Hyperammonemia and chronic hepatic encephalopathy: an in vivo PMRS study of the rat brain

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.

Recent advances in understanding and managing hepatic encephalopathy in chronic liver disease

Hepatic encephalopathy (HE) is a common, severe complication of advanced chronic liver disease (CLD) and has a devastating impact on the patient’s quality of life and prognosis. The neurotoxin ammonia and the presence of systemic and neurological inflammation are considered the key drivers of this neuropsychiatric syndrome. Treatment options available in routine clinical practice are limited, and the development of novel therapies is hampered owing to the complexity and heterogeneity of HE. This review article aims to outline the current understanding of the pathomechanisms of HE and the recent advances in the identification and development of novel therapeutic targets.

Bioenergetic dysfunction in a zebrafish model of acute hyperammonemic decompensation

Acute hyperammonemic encephalopathy is a life-threatening manifestation of individuals with urea cycle disorders, which is associated with high mortality rates and severe neurological sequelae in survivors. Cerebral bioenergetic failure has been proposed as one of the key mechanisms underlying hyperammonemia-induced brain damage, but data supporting this hypothesis remain inconclusive and partially contradictory. Using a previously established zebrafish model of acute hyperammonemic decompensation, we unraveled that acute hyperammonemia leads to a transamination-dependent withdrawal of 2-oxoglutarate (alpha-ketoglutarate) from the tricarboxylic acid (TCA) cycle with consecutive TCA cycle dysfunction, ultimately causing impaired oxidative phosphorylation with ATP shortage, decreased ATP/ADP-ratio and elevated lactate concentrations. Thus, our study supports and extends the hypothesis that cerebral bioenergetic dysfunction is an important pathophysiological hallmark of hyperammonemia-induced neurotoxicity.

Mitochondrial dysfunctions contribute to energy deficits in rodent model of hepatic encephalopathy

Perturbations in the cerebral energy metabolism are anticipated to be an important factor by which ammonia may exert its toxic effects on the central nervous system. The present study was designed to investigate the role of impaired mitochondrial functions and cerebral energy metabolism in the development hepatic encephalopathy (HE) induced by of bile duct ligation (BDL). After four weeks of BDL, a significant increase in hepatic hydroxyproline and collagen content was observed which confirmed biliary fibrosis. Brain regions viz. cortex, hippocampus, striatum and cerebellum of BDL rats had impaired activity of mitochondrial respiratory chain enzymes. This was accompanied by increase in mitochondrial reactive oxygen species (ROS), malondialdehyde (MDA) and protein carbonyl levels in the brain. Mitochondrial redox ratio was significantly reduced in the brain of BDL rats. In addition, mitochondria from brain of BDL rats were depolarized and swollen compared to the sham controls. Ultrastructure analysis of mitochondria from cortex and hippocampus of BDL animals revealed aberrant cristae, ruptured membranes and non-dense matrix. Further, a significant decrease was observed in creatine kinase activity, glucose uptake and CO₂ production in the brain regions of BDL rats. ATP/ADP ratio, a critical parameter of cellular energy status, was also significantly reduced in brain regions of rats with HE. Overall, the findings clearly demonstrate that BDL induced HE involves mitochondrial respiratory chain dysfunctions, mitochondrial depolarization and swelling that accentuates oxidative stress which in turn leads to compromise in cerebral energy metabolism thereby contributing to the pathophysiology of chronic HE.

Brain energy metabolism and mitochondrial dysfunction in acute and chronic hepatic encephalopathy

One proposed mechanism for acute and chronic hepatic encephalopathy (HE) is a disturbance in cerebral energy metabolism. It also reviews the current status of this mechanism in both acute and chronic HE, as well as in other hyperammonemic disorders. It also reviews abnormalities in glycolysis, lactate metabolism, citric acid cycle, and oxidative phosphorylation as well as associated energy impairment. Additionally, the role of mitochondrial permeability transition (mPT), a recently established factor in the pathogenesis of HE and hyperammonemia, is emphasized. Energy failure appears to be an important pathogenetic component of both acute and chronic HE and a potential target for therapy.

Mechanisms of hyperammonemia

Hyperammonemia is mainly found in hepatic encephalopathy and in genetic defects of the urea cycle or other pathways of the intermediary metabolism. Clinically a difference has to be made between chronic moderate hyperammonemia and acutely increased concentrations. Pathogenetic mechanisms of ammonia toxicity to the brain are partly unraveled. In some animal models confounding variables, such as the reduced intake of food and amino acid imbalance due to liver insufficiency, do not allow to establish unequivocal causal relationships between the ammonia concentration and measured effects. In chronic moderate hyperammonemia an increased flux through the serotonin pathway is a key factor. It is caused by an increased transport of large neutral amino acids (including tryptophan) through the blood-brain barrier, accentuated by the imbalance of plasma amino acids in hepatic insufficiency. It is stimulated by D- or L-glutamine. Evidence is presented showing that a functioning gamma-glutamyl cycle (glutathione formation) is a prerequisite. In acute hyperammonemia involvement of NMDA receptors, glutamate, NO and cGMP plays an additional role. In hyperammonemic crises the increased cerebral blood flow leads to brain edema; factors discussed here are increased osmolytes in astrocytes and serotoninergic activity. Recent data indicate that axonal development is affected by ammonia and can be normalized in vitro by creatine supplementation in developing mixed brain cell aggregate cultures, thus reviving the old hypothesis of the impact of hyperammonemia on energy metabolism in the developing brain that could cause mental retardation.

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.