Extracellular matrix elastin

The first elastomeric protein is elastin, this structural protein is one of the main components of the extracellular matrix, which provides stmctural integrity to the tissues and organs of the body. This highly crosslinked and therefore insoluble protein is the essential element of elastic fibers, which induce elasticity to tissue of lung, skin, and arteries. In these fibers, elastin forms the internal core, which is interspersed with microfibrils [1,2]. Not only this biopolymer but also its precursor material, tropoelastin, have inspired materials scientists for many years. The most interesting characteristic of the precursor is its ability to self-assemble under physiological conditions, thereby demonstrating a lower critical solution temperature (LCST) behavior. This specific property has led to the development of a new class of synthetic polypeptides that mimic elastin in its composition and are therefore also known as elastin-like polypeptides (ELPs). 

Elastin is a heavily crosslinked biopolymer that is formed in a process named elastogenesis. In this section, the role of elastin and the different steps of elastin production will be described, starting with transcription of the genetic code and processing of the primary transcript, followed by translation into the elastin precursor protein and its transport to the extracellular matrix. Finally, the crosslinking and fiber formation, which result in the transition from tropoelastin to elastin, are described. 

Fig. 15 Amino acid sequences of artificial extracellular matrix (aECM) proteins. Each protein contains a TV tag, a histidine tag, a cleavage site, and elastin-like domains with lysine residues for crosslinking. The RGD cell-binding domain is found in aECM 1, whereas aECM 3 contains the CS5 cell-binding domain. aECM 2 and aECM 4 are the negative controls with scrambled binding domains for aECM 1 and aECM 3, respectively. Reprinted from [121] with permission from American Chemical Society, copyright 2004...

It has been demonstrated in other cell types that lutein can inhibit expression of MMPs and/ or activity (Philips et al., 2007). For example, in dermal fibroblasts lutein inhibits expression of MMP-1 and decreases levels of MMP-2 protein (Philips et al., 2007). In melanoma cells, lutein inhibits MMP-1 expression while stimulating TIMP-2 (Philips et al., 2007). Moreover it has been shown that lutein inhibits elastin expression in fibroblasts subjected to oxidative stress by exposure to ultraviolet light (Philips et al., 2007). These results clearly indicate that lutein can play an important role in remodeling of the extracellular matrix. 

Therefore it is of interest to determine whether carotenoids can modulate the turnover of the extracellular matrix by the RPE by affecting the expression of MMPs, elastin, and/or collagen. Cultured RPE cells are a suitable model for such investigations. 

Structure of tropoelastin Elastin is synthesized from a precursor, tropoelastin, that is rich in proline and lysine, but contains only a little hydroxyproline and no hydroxylysine. In the extracellular matrix, tropoelastin is converted to elastin. 

Fibrillin microfibrils are widely distributed extracellular matrix assemblies that endow elastic and non elastic connective tissues with long-range elasticity. They direct tropoelastin deposition during elastic fibrillogenesis and form an outer mantle for mature elastic fibers. Microfibril arrays are also abundant in dynamic tissues that do not express elastin, such as the ciliary zonules of the eye. Mutations in fibrillin-1—the principal structural component of microfibrils—cause Marfan syndrome, a heritable disease with severe aortic, ocular, and skeletal defects. Isolated fibrillin-rich microfibrils have a complex 56 nm beads-on-a-string appearance the molecular basis of their assembly and... 

In developing elastic tissue, the microfibrils are the first components to appear in the extracellular matrix. They are then thought to act as a scaffold for deposition, orientation, and assembly of tropoelastin monomers. They are 10—12 nm in diameter, and lie adjacent to cells producing elastin and parallel to the long axis of the developing elastin fiber (Cleary, 1987). 

The process of coacervation is finely tuned to the physiological conditions of the extracellular matrix. Optimal coacervation of human tropo-elastin occurs at 37 °G, 150 mM NaCl, and pH 7-8 (Vrhovski et al, 1997). The arrangement of sequences in tropoelastin is critical to this process of coacervation, where association through hydrophobic domains depends on their contextual location in the molecule (Toonkool et al., 2001b). Tropoelastin association rapidly proceeds through a monomer to tuner transition, with little evidence of intermediate forms (Toonkool et al, 2001a). 

Ito, S., Ishimaru, S., and Wilson, S. E. (1998). Effect of coacervated alpha-elastin on proliferation of vascular smooth muscle and endothelial cells. Angiology 49, 289-297. Jacob, M. P., Badier-Commander, C., Fontaine, V., Benazzoug, Y., Feldman, L., and Michel, J. B. (2001). Extracellular matrix remodeling in the vascular wall. Pathol. Biol. 49, 326-332. 

Panitch, A., Yamaoka, T., Fournier, M. J., Mason, T. L., and Tirrell, D. A. (1999). Design and biosynthesis of elastin-like artificial extracellular matrix proteins containing periodically spaced fibronectin CS5 domains. Macromolecules 32, 1701-1703. 

Tissue also contains some endogenous species that exhibit fluorescence, such as aromatic amino acids present in proteins (phenylalanine, tyrosine, and tryptophan), pyridine nucleotide enzyme cofactors (e.g., oxidized nicotinamide adenine dinucleotide, NADH pyridoxal phosphate flavin adenine dinucleotide, FAD), and cross-links between the collagen and the elastin in extracellular matrix.100 These typically possess excitation maxima in the ultraviolet, short natural lifetimes, and low quantum yields (see Table 10.1 for examples), but their characteristics strongly depend on whether they are bound to proteins. Excitation of these molecules would elicit background emission that would contaminate the emission due to implanted sensors, resulting in baseline offsets or even major spectral shifts in extreme cases therefore, it is necessary to carefully select fluorophores for implants. It is also noteworthy that the lifetimes are fairly short, such that use of longer lifetime emitters in sensors would allow lifetime-resolved measurements to extract sensor emission from overriding tissue fluorescence. 

The MMPs are a family of zinc-dependent neutral endopep-tidases that share structural domains but differ in substrate specificity, cellular sources, and inductivity (Table I). All the MMPs are important for remodeling of the extra cellular matrix and share the following functional features (/) they degrade extracellular matrix components, including fibronectin, collagen, elastin, proteoglycans, and laminin, (//) they are secreted in a latent proform and require activation for proteolytic activity, (///) they contain zinc at their active site and need calcium for stability, (/V) they function at neutral pH, and (v) they are inhibited by specific tissue inhibitors of metalloproteinases (TIMPs). 

Elastic fibers form the network in skin and cardiovascular tissue (elastic arteries) that is associated with elastic recovery. Historically the recovery of skin and vessel wall on removal of mechanical loads at low strains has been attributed to elastic fibers. Elastic fibers are composed of a core of elastin surrounded by microfibrils 10 to 15 nm in diameter composed of a family of glycoproteins recently termed fibrillins. Fibrillins are a family of extracellular matrix glycoproteins (MW about 350,000) containing a large number of cysteine residues (cysteine residues form disulfide crosslinks). Several members of the family have been described. The common molecular features include N and C terminal ends with 47 tandemly repeated epi-... 

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