SPLUNC1 expression reduces surface levels of the epithelial sodium channel (ENaC) inXenopus laevisoocytes

Abstract

Throughout the body, the epithelial Na+ channel (ENaC) plays a critical role in salt and liquid homeostasis. In cystic fibrosis airways, for instance, improper regulation of ENaC results in hyperabsorption of sodium that causes dehydration of airway surface liquid. This dysregulation then contributes to mucus stasis and chronic lung infections. ENaC is known to undergo proteolytic cleavage, which is required for its ability to conduct Na+ ions. We have previously shown that the short, palate lung and nasal epithelial clone (SPLUNC1) binds to and inhibits ENaC in both airway epithelia and in Xenopus laevis oocytes. In this study, we found that SPLUNC1 was more potent at inhibiting ENaC than either SPLUNC2 or long PLUNC1 (LPLUNC1), two other PLUNC family proteins that are also expressed in airway epithelia. Furthermore, we were able to shed light on the potential mechanism of SPLUNC1's inhibition of ENaC. While SPLUNC1 did not inhibit proteolytic activity of trypsin, it significantly reduced ENaC currents by reducing the number of ENaCs in the plasma membrane. A better understanding of ENaC's regulation by endogenous inhibitors may aid in the development of novel therapies designed to inhibit hyperactive ENaC in cystic fibrosis epithelia.

Short Palate, Lung, and Nasal Epithelial Clone–1 Is a Tightly Regulated Airway Sensor in Innate and Adaptive Immunity

“…The rapid down-modulation and loss of SPLUNC1, as these data show, imply that the fluctuation of SPLUNC1 also plays a role in host defense. In addition to its antimicrobial functions, SPLUNC1 suppresses allergic airway inflammation, has surfactant properties, regulates the epithelial sodium channel ENaC that modulates the airway surface liquid, and enhances mucociliary clearance (10,23,24,38). These studies strongly suggested that the loss of SPLUNC1 during inflammation and infection is important in host defense.…”

Bactericidal/Permeability-Increasing Protein Fold–Containing Family Member A1 in Airway Host Protection and Respiratory Disease

“…Initial studies suggested that BPIFA1 regulated ENaC through inhibition of its cleavage by serine proteases present in the ASL. However, subsequent studies showed that BPIFA1 did not directly inhibit extracellular proteases but rather controlled ENaC function through multiple other mechanisms, including binding to ENaC and preventing its cleavage, reducing the amount of ENaC on the epithelial cell surface, and possibly by maintaining ENaC in a low-opening probability state (64,65). BPIFA1 modulates sodium transport through ENaC in a pH-dependent manner.…”

New Pulmonary Therapies Directed at Targets Other than CFTR

“…For example, SPLUNC1-a highly expressed, secreted protein-was found to bind to ENaC and protect it from proteolytic cleavage/activation (Garcia-Caballero et al 2009;Rollins et al 2010). While not possessing intrinsic antiprotease activity itself, alterations in the expression or binding of SPLUNC to ENaC could also result in constitutive activation of ENaC by endogenous proteases.…”