Elastin

Elastin is a highly elastic protein in connective tissue and allows 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]

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]

Composition

Stretched elastin isolated from bovine aorta

Elastic fiber is composed mainly of an amorphous component, which is extensively cross-linked elastin, and a fibrillar component, which are primarily the microfibrils such as fibrillin, both of which are made of simple amino acids such as glycine, valine, alanine, and proline.^[6][7] The total elastin ranges from 58 to 75% of the weight of the dry defatted artery in normal canine arteries.^[8] 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.^[9] Elastin is made by linking many soluble tropoelastin protein molecules, in a reaction catalyzed by lysyl oxidase, to make a massive insoluble, durable complex cross-linked by desmosine and isodesmosine in an in vivo Chichibabin pyridine synthesis reaction.^[10] The amino acid responsible for these cross-links is lysine. Tropoelastin is a specialized protein with a molecular weight of 64 to 66 kDa, and an irregular or random coil conformation made up of 830 amino acids.

Molecular biology

In mammals, only a single gene for ELN is present. In humans, the ELN gene is a 45 kb segment that lies 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. Due to alternative splicing, there are at least 11 known human tropoelastin isoforms, and are under developmental regulation. However, there are minimal differences among tissues at the same developmental stage.^[6]

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.^[11]

See also

• Elastic fibers
• Elastin receptor
• Williams syndrome

References

Further reading

External links

• Elastin at the US National Library of Medicine Medical Subject Headings (MeSH)
• Histology image: 21402loa – Histology Learning System at Boston University
• GeneReviews/NIH/NCBI/UW entry on Williams or Williams-Beuren Syndrome
• The Elastin Protein
• Microfibril

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