The C5 domain of the collagen VI alpha3(VI) chain is critical for extracellular microfibril formation and is present in the extracellular matrix of cultured cells

Matrix metalloproteinase-11/stromelysin-3 exhibits collagenolytic function against collagen VI under normal and malignant conditions

The substrate of matrix metalloproteinase 11 (MMP11) remains unknown. We have recently shown that MMP11 is a negative regulator of adipogenesis, able to reduce and even to revert mature adipocyte differentiation. Here, we have used mouse 3T3L1 cells and human U87MG and SaOS cells to show that MMP11 cleaves the native alpha3 chain of collagen VI, which is an adipocyte-related extracellular matrix component. It is known that extracellular proteolytic processing of this chain is required for correct collagen VI folding. Interestingly, MMP11-deficient fat tissue is less cohesive and exhibits collagen VI alteration, dramatic adipocyte plasma and basement membrane abnormalities and lipid leakage. MMP11 is thus required for correct collagen VI folding and therefore for fat tissue cohesion and adipocyte function. Both MMP11 and collagen VI favor tumor progression. Similar spatio-temporal overexpression at the adipocyte-cancer cell interface has been reported for the two proteins. MMP11-dependent collagen VI processing might therefore be expected to occur during malignancy. Accordingly, collagen VI no longer delineates adipocytes located at the invasive front of breast carcinomas. In conclusion, the native alpha3 chain of collagen VI constitutes a specific MMP11 substrate. This MMP11 collagenolytic activity is functional in fat tissue ontogenesis as well as during cancer invasive steps.

Three novel collagen VI chains, alpha4(VI), alpha5(VI), and alpha6(VI)

We report the identification of three new collagen VI genes at a single locus on human chromosome 3q22.1. The three new genes are COL6A4, COL6A5, and COL6A6 that encode the alpha4(VI), alpha5(VI), and alpha6(VI) chains. In humans, the COL6A4 gene has been disrupted by a chromosome break. Each of the three new collagen chains contains a 336-amino acid triple helix flanked by seven N-terminal von Willebrand factor A-like domains and two (alpha4 and alpha6 chains) or three (alpha5 chain) C-terminal von Willebrand factor A-like domains. In humans, mRNA expression of COL6A5 is restricted to a few tissues, including lung, testis, and colon. In contrast, the COL6A6 gene is expressed in a wide range of fetal and adult tissues, including lung, kidney, liver, spleen, thymus, heart, and skeletal muscle. Antibodies to the alpha6(VI) chain stained the extracellular matrix of human skeletal and cardiac muscle, lung, and the territorial matrix of articular cartilage. In cell transfection and immunoprecipitation experiments, mouse alpha4(VI)N6-C2 chain co-assembled with endogenous alpha1(VI) and alpha2(VI) chains to form trimeric collagen VI molecules that were secreted from the cell. In contrast, alpha5(VI)N5-C1 and alpha6(VI)N6-C2 chains did not assemble with alpha1(VI) and alpha2(VI) chains and accumulated intracellularly. We conclude that the alpha4(VI)N6-C2 chain contains all the elements necessary for trimerization with alpha1(VI) and alpha2(VI). In summary, the discovery of three additional collagen VI chains doubles the collagen VI family and adds a layer of complexity to collagen VI assembly and function in the extracellular matrix.

Three novel collagen VI chains with high homology to the alpha3 chain

Here we describe three novel collagen VI chains, alpha4, alpha5, and alpha6. The corresponding genes are arranged in tandem on mouse chromosome 9. The new chains structurally resemble the collagen VI alpha3 chain. Each chain consists of seven von Willebrand factor A domains followed by a collagenous domain, two C-terminal von Willebrand factor A domains, and a unique domain. In addition, the collagen VI alpha4 chain carries a Kunitz domain at the C terminus, whereas the collagen VI alpha5 chain contains an additional von Willebrand factor A domain and a unique domain. The size of the collagenous domains and the position of the structurally important cysteine residues within these domains are identical between the collagen VI alpha3, alpha4, alpha5, and alpha6 chains. In mouse, the new chains are found in or close to basement membranes. Collagen VI alpha1 chain-deficient mice lack expression of the new collagen VI chains implicating that the new chains may substitute for the alpha3 chain, probably forming alpha1alpha2alpha4, alpha1alpha2alpha5, or alpha1alpha2alpha6 heterotrimers. Due to a large scale pericentric inversion, the human COL6A4 gene on chromosome 3 was broken into two pieces and became a non-processed pseudogene. Recently COL6A5 was linked to atopic dermatitis and designated COL29A1. The identification of novel collagen VI chains carries implications for the etiology of atopic dermatitis as well as Bethlem myopathy and Ullrich congenital muscular dystrophy.

Molecular consequences of dominant Bethlem myopathy collagen VI mutations

OBJECTIVE

Dominant mutations in the three collagen VI genes cause Bethlem myopathy, a disorder characterized by proximal muscle weakness and commonly contractures of the fingers, wrists, and ankles. Although more than 20 different dominant mutations have been identified in Bethlem myopathy patients, the biosynthetic consequences of only a subset of these have been studied, and in many cases, the pathogenic mechanisms remain unknown.

METHODS

We have screened fourteen Bethlem myopathy patients for collagen VI mutations and performed detailed analyses of collagen VI biosynthesis and intracellular and extracellular assembly.

RESULTS

Collagen VI abnormalities were identified in eight patients. One patient produced around half the normal amount of alpha1(VI) messenger RNA and reduced amounts of collagen VI protein. Two patients had a previously reported mutation causing skipping of COL6A1 exon 14, and three patients had novel mutations leading to in-frame deletions toward the N-terminal end of the triple-helical domain. These mutations have different and complex effects on collagen VI intracellular and extracellular assembly. Two patients had single amino acid substitutions in the A-domains of COL6A2 and COL6A3. Collagen VI intracellular and extracellular assembly was normal in one of these patients.

INTERPRETATION

The key to dissecting the pathogenic mechanisms of collagen VI mutations lies in detailed analysis of collagen VI biosynthesis and assembly. The majority of mutations result in secretion and deposition of structurally abnormal collagen VI. However, one A-domain mutation had no detectable effect on assembly, suggesting that it acts by compromising collagen VI interactions in the extracellular matrix of muscle.

Secretion and assembly of type IV and VI collagens depend on glycosylation of hydroxylysines

Most lysines in type IV and VI collagens are hydroxylated and glycosylated, but the functions of these unique galactosylhydroxylysyl and glucosylgalactosylhydroxylysyl residues are poorly understood. The formation of glycosylated hydroxylysines is catalyzed by multifunctional lysyl hydroxylase 3 (LH3) in vivo, and we have used LH3-manipulated mice and cells as models to study the function of these carbohydrates. These hydroxylysine-linked carbohydrates were shown recently to be indispensable for the formation of basement membranes (Ruotsalainen, H., Sipilä, L., Vapola, M., Sormunen, R., Salo, A. M., Uitto, L., Mercer, D. K., Robins, S. P., Risteli, M., Aszodi, A., Fässler, R., and Myllylä, R. (2006) J. Cell Sci. 119, 625-635). Analysis of LH3 knock-out embryos and cells in this work indicated that loss of glycosylated hydroxylysines prevents the intracellular tetramerization of type VI collagen and leads to impaired secretion of type IV and VI collagens. Mice lacking the LH activity of LH3 produced slightly underglycosylated type IV and VI collagens with abnormal distribution. The altered distribution and aggregation of type VI collagen led to similar ultrastructural alterations in muscle to those detected in collagen VI knockout and some Ullrich congenital muscular dystrophy patients. Our results provide new information about the function of hydroxylysine-linked carbohydrates of collagens, indicating that they play an important role in the secretion, assembly, and distribution of highly glycosylated collagen types.

Molecular recognition in the assembly of collagens: terminal noncollagenous domains are key recognition modules in the formation of triple helical protomers

The alpha-chains of the collagen superfamily are encoded with information that specifies self-assembly into fibrils, microfibrils, and networks that have diverse functions in the extracellular matrix. A key self-organizing step, common to all collagen types, is trimerization that selects, binds, and registers cognate alpha-chains for assembly of triple helical protomers that subsequently oligomerize into specific suprastructures. In this article, we review recent findings on the mechanism of chain selection and infer that terminal noncollagenous domains function as recognition modules in trimerization and are therefore key determinants of specificity in the assembly of suprastructures. This mechanism is also illustrated with computer-generated animations.