Ursula Seidler

Diarrhea is a hallmark of Crohn’s disease (CD) and ulcerative colitis (UC), often linked to a leaky epithelial barrier and impaired fluid and electrolyte absorption. The present study investigates whether a defect in enterocyte differentiation contributes to reduced absorptive protein expression but increased leak protein expression in patients with inflammatory bowel disease (IBD). Segmental biopsies from the terminal ileum to the rectum were collected from IBD patients and healthy controls (HC). mRNA and protein levels of key absorptive ion transporters and markers of cell proliferation and differentiation were analyzed. Disease activity and diarrhea severity were assessed using clinical data, endoscopy, histopathology, and proinflammatory cytokine expression. In healthy subjects, ion transporters such as Na+/H+ exchanger NHE3 (SLC9a3), Cl−/HCO3− exchanger DRA (SLC26A3), short-chain fatty acid transporter MCT1 (SLC16A1), and epithelial Na+ channel showed distinct segment-specific expression patterns. Compared with HC, inflamed segments from IBD patients exhibited significantly reduced ion transporter expression and increased expression of the leak-promoting tight junction protein claudin 2. Markers of absorptive enterocyte differentiation—including villin, intestinal alkaline phosphatase, and HES1—were reduced, whereas the proliferative marker Ki67 was elevated. Goblet cell differentiation was also impaired in highly inflamed areas. High diarrhea scores correlated with lower transporter expression. Together, our findings suggest that inflammation-driven defects in epithelial differentiation may represent an important contributing mechanism to IBD-associated diarrhea, alongside other established mechanisms, including direct inflammation- and cytokine-mediated regulation of epithelial ion transporters. Mucosal healing restored normal segmental transporter expression, highlighting the reversibility of these defects.

Background and Aims: Colonic bicarbonate secretion is mediated by the chloride/bicarbonate exchanger SLC26A3 and the cystic fibrosis transmembrane conductance regulator (CFTR). Dysfunction of either causes luminal acidosis, altered mucus properties, and inflammation. While physical and functional interactions have been demonstrated in heterologous systems, their relationship in native epithelium is not fully established. We investigated the distinct roles of SLC26A3 and CFTR using human intestinal organoids with inducible SLC26A3 overexpression. Methods: Human colonic and rectal organoids from healthy controls and cystic fibrosis patients with F508del mutations were studied in the proliferative state with high endogenous CFTR expression and inducible SLC26A3 overexpression. Real‐time surface pH measurements, electrophysiological analysis, forskolin‐induced swelling assays, and confocal microscopy were employed. Results: Steady‐state surface pH was lower in CF versus healthy organoids (7.23 ± 0.03 vs. 7.34 ± 0.03). SLC26A3 overexpression normalized surface pH in CF organoids and CFTR‐inhibited organoids, equalizing responses between genotypes. SLC26A3 overexpression corrected abnormal morphology and significantly improved intracellular MUC2 distribution in CF organoids. However, SLC26A3 did not restore fluid secretion in CF organoids or enhance CFTR‐mediated electrogenic anion secretion in Ussing chambers. Conclusions: SLC26A3 and CFTR perform distinct yet complementary functions. SLC26A3 dominates surface pH regulation and maintains bicarbonate efflux independently of CFTR, while CFTR drives agonist‐stimulated fluid secretion. SLC26A3's ability to restore pH homeostasis and normalize mucin intracellular distribution in CF organoids demonstrates its critical importance for maintaining colonic mucosal health.

Background and Purpose: Guanylate cyclase‐C (GC‐C) is the receptor for endogenous (uro)guanylin peptides, bacterial toxins and pharmacological analogues. Receptor activation leads to intestinal fluid loss, but also activates an antiproliferative pathway and is a promising target in colorectal cancer therapy. The present study delineates the molecular mechanisms that regulate the intensity of the anion secretory response (ASR) to GC‐C activation in the different segments of the murine ileocolon. Experimental Approach: To assess the ASR to the guanylin analogue linaclotide, isolated mucosa from the different segments of the ileocolon of cGMP‐dependent kinase II (cGKII)‐deficient and WT mice were studied electrophysiologically in Ussing‐chambers. The mucosal expression pattern of different phosphodiesterases (PDEs) was measured by RT‐PCR. Key Results: The ASR to linaclotide and 8‐pCPT‐cGMP was strongly dependent on the presence of cGKII in the ileum and proximal colon, but not in the mid and distal colon, where cGKII expression is low. The inhibition of cGMP‐sensitive PDE3 completely prevented the linaclotide‐induced ASR only in the mid and distal colon, demonstrating that GC‐C dependent ASR occurs via an increase in cAMP in these segments, mediated by the cGMP‐induced inhibition of PDE3. In addition, the cGMP‐specific PDE9 was highly expressed in the distal colon, and its inhibition strongly enhanced the ASR to linaclotide. Conclusions and implications: GC‐C activation leads to CFTR‐mediated ASR in all segments of the intestine. In the mid and distal colon ASR is strongly reduced by the low cGKII and high PDE9 expression, with the CFTR channel being activated via cAMP‐mediated phosphorylation.

The Na⁺/H⁺ exchanger 8 (NHE8) plays an important role in the regulation of intracellular pH_i, 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.

Background: 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. Methods: 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. Results: 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. Conclusions: 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.

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?

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.

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.

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.