Inflammatory cytokines induce protein tyrosine nitration in rat astrocytes

Treatment with hydrogen-rich water protects against thioacetamide-induced hepatic encephalopathy in rats through stabilizing liver-brain disturbance

Hepatic encephalopathy (HE), a neuropsychiatric complication secondary to liver cirrhosis and hepatic failure, represents the leading cause of mortality in end-stage liver disease. While hyperammonemia remains the central pathogenic factor in HE progression, emerging evidence implicates oxidative stress, neuroinflammation, and neuronal apoptosis as critical synergistic contributors to HE pathogenesis. Hydrogen-rich water, known for its antioxidant, anti-inflammatory, and anti-apoptotic properties, has not been systematically investigated for therapeutic efficacy in HE management. In the current investigation, we successfully established a HE rat model by administering thioacetamide via intraperitoneal injection. By observing the general state and behavioral changes of the rats, detecting liver function and blood ammonia, and observing the pathological changes of liver and brain tissue, it was discussed whether hydrogen-rich water had a preventive and therapeutic effect on hepatic encephalopathy. Oxidative stress, inflammation and neuronal apoptosis were detected in plasma, prefrontal cortex and hippocampus to explore the possible mechanism of its protective effect. The results showed that hydrogen-rich water can improve the behavioral changes of the HE rats, reduce blood ammonia, reduce liver function damage, alleviate the pathological changes of liver and brain tissue, significantly inhibit the systemic and local oxidative stress and inflammation of the brain tissue of the HE rats, and reduce neuronal apoptosis. In summary, hydrogen-rich water might stabilize liver-brain disturbance in thioacetamide-induced HE rats by anti-inflammation, anti-oxidative stress and reducing neuronal apoptosis.

Neuroglia in hepatic encephalopathy

Neuroglia contribute to the pathophysiology of hepatic encephalopathy (HE) either beneficially or detrimentally. Pathogenesis of HE is linked to damage triggered by blood-derived toxins, with ammonia being the main causative factor. Neuroglial cells, especially astrocytes and microglia, respond to HE-associated systemic and central signals and undergo complex and variable changes in their metabolism, morphology, and function, which include ion and water dyshomeostasis in conjunction with neurotransmission imbalance and neuroinflammation. HE-induced alterations of astrocytes are defined as astrocytopathy, with aberrant astrocytes resulting in either gain or loss of functions. In the chronic HE, the presence of Alzheimer type II cells is a histologic hallmark, with asthenic astrocytes emerging as a newcomer. In acute HE, rapid swelling of astrocytes is a primary cause of cerebral edema and mortality. This chapter reviews the dominant role of astrocytes in the pathogenesis of HE resulting from acute and chronic liver failure, mainly in experimental models. The focus is on the loss of homeostatic function bearing upon the functioning of the glymphatic system, aberrant neurotransmission as a consequence of astrocyte-neuron miscommunication, and the concordant neuroinflammatory response of astrocytes and microglia. The chapter concludes with a delineation of concepts for future research.

Astrocytic NMDA Receptors

Astrocytic NMDA receptors (NMDARs) are heterotetramers, whose expression and properties are largely determined by their subunit composition. Astrocytic NMDARs are characterized by a low sensitivity to magnesium ions and low calcium conductivity. Their activation plays an important role in the regulation of various intracellular processes, such as gene expression and mitochondrial function. Astrocytic NMDARs are involved in calcium signaling in astrocytes and can act through the ionotropic and metabotropic pathways. Astrocytic NMDARs participate in the interactions of the neuroglia, thus affecting synaptic plasticity. They are also engaged in the astrocyte-vascular interactions and contribute to the regulation of vascular tone. Astrocytic NMDARs are involved in various pathologies, such as ischemia and hyperammonemia, and their blockade prevents negative changes in astrocytes during these diseases.

Nitroproteomics is instrumental for stratification and targeted treatments of astrocytoma patients: expert recommendations for advanced 3PM approach with improved individual outcomes

Protein tyrosine nitration is a selectively and reversible important post-translational modification, which is closely related to oxidative stress. Astrocytoma is the most common neuroepithelial tumor with heterogeneity and complexity. In the past, the diagnosis of astrocytoma was based on the histological and clinical features, and the treatment methods were nothing more than surgery-assisted radiotherapy and chemotherapy. Obviously, traditional methods short falls an effective treatment for astrocytoma. In late 2021, the World Health Organization (WHO) adopted molecular biomarkers in the comprehensive diagnosis of astrocytoma, such as IDH-mutant and DNA methylation, which enabled the risk stratification, classification, and clinical prognosis prediction of astrocytoma to be more correct. Protein tyrosine nitration is closely related to the pathogenesis of astrocytoma. We hypothesize that nitroproteome is significantly different in astrocytoma relative to controls, which leads to establishment of nitroprotein biomarkers for patient stratification, diagnostics, and prediction of disease stages and severity grade, targeted prevention in secondary care, treatment algorithms tailored to individualized patient profile in the framework of predictive, preventive, and personalized medicine (PPPM; 3P medicine). Nitroproteomics based on gel electrophoresis and tandem mass spectrometry is an effective tool to identify the nitroproteins and effective biomarkers in human astrocytomas, clarifying the biological roles of oxidative/nitrative stress in the pathophysiology of astrocytomas, functional characteristics of nitroproteins in astrocytomas, nitration-mediated signal pathway network, and early diagnosis and treatment of astrocytomas. The results finds that these nitroproteins are enriched in mitotic cell components, which are related to transcription regulation, signal transduction, controlling subcellular organelle events, cell perception, maintaining cell homeostasis, and immune activity. Eleven statistically significant signal pathways are identified in astrocytoma, including remodeling of epithelial adherens junctions, germ cell-sertoli cell junction signaling, 14-3-3-mediated signaling, phagosome maturation, gap junction signaling, axonal guidance signaling, assembly of RNA polymerase III complex, and TREM1 signaling. Furthermore, protein tyrosine nitration is closely associated with the therapeutic effects of protein drugs, and molecular mechanism and drug targets of cancer. It provides valuable data for studying the protein nitration biomarkers, molecular mechanisms, and therapeutic targets of astrocytoma towards PPPM (3P medicine) practice.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13167-023-00348-y.

Cellular Pathogenesis of Hepatic Encephalopathy: An Update

Hepatic encephalopathy (HE) is a neuropsychiatric syndrome derived from metabolic disorders due to various liver failures. Clinically, HE is characterized by hyperammonemia, EEG abnormalities, and different degrees of disturbance in sensory, motor, and cognitive functions. The molecular mechanism of HE has not been fully elucidated, although it is generally accepted that HE occurs under the influence of miscellaneous factors, especially the synergistic effect of toxin accumulation and severe metabolism disturbance. This review summarizes the recently discovered cellular mechanisms involved in the pathogenesis of HE. Among the existing hypotheses, ammonia poisoning and the subsequent oxidative/nitrosative stress remain the mainstream theories, and reducing blood ammonia is thus the main strategy for the treatment of HE. Other pathological mechanisms mainly include manganese toxicity, autophagy inhibition, mitochondrial damage, inflammation, and senescence, proposing new avenues for future therapeutic interventions.

Multiple oxidative post-translational modifications of human glutamine synthetase mediate peroxynitrite-dependent enzyme inactivation and aggregation

Glutamine synthetase (GS), which catalyzes the ATP-dependent synthesis of L-glutamine from L-glutamate and ammonia, is a ubiquitous and conserved enzyme that plays a pivotal role in nitrogen metabolism across all life domains. In vertebrates, GS is highly expressed in astrocytes, where its activity sustains the glutamate-glutamine cycle at glutamatergic synapses and is thus essential for maintaining brain homeostasis. In fact, decreased GS levels or activity have been associated with neurodegenerative diseases, with these alterations attributed to oxidative post-translational modifications of the protein, in particular tyrosine nitration. In this study, we expressed and purified human GS (HsGS) and performed an in-depth analysis of its oxidative inactivation by peroxynitrite (ONOO^-) in vitro. We found that ONOO^- exposure led to a dose-dependent loss of HsGS activity, the oxidation of cysteine, methionine, and tyrosine residues and also the nitration of tryptophan and tyrosine residues. Peptide mapping by LC-MS/MS through combined H₂¹⁶O/H₂¹⁸O trypsin digestion identified up to 10 tyrosine nitration sites and five types of dityrosine cross-links; these modifications were further scrutinized by structural analysis. Tyrosine residues 171, 185, 269, 283, and 336 were the main nitration targets; however, tyrosine-to-phenylalanine HsGS mutants revealed that their sole nitration was not responsible for enzyme inactivation. In addition, we observed that ONOO^- induced HsGS aggregation and activity loss. Thiol oxidation was a key modification to elicit aggregation, as it was also induced by hydrogen peroxide treatment. Taken together, our results indicate that multiple oxidative events at various sites are responsible for the inactivation and aggregation of human GS.

Hepatic encephalopathy

Hepatic encephalopathy (HE) is a prognostically relevant neuropsychiatric syndrome that occurs in the course of acute or chronic liver disease. Besides ascites and variceal bleeding, it is the most serious complication of decompensated liver cirrhosis. Ammonia and inflammation are major triggers for the appearance of HE, which in patients with liver cirrhosis involves pathophysiologically low-grade cerebral oedema with oxidative/nitrosative stress, inflammation and disturbances of oscillatory networks in the brain. Severity classification and diagnostic approaches regarding mild forms of HE are still a matter of debate. Current medical treatment predominantly involves lactulose and rifaximin following rigorous treatment of so-called known HE precipitating factors. New treatments based on an improved pathophysiological understanding are emerging.

Glutamine synthetase as a central element in hepatic glutamine and ammonia metabolism: novel aspects

Glutamine synthetase (GS) in the liver is expressed in a small perivenous, highly specialized hepatocyte population and is essential for the maintenance of low, non-toxic ammonia levels in the organism. However, GS activity can be impaired by tyrosine nitration of the enzyme in response to oxidative/nitrosative stress in a pH-sensitive way. The underlying molecular mechanism as investigated by combined molecular simulations and in vitro experiments indicates that tyrosine nitration can lead to a fully reversible and pH-sensitive regulation of protein function. This approach was also used to understand the functional consequences of several recently described point mutations of human GS with clinical relevance and to suggest an approach to restore impaired GS activity.

Rapid metabolic and bioenergetic adaptations of astrocytes under hyperammonemia - a novel perspective on hepatic encephalopathy

Hepatic encephalopathy (HE) is a well-studied, neurological syndrome caused by liver dysfunctions. Ammonia, the major toxin during HE pathogenesis, impairs many cellular processes within astrocytes. Yet, the molecular mechanisms causing HE are not fully understood. Here we will recapitulate possible underlying mechanisms with a clear focus on studies revealing a link between altered energy metabolism and HE in cellular models and in vivo. The role of the mitochondrial glutamate dehydrogenase and its role in metabolic rewiring of the TCA cycle will be discussed. We propose an updated model of ammonia-induced toxicity that may also be exploited for therapeutic strategies in the future.

Pathomechanisms in hepatic encephalopathy

Hepatic encephalopathy (HE) is a frequent neuropsychiatric complication in patients with acute or chronic liver failure. Symptoms of HE in particular include disturbances of sensory and motor functions and cognition. HE is triggered by heterogeneous factors such as ammonia being a main toxin, benzodiazepines, proinflammatory cytokines and hyponatremia. HE in patients with liver cirrhosis is triggered by a low-grade cerebral edema and cerebral oxidative/nitrosative stress which bring about a number of functionally relevant alterations including posttranslational protein modifications, oxidation of RNA, gene expression changes and senescence. These alterations are suggested to impair astrocyte/neuronal functions and communication. On the system level, a global slowing of oscillatory brain activity and networks can be observed paralleling behavioral perceptual and motor impairments. Moreover, these changes are related to increased cerebral ammonia, alterations in neurometabolite and neurotransmitter concentrations and cortical excitability in HE patients.

Mechanism of Fully Reversible, pH-Sensitive Inhibition of Human Glutamine Synthetase by Tyrosine Nitration

Glutamine synthetase (GS) catalyzes an ATP-dependent condensation of glutamate and ammonia to form glutamine. This reaction-and therefore GS-are indispensable for the hepatic nitrogen metabolism. Nitration of tyrosine 336 (Y336) inhibits human GS activity. GS nitration and the consequent loss of GS function are associated with a broad range of neurological diseases. The mechanism by which Y336 nitration inhibits GS, however, is not understood. Here, we show by means of unbiased MD simulations, binding, and configurational free energy computations that Y336 nitration hampers ATP binding but only in the deprotonated and negatively charged state of residue 336. By contrast, for the protonated and neutral state, our computations indicate an increased binding affinity for ATP. pK_a computations of nitrated Y336 within GS predict a pK_a of ∼5.3. Thus, at physiological pH, nitrated Y336 exists almost exclusively in the deprotonated and negatively charged state. In vitro experiments confirm these predictions, in that, the catalytic activity of nitrated GS is decreased at pH 7 and 6 but not at pH 4. These results indicate a novel, fully reversible, pH-sensitive mechanism for the regulation of GS activity by tyrosine nitration.

Astrocyte swelling in hepatic encephalopathy: molecular perspective of cytotoxic edema

Hepatic encephalopathy (HE) may occur in patients with liver failure. The most critical pathophysiologic mechanism of HE is cerebral edema following systemic hyperammonemia. The dysfunctional liver cannot eliminate circulatory ammonia, so its plasma and brain levels rise sharply. Astrocytes, the only cells that are responsible for ammonia detoxification in the brain, are dynamic cells with unique phenotypic properties that enable them to respond to small changes in their environment. Any pathological changes in astrocytes may cause neurological disturbances such as HE. Astrocyte swelling is the leading cause of cerebral edema, which may cause brain herniation and death by increasing intracranial pressure. Various factors may have a role in astrocyte swelling. However, the exact molecular mechanism of astrocyte swelling is not fully understood. This article discusses the possible mechanisms of astrocyte swelling which related to hyperammonia, including the possible roles of molecules like glutamine, lactate, aquaporin-4 water channel, 18 KDa translocator protein, glial fibrillary acidic protein, alanine, glutathione, toll-like receptor 4, epidermal growth factor receptor, glutamate, and manganese, as well as inflammation, oxidative stress, mitochondrial permeability transition, ATP depletion, and astrocyte senescence. All these agents and factors may be targeted in therapeutic approaches to HE.

TNFα increases STAT3-mediated expression of glutaminase isoform KGA in cultured rat astrocytes

Glutamate related excitotoxicity and excess of cerebral levels of tumor necrosis factor alpha (TNFα) are interrelated and well documented abnormalities noticed in many central nervous system diseases. Contribution of kidney type glutaminase (KGA) and shorter alternative splicing form (GAC) to glutamine degradation in astrocytes has been recently a matter of dispute and extensive study but the regulation of the GLS isoforms by inflammatory factors is still not well known. Here we show that treatment of cultured rat cortical astrocytes with pathophysiologically relevant (50 ng/ml) concentration of TNFα specifically increases the expression of KGA but not GAC and increases activity of GLS. No changes in the expression of either of two GLS isoforms were observed following treatment with other tested cytokines IL-1β and IL-6. The TNFα mediated KGA expression was associated with increased phosphorylation of signal transducer and activator of transcription 3 (STAT3). Stimulatory effect of TNF-α on KGA expression was reduced by selective inhibition of (STAT3) but not by inhibition of STAT1 nor nuclear transcription factor kappa. Additionally, the role of miRNA in TNFα-induced expression of KGA in astrocytes was excluded, since the expression of miR-23a/b and miR-200c, potential regulators of KGA expression, was unchanged. This study documents increased KGA expression in the astrocytes under inflammatory stimulation, identifying TNFα as a cytokine mediating this response, and demonstrates the specific and selective involvement of STAT3.

Hepatic Encephalopathy and Astrocyte Senescence

Hepatic Encephalopathy (HE) is a severe complication of acute or chronic liver diseases with a broad spectrum of neurological symptoms including motor disturbances and cognitive impairment of different severity. Contrary to former beliefs, a growing number of studies suggest that cognitive impairment may not fully reverse after an acute episode of overt HE in patients with liver cirrhosis. The reasons for persistent cognitive impairment in HE are currently unknown but recent observations raise the possibility that astrocyte senescence may play a role here. Astrocyte senescence is closely related to oxidative stress and correlate with irreversible cognitive decline in aging and neurodegenerative diseases. In line with this, surrogate marker for oxidative stress and senescence were upregulated in ammonia-exposed cultured astrocytes and in post mortem brain tissue from patients with liver cirrhosis with but not without HE. Ammonia-induced senescence in astrocytes involves glutamine synthesis-dependent formation of reactive oxygen species (ROS), p53 activation and upregulation of cell cycle inhibitory factors p21 and GADD45α. More recent studies also suggest a role of ROS-induced downregulation of Heme Oxygenase (HO)1-targeting micro RNAs and upregulation of HO1 for ammonia-induced proliferation inhibition in cultured astrocytes. Further studies are required to identify the precise sequence of events that lead to astrocyte senescence and to elucidate functional implications of senescence for cognitive performance in patients with liver cirrhosis and HE.

Memory Improvement Effect of Ethanol Garlic (A. sativum) Extract in Streptozotocin-Nicotinamide Induced Diabetic Wistar Rats Is Mediated through Increasing of Hippocampal Sodium-Potassium ATPase, Glutamine Synthetase, and Calcium ATPase Activities

Studies suggest that garlic (A. sativum) improves memory dependent on the hippocampus. However, the effect of ethanol garlic extract on hippocampus Na⁺/K⁺ ATPase, Ca²⁺ ATPase, and glutamine synthetase (GS) activities as possible mechanisms in memory improvement in diabetic Wistar rats has not been reported. Twenty-four male Wistar rats weighing 200-250 g were divided into three groups with 8 rats each. Group (A), normal control rats, and Group (B), diabetic rats, received 1 ml of normal saline; diabetic rats in Group (C) received 1000 mg/kg of garlic extract orally for 21 days. Hyperglycemia was induced by a single intraperitoneal injection of streptozotocin 60 mg/kg followed by 120 mg/kg nicotinamide while extraction of garlic was done by cold maceration method. Memory was tested in all groups. After that, the rats were sacrificed, the brain was removed, and the hippocampi were carefully excised and then homogenized. Activities of Na⁺/K⁺ ATPase, calcium ATPase, and GS were analyzed from the homogenate. Results showed improvement in memory and a significant increase (P < 0.05) in hippocampus Na⁺/K⁺ ATPase, Ca²⁺ ATPase, and GS activities in diabetic rats treated with garlic extract. In conclusion, the increased activity of hippocampus Na⁺/K⁺ ATPase, calcium ATPase, and glutamine synthetase may account for the memory improvement.

Ammonia-induced miRNA expression changes in cultured rat astrocytes

Hepatic encephalopathy is a neuropsychiatric syndrome evolving from cerebral osmotic disturbances and oxidative/nitrosative stress. Ammonia, the main toxin of hepatic encephalopathy, triggers astrocyte senescence in an oxidative stress-dependent way. As miRNAs are critically involved in cell cycle regulation and their expression may be regulated by oxidative stress, we analysed, whether astrocyte senescence is a consequence of ammonia-induced miRNA expression changes. Using a combined miRNA and gene microarray approach, 43 miRNA species which were downregulated and 142 genes which were upregulated by NH₄Cl (5 mmol/l, 48 h) in cultured rat astrocytes were found. Ammonia-induced miRNA and gene expression changes were validated by qPCR and 43 potential miRNA target genes, including HO-1, were identified by matching upregulated mRNA species with predicted targets of miRNA species downregulated by ammonia. Inhibition of HO-1 targeting miRNAs which were downregulated by NH₄Cl strongly upregulated HO-1 mRNA and protein levels and inhibited astrocyte proliferation in a HO-1-dependent way. Preventing ammonia-induced upregulation of HO-1 by taurine (5 mmol/l) as well as blocking HO-1 activity by tin-protoporphyrine IX fully prevented ammonia-induced proliferation inhibition and senescence. The data suggest that ammonia induces astrocyte senescence through NADPH oxidase-dependent downregulation of HO-1 targeting miRNAs and concomitant upregulation of HO-1 at both mRNA and protein level.

Multidrug resistance-associated protein 4 expression in ammonia-treated cultured rat astrocytes and cerebral cortex of cirrhotic patients with hepatic encephalopathy

Hepatic encephalopathy (HE) is a neuropsychiatric syndrome frequently accompanying liver cirrhosis and reflects the clinical manifestation of a low grade cerebral edema associated with cerebral oxidative/nitrosative stress. The multidrug resistance-associated protein (Mrp) 4 is an export pump which transports metabolites that were recently suggested to play a major role in the pathogenesis of HE such as neurosteroids and cyclic nucleotides. We therefore studied Mrp4 expression changes in ammonia-exposed cultured astrocytes and postmortem human brain samples of cirrhotic patients with HE. NH₄ Cl increased Mrp4 mRNA and protein levels in astrocytes in a dose- and time-dependent manner up to threefold after 72 h of exposure and concurrently inhibited N-glycosylation of Mrp4 protein. Upregulation of Mrp4 mRNA and protein as well as impaired N-glycosylation of Mrp4 protein by ammonia were sensitive towards the glutamine-synthetase inhibitor l-methionine-S-sulfoximine and were not induced by CH₃ NH₃ Cl (5 mmol/L). Upregulation of Mrp4 mRNA required ammonia-induced activation of nitric oxide synthases or NADPH oxidase and p38^MAPK -dependent activation of PPARα. Inhibition of Mrp4 by ceefourin 1 synergistically enhanced both, inhibition of astrocyte proliferation as well as transcription of the oxidative stress surrogate marker heme oxygenase 1 by forskolin (10 µmol/L, 72 h) or NH₄ Cl (5 mmol/L, 72 h) in cultured rat astrocytes. Increased Mrp4 mRNA and protein levels were also found in postmortem brain samples from patients with liver cirrhosis with HE but not in those without HE. The data show that Mrp4 is upregulated in HE, which may be relevant for the handling of neurosteroids and cyclic nucleotides in response to ammonia. GLIA 2015;63:2092-2105.

Management in acute liver failure

Acute liver failure (ALF) is a rare, potentially fatal complication of severe hepatic illness resulting from various causes. In a clinical setting, severe hepatic injury is usually recognised by the appearance of jaundice, encephalopathy and coagulopathy. The central and most important clinical event in ALF is occurrence of hepatic encephalopathy (HE) and cerebral edema which is responsible for most of the fatalities in this serious clinical syndrome. The pathogenesis of encephalopathy and cerebral edema in ALF is unique and multifactorial. Ammonia plays a central role in the pathogenesis. The role of newer ammonia lowering agents is still evolving. Liver transplant is the only effective therapy that has been identified to be of promise in those with poor prognostic factors, whereas in the others, aggressive intensive medical management has been documented to salvage a substantial proportion of patients. A small fraction of patients undergo liver transplant and the remaining are usually treated with medical therapy. Therefore, identification of the complications and causes of death in such patients, and use of appropriate prognostic models to identify those who need liver transplant and those who can be managed with medical treatment is a vital component of therapeutic strategy. In this review, we discuss the various pathogenetic mechanisms and treatment options available.

Pathophysiology of cerebral oedema in acute liver failure

Cerebral oedema is a devastating consequence of acute liver failure (ALF) and may be associated with the development of intracranial hypertension and death. In ALF, some patients may develop cerebral oedema and increased intracranial pressure but progression to life-threatening intracranial hypertension is less frequent than previously described, complicating less than one third of cases who have proceeded to coma since the advent of improved clinical care. The rapid onset of encephalopathy may be dramatic with the development of asterixis, delirium, seizures and coma. Cytotoxic and vasogenic oedema mechanisms have been implicated with a preponderance of experimental data favouring a cytotoxic mechanism. Astrocyte swelling is the most consistent neuropathological finding in humans with ALF and ammonia plays a definitive role in the development of cytotoxic brain oedema. The mechanism(s) by which ammonia induces astrocyte swelling remains unclear but glutamine accumulation within astrocytes has led to the osmolyte hypothesis. Current evidence also supports an alternate ‘Trojan horse’ hypothesis, with glutamine as a carrier of ammonia into mitochondria, where its accumulation results in oxidative stress, energy failure and ultimately astrocyte swelling. Although a complete breakdown of the blood-brain barrier is not evident in human ALF, increased permeation to water and other small molecules such as ammonia has been demonstrated resulting from subtle alterations in the protein composition of paracellular tight junctions. At present, there is no fully efficacious therapy for cerebral oedema other than liver transplantation and this reflects our incomplete knowledge of the precise mechanisms underlying this process which remain largely unknown.

Relationship of plasma cytokines and clinical biomarkers to memory performance in HIV

Chronic systemic immune activation and inflammatory processes have been linked to brain dysfunction in medically stable HIV-infected people. We investigated the association between verbal memory performance and plasma concentrations of 13 cytokines measured using multiplexed bead array immunoassay in 74 HIV-seropositive individuals and 50 HIV-seronegative controls. Memory performance was positively related to levels of IL-8 and IFN-γ, and negatively related to IL-10 and IL-18 and to hepatitis C infection. Memory performance was not significantly related to HIV disease markers. The results indicate the importance of systemic immune and inflammatory markers to neurocognitive function in chronic and stable HIV disease.

Oxidative stress and redistribution of glutamine synthetase in California sea lions (Zalophus californianus) with domoic acid toxicosis

The aim of this study was to test the hypothesis that oxidative stress and glutamine synthetase (GS) redistribution occur in domoic acid (DA) toxicosis in California sea lions (CSLs, Zalophus californianus). Sections of archived hippocampi from seven control and 13 CSLs diagnosed with DA toxicosis were labelled immunohistochemically for GS and for two markers of oxidative stress, malondialdehyde (MDA) and 3-nitrotyrosine (NT). The distribution and intensity of labelling were compared with the pathological changes seen in haematoxylin and eosin-stained sections. Increased expression of MDA and NT occurred in neurons of the hippocampal formation of CSLs with lesions consistent with DA toxicosis. The degree of oxidative stress was not affected significantly by the chronicity or severity of hippocampal damage. In six out of seven CSLs with chronic effects of DA toxicosis, in addition to the normal glial distribution of GS, GS expression was very strong in some neurons of the subiculum. However, neuronal GS labelling was also seen in one control CSL, an effect that may have been due to previous exposure to DA. GS expression in neurons was associated with decreases in GS labelling in neighbouring glial cell processes. DA toxicosis therefore induces increased expression of markers of oxidative stress in neurons consistent with oxidative stress contributing to the initial DA insult and also the epilepsy that often develops in chronic DA toxicosis. GS redistribution occurred primarily in chronic DA toxicosis, perhaps leading to alterations of the glutamine-glutamate-GABA (gamma-aminobutyric acid) cycle and contributing to the excitotoxicity and seizures often seen in DA toxicosis.

Precipitants of hepatic encephalopathy induce rapid astrocyte swelling in an oxidative stress dependent manner

Hepatic encephalopathy (HE) is seen as the clinical manifestation of a low grade cerebral edema with formation of reactive oxygen and nitrogen species (RNOS). Astrocyte swelling is a crucial event and in cultured astrocytes HE-relevant factors almost instantaneously induce the formation of RNOS. However, short term effects of ammonia, inflammatory cytokines and RNOS on the volume of astrocytes and other brain cells as well as the underlying mechanisms are largely unknown, although a pathogenic link between RNOS formation and swelling in HE has been proposed. This issue was addressed in the present study by means of live-cell volume microscopy of brain cells in vitro. Ammonia, diazepam and pro-inflammatory cytokines such as tumor-necrosis factor-α (TNF-α), interferon-γ, interleukin-1β induced within 20min astrocyte swelling by about 25% accompanied by nuclear swelling of similar magnitude. Astrocyte swelling in response to NH4Cl, TNF-α or diazepam was abolished by the antioxidant epigallocatechin-gallate pointing to an involvement of RNOS. NH4Cl-induced astrocyte swelling was sensitive to inhibition of glutamine synthetase, NADPH oxidase or nitric oxide synthases. In line with a NMDA receptor-, prostanoid- and Ca(2+)-dependence of NH4Cl-induced RNOS formation, Ca(2+) chelation and inhibition of NMDA receptors or cyclooxygenase suppressed NH4Cl-induced astrocyte swelling, whereas the Ca(2+)-ionophore ionomycin, NMDA, glutamate and prostanoids induced rapid astrocyte swelling. NH4Cl also induced swelling of cultured microglia in a glutamine-synthesis dependent way, but had no effect on cell volume of cultured neurons. It is concluded that the pathways which trigger RNOS formation in astrocytes also trigger astrocyte swelling, whereas conversely and as shown previously hypoosmotic astrocyte swelling can induce RNOS formation. This establishes a complex interplay with an auto-amplificatory loop between RNOS formation and astrocyte swelling as an important event in the pathogenesis of HE.

Inflammation and hepatic encephalopathy

Hepatic encephalopathy (HE) is a neuropsychiatric syndrome associated with both acute and chronic liver dysfunction, spanning a spectrum that ranges from mild neuropsychological disturbances to coma. The central role of ammonia in the pathogenesis of HE remains incontrovertible however, there is a robust evidence base indicating the important role of inflammation in exacerbating the neurological effects of HE. Inflammation can arise directly within the brain itself as a result of deranged nitrogen and energy homeostasis, with resultant neuronal, astrocyte and microglial dysfunction. Inflammation may also originate in the peripheral circulation and exert effects on the brain indirectly, via the release of pro-inflammatory mediators which directly signal to the brain via the vagus nerve. This review summarises the data that demonstrate the synergistic relationship of inflammation and ammonia that culminates in the manifestation of HE. Sterile inflammation arising from the inflamed or necrotic liver, circulating endotoxin arising from the gut (bacterial translocation) inducing immune dysfunction, and superimposed sepsis will be comprehensively discussed. Finally, this review will provide an overview of the existing and novel treatments on the horizon which can target the inflammatory response, and how they might translate into clinical practise as therapies in the prophylaxis and treatment of HE.

Osmotic and oxidative/nitrosative stress in ammonia toxicity and hepatic encephalopathy

Hepatic encephalopathy (HE) is a neuropsychiatric complication of acute or chronic liver failure. Currently, HE in cirrhotic patients is seen as a clinical manifestation of a low grade cerebral edema which exacerbates in response to a variety of precipitating factors after an ammonia-induced exhaustion of the volume-regulatory capacity of the astrocyte. Astrocyte swelling triggers a complex signaling cascade which relies on NMDA receptor activation, elevation of intracellular Ca(2+) concentration and prostanoid-driven glutamate exocytosis, which result in increased formation of reactive nitrogen and oxygen species (RNOS) through activation of NADPH oxidase and nitric oxide synthase. Since RNOS in turn promote astrocyte swelling, a self-amplifying signaling loop between osmotic- and oxidative stress ensues, which triggers a variety of downstream consequences. These include protein tyrosine nitration (PTN), oxidation of RNA, mobilization of zinc, alterations in intra- and intercellular signaling and multiple effects on gene transcription. Whereas PTN can affect the function of a variety of proteins, such as glutamine synthetase, oxidized RNA may affect local protein synthesis at synapses, thereby potentially interfering with protein synthesis-dependent memory formation. PTN and RNA oxidation are also found in post mortem human cerebral cortex of cirrhotic patients with HE but not in those without HE, thereby confirming a role for oxidative stress in the pathophysiology of HE. Evidence derived from animal experiments and human post mortem brain tissue also indicates an up-regulation of microglia activation markers in the absence of increased synthesis of pro-inflammatory cytokines. However, the role of activated microglia in the pathophysiology of HE needs to be worked out in more detail. Most recent observations made in whole genome micro-array analyses of post mortem human brain tissue point to a hitherto unrecognized activation of multiple anti-inflammatory signaling pathways.

Ammonia increases nitric oxide, free Zn(2+), and metallothionein mRNA expression in cultured rat astrocytes

Ammonia is a major player in the pathogenesis of hepatic encephalopathy (HE) and affects astrocyte function by triggering a self-amplifying cycle between osmotic and oxidative stress. We recently demonstrated that hypoosmotic astrocyte swelling rapidly stimulates nitric oxide (NO) production and increases intracellular free Zn(2+) concentration ([Zn(2+)](i)). Here we report effects of ammonia on [Zn(2+)](i) homeostasis and NO synthesis. In cultured rat astrocytes, NH(4)Cl (5 mm) increased within 6 h both cytosolic and mitochondrial [Zn(2+)]. The [Zn(2+)](i) increase was transient and was mimicked by the nonmetabolizable CH(3)NH(3)Cl, and it was dependent on NO formation, as evidenced by the sensitivity toward the nitric oxide synthase inhibitor N(G)-monomethyl-l-arginine. The NH(4)Cl-induced NO formation was sensitive to the Ca(2+) chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl) ester and increases in both NO and [Zn(2+)](i) were blocked by the N-methyl-d-aspartate receptor antagonist MK-801. The NH(4)Cl-triggered increase in [Zn(2+)](i) was followed by a Zn(2+)-dependent nuclear appearance of the metal response element-binding transcription factor and metallothionein messenger RNA (mRNA) induction. Metallothionein mRNA was also increased in vivo in rat cerebral cortex 6 h after an NH(4)Ac challenge. NH(4)Cl increased peripheral-type benzodiazepine receptor (PBR) protein expression, whereas PBR mRNA levels were decreased in a Zn(2+)-independent manner. The Zn(2+)-dependent upregulation of metallothionein following ammonia intoxication may reflect a cytoprotective response, whereas the increase in PBR expression may augment HE development.

HIV-1, methamphetamine and astrocyte glutamate regulation: combined excitotoxic implications for neuro-AIDS

Glutamate, the most abundant excitatory transmitter in the brain can lead to neurotoxicity when not properly regulated. Excitotoxicity is a direct result of abnormal regulation of glutamate concentrations in the synapse, and is a common neurotoxic mediator associated with neurodegenerative disorders. It is well accepted that methamphetamine (METH), a potent central nervous stimulant with high abuse potential, and human immunodeficiency virus (HIV)-1 are implicated in the progression of neurocognitive malfunction. Both have been shown to induce common neurodegenerative effects such as astrogliosis, compromised blood brain barrier integrity, and excitotoxicity in the brain. Reduced glutamate uptake from neuronal synapses likely leads to the accumulation of glutamate in the extracellular spaces. Astrocytes express the glutamate transporters responsible for majority of the glutamate uptake from the synapse, as well as for vesicular glutamate release. However, the cellular and molecular mechanisms of astrocyte-mediated excitotoxicity in the context of METH and HIV-1 are undefined. Topics reviewed include dysregulation of the glutamate transporters, specifically excitatory amino acid transporter-2, metabotropic glutamate receptor(s) expression and the release of glutamate by vesicular exocytosis. We also discuss glutamate concentration dysregulation through astrocytic expression of enzymes for glutamate synthesis and metabolism. Lastly, we discuss recent evidence of various astrocyte and neuron crosstalk mechanisms implicated in glutamate regulation. Astrocytes play an essential role in the neuropathologies associated with METH/HIV-1-induced excitotoxicity. We hope to shed light on common cellular and molecular pathways astrocytes share in glutamate regulation during drug abuse and HIV-1 infection.

Ammonia increases nitric oxide, free Zn(2+), and metallothionein mRNA expression in cultured rat astrocytes

Ammonia is a major player in the pathogenesis of hepatic encephalopathy (HE) and affects astrocyte function by triggering a self-amplifying cycle between osmotic and oxidative stress. We recently demonstrated that hypoosmotic astrocyte swelling rapidly stimulates nitric oxide (NO) production and increases intracellular free Zn(2+) concentration ([Zn(2+)](i)). Here we report effects of ammonia on [Zn(2+)](i) homeostasis and NO synthesis. In cultured rat astrocytes, NH(4)Cl (5 mm) increased within 6 h both cytosolic and mitochondrial [Zn(2+)]. The [Zn(2+)](i) increase was transient and was mimicked by the nonmetabolizable CH(3)NH(3)Cl, and it was dependent on NO formation, as evidenced by the sensitivity toward the nitric oxide synthase inhibitor N(G)-monomethyl-l-arginine. The NH(4)Cl-induced NO formation was sensitive to the Ca(2+) chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl) ester and increases in both NO and [Zn(2+)](i) were blocked by the N-methyl-d-aspartate receptor antagonist MK-801. The NH(4)Cl-triggered increase in [Zn(2+)](i) was followed by a Zn(2+)-dependent nuclear appearance of the metal response element-binding transcription factor and metallothionein messenger RNA (mRNA) induction. Metallothionein mRNA was also increased in vivo in rat cerebral cortex 6 h after an NH(4)Ac challenge. NH(4)Cl increased peripheral-type benzodiazepine receptor (PBR) protein expression, whereas PBR mRNA levels were decreased in a Zn(2+)-independent manner. The Zn(2+)-dependent upregulation of metallothionein following ammonia intoxication may reflect a cytoprotective response, whereas the increase in PBR expression may augment HE development.

Microglia activation in hepatic encephalopathy in rats and humans

Astrocytes play an important role in the pathogenesis of hepatic encephalopathy (HE) and ammonia toxicity, whereas little is known about microglia and neuroinflammation under these conditions. We therefore studied the effects of ammonia on rat microglia in vitro and in vivo and analyzed markers of neuroinflammation in post mortem brain tissue from patients with cirrhosis with and without HE and non-cirrhotic controls. In cultured rat microglia, ammonia stimulated cell migration and induced oxidative stress and an up-regulation of the microglial activation marker ionized calcium-binding adaptor molecule-1 (Iba-1). Up-regulation of Iba-1 was also found in the cerebral cortex from acutely ammonia-intoxicated rats and in the cerebral cortex from patients with cirrhosis who have HE, but not from patients with cirrhosis who do not have HE. However, ammonia had no effect on microglial glutamate release, prostaglandin synthesis, and messenger RNA (mRNA) levels of inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2), and the proinflammatory cytokines interleukin (IL)-1α/β, tumor necrosis factor α, or IL-6, whereas in cultured astrocytes ammonia induced the release of glutamate, prostaglandins, and increased IL-1β mRNA. mRNA and protein expression of iNOS and COX-2 or mRNA expression of proinflammatory cytokines and chemokine monocyte chemoattractive protein-1 in cerebral cortex from patients with liver cirrhosis and HE were not different from those found in patients with cirrhosis who did not have HE or control patients without cirrhosis.

CONCLUSION

These data suggest that microglia become activated in experimental hyperammonemia and HE in humans and may contribute to the generation of oxidative stress. However, HE in patients with liver cirrhosis is not associated with an up-regulation of inflammatory cytokines in cerebral cortex, despite microglia activation.

Plasma cytokine concentrations associated with HIV/hepatitis C coinfection are related to attention, executive and psychomotor functioning

Cytokine disturbances have been linked to brain dysfunction among HIV-infected people. Past studies have not simultaneously examined a large set of cytokine measures and their relationships to HIV-associated neurocognitive deficits. We hypothesized that performance on measures of attention and executive and psychomotor functions would be associated with plasma cytokine concentrations in HIV-infected individuals. Plasma samples drawn from 30 HIV-infected and 37 HIV seronegative individuals were analyzed via xMAP multiplexed bead array immunoassay to determine concentrations of 13 cytokines. Performance on Trail Making A/B, Stroop Test, Letter-Number Sequencing, Digit Symbol Coding, Symbol Search, and Grooved Pegboard tests was assessed. Statistical analyses were performed to examine group differences in cytokine concentrations, and associations between cytokine and HIV clinical variables and neurocognitive performance. Significant HIV effects were found on 7 of the 13 cytokines, primarily with respect to interleukins. HIV clinical factors (CD4 and HIV RNA levels, duration of illness, antiretroviral treatment) and hepatitis C status were associated with specific plasma cytokine concentrations. Neurocognitive measures were associated with cytokine concentrations, most consistently among the interleukins and IP-10. Generally, cytokine concentrations were among the strongest predictors of neurocognitive function relative to other clinical factors, which reinforces their potential importance in examining the neuropathological processes of HIV. The findings also point to the potential value of simultaneously examining a panel of biomarkers. The current results suggest that a complex relationship likely exists among cytokines [how?] and that these relationships are mediated not only by HIV infection but also by antiretroviral treatment and other comorbid conditions.

Locomotor impairment and cerebrocortical oxidative stress in portal vein ligated rats in vivo

BACKGROUND & AIMS

Oxidative/nitrosative stress plays an important role in the pathogenesis of hepatic encephalopathy and ammonia toxicity. The present study was undertaken in order to investigate the impact of portal vein ligation on cerebrocortical oxidative stress and its relation to locomotor activity.

METHODS

Cerebral protein tyrosine nitration, RNA oxidation, locomotor activity, and microglia activation were studied in rats that underwent portal vein ligation (PVL).

RESULTS

Two weeks after PVL, increased levels of protein tyrosine nitration and RNA oxidation were found in the brain. PVL rats exhibited hyperammonemia and reduced locomotor behaviour, but displayed no signs of microglia activation or upregulation of the mRNAs for interleukin-1ß and tumor necrosis factor-α. PVL also had no effect on astrocytic glutamate transporter or inducible nitric-oxide synthase expression. Only cerebral Il-6 mRNA levels were increased. Daily administration of indomethacin prevented PVL-induced protein tyrosine nitration, RNA oxidation, Il-6 mRNA increase, and the impairment of locomotor activity, but did not prevent PVL-induced hyperammonemia.

CONCLUSIONS

The data suggest that PVL triggers oxidative/nitrosative stress in the brain without activation of microglia and neuroinflammation. Prevention of protein tyrosine nitration and RNA oxidation by indomethacin also prevents the disturbances in locomotor activity pointing to a relevance of oxidative stress in the pathophysiology of HE.

Changing face of hepatic encephalopathy: Role of inflammation and oxidative stress

The face of hepatic encephalopathy (HE) is changing. This review explores how this neurocognitive disorder, which is associated with both acute and chronic liver injury, has grown to become a dynamic syndrome that spans a spectrum of neuropsychological impairment, from normal performance to coma. The central role of ammonia in the pathogenesis of HE remains incontrovertible. However, over the past 10 years, the HE community has begun to characterise the key roles of inflammation, infection, and oxidative/nitrosative stress in modulating the pathophysiological effects of ammonia on the astrocyte. This review explores the current thoughts and evidence base in this area and discusses the potential role of existing and novel therapies that might abrogate the oxidative and nitrosative stresses inflicted on the brain in patients with, or at risk of developing, HE.

Peroxynitrite-An ugly biofactor?

Cellular damage occurring under oxidative conditions has been ascribed mainly to the formation of peroxynitrite (ONOOH/ONOO(-)) that originates from the reaction of NO(*) with O(2) (*-). The detrimental effects of peroxynitrite are exacerbated by the reaction with CO(2) that leads to ONOOC(O)O(-), which further decays to the strong oxidant radicals NO(2) (*) and CO(3) (*-). The reaction with CO(2), however, may redirect peroxynitrite specificity. An excessive formation of peroxynitrite represents an important mechanism contributing to the DNA damage, the inactivation of metabolic enzymes, ionic pumps, and structural proteins, and the disruption of cell membranes. Because of its ability to oxidize biomolecules, peroxynitrite is implicated in an increasing list of diseases, including neurodegenerative and cardiovascular disorders, inflammation, pain, autoimmunity, cancer, and aging. However, peroxynitrite displays also protective activities: (i) at high concentrations, it shows anti-viral, anti-microbial, and anti-parasitic actions; and (ii) at low concentrations, it stimulates protective mechanisms in the cardiovascular, nervous, and respiratory systems. The detrimental effects of peroxynitrite and related reactive species are impaired by (pseudo-) enzymatic systems, mainly represented by heme-proteins (e.g., hemoglobin and myoglobin). Here, we report biochemical aspects of peroxynitrite actions being at the root of its biomedical effects.

Interaction of oxidative stress, astrocyte swelling and cerebral ammonia toxicity

PURPOSE OF REVIEW

Description of the role of oxidative stress in the pathogenesis of cerebral ammonia toxicity and hepatic encephalopathy.

RECENT FINDINGS

Ammonia plays a key role in the pathogenesis of hepatic encephalopathy, which manifests as a neuropsychiatric syndrome accompanying acute and chronic liver failure. One consequence of ammonia action on the brain is astrocyte swelling, which triggers the generation of oxidative/nitrosative stress at the level of NADPH oxidase, nitric oxide synthases and the mitochondria. A self-amplifying signaling loop between oxidative stress and astrocyte swelling has been proposed. Consequences of the ammonia-induced oxidative/nitrosative stress response are protein modifications through nitration of tyrosine residues and oxidation of astrocytic and neuronal RNA. Nitrosative stress also mobilizes zinc from intracellular stores with impact on gene expression. These alterations may at least in part mediate cerebral ammonia toxicity through disturbances of intracellular and intercellular signaling and of synaptic plasticity.

SUMMARY

Oxidative/nitrosative stress and a low-grade cerebral edema as key events in the pathogenesis of ammonia toxicity and hepatic encephalopathy may offer potential new strategies for treatment. Ammonia-induced oxidation of RNA and proteins may impair postsynaptic protein synthesis, which is critically involved in learning and memory consolidation. RNA oxidation offers a novel explanation for multiple disturbances of neurotransmitter systems and gene expression and the cognitive deficits observed in hepatic encephalopathy.

The freshwater Amazonian stingray, Potamotrygon motoro, up-regulates glutamine synthetase activity and protein abundance, and accumulates glutamine when exposed to brackish (15‰) water

This study aimed to examine whether the stenohaline freshwater stingray, Potamotrygon motoro, which lacks a functional ornithine—urea cycle, would up-regulate glutamine synthetase (GS) activity and protein abundance, and accumulate glutamine during a progressive transfer from freshwater to brackish (15‰) water with daily feeding. Our results revealed that, similar to other freshwater teleosts, P. motoro performed hyperosmotic regulation, with very low urea concentrations in plasma and tissues, in freshwater. In 15‰ water, it was non-ureotelic and non-ureoosmotic, acting mainly as an osmoconformer with its plasma osmolality, [Na+] and [Cl−] comparable to those of the external medium. There were significant increases in the content of several free amino acids (FAAs), including glutamate, glutamine and glycine, in muscle and liver, but not in plasma, indicating that FAAs could contribute in part to cell volume regulation. Furthermore, exposure of P. motoro to 15‰ water led to up-regulation of GS activity and protein abundance in both liver and muscle. Thus, our results indicate for the first time that, despite the inability to synthesize urea and the lack of functional carbamoyl phosphate synthetase III (CPS III) which uses glutamine as a substrate, P. motoro retained the capacity to up-regulate the activity and protein expression of GS in response to salinity stress. Potamotrygon motoro was not nitrogen (N) limited when exposed to 15‰ water with feeding, and there were no significant changes in the amination and deamination activities of hepatic glutamate dehydrogenase. In contrast, P. motoro became N limited when exposed to 10‰ water with fasting and could not survive well in 15‰ water without food.

alpha-Luminol prevents decreases in glutamate, glutathione, and glutamine synthetase in the retinas of glaucomatous DBA/2J mice

OBJECTIVE

To test the hypothesis that in DBA/2J mice, oxidative stress decreases glutamine synthetase (GS) levels resulting in a loss of neuronal glutamate and that the antioxidant alpha-luminol (GVT) decreases this stress and glutamate loss in some types of glaucoma.

ANIMALS

DBA/2J mice were separated into two groups, of which one was not treated, and the other treated with GVT in the drinking water. At 7 months of age, retinas were examined from five untreated DBA/2J mice, seven GVT-treated mice, and five C57BL/6 mice (negative controls).

METHODS

Serial 0.5 microm plastic sections were immunogold stained for glutamate, GS, and total glutathione, followed by image analysis for staining patterns and density.

RESULTS

Focal decreases in glutamate immunostaining were common in the inner nuclear layer (INL) of DBA/2J retinas, but not in C57BL/6 or GVT-treated DBA/2J retinas. Decreases in glutathione and GS immunostaining were found in DBA/2J retinal regions where neuronal glutamate immunostaining was reduced. Retinas from GVT-treated DBA/2J had no significant decreases in INL levels of glutamate, glutathione, or GS.

CONCLUSIONS

Retinas of dogs with primary glaucoma are reported to have focal depletion of neuronal glutamate. In DBA/2J mice, similar changes occur prior to the development of clinical disease. In these focal glutamate-depleted regions, levels of glutathione and GS are also reduced, consistent with the hypothesis that oxidative stress contributes to retinal changes in glaucoma. The ability of GVT, an antioxidant, to inhibit retinal abnormalities in DBA/2J mice provides further support for this hypothesis.

Glutamine accumulation and up-regulation of glutamine synthetase activity in the swamp eel, Monopterus albus (Zuiew), exposed to brackish water

The swamp eel, Monopterus albus, is an air-breathing teleost which typically lives in freshwater but can also be found in estuaries, where it has to deal with ambient salinity fluctuations. Unlike other teleosts, its gills are highly degenerate. Hence, it may have uncommon osmoregulatory adaptations, but no information is available on its osmoregulatory capacity and mechanisms at present. In this study M. albus was exposed to a 5 day progressive increase in salinity from freshwater (1 per thousand) to brackish water (25 per thousand) and subsequently kept in 25 per thousand water for a total of 4 days. The results indicate that M. albus switched from hyperosmotic hyperionic regulation in freshwater to a combination of osmoconforming and hypoosmotic hypoionic regulation in 25 per thousand water. Exposure to 25 per thousand water resulted in relatively large increases in plasma osmolality, [Na(+)] and [Cl(-)]. Consequently, fish exposed to 25 per thousand water had to undergo cell volume regulation through accumulation of organic osmolytes and inorganic ions. Increases in tissue free amino acid content were apparently the result of increased protein degradation, decreased amino acid catabolism, and increased synthesis of certain non-essential amino acids. Here we report for the first time that glutamine is the major organic osmolyte in M. albus. Glutamine content increased to a phenomenal level of > 12 micromol g(-1) and > 30 micromol g(-1) in the muscle and liver, respectively, of fish exposed to 25 per thousand water. There were significant increases in glutamine synthetase (GS) activity in muscle and liver of these fish. In addition, exposure to 25 per thousand water for 4 days led to significant increases in GS protein abundance in both muscle and liver, indicating that increases in the expression of GS mRNA could have occurred.

The brain in acute liver failure. A tortuous path from hyperammonemia to cerebral edema

Acute liver failure (ALF) is a condition with an unfavourable prognosis. Multiorgan failure and circulatory collapse are frequent causes of death, but cerebral edema and intracranial hypertension (ICH) are also common complications with a high risk of fatal outcome. The underlying pathogenesis has been extensively studied and although the development of cerebral edema and ICH is of a complex and multifactorial nature, it is well established that ammonia plays a pivotal role. This review will focus on the effects of hyperammonemia on neurotransmission, mitochondrial function, oxidative stress, inflammation and regulation of cerebral blood flow. Finally, potential therapeutic targets and future perspectives are briefly discussed.

RNA oxidation and zinc in hepatic encephalopathy and hyperammonemia

Hepatic encephalopathy is a neuropsychiatric manifestation of acute and chronic liver failure. Ammonia plays a key role in the pathogenesis of hepatic encephalopathy by inducing astrocyte swelling and/or sensitizing astrocytes to swelling by a heterogeneous panel of precipitating factors and conditions. Whereas astrocyte swelling in acute liver failure contributes to a clinically overt brain edema, a low grade glial edema without clinically overt brain edema is observed in hepatic encephalopathy in liver cirrhosis. Astrocyte swelling produces reactive oxygen and nitrogen oxide species (ROS/RNOS), which again increase astrocyte swelling, thereby creating a self-amplifying signaling loop. Astroglial swelling and ROS/RNOS increase protein tyrosine nitration and may account for neurotoxic effects of ammonia and other precipitants of hepatic encephalopathy. Recently, RNA oxidation and an increase of free intracellular zinc ([Zn(2+)](i)) were identified as further consequences of astrocyte swelling and ROS/RNOS production. An elevation of [Zn(2+)](i) mediates mRNA expression of metallothionein and the peripheral benzodiazepine receptor (PBR) induced by hypoosmotic astrocyte swelling. Further, Zn(2+) mediates RNA oxidation in ammonia-treated astrocytes. In the brain of hyperammonemic rats oxidized RNA localizes in part to perivascular astrocyte processes and to postsynaptic dendritic spines. RNA oxidation may impair postsynaptic protein synthesis, which is critically involved in learning and memory consolidation. RNA oxidation offers a novel explanation for multiple disturbances of neurotransmitter systems and gene expression and the cognitive deficits observed in hepatic encephalopathy.

Reversal of axonal loss and disability in a mouse model of progressive multiple sclerosis

Axonal degeneration is an important determinant of progressive neurological disability in multiple sclerosis (MS). Thus, therapeutic approaches promoting neuroprotection could aid the treatment of progressive MS. Here, we used what we believe is a novel water-soluble fullerene derivative (ABS-75) attached to an NMDA receptor antagonist, which combines antioxidant and anti-excitotoxic properties, to block axonal damage and reduce disease progression in a chronic progressive EAE model. Fullerene ABS-75 treatment initiated after disease onset reduced the clinical progression of chronic EAE in NOD mice immunized with myelin-oligodendrocyte glycoprotein (MOG). Reduced disease progression in ABS-75-treated mice was associated with reduced axonal loss and demyelination in the spinal cord. Fullerene ABS-75 halted oxidative injury, CD11b+ infiltration, and CCL2 expression in the spinal cord of mice without interfering with antigen-specific T cell responses. In vitro, fullerene ABS-75 protected neurons from oxidative and glutamate-induced injury and restored glutamine synthetase and glutamate transporter expression in astrocytes under inflammatory insult. Glutamine synthetase expression was also increased in the white matter of fullerene ABS-75-treated animals. Our data demonstrate the neuroprotective effect of treatment with a fullerene compound combined with a NMDA receptor antagonist, which may be useful in the treatment of progressive MS and other neurodegenerative diseases.

Aquaporin-4 in hepatic encephalopathy

Brain edema is a critical component of hepatic encephalopathy (HE) associated with acute liver failure and such edema appears to be principally due to astrocyte swelling (cytotoxic edema). Ammonia is believed to represent a major factor responsible for astrocyte swelling, although the mechanisms by which ammonia causes such swelling are not completely understood. Recent studies have implicated potential role of oxidative stress, and the mitochondrial permeability transition (mPT). While it is not known how oxidative stress and the mPT cause astrocyte swelling, it is reasonable to suggest that these events may affect one or more plasma membrane proteins involved in water and ion homeostasis in astrocytes. One such protein strongly implicated in brain edema in other neurological conditions is the water channel protein aquaporin-4 (AQP-4), which is abundantly expressed in astrocytes. This article summarizes the potential role of AQP-4 in brain edema in in vivo models of HE, as well as in ammonia-induced cell swelling in cultured astrocytes. The involvement of AQP-4 in the effects of manganese, another toxin implicated in HE, will also be discussed.

17β-Estradiol reduces nitrotyrosine immunoreactivity and increases SOD1 and SOD2 immunoreactivity in nigral neurons in male mice following MPTP insult

Emerging evidence suggests the beneficial effects of estrogen on Parkinson's disease (PD), yet the mechanisms of action implicated remain elusive. While experimental evidence suggests that estrogen possesses potent antioxidative properties, it is still unknown whether the hormone exhibits a neuroprotection in a PD animal model through its antioxidant activities. This study therefore investigated the effects of 17beta-estradiol (E2) on the immunoreactivity of nigral neurons and glia for nitrotyrosine (NT, a stable marker for oxidative stress), Cu/Zn superoxide dismutase (SOD1) and Mn superoxide dismutase (SOD2) in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model. Adult male mice were treated with E2 or vehicle for 11 days during which they were injected with MPTP or saline on the sixth day. The brains were collected on day 11 and quantitative immunohistochemistry was used to assess the number of NT-, SOD1- and SOD2-immunoreactive (IR) cells in the substantia nigra pars compacta (SNpc). In saline-treated group, E2 decreased NT-IR neuronal number and raised SOD1 and SOD2 expression in neurons and glia in the SNpc. MPTP induced a significant increase in the number of NT- and SOD2-IR neurons, but decreased the number of SOD1-IR neurons. MPTP also triggered a significant increase of SOD2- and SOD1-IR glial number. E2 pretreatment in MPTP mice reduced the number of NT-IR neurons, increased the number of SOD1- and SOD2-IR neurons, but did not alter the MPTP effect on glia immunoreactive to either SOD. Stimulation of SOD1 and SOD2 expression in nigral neurons suggests that E2 provides neuroprotection against MPTP-induced oxidative stress, partly through its ability to act as an antioxidant.