Identification of tyrosine-nitrated proteins in HT22 hippocampal cells during glutamate-induced oxidative stress

Method for detection of hydrogen peroxide in HT22 cells

We have proposed a new method which can be applied in assessing the intracellular production of hydrogen peroxide. Using this assay we have examined the hydrogen peroxide generation during the L-glutamate induced oxidative stress in the HT22 hippocampal cells. The detection of hydrogen peroxide is based on two crucial reagents cis-[Cr(C₂O₄)(pm)(OH₂)₂]⁺ (pm denotes pyridoxamine) and 2-ketobutyrate. The results obtained indicate that the presented method can be a promising tool to detect hydrogen peroxide in biological samples, particularly in cellular experimental models.

Effects of radiofrequency field exposure on glutamate-induced oxidative stress in mouse hippocampal HT22 cells

PURPOSE

To define the impact of radiofrequency (RF) under in vitro experimental Alzheimer's disease conditions, we investigated the effect of RF radiation on glutamate-induced oxidative stress in mouse hippocampal neuronal HT22 cells.

MATERIALS AND METHODS

Cell survival rate was measured by MTT and trypan blue exclusion assays. Cell cycle distribution, cell death, and ROS production were analyzed using flow cytometry. Expression of proteins was analyzed by Western blot.

RESULTS

RF exposure alone had a marginal impact on cell proliferation; however, it significantly enhanced glutamate-induced cytotoxicity in HT22 cells. Glutamate augmented the subG1 fraction of cell cycle, annexin/propidium iodide positive cell population, and expression of cleaved poly (ADP ribose) polymerase, which were further increased by RF exposure. Glutamate induced reactive oxygen species (ROS) generation and RF exposure further upregulated it. N-acetylcysteine (NAC) treatment completely abrogated glutamate- and RF-induced ROS production followed by cell death and restored cell proliferation in HT22 cells. Finally, glutamate phosphorylated c-Jun N-terminal kinase (JNK) and RF increased this event further. Treatment with NAC and inhibitor of JNK decreased JNK phosphorylation and restored cell proliferation, respectively.

CONCLUSIONS

Our results demonstrate that RF exposure enhanced glutamate-induced cytotoxicity by further increase of ROS production in HT22 cells.

Mass spectrometry analysis of nitrotyrosine-containing proteins

Oxidative stress plays important roles in a wide range of diseases such as cancer, inflammatory disease, neurodegenerative disorders, etc. Tyrosine nitration in a protein is a chemically stable oxidative modification, and a marker of oxidative injuries. Mass spectrometry (MS) is a key technique to identify nitrotyrosine-containing proteins and nitrotyrosine sites in endogenous and synthetic nitroproteins and nitropeptides. However, in vivo nitrotyrosine-containing proteins occur with extreme low-abundance to severely challenge the use of MS to identify in vivo nitroproteins and nitrotyrosine sites. A preferential enrichment of nitroproteins and/or nitropeptides is necessary before MS analysis. Current enrichment methods include immuno-affinity techniques, chemical derivation of the nitro group plus target isolations, followed with tandem mass spectrometry analysis. This article reviews the MS techniques and pertinent before-MS enrichment techniques for the identification of nitrotyrosine-containing proteins. This article reviews future trends in the field of nitroproteomics, including quantitative nitroproteomics, systems biological networks of nitroproteins, and structural biology study of tyrosine nitration to completely clarify the biological functions of tyrosine nitration.

Differentiation renders susceptibility to excitotoxicity in HT22 neurons

HT22 is an immortalized mouse hippocampal neuronal cell line that does not express cholinergic and glutamate receptors like mature hippocampal neurons in vivo. This in part prevents its use as a model for mature hippocampal neurons in memory-related studies. We now report that HT22 cells were appropriately induced to differentiate and possess properties similar to those of mature hippocampal neurons in vivo, such as becoming more glutamate-receptive and excitatory. Results showed that sensitivity of HT22 cells to glutamate-induced toxicity changed dramatically when comparing undifferentiated with differentiated cells, with the half-effective concentration for differentiated cells reducing approximately two orders of magnitude. Moreover, glutamate-induced toxicity in differentiated cells, but not undifferentiated cells, was inhibited by the N-methyl-D- aspartate receptor antagonists MK-801 and memantine. Evidently, differentiated HT22 cells expressed N-methyl-D-aspartate receptors, while undifferentiated cells did not. Our experimental findings indicated that differentiation is important for immortalized cell lines to render post-mitotic neuronal properties, and that differentiated HT22 neurons represent a better model of hippocampal neurons than undifferentiated cells.

Pretreatment with 2-(4-methoxyphenyl)ethyl-2-acetamido-2-deoxy-β-D-pyranoside attenuates cerebral ischemia/reperfusion-induced injury in vitro and in vivo

Salidroside, extracted from the root of Rhodiola rosea L, is known for its pharmacological properties, in particular its neuroprotective effects. 2-(4-Methoxyphenyl) ethyl-2-acetamido-2-deoxy-β-D-pyranoside (GlcNAc-Sal), an analog of salidroside, was recently synthesized and shown to possess neuroprotective properties. The purpose of the current study was to investigate the neuroprotective effects of GlcNAc-Sal against oxygen-glucose deprivation-reperfusion (OGD-R)-induced neurotoxicity in vitro and global cerebral ischemia-reperfusion (GCI-R) injury in vivo. Cell viability tests and Hoechst 33342 staining confirmed that GlcNAc-Sal pretreatment markedly attenuated OGD-R induced apoptotic cell death in immortalized mouse hippocampal HT22 cells. Western blot, immunofluorescence and PCR analyses revealed that GlcNAc-Sal pretreatment restored the balance of pro- and anti-apoptotic proteins and inhibited the activation of caspase-3 and PARP induced by OGD-R treatment. Further analyses showed that GlcNAc-Sal pretreatment antagonized reactive oxygen species (ROS) generation, iNOS-derived NO production and NO-related apoptotic cell death during OGD-R stimulation. GCI-R was induced by bilateral common carotid artery occlusion (BCCAO) and reperfusion in mice in vivo. Western blot analysis showed that GlcNAc-Sal pretreatment decreased the expression of caspase-3 and increased the expression of Bcl-2 (B-cell lymphoma 2)/Bax (Bcl-2-associated X protein) induced by GCI-R treatment. Our findings suggest that GlcNAc-Sal pretreatment prevents brain ischemia reperfusion injury by the direct or indirect suppression of cell apoptosis and GlcNAc-Sal could be developed as a broad-spectrum agent for the prevention and/or treatment of cerebral ischemic injury.

Mass spectrometry and 3-nitrotyrosine: strategies, controversies, and our current perspective

Reactive-nitrogen species (RNS) such as peroxynitrite (ONOO(-)), that is, the reaction product of nitric oxide ((•)NO) and superoxide (O2(-•)), nitryl chloride (NO2Cl) and (•)NO2 react with the activated aromatic ring of tyrosine to form 3-nitrotyrosine. This modification, which has been known for more than a century, occurs to both the free form of the amino acid (i.e., soluble/free tyrosine) and to tyrosine residues covalently bound within the backbone of peptides and proteins. Nitration of tyrosine is thought to be of biological significance and has been linked to health and disease, but determining its role has proved challenging. Several key questions have been the focus of much of the research activity: (a) to what extent is free/soluble tyrosine nitrated in biological tissues and fluids, and (b) are there specific site(s) of nitration within peptides/proteins and to what extent (i.e., stoichiometry) does this modification occur? These issues have been addressed in a wide range of sample types (e.g., blood, urine, CSF, exhaled breath condensate and various tissues) and a diverse array of physiological/pathophysiological scenarios. The accurate determination of nitrated tyrosine is, however, a stumbling block. Despite extensive study, the extent to which nitration occurs in vivo, the specificity of the nitration reaction, and its importance in health and disease, remain unclear. In this review, we highlight the analytical challenges and discuss the approaches adopted to address them. Mass spectrometry, in combination with either gas chromatography (GC-MS, GC-MS/MS) or liquid chromatography (LC-MS/MS), has played the central role in the analysis of 3-nitrotyrosine and tyrosine-nitrated biological macromolecules. We discuss its unique attributes and highlight the role of stable-isotope labeled 3-nitrotyrosine analogs in both accurate quantification, and in helping to define the biological relevance of tyrosine nitration. We show that the application of sophisticated mass spectrometric techniques is advantageous if not essential, but that this alone is by no means a guarantee of accurate findings. We discuss the important analytical challenges in quantifying 3-nitrotyrosine, possible workarounds, and we attempt to make sense of the disparate findings that have been reported so far.

Cis-[Cr(C2O4)(pm)(OH2)2]+ coordination ion as a specific sensing ion for H2O2 detection in HT22 cells

The purpose of this study was to examine the application of the coordinated cis-[Cr(C₂O₄)(pm)(OH)₂]⁺ cation where pm denotes pyridoxamine, as a specific sensing ion for the detection of hydrogen peroxide (H₂O₂). The proposed method for H₂O₂ detection includes two key steps. The first step is based on the nonenzymatic decarboxylation of pyruvate upon reaction with H₂O₂, while the second step is based on the interaction of cis-[Cr(C₂O₄)(pm)(OH₂)₂]⁺ with the CO₂ released in the previous step. Using this method H₂O₂ generated during glutamate-induced oxidative stress was detected in HT22 hippocampal cells. The coordination ion cis-[Cr(C₂O₄)(pm)(OH₂)₂]⁺ and the spectrophotometric stopped-flow technique were applied to determine the CO₂ concentration in cell lysates, supernatants and cell-free culture medium. Prior to CO₂ assessment pyruvate was added to all samples studied. Pyruvate reacts with H₂O₂ with 1:1 stoichiometry, and consequently the amount of CO₂ released in this reaction is equivalent to the amount of H₂O₂.

Curcumin abates hypoxia-induced oxidative stress based-ER stress-mediated cell death in mouse hippocampal cells (HT22) by controlling Prdx6 and NF-κB regulation

Oxidative stress and endoplasmic reticulum (ER) stress are emerging as crucial events in the etiopathology of many neurodegenerative diseases. While the neuroprotective contributions of the dietary compound curcumin has been recognized, the molecular mechanisms underlying curcumin's neuroprotection under oxidative and ER stresses remains elusive. Herein, we show that curcumin protects HT22 from oxidative and ER stresses evoked by the hypoxia (1% O(2) or CoCl(2) treatment) by enhancing peroxiredoxin 6 (Prdx6) expression. Cells exposed to CoCl(2) displayed reduced expression of Prdx6 with higher reactive oxygen species (ROS) expression and activation of NF-κB with IκB phosphorylation. When NF-κB activity was blocked by using SN50, an inhibitor of NF-κB, or cells treated with curcumin, the repression of Prdx6 expression was restored, suggesting the involvement of NF-κB in modulating Prdx6 expression. These cells were enriched with an accumulation of ER stress proteins, C/EBP homologous protein (CHOP), GRP/78, and calreticulin, and had activated states of caspases 12, 9, and 3. Reinforced expression of Prdx6 in HT22 cells by curcumin reestablished survival signaling by reducing propagation of ROS and blunting ER stress signaling. Intriguingly, knockdown of Prdx6 by antisense revealed that loss of Prdx6 contributed to cell death by sustaining enhanced levels of ER stress-responsive proapoptotic proteins, which was due to elevated ROS production, suggesting that Prdx6 deficiency is a cause of initiation of ROS-mediated ER stress-induced apoptosis. We propose that using curcumin to reinforce the naturally occurring Prdx6 expression and attenuate ROS-based ER stress and NF-κB-mediated aberrant signaling improves cell survival and may provide an avenue to treat and/or postpone diseases associated with ROS or ER stress.

Effects of [6]-shogaol on cholinergic signaling in HT22 cells following neuronal damage induced by hydrogen peroxide

Cholinergic neurons play a major role in memory and attention. The dysfunction and death of these neurons, especially in the hippocampus, are thought to contribute to the pathophysiology of memory deficits associated with Alzheimer's disease (AD). Therefore, studying the cholinergic properties and cell survival may help in treating this disease. We investigated the possible effects of [6]-shogaol on cholinergic signaling in HT22 hippocampal neuronal cells. HT22 cells express essential cholinergic markers, including choline acetyltransferase (ChAT) and choline transporter (ChTp). HT22 cells treated with H(2)O(2) for 3h showed an increase in ROS production (35%). These features were partly recovered by [6]-shogaol. Treating H(2)O(2)-treated HT22 cells with [6]-shogaol markedly increased the expression of ChAT and ChTp, an effect similar to that of brain-derived neurotrophic factor (BDNF). Furthermore, K-252a, an inhibitor of the BDNF receptor Trk B, attenuated the effects of both [6]-shogaol and BDNF. These data suggest that [6]-shogaol protects neurons by increasing ChAT and ChTp expression through a BDNF increase and thus may be useful for treating neurodegenerative diseases.