Altered fluid transport across airway epithelium in cystic fibrosis

Differential effects of cyclic and constant stress on ATP release and mucociliary transport by human airway epithelia

In the lungs, the first line of defence against bacterial infection is the thin layer of airway surface liquid (ASL) lining the airway surface. The superficial airway epithelium exhibits complex regulatory pathways that blend ion transport to adjust ASL volume to maintain proper mucociliary clearance (MCC). We hypothesized that stresses generated by airflow and transmural pressures during breathing govern ASL volume by regulating the rate of epithelial ATP release. Luminal ATP, via interactions with apical membrane P2-purinoceptors, regulates the balance of active ion secretion versus absorption to maintain ASL volume at optimal levels for MCC. In this study we tested the hypothesis that cyclic compressive stress (CCS), mimicking normal tidal breathing, regulates ASL volume in airway epithelia. Polarized tracheobronchial epithelial cultures from normal and cystic fibrosis (CF) subjects responded to a range of CCS by increasing the rate of ATP release. In normal airway epithelia, the CCS-induced increase in ASL ATP concentration was sufficient to induce purinoceptor-mediated increases in ASL height and MCC, via inhibition of epithelial Na(+)-channel-mediated Na(+) absorption and stimulation of Cl(-) secretion through CFTR and the Ca(2+)-activated chloride channels. In contrast, static, non-oscillatory stress did not stimulate ATP release, ion transport or MCC, emphasizing the importance of rhythmic mechanical stress for airway defence. In CF airway cultures, which exhibit basal ASL depletion, CCS was partially effective, producing less ASL volume secretion than in normal cultures, but a level sufficient to restore MCC. The present data suggest that CCS may (1) regulate ASL volume in the normal lung and (2) improve clearance in the lungs of CF patients, potentially explaining the beneficial role of exercise in lung defence.

Differentiated structure and function of primary cultures of monkey oviductal epithelium

We have established well-differentiated, polarized cultures of monkey oviductal epithelium. Oviductal epithelial cells were isolated by protease digestion and plated on collagen-coated, porous cell culture inserts. About 5 d after plating, cells developed detectable transepithelial electrical resistance of up to 2000 Omega.cm(2) (an index of tight junction formation) and transepithelial voltages of up to 20 mV (an index of vectorial transepithelial ion transport). Measurements of short-circuit current in Ussing chambers indicated that active secretion of Cl was the major transepithelial active ion transport process, and that this was stimulated by elevation of either cAMP or Ca(i). Furthermore, estimates of the volume of mucosal liquid were consistent with Cl secretion mediating fluid secretion. Various microscopical methods showed that the cultures were densely ciliated and contained mature secretory cells. Transport across the oviductal epithelium determines the composition of the oviductal fluid, and the study of the relevant transport processes will be greatly enhanced by well-differentiated cultures of oviductal epithelium of the kind established here.

Confluent monolayers of cultured human fetal retinal pigment epithelium exhibit morphology and physiology of native tissue

PURPOSE

Provide a reproducible method for culturing confluent monolayers of hfRPE cells that exhibit morphology, physiology, polarity, and protein expression patterns similar to native tissue.

METHODS

Human fetal eyes were dissected on arrival, and RPE cell sheets were mechanically separated from the choroid and cultured in a specifically designed medium comprised entirely of commercially available components. Physiology experiments were performed with previously described techniques. Standard techniques were used for immunohistochemistry, electron microscopy, and cytokine measurement by ELISA.

RESULTS

Confluent monolayers of RPE cell cultures exhibited epithelial morphology and heavy pigmentation, and electron microscopy showed extensive apical membrane microvilli. The junctional complexes were identified with immunofluorescence labeling of various tight junction proteins. The mean transepithelial potential (TEP) was 2.6 +/- 0.8 mV, apical positive, and the mean transepithelial resistance (R(T)) was 501 +/- 138 Omega . cm(2) (mean +/- SD; n = 35). Addition of 100 microM adenosine triphosphate (ATP) to the apical bath increased net fluid absorption from 13.6 +/- 2.6 to 18.8 +/- 4.6 microL . cm(-2) per hour (mean +/- SD; n = 4). In other experiments, VEGF was mainly secreted into the basal bath (n = 10), whereas PEDF was mainly secreted into the apical bath (n = 10).

CONCLUSIONS

A new cell culture procedure has been developed that produces confluent primary hfRPE cultures with morphological and physiological characteristics of the native tissue. Epithelial polarity and function of these easily reproducible primary cultures closely resemble previously studied native human fetal and bovine RPE-choroid explants.

Airway surface liquid volume regulates ENaC by altering the serine protease-protease inhibitor balance: a mechanism for sodium hyperabsorption in cystic fibrosis

Efficient clearance of mucus and inhaled pathogens from the lung is dependent on an optimal airway surface liquid (ASL) volume, which is maintained by the regulated transport of sodium and chloride across the airway epithelium. Accumulating evidence suggests that impaired mucus clearance in cystic fibrosis (CF) airways is a result of ASL depletion caused by excessive Na(+) absorption through the epithelial sodium channel (ENaC). However, the cellular mechanisms that result in increased ENaC activity in CF airways are not completely understood. Recently, proteases were shown to modulate the activity of ENaC, but the relevance of this mechanism to the physiologic regulation of ASL volume is unknown. Using primary human airway epithelial cells, we demonstrate that: (i) protease inhibitors are present in the ASL and prevent the activation of near-silent ENaC, (ii) when the ASL volume is increased, endogenous protease inhibitors become diluted, allowing for proteolytic activation of near-silent channels, and (iii) in CF, the normally present near-silent pool of ENaC is constitutively active and the alpha subunit undergoes increased proteolytic processing. These findings indicate that the ASL volume modulates the activity of ENaC by modification of the serine protease-protease inhibitor balance and that alterations in this balance contribute to excessive Na(+) absorption in cystic fibrosis.

Prenatal estrogen and progesterone deprivation impairs alveolar formation and fluid clearance in newborn piglets

Exposure to high levels of estradiol (E2) and progesterone (P) derived from the fetoplacentomaternal unit during the last trimester of pregnancy may play a crucial role in prenatal lung development and immediate postnatal alveolar fluid clearance (AFC). To measure prenatal alveolar formation and postnatal amiloride-sensitive AFC after pharmacological deprivation of E2 and P in utero, fetuses from five sows received an intramuscular depot injection of the E2 receptor blocker ICI 182.780 (ICI) and the P receptor blocker RTI 3021-022 (RTI) and fetuses of five other sows received a placebo injection (control group) during a laparotomy at 90 d of gestation (term gestation, 115 d). Piglets were delivered by cesarean section on d 114 of gestation. Of 95 live-born piglets, 35 were mechanically ventilated. The airways of the right lower lobe were isolated by a balloon catheter wedged in the bronchus and 5% albumin in 0.9% NaCl with or without 1 mmol/L amiloride was instilled. Amiloride-sensitive AFC was calculated from the protein concentration changes in fluid recovered after 120 min as the percentage of absorbed fluid. Lungs were removed under standardized conditions to perform alveolar counts. Prenatal treatment with ICI and RTI resulted in a significantly lower amiloride-sensitive AFC (median, 31%; min-max, -4-58) than placebo (74%, 18-231). Median alveolar counts per visual field were significantly lower in piglets that were exposed to ICI and RTI (38, 21-78) compared with placebo (56, 32-113). We conclude that prenatal E2 and P deprivation significantly impaired alveolar formation and amiloride-sensitive AFC.

Expression of CLC-K chloride channels in the rat cochlea

Current models of the lateral K+ recycling pathway in the mammalian cochlea include two multicellular transport networks separated from one another by three interstitial gaps. The first gap is between outer hair cells and Deiters cells, the second is between outer sulcus cells and type II spiral ligament fibrocytes and the third is between intermediate and marginal cells in the stria vascularis. K+ taken up by cells bordering these interstitial spaces is accompanied by Cl-. Maintaining appropriate electrolyte balance and membrane potentials in these cells requires a mechanism for exit of the resorbed Cl-. One possible candidate for regulating this Cl- efflux is ClC-K, a chloride channel previously thought to be kidney specific. Here, we demonstrate the expression of both known isoforms of ClC-K in the organ of Corti, spiral ligament and stria vascularis of the rat cochlea by immunohistochemical, Western blot and RT-PCR analysis. These results indicate a role for ClC-K in mediating Cl- recycling in the cochlea. The widespread expression of both ClC-K isoforms in the cochlea may help to explain the symptoms of Bartter's syndrome Type III, a mutation in the hClC-Kb gene (human homologue of ClC-K2), which results in renal salt wasting without deafness. These data support the hypothesis that both isoforms of ClC-K are co-expressed in some cell membranes and account for the preservation of hearing in the presence of a mutation in only one channel isoform.

Expansion of cultures of human tracheal epithelium with maintenance of differentiated structure and function

We have developed a technique for expanding primary cultures of human tracheal epithelium while minimizing loss of differentiated structure and function. Cells were seeded at 2 x 10(4) cells/cm2 into T75 flasks and trypsinized when approximately 80% confluent. The dispersed cells were then passaged at the same plating density into further T75 flasks or seeded at 5 x 10(5) cells/cm2 on porous-bottomed inserts and maintained with an air-interface. Differentiation of cells on inserts was assessed from transepithelial electrical resistance (an index of tight junction formation), short-circuit current (an index of transepithelial salt transport), cell numbers, total cell protein, and histology. Unpassaged cells (P0) and cells passaged once (P1) took about a week to become 80% confluent on T75 flasks, with 10-fold and 5-fold increases in cell numbers, respectively. Confluence was achieved in approximately 3 days following plating to inserts. Functionally and structurally, P1 and P2 cells (cells passaged twice) were little different from P0 cells. Thus, within a little over 2 weeks, the numbers of confluent cell sheets can be increased 50-fold with minimal change in function. However, there was a marked decline in differentiation by cells passaged three times (P3), and not all cell preparations could be taken to P4 (cells passaged four times).

Cystic fibrosis and airway submucosal glands

The chronic pulmonary infections and inflammation associated with cystic fibrosis (CF) are responsible for almost all the morbidity and mortality of this disease. Our understanding of the mechanisms that underlie the very early stages of CF lung disease, that result directly from mutations in the CF gene, is relatively poor. However, the demonstration that the predominant sites of expression of the CF gene in normal lungs are the submucosal glands, together with the histological observations showing that hyperplasia of these glands and mucin occlusion of the gland ducts are the earliest signs of disease in the CF lung, suggest that malfunction of the submucosal glands may be an important factor contributing to the early pathophysiology of CF lung disease. This review describes the function of submucosal glands in normal lungs, and the way in which their function may be disrupted in CF and may thus contribute to the early stages of CF lung disease.

Innate host defense of the lung: effects of lung-lining fluid pH

Lung-lining fluid (LLF) is a primary constituent of the pulmonary host defense system. It is distributed continuously throughout the respiratory tract but is heterogeneous regarding its chemistry and physiology between the conducting airways and alveoli. The conducting airways are lined with airway surface liquid (ASL), a mucus gel-aqueous sol complex that interacts functionally with epithelial cilia as the mucociliary escalator. The alveoli are lined with alveolar subphase fluid (AVSF) and pulmonary surfactant. AVSF sterility is maintained in part by the phagocytic activity of resident alveolar macrophages. Normal ASL and AVSF are both more acidic than blood plasma. However, the details of acid-base regulation differ between the two media. Appreciable transepithelial acid-base flux is possible across the airway epithelium, whereas the alveolar epithelium is relatively impermeable to transepithelial acid-base flux. Moreover, one must consider the influence of resident macrophages on AVSF pH. Resident macrophages occupy a sizable fraction of AVSF by volume and are a substantial source of metabolic H+. The buffering capacities of ASL and AVSF probably are largely due to secreted peptides (e.g., ASL mucins and AVSF surfactant proteins). Acid-base exchange between the extracellular hydrophase and intracellular buffering systems of resident macrophages represents an additional buffer pool for AVSF. The pH of ASL and AVSF can be depressed by disease or inflammation. Low pH is predicted to suppress microbe clearance from the airways and alveoli, increase pathogen survival in both regions, and alter mediator release by resident macrophages and recruited leukocytes thereby increasing the propensity for bystander cell injury. Overall, ASL/AVSF pH is expected to be a major determinant of lung host defense responses.

Effect of static growth and different levels of environmental oxygen on toxA and ptxR expression in the Pseudomonas aeruginosa strain PAO1

Within certain infection sites, such as the lung of cystic fibrosis patients, Pseudomonas aeruginosa grows statically under either decreased oxygen tension or anaerobic conditions, a situation that is likely to influence the production of virulence factors. The goal of this study was to determine the effect of static growth under microaerobic (decreased oxygen) and anaerobic conditions on the expression of the P. aeruginosa exotoxin A (ETA) gene toxA and its positive regulator ptxR. Using toxA-lacZ and ptxR-lacZ fusion plasmids, the level of toxA and ptxR expression was measured throughout the growth cycle of strain PAO1, which was grown in either iron-deficient or iron-sufficient medium under four different conditions: 20%-SH (aerobic, shaking), 20%-ST (aerobic, static), 10%-ST (microaerobic, static) and 0%-ST (anaerobic, static). In iron-deficient medium, toxA expression was higher under 20%-ST and 10%-ST than under 20%-SH. However, the highest level of toxA expression occurred under 0%-ST. Analysis of ETA protein using sandwich ELISA revealed that at time points between 8 and 24 h of the growth curve, PAO1 produced higher levels of ETA under 0%-ST than under 20%-SH. In iron-sufficient medium, toxA expression was significantly repressed under all conditions. Additional analyses using PAO1 strains that carry lacZ fusions with the toxA regulatory genes regA and pvdS revealed that the expression of regA and pvdS is reduced rather than increased at 0%-ST. ptxR expression under different conditions paralleled that of toxA expression, except that it was repressed by iron under 20 %-SH only. Between 6 and 24 h of growth, and under all conditions, the level of dissolved oxygen (DO) within the PAO1 cultures was sharply reduced. These results suggest that (1) the combined effect of static growth and anaerobic conditions produce a significant increase in toxA and ptxR expression in PAO1; (2) this effect appears to be unique to toxA and ptxR, since the level of regA and pvdS expression was reduced under the same conditions; (3) neither static growth nor anaerobic conditions interfere with the repression of toxA expression by iron, although static growth deregulates ptxR expression with respect to iron; and (4) the enhanced expression of toxA and ptxR is not related to the reduced levels of DO in PAO1 cultures.

Inositol polyphosphate derivative inhibits Na+ transport and improves fluid dynamics in cystic fibrosis airway epithelia

Amiloride-sensitive, epithelial Na(+) channel (ENaC)-mediated, active absorption of Na(+) is elevated in the airway epithelium of cystic fibrosis (CF) patients, resulting in excess fluid removal from the airway lumen. This excess fluid/volume absorption corresponds to CF transmembrane regulator-linked defects in ENaC regulation, resulting in the reduced mucociliary clearance found in CF airways. Herein we show that INO-4995, a synthetic analog of the intracellular signaling molecule, D-myo-inositol 3,4,5,6-tetrakisphosphate, inhibits Na(+) and fluid absorption across CF airway epithelia, thus alleviating this critical pathology. This conclusion was based on electrophysiological studies, fluid absorption, and (22)Na(+) flux measurements in CF airway epithelia, contrasted with normal epithelia, and on electrophysiological studies in Madin-Darby canine kidney cells and 3T3 cells overexpressing ENaC. The effects of INO-4995 were long-lasting, dose-dependent, and more pronounced in epithelia from CF patients vs. controls. These findings support preclinical development of INO-4995 for CF treatment and demonstrate for the first time the therapeutic potential of inositol polyphosphate derivatives.

Adult stem cells from bone marrow stroma differentiate into airway epithelial cells: potential therapy for cystic fibrosis

Cystic fibrosis (CF), the most prevalent, fatal genetic disorder in the Caucasian population, is caused by mutations of CF transmembrane conductance regulator (CFTR). The mutations of this chloride channel alter the transport of chloride and associated liquid and thereby impair lung defenses. Patients typically succumb to chronic bacterial infections and respiratory failure. Restoration of the abnormal CFTR function to CF airway epithelium is considered the most direct way to treat the disease. In this report, we explore the potential of adult stem cells from bone marrow, referred to as mesenchymal or marrow stromal stem cells (MSCs), to provide a therapy for CF. We found that MSCs possess the capacity of differentiating into airway epithelia. MSCs from CF patients are amenable to CFTR gene correction, and expression of CFTR does not influence the pluripotency of MSCs. Moreover, the CFTR-corrected MSCs from CF patients are able to contribute to apical Cl(-) secretion in response to cAMP agonist stimulation, suggesting the possibility of developing cell-based therapy for CF. The ex vivo coculture system established in this report offers an invaluable approach for selection of stem-cell populations that may have greater potency in lung differentiation.

Purinergic signaling underlies CFTR control of human airway epithelial cell volume

BACKGROUND

Loss of cystic fibrosis transmembrane conductance regulator (CFTR) function in cystic fibrosis (CF) causes dysregulation of multiple ion channels, water channels, and acid-base transporters in epithelia. As such, we hypothesized that dysregulation of many critical ion channels and transporters may cause defects in human airway epithelial cell volume regulation.

METHODS

Cell volume, regulatory volume decrease, and its regulation was assessed in real-time via Coulter Counter Multisizer III-driven electronic cell sizing in non-CF, CF, and CFTR-complemented CF human airway epithelial cells. SPQ halide fluorescence assay of hypotonicity-induced chloride efflux provided indirect validation of the cell volume assays.

RESULTS

CFTR, via autocrine ATP signaling, governs human airway epithelial cell volume regulation. Non-CF cells and wild-type (WT)-CFTR-transfected CF cells had normal regulatory volume decrease (RVD) responses that were attenuated by blockade of autocrine and paracrine purinergic signaling. In contrast, parental IB3-1 CF cells or IB3-1 cells expressing CFTR mutants (DeltaF508, G551D, and S1455X) failed to RVD. CF cell RVD was rescued by agonists to P2Y G protein-coupled receptors and, more robustly, by agonists to P2X purinergic receptor channels.

CONCLUSIONS

Loss of CFTR and CFTR-driven autocrine ATP signaling may underlie defective cell volume regulation and dysregulated ion, water, and acid-base transport in CF airway epithelia.

Thioredoxin liquefies and decreases the viscoelasticity of cystic fibrosis sputum

The persistent and viscous nature of airway secretions in cystic fibrosis (CF) disease leads to airway obstruction, opportunistic infection, and deterioration of lung function. Thioredoxin (Trx) is a protein disulfide reductase that catalyzes numerous thiol-dependent cellular reductive processes. To determine whether Trx can alter the rheological properties of mucus, sputum obtained from CF patients was treated with TRX and its reducing system (0.1 μM thioredoxin reductase + 2 mM NADPH), and liquid phase-gel phase ratio (percent liquid phase) was assessed by compaction assay. Exposure to low Trx concentrations (1 μM) caused significant increases in the percentage of liquid phase of sputum. Maximal increases in percent liquid phase occurred with 30 μM Trx. Additional measurements revealed that sputum liquefaction by the Trx reducing system is dependent on NADPH concentration. The relative potency of the Trx reducing system also was compared with other disulfide-reducing agents. In contrast with Trx, glutathione and N-acetylcysteine were ineffective in liquefying sputum when used at concentrations <1 mM. Sputum viscoelasticity, measured by magnetic microrheometry, also was diminished significantly following 20-min treatment with 3, 10, or 30 μM Trx. Similarly, this reduction in viscoelasticty also was dependent on NADPH concentration. Further investigation has indicated that Trx treatment increases the solubility of high-molecular-weight glycoproteins and causes redistribution of extracellular DNA into the liquid phase of sputum. Recognizing that mucins are the major gel-forming glycoproteins in mucus, we suggest that Trx alters sputum rheology by enzymatic reduction of glycoprotein polymers present in sputum.

Secretion of lactoferrin and lysozyme by cultures of human airway epithelium

Lactoferrin and lysozyme are important antimicrobial compounds of airway surface liquid, derived predominantly from serous cells of submucosal glands but also from surface epithelium. Here we compared release of these compounds from the following human cell cultures: primary cultures of tracheal epithelium (HTE), Calu-3 cells (a lung adenocarcinoma cell line frequently used as a model of serous gland cells), 16HBE14o- cells (an SV40 transformed line from airway surface epithelium), T84 cells (a colon carcinoma cell line), and human foreskin fibroblasts (HFF). For lysozyme, baseline secretory rates were in the order Calu-3 > 16HBE14o- > HTE ≈ T84 > HFF = 0; for lactoferrin, the only cell type showing measurable release was HTE; for mucus, HTE > Calu-3 > 16HBE14o- ≈ T84 > HFF = 0. A wide variety of neurohumoral agents and inflammatory stimuli was without effect on lactoferrin and lysozyme release from HTE or Calu-3 cells, although forskolin did stimulate secretion of water and lysozyme from Calu-3 cells. However, the concentration of lysozyme in the forskolin-induced secretions was much less than in airway gland secretions. Thus our data cast doubt on the utility of Calu-3 cells as a model of airway serous gland cells but do suggest that HTE could prove highly suitable for studies of mucin synthesis and release.

Fluid transport across cultured rat alveolar epithelial cells: a novel in vitro system

Previous studies have used fluid-instilled lungs to measure net alveolar fluid transport in intact animal and human lungs. However, intact lung studies have two limitations: the contribution of different distal lung epithelial cells cannot be studied separately, and the surface area for fluid absorption can only be approximated. Therefore, we developed a method to measure net vectorial fluid transport in cultured rat alveolar type II cells using an air-liquid interface. The cells were seeded on 0.4-microm microporous inserts in a Transwell system. At 96 h, the transmembrane electrical resistance reached a peak level (1,530 +/- 115 Omega.cm(2)) with morphological evidence of tight junctions. We measured net fluid transport by placing 150 microl of culture medium containing 0.5 microCi of (131)I-albumin on the apical side of the polarized cells. Protein permeability across the cell monolayer, as measured by labeled albumin, was 1.17 +/- 0.34% over 24 h. The change in concentration of (131)I-albumin in the apical fluid was used to determine the net fluid transported across the monolayer over 12 and 24 h. The net basal fluid transport was 0.84 microl.cm(-2).h(-1). cAMP stimulation with forskolin and IBMX increased fluid transport by 96%. Amiloride inhibited both the basal and stimulated fluid transport. Ouabain inhibited basal fluid transport by 93%. The cultured cells retained alveolar type II-like features based on morphologic studies, including ultrastructural imaging. In conclusion, this novel in vitro system can be used to measure net vectorial fluid transport across cultured, polarized alveolar epithelial cells.

Cystic fibrosis airway epithelial cell polarity and bacterial flagellin determine host response toPseudomonas aeruginosa

The role of epithelial polarity and bacterial factors in the control of the innate immune response of airway epithelial cells to Pseudomonas aeruginosa PAK was investigated using a human, nasal cystic fibrosis (DeltaF508/DeltaF508) epithelial cell line CF15 grown as confluent layers on permeable supports. Addition of PAK to the basal surface of CF15 layers caused significant expression changes in 1525 different genes (out of 12 625 examined), including the cytokines IL-6, IL-8, IL-1beta and TNF-alpha, as well as genes associated with leucocyte adhesion, antibacterial factors, and NF-kappaB signalling. Confocal microscopy showed that nuclear migration of NF-kappaB in all CF15 cells was preceded by PAK binding to the basal and lateral surfaces of some cells. Addition of PAK to the apical surface of CF15 monolayers elicited changes in expression of only 602 genes, including 256 not affected during basolateral PAK exposure. Over time, cytokine expression during apical PAK was similar to that exhibited by basal PAK, but the magnitudes during apical treatment were much smaller with little/no nuclear migration of NF-kappaB in CF15 cells. Furthermore, these responses depended on the presence of flagellin, but not pili on the bacteria. Thus, P. aeruginosa triggered a strong innate immune response that depended on the apical versus basolateral polarity of CF15 cells and the presence of flagellin on the bacteria.

Extracellular ATP as a signaling molecule for epithelial cells

The charge of this invited review is to present a convincing case for the fact that cells release their ATP for physiological reasons. Many of our "purinergic" colleagues as well as ourselves have experienced resistance to this concept, because it is teleologically counter-intuitive. This review serves to integrate the three main tenets of extracellular ATP signaling: ATP release from cells, ATP receptors on cells, and ATP receptor-driven signaling within cells to affect cell or tissue physiology. First principles will be discussed in the Introduction concerning extracellular ATP signaling. All possible cellular mechanisms of ATP release will then be presented. Use of nucleotide and nucleoside scavengers as well as broad-specificity purinergic receptor antagonists will be presented as a method of detecting endogenous ATP release affecting a biological endpoint. Innovative methods of detecting released ATP by adapting luciferase detection reagents or by using "biosensors" will be presented. Because our laboratory has been primarily interested in epithelial cell physiology and pathophysiology for several years, the role of extracellular ATP in regulation of epithelial cell function will be the focus of this review. For ATP release to be physiologically relevant, receptors for ATP are required at the cell surface. The families of P2Y G protein-coupled receptors and ATP-gated P2X receptor channels will be introduced. Particular attention will be paid to P2X receptor channels that mediate the fast actions of extracellular ATP signaling, much like neurotransmitter-gated channels versus metabotropic heptahelical neurotransmitter receptors that couple to G proteins. Finally, fascinating biological paradigms in which extracellular ATP signaling has been implicated will be highlighted. It is the goal of this review to convert and attract new scientists into the exploding field of extracellular nucleotide signaling and to convince the reader that extracellular ATP is indeed a signaling molecule.

Regulation of mucin secretion from human bronchial epithelial cells grown in murine hosted xenografts

Studies of regulated mucin secretion from goblet cells in primary cultures of human bronchial epithelial (HBE) cells have suffered, generally, from poor signal-to-noise ratios, with reported secretory responses of <100% (less than onefold) relative to baseline. Using, instead, HBE cells grown as xenografts in the backs of nude mice, we found that UTP (100 micro M) stimulated strong mucin secretory responses from isolated, luminally perfused preparations. The peak response (10 min) for 11 control experiments (37 xenografts) was 3.3 +/- 0.05-fold relative to baseline, and the time-integrated response (60 min) was 23.4 +/- 0.5-fold. Because responses to ATP and UTP were approximately equal, an apical membrane P2Y(2)-receptor (R) is suggested. Additionally, ADP activated mucin release from HBE xenografts, whereas UDP and 2-methlythio-ADP did not, a pattern of response inconsistent with known purinoceptors. Hence, either a novel receptor to ADP is suggested or there is significant conversion of ADP to ATP by ecto-adenylate kinase activity. Adenosine and a nitric oxide donor were without effect. Consistent with P2Y(2)-R coupling to phospholipase C, HBE xenografts responded to ionomycin and PMA; however, they were recalcitrant to forskolin and chlorophenylthio-cAMP, and to 8-bromo-cGMP. Hence, human airway goblet cells, like those of other species, appear to be regulated primarily via phospholipase C pathways, activated particularly by apical membrane P2Y(2)-R agonists.

P2 purinoceptors regulate calcium-activated chloride and fluid transport in 31EG4 mammary epithelia

It has been reported that secretory mammary epithelial cells (MEC) release ATP, UTP, and UDP upon mechanical stimulation. Here we examined the physiological changes caused by ATP/UTP in nontransformed, clonal mouse mammary epithelia (31EG4 cells). In control conditions, transepithelial potential (apical side negative) and resistance were -4.4 +/- 1.3 mV (mean +/- SD, n = 12) and 517.7 +/- 39.4 Omega. cm(2), respectively. The apical membrane potential was -43.9 +/- 1.7 mV, and the ratio of apical to basolateral membrane resistance (R(A)/R(B)) was 3.5 +/- 0.2. Addition of ATP or UTP to the apical or basolateral membranes caused large voltage and resistance changes with an EC(50) of approximately 24 microM (apical) and approximately 30 microM (basal). Apical ATP/UTP (100 microM) depolarized apical membrane potential by 17.6 +/- 0.8 mV (n = 7) and decreased R(A)/R(B) by a factor of approximately 3. The addition of adenosine to either side (100 microM) had no effect on any of these parameters. The ATP/UTP responses were partially inhibited by DIDS and suramin and mediated by a transient increase in free intracellular Ca(2+) concentration (427 +/- 206 nM; 15-25 microM ATP, apical; n = 6). This Ca(2+) increase was blocked by cyclopiazonic acid, by BAPTA, or by xestospongin C. 31EG4 MEC monolayers also secreted or absorbed fluid in the resting state, and ATP or UTP increased fluid secretion by 5.6 +/- 3 microl x cm(-2) x h(-1) (n = 10). Pharmacology experiments indicate that 31EG4 epithelia contain P2Y(2) purinoceptors on the apical and basolateral membranes, which upon activation stimulate apical Ca(2+)-dependent Cl channels and cause fluid secretion across the monolayer. This suggests that extracellular nucleotides could play a fundamental role in mammary gland paracrine signaling and the regulation of milk composition in vivo.

Retinal pigment epithelial function: a role for CFTR?

In the vertebrate eye, the photoreceptor outer segments and the apical membrane of the retinal pigment epithelium (RPE) are separated by a small extracellular (subretinal) space whose volume and chemical composition varies in the light and dark. Light onset triggers relatively fast (ms) retinal responses and much slower voltage and resistance changes (s to min) at the apical and basolateral membranes of the RPE. Two of these slow RPE responses, the fast oscillation (FO) and the light peak, are measured clinically as part of the electrooculogram (EOG). Both EOG responses are mediated in part by apical and basolateral membranes proteins that form a pathway for the movement of salt and osmotically obliged fluid across the RPE, from retina to choroid. This transport pathway serves to control the volume and chemical composition of the subretinal and choroidal extracellular spaces. In human fetal RPE, we have identified one of these proteins, the cystic fibrosis transmembrane conductance regulator (CFTR) by RT-PCR, immunolocalization, and electrophysiological techniques. Evidence is presented to suggest that the FO component of the EOG is mediated directly or indirectly by CFTR.

Current status of gene therapy for inherited lung diseases

Gene therapy as a treatment modality for pulmonary disorders has attracted significant interest over the past decade. Since the initiation of the first clinical trials for cystic fibrosis lung disease using recombinant adenovirus in the early 1990s, the field has encountered numerous obstacles including vector inflammation, inefficient delivery, and vector production. Despite these obstacles, enthusiasm for lung gene therapy remains high. In part, this enthusiasm is fueled through the diligence of numerous researchers whose studies continue to reveal great potential of new gene transfer vectors that demonstrate increased tropism for airway epithelia. Several newly identified serotypes of adeno-associated virus have demonstrated substantial promise in animal models and will likely surface soon in clinical trials. Furthermore, an increased understanding of vector biology has also led to the development of new technologies to enhance the efficiency and selectivity of gene delivery to the lung. Although the promise of gene therapy to the lung has yet to be realized, the recent concentrated efforts in the field that focus on the basic virology of vector development will undoubtedly reap great rewards over the next decade in treating lung diseases.

Effects of media on differentiation of cultured human tracheal epithelium

The purpose of the this study was to find media that supported high levels of differentiation in primary cultures of human tracheal epithelium. We tested six previously described, partially defined media and three nondefined media. Cells were grown with an air interface on porous-bottomed inserts, and differentiation was assessed from electrophysiological properties, levels of total protein and deoxyribonucleic acid, and histology. In all media, cells polarized and developed tight junctions, as assessed from transepithelial electrical resistance and were better differentiated at 14 d after plating than at 7 d. The partially defined media described previously by Gray et al. (Am. J. Respir. Cell. Mol. Biol. 14:104-112; 1996) and Matsui et al. (J. Clin. Invest. 102:1125-1131; 1998) and an undefined medium containing Ultroser G serum substitute produced the most highly differentiated epithelial cells, as revealed by a high short-circuit current (I(sc)) and a ciliated, pseudostratified appearance. In other media, cells tended to be either squamous or stratified squamous, with I(sc) levels <25% of those obtained with the three optimal media. Though no key factor in the composition of the partially defined media could be identified, two of the four media with high concentrations of retinoic acid produced good differentiation. In contrast, the two media with the lowest [Ca] (0.11 mM) produced poorly differentiated cells, as did the two partially defined media with low or no retinoic acid concentration.

Absent secretion to vasoactive intestinal peptide in cystic fibrosis airway glands

We are testing the hypothesis that the malfunctioning of airway gland serous cells is a component of cystic fibrosis (CF) airway disease. CF is caused by mutations that disrupt CF transmembrane conductance regulator, an anion channel essential for proper fluid secretion in some epithelia. Submucosal glands supply most of the mucus in upper airways, and gland serous cells are the primary site of CF transmembrane conductance regulator expression in airways. We have discovered a major defect in CF glands by in situ optical monitoring of secretions from single human airway glands. CF glands did not secrete to agents that elevated [cAMP](i) (0 responses/450 glands, 8 subjects), whereas glands were responsive in all donor tracheas (605/827 glands, 15 subjects) and in bronchi from subjects who were transplanted because of other lung diseases (148/166 glands, n = 10). CF glands secreted to cholinergic stimulation, and serous cells were abundant in glands from all CF subjects. The complete absence of secretion to agents that elevate [cAMP](i) suggests that altered secretion of gland mucus could contribute to CF lung disease.

An overview of the pathogenesis of cystic fibrosis lung disease

The pathogenesis of cystic fibrosis (CF) lung disease is reviewed, focusing on an overview of the physiologic mechanisms that regulate mucus transport. A major emphasis is placed on the active transport systems that regulate the airway surface liquid (ASL) volume and, particularly, regulate the volume of the periciliary liquid (PCL) layer. A sequence is developed for CF whereby there is a depletion of the PCL that reflects the combined dysfunctions of accelerated Na(+)-dependent volume absorption and failure to secrete Cl(-). Both dysfunctions are a direct consequence of missing cystic fibrosis transmembrane conductance regulator (CFTR) at the apical membrane of airway epithelial cells. PCL depletion leads to failure of mucus transport, which is associated with persistent mucin secretion and formation of adherent mucus plaques and plugs. These plugs become the nidus for persistent bacterial airway infections that ultimately lead to a markedly anaerobic luminal environment.

Pulmonary oedema fluid induces non-alpha-ENaC-dependent Na(+) transport and fluid absorption in the distal lung

To determine if pulmonary oedema fluid (EF) alters ion and fluid transport of distal lung epithelium (DLE), EF was collected from rats in acute heart failure. EF, but not plasma, increased amiloride-insensitive short circuit current (I(sc)) and Na(+)-K(+) ATPase protein content and pump activity of DLE grown in primary culture. Inhibitors of Cl(-) transport or cGMP-gated cation channels had a significant (P < 0.05), but limited ability to block the increased I(sc). EF increased amiloride-insensitive, but not amiloride-sensitive, DLE apical membrane Na(+) conductance. The level of mRNA encoding epithelial sodium channel (ENaC) subunits was unchanged (alpha, beta), or decreased (gamma, P < 0.05) in EF-exposed DLE. EF also induced an amiloride-insensitive increase in the potential difference across murine tracheal cysts. Distal lung explants from late gestation wild-type and alpha-ENaC-deficient fetal mice, which normally expand due to liquid secretion, decreased in size due to liquid absorption when exposed to EF. Trypsin digestion or heat treatment of EF abrogated the ability of EF to increase amiloride-insensitive I(sc) in DLE and liquid absorption by distal lung explants. Thus proteins or protein-dependent factors within cardiogenic EF induce an alpha-ENaC-independent and amiloride-insensitive apical membrane Na(+) conductance and liquid absorption in the distal lung.

Characterization of ion and fluid transport in human bronchioles

The regulation of the volume and composition of airway surface liquid is achieved through epithelial ion transport processes. In humans, these processes have been characterized in proximal but not distal airways. Segments of human bronchioles were dissected from surgically removed lung pieces. The transmural potential difference of microperfused bronchioles was inhibited by luminal exposure to amiloride and increased when exposed to the Cl secretagogues forskolin and ATP in the presence of amiloride. Human bronchiolar epithelial cells were cultured on permeable supports and studied in Ussing chambers. They generated a short circuit current (Isc) that decreased in response to amiloride and increased in response to forskolin and to ATP in the presence of amiloride. In low-Cl Kreb's Ringer bicarbonate, the baseline Isc and amiloride-induced decrease in Isc were not different, whereas the forskolin- and ATP-induced increases in Isc were smaller. Fluid transport measurement in excised bronchioles revealed a basal absorptive flow that was reduced by amiloride, whereas forskolin and ATP combined induced a secretory flow in the presence of amiloride. We conclude that human bronchioles actively absorb Na and fluid in unstimulated conditions and are capable of active Cl and fluid secretion when exposed to forskolin and to ATP.

Regulation of the depth and composition of airway surface liquid

The airways are lined with a film of liquid about 10 microm deep that is in two layers. Around the cilia is the watery periciliary sol. Over this is a mucous blanket that traps inhaled particles. The low viscosity of the periciliary sol allows the cilia to beat and propel the mucous blanket to the mouth. In large airways, mucus comes predominantly from the mucous glands but also from goblet cells in the surface epithelium. Water is added to the airway surface by gland secretion that is driven by active Cl secretion by serous cells. During inflammation elevation of the subepithelial hydrostatic pressure may also add significant volumes of water to the airway lumen. Water is removed by active Na transport across the surface epithelium. In airway diseases, the balance is shifted from water secretion to mucus secretion. In bronchitis and asthma this is due mainly to conversion of gland serous to mucous cells. In cystic fibrosis, gland serous cells cannot secrete water because they lack functioning CFTR in their apical membranes (CFTR is the cystic fibrosis transmembrane conductance regulator, a Cl channel that is abundant in serous cells). In all three diseases, the result is secretion of excessively concentrated gland secretions that are poorly moved by the cilia and accumulate. Altered salt and water transport by the surface epithelium may also contribute to the pathology of cystic fibrosis.

INS365 suppresses loss of corneal epithelial integrity by secretion of mucin-like glycoprotein in a rabbit short-term dry eye model

P2Y2 receptor agonists, like UTP and ATP, stimulate mucin secretion from goblet cells in vitro. Therefore, mucin stimulants could be good candidates for the treatment of dry eye syndrome because mucin increases the tear film stability and protects against desiccation of ocular surface. INS365 is a more stable P2Y2 receptor agonist than UTP. In the present study, we evaluated, in normal rabbit eyes, its effectiveness to release mucin from goblet cells and to protect the corneal damage induced by desiccation. For mucin secretion, impression cytology was performed following the instillation of INS365 solution or saline into the conjunctival sac. The specimens were stained with periodic acid and Schiff (PAS) reagent, and then the staining area was calculated using computer software. INS365 dose-dependently decreased the PAS staining area of conjunctival goblet cells from 2 to 15 min post-application. Furthermore, we utilized the rabbit short-term dry eye model to evaluate if INS365 eyedrops could protect against any of the damage produced by blockage of blinking with ocular speculum. INS365 significantly suppressed corneal damage at concentrations of more than 0.1% w/v. These results suggest that this P2Y2 agonist is a good candidate for the treatment of dry eye disease.

Regulation of the epithelial sodium channel by serine proteases in human airways

The epithelial sodium channel (ENaC) constitutes the rate-limiting step for sodium absorption across airway epithelia, which in turn regulates airway surface liquid (ASL) volume and the efficiency of mucociliary clearance. This role in ASL volume regulation suggests that ENaC activity is influenced by local factors rather than systemic signals indicative of total body volume homeostasis. Based on reports that ENaC may be regulated by extracellular serine protease activity in Xenopus and mouse renal epithelia, we sought to identify proteases that serve similar functions in human airway epithelia. Homology screening of a human airway epithelial cDNA library identified two trypsin-like serine proteases (prostasin and TMPRSS2) that, as revealed by in situ hybridization, are expressed in airway epithelia. Functional studies in the Xenopus oocyte expression system demonstrated that prostasin increased ENaC currents 60--80%, whereas TMPRSS2 markedly decreased ENaC currents and protein levels. Studies of primary nasal epithelial cultures in Ussing chambers revealed that inhibition of endogenous serine protease activity with aprotinin markedly decreased ENaC-mediated currents and sensitized the epithelia to subsequent channel activation by exogenous trypsin. These data, therefore, suggest that protease-mediated regulation of sodium absorption is a function of human airway epithelia, and prostasin is a likely candidate for this activity.

Effects of reduced mucus oxygen concentration in airway Pseudomonas infections of cystic fibrosis patients

Current theories of CF pathogenesis predict different predisposing "local environmental" conditions and sites of bacterial infection within CF airways. Here we show that, in CF patients with established lung disease, Pseudomonas aeruginosa was located within hypoxic mucopurulent masses in airway lumens. In vitro studies revealed that CF-specific increases in epithelial O(2) consumption, linked to increased airway surface liquid (ASL) volume absorption and mucus stasis, generated steep hypoxic gradients within thickened mucus on CF epithelial surfaces prior to infection. Motile P. aeruginosa deposited on CF airway surfaces penetrated into hypoxic mucus zones and responded to this environment with increased alginate production. With P. aeruginosa growth in oxygen restricted environments, local hypoxia was exacerbated and frank anaerobiosis, as detected in vivo, resulted. These studies indicate that novel therapies for CF include removal of hypoxic mucus plaques and antibiotics effective against P. aeruginosa adapted to anaerobic environments.

Perspectives on gene therapy for cystic fibrosis airway disease

Since the discovery of the cystic fibrosis transmembrane conductance regulator (CFTR) gene nearly 12 years ago, cystic fibrosis (CF) has become one of the most intensively investigated monogenetic disorders considered approachable by gene therapy. This has resulted in over 20 clinical trials currently under way, concluded or awaiting approval. Despite the initial promise of gene therapy for CF, and the demonstration of successful gene transfer to the nose and airways of individuals, it has not so far been as effective as initially projected. Here we discuss the rationale behind CF gene therapy and dissect the vast array of literature representing the work that ultimately brought about the current phase I/II clinical trials. In the context of human trials, we review the limitations of current vector systems for CF gene therapy. We come to the conclusion that at present none of the application methods and vector systems are able to achieve the level and persistence of CFTR gene expression in the affected epithelia of CF patients that is required for therapeutic success. We also outline the challenges that must be overcome and describe some of the novel approaches to be taken in order to attain the curative therapy that was originally envisaged for this disease.

Partial correction of defective Cl(-) secretion in cystic fibrosis epithelial cells by an analog of squalamine

Defective cystic fibrosis (CF) transmembrane conductance regulator (CFTR)-mediated Cl(-) transport across the apical membrane of airway epithelial cells is implicated in the pathophysiology of CF lungs. A strategy to compensate for this loss is to augment Cl(-) transport through alternative pathways. We report here that partial correction of this defect could be attained through the incorporation of artificial anion channels into the CF cells. Introduction of GL-172, a synthetic analog of squalamine, into CFT1 cells increased cell membrane halide permeability. Furthermore, when a Cl(-) gradient was generated across polarized monolayers of primary human airway or Fischer rat thyroid cells in an Ussing chamber, addition of GL-172 caused an increase in the equivalent short-circuit current. The magnitude of this change in short-circuit current was ~30% of that attained when CFTR was maximally stimulated with cAMP agonists. Patch-clamp studies showed that addition of GL-172 to CFT1 cells also increased whole cell Cl(-) currents. These currents displayed a linear current-voltage relationship and no time dependence. Additionally, administration of GL-172 to the nasal epithelium of transgenic CF mice induced a hyperpolarization response to perfusion with a low-Cl(-) solution, indicating restoration of Cl(-) secretion. Together, these results demonstrate that in CF airway epithelial cells, administration of GL-172 is capable of partially correcting the defective Cl(-) secretion.

Liquid transport properties of porcine tracheal epithelium

Because of its possible importance to the etiology of cystic fibrosis lung disease, the ion and water transport properties of tracheal epithelium were studied. Net liquid flux (J(V)) across porcine tracheal epithelium was measured in vitro using blue dextran as a volume probe. Luminal instillation of isosmotic sucrose solution (280 mM) induced a small net secretion of liquid (7.0 +/- 1.7 nl x cm(-2) x s(-1)), whereas luminal hyposmotic sucrose solutions (220 or 100 mM) induced substantial and significant (P < 0.05) liquid absorption (34.5 +/- 12 and 38.1 +/- 7.3 nl x cm(-2) x s(-1), respectively). When the luminal solution was normal (isosmotic) Krebs buffer, liquid was absorbed at 10.2 +/- 1.1 nl x cm(-2) x s(-1). Absorptive J(V) was abolished by 100 microM amiloride in the luminal solution and significantly reduced when the luminal solution was Na(+)-free Krebs solution. Absorptive J(V) was not significantly affected by 300 microM 5-nitro-2-(3-phenylpropylamino)benzoate or 100 microM diphenylamine-2-carboxylic acid, both cystic fibrosis transmembrane conductance regulator protein (CFTR) inhibitors, in the instillate but was significantly reduced by 60% when the luminal solution was Cl(-)-free Krebs solution. We conclude that water freely permeates porcine tracheal epithelium and that absorption of liquid is normally driven by active transcellular Na(+) transport and does not require the CFTR.

ENaC- and CFTR-dependent ion and fluid transport in mammary epithelia

Mammary epithelial 31EG4 cells (MEC) were grown as monolayers on filters to analyze the apical membrane mechanisms that help mediate ion and fluid transport across the epithelium. RT-PCR showed the presence of cystic fibrosis transmembrane conductance regulator (CFTR) and epithelial Na(+) channel (ENaC) message, and immunomicroscopy showed apical membrane staining for both proteins. CFTR was also localized to the apical membrane of native human mammary duct epithelium. In control conditions, mean values of transepithelial potential (apical-side negative) and resistance (R(T)) are -5.9 mV and 829 Omega x cm(2), respectively. The apical membrane potential (V(A)) is -40.7 mV, and the mean ratio of apical to basolateral membrane resistance (R(A)/R(B)) is 2.8. Apical amiloride hyperpolarized V(A) by 19.7 mV and tripled R(A)/R(B). A cAMP-elevating cocktail depolarized V(A) by 17.6 mV, decreased R(A)/R(B) by 60%, increased short-circuit current by 6 microA/cm(2), decreased R(T) by 155 Omega x cm(2), and largely eliminated responses to amiloride. Whole cell patch-clamp measurements demonstrated amiloride-inhibited Na(+) currents [linear current-voltage (I-V) relation] and forskolin-stimulated Cl(-) currents (linear I-V relation). A capacitance probe method showed that in the control state, MEC monolayers either absorbed or secreted fluid (2--4 microl x cm(-2) x h(-1)). Fluid secretion was stimulated either by activating CFTR (cAMP) or blocking ENaC (amiloride). These data plus equivalent circuit analysis showed that 1) fluid absorption across MEC is mediated by Na(+) transport via apical membrane ENaC, and fluid secretion is mediated, in part, by Cl(-) transport via apical CFTR; 2) in both cases, appropriate counterions move through tight junctions to maintain electroneutrality; and 3) interactions among CFTR, ENaC, and tight junctions allow MEC to either absorb or secrete fluid and, in situ, may help control luminal [Na(+)] and [Cl(-)].

Rabbit conjunctival epithelium transports fluid, and P2Y2(2) receptor agonists stimulate Cl(-) and fluid secretion

Rabbit conjunctival epithelium exhibits UTP-dependent Cl(-) secretion into the tears. We investigated whether fluid secretion also takes place. Short-circuit current (I(sc)) was 14.9 +/- 1.4 microA/cm(2) (n = 16). Four P2Y(2) purinergic receptor agonists [UTP and the novel compounds INS365, INS306, and INS440 (Inspire Pharmaceuticals)] added apically (10 microM) resulted in temporary (approximately 30 min) I(sc) increases (88%, 66%, 57%, and 28%, respectively; n = 4 each). Importantly, the conjunctiva transported fluid from serosa to mucosa at a rate of 6.5 +/- 0.7 microl x h(-1) x cm(-2) (range 2.1--15.3, n = 20). Fluid transport was stimulated by mucosal additions of 10 microM: 1) UTP, from 7.4 +/- 2.3 to 10.7 +/- 3.3 microl x h(-1) x cm(-2), n = 5; and 2) INS365, from 6.3 +/- 1.0 to 9.8 +/- 2.5 microl. h(-1) x cm(-2), n = 5. Fluid transport was abolished by 1 mM ouabain (n = 5) and was drastically inhibited by 300 microM quinidine (from 6.4 +/- 1.2 to 3.6 +/- 1.0 microl x h(-1) x cm(-2), n = 4). We conclude that this epithelium secretes fluid actively and that P2Y(2) agonists stimulate both Cl(-) and fluid secretions.

Ocular safety of INS365 ophthalmic solution: a P2Y(2) agonist in healthy subjects

The purpose of this study was to evaluate the ocular safety and tolerability of the P2Y(2) receptor agonist, INS365, when applied as eye drops in normal human subjects. This study was a double-masked, placebo-controlled, randomized, within subject paired-comparison, dose-escalation study in five cohorts of ten healthy subjects. The concentrations of INS365 ophthalmic solution were 0.5, 1.0, 2.0, and 5.0% given three times over six hours. Safety was assessed by general and ophthalmic examination and symptomatology. Unanesthetized Schirmer tests were performed in the last cohort of 10 subjects to evaluate the acute effects of INS365 on tear secretion. There were no significant differences in the number of subjects with ocular events reported in placebo-treated eyes compared to INS365-treated eyes. Two adverse events were possibly related to INS365: painless blepharospasm and an increase in lacrimation after 5.0% INS365 instillation. Unanesthetized Schirmer testing showed no acute effects of INS365 on tear secretion, compared to its vehicle, in healthy subjects, in which reflex tearing often produced maximal Schirmer values. INS365 ophthalmic solution was well-tolerated when administered by ocular instillation. Stimulation of ocular surface P2Y(2) receptors was not associated with ocular tolerability issues in healthy subjects.

Physiological evidence for a P2Y receptor responsive to diadenosine polyphosphates in human lung via Ca(2+) release studies in bronchial epithelial cells

P2Y(2) receptors that are activated by the extracellular nucleotides ATP or UTP mediate Cl(-) secretion via an increase in [Ca(2+)](i) (intracellular calcium concentration). Therefore, in the lung of patients suffering from cystic fibrosis, inhalation of aerosolized UTP offers a way to circumvent the defect in Cl(-) secretion by the cystic fibrosis transmembrane conductance regulator. A possible alternative for the relatively unstable UTP in inhalation therapy is the more resistant diadenosine tetraphosphate (Ap(4)A). In human and rat lung membranes, Ap(4)A binds to P2 receptor sites coupled to G proteins. Here, we showed that Ap(4)A caused an increase in [Ca(2+)](i) with an EC(50) of 17 microM in human bronchial epithelial cells (HBE1). The [Ca(2+)](i) rise evoked by ATP and UTP was completely, but that induced by Ap(4)A only partially, caused by release of Ca(2+) from internal stores. Moreover, the potency of Ap(4)A to mobilize Ca(2+) was lower than that of ATP and UTP (EC(50) 1.5 and 1.8 microM, respectively), and the maximal increase in [Ca(2+)](i) was considerably smaller than that after ATP or UTP. In accordance with our previous results providing evidence for a common binding site for various diadenosine polyphosphates in lung membranes, all Ap(n)A analogues tested (n = 3 to 6) caused a comparable [Ca(2+)](i) increase. Homologous or heterologous prestimulation largely diminished the increase in [Ca(2+)](i) found after a second pulse of either UTP or Ap(4)A. Although specific binding characteristics and functional responses of Ap(4)A on lung cells are in favor of a distinct receptor for Ap(4)A, the cross-talk between UTP and Ap(4)A in HBE1 cells and the only slight differences in Ca(2+) mobilization by ATP or UTP and Ap(4)A render it impossible at this point to state unequivocally whether there exists a distinct P2Y receptor specific for diadenosine polyphosphates in lung epithelia or whether Ap(4)A activates one of the nucleotide receptors already described.

Staphylococcus aureus RN6390 replicates and induces apoptosis in a pulmonary epithelial cell line

Staphylococcus aureus frequently colonizes the airways of patients with compromised airway defenses (e.g., cystic fibrosis [CF] patients) for extended periods. Persistent and relapsing infections may be related to live S. aureus bacteria actively residing inside epithelial cells. In this study, we infected a respiratory epithelial cell line, which was derived from a CF patient, with S. aureus RN6390. Internalization of S. aureus was found to be time and dose dependent and could be blocked by cytochalasin D. Transmission electron microscopy revealed that internalized bacteria resided within endocytic vacuoles without any evidence of lysosomal fusion in a 24-h period. The results of internalization experiments and time-lapse fluorescence microscopy of epithelial cells infected with green fluorescent S. aureus indicate that, after an initial lag period of 7 to 9 h, intracellular bacteria began to replicate, with three to five divisions in a 24-h period, leading to apoptosis of infected cells. Induction of apoptosis required bacterial internalization and is associated with intracellular replication. The slow and gradual replication of S. aureus inside epithelial cells hints at the role of host factors or signals in bacterial growth and further suggests possible cross talk between host cells and S. aureus.

Distribution of ion transport mRNAs throughout murine nose and lung

Evidence of absorptive or secretory ion transport in different respiratory regions of the mouse was sought by assessing the regional distribution of alpha-, beta-, and gamma-epithelial sodium channel (ENaC; Na(+) absorptive), cystic fibrosis transmembrane conductor regulator (CFTR), and Na(+)-K(+)-2Cl(-) cotransporter mRNAs. High levels of ENaC subunit expression were found in nasal surface epithelium and gland ducts. CFTR was expressed in both superficial nasal respiratory epithelium and glands. These results are consistent with basal amiloride-sensitive Na(+) absorption and cAMP-dependent Cl(-) secretion in murine nasal epithelia. Expression of all three ENaC subunits increased progressively from trachea to terminal bronchioles. Intermediate levels of CFTR and cotransporter expression in bronchial epithelium diminished in bronchioles. The low abundance of CFTR mRNA throughout murine pulmonary epithelium is consistent with functional data that attributes Cl(-) secretion predominantly to an alternative Cl(-) channel. alpha-ENaC as the only mRNA found in all regions of airway epithelia is consistent with the alpha-subunit as requisite for Na(+) absorption, and the increased expression of alpha-, beta-, and gamma-ENaC in distal airways suggests a greater absorptive capability in this region.

Extracellular UTP stimulates electrogenic bicarbonate secretion across CFTR knockout gallbladder epithelium

The loss of cystic fibrosis transmembrane conductance regulator (CFTR)-mediated transepithelial HCO(3)(-) secretion contributes to the pathogenesis of pancreatic and biliary disease in cystic fibrosis (CF) patients. Recent studies have investigated P2Y(2) nucleotide receptor agonists, e.g., UTP, as a means to bypass the CFTR defect by stimulating Ca(2+)-activated Cl(-) secretion. However, the value of this treatment in facilitating transepithelial HCO(3)(-) secretion is unknown. Gallbladder mucosae from CFTR knockout mice were used to isolate the Ca(2+)-dependent anion conductance during activation of luminal P2Y(2) receptors. In Ussing chamber studies, UTP stimulated a transient peak in short-circuit current (I(sc)) that declined to a stable plateau phase lasting 30-60 min. The plateau I(sc) after UTP was Cl(-) independent, HCO(3)(-) dependent, insensitive to bumetanide, and blocked by luminal DIDS. In pH stat studies, luminal UTP increased both I(sc) and serosal-to-mucosal HCO(3)(-) flux (J(s-->m)) during a 30-min period. Substitution of Cl(-) with gluconate in the luminal bath to inhibit Cl(-)/HCO(3)(-) exchange did not prevent the increase in J(s-->m) and I(sc) during UTP. In contrast, luminal DIDS completely inhibited UTP-stimulated increases in J(s-->m) and I(sc). We conclude that P2Y(2) receptor activation results in a sustained (30-60 min) increase in electrogenic HCO(3)(-) secretion that is mediated via an intracellular Ca(2+)-dependent anion conductance in CF gallbladder.

Advances in signalling by extracellular nucleotides. the role and transduction mechanisms of P2Y receptors

Nucleotides are ubiquitous intercellular messengers whose actions are mediated by specific receptors. Since the first clonings in 1993, it is known that nucleotide receptors belong to two families: the ionotropic P2X receptors and the metabotropic P2Y receptors. Five human P2Y receptor subtypes have been cloned so far and a sixth one must still be isolated. In this review we will show that they differ by their preference for adenine versus uracil nucleotides and triphospho versus diphospho nucleotides, as well as by their transduction mechanisms and cell expression.

Effects of methacholine and uridine 5'-triphosphate on tracheal mucus rheology in mice

We compared the action of methacholine (MCh) and uridine 5'-triphosphate (UTP) with and without pretreatment with the chloride channel blocker 4,4'-diisothiocyano-2,2'-stilbenedisulfonate (DIDS) on the transepithelial potential difference (PD), the mucus collection rate (MCR), and tracheal mucus rheology using anesthetized C57BL/6 mice. The cystic fibrosis transmembrane conductance regulator (CFTR) blocker 5-nitro-2-(3-phenylpropylamino)benzoate (NPPB) was also used as a pretreatment for MCh. After collecting baseline mucus for 1.5 h, mucus secretion was stimulated by instilling 5 microl of 10(-2) M MCh or UTP around the upper trachea. There was a significant increase in PD after MCh or UTP stimulation (-21.3+/-2.0 mV MCh versus -14.1+/-1.6 mV control; -25.4+/-2.5 mV UTP versus -19.2+/-1.9 mV control). When UTP administration was preceded by DIDS, PD shifted from -15.2+/-2.9 to -12.0+/-2.2 mV. When MCh was preceded by DIDS or by NPPB, there was no change in PD. There was a significant decrease in mucus rigidity index, logG*, with MCh (2.54+/-0.09 versus 2.99+/-0.14 for control), similar to that previously reported in other species. With UTP, 14 of 16 mice responded in terms of PD becoming more negative, and of these, there was a significant difference in logG* after UTP administration (2.29 +/-0.10 versus 2.57+/-0.10 for control), whereas there was no change in logG* with DIDS administration before UTP. When DIDS administration preceded MCh, there was a diminished but still significant decrease in logG* from control, whereas there was no change in logG* when NPPB was preadministered. The control mucus collection rate was 0.19+/-0.09 mg/h, whereas after MCh stimulation, it increased to 2.83+/-0.78 mg/h. No significant difference was measured in the MCR after either UTP or DIDS+UTP stimulation. DIDS+MCh and NPPB+MCh both resulted in significant increases in MCR, but of a much smaller magnitude than that for MCh alone. We conclude that hypersecretion owing to UTP in C57BL/6 mice is less vigorous than with MCh, reflecting the limited population of Ca(2+)-dependent Cl(-) channels stimulated by UTP P(2) receptors. The action of MCh on tracheal mucus secretion in mice appears to involve both CFTR- and non-CFTR-dependent chloride channels.