Gara, Sudheer Kumar
(2010).
Identification and Characterization of Novel Collagen Chains.
PhD thesis, Universität zu Köln.
Abstract
Collagen VI and collagen XXVIII are two extracellular matrix proteins that belong to the superfamily of von Willebrand Factor A (VWA) domain containing molecules. Earlier studies on collagen VI indicated that this widely distributed protein is composed of α1, α2 and α3 polypeptide chains, which form a microfibrillar network in close association with basement membranes in muscle and other tissues. In contrast, an initial study on collagen XXVIII reported that it forms a homotrimer and has a very restricted localization at specific basement membranes of peripheral nerves. In this dissertation, the identification and characterization of three novel collagen VI chains, α4, α5 and α6, that show similarity to the collagen VI α3 chain is described. The genes coding for the new chains are arranged in tandem on mouse chromosome 9. The proteins contain seven N-terminal VWA domains followed by a collagenous domain, two C-terminal VWA domains and a unique domain. In addition the collagen VI α4 chain carries a Kunitz domain at the C-terminus whereas the collagen VI α5 chain contains an additional VWA domain and unique domain. The lengths of the collagenous domains and the positions of the structurally important cysteine residues are identical in the collagen VI α3, α4, α5 and α6 chains. In mouse, the new chains show a very restricted and differential expression mainly associated with basement membranes. They are sometimes detected in regions where the collagen VI α3 chain is not expressed, suggesting that the α3 chain is not required for their assembly. Analysis of the collagen VI α1 chain deficient mouse strain, confirmed that the new chains require the α1 chain and may substitute for the α3 chain, probably forming α1α2α4, α1α2α5 and α1α2α6 heterotrimers. In humans, only the genes coding for the collagen VI α5 and α6 chains are preserved. The COL6A4 gene has been inactivated due to large pericentric inversion on chromosome 3 that split the gene in two pieces and transformed it into two non-processed pseudogenes. In humans, the collagen VI α5 and α6 chains are present in close association with the basement membranes of skeletal muscle and skin. Ullrich Congenital Muscular Dystrophy (UCMD) and Bethlem Myopathy (BM) patients carrying mutations in COL6A1, COL6A2 and COL6A3 show also skin phenotypes like keloid scarring or keratosis pilaris. Immunohistochemical analysis of the new chains in the skin of UCMD and BM patients showed a disturbed staining pattern only when the COL6A1 or COL6A2 genes are affected. This indicates that the new chains may substitute for the collagen VI α3 chain forming α1α2α5 and α1α2α6 heterotrimers. However the exact role of new chains for the development of skin phenotypes in myopathy patients remains to be elucidated. The functional role of collagen XXVIII is not known. Therefore, the inactivation of the Col28a1 gene in mouse was initiated. A targeting vector disrupting the exon 2 of Col28a1 was generated in vitro, followed by ES cell targeting in vivo. Positive ES clones were injected into blastocysts and transferred to surrogate mothers, which resulted in a chimeric mice carrying both the wild type and the targeted allele. However, the targeted allele did so far not enter the germline.
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