Abnormal brain oxygen homeostasis in an animal model of liver disease

Background & Aims

Increased plasma ammonia concentration and consequent disruption of brain energy metabolism could underpin the pathogenesis of hepatic encephalopathy (HE). Brain energy homeostasis relies on effective maintenance of brain oxygenation, and dysregulation impairs neuronal function leading to cognitive impairment. We hypothesised that HE is associated with reduced brain oxygenation and we explored the potential role of ammonia as an underlying pathophysiological factor.

Methods

In a rat model of chronic liver disease with minimal HE (mHE; bile duct ligation [BDL]), brain tissue oxygen measurement, and proton magnetic resonance spectroscopy were used to investigate how hyperammonaemia impacts oxygenation and metabolic substrate availability in the central nervous system. Ornithine phenylacetate (OP, OCR-002; Ocera Therapeutics, CA, USA) was used as an experimental treatment to reduce plasma ammonia concentration.

Results

In BDL animals, glucose, lactate, and tissue oxygen concentration in the cerebral cortex were significantly lower than those in sham-operated controls. OP treatment corrected the hyperammonaemia and restored brain tissue oxygen. Although BDL animals were hypotensive, cortical tissue oxygen concentration was significantly improved by treatments that increased arterial blood pressure. Cerebrovascular reactivity to exogenously applied CO2 was found to be normal in BDL animals.

Conclusions

These data suggest that hyperammonaemia significantly decreases cortical oxygenation, potentially compromising brain energy metabolism. These findings have potential clinical implications for the treatment of patients with mHE.

Lay summary

Brain dysfunction is a serious complication of cirrhosis and affects approximately 30% of these patients; however, its treatment continues to be an unmet clinical need. This study shows that oxygen concentration in the brain of an animal model of cirrhosis is markedly reduced. Low arterial blood pressure and increased ammonia (a neurotoxin that accumulates in patients with liver failure) are shown to be the main underlying causes. Experimental correction of these abnormalities restored oxygen concentration in the brain, suggesting potential therapeutic avenues to explore.

Sex-specific metabolic alterations in the type 1 diabetic brain of mice revealed by an integrated method of metabolomics and mixed-model

Type 1 diabetes (T1D) can cause brain region-specific metabolic disorders, but whether gender influences T1D-related brain metabolic changes is rarely reported. Therefore, here we examined metabolic changes in six different brain regions of male and female mice under normal and T1D conditions using an integrated method of NMR-based metabolomics and linear mixed-model, and aimed to explore sex-specific metabolic changes from normal to T1D. The results demonstrate that metabolic differences occurred in all brain regions between two genders, while the hippocampal metabolism is more likely to be affected by T1D. At the 4th week after streptozotocin treatment, brain metabolic disorders mainly occurred in the cortex and hippocampus in female T1D mice, but the striatum and hippocampus in male T1D mice. In addition, anaerobic glycolysis was significantly altered in male mice, mainly in the striatum, midbrain, hypothalamus and hippocampus, but not in female mice. We also found that female mice exhibited a hypometabolism status relative to male mice from normal to T1D. Collectively, this study suggests that T1D affected brain region-specific metabolic alterations in a sex-specific manner, and may provide a metabolic view on diabetic brain diseases between genders.

Using thermodynamic parameters to calibrate a mechanistic dose-response for infection of a host by a virus

Assessing the risk of infection from emerging viruses or of existing viruses jumping the species barrier into novel hosts is limited by the lack of dose response data. The initial stages of the infection of a host by a virus involve a series of specific contact interactions between molecules in the host and on the virus surface. The strength of the interaction is quantified in the literature by the dissociation constant (K_d) which is determined experimentally and is specific for a given virus molecule/host molecule combination. Here, two stages of the initial infection process of host intestinal cells are modelled, namely escape of the virus in the oral challenge dose from the innate host defenses (e.g. mucin proteins in mucus) and the subsequent binding of any surviving virus to receptor molecules on the surface of the host epithelial cells. The strength of virus binding to host cells and to mucins may be quantified by the association constants, K_a and K_mucin, respectively. Here, a mechanistic dose-response model for the probability of infection of a host by a given virus dose is constructed using K_a and K_mucin which may be derived from published K_d values taking into account the number of specific molecular interactions. It is shown that the effectiveness of the mucus barrier is determined not only by the amount of mucin but also by the magnitude of K_mucin. At very high K_mucin values, slight excesses of mucin over virus are sufficient to remove all the virus according to the model. At lower K_mucin values, high numbers of virus may escape even with large excesses of mucin. The output from the mechanistic model is the probability (p₁) of infection by a single virion which is the parameter used in conventional dose-response models to predict the risk of infection of the host from the ingested dose. It is shown here how differences in K_a (due to molecular differences in an emerging virus strain or new host) affect p₁, and how these differences in K_a may be quantified in terms of two thermodynamic parameters, namely enthalpy and entropy. This provides the theoretical link between sequencing data and risk of infection. Lack of data on entropy is a limitation at present and may also affect our interpretation of K_d in terms of infectivity. It is concluded that thermodynamic approaches have a major contribution to make in developing dose-response models for emerging viruses.

Obstructive sleep apnea is associated with altered midbrain chemical concentrations

Obstructive sleep apnea (OSA) is accompanied by altered structure and function in cortical, limbic, brainstem, and cerebellar regions. The midbrain is relatively unexamined, but contains many integrative nuclei which mediate physiological functions that are disrupted in OSA. We therefore assessed the chemistry of the midbrain in OSA in this exploratory study. We used a recently developed accelerated 2D magnetic resonance spectroscopy (2D-MRS) technique, compressed sensing-based 4D echo-planar J-resolved spectroscopic imaging (4D-EP-JRESI), to measure metabolites in the midbrain of 14 OSA (mean age±SD:54.6±10.6years; AHI:35.0±19.4; SAO2 min:83±7%) and 26 healthy control (50.7±8.5years) subjects. High-resolution T1-weighted scans allowed voxel localization. MRS data were processed with custom MATLAB-based software, and metabolite ratios calculated with respect to the creatine peak using a prior knowledge fitting (ProFit) algorithm. The midbrain in OSA showed decreased N-acetylaspartate (NAA; OSA:1.24±0.43, Control:1.47±0.41; p=0.03; independent samples t-test), a marker of neuronal viability. Increased levels in OSA over control subjects appeared in glutamate (Glu; OSA:1.23±0.57, Control:0.98±0.33; p=0.03), ascorbate (Asc; OSA:0.56±0.28, Control:0.42±0.20; (50.7±8.5years; p=0.03), and myo-inositol (mI; OSA:0.96±0.48, Control:0.72±0.35; p=0.03). No differences between groups appeared in γ-aminobutyric acid (GABA) or taurine. The midbrain in OSA patients shows decreased NAA, indicating neuronal injury or dysfunction. Higher Glu levels may reflect excitotoxic processes and astrocyte activation, and higher mI is also consistent with glial activation. Higher Asc levels may result from oxidative stress induced by intermittent hypoxia in OSA. Additionally, Asc and Glu are involved with glutamatergic processes, which are likely upregulated in the midbrain nuclei of OSA patients. The altered metabolite levels help explain dysfunction and structural deficits in the midbrain of OSA patients.

Glutamine up-regulates MAPK phosphatase-1 induction via activation of Ca2+→ ERK cascade pathway

The non-essential amino acid L-glutamine (Gln) displays potent anti-inflammatory activity by deactivating p38 mitogen activating protein kinase and cytosolic phospholipase A₂ via induction of MAPK phosphatase-1 (MKP-1) in an extracellular signal-regulated kinase (ERK)-dependent way. In this study, the mechanism of Gln-mediated ERK-dependency in MKP-1 induction was investigated. Gln increased ERK phosphorylation and activity, and phosphorylations of Ras, c-Raf, and MEK, located in the upstream pathway of ERK, in response to lipopolysaccharidein vitro and in vivo. Gln-induced dose-dependent transient increases in intracellular calcium ([Ca²⁺]_i) in MHS macrophage cells. Ionomycin increased [Ca²⁺]_i and activation of Ras → ERK pathway, and MKP-1 induction, in the presence, but not in the absence, of LPS. The Gln-induced pathways involving Ca²⁺→ MKP-1 induction were abrogated by a calcium blocker. Besides Gln, other amino acids including L-phenylalanine and l-cysteine (Cys) also induced Ca²⁺ response, activation of Ras → ERK, and MKP-1 induction, albeit to a lesser degree. Gln and Cys were comparable in suppression against 2, 4-dinitrofluorobenzene-induced contact dermatitis. Gln-mediated, but not Cys-mediated, suppression was abolished by MKP-1 small interfering RNA. These data indicate that Gln induces MKP-1 by activating Ca²⁺→ ERK pathway, which plays a key role in suppression of inflammatory reactions.

Ameliorating effects of traditional Chinese medicine preparation, Chinese materia medica and active compounds on ischemia/reperfusion-induced cerebral microcirculatory disturbances and neuron damage

Ischemic stroke and ischemia/reperfusion (I/R) injury induced by thrombolytic therapy are conditions with high mortality and serious long-term physical and cognitive disabilities. They have a major impact on global public health. These disorders are associated with multiple insults to the cerebral microcirculation, including reactive oxygen species (ROS) overproduction, leukocyte adhesion and infiltration, brain blood barrier (BBB) disruption, and capillary hypoperfusion, ultimately resulting in tissue edema, hemorrhage, brain injury and delayed neuron damage. Traditional Chinese medicine (TCM) has been used in China, Korea, Japan and other Asian countries for treatment of a wide range of diseases. In China, the usage of compound TCM preparation to treat cerebrovascular diseases dates back to the Han Dynasty. Even thousands of years earlier, the medical formulary recorded many classical prescriptions for treating cerebral I/R-related diseases. This review summarizes current information and underlying mechanisms regarding the ameliorating effects of compound TCM preparation, Chinese materia medica, and active components on I/R-induced cerebral microcirculatory disturbances, brain injury and neuron damage.