The conversion of primary bile acids to secondary bile acids by the gut microbiota has been implicated in colonic inflammation. This study investigated the role of gut microbiota related bile acid metabolism in colonic inflammation in both patients with inflammatory bowel disease (IBD) and a murine model of dextran sulfate sodium (DSS)-induced colitis. Bile acids in fecal samples from patients with IBD and DSS-induced colitis mice, with and without antibiotic treatment, were analyzed using ultraperformance liquid chromatography-mass spectrometry (UPLC-MS). The composition of the microbiota in fecal samples from IBD patients and DSS-colitis mice was characterized via Illumina MiSeq sequencing of the bacterial 16S rRNA gene V3-V4 region. Metagenomic profiling further identified metabolism-related gene signatures in stool samples from DSS-colitis mice. Histological analysis, quantitative PCR (qPCR) and Western Blotting were conducted on colonic samples from DSS-induced colitis mice to assess colonic inflammation, mucosal barrier integrity, and associated signaling pathways. The multivariate analysis of bile acids was conducted using Soft Independent Modelling of Class Analogy (SIMCA, Umetrics, Sweden). The relation between the relative abundance of specific phyla/genera and bile acid concentration was assess through Spearman's correlation analyses. Finally, lithocholic acid (LCA), the key bile acid, was administered via gavage to evaluate its effect on colonic inflammation and mucosal barrier integrity. In patients with IBD, the composition of colonic bile acids and gut microbiota was altered. Moreover, changes in the gut microbiota further modulate the composition of bile acids in the intestine. As the gut microbiota continues to shift, the bile acid profile undergoes additional alterations. The aforementioned alterations were also observed in mice with DSS-induced colitis. The study revealed a correlation between dysbiosis of the gut microbiota and modifications in the profile of colonic bile acids, notably LCA observed in both patients with IBD and mice with DSS-induced colitis. Through multivariate analysis, LCA was identified as the key bile acid that significantly affects colonic inflammation and the integrity of mucosal barrier. Subsequent experiments confirmed that LCA supplementation effectively mitigated the inhibitory effects of gut microbiota on colitis progression in mice, primarily through the activation of the sphingosine-1-phosphate receptor 2 (S1PR2)/NF-κB p65 signaling pathway. Analysis of the microbiome and metagenomic data revealed changes in the gut microbiota, notably an increased abundance of an unclassified genus within the family Prevotellaceae in DSS-induced colitis mice. Furthermore, a positive correlation was observed between the relative abundance of Prevotellaceae and bile acid biosynthesis pathways, as well as colonic LCA level. These findings suggest that LCA and its positively correlated gut bacteria, Prevotellaceae, are closely associated with intestinal inflammation. Targeting colonic inflammation may involve inhibiting LCA and members of the Prevotellaceae family as potential therapeutic strategies.

The gastrointestinal Na+/H+ transporter 8 (NHE8) is downregulated in the mucosa of patients with ulcerative colitis, and its deletion in the murine intestine causes a "colitis-like" phenotype. Since ulcerative colitis is characterized by repeated mucosal injury, we investigated the role of NHE8 in intestinal wound repair, by accessing its effect on intracellular pH (pHi) regulation, cell proliferation, and migration. NHE8 expression was downregulated via shRNA lentiviral transduction in HT29MTX cells. The selected clonal cell line (HT29/shNHE8) with ∼80% reduced NHE8 mRNA expression displayed an increased proliferative but reduced migratory rate compared with the mock-transduced cells (HT29/pLKO1). The wound front of the HT29/shNHE8 cells consisted of both migrating and nonmigrating cells, and pHi measured in this segment displayed the following values: pHi(HT29/pLKO.1) = 7.35, pHi(HT29/shNHE8migrating) = 7.27, and pHi(HT29/shNHE8nonmigrating) = 7.1. The migrating HT29/shNHE8 cells exhibited a significantly increased NHE activity compared with migrating mock-transfected and nonmigrating cells, which was abolished by pharmacological NHE3 inhibition. NHE3 localized to the wound front of HT29/shNHE8, but not to that of HT29/pLKO.1 cells. Cell flattening and lamellipodia development were observed at the wound front in HT29/pLKO.1 cells, whereas the HT29/shNHE8 cells formed tight actin bundles and retained their apical-basal architecture. RAC1 and Cofilin-1, required for the generation of actin-based membrane protrusions, were absent at the wound front of HT29/shNHE8 cells but were expressed in migrating HT29/pLKO.1 cells, where RAC1 partially colocalized with NHE8. The results show that NHE8 downregulation reduces the pHi and leads to enhanced epithelial cell proliferation, but impairs migration likely due to altered actin polymerization.NEW & NOTEWORTHY The Na+/H+ exchanger 8 (NHE8) plays an important role in the regulation of intracellular pHi, cell proliferation, and epithelial sheet migration in HT29MTX intestinal cells. NHE8 knockdown cells lack the ability to dynamically rearrange the actin cytoskeleton at the wound front, resulting in a reduced migration rate. These observations provide insights into the molecular mechanism of the downregulation of this transporter in human ulcerative colitis and its role in epithelial restitution.

SLC26A6, a member of the SLC26 family of multifunctional anion transporters, has been particularly enigmatic because of its multiple modes of transport, its expression in organs that are difficult to study physiologically, and the lack of specific antibodies and inhibitors. This has recently changed. SLC26A6 is expressed in the human pancreas, kidney, intestine, heart and some other organs and is involved in fluid absorption, anion secretion, regulation of intracellular pH and elimination of waste products such as oxalate. This review will focus on three topics: Firstly, a molecular structure of human SLC26A6 has recently been obtained by cryo-electron microscopy. Structure-function studies of the reconstituted SLC26A6 in proteoliposomes suggested a 1:1 stoichiometry, resulting in electroneutral Cl−/HCO3− exchange and electrogenic Cl−/oxalate2− exchange. How do these data help to understand the published functional studies? Secondly, whole exon sequencing of a kidney stone cohort from the United Kingdom database revealed a dominant negative SLC26A6 mutation in a patient with enteric hyperoxaluria, oxalate kidney stones and a low calcium diet. How does this finding fit with previous genetic studies in mice and humans of SLC26A6 gene mutations? Thirdly, progress has been made in identifying specific inhibitors for SLC26A6. Where might this be of clinical relevance?

Diagnostic delay (DD) is a common finding in inflammatory bowel diseases (IBD). The reasons and effects of this delay are frequently underestimated, particularly in the context of sex. Our aims were to examine the impact of delayed diagnosis in IBD, with a particular focus on sex disparities. We performed a single-center, cross-sectional study at a tertiary referral center, including patients with IBD. The data was collected between August 2020 and June 2024. A total of 247 individuals with IBD were included in this study, with 53% identifying as female and 51% having Crohn's disease. Probability estimators revealed an effect of a DD on the cumulative advanced drug therapy probability in women (pLog-Rank = 0.045). Further analysis of the interaction between therapeutic regimens and DD revealed significant differences between the sexes. Women with a longer latency in their diagnosis were more frequently treated with steroids only compared to men. Entity-specific DD was further identified as a risk factor for steroid-only treatment in women with IBD (OR: 2.6; 95% CI, 1.11-5.98; P = .028). Additionally, a notable disparity in quality of life was observed between women who exhibited DD and men, with the former demonstrating a significantly reduced quality of life. A delayed diagnosis has a significant impact on IBD treatment trajectories, with a notable sex-related effect observed especially in women. Therapeutic needs in female patients with IBD seem underestimated, particularly in instances where a DD is present.

Butyrate may decrease intestinal inflammation and diarrhea. This study investigates the impact of oral application of sodium butyrate (NaB) and tributyrin (TB) on colonic butyrate concentration, SCFA transporter expression, colonic absorptive function, barrier properties, inflammation, and microbial composition in the colon of slc26a3-/- mice, a mouse model for inflammatory diarrhea. In vivo fluid absorption and bicarbonate secretory rates were evaluated in the cecum and mid-colon of slc26a3+/+ and slc26a3-/- mice before and during luminal perfusion of NaB-containing saline and were significantly stimulated in both slc26a3+/+ and slc26a3-/- colon by NaB. Age-matched slc26a3+/+ and slc26a3-/- mice were either fed chow containing 5% NaB or gavaged twice daily with TB for 21 d. Food and water intake, weight, and stool water content were assessed daily. Stool and tissues were collected for further analysis of SCFA production, barrier integrity, mucosal inflammation, and microbiome analysis by 16S rRNA gene sequencing. 5% NaB diet did not exert a significant impact on SCFA levels, mucus barrier, or inflammatory markers, but significantly increased oral water intake. TB gavage treatment increased the expression of SCFA transporters Mct1 and Smct1, mucus content and microbial diversity, and decreased the neutrophil marker Lipocalin 2, Phospholipase A2, and the antimicrobial peptide Reg3b in the slc26a3-/- cecum. However, TB treatment also resulted in an increase in inflammatory markers such as TNFα, Il-1β and CD3e in the wildtype mucosa. While there are some benefits with TB ingestion for barrier properties and microbial composition in the diseased cecum, potentially detrimental effects were noted in the healthy colon.

BACKGROUND & AIMS:Subcutaneous (SC) induction and maintenance with guselkumab was evaluated in adult participants with moderately to severely active Crohn's disease. METHODS:The Phase 3 double-blind, placebo-controlled, treat-through GRAVITI study randomized 347 participants 1:1:1 to guselkumab 400 mg SC every 4 weeks→100 mg SC every 8 weeks (n = 115), guselkumab 400 mg SC every 4 weeks→200 mg SC every 4 weeks (n = 115), or placebo (n = 117). Placebo participants meeting rescue criteria received guselkumab from week 16 onward. Co-primary endpoints were clinical remission at week 12 and endoscopic response at week 12. Additional multiplicity-controlled endpoints were Patient-Reported Outcome-2 remission (week 12), clinical response (week 12), clinical remission (week 24), clinical remission (week 48), and endoscopic response (week 48). Safety was assessed through week 48. RESULTS:All multiplicity-controlled endpoints were met. At week 12, significantly greater proportions of participants receiving guselkumab 400 mg achieved clinical remission vs placebo (56.1% vs 21.4%; Δ = 34.9; P < .001), and endoscopic response vs placebo (41.3% vs 21.4%; Δ = 19.9; P < .001). At week 48, significantly greater proportions of participants in both guselkumab groups (100 mg SC every 8 weeks: 60.0%, Δ = 42.8; 200 mg SC every 4 weeks: 66.1%, Δ = 48.9) achieved clinical remission vs placebo (17.1%; P < .001 each) and endoscopic response (44.3%, Δ = 37.5; 51.3%, Δ = 44.6; vs placebo 6.8%; P < .001 each). Efficacy was observed in both bionaive participants and those with inadequate response or intolerance to biologics. Adverse event rates were not greater in guselkumab groups vs placebo. CONCLUSION:Subcutaneous guselkumab for both induction and maintenance was efficacious in treating participants with moderately to severely active Crohn's disease. Safety findings were consistent with those of guselkumab in approved indications, including ulcerative colitis. (ClinicalTrials.gov, Number: NCT05197049.).

AIM:Trafficking, membrane retention, and signal-specific regulation of the Na+/H+ exchanger 3 (NHE3) are modulated by the Na+/H+ Exchanger Regulatory Factor (NHERF) family of PDZ-adapter proteins. This study explored the assembly of NHE3 and NHERF2 with the cGMP-dependent kinase II (cGKII) within detergent-resistant membrane microdomains (DRMs, "lipid rafts") during in vivo guanylate cycle C receptor (Gucy2c) activation in murine small intestine. METHODS:Small intestinal brush border membranes (siBBMs) were isolated from wild type, NHE3-deficient, cGMP-kinase II-deficient, and NHERF2-deficient mice, after oral application of the heat-stable Escherichia coli toxin (STa) analog linaclotide. Lipid raft and non-raft fractions were separated by Optiprep density gradient centrifugation of Triton X-solubilized siBBMs. Confocal microscopy was performed to study NHE3 redistribution after linaclotide application in vivo. RESULTS:In the WT siBBM, NHE3, NHERF2, and cGKII were strongly raft associated. The raft association of NHE3, but not of cGKII, was NHERF2 dependent. After linaclotide application to WT mice, lipid raft association of NHE3 decreased, that of cGKII increased, while that of NHERF2 did not change. NHE3 expression in the BBM shifted from a microvillar to a terminal web region. The linaclotide-induced decrease in NHE3 raft association and in microvillar abundance was abolished in cGKII-deficient mice, and strongly reduced in NHERF2-deficient mice. CONCLUSION:NHE3, cGKII, and NHERF2 form a lipid raft-associated signal complex in the siBBM, which mediates the inhibition of salt and water absorption by Gucy2c activation. NHERF2 enhances the raft association of NHE3, which is essential for its close interaction with the exclusively raft-associated activated cGKII.

Background Myeloid differentiation factor 88 (MyD88) is the core adaptor for Toll-like receptors defending against microbial invasion and initiating a downstream immune response during microbiota-host interaction. However, the role of MyD88 in the pathogenesis of inflammatory bowel disease is controversial. This study aims to investigate the impact of MyD88 on intestinal inflammation and the underlying mechanism.Methods MyD88 knockout (MyD88-/-) mice and the MyD88 inhibitor (TJ-M2010-5) were used to investigate the impact of MyD88 on acute dextran sodium sulfate (DSS)-induced colitis. Disease activity index, colon length, histological score, and inflammatory cytokines were examined to evaluate the severity of colitis. RNA transcriptome analysis and 16S rDNA sequencing were used to detect the potential mechanism.Results In an acute DSS-colitis model, the severity of colitis was not alleviated in MyD88-/- mice and TJ-M2010-5-treated mice, despite significantly lower levels of NF-kappa B activation being exhibited compared to control mice. Meanwhile, 16S rDNA sequencing and RNA transcriptome analysis revealed a higher abundance of intestinal Proteobacteria and an up-regulation of the nucleotide oligomerization domain-like receptors (NLRs) signaling pathway in colitis mice following MyD88 suppression. Further blockade of the NLRs signaling pathway or elimination of gut microbiota with broad-spectrum antibiotics in DSS-induced colitis mice treated with TJ-M2010-5 ameliorated the disease severity, which was not improved solely by MyD88 inhibition. After treatment with broad-spectrum antibiotics, downregulation of the NLR signaling pathway was observed.Conclusion Our study suggests that the suppression of MyD88 might be associated with unfavorable changes in the composition of gut microbiota, leading to NLR-mediated immune activation and intestinal inflammation.

NBCn1 (SLC4A7) is one of the two major Na+-HCO3- cotransporters in the human colonic epithelium, expressed predominantly in the highly proliferating colonocytes at the cryptal base. Increased NBCn1 expression levels are reported in tumors, including colorectal cancer. The study explores its importance for maintenance of the intracellular pH (pHi), as well as the proliferative, adhesive, and migratory behavior of the self-differentiating Caco2BBe colonic tumor cell line. In the self-differentiating Caco2BBe cells, NBCn1 mRNA was highly expressed from the proliferative stage until full differentiation. The downregulation of NBCn1 expression by RNA interference affected proliferation and differentiation and decreased intracellular pH (pHi) of the cells in correlation with the degree of knockdown. In addition, a disturbed cell adhesion and reduced migratory speed were associated with NBCn1 knockdown. Murine colonic Nbcn1-/- enteroids also displayed reduced proliferative activity. In the migrating Caco2BBe cells, NBCn1 was found at the leading edge and in colocalization with the focal adhesion markers vinculin and paxillin, which suggests that NBCn1 is involved in the establishment of cell-matrix adhesion. Our data highlight the physiological significance of NBCn1 in modulating epithelial pH homeostasis and cell-matrix interactions in the proliferative region of the colonic epithelium and unravel the molecular mechanism behind pathological overexpression of this transporter in human colorectal cancers.NEW & NOTEWORTHY The transporter NBCn1 plays a central role in maintaining homeostasis within Caco2BBe colonic epithelial cells through its regulation of intracellular pH, matrix adhesion, migration, and proliferation. These observations yield valuable insights into the molecular mechanism of the aberrant upregulation of this transporter in human colorectal cancers.

Mutations in LRBA, a BEACH domain protein, cause severe immune deficiency in humans. LRBA is expressed in many tissues and organs according to biochemical analysis, but little is known about its cellular and subcellular localization, and its deficiency phenotype outside the immune system. By LacZ histochemistry of Lrba gene-trap mice, we performed a comprehensive survey of LRBA expression in numerous tissues, detecting it in many if not all epithelia, in exocrine and endocrine cells, and in subpopulations of neurons. Immunofluorescence microscopy of the exocrine and endocrine pancreas, salivary glands, and intestinal segments, confirmed these patterns of cellular expression and provided information on the subcellular localizations of the LRBA protein. Immuno-electron microscopy demonstrated that in neurons and endocrine cells, which co-express LRBA and its closest relative, neurobeachin, both proteins display partial association with endomembranes in complementary, rather than overlapping, subcellular distributions. Prominent manifestations of human LRBA deficiency, such as inflammatory bowel disease or endocrinopathies, are believed to be primarily due to immune dysregulation. However, as essentially all affected tissues also express LRBA, it is possible that LRBA deficiency enhances their vulnerability and contributes to the pathogenesis.

The Ca2+ activated Cl− channel TMEM16A (anoctamin 1; ANO1) is expressed in secretory epithelial cells of airways and intestine. Previous studies provided evidence for a role of ANO1 in mucus secretion. In the present study we investigated the effects of the two ANO1-inhibitors niclosamide (Niclo) and benzbromarone (Benz) in vitro and in vivo in mouse models for cystic fibrosis (CF) and asthma. In human CF airway epithelial cells (CFBE), Ca2+ increase and activation of ANO1 by adenosine triphosphate (ATP) or ionomycin was strongly inhibited by 200 nM Niclo and 1 µM Benz. In asthmatic mice airway mucus secretion was inhibited by intratracheal instillation of Niclo or Benz. In homozygous F508del-cftr mice, intestinal mucus secretion and infiltration by CD45-positive cells was inhibited by intraperitoneal injection of Niclo (13 mg/kg/day for 7 days). In homozygous F508del-cftr rats intestinal mucus secretion was inhibited by oral application of Benz (5 mg/kg/day for 60 days). Taken together, well tolerated therapeutic concentrations of niclosamide and benzbromarone corresponding to plasma levels of treated patients, inhibit ANO1 and intracellular Ca2+ signals and may therefore be useful in inhibiting mucus hypersecretion and mucus obstruction in airways and intestine of patients suffering from asthma and CF, respectively.

Loss of function mutations in the SLC26A3 gene cause chloride-losing diarrhea in mice and humans. Although systemic adaptive changes have been documented in these patients and in the corresponding knockout mice, how colonic enterocytes adapt to loss of this highly expressed and highly regulated luminal membrane anion exchanger remains unclear. To address this question, SLC26A3 was deleted in the self-differentiating Caco2BBe colonic cell line by the CRISPR/Cas9 technique. We selected a clone with loss of SLC26A3 protein expression and morphological features indistinguishable from those of the native cell line. Neither growth curves nor development of transepithelial electrical resistance (TEER) differed between wild-type (WT) and SLC26A3 knockout (KO) cells. Real-time qPCR and Western analysis in SLC26A3-KO cells revealed an increase in AE2 expression without significant change in NHE3 expression or localization. Steady-state pHi and apical and basolateral Cl-/HCO3- exchange activities were assessed fluorometrically in a dual perfusion chamber with independent perfusion of luminal and serosal baths. Apical Cl-/HCO3- exchange rates were strongly reduced in SLC26A3-KO cells, accompanied by a surface pH more acidic than that of WT cells. Steady-state pHi was not significantly different from that of WT cells, but basolateral Cl-/HCO3- exchange rates were higher in SLC26A3-KO than in WT cells. The data show that CRISPR/Cas9-mediated SLC26A3 deletion strongly reduced apical Cl-/HCO3- exchange rate and apical surface pH, but sustained a normal steady-state pHi due to increased expression and function of basolateral AE2. The low apical surface pH resulted in functional inhibition of NHE-mediated fluid absorption despite normal expression of NHE3 polypeptide.NEW & NOTEWORTHY SLC26A3 gene mutations cause chloride-losing diarrhea. To understand how colonic enterocytes adapt, SLC26A3 was deleted in Caco2BBe cells using CRISPR/Cas9. In comparison to the wild-type cells, SLC26A3 knockout cells showed similar growth and transepithelial resistance but substantially reduced apical Cl-/HCO3- exchange rates, and an acidic surface pH. Steady-state intracellular pH was comparable between the WT and KO cells due to increased basolateral AE2 expression and function.

The transport of bicarbonate across the enterocyte cell membrane regulates the intracellular as well as the luminal pH and is an essential part of directional fluid movement in the gut. Since the first description of “active” transport of HCO 3 − ions against a concentration gradient in the 1970s, the fundamental role of HCO 3 − transport for multiple intestinal functions has been recognized. The ion transport proteins have been identified and molecularly characterized, and knockout mouse models have given insight into their individual role in a variety of functions. This review describes the progress made in the last decade regarding novel techniques and new findings in the molecular regulation of intestinal HCO 3 − transport in the different segments of the gut. We discuss human diseases with defects in intestinal HCO 3 − secretion and potential treatment strategies to increase luminal alkalinity. In the last part of the review, the cellular and organismal mechanisms for acid/base sensing in the intestinal tract are highlighted.

Abstract Background Inflammatory bowel diseases (IBD), including Ulcerative colitis (UC) and Crohn's disease (CD), are chronic intestinal disorders characterised by varied intestinal symptoms and a negative impact on the patients` quality of life. Increasing evidence demonstrates sex differences regarding the prevalence, pathophysiology, clinical presentation and treatment outcome of IBD. While for some factors evidence is rather good, for others data are conflicting – thus a deeper understanding of sex related differences in IBD courses is still needed. Methods In this monocentric, cross-sectional study 221 IBD patients were stratified into 2 groups according to gender/biological sex (male [n=106]; female [n=113]). We evaluated patients personal and medical data including disease-specific quality of life (IBDQ) as well as clinical (HBI, PMS) and biochemical (CRP, fecal calprotectin) parameters of disease activity. Furthermore, we isolated and stimulated PBMCs with various pathogens to test ex vivo cytokine responses of this heterogeneous IBD cohort. Results Binary logistic regression analysis suggested an independent association of diagnostic delay (p= 0.039), IBD entity (p= 0.001), IBDQ (p= 0.001), and vitamin D3-25-OH serum level (p= 0.035) with disease activity for female IBD patients, while for male IBD patients an independent association was only suggested for IBD entity (p= 0.004) and IBDQ (p= 0.001) (figure 1). Next, we tested the association between the number of ADT (Advanced Therapies) taken during the overall course of the disease with both disease activity and diagnostic delay. We found a significant association between ADT and disease activity in both sexes (male-p = 0.015, female-p=0.037), but ADT was only significantly associated with diagnostic delay in female IBD patients (p=0.044) (figure 2). Regarding the frequency of therapies used in both sexes, we found that male patients are more frequently subscribed prednisolone (Mann-Whitney-U, p= 0.009) as well as 5-ASA (p= 0.004) (figure 3). Cytokine responses upon PBMC stimulations with various pathogens showed a significant trend in Interferon gamma levels in male CD patients (p< 0.05) (figure 5A). A significant negative correlation between Interferon gamma levels and CD activity has only been found in male CD patients, but not in female CD patients (figure 5B). Conclusion In this analysis of a cross-sectional study, we are able to illustrate sex-specific aspects of not only clinical courses in IBD patients but also within specific immunological responses – highlighting the importance to closely take the IBD patients sex into consideration in the upcoming era of personalized medicine.

Studies in human colonic cell lines and murine intestine suggest the presence of a Ca2+-activated anion channel, presumably TMEM16a. Is there a potential for fluid secretion in patients with severe cystic fibrosis transmembrane conductance regulator (CFTR) mutations by activating this alternative pathway? Two-dimensional nondifferentiated colonoid–myofibroblast cocultures resembling transit amplifying/progenitor (TA/PE) cells, as well as differentiated monolayer (DM) cultures resembling near-surface cells, were established from both healthy controls (HLs) and patients with severe functional defects in the CFTR gene (PwCF). F508del mutant and CFTR knockout (null) mice ileal and colonic mucosa was also studied. HL TA/PE monolayers displayed a robust short-circuit current response (ΔIeq) to UTP (100 µM), forskolin (Fsk, 10 µM) and carbachol (CCH, 100 µM), while ΔIeq was much smaller in differentiated monolayers. The selective TMEM16a inhibitor Ani9 (up to 30 µM) did not alter the response to luminal UTP, significantly decreased Fsk-induced ΔIeq, and significantly increased CCH-induced ΔIeq in HL TA/PE colonoid monolayers. The PwCF TA/PE and the PwCF differentiated monolayers displayed negligible agonist-induced ΔIeq, without a significant effect of Ani9. When TMEM16a was localized in intracellular structures, a staining in the apical membrane was not detected. TMEM16a is highly expressed in human colonoid monolayers resembling transit amplifying cells of the colonic cryptal neck zone, from both HL and PwCF. While it may play a role in modulating agonist-induced CFTR-mediated anion currents, it is not localized in the apical membrane, and it has no function as an apical anion channel in cystic fibrosis (CF) and healthy human colonic epithelium.

SLC26A9 is one out of 11 proteins that belong to the SLC26A family of anion transporters. Apart from expression in the gastrointestinal tract, SLC26A9 is also found in the respiratory system, in male tissues and in the skin. SLC26A9 has gained attention because of its modifier role in the gastrointestinal manifestation of cystic fibrosis (CF). SLC26A9 appears to have an impact on the extent of intestinal obstruction caused by meconium ileus. SLC26A9 supports duodenal bicarbonate secretion, but was assumed to provide a basal Cl- secretory pathway in airways. However, recent results show that basal airway Cl- secretion is due to cystic fibrosis conductance regulator (CFTR), while SLC26A9 may rather secrete HCO3-, thereby maintaining proper airway surface liquid (ASL) pH. Moreover, SLC26A9 does not secrete but probably supports reabsorption of fluid particularly in the alveolar space, which explains early death by neonatal distress in Slc26a9-knockout animals. While the novel SLC26A9 inhibitor S9-A13 helped to unmask the role of SLC26A9 in the airways, it also provided evidence for an additional role in acid secretion by gastric parietal cells. Here we discuss recent data on the function of SLC26A9 in airways and gut, and how S9-A13 may be useful in unraveling the physiological role of SLC26A9.