Hydrokinetic pancreatic function and insulin secretion are moduled by Cl- uniporter Slc26a9 in mice

AIM

Slc26a9 is a member of the Slc26 multifunctional anion transporter family. Polymorphisms in Slc26a9 are associated with an increased incidence of meconium ileus and diabetes in cystic fibrosis patients. We investigated the expression of Slc26a9 in the murine pancreatic ducts, islets and parenchyma, and elucidated its role in pancreatic ductal electrolyte and fluid secretion and endocrine function.

METHODS

Pancreatic Slc26a9 and CFTR mRNA expression, fluid and bicarbonate secretion were assessed in slc26a9^-/- mice and their age- and sex-matched wild-type (wt) littermates. Glucose and insulin tolerance tests were performed.

RESULTS

Compared with stomach, the mRNA expression of Slc26a9 was low in pancreatic parenchyma, 20-fold higher in microdissected pancreatic ducts than parenchyma, and very low in islets. CFTR mRNA was ~10 fold higher than Slc26a9 mRNA expression in each pancreatic cell type. Significantly reduced pancreatic fluid secretory rates and impaired glucose tolerance were observed in female slc26a9^-/- mice, whereas alterations in male mice did not reach statistical significance. No significant difference was observed in peripheral insulin resistance in slc26a9^-/- compared to sex- and aged-matched wt controls. In contrast, isolated slc26a9^-/- islets in short term culture displayed no difference in insulin content, but a significantly reduced glucose-stimulated insulin secretion compared to age- and sex-matched wt islets, suggesting that the impaired glucose tolerance in the absence of Slc26a9 expression these is a pancreatic defect.

CONCLUSIONS

Deletion of Slc26a9 is associated with a reduction in pancreatic fluid secretion and impaired glucose tolerance in female mice. The results underline the importance of Slc26a9 in pancreatic physiology.

A15 EFFICACY AND SAFETY OF FILGOTINIB AS INDUCTION THERAPY FOR PATIENTS WITH MODERATELY TO SEVERELY ACTIVE ULCERATIVE COLITIS: RESULTS FROM THE PHASE 2B/3 SELECTION STUDY

Aims

The SELECTION (NCT02914522) Induction Studies evaluated the efficacy/safety of filgotinib (FIL), a preferential JAK1 inhibitor, as induction therapy for patients (pts) with moderately to severely active ulcerative colitis (UC) who were biologic-naïve but failed conventional therapy (Induction Study A) or failed prior biologics (Induction Study B).

Methods

Pts were randomized 2:2:1 to once–daily FIL 200mg, FIL 100mg or placebo (PBO). The primary (clinical remission), key secondary (Mayo Clinic Score [MCS] remission, endoscopic remission, and histologic remission), and exploratory endpoints (MCS response and endoscopic improvement) were assessed at Week 10.

Results

In both studies, baseline demographics and disease characteristics were similar across treatment groups. In Study A, 659 pts were randomized and treated. Baseline mean MCS was 8.6 and 56% had a Mayo endoscopic subscore (ES)=3. A significantly higher proportion of biologic-naïve pts on FIL 200mg achieved clinical remission vs PBO and all key secondary endpoints (Table). In Study B, 689 pts were randomized and treated. Baseline mean MCS was 9.3 and 78% had ES=3. Prior treatment failures were: anti-TNF (86%), vedolizumab (52%) and both (dual-refractory; 43%). A significantly higher proportion of biologic-experienced pts on FIL 200mg achieved clinical remission vs PBO. In Studies A and B, a greater proportion of pts on FIL 200 mg achieved an MCS response and endoscopic improvement vs PBO.

The rates of AEs, serious AEs and discontinuations due to AEs were similar across FIL and PBO groups during induction. In the PBO, FIL 100mg and FIL 200mg groups, serious infections occurred in 0.7%, 0.7% and 0.4% pts in Study A and 1.4%, 1.4% and 0.8% pts in Study B; H Zoster occurred in <1% in both groups for both cohorts.

Conclusions

SELECTION included a high proportion of dual-refractory pts, and pts with severe endoscopic disease. Both doses of FIL were well tolerated. Filgotinib 200mg was effective induction therapy for both biologic-naïve/-experienced pts with moderately to severely active UC.

Funding Agencies

None

Genetic ablation of carbonic anhydrase IX disrupts gastric barrier function via claudin-18 downregulation and acid backflux

Aim

This study aimed to explore the molecular mechanisms for the parietal cell loss and fundic hyperplasia observed in gastric mucosa of mice lacking the carbonic anhydrase 9 (CAIX).

Methods

We assessed the ability of CAIX‐knockout and WT gastric surface epithelial cells to withstand a luminal acid load by measuring the pH_i of exteriorized gastric mucosa in vivo using two‐photon confocal laser scanning microscopy. Cytokines and claudin‐18A2 expression was analysed by RT‐PCR.

Results

CAIX‐knockout gastric surface epithelial cells showed significantly faster pH_i decline after luminal acid load compared to WT. Increased gastric mucosal IL‐1β and iNOS, but decreased claudin‐18A2 expression (which confer acid resistance) was observed shortly after weaning, prior to the loss of parietal and chief cells. At birth, neither inflammatory cytokines nor claudin‐18 expression were altered between CAIX and WT gastric mucosa. The gradual loss of acid secretory capacity was paralleled by an increase in serum gastrin, IL‐11 and foveolar hyperplasia. Mild chronic proton pump inhibition from the time of weaning did not prevent the claudin‐18 decrease nor the increase in inflammatory markers at 1 month of age, except for IL‐1β. However, the treatment reduced the parietal cell loss in CAIX‐KO mice in the subsequent months.

Conclusions

We propose that CAIX converts protons that either backflux or are extruded from the cells rapidly to CO₂ and H₂O, contributing to tight junction protection and gastric epithelial pH_i regulation. Lack of CAIX results in persistent acid backflux via claudin‐18 downregulation, causing loss of parietal cells, hypergastrinaemia and foveolar hyperplasia.

Host and microbiota interactions are critical for development of murine Crohn's-like ileitis

Deregulation of host-microbiota interactions in the gut is a pivotal characteristic of Crohn's disease. It remains unclear, however, whether commensals and/or the dysbiotic microbiota associated with pathology in humans are causally involved in Crohn's pathogenesis. Here, we show that Crohn's-like ileitis in Tnf(ΔARE/+) mice is microbiota-dependent. Germ-free Tnf(ΔARE/+) mice are disease-free and the microbiota and its innate recognition through Myd88 are indispensable for tumor necrosis factor (TNF) overexpression and disease initiation in this model. The epithelium of diseased mice shows no major defects in mucus barrier and paracellular permeability. However, Tnf(ΔARE/+) ileitis associates with the reduction of lysozyme-expressing Paneth cells, mediated by adaptive immune effectors. Furthermore, we show that established but not early ileitis in Tnf(ΔARE/+) mice involves defective expression of antimicrobials and dysbiosis, characterized by Firmicutes expansion, including epithelial-attaching segmented filamentous bacteria, and decreased abundance of Bacteroidetes. Microbiota modulation by antibiotic treatment at an early disease stage rescues ileitis. Our results suggest that the indigenous microbiota is sufficient to drive TNF overexpression and Crohn's ileitis in the genetically susceptible Tnf(ΔARE/+) hosts, whereas dysbiosis in this model results from disease-associated alterations including loss of lysozyme-expressing Paneth cells.

Randomised clinical trial: vercirnon, an oral CCR9 antagonist, vs. placebo as induction therapy in active Crohn's disease

BACKGROUND

Many patients with active Crohn's disease do not adequately respond to therapies, highlighting the need for new treatments.

AIMS

To conduct a randomised, double-blind, placebo-controlled phase 3 study to assess the efficacy and safety of vercirnon, an oral inhibitor of CC chemokine receptor-9, for the treatment of patients with moderately-to-severely active Crohn's disease.

METHODS

Patients with a Crohn's Disease Activity Index (CDAI) of 220-450, plus evidence of active disease (endoscopically confirmed or elevation of both C-reactive protein and faecal calprotectin), who had failed corticosteroid or immunosuppressant therapy were enrolled. Patients were equally randomised to receive placebo, vercirnon 500 mg once daily or vercirnon 500 mg twice daily. The primary endpoint was clinical response, defined as a 100-point decrease in CDAI from baseline to week 12.

RESULTS

Six hundred and eight patients were randomised. Patient characteristics and baseline demographics were similar among the groups. The proportions of patients achieving a clinical response were 25.1%, 27.6% and 27.2% for placebo, once daily and twice daily respectively; treatment differences were not significant (2.5%; 95% confidence interval, CI -6.1% to 11.0%, P = 0.546 for once daily vs. placebo, and 2.1%; 95% CI -6.5% to 10.7%, P = 0.648 for twice daily vs. placebo). Adverse events were reported in 69.8%, 73.3% and 78.1% with serious adverse events in 8.9%, 5.9%, and 6.0% of patients in the placebo, once-daily and twice-daily groups, respectively.

CONCLUSIONS

We did not demonstrate efficacy of vercirnon as an induction therapy in patients with moderately-to-severely active Crohn's disease; its effect in maintenance therapy was not addressed.

Slc26a3/Dra and Slc26a6 in Murine Ameloblasts

Formation of apatite crystals during enamel development generates protons. To sustain mineral accretion, maturation ameloblasts need to buffer these protons. The presence of cytosolic carbonic anhydrases, the basolateral Na(+) bicarbonate cotransporter Nbce1, and the basolateral anion exchanger Ae2a,b in maturation ameloblasts suggests that these cells secrete bicarbonates into the forming enamel, but it is unknown by which mechanism. Solute carrier (Slc) family 26A encodes different anion exchangers that exchange Cl(-)/HCO3 (-), including Slc26a3/Dra, Slc26a6/Pat-1, and Slc26a4/pendrin. Previously, we showed that pendrin is expressed in ameloblasts but is not critical for enamel formation. In this study, we tested the hypothesis that maturation ameloblasts express Dra and Slc26a6 to secrete bicarbonate into the enamel space in exchange for Cl(-). Real-time polymerase chain reaction detected mRNA transcripts for Dra and Slc26a6 in mouse incisor enamel organs, and Western blotting confirmed their translation into protein. Both isoforms were immunolocalized in ameloblasts, principally at maturation stage. Mice with null mutation of either Dra or Slc26a6 had a normal dental or skeletal phenotype without changes in mineral density, as measured by micro-computed tomography. In enamel organs of Slc26a6-null mice, Dra and pendrin protein levels were both elevated by 52% and 55%, respectively. The amount of Slc26a6 protein was unchanged in enamel organs of Ae2a,b- and Cftr-null mice but reduced in Dra-null mice by 36%. Our data show that ameloblasts express Dra, pendrin, or Slc26a6 but each of these separately is not critical for formation of dental enamel. The data suggest that in ameloblasts, Slc26a isoforms can functionally compensate for one another.

Slc26a3 deficiency is associated with loss of colonic HCO3 (-) secretion, absence of a firm mucus layer and barrier impairment in mice

AIM

Downregulated in adenoma (DRA, Slc26a3) is a member of the solute carrier family 26 (SLC26), family of anion transporters, which is mutated in familial chloride-losing diarrhoea (CLD). Besides Cl(-) -rich diarrhoea, CLD patients also have a higher-than-average incidence of intestinal inflammation. In a search for potential explanations for this clinical finding, we investigated colonic electrolyte transport, the mucus layer and susceptibility against dextran sodium sulphate (DSS)-induced colitis in Slc26a3(-/-) mice.

METHODS

HCO3 (-) secretory (JHCO3 (-) ) and fluid absorptive rates were measured by single-pass perfusion in vivo and in isolated mid-distal colonic mucosa in Ussing chambers in vitro. Colonocyte intracellular pH (pHi ) was assessed fluorometrically, the mucus layer by immunohistochemistry and colitis susceptibility by the addition of DSS to the drinking water.

RESULTS

HCO3 (-) secretory (JHCO3- ) and fluid absorptive rates were strongly reduced in Slc26a3(-/-) mice compared to wild-type (WT) littermates. Despite an increase in sodium/hydrogen exchanger 3 (NHE3) mRNA and protein expression, and intact acid-activation of NHE3, the high colonocyte pH in Slc26a3(-/-) mice prevented Na(+) /H(+) exchange-mediated fluid absorption in vivo. Mucin 2 (MUC2) immunohistochemistry revealed the absence of a firm mucus layer, implying that alkaline secretion and/or an absorptive flux may be necessary for optimal mucus gel formation. Slc26a3(-/-) mice were highly susceptible to DSS damage.

CONCLUSIONS

Deletion of DRA results in severely reduced colonic HCO3 (-) secretory rate, a loss of colonic fluid absorption, a lack of a firmly adherent mucus layer and a severely reduced colonic mucosal resistance to DSS damage. These data provide potential pathophysiological explanations for the increased susceptibility of CLD patients to intestinal inflammation.

The PDZ-interaction of the intestinal anion exchanger downregulated in adenoma (DRA; SLC26A3) facilitates its movement into Rab11a-positive recycling endosomes

Electroneutral NaCl absorption in the ileum and colon is mediated by downregulated in adenoma (DRA) (Cl⁻/HCO₃⁻ exchanger; SLC26A3) and Na⁺/H⁺ exchanger 3 (NHE3, SLC9A3). Surface expression of transport proteins undergoes basal and regulated recycling by endo- and exocytosis. Expression and activity of DRA in the plasma membrane depend on intact lipid rafts, phosphatidylinositol 3-kinase (PI3-kinase), and the PDZ interaction of DRA. However, it is unknown how the PDZ interaction of DRA affects its trafficking to the cell surface. Therefore, the (re)cycling pathway of DRA was investigated in HEK cells stably expressing enhanced green fluorescent protein (EGFP)-DRA or EGFP-DRA-ETKFminus (a mutant lacking the PDZ interaction motif). Early, late, and recycling endosomes were immunoisolated by precipitating stably transfected mCherry-hemagglutinin (HA)-Rab5a, -7a, or -11a. EGFP-DRA and EGFP-DRA-ETKFminus were equally present in early endosomes. In recycling endosomes, wild-type DRA was preferentially present, whereas, in late endosomes, DRA-ETKF-minus dominated. Correspondingly, EGFP-DRA colocalized with mCherry-HA-Rab11a in recycling endosomes, whereas EGFP-DRA-ETKFminus colocalized with mCherry-HA-Rab7a in late endosomes. Functionally, this different distribution was reflected by a shorter half-life of the mutant DRA. Transient expression of dominant-negative Rab11a(S25N) inhibited the activity (-17%, P < 0.05) and the cell surface expression of DRA (-30%, P < 0.05). Transient transfection of Rab4a or its dominant-negative mutant Rab4a(S22N) was without effect and thus excluded participation of the rapid recycling pathway. Taken together, the PDZ interaction of DRA facilitates its movement into Rab11a-positive recycling endosomes, from where it is inserted in the plasma membrane. A scenario emerges where specific PDZ adaptor proteins are present along several compartments of the endocytosis-recycling pathway.

Alterations in the proteome of the NHERF2 knockout mouse jejunal brush border membrane vesicles

To identify additional potential functions for the multi-PDZ domain containing protein Na+/H+ exchanger regulatory factor 2 (NHERF2), which is present in the apical domain of intestinal epithelial cells, proteomic studies of mouse jejunal villus epithelial cell brush border membrane vesicles compared wild-type to homozygous NHERF2 knockout FVB mice by a two-dimensional liquid chromatography-tandem mass spectrometry (LC-MS/MS)-iTRAQ approach. Jejunal architecture appeared normal in NHERF2 null in terms of villus length and crypt depth, Paneth cell number, and microvillus structure by electron microscopy. There was also no change in proliferative activity based on BrdU labeling. Four brush border membrane vesicles (BBMV) preparations from wild-type mouse jejunum were compared with four preparations from NHERF2 knockout mice. LC-MS/MS identified 450 proteins in both matched wild-type and NHERF2 null BBMV; 13 proteins were changed in two or more separate BBMV preparations (9 increased and 4 decreased in NHERF2 null mice), while an additional 92 proteins were changed in a single BBMV preparation (68 increased and 24 decreased in NHERF2 null mice). These proteins were categorized as 1) transport proteins (one increased and two decreased in NHERF2 null); 2) signaling molecules (2 increased in NHERF2 null); 3) cytoskeleton/junctional proteins (4 upregulated and 1 downregulated in NHERF2 null); and 4) metabolic proteins/intrinsic BB proteins) (2 upregulated and 1 downregulated in NHERF2 null). Immunoblotting of BBMV was used to validate or extend the findings, demonstrating increase in BBMV of NHERF2 null of MCT1, coronin 3, and ezrin. The proteome of the NHERF2 null mouse small intestinal BB demonstrates up- and downregulation of multiple transport proteins, signaling molecules, cytoskeletal proteins, tight junctional and adherens junction proteins, and proteins involved in metabolism, suggesting involvement of NHERF2 in multiple apical regulatory processes and interactions with luminal contents.

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.

Recent advances in the molecular and functional characterization of acid/base and electrolyte transporters in the basolateral membranes of gastric and duodenal epithelial cells

All segments of the gastrointestinal tract are comprised of an elaborately folded epithelium that expresses a variety of cell types and performs multiple secretory and absorptive functions. While the apical membrane expresses the electrolyte transporters that secrete or absorb electrolytes and water, basolateral transporters regulate the secretory or absorptive rates. During gastric acid formation, Cl⁻/HCO₃⁻ and Na(+) /H(+) exchange and other transporters secure Cl⁻ re-supply as well as pH and volume regulation. Gastric surface cells utilize ion transporters to secrete HCO₃⁻, maintain pH(i) during a luminal acid load and repair damaged surface areas during the process of epithelial restitution. Na(+)/H(+) exchange and Na(+)/HCO₃⁻ cotransport serve basolateral acid/base import for gastroduodenal HCO₃⁻ secretion. The gastric and duodenal epithelium also absorbs salt and water. Recent molecular information on novel ion transporters expressed in the gastric and duodenal epithelium has exploded; however, a function has not been found yet for all transporters. The purpose of this review is to summarize current knowledge on the molecular identity and cellular function of basolateral ion transporters in the gastric and duodenal epithelium.

Motility-induced but not vasoactive intestinal peptide-induced increase in luminal alkalinization in rat duodenum is dependent on luminal Cl(-)

AIM

to investigate whether the motility- and the vasoactive intestinal peptide (VIP)-induced increase in luminal alkalinization in the duodenum is dependent on luminal Cl(-).

METHODS

  experiments were performed in anaesthetized rats in vivo. The proximal duodenum was perfused luminally with an isotonic solution, containing zero or low Cl(-) and the effects on luminal alkalinization, motility, fluid flux and epithelial permeability were determined. Parecoxib, a COX-2 inhibitor, was used to induce duodenal contractions.

RESULTS

control rats lacked duodenal wall contractions while parecoxib-treated ones exhibited contractions throughout the experiment. Most animals had a net fluid absorption during the perfusion with isotonic NaCl. Luminal alkalinization was about 100% higher in parecoxib-treated rats than in controls. Cl(-) -free solutions did not affect epithelial permeability or motility but decreased luminal alkalinization by ≥50% and decreased net fluid absorption in both control and parecoxib-treated animals. Reduction in luminal Cl(-) decreased alkalinization in a concentration-dependent manner. The parecoxib-induced increase in alkalinization was markedly reduced in the absence of luminal Cl(-) . VIP increased luminal alkalinization and induced fluid secretion. The lack of luminal Cl(-) did not affect the VIP-induced increase in alkalinization but reduced fluid secretion.

CONCLUSIONS

the parecoxib-induced increase in luminal alkalinization is highly dependent on luminal Cl(-) and it is proposed that COX-2 inhibition, via induction of duodenal motility, enhances HCO(3) (-) efflux through stimulation of apical Cl(-) /HCO(3) (-) exchange in duodenal epithelial cells. Although the VIP-induced stimulation of fluid secretion is partly dependent on luminal Cl(-) , the VIP-induced increase in luminal alkalinization is not.

Alterations in the proteome of the NHERF1 knockout mouse jejunal brush border membrane vesicles

Na/H exchanger regulatory factor 1 (NHERF1) is a scaffold protein made up of two PDZ domains and an ERM binding domain. It is in the brush border of multiple epithelial cells where it modulates 1) Na absorption by regulating NHE3 complexes and cytoskeletal association, 2) Cl secretion through trafficking of CFTR, and 3) Na-coupled phosphate absorption through membrane retention of NaPi2a. To further understand the role of NHERF1 in regulation of small intestinal Na absorptive cell function, with emphasis on apical membrane transport regulation, quantitative proteomic analysis was performed on brush border membrane vesicles (BBMV) prepared from wild-type (WT) and homozygous NHERF1 knockout mouse jejunal villus Na absorptive cells. Jejunal architecture appeared normal in NHERF1 null; however, there was increased proliferative activity, as indicated by increased crypt BrdU staining. LC-MS/MS analysis using iTRAQ to compare WT and NHERF1 null BBMV identified 463 proteins present in both WT and NHERF1 null BBMV of simultaneously prepared and studied samples. Seventeen proteins had an altered amount of expression between WT and NHERF1 null in two or more separate preparations, and 149 total proteins were altered in at least one BBMV preparation. The classes of the majority of proteins altered included transport proteins, signaling and trafficking proteins, and proteins involved in proliferation and cell division. Affected proteins also included tight junction and adherens junction proteins, cytoskeletal proteins, as well as metabolic and BB digestive enzymes. Changes in abundance of several proteins were confirmed by immunoblotting [increased CEACAM1, decreased ezrin (p-ezrin), NHERF3, PLCβ3, E-cadherin, p120, β-catenin]. The changes in the jejunal BBMV proteome of NHERF1 null mice are consistent with a more complex role of NHERF1 than just forming signaling complexes and anchoring proteins to the apical membrane and include at least alterations in proteins involved in transport, signaling, and proliferation.

Do we need gastric acid?

Evidence from comparative anatomy and physiology studies indicates that gastric acid secretion developed during the evolution of vertebrates approximately 350 million years ago. The cellular mechanisms that produce gastric acid have been conserved over the millennia and therefore proton pump inhibitors have pharmacological effects in almost all relevant species. These observations suggest that gastric acid provides an important selective advantage; however, in modern-day humans the need for gastric acid can be questioned in light of the widespread use of safe and effective pharmacologic acid suppression. The Kandahar Working Group addressed questions concerning the need, production and effects of gastric acid, specifically: (1) motility in the upper gastrointestinal (GI) tract; (2) neuroendocrine factors; (3) digestive and mucosal processes; (4) microbiology, and (5) central processes and psychological involvement. We addressed each topic with the individual models available to answer our questions including animal versus human studies, pharmacologic, surgical as well as pathophysiologic states of acid suppression.

Defective jejunal and colonic salt absorption and alteredNa(+)/H (+) exchanger 3 (NHE3) activity in NHE regulatory factor 1 (NHERF1) adaptor protein-deficient mice

We investigated the role of the Na(+)/H(+) exchanger regulatory factor 1 (NHERF1) on intestinal salt and water absorption, brush border membrane (BBM) morphology, and on the NHE3 mRNA expression, protein abundance, and transport activity in the murine intestine. NHERF1-deficient mice displayed reduced jejunal fluid absorption in vivo, as well as an attenuated in vitro Na(+) absorption in isolated jejunal and colonic, but not of ileal, mucosa. However, cAMP-mediated inhibition of both parameters remained intact. Acid-activated NHE3 transport rate was reduced in surface colonocytes, while its inhibition by cAMP and cGMP was normal. Immunodetection of NHE3 revealed normal NHE3 localization in the BBM of NHERF1 null mice, but NHE3 abundance, as measured by Western blot, was significantly reduced in isolated BBM from the small and large intestines. Furthermore, the microvilli in the proximal colon, but not in the small intestine, were significantly shorter in NHERF1 null mice. Additional knockout of PDZK1 (NHERF3), another member of the NHERF family of adaptor proteins, which binds to both NHE3 and NHERF1, further reduced basal NHE3 activity and caused complete loss of cAMP-mediated NHE3 inhibition. An activator of the exchange protein activated by cAMP (EPAC) had no effect on jejunal fluid absorption in vivo, but slightly inhibited NHE3 activity in surface colonocytes in vitro. In conclusion, NHERF1 has segment-specific effects on intestinal salt absorption, NHE3 transport rates, and NHE3 membrane abundance without affecting mRNA levels. However, unlike PDZK1, NHERF1 is not required for NHE3 regulation by cyclic nucleotides.

CFTR and its key role in in vivo resting and luminal acid-induced duodenal HCO3- secretion

BACKGROUND AND AIMS

We investigated the role of the recently discovered, villous-expressed anion exchanger Slc26a6 (PAT1) and the predominantly crypt-expressed cystic fibrosis transmembrane regulator (CFTR) in basal and acid-stimulated murine duodenal HCO(3)(-) secretion in vivo, and the influence of blood HCO(3)(-) concentration on both.

METHODS

The proximal duodenum of anaesthetized mice was perfused in situ, and HCO(3)(-) secretion was determined by back-titration. Duodenal mucosal permeability was assessed by determining (51)Cr-EDTA leakage from blood to lumen.

RESULTS

Compared with wild type (WT) littermates basal duodenal HCO(3)(-) secretory rates were slightly reduced in Slc26-deficient mice at low ( approximately 21 mm), and markedly reduced at high blood HCO(3)(-) concentration ( approximately 29 mm). In contrast, basal HCO(3)(-) secretion was markedly reduced in CFTR-deficient mice compared with WT littermates both at high and low blood HCO(3)(-) concentration. A short-term application of luminal acid increased duodenal HCO(3)(-) secretory rate in Slc26a6-deficient and WT mice to the same degree, but had no stimulatory effect in the absence of CFTR. Luminal acidification to pH 2.5 did not alter duodenal permeability.

CONCLUSIONS

The involvement of Slc26a6 in basal HCO(3)(-) secretion in murine duodenum in vivo is critically dependent on the systemic acid/base status, and this transporter is not involved in acid-stimulated HCO(3)(-) secretion. The presence of CFTR is essential for basal and acid-induced HCO(3)(-) secretion irrespective of acid/base status. This suggests a coupled action of Slc26a6 with CFTR for murine basal duodenal HCO(3)(-) secretion, but not acid-stimulated secretion, in vivo.

The emerging role of PDZ adapter proteins for regulation of intestinal ion transport

In the gastrointestinal tract, CFTR, in conjunction with one or several members of the SLC26 anion exchanger family, mediates electrogenic Cl- and HCO3- secretion. Na+/H+ exchanger isoform NHE3, on the other hand, coupled to one or several of the SLC26 isoforms, mediates electroneutral NaCl absorption. The agonist-induced activation of anion secretion and inhibition of salt absorption causes secretory diarrhea. Current dogma sees the formation of a multiprotein complex of transport proteins, postsynaptic density-95/discs large/zonula occludens-1 (PDZ) adapter proteins, anchoring proteins, the cytoskeleton, and the involved protein kinases as one crucial step in the regulation of these transport processes. Data obtained in heterologous expression studies suggest an important role of these PDZ adapter proteins in trafficking, endocytic recycling, and membrane retention of the respective transmembrane proteins. This article reviews recent advances in our understanding of the role of the PDZ adapter proteins NHERF, E3KARP, PDZK1, IKEPP (NHERF-1 to NHERF-4), CAL, and Shank-2 that bind to CFTR, NHE3, and the intestinal SLC26 members in the regulation of intestinal fluid transport. Current concepts are mostly derived from heterologous expression studies and studies on their role in organ physiology are still in infancy. Recently, however, PDZ adapter protein-deficient mice and organ-specific cell lines have become available, and the first results suggest a more cell-type and possibly signal-specific role of these adapter proteins. This opens the potential for drug development targeted to PDZ domain interactions, which is, in theory, one of the most efficient antidiarrheal strategies.

Carbachol increases Na+-HCO3- cotransport activity in murine colonic crypts in a M3-, Ca2+/calmodulin-, and PKC-dependent manner

The Na(+)-HCO(3)(-) cotransporter (NBC) mediates HCO(3)(-) import into the colonocyte via its pNBC1 isoform. Whereas renal kNBC1 is inhibited by increased cAMP levels, pNBC1 is stimulated. Cholinergic stimulation activates renal NBC, but the effect on intestinal NBC is unknown. Therefore, crypts were isolated from the murine proximal colon by Ca(2+) chelation and loaded with the pH-sensitive dye 2',7'-bis-carboxyethyl-5,6-carboxyfluorescein. Na(+)-HCO(3)(-) cotransport activity was calculated from the dimethylamiloride-insensitive (500 microM) intracellular pH recovery from an acid load in the presence of CO(2)-HCO(3)(-) and the intracellular buffering capacity. Carbachol strongly increased Na(+)-HCO(3)(-) cotransport activity compared with control rates. Ca(2+) chelation with BAPTA-AM, blockade of the M(3) subtype of muscarinergic receptors with 4-diphenylacetoxy-N-methylpiperidine methiodide, and inhibition of Ca(2+)/calmodulin kinase II with KN-62 all caused significant inhibition of the carbachol-induced NBC activity increase. Furthermore, PKC inhibition with Gö-6976 and Gö-6850 significantly reduced the carbachol effect, which may be related to the unique NH(2)-terminal consensus site for PKC-dependent phosphorylation of pNBC1. We conclude that NBC in the murine colon is thus activated by carbachol, consistent with its presumed function as an anion uptake pathway during intestinal anion secretion, but that the signal transductions pathways are distinct from those involved in the cholinergic activation of renal NBC1.

Functional role of Na+-HCO3- cotransport in migration of transformed renal epithelial cells

Cell migration is crucial for immune defence, wound healing or formation of tumour metastases. It has been shown that the activity of the Na(+)-H(+) exchanger (NHE1) plays an important role in cell migration. However, so far it is unknown whether Na(+)- HCO(3)(-) cotransport (NBC), which has similar functions in the regulation of intracellular pH (pH(i)) as NHE1, is also involved in cell migration. We therefore isolated NHE-deficient Madin-Darby canine kidney (MDCK-F) cells and tested whether NBC compensates for NHE in pH(i) and cell volume regulation as well as in migration. Intracellular pH was measured with the fluorescent pH indicator 2'7'-bis(carboxyethyl)-5-carboxyfluorescein (BCECF). The expression of NBC isoforms was determined with semiquantitative PCR. Migration was monitored with time-lapse video microscopy and quantified as the displacement of the cell centre. We found that MDCK-F cells express the isoform NBC1 (SLCA4A gene product) at a much higher level than the isoform kNBC3 (SLCA4A8 gene product). This difference is even more pronounced in NHE-deficient cells so that NBC1 is likely to be the major acid extruder in these cells and the major mediator of propionate-induced cell volume increase. NHE-deficient MDCK-F cells migrate more slowly than normal MDCK-F cells. NBC activity promotes migration during an acute intracellular acid load and increases migratory speed and displacement on a short timescale (< 30 min) whereas it has no effect on the long-term behaviour of migrating MDCK-F cells. Taken together, our results show that NBC actvity, despite many functional similarities, does not have the same importance for cell migration as NHE1 activity.

Epithelial transport and barrier function in occludin-deficient mice

BACKGROUND AND AIMS

This study aimed at functional characterization of the tight junction protein occludin using the occludin-deficient mouse model.

METHODS

Epithelial transport and barrier functions were characterized in Ussing chambers. Impedance analysis revealed the ionic permeability of the epithelium (Re, epithelial resistance). Conductance scanning differentiated transcellular (Gc) and tight junctional conductance (Gtj). The pH-stat technique quantified gastric acid secretion.

RESULTS

In occludin+/+ mice, Re was 23+/-5 Omega cm2 in jejunum, 66+/-5 Omega cm2 in distal colon and 33+/-6 Omega cm2 in gastric corpus and was not altered in heterozygotic occludin+/- or homozygotic occludin-/- mice. Additionally, [3H]mannitol fluxes were unaltered. In the control colon, Gc and Gtj were 7.6+/-1.0 and 0.3+/-0.1 mS/cm2 and not different in occludin deficiency. Epithelial resistance after mechanical perturbation or EGTA exposition (low calcium switch) was not more affected in occludin-/- mice than in control. Barrier function was measured in the urinary bladder, a tight epithelium, and in the stomach. Control Rt was 5.8+/-0.8 kOmega cm2 in urinary bladder and 33+/-6 Omega cm2 in stomach and not altered in occludin-/- mice. In gastric corpus mucosa, the glandular structure exhibited a complete loss of parietal cells and mucus cell hyperplasia, as a result of which acid secretion was virtually abolished in occludin-/- mice.

CONCLUSION

Epithelial barrier characterization in occludin-deficiency points against an essential barrier function of occludin within the tight junction strands or to a substitutional redundancy of single tight junction molecules like occludin. A dramatic change in gastric morphology and secretory function indicates that occludin is involved in gastric epithelial differentiation.

The Na+/H+ exchanger isoform 2 is the predominant NHE isoform in murine colonic crypts and its lack causes NHE3 upregulation

The Na(+)/H(+) exchanger isoform NHE2 is highly expressed in the intestinal tract, but its physiological role has remained obscure. The aim of this study was to define its expression, location, and regulatory properties in murine colon and to look for the compensatory changes in NHE2 (-/-) colon that allow normal histology and absorptive function. To this end, we measured murine proximal colonic surface and crypt cell NHE1, NHE2, and NHE3 expression levels, transport rates in response to acid, hyperosmolarity and cAMP in murine proximal colonic crypts, as well as changes in transcript levels and acid-activated NHE activity in NHE2 (-/-) crypts. We found that NHE2 was expressed most abundantly in crypts, NHE1 equally in crypts and surface cells, and NHE3 much stronger in surface cells. NHE2, like NHE1, was activated by low intracellular pH (pH(i)), hyperosmolarity, and cAMP, whereas NHE3 was activated only by low pH(i). Crypts isolated from NHE2 (-/-) mice displayed increased acid-activated NHE1- and NHE3-attributable Na(+)/H(+) exchange activity, no change in NHE1 expression, and NHE3 expression levels twice as high as in normal littermates. No change in cellular ultrastructure was found in NHE2 (-/-) colon. Our results demonstrate high NHE2 expression in the crypts and suggest a role for NHE2 in cryptal pH(i) and volume homeostasis.

Mechanisms of diarrhea in the interleukin-2-deficient mouse model of colonic inflammation

Colitis in interleukin-2-deficient (IL-2(-/-)) mice resembles ulcerative colitis in humans. We studied epithelial transport and barrier function in IL-2(-/-) mice and used this model to characterize mechanisms of diarrhea during intestinal inflammation. (22)Na(+) and (36)Cl(-) fluxes were measured in proximal colon. Net Na(+) flux was reduced from 4.0 +/- 0.5 to 0.8 +/- 0.5 micromol.h(-1).cm(-2), which was paralleled by diminished mRNA and protein expression of the Na(+)/H(+) exchanger NHE3. Net Cl(-) flux was also decreased from 2.2 +/- 1.6 to -2.7 +/- 0.6 micromol.h(-1).cm(-2), indicating impaired Na(+)-Cl(-) absorption. In distal colon, aldosterone-induced electrogenic Na(+) absorption was 6.1 +/- 0.9 micromol.h(-1).cm(-2) in controls and was abolished in IL-2(-/-) mice. Concomitantly, mRNA expression of beta- and gamma-subunits of the epithelial sodium channel (ENaC) was reduced. Epithelial barrier was studied in proximal colon by impedance technique and mannitol fluxes. In contrast to ulcerative colitis, epithelial resistance was increased and mannitol fluxes were decreased in IL-2(-/-) mice. This was in accord with the findings of reduced ion transport as well as increased expression of tight junction proteins occludin and claudin-1, -2, -3, and -5. In conclusion, the IL-2(-/-) mucosa exhibits impaired electroneutral Na(+)-Cl(-) absorption and electrogenic Na(+) transport due to reduced mRNA and protein expression of NHE3 and ENaC beta- and gamma-subunit mRNA. This represents a model of early intestinal inflammation with absorptive dysfunction due to impaired transport protein expression/function while epithelial barrier is still intact. Therefore, this model is ideal to study regulation of transporter expression independent of barrier defects.

Independence of apical Cl-/HCO3- exchange and anion conductance in duodenal HCO3- secretion

Reduced gastrointestinal HCO3- secretion contributes to malabsorption and obstructive syndromes in cystic fibrosis. The apical HCO3- transport pathways in these organs have not been defined. We therefore assessed the involvement of apical Cl-/HCO3- exchangers and anion conductances in basal and cAMP-stimulated duodenal HCO3- secretion. Muscle-stripped rat and rabbit proximal duodena were mounted in Ussing chambers, and electrical parameters, HCO3- secretion rates, and 36Cl-, 22Na+, and 3H+ mannitol fluxes were assessed. mRNA expression levels were measured by a quantitative PCR technique. Removal of Cl- from or addition of 1 mM DIDS to the luminal perfusate markedly decreased basal HCO3- secretion but did not influence the HCO3- secretory response to 8-bromo-cAMP, which was inhibited by luminal 5-nitro-2-(3-phenylpropylamino)-benzoate. Bidirectional 22Na+ and 36Cl- flux measurements demonstrated an inhibition rather than a stimulation of apical anion exchange during cAMP-stimulated HCO3- secretion. The ratio of Cl- to HCO3- in the anion secretory response was compatible with both Cl- and HCO3- being secreted via the CFTR anion channel. CFTR expression was very high in the duodenal mucosa of both species. We conclude that in rat and rabbit duodena, an apical Cl-/HCO3- exchanger mediates a significant part of basal HCO3- secretion but is not involved in the HCO3- secretory response to cAMP analogs. The inhibitor profile, the strong predominance of Cl- over HCO3- in the anion secretory response, and the high duodenal CFTR expression levels suggest that a major portion of cAMP-stimulated duodenal HCO3- secretion is directly mediated by CFTR.

Expression and regulation of the Na+-K+-2Cl- cotransporter NKCC1 in the normal and CFTR-deficient murine colon

Defective regulation and/or reduced expression of the Na+-K+-2Cl- cotransporter NKCC1 may contribute to the severe secretory defect that is observed in cystic fibrosis, but data concerning the expression and function of NKCC1 in cystic fibrosis transmembrane conductance regulator (CFTR)-deficient cells are equivocal. We therefore investigated NKCC1 mRNA expression, Na+-K+-2Cl- cotransport activity and regulation by cAMP in crypts isolated from the proximal colon of CFTR-containing (CFTR (+/+)) and CFTR-deficient (CFTR (-/-)) mice. mRNA expression levels were determined by semiquantitative PCR, transport rates were measured fluorometrically in 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein acetomethylester (BCECF)-loaded crypts, cytoplasmic volume changes were assessed by confocal microscopy, and [Cl-]i changes were examined by N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide (MQAE) quenching. NKCC1 mRNA expression levels were not significantly reduced in CFTR (-/-) crypts compared to controls. Azosemide-sensitive NH4+ influx (used as a measure of Na+-K+-2Cl- cotransport) was 2.23 +/- 0.72 vs. 1.56 +/- 0.16 mM min-1, and increased by 63.6 % in (+/+) and 87.3 % in (-/-) crypts upon stimulation for 5 min with forskolin. After 20 min of stimulation with forskolin, the Na+-K+-2Cl- cotransport rates in (-/-) and (+/+) crypts were identical. Crypt cross-sectional area and [Cl-]i decreased only in (+/+) crypts upon stimulation. In conclusion, normal NKCC1 expression levels, somewhat reduced Na+-K+-2Cl- cotransport rates, but preserved activation by cAMP were found in colonic crypts from CFTR (-/-) mice, ruling out a severe dysfunction of the Na+-K+-2Cl- cotransporter in the CF intestine. Furthermore, these studies establish the existence of a direct, cell-volume- and [Cl-]i-independent activation of colonic NKCC1 by an increase in intracellular cAMP.

cAMP-mediated regulation of murine intestinal/pancreatic Na+/HCO3- cotransporter subtype pNBC1

Basolateral Na(+)-HCO(3)(-) cotransport is essential for intestinal anion secretion, and indirect evidence suggests that it may be stimulated by a rise of intracellular cAMP. We therefore investigated the expression, activity, and regulation by cAMP of the Na(+)-HCO(3)(-) cotransporter isoforms NBC1 and NBCn1 in isolated murine colonic crypts. Na(+)-HCO(3)(-) transport rates were measured fluorometrically in BCECF-loaded crypts, and mRNA expression levels and localization were determined by semiquantitative PCR and in situ hybridization. Acid-activated Na(+)-HCO(3)(-) cotransport rates were 5.07 +/- 0.7 mM/min and increased by 62% after forskolin stimulation. NBC1 mRNA was more abundant in colonic crypts than in surface cells, and crypts expressed far more NBC1 than NBCn1. To investigate whether the cAMP-induced Na(+)-HCO(3)(-) cotransport activation was secondary to secretion-associated changes in HCO(3)(-) or cell volume, we measured potential forskolin-induced changes in intracellular pH and assessed Na(+)-HCO(3)(-) transport activity in CFTR -/- crypts (in which no forskolin-induced cell shrinkage occurs). We found 30% reduced Na(+)-HCO(3)(-) transport rates in CFTR -/- compared with CFTR +/+ crypts but similar Na(+)-HCO(3)(-) cotransport activation by forskolin. These studies establish the existence of an intracellular HCO(3)(-) concentration- and cell volume-independent activation of colonic NBC by an increase in intracellular cAMP.

Differential expression and regulation of Na(+)/H(+) exchanger isoforms in rabbit parietal and mucous cells

Several Na(+)/H(+) exchanger (NHE) isoforms are expressed in the stomach, and NHE1 and NHE2 knockout mice display gastric mucosal atrophy. This study investigated the cellular distribution of the NHE isoforms NHE1, NHE2, NHE3, and NHE4 in rabbit gastric epithelial cells and their regulation by intracellular pH (pH(i)), hyperosmolarity, and an increase in cAMP. Semiquantitative RT-PCR and Northern blot experiments showed high NHE1 and NHE2 mRNA levels in mucous cells and high NHE4 mRNA levels in parietal and chief cells. Fluorescence optical measurements in cultured rabbit parietal and mucous cells using the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein and NHE isoform-specific inhibitors demonstrated that in both cell types, intracellular acidification activates NHE1 and NHE2, whereas hyperosmolarity activates NHE1 and NHE4. The relative contribution of the different isoforms to pH(i)- and hyperosmolarity-activated Na(+)/H(+) exchange in the different cell types paralleled their relative expression levels. cAMP elevation also stimulated NHE4, whereas an increase in osmolarity above a certain threshold further increased NHE1 and not NHE4 activity. We conclude that in rabbit gastric epithelium, NHE1 and NHE4 regulate cell volume and NHE1 and NHE2 regulate pH(i). The high NHE1 and NHE2 expression levels in mucous cells may reflect their special need for pH(i) regulation during high gastric acidity. NHE4 is likely involved in volume regulation during acid secretion.

Differential expression and regulation of AE2 anion exchanger subtypes in rabbit parietal and mucous cells

1. The anion exchanger isoform 2 (AE2) gene encodes three subtypes (AE2a, b and c), which have different N-termini and tissue distributions. AE2 is expressed at high levels in the stomach, where it is thought to mediate basolateral base exit during acid production. The present study investigated if the three AE2 subtypes are differentially expressed and regulated in different cell types within the gastric mucosa. 2. The cloning strategy to obtain rabbit AE2a, b and c cDNAs combined genomic PCR and RT-PCR based on primers deduced from the rat sequences. Semiquantitative RT-PCR using homologous primers revealed much higher AE2 mRNA expression in rabbit parietal cells (PCs) than in mucous cells (MCs). The subtype expression pattern was AE2b >> AE2c > or = AE2a in PCs and AE2a >AE2b >> AE2c in MCs. Sequence analysis revealed the presence of a highly conserved protein kinase C (PKC) consensus sequence in the AE2a alternative N-terminus. 3. Maximal Cl(-)-HCO(3)(-) exchange rates, measured fluorometrically in BCECF-loaded cultured gastric cells, were much higher in PCs than MCs. PKC activation by phorbol ester stimulated maximal Cl(-)-HCO(3)(-) exchange rates in MCs but not in PCs, whereas forskolin had no effect in each cell type. 4. In summary, rabbit PCs and MCs, which originate from the same gastric stem cell population, display a completely different AE2 subtype expression pattern. Therefore, AE2 subtype expression is not organ specific but cell type specific. The different regulation of anion exchange in parietal and mucous cells suggests that AE2 subtypes may be differentially regulated.

Na+/HCO3- cotransport in normal and cystic fibrosis intestine

In a search for the HCO(3)(-) supply mechanisms to the enterocyte we cloned and sequenced an intestinal subtype of the Na(+)HCO(3)(-) cotransporter isoform I (dNBC1), which turned out to be identical to the pancreatic NBC1 subtype (pNBC1). Within the intestine, we found particularly high NBC1 expression levels in the duodenum and proximal colon. Experiments with stripped rabbit duodenum in Ussing-chambers revealed that Na(+)HCO(3)(-) cotransport (NBC) and CO(2) hydration/Na(+)/H(+) exchange were equally important duodenal HCO(3)(-) supply pathways and were both upregulated during cAMP-mediated secretion. In the proximal colon, however, HCO(3)(-) secretion was low but NBC1 expression even higher than in the duodenum. Ussing-chamber experiments with an NBC-specific inhibitor revealed that NBC, coupled to basolateral Cl(-)/HCO(3)(-) exchange, was an important alternative Cl(-) supply pathway to Na(+)K(+)2Cl(-) cotransport (NKCC) during cAMP-stimulated colonic Cl(-) secretion. To investigate the functional integrity of anion uptake pathways in the absence of cystic fibrosis transmembrane conductance regulator (CFTR), we fluorometrically assessed NBC and NKCC transport rates and cell volume before and during forskolin-stimulation in isolated colonic crypts from normal and CFTR (-/-) mice. Although forskolin stimulation decreased cell volume only in normal, not in CFTR (-/-) crypts, it activated NBC and NKCC to a similar degree in both normal and CFTR (-/-) crypts. We conclude that, depending on the intestinal segment, NBC1 plays an important role in basolateral HCO(3)(-) or Cl(-) uptake. Expression and activation by cAMP is preserved in CFTR (-/-) intestine.

Three 5'-variant mRNAs of anion exchanger AE2 in stomach and intestine of mouse, rabbit, and rat

AE2 is one of three known isoforms of the anion exchanger (AE) gene family. The use of alternative promoters, resulting in a tissue-specific transcript pattern, was reported for all AE genes. Three N-terminal variant AE2 subtypes are described: AE2a, AE2b, and AE2c. Although the basolaterally located parietal cell anion exchanger is known to be an AE2, the molecular identity of the basolateral and apical anion exchangers throughout the gut are still unknown. This article summarizes functional, immunohistochemical, and Western blot data demonstrating the basolateral localization of the gastric and intestinal AE2 in rabbit, mouse, and rat, and showing the AE2 subtype mRNA expression pattern in the stomach and along the intestine of rabbit and mouse: AE2a is expressed in all studied tissues, but most strongly in the colon; AE2b is expressed mainly in the stomach; and AE2c is detected nearly exclusively in the stomach. Further investigation is necessary to characterize the apical anion transport protein involved in NaCl absorption and HCO3- secretion in the gut.

Expression and function of Na+HCO3- cotransporters in the gastrointestinal tract

The stomach, duodenum, colon, and pancreas secrete HCO3- ions into the lumen. Although the importance of HCO3- secretion for the maintenance of mucosal integrity, a normal digestion, and the reabsorption of Cl- has been well established, the molecular nature of the apical and basolateral HCO3- transporting proteins has remained largely unknown. Functional studies have suggested that a Na+HCO3- cotransport system, similar but not identical to the well-characterized Na+HCO3- cotransporter in the basolateral membrane of the kidney proximal tubule, is present in duodenal and colonic enterocytes, pancreatic ducts cells, and gastric cells and involved in HCO3- uptake from the interstitium. This report describes our work towards understanding the molecular nature, cellular origin, and functional relevance of the Na+HCO3- cotransporter(s) in the stomach and intestine and reviews work by others on the function and localization of Na+HCO3- cotransport processes in the gastrointestinal tract.

Role of Na(+)HCO(3)(-) cotransporter NBC1, Na(+)/H(+) exchanger NHE1, and carbonic anhydrase in rabbit duodenal bicarbonate secretion

BACKGROUND & AIMS

HCO(3)(-) supply to the enterocyte is rate limiting for duodenal HCO(3)(-) secretion (J(HCO3-)). This study defines the molecular nature of the major HCO(3)(-) uptake pathways in rabbit duodenocytes and investigates their physiologic significance and regulation during basal and stimulated J(HCO3-).

METHODS & RESULTS

pH gradient-driven (22)Na(+) uptake into duodenal basolateral membrane vesicles was partly HCO(3)(-) dependent, stilbene sensitive, and therefore mediated by Na(+)HCO(3)(-) cotransport, and partly HCO(3)(-) independent, Hoechst 642 sensitive, and therefore mediated by the Na(+)/H(+) exchanger isoform NHE1. Semiquantitative polymerase chain reaction (PCR) revealed high duodenal expression levels for the NBC1 isoform of the Na(+)HCO(3)(-) cotransporter gene family and NHE1. Cloning and comparison of full-length rabbit with human gastrointestinal and kidney NBC1 subtype revealed a conserved protein kinase A consensus sequence in the cytoplasmic N-terminus of the gastrointestinal NBC1. Inhibition of either Na(+)HCO(3)(-) cotransport or carbonic anhydrase reduced ouabain-sensitive J(HCO3-) in in vitro rabbit duodenal mucosae by approximately 50%, but did not affect 8-Br-cAMP-induced DeltaJ(HCO3-), suggesting cAMP-mediated up-regulation of the alternative pathway. However, inhibition of both Na(+)HCO(3)(-) cotransport and either carbonic anhydrase or NHE1 strongly reduced DeltaJ(HCO3-).

CONCLUSIONS

NBC1 and NHE1 are the major base importers in rabbit duodenocytes. Na(+)HCO(3)(-) cotransport and CO(2) hydration/Na(+)/H(+) exchange are equally important pathways for duodenal HCO(3)(-) supply and are up-regulated during cAMP-mediated stimulation.

Agonist-induced cytoplasmic volume changes in cultured rabbit parietal cells

Concomitant Na(+)/H(+) and Cl(-)/HCO(3)(-) exchange activation occurs during stimulation of acid secretion in cultured rabbit parietal cells, possibly related to a necessity for volume regulation during the secretory process. We investigated whether cytoplasmic volume changes occur during secretagogue stimulation of cultured rabbit parietal cells. Cells were loaded with the fluorescent dye calcein, and the calcein concentration within a defined cytoplasmic volume was recorded by confocal microscopy. Forskolin at 10(-5) M, carbachol at 10(-4) M, and hyperosmolarity (400 mosmol) resulted in a rapid increase in the cytoplasmic dye concentration by 21 +/- 6, 9 +/- 4, and 23 +/- 5%, respectively, indicative of cell shrinkage, followed by recovery to baseline within several minutes, indicative of regulatory volume increase (RVI). Depolarization by 5 mM barium resulted in a decrease of the cytoplasmic dye concentration by 10 +/- 2%, indicative of cell swelling, with recovery within 15 min, and completely prevented forskolin- or carbachol-induced cytoplasmic shrinkage. Na(+)/H(+) exchange inhibitors slightly reduced the initial cell shrinkage and significantly slowed the RVI, whereas 100 microM bumetanide had no significant effect on either parameter. We conclude that acid secretagoguges induce a rapid loss of parietal cell cytoplasmic volume, followed by RVI, which is predominantly mediated by Na(+)/H(+) and Cl(-)/HCO(3)(-) exchange.

Ca2+/calmodulin-dependent protein kinase II isoenzymes gamma and delta are both present in H+/K+-ATPase-containing rabbit gastric tubulovesicles

Ca2+/calmodulin-dependent protein kinase II is thought to participate in M3 muscarinic receptor-mediated acid secretion in gastric parietal cells. During acid secretion tubulovesicles carrying H+/K+-ATPase fuse with the apical membrane. We localized Ca2+/calmodulin-dependent protein kinase II from highly purified rabbit gastric tubulovesicles using Ca2+/calmodulin-dependent protein kinase II isoform-specific antibodies, in vitro phosphorylation and pharmacological inhibition of Ca2+/calmodulin-dependent protein kinase II activity by the potent Ca2+/calmodulin-dependent protein kinase II inhibitor KN-62. The presence of Ca2+/calmodulin-dependent protein kinase II in tubulovesicles was shown by immunoblot detection of both Ca2+/calmodulin-dependent protein kinase II-gamma (54 kDa) and Ca2+/calmodulin-dependent protein kinase II-delta (56.5 kDa). The immunoprecipitated Ca2+/calmodulin-dependent protein kinase II from tubulovesicles showed Ca2+/calmodulin-dependent protein kinase activity by phosphorylating autocamtide-II, a specific synthetic Ca2+/calmodulin-dependent protein kinase II substrate. KN-62 inhibited the in vitro autophosphorylation of tubulovesicle-associated Ca2+/calmodulin-dependent protein kinase II (IC50 = 11 nM). During the search for potential Ca2+/calmodulin-dependent protein kinase II substrates we identified different proteins associated with tubulovesicles, such as synaptophysin and beta-tubulin immunoreactivity, which were identified using specific antibodies. These targets are known to participate in intracellular membrane traffic. Ca2+/calmodulin-dependent protein kinase II is thought to play an important role in regulating tubulovesicular motor activity and therefore in acid secretion.

Expression of AE2 anion exchanger in mouse intestine

We have characterized expression of anion exchanger 2 (AE2) mRNA and protein in the mouse intestine. AE2 mRNA abundance was higher in colon than in more proximal segments. AE2a mRNA was more abundant than AE2b mRNA throughout the intestine, and AE2c mRNA was expressed at very low levels. This AE2 mRNA pattern contrasted with that in mouse stomach, in which AE2c > AE2b > AE2a. AE2 polypeptide abundance as detected by immunoblot qualitatively paralleled that of mRNA, whereas AE2 immunostaining exhibited a more continuous decrease in intensity from colon to duodenum. AE2 polypeptide was more abundant in colonic surface cells than in crypts, whereas ileal crypts and villi exhibited similar AE2 abundance. AE2 was also observed in mural and vascular smooth muscle. Localization of AE2 epitopes was restricted to the basolateral membranes of epithelial cells throughout the intestine with three exceptions. Under mild fixation conditions, anti-AE2 amino acids (aa) 109-122 detected nonpolarized immunostaining of ileal enterocytes and of Paneth cell granule membranes. An epitope detected by anti-AE2 aa 1224-1237 was also localized to subapical regions of Brunner's gland ducts of duodenum and upper jejunum. These localization studies will aid in the interpretation of anion exchanger function measured in epithelial sheets, isolated cells, and membrane vesicles.

Na+/HCO3- cotransport and expression of NBC1 and NBC2 in rabbit gastric parietal and mucous cells

BACKGROUND & AIMS

The gastric epithelium protects itself against luminal acid by secreting HCO3--rich fluid into the mucous layer and by HCO3--dependent intracellular pH regulation, but the basolateral HCO3- uptake mechanisms are incompletely characterized. This study examined the expression and functional significance of the Na+/HCO3- cotransporters NBC1 and NBC2 in rabbit gastric epithelial cells.

METHODS

Rabbit NBC1 and NBC2 complementary DNA fragments were cloned and sequenced, and cellular expression levels were assessed by semiquantitative polymerase chain reaction. Na+/HCO3- cotransport activity was measured fluorometrically in cultured rabbit parietal and mucous cells.

RESULTS

NBC1 expression was 4.5-fold lower in the stomach than kidney cortex and 5.5-fold higher in mucous than parietal cells. NBC2 expression in the stomach was much lower than in the eye, approximately 4-fold lower than NBC1 expression in the stomach, and 2.5-fold higher in mucous than parietal cells. The Na+- and HCO3--dependent, dimethylamiloride-insensitive (which at 500 micromol/L completely inhibits all Na+/H+ exchanger isoforms) base influx rates were 4.6 +/- 0.02 and 16.2 +/- 0.04 mmol/L/min in acidified parietal and mucous cells, respectively, and were not significantly different in the absence of Cl-.

CONCLUSIONS

This study shows that NBC1 and NBC2 are expressed in rabbit stomach, with high levels in mucous cells where Na+/HCO3- cotransport is the major base-importing mechanism in the presence of CO2/HCO3-.

Different acid secretagogues activate different Na+/H+ exchanger isoforms in rabbit parietal cells

Rabbit parietal cells express three Na+/H+ exchanger isoforms (NHE1, NHE2, and NHE4). We investigated the effects of carbachol, histamine, and forskolin on Na+/H+ exchange activity and acid formation in cultured rabbit parietal cells and tested the effect of NHE isoform-specific inhibition on agonist-induced Na+/H+ exchange. Carbachol (10(-4) M) was the weakest acid secretagogue but caused the strongest Na+/H+ exchange activation, which was completely blocked by 1 microM HOE-642 (selective for NHE1); histamine (10(-4) M) and forskolin (10(-5) M) were stronger stimulants of [14C]aminopyrine accumulation but weaker stimulants of Na+/H+ exchange activity. HOE-642 (1 microM) reduced forskolin-stimulated Na+/H+ exchange activity by 35%, and 25 microM HOE-642 (inhibits NHE1 and -2) inhibited an additional 13%, but 500 microM dimethyl amiloride (inhibits NHE1, -2, and -4) caused complete inhibition. The presence of 5% CO2-HCO-3 markedly reduced agonist-stimulated H+ efflux rates, suggesting that the anion exchanger is also activated. Hyperosmolarity also activated Na+/H+ exchange. Our data suggest that, in rabbit parietal cells, Ca2+-dependent stimulation causes a selective activation of NHE1, whereas cAMP-dependent stimulation activates NHE1, NHE2, and more strongly NHE4. Because intracellular pH (pHi) did not change in the presence of CO2-HCO-3 and concomitant activation of Na+/H+ and anion exchange is one of the volume regulatory mechanisms, we speculate that the physiological significance of secretagogue-induced Na+/H+ exchange activation may not be related to pHi but to volume regulation during acid secretion.

Effect of inhibitors of Na+/H+-exchange and gastric H+/K+ ATPase on cell volume, intracellular pH and migration of human polymorphonuclear leucocytes

1. Stimulation of chemotaxis of human polymorphonuclear leucocytes (PMNs) with the chemoattractive peptide fMLP (N-formyl-Met-Leu-Phe) is paralleled by profound morphological and metabolic alterations like changes of intracellular pH (pHi) and cell shape. The present study was performed to investigate the interrelation of cell volume (CV) regulatory ion transport, pHi and migration of fMLP stimulated PMNs. 2. Addition of fMLP to PMNs stimulated directed migration in Boyden chamber assays and was accompanied by rapid initial intracellular acidification and cell swelling. 3. Inhibition of the Na+/H+ exchanger suppressed fMLP stimulated cell migration, accelerated the intracellular acidification and inhibited the fMLP-induced cell swelling. 4. Step omission of extracellular Na+ caused intracellular acidification, which was accelerated by subsequent addition of gastric H+/K+ ATPase inhibitor SCH 28080, or by omission of extracellular K+ ions. In addition Na+ removal caused cell swelling, which was further enhanced by fMLP. 5. H+/K+ATPase inhibitors omeprazole and SCH 28080 inhibited stimulated migration and blunted the fMLP-induced increase in CV. 6. Increasing extracellular osmolarity by addition of mannitol to the extracellular solution caused cell shrinkage followed by regulatory volume increase, partially due to activation of the Na+/H+ exchanger. In fMLP-stimulated cells the CV increase was counteracted by simultaneous addition of mannitol. Under these conditions the fMLP stimulated migration was inhibited. 7. The antibacterial activity of PMNs was not modified by Hoe 694 or omeprazole. 8. Western analysis with a monoclonal anti gastric H+/K+ ATPase beta-subunit antibody detected a glycosylated 35 kD core protein in lysates of mouse and human gastric mucosa as well as in human PMNs. 9. The results indicate that fMLP leads to cell swelling of PMNs due to activation of the Na+/H+ exchanger and a K+-dependent H+-extruding mechanism, presumably an H+/K+ ATPase. Inhibition of these ion transporters suppresses the increase in CV and precludes PMNs from stimulated migration.

Pseudotype formation of Moloney murine leukemia virus with Sendai virus glycoprotein F

Mixed infection of cells with both Moloney murine leukemia virus (MoMLV) and related or heterologous viruses produces progeny pseudotype virions bearing the MoMLV genome encapsulated by the envelope of the other virus. In this study, pseudotype formation between MoMLV and the prototype parainfluenza virus Sendai virus (SV) was investigated. We report for the first time that SV infection of MoMLV producer cells results in the formation of MoMLV(SV) pseudotypes, which display a largely extended host range compared to that of MoMLV particles. This could be associated with SV hemagglutinin-neuraminidase (SV-HN) glycoprotein incorporation into MoMLV envelopes. In contrast, solitary incorporation of the other SV glycoprotein, SV fusion protein (SV-F), resulted in a distinct and narrow extension of the MoMLV host range to asialoglycoprotein receptor (ASGP-R)-positive cells (e.g., cultured human hepatoma cells). Since stably ASGP-R cDNA-transfected MDCK cells, but not parental ASGP-R-negative MDCK cells, were found to be transduced by MoMLV(SV-F) pseudotypes and transduction of ASGP-R-expressing cells was found to be inhibited by ASGP-R antiserum, a direct proof for the ASGP-R-restricted tropism of MoMLV(SV-F) pseudotypes was provided. Cultivation of ASGP-R-positive HepG2 hepatoma cells on Transwell-COL membranes led to a significant enhancement of MoMLV(SV-F) titers in subsequent flowthrough transduction experiments, thereby suggesting the importance of ASGP-R accessibility at the basolateral domain for MoMLV(SV-F) pseudotype transduction. The availability of such ASGP-R-restricted MoMLV(SV-F)-pseudotyped vectors opens up new perspectives for future liver-restricted therapeutic gene transfer applications.

A functional CFTR protein is required for mouse intestinal cAMP-, cGMP- and Ca(2+)-dependent HCO3- secretion

1. Most segments of the gastrointestinal tract secrete HCO3-, but the molecular nature of the secretory mechanisms has not been identified. We had previously speculated that the regulator for intestinal electrogenic HCO3- secretion is the cystic fibrosis transmembrane regulator (CFTR) channel. To prove this hypothesis, we have now measured HCO3- secretion by pH-stat titration, and recorded the electrical parameters of in vitro duodenum, jejunum and ileum of mice deficient in the gene for the CFTR protein ('CF-mice') and their normal littermates. 2. Basal HCO3- secretory rates were reduced in all small intestinal segments of CF mice. Forskolin, PGE2, 8-bromo-cAMP and VIP (cAMP-dependent agonists), heat-stable enterotoxin of Escherichia coli (STa), guanylin and 8-bromo-cGMP (cGMP-dependent agonists) and carbachol (Ca2+ dependent) stimulated both the short-circuit current (Isc) and the HCO3- secretory rate (JHCO3-) in all intestinal segments in normal mice, whereas none of these agonists had any effect on JHCO3- in the intestine of CF mice. 3. To investigate whether Cl(-)-HCO3- exchangers, which have been implicated in mediating the response to some of these agonists in the intestine, were similarly active in the small intestine of normal and CF mice, we studied Cl- gradient-driven 36Cl- uptake into brush-border membrane (BBM) vesicles isolated from normal and CF mouse small intestine. Both the time course and the peak value for 4,4'-diisothiocyanostilbene-2',2-disulphonic acid (DIDS)-inhibited 36Cl- uptake was similar in normal and CF mice BBM vesicles. 4. In summary, the results demonstrate that the presence of the CFTR channel is necessary for agonist-induced stimulation of electrogenic HCO3- secretion in all segments of the small intestine, and all three intracellular signal transduction pathways stimulate HCO3- secretion exclusively via activation of the CFTR channel.

Intestinal secretory defects and dwarfism in mice lacking cGMP-dependent protein kinase II

Cyclic guanosine 3',5'-monophosphate (cGMP)-dependent protein kinases (cGKs) mediate cellular signaling induced by nitric oxide and cGMP. Mice deficient in the type II cGK were resistant to Escherichia coli STa, an enterotoxin that stimulates cGMP accumulation and intestinal fluid secretion. The cGKII-deficient mice also developed dwarfism that was caused by a severe defect in endochondral ossification at the growth plates. These results indicate that cGKII plays a central role in diverse physiological processes.

Guanylin strongly stimulates rat duodenal HCO3- secretion: proposed mechanism and comparison with other secretagogues

BACKGROUND & AIMS

Guanylin and heat-stable enterotoxin (STa) stimulate intestinal Cl- secretion via activation of the cystic fibrosis transmembrane regulator (CFTR)-encoded Cl- channel. It was speculated that CFTR activation also regulates electrogenic duodenal HCO3- secretion. Therefore, the effect of guanylin/STa and other secretagogues on rat duodenal HCO3- secretion was studied.

METHODS

The HCO3- secretory rate of in vitro rat proximal duodenum was determined by pH stat titration and paracellular permeability by 3H-mannitol fluxes, bidirectional 36Cl- fluxes were measured, and the short-circuit current (Isc) was recorded.

RESULTS

Luminal guanylin and STa concentration dependently stimulated the HCO3- secretory rate and Isc. Guanylin-stimulated HCO3- secretion was independent of luminal Cl-, inhibited by the Cl- channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoate, and additive to the HCO3- secretory rate stimulated by glucagon and carbachol but not by the tested adenosine 3',5'-cyclic monophosphate (cAMP)-dependent agonists. The ratio of the HCO3- secretory rate/Isc stimulated by the tested guanosine 3',5'-cyclic monophosphate (cGMP)-dependent agonists was markedly higher than the cAMP-dependent agonists. Prostaglandin E2 and 8-bromo-cAMP but not STa/guanylin also transiently increased paracellular permeability.

CONCLUSIONS

Guanylin and STa stimulate electrogenic HCO3- secretion in rat duodenum, most likely via CFTR Cl- channel activation, but the different relationship for HCO3- to Isc in cGMP-than in cAMP-stimulated anion secretion suggests a different cellular source and/or signaling pathways.

Interstitial buffer capacity influences Na+/H+ exchange kinetics and oxyntic cell pHi in intact frog gastric mucosa

We studied the influence of interstitial buffer capacity and CO2-HCO3- on oxyntic cell intracellular pH (pHi) in intact frog gastric mucosa. Oxyntic cells in stripped gastric mucosa of Rana esculenta were loaded with the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, and pHi was assessed fluorometrically. In the presence of a constant serosal and luminal pH, oxyntic cell pHi was dependent on the serosal but not the luminal concentration of cell-impermeable buffer ions such as N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES), tris(hydroxymethyl)aminomethane, N-tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid, and 3-(N-morpholino)propanesulfonic acid. The stepwise increase in oxyntic cell pHi from 6.74 +/- 0.05 with 1 mM HEPES to 7.23 +/- 0.08 was almost completely inhibited by removal of serosal Na+ and by amiloride and dimethyl amiloride, suggesting that it was largely due to Na+/H+ exchange. Increasing the serosal concentration of a CO2-HCO3- buffer from 1% CO2-4 mM HCO3- to 10% CO2-40 mM HCO3- increased oxyntic cell pHi from 7.03 +/- 0.06 to 7.39 +/- 0.07. This CO2-HCO(3-)-dependent pHi increase was also Na+ and amiloride sensitive, but high HCO3- concentrations increased pHi even in the absence of Na+, K+, or Cl-, and in the presence of omeprazole, bafilomycin A1, or acetazolamide. We suggest that in intact frog gastric mucosa, cellular proton extrusion acidifies the interstitial pH immediately adjacent to the basolateral membrane of the oxyntic cells in the absence of a high interstitial buffer capacity.(ABSTRACT TRUNCATED AT 250 WORDS)

Different regulation by pHi and osmolarity of the rabbit ileum brush-border and parietal cell basolateral anion exchanger

1. The purpose of this study was to look for evidence of a pH-sensitive modifier site on the parietal cell basolateral anion exchanger, determine the pH range in which allosteric regulation takes place, investigate the effect of the osmolarity on internal pH (pHi) dependence and compare it with that of the ileum brush-border anion exchanger. 2. When the pHi in parietal cell basolateral membrane (BLM) vesicles was increased, the rate of Cl(-)-gradient-driven 36Cl- uptake increased from 6.03 +/- 2.24 to 38.09 +/- 3.33 nmol (mg protein)-1 with the steep increase in anion exchange rates occurring within a narrow pH range between pHi 7.0 and 7.5. This was due to allosteric activation by internal OH- and not due to a change in driving force, since the driving force for maximal exchange rates was provided by the outwardly directed Cl- gradient. 3. The pHi dependency curve of parietal cell BLM anion exchange rates was shifted to the left by 0.25 pH units by increasing the osmolarity of the intra- and extravesicular solutions from 300 to 380 mosmol l-1. Thus cell shrinking may activate the parietal cell anion exchanger without a change in pHi and without phosphorylation of the anion exchanger protein. 4. In ileum brush-border membranes, the pHi-dependent increase in the rate of Cl(-)-gradient-driven 36Cl- uptake was more gradual and the half-maximal anion exchange rate was attained at lower pHi (pH 6.5). Increasing the osmolarity from 300 to 500 mosmol l-1 had no effect on pH dependence. 5. We conclude that the parietal cell basolateral and ileum brush-border anion exchangers possess an internal modifier site for allosteric activation by OH-, but the pH range in which allosteric regulation occurs differs between the two exchangers, as does the effect of an increase in osmolarity. Since current evidence suggests that both the parietal cell basolateral and the ileum brush-border anion exchanger are encoded by the AE2 gene, the differences in pHi dependence between the two may be due to alternative splicing, post-transcriptional modification, or the different membrane environment. 6. The pHi range for allosteric activation found in this study would suggest that for both the ileum and the parietal cell anion exchanger, but especially for the latter, a potentiating effect of the allosteric activation and the HCO3- availability occurs within the physiological pHi range and can cause dramatic increases in maximal anion exchange rates with increasing pHi.

pHi and HCO3- dependence of proton extrusion and Cl(-)-base exchange rates in isolated rabbit parietal cells

In many cell types, the regulation of intracellular pH (pHi) is different in the presence vs. absence of HCO3-. We investigated the pHi and HCO3- dependence of proton extrusion and anion exchange rates in isolated rabbit parietal cells loaded with the pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). In Cl(-)-depleted parietal cells, the dependence of maximal proton efflux rate on pHi showed a strong inverse correlation but was identical in the presence and absence of HCO3-. Amiloride and Na+ removal inhibited proton extrusion rates to a similar degree with or without HCO3-, whereas 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) had no effect. This suggests that a Na(+)-H+ exchanger is the major acid extrusion ion transporter under these experimental conditions. In Cl(-)-containing cells, there was also some Na(+)-independent, extracellular HCO(3-)- and intracellular Cl(-)-dependent, DIDS-inhibitable pHi recovery from an acid load, most likely due to intracellular Cl(-)-extracellular HCO3- exchange. Recovery from an alkaline load was primarily mediated by anion exchange, and the dependence of maximal anion exchange rates on pHi was very different in the absence and presence of HCO3-. In its absence, maximal anion exchange (Cl(-)-OH-) rates increased slowly over the tested pHi range from 6.4 to 7.8. In the presence of HCO3-, however, there was an S-shaped dependence of maximal flux rates on pHi, with a steep increase in flux rates between 6.8 and 7.5.(ABSTRACT TRUNCATED AT 250 WORDS)