Abstract

The epithelial Na(+) channel (ENaC) is a heteromultimeric ion channel that plays a key role in Na(+) reabsorption across tight epithelia. The canonical ENaC is formed by three analogous subunits, α, β and γ. A fourth ENaC subunit, named δ, is expressed in the nervous system of primates, where its role is unknown. The human δ ENaC gene generates at least two splice isoforms, δ1 and δ2, differing in the N-terminal sequence. Neurons in diverse areas of the human and monkey brain differentially express either δ1 or δ2, with few cells co-expressing both isoforms, which suggests that they may play specific physiological roles. Here we show that heterologous expression of δ1 in Xenopus oocytes and HEK293 cells produces higher current levels than δ2. Patch-clamp experiments showed no differences in single channel current magnitude and open probability between isoforms. Steady state plasma membrane abundance accounts for the dissimilarity in macroscopic current levels. Differential trafficking between isoforms is independent of β and γ subunits, PY-motif mediated endocytosis or the presence of additional lysine residues in δ2 N-terminus. Analysis of δ2 N-terminus identified two sequences that independently reduce channel abundance in the plasma membrane. δ1 higher abundance is consistent with an increased insertion rate into the membrane, since endocytosis rates of both isoforms are indistinguishable. Finally, we conclude that δ ENaC undergoes dynamin-independent endocytosis as opposed to αβγ channels.