Maintenance Of Intestinal Mucosal Integrity Research Articles

Influence of gut microbiome on the human physiology

Trillions of microbes harbor the gastrointestinal tract and co-exist peacefully with the human host. Microbial diversity and composition among individuals depend upon human age, diet, and environmental factors. The dynamic population of the gut microbiome, majorly consisting of Bacteroidetes and Firmicutes, forms a complex ecological community. The collective metabolic activities and interaction of gut microbiota with the host exert a marked influence on human physiology. The gut microbiota help to perform various functions, such as maintenance of intestinal mucosal integrity, production of anti-microbial peptides, protection against foreign invaders, and immunity development. In addition, they also provide essential nutrients, such as enzymes and vitamins, and also take part in metabolite synthesis, which influences both cognitive and behavioral functions of the human host. Homeostatic equilibrium among gut microorganisms, and between the microbes and intestinal interface of the host allows the maintenance of beneficial microbiota. Any alteration or dysbiosis in gut microbial composition, due to a sedentary lifestyle or intake of an unbalanced diet, plays a crucial role in the development of diseases like systemic inflammation, insulin resistance, auto-immune and metabolic disorders. This review summarizes our current understanding of the gut microbial organization, its importance in the fundamental biological processes and the pathogenesis of various human diseases and infections, and also the prognostic, diagnostic, and therapeutic potential of the gut microbiota.

Role of the sodium-dependent multivitamin transporter (SMVT) in the maintenance of intestinal mucosal integrity.

Utilizing a conditional (intestinal-specific) knockout (cKO) mouse model, we have recently shown that the sodium-dependent multivitamin transporter (SMVT) (SLC5A6) is the only biotin uptake system that operates in the gut and that its deletion leads to biotin deficiency. Unexpectedly, we also observed that all SMVT-cKO mice develop chronic active inflammation, especially in the cecum. Our aim here was to examine the role of SMVT in the maintenance of intestinal mucosal integrity [permeability and expression of tight junction (TJ) proteins]. Our results showed that knocking out the mouse intestinal SMVT is associated with a significant increase in gut permeability and with changes in the level of expression of TJ proteins. To determine whether these changes are related to the state of biotin deficiency that develops in SMVT-cKO mice, we induced (by dietary means) biotin deficiency in wild-type mice and examined its effect on the above-mentioned parameters. The results showed that dietary-induced biotin deficiency leads to a similar development of chronic active inflammation in the cecum with an increase in the level of expression of proinflammatory cytokines, as well as an increase in intestinal permeability and changes in the level of expression of TJ proteins. We also examined the effect of chronic biotin deficiency on permeability and expression of TJ proteins in confluent intestinal epithelial Caco-2 monolayers but observed no changes in these parameters. These results show that the intestinal SMVT plays an important role in the maintenance of normal mucosal integrity, most likely via its role in providing biotin to different cells of the gut mucosa.

Specific roles of threonine in intestinal mucosal integrity and barrier function

Threonine is the second or third limiting amino acid in swine or poultry diets. This nutrient plays a critical role in the maintenance of intestinal mucosal integrity and barrier function, which can be indicated by intestinal morphology, mucus production (number of goblet cells), transepithelial permeability, brush border enzyme activity, and growth performance. Dietary threonine restriction may decrease the production of digestive enzymes and increase mucosal paracellular permeability. A large proportion of dietary threonine is utilized for intestinal-mucosal protein synthesis, especially for mucin synthesis, and there is no oxidation of threonine by enterocytes. Because mucin proteins cannot be digested and reused, intestinal mucin secretion is a net loss of threonine from the body. Luminal threonine availability can influence synthesis of intestinal mucins and other proteins. Under pathological conditions, such as ileitis and sepsis, threonine requirement may be increased to maintain intestinal morphology and physiology. Collectively, knowledge about the role of threonine in mucin synthesis is critical for improving gut health under physiological and pathological conditions in animals and humans.

Toll-like receptor 2 plays a critical role in maintaining mucosal integrity during Citrobacter rodentium-induced colitis

Inflammatory bowel diseases and infectious gastroenteritis likely occur when the integrity of intestinal barriers is disrupted allowing luminal bacterial products to cross into the intestinal mucosa, stimulating immune cells and triggering inflammation. While specific Toll-like receptors (TLR) are involved in the generation of inflammatory responses against enteric bacteria, their contributions to the maintenance of intestinal mucosal integrity are less clear. These studies investigated the role of TLR2 in a model of murine colitis induced by the bacterial pathogen Citrobacter rodentium. C. rodentium supernatants specifically activated TLR2 in vitro while infected TLR2-/- mice suffered a lethal colitis coincident with colonic mucosal ulcerations, bleeding and increased cell death but not increased pathogen burden. TLR2-/- mice suffered impaired epithelial barrier function mediated via zonula occludens (ZO)-1 in naïve mice and claudin-3 in infected mice, suggesting this could underlie their susceptibility. TLR2 deficiency was also associated with impaired production of IL-6 by bone marrow-derived macrophages and infected colons cultured ex vivo. As IL-6 has antiapoptotic and epithelial repair capabilities, its reduced expression could contribute to the impaired mucosal integrity. These studies report for the first time that TLR2 plays a critical role in maintaining intestinal mucosal integrity during infection by a bacterial pathogen.

Bile salts regulate intestinal epithelial cell migration by nuclear factor–κB–induced expression of transforming growth factor–β

Mucosal restitution is an important repair modality in the gastrointestinal tract. We have shown that taurodeoxycholate increases intestinal epithelial cell migration by increasing TGF-beta expression, and that the transcription factor NF-kappa B regulates TDCA induced cell migration after injury. The objectives of this study were to determine if this is a property shared by other bile salts or an effect specific to TDCA, and to determine if NF-kappa B regulates TGF-beta expression. Studies were conducted in IEC-6 cells. Cell migration was examined using an in vitro model. TGF-beta protein and mRNA expression was determined by ELISA and Northern blot analysis. Sequence-specific NF-kappa B binding activity was measured by gel shift assays. Taurocholate and deoxycholate at physiologic concentrations significantly increased intestinal epithelial cell migration 6 hours after wounding (p < 0.01), and was associated with a significant increase in specific NF-kappa B binding activity. Inhibition of NF-kappa B activity significantly inhibited cell migration during restitution and resulted in a significant decrease in TGF-beta mRNA expression and protein expression. We conclude that bile salts at physiologic conditions increase cell migration after injury, an effect regulated by NF-kappa B. Further, NF-kappa B elicits TGF-beta gene transcription during cell migration. These data support a physiologic role of bile salts in the maintenance of intestinal mucosal integrity.

NF-kappaB regulates intestinal epithelial cell and bile salt-induced migration after injury.

To determine if NF-kappa B regulates intestinal epithelial cell migration and if it has a role during bile salt-induced migration. Mucosal restitution is an important repair modality in the gastrointestinal tract. The authors have shown that taurodeoxycholate (TDCA) increases intestinal epithelial cell migration. NF-kappa B regulates activation of a number of genes involved in inflammatory responses. Studies were conducted in IEC-6 cells. I kappa B protein expression was determined by Western blot analysis. Sequence-specific NF-kappa B binding activity was measured by EMSA shift assays and nuclear localization by immunohistochemistry. Cell migration was examined by using an in vitro model that mimics the early cell division-independent stages of epithelial restitution. The process of cell migration over the wounded area was associated with a significant increase in NF-kappa B binding activity in IEC-6 cells. Immunohistochemistry revealed translocation of NF-kappa B into the nucleus. Western blot analysis showed that injury decreased I kappa B protein expression. Inhibition of the binding activity by treatment with a specific NF-kappa B inhibitor, MG-132, inhibited cell migration during restitution. Further, exposure to TDCA at the physiologic concentration that induces intestinal epithelial cell migration increased NF-kappa B binding activity, induced NF-kappa B translocation into the nucleus, and decreased I kappa B protein expression. MG-132 also inhibits bile salt-induced cell migration. NF-kappa B regulates intestinal epithelial cell migration. Bile salts at physiologic concentrations increase cell migration by activation of NF-kappa B. These data show that bile salts may have a role in the maintenance of intestinal mucosal integrity.

SELENIUM PRETREATMENT PREVENTS BACTERIAL TRANSLOCATION IN RAT INTESTINAL ISCHEMIA/REPERFUSION MODEL

Protective role of selenium against free radical damage was first demonstrated in the heart and this effect was further questioned in other systems. In the present study, the effects of exogenously administered selenium on intestinal fine morphology, lipid peroxidation, and bacterial translocation (BT) in experimental intestinal ischemia/reperfusion (I/R) model were examined. Thirty-two male Wistar rats weighing 250–300 g were randomized into four groups. Sham group ( n=8) underwent laparotomy only. In the I/R group ( n=8), laparotomy was performed and the superior mesenteric artery was occluded using an atraumatic microvascular clamp for 30 min. In corresponding selenium-treated groups ( n=8 each), sodium selenate was given 0.2 mg kg −1 day −1 intraperitoneally (i.p.) for 3 consecutive days, prior to surgery for either laparotomy only or with I/R. Twenty-four hours later, tissue samples from liver, spleen, and mesenteric lymph nodes were obtained under sterile conditions for microbiological analysis and further evaluation of I/R-induced intestinal injury. Ileum samples were fixed in 10% formaldehyde for histopathological evaluation. In the I/R group, the incidence of bacteria-isolated mesenteric lymph nodes, spleen, and liver was significantly higher than other groups ( P<0.05). Selenium supplementation prevented I/R-induced BT and significantly reduced the I/R-induced intestinal injury ( P<0.05). Tissue MDA levels from the ileum specimens of selenium-treated rats were significantly lower than that of the I/R group ( P<0.05). Our results provide evidence that the relationship between BT and lipid peroxidation in intestinal tissue is crucial. Selenium pretreatment reduces lipid peroxidation which contributes to the maintenance of intestinal mucosal integrity.

NF-kappaB activation and susceptibility to apoptosis after polyamine depletion in intestinal epithelial cells.

The maintenance of intestinal mucosal integrity depends on a balance between cell renewal and cell death, including apoptosis. The natural polyamines, putrescine, spermidine, and spermine, are essential for mucosal growth, and decreasing polyamine levels cause G(1) phase growth arrest in intestinal epithelial (IEC-6) cells. The present study was done to determine changes in susceptibility of IEC-6 cells to apoptosis after depletion of cellular polyamines and to further elucidate the role of nuclear factor-kappaB (NF-kappaB) in this process. Although depletion of polyamines by alpha-difluoromethylornithine (DFMO) did not directly induce apoptosis, the susceptibility of polyamine-deficient cells to staurosporine (STS)-induced apoptosis increased significantly as measured by changes in morphological features and internucleosomal DNA fragmentation. In contrast, polyamine depletion by DFMO promoted resistance to apoptotic cell death induced by the combination of tumor necrosis factor-alpha (TNF-alpha) and cycloheximide. Depletion of cellular polyamines also increased the basal level of NF-kappaB proteins, induced NF-kappaB nuclear translocation, and activated the sequence-specific DNA binding activity. Inhibition of NF-kappaB binding activity by sulfasalazine or MG-132 not only prevented the increased susceptibility to STS-induced apoptosis but also blocked the resistance to cell death induced by TNF-alpha in combination with cycloheximide in polyamine-deficient cells. These results indicate that 1) polyamine depletion sensitizes intestinal epithelial cells to STS-induced apoptosis but promotes the resistance to TNF-alpha-induced cell death, 2) polyamine depletion induces NF-kappaB activation, and 3) disruption of NF-kappaB function is associated with altered susceptibility to apoptosis induced by STS or TNF-alpha. These findings suggest that increased NF-kappaB activity after polyamine depletion has a proapoptotic or antiapoptotic effect on intestinal epithelial cells determined by the nature of the death stimulus.