Expression of PSACH-associated mutant COMP in tendon fibroblasts leads to increased apoptotic cell death irrespective of the secretory characteristics of mutant COMP

Novel therapeutic interventions for pseudoachondroplasia

Pseudoachondroplasia (PSACH), a severe short-limbed dwarfing condition, is associated with life-long joint pain and early onset osteoarthritis. PSACH is caused by mutations in cartilage oligomeric matrix protein (COMP), a pentameric matricellular protein expressed primarily in cartilage and other musculoskeletal tissues. Mutations in COMP diminish calcium binding and as a result perturb protein folding and export to the extracellular matrix. Mutant COMP is retained in the endoplasmic reticulum (ER) of growth plate chondrocytes resulting in massive intracellular COMP retention. COMP trapped in the ER builds an intracellular matrix network that may prevent the normal cellular clearance mechanisms. We have shown that accumulation of intracellular matrix in mutant-COMP (MT-COMP) mice stimulates intense unrelenting ER stress, inflammation and oxidative stress. This cytotoxic stress triggers premature death of growth plate chondrocytes limiting long-bone growth. Here, we review the mutant COMP pathologic mechanisms and anti-inflammatory/antioxidant therapeutic approaches to reduce ER stress. In MT-COMP mice, aspirin and resveratrol both dampen the mutant COMP chondrocyte phenotype by decreasing intracellular accumulation, chondrocyte death and inflammatory marker expression. This reduction in chondrocyte stress translates into an improvement in long-bone growth in the MT-COMP mice. Our efforts now move to translational studies targeted at reducing the clinical consequences of MT-COMP and painful sequelae associated with PSACH.

Dissection of Thrombospondin-4 Domains Involved in Intracellular Adaptive Endoplasmic Reticulum Stress-Responsive Signaling

Thrombospondins are a family of stress-inducible secreted glycoproteins that underlie tissue remodeling. We recently reported that thrombospondin-4 (Thbs4) has a critical intracellular function, regulating the adaptive endoplasmic reticulum (ER) stress pathway through activating transcription factor 6α (Atf6α). In the present study, we dissected the domains of Thbs4 that mediate interactions with ER proteins, such as BiP (Grp78) and Atf6α, and the domains mediating activation of the ER stress response. Functionally, Thbs4 localized to the ER and post-ER vesicles and was actively secreted from cardiomyocytes, as were the type III repeat (T3R) and TSP-C domains, while the LamG domain localized to the Golgi apparatus. We also mutated the major calcium-binding motifs within the T3R domain of full-length Thbs4, causing ER retention and secretion blockade. The T3R and TSP-C domains as well as wild-type Thbs4 and the calcium-binding mutant interacted with Atf6α, induced an adaptive ER stress response, and caused expansion of intracellular vesicles. In contrast, overexpression of a related secreted oligomeric glycoprotein, Nell2, which lacks only the T3R and TSP-C domains, did not cause these effects. Finally, deletion of Atf6α abrogated Thbs4-induced vesicular expansion. Taken together, these data identify the critical intracellular functional domains of Thbs4, which was formerly thought to have only extracellular functions.

MED and PSACH COMP mutations affect chondrogenesis in chicken limb bud micromass cultures

Mutations in cartilage oligomeric matrix protein (COMP) cause pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED). We studied the effects of over-expression of wild type and mutant COMP on early stages of chondrogenesis in chicken limb bud micromass cultures. Cells were transduced with RCAS virus harboring wild type or mutant (C328R, PSACH; T585R, MED) COMP cDNAs and cultured for 3, 4, and 5 days. The effect of COMP constructs on chondrogenesis was assessed by analyzing mRNA and protein expression of several COMP binding partners. Cell viability was assayed, and evaluation of apoptosis was performed by monitoring caspase 3 processing. Over-expression of COMP, and especially expression of COMP mutants, had a profound affect on the expression of syndecan 3 and tenascin C, early markers of chondrogenesis. Over-expression of COMP did not affect levels of type II collagen or matrilin-3; however, there were increases in type IX collagen expression and sulfated proteoglycan synthesis, particularly at day 5 of harvest. In contrast to cells over-expressing COMP, cells with mutant COMP showed reduction in type IX collagen expression and increased matrilin 3 expression. Finally, reduction in cell viability, and increased activity of caspase 3, at days 4 and 5, were observed in cultures expressing either wild type or mutant COMP. MED, and PSACH mutations, despite displaying phenotypic differences, demonstrated only subtle differences in their cellular viability and mRNA and protein expression of components of the extracellular matrix, including those that interact with COMP. These results suggest that COMP mutations, by disrupting normal interactions between COMP and its binding partners, significantly affect chondrogenesis.

A mouse model offers novel insights into the myopathy and tendinopathy often associated with pseudoachondroplasia and multiple epiphyseal dysplasia

Pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED) are relatively common skeletal dysplasias belonging to the same bone dysplasia family. PSACH is characterized by generalized epi-metaphyseal dysplasia, short-limbed dwarfism, joint laxity and early onset osteoarthritis. MED is a milder disease with radiographic features often restricted to the epiphyses of the long bones. PSACH and some forms of MED result from mutations in cartilage oligomeric matrix protein (COMP), a pentameric glycoprotein found in cartilage, tendon, ligament and muscle. PSACH-MED patients often have a mild myopathy characterized by mildly increased plasma creatine kinase levels, a variation in myofibre size and/or small atrophic fibres. In some instances, patients are referred to neuromuscular clinics prior to the diagnosis of an underlying skeletal dysplasia; however, the myopathy associated with PSACH-MED has not previously been studied. In this study, we present a detailed study of skeletal muscle, tendon and ligament from a mouse model of mild PSACH harbouring a COMP mutation. Mutant mice exhibited a progressive muscle weakness associated with an increased number of muscle fibres with central nuclei at the perimysium and at the myotendinous junction. Furthermore, the distribution of collagen fibril diameters in the mutant tendons and ligaments was altered towards thicker collagen fibrils, and the tendons became more lax in cyclic strain tests. We hypothesize that the myopathy in PSACH-MED originates from an underlying tendon and ligament pathology that is a direct result of structural abnormalities to the collagen fibril architecture. This is the first comprehensive characterization of the musculoskeletal phenotype of PSACH-MED and is directly relevant to the clinical management of these patients.

Cartilage oligomeric matrix protein promotes cell attachment via two independent mechanisms involving CD47 and alphaVbeta3 integrin

Cartilage oligomeric matrix protein (COMP) is a pentameric approximately 524 kDa multidomain extracellular matrix protein and is the fifth member of the thrombospondin family. COMP is abundantly expressed in proliferating and hypertrophic chondrocytes of the growth plate, articular cartilage, synovium, tendon, and ligament. The spatial localization of COMP highlights its importance in the phenotypes of pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED), COMP disorders that are characterized by disproportionate short stature, brachydactyly, scoliosis, early-onset osteoarthritis, and joint hypermobility. In this study, the role of COMP in ligament was investigated with a series of cell attachment assays using ligament cells binding to COMP. A dose-dependent cell attachment activity was found, which was inhibited by a peptide containing the SFYVVMWK amino acid sequence derived from the globular C-terminal domain of COMP. This activity was independent of the recently described RGD-dependent attachment activity. Function-blocking antibodies to CD47 and alphaVbeta3 integrin reduced cell attachment to COMP, implicating the participation of these cell surface molecules in COMP cell binding. Immunofluorescence studies showed that cell attachment to COMP induced the formation of lamellae containing F-actin microspikes associated with fascin. We propose that COMP promotes cell attachment via two independent mechanisms involving cell surface CD47 and alphaVbeta3 integrin and that a consequence of cell attachment to COMP is the specific induction of fascin-stabilized actin microspikes.

An inducible cartilage oligomeric matrix protein mouse model recapitulates human pseudoachondroplasia phenotype

Cartilage oligomeric matrix protein (COMP) is a pentameric extracellular protein expressed in cartilage and other musculoskeletal tissues. Mutations in the COMP gene cause pseudoachondroplasia (PSACH), a severe dwarfing condition that has a growth plate chondrocyte pathology. PSACH is characterized by intracellular retention of COMP and other extracellular matrix (ECM) proteins, which form an ordered matrix within large rough endoplasmic reticulum cisternae. This accumulation is cytotoxic and causes premature chondrocyte cell death, thereby depleting chondrocytes needed for normal long bone growth. Research to define the underlying molecular mechanisms of PSACH has been hampered by the lack of a suitable model system. In this study, we achieved robust expression of human mutant (MT) or wild-type (WT) COMP in mice by using a tetracycline-inducible promoter. Normal growth plate distribution of ECM proteins was observed in 1-month-old WT-COMP and C57BL\6 control mice. In contrast, the structure of the MT-COMP growth plate recapitulated the findings of human PSACH growth plate morphology, including (1) retention of ECM proteins, (2) intracellular matrix formation in the rER cisternae, and (3) increased chondrocyte apoptosis. Therefore, we have generated the first mouse model to show extensive intracellular retention of ECM proteins recapitulating the human PSACH disease process at the cellular level.

The crystal structure of the signature domain of cartilage oligomeric matrix protein: implications for collagen, glycosaminoglycan and integrin binding

Cartilage oligomeric matrix protein (COMP), or thrombospondin-5 (TSP-5), is a secreted glycoprotein that is important for growth plate organization and function. Mutations in COMP cause two skeletal dysplasias, pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (EDM1). In this study, we determined the structure of a recombinant protein that contains the last epidermal growth factor repeat, the type 3 repeats and the C-terminal domain (CTD) of COMP to 3.15-A resolution limit by X-ray crystallography. The CTD is a beta-sandwich that is composed of 15 antiparallel beta-strands, and the type 3 repeats are a contiguous series of calcium binding sites that associate with the CTD at multiple points. The crystal packing reveals an exposed potential metal-ion-dependent adhesion site (MIDAS) on one edge of the beta-sandwich that is common to all TSPs and may serve as a binding site for collagens and other ligands. Disease-causing mutations in COMP disrupt calcium binding, disulfide bond formation, intramolecular interactions, or sites for potential ligand binding. The structure presented here and its unique molecular packing in the crystal identify potential interactive sites for glycosaminoglycans, integrins, and collagens, which are key to cartilage structure and function.

Genetic diseases of connective tissues: cellular and extracellular effects of ECM mutations

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.

Inverse association of general joint hypermobility with hand and knee osteoarthritis and serum cartilage oligomeric matrix protein levels

OBJECTIVE

Extensive joint hypermobility, lower serum cartilage oligomeric matrix protein (COMP) levels, and early-onset osteoarthritis (OA) are phenotypes of inherited pseudoachondroplasia and multiple epiphyseal dysplasia. However, few studies have evaluated the association between articular hypermobility and primary OA. We undertook the present study to evaluate this association and to test the hypothesis that COMP levels are associated with hypermobility in patients with OA and individuals without OA.

METHODS

Two separate cohorts were available for analysis, the CARRIAGE (CARolinas Region Interaction of Aging Genes and Environment) extended family and a subset of the GOGO (Genetics of Generalized Osteoarthritis) sibpair cohort. In the CARRIAGE family, we performed hand and knee examinations and hypermobility evaluations (Beighton criteria) and obtained sera for measurement of COMP and hyaluronan (HA). Data on COMP and HA levels and extensive joint radiographic and hypermobility data were also available for the GOGO cohort.

RESULTS

The prevalence of hypermobility was 13% in the CARRIAGE family and 5% in the GOGO cohort. In the CARRIAGE family, hypermobility was associated with a significantly reduced prevalence of hand (especially proximal interphalangeal joint) and knee OA and lower mean serum COMP levels, both in the total cohort and in non-hand-OA subgroups. These results were further validated in the GOGO subsets without radiographic OA, in which hypermobility was also associated with a significantly reduced mean serum COMP level (P < 0.0001 adjusted for age). Serum HA levels did not differ in relation to hypermobility in either cohort.

CONCLUSION

The present results indicate that there is an inverse relationship between hypermobility and hand and knee OA, and that hypermobility is associated with lower serum COMP levels. Genetic variations of the COMP gene may account for some subgroups of benign joint hypermobility.

Mutations targeting intermodular interfaces or calcium binding destabilize the thrombospondin-2 signature domain

Thrombospondins (THBSs) are a family of secreted calcium-binding glycoproteins with roles in angiogenesis, cell motility, apoptosis, cytoskeletal organization, and extracellular matrix organization. The THBS-2 signature domain (three epidermal growth factor (EGF)-like modules, a wire module with 13 calcium-binding repeats, and a lectin-like module) binds 30 calcium ions and forms extensive interactions among its parts. We explored the significance of these structural elements by examining the impact of 10 different mutations known to result in pseudoachondrodysplasia or multiple epiphyseal dysplasia when found in the homologous wire and lectin-like modules of thrombospondin-5 (THBS-5). A variety of observations indicate that the mutations result in unstable THBS-5 proteins that aggregate in the endoplasmic reticulum. We introduced the mutations into homologous sites of a THBS-2 construct, for which the crystal structure is known, and determined the effects of the mutations on structure as assayed by differential scanning calorimetry and expression of the epitope for the 4B6.13 conformation-sensitive antibody. Abnormalities were found in one or more of several readouts: stability of interactions between the wire and lectin-like modules, stabilities of the EGF-like and wire modules, expression of the 4B6.13 epitope in soluble protein, and expression of the 4B6.13 epitope in substrate-adsorbed protein at different calcium concentrations. The patterns of abnormalities support the idea that the EGF-like, wire, and lectin-like modules constitute a dynamic and interactive calcium-sensitive structure in which a distortion at one site is transmitted to distal sites, leading to global changes in the protein.