Chapter 2. Review of the literature
- Table of Contents
- 2.1. Collagens
- 2.2. Hydroxylysine
- 2.3. Lysyl hydroxylase
- 2.4. Ehlers-Danlos syndrome type VI
- 2.5. Human diseases and animal models
2.1. Collagens
Collagens, the most abundant proteins in the human body, constitute a multigene family of extracellular matrix proteins. In addition to providing mechanical strength for various organs and tissues, they have a number of other important biological functions. Currently 27 collagen types with at least 42 distinct polypeptide chains have been identified, and their genes are dispersed among at least 15 chromosomes. In addition, more than 20 proteins with collagen-like domains have been identified (Kivirikko 1993, Brown & Timpl 1995, Prockop & Kivirikko 1995, Fitzgerald & Bateman 2001, Koch et al. 2001, 2003, Myllyharju & Kivirikko 2001, 2003, Hashimoto et al. 2002, Kielty & Grant 2002, Sato et al. 2002, Tuckwell 2002, Banyard et al. 2003, Boot-Handford et al. 2003, Pace et al. 2003).
All collagen molecules are built up of three polypeptide chains, called α-chains, containing the repeating triplet sequence Gly-X-Y, where X and Y represent amino acids other than glycine. These α-chains then wind together to form a triple helix. The presence of glycine in every third position is essential because it is small enough to fit into the restricted space in the center of the triple helix. Proline is commonly found in the X position and 4-hydroxyproline in the Y position of the Gly-X-Y triplets. These two amino acids are essential for the collagen molecule, in that they provide stability for the triple helix (van der Rest & Garrone 1991, Prockop & Kivirikko 1995, Myllyharju & Kivirikko 2001, Jenkins & Raines 2002).
2.1.1. Collagen types
Most collagen molecules form supramolecular assemblies, and the superfamily can be divided into eight families based on their polymeric structures or other features: collagens that form fibrils (types I-III, V, XI, XXIV and XXVII); collagens that are located on the surfaces of fibrils and are called fibril-associated collagens with interrupted triple helices (FACIT and structurally related collagens, types IX, XII, XIV, XVI, XIX, XX, XXI, XXII and XXVI); collagens that form hexagonal networks (types VIII and X); the family of type IV collagens found in basement membranes; type VI collagen, forming beaded filaments; type VII collagen, forming anchoring fibrils; collagens with transmembrane domains (types XIII, XVII, XXIII and XXV) and the family of type XV and XVIII collagens (Byers PH 2001, Myllyharju & Kivirikko 2001, 2003).
Increasing numbers of secreted proteins containing triple-helical collagenous domains but not defined as collagens are also being included within the collagen superfamily. According to Kielty and Grant (2002) these proteins can be divided into five categories: the collectin subgroup of C-type animal lectins, complement and related factors, metabolic molecules, cytokines and related molecules, and elastic fibre-associated molecules (Prockop & Kivirikko 1995, Beck & Brodsky 1998, Kivirikko & Pihlajaniemi 1998, Chung et al. 1999, Ezer et al. 1999, Myllyharju & Kivirikko 2001, 2003, Kielty & Grant 2002).
2.1.2. Collagen biosynthesis
Collagen biosynthesis is characterized by a large number of cotranslational and post-translational modifications, many of which are unique to collagens and other proteins with collagen-like amino acid sequences.
The post-translational processing of fibril-forming collagens can be regarded as occurring in two stages (see Figure 1). Intracellular modifications, together with the synthesis of the polypeptide chains, result in the formation of the triple-helical procollagen molecule. These modifications, which take place when the procollagen chains are translocated across the ER membrane into the lumen and are being synthesized, include cleavage of the signal peptides; hydroxylation of certain proline and lysine residues to 4-hydroxyproline, 3-hydroxyproline and hydroxylysine; glycosylation of some of the hydroxylysine residues to galactosyl-hydroxylysine and glucosylgalactosyl-hydroxylysine; glycosylation of certain asparagine residues in one or both of the propeptides; chain association; disulphide bonding and formation of the triple helix (Prockop & Kivirikko 1995, Kadler 1995, Myllyharju & Kivirikko 2001, Kielty & Grant 2002).
Extracellular processing then converts the procollagens to collagens and incorporates the collagen molecules into stable, crosslinked fibrils or other supramolecular aggregates. The extracellular steps include cleavage of the N and C propeptides, self-assembly of the collagen molecules into fibrils by nucleation and propagation, and formation of covalent crosslinks (Prockop & Kivirikko 1995, Kadler et al. 1996, Prockop et al. 1998, Myllyharju & Kivirikko 2001).
The intracellular modifications require five specific enzymes: prolyl 4-hydroxylase, prolyl 3-hydroxylase, lysyl hydroxylase, hydroxylysyl galactosyltransferase and galactosyl-hydroxylysyl glucosyltransferase, while the extracellular modifications require procollagen N-proteinase, procollagen C-proteinase and lysyl oxidase (Kivirikko & Pihlajaniemi 1998, Prockop et al. 1998, Smith-Mungo & Kagan 1998, Myllyharju & Kivirikko 2001, Kagan & Li 2003, Myllyharju 2003). Additional important enzymes are peptidyl proline cis-trans isomerases and protein disulphide isomerase, and collagen synthesis also involves a specific molecular chaperone, Hsp47 (Nagata 1998, Lamande & Bateman 1999, Hendershot & Bulleid 2000).
Figure 1. Biosynthesis of a fibril-forming collagen. Procollagen polypeptide chains are synthesized on the ribosomes of the rough endoplasmic reticulum and secreted into the lumen, where they are modified by hydroxylation of certain proline and lysine residues and glycosylation before chain association and triple helix formation. The procollagen molecules are secreted into the extracellular space where the N and C propeptides are cleaved by specific proteases. The collagen molecules then assemble into fibrils, which are stabilized by the formation of covalent crosslinks (Reproduced from Myllyharju and Kivirikko 2001, with the permission of Taylor & Francis AB).
2.1.3. Extracellular matrix and connective tissues
The extracellular matrix (ECM) is a complex mixture of structural and functional proteins arranged into a unique, tissue-specific three-dimensional ultrastructure. These proteins provide a natural scaffold for tissue and organ morphogenesis, maintenance, and regeneration following injury (Alberts et al. 2002).
Animal connective tissues consist mostly of extracellular matrix, with collagens and elastin providing the tensile strength of the tissue. The cells producing collagen have different names in different tissues, being known as chondrocytes in cartilage, osteoblasts in bone and fibroblasts in skin and some other tissues (Alberts et al. 2002).
2.1.3.1. Basement membranes
Basement membranes are specialized extracellular matrices that underlie all epithelial cell sheets and tubes. They also surround individual muscle cells, fat cells and Schwann cells, separating these cells and cell sheets from the underlying or surrounding connective tissue. All basement membranes contain type IV collagen together with proteoglycans (primarily heparan sulphates) and the glycoproteins laminin and entactin (Alberts et al. 2002).
Basement membranes are largely synthesized by the cells that rest on them, and they provide a strong connection between the epithelia and the underlying connective tissue. Basement membranes also act as filtration barriers for substances moving between parenchymal cells and the connective tissue space, and provide a scaffold for the migration of cells during embryogenesis and regeneration (Leeson et al. 1988).
The best-studied basement membrane proteins are the laminins, which constitute the major non-collagenous basement membrane component (Timpl 1996). In order to study the initial function of basement membranes in tissue development during embryogenesis, Smyth et al. (1999) used homologous recombination to inactivate one or both of the alleles coding for the laminin γ 1 subunit. Mice that where heterozygous for the mutation had a normal phenotype and were fertile, whereas homozygous mutant embryos did not survive beyond embryonic day 5.5. These embryos lacked basement membranes, having disorganized extracellular deposits of the basement membrane proteins collagen IV and perlecan. Surprisingly, basement membranes were not necessary for the formation of the first epithelium during embryogenesis, but were first required for extra-embryonic endoderm differentiation (Smyth et al. 1999).