Airway epithelial cells: 'Bicarbonate in' ≠ 'Bicarbonate out'

Transcriptome Analysis Identifies Doublesex and Mab-3 Related Transcription Factor (DMRT3) in Nasal Polyp Epithelial Cells of Patients Suffering from Non-Steroidal Anti-Inflammatory Drug-Exacerbated Respiratory Disease (AERD)

Background: Aspirin-exacerbated respiratory disease (AERD) is a syndrome characterised by chronic rhinosinusitis, nasal polyps, asthma and aspirin intolerance. An imbalance of eicosanoid metabolism with anover-production of cysteinyl leukotrienes (CysLTs) has been associated with AERD. However, the precise mechanisms underlying AERD are unknown. Objective: To establish the transcriptome of the nasal polyp airway epithelial cells derived from AERD patients to discover gene expression patterns in this disease. Methods: Nasal airway epithelial cells were isolated from 12 AERD polyps and 8 AERD non-polyp nasal mucosa samples as controls from the same subjects. Utilising the Illumina HiSeq 2500 platform, RNA samples were sequenced. Potential gene candidate DMRT3 was selected from the differentially-expressed genes for validation. Results: Comparative transcriptome profiling of nasal epithelial cells was accomplished in AERD. A total of 20 genes had twofold mean regulation expression differences or greater. In addition, 8 genes were upregulated, including doublesex and mab-3 related transcription factor 3 (DMRT3), and 12 genes were downregulated. Differentially regulated genes comprised roles in inflammation, defence and immunity. Metabolic process and embryonic development pathways were significantly enriched. Enzyme-linked immune sorbent assay (ELISA) results of DMRT3 in AERD patients were significantly upregulated compared to controls (p = 0.03). Immunohistochemistry (IHC) of AERD nasal polyps localised DMRT3 and was predominantly released in the airway epithelia. Conclusion: Findings suggest that DMRT3 could be potentially involved in nasal polyp development in AERD patients. Furthermore, several genes are downregulated, hinting at the dedifferentiation phenomenon in AERD polyps. However, further studies are imperative to confirm the exact mechanism of polyp formation in AERD patients.

Is CFTR an exchanger?: Regulation of HCO3 -Transport and extracellular pH by CFTR

The disease, cystic fibrosis, is caused by the malfunction of the cystic fibrosis transmembrane conductance regulator. Expression of functional CFTR may normally regulate extracellular pH via control of bicarbonate efflux. Reports also suggest that the CFTR may be a Cl-/HCO3- exchanger. If true, this could be very important for treatment of CF given the airway host defense system is quite sensitive to pH, and acidic pH been found to increase mucus viscosity. We compared evidentiary support of four possible models of CFTR's role in the transport of bicarbonate: 1) CFTR as a Cl-channel that permits bicarbonate conductance, 2) CFTR as an anion Cl-/HCO3- exchanger (AE), 3.) CFTR as both a Cl-channel and an AE, and 4.) CFTR as a Cl-channel that allows for transport of bicarbonate and regulates an independent AE. The effect of stimulators and inhibitors of CFTR and AEs were evaluated via iodide efflux and studies of extracellular pH. This data, as well as that published by others, suggest that while CFTR may support and regulate bicarbonate flux it is unlikely it directly performs Cl-/HCO3- anion exchange.

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Four possible models of CFTR's role in the transport of HC03- were evaluated.

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Cells exposed to modulators of CFTR/AE activity were studied by efflux and pHo.

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Efflux: While CFTR inhibition reduced anion efflux, AE inhibition did not.

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pHo: In Cl-free buffer conditions CFTR activation reduced pHo acidification.

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CFTR may regulate but unlikely it directly performs Cl-/HCO3- exchange.

Extracellular pH and lung infections in cystic fibrosis

Cystic fibrosis (CF) is an autosomal recessive disease caused by CFTR mutations. It is characterized by high NaCl concentration in sweat and the production of a thick and sticky mucus, occluding secretory ducts, intestine and airways, accompanied by chronic inflammation and infections of the lungs. This causes a progressive and lethal decline in lung function. Therefore, finding the mechanisms driving the high susceptibility to lung infections has been a key issue. For decades the prevalent hypothesis was that a reduced airway surface liquid (ASL) volume and composition, and the consequent increased mucus concentration (dehydration), create an environment favoring infections. However, a few years ago, in a pig model of CF, the Na⁺/K⁺ concentrations and the ASL volume were found intact. Immediately a different hypothesis arose, postulating a reduced ASL pH as the cause for the increased susceptibility to infections, due to a diminished bicarbonate secretion through CFTR. Noteworthy, a recent report found normal ASL pH values in CF children and in cultured primary airway cells, challenging the ASL pH hypothesis. On the other hand, recent evidences revitalized the hypothesis of a reduced ASL secretion. Thus, the role of the ASL pH in the CF is still a controversial matter. In this review we discuss the basis that sustain the role of CFTR in modulating the extracellular pH, and the recent results sustaining the different points of view. Finding the mechanisms of CFTR signaling that determine the susceptibility to infections is crucial to understand the pathophysiology of CF and related lung diseases.

Development of an airway mucus defect in the cystic fibrosis rat

The mechanisms underlying the development and natural progression of the airway mucus defect in cystic fibrosis (CF) remain largely unclear. New animal models of CF, coupled with imaging using micro-optical coherence tomography, can lead to insights regarding these questions. The Cftr-/- (KO) rat allows for longitudinal examination of the development and progression of airway mucus abnormalities. The KO rat exhibits decreased periciliary depth, hyperacidic pH, and increased mucus solid content percentage; however, the transport rates and viscoelastic properties of the mucus are unaffected until the KO rat ages. Airway submucosal gland hypertrophy develops in the KO rat by 6 months of age. Only then does it induce increased mucus viscosity, collapse of the periciliary layer, and delayed mucociliary transport; stimulation of gland secretion potentiates this evolution. These findings could be reversed by bicarbonate repletion but not pH correction without counterion donation. These studies demonstrate that abnormal surface epithelium in CF does not cause delayed mucus transport in the absence of functional gland secretions. Furthermore, abnormal bicarbonate transport represents a specific target for restoring mucus clearance, independent of effects on periciliary collapse. Thus, mature airway secretions are required to manifest the CF defect primed by airway dehydration and bicarbonate deficiency.

Role of anion exchangers in Cl− and HCO3− secretion by the human airway epithelial cell line Calu-3

Despite the importance of airway surface liquid pH in the lung's defenses against infection, the mechanism of airway HCO3− secretion remains unclear. Our aim was to assess the contribution of apical and basolateral Cl−/HCO3− exchangers to Cl− and HCO3− transport in the Calu-3 cell line, derived from human airway submucosal glands. Changes in intracellular pH (pHi) were measured following substitution of Cl− with gluconate. Apical Cl− substitution led to an alkalinization in forskolin-stimulated cells, indicative of Cl−/HCO3− exchange. This was unaffected by the anion exchange inhibitor DIDS but inhibited by the CFTR blocker CFTRinh-172, suggesting that the HCO3− influx might occur via CFTR, rather than a solute carrier family 26 (SLC26) exchanger, as recently proposed. The anion selectivity of the recovery process more closely resembled that of CFTR than an SLC26 exchanger, and quantitative RT-PCR showed only low levels of SLC26 exchanger transcripts relative to CFTR and anion exchanger 2 (AE2). For pHi to rise to observed values (∼7.8) through HCO3− entry via CFTR, the apical membrane potential must reverse to at least +20 mV following Cl− substitution; this was confirmed by perforated-patch recordings. Substitution of basolateral Cl− evoked a DIDS-sensitive alkalinization, attributed to Cl−/HCO3− exchange via AE2. This appeared to be abolished in forskolin-stimulated cells but was unmasked by blocking apical efflux of HCO3− via CFTR. We conclude that Calu-3 cells secrete HCO3− predominantly via CFTR, and, contrary to previous reports, the basolateral anion exchanger AE2 remains active during stimulation, providing an important pathway for basolateral Cl− uptake.