ENaC, also called the amiloride-sensitive sodium channel (ASC), is an epithelial Na+ channel found on the apical side of polar epithelial cells of the kidney, colon, lung and sweat glands. It is a non-voltage-sensitive ion channel permeable to Na+ ions. The Na+ ions move from the lumen to the blood side of epithelial cells, ie. they are reabsorbed.
In the kidney, ENaC is located on principal cells in the distal convoluted tubule and collecting duct where its role is to retain Na+. In the colon, the ENaC reabsorbs Na+ from the diet in the lumen and contributes to the body’s overall Na+ balance. In the lungs, ENaC is vital for neonates where it reabsorbs and removes the amniotic fluid that fills the interior of the lungs allowing them to fill with air for the first breath.
The ring in between subunits α, β and γ determines the selectivity of the channel. Amino acids 587 to 589 (glycine to serine) make up the selectivity filter. Amino acid at position 583 (serine) is the amiloride binding site.
The channel has an intracellular N-terminus in the cytoplasm that is followed by the first transmembrane domain which extends into a large extracellular loop and goes back into the membrane as the second transmembrane domain and ends at a C-terminal intracellular tail in the cytoplasm.
The large extracellular loop has cysteine rich domains (CRDs) that help regulate the channel.
Regulation of Na+ Absorption
1 – ENaC and Na+ entry is the rate limiting step of Na+ absorption
An increase in the external Na+ means there is a direct increase in the Na+ moving into the cell but only to a certain extent after which ENaC intrinsically down regulates and inhibits itself using its tertiary structure, leading to a decrease in the open state probability (O.S.P) and the Na+ influx becomes steady.
2 – Activation of ENaC by proteolitic cleavage
a) ENaC moves to the membrane in its inactive form. The first step to activation involves a protease furin to cleave a 26mer with a 8mer inhibitory tract from the α subunit. This makes ENaC partially activated.
b) Another protease, usually prostatin, cleaves a 43mer peptide from the γ subunit of ENaC fully activating the channel.
1 – In the colon, late distal tubule and collecting duct when Na+ absorption increases, K+ secretion increases as a result due to the lumen being more electronegative than the blood basolateral side of the cells.
2 – Hormone Control
The steroid hormone aldosterone increases the insertion of ENaCs into the membrane and their open state probability but usually not the channel’s synthesis unless in the colon. When blood pressure and volume in the body are low, the RAAS system will be activated by Renin release from the juxtaglomerular cells in the kidney in response to low afferent tension and Na+ flow. As this eventually results in a production of aldosterone, ENaC activity will increase and more Na+ will be retained so osmolarity and volume increase, resulting in an increase in blood pressure that returns it back to a normal range.
From the bloodstream, aldosterone crosses the cell membrane and binds its corticosteroid receptor found in the cytoplasm. The two travel to the nucleus where they act as a transcription factor and increase the transcription of mRNA that encodes aldosterone induced/regulated proteins (AIT/ARTs). These proteins increase cell surface ENaC and Na+/K+ ATP-ase density.
One way this is done is by up regulating serum and glucocorticoid regulated kinase (SGK) which is the first protein translated from mRNA. SGK phosphorylates a serine on Nedd4 which disables it from marking ENaC for degradation and thus the channel stays on the membrane.
Nedd4 is a ubiquitin ligase which marks ENaC for degradation by binding to the C terminal of the channel that is rich in proline. When bound, Nedd4 will ligate Ubiquitin to ENaC’s N terminus which marks the channel for retrieval.
Disease and Treatment
The gene encoding for ENaC is found on chromosome 4 at map 4q31.3-q32. Mutations in the genes encoding the cytoplasmic C-terminal of either the β or γ subunit will result in Liddle’s Syndrome. The faulty C-terminal of ENaC means that Nedd4 is unable to bind to it and cannot ligate Ubiquitin so the channel is not marked for retrieval and ENaC activity stays high in the cell. The condition results in hypertension, hypokalemia and sometimes alkalosis. This is because too much Na+ is retained, elevating blood volume and thus pressure (hypertension) which suppresses the RAAS system. The elevated Na+ means that more K+ is also secreted, leaving the blood with low K+ levels (hypokalemia). The blood is also more electropositive than normal so H+ ions sometimes expelled from the cell via the apical side leaving the blood alkaline (alkalosis).
Treatment for Liddle’s Syndrome comes in K+-sparing diuretics that act on the late distal tubule and collecting duct. Common drugs that fall into this category are amiloride or triamterene. Amiloride is a cationic drug at physiological state and acts as a high affinity physical blocker to the channel by binding amino acid position 583. The drug is orally absorbed (15-25%) and has a half life of 21 hours. By blocking the channel is decreases Na+ retention and creates a more electropositive lumen thus reduce K+ and H+ secretion into it which makes the drug “K+-sparing”.
ENaC inhibition in the lungs can be useful for the treatment of cystic fibrosis. In normal individuals CFTR inhibits ENaC and controls Na+ absorption but in CF patients, there is either no CFTR or it is faulty, which results in no inhibition of ENaC and thus too much Na+ being reabsorbed and decrease in airway surface liquid (ASL) . A potential treatment to avoid this is to block ENaC with amiloride-like drugs such as GS9411.
- ↑ http://prosite.expasy.org/PDOC00926#ref4
- ↑ Canessa et al Nature 367, 3rd Feb, 1994
- ↑ Stockland JD et al, Life, 60(9): 620–628
- ↑ Pflugers Arch. 2010 June ; 460(1): 1–17. doi:10.1007/s00424-010-0827-z
- ↑ Soundararajan R et al. J. Biol. Chem. 2010;285:30363-30369
- ↑ NP_059115.1
- ↑ PubChem CID 16231
- ↑ Nat. Med. May 2004. 10:452-453