SUMMARY Activation of the immune response during injury is a critical early event that determines whether the outcome of tissue restoration is regeneration or replacement of the damaged tissue with a scar. The mechanisms by which immune signals control these fundamentally different regenerative pathways are largely unknown. We have demonstrated that, during skin repair in mice, interleukin-4 receptor α (IL-4Rα)-dependent macrophage activation controlled collagen fibril assembly and that this process was important for effective repair while having adverse pro-fibrotic effects. We identified Relm-α as one important player in the pathway from IL-4Rα signaling in macrophages to the induction of lysyl hydroxylase 2 (LH2), an enzyme that directs persistent pro-fibrotic collagen cross-links, in fibroblasts. Notably, Relm-β induced LH2 in human fibroblasts, and expression of both factors was increased in lipodermatosclerosis, a condition of excessive human skin fibrosis. Collectively, our findings provide mechanistic insights into the link between type 2 immunity and initiation of pro-fibrotic pathways.

Native supramolecular assemblies containing collagen VI microfibrils and associated extracellular matrix proteins were isolated from Swarm rat chondrosarcoma tissue. Their composition and spatial organization were characterized by electron microscopy and immunological detection of molecular constituents. The small leucine-rich repeat (LRR) proteoglycans biglycan and decorin were bound to the N-terminal region of collagen VI. Chondroadherin, another member of the LRR family, was identified both at the N and C termini of collagen VI. Matrilin-1, -3, and -4 were found in complexes with biglycan or decorin at the N terminus. The interactions between collagen VI, biglycan, decorin, and matrilin-1 were studied in detail and revealed a biglycan/matrilin-1 or decorin/matrilin-1 complex acting as a linkage between collagen VI microfibrils and aggrecan or alternatively collagen II. The complexes between matrilin-1 and biglycan or decorin were also reconstituted in vitro. Colocalization of collagen VI and the different ligands in the pericellular matrix of cultured chondrosarcoma cells supported the physiological relevance of the observed interactions in matrix assembly.Connective tissues are characterized by an abundant extracellular matrix in which a wide variety of different proteins and proteoglycans assemble into multimolecular complexes, often in the form of networks. The fibrillar collagens are major components (for review, see Ref. 1) and, in cartilage, collagen II forms cross-striated fibrils in association with collagen IX and XI (2). Collagen VI is another member of the collagen family that distinguishes itself by containing large globular domains at its N and C termini (3-6). The molecule consists of three genetically distinct ␣-chains, ␣1(VI), ␣2(VI), and ␣3(VI). The N-terminal globular region is composed of nine or ten von Willebrand factor (vWF) 1 A-like domains derived from the ␣3-chain. Collagen VI molecules associate laterally in an antiparallel fashion into dimers that are stabilized by disulfide bridges (3, 4, 7). The dimers aggregate further into tetramers that are secreted into the extracellular matrix (7), where they join end to end into microfibrils. These subsequently form characteristic thin beaded filaments that are found in a variety of tissues (3,8,9). The formation of microfibrils was recently shown to depend on the N5 vWFA-like domain of ␣3(VI) (10). In addition to the collagens, the large hyaluronan-binding proteoglycan, aggrecan is a major constituent of the cartilage extracellular matrix. The aggrecan core protein has a molecular weight of ϳ220 kDa (11) These major constituents provide the basic organization of the extracellular matrix, while other molecules modulate its assembly and structure. The matrilins are a family of oligomeric matrix proteins containing common structural motifs such as vWFA-like domains, epidermal growth factor-like EGF modules and coiled-coil regions (reviewed in Ref. 18). Matrilin-1 (also known as cartilage matrix protein, CMP), and matrilin-3 are abundant in ca...

Using forward genetics, we have identified the genes mutated in two classes of zebrafish fin mutants. The mutants of the first class are characterized by defects in embryonic fin morphogenesis, which are due to mutations in a Laminin subunit or an Integrin alpha receptor, respectively. The mutants of the second class display characteristic blistering underneath the basement membrane of the fin epidermis. Three of them are due to mutations in zebrafish orthologues of FRAS1, FREM1, or FREM2, large basement membrane protein encoding genes that are mutated in mouse bleb mutants and in human patients suffering from Fraser Syndrome, a rare congenital condition characterized by syndactyly and cryptophthalmos. Fin blistering in a fourth group of zebrafish mutants is caused by mutations in Hemicentin1 (Hmcn1), another large extracellular matrix protein the function of which in vertebrates was hitherto unknown. Our mutant and dose-dependent interaction data suggest a potential involvement of Hmcn1 in Fraser complex-dependent basement membrane anchorage. Furthermore, we present biochemical and genetic data suggesting a role for the proprotein convertase FurinA in zebrafish fin development and cell surface shedding of Fras1 and Frem2, thereby allowing proper localization of the proteins within the basement membrane of forming fins. Finally, we identify the extracellular matrix protein Fibrillin2 as an indispensable interaction partner of Hmcn1. Thus we have defined a series of zebrafish mutants modelling Fraser Syndrome and have identified several implicated novel genes that might help to further elucidate the mechanisms of basement membrane anchorage and of the disease's aetiology. In addition, the novel genes might prove helpful to unravel the molecular nature of thus far unresolved cases of the human disease.

Here we describe three novel collagen VI chains, ␣4, ␣5, and ␣6. The corresponding genes are arranged in tandem on mouse chromosome 9. The new chains structurally resemble the collagen VI ␣3 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 ␣4 chain carries a Kunitz domain at the C terminus, whereas the collagen VI ␣5 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 ␣3, ␣4, ␣5, and ␣6 chains. In mouse, the new chains are found in or close to basement membranes. Collagen VI ␣1 chain-deficient mice lack expression of the new collagen VI chains implicating that the new chains may substitute for the ␣3 chain, probably forming ␣1␣2␣4, ␣1␣2␣5, or ␣1␣2␣6 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.Members of the collagen protein superfamily play important roles in maintaining extracellular matrix structure and function. To date 28 family members are known (1, 2), among which the fibril-forming collagens and the FACIT collagens form large subgroups. In addition, several collagens exist that have highly specific functions. Among these, collagen VI forms a distinct network of microfibrils in most connective tissues. Electron microscopy revealed a beaded filament structure of the microfibrils (3). The ␣1, ␣2, and ␣3 chains of collagen VI form heterotrimeric monomers that already intracellularly assemble to dimers and tetramers (4, 5). After secretion, filaments are formed by end to end interactions of the preassembled tetramers.The three previously known collagen VI chains contain a relatively short collagenous domain of about 335 residues together with VWA 3 domains, which are the characteristic non-collagenous domains of collagen VI. A common feature of VWA domains is their involvement in the formation of multiprotein complexes (6). Whereas all three collagen VI chains contain two C-terminal VWA domains, the ␣1 and ␣2 chains carry only one and the ␣3 chain ten VWA domains at the N terminus (7,8). In addition, the ␣3 chain contains a unique domain with similarities to salivary gland proteins, a fibronectin type III repeat, and a bovine pancreatic trypsin inhibitor/ Kunitz family of serine protease inhibitor domain (Kunitz domain) at the C terminus (8). It was suggested that the VWA domains play a role in the assembly of collagen VI (9 -11). However, recently the analysis of lysyl hydroxylase 3-deficient mouse embryos indicated...

The matrilins are a family of four noncollagenous oligomeric extracellular matrix proteins with a modular structure. Matrilins can act as adapters which bridge different macromolecular networks. We therefore investigated the effect of collagen IX deficiency on matrilin-3 integration into cartilage tissues. Mice harboring a deleted Col9a1 gene lack synthesis of a functional protein and produce cartilage fibrils completely devoid of collagen IX. Newborn collagen IX knockout mice exhibited significantly decreased matrilin-3 and cartilage oligomeric matrix protein (COMP) signals, particularly in the cartilage primordium of vertebral bodies and ribs. In the absence of collagen IX, a substantial amount of matrilin-3 is released into the medium of cultured chondrocytes instead of being integrated into the cell layer as in wild-type and COMP-deficient cells. Gene expression of matrilin-3 is not affected in the absence of collagen IX, but protein extraction from cartilage is greatly facilitated. Matrilin-3 interacts with collagen IX-containing cartilage fibrils, while fibrils from collagen IX knockout mice lack matrilin-3, and COMP-deficient fibrils exhibit an intermediate integration. In summary, the integration of matrilin-3 into cartilage fibrils occurs both by a direct interaction with collagen IX and indirectly with COMP serving as an adapter. Matrilin-3 can be considered as an interface component, capable of interconnecting macromolecular networks and mediating interactions between cartilage fibrils and the extrafibrillar matrix.

Disruption to endochondral ossification leads to delayed and irregular bone formation and can result in a heterogeneous group of genetic disorders known as the chondrodysplasias. One such disorder, multiple epiphyseal dysplasia (MED), is characterized by mild dwarfism and early-onset osteoarthritis and can result from mutations in the gene encoding matrilin-3 (MATN3). To determine the disease mechanisms that underpin the pathophysiology of MED we generated a murine model of epiphyseal dysplasia by knocking-in a matn3 mutation. Mice that are homozygous for the mutation develop a progressive dysplasia and have short-limbed dwarfism that is consistent in severity with the relevant human phenotype. Mutant matrilin-3 is retained within the rough endoplasmic reticulum of chondrocytes and is associated with an unfolded protein response. Eventually, there is reduced proliferation and spatially dysregulated apoptosis of chondrocytes in the cartilage growth plate, which is likely to be the cause of disrupted linear bone growth and the resulting short-limbed dwarfism in the mutant mice.

Matrilin-3 is a recently identified member of the superfamily of proteins containing von Willebrand factor A-like domains and is able to form hetero-oligomers with matrilin-1 (cartilage matrix protein) via a C-terminal coiled-coil domain. Full-length matrilin-3 and a fragment lacking the assembly domain were expressed in 293-EBNA cells, purified, and subjected to biochemical characterization. Recombinantly expressed full-length matrilin-3 occurs as monomers, dimers, trimers, and tetramers, as detected by electron microscopy and SDSpolyacrylamide gel electrophoresis, whereas matrilin-3, purified from fetal calf cartilage, forms homotetramers as well as hetero-oligomers of variable stoichiometry with matrilin-1. In the matrix formed by cultured chondrosarcoma cells, matrilin-3 is found in a filamentous, collagen-dependent network connecting cells and in a collagen-independent pericellular network. Affinity-purified antibodies detect matrilin-3 expression in a variety of mouse cartilaginous tissues, such as sternum, articular, and epiphyseal cartilage, and in the cartilage anlage of developing bones. It is found both inside the lacunae and in the interterritorial matrix of the resting, proliferating, hypertrophic, and calcified cartilage zones, whereas the expression is lower in the superficial articular cartilage. In trachea and in costal cartilage of adult mice, an expression was seen in the perichondrium. Furthermore, matrilin-3 is found in bone, and its expression is, therefore, not restricted to chondroblasts and chondrocytes.The matrilins constitute a recently discovered family of noncollagenous proteins (1) belonging to the von Willebrand factor A (vWFA) 1 -like domain superfamily. To date, there are four matrilins known. Matrilin-2 (2, 3) and matrilin-4 (4, 5) have a broad tissue distribution, whereas the expression of matrilin-1 (also known as cartilage matrix protein) (6 -8) and matrilin-3 (9 -11) is more restricted to skeletal tissues. The division of the family into two subgroups can also be concluded from evolutionary studies (1). The descent from a common ancestor and the divergence through duplication of whole domains indicates the possibility of the different family members providing similar functions in different tissues. The at least partially coordinated expression of matrilin-1 and -3 gains further functional significance through the recent discovery of hetero-oligomers formed by matrilin-1 and -3 in epiphyseal cartilage of fetal calf femur (12).Matrilin-3 has most features of the modular structure typical for matrilins and consists of an N-terminal vWFA-like domain, four EGF-like domains, and a C-terminal ␣-helical coiled-coil oligomerization domain (9 -11), but it lacks the second vWFAlike domain that is present in all the other matrilins. Similarly, a unique mouse matrilin-4 splice variant lacking the N-terminal vWFA-like domain was recently identified (4). In addition, matrilin-3 possesses a domain with a high content of positively charged amino acids between the N-terminal vWFA-like domain ...

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