Effect of ammonium chloride on energy metabolism of astrocytes and C6-glioma cells in vitro

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.

New Insight Into Mechanisms of Hepatic Encephalopathy: An Integrative Analysis Approach to Identify Molecular Markers and Therapeutic Targets

Hepatic encephalopathy (HE) is a set of complex neurological complications that arise from advanced liver disease. The precise molecular and cellular mechanism of HE is not fully understood. Differentially expressed genes (DEGs) from microarray technologies are powerful approaches to obtain new insight into the pathophysiology of HE. We analyzed microarray data sets of cirrhotic patients with HE from Gene Expression Omnibus to identify DEGs in postmortem cerebral tissues. Consequently, we uploaded significant DEGs into the STRING to specify protein-protein interactions. Cytoscape was used to reconstruct the genetic network and identify hub genes. Target genes were uploaded to different databases to perform comprehensive enrichment analysis and repurpose new therapeutic options for HE. A total of 457 DEGs were identified in 2 data sets totally from 12 cirrhotic patients with HE compared with 12 healthy subjects. We found that 274 genes were upregulated and 183 genes were downregulated. Network analyses on significant DEGs indicated 12 hub genes associated with HE. Enrichment analysis identified fatty acid beta-oxidation, cerebral organic acidurias, and regulation of actin cytoskeleton as main involved pathways associated with upregulated genes; serotonin receptor 2 and ELK-SRF/GATA4 signaling, GPCRs, class A rhodopsin-like, and p38 MAPK signaling pathway were related to downregulated genes. Finally, we predicted 39 probable effective drugs/agents for HE. This study not only confirms main important involved mechanisms of HE but also reveals some yet unknown activated molecular and cellular pathways in human HE. In addition, new targets were identified that could be of value in the future study of HE.

Non-hepatic Hyperammonemia: A Potential Therapeutic Target for Sepsis-associated Encephalopathy

Sepsis-Associated Encephalopathy (SAE) is a common complication in the acute phase of sepsis, and patients who develop SAE have a higher mortality rate, longer hospital stay, and worse quality of life than other sepsis patients. Although the incidence of SAE is as high as 70% in sepsis patients, no effective treatment is available for this condition. To develop an effective treatment for SAE, it is vital to explore its pathogenesis. It is known that hyperammonemia is a possible factor in the pathogenesis of hepatic encephalopathy as ammonia is a potent neurotoxin. Furthermore, our previous studies indicate that non-hepatic hyperammonemia seems to occur more often in sepsis patients; it was also found that >50% of sepsis patients with non-hepatic hyperammonemia exhibited encephalopathy and delirium. Substatistical analyses indicate that non-hepatic hyperammonemia is an independent risk factor for SAE. This study updates the definition, clinical manifestations, and diagnosis of SAE; it also investigates the possible treatment options available for non-hepatic hyperammonemia in patients with sepsis and the mechanisms by which non-hepatic hyperammonemia causes encephalopathy.

Protective Effects of Probucol on Different Brain Cells Exposed to Manganese

Manganese (Mn) is an essential metal for many functions in the body. However, in excess, it can be neurotoxic and cause a Parkinson-like syndrome, known as manganism. Here, we aimed to identify a protective effect of probucol, a lipid-lowering agent with anti-inflammatory and antioxidant properties, against Mn-induced toxicity in human neuroblastoma (SH-SY5Y) and glioblastoma (C6) cell lines. The cells were incubated with increasing concentrations of Mn followed by probucol addition 1, 3, 6, and/or 24 h to assess the metal toxic doses and measure the protective effect of probucol against Mn-induced oxidative damage. Longer exposition to Mn showed decreased SH-SY5Y cellular viability in concentrations higher than 100 µM, and probucol was able to prevent this effect. The C6 cells were more sensitive to the Mn deleterious actions, decreasing the cell viability after 6 h of 500 µM Mn exposure. In addition, probucol prevents the complex I and II of the mitochondrial respiratory chain (MRC) inhibition caused by Mn and decreased the intracellular ROS production. Taken together, our results showed that Mn toxicity affects differently both cell lines and probucol has a protective effect against the oxidative imbalance in the central nervous system.

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.

Astrocyte culture models: Molecular and function characterization of primary culture, immortalized astrocytes and C6 glioma cells

The understanding of the physiology of astrocytes and their role in brain function progresses continuously. Primary astrocyte culture is an alternative method to study these cells in an isolated system: in their physiologic and pathologic states. Cell lines are often used as an astrocyte model, since they are easier and faster to manipulate and cost less. However, there are a few studies evaluating the different features of these cells which may put into question the validity of using them as astrocyte models. The aim of this study was to compare primary cultures (PC) with two cell lines - immortalized astrocytes and C6 cells, in terms of protein characterization, morphology and metabolic functional activity. Our results showed, under the same culture condition, that immortalized astrocytes and C6 are positive for differentiated astrocytic markers (eg. GFAP, S100B, AQP4 and ALDH1L1), although expressing them in less quantities then primary astrocyte cultures. Glutamate metabolism and cell communication are reduced in proliferative cells. However, glucose uptake is elevated in C6 lineage cells in comparison with primary astrocytes, probably due to their tumorigenic origin and high proliferation rate. Immortalized astrocytes presented a lower growth rate than C6 cells, and a similar basal morphology as primary astrocytes. However, they did not prove to be as good reproductive models of some of the classic astrocytic functions, such as S100B secretion and GFAP content, especially while under stimulation. In contrast, C6 cells presented similar results in comparison to primary astrocytes in response to stimuli. Here we provide a functional comparison of three astrocytic models, in an attempt to select the most suitable model for the study of astrocytes, optimizing the research in this area of knowledge.

L-carnitine prevents ammonia-induced cytotoxicity and disturbances in intracellular amino acid levels in human astrocytes

BACKGROUND AND AIM

L-carnitine (L-CA) has been used therapeutically to treat hepatic encephalopathy with hyperammonemia, but the mechanism by which L-CA contributes to ammonia detoxification in the brain is still unclear. Thus, the cytotoxicity and changes in intracellular amino acids (AAs) in astrocytes with hyperammonemia following L-CA administration were studied.

METHODS

Human astrocytes were treated with ammonium chloride (NH₄ Cl), L-CA or a mixture of NH₄ Cl, and L-CA under defined conditions. Total intracellular reactive oxygen species and lactate dehydrogenase leakage were measured following different treatment periods. The intracellular levels of AAs in astrocytes were determined using metabolomic analysis.

RESULTS

Intracellular total reactive oxygen species and lactate dehydrogenase leakage were significantly increased after treatment with NH₄ Cl. In contrast, co-treatment with L-CA significantly inhibited the cytotoxic effects of NH₄ Cl. The intracellular levels of almost all AAs involving glutamine and branched-chain AAs (BCAAs) were significantly increased in the NH₄ Cl-treated cells compared with in the control cells; these changes in BCAA levels were reduced with L-CA co-treatment. Additionally, the level of 3-methyl-2-oxovaleric acid, which is a metabolite from isoleucine and plays a critical role in neurological damage, was significantly increased in the NH₄ Cl-treated cells, but this metabolite was significantly decreased with L-CA co-treatment.

CONCLUSION

L-CA protects human astrocytes from ammonia-induced acute cytotoxic effects and the increased intracellular levels of glutamine and BCAAs.

Neurotoxicity of Ammonia

Abnormal liver function has dramatic effects on brain functions. Hyperammonemia interferes profoundly with brain metabolism, astrocyte volume regulation, and in particular mitochondrial functions. Gene expression in the brain and excitatory and inhibitory neurotransmission circuits are also affected. Experiments with a number of pertinent animal models have revealed several potential mechanisms which could underlie the pathological phenomena occurring in hepatic encephalopathy.

Effect of alpha-ketoglutarate on growth and metabolism of cells cultured on three-dimensional cryogel matrix

Alpha- ketoglutarate (α-KG) is a well-known intermediate of Kreb's cycle and thus one of the important candidates in the role of cellular metabolism. The aim was to investigate the effect of α-KG on the growth and proliferation of different cells (fibroblasts and chondrocytes). Further change in the consumption of glucose and release of ammonia in the media, upon the addition of α-KG was also examined. NIH3T3 and chondrocytes were seeded on two different macroporous cryogel matrices synthesized by poly (hydroxyethylmethaacrylate)-gelatin (HG) and alginate-gelatin (AG). Higher proliferation was observed for both the cell lines in 2-D as well as 3-D where α-KG was added to the media compared to the controls. It was evident that the chondrocytes were metabolically more active in the case of α-KG containing samples as early extracellular matrix (ECM) accumulation was observed. In comparison to the 6-8 weeks duration required by the chondrocytes for ECM accumulation in normal in-vitro culture conditions, α-KG containing samples showed an earlier accumulation within 3 weeks. In order to further validate the results, scanning electron microscopic (SEM) analysis was performed showed high ECM deposition and cells embedded in the matrix. Homogenous distribution of cells on both the synthesized matrix was reported using 4'-6-diamidino-2-phenylindole (DAPI) and propidium iodide (PI) staining. The results clearly showed reduction in ammonia concentration in α-KG containing samples thus leading to decreased ammonia toxicity that builds up during long cell culture conditions thereby enhancing the proliferation and metabolic activity. Thus α-KG can be used potentially for long batch-cultures for the production of vaccines or antibody as well as can play an important role as a bioactive molecule for in vitro neo-cartilage generation.

Glutamine synthetase gene expression and glutamate transporters in C6-glioma cells

Glutamine synthetase (GS) is the major glutamate-forming enzyme of vertebrae and is accepted to be a marker of astroglial cells. Maturation of astroglial cells is characterized by an increase in GS activity, and the regulation of this enzyme is the topic of many publications. The amino acid glutamate is the major excitatory neurotransmitter in the brain and mediates normal excitatory synaptic transmission by interaction with postsynaptic receptors. Glutamate also acts as a potent neurotoxin when present at high concentration. Glutamate neurotoxicity plays an important role in the pathophysiology of many neurological disorders, such as Alzheimer's disease, Huntington's disease and amyotrophic lateral sclerosis. In the normal condition, L-glutamate is predominantly taken up, metabolized and recycled by astrocytes through the glutamate transporters (GLAST/GLT1) and glutamine synthetase (GS) catalytic activity. Because of the fundamental role of these glutamate transporters and the glutamine synthetase enzyme in controlling cerebral glutamate level, regulation of GS and studying of the glutamate transporters in glial cells is important. Astrocytes are supportive cells and act as the site of detoxification of glutamate in the brain. However, their isolation from the brain is a tedious, costly and time consuming procedure. On the other hand, the C6-glioma cells are readily available on the market. They are well characterized and have been a useful model for CNS glia in many laboratories. For this study, we used the C6-glioma cell line as a model system. We examined the presence or absence of glial specific glutamate transporters (GLTI and GLAST) in C6-glioma cells, which was done by immunocytochemistry. We also examined glutamine synthetase gene expression in these cells by treatment of the C6-glioma cells with estrogen (17ß estradiol). The findings from this study provide useful information about C6-glioma cells which makes the study of the CNS tremendously inexpensive.

Acute ethanol exposure disrupts actin cytoskeleton and generates reactive oxygen species in c6 cells

Central nervous system dysfunctions are among the most significant effects of exposure to ethanol and the glial cells that play an important role in maintaining neuronal function, are extremely involved with these effects. The actin cytoskeleton plays a crucial role in a wide variety of cellular functions, especially when there is some injury. Therefore the aim of the present study was to analyze the short-term effects of ethanol (50, 100 and 200 mM) on the cytoskeleton of C6 glioma cells. Here we report that acute ethanol exposure profoundly disrupts the actin cytoskeleton in C6 cells decreasing stress fiber formation and downregulating RhoA and vinculin immunocontent. In contrast, microtubule and GFAP networks were not altered. We further demonstrate that anti-oxidants prevent ethanol-induced actin alterations, suggesting that the actions of ethanol on the actin cytoskeleton are related with generation of reactive oxygen species (ROS) in these cells. Our results show that ethanol at concentrations described to be toxic to the central nervous system was able to target the cytoskeleton of C6 cells and this effect could be related with increased ROS generation. Therefore, we propose that the dynamic restructuring of the cytoskeleton of glial cells might contribute to the response to the injury provoked by binge-like ethanol exposure in brain.

Induction of S100B secretion in C6 astroglial cells by the major metabolites accumulating in glutaric acidemia type I

Glutaryl-CoA dehydrogenase deficiency or glutaric acidemia type I (GA I) is an inherited neurometabolic disorder biochemically characterized by tissue accumulation of predominantly glutaric (GA) and 3-hydroxyglutaric (3OHGA) acids and clinically by severe neurological symptoms and structural brain abnormalities, manifested as progressive cerebral atrophy and acute striatum degeneration following encephalopathic crises, whose pathophysiology is still in debate. Considering that reactive astrogliosis is a common finding in brain of GA I patients, in the present study we investigated the effects of GA and 3OHGA on glial activity determined by S100B release by rat C6-glioma cells. We also evaluated the effects of these organic acids on some parameters of oxidative stress in these astroglial cells. We observed that GA and 3OHGA significantly increased S100B secretion and thiobarbituric acid-reactive substances (lipid peroxidation), whereas GA markedly decreased reduced glutathione levels in these glioma cells. This is the first report demonstrating that the major metabolites accumulating in GA I activate S100B secretion in astroglial cells, indicating activation of these cells. We also showed that GA and 3OHGA induced oxidative stress in C6 lineage cells, confirming previous findings observed in brain fresh tissue. It is therefore presumed that reactive glial cells and oxidative damage may underlie at least in part the neuropathology of GA I.

Detoxification of ammonia in mouse cortical GABAergic cell cultures increases neuronal oxidative metabolism and reveals an emerging role for release of glucose-derived alanine

Cerebral hyperammonemia is believed to play a pivotal role in the development of hepatic encephalopathy (HE), a debilitating condition arising due to acute or chronic liver disease. In the brain, ammonia is thought to be detoxified via the activity of glutamine synthetase, an astrocytic enzyme. Moreover, it has been suggested that cerebral tricarboxylic acid (TCA) cycle metabolism is inhibited and glycolysis enhanced during hyperammonemia. The aim of this study was to characterize the ammonia-detoxifying mechanisms as well as the effects of ammonia on energy-generating metabolic pathways in a mouse neuronal-astrocytic co-culture model of the GABAergic system. We found that 5 mM ammonium chloride affected energy metabolism by increasing the neuronal TCA cycle activity and switching the astrocytic TCA cycle toward synthesis of substrate for glutamine synthesis. Furthermore, ammonia exposure enhanced the synthesis and release of alanine. Collectively, our results demonstrate that (1) formation of glutamine is seminal for detoxification of ammonia; (2) neuronal oxidative metabolism is increased in the presence of ammonia; and (3) synthesis and release of alanine is likely to be important for ammonia detoxification as a supplement to formation of glutamine.

Signaling factors in the mechanism of ammonia neurotoxicity

Mechanisms involved in hepatic encephalopathy (HE) still remain poorly understood. It is generally accepted that ammonia plays a major role in this disorder, and that astrocytes represent the principal target of ammonia neurotoxicity. In recent years, studies from several laboratories have uncovered a number of factors and pathways that appear to be critically involved in the pathogenesis of this disorder. Foremost is oxidative and nitrosative stress (ONS), which is largely initiated by an ammonia-induced increase in intracellular Ca(2+). Such increase in Ca(2+) activates a number of enzymes that promote the synthesis of reactive oxygen-nitrogen species, including constitutive nitric oxide synthase, NADPH oxidase and phospholipase A2. ONS subsequently induces the mitochondrial permeability transition, and activates mitogen-activated protein kinases and the transcription factor, nuclear factor-kappaB (NF-kappaB). These factors act to generate additional reactive oxygen-nitrogen species, to phosphorylate various proteins and transcription factors, and to cause mitochondrial dysfunction. This article reviews the role of these factors in the mechanism of HE and ammonia toxicity with a focus on astrocyte swelling and glutamate uptake, which are important consequences of ammonia neurotoxicity. These pathways and factors provide attractive targets for identifying agents potentially useful in the therapy of HE and other hyperammonemic disorders.

Energy metabolism in brain cells: effects of elevated ammonia concentrations

Both neurons and astrocytes have high rates of glucose utilization and oxidative metabolism. Fully 20% of glucose consumption is used for astrocytic production of glutamate and glutamine, which during intense glutamatergic activity leads to an increase in glutamate content, but at steady state is compensated for by an equally intense oxidation of glutamate. The amounts of ammonia used for glutamine synthesis and liberated during glutamine hydrolysis are large, compared to the additional demand for glutamine synthesis in hyperammonemic animals and patients with hepatic encephalopathy. Nevertheless, elevated ammonia concentrations lead to an increased astrocytic glutamine production and an elevated content of glutamine combined with a decrease in glutamate content, probably mainly in a cytosolic pool needed for normal activity of the malate-asparate shuttle (MAS); another compartment generated by glutamine hydrolysis is increased. As a result of reduced MAS activity the pyruvate/lactate ratio is decreased in astrocytes but not in neurons and decarboxylation of pyruvate to form acetyl coenzyme A is reduced. Elevated ammonia concentrations also inhibit decarboxylation of alpha-ketoglutarate in the TCA cycle. This effect occurs in both neurons and astrocytes, is unrelated to MAS activity and seen after chronic treatment with ammonia even in the absence of elevated ammonia concentrations.

Effect of the branched-chain alpha-keto acids accumulating in maple syrup urine disease on S100B release from glial cells

Accumulation of the branched-chain alpha-keto acids (BCKA), alpha-ketoisocaproic acid (KIC), alpha-keto-beta-methylvaleric acid (KMV) and alpha-ketoisovaleric acid (KIV) and their respective branched-chain alpha-amino acids (BCAA) occurs in tissues and biological fluids of patients affected by the neurometabolic disorder maple syrup urine disease (MSUD). The objective of this study was to verify the effect of the BCKA on S100B release from C6 glioma cells. The cells were exposed to 1, 5 or 10 mM BCKA for different periods and the S100B release was measured afterwards. The results indicated that KIC and KIV, but not KMV, significantly enhanced S100B liberation after 6 h of exposure. Furthermore, the stimulatory effect of the BCKA on S100B release was prevented by coincubation with the energetic substrate creatine and with the N-nitro-l-arginine methyl ester (l-NAME), a nitric oxide synthase inhibitor, indicating that energy deficit and nitric oxide (NO) were probably involved in this effect. Furthermore, the increase of S100B release was prevented by preincubation with the protein kinase inhibitors KN-93 and H-89, indicating that KIC and KIV altered Ca2+/calmodulin (PKCaMII)- and cAMP (PKA)-dependent protein kinases activities, respectively. In contrast, other antioxidants such as glutathione (GSH) and trolox (soluble vitamin E) were not able to prevent KIC- and KIV-induced increase of S100B liberation, suggesting that the alteration of S100B release caused by the BCKA is not mediated by oxidation of sulfydryl or other essential groups of the enzyme as well as by lipid peroxyl radicals. Considering the importance of S100B for brain regulation, it is conceivable that enhanced liberation of this protein by increased levels of BCKA may contribute to the neurodegeneration characteristic of MSUD patients.

Branched‐chain amino acids accumulating in maple syrup urine disease induce morphological alterations in C6 glioma cells probably through reactive species

In the present study, we investigated the effects of the branched-chain amino acids (BCAA) leucine (Leu), isoleucine (Ile) and valine (Val), which accumulate in maple syrup urine disease (MSUD), on C6 glioma cell morphology and cytoskeletal reorganization by exposing the cultured cells to 1 and 5 mM BCAA. We observed that cells showed a fusiform shape with processes after 3 h treatment. Cell death was also observed when cells were incubated in the presence of the BCAA for 3 and 24 h. Val-treated cells presented the most dramatic morphological alterations. Immunocytochemistry with anti-actin and anti-GFAP antibodies revealed that all BCAA induced reorganization of actin and GFAP cytoskeleton. Although phosphorylation regulates intermediate filament (IF) assembly/disassembly, we verified that the BCAA did not change the in vitro phosphorylation of IF proteins either in C6 cells or in slices of cerebral cortex of rats during development (9-, 12-, 17- and 21-day-old). Furthermore, we observed that 3 h cell exposure to 5 mM of each BCAA resulted in a marked reduction of reduced glutathione (GSH) levels and significantly increased nitric oxide production. Finally, we observed that the morphological features caused by the BCAA on C6 cells were prevented by the use of the antioxidants GSH (1 mM) and N(omega)-nitro-L-arginine methyl ester (L-NAME, 0.5 mM). On the basis of the present results, we conclude that free radical attack might be involved in the cell morphological alterations, as well as, in the cytoskeletal reorganization elicited by the BCAA. It is therefore presumed that these findings could be involved in the neuropathological features observed in patients affected by MSUD.

Morphological alterations and induction of oxidative stress in glial cells caused by the branched-chain alpha-keto acids accumulating in maple syrup urine disease

Maple syrup urine disease (MSUD) is an inherited neurometabolic disorder biochemically characterized by the accumulation of the branched-chain alpha-keto acids (BCKA) alpha-ketoisocaproic (KIC), alpha-keto-beta-methylvaleric (KMV) and alpha-ketoisovaleric (KIV) and their respective branched-chain alpha-amino acids in body fluids and tissues. Affected MSUD patients have predominantly neurological features, including cerebral edema and atrophy whose pathophysiology is not well established. In the present study we investigated the effects of KIC, KMV and KIV on cell morphology, cytoskeleton reorganization, actin immunocontent and on various parameters of oxidative stress, namely total antioxidant reactivity (TAR), glutathione (GSH) and nitric oxide concentrations, and on the activities of catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GPx) in C6 glioma cells. We initially observed that C6 cultivated cells exposed for 3 h to the BCKA (1 and 10 mM) changed their usual rounded morphology to a fusiform or process-bearing cell appearance, while 24 h exposure to these organic acids elicited massive cell death. Rhodamine-labelled phalloidin analysis revealed that these organic acids induced reorganization of the actin cytoskeleton with no modifications on total actin content. It was also observed that 3h cell exposure to low doses of all BCKA (1 mM) resulted in a marked reduction of the non-enzymatic antioxidant defenses, as determined by TAR and GSH measurements. In addition, KIC provoked a reduced activity of SOD and GPx, whereas KMV caused a diminution of SOD activity. In contrast, CAT activity was not modified by the metabolites. Furthermore, nitric oxide production was significantly increased by all BCKA. Finally, we observed that the morphological features caused by BCKA on C6 cells were prevented by the use of the antioxidants GSH (1.0 mM), alpha-tocopherol (trolox; 10 microM) and Nomega-nitro-L-arginine methyl ester (L-NAME; 500 microM). These results strongly indicate that oxidative stress might be involved in the cell morphological alterations and death, as well as in the cytoskeletal reorganization elicited by the BCKA. It is presumed that these findings are possibly implicated in the neuropathological features observed in patients affected by MSUD.

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.

Induction of the mitochondrial permeability transition in cultured astrocytes by glutamine

Ammonia is a toxin that has been strongly implicated in the pathogenesis of hepatic encephalopathy (HE), and astrocytes appear to be the principal target of ammonia toxicity. Glutamine, a byproduct of ammonia metabolism, has been implicated in some of the deleterious effects of ammonia on the CNS. We have recently shown that ammonia induces the mitochondrial permeability transition (MPT) in cultured astrocytes, but not in neurons. We therefore determined whether glutamine is also capable of inducing the MPT in cultured astrocytes. Astrocytes were treated with glutamine (4.5 mM) for various time periods and the MPT was assessed by changes in 2-deoxyglucose (2-DG) mitochondrial permeability, calcein fluorescence assay, and by changes in cyclosporin A (CsA)-sensitive inner mitochondrial membrane potential (deltapsi(m)) using the potentiometric dye, JC-1. Astrocytes treated with glutamine significantly increased 2-DG permeability (120%, P<0.01), decreased mitochondrial calcein fluorescence, and concomitantly dissipated the deltapsi(m). All of these effects were blocked by CsA. These data indicate that glutamine induces the MPT in cultured astrocytes. The induction of the MPT by glutamine in astrocytes, and the subsequent development of mitochondrial dysfunction, may partially explain the deleterious affects of glutamine on the CNS in the setting of hyperammonemia.

Ammonia neurotoxicity: role of the mitochondrial permeability transition

Hepatic encephalopathy (HE) is an important cause of morbidity and mortality in patients with severe liver disease. Although the mechanisms responsible for HE remain elusive, ammonia is generally considered to be involved in its pathogenesis, and astrocytes are thought to be the principal target of ammonia neurotoxicity. Altered bioenergetics and oxidative stress are also thought to play a major role in this disorder. In this paper, we present data invoking the mitochondrial permeability transition (MPT) as a factor in the pathogenesis of HE/hyperammonemia. The MPT is a Ca2+-dependent, cyclosporin A (CsA) sensitive process due to the opening of a pore in the inner mitochondrial membrane that leads to a collapse of ionic gradients and ultimately to mitochondrial dysfunction. Many of the factors that facilitate the induction of the MPT are also known to be implicated in the mechanism of HE, including free radicals, Ca2+, nitric oxide, alkaline pH, and glutamine. We have recently shown that treatment of cultured astrocytes with 5 mM NH4Cl resulted in a dissipation of the mitochondrial membrane potential (delta(psi)m), which was sensitive to CsA. Similarly treated cultured neurons failed to show a loss of the delta(psi)m. Further support for the ammonia induction of the MPT was obtained by observing an increase in mitochondrial permeability to 2-deoxyglucose-6-phosphate, and a decrease in calcein fluorescence in astrocytes after ammonia treatment, both of which were also blocked by CsA. CsA was likewise capable of exerting a protective effect against hyperammonemia in mice. Taken together, our data suggest that the MPT represents an important component of the pathogenesis of HE and other hyperammonemic states.

Cerebral energy metabolism in hepatic encephalopathy and hyperammonemia

Hepatic encephalopathy (HE) is an important cause of morbidity and mortality in patients with severe liver disease. Although the molecular basis for the neurological disorder in HE remains elusive, elevated ammonia and its chief metabolite glutamine are believed to be important factors responsible for altered cerebral functions, including multiple neurotransmitter system(s) failure, altered bioenergetics, and more recently oxidative stress. Accumulated evidence suggests that direct interference of ammonia at several points in cerebral energy metabolism, including glycolysis, TCA cycle, and the electron transport chain, could lead to energy depletion. Additionally, recent studies from our laboratory have invoked the possibility that ammonia and glutamine may induce the mitochondrial permeability transition in astrocytes, a process capable of causing mitochondrial dysfunction. Altered mitochondrial metabolism appears to be an important mechanism responsible for the cerebral abnormalities associated with HE and other hyperammonemic states.

The effect of ammonium chloride on metabolism of primary neurons and neuroblastoma cells in vitro

Hyperammonemia is a consistent finding in many metabolic disorders. The excess ammonia (NH4Cl) interferes with brain energy metabolism possibly in part by inhibiting the tricarboxylic acid (TCA) cycle. Inhibition of the TCA cycle may result in depletion of ATP in the brain cells. In this study, the acute and chronic effect of NH4Cl (7.5 mM and 15 mM) on the metabolism of isolated neurons and neuroblastoma cells was examined. These cells were treated with NH4Cl for 15 minutes and 24 hours. Morphologic and metabolic toxicity were greater in neuroblastoma cells than in primary neurons. Following 15 minutes treatment, concentration of lactate increased significantly in neuroblastoma cells but, the concentration of other metabolites did not change significantly in neuroblastoma cells and in primary neurons. Following 24 hours treatment, the glucose utilization increased in both cell types. This high utilization of glucose in neuroblastoma cells was in concert with an increase in lactate and decrease in glutamate and ATP. In primary neurons, following 24 hours treatment, the glucose utilization significantly increased, but the concentration of the other metabolites did not change significantly. Neuroblastoma cells consumed more glucose than primary neurons in absence of NH4Cl, but generated the same amount of lactate as neurons.

Responses in primary astrocytes and C6-glioma cells to ammonium chloride and dibutyryl cyclic-AMP

Elevated brain ammonia levels are a major factor in the genesis of hepatic encephalopathy (HE). The mechanism of ammonium chloride (NH4Cl) neurotoxicity involves interruption of oxidative metabolism. This leads to decreased levels of ATP concentration and subsequent glial fibrillary acidic protein (GFAP) degradation of astrocytes and fibrous C6-glioma cells. Our study investigates NH4Cl toxicity by evaluating changes in ATP concentration and mitochondrial function as well as by evaluating alterations in GFAP expression. NH4Cl induced decreases in ATP were detected after 15 minutes in C6-glioma cells and 24 hours in both cell types. Mitochondrial function, assessed by MTT (2-4,5-dimethylthiazol A-yl)-2, 5-diphenyltetrazolium bromide) assay, was impaired in both cell types at 24 hours following NH4Cl treatment. GFAP was also significantly decreased in both cell types. Morphologic and metabolic toxicities were greater in C6-glioma cells. The data clearly indicate that NH4Cl interrupts oxidative metabolism. The greater toxicity seen in C6-glioma cells may be due to their greater dependence on oxidative metabolism. Lastly, the decrease in GFAP is probably a consequence of diminished ATP.

The role of astrocytes in the development of hepatic encephalopathy

Thioacetamide (TAA), a hepatotoxin used to ascertain the role of astrocytes in hepatic encephalopathy, was administered to prepare four experimental groups of rats. (The TAA1D, TAA1.5D, TAA2D, and TAA2.5D group rats were perfusion fixated with formalin at 1, 1.5, 2, and 2.5 days, respectively, after initial administration of TAA. In addition, TAA was readministered to the TAA2D and TAA2.5D rats 24 h after the first dose.) Abnormalities of higher brain function and equilibrium that progressed with time were apparent in the rats receiving TAA. On the other hand, innate reflexes (e.g. pupillary reflex) were similar to those in the normal control group. Astrocyte cell areas in the hippocampus, neocortex, hypothalamus, cerebellum, and basal ganglia (striatum) from the TAA rats were significantly larger than in corresponding sites from the normal rats (maximum in TAA1D and TAA1.5D groups). However, there were no differences with respect to the midbrain. Any morphological difference was not observed in neurons between the hepatic encephalopathy and normal rats. Administration of TAA caused hepatic tissue injury that progressed over time. Surprisingly, encephalopathy was apparent even when hepatic injury was mild. These findings suggest that abnormalities in astrocytes, which precede any abnormal change in neurons, play a role in the development of hepatic encephalopathy.

Astroglial dysfunction in hepatic encephalopathy

While the pathogenesis of hepatic encephalopathy (HE) remains elusive, there is considerable evidence pointing to a key role of ammonia-induced dysfunction of astrocytes in this condition. Deficits in the ability of astrocytes to take up glutamate from the extracellular space may lead to abnormal glutamatergic neurotransmission. Furthermore, excessive stimulation of neuronal and glial glutamate receptors by elevated extracellular levels of glutamate may lead to excitotoxicity and greater glial dysfunction. Ammonia also causes upregulation of astroglial peripheral-type benzodiazepine receptors (PBRs) which is associated with increased production of neurosteroids. These neurosteroids have potent positive modulatory effects on the neuronal GABA(A) receptor which, combined with an ammonia-induced astroglial defect in GABA uptake, may result in enhanced GABAergic tone. Brain edema, associated with fulminant hepatic failure, may also result from astroglial abnormalities as the edema appears to be principally caused by swelling of these cells. Increased amounts of glutamine in astrocytes resulting from elevated brain ammonia levels may be a factor in this swelling. Other osmolytes such as glutathione may also be involved. Glial swelling may also result from NH4+ - and K+ -mediated membrane depolarization as well as by the actions of PBR agonists and neurosteroids. These findings show that an ammonia-induced gliopathy is a major factor in the pathogenesis of HE.