The active transport of sodium in epithelial tissue has been stated since 1950s (Leaf et al. 1958). In 1962, scientists found that the movement of Na+ from the liquid of lumen side into epithelial cell was not in the manner of free diffusion, but was possibly driven by electrical forces through the interaction with a saturable permease on the plasma membrane (Frazier 1962; Frazier et al. 1962; Park and Fanestil 1980). In 1970s, this permease was found to be the target of amiloride, the specific blocker of epithelial Na+ channel (ENaC), and active as a channel rather than a carrier (Lindemann and Van Driessche 1977; Park and Fanestil 1980). Epithelial Na+ channel (ENaC) was characterized as to be composed of several subunits in 1987 (Benos et al. 1987), and the gene of an amiloride-sensitive subunit was cloned in 1990 (Barbry et al. 1990). Canessa and colleagues indicated that ENaC was made of three subunits and identified α-, β-, and γ-subunit in 1993 and 1994 separately (Canessa et al. 1993; Canessa et al. 1994), and δ-subunit was identified in 1994 too (Lingueglia et al. 1993) and cloned in 1995 (Waldmann et al. 1995).
Molecular Biology of ENaC
The gene SCNN1A, SCNN1B, SCNN1G, and SCNN1D in mammals, which are the paralogs, belong to ENaC/Degenerin superfamily, encode 650–700 amino acids, respectively, for α-, β-, γ-, and δ-subunit of ENaC. These paralogs are highly conserved in all vertebrates. For mouse, the gene SCNN1D is assumed as a pseudogene (Ji et al. 2012). Fully functional ENaC is a heterotrimer composed of α (or δ), β, and γ subunits (Hanukoglu and Hanukoglu 2016). Each subunit has two transmembrane segments, which is a distinctive feature of ENaC paralogs, and two intracellular amino (N) and the carboxyl (C) termini which is about 8–10 kDa and a large 50-kDa ectodomain containing about 70% of the amino acids and connecting the transmembrane segments (Hanukoglu and Hanukoglu 2016). Only one α- or δ-subunit is adequate to enable the transmembrane Na+ transport, but covalent binding with β- and γ-subunit would up-regulate the channel activity dramatically (Ji et al. 2012).
In many organs, ENaCs are critical for the transepithelial fluid and electrolyte transport. The α-, β-, and γ-subunits of ENaC are mainly expressed in epithelia in kidney, lung, sweat glands, salivary glands, skin, placenta, and the colon (Rossier et al. 2015). Through the approach of large-scale microarray and high-throughput RNA sequencing experiments, many other tissues including fallopian tube, esophagus, prostate, skin, stomach, thyroid, tongue, and vagina were thought to have ENaCs expressing in them (Hanukoglu and Hanukoglu 2016). In human eye, nasal mucosa, mouse ear, rat muscle, vascular endothelium, vascular smooth muscle cells, lymphocytes, platelets, mammary epithelia, and the astrocytes in the brain, ENaC subunits was detected through immunolocalization technique in some recent studies as well (Hanukoglu and Hanukoglu 2016).
The tissue distribution pattern of the δ subunit is quite different from that of other three subunits. The δ-ENaC is expressed in neural tissues (including brain, cerebral cortex, cerebellum, hippocampus, hypothalamus, and pituitary gland), pancreas, testis, and ovary. However, in kidney and lung, the expression of δ-ENaC is relatively lower. The δ subunit expression has been also detected in the human nasal epithelium and eye (Hanukoglu and Hanukoglu 2016).
In respiratory airway, ENaCs play roles in regulation of airway surface liquid (ASL) volume, composition, and mucociliary clearance. The reabsorption of Na+ in the kidney tubules, which is driven by ENaC, regulate the extracellular fluid volume and influence the blood pressure and electrolyte homeostasis by modulating osmolarity. In reproductive tract, the cilial transport of gemmates, fertilization, and implantation would be influenced by the activity of ENaC through regulating epithelial fluid volume. ENaCs also serve as salt taste receptors in taste buds (Hanukoglu and Hanukoglu 2016).
Regulation of ENaC
The activity of ENaC is regulated by varieties of factors including shear stress, ions, proteases, hormones, and many other types of cytokines through the controls on the inherent activity or the membrane abundance of this channel (Hanukoglu and Hanukoglu 2016). The open probability of ENaC is remarkably related to the proteolytic processing and regulated by the interactions of specific membrane acidic phospholipids with channel cytoplasmic domains. It was recently found that in a multiprotein complex, SGK1 and Nedd4-2, along with other ENaC-regulatory proteins, could physically interact with each other and ENaC. The diversity of the complex is regulated by several signaling pathways, involving PI3K, mTOR, steroid receptor, and Raf-MEK-ERK (Soundararajan et al. 2012). In the epithelium of kidney, colon, and taste buds, the activity and expression of ENaC are responsive to mineralocorticoid, but they are rather regulated by glucocorticoid in distal airway epithelium (Rossier et al. 2002). For example, aldosterone can stimulate the expression and activity of ENaC in connecting tubule or cortical collecting duct principal cell in kidney, and dexamethasone can regulate ENaC activity and water transport in H441 cells and A549 cells, which are derived from human club cells and alveolar epithelial cells, respectively. The downstream of mineralocorticoid or glucocorticoid is a complex network associated with the pathway of cAMP/PKA/PI3K, SGK1, and Nedd4-2.
Fully functional ENaC is composed of three subunits α-(or δ- ), β-, and γ-subunit. The α or α-like subunit (δ-subunit) is essential for the Na+ transport, and covalent binding with β- and γ-subunit would up-regulate the channel activity dramatically. ENaC is generally located at the apical surface of polarized epithelial cells in tight or high-resistance epithelia in the organs including kidney, lung, sweat glands, salivary glands, skin, placenta, colon, etc. ENaC drives the active Na+ transport across the cell membrane, consequently regulates the flow and volume of the extracellular fluid in the lumen through the modulation on the osmolarity, and maintains body salt and water homeostasis. The activity of ENaC is regulated by varieties of factors including shear stress, ions, proteases, hormones, and many other types of cytokines through the controls on the inherent activity or the membrane abundance of this channel. A complex network composed of corticoid, cAMP/PKA/PI3K, SGK1, Nedd4-2, and ERK plays important regulatory role on the activity of ENaC.