Increased neuronal nitric oxide synthase expression in brain following portacaval anastomosis

Dysfunction of cerebellar microglia in Ataxia-telangiectasia

Ataxia-telangiectasia (A-T) is a multisystem autosomal recessive disease caused by mutations in the ATM gene and characterized by cerebellar atrophy, progressive ataxia, immunodeficiency, male and female sterility, radiosensitivity, cancer predisposition, growth retardation, insulin-resistant diabetes, and premature aging. ATM phosphorylates more than 1500 target proteins, which are involved in cell cycle control, DNA repair, apoptosis, modulation of chromatin structure, and other cytoplasmic as well as mitochondrial processes. In our quest to better understand the mechanisms by which ATM deficiency causes cerebellar degeneration, we hypothesized that specific vulnerabilities of cerebellar microglia underlie the etiology of A-T. Our hypothesis is based on the recent finding that dysfunction of glial cells affect a variety of process leading to impaired neuronal functionality (Song et al., 2019). Whereas astrocytes and neurons descend from the neural tube, microglia originate from the hematopoietic system, invade the brain at early embryonic stage, and become the innate immune cells of the central nervous system and important participants in development of synaptic plasticity. Here we demonstrate that microglia derived from Atm^-/- mouse cerebellum display accelerated cell migration and are severely impaired in phagocytosis, secretion of neurotrophic factors, and mitochondrial activity, suggestive of apoptotic processes. Interestingly, no microglial impairment was detected in Atm-deficient cerebral cortex, and Atm deficiency had less impact on astroglia than microglia. Collectively, our findings validate the roles of glial cells in cerebellar attrition in A-T.

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.

Cerebral edema and liver disease: Classic perspectives and contemporary hypotheses on mechanism

Liver disease is a growing public health concern. Hepatic encephalopathy, the syndrome of brain dysfunction secondary to liver disease, is a frequent complication of both acute and chronic liver disease and cerebral edema (CE) is a key feature. While altered ammonia metabolism is a key contributor to hepatic encephalopathy and CE in liver disease, there is a growing appreciation that additional mechanisms contribute to CE. In this review we will begin by presenting three classic perspectives that form a foundation for a discussion of CE in liver disease: 1) CE is unique to acute liver failure, 2) CE in liver disease is only cytotoxic, and 3) CE in liver disease is primarily an osmotically mediated consequence of ammonia and glutamine metabolism. We will present each classic perspective along with more recent observations that call in to question that classic perspective. After highlighting these areas of debate, we will explore the leading contemporary mechanisms hypothesized to contribute to CE during liver disease.

Portacaval shunting causes differential mitochondrial superoxide production in brain regions

The portacaval shunting (PCS) prevents portal hypertension and recurrent bleeding of esophageal varices. On the other hand, it can induce chronic hyperammonemia and is considered to be the best model of mild hepatic encephalopathy (HE). Pathogenic mechanisms of HE and dysfunction of the brain in hyperammonemia are not fully elucidated, but it was originally suggested that the pathogenetic defect causes destruction of antioxidant defense which leads to an increase in the production of reactive oxygen species (ROS) and the occurrence of oxidative stress. In order to gain insight into the pathogenic mechanisms of HE in the brain tissue, we investigated the effects of PCS in rats on free radicals production and activity levels of antioxidant and prooxidant enzymes in mitochondria isolated from different brain areas. We found that O₂^·- production, activities of Mn-superoxide dismutase (Mn-SOD), glutathione peroxidase (GPx), glutathione reductase (GR), glutathione transferase (GT), nitric oxide synthase (NOS), and levels of carbonylated proteins differed between the four brain regions both in the amount and response to PCS. In PCS rats, Mn-SOD activity in the cerebellum was significantly decreased, and remained unchanged in the neocortex, hippocampus and striatum compared with that in sham-operated animals. Among the four brain regions in control rats, the levels of the carbonyl groups in mitochondrial proteins were maximal in the cerebellum. 4 weeks after PCS, the content of carbonylated proteins were higher only in mitochondria of this brain region. Under control conditions, O₂^·- production by submitochondrial particles in the cerebellum was significantly higher than in other brain regions, but was significantly increased in each brain region from PCS animals. Indeed, the production of O₂^·- by submitochondrial particles correlated with mitochondrial ammonia levels in the four brain regions of control and PCS-animals. These findings are the first to suggest that in vivo levels of ammonia in the brain directly affect the rate of mitochondrial O₂^·- production.

Naringin regulates glutamate-nitric oxide cGMP pathway in ammonium chloride induced neurotoxicity

Naringin, plant bioflavonoid extracted mainly from grapefruit and other related citrus species. This study was designed to assess the neuroprotective effect of naringin on ammonium chloride (NH₄Cl) induced hyperammonemic rats. Experimental hyperammonemia was induced by intraperitonial injection (i.p) of NH₄Cl (100mg/kg body weight (b.w.)) thrice a week for 8 consecutive weeks. Hyperammonemic rats were treated with naringin (80mg/kg b.w.) via oral gavage. Naringin administration drastically restored the levels of blood ammonia, plasma urea, nitric oxide (NO), glutamate, glutamine, lipid peroxidation, lipid profile, activities of liver marker enzymes, antioxidant status and sodium/potassium-ATPase (Na⁺/K⁺-ATPase). In addition, naringin supplementation reverted back the pathological changes of liver, brain and kidney tissues, the expressions of Glutamine synthetase (GS), Na⁺/K⁺-ATPase, neuronal nitric oxide (nNOS) and soluble guanylate cyclase (sGC) in hyperammonemic rats. Hence, this study suggested that nargingin exhibited their protective effect against NH₄Cl induced toxicity via enhancing the activities of antioxidant enzymes and inhibiting the lipid peroxidation process. Take together, this study provides data that naingin effectively reduced neurotoxicity by attenuating hyperammonemia, suggesting that naringin act as a potential therapeutic agent to treat hyperammonemic rats.

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.

Transjugular intrahepatic portosystemic shunt induced short- and long-term cerebral blood flow variations in cirrhotic patients: an arterial spin labeling MRI study

Short- and long-term effects of transjugular intrahepatic portosystemic shunt (TIPS) on cerebral blood flow (CBF) in patients with cirrhosis are still unclear. The purpose of this longitudinal study was to explore CBF alteration patterns in cirrhotic patients after TIPS. Thirteen cirrhotic patients (7 male, 6 female, mean age 50.0 ± 9.3 years) underwent arterial-spin labeling (ASL) MRI 1-9 days (median 1 days) before TIPS. Follow-up MR examinations were performed about 1 week (median 6 days), 3 months (n = 6), 6-9 months (n = 5) and 12-18 months (n = 5) after TIPS. CBF, ammonia level, Child-Pugh score, number connection test type A (NCT-A) and digit symbol test (DST) scores were converted into relative values by dividing by his/her pre-TIPS values, and then, compared via one-way analysis of variance (ANOVA). Correlations between the pre- and post-TIPS changes of relative CBF (rCBF) and the changes of relative ammonia (rAmmonia), Child-Pugh (rChild-Pugh), and NCT-A/DST (rNCT-A/rDST) scores were calculated by crossing subjects. Compared with the pre-TIPS level, the global rCBF slightly increased by 10.9 % about 1 week later, then rapidly decreased by 14.2 % 3 months later, and flatly decreased by 17.2 % in 6-9 months and 18.0 % in 12-18 months following TIPS. The changes of 3-month rDST score were slightly correlated with 3-month rCBF rather than 1-week rCBF, (P < 0.1, FDR-corrected) No difference was found between the pre- and post-TIPS rAmmonia levels, rChild-Pugh and rNCT-A/rDST scores (Post-hoc P > 0.05). CBF measured at different time points after TIPS insertion showed different patterns, indicating varying longitudinal effects of TIPS on CBF. A sharp decline of rCBF was found in the 1 week to 3 months period after insertion, indicating that high event rate of hepatic encephalopathy might relate with the unadaptable CBF in patients undergoing TIPS insertion.

The C57BL/6J mouse exhibits sporadic congenital portosystemic shunts

C57BL/6 mice are the most widely used strain of laboratory mice. Using in vivo proton Magnetic Resonance Spectroscopy (¹H MRS), we have repeatedly observed an abnormal neurochemical profile in the brains of both wild-type and genetically modified mice derived from the C57BL/6J strain, consisting of a several fold increase in cerebral glutamine and two fold decrease in myo-inositol. This strikingly abnormal neurochemical “phenotype” resembles that observed in chronic liver disease or portosystemic shunting and appeared to be independent of transgene, origin or chow and was not associated with liver failure. As many as 25% of animals displayed the abnormal neurochemical profile, questioning the reliability of this model for neurobiology. We conducted an independent study to determine if this neurochemical profile was associated with portosystemic shunting. Our results showed that 100% of the mice with high brain glutamine displayed portosystemic shunting by concomitant portal angiography while all mice with normal brain glutamine did not. Since portosystemic shunting is known to cause alterations in gene expression in many organs including the brain, we conclude that portosystemic shunting may be the most significant problem associated with C57BL/6J inbreeding both for its effect on the central nervous system and for its systemic repercussions.

Cerebral blood flow measured by arterial-spin labeling MRI: a useful biomarker for characterization of minimal hepatic encephalopathy in patients with cirrhosis

PURPOSE

To investigate the role of arterial-spin labeling (ASL) MRI to non-invasively characterize the patterns of cerebral blood flow (CBF) changes in cirrhotic patients and to assess the potential of ASL MRI to characterize minimal hepatic encephalopathy (MHE).

MATERIALS AND METHODS

This study was approved by the local ethics committee, and written informed consent was obtained from all participants. Thirty six cirrhosis patients without overt hepatic encephalopathy (16 MHE patients and 20 non hepatic encephalopathy (non-HE) patients) and 25 controls underwent ASL MRI, and CBF was measured for each subject. One-way ANOCOVA test with age and gender as covariences was used to compare CBF difference among three groups, and post hoc analysis was performed between each two groups. Region-based correlation analysis was applied between Child-Pugh score, venous blood ammonia level, neuropsychological tests and CBF values in cirrhosis patients. Receiver operator characteristic (ROC) analysis was used for assessing CBF measurements in ASL MRI to differentiate MHE from non-HE patients.

RESULTS

The gray matter CBF of MHE patients (71.09 ± 11.88 mL min(-1)100g(-1)) was significantly higher than that of non-HE patients (55.28 ± 12.30 mL min(-1)100g(-1), P<0.01) and controls (52.09 ± 9.27 mL min(-1)100g(-1), P<0.001). Voxel-wise ANOCOVA results showed that CBFs were significantly different among three groups in multiple gray matter areas (P<0.05, Bonferroni corrected). Post hoc comparisons showed that CBF of these brain regions was increased in MHE patients compared with controls and non-HE patients (P<0.05, Bonferroni corrected). CBF of the right putamen was of the highest sensitivity (93.8%) and moderate specificity (75.0%) for characterization of MHE when using the cutoff value of 50.57 mL min(-1)100g(-1). CBFs in the bilateral median cingulate gyri, left supramarginal gyrus, right angular gyrus, right heschl gyrus and right superior temporal gyrus have both sensitivity and specificity of approximately 80% for the diagnosis of MHE.

CONCLUSION

Higher CBF was found in many brain regions in cirrhotic patients than controls and gradually increased with the progress of disease. CBF measured with ASL MRI can be a useful marker for differentiating MHE from non-HE patients.

Changes in cerebral blood flow after transjugular intrahepatic portosystemic shunt can help predict the development of hepatic encephalopathy: an arterial spin labeling MR study

BACKGROUND AND PURPOSE

Cerebral blood flow (CBF) changes after transjugular intrahepatic portosystemic shunt (TIPS) are still unclear. Our aim is to assess the TIPS-induced CBF changes and their potential clinical significance using the arterial spin labeling (ASL) perfusion magnetic resonance imaging.

MATERIALS AND METHODS

Nine cirrhotic patients underwent ASL 1-8 days before and 4-7 days after TIPS. CBF was calculated at each voxel and mean CBF values were computed in the whole brain, gray matter and white matter. Changes of CBFs before and after TIPS were compared by paired t-test.

RESULTS

Voxel-wise results showed CBF diffusely increased in patients after TIPS, but no region with significant decrease in CBF was found, nor was any significant mean CBF difference detected in the whole brain, gray matter and white matter. Six patients out of nine showed a global CBF increase of 9-39%; one patient presented a global CBF decrease of 6%; another two showed a global CBF decrease of 16% and 31% respectively. Follow-up studies showed that the two patients with greatly decreased global CBF suffered from multiple episodes of overt hepatic encephalopathy (OHE) after TIPS and one died of OHE.

CONCLUSIONS

CBF derived from noninvasive ASL MRI could be used as a useful biomarker to predict the development of OHE through consecutively tracking CBF changes in patients with inserted TIPS. Increased CBFs in many cortical regions could be common effects of the TIPS procedure, while decreased global CBF following TIPS might indicate the development of OHE.

Oxidative stress markers in the brain of patients with cirrhosis and hepatic encephalopathy

Cell culture studies and animal models point to an important role of oxidative/nitrosative stress in the pathogenesis of cerebral ammonia toxicity. However, it is unknown whether oxidative/nitrosative stress in the brain is also characteristic of hepatic encephalopathy (HE) in humans. We therefore analyzed post mortem cortical brain tissue samples from patients with cirrhosis dying with or without HE in comparison with brains from patients without liver disease. Significantly elevated levels of protein tyrosine-nitrated proteins, heat shock protein-27, and 8-hydroxyguanosine as a marker for RNA oxidation were found in the cerebral cortex of HE patients, but not of patients with cirrhosis but without HE. Glutamine synthetase (GS) activity was significantly decreased, whereas GS protein expression was not significantly affected. Protein expression of the glutamate/aspartate cotransporter was up-regulated in HE, whereas protein expression of neuronal and inducible nitric oxide synthases, manganese-dependent and copper/zinc-dependent superoxide dismutase, and glial glutamate transporter-1 were not significantly increased.

CONCLUSION

These data indicate that HE in patients with cirrhosis is associated with oxidative/nitrosative stress, protein tyrosine nitration, and RNA oxidation, suggesting a role of oxidative stress in the pathogenesis of HE in patients with cirrhosis.

Induction of NOS and nitrotyrosine expression in the rat striatum following experimental hepatic encephalopathy

Hepatic encephalopathy (HE) is a neurologic disease associated with hepatic dysfunction. Astroglial and neuronal alterations have been described in the basal ganglia in HE. Our study was performed to determine whether such alterations are mediated by nitric oxide (NO), by using an experimental model of HE (portacaval anastomosis [PCA]). The expression of the NO synthases (nNOS and iNOS) and the production of nitrotyrosine (NT) were evaluated in the striatum of rats exposed to PCA for 1 and 6 months. The expression of nNOS in the striatal neurons of PCA rats was increased compared to controls. nNOS expression was also detectable in astrocytes after 6 months of exposure to PCA. Whereas astroglial cells in the normal striatum showed no iNOS expression, iNOS was expressed in the astrocytes of PCA brains, mainly in perivascular processes at 6 months PCA exposure (demonstrated by colocalization with GFAP). The increased expression of both the nNOS and iNOS isoforms in PCA rats might indicate a critical role for NO in the pathomechanism of HE. To study the potential cell damage caused by NO, the deposition of NT in PCA-rats was analysed. Nitrotyrosine was detected in neurons although it was mainly seen in the astrocytes of PCA brains, in which double immunolabelling showed NT to be colocalized with GFAP. Thus, the present study shows the induction of iNOS and NT in astrocytes, which increases with the duration of PCA exposure. This suggests that the induced astroglial production of NO during PCA might be one of the main factors contributing to HE.

Glutamine inhibits ammonia-induced accumulation of cGMP in rat striatum limiting arginine supply for NO synthesis

Brain L-glutamine (Gln) accumulation and increased activity of the NO/cGMP pathway are immediate consequences of acute exposure to ammonia. This study tested whether excess Gln may influence NO and/or cGMP synthesis. Intrastriatal administration of the glutaminase inhibitor 6-diazo-5-oxo-L-norleucine or the system A-specific Gln uptake inhibitor methylaminoisobutyrate increased microdialysate Gln concentration and reduced basal and ammonia-induced NO and cGMP accumulation. Gln applied in vivo (via microdialysis) or in vitro (to rat brain cortical slices) reduced NO and cGMP accumulation in the presence and/or absence of ammonia, but not cGMP synthesis induced by the NO donor sodium nitroprusside. Attenuation of cGMP synthesis by Gln was prevented by administration of L-arginine (Arg). The L-arginine co-substrates of y(+)LAT2 transport system, L-leucine and cyclo-leucine, mimicked the effect of exogenous Gln, suggesting that Gln limits Arg supply for NO synthesis by interfering with y+LAT2-mediated Arg uptake across the cell membrane.

Basal and learning task-related brain oxidative metabolism in cirrhotic rats

Hepatic encephalopathy is a neurological complication observed in patients with liver disease. Subjects with hepatic encephalopathy can develop memory alterations. In order to investigate brain oxidative metabolism in an animal model of chronic cirrhosis and its modification after spatial working memory task, we determined the neural metabolic activity of several brain limbic system regions by cytochrome oxidase (COx) histochemistry and assessed the spatial working memory in the Morris water maze of rats with cirrhosis by administration of thioacetamide. This COx histochemistry was done in cirrhotic and control rats under basal conditions and after the spatial working memory task. The histochemical results showed differences in basal COx activity between control and cirrhotic rats in hippocampal and thalamic regions. In cirrhotic rats basal COx activity was increased in the CA1 and CA3 areas of the hippocampus and reduced in the anterodorsal and anteroventral thalamic nuclei. We found impaired spatial working memory in animals with cirrhosis. These animals showed absence of metabolic activation of the CA3 hippocampal subfield and the lateral mammillary nucleus and disturbance of COx activity in the medial mammillary nucleus and the anteroventral thalamus. These findings suggest that cirrhotic rats show spatial working memory deficits that could be related to the alteration of metabolic activity of neural regions thought to be involved in the processing of spatial memories.

Profiling of astrocyte properties in the hyperammonaemic brain: shedding new light on the pathophysiology of the brain damage in hyperammonaemia

Acute hyperammonaemia (HA) causes cerebral oedema and severe brain damage in patients with urea cycle disorders (UCDs) or acute liver failure (ALF). Chronic HA is associated with developmental delay and intellectual disability in patients with UCDs and with neuropsychiatric symptoms in patients with chronic liver failure. Treatment often cannot prevent severe brain injury and neurological sequelae. The causes of the brain oedema in hyperammonaemic encephalopathy (HAE) have been subject of intense controversy among physicians and scientists working in this field. Currently favoured hypotheses are astrocyte swelling due to increased intracellular glutamine content and neuronal cell death due to excitotoxicity caused by elevated extracellular glutamate levels. While many researchers focus on these mechanisms of cytotoxicity, others emphasize vascular causes of brain oedema. New data gleaned from expression profiling of astrocytes acutely isolated from hyperammonaemic mouse brains point to disturbed water and potassium homeostasis as regulated by astrocytes at the brain microvasculature and in the perisynaptic space as a potential mechanism of brain oedema development in hyperammonaemia.

NFkappaB in the mechanism of ammonia-induced astrocyte swelling in culture

Astrocyte swelling and brain edema are major neuropathological findings in the acute form of hepatic encephalopathy (fulminant hepatic failure), and substantial evidence supports the view that elevated brain ammonia level is an important etiological factor in this condition. Although the mechanism by which ammonia brings about astrocyte swelling remains to be determined, oxidative/nitrosative stress and mitogen-activated protein kinases (MAPKs) have been considered as important elements in this process. One factor known to be activated by both oxidative stress and MAPKs is nuclear factor kappaB (NFkappaB), a transcription factor that activates many genes, including inducible nitric oxide synthase (iNOS). As the product of iNOS, nitric oxide (NO), is known to cause astrocyte swelling, we examined the potential involvement of NFkappaB in ammonia-induced astrocyte swelling. Western blot analysis of cultured astrocytes showed a significant increase in NFkappaB nuclear translocation (a measure of NFkappaB activation) from 12 h to 2 days after treatment with NH(4)Cl (5 mM). Cultures treated with anti-oxidants, including superoxide dismutase, catalase, and vitamin E as well as the MAPKs inhibitors, SB239063 (an inhibitor of p38-MAPK) and SP600125 (an inhibitor of c-Jun N-terminal kinase), significantly diminished NFkappaB activation by ammonia, supporting a role of oxidative stress and MAPKs in NFkappaB activation. The activation of NFkappaB was associated with increased iNOS protein expression and NO generation, and these changes were blocked by BAY 11-7082, an inhibitor of NFkappaB. Additionally, ammonia-induced astrocyte swelling was inhibited by the NFkappaB inhibitors, BAY 11-7082 and SN-50, thereby implicating NFkappaB in the mechanism of astrocyte swelling. Our studies indicate that cultured astrocytes exposed to ammonia display NFkappaB activation, which is likely to be a consequence of oxidative stress and activation of MAPKs. NFkappaB activation appears to contribute to the mechanism of ammonia-induced astrocyte swelling, apparently through its up-regulation of iNOS protein expression and the subsequent generation of NO.

Pros and cons of L-arginine supplementation in disease

The amino acid arginine and one of its metabolites NO have gathered broad attention in the last decade. Although arginine is regarded as a conditionally essential amino acid in disease, L-arginine supplementation in severe illness has not found its way into clinical practice. This might be due to the invalid interpretation of results from studies with immune-enhancing diets containing L-arginine amongst other pharmaconutrients. However, not much attention is given to research using L-arginine as a monotherapy and the possibility of the alternative hypothesis: that L-arginine supplementation is beneficial in disease. The present review will discuss data from studies in healthy and diseased animals and patients with monotherapy of L-arginine to come to an objective overview of positive and negative aspects of L-arginine supplementation in disease with special emphasis on sepsis, cancer, liver failure and wound healing.

New concepts in the mechanism of ammonia-induced astrocyte swelling

It is generally accepted that astrocyte swelling forms the major anatomic substrate of the edema associated with acute liver failure (ALF) and that ammonia represents a major etiological factor in its causation. The mechanisms leading to such swelling, however, remain elusive. Recent studies have invoked the role of oxidative stress in the mechanism of hepatic encephalopathy (HE), as well as in the brain edema related to ALF. This article summarizes the evidence for oxidative stress as a major pathogenetic factor in HE/ALF and discusses mechanisms that are triggered by oxidative stress, including the induction of the mitochondrial permeability transition (MPT) and activation of signaling kinases. We propose that a cascade of events initiated by ammonia-induced oxidative stress results in cell volume dysregulation leading to cell swelling/brain edema. Blockade of this cascade may provide novel therapies for the brain edema associated with ALF.

Glutamate-induced activation of nitric oxide synthase is impaired in cerebral cortex in vivo in rats with chronic liver failure

It has been proposed that impairment of the glutamate-nitric oxide-cyclic guanosine monophosphate (cGMP) pathway in brain contributes to cognitive impairment in hepatic encephalopathy. The aims of this work were to assess whether the function of this pathway and of nitric oxide synthase (NOS) are altered in cerebral cortex in vivo in rats with chronic liver failure due to portacaval shunt (PCS) and whether these alterations are due to hyperammonemia. The glutamate-nitric oxide-cGMP pathway function and NOS activation by NMDA was analysed by in vivo microdialysis in cerebral cortex of PCS and control rats and in rats with hyperammonemia without liver failure. Similar studies were done in cortical slices from these rats and in cultured cortical neurons exposed to ammonia. Basal NOS activity, nitrites and cGMP are increased in cortex of rats with hyperammonemia or liver failure. These increases seem due to increased inducible nitric oxide synthase expression. NOS activation by NMDA is impaired in cerebral cortex in both animal models and in neurons exposed to ammonia. Chronic liver failure increases basal NOS activity, nitric oxide and cGMP but reduces activation of NOS induced by NMDA receptors activation. Hyperammonemia is responsible for both effects which will lead, independently, to alterations contributing to neurological alterations in hepatic encephalopathy.

The expression of nNOS, iNOS and nitrotyrosine is increased in the rat cerebral cortex in experimental hepatic encephalopathy

The changes in the distribution and amount of nitric oxide (NO) synthases (nNOS and iNOS) and the appearance of nitrotyrosine (NT) in the rat cerebral cortex were investigated following portacaval anastomosis (PCA), an experimental hepatic encephalopathy (HE) model. One month after PCA, rats showed more neurones immunoreactive to nNOS than did control animals. At 6 months post PCA, the number of neurones expressing nNOS had again increased and the intensity of the immunoreactions was stronger. Immunohistochemical analysis also showed that iNOS was increasingly expressed in pyramidal-like cortical neurones and in perivascular astrocytes from 1 to 6 months post PCA. In addition, a significant increase in cerebral iNOS concentration, at both post-PCA periods, was determined by Western blotting. The iNOS induction appears to be correlated with the length of the post-PCA period. PCA also induced the expression of NT, a nitration product of peroxynitrite. NT immunoreactivity was found in pyramidal-like cortical neurones. At 6 months, NT immunoreactivity was also evident in perivascular astrocytes, which was concomitant with a significant increase in NT protein level. PCA therefore not only increases the expression of nNOS but also induces the expression of iNOS and NT in both neurones and astrocytes. Taken together, these findings indicate that the induction of iNOS in pyramidal neurones and cortical astrocytes 6 months after PCA contributes to the generation of NT, and demonstrate the clear participation of NO in the pathogenic process of HE in this model.

Glutamine: a Trojan horse in ammonia neurotoxicity

Mechanisms involved in hepatic encephalopathy still remain to be defined. Nonetheless, it is well recognized that ammonia is a major factor in its pathogenesis, and that the astrocyte represents a major target of its CNS toxicity. In vivo and in vitro studies have shown that ammonia evokes oxidative/nitrosative stress, mitochondrial abnormalities (the mitochondrial permeability transition, MPT) and astrocyte swelling, a major component of the brain edema associated with fulminant hepatic failure. How ammonia brings about these changes in astrocytes is not well understood. It has long been accepted that the conversion of glutamate to glutamine, catalyzed by glutamine synthetase, a cytoplasmic enzyme largely localized to astrocytes in brain, represented the principal means of cerebral ammonia detoxification. Yet, the "benign" aspect of glutamine synthesis has been questioned. This article highlights evidence that, at elevated levels, glutamine is indeed a noxious agent. We also propose a mechanism by which glutamine executes its toxic effects in astrocytes, the "Trojan horse" hypothesis. Much of the newly synthesized glutamine is subsequently metabolized in mitochondria by phosphate-activated glutaminase, yielding glutamate and ammonia. In this manner, glutamine (the Trojan horse) is transported in excess from the cytoplasm to mitochondria serving as a carrier of ammonia. We propose that it is the glutamine-derived ammonia within mitochondria that interferes with mitochondrial function giving rise to excessive production of free radicals and induction of the MPT, two phenomena known to bring about astrocyte dysfunction, including cell swelling. Future therapeutic approaches might include controlling excessive transport of newly synthesized glutamine to mitochondria and its subsequent hydrolysis.

The brain glutamate system in liver failure

Liver failure results in significant alterations of the brain glutamate system. Ammonia and the astrocyte play major roles in such alterations, which affect several components of the brain glutamate system, namely its synthesis, intercellular transport (uptake and release), and function. In addition to the neurological symptoms of hepatic encephalopathy, modified glutamatergic regulation may contribute to other cerebral complications of liver failure, such as brain edema, intracranial hypertension and changes in cerebral blood flow. A better understanding of the cause and precise nature of the alterations of the brain glutamate system in liver failure could lead to new therapeutic avenues for the cerebral complications of liver disease.

Nitric oxide production in striatum and pallidum of cirrhotic rats

Ammonium and manganese are neurotoxic agents related to brain metabolic disturbances observed after prolonged liver damage. The aim of this study was to assess the production of nitric oxide (NO) in the brain of cirrhotic rats exposed to manganese. We induced cirrhosis by bile duct ligation for 4 weeks in rats. From brain, striatum and globus pallidus were dissected out, and NO synthase activity and the content of nitrites plus nitrates (NOx) were determined. In pallidum we found a diminished constitutive NO synthase activity from cirrhotic rats, independently of manganese exposure. This result was confirmed by low levels of NOx in the same brain area (P<0.05, two-way ANOVA). This finding was not related to protein expression of NO synthase since no differences were observed in immunoblot signals between cirrhotic and sham-operated animals. Results from present study suggest that the production of NO is reduced in basal ganglia during cirrhosis.

Oxidative stress and mitogen-activated protein kinase phosphorylation mediate ammonia-induced cell swelling and glutamate uptake inhibition in cultured astrocytes

Hepatic encephalopathy (HE) is a major neurological complication in patients with severe liver failure. Elevated levels of ammonia have been strongly implicated as a factor in HE, and astrocytes appear to be the primary target of its neurotoxicity. Mechanisms mediating key aspects of ammonia-induced astrocyte dysfunction such as cell swelling and inhibition of glutamate uptake are not clear. We demonstrated previously that cultured astrocytes exposed to ammonia increase free radical production. We now show that treatment with antioxidants significantly prevents ammonia-induced astrocyte swelling as well as glutamate uptake inhibition. Because one consequence of oxidative stress is the phosphorylation of mitogen-activated protein kinases (MAPKs), we investigated whether phosphorylation of MAPKs may mediate astrocyte dysfunction. Primary cultured astrocytes exposed to 5 mm NH4Cl for different time periods (1-72 h) significantly increased phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), p38(MAPK), and c-Jun N-terminal kinase (JNK) 1/2/3, which was inhibited by appropriate MAPK inhibitors 1, 4-diamino-2, 3-dicyano-1, 4-bis (2-aminophenylthio) butadiene (UO126; for ERK1/2), trans-1-(4-hydroxyclyclohexyl)-4-(4-fluorophenyl)-5-(2-methoxypyrimidin-4-yl)imidazole (SB 239063; for p38(MAPK)), and anthra[1,9-cd]pyrazol-6(2H)-one (SP600125; for JNK1/2/3), as well as by antioxidants. Kinase inhibitors partially or completely prevented astrocyte swelling. Although SB239063 and SP600125 significantly reversed glutamate uptake inhibition and ammonia-induced decline in glutamate-aspartate transporter protein levels, UO126 did not, indicating a differential effect of these kinases in ammonia-induced astrocyte swelling and glutamate transport impairment. These studies strongly suggest the involvement of oxidative stress and phosphorylation of MAPKs in the mechanism of ammonia-induced astrocyte dysfunction associated with ammonia neurotoxicity.

Role of ammonia and nitric oxide in the decrease in plasma prolactin levels in prehepatic portal hypertensive male rats

OBJECTIVES

Since very little is known about neuroendocrine changes that occur in portal-systemic hepatic encephalopathy, we studied plasma prolactin (PRL) levels and the involvement of hyperammonemia, nitric oxide (NO) and dopaminergic and adrenergic systems in the control of this hormone secretion in a male rat model of prehepatic portal hypertension (PH).

METHODS

We conducted in vivo studies to determine plasma ammonia and PRL levels. Dopamine (DA), dihydroxyphenylacetic acid (DOPAC), epinephrine and norepinephrine content in medial basal hypothalamus (MBH) and anterior pituitary (AP) were measured. In addition, NO synthase (NOS) activity and protein expression were evaluated in APs. In in vitro studies, the APs from intact rats were incubated with different doses of ammonia and PRL secretion was determined. In ex vivo studies, the APs from normal and PH rats were incubated in the presence of ammonia and/or a NOS inhibitor, NG-nitro-L-arginine-methyl ester (L-NAME) and PRL secretion was determined.

RESULTS

PH rats had a significant increase in plasma ammonia levels (p < 0.001) and a decrease in plasma PRL levels (p < 0.05). Neither DA nor DOPAC content or DOPAC/DA ratios were modified in both MBH and APs; however, we observed a significant increase in norepinephrine content in both MBH and AP (p < 0.001 and p < 0.05, respectively) and a significant increase in epinephrine in APs (p < 0.001). Moreover, PH produced an increase in NOS activity (p < 0.01) and NOS protein expression (p < 0.0001) in APs. The ammonia (100 microM) significantly reduced PRL secretion from APs in vitro (p < 0.05). The presence of L-NAME, an inhibitor of NOS, abrogated the inhibitory effect of ammonia on PRL secretion from APs from control and PH rats.

CONCLUSIONS

We found that plasma PRL levels were decreased in PH rats probably due to the high ammonia levels. The central noradrenergic system could also mediate this decrease. Also, the increase in NOS activity and/or content in AP induced NO production that directly inhibited PRL secretion from the AP, without the participation of the dopaminergic system.

Limited capacity for ammonia removal by brain in chronic liver failure: potential role of nitric oxide

Chronic liver failure leads to hyperammonemia and consequently increased brain ammonia concentrations, resulting in hepatic encephalopathy. When the liver fails to regulate ammonia concentrations, the brain, devoid of a urea cycle, relies solely on the amidation of glutamate to glutamine through glutamine synthetase, to efficiently clear ammonia. Surprisingly, under hyperammonemic conditions, the brain is not capable of increasing its capacity to remove ammonia, which even decreases in some regions of the brain. This non-induction of glutamine synthetase in astrocytes could result from possible limiting substrates or cofactors for the enzyme, or an indirect effect of ammonia on glutamine synthetase expression. In addition, there is evidence that nitration of the enzyme resulting from exposure to nitric oxide could also be implicated. The present review summarizes these possible factors involved in limiting the increase in capacity of glutamine synthetase in brain, in chronic liver failure.

Mechanisms of ammonia-induced astrocyte swelling

Astrocyte swelling represents the major factor responsible for the brain edema associated with fulminant hepatic failure (FHF). The edema may be of such magnitude as to increase intracranial pressure leading to brain herniation and death. Of the various agents implicated in the generation of astrocyte swelling, ammonia has had the greatest amount of experimental support. This article reviews mechanisms of ammonia neurotoxicity that contribute to astrocyte swelling. These include oxidative stress and the mitochondrial permeability transition (MPT). The involvement of glutamine in the production of cell swelling will be highlighted. Evidence will be provided that glutamine induces oxidative stress as well as the MPT, and that these events are critical in the development of astrocyte swelling in hyperammonemia.

Neuronal and inducible nitric oxide synthase expression in the rat cerebellum following portacaval anastomosis

In order to determine the role of neuronal nitric oxide synthase (nNOS) and inducible nitric oxide synthase (iNOS) in the pathogenesis of experimental hepatic encephalopathy (HE), the expression of both was analyzed in the cerebellum of rats 1 month and 6 months after performing portacaval anastomosis (PCA). In control cerebella, nNOS immunoreactivity was mainly observed in the molecular layer (ML), whereas the Purkinje cells did not express nNOS. However, nNOS expression was detected in the Purkinje cells at 1 month after PCA, correlating with a decrease in nNOS expression in the ML--part of an overall reduction in cerebellar nNOS concentrations (as determined by Western blotting). At 6 months post-PCA, a significant increase in nNOS expression was observed in the ML, as well as increased nNOS immunoreactivity in the Purkinje cells. nNOS immunoreactivity was also observed in the Bergmann glial cells of PCA-treated rats. While no immunoreactivity for iNOS was seen in the cerebella of control rats, iNOS immunoreactivity was significantly induced in the cerebellum 1 month after PCA. In addition, the expression of iNOS was greater at 6 months than at 1 month post-PCA. Immunohistochemical analysis revealed this iNOS to be localized in the Purkinje cells and Bergmann glial cells. The induction of iNOS in astroglial cells has been associated with pathological conditions. Therefore, the iNOS expression observed in the Bergmann glial cells might play a role in the pathogenesis of HE, the harmful effects of PCA being caused by them via the production of excess nitric oxide. These results show that nNOS and iNOS are produced in the Purkinje cells and Bergmann glial cells following PCA, implicating nitric oxide in the pathology of HE.

Bile duct ligation plus hyperammonemia in rats reproduces the alterations in the modulation of soluble guanylate cyclase by nitric oxide in brain of cirrhotic patients

Modulation of soluble guanylate cyclase (sGC) by nitric oxide (NO) is altered in brain from cirrhotic patients. The aim of this work was to assess whether an animal model of cirrhosis, bile duct ligation, alone or combined with diet-induced hyperammonemia for 7-10 days reproduces the alterations in NO modulation of sGC found in brains from cirrhotic patients. sGC activity was measured under basal conditions and in the presence of NO in cerebellum and cerebral cortex of the following groups of rats: controls, bile duct ligation without or with hyperammonemia and hyperammonemia without bile duct ligation. In cerebellum activation of sGC by NO was significantly lower in bile duct ligated rats with (12 +/- five-fold) or without (14 +/- six-fold) hyperammonemia than in control rats (23 +/- seven-fold). In cerebral cortex activation of sGC by NO was higher in rats with bile duct ligation with hyperammonemia (124 +/- 30-fold) but not without hyperammonemia (59 +/- 15-fold) than in control rats (66 +/- 11-fold). The combination of bile duct ligation and hyperammonemia reproduces the alterations in the modulation of soluble guanylate cyclase by NO found in cerebral cortex and cerebellum of cirrhotic patients while bile duct ligation or hyperammonemia alone reproduces the effects in cerebellum but not in cerebral cortex.

Moderate hypothermia in patients with acute liver failure and uncontrolled intracranial hypertension

BACKGROUND AND AIMS

About 20% of patients with acute liver failure (ALF) die from increased intracranial pressure (ICP) while awaiting transplantation. This study evaluates the clinical effects and pathophysiologic basis of hypothermia in patients with ALF and intracranial hypertension that is unresponsive to standard medical therapy.

METHODS

Fourteen patients with ALF who were awaiting orthotopic liver transplantation (OLT) and had increased ICP that was unresponsive to standard medical therapy were studied. Core temperature was reduced to 32 degrees C-33 degrees C using cooling blankets.

RESULTS

Thirteen patients were successfully bridged to OLT with a median of 32 hours (range, 10-118 hours) of cooling. They underwent OLT with no significant complications related to cooling either before or after OLT and had complete neurologic recovery. ICP before cooling was 36.5 +/- 2.7 mm Hg and was reduced to 16.3 +/- .7 mm Hg at 4 hours, which was sustained at 24 hours (16.8 +/- 1.5 mm Hg) ( P < .0001). Mean arterial pressure and cerebral perfusion pressure increased significantly, and the requirement for inotropes was reduced significantly. Hypothermia produced sustained and significant reduction in arterial ammonia concentration and its brain metabolism, cerebral blood flow, brain cytokine production, and markers of oxidative stress.

CONCLUSIONS

Moderate hypothermia is an effective and safe bridge to OLT in patients with ALF who have increased ICP that is resistant to standard medical therapy. Hypothermia reduces ICP by impacting on multiple pathophysiologic mechanisms that are believed to be important in its pathogenesis. A large multicenter trial of hypothermia in ALF is justified.

Upregulation of endothelial nitric oxide synthase maintains nitric oxide production in the cerebellum of thioacetamide cirrhotic rats

This study examines the expression and cellular distribution pattern of nitric oxide synthase (NOS) isoforms, nitrotyrosine-derived complexes, and the nitric oxide (NO) production in the cerebellum of rats with cirrhosis induced by thioacetamide (TAA). The results showed local changes in the tissue distribution pattern of the NOS isoforms and nitrated proteins in the cerebellum of these animals. Particularly, eNOS immunoreactivity in perivascular glial cells of the white matter was detected only in TAA-treated animals. In addition, although neither neuronal NOS (nNOS) nor inducible NOS (iNOS) cerebellar protein levels appeared to be affected, the endothelial NOS (eNOS) isoform significantly increased its expression, and NO production slightly augmented in TAA-treated rats. These NOS/NO changes may contribute differently to the evolution of the hepatic disease either by maintaining the guanosine monophosphate-NO signal transduction pathways and the physiological cerebellar functions or by inducing oxidative stress and cell damage. This model gives rise to the hypothesis that the upregulation of the eNOS maintains the physiological production of NO, while the iNOS is silenced and the nNOS remains unchanged. The differential NOS-distribution and expression pattern may be one of the mechanisms involved to balance cerebellar NO production in order to minimize TAA toxic injury. These data help elucidate the role of the NOS/NO system in the development and progress of hepatic encephalopathy associated with TAA cirrhosis.

Role of circulating neurotoxins in the pathogenesis of hepatic encephalopathy: potential for improvement following their removal by liver assist devices

Both acute and chronic liver failure result in impaired cerebral function known as hepatic encephalopathy (HE). Evidence suggests that HE is the consequence of the accumulation in brain of neurotoxic and/or neuroactive substance including ammonia, manganese, aromatic amino acids, mercaptans, phenols, short-chain fatty acids, bilirubin and a variety of neuroactive medications prescribed as sedatives to patients with liver failure. Brain ammonia concentrations may attain levels in excess of 2 mm, concentrations which are known to adversely affect both excitatory and inhibitory neurotransmission as well as brain energy metabolism. Manganese exerts toxic effects on dopaminergic neurones. Prevention and treatment of HE continues to rely heavily on the reduction of circulating ammonia either by reduction of gut production using lactulose or antibiotics or by increasing its metabolism using L-ornithine-L-aspartate. No specific therapies have so far been designed to reduce circulating concentrations of other toxins. Liver assist devices offer a potential new approach to the reduction of circulating neurotoxins generated in liver failure. In this regard, the Molecular Adsorbents Recirculating System (MARS) appears to offer distinct advantages over hepatocyte-based systems.

Lack of detrimental effects of nitric oxide inhibition in bile duct-ligated rats with hepatic encephalopathy

BACKGROUND

The pathogenetic mechanisms of hepatic encephalopathy (HE) are not fully understood. Vasodilatation induced by nitric oxide (NO) may be involved in the development of HE. There is no comprehensive data concerning the effects of NO inhibition on HE in chronic liver disease.

METHODS

Male Sprague-Dawley rats weighing 240-270 g at the time of surgery were selected for experiments. Secondary biliary cirrhosis was induced by bile duct ligation (BDL). Counts of movements were compared between BDL rats and rats receiving a sham operation. In another series of experiments, BDL rats received either Nomega-nitro-L-arginine methyl ester (L-NAME, 25 mg kg-1 day-1 in tap water) or tap water (control) from the 36th to 42nd days after BDL. Besides motor activities, plasma levels of tumour necrosis factor (TNF)-alpha and nitrate/nitrite, liver biochemistry tests and haemodynamics were determined after treatment.

RESULTS

Compared with the sham-operated rats, the total, ambulatory and vertical movements were significantly decreased in the BDL rats (P </= 0.001). The L-NAME group had a significantly higher mean arterial pressure than that of the control group (119.0 +/- 2.5 mmHg vs. 97.3 +/- 2.8 mmHg, P = 0.002). However, the counts of motor activities, plasma levels of TNF-alpha and nitrate/nitrite, and serum biochemistry tests were not significantly different between the L-NAME and control groups.

CONCLUSIONS

Bile duct ligation may induce HE evidenced by a decrease in motor activities. However, chronic L-NAME administration did not have significantly detrimental or therapeutic effects on the severity of encephalopathy in BDL rats.

Cell-selective effects of ammonia on glutamate transporter and receptor function in the mammalian brain

Increased brain ammonia concentrations are a hallmark feature of several neurological disorders including congenital urea cycle disorders, Reye's syndrome and hepatic encephalopathy (HE) associated with liver failure. Over the last decade, increasing evidence suggests that hyperammonemia leads to alterations in the glutamatergic neurotransmitter system. Studies utilizing in vivo and in vitro models of hyperammonemia reveal significant changes in brain glutamate levels, glutamate uptake and glutamate receptor function. Extracellular brain glutamate levels are consistently increased in rat models of acute liver failure. Furthermore, glutamate transport studies in both cultured neurons and astrocytes demonstrate a significant suppression in the high affinity uptake of glutamate following exposure to ammonia. Reductions in NMDA and non-NMDA glutamate receptor sites in animal models of acute liver failure suggest a compensatory decrease in receptor levels in the wake of rising extracellular levels of glutamate. Ammonia exposure also has significant effects on metabotropic glutamate receptor activation with implications, although less clear, that may relate to the brain edema and seizures associated with clinical hyperammonemic pathologies. Therapeutic measures aimed at these targets could result in effective measures for the prevention of CNS consequences in hyperammonemic syndromes.

Effects of ammonia on high affinity glutamate uptake and glutamate transporter EAAT3 expression in cultured rat cerebellar granule cells

Increased levels of extracellular glutamate are a consistent feature of hepatic encephalopathy (HE) associated with liver failure and other hyperammonemic pathologies. Reduction of glutamate uptake has been described in ammonia-exposed cultured astrocytes, synaptosomes, and in animal models of hyperammonemia. In the present study, we examine the effects of pathophysiological concentrations of ammonia on D-aspartate (a non-metabolizable analog of glutamate) uptake by cultured rat cerebellar granule neurons. Exposure of these cells to ammonia resulted in time-dependent (24% reduction at 24h and 60% reduction at 5 days, P<0.001) and dose-dependent (21, 37, and 57% reduction at 1, 2.5, and 5mM for 5 days, P<0.01) suppression of D-aspartate uptake. Kinetic analyses revealed significant decreases in the velocity of uptake (V(max)) (37% decrease at 2.5mM NH(4)Cl, P<0.05 and 52% decrease at 5mM NH(4)Cl, P<0.001) as well as significant reductions in K(m) values (25% reduction at 2.5mM NH(4)Cl, P<0.05 and 45% reduction at 5mM NH(4)Cl, P<0.001). Western blotting, on the other hand, showed no significant changes in the neuronal glutamate transporter EAAC1/EAAT3 protein, the only glutamate transporter currently known to be expressed by these cells. In addition, 1H combined with 13C-NMR spectroscopy studies using the stable isotope [1-13C]-glucose demonstrated a significant increase in intracellular glutamate levels derived from the oxidative metabolism of glucose, rather than from the deamidation of exogenous glutamine in cultured granule neurons exposed to ammonia. The present study provides evidence that the effects of ammonia on glutamate uptake are not solely an astrocytic phenomenon and that unlike the astrocytic glutamate transporter counterpart, EAAT3 protein expression in cultured cerebellar granule cells is not down-regulated when exposed to ammonia. Decrease of glutamate uptake in these cellular preparations may afford an additional regulatory mechanism aimed at controlling intracellular levels of glutamate and ultimately the releasable pool of glutamate in neurons.

The pathophysiological basis of acute-on-chronic liver failure

The vast majority of patients that are referred to a specialist hepatological centre suffer from acute deterioration of their chronic liver disease. Yet, this entity of acute-on-chronic liver failure remains poorly defined. With the emergence of newer liver support strategies, it has become necessary to define this entity, its pathophysiology and the short and long-term prognosis. This review focuses upon how a precipitant such as an episode of gastrointestinal bleeding or sepsis may start a cascade of events that culminate in end-organ dysfunction and liver failure. We briefly review the pathophysiological basis of the therapeutic modalities that are available. Our current strategy for the management of liver failure involves supportive therapy for the end-organs with the hope that the liver function would recover if sufficient time for such a recovery is allowed. Because liver failure, whether of the acute or acute-on-chronic variety, is potentially reversible, the stage is set for the application of newer liver support strategies to enhance the recovery process.

Nitric oxide in liver transplantation: pathobiology and clinical implications

The gaseous molecule nitric oxide is involved in a variety of liver transplant-relevant processes, including ischemia-reperfusion injury, acute cellular rejection, and circulatory changes characteristic of advanced liver disease. This review article focuses on new advances relating to the role of nitric oxide in these syndromes with an emphasis on pathobiology and potential clinical implications.

Ammonia-induced heme oxygenase-1 expression in cultured rat astrocytes and rat brain in vivo

Ammonia is a key factor in the pathogenesis of hepatic encephalopathy (HE), which is a major complication in acute and chronic liver failure and other hyperammonemic states. The molecular mechanisms underlying ammonia neurotoxicity and the functional consequences of ammonia on gene expression in astrocytes are incompletely understood. Using cDNA array hybridization technique we identified ammonia as a trigger of heme oxygenase-1 (HO-1) mRNA levels in cultured rat astrocytes. As shown by Northern and Western blot analysis, HO-1 mRNA levels were upregulated by ammonia (0.1-5 mmol/L) after 24 h and protein expression after 72 h in astrocytes. These ammonia effects on HO-1 are probably triggered to a minor extent by ammonia-induced glutamine synthesis or by astrocyte swelling, because HO-1 expression was not inhibited by the glutamine synthetase inhibitor methionine sulfoximine (which abrogated ammonia-induced cell swelling in cultured astrocytes), and ammonia-induced HO-1 expression could only partly be mimicked by hypoosmotic astrocyte swelling. Hypoosmotic (205 mOsm/L) exposure of astrocytes led even to a decrease in HO-1 mRNA levels within 4 h, whereas hyperosmotic (405 mOsm/L) exposure increased HO-1 mRNA expression. After 24 h, hypoosmolarity slightly raised HO-1 mRNA expression. Taurine and melatonin diminished ammonia-induced HO-1 mRNA or protein expression, whereas other antioxidants (dimethylthiourea, butylated hydroxytoluene, N-acetylcysteine, and reduced glutathione) increased HO-1 mRNA levels under ammonia-free conditions. An in vivo relevance is suggested by the finding that increased HO-1 expression occurs in the brain cortex from acutely ammonia-intoxicated rats. It is concluded that ammonia-induced HO-1 expression may contribute to cerebral hyperemia in hyperammonic states.

Region selective alterations of soluble guanylate cyclase content and modulation in brain of cirrhotic patients

Modulation of soluble guanylate cyclase (sGC) by nitric oxide (NO) is altered in brain from experimental animals with hyperammonemia with or without liver failure. The aim of this work was to assess the content and modulation of sGC in brain in chronic liver failure in humans. Expression of the alpha-1, alpha-2, and beta-1 subunits of sGC was measured by immunoblotting in autopsied frontal cortex and cerebellum from cirrhotic patients and controls. The contents of alpha-1 and alpha-2 subunits of guanylate cyclase was increased both in cortex and cerebellum, whereas the beta-1 subunit was not affected. Addition of the NO-generating agent S-nitroso-N-acetyl-penicillamine (SNAP) to homogenates of frontal cortex from controls increased the activity of sGC 87-fold, whereas, in homogenates from cirrhotic patients, the increase was significantly higher (183-fold). In contrast, in cerebellum, activation of guanylate cyclase by NO was significantly lower in patients (156-fold) than in controls (248-fold). A similar regional difference was found in rats with portacaval anastomosis. In conclusion, these findings show that the NO-guanylate cyclase signal transduction pathway is strongly altered in brain in patients with chronic liver failure and that the effects are different in different brain areas. Given that activation of sGC by NO in brain is involved in the modulation of important cerebral processes such as intercellular communication, learning and memory, and the sleep-wake cycle, these changes could be implicated in the pathogenesis of hepatic encephalopathy in these patients.

Glutamine synthetase in brain: effect of ammonia

Glutamine synthetase (GS) in brain is located mainly in astrocytes. One of the primary roles of astrocytes is to protect neurons against excitotoxicity by taking up excess ammonia and glutamate and converting it into glutamine via the enzyme GS. Changes in GS expression may reflect changes in astroglial function, which can affect neuronal functions. Hyperammonemia is an important factor responsible of hepatic encephalopathy (HE) and causes astroglial swelling. Hyperammonemia can be experimentally induced and an adaptive astroglial response to high levels of ammonia and glutamate seems to occur in long-term studies. In hyperammonemic states, astroglial cells can experience morphological changes that may alter different astrocyte functions, such as protein synthesis or neurotransmitters uptake. One of the observed changes is the increase in the GS expression in astrocytes located in glutamatergic areas. The induction of GS expression in these specific areas would balance the increased ammonia and glutamate uptake and protect against neuronal degeneration, whereas, decrease of GS expression in non-glutamatergic areas could disrupt the neuron-glial metabolic interactions as a consequence of hyperammonemia. Induction of GS has been described in astrocytes in response to the action of glutamate on active glutamate receptors. The over-stimulation of glutamate receptors may also favour nitric oxide (NO) formation by activation of NO synthase (NOS), and NO has been implicated in the pathogenesis of several CNS diseases. Hyperammonemia could induce the formation of inducible NOS in astroglial cells, with the consequent NO formation, deactivation of GS and dawn-regulation of glutamate uptake. However, in glutamatergic areas, the distribution of both glial glutamate receptors and glial glutamate transporters parallels the GS location, suggesting a functional coupling between glutamate uptake and degradation by glutamate transporters and GS to attenuate brain injury in these areas. In hyperammonemia, the astroglial cells located in proximity to blood-vessels in glutamatergic areas show increased GS protein content in their perivascular processes. Since ammonia freely crosses the blood-brain barrier (BBB) and astrocytes are responsible for maintaining the BBB, the presence of GS in the perivascular processes could produce a rapid glutamine synthesis to be released into blood. It could, therefore, prevent the entry of high amounts of ammonia from circulation to attenuate neurotoxicity. The changes in the distribution of this critical enzyme suggests that the glutamate-glutamine cycle may be differentially impaired in hyperammonemic states.

Nitric oxide in hepatic encephalopathy and hyperammonemia

Chronic alcoholism, viral hepatitis or hepatotoxic drug overdose result in liver dysfunction which may lead to a neuropsychiatric disorder termed hepatic encephalopathy (HE). Although, the exact molecular mechanisms underlying the pathophysiology of HE are not known, excitatory/inhibitory neurotransmitter imbalance leading to dysfunction of the glutamate-nitric oxide (NO) system is thought to play a major role. Activation of the NMDA subtype of glutamate receptors leads to increase in intracellular calcium, which initiates several calcium-dependent processes including NO formation. NO is a gaseous, highly reactive, freely diffusible molecule with a short half-life. Recent studies demonstrate increased expression of the neuronal isoform of NO synthase (NOS) and the uptake of L-arginine (the obligate precursor of NO) in both chronic and acute HE. Hyperammonemia associated with liver dysfunction results in increased NO, which may lead to learning and memory impairments and cerebral edema commonly seen, particularly in acute hyperammonemia.

GeneChip analysis shows altered mRNA expression of transcripts of neurotransmitter and signal transduction pathways in the cerebral cortex of portacaval shunted rats

Identifying the gene expression changes induced by hepatic encephalopathy (HE) leads to a better understanding of the molecular mechanisms of HE-induced neurological dysfunction. Using GeneChip and real-time PCR, the present study evaluated the gene expression profile of rat cerebral cortex at 4 weeks after portacaval shunting. Among 1,263 transcripts represented on the chip, mRNA levels of 31 transcripts were altered (greater than twofold; 16 increased and 15 decreased) in the portacaval shunted (PCS) rat compared to sham control. Changes observed by GeneChip analysis were confirmed for 20 transcripts (8 increased, 7 decreased, and 5 unchanged in PCS rat brain) by real-time PCR. Neurotransmitter receptors, transporters, and members of the second messenger signal transduction are the major groups of genes altered in PCS rat brain. Of importance was that the increased heme oxygenase-1 and decreased Cu,Zn-superoxide dismutase expression observed raise the possibility of oxidative stress playing a pathogenic role in chronic HE.

Altered content and modulation of soluble guanylate cyclase in the cerebellum of rats with portacaval anastomosis

It is shown that the glutamate-NO-cGMP pathway is impaired in cerebellum of rats with portacaval anastomosis in vivo as assessed by in vivo brain microdialysis in freely moving rats. NMDA-induced increase in extracellular cGMP in the cerebellum was significantly reduced (by 27%) in rats with portacaval anastomosis. Activation of soluble guanylate cyclase by the NO-generating agent S-nitroso-N-acetyl-penicillamine and by the NO-independent activator YC-1 was also significantly reduced (by 35-40%), indicating that portacaval anastomosis leads to remarkable alterations in the modulation of guanylate cyclase in cerebellum. Moreover, the content of soluble guanylate cyclase was increased ca. two-fold in the cerebellum of rats with portacaval anastomosis. Activation of soluble guanylate cyclase by NO was higher in lymphocytes isolated from rats with portacaval anastomosis (3.3-fold) than in lymphocytes from control rats (2.1-fold). The results reported show that the content and modulation of soluble guanylate cyclase are altered in brain of rats with hepatic failure, resulting in altered function of the glutamate-NO-cGMP pathway in the rat in vivo. This may lead to alterations in cerebral processes such as intercellular communication, circadian rhythms, including the sleep-waking cycle, long-term potentiation, and some forms of learning and memory.

Identification of osmosensitive and ammonia-regulated genes in rat astrocytes by Northern blotting and differential display reverse transcriptase-polymerase chain reaction

BACKGROUND/AIMS

The hepatic encephalopathy (HE) is in part the result of astrocyte swelling with alterations of glial function. Detoxification of ammonia may be one mechanism by which astrocyte swelling is triggered in HE.

METHODS

The differential display polymerase chain reaction (DDRT-PCR) and Northern blot analysis were used for study the functional consequence of cell volume changes and ammonia on gene expression in primary rat astrocytes.

RESULTS

Differentially expressed cDNA products were identified with about 92% homology to genes coding for mouse proline rich protein expressed in brain (PRTB), rat clusterin, elongin, and human Kelch motif containing protein. As shown by Northern blot analysis, PRTB and clusterin mRNA levels were upregulated by 19-64% after 4-8 h by both ammonia and hypoosmolarity. Elongin mRNA expression increased by 97% in response to ammonia but slightly by hypoosmolarity. Further, hypoosmotic exposure for 1-24 hours but not ammonia led to an increase of 80% in KMCP mRNA levels.

CONCLUSIONS

The identification of these genes offers the opportunity to identify unrecognized molecular mechanisms of HE. The finding that several genes are induced by both, hypoosmolarity and ammonia, supports the view that astrocyte swelling is a major, but not the only pathogenetic event in HE.