Aquaporin-4 in hepatic encephalopathy

Brain edema is a critical component of hepatic encephalopathy (HE) associated with acute liver failure and such edema appears to be principally due to astrocyte swelling (cytotoxic edema). Ammonia is believed to represent a major factor responsible for astrocyte swelling, although the mechanisms by which ammonia causes such swelling are not completely understood. Recent studies have implicated potential role of oxidative stress, and the mitochondrial permeability transition (mPT). While it is not known how oxidative stress and the mPT cause astrocyte swelling, it is reasonable to suggest that these events may affect one or more plasma membrane proteins involved in water and ion homeostasis in astrocytes. One such protein strongly implicated in brain edema in other neurological conditions is the water channel protein aquaporin-4 (AQP-4), which is abundantly expressed in astrocytes. This article summarizes the potential role of AQP-4 in brain edema in in vivo models of HE, as well as in ammonia-induced cell swelling in cultured astrocytes. The involvement of AQP-4 in the effects of manganese, another toxin implicated in HE, will also be discussed.     

Keeping cool in acute liver failure: rationale for the use of mild hypothermia

Encephalopathy, brain edema and intracranial hypertension are neurological complications responsible for substantial morbidity/mortality in patients with acute liver failure (ALF), where, aside from liver transplantation, there is currently a paucity of effective therapies. Mirroring its cerebro-protective effects in other clinical conditions, the induction of mild hypothermia may provide a potential therapeutic approach to the management of ALF. A solid mechanistic rationale for the use of mild hypothermia is provided by clinical and experimental studies showing its beneficial effects in relation to many of the key factors that determine the development of brain edema and intracranial hypertension in ALF, namely the delivery of ammonia to the brain, the disturbances of brain organic osmolytes and brain extracellular amino acids, cerebro-vascular haemodynamics, brain glucose metabolism, inflammation, subclinical seizure activity and alterations of gene expression. Initial uncontrolled clinical studies of mild hypothermia in patients with ALF suggest that it is an effective, feasible and safe approach. Randomized controlled clinical trials are now needed to adequately assess its efficacy, safety, clinical impact on global outcomes and to provide the guidelines for its use in ALF.     

Direct molecular and spectroscopic evidence for increased ammonia removal capacity of skeletal muscle in acute liver failure

BACKGROUND/AIMS: It has been proposed that, in acute liver failure, skeletal muscle adapts to become the principle organ responsible for removal of blood-borne ammonia by increasing glutamine synthesis, a reaction that is catalyzed by the cytosolic ATP-dependent enzyme glutamine synthetase. To address this issue, glutamine synthetase expression and activities were measured in skeletal muscle of rats with acute liver failure resulting from hepatic devascularization. METHODS: Glutamine synthetase protein and gene expression were investigated using immunoblotting and semi-quantitative RT-PCR analysis. Glutamine synthetase activity and glutamine de novo synthesis were measured using, respectively, a standard enzymatic assay and [13C]-nuclear magnetic resonance spectroscopy. RESULTS: Glutamine synthetase protein (but not gene) expression and enzyme activities were significantly up-regulated leading to increased de novo synthesis of glutamine and increased skeletal muscle capacity for ammonia removal in acute liver failure. In contrast to skeletal muscle, expression and activities of glutamine synthetase in the brain were significantly decreased. CONCLUSIONS: These findings demonstrate that skeletal muscle adapts, through a rapid induction of glutamine synthetase, to increase its capacity for removal of blood-borne ammonia in acute liver failure. Maintenance of muscle mass together with the development of agents with the capacity to stimulate muscle glutamine synthetase could provide effective ammonia-lowering strategies in this disorder.     

线粒体损伤在肝功能衰竭发生中的分子机制

肝功能衰竭是由多种因素引起的,以肝细胞大量凋亡、坏死为特征的临床综合征.其病情危重,发展迅速,病死率高达60％～80％.线粒体是真核细胞中一种非常重要的细胞器,与肝脏的生物合成、代谢和解毒功能密不可分.三羧酸循环、脂肪酸β-氧化、氧化磷酸化及鸟氨酸循环等均在线粒体内进行.此外,线粒体在调节细胞内钙离子浓度、信息传递及细胞死亡等过程中也具有重要作用.线粒体是肝细胞内较敏感的细胞器之一,多种急、慢性肝病均存在线粒体结构和功能的异常.有研究结果显示,线粒体与肝衰竭的发病关系密切,可能的作用途径包括:呼吸链被抑制导致三磷酸腺苷(adenosine-triphosphate,ATP)耗竭、氧化应激、脂肪酸β-氧化的抑制、线粒体通透性转换(mitochondrial permeability transition,MPT)所致肝细胞凋亡或坏死等,但其作用机制尚未完全阐明[1].现就目前的相关研究进展作一综述.     

肝性脑病发病机制的研究进展

肝性脑病(hepatic encephalopathy,HE)是肝功能严重障碍和/或门体分流术后患者发生的以代谢紊乱为基础,神经、精神症状为主要表现的综合征.其症状包括意识混乱、定向力障碍、协调能力降低,甚至昏迷.其发病机制被认为是高氨血症使脑星形胶质细胞内谷氨酰胺浓度升高、钙离子内流启动氧化应激、破坏线粒体功能,干扰能量代谢并诱发炎症反应,破坏血脑屏障使内皮细胞、脑星形胶质细胞对水通透性增加,引发脑水肿.炎症反应又反过来升高脑内氨浓度,增加其中枢神经系统毒性,而锰是参与上述过程的重要组成成分,故目前公认为高氨血症和炎症反应的协同作用导致星形胶质细胞肿胀,进而引起脑水肿导致HE.本文就HE发病机制的研究进展作一综述.     

急性肝衰竭大鼠血浆神经降压素和降钙素基因相关肽水平的变化

目的探讨急性肝衰竭(ALF)大鼠血浆神经降压素(NT)和降钙素基因相关肽(CGRP)含量的变化。方法采用硫代乙酰胺制备ALF大鼠模型。采用RIA法测定血浆NT和CGRP含量。结果模型组大鼠血浆NT和CGRP含量明显升高,与正常对照组比,差异有统计学意义(P0.01);模型组大鼠分别于造模后12h、24h、36h和48h,肝性脑病逐渐进展,血浆NT和CGRP含量也呈进行性升高。结论测定血浆NT和CGRP含量可作为判断肝性脑病早期诊断的指标之一,其含量的变化对肝性脑病严重程度的判断有重要的参考价值。     

Neuropathological changes in the brain of pigs with acute liver failure

Abstract

Objective. Cerebral edema is a serious complication of acute liver failure (ALF), which may lead to intracranial hypertension and death. An accepted tenet has been that the blood-brain barrier is intact and that brain edema is primarily caused by a cytotoxic etiology due to hyperammonemia. However, the neuropathological changes in ALF have been poorly studied. Using a well characterized porcine model we aimed to investigate ultrastructural changes in the brain from pigs suffering from ALF. Materials and methods. Sixteen female Norwegian Landrace pigs weighing 27–35 kg were randomised into two groups: ALF (n = 8) and sham operated controls (n = 8). ALF was induced with an end-to-side portacaval shunt followed by ligation of the hepatic arteries. Biopsies were harvested from three different areas of the brain (frontal lobe, cerebellum, and brain stem) following eight hours of ALF and analyzed using electron microscopy. Results. Profound perivascular and interstitial edema were found in all three areas. Disruption of pericytic and astrocytic processes were seen, reflecting breakdown/lesion of the blood-brain barrier in animals suffering from ALF. Furthermore, neurons and axons were edematous and surrounded by vesicles. Severe damage to Purkinje neuron (necrosis) and damaged myelin were seen in the cerebellum and brain stem, respectively. Biopsies from sham operated animals were normal. Conclusions. Our data support the concept that vasogenic brain edema plays an important role in the development of intracranial hypertension in pigs with ALF.     

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 lifethreatening 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.     

曲美他嗪对急性肝衰竭小鼠肝组织NOX2和NOX4表达的影响

目的 探讨曲美他嗪对急性肝衰竭小鼠肝组织NOX2和NOX4表达的影响。方法 随机将65只C57BL/6小鼠分为对照组、模型组、小、中、大剂量曲美他嗪处理组和还原型谷胱甘肽处理组,除模型组15只外,其他组均为10只。采用D-氨基半乳糖联合脂多糖腹腔注射建立急性肝衰竭模型。取肝组织匀浆检测丙二醛(MDA)和过氧化物酶(CAT)含量,采用RT-PCR法和Western blot法分别检测肝组织还原型烟酰胺腺嘌呤二核苷酸磷酸氧化酶( NOX2/4) mRNA水平和蛋白表达。结果 模型组血清ALT、AST和肝匀浆MDA水平分别为(121.4±3.7)U/L、(208.9±27.4)U/L和(51.0±20.5)nmol/mg,均显著高于对照组【分别为(35.3±3.2)U/L、(49.9±4.4)U/L和(14.1±5.2)nmol/mg,P＜0.05】,而模型组肝匀浆CAT水平为(51.7±16.8)U/mg,显著低于对照组【(110.2±33.7)U/mg,P＜0.05】;模型组NOX2/4 mRNA和蛋白相对水平分别为(8.2±2.0)/(1.2±0.1)和(2.6±0.1)/(1.3±0.1),均显著高于对照组【分别为(1.0±0.2)/(0.5±0.1)和(1.0±0.1)/(0.4±0.1),P＜0.05】,中、大剂量曲美他嗪处理组血清ALT和AST及肝匀浆MDA水平分别为(86.4±1.00)U/L、(154.0±6.2)U/L和(22.5±1.9)nmol/mg及(81.1±1.5)U/L、(134.7±5.3)U/L和(20.1±3.7)nmol/mg,均显著低于模型组【分别为(121.4±3.7)U/L、(208.9±27.4)U/L和(51.0±20.5)nmol/mg,P＜0.05】,肝匀浆CAT水平分别为(99.4±15.5)和(102.3±15.5),均显著高于模型组【(51.6±16.8),P＜0.05】;肝组织NOX2/4 mRNA及蛋白相对表达量分别为(5.6±0.2)/(0.7±0.0)和(5.2±1.4/0.6±0.1)及(1.7±0.2)/(0.7±0.2)和(1.5±0.1)/(0.6±0.2),均显著低于模型组【(8.2±2.0)/(1.2±0.1)和(2.6±0.1)/(1.3±0.1),P＜0.05】。结论 曲美他嗪可能通过下调肝组织NOX2/4表达减轻氧化应激损伤,改善D-Galn/LPS诱导的急性肝衰竭小鼠肝损伤。