Glutamine-induced free radical production in cultured astrocytes

Future Therapies of Hepatic Encephalopathy

Hepatic encephalopathy, either covert or overt, affects more than half of patients with cirrhosis and has lasting effects even after portal hypertension is corrected. Unfortunately, the current therapeutic options still result in high rates of relapse and progression, in part owing to cost barriers and side effects, leading to poor adherence. This review summarizes emerging treatment options, which could take advantage of alternative disease pathways to improve future care of those with hepatic encephalopathy.

Effects of oxidative stress on hepatic encephalopathy pathogenesis in mice

Oxidative stress plays a crucial role in the pathogenesis of hepatic encephalopathy (HE), but the mechanism remains unclear. GABAergic neurons in substantia nigra pars reticulata (SNr) contribute to the motor deficit of HE. The present study aims to investigate the effects of oxidative stress on HE in male mice. The results validate the existence of oxidative stress in both liver and SNr across two murine models of HE induced by thioacetamide (TAA) and bile duct ligation (BDL). Systemic mitochondria-targeted antioxidative drug mitoquinone (Mito-Q) rescues mitochondrial dysfunction and oxidative injury in SNr, so as to restore the locomotor impairment in TAA and BDL mice. Furthermore, the GAD2-expressing SNr population (SNr^GAD2) is activated by HE. Both overexpression of mitochondrial uncoupling protein 2 (UCP2) targeted to SNr^GAD2 and SNr^GAD2-targeted chemogenetic inhibition targeted to SNr^GAD2 rescue mitochondrial dysfunction in TAA-induced HE. These results define the key role of oxidative stress in the pathogenesis of HE.

Glial Glutamine Homeostasis in Health and Disease

Glutamine is an essential cerebral metabolite. Several critical brain processes are directly linked to glutamine, including ammonia homeostasis, energy metabolism and neurotransmitter recycling. Astrocytes synthesize and release large quantities of glutamine, which is taken up by neurons to replenish the glutamate and GABA neurotransmitter pools. Astrocyte glutamine hereby sustains the glutamate/GABA-glutamine cycle, synaptic transmission and general brain function. Cerebral glutamine homeostasis is linked to the metabolic coupling of neurons and astrocytes, and relies on multiple cellular processes, including TCA cycle function, synaptic transmission and neurotransmitter uptake. Dysregulations of processes related to glutamine homeostasis are associated with several neurological diseases and may mediate excitotoxicity and neurodegeneration. In particular, diminished astrocyte glutamine synthesis is a common neuropathological component, depriving neurons of an essential metabolic substrate and precursor for neurotransmitter synthesis, hereby leading to synaptic dysfunction. While astrocyte glutamine synthesis is quantitatively dominant in the brain, oligodendrocyte-derived glutamine may serve important functions in white matter structures. In this review, the crucial roles of glial glutamine homeostasis in the healthy and diseased brain are discussed. First, we provide an overview of cellular recycling, transport, synthesis and metabolism of glutamine in the brain. These cellular aspects are subsequently discussed in relation to pathological glutamine homeostasis of hepatic encephalopathy, epilepsy, Alzheimer's disease, Huntington's disease and amyotrophic lateral sclerosis. Further studies on the multifaceted roles of cerebral glutamine will not only increase our understanding of the metabolic collaboration between brain cells, but may also aid to reveal much needed therapeutic targets of several neurological pathologies.

Hippocampus mitochondrial MnSOD activation by a SIRT3 activator, honokiol, correlates with its deacetylation and upregulation of FoxO3a and PGC1α in a rat model of ammonia neurotoxicity

We have recently reported that honokiol (HKL), by activating mitochondrial SIRT3, normalizes reactive oxygen species level and mitochondrial integrity in hippocampus of the moderate grade hepatic encephalopathy (MoHE) rat model of ammonia neurotoxicity. To delineate the mechanism by which HKL does so, the present study describes activity versus level of the deacetylated mitochondrial Mn-superoxide dismutase (MnSOD) and expression of MnSOD versus levels of its main transcription regulators, FoxO3a and PGC1α, in the hippocampus of the MoHE rats. MoHE in rat was developed by administration of 100 mg/kg bw thioacetamide i.p. for 10 days. The study parameters were compared between the control, the MoHE rats and the MoHE rats treated with HKL (10 mg/Kg b.w.) for 7 days. As compared to control, the hippocampus mitochondria from MoHE rats showed a significantly declined activity of MnSOD vs enhanced lipid peroxidation coinciding with the increased level of its acetylated form. The HKL treatment could, however, normalize all these parameters in those MoHE rats. Also, a significantly reduced expression of MnSOD in the hippocampus of the MoHE rats coincided with a similar decline in transcript level of Foxo3a and Pgc1α. This was consistent with the reduced level of immuno-stained Foxo3a and Pgc1α proteins in hippocampus DG, CA1 and CA3 regions of those MoHE rats. However, all these factors were observed to be restored back to their normal levels due to the treatment with HKL. As HKL is a specific activator of mitochondrial SIRT3, these findings suggest involvement of Sirt3 activation led deacetylation of MnSOD and upregulation of its transcription activators, FoxO3a and PGC1α, in restoring mitochondrial MnSOD level in the hippocampus of the MoHE rat model of ammonia neurotoxicity.

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.

Overview of oxidative stress findings in hepatic encephalopathy: From cellular and ammonium-based animal models to human data

Oxidative stress is a natural phenomenon in the body. Under physiological conditions intracellular reactive oxygen species (ROS) are normal components of signal transduction cascades, and their levels are maintained by a complex antioxidants systems participating in the in-vivo redox homeostasis. Increased oxidative stress is present in several chronic diseases and interferes with phagocytic and nervous cell functions, causing an up-regulation of cytokines and inflammation. Hepatic encephalopathy (HE) occurs in both acute liver failure (ALF) and chronic liver disease. Increased blood and brain ammonium has been considered as an important factor in pathogenesis of HE and has been associated with inflammation, neurotoxicity, and oxidative stress. The relationship between ROS and the pathophysiology of HE is still poorly understood. Therefore, sensing ROS production for a better understanding of the relationship between oxidative stress and functional outcome in HE pathophysiology is critical for determining the disease mechanisms, as well as to improve the management of patients. This review is emphasizing the important role of oxidative stress in HE development and documents the changes occurring as a consequence of oxidative stress augmentation based on cellular and ammonium-based animal models to human data.

Dysregulation of Astrocytic Glutamine Transport in Acute Hyperammonemic Brain Edema

Acute liver failure (ALF) impairs ammonia clearance from blood, which gives rise to acute hyperammonemia and increased ammonia accumulation in the brain. Since in brain glutamine synthesis is the only route of ammonia detoxification, hyperammonemia is as a rule associated with increased brain glutamine content (glutaminosis) which correlates with and contributes along with ammonia itself to hyperammonemic brain edema-associated with ALF. This review focuses on the effects of hyperammonemia on the two glutamine carriers located in the astrocytic membrane: Slc38a3 (SN1, SNAT3) and Slc7a6 (y + LAT2). We emphasize the contribution of the dysfunction of either of the two carriers to glutaminosis- related aspects of brain edema: retention of osmotically obligated water (Slc38a3) and induction of oxidative/nitrosative stress (Slc7a6). The changes in glutamine transport link glutaminosis- evoked mitochondrial dysfunction to oxidative-nitrosative stress as formulated in the “Trojan Horse” hypothesis.

Hyperammonemia in Inherited Metabolic Diseases

Ammonia is a neurotoxic compound which is detoxified through liver enzymes from urea cycle. Several inherited or acquired conditions can elevate ammonia concentrations in blood, causing severe damage to the central nervous system due to the toxic effects exerted by ammonia on the astrocytes. Therefore, hyperammonemic patients present potentially life-threatening neuropsychiatric symptoms, whose severity is related with the hyperammonemia magnitude and duration, as well as the brain maturation stage. Inherited metabolic diseases caused by enzymatic defects that compromise directly or indirectly the urea cycle activity are the main cause of hyperammonemia in the neonatal period. These diseases are mainly represented by the congenital defects of urea cycle, classical organic acidurias, and the defects of mitochondrial fatty acids oxidation, with hyperammonemia being more severe and frequent in the first two groups mentioned. An effective and rapid treatment of hyperammonemia is crucial to prevent irreversible neurological damage and it depends on the understanding of the pathophysiology of the diseases, as well as of the available therapeutic approaches. In this review, the mechanisms underlying the hyperammonemia and neurological dysfunction in urea cycle disorders, organic acidurias, and fatty acids oxidation defects, as well as the therapeutic strategies for the ammonia control will be discussed.

Mitochondrial Targeting of the Ammonia-Sensitive Uncoupler SLC4A11 by the Chaperone-Mediated Carrier Pathway in Corneal Endothelium

Purpose

SLC4A11, an electrogenic H⁺ transporter, is found in the plasma membrane and mitochondria of corneal endothelium. However, the underlying mechanism of SLC4A11 targeting to mitochondria is unknown.

Methods

The presence of mitochondrial targeting sequences was examined using in silico mitochondrial proteomic analyses. Thiol crosslinked peptide binding to SLC4A11 was screened by untargeted liquid chromatography/tandem mass spectrometry (LC-MS/MS) analysis. Direct protein interactions between SLC4A11 and chaperones were examined using coimmunoprecipitation analysis and proximity ligation assay. Knockdown or pharmacologic inhibition of chaperones in human corneal endothelial cells (HCECs) or mouse corneal endothelial cells (MCECs), ex vivo kidney, or HA-SLC4A11–transfected fibroblasts was performed to investigate the functional consequences of interfering with mitochondrial SLC4A11 trafficking.

Results

SLC4A11 does not contain canonical N-terminal mitochondrial targeting sequences. LC-MS/MS analysis showed that HSC70 and/or HSP90 are bound to HA-SLC4A11–transfected PS120 fibroblast whole-cell lysates or isolated mitochondria, suggesting trafficking through the chaperone-mediated carrier pathway. SLC4A11 and either HSP90 or HSC70 complexes are directly bound to the mitochondrial surface receptor, TOM70. Interference with this trafficking leads to dysfunctional mitochondrial glutamine catabolism and increased reactive oxygen species production. In addition, glutamine (Gln) use upregulated SLC4A11, HSP70, and HSP90 expression in whole-cell lysates or purified mitochondria of HCECs and HA-SLC4A11–transfected fibroblasts.

Conclusions

HSP90 and HSC70 are critical in mediating mitochondrial SLC4A11 translocation in corneal endothelial cells and kidney. Gln promotes SLC4A11 import to the mitochondria, and the continuous oxidative stress derived from Gln catabolism induced HSP70 and HSP90, protecting cells against oxidative stress.

Altered motor cortical plasticity in patients with hepatic encephalopathy: A paired associative stimulation study

OBJECTIVE

Hepatic encephalopathy (HE) is a potentially reversible brain dysfunction caused by liver failure. Altered synaptic plasticity is supposed to play a major role in the pathophysiology of HE. Here, we used paired associative stimulation with an inter-stimulus interval of 25 ms (PAS25), a transcranial magnetic stimulation (TMS) protocol, to test synaptic plasticity of the motor cortex in patients with manifest HE.

METHODS

23 HE-patients and 23 healthy controls were enrolled in the study. Motor evoked potential (MEP) amplitudes were assessed as measure for cortical excitability. Time courses of MEP amplitude changes after the PAS25 intervention were compared between both groups.

RESULTS

MEP-amplitudes increased after PAS25 in the control group, indicating PAS25-induced synaptic plasticity in healthy controls, as expected. In contrast, MEP-amplitudes within the HE group did not change and were lower than in the control group, indicating no induction of plasticity.

CONCLUSIONS

Our study revealed reduced synaptic plasticity of the primary motor cortex in HE.

SIGNIFICANCE

Reduced synaptic plasticity in HE provides a link between pathological changes on the molecular level and early clinical symptoms of the disease. This decrease may be caused by disturbances in the glutamatergic neurotransmission due to the known hyperammonemia in HE patients.

Pathomechanisms in hepatic encephalopathy

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

SOFAMONIA: Comparison of the original SOFA score with the proposed new score including serum ammonia

Introduction:

Hyperammonemia can represent organic dysfunction of the brain, kidney, or liver. Evaluation of serum ammonia concentrations as a parameter for organ dysfunction may be justified.

Objective:

To evaluate the performance of serum ammonia as an additional or substitute variable for organ systems in the Sequential Organ Failure Assessment (SOFA) score.

Methods:

A prospective cohort study including 173 patients admitted to the intensive care unit between March 2015 and February 2016. SOFAMONIA scores were defined as follows: SOFAMONIA1 (Glasgow coma scale replaced by serum ammonia), SOFAMONIA2 (serum bilirubin replaced by serum ammonia), SOFAMONIA3 (renal system score replaced by serum ammonia), and SOFAMONIA4 (addition of serum ammonia to the original SOFA as the seventh variable, changing the maximum score from 24 to 28).

Results:

The original SOFA presented an area under the curve–receiver operating characteristic of 0.697 to predict hospital mortality. There was a positive correlation between SOFA and SOFAMONIA scores. SOFAMONIA1 presented a cut-off point of 5 for area under the curve 0.684 (0.610–0.753, 95% confidence interval), SOFAMONIA2 presented a cut-off point of 9 for area under the curve 0.701 (0.626–0.768, 95% confidence interval), SOFAMONIA3 presented a cut-off point of 8 for area under the curve 0.674 (0.598–0.743, 95% confidence interval), and SOFAMONIA4 presented a cut-off point of 8 for area under the curve 0.702 (0.628–0.769, 95% confidence interval).

Conclusions:

The addition of ammonia as the seventh parameter of the SOFA score showed the best performance to predict hospital mortality. The addition of ammonia as a representative of metabolic dysfunction may be useful in the follow-up of critically ill patients.

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.

Hepatic Encephalopathy and Nutrition Influences: A Narrative Review

Hepatic encephalopathy (HE) is a potentially reversible neurocognitive condition seen in patients with advanced liver disease. The overt form of HE has been reported in up to 45% of patients with cirrhosis. This debilitating condition is associated with increased morbidity and mortality and imposes a significant burden on the caregivers and healthcare system. After providing an overview of HE epidemiology and pathophysiology, this review focuses on the interaction of HE and frailty, nutrition requirements and recommendations in cirrhotic patients with HE, and current dietary and pharmacologic options for HE treatment.

Ammonia sensitive SLC4A11 mitochondrial uncoupling reduces glutamine induced oxidative stress

SLC4A11 is a NH₃ sensitive membrane transporter with H⁺ channel-like properties that facilitates Glutamine catabolism in Human and Mouse corneal endothelium (CE). Loss of SLC4A11 activity induces oxidative stress and cell death, resulting in Congenital Hereditary Endothelial Dystrophy (CHED) with corneal edema and vision loss. However, the mechanism by which SLC4A11 prevents ROS production and protects CE is unknown. Here we demonstrate that SLC4A11 is localized to the inner mitochondrial membrane of CE and SLC4A11 transfected PS120 fibroblasts, where it acts as an NH₃-sensitive mitochondrial uncoupler that enhances glutamine-dependent oxygen consumption, electron transport chain activity, and ATP levels by suppressing damaging Reactive Oxygen Species (ROS) production. In the presence of glutamine, Slc4a11^-/- (KO) mouse CE generate significantly greater mitochondrial superoxide, a greater proportion of damaged depolarized mitochondria, and more apoptotic cells than WT. KO CE can be rescued by MitoQ, reducing NH₃ production by GLS1 inhibition or dimethyl αKetoglutarate supplementation, or by BAM15 mitochondrial uncoupling. Slc4a11 KO mouse corneal edema can be partially reversed by αKetoglutarate eye drops. Moreover, we demonstrate that this role for SLC4A11 is not specific to CE cells, as SLC4A11 knockdown in glutamine-addicted colon carcinoma cells reduced glutamine catabolism, increased ROS production, and inhibited cell proliferation. Overall, our studies reveal a unique metabolic mechanism that reduces mitochondrial oxidative stress while promoting glutamine catabolism.

Updates on the pathophysiology and therapeutic targets for hepatic encephalopathy

PURPOSE OF REVIEW

Hepatic encephalopathy is one of the most debilitating clinical manifestations of cirrhosis and associated with increased morbidity and mortality. Treatment modalities available include the nonabsorbable disaccharides (lactulose) and the nonabsorbable antibiotics (rifaximin).

RECENT FINDINGS

Newer therapeutic targets under evaluation include ammonia scavengers (ornithine phenylacetate) and modulation of gut microbiota (fecal microbiota transplantation).

SUMMARY

This review will focus on the pathophysiology of hepatic encephalopathy along with an update on therapeutic targets under investigation.

Organ-specific responses to total ammonia nitrogen stress on juvenile grass carp (Ctenopharyngodon idellus)

Fish are important in constructed wetland (CW) ecosystem. An 80-day experiment was conducted by exposing juvenile grass carp (Ctenopharyngodon idellus) to 0, 0.5, 2.0, 4.5, 9.0, and 18.0 mg L^-1 total ammonia nitrogen (TAN) stress to determine the severity of physiological changes in fish organs (liver, gills, and muscle) in CW. Specific growth rate results indicated that low TAN (≤ 2.0 mg L^-1) help maintain or enhance grass carp growth. Fish physiological indexes did not significantly change during exposure, except for the gill's reactive oxygen species (ROS) level that is susceptible to TAN exposure. Under high TAN (≥ 4.5 mg L^-1), physiological changes and organ-specific responses were revealed. The ROS and malondialdehyde levels were higher in the gills than in the liver. At 9.0 mg L^-1 TAN, the muscle cells manifested toxicity. The antioxidant system of different organs responded differently because the gills were more susceptible to low TAN than other organs. After TAN removal from the low TAN system, the antioxidative enzymes and antioxidants were increased to scavenge extra ROS and reverted to the normal level. However, grass carp cannot recover from the oxidative damage at ≥ 9.0 mg L^-1 external TAN, resulting in organ dysfunction and failed ROS scavenging. This study provides information in maintaining CW sustainability.

Ginkgo biloba extract and its diterpene ginkgolide constituents ameliorate the metabolic disturbances caused by recombinant tissue plasminogen activator in rat prefrontal cortex

INTRODUCTION

Although recombinant tissue plasminogen activator (rtPA) is a widely used therapy in patients with acute ischemic stroke, rtPA-induced toxicity or its adverse effects have been reported in our previous studies. However, Ginkgo biloba extract (GBE) may provide neuroprotective effects against rtPA-induced toxicity. Thus, in the present study, we investigated whether a single administration of rtPA caused neurotoxicity in the prefrontal cortex (PFC) of rats and determined whether GBE or its diterpene ginkgolide (DG) constituents were neuroprotective against any rtPA-induced toxicity.

MATERIALS AND METHODS

We randomly divided adult Sprague-Dawley rats into four groups that were intravenously administered saline, rtPA, rtPA+DG, or rtPA+GBE. The rats were sacrificed 24 hours later and the whole brain removed. A gas chromatography-mass spectrometry metabolomic approach was used to detect molecular changes in the PFC among the groups. Multivariate statistical and pathway analyses were used to determine the relevant metabolites as well as their functions and pathways.

RESULTS

We found 32 metabolites differentially altered in the four groups that were primarily involved in neurotransmitter, amino acid, energy, lipid, and nucleotide metabolism. Our results indicated that a single rtPA administration caused metabolic disturbances in the PFC. Both GBE and DG effectively ameliorated these rtPA-induced disturbances, although DG better controlled the rtPA-induced glutamate and aspartate excitotoxicity and the activation of NMDA receptor.

CONCLUSION

Our results provide important novel mechanistic insights into the adverse effects of rtPA and offer directions for future exploration on the thrombolytic effects of rtPA combined with the administration of DG or GBE for the treatment of acute ischemic stroke in humans.

Investigation of dermal toxicity of ionic liquids in monolayer-cultured skin cells and 3D reconstructed human skin models

Ionic liquids have gained increasing attention in the chemical industry as potential green substitutes for traditional solvents. However, little is known about toxicity of ionic liquids on the skin, a major exposure portal to toxic substances. Here, we evaluated dermal toxicity of ionic liquids using human keratinocyte and fibroblast cell line, 3D reconstructed human epidermis, and full-thickness model to investigate underlying mechanisms. Cytotoxicity of ionic liquids was evaluated for representative anions, [TFSI], [PF₆], [BF₄], and [DCA], as well as for cations, [EMIM], [BMPY], [TBA] and [Zn], in human keratinocyte cell line, HaCaT, and human dermal fibroblasts. In our results, significant cytotoxicity was induced by ionic liquids with [TFSI] in both cell lines. Notably, cytotoxicity of [TFSI] containing ionic liquids was comparable to xylene, a toxic conventional organic solvent. Fluorescent and flow cytometric analysis revealed that [TFSI]-exposed cells underwent necrotic cell death. Reactive oxygen species (ROS) was increased while the amount of glutathione was decreased by [TFSI] in dose-dependent manner, which was reversed by antioxidant, N-acetylcysteine. In 3D reconstructed human epidermis and full-thickness model, a single application of [TFSI] induced toxicity although it was minimal and largely limited to epidermal layer. Collectively, these results demonstrated potential dermal toxicity of ionic liquids.

Hepatic Encephalopathy: An Update on the Pathophysiology and Therapeutic Options

Hepatic encephalopathy is a spectrum of reversible neuropsychiatric abnormalities, seen in patients with liver dysfunction and/or portosystemic shunting. One of the most debilitating complications of cirrhosis, encephalopathy affects 30-45% of cirrhotics. In addition to significantly affecting the lives of patients and their caregivers, it is also associated with increased morbidity and mortality as well as significant utilization of health care resources. In this paper, we provide an overview on the pathophysiology, diagnosis, management and newer therapies of hepatic encephalopathy.

Influence of clove oil and eugenol on muscle contraction of silkworm (Bombyx mori)

Clove oil is used in fish anesthesia and expected to have a mechanism via glutamic receptor. The present study explores the activities of clove oil and its major compound, eugenol, in comparison with L-glutamic acid on glutamic receptor of silkworm muscle and fish anesthesia. It was found that clove oil and eugenol had similar effects to L-glutamic acid on inhibition of silkworm muscle contraction after treated with D-glutamic acid and kainic acid. Anesthetic activity of the test samples was investigated in goldfish. The results demonstrated that L-glutamic acid at 20 and 40 mM could induce the fish to stage 3 of anesthesia that the fish exhibited total loss of equilibrium and muscle tone, whereas clove oil and eugenol at 60 ppm could induce the fish to stage 4 of anesthesia that the reflex activity of the fish was lost. These results suggest that clove oil and eugenol have similar functional activities and mechanism to L-glutamic acid on muscle contraction and fish anesthesia.

Accumulation of lactate in the rat brain during hyperammonaemia is not associated with impaired mitochondrial respiratory capacity

In acute liver failure (ALF) cerebral oedema and high intracranial pressure (ICP) are potentially deadly complications. Astrocytes cultured in ammonia have shown mitochondrial dysfunction and in rat models of liver failure, de novo lactate production in the brain has been observed and has led to a hypothesis of compromised brain metabolism during ALF. In contrast, normal lactate levels are found in cerebral microdialysate of ALF patients and the oxygen: glucose ratio of cerebral metabolic rates remains normal. To investigate this inconsistency we studied the mitochondrial function in brain tissue with respirometry in animal models of hyperammonaemia. Wistar rats with systemic inflammation induced by lipopolysaccharide or liver insufficiency induced by 90% hepatectomy were given ammonium or sodium acetate for 120 min. A cerebral cortex homogenate was studied with respirometry and substrates of the citric acid cycle, uncouplers and inhibitors of the mitochondrial complexes were successively added to investigate the mitochondrial function in detail. In a separate dose-response experiment cortex from healthy rats was incubated for 120 min in ammonium acetate in concentrations up to 80 mM prior to respirometry. Hyperammonaemia was associated with elevated ICP and increased tissue lactate concentration. No difference between groups was found in total respiratory capacity or the function of individual mitochondrial complexes. Ammonium in concentrations of 40 and 80 mM reduced the respiratory capacity in vitro. In conclusion, acute hyperammonaemia leads to elevated ICP and cerebral lactate accumulation. We found no indications of impaired oxidative metabolism in vivo but only in vitro at extreme concentrations of ammonium.

Neuroinflammation in liver disease: sessional talks from ISHEN

At the recent ISHEN ('International Symposium of Hepatic Encephalopathy & Nitrogen Metabolism') conference in London, a whole session was dedicated to our increasing awareness of the importance of inflammation in the brain - termed 'neuroinflammation', in the development of Hepatic Encephalopathy (HE) - the neurological manifestations of advanced liver disease. In this review our ISHEN speakers further discuss the content of their sessional presentations and more broadly we discuss our understanding of the role of neuroinflammation in HE pathogenesis.

The Role of RRT in Hyperammonemic Patients

Hyperammonemia is an important cause of cerebral edema in both adults with liver failure and children with inborn errors of metabolism. There are few studies that have analyzed the role of extracorporeal dialysis in reducing blood ammonia levels in the adult population. Furthermore, there are no firm guidelines about when to implement RRT, because many of the conditions that are characterized by hyperammonemia are extremely rare. In this review of existing literature on RRT, we present the body's own mechanisms for clearing ammonia as well as the dialytic properties of ammonia. We review the available literature on the use of continuous venovenous hemofiltration, peritoneal dialysis, and hemodialysis in neonates and adults with conditions characterized by hyperammonemia and discuss some of the controversies that exist over selecting one modality over another.

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.

Multifactorial Effects on Different Types of Brain Cells Contribute to Ammonia Toxicity

Effects of ammonia on astrocytes play a major role in hepatic encephalopathy, acute liver failure and other diseases caused by increased arterial ammonia concentrations (e.g., inborn errors of metabolism, drug or mushroom poisoning). There is a direct correlation between arterial ammonia concentration, brain ammonia level and disease severity. However, the pathophysiology of hyperammonemic diseases is disputed. One long recognized factor is that increased brain ammonia triggers its own detoxification by glutamine formation from glutamate. This is an astrocytic process due to the selective expression of the glutamine synthetase in astrocytes. A possible deleterious effect of the resulting increase in glutamine concentration has repeatedly been discussed and is supported by improvement of some pathologic effects by GS inhibition. However, this procedure also inhibits a large part of astrocytic energy metabolism and may prevent astrocytes from responding to pathogenic factors. A decrease of the already low glutamate concentration in astrocytes due to increased synthesis of glutamine inhibits the malate-aspartate shuttle and energy metabolism. A more recently described pathogenic factor is the resemblance between NH₄⁺ and K⁺ in their effects on the Na⁺,K⁺-ATPase and the Na⁺,K⁺, 2 Cl^- and water transporter NKCC1. Stimulation of the Na⁺,K⁺-ATPase driven NKCC1 in both astrocytes and endothelial cells is essential for the development of brain edema. Na⁺,K⁺-ATPase stimulation also activates production of endogenous ouabains. This leads to oxidative and nitrosative damage and sensitizes NKCC1. Administration of ouabain antagonists may accordingly have therapeutic potential in hyperammonemic diseases.

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

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

Refining the ammonia hypothesis: a physiology-driven approach to the treatment of hepatic encephalopathy

Hepatic encephalopathy (HE) is one of the most important complications of cirrhosis and portal hypertension. Although the etiology is incompletely understood, it has been linked to ammonia directly and indirectly. Our goal is to review for the clinician the mechanisms behind hyperammonemia and the pathogenesis of HE to explain the rationale for its therapy. We reviewed articles collected through a search of MEDLINE/PubMed, Cochrane Database of Systematic Reviews, and Google Scholar between October 1, 1948, and December 8, 2014, and by a manual search of citations within retrieved articles. Search terms included hepatic encephalopathy, ammonia hypothesis, brain and ammonia, liver failure and ammonia, acute-on-chronic liver failure and ammonia, cirrhosis and ammonia, portosytemic shunt, ammonia and lactulose, rifaximin, zinc, and nutrition. Ammonia homeostatsis is a multiorgan process involving the liver, brain, kidneys, and muscle as well as the gastrointestinal tract. Indeed, hyperammonemia may be the first clue to poor functional reserves, malnutrition, and impending multiorgan dysfunction. Furthermore, the neuropathology of ammonia is critically linked to states of systemic inflammation and endotoxemia. Given the complex interplay among ammonia, inflammation, and other factors, ammonia levels have questionable utility in the staging of HE. The use of nonabsorbable disaccharides, antibiotics, and probiotics reduces gut ammoniagenesis and, in the case of antibiotics and probiotics, systemic inflammation. Nutritional support preserves urea cycle function and prevents wasting of skeletal muscle, a significant site of ammonia metabolism. Correction of hypokalemia, hypovolemia, and acidosis further assists in the reduction of ammonia production in the kidney. Finally, early and aggressive treatment of infection, avoidance of sedatives, and modification of portosystemic shunts are also helpful in reducing the neurocognitive effects of hyperammonemia. Refining the ammonia hypothesis to account for these other factors instructs a solid foundation for the effective treatment and prevention of hepatic encephalopathy.

Dysregulation of glutamine transporter SNAT1 in Rett syndrome microglia: a mechanism for mitochondrial dysfunction and neurotoxicity

Rett syndrome (RTT) is an autism spectrum disorder caused by loss-of-function mutations in the gene encoding MeCP2, an epigenetic modulator that binds the methyl CpG dinucleotide in target genes to regulate transcription. Previously, we and others reported a role of microglia in the pathophysiology of RTT. To understand the mechanism of microglia dysfunction in RTT, we identified a MeCP2 target gene, SLC38A1, which encodes a major glutamine transporter (SNAT1), and characterized its role in microglia. We found that MeCP2 acts as a microglia-specific transcriptional repressor of SNAT1. Because glutamine is mainly metabolized in the mitochondria, where it is used as an energy substrate and a precursor for glutamate production, we hypothesize that SNAT1 overexpression in MeCP2-deficient microglia would impair the glutamine homeostasis, resulting in mitochondrial dysfunction as well as microglial neurotoxicity because of glutamate overproduction. Supporting this hypothesis, we found that MeCP2 downregulation or SNAT1 overexpression in microglia resulted in (1) glutamine-dependent decrease in microglial viability, which was corroborated by reduced microglia counts in the brains of MECP2 knock-out mice; (2) proliferation of mitochondria and enhanced mitochondrial production of reactive oxygen species; (3) increased oxygen consumption but decreased ATP production (an energy-wasting state); and (4) overproduction of glutamate that caused NMDA receptor-dependent neurotoxicity. The abnormalities could be rectified by mitochondria-targeted expression of catalase and a mitochondria-targeted peptide antioxidant, Szeto-Schiller 31. Our results reveal a novel mechanism via which MeCP2 regulates bioenergetic pathways in microglia and suggest a therapeutic potential of mitochondria-targeted antioxidants for RTT.

Expression of glutamine transporter isoforms in cerebral cortex of rats with chronic hepatic encephalopathy

Hepatic encephalopathy (HE) is a neuropsychiatric disorder that occurs due to acute and chronic liver diseases, the hallmark of which is the increased levels of ammonia and subsequent alterations in glutamine synthesis, i.e. conditions associated with the pathophysiology of HE. Under physiological conditions, glutamine is fundamental for replenishment of the neurotransmitter pools of glutamate and GABA. The different isoforms of glutamine transporters play an important role in the transfer of this amino acid between astrocytes and neurons. A disturbance in the GABA biosynthetic pathways has been described in bile duct ligated (BDL) rats, a well characterized model of chronic HE. Considering that glutamine is important for GABA biosynthesis, altered glutamine transport and the subsequent glutamate/GABA-glutamine cycle efficacy might influence these pathways. Given this potential outcome, the aim of the present study was to investigate whether the expression of the glutamine transporters SAT1, SAT2, SN1 and SN2 would be affected in chronic HE. We verified that mRNA expression of the neuronal glutamine transporters SAT1 and SAT2 was found unaltered in the cerebral cortex of BDL rats. Similarly, no changes were found in the mRNA level for the astrocytic transporter SN1, whereas the gene expression of SN2 was increased by two-fold in animals with chronic HE. However, SN2 protein immuno-reactivity did not correspond with the increase in gene transcription since it remained unaltered. These data indicate that the expression of the glutamine transporter isoforms is unchanged during chronic HE, and thus likely not to participate in the pathological mechanisms related to the imbalance in the GABAergic neurotransmitter system observed in this neurologic condition.

Cerebral effects of ammonia in liver disease: current hypotheses

Hyperammonemia is necessary for development of the cerebral complications to liver disease including hepatic encephalopathy and cerebral edema but the mechanisms are unclear. Ammonia is taken up by the brain in proportion to its arterial concentration. The flux into the brain is most likely by both diffusion of NH3 and mediated transport of NH4 (+) . Astrocytic detoxification of ammonia involves formation of glutamine at concentrations high enough to produce cellular edema, but compensatory mechanisms reduce this effect. Glutamine can be taken up by astrocytic mitochondria and initiate the mitochondrial permeability transition but the clinical relevance is uncertain. Elevated astrocytic glutamine interferes with neurotransmission. Thus, animal studies show enhanced glutamatergic neurotransmission via the NMDA receptor which may be related to the acute cerebral complications to liver failure, while impairment of the NMDA activated glutamate-NO-cGMP pathway could relate to the behavioural changes seen in hepatic encephalopathy. Elevated glutamine also increases GABA-ergic tone, an effect which is aggravated by mitochondrial production of neurosteroids; this may relate to decreased neurotransmission and precipitation of encephalopathy by GABA targeting drugs. Hyperammonemia may compromise cerebral energy metabolism as elevated cerebral lactate is generally reported. Hypoxia is unlikely since cerebral oxygen:glucose utilisation and lactate:pyruvate ratio are both normal in clinical studies. Ammonia inhibits α-ketoglutaratedehydrogenase in isolated mitochondria, but the clinical relevance is dubious due to the observed normal cerebral oxygen:glucose utilization. Recent studies suggest that ammonia stimulates glycolysis in excess of TCA cycle activity, a hypothesis that may warrant further testing, in being in accordance with the limited clinical observations.

Ammonia metabolism and hyperammonemic disorders

Human adults produce around 1000 mmol of ammonia daily. Some is reutilized in biosynthesis. The remainder is waste and neurotoxic. Eventually most is excreted in urine as urea, together with ammonia used as a buffer. In extrahepatic tissues, ammonia is incorporated into nontoxic glutamine and released into blood. Large amounts are metabolized by the kidneys and small intestine. In the intestine, this yields ammonia, which is sequestered in portal blood and transported to the liver for ureagenesis, and citrulline, which is converted to arginine by the kidneys. The amazing developments in NMR imaging and spectroscopy and molecular biology have confirmed concepts derived from early studies in animals and cell cultures. The processes involved are exquisitely tuned. When they are faulty, ammonia accumulates. Severe acute hyperammonemia causes a rapidly progressive, often fatal, encephalopathy with brain edema. Chronic milder hyperammonemia causes a neuropsychiatric illness. Survivors of severe neonatal hyperammonemia have structural brain damage. Proposed explanations for brain edema are an increase in astrocyte osmolality, generally attributed to glutamine accumulation, and cytotoxic oxidative/nitrosative damage. However, ammonia neurotoxicity is multifactorial, with disturbances also in neurotransmitters, energy production, anaplerosis, cerebral blood flow, potassium, and sodium. Around 90% of hyperammonemic patients have liver disease. Inherited defects are rare. They are being recognized increasingly in adults. Deficiencies of urea cycle enzymes, citrin, and pyruvate carboxylase demonstrate the roles of isolated pathways in ammonia metabolism. Phenylbutyrate is used routinely to treat inherited urea cycle disorders, and its use for hepatic encephalopathy is under investigation.

Ammonia-induced senescence in cultured rat astrocytes and in human cerebral cortex in hepatic encephalopathy

Hepatic encephalopathy (HE) is a frequent complication of liver cirrhosis and is due to a low-grade cerebral edema associated with oxidative/nitrosative stress. Recent reports suggest that cognitive impairment in cirrhotic patients may not resolve completely after an attack of manifest HE. As astrocyte dysfunction is central to the pathogenesis of HE and astrocytes are critically involved in synaptic plasticity, we tested for sustained impairment of astrocyte function by analyzing expression levels of senescence biomarkers in ammonia-treated cultured rat astrocytes and in postmortem brain samples from cirrhotic patients with or without HE. NH4 Cl time- and dose-dependently inhibited proliferation of cultured astrocytes by up to 45% (5 mmol/L, 72 h) and strongly increased senescence-associated β-galactosidase activity. Inhibition of astrocyte proliferation by ammonia was mediated by a l-methionine sulfoximine-, oxidative stress-, and p38(MAPK) -dependent activation of p53 associated with enhanced transcription of cell cycle inhibitory genes GADD45α and p21. Mitochondria and the nucleus were identified as sources of oxygen radical formation after prolonged NH4 Cl exposure. Concurrently, NH4 Cl (5 mmol/L) treatment inhibited both epidermal growth factor- and brain-derived neurotrophic factor (BDNF)-induced proliferation as well as BDNF-mediated astrocyte morphology changes through downregulation of the respective growth factor receptors epidermal growth factor receptor and truncated tyrosine receptor kinase B. Increased mRNA expression levels of senescence-associated genes were also found in post mortem brain samples from patients with liver cirrhosis with HE, but not in those without HE. The data suggest that ammonia toxicity and HE are associated with premature astrocyte senescence, which may impair neurotransmission and contribute to persistence of cognitive disturbances after resolution of episodes of overt HE.

Ephrin/Ephrin receptor expression in ammonia-treated rat astrocytes and in human cerebral cortex in hepatic encephalopathy

Hepatic encephalopathy (HE) represents a neuropsychiatric syndrome, which evolves as a consequence of a low grade cerebral edema and a concomitant oxidative/nitrosative stress response. Ephrin receptors (EphR) and their ligands (ephrins) regulate astrocytic glutamate uptake and gliotransmitter release thereby governing neurotransmission, but their role in HE and ammonia toxicity is unclear. We therefore tested effects of ammonia on expression levels of EphR/ephrin isoforms in cultured rat astrocytes and analysed underlying mechanisms. NH4Cl induced mRNA expression changes of several EphR/ephrin isoforms in a methionine sulfoximine-, NADPH oxidase- and NO synthase-dependent manner in cultured astrocytes. A prominent upregulation was noted for EphR A4 mRNA and protein in NH4Cl-treated astrocytes. NH4Cl-treatment decreased EphR A4 molecular mass to similar extent as found in astrocytes treated with the N-glycosylation inhibitor tunicamycin. Knockdown of EphR A4 by siRNA, or treating astrocytes with NH4Cl or tunicamycin abolished fibroblast growth factor-induced and EphR A4-dependent astrocyte proliferation. NH4Cl-treatment also decreased GLAST mRNA levels in cultured astrocytes. This effect was sensitive to inhibitors of NAPDH oxidase or glutamine synthetase, but was insensitive to siRNA-mediated EphR A4 knockdown. Eph/ephrin gene expression changes were also found in post mortem brain samples of cirrhotic patients without or with HE compared to controls suggesting a potential in vivo relevance of the present findings. The present study suggests that ammonia modulates EphR/ephrin signaling in astrocytes and in the brain of cirrhotic patients with HE with potential implications for deranged neurotransmission in HE.

Up-regulation of brain cytokines and chemokines mediates neurotoxicity in early acute liver failure by a mechanism independent of microglial activation

The neurological involvement in acute liver failure (ALF) is characterized by arousal impairment with progression to coma. There is a growing body of evidence that neuroinflammatory mechanisms play a role in this process, including production of inflammatory cytokines and microglial activation. However, it is still uncertain whether brain-derived cytokines and glial cells are crucial to the pathophysiology of ALF at the early stage, before coma development. Here, we investigated the influence of cytokines and microglia in ALF-induced encephalopathy in mice as soon as neurological symptoms were identifiable. Behavior was assessed at 12, 24, 36 and 48 h post-injection of thioacetamide, a hepatotoxic drug, through locomotor activity by an open field test. Brain concentration of cytokines (TNF-α and IL-1β) and chemokines (CXCL1, CCL2, CCL3 and CCL5) were assessed by ELISA. Microglial activation in brain sections was investigated through immunohistochemistry, and cellular ultrastructural changes were observed by transmission electron microscopy. We found that ALF-induced animals presented a significant decrease in locomotor activity at 24 h, which was accompanied by an increase in IL-1β, CXCL1, CCL2, CCL3 and CCL5 in the brain. TNF-α level was significantly increased only at 36 h. Despite marked morphological changes in astrocytes and brain endothelial cells, no microglial activation was observed. These findings suggest an involvement of brain-derived chemokines and IL-1β in early pathophysiology of ALF by a mechanism independent of microglial activation.

Ammonia, like K(+), stimulates the Na(+), K(+), 2 Cl(-) cotransporter NKCC1 and the Na(+),K(+)-ATPase and interacts with endogenous ouabain in astrocytes

Brain edema during hepatic encephalopathy or acute liver failure as well as following brain ischemia has a multifactorial etiology, but it is a dangerous and occasionally life-threatening complication because the brain is enclosed in the rigid skull. During ischemia the extracellular K(+) concentration increases to very high levels, which when energy becomes available during reperfusion stimulate NKCC1, a cotransporter driven by the transmembrane ion gradients established by the Na(+),K(+)-ATPase and accumulating Na(+), K(+) and 2 Cl(-) together with water. This induces pronounced astrocytic swelling under pathologic conditions, but NKCC1 is probably also activated, although to a lesser extent, during normal brain function. Redistribution of ions and water between extra- and intracellular phases does not create brain edema, which in addition requires uptake across the blood-brain barrier. During hepatic encephalopathy and acute liver failure a crucial factor is the close resemblance between K(+) and NH4(+) in their effects not only on NKCC1 and Na(+),K(+)-ATPase but also on Na(+),K(+)-ATPase-induced signaling by endogenous ouabains. These in turn activate production of ROS and nitrosactive agents which slowly sensitize NKCC1, explaining why cell swelling and brain edema generally are delayed under hyperammonemic conditions, although very high ammonia concentrations can cause immediate NKCC1 activation.

Hepatic encephalopathy: how to test and treat

PURPOSE OF REVIEW

Hepatic encephalopathy causes significant cognitive impairment and morbidity in patients with cirrhosis; however, hepatic encephalopathy is considered a reversible syndrome once recognized clinically. Although hepatic encephalopathy is not a single clinical entity, the pathophysiology resulting in brain dysfunction is not fully understood, although it is believed that ammonia production is an important contributing factor. The purpose of this review is to highlight studies used to test for hepatic encephalopathy and those utilizing specific new treatments.

RECENT FINDINGS

A 'STROOP' smartphone app has been developed to allow clinicians to test for covert hepatic encephalopathy (CHE). Lactulose therapy was effective for cirrhotic patients as primary prophylaxis to prevent overt hepatic encephalopathy (OHE) episodes. In patients without prior OHE, probiotics can be useful in preventing OHE. Lactulose, probiotics, L-ornithine-L-aspartate, and potassium-iron-phosphate-citrate have been studied in the treatment of CHE. Rifaximin was found to be safe and well tolerated in long-term maintenance of remission from OHE; however, compared to lactulose therapy in CHE, it is not cost-effective.

SUMMARY

Refinement in clinical management strategies for patients with cirrhosis and hepatic encephalopathy appears to continue to contribute to improved patient outcomes.

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.

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

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

Brain edema in acute liver failure and chronic liver disease: similarities and differences

Hepatic encephalopathy (HE) is a complex neuropsychiatric syndrome that typically develops as a result of acute liver failure or chronic liver disease. Brain edema is a common feature associated with HE. In acute liver failure, brain edema contributes to an increase in intracranial pressure, which can fatally lead to brain stem herniation. In chronic liver disease, intracranial hypertension is rarely observed, even though brain edema may be present. This discrepancy in the development of intracranial hypertension in acute liver failure versus chronic liver disease suggests that brain edema plays a different role in relation to the onset of HE. Furthermore, the pathophysiological mechanisms involved in the development of brain edema in acute liver failure and chronic liver disease are dissimilar. This review explores the types of brain edema, the cells, and pathogenic factors involved in its development, while emphasizing the differences in acute liver failure versus chronic liver disease. The implications of brain edema developing as a neuropathological consequence of HE, or as a cause of HE, are also discussed.

Does microglial dysfunction play a role in autism and Rett syndrome?

Autism spectrum disorders (ASDs) including classic autism is a group of complex developmental disabilities with core deficits of impaired social interactions, communication difficulties and repetitive behaviors. Although the neurobiology of ASDs has attracted much attention in the last two decades, the role of microglia has been ignored. Existing data are focused on their recognized role in neuroinflammation, which only covers a small part of the pathological repertoire of microglia. This review highlights recent findings on the broader roles of microglia, including their active surveillance of brain microenvironments and regulation of synaptic connectivity, maturation of brain circuitry and neurogenesis. Emerging evidence suggests that microglia respond to pre- and postnatal environmental stimuli through epigenetic interface to change gene expression, thus acting as effectors of experience-dependent synaptic plasticity. Impairments of these microglial functions could substantially contribute to several major etiological factors of autism, such as environmental toxins and cortical underconnectivity. Our recent study on Rett syndrome, a syndromic autistic disorder, provides an example that intrinsic microglial dysfunction due to genetic and epigenetic aberrations could detrimentally affect the developmental trajectory without evoking neuroinflammation. We propose that ASDs provide excellent opportunities to study the influence of microglia on neurodevelopment, and this knowledge could lead to novel therapies.

Aspects of astrocyte energy metabolism, amino acid neurotransmitter homoeostasis and metabolic compartmentation

Astrocytes are key players in brain function; they are intimately involved in neuronal signalling processes and their metabolism is tightly coupled to that of neurons. In the present review, we will be concerned with a discussion of aspects of astrocyte metabolism, including energy-generating pathways and amino acid homoeostasis. A discussion of the impact that uptake of neurotransmitter glutamate may have on these pathways is included along with a section on metabolic compartmentation.

Mechanisms of glutamate efflux at the blood-brain barrier: involvement of glial cells

At high concentrations, glutamate (Glu) exerts potent neurotoxic properties, leading to irreversible brain damages found in numerous neurological disorders. The accepted notion that Glu homeostasis in brain interstitial fluid is maintained primarily through the activity of Glu transporters present on glial cells does not take into account the possible contribution of endothelial cells constituting the blood-brain barrier (BBB) to this process. Here, we present evidence for the presence of the Glu transporters, excitatory amino-acid transporters (EAATs) 1 to 3, in porcine brain endothelial cells (PBECs) and show their participation in Glu uptake into PBECs. Moreover, transport of Glu across three in vitro models of the BBB is investigated for the first time, and evidence for Glu transport across the BBB in both directions is presented. Our results provide evidence that the BBB can function in the efflux mode to selectively remove Glu, via specific transporters, from the abluminal side (brain) into the luminal compartment (blood). Furthermore, we found that glial cells lining the BBB have an active role in the efflux process by taking up Glu and releasing it, through hemichannels, anion channels, and possibly the reversal of its EAATs, in close proximity to ECs, which in turn take up Glu and release it to the blood.

Inhibition of glutamine synthesis induces glutamate dehydrogenase-dependent ammonia fixation into alanine in co-cultures of astrocytes and neurons

It has been previously demonstrated that ammonia exposure of neurons and astrocytes in co-culture leads to net synthesis not only of glutamine but also of alanine. The latter process involves the concerted action of glutamate dehydrogenase (GDH) and alanine aminotransferase (ALAT). In the present study it was investigated if the glutamine synthetase (GS) inhibitor methionine sulfoximine (MSO) would enhance alanine synthesis by blocking the GS-dependent ammonia scavenging process. Hence, co-cultures of neurons and astrocytes were incubated for 2.5h with [U-(13)C]glucose to monitor de novo synthesis of alanine and glutamine in the absence and presence of 5.0 mM NH(4)Cl and 10 mM MSO. Ammonia exposure led to increased incorporation of label but not to a significant increase in the amount of these amino acids. However, in the presence of MSO, glutamine synthesis was blocked and synthesis of alanine increased leading to an elevated content intra- as well as extracellularly of this amino acid. Treatment with MSO led to a dramatic decrease in glutamine content and increased the intracellular contents of glutamate and aspartate. The large increase in alanine during exposure to MSO underlines the importance of the GDH and ALAT biosynthetic pathway for ammonia fixation, and it points to the use of a GS inhibitor to ameliorate the brain toxicity and edema induced by hyperammonemia, events likely related to glutamine synthesis.

Role of astrocytes in brain function and disease

Astrocytes assume multiple roles in maintaining an optimally suited milieu for neuronal function. Select astrocytic functions include the maintenance of redox potential, the production of trophic factors, the regulation of neurotransmitter and ion concentrations, and the removal of toxins and debris from the cerebrospinal fluid (CSF). Impairments in these and other functions, as well as physiological reactions of astrocytes to injury, can trigger or exacerbate neuronal dysfunction. This review addresses select metabolic interactions between neurons and astrocytes and emphasizes the role of astrocytes in mediating and amplifying the progression of several neurodegenerative disorders, such as Parkinson's disease (PD), hepatic encephalopathy (HE), hyperammonemia (HA), Alzheimer's disease (AD), and ischemia.