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Shamsuddin AKM, Quinton PM. Concurrent absorption and secretion of airway surface liquids and bicarbonate secretion in human bronchioles. Am J Physiol Lung Cell Mol Physiol 2019; 316:L953-L960. [PMID: 30838869 PMCID: PMC6589593 DOI: 10.1152/ajplung.00545.2018] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/13/2018] [Revised: 03/01/2019] [Accepted: 03/01/2019] [Indexed: 01/15/2023] Open
Abstract
Although small airways account for the largest fraction of the total conducting airway surfaces, the epithelial fluid and electrolyte transport in small, native airway epithelia has not been well characterized. Investigations have been limited, no doubt, by the complex tissue architecture as well as by its inaccessibility, small dimensions, and lack of applicable assays, especially in human tissues. To better understand how the critically thin layer of airway surface liquid (ASL) is maintained, we applied a "capillary"-Ussing chamber (area ≈1 mm2) to measure ion transport properties of bronchioles with diameters of ~2 mm isolated from resected specimens of excised human lungs. We found that the small human airway, constitutively and concurrently, secretes and absorbs fluid as observed in porcine small airways (50). We found that the human bronchiolar epithelium is also highly anion selective and constitutively secretes bicarbonate ( HCO 3 - ), which can be enhanced pharmacologically by cAMP as well as Ca2+-mediated agonists. Concurrent secretion and absorption of surface liquid along with HCO 3 - secretion help explain how the delicate volume of the fluid lining the human small airway is physiologically buffered and maintained in a steady state that avoids desiccating or flooding the small airway with ASL.
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Affiliation(s)
- A K M Shamsuddin
- Department of Pediatrics, University of California , San Diego, California
| | - Paul M Quinton
- Department of Pediatrics, University of California , San Diego, California
- Division of Biomedical Sciences, University of California , Riverside, California
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2
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Sweat rate analysis of ivacaftor potentiation of CFTR in non-CF adults. Sci Rep 2018; 8:16233. [PMID: 30389955 PMCID: PMC6214959 DOI: 10.1038/s41598-018-34308-8] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2018] [Accepted: 10/15/2018] [Indexed: 12/14/2022] Open
Abstract
To determine if ivacaftor (Kalydeco) influences non-CF human CFTR function in vivo, we measured CFTR-dependent (C-sweat) and CFTR-independent (M-sweat) rates from multiple identified sweat glands in 8 non-CF adults. The two types of sweating were stimulated sequentially with intradermal injections of appropriate reagents; each gland served as its own control via alternating off-on drug tests on both arms, given at weekly intervals with 3 off and 3 on tests per subject. We compared drug effects on C-sweating stimulated by either high or low concentrations of β-adrenergic cocktail, and on methacholine-stimulated M-sweating. For each subject we measured ~700 sweat volumes from ~75 glands per arm (maximum 12 readings per gland), and sweat volumes were log-transformed for statistical analysis. T-tests derived from linear mixed models (LMMs) were more conservative than the familiar paired sample t-tests, and show that ivacaftor significantly increased C-sweating stimulated by both levels of agonist, with a larger effect in the low cocktail condition; ivacaftor did not increase M-sweat. Concurrent sweat chloride tests detected no effect of ivacaftor. We conclude that ivacaftor in vivo increases the open channel probability (PO) of WT CFTR, provided it is not already maximally stimulated.
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Wilson TE. Renal sympathetic nerve, blood flow, and epithelial transport responses to thermal stress. Auton Neurosci 2016; 204:25-34. [PMID: 28043810 DOI: 10.1016/j.autneu.2016.12.007] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/14/2016] [Revised: 11/28/2016] [Accepted: 12/20/2016] [Indexed: 12/12/2022]
Abstract
Thermal stress is a profound sympathetic stress in humans; kidney responses involve altered renal sympathetic nerve activity (RSNA), renal blood flow, and renal epithelial transport. During mild cold stress, RSNA spectral power but not total activity is altered, renal blood flow is maintained or decreased, and epithelial transport is altered consistent with a sympathetic stress coupled with central volume loaded state. Hypothermia decreases RSNA, renal blood flow, and epithelial transport. During mild heat stress, RSNA is increased, renal blood flow is decreased, and epithelial transport is increased consistent with a sympathetic stress coupled with a central volume unloaded state. Hyperthermia extends these directional changes, until heat illness results. Because kidney responses are very difficult to study in humans in vivo, this review describes and qualitatively evaluates an in vivo human skin model of sympathetically regulated epithelial tissue compared to that of the nephron. This model utilizes skin responses to thermal stress, involving 1) increased skin sympathetic nerve activity (SSNA), decreased skin blood flow, and suppressed eccrine epithelial transport during cold stress; and 2) increased SSNA, skin blood flow, and eccrine epithelial transport during heat stress. This model appears to mimic aspects of the renal responses. Investigations of skin responses, which parallel certain renal responses, may aid understanding of epithelial-sympathetic nervous system interactions during cold and heat stress.
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Affiliation(s)
- Thad E Wilson
- Division of Biomedical Sciences, Marian University College of Osteopathic Medicine, Indianapolis, IN, USA.
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Vijftigschild LAW, Berkers G, Dekkers JF, Zomer-van Ommen DD, Matthes E, Kruisselbrink E, Vonk A, Hensen CE, Heida-Michel S, Geerdink M, Janssens HM, van de Graaf EA, Bronsveld I, de Winter-de Groot KM, Majoor CJ, Heijerman HGM, de Jonge HR, Hanrahan JW, van der Ent CK, Beekman JM. β2-Adrenergic receptor agonists activate CFTR in intestinal organoids and subjects with cystic fibrosis. Eur Respir J 2016; 48:768-79. [PMID: 27471203 DOI: 10.1183/13993003.01661-2015] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/08/2015] [Accepted: 05/11/2016] [Indexed: 01/12/2023]
Abstract
We hypothesized that people with cystic fibrosis (CF) who express CFTR (cystic fibrosis transmembrane conductance regulator) gene mutations associated with residual function may benefit from G-protein coupled receptor (GPCR)-targeting drugs that can activate and enhance CFTR function.We used intestinal organoids to screen a GPCR-modulating compound library and identified β2-adrenergic receptor agonists as the most potent inducers of CFTR function.β2-Agonist-induced organoid swelling correlated with the CFTR genotype, and could be induced in homozygous CFTR-F508del organoids and highly differentiated primary CF airway epithelial cells after rescue of CFTR trafficking by small molecules. The in vivo response to treatment with an oral or inhaled β2-agonist (salbutamol) in CF patients with residual CFTR function was evaluated in a pilot study. 10 subjects with a R117H or A455E mutation were included and showed changes in the nasal potential difference measurement after treatment with oral salbutamol, including a significant improvement of the baseline potential difference of the nasal mucosa (+6.35 mV, p<0.05), suggesting that this treatment might be effective in vivo Furthermore, plasma that was collected after oral salbutamol treatment induced CFTR activation when administered ex vivo to organoids.This proof-of-concept study suggests that organoids can be used to identify drugs that activate CFTR function in vivo and to select route of administration.
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Affiliation(s)
- Lodewijk A W Vijftigschild
- Dept of Pediatric Pulmonology, University Medical Center, Utrecht, The Netherlands Laboratory of Translational Immunology, University Medical Center, Utrecht, The Netherlands These two authors contributed equally to this work
| | - Gitte Berkers
- Dept of Pediatric Pulmonology, University Medical Center, Utrecht, The Netherlands These two authors contributed equally to this work
| | - Johanna F Dekkers
- Dept of Pediatric Pulmonology, University Medical Center, Utrecht, The Netherlands Laboratory of Translational Immunology, University Medical Center, Utrecht, The Netherlands These two authors contributed equally to this work
| | - Domenique D Zomer-van Ommen
- Dept of Pediatric Pulmonology, University Medical Center, Utrecht, The Netherlands Laboratory of Translational Immunology, University Medical Center, Utrecht, The Netherlands These two authors contributed equally to this work
| | - Elizabeth Matthes
- CF Translational Research Centre, Dept of Physiology, McGill University, Montréal, QC, Canada
| | - Evelien Kruisselbrink
- Dept of Pediatric Pulmonology, University Medical Center, Utrecht, The Netherlands Laboratory of Translational Immunology, University Medical Center, Utrecht, The Netherlands
| | - Annelotte Vonk
- Dept of Pediatric Pulmonology, University Medical Center, Utrecht, The Netherlands Laboratory of Translational Immunology, University Medical Center, Utrecht, The Netherlands
| | - Chantal E Hensen
- Dept of Pediatric Pulmonology, University Medical Center, Utrecht, The Netherlands
| | - Sabine Heida-Michel
- Dept of Pediatric Pulmonology, University Medical Center, Utrecht, The Netherlands
| | - Margot Geerdink
- Dept of Pediatric Pulmonology, University Medical Center, Utrecht, The Netherlands
| | - Hettie M Janssens
- Dept of Pediatric Pulmonology, Erasmus Medical Center/Sophia Children's Hospital, Rotterdam, The Netherlands
| | | | - Inez Bronsveld
- Dept of Pulmonology, University Medical Center, Utrecht, The Netherlands
| | | | - Christof J Majoor
- Dept of Respiratory Medicine, Academic Medical Center, Amsterdam, The Netherlands
| | - Harry G M Heijerman
- Dept of Pulmonology and Cystic Fibrosis, Haga Teaching Hospital, The Hague, The Netherlands
| | - Hugo R de Jonge
- Dept of Gastroenterology and Hepatology, Erasmus University Medical Center, Rotterdam, The Netherlands
| | - John W Hanrahan
- CF Translational Research Centre, Dept of Physiology, McGill University, Montréal, QC, Canada
| | | | - Jeffrey M Beekman
- Dept of Pediatric Pulmonology, University Medical Center, Utrecht, The Netherlands Regenerative Medicine Center Utrecht, University Medical Center, Utrecht, The Netherlands
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Reddy MM, Stutts MJ. Status of fluid and electrolyte absorption in cystic fibrosis. Cold Spring Harb Perspect Med 2013; 3:a009555. [PMID: 23284077 DOI: 10.1101/cshperspect.a009555] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
Salt and fluid absorption is a shared function of many of the body's epithelia, but its use is highly adapted to the varied physiological roles of epithelia-lined organs. These functions vary from control of hydration of outward-facing epithelial surfaces to conservation and regulation of total body volume. In the most general context, salt and fluid absorption is driven by active Na(+) absorption. Cl(-) is absorbed passively through various available paths in response to the electrical driving force that results from active Na(+) absorption. Absorption of salt creates a concentration gradient that causes water to be absorbed passively, provided the epithelium is water permeable. Key differences notwithstanding, the transport elements used for salt and fluid absorption are broadly similar in diverse epithelia, but the regulation of these elements enables salt absorption to be tailored to very different physiological needs. Here we focus on salt absorption by exocrine glands and airway epithelia. In cystic fibrosis, salt and fluid absorption by gland duct epithelia is effectively prevented by the loss of cystic fibrosis transmembrane conductance regulator (CFTR). In airway epithelia, salt and fluid absorption persists, in the absence of CFTR-mediated Cl(-) secretion. The contrast of these tissue-specific changes in CF tissues is illustrative of how salt and fluid absorption is differentially regulated to accomplish tissue-specific physiological objectives.
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Affiliation(s)
- M M Reddy
- Department of Pediatrics, UCSD School of Medicine, University of California, San Diego, La Jolla, CA 92093, USA
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7
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Fan S, Harfoot N, Bartolo RC, Butt AG. CFTR is restricted to a small population of high expresser cells that provide a forskolin-sensitive transepithelial Cl- conductance in the proximal colon of the possum, Trichosurus vulpecula. ACTA ACUST UNITED AC 2012; 215:1218-30. [PMID: 22399668 DOI: 10.1242/jeb.061176] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
The cystic fibrosis transmembrane conductance regulator (CFTR) is central to anion secretion in both the possum and eutherian small intestine. Here, we investigated its role in the possum proximal colon, which has novel transport properties compared with the eutherian proximal colon. Despite considerable CFTR expression, high doses of the CFTR activator forskolin (EC(50)≈10 μmol l(-1)) were required for a modest, CFTR-dependent increase in short-circuit current (I(sc)) in the proximal colon. Presumably, this is because CFTR is restricted to the apical membrane of a small population of CFTR high expresser (CHE) cells in the surface and upper crypt epithelium. Furthermore, although the forskolin-stimulated I(sc) was dependent on serosal Na(+), Cl(-) and HCO(3)(-), consistent with anion secretion, inhibition of the basolateral Na-K-2Cl(-) (NKCC1) or Na-HCO(3) (pNBCe1) cotransporters did not prevent it. Therefore, although NKCC1 and pNBCe1 are expressed in the colonic epithelium they do not appear to be expressed in CHE cells. At low doses (IC(50)≈1 μmol l(-1)), forskolin also decreased the transepithelial conductance (G(T)) of the colon through inhibition of a 4,4'-diisothiocyano-2,2'-stilbenedisulphonic acid-sensitive anion conductance in the basolateral membrane of the CHE cells. This conductance is arranged in series with CFTR in the CHE cells and, therefore, the CHE cells provide a transepithelial Cl(-) conductance for passive Cl(-) absorption across the epithelium. Inhibition of the basolateral Cl(-) conductance of the CHE cells by forskolin will inhibit Na(+) absorption by restricting the movement of its counter-ion Cl(-), assisting in the conversion of the tissue from an absorptive to a secretory state.
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Affiliation(s)
- Shujun Fan
- Department of Physiology, School of Medical Sciences, University of Otago, Dunedin, New Zealand
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Abstract
Native small airways must remain wet enough to be pliable and support ciliary clearance, but dry enough to remain patent for gas flow. The airway epithelial lining must both absorb and secrete ions to maintain a critical level of fluid on its surface. Despite frequent involvement in lung diseases, the minuscule size has limited studies of peripheral airways. To meet this challenge, we used a capillary to construct an Ussing chamber (area <1 mm(2)) to measure electrolyte transport across small native airways (∼1 mm ø) from pig lung. Transepithelial potentials (V(t)) were recorded in open circuit conditions while applying constant current pulses across the luminal surface of dissected airways to calculate transepithelial electrical conductance (G(t)) and equivalent short circuit current (I(eq)(sc)) in the presence and absence of selected Na(+) and Cl(-) transport inhibitors (amiloride, GlyH-101, Niflumic acid) and agonists (Forskolin + IBMX, UTP). Considered together the responses suggest an organ composed of both secreting and absorbing epithelia that constitutively and concurrently transport fluids into and out of the airway, i.e. in opposite directions. Since the epithelial lining of small airways is arranged in long, accordion-like rows of pleats and folds that run axially down the lumen, we surmise that cells within the pleats are mainly secretory while the cells of the folds are principally absorptive. This structural arrangement could provide local fluid transport from within the pleats toward the luminal folds that may autonomously regulate the local surface fluid volume for homeostasis while permitting acute responses to maintain clearance.
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Affiliation(s)
- A K M Shamsuddin
- Department of Pediatrics, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0830, USA
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9
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Brown MB, Haack KKV, Pollack BP, Millard-Stafford M, McCarty NA. Low abundance of sweat duct Cl- channel CFTR in both healthy and cystic fibrosis athletes with exceptionally salty sweat during exercise. Am J Physiol Regul Integr Comp Physiol 2011; 300:R605-15. [PMID: 21228336 PMCID: PMC3064278 DOI: 10.1152/ajpregu.00660.2010] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/04/2010] [Accepted: 01/06/2011] [Indexed: 11/22/2022]
Abstract
To understand potential mechanisms explaining interindividual variability observed in human sweat sodium concentration ([Na(+)]), we investigated the relationship among [Na(+)] of thermoregulatory sweat, plasma membrane expression of Na(+) and Cl(-) transport proteins in biopsied human eccrine sweat ducts, and basal levels of vasopressin (AVP) and aldosterone. Lower ductal luminal membrane expression of the Cl(-) channel cystic fibrosis transmembrane conductance regulator (CFTR) was observed in immunofluorescent staining of sweat glands from healthy young adults identified as exceptionally "salty sweaters" (SS) (n = 6, P < 0.05) and from patients with cystic fibrosis (CF) (n = 6, P < 0.005) compared with ducts from healthy young adults with "typical" sweat [Na(+)] (control, n = 6). Genetic testing of healthy subjects did not reveal any heterozygotes ("carriers") for any of the 39 most common disease-causing CFTR mutations in the United States. SS had higher baseline plasma [AVP] compared with control (P = 0.029). Immunostaining to investigate a potential relationship between higher plasma [AVP] (and sweat [Na(+)]) and ductal membrane aquaporin-5 revealed for all groups a relatively sparse and location-dependent ductal expression of the water channel with localization primarily to the secretory coil. Availability of CFTR for NaCl transport across the ductal membrane appears related to the significant physiological variability observed in sweat salt concentration in apparently healthy humans. At present, a heritable link between healthy salty sweaters and the most prevalent disease-causing CFTR mutations cannot be established.
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Affiliation(s)
- Mary Beth Brown
- School of Applied Physiology, Georgia Institute of Technology, Atlanta, USA.
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Reddy MM, Quinton PM. PKA mediates constitutive activation of CFTR in human sweat duct. J Membr Biol 2009; 231:65-78. [PMID: 19865788 PMCID: PMC2776937 DOI: 10.1007/s00232-009-9205-1] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2009] [Accepted: 09/23/2009] [Indexed: 11/29/2022]
Abstract
The cystic fibrosis transmembrane conductance regulator (CFTR) Cl− channels are constitutively activated in sweat ducts. Since phosphorylation-dependent and -independent mechanisms can activate CFTR, we sought to determine the actual mechanism responsible for constitutive activation of these channels in vivo. We show that the constitutively activated CFTR Cl− conductance (gCFTR) in the apical membrane is completely deactivated following α-toxin permeabilization of the basolateral membrane. We investigated whether such inhibition of gCFTR following permeabilization is due to the loss of cytoplasmic glutamate or due to dephosphorylation of CFTR by an endogenous phosphatase in the absence of kinase activity (due to the loss of kinase agonist cAMP, cGMP or GTP through α-toxin pores). In order to distinguish between these two possibilities, we examined the effect of inhibiting the endogenous phosphatase activity with okadaic acid (10−8 M) on the permeabilization-induced deactivation of gCFTR. We show that okadaic acid (1) inhibits an endogenous phosphatase responsible for dephosphorylating cAMP but not cGMP or G protein-activated CFTR and (2) prevents deactivation of CFTR following permeabilization of the basolateral membrane. These results indicate that distinctly different phosphatases may be responsible for dephosphorylating different kinase-specific sites on CFTR. We conclude that the phosphorylation by PKA alone appears to be primarily responsible for constitutive activation of gCFTR in vivo.
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Affiliation(s)
- M M Reddy
- Department of Pediatrics, UCSD School of Medicine, University of California, San Diego, 9500 Gilman Drive 0830, La Jolla, CA 92093, USA.
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Shamsuddin AKM, Reddy MM, Quinton PM. Iontophoretic β-adrenergic stimulation of human sweat glands: possible assay for cystic fibrosis transmembrane conductance regulator activityin vivo. Exp Physiol 2008; 93:969-81. [DOI: 10.1113/expphysiol.2008.042283] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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Rowe SM, Accurso F, Clancy JP. Detection of cystic fibrosis transmembrane conductance regulator activity in early-phase clinical trials. Ann Am Thorac Soc 2007; 4:387-98. [PMID: 17652506 PMCID: PMC2647604 DOI: 10.1513/pats.200703-043br] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023] Open
Abstract
Advances in our understanding of cystic fibrosis pathogenesis have led to strategies directed toward treatment of underlying causes of the disease rather than treatments of disease-related symptoms. To expedite evaluation of these emerging therapies, early-phase clinical trials require extension of in vivo cystic fibrosis transmembrane conductance regulator (CFTR)-detecting assays to multicenter trial formats, including nasal potential difference and sweat chloride measurements. Both of these techniques can be used to fulfill diagnostic criteria for the disease, and can discriminate various levels of CFTR function. Full realization of these assays in multicenter clinical trials requires identification of sources of nonbiological intra- and intersite variability, and careful attention to study design and statistical analysis of study-generated data. In this review, we discuss several issues important to the performance of these assays, including efforts to identify and address aspects that can contribute to inconsistent and/or potentially erroneous results. Adjunctive means of detecting CFTR including mRNA expression, immunocytochemical localization, and other methods are also discussed. Recommendations are presented to advance our understanding of these biomarkers and to improve their capacity to predict cystic fibrosis outcomes.
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Affiliation(s)
- Steven M Rowe
- Department of Medicine, University of Alabama at Birmingham, Birmingham, Alabama 35294-0006, USA.
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Servetnyk Z, Roomans GM. Chloride transport in NCL-SG3 sweat gland cells: Channels involved. Exp Mol Pathol 2007; 83:47-53. [PMID: 17383636 DOI: 10.1016/j.yexmp.2007.02.003] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/11/2006] [Revised: 01/22/2007] [Accepted: 02/16/2007] [Indexed: 11/18/2022]
Abstract
The aim of the study was to assess whether NCL-SG3, the only immortalized sweat gland cell line available, can be used as an in vitro model to study chloride ion transport in cultured sweat gland cells. Cl(-) efflux was measured using the MQAE dye fluorescence technique after stimulating the cells with different agonists. A significant stimulation of chloride efflux was achieved with the calcium ionophore A23187 resulting in an efflux rate of 0.9 mM/s. Both ATP and UTP activated chloride efflux in these cells, with the ATP response being larger. IBMX and forskolin stimulation did not induce a rate of chloride efflux above the basal level. Immunocytochemistry showed no detectable CFTR in NCL-SG3 cells. This finding was confirmed with flow cytometry analysis. Niflumic acid (20 and 100 microM NFA) and 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonic acid (H2DIDS) (100 ìM) decreased the rate of ATP-stimulated chloride efflux significantly (0.40 and 0.31 mM/s with NFA, 0.37 mM/s with H2DIDS). Gadolinium (20 ìM) had no effect on the chloride transport rate. In conclusion, the NCL-SG3 cells retain some of the aspects of human sweat gland epithelium, such as the ability to form cell-cell contacts. The CFTR protein is neither functional nor expressed in cultured NCL-SG3 sweat gland cells. Ca(2+)-activated chloride conductance is confirmed and the putative Ca(2+)-activated chloride channel (CaCC) is further characterized in term of its pharmacological sensitivity. The NCL-SG3 sweat gland cell line can be used to investigate the characteristics of the CaCC and to identify the channel.
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Affiliation(s)
- Z Servetnyk
- Department of Medical Cell Biology, Uppsala University, Box 571, SE-751 23 Uppsala, Sweden.
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Reddy MM, Quinton PM. ENaC activity requires CFTR channel function independently of phosphorylation in sweat duct. J Membr Biol 2007; 207:23-33. [PMID: 16463140 DOI: 10.1007/s00232-005-0798-8] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/23/2005] [Accepted: 10/01/2005] [Indexed: 12/13/2022]
Abstract
We previously showed that activation of the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) Cl- conductance (gCFTR) supports parallel activation of amiloride-sensitive epithelial Na+ channel (ENaC) in the native human sweat duct. However, it is not clear whether phosphorylated CFTR, phosphorylated ENaC, or only Cl(-) -channel function is required for activation. We used basilaterally alpha-toxin-permeabilized human sweat ducts to test the hypothesis that ENaC activation depends only on Cl(-) -channel function and not on phosphorylation of either CFTR or ENaC. CFTR is classically activated by PKA plus millimolar ATP, but cytosolic glutamate activation of gCFTR is independent of ATP and phosphorylation. We show here that both phosphorylation-dependent (PKA) and phosphorylation-independent (glutamate) activation of CFTR Cl- channel function support gENaC activation. We tested whether cytosolic application of 5 mM ATP alone, phosphorylation by cAMP, cGMP, G-protein dependent kinases (all in the presence of 100 microM ATP), or glutamate could support ENaC activation in the absence of gCFTR. We found that none of these agonists activated gENaC by themselves when Cl- current (I(Cl-)) through CFTR was blocked by: 1) Cl- removal, 2) DIDS inhibition, 3) lowering the ATP concentration to 100 microM (instead of 5 mM required to support CFTR channel function), or 4) mutant CFTR (homozygous DeltaF508 CF ducts). However, Cl- gradients in the direction of absorption supported, while Cl- gradients in the direction of secretion prevented ENaC activation. We conclude that the interaction between CFTR and ENaC is dependent on activated I(Cl-) through CFTR in the direction of absorption (Cl- gradient from lumen to cell). But such activation of ENaC is independent of phosphorylation and ATP. However, reversing I(Cl-) through CFTR in the direction of secretion (Cl- gradient from cell to lumen) prevents ENaC activation even in the presence of I(Cl-) through CFTR.
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Affiliation(s)
- M M Reddy
- Department of Pediatrics, University of California, San Diego, UCSD School of Medicine, 9500 Gillman Drive, La Jolla, CA 92103-0831, USA.
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Outside Neurons/Inside Epithelia: Novel Activation of CFTR Cl− and HCO3 − Conductances. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2007. [DOI: 10.1007/0-387-23250-8_1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
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Abstract
Lessons from the sweat gland on cystic fibrosis (CF) began long before modern medicine became a science. In European folklore, the curse that “a child that taste salty when kissed will soon die” (Alonso y de los Ruyzes de Fonteca J. Diez Previlegios para Mugeres Prenadas. Henares, Spain, 1606) has been taken by many as a direct reference to cystic fibrosis [Busch R. Acta Univ Carol Med (Praha) 36: 13–15, 1990]. The high salt concentration in sweat from patients with CF is now accepted as almost pathognomonic with this fatal genetic disease, but the earliest descriptions of cystic fibrosis as a disease entity did not mention sweat or sweat glands (Andersen DH. Am J Dis Child 56: 344–399, 1938; Andersen DH, Hodges RG. Am J Dis Child 72: 62–80, 1946). Nonetheless, defective sweating soon became an inseparable, and major, component of the constellation of symptoms that diagnose “cystic fibrosis” (Davis PB. Am J Respir Crit Care Med 173: 475–482, 2006). The sweat gland has played a foremost role in diagnosing, defining pathophysiology, debunking misconceptions, and increasing our understanding of the effects of the disease on organs, tissues, cells, and molecules. The sweat gland has taught us much.
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Affiliation(s)
- Paul M Quinton
- Biomedical Sciences, University of California, Riverside, USA.
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Reddy MM, Wang XF, Gottschalk M, Jones K, Quinton PM. Normal CFTR Activity and Reversed Skin Potentials in Pseudohypoaldosteronism. J Membr Biol 2005; 203:151-9. [PMID: 15986094 DOI: 10.1007/s00232-005-0740-0] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/26/2004] [Accepted: 02/08/2005] [Indexed: 10/25/2022]
Abstract
Cystic fibrosis (CF) transmembrane conductance regulator (CFTR) Cl(-) channel function is required for activating amiloride-sensitive epithelial Na(+) channels (ENaC) in salt-absorbing human sweat duct. It is unclear whether ENaC channel function is also required for CFTR activation. The dysfunctional ENaC mutations in type-1 pseudohypoaldosteronism (PHA-1) provided a good opportunity to study this phenomenon of ion channel interaction between CFTR and ENaC. The PHA-1 ducts completely lacked spontaneous ENaC conductance (gENaC). In contrast, the normal ducts showed large spontaneous gENaC (46 +/- 10 ms, mean +/- SE: ). After permeabilization of the basolateral membrane with alpha-toxin, cAMP + ATP activation of CFTR Cl(-) conductance (gCFTR) or alkalinization of cytosolic pH (6.8 to 8.5) stimulated gENaC of normal but not PHA-1 ducts. In contrast, both spontaneous gCFTR in intact ducts and (cAMP + ATP)-activated gCFTR of permeabilized ducts appeared to be similar in normal and PHA-1 subjects. Lack of gENaC completely blocked salt absorption and caused dramatic reversal of skin potentials associated with pilocarpine-induced sweat secretion from significantly negative in normal subjects (-13 +/- 7.0 mV) to significantly positive (+22 +/- 11.0 mV) in PHA-1 patients. We conclude that virtual lack of ENaC in PHA-1 ducts had little effect on CFTR activity and that the positive skin potentials could potentially serve as a diagnostic tool to identify type-1 pseudohypoaldosteronism.
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Affiliation(s)
- M M Reddy
- Department of Pediatrics, UCSD School of Medicine, University of California, San Diego, La-Jolla, 92093-0831, USA. mmr@ ucsd.edu
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Wang XF, Reddy MM, Quinton PM. Effects of a new cystic fibrosis transmembrane conductance regulator inhibitor on Cl- conductance in human sweat ducts. Exp Physiol 2004; 89:417-25. [PMID: 15131065 DOI: 10.1113/expphysiol.2003.027003] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Effective and specific inhibition of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel in epithelia has long been needed to better understand the role of anion movements in fluid and electrolyte transport. Until now, available inhibitors have required high concentrations, usually in the millimolar or high micromolar range, to effect even an incomplete block of channel conductance. These inhibitors, including 5-nitro-2(3-phenylpropyl-amino)benzoate (NPPB), bumetamide, glibenclamide and DIDS, are also relatively non-specific. Recently a new anion channel inhibitor, a thiazolidinone derivative, termed CFTRInh-172 has been synthesized and introduced with apparently improved inhibitory properties as shown by effects on anion conductance expressed in cell lines and on secretion in vivo. Here, we assay the effect of this inhibitor on a purely salt absorbing native epithelial tissue, the freshly isolated microperfused human sweat duct, known for its inherently high expression of CFTR. We found that the inhibitor at a maximum dose limited by its aqueous solubility of 5 microm partially blocked CFTR when applied to either surface of the membrane; however, it may be somewhat more effective from the cytosolic side (approximately 70% inhibition). It may also partially inhibit Na+ conductance. The inhibition was relatively slow, with a half time for maximum effect of about 3 min, and showed very slow reversibility. Results also suggest that CFTR Cl- conductance (GCl) was blocked in both apical and basal membranes. The inhibitor appears to exert some effect on Na+ transport as well.
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Affiliation(s)
- X F Wang
- Department of Pediatrics, UCSD, 9500 Gilman Drive, La Jolla, CA 92093-0831, USA.
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Kunzelmann K, Mall M. Electrolyte transport in the mammalian colon: mechanisms and implications for disease. Physiol Rev 2002; 82:245-89. [PMID: 11773614 DOI: 10.1152/physrev.00026.2001] [Citation(s) in RCA: 449] [Impact Index Per Article: 20.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022] Open
Abstract
The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.
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Affiliation(s)
- Karl Kunzelmann
- Department of Physiology and Pharmacology, University of Queensland, St. Lucia, Queensland, Brisbane, Australia.
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Boucherot A, Schreiber R, Kunzelmann K. Role of CFTR's PDZ1-binding domain, NBF1 and Cl(-) conductance in inhibition of epithelial Na(+) channels in Xenopus oocytes. BIOCHIMICA ET BIOPHYSICA ACTA 2001; 1515:64-71. [PMID: 11597353 DOI: 10.1016/s0005-2736(01)00396-0] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
The cystic fibrosis transmembrane conductance regulator (CFTR) inhibits epithelial Na(+) channels (ENaC). Evidence has accumulated that both Cl(-) transport through CFTR Cl(-) channels and the first nucleotide binding domain (NBF1) of CFTR are crucial for inhibition of ENaC. A PDZ binding domain (PDZ-BD) at the C-terminal end links CFTR to scaffolding and cytoskeletal proteins, which have been suggested to play an important role in activation of CFTR and eventually inhibition of ENaC. We eliminated the PDZ-BD of CFTR and coexpressed Na(+)/H(+)-exchange regulator factors together with CFTR and ENaC. The results do not support a role of PDZ-BD in inhibition of ENaC by CFTR. However, inhibition of ENaC was closely linked to Cl(-) currents generated by CFTR and was observed in the presence of Cl(-), I(-) or Br(-) but not gluconate. Therefore, functional NBF1 and Cl(-) transport are required for inhibition of ENaC in Xenopus oocytes, while the PDZ-BD is not essential.
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Affiliation(s)
- A Boucherot
- School of Biomedical Sciences, Department of Physiology and Pharmacology, University of Queensland, St. Lucia, 4072, Brisbane, Qld, Australia
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Reddy MM, Quinton PM. cAMP-independent phosphorylation activation of CFTR by G proteins in native human sweat duct. Am J Physiol Cell Physiol 2001; 280:C604-13. [PMID: 11171580 DOI: 10.1152/ajpcell.2001.280.3.c604] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
It is generally believed that cAMP-dependent phosphorylation is the principle mechanism for activating cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channels. However, we showed that activating G proteins in the sweat duct stimulated CFTR Cl(-) conductance (G(Cl)) in the presence of ATP alone without cAMP. The objective of this study was to test whether the G protein stimulation of CFTR G(Cl) is independent of protein kinase A. We activated G proteins and monitored CFTR G(Cl) in basolaterally permeabilized sweat duct. Activating G proteins with guanosine 5'-O-(3-thiotriphosphate) (10-100 microM) stimulated CFTR G(Cl) in the presence of 5 mM ATP alone without cAMP. G protein activation of CFTR G(Cl) required Mg(2+) and ATP hydrolysis (5'-adenylylimidodiphosphate could not substitute for ATP). G protein activation of CFTR G(Cl) was 1) sensitive to inhibition by the kinase inhibitor staurosporine (1 microM), indicating that the activation process requires phosphorylation; 2) insensitive to the adenylate cyclase (AC) inhibitors 2',5'-dideoxyadenosine (1 mM) and SQ-22536 (100 microM); and 3) independent of Ca(2+), suggesting that Ca(2+)-dependent protein kinase C and Ca(2+)/calmodulin-dependent kinase(s) are not involved in the activation process. Activating AC with 10(-6) M forskolin plus 10(-6) M IBMX (in the presence of 5 mM ATP) did not activate CFTR, indicating that cAMP cannot accumulate sufficiently to activate CFTR in permeabilized cells. We concluded that heterotrimeric G proteins activate CFTR G(Cl) endogenously via a cAMP-independent pathway in this native absorptive epithelium.
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Affiliation(s)
- M M Reddy
- Department of Pediatrics, School of Medicine, University of California, San Diego, La Jolla, California 92093-0831, USA
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Kunzelmann K. The cystic fibrosis transmembrane conductance regulator and its function in epithelial transport. Rev Physiol Biochem Pharmacol 1999; 137:1-70. [PMID: 10207304 DOI: 10.1007/3-540-65362-7_4] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
CF is a well characterized disease affecting a variety of epithelial tissues. Impaired function of the cAMP activated CFTR Cl- channel appears to be the basic defect detectable in epithelial and non-epithelial cells derived from CF patients. Apart from cAMP-dependent Cl- channels also Ca2+ and volume activated Cl- currents may be changed in the presence of CFTR mutations. This is supported by recent additional findings showing that different intracellular messengers converge on the CFTR Cl- channel. Analysis of the ion transport in CF airways and intestinal epithelium identified additional defects in Na+ transport. It became clear recently that mutations of CFTR may also affect the activity of other membrane conductances including epithelial Na+ channels, KvLQT-1 K+ channels and aquaporins (Fig. 7). Several additional, initially unexpected effects of CFTR on cellular functions, such as exocytosis, mucin secretion and regulation of the intracellular pH were reported during the past. Taken together, these results clearly indicate that CFTR not only acts as a cAMP regulated Cl- channel, but may fulfill several other cellular functions, particularly by regulating other membrane conductances. Failure in CFTR dependent regulation of these membrane conductances is likely to contribute to the defects observed in CF. Currently, no general concept is available that can explain how CFTR controls this variety of cellular functions. Further studies will have to verify whether direct protein interaction, specific effects on membrane turnover, changes of the intracellular ion concentration or additional proteins are involved in these regulatory loops. At the end of this review one cannot share the provocative and reassuring title "CFTR!" of a review written a few years ago [114]. Today one might rather finish with the statement "CFTR?".
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Affiliation(s)
- K Kunzelmann
- Physiologisches Institut, Albert-Ludwigs-Universität Freiburg, Germany
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Reddy MM, Quinton PM. Bumetanide blocks CFTR GCl in the native sweat duct. THE AMERICAN JOURNAL OF PHYSIOLOGY 1999; 276:C231-7. [PMID: 9886939 DOI: 10.1152/ajpcell.1999.276.1.c231] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Bumetanide is well known for its ability to inhibit the nonconductive Na+-K+-2Cl- cotransporter. We were surprised in preliminary studies to find that bumetanide in the contraluminal bath also inhibited NaCl absorption in the human sweat duct, which is apparently poor in cotransporter activity. Inhibition was accompanied by a marked decrease in the transepithelial electrical conductance. Because the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is richly expressed in the sweat duct, we asked whether bumetanide acts by blocking this anion channel. We found that bumetanide 1) significantly increased whole cell input impedance, 2) hyperpolarized transepithelial and basolateral membrane potentials, 3) depolarized apical membrane potential, 4) increased the ratio of apical-to-basolateral membrane resistance, and 5) decreased transepithelial Cl- conductance (GCl). These results indicate that bumetanide inhibits CFTR GCl in both cell membranes of this epithelium. We excluded bumetanide interference with the protein kinase A phosphorylation activation process by "irreversibly" phosphorylating CFTR [by using adenosine 5'-O-(3-thiotriphosphate) in the presence of a phosphatase inhibition cocktail] before bumetanide application. We then activated CFTR GCl by adding 5 mM ATP. Bumetanide in the cytoplasmic bath (10(-3) M) inhibited approximately 71% of this ATP-activated CFTR GCl, indicating possible direct inhibition of CFTR GCl. We conclude that bumetanide inhibits CFTR GCl in apical and basolateral membranes independent of phosphorylation. The results also suggest that >10(-5) M bumetanide cannot be used to specifically block the Na+-K+-2Cl- cotransporter.
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Affiliation(s)
- M M Reddy
- Department of Pediatrics, University of California, San Diego, Medical Center, San Diego, California 92103-0831, USA
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Abstract
Intracellular CFTR: Localization and Function. Physiol. Rev. 79, Suppl.: S175-S191, 1999. - There is considerable evidence that CFTR can function as a chloride-selective anion channel. Moreover, this function has been localized to the apical membrane of chloride secretory epithelial cells. However, because cystic fibrosis transmembrane conductance regulator (CFTR) is an integral membrane protein, it will also be present, to some degree, in a variety of other membrane compartments (including endoplasmic reticulum, Golgi stacks, endosomes, and lysosomes). An incomplete understanding of the molecular mechanisms by which alterations in an apical membrane chloride conductance could give rise to the various clinical manifestations of cystic fibrosis has prompted the suggestion that CFTR may also play a role in the normal function of certain intracellular compartments. A variety of intracellular functions have been attributed to CFTR, including regulation of membrane vesicle trafficking and fusion, acidification of organelles, and transport of small anions. This paper aims to review the evidence for localization of CFTR in intracellular organelles and the potential physiological consequences of that localization.
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Affiliation(s)
- N A Bradbury
- Department of Cell Biology and Physiology, University of Pittsburgh School of Medicine, Pennsylvania, USA
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Reddy MM, Kopito RR, Quinton PM. Cytosolic pH regulates GCl through control of phosphorylation states of CFTR. THE AMERICAN JOURNAL OF PHYSIOLOGY 1998; 275:C1040-7. [PMID: 9755057 DOI: 10.1152/ajpcell.1998.275.4.c1040] [Citation(s) in RCA: 149] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Our objective in this study was to determine the effect of changes in luminal and cytoplasmic pH on cystic fibrosis transmembrane regulator (CFTR) Cl- conductance (GCl). We monitored CFTR GCl in the apical membranes of sweat ducts as reflected by Cl- diffusion potentials (VCl) and transepithelial conductance (GCl). We found that luminal pH (5.0-8.5) had little effect on the cAMP/ATP-activated CFTR GCl, showing that CFTR GCl is maintained over a broad range of extracellular pH in which it functions physiologically. However, we found that phosphorylation activation of CFTR GCl is sensitive to intracellular pH. That is, in the presence of cAMP and ATP [adenosine 5'-O-(3-thiotriphosphate)], CFTR could be phosphorylated at physiological pH (6.8) but not at low pH (approximately 5.5). On the other hand, basic pH prevented endogenous phosphatase(s) from dephosphorylating CFTR. After phosphorylation of CFTR with cAMP and ATP, CFTR GCl is normally deactivated within 1 min after cAMP is removed, even in the presence of 5 mM ATP. This deactivation was due to an increase in endogenous phosphatase activity relative to kinase activity, since it was reversed by the reapplication of ATP and cAMP. However, increasing cytoplasmic pH significantly delayed the deactivation of CFTR GCl in a dose-dependent manner, indicating inhibition of dephosphorylation. We conclude that CFTR GCl may be regulated via shifts in cytoplasmic pH that mediate reciprocal control of endogenous kinase and phosphatase activities. Luminal pH probably has little direct effect on these mechanisms. This regulation of CFTR may be important in shifting electrolyte transport in the duct from conductive to nonconductive modes.
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Affiliation(s)
- M M Reddy
- Department of Pediatrics, School of Medicine, University of California, San Diego, La Jolla CA 92093-0831, USA
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26
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Bell CL, Quinton PM. An immortal cell line to study the role of endogenous CFTR in electrolyte absorption. In Vitro Cell Dev Biol Anim 1995; 31:30-6. [PMID: 7535636 DOI: 10.1007/bf02631335] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
The intact human reabsorptive sweat duct (RD) has been a reliable model for investigations of the functional role of "endogenous" CFTR (cystic fibrosis transmembrane conductance regulator) in normal and abnormal electrolyte absorptive function. But to overcome the limitations imposed by the use of fresh, intact tissue, we transformed cultured RD cells using the chimeric virus Ad5/SV40 1613 ori-. The resultant cell line, RD2(NL), has remained differentiated forming a polarized epithelium that expressed two fundamental components of absorption, a cAMP activated Cl- conductance (GCl) and an amiloride-sensitive Na+ conductance (GNa). In the unstimulated state, there was a low level of transport activity; however, addition of forskolin (10(-5) M) significantly increased the Cl- diffusion potential (Vt) generated by a luminally directed Cl- gradient from -15.3 +/- 0.7 mV to -23.9 +/- 1.1 mV, n = 39; and decreased the transepithelial resistance (Rt) from 814.8 +/- 56.3 omega.cm2 to 750.5 +/- 47.5 omega.cm2, n = 39, (n = number of cultures). cAMP activation, anion selectivity (Cl- > I- > gluconate), and a dependence upon metabolic energy (metabolic poisoning inhibited GCl), all indicate that the GCl expressed in RD2(NL) is in fact CFTR-GCl. The presence of an apical amiloride-sensitive GNa was shown by the amiloride (10(-5) M) inhibition of GNa as indicated by a reduction of Vt and equivalent short circuit current by 78.0 +/- 3.1% and 77.9 +/- 2.6%, respectively, and an increase in Rt by 7.2 +/- 0.8%, n = 36. In conclusion, the RD2(NL) cell line presents the first model system in which CFTR-GCl is expressed in a purely absorptive tissue.(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- C L Bell
- Division of Biomedical Sciences, University of California, Riverside 92521, USA
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Abstract
Even though the same Cl channel (CFTR) is common to certain fluid transport functions that are oppositely directed, i.e., secretion and absorption, only fluid secretion has clearly been shown to be acutely regulated. It is now clear that fluid secretion activated by beta-adrenergic stimulation is controlled by cAMP-mediated opening and closing of CFTR-Cl channels. Since the conductance of the human sweat duct is almost wholly due to CFTR-Cl conductance (CFTR-GCl), we sought to determine whether salt absorption via CFTR-Cl channels could also be subject to acute regulation in this purely absorptive epithelium. After alpha-toxin permeabilization, we found that addition of cAMP resulted in a large increase in Cl diffusion potentials across the apical membrane and a more than twofold increase in the average membrane conductance. Since the cAMP effects were dependent on Cl alone, not on Na, and since apical Cl conductance appears to be almost exclusively comprised of CFTR-GCl, we surmise that this form of electrolyte absorption like secretion is also subject to acute control through CFTR-GCl. Acute regulation of absorption involves both activation by phosphorylation (PKA) and inactivation by dephosphorylation (unknown endogenous phosphatase) of CFTR. Phosphorylation of CFTR was shown by the facts that CFTR-GCl could be activated by cAMP and inhibited by the kinase antagonist staurosporine, or by removal of either substrate ATP or Mg2+ cofactor. Inactivation of CFTR-GCl by endogenous phosphatase(s) was indicated by a spontaneous but reversible loss of CFTR-GCl upon removal of cAMP. Such loss of CFTR-GCl activity could be prevented either by application of phosphatase inhibitors or by using phosphatase-resistant ATP-gamma-S as substrate to phosphorylate CFTR. We surmise that absorptive function is subject to rapid regulation which can be switched "on" and "off" acutely by a control system that is common to both absorptive and secretory processes and that this control is crucial to switching between conductive and nonconductive transport mechanisms during salt absorption.
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Affiliation(s)
- M M Reddy
- Division of Biomedical Sciences, University of California, Riverside 92521-0121
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