Chloride channel-targeted therapy for secretory diarrheas

Nutrition strategies to control post-weaning diarrhea of piglets: From the perspective of feeds

Post-weaning diarrhea (PWD) is a globally significant threat to the swine industry. Historically, antibiotics as well as high doses of zinc oxide and copper sulfate have been commonly used to control PWD. However, the development of bacterial resistance and environmental pollution have created an interest in alternative strategies. In recent years, the research surrounding these alternative strategies and the mechanisms of piglet diarrhea has been continually updated. Mechanically, diarrhea in piglets is a result of an imbalance in intestinal fluid and electrolyte absorption and secretion. In general, enterotoxigenic Escherichia coli (ETEC) and diarrheal viruses are known to cause an imbalance in the absorption and secretion of intestinal fluids and electrolytes in piglets, resulting in diarrhea when Cl- secretion-driven fluid secretion surpasses absorptive capacity. From a perspective of feedstuffs, factors that contribute to imbalances in fluid absorption and secretion in the intestines of weaned piglets include high levels of crude protein (CP), stimulation by certain antigenic proteins, high acid-binding capacity (ABC), and contamination with deoxynivalenol (DON) in the diet. In response, efforts to reduce CP levels in diets, select feedstuffs with lower ABC values, and process feedstuffs using physical, chemical, and biological approaches are important strategies for alleviating PWD in piglets. Additionally, the diet supplementation with additives such as vitamins and natural products can also play a role in reducing the diarrhea incidence in weaned piglets. Here, we examine the mechanisms of absorption and secretion of intestinal fluids and electrolytes in piglets, summarize nutritional strategies to control PWD in piglets from the perspective of feeds, and provide new insights towards future research directions.

Purinergic Signaling Drives Multiple Aspects of Rotavirus Pathophysiology

Rotavirus causes life-threatening diarrhea in children, resulting in ∼200,000 deaths/year. The current treatment during infection is Oral Rehydration Solution which successfully replenishes fluids but does not alleviate diarrhea volume or severity. As a result, there is an urgent need to better understand rotavirus pathophysiology and develop more effective pediatric therapeutics. Rotavirus primarily infects the tips of small intestinal villi, yet has far-reaching effects on cell types distant from infected cells. We recently identified that rotavirus infected cells release the purinergic signaling molecule ADP, which activates P2Y1 receptors on nearby uninfected cells in vitro . To elucidate the role of purinergic signaling via P2Y1 receptors during rotavirus infection in vivo , we used the mouse-like rotavirus strain D6/2 which generates a severe infection in mice. C57BL/6J mouse pups were given an oral gavage of D6/2 rotavirus and assessed over the course of 5-7 days. Beginning at day 1 post infection, infected pups were treated daily by oral gavage with saline or 4 mg/kg MRS2500, a selective P2Y1 antagonist. Mice were monitored for diarrhea severity, diarrhea incidence, and viral shedding. Neonatal mice were euthanized at days 3 and 5 post-infection and small intestine was collected to observe infection. MRS2500 treatment decreased the severity, prevalence, and incidence of rotavirus diarrhea. Viral stool shedding, assessed by qPCR for rotavirus gene levels, revealed that MRS2500 treated pups had significantly lower viral shedding starting at day 4 post infection compared to saline treated pups, which suggests P2Y1 signaling may enhance rotavirus replication. Finally, we found that inhibition of P2Y1 with MRS2500 limited transmitted rotavirus diarrhea to uninfected pups within a litter. Together, these results suggest that P2Y1 signaling is involved in the pathogenesis of a homologous murine rotavirus strain, making P2Y1 receptors a promising anti-diarrheal, anti-viral therapeutic target to reduce rotavirus disease burden.

Loureirin C extracted from Dracaena cochinchinensis S.C. Chen prevents rotaviral diarrhea in mice by inhibiting the intestinal Ca2+-activated Cl- channels

ETHNOPHARMACOLOGICAL RELEVANCE

Resina Draconis (RD) is the red resin of Dracaena cochinchinensis (Lour.) S.C. Chen and most used as a hemostatic drug in traditional Chinese medicine. Recent studies have reported that RD has a therapeutic effect on gastrointestinal diseases. Loureirin A, B, and C (LA, LB, and LC) are dihydrochalcone compounds isolated from RD.

AIM OF THE STUDY

Dehydration is the primary cause of death in rotaviral diarrhea. Inhibition of Ca2+-activated Cl- channels (CaCCs)-mediated Cl- secretion significantly reduced fluid secretion in rotaviral diarrhea. RD was used to treat digestive diseases such as diarrhea and abdominal pain; however, the pharmacological mechanism remains unclear. This study investigated the effects of RD and loureirin on intestinal Cl- channels and their therapeutic effects on rotavirus-induced diarrhea, aiming to reveal RD's molecular basis, targets, and mechanisms for treating rotaviral diarrhea.

MATERIALS AND METHODS

Cell-based fluorescence quenching assays were used to examine the effect of RD and loureirin on Cl- channels activity. Electrophysiological properties were tested using short-circuit current experiments in epithelial cells or freshly isolated mouse intestinal tissue. Fecal water content, intestinal peristalsis rate, and smooth muscle contraction were measured in neonatal mice infected with SA-11 rotavirus before and after LC treatment or adult mice.

RESULTS

RD, LA, LB, and LC inhibited CaCCs-mediated Cl- current in HT-29 cells and colonic epithelium. The inhibitory effect of LC on CaCCs was primarily on the apical side in epithelial cells, which may be partially produced by affecting cytoplasmic Ca2+ levels. LC significantly inhibited TMEM16A-mediated Cl- current. Characterization studies revealed that LC inhibited basolateral K+ channel activity without affecting Na+/K+-ATPase activity in the colonic epithelium. Although LC activated the cystic fibrosis transmembrane regulator in epithelial cells, its effect was not apparent in colonic epithelium. In vivo, LC significantly reduced the fecal water content, intestinal peristalsis rate, and smooth muscle contraction of mice infected with rotavirus.

CONCLUSION

RD and its active compound LC inhibit intestinal CaCCs activity, which might mediate the anti-rotaviral diarrheal effect of RD.

Epithelial Transport in Disease: An Overview of Pathophysiology and Treatment

Epithelial transport is a multifaceted process crucial for maintaining normal physiological functions in the human body. This comprehensive review delves into the pathophysiological mechanisms underlying epithelial transport and its significance in disease pathogenesis. Beginning with an introduction to epithelial transport, it covers various forms, including ion, water, and nutrient transfer, followed by an exploration of the processes governing ion transport and hormonal regulation. The review then addresses genetic disorders, like cystic fibrosis and Bartter syndrome, that affect epithelial transport. Furthermore, it investigates the involvement of epithelial transport in the pathophysiology of conditions such as diarrhea, hypertension, and edema. Finally, the review analyzes the impact of renal disease on epithelial transport and highlights the potential for future research to uncover novel therapeutic interventions for conditions like cystic fibrosis, hypertension, and renal failure.

Sesquiterpene lactones improve secretory diarrhea symptoms by inhibiting intestinal Ca2+-activated Cl- channel activities directly and indirectly

Secretory diarrhea caused by bacteria and viruses is usually accompanied by activation of the cystic fibrosis transmembrane conductance regulator (CFTR) and calcium-activated Cl- channels (CaCCs) in the intestinal epithelium. Inhibition of CFTR and CaCCs activities significantly reduces fluid losses and intestinal motility in diarrheal diseases. For this reason, CFTR and CaCCs are potential targets of therapeutic drug screening. Here, we reported that the sesquiterpene lactones, alantolactone (AL) and isoalantolactone (iAL), significantly inhibited ATP and Eact-induced short-circuit currents in T84, HT-29 and Fischer rat thyroid (FRT) cells expressing transmembrane protein 16A (TMEM16A) in a concentration-dependent manner. AL and iAL also inhibited the CaCC-mediated short-circuit currents induced by carbachol in the mouse colons. Both compounds inhibited forskolin-induced currents in T84 cells but did not significantly affect mouse colons. In vivo studies indicated that AL and iAL attenuated gastrointestinal motility and decreased watery diarrhea in rotavirus-infected neonatal mice. Preliminary mechanism studies showed that AL and iAL inhibited CaCCs at least partially by inhibiting Ca2+ release and basolateral membrane K+ channels activity. These findings suggest a new pharmacological activity of sesquiterpene lactone compounds that might lead to the development of treatments for rotaviral secretory diarrhea.

Epithelial dysfunction is prevented by IL-22 treatment in a Citrobacter rodentium-induced colitis model that shares similarities with inflammatory bowel disease

Inflammatory bowel disease (IBD) is characterized by a dysregulated intestinal epithelial barrier leading to breach of barrier immunity. Here we identified similar protein expression changes between IBD and Citrobacter rodentium-infected FVB mice with respect to dysregulation of solute transporters as well as components critical for intestinal barrier integrity. We attribute the disease associated changes in the model to the emergence of undifferentiated intermediate intestinal epithelial cells. Prophylactic treatment with IL-22.Fc in C. rodentium-infected FVB mice reduced disease severity and rescued the mice from lethality. Multi-omics and solute analyses revealed that IL-22.Fc treatment prevented disease-associated changes including disruption of the solute transporter machinery and restored proper physiological functions of the intestine, respectively. Taken together, we established the disease relevance of the C. rodentium-induced colitis model to IBD, demonstrated the protective role of IL-22 in amelioration of epithelial dysfunction and elucidated the molecular mechanisms with IL-22's effect on intestinal epithelial cells.

Ebola virus delta peptide is an enterotoxin

During the 2013-2016 West African (WA) Ebola virus (EBOV) outbreak, severe gastrointestinal symptoms were common in patients and associated with poor outcome. Delta peptide is a conserved product of post-translational processing of the abundant EBOV soluble glycoprotein (sGP). The murine ligated ileal loop model was used to demonstrate that delta peptide is a potent enterotoxin. Dramatic intestinal fluid accumulation follows injection of biologically relevant amounts of delta peptide into ileal loops, along with gross alteration of villous architecture and loss of goblet cells. Transcriptomic analyses show that delta peptide triggers damage response and cell survival pathways and downregulates expression of transporters and exchangers. Induction of diarrhea by delta peptide occurs via cellular damage and regulation of genes that encode proteins involved in fluid secretion. While distinct differences exist between the ileal loop murine model and EBOV infection in humans, these results suggest that delta peptide may contribute to EBOV-induced gastrointestinal pathology.

Flavonoids containing Vitex negundo extract displayed calcium channels blocking property underlying its anti-diarrheal and anti-spasmodic activities

Diarrhea is a major health problem and despite of interventions, it remains a substantial cause of mortality and morbidity. In this study, a flavonoids-rich Vitex negundo extract was mechanistically evaluated for its effectiveness in diarrheal diseases. The Vitex negundo ethanolic extract was subjected to extraction for isolation of total flavonoids and qualitative phytochemical analysis. The acute toxicity of flavonoids-extract was done in mice to assess its safety and tolerability. The anti-diarrhoeal activity was determined using the castor oil induced diarrhea mouse model at doses of 10, 30 and 100 mg/kg in relation to loperamide (10 mg/kg, oral). The antispasmodic profile was determined by using cut pieces of rabbit’s jejunum in Tyrode’s solutions, employing acetylcholine (0.03μM) as a tissue stabilizer and verapamil, as a calcium channel blocker. The preliminary qualitative analysis of extract revealed different phytochemicals, in addition to flavonoids. The acute toxicity profile showed that the flavonoids-extract is safe and tolerable (LD50 = 1678 mg/kg). In the tissue experiments, the total flavonoids exhibited an EC50 of 1.52 mg/mL and showed maximum relaxation of spontaneous contractions at 5.0 mg/mL and against high-K + induced contractions at 3 mg/mL (EC50 = 0.43 mg/mL). The spasmolytic activity of total flavonoids was comparable to verapamil which suggests that the activity might be due to the blockade of calcium channels. The flavonoids extract (0.1, 0.3 and 1.0 mg/mL) produced an adequate right shift in the calcium concentration response curve as compared to the control (EC50 value = –2.67 mg/mL), which confirmed that the extract has calcium channel blocking activity.

The Role of Intestinal Permeability in Gastrointestinal Disorders and Current Methods of Evaluation

An increased intestinal permeability has been described in various gastrointestinal and non-gastrointestinal disorders. Nevertheless, the concept and definition of intestinal permeability is relatively broad and includes not only an altered paracellular route, regulated by tight junction proteins, but also the transcellular route involving membrane transporters and channels, and endocytic mechanisms. Paracellular intestinal permeability can be assessed in vivo by using different molecules (e.g., sugars, polyethylene glycols, ⁵¹Cr-EDTA) and ex vivo in Ussing chambers combining electrophysiology and probes of different molecular sizes. The latter is still the gold standard technique for assessing the epithelial barrier function, whereas in vivo techniques, including putative blood biomarkers such as intestinal fatty acid-binding protein and zonulin, are broadly used despite limitations. In the second part of the review, the current evidence of the role of impaired barrier function in the pathophysiology of selected gastrointestinal and liver diseases is discussed. Celiac disease is one of the conditions with the best evidence for impaired barrier function playing a crucial role with zonulin as its proposed regulator. Increased permeability is clearly present in inflammatory bowel disease, but the question of whether this is a primary event or a consequence of inflammation remains unsolved. The gut-liver axis with a crucial role in impaired intestinal barrier function is increasingly recognized in chronic alcoholic and metabolic liver disease. Finally, the current evidence does not support an important role for increased permeability in bile acid diarrhea.

β-eudesmol but not atractylodin exerts an inhibitory effect on CFTR-mediated chloride transport in human intestinal epithelial cells

Oriental herbal medicine with the two bioactive constituents, β-eudesmol (BE) and atractylodin (AT), has been used as a remedy for gastrointestinal disorders. There was no scientific evidence reporting their antidiarrheal effect and underpinning mechanisms. Therefore, we aimed to investigate the anti-secretory activity of these two compounds in vitro. The inhibitory effect of BE and AT on cAMP-induced Cl^- secretion was evaluated by Ussing chamber in human intestinal epithelial (T84) cells. Short-circuit current (I_SC) and apical Cl^- current (I_Cl-) were measured after adding indirect and direct cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel activator. MTT assay was used to determine cellular cytotoxicity. Protein-ligand interaction was investigated by in silico molecular docking analysis. BE, but not AT concentration-dependently (IC₅₀ of ~1.05 µM) reduced cAMP-mediated, CFTR_inh-172 inhibitable Cl^- secretion as determined by transepithelial I_SC across a monolayer of T84 cells. Potency of CFTR-mediated I_Cl- inhibition by BE did not change with the use of different CFTR activators suggesting a direct blockage of the channel active site(s). Pretreatment with BE completely prevented cAMP-induced I_Cl-. Furthermore, BE at concentrations up to 200 µM (24 h) had no effect on T84 cell viability. In silico studies indicated that BE could best dock onto dephosphorylated structure of CFTR at ATP-binding pockets in nucleotide-binding domain (NBD) 2 region. These findings provide the first evidence for the anti-secretory effect of BE involving inhibition of CFTR function. BE represents a promising candidate for the therapeutic or prophylactic intervention of diarrhea resulted from intestinal hypersecretion of Cl^.

Calcium-activated chloride channel is involved in the onset of diarrhea triggered by EGFR tyrosine kinase inhibitor treatment in rats

EGFR tyrosine kinase inhibitors (TKIs) are mainly used to treat non-small cell lung cancer; however, adverse effects such as severe diarrhea represent a major obstacle towards the continuation of EGFR-TKIs therapy. Chloride channels, which control the fluid flow in the intestinal lumen, are proposed as an important target to remediate EGFR-TKIs-induced diarrhea, but the mechanism remains unclear. The aim of this study was to clarify the mechanism underlying EGFR-TKIs-induced diarrhea with a particular focus on the role of intestinal chloride channels. Here, we show that osimertinib-treated rats exhibit diarrhea and an increase in fecal water content without showing any severe histopathological changes. This diarrhea was attenuated by intraperitoneal treatment with the calcium-activated chloride channel (CaCC) inhibitor CaCCinh-A01. These findings were confirmed in afatinib-treated rats with diarrhea. Moreover, treatment with the Japanese traditional herbal medicine, hangeshashinto (HST), decreased fecal water content and improved fecal appearance in rats treated with EGFR-TKIs. HST inhibited the ionomycin-induced CaCC activation in HEK293 cells in patch-clamp current experiments and its active ingredients were identified. In conclusion, secretory diarrhea induced by treatment with EGFR-TKIs might be partially mediated by the activation of CaCC. Therefore, blocking the CaCC could be a potential new treatment for EGFR-TKI-induced diarrhea.

Fluorescence assay for simultaneous quantification of CFTR ion-channel function and plasma membrane proximity

The cystic fibrosis transmembrane conductance regulator (CFTR) is a plasma membrane anion channel that plays a key role in controlling transepithelial fluid movement. Excessive activation results in intestinal fluid loss during secretory diarrheas, whereas CFTR mutations underlie cystic fibrosis (CF). Anion permeability depends both on how well CFTR channels work (permeation/gating) and on how many are present at the membrane. Recently, treatments with two drug classes targeting CFTR-one boosting ion-channel function (potentiators) and the other increasing plasma membrane density (correctors)-have provided significant health benefits to CF patients. Here, we present an image-based fluorescence assay that can rapidly and simultaneously estimate both CFTR ion-channel function and the protein's proximity to the membrane. We monitor F508del-CFTR, the most common CF-causing variant, and confirm rescue by low temperature, CFTR-targeting drugs and second-site revertant mutation R1070W. In addition, we characterize a panel of 62 CF-causing mutations. Our measurements correlate well with published data (electrophysiology and biochemistry), further confirming validity of the assay. Finally, we profile effects of acute treatment with approved potentiator drug VX-770 on the rare-mutation panel. Mapping the potentiation profile on CFTR structures raises mechanistic hypotheses on drug action, suggesting that VX-770 might allow an open-channel conformation with an alternative arrangement of domain interfaces. The assay is a valuable tool for investigation of CFTR molecular mechanisms, allowing accurate inferences on gating/permeation. In addition, by providing a two-dimensional characterization of the CFTR protein, it could better inform development of single-drug and precision therapies addressing the root cause of CF disease.

Strategies for cystic fibrosis transmembrane conductance regulator inhibition: from molecular mechanisms to treatment for secretory diarrhoeas

Cystic fibrosis transmembrane conductance regulator (CFTR) is an unusual ABC transporter. It acts as an anion‐selective channel that drives osmotic fluid transport across many epithelia. In the gut, CFTR is crucial for maintaining fluid and acid‐base homeostasis, and its activity is tightly controlled by multiple neuro‐endocrine factors. However, microbial toxins can disrupt this intricate control mechanism and trigger protracted activation of CFTR. This results in the massive faecal water loss, metabolic acidosis and dehydration that characterize secretory diarrhoeas, a major cause of malnutrition and death of children under 5 years of age. Compounds that inhibit CFTR could improve emergency treatment of diarrhoeal disease. Drawing on recent structural and functional insight, we discuss how existing CFTR inhibitors function at the molecular and cellular level. We compare their mechanisms of action to those of inhibitors of related ABC transporters, revealing some unexpected features of drug action on CFTR. Although challenges remain, especially relating to the practical effectiveness of currently available CFTR inhibitors, we discuss how recent technological advances might help develop therapies to better address this important global health need.

Inhibitory activities of curzerenone, curdione, furanodienone, curcumol and germacrone on Ca2+-activated chloride channels

Calcium-activated chloride channels (CaCCs) as a kind of widely expressed ion channels play crucial roles in a variety of physiological regulation. TMEM16A has been identified as the molecular basis of CaCCs in numerous cell types and is considered a new drug target for many diseases. Regulating the function of TMEM16A through small molecule modulators has become a new strategy to improve respiratory and digestive dysfunction and even tumor therapy. Herein, we obtained 5 sesquiterpenoids, named curzerenone, curdione, furanodienone, curcumol and germacrone with TMEM16A inhibition and revealed their mechanism of action by fluorescent and electrophysiological assays. Cell-based YFP fluorescence data demonstrated that 5 compounds inhibited TMEM16A-mediated I^- influx in a dose-dependent manner. To explore the mechanism of 5 compounds on CaCCs, FRT cells with high expression of TMEM16A, HBE, HT-29 and T84 cells and mouse colons were used in short-circuit current assay. Our results showed that 5 compounds inhibited the Ca²⁺-activated Cl^- currents generated by the E_act, ATP and UTP stimulation, and this inhibitory effect was related not only to the direct inhibition of channel opening, but also the inhibition of intracellular Ca²⁺ concentration and K⁺ channel activity. In addition to CaCCs, these 5 compounds also had definite inhibitory activities against cystic fibrosis transmembrane regulator (CFTR) at the cellular level. In summary, these compounds have the potential to regulate the activites of TMEM16A/CaCCs and CFTR channels in vitro, providing a new class of lead compounds for the development of drugs for diseases related to chloride channel dysfunction.

Application of the Nested Enzyme-Within-Enterocyte (NEWE) Turnover Model for Predicting the Time Course of Pharmacodynamic Effects

The gut wall consists of many biological elements, including enterocytes. Rapid turnover, a prominent feature of the enterocytes, has generally been ignored in the development of enterocyte-targeting drugs, although it has a comparable rate to other kinetic rates. Here, we investigated the impact of enterocyte turnover on the pharmacodynamics of enterocyte-targeting drugs by applying a model accounting for turnover of enterocytes and target proteins. Simulations showed that the pharmacodynamics depend on enterocyte lifespan when drug-target affinity is strong and half-life of target protein is long. Interindividual variability of enterocyte lifespan, which can be amplified by disease conditions, has a substantial impact on the variability of response. However, our comprehensive literature search showed that the enterocyte turnover causes a marginal impact on currently approved enterocyte-targeting drugs due to their relatively weak target affinities. This study proposes a model-informed drug development approach for selecting enterocyte-targeting drugs and their optimal dosage regimens.

Sodium-coupled glucose transport, the SLC5 family, and therapeutically relevant inhibitors: from molecular discovery to clinical application

Sodium glucose transporters (SGLTs) belong to the mammalian solute carrier family SLC5. This family includes 12 different members in human that mediate the transport of sugars, vitamins, amino acids, or smaller organic ions such as choline. The SLC5 family belongs to the sodium symporter family (SSS), which encompasses transporters from all kingdoms of life. It furthermore shares similarity to the structural fold of the APC (amino acid-polyamine-organocation) transporter family. Three decades after the first molecular identification of the intestinal Na+-glucose cotransporter SGLT1 by expression cloning, many new discoveries have evolved, from mechanistic analysis to molecular genetics, structural biology, drug discovery, and clinical applications. All of these advances have greatly influenced physiology and medicine. While SGLT1 is essential for fast absorption of glucose and galactose in the intestine, the expression of SGLT2 is largely confined to the early part of the kidney proximal tubules, where it reabsorbs the bulk part of filtered glucose. SGLT2 has been successfully exploited by the pharmaceutical industry to develop effective new drugs for the treatment of diabetic patients. These SGLT2 inhibitors, termed gliflozins, also exhibit favorable nephroprotective effects and likely also cardioprotective effects. In addition, given the recent finding that SGLT2 is also expressed in tumors of pancreas and prostate and in glioblastoma, this opens the door to potential new therapeutic strategies for cancer treatment by specifically targeting SGLT2. Likewise, further discoveries related to the functional association of other SGLTs of the SLC5 family to human pathologies will open the door to potential new therapeutic strategies. We furthermore hope that the herein summarized information about the physiological roles of SGLTs and the therapeutic benefits of the gliflozins will be useful for our readers to better understand the molecular basis of the beneficial effects of these inhibitors, also in the context of the tubuloglomerular feedback (TGF), and the renin-angiotensin system (RAS). The detailed mechanisms underlying the clinical benefits of SGLT2 inhibition by gliflozins still warrant further investigation that may serve as a basis for future drug development.

Intestinal Permeability, Inflammation and the Role of Nutrients

The interaction between host and external environment mainly occurs in the gastrointestinal tract, where the mucosal barrier has a critical role in many physiologic functions ranging from digestion, absorption, and metabolism. This barrier allows the passage and absorption of nutrients, but at the same time, it must regulate the contact between luminal antigens and the immune system, confining undesirable products to the lumen. Diet is an important regulator of the mucosal barrier, and the cross-talk among dietary factors, the immune system, and microbiota is crucial for the modulation of intestinal permeability and for the maintenance of gastrointestinal tract (GI) homeostasis. In the present review, we will discuss the role of a number of dietary nutrients that have been proposed as regulators of inflammation and epithelial barrier function. We will also consider the metabolic function of the microbiota, which is capable of elaborating the diverse nutrients and synthesizing products of great interest. Better knowledge of the influence of dietary nutrients on inflammation and barrier function can be important for the future development of new therapeutic approaches for patients with mucosal barrier dysfunction, a critical factor in the pathogenesis of many GI and non-GI diseases.

Activation of constitutive androstane receptor inhibits intestinal CFTR-mediated chloride transport

Constitutive androstane receptor (CAR) belonging to the nuclear receptor superfamily plays an important role in the xenobiotic metabolism and disposition. It has been reported that CAR regulates the expression of the ATP-binding cassette (ABC) transporters in the intestine, such as multidrug resistance protein 1 (MDR1) and multidrug resistance-associated protein 2/3 (MRP2 and MRP3). In this study, we investigated the role of CAR in the regulation of cystic fibrosis transmembrane conductance regulator (CFTR)-mediated chloride transport in T84 human colonic epithelial cells and mouse intestinal tissues. Treatments of T84 cell monolayers with specific CAR agonists (CITCO and phenytoin at concentrations of 1 μM and 5 μM, respectively) for 24 h decreased transepithelial Cl^- secretion in response to cAMP-dependent agonist. This inhibition was abolished by coincubation of CITCO with a CAR antagonist, CINPA1. We confirmed that an inhibitory effect of CAR agonists was not due to their cytotoxicity. Basolateral membrane permeabilization experiments also revealed that activation of CAR decreased apical Cl^- current stimulated by both CPT-cAMP and genistein (a direct CFTR activator). Such activation also reduced both mRNA and protein expression of CFTR. Furthermore, CITCO decreased cholera toxin (CT)-induced Cl^- secretion across T84 cell monolayers. In ICR mice, administration of TCPOBOP (3 mg/kgBW), a murine-specific CAR agonist, for 7 days produced significant decreases in CFTR mRNA and protein expressions in intestinal tissues. Interestingly, TCPOBOP also inhibited CT-induced intestinal fluid accumulation in mice. This is the first evidence showing that CFTR was downregulated by CAR activation in the intestine. Our findings suggest that CAR has potential as a new drug target for treatment of condition with hyperactivity/ hyperfunction of CFTR especially secretory diarrheas.

Prevention and treatment of secretory diarrhea by the lysophosphatidic acid analog Rx100

IMPACT STATEMENT

A critical barrier in treating diarrheal disease is easy-to-use effective treatments. Rx100 is a first in class, novel small molecule that has shown efficacy after both subcutaneous and oral administration in a mouse cholera-toxin- and Citrobacter rodentium infection-induced diarrhea models. Our findings indicate that Rx100 a metabolically stable analog of the lipid mediator lysophosphatidic acid blocks activation of CFTR-mediated secretion responsible for fluid discharge in secretory diarrhea. Rx100 represents a new treatment modality which does not directly block CFTR but attenuates its activation by bacterial toxins. Our results provide proof-of-principle that Rx100 can be developed for use as an effective oral or injectable easy-to-use drug for secretory diarrhea which could significantly improve care by eliminating the need for severely ill patients to regularly consume large quantities of oral rehydration therapies and offering options for pediatric patients.

Slippery When Wet: Airway Surface Liquid Homeostasis and Mucus Hydration

The ability to regulate cell volume is crucial for normal physiology; equally the regulation of extracellular fluid homeostasis is of great importance. Alteration of normal extracellular fluid homeostasis contributes to the development of several diseases including cystic fibrosis. With regard to the airway surface liquid (ASL), which lies apically on top of airway epithelia, ion content, pH, mucin and protein abundance must be tightly regulated. Furthermore, airway epithelia must be able to switch from an absorptive to a secretory state as required. A heterogeneous population of airway epithelial cells regulate ASL solute and solvent composition, and directly secrete large mucin molecules, antimicrobials, proteases and soluble mediators into the airway lumen. This review focuses on how epithelial ion transport influences ASL hydration and ASL pH, with a specific focus on the roles of anion and cation channels and exchangers. The role of ions and pH in mucin expansion is also addressed. With regard to fluid volume regulation, we discuss the roles of nucleotides, adenosine and the short palate lung and nasal epithelial clone 1 (SPLUNC1) as soluble ASL mediators. Together, these mechanisms directly influence ciliary beating and in turn mucociliary clearance to maintain sterility and to detoxify the airways. Whilst all of these components are regulated in normal airways, defective ion transport and/or mucin secretion proves detrimental to lung homeostasis as such we address how defective ion and fluid transport, and a loss of homeostatic mechanisms, contributes to the development of pathophysiologies associated with cystic fibrosis.

Development of Arrayed Colonic Organoids for Screening of Secretagogues Associated with Enterotoxins

Enterotoxins increase intestinal fluid secretion through modulation of ion channels as well as activation of the enteric nervous and immune systems. Colonic organoids, also known as colonoids, are functionally and phenotypically similar to in vivo colonic epithelium and have been used to study intestinal ion transport and subsequent water flux in physiology and disease models. In conventional cultures, organoids exist as spheroids embedded within a hydrogel patty of extracellular matrix, and they form at multiple depths, impairing efficient imaging necessary to capture data from statistically relevant sample sizes. To overcome these limitations, an analytical platform with colonic organoids localized to the planar surface of a hydrogel layer was developed. The arrays of densely packed colonoids (140 μm average diameter, 4 colonoids/mm2) were generated in a 96-well plate, enabling assay of the response of hundreds of organoids so that organoid subpopulations with distinct behaviors were identifiable. Organoid cell types, monolayer polarity, and growth were similar to those embedded in hydrogel. An automated imaging and analysis platform efficiently tracked over time swelling due to forskolin and fluid movement across the cell monolayer stimulated by cholera toxin. The platform was used to screen compounds associated with the enteric nervous and immune systems for their effect on fluid movement across epithelial cells. Prostaglandin E2 promoted increased water flux in a subset of organoids that resulted in organoid swelling, confirming a role for this inflammatory mediator in diarrheal conditions but also illustrating organoid differences in response to an identical stimulus. By allowing sampling of a large number of organoids, the arrayed organoid platform permits identification of organoid subpopulations intermixed within a larger group of nonresponding organoids. This technique will enable automated, large-scale screening of the impact of drugs, toxins, and other compounds on colonic physiology.

Epithelial immunity: priming defensive responses in the intestinal mucosa

As the largest interface between the outside and internal milieu, the intestinal epithelium constitutes the first structural component facing potential luminal threats to homeostasis. This single-cell layer is the epicenter of a tightly regulated communication network between external and internal factors that converge to prime defensive responses aimed at limiting antigen penetration and the maintenance of intestinal barrier function. The defensive role developed by intestinal epithelial cells (IEC) relies largely on the variety of receptors they express at both extracellular (apical and basolateral) and intracellular compartments, and the capacity of IEC to communicate with immune and nervous systems. IEC recognize pathogen-associated molecules by innate receptors that promote the production of mucus, antimicrobial substances, and immune mediators. Epithelial cells are key to oral tolerance maintenance and also participate in adaptive immunity through the expression of immunoglobulin (Ig) receptors and by promoting local Ig class switch recombination. In IEC, different types of antigens can be sensed by multiple immune receptors that share signaling pathways to assure effective responses. Regulated defensive activity maintains intestinal homeostasis, whereas a breakdown in the control of epithelial immunity can increase the intestinal passage of luminal content and microbial invasion, leading to inflammation and tissue damage. In this review, we provide an updated overview of the type of immune receptors present in the human intestinal epithelium and the responses generated to promote effective barrier function and maintain mucosal homeostasis.

Targeting Small Bowel Receptors to Treat Constipation and Diarrhea

BACKGROUND

The options for the treatment of diarrhea and constipation are evolving as emerging therapies target small bowel receptors. The goal of this review is to discuss small bowel receptors involved in intestinal absorption, secretion, and motility. The review highlights therapies already approved or currently being studied for the modulation of these receptors.

METHODS

The articles cited in this review focus on the molecular level of pathways involved in diarrhea and constipation, and highlight the respective pharmacotherapies.

RESULTS

The majority of the studies in the current literature investigate the effects of both the small and large intestine receptors on diarrhea and constipation. There are fewer studies that isolate the effects of these receptors solely on the small bowel, and focusing more on the receptors found distinctly in the small intestine may be an area of interest for future studies as this can inspire more targeted therapies.

Cystic Fibrosis Transmembrane Conductance Regulator Reduces Microtubule-Dependent Campylobacter jejuni Invasion

Campylobacterjejuni is a foodborne pathogen that induces gastroenteritis. Invasion and adhesion are essential in the process of C. jejuni infection leading to gastroenteritis. The mucosal layer plays a key role in the system of defense against efficient invasion and adhesion by bacteria, which is modulated by several ion channels and transporters mediated by water flux in the intestine. The cystic fibrosis transmembrane conductance regulator (CFTR) plays the main role in water flux in the intestine, and it is closely associated with bacterial clearance. We previously reported that C. jejuni infection suppresses CFTR channel activity in intestinal epithelial cells; however, the mechanism and importance of this suppression are unclear. This study sought to elucidate the role of CFTR in C. jejuni infection. Using HEK293 cells that stably express wild-type and mutated CFTR, we found that CFTR attenuated C. jejuni invasion and that it was not involved in bacterial adhesion or intracellular survival but was associated with microtubule-dependent intracellular transport. Moreover, we revealed that CFTR attenuated the function of the microtubule motor protein, which caused inhibition of C. jejuni invasion, but did not affect microtubule stability. Meanwhile, the CFTR mutant G551D-CFTR, which had defects in channel activity, suppressed C. jejuni invasion, whereas the ΔF508-CFTR mutant, which had defects in maturation, did not suppress C. jejuni invasion, suggesting that CFTR suppression of C. jejuni invasion is related to CFTR maturation but not channel activity. When these findings are taken together, it may be seen that mature CFTR inhibits C. jejuni invasion by regulating microtubule-mediated pathways. We suggest that CFTR plays a critical role in cellular defenses against C. jejuni invasion and that suppression of CFTR may be an initial step in promoting cell invasion during C. jejuni infection.

Prophylactic use of a standardized botanical extract for the prevention of naturally occurring diarrhea in newborn Holstein calves

The objectives of this study were to evaluate the prophylactic use of SB-300 (Jaguar Animal Health, San Francisco, CA), a standardized botanical extract isolated from the bark latex of Croton lechleri, on reducing fecal water losses and diarrhea events in Holstein bull calves individually housed under a restricted whole-milk feeding regimen (6 L/d) from 1 to 25 d of life. Fluid therapy administration due to dehydration, average weight gain, and the fecal microbiome were also evaluated. Bull calves used in this study were born from normal parturition, fed 4 L of pooled pasteurized colostrum by esophageal feeder, and moved to a research facility at Cornell University (Ithaca, NY). A double-blinded randomized clinical trial was designed to allocate a total of 40 newborn calves into 1 of 2 treatment groups: calves receiving (twice daily) a solution containing 500 mg of SB-300 added to the whole milk for the first 15 d of life (SB-300, n = 20) or a control group receiving sterile water added to whole milk for the same period (CTR, n = 20). Treatment solutions had a total volume of 10 mL per treatment. Data regarding fecal dry matter were collected to precisely measure water content in fecal samples and to define diarrhea events; the SB-300 group had significantly increased fecal dry matter during the study period. Additionally, significantly fewer events of diarrhea were observed for calves in the SB-300 group (16.9%) compared with calves in the CTR group (46.5%). Dehydration status was evaluated and treated accordingly; calves with moderate dehydration were offered oral electrolytes, and calves with severe dehydration were rescued with intravenous fluid therapy. Calves in the SB-300 group had fewer intravenous fluid therapies administered during the study period (1.6%) compared with the CTR group (3.1%). Overall fluid therapy administered (oral electrolytes plus intravenous fluids) was significantly higher for the CTR group (9.2%) compared with the SB-300 group (6.1%) during the study period. No differences in milk consumption, calf starter intake, or weight gain were observed between treatment groups. A single time increase in Bifidobacterium was observed on d 20 of life for the SB-300 group; otherwise, no differences in fecal microbiome profile were detected between treatment groups. These results suggest that 500 mg of SB-300 added to the milk for 15 d can reduce the incidence of diarrhea and reduce severe dehydration in milk-fed calves.

Pathophysiology, Evaluation, and Management of Chronic Watery Diarrhea

Chronic watery diarrhea poses a diagnostic and therapeutic challenge and is often a disabling condition for patients. Although acute diarrhea is likely to be caused by infection, the causes of chronic diarrhea (>4 weeks in duration) are more elusive. We review the pathophysiology, diagnosis, and treatment of chronic diarrhea. Drawing on recent insights into the molecular mechanisms of intestinal epithelial transport and barrier function, we discuss how diarrhea can result from a decrease in luminal solute absorption, an increase in secretion, or both, as well as derangements in barrier properties. We also describe the various extraepithelial factors that activate diarrheal mechanisms. Finally, clinical evaluation and tests used in the assessment of patients presenting with chronic diarrhea are reviewed, and an algorithm guiding therapeutic decisions and pharmacotherapy is presented.

CFTR pharmacology

CFTR protein is an ion channel regulated by cAMP-dependent phosphorylation and expressed in many types of epithelial cells. CFTR-mediated chloride and bicarbonate secretion play an important role in the respiratory and gastrointestinal systems. Pharmacological modulators of CFTR represent promising drugs for a variety of diseases. In particular, correctors and potentiators may restore the activity of CFTR in cystic fibrosis patients. Potentiators are also potentially useful to improve mucociliary clearance in patients with chronic obstructive pulmonary disease. On the other hand, CFTR inhibitors may be useful to block fluid and electrolyte loss in secretory diarrhea and slow down the progression of polycystic kidney disease.

PGE2 is a direct and robust mediator of anion/fluid secretion by human intestinal epithelial cells

Intestinal epithelial cells (IECs) play an indispensable role in maintaining body fluid balance partly through their ability to regulate anion/fluid secretion. Yet in various inflammatory gastrointestinal diseases, over-secretion of anions results in symptoms such as severe diarrhoea. Endogenous mediators, such as vasoactive intestinal peptide or prostaglandin E₂ (PGE₂), regulate intestinal anion/fluid secretion, but their direct effect on purified human IECs has never been described in detail. Based on a previously described intestinal organoid swelling model, we established a 3D-scanner-assisted quantification method to evaluate the anion/fluid secretory response of cultured human IECs. Among various endogenous secretagogues, we found that PGE₂ had the lowest EC₅₀ value with regard to the induction of swelling of the jejunal and colonic organoids. This PGE₂-mediated swelling response was dependent on environmental Cl^- concentrations as well as on several channels and transporters as shown by a series of chemical inhibitor studies. The concomitant presence of various inflammatory cytokines with PGE₂ failed to modulate the PGE₂-mediated organoid swelling response. Therefore, the present study features PGE₂ as a direct and robust mediator of anion/fluid secretion by IECs in the human intestine.

Chemotherapy-Induced Constipation and Diarrhea: Pathophysiology, Current and Emerging Treatments

Gastrointestinal (GI) side-effects of chemotherapy are a debilitating and often overlooked clinical hurdle in cancer management. Chemotherapy-induced constipation (CIC) and Diarrhea (CID) present a constant challenge in the efficient and tolerable treatment of cancer and are amongst the primary contributors to dose reductions, delays and cessation of treatment. Although prevalence of CIC is hard to estimate, it is believed to affect approximately 16% of cancer patients, whilst incidence of CID has been estimated to be as high as 80%. Despite this, the underlying mechanisms of both CID and CIC remain unclear, but are believed to result from a combination of intersecting mechanisms including inflammation, secretory dysfunctions, GI dysmotility and alterations in GI innervation. Current treatments for CIC and CID aim to reduce the severity of symptoms rather than combating the pathophysiological mechanisms of dysfunction, and often result in worsening of already chronic GI symptoms or trigger the onset of a plethora of other side-effects including respiratory depression, uneven heartbeat, seizures, and neurotoxicity. Emerging treatments including those targeting the enteric nervous system present promising avenues to alleviate CID and CIC. Identification of potential targets for novel therapies to alleviate chemotherapy-induced toxicity is essential to improve clinical outcomes and quality of life amongst cancer sufferers.

Cytokine responses and epithelial function in the intestinal mucosa

Inflammatory diseases of mucosal organs are significantly influenced by the microenvironment in which they reside. Cytokines found within this microenvironment contribute significantly to endpoint functions of the mucosa. Studies dating back to the 1990s have revealed that epithelial cells are both a source as well as a target for numerous cytokines and that such signaling can substantially influence the outcome of mucosal disease, such as inflammatory bowel disease. Here, we will review literature regarding intestinal epithelial cells as sources and responders to cytokines found in the intestinal milieu. These studies highlight the dynamic nature of these pathways and lend insight into the complexity of treating mucosal inflammation.

Individualized medicine using intestinal responses to CFTR potentiators and correctors

Cystic fibrosis transmembrane conductance regulator (CFTR) modulators that target the mutant CFTR protein are being introduced for treatment of cystic fibrosis. Stratification of subjects based on their CFTR genotype has been proven essential to demonstrate clinical efficacy of these novel treatments. Despite this stratification, considerable heterogeneity between subjects receiving CFTR modulators is still observed which remains largely uncharacterized. The CFTR genotype, and additional genetic and environmental factors that impact either tissue-specific CFTR protein characteristics or the pharmacokinetic properties of treatments will likely determine the individual response to therapy. The development of intestinal biomarkers for CFTR modulators may help to better quantitate individual responses to treatment, with potential to optimize treatments for subjects with limited responses, and the selection of responsive subjects that currently do not receive treatments. Here, recent advances concerning the use of intestinal biomarkers for CFTR modulator treatments are reviewed, with a focus on biomarkers of CFTR function in ex vivo rectal biopsies and in vitro cultured primary intestinal organoids. Their potential value is considered in the context of the current unmet needs for better treatments for the majority of subjects with CF, and individual biomarkers that enable the prediction of long term therapeutic responses to CFTR modulators. Pediatr Pulmonol. 2016;51:S23-S34. © 2016 Wiley Periodicals, Inc.

Bioactivity-guided fractionation of an antidiarrheal Chinese herb Rhodiola kirilowii (Regel) Maxim reveals (-)-epicatechin-3-gallate and (-)-epigallocatechin-3-gallate as inhibitors of cystic fibrosis transmembrane conductance regulator

Cystic fibrosis transmembrane conductance regulator (CFTR) is the principal apical route for transepithelial fluid transport induced by enterotoxin. Inhibition of CFTR has been confirmed as a pharmaceutical approach for the treatment of secretory diarrhea. Many traditional Chinese herbal medicines, like Rhodiola kirilowii (Regel) Maxim, have long been used for the treatment of secretory diarrhea. However, the active ingredients responsible for their therapeutic effectiveness remain unknown. The purpose of this study is to identify CFTR inhibitors from Rhodiola kirilowii (Regel) Maxim via bioactivity-directed isolation strategy. We first identified fractions of Rhodiola kirilowii (Regel) Maxim that inhibited CFTR Cl^- channel activity. Further bioactivity-directed fractionation led to the identification of (-)–epigallocatechin-3-gallate (EGCG) as CFTR Cl^- channel inhibitor. Analysis of 5 commercially available EGCG analogs including (+)–catechins (C), (-)–epicatechin (EC), (-)–epigallocatechin (EGC), (-)–epicatechin-3-gallate (ECG) and EGCG revealed that ECG also had CFTR inhibitory activity. EGCG dose-dependently and reversibly inhibited CFTR Cl^- channel activity in transfected FRT cells with an IC₅₀ value around 100 μM. In ex vivo studies, EGCG and ECG inhibited CFTR-mediated short-circuit currents in isolated rat colonic mucosa in a dose-dependent manner. In an intestinal closed-loop model in mice, intraluminal application of EGCG (10 μg) and ECG (10 μg) significantly reduced cholera toxin-induced intestinal fluid secretion. CFTR Cl^- channel is a molecular target of natural compounds EGCG and ECG. CFTR inhibition may account, at least in part, for the antidiarrheal activity of Rhodiola kirilowii (Regel) Maxim. EGCG and ECG could be new lead compounds for development of CFTR-related diseases such as secretory diarrhea.

Three charged amino acids in extracellular loop 1 are involved in maintaining the outer pore architecture of CFTR

Disease-associated mutation of charged amino acids in extracellular loop 1 of CFTR may reduce chloride flow by damaging the outer pore architecture.

The cystic fibrosis (CF) transmembrane conductance regulator (CFTR) bears six extracellular loops (ECL1–6); ECL1 is the site of several mutations associated with CF. Mutation R117H has been reported to reduce current amplitude, whereas D110H, E116K, and R117C/L/P may impair channel stability. We hypothesized that these amino acids might not be directly involved in ion conduction and permeation but may contribute to stabilizing the outer vestibule architecture in CFTR. We used cRNA injected oocytes combined with electrophysiological techniques to test this hypothesis. Mutants bearing cysteine at these sites were not functionally modified by extracellular MTS reagents and were blocked by GlyH-101 similarly to WT-CFTR. These results suggest that these three residues do not contribute directly to permeation in CFTR. In contrast, mutants D110R-, E116R-, and R117A-CFTR exhibited instability of the open state and significantly shortened burst duration compared with WT-CFTR and failed to be locked into the open state by AMP-PNP (adenosine 5′-(β,γ-imido) triphosphate); charge-retaining mutants showed mainly the full open state with comparably longer open burst duration. These interactions suggest that these ECL1 residues might be involved in maintaining the outer pore architecture of CFTR. A CFTR homology model suggested that E116 interacts with R104 in both the closed and open states, D110 interacts with K892 in the fully closed state, and R117 interacts with E1126 in the open state. These interactions were confirmed experimentally. The results suggest that D110, E116, and R117 may contribute to stabilizing the architecture of the outer pore of CFTR by interactions with other charged residues.