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

► Changes in brain energy metabolism in HE and hyperammonemia have been highlighted in this review. ► The role of mitochondrial permeability transition in the pathogenesis of HE has been emphasized. ► Impact of altered bioenergetics on brain edema in acute liver failure has also been presented. 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

Manganese induces the mitochondrial permeability transition in cultured astrocytes

Manganese is known to cause central nervous system injury leading to parkinsonism and to contribute to the pathogenesis of hepatic encephalopathy. Although mechanisms of manganese neurotoxicity are not completely understood, chronic exposure of various cell types to manganese has shown oxidative stress and mitochondrial energy failure, factors that are often implicated in the induction of the mitochondrial permeability transition (MPT). In this study, we examined whether exposure of cultured neurons and astrocytes to manganese induces the MPT. Cells were treated with manganese acetate (10-100 microM), and the MPT was assessed by changes in the mitochondrial membrane potential and in mitochondrial calcein fluorescence. In astrocytes, manganese caused a dissipation of the mitochondrial membrane potential and decreased the mitochondrial calcein fluorescence in a concentration- and time-dependent manner. These changes were completely blocked by pretreatment with cyclosporin A, consistent with induction of the MPT. On the other hand, similarly treated cultured cortical neurons had a delayed or reduced MPT as compared with astrocytes. The manganese-induced MPT in astrocytes was blocked by pretreatment with antioxidants, suggesting the potential involvement of oxidative stress in this process. Induction of the MPT by manganese and associated mitochondrial dysfunction in astrocytes may represent key mechanisms in manganese neurotoxicity

Inhibitors of the mitochondrial permeability transition reduce ammonia-induced cell swelling in cultured astrocytes

Ammonia is the principal neurotoxin implicated in the pathogenesis of hepatic encephalopathy, and astrocytes are the neural cells predominantly affected in this condition. Astrocyte swelling (cytotoxic edema) represents a critical component of the brain edema in acute form of hepatic encephalopathy (acute liver failure, ALF). Although mechanisms of astrocyte swelling by ammonia are not completely understood, cultured astrocytes exposed to pathophysiological levels of ammonia develop the mitochondrial permeability transition (mPT), a process that was shown to result in astrocyte swelling. Cyclosporin A (CsA), a traditional inhibitor of the mPT, was previously shown to completely block ammonia-induced astrocyte swelling in culture. However, the efficacy of CsA to protect cytotoxic brain edema in ALF is problematic because it poorly crosses the blood-brain barrier, which is relatively intact in ALF. We therefore examined the effect of agents that block the mPT but are also known to cross the blood-brain barrier, including pyruvate, magnesium, minocycline, and trifluoperazine on the ammonia-induced mPT, as well as cell swelling. Cultured astrocytes exposed to ammonia for 24 hr displayed the mPT as demonstrated by a CsA-sensitive dissipation of the mitochondrial inner membrane potential. Pyruvate, minocycline, magnesium, and trifluoperazine significantly blocked the ammonia-induced mPT. Ammonia resulted in a significant increase in cell volume, which was blocked by the above-mentioned agents to a variable degree. A regression analysis indicated a high correlation between the effectiveness of reducing the mPT and cell swelling. Our data suggest that all these agents have therapeutic potential in mitigating brain edema in ALF

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

Glutamine in the pathogenesis of acute hepatic encephalopathy

► Brain levels of glutamine are elevated in hepatic encephalopathy (HE). ► The formation of glutamine represents a toxic product of ammonia metabolism. ► Glutamine is hydrolized in mitochondria and produces high levels of ammonia to cause toxicity. ► Oxidative stress and mitochondrial permeability transition are major factors in glutamine toxicity. Hepatic encephalopathy (HE) is the major neurological disorder associated with liver disease. It presents in chronic and acute forms, and astrocytes are the major neural cells involved. While the principal etiological factor in the pathogenesis of HE is increased levels of blood and brain ammonia, glutamine, a byproduct of ammonia metabolism, has also been implicated in its pathogenesis. This article reviews the current status of glutamine in the pathogenesis of HE, particularly its involvement in some of the events triggered by ammonia, including mitochondrial dysfunction, generation of oxidative stress, and alterations in signaling mechanisms, including activation of mitogen-activated protein kinases (MAPKs) and nuclear factor-kappaB (NF-κB). Mechanisms by which glutamine contributes to astrocyte swelling/brain edema associated with acute liver failure (ALF) will also be described

Aquaporin-4 deletion in mice reduces encephalopathy and brain edema in experimental acute liver failure

Brain edema and associated astrocyte swelling leading to increased intracranial pressure are hallmarks of acute liver failure (ALF). Elevated blood and brain levels of ammonia have been implicated in the development of brain edema in ALF. Cultured astrocytes treated with ammonia have been shown to undergo cell swelling and such swelling was associated with an increase in the plasma membrane expression of aquaporin-4 (AQP4) protein. Further, silencing the AQP4 gene in cultured astrocytes was shown to prevent the ammonia-induced cell swelling. Here, we examined the evolution of brain edema in AQP4-null mice and their wild type counterparts (WT-mice) in different models of ALF induced by thioacetamide (TAA) or acetaminophen (APAP). Induction of ALF with TAA or APAP significantly increased brain water content in WT mice (by 1.6% ± 0.3 and 2.3 ± 0.4%, respectively). AQP4 protein was significantly increased in brain plasma membranes of WT mice with ALF induced by either TAA or APAP. In contrast to WT-mice, brain water content did not increase in AQP4-null mice. Additionally, AQP4-null mice treated with either TAA or APAP showed a remarkably lesser degree of neurological deficits as compared to WT mice; the latter displayed an inability to maintain proper gait, and demonstrated a markedly reduced exploratory behavior, with the mice remaining in one corner of the cage with its head tilted downwards. These results support a central role of AQP4 in the brain edema associated with ALF

Brain edema in acute liver failure: mechanisms and concepts

Brain edema and associated increase in intracranial pressure continue to be lethal complications of acute liver failure (ALF). Abundant evidence suggests that the edema in ALF is largely cytotoxic brought about by swelling of astrocytes. Elevated blood and brain ammonia levels have been strongly implicated in the development of the brain edema. Additionally, inflammation and sepsis have been shown to contribute to the astrocyte swelling/brain edema in the setting of ALF. We posit that ammonia initiates a number of signaling events, including oxidative/nitrative stress (ONS), the mitochondrial permeability transition (mPT), activation of the transcription factor (NF-κB) and signaling kinases, all of which have been shown to contribute to the mechanism of astrocyte swelling. All of these factors also impact ion-transporters, including Na(+), K(+), Cl(-) cotransporter and the sulfonylurea receptor 1, as well as the water channel protein aquaporin-4 resulting in a perturbation of cellular ion and water homeostasis, ultimately resulting in astrocyte swelling/brain edema. All of these events are also potentiated by inflammation. This article reviews contemporary knowledge regarding mechanisms of astrocyte swelling/brain edema formation which hopefully will facilitate the identification of therapeutic targets capable of mitigating the brain edema associated with ALF

The mitochondrial permeability transition, and oxidative and nitrosative stress in the mechanism of copper toxicity in cultured neurons and astrocytes

Copper is an essential element and an integral component of various enzymes. However, excess copper is neurotoxic and has been implicated in the pathogenesis of Wilson's disease, Alzheimer's disease, prion conditions, and other disorders. Although mechanisms of copper neurotoxicity are not fully understood, copper is known to cause oxidative stress and mitochondrial dysfunction. As oxidative stress is an important factor in the induction of the mitochondrial permeability transition (mPT), we determined whether mPT plays a role in copper-induced neural cell injury. Cultured astrocytes and neurons were treated with 20 microM copper and mPT was measured by changes in the cyclosporin A (CsA)-sensitive inner mitochondrial membrane potential (Delta Psi m), employing the potentiometric dye TMRE. In astrocytes, copper caused a 36% decrease in the Delta Psi m at 12 h, which decreased further to 48% by 24 h and remained at that level for at least 72 h. Cobalt quenching of calcein fluorescence as a measure of mPT similarly displayed a 45% decrease at 24 h. Pretreatment with antioxidants significantly blocked the copper-induced mPT by 48-75%. Copper (24 h) also caused a 30% reduction in ATP in astrocytes, which was completely blocked by CsA. Copper caused death (42%) in astrocytes by 48 h, which was reduced by antioxidants (35-60%) and CsA (41%). In contrast to astrocytes, copper did not induce mPT in neurons. Instead, it caused early and extensive death with a concomitant reduction (63%) in ATP by 14 h. Neuronal death was prevented by antioxidants and nitric oxide synthase inhibitors but not by CsA. Copper increased protein tyrosine nitration in both astrocytes and neurons. These studies indicate that mPT, and oxidative and nitrosative stress represent major factors in copper-induced toxicity in astrocytes, whereas oxidative and nitrosative stress appears to play a major role in neuronal injury

Interaction between cytokines and ammonia in the mitochondrial permeability transition in cultured astrocytes

Hepatic encephalopathy (HE) is the major neurological complication occurring in patients with acute and chronic liver failure. Elevated levels of blood and brain ammonia are characteristic of HE, and astrocytes are the primary target of ammonia toxicity. In addition to ammonia, recent studies suggest that inflammation and associated cytokines (CKs) also contribute to the pathogenesis of HE. It was previously established that ammonia induces the mitochondrial permeability transition (mPT) in cultured astrocytes. As CKs have been shown to cause mitochondrial dysfunction in other conditions, we examined whether CKs induce the mPT in cultured astrocytes. Cultures treated with tumor necrosis factor-α, interleukin-1β, interleukin-6, and interferon-γ, individually or in a mixture, resulted in the induction of the mPT in a time-dependent manner. Simultaneous treatment of cultures with a mixture of CKs and ammonia showed a marked additive effect on the mPT. As oxidative stress (OS) is known to induce the mPT, so we examined the effect of CKs and ammonia on hemeoxygenase-1 (HO-1) protein expression, a marker of OS. Such treatment displayed a synergistic effect in the upregulation of HO-1. Antioxidants significantly blocked the additive effects on the mPT by CKs and ammonia, suggesting that OS represents a major mechanism in the induction of the mPT. Treatment of cultures with minocycline, an antiinflammatory agent, which is known to inhibit OS, also diminished the additive effects on the mPT caused by CKs and ammonia. Induction of the mPT in astrocytes appears to represent a major pathogenetic factor in HE, in which CKs and ammonia are critically involved