SCNN1D

The SCNN1D gene encodes for the δ (delta) subunit of the epithelial sodium channel ENaC in vertebrates. ENaC is assembled as a heterotrimer composed of three homologous subunits α, β, and γ or δ, β, and γ.[3] The other ENAC subunits are encoded by SCNN1A, SCNN1B, and SCNN1G.

SCNN1D
Identifiers
AliasesSCNN1D, ENaCd, ENaCdelta, SCNED, dNaCh, sodium channel epithelial 1 delta subunit, sodium channel epithelial 1 subunit delta
External IDsOMIM: 601328 HomoloGene: 48152 GeneCards: SCNN1D
Gene location (Human)
Chr.Chromosome 1 (human)[1]
Band1p36.33Start1,280,436 bp[1]
End1,292,029 bp[1]
RNA expression pattern
More reference expression data
Orthologs
SpeciesHumanMouse
Entrez

6339

n/a

Ensembl

ENSG00000162572

n/a

UniProt

P51172

n/a

RefSeq (mRNA)

NM_001130413
NM_002978

n/a

RefSeq (protein)

NP_001123885

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Location (UCSC)Chr 1: 1.28 – 1.29 Mbn/a
PubMed search[2]n/a
Wikidata
View/Edit Human

ENaC is expressed in epithelial cells and is different from the voltage-gated sodium channel that is involved in the generation of action potentials in neurons. The abbreviation for the genes encoding for voltage-gated sodium channel starts with three letters: SCN. In contrast to these sodium channels, ENaC is constitutively active and is not voltage-dependent. The second N in the abbreviation (SCNN1D) represents that these are NON-voltage-gated channels.

In most vertebrates, sodium ions are the major determinant of the osmolarity of the extracellular fluid.[4] ENaC allows transfer of sodium ions across the epithelial cell membrane in so-called "tight-epithelia" that have low permeability. The flow of sodium ions across epithelia affects osmolarity of the extracellular fluid. Thus, ENaC plays a central role in the regulation of body fluid and electrolyte homeostasis and consequently affects blood pressure.[5]

As ENaC is strongly inhibited by amiloride, it is also referred to as an "amiloride-sensitive sodium channel".

History

The first cDNA encoding the delta subunit of ENaC was cloned and sequenced by Waldmann et al. from human kidney mRNA.[6]

Gene structure

The sequence of the SCNN1D gene was first revealed by the human genome project. SCNN1D is located in the short arm of chromosome 1 (Ensembl database code: ENSG00000162572) and starts at nucleotide 1,280,436 on theg forward strand. Its length is about 11,583 bp. The gene encodes several alternative transcripts with different transcription and translation initiation sites (see Fig. 1 below). In mRNA samples from human brain, alternative splicing products have been detected, cloned and characterized.[7][8] ffebhe SCNN1D gene is found in most vertebrates.[3] But the gene has been lost in the mouse and rat genomes. [9] [10]

Tissue-specific expression

The tissue specific expression of the δ-subunit is very different from that of the other three subunits encoded by SCNN1A, SCNN1B, and SCNN1G. While the α, β, and γ subunits are expressed mainly in the kidney tubular epithelia, the respiratory airway,[11] the female reproductive tract,[11] colon, salivary and sweat glands,[12] the δ-subunit is expressed mainly in the brain, pancreas, testis and ovary.[10]

Protein structure

The primary structures of all four ENaC subunits show strong similarity. Thus, these four proteins represent a family of proteins that share a common ancestor. In global alignment (meaning alignments of sequences along their entire length and not just a partial segment), the human δ subunit shares 34% identity with the α subunit and 23% identity with the β and γ subunits.[3]

All four ENaC subunit sequences have two hydrophobic stretches that form two transmembrane segments named as TM1 and TM2.[13] In the membrane-bound form, the TM segments are embedded in the membrane bilayer, the amino- and carboxy-terminal regions are located inside the cell, and the segment between the two TMs remains outside of the cell as the extracellular region of ENaC. This extracellular region includes about 70% of the residues of each subunit. Thus, in the membrane-bound form, the bulk of each subunit is located outside of the cell.

The structure of ENaC has not been yet determined. Yet, the structure of a homologous protein ASIC1 has been resolved.[14][15] The chicken ASIC1 structure revealed that ASIC1 is assembled as a homotrimer of three identical subunits. The authors of the original study suggested that the ASIC1 trimer resembles a hand holding a ball.[14] Hence distinct domains of ASIC1 have been referred to as palm, knuckle, finger, thumb, and β-ball.[14]

Alignment of ENaC subunit sequences with ASIC1 sequence reveals that TM1 and TM2 segments and palm domain are conserved, and the knuckle, finger and thumb domains have insertions in ENaC. Site-directed mutagenesis studies on ENaC subunits provide evidence that many basic features of the ASIC1 structural model apply to ENaC as well.[3] Yet, ENaC is an obligate heterotrimer composed of three subunits as an αβγ or a βγδ trimer.[16]

Associated diseases

So far mutations in the delta subunit have not been associated with a specific disease.

Notes

References
  1. GRCh38: Ensembl release 89: ENSG00000162572 - Ensembl, May 2017
  2. "Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  3. Hanukoglu I, Hanukoglu A (January 2016). "Epithelial sodium channel (ENaC) family: Phylogeny, structure-function, tissue distribution, and associated inherited diseases". Gene. 579 (2): 95–132. doi:10.1016/j.gene.2015.12.061. PMC 4756657. PMID 26772908.
  4. Bourque CW (July 2008). "Central mechanisms of osmosensation and systemic osmoregulation". Nature Reviews. Neuroscience. 9 (7): 519–31. doi:10.1038/nrn2400. PMID 18509340.
  5. Rossier BC, Baker ME, Studer RA (January 2015). "Epithelial sodium transport and its control by aldosterone: the story of our internal environment revisited". Physiological Reviews. 95 (1): 297–340. doi:10.1152/physrev.00011.2014. PMID 25540145.
  6. Waldmann R, Champigny G, Bassilana F, Voilley N, Lazdunski M (November 1995). "Molecular cloning and functional expression of a novel amiloride-sensitive Na+ channel". The Journal of Biological Chemistry. 270 (46): 27411–4. doi:10.1074/jbc.270.46.27411. PMID 7499195.
  7. Yamamura H, Ugawa S, Ueda T, Nagao M, Shimada S (October 2006). "A novel spliced variant of the epithelial Na+ channel delta-subunit in the human brain". Biochemical and Biophysical Research Communications. 349 (1): 317–21. doi:10.1016/j.bbrc.2006.08.043. PMID 16930535.
  8. Giraldez T, Afonso-Oramas D, Cruz-Muros I, Garcia-Marin V, Pagel P, González-Hernández T, Alvarez de la Rosa D (August 2007). "Cloning and functional expression of a new epithelial sodium channel delta subunit isoform differentially expressed in neurons of the human and monkey telencephalon". Journal of Neurochemistry. 102 (4): 1304–15. doi:10.1111/j.1471-4159.2007.04622.x. PMID 17472699.
  9. Ji HL, Zhao RZ, Chen ZX, Shetty S, Idell S, Matalon S (December 2012). "δ ENaC: a novel divergent amiloride-inhibitable sodium channel". American Journal of Physiology. Lung Cellular and Molecular Physiology. 303 (12): L1013–26. doi:10.1152/ajplung.00206.2012. PMC 3532584. PMID 22983350.
  10. Giraldez T, Rojas P, Jou J, Flores C, Alvarez de la Rosa D (August 2012). "The epithelial sodium channel δ-subunit: new notes for an old song". American Journal of Physiology. Renal Physiology. 303 (3): F328–38. doi:10.1152/ajprenal.00116.2012. PMID 22573384.
  11. Enuka Y, Hanukoglu I, Edelheit O, Vaknine H, Hanukoglu A (March 2012). "Epithelial sodium channels (ENaC) are uniformly distributed on motile cilia in the oviduct and the respiratory airways". Histochemistry and Cell Biology. 137 (3): 339–53. doi:10.1007/s00418-011-0904-1. PMID 22207244.
  12. Duc C, Farman N, Canessa CM, Bonvalet JP, Rossier BC (December 1994). "Cell-specific expression of epithelial sodium channel alpha, beta, and gamma subunits in aldosterone-responsive epithelia from the rat: localization by in situ hybridization and immunocytochemistry". The Journal of Cell Biology. 127 (6 Pt 2): 1907–21. doi:10.1083/jcb.127.6.1907. PMC 2120291. PMID 7806569.
  13. Canessa CM, Merillat AM, Rossier BC (December 1994). "Membrane topology of the epithelial sodium channel in intact cells". The American Journal of Physiology. 267 (6 Pt 1): C1682–90. doi:10.1152/ajpcell.1994.267.6.C1682. PMID 7810611.
  14. Jasti J, Furukawa H, Gonzales EB, Gouaux E (September 2007). "Structure of acid-sensing ion channel 1 at 1.9 A resolution and low pH". Nature. 449 (7160): 316–23. doi:10.1038/nature06163. PMID 17882215.
  15. Baconguis I, Bohlen CJ, Goehring A, Julius D, Gouaux E (February 2014). "X-ray structure of acid-sensing ion channel 1-snake toxin complex reveals open state of a Na(+)-selective channel". Cell. 156 (4): 717–29. doi:10.1016/j.cell.2014.01.011. PMC 4190031. PMID 24507937.
  16. Hanukoglu I (2017). "ASIC and ENaC type sodium channels: Conformational states and the structures of the ion selectivity filters". The FEBS Journal. 284 (4): 525–545. doi:10.1111/febs.13840. PMID 27580245.

Further reading
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