A novel spliced variant of the epithelial Na+ channel δ-subunit in the human brain

Structural Insights into Subunit-Dependent Functional Regulation in Epithelial Sodium Channels

Epithelial sodium channels (ENaC) play a crucial role in Na + reabsorption in mammals. To date, four subunits have been identified-α, β, γ, and δ-believed to form different heteromeric complexes. Currently, only the structure of the αβγ complex is known. To understand how these channels form with varying subunit compositions and define the contribution of each subunit to distinct properties, we co-expressed human δ, β, and γ. Using single-particle cryo-electron microscopy, we observed three distinct ENaC complexes. The structures unveil a pattern in which β and γ positions are conserved among the different complexes while the α position in αβγ trimer is occupied by either δ or another β. The presence of δ induces structural rearrangements in the γ subunit explaining the differences in channel activity observed between αβγ and δβγ channels. These structures define the mechanism by which ENaC subunit composition tunes ENaC function.

Genetic variants in SCNN1B and AHCYL1 are associated with eggshell quality in Chinese domestic laying ducks (Anas platyrhynchos)

1. The objective of this study was to investigate the evolution of SCNN1B and AHCYL1 proteins among 10 domestic and mammalian animals, to uncover the expression patterns of SCNN1B and AHCYL1 genes in ducks, identify the genetic variants of the SCNN1B and AHCYL1 genes and analyse their effects on eggshell quality.2. Expression profiles of the SCNN1B and AHCYL1 genes in Sansui female ducks were determined using real-time fluorescence quantitative PCR to identify SNPs. The duck SCNN1B and AHCYL1 genes were amplified to identify SNPs. A total of 502 Sansui female ducks were genotyped by sequencing, and the associations between the mRNA expression/SNP genotypes and six eggshell quality indices were analysed using PASW Statistics 18.0.3. The results showed that the SCNN1B and AHCYL1 proteins are highly conserved in different mammalian or domestic animals, especially the AHCYL1 protein. The SCNN1B and AHCYL1 genes were widely expressed in different tissues of male and female ducks, and expression level in the uterus was greater than in other tissues. The expression of SCNN1B and AHCYL1 during oviposition cycle indicated that expression levels were related to the eggshell mineralisation stage.4. The mRNA expression levels of uterine SCNN1B and AHCYL1 genes were positively correlated with eggshell strength (ESS), percentage (ESP) and weight (ESW) (P<0.05), respectively. Ten novel SNPs in SCNN1B and AHCYL1 genes from Chinese domestic laying ducks were identified through PCR amplicon sequencing.5. Genetic association analysis indicated g.797509 C > T, g.797573 C > T and g.797834 C > T in SCNN1B gene and g.169244 T > A, g.169265 T > C and g.175311T > C in AHCYL1 gene had a significant effect on eggshell quality. Correlation analysis between the SNP genotype and SCNN1B and AHCYL1 genes expression in the uterus showed that the genotypes of g.797509 C>T, g.797573 C>T, g.797834 C>T, g.169244 T>A and g.175311T>C sites affected the expression of SCNN1B and AHCYL1 genes in utero (P<0.05).6. The study indicated SCNN1B and AHCYL1 as candidate genes to improve eggshell traits in ducks.

Controlling epithelial sodium channels with light using photoswitchable amilorides

Amiloride is a widely used diuretic that blocks epithelial sodium channels (ENaCs). These heterotrimeric transmembrane proteins, assembled from β, γ and α or δ subunits, effectively control water transport across epithelia and sodium influx into non-epithelial cells. The functional role of δβγENaC in various organs, including the human brain, is still poorly understood and no pharmacological tools are available for the functional differentiation between α- and δ-containing ENaCs. Here we report several photoswitchable versions of amiloride. One compound, termed PA1, enables the optical control of ENaC channels, in particular the δβγ isoform, by switching between blue and green light, or by turning on and off blue light. PA1 was used to modify functionally δβγENaC in amphibian and mammalian cells. We also show that PA1 can be used to differentiate between δβγENaC and αβγENaC in a model for the human lung epithelium.

The sodium channel gene family is specifically expressed in hen uterus and associated with eggshell quality traits

BACKGROUND

Eggshell quality is important for the poultry industry. During eggshell formation a mass of inorganic minerals is deposited. The Sodium Channel (SCNN1) gene family plays an essential role in cation transportation. The objective of this study was to investigate the pattern of expression of members of the SCNN1 gene family, their variation and their effects on eggshell quality.

RESULT

The highest expression of SCNN1a, SCNN1b, and SCNN1g genes were in the active uterus during eggshell mineralization, while SCNN1d showed its highest expression level in the quiescent uterus (no egg present). Nineteen candidate SNPs from the four genes were genotyped in a population of 338 White Leghorn layers. Association analysis between SNPs (haplotypes/diplotypes) and eggshell traits was performed. Among seven significant SNPs, five SNPs were associated with eggshell strength, eggshell thickness, eggshell percentage or/and egg weight, while the other two SNPs within SCNN1d were only associated with eggshell percentage. These SNPs had a 0.25-6.99% contribution to phenotypic variance, depending on the trait. In haplotype analysis, SCNN1b and SCNN1d were associated with egg weight. The SCNN1b and SCNN1g were significantly associated with eggshell weight while only SCNN1g explained 2.04% of phenotypic variance. All the alleles of the members of SCNN1 gene family were associated with eggshell percentage and eggshell thickness, and others members had an association with eggshell strength except for SCNN1a. The contribution of different haplotypes of the SCNN1 gene family to eggshell phenotypic variance ranged from 0.09% to 5.74%.

CONCLUSIONS

Our study indicated that the SCNN1 gene family showed tissue expression specificity and was significantly associated with eggshell traits in chicken. This study provides evidence that genetic variation in members of the sodium channel can influence eggshell quality.

δ ENaC: a novel divergent amiloride-inhibitable sodium channel

The fourth subunit of the epithelial sodium channel, termed delta subunit (δ ENaC), was cloned in human and monkey. Increasing evidence shows that this unique subunit and its splice variants exhibit biophysical and pharmacological properties that are divergent from those of α ENaC channels. The widespread distribution of epithelial sodium channels in both epithelial and nonepithelial tissues implies a range of physiological functions. The altered expression of SCNN1D is associated with numerous pathological conditions. Genetic studies link SCNN1D deficiency with rare genetic diseases with developmental and functional disorders in the brain, heart, and respiratory systems. Here, we review the progress of research on δ ENaC in genomics, biophysics, proteomics, physiology, pharmacology, and clinical medicine.

The epithelial sodium channel δ-subunit: new notes for an old song

Amiloride-sensitive epithelial Na(+) channels (ENaCs) can be formed by different combinations of four homologous subunits, named α, β, γ, and δ. In addition to providing an apical entry pathway for transepithelial Na(+) reabsorption in tight epithelia such as the kidney distal tubule and collecting duct, ENaCs are also expressed in nonepithelial cells, where they may play different functional roles. The δ-subunit of ENaC was originally identified in humans and is able to form amiloride-sensitive Na(+) channels alone or in combination with β and γ, generally resembling the canonical kidney ENaC formed by α, β, and γ. However, δ differs from α in its tissue distribution and channel properties. Despite the low sequence conservation between α and δ (37% identity), their similar functional characteristics provide an excellent model for exploring structural correlates of specific ENaC biophysical and pharmacological properties. Moreover, the study of cellular mechanisms modulating the activity of different ENaC subunit combinations provides an opportunity to gain insight into the regulation of the channel. In this review, we examine the evolution of ENaC genes, channel subunit composition, the distinct functional and pharmacological features that δ confers to ENaC, and how this can be exploited to better understand this ion channel. Finally, we briefly consider possible functional roles of the ENaC δ-subunit.

Characterization of a novel splice variant of δ ENaC subunit in human lungs

Salt absorption via apical epithelial sodium channels (ENaC) is a critical rate-limiting process in maintaining airway and lung lining fluid at the physiological level. δ ENaC (termed δ1 in this article) has been detected in human lung epithelial cells in addition to α, β, and γ subunits (Ji HL, Su XF, Kedar S, Li J, Barbry P, Smith PR, Matalon S, Benos DJ. J Biol Chem 281: 8233-8241, 2006; Nie HG, Chen L, Han DY, Li J, Song WF, Wei SP, Fang XH, Gu X, Matalon S, Ji HL, J Physiol 587: 2663-2676, 2009) and may contribute to the differences in the biophysical properties of amiloride-inhibitable cation channels in pulmonary epithelial cells. Here we cloned a splicing variant of the δ1 ENaC, namely, δ2 ENaC in human bronchoalveolar epithelial cells (16HBEo). δ2 ENaC possesses 66 extra amino acids attached to the distal amino terminal tail of the δ1 ENaC. δ2 ENaC was expressed in both alveolar type I and II cells of human lungs as revealed by in situ hybridization and real-time RT-PCR. To characterize the biophysical and pharmacological features of the splicing variant, we injected Xenopus oocytes with human ENaC cRNAs and measured whole cell and single channel currents of δ1βγ, δ2βγ, and αβγ channels. Oocytes injected with δ2βγ cRNAs exhibited whole cell currents significantly greater than those expressing δ1βγ and αβγ channels. Single channel activity, unitary conductance, and open probability of δ2βγ channels were significantly greater compared with δ1βγ and αβγ channels. In addition, δ2βγ and δ1βγ channels displayed significant differences in apparent Na(+) affinity, dissociation constant for amiloride (K(i)(amil)), the EC(50) for capsazepine activation, and gating kinetics by protons. Channels comprising of this novel splice variant may contribute to the diversities of native epithelial Na(+) channels.

Differential N termini in epithelial Na+ channel δ-subunit isoforms modulate channel trafficking to the membrane

The epithelial Na(+) channel (ENaC) is a heteromultimeric ion channel that plays a key role in Na(+) reabsorption across tight epithelia. The canonical ENaC is formed by three analogous subunits, α, β, and γ. A fourth ENaC subunit, named δ, is expressed in the nervous system of primates, where its role is unknown. The human δ-ENaC gene generates at least two splice isoforms, δ(1) and δ(2) , differing in the N-terminal sequence. Neurons in diverse areas of the human and monkey brain differentially express either δ(1) or δ(2) , with few cells coexpressing both isoforms, which suggests that they may play specific physiological roles. Here we show that heterologous expression of δ(1) in Xenopus oocytes and HEK293 cells produces higher current levels than δ(2) . Patch-clamp experiments showed no differences in single channel current magnitude and open probability between isoforms. Steady-state plasma membrane abundance accounts for the dissimilarity in macroscopic current levels. Differential trafficking between isoforms is independent of β- and γ-subunits, PY-motif-mediated endocytosis, or the presence of additional lysine residues in δ(2)-N terminus. Analysis of δ(2)-N terminus identified two sequences that independently reduce channel abundance in the plasma membrane. The δ(1) higher abundance is consistent with an increased insertion rate into the membrane, since endocytosis rates of both isoforms are indistinguishable. Finally, we conclude that δ-ENaC undergoes dynamin-independent endocytosis as opposed to αβγ-channels.

The neuronal-specific SGK1.1 kinase regulates {delta}-epithelial Na+ channel independently of PY motifs and couples it to phospholipase C signaling

The δ-subunit of the epithelial Na(+) channel (ENaC) is expressed in neurons of the human and monkey central nervous system and forms voltage-independent, amiloride-sensitive Na(+) channels when expressed in heterologous systems. It has been proposed that δ-ENaC could affect neuronal excitability and participate in the transduction of ischemic signals during hypoxia or inflammation. The regulation of δ-ENaC activity is poorly understood. ENaC channels in kidney epithelial cells are regulated by the serum- and glucocorticoid-induced kinase 1 (SGK1). Recently, a new isoform of this kinase (SGK1.1) has been described in the central nervous system. Here we show that δ-ENaC isoforms and SGK1.1 are coexpressed in pyramidal neurons of the human and monkey (Macaca fascicularis) cerebral cortex. Coexpression of δβγ-ENaC and SGK1.1 in Xenopus oocytes increases amiloride-sensitive current and channel plasma membrane abundance. The kinase also exerts its effect when δ-subunits are expressed alone, indicating that the process is not dependent on accessory subunits or the presence of PY motifs in the channel. Furthermore, SGK1.1 action depends on its enzymatic activity and binding to phosphatidylinositol(4,5)-bisphosphate. Physiological or pharmacological activation of phospholipase C abrogates SGK1.1 interaction with the plasma membrane and modulation of δ-ENaC. Our data support a physiological role for SGK1.1 in the regulation of δ-ENaC through a pathway that differs from the classical one and suggest that the kinase could serve as an integrator of different signaling pathways converging on the channel.

The delta-subunit of the epithelial sodium channel (ENaC) enhances channel activity and alters proteolytic ENaC activation

The epithelial sodium channel (ENaC) is probably a heterotrimer with three well characterized subunits (alphabetagamma). In humans an additional delta-subunit (delta-hENaC) exists but little is known about its function. Using the Xenopus laevis oocyte expression system, we compared the functional properties of alphabetagamma- and deltabetagamma-hENaC and investigated whether deltabetagamma-hENaC can be proteolytically activated. The amiloride-sensitive ENaC whole-cell current (DeltaI(ami)) was about 11-fold larger in oocytes expressing deltabetagamma-hENaC than in oocytes expressing alphabetagamma-hENaC. The 2-fold larger single-channel Na(+) conductance of deltabetagamma-hENaC cannot explain this difference. Using a chemiluminescence assay, we demonstrated that an increased channel surface expression is also not the cause. Thus, overall channel activity of deltabetagamma-hENaC must be higher than that of alphabetagamma-hENaC. Experiments exploiting the properties of the known betaS520C mutant ENaC confirmed this conclusion. Moreover, chymotrypsin had a reduced stimulatory effect on deltabetagamma-hENaC whole-cell currents compared with its effect on alphabetagamma-hENaC whole-cell currents (2-fold versus 5-fold). This suggests that the cell surface pool of so-called near-silent channels that can be proteolytically activated is smaller for deltabetagamma-hENaC than for alphabetagamma-hENaC. Proteolytic activation of deltabetagamma-hENaC was associated with the appearance of a delta-hENaC cleavage product at the cell surface. Finally, we demonstrated that a short inhibitory 13-mer peptide corresponding to a region of the extracellular loop of human alpha-ENaC inhibited DeltaI(ami) in oocytes expressing alphabetagamma-hENaC but not in those expressing deltabetagamma-hENaC. We conclude that the delta-subunit of ENaC alters proteolytic channel activation and enhances base-line channel activity.

Characterization of the epithelial sodium channel delta-subunit in human nasal epithelium

The epithelial sodium channel (ENaC) mediates the first step in Na+ reabsorption in epithelial cells such as kidney, colon, and airways and may consist of four homologous subunits (alpha, beta, gamma, delta). Predominantly, the alpha-subunit is expressed in these epithelia, and it usually forms functional channels with the beta- and gamma-subunits. The delta-subunit was first found in human brain and kidney, but the expression was also detected in human cell lines of lung, pancreatic, and colonic origin. When co-expressed with beta and gamma accessory subunits in heterologous systems, the two known isoforms of the delta-ENaC subunit (delta1 and delta2) can build amiloride-sensitive Na+ channels. In the present study we demonstrate the expression and function of the delta-subunit in human nasal epithelium (HNE). We cloned and sequenced the full-length cDNA of the delta-ENaC subunit and were able to show that in nasal tissue at least isoform 1 is expressed. Furthermore, we performed Western blot analyses and compared the cell surface expression of the delta-subunit with the classically expressed alpha-subunit by using immunofluorescence experiments. Thereby, we could show that the quantity of both subunits is almost similar. In addition, we show the functional expression of the delta-ENaC subunit with measurements in modified Ussing chambers, and demonstrate that in HNE a large portion of the Na+ transport is mediated by the delta-ENaC subunit. Therefore, we suppose that the delta-subunit may possess an important regulatory function and might interact with other ENaC subunits or members of the DEG/ENaC family in the human respiratory epithelium.

alpha-ENaC is a functional element of the hypertonicity-induced cation channel in HepG2 cells and it mediates proliferation

The molecular correlate of hypertonicity-induced cation channels (HICCs) and their role in proliferation vs. apoptosis is a matter of debate. We report in this paper that, in whole-cell patch-clamp recordings, hypertonic stress (340→450 mosM) reversibly increased the Na⁺ conductance of HepG2 cells from 0.8 to 5.8 nS. The effect was dose-dependently inhibited by flufenamate and amiloride, known blockers of HICCs, with some 50% efficiency at 300 μM. In parallel, both drugs decreased HepG2 cell proliferation [in 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays and with automatic cell counting]. Small interfering RNA (siRNA) silencing of the α-subunit of the epithelial Na⁺ channel (ENaC) reduced hypertonicity-induced Na⁺ currents to 60%, whereas the rate of HepG2 cell proliferation was approximately half of that of the control. Moreover, α-ENaC siRNA inhibited the regulatory volume increase of HepG2 cells (measured with scanning acoustic microscopy) by 60%. In florescence-activated cell sorting measurements, silencing of α-ENaC led to a significant decrease in the G1 and an increase in the G2/M phase of the cell cycle, whereas the S phase was not changing. Finally (determined by a caspase 3/7 assay), HICC inhibition by flufenamate and silencing of α-ENaC increased the rate of apoptosis in HepG2 cells. It is concluded that α-ENaC is one functional element of the HICC in HepG2 cells and that the channel is an important mediator of cell proliferation; likewise, HICC blockage shifts the system from a proliferative into a rather apoptotic one. This is the first report of a role of α-ENaC in cell proliferation.

Epithelial Na+ channel delta subunit is an acid sensor in the human oesophagus

Gastro-oesophageal reflux disease is caused by the reflux of gastric contents into the oesophagus, and thus the oesophageal lumen is damaged by gastric acid. The acid sensor involved in oesophageal epithelial defense is still unclear. Recently, we described that the epithelial Na(+) channel delta subunit (ENaCdelta) is a candidate molecule for a pH sensor in the human brain. Here, using reverse transcription-polymerase chain reaction and in situ hybridization methods, we showed that the proton-sensitive ENaCdelta was strongly expressed in the epithelial layer of the human oesophagus, representative peripheral tissue that can be exposed to an acidic environment. Other ENaC subunits (alpha, beta, and gamma) were also localized there. Based on the expression pattern, human oesophageal ENaC complex was mimicked in the Xenopus oocyte expression system and the response to acidic pH was recorded using a two-electrode voltage-clamp technique. The human oesophageal-mimicking ENaCdeltabetagammaalpha complex generated an amiloride-sensitive inward current at the holding potential of -60 mV. The ENaCdeltabetagammaalpha current was significantly activated by acidic pH (pH 4.0), approximately equal to the luminal value when gastric acid refluxes into the oesophagus. In conclusion, ENaCdelta is a candidate molecule for pH sensing in the gastrointestinal system in humans, providing a novel therapeutic target for gastro-oesophageal reflux disease.

TRPM8 activation suppresses cellular viability in human melanoma

The transient receptor potential melastatin subfamily (TRPM), which is a mammalian homologue of cell death-regulated genes in Caenorhabditis elegans and Drosophila, has potential roles in the process of the cell cycle and regulation of Ca2+signaling. Among this subfamily, TRPM8 (also known as Trp-p8) is a Ca2+-permeable channel that was originally identified as a prostate-specific gene upregulated in tumors. Here we showed that the TRPM8 channel was expressed in human melanoma G-361 cells, and activation of the channel produced sustainable Ca2+influx. The application of menthol, an agonist for TRPM8 channel, elevated cytosolic Ca2+concentration in a concentration-dependent manner with an EC50value of 286 μM in melanoma cells. Menthol-induced responses were significantly abolished by the removal of external Ca2+. Moreover, inward currents at a holding potential of −60 mV in melanoma cells were markedly potentiated by the addition of 300 μM menthol. The most striking finding was that the viability of melanoma cells was dose-dependently depressed in the presence of menthol. These results reveal that a functional TRPM8 protein is expressed in human melanoma cells to involve the mechanism underlying tumor progression via the Ca2+handling pathway, providing us with a novel target of drug development for malignant melanoma.

Epithelial Na+ channel delta subunit mediates acid-induced ATP release in the human skin

The amiloride-sensitive epithelial Na(+) channel (ENaC) regulates Na(+) homeostasis in cells and across epithelia. Although we described that ENaCdelta is a candidate molecule for a pH sensor in the human brain, the physiological and pathological roles of ENaCdelta in non-neuronal tissues are still unknown. Here we show a novel physiological function of ENaCdelta in peripheral tissues in humans. Expression analyses at the level of mRNA clearly revealed that ENaCdelta was abundantly expressed in human epidermis and keratinocytes. In addition, ENaCdelta protein was detected in there. In cultured keratinocytes, acidic stress (pH 5.0) evoked ATP release, which was significantly reduced in the presence of 100 microM amiloride or 10 microM benzamil. In conclusion, ENaCdelta may be involved in the mechanism underlying pH sensing followed by the regulation of cell viability in the human skin.

Small molecule activator of the human epithelial sodium channel

The epithelial sodium channel (ENaC), a heterotrimeric complex composed of alpha, beta, and gamma subunits, belongs to the ENaC/degenerin family of ion channels and forms the principal route for apical Na(+) entry in many reabsorbing epithelia. Although high affinity ENaC blockers, including amiloride and derivatives, have been described, potent and specific small molecule ENaC activators have not been reported. Here we describe compound S3969 that fully and reversibly activates human ENaC (hENaC) in an amiloride-sensitive and dose-dependent manner in heterologous cells. Mechanistically, S3969 increases hENaC open probability through interactions requiring the extracellular domain of the beta subunit. hENaC activation by S3969 did not require cleavage by the furin protease, indicating that nonproteolyzed channels can be opened. Function of alphabetaG37Sgamma hENaC, a channel defective in gating that leads to the salt-wasting disease pseudohypoaldosteronism type I, was rescued by S3969. Small molecule activation of hENaC may find application in alleviating human disease, including pseudohypoaldosteronism type I, hypotension, and neonatal respiratory distress syndrome, when improved Na(+) flux across epithelial membranes is clinically desirable.

Expression analysis of the epithelial Na+ channel delta subunit in human melanoma G-361 cells

Malignant melanoma is the most deadly form of skin cancer and its incidence is steadily increasing worldwide. The plasma membrane in melanoma cells possesses a variety of ion channels, so its profile is thought to lead to a novel target for medical treatment for malignant melanoma. Here we showed that human melanoma G-361 cells expressed the epithelial Na(+) channel delta subunit (ENaC delta), which is largely unknown in physiological and pathological functions in non-neuronal tissues. Expression analyses at the level of mRNA clearly revealed that ENaC delta transcript was strongly expressed in human melanoma cells using reverse transcription-polymerase chain reaction and cell-based in situ hybridization techniques. Other ENaC subunits (alpha, beta, and gamma) were also distributed in human melanoma cells. In addition, human melanoma cells possessed an abundant expression of ENaC delta protein by immunocytochemistry. These results provide an attractive target for drug development of malignant melanoma.

Cloning and functional expression of a new epithelial sodium channel delta subunit isoform differentially expressed in neurons of the human and monkey telencephalon

Epithelial sodium channel (ENaC) is a member of the ENaC/degenerin family of amiloride-sensitive, non-voltage gated sodium ion channels. ENaC alpha, beta and gamma subunits are abundantly expressed in epithelial tissues, where they have been well characterized. An ENaC delta subunit has also been described in the human nervous system, although its histological distribution pattern remains unexplored. We have now isolated a novel ENaC delta isoform (delta2) from human brain and studied the expression pattern of both the known (delta1) and the new (delta2) isoforms in the human and monkey telencephalon. ENaC delta2 is produced by a combination of alternative transcription start sites, a frame shift in exon 3 and alternative splicing of exon 4. It forms functional amiloride-sensitive sodium channels when co-expressed with ENaC beta and gamma accessory subunits. Comparison with the classical ENaC channel (alphabetagamma) indicates that the interaction between delta2, beta and gamma is functionally inefficient. Both ENaC delta isoforms are widely expressed in pyramidal cells of the human and monkey cerebral cortex and in different neuronal populations of telencephalic subcortical nuclei, but double-labelling experiments demonstrated a low level of co-localization between isoforms (5-8%), suggesting specific functional roles for each of them.