Alternative splicing of VWFA modules generates variants of type VI collagen alpha 3 chain with a distinctive expression pattern in embryonic chicken tissues and potentially different adhesive function

Collagen VI microfibril formation is abolished by an {alpha}2(VI) von Willebrand factor type A domain mutation in a patient with Ullrich congenital muscular dystrophy

Collagen VI is an extracellular protein that most often contains the three genetically distinct polypeptide chains, α1(VI), α2(VI), and α3(VI), although three recently identified chains, α4(VI), α5(VI), and α6(VI), may replace α3(VI) in some situations. Each chain has a triple helix flanked by N- and C-terminal globular domains that share homology with the von Willebrand factor type A (VWA) domains. During biosynthesis, the three chains come together to form triple helical monomers, which then assemble into dimers and tetramers. Tetramers are secreted from the cell and align end-to-end to form microfibrils. The precise molecular mechanisms responsible for assembly are unclear. Mutations in the three collagen VI genes can disrupt collagen VI biosynthesis and matrix organization and are the cause of the inherited disorders Bethlem myopathy and Ullrich congenital muscular dystrophy. We have identified a Ullrich congenital muscular dystrophy patient with compound heterozygous mutations in α2(VI). The first mutation causes skipping of exon 24, and the mRNA is degraded by nonsense-mediated decay. The second mutation is a two-amino acid deletion in the C1 VWA domain. Recombinant C1 domains containing the deletion are insoluble and retained intracellularly, indicating that the mutation has detrimental effects on domain folding and structure. Despite this, mutant α2(VI) chains retain the ability to associate into monomers, dimers, and tetramers. However, we show that secreted mutant tetramers containing structurally abnormal C1 VWA domains are unable to associate further into microfibrils, directly demonstrating the critical importance of a correctly folded α2(VI) C1 domain in microfibril formation.

Alternative splicing of transcripts for the alpha 3 chain of mouse collagen VI: identification of an abundant isoform lacking domains N7-N10 in mouse and human

Three distinct alpha chains form the collagen VI monomer, the alpha 3(VI) chain being much larger than the alpha 1(VI) and alpha 2(VI) chains. The alpha 3(VI) chain has 10 von Willebrand Factor type A domains of approximately 200 amino acids at the N-terminus (N1-N10) compared with only one such domain in the alpha 1(VI) and alpha 2(VI) chains. Domains N10, N9, N7 and N3 of the alpha 3(VI) chain are subject to alternative splicing in chick and/or human tissues, indicating the possibility of isoforms that have different functions depending on which N-terminal domains are included or excluded. In this study we have PCR amplified and sequenced mouse alpha 3(VI) cDNA encoding the N2-N10 domains. By reverse transcription-PCR using oligonucleotides spanning different regions of the cDNA we have undertaken a comprehensive analysis of alternative splicing of the alpha 3(VI) mRNA in embryonic and adult mouse tissues. We demonstrate that domains N10, N9 and N7 are also subject to alternative splicing in mouse tissues and in addition identify an abundant novel variant transcript that lacks all four N-terminal domains (N7-N10) in mouse tissues and human cells. We also identify less abundant transcripts that lack a large part of the N3 domain, and transcripts lacking the entire N5 domain. Using specific RNase protection assays we show that the shorter transcripts containing domains (N8+N7+N6), (N8+N6) and N6 are present at higher levels than transcripts containing the N10 and/or N9 domains, with tissue-specific variation in the levels of variant transcripts. These studies demonstrate a larger range of collagen VI protein variants than previously described.

The distribution of type VI collagen in the developing tissues of the bovine femoral head

Type VI collagen appears central to the maintenance of tissue integrity. In adult articular cartilage, type VI collagen is preferentially localised in the chondron where it may be involved in cell attachment. In actively remodelling developing cartilage, the distribution is less certain. We have used confocal immunohistochemistry and in situ hybridisation to investigate type VI collagen distribution in third trimester bovine proximal femoral epiphyses. In general, type VI collagen immunofluorescence was concentrated in the chondrocyte pericellular matrix, with staining intensity strongest in regions which persist to maturity and weakest in regions that remodel during development. Type VI collagen was also present in cartilage canals. In the growth plate and around the secondary centre of ossification, the intensity of type VI collagen stain rapidly decreased with chondrocyte maturation and was absent at hypertrophy, except where canal branches penetrated the growth plate and stain was retained around the adjacent chondrocytes. In situ hybridisation confirmed the presence of type VI collagen mRNA in cartilage canal mesenchymal cells but the signal was low in chondrocytes, suggesting minimal levels of synthesis and turnover. The results are consistent with a role for type VI collagen in stabilising the extracellular matrix during development.

Identification of domains responsible for von Willebrand factor type VI collagen interaction mediating platelet adhesion under high flow

We have identified type VI collagen (Col VI) as a primary subendothelial extracellular matrix component responsible for von Willebrand factor (vWF)-dependent platelet adhesion and aggregation under high tensile strength. Intact tetrameric Col VI was the form of the collagen found to be capable of promoting vWF-mediated platelet adhesion/aggregation under this shear condition, whereas removal of the predominant portion of the terminal globules by pepsin treatment abrogated its activity. The inability of the pepsin-digested Col VI to support any platelet interaction at high flow was because of the failure of the A3(vWF) domain to bind to this form of collagen, suggesting a stringent requirement of a tridimensional conformation or of intactness of its macromolecular structure. In contrast, the A1(vWF) domain bound to both intact and pepsin-digested Col VI tetramers but, in accordance with the cooperating function of the two vWF domains, failed to support platelet adhesion/aggregation under high shear onto Col VI by itself. The putative A1(vWF) binding site resided within the A7(VI) module (residues 413-613) of the globular amino-terminal portion of the alpha3(VI) chain. Soluble recombinant A7(VI) polypeptide strongly perturbed the vWF-mediated platelet adhesion to Col VI under high shear rates, without affecting the binding of the vWF platelet receptor glycoprotein Ibalpha to its cognate ligand A1(vWF). The findings provide evidence for a concerted action of the A1(vWF) and A3(vWF) domains in inducing platelet arrest on Col VI. This is accomplished via an interaction of the A1(vWF) domain with a site contained in the alpha3 chain A7(VI) domain and via a conformation-dependent interaction of the A3(vWF) domain with the intact tetrameric collagen. The data further emphasize that Col VI microfilaments linking the subendothelial basement membrane to the interstitial collagenous network may play a pivotal role in the hemostatic process triggered upon damage of the blood vessel wall.

Mutations in extracellular matrix molecules

Genetic studies of humans and mice continue to highlight the nonredundant mechanical role of components in complexes that anchor cells to extracellular matrices. At the same time, recent data provide exciting insights into nonredundant, critical roles of transcription factors in regulating differentiation and function of matrix-producing cells.