“…Further, alternative CF-relevant inflammatory pathways also increase pendrin expression in freshly isolated airway epithelial cells from CF subjects (23). There is considerable interest in identifying ion transport modulators to increase ASL depth in CF, with ENaC inhibitors and activators of CaCC under development (47). The current study confirms that pendrin is an alternative target for ASL rehydration.…”
Pendrin (SLC26A4) is a Cl 2 /anion exchanger expressed in the epithelium of inflamed airways where it is thought to facilitate Cl 2 absorption and HCO 3 2 secretion. Studies using pendrin knockout mice and airway epithelial cells from hearing-impaired subjects with pendrin loss of function suggest involvement of pendrin in inflammatory lung diseases, including cystic fibrosis (CF), perhaps by regulation of airway surface liquid (ASL) volume. Here we identified small-molecule pendrin inhibitors and demonstrated their efficacy in increasing ASL volume. A cell-based, functional high-throughput screen of ∼36,000 synthetic small molecules produced 3 chemical classes of inhibitors of human pendrin. After structure-activity studies, tetrahydropyrazolopyridine and pyrazolothiophenesulfonamide compounds reversibly inhibited pendrin-facilitated Cl 2 exchange with SCN 2 , I 2 , NO 3 2 , and HCO 3 2 with drug concentration causing 50% inhibition down to ∼2.5 mM. In well-differentiated primary cultures of human airway epithelial cells from non-CF and CF subjects, treatment with IL-13, which causes inflammation with strong pendrin up-regulation, strongly increased Cl 2 /HCO 3 2 exchange and the increase was blocked by pendrin inhibition. Pendrin inhibition significantly increased ASL depth (by ∼8 mm) in IL-13-treated non-CF and CF cells but not in untreated cells. These studies implicate the involvement of pendrin-facilitated Cl 2 / HCO 3 2 in the regulation of ASL volume and suggest the utility of pendrin inhibitors in inflammatory lung diseases, including CF
“…Further, alternative CF-relevant inflammatory pathways also increase pendrin expression in freshly isolated airway epithelial cells from CF subjects (23). There is considerable interest in identifying ion transport modulators to increase ASL depth in CF, with ENaC inhibitors and activators of CaCC under development (47). The current study confirms that pendrin is an alternative target for ASL rehydration.…”
Pendrin (SLC26A4) is a Cl 2 /anion exchanger expressed in the epithelium of inflamed airways where it is thought to facilitate Cl 2 absorption and HCO 3 2 secretion. Studies using pendrin knockout mice and airway epithelial cells from hearing-impaired subjects with pendrin loss of function suggest involvement of pendrin in inflammatory lung diseases, including cystic fibrosis (CF), perhaps by regulation of airway surface liquid (ASL) volume. Here we identified small-molecule pendrin inhibitors and demonstrated their efficacy in increasing ASL volume. A cell-based, functional high-throughput screen of ∼36,000 synthetic small molecules produced 3 chemical classes of inhibitors of human pendrin. After structure-activity studies, tetrahydropyrazolopyridine and pyrazolothiophenesulfonamide compounds reversibly inhibited pendrin-facilitated Cl 2 exchange with SCN 2 , I 2 , NO 3 2 , and HCO 3 2 with drug concentration causing 50% inhibition down to ∼2.5 mM. In well-differentiated primary cultures of human airway epithelial cells from non-CF and CF subjects, treatment with IL-13, which causes inflammation with strong pendrin up-regulation, strongly increased Cl 2 /HCO 3 2 exchange and the increase was blocked by pendrin inhibition. Pendrin inhibition significantly increased ASL depth (by ∼8 mm) in IL-13-treated non-CF and CF cells but not in untreated cells. These studies implicate the involvement of pendrin-facilitated Cl 2 / HCO 3 2 in the regulation of ASL volume and suggest the utility of pendrin inhibitors in inflammatory lung diseases, including CF
“…On the other hand, the recent generation of CF pigs (both CFTR−/− and F508del) do not show any increased Na + or water absorption compared to wild-type animals, in spite of an increased amiloride-sensitive voltage, and short-circuit current, observed in airway epithelial cultures from these animals [44]. Although the direct or indirect regulatory effect of CFTR on ENaC still needs to be confirmed, several groups are pursuing ENaC inhibitors as a therapeutic approach to increase ASL depth, and the available data show that interfering with ENaC activity could potentially be beneficial in CF to rehydrate the ASL [45]. Fig.…”
Salt and fluid absorption and secretion are two processes that are fundamental to epithelial function and whole body fluid homeostasis, and as such are tightly regulated in epithelial tissues. The CFTR anion channel plays a major role in regulating both secretion and absorption in a diverse range of epithelial tissues, including the airways, the GI and reproductive tracts, sweat and salivary glands. It is not surprising then that defects in CFTR function are linked to disease, including life-threatening secretory diarrhoeas, such as cholera, as well as the inherited disease, cystic fibrosis (CF), one of the most common life-limiting genetic diseases in Caucasian populations. More recently, CFTR dysfunction has also been implicated in the pathogenesis of acute pancreatitis, chronic obstructive pulmonary disease (COPD), and the hyper-responsiveness in asthma, underscoring its fundamental role in whole body health and disease. CFTR regulates many mechanisms in epithelial physiology, such as maintaining epithelial surface hydration and regulating luminal pH. Indeed, recent studies have identified luminal pH as an important arbiter of epithelial barrier function and innate defence, particularly in the airways and GI tract. In this chapter, we will illustrate the different operational roles of CFTR in epithelial function by describing its characteristics in three different tissues: the airways, the pancreas, and the sweat gland.
“…In CF, loss of function of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel is possibly paralleled by ENaC dysregulation, consisting of enhanced activity that contributes to airway surface dehydration. Although there is some controversy about the upregulation of ENaC in CF (Chen et al., ; Itani et al., ), it is clear that inhibition of ENaC function is a valid therapeutic option to counteract CFTR defects in CF (Åstrand et al., ; Donaldson & Galietta, ).…”
New Findings
What is the central question of this study?What is the precise subcellular localization of the epithelial sodium channel (ENaC) in human airway epithelium?
What is the main finding and its importance?ENaC protein has an unexpected localization in the peripheral region of the apical membrane of bronchial epithelial cells, very close to tight junctions. This may be important for the mechanism of Na+ absorption
Abstract
The epithelial sodium channel (ENaC) has a key role in absorbing fluid across the human airway epithelium. Altered activity of ENaC may perturb the process of mucociliary clearance, thus impairing the innate defence mechanisms against microbial agents. The proteins forming ENaC are present on the apical membrane of the epithelium. However, their precise localization is unknown. In the present study, we used two antibodies recognizing the α and β ENaC subunits. Both antibodies revealed a restricted localization of ENaC in the peripheral region of the apical membrane of cultured bronchial epithelial cells, close to but not overlapping with tight junctions. In contrast, the cystic fibrosis transmembrane conductance regulator chloride channel was more diffusely expressed on the whole apical membrane. Modulation of ENaC activity by aprotinin or elastase resulted in a decrease or increase in the peripheral localization, respectively. Our results suggest that sodium absorption is mainly occurring close to tight junctions where this cation may be rapidly expelled by the Na+/K+ pump present in lateral membranes. This arrangement of channels and pumps may limit Na+ build‐up in other regions of the cells.
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