Cellular basis for defective electrolyte transport in inflamed human colon

Colonic Epithelial Permeability to Ions Is Restored after Vedolizumab Treatment and May Predict Clinical Response in Inflammatory Bowel Disease Patients

Vedolizumab (VDZ) is used for treating inflammatory bowel disease (IBD) patients. A study investigating colonic epithelial barrier function ex vivo following VDZ is lacking. This work aims to evaluate ex vivo the colonic epithelial barrier function in IBD patients at baseline and during VDZ treatment, and to investigate the relationships between barrier function and clinical parameters. Colonic specimens were obtained from 23 IBD patients before, and at 24 and 52 weeks after VDZ treatment, and from 26 healthy volunteers (HV). Transepithelial electrical resistance (TEER, permeability to ions) and paracellular permeability were measured in Ussing chambers. IBD patients showed increased epithelial permeability to ions (TEER, 13.80 ± 1.04 Ω × cm2 vs. HV 20.70 ± 1.52 Ω × cm2, p < 0.001) without changes in paracellular permeability of a 4 kDa probe. VDZ increased TEER (18.09 ± 1.44 Ω × cm2, p < 0.001) after 52 weeks. A clinical response was observed in 58% and 25% of patients at week 24, and in 62% and 50% at week 52, in ulcerative colitis and Crohn's disease, respectively. Clinical and endoscopic scores were strongly associated with TEER. TEER < 14.65 Ω × cm2 predicted response to VDZ (OR 11; CI 2-59). VDZ reduces the increased permeability to ions observed in the colonic epithelium of IBD patients before treatment, in parallel to a clinical, histological (inflammatory infiltrate), and endoscopic improvement. A low TEER predicts clinical response to VDZ therapy.

Dysregulation of intestinal epithelial electrolyte transport in canine chronic inflammatory enteropathy and the role of the renin-angiotensin-aldosterone-system

Chronic diarrhea is a hallmark sign of canine chronic inflammatory enteropathy (CIE), leading to fluid and electrolyte losses. Electrolyte homeostasis is regulated by the renin-angiotensin-aldosterone-system (RAAS), which might be involved in (counter-)regulating electrolyte losses in canine CIE. Whether and which electrolyte transporters are affected or if RAAS is activated in canine CIE is unknown. Thus, intestinal electrolyte transporters and components of the RAAS were investigated in dogs with CIE. Serum RAAS fingerprint analysis by mass spectrometry was performed in 5 CIE dogs and 5 healthy controls, and mRNA levels of intestinal electrolyte transporters and local RAAS pathway components were quantified by RT-qPCR in tissue biopsies from the ileum (7 CIE, 10 controls) and colon (6 CIE, 12 controls). Concentrations of RAAS components and mRNA expression of electrolyte transporters were compared between both groups of dogs and were tested for associations among each other. In dogs with CIE, associations with clinical variables were also tested. Components of traditional and alternative RAAS pathways were higher in dogs with CIE than in healthy controls, with statistical significance for Ang I, Ang II, and Ang 1-7 (all p < 0.05). Expression of ileal, but not colonic electrolyte transporters, such as Na+/K+-ATPase, Na+/H+-exchanger 3, Cl- channel 2, down-regulated in adenoma, and Na+-glucose-cotransporter (all p < 0.05) was increased in CIE. Our results suggest that the dys- or counter-regulation of intestinal electrolyte transporters in canine CIE might be associated with a local influence of RAAS. Activating colonic absorptive reserve capacities may be a promising therapeutic target in canine CIE.

Diarrheal Mechanisms and the Role of Intestinal Barrier Dysfunction in Campylobacter Infections

Campylobacter enteritis is the most common cause of foodborne bacterial diarrhea in humans. Although various studies have been performed to clarify the pathomechanism in Campylobacter infection, the mechanism itself and bacterial virulence factors are yet not completely understood. The purpose of this chapter is to (i) give an overview on Campylobacter-induced diarrheal mechanisms, (ii) illustrate underlying barrier defects, (iii) explain the role of the mucosal immune response and (iv) weigh preventive and therapeutic approaches. Our present knowledge of pathogenetic and diarrheal mechanisms of Campylobacter jejuni is explained in the first part of this chapter. In the second part, the molecular basis for the Campylobacter-induced barrier dysfunction is compared with that of other species in the Campylobacter genus. The bacteria are capable of overcoming the intestinal epithelial barrier. The invasion into the intestinal mucosa is the initial step of the infection, followed by a second step, the epithelial barrier impairment. The extent of the impairment depends on various factors, including tight junction dysregulation and epithelial apoptosis. The disturbed intestinal epithelium leads to a loss of water and solutes, the leak flux type of diarrhea, and facilitates the uptake of harmful antigens, the leaky gut phenomenon. The barrier dysfunction is accompanied by increased pro-inflammatory cytokine secretion, which is partially responsible for the dysfunction. Moreover, cytokines also mediate ion channel dysregulation (e.g., epithelial sodium channel, ENaC), leading to another diarrheal mechanism, which is sodium malabsorption. Future perspectives of Campylobacter research are the clarification of molecular pathomechanisms and the characterization of therapeutic and preventive compounds to combat and prevent Campylobacter infections.

Pathology of Aldosterone Biosynthesis and its Action

Aldosterone plays pivotal roles in renin-angiotensin-aldosterone system in order to maintain the equilibrium of liquid volume and electrolyte metabolism. Aldosterone action is mediated by both mineralocorticoid receptor and 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2). Its excessive actions directly induced tissue injuries in its target organs such as myocardial and vascular fibrosis in addition to chronic kidney diseases. Excessive aldosterone actions were also reported to be involved in unbalanced electrolyte metabolism in inflammatory bowel disease and development of pulmonary diseases. Hyperaldosteronism is tentatively classified into primary and secondary types. Primary aldosteronism is more frequent and has been well known to result in secondary hypertension with subsequent cardiovascular damages. Primary aldosteronism is also further classified into distinctive subtypes and among those, aldosterone-producing adenoma is the most frequent one accounting for the great majority of unilateral primary aldosteronism cases. In bilateral hyperaldosteronism, aldosterone-producing diffuse hyperplasia and aldosterone-producing micronodules or nodules are the major subtypes. All these aldosterone-producing lesions were reported to harbor somatic mutations including KCNJ5, CACNA1D, ATP1A1 and ATP2B3, which were all related to excessive aldosterone production. Among those mutations above, somatic mutation of KCNJ5 is the most frequent in aldosterone-producing adenoma and mostly composed of clear cells harboring abundant aldosterone synthase expression. In contrast, CACNA1D-mutated aldosterone-producing micronodules or aldosterone-producing nodules were frequently detected not only in primary aldosteronism patients but also in the zona glomerulosa of normal adrenal glands, which could eventually lead to an autonomous aldosterone production resulting in normotensive or overt primary aldosteronism, but their details have remained unknown.

Hyperosmolar environment and intestinal epithelial cells: impact on mitochondrial oxygen consumption, proliferation, and barrier function in vitro

The aim of the present study was to elucidate the in vitro short-term (2-h) and longer-term (24-h) effects of hyperosmolar media (500 and 680 mOsm/L) on intestinal epithelial cells using the human colonocyte Caco-2 cell line model. We found that a hyperosmolar environment slowed down cell proliferation compared to normal osmolarity (336 mOsm/L) without inducing cell detachment or necrosis. This was associated with a transient reduction of cell mitochondrial oxygen consumption, increase in proton leak, and decrease in intracellular ATP content. The barrier function of Caco-2 monolayers was also transiently affected since increased paracellular apical-to-basal permeability and modified electrolyte permeability were measured, allowing partial equilibration of the trans-epithelial osmotic difference. In addition, hyperosmotic stress induced secretion of the pro-inflammatory cytokine IL-8. By measuring expression of genes involved in energy metabolism, tight junction forming, electrolyte permeability and intracellular signaling, different response patterns to hyperosmotic stress occurred depending on its intensity and duration. These data highlight the potential impact of increased luminal osmolarity on the intestinal epithelium renewal and barrier function and point out some cellular adaptive capacities towards luminal hyperosmolar environment.

AMPK mediates inhibition of electrolyte transport and NKCC1 activity by reactive oxygen species

Reactive oxygen species such as H₂O₂ are believed to play a prominent role in the injury and loss of transport function that affect the intestinal epithelium in inflammatory conditions such as inflammatory bowel diseases. Defects in intestinal epithelial ion transport regulation contribute to dysbiosis and inflammatory phenotypes. We previously showed that H₂O₂ inhibits Ca²⁺-dependent Cl^- secretion across intestinal epithelial cells (IECs) via a phosphatidylinositol 3-kinase (PI3K)- and extracellular signal-regulated kinase (ERK)-dependent mechanism that occurs, at least in part, through inhibition of the basolateral Na⁺-K⁺-2Cl^- cotransporter NKCC1. NKCC1 governs Cl^- entry into crypt IECs and thus plays a critical role in maintaining the driving force for Cl^- secretion. Electrolyte transport consumes large amounts of cellular energy, and direct pharmacological activation of the cellular energy sensor AMP-activated protein kinase (AMPK) has been shown to inhibit a number of ion transport proteins. Here, we show that H₂O₂ activates AMPK in human IEC lines and ex vivo human colon. Moreover, we demonstrate that the inhibitory effect of H₂O₂ on Ca²⁺-dependent Cl^- secretion and NKCC1 activity is AMPK-dependent. This inhibitory effect is associated with a physical interaction between AMPK and NKCC1, as well as increased phosphorylation (Thr^212,217) of NKCC1, without causing NKCC1 internalization. These data identify a key role for AMPK-NKCC1 interaction as a point of convergence for suppression of colonic epithelial ion transport by inflammatory reactive oxygen species.NEW & NOTEWORTHY H₂O₂ inhibition of intestinal epithelial Ca²⁺-dependent Cl^- secretion involves recruitment of AMP-activated protein kinase (AMPK) downstream of ERK and phosphatidylinositol 3-kinase signaling pathways, physical interaction of AMPK with the Na⁺-K⁺-2Cl^- cotransporter NKCC1, and AMPK-dependent suppression of NKCC1-mediated electrolyte influx without causing NKCC1 internalization. It is intriguing that, in human intestinal epithelial cell lines and human colon, H₂O₂ activation of AMPK increased phosphorylation of NKCC1 residues required for promoting, not inhibiting, NKCC1 activity. These data identify an elevated complexity of AMPK regulation of NKCC1 in the setting of an inflammatory stimulus.

Physiology of Electrolyte Transport in the Gut: Implications for Disease

We now have an increased understanding of the genetics, cell biology, and physiology of electrolyte transport processes in the mammalian intestine, due to the availability of sophisticated methodologies ranging from genome wide association studies to CRISPR-CAS technology, stem cell-derived organoids, 3D microscopy, electron cryomicroscopy, single cell RNA sequencing, transgenic methodologies, and tools to manipulate cellular processes at a molecular level. This knowledge has simultaneously underscored the complexity of biological systems and the interdependence of multiple regulatory systems. In addition to the plethora of mammalian neurohumoral factors and their cross talk, advances in pyrosequencing and metagenomic analyses have highlighted the relevance of the microbiome to intestinal regulation. This article provides an overview of our current understanding of electrolyte transport processes in the small and large intestine, their regulation in health and how dysregulation at multiple levels can result in disease. Intestinal electrolyte transport is a balance of ion secretory and ion absorptive processes, all exquisitely dependent on the basolateral Na⁺ /K⁺ ATPase; when this balance goes awry, it can result in diarrhea or in constipation. The key transporters involved in secretion are the apical membrane Cl^- channels and the basolateral Na⁺ -K⁺ -2Cl^- cotransporter, NKCC1 and K⁺ channels. Absorption chiefly involves apical membrane Na⁺ /H⁺ exchangers and Cl^- /HCO₃ ^- exchangers in the small intestine and proximal colon and Na⁺ channels in the distal colon. Key examples of our current understanding of infectious, inflammatory, and genetic diarrheal diseases and of constipation are provided. © 2019 American Physiological Society. Compr Physiol 9:947-1023, 2019.

Decreased electrogenic anionic secretory response in the porcine colon following in vivo challenge with Brachyspira spp. supports an altered mucin environment

Brachyspira spp. cause diarrheal disease in multiple animal species by colonization of the colon, resulting in colitis, mucus induction, and disrupted ion transport. Unique to spirochete pathogenesis is the immense production of mucus, resulting in a niche mucin environment likely favoring spirochete colonization. Mucin rheological properties are heavily influenced by anionic secretion, and loss of secretory function has been implicated in diseases such as cystic fibrosis. Here, the effects on the agonist-induced electrogenic anionic secretory response by infectious colonic spirochete bacteria Brachyspira hyodysenteriae and Brachyspira hampsonii were assessed in the proximal, apex, and distal sections of colon in Ussing chambers. Activation of secretion via isoproterenol, carbachol, and forskolin/3-isobutyl-1-methylxanthine demonstrated a significantly decreased change in short-circuit current ( I_sc) in Brachyspira-infected pigs in all sections. Tissue resistances did not account for this difference, rather, it was attributed to a decrease in anionic secretion as indicated by a decrease in bumetanide inhibitable I_sc. Quantitative RT-PCR and Western blot analyses determined that the major anionic channels of the epithelium were downregulated in diarrheic pigs paired with altered mucin gene expression. The investigated cytokines were not responsible for the downregulation of anion channel gene transcripts. Although IL-1α was upregulated in all segments, it did not alter cystic fibrosis transmembrane conductance regulator (CFTR) mRNA expression in Caco-2 monolayers. However, a whole cell Brachyspira hampsonii lysate significantly reduced CFTR mRNA expression in Caco-2 monolayers. Together, these findings indicate that these two Brachyspira spp. may directly cause a decreased anionic secretory response in the porcine colon, supporting an altered mucin environment likely favoring spirochete colonization. NEW & NOTEWORTHY This research demonstrates for the first time that the niche mucin environment produced by two infectious spirochete spp. is supported by a decrease in the electrogenic anionic secretory response throughout the porcine colon. Our findings suggest that the host's cytokine response is not likely responsible for the decrease in anionic secretory function. Rather, it appears that Brachyspira spp. directly impede ion channel transcription and translation, potentially altering colonic mucin rheological properties, which may favor spirochete colonization.

Colonic Potassium Absorption and Secretion in Health and Disease

The colon has large capacities for K⁺ absorption and K⁺ secretion, but its role in maintaining K⁺ homeostasis is often overlooked. For many years, passive diffusion and/or solvent drag were thought to be the primary mechanisms for K⁺ absorption in human and animal colon. However, it is now clear that apical H⁺ ,K⁺ -ATPase, in coordination with basolateral K⁺ -Cl^- cotransport and/or K⁺ and Cl^- channels operating in parallel, mediate electroneutral K⁺ absorption in animal colon. We now know that K⁺ absorption in rat colon reflects ouabain-sensitive and ouabain-insensitive apical H⁺ ,K⁺ -ATPase activities. Ouabain-insensitive and ouabain-sensitive H⁺ ,K⁺ -ATPases are localized in surface and crypt cells, respectively. Colonic H⁺ ,K⁺ -ATPase consists of α- (HKC_α ) and β- (HKC_β ) subunits which, when coexpressed, exhibit ouabain-insensitive H⁺ ,K⁺ -ATPase activity in HEK293 cells, while HKC_α coexpressed with the gastric β-subunit exhibits ouabain-sensitive H⁺ ,K⁺ -ATPase activity in Xenopus oocytes. Aldosterone enhances apical H⁺ ,K⁺ -ATPase activity, HKC_α specific mRNA and protein expression, and K⁺ absorption. Active K⁺ secretion, on the other hand, is mediated by apical K⁺ channels operating in a coordinated way with the basolateral Na⁺ -K⁺ -2Cl^- cotransporter. Both Ca²⁺ -activated intermediate conductance K⁺ (IK) and large conductance K⁺ (BK) channels are located in the apical membrane of colonic epithelia. IK channel-mediated K⁺ efflux provides the driving force for Cl^- secretion, while BK channels mediate active (e.g., cAMP-activated) K⁺ secretion. BK channel expression and activity are increased in patients with end-stage renal disease and ulcerative colitis. This review summarizes the role of apical H⁺ ,K⁺ -ATPase in K⁺ absorption, and apical BK channel function in K⁺ secretion in health and disease. © 2018 American Physiological Society. Compr Physiol 8:1513-1536, 2018.

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.

Hydrogen peroxide scavenger, catalase, alleviates ion transport dysfunction in murine colitis

Reactive oxygen species (ROS) such as hydrogen peroxide (H₂ O₂ ) contribute to epithelial damage and ion transport dysfunction (key events in inflammatory diarrhoea) in inflammatory bowel disease (IBD). The aim of this study was to identify if H₂ O₂ mediates suppression of colonic ion transport function in the murine dextran sulfate sodium (DSS) colitis model by using the H₂ O₂ degrading enzyme, catalase. Colitis was induced by administering DSS (4%) in drinking water for 5 days followed by 3 days on normal H₂ O. Mice were administered either pegylated catalase or saline at day -1, 0 and +1 of DSS treatment. Ion transport responses to the Ca²⁺ -dependent agonist, carbachol (CCh), or the cAMP-dependent agonist, forskolin, were measured across distal colonic mucosa mounted in Ussing chambers. Parameters of DSS-induced inflammation (loss in body weight, decreased colon length, altered stool consistency), were only partially alleviated by catalase while histology was only minimally improved. However, catalase significantly reversed the DSS-induced reduction in baseline ion transport as well as colonic I_sc responses to CCh. However, ion transport responses to forskolin were not significantly restored. Catalase also reduced activation of ERK MAP kinase in the setting of colitis, and increased expression of the Na⁺ -K⁺ -2Cl^- cotransporter, NKCC1, consistent with restoration of ion transport function. Ex vivo treatment of inflamed colonic mucosae with catalase also partially restored ion transport function. Therefore, catalase partially prevents, and rescues, the loss of ion transport properties in DSS colitis even in the setting of unresolved tissue inflammation. These findings indicate a prominent role for ROS in ion transport dysfunction in colitis and may suggest novel strategies for the treatment of inflammatory diarrhoea.

Modulation of epithelial cell polarity by bacterial pathogens

Epithelial cells constitute a physical barrier that aids in protecting the host from microbial pathogens. Polarized epithelial cells contain distinct apical and basolateral membrane domains separated by intercellular junctions, including tight junctions (TJs), which contribute to the maintenance of apical-basal polarity. Polarity complexes also contribute to the establishment of TJ formation. Several pathogens perturb epithelial TJ barrier function and structure in addition to causing a loss of apical-basal polarity. Here, we review the impact of pathogenic bacteria on the disruption of cell-cell junctions and epithelial polarity.

Rebamipide ameliorates radiation-induced intestinal injury in a mouse model

Radiation-induced enteritis is a major side effect in cancer patients undergoing abdominopelvic radiotherapy. Radiation exposure produces an uncontrolled inflammatory cascade and epithelial cell loss leading to impaired epithelial barrier function. The goal of this study was to determine the effect of rebamipide on regeneration of the intestinal epithelia after radiation injury. The abdomens of C57BL/6 mice were exposed to 13Gy of irradiation (IR) and then the mice were treated with rebamipide. Upon IR, intestinal epithelia were destroyed structurally at the microscopic level and bacterial translocation was increased. The intestinal damage reached a maximum level on day 6 post-IR and intestinal regeneration occurred thereafter. We found that rebamipide significantly ameliorated radiation-induced intestinal injury. In mice treated with rebamipide after IR, intestinal barrier function recovered and expression of the tight junction components of the intestinal barrier were upregulated. Rebamipide administration reduced radiation-induced intestinal mucosal injury. The levels of proinflammatory cytokines and matrix metallopeptidase 9 (MMP9) were significantly reduced upon rebamipide administration. Intestinal cell proliferation and β-catenin expression also increased upon rebamipide administration. These data demonstrate that rebamipide reverses impairment of the intestinal barrier by increasing intestinal cell proliferation and attenuating the inflammatory response by inhibiting MMP9 and proinflammatory cytokine expression in a murine model of radiation-induced enteritis.

Mechanism of down regulation of Na-H exchanger-2 in experimental colitis

BACKGROUND

The Na-H exchanger [NHE] performs an electroneutral uptake of NaCl and water from the lumen of the gastrointestinal tract. There are several distinct NHE isoforms, some of which show an altered expression in the inflammatory bowel diseases (IBD). In this study, we examined a role of NHE-2 in experimental colitis.

METHODS

Colitis was induced in male Sprague-Dawley rats by intra-rectal administration of trinitrobenzenesulphonic acid (TNBS). On day 6 post-TNBS, the animals were sacrificed, colonic and ileal segments were taken out, cleaned with phosphate buffered saline and used in this study.

RESULTS

There was a significant decrease in the level of NHE-2 protein as measured by ECL western blot analysis and confocal immunofluorescence microscopy. The levels of NHE-2 mRNA and heteronuclear RNA measured by an end-point RT-PCR and a real time PCR were also decreased significantly in the inflamed colon. However, there was no change in the level of NHE-2 protein in response to in vitro TNF-α treatment of uninflamed rat colonic segment. These changes were selective and localized to the colon as actin, an internal control, remained unchanged. Confocal immunofluorescence microscopy revealed co-localization of NHE-2 and NHE-3 in the brush borders of colonic epithelial cells. Inflamed colon showed a significant increase in myeloperoxidase activity and colon hypertrophy. In addition, there was a significant decrease in body weight and goblet cells' mucin staining in the TNBS treated colon. These changes were not conspicuous in the non-inflamed ileum.

CONCLUSIONS

These findings demonstrate suppression of NHE-2 expression on the brush borders in the colonic epithelial cells which is regulated transcriptionally. However a role of TNF-α in the regulation of NHE-2 is discounted in the present model of colitis. This decrease in the NHE-2 expression will lead to a loss of electrolyte and water uptake thus contributing to the symptoms associated with IBD.

Active and passive involvement of claudins in the pathophysiology of intestinal inflammatory diseases

Intestinal inflammatory diseases, four of which are discussed here, are associated with alterations of claudins. In ulcerative colitis, diarrhea and antigen entry into the mucosa occurs. Claudin-2 is upregulated but data on other claudins are still limited or vary (e.g., claudin-1 and -4). Apart from that, tight junction changes contribute to diarrhea via a leak flux mechanism, while protection against antigen entry disappears behind epithelial gross lesions (erosions) and apoptotic foci. Crohn's disease is additionally characterized by a claudin-5 and claudin-8 reduction which plays an active role in antigen uptake already before gross lesions appear. In microscopic colitis (MC), upregulation of claudin-2 expression is weak and a reduction in claudin-4 may be only passively involved, while sodium malabsorption represents the main diarrheal mechanism. However, claudin-5 is removed from MC tight junctions which may be an active trigger for inflammation through antigen uptake along the so-called leaky gut concept. In celiac disease, primary barrier defects are discussed in the context of candidate genes as PARD3 which regulate cell polarity and tight junctions. The loss of claudin-5 allows small antigens to invade, while the reductions in others like claudin-3 are rather passive events. Taken together, the specific role of single tight junction proteins for the onset and perpetuation of inflammation and the recovery from these diseases is far from being fully understood and is clearly dependent on the stage of the disease, the background of the other tight junction components, the transport activity of the mucosa, and the presence of other barrier features like gross lesions, an orchestral interplay which is discussed in this article.

Pathophysiology of Intestinal Na+/H+ exchange

Several members of the SLC9A family of Na⁺/H⁺ exchangers are expressed in the gut, with varying expression patterns and cellular localization. Not only do they participate in the regulation of basic epithelial cell functions, including control of transepithelial Na⁺ absorption, intracellular pH (pH _i ), cell volume, and nutrient absorption, but also in cellular proliferation, migration, and apoptosis. Additionally, they modulate the extracellular milieu in order to facilitate other nutrient absorption and to regulate the intestinal microbial microenvironment. Na⁺/H⁺ exchangers are frequent targets of inhibition in gastrointestinal pathologies, either by intrinsic factors (e.g. bile acids, inflammatory mediators) or infectious agents and associated microbial toxins. Based on emerging evidence, disruption of NHE activity via impaired expression or function of respective isoforms may contribute not only to local and systemic electrolyte imbalance, but also to the disease severity via multiple mechanisms. Here, we review the current state of knowledge about the roles Na⁺/H⁺ exchangers play in the pathogenesis of disorders of diverse origin and affecting a range of GI tissues.

Regulation of antiapoptotic and cytoprotective pathways in colonic epithelial cells in ulcerative colitis

Ulcerative colitis is an inflammatory bowel disease involving the colon resulting in bloody diarrhea and increased risk of colorectal cancer in certain patient subgroups. Increased apoptosis in the epithelial cell layer causes increased permeability, especially during flares; this leads to translocation of luminal pathogens resulting in a continued inflammatory drive. The present work investigates how epithelial apoptosis is regulated in ulcerative colitis. The main results are that Fas mediated apoptosis is inhibited during flares of ulcerative colitis, probably by an upregulation of cellular inhibitor of apoptosis protein 2 (cIAP2) and cellular FLICE-like inhibitory protein. cIAP2 is upregulated in regenerative epithelial cells both in ulcerative colitis and in experimental intestinal wounds. Inhibition of cIAP2 decreases wound healing in vitro possibly through inhibition of migration. Altogether, it is shown that epithelial cells in ulcerative colitis responds to the hostile microenvironment by activation of cytoprotective pathways that tend to counteract the cytotoxic effects of inflammation. However, the present studies also show that epithelial cells produce increased amounts of reactive oxygen species during stimulation with tumor necrosis factor-α and interferon-γ resulting in DNA instability. The combined effect of increased DNA-instability and decreased apoptosis responses could lead to neoplasia.

Role of epithelial ion transports in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic inflammatory disorder with a complex pathogenesis. Diarrhea is a highly prevalent and often debilitating symptom of IBD patients that results, at least in part, from an intestinal hydroelectrolytic imbalance. Evidence suggests that reduced electrolyte absorption is more relevant than increased secretion to this disequilibrium. This systematic review analyses and integrates the current evidence on the roles of epithelial Na(+)-K(+)-ATPase (NKA), Na(+)/H(+) exchangers (NHEs), epithelial Na(+) channels (ENaC), and K(+) channels (KC) in IBD-associated diarrhea. NKA is the key driving force of the transepithelial ionic transport and its activity is decreased in IBD. In addition, the downregulation of apical NHE and ENaC and the upregulation of apical large-conductance KC all contribute to the IBD-associated diarrhea by lowering sodium absorption and/or increasing potassium secretion.

Upregulation of basolateral small conductance potassium channels (KCNQ1/KCNE3) in ulcerative colitis

Background

Basolateral K⁺ channels hyperpolarize colonocytes to ensure Na⁺ (and thus water) absorption. Small conductance basolateral (KCNQ1/KCNE3) K⁺ channels have never been evaluated in human colon. We therefore evaluated KCNQ1/KCNE3 channels in distal colonic crypts obtained from normal and active ulcerative colitis (UC) patients.

Methods

KCNQ1 and KCNE3 mRNA levels were determined by qPCR, and KCNQ1/KCNE3 channel activity in normal and UC crypts, and the effects of forskolin (activator of adenylate cyclase) and UC-related proinflammatory cytokines on normal crypts, studied by patch clamp recording.

Results

Whereas KCNQ1 and KCNE3 mRNA expression was similar in normal and UC crypts, single 6.8 pS channels were seen in 36% of basolateral patches in normal crypts, and to an even greater extent (74% of patches, P < 0.001) in UC crypts, with two or more channels per patch. Channel activity was 10-fold higher (P < 0.001) in UC crypts, with a greater contribution to basolateral conductance (5.85 ± 0.62 mS cm⁻²) than in controls (0.28 ± 0.04 mS cm⁻², P < 0.001). In control crypts, forskolin and thromboxane A₂ stimulated channel activity 30-fold and 10-fold respectively, while PGE₂, IL-1β, and LTD₄ had no effect.

Conclusions

KCNQ1/KCNE3 channels make only a small contribution to basolateral conductance in normal colonic crypts, with increased channel activity in UC appearing insufficient to prevent colonic cell depolarization in this disease. This supports the proposal that defective Na⁺ absorption rather than enhanced Cl⁻ secretion, is the dominant pathophysiological mechanism of diarrhea in UC.

Mechanisms Underlying Dysregulation of Electrolyte Absorption in Inflammatory Bowel Disease-Associated Diarrhea

Inflammatory bowel diseases (IBDs), including Crohn's disease and ulcerative colitis, are chronic relapsing inflammatory disorders of the gastrointestinal tract. Chronic inflammation of the intestine affects the normal fluid and electrolyte absorption leading to diarrhea, the hallmark symptom of IBD. The management of IBD-associated diarrhea still remains to be a challenge, and extensive studies over the last 2 decades have focused on investigating the molecular mechanisms underlying IBD-associated diarrhea. These studies have shown that the predominant mechanism of diarrhea in IBD involves impairment of electroneutral NaCl absorption, with very little role if any played by anion secretion. The electroneutral NaCl absorption involves coupled operation of Na/H exchanger 3 (NHE3 or SLC9A3) and Cl/HCO3 exchanger DRA (Down Regulated in Adenoma, or SLC26A3). Increasing evidence now supports the critical role of a marked decrease in NHE3 and DRA function and/or expression in IBD-associated diarrhea. This review provides a detailed analysis of the current knowledge related to alterations in NHE3 and DRA function and expression in IBD including the mechanisms underlying these observations and highlights the potential of these transporters as important and novel therapeutic targets.

Na-H Exchanger Isoform-2 (NHE2) Mediates Butyrate-dependent Na+ Absorption in Dextran Sulfate Sodium (DSS)-induced Colitis

Diarrhea associated with ulcerative colitis (UC) occurs primarily as a result of reduced Na(+) absorption. Although colonic Na(+) absorption is mediated by both epithelial Na(+) channels (ENaC) and Na-H exchangers (NHE), inhibition of NHE-mediated Na(+) absorption is the primary cause of diarrhea in UC. As there are conflicting observations reported on NHE expression in human UC, the present study was initiated to identify whether NHE isoforms (NHE2 and NHE3) expression is altered and how Na(+) absorption is regulated in DSS-induced inflammation in rat colon, a model that has been used to study UC. Western blot analyses indicate that neither NHE2 nor NHE3 expression is altered in apical membranes of inflamed colon. Na(+) fluxes measured in vitro under voltage clamp conditions in controls demonstrate that both HCO3 (-)-dependent and butyrate-dependent Na(+) absorption are inhibited by S3226 (NHE3-inhibitor), but not by HOE694 (NHE2-inhibitor) in normal animals. In contrast, in DSS-induced inflammation, butyrate-, but not HCO3 (-)-dependent Na(+) absorption is present and is inhibited by HOE694, but not by S3226. These observations indicate that in normal colon NHE3 mediates both HCO3 (-)-dependent and butyrate-dependent Na(+) absorption, whereas DSS-induced inflammation activates NHE2, which mediates butyrate-dependent (but not HCO3 (-)-dependent) Na(+) absorption. In in vivo loop studies HCO3 (-)-Ringer and butyrate-Ringer exhibit similar rates of water absorption in normal rats, whereas in DSS-induced inflammation luminal butyrate-Ringer reversed water secretion observed with HCO3 (-)-Ringer to fluid absorption. Lumen butyrate-Ringer incubation activated NHE3-mediated Na(+) absorption in DSS-induced colitis. These observations suggest that the butyrate activation of NHE2 would be a potential target to control UC-associated diarrhea.

Anti-colitic effects of kanjangs (fermented soy sauce and sesame sauce) in dextran sulfate sodium-induced colitis in mice

This study was conducted to investigate the preventive effects of different kanjangs (Korean soy sauces), including acid-hydrolyzed soy sauce (AHSS), fermented soy sauce (FSS), and fermented sesame sauce (FSeS), on 2% dextran sulfate sodium (DSS)-induced ulcerative colitis in C57BL/6J mice. The fermented sauces, particularly FSeS, significantly suppressed DSS-induced body weight loss, increased colon length, and decreased colon weight/length ratios. Histological observations suggested that the fermented sauces prevented edema, mucosal damage, and the loss of crypts induced by DSS compared to the control mice and animals fed AHSS. FSeS and FSS decreased the serum levels of tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ), interleukin (IL)-6, and IL-17α. mRNA expression of these cytokines as well as that of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) in colon mucosa was also inhibited by the two sauces. Our results suggest that fermented sauces, especially FSeS, exert an anticolitic effect partially by reducing the serum levels of proinflammatory cytokines and inhibiting the mRNA expression of these factors in the colon tissue of mice treated with DSS. However, AHSS did not protect against DSS-induced colitis. In addition, low-dose treatment (4 mL/kg) with the fermented sauces resulted in greater anticolitic effects than consumption of a high quantity (8 mL/kg) of the sauces.

Diarrhea in Crohn’s disease: investigating the role of the ileal hormone fibroblast growth factor 19

BACKGROUND

Bile acids [BA] are usually reabsorbed by the terminal ileum, but this process is frequently abnormal in Crohn’s disease [CD]. BA malabsorption occurs, and excess colonic BA cause secretory diarrhea. Furthermore, the hormone fibroblast growth factor 19 [FGF19] is synthesized in the ileum in response to BA absorption and regulates BA synthesis. We hypothesized that reduced serum FGF19 levels will be associated with diarrheal symptoms and disease activity in both ileal resected[IR-CD] and non-resected CD [NR-CD] patients.

METHODS

Fasting serum FGF19 levels were measured in 58 patients [23 IR-CD patients and 35NR-CD patients]. Disease activity was assessed using the Harvey Bradshaw Index and C-reactive protein [CRP]. Stool frequency, Bristol Stool Form Scale and length of previous ileal resection were recorded. FGF19 levels were also compared with healthy and diarrhea control patients.

RESULTS

FGF19 levels were inversely correlated with ileal resection length in IR-CD patients[r = -0.54, p = 0.02]. In NR-CD patients, median FGF19 levels were significantly lower in patients with active disease compared with inactive disease [103 vs. 158 pg/ml, p = 0.04] and in those with symptoms of diarrhea compared with those without [86 vs. 145 pg/ml, p = 0.035]. FGF19 levels were inversely correlated with stool frequency, Bristol stool form and CRP in NR-CD patients with ileal disease.

CONCLUSIONS

Reduced FGF19 levels are associated with ileal resection, diarrhea and disease activity. FGF19 may have utility as a biomarker for functioning ileum in CD. This study supports a potential role of FGF19 in guiding treatments for diarrhea in Crohn’s disease.

Epithelial transport in inflammatory bowel diseases

The epithelium of the gastrointestinal tract is one of the most versatile tissues in the organism, responsible for providing a tight barrier between dietary and bacterial antigens and the mucosal and systemic immune system while maintaining efficient digestive and absorptive processes to ensure adequate nutrient and energy supply. Inflammatory bowel diseases (Crohn's disease and ulcerative colitis) are associated with a breakdown of both functions, which in some cases are clearly interrelated. In this updated literature review, we focus on the effects of intestinal inflammation and the associated immune mediators on selected aspects of the transepithelial transport of macronutrients and micronutrients. The mechanisms responsible for nutritional deficiencies are not always clear and could be related to decreased intake, malabsorption, and excess losses. We summarize the known causes of nutrient deficiencies and the mechanism of inflammatory bowel disease-associated diarrhea. We also overview the consequences of impaired epithelial transport, which infrequently transcend its primary purpose to affect the gut microbial ecology and epithelial integrity. Although some of those regulatory mechanisms are relatively well established, more work needs to be done to determine how inflammatory cytokines can alter the transport process of nutrients across the gastrointestinal and renal epithelia.

TGF-β directs trafficking of the epithelial sodium channel ENaC which has implications for ion and fluid transport in acute lung injury

TGF-β is a pathogenic factor in patients with acute respiratory distress syndrome (ARDS), a condition characterized by alveolar edema. A unique TGF-β pathway is described, which rapidly promoted internalization of the αβγ epithelial sodium channel (ENaC) complex from the alveolar epithelial cell surface, leading to persistence of pulmonary edema. TGF-β applied to the alveolar airspaces of live rabbits or isolated rabbit lungs blocked sodium transport and caused fluid retention, which--together with patch-clamp and flow cytometry studies--identified ENaC as the target of TGF-β. TGF-β rapidly and sequentially activated phospholipase D1, phosphatidylinositol-4-phosphate 5-kinase 1α, and NADPH oxidase 4 (NOX4) to produce reactive oxygen species, driving internalization of βENaC, the subunit responsible for cell-surface stability of the αβγENaC complex. ENaC internalization was dependent on oxidation of βENaC Cys(43). Treatment of alveolar epithelial cells with bronchoalveolar lavage fluids from ARDS patients drove βENaC internalization, which was inhibited by a TGF-β neutralizing antibody and a Tgfbr1 inhibitor. Pharmacological inhibition of TGF-β signaling in vivo in mice, and genetic ablation of the nox4 gene in mice, protected against perturbed lung fluid balance in a bleomycin model of lung injury, highlighting a role for both proximal and distal components of this unique ENaC regulatory pathway in lung fluid balance. These data describe a unique TGF-β-dependent mechanism that regulates ion and fluid transport in the lung, which is not only relevant to the pathological mechanisms of ARDS, but might also represent a physiological means of acutely regulating ENaC activity in the lung and other organs.

The application of Ussing chambers for determining the impact of microbes and probiotics on intestinal ion transport

Host–microbe interactions have gained considerable attention in recent years with regards to their role in various organic disorders and diseases. In particular, research efforts have focused on the intestinal microbiota, where the largest and most diverse populations not only co-exist with the host, but also directly influence the state and function of the gastrointestinal (GI) tract. Moreover, both human and animal studies alike are now beginning to show a positive influence of probiotic bacteria on GI disorders associated with diarrhoea or constipation. Diarrheagenic GI diseases, such as those caused by Vibreo cholera or enterpathogenic Eschericia coli, have well-characterised interactions with the host that explain much of the observed symptoms, in particular severe diarrhoea. However, the mechanisms of action of nonpathogenic bacteria or probiotics on host physiology are less clearly understood. In the context of defining the mechanisms of action of probiotics in vitro, the Ussing chamber has proven to be a particularly useful tool. Here, we will present data from several studies that have defined molecular targets for microbes and putative probiotics in the regulation of intestinal secretory and absorptive function, and we will discuss these in the context of their application in pathogen- or inflammation-induced alterations in intestinal ion transport.

Pharmacology and clinical potential of guanylyl cyclase C agonists in the treatment of ulcerative colitis

Agonists of the transmembrane intestinal receptor guanylyl cyclase C (GCC) have recently attracted interest as promising human therapeutics. Peptide ligands that can specifically induce GCC signaling in the intestine include endogenous hormones guanylin and uroguanylin, diarrheagenic bacterial enterotoxins (ST), and synthetic drugs linaclotide, plecanatide, and SP-333. These agonists bind to GCC at intestinal epithelial surfaces and activate the receptor’s intracellular catalytic domain, an event initiating discrete biological responses upon conversion of guanosine-5′-triphosphate to cyclic guanosine monophosphate. A principal action of GCC agonists in the colon is the promotion of mucosal homeostasis and its dependent barrier function. Herein, GCC agonists are being developed as new medications to treat inflammatory bowel diseases, pathological conditions characterized by mucosal barrier hyperpermeability, abnormal immune reactions, and chronic local inflammation. This review will present important concepts underlying the pharmacology and therapeutic utility of GCC agonists for patients with ulcerative colitis, one of the most prevalent inflammatory bowel disease disorders.

Diarrhea in chronic inflammatory bowel diseases

Diarrhea is a common clinical feature of inflammatory bowel diseases and may be accompanied by abdominal pain, urgency, and fecal incontinence. The pathophysiology of diarrhea in these diseases is complex, but defective absorption of salt and water by the inflamed bowel is the most important mechanism involved. In addition to inflammation secondary to the disease, diarrhea may arise from a variety of other conditions. It is important to differentiate the pathophysiologic mechanisms involved in the diarrhea in the individual patient to provide the appropriate therapy. This article reviews microscopic colitis, ulcerative colitis, and Crohn's disease, focusing on diarrhea.

Ulcerative colitis patients in remission have an altered secretory capacity in the proximal colon despite macroscopically normal mucosa

BACKGROUND

One of the hallmarks of acute colitis is loss of epithelial transport. For unknown reasons, many patients still suffer from GI symptoms during remission, indicating a sustained imbalance between absorption and secretion. We hypothesize that the colonic epithelium becomes more reactive to secretagogues to compensate for a failing barrier.

METHODS

Biopsies from ascending colon and sigmoid colon of UC patients in remission and controls were mounted in Ussing chambers. Membrane current (Im) and epithelial capacitance (Cp) were used as markers for anion secretion and mucus exocytosis. Carbachol (1 mmol L(-1) ) and forskolin (10 μmol L(-1) ) were used to study Ca(2+) and cAMP-mediated secretion.

KEY RESULTS

Baseline values showed segmental patterns with higher Im in ascending colon and higher Cp in sigmoid colon of both UC patients and controls, but the patterns did not differ between the groups. The Im response to forskolin was increased (+35%) in the ascending colon of UC patients and the Im response to carbachol was decreased (-40%) in the same segment. No group differences were seen in the distal colon for either the forskolin or carbachol-induced Im responses. The Cp response to carbachol was instead up-regulated in the distal colon of UC patients, but remained unaffected in the proximal colon.

CONCLUSIONS & INFERENCES

The proximal colonic mucosa of UC patients in remission seems to shift its reactivity to secretagogues, becoming more sensitive to cAMP-dependent secretion and less sensitive to Ca(2+) -dependent secretion. This phenomenon may contribute to residual diarrhea in this patient group, despite resolution of inflammation.

Adapter-modified Ussing chamber enables evaluation of endoscopically-obtained colonic biopsy samples from cats and dogs

Adapter-modified Ussing chambers have been used for assessment of endoscopically obtained intestinal biopsies in humans. The aim of this study was to evaluate the feasibility of an adapter-modified Ussing chamber for assessment of intestinal transport physiology in endoscopically-obtained colonic biopsies from cats and dogs. Fifteen colonic biopsies from four cats and 13 colonic biopsies from four dogs were transferred into a modified Ussing chamber and sequentially exposed to several compounds. Baseline mean±SD conductance was measured. Changes of short circuit current (ΔIsc) were observed after exposure to glucose (number of feline biopsies that responded=0/number of canine biopsies that responded=4), phloridzin (n=0/n=7), histamine (n=5/n=12), serotonin (n=7/n=12), prostaglandin (n=5/n=7), forskolin (n=7/n=7), and ouabain (n=9/n=7). The adapter-modified Ussing chamber studied here enables investigation of transport physiology of endoscopically-obtained colonic biopsies from companion animals. However, we observed a large variability of results, suggesting that clinical use of this method is limited.

News from the end of the gut--how the highly segmental pattern of colonic HCO₃⁻ transport relates to absorptive function and mucosal integrity

A number of transport mechanisms in the colonic epithelium contribute to HCO₃⁻ movement across the apical and basolateral membranes, but this ion has been largely regarded as a by-product of the transport functions it is involved in, such as NaCl or short chain fatty acid (SCFA) absorption. However, emerging data points to several specific roles of HCO₃⁻ for colonic epithelial physiology, including pH control in the colonic surface microenvironment, which is important for transport and immune functions, as well as the secretion and the rheological properties of the mucus gel. Furthermore, recent studies have demonstrated that colonic HCO₃⁻ transporters are expressed in a highly segmental as well as species-specific manner. This review summarizes recently gathered information on the functional anatomy of the colon, the roles of HCO₃⁻ in the colonic epithelium, colonic mucosal integrity, and the expression and function of HCO₃⁻ transporting mechanisms in health and disease.

Basolateral ion transporters involved in colonic epithelial electrolyte absorption, anion secretion and cellular homeostasis

Electrolyte transporters located in the basolateral membrane of the colonic epithelium are increasingly appreciated as elaborately regulated components of specific transport functions and cellular homeostasis: During electrolyte absorption, Na(+) /K(+) ATPase, Cl⁻ conductance, Cl⁻/HCO₃⁻ exchange, K(+) /Cl⁻ cotransport and K(+) channels are candidates for basolateral Na(+) , Cl⁻ and K(+) extrusion. The process of colonic anion secretion involves basolateral Na(+) /K(+) /2Cl⁻ , and probably also Na(+) /HCO₃⁻ cotransport, as well as Na(+) /K(+) ATPase and K(+) channels to supply substrate, stabilize the membrane potential and generate driving force respectively. Together with a multitude of additional transport systems, Na(+) /H(+) exchange and Na(+) /HCO₃⁻ cotransport have been implicated in colonocyte pH(i) and volume homeostasis. The purpose of this article is to summarize recently gathered information on the molecular identity, function and regulation of the involved basolateral transport systems in native tissue. Furthermore, we discuss how these findings can help to integrate these systems into the transport function and the cellular homoeostasis of colonic epithelial cells. Finally, disturbances of basolateral electrolyte transport during disease states such as mucosal inflammation will be reviewed.

Loss of protein tyrosine phosphatase N2 potentiates epidermal growth factor suppression of intestinal epithelial chloride secretion

The Crohn's disease candidate gene, protein tyrosine phosphatase nonreceptor type 2 (PTPN2), has been shown to regulate epidermal growth factor (EGF)-induced phosphatidylinositol 3-kinase (PI3K) activation in fibroblasts. In intestinal epithelial cells (IECs), EGF-induced EGF receptor (EGFR) activation and recruitment of PI3K play a key role in regulating many cellular functions including Ca(2+)-dependent Cl(-) secretion. Moreover, EGFR also serves as a conduit for signaling by other non-growth factor receptor ligands such as the proinflammatory cytokine, IFN-γ. Here we investigated a possible role for PTPN2 in the regulation of EGFR signaling and Ca(2+)-dependent Cl(-) secretion in IECs. PTPN2 knockdown enhanced EGF-induced EGFR tyrosine phosphorylation in T(84) cells. In particular, PTPN2 knockdown promoted EGF-induced phosphorylation of EGFR residues Tyr-992 and Tyr-1068 and led subsequently to increased association of the catalytic PI3K subunit, p110, with EGFR and elevated phosphorylation of the downstream marker, Akt. As a functional consequence, loss of PTPN2 potentiated EGF-induced inhibition of carbachol-stimulated Ca(2+)-dependent Cl(-) secretion. In contrast, PTPN2 knockdown affected neither IFN-γ-induced EGFR transactivation nor EGF- or IFN-γ-induced phosphorylation of ERK1/2. In summary, our data establish a role for PTPN2 in the regulation of EGFR signaling in IECs in response to EGF but not IFN-γ. Knockdown of PTPN2 directs EGFR signaling toward increased PI3K activation and increased suppression of epithelial chloride secretory responses. Moreover, our findings suggest that PTPN2 dysfunction in IECs leads to altered control of intestinal epithelial functions regulated by EGFR.

Preserved Na(+)/H(+) exchanger isoform 3 expression and localization, but decreased NHE3 function indicate regulatory sodium transport defect in ulcerative colitis

BACKGROUND

A major causative factor of diarrhea in ulcerative colitis (UC) patients is the loss of Na(+) absorptive capacity of the inflamed colonic mucosa. Potential contributing mechanisms include reduced driving force for active transport, and impaired expression, mislocalization, or defective transport function of Na(+) absorptive proteins. We therefore studied the expression, brush border membrane (BBM) localization, and transport capacity of the major intestinal Na(+) absorptive protein, the Na(+)/H(+) exchanger isoform 3 (NHE3) in biopsies from UC patients.

METHODS

In UC and control biopsies, inflammation was graded histologically, NHE3, tumor necrosis factor alpha (TNF-alpha), villin, as well as other housekeeping genes were analyzed by quantitative real-time polymerase chain reaction (PCR), BBM localization of NHE3 determined by immunohistochemistry, and confocal microscopy. Na(+) absorptive capacity was assessed by (22)Na(+) isotope fluxes and NHE3 transport activity measured microfluorometrically in BCECF-loaded surface colonocytes within isolated crypts.

RESULTS

In mildly, moderately, and severely inflamed sigmoid colon of UC patients, neither NHE3 mRNA expression nor the abundance of NHE3 in the BBM was significantly altered compared to other structural components of the BBM. However, Na(+) absorption was strongly reduced by approximately 80% and acid-activated NHE3 transport activity was significantly decreased in the surface cells of sigmoid colonic crypts even in moderately inflamed mucosa.

CONCLUSIONS

In the colonic mucosa of patients with active UC, NHE3 transport capacity was found significantly decreased despite correct NHE3 location and abundance in the brush border, independent of current treatment. These findings suggest functional NHE3 transport as a novel factor for inflammatory diarrhea in UC patients.

Effect of natural commensal-origin DNA on toll-like receptor 9 (TLR9) signaling cascade, chemokine IL-8 expression, and barrier integritiy of polarized intestinal epithelial cells

BACKGROUND AND AIM

The intestinal epithelium is constantly exposed to high levels of genetic material like bacterial DNA. Under normal physiological conditions, the intestinal epithelial monolayer as a formidable dynamic barrier with a high-polarity structure facilitates only a controlled and selective flux on components between the lumen and the underlining mucosa and even is able to facilitate structure-based macromolecules movement. The aim of this study was to test the effect of natural commensal-origin DNA on the TLR9 signaling cascade and the barrier integrity of polarized intestinal epithelial cells (IECs).

METHODS

: Polarized HT-29 and T84 cells were treated with TNF-alpha in the presence or absence of DNA from Lactobacillus rhamnosus GG (LGG) and Bifidobacterium longum. TLR9 and interleukin-8 (IL-8) mRNA expression was assessed by semiquantitative and TaqMan real-time reverse-transcription polymerase chain reaction. Expression of TLR9 protein, degradation of inhibitor of kappa B alpha (IkappaBalpha), and p38 mitogen-activated protein kinase (p38 MAP) phosphorylation were assessed by Western blotting. To further reveal the role of TLR9 signaling, the TLR9 gene was silenced by siRNA. IL-8 secretion was measured by an enzyme-linked immunosorbent assay. Nuclear factor-kappa B (NF-kappaB) activity was assessed by the electrophoretic mobility shift assay (EMSA) and NF-kappaB-dependent luciferase reporter gene assays. As an indicator of tight junction formation and monolayer integrity of epithelial cell monolayers, transepithelial electrical resistance (TER) was repetitively monitored. Transmonolayer movement of natural commensal-origin DNA across monolayers was monitored using qRT-PCR and nested PCR based on bacterial 16S rRNA genes.

RESULTS

In response to apically applied natural commensal-origin DNA, polarized HT-29 and T84 cells enhanced expression of TLR9 in a specific manner, which was subsequently associated with attenuation of TNF-alpha-induced NF-kappaB activation and NF-kappaB-mediated IL-8 expression. TLR9 silencing abolished this inhibitory effect. Apically applied LGG DNA attenuated TNF-alpha-enhanced NF-kappaB activity by reducing IkappaBalpha degradation and p38 phosphorylation. LGG DNA did not decrease the TER but rather diminished the TNF-alpha-induced TER reduction. Translocation of natural commensal-origin DNA into basolateral compartments did not occur under tested conditions.

CONCLUSIONS

Our study indicates that TLR9 signaling mediates, at least in part, the anti-inflammatory effects of natural commensal-origin DNA on the gut because TLR9 silencing abolished the inhibitory effect of natural commensal-origin DNA on TNF-alpha-induced IL-8 secretion in polarized IECs. The nature of the TLR9 agonist, the polarity of cells, and the tight junction integrity of IECs has to be taken into account in order to predict the outcome of TLR9 signaling. (Inflamm Bowel Dis 2010).

Altered distribution of tight junction proteins after intestinal ischaemia/reperfusion injury in rats

Tight junction (TJ) disruptions have been demonstrated both in vitro and more recently in vivo in infection. However, the molecular basis for changes of TJ during ischaemia-reperfusion (I/R) injury is poorly understood. In the present study, intestinal damage was induced by I/R in an animal model. As assessed by TUNEL and propidium iodide uptake, we showed that I/R injury induced apoptosis as well as necrosis in rat colon, and the frequency of apoptotic and necrotic cells reached the maximum at 5 hrs of reperfusion. Immunofluorescence microscopy revealed that claudins 1, 3 and 5 are strongly expressed in the surface epithelial cells of the colon; however, labelling of all three proteins was present diffusely within cells and no longer focused at the lateral cell boundaries after I/R. Using Western blot analysis, we found that distribution of TJ proteins in membrane microdomains of TJ was markedly affected in I/R injury rats. Occludin, ZO-1, claudin-1 and claudin-3 were completely displaced from TX-100 insoluble fractions to TX-100 soluble fractions, and claudin-5 was partly displaced. The distribution of lipid raft marker protein caveolin-1 was also changed after I/R. I/R injury results in the disruption of TJs, which characterized by relocalization of the claudins 1, 3 and 5 and an increase in intestinal permeability using molecular tracer measurement. I/R injury altered distribution of TJ proteins in vivo that was associated with functional TJ deficiencies.

Effect of probiotics on intestinal barrier function

Impairment of the intestinal barrier is a key event in various gastrointestinal diseases, including inflammatory bowel diseases, celiac disease, gastrointestinal infections, diarrhea, and critical illness. Recent studies demonstrated that probiotic bacteria have beneficial effects in these diseases by effectively improving intestinal barrier function. This article reviews available data on the effect of probiotics on intestinal barrier function in vitro, in animal models, and in clinical studies.

Mechanisms of diarrhea in inflammatory bowel diseases

Diarrhea is a frequent symptom/sign in patients with ulcerative colitis and Crohn's disease (inflammatory bowel diseases), and several different mechanisms likely account for this diarrhea. As treatment of diarrhea is dependent on the pathogenetic process(es) responsible for diarrhea, increased understanding of the pathophysiology of diarrhea will help improve therapy. Inflammation is central to the diarrhea in patients with ulcerative colitis, while in Crohn's disease both inflammatory and noninflammatory mechanisms are responsible. This presentation summarizes the pathophysiology of diarrhea in both ulcerative colitis and Crohn's disease.

Ste20-related proline/alanine-rich kinase (SPAK) regulated transcriptionally by hyperosmolarity is involved in intestinal barrier function

The Ste20-related protein proline/alanine-rich kinase (SPAK) plays important roles in cellular functions such as cell differentiation and regulation of chloride transport, but its roles in pathogenesis of intestinal inflammation remain largely unknown. Here we report significantly increased SPAK expression levels in hyperosmotic environments, such as mucosal biopsy samples from patients with Crohn's disease, as well as colon tissues of C57BL/6 mice and Caco2-BBE cells treated with hyperosmotic medium. NF-kappaB and Sp1-binding sites in the SPAK TATA-less promoter are essential for SPAK mRNA transcription. Hyperosmolarity increases the ability of NF-kappaB and Sp1 to bind to their binding sites. Knock-down of either NF-kappaB or Sp1 by siRNA reduces the hyperosmolarity-induced SPAK expression levels. Furthermore, expression of NF-kappaB, but not Sp1, was upregulated by hyperosmolarity in vivo and in vitro. Nuclear run-on assays showed that hyperosmolarity increases SPAK expression levels at the transcriptional level, without affecting SPAK mRNA stability. Knockdown of SPAK expression by siRNA or overexpression of SPAK in cells and transgenic mice shows that SPAK is involved in intestinal permeability in vitro and in vivo. Together, our data suggest that SPAK, the transcription of which is regulated by hyperosmolarity, plays an important role in epithelial barrier function.

Glucocorticoids and tumor necrosis factor-alpha synergize to induce absorption by the epithelial sodium channel in the colon

BACKGROUND & AIMS

The epithelial sodium channel (ENaC) mediates electrogenic sodium absorption in distal colon. In patients with inflammatory bowel disease (IBD), ENaC induction is impaired, mainly through transcriptional suppression by proinflammatory cytokines such as tumor necrosis factor (TNF)-alpha. Glucocorticoid therapy rapidly increases sodium absorption; we investigated the molecular mechanisms underlying the interaction among TNF-alpha, glucocorticoids, and ENaC induction.

METHODS

ENaC-mediated sodium transport in glucocorticoid receptor (GR)-expressing HT-29/B6 cells and rat distal colon, under the influence of the synthetic glucocorticoid dexamethasone and TNF-alpha, was quantified in Ussing chambers. ENaC messenger RNA (mRNA) levels were monitored by real-time polymerase chain reaction. GR transactivation and expression were investigated by gene reporter, immunoblot, and confocal immunofluorescence microscopy analyses. The GR mRNA half-life was determined. Signaling pathways were characterized using mitogen-activated protein kinase inhibitors.

RESULTS

Dexamethasone not only prevented TNF-alpha-mediated ENaC suppression but caused synergistic induction of ENaC-dependent sodium absorption in HT-29/B6-GR cells and rat distal colon. This synergy resulted from TNF-alpha-mediated increases in GR protein levels because of GR mRNA stabilization and subsequent GR transactivation by dexamethasone. As a consequence, transcription of the ENaC beta- and gamma-subunits was up-regulated, increasing ENaC-dependent sodium absorption. p38 Mitogen-activated protein kinase is required for this synergistic effect: p38 inhibition blocked the increase in GR protein expression and ENaC-dependent sodium absorption.

CONCLUSIONS

TNF-alpha and dexamethasone induce ENaC, explaining the rapid and intense proabsorptive effect of glucocorticoid therapies.

Increased transmucosal uptake of E. coli K12 in collagenous colitis persists after budesonide treatment

OBJECTIVES

Collagenous colitis is increasingly recognized as a common diarrheal disorder of inflammatory origin. Intestinal inflammation is generally associated with increased mucosal permeability, but little is known about barrier function in microscopic colitis. Our aim was to investigate the mucosal barrier to nonpathogenic bacteria in collagenous colitis.

METHODS

The study included 33 individuals, 25 with collagenous colitis (14 in clinical remission, 11 with active disease, and 8 of these again after 6 weeks budesonide treatment) and 8 control patients. Bowel movements were registered for 1 week. Endoscopic biopsies from the sigmoid colon were mounted in modified Ussing chambers and assessed for short-circuit current (I(sc)), transepithelial resistance (TER), and transmucosal passage of chemically killed Escherichia coli K12.

RESULTS

Bacterial uptake was increased in patients in remission, 1.6 U (1.1-3.0) and in those with active disease, 4.6 U (2.5-5.8; median (IQR)), compared to controls, 0.7 U (0.1-1.1; P=0.004 and P-0.001, respectively). Active disease also had significant decrease in transepithelial resistance (TER) after 120 min, -9.7 Omega cm(2) ((-13)-(-4.3)), compared to controls, -5.2 Omega cm(2) ((-7.2)-(-3.1)), P-0.03; or patients in remission, -4.8 Omega cm(2) ((-8.0)-(-1.2)), P=0.04. Budesonide decreased median stool frequency to 1.9 (1.3-2.2) compared to 3.8 (3.7-4.2) before treatment (P=0.01), but bacterial uptake was still increased after budesonide 2.9 U (1.5-3.8), (P=0.006 compared to controls), and there were no significant changes in histology.

CONCLUSIONS

Collagenous colitis presents with significantly increased uptake and altered mucosal reactivity to nonpathogenic bacteria. Budesonide induces clinical remission and restores mucosal reactivity but does not abolish the increased bacterial uptake. An underlying barrier dysfunction may explain the frequent and rapid relapses in CC.

Characterization of the 5'-flanking region and regulation of expression of human anion exchanger SLC26A6

SLC26A6 (putative anion transporter 1, PAT1) has been shown to play an important role in mediating the luminal Cl(-)/OH(-)(HCO(3)(-)) exchange process in the intestine. Very little is known about the molecular mechanisms involved in the transcriptional regulation of intestinal SLC26A6 gene expression in the intestine. Current studies were, therefore, designed to clone and characterize the 5'-regulatory region of the human SLC26A6 gene and determine the mechanisms involved in its regulation. A 1,120 bp (p-964/+156) SLC26A6 promoter fragment cloned upstream to the luciferase reporter gene in pGL2-basic exhibited high promoter activity when transfected in Caco2 cells. Progressive deletions of the 5'-flanking region demonstrated that -214/-44 region of the promoter harbors cis-acting elements important for maximal SLC26A6 promoter activity. Since, diarrhea associated with inflammatory bowel diseases is attributed to increased secretion of pro-inflammatory cytokines, we examined the effects of IFNgamma (30 ng/ml, 24 h) on SLC26A6 function, expression and promoter activity. IFNgamma decreased both SLC26A6 mRNA and function and repressed SLC26A6 promoter activity. Deletion analysis indicated that IFNgamma response element is located between -414/-214 region and sequence analysis of this region revealed the presence of potential Interferon Stimulated Responsive Element (ISRE), a binding site (-318/-300 bp) for interferon regulatory factor-1 transcription factor (IRF-1). Mutations in the potential ISRE site abrogated the inhibitory effects of IFNgamma. These studies provided novel evidence for the involvement of IRF-1 in the regulation of SLC26A6 gene expression by IFNgamma in the human intestine.

Molecular bases of impaired water and ion movements in inflammatory bowel diseases

The intestine is dedicated to the absorption of water and nutrients. Fine tuning of this process is necessary to maintain an adequate balance and inflammation disrupts the equilibrium. This review summarizes the current evidence in this field. Classical mechanisms proposed include alteration of epithelial integrity, augmented secretion, and reduced absorption. In addition, intestinal inflammation is associated with defects in epithelial barrier function. However, our understanding of the phenomenon has been complicated by the fact that ionic secretion is in fact diminished in vivo, even after inflammation has subsided. Inhibited ionic secretion can be reversed partially or totally in vitro by maneuvers such as blockade of inducible nitric oxide synthase or removal of the submucosal layer. Disturbances in ionic absorption are less well characterized but clearly involve both electroneutral and electrogenic Na(+) absorption. Altered ionic transport is associated with changes in the expression and function of the transporters, including the Na(+)/K(+) ATPase, the sodium/potassium/chloride cotransporter 1 (NKCC1), the sodium/hydrogen exchanger 3 (NHE3), and the epithelial sodium channel (ENaC), as well as to the modulation of intracellular signaling. Further investigation is needed in this area in order to provide an integrated paradigm of ionic transport in the inflamed intestine. In particular, we do not know exactly how diarrhea ensues in inflammation and, consequently, we do not have specific pharmacological tools to combat this condition effectively and without side effects. Moreover, whether transport disturbances are reversible independently of inflammatory control is unknown.

Decoding epithelial signals: critical role for the epidermal growth factor receptor in controlling intestinal transport function

The intestinal epithelium engages in bidirectional transport of fluid and electrolytes to subserve the physiological processes of nutrient digestion and absorption, as well as the elimination of wastes, without excessive losses of bodily fluids that would lead to dehydration. The overall processes of intestinal ion transport, which in turn drive the secretion or absorption of water, are accordingly carefully regulated. We and others have identified the epidermal growth factor receptor (EGFr) as a critical regulator of mammalian intestinal ion transport. In this article, we focus on our studies that have uncovered the intricate signalling mechanisms downstream of EGFr that regulate both chloride secretion and sodium absorption by colonocytes. Emphasis will be placed on the EGFr-associated regulatory pathways that dictate the precise outcome to receptor activation in response to signals that may seem, on their face, to be quite similar if not identical. The concepts to be discussed underlie the ability of the intestinal epithelium to utilize a limited set of signalling effectors to produce a variety of outcomes suitable for varying physiological and pathophysiological demands. Our findings therefore are relevant not only to basic biological principles, but also may ultimately point to new therapeutic targets in intestinal diseases where ion transport is abnormal.

Altered ENaC expression leads to impaired sodium absorption in the noninflamed intestine in Crohn's disease

BACKGROUND & AIMS

Crohn's disease (CD) is a chronic inflammatory bowel disease. In this study, we have investigated sodium absorption via epithelial sodium channels (ENaC) in the macroscopically noninflamed colon in active CD.

METHODS

Sodium transport via ENaC was investigated in Ussing chambers using biopsy specimens of sigmoid colon from controls and active CD limited to the small intestine. ENaC messenger RNA expression and subcellular localization were studied by real-time polymerase chain reaction and confocal microscopy. Effects of proinflammatory cytokines on ENaC and signaling via mitogen-activated protein kinases were investigated in rat distal colon. Therapeutic inhibition of mitogen-activated protein kinases was studied in CD biopsy specimens.

RESULTS

Electrogenic sodium absorption via ENaC was strongly impaired in the macroscopically noninflamed CD colon because of reduced gamma-ENaC transcription, whereas subcellular localization of ENaC was not changed. In contrast to impaired epithelial sodium transport, epithelial barrier function was not altered in noninflamed CD colon, indicating that paracellular leak flux of ions did not contribute to decreased sodium absorption. Exposure of rat distal colon to tumor necrosis factor alpha led to reduced electrogenic sodium absorption because of impaired transcriptional gamma-ENaC induction, which resembled the changes found in CD. Tumor necrosis factor alpha effects were dependent on extracellular signal-regulated kinase 1/2 but not p38 or c-Jun-N-terminal kinase because inhibition of mitogen-activated protein kinase/extracellular regulated kinase (MEK)1/2 but not inhibition of p38 or c-Jun-N-terminal kinase prevented suppression of ENaC. Finally, therapeutic inhibition of MEK1/2 restored electrogenic sodium absorption in CD.

CONCLUSIONS

In CD, macroscopically noninflamed colon contributes to diarrhea via impaired ENaC-mediated sodium absorption. Inhibition of extracellular signal-regulated kinase might serve as a potential therapeutic strategy for CD diarrhea.

Hydrogen peroxide inhibits Ca2+-dependent chloride secretion across colonic epithelial cells via distinct kinase signaling pathways and ion transport proteins

Reactive oxygen species (ROS) are key mediators in a number of inflammatory conditions, including inflammatory bowel disease (IBD). ROS, including hydrogen peroxide (H(2)O(2)), modulate intestinal epithelial ion transport and are believed to contribute to IBD-associated diarrhea. Intestinal crypt fluid secretion, driven by electrogenic Cl(-) secretion, hydrates and sterilizes the crypt, thus reducing bacterial adherence. Here, we show that pathophysiological concentrations of H(2)O(2) inhibit Ca(2+)-dependent Cl(-) secretion across T(84) colonic epithelial cells by elevating cytosolic Ca(2+), which contributes to activation of two distinct signaling pathways. One involves recruitment of the Ca(2+)-responsive kinases, Src and Pyk-2, as well as extracellular signal-regulated kinase (ERK). A separate pathway recruits p38 MAP kinase and phosphoinositide 3-kinase (PI3-K) signaling. The ion transport response to Ca(2+)-dependent stimuli is mediated in part by K(+) efflux through basolateral K(+) channels and Cl(-) uptake by the Na(+)-K(+)-2Cl(-) cotransporter, NKCC1. We demonstrate that H(2)O(2) inhibits Ca(2+)-dependent basolateral K(+) efflux and also inhibits NKCC1 activity independently of inhibitory effects on apical Cl(-) conductance. Thus, we have demonstrated that H(2)O(2) inhibits Ca(2+)-dependent Cl(-) secretion through multiple negative regulatory signaling pathways and inhibition of specific ion transporters. These findings increase our understanding of mechanisms by which inflammation disturbs intestinal epithelial function and contributes to intestinal pathophysiology.

Role of the intestinal barrier in inflammatory bowel disease

A critical function of the intestinal mucosa is to form a barrier that separates luminal contents from the interstitium. The single layer of intestinal epithelial cells (IECs) serves as a dynamic interface between the host and its environment. Cell polarity and structural properties of the epithelium is complex and is important in the development of epithelial barrier function. Epithelial cells associate with each other via a series of intercellular junctions. The apical most intercellular junctional complex referred to as the Apical Junction Complex (AJC) is important in not only cell-cell recognition, but also in the regulation of paracellular movement of fluid and solutes. Defects in the intestinal epithelial barrier function have been observed in a number of intestinal disorders such as inflammatory bowel disease (IBD). It is now becoming evident that an aberrant epithelial barrier function plays a central role in the pathophysiology of IBD. Thus, a better understanding of the intestinal epithelial barrier structure and function in healthy and disease states such as IBD will foster new ideas for the development of therapies for such chronic disorders.