Toll-like receptor 4 increases intestinal permeability through up-regulation of membrane PKC activity in alcoholic steatohepatitis

Intestinal barrier permeability: the influence of gut microbiota, nutrition, and exercise

The intestinal wall is a selectively permeable barrier between the content of the intestinal lumen and the internal environment of the body. Disturbances of intestinal wall permeability can potentially lead to unwanted activation of the enteric immune system due to excessive contact with gut microbiota and its components, and the development of endotoxemia, when the level of bacterial lipopolysaccharides increases in the blood, causing chronic low-intensity inflammation. In this review, the following aspects are covered: the structure of the intestinal wall barrier; the influence of the gut microbiota on the permeability of the intestinal wall via the regulation of functioning of tight junction proteins, synthesis/degradation of mucus and antioxidant effects; the molecular mechanisms of activation of the pro-inflammatory response caused by bacterial invasion through the TLR4-induced TIRAP/MyD88 and TRAM/TRIF signaling cascades; the influence of nutrition on intestinal permeability, and the influence of exercise with an emphasis on exercise-induced heat stress and hypoxia. Overall, this review provides some insight into how to prevent excessive intestinal barrier permeability and the associated inflammatory processes involved in many if not most pathologies. Some diets and physical exercise are supposed to be non-pharmacological approaches to maintain the integrity of intestinal barrier function and provide its efficient operation. However, at an early age, the increased intestinal permeability has a hormetic effect and contributes to the development of the immune system.

Efficacy of high-intensity interval and continuous endurance trainings on cecal microbiota metabolites and inflammatory factors in diabetic rats induced by high-fat diet

Physical exercise is known to modulate the intestinal microbiota composition and control the symptoms of metabolic syndrome. In this research, we intend to investigate and compare the effect of high-intensity interval and continuous endurance trainings (HIIT and CET) on cecal microbiota metabolites and inflammatory factors in diabetic rats. A number of Wistar rats were made diabetic by a high-fat diet and trained under two types of exercise protocols, HIIT and CET. After taking samples from the cecal tissue and serum of rats to reveal the effect of exercise, three microbial species from the Firmicute and Bacteroid phyla, which are the main types of intestinal microbes, and their metabolites include two short-chain fatty acids (SCFAs), butyrate and propionate and also, the inflammatory factors TLR4 and IL6 were analyzed through quantitative polymerase chain reaction (qPCR), high-performance liquid chromatography (HPLC), and Enzyme-linked immunosorbent assay (ELISA) methods. In general, exercise while increasing the representative of Firmicute has caused a relative reduction of Bacteroides and improved the concentration of SCFAs. In this regard, HIIT outperforms CET in up-regulating Akkermansia and Butyrivibrio expression, and butyrate and propionate metabolites concentration. Also, both exercises significantly reduced cecal expression of TLR4 and sera concentration of IL6 compared to the diabetic group, although the reduction rate was higher in the CET group than in HIIT. Our findings suggest that some symptoms of metabolic syndrome such as intestinal dysbiosis and the resulting metabolic disorders are better controlled by HIIT and inflammation by CET. Certainly, more extensive research on other contributing factors could help clarify the results.

Cell-free hemoglobin triggers macrophage cytokine production via TLR4 and MyD88

Cell-free hemoglobin (CFH) is elevated in the airspace of patients with acute respiratory distress syndrome (ARDS) and is sufficient to cause acute lung injury in a murine model. However, the pathways through which CFH causes lung injury are not well understood. Toll-like receptor 4 (TLR4) is a mediator of inflammation after detection of damage- and pathogen-associated molecular patterns. We hypothesized that TLR4 signaling mediates the proinflammatory effects of CFH in the airspace. After intratracheal CFH, BALBc mice deficient in TLR4 had reduced inflammatory cell influx into the airspace [bronchoalveolar lavage (BAL) cell counts, median TLR4 knockout (KO): 0.8 × 104/mL [IQR 0.4-1.2 × 104/mL], wild-type (WT): 3.0 × 104/mL [2.2-4.0 × 104/mL], P < 0.001] and attenuated lung permeability (BAL protein, TLR4KO: 289 µg/mL [236-320], WT: 488 µg/mL [422-536], P < 0.001). These mice also had attenuated production of interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α in the airspace. C57Bl/6 mice lacking TLR4 on myeloid cells only (LysM.Cre+/-TLR4fl/fl) had reduced cytokine production in the airspace after CFH, without attenuation of lung permeability. In vitro studies confirm that WT primary murine alveolar macrophages exposed to CFH (0.01-1 mg/mL) had dose-dependent increases in IL-6, IL-1 β, CXC motif chemokine ligand 1 (CXCL-1), TNF-α, and IL-10 (P < 0.001). Murine MH-S alveolar-like macrophages show TLR4-dependent expression of IL-1β, IL-6, and CXCL-1 in response to CFH. Primary alveolar macrophages from mice lacking TLR4 adaptor proteins myeloid differentiation primary response 88 (MyD88) or TIR-domain-containing adapter-inducing interferon-β (TRIF) revealed that MyD88KO macrophages had 71-96% reduction in CFH-dependent proinflammatory cytokine production (P < 0.001), whereas macrophages from TRIFKO mice had variable changes in cytokine responses. These data demonstrate that myeloid TLR4 signaling through MyD88 is a key regulator of airspace inflammation in response to CFH.NEW & NOTEWORTHY Cell-free hemoglobin (CFH) is elevated in the airspace of most patients with acute respiratory distress syndrome and causes severe inflammation. Here, we identify that CFH contributes to macrophage-induced cytokine production via Toll-like receptor 4 (TLR4) and myeloid differentiation primary response 88 (MyD88) signaling. These data increase our knowledge of the mechanisms through which CFH contributes to lung injury and may inform development of targeted therapeutics to attenuate inflammation.

Fermentation of Plant Extracts Supplemented with Milk Components by Lactic Acid Bacteria Produces Soluble Agonists for Toll-like Receptor 2 Possibly Suitable for Cosmetics

Stimulation of Toll-like receptor 2 (TLR2) on epidermal keratinocytes results in the tightening of cell–cell junctions between keratinocytes; therefore, appropriate agonists for TLR2 could be promising ingredients for cosmetics. However, a method to produce significant amounts of soluble TLR2 agonists using materials that are suitable for preparing cosmetics has not yet been developed. In this study, we tried to identify appropriate lactic acid bacterial strains and media for fermentation to obtain soluble TLR2 agonists from traditional fermented foods and natural food sources. We found that Lactobacillus delbrueckii subsp. lactis TL24 (TL24) and a combination of hot water extracts of asparagus edible stem and cow skimmed milk were the best strain and culture medium, respectively, for this purpose. The TL24 ferments effectively stimulated TLR2 in HEK293 reporter cells expressing human TLR2 on their surface and also inhibited paracellular molecular transfer in a cell sheet of human primary keratinocytes. Since these effects of the TL24 ferments were suppressed by anti-TLR2 neutralizing antibodies, it is proposed that TL24 ferments elicit these effects via TLR2. Taken together, these results suggest that TL24 ferments containing soluble TLR2 agonists are potential ingredients for cosmetics.

The dietary fibre rhamnogalacturonan improves intestinal epithelial barrier function in a microbiota-independent manner

BACKGROUND AND PURPOSE

Dietary fibre comprises a complex group of polysaccharides that are indigestible but are fermented by gut microbiota, promoting beneficial effects to the intestinal mucosa indirectly through the production of short chain fatty acids. We found that a polysaccharide, rhamnogalacturonan (RGal), from the plant Acmella oleracea, has direct effects on intestinal epithelial barrier function. Our objective was to determine the mechanism whereby RGal enhances epithelial barrier function.

EXPERIMENTAL APPROACH

Monolayers of colonic epithelial cell lines (Caco-2, T84) and of human primary cells from organoids were mounted in Ussing chambers to assess barrier function. The cellular mechanism of RGal effects on barrier function was determined using inhibitors of TLR-4 and PKC isoforms.

KEY RESULTS

Apically applied RGal (1000 μg/ml) significantly enhanced barrier function as shown by increased transepithelial electrical resistance (TER) and reduced fluorescein isothiocyanate (FITC)-dextran flux in Caco-2, T84 and human primary cell monolayers, and accelerated tight junction reassembly in Caco-2 cells in a calcium switch assay. RGal also reversed the barrier-damaging effects of inflammatory cytokines on FITC-dextran flux and preserved the tight junction distribution of occludin. RGal activated TLR4 in TLR4-expressing HEK reporter cells, an effect that was significantly inhibited by the TLR4 inhibitor, C34. The effect of RGal was also dependent on PKC, specifically the isoforms PKCd and PKCζ.

CONCLUSION AND IMPLICATIONS

RGal enhances intestinal epithelial barrier function through activation of TLR4 and PKC signaling pathways. Elucidation of RGal mechanisms of action could lead to new, dietary approaches to enhance mucosal healing in inflammatory bowel diseases.

Acute Alcohol Intoxication Modulates Monocyte Subsets and Their Functions in a Time-Dependent Manner in Healthy Volunteers

Background

Excessive alcohol intake is associated with adverse immune response-related effects, however, acute and chronic abuse differently modulate monocyte activation. In this study, we have evaluated the phenotypic and functional changes of monocytes in acutely intoxicated healthy volunteers (HV).

Methods

Twenty-two HV consumed individually adjusted amounts of alcoholic beverages until reaching a blood alcohol level of 1‰ after 4h (T4). Peripheral blood was withdrawn before and 2h (T2), 4h (T4), 6h (T6), 24h (T24), and 48h (T48) after starting the experiment and stained for CD14, CD16 and TLR4. CD14^brightCD16^-, CD14^brightCD16⁺ and CD14^dimCD16⁺ monocyte subsets and their TLR4 expression were analyzed by flow cytometry. Inflammasome activation via caspase-1 in CD14⁺ monocytes was measured upon an ex vivo in vitro LPS stimulation. Systemic IL-1β and adhesion capacity of isolated CD14⁺ monocytes upon LPS stimulation were evaluated.

Results

The percentage of CD14⁺ monocyte did not change following alcohol intoxication, whereas CD14^brightCD16^- monocyte subset significantly increased at T2 and T24, CD14^brightCD16⁺ at T2, T4 and T6 and CD14^dimCD16⁺ at T4 and T6. The relative fraction of TLR4 expressing CD14⁺ monocytes as well as the density of TLR4 surface presentation increased at T2 and decreased at T48 significantly. TLR4⁺CD14⁺ monocytes were significantly enhanced in all subsets at T2. TLR4 expression significantly decreased in CD14^brightCD16^- at T48, in CD14^brightCD16⁺ at T24 and T48, increased in CD14^dimCD16⁺ at T2. IL-1β release upon LPS stimulation decreased at T48, correlating with TLR4 receptor expression. Alcohol downregulated inflammasome activation following ex vivo in vitro stimulation with LPS between T2 and T48 vs. T0. The adhesion capacity of CD14⁺ monocytes decreased from T2 with significance at T4, T6 and T48. Following LPS administration, a significant reduction of adhesion was observed at T4 and T6.

Conclusions

Alcohol intoxication immediately redistributes monocyte subsets toward the pro-inflammatory phenotype with their subsequent differentiation into the anti-inflammatory phenotype. This is paralleled by a significant functional depression, suggesting an alcohol-induced time-dependent hyporesponsiveness of monocytes to pathogenic triggers.

Fenretinide Improves Intestinal Barrier Function and Mitigates Alcohol Liver Disease

Alcohol liver disease (ALD) is a major cause of liver-related mortality globally, yet there remains an unmet demand for approved ALD drugs. The pathogenesis of ALD involves perturbations to the intestinal barrier and subsequent translocation of bacterial endotoxin that, acting through toll-like receptor 4 (TLR4), promotes hepatic inflammation and progression of ALD. In the present study we investigated the ability of fenretinide (Fen) [N-(4-hydroxyphenyl) retinamide], a synthetic retinoid with known anti-cancer and anti-inflammatory properties, to modulate intestinal permeability and clinical hallmarks of ALD in a mouse model of chronic ethanol (EtOH) exposure. Our results show that EtOH-treated mice had reductions in mRNA and protein expression of intestinal tight junction proteins, including claudin one and occludin, and increases in intestinal permeability and endotoxemia compared to pair-fed mice. Also, EtOH-treated mice had marked increases in hepatic steatosis, liver injury, and expression of pro-inflammatory mediators, including TNF-α, and TLR4-positive macrophages, Kupffer cells, and hepatocytes in the intestines and liver, respectively. In contrast, EtOH + Fen-treated mice were resistant to the effects of EtOH on promoting intestinal permeability and had higher intestinal protein levels of claudin one and occludin. Also, EtOH + Fen-treated mice had significantly lower plasma levels of endotoxin, and reductions in expression of TNF-α and TLR4 positive macrophages, Kupffer cells, and hepatocytes in the intestine and liver. Lastly, we found that EtOH + Fen-treated mice exhibited major reductions in hepatic triglycerides, steatosis, and liver injury compared to EtOH-treated mice. Our findings are the first to demonstrate that Fen possesses anti-ALD properties, potentially through modulation of the intestinal barrier function, endotoxemia, and TLR4-mediated inflammation. These data warrant further pre-clinical investigations of Fen as a potential anti-ALD drug.

Small Bowel Resection Increases Paracellular Gut Barrier Permeability via Alterations of Tight Junction Complexes Mediated by Intestinal TLR4

BACKGROUND

Short bowel syndrome resulting from small bowel resection (SBR) is associated with significant morbidity and mortality. Many adverse sequelae including steatohepatitis and bacterial overgrowth are thought to be related to increased bacterial translocation, suggesting alterations in gut permeability. We hypothesized that after intestinal resection, the intestinal barrier is altered via toll-like receptor 4 (TLR4) signaling at the intestinal level.

METHODS

B6 and intestinal-specific TLR4 knockout (iTLR4 KO) mice underwent 50% SBR or sham operation. Transcellular permeability was evaluated by measuring goblet cell associated antigen passages via two-photon microscopy. Fluorimetry and electron microscopy evaluation of tight junctions (TJ) were used to assess paracellular permeability. In parallel experiments, single-cell RNA sequencing measured expression of intestinal integral TJ proteins. Western blot and immunohistochemistry confirmed the results of the single-cell RNA sequencing.

RESULTS

There were similar number of goblet cell associated antigen passages after both SBR and sham operation (4.5 versus 5.0, P > 0.05). Fluorescein isothiocyanate-dextran uptake into the serum after massive SBR was significantly increased compared with sham mice (2.13 ± 0.39 ng/μL versus 1.62 ± 0.23 ng/μL, P < 0.001). SBR mice demonstrated obscured TJ complexes on electron microscopy. Single-cell RNA sequencing revealed a decrease in TJ protein occludin (21%) after SBR (P < 0.05), confirmed with immunostaining and western blot analysis. The KO of iTLR4 mitigated the alterations in permeability after SBR.

CONCLUSIONS

Permeability after SBR is increased via changes at the paracellular level. However, these alterations were prevented in iTLR4 mice. These findings suggest potential protein targets for restoring the intestinal barrier and obviating the adverse sequelae of short bowel syndrome.

Changes in inflammatory factors in the Brown Norway rat model of food allergy

BACKGROUND

The role of serum S100A8/A9 in intestinal inflammation has been confirmed, and its role in food allergy is currently being investigated.

OBJECTIVE

To explore the levels of S100A8/A9 and inflammatory factors, including Toll-like receptors 4 (TLR4), Nuclear transcription factors (NF-κB) and Tumor necrosis factor α (TNF-α), in mild food allergies.

METHODS

Eighty 3-week-old male Brown Norway rats were used. Forty rats were randomly assigned to the ovalbumin-sensitized experimental group, while 40 rats were assigned to the normal saline sham-sensitized control group. Body weight and length and the levels of serum ovalbumin-specific IgE (OVA-IgE), histamine, Th1-associated and Th2-associated factors, S100A8/A9 and inflammation-associated cytokines were compared.

RESULTS

Through the evaluation of OVA-IgE level and Th1/Th2 balance in the experimental group, a successful IgE-mediated food allergy model was constructed. Compared with the control group, the experimental group had higher serum S100A8/A9 levels on days 21, 28, 35 and 42 (all P < 0.05); higher TLR4 levels on days 28, 35 and 42 (all P < 0.05); higher TNF-α levels on days 28, 35 and 42 (all P < 0.05); higher NF-κB levels on days 35 and 42 (all P < 0.05); and higher IL-1β and IL-6 levels on days 7 to 42 (all P < 0.05). Moreover, positive correlations were found between the serum levels of S100A8/A9 and inflammation-associated cytokines [TNF-α: r = 0.378, P = 0.039; IL-1β: r = 0.679, P = 0.000; IL-6: r = 0.590, P = 0.001].

CONCLUSION

S100A8/A9 and inflammatory-related factors, including TLR4, NF-κB, TNF-α, IL-6 and IL-1β, is closely related to food allergies. Moreover, immune and inflammatory factors interact with each other in food allergies, which may provide insight into food allergy causes and treatments.

Implication of the Gut Microbiota in Metabolic Inflammation Associated with Nutritional Disorders and Obesity

SCOPE

More than a decade ago, the concept of "metabolic endotoxemia" is elaborated on the fact that some bacterial components, classified as microbial associated membrane pathogens (MAMPs) can pass through the gut barrier and create a systemic low tone inflammation.

METHODS AND RESULTS

The translocation of lipopolysaccharides and its contribution to systemic inflammation are largely studied in murine models of obesity, allowing to unravel the molecular pathways involved in the process. Many different pathological contexts evoke the loss of gut barrier as an event contributing to inflammation and thereby driving metabolic and behavioral alterations.

CONCLUSION

This review describes the role of nutrition as a modulator of metabolic regulation and focuses on the contribution of the gut microbiota in the process of the production of a large diversity of bioactive metabolites. The two first sections of the review will be dedicated to the impact of nutritional disorders on both the gut microbiota composition and on metabolic inflammation. The last and more prominent section will describe the role of different nutrient-derived gut metabolites on the gut barrier integrity, metabolic inflammation, and peripheral tissue alterations during obesity or associated complications.

Site-specific contribution of Toll-like receptor 4 to intestinal homeostasis and inflammatory disease

Toll-like receptor 4 (TLR4) is a highly conserved protein of innate immunity, responsible for the regulation and maintenance of homeostasis, as well as immune recognition of external and internal ligands. TLR4 is expressed on a variety of cell types throughout the gastrointestinal tract, including on epithelial and immune cell populations. In a healthy state, epithelial cell expression of TLR4 greatly assists in homeostasis by shaping the host microbiome, promoting immunoglobulin A production, and regulating follicle-associated epithelium permeability. In contrast, immune cell expression of TLR4 in healthy states is primarily centred on the maturation of dendritic cells in response to stimuli, as well as adequately priming the adaptive immune system to fight infection and promote immune memory. Hence, in a healthy state, there is a clear distinction in the site-specific roles of TLR4 expression. Similarly, recent research has indicated the importance of site-specific TLR4 expression in inflammation and disease, particularly the impact of epithelial-specific TLR4 on disease progression. However, the majority of evidence still remains ambiguous for cell-specific observations, with many studies failing to provide the distinction of epithelial versus immune cell expression of TLR4, preventing specific mechanistic insight and greatly impacting the translation of results. The following review provides a critical overview of the current understanding of site-specific TLR4 activity and its contribution to intestinal/immune homeostasis and inflammatory diseases.

Occludin endocytosis is involved in the disruption of the intestinal epithelial barrier in a mouse model of alcoholic steatohepatitis

OBJECTIVE

We aimed to investigate the involvement of the endocytosis of occludin, a key component of tight junction (TJ), in the ethanol-induced disassembly of TJ in a model of alcoholic steatohepatitis.

METHODS

Wild-type mice were fed an ethanol-containing or isocaloric liquid diet for 8 weeks and then assessed for liver injury (histopathology and measurement of serum enzymes), gut permeability (in vivo lactulose/mannitol and ex vivo dye leakage assays), intestinal epithelium ultrastructure (transmission electron microscopy), and intestinal occludin localization (immunofluorescence microscopy). The human intestinal epithelial cell line Caco-2 was also analyzed in vitro for the effects of ethanol on the barrier function (transepithelial electrical resistance), occludin localization (immunofluorescence microscopy and Western blotting), and endocytosis pathways (double-labeling immunofluorescence microscopy with selective pathway inhibitors).

RESULTS

The ethanol-fed mice developed steatohepatitis and displayed intestinal barrier dysfunction, the disruption of intestinal TJ, and enhanced intestinal endocytosis of occluding compared with the control mice. In the Caco-2 monolayers, ethanol treatment decreased transepithelial electrical resistance, disrupted TJ formation, and enhanced occludin endocytosis in a dose- and time-dependent manner. These deleterious events were reversed by pretreating the Caco-2 cells with a selective pharmacological inhibitor of macropinocytosis, but not with the inhibitors of clathrin or caveolin-mediated endocytic pathways.

CONCLUSION

Chronic ethanol exposure may increase intestinal permeability by inducing the micropinocytosis of occludin, resulting in the disruption of intestinal TJ.

Intestinal microbiome and autoimmune liver disease

At present, it has been proved that intestinal microbial-related disorders are involved in the development and progression of multi-organ system diseases. Intestinal microflora is the accumulation of microbial antigens and activated immune cells. Changes in the composition of intestinal microflora (biological disorders) can destroy the systemic immune tolerance of intestinal and symbiotic bacteria. Toll-like receptors in the intestine recognize microbial-related molecular patterns and T helper lymphocyte subpopulations that can cross-react with host antigens (molecular mimics). Activated enterogenous lymphocytes can migrate to lymph nodes, and enterogenous microbial antigens can migrate to extraintestinal sites. Inflammasomes can form in hepatocytes and hepatic stellate cells, which can drive inflammatory, immune-mediated and fibrotic responses. This article reviews and evaluates the role of intestinal microorganisms in the pathogenesis and treatment of autoimmune liver disease.

Alcohol binge disrupts the rat intestinal barrier: the partial protective role of oleoylethanolamide

BACKGROUND AND PURPOSE

Chronic alcohol consumption alters the gut-brain axis, but little is known about alcohol binge episodes on the functioning of the intestinal barrier. We investigated the influence of ethanol binges on bacterial translocation, gut inflammation and immunity, and tight junction (TJ) structure and the ability of the biolipid oleoylethanolamide (OEA) to prevent ethanol binge-induced intestinal barrier dysfunction.

EXPERIMENTAL APPROACH

OEA was injected i.p. before repeated ethanol administration by oral gavage. Plasma, spleen, liver and mesenteric lymph nodes (MLN) were collected in sterile conditions for determination of bacterial load. Immune/inflammatory parameters, TJ proteins and apoptotic markers were determined in colonic tissue by RT-PCR and Western blotting. TJ ultrastructure was examined by transmission electron microscopy.

KEY RESULTS

Ethanol binges induced bacterial translocation to the MLN (mainly) and spleen. Colonic tissues showed signs of inflammation, and activation of innate (Toll-like receptor-4) and adaptive (IgA) immune systems and TJ proteins (occludin and claudin-3) were decreased after ethanol binges. Pretreatment with OEA reduced intestinal inflammation and immune activation and partially preserved the TJ structure affected by alcohol binges but had no effect on alcohol-induced apoptosis. Ultrastructural analyses of colonic TJs revealed dilated TJs in all ethanol groups, with less electron-dense material in non-pretreated rats. The protective effects of i.p. OEA did not reduce bacterial translocation to the MLN. However, intragastric OEA administration significantly reduced plasma LPS levels and bacterial translocation to the MLN.

CONCLUSION AND IMPLICATIONS

OEA-based pharmacotherapies could potentially be useful to treat disorders characterized by intestinal barrier dysfunction, including alcohol abuse.

Hematopoietic reconstitution of neonatal immunocompetent mice to study conditions with a perinatal window of susceptibility

Efficient hematopoietic reconstitution of wild type mice requires preconditioning. Established experimental protocols exist to transplant hematopoietic stem cells into lethally irradiated or chemically myeloablated adult mice or unirradiated immunodeficient mice. We sought to develop a protocol to reconstitute immuno-replete neonatal mice. We describe irradiation and injection procedures for two-day old mice that lead to efficient long-term reconstitution of primary and secondary lymphoid organs. We demonstrate that the frequencies of lymphoid and myeloid cells in primary and secondary lymphoid organs are indistinguishable from unirradiated uninjected sex- and age-matched control animals by 5 weeks post-reconstitution. Thus, this system will facilitate studies aimed at understanding the developmental and environmental mechanisms that contribute to conditions that have a window of susceptibility during the perinatal period.

Environmental Subconcussive Injury, Axonal Injury, and Chronic Traumatic Encephalopathy

Brain injury occurs in two phases: the initial injury itself and a secondary cascade of precise immune-based neurochemical events. The secondary phase is typically functional in nature and characterized by delayed axonal injury with more axonal disconnections occurring than in the initial phase. Axonal injury occurs across the spectrum of disease severity, with subconcussive injury, especially when repetitive, now considered capable of producing significant neurological damage consistent with axonal injury seen in clinically evident concussion, despite no observable symptoms. This review is the first to introduce the concept of environmental subconcussive injury (ESCI) and sets out how secondary brain damage from ESCI once past the juncture of microglial activation appears to follow the same neuron-damaging pathway as secondary brain damage from conventional brain injury. The immune response associated with ESCI is strikingly similar to that mounted after conventional concussion. Specifically, microglial activation is followed closely by glutamate and calcium flux, excitotoxicity, reactive oxygen species and reactive nitrogen species (RNS) generation, lipid peroxidation, and mitochondrial dysfunction and energy crisis. ESCI damage also occurs in two phases, with the primary damage coming from microbiome injury (due to microbiome-altering events) and secondary damage (axonal injury) from progressive secondary neurochemical events. The concept of ESCI and the underlying mechanisms have profound implications for the understanding of chronic traumatic encephalopathy (CTE) etiology because it has previously been suggested that repetitive axonal injury may be the primary CTE pathogenesis in susceptible individuals and it is best correlated with lifetime brain trauma load. Taken together, it appears that susceptibility to brain injury and downstream neurodegenerative diseases, such as CTE, can be conceptualized as a continuum of brain resilience. At one end is optimal resilience, capable of launching effective responses to injury with spontaneous recovery, and at the other end is diminished resilience with a compromised ability to respond and/or heal appropriately. Modulating factors such as one's total cumulative and synergistic brain trauma load, bioavailability of key nutrients needed for proper functioning of restorative metabolic pathways (specifically those involved in the deactivation and clearance of metabolic by-products of brain injury) are key to ultimately determining one's brain resilience.

Toll-Like Receptor 4 Reduces Occludin and Zonula Occludens 1 to Increase Retinal Permeability Both in vitro and in vivo

We reported that β-adrenergic receptors regulate toll-like receptor 4 (TLR4) signaling in the retina of diabetic mice and in retinal endothelial cells (REC) and Müller cells. We hypothesized that TLR4 regulates retinal permeability both in vitro and in vivo in the retinal vasculature. We used REC cultured in normal and high-glucose conditions and TLR4 siRNA treatments for cell culture studies of permeability and protein analyses of tumor necrosis factor α, occludin, and zonula occludens 1 (ZO-1). We used endothelial cell (EC)-specific Cre-Lox TLR4 knockout mice to study retinal permeability and neuronal and vascular changes following exposure to ocular ischemia/reperfusion (I/R) used as a retinal stressor. We found that the loss of TLR4 in the EC led to the reduced permeability following I/R and fewer degenerate capillaries. Retinal permeability was increased in REC grown in high-glucose conditions but was inhibited by TLR4 siRNA treatment. High-glucose culture conditions significantly reduced occludin and ZO-1 levels in REC, and TLR4 siRNA treatment restored levels to baseline. In conclusion, these studies demonstrate that TLR4 in EC strongly regulates retinal permeability and neuronal and vascular changes following exposure to stressors such as I/R.

Toll-like receptor 4 is critical for the development of resection-associated hepatic steatosis

BACKGROUND

A significant number of children with short bowel syndrome experience intestinal failure-associated liver disease. We recently demonstrated accelerated hepatic steatosis after 50% small bowel resection (SBR) in mice. Since SBR is associated with alterations in the gut microbiome, the purpose of this study was to determine whether TLR4 signaling is critical to the development of resection-associated hepatic steatosis.

METHODS

Male C57BL6 (control) and TLR4-knockout (KO) mice underwent 50% proximal SBR. Liver sections were analyzed to obtain the percent lipid content, and Ileal sections were assessed for morphological adaptation. Intestinal TLR4 mRNA expression was measured at 7days and 10weeks.

RESULTS

Compared to controls, TLR4 KO mice demonstrated similar weight gain and morphological adaptation after SBR. Hepatic steatosis was decreased 32-fold in the absence of TLR4. Intestinal TLR4 mRNA expression was significantly elevated 7days after SBR. We also found that TLR4 expression in the intestine was 20-fold higher in whole bowel sections compared with isolated enterocytes.

CONCLUSIONS

TLR4 signaling is critical for the development of resection-associated steatosis, but not involved in intestinal adaptation after massive SBR. Further studies are needed to delineate the mechanism for TLR4 signaling in the genesis of resection-associated liver injury.

LEVEL OF EVIDENCE

Animal study, not clinical.

Precision medicine in alcoholic and nonalcoholic fatty liver disease via modulating the gut microbiota

Alcoholic liver disease (ALD) and nonalcoholic fatty liver disease (NAFLD) represent a major health burden in industrialized countries. Although alcohol abuse and nutrition play a central role in disease pathogenesis, preclinical models support a contribution of the gut microbiota to ALD and NAFLD. This review describes changes in the intestinal microbiota compositions related to ALD and NAFLD. Findings from in vitro, animal, and human studies are used to explain how intestinal pathology contributes to disease progression. This review summarizes the effects of untargeted microbiome modifications using antibiotics and probiotics on liver disease in animals and humans. While both affect humoral inflammation, regression of advanced liver disease or mortality has not been demonstrated. This review further describes products secreted by Lactobacillus- and microbiota-derived metabolites, such as fatty acids and antioxidants, that could be used for precision medicine in the treatment of liver disease. A better understanding of host-microbial interactions is allowing discovery of novel therapeutic targets in the gut microbiota, enabling new treatment options that restore the intestinal ecosystem precisely and influence liver disease. The modulation options of the gut microbiota and precision medicine employing the gut microbiota presented in this review have excellent prospects to improve treatment of liver disease.

Factoring the intestinal microbiome into the pathogenesis of autoimmune hepatitis

The intestinal microbiome is a reservoir of microbial antigens and activated immune cells. The aims of this review were to describe the role of the intestinal microbiome in generating innate and adaptive immune responses, indicate how these responses contribute to the development of systemic immune-mediated diseases, and encourage investigations that improve the understanding and management of autoimmune hepatitis. Alterations in the composition of the intestinal microflora (dysbiosis) can disrupt intestinal and systemic immune tolerances for commensal bacteria. Toll-like receptors within the intestine can recognize microbe-associated molecular patterns and shape subsets of T helper lymphocytes that may cross-react with host antigens (molecular mimicry). Activated gut-derived lymphocytes can migrate to lymph nodes, and gut-derived microbial antigens can translocate to extra-intestinal sites. Inflammasomes can form within hepatocytes and hepatic stellate cells, and they can drive the pro-inflammatory, immune-mediated, and fibrotic responses. Diet, designer probiotics, vitamin supplements, re-colonization methods, antibiotics, drugs that decrease intestinal permeability, and molecular interventions that block signaling pathways may emerge as adjunctive regimens that complement conventional immunosuppressive management. In conclusion, investigations of the intestinal microbiome are warranted in autoimmune hepatitis and promise to clarify pathogenic mechanisms and suggest alternative management strategies.

Cytokine-mediated blood brain barrier disruption as a conduit for cancer/chemotherapy-associated neurotoxicity and cognitive dysfunction

Neurotoxicity is a common side effect of chemotherapy treatment, with unclear molecular mechanisms. Clinical studies suggest that the most frequent neurotoxic adverse events affect memory and learning, attention, concentration, processing speeds and executive function. Emerging preclinical research points toward direct cellular toxicity and induction of neuroinflammation as key drivers of neurotoxicity and subsequent cognitive impairment. Emerging data now show detectable levels of some chemotherapeutic agents within the CNS, indicating potential disruption of blood brain barrier integrity or transport mechanisms. Blood brain barrier disruption is a key aspect of many neurocognitive disorders, particularly those characterized by a proinflammatory state. Importantly, many proinflammatory mediators able to modulate the blood brain barrier are generated by tissues and organs that are targets for chemotherapy-associated toxicities. This review therefore aims to explore the hypothesis that peripherally derived inflammatory cytokines disrupt blood brain barrier permeability, thereby increasing direct access of chemotherapeutic agents into the CNS to facilitate neuroinflammation and central neurotoxicity.

Recombinant Human Annexin A5 Can Repair the Disrupted Cardiomyocyte Adherens Junctions in Endotoxemia

Recombinant human annexin A5 (Anx5) is known to protect cardiac function during endotoxemia, although the underlying mechanisms have yet to be elucidated. In this study, we demonstrated that Anx5 could repair the disrupted cardiomyocyte adherens junctions and improve the myocardial contractile function in lipopolysaccharide (LPS)-induced endotoxemia. Mechanistic studies revealed that Anx5 could antagonize the disassociation between p120-catenin (p120) and N-cadherin as well as the dephosphorylation of p120 in LPS-treated cardiomyocytes. Small interference RNA and specific inhibitors experiment demonstrated that Anx5 regulated p120 functions by inhibition of p21-activated kinase 5 in a protein kinase Cα-dependent way. Moreover, Anx5 could inhibit nuclear factor κB activation and downregulate the level of inflammatory cytokines, such as tumor necrosis factor α and interleukin 1β, which contributed to improving tissue pathological damage in LPS-induced mouse endotoxemia model. Taken together, Anx5 could protect cardiomyocytes adherens junctions and improve myocardial contractile function via regulation of p120 and anti-inflammation in LPS-induced endotoxemia. This study provided novel insights in the prevention and treatment of septic shock.

Beneficial effects of combined ursodeoxycholic acid and angiotensin-II type 1 receptor blocker on hepatic fibrogenesis in a rat model of nonalcoholic steatohepatitis

BACKGROUND AND AIMS

Ursodeoxycholic acid (UDCA) is considered to be effective in the treatment of nonalcoholic steatohepatitis (NASH), particularly in combination with other pharmacological agents. UDCA reportedly counteracts the effects of endotoxemia. Previously, we demonstrated attenuated hepatic fibrogenesis and suppression of activated hepatic stellate cells (Ac-HSC) with an angiotensin-II (AT-II) type 1 receptor blocker (ARB). Here we evaluated the simultaneous effect of both agents on hepatic fibrogenesis in a rat model of NASH.

METHODS

Fischer 344 rats were fed a choline-deficient L-amino-acid-defined (CDAA) diet for 8 weeks. The therapeutic effect of UDCA and ARB was evaluated along with hepatic fibrogenesis, lipopolysaccharide (LPS)-Toll-like receptor 4 (TLR4) regulatory cascade, and intestinal barrier function. The direct inhibitory effect of both UDCA and ARB on Ac-HSC was assessed in vitro.

RESULTS

Both UDCA and ARB had a potent inhibitory effect on hepatic fibrogenesis with suppression of the HSC activation and hepatic expression of transforming growth factor (TGF)-β1 and TLR4. UDCA decreased intestinal permeability by ameliorating zonula occuludens-1 (ZO-1) disruption induced by the CDAA diet. ARB was found to directly suppress regulation of Ac-HSC.

CONCLUSIONS

UDCA and ARB have a synergistic repressive effect on hepatic fibrogenesis by counteracting endotoxemia induced by intestinal barrier dysfunction and suppressing the activation of Ac-HSC. Because both agents are currently used in clinical practice, combined UDCA and ARB may represent a promising novel therapeutic approach for NASH.

Role of defective methylation reactions in ethanol-induced dysregulation of intestinal barrier integrity

Alcoholic liver disease (ALD) is a major healthcare challenge worldwide. Emerging evidence reveals that ethanol administration disrupts the intestinal epithelial tight junction (TJ) complex; this defect allows for the paracellular translocation of gut-derived pathogenic molecules to reach the liver to cause inflammation and progressive liver injury. We have previously demonstrated a causative role of impairments in liver transmethylation reactions in the pathogenesis of ALD. We have further shown that treatment with betaine, a methylation agent that normalizes liver methylation potential, can attenuate ethanol-induced liver injury. Herein, we explored whether alterations in methylation reactions play a causative role in disrupting intestinal mucosal barrier function by employing an intestinal epithelial cell line. Monolayers of Caco-2 cells were exposed to ethanol or a-pan methylation reaction inhibitor, tubercidin, in the presence and absence of betaine. The structural and functional integrity of intestinal epithelial barrier was then examined. We observed that exposure to either ethanol or tubercidin disrupted TJ integrity and function by decreasing the localization of TJ protein occludin-1 to the intracellular junctions, reducing transepithelial electrical resistance and increasing dextran influx. All these detrimental effects of ethanol and tubercidin were attenuated by co-treatment with betaine. We further show that the mechanism of betaine protection was through BHMT-mediated catalysis. Collectively, our data suggest a novel mechanism for alcohol-induced gut leakiness and identifies the importance of normal methylation reactions in maintaining TJ integrity. We also propose betaine as a potential therapeutic option for leaky gut in alcohol-consuming patients who are at the risk of developing ALD.

Toll-like receptor mediated modulation of T cell response by commensal intestinal microbiota as a trigger for autoimmune arthritis

In autoimmune diseases, a disturbance of the balance between T helper 17 (Th17) and regulatory T cells (Tregs) is often observed. This disturbed balance is also the case in rheumatoid arthritis (RA). Genetic predisposition to RA confers the presence of several polymorphisms mainly regulating activation of T lymphocytes. However, the presence of susceptibility factors is neither necessary nor sufficient to explain the disease development, emphasizing the importance of environmental factors. Multiple studies have shown that commensal gut microbiota is of great influence on immune homeostasis and can trigger the development of autoimmune diseases by favoring induction of Th17 cells over Tregs. However the mechanism by which intestinal microbiota influences the Th cell balance is not completely understood. Here we review the current evidence supporting the involvement of commensal intestinal microbiota in rheumatoid arthritis, along with a potential role of Toll-like receptors (TLRs) in modulating the relevant Th cell responses to trigger autoimmunity. A better understanding of TLR triggering by intestinal microbiota and subsequent T cell activation might offer new perspectives for manipulating the T cell response in RA patients and may lead to the discovery of new therapeutic targets or even preventive measures.

Abnormal ultrastructure of intestinal epithelial barrier in mice with alcoholic steatohepatitis

Intestinal barrier dysfunction caused by chronic alcohol consumption is closely associated with disruption of the intestinal epithelial apical junction complex. The present study was undertaken to directly display by transmission electron microscopy the abnormal ultrastructure of the intestinal epithelial barrier in mice with alcoholic steatohepatitis. The results showed that chronic alcohol consumption could induce obvious liver injury, with diffuse lipid accumulation and focal inflammatory cell infiltration in the liver, assessed by hematoxylin and eosin staining. The indicators of intestinal barrier dysfunction, d-lactic acid and lipopolysaccharide, were significantly higher in alcohol-fed mice than in control mice. Alcohol exposure obviously caused high permeability in the ileum to fluorescein isothiocyanate-dextran (FD-4; molecular weight 4000). Transmission electron microscopy demonstrated that tight junctions and adherens junctions expanded noticeably in alcohol-fed mice. Although the tight junction (TJ) length of alcohol-fed mice had no significant difference compared with control mice, the adherens junction (AJ) length of alcohol-fed mice significantly decreased compared with control mice. Additionally, the ratios of both TJmax/TJmin and AJmax/AJmin were significantly larger in alcohol-fed mice than in control liquid-fed mice. In conclusion, high intestinal permeability caused by alcohol attributes to the irregular ultrastructure of the intestinal epithelial barrier.

Toll-like receptor-mediated signaling cascade as a regulator of the inflammation network during alcoholic liver disease

Chronic abuse of alcohol leads to various histological abnormalities in the liver. These are conditions collectively known as alcoholic liver disease (ALD). Currently, ALD is considered to be one of the major causes of death worldwide. An impaired intestinal barrier with related endotoxemia is among the various pathogenetic factors. This is mainly characterized by circulating levels of lipopolysaccharide (LPS), considered critical for the onset of intra-hepatic inflammation. This in turn promotes hepatocellular damage and fibrosis in ALD. Elevated levels of LPS exert their effects by binding to Toll-like receptors (TLRs) which are expressed by all liver-resident cells. The activation of TLR signaling triggers an overproduction and release of some cytokines, which promote an autocatalytic cascade of other pro-inflammatory signals. In this review, we provide an overview of the mechanisms that sustain LPS-mediated activation of TLR signaling, reporting current experimental and clinical evidence of its role during inflammation in ALD.

In vitro evaluation of intestinal epithelial TLR activation in preventing food allergic responses

Alterations in the gut microbiota composition are associated with food allergy. Toll-like receptors (TLRs) respond to microbial stimuli. We studied the effects of the ligation of TLRs on intestinal epithelial cells (IECs) in preventing an allergic effector response. IEC monolayers (T84 cells) were co-cultured with CD3/28-activated PBMCs from healthy controls or atopic patients and simultaneously apically exposed to TLR2, TLR4 or TLR9 ligands. The barrier integrity of T84 cell monolayers was significantly reduced upon co-culture with PBMCs of food allergic subjects compared to healthy subjects. Apical exposure of IECs to a TLR9 ligand prevented PBMC-induced epithelial barrier disruption. Using PBMCs from food allergic subjects, apical TLR9 activation on IECs increased the IFN-γ/IL-13 and IL-10/IL-13 ratio, while suppressing pro-inflammatory IL-6, IL-8 and TNF-α production in an IEC-dependent manner. Hence, the activation of apical TLR9 on IECs, potentially by microbiota-derived signals, may play an important role in the prevention of allergic inflammation.

Regulation of Intestinal Immune Responses through TLR Activation: Implications for Pro- and Prebiotics

The intestinal mucosa is constantly facing a high load of antigens including bacterial antigens derived from the microbiota and food. Despite this, the immune cells present in the gastrointestinal tract do not initiate a pro-inflammatory immune response. Toll-like receptors (TLRs) are pattern recognition receptors expressed by various cells in the gastrointestinal tract, including intestinal epithelial cells (IEC) and resident immune cells in the lamina propria. Many diseases, including chronic intestinal inflammation (e.g., inflammatory bowel disease), irritable bowel syndrome (IBS), allergic gastroenteritis (e.g., eosinophilic gastroenteritis and allergic IBS), and infections are nowadays associated with a deregulated microbiota. The microbiota may directly interact with TLR. In addition, differences in intestinal TLR expression in health and disease may suggest that TLRs play an essential role in disease pathogenesis and may be novel targets for therapy. TLR signaling in the gut is involved in either maintaining intestinal homeostasis or the induction of an inflammatory response. This mini review provides an overview of the current knowledge regarding the contribution of intestinal epithelial TLR signaling in both tolerance induction or promoting intestinal inflammation, with a focus on food allergy. We will also highlight a potential role of the microbiota in regulating gut immune responses, especially through TLR activation.

TLR4/PKC-mediated tight junction modulation: a clinical marker of chemotherapy-induced gut toxicity?

Chemotherapy-induced gut toxicity is a major clinical and economic burden to oncology practice. The mechanisms responsible for its development are ill defined, hampering the development of therapeutic interventions. In light of newly published research foci and clinical practice guidelines in supportive care in cancer, there has been renewed interest in the role tight junctions play in the pathobiology of chemotherapy-induced gut toxicity. Several preclinical studies have identified molecular defects in intestinal tight junctions following chemotherapy. Despite these findings, the mechanisms responsible for chemotherapy-induced tight junction disruption remain unclear. Recent research has highlighted roles for toll-like receptor 4 and protein kinase C signalling in the regulation of tight junctions. This critical review therefore aims to provide evidence linking toll-like receptor 4 expression, protein kinase C activation and tight junction disruption and their relationship to clinical toxicity.