The mucosal barrier at a glance

E-cadherin Beyond Structure: A Signaling Hub in Colon Homeostasis and Disease

E-cadherin is the core component of epithelial adherens junctions, essential for tissue development, differentiation, and maintenance. It is also fundamental for tissue barrier formation, a critical function of epithelial tissues. The colon or large intestine is lined by an epithelial monolayer that encompasses an E-cadherin-dependent barrier, critical for the homeostasis of the organ. Compromised barriers of the colonic epithelium lead to inflammation, fibrosis, and are commonly observed in colorectal cancer. In addition to its architectural role, E-cadherin is also considered a tumor suppressor in the colon, primarily a result of its opposing function to Wnt signaling, the predominant driver of colon tumorigenesis. Beyond these well-established traditional roles, several studies have portrayed an evolving role of E-cadherin as a signaling epicenter that regulates cell behavior in response to intra- and extra-cellular cues. Intriguingly, these recent findings also reveal tumor-promoting functions of E-cadherin in colon tumorigenesis and new interacting partners, opening future avenues of investigation. In this Review, we focus on these emerging aspects of E-cadherin signaling, and we discuss their implications in colon biology and disease.

Probing paracellular -versus transcellular tissue barrier permeability using a gut mucosal explant culture system

Intestinal permeation enhancers (PEs), i.e. agents improving oral delivery of therapeutic drugs with poor bioavailability, may typically act by two principally different mechanisms: to increase either transcellular -or paracellular passage across the epithelium. With the aim to define these different modes of action in a small intestinal mucosal explant system, the transcellular-acting PE sodium dodecyl sulfate (SDS) was compared to the paracellular-acting PE ethylenediaminetetraacetic acid (EDTA), using several fluorescent polar - and lipophilic probes. Here, SDS rendered the enterocyte cell membranes leaky for the relatively small polar tracers Lucifer yellow and a 3 kD Texas red-conjugated dextran, but most conspicuously SDS blocked constitutive endocytosis from the brush border. In contrast, the main action of EDTA was to increase paracellular passage across the epithelium of both polar probes, including 10 - and 70 kDa dextrans and lipophilic probes, visualized by distinct stripy lateral staining of enterocytes and/or accumulation in the lamina propria. In addition, EDTA caused a loss of epithelial cell polarity by opening tight junctions for diffusion of domain-specific basolateral/apical cell membrane protein markers into the opposite domains. By transmission electron microscopy, SDS caused the formation of vacuoles and vesicle-like structures at the lateral cell membranes. In contrast, EDTA led to a bulging of the whole enterocyte apex, resulting in a "cobblestone" appearance of the epithelium, probably caused by an extreme contraction of the perijunctional actomyosin ring. We conclude that the mucosal explant system is a convenient model for predicting transcellular/paracellular modes of action of novel prospective PEs.

The Sandwich Assay: A Method for Subcellular Visualization of Paracellular Macromolecule Passage in Epithelial Sheets

To date, the permeability of epithelia to larger solutes (greater than ∼4 Å in diameter) has been analyzed by flux measurements using various tracers that cannot spatially resolve the permeation sites. This unit describes a method for localizing such sites of passage in epithelial sheets with subcellular resolution. The method makes use of avidin as a basolateral capture probe in epithelial monolayers or mucosae to unmask the passage of biotinylated and fluorophore-labeled tracer molecules as they go through the junctional barrier. Once bound to avidin, the tracers are immobilized at the site of a barrier leak. The localization, the distribution, and the extent of passage are eventually evaluated by imaging. The assay detects single leaks and is hence able to spatially resolve rarely occurring changes. It is also modular and flexible to use with various macromolecular tracers, and its sensitivity is adjustable. If designed as a chase experiment, the method allows for analysis of temporal barrier openings. If performed at low temperatures, this assay will block transcellular passage and, combined with global flux measurement, unambiguously determine paracellular passage. © 2018 by John Wiley & Sons, Inc.

IFN-λ1 enhances Staphylococcus aureus clearance in healthy nasal mucosa but not in nasal polyps

BACKGROUND

Chronic rhinosinusitis with nasal polyps (CRSwNP) is characterized by TH2-skewed inflammation and increased colonization by Staphylococcus aureus. IFN-λ1 is known for its antiviral activity, but there is little information on its antibacterial role.

OBJECTIVE

We sought to determine the expression and release of IFN-λ1 from nasal mucosal tissue of healthy subjects and patients with CRSwNP on exposure to S aureus and assess its potential role in antibacterial defense mechanisms.

METHODS

Nasal tissue from healthy subjects and patients with CRSwNP was exposed to S aureus, and we assessed expression of IFN-λ1, MUC5AC, and MUC5B. THP1-derived macrophages incubated with or without IFN-λ1 were assessed for uptake and killing of S aureus and expression of lysosomal-associated membrane protein 1 and intracellular reactive oxidase substrate (ROS), the IFN-λ1 receptor IL-28 receptor (IL-28R), and the Janus kinase/signal transducer and activator of transcription (STAT) 1 pathway by means of immunofluorescence staining.

RESULTS

S aureus infection increased IFN-λ1 expression in tissue from patients with CRSwNP. IFN-λ1 (10 ng/mL) significantly decreased the number of S aureus colony-forming units in healthy control tissue but not in tissue from patients with CRSwNP and upregulated MUC5AC and MUC5B expression in control tissue on S aureus infection. IFN-λ1 stimulation increased intracellular killing of S aureus in THP1-derived macrophages and substantially increased lysosomal-associated membrane protein 1, IL-28R, ROS, and STAT signaling in macrophages incubated with S aureus. All of these effects were attenuated by blocking IL-28R and ROS activities.

CONCLUSIONS

IFN-λ1 favors clearance of S aureus in healthy nasal mucosa and enhances antibacterial function of macrophages through IFN-λ1-IL-28R-ROS-Janus kinase-STAT signaling pathways.

Meat proteins in a high-fat diet have a substantial impact on intestinal barriers through mucus layer and tight junction protein suppression in C57BL/6J mice

Protein diets are well known for body maintenance and weight loss.

Epithelial stratification shapes infection dynamics

Infections of stratified epithelia contribute to a large group of common diseases, such as dermatological conditions and sexually transmitted diseases. To investigate how epithelial structure affects infection dynamics, we develop a general ecology-inspired model for stratified epithelia. Our model allows us to simulate infections, explore new hypotheses and estimate parameters that are difficult to measure with tissue cell cultures. We focus on two contrasting pathogens: Chlamydia trachomatis and Human papillomaviruses (HPV). Using cervicovaginal parameter estimates, we find that key infection symptoms can be explained by differential interactions with the layers, while clearance and pathogen burden appear to be bottom-up processes. Cell protective responses to infections (e.g. mucus trapping) generally lowered pathogen load but there were specific effects based on infection strategies. Our modeling approach opens new perspectives for 3D tissue culture experimental systems of infections and, more generally, for developing and testing hypotheses related to infections of stratified epithelia.

The relationship between vitamin C status, the gut-liver axis, and metabolic syndrome

Metabolic syndrome (MetS) is a constellation of cardiometabolic risk factors, which together predict increased risk of more serious chronic diseases. We propose that one consequence of dietary overnutrition is increased abundance of Gram-negative bacteria in the gut that cause increased inflammation, impaired gut function, and endotoxemia that further dysregulate the already compromised antioxidant vitamin status in MetS. This discussion is timely because "healthy" individuals are no longer the societal norm and specialized dietary requirements are needed for the growing prevalence of MetS. Further, these lines of evidence provide the foundational basis for investigation that poor vitamin C status promotes endotoxemia, leading to metabolic dysfunction that impairs vitamin E trafficking through a mechanism involving the gut-liver axis. This report will establish a critical need for translational research aimed at validating therapeutic approaches to manage endotoxemia-an early, but inflammation-inducing phenomenon, which not only occurs in MetS, but is also prognostic of more advanced metabolic disorders including type 2 diabetes mellitus, as well as the increasing severity of nonalcoholic fatty liver diseases.

A dietary carbohydrase blend improved intestinal barrier function and growth rate in weaned pigs fed higher fiber diets

The objective of this study was to evaluate the effects of dietary xylanase (X) and a carbohydrase enzyme blend (EB: cellulase, β-glucanase, and xylanase) on nutrient digestibility, intestinal barrier integrity, inflammatory status, and growth performance in weaned piglets fed higher fiber diets. A total of 460 pigs (6.43 ± 0.06 kg BW; F25 × 6.0 Genetiporc) were blocked by initial BW and pens (n = 12 per treatment) were randomly assigned to 1 of 4 dietary treatments. The diets included a higher fiber unsupplemented control diet (CON) and the CON supplemented with 0.01% X, 0.01% EB, or both enzymes, arranged in a 2 × 2 factorial. The diets were based on corn, soybean meal, corn distillers dried grains with solubles (DDGS), and wheat middlings. Pigs had 7 d to adapt to the environment and consumed the same commercial diet. Pigs were fed the experimental diets for 28 d with free access to feed and water. Body weight and feed disappearance were recorded weekly. One pig with BW closest to the pen average from each pen was selected and moved to metabolism crates on day 16 and intragastric gavaged a solution of lactulose and mannitol on day 22 followed by 12-h urine collection. Feces were collected from day 23 to 25. Intestinal tissues and mucosal scrapings were collected on day 28. Data were analyzed using PROC MIXED of SAS (9.4). Xylanase, EB, and their interaction were fixed effects and block was a random effect. The EB, but not X, increased pig BW and improved ADG over 28 d (P < 0.05). Neither carbohydrase impacted ADFI, G:F, or apparent total tract digestibility (ATTD) of DM, GE, or CP. The EB improved ATTD of ADF (32.45 vs. 26.57%; P < 0.01), but had no effect on NDF. Unexpectedly, X reduced ATTD of NDF and ADF (P < 0.01). The EB reduced urinary lactulose:mannitol and increased ileal claudin-3 mRNA abundance (P < 0.05), indicating improved small intestinal barrier integrity. There was a X × EB interaction on ileal secretory immunoglobulin A (sIgA) concentration (P < 0.05); in the absence of X, EB decreased sIgA compared to CON, but this effect disappeared in the presence of X. The EB also reduced ileal IL-22 mRNA abundance (P < 0.05), probably indicating decreased immune activation. In conclusion, EB but not X enhanced growth rate of weaned pigs fed higher fiber diets, which may be partly explained by the improved small intestinal barrier integrity and reduced immune activation, rather than improvement in nutrient digestibility.

The effect of dietary supplementation of low crude protein on intestinal morphology in pigs

To explore the effects of reducing the Cp levels on intestinal barrier function, low Cp (LP) and NRC standard Cp (NP) diets were fed to pigs from 45 to 160 days, and in vitro experiments were performed using monolayers of IPEC-J2 cells. The number of goblet cells, expression of proteins related to cell junction, amino acid transport, glucose transport, transepithelial electrical resistance (TEER), dextran permeability, and IL-6 secretion level were detected in pigs. The results demonstrated that a moderate reduction of Cp levels did not affect intestinal morphology, as demonstrated by a normal villi height, crypt depth and normal numbers of goblet cells. The maintenance of the intestinal structure obtained with LP was also confirmed by stable mRNA expression levels of muc2 and E-cadherin in the jejunum. We also found that LP did not affect the protein expression of cationic amino acid transporter 1 (CAT-1) and alanine serine cysteine transporter 1 (ASCT1) from 45 to 160 days. Moreover, the excitatory amino acid transporter 3 (EAAT3), sodium-glucose cotransporter 1 (SGLT1) and glucose transporter (GLUT2) protein expression levels in the jejunum were significantly increased at a certain age during the rearing period. Furthermore, we also demonstrated that a reduction in protein concentration up to 15% in the cultural medium of IPEC-J2 cells did not impact the mucosal barrier function. This study demonstrated that a moderate reduction of the protein level did not affect intestinal mucosal barrier function and morphology in the jejunum.

Liver Injury, Endotoxemia, and Their Relationship to Intestinal Microbiota Composition in Alcohol-Preferring Rats

BACKGROUND

There is strong evidence that alcoholism leads to dysbiosis in both humans and animals. However, it is unclear how changes in the intestinal microbiota (IM) relate to ethanol (EtOH)-induced disruption of gut-liver homeostasis. We investigated this issue using selectively bred Sardinian alcohol-preferring (sP) rats, a validated animal model of excessive EtOH consumption.

METHODS

Independent groups of male adult sP rats were exposed to the standard, home-cage 2-bottle "EtOH (10% v/v) versus water" choice regimen with unlimited access for 24 h/d (Group Et) for 3 (T1), 6 (T2), and 12 (T3) consecutive months. Control groups (Group Ct) were composed of matched-age EtOH-naïve sP rats. We obtained samples from each rat at the end of each experimental time, and we used blood and colon tissues for intestinal barrier integrity and/or liver pathology assessments and used stool samples for IM analysis with 16S ribosomal RNA gene sequencing.

RESULTS

Rats in Group Et developed hepatic steatosis and elevated serum transaminases and endotoxin/lipopolysaccharide (LPS) levels but no other liver pathological changes (i.e., necrosis/inflammation) or systemic inflammation. While we did not find any apparent alteration of the intestinal colonic mucosa, we found that rats in Group Et exhibited significant changes in IM composition compared to the rats in Group Ct. These changes were sustained throughout T1, T2, and T3. In particular, Ruminococcus, Coprococcus, and Streptococcus were the differentially abundant microbial genera at T3. The KEGG Ortholog profile revealed that IM functional modules, such as biosynthesis, transport, and export of LPS, were also enriched in Group Et rats at T3.

CONCLUSIONS

We showed that chronic, voluntary EtOH consumption induced liver injury and endotoxemia together with dysbiotic changes in sP rats. This work sets the stage for improving our knowledge of the prevention and treatment of EtOH-related diseases.

Protective effect of rhubarb against intestinal mucosal barrier injury in rats with obstructive jaundice

Objective: To investigate the effects of raw rhubarb (RR) on the intestinal barrier dysfunction and endotoxemia in rat models with obstructive jaundice (OJ).

Methods: Twenty-seven Sprague–Dawley rats were randomly allocated to four groups: control ([Formula: see text]), sham operation ([Formula: see text]), model ([Formula: see text]), and treatment ([Formula: see text]). Rat models with OJ were used in the model and treatment groups. In the treatment group, rats were intragastrically administered with RR granular solution. After treatment, serum endotoxin (ET), serum alanine aminotransferase (ALT), aspartate aminotransferase (AST), and total bilirubin (TB) levels were determined. The liver tissue, bile duct tissue above the obstruction bile duct site, and parts of the ileum and colon tissues were stained with hematoxylin and eosin and observed by light microscopy, and the histopathological changes in the ileum were observed by electron microscopy.

Results: Fourteen days after the rats in the treatment group were intragastrically administered with RR granular solution, the ALT, AST, and TB levels showed no significant difference between the control and sham operation groups ([Formula: see text]). Serum ET level was significantly lower in the treatment group than in the model group ([Formula: see text]). Histopathology of the liver and bile duct revealed that RR might alleviate OJ-associated hepatocyte degeneration/necrosis, infiltration of inflammatory cells, and hepatic fibrosis, and reduce the damage to parietal cells and bile duct mucosa. In OJ rats, RR might also have a protective effect against colonic wall edema and necrosis and infiltration of inflammatory cells.

Conclusions: RR may improve the structural changes in the intestinal mucosa caused by OJ and decrease serum ET level in OJ rats.

Bifidobacterium animalis subspecies lactis modulates the local immune response and glucose uptake in the small intestine of juvenile pigs infected with the parasitic nematode Ascaris suum

An evaluation of a localized intestinal allergic type-2 response concomitant with consumption of probiotic bacteria is not well documented. This study investigated the effect of feeding probiotic Bifidobacterium animalis subspecies lactis (Bb12) or a placebo in weaned pigs that were also inoculated with Ascaris suum (A. suum) eggs to induce a strong Th2-dependent allergic type 2 immune response. Sections of jejunal mucosa were mounted in Ussing chambers to determine changes in permeability and glucose absorption, intestine and liver samples were collected for analysis of type-2 related gene expression, jejunum examined histologically, and sera and intestinal fluid were assayed for parasite antigen specific antibody. The prototypical parasite-induced secretory response to histamine and reduced absorption of glucose in the jejunum were attenuated by feeding Bb12 without a change in mucosal resistance. Parasite antigen-specific IgA response in the serum and IgG1 and IgG2 response in the ileal fluid were significantly increased in A. suum-infected pigs treated with Bb12 compared to infected pigs given the placebo. Ascaris suum-induced eosinophilia in the small intestinal mucosa was inhibited by Bb12 treatment without affecting the normal expulsion of A. suum 4th stage larvae (L4) or the morphometry of the intestine. Expression of genes associated with Th1/Th2 cells, Treg cells, mast cells, and physiological function in the intestine were modulated in A. suum infected-pigs treated with Bb12. These results suggested that Bb12 can alter local immune responses and improve intestinal function during a nematode infection by reducing components of a strong allergenic type-2 response in the pig without compromising normal parasite expulsion.

Exosomes derived from high-glucose-stimulated Schwann cells promote development of diabetic peripheral neuropathy

Schwann cells actively interact with axons of dorsal root ganglia (DRG) neurons. Exosomes mediate intercellular communication by transferring their biomaterials, including microRNAs (miRs) into recipient cells. We hypothesized that exosomes derived from Schwann cells stimulated by high glucose (HG) exosomes accelerate development of diabetic peripheral neuropathy and that exosomal cargo miRs contribute to this process. We found that HG exosomes contained high levels of miR-28, -31a, and -130a compared to exosomes derived from non-HG-stimulated Schwann cells. In vitro, treatment of distal axons with HG exosomes resulted in reduction of axonal growth, which was associated with elevation of miR-28, -31a, and -130a and reduction of their target proteins of DNA methyltransferase-3α, NUMB (an endocytic adaptor protein), synaptosome associated protein 25, and growth-associated protein-43 in axons. In vivo, administration of HG exosomes to sciatic nerves of diabetic db/db mice at 7 wk of age promoted occurrence of peripheral neuropathy characterized by impairment of nerve conduction velocity and induction of mechanic and thermal hypoesthesia, which was associated with substantial decreases in intraepidermal nerve fibers. Our findings demonstrate a functional role of exosomes derived from HG-stimulated Schwann cells in mediating development of diabetic peripheral neuropathy.-Jia, L., Chopp, M., Wang, L., Lu, X., Szalad, A., Zhang, Z. G. Exosomes derived from high-glucose-stimulated Schwann cells promote development of diabetic peripheral neuropathy.

Mechanisms and Regulation of Intestinal Phosphate Absorption

States of hypo- and hyperphosphatemia have deleterious consequences including rickets/osteomalacia and renal/cardiovascular disease, respectively. Therefore, the maintenance of appropriate plasma levels of phosphate is an essential requirement for health. This control is executed by the collaborative action of intestine and kidney whose capacities to (re)absorb phosphate are regulated by a number of hormonal and metabolic factors, among them parathyroid hormone, fibroblast growth factor 23, 1,25(OH)₂ vitamin D₃ , and dietary phosphate. The molecular mechanisms responsible for the transepithelial transport of phosphate across enterocytes are only partially understood. Indeed, whereas renal reabsorption entirely relies on well-characterized active transport mechanisms of phosphate across the renal proximal epithelia, intestinal absorption proceeds via active and passive mechanisms, with the molecular identity of the passive component still unknown. The active absorption of phosphate depends mostly on the activity and expression of the sodium-dependent phosphate cotransporter NaPi-IIb (SLC34A2), which is highly regulated by many of the factors, mentioned earlier. Physiologically, the contribution of NaPi-IIb to the maintenance of phosphate balance appears to be mostly relevant during periods of low phosphate availability. Therefore, its role in individuals living in industrialized societies with high phosphate intake is probably less relevant. Importantly, small increases in plasma phosphate, even within normal range, associate with higher risk of cardiovascular disease. Therefore, therapeutic approaches to treat hyperphosphatemia, including dietary phosphate restriction and phosphate binders, aim at reducing intestinal absorption. Here we review the current state of research in the field. © 2017 American Physiological Society. Compr Physiol 8:1065-1090, 2018.

Sex differences in NSAID-induced perturbation of human intestinal barrier function and microbiota

Intestinal barrier function and microbiota are integrally related and play critical roles in maintenance of host physiology. Sex is a key biologic variable for several disorders. Our aim was to determine sex-based differences in response to perturbation and subsequent recovery of intestinal barrier function and microbiota in healthy humans. Twenty-three volunteers underwent duodenal biopsies, mucosal impedance, and in vivo permeability measurement. Permeability testing was repeated after administration of indomethacin, then 4 to 6 wk after its discontinuation. Duodenal and fecal microbiota composition was determined using 16S rRNA amplicon sequencing. Healthy women had lower intestinal permeability and higher duodenal and fecal microbial diversity than healthy men. Intestinal permeability increases after indomethacin administration in both sexes. However, only women demonstrated decreased fecal microbial diversity, including an increase in Prevotella abundance, after indomethacin administration. Duodenal microbiota composition did not show sex-specific changes. The increase in permeability and microbiota changes normalized after discontinuation of indomethacin. In summary, women have lower intestinal permeability and higher microbial diversity. Intestinal permeability is sensitive to perturbation but recovers to baseline. Gut microbiota in women is sensitive to perturbation but appears to be more stable in men. Sex-based differences in intestinal barrier function and microbiome should be considered in future studies.-Edogawa, S., Peters, S. A., Jenkins, G. D., Gurunathan, S. V., Sundt, W. J., Johnson, S., Lennon, R. J., Dyer, R. B., Camilleri, M., Kashyap, P. C., Farrugia, G., Chen, J., Singh, R. J., Grover, M. Sex differences in NSAID-induced perturbation of human intestinal barrier function and microbiota.

Cryptosporidium parvum disrupts intestinal epithelial barrier function via altering expression of key tight junction and adherens junction proteins

Infection with the protozoan parasite Cryptosporidium parvum (CP) causes cryptosporidiosis, a widespread diarrhoeal disease. Impaired intestinal epithelial barrier function and increased permeability are most commonly associated with diarrhoeal diseases caused by enteric infections. However, studies on barrier disruption and underlying mechanisms in cryptosporidiosis are extremely limited. Epithelial tight junctions (TJs) and adherens junctions (AJs) are important in maintaining barrier integrity. Therefore, we examined the effects of CP infection on paracellular permeability and on the expression of the major TJ and AJ proteins utilising in vitro, ex vivo, and in vivo models. CP infection (0.5 × 106  oocysts/well in Transwell inserts, 24 hr) increased paracellular permeability (FITC-dextran flux) in Caco-2 cell monolayers and substantially decreased the protein levels of occludin, claudin 4, and E-cadherin. Claudin 3, zonula occludens-1 (ZO1) and α-catenin were also significantly decreased, whereas claudins 1 and 2 and β-catenin were not altered. Substantial downregulation of occludin, claudin 4, and E-cadherin was also observed in response to CP infection ex vivo in mouse enteroid-derived monolayers and in vivo in the ileal and jejunal mocosa of C57BL/6 mice. The mRNA levels of these proteins were also significantly decreased in CP-infected mouse ileum and jejunum but were unaltered in Caco-2 cells. Further, bafilomycin-A, an inhibitor of lysosomal proton pump, partially abrogated CP effects on occludin expression in Caco-2 cells, suggesting a potential role of posttranslational mechanisms, such as induction of protein degradation pathways, in mediating the effects of the parasite. Our studies suggest that disruption of barrier function via downregulation of specific key components of TJ and AJ could be a major mechanism underlying CP infection-induced diarrhoea.

Role of mucins in lung homeostasis: regulated expression and biosynthesis in health and disease

In humans and mice, the first line of innate defense against inhaled pathogens and particles in the respiratory tract is airway mucus. The primary solid components of the mucus layer are the mucins MUC5AC and MUC5B, polymeric glycoproteins whose changes in abundance and structure can dramatically affect airway defense. Accordingly, MUC5AC/Muc5ac and MUC5B/Muc5b are tightly regulated at a transcriptional level by tissue-specific transcription factors in homeostasis and in response to injurious and inflammatory triggers. In addition to modulated levels of mucin gene transcription, translational and post-translational biosynthetic processes also exert significant influence upon mucin function. Mucins are massive macromolecules with numerous functional domains that contribute to their structural composition and biophysical properties. Single MUC5AC and MUC5B apoproteins have molecular masses of >400 kDa, and von Willebrand factor D-like as well as other cysteine-rich domain segments contribute to mucin polymerization and flexibility, thus increasing apoprotein length and complexity. Additional domains serve as sites for O-glycosylation, which increase further mucin mass several-fold. Glycosylation is a defining process for mucins that is specific with respect to additions of glycans to mucin apoprotein backbones, and glycan additions influence the physical properties of the mucins via structural modifications as well as charge interactions. Ultimately, through their tight regulation and complex assembly, airway mucins follow the biological rule of 'form fits function' in that their structural organization influences their role in lung homeostatic mechanisms.

Pathophysiology of IBD associated diarrhea

Inflammatory bowel diseases broadly categorized into Crohn's disease (CD) and ulcerative colitis (UC), are chronic inflammatory disorders of the gastrointestinal tract with increasing prevalence worldwide. The etiology of the disease is complex and involves a combination of genetic, environmental, immunological and gut microbial factors. Recurring and bloody diarrhea is the most prevalent and debilitating symptom in IBD. The pathogenesis of IBD-associated diarrhea is multifactorial and is essentially an outcome of mucosal damage caused by persistent inflammation resulting in dysregulated intestinal ion transport, impaired epithelial barrier function and increased accessibility of the pathogens to the intestinal mucosa. Altered expression and/or function of epithelial ion transporters and channels is the principle cause of electrolyte retention and water accumulation in the intestinal lumen leading to diarrhea in IBD. Aberrant barrier function further contributes to diarrhea via leak-flux mechanism. Mucosal penetration of enteric pathogens promotes dysbiosis and exacerbates the underlying immune system further perpetuating IBD associated-tissue damage and diarrhea. Here, we review the mechanisms of impaired ion transport and loss of epithelial barrier function contributing to diarrhea associated with IBD.

Intestinal surfactant permeation enhancers and their interaction with enterocyte cell membranes in a mucosal explant system

Intestinal permeation enhancers (PEs) are agents aimed to improve oral delivery of therapeutic drugs with poor bioavailability. The main permeability barrier for oral delivery is the intestinal epithelium, and PEs act to increase the paracellular and/or transcellular passage of drugs. Transcellular passage can be achieved by cell membrane permeabilization and/or by endocytic uptake and subsequent transcytosis. One broad class of PEs is surfactants which act by inserting into the cell membrane, thereby perturbing its integrity, but little is known about how the dynamics of the membrane are affected. In the present work, the interaction of the surfactants lauroyl-L-carnitine, 1-decanoyl-rac-glycerol, and nonaethylene glycol monododecyl ether with the intestinal epithelium was studied in organ cultured pig jejunal mucosal explants. As expected, at 2 mM, these agents rapidly permeabilized the enterocytes for the fluorescent polar tracer lucifer yellow, but surprisingly, they all also blocked both constitutive -and receptor-mediated pathways of endocytosis from the brush border, indicating a complete arrest of apical membrane trafficking. At the ultrastructural level, the PEs caused longitudinal fusion of brush border microvilli. Such a membrane fusogenic activity could also explain the observed formation of vesicle-like structures and large vacuoles along the lateral cell membranes of the enterocytes induced by the PEs. We conclude that the surfactant action of the PEs selected in this study not only permeabilized the enterocytes, but profoundly changed the dynamic properties of their constituent cell membranes.

Advances in Probiotic Regulation of Bone and Mineral Metabolism

Probiotics have been consumed by humans for thousands of years because they are beneficial for long-term storage of foods and promote the health of their host. Ingested probiotics reside in the gastrointestinal tract where they have many effects including modifying the microbiota composition, intestinal barrier function, and the immune system which result in systemic benefits to the host, including bone health. Probiotics benefit bone growth, density, and structure under conditions of dysbiosis, intestinal permeability, and inflammation (recognized mediators of bone loss and osteoporosis). It is likely that multiple mechanisms are involved in mediating probiotic signals from the gut to the bone. Studies indicate a role for the microbiota (composition and activity), intestinal barrier function, and immune cells in the signaling process. These mechanisms are not mutually exclusive, but rather, may synergize to provide benefits to the skeletal system of the host and serve as a starting point for investigation. Given that probiotics hold great promise for supporting bone health and are generally regarded as safe, future studies identifying mechanisms are warranted.

EB 2017 Article: Interpretation of the lactulose:mannitol test in rural Malawian children at risk for perturbations in intestinal permeability

The dual sugar absorption test, specifically the lactulose:mannitol test, is used to assess gut health. Lactulose absorption is said to represent gut damage and mannitol absorption is used as a measure of normal small bowel function and serves as normalizing factor for lactulose. A underappreciated limitation of this common understanding of the lactulose:mannitol test is that mannitol is not absorbed to any substantial extent by a transcellular process. Additionally, this interpretation of lactulose:mannitol is not consistent with current understanding of paracellular pathways, where three pathway types exist: pore, leak, and unrestricted. Pore and leak pathways are regulated biological constructions of the small bowel barrier, and unrestricted pathways represent micropathological damage. We analyzed 2334 lactulose:mannitol measurements rigorously collected from 622 young rural Malawian children at high risk for poor gut health in light of the pathway model. An alternative method of normalizing for gut length utilizing autopsy data is described. In our population, absorbed lactulose and mannitol are strongly correlated, r = 0.68 P <0.0001, suggesting lactulose and mannitol are traversing the gut barrier via the same pathways. Considering measurements where pore pathways predominate, mannitol flux is about 14 times that of lactulose. As more leak pathways are present, this differential flux mannitol:lactulose falls to 8:1 and when increased numbers of unrestricted pathways are present, the differential flux of mannitol:lactulose is 6:1. There was no substantial correlation between the lactulose:mannitol and linear growth. Given that mannitol will always pass through a given pathway at a rate at least equal to that of lactulose, and lactulose absorption is a composite measure of flux through both physiologic and pathologic pathways, we question the utility of the lactulose:mannitol test. We suggest using lactulose alone is as informative as lactulose:mannitol in a sugar absorption testing in subclinical gut inflammation. Impact statement Our work integrates the standard interpretation of the lactulose:mannitol test (L:M), with mechanistic insight of intestinal permeability. There are three paracellular pathways in the gut epithelium; pore, leak, and unrestricted. Using thousands of L:M measurements from rural Malawian children at risk for increased intestinal permeability, we predict the differential flux of L and M through the pathways. Our findings challenge the traditional notions that little L is absorbed through a normal epithelial barrier and that M is a normalizing factor for L. Our observations are consistent with pore pathways allowing only M to pass. And that substantial amounts of L and M pass through leak pathways which are normal, regulated, cell-junctional adaptations. So M is a composite measure of all pathways, and L is not a measure solely of pathologic gut damage. Using L alone as a probe will yield more information about gut health than L:M.

Modulation of Intestinal Paracellular Transport by Bacterial Pathogens

The passive and regulated movement of ions, solutes, and water via spaces between cells of the epithelial monolayer plays a critical role in the normal intestinal functioning. This paracellular pathway displays a high level of structural and functional specialization, with the membrane-spanning complexes of the tight junctions, adherens junctions, and desmosomes ensuring its integrity. Tight junction proteins, like occludin, tricellulin, and the claudin family isoforms, play prominent roles as barriers to unrestricted paracellular transport. The past decade has witnessed major advances in our understanding of the architecture and function of epithelial tight junctions. While it has been long appreciated that microbes, notably bacterial and viral pathogens, target and disrupt junctional complexes and alter paracellular permeability, the precise mechanisms remain to be defined. Notably, renewed efforts will be required to interpret the available data on pathogen-mediated barrier disruption in the context of the most recent findings on tight junction structure and function. While much of the focus has been on pathogen-induced dysregulation of junctional complexes, commensal microbiota and their products may influence paracellular permeability and contribute to the normal physiology of the gut. Finally, microbes and their products have become important tools in exploring host systems, including the junctional properties of epithelial cells. © 2018 American Physiological Society. Compr Physiol 8:823-842, 2018.

Targeting the gut barrier for the treatment of alcoholic liver disease

Alcohol consumption remains one of the predominant causes of liver disease and liver-related death worldwide. Intriguingly, dysregulation of the gut barrier is a key factor promoting the pathogenesis of alcoholic liver disease (ALD). A functional gut barrier, which consists of a mucus layer, an intact epithelial monolayer and mucosal immune cells, supports nutrient absorption and prevents bacterial penetration. Compromised gut barrier function is associated with the progression of ALD. Indeed, alcohol consumption disrupts the gut barrier, increases gut permeability, and induces bacterial translocation both in ALD patients and in experimental models with ALD. Moreover, alcohol consumption also causes enteric dysbiosis with both numerical and proportional perturbations. Here, we review and discuss mechanisms of alcohol-induced gut barrier dysfunction to better understand the contribution of the gut-liver axis to the pathogenesis of ALD. Unfortunately, there is no effectual Food and Drug Administration-approved treatment for any stage of ALD. Therefore, we conclude with a discussion of potential strategies aimed at restoring the gut barrier in ALD. The principle behind antibiotics, prebiotics, probiotics and fecal microbiota transplants is to restore microbial symbiosis and subsequently gut barrier function. Nutrient-based treatments, such as dietary supplementation with zinc, niacin or fatty acids, have been shown to regulate tight junction expression, reduce intestinal inflammation, and prevent endotoxemia as well as liver injury caused by alcohol in experimental settings. Interestingly, saturated fatty acids may also directly control the gut microbiome. In summary, clinical and experimental studies highlight the significance and efficacy of the gut barrier in treating ALD.

MicroRNAs in intestinal barrier function, inflammatory bowel disease and related cancers-their effects and therapeutic potentials

The initiation and development or inflammatory bowel disease (IBD) and associated colorectal cancers, have been linked to inflammation. MicroRNAs are non-coding regulators of gene expression that have gained great attention due to their capability to regulate the expression of a number of target transcripts. It is now generally admitted that microRNAs are instrumental in gut pathologies, in particular through their targeting of transcripts encoding proteins of the intestinal barrier (IB) and their regulators. Intense research is conducted to identify microRNAs susceptible to be used as biomarkers and to design new therapeutic approaches based upon using synthetic microRNA mimics and inhibitors as well as finding new drugs capable to restore or modify microRNA expression in the context of gut pathologies.

Transcription factor TFEB cell-autonomously modulates susceptibility to intestinal epithelial cell injury in vivo

Understanding the transcription factors that modulate epithelial resistance to injury is necessary for understanding intestinal homeostasis and injury repair processes. Recently, transcription factor EB (TFEB) was implicated in expression of autophagy and host defense genes in nematodes and mammalian cells. However, the in vivo roles of TFEB in the mammalian intestinal epithelium were not known. Here, we used mice with a conditional deletion of Tfeb in the intestinal epithelium (Tfeb ΔIEC) to examine its importance in defense against injury. Unperturbed Tfeb ΔIEC mice exhibited grossly normal intestinal epithelia, except for a defect in Paneth cell granules. Tfeb ΔIEC mice exhibited lower levels of lipoprotein ApoA1 expression, which is downregulated in Crohn's disease patients and causally linked to colitis susceptibility. Upon environmental epithelial injury using dextran sodium sulfate (DSS), Tfeb ΔIEC mice exhibited exaggerated colitis. Thus, our study reveals that TFEB is critical for resistance to intestinal epithelial cell injury, potentially mediated by APOA1.

Vitamin D signaling in intestinal innate immunity and homeostasis

The lumen of the gut hosts a plethora of microorganisms that participate in food assimilation, inactivation of harmful particles and in vitamin synthesis. On the other hand, enteric flora, a number of food antigens, and toxins are capable of triggering immune responses causing inflammation, which, when unresolved, may lead to chronic conditions such as inflammatory bowel disease (IBD). It is important, therefore, to contain the gut bacteria within the lumen, control microbial load and composition, as well as ensure adequate innate and adaptive immune responses to pathogenic threats. There is growing evidence that vitamin D signaling has impacts on all these aspects of intestinal physiology, contributing to healthy enteric homeostasis. VD was first discovered as the curative agent for nutritional rickets, and its classical actions are associated with calcium absorption and bone health. However, vitamin D exhibits a number of extra-skeletal effects, particularly in innate immunity. Notably, it stimulates production of pattern recognition receptors, anti-microbial peptides, and cytokines, which are at the forefront of innate immune responses. They play a role in sensing the microbiota, in preventing excessive bacterial overgrowth, and complement the actions of vitamin D signaling in enhancing intestinal barrier function. Vitamin D also favours tolerogenic rather than inflammogenic T cell differentiation and function. Compromised innate immune function and overactive adaptive immunity, as well as defective intestinal barrier function, have been associated with IBD. Importantly, observational and intervention studies support a beneficial role of vitamin D supplementation in patients with Crohn's disease, a form of IBD. This review summarizes the effects of vitamin D signaling on barrier integrity and innate and adaptive immunity in the gut, as well as on microbial load and composition. Collectively, studies to date reveal that vitamin D signaling has widespread effects on gut homeostasis, and provide a mechanistic basis for potential therapeutic benefit of vitamin D supplementation in IBD.

Dynamic modeling of the tight junction pore pathway

Claudins define paracellular permeability to small molecules by forming ion-selective pores within the tight junction. We recently demonstrated that claudin-2 channels are gated and open and close on a submillisecond timescale. To determine if and how the ensemble behavior of this unique class of entirely extracellular gated ion channels could define global epithelial barrier function, we have developed an in silico model of local claudin-2 behavior. This model considers the complex anastomosing ultrastructure of tight junction strands and can be scaled to show that local behavior defines global epithelial barrier function of epithelial monolayers expressing different levels of claudin-2. This is the first mathematical model to describe global epithelial barrier function in terms of the dynamic behavior of single tight junction channels and establishes a framework to consider gating kinetics as a means to regulate barrier function.