EXTRACELLULAR-MATRIX .4. THE ELASTIC FIBER

The clastic properties of many tissues such as the lung, dermis, and large blood vessels are due to the presence of elastic fibers in the extracellular space. These fibers have been shown by biochemical and ultrastructural analysis to be composed of two distinct components, a more abundant amorphous component and a 10-12 nm microfibrillar component, which is located primarily around the periphery of the amorphous component. The protein elastin makes up the highly insoluble amorphous component and is responsible for the elastic properties. Elastin is found throughout the vertebrate kingdom and possesses an unusual chemical composition rich in glycine, proline, and hydrophobic amino acids, consonant with its characteristic physical properties. The 72-kDa biosynthetic precursor, tropoelastin, is secreted into the extracellular space where it becomes highly cross-linked into a rubber-like network through the activity of the copper-requiring enzyme lysyl oxidase. Analysis of the elastin gene has demonstrated that hydrophobic and cross-linking domains are encoded in separate exons and that there is significant alternative splicing, resulting in multiple isoforms of tropoelastin. The elastin gene promoter contains many potential binding sites for various modulating factors indicative of a complex pattern of transcriptional regulation. The microfibrils contain several proteins, including fibrillin, and probably act as an organizing scaffold in the formation of the elastin network. There appears to be a fibrillin gene family in which each protein contains multiple repeats of a motif previously found in epidermal growth factor and a second motif observed in transforming growth factor beta1-binding protein. Mutations in the fibrillin gene located on human chromosome 15 have been strongly implicated as the cause of the Marfan syndrome.