cAMP Stimulates SLC26A3 Activity in Human Colon by a CFTR-Dependent Mechanism That Does Not Require CFTR Activity

Metabolic alkalosis in cystic fibrosis: from vascular volume depletion to impaired bicarbonate excretion

Cystic fibrosis (CF) is the most common life-threatening genetic disease in the United States and among people of European descent. Despite the widespread distribution of the cystic fibrosis transmembrane conductance regulator (CFTR) along kidney tubules, specific renal phenotypes attributable to CF have not been well documented. Recent studies have demonstrated the downregulation of the apical Cl-/HCO3 - exchanger pendrin (Slc26a4) in kidney B-intercalated cells of CF mouse models. These studies have shown that kidneys of both mice and humans with CF have an impaired ability to excrete excess HCO3 -, thus developing metabolic alkalosis when subjected to excess HCO3 - intake. The purpose of this minireview is to discuss the latest advances on the role of pendrin as a molecule with dual critical roles in acid base regulation and systemic vascular volume homeostasis, specifically in CF. Given the immense prevalence of vascular volume depletion, which is primarily precipitated via enhanced chloride loss through perspiration, we suggest that the dominant presentation of metabolic alkalosis in CF is due to the impaired function of pendrin, which plays a critical role in systemic vascular volume and acid base homeostasis.

DRA involvement in linaclotide-stimulated bicarbonate secretion during loss of CFTR function

Duodenal bicarbonate secretion is critical to epithelial protection, as well as nutrient digestion and absorption, and is impaired in cystic fibrosis (CF). We examined if linaclotide, typically used to treat constipation, may also stimulate duodenal bicarbonate secretion. Bicarbonate secretion was measured in vivo and in vitro using mouse and human duodenum (biopsies and enteroids). Ion transporter localization was identified with confocal microscopy, and de novo analysis of human duodenal single-cell RNA sequencing (scRNA-Seq) data sets was performed. Linaclotide increased bicarbonate secretion in mouse and human duodenum in the absence of cystic fibrosis transmembrane conductance regulator (CFTR) expression (Cftr-knockout mice) or function (CFTRinh-172). Na+/H+ exchanger 3 inhibition contributed to a portion of this response. Linaclotide-stimulated bicarbonate secretion was eliminated by down-regulated in adenoma (DRA, SLC26A3) inhibition during loss of CFTR activity. ScRNA-Seq identified that 70% of villus cells expressed SLC26A3, but not CFTR, mRNA. Loss of CFTR activity and linaclotide increased apical brush border expression of DRA in non-CF and CF differentiated enteroids. These data provide further insights into the action of linaclotide and how DRA may compensate for loss of CFTR in regulating luminal pH. Linaclotide may be a useful therapy for CF individuals with impaired bicarbonate secretion.

Type 1 diabetes human enteroid studies reveal major changes in the intestinal epithelial compartment

Lack of understanding of the pathophysiology of gastrointestinal (GI) complications in type 1 diabetes (T1D), including altered intestinal transcriptomes and protein expression represents a major gap in the management of these patients. Human enteroids have emerged as a physiologically relevant model of the intestinal epithelium but establishing enteroids from individuals with long-standing T1D has proven difficult. We successfully established duodenal enteroids using endoscopic biopsies from pediatric T1D patients and compared them with aged-matched enteroids from healthy subjects (HS) using bulk RNA sequencing (RNA-seq), and functional analyses of ion transport processes. RNA-seq analysis showed significant differences in genes and pathways associated with cell differentiation and proliferation, cell fate commitment, and brush border membrane. Further validation of these results showed higher expression of enteroendocrine cells, and the proliferating cell marker Ki-67, significantly lower expression of NHE3, lower epithelial barrier integrity, and higher fluid secretion in response to cAMP and elevated calcium in T1D enteroids. Enteroids established from pediatric T1D duodenum identify characteristics of an abnormal intestinal epithelium and are distinct from HS. Our data supports the use of pediatric enteroids as an ex-vivo model to advance studies of GI complications and drug discovery in T1D patients.

Secretin: a hormone for HCO3- homeostasis

Secretin is a key hormone of the intestinal phase of digestion which activates pancreatic, bile duct and Brunner gland HCO3- secretion. Recently, the secretin receptor (SCTR) was also found in the basolateral membrane of the beta-intercalated cell (B-IC) of the collecting duct. Experimental addition of secretin triggers a pronounced activation of urinary HCO3- excretion, which is fully dependent on key functional proteins of the B-IC, namely apical pendrin and CFTR and the basolateral SCTR. Recent studies demonstrated that the SCTR knock-out mouse is unable to respond to an acute base load. Here, SCTR KO mice could not rapidly increase urine base excretion, developed prolonged metabolic alkalosis and exhibited marked compensatory hypoventilation. Here, we review the physiological effects of secretin with distinct focus on how secretin activates renal HCO3- excretion. We describe its new function as a hormone for HCO3- homeostasis.

Bicarbonate secretion and acid/base sensing by the intestine

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 HCO3- ions against a concentration gradient in the 1970s, the fundamental role of HCO3- 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 HCO3- transport in the different segments of the gut. We discuss human diseases with defects in intestinal HCO3- 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.

Mechanistic insights into auxin-enhancing polycyclic aromatic hydrocarbon uptake by wheat roots: Evidence from in situ intracellular pH and root-surface H+ flux

Polycyclic aromatic hydrocarbons (PAHs) are a group of extremely carcinogenic organic pollutants. Our previous findings have demonstrated that plant roots actively take up PAHs through co-transport with H+ ions. Auxin serves as a pivotal regulator of plant growth and development. However, it remains unclear whether the hormone can enhance the uptake of PAHs by plant roots. Hence, the wheat root exposed to PAHs with/without auxins was set to investigate how the auxin promotes the PAHs uptake by roots. In our study, auxin could significantly enhance the uptake of PAHs after 4 h of exposure. After the addition of auxin, the root tissue cytoplasmic pH value was decreased and the H+ influx was observed, indicating that the extracellular space was alkalinized in a short time. The increased H+ influx rate enhanced the uptake of PAHs. In addition, the H+-ATPase activity was also increased, suggesting that auxin activated two distinct and antagonistic H+ flux pathways, and the H+ influx pathway was dominant. Our findings offer important information for exploring the mechanism underlying auxin regulation of PAHs uptake and the phytoremediation of PAH-contaminated soil and water.

Adenosine in Intestinal Epithelial Barrier Function

At the intestinal front, several lines of defense are in place to resist infection and injury, the mucus layer, gut microbiome and strong epithelial junctions, to name a few. Their collaboration creates a resilient barrier. In intestinal disorders, such as inflammatory bowel disease (IBD), barrier function is compromised, which results in rampant inflammation and tissue injury. In response to the destruction, the intestinal epithelium releases adenosine, a small but powerful nucleoside that functions as an alarm signal. Amidst the chaos of inflammation, adenosine aims to restore order. Within the scope of its effects is the ability to regulate intestinal epithelial barrier integrity. This review aims to define the contributions of adenosine to mucus production, microbiome-dependent barrier protection, tight junction dynamics, chloride secretion and acid-base balance to reinforce its importance in the intestinal epithelial barrier.

Calcium-sensing receptor activator cinacalcet for treatment of cyclic nucleotide-mediated secretory diarrheas

Cyclic nucleotide elevation in intestinal epithelial cells is the key pathology causing intestinal fluid loss in secretory diarrheas such as cholera. Current secretory diarrhea treatment is primarily supportive, and oral rehydration solution is the mainstay of cholera treatment. There is an unmet need for safe, simple and effective diarrhea treatments. By promoting cAMP hydrolysis, extracellular calcium-sensing receptor (CaSR) is a regulator of intestinal fluid transport. We studied the antidiarrheal mechanisms of FDA-approved CaSR activator cinacalcet and tested its efficacy in clinically relevant human cell, mouse and intestinal organoid models of secretory diarrhea. By using selective inhibitors, we found that cAMP agonists-induced secretory short-circuit currents (Isc) in human intestinal T84 cells are mediated by collective actions of apical membrane cystic fibrosis transmembrane conductance regulator (CFTR) and Clc-2 Cl- channels, and basolateral membrane K+ channels. 30 μM cinacalcet pretreatment inhibited all 3 components of forskolin and cholera toxin-induced secretory Isc by ∼75%. In mouse jejunal mucosa, cinacalcet inhibited forskolin-induced secretory Isc by ∼60% in wild type mice, with no antisecretory effect in intestinal epithelia-specific Casr knockout mice (Casr-flox; Vil1-cre). In suckling mouse model of cholera induced by oral cholera toxin, single dose (30 mg/kg) oral cinacalcet treatment reduced intestinal fluid accumulation by ∼55% at 20 hours. Lastly, cinacalcet inhibited forskolin-induced secretory Isc by ∼75% in human colonic and ileal organoids. Our findings suggest that CaSR activator cinacalcet has antidiarrheal efficacy in distinct human cell, organoid and mouse models of secretory diarrhea. Considering its excellent clinical safety profile, cinacalcet can be repurposed as a treatment for cyclic nucleotide-mediated secretory diarrheas including cholera.

SLC26A3 (DRA) is stimulated in a synergistic, intracellular Ca2+-dependent manner by cAMP and ATP in intestinal epithelial cells

In polarized intestinal epithelial cells, downregulated in adenoma (DRA) is an apical Cl-/[Formula: see text] exchanger that is part of neutral NaCl absorption under baseline conditions, but in cyclic adenosine monophosphate (cAMP)-driven diarrheas, it is stimulated and contributes to increased anion secretion. To further understand the regulation of DRA in conditions mimicking some diarrheal diseases, Caco-2/BBE cells were exposed to forskolin (FSK) and adenosine 5'-triphosphate (ATP). FSK and ATP stimulated DRA in a concentration-dependent manner, with ATP acting via P2Y1 receptors. FSK at 1 µM and ATP at 0.25 µM had minimal to no effect on DRA given individually; however, together, they stimulated DRA to levels seen with maximum concentrations of FSK and ATP alone. In Caco-2/BBE cells expressing the Ca2+ indicator GCaMP6s, ATP increased intracellular Ca2+ (Ca2+i) in a concentration-dependent manner, whereas FSK (1 µM), which by itself did not significantly alter Ca2+i, followed by 0.25 µM ATP produced a large increase in Ca2+ that was approximately equal to the elevation caused by 1 µM ATP. 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) (BAPTA-AM) pretreatment prevented the ATP and FSK/ATP synergistically increased the DRA activity and the increase in Ca2+i caused by FSK/ATP. FSK/ATP synergistic stimulation of DRA was similarly observed in human colonoids. In Caco-2/BBE cells, subthreshold concentrations of FSK (cAMP) and ATP (Ca2+) synergistically increased Ca2+i and stimulated DRA activity with both being blocked by BAPTA-AM pretreatment. Diarrheal diseases, such as bile acid diarrhea, in which both cAMP and Ca2+ are elevated, are likely to be associated with stimulated DRA activity contributing to increased anion secretion, whereas separation of DRA from Na+/H+ exchanger isoform-3 (NHE3) contributes to reduced NaCl absorption.NEW & NOTEWORTHY The BB Cl-/[Formula: see text] exchanger DRA takes part in both neutral NaCl absorption and stimulated anion secretion. Using intestinal cell line, Caco-2/BBE high concentrations of cAMP and Ca2+ individually stimulated DRA activity, whereas low concentrations, which had no/minimal effect, synergistically stimulated DRA activity that required a synergistic increase in intracellular Ca2+. This study increases understanding of diarrheal diseases, such as bile salt diarrhea, in which both cAMP and elevated Ca2+ are involved.

The Air-Liquid Interface Reorganizes Membrane Lipids and Enhances the Recruitment of Slc26a3 to Lipid-Rich Domains in Human Colonoid Monolayers

Cholesterol-rich membrane domains, also called lipid rafts (LRs), are specialized membrane domains that provide a platform for intracellular signal transduction. Membrane proteins often cluster in LRs that further aggregate into larger platform-like structures that are enriched in ceramides and are called ceramide-rich platforms (CRPs). The role of CRPs in the regulation of intestinal epithelial functions remains unknown. Down-regulated in adenoma (DRA) is an intestinal Cl-/HCO3- antiporter that is enriched in LRs. However, little is known regarding the mechanisms involved in the regulation of DRA activity. The air-liquid interface (ALI) was created by removing apical media for a specified number of days; from 12-14 days post-confluency, Caco-2/BBe cells or a colonoid monolayer were grown as submerged cultures. Confocal imaging was used to examine the dimensions of membrane microdomains that contained DRA. DRA expression and activity were enhanced in Caco-2/BBe cells and human colonoids using an ALI culture method. ALI causes an increase in acid sphingomyelinase (ASMase) activity, an enzyme responsible for enhancing ceramide content in the plasma membrane. ALI cultures expressed a larger number of DRA-containing platforms with dimensions >2 µm compared to cells grown as submerged cultures. ASMase inhibitor, desipramine, disrupted CRPs and reduced the ALI-induced increase in DRA expression in the apical membrane. Exposing normal human colonoid monolayers to ALI increased the ASMase activity and enhanced the differentiation of colonoids along with basal and forskolin-stimulated DRA activities. ALI increases DRA activity and expression by increasing ASMase activity and platform formation in Caco-2/BBe cells and by enhancing the differentiation of colonoids.

Impaired distal colonic pH in adults with cystic fibrosis

Previous wireless motility capsule (WMC) studies demonstrated decreased small intestinal pH in people with CF (PwCF) however the data is lacking on the colonic pH profile. We re-analyzed previously published WMC data to determine colonic pH/bicarbonate concentration and single cell RNA sequencing (sc-RNAseq) to examine the normal expression of acid-base transporters in the colon/rectum.CF patients showed significantly lower pH and bicarbonate concentration values, particularly in the distal rectosigmoid region. There was no difference in colonic motility parameters between CF and non-CF subjects. SLC26A3 is highly expressed bicarbonate transporter in the colon and rectum, more so than CFTR. While dysmotility can alter intraluminal pH, observed changes likely originate from alterations in intestinal ion transport rather than colonic dysmotility. SLC26A3 is abundantly expressed in the human colon and rectum and may be a therapeutic target for restoration of bicarbonate transport. These findings may help better understand the gastrointestinal symptoms in PwCF.

Human Colonoid-Myofibroblast Coculture for Study of Apical Na+/H+ Exchangers of the Lower Cryptal Neck Region

Cation and anion transport in the colonocyte apical membrane is highly spatially organized along the cryptal axis. Because of lack of experimental accessibility, information about the functionality of ion transporters in the colonocyte apical membrane in the lower part of the crypt is scarce. The aim of this study was to establish an in vitro model of the colonic lower crypt compartment, which expresses the transit amplifying/progenitor (TA/PE) cells, with accessibility of the apical membrane for functional study of lower crypt-expressed Na⁺/H⁺ exchangers (NHEs). Colonic crypts and myofibroblasts were isolated from human transverse colonic biopsies, expanded as three-dimensional (3D) colonoids and myofibroblast monolayers, and characterized. Filter-grown colonic myofibroblast-colonic epithelial cell (CM-CE) cocultures (myofibroblasts on the bottom of the transwell and colonocytes on the filter) were established. The expression pattern for ion transport/junctional/stem cell markers of the CM-CE monolayers was compared with that of nondifferentiated (EM) and differentiated (DM) colonoid monolayers. Fluorometric pH_i measurements were performed to characterize apical NHEs. CM-CE cocultures displayed a rapid increase in transepithelial electrical resistance (TEER), paralleled by downregulation of claudin-2. They maintained proliferative activity and an expression pattern resembling TA/PE cells. The CM-CE monolayers displayed high apical Na⁺/H⁺ exchange activity, mediated to >80% by NHE2. Human colonoid-myofibroblast cocultures allow the study of ion transporters that are expressed in the apical membrane of the nondifferentiated colonocytes of the cryptal neck region. The NHE2 isoform is the predominant apical Na⁺/H⁺ exchanger in this epithelial compartment.

Oxalate secretion is stimulated by a cAMP-dependent pathway in the mouse cecum

Elevated levels of the intracellular second messenger cAMP can stimulate intestinal oxalate secretion however the membrane transporters responsible are unclear. Oxalate transport by the chloride/bicarbonate (Cl-/HCO3-) exchanger Slc26a6 or PAT-1 (Putative Anion Transporter 1), is regulated via cAMP when expressed in Xenopus oocytes and cultured cells but whether this translates to the native epithelia is unknown. This study investigated the regulation of oxalate transport by the mouse intestine focusing on transport at the apical membrane hypothesizing PAT-1 is the target of a cAMP-dependent signaling pathway. Adopting the Ussing chamber technique we measured unidirectional 14C-oxalate and 36Cl- flux ([Formula: see text] and [Formula: see text]) across distal ileum, cecum and distal colon, employing forskolin (FSK) and 3-isobutyl-1-methylxanthine (IBMX) to trigger cAMP production. FSK/IBMX initiated a robust secretory response by all segments but the stimulation of net oxalate secretion was confined to the cecum only involving activation of [Formula: see text] and distinct from net Cl- secretion produced by inhibiting [Formula: see text]. Using the PAT-1 knockout (KO) mouse we determined cAMP-stimulated [Formula: see text] was not directly dependent on PAT-1, but it was sensitive to mucosal DIDS (4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid), although unlikely to be another Cl-/HCO3- exchanger given the lack of trans-stimulation or cis-inhibition by luminal Cl- or HCO3-. The cAMP-activated oxalate efflux was reliant on CFTR (Cystic Fibrosis Transmembrane conductance Regulator) activity, but only in the presence of PAT-1, leading to speculation on the involvement of a multi-transporter regulatory complex. Further investigations at the cellular and molecular level are necessary to define the mechanism and transporter(s) responsible.

Regulation of nutrient and electrolyte absorption in human organoid-derived intestinal epithelial cell monolayers

Recently developed human intestinal epithelial 3D organoid cultures are a useful cell culture model to study intestinal transport physiology. From these, 2D monolayer cultures can be generated in which apical transporters are exposed to the medium, thereby better facilitating in vitro investigation of intestinal absorption processes. However, whether nutrient and electrolyte absorption can be physiologically regulated in human organoid-derived monolayers has not been determined. Constitutive nitric oxide (cNO) is known to regulate multiple gastrointestinal physiological functions. Previous studies using in vivo and in vitro mammalian animal models indicate that enhanced intracellular cNO differentially regulates the two primary apical Na transporters in small intestinal epithelial cells. Here, we generated human jejunal organoid-derived monolayers to determine whether apical nutrient and electrolyte transporter function is regulated by cNO in human enterocytes. Western blot analysis and immunocytochemical staining showed that organoid-derived 2D cultures express markers of enterocyte differentiation and form intact monolayers of apical-basal polarized epithelial cells. Uptake studies demonstrated that jejunal monolayers exhibit functional activity of Na-glucose cotransporter 1 (SGLT1; SLC5A1) and Na-H exchanger 3 (NHE3; SLC9A3). In response to physiological increases in cNO, the two primary apical Na transporters were differentially regulated in human intestinal organoid-derived monolayers, across multiple human specimens. An increase in cNO stimulated SGLT1, while NHE3 was inhibited. These results are similar to what is seen in vivo and in vitro in different animal intestinal models. Thus, human jejunal organoid-derived monolayers are an ideal in vitro model to better understand how intestinal nutrient absorption is regulated.

An integrative transcriptome analysis reveals potential predictive, prognostic biomarkers and therapeutic targets in colorectal cancer

BACKGROUND

A deep understanding of potential molecular biomarkers and therapeutic targets related to the progression of colorectal cancer (CRC) from early stages to metastasis remain mostly undone. Moreover, the regulation and crosstalk among different cancer-driving molecules including messenger RNAs (mRNAs), long non-coding RNAs (lncRNAs) and micro-RNAs (miRNAs) in the transition from stage I to stage IV remain to be clarified, which is the aim of this study.

METHODS

We carried out two separate differential expression analyses for two different sets of samples (stage-specific samples and tumor/normal samples). Then, by the means of robust dataset analysis we identified distinct lists of differently expressed genes (DEGs) for Robust Rank Aggregation (RRA) and weighted gene co-expression network analysis (WGCNA). Then, comprehensive computational systems biology analyses including mRNA-miRNA-lncRNA regulatory network, survival analysis and machine learning algorithms were also employed to achieve the aim of this study. Finally, we used clinical samples to carry out validation of a potential and novel target in CRC.

RESULTS

We have identified the most significant stage-specific DEGs by combining distinct results from RRA and WGCNA. After finding stage-specific DEGs, a total number of 37 DEGs were identified to be conserved across all stages of CRC (conserved DEGs). We also found DE-miRNAs and DE-lncRNAs highly associated to these conserved DEGs. Our systems biology approach led to the identification of several potential therapeutic targets, predictive and prognostic biomarkers, of which lncRNA LINC00974 shown as an important and novel biomarker.

CONCLUSIONS

Findings of the present study provide new insight into CRC pathogenesis across all stages, and suggests future assessment of the functional role of lncRNA LINC00974 in the development of CRC.

Identification of Intestinal NaCl Absorptive-Anion Secretory Cells: Potential Functional Significance

Use of human enteroids studied in the undifferentiated and differentiated state that mimic the intestinal crypt and villus, respectively, has allowed studies of multiple enterocyte populations, including a large population of enterocytes that are transitioning from the crypt to the villus. This population expresses NHE3, DRA, and CFTR, representing a combination of Na absorptive and anion secretory functions. In this cell population, these three transporters physically interact, which affects their baseline and regulated activities. A study of this cell population and differentiated Caco-2 cells transduced with NHE3 and endogenously expressing DRA and CFTR has allowed an understanding of previous studies in which cAMP seemed to stimulate and inhibit DRA at the same time. Understanding the contributions of these cells to overall intestinal transport function as part of the fasting and post-prandial state and their contribution to the pathophysiology of diarrheal diseases and some conditions with constipation will allow new approaches to drug development.

Colonic Fluid and Electrolyte Transport 2022: An Update

Colonic epithelial cells are responsible for maintaining a delicate balance between luminal secretion and the absorption of fluids and ions. This review aims to discuss and update the model of colonic electrolyte secretion and absorption via the cystic fibrosis transmembrane regulator (CFTR), epithelial sodium channel (ENaC), Na-K-Cl cotransporters (NKCC1 and 2), Na-H exchangers (NHE1–4), colonic H,KATPase, and several other key components involved in multi-level transepithelial ion transport. Developments in our understanding of the activity, regulation, localization, and relationships of these ion transporters and their interactions have helped forge a more robust understanding of colonic ion movement that accounts for the colonic epithelium’s role in mucosal pH modulation, the setting of osmotic gradients pivotal for fluid retention and secretion, and cell death regulation. Deviations from homeostatic ion transport cause diarrhea, constipation, and epithelial cell death and contribute to cystic fibrosis, irritable bowel syndrome (IBS), ulcerative colitis, and cancer pathologies. Signal transduction pathways that regulate electrolyte movement and the regulatory relationships between various sensors and transporters (CFTR as a target of CaSR regulation and as a regulator of ENaC and DRA, for example) are imperative aspects of a dynamic and comprehensive model of colonic ion homeostasis.

Role of SLC4 and SLC26 solute carriers during oxidative stress

Bicarbonate is one of the major anions in mammalian tissues and fluids, is utilized by various exchangers to transport other ions and organic substrates across cell membranes and plays a critical role in cell and systemic pH homoeostasis. Chloride/bicarbonate (Cl⁻/HCO₃⁻) exchangers are abundantly expressed in erythrocytes and epithelial cells and, as a consequence, are particularly exposed to oxidants in the systemic circulation and at the interface with the external environment. Here, we review the physiological functions and pathophysiological alterations of Cl⁻/HCO₃⁻ exchangers belonging to the solute carriers SLC4 and SLC26 superfamilies in relation to oxidative stress. Particularly well studied is the impact of oxidative stress on the red blood cell SLC4A1/AE1 (Band 3 protein), of which the function seems to be directly affected by oxidative stress and possibly involves oxidation of the transporter itself or its interacting proteins, with detrimental consequences in oxidative stress‐related diseases including inflammation, metabolic dysfunctions and ageing. The effect of oxidative stress on SLC26 members was less extensively explored. Indirect evidence suggests that SLC26 transporters can be target as well as determinants of oxidative stress, especially when their expression is abolished or dysregulated.

mOrange2, a Genetically Encoded, pH Sensitive Fluorescent Protein, is an Alternative to BCECF-AM to Measure Intracellular pH to Determine NHE3 and DRA Activity

BACKGROUND/AIMS

NHE3 (Na+/H+ exchanger3) and SLC26A3 (Cl-/HCO3- exchanger, DRA) are the major components of the intestinal neutral NaCl absorptive process and based on the intestinal segment, contribute to HCO3- absorption and HCO3- secretion. NHE3 and DRA are highly regulated by changes in second messengers, cAMP, cGMP and Ca2+. Precise and convenient measurement of exchanger activity is necessary to allow rapid study of physiologic and pharmacologic functions. Some epithelial cells are difficult to load with AM ester dyes and loading may not be uniform.

METHODS

The use of a genetically modified fluorescent protein, mOrange2 was explored as an intracellular pH sensor protein to measure exchange activity of NHE3 and DRA. The model used was FRT cells stably expressing NHE3 or DRA with intracellular pH measured by changes of mOrange2 fluorescence intensity. Intracellular pH was monitored using a) Isolated single clones of FRT/mOrange2/HA-NHE3 cells studied in a confocal microscope with time-lapse live cell imaging under basal conditions and when NHE3 was inhibited by exposure to forskolin and stimulated by dexamethasone, b) coverslip grown FRT/mOrange2 cells expressing NHE3 or DRA using a computerized fluorometer with a perfused cuvette with standardization of the mOrange2 absorption and emission signal using K+/Nigericin as an internal standard in each experiment.

RESULTS

A similar rate of intracellular alkalization by Na+ addition in cells expressing NHE3 and by Cl- removal in cells expressing DRA was found in mOrange2 expressing cells compared to the same cells loaded with BCECF-AM,
both using the same pH calibration with K+/Nigericin. Using mOrange2 as the pH sensor, NHE3 basal activity was quantitated and shown to be inhibited by forskolin and stimulated by dexamethasone, and DRA was oppositely shown to be stimulated by forskolin, responses similar to results found using BCECF-AM.

CONCLUSION

This study demonstrates that mOrange2 protein can be an effective alternate to BCECF-AM in measuring intracellular pH (preferred setting Ex520nm, Em 563nm) as affected by NHE3 and DRA activity, with the advantage, compared to AM ester dyes, that genetic expression can provide uniform expression of the pH sensor.

SATB2 Defect Promotes Colitis and Colitis-associated Colorectal Cancer by Impairing Cl-/HCO3- Exchange and Homeostasis of Gut Microbiota

BACKGROUND

SATB2 is a diagnostic biomarker and a favourable prognostic marker for colorectal cancer [CRC], but its role in colitis and colitis-associated colorectal cancer [CAC] is unknown.

METHODS

Colitis was induced in intestinal epithelial-specific Satb2 knockout [Satb2 IEC-KO] and control mice using dextran sulphate sodium [DSS]. RNA-seq analysis was performed on colonic tissues, and 16S rDNA-Seq on faecal bacterial DNA from Satb2 IEC-KO and control mice. Immunohistochemistry and flow cytometry were performed to reveal the proportions of different immune cells. Chromatin immunoprecipitation [ChIP] and luciferase reporter were applied to show the regulatory role of SATB2 on SLC26A3, of which the Cl-/HCO3- exchange activity was measured fluorometrically by the pHi-sensitive dye. Bacteroides were detected by fluorescence in situ hybridisation [FISH] on colonic tissue.

RESULTS

Satb2 IEC-KO mice suffered from intestinal epithelial damage spontaneously, and developed more severe colitis and CAC. The expression of SLC26A3 correlated well with SATB2 revealed by RNA-seq and The Cancer Genome Atlas [TCGA] data, and was governed by SATB2 confirmed by ChIP and luciferase reporter experiments. Decreased intestinal flora diversity was seen in Satb2 IEC-KO mice. Bacteroides were more abundant and could colonise into the inner layer of colonic mucosa in Satb2 IEC-KO mice. Faecal microbiome transplantation from Satb2 IEC-KO mice aggravated colitis and M1 macrophages infiltration.

CONCLUSIONS

SATB2 plays a vital role in maintaining intestinal homeostasis, and its deficiency promotes the development of colitis and CAC by influencing the intestinal luminal environment and gut flora.

Chronic Inflammation in Ulcerative Colitis Causes Long-Term Changes in Goblet Cell Function

BACKGROUND & AIMS

One of the features of ulcerative colitis (UC) is a defect in the protective mucus layer. This has been attributed to a reduced number of goblet cells (GCs). However, it is not known whether abnormal GC mucus secretion also contributes to the reduced mucus layer. Our aims were to investigate whether GC secretion was abnormal in UC and exists as a long-term effect of chronic inflammation.

METHODS

Colonoids were established from intestinal stem cells of healthy subjects (HS) and patients with UC. Colonoids were maintained as undifferentiated (UD) or induced to differentiate (DF) and studied as three-dimensional or monolayers on Transwell filters. Total RNA was extracted for quantitative real-time polymerase chain reaction analysis. Carbachol and prostaglandin E2 mediated mucin stimulation was examined by MUC2 IF/confocal microscopy and transmission electron microscopy.

RESULTS

Colonoids from UC patients can be propagated over many passages; however, they exhibit a reduced rate of growth and transepithelial electrical resistance compared with HS. Differentiated UC colonoid monolayers form a thin and non-continuous mucus layer. UC colonoids have increased expression of secretory lineage markers ATOH1 and SPDEF, along with MUC2 positive GCs, but failed to secrete mucin in response to the cholinergic agonist carbachol and prostaglandin E2, which caused increased secretion in HS. Exposure to tumor necrosis factor α (5 days) reduced the number of GCs, with a greater percentage decrease in UC colonoids compared with HS.

CONCLUSIONS

Chronic inflammation in UC causes long-term changes in GCs, leading to abnormal mucus secretion. This continued defect in GC mucus secretion may contribute to the recurrence in UC.

A Novel Microphysiological Colon Platform to Decipher Mechanisms Driving Human Intestinal Permeability

BACKGROUND & AIMS

The limited availability of organoid systems that mimic the molecular signatures and architecture of human intestinal epithelium has been an impediment to allowing them to be harnessed for the development of therapeutics as well as physiological insights. We developed a microphysiological Organ-on-Chip (Emulate, Inc, Boston, MA) platform designed to mimic properties of human intestinal epithelium leading to insights into barrier integrity.

METHODS

We combined the human biopsy-derived leucine-rich repeat-containing G-protein-coupled receptor 5-positive organoids and Organ-on-Chip technologies to establish a micro-engineered human Colon Intestine-Chip (Emulate, Inc, Boston, MA). We characterized the proximity of the model to human tissue and organoids maintained in suspension by RNA sequencing analysis, and their differentiation to intestinal epithelial cells on the Colon Intestine-Chip under variable conditions. Furthermore, organoids from different donors were evaluated to understand variability in the system. Our system was applied to understanding the epithelial barrier and characterizing mechanisms driving the cytokine-induced barrier disruption.

RESULTS

Our data highlight the importance of the endothelium and the in vivo tissue-relevant dynamic microenvironment in the Colon Intestine-Chip in the establishment of a tight monolayer of differentiated, polarized, organoid-derived intestinal epithelial cells. We confirmed the effect of interferon-γ on the colonic barrier and identified reorganization of apical junctional complexes, and induction of apoptosis in the intestinal epithelial cells as mediating mechanisms. We show that in the human Colon Intestine-Chip exposure to interleukin 22 induces disruption of the barrier, unlike its described protective role in experimental colitis in mice.

CONCLUSIONS

We developed a human Colon Intestine-Chip platform and showed its value in the characterization of the mechanism of action of interleukin 22 in the human epithelial barrier. This system can be used to elucidate, in a time- and challenge-dependent manner, the mechanism driving the development of leaky gut in human beings and to identify associated biomarkers.

The molecular mechanism of CFTR- and secretin-dependent renal bicarbonate excretion

This review summarizes the newly discovered molecular mechanism of secretin-stimulated urine HCO₃ ^- excretion and the role of cystic fibrosis transmembrane conductance regulator (CFTR) in renal HCO₃ ^- excretion. The secretin receptor is functionally expressed in the basolateral membrane of the HCO₃ ^- -secreting β-intercalated cells of the collecting duct. Here it activates a fast and efficient secretion of HCO₃ ^- into the urine serving to normalize metabolic alkalosis. The ability to acutely increase renal base excretion is entirely dependent on functional pendrin (SLC26A4) and CFTR, and both proteins localize to the apical membrane of the β-intercalated cells. In cystic fibrosis mice and patients, this function is absent or markedly reduced. We discuss that the alkaline tide, namely the transient urine alkalinity after a meal, has now received a clear physiological explanation.

Slc26a3 (DRA) in the Gut: Expression, Function, Regulation, Role in Infectious Diarrhea and Inflammatory Bowel Disease

BACKGROUND

The transport of transepithelial Cl- and HCO3- is crucial for the function of the intestinal epithelium and maintains the acid-based homeostasis. Slc26a3 (DRA), as a key chloride-bicarbonate exchanger protein in the intestinal epithelial luminal membrane, participates in the electroneutral NaCl absorption of intestine, together with Na+/H+ exchangers. Increasing recent evidence supports the essential role of decreased DRA function or expression in infectious diarrhea and inflammatory bowel disease (IBD).

METHOD

In this review, we give an overview of the current knowledge of Slc26a3, including its cloning and expression, function, roles in infectious diarrhea and IBD, and mechanisms of actions. A better understanding of the physiological and pathophysiological relevance of Slc26a3 in infectious diarrhea and IBD may reveal novel targets for future therapy.

CONCLUSION

Understanding the physiological function, regulatory interactions, and the potential mechanisms of Slc26a3 in the pathophysiology of infectious diarrhea and IBD will define novel therapeutic approaches in future.

Bayesian Model Infers Drug Repurposing Candidates for Treatment of COVID-19

The emergence of COVID-19 progressed into a global pandemic that has functionally put the world at a standstill and catapulted major healthcare systems into an overburdened state. The dire need for therapeutic strategies to mitigate and successfully treat COVID-19 is now a public health crisis with national security implications for many countries. The current study employed Bayesian networks to a longitudinal proteomic dataset generated from Caco-2 cells transfected with SARS-CoV-2 (isolated from patients returning from Wuhan to Frankfurt). Two different approaches were employed to assess the Bayesian models, a titer-center topology analysis and a drug signature enrichment analysis. Topology analysis identified a set of proteins directly linked to the SAR-CoV2 titer, including ACE2, a SARS-CoV-2 binding receptor, MAOB and CHECK1. Aligning with the topology analysis, MAOB and CHECK1 were also identified within the enriched drug-signatures. Taken together, the data output from this network has identified nodal host proteins that may be connected to 18 chemical compounds, some already marketed, which provides an immediate opportunity to rapidly triage these assets for safety and efficacy against COVID-19.

Fluid shear stress enhances differentiation of jejunal human enteroids in Intestine-Chip

There is increasing evidence that the study of normal human enteroids duplicates many known aspects of human intestinal physiology. However, this epithelial cell-only model lacks the many nonepithelial intestinal cells present in the gastrointestinal tract and exposure to the mechanical forces to which the intestine is exposed. We tested the hypothesis that physical shear forces produced by luminal and blood flow would provide an intestinal model more closely resembling normal human jejunum. Jejunal enteroid monolayers were studied in the Emulate, Inc. Intestine-Chip under conditions of constant luminal and basolateral flow that was designed to mimic normal intestinal fluid flow, with human umbilical vein endothelial cells (HUVECs) on the basolateral surface and with Wnt3A, R-spondin, and Noggin only on the luminal surface. The jejunal enteroids formed monolayers that remained confluent for 6-8 days, began differentiating at least as early as day 2 post plating, and demonstrated continuing differentiation over the entire time of the study, as shown by quantitative real-time polymerase chain reaction and Western blot analysis. Differentiation impacted villus genes and proteins differently with early expression of regenerating family member 1α (REG1A), early reduction to a low but constant level of expression of Na⁺-K⁺-2Cl^- cotransporter 1 (NKCC1), and increasing expression of sucrase-isomaltase (SI) and downregulated in adenoma (DRA). These results were consistent with continual differentiation, as was shown to occur in mouse villus enterocytes. Compared with differentiated enteroid monolayers grown on Transwell inserts, enteroids exposed to flow were more differentiated but exhibited increased apoptosis and reduced carbohydrate metabolism, as shown by proteomic analysis. This study of human jejunal enteroids-on-chip suggests that luminal and basolateral flow produce a model of continual differentiation over time and NaCl absorption that mimics normal intestine and should provide new insights in intestinal physiology.NEW & NOTEWORTHY This study showed that polarized enteroid models in which there is no basolateral Wnt3a, are differentiated, regardless of the Wnt3a status of the apical media. The study supports the concept that in the human intestine villus differentiation is not an all or none phenomenon, demonstrating that at different days after lack of basolateral Wnt exposure, clusters of genes and proteins exist geographically along the villus with different domains having different functions.

Pharmacological and Parenteral Nutrition-Based Interventions in Microvillus Inclusion Disease

Microvillus inclusion disease (MVID) is a rare inherited and invariably fatal enteropathy, characterized by severe intractable secretory diarrhea and nutrient malabsorption. No cure exists, and patients typically die during infancy because of treatment-related complications. The need for alternative treatment strategies is evident. Several pharmacological interventions with variable successes have been tried and reported for individual patients as part of their clinical care. Unfortunately, these interventions and their outcomes have remained hidden in case reports and have not been reviewed. Further, recent advances regarding MVID pathogenesis have shed new light on the outcomes of these pharmacological interventions and offer suggestions for future clinical research and trials. Hence, an inventory of reported pharmacological interventions in MVID, their rationales and outcomes, and a discussion of these in the light of current knowledge is opportune. Together with a discussion on MVID-specific pharmacokinetic, -dynamic, and -genetic concerns that pose unique challenges regarding pharmacological strategies, we envision that this paper will aid researchers and clinicians in their efforts to develop pharmacological interventions to combat this devastating disease.

Identification of microRNA-451a as a Novel Circulating Biomarker for Colorectal Cancer Diagnosis

Background

Colorectal cancer (CRC) is one of the leading causes of cancer death worldwide. Successful treatment of CRC relies on accurate early diagnosis, which is currently a challenge due to its complexity and personalized pathologies. Thus, novel molecular biomarkers are needed for early CRC detection.

Methods

Gene and microRNA microarray profiling of CRC tissues and miRNA-seq data were analyzed. Candidate microRNA biomarkers were predicted using both CRC-specific network and miRNA-BD tool. Validation analyses were carried out to interrogate the identified candidate CRC biomarkers.

Results

We identified miR-451a as a potential early CRC biomarker circulating in patient's serum. The dysregulation of miR-451a was revealed both in primary tumors and in patients' sera. Downstream analysis validated the tumor suppressor role of miR-451a and high sensitivity of miR-451a in CRC patients, further confirming its potential role as CRC circulation biomarker.

Conclusion

The miR-451a is a potential circulating biomarker for early CRC diagnosis.

Quantitative phase imaging to study transmembrane water fluxes regulated by CFTR and AQP3 in living human airway epithelial CFBE cells and CHO cells

In epithelial cells, the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-regulated Cl^- channel, plays a key role in water and electrolytes secretion. A dysfunctional CFTR leads to the dehydration of the external environment of the cells and to the production of viscous mucus in the airways of cystic fibrosis patients. Here, we applied the quadriwave lateral shearing interferometry (QWLSI), a quantitative phase imaging technique based on the measurement of the light wave shift when passing through a living sample, to study water transport regulation in human airway epithelial CFBE and CHO cells expressing wild-type, G551D- and F508del-CFTR. We were able to detect phase variations during osmotic challenges and confirmed that cellular volume changes reflecting water fluxes can be detected with QWLSI. Forskolin stimulation activated a phase increase in all CFBE and CHO cell types. This phase variation was due to cellular volume decrease and intracellular refractive index increase and was completely blocked by mercury, suggesting an activation of a cAMP-dependent water efflux mediated by an endogenous aquaporin (AQP). AQP3 mRNAs, not AQP1, AQP4 and AQP5 mRNAs, were detected by RT-PCR in CFBE cells. Readdressing the F508del-CFTR protein to the cell surface with VX-809 increased the detected water efflux in CHO but not in CFBE cells. However, VX-770, a potentiator of CFTR function, failed to further increase the water flux in either G551D-CFTR or VX-809-corrected F508del-CFTR expressing cells. Our results show that QWLSI could be a suitable technique to study water transport in living cells. We identified a CFTR and cAMP-dependent, mercury-sensitive water transport in airway epithelial and CHO cells that might be due to AQP3. This water transport appears to be affected when CFTR is mutated and independent of the chloride channel function of CFTR.

Cystic Fibrosis, CFTR, and Colorectal Cancer

Cystic fibrosis (CF), caused by biallelic inactivating mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, has recently been categorized as a familial colorectal cancer (CRC) syndrome. CF patients are highly susceptible to early, aggressive colorectal tumor development. Endoscopic screening studies have revealed that by the age of forty 50% of CF patients will develop adenomas, with 25% developing aggressive advanced adenomas, some of which will have already advanced to adenocarcinomas. This enhanced risk has led to new CF colorectal cancer screening recommendations, lowering the initiation of endoscopic screening to age forty in CF patients, and to age thirty in organ transplant recipients. The enhanced risk for CRC also extends to the millions of people (more than 10 million in the US) who are heterozygous carriers of CFTR gene mutations. Further, lowered expression of CFTR is reported in sporadic CRC, where downregulation of CFTR is associated with poor survival. Mechanisms underlying the actions of CFTR as a tumor suppressor are not clearly understood. Dysregulation of Wnt/β-catenin signaling and disruption of intestinal stem cell homeostasis and intestinal barrier integrity, as well as intestinal dysbiosis, immune cell infiltration, stress responses, and intestinal inflammation have all been reported in human CF patients and in animal models. Notably, the development of new drug modalities to treat non-gastrointestinal pathologies in CF patients, especially pulmonary disease, offers hope that these drugs could be repurposed for gastrointestinal cancers.

Adaptation to inflammatory acidity through neutrophil-derived adenosine regulation of SLC26A3

Acute intestinal inflammation includes the early accumulation of neutrophils (PMN). Based on recent evidence that PMN infiltration "imprints" changes in the local tissue environment through local oxygen depletion and the release of adenine nucleotides, we hypothesized that the interaction between transmigrating PMN and intestinal epithelial cells (IECs) results in inflammatory acidification of the tissue. Using newly developed tools, we revealed that active PMN transepithelial migration (TEM) significantly acidifies the local microenvironment, a decrease of nearly 2 pH units. Using unbiased approaches, we sought to define acid-adaptive pathways elicited by PMN TEM. Given the significant amount of adenosine (Ado) generated during PMN TEM, we profiled the influence of Ado on IECs gene expression by microarray and identified the induction of SLC26A3, the major apical Cl^-/HCO₃^- exchanger in IECs. Utilizing loss- and gain-of-function approaches, as well as murine and human colonoids, we demonstrate that Ado-induced SLC26A3 promotes an adaptive IECs phenotype that buffers local pH during active inflammation. Extending these studies, chronic murine colitis models were used to demonstrate that SLC26A3 expression rebounds during chronic DSS-induced inflammation. In conclusion, Ado signaling during PMN TEM induces an adaptive tissue response to inflammatory acidification through the induction of SLC26A3 expression, thereby promoting pH homeostasis.

Cyclic nucleotides, gut physiology and inflammation

Misregulation of gut function and homeostasis impinges on the overall well-being of the entire organism. Diarrheal disease is the second leading cause of death in children under 5 years of age, and globally, 1.7 billion cases of childhood diarrhea are reported every year. Accompanying diarrheal episodes are a number of secondary effects in gut physiology and structure, such as erosion of the mucosal barrier that lines the gut, facilitating further inflammation of the gut in response to the normal microbiome. Here, we focus on pathogenic bacteria-mediated diarrhea, emphasizing the role of cyclic adenosine 3',5'-monophosphate and cyclic guanosine 3',5'-monophosphate in driving signaling outputs that result in the secretion of water and ions from the epithelial cells of the gut. We also speculate on how this aberrant efflux and influx of ions could modulate inflammasome signaling, and therefore cell survival and maintenance of gut architecture and function.

What's in the pipeline for lower functional gastrointestinal disorders in the next 5 years?

The overall objectives of this review are to summarize actionable biomarkers for organic etiology of lower functional gastrointestinal disorders (FGIDs) that lead to individualized treatment for their FGIDs and to assess the pipeline for novel approaches to the management of constipation, diarrhea, and chronic abdominal pain in lower FGIDs. The new approaches to therapy include ion exchangers/transporters for functional constipation (sodium-glucose cotransporter 1, Na+/H+ exchanger 3, and solute carrier family 26 member 3 inhibitors), bile acid modulators for constipation such as ileal bile acid transporter inhibitors and fibroblast growth factor 19 analog for functional constipation, and bile acid sequestrants or farnesoid X receptor agonists for functional diarrhea. Treatment for chronic abdominal pain remains an unmet need in patients with lower FGIDs, and promising novel approaches include delayed-release linaclotide, nonclassical opioid visceral analgesics, and selective cannabinoid receptor agonists. The role of probiotics, fecal microbial transplantation, and possible future microbiome therapies is discussed.

Role of ion channels in gastrointestinal cancer

In their seminal papers Hanahan and Weinberg described oncogenic processes a normal cell undergoes to be transformed into a cancer cell. The functions of ion channels in the gastrointestinal (GI) tract influence a variety of cellular processes, many of which overlap with these hallmarks of cancer. In this review we focus on the roles of the calcium (Ca2+), sodium (Na+), potassium (K+), chloride (Cl-) and zinc (Zn2+) transporters in GI cancer, with a special emphasis on the roles of the KCNQ1 K+ channel and CFTR Cl- channel in colorectal cancer (CRC). Ca2+ is a ubiquitous second messenger, serving as a signaling molecule for a variety of cellular processes such as control of the cell cycle, apoptosis, and migration. Various members of the TRP superfamily, including TRPM8, TRPM7, TRPM6 and TRPM2, have been implicated in GI cancers, especially through overexpression in pancreatic adenocarcinomas and down-regulation in colon cancer. Voltage-gated sodium channels (VGSCs) are classically associated with the initiation and conduction of action potentials in electrically excitable cells such as neurons and muscle cells. The VGSC NaV1.5 is abundantly expressed in human colorectal CRC cell lines as well as being highly expressed in primary CRC samples. Studies have demonstrated that conductance through NaV1.5 contributes significantly to CRC cell invasiveness and cancer progression. Zn2+ transporters of the ZIP/SLC39A and ZnT/SLC30A families are dysregulated in all major GI organ cancers, in particular, ZIP4 up-regulation in pancreatic cancer (PC). More than 70 K+ channel genes, clustered in four families, are found expressed in the GI tract, where they regulate a range of cellular processes, including gastrin secretion in the stomach and anion secretion and fluid balance in the intestinal tract. Several distinct types of K+ channels are found dysregulated in the GI tract. Notable are hERG1 upregulation in PC, gastric cancer (GC) and CRC, leading to enhanced cancer angiogenesis and invasion, and KCNQ1 down-regulation in CRC, where KCNQ1 expression is associated with enhanced disease-free survival in stage II, III, and IV disease. Cl- channels are critical for a range of cellular and tissue processes in the GI tract, especially fluid balance in the colon. Most notable is CFTR, whose deficiency leads to mucus blockage, microbial dysbiosis and inflammation in the intestinal tract. CFTR is a tumor suppressor in several GI cancers. Cystic fibrosis patients are at a significant risk for CRC and low levels of CFTR expression are associated with poor overall disease-free survival in sporadic CRC. Two other classes of chloride channels that are dysregulated in GI cancers are the chloride intracellular channels (CLIC1, 3 & 4) and the chloride channel accessory proteins (CLCA1,2,4). CLIC1 & 4 are upregulated in PC, GC, gallbladder cancer, and CRC, while the CLCA proteins have been reported to be down-regulated in CRC. In summary, it is clear, from the diverse influences of ion channels, that their aberrant expression and/or activity can contribute to malignant transformation and tumor progression. Further, because ion channels are often localized to the plasma membrane and subject to multiple layers of regulation, they represent promising clinical targets for therapeutic intervention including the repurposing of current drugs.