Epithelial sodium channels (ENaC): Introduction

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Overview

The epithelial sodium channels (ENaC) are located on the apical membrane of epithelial cells in the distal kidney tubules, lung, respiratory tract, male and female reproductive tracts, sweat and salivary glands, placenta, colon and some other organs [6,10,19]. In these epithelia, ENaC allows flow of Na+ ions from the extracellular fluid in the lumen into the epithelial cell. Na+ ions are then pumped out of the cytoplasm into the interstitial fluid by the Na+/K+ ATPase located on the basolateral membrane [34]. As Na+ is one of the major electrolytes in the extracellular fluid (ECF), osmolarity change initiated by the Na+ flow is accompanied by a flow of water accompanying Na+ ions [5]. Thus, ENaC has a central role in the regulation of ECF volume and blood pressure, especially via its function in the kidney [23,27]. The expression of ENaC subunits, hence its activity, is regulated by the renin-angotensin-aldosterone system, and other factors that are involved in electrolyte homeostasis [1,26-27]. In the respiratory tract and female reproductive tract large segments of the tracts are covered by multi-ciliated cells. In these cells ENaC has been shown to be located along the entire length of the cilia [13]. Cilial location greatly increases ENaC density per cell surface and allows ENaC to serve as a sensitive regulator of osmolarity of the periciliary fluid throughout the whole depth of the fluid bathing the cilia [13]. In contrast to ENaC, CFTR that is defective in cystic fibrosis is not located on non-cilial cell-surface [13]. Thus, ENaC function is also essential for the clearance of respiratory airways, transport of germ cells, fertilization, implantation and cell migration [13,30].

Genes and Phylogeny

In the human genome there are four homologous genes (SCNN1A, SCNN1B, SCNN1D, and SCNN1G) that encode for four subunits, α-, β-, γ-, and δ-ENaC that may be involved in the assembly of ENaC [7,25,31,35-36]. These four subunits share 23-34% sequence identity among themselves and <20% identity with ASIC subunits [19]. The genes coding for all four ENaC subunits are present in all bony vertebrates with the exception of ray-finned fish genomes that have apparently lost all ENaC genes, and mouse genome that has lost the gene SCNN1D coding for &delta-ENaC [14,19]. The α-, β-, and γ-ENaC genes are also present in jawless vertebrates (e.g. lampreys) and cartilaginous fishes (e.g. shark) [19].

Diseases associated with ENaC mutations

Mutations in any of the three genes (SCNN1A, SCNN1B, and SCNN1G) may cause partial or complete loss of ENaC activity, depending on the mutation [8,17]. Such inherited loss-of-function mutations are associated with a syndrome of so-called "systemic" or "multi-system" pseudohypoaldosteronism (PHA) [12,16,19,37]. These observations indicate that all three subunits are essential for the function of ENaC. So far, no mutation has been found in the SCNN1D gene that causes PHA.

The carboxy terminal of ENaC includes a short consensus sequence called the PY motif. Mutations in this motif in SCNN1B and SCNN1G are associated with Liddle syndrome that is characterized by early-onset hypertension [4,33]. The PY motif is recognized by Nedd4-2 that is a ubiquitin ligase. Thus, mutations in the PY motif reduces ubiquitylation of ENaC leading to accumulation of ENaC in the membrane consequently enhanced activity of ENaC [29].

Channel structure and function

The sequences of all ENaC subunits include two hydrophobic domains at the amino- and carboxy terminal regions that anchor the proteins to the membrane as transmembrane (TM) segments [6,10,19]. Within the superfamily that includes ENaC, ASIC is the only channel the crystal structure of which has been determined in several conformations [2,18,21]. Models of ASIC1 revealed a trimeric structure that is embedded in the lipid bilayer by six transmembrane (TM) helices composed of two TM helices of each subunit [2,15,21].

Modeling and site directed mutagenesis studies indicate that ENaC has a 3D quaternary structure similar to ASIC [18,23]. In contrast to ASIC1 that can assemble into a functional homotrimer, ENaC activity can be reconstituted fully only as a heterotrimer with an αβγ or a δβγ composition [19,23,36].

ENaC is a constitutively active channel, i.e., the flow of Na+ ions is not dependent on an activating factor. Hence, in heterologous cells that express ENaC (e.g. Xenopus oocytes), the cells have to be maintained in a solution that contains amiloride to fully inhibit ENaC. To measure ENaC activity, the bath solution is switched to a solution without amiloride (cf. with ASIC [18]). ENaC has two major states: 1) Open, and 2) Closed. The probability of ENaC being in the open state is called ENaC open probability (Po). ENaC activity is regulated by a diverse array of hormones, ions and signal transduction systems [9,23]. These factors exert their effects by modifying, directly or indirectly, two major parameters: 1) The density of ENaC in the membrane; and 2) The channel open probability [22,26]. The open probability (Po) of ENaC is greatly decreased by external Na+ and this response is called Na+ self-inhibition [3,11,20,32].

An important aspect of ENaC regulation is that the α and the γ subunits have conserved serine protease cleavage sites in the extracellular segment [19]. Cleavage of these subunits by proteases such as furin, prostasin, and plasmin leads to activation of ENaC [24,28].

References

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To cite this family introduction, please use the following:

Israel Hanukoglu.
Epithelial sodium channels (ENaC), introduction. Last modified on 22/05/2017. Accessed on 19/08/2017. IUPHAR/BPS Guide to PHARMACOLOGY, http://guidetopharmacology.org/GRAC/FamilyIntroductionForward?familyId=122.