Collagen & Elastin

References: Molecular Biology of the Cell: The Extracellular Matrix Components and Cytoskeletal by EMD and Merck, the research reagents group of the Life Science Products division of Merck KGaA and their Calbiochem, Tools for Mitochondrial Metabolism, which offers a wide range of biochemicals and kits for the study of disease states, signal transduction, apoptosis, cell cycle and protein chemistry research.

The human skin includes two compartments, namely, a superficial compartment, the epidermis, and a deep compartment, the dermis.

The natural human epidermis principally comprises three types of cells which are the keratinocytes, which are in the great majority, the melanocytes and the Langerhans' cells. Each of these cell types contributes, through its specific functions, to the essential role served in the body by the skin.

The dermis provides the epidermis with a solid support. It is also its nourishing component. It principally comprises fibroblasts and an extracellular matrix which itself comprises various extracellular proteins, among which are, in particular, collagen fibers, elastin and various glycoproteins. All of these extracellular components are synthesized by the fibroblasts. Also present in the dermis are leukocytes, mastocytes and tissue macrophages. Finally, the dermis contains blood vessels and nerve fibers.

The fibroblast, by virtue of its activity in the synthesis of the extracellular matrix proteins (proteoglycans, collagen fibers and other structural glycoproteins) is the principal factor in the structural formation of the dermis.

Collagen fibers are responsible for the strength of the dermis. These are very resistant but sensitive to certain enzymes generally designated collagenases. In the dermis, the collagen fibers are fibrils which are firmly attached to each other, thus forming more than ten types of different structures. The structure of the dermis is in great part due to the entanglement of the packed collagen fibers. The collagen fibers participate in the tonicity of the skin.

The extracellular matrix (ECM) is composed of fibrillar and non-fibrillar components. The two main groups of macromolecules that form ECM are fibrous proteins and glycosaminoglycans (GAGs). The fibrous proteins can be grouped as (i) structural proteins, such as collagens and elastin, and (ii) adhesive proteins, which include fibronectin and laminin.

Collagens are the most abundant protein in the body and are predominantly synthesized by fibroblasts; however, epithelial cells may also synthesize small amounts of collagen.

Collagen proteins are a major component of skin and bone and constitute ~25% of total protein mass in mammals. Collagen is rich in glycine and proline.

Collagen fibers are regularly renewed, but this renewal decreases with age, which causes, in particular, thinning of the dermis. It is also accepted that extrinsic factors such as ultraviolet radiation, tobacco and certain treatments (retinoic acid and derivatives, glucocorticoids, vitamin D and derivatives thereof, for example) also elicit an adverse effect on the skin and on its collagen level.

Glycosaminoglycans (GAGs) generally form a highly hydrated, gel-like substance, in which fibrous proteins are embedded. GAGs are heteropolysaccharides consisting of long unbranched polysaccharides containing a repeating disaccharide unit. The disaccharide units contain either N-acetylgalactosamine (GalNAc) or N-acetylglucosamine (GlcNAc) and an uronic acid such as glucuronate or iduronate. GAGs are located either on the cell surface or in ECM. They exhibit high degree of viscosity and low compressibility, which makes them suitable for lubricating joints. The majority of GAGs are linked to core proteins and form proteoglycans (mucopolysaccharides). This linkage involves a specific trisaccharide consisting of two galactose and a xylulose residue (GAG-GalGalXyl-O-CH2-protein).

Defects in the synthesis and degradation of collagen and elastin contribute to a number of diseases.

Some of the well-known collagen related diseases include osteogenesis imperfecta, many chondrodysplasias, some cases of osteoporosis, and several subtypes of the Ehlers-Danlos syndromes. The adhesive proteins assist cells to attach to the extracellular matrix. For example, fibronectin, a large dimeric glycoprotein, promotes the attachment of fibroblasts and other cells to the matrix in connective tissue. It interacts with a wide variety of proteins, including collagen, heparin, fibrin, gelatin, DNA and cell-surface receptors of integrins. Fibronectins can also regulate the shape of cells and organize cytoskeleton.

By facilitating the migration of immune cells to the site of injury fibronectins play an important role in wound healing. They bind to integrins on platelets via RGD domains and cause localization of platelets to the site of injury. They also bind to fibrin and assist in blood clotting. Other adhesive molecule, such as laminin, promote the attachment of epithelial cells to the basal lamina. Laminin is a hetrotrimer, consisting of A (400 kDa), B1 and B2 subunits (200 kDa each). Each subunit contains at least 12 repeats of EGF-like domains. The number of EGF repeats varies between different species. Laminin also stimulates spreading of many cell types and promotes the outgrowth of neurites in culture.

From: Glucocorticoid Regulation of Elastin Synthesis in Human Fibroblasts: Down-Regulation in Fibroblasts from Normal Dermis But Not From Keloids

Keloids arise as benign connective tissue masses at sites of injury in genetically predisposed individuals, In addition to excessive collagen accumulation, there is biochemical and histologic evidence of elastic tissue. Previous studies showed that glucocorticoid regulation of collagen synthesis differs in fibroblasts from normal adult dermis and keloids, To define further the abnormal regulation of matrix synthesis in keloid fibroblasts, we examined glucocorticoid regulation of elastin synthesis. The basal level of elastin synthesis was significantly higher in keloid than in normal cells, and hydrocortisone reduced synthesis of elastin and elastin mRNA in normal but not in keloid fibroblasts. We had shown previously that fibroblasts from fetal dermis resembled keloid fibroblasts in glucocorticoid regulation of growth and collagen synthesis. In this study, glucocorticoids failed to down-regulate elastin synthesis in fetal cells that had not differentiated to produce normal levels of elastin, whereas fetal cells with normal elastin production exhibited glucocorticoid down-regulation. Abnormal regulation in keloid cells was independent of cell density and was confined to fibroblasts cultured from the keloid nodule. These findings reinforce the conclusion that a matrix-regulatory pathway is deranged in these focal lesions. Coordinate down-regulation of collagen and elastin by hydrocortisone in normal adult denial fibroblasts and the failure of hydrocortisone to down-regulate synthesis of either protein in keloid cells support the existence of common elements in the regulatory pathways of these two matrix proteins.

What is Elastin?

Elastin is our body's structural protein that gives elasticity to our tissues and organs. Elastin is found predominantly in the walls of our arteries, in our lungs, intestines, and skin, as well as in other elastic tissues. It functions in connective tissue in partnership with collagen. Whereas collagen provides rigidity, elastin is the protein which allows the connective tissues in our blood vessels and heart tissues, for example, to stretch and then recoil to their original positions.

Imagine elastin within the body's connective tissue to act like a bunch of rubber bands that are tied together at a number of places. When the elastic bands are pulled, they will stretch, and when there is no longer a pull, they will return to their original relaxed state. You can't pull the elastin chain too far because the companion stiff collagen fibers in the connective tissue limit the stretching of the elastin fibers in the tissue.

Why is elastin a particularly unusual protein? Elastin is considered by scientists to be a very tough and relatively stable protein because it has many internal linkages. Those linkages make elastin resistant to the normal breakdown characteristic of most proteins.

A Basic Introduction
The following websites contain a basic introduction to elastin:
Williams Syndrome Foundation
Elastin and collagen diagram

Last modified: October 6, 2008

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