Modelling dysregulated Na+ absorption in airway epithelial cells with mucosal nystatin treatment.

In cystic fibrosis (CF), the absence of functional CFTR leads to dysregulated Na(+) absorption across airway epithelia. We established an in vitro model of dysregulated Na(+) absorption by treating polarized normal human bronchial epithelial cells (HBEs) with nystatin (Nys), a polyene antibiotic that enables monovalent cations to permeate biological membranes. Acute mucosal Nys produced a rapid increase in short circuit current (I(sc)) that reflected increased transepithelial Na(+) absorption and required Na(+)/K(+)ATPase activity. The acute increase in I(sc) was associated with increased mucosal liquid absorption. Prolonged mucosal Nys treatment resulted in sustained Na(+) hyperabsorption, associated with increased mucosal liquid absorption in comparison with naïve (nontreated, kept under air-liquid interface conditions) or vehicle-treated cultures. Nys treatment was not toxic. Increased lactate accumulation in Nys-treated culture media suggested a higher metabolic rate associated with the higher energy demand for Na(+) transport. After chronic Nys treatment, the increased I(sc) was rapidly lost when the cultures were mounted in Ussing chambers, indicating that Nys could be rapidly removed from the apical membrane. Importantly, chronic Nys treatment promoted sustained mucosal liquid depletion and caused mucus dehydration, compaction, and adhesion to the apical surface of Nys-treated cultures. We conclude that mucosal Nys treatment of HBEs provides a simple in vitro model to recapitulate the Na(+) and volume hyperabsorptive features of CF airway epithelia.

The mitochondrial barriers segregate agonist-induced calcium-dependent functions in human airway epithelia.

In airway epithelia, purinergic receptor (P2Y2-R) stimulation of intracellular calcium (Ca2+i)-regulated ion transport is restricted to the membrane domain ipsilateral to receptor activation, implying compartmentalization of Ca2+i signaling. Because mitochondria can spatially restrict cellular Ca2+i signals, immunocytochemical, electron microscopic, and fluorescent studies of mitochondria localization were performed in human airway epithelia. Although concentrated at the apical domain, mitochondria were found distributed at both the apical and the basolateral poles and in close association with the endoplasmic reticulum. The role of mitochondria in locally restricting P2Y2-R-induced Ca2+i signals was investigated by measuring changes in mitochondrial Ca2+ (Ca2+m) in human airway epithelial monolayers. P2Y2-R activation induced Ca2+m accumulation in mitochondria confined to the domain ipsilateral to P2Y2-R stimulation, which was blocked by mitochondrial uncoupling with 1 microM CCCP and 2.5 microg/ml oligomycin. The role of mitochondria in restricting the cellular cross-talk between basolateral P2Y2-R-dependent Ca2+i mobilization and apical membrane Ca2+-activated Cl- secretion was investigated in studies simultaneously measuring Ca2+i and Cl- secretion in cystic fibrosis human airway epithelial monolayers. Activation of basolateral P2Y2-Rs produced similar increases in Ca2+i in monolayers without and with pretreatment with uncouplers, whereas Ca2+i-activated Cl- secretion was only efficiently triggered in mitochondria-uncoupled conditions. We conclude that (a) mitochondria function as a Ca2+i-buffering system in airway epithelia, compartmentalizing Ca2+i-dependent functions to the membrane ipsilateral to receptor stimulation; and (b) the mitochondria provide structural barriers that protect the airway epithelia against nonspecific activation of Ca2+i-modulated functions associated with Ca2+i signals emanating from the apical or the basolateral membrane domains.

Regulation of murine airway surface liquid volume by CFTR and Ca2+-activated Cl- conductances.

Two Cl(-) conductances have been described in the apical membrane of both human and murine proximal airway epithelia that are thought to play predominant roles in airway hydration: (1) CFTR, which is cAMP regulated and (2) the Ca(2+)-activated Cl(-) conductance (CaCC) whose molecular identity is uncertain. In addition to second messenger regulation, cross talk between these two channels may also exist and, whereas CFTR is absent or defective in cystic fibrosis (CF) airways, CaCC is preserved, and may even be up-regulated. Increased CaCC activity in CF airways is controversial. Hence, we have investigated the effects of CFTR on CaCC activity and have also assessed the relative contributions of these two conductances to airway surface liquid (ASL) height (volume) in murine tracheal epithelia. We find that CaCC is up-regulated in intact murine CF tracheal epithelia, which leads to an increase in UTP-mediated Cl(-)/volume secretion. This up-regulation is dependent on cell polarity and is lost in nonpolarized epithelia. We find no role for an increased electrical driving force in CaCC up-regulation but do find an increased Ca(2+) signal in response to mucosal nucleotides that may contribute to the increased Cl(-)/volume secretion seen in intact epithelia. CFTR plays a critical role in maintaining ASL height under basal conditions and accordingly, ASL height is reduced in CF epithelia. In contrast, CaCC does not appear to significantly affect basal ASL height, but does appear to be important in regulating ASL height in response to released agonists (e.g., mucosal nucleotides). We conclude that both CaCC and the Ca(2+) signal are increased in CF airway epithelia, and that they contribute to acute but not basal regulation of ASL height.

Cystic fibrosis airway epithelial Ca2+ i signaling: the mechanism for the larger agonist-mediated Ca2+ i signals in human cystic fibrosis airway epithelia.

In cystic fibrosis (CF) airways, abnormal epithelial ion transport likely initiates mucus stasis, resulting in persistent airway infections and chronic inflammation. Mucus clearance is regulated, in part, by activation of apical membrane receptors coupled to intracellular calcium (Ca(2+)(i)) mobilization. We have shown that Ca(2+)(i) signals resulting from apical purinoceptor (P2Y(2)-R) activation are increased in CF compared with normal human airway epithelia. The present study addressed the mechanism for the larger apical P2Y(2)-R-dependent Ca(2+)(i) signals in CF human airway epithelia. We show that the increased Ca(2+)(i) mobilization in CF was not specific to P2Y(2)-Rs because it was mimicked by apical bradykinin receptor activation, and it did not result from a greater number of P2Y(2)-R or a more efficient coupling between P2Y(2)-Rs and phospholipase C-generated inositol 1,4,5-trisphosphate. Rather, the larger apical P2Y(2)-R activation-promoted Ca(2+)(i) signals in CF epithelia resulted from an increased density and Ca(2+) storage capacity of apically confined endoplasmic reticulum (ER) Ca(2+) stores. To address whether the ER up-regulation resulted from ER retention of misfolded DeltaF508 CFTR or was an acquired response to chronic luminal airway infection/inflammation, three approaches were used. First, ER density was studied in normal and CF sweat duct human epithelia expressing high levels of DeltaF508 CFTR, and it was found to be the same in normal and CF epithelia. Second, apical ER density was morphometrically analyzed in airway epithelia from normal subjects, DeltaF508 homozygous CF patients, and a disease control, primary ciliary dyskinesia; it was found to be greater in both CF and primary ciliary dyskinesia. Third, apical ER density and P2Y(2)-R activation-mobilized Ca(2+)(i), which were investigated in airway epithelia in a long term culture in the absence of luminal infection, were similar in normal and CF epithelia. To directly test whether luminal infection/inflammation triggers an up-regulation of the apically confined ER Ca(2+) stores, normal airway epithelia were chronically exposed to supernatant from mucopurulent material from CF airways. Supernatant treatment expanded the apically confined ER, resulting in larger apical P2Y(2)-R activation-dependent Ca(2+)(i) responses, which reproduced the increased Ca(2+)(i) signals observed in CF epithelia. In conclusion, the mechanism for the larger Ca(2+)(i) signals elicited by apical P2Y(2)-R activation in CF airway epithelia is an expansion of the apical ER Ca(2+) stores triggered by chronic luminal airway infection/inflammation. Greater ER-derived Ca(2+)(i) signals may provide a compensatory mechanism to restore, at least acutely, mucus clearance in CF airways.

Chronic airway infection/inflammation induces a Ca2+i-dependent hyperinflammatory response in human cystic fibrosis airway epithelia.

Hyperinflammatory responses to infection have been postulated as a component of cystic fibrosis (CF) lung disease. Studies have linked intracellular calcium (Ca(2+)(i)) mobilization with inflammatory responses in several systems. We have reported that the pro-inflammatory mediator bradykinin (BK) promotes larger Ca(2+)(i) signals in CF compared with normal bronchial epithelia, a response that reflects endoplasmic reticulum (ER)/Ca(2+) store expansion induced by chronic luminal airway infection/inflammation. The present study investigated whether CF airway epithelia were hyperinflammatory and, if so, whether the hyperinflammatory CF phenotype was linked to larger Ca(2+) stores in the ER. We found that DeltaF508 CF bronchial epithelia were hyperinflammatory as defined by an increased basal and mucosal BK-induced interleukin (IL)-8 secretion. However, the CF hyperinflammation expressed in short-term (6-11-day-old) primary cultures of DeltaF508 bronchial epithelia was lost in long-term (30-40-day-old) primary cultures of DeltaF508 bronchial epithelia, indicating this response was independent of mutant cystic fibrosis transmembrane conductance regulator. Exposure of 30-40-day-old cultures of normal airway epithelia to supernatant from mucopurulent material (SMM) from CF airways reproduced the increased basal and mucosal BK-stimulated IL-8 secretion of short-term CF cultures. The BK-triggered increased IL-8 secretion in SMM-treated cultures was mediated by an increased Ca(2+)(i) mobilization consequent to an ER expansion associated with increases in protein synthesis (total, cytokines, and antimicrobial factors). The increased ER-dependent, Ca(2+)(i)-mediated hyperinflammatory epithelial response may represent a general beneficial airway epithelial adaptation to transient luminal infection. However, in CF airways, the Ca(2+)(i)-mediated hyperinflammation may be ineffective in promoting the eradication of infection in thickened mucus and, consequently, may have adverse effects in the lung.

Normal and cystic fibrosis airway surface liquid homeostasis. The effects of phasic shear stress and viral infections.

Mammalian airways normally regulate the volume of a thin liquid layer, the periciliary liquid (PCL), to facilitate the mucus clearance component of lung defense. Studies under standard (static) culture conditions revealed that normal airway epithelia possess an adenosine-regulated pathway that blends Na+ absorption and Cl- secretion to optimize PCL volume. In cystic fibrosis (CF), the absence of CF transmembrane conductance regulator results in a failure of adenosine regulation of PCL volume, which is predicted to initiate mucus stasis and infection. However, under conditions that mimic the phasic motion of the lung in vivo, ATP release into PCL was increased, CF ion transport was rebalanced, and PCL volume was restored to levels adequate for lung defense. This ATP signaling system was vulnerable, however, to insults that trigger CF bacterial infections, such as viral (respiratory syncytial virus) infections, which up-regulated extracellular ATPase activity and abolished motion-dependent ATP regulation of CF PCL height. These studies demonstrate (i) how the normal coordination of opposing ion transport pathways to maintain PCL volume is disrupted in CF, (ii) the hitherto unknown role of phasic motion in regulating key aspects of normal and CF innate airways defense, and (iii) that maneuvers directed at increasing motion-induced nucleotide release may be therapeutic in CF patients.

Polarized distribution of HCO3- transport in human normal and cystic fibrosis nasal epithelia.

The polarized distribution of HCO3- transport was investigated in human nasal epithelial cells from normal and cystic fibrosis (CF) tissues. To test for HCO3- transport via conductive versus electroneutral Cl-/HCO3- exchange (anion exchange, AE) pathways, nasal cells were loaded with the pH probe 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein and mounted in a bilateral perfusion chamber. In normal, but not CF, epithelia, replacing mucosal Cl- with gluconate caused intracellular pH (pHi) to increase, and the initial rates (Delta pH min-1) of this increase were modestly augmented (approximately 26 %) when normal cells were pretreated with forskolin (10 microM). Recovery from this alkaline shift was dependent on mucosal Cl-, was insensitive to the AE inhibitor 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonic acid (H2DIDS; 1.5 mM), but was sensitive to the cystic fibrosis transmembrane conductance regulator (CFTR) channel inhibitor diphenylamine-2-carboxylate (DPC; 100 microM). In contrast, removal of serosal Cl- caused pHi to alkalinize in both normal and CF epithelia. Recovery from this alkaline challenge was dependent on serosal Cl- and blocked by H2DIDS. Additional studies showed that serosally applied Ba2+ (5.0 mM) in normal, but not CF, cells induced influx of HCO3- across the apical membrane that was reversibly blocked by mucosal DPC. In a final series of studies, normal and CF cells acutely alkaline loaded by replacing bilateral Krebs bicarbonate Ringer (KBR) with Hepes-buffered Ringer solution exhibited basolateral, but not apical, recovery from an alkaline challenge that was dependent on Cl-, independent of Na+ and blocked by H2DIDS. We conclude that: (1) normal, but not CF, nasal epithelia have a constitutively active DPC-sensitive HCO3- influx/efflux pathway across the apical membrane of cells, consistent with the movement of HCO3- via CFTR; and (2) both normal and CF nasal epithelia have Na+-independent, H2DIDS-sensitive AE at their basolateral domain.

Abnormal surface liquid pH regulation by cultured cystic fibrosis bronchial epithelium.

Cystic fibrosis (CF) transmembrane conductance regulator (CFTR)-dependent airway epithelial bicarbonate transport is hypothesized to participate in airway surface liquid pH regulation and contribute to lung defense. We measured pH and ionic composition in apical surface liquid (ASL) on polarized normal (NL) and CF primary bronchial epithelial cell cultures under basal conditions, after cAMP stimulation, and after challenge with luminal acid loads. Under basal conditions, CF epithelia acidified ASL more rapidly than NL epithelia. Two ASL pH regulatory paths that contributed to basal pH were identified in the apical membrane of airway epithelia, and their activities were measured. We detected a ouabain-sensitive (nongastric) H+,K+-ATPase that acidified ASL, but its activity was not different in NL and CF cultures. We also detected the following evidence for a CFTR-dependent HCO3- secretory pathway that was defective in CF: (i). ASL [HCO3-] was higher in NL than CF ASL; (ii). activating CFTR with forskolin/3-isobutyl-1-methylxanthine alkalinized NL ASL but acidified CF ASL; and (iii). NL airway epithelia more rapidly and effectively alkalinized ASL in response to a luminal acid challenge than CF epithelia. We conclude that cultured human CF bronchial epithelial pHASL is abnormally regulated under basal conditions because of absent CFTR-dependent HCO3- secretion and that this defect can lead to an impaired capacity to respond to airway conditions associated with acidification of ASL.

Evaluation of second generation amiloride analogs as therapy for cystic fibrosis lung disease.

Epithelial sodium channel (ENaC) blockers have been proposed as a therapy to restore mucus clearance (MC) in cystic fibrosis (CF) airways. The therapeutic effects of the first generation ENaC blocker, amiloride, in CF patients, however, were minimal. Because the failure of amiloride reflected both its low potency and short duration of action on airway surfaces, we investigated whether the increased potency of benzamil and phenamil would produce more favorable pharmacodynamic properties. In vitro potency, maximal efficacy, rate of recovery from maximal block of ENaC, and rate of drug absorption were compared for amiloride, benzamil, and phenamil in cultured human and ovine bronchial epithelial cells. In both human and ovine bronchial epithelia, the rank order of potency was benzamil > phenamil >> amiloride, the maximal efficacy was benzamil = phenamil = amiloride, the recovery to baseline sodium transport was phenamil < benzamil << amiloride, and the rate of drug absorption was phenamil > benzamil >> amiloride. Based on greater potency, benzamil was compared with amiloride in in vivo pharmacodynamic studies in sheep, including tracheal mucus velocity (TMV) and MC. Benzamil enhanced MC and TMV, but acute potency or duration of effect did not exceed that of amiloride. In conclusion, our data support the hypothesis that ENaC blocker aerosol therapy increases MC. However, rapid absorption of benzamil from the mucosal surface offset its greater potency, making it equieffective with amiloride in vivo. More potent, less absorbable, third generation ENaC blockers will be required for an effective aerosol CF pharmacotherapy.