Cerebellar inhibition in hepatic encephalopathy

Increased levels and activation of the IL-17 receptor in microglia contribute to enhanced neuroinflammation in cerebellum of hyperammonemic rats

BACKGROUND

Patients with liver cirrhosis may show minimal hepatic encephalopathy (MHE) with mild cognitive impairment and motor incoordination. Rats with chronic hyperammonemia reproduce these alterations. Motor incoordination in hyperammonemic rats is due to increased GABAergic neurotransmission in cerebellum, induced by neuroinflammation, which enhances TNFα-TNFR1-S1PR2-CCL2-BDNF-TrkB pathway activation. The initial events by which hyperammonemia triggers activation of this pathway remain unclear. MHE in cirrhotic patients is triggered by a shift in inflammation with increased IL-17. The aims of this work were: (1) assess if hyperammonemia increases IL-17 content and membrane expression of its receptor in cerebellum of hyperammonemic rats; (2) identify the cell types in which IL-17 receptor is expressed and IL-17 increases in hyperammonemia; (3) assess if blocking IL-17 signaling with anti-IL-17 ex-vivo reverses activation of glia and of the TNFα-TNFR1-S1PR2-CCL2-BDNF-TrkB pathway.

RESULTS

IL-17 levels and membrane expression of the IL-17 receptor are increased in cerebellum of rats with hyperammonemia and MHE, leading to increased activation of IL-17 receptor in microglia, which triggers activation of STAT3 and NF-kB, increasing IL-17 and TNFα levels, respectively. TNFα released from microglia activates TNFR1 in Purkinje neurons, leading to activation of NF-kB and increased IL-17 and TNFα also in these cells. Enhanced TNFR1 activation also enhances activation of the TNFR1-S1PR2-CCL2-BDNF-TrkB pathway which mediates microglia and astrocytes activation.

CONCLUSIONS

All these steps are triggered by enhanced activation of IL-17 receptor in microglia and are prevented by ex-vivo treatment with anti-IL-17. IL-17 and IL-17 receptor in microglia would be therapeutic targets to treat neurological impairment in patients with MHE.

Neuroinflammation alters GABAergic neurotransmission in hyperammonemia and hepatic encephalopathy, leading to motor incoordination. Mechanisms and therapeutic implications

Enhanced GABAergic neurotransmission contributes to impairment of motor coordination and gait and of cognitive function in different pathologies, including hyperammonemia and hepatic encephalopathy. Neuroinflammation is a main contributor to enhancement of GABAergic neurotransmission through increased activation of different pathways. For example, enhanced activation of the TNFα-TNFR1-NF-κB-glutaminase-GAT3 pathway and the TNFα-TNFR1-S1PR2-CCL2-BDNF-TrkB pathway in cerebellum of hyperammonemic rats enhances GABAergic neurotransmission. This is mediated by mechanisms affecting GABA synthesizing enzymes GAD67 and GAD65, total and extracellular GABA levels, membrane expression of GABAA receptor subunits, of GABA transporters GAT1 and GAT three and of chloride co-transporters. Reducing neuroinflammation reverses these changes, normalizes GABAergic neurotransmission and restores motor coordination. There is an interplay between GABAergic neurotransmission and neuroinflammation, which modulate each other and altogether modulate motor coordination and cognitive function. In this way, neuroinflammation may be also reduced by reducing GABAergic neurotransmission, which may also improve cognitive and motor function in pathologies associated to neuroinflammation and enhanced GABAergic neurotransmission such as hyperammonemia, hepatic encephalopathy or Parkinson's disease. This provides therapeutic targets that may be modulated to improve cognitive and motor function and other alterations such as fatigue in a wide range of pathologies. As a proof of concept it has been shown that antagonists of GABAA receptors such as bicuculline reduces neuroinflammation and improves cognitive and motor function impairment in rat models of hyperammonemia and hepatic encephalopathy. Antagonists of GABAA receptors are not ideal therapeutic tools because they can induce secondary effects. As a more effective treatment to reduce GABAergic neurotransmission new compounds modulating it by other mechanisms are being developed. Golexanolone reduces GABAergic neurotransmission by reducing the potentiation of GABAA receptor activation by neurosteroids such as allopregnanolone. Golexanolone reduces neuroinflammation and GABAergic neurotransmission in animal models of hyperammonemia, hepatic encephalopathy and cholestasis and this is associated with improvement of fatigue, cognitive impairment and motor incoordination. This type of compounds may be useful therapeutic tools to improve cognitive and motor function in different pathologies associated with neuroinflammation and increased GABAergic neurotransmission.

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.

Regulation of mild cognitive impairment associated with liver disease by humoral factors derived from the gastrointestinal tract and MRI research progress: a literature review

Patients with liver disease are prone to various cognitive impairments. It is undeniable that cognitive impairment is often regulated by both the nervous system and the immune system. In this review our research focused on the regulation of mild cognitive impairment associated with liver disease by humoral factors derived from the gastrointestinal tract, and revealed that its mechanisms may be involved with hyperammonemia, neuroinflammation, brain energy and neurotransmitter metabolic disorders, and liver-derived factors. In addition, we share the emerging research progress in magnetic resonance imaging techniques of the brain during mild cognitive impairment associated with liver disease, in order to provide ideas for the prevention and treatment of mild cognitive impairment in liver disease.

J-difference GABA-edited MRS reveals altered cerebello-thalamo-cortical metabolism in patients with hepatic encephalopathy

Hepatic encephalopathy (HE) is a common neurological manifestation of liver cirrhosis and is characterized by an increase of ammonia in the brain accompanied by a disrupted neurotransmitter balance, including the GABAergic and glutamatergic systems. The aim of this study is to investigate metabolic abnormalities in the cerebello-thalamo-cortical system of HE patients using GABA-edited MRS and links between metabolite levels, disease severity, critical flicker frequency (CFF), motor performance scores, and blood ammonia levels. GABA-edited MRS was performed in 35 participants (16 controls, 19 HE patients) on a clinical 3 T MRI system. MRS voxels were placed in the right cerebellum, left thalamus, and left motor cortex. Levels of GABA+ and of other metabolites of interest (glutamine, glutamate, myo-inositol, glutathione, total choline, total NAA, and total creatine) were assessed. Group differences in metabolite levels and associations with clinical metrics were tested. GABA+ levels were significantly increased in the cerebellum of patients with HE. GABA+ levels in the motor cortex were significantly decreased in HE patients, and correlated with the CFF (r = 0.73; p < .05) and motor performance scores (r = -0.65; p < .05). Well-established HE-typical metabolite patterns (increased glutamine, decreased myo-inositol and total choline) were confirmed in all three regions and were closely linked to clinical metrics. In summary, our findings provide further evidence for alterations in the GABAergic system in the cerebellum and motor cortex in HE. These changes were accompanied by characteristic patterns of osmolytes and oxidative stress markers in the cerebello-thalamo-cortical system. These metabolic disturbances are a likely contributor to HE motor symptoms in HE. In patients with hepatic encephalopathy, GABA+ levels in the cerebello-thalamo-cortical loop are significantly increased in the cerebellum and significantly decreased in the motor cortex. GABA+ levels in the motor cortex strongly correlate with critical flicker frequency (CFF) and motor performance score (pegboard test tPEG), but not blood ammonia levels (NH3).

Sustained Hyperammonemia Activates NF-κB in Purkinje Neurons Through Activation of the TrkB-PI3K-AKT Pathway by Microglia-Derived BDNF in a Rat Model of Minimal Hepatic Encephalopathy

Chronic hyperammonemia is a main contributor to the cognitive and motor impairment in patients with hepatic encephalopathy. Sustained hyperammonemia induces the TNFα expression in Purkinje neurons, mediated by NF-κB activation. The aims were the following: (1) to assess if enhanced TrkB activation by BDNF is responsible for enhanced NF-κB activation in Purkinje neurons in hyperammonemic rats, (2) to assess if this is associated with increased content of NF-κB modulated proteins such as TNFα, HMGB1, or glutaminase I, (3) to assess if these changes are due to enhanced activation of the TNFR1-S1PR2-CCR2-BDNF-TrkB pathway, (4) to analyze if increased activation of NF-κB is mediated by the PI3K-AKT pathway. It is shown that, in the cerebellum of hyperammonemic rats, increased BDNF levels enhance TrkB activation in Purkinje neurons leading to activation of PI3K, which enhances phosphorylation of AKT and of IκB, leading to increased nuclear translocation of NF-κB which enhances TNFα, HMGB1, and glutaminase I content. To assess if the changes are due to enhanced activation of the TNFR1-S1PR2-CCR2 pathway, we blocked TNFR1 with R7050, S1PR2 with JTE-013, and CCR2 with RS504393. These changes are reversed by blocking TrkB, PI3K, or the TNFR1-SP1PR2-CCL2-CCR2-BDNF-TrkB pathway at any step. In hyperammonemic rats, increased levels of BDNF enhance TrkB activation in Purkinje neurons, leading to activation of the PI3K-AKT-IκB-NF-κB pathway which increased the content of glutaminase I, HMGB1, and TNFα. Enhanced activation of this TrkB-PI3K-AKT-NF-κB pathway would contribute to impairing the function of Purkinje neurons and motor function in hyperammonemic rats and likely in cirrhotic patients with minimal or clinical hepatic encephalopathy.

Enhanced BDNF and TrkB Activation Enhance GABA Neurotransmission in Cerebellum in Hyperammonemia

Background: Hyperammonemia is a main contributor to minimal hepatic encephalopathy (MHE) in cirrhotic patients. Hyperammonemic rats reproduce the motor incoordination of MHE patients, which is due to enhanced GABAergic neurotransmission in the cerebellum as a consequence of neuroinflammation. In hyperammonemic rats, neuroinflammation increases BDNF by activating the TNFR1–S1PR2–CCR2 pathway. (1) Identify mechanisms enhancing GABAergic neurotransmission in hyperammonemia; (2) assess the role of enhanced activation of TrkB; and (3) assess the role of the TNFR1–S1PR2–CCR2–BDNF pathway. In the cerebellum of hyperammonemic rats, increased BDNF levels enhance TrkB activation in Purkinje neurons, leading to increased GAD65, GAD67 and GABA levels. Enhanced TrkB activation also increases the membrane expression of the γ2, α2 and β3 subunits of GABA_A receptors and of KCC2. Moreover, enhanced TrkB activation in activated astrocytes increases the membrane expression of GAT3 and NKCC1. These changes are reversed by blocking TrkB or the TNFR1–SP1PR2–CCL2–CCR2–BDNF–TrkB pathway. Hyperammonemia-induced neuroinflammation increases BDNF and TrkB activation, leading to increased synthesis and extracellular GABA, and the amount of GABA_A receptors in the membrane and chloride gradient. These factors enhance GABAergic neurotransmission in the cerebellum. Blocking TrkB or the TNFR1–SP1PR2–CCL2–CCR2–BDNF–TrkB pathway would improve motor function in patients with hepatic encephalopathy and likely with other pathologies associated with neuroinflammation.

A comprehensive review of transcranial magnetic stimulation in secondary dementia

Although primary degenerative diseases are the main cause of dementia, a non-negligible proportion of patients is affected by a secondary and potentially treatable cognitive disorder. Therefore, diagnostic tools able to early identify and monitor them and to predict the response to treatment are needed. Transcranial magnetic stimulation (TMS) is a non-invasive neurophysiological technique capable of evaluating in vivo and in “real time” the motor areas, the cortico-spinal tract, and the neurotransmission pathways in several neurological and neuropsychiatric disorders, including cognitive impairment and dementia. While consistent evidence has been accumulated for Alzheimer’s disease, other degenerative cognitive disorders, and vascular dementia, to date a comprehensive review of TMS studies available in other secondary dementias is lacking. These conditions include, among others, normal-pressure hydrocephalus, multiple sclerosis, celiac disease and other immunologically mediated diseases, as well as a number of inflammatory, infective, metabolic, toxic, nutritional, endocrine, sleep-related, and rare genetic disorders. Overall, we observed that, while in degenerative dementia neurophysiological alterations might mirror specific, and possibly primary, neuropathological changes (and hence be used as early biomarkers), this pathogenic link appears to be weaker for most secondary forms of dementia, in which neurotransmitter dysfunction is more likely related to a systemic or diffuse neural damage. In these cases, therefore, an effort toward the understanding of pathological mechanisms of cognitive impairment should be made, also by investigating the relationship between functional alterations of brain circuits and the specific mechanisms of neuronal damage triggered by the causative disease. Neurophysiologically, although no distinctive TMS pattern can be identified that might be used to predict the occurrence or progression of cognitive decline in a specific condition, some TMS-associated measures of cortical function and plasticity (such as the short-latency afferent inhibition, the short-interval intracortical inhibition, and the cortical silent period) might add useful information in most of secondary dementia, especially in combination with suggestive clinical features and other diagnostic tests. The possibility to detect dysfunctional cortical circuits, to monitor the disease course, to probe the response to treatment, and to design novel neuromodulatory interventions in secondary dementia still represents a gap in the literature that needs to be explored.

Golexanolone, a GABAA receptor modulating steroid antagonist, restores motor coordination and cognitive function in hyperammonemic rats by dual effects on peripheral inflammation and neuroinflammation

Aims

Hyperammonemic rats show peripheral inflammation, increased GABAergic neurotransmission and neuroinflammation in cerebellum and hippocampus which induce motor incoordination and cognitive impairment. Neuroinflammation enhances GABAergic neurotransmission in cerebellum by enhancing the TNFR1‐glutaminase‐GAT3 and TNFR1‐CCL2‐TrkB‐KCC2 pathways. Golexanolone reduces GABA_A receptors potentiation by allopregnanolone. This work aimed to assess if treatment of hyperammonemic rats with golexanolone reduces peripheral inflammation and neuroinflammation and restores cognitive and motor function and to analyze underlying mechanisms.

Methods

Rats were treated with golexanolone and effects on peripheral inflammation, neuroinflammation, TNFR1‐glutaminase‐GAT3 and TNFR1‐CCL2‐TrkB‐KCC2 pathways, and cognitive and motor function were analyzed.

Results

Hyperammonemic rats show increased TNFα and reduced IL‐10 in plasma, microglia and astrocytes activation in cerebellum and hippocampus, and impaired motor coordination and spatial and short‐term memories. Treating hyperammonemic rats with golexanolone reversed changes in peripheral inflammation, microglia and astrocytes activation and restored motor coordination and spatial and short‐term memory. This was associated with reversal of the hyperammonemia‐enhanced activation in cerebellum of the TNFR1‐glutaminase‐GAT3 and TNFR1‐CCL2‐TrkB‐KCC2 pathways.

Conclusion

Reducing GABA_A receptors activation with golexanolone reduces peripheral inflammation and neuroinflammation and improves cognitive and motor function in hyperammonemic rats. The effects identified would also occur in patients with hepatic encephalopathy and, likely, in other pathologies associated with neuroinflammation.

Extracellular Vesicles From Hyperammonemic Rats Induce Neuroinflammation in Cerebellum of Normal Rats: Role of Increased TNFα Content

Hyperammonemia plays a main role in the neurological impairment in cirrhotic patients with hepatic encephalopathy. Rats with chronic hyperammonemia reproduce the motor incoordination of patients with minimal hepatic encephalopathy, which is due to enhanced GABAergic neurotransmission in cerebellum as a consequence of neuroinflammation. Extracellular vesicles (EVs) could play a key role in the transmission of peripheral alterations to the brain to induce neuroinflammation and neurological impairment in hyperammonemia and hepatic encephalopathy. EVs from plasma of hyperammonemic rats (HA-EVs) injected to normal rats induce neuroinflammation and motor incoordination, but the underlying mechanisms remain unclear. The aim of this work was to advance in the understanding of these mechanisms. To do this we used an ex vivo system. Cerebellar slices from normal rats were treated ex vivo with HA-EVs. The aims were: 1) assess if HA-EVs induce microglia and astrocytes activation and neuroinflammation in cerebellar slices of normal rats, 2) assess if this is associated with activation of the TNFR1-NF-kB-glutaminase-GAT3 pathway, 3) assess if the TNFR1-CCL2-BDNF-TrkB pathway is activated by HA-EVs and 4) assess if the increased TNFα levels in HA-EVs are responsible for the above effects and if they are prevented by blocking the action of TNFα. Our results show that ex vivo treatment of cerebellar slices from control rats with extracellular vesicles from hyperammonemic rats induce glial activation, neuroinflammation and enhance GABAergic neurotransmission, reproducing the effects induced by hyperammonemia in vivo. Moreover, we identify in detail key underlying mechanisms. HA-EVs induce the activation of both the TNFR1-CCL2-BDNF-TrkB-KCC2 pathway and the TNFR1-NF-kB-glutaminase-GAT3 pathway. Activation of these pathways enhances GABAergic neurotransmission in cerebellum, which is responsible for the induction of motor incoordination by HA-EVs. The data also show that the increased levels of TNFα in HA-EVs are responsible for the above effects and that the activation of both pathways is prevented by blocking the action of TNFα. This opens new therapeutic options to improve motor incoordination in hyperammonemia and also in cirrhotic patients with hepatic encephalopathy and likely in other pathologies in which altered cargo of extracellular vesicles contribute to the propagation of the pathology.

Hepatic encephalopathy

Hepatic encephalopathy (HE) is a prognostically relevant neuropsychiatric syndrome that occurs in the course of acute or chronic liver disease. Besides ascites and variceal bleeding, it is the most serious complication of decompensated liver cirrhosis. Ammonia and inflammation are major triggers for the appearance of HE, which in patients with liver cirrhosis involves pathophysiologically low-grade cerebral oedema with oxidative/nitrosative stress, inflammation and disturbances of oscillatory networks in the brain. Severity classification and diagnostic approaches regarding mild forms of HE are still a matter of debate. Current medical treatment predominantly involves lactulose and rifaximin following rigorous treatment of so-called known HE precipitating factors. New treatments based on an improved pathophysiological understanding are emerging.

Transcranial Magnetic Stimulation and Neocortical Neurons: The Micro-Macro Connection

Understanding the operation of cortical circuits is an important and necessary task in both neuroscience and neurorehabilitation. The functioning of the neocortex results from integrative neuronal activity, which can be probed non-invasively by transcranial magnetic stimulation (TMS). Despite a clear indication of the direct involvement of cortical neurons in TMS, no explicit connection model has been made between the microscopic neuronal landscape and the macroscopic TMS outcome. Here we have performed an integrative review of multidisciplinary evidence regarding motor cortex neurocytology and TMS-related neurophysiology with the aim of elucidating the micro–macro connections underlying TMS. Neurocytological evidence from animal and human studies has been reviewed to describe the landscape of the cortical neurons covering the taxonomy, morphology, circuit wiring, and excitatory–inhibitory balance. Evidence from TMS studies in healthy humans is discussed, with emphasis on the TMS pulse and paradigm selectivity that reflect the underlying neural circuitry constitution. As a result, we propose a preliminary neuronal model of the human motor cortex and then link the TMS mechanisms with the neuronal model by stimulus intensity, direction of induced current, and paired-pulse timing. As TMS bears great developmental potential for both a probe and modulator of neural network activity and neurotransmission, the connection model will act as a foundation for future combined studies of neurocytology and neurophysiology, as well as the technical advances and application of TMS.

Hyperammonemia Enhances GABAergic Neurotransmission in Hippocampus: Underlying Mechanisms and Modulation by Extracellular cGMP

Rats with chronic hyperammonemia reproduce the cognitive and motor impairment present in patients with hepatic encephalopathy. It has been proposed that enhanced GABAergic neurotransmission in hippocampus may contribute to impaired learning and memory in hyperammonemic rats. However, there are no direct evidences of the effects of hyperammonemia on GABAergic neurotransmission in hippocampus or on the underlying mechanisms. The aims of this work were to assess if chronic hyperammonemia enhances the function of GABA_A receptors in hippocampus and to identify the underlying mechanisms. Activation of GABA_A receptors is enhanced in hippocampus of hyperammonemic rats, as analyzed in a multielectrode array system. Hyperammonemia reduces membrane expression of the GABA transporters GAT1 and GAT3, which is associated with increased extracellular GABA concentration. Hyperammonemia also increases gephyrin levels and phosphorylation of the β3 subunit of GABA_A receptor, which are associated with increased membrane expression of the GABA_A receptor subunits α1, α2, γ2, β3, and δ. Enhanced levels of extracellular GABA and increased membrane expression of GABA_A receptors would be responsible for the enhanced GABAergic neurotransmission in hippocampus of hyperammonemic rats. Increasing extracellular cGMP reverses the increase in GABA_A receptors activation by normalizing the membrane expression of GABA transporters and GABA_A receptors. The increased GABAergic neurotransmission in hippocampus would contribute to cognitive impairment in hyperammonemic rats. The results reported suggest that reducing GABAergic tone in hippocampus by increasing extracellular cGMP or by other means may be useful to improve cognitive function in hyperammonemia and in cirrhotic patients with minimal or clinical hepatic encephalopathy.

Diagnostic contribution and therapeutic perspectives of transcranial magnetic stimulation in dementia

Transcranial magnetic stimulation (TMS) is a powerful tool to probe in vivo brain circuits, as it allows to assess several cortical properties such asexcitability, plasticity and connectivity in humans. In the last 20 years, TMS has been applied to patients with dementia, enabling the identification of potential markers of thepathophysiology and predictors of cognitive decline; moreover, applied repetitively, TMS holds promise as a potential therapeutic intervention. The objective of this paper is to present a comprehensive review of studies that have employed TMS in dementia and to discuss potential clinical applications, from the diagnosis to the treatment. To provide a technical and theoretical framework, we first present an overview of the basic physiological mechanisms of the application of TMS to assess cortical excitability, excitation and inhibition balance, mechanisms of plasticity and cortico-cortical connectivity in the human brain. We then review the insights gained by TMS techniques into the pathophysiology and predictors of progression and response to treatment in dementias, including Alzheimer's disease (AD)-related dementias and secondary dementias. We show that while a single TMS measure offers low specificity, the use of a panel of measures and/or neurophysiological index can support the clinical diagnosis and predict progression. In the last part of the article, we discuss the therapeutic uses of TMS. So far, only repetitive TMS (rTMS) over the left dorsolateral prefrontal cortex and multisite rTMS associated with cognitive training have been shown to be, respectively, possibly (Level C of evidence) and probably (Level B of evidence) effective to improve cognition, apathy, memory, and language in AD patients, especially at a mild/early stage of the disease. The clinical use of this type of treatment warrants the combination of brain imaging techniques and/or electrophysiological tools to elucidate neurobiological effects of neurostimulation and to optimally tailor rTMS treatment protocols in individual patients or specific patient subgroups with dementia or mild cognitive impairment.

The reliability of cerebellar brain inhibition

OBJECTIVE

Connectivity between the cerebellum and primary motor cortex (M1) can be assessed by using transcranial magnetic stimulation to measure cerebellar brain inhibition (CBI). The aim of the present study was to determine the intra- and inter-day measurment error and relative reliability of CBI. The former informs the degree to which repeated measurements vary, whereas the latter informs how well the measure can distinguish individuals from one another within a sample.

METHODS

We obtained CBI data from 83 healthy young participants (n = 55 retrospective). Intra-day measurements were separated by ~ 30 min. Inter-day measurmenets were separated by a minimum of 24 h.

RESULTS

We show that CBI has low measurement error (~15%) within and between sessions. Using the measurment error, we demonstrate that change estimates which exceed measurment noise are large at an individual level, but can be detected with modest sample sizes. Finally, we demonstrate that the CBI measurement has fair to good relative reliability in healthy individuals, which may be deflated by low sample heterogeneity.

CONCLUSIONS

CBI has low measurement error supporting its use for tracking intra- and inter-day changes in cerebellar-M1 connectivity.

SIGNIFICANCE

Our findings provide clear reliability guidelines for future studies assessing modulation of cerebellar-M1 connectivity with intervention or disease progression.

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.

The Role of Intestinal Bacteria and Gut-Brain Axis in Hepatic Encephalopathy

Hepatic encephalopathy (HE) is a neurological disorder that occurs in patients with liver insufficiency. However, its pathogenesis has not been fully elucidated. Pharmacotherapy is the main therapeutic option for HE. It targets the pathogenesis of HE by reducing ammonia levels, improving neurotransmitter signal transduction, and modulating intestinal microbiota. Compared to healthy individuals, the intestinal microbiota of patients with liver disease is significantly different and is associated with the occurrence of HE. Moreover, intestinal microbiota is closely associated with multiple links in the pathogenesis of HE, including the theory of ammonia intoxication, bile acid circulation, GABA-ergic tone hypothesis, and neuroinflammation, which contribute to cognitive and motor disorders in patients. Restoring the homeostasis of intestinal bacteria or providing specific probiotics has significant effects on neurological disorders in HE. Therefore, this review aims at elucidating the potential microbial mechanisms and metabolic effects in the progression of HE through the gut–brain axis and its potential role as a therapeutic target in HE.

Hyperammonemia Post Lung Transplantation: A Review

Hyperammonemia is the pathological accumulation of ammonia in the blood, which can occur in many different clinical settings. Most commonly in adults, hyperammonemia occurs secondary to hepatic dysfunction; however, it is also known to be associated with other pathologies, surgeries, and medications. Although less common, hyperammonemia has been described as a rare, but consistent complication of solid organ transplantation. Lung transplantation is increasingly recognized as a unique risk factor for the development of this condition, which can pose grave health risks-including long-term neurological sequelae and even death. Recent clinical findings have suggested that patients receiving lung transplantations may experience postoperative hyperammonemia at rates as high as 4.1%. A wide array of etiologies has been attributed to this condition. A growing number of case studies and investigations suggest disseminated opportunistic infection with Ureaplasma or Mycoplasma species may drive this metabolic disturbance in lung transplant recipients. Regardless of the etiology, hyperammonemia presents a severe clinical problem with reported mortality rates as high as 75%. Typical treatment regimens are multimodal and focus on 3 main avenues of management: (1) the reduction of impact on the brain through the use of neuroprotective medications and decreasing cerebral edema, (2) augmentation of mechanisms for the elimination of ammonia from the blood via hemodialysis, and (3) the diminishment of processes producing predominantly using antibiotics. The aim of this review is to detail the pathophysiology of hyperammonemia in the setting of orthotopic lung transplantation and discuss methods of identifying and managing patients with this condition.

Ammonia induces calpain-dependent cleavage of CRMP-2 during neurite degeneration in primary cultured neurons

Hyperammonemia in the CNS induces irreversible damages to neurons due to ultimate cell loss. Neurite degeneration, a primary event that leads to neuronal cell death, remains less elucidated especially in hyperammonemia circumstances. Here, we found that the administration of ammonia induced neurite degeneration in cultured cerebellar granule neurons. The resulting altered neuronal morphology, rupture of neurites, and disassembly of the cytoskeleton led to cell death. Calcein and Fluo-4 staining revealed that ammonia induced intracellular calcium dysregulation. Subsequently activated calpain cleaved CRMP-2, a microtubule assembly protein. Pharmacologically inhibition of calpain, but not caspases or GSK-3, suppressed the cleavage of CRMP-2 and reversed neurite degeneration under ammonia treatment. Exposure to ammonia decreased whereas inhibition of calpain restored the amplitude and frequency of miniature excitatory postsynaptic currents. These data suggest a mechanism by which elevated ammonia level may induce neuronal dysfunction via abnormal calcium influx and calpain-dependent CRMP-2 cleavage, leading to abnormal synaptic transmission, cytoskeletal collapse, and neurite degeneration.