Trafficking and cell surface stability of the epithelial Na+ channel expressed in epithelial Madin-Darby canine kidney cells

Abstract

The apically located epithelial Na+ channel (αβγ-ENaC) plays a key role in the regulation of salt and fluid transport in the kidney and other epithelia, yet its mode of trafficking to the plasma membrane and its cell surface stability in mammalian cells are poorly understood. Because the expression of ENaC in native tissues/cells is very low, we generated epithelial Madin-Darby canine kidney (MDCK) cells stably expressing αβγ-ENaC, where each subunit is tagged differentially at the intracellular C terminus and the β-subunit is also Myc-tagged at the ectodomain (αHAβMyc,T7γFLAG). ENaC expression in these cells was verified by immunoblotting with antibodies to the tags, and patch clamp analysis has confirmed that the tagged channel is functional. Moreover, using electron microscopy, we demonstrated apical, but not basal, membrane localization of ENaC in these cells. The glycosylation pattern of the intracellular pool of ENaC revealed peptide N-glycosidase F and endoglycosidase H sensitivity. Surprisingly, the cell surface pool of ENaC, analyzed by surface biotinylation, was also core glycosylated and lacked detectable endoglycosidase H-resistant channels. Extraction of the channel from cells in Triton X-100 demonstrated that both intracellular and cell surface pools of ENaC are largely soluble. Moreover, floatation assays to analyze the presence of ENaC in lipid rafts showed that both intracellular and cell surface pools of this channel are not associated with rafts. We have shown previously that the total cellular pool of ENaC is turned over rapidly (t1/2 ∼ 1-2 h). Using cycloheximide treatment and surface biotinylation we now demonstrate that the cell surface pool of ENaC has a similarly short half-life (t1/2 ∼1 h), unlike the long half-life reported recently for the Xenopus A6 cells. Collectively, these results help elucidate key aspects of ENaC trafficking and turnover rates in mammalian kidney epithelial cells.