cord of rats from control, CONPs, and oxaliplatin groups were used for immunohistochemical evaluations of nitrotyrosine, myelin basic protein (MBP), and glial fibrillary acidic protein (GFAP). Cisplatin significantly increased serum urea and creatinine levels, significantly decreased the kidney level of IL10 and TAO with marked tubular necrosis, hemorrhage and renal damage. Also, it decreased IL10 immunohistochemical expression. CONPs significantly decreased the serum urea and creatinine level and increased IL10 and TAO with lower renal damage and strong IL10 expression compared with cisplatin group. Oxaliplatin significantly decreased MBP immunoreactivity and increased nitrotyrosine immunoreactivity. In the lumbar spinal cord, GFAP immunoreactivity was significantly increased. CONPs significantly increased MBP and decreased nitrotyrosine immunoreactivity. GFAP immunoreactivity was significantly decreased. CONPs ameliorated cisplatin and oxaliplatin primary toxicities through anti-inflammatory and antioxidant characteristics.

Keap1 controls protein S-nitrosation and apoptosis-senescence switch in endothelial cells

Premature senescence, a death escaping pathway for cells experiencing stress, is conducive to aging and cardiovascular diseases. The molecular switch between senescent and apoptotic fate remains, however, poorly recognized. Nrf2 is an important transcription factor orchestrating adaptive response to cellular stress. Here, we show that both human primary endothelial cells (ECs) and murine aortas lacking Nrf2 signaling are senescent but unexpectedly do not encounter damaging oxidative stress. Instead, they exhibit markedly increased S-nitrosation of proteins. A functional role of S-nitrosation is protection of ECs from death by inhibition of NOX4-mediated oxidative damage and redirection of ECs to premature senescence. S-nitrosation and senescence are mediated by Keap1, a direct binding partner of Nrf2, which colocalizes and precipitates with nitric oxide synthase (NOS) and transnitrosating protein GAPDH in ECs devoid of Nrf2. We conclude that the overabundance of this "unrestrained" Keap1 determines the fate of ECs by regulation of S-nitrosation and propose that Keap1/GAPDH/NOS complex may serve as an enzymatic machinery for S-nitrosation in mammalian cells.

Detection, identification, and quantification of oxidative protein modifications

Exposure of biological molecules to oxidants is inevitable and therefore commonplace. Oxidative stress in cells arises from both external agents and endogenous processes that generate reactive species, either purposely (e.g. during pathogen killing or enzymatic reactions) or accidentally (e.g. exposure to radiation, pollutants, drugs, or chemicals). As proteins are highly abundant and react rapidly with many oxidants, they are highly susceptible to, and major targets of, oxidative damage. This can result in changes to protein structure, function, and turnover and to loss or (occasional) gain of activity. Accumulation of oxidatively-modified proteins, due to either increased generation or decreased removal, has been associated with both aging and multiple diseases. Different oxidants generate a broad, and sometimes characteristic, spectrum of post-translational modifications. The kinetics (rates) of damage formation also vary dramatically. There is a pressing need for reliable and robust methods that can detect, identify, and quantify the products formed on amino acids, peptides, and proteins, especially in complex systems. This review summarizes several advances in our understanding of this complex chemistry and highlights methods that are available to detect oxidative modifications-at the amino acid, peptide, or protein level-and their nature, quantity, and position within a peptide sequence. Although considerable progress has been made in the development and application of new techniques, it is clear that further development is required to fully assess the relative importance of protein oxidation and to determine whether an oxidation is a cause, or merely a consequence, of injurious processes.

Dysregulation of stress systems and nitric oxide signaling underlies neuronal dysfunction in Alzheimer's disease

Stress is a multimodal response involving the coordination of numerous body systems in order to maximize the chance of survival. However, long term activation of the stress response results in neuronal oxidative stress via reactive oxygen and nitrogen species generation, contributing to the development of depression. Stress-induced depression shares a high comorbidity with other neurological conditions including Alzheimer's disease (AD) and dementia, often appearing as one of the earliest observable symptoms in these diseases. Furthermore, stress and/or depression appear to exacerbate cognitive impairment in the context of AD associated with dysfunctional catecholaminergic signaling. Given there are a number of homologous pathways involved in the pathophysiology of depression and AD, this article will highlight the mechanisms by which stress-induced perturbations in oxidative stress, and particularly NO signaling, contribute to neurodegeneration.

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.

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.

The Interplay among PINK1/PARKIN/Dj-1 Network during Mitochondrial Quality Control in Cancer Biology: Protein Interaction Analysis

PARKIN (E3 ubiquitin ligase PARK2), PINK1 (PTEN induced kinase 1) and DJ-1 (PARK7) are proteins involved in autosomal recessive parkinsonism, and carcinogenic processes. In damaged mitochondria, PINK1’s importing into the inner mitochondrial membrane is prevented, PARKIN presents a partial mitochondrial localization at the outer mitochondrial membrane and DJ-1 relocates to mitochondria when oxidative stress increases. Depletion of these proteins result in abnormal mitochondrial morphology. PINK1, PARKIN, and DJ-1 participate in mitochondrial remodeling and actively regulate mitochondrial quality control. In this review, we highlight that PARKIN, PINK1, and DJ-1 should be regarded as having an important role in Cancer Biology. The STRING database and Gene Ontology (GO) enrichment analysis were performed to consolidate knowledge of well-known protein interactions for PINK1, PARKIN, and DJ-1 and envisage new ones. The enrichment analysis of KEGG pathways showed that the PINK1/PARKIN/DJ-1 network resulted in Parkinson disease as the main feature, while the protein DJ-1 showed enrichment in prostate cancer and p53 signaling pathway. Some predicted transcription factors regulating PINK1, PARK2 (PARKIN) and PARK7 (DJ-1) gene expression are related to cell cycle control. We can therefore suggest that the interplay among PINK1/PARKIN/DJ-1 network during mitochondrial quality control in cancer biology may occur at the transcriptional level. Further analysis, like a systems biology approach, will be helpful in the understanding of PINK1/PARKIN/DJ-1 network.

Nitrosylation of GAPDH augments pathological tau acetylation upon exposure to amyloid-β

Acetylation of the microtubule-associated protein tau promotes its polymerization into neurofibrillary tangles that are implicated in the pathology of Alzheimer's disease (AD). The gaseous neurotransmitter nitric oxide (NO) regulates cell signaling through the nitrosylation of proteins. We found that NO production and tau acetylation at Lys²⁸⁰ occurred in the brain tissue in mice and in cultured mouse cortical neurons in response to exposure to amyloid-β_1-42 (Aβ_1-42), a peptide that is also implicated in AD. An increased abundance of NO facilitated the S-nitrosylation (SNO) of glyceraldehyde-3-phosphate dehydrogenase (GAPDH). S-nitrosylated GAPDH (GAPDH-SNO) promoted the acetylation and activation of the acetyltransferase p300 and facilitated the nitrosylation and inactivation of the deacetylase sirtuin 1 (SIRT1). The abundance of GAPDH-SNO was increased in postmortem brain samples from AD patients. Preventing the increase in GAPDH-SNO abundance in both cultured neurons and mice, either by overexpression of the nitrosylation mutant of GAPDH (GAPDH C150S) or by treatment with the GAPDH nitrosylation inhibitor CGP3466B (also known as omigapil), abrogated Aβ_1-42-induced tau acetylation, memory impairment, and locomotor dysfunction in mice, suggesting that this drug might be repurposed to treat patients with AD.

Dual Maleimide Tagging for Relative and Absolute Quantitation of Cysteine-Containing Peptides by MALDI-TOF MS

A dual maleimide (DuMal) tagging method has been developed for both relative and absolute quantitation of cysteine-containing peptides (CCPs) in combination with MALDI-TOF mass spectrometry. A pair of maleimides with minimal differences in their chemical structures, N-methylmaleimide and Nethylmaleimide, have been chosen to allow for the rapid (≈minutes) tagging of CCPs in the Michael addition reaction with high efficiency. It has been validated that the DuMal tagging technique is sensitive and reliable in the quantitative analysis of CCPs. Absolute quantitation of CCPs can be achieved with a detection limit as low as 7.3 nm. Relative quantitation of CCPs can be performed in various sample mixtures with consistent results (coefficient of variation <5 %). The DuMal tagging technique provides a sensitive and accurate approach for the quantitation of biomolecules containing thiol reactive sites; thus it is promising for protein detection, disease diagnosis, and biomarker discovery associated with post-translational modifications of cysteines.

Recent Progress in Alzheimer's Disease Research, Part 1: Pathology

The field of Alzheimer's disease (AD) research has grown exponentially over the past few decades, especially since the isolation and identification of amyloid-β from postmortem examination of the brains of AD patients. Recently, the Journal of Alzheimer's Disease (JAD) put forth approximately 300 research reports which were deemed to be the most influential research reports in the field of AD since 2010. JAD readers were asked to vote on these most influential reports. In this 3-part review, we review the results of the 300 most influential AD research reports to provide JAD readers with a readily accessible, yet comprehensive review of the state of contemporary research. Notably, this multi-part review identifies the "hottest" fields of AD research providing guidance for both senior investigators as well as investigators new to the field on what is the most pressing fields within AD research. Part 1 of this review covers pathogenesis, both on a molecular and macro scale. Part 2 review genetics and epidemiology, and part 3 covers diagnosis and treatment. This part of the review, pathology, reviews amyloid-β, tau, prions, brain structure, and functional changes with AD and the neuroimmune response of AD.

S-nitrosylation of the IGF-1 receptor disrupts the cell proliferative action of IGF-1

The insulin-like growth factor 1 receptor (IGF-1R) is a disulfide-linked heterotetramer containing two α-subunits and two β-subunits. Earlier studies demonstrate that nitric oxide (NO) can adversely affect IGF-1 action in the central nervous system. It is known that NO can induce S-nitrosylation of the cysteine residues in proteins, thereby partly contributing to the regulation of protein function. In the present study, we sought to determine whether S-nitrosylation of the cysteine residues in IGF-1R is an important post-translational modification that regulates its response to IGF-1. Using cultured SH-SY5Y human neuroblastoma cells as an in vitro model, we found that treatment of cells with S-nitroso-cysteine (SNOC), a NO donor that can nitrosylate the cysteine residues in proteins, induces S-nitrosylation of the β subunit of IGF-1R but not its α-subunit. IGF-1Rβ S-nitrosylation by SNOC is coupled with increased dissociation of the IGF-1R protein complex. In addition, disruption of the IGF-1R function resulting from S-nitrosylation of the IGF-1Rβ subunit is associated with disruption of the phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) signaling pathways. Further, we observed that SNOC-induced IGF-1Rβ S-nitrosylation results in a dose-dependent inhibition of cell proliferation and survival. Together, these results suggest that elevated nitrosative stress may result in dysfunction of cellular IGF-1R signaling through S-nitrosylation of the cysteine residues in the IGF-1Rβ subunit, thereby disrupting the downstream PI3K and MAPK signaling functions and ultimately resulting in inhibition of cell proliferation and survival.

Resveratrol alleviates oxidative damage in enteric neurons and associated gastrointestinal dysfunction caused by chemotherapeutic agent oxaliplatin

Oxaliplatin is a first-line chemotherapeutic agent used for the treatment of colorectal cancer. Its use is associated with severe gastrointestinal (GI) side-effects, associated with oxidative damage and neurotoxicity to the enteric neurons. Resveratrol is a potent anti-oxidant that has been shown to protect against oxidative damage and neurotoxicity in other neurons and could therefore prevent oxaliplatin-induced damage to enteric neurons. We determined whether co-administration of resveratrol with oxaliplatin alleviates enteric neuron toxicity and GI dysfunction in mice. Colons were collected for immunohistochemical analysis of myenteric neurons and assessment of motor activity in organ-bath experiments. Morphological damage to the colonic mucosa and muscles was analysed. Oxaliplatin treatment induced translocation of nitrated proteins into the nuclei of myenteric neurons and significant damage to the mucosal lining, vacuolisation and a decrease in muscle thickness. This damage is linked to motor dysfunction due to inhibition of the amplitude of colonic contractions, leading to chronic constipation. Co-treatment with resveratrol prevented oxaliplatin-induced neurotoxicity, alleviated damage to GI mucosa, crypts and muscle layer, resulting in improved contractility and a decrease in constipation. Resveratrol could be integrated as part of a therapeutic regimen to help alleviate oxaliplatin-induced GI dysfunction.

Role of oxidative stress in oxaliplatin-induced enteric neuropathy and colonic dysmotility in mice

BACKGROUND AND PURPOSE

Oxaliplatin is a platinum-based chemotherapeutic drug used as a first-line therapy for colorectal cancer. However, its use is associated with severe gastrointestinal side-effects resulting in dose limitations and/or cessation of treatment. In this study, we tested whether oxidative stress, caused by chronic oxaliplatin treatment, induces enteric neuronal damage and colonic dysmotility.

EXPERIMENTAL APPROACH

Oxaliplatin (3 mg·kg^-1 per day) was administered in vivo to Balb/c mice intraperitoneally three times a week. The distal colon was collected at day 14 of treatment. Immunohistochemistry was performed in wholemount preparations of submucosal and myenteric ganglia. Neuromuscular transmission was studied by intracellular electrophysiology. Circular muscle tone was studied by force transducers. Colon propulsive activity studied in organ bath experiments and faeces were collected to measure water content.

KEY RESULTS

Chronic in vivo oxaliplatin treatment resulted in increased formation of reactive oxygen species (O₂ -), nitration of proteins, mitochondrial membrane depolarisation resulting in the release of cytochrome c, loss of neurons, increased inducible NOS expression and apoptosis in both the submucosal and myenteric plexuses of the colon. Oxaliplatin treatment enhanced NO-mediated inhibitory junction potentials and altered the response of circular muscles to the NO donor, sodium nitroprusside. It also reduced the frequency of colonic migrating motor complexes and decreased circular muscle tone, effects reversed by the NO synthase inhibitor, Nω-Nitro-L-arginine.

CONCLUSION AND IMPLICATIONS

Our study is the first to provide evidence that oxidative stress is a key player in enteric neuropathy and colonic dysmotility leading to symptoms of chronic constipation observed in oxaliplatin-treated mice.

Hydrogen Sulfide Modulates the S-Nitrosoproteome and the Mitochondrial Morphology in Endothelial Cells

BACKGROUND

Hydrogen sulfide (H₂S) is one of the endogenous gaseous molecules promoting the production of nitric oxide (NO) which has cardioprotective functions. However, the role of the H₂S-mediated protein S-nitrosoproteome and its subsequent physiological response remains unclear.

METHODS

Endothelial cells EAhy 926 were treated with 50 μM of H₂S for 2 hours. The NO bound S-nitrosoproteins were purified by a biotin-switch and then digested by trypsin. Resulting peptides from control and H₂S treatment were separately labeled by isobaric tag for relative and absolute quantitation 114/115, quantified by liquid chromatography tandem-mass spectrometry and analyzed by ingenuity pathway analysis (IPA) software. The microP software was applied to analyze the morphological changes of mitochondria.

RESULTS

With the treatment of H₂S, 416 S-nitrosylated proteins were identified. IPA analysis showed that these proteins were involved in five signaling pathways. The NO-bound cysteine residues and the S-nitrosylation levels (115/114) were shown for ten S-nitrosoproteins. Western blot further verified the S-nitrosylation of thioredoxin-dependant peroxide reductase, cytochrome c oxidase and cytochrome b-c1 complex that are involved in the mitochondrial signaling pathway. H₂O₂-induced mitochondrial swelling can be reduced by the pretreatment of H₂S.

CONCLUSIONS

The H₂S-mediated endothelial S-nitrosoproteome has been confirmed. In the present study, we have proposed the cardioprotective role of H₂S via maintaining mitochondrial homeostasis.

Indazole, Pyrazole, and Oxazole Derivatives Targeting Nitric Oxide Synthases and Carbonic Anhydrases

Nitric oxide (NO) is an essential endogenous mediator with a physiological role in the central nervous system as neurotransmitter and neuromodulator. A growing number of studies have demonstrated that abnormal nitrergic signaling is a crucial event in the development of neurodegeneration. In particular, the uncontrolled production of NO by neuronal nitric oxide synthase (nNOS) is observed in several neurodegenerative diseases. Moreover, it is well recognized that specific isoforms of human carbonic anhydrase (hCA) physiologically modulate crucial pathways of signal processing and that low expression of CA affects cognition, leading to mental retardation, Alzheimer's disease, and aging-related cognitive impairments. In light of this, dual agents that are able to target both NOS (inhibition) and CA (activation) could be useful drug candidates for the treatment of Alzheimer's disease, aging, and other neurodegenerative diseases. In the present work, we show the design, synthesis, and in vitro biological evaluation of new nitrogen-based heterocyclic compounds. Among the tested molecules, 2-amino-3-(4-hydroxyphenyl)-N-(1H-indazol-5-yl)propanamide hydrochloride (10 b) was revealed to be a potent dual agent, able to act as a selective nNOS inhibitor and activator of the hCA I isoform.

Procaspase-9 induces its cleavage by transnitrosylating XIAP via the Thioredoxin system during cerebral ischemia-reperfusion in rats

Transnitrosylation is an important mechanism by which nitric oxide (NO) modulates cell signaling pathways. For instance, SNO-caspase-3 can transnitrosylate the X-linked inhibitor of apoptosis (XIAP) to enhance apoptosis. XIAP is a potent antagonist of caspase apoptotic activity. Decrease in XIAP activity via nitrosylation results in SNO-XIAP-mediated caspase activation. Considering the functional liaison of procaspase-9 and XIAP, we hypothesized that procaspase-9 nitrosylates XIAP directly. Our data confirmed that cerebral ischemia-reperfusion induced XIAP nitrosylation, procaspase-9 denitrosylation and cleavage. Interestingly, the time courses of the nitrosylation of procaspase-9 and XIAP were negatively correlated, which was more prominent after cerebral ischemia-reperfusion, suggesting a direct interaction. The nitrosylation of XIAP, as well as the denitrosylation and cleavage of procaspase-9, were inhibited by DNCB, TrxR1 AS-ODNs, or TAT-AVPY treatment. Meanwhile, DNCB, TrxR1 AS-ODNs, or TAT-AVPY also inhibited the decrease in hippocampal CA1 neurons induced by ischemia-reperfusion in rats. The denitrosylation and cleavage of procaspase-9 induced by OGD/reoxygenation in SH-SY5Y cells were inhibited when cells were co-transfected with wild-type procaspase-9 and XIAP mutant (C449G). These data suggest that cerebral ischemia-reperfusion induces a transnitrosylation from procaspase-9 to XIAP via the Trx system to consequently cause apoptosis. Additionally, Cys325 is a critical S-nitrosylation site of procaspase-9.

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.

Nitric Oxide-Mediated Posttranslational Modifications: Impacts at the Synapse

Nitric oxide (NO) is an important gasotransmitter molecule that is involved in numerous physiological processes throughout the nervous system. In addition to its involvement in physiological plasticity processes (long-term potentiation, LTP; long-term depression, LTD) which can include NMDAR-mediated calcium-dependent activation of neuronal nitric oxide synthase (nNOS), new insights into physiological and pathological consequences of nitrergic signalling have recently emerged. In addition to the canonical cGMP-mediated signalling, NO is also implicated in numerous pathways involving posttranslational modifications. In this review we discuss the multiple effects of S-nitrosylation and 3-nitrotyrosination on proteins with potential modulation of function but limit the analyses to signalling involved in synaptic transmission and vesicular release. Here, crucial proteins which mediate synaptic transmission can undergo posttranslational modifications with either pre- or postsynaptic origin. During normal brain function, both pathways serve as important cellular signalling cascades that modulate a diverse array of physiological processes, including synaptic plasticity, transcriptional activity, and neuronal survival. In contrast, evidence suggests that aging and disease can induce nitrosative stress via excessive NO production. Consequently, uncontrolled S-nitrosylation/3-nitrotyrosination can occur and represent pathological features that contribute to the onset and progression of various neurodegenerative diseases, including Parkinson's, Alzheimer's, and Huntington's.

Mechanism of Inactivation of Neuronal Nitric Oxide Synthase by (S)-2-Amino-5-(2-(methylthio)acetimidamido)pentanoic Acid

Nitric oxide synthase (NOS) catalyzes the conversion of l-arginine to l-citrulline and the second messenger nitric oxide. Three mechanistic pathways are proposed for the inactivation of neuronal NOS (nNOS) by (S)-2-amino-5-(2-(methylthio)acetimidamido)pentanoic acid (1): sulfide oxidation, oxidative dethiolation, and oxidative demethylation. Four possible intermediates were synthesized. All compounds were assayed with nNOS, their IC50, KI, and kinact values were obtained, and their crystal structures were determined. The identification and characterization of the products formed during inactivation provide evidence for the details of the inactivation mechanism. On the basis of these studies, the most probable mechanism for the inactivation of nNOS involves oxidative demethylation with the resulting thiol coordinating to the cofactor heme iron. Although nNOS is a heme-containing enzyme, this is the first example of a NOS that catalyzes an S-demethylation reaction; the novel mechanism of inactivation described here could be applied to the design of inactivators of other heme-dependent enzymes.

S-nitrosylation of the thioredoxin-like domains of protein disulfide isomerase and its role in neurodegenerative conditions

Correct protein folding and inhibition of protein aggregation is facilitated by a cellular “quality control system” that engages a network of protein interactions including molecular chaperones and the ubiquitin proteasome system. Key chaperones involved in these regulatory mechanisms are the protein disulfide isomerases (PDI) and their homologs, predominantly expressed in the endoplasmic reticulum of most tissues. Redox changes that disrupt ER homeostasis can lead to modification of these enzymes or chaperones with the loss of their proposed neuroprotective role resulting in an increase in protein misfolding. Misfolded protein aggregates have been observed in several disease states and are considered to play a pivotal role in the pathogenesis of neurodegenerative conditions such as Alzheimer's disease, Parkinson's disease, and Amyotrophic Lateral sclerosis. This review will focus on the importance of the thioredoxin-like CGHC active site of PDI and how our understanding of this structural motif will play a key role in unraveling the pathogenic mechanisms that underpin these neurodegenerative conditions.

Mechanistic studies of inactivation of inducible nitric oxide synthase by amidines

Nitric oxide synthase (NOS) catalyzes the conversion of L-arginine to L-citrulline and nitric oxide. N(5)-(1-Iminoethyl)-L-ornithine (L-NIO), an amidine-containing molecule, is a natural product known to be an inactivator of inducible NOS (iNOS). Because of the presence of the amidine methyl group in place of the guanidine amino group of substrate L-arginine, the active site heme peroxy intermediate sometimes cannot be protonated, thereby preventing its conversion to the heme oxo intermediate; instead, a heme oxygenase-type mechanism occurs, leading to conversion of the heme to biliverdin. This might be a new and general inactivation mechanism for heme-containing enzymes. In the studies described here, we attempted to provide support for amidines as substrates and inactivators of iNOS by the design and synthesis of amidine analogues of L-NIO having groups other than the amidine methyl group. No nitric oxide- or enzyme-catalyzed products could be detected by incubation of these amidines with iNOS. Although none of the L-NIO analogues acted as substrates, they all inhibited iNOS; increased inhibitory potency correlated with decreased substituent size. Computer modeling and molecular dynamics simulations were run on 10 and 11 to rationalize why these compounds do not act as substrates. Unlike the methyl amidine (L-NIO), the other alkyl groups block binding of O2 at the heme iron. Compounds 8, 9, and 11 were inactivators; however, no heme was lost, and no biliverdin was formed. No kinetic isotope effect on inactivation was observed with perdeuterated ethyl 8. A small amount of dimer disruption occurred with these inactivators, although the amount would not account for complete enzyme inactivation. The L-NIO analogues inactivate iNOS by a yet unknown mechanism; however, it is different from that of L-NIO, and the inactivation mechanism previously reported for L-NIO appears to be unique to methyl amidines.

Characterisation and comparison of temporal release profiles of nitric oxide generating donors

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Nitric oxide release profiles were characterised for commonly used donors.

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Released NO differs greatly between donors and depends on storage conditions.

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High release donors (NOC-5, PAPA NONOate) decay quickly.

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SNP and GSNO show greater stability releasing consistent lower NO levels.

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This comprehensive characterisation provides knowledge to define NO concentrations released in vitro.

Background

Nitric oxide (NO) is a vital signalling molecule in a variety of tissues including the neuronal, vascular and reproductive system. However, its high diffusibility and inactivation make characterisation of nitrergic signalling difficult. The use of NO donors is essential to characterise downstream signalling pathways but knowledge of donor release capacities is lacking, thus making comparisons of donor responses difficult.

New method

This study characterises NO profiles of commonly used NO donors. Donors were stored under defined conditions and temporal release profiles detected to allow determination of released NO concentrations.

Results

Using NO-sensitive microsensors we assessed release profiles of NO donors following different storage times and conditions. We found that donors such as NOC-5 and PAPA-NONOate decayed substantially within days, whereas SNP and GSNO showed greater stability releasing consistent levels of NO over days. In all donors tested, the amount of released NO differs between frozen and unfrozen stocks.

Comparison with existing method(s)

Fluorescent and amperometric approaches to measure NO concentrations yield a wide range of levels. However, due to a lack of characterisation of the release profiles, inconsistent effects on NO signalling have been widely documented. Our systematic assessment of release profiles of a range of NO donors therefore provides new essential data allowing for improved and defined investigations of nitrergic signalling.

Conclusions

This is the first systematic comparison of temporal release profiles of different NO donors allowing researchers to compare conditions across different studies and the use of defined NO levels by choosing specific donors and concentrations.

Conversion of S-phenylsulfonylcysteine residues to mixed disulfides at pH 4.0: utility in protein thiol blocking and in protein-S-nitrosothiol detection

A three step protocol for protein S-nitrosothiol conversion to fluorescent mixed disulfides with purified proteins, referred to as the thiosulfonate switch, is explored which involves: (1) thiol blocking at pH 4.0 using S-phenylsulfonylcysteine (SPSC); (2) trapping of protein S-nitrosothiols as their S-phenylsulfonylcysteines employing sodium benzenesulfinate; and (3) tagging the protein thiosulfonate with a fluorescent rhodamine based probe bearing a reactive thiol (Rhod-SH), which forms a mixed disulfide between the probe and the formerly S-nitrosated cysteine residue. S-Nitrosated bovine serum albumin and the S-nitrosated C-terminally truncated form of AdhR-SH (alcohol dehydrogenase regulator) designated as AdhR*-SNO were selectively labelled by the thiosulfonate switch both individually and in protein mixtures containing free thiols. This protocol features the facile reaction of thiols with S-phenylsulfonylcysteines forming mixed disulfides at mild acidic pH (pH = 4.0) in both the initial blocking step as well as in the conversion of protein-S-sulfonylcysteines to form stable fluorescent disulfides. Labelling was monitored by TOF-MS and gel electrophoresis. Proteolysis and peptide analysis of the resulting digest identified the cysteine residues containing mixed disulfides bearing the fluorescent probe, Rhod-SH.

Optimization of the isolation and enrichment of S-nitrosylated proteins from brain tissues of rodents and humans with various prion diseases for iTRAQ-based proteomics

Accumulating evidence indicates that S-nitrosylation plays an important role in protein function either physiologically or pathologically. In the present study, we describe a modified method for the isolation and enrichment of S-nitrosylated (SNO) proteins from brain tissue based on a biotin labeling system using the biotin switch technique (BST). Various working conditions for the incubation of biotin-labeled samples with streptavidin beads and for the elution of SNO proteins from streptavidin beads were comparatively evaluated. The working conditions were optimized with incubation at a ratio of 1:3 (streptavidin beads/brain homogenates) at 25˚C for 120 min, and the elution conditions were optimized using buffer containing 0.5% sodium dodecyl sulfate. Under these conditions, we found that at least 12 rounds of successive incubation were required in order to recover all the SNO proteins in the human and rodent brain homogenates. Western blot analyses of some of the eluted products confirmed the reliable immunoreactivity of the isolated SNO proteins. iTRAQ-based mass spectrometric (MS) analysis of the eluted products from the brain tissues of a normal healthy subject and patients with various prion diseases identified 1,509 SNO proteins with high confidence [false discovery rate (FDR) <1%]. These data indicate that with this optimized method, the endogenous SNO proteins from the brain tissue of humans and rodents can be sufficiently isolated, which can then be used directly in further assays, such as large-scale analysis of the S-nitrosoproteome in complex backgrounds.

Enzymatic and cryoreduction EPR studies of the hydroxylation of methylated N(ω)-hydroxy-L-arginine analogues by nitric oxide synthase from Geobacillus stearothermophilus

Nitric oxide synthase (NOS) catalyzes the conversion of l-arginine to l-citrulline and NO in a two-step process involving the intermediate N^ω-hydroxy-l-arginine (NHA). It was shown that Cpd I is the oxygenating species for l-arginine; the hydroperoxo ferric intermediate is the reactive intermediate with NHA. Methylation of the N^ω-OH and N^ω-H of NHA significantly inhibits the conversion of NHA into NO and l-citrulline by mammalian NOS. Kinetic studies now show that N^ω-methylation of NHA has a qualitatively similar effect on H₂O₂-dependent catalysis by bacterial gsNOS. To elucidate the effect of methylating N^ω-hydroxy l-arginine on the properties and reactivity of the one-electron-reduced oxy-heme center of NOS, we have applied cryoreduction/annealing/EPR/ENDOR techniques. Measurements of solvent kinetic isotope effects during 160 K cryoannealing cryoreduced oxy-gsNOS/NHA confirm the hydroperoxo ferric intermediate as the catalytically active species of step two. Product analysis for cryoreduced samples with methylated NHA’s, NHMA, NMOA, and NMMA, annealed to 273 K, show a correlation of yields of l-citrulline with the intensity of the g 2.26 EPR signal of the peroxo ferric species trapped at 77 K, which converts to the reactive hydroperoxo ferric state. There is also a correlation between the yield of l-citrulline in these experiments and k_obs for the H₂O₂-dependent conversion of the substrates by gsNOS. Correspondingly, no detectable amount of cyanoornithine, formed when Cpd I is the reactive species, was found in the samples. Methylation of the NHA guanidinium N^ω-OH and N^ω-H inhibits the second NO-producing reaction by favoring protonation of the ferric-peroxo to form unreactive conformers of the ferric-hydroperoxo state. It is suggested that this is caused by modification of the distal-pocket hydrogen-bonding network of oxy gsNOS and introduction of an ordered water molecule that facilitates delivery of the proton(s) to the one-electron-reduced oxy-heme moiety. These results illustrate how variations in the properties of the substrate can modulate the reactivity of a monooxygenase.

Importance of NO and its related compounds in enteric nervous system regulation of gut homeostasis and disease susceptibility

Nitric oxide (NO) produced by the enteric nervous system represents an important regulatory mechanism in gut homeostasis. Aberrant NO signaling contributes significantly toward enteric disease by altering gut motility, vascular tone, blood supply, mucosal barrier function, secretions and immunity. Consequently, there is much interest in therapeutically targeting NO production and its bioactive intermediates. This article highlights recent advances in NO signaling and therapeutics as it relates to the gastrointestinal tract and its associated NO producing microbiota. Because of its limited scope, a particular emphasis is placed on S-nitrosylation as the emerging physiologic mechanism for NO signal transduction, and how such signals are modulated by other gaseous transmitters - notably hydrogen disulfide and carbon monoxide - that are produced by the enteric nervous system and share common molecular targets. Recent findings also indicate that druggable regulators of S-nitrosylation, for example S-nitrosoglutathione (GSNO) reductase, provide for a superior pharmacology and finer therapeutic control over classical NO donors, and may be better suited for oral delivery to the gastrointestinal tract.

Protein S-nitrosylation in Plasmodium falciparum

AIMS

Due to its life in different hosts and environments, the human malaria parasite Plasmodium falciparum is exposed to oxidative and nitrosative challenges. Nitric oxide (NO) and NO-derived reactive nitrogen species can constitute nitrosative stress and play a major role in NO-related signaling. However, the mode of action of NO and its targets in P. falciparum have hardly been characterized. Protein S-nitrosylation (SNO), a posttranslational modification of protein cysteine thiols, has emerged as a principal mechanism by which NO exerts diverse biological effects. Despite its potential importance, SNO has hardly been studied in human malaria parasites. Using a biotin-switch approach coupled to mass spectrometry, we systemically studied SNO in P. falciparum cell extracts.

RESULTS

We identified 319 potential targets of SNO that are widely distributed throughout various cellular pathways. Glycolysis in the parasite was found to be a major target, with glyceraldehyde-3-phosphate dehydrogenase being strongly inhibited by S-nitrosylation of its active site cysteine. Furthermore, we show that P. falciparum thioredoxin 1 (PfTrx1) can be S-nitrosylated at its nonactive site cysteine (Cys43). Mechanistic studies indicate that PfTrx1 possesses both denitrosylating and transnitrosylating activities mediated by its active site cysteines and Cys43, respectively.

INNOVATION

This work provides first insights into the S-nitrosoproteome of P. falciparum and suggests that the malaria parasite employs the thioredoxin system to deal with nitrosative challenges.

CONCLUSION

Our results indicate that SNO may influence a variety of metabolic processes in P. falciparum and contribute to our understanding of NO-related signaling processes and cytotoxicity in the parasites.

Hydrogen sulfide increases nitric oxide production and subsequent S-nitrosylation in endothelial cells

Hydrogen sulfide (H₂S) and nitric oxide (NO), two endogenous gaseous molecules in endothelial cells, got increased attention with respect to their protective roles in the cardiovascular system. However, the details of the signaling pathways between H₂S and NO in endothelia cells remain unclear. In this study, a treatment with NaHS profoundly increased the expression and the activity of endothelial nitric oxide synthase. Elevated gaseous NO levels were observed by a novel and specific fluorescent probe, 5-amino-2-(6-hydroxy-3-oxo-3H-xanthen-9-yl)benzoic acid methyl ester (FA-OMe), and quantified by flow cytometry. Further study indicated an increase of upstream regulator for eNOS activation, AMP-activated protein kinase (AMPK), and protein kinase B (Akt). By using a biotin switch, the level of NO-mediated protein S-nitrosylation was also enhanced. However, with the addition of the NO donor, NOC-18, the expressions of cystathionine-γ-lyase, cystathionine-β-synthase, and 3-mercaptopyruvate sulfurtransferase were not changed. The level of H₂S was also monitored by a new designed fluorescent probe, 4-nitro-7-thiocyanatobenz-2-oxa-1,3-diazole (NBD-SCN) with high specificity. Therefore, NO did not reciprocally increase the expression of H₂S-generating enzymes and the H₂S level. The present study provides an integrated insight of cellular responses to H₂S and NO from protein expression to gaseous molecule generation, which indicates the upstream role of H₂S in modulating NO production and protein S-nitrosylation.

Potential effect of S-nitrosylated protein disulfide isomerase on mutant SOD1 aggregation and neuronal cell death in amyotrophic lateral sclerosis

Aggregation of misfolded protein and resultant intracellular inclusion body formation are common hallmarks of mutant superoxide dismutase (mSOD1)-linked familial amyotrophic lateral sclerosis (FALS) and have been associated with the selective neuronal death. Protein disulfide isomerase (PDI) represents a family of enzymatic chaperones that can fold nascent and aberrant proteins in the endoplasmic reticulum (ER) lumen. Recently, our group found that S-nitrosylated PDI could contribute to protein misfolding and subsequent neuronal cell death. However, the exact role of PDI in the pathogenesis of ALS remains unclear. In this study, we propose that PDI attenuates aggregation of mutant/misfolded SOD1 and resultant neurotoxicity associated with ER stress. ER stress resulting in PDI dysfunction therefore provides a mechanistic link between deficits in molecular chaperones, accumulation of misfolded proteins, and neuronal death in neurodegenerative diseases. In contrast, S-nitrosylation of PDI inhibits its activity, increases mSOD1 aggregation, and increases neuronal cell death. Specifically, our data show that S-nitrosylation abrogates PDI-mediated attenuation of neuronal cell death triggered by thapsigargin. Biotin switch assays demonstrate S-nitrosylated PDI both in the spinal cords of SOD1 (G93A) mice and human patients with sporadic ALS. Therefore, denitrosylation of PDI may have therapeutic implications. Taken together, our results suggest a novel strategy involving PDI as a therapy to prevent mSOD1 aggregation and neuronal degeneration. Moreover, the data demonstrate that inactivation of PDI by S-nitrosylation occurs in both mSOD1-linked and sporadic forms of ALS in humans as well as mice.

Selective vulnerability of synaptic signaling and metabolism to nitrosative stress

SIGNIFICANCE

Nitric oxide (NO) plays diverse physiological roles in the central nervous system, where it modulates neuronal communication, regulates blood flow, and contributes to the innate immune responses. In a number of brain pathologies, the excessive production of NO also leads to the formation of reactive and toxic intermediates generically termed reactive nitrogen species (RNS). RNS cause irreversible or poorly reversible damage to brain cells.

RECENT ADVANCES

Recent work in the field focused on the ability of NO and RNS to yield protein modifications, including the S-nitrosation of cysteine residues, which, in many instances, impact cellular functions and viability.

CRITICAL ISSUES

The vast majority of neuropathological studies focus on the loss of cell viability, but nitrosative stress may also strongly impair the functions of neuronal processes: axonal projections and dendritic trees. The functional integrity of axons and dendrites critically depends on local metabolism and effective delivery of metabolic enzymes and organelles. Here, we summarize the existing literature describing the effects of nitrosative stress on the major pathways of energetic metabolism: glycolysis, tricarboxylic acid cycle, and mitochondrial respiration, with the emphasis on modifications of protein thiols.

FUTURE DIRECTIONS

We propose that axons and dendrites are highly vulnerable to nitrosative stress because of their low glycolytic capacity and high dependence on timely delivery of metabolic enzymes and organelles from the cell body. Thus, supplementation with the end products of glycolysis, pyruvate or lactate, may help preserve metabolism in distal neuronal processes and protect or restore synaptic function in the ailing brain.

Thiol-based posttranslational modifications in parasites

SIGNIFICANCE

Cysteine residues of proteins participate in the catalysis of biochemical reactions, are crucial for redox reactions, and influence protein structure by the formation of disulfide bonds. Covalent posttranslational modifications (PTMs) of cysteine residues are important mediators of redox regulation and signaling by coupling protein activity to the cellular redox state, and moreover influence stability, function, and localization of proteins. A diverse group of protozoan and metazoan parasites are a major cause of diseases in humans, such as malaria, African trypanosomiasis, leishmaniasis, toxoplasmosis, filariasis, and schistosomiasis.

RECENT ADVANCES

Human parasites undergo dramatic morphological and metabolic changes while they pass complex life cycles and adapt to changing environments in host and vector. These processes are in part regulated by PTMs of parasitic proteins. In human parasites, posttranslational cysteine modifications are involved in crucial cellular events such as signal transduction (S-glutathionylation and S-nitrosylation), redox regulation of proteins (S-glutathionylation and S-nitrosylation), protein trafficking and subcellular localization (palmitoylation and prenylation), as well as invasion into and egress from host cells (palmitoylation). This review focuses on the occurrence and mechanisms of these cysteine modifications in parasites.

CRITICAL ISSUES

Studies on cysteine modifications in human parasites are so far largely based on in vitro experiments.

FUTURE DIRECTIONS

The in vivo regulation of cysteine modifications and their role in parasite development will be of great interest in order to understand redox signaling in parasites.

Established Principles and Emerging Concepts on the Interplay between Mitochondrial Physiology and S-(De)nitrosylation: Implications in Cancer and Neurodegeneration

S-nitrosylation is a posttranslational modification of cysteine residues that has been frequently indicated as potential molecular mechanism governing cell response upon redox unbalance downstream of nitric oxide (over)production. In the last years, increased levels of S-nitrosothiols (SNOs) have been tightly associated with the onset of nitroxidative stress-based pathologies (e.g., cancer and neurodegeneration), conditions in which alterations of mitochondrial homeostasis and activation of cellular processes dependent on it have been reported as well. In this paper we aim at summarizing the current knowledge of mitochondria-related proteins undergoing S-nitrosylation and how this redox modification might impact on mitochondrial functions, whose impairment has been correlated to tumorigenesis and neuronal cell death. In particular, emphasis will be given to the possible, but still neglected implication of denitrosylation reactions in the modulation of mitochondrial SNOs and how they can affect mitochondrion-related cellular process, such as oxidative phosphorylation, mitochondrial dynamics, and mitophagy.

The role of nitric oxide on rosuvastatin-mediated S-nitrosylation and translational proteomes in human umbilical vein endothelial cells

Background

The pleiotropic effects of 3-Hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins), which are independent from their cholesterol-lowering action, have been widely recognized in various biological systems. Statins can affect endothelial homeostasis, which is partly modulated by the production of nitric oxide (NO). However, it is unclear how statin/NO-mediated posttranslational S-nitrosylation of endothelial proteins and changes in translational profiles may benefit endothelial integrity. Therefore, it is important to understand the statin/NO-mediated S-nitrosylation in endothelial cells.

Results

Rosuvastatin treatment of human umbilical vein endothelial cells (ECs) enhanced the enzymatic activity of endothelial nitric oxide synthase (eNOS) and the expression of 78 S-nitrosoproteins. Among these S-nitrosoproteins, we identified 17 proteins, including protein disulfide bond isomerase, phospholipase C, transaldolase and heat shock proteins. Furthermore, a hydrophobic Cys66 was determined as the S-nitrosylation site of the mitochondrial HSP70. In addition to the statin-modulated posttranslational S-nitrosylation, changes in the NO-mediated translational proteome were also observed. Seventeen major proteins were significantly upregulated after rosuvastatin treatment. However, 12 of these proteins were downregulated after pretreating ECs with an eNOS inhibitor (L-NAME), which indicated that their expression was modulated by NO.

Conclusions

ECs treated with rosuvastatin increase eNOS activation. The increased NO production is involved in modulating S-nitrosylation and translation of proteins. We provide further evidence of the pleiotropic effect of rosuvastatin on endothelial physiology.

The protein disulfide isomerase family: key players in health and disease

SIGNIFICANCE

Protein disulfide isomerase (PDI) and its homologs have essential roles in the oxidative folding and chaperone-mediated quality control of proteins in the secretory pathway. In this review, the importance of PDI in health and disease will be examined, using examples from the fields of lipid homeostasis, hemostasis, infectious disease, cancer, neurodegeneration, and infertility.

RECENT ADVANCES

Recent structural studies, coupled with cell biological, biochemical, and clinical approaches, have demonstrated that PDI family proteins are involved in a wide range of physiological and disease processes.

CRITICAL ISSUES

Critical issues in the field include understanding how and why a PDI family member is involved in a given disease, and defining the physiological client specificity of the various PDI proteins when they are expressed in different tissues.

FUTURE DIRECTIONS

Future directions are likely to include the development of new and more specific reagents to study and manipulate PDI family function. The development of conditional mouse models in concert with clinical data will help us to understand the in vivo function of the different PDIs at the organism level. Taken together with advances in structural biology and biochemical studies, this should help us to further understand and modify PDIs' functional interactions.

Post-translational modification in the gas phase: mechanism of cysteine S-nitrosylation via ion-molecule reactions

The gas-phase mechanism of S-nitrosylation of thiols was studied in a quadrupole ion trap mass spectrometer. This was done via ion-molecule reactions of protonated cysteine and many of its derivatives and other thiol ions with neutral tert-butyl nitrite or nitrous acid. Our results showed that the presence of the carboxylic acid functional group, -COOH, in the vicinity of the thiol group is essential for the gas-phase nitrosylation of thiols. When the carboxyl proton is replaced by a methyl group (cysteine methyl ester) no nitrosylation was observed. Other thiols lacking a carboxylic acid functional group displayed no S-nitrosylation, strongly suggesting that the carboxyl hydrogen plays a key role in the nitrosylation process. These results are in excellent agreement with a solution-phase mechanism proposed by Stamler et al. (J. S. Stamler, E. J. Toone, S. A. Lipton, N. J. Sucher. Neuron 1997, 18, 691-696) who suggested a catalytic role for the carboxylic acid group adjacent to cysteine residues and with later additions by Ascenzi et al. (P. Ascenzi, M. Colasanti, T. Persichini, M. Muolo, F. Polticelli, G. Venturini, D. Bordo, M. Bolognesi. Biol. Chem. 2000, 381, 623-627) who postulated that the presence of the carboxyl in the cysteine microenvironment in proteins is crucial for S-nitrosylation. A concerted mechanism for the gas-phase S-nitrosylation was proposed based on our results and was further studied using theoretical calculations. Our calculations showed that this proposed pathway is exothermic by 44.0 kJ mol(-1). This is one of the few recent examples when a gas-phase mechanism matches one in solution.

Thioredoxin 1-mediated post-translational modifications: reduction, transnitrosylation, denitrosylation, and related proteomics methodologies

Despite the significance of redox post-translational modifications (PTMs) in regulating diverse signal transduction pathways, the enzymatic systems that catalyze reversible and specific oxidative or reductive modifications have yet to be firmly established. Thioredoxin 1 (Trx1) is a conserved antioxidant protein that is well known for its disulfide reductase activity. Interestingly, Trx1 is also able to transnitrosylate or denitrosylate (defined as processes to transfer or remove a nitric oxide entity to/from substrates) specific proteins. An intricate redox regulatory mechanism has recently been uncovered that accounts for the ability of Trx1 to catalyze these different redox PTMs. In this review, we will summarize the available evidence in support of Trx1 as a specific disulfide reductase, and denitrosylation and transnitrosylation agent, as well as the biological significance of the diverse array of Trx1-regulated pathways and processes under different physiological contexts. The dramatic progress in redox proteomics techniques has enabled the identification of an increasing number of proteins, including peroxiredoxin 1, whose disulfide bond formation and nitrosylation status are regulated by Trx1. This review will also summarize the advancements of redox proteomics techniques for the identification of the protein targets of Trx1-mediated PTMs. Collectively, these studies have shed light on the mechanisms that regulate Trx1-mediated reduction, transnitrosylation, and denitrosylation of specific target proteins, solidifying the role of Trx1 as a master regulator of redox signal transduction.

S-Nitrosylation activates Cdk5 and contributes to synaptic spine loss induced by beta-amyloid peptide

The activity of Cdk5 and its regulatory subunit p35 is thought to be important in both normal brain function and neurodegenerative disease pathogenesis. Increased Cdk5 activity, via proteolytic cleavage of p35 to a p25 fragment by the calcium-activated protease calpain or by phosphorylation at Cdk5(Tyr15), can contribute to neurotoxicity. Nonetheless, our knowledge of regulation of Cdk5 activity in disease states is still emerging. Here we demonstrate that Cdk5 is activated by S-nitrosylation or reaction of nitric oxide (NO)-related species with the thiol groups of cysteine residues 83 and 157, to form SNO-Cdk5. We then show that S-nitrosylation of Cdk5 contributes to amyloid-β (Aβ) peptide-induced dendritic spine loss. Furthermore, we observed significant levels of SNO-Cdk5 in postmortem Alzheimer's disease (AD) but not in normal human brains. These findings suggest that S-nitrosylation of Cdk5 is an aberrant regulatory mechanism of enzyme activity that may contribute to the pathogenesis of AD.

Protein S-nitrosylation: role for nitric oxide signaling in neuronal death

BACKGROUND

One of the signaling mechanisms mediated by nitric oxide (NO) is through S-nitrosylation, the reversible redox-based modification of cysteine residues, on target proteins that regulate a myriad of physiological and pathophysiological processes. In particular, an increasing number of studies have identified important roles for S-nitrosylation in regulating cell death.

SCOPE OF REVIEW

The present review focuses on different targets and functional consequences associated with nitric oxide and protein S-nitrosylation during neuronal cell death.

MAJOR CONCLUSIONS

S-Nitrosylation exhibits double-edged effects dependent on the levels, spatiotemporal distribution, and origins of NO in the brain: in general Snitrosylation resulting from the basal low level of NO in cells exerts anti-cell death effects, whereas S-nitrosylation elicited by induced NO upon stressed conditions is implicated in pro-cell death effects.

GENERAL SIGNIFICANCE

Dysregulated protein S-nitrosylation is implicated in the pathogenesis of several diseases including degenerative diseases of the central nervous system (CNS). Elucidating specific targets of S-nitrosylation as well as their regulatory mechanisms may aid in the development of therapeutic intervention in a wide range of brain diseases.