Patient-derived enteroids provide a platform for the development of therapeutic approaches in microvillus inclusion disease

Microvillus inclusion disease (MVID), caused by loss-of-function mutations in the motor protein myosin Vb (MYO5B), is a severe infantile disease characterized by diarrhea, malabsorption, and acid/base instability, requiring intensive parenteral support for nutritional and fluid management. Human patient-derived enteroids represent a model for investigation of monogenic epithelial disorders but are a rare resource from MVID patients. We developed human enteroids with different loss-of function MYO5B variants and showed that they recapitulated the structural changes found in native MVID enterocytes. Multiplex immunofluorescence imaging of patient duodenal tissues revealed patient-specific changes in localization of brush border transporters. Functional analysis of electrolyte transport revealed profound loss of Na+/H+ exchange (NHE) activity in MVID patient enteroids with near-normal chloride secretion. The chloride channel-blocking antidiarrheal drug crofelemer dose-dependently inhibited agonist-mediated fluid secretion. MVID enteroids exhibited altered differentiation and maturation versus healthy enteroids. γ-Secretase inhibition with DAPT recovered apical brush border structure and functional Na+/H+ exchange activity in MVID enteroids. Transcriptomic analysis revealed potential pathways involved in the rescue of MVID cells including serum/glucocorticoid-regulated kinase 2 (SGK2) and NHE regulatory factor 3 (NHERF3). These results demonstrate the utility of patient-derived enteroids for developing therapeutic approaches to MVID.

A Roadmap for the Human Gut Cell Atlas

The number of studies investigating the human gastrointestinal tract using various single-cell profiling methods has increased substantially in the past few years. Although this increase provides a unique opportunity for the generation of the first comprehensive Human Gut Cell Atlas (HGCA), there remains a range of major challenges ahead. Above all, the ultimate success will largely depend on a structured and coordinated approach that aligns global efforts undertaken by a large number of research groups. In this Roadmap, we discuss a comprehensive forward-thinking direction for the generation of the HGCA on behalf of the Gut Biological Network of the Human Cell Atlas. Based on the consensus opinion of experts from across the globe, we outline the main requirements for the first complete HGCA by summarizing existing data sets and highlighting anatomical regions and/or tissues with limited coverage. We provide recommendations for future studies and discuss key methodologies and the importance of integrating the healthy gut atlas with related diseases and gut organoids. Importantly, we critically overview the computational tools available and provide recommendations to overcome key challenges.

Perspectives of Rare Disease Experts on Newborn Genome Sequencing

Importance

Newborn genome sequencing (NBSeq) can detect infants at risk for treatable disorders currently undetected by conventional newborn screening. Despite broad stakeholder support for NBSeq, the perspectives of rare disease experts regarding which diseases should be screened have not been ascertained.

Objective

To query rare disease experts about their perspectives on NBSeq and which gene-disease pairs they consider appropriate to evaluate in apparently healthy newborns.

Design, Setting, and Participants

This survey study, designed between November 2, 2021, and February 11, 2022, assessed experts' perspectives on 6 statements related to NBSeq. Experts were also asked to indicate whether they would recommend including each of 649 gene-disease pairs associated with potentially treatable conditions in NBSeq. The survey was administered between February 11 and September 23, 2022, to 386 experts, including all 144 directors of accredited medical and laboratory genetics training programs in the US.

Exposures

Expert perspectives on newborn screening using genome sequencing.

Main Outcomes and Measures

The proportion of experts indicating agreement or disagreement with each survey statement and those who selected inclusion of each gene-disease pair were tabulated. Exploratory analyses of responses by gender and age were conducted using t and χ2 tests.

Results

Of 386 experts invited, 238 (61.7%) responded (mean [SD] age, 52.6 [12.8] years [range 27-93 years]; 126 [52.9%] women and 112 [47.1%] men). Among the experts who responded, 161 (87.9%) agreed that NBSeq for monogenic treatable disorders should be made available to all newborns; 107 (58.5%) agreed that NBSeq should include genes associated with treatable disorders, even if those conditions were low penetrance; 68 (37.2%) agreed that actionable adult-onset conditions should be sequenced in newborns to facilitate cascade testing in parents, and 51 (27.9%) agreed that NBSeq should include screening for conditions with no established therapies or management guidelines. The following 25 genes were recommended by 85% or more of the experts: OTC, G6PC, SLC37A4, CYP11B1, ARSB, F8, F9, SLC2A1, CYP17A1, RB1, IDS, GUSB, DMD, GLUD1, CYP11A1, GALNS, CPS1, PLPBP, ALDH7A1, SLC26A3, SLC25A15, SMPD1, GATM, SLC7A7, and NAGS. Including these, 42 gene-disease pairs were endorsed by at least 80% of experts, and 432 genes were endorsed by at least 50% of experts.

Conclusions and Relevance

In this survey study, rare disease experts broadly supported NBSeq for treatable conditions and demonstrated substantial concordance regarding the inclusion of a specific subset of genes in NBSeq.

WNT2B Deficiency Causes Increased Susceptibility to Colitis in Mice and Impairs Intestinal Epithelial Development in Humans

Background and aims

WNT2B is a canonical Wnt ligand previously thought to be fully redundant with other Wnts in the intestinal epithelium. However, humans with WNT2B deficiency have severe intestinal disease, highlighting a critical role for WNT2B. We sought to understand how WNT2B contributes to intestinal homeostasis.

Methods

We investigated the intestinal health of Wnt2b knock out (KO) mice. We assessed the impact of inflammatory challenge to the small intestine, using anti-CD3χ antibody, and to the colon, using dextran sodium sulfate (DSS). In addition, we generated human intestinal organoids (HIOs) from WNT2B-deficient human iPSCs for transcriptional and histological analyses.

Results

Mice with WNT2B deficiency had significantly decreased Lgr5 expression in the small intestine and profoundly decreased expression in the colon, but normal baseline histology. The small intestinal response to anti-CD3χ antibody was similar in Wnt2b KO and wild type (WT) mice. In contrast, the colonic response to DSS in Wnt2b KO mice showed an accelerated rate of injury, featuring earlier immune cell infiltration and loss of differentiated epithelium compared to WT. WNT2B-deficient HIOs showed abnormal epithelial organization and an increased mesenchymal gene signature.

Conclusion

WNT2B contributes to maintenance of the intestinal stem cell pool in mice and humans. WNT2B deficient mice, which do not have a developmental phenotype, show increased susceptibility to colonic injury but not small intestinal injury, potentially due to a higher reliance on WNT2B in the colon compared to the small intestine.WNT2B deficiency causes a developmental phenotype in human intestine with HIOs showing a decrease in their mesenchymal component and WNT2B-deficient patients showing epithelial disorganization.

Data Transparency Statement

All RNA-Seq data will be available through online repository as indicated in Transcript profiling. Any other data will be made available upon request by emailing the study authors.

Gasdermin D pore-forming activity is redox-sensitive

Reactive oxygen species (ROS) regulate the activities of inflammasomes, which are innate immune signaling organelles that induce pyroptosis. The mechanisms by which ROS control inflammasome activities are unclear and may be multifaceted. Herein, we report that the protein gasdermin D (GSDMD), which forms membrane pores upon cleavage by inflammasome-associated caspases, is a direct target of ROS. Exogenous and endogenous sources of ROS, and ROS-inducing stimuli that prime cells for pyroptosis induction, promote oligomerization of cleaved GSDMD, leading to membrane rupture and cell death. We find that ROS enhance GSDMD activities through oxidative modification of cysteine 192 (C192). Within macrophages, GSDMD mutants lacking C192 show impaired ability to form membrane pores and induce pyroptosis. Reciprocal mutagenesis studies reveal that C192 is the only cysteine within GSDMD that mediates ROS responsiveness. Cellular redox state is therefore a key determinant of GSDMD activities.

Therapy Development for Microvillus Inclusion Disease using Patient-derived Enteroids

Microvillus Inclusion Disease (MVID), caused by loss-of-function mutations in the motor protein Myosin Vb (MYO5B), is a severe infantile disease characterized by diarrhea, malabsorption, and acid-base instability, requiring intensive parenteral support for nutritional and fluid management. Human patient-derived enteroids represent a model for investigation of monogenic epithelial disorders but are a rare resource from MVID patients. We developed human enteroids with different loss-of function MYO5B variants and showed that they recapitulated the structural changes found in native MVID enterocytes. Multiplex Immunofluorescence imaging of patient duodenal tissues revealed patient-specific changes in localization of brush border transporters. Functional analysis of electrolyte transport revealed profound loss of Na + /H + exchange (NHE) activity in MVID patient enteroids with near-normal chloride secretion. The chloride channel-blocking anti-diarrheal drug, Crofelemer, dose-dependently inhibited agonist-mediated fluid secretion. MVID enteroids exhibited altered differentiation and maturation versus healthy enteroids. Inhibition of Notch signaling with the γ-secretase inhibitor, DAPT, recovered apical brush border structure and functional Na + /H + exchange activity in MVID enteroids. Transcriptomic analysis revealed potential pathways involved in the rescue of MVID cells including serum- and glucocorticoid-induced protein kinase 2 (SGK2), and NHE regulatory factor 3 (NHERF3). These results demonstrate the utility of patient-derived enteroids for developing therapeutic approaches to MVID.

Conflict-of-interest statement

The authors have declared that no conflict of interest exists.

Nociceptor neurons direct goblet cells via a CGRP-RAMP1 axis to drive mucus production and gut barrier protection

Neuroepithelial crosstalk is critical for gut physiology. However, the mechanisms by which sensory neurons communicate with epithelial cells to mediate gut barrier protection at homeostasis and during inflammation are not well understood. Here, we find that Nav1.8+CGRP+ nociceptor neurons are juxtaposed with and signal to intestinal goblet cells to drive mucus secretion and gut protection. Nociceptor ablation led to decreased mucus thickness and dysbiosis, while chemogenetic nociceptor activation or capsaicin treatment induced mucus growth. Mouse and human goblet cells expressed Ramp1, receptor for the neuropeptide CGRP. Nociceptors signal via the CGRP-Ramp1 pathway to induce rapid goblet cell emptying and mucus secretion. Notably, commensal microbes activated nociceptors to control homeostatic CGRP release. In the absence of nociceptors or epithelial Ramp1, mice showed increased epithelial stress and susceptibility to colitis. Conversely, CGRP administration protected nociceptor-ablated mice against colitis. Our findings demonstrate a neuron-goblet cell axis that orchestrates gut mucosal barrier protection.

The epithelial-specific ER stress sensor ERN2/IRE1β enables host-microbiota crosstalk to affect colon goblet cell development

Epithelial cells lining mucosal surfaces of the gastrointestinal and respiratory tracts uniquely express ERN2/IRE1β, a paralogue of the most evolutionarily conserved endoplasmic reticulum stress sensor, ERN1/IRE1α. How ERN2 functions at the host-environment interface and why a second paralogue evolved remain incompletely understood. Using conventionally raised and germ-free Ern2-/- mice, we found that ERN2 was required for microbiota-induced goblet cell maturation and mucus barrier assembly in the colon. This occurred only after colonization of the alimentary tract with normal gut microflora, which induced Ern2 expression. ERN2 acted by splicing Xbp1 mRNA to expand ER function and prevent ER stress in goblet cells. Although ERN1 can also splice Xbp1 mRNA, it did not act redundantly to ERN2 in this context. By regulating assembly of the colon mucus layer, ERN2 further shaped the composition of the gut microbiota. Mice lacking Ern2 had a dysbiotic microbial community that failed to induce goblet cell development and increased susceptibility to colitis when transferred into germ-free WT mice. These results show that ERN2 evolved at mucosal surfaces to mediate crosstalk between gut microbes and the colonic epithelium required for normal homeostasis and host defense.

Pediatric Gastrointestinal Histopathology in Patients With Tetratricopeptide Repeat Domain 7A (TTC7A) Germline Mutations: A Rare Condition Leading to Multiple Intestinal Atresias, Severe Combined Immunodeficiency, and Congenital Enteropathy

Mutations in the tetratricopeptide repeat domain 7A (TTC7A) gene are a rare cause of congenital enteropathy that can result in significant morbidity. TTC7A deficiency leads to disruption of the intestinal epithelium. The histopathology of this condition has been partly described in case reports and clinical studies. This manuscript describes an in-depth investigation of the pediatric gastrointestinal pathology of the largest histologically examined cohort with confirmed TTC7A mutations reported to date and, for the first time, compared the findings to age-matched and sex-matched control patients with intestinal atresia not thought to be associated with TTC7A mutations. Hematoxylin and eosin-stained slides of endoscopically obtained mucosal biopsies and surgical resection specimens from 7 patients with known TTC7A mutations were examined retrospectively. The microscopic findings were found to be on a spectrum from atresia-predominant to those with predominantly epithelial abnormalities. Several unique histopathologic characteristics were observed when compared with controls. These included neutrophilic colitis and prominent lamina propria eosinophilia throughout the gastrointestinal tract. Striking architectural abnormalities of the epithelium were observed in 4 of the 7 patients. The 5 patients with intestinal atresia demonstrated hypertrophy and disorganization of the colonic muscularis mucosae accompanied by bland spindle cell nodules within the intestinal wall. The components of the latter were further elucidated using immunohistochemistry, and we subsequently hypothesize that they represent obliterated mucosa with remnants of the muscularis mucosae. Finally, atrophic gastritis was noted in 4 patients. In conclusion, the unique histopathologic characteristics of TTC7A mutation-associated enteropathy described herein more fully describe this novel disease entity in infants who present with congenital enteropathy or enterocolitis.

Membrane Lipids in Epithelial Polarity: Sorting out the PIPs

The development of cell polarity in epithelia, is critical for tissue morphogenesis and vectorial transport between the environment and the underlying tissue. Epithelial polarity is defined by the development of distinct plasma membrane domains: the apical membrane interfacing with the exterior lumen compartment, and the basolateral membrane directly contacting the underlying tissue. The de novo generation of polarity is a tightly regulated process, both spatially and temporally, involving changes in the distribution of plasma membrane lipids, localization of apical and basolateral membrane proteins, and vesicular trafficking. Historically, the process of epithelial polarity has been primarily described in relation to the localization and function of protein 'polarity complexes.' However, a critical and foundational role is emerging for plasma membrane lipids, and in particular phosphoinositide species. Here, we broadly review the evidence for a primary role for membrane lipids in the generation of epithelial polarity and highlight key areas requiring further research. We discuss the complex interchange that exists between lipid species and briefly examine how major membrane lipid constituents are generated and intersect with vesicular trafficking to be preferentially localized to different membrane domains with a focus on some of the key protein-enzyme complexes involved in these processes.

Minimally Invasive Image-Guided Gut Transport Function Measurement Probe

INTRODUCTION

Diseases such as celiac disease, environmental enteric dysfunction, infectious gastroenteritis, type II diabetes and inflammatory bowel disease are associated with increased gut permeability. Dual sugar absorption tests, such as the lactulose to rhamnose ratio (L:R) test, are the current standard for measuring gut permeability. Although easy to administer in adults, the L:R test has a number of drawbacks. These include an inability to assess for spatial heterogeneity in gut permeability that may distinguish different disease severity or pathology, additional sample collection for immunoassays, and challenges in carrying out the test in certain populations such as infants and small children. Here, we demonstrate a minimally invasive probe for real-time localized gut permeability evaluation through gut potential difference (GPD) measurement.

MATERIALS AND METHODS

The probe has an outer diameter of 1.2 mm diameter and can be deployed in the gut of unsedated subjects via a transnasal introduction tube (TNIT) that is akin to an intestinal feeding tube. The GPD probe consists of an Ag/AgCl electrode, an optical probe and a perfusion channel all housed within a transparent sheath. Lactated Ringer's (LR) solution is pumped through the perfusion channel to provide ionic contact between the electrodes and the gut lining. The optical probe captures non-scanning (M-mode) OCT images to confirm electrode contact with the gut lining. A separate skin patch probe is placed over an abraded skin area to provide reference for the GPD measurements. Swine studies were conducted to validate the GPD probe. GPD in the duodenum was modulated by perfusing 45 ml of 45 mM glucose.

RESULTS

GPD values of -13.1 ± 2.8 mV were measured in the duodenum across four swine studies. The change in GPD in the duodenum with the addition of glucose was -10.5 ± 2.4 mV (p < 0.001). M-mode OCT images provided electrode-tissue contact information, which was vital in ascertaining the probe's proximity to the gut mucosa.

CONCLUSION

We developed and demonstrated a minimally invasive method for investigating gastrointestinal permeability consisting of an image guided GPD probe that can be used in unsedated subjects.

Control of gasdermin D oligomerization and pyroptosis by the Ragulator-Rag-mTORC1 pathway

The process of pyroptosis is mediated by inflammasomes and a downstream effector known as gasdermin D (GSDMD). Upon cleavage by inflammasome-associated caspases, the N-terminal domain of GSDMD forms membrane pores that promote cytolysis. Numerous proteins promote GSDMD cleavage, but none are known to be required for pore formation after GSDMD cleavage. Herein, we report a forward genetic screen that identified the Ragulator-Rag complex as being necessary for GSDMD pore formation and pyroptosis in macrophages. Mechanistic analysis revealed that Ragulator-Rag is not required for GSDMD cleavage upon inflammasome activation but rather promotes GSDMD oligomerization in the plasma membrane. Defects in GSDMD oligomerization and pore formation can be rescued by mitochondrial poisons that stimulate reactive oxygen species (ROS) production, and ROS modulation impacts the ability of inflammasome pathways to promote pore formation downstream of GSDMD cleavage. These findings reveal an unexpected link between key regulators of immunity (inflammasome-GSDMD) and metabolism (Ragulator-Rag).

Novel variants in the stem cell niche factor WNT2B define the disease phenotype as a congenital enteropathy with ocular dysgenesis

WNT2B is a member of the Wnt family, a group of signal transduction proteins involved in embryologic development and stem cell renewal and maintenance. We recently reported homozygous nonsense variants in WNT2B in three individuals with severe, neonatal-onset diarrhea, and intestinal failure. Here we present a fourth case, from a separate family, with neonatal diarrhea associated with novel compound heterozygous WNT2B variants. One of the two variants was a frameshift variant (c.423del [p.Phe141fs]), while the other was a missense change (c.722 G > A [p.G241D]) that we predict through homology modeling to be deleterious, disrupting post-translational acylation. This patient presented as a neonate with severe diet-induced (osmotic) diarrhea and growth failure resulting in dependence on parenteral nutrition. Her gastrointestinal histology revealed abnormal cellular architecture particularly in the stomach and colon, including oxyntic atrophy, abnormal distribution of enteroendocrine cells, and a paucity of colonic crypt glands. In addition to her gastrointestinal findings, she had bilateral corneal clouding and atypical genital development later identified as a testicular 46,XX difference/disorder of sexual development. Upon review of the previously reported cases, two others also had anterior segment ocular anomalies though none had atypical genital development. This growing case series suggests that variants in WNT2B are associated with an oculo-intestinal (and possibly gonadal) syndrome, due to the protein's putative involvement in multiple developmental and stem cell maintenance pathways.

Extracellular cyclic dinucleotides induce polarized responses in barrier epithelial cells by adenosine signaling

Cyclic dinucleotides (CDNs) are secondary messengers used by prokaryotic and eukaryotic cells. In mammalian cells, cytosolic CDNs bind STING (stimulator of IFN gene), resulting in the production of type I IFN. Extracellular CDNs can enter the cytosol through several pathways but how CDNs work from outside eukaryotic cells remains poorly understood. Here, we elucidate a mechanism of action on intestinal epithelial cells for extracellular CDNs. We found that CDNs containing adenosine induced a robust CFTR-mediated chloride secretory response together with cAMP-mediated inhibition of Poly I:C-stimulated IFNβ expression. Signal transduction was strictly polarized to the serosal side of the epithelium, dependent on the extracellular and sequential hydrolysis of CDNs to adenosine by the ectonucleosidases ENPP1 and CD73, and occurred via activation of A2B adenosine receptors. These studies highlight a pathway by which microbial and host produced extracellular CDNs can regulate the innate immune response of barrier epithelial cells lining mucosal surfaces.

NOX1 Regulates Collective and Planktonic Cell Migration: Insights From Patients With Pediatric-Onset IBD and NOX1 Deficiency

BACKGROUND

Genetic defects of pediatric-onset inflammatory bowel disease (IBD) provide critical insights into molecular factors controlling intestinal homeostasis. NOX1 has been recently recognized as a major source of reactive oxygen species (ROS) in human colonic epithelial cells. Here we assessed the functional consequences of human NOX1 deficiency with respect to wound healing and epithelial migration by studying pediatric IBD patients presenting with a stop-gain mutation in NOX1.

METHODS

Functional characterization of the NOX1 variant included ROS generation, wound healing, 2-dimensional collective chemotactic migration, single-cell planktonic migration in heterologous cell lines, and RNA scope and immunohistochemistry of paraffin-embedded patient tissue samples.

RESULTS

Using exome sequencing, we identified a stop-gain mutation in NOX1 (c.160C>T, p.54R>*) in patients with pediatric-onset IBD. Our studies confirmed that loss-of-function of NOX1 causes abrogated ROS activity, but they also provided novel mechanistic insights into human NOX1 deficiency. Cells that were NOX1-mutant showed impaired wound healing and attenuated 2-dimensional collective chemotactic migration. High-resolution microscopy of the migrating cell edge revealed a reduced density of filopodial protrusions with altered focal adhesions in NOX1-deficient cells, accompanied by reduced phosphorylation of p190A. Assessment of single-cell planktonic migration toward an epidermal growth factor gradient showed that NOX1 deficiency is associated with altered migration dynamics with loss of directionality and altered cell-cell interactions.

CONCLUSIONS

Our studies on pediatric-onset IBD patients with a rare sequence variant in NOX1 highlight that human NOX1 is involved in regulating wound healing by altering epithelial cytoskeletal dynamics at the leading edge and directing cell migration.

Drug Screen Identifies Leflunomide for Treatment of Inflammatory Bowel Disease Caused by TTC7A Deficiency

BACKGROUND & AIMS

Mutations in the tetratricopeptide repeat domain 7A gene (TTC7A) cause intestinal epithelial and immune defects. Patients can become immune deficient and develop apoptotic enterocolitis, multiple intestinal atresia, and recurrent intestinal stenosis. The intestinal disease in patients with TTC7A deficiency is severe and untreatable, and it recurs despite resection or allogeneic hematopoietic stem cell transplant. We screened drugs for those that prevent apoptosis of in cells with TTC7A deficiency and tested their effects in an animal model of the disease.

METHODS

We developed a high-throughput screen to identify compounds approved by the US Food and Drug Administration that reduce activity of caspases 3 and 7 in TTC7A-knockout (TTC7A-KO) HAP1 (human haploid) cells and reduce the susceptibility to apoptosis. We validated the effects of identified agents in HeLa cells that stably express TTC7A with point mutations found in patients. Signaling pathways in cells were analyzed by immunoblots. We tested the effects of identified agents in zebrafish with disruption of ttc7a, which develop intestinal defects, and colonoids derived from biopsy samples of patients with and without mutations in TTC7A. We performed real-time imaging of intestinal peristalsis in zebrafish and histologic analyses of intestinal tissues from patients and zebrafish. Colonoids were analyzed by immunofluorescence and for ion transport.

RESULTS

TTC7A-KO HAP1 cells have abnormal morphology and undergo apoptosis, due to increased levels of active caspases 3 and 7. We identified drugs that increased cell viability; leflunomide (used to treat patients with inflammatory conditions) reduced caspase 3 and 7 activity in cells by 96%. TTC7A-KO cells contained cleaved caspase 3 and had reduced levels of phosphorylated AKT and X-linked inhibitor of apoptosis (XIAP); incubation of these cells with leflunomide increased levels of phosphorylated AKT and XIAP and reduced levels of cleaved caspase 3. Administration of leflunomide to ttc7a^-/- zebrafish increased gut motility, reduced intestinal tract narrowing, increased intestinal cell survival, increased sizes of intestinal luminal spaces, and restored villi and goblet cell morphology. Exposure of patient-derived colonoids to leflunomide increased cell survival, polarity, and transport function.

CONCLUSIONS

In a drug screen, we identified leflunomide as an agent that reduces apoptosis and activates AKT signaling in TTC7A-KO cells. In zebrafish with disruption of ttc7a, leflunomide restores gut motility, reduces intestinal tract narrowing, and increases intestinal cell survival. This drug might be repurposed for treatment of TTC7A deficiency.

IRE1β negatively regulates IRE1α signaling in response to endoplasmic reticulum stress

IRE1β is an ER stress sensor uniquely expressed in epithelial cells lining mucosal surfaces. Here, we show that intestinal epithelial cells expressing IRE1β have an attenuated unfolded protein response to ER stress. When modeled in HEK293 cells and with purified protein, IRE1β diminishes expression and inhibits signaling by the closely related stress sensor IRE1α. IRE1β can assemble with and inhibit IRE1α to suppress stress-induced XBP1 splicing, a key mediator of the unfolded protein response. In comparison to IRE1α, IRE1β has relatively weak XBP1 splicing activity, largely explained by a nonconserved amino acid in the kinase domain active site that impairs its phosphorylation and restricts oligomerization. This enables IRE1β to act as a dominant-negative suppressor of IRE1α and affect how barrier epithelial cells manage the response to stress at the host-environment interface.

A quantitative single-cell assay for retrograde membrane traffic enables rapid detection of defects in cellular organization

Retrograde membrane trafficking from plasma membrane to Golgi and endoplasmic reticulum typifies one of the key sorting steps emerging from the early endosome that affects cell surface and intracellular protein dynamics underlying cell function. While some cell surface proteins and lipids are known to sort retrograde, there are few effective methods to quantitatively measure the extent or kinetics of these events. Here we took advantage of the well-known retrograde trafficking of cholera toxin and newly defined split fluorescent protein technology to develop a quantitative, sensitive, and effectively real-time single-cell flow cytometry assay for retrograde membrane transport. The approach can be applied in high throughput to elucidate the underlying biology of membrane traffic and how endosomes adapt to the physiologic needs of different cell types and cell states.

Intestinal epithelial potassium channels and CFTR chloride channels activated in ErbB tyrosine kinase inhibitor diarrhea

Diarrhea is a major side effect of ErbB receptor tyrosine kinase inhibitors (TKIs) in cancer chemotherapy. Here, we show that the primary mechanism of ErbB TKI diarrhea is activation of basolateral membrane potassium (K+) channels and apical membrane chloride (Cl-) channels in intestinal epithelia and demonstrate the efficacy of channel blockers in a rat model of TKI diarrhea. Short-circuit current in colonic epithelial cells showed that the TKIs gefitinib, lapatinib, and afatinib do not affect basal secretion but amplify carbachol-stimulated secretion by 2- to 3-fold. Mechanistic studies with the second-generation TKI afatinib showed that the amplifying effect on Cl- secretion was Ca2+ and cAMP independent, was blocked by CF transmembrane conductance regulator (CFTR) and K+ channel inhibitors, and involved EGFR binding and ERK signaling. Afatinib-amplified activation of basolateral K+ and apical Cl- channels was demonstrated by selective membrane permeabilization, ion substitution, and channel inhibitors. Rats that were administered afatinib orally at 60 mg/kg/day developed diarrhea with increased stool water from approximately 60% to greater than 80%, which was reduced by up to 75% by the K+ channel inhibitors clotrimazole or senicapoc or the CFTR inhibitor (R)-BPO-27. These results indicate a mechanism for TKI diarrhea involving K+ and Cl- channel activation and support the therapeutic efficacy of channel inhibitors.

INAVA-ARNO complexes bridge mucosal barrier function with inflammatory signaling

Homeostasis at mucosal surfaces requires cross-talk between the environment and barrier epithelial cells. Disruption of barrier function typifies mucosal disease. Here we elucidate a bifunctional role in coordinating this cross-talk for the inflammatory bowel disease risk-gene INAVA. Both activities require INAVA’s DUF3338 domain (renamed CUPID). CUPID stably binds the cytohesin ARF-GEF ARNO to effect lateral membrane F-actin assembly underlying cell-cell junctions and barrier function. Unexpectedly, when bound to CUPID, ARNO affects F-actin dynamics in the absence of its canonical activity as a guanine nucleotide-exchange factor. Upon exposure to IL-1β, INAVA relocates to form cytosolic puncta, where CUPID amplifies TRAF6-dependent polyubiquitination and inflammatory signaling. In this case, ARNO binding to CUPID negatively-regulates polyubiquitination and the inflammatory response. INAVA and ARNO act similarly in primary human macrophages responding to IL-1β and to NOD2 agonists. Thus, INAVA-CUPID exhibits dual functions, coordinated directly by ARNO, that bridge epithelial barrier function with extracellular signals and inflammation.

Neonatal-Onset Chronic Diarrhea Caused by Homozygous Nonsense WNT2B Mutations

Homozygous nonsense mutations in WNT2B were identified in three individuals from two unrelated families with severe, neonatal-onset osmotic diarrhea after whole-exome sequencing was performed on trios from the two families. Intestinal biopsy samples from affected individuals were used for histology and immunofluorescence and to generate enteroids ex vivo. Histopathologic evaluation demonstrated chronic inflammatory changes in the stomach, duodenum, and colon. Immunofluorescence demonstrated diminished staining for OLFM4, a marker for intestinal stem cells (ISCs). The enteroids generated from WNT2B-deficient intestinal epithelium could not be expanded and did not survive passage. Addition of CHIR-99021 (a GSK3A and GSK3B inhibitor and activator of canonical WNT/β-CATENIN signaling) could not rescue WNT2B-deficient enteroids. Addition of supplemental recombinant murine WNT2B was able to perpetuate small enteroids for multiple passages but failed to expand their number. Enteroids showed a 10-fold increase in the expression of LEF1 mRNA and a 100-fold reduction in TLR4 expression, compared with controls by quantitative RT-PCR, indicating alterations in canonical WNT and microbial pattern-recognition signaling. In summary, individuals with homozygous nonsense mutations in WNT2B demonstrate severe intestinal dysregulation associated with decreased ISC number and function, likely explaining their diarrheal phenotype. WNT2B deficiency should be considered for individuals with neonatal-onset diarrhea.

Advances in Evaluation of Chronic Diarrhea in Infants

Diarrhea is common in infants (children less than 2 years of age), usually acute, and, if chronic, commonly caused by allergies and occasionally by infectious agents. Congenital diarrheas and enteropathies (CODEs) are rare causes of devastating chronic diarrhea in infants. Evaluation of CODEs is a lengthy process and infrequently leads to a clear diagnosis. However, genomic analyses and the development of model systems have increased our understanding of CODE pathogenesis. With these advances, a new diagnostic approach is needed. We propose a revised approach to determine causes of diarrhea in infants, including CODEs, based on stool analysis, histologic features, responses to dietary modifications, and genetic tests. After exclusion of common causes of diarrhea in infants, the evaluation proceeds through analyses of stool characteristics (watery, fatty, or bloody) and histologic features, such as the villus to crypt ratio in intestinal biopsies. Infants with CODEs resulting from defects in digestion, absorption, transport of nutrients and electrolytes, or enteroendocrine cell development or function have normal villi to crypt ratios; defects in enterocyte structure or immune-mediated conditions result in an abnormal villus to crypt ratios and morphology. Whole-exome and genome sequencing in the early stages of evaluation can reduce the time required for a definitive diagnosis of CODEs, or lead to identification of new variants associated with these enteropathies. The functional effects of gene mutations can be analyzed in model systems such as enteroids or induced pluripotent stem cells and are facilitated by recent advances in gene editing procedures. Characterization and investigation of new CODE disorders will improve management of patients and advance our understanding of epithelial cells and other cells in the intestinal mucosa.

Microbial sphingomyelinase induces RhoA-mediated reorganization of the apical brush border membrane and is protective against invasion

Both commensal and pathogenic microbes that colonize the GI tract can synthesize and secrete spingomyelinase enzymes that cleave membrane sphingomyelin, leaving the ceramide component intact in the cell membrane. This study examines how this reaction affects the structure and function of host enterocytes and mucosal defense.

The apical brush border membrane (BBM) of intestinal epithelial cells forms a highly structured and dynamic environmental interface that serves to regulate cellular physiology and block invasion by intestinal microbes and their products. How the BBM dynamically responds to pathogenic and commensal bacterial signals can define intestinal homeostasis and immune function. We previously found that in model intestinal epithelium, the conversion of apical membrane sphingomyelin to ceramide by exogenous bacterial sphingomyelinase (SMase) protected against the endocytosis and toxicity of cholera toxin. Here we elucidate a mechanism of action by showing that SMase induces a dramatic, reversible, RhoA-dependent alteration of the apical cortical F-actin network. Accumulation of apical membrane ceramide is necessary and sufficient to induce the actin phenotype, and this coincides with altered membrane structure and augmented innate immune function as evidenced by resistance to invasion by Salmonella.

CFTR activator increases intestinal fluid secretion and normalizes stool output in a mouse model of constipation

BACKGROUND & AIMS

Constipation is a common clinical problem that negatively impacts quality of life and is associated with significant health care costs. Activation of the cystic fibrosis transmembrane regulator (CFTR) chloride channel is the primary pathway that drives fluid secretion in the intestine, which maintains lubrication of luminal contents. We hypothesized that direct activation of CFTR would cause fluid secretion and reverse the excessive dehydration of stool found in constipation.

METHODS

A cell-based high-throughput screen was done for 120,000 drug-like, synthetic small molecules. Active compounds were characterized for mechanism of action and one lead compound was tested in a loperamide-induced constipation model in mice.

RESULTS

Several classes of novel CFTR activators were identified, one of which, the phenylquinoxalinone CFTR_act-J027, fully activated CFTR chloride conductance with EC₅₀ ~ 200 nM, without causing elevation of cytoplasmic cAMP. Orally administered CFTR_act-J027 normalized stool output and water content in a loperamide-induced mouse model of constipation with ED₅₀ ~0.5 mg/kg; CFTR_act-J027 was without effect in cystic fibrosis mice lacking functional CFTR. Short-circuit current, fluid secretion and motility measurements in mouse intestine indicated a pro-secretory action of CFTR_act-J027 without direct stimulation of intestinal motility. Oral administration of 10 mg/kg CFTR_act-J027 showed minimal bioavailability, rapid hepatic metabolism and blood levels <200 nM, and without apparent toxicity after chronic administration.

CONCLUSIONS

CFTR_act-J027 or alternative small-molecule CFTR-targeted activators may be efficacious for the treatment of constipation.

Secretory diarrhoea: mechanisms and emerging therapies

Diarrhoeal disease remains a major health burden worldwide. Secretory diarrhoeas are caused by certain bacterial and viral infections, inflammatory processes, drugs and genetic disorders. Fluid secretion across the intestinal epithelium in secretory diarrhoeas involves multiple ion and solute transporters, as well as activation of cyclic nucleotide and Ca(2+) signalling pathways. In many secretory diarrhoeas, activation of Cl(-) channels in the apical membrane of enterocytes, including the cystic fibrosis transmembrane conductance regulator and Ca(2+)-activated Cl(-) channels, increases fluid secretion, while inhibition of Na(+) transport reduces fluid absorption. Current treatment of diarrhoea includes replacement of fluid and electrolyte losses using oral rehydration solutions, and drugs targeting intestinal motility or fluid secretion. Therapeutics in the development pipeline target intestinal ion channels and transporters, regulatory proteins and cell surface receptors. This Review describes pathogenic mechanisms of secretory diarrhoea, current and emerging therapeutics, and the challenges in developing antidiarrhoeal therapeutics.

Congenital chloride-losing diarrhea in a Mexican child with the novel homozygous SLC26A3 mutation G393W

Congenital chloride diarrhea is an autosomal recessive disease caused by mutations in the intestinal lumenal membrane Cl⁻/HCO⁻₃ exchanger, SLC26A3. We report here the novel SLC26A3 mutation G393W in a Mexican child, the first such report in a patient from Central America. SLC26A3 G393W expression in Xenopus oocytes exhibits a mild hypomorphic phenotype, with normal surface expression and moderately reduced anion transport function. However, expression of HA-SLC26A3 in HEK-293 cells reveals intracellular retention and greatly decreased steady-state levels of the mutant polypeptide, in contrast to peripheral membrane expression of the wildtype protein. Whereas wildtype HA-SLC26A3 is apically localized in polarized monolayers of filter-grown MDCK cells and Caco2 cells, mutant HA-SLC26A3 G393W exhibits decreased total polypeptide abundance, with reduced or absent surface expression and sparse punctate (or absent) intracellular distribution. The WT protein is similarly localized in LLC-PK1 cells, but the mutant fails to accumulate to detectable levels. We conclude that the chloride-losing diarrhea phenotype associated with homozygous expression of SLC26A3 G393W likely reflects lack of apical surface expression in enterocytes, secondary to combined abnormalities in polypeptide trafficking and stability. Future progress in development of general or target-specific folding chaperonins and correctors may hold promise for pharmacological rescue of this and similar genetic defects in membrane protein targeting.

Chloride channel inhibition by a red wine extract and a synthetic small molecule prevents rotaviral secretory diarrhoea in neonatal mice

BACKGROUND

Rotavirus is the most common cause of severe secretory diarrhoea in infants and young children globally. The rotaviral enterotoxin, NSP4, has been proposed to stimulate calcium-activated chloride channels (CaCC) on the apical plasma membrane of intestinal epithelial cells. We previously identified red wine and small molecule CaCC inhibitors.

OBJECTIVE

To investigate the efficacy of a red wine extract and a synthetic small molecule, CaCCinh-A01, in inhibiting intestinal CaCCs and rotaviral diarrhoea.

DESIGN

Inhibition of CaCC-dependent current was measured in T84 cells and mouse ileum. The effectiveness of an orally administered wine extract and CaCCinh-A01 in inhibiting diarrhoea in vivo was determined in a neonatal mouse model of rotaviral infection.

RESULTS

Screening of ∼150 red wines revealed a Cabernet Sauvignon that inhibited CaCC current in T84 cells with IC50 at a ∼1:200 dilution, and higher concentrations producing 100% inhibition. A >1 kdalton wine extract prepared by dialysis, which retained full inhibition activity, blocked CaCC current in T84 cells and mouse intestine. In rotavirus-inoculated mice, oral administration of the wine extract prevented diarrhoea by inhibition of intestinal fluid secretion without affecting rotaviral infection. The wine extract did not inhibit the cystic fibrosis chloride channel (CFTR) in cell cultures, nor did it prevent watery stools in neonatal mice administered cholera toxin, which activates CFTR-dependent fluid secretion. CaCCinh-A01 also inhibited rotaviral diarrhoea.

CONCLUSIONS

Our results support a pathogenic role for enterocyte CaCCs in rotaviral diarrhoea and demonstrate the antidiarrhoeal action of CaCC inhibition by an alcohol-free, red wine extract and by a synthetic small molecule.

Altered goblet cell differentiation and surface mucus properties in Hirschsprung disease

Hirschsprung disease-associated enterocolitis (HAEC) leads to significant mortality and morbidity, but its pathogenesis remains unknown. Changes in the colonic epithelium related to goblet cells and the luminal mucus layer have been postulated to play a key role. Here we show that the colonic epithelium of both aganglionic and ganglionic segments are altered in patients and in mice with Hirschsprung disease (HSCR). Structurally, goblet cells were altered with increased goblet cell number and reduced intracellular mucins in the distal colon of biopsies from patients with HSCR. Endothelin receptor B (Ednrb) mutant mice showed increased goblet cell number and size and increased cell proliferation compared to wild-type mice in aganglionic segments, and reduced goblet cell size and number in ganglionic segments. Functionally, compared to littermates, Ednrb^−/− mice showed increased transepithelial resistance, reduced stool water content and similar chloride secretion in the distal colon. Transcript levels of goblet cell differentiation factors SPDEF and Math1 were increased in the distal colon of Ednrb^−/− mice. Both distal colon from Ednrb mice and biopsies from HSCR patients showed reduced Muc4 expression as compared to controls, but similar expression of Muc2. Particle tracking studies showed that mucus from Ednrb^−/− mice provided a more significant barrier to diffusion of 200 nm nanoparticles as compared to wild-type mice. These results suggest that aganglionosis is associated with increased goblet cell proliferation and differentiation and subsequent altered surface mucus properties, prior to the development of inflammation in the distal colon epithelium. Restoration of normal goblet cell function and mucus layer properties in the colonic epithelium may represent a therapeutic strategy for prevention of HAEC.

Discovery and development of antisecretory drugs for treating diarrheal diseases

Diarrheal diseases constitute a significant global health burden and are a major cause of childhood mortality and morbidity. Treatment of diarrheal disease has centered on the replacement of fluid and electrolyte losses using oral rehydration solutions. Although oral rehydration solutions have been highly successful, significant mortality and morbidity due to diarrheal disease remains. Secretory diarrheas, such as those caused by bacterial and viral enterotoxins, result from activation of cyclic nucleotide and/or Ca(2+) signaling pathways in intestinal epithelial cells, enterocytes, which increase the permeability of Cl(-) channels at the lumen-facing membrane. Additionally, there is often a parallel reduction in intestinal Na(+) absorption. Inhibition of enterocyte Cl(-) channels, including the cystic fibrosis transmembrane conductance regulator and Ca(2+)-activated Cl(-) channels, represents an attractive strategy for antisecretory drug therapy. High-throughput screening of synthetic small-molecule collections has identified several classes of Cl(-) channel inhibitors that show efficacy in animal models of diarrhea but remain to be tested clinically. In addition, several natural product extracts with Cl(-) channel inhibition activity have shown efficacy in diarrhea models. However, a number of challenges remain to translate the promising bench science into clinically useful therapeutics, including efficiently targeting orally administered drugs to enterocytes during diarrhea, funding development costs, and carrying out informative clinical trials. Nonetheless, Cl(-) channel inhibitors may prove to be effective adjunctive therapy in a broad spectrum of clinical diarrheas, including acute infectious and drug-related diarrheas, short bowel syndrome, and congenital enteropathies.

CFTR inhibitors

The cystic fibrosis transmembrane conductance regulator (CFTR) protein is a cAMP-regulated Cl- channel whose major function is to facilitate epithelial fluid secretion. Loss-of-function mutations in CFTR cause the genetic disease cystic fibrosis. CFTR is required for transepithelial fluid transport in certain secretory diarrheas, such as cholera, and for cyst expansion in autosomal dominant polycystic kidney disease. High-throughput screening has yielded CFTR inhibitors of the thiazolidinone, glycine hydrazide and quinoxalinedione chemical classes. The glycine hydrazides target the extracellular CFTR pore, whereas the thiazolidinones and quinoxalinediones act at the cytoplasmic surface. These inhibitors have been widely used in cystic fibrosis research to study CFTR function at the cell and organ levels. The most potent CFTR inhibitor has IC50 of approximately 4 nM. Studies in animal models support the development of CFTR inhibitors for antisecretory therapy of enterotoxin-mediated diarrheas and polycystic kidney disease.

Convective washout reduces the antidiarrheal efficacy of enterocyte surface-targeted antisecretory drugs

Secretory diarrheas such as cholera are a major cause of morbidity and mortality in developing countries. We previously introduced the concept of antisecretory therapy for diarrhea using chloride channel inhibitors targeting the cystic fibrosis transmembrane conductance regulator channel pore on the extracellular surface of enterocytes. However, a concern with this strategy is that rapid fluid secretion could cause convective drug washout that would limit the efficacy of extracellularly targeted inhibitors. Here, we developed a convection-diffusion model of washout in an anatomically accurate three-dimensional model of human intestine comprising cylindrical crypts and villi secreting fluid into a central lumen. Input parameters included initial lumen flow and inhibitor concentration, inhibitor dissociation constant (K(d)), crypt/villus secretion, and inhibitor diffusion. We modeled both membrane-impermeant and permeable inhibitors. The model predicted greatly reduced inhibitor efficacy for high crypt fluid secretion as occurs in cholera. We conclude that the antisecretory efficacy of an orally administered membrane-impermeant, surface-targeted inhibitor requires both (a) high inhibitor affinity (low nanomolar K(d)) to obtain sufficiently high luminal inhibitor concentration (>100-fold K(d)), and (b) sustained high luminal inhibitor concentration or slow inhibitor dissociation compared with oral administration frequency. Efficacy of a surface-targeted permeable inhibitor delivered from the blood requires high inhibitor permeability and blood concentration (relative to K(d)).

Airway surface liquid depth imaged by surface laser reflectance microscopy

The thin layer of liquid at the surface of airway epithelium, the airway surface liquid (ASL), is important in normal airway physiology and in the pathophysiology of cystic fibrosis. At present, the best method to measure ASL depth involves scanning confocal microscopy after staining with an aqueous-phase fluorescent dye. We describe here a simple, noninvasive imaging method to measure ASL depth by reflectance imaging of an epithelial mucosa in which the surface is illuminated at a 45-degree angle by an elongated 13-µm wide rectangular beam produced by a 670-nm micro-focus laser. The principle of the method is that air–liquid, liquid–liquid, and liquid–cell interfaces produce distinct specular or diffuse reflections that can be imaged to give a micron-resolution replica of the mucosal surface. The method was validated using fluid layers of specified thicknesses and applied to measure ASL depth in cell cultures and ex vivo fragments of pig trachea. In addition, the method was adapted to measure transepithelial fluid transport from the dynamics of fluid layer depth. Compared with confocal imaging, ASL depth measurement by surface laser reflectance microscopy does not require dye staining or costly instrumentation, and can potentially be adapted for in vivo measurements using fiberoptics.

Inhibition of Ca2+-activated Cl- channels by gallotannins as a possible molecular basis for health benefits of red wine and green tea

TMEM16A was found recently to be a calcium-activated Cl(-) channel (CaCC). CaCCs perform important functions in cell physiology, including regulation of epithelial secretion, cardiac and neuronal excitability, and smooth muscle contraction. CaCC modulators are of potential utility for treatment of hypertension, diarrhea, and cystic fibrosis. Screening of drug and natural product collections identified tannic acid as an inhibitor of TMEM16A, with IC(50) ∼ 6 μM and ∼100% inhibition at higher concentrations. Tannic acid inhibited CaCCs in multiple cell types but did not affect CFTR Cl(-) channels. Structure-activity analysis indicated the requirement of gallic or digallic acid substituents on a macromolecular scaffold (gallotannins), as are present in green tea and red wine. Other polyphenolic components of teas and wines, including epicatechin, catechin, and malvidin-3-glucoside, poorly inhibited CaCCs. Remarkably, a 1000-fold dilution of red wine and 100-fold dilution of green tea inhibited CaCCs by >50%. Tannic acid, red wine, and green tea inhibited arterial smooth muscle contraction and intestinal Cl(-) secretion. Gallotannins are thus potent CaCC inhibitors whose biological activity provides a potential molecular basis for the cardioprotective and antisecretory benefits of red wine and green tea.

Impaired enterocyte proliferation in aquaporin-3 deficiency in mouse models of colitis

BACKGROUND/AIMS

Recent evidence has implicated the involvement of aquaporins (AQPs) in cellular functions that are unrelated to transepithelial water transport. Although AQPs are expressed in the gastrointestinal tract, their importance has so far been unclear. AQP3 is a water/glycerol transporter expressed at the basolateral membrane of colonic epithelial cells. The aim of this study was to investigate the involvement of AQP3 in enterocyte proliferation using mouse models of inflammatory bowel disease.

METHODS

Expression and function of AQP3 in mouse colonic epithelium were established. Colitis was induced in wild-type and AQP3 null mice by oral dextran sulphate administration or intracolonic acetic acid administration. Outcome measures included clinical disease severity, survival, pathology and cellular responses. Some mice were administered glycerol to test whether disease progression could be altered.

RESULTS

AQP3 null mice given dextran sulphate developed severe colitis after 3 days, with colonic haemorrhage, marked epithelial cell loss and death. Wild-type mice, which had comparable initial colonic damage as assessed by cell apoptosis, developed remarkably less severe colitis, surviving to >8 days. Cell proliferation was greatly reduced in AQP3 null mice. Oral glycerol administration significantly improved survival and reduced the severity of colitis in AQP3 null mice. Survival was also reduced in AQP3 null mice in the acetic acid model.

CONCLUSIONS

The results implicate a novel role for AQP3 in enterocyte proliferation that is probably related to its glycerol-transporting function. AQP3 is thus a potential target for therapy of intestinal diseases associated with enterocyte destruction.

Slowed diffusion in tumors revealed by microfiberoptic epifluorescence photobleaching

It has not been possible to measure diffusion deep in solid tissues such as tumors because of the limited light penetration of conventional optical techniques. Here we report a microfiberoptic epifluorescence photobleaching (MFEP) method in which photobleaching is done by laser epi-illumination through a multimode fiberoptic whose micron-sized tip can be introduced deep into tissues. We applied MFEP to measure the diffusion of fluorescent macromolecules in tumors in living mice, at depths well beyond those accessible by surface optical measurements. Macromolecule diffusion was slowed about twofold within 200 microm of the surface of a solid tumor, but was slowed greater than tenfold beyond 500 microm. Our results reveal a remarkable and previously unrecognized slowing of diffusion deep in tumors, which correlated with the differing tissue architectures of tumor periphery versus core, and with altered tumor vasculature produced by aquaporin-1 deletion. MFEP should have wide applications for measuring diffusion in organs, solid tumors and other light-inaccessible tissue masses.

New drug targets for cholera therapy

Intestinal infection with Vibrio cholerae results in secretory diarrhea with potentially massive fluid losses and volume depletion. Morbidity and mortality associated with cholera remain a major problem in the developing world despite the success of oral rehydration therapy. New research aiming to inhibit cholera toxin binding to receptors in the intestine provides an attractive strategy for cholera therapy. Together with anti-secretory agents, including inhibitors of enkephalinase and of the cystic fibrosis transmembrane conductance regulator, new treatment options for managing severe diarrhea in cholera could soon be available.

Noninvasive early detection of brain edema in mice by near-infrared light scattering

Brain edema accounts for significant morbidity and mortality in many neurologic conditions such as head trauma, stroke, meningitis, and brain tumor. The water channel aquaporin-4 (AQP4) has been found to be an important determinant of brain water accumulation and clearance of excess brain water. We report the development of a noninvasive near-infrared (NIR) light-scattering method to compare the early kinetics of brain swelling in normal and AQP4-deficient mice. Brain tissue was illuminated through the intact skull with NIR light at 850 nm, and steady-state scattered light intensity was monitored at an angle of 90 degrees at a position on the skull approximately 10 mm from the illuminated site. NIR light scattering reversibly increased with brain swelling (DeltaI/Io approximately 25% per 1% increase in brain water content), but was insensitive to changes in cerebral blood flow, blood oxygenation, or blood flow-related changes in intracranial pressure (ICP). DeltaI/Io increased approximately linearly with brain water content as measured by wet-to-dry weight ratios. Acute water intoxication (intraperitoneal water, 20% body weight) produced a gradual increase in DeltaI/Io of 12 +/- 4% in wild-type mice at 5 min, much greater than that of 2 +/- 1% in AQP4-null mice. Correlation of the NIR signal with ICP showed that increased DeltaI/Io preceded measurable increases in ICP, indicating the ability of the NIR method to detect early brain edema before ICP elevation. NIR light scattering provides a simple noninvasive method to monitor brain edema in mice, with potential clinical applications.

Submucosal gland dysfunction as a primary defect in cystic fibrosis

It has been proposed that defective submucosal gland function in CF airways is a major determinant of CF airway disease. We tested the hypothesis that submucosal gland function is defective early in CF subjects with minimal clinical disease. Functional assays of gland fluid secretion rate and viscosity were performed on freshly obtained nasal biopsies from 6 CF subjects and 5 non-CF controls (age range 2-22 years). Secretions from individual submucosal glands were visualized by light/fluorescence microscopy after orienting and immobilizing biopsy specimens in a custom chamber. The viscosity of freshly secreted gland fluid after pilocarpine, measured by fluorescence recovery after photobleaching of microinjected FITC-dextran, was 4.9 +/- 0.2- vs. 2.2 +/- 0.2-fold greater than water viscosity in CF vs. non-CF specimens, respectively (SE, P<10(-4)). Gland fluid secretion rate in CF specimens, measured by video imaging (4.5+/-0.5 nL/min/gland, n=6), was 2.7-fold reduced compared to non-CF specimens (n=3, P<0.05). Quantitative histology revealed similar size and morphology of submucosal glands in CF and non-CF specimens. Our results suggest that defective airway submucosal gland function is an early, primary defect in CF. Therapies directed at normalizing gland fluid secretion early in CF may thus reduce lung disease.

CFTR pharmacology and its role in intestinal fluid secretion

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-activated Cl(-) channel expressed in epithelial cells in the airways, pancreas, intestine and other fluid-transporting tissues. Cystic fibrosis is caused by mutations in the CFTR, resulting in impaired Cl(-) transport and plasma membrane targeting. CFTR is expressed in the lumenal membrane of enterocytes, where it functions as the principal pathway for secretion of Cl(-) and fluid in enterotoxin-induced secretory diarrheas such as cholera. Small-molecule CFTR inhibitors reduce enterotoxin-induced intestinal fluid secretion in animal models. CFTR inhibition might also reduce intestinal fluid losses in cholera and possibly in other infectious and non-infectious diarrheas.

A small molecule CFTR inhibitor produces cystic fibrosis-like submucosal gland fluid secretions in normal airways

Airway submucosal glands have been proposed as a primary site for initiating and sustaining airway disease in cystic fibrosis (CF). However, it has been difficult to define the role of CFTR in gland fluid secretion because of concerns in interpreting experiments on diseased CF human airways subjected to chronic infection and inflammation. Here, we test the role of CFTR in gland fluid secretion by using a selective CFTR inhibitor (CFTRinh-172) in pig and human airways. Measurements of single-gland fluid secretion rates showed inhibition of both cholinergic and cAMP-stimulated fluid secretion by CFTRinh-172. Secreted fluid [Na+] and [Cl-] measured by fluorescence ratio imaging were 101 and 116 mM, respectively, and not significantly altered by secretory agonists or CFTR inhibition. Gland fluid pH was 7.1 and reduced by 0.4 units after CFTR inhibition. Gland fluid viscosity, determined by photobleaching of FITC-dextran, was threefold increased in pilocarpine-stimulated gland fluid after CFTR inhibition, and protein concentration was increased from 12 to 20 mg/ml. Our data provide strong evidence that gland fluid secretion is CFTR-dependent. The relatively hyper-viscous and acidic fluid secretions produced by acute CFTR inhibition support a role for defective gland function in CF lung disease and provide a rational basis for pharmacological creation of a large animal model of CF.

Prevention of toxin-induced intestinal ion and fluid secretion by a small-molecule CFTR inhibitor

BACKGROUND & AIMS

The cystic fibrosis transmembrane conductance regulator (CFTR) provides an important apical route for Cl(-) secretion across intestinal epithelia. A thiazolidinone-type CFTR blocker (CFTR(inh)-172) reduced cholera toxin-induced fluid accumulation in mouse intestinal loops. Here, we characterize the efficacy and pharmacodynamics of CFTR(inh)-172 in blocking cAMP and cGMP induced Cl(-)/fluid secretion in rodent and human intestine.

METHODS & RESULTS

CFTR(inh)-172 inhibited cAMP and cGMP agonist induced short-circuit current by >95% in T84 colonic epithelial cells (K(I) approximately 3 micromol/L) and in mouse and human intestinal sheets (K(I) approximately 9 micromol/L). A single intraperitoneal injection of CFTR(inh)-172 (200 microg) blocked intestinal fluid secretion in a rat closed-loop model by >90% for cholera toxin and >70% for STa Escherichia coli toxin. In mice, CFTR(inh)-172 (20 microg) inhibited cholera toxin-induced intestinal fluid secretion by 90% (persistence t(1/2) approximately 10 hours, K(I) approximately 5 microg) and STa toxin by 75% (K(I) approximately 10 microg). Tissue distribution and pharmacokinetic studies indicated intestinal CFTR(inh)-172 accumulation facilitated by enterohepatic circulation. An oral CFTR(inh)-172 preparation reduced fluid secretion by >90% in a mouse open-loop cholera model.

CONCLUSIONS

A small molecule CFTR blocker markedly reduced intestinal ion and fluid secretion caused by cAMP/cGMP-dependent bacterial enterotoxins. CFTR inhibition may thus reduce fluid secretion in infectious secretory diarrheas.

Role of airway surface liquid and submucosal glands in cystic fibrosis lung disease

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) protein, an epithelial chloride channel expressed in the airways, pancreas, testis, and other tissues. A central question is how defective CFTR function in CF leads to chronic lung infection and deterioration of lung function. Several mechanisms have been proposed to explain lung disease in CF, including abnormal airway surface liquid (ASL) properties, defective airway submucosal gland function, altered inflammatory response, defective organellar acidification, loss of CFTR regulation of plasma membrane ion transporters, and others. This review focuses on the physiology of the ASL and submucosal glands with regard to their proposed role in CF lung disease. Experimental evidence for defective ASL properties and gland function in CF is reviewed, and deficiencies in understanding ASL/gland physiology are identified as areas for further investigation. New model systems and measurement technologies are being developed to make progress in establishing lung disease mechanisms in CF, which should facilitate mechanism-based design of therapies for CF.

Thiazolidinone CFTR inhibitor identified by high-throughput screening blocks cholera toxin-induced intestinal fluid secretion

Secretory diarrhea is the leading cause of infant death in developing countries and a major cause of morbidity in adults. The cystic fibrosis transmembrane conductance regulator (CFTR) protein is required for fluid secretion in the intestine and airways and, when defective, causes the lethal genetic disease cystic fibrosis. We screened 50,000 chemically diverse compounds for inhibition of cAMP/flavone-stimulated Cl(-) transport in epithelial cells expressing CFTR. Six CFTR inhibitors of the 2-thioxo-4-thiazolidinone chemical class were identified. The most potent compound discovered by screening of structural analogs, CFTR(inh)-172, reversibly inhibited CFTR short-circuit current in less than 2 minutes in a voltage-independent manner with K(I) approximately 300 nM. CFTR(inh)-172 was nontoxic at high concentrations in cell culture and mouse models. At concentrations fully inhibiting CFTR, CFTR(inh)-172 did not prevent elevation of cellular cAMP or inhibit non-CFTR Cl(-) channels, multidrug resistance protein-1 (MDR-1), ATP-sensitive K(+) channels, or a series of other transporters. A single intraperitoneal injection of CFTR(inh)-172 (250 micro g/kg) in mice reduced by more than 90% cholera toxin-induced fluid secretion in the small intestine over 6 hours. Thiazolidinone CFTR inhibitors may be useful in developing large-animal models of cystic fibrosis and in reducing intestinal fluid loss in cholera and other secretory diarrheas.