Furin Inhibition as a Mechanism to Reduce Aberrant ENaC‐Mediated Sodium Transport and Rehydrate the Airways in Cystic Fibrosis Lung Disease

Abstract

The epithelial sodium channel (ENaC) is found in a number of tissues including lung epithelia and is activated through proteolytic cleavage of its α and γ subunits. Dysregulation of ENaC, secondary to the loss of functional CFTR in cystic fibrosis (CF), leads to increased Na+ absorption which contributes to airways dehydration, thickened mucus and impaired mucociliary clearance (MCC). Pro‐protein convertases such as furin are implicated in the activation of a distinct pool of ENaC as it passes through the biosynthetic pathway (1). Furin cleaves α‐ENaC at two sites releasing a small inhibitory peptide from the extracellular loop, partially activating the channel, and subsequently cleaves γ‐ENaC. These actions ‘prime’ the channel for cleavage at a secondary site on the γ subunit by cell‐surface channel activating proteases such as prostasin, trypsin and neutrophil elastase (2–4). Furin, therefore plays a key role in the proteolytic regulation of ENaC‐mediated sodium absorption and presents a promising therapeutic target for CF lung disease.The aim of this first‐in‐kind study was to determine the effect of two chemically different, highly selective furin inhibitors (BOS‐981 and BOS‐318) on ENaC signaling and airway surface liquid (ASL) height, using primary human CF bronchial epithelial cells (HBEs).Fully differentiated HBEs homozygous for F508‐del, grown at air‐liquid interface, were used to determine ENaC activity via equivalent current (Ieq) readings obtained using a 24‐well Transepithelial Current Clamp (TECC‐24) system. ASL height was measured using confocal microscopy.We report BOS‐981 and BOS‐318 to be potent inhibitors of furin. Both compounds, applied basolaterally, significantly reduced ENaC‐mediated Na+ absorption as measured by an acute reduction in amiloride‐sensitive Ieq after immediate treatment, which was sustained after longer (48 hr) exposure. The compounds also protected ENaC against cell surface activation by neutrophil elastase and CF sputum supernatant. Furthermore, the reduction in ENaC activity after a 48 hr treatment correlated with an increase in ASL height.These ground‐breaking results demonstrate that highly selective inhibition of furin by the novel inhibitors BOS‐981 and BOS‐318 presents a promising mechanism for the reduction of aberrant ENaC‐mediated Na+ absorption. This strategy has the potential to rehydrate the airways, restore effective MCC and reduce the devastating pulmonary decline observed in CF.Support or Funding InformationThis work was supported by funding from the MRC Proximity to Discovery: Industry Engagement FundThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.