SPLUNC1 regulates airway surface liquid volume by protecting ENaC from proteolytic cleavage

Abstract

Many epithelia, including the superficial epithelia of the airways, are thought to secrete “volume sensors,” which regulate the volume of the mucosal lining fluid. The epithelial Na + channel (ENaC) is often the rate limiting factor in fluid absorption, and must be cleaved by extracellular and/or intracellular proteases before it can conduct Na + and absorb excess mucosal liquid, a process that can be blocked by proteases inhibitors. In the airways, airway surface liquid dilution or removal activates ENaC. Therefore, we hypothesized that endogenous proteases are membrane-anchored, whereas endogenous proteolysis inhibitors are soluble and can function as airway surface liquid volume sensors to inhibit ENaC activity. Using a proteomic approach, we identified short palate, lung, and nasal epithelial clone (SPLUNC)1 as a candidate volume sensor. Recombinant SPLUNC1 inhibited ENaC activity in both human bronchial epithelial cultures and Xenopus oocytes. Knockdown of SPLUNC1 by shRNA resulted in a failure of bronchial epithelial cultures to regulate ENaC activity and airway surface liquid volume, which was restored by adding recombinant SPLUNC1 to the airway surface liquid. Despite being able to inhibit ENaC, recombinant SPLUNC1 had little effect on extracellular serine protease activity. However, SPLUNC1 specifically bound to ENaC, preventing its cleavage and activation by serine proteases. SPLUNC1 is highly expressed in the airways, as well as in colon and kidney. Thus, we propose that SPLUNC1 is secreted onto mucosal surfaces as a soluble volume sensor whose concentration and dilution can regulate ENaC activity and mucosal volumes, including that of airway surface liquid.

An innate defense peptide BPIFA1/SPLUNC1 restricts influenza A virus infection

“…The mouse and human BPIFA1 sequences are homologous 8, 10, 11 . 7 Previous studies have demonstrated that BPIFA1 acts as a surfactant 12 , can 8 regulate the amiloride-sensitive epithelial sodium channel, ENaC 13 and affects 9 mucociliary clearance in the upper airways 14 . It has also been shown to have anti-10 bacterial roles.…”

SPLUNC1 Promotes Lung Innate Defense Against Mycoplasma pneumoniae Infection in Mice

“…In addition to its potential antimi-crobial function, SPLUNC1 regulates airway surface liquid volume in vitro. 7 Each year, approximately 2 million pneumonia cases due to infection with Mycoplasma pneumoniae (Mp), including 100,000 hospitalizations, are reported in the United States. 8 Mp is an atypical bacterium that lacks a cell wall and relies on adhesion to host cells for colonization and infection, which is generally restricted to airway epithelium.…”

SPLUNC1 Is a Significant Marker in Pleural Effusion from Lung Cancer Compared to Tuberculosis

“…(17) SPLUNC1 has also been shown to have an anti-inflammatory function in exogenous microbe-induced respiratory infection, such as K. pneumoniae, (18) Mycoplasma pneumoniae, (19) P. aeruginosa, and Epstein-Barr virus. (20) Additionally, SPLUNC1 could also regulate ion/mucus transport and hydration levels in the lung, (21) lower surface tension by its surfactant-like properties, (22) and act as an inflammatory mediator.…”