SCNN1A explained

The SCNN1A gene encodes for the α 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.[1] The other ENAC subunits are encoded by SCNN1B, SCNN1G, and SCNN1D.

ENaC is expressed in epithelial cells[1] 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 (SCNN1A) represents that these are NON-voltage-gated channels.

In most vertebrates, sodium ions are the major determinant of the osmolarity of the extracellular fluid.[2] 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.[3]

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

History

The first mRNA encoding the alpha subunit of ENaC was isolated by two independent groups by screening a rat colon cDNA library.[4] [5]

Gene structure

The human gene SCNN1A is located in the short arm of chromosome 12 (12p3).[6] [7] Human SCNN1A includes 13 exons spanning about 29,000 bp. The protein coding region is located in exons 2-13. The positions of introns are conserved in all four human ENaC genes.[8] The positions of the introns are also highly conserved across vertebrates See: Ensembl GeneTree.

Analysis of α subunit mRNA from human lung and kidney showed that during transcription of SCNN1A gene different mRNAs are produced as a result of alternative translation initiation and splicing sites. The isoforms translated from these differ in their activities.[9] [10] [11] [12]

Tissue-specific expression

SCNN1A, SCNN1B, and SCNN1G are commonly expressed in tight epithelia that have low water permeability. The major organs where ENaC is expressed include parts of the kidney tubular epithelia,[1] [13] the respiratory airway,[14] the female reproductive tract, testis, including, spermatogonia in the seminiferous tubules, Sertoli cells, and spermatozoa,[15] colon and salivary glands. In the skin, SCNN1A is expressed in the keratinocytes in the epidermal layer, in the sebaceous sweat glands, and the smooth muscle cells mostly within the cytoplasm.[16] In contrast, in the eccrine sweat glands ENaC is mostly located on the luminal surface of eccrine duct epithelia.

ENaC is also expressed in the tongue, where it has been shown to be essential for the perception of salt taste.[13]

The expression of ENaC subunit genes is regulated mainly by the mineralocorticoid hormone aldosterone that is activated by the renin-angiotensin system.[17] [18] [19]

Protein structure

The primary structures of all four ENaC subunits show strong similarity.[1] 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 α subunit shares 34% identity with the δ subunit and 26-27% identity with the β and γ subunits.

All four ENaC subunit sequences have two hydrophobic stretches that form two transmembrane segments named as TM1 and TM2.[20] 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.[21] [22] 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. Hence distinct domains of ASIC1 have been referred to as palm, knuckle, finger, thumb, and β-ball.

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.[23] [24] [25]

Associated diseases

The disease most commonly associated with mutations in SCNN1A is the multi-system form of type I pseudohypoaldosteronism (PHA1B) that was first characterized by A. Hanukoglu as an autosomal recessive disease.[26] This is a syndrome of unresponsiveness to aldosterone in patients that have high serum levels of aldosterone but suffer from symptoms of aldosterone deficiency with a high risk of mortality due to severe salt loss.[1] Initially, this disease was thought to be a result of a mutation in the mineralocorticoid receptor (NR3C2) that binds aldosterone. But homozygosity mapping in 11 affected families revealed that the disease is associated with two loci on chromosome 12p13.1-pter and chromosome 16p12.2-13 that include the genes for SCNN1A and SCNN1B and SCNN1G respectively.[27] Sequencing of the ENaC genes identified mutation in affected patients, and functional expression of the mutated cDNAs further confirmed that identified mutations lead to the loss of activity of ENaC.[28]

In the majority of the patients with multi-system PHA1B a homozygous mutation or two compound heterozygous mutations have been detected.[29] [30] [31] [32]

A stop mutation in the SCNN1A gene has been shown to be associated with female infertility.[33]

Interactions

SCNN1A has been shown to interact with:

See also

Further reading

Notes and References

  1. Hanukoglu I, Hanukoglu A . Epithelial sodium channel (ENaC) family: Phylogeny, structure-function, tissue distribution, and associated inherited diseases . Gene . 579 . 2 . 95–132 . April 2016 . 26772908 . 4756657 . 10.1016/j.gene.2015.12.061 .
  2. Bourque CW . Central mechanisms of osmosensation and systemic osmoregulation . Nature Reviews. Neuroscience . 9 . 7 . 519–31 . July 2008 . 18509340 . 10.1038/nrn2400 . 205504313 .
  3. Rossier BC, Baker ME, Studer RA . Epithelial sodium transport and its control by aldosterone: the story of our internal environment revisited . Physiological Reviews . 95 . 1 . 297–340 . January 2015 . 25540145 . 10.1152/physrev.00011.2014 .
  4. Lingueglia E, Voilley N, Waldmann R, Lazdunski M, Barbry P . Expression cloning of an epithelial amiloride-sensitive Na+ channel. A new channel type with homologies to Caenorhabditis elegans degenerins . FEBS Letters . 318 . 1 . 95–9 . February 1993 . 8382172 . 10.1016/0014-5793(93)81336-x . 43316314 . free .
  5. Canessa CM, Horisberger JD, Rossier BC . Epithelial sodium channel related to proteins involved in neurodegeneration . Nature . 361 . 6411 . 467–70 . February 1993 . 8381523 . 10.1038/361467a0 . 1993Natur.361..467C . 4229554 .
  6. Meisler MH, Barrow LL, Canessa CM, Rossier BC . SCNN1, an epithelial cell sodium channel gene in the conserved linkage group on mouse chromosome 6 and human chromosome 12 . Genomics . 24 . 1 . 185–6 . November 1994 . 7896277 . 10.1006/geno.1994.1599 . 2027.42/31204 . free .
  7. Ludwig M, Bolkenius U, Wickert L, Marynen P, Bidlingmaier F . Structural organisation of the gene encoding the alpha-subunit of the human amiloride-sensitive epithelial sodium channel . Human Genetics . 102 . 5 . 576–81 . May 1998 . 9654208 . 10.1007/s004390050743 . 22547152 .
  8. Saxena A, Hanukoglu I, Strautnieks SS, Thompson RJ, Gardiner RM, Hanukoglu A . Gene structure of the human amiloride-sensitive epithelial sodium channel beta subunit . Biochemical and Biophysical Research Communications . 252 . 1 . 208–13 . November 1998 . 9813171 . 10.1006/bbrc.1998.9625 .
  9. Thomas CP, Auerbach S, Stokes JB, Volk KA . 5' heterogeneity in epithelial sodium channel alpha-subunit mRNA leads to distinct NH2-terminal variant proteins . The American Journal of Physiology . 274 . 5 Pt 1 . C1312–23 . May 1998 . 9612219 . 10.1152/ajpcell.1998.274.5.C1312 . free .
  10. Chow YH, Wang Y, Plumb J, O'Brodovich H, Hu J . Hormonal regulation and genomic organization of the human amiloride-sensitive epithelial sodium channel alpha subunit gene . Pediatric Research . 46 . 2 . 208–14 . August 1999 . 10447117 . 10.1203/00006450-199908000-00014 . free .
  11. Tucker JK, Tamba K, Lee YJ, Shen LL, Warnock DG, Oh Y . Cloning and functional studies of splice variants of the alpha-subunit of the amiloride-sensitive Na+ channel . The American Journal of Physiology . 274 . 4 Pt 1 . C1081–9 . April 1998 . 9575806 . 10.1152/ajpcell.1998.274.4.C1081 .
  12. Berman JM, Brand C, Awayda MS . A long isoform of the epithelial sodium channel alpha subunit forms a highly active channel . Channels . 9 . 1 . 30–43 . 2015 . 25517724 . 4594420 . 10.4161/19336950.2014.985478 .
  13. Duc C, Farman N, Canessa CM, Bonvalet JP, Rossier BC . 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 . December 1994 . 7806569 . 2120291 . 10.1083/jcb.127.6.1907 .
  14. Enuka Y, Hanukoglu I, Edelheit O, Vaknine H, Hanukoglu A . 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 . March 2012 . 22207244 . 10.1007/s00418-011-0904-1 . 15178940 .
  15. Sharma S, Hanukoglu A, Hanukoglu I . Localization of epithelial sodium channel (ENaC) and CFTR in the germinal epithelium of the testis, Sertoli cells, and spermatozoa . J. Mol. Histol. . 49 . 2 . 195–208 . April 2018 . 29453757 . 10.1007/s10735-018-9759-2 . 3761720 .
  16. Hanukoglu I, Boggula VR, Vaknine H, Sharma S, Kleyman T, Hanukoglu A . Expression of epithelial sodium channel (ENaC) and CFTR in the human epidermis and epidermal appendages . Histochemistry and Cell Biology . 147 . 6 . 733–748 . January 2017 . 28130590 . 10.1007/s00418-016-1535-3 . 8504408 .
  17. Mick VE, Itani OA, Loftus RW, Husted RF, Schmidt TJ, Thomas CP . The alpha-subunit of the epithelial sodium channel is an aldosterone-induced transcript in mammalian collecting ducts, and this transcriptional response is mediated via distinct cis-elements in the 5'-flanking region of the gene . Molecular Endocrinology . 15 . 4 . 575–88 . April 2001 . 10.1210/mend.15.4.0620 . 11266509 . free .
  18. Palmer LG, Patel A, Frindt G . Regulation and dysregulation of epithelial Na+ channels . Clinical and Experimental Nephrology . 16 . 1 . 35–43 . February 2012 . 22038262 . 10.1007/s10157-011-0496-z . 19437696 .
  19. Thomas W, Harvey BJ . Mechanisms underlying rapid aldosterone effects in the kidney . Annual Review of Physiology . 73 . 335–57 . 2011 . 20809792 . 10.1146/annurev-physiol-012110-142222 .
  20. Canessa CM, Merillat AM, Rossier BC . Membrane topology of the epithelial sodium channel in intact cells . The American Journal of Physiology . 267 . 6 Pt 1 . C1682–90 . December 1994 . 7810611 . 10.1152/ajpcell.1994.267.6.C1682 .
  21. Jasti J, Furukawa H, Gonzales EB, Gouaux E . Structure of acid-sensing ion channel 1 at 1.9 A resolution and low pH . Nature . 449 . 7160 . 316–23 . September 2007 . 17882215 . 10.1038/nature06163 . 2007Natur.449..316J .
  22. Baconguis I, Bohlen CJ, Goehring A, Julius D, Gouaux E . 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 . February 2014 . 24507937 . 4190031 . 10.1016/j.cell.2014.01.011 .
  23. Edelheit O, Hanukoglu I, Dascal N, Hanukoglu A . Identification of the roles of conserved charged residues in the extracellular domain of an epithelial sodium channel (ENaC) subunit by alanine mutagenesis . American Journal of Physiology. Renal Physiology . 300 . 4 . F887–97 . April 2011 . 21209000 . 10.1152/ajprenal.00648.2010 . 869654 .
  24. Edelheit O, Ben-Shahar R, Dascal N, Hanukoglu A, Hanukoglu I . Conserved charged residues at the surface and interface of epithelial sodium channel subunits--roles in cell surface expression and the sodium self-inhibition response . The FEBS Journal . 281 . 8 . 2097–111 . April 2014 . 24571549 . 10.1111/febs.12765 . 5807500 . free .
  25. Hanukoglu I . ASIC and ENaC type sodium channels: conformational states and the structures of the ion selectivity filters . The FEBS Journal . 284 . 4 . 525–545 . February 2017 . 27580245 . 10.1111/febs.13840 . 24402104 .
  26. Hanukoglu A . Type I pseudohypoaldosteronism includes two clinically and genetically distinct entities with either renal or multiple target organ defects . The Journal of Clinical Endocrinology and Metabolism . 73 . 5 . 936–44 . November 1991 . 1939532 . 10.1210/jcem-73-5-936 .
  27. Strautnieks SS, Thompson RJ, Hanukoglu A, Dillon MJ, Hanukoglu I, Kuhnle U, Seckl J, Gardiner RM, Chung E . Localisation of pseudohypoaldosteronism genes to chromosome 16p12.2-13.11 and 12p13.1-pter by homozygosity mapping . Human Molecular Genetics . 5 . 2 . 293–9 . February 1996 . 8824886 . 10.1093/hmg/5.2.293 . free .
  28. Chang SS, Grunder S, Hanukoglu A, Rösler A, Mathew PM, Hanukoglu I, Schild L, Lu Y, Shimkets RA, Nelson-Williams C, Rossier BC, Lifton RP . Mutations in subunits of the epithelial sodium channel cause salt wasting with hyperkalaemic acidosis, pseudohypoaldosteronism type 1 . Nature Genetics . 12 . 3 . 248–53 . March 1996 . 8589714 . 10.1038/ng0396-248 . 8185511 .
  29. Strautnieks SS, Thompson RJ, Gardiner RM, Chung E . A novel splice-site mutation in the gamma subunit of the epithelial sodium channel gene in three pseudohypoaldosteronism type 1 families . Nature Genetics . 13 . 2 . 248–50 . June 1996 . 8640238 . 10.1038/ng0696-248 . 21124946 .
  30. Saxena A, Hanukoglu I, Saxena D, Thompson RJ, Gardiner RM, Hanukoglu A . Novel mutations responsible for autosomal recessive multisystem pseudohypoaldosteronism and sequence variants in epithelial sodium channel alpha-, beta-, and gamma-subunit genes . Journal of Clinical Endocrinology and Metabolism . 87 . 7 . 3344–3350 . Jul 2002 . 12107247 . 10.1210/jcem.87.7.8674 . free .
  31. Edelheit O, Hanukoglu I, Gizewska M, Kandemir N, Tenenbaum-Rakover Y, Yurdakök M, Zajaczek S, Hanukoglu A . Novel mutations in epithelial sodium channel (ENaC) subunit genes and phenotypic expression of multisystem pseudohypoaldosteronism . Clinical Endocrinology . 62 . 5 . 547–53 . May 2005 . 15853823 . 10.1111/j.1365-2265.2005.02255.x . 2749562 .
  32. Zennaro MC, Hubert EL, Fernandes-Rosa FL . Aldosterone resistance: structural and functional considerations and new perspectives . Molecular and Cellular Endocrinology . 350 . 2 . 206–15 . March 2012 . 21664233 . 10.1016/j.mce.2011.04.023 . 24896754 .
  33. Boggula VR, Hanukoglu I, Sagiv R, Enuka Y, Hanukoglu A . Expression of the epithelial sodium channel (ENaC) in the endometrium - Implications for fertility in a patient with pseudohypoaldosteronism . The Journal of Steroid Biochemistry and Molecular Biology . 183 . 137–141 . October 2018 . 29885352 . 10.1016/j.jsbmb.2018.06.007 . 47010706 .
  34. Harvey KF, Dinudom A, Cook DI, Kumar S . The Nedd4-like protein KIAA0439 is a potential regulator of the epithelial sodium channel . The Journal of Biological Chemistry . 276 . 11 . 8597–601 . March 2001 . 11244092 . 10.1074/jbc.C000906200 . free .
  35. Malbert-Colas L, Nicolas G, Galand C, Lecomte MC, Dhermy D . Identification of new partners of the epithelial sodium channel alpha subunit . Comptes Rendus Biologies . 326 . 7 . 615–24 . July 2003 . 14556380 . 10.1016/s1631-0691(03)00154-9 .
  36. Farr TJ, Coddington-Lawson SJ, Snyder PM, McDonald FJ . Human Nedd4 interacts with the human epithelial Na+ channel: WW3 but not WW1 binds to Na+-channel subunits . The Biochemical Journal . 345 Pt 3 . 3 . 503–9 . February 2000 . 10642508 . 1220784 . 10.1042/0264-6021:3450503 .
  37. McDonald FJ, Western AH, McNeil JD, Thomas BC, Olson DR, Snyder PM . Ubiquitin-protein ligase WWP2 binds to and downregulates the epithelial Na(+) channel . American Journal of Physiology. Renal Physiology . 283 . 3 . F431–6 . September 2002 . 12167593 . 10.1152/ajprenal.00080.2002 .
  38. Boulkroun S, Ruffieux-Daidié D, Vitagliano JJ, Poirot O, Charles RP, Lagnaz D, Firsov D, Kellenberger S, Staub O . Vasopressin-inducible ubiquitin-specific protease 10 increases ENaC cell surface expression by deubiquitylating and stabilizing sorting nexin 3 . American Journal of Physiology. Renal Physiology . 295 . 4 . F889–900 . October 2008 . 18632802 . 10.1152/ajprenal.00001.2008 .
  39. Raikwar NS, Thomas CP . Nedd4-2 isoforms ubiquitinate individual epithelial sodium channel subunits and reduce surface expression and function of the epithelial sodium channel . American Journal of Physiology. Renal Physiology . 294 . 5 . F1157–65 . May 2008 . 18322022 . 2424110 . 10.1152/ajprenal.00339.2007 .

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