A long isoform of the epithelial sodium channel alpha subunit forms a highly active channel

Human epithelial Na+ channel missense variants identified in the GenSalt study alter channel activity

Mutations in genes encoding subunits of the epithelial Na⁺ channel (ENaC) can cause early onset familial hypertension, demonstrating the importance of this channel in modulating blood pressure. It remains unclear whether other genetic variants resulting in subtler alterations of channel function result in hypertension or altered sensitivity of blood pressure to dietary salt. This study sought to identify functional human ENaC variants to examine how these variants alter channel activity and to explore whether these variants are associated with altered sensitivity of blood pressure to dietary salt. Six-hundred participants of the Genetic Epidemiology Network of Salt Sensitivity (GenSalt) study with salt-sensitive or salt-resistant blood pressure underwent sequencing of the genes encoding ENaC subunits. Functional effects of identified variants were examined in a Xenopus oocyte expression system. Variants that increased channel activity included three in the gene encoding the α-subunit (αS115N, αR476W, and αV481M), one in the β-subunit (βS635N), and one in the γ-subunit (γL438Q). One α-subunit variant (αA334T) and one γ-subunit variant (βD31N) decreased channel activity. Several α-subunit extracellular domain variants altered channel inhibition by extracellular Na⁺ (Na⁺ self-inhibition). One variant (αA334T) decreased and one (αV481M) increased cell surface expression. Association between these variants and salt sensitivity did not reach statistical significance. This study identifies novel functional human ENaC variants and demonstrates that some variants alter channel cell surface expression and/or Na⁺ self-inhibition.

High Salt Diet Affects Renal Sodium Excretion and ERRα Expression

Kidneys regulate the balance of water and sodium and therefore are related to blood pressure. It is unclear whether estrogen-related receptor α (ERRα), an orphan nuclear receptor and transcription factor highly expressed in kidneys, affects the reabsorption of water and sodium. The aim of this study was to determine whether changes in the expressions of ERRα, Na⁺/K⁺-ATPase and epithelial sodium channel (ENaC) proteins affected the reabsorption of water and sodium in kidneys of Dahl salt-sensitive (DS) rats. SS.13BN rats, 98% homologous to the DS rats, were used as a normotensive control group. The 24 h urinary sodium excretion of the DS and SS.13BN rats increased after the 6-week high salt diet intervention, while sodium excretion was increased in DS rats with daidzein (agonist of ERRα) treatment. ERRα expression was decreased, while β- and γ-ENaC mRNA expressions were increased upon high sodium diet treatment in the DS rats. In the chromatin immunoprecipitation (CHIP) assay, positive PCR signals were obtained in samples treated with anti-ERRα antibody. The transcriptional activity of ERRα was decreased upon high salt diet intervention. ERRα reduced the expressions of β- and γ-ENaC by binding to the ENaC promoter, thereby increased Na+ reabsorption. Therefore, ERRα might be one of the factors causing salt-sensitive hypertension.

Epithelial sodium channel (ENaC) family: Phylogeny, structure-function, tissue distribution, and associated inherited diseases

The epithelial sodium channel (ENaC) is composed of three homologous subunits and allows the flow of Na(+) ions across high resistance epithelia, maintaining body salt and water homeostasis. ENaC dependent reabsorption of Na(+) in the kidney tubules regulates extracellular fluid (ECF) volume and blood pressure by modulating osmolarity. In multi-ciliated cells, ENaC is located in cilia and plays an essential role in the regulation of epithelial surface liquid volume necessary for cilial transport of mucus and gametes in the respiratory and reproductive tracts respectively. The subunits that form ENaC (named as alpha, beta, gamma and delta, encoded by genes SCNN1A, SCNN1B, SCNN1G, and SCNN1D) are members of the ENaC/Degenerin superfamily. The earliest appearance of ENaC orthologs is in the genomes of the most ancient vertebrate taxon, Cyclostomata (jawless vertebrates) including lampreys, followed by earliest representatives of Gnathostomata (jawed vertebrates) including cartilaginous sharks. Among Euteleostomi (bony vertebrates), Actinopterygii (ray finned-fishes) branch has lost ENaC genes. Yet, most animals in the Sarcopterygii (lobe-finned fish) branch including Tetrapoda, amphibians and amniotes (lizards, crocodiles, birds, and mammals), have four ENaC paralogs. We compared the sequences of ENaC orthologs from 20 species and established criteria for the identification of ENaC orthologs and paralogs, and their distinction from other members of the ENaC/Degenerin superfamily, especially ASIC family. Differences between ENaCs and ASICs are summarized in view of their physiological functions and tissue distributions. Structural motifs that are conserved throughout vertebrate ENaCs are highlighted. We also present a comparative overview of the genotype-phenotype relationships in inherited diseases associated with ENaC mutations, including multisystem pseudohypoaldosteronism (PHA1B), Liddle syndrome, cystic fibrosis-like disease and essential hypertension.

Effects of urine composition on epithelial Na+ channel-targeted protease activity

We examined human urinary proteolytic activity toward the Epithelial Sodium Channel (ENaC). We focused on two sites in each of alpha and gamma ENaC that are targets of endogenous and exogenous proteases. We examined the effects of ionic strength, pH and urinary H⁺-buffers, metabolic intermediates, redox molecules, and large urinary proteins. Monoatomic cations caused the largest effect, with sodium inhibiting activity in the 15–515 mEq range. Multivalent cations zinc and copper inhibited urinary proteolytic activity at concentrations below 100 μmol/L. Similar to sodium, urea caused a 30% inhibition in the 0–500 mmol/L range. This was not observed with acetone and ethanol. Modulating urinary redox status modified activity with H₂O₂ stimulated and ascorbate inhibited activity. Minimal effects (<10%) were observed with caffeine, glucose, several TCA cycle intermediates, salicylic acid, inorganic phosphate, albumin, creatinine, and Tamm–Horsfall protein. The cumulative activity of ENaC-cleaving proteases was highest at neutral pH, however, alpha and gamma proteases exhibited an inverse dependence with alpha stimulated at acidic and gamma stimulated at alkaline pH. These data indicate that ENaC-targeting urinary proteolytic activity is sensitive to sodium, urea and pH and changes in these components can modify channel cleavage and activation status, and likely downstream sodium absorption unrelated to changes in protein or channel density.

Interacting domains in the epithelial sodium channel that mediate proteolytic activation

Epithelial Sodium Channel (ENaC) proteolysis at sites in the extracellular loop of the α and γ subunits leads to marked activation. The mechanism of this effect remains debated, as well as the role of the N- and C-terminal fragments of these subunits created by cleavage. We introduced cysteines at sites bracketing upstream and downstream the cleavage regions in α and γ ENaC to examine the role of these fragments in the activated channel. Using thiol modifying reagents, as well as examining the effects of cleavage by exogenous proteases we constructed a functional model that determines the potential interactions of the termini near the cleavage regions. We report that the N-terminal fragments of both α and γ ENaC interact with the channel complex; with interactions between the N-terminal γ and the C-terminal α fragments being the most critical to channel function and activation by exogenous cleavage by subtilisin. Positive charge modification at a.a.135 in the N-terminal fragment of γ exhibited the largest inhibition of channel function. This region was found to interact with the C-terminal α fragment between a.a. 205 and 221; a tract which was previously identified to be the site of subtilisin's action. These data provide the first evidence for the functional channel rearrangement caused by proteolysis of the α and γ subunit and indicate that the untethered N-terminal fragments of these subunits interact with the channel complex.