Differentiating Between Tight Junction-Dependent and Tight Junction-Independent Intestinal Barrier Loss In Vivo

Antigen Uptake in the Gut: An Underappreciated Piece to the Puzzle?

The mammalian gut is a vast, diverse, and dynamic single-layer epithelial surface exposed to trillions of microbes, microbial products, and the diet. Underlying this epithelium lies the largest collection of immune cells in the body; these cells encounter luminal substances to generate antigen-specific immune responses characterized by tolerance at homeostasis and inflammation during enteric infections. How the outcomes of antigen-specific tolerance and inflammation are appropriately balanced is a central question in mucosal immunology. Furthermore, how substances large enough to generate antigen-specific responses cross the epithelium and encounter the immune system in homeostasis and during inflammation remains largely unexplored. Here we discuss the challenges presented to the gut immune system, the identified pathways by which luminal substances cross the epithelium, and insights suggesting that the pathways used by substances to cross the epithelium affect the ensuing immune response.

Helicobacter hepaticus CdtB Triggers Colonic Mucosal Barrier Disruption in Mice via Epithelial Tight Junction Impairment Mediated by MLCK/pMLC2 Signaling Pathway

Background:Helicobacter hepaticus (H. hepaticus) has been demonstrated to have clinical relevance to the development of colitis in rodents. H. hepaticus produces cytolethal distending toxins (CDTs), which are identified as the most important virulence factors to the pathogenicity of CDT-producing bacteria in animals. However, the precise relationship between CDTs of H. hepaticus and intestinal barrier dysfunction remains unclear. The objective of the present study was to ascertain the impact of CdtB, the active subunit of CDTs, on the colonic mucosal barrier during H. hepaticus infection. Materials and Methods: We investigated the infection of male BALB/c mice, intestinal organoids, and IEC-6 cell monolayers by H. hepaticus or CdtB-deficient H. hepaticus (ΔCdtB). A comprehensive histopathological examination was conducted, encompassing the assessment of H. hepaticus colonization, the levels of mRNA expression for inflammatory cytokines, the expression levels of tight junction proteins, and the related signaling pathways. Results: The results demonstrate that the presence of ΔCdtB led to a mitigation of the symptoms associated with H. hepaticus-induced colitis, as evidenced by colon length shortening and the colon histological inflammation score. In addition, the levels of pro-inflammatory cytokines were reduced in the ΔCdtB group. Moreover, a downward trend was observed in the phosphorylation levels of STAT3 and nuclear factor-κB (p65). In vitro, the presence of H. hepaticus resulted in a reduction in the expression of tight junction (TJ) markers (ZO-1 and occludin) and an impairment of the F-actin structure in either the intestinal epithelium or intestinal organoids. However, these effects were reversed by CdtB deletion. Concurrently, both ROS levels and apoptosis levels were found to be significantly reduced in cells treated with the ΔCdtB strain. Mechanistically, myosin light chain kinase (MLCK) activation was observed in the H. hepaticus-infected group in vivo, whereas the MLCK inhibitor ML-7 was found to reverse the CdtB-induced alterations in TJ proteins in IEC6 cells. Conclusions: The collective findings demonstrate that CdtB plays a pivotal role in the H. hepaticus-induced colonic mucosal barrier. This is achieved through the regulation of TJs via the MLCK/pMLC2 signaling pathway, which is linked to elevations in oxidative stress and inflammation within intestinal epithelial cells.

Tight junction regulation, intestinal permeability, and mucosal immunity in gastrointestinal health and disease

PURPOSE OF REVIEW

The contributions of intestinal barrier loss, that is, increased permeability, to multiple disorders, including inflammatory bowel disease (IBD), have been a topic of speculation for many years, and the literature is replete with conclusions based on correlation and speculation. The goal of this article is to critically review recent advances in mechanistic understanding of barrier regulation and the evidence for and against contributions of intestinal barrier loss to disease pathogenesis.

RECENT FINDINGS

It is now recognized that intestinal permeability reflects the combined effects of two distinct routes across tight junctions, which form selectively permeable seals between adjacent epithelial cells, and mucosal damage that leads to nonselective barrier loss. These are referred to as pore and leak pathways across the tight junction and an unrestricted pathway at sites of damage. Despite advances in phenotypic and mechanistic characterization of three distinct permeability pathways, development of experimental agents that specifically target these pathways, and remarkable efficacy in preclinical models, pathway-targeted therapies have not been tested in human subjects.

SUMMARY

After decades of speculation, therapeutic interventions that target the intestinal barrier are nearly within reach. More widespread use of available tools and development of new tools that discriminate between pore, leak, and unrestricted pathway permeabilities and underlying regulatory mechanisms will be essential to understanding the local and systemic consequences of intestinal barrier loss.

Exertional Stress-induced Pathogenic Luminal Content Translocation - Friend or Foe?

The incidence of perturbed gastrointestinal integrity, as well as resulting systemic immune responses and gastrointestinal symptoms, otherwise known as exercised-induced gastrointestinal syndrome (EIGS), is common among individuals who partake in prolonged exercise. EIGS may cause the translocation of pathogenic material, including whole bacteria and bacterial endotoxins, from the lumen into circulation, which may progress into clinical consequences such as sepsis, and potentially subsequent fatality. However, further investigation is warranted to assess the possibility of food allergen and/or digestive enzyme luminal to circulatory translocation in response to exercise, and the clinical consequences. Findings from this narrative literature review demonstrate evidence that whole bacteria and bacterial endotoxins translocation from the gastrointestinal lumen to systemic circulation occurs in response to exercise stress, with a greater propensity of translocation occurring with accompanying heat exposure. It has also been demonstrated that food allergens can translocate from the lumen to systemic circulation in response to exercise stress and initiate anaphylaxis. To date, no research investigating the effect of exercise on the translocation of digestive enzymes from the lumen into systemic circulation exists. It is evident that EIGS and consequential pathogenic translocation presents life-threatening clinical implications, warranting the development and implementation of effective management strategies in at-risk populations.

Development of an in vitro co-culture model using Caco-2 and J774A.1 cells to mimic intestinal inflammation

In vitro models that mimic the pathophysiology in vivo are important tools to study mechanisms of disease and assess the pharmacology and toxicity of drugs. In this work, we report the development of a novel model of intestinal inflammation. This model is based on the co-culture of intestinal epithelial Caco-2 cells and murine J774A.1 macrophages. The model is shown to mimic the intestinal barrier in both healthy and inflamed state. In the healthy state, without external stimulation, Caco-2 and J774A.1 cells were co-cultured in one system without affecting the barrier integrity of intestinal epithelial cells and without inducing release of cytokines from macrophages. To mimic the inflamed intestine, Caco-2 cells were primed with an optimised cytokine cocktail (TNF-⍺, IFN-γ and IL-1β) and J774A.1 cells were pre-exposed to lipopolysaccharide (LPS) and IFN-γ for 24 h before combining the two cell lines into co-culture. In these conditions, a significant disruption of the epithelial barrier and an increase in pro-inflammatory cytokine (TNF-⍺ and IL-6) levels released from macrophages were detected. The data also show that inflammation in the co-culture model was temporary and reversible upon the removal of the inflammatory stimulus. This new in vitro model could be a valuable tool for investigating the safety and efficacy of drugs in the context of intestinal inflammation and provides advantages over other reported co-culture models of intestinal inflammation in terms of cost and simplicity.

Tmprss2 maintains epithelial barrier integrity and transepithelial sodium transport

The mouse cortical collecting duct cell line presents a tight epithelium with regulated ion and water transport. The epithelial sodium channel (ENaC) is localized in the apical membrane and constitutes the rate-limiting step for sodium entry, thereby enabling transepithelial transport of sodium ions. The membrane-bound serine protease Tmprss2 is co-expressed with the alpha subunit of ENaC. αENaC gene expression followed the Tmprss2 expression, and the absence of Tmprss2 resulted not only in down-regulation of αENaC gene and protein expression but also in abolished transepithelial sodium transport. In addition, RNA-sequencing analyses unveiled drastic down-regulation of the membrane-bound protease CAP3/St14, the epithelial adhesion molecule EpCAM, and the tight junction proteins claudin-7 and claudin-3 as also confirmed by immunohistochemistry. In summary, our data clearly demonstrate a dual role of Tmprss2 in maintaining not only ENaC-mediated transepithelial but also EpCAM/claudin-7-mediated paracellular barrier; the tight epithelium of the mouse renal mCCD cells becomes leaky. Our working model proposes that Tmprss2 acts via CAP3/St14 on EpCAM/claudin-7 tight junction complexes and through regulating transcription of αENaC on ENaC-mediated sodium transport.

IL-17 receptor A functions to help maintain barrier integrity and limit activation of immunopathogenic response to H. pylori infection

Activation of Th17 cell responses, including the production of IL-17A and IL-21, contributes to host defense and inflammatory responses by coordinating adaptive and innate immune responses. IL-17A and IL-17F signal through a multimeric receptor, which includes the IL-17 receptor A (IL-17RA) subunit and the IL-17RC subunit. IL-17RA is expressed by many cell types, and data from previous studies suggest that loss of IL-17 receptor is required to limit immunopathology in the Helicobacter pylori model of infection. Here, an Il17ra-/- mouse was generated on the FVB/n background, and the role of IL-17 signaling in the maintenance of barrier responses to H. pylori was investigated. Generating the Il17ra-/- on the FVB/n background allowed for the examination of responses in the paragastric lymph node and will allow for future investigation into carcinogenesis. While uninfected Il17ra-/- mice do not develop spontaneous gastritis following H. pylori infection, Il17ra-/- mice develop severe gastric inflammation accompanied by lymphoid follicle production and exacerbated production of Th17 cytokines. Increased inflammation in the tissue, increased IgA levels in the lumen, and reduced production of Muc5ac in the corpus correlate with increased H. pylori-induced paragastric lymph node activation. These data suggest that the cross talk between immune cells and epithelial cells regulates mucin production, IgA production, and translocation, impacting the integrity of the gastric mucosa and therefore activating of the adaptive immune response.

Respiratory exposure to agricultural dust extract promotes increased intestinal Tnfα expression, gut barrier dysfunction, and endotoxemia in mice

Concentrated animal feeding operations (CAFOs) are responsible for the production of global greenhouse gases and harmful environmental pollutants including hydrogen sulfide, ammonia, and particulate matter. Swine farmers are frequently exposed to organic dust that is proinflammatory in the lung and are thus at greater risk of developing pneumonia, asthma, and other respiratory conditions. In addition to respiratory disease, air pollutants are directly associated with altered gastrointestinal (GI) physiology and the development of GI diseases, thereby highlighting the gut-lung axis in disease progression. Instillation of hog dust extract (HDE) for 3 wk has been reported to promote the development of chronic airway inflammation in mice, however, the impact of HDE exposure on intestinal homeostasis is poorly understood. We report that 3-wk intranasal exposure of HDE is associated with increased intestinal macromolecule permeability and elevated serum endotoxin concentrations in C57BL/6J mice. In vivo studies also indicated mislocalization of the epithelial cell adhesion protein, E-cadherin, in the colon as well as an increase in the proinflammatory cytokine, Tnfα, in the proximal colon. Moreover, mRNA expression of the Paneth cell-associated marker, Lyz1, was increased the proximal colon, whereas the expression of the goblet cell marker, Muc2, was unchanged in the epithelial cells of the ileum, cecum, and distal colon. These results demonstrate that airway exposure to CAFOs dusts promote airway inflammation and modify the gastrointestinal tract to increase intestinal permeability, induce systemic endotoxemia, and promote intestinal inflammation. Therefore, this study identifies complex physiological consequences of chronic exposure to organic dusts derived from CAFOs on the gut-lung axis.NEW & NOTEWORTHY Agricultural workers have a higher prevalence of occupational respiratory symptoms and are at greater risk of developing respiratory diseases. However, gastrointestinal complications have also been reported, yet the intestinal pathophysiology is understudied. This work is novel because it emphasizes the role of an inhaled environmental pollutant on the development of intestinal pathophysiological outcomes. This work will provide foundation for other studies evaluating how agricultural dusts disrupts host physiology and promotes debilitating gastrointestinal and systemic disorders.

Deciphering internal and external factors influencing intestinal junctional complexes

The intestinal barrier, an indispensable guardian of gastrointestinal health, mediates the intricate exchange between internal and external environments. Anchored by evolutionarily conserved junctional complexes, this barrier meticulously regulates paracellular permeability in essentially all living organisms. Disruptions in intestinal junctional complexes, prevalent in inflammatory bowel diseases and irritable bowel syndrome, compromise barrier integrity and often lead to the notorious "leaky gut" syndrome. Critical to the maintenance of the intestinal barrier is a finely orchestrated network of intrinsic and extrinsic factors that modulate the expression, composition, and functionality of junctional complexes. This review navigates through the composition of key junctional complex components and the common methods used to assess intestinal permeability. It also explores the critical intracellular signaling pathways that modulate these junctional components. Lastly, we delve into the complex dynamics between the junctional complexes, microbial communities, and environmental chemicals in shaping the intestinal barrier function. Comprehending this intricate interplay holds paramount importance in unraveling the pathophysiology of gastrointestinal disorders. Furthermore, it lays the foundation for the development of precise therapeutic interventions targeting barrier dysfunction.

Defects in microvillus crosslinking sensitize to colitis and inflammatory bowel disease

Intestinal epithelial cells are covered by the brush border, which consists of densely packed microvilli. The Intermicrovillar Adhesion Complex (IMAC) links the microvilli and is required for proper brush border organization. Whether microvillus crosslinking is involved in the intestinal barrier function or colitis is currently unknown. We investigate the role of microvillus crosslinking in colitis in mice with deletion of the IMAC component CDHR5. Electron microscopy shows pronounced brush border defects in CDHR5-deficient mice. The defects result in severe mucosal damage after exposure to the colitis-inducing agent DSS. DSS increases the permeability of the mucus layer and brings bacteria in direct contact with the disorganized brush border of CDHR5-deficient mice. This correlates with bacterial invasion into the epithelial cell layer which precedes epithelial apoptosis and inflammation. Single-cell RNA sequencing data of patients with ulcerative colitis reveals downregulation of CDHR5 in enterocytes of diseased areas. Our results provide experimental evidence that a combination of microvillus crosslinking defects with increased permeability of the mucus layer sensitizes to inflammatory bowel disease.

The Impact of a 24-h Low and High Fermentable Oligo- Di- Mono-Saccharides and Polyol (FODMAP) Diet on Plasma Bacterial Profile in Response to Exertional-Heat Stress

Exertional-heat stress (EHS) compromises intestinal epithelial integrity, potentially leading to the translocation of pathogenic agents into circulation. This study aimed to explore the impact of EHS on the systemic circulatory bacterial profile and to determine the impact of a short-term low (LFOD) and high (HFOD) fermentable oligo- di- mono-saccharide and polyol dietary intervention before EHS on this profile. Using a double-blind randomized cross-over design, thirteen endurance runners (n = 8 males, n = 5 females), with a history of exercise-associated gastrointestinal symptoms (Ex-GIS), consumed a 24 h LFOD and HFOD before 2 h running at 60% V.O2max in 35.6 °C. Blood and fecal samples were collected pre-EHS to determine plasma microbial DNA concentration, and sample bacteria and short chain fatty acid (SCFA) profiles by fluorometer quantification, 16S rRNA amplicon gene sequencing, and gas chromatography, respectively. Blood samples were also collected post-EHS to determine changes in plasma bacteria. EHS increased plasma microbial DNA similarly in both FODMAP trials (0.019 ng·μL-1 to 0.082 ng·μL-1) (p < 0.01). Similar pre- to post-EHS increases in plasma Proteobacteria (+1.6%) and Firmicutes (+0.6%) phyla relative abundance were observed in both FODMAP trials. This included increases in several Proteobacteria genus (Delftia and Serratia) groups. LFOD presented higher fecal Firmicutes (74%) and lower Bacteroidota (10%) relative abundance pre-EHS, as a result of an increase in Ruminococcaceae and Lachnospiraceae family and respective genus groups, compared with HFOD (64% and 25%, respectively). Pre-EHS plasma total SCFA (p = 0.040) and acetate (p = 0.036) concentrations were higher for HFOD (188 and 178 μmol·L-1, respectively) vs. LFOD (163 and 153 μmol·L-1, respectively). Pre-EHS total fecal SCFA concentration (119 and 74 μmol·g-1; p < 0.001), including acetate (74 and 45 μmol·g-1; p = 0.001), butyrate (22 and 13 μmol·g-1; p = 0.002), and propionate (20 and 13 μmol·g-1; p = 0.011), were higher on HFOD vs LFOD, respectively. EHS causes the translocation of whole bacteria into systemic circulation and alterations to the plasma bacterial profile, but the FODMAP content of a 24 h diet beforehand does not alter this outcome.

From Non-Alcoholic Fatty Liver Disease to Liver Cancer: Microbiota and Inflammation as Key Players

It is estimated that 25% of the world's population has non-alcoholic fatty liver disease. This disease can advance to a more severe form, non-alcoholic steatohepatitis (NASH), a disease with a greater probability of progression to cirrhosis and hepatocellular carcinoma (HCC). NASH could be characterized as a necro-inflammatory complication of chronic hepatic steatosis. The combination of factors that lead to NASH and its progression to HCC in the setting of inflammation is not clearly understood. The portal vein is the main route of communication between the intestine and the liver. This allows the transfer of products derived from the intestine to the liver and the hepatic response pathway of bile and antibody secretion to the intestine. The intestinal microbiota performs a fundamental role in the regulation of immune function, but it can undergo changes that alter its functionality. These changes can also contribute to cancer by disrupting the immune system and causing chronic inflammation and immune dysfunction, both of which are implicated in cancer development. In this article, we address the link between inflammation, microbiota and HCC. We also review the different in vitro models, as well as recent clinical trials addressing liver cancer and microbiota.

Claudin Barriers on the Brink: How Conflicting Tissue and Cellular Priorities Drive IBD Pathogenesis

Inflammatory bowel diseases (IBDs) are characterized by acute or chronic recurring inflammation of the intestinal mucosa, often with increasing severity over time. Life-long morbidities and diminishing quality of life for IBD patients compel a search for a better understanding of the molecular contributors to disease progression. One unifying feature of IBDs is the failure of the gut to form an effective barrier, a core role for intercellular complexes called tight junctions. In this review, the claudin family of tight junction proteins are discussed as they are a fundamental component of intestinal barriers. Importantly, claudin expression and/or protein localization is altered in IBD, leading to the supposition that intestinal barrier dysfunction exacerbates immune hyperactivity and disease. Claudins are a large family of transmembrane structural proteins that constrain the passage of ions, water, or substances between cells. However, growing evidence suggests non-canonical claudin functions during mucosal homeostasis and healing after injury. Therefore, whether claudins participate in adaptive or pathological IBD responses remains an open question. By reviewing current studies, the possibility is assessed that with claudins, a jack-of-all-trades is master of none. Potentially, a robust claudin barrier and wound restitution involve conflicting biophysical phenomena, exposing barrier vulnerabilities and a tissue-wide frailty during healing in IBD.

Tacrolimus-binding protein FKBP8 directs myosin light chain kinase-dependent barrier regulation and is a potential therapeutic target in Crohn's disease

OBJECTIVE

Intestinal barrier loss is a Crohn's disease (CD) risk factor. This may be related to increased expression and enzymatic activation of myosin light chain kinase 1 (MLCK1), which increases intestinal paracellular permeability and correlates with CD severity. Moreover, preclinical studies have shown that MLCK1 recruitment to cell junctions is required for tumour necrosis factor (TNF)-induced barrier loss as well as experimental inflammatory bowel disease progression. We sought to define mechanisms of MLCK1 recruitment and to target this process pharmacologically.

DESIGN

Protein interactions between FK506 binding protein 8 (FKBP8) and MLCK1 were assessed in vitro. Transgenic and knockout intestinal epithelial cell lines, human intestinal organoids, and mice were used as preclinical models. Discoveries were validated in biopsies from patients with CD and control subjects.

RESULTS

MLCK1 interacted specifically with the tacrolimus-binding FKBP8 PPI domain. Knockout or dominant negative FKBP8 expression prevented TNF-induced MLCK1 recruitment and barrier loss in vitro. MLCK1-FKBP8 binding was blocked by tacrolimus, which reversed TNF-induced MLCK1-FKBP8 interactions, MLCK1 recruitment and barrier loss in vitro and in vivo. Biopsies of patient with CD demonstrated increased numbers of MLCK1-FKBP8 interactions at intercellular junctions relative to control subjects.

CONCLUSION

Binding to FKBP8, which can be blocked by tacrolimus, is required for MLCK1 recruitment to intercellular junctions and downstream events leading to immune-mediated barrier loss. The observed increases in MLCK1 activity, MLCK1 localisation at cell junctions and perijunctional MLCK1-FKBP8 interactions in CD suggest that targeting this process may be therapeutic in human disease. These new insights into mechanisms of disease-associated barrier loss provide a critical foundation for therapeutic exploitation of FKBP8-MLCK1 interactions.

Paracellular permeability and tight junction regulation in gut health and disease

Epithelial tight junctions define the paracellular permeability of the intestinal barrier. Molecules can cross the tight junctions via two distinct size-selective and charge-selective paracellular pathways: the pore pathway and the leak pathway. These can be distinguished by their selectivities and differential regulation by immune cells. However, permeability increases measured in most studies are secondary to epithelial damage, which allows non-selective flux via the unrestricted pathway. Restoration of increased unrestricted pathway permeability requires mucosal healing. By contrast, tight junction barrier loss can be reversed by targeted interventions. Specific approaches are needed to restore pore pathway or leak pathway permeability increases. Recent studies have used preclinical disease models to demonstrate the potential of pore pathway or leak pathway barrier restoration in disease. In this Review, we focus on the two paracellular flux pathways that are dependent on the tight junction. We discuss the latest evidence that highlights tight junction components, structures and regulatory mechanisms, their impact on gut health and disease, and opportunities for therapeutic intervention.

Establishment of a novel in vitro co-culture system of enteric neurons and Caco-2 cells for evaluating the effect of enteric nervous system on transepithelial transport of drugs

The gastrointestinal tract is innervated by extrinsic autonomic nerves and intrinsic enteric nervous system (ENS). However, the role of ENS in drug absorption has remained to be clarified. To investigate the effect of ENS on drug transport across the intestinal epithelial cells, we established a novel co-culture system of Caco-2 cells and enteric neurons differentiated from neural crest stem (NCS)-like cells isolated from mouse longitudinal muscle/myenteric plexus (LMMP). Immunostaining analysis revealed that the proportions of neuron, glia, and NCS-like cells were only <5 % at population in the primary culture of LMMP cells. Therefore, we proliferated NCS-like cells and differentiated them into neuronal cells and successfully increased the neuronal cell population upto about 40 %. Then, the differentiated neuronal cells were co-cultured with Caco-2 cell monolayers, and we found that the co-culture significantly decreased the transepithelial electrical resistance and enhanced the transport of fluorescein isothiocyanate-labeled dextran-4 across Caco-2 cell monolayers, suggesting that the enteric neurons would function to open the tight junction and facilitate the drug transport via the paracellular route. On the other hand, no changes in the permeability of antipyrine were observed, suggesting that the enteric neurons would not affect the passive transport via the transcellular pathway.

Increased intestinal permeability and downregulation of absorptive ion transporters Nhe3, Dra, and Sglt1 contribute to diarrhea during Clostridioides difficile infection

BACKGROUND & AIM

Clostridioides difficile infection (CDI) is the leading cause of hospital-acquired diarrhea and pseudomembranous colitis. Two protein toxins, TcdA and TcdB, produced by C. difficile are the major determinants of disease. However, the pathophysiological causes of diarrhea during CDI are not well understood. Here, we investigated the effects of C. difficile toxins on paracellular permeability and apical ion transporters in the context of an acute physiological infection.

METHODS

We studied intestinal permeability and apical membrane transporters in female C57BL/6J mice. Üssing chambers were used to measure paracellular permeability and ion transporter function across the intestinal tract. Infected intestinal tissues were analyzed by immunofluorescence microscopy and RNA-sequencing to uncover mechanisms of transporter dysregulation.

RESULTS

Intestinal permeability was increased through the size-selective leak pathway in vivo during acute CDI in a 2-day-post infection model. Chloride secretory activity was reduced in the cecum and distal colon during infection by decreased CaCC and CFTR function, respectively. SGLT1 activity was significantly reduced in the cecum and colon, accompanied by ablated SGLT1 expression in colonocytes and increased luminal glucose concentrations. SGLT1 and DRA expression was ablated by either TcdA or TcdB during acute infection, but NHE3 was decreased in a TcdB-dependent manner. The localization of key proteins that link filamentous actin to the ion transporters in the apical plasma membrane was unchanged. However, Sglt1, Nhe3, and Dra were drastically reduced at the transcript level, implicating downregulation of ion transporters in the mechanism of diarrhea during CDI.

CONCLUSIONS

CDI increases intestinal permeability and decreases apical abundance of NHE3, SGLT1, and DRA. This combination likely leads to dysfunctional water and solute absorption in the large bowel, causing osmotic diarrhea. These findings provide insights into the pathophysiological mechanisms underlying diarrhea and may open novel avenues for attenuating CDI-associated diarrhea.

Development of Paeonol Liposomes: Design, Optimization, in vitro and in vivo Evaluation

BACKGROUND

Ulcerative colitis (UC) is one of the intractable diseases recognized by the World Health Organization, and paeonol has been proven to have therapeutic effects. However, the low solubility of paeonol limits its clinical application. To prepare and optimize paeonol liposome, study its absorption mechanism and the anti-inflammatory activity in vitro and in vivo, in order to provide experimental basis for the further development of paeonol into an anti-inflammatory drug in the future.

METHODS

Paeonol loaded liposomes were prepared and optimized by thin film dispersion-ultrasonic method. The absorption mechanism of paeonol-loaded liposomes was studied by pharmacokinetics, in situ single-pass intestinal perfusion and Caco-2 cell monolayer model, the anti-inflammatory activity was studied in a mouse ulcerative model.

RESULTS

Box-Behnken response surface methodology permits to screen the best formulations. The structural and morphological characterization showed that paeonol was entrapped inside the bilayer in liposomes. Pharmacokinetic studies found that the AUC_0-t of Pae-Lips was 2.78 times than that of paeonol suspension, indicating that Pae-Lips significantly improved the absorption of paeonol. In situ single intestinal perfusion and Caco-2 monolayer cell model results showed that paeonol was passively transported and absorbed, and was the substrate of P-gp, MRP2 and BCRP, and the Papp value of Pae-Lips was significantly higher than that of paeonol. In vitro and in vivo anti-inflammatory experiments showed that compared with paeonol, Pae-Lips exhibited excellent anti-inflammatory activity.

CONCLUSION

In this study, Pae-Lips were successfully prepared to improve the oral absorption of paeonol. Absorption may involve passive diffusion and efflux transporters. Moreover, Pae-Lips have excellent anti-inflammatory activity in vitro and in vivo, which preliminarily clarifies the feasibility of further development of Pae-Lips into oral anti-inflammatory drugs.

Special Issue on the "Regulation and Physiopathology of the Gut Barrier"

The importance of gut barrier integrity in intestinal homeostasis and the consequences of its alteration in the etiology of human pathologies have been subjects of exponentially growing interest during the last decade [...].

Tight junctions: from molecules to gastrointestinal diseases

Intestinal epithelium functions as a tissue barrier to prevent interaction between the internal compartment and the external milieu. Intestinal barrier function also determines epithelial polarity for the absorption of nutrients and the secretion of waste products. These vital functions require strong integrity of tight junction proteins. In fact, intestinal tight junctions that seal the paracellular space can restrict mucosal-to-serosal transport of hostile luminal contents. Tight junctions can form both an absolute barrier and a paracellular ion channel. Although defective tight junctions potentially lead to compromised intestinal barrier and the development and progression of gastrointestinal (GI) diseases, no FDA-approved therapies that recover the epithelial tight junction barrier are currently available in clinical practice. Here, we discuss the impacts and regulatory mechanisms of tight junction disruption in the gut and related diseases. We also provide an overview of potential therapeutic targets to restore the epithelial tight junction barrier in the GI tract.

Tight junction proteins occludin and ZO-1 as regulators of epithelial proliferation and survival

Epithelial cells are the first line of mucosal defense. In the intestine, a single layer of epithelial cells must establish a selectively permeable barrier that supports nutrient absorption and waste secretion while preventing the leakage of potentially harmful luminal materials. Key to this is the tight junction, which seals the paracellular space and prevents unrestricted leakage. The tight junction is a protein complex established by interactions between members of the claudin, zonula occludens, and tight junction-associated MARVEL protein (TAMP) families. Claudins form the characteristic tight junction strands seen by freeze-fracture microscopy and create paracellular channels, but the functions of ZO-1 and occludin, founding members of the zonula occludens and TAMP families, respectively, are less well defined. Recent studies have revealed that these proteins have essential noncanonical (nonbarrier) functions that allow them to regulate epithelial apoptosis and proliferation, facilitate viral entry, and organize specialized epithelial structures. Surprisingly, neither is required for intestinal barrier function or overall health in the absence of exogenous stressors. Here, we provide a brief overview of ZO-1 and occludin canonical (barrier-related) functions, and a more detailed examination of their noncanonical functions.

L-glutamine for sickle cell disease: more than reducing redox

Oxidative stress is a major contributor to the pathophysiology of sickle cell disease (SCD) including hemolysis and vaso-occlusive crisis (VOC). L-glutamine is a conditionally essential amino acid with important roles, including the synthesis of antioxidants, such as reduced glutathione and the cofactors NAD(H) and NADP(H), as well as nitric oxide. Given the increased levels of oxidative stress and lower (NADH):(NAD +  + NADH) ratio in sickle erythrocytes that adversely affects the blood rheology compared to normal red blood cells, L-glutamine was investigated for its therapeutic potential to reduce VOC. While L-glutamine was approved by the United States (US) Food and Drug Administration to treat SCD, its impact on the redox environment in sickle erythrocytes is not fully understood. The mechanism through which L-glutamine reduces VOC in SCD is also not clear. In this paper, we will summarize the results of the Phase 3 study that led to the approval of L-glutamine for treating SCD and discuss its assumed mechanisms of action. We will examine the role of L-glutamine in health and propose how the extra-erythrocytic functions of L-glutamine might contribute to its beneficial effects in SCD. Further research into the role of L-glutamine on extra-erythrocyte functions might help the development of an improved formulation with more efficacy.

Zbtb20 identifies and controls a thymus-derived population of regulatory T cells that play a role in intestinal homeostasis

The expression of BTB-ZF transcription factors such as ThPOK in CD4⁺ T cells, Bcl6 in T follicular helper cells, and PLZF in natural killer T cells defines the fundamental nature and characteristics of these cells. Screening for lineage-defining BTB-ZF genes led to the discovery of a subset of T cells that expressed Zbtb20. About half of Zbtb20⁺ T cells expressed FoxP3, the lineage-defining transcription factor for regulatory T cells (T_regs). Zbtb20⁺ T_regs were phenotypically and genetically distinct from the larger conventional T_reg population. Zbtb20⁺ T_regs constitutively expressed mRNA for interleukin-10 and produced high levels of the cytokine upon primary activation. Zbtb20⁺ T_regs were enriched in the intestine and specifically expanded when inflammation was induced by the use of dextran sodium sulfate. Conditional deletion of Zbtb20 in T cells resulted in a loss of intestinal epithelial barrier integrity. Consequently, knockout (KO) mice were acutely sensitive to colitis and often died because of the disease. Adoptive transfer of Zbtb20⁺ T_regs protected the Zbtb20 conditional KO mice from severe colitis and death, whereas non-Zbtb20 T_regs did not. Zbtb20 was detected in CD24^hi double-positive and CD62L^lo CD4 single-positive thymocytes, suggesting that expression of the transcription factor and the phenotype of these cells were induced during thymic development. However, Zbtb20 expression was not induced in "conventional" T_regs by activation in vitro or in vivo. Thus, Zbtb20 expression identified and controlled the function of a distinct subset of T_regs that are involved in intestinal homeostasis.

The Influence of Nutrition on Intestinal Permeability and the Microbiome in Health and Disease

The leakage of the intestinal barrier and the disruption of the gut microbiome are increasingly recognized as key factors in different pathophysiological conditions, such as irritable bowel syndrome (IBS), inflammatory bowel disease (IBD), chronic liver diseases, obesity, diabetes mellitus, types of cancer, and neuropsychiatric disorders. In this study, the mechanisms leading to dysbiosis and "leaky gut" are reviewed, and a short summary of the current knowledge regarding different diseases is provided. The simplest way to restore intestinal permeability and the microbiota could be ideal nutrition. Further therapeutic options are also available, such as the administration of probiotics or postbiotics or fecal microbiota transplantation.

Gut-Derived Sterile Inflammation and Parkinson's Disease

The etiology of Parkinson's disease (PD) is unknown, but evidence is increasing that there is a prominent inflammatory component to the illness. Epidemiological, genetic, and preclinical evidence support a role for gut-derived sterile inflammation. Pro-inflammatory bacteria are over-represented in the PD gut microbiota. There is evidence for decreased gut barrier function and leak of bacterial antigen across the gut epithelium with sub-mucosal inflammation and systemic exposure to the bacterial endotoxin lipopolysaccharide. Preclinical evidence supports these clinical findings and suggests that systemic inflammation can affect the CNS through vagal pathways or the systemic circulation. We will review recent preclinical and clinical evidence to support this mechanism and suggest possible treatments directed at the gut-brain axis.

The Tight Junction Protein ZO-1 Is Dispensable for Barrier Function but Critical for Effective Mucosal Repair

BACKGROUNDS & AIMS

Increased permeability is implicated in the pathogenesis of intestinal disease. In vitro and in vivo studies have linked down-regulation of the scaffolding protein ZO-1, encoded by the TJP1 gene, to increased tight junction permeability. This has not, however, been tested in vivo. Here, we assessed the contributions of ZO-1 to in vivo epithelial barrier function and mucosal homeostasis.

METHODS

Public Gene Expression Omnibus data sets and biopsy specimens from patients with inflammatory bowel disease (IBD) and healthy control individuals were analyzed. Tjp1f/f;vil-CreTg mice with intestinal epithelial-specific ZO-1 knockout (ZO-1KO.IEC) mice and Tjp1f/f mice littermates without Cre expression were studied using chemical and immune-mediated models of disease as well as colonic stem cell cultures.

RESULTS

ZO-1 transcript and protein expression were reduced in biopsy specimens from patients with IBD. Despite mildly increased intestinal permeability, ZO-1KO.IEC mice were healthy and did not develop spontaneous disease. ZO-1KO.IEC mice were, however, hypersensitive to mucosal insults and displayed defective repair. Furthermore, ZO-1-deficient colonic epithelia failed to up-regulate proliferation in response to damage in vivo or Wnt signaling in vitro. ZO-1 was associated with centrioles in interphase cells and mitotic spindle poles during division. In the absence of ZO-1, mitotic spindles failed to correctly orient, resulting in mitotic catastrophe and abortive proliferation. ZO-1 is, therefore, critical for up-regulation of epithelial proliferation and successful completion of mitosis.

CONCLUSIONS

ZO-1 makes critical, tight junction-independent contributions to Wnt signaling and mitotic spindle orientation. As a result, ZO-1 is essential for mucosal repair. We speculate that ZO-1 down-regulation may be one cause of ineffective mucosal healing in patients with IBD.