Tissue-specific extracellular matrices (ECMs) are crucial for normal development and tissue function, and mutations in ECM genes result in a wide range of serious inherited connective tissue disorders. Mutations cause ECM dysfunction by combinations of two mechanisms. First, secretion of the mutated ECM components can be reduced by mutations affecting synthesis or by structural mutations causing cellular retention and/or degradation. Second, secretion of mutant protein can disturb crucial ECM interactions, structure and stability. Moreover, recent experiments suggest that endoplasmic reticulum (ER) stress, caused by mutant misfolded ECM proteins, contributes to the molecular pathology. Targeting ER stress might offer a new therapeutic strategy.

Pathologies caused by mutations in extracellular matrix proteins are generally considered to result from the synthesis of extracellular matrices that are defective. Mutations in type X collagen cause metaphyseal chondrodysplasia type Schmid (MCDS), a disorder characterised by dwarfism and an expanded growth plate hypertrophic zone. We generated a knock-in mouse model of an MCDS–causing mutation (COL10A1 p.Asn617Lys) to investigate pathogenic mechanisms linking genotype and phenotype. Mice expressing the collagen X mutation had shortened limbs and an expanded hypertrophic zone. Chondrocytes in the hypertrophic zone exhibited endoplasmic reticulum (ER) stress and a robust unfolded protein response (UPR) due to intracellular retention of mutant protein. Hypertrophic chondrocyte differentiation and osteoclast recruitment were significantly reduced indicating that the hypertrophic zone was expanded due to a decreased rate of VEGF–mediated vascular invasion of the growth plate. To test directly the role of ER stress and UPR in generating the MCDS phenotype, we produced transgenic mouse lines that used the collagen X promoter to drive expression of an ER stress–inducing protein (the cog mutant of thyroglobulin) in hypertrophic chondrocytes. The hypertrophic chondrocytes in this mouse exhibited ER stress with a characteristic UPR response. In addition, the hypertrophic zone was expanded, gene expression patterns were disrupted, osteoclast recruitment to the vascular invasion front was reduced, and long bone growth decreased. Our data demonstrate that triggering ER stress per se in hypertrophic chondrocytes is sufficient to induce the essential features of the cartilage pathology associated with MCDS and confirm that ER stress is a central pathogenic factor in the disease mechanism. These findings support the contention that ER stress may play a direct role in the pathogenesis of many connective tissue disorders associated with the expression of mutant extracellular matrix proteins.

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...