Elastin

Elastin is a key protein of the extracellular matrix.[1] It is highly elastic and present in connective tissue allowing many tissues in the body to resume their shape after stretching or contracting. Elastin helps skin to return to its original position when it is poked or pinched. Elastin is also an important load-bearing tissue in the bodies of vertebrates and used in places where mechanical energy is required to be stored. In humans, elastin is encoded by the ELN gene.[2]

Elastin
Identifiers
AliasestropoelastinelastinELN
External IDsGeneCards:
Orthologs
SpeciesHumanMouse
Entrez

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Ensembl

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UniProt

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RefSeq (protein)

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Function

The ELN gene encodes a protein that is one of the two components of elastic fibers. The encoded protein is rich in hydrophobic amino acids such as glycine and proline, which form mobile hydrophobic regions bounded by crosslinks between lysine residues.[3] Multiple transcript variants encoding different isoforms have been found for this gene.[3] Elastin's soluble precursor is tropoelastin.[4] The characterization of disorder is consistent with an entropy-driven mechanism of elastic recoil. It is concluded that conformational disorder is a constitutive feature of elastin structure and function.[5]

Clinical significance

Deletions and mutations in this gene are associated with supravalvular aortic stenosis (SVAS) and the autosomal dominant cutis laxa.[3] Other associated defects in elastin include Marfan syndrome, emphysema caused by α1-antitrypsin deficiency, atherosclerosis, Buschke-Ollendorff syndrome, Menkes syndrome, pseudoxanthoma elasticum, and Williams syndrome.[6]

Elastosis

Elastosis is the buildup of elastin in tissues, and is a form of degenerative disease.[7] There are a multitude of causes, but the most commons cause is actinic elastosis of the skin, also known as solar elastosis, which is caused by prolonged and excessive sun exposure, a process known as photoaging. Uncommon causes of skin elastosis include elastosis perforans serpiginosa, perforating calcific elastosis and linear focal elastosis.[7]

Skin elastosis causes
ConditionDistinctive featuresHistopathology
Actinic elastosis
(most common, also called solar elastosis)		 Elastin replacing collagen fibers of the papillary dermis and reticular dermis
Elastosis perforans serpiginosa Degenerated elastic fibers and transepidermal perforating canals (arrow in image points at one of them)[8]
Perforating calcific elastosis Clumping of short elastic fibers in the dermis.[8]
Linear focal elastosis Accumulation of fragmented elastotic material within the papillary dermis and transcutaneous elimination of elastotic fibers.[8]

Composition
Stretched elastin isolated from bovine aorta

In the body, elastin is usually associated with other proteins in connective tissues. Elastic fiber in the body is a mixture of amorphous elastin and fibrous fibrillin. Both components are primarily made of smaller amino acids such as glycine, valine, alanine, and proline.[6][9] The total elastin ranges from 58 to 75% of the weight of the dry defatted artery in normal canine arteries.[10] Comparison between fresh and digested tissues shows that, at 35% strain, a minimum of 48% of the arterial load is carried by elastin, and a minimum of 43% of the change in stiffness of arterial tissue is due to the change in elastin stiffness.[11]

Tissue distribution

Elastin serves an important function in arteries as a medium for pressure wave propagation to help blood flow and is particularly abundant in large elastic blood vessels such as the aorta. Elastin is also very important in the lungs, elastic ligaments, elastic cartilage, the skin, and the bladder. It is present in all vertebrates above the jawless fish.[12]

Characteristics

Elastin is a very long-lived protein, with a half-life of over 78 years in humans.[13]

Clinical research

The feasibility of using recombinant human tropoelastin to enable elastin fiber production to improve skin flexibility in wounds and scarring has been studied.[14][15] After subcutaneous injections of recombinant human tropoelastin into fresh wounds it was found there was no improvement in scarring or the flexibility of the eventual scarring.[14][15]

Biosynthesis

Tropoelastin precursors

Elastin is made by linking together many small soluble precursor tropoelastin protein molecules (50-70 kDa), to make the final massive insoluble, durable complex. The unlinked tropoelastin molecules are not normally available in the cell, since they become crosslinked into elastin fibres immediately after their synthesis by the cell and during their export into the extracellular matrix.

Each tropoelastin consists of a string of 36 small domains, each weighing about 2 kDa in a random coil conformation. The protein consists of alternating hydrophobic and hydrophilic domains, which are encoded by separate exons, so that the domain structure of tropoelastin reflects the exon organization of the gene. The hydrophilic domains contain Lys-Ala (KA) and Lys-Pro (KP) motifs that are involved in crosslinking during the formation of mature elastin. In the KA domains, lysine residues occur as pairs or triplets separated by two or three alanine residues (e.g. AAAKAAKAA) whereas in KP domains the lysine residues are separated mainly by proline residues (e.g. KPLKP).

Aggregation

Tropoelastin aggregates at physiological temperature due to interactions between hydrophobic domains in a process called coacervation. This process is reversible and thermodynamically controlled and does not require protein cleavage. The coacervate is made insoluble by irreversible crosslinking.

Crosslinking

To make mature elastin fibres, the tropoelastin molecules are cross-linked via their lysine residues with desmosine and isodesmosine cross-linking molecules. The enzyme that performs the crosslinking is lysyl oxidase, using an in vivo Chichibabin pyridine synthesis reaction.[16]

Molecular biology
Domain structure of human tropoelastin

In mammals, the genome only contains one gene for tropoelastin, called ELN. The human ELN gene is a 45 kb segment on chromosome 7, and has 34 exons interrupted by almost 700 introns, with the first exon being a signal peptide assigning its extracellular localization. The large number of introns suggests that genetic recombination may contribute to the instability of the gene, leading to diseases such as SVAS. The expression of tropoelastin mRNA is highly regulated under at least eight different transcription start sites.

Tissue specific variants of elastin are produced by alternative splicing of the tropoelastin gene. There are at least 11 known human tropoelastin isoforms. these isoforms are under developmental regulation, however there are minimal differences among tissues at the same developmental stage.[6]

See also

References
  1. Mithieux, SM; Weiss, AS (2005). "Elastin". Advances in Protein Chemistry. 70: 437–61. doi:10.1016/S0065-3233(05)70013-9. ISBN 9780120342709. PMID 15837523.
  2. Curran, Mark E.; Atkinson, Donald L.; Ewart, Amanda K.; Morris, Colleen A.; Leppert, Mark F.; Keating, Mark T. (9 April 1993). "The elastin gene is disrupted by a translocation associated with supravalvular aortic stenosis". Cell. 73 (1): 159–168. doi:10.1016/0092-8674(93)90168-P. PMID 8096434. S2CID 8274849.
  3. "Entrez Gene: elastin".
  4. "Elastin (ELN)". Retrieved 31 October 2011.
  5. Muiznieks LD, Weiss AS, Keeley FW (Apr 2010). "Structural disorder and dynamics of elastin". Biochemistry and Cell Biology. 88 (2): 239–50. doi:10.1139/o09-161. PMID 20453927.
  6. Vrhovski, Bernadette; Weiss, Anthony S. (15 November 1998). "Biochemistry of tropoelastin". European Journal of Biochemistry. 258 (1): 1–18. doi:10.1046/j.1432-1327.1998.2580001.x. PMID 9851686.
  7. Beth Wright. "Elastosis". DermNet NZ.
  8. Hosen, Mohammad J.; Lamoen, Anouck; De Paepe, Anne; Vanakker, Olivier M. (2012). "Histopathology of Pseudoxanthoma Elasticum and Related Disorders: Histological Hallmarks and Diagnostic Clues". Scientifica. 2012: 1–15. doi:10.6064/2012/598262. ISSN 2090-908X. PMC 3820553. PMID 24278718.
    -Creative Commons Attribution 3.0 Unported license
  9. Kielty CM, Sherratt MJ, Shuttleworth CA (Jul 2002). "Elastic fibres". Journal of Cell Science. 115 (Pt 14): 2817–28. PMID 12082143.
  10. Fischer GM, Llaurado JG (Aug 1966). "Collagen and elastin content in canine arteries selected from functionally different vascular beds". Circulation Research. 19 (2): 394–399. doi:10.1161/01.res.19.2.394. PMID 5914851.
  11. Lammers SR, Kao PH, Qi HJ, Hunter K, Lanning C, Albietz J, Hofmeister S, Mecham R, Stenmark KR, Shandas R (Oct 2008). "Changes in the structure-function relationship of elastin and its impact on the proximal pulmonary arterial mechanics of hypertensive calves". American Journal of Physiology. Heart and Circulatory Physiology. 295 (4): H1451–9. doi:10.1152/ajpheart.00127.2008. PMC 2593497. PMID 18660454.
  12. Sage EH, Gray WR (1977). "Evolution of elastin structure". Elastin and Elastic Tissue. Advances in Experimental Medicine and Biology. 79. pp. 291–312. doi:10.1007/978-1-4684-9093-0_27. ISBN 978-1-4684-9095-4. PMID 868643.
  13. Toyama, Brandon H.; Hetzer, Martin W. (January 2013). "Protein homeostasis: live long, won't prosper". Nature Reviews. Molecular Cell Biology. 14 (1): 55–61. doi:10.1038/nrm3496. ISSN 1471-0072. PMC 3570024. PMID 23258296.
  14. Souto, Maria C. (9 January 2020). "New Nanotechnologies for the Treatment and Repair of Skin Burns Infections". International Journal of Molecular Sciences. 21 (2): 393. doi:10.3390/ijms21020393. PMC 7013843. PMID 31936277. Retrieved 20 August 2020.
  15. Xie, Hua (1 September 2017). "Treatment of Burn and Surgical Wounds With Recombinant Human Tropoelastin Produces New Elastin Fibers in Scars". Journal of Burn Care & Research. 38 (5): e859–e867. doi:10.1097/BCR.0000000000000507. PMID 28221299. S2CID 39251937. Retrieved 20 August 2020.
  16. Umeda H, Takeuchi M, Suyama K (Apr 2001). "Two new elastin cross-links having pyridine skeleton. Implication of ammonia in elastin cross-linking in vivo". The Journal of Biological Chemistry. 276 (16): 12579–12587. doi:10.1074/jbc.M009744200. PMID 11278561.

Further reading

This article incorporates text from the United States National Library of Medicine, which is in the public domain.

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