Epithelial sodium channel

The epithelial sodium channel (short: ENaC, also: amiloride-sensitive sodium channel) is a membrane-bound ion-channel that is selectively permeable to Na+ ions and that is assembled as a heterotrimer composed of three homologous subunits α, β, and γ or δ, β, and γ. These subunits are encoded by four genes: SCNN1A, SCNN1B, SCNN1G, and SCNN1D. It is involved primarily in the reabsorption of sodium ions in the collecting ducts of the kidney's nephrons.

The apical membranes of many tight epithelia contains sodium channels that are characterized primarily by their high affinity for the diuretic blocker amiloride. These channels mediate the first step of active sodium reabsorption essential for the maintenance of body salt and water homeostasis. In vertebrates, the channels control reabsorption of sodium in kidney, colon, lung and sweat glands; they also play a role in taste perception.

Location and function

ENaC is located in the apical membrane of polarized epithelial cells in particular in the kidney (primarily in the distal tubule), the lung, and the colon. Epithelial sodium channels facilitate Na⁺ reabsorption across the apical membranes of epithelia in the distal nephron, respiratory and reproductive tracts and exocrine glands, and hence they have a role in fluid volume homeostasis, osmolarity and arterial blood pressure regulation.

It plays a major role in the Na+- and K+-ion homeostasis of blood and epithelia and extraepithelial fluids by resorption of Na+-ions. The activity of ENaC in colon and kidney is modulated by the mineralcorticoid aldosterone. It can be blocked by either triamterene or amiloride, which are used medically to serve as diuretics. In the kidney, it is inhibited by atrial natriuretic peptide, causing natriuresis and diuresis.

ENaC can furthermore be found in taste receptor cells, where it plays an important role in salt taste perception. In rodents, virtually the entire salt taste is mediated by ENaC, whereas it seems to play a less significant role in humans: About 20 percent can be accredited to the epithelial sodium channel.

In cells with motile cilia, ENaC is located along the entire length of the cilia indicating that ENaC functions as a regulator of osmolarity of the periciliary fluid. In contrast to ENaC, CFTR is not found on cilia. These findings contradict previous hypothesis that stated that, under normal circumstances, ENaC is downregulated by direct interaction with CFTR and that, in CF patients, CFTR cannot downregulate ENaC, causing hyper-absorption in the lungs and recurrent lung infections. It has been suggested that it may be a ligand-gated ion channel.

Other members of the ENaC family, the acid-sensing and/or mechanosensory ion channels, ASIC1-4, are homo- or hetero-oligomeric neuronal Zn2+ and H+-gated, mechanosensitive channels that mediate pain sensation in response to tissue acidosis. ASIC1-4 also mediate light touch sensation and are excited by hair movement.

Ion selectivity

Studies show that the ENaC channel is permeable to Na+ and Li+ ions, but has very little permeability to K+, Cs+ or Rb+ ions.

Transport reaction

The generalized transport reaction for Na+ channels is:

   Na+ (out) → Na+ (in)

That for the degenerins is:

   Cation (out) → cation (in)

Structure

ENaC consists of three different subunits: α, β, γ. All three subunits are essential for transport to the membrane assembly of functional channels on the membrane. The C-terminus of each ENaC subunit contains a PPXY motif which when mutated or deleted in either the β- or γ-ENaC subunit leads to Liddle's syndrome, a human autosomal dominant form of hypertension. The stoichiometry of these subunits is still to be verified, but ENaC is very likely to be a heterotrimeric protein like the recently analyzed acid-sensing ion channel 1 (ASIC1), which belongs to the same family. Each of the subunits consists of two transmembrane helices and an extracellular loop. The amino- and carboxy-termini of all polypeptides are located in the cytosol.

Crystal structure of ASIC1 and site-directed mutagenesis studies suggest that ENaC has a central ion channel located along the central symmetry axis in between the three subunits.

In terms of structure, the proteins that belong to this family consist of about 510 to 920 amino acid residues. They are made of an intracellular N-terminus region followed by a transmembrane domain, a large extracellular loop, a second transmembrane segment, and a C-terminal intracellular tail. δ-subunit In addition there is a fourth, so-called δ-subunit, that shares considerable sequence similarity with the α-subunit and can form a functional ion-channel together with the β- and γ-subunits. Such δ-, β-, γ-ENaC appear in pancreas, testes, and ovaries. Their function is yet unknown.

Families

Members of the epithelial Na+ channel (ENaC) family fall into four subfamilies, termed alpha, beta, gamma and delta. The proteins exhibit the same apparent topology, each with two transmembrane (TM)-spanning segments (TMS), separated by a large extracellular loop. In most ENaC proteins studied to date, the extracellular domains are highly conserved and contain numerous cysteine residues, with flanking C-terminal amphipathic TM regions, postulated to contribute to the formation of the hydrophilic pores of the oligomeric channel protein complexes. It is thought that the well-conserved extracellular domains serve as receptors to control the activities of the channels.

The vertebrate ENaC proteins from epithelial cells cluster tightly together on the phylogenetic tree; voltage-insensitive ENaC homologues are also found in the brain. The many sequenced C. elegans proteins, including the worm degenerins, are distantly related to the vertebrate proteins as well as to each other. Vertebrate ENaC proteins are similar to degenerins of Caenorhabditis elegans: deg-1, del-1, mec-4, mec-10 and unc-8. These proteins can be mutated to cause neuronal degradation, and are also thought to form sodium channels.

Superfamily

The epithelial sodium(Na+) channel (ENac) family belongs to the ENaC/P2X superfamily. ENaC and P2X receptors have similar 3-d structures and are homologous.

Genes

The exon–intron architecture of the three genes encoding the three subunits of ENaC have remained highly conserved despite the divergence of their sequences.

• SCNN1A, SCNN1B, SCNN1G, SCNN1D

There are four related amiloride sensitive sodium channels:

• ACCN1, ACCN2, ACCN3, ACCN4

Clinical significance

ENaC interaction with CFTR is of important pathophysiological relevance in cystic fibrosis. CFTR is a transmembrane channel responsible for chloride transport and defects in this protein cause cystic fibrosis, partly through upregulation of the ENaC channel in the absence of functional CFTR.

In the airways, CFTR allows for the secretion of chloride, and sodium ions and water follow passively. However, in the absence of functional CFTR, the ENaC channel is upregulated, and further decreases salt and water secretion by reabsorbing sodium ions. As such, the respiratory complications in cystic fibrosis are not solely caused by the lack of chloride secretion but instead by the increase in sodium and water reabsorption. This results in the deposition of thick, dehydrated mucus, which collects in the respiratory tract, interfering with gas exchange and allowing for the collection of bacteria. Nevertheless, an upregulation of CFTR does not correct the influence of high-activity ENaC. Probably other interacting proteins are necessary to maintain a functional ion homeostasis in epithelial tissue of the lung, like potassium channels, aquaporins or Na/K-ATPase.

In sweat glands, CFTR is responsible for the reabsorption of chloride in the sweat duct. Sodium ions follow passively through ENaC as a result of the electrochemical gradient caused by chloride flow. This reduces salt and water loss. In the absence of chloride flow in cystic fibrosis, sodium ions do not flow through ENaC, leading to greater salt and water loss. (This is true despite upregulation of the ENaC channel, as flow in the sweat ducts is limited by the electrochemical gradient set up by chloride flow through CFTR.) As such, patients' skin tastes salty, and this is commonly used to help diagnose the disease, both in the past and today by modern electrical tests.[citation needed]

The β and γ subunits are associated with Liddle's syndrome.

Amiloride and triamterene are potassium-sparing diuretics that act as epithelial sodium channel blockers.