FXYD3 increases Na+ transport across human airway epithelia

Abstract

The Na/K ATPase localizes within the basolateral membrane of most epithelia, where Na/K ATPase activity mediates Na+ absorption. The Na/K ATPase requires its alpha subunit for ion transport and its beta subunit for proper trafficking to the plasma membrane. The non‐essential gamma subunit of the Na/K ATPase modifies Na/K ATPase activity. Datasets within the NIH Gene Expression Omnibus indicate that FXYD3, a Na/K ATPase gamma subunit, is highly expressed by human airway epithelia. However, how FXYD3 affects airway epithelial function remains unstudied. In addition to binding the Na/K ATPase, FXYD3 can bind and modify H/K ATPases in heterologous expression systems. The H/K ATPase ATP12A acidifies human airway surface liquid. Therefore, we evaluated the role of FXYD3 in both acid secretion and Na+ absorption across human airway epithelia. Single cell RNA sequencing revealed high FXYD3 expression for all the airway cell types, and immunocytochemistry revealed FXYD3 expression within the basolateral membrane of airway epithelia. Consistent with basolateral FXYD3 localization, the airway surface liquid pH, established in part by apical H/K ATPase, was unaffected by siRNA‐mediated knockdown of FXYD3. FXYD3 knockdown decreased amiloride‐sensitive short‐circuit currents (~20% in NaCl and ~40% in NaGluconate solutions), a finding that is consistent with FXYD3 increasing Na/K ATPase activity. Consequently, fluid absorption across airway epithelia was reduced by ~20% with FXYD3 knockdown. FXYD3 may be required to efficiently return the airway surface liquid to homeostatic volumes after fluid secretion into the proximal airways.