Evidence of a functional CFTR Cl(-) channel in adult alveolar epithelial cells

Function of KvLQT1 potassium channels in a mouse model of bleomycin-induced acute lung injury

Acute respiratory distress syndrome (ARDS) is characterized by an exacerbated inflammatory response, severe damage to the alveolar-capillary barrier and a secondary infiltration of protein-rich fluid into the airspaces, ultimately leading to respiratory failure. Resolution of ARDS depends on the ability of the alveolar epithelium to reabsorb lung fluid through active transepithelial ion transport, to control the inflammatory response, and to restore a cohesive and functional epithelium through effective repair processes. Interestingly, several lines of evidence have demonstrated the important role of potassium (K+) channels in the regulation of epithelial repair processes. Furthermore, these channels have previously been shown to be involved in sodium/fluid absorption across alveolar epithelial cells, and we have recently demonstrated the contribution of KvLQT1 channels to the resolution of thiourea-induced pulmonary edema in vivo. The aim of our study was to investigate the role of the KCNQ1 pore-forming subunit of KvLQT1 channels in the outcome of ARDS parameters in a model of acute lung injury (ALI). We used a molecular approach with KvLQT1-KO mice challenged with bleomycin, a well-established ALI model that mimics the key features of the exudative phase of ARDS on day 7. Our data showed that KvLQT1 deletion exacerbated the negative outcome of bleomycin on lung function (resistance, elastance and compliance). An alteration in the profile of infiltrating immune cells was also observed in KvLQT1-KO mice while histological analysis showed less interstitial and/or alveolar inflammatory response induced by bleomycin in KvLQT1-KO mice. Finally, a reduced repair rate of KvLQT1-KO alveolar cells after injury was observed. This work highlights the complex contribution of KvLQT1 in the development and resolution of ARDS parameters in a model of ALI.

Rescue of alveolar wall liquid secretion blocks fatal lung injury due to influenza-staphylococcal coinfection

Secondary lung infection by inhaled Staphylococcus aureus (SA) is a common and lethal event for individuals infected with influenza A virus (IAV). How IAV disrupts host defense to promote SA infection in lung alveoli, where fatal lung injury occurs, is not known. We addressed this issue using real-time determinations of alveolar responses to IAV in live, intact, perfused lungs. Our findings show that IAV infection blocked defensive alveolar wall liquid (AWL) secretion and induced airspace liquid absorption, thereby reversing normal alveolar liquid dynamics and inhibiting alveolar clearance of inhaled SA. Loss of AWL secretion resulted from inhibition of the cystic fibrosis transmembrane conductance regulator (CFTR) ion channel in the alveolar epithelium, and airspace liquid absorption was caused by stimulation of the alveolar epithelial Na+ channel (ENaC). Loss of AWL secretion promoted alveolar stabilization of inhaled SA, but rescue of AWL secretion protected against alveolar SA stabilization and fatal SA-induced lung injury in IAV-infected mice. These findings reveal a central role for AWL secretion in alveolar defense against inhaled SA and identify AWL inhibition as a critical mechanism of IAV lung pathogenesis. AWL rescue may represent a new therapeutic approach for IAV-SA coinfection.

Airway and Lung Organoids from Human-Induced Pluripotent Stem Cells Can Be Used to Assess CFTR Conductance

Airway and lung organoids derived from human-induced pluripotent stem cells (hiPSCs) are current models for personalized drug screening, cell–cell interaction studies, and lung disease research. We analyzed the existing differentiation protocols and identified the optimal conditions for obtaining organoids. In this article, we describe a step-by-step protocol for differentiating hiPSCs into airway and lung organoids. We obtained airway and lung organoids from a healthy donor and from five donors with cystic fibrosis. Analysis of the cellular composition of airway and lung organoids showed that airway organoids contain proximal lung epithelial cells, while lung organoids contain both proximal and distal lung epithelial cells. Forskolin-induced swelling of organoids derived from a healthy donor showed that lung organoids, as well as airway organoids, contain functional epithelial cells and swell after 24 h exposure to forskolin, which makes it a suitable model for analyzing the cystic fibrosis transmembrane conductance regulator (CFTR) channel conductance in vitro. Thus, our results demonstrate the feasibility of generating and characterizing airway and lung organoids from hiPSCs, which can be used for a variety of future applications.

Impact of KvLQT1 potassium channel modulation on alveolar fluid homeostasis in an animal model of thiourea-induced lung edema

Alveolar ion and fluid absorption is essential for lung homeostasis in healthy conditions as well as for the resorption of lung edema, a key feature of acute respiratory distress syndrome. Liquid absorption is driven by active transepithelial sodium transport, through apical ENaC Na⁺ channels and basolateral Na⁺/K⁺-ATPase. Our previous work unveiled that KvLQT1 K⁺ channels also participate in the control of Na⁺/liquid absorption in alveolar epithelial cells. Our aim was to further investigate the function of KvLQT1 channels and their interplay with other channels/transporters involved in ion/liquid transport in vivo using adult wild-type (WT) and KvLQT1 knock-out (KO) mice under physiological conditions and after thiourea-induced lung edema. A slight but significant increase in water lung content (WLC) was observed in naïve KvLQT1-KO mice, relative to WT littermates, whereas lung function was generally preserved and histological structure unaltered. Following thiourea-induced lung edema, KvLQT1-KO did not worsen WLC or lung function. Similarly, lung edema was not aggravated by the administration of a KvLQT1 inhibitor (chromanol). However, KvLQT1 activation (R-L3) significantly reduced WLC in thiourea-challenged WT mice. The benefits of R-L3 were prevented in KO or chromanol-treated WT mice. Furthermore, R-L3 treatment had no effect on thiourea-induced endothelial barrier alteration but restored or enhanced the levels of epithelial alveolar AQP5, Na⁺/K⁺-ATPase, and ENaC expressions. Altogether, the results indicate the benefits of KvLQT1 activation in the resolution of lung edema, probably through the observed up-regulation of epithelial alveolar channels/transporters involved in ion/water transport.

Loss of endothelial CFTR drives barrier failure and edema formation in lung infection and can be targeted by CFTR potentiation

Pneumonia is the most common cause of the acute respiratory distress syndrome (ARDS). Here, we identified loss of endothelial cystic fibrosis transmembrane conductance regulator (CFTR) as an important pathomechanism leading to lung barrier failure in pneumonia-induced ARDS. CFTR was down-regulated after Streptococcus pneumoniae infection ex vivo or in vivo in human or murine lung tissue, respectively. Analysis of isolated perfused rat lungs revealed that CFTR inhibition increased endothelial permeability in parallel with intracellular chloride ion and calcium ion concentrations ([Cl^-]_i and [Ca²⁺]_i). Inhibition of the chloride ion-sensitive with-no-lysine kinase 1 (WNK1) protein with tyrphostin 47 or WNK463 replicated the effect of CFTR inhibition on endothelial permeability and endothelial [Ca²⁺]_i, whereas WNK1 activation by temozolomide attenuated it. Endothelial [Ca²⁺]_i transients and permeability in response to inhibition of either CFTR or WNK1 were prevented by inhibition of the cation channel transient receptor potential vanilloid 4 (TRPV4). Mice deficient in Trpv4 (Trpv4^-/-) developed less lung edema and protein leak than their wild-type littermates after infection with S. pneumoniae. The CFTR potentiator ivacaftor prevented lung CFTR loss, edema, and protein leak after S. pneumoniae infection in wild-type mice. In conclusion, lung infection caused loss of CFTR that promoted lung edema formation through intracellular chloride ion accumulation, inhibition of WNK1, and subsequent disinhibition of TRPV4, resulting in endothelial calcium ion influx and vascular barrier failure. Ivacaftor prevented CFTR loss in the lungs of mice with pneumonia and may, therefore, represent a possible therapeutic strategy in people suffering from ARDS due to severe pneumonia.

Establishing an economical and widely accessible donation after circulatory death animal abattoir model for lung research using ex vivo lung perfusion

BACKGROUND

Ex vivo lung perfusion (EVLP), is a platform that allows simultaneous testing and treatment of the lungs. However, use of EVLP is costly and requires access to lab animals and accompanying facilities. To increase the use of EVLP for research, we developed a method to perform EVLP using abattoir procured lungs. Furthermore, we were also able to significantly decrease costs.

METHODS

Six pair of lungs were procured from abattoir sheep. The lungs were then flushed and stored in ice for 3 h. A low-flow (20% of cardiac output) approach, a tidal volume of 6 ml/kg bodyweight and total perfusion time of 3 h were chosen. Perfusion fluids and circuits were self-made. Lung biopsies, perfusate collection, respiratory values, circulatory pressures were recorded and hourly blood gas analyses were performed.

RESULTS

Mean pO₂ remained stable from 60 min (49.3 ± 7.1 kPa) to 180 min (51.5 kPa ± 8.0), p = 0.66. Pulmonary artery pressure remained ≤15 mm Hg and the left atrial pressure remained between 3 and 5 mm Hg and peak respiratory pressures ≤20 cmH₂ O. Lactate dehydrogenase increased from start (96.3 ± 56.4 U/L) to the end of perfusion (315.8 ± 85.0 U/L), p < 0.05. No difference was observed in ATP between procurement and post-EVLP, 129.7 ± 37.4 μmol/g protein to 132.0 ± 23.4 μmol/g, p = 0.92.

CONCLUSIONS

Sheep lungs, acquired from an abattoir, can be ex vivo perfused under similar conditions as lab animal lungs with similar results regarding e.g., oxygenation and ATP restoration. Furthermore, costs can be significantly reduced by making use of this abattoir model. By increasing accessibility and lowering costs for experiments using lung perfusion, more results may be achieved in the field of lung diseases.

Impact of e-cigarette aerosol on primary human alveolar epithelial type 2 cells

Electronic cigarettes (e-cigarettes) are designed to simulate combustible cigarette smoking and to aid in smoking cessation. Although the number of e-cigarette users has been increasing, the potential health impacts and biological effects of e-cigarettes are still not fully understood. Previous research has focused on the biological effects of e-cigarettes on lung cancer cell lines and distal airway epithelial cells; however, there have been few published studies on the effect of e-cigarettes on primary lung alveolar epithelial cells. The primary purpose of this study was to investigate the direct effect of e-cigarette aerosol on primary human lung alveolar epithelial type 2 (AT2) cells, both alone and in the presence of viral infection. The Melo-3 atomizer caused direct AT2 cell toxicity, whereas the more popular Juul pod's aerosol did not have a detectable cytotoxic effect on AT2 cells. Juul nicotine aerosol also did not increase short-term susceptibility to viral infection. However, 3 days of exposure upregulated genes central to the generation of reactive oxygen species, lipid peroxidation, and carcinogen metabolism and downregulated key innate immune system genes related to cytokine and chemokine signaling. These findings have implications for the potentially injurious impact of long-term use of popular low-power e-cigarette pods on the human alveolar epithelium. Gene expression data might be an important endpoint for evaluating the potential harmful effects of vaping devices that do not cause overt toxicity.

The role of pulmonary ORCC and CLC-2 channels in the response to oxidative stress

Background

Exposure of human lung epithelial cells to the oxidant pollutant ozone (O3) alters cell Cl− currents inducing an outward rectifier effect. Among the various Cl− channels, ClC-2 and ORCC seemed to be involved in this response.

Objectives

To identify the channel related to O3 induced current changes.

Results

Down regulating the expression of ORCC and ClC-2 genes and analyzing the membrane current show that the enhancement of the current disappeared when ORCC was silenced. The contribution of ORCC and ClC-2 channels in control and O3 treated cells was obtained by a mathematical approach.

Conclusion

We suggest that O3 activates ORCC channels and slightly inhibited ClC-2 channels in the negative voltage range. These findings open the possibility of identifying the biomolecular changes induced by O3 allowing a possible pharmacological intervention towards chloride current due to oxidative stress.

Dexamethasone fails to improve bleomycin-induced acute lung injury in mice

Acute respiratory distress syndrome (ARDS) features an exudative phase characterized by alveolar damage, lung edema and exacerbated inflammatory response. Given their anti‐inflammatory properties, the potential therapeutic effect of corticosteroids has been evaluated in ARDS clinical trials and experimental models of ALI. These studies produced contradictory results. Therefore, our aim was to investigate the effects of dexamethasone in an animal model of bleomycin‐induced acute lung injury and then to determine if the lack of response could be related to an impairment in repair ability of alveolar epithelial cells after injury. NMRI mice were challenged with bleomycin and then treated daily with dexamethasone or saline. Bronchoalveolar lavages (BAL) and lungs were collected for assessment of the inflammatory response and wet/dry ratio (lung edema) and for histological analyses. The effect of bleomycin and dexamethasone on wound repair was also evaluated in vitro on primary alveolar epithelial cell (ATII) cultures. Our data first showed that dexamethasone treatment did not reduce the weight loss or mortality rates induced by bleomycin. Although the TNF‐α level in BAL of bleomycin‐treated mice was reduced by dexamethasone, the neutrophil infiltration remained unchanged. Dexamethasone also failed to reduce lung edema and damage scores. Finally, bleomycin elicited a time‐ and dose‐dependent reduction in repair rates of ATII cell cultures. This inhibitory effect was further enhanced by dexamethasone, which also affected the expression of β3‐ and β6‐integrins, key proteins of alveolar repair. Altogether, our data indicate that the inability of dexamethasone to improve the resolution of ALI might be due to his deleterious effect on the alveolar epithelium repair.

PCTR1 improves pulmonary edema fluid clearance through activating the sodium channel and lymphatic drainage in lipopolysaccharide-induced ARDS

Acute respiratory distress syndrome (ARDS) is a lethal clinical syndrome characterized by damage of the epithelial barriers and accumulation of pulmonary edema fluid. Protectin conjugates in tissue regeneration 1 (PCTR1), an endogenously produced lipid mediator, are believed to exert anti-inflammatory and pro-resolution effects. PCTR1 (1 µg/kg) was injected at 8 hr after lipopolysaccharide (LPS; 14 mg/kg) administration, and the rate of pulmonary fluid clearance was measured in live rats at 1 hr after PCTR1 treatment. The primary type II alveolar epithelial cells were cultured with PCTR1 (10 nmol/ml) and LPS (1 μg/ml) for 8 hr. PCTR1 effectively improved pulmonary fluid clearance and ameliorated morphological damage and reduced inflammation of lung tissue, as well as improved the survival rate in the LPS-induced acute lung injury (ALI) model. Moreover, PCTR1 markedly increased sodium channel expression as well as Na, K-ATPase expression and activity in vivo and in vitro. In addition, PCTR1i also upregulated the expression of LYVE-1 in vivo. Besides that, BOC-2, HK7, and LY294002 blocked the promoted effect of PCTR1 on pulmonary fluid clearance. Taken together, PCTR1 upregulates sodium channels' expression via activating the ALX/cAMP/P-Akt/Nedd4-2 pathway and increases Na, K-ATPase expression and activity to promote alveolar fluid clearance. Moreover, PCTR1 also promotes the expression of LYVE-1 to recover the lymphatic drainage resulting in the increase of lung interstitial fluid clearance. In summary, these results highlight a novel systematic mechanism for PCTR1 in pulmonary edema fluid clearance after ALI/ARDS, suggesting its potential role in a therapeutic approach for ALI/ARDS.

Type 2 secretory cells are primary source of ATP release in mechanically stretched lung alveolar cells

Extracellular ATP and its metabolites are potent paracrine modulators of lung alveolar cell function, including surfactant secretion and fluid transport, but the sources and mechanism of intra-alveolar ATP release remain unclear. To determine the contribution of gas-exchanging alveolar type 1 (AT1) and surfactant-secreting type 2 (AT2) cells to stretch-induced ATP release, we used quantitative real-time luminescence ATP imaging and rat primary alveolar cells cultured on silicon substrate for 2-7 days. When cultured on solid support, primary AT2 cells progressively transdifferentiated into AT1-like cells with ~20% of cells showing AT1 phenotype by day 2-3 (AT2:AT1 ≈ 4:1), while on day 7, the AT2:AT1 cell ratio was reversed with up to 80% of the cells displaying characteristics of AT1 cells. Stretch (1 s, 5-35%) induced ATP release from AT2/AT1 cell cultures, and it was highest on days 2 and 3 but declined in older cultures. ATP release tightly correlated with the number of remaining AT2 cells in culture, consistent with ~10-fold lower ATP release by AT1 than AT2 cells. ATP release was unaffected by inhibitors of putative ATP channels carbenoxolone and probenecid but was significantly diminished in cells loaded with calcium chelator BAPTA. These pharmacological modulators had similar effects on stretch-induced intracellular Ca²⁺ responses measured by Fura2 fluorescence. The study revealed that AT2 cells are the primary source of stretch-induced ATP release in heterocellular AT2/AT1 cell cultures, suggesting similar contribution in intact alveoli. Our results support a role for calcium-regulated mechanism but not ATP-conducting channels in ATP release by alveolar epithelial cells.

Transepithelial Fluid and Salt Re-Absorption Regulated by cGK2 Signals

Transepithelial fluid and salt re-absorption in epithelial tissues play an important role in fluid and salt homeostasis. In absorptive epithelium, fluid and salt flux is controlled by machinery mainly composed of epithelial sodium channels (ENaC), cystic fibrosis transmembrane conductance regulator (CFTR), Na⁺/H⁺ exchanger (NHE), aquaporin, and sodium potassium adenosine triphosphatase (Na⁺/K⁺-ATPase). Dysregulation of fluid and salt transport across epithelium contributes to the pathogenesis of many diseases, such as pulmonary edema and cystic fibrosis. Intracellular and extracellular signals, i.e., hormones and protein kinases, regulate fluid and salt turnover and resolution. Increasing evidence demonstrates that transepithelial fluid transport is regulated by cyclic guanosine monophosphate-dependent protein kinase (cGK) signals. cGK2 was originally identified and cloned from intestinal specimens, the presence of which has also been confirmed in the kidney and the lung. cGK2 regulates fluid and salt through ENaC, CFTR and NHE. Deficient cGK2 regulation of transepithelial ion transport was seen in acute lung injury, and cGK2 could be a novel druggable target to restore edematous disorder in epithelial tissues.

Ion channels of the lung and their role in disease pathogenesis

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

Complementary roles of KCa3.1 channels and β1-integrin during alveolar epithelial repair

BACKGROUND

Extensive alveolar epithelial injury and remodelling is a common feature of acute lung injury and acute respiratory distress syndrome (ARDS) and it has been established that epithelial regeneration, and secondary lung oedema resorption, is crucial for ARDS resolution. Much evidence indicates that K(+) channels are regulating epithelial repair processes; however, involvement of the KCa3.1 channels in alveolar repair has never been investigated before.

RESULTS

Wound-healing assays demonstrated that the repair rates were increased in primary rat alveolar cell monolayers grown on a fibronectin matrix compared to non-coated supports, whereas an anti-β1-integrin antibody reduced it. KCa3.1 inhibition/silencing impaired the fibronectin-stimulated wound-healing rates, as well as cell migration and proliferation, but had no effect in the absence of coating. We then evaluated a putative relationship between KCa3.1 channel and the migratory machinery protein β1-integrin, which is activated by fibronectin. Co-immunoprecipitation and immunofluorescence experiments indicated a link between the two proteins and revealed their cellular co-distribution. In addition, we demonstrated that KCa3.1 channel and β1-integrin membrane expressions were increased on a fibronectin matrix. We also showed increased intracellular calcium concentrations as well as enhanced expression of TRPC4, a voltage-independent calcium channel belonging to the large TRP channel family, on a fibronectin matrix. Finally, wound-healing assays showed additive effects of KCa3.1 and TRPC4 inhibitors on alveolar epithelial repair.

CONCLUSION

Taken together, our data demonstrate for the first time complementary roles of KCa3.1 and TRPC4 channels with extracellular matrix and β1-integrin in the regulation of alveolar repair processes.

Novel role for cystic fibrosis transmembrane conductance regulator in alveolar fluid clearance in lipopolysaccharide-induced acute lung injury in mice

BACKGROUND AND OBJECTIVE

Alveolar fluid clearance (AFC) is important for the resolution of acute lung injury (ALI). The role of cystic fibrosis transmembrane conductance regulator (CFTR) in AFC has not been entirely elucidated in animal models of ALI. The aim of this study was to investigate the role of CFTR and its mechanisms in AFC in normal and ALI mice.

METHODS

Seventy mice were randomly divided into 14 groups and ALI was established by intratracheal instillation of lipopolysaccharide (LPS). After 48 h, CFTR activator CFTRact-16 or inhibitor CFinh-172 with or without β-agonist was instillated intratracheally and AFC was measured with radioisotopic tracer.

RESULTS

Although there was no effects of CFTRact-16 on AFC in mice with or without isoproterenol, CFinh-172 markedly decreased isoproterenol-stimulated AFC in both normal (P < 0.01) and LPS-induced ALI mice (P < 0.01) and there was significantly decreased basal AFC in ALI mice (P < 0.05).

CONCLUSIONS

These results provide direct functional evidence for CFTR in cAMP-mediated AFC in both normal and ALI mice.

Hypotonic shock modulates Na(+) current via a Cl(-) and Ca(2+)/calmodulin dependent mechanism in alveolar epithelial cells

Alveolar epithelial cells are involved in Na⁺ absorption via the epithelial Na⁺ channel (ENaC), an important process for maintaining an appropriate volume of liquid lining the respiratory epithelium and for lung oedema clearance. Here, we investigated how a 20% hypotonic shock modulates the ionic current in these cells. Polarized alveolar epithelial cells isolated from rat lungs were cultured on permeant filters and their electrophysiological properties recorded. A 20% bilateral hypotonic shock induced an immediate, but transient 52% rise in total transepithelial current and a 67% increase in the amiloride-sensitive current mediated by ENaC. Amiloride pre-treatment decreased the current rise after hypotonic shock, showing that ENaC current is involved in this response. Since Cl^- transport is modulated by hypotonic shock, its contribution to the basal and hypotonic-induced transepithelial current was also assessed. Apical NPPB, a broad Cl^- channel inhibitor and basolateral DIOA a potassium chloride co-transporter (KCC) inhibitor reduced the total and ENaC currents, showing that transcellular Cl^- transport plays a major role in that process. During hypotonic shock, a basolateral Cl^- influx, partly inhibited by NPPB is essential for the hypotonic-induced current rise. Hypotonic shock promoted apical ATP secretion and increased intracellular Ca²⁺. While apyrase, an ATP scavenger, did not inhibit the hypotonic shock current response, W7 a calmodulin antagonist completely prevented the hypotonic current rise. These results indicate that a basolateral Cl^- influx as well as Ca²⁺/calmodulin, but not ATP, are involved in the acute transepithelial current rise elicited by hypotonic shock.

Influenza A virus (H1N1) increases airway epithelial cell secretion by up-regulation of potassium channel KCNN4

Influenza infects the epithelial cells lining the airways. Normally epithelial cells move solutes through ion channels to create the osmotic drive to hydrate the airways. Viral alteration of this process could explain, in part, the fluid imbalance in the lungs and the resulting pulmonary edema that occurs during severe influenza infections. Using western blot and RT-qPCR, we measured ion channel and cytokine expression in the Calu3 airway cell line after infection with influenza virus (H1N1) for 48 h. We simultaneously measured chloride and potassium channel function by means of a short-circuit current (I(sc)) produced in an Ussing chamber. At a multiplicity of infection (MOI) of 10, viral M1 protein and pro-inflammatory cytokine expression was observed 24h post-infection, despite a lack of measurable change in Isc. However, we observed a decreased secretory response in cAMP- and calcium-induced Isc 48 h post-infection. This correlated with a decrease in CFTR and KCNN4 protein levels. Interestingly, a viral dose of an MOI 0.6 revealed an increased secretory response that correlated with pro-inflammatory cytokine expression. This increased secretory response seemed to be primarily driven through KCNN4. We detected an increase in KCNN4 mRNA and protein, while CFTR function and expression remained unchanged. Furthermore, inhibition of the KCNN4-stimulated I(sc) with TRAM-34, a specific inhibitor, ameliorated the response, implicating KCNN4 as the main driving force behind the secretory phenotype.

K+ channels regulate ENaC expression via changes in promoter activity and control fluid clearance in alveolar epithelial cells

Active Na+ absorption by alveolar ENaC is the main driving force of liquid clearance at birth and lung edema resorption in adulthood. We have demonstrated previously that long-term modulation of KvLQT1 and KATP K+ channel activities exerts sustained control in Na+ transport through the regulation of ENaC expression in primary alveolar type II (ATII) cells. The goal of the present study was: 1) to investigate the role of the alpha-ENaC promoter, transfected in the A549 alveolar cell line, in the regulation of ENaC expression by K+ channels, and 2) to determine the physiological impact of K+ channels and ENaC modulation on fluid clearance in ATII cells. KvLQT1 and KATP channels were first identified in A549 cells by PCR and Western blotting. We showed, for the first time, that KvLQT1 activation by R-L3 (applied for 24 h) increased alpha-ENaC expression, similarly to KATP activation by pinacidil. Conversely, pharmacological KvLQT1 and KATP inhibition or silencing with siRNAs down-regulated alpha-ENaC expression. Furthermore, K+ channel blockers significantly decreased alpha-ENaC promoter activity. Our results indicated that this decrease in promoter activity could be mediated, at least in part, by the repressor activity of ERK1/2. Conversely, KvLQT1 and KATP activation dose-dependently enhanced alpha-ENaC promoter activity. Finally, we noted a physiological impact of changes in K+ channel functions on ERK activity, alpha-, beta-, gamma-ENaC subunit expression and fluid absorption through polarized ATII cells. In summary, our results disclose that K+ channels regulate alpha-ENaC expression by controlling its promoter activity and thus affect the alveolar function of fluid clearance.

Induction of nitric oxide synthase expression by lipopolysaccharide is mediated by calcium-dependent PKCα-β1 in alveolar epithelial cells

Nitric oxide (NO) plays an important role in innate host defense and inflammation. In response to infection, NO is generated by inducible nitric oxide synthase (iNOS), a gene product whose expression is highly modulated by different stimuli, including lipopolysaccharide (LPS) from gram-negative bacteria. We reported recently that LPS from Pseudomonas aeruginosa altered Na⁺ transport in alveolar epithelial cells via a suramin-dependent process, indicating that LPS activated a purinergic response in these cells. To further study this question, in the present work, we tested whether iNOS mRNA and protein expression were modulated in response to LPS in alveolar epithelial cells. We found that LPS induced a 12-fold increase in iNOS mRNA expression via a transcription-dependent process in these cells. iNOS protein, NO, and nitrotyrosine were also significantly elevated in LPS-treated cells. Ca²⁺ chelation and protein kinase C (PKCα-β1) inhibition suppressed iNOS mRNA induction by LPS, implicating Ca²⁺-dependent PKC signaling in this process. LPS evoked a significant increase of extracellular ATP. Because PKC activation is one of the signaling pathways known to mediate purinergic signaling, we evaluated the hypothesis that iNOS induction was ATP dependent. Although high suramin concentration inhibited iNOS mRNA induction, the process was not ATP dependent, since specific purinergic receptor antagonists could not inhibit the process. Altogether, these findings demonstrate that iNOS expression is highly modulated in alveolar epithelial cells by LPS via a Ca²⁺/PKCα-β1 pathway independent of ATP signaling.

Contribution of CFTR to alveolar fluid clearance by lipoxin A4 via PI3K/Akt pathway in LPS-induced acute lung injury

The lipoxins are the first proresolution mediators to be recognized and described as the endogenous “braking signals” for inflammation. We evaluated the anti-inflammatory and proresolution bioactions of lipoxin A₄ in our lipopolysaccharide (LPS-)induced lung injury model. We demonstrated that lipoxin A₄ significantly improved histology of rat lungs and inhibited IL-6 and TNF-α in LPS-induced lung injury. In addition, lipoxin A₄ increased alveolar fluid clearance (AFC) and the effect of lipoxin A₄ on AFC was abolished by CFTR_inh-172 (a specific inhibitor of CFTR). Moreover, lipoxin A₄ could increase cystic fibrosis transmembrane conductance regulator (CFTR) protein expression in vitro and in vivo. In rat primary alveolar type II (ATII) cells, LPS decreased CFTR protein expression via activation of PI3K/Akt, and lipoxin A₄ suppressed LPS-stimulated phosphorylation of Akt. These results showed that lipoxin A₄ enhanced CFTR protein expression and increased AFC via PI3K/Akt pathway. Thus, lipoxin A₄ may provide a potential therapeutic approach for acute lung injury.

CFTR, mucins, and mucus obstruction in cystic fibrosis

Mucus pathology in cystic fibrosis (CF) has been known for as long as the disease has been recognized and is sometimes called mucoviscidosis. The disease is marked by mucus hyperproduction and plugging in many organs, which are usually most fatal in the airways of CF patients, once the problem of meconium ileus at birth is resolved. After the CF gene, CFTR, was cloned and its protein product identified as a cAMP-regulated Cl(-) channel, causal mechanisms underlying the strong mucus phenotype of the disease became obscure. Here we focus on mucin genes and polymeric mucin glycoproteins, examining their regulation and potential relationships to a dysfunctional cystic fibrosis transmembrane conductance regulator (CFTR). Detailed examination of CFTR expression in organs and different cell types indicates that changes in CFTR expression do not always correlate with the severity of CF disease or mucus accumulation. Thus, the mucus hyperproduction that typifies CF does not appear to be a direct cause of a defective CFTR but, rather, to be a downstream consequence. In organs like the lung, up-regulation of mucin gene expression by inflammation results from chronic infection; however, in other instances and organs, the inflammation may have a non-infectious origin. The mucus plugging phenotype of the β-subunit of the epithelial Na(+) channel (βENaC)-overexpressing mouse is proving to be an archetypal example of this kind of inflammation, with a dehydrated airway surface/concentrated mucus gel apparently providing the inflammatory stimulus. Data indicate that the luminal HCO(3)(-) deficiency recently described for CF epithelia may also provide such a stimulus, perhaps by causing a mal-maturation of mucins as they are released onto luminal surfaces. In any event, the path between CFTR dysfunction and mucus hyperproduction has proven tortuous, and its unraveling continues to offer its own twists and turns, along with fascinating glimpses into biology.

The role of Cl- in the regulation of ion and liquid transport in the intact alveolus during β-adrenergic stimulation

The epithelium of the developing lung displays an evolving liquid transport phenotype, in which Cl(-) secretion during fetal life is rapidly switched to Na(+) absorption perinatally. However, the mechanisms underlying the homeostasis of the thin layer of liquid lining the postnatal pulmonary epithelium remain elusive. In particular, it remains unclear whether the stimulated clearance of excess alveolar liquid is mediated via transepithelial Cl(-) transport. Our study is a pharmacological analysis with the aim of addressing this issue, which is of major physiological significance in cases of pulmonary oedema from any cause. We measured the rate of transepithelial liquid movement (J(v)) with (125)I-albumin, in the in situ perfused adult rat lung. Transepithelial Cl(-) transport was studied with the use of the Cl(-) channel inhibitor NPPB in the resting state and during stimulation with the β(2)-adrenergic agonist terbutaline. The study of J(v) in these conditions revealed the following findings: (1) there is net absorption of excess of alveolar liquid in the resting, unstimulated state, which is predominantly amiloride sensitive; (2) inhibition of Cl(-) transport with NPPB in the resting state results in a 1.6-fold increase in net absorption of alveolar liquid; and (3) the terbutaline-stimulated net absorption of the excess liquid is enhanced by 2.8-fold in the presence of NPPB. Our results are suggestive of the functional presence of secretory, but not absorptive, Cl(-) mechanisms and show that transepithelial Cl(-) transport is not part of the mechanism underlying lung liquid clearance in response to β-adrenergic stimulation.

Ion transport by pulmonary epithelia

The lung surface of air-breathing vertebrates is formed by a continuous epithelium that is covered by a fluid layer. In the airways, this epithelium is largely pseudostratified consisting of diverse cell types such as ciliated cells, goblet cells, and undifferentiated basal cells, whereas the alveolar epithelium consists of alveolar type I and alveolar type II cells. Regulation and maintenance of the volume and viscosity of the fluid layer covering the epithelium is one of the most important functions of the epithelial barrier that forms the outer surface area of the lungs. Therefore, the epithelial cells are equipped with a wide variety of ion transport proteins, among which Na⁺, Cl⁻, and K⁺ channels have been identified to play a role in the regulation of the fluid layer. Malfunctions of pulmonary epithelial ion transport processes and, thus, impairment of the liquid balance in our lungs is associated with severe diseases, such as cystic fibrosis and pulmonary oedema. Due to the important role of pulmonary epithelial ion transport processes for proper lung function, the present paper summarizes the recent findings about composition, function, and ion transport properties of the airway epithelium as well as of the alveolar epithelium.

Nasal potential difference to detect Na+ channel dysfunction in acute lung injury

Pulmonary fluid clearance is regulated by the active transport of Na(+) and Cl(-) through respiratory epithelial ion channels. Ion channel dysfunction contributes to the pathogenesis of various pulmonary fluid disorders including high-altitude pulmonary edema (HAPE) and neonatal respiratory distress syndrome (RDS). Nasal potential difference (NPD) measurement allows an in vivo investigation of the functionality of these channels. This technique has been used for the diagnosis of cystic fibrosis, the archetypal respiratory ion channel disorder, for over a quarter of a century. NPD measurements in HAPE and RDS suggest constitutive and acquired dysfunction of respiratory epithelial Na(+) channels. Acute lung injury (ALI) is characterized by pulmonary edema due to alveolar epithelial-interstitial-endothelial injury. NPD measurement may enable identification of critically ill ALI patients with a susceptible phenotype of dysfunctional respiratory Na(+) channels and allow targeted therapy toward Na(+) channel function.

Potential application of mesenchymal stem cells in acute lung injury

Despite extensive research into the pathogenesis of acute lung injury and the acute respiratory distress syndrome (ALI/ARDS), mortality remains high at approximately 40%. Current treatment is primarily supportive, with lung-protective ventilation and a fluid conservative strategy. Pharmacologic therapies that reduce the severity of lung injury in experimental studies have not yet been translated into effective clinical treatment options. Therefore, innovative therapies are needed. Recent studies have suggested that bone-marrow-derived multipotent mesenchymal stem cells (MSC) may have therapeutic applications in multiple clinical disorders including myocardial infarction, diabetes, sepsis, hepatic and acute renal failure. Recently, MSC have been studied in several in vivo models of lung disease. This review focuses on first describing the existing experimental literature that has tested the use of MSC in models of ALI/ARDS, and then the potential mechanisms underlying their therapeutic use with an emphasis on secreted paracrine soluble factors. The review concludes with a discussion of future research directions required for potential clinical trials.

UTP regulation of ion transport in alveolar epithelial cells involves distinct mechanisms

UTP is known to regulate alveolar fluid clearance. However, the relative contribution of alveolar type I cells and type II cells to this process is unknown. In this study, we investigated the effects of UTP on ion transport in type I-like cell (AEC I) and type II-like cell (AEC II) monolayers. Luminal treatment of cell monolayers with UTP increased short-circuit current (I(sc)) of AEC II but decreased I(sc) of AEC I. The Cl(-) channel blockers NPPB and DIDS inhibited the UTP-induced changes in I(sc) (DeltaIsc) in both types of cells. Amiloride, an inhibitor of epithelial Na(+) channels (ENaC), abolished the UTP-induced DeltaI(sc) in AEC I, but not in AEC II. The general blocker of K(+) channels, BaCl(2), eliminated the UTP-induced DeltaI(sc) in AEC II, but not in AEC I. The intermediate conductance (IK(Ca)) blocker, clofilium, also blocked the UTP effect in AEC II. The signal transduction pathways mediated by UTP were the same in AEC I and AEC II. Furthermore, UTP increased Cl(-) secretion in AEC II and Cl(-) absorption in AEC I. Our results suggest that UTP induces opposite changes in I(sc) in AEC I and AEC II, likely due to the reversed Cl(-) flux and different contributions of ENaC and IK(Ca). Our results further imply a new concept that type II cells contribute to UTP-induced fluid secretion and type I cells contribute to UTP-induced fluid absorption in alveoli.

Transient in utero disruption of cystic fibrosis transmembrane conductance regulator causes phenotypic changes in alveolar type II cells in adult rats

Background

Mechanicosensory mechanisms regulate cell differentiation during lung organogenesis. We have previously demonstrated that cystic fibrosis transmembrane conductance regulator (CFTR) was integral to stretch-induced growth and development and that transient expression of antisense-CFTR (ASCFTR) had negative effects on lung structure and function. In this study, we examined adult alveolar type II (ATII) cell phenotype after transient knock down of CFTR by adenovirus-directed in utero expression of ASCFTR in the fetal lung.

Results

In comparison to (reporter gene-treated) Controls, ASCFTR-treated adult rat lungs showed elevated phosphatidylcholine (PC) levels in the large but not in the small aggregates of alveolar surfactant. The lung mRNA levels for SP-A and SP-B were lower in the ASCFTR rats. The basal PC secretion in ATII cells was similar in the two groups. However, compared to Control ATII cells, the cells in ASCFTR group showed higher PC secretion with ATP or phorbol myristate acetate. The cell PC pool was also larger in the ASCFTR group. Thus, the increased surfactant secretion in ATII cells could cause higher PC levels in large aggregates of surfactant. In freshly isolated ATII cells, the expression of surfactant proteins was unchanged, suggesting that the lungs of ASCFTR rats contained fewer ATII cells. Gene array analysis of RNA of freshly isolated ATII cells from these lungs showed altered expression of several genes including elevated expression of two calcium-related genes, Ca²⁺-ATPase and calcium-calmodulin kinase kinase1 (CaMkk1), which was confirmed by real-time PCR. Western blot analysis showed increased expression of calmodulin kinase I, which is activated following phosphorylation by CaMkk1. Although increased expression of calcium regulating genes would argue in favor of Ca²⁺-dependent mechanisms increasing surfactant secretion, we cannot exclude contribution of alternate mechanisms because of other phenotypic changes in ATII cells of the ASCFTR group.

Conclusion

Developmental changes due to transient disruption of CFTR in fetal lung reflect in altered ATII cell phenotype in the adult life.

Inhibition of the KCa3.1 channels by AMP-activated protein kinase in human airway epithelial cells

The vectorial transport of ions and water across epithelial cells depends to a large extent on the coordination of the apical and basolateral ion fluxes with energy supply. In this work we provide the first evidence for a regulation by the 5'-AMP-activated protein kinase (AMPK) of the calcium-activated potassium channel KCa3.1 expressed at the basolateral membrane of a large variety of epithelial cells. Inside-out patch-clamp experiments performed on human embryonic kidney (HEK) cells stably transfected with KCa3.1 first revealed a decrease in KCa3.1 activity following the internal addition of AMP at a fixed ATP concentration. This effect was dose dependent with half inhibition at 140 muM AMP in 1 mM ATP. Evidence for an interaction between the COOH-terminal region of KCa3.1 and the gamma1-subunit of AMPK was next obtained by two-hybrid screening and pull-down experiments. Our two-hybrid analysis confirmed in addition that the amino acids extending from Asp(380) to Ala(400) in COOH-terminal were essential for the interaction AMPK-gamma1/KCa3.1. Inside-out experiments on cells coexpressing KCa3.1 with the dominant negative AMPK-gamma1-R299G mutant showed a reduced sensitivity of KCa3.1 to AMP, arguing for a functional link between KCa3.1 and the gamma1-subunit of AMPK. More importantly, coimmunoprecipitation experiments carried out on bronchial epithelial NuLi cells provided direct evidence for the formation of a KCa3.1/AMPK-gamma1 complex at endogenous AMPK and KCa3.1 expression levels. Finally, treating NuLi monolayers with the membrane permeant AMPK activator 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) caused a significant decrease of the KCa3.1-mediated short-circuit currents, an effect reversible by coincubation with the AMPK inhibitor Compound C. These observations argue for a regulation of KCa3.1 by AMPK in a functional epithelium through protein/protein interactions involving the gamma1-subunit of AMPK.

The porcine lung as a potential model for cystic fibrosis

Airway disease currently causes most of the morbidity and mortality in patients with cystic fibrosis (CF). However, understanding the pathogenesis of CF lung disease and developing novel therapeutic strategies have been hampered by the limitations of current models. Although the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) has been targeted in mice, CF mice fail to develop lung or pancreatic disease like that in humans. In many respects, the anatomy, biochemistry, physiology, size, and genetics of pigs resemble those of humans. Thus pigs with a targeted CFTR gene might provide a good model for CF. Here, we review aspects of porcine airways and lung that are relevant to CF.

Ionotropic GABA receptor expression in the lung during development

Cl(-) transport is essential for lung development. Because gamma-aminobutyric acid (GABA) receptors allow the flow of negatively-charged Cl(-) ions across the cell membrane, we hypothesized that the expression of ionotropic GABA receptors are regulated in the lungs during development. We identified 17 GABA receptor subunits in the lungs by real-time PCR. These subunits were categorized into four groups: Group 1 had high mRNA expression during fetal stages and low in adults; Group 2 had steady expression to adult stages with a slight up-regulation at birth; Group 3 showed an increasing expression from fetal to adult lungs; and Group 4 displayed irregular mRNA fluctuations. The protein levels of selected subunits were also determined by Western blots and some subunits had protein levels that corresponded to mRNA levels. Further studied subunits were primarily localized in epithelial cells in the developing lung with differential mRNA expression between isolated cells and whole lung tissues. Our results add to the knowledge of GABA receptor expression in the lung during development.

DETANO and nitrated lipids increase chloride secretion across lung airway cells

We investigated the cellular mechanisms by which nitric oxide (NO) increases chloride (Cl-) secretion across lung epithelial cells in vitro and in vivo. Addition of (Z)-1-[2-(2-aminoethyl)-N-(2-ammonioethyl) amino] diazen-1-ium-1, 2-diolate (DETANONOate [DETANO];1-1,000 microM) into apical compartments of Ussing chambers containing Calu-3 cells increased short-circuit currents (I(sc)) from 5.2 +/- 0.8 to 15.0 +/- 2.1 microA/cm(2) (X +/- 1 SE; n = 7; P < 0.001). NO generated from two nitrated lipids (nitrolinoleic and nitrooleic acids; 1-10 microM) also increased I(sc) by about 100%. Similar effects were noted across basolaterally, but not apically, permeabilized Calu-3 cells. None of these NO donors increased I(sc) in Calu-3 cells pretreated with 10 microM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (an inhibitor of soluble guanylyl cyclase). Scavenging of NO either prevented or reversed the increase of I(sc). These data indicate that NO stimulation of soluble guanylyl cyclase was sufficient and necessary for the increase of I(sc) via stimulation of the apical cystic fibrosis transmembrane regulator (CFTR). Both Calu-3 and alveolar type II (ATII) cells contained CFTR, as demonstrated by in vitro phosphorylation of immunoprecipitated CFTR by protein kinase (PK) A. PKGII (but not PKGI) phosphorylated CFTR immuniprecipitated from Calu-3 cells. Corresponding values in ATII cells were below the threshold of detection. Furthermore, DETANO, 8-Br-cGMP, or 8-(4-chlorophenylthio)-cGMP (up to 2 mM each) did not increase Cl- secretion across amiloride-treated ATII cells in vitro. Measurements of nasal potential differences in anesthetized mice showed that perfusion of the nares with DETANO activated glybenclamide-sensitive Cl- secretion. These findings suggest that small concentrations of NO donors may prove beneficial in stimulating Cl- secretion across airway cells without promoting alveolar edema.

Volumetric and ionic regulation during the in vitro development of a corneal endothelial barrier

Corneal endothelium is responsible for generating an ion flux between the corneal stroma and the anterior chamber of the eye that is necessary for the cornea to remain transparent. However, the ion transport regulatory mechanisms that develop during the formation of the endothelial barrier are not known. In this study, we determined the influence of cell confluence on cell volume and intracellular ionic content on the corneal endothelial cells of rabbits. Our results demonstrate that non-confluent endothelial cells display a hypertrophic volume increase, with higher intracellular contents of potassium and chlorine than those of confluent cells. In contrast, when cells reach confluence and the endothelial barrier forms, cell volume decreases and the intracellular contents of potassium and chlorine decrease. Our genetic analysis showed a higher expression of CFTR and CA2 genes in non-confluent cells, and of the gene KCNC3 in confluent cells. These results suggest that the normal ionic current that keeps the corneal stroma dehydrated and transparent is regulated by cell-cell contacts and endothelial cell confluence, and could explain why the loss of corneal endothelial cells is often associated with corneal edema and even blindness.

Electrophysiological characterization of rat type II pneumocytes in situ

Optimal aeration of the lungs is dependent on an alveolar fluid clearance, a process that is governed by Na+ and Cl- transport. However, the specific contribution of various ion channels in different alveolar cell types under basal or stimulated conditions is not exactly known. We established a novel functional model of rat lung slices suitable for nystatin-perforated whole-cell patch-clamp experiments. Lung slices retained a majority of live cells for up to 72 hours. Type II pneumocytes in situ had a mean capacitance of 8.8 +/- 2.5 pF and a resting membrane potential of -4.4 +/- 1.9 mV. Bath replacement of Na+ with NMDG+ decreased inward whole-cell currents by 70%, 21% and 52% of which were sensitive to 10 microM and 1 mM of amiloride, respectively. Exposure of slices to 0.5 microM dexamethasone for 1 hour did not affect ion currents, while chronic exposure (0.5 microM, 24-72 h) induced an increase in both total Na+-entry currents and amiloride-sensitive currents. Under acute exposure to 100 microM cpt-cAMP, Type II cells in situ rapidly hyperpolarized by 25-30 mV, due to activation of whole-cell Cl- currents sensitive to 0.1 mM of 5-Nitro-2-(3-phenylpropylamino)benzoic acid. In addition, in the presence of cpt-cAMP, total sodium currents and currents sensitive to 10 microM amiloride increased by 32% and 70%, respectively. Thus, in Type II pneumocytes in situ: (1) amiloride-sensitive sodium channels contribute to only half of total Na+-entry and are stimulated by chronic exposure to glucocorticoids; (2) acute increase in cellular cAMP content simultaneously stimulates the entry of Cl- and Na+ ions.

Alveolar, but not bronchial nitric oxide production is elevated in cystic fibrosis

Exhaled nitric oxide (NO) remains a promising non-invasive marker for measuring inflammation in lung diseases. In cystic fibrosis (CF), exhaled NO measured at a single expiratory flow has been found to be normal or low. However, this measure cannot localize the anatomical site of NO production. The aims of this study were to apply a multiple-flow NO analysis to compare alveolar NO concentration and bronchial NO flux in CF children with healthy controls. Twenty-two children with CF and 17 healthy controls had exhaled NO measured at four different expiratory flows to calculate bronchial NO flux and alveolar NO concentration. Median (range) alveolar NO concentration was 2.2 (0.6-5.6) ppb for children with CF and 1.5 (0.4-2.6) ppb for healthy controls. Median (range) bronchial NO flux was 445 (64-1,256) pL/sec for children with CF and 509 (197-1,913) pL/sec for healthy controls. Children with CF had a significantly higher alveolar NO concentration, but no significant difference in bronchial NO flux compared to healthy children. In conclusion, children with CF have increased alveolar NO production, but not bronchial NO flux compared to healthy controls. The distal airway is a major site of inflammation in CF, and measuring alveolar NO may be a marker of distal inflammation in this disease.

Involvement of KATP and KvLQT1 K+ channels in EGF-stimulated alveolar epithelial cell repair processes

Several respiratory diseases are associated with extensive damage of lung epithelia, and the regulatory mechanisms involved in their regeneration are not clearly defined. Growth factors released by epithelial cells or fibroblasts from injured lungs are important regulators of alveolar repair by stimulating cell motility, proliferation, and differentiation. In addition, K(+) channels regulate cell proliferation/migration and are coupled with growth factor signaling in several tissues. We decided to explore the hypothesis, never investigated before, that K(+) could play a prominent role in alveolar repair. We employed a model of mechanical wounding of rat alveolar type II epithelia, in primary culture, to study their response to injury. Wound healing was suppressed by one-half upon epidermal growth factor (EGF) titration with EGF-antibody (Ab) or erbB1/erbB2 tyrosine-kinase inhibition with AG-1478/AG-825. The addition of exogenous EGF slightly stimulated the alveolar wound healing and enhanced, by up to five times, alveolar cell migration measured in a Boyden-type chamber. Conditioned medium collected from injured alveolar monolayers also stimulated cell migration; this effect was abolished in the presence of EGF-Ab. The impact of K(+) channel modulators was examined in basal and EGF-stimulated conditions. Wound healing was stimulated by pinacidil, an ATP-dependent K(+) channel (K(ATP)) activator, which also increased cell migration, by twofold, in basal conditions and potentiated the stimulatory effect of EGF. K(ATP) or KvLQT1 inhibitors (glibenclamide, clofilium) reduced EGF-stimulated wound healing, cell migration, and proliferation. Finally, EGF stimulated K(ATP) and KvLQT1 currents and channel expression. In summary, stimulation of K(+) channels through autocrine activation of EGF receptors could play a crucial role in lung epithelia repair processes.

Alveolar epithelial transport in the adult lung

The alveolar surface comprises >99% of the internal surface area of the lungs. At birth, the fetal lung rapidly converts from a state of net fluid secretion, which is necessary for normal fetal lung development, to a state in which there is a minimal amount of alveolar liquid. The alveolar surface epithelium facing the air compartment is composed of TI and TII cells. The morphometric characteristics of both cell types are fairly constant over a range of mammalian species varying in body weight by a factor of approximately 50,000. From the conservation of size and shape across species, one may infer that both TI and TII cells also have important conserved functions. The regulation of alveolar ion and liquid transport has been extensively investigated using a variety of experimental models, including whole animal, isolated lung, isolated cell, and cultured cell model systems, each with their inherent strengths and weaknesses. The results obtained with different model systems and a variety of different species point to both interesting parallels and some surprising differences. Sometimes it has been difficult to reconcile results obtained with different model systems. In this section, the primary focus will be on aspects of alveolar ion and liquid transport under normal physiologic conditions, emphasizing newer data and describing evolving paradigms of lung ion and fluid transport. We will highlight some of the unanswered questions, outline the similarities and differences in results obtained with different model systems, and describe some of the complex and interweaving regulatory networks.

Acute lung injury edema fluid decreases net fluid transport across human alveolar epithelial type II cells

Most patients with acute lung injury (ALI) have reduced alveolar fluid clearance that has been associated with higher mortality. Several mechanisms may contribute to the decrease in alveolar fluid clearance. In this study, we tested the hypothesis that pulmonary edema fluid from patients with ALI might reduce the expression of ion transport genes responsible for vectorial fluid transport in primary cultures of human alveolar epithelial type II cells. Following exposure to ALI pulmonary edema fluid, the gene copy number for the major sodium and chloride transport genes decreased. By Western blot analyses, protein levels of alphaENaC, alpha1Na,K-ATPase, and cystic fibrosis transmembrane conductance regulator decreased as well. In contrast, the gene copy number for several inflammatory cytokines increased markedly. Functional studies demonstrated that net vectorial fluid transport was reduced for human alveolar type II cells exposed to ALI pulmonary edema fluid compared with plasma (0.02 +/- 0.05 versus 1.31 +/- 0.56 microl/cm2/h, p < 0.02). An inhibitor of p38 MAPK phosphorylation (SB202190) partially reversed the effects of the edema fluid on net fluid transport as well as gene and protein expression of the main ion transporters. In summary, alveolar edema fluid from patients with ALI induced a significant reduction in sodium and chloride transport genes and proteins in human alveolar epithelial type II cells, effects that were associated with a decrease in net vectorial fluid transport across human alveolar type II cell monolayers.

Alveolar edema fluid clearance and acute lung injury

Although lung-protective ventilation strategies have substantially reduced mortality of acute lung injury patients there is still a need for new therapies that can further decrease mortality in patients with acute lung injury. Studies of epithelial ion and fluid transport across the distal pulmonary epithelia have provided important new concepts regarding potential new therapies for acute lung injury. Overall, there is convincing evidence that the alveolar epithelium is not only a tight epithelial barrier that resists the movement of edema fluid into the alveoli, but it is also actively involved in the transport of ions and solutes, a process that is essential for edema fluid clearance and the resolution of acute lung injury. The objective of this article is to consider some areas of recent progress in the field of alveolar fluid transport under normal and pathologic conditions. Vectorial ion transport across the alveolar and distal airway epithelia is the primary determinant of alveolar fluid clearance. The general paradigm is that active Na(+) and Cl(-) transport drives net alveolar fluid clearance, as demonstrated in several different species, including the human lung. Although these transport processes can be impaired in severe lung injury, multiple experimental studies suggest that upregulation of Na(+) and Cl(-) transport might be an effective therapy in acute lung injury. We will review mechanisms involved in pharmacological modulation of ion transport in lung injury with a special focus on the use of beta-adrenergic agonists which has generated considerable interest and is a promising therapy for clinical acute lung injury.

A pure population of lung alveolar epithelial type II cells derived from human embryonic stem cells

Alveolar epithelial type II (ATII) cells are small, cuboidal cells that constitute approximately 60% of the pulmonary alveolar epithelium. These cells are crucial for repair of the injured alveolus by differentiating into alveolar epithelial type I cells. ATII cells derived from human ES (hES) cells are a promising source of cells that could be used therapeutically to treat distal lung diseases. We have developed a reliable transfection and culture procedure, which facilitates, via genetic selection, the differentiation of hES cells into an essentially pure (>99%) population of ATII cells (hES-ATII). Purity, as well as biological features and morphological characteristics of normal ATII cells, was demonstrated for the hES-ATII cells, including lamellar body formation, expression of surfactant proteins A, B, and C, alpha-1-antitrypsin, and the cystic fibrosis transmembrane conductance receptor, as well as the synthesis and secretion of complement proteins C3 and C5. Collectively, these data document the successful generation of a pure population of ATII cells derived from hES cells, providing a practical source of ATII cells to explore in disease models their potential in the regeneration and repair of the injured alveolus and in the therapeutic treatment of genetic diseases affecting the lung.

Regulation of ENaC and CFTR expression with K+channel modulators and effect on fluid absorption across alveolar epithelial cells

In a recent study (Leroy C, Dagenais A, Berthiaume Y, and Brochiero E. Am J Physiol Lung Cell Mol Physiol 286: L1027–L1037, 2004), we identified an ATP-sensitive K+(KATP) channel in alveolar epithelial cells, formed by inwardly rectifying K+channel Kir6.1/sulfonylurea receptor (SUR)2B subunits. We found that short applications of KATP, voltage-dependent K+channel KvLQT1, and calcium-activated K+(KCa) channel modulators modified Na+and Cl−currents in alveolar monolayers. In addition, it was shown previously that a KATPopener increased alveolar liquid clearance in human lungs by a mechanism possibly related to epithelial sodium channels (ENaC). We therefore hypothesized that prolonged treatment with K+channel modulators could induce a sustained regulation of ENaC activity and/or expression. Alveolar monolayers were treated for 24 h with inhibitors of KATP, KvLQT1, and KCachannels identified by PCR. Glibenclamide and clofilium (KATPand KvLQT1 inhibitors) strongly reduced basal transepithelial current, amiloride-sensitive Na+current, and forskolin-activated Cl−currents, whereas pinacidil, a KATPactivator, increased them. Interestingly, K+inhibitors or membrane depolarization (induced by valinomycin in high-K+medium) decreased α-, β-, and γ-ENaC and CFTR mRNA. α-ENaC and CFTR proteins also declined after glibenclamide or clofilium treatment. Conversely, pinacidil augmented ENaC and CFTR mRNAs and proteins. Since alveolar fluid transport was found to be driven, at least in part, by Na+transport through ENaC, we tested the impact of K+channel modulators on fluid absorption across alveolar monolayers. We found that glibenclamide and clofilium reduced fluid absorption to a level similar to that seen in the presence of amiloride, whereas pinacidil slightly enhanced it. Long-term regulation of ENaC and CFTR expression by K+channel activity could benefit patients with pulmonary diseases affecting ion transport and fluid clearance.

Src kinase Lyn is crucial for Pseudomonas aeruginosa internalization into lung cells

Lyn is an important B cell signaling kinase of the Src tyrosine kinase family with a broad range of functions from cytoskeletal changes to induction of apoptosis. However, the role of Lyn in infectious diseases is not clear. Here, we demonstrate that Lyn activation by phosphorylation significantly impacted invasion of an alveolar epithelial cell line, primary lung cells, and rat lungs by Pseudomonas aeruginosa (PA), a common opportunistic lung pathogen affecting individuals with deficient lung immunity. Our results indicate that activation of Lyn and its interaction with rafts and TLR2, played an important role in the initial stages of PA interaction with host cells. The role of Lyn was further evaluated using the pharmacologic Src-specific inhibitor PP2, a dominant negative mutant, and finally confirmed with Lyn-deficient (Lyn(-/-)) bone marrow-derived mast cells. Inhibition of Lyn's function by above approaches prevented PA internalization. Moreover, blocking of Lyn also affected downstream events: induction of inflammatory cytokines and apoptosis. This report brings out a new role of Lyn in infectious diseases and indicates potential new targets for prevention and treatment of infections.

Regulation of chemokine expression by NaCl occurs independently of cystic fibrosis transmembrane conductance regulator in macrophages

Chronic pulmonary inflammation and infection are the leading causes of morbidity and mortality in cystic fibrosis (CF). While the effect of mutation of the cystic fibrosis transmembrane conductance regulator (CFTR) on airways remains controversial, some groups have demonstrated increases in Na(+) and Cl(-) in CF airway surface liquid compared to normal airways. We investigated the consequences of NaCl on pro-inflammatory chemokine and cytokine production by macrophages. Stimulation of mouse macrophages with increasing amounts of NaCl induced macrophage inflammatory protein-2 (MIP-2) and tumor necrosis factor-alpha (TNF-alpha) production. Further, co-incubation of macrophages with NaCl in the presence of either lipopolysaccharide (LPS) or TNF-alpha synergistically increased MIP-2 production. Both the NaCl and NaCl plus LPS responses were partially dependent on endogenous production and autocrine signaling by TNF-alpha. To investigate the role of CFTR in MIP-2 production, we compared the responses of wild-type and DeltaF508 CF mouse macrophages to NaCl and LPS. The responses of macrophages from both strains were indistinguishable. In addition, CFTR mRNA was not expressed in macrophages. Taken together, these findings suggest that NaCl stimulates MIP-2 production by macrophages through a mechanism that is partially dependent on TNF-alpha but independent of macrophage CFTR expression.

Dexamethasone inhibits the action of TNF on ENaC expression and activity

We have reported that TNF, a proinflammatory cytokine present in several lung pathologies, decreases the expression and activity of the epithelial Na(+) channel (ENaC) by approximately 70% in alveolar epithelial cells. Because dexamethasone has been shown to upregulate ENaC mRNA expression and is well known to downregulate proinflammatory genes, we tested if it could alleviate the effect of TNF on ENaC expression and activity. In cotreatment with TNF, we found that dexamethasone reversed the inhibitory effect of TNF and upregulated alpha, beta, and gammaENaC mRNA expression. When the cells were pretreated for 24 h with TNF before cotreatment, dexamethasone was still able to increase alphaENaC mRNA expression to 1.8-fold above control values. However, in these conditions, beta and gammaENaC mRNA expression was reduced to 47% and 14%, respectively. The potential role of TNF and dexamethasone on alphaENaC promoter activity was tested in A549 alveolar epithelial cells. TNF decreased luciferase (Luc) expression by approximately 25% in these cells, indicating that the strong diminution of alphaENaC mRNA must be related to posttranscriptional events. Dexamethasone raised Luc expression by fivefold in the cells and augmented promoter activity by 2.77-fold in cotreatment with TNF. In addition to its effect on alphaENaC gene expression, dexamethasone was able to maintain amiloride-sensitive current as well as the liquid clearance abilities of TNF-treated cells within the normal range. All these results suggest that dexamethasone alleviates the downregulation of ENaC expression and activity in TNF-treated alveolar epithelial cells.

Functional ion channels in pulmonary alveolar type I cells support a role for type I cells in lung ion transport

Efficient gas exchange in the lungs depends on regulation of the amount of fluid in the thin (average 0.2 mum) liquid layer lining the alveolar epithelium. Fluid fluxes are regulated by ion transport across the alveolar epithelium, which is composed of alveolar type I (TI) and type II (TII) cells. The accepted paradigm has been that TII cells, which cover <5% of the internal surface area of the lung, transport Na(+) and Cl(-) and that TI cells, which cover >95% of the surface area, provide a route for water absorption. Here we present data that TI cells contain functional epithelial Na(+) channels (ENaC), pimozide-sensitive cation channels, K(+) channels, and the cystic fibrosis transmembrane regulator. TII cells contain ENaC and cystic fibrosis transmembrane regulator, but few pimozide-sensitive cation channels. These findings lead to a revised paradigm of ion and water transport in the lung in which (i) Na(+) and Cl(-) transport occurs across the entire alveolar epithelium (TI and TII cells) rather than only across TII cells; and (ii) by virtue of their very large surface area, TI cells are responsible for the bulk of transepithelial Na(+) transport in the lung.

Contribution of CFTR to apical-basolateral fluid transport in cultured human alveolar epithelial type II cells

Previous studies in intact lung suggest that CFTR may play a role in cAMP-regulated fluid transport from the distal air spaces of the lung. However, the potential contribution of different epithelial cells (alveolar epithelial type I, type II, or bronchial epithelial cells) to CFTR-regulated fluid transport is unknown. In this study we determined whether the CFTR gene is expressed in human lung alveolar epithelial type II (AT II) cells and whether the CFTR chloride channel contributes to cAMP-regulated fluid transport in cultured human AT II cells. Human AT II cells were isolated and cultured on collagen I-coated Transwell membranes for 120–144 h with an air-liquid interface. The cultured cells retained typical AT II-like features based on morphologic studies. Net basal fluid transport was 0.9 ± 0.1 μl·cm−2·h−1and increased to 1.35 ± 0.11 μl·cm−2·h−1(mean ± SE, n = 18, P < 0.05) by stimulation with cAMP agonists. The CFTR inhibitor, CFTRinh-172, inhibited cAMP stimulated but not basal fluid transport. In short-circuit current ( Isc) studies with an apical-to-basolateral transepithelial Cl−gradient, apical application of CFTRinh-172 reversed the forskolin-induced decrease in Isc. Real time RT-PCR demonstrated CFTR transcript expression in human AT II cells at a level similar to that in airway epithelial cells. We conclude that CFTR is expressed in cultured human AT II cells and may contribute to cAMP-regulated apical-basolateral fluid transport.