Proteases, ENaCs and cystic fibrosis.

Airway epithelia maintain a fluid cushion supporting a mucous layer that traps inhaled particulates. This fluid layer facilitates ciliary beating that propels mucus out of the airway. The height of this fluid cushion is carefully regulated by balancing rates of fluid secretion mediated by the cystic fibrosis transmembrane conductance regulator (CFTR) and other anion transporters, and fluid absorption mediated primarily by the epithelial Na+ channel (ENaC). Individuals with cystic fibrosis (CF) have reduced airway fluid secretion as a result of mutations that impair CFTR trafficking and/or gating, and also appear to have increased ENaC activity that enhances airway fluid absorption. The net result is a reduction in airway surface liquid volume and impaired mucociliary clearance (Hobbs et al. 2013). The increase in ENaC activity found in CF airway is thought to reflect an increase in channel open probability (Po). There are a number of factors that increase ENaC Po, including inositol phospholipids, extracellular acidification, and modification of channel subunits by palmitoylation and by proteolytic cleavage (Kashlan & Kleyman, 2011). Several of these factors may have a role in activating ENaCs in CF airway. For example, the reduced pH of CF human airway fluids would be predicted to increase ENaC Po. It has been suggested that enhanced ENaC proteolysis also contributes to channel activation in the setting of CF (Hobbs et al. 2013). How are ENaCs activated by proteases? These channels are composed of three structurally related subunits. Two of these subunits (α and γ) have short imbedded inhibitory tracts in their extracellular regions that can be released by proteases that cleave at sites flanking the tracts. As channels transit though the trans-Golgi network, the α subunit is cleaved twice by the serine protease furin, releasing an inhibitory tract and partially activating the channel. The γ subunit is cleaved once by furin at a site preceding its inhibitory tract. Subsequent cleavage by a second protease at a site distal to the tract transitions channels to a high Po state (Kleyman et al. 2009). There are an increasing number of proteases that can cleave the γ subunit and activate ENaCs, and some may be relevant in the CF airway. For example, there are high levels of elastase in the CF airway, and elastase can cleave and activate ENaCs. The work of Da Tan et al., published in this issue of The Journal of Physiology, provides another piece of this puzzle (Da Tan et al. 2014). They show that the cysteine protease cathepsin B is capable of activating ENaCs expressed in Xenopus oocytes, in agreement with previous work performed in a renal epithelial cell line (Alli et al. 2012). Furthermore, cathepsin B induced a shift in the size of a C-terminal (presumably furin cleaved) γ subunit fragment, consistent with cleavage at a site distal to the furin cleavage site. While the reported cathepsin B-induced shift in molecular mass (2.4 kDa) noted when channels were expressed in Xenopus oocytes might not be sufficient to disrupt or release the inhibitory tract, it is difficult to accurately assess small changes in molecular mass. The cathepsin B-induced shift in molecular mass (7.0 kDa) noted when channels were expressed in cells from the human embryonic kidney (HEK) cell line is consistent with the release of the inhibitory tract. Cathepsin B treatment did not alter the surface expression of wild-type channels in HEK cells, in agreement with channel activation being due to an increase in Po. Moreover, mutation of the α and γ subunit furin cleavage sites blunted channel activation by cathepsin B. The authors showed that cathepsin B is an acid-activated protease that is expressed at the apical membrane of normal and CF airway epithelia and cultured airway cells. Perhaps the most interesting observation was that CA074, a cell-permeant inhibitor of cathepsin B, prevented the reduction in the height of the apical surface liquid in human airway epithelial cells derived from controls or individuals with CF. The reduction in apical surface liquid volume also required that this fluid was acidic (pH 6), consistent with a role of acidification in activating ENaCs, either directly (as reviewed in (Kashlan & Kleyman, 2011) or indirectly, via an acid-activated protease. A surprising, and unexplained finding was that exposure of human airway epithelial cells to cathepsin B for 60 min led to an increase in surface expression of ENaC α and γ subunits. In summary, the work of Da Tan et al. provides new insights regarding cathepsin B in regulating both ENaCs and the volume of the apical surface liquid in cultured airway cells (Da Tan et al. 2014). Future studies are needed to address whether cathepsin B contributes to the marked reduction in airway surface liquid volume and impaired mucociliary clearance in individuals with CF, where it is likely that other proteases that can cleave the γ subunit and activate ENaC are present (Hobbs et al. 2013). It will also be interesting to see whether cathepsin B contributes to changes in airway surface liquid volume and mucociliary clearance in other pulmonary disorders.