Differential effects of ethanol on glial signal transduction initiated by lipopolysaccharide and interferon-gamma

Relationship between sphingolipids-mediated neuroinflammation and alcohol use disorder

BACKGROUND

Alcohol use disorder is a chronic recurrent encephalopathy, and its pathogenesis has not been fully understood. Among possible explanations, neuroinflammation caused by the disorders of brain central immune signaling has been identified as one possible mechanism of alcohol use disorder. As the basic components of cells and important bioactive molecules, sphingolipids are essential in regulating many cellular activities. Recent studies have shown that sphingolipids-mediated neuroinflammation may be involved in the development of alcohol use disorder.

METHODS

PubMed databases were searched for literature on sphingolipids and alcohol use disorder (alcohol abuse, alcohol addiction, alcohol dependence, and alcohol misuse) including evidence of the relationship between sphingolipids-mediated neuroinflammation and alcohol use disorder (formation, withdrawal, treatment).

RESULTS

Disorders of sphingolipid metabolism, including the different types of sphingolipids and regulatory enzyme activity, have been found in patients with alcohol use disorder as well as animal models, which in turn cause neuro-inflammation in the central nervous system. Thus, these disorders may also be an important mechanism in the development of alcohol use disorder in patients. In addition, different sphingolipids may have different or even reverse effects on alcohol use disorder.

CONCLUSIONS

The sphingolipids-mediated neuroinflammation plays an important role in the development of alcohol use disorder. This review proposes a potential approach to prevent and treat alcohol use disorders by manipulating sphingolipid metabolism.

Prevention and mitigation of alcohol-induced neuroinflammation by Lactobacillus plantarum by an EGF receptor-dependent mechanism

Neuroinflammation is implicated in the pathogenesis of alcohol use disorders. We investigated the role of Gut-Brain interactions in alcohol-induced neuroinflammation by probiotic-mediated manipulation of intestinal dysbiosis in mice. Chronic ethanol feeding induced dysbiosis, as evidenced by an increase in Firmicutes/Bacteroidetes ratio and depletion of Lactobacillus species in the colon. Ethanol increased the levels of IL-1β, IL-6, and TNFα in plasma and the mRNA for IL-1β, IL-6, TNFα, and MCP1 genes in the cerebral cortex and hippocampus. Ethanol feeding increased inulin flux from the circulation into different brain regions, accompanied by the increase in TLR4 mRNA levels in the cerebral cortex and hippocampus. The immunofluorescence confocal microscopy showed that ethanol elevates the expression of microglial activation marker TMEM119 in the cerebral cortex. Feeding L. plantarum suppressed the ethanol-induced dysbiosis to some extent, as evidenced by attenuation of ethanol effects on Firmicutes/Bacteroidetes ratio and abundance of Lactobacillus spp. L. plantarum blocked ethanol-induced elevation of plasma cytokines, inulin permeability to the brain, mRNA for TLR4, IL-1β, IL-6, TNFα, and MCP1 in brain regions, and the expression of TMEM119 in the cerebral cortex. The L. plantarum effect was absent in mice that express a dominant-negative EGFR, suggesting that the EGFR receptor plays an essential role in the protective effect of L. plantarum against ethanol-induced neuroinflammation. L. plantarum, when administered after chronic ethanol-induced injury, rescued the ethanol-induced systemic inflammation and neuroinflammation. This study demonstrates that L. plantarum in the gut prevents and mitigates ethanol-induced neuroinflammation by an EGFR-dependent mechanism.

Delivering alcohol neurotoxicity into nucleus, when clusterin meets BclXL: a commentary

The study by Kim and colleagues (in press) demonstrated that the expression of nuclear isoform of clusterin is induced by ethanol (EtOH) to participate in apoptotic cell death of neurons in developing rodent brain. EtOH-induced nuclear clusterin interacts with Bcl(XL), thereby liberating proapoptotic Bax. This study indicates the proapoptotic role of nuclear clusterin in EtOH-exposed neurons, linking specific nuclear events to alcohol neurotoxicity. The study provides novel insights into the molecular mechanisms underlying fetal alcohol spectrum disorders.

Measurement of serum, liver, and brain cytokine induction, thiamine levels, and hepatopathology in rats exposed to a 4-day alcohol binge protocol

BACKGROUND

 In rodent and human studies, ethanol (EtOH) exposure is associated with elevated brain levels of the magnetic resonance spectroscopy (MRS) signal representing choline-containing compounds (Cho). One interpretation of elevated brain Cho is that it is a marker of neuroinflammation, and some evidence suggests that EtOH exposure promotes neuroinflammation. This study aimed to determine whether binge EtOH exposure (intragastric 3 g/kg 25% EtOH every 8 hours for 4 days) would induce the expression of certain cytokines in blood, liver, or brain, thereby supporting the neuroinflammation hypothesis of elevated Cho.

METHODS

Ten of 18 wild-type male Wistar rats (~322 g at baseline) were exposed to EtOH and attained average blood alcohol levels of ~315 mg/dl across 4 days. Blood for cytokine immunoassays was collected at baseline, after 5 doses of EtOH (binge), and immediately preceding euthanasia either 4 or 24 hours after the last dose of EtOH. Blood was additionally assayed for the levels of thiamine and liver enzymes; liver histopathology was performed postmortem; and tissue from liver and 6 brain regions was assayed for the potential induction of 7 cytokines.

RESULTS

There were no group effects on the levels of thiamine or its phosphate derivatives, thiamine monophosphate or thiamine diphosphate. ANOVAs of liver enzyme levels indicated that only alkaline phosphatase (ALP) levels were higher in the EtOH group than in control group at binge; ALP elevations, however, are difficult to explain in the absence of changes in the levels of additional liver enzymes. Postmortem liver pathology provided evidence for minimal microvesicular lipidosis and portocentric fibrosis in the EtOH group. Group effects on the levels of the measured cytokines in the blood (TNF-α, IFN-γ, IL-1β, IL-4, IL-5, IL-13, and GRO/CXCL1) were not significant. Similarly, postmortem evaluation of liver cytokines did not reveal group effects. Postmortem evaluation of the 7 cytokines in 6 brain regions (anterior cerebellar vermis, cingulate cortex, frontal cortex, hippocampus, hypothalamus, striatum) also failed to identify group effects.

CONCLUSIONS

A single 4-day bout of binge EtOH exposure alone was insufficient to induce the expression of 7 cytokines in blood, liver, or 6 brain regions of wild-type Wistar rats. Alternative interpretations for elevations in brain Cho in response to a 4-day binge EtOH treatment are therefore necessary and may include induction of cytokines not measured herein or other noninflammatory mechanisms.

TNF is a key mediator of septic encephalopathy acting through its receptor, TNF receptor-1

In this study, we demonstrate that mice deficient in TNFR1 (TNFR1(-/-)) were resistant to LPS-induced encephalopathy. Systemic administration of lipopolysaccharide (LPS) induces a widespread inflammatory response similar to that observed in sepsis. Following LPS administration TNFR1(-/-) mice had less caspase-dependent apoptosis in brain cells and fewer neutrophils infiltrating the brain (p<0.039), compared to control C57Bl6 (TNFR1(+/+)) mice. TNFR1-dependent increase in aquaporin (AQP)-4 mRNA and protein expression was observed with a concomitant increase in water content, in brain (18% increase in C57Bl6 mice treated with LPS versus those treated with saline), similar to cerebral edema observed in sepsis. Furthermore, absence of TNFR1 partially but significantly reduced the activation of astrocytes, as shown by immunofluorescence and markedly inhibited iNOS mRNA expression (p<0.01). Septic encephalopathy is a devastating complication of sepsis. Although, considerable work has been done to identify the mechanism causing the pathological alterations in this setting, the culprit still remains an enigma. Our results demonstrate for the first time that endotoxemia leads to inflammation in brain, with alteration in blood-brain barrier, up-regulation of AQP4 and associated edema, neutrophil infiltration, astrocytosis, as well as apoptotic cellular death, all of which appear to be mediated by TNF-alpha signaling through TNFR1.

Regulation of Toll-like receptor 4 expression and its signaling by hypoxia in cultured microglia

Hypoxia is an important biological signal that regulates a wide variety of physiological responses. At the same time, hypoxia is involved in multiple pathological situations. In particular, hypoxia is closely associated with neural injury in the brain. Hypoxia has been recently proposed as a neuroinflammatogen, as it can induce the inflammatory activation of microglia, a major cellular source of inflammatory mediators in the brain. In this article, we present evidence that hypoxia enhances Toll-like receptor 4 (TLR4) expression in cultured microglia and differentially regulates the downstream signaling pathways of TLR4. Hypoxia up-regulated TLR4 expression at the mRNA and protein levels in a microglia cell line, as well as in primary microglia cultures. Hypoxia, however, differentially regulated MyD88-dependent and -independent pathways of TLR4 signaling: Hypoxia enhanced lipopolysaccharide (LPS)-induced interferon regulatory factor-3 (IRF-3) activation and the subsequent expression of IFNbeta (MyD88-independent pathway), whereas it suppressed LPS-induced NF-kappaB activation (MyD88-dependent pathway). Hypoxia did not affect IFNgamma signaling, which was represented by signal transducer and activator of transcription-1 (STAT1) activation and interferon-regulatory factor-1 (IRF-1) induction. Taken together, although hypoxia up-regulates TLR4 expression, its downstream signaling pathways appear to be differentially modulated by hypoxia.