Studies in cystic fibrosis patients and mice overexpressing the epithelial Na+ channel β-subunit (βENaC-Tg) suggest that raised airway Na+ transport and airway surface liquid (ASL) depletion are central to the pathogenesis of cystic fibrosis lung disease. However,
patients or mice with Liddle gain-of-function βENaC mutations exhibit hypertension but no lung disease. To investigate this
apparent paradox, we compared the airway phenotype (nasal versus tracheal) of Liddle with CFTR-null, βENaC-Tg, and double mutant mice. In mouse nasal epithelium, the region that functionally
mimics human airways, high levels of CFTR expression inhibited Liddle epithelial Nat channel (ENaC) hyperfunction. Conversely,
in mouse trachea, low levels of CFTR failed to suppress Liddle ENaC hyperfunction. Indeed, Na+ transport measured in Ussing chambers (“flooded” conditions) was raised in both Liddle and βENaC-Tg mice. Because enhanced
Na+ transport did not correlate with lung disease in these mutant mice, measurements in tracheal cultures under physiologic “thin
film” conditions and in vivo were performed. Regulation of ASL volume and ENaC-mediated Na+ absorption were intact in Liddle but defective in βENaC-Tg mice. We conclude that the capacity to regulate Na+ transport and ASL volume, not absolute Na+ transport rates in Ussing chambers, is the key physiologic function protecting airways from dehydration-induced lung disease.
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"In contrast, an increase in Na + absorption by ENaC overexpression in a genetic mouse model leads to a cystic fibrosis lung phenotype (Mall et al., 2010). In airway epithelia of cystic fibrosis patients, the loss of cystic fibrosis transmembrane conductance regulator (CFTR) activity is accompanied by an increased ENaC activity resulting in Na + hyperabsorption, suggesting that ENaC upregulation may contribute to the cystic fibrosis lung phenotype (Mall et al., 2010; Hobbs et al., 2013). A cystic fibrosis-like lung phenotype, however, has not been consistently observed in Liddle patients. "
[Show abstract][Hide abstract]ABSTRACT: The epithelial Na(+) channel (ENaC) and the acid-sensing ion channels (ASICs) form subfamilies within the ENaC/degenerin family of Na(+) channels. ENaC mediates transepithelial Na(+) transport, thereby contributing to Na(+) homeostasis and the maintenance of blood pressure and the airway surface liquid level. ASICs are H(+)-activated channels found in central and peripheral neurons, where their activation induces neuronal depolarization. ASICs are involved in pain sensation, the expression of fear, and neurodegeneration after ischemia, making them potentially interesting drug targets. This review summarizes the biophysical properties, cellular functions, and physiologic and pathologic roles of the ASIC and ENaC subfamilies. The analysis of the homologies between ENaC and ASICs and the relation between functional and structural information shows many parallels between these channels, suggesting that some mechanisms that control channel activity are shared between ASICs and ENaC. The available crystal structures and the discovery of animal toxins acting on ASICs provide a unique opportunity to address the molecular mechanisms of ENaC and ASIC function to identify novel strategies for the modulation of these channels by pharmacologic ligands.
Preview · Article · Jan 2015 · Pharmacological reviews
"Dysregulation of ENaC results in increased Na+ flux and an increase in fluid absorption in isolated murine trachea overexpressing β-ENaC under thin film conditions. Similarly, studies of fluid secretion using isolated pig and human trachea and specific channel blockers for CFTR and ENaC demonstrate that both channels contribute to secretion and ASL fluid maintenance , , . Thus, either inhibition or hyper-activation of these channels would potentially alter fluid balance in the airway . "
[Show abstract][Hide abstract]ABSTRACT: The serralysin family of metalloproteases is associated with the virulence of multiple gram-negative human pathogens, including Pseudomonas aeruginosa and Serratia marcescens. The serralysin proteases share highly conserved catalytic domains and show evolutionary similarity to the mammalian matrix metalloproteases. Our previous studies demonstrated that alkaline protease (AP) from Pseudomonas aeruginosa is capable of activating the epithelial sodium channel (ENaC), leading to an increase in sodium absorption in airway epithelia. The serralysin proteases are often co-expressed with endogenous, intracellular or periplasmic inhibitors, which putatively protect the bacterium from unwanted or unregulated protease activities. To evaluate the potential use of these small protein inhibitors in regulating the serralysin induced activation of ENaC, proteases from Pseudomonas aeruginosa and Serratia marcescens were purified for characterization along with a high affinity inhibitor from Pseudomonas. Both proteases showed activity against in vitro substrates and could be blocked by near stoichiometric concentrations of the inhibitor. In addition, both proteases were capable of activating ENaC when added to the apical surfaces of multiple epithelial cells with similar slow activation kinetics. The high-affinity periplasmic inhibitor from Pseudomonas effectively blocked this activation. These data suggest that multiple metalloproteases are capable of activating ENaC. Further, the endogenous, periplasmic bacterial inhibitors may be useful for modulating the downstream effects of the serralysin virulence factors under physiological conditions.
"One possible explanation (although likely not the only one) lies in the difference between CFTR expression levels in the lower airways of mice vs. humans. Unlike in humans, CFTR is only weakly expressed in the distal airways of mice (Mall et al., 2010), thus it may not be able adequately suppress the elevated levels of ENaC obtained upon overexpression of its β subunit (Mall et al., 2004) or upon knockout of Nedd4-2 (Kimura et al., 2011). "
[Show abstract][Hide abstract]ABSTRACT: Nedd4-2 is a ubiquitin ligase previously demonstrated to regulate endocytosis and lysosomal degradation of the epithelial Na(+) channel (ENaC) and other ion channels and transporters. Recent studies using Nedd4-2 knockout mice specifically in kidney or lung epithelia has revealed a critical role for this E3 ubiquitin ligase in regulating salt and fluid transport in these tissues/organs and in maintaining homeostasis of body blood pressure. Interestingly, the primary targets for Nedd4-2 may differ in these two organs: in the lung Nedd4-2 targets ENaC, and loss of Nedd4-2 leads to excessive ENaC function and to cystic fibrosis - like lung disease, whereas in the kidney, Nedd4-2 targets the Na(+)/Cl(-) cotransporter (NCC) in addition to targeting ENaC. In accord, loss of Nedd4-2 in the distal nephron leads to increased NCC abundance and function. The aldosterone-responsive kinase, Sgk1, appears to be involved in the regulation of NCC by Nedd4-2 in the kidney, similar to its regulation of ENaC. Collectively, these new findings underscore the physiological importance of Nedd4-2 in regulating epithelial salt and fluid transport and balance.
Preview · Article · Jun 2012 · Frontiers in Physiology