2.4. Collagen synthesis in skeletal muscle
A characteristic property of collagens is the formation of triple helixes composed of three polypeptide chains. Fibril forming collagens consist of uninterrupted triple helixes, but other collagens have one or more triple helical domain of various length. In skeletal muscle, collagens are expressed principally by fibroblasts, and their biosynthesis is characterized by the presence of an extensive number of co- and posttranslational modifications of the polypeptide chains (Fig. 3). (Prockop & Kivirikko 1995) Gross fractional synthesis rate for collagen is about 5 %/day in skeletal muscles of young adult rats (Mays et al. 1991), whereas the fractional synthesis rate for total protein is about 11-15 %/day (Goldspink et al. 1986).
Fibril forming collagens share numerous genetic features. One common feature of the genes is the major triple helical domain of each chain coded by 42 exons, each beginning with a complete codon for glycine. Repeated -Gly-X-Y- tripeptide units enable triple helix formation. Type I collagen consists of two α1(I) chains and one α2(I) chain, while type III collagen is a homotrimer of three α1(III) chains. The structure of type IV collagen genes is distinctly different from those of fibril forming collagens. The collagenous domain of type IV collagen is longer than in the fibril forming collagens; it is however frequently interrupted with noncollagenous sequences. Most type IV collagen molecules consist of a combination of α1(IV) and α2(IV) chains, although combinations of α3(IV) and α4(IV), as well as α5(IV) and α6(IV) chains are found in some basement membranes. (Prockop & Kivirikko 1995)
Figure 2. Collagens in skeletal muscle. (Kuo et al.1997, Aumailley & Gayraud 1998 and Tu et al. 2002)
2.4.1. Intracellular events
After transcription, mRNA is extensively processed and then translated in rough endoplasmic reticulum (Fig. 3). The first step in intracellular processing of the polypeptide chain is the cleavage of signal peptides. Then the proline and lysine residues in Y-position are hydroxylated to 4-hydroxyproline and hydroxylysine by prolyl 4-hydroxylase and lysyl hydroxylase. A few X-position proline residues are hydroxylated to 3-hydroxyproline. Galactose and/or glucose are added to some of the hydroxylysine by hydroxylysyl galactosyltransferase and galactosylhydroxylysyl glucosyltransferase (GGT). Fibrillar collagens have C- and N-terminal propeptides, in which mannose-rich oligosaccharides are added while processing. Formation of the collagen trimer begins by association of the C-terminal propeptides. Triple helix is formed from a C-terminal nucleus towards the N-terminus in a zipper-like manner. Formation of intra- and interchain disulfide bonds stabilizes the structure. The processing and assembly of fibrillar and nonfibrillar collagens is principally the same, although many nonfibrillar collagens contain N- and /or C-terminal domains that are not removed and therefore not called propeptides. Different collagens may have special features in their synthesis, e.g. chain association and folding of type I and IV collagens may involve a collagen-specific stress protein, heat shock protein 47 (Hsp47). (Laurent 1987, Prockop & Kivirikko 1995, Kadler et al. 1996)
2.4.2. Extracellular events
Fibrillar collagens are secreted as soluble procollagens, which have a trimeric globular C-propeptide domain and a trimeric helical N-propeptide domain. These procollagens are converted to collagen by cleavage of terminal propeptides by procollagen N- and C-proteinases (Fig. 3). Collagen then spontaneously self-assembles into fibrils. Stabilization of the fibrils is provided by covalent cross-links generated by conversion of some of the lysine and hydroxylysine residues to aldehyde derivates by lysyl oxidase. (Laurent 1987, Prockop & Kivirikko 1995, Kadler et al. 1996)
Type IV collagen molecules form their network with different processes. The N-terminal 7-S domains of four type IV collagens are covalently joined together, while the C-terminal globular domains (NC1) of two separate type IV collagen molecules are joined together by disulfide bonds. Tight meshwork is formed by irregularly branching lateral associations of the triple helical regions. (Yurchenco & O´Rear 1994)