Some assembly required: putting the epithelial sodium channel together.

The epithelial sodium channel (ENaC)2 constitutes the rate-limiting step in Na+ re-absorption in the apical membrane of epithelia, including kidney, lung, colon, salivary glands, and sweat glands (1). The channel is integral to homeostasis as knock-out of the α-subunit of ENaC is lethal in mice. Abnormal regulation of the channel that results in either gain or loss of function has been implicated in the pathogenesis of several disease states, including forms of salt-sensitive hypertension and pseudohypoaldosteronism type II, and may also contribute to the progression of pulmonary disease in cystic fibrosis (1–3). ENaC is a highly Na+-selective amiloride-sensitive channel of relatively low conductance that appears to be constitutively active when present in cell membranes (1). The channel is a heterotrimeric protein, made up of three homologous subunits (α, β, and γ) with a likely stoichiometry of 1:1:1 (4, 5). A δ-subunit that can co-assemble with α-, β-, and γ-ENaC has also been described (6). Given its essential roles in fluid clearance and regulation of blood volume, it is not surprising that ENaC activity is under strict control. The apical membrane abundance and open probability of ENaC are altered by a wide variety of hormonal and cellular effectors (1, 7). Regulation of channel expression at the translational and post-translational levels is tissue-specific, and these differences are apparent for either whole channel levels or expression of individual subunits (8). For example, message for the individual subunits is differentially expressed not only in distinct tissues such as lung and kidney, but also temporally, so the message for some subunits is expressed constitutively, whereas that for others is induced hormonally (8). Differential translational efficiency of ENaC subunit message has also been noted. In endogenously expressing tissues and cell lines, it is clear that total cellular and apical membrane levels of some subunits remain unchanged, whereas others are either up- or down-regulated in response to factors that alter regulation of transport rate (9). This phenomenon has been referred to as non-coordinate regulation of ENaC, a term first applied by Farman and co-workers (10, 11) to the differential steroid regulation of ENaC subunit mRNA in lung, kidney, and colon. We have previously proposed three possible mechanisms to explain this phenomenon: 1) post-translational assembly of channels, 2) the existence of channels with alternative stoichiometry that may be differentially regulated, and 3) disassembly and recombination of subunits at some post-translational site (8). These mechanisms are not mutually exclusive, which highlights the likelihood that more than one pathway participates in the generation of the phenomenon broadly described as non-coordinate regulation. The purpose of this review is to consider the status of non-coordinate regulation of ENaC with respect to several recent developments in the field, in particular the appreciation of the probable trimeric stoichiometry of the channel and the accumulating evidence that sub-apical pools of ENaC subunits exist that can participate in recycling of the channel. Finally, we will discuss the impact that differing molecular mass forms of the ENaC subunits present at the apical membrane following proteolytic activation of the channel may have on our understanding of non-coordinate regulation.