Dual activation of CFTR and CLCN2 by lubiprostone in murine nasal epithelia

The ClC-2 Chloride Channel Activator, Lubiprostone, Improves Intestinal Barrier Function in Biopsies from Crohn’s Disease but Not Ulcerative Colitis Patients

The prostone analog, lubiprostone, is approved to manage constipation-predominant irritable bowel syndrome. Lubiprostone also protects intestinal mucosal barrier function in animal models of colitis. The aim of this study was to determine if lubiprostone improves barrier properties in isolated colonic biopsies from Crohn’s disease (CD) and ulcerative colitis (UC) patients. Sigmoid colon biopsies from healthy subjects, CD and UC patients in remission, and CD patients with active disease were mounted in Ussing chambers. Tissues were treated with lubiprostone or vehicle to determine the effects on transepithelial electrical resistance (TER), FITC-dextran 4kD (FD4) permeability, and electrogenic ion transport responses to forskolin and carbachol. Localization of the tight junction protein, occludin, was determined by immunofluorescence. Lubiprostone significantly increased ion transport across control, CD and UC remission biopsies but not active CD. Lubiprostone selectively improved TER in both CD remission and active disease biopsies but not in control or UC biopsies. The improved TER was associated with increased membrane localization of occludin. Lubiprostone selectively improved barrier properties of biopsies from CD patients vs. UC and independent of an ion transport response. These data indicate that lubiprostone has potential efficacy in improving mucosal integrity in Crohn’s disease.

Modulator Combination Improves In Vitro the Microrheological Properties of the Airway Surface Liquid of Cystic Fibrosis Airway Epithelia

Cystic fibrosis (CF) is a genetic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) protein, a plasma membrane protein expressed on the apical surface of secretory epithelia of the airways. In the airways, defective or absent function of the CFTR protein determines abnormalities of chloride and bicarbonate secretion and, in general, of the transepithelial homeostasis that lead to alterations of airway surface liquid (ASL) composition and properties. The reduction of ASL volume impairs ciliary beating with the consequent accumulation of a sticky mucus. This situation prevents normal mucociliary clearance, favoring the survival and proliferation of bacteria and contributing to the genesis of the CF pulmonary disease. We explored the potential of some CFTR modulators, namely ivacaftor, tezacaftor, elexacaftor and their combination Kaftrio^TM, capable of partially recovering the basic defects of the CFTR protein, to ameliorate the transepithelial fluid transport and the viscoelastic properties of the mucus when used singly or in combination. Primary human bronchial epithelial cells obtained from CF and non-CF patients were differentiated into a mucociliated epithelia in order to assess the effects of correctors tezacaftor, elexacaftor and their combination with potentiator ivacaftor on the key properties of ASL, such as fluid reabsorption, viscosity, protein content and pH. The treatment of airway epithelia bearing the deletion of a phenylalanine at position 508 (F508del) in the CFTR gene with tezacaftor and elexacaftor significantly improved the pericilial fluid composition, reducing the fluid reabsorption, correcting the ASL pH and reducing the viscosity of the mucus. Kaftrio^TM was more effective than single modulators in improving all the evaluated parameters, demonstrating once more that this combination recently approved for patients 6 years and older with cystic fibrosis who have at least one F508del mutation in the CFTR gene represents a valuable tool to defeat CF.

Integrated Omics Approaches Revealed the Osmotic Stress-Responsive Genes and Microbiota in Gill of Marine Medaka

This is the first study using the transcriptome and 16S rRNA gene sequencing to report the hypotonic responsive genes in gill cells and the compositions of gill microbiota in marine medaka. The overlapped glycosaminoglycan- and chitin-related pathways suggest host-bacterium interaction in fish gill during osmotic stress.

Receptor-mediated activation of CFTR via prostaglandin signaling pathways in the airway

Cystic fibrosis (CF) is a genetic disease caused by mutations of the gene encoding a cAMP-activated Cl^- channel, the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR modulator therapies consist of small-molecule drugs that rescue mutant CFTR. Regimens of single or combinations of CFTR modulators still rely on endogenous levels of cAMP to regulate CFTR activity. We investigated CFTR activation by the natural mediator prostaglandin E2 (PGE2) and lubiprostone (a Food and Drug Administration-approved drug known to target prostaglandin receptors) and tested the hypothesis that receptor-mediated CFTR activators can be used in combination with currently available CFTR modulators to increase function of mutant CFTR. Primary-cultured airway epithelia were assayed in Ussing chambers. Experimental CFTR activators and established CFTR modulators were applied for 24 h and/or acutely and analyzed for their effect on CFTR activity as measured by changes in short-circuit current (I_SC). In non-CF airway epithelia, acute application of lubiprostone and PGE2 activated CFTR to the levels comparable to forskolin (Fsk). Pretreatment (24 h) with antagonists to prostaglandin receptors EP2 and EP4 abolished the ability of lubiprostone to acutely activate CFTR. In F508del homozygous airway epithelia pretreated with the triple combination of elexacaftor, tezacaftor, and ivacaftor (ELEXA/TEZ/IVA; i.e., Trikafta), acute application of lubiprostone was able to maximally activate CFTR. Prolonged (24 h) cotreatment of F508del homozygous epithelia with ELEXA/TEZ/IVA and lubiprostone increased acute CFTR activation by ∼60% compared with the treatment with ELEXA/TEZ/IVA alone. This work establishes the feasibility of targeting prostaglandin receptors to activate CFTR on the airway epithelia and demonstrates that cotreatment with lubiprostone can further restore modulator-rescued CFTR.

Functional Analysis and Clinical Significance of Chloride Channel 2 Expression in Esophageal Squamous Cell Carcinoma

BACKGROUND

Chloride channel 2 (CLCN2) was recently shown to affect tumor behavior. The present study examined the functions of CLCN2 in the regulation of genes that play a role in tumor progression, as well as its clinicopathological significance in esophageal squamous cell carcinoma (ESCC).

METHODS

Knockdown experiments were conducted using CLCN2-small-interfering RNA, and changes in proliferation, survival, and cellular movement in human ESCC cell lines were investigated. A microarray analysis of gene expression profiles in CLCN2-depleted ESCC cells was conducted. Fifty-four primary ESCC samples were examined by immunohistochemistry (IHC).

RESULTS

The strong expression of CLCN2 was detected in TE5 and KYSE70 cells. Downregulated expression of CLCN2 enhanced proliferation and decreased apoptosis, whereas its upregulation inhibited proliferation and increased apoptosis. The effects of lubiprostone, a CLCN2 activator, were also investigated. In lubiprostone-treated cells, proliferation was inhibited and apoptosis was increased. The microarray analysis demonstrated that interferon (IFN) signaling-related genes were downregulated in CLCN2-depleted cells. IHC showed the presence of CLCN2 in the cytoplasm and cell membranes of ESCC cells. The prognostic analysis revealed a relationship between weak CLCN2 expression and shorter overall survival.

CONCLUSIONS

The present results indicate that tumor progression is regulated by CLCN2 through its effects on IFN signaling. Furthermore, weak CLCN2 expression was associated with poorer outcomes in ESCC patients. The present study will contribute to a clearer understanding of the role of CLCN2 as a mediator of ESCC, as well as its use as a biomarker for this cancer.

Prognosis-Based Early Intervention Strategies to Resolve Exacerbation and Progressive Lung Function Decline in Cystic Fibrosis

Cystic fibrosis (CF) is a genetic disease caused by a mutation(s) in the CF transmembrane regulator (CFTR), where progressive decline in lung function due to recurring exacerbations is a major cause of mortality. The initiation of chronic obstructive lung disease in CF involves inflammation and exacerbations, leading to mucus obstruction and lung function decline. Even though clinical management of CF lung disease has prolonged survival, exacerbation and age-related lung function decline remain a challenge for controlling the progressive lung disease. The key to the resolution of progressive lung disease is prognosis-based early therapeutic intervention; thus, the development of novel diagnostics and prognostic biomarkers for predicting exacerbation and lung function decline will allow optimal management of the lung disease. Hence, the development of real-time lung function diagnostics such as forced oscillation technique (FOT), impulse oscillometry system (IOS), and electrical impedance tomography (EIT), and novel prognosis-based intervention strategies for controlling the progression of chronic obstructive lung disease will fulfill a significant unmet need for CF patients. Early detection of CF lung inflammation and exacerbations with the timely resolution will not only prolong survival and reduce mortality but also improve quality of life while reducing significant health care costs due to recurring hospitalizations.

Negative chronotropic and inotropic effects of lubiprostone on iPS cell-derived cardiomyocytes via activation of CFTR

Background

Lubiprostone (LBP) is a novel chloride channel opener that has been reported to activate chloride channel protein 2 (ClC-2) and cystic fibrosis transmembrane conductance regulator (CFTR). LBP facilitates fluid secretion by activating CFTR in the intestine and is used as a drug for treating chronic constipation. While ClC-2 and CFTR expression has been confirmed in cardiomyocytes (CMs), the effect of LBP on CMs has not yet been investigated. Thus, the present study aimed to investigate the effect of LBP on CMs using mouse-induced pluripotent stem (iPS) cell-derived CMs (iPS-CMs).

Methods

We induced mouse iPS cells into CMs through embryoid body (EB) formation. We compared the differentiated cells to CMs isolated from adult and fetal mice using gene expression, spontaneous beating rate, and contraction ratio analyses.

Results

Gene expression analysis revealed that, in the iPS-CMs, the mRNA expression of the undifferentiated cell markers Rex1 and Nanog decreased, whereas the expression of the unique cardiomyocyte markers cardiac troponin I (cTnI) and cardiac troponin T (cTNT), increased. Immunostaining showed that the localization of cTnI and connexin-43 in the iPS-CMs was similar to that in the primary fetal CMs (FCMs) and adult CMs (ACMs). LBP decreased the spontaneous beating rate of the iPS-CMs and FCMs, and decreased the contraction ratio of the iPS-CMs and ACMs. The reduction in the beating rate and contraction ratio caused by LBP was inhibited by glycine hydrazide (GlyH), which is a CFTR inhibitor.

Conclusion

These results suggest that LBP stimulates CFTR in CMs and that LBP has negative chronotropic and inotropic effects on CMs. LBP may be useful for treating cardiac diseases such as heart failure, ischemia, and arrhythmia.

An Intriguing Involvement of Mitochondria in Cystic Fibrosis

Cystic fibrosis (CF) occurs when the cystic fibrosis transmembrane conductance regulator (CFTR) protein is not synthetized and folded correctly. The CFTR protein helps to maintain the balance of salt and water on many body surfaces, such as the lung surface. When the protein is not working correctly, chloride becomes trapped in cells, then water cannot hydrate the cellular surface and the mucus covering the cells becomes thick and sticky. Furthermore, a defective CFTR appears to produce a redox imbalance in epithelial cells and extracellular fluids and to cause an abnormal generation of reactive oxygen species: as a consequence, oxidative stress has been implicated as a causative factor in the aetiology of the process. Moreover, massive evidences show that defective CFTR gives rise to extracellular GSH level decrease and elevated glucose concentrations in airway surface liquid (ASL), thus encouraging lung infection by pathogens in the CF advancement. Recent research in progress aims to rediscover a possible role of mitochondria in CF. Here the latest new and recent studies on mitochondrial bioenergetics are collected. Surprisingly, they have enabled us to ascertain that mitochondria have a leading role in opposing the high ASL glucose level as well as oxidative stress in CF.

Ion channels of the lung and their role in disease pathogenesis

Maintenance of normal epithelial ion and water transport in the lungs includes providing a thin layer of surface liquid that coats the conducting airways. This airway surface liquid is critical for normal lung function in a number of ways but, perhaps most importantly, is required for normal mucociliary clearance and bacterial removal. Preservation of the appropriate level of hydration, pH, and viscosity for the airway surface liquid requires the proper regulation and function of a battery of different types of ion channels and transporters. Here we discuss how alterations in ion channel/transporter function often lead to lung pathologies.

Analysis of nasal potential in murine cystic fibrosis models

The nasal epithelium of the mouse closely mimics the bioelectrical phenotype of the human airways. Ion transport across the nasal epithelium induces a nasal transepithelial potential difference. Its measurement by a relatively non-invasive method adapted from humans allows in vivo longitudinal measurements of CFTR-dependent ionic transport in the murine nasal mucosa. This test offers a useful tool to assess CFTR function in preclinical studies for novel therapeutics modulating CFTR activity. Here we extensively review work done to assess transepithelial transport in the murine respiratory epithelium in the basal state and after administration of CFTR modulators. Factors of variability and discriminative threshold between the CF and the WT mice for different readouts are discussed.

Chronic Constipation: a Review of Current Literature

Chronic constipation is a common health condition representing a substantial proportion of primary care visits and referrals to specialist providers. Chronic constipation can have a significant negative effect on health-related quality of life and has been associated with psychological distress in severely affected patients. It has the potential to cause patients to curtail work, school, and social activities. While different pathophysiological mechanisms have been implicated in the development of chronic constipation, in some instances, the causes of chronic constipation are not easily determined. Expenditures for the evaluation and management of chronic constipation represent a significant burden on patients and payers, and it is important for clinicians to have a clear understanding of the different pathophysiological mechanisms associated with constipation, understand the different testing modalities and treatments that are available including their appropriateness and limitations, and tailor that knowledge to the management of individual patients.

Opioid-induced constipation: advances and clinical guidance

Currently opioids are the most frequently used medications for chronic noncancer pain. Opioid-induced constipation is the most common adverse effect associated with prolonged use of opioids, having a major impact on quality of life. There is an increasing need to treat opioid-induced constipation. With the recent approval of medications for the treatment of opioid-induced constipation, there are several therapeutic approaches. This review addresses the clinical presentation and diagnosis of opioid-induced constipation, barriers to its diagnosis, effects of opioids in the gastrointestinal tract, differential tolerance to opiates in different gastrointestinal organs, medications approved and in development for the treatment of opioid-induced constipation, and a proposed clinical management algorithm for treating opioid-induced constipation in patients with noncancer pain.

Transcriptomic analysis reveals specific osmoregulatory adaptive responses in gill mitochondria-rich cells and pavement cells of the Japanese eel

BACKGROUND

Homeostasis of ions and water is important for the maintenance of cellular functions. The regulation of the homeostasis is particularly important in euryhaline fish that migrate between freshwater (FW) and seawater (SW) environments. The fish gill, the major tissue that forms an interface separating the extracellular fluids and external water environment, has an effective transport system to maintain and regulate a constant body osmolality. In fish gills, the two major epithelial cells, pavement cells (PVCs) and mitochondria-rich cells (MRCs), are known to play key and complementary roles in ion transport at the interface. Discovering the robust mechanisms underlying the two cell types' response to osmotic stress would benefit our understanding of the fundamental mechanism allowing PVCs and MRCs to handle osmotic stress. Owing to the limited genomic data available on estuarine species, existing knowledge in this area is slim. In this study, transcriptome analyses were conducted using PVCs and MRCs isolated from Japanese eels adapted to FW or SW environments to provide a genome-wide molecular study to unravel the fundamental processes at work.

RESULTS

The study identified more than 12,000 transcripts in the gill cells. Interestingly, remarkable differential expressed genes (DEGs) were identified in PVCs (970 transcripts) instead of MRCs (400 transcripts) in gills of fish adapted to FW or SW. Since PVCs cover more than 90 % of the gill epithelial surface, the greater change in gene expression patterns in PVCs in response to external osmolality is anticipated. In the integrity pathway analysis, 19 common biological functions were identified in PVCs and MRCs. In the enriched signaling pathways analysis, most pathways differed between PVCs and MRCs; 14 enriched pathways were identified in PVCs and 12 in MRCs. The results suggest that the osmoregulatory responses in PVCs and MRCs are cell-type specific, which supports the complementary functions of the cells in osmoregulation.

CONCLUSIONS

This is the first study to provide transcriptomic analysis of PVCs and MRCs in gills of eels adapted to FW or SW environments. It describes the cell-type specific transcriptomic network in different tonicity. The findings consolidate the known osmoregulatory pathways and provide molecular insight in osmoregulation. The presented data will be useful for researchers to select their targets for further studies.

Pharmaceutical Activation or Genetic Absence of ClC-2 Alters Tight Junctions During Experimental Colitis

BACKGROUND

We have previously reported that the ClC-2 chloride channel has an important role in regulation of tight junction barrier function during experimental colitis, and the pharmaceutical ClC-2 activator lubiprostone initiates intestinal barrier repair in ischemic-injured intestine. Thus, we hypothesized that pharmaceutical ClC-2 activation would have a protective and therapeutic effect in murine models of colitis, which would be absent in ClC-2 mice.

METHODS

We administered lubiprostone to wild-type or ClC-2 mice with dextran sulfate sodium (DSS) or 2, 4, 5-trinitrobenzene sulfonic acid-induced colitis. We determined the severity of colitis and assessed intestinal permeability. Selected tight junction proteins were analyzed by Western blotting and immunofluorescence/confocal microscopy, whereas proliferative and differentiated cells were examined with special staining and immunohistochemistry.

RESULTS

Oral preventive or therapeutic administration of lubiprostone significantly reduced the severity of colitis and reduced intestinal permeability in both DSS and trinitrobenzene sulfonic acid-induced colitis. Preventive treatment with lubiprostone induced significant recovery of the expression and distribution of selected sealing tight junction proteins in mice with DSS-induced colitis. In addition, lubiprostone reduced crypt proliferation and increased the number of differentiated epithelial cells. Alternatively, when lubiprostone was administered to ClC-2 mice, the protective effect against DSS colitis was limited.

CONCLUSIONS

This study suggests a central role for ClC-2 in restoration of barrier function and tight junction architecture in experimental murine colitis, which can be therapeutically targeted with lubiprostone.

ClC-2 regulation of intestinal barrier function: Translation of basic science to therapeutic target

The ClC-2 chloride channel is a member of the voltage-gated chloride channel family. ClC-2 is involved in various physiological processes, including fluid transport and secretion, regulation of cell volume and pH, maintaining the membrane potential of the cell, cell-to-cell communication, and tissue homeostasis. Recently, our laboratory has accumulated evidence indicating a critical role of ClC-2 in the regulation of intestinal barrier function by altering inter-epithelial tight junction composition. This review will detail the role of ClC-2 in intestinal barrier function during intestinal disorders, including experimental ischemia/reperfusion injury and dextran sodium sulfate (DSS)-induced inflammatory bowel disease. Details of pharmacological manipulation of ClC-2 via prostone agonists will also be provided in an effort to show the potential therapeutic relevance of ClC-2 regulation, particularly during intestinal barrier disruption.

Role of epithelial sodium channels in the regulation of lung fluid homeostasis

In utero, fetal lung epithelial cells actively secrete Cl(-) ions into the lung air spaces while Na(+) ions follow passively to maintain electroneutrality. This process, driven by an electrochemical gradient generated by the Na(+)-K(+)-ATPase, is responsible for the secretion of fetal fluid that is essential for normal lung development. Shortly before birth, a significant upregulation of amiloride-sensitive epithelial channels (ENaCs) on the apical side of the lung epithelial cells results in upregulation of active Na(+) transport. This process is critical for the reabsorption of fetal lung fluid and the establishment of optimum gas exchange. In the adult lung, active Na(+) reabsorption across distal lung epithelial cells limits the degree of alveolar edema in patients with acute lung injury and cardiogenic edema. Cl(-) ions are transported either paracellularly or transcellularly to preserve electroneutrality. An increase in Cl(-) secretion across the distal lung epithelium has been reported following an acute increase in left atrial pressure and may result in pulmonary edema. In contrast, airway epithelial cells secrete Cl(-) through apical cystic fibrosis transmembrane conductance regulator and Ca(2+)-activated Cl(-) channels and absorb Na(+). Thus the coordinated action of Cl(-) secretion and Na(+) absorption is essential for maintenance of the volume of epithelial lining fluid that, in turn, maximizes mucociliary clearance and facilitates clearance of bacteria and debris from the lungs. Any factor that interferes with Na(+) or Cl(-) transport or dramatically upregulates ENaC activity in airway epithelial cells has been associated with lung diseases such as cystic fibrosis or chronic obstructive lung disease. In this review we focus on the role of the ENaC, the mechanisms involved in ENaC regulation, and how ENaC dysregulation can lead to lung pathology.

Lubiprostone in constipation: clinical evidence and place in therapy

Constipation is one of the most common function bowel disorders encountered by primary care providers and gastroenterologists. Disorders of chronic constipation, including irritable bowel syndrome with constipation, chronic idiopathic constipation, and opioid-induced chronic constipation, are associated with significant medical costs and a negative impact on quality of life. Although there is evidence supporting the effectiveness of some over-the-counter laxatives in chronic constipation, currently there is no evidence supporting lifestyle modification, dietary change or over-the-counter laxatives as effective long-term therapy for patients with chronic constipation. Lubiprostone is a prostaglandin-derived bicyclic fatty acid available to use for long-term treatment of constipation. Lubiprostone works by increasing intraluminal chloride ion secretion, which results in a passive influx of water and sodium, leading to increased intestinal peristalsis and colonic laxation with decreased intestinal stool transit time. Randomized, double-blind, placebo-controlled trials of lubiprostone in patients with chronic constipation, irritable bowel syndrome and opioid-induced constipation have shown it to be effective and free of serious adverse effects. The most common side effects associated with lubiprostone are mild to moderate nausea and diarrhea. Currently lubiprostone is approved for treatment of chronic constipation and opioid-induced constipation for men and women at 24 µg twice daily and for treatment of irritable bowel syndrome with constipation in women at 8 µg twice daily. Additional research continues to shed light on the molecular mechanisms of lubiprostone and further work may expand its clinical applications.

Direct interactions between ENaC gamma subunit and ClCN2 in cystic fibrosis epithelial cells

Cystic fibrosis (CF) is a lethal disease caused by mutations in the chloride channel CFTR gene. The disease is characterized by decreased chloride secretion and unregulated sodium absorption through the epithelial sodium channel (ENaC) in the airway epithelium and other affected organs. We hypothesize that a non‐CFTR alternative chloride channel ClCN2 can be activated to negatively regulate ENaC in CF epithelial cell cultures. We identified a novel interaction between ClCN2 and the ENaCγ subunit in CF airway epithelial cells and show that the upregulation of ClCN2 leads to decreased expression of ENaCγ via a K63 ubiquitination mechanism. These regulatory effects of ClCN2 on ENaCγ appear to be dependent on the CBS‐1 domain located within the c‐terminus of ClCN2, which is necessary for the targeting of ClCN2 to the apical surface. In sum, these results suggest the ability of ClCN2 to negatively regulate sodium absorption through ENaC, supporting its role as a therapeutic target for the treatment of CF.

Cystic Fibrosis is caused by mutations in the chloride channel CFTR gene which secondarily increases sodium reabsorption in the airways. ClCN2 is an epithelial chloride channel in the airways that can be activated in CF. We show an interaction between ClCN2 and the epithelial sodium channel subunit gamma that affects subunit expression and targeting to the plasma membrane.

CFTR and lung homeostasis

CFTR is a cAMP-activated chloride and bicarbonate channel that is critical for lung homeostasis. Decreases in CFTR expression have dire consequences in cystic fibrosis (CF) and have been suggested to be a component of the lung pathology in chronic obstructive pulmonary disease. Decreases or loss of channel function often lead to mucus stasis, chronic bacterial infections, and the accompanying chronic inflammatory responses that promote progressive lung destruction, and, eventually in CF, lung failure. Here we discuss CFTR's functional role airway surface liquid hydration and pH, in regulation of other channels such as the epithelial sodium channel, and in regulating inflammatory responses in the lung.

Chloride channels in stellate cells are essential for uniquely high secretion rates in neuropeptide-stimulated Drosophila diuresis

Epithelia frequently segregate transport processes to specific cell types, presumably for improved efficiency and control. The molecular players underlying this functional specialization are of particular interest. In Drosophila, the renal (Malpighian) tubule displays the highest per-cell transport rates known and has two main secretory cell types, principal and stellate. Electrogenic cation transport is known to reside in the principal cells, whereas stellate cells control the anion conductance, but by an as-yet-undefined route. Here, we resolve this issue by showing that a plasma membrane chloride channel, encoded by ClC-a, is exclusively expressed in the stellate cell and is required for Drosophila kinin-mediated induction of diuresis and chloride shunt conductance, evidenced by chloride ion movement through the stellate cells, leading to depolarization of the transepithelial potential. By contrast, ClC-a knockdown had no impact on resting secretion levels. Knockdown of a second CLC gene showing highly abundant expression in adult Malpighian tubules, ClC-c, did not impact depolarization of transepithelial potential after kinin stimulation. Therefore, the diuretic action of kinin in Drosophila can be explained by an increase in ClC-a-mediated chloride conductance, over and above a resting fluid transport level that relies on other (ClC-a-independent) mechanisms or routes. This key segregation of cation and anion transport could explain the extraordinary fluid transport rates displayed by some epithelia.

Differentiation between human ClC-2 and CFTR Cl− channels with pharmacological agents

It has been difficult to separate/identify the roles of ClC-2 and CFTR in Cl− transport studies. Using pharmacological agents, we aimed to differentiate functionally between ClC-2 and CFTR Cl− channel currents. Effects of CFTR inhibitor 172 (CFTRinh172), N-(4-methylphenylsulfonyl)- N′-(4-trifluoromethylphenyl)urea (DASU-02), and methadone were examined by whole cell patch clamp on Cl− currents in recombinant human ClC-2/human embryonic kidney 293 (ClC-2/HEK293) cells stably transformed with Epstein-Barr nuclear antigen 1 (hClC-2/293EBNA) and human CFTR/HEK293 (hCFTR/HEK293) cells and by short-circuit current ( Isc) measurements in T84 cells. Lubiprostone and forskolin-IBMX were used as activators. CFTRinh172 inhibited forskolin-IBMX-stimulated recombinant human CFTR (hCFTR) and lubiprostone-stimulated recombinant human ClC-2 (hClC-2) Cl− currents in a concentration-dependent manner equipotently. DASU-02 inhibited forskolin-IBMX-stimulated Cl− currents in hCFTR/HEK293 cells, but not lubiprostone-stimulated Cl− currents in hClC-2/293EBNA cells. In T84 cells with basolateral nystatin or 1-ethyl-2-benzimidazolinone (1-EBIO), lubiprostone-stimulated and forskolin-IBMX-cyclosporin A (FICA)-stimulated Isc components were observed. CFTRinh172 inhibited major portions of both components. DASU-02 had no effect on lubiprostone-stimulated Isc but partially inhibited FICA-stimulated Isc. T84 cells in which ClC-2 or CFTR was knocked down using siRNAs were constructed. T84 ClC-2 knockdown cells did not respond to lubiprostone but did respond to forskolin-IBMX in a methadone-insensitive, DASU-02-sensitive manner, indicating CFTR function. T84 CFTR knockdown cells responded separately to lubiprostone and forskolin-IBMX in a methadone-sensitive and DASU-02-insensitive manner, indicating ClC-2 function. Low lubiprostone concentrations activated ClC-2, but not CFTR, and both channels were activated by forskolin-IBMX but have different inhibitor sensitivities. Methadone, but not DASU-02, inhibited ClC-2. DASU-02, but not methadone, inhibited CFTR. In T84 cells, both ClC-2 and CFTR are present and likely play roles in Cl− secretion.