Oliver Bachmann

Apoptosis of regulatory T lymphocytes is increased in chronic inflammatory bowel disease and reversed by anti-TNFα treatment

Background and aims Inappropriate immune responses contribute to the continuous stimulation of the intestinal immune system in chronic inflammatory bowel disease (IBD). Among several pathogenic factors, a numerical deficiency of regulatory T (Treg) cells has been suggested to lead to an insufficient compensation of chronically activated T lymphocytes. This study was conducted to investigate whether increased apoptosis contributes to Treg cell deficiency in IBD and whether successful treatment with antitumour necrosis factor α (TNFα) is achieved by reducing of Treg cell apoptosis. Methods Apoptosis of CD4+Foxp3+ Treg cells in tissue sections of patients with active IBD was analysed by immunohistochemistry and TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labelling) staining. Apoptosis of peripheral blood CD4+CD25+Foxp3+ Treg cells was investigated by flow cytometry and annexin-V staining. In addition, caspase activity and apoptosis were measured in sera of patients with IBD treated with anti-TNFα by a luminometric caspase enzyme assay. Results It is demonstrated that patients with active IBD revealed increased apoptosis of local CD4+Foxp3+ Treg cells in the inflamed mucosa compared with non-inflamed control colon tissue. Moreover, in peripheral blood a reduced frequency and increased apoptosis of Treg cells were found and accompanied by elevated caspase activity in the serum. During anti-TNFα treatment, Treg cell apoptosis declined in close correlation with elevated peripheral Treg cell numbers and a decrease of caspase activation and disease activity. Conclusions These data suggest that increased apoptosis of Treg cells plays a potentially important role in the pathogenesis of IBD and can be reversed by anti-TNFα treatment. Measurement of Treg cell apoptosis and serum caspase activity might therefore represent promising tools for monitoring disease activity and treatment response in patients with IBD.

NHE3 inhibition by cAMP and Ca2+ is abolished in PDZ-domain protein PDZK1-deficient murine enterocytes.

The PDZ‐binding protein PDZK1 (NHERF3/CAP70/PDZ‐dc‐1) in vitro binds to NHE3, but its role in the regulation of NHE3 activity in native enterocytes is unknown. This study was undertaken to understand the physiological role of PDZK1 in regulating NHE3 activity in native murine colonic enterocytes. NHE3 transport rates were assessed fluorometrically in BCECF‐loaded colonic crypts in the NHE3‐expressing cryptal openings by measuring acid‐activated, Na+‐dependent, Hoe 642‐insensitive proton efflux rates. NHE3 mRNA expression levels and NHE3 total enterocyte and brush border membrane (BBM) protein abundance were determined by quantitative PCR and Western analysis and immunohistochemistry. In pdzk1−/− colonic surface cells, acid‐activated NHE3 transport rates were strongly reduced, and the inhibitory effect of forskolin and ionomcyin was virtually abolished. Hyperosmolarity, on the other hand, still had an inhibitory effect. In addition, the NHE3‐selective inhibitor S1611 inhibited acid‐activated NHE3 activity in pdzk1−/− and +/+ mice, suggesting that functional NHE3 is present in pdzk1‐deficient colonocytes. NHE1 and NHE2 activity was not altered in pdzk1−/− colonic crypts. Immunohistochemistry revealed apical NHE3 staining in pdzk1−/− and +/+ proximal colon, and Western blot analysis revealed no difference in NHE3 abundance in colonic enterocyte homogenate as well as brush border membrane. Lack of the PDZ‐adaptor protein PDZK1 in murine proximal colonic enterocytes does not influence NHE3 abundance or targeting to the apical membrane, but abolishes NHE3 regulation by cAMPergic and Ca2+ ‐dependent pathways. It leaves NHE3 inhibition by hyperosmolarity intact, suggesting an important and selective role for PDZK1 in the agonist‐mediated regulation of intestinal NHE3 activity.

Loss of downregulated in adenoma (DRA) impairs mucosal HCO3− secretion in murine ileocolonic inflammation

Background: Ileocolonic luminal pH has been reported to be abnormally low in inflammatory bowel disease (IBD) patients, and one of the causative factors may be reduced epithelial HCOSymbol secretory rate (JSymbol). Disturbances in JSymbol may occur due to inflammation‐induced changes in the crypt and villous architecture, or due to the effect of proinflammatory cytokines on epithelial ion transporters. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Methods: To discriminate between these possibilities, the tumor necrosis factor alpha (TNF‐&agr;) overexpressing (TNF+/&Dgr;ARE) mouse model was chosen, which displays high proinflammatory cytokine levels in both ileum and colon, but develops only mild colonic histopathology and diarrhea. HCOSymbol secretion, mRNA expression, immunohistochemistry, and fluid absorptive capacity were measured in ileal and mid‐colonic mucosa of TNF+/&Dgr;ARE and wildtype (WT) (TNF+/+) mice in Ussing chambers, and in anesthetized mice in vivo. Symbol. No caption available. Results: The high basal JSymbol observed in WT ileal and mid‐colonic mucosa were luminal Cl−‐dependent and strongly decreased in TNF+/&Dgr;ARE mice. Downregulated in adenoma (DRA) mRNA and protein expression was strongly decreased in TNF+/&Dgr;ARE ileocolon, whereas cystic fibrosis transmembrane conductance regulator (CFTR), Na+/H+ exchanger 3 (NHE3), Na+/HCOSymbol cotransporter (NBC), and epithelial sodium channel (ENaC) expression was not significantly altered. This indicates that the severe defect in ileocolonic JSymbol was due to DRA downregulation. Fluid absorption was severely depressed in the ileum but only mildly affected in the mid‐distal colon, preventing the development of overt diarrhea. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Conclusions: Even mild ileocolonic inflammation may result in a decrease of epithelial HCOSymbol secretion, which may contribute to alterations in surface pH, intestinal flora, and mucus barrier properties. (Inflamm Bowel Dis 2011;) Symbol. No caption available.

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‐&agr;), 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 22Na+ 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 ≈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. (Inflamm Bowel Dis 2010)

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 pHi 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.

Knockout mouse models for intestinal electrolyte transporters and regulatory PDZ adaptors: new insights into cystic fibrosis, secretory diarrhoea and fructose-induced hypertension.

Knockout mouse models have provided key insights into the physiological significance of many intestinal electrolyte transporters. This review has selected three examples to highlight the importance of knockout mouse technology in unravelling complex regulatory relationships important for the understanding of human diseases. Genetic ablation of the cystic fibrosis transmembrane conductance regulator (CFTR) has created one of the most useful mouse models for understanding intestinal transport. Recent work has provided an understanding of the key role of the CFTR anion channel in the regulation of HCO3− secretion, and the important consequences that a defect in HCO3− output may have on the viscoelastic properties of mucus, on lipid absorption and on male and female reproductive function. The regulation of CFTR activity, and also that of the intestinal salt absorptive transporter NHE3, occurs via the formation of PSD95‐Drosophila homologue Discs‐large‐tight junction protein ZO‐1 (PDZ) adaptor protein‐mediated multiprotein complexes. The recent generation of knockout mice for three members of the sodium‐hydrogen regulatory factor (NHERF) family of PDZ adaptor proteins, namely NHERF1 (EBP50), NHERF2 (E3KARP) and NHERF3 (PDZK1), has helped to explain why NHERF1 is essential for both normal and mutant CFTR function. In addition, they have provided new insight into the molecular mechanisms of secretory diarrhoeas. Genetic ablation of members of the recently discovered Slc26 anion transporter gene family not only reproduced the phenotype of the genetic diseases that led to the discovery of the gene family, but also resulted in new insights into complex human diseases such as secretory diarrhoea, fructose‐induced hypertension and urolithiasis.

Serotonin 5-HT7 receptor is critically involved in acute and chronic inflammation of the gastrointestinal tract.

Background:Intestinal inflammation is often associated with an increased level of serotonin (5-HT), an important gastrointestinal signaling molecule involved in gut homeostasis through stimulation of specific receptors. In this study, we investigated the role of 5-HT7 receptor (5-HT7R) in the induction and development of intestinal inflammation using a mouse model of acute and chronic colitis and human patients with Crohns disease (CD). Methods:Acute colitis was induced through administration of dextran sodium sulfate to wild-type, 5-HT7R–deficient mice and hematopoietic bone marrow chimera. Chronic colitis was induced in interleukin 10–deficient mice. The role of 5-HT7R in gut inflammation was assessed using agonist/antagonist treatment. We investigated expression and distribution of 5-HT7R, extent of gut inflammation with magnetic resonance imaging and histological analysis, survival rate, and disease activity index. Finally, biopsies from the large intestine of patients with CD were analyzed. Results:Under basal conditions, 5-HT7R is expressed both in enteric neurons and CD11c+ cells of the large intestine. Expression of 5-HT7R significantly increased after induction of colitis in mice and in inflamed intestinal regions of patients with CD in CD11c/CD86 double-positive cells. Pharmacological blockade or genetic ablation of 5-HT7R resulted in increased severity of both acute and chronic dextran sodium sulfate-induced colitis, whereas receptor stimulation showed an anti-inflammatory effect. Analysis of bone marrow chimera indicated importance of 5-HT7R expressed by hematopoietic cells in intestinal inflammation. Conclusions:The 5-HT7R expressed on CD11c/CD86-positive myeloid cells modulates the severity of intestinal inflammation in an acute and chronic colitis and thus represents a potential therapeutic target for the treatment of inflammatory disorders such as CD.

Secretagogue stimulation enhances NBCe1 (electrogenic Na+/HCO3− cotransporter) surface expression in murine colonic crypts

A Na(+)/HCO(3)(-) cotransporter (NBC) is located in the basolateral membrane of the gastrointestinal epithelium, where it imports HCO(3)(-) during stimulated anion secretion. Having previously demonstrated secretagogue activation of NBC in murine colonic crypts, we now asked whether vesicle traffic and exocytosis are involved in this process. Electrogenic NBCe1-B was expressed at significantly higher levels than electroneutral NBCn1 in colonic crypts as determined by QRT-PCR. In cell surface biotinylation experiments, a time-dependent increase in biotinylated NBCe1 was observed, which occurred with a peak of +54.8% after 20 min with forskolin (P < 0.05) and more rapidly with a peak of +59.8% after 10 min with carbachol (P < 0.05) and which corresponded well with the time course of secretagogue-stimulated colonic bicarbonate secretion in Ussing chamber experiments. Accordingly, in isolated colonic crypts pretreated with forskolin and carbachol for 10 min, respectively, and subjected to immunohistochemistry, the NBCe1 signal showed a markedly stronger colocalization with the E-cadherin signal, which was used as a membrane marker, compared with the untreated control. Cytochalasin D did not change the observed increase in membrane abundance, whereas colchicine alone enhanced NBCe1 membrane expression without an additional increase after carbachol or forskolin, and LY294002 had a marked inhibitory effect. Taken together, our results demonstrate a secretagogue-induced increase of NBCe1 membrane expression. Vesicle traffic and exocytosis might thus represent a novel mechanism of intestinal NBC activation by secretagogues.

Increased epithelial permeability is the primary cause for bicarbonate loss in inflamed murine colon.

Background:Bicarbonate loss into the lumen occurs during intestinal inflammation in different species. However, candidate pathways like CFTR or DRA are inhibited in the inflamed gut. This study addressed the question whether and how inflammation-associated increased intestinal permeability may result in epithelial HCO3− loss. Methods:Murine proximal colon was studied because it does not express functional DRA but is inflamed in the tumor necrosis factor &agr; overexpressing mouse model (TNF&Dgr;ARE). Luminal alkalization, 3H-mannitol fluxes, impedance spectroscopy, and dilution potentials were measured in Ussing chambers, whereas expression and localization of tight junction–associated proteins were analyzed by Western blots and immunohistochemistry. Results:Luminal alkalization rates and 3H-mannitol fluxes were increased in TNF+/&Dgr;ARE proximal colon, whereas forskolin-stimulated Isc was not altered. Epithelial resistance was reduced, but subepithelial resistance increased. The epithelial lining was intact, and enterocyte apoptosis rate was not increased despite massively increased Th1 cytokine levels and lymphoplasmacellular infiltration. Measurement of dilution potentials suggested a loss of cation selectivity with increased anion permeability. Western analysis revealed a downregulation of occludin expression and an upregulation of both claudin-2 and claudin-5, with no change in ZO-1, E-cadherin, claudin-4, and claudin-8. Immunohistochemistry suggested correct occludin localization but reduced tight junction density in TNF+/&Dgr;ARE surface epithelium. Conclusions:Inflammation during TNF-&agr; overexpression leads to increased epithelial permeability in murine proximal colon, decreased tight junctional cation selectivity, and increased HCO3− loss into the lumen. Inflammation-associated colonic HCO3− loss may occur through leaky tight junctions rather than through HCO3− secreting ion transporters.

Evidence for a causal link between adaptor protein PDZK1 downregulation and Na + /H + exchanger NHE3 dysfunction in human and murine colitis

A dysfunction of the Na+/H+ exchanger isoform 3 (NHE3) significantly contributes to the reduced salt absorptive capacity of the inflamed intestine. We previously reported a strong decrease in the NHERF family member PDZK1 (NHERF3), which binds to NHE3 and regulates its function in a mouse model of colitis. The present study investigates whether a causal relationship exists between the decreased PDZK1 expression and the NHE3 dysfunction in human and murine intestinal inflammation. Biopsies from the colon of patients with ulcerative colitis, murine inflamed ileal and colonic mucosa, NHE3-transfected Caco-2BBe colonic cells with short hairpin RNA (shRNA) knockdown of PDZK1, and Pdzk1-gene-deleted mice were studied. PDZK1 mRNA and protein expression was strongly decreased in inflamed human and murine intestinal tissue as compared to inactive disease or control tissue, whereas that of NHE3 or NHERF1 was not. Inflamed human and murine intestinal tissues displayed correct brush border localization of NHE3 but reduced acid-activated NHE3 transport activity. A similar NHE3 transport defect was observed when PDZK1 protein content was decreased by shRNA knockdown in Caco-2BBe cells or when enterocyte PDZK1 protein content was decreased to similar levels as found in inflamed mucosa by heterozygote breeding of Pdzk1-gene-deleted and WT mice. We conclude that a decrease in PDZK1 expression, whether induced by inflammation, shRNA-mediated knockdown, or heterozygous breeding, is associated with a decreased NHE3 transport rate in human and murine enterocytes. We therefore hypothesize that inflammation-induced loss of PDZK1 expression may contribute to the NHE3 dysfunction observed in the inflamed intestine.