A mouse model of hepatic encephalopathy: bile duct ligation induces brain ammonia overload, glial cell activation and neuroinflammation

Farnesoid X receptor agonist tropifexor detoxifies ammonia by regulating the glutamine metabolism and urea cycles in cholestatic livers

Hyperammonemia refers to elevated levels of ammonia in the blood, which is an important pathological feature of liver cirrhosis and hepatic failure. Preclinical studies suggest tropifexor (TXR), a novel non-bile acid agonist of Farnesoid X Receptor (FXR), has shown promising effects on reducing hepatic steatosis, inflammation, and fibrosis. This study evaluates the impact of TXR on hyperammonemia in a piglet model of cholestasis. We here observed blood ammonia significantly elevated in patients with biliary atresia (BA) and was positively correlated with liver injury. Targeted metabolomics and immunblotting showed glutamine metabolism and urea cycles were impaired in BA patients. Next, we observed that TXR potently suppresses bile duct ligation (BDL)-induced injuries in liver and brain with improving the glutamine metabolism and urea cycles. Within the liver, TXR enhances glutamine metabolism and urea cycles by up-regulation of key regulatory enzymes, including glutamine synthetase (GS), carbamoyl-phosphate synthetase 1 (CPS1), argininosuccinate synthetase (ASS1), argininosuccinate lyase (ASL), and arginase 1 (ARG1). In primary mice hepatocytes, TXR detoxified ammonia via increasing ureagenesis. Mechanically, TXR activating FXR to increase express enzymes that regulating ureagenesis and glutamine synthesis through a transcriptional approach. Together, these results suggest that TXR may have therapeutic implications for hyperammonemic conditions in cholestatic livers.

Interaction between Per- and Polyfluorinated Substances (PFAS) and Acetaminophen in Disease Exacerbation-Focusing on Autism and the Gut-Liver-Brain Axis

This review presents a new perspective on the exacerbation of autism spectrum disorder (ASD) by per- and polyfluoroalkyl substances (PFAS) through the gut-liver-brain axis. We have summarized evidence reported on the involvement of the gut microbiome and liver inflammation that led to the onset and exacerbation of ASD symptoms. As PFAS are toxicants that particularly target liver, this review has comprehensively explored the possible interaction between PFAS and acetaminophen, another liver toxicant, as the chemicals of interest for future toxicology research. Our hypothesis is that, at acute dosages, acetaminophen has the ability to aggravate the impaired conditions of the PFAS-exposed liver, which would further exacerbate neurological symptoms such as lack of social communication and interest, and repetitive behaviors using mechanisms related to the gut-liver-brain axis. This review discusses their potential interactions in terms of the gut-liver-brain axis and signaling pathways that may contribute to neurological diseases.

Ammonia scavenger and glutamine synthetase inhibitors cocktail in targeting mTOR/β-catenin and MMP-14 for nitrogen homeostasis and liver cancer

The glutamine synthetase (GS) facilitates cancer cell growth by catalyzing de novo glutamine synthesis. This enzyme removes ammonia waste from the liver following the urea cycle. Since cancer development is associated with dysregulated urea cycles, there has been no investigation of GS's role in ammonia clearance. Here, we demonstrate that, although GS expression is increased in the setting of β-catenin oncogenic activation, it is insufficient to clear the ammonia waste burden due to the dysregulated urea cycle and may thus be unable to prevent cancer formation. In vivo study, a total of 165 male Swiss albino mice allocated in 11 groups were used, and liver cancer was induced by p-DAB. The activity of GS was evaluated along with the relative expression of mTOR, β-catenin, MMP-14, and GS genes in liver samples and HepG2 cells using qRT-PCR. Moreover, the cytotoxicity of the NH3 scavenger phenyl acetate (PA) and/or GS-inhibitor L-methionine sulfoximine (MSO) and the migratory potential of cells was assessed by MTT and wound healing assays, respectively. The Swiss target prediction algorithm was used to screen the mentioned compounds for probable targets. The treatment of the HepG2 cell line with PA plus MSO demonstrated strong cytotoxicity. The post-scratch remaining wound area (%) in the untreated HepG2 cells was 2.0%. In contrast, the remaining wound area (%) in the cells treated with PA, MSO, and PA + MSO for 48 h was 61.1, 55.8, and 78.5%, respectively. The combination of the two drugs had the greatest effect, resulting in the greatest decrease in the GS activity, β-catenin, and mTOR expression. MSO and PA are both capable of suppressing mTOR, a key player in the development of HCC, and MMP-14, a key player in the development of HCC. PA inhibited the MMP-14 enzyme more effectively than MSO, implying that PA might be a better way to target HCC as it inhibited MMP-14 more effectively than MSO. A large number of abnormal hepatocytes (5%) were found to be present in the HCC mice compared to mice in the control group as determined by the histopathological lesions scores. In contrast, PA, MSO, and PA + MSO showed a significant reduction in the hepatic lesions score either when protecting the liver or when treating the liver. The molecular docking study indicated that PA and MSO form a three-dimensional structure with NF-κB and COX-II, blocking their ability to promote cancer and cause gene mutations. PA and MSO could be used to manipulate GS activities to modulate ammonia levels, thus providing a potential treatment for ammonia homeostasis.

Limitations of PLX3397 as a microglial investigational tool: peripheral and off-target effects dictate the response to inflammation

Microglia, the resident macrophages of the central nervous system (CNS), play a critical role in CNS homeostasis and neuroinflammation. Pexidartinib (PLX3397), a colony-stimulating factor 1 (CSF1) receptor inhibitor, is widely used to deplete microglia, offering flexible options for both long-term depletion and highly versatile depletion-repopulation cycles. However, the potential impact of PLX3397 on peripheral (immune) cells remains controversial. Until now, the microglia-specificity of this type of compounds has not been thoroughly evaluated, particularly in the context of peripherally derived neuroinflammation. Our study addresses this gap by examining the effects of PLX3397 on immune cells in the brain, liver, circulation and bone marrow, both in homeostasis and systemic inflammation models. Intriguingly, we demonstrate that PLX3397 treatment not only influences the levels of tissue-resident macrophages, but also affects circulating and bone marrow immune cells beyond the mononuclear phagocyte system (MPS). These alterations in peripheral immune cells disrupt the response to systemic inflammation, consequently impacting the phenotype irrespective of microglial depletion. Furthermore, we observed that a lower dose of PLX3397, which does not deplete microglia, demonstrates similar (non-)MPS effects, both in the periphery and the brain, but fails to fully replicate the peripheral alterations seen in the higher doses, questioning lower doses as a 'peripheral control' strategy. Overall, our data highlight the need for caution when interpreting studies employing this compound, as it may not be suitable for specific investigation of microglial function in the presence of systemic inflammation.

Hyperammonemia induces microglial NLRP3 inflammasome activation via mitochondrial oxidative stress in hepatic encephalopathy

BACKGROUND

The role of nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) inflammasome in the pathogenesis of hepatic encephalopathy (HE) is unclear. Mitochondrial reactive oxygen species (mtROS) is a signal for NLRP3 inflammasome activation. Therefore, we aimed to determine whether mtROS-dependent NLRP3 inflammasome activation is involved in HE, using in vivo and in vitro models.

METHODS

Bile duct ligation (BDL) in C57/BL6 mice was used as an in vivo HE model. NLRP3 activation was assessed in the hippocampus. Immunofluorescence staining was performed to determine the cellular source of NLRP3 in the hippocampal tissue. For the in vitro experiment, BV-2 microglial cells were primed with lipopolysaccharide (LPS), followed by ammonia treatment. NLRP3 activation and mitochondrial dysfunction were measured. Mito-TEMPO was used to suppress mtROS production.

RESULTS

BDL mice showed cognitive impairment with hyperammonemia. Both the priming and activation steps of NLRP3 inflammasome activation were processed in the hippocampus of BDL mice. Moreover, intracellular ROS levels increased in the hippocampus, and NLRP3 was mainly expressed in the microglia of the hippocampus. In LPS-primed BV-2 cells, ammonia treatment induced NLRP3 inflammasome activation and pyroptosis, with elevation of mtROS and altered mitochondrial membrane potential. Pretreatment with Mito-TEMPO suppressed mtROS production and the subsequent NLRP3 inflammasome activation and pyroptosis under LPS and ammonia treatment in BV-2 cells.

CONCLUSIONS

Hyperammonemia in HE may be involved in mtROS overproduction and subsequent NLRP3 inflammasome activation. Further studies using NLRP3-specific inhibitor or NLRP3 knockout mice are needed to elucidate the important role of NLRP3 inflammasome in HE development.

Therapeutic potential of infliximab for pruritus in mice model of cholestasis induced by bile duct ligation: Possible involvement of IL-31

BACKGROUND

Cholestatic pruritus is a distressful sensation that can cause a massive desire of scratching skin. Despite maximum medication therapy, some patients still experience pruritus. In this study, we evaluated the effect of infliximab on cholestatic pruritus induced in mice by bile duct ligation.

METHODS

Twenty-four balb/c mice were randomly assigned to three groups; sham, control, and treatment. The bile duct ligation procedure was performed on mice in the control and treatment groups. After six days, mice in the treatment group received subcutaneous administration of infliximab, and the next day all mice were subjected to the scratching behavior test. Skin, dorsal root ganglia (DRG), and blood samples of mice were collected and evaluated by histopathological, molecular, and biochemical tests.

RESULTS

The scratching behavior has significantly decreased in mice with cholestasis after the administration of infliximab. The levels of TNFα, TNFR1, TNFR2, NF-κB, and IL-31were higher in control mice compared to sham. In addition, expression levels of TNFR1, NF-κB, and IL-31 were decreased in the treatment group compared to the controls in skin and DRG, while TNFR2 levels were decreased only in DRG.

CONCLUSION

Infliximab can block TNFα interaction with receptors and inhibit further inflammatory response. Also, our results suggested that infliximab can suppress IL-31 expression indirectly, which is a well-known cytokine in pruritus pathophysiology Infliximab can be a potential therapeutic approach in resistant pruritus in cholestatic disorders.

Cognitive Functions, Neurotransmitter Alterations, and Hippocampal Microstructural Changes in Mice Caused by Feeding on Western Diet

Metabolic Dysfunction Associated Steatotic Liver Disease (MASLD) is the most common chronic liver disease in Western countries. It is becoming increasingly evident that peripheral organ-centered inflammatory diseases, including liver diseases, are linked with brain dysfunctions. Therefore, this study aims to unravel the effect of MASLD on brain histology, cognitive functions, and neurotransmitters. For this purpose, mice fed for 48 weeks on standard (SD) or Western diet (WD) were evaluated by behavioral tests, followed by sacrifice and analysis of the liver-brain axis including histopathology, immunohistochemistry, and biochemical analyses. Histological analysis of the liver showed features of Metabolic Dysfunction-Associated Steatohepatitis (MASH) in the WD-fed mice including lipid droplet accumulation, inflammation, and fibrosis. This was accompanied by an elevation of transaminase and alkaline phosphatase activities, increase in inflammatory cytokine and bile acid concentrations, as well as altered amino acid concentrations in the blood. Interestingly, compromised blood capillary morphology coupled with astrogliosis and microgliosis were observed in brain hippocampus of the WD mice, indicating neuroinflammation or a disrupted neurovascular unit. Moreover, attention was impaired in WD-fed mice along with the observations of impaired motor activity and balance, enhanced anxiety, and stereotyped head-twitch response (HTR) behaviors. Analysis of neurotransmitters and modulators including dopamine, serotonin, GABA, glutamate, and acetylcholine showed region-specific dysregulation in the brain of the WD-fed mice. In conclusion, the induction of MASH in mice is accompanied by the alteration of cellular morphology and neurotransmitter expression in the brain, associated with compromised cognitive functions.

Gut Microbiota and Neuroinflammation in Acute Liver Failure and Chronic Liver Disease

Acute liver failure and chronic liver disease are associated with a wide spectrum of neurological changes, of which the best known is hepatic encephalopathy (HE). Historically, hyperammonemia, causing astrocyte swelling and cerebral oedema, was considered the main etiological factor in the pathogenesis of cerebral dysfunction in patients with acute and/or chronic liver disease. However, recent studies demonstrated a key role of neuroinflammation in the development of neurological complications in this setting. Neuroinflammation is characterized by activation of microglial cells and brain secretion of pro-inflammatory cytokines, such as tumor necrosis factor (TNF)-α, interleukin (IL)-1β, and IL-6, which alter neurotransmission, leading to cognitive and motor dysfunction. Changes in the gut microbiota resulting from liver disease play a crucial role in the pathogenesis of neuroinflammation. Dysbiosis and altered intestinal permeability, resulting in bacterial translocation and endotoxemia, are responsible for systemic inflammation, which can spread to brain tissue and trigger neuroinflammation. In addition, metabolites derived from the gut microbiota can act on the central nervous system and facilitate the development of neurological complications, exacerbating clinical manifestations. Thus, strategies aimed at modulating the gut microbiota may be effective therapeutic weapons. In this review, we summarize the current knowledge on the role of the gut-liver-brain axis in the pathogenesis of neurological dysfunction associated with liver disease, with a particular focus on neuroinflammation. In addition, we highlight emerging therapeutic approaches targeting the gut microbiota and inflammation in this clinical setting.

Experimental hepatic encephalopathy causes early but sustained glial transcriptional changes

Hepatic encephalopathy (HE) is a common complication of liver cirrhosis, associated with high morbidity and mortality, for which no brain-targeted therapies exist at present. The interplay between hyperammonemia and inflammation is thought to drive HE development. As such, astrocytes, the most important ammonia-metabolizing cells in the brain, and microglia, the main immunomodulatory cells in the brain, have been heavily implicated in HE development. As insight into cellular perturbations driving brain pathology remains largely elusive, we aimed to investigate cell-type specific transcriptomic changes in the HE brain. In the recently established mouse bile duct ligation (BDL) model of HE, we performed RNA-Seq of sorted astrocytes and microglia at 14 and 28 days after induction. This revealed a marked transcriptional response in both cell types which was most pronounced in microglia. In both cell types, pathways related to inflammation and hypoxia, mechanisms commonly implicated in HE, were enriched. Additionally, astrocytes exhibited increased corticoid receptor and oxidative stress signaling, whereas microglial transcriptome changes were linked to immune cell attraction. Accordingly, both monocytes and neutrophils accumulated in the BDL mouse brain. Time-dependent changes were limited in both cell types, suggesting early establishment of a pathological phenotype. While HE is often considered a unique form of encephalopathy, astrocytic and microglial transcriptomes showed significant overlap with previously established gene expression signatures in other neuroinflammatory diseases like septic encephalopathy and stroke, suggesting common pathophysiological mechanisms. Our dataset identifies key molecular mechanisms involved in preclinical HE and provides a valuable resource for development of novel glial-directed therapeutic strategies.

The Glymphatic System May Play a Vital Role in the Pathogenesis of Hepatic Encephalopathy: A Narrative Review

Hepatic encephalopathy (HE) is a neurological complication of liver disease resulting in cognitive, psychiatric, and motor symptoms. Although hyperammonemia is a key factor in the pathogenesis of HE, several other factors have recently been discovered. Among these, the impairment of a highly organized perivascular network known as the glymphatic pathway seems to be involved in the progression of some neurological complications due to the accumulation of misfolded proteins and waste substances in the brain interstitial fluids (ISF). The glymphatic system plays an important role in the clearance of brain metabolic derivatives and prevents aggregation of neurotoxic agents in the brain ISF. Impairment of it will result in aggravated accumulation of neurotoxic agents in the brain ISF. This could also be the case in patients with liver failure complicated by HE. Indeed, accumulation of some metabolic by-products and agents such as ammonia, glutamine, glutamate, and aromatic amino acids has been reported in the human brain ISF using microdialysis technique is attributed to worsening of HE and correlates with brain edema. Furthermore, it has been reported that the glymphatic system is impaired in the olfactory bulb, prefrontal cortex, and hippocampus in an experimental model of HE. In this review, we discuss different factors that may affect the function of the glymphatic pathways and how these changes may be involved in HE.

Transcriptome Profile in the Mouse Brain of Hepatic Encephalopathy and Alzheimer's Disease

Hepatic encephalopathy (HE) is a chronic metabolic disease accompanied by neuropathological and neuropsychiatric features, including memory deficits, psychomotor dysfunction, depression, and anxiety. Alzheimer's disease (AD), the most common neurodegenerative disease, is characterized by tau hyperphosphorylation, excessive amyloid beta (Aβ) accumulation, the formation of fibrillary tangles, hippocampus atrophy, and neuroinflammation. Recent studies have suggested a positive correlation between HE and AD. Some studies reported that an impaired cholesterol pathway, abnormal bile acid secretion, excessive ammonia level, impaired Aβ clearance, astrocytic dysfunction, and abnormal γ-aminobutyric acid GABAergic neuronal signaling in HE may also be involved in AD pathology. However, the mechanisms and related genes involved in AD-like pathology in the HE brain are unclear. Thus, we compared the cortical transcriptome profile between an HE mouse model, bile duct ligation (BDL), and an AD mouse model, the 5×FAD. Our study showed that the expression of many genes implicated in HE is associated with neuronal dysfunction in AD mice. We found changes in various protein-coding RNAs, implicated in synapses, neurogenesis, neuron projection, neuron differentiation, and neurite outgrowth, and non-coding RNAs possibly associated with neuropathology. Our data provide an important resource for further studies to elucidate AD-like pathophysiology in HE patients.