Tau is Endogenously Nitrated in Mouse Brain: Identification of a Tyrosine Residue Modified In vivo by NO

Non-enzymatic posttranslational protein modifications in protein aggregation and neurodegenerative diseases

Highly reactive metabolic intermediates and other small molecules frequently react with amino acid side chains, leading to non-enzymatic posttranslational modifications (nPTMs) of proteins. The abundance of these modifications increases under high metabolic activity or stress conditions and can dramatically impact protein structure and function. Although protein quality control mechanisms typically mitigate the effects of these impaired proteins, in long-lived and degradation-resistant proteins, nPTMs accumulate. In some cases, such as cataract development and diabetes, clear links between nPTMs, aging, and disease progression have been established. In neurodegenerative diseases such as Alzheimer's and Parkinson's disease, a key question is whether accumulation of nPTMs is a cause or consequence of protein aggregation. This review focuses on major nPTMs found on proteins with central roles in neurodegenerative diseases such as α-synuclein, β-amyloid, and tau. We summarize current knowledge on the formation of these modifications and discuss their potential impact on disease onset and progression. Additionally, we examine what is known to date about how nPTMs impair cellular detoxification, repair, and degradation systems. Finally, we critically discuss the available methodologies to systematically investigate nPTMs at the molecular level and outline suitable approaches to study their effects on protein aggregation. We aim to foster more research into the role of nPTMs in neurodegeneration by adapting methodologies that have proven successful in studying enzymatic posttranslational modifications. Specifically, we advocate for site-specific incorporation of these modifications into target proteins using advanced chemical and molecular biology techniques.

Comparing Plasma Phospho Tau, Total Tau, and Phospho Tau-Total Tau Ratio as Acute and Chronic Traumatic Brain Injury Biomarkers

Importance

Annually in the United States, at least 3.5 million people seek medical attention for traumatic brain injury (TBI). The development of therapies for TBI is limited by the absence of diagnostic and prognostic biomarkers. Microtubule-associated protein tau is an axonal phosphoprotein. To date, the presence of the hypophosphorylated tau protein (P-tau) in plasma from patients with acute TBI and chronic TBI has not been investigated.

Objective

To examine the associations between plasma P-tau and total-tau (T-tau) levels and injury presence, severity, type of pathoanatomic lesion (neuroimaging), and patient outcomes in acute and chronic TBI.

Design, Setting, and Participants

In the TRACK-TBI Pilot study, plasma was collected at a single time point from 196 patients with acute TBI admitted to 3 level I trauma centers (<24 hours after injury) and 21 patients with TBI admitted to inpatient rehabilitation units (mean [SD], 176.4 [44.5] days after injury). Control samples were purchased from a commercial vendor. The TRACK-TBI Pilot study was conducted from April 1, 2010, to June 30, 2012. Data analysis for the current investigation was performed from August 1, 2015, to March 13, 2017.

Main Outcomes and Measures

Plasma samples were assayed for P-tau (using an antibody that specifically recognizes phosphothreonine-231) and T-tau using ultra-high sensitivity laser-based immunoassay multi-arrayed fiberoptics conjugated with rolling circle amplification.

Results

In the 217 patients with TBI, 161 (74.2%) were men; mean (SD) age was 42.5 (18.1) years. The P-tau and T-tau levels and P-tau-T-tau ratio in patients with acute TBI were higher than those in healthy controls. Receiver operating characteristic analysis for the 3 tau indices demonstrated accuracy with area under the curve (AUC) of 1.000, 0.916, and 1.000, respectively, for discriminating mild TBI (Glasgow Coma Scale [GCS] score, 13-15, n = 162) from healthy controls. The P-tau level and P-tau-T-tau ratio were higher in individuals with more severe TBI (GCS, ≤12 vs 13-15). The P-tau level and P-tau-T-tau ratio outperformed the T-tau level in distinguishing cranial computed tomography-positive from -negative cases (AUC = 0.921, 0.923, and 0.646, respectively). Acute P-tau levels and P-tau-T-tau ratio weakly distinguished patients with TBI who had good outcomes (Glasgow Outcome Scale-Extended GOS-E, 7-8) (AUC = 0.663 and 0.658, respectively) and identified those with poor outcomes (GOS-E, ≤4 vs >4) (AUC = 0.771 and 0.777, respectively). Plasma samples from patients with chronic TBI also showed elevated P-tau levels and a P-tau-T-tau ratio significantly higher than that of healthy controls, with both P-tau indices strongly discriminating patients with chronic TBI from healthy controls (AUC = 1.000 and 0.963, respectively).

Conclusions and Relevance

Plasma P-tau levels and P-tau-T-tau ratio outperformed T-tau level as diagnostic and prognostic biomarkers for acute TBI. Compared with T-tau levels alone, P-tau levels and P-tau-T-tau ratios show more robust and sustained elevations among patients with chronic TBI.

NPY binds with heme to form a NPY–heme complex: enhancing peroxidase activity in free heme and promoting NPY nitration and inactivation

NPY binding with heme enhances the peroxidase activity of free heme, resulting in the important tyrosine nitration, which will attenuate its bioactivity.

Mass spectrometric analysis of protein tyrosine nitration in aging and neurodegenerative diseases

This review highlights the significance of protein tyrosine nitration (PTN) in signal transduction pathways, the progress achieved in analytical methods, and the implication of nitration in the cellular pathophysiology of aging and age-related neurodegenerative diseases. Although mass spectrometry of nitrated peptides has become a powerful tool for the characterization of nitrated peptides, the low stoichiometry of this modification clearly necessitates the use of affinity chromatography to enrich modified peptides. Analysis of nitropeptides involves identification of endogenous, intact modification as well as chemical conversion of the nitro group to a chemically reactive amine group and further modifications that enable affinity capture and enhance detectability by altering molecular properties. In this review, we focus on the recent progress in chemical derivatization of nitropeptides for enrichment and mass analysis, and for detection and quantification using various analytical tools. PTN participates in physiological processes, such as aging and neurodegenerative diseases. Accumulation of 3-nitrotyrosine has been found to occur during the aging process; this was identified through mass spectrometry. Further, there are several studies implicating the presence of nitrated tyrosine in age-related diseases such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis.

Nitric oxide synthase mediates PC12 differentiation induced by the surface topography of nanostructured TiO2

Background

Substrate nanoscale topography influences cell proliferation and differentiation through mechanisms that are at present poorly understood. In particular the molecular mechanism through which cells 'sense’ and adapt to the substrate and activate specific intracellular signals, influencing cells survival and behavior, remains to be clarified.

Results

To characterize these processes at the molecular level we studied the differentiation of PC12 cells on nanostructured TiO₂ films obtained by supersonic cluster beam deposition.

Our findings indicate that, in PC12 cells grown without Nerve Growth Factor (NGF), the roughness of nanostructured TiO₂ triggers neuritogenesis by activating the expression of nitric oxide synthase (NOS) and the phospho-extracellular signal-regulated kinase 1/2 (pERK1/2) signaling. Differentiation is associated with an increase in protein nitration as observed in PC12 cells grown on flat surfaces in the presence of NGF. We demonstrate that cell differentiation and protein nitration induced by topography are not specific for PC12 cells but can be regarded as generalized effects produced by the substrate on different neuronal-like cell types, as shown by growing the human neuroblastoma SH-SY5Y cell line on nanostructured TiO₂.

Conclusion

Our data provide the evidence that the nitric oxide (NO) signal cascade is involved in the differentiation process induced by nanotopography, adding new information on the mechanism and proteins involved in the neuritogenesis triggered by the surface properties.

Protein nitration as footprint of oxidative stress-related nitric oxide signaling pathways in developing Ciona intestinalis

Developmental processes in the ascidian Ciona intestinalis depend on a complex interplay of events including, during metamorphosis, a caspase-dependent apoptosis which is regulated by the nitric oxide (NO)-cGMP signaling pathway. Herein we disclose an alternate NO-mediated signaling pathway during Ciona development which appears to be critically dependent on local redox control. Evidence in support of this conclusion includes: (a) inhibitors of NO synthase (NOS) and scavengers of NO-derived nitrating agents markedly decrease the rate of Ciona metamorphosis; (b) an NO donor or peroxynitrite caused an opposite effect; (c) increased protein nitration is observed at larva stage. Integrated proteomic and immunochemical methodologies identified nitrated tyrosine residues in ERK and snail. Overall, these results point to protein nitration as a hitherto overlooked NO-dependent regulatory mechanism in Ciona which is specifically triggered by elevated ROS production during developmental processes.

Selective tau tyrosine nitration in non-AD tauopathies

Previously, we reported the characterization of two novel antibodies that react with tau nitrated at tyrosine 197 (Tau-nY197) and tyrosine 394 (Tau-nY394) in Alzheimer's disease (AD). In this report, we examined whether tau nitration at these sites also occurs in corticobasal degeneration (CBD), progressive supranuclear palsy (PSP) and Pick's disease (PiD), three neurodegenerative tauopathies that contain abundant tau deposits within glial and neuronal cell types but lack amyloid deposition. The reactivity of these antibodies was also compared to two previously characterized antibodies Tau-nY18 and Tau-nY29, specific for tau nitrated at tyrosine 18 and tyrosine 29, respectively. In the present experiments, Tau-nY18 did not label the classical pathological lesions of CBD or PSP but did label the neuronal lesions associated with PiD to a limited extent. In contrast, Tau-nY29 revealed some, but not all classes of tau inclusions associated with both CBD and PSP but did label numerous Pick body inclusions in PiD. Tau-nY197 was restricted to the neuropil threads in both CBD and PSP; however, similar to Tau-nY29, extensive Pick body pathology was clearly labeled. Tau-nY394 did not detect any of the lesions associated with these disorders. In contrast, extensive neuronal and glial tau pathology within these diseases was labeled by Tau-Y197, a monoclonal antibody that reacts within the Y-197-containing proline-rich region of the molecule. Based on our Western and IHC experiments, it appears that nitration of tau at tyrosine 29 is a pathological modification that might be associated with neurodegeneration. Collectively, our data suggest that site-specific tau tyrosine nitration events occur in a disease and lesion-specific manner, indicating that nitration appears to be a highly controlled modification in AD and non-AD tauopathies.

Ferulic acid ethyl ester as a potential therapy in neurodegenerative disorders

Oxidative stress is involved in the onset, progression and pathogenesis of a number of diseases including neurodegenerative diseases. It is critical to develop a pharmacological approach to combat oxidative stress which may reduce the risk of diseases and help in promoting healthy life. In an attempt to reduce the side effects associated with allopathic medicines a number of studies are now focusing on developing treatment regimens from naturally occurring plant products. In this review, the protective role of ferulic acid (4-hydroxy-3-methoxycinnamic acid) (FA), a naturally occurring antioxidant compound found in fruit, some vegetables, and grains, and its ethyl ester derivative are discussed with respect to neurodegeneration. This article is part of a Special Issue entitled: Antioxidants and Antioxidant Treatment in Disease.

Tyrosine nitration within the proline-rich region of Tau in Alzheimer's disease

A substantial body of evidence suggests that nitrative injury contributes to neurodegeneration in Alzheimer's disease (AD) and other neurodegenerative disorders. Previously, we showed in vitro that within the tau protein the N-terminal tyrosine residues (Y18 and Y29) are more susceptible to nitrative modifications than other tyrosine sites (Y197 and Y394). Using site-specific antibodies to nitrated tau at Y18 and Y29, we identified tau nitrated in both glial (Y18) and neuronal (Y29) tau pathologies. In this study, we report the characterization of two novel monoclonal antibodies, Tau-nY197 and Tau-nY394, recognizing tau nitrated at Y197 and Y394, respectively. By Western blot analysis, Tau-nY197 labeled soluble tau and insoluble paired helical filament proteins (PHF-tau) nitrated at Y197 from control and AD brain samples. Tau-nY394 failed to label soluble tau isolated from control or severe AD samples, but labeled insoluble PHF-tau to a limited extent. Immunohistochemical analysis using Tau-nY197 revealed the hallmark tau pathology associated with AD; Tau-nY394 did not detect any pathological lesions characteristic of the disorder. These data suggest that a subset of the hallmark pathological inclusions of AD contain tau nitrated at Y197. However, nitration at Y197 was also identified in soluble tau from all control samples, including those at Braak stage 0, suggesting that nitration at this site in the proline-rich region of tau may have normal biological functions in the human brain.

Protein pattern of Xenopus laevis embryos grown in simulated microgravity

Numerous studies indicate that microgravity affects cell growth and differentiation in many living organisms, and various processes are modified when cells are placed under conditions of weightlessness. However, until now, there is no coherent explanation for these observations, and little information is available concerning the biomolecules involved. Our aim has been to investigate the protein pattern of Xenopus laevis embryos exposed to simulated microgravity during the first 6 days of development. A proteomic approach was applied to compare the protein profiles of Xenopus embryos developed in simulated microgravity and in normal conditions. Attention was focused on embryos that do not present visible malformations in order to investigate if weightlessness has effects at protein level in the absence of macroscopic alterations. The data presented strongly suggest that some of the major components of the cytoskeleton vary in such conditions. Three major findings are described for the first time: (i) the expression of important factors involved in the organization and stabilization of the cytoskeleton, such as Arp (actin-related protein) 3 and stathmin, is heavily affected by microgravity; (ii) the amount of the two major cytoskeletal proteins, actin and tubulin, do not change in such conditions; however, (iii) an increase in the tyrosine nitration of these two proteins can be detected. The data suggest that, in the absence of morphological alterations, simulated microgravity affects the intracellular movement system of cells by altering cytoskeletal proteins heavily involved in the regulation of cytoskeleton remodelling.

Role of protein tyrosine nitration in neurodegenerative diseases and atherosclerosis

Nitric oxide generates reactive nitrosative species, such as peroxynitrite (ONOO(-)) that may be involved in a number of diseases. ONOO(-) can mediate protein tyrosine nitration which causes structural changes of affected proteins and leads to their inactivation. Various proteomics and immunological methods including mass spectrometry combined with both liquid and 2-D PAGE, and immunodetection have been employed to identify and characterize nitrated proteins from pathological samples. This review presents the pahtobiological roles of the pathogenic posttranslational modification in neurodegenerative diseases and atherosclerosis.