Cartilage oligomeric matrix protein/thrombospondin 5 supports chondrocyte attachment through interaction with integrins

Integrin signalling in joint development, homeostasis and osteoarthritis

Integrins are key regulators of cell-matrix interactions during joint development and joint tissue homeostasis, as well as in the development of osteoarthritis (OA). The signalling cascades initiated by the interactions of integrins with a complex network of extracellular matrix (ECM) components and intracellular adaptor proteins orchestrate cellular responses necessary for maintaining joint tissue integrity. Dysregulated integrin signalling, triggered by matrix degradation products such as matrikines, disrupts this delicate balance, tipping the scales towards an environment conducive to OA pathogenesis. The interplay between integrin signalling and growth factor pathways further underscores the multifaceted nature of OA. Moreover, emerging insights into the role of endocytic trafficking in regulating integrin signalling add a new layer of complexity to the understanding of OA development. To harness the therapeutic potential of targeting integrins for mitigation of OA, comprehensive understanding of their molecular mechanisms across joint tissues is imperative. Ultimately, deciphering the complexities of integrin signalling will advance the ability to treat OA and alleviate its global burden.

Gill M, Page GL, Pope A, Measom GJ, Hager RL, Seeley MK. Serum Cartilage Oligomeric Matrix Protein Concentration Increases More After Running Than Swimming for Older People

BACKGROUND

Knee osteoarthritis is common in older people. Serum cartilage oligomeric matrix protein (sCOMP) is a biomarker of knee articular cartilage metabolism. The purpose of this study was 2-fold: to (1) determine acute effects of running and swimming on sCOMP concentration in older people; and (2) investigate relationships between sCOMP concentration change due to running and swimming and measures of knee health in older people.

HYPOTHESES

Running would result in greater increase in sCOMP concentration than swimming, and increase in sCOMP concentration due to running and swimming would associate positively with measures of poor knee health.

STUDY DESIGN

Cross-sectional.

LEVEL OF EVIDENCE

Level 3.

METHODS

A total of 20 participants ran 5 km and 19 participants swam 1500 m. sCOMP concentration was measured immediately before, immediately after, and 15, 30, and 60 minutes after running or swimming. sCOMP concentration change due to running and swimming was compared. Correlations between sCOMP concentration change due to running and swimming, and other measures of knee health were evaluated, including the Tegner Activity Scale and Knee injury and Osteoarthritis Outcome Score.

RESULTS

sCOMP concentration increased 29% immediately after running, relative to baseline, but only 6% immediately after swimming (P < 0.01). No significant relationship was observed between acute sCOMP change due to running and swimming, and observed measures of knee health (P > 0.05). Participants with clinically relevant knee symptoms exhibited greater sCOMP concentration before and after running and swimming (P = 0.03) and had greater body mass (P = 0.04).

CONCLUSION

Running results in greater acute articular cartilage metabolism than swimming; however, the chronic effects of this are unclear. Older people with clinically relevant knee symptoms possess greater sCOMP concentration and are heavier, independent of exercise mode and physical activity level.

CLINICAL RELEVANCE

These results describe the effects of exercise (running and swimming) for older physically active persons, with and without knee pain.

Thrombospondins in the tumor microenvironment

Many cancers begin with the formation of a small nest of transformed cells that can remain dormant for years. Thrombospondin-1 (TSP-1) initially promotes dormancy by suppressing angiogenesis, a key early step in tumor progression. Over time, increases in drivers of angiogenesis predominate, and vascular cells, immune cells, and fibroblasts are recruited to the tumor mass forming a complex tissue, designated the tumor microenvironment. Numerous factors, including growth factors, chemokine/cytokine, and extracellular matrix, participate in the desmoplastic response that in many ways mimics wound healing. Vascular and lymphatic endothelial cells, and cancer-associated pericytes, fibroblasts, macrophages and immune cells are recruited to the tumor microenvironment, where multiple members of the TSP gene family promote their proliferation, migration and invasion. The TSPs also affect the immune signature of tumor tissue and the phenotype of tumor-associated macrophages. Consistent with these observations, expression of some TSPs has been established to correlate with poor outcomes in specific types of cancer.

Functional characterization of a rare pathogenic variant c.875G > A, p.(Cys292Tyr) in COMP

BACKGROUND

The protein encoded by the cartilage oligomeric matrix protein (COMP) gene is a noncollagenous extracellular matrix (ECM) protein that is important for chondrocyte formation and growth. Variations in the COMP gene cause pseudoachondroplasia (PSACH), which is mainly characterized by short-limbed dwarfing in the clinic.

AIMS

To characterize the function of a rare pathogenic variant in the COMP gene (c.875G > A, p.Cys292Tyr).

MATERIALS & METHODS

We performed 3D structural analysis, in vitro expression analysis, and immunofluorescence to characterize the effects of the variant on protein structure, expression, and cellular localization respectively.

RESULTS

Variation modeling showed that the interactions between amino acids were changed after the variation, and there were 31 changes in the secondary structure of mutant COMP (MT-COMP). Western blot showed that the intracellular quantity of MT-COMP was higher than the wild-type COMP (WT-COMP). Cellular immunofluorescence results showed that WT-COMP was less abundant and homogenously distributed in cells, while the MT-COMP accumulated in the cytoplasm.

DISCUSSION

Herein, we report a variant of COMP in a Chinese family with PSACH. We have shown that the rare missense variant, COMP c.875G > A, previously reported in ClinVar and identified in our patient, results in excessive accumulation of mutant protein in the cytoplasm, and is therefore pathogenic.

CONCLUSION

Through in silico and experimental analyses, we provide evidence that COMP c.875G > A is the likely cause of PSACH in a Chinese family.

Matricellular proteins in atherosclerosis development

The extracellular matrix (ECM) is an intricate network composed of various multi-domain macromolecules like collagen, proteoglycans, and fibronectin, etc., that form a structurally stable composite, contributing to the mechanical properties of tissue. However, matricellular proteins are non-structural, secretory extracellular matrix proteins, which modulate various cellular functions via interacting with cell surface receptors, proteases, hormones, and cell-matrix. They play essential roles in maintaining tissue homeostasis by regulating cell differentiation, proliferation, adhesion, migration, and several signal transduction pathways. Matricellular proteins display a broad functionality regulated by their multiple structural domains and their ability to interact with different extracellular substrates and/or cell surface receptors. The expression of these proteins is low in adults, however, gets upregulated following injuries, inflammation, and during tumor growth. The marked elevation in the expression of these proteins during atherosclerosis suggests a positive association between their expression and atherosclerotic lesion formation. The role of matricellular proteins in atherosclerosis development has remained an area of research interest in the last two decades and studies revealed these proteins as important players in governing vascular function, remodeling, and plaque formation. Despite extensive research, many aspects of the matrix protein biology in atherosclerosis are still unknown and future studies are required to investigate whether targeting pathways stimulated by these proteins represent viable therapeutic approaches for patients with atherosclerotic vascular diseases. This review summarizes the characteristics of distinct matricellular proteins, discusses the available literature on the involvement of matrix proteins in the pathogenesis of atherosclerosis and suggests new avenues for future research.

Anatomy, molecular structures, and hyaluronic acid - Gelatin injectable hydrogels as a therapeutic alternative for hyaline cartilage recovery: A review

Cartilage damage caused by trauma or osteoarthritis is a common joint disease that can increase the social and economic burden in society. Due to its avascular characteristics, the poor migration ability of chondrocytes, and a low number of progenitor cells, the self-healing ability of cartilage defects has been significantly limited. Hydrogels have been developed into one of the most suitable biomaterials for the regeneration of cartilage because of its characteristics such as high-water absorption, biodegradation, porosity, and biocompatibility similar to natural extracellular matrix. Therefore, the present review article presents a conceptual framework that summarizes the anatomical, molecular structure and biochemical properties of hyaline cartilage located in long bones: articular cartilage and growth plate. Moreover, the importance of preparation and application of hyaluronic acid - gelatin hydrogels for cartilage tissue engineering are included. Hydrogels possess benefits of stimulating the production of Agc1, Col2α1-IIa, and SOX9, molecules important for the synthesis and composition of the extracellular matrix of cartilage. Accordingly, they are believed to be promising biomaterials of therapeutic alternatives to treat cartilage damage.

CurQ+, a Next-Generation Formulation of Curcumin, Ameliorates Growth Plate Chondrocyte Stress and Increases Limb Growth in a Mouse Model of Pseudoachondroplasia

Mutations in cartilage oligomeric matrix protein (COMP) causes protein misfolding and accumulation in chondrocytes that compromises skeletal growth and joint health in pseudoachondroplasia (PSACH), a severe dwarfing condition. Using the MT-COMP mice, a murine model of PSACH, we showed that pathological autophagy blockage was key to the intracellular accumulation of mutant-COMP. Autophagy is blocked by elevated mTORC1 signaling, preventing ER clearance and ensuring chondrocyte death. We demonstrated that resveratrol reduces the growth plate pathology by relieving the autophagy blockage allowing the ER clearance of mutant-COMP, which partially rescues limb length. To expand potential PSACH treatment options, CurQ+, a uniquely absorbable formulation of curcumin, was tested in MT-COMP mice at doses of 82.3 (1X) and 164.6 mg/kg (2X). CurQ+ treatment of MT-COMP mice from 1 to 4 weeks postnatally decreased mutant COMP intracellular retention, inflammation, restoring both autophagy and chondrocyte proliferation. CurQ+ reduction of cellular stress in growth plate chondrocytes dramatically reduced chondrocyte death, normalized femur length at 2X 164.6 mg/kg and recovered 60% of lost limb growth at 1X 82.3 mg/kg. These results indicate that CurQ+ is a potential therapy for COMPopathy-associated lost limb growth, joint degeneration, and other conditions involving persistent inflammation, oxidative stress, and a block of autophagy.

Clinical, Biochemical, Radiological, Genetic and Therapeutic Analysis of Patients with COMP Gene Variants

Pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia type 1 (MED1) are two rare skeletal disorders caused by cartilage oligomeric matrix protein (COMP) variants. This study aims to analyze the genotype and phenotype of patients with COMP variants. Clinical information for 14 probands was collected; DNA was extracted from blood for COMP variant detection. Clinical manifestations and radiology scoring systems were established to evaluate the severity of each patient's condition. Serum COMP levels in PSACH patients and healthy subjects were measured. Thirty-nine patients were included, along with 12 PSACH probands and two MED1 probands. Disproportionate short stature, waddling gait, early-onset osteoarthritis and skeletal deformities were the most common features. The height Z-score of PSACH patients correlated negatively with age at evaluation (r =  - 0.603, p = 0.01) and the clinical manifestation score (r =  - 0.556, p = 0.039). Over 50% of the PSACH patients were overweight/obese. The median serum COMP level in PSACH patients was 16.75 ng/ml, which was significantly lower than that in healthy controls (98.53 ng/ml; p < 0.001). The condition of MED1 patients was better than that of PSACH patients. Four novel variants of COMP were detected: c.874T>C, c.1123_1134del, c.1531G>A, and c.1576G>T. Height Z-scores and serum COMP levels were significantly lower in patients carrying mutations located in calmodulin-like domains 6, 7, and 8. As the two phenotypes overlap to different degrees, PSACH and MED1 are suggested to combine to produce "spondyloepiphyseal dysplasia, COMP type". Clinical manifestations and radiology scoring systems, serum COMP levels and genotype are important for evaluating patient condition severity.

Thrombospondin 2 Promotes IL-6 Production in Osteoarthritis Synovial Fibroblasts via the PI3K/AKT/NF-κB Pathway

Background

It is known that osteoarthritis (OA) pathogenesis involves inflammation that drives pathologic changes and that the matricellular protein, thrombospondin-2 (TSP2), is involved in angiogenesis, carcinogenesis, and inflammation. However, how TSP2 contributes to OA inflammatory processes is unclear.

Objective

The aim of current study was to elucidate whether TSP2 could promote interleukin-6 (IL-6), a pro-inflammatory cytokine, expression in osteoarthritis synovial fibroblasts (OASFs).

Methods

The synovial fibroblasts isolated from osteoarthritis and healthy donors were incubated with recombinant TSP2 to evaluate its effect in OA pathogenesis. The SFs were incubated with recombinant TSP2, followed by determining the IL-6 expression by qPCR and Western blot. After SFs were incubated with TSP2 for different time interval, the Western blot was performed to investigate the activation of signal pathway. The different strategies including neutralizing antibodies, siRNAs, and chemical inhibitors were used to discover the signal transduction in response to TSP2 incubation in OASFs. To evaluate the therapeutic potential of TSP2 in osteoarthritis, the anterior cruciate ligament transection (ACLT) in SD rats was performed in the presence or absence of TSP neutralizing antibody treatment.

Results

Our investigations have revealed that TSP2 promoted IL-6 expression in OASFs in a dose-dependent manner, especially in 30 and 100 ng/mL concentration (p < 0.05). Using different strategies including neutralizing antibodies, siRNAs, and chemical inhibitors, all of which attenuated signal pathway components in OASFs, we found evidence for the involvement of integrin α_vβ₃, PI3K, Akt, and NF-κB in TSP2-mediated upregulation of IL-6 (p < 0.05). Finally, in the result of rat ACLT surgical model, we found that TSP2 neutralizing antibody had protective effects in cartilage destruction during OA progression.

Conclusion

Thrombospondin-2 palys an important role in osteoarthritis pathogenesis and provides an opportunity to deal with osteoarthritis.

Spatiotemporal distribution of thrombospondin-4 and -5 in cartilage during endochondral bone formation and repair

During skeletal development most bones are first formed as cartilage templates, which are gradually replaced by bone. If later in life those bones break, temporary cartilage structures emerge to bridge the fractured ends, guiding the regenerative process. This bone formation process, known as endochondral ossification (EO), has been widely studied for its potential to reveal factors that might be used to treat patients with large bone defects. The extracellular matrix of cartilage consists of different types of collagens, proteoglycans and a variety of non-collagenous proteins that organise the collagen fibers in complex networks. Thrombospondin-5, also known as cartilage oligomeric matrix protein (TSP-5/COMP) is abundant in cartilage, where it has been described to enhance collagen fibrillogenesis and to interact with a variety of growth factors, matrix proteins and cellular receptors. However, very little is known about the skeletal distribution of its homologue thrombospondin-4 (TSP-4). In our study, we compared the spatiotemporal expression of TSP-5 and TSP-4 during postnatal bone formation and fracture healing. Our results indicate that in both these settings, TSP-5 distributes across all layers of the transient cartilage, while the localisation of TSP-4 is restricted to the population of hypertrophic chondrocytes. Furthermore, in fractured bones we observed TSP-4 sparsely distributed in the periosteum, while TSP-5 was absent. Last, we analysed the chemoattractant effects of the two proteins on endothelial cells and bone marrow stem cells and hypothesised that, of the two thrombospondins, only TSP-4 might promote blood vessel invasion during ossification. We conclude that TSP-4 is a novel factor involved in bone formation. These findings reveal TSP-4 as an attractive candidate to be evaluated for bone tissue engineering purposes.

Mechanistic Insight Into the Roles of Integrins in Osteoarthritis

Osteoarthritis (OA), one of the most common degenerative diseases, is characterized by progressive degeneration of the articular cartilage and subchondral bone, as well as the synovium. Integrins, comprising a family of heterodimeric transmembrane proteins containing α subunit and β subunit, play essential roles in various physiological functions of cells, such as cell attachment, movement, growth, differentiation, and mechanical signal conduction. Previous studies have shown that integrin dysfunction is involved in OA pathogenesis. This review article focuses on the roles of integrins in OA, especially in OA cartilage, subchondral bone and the synovium. A clear understanding of these roles may influence the future development of treatments for OA.

COMP and TSP-4: Functional Roles in Articular Cartilage and Relevance in Osteoarthritis

Osteoarthritis (OA) is a slow-progressing joint disease, leading to the degradation and remodeling of the cartilage extracellular matrix (ECM). The usually quiescent chondrocytes become reactivated and accumulate in cell clusters, become hypertrophic, and intensively produce not only degrading enzymes, but also ECM proteins, like the cartilage oligomeric matrix protein (COMP) and thrombospondin-4 (TSP-4). To date, the functional roles of these newly synthesized proteins in articular cartilage are still elusive. Therefore, we analyzed the involvement of both proteins in OA specific processes in in vitro studies, using porcine chondrocytes, isolated from femoral condyles. The effect of COMP and TSP-4 on chondrocyte migration was investigated in transwell assays and their potential to modulate the chondrocyte phenotype, protein synthesis and matrix formation by immunofluorescence staining and immunoblot. Our results demonstrate that COMP could attract chondrocytes and may contribute to a repopulation of damaged cartilage areas, while TSP-4 did not affect this process. In contrast, both proteins similarly promoted the synthesis and matrix formation of collagen II, IX, XII and proteoglycans, but inhibited that of collagen I and X, resulting in a stabilized chondrocyte phenotype. These data suggest that COMP and TSP-4 activate mechanisms to protect and repair the ECM in articular cartilage.

Comprehensive RNA expression profile of therapeutic adipose‑derived mesenchymal stem cells co‑cultured with degenerative nucleus pulposus cells

Stem cell-based therapy is a promising alternative to conventional approaches to treating intervertebral disc degeneration (IDD). However, comprehensive understanding of stem cell-based therapy at the gene level is still lacking. In the present study, we identified the expression profiles of messenger RNAs (mRNAs) and long non-coding RNAs (lncRNAs) expressed within a co-culture system of adipose-derived mesenchymal stem cells (ASCs) and degenerative nucleus pulposus cells (NPCs) and explored the signaling pathways involved and their regulatory networks. Microarray analysis was used to compare ASCs co-cultured with degenerative NPCs to ASCs cultured alone, and the underlying regulatory pattern, including the signaling pathways and competing endogenous RNA (ceRNA) network, was analyzed with robust bioinformatics methods. The results showed that 360 lncRNAs and 1757 mRNAs were differentially expressed by ASCs, and the microarray results were confirmed by quantitative PCR. Moreover, 589 Gene Ontology terms were upregulated, whereas 661 terms were downregulated. A total of 299 signaling pathways were significantly altered. A Path-net and a Signal-net were built to show interactions among differentially expressed genes. An mRNA-lncRNA co-expression network was constructed to reveal the interplay among differentially expressed mRNAs and lncRNAs, whereas a ceRNA network was built to investigate their connections with microRNAs involved in IDD. To the best of our knowledge, this original and comprehensive exploration reveals differentially expressed lncRNAs and mRNAs of ASCs stimulated by degenerative NPCs, underscoring the regulation pattern within the co-culture system at the gene level. These data may further understanding of NPC-directed differentiation of ASCs and facilitate the application of ASCs in future treatments for IDD.

Emerging Roles of Matricellular Proteins in Systemic Sclerosis

Systemic sclerosis is a rare chronic heterogenous disease that involves inflammation and vasculopathy, and converges in end-stage development of multisystem tissue fibrosis. The loss of tight spatial distribution and temporal expression of proteins in the extracellular matrix (ECM) leads to progressive organ stiffening, which is a hallmark of fibrotic disease. A group of nonstructural matrix proteins, known as matricellular proteins (MCPs) are implicated in dysregulated processes that drive fibrosis such as ECM remodeling and various cellular behaviors. Accordingly, MCPs have been described in the context of fibrosis in sclerosis (SSc) as predictive disease biomarkers and regulators of ECM synthesis, with promising therapeutic potential. In this present review, an informative summary of major MCPs is presented highlighting their clear correlations to SSc- fibrosis.

A novel COMP mutation in a Chinese family with multiple epiphyseal dysplasia

Background

Multiple epiphyseal dysplasia (MED) is a skeletal disorder characterized by delayed and irregular ossification of the epiphyses and early-onset osteoarthritis. At least 66% of the reported autosomal dominant MED (AD-MED) cases are caused by COMP mutations.

Methods

We recruited a four-generation Chinese family with early-onset hip osteoarthritis, flatfoot, brachydactyly, and mild short stature. An assessment of the family history, detailed physical examinations, and radiographic evaluations were performed on the proband and other family members, followed by the performance of whole-exome sequencing (WES). The pathogenicity of the candidate mutation was also analyzed.

Results

An AD-MED family with 10 affected members and 17 unaffected members was recruited. The main radiographic findings were symmetrical changes in the dysplastic acetabulum and femoral heads, irregular contours of the epiphyses, a shortened femoral neck, and flatfoot. Lower bone density was also observed in the ankle joints, wrist joints, and knees, as well as irregular vertebral end plates. In the proband, we identified the missense mutation c.1153G > T (p. Asp385Tyr), located in exon 11 of the COMP gene. This mutation was assessed as 'pathogenic' because of its low allele frequency and its high likelihood of co-segregation with disease in the reported family. Sanger sequencing validated the novel heterozygous mutation c.1153G > T (p. Asp385Tyr) in exon 11 of COMP in all affected individuals in the family.

Conclusions

Our results underlined a key role of the Asp385 amino acid in the protein function of COMP and confirmed the pathogenicity of the COMP (c.1153G > T; p. Asp385Tyr) mutation in AD-MED disease. We have therefore expanded the known mutational spectrum of COMP and revealed new phenotypic information for AD-MED.

Cartilage Oligomeric Matrix Protein Negatively Influences Keratinocyte Proliferation via α5β1-Integrin: Potential Relevance of Altered Cartilage Oligomeric Matrix Protein Expression in Psoriasis

In psoriasis, nonlesional skin shows alterations at the dermal-epidermal junction compared with healthy skin. Cartilage oligomeric matrix protein (COMP) is part of the papillary dermis of healthy skin, and its expression has not yet been studied in psoriatic skin. In this study, we found that COMP localization extended deeper into the dermis and formed a more continuous layer in psoriatic nonlesional skin compared with healthy skin, whereas in psoriatic lesions, COMP showed a partially discontinuous deposition at the dermal-epidermal junction. COMP and β1-integrin showed strong colocalization in nonlesional skin, where the laminin layer within the basement membrane is discontinuous. In in vitro models, the presence of exogenous COMP decreased the proliferation rate of keratinocytes, and this proliferation-suppressing effect was diminished by blocking α5β1-integrin. Our results suggest that COMP can interact with α5β1-integrin of basal keratinocytes through the disrupted basement membrane, and this interaction might stabilize the epidermis in the nonlesional state by contributing to the suppression of keratinocyte proliferation. The antiproliferative effect of COMP is likely to be relevant to other skin diseases in which chronic nonhealing wounds are coupled with massive COMP accumulation.

Novel mTORC1 Mechanism Suggests Therapeutic Targets for COMPopathies

Cartilage oligomeric matrix protein (COMP) is a large, multifunctional extracellular protein that, when mutated, is retained in the rough endoplasmic reticulum (ER). This retention elicits ER stress, inflammation, and oxidative stress, resulting in dysfunction and death of growth plate chondrocytes. While identifying the cellular pathologic mechanisms underlying the murine mutant (MT)-COMP model of pseudoachondroplasia, increased midline-1 (MID1) expression and mammalian target of rapamycin complex 1 (mTORC1) signaling was found. This novel role for MID1/mTORC1 signaling was investigated since treatments shown to repress the pathology also reduced Mid1/mTORC1. Although ER stress-inducing drugs or tumor necrosis factor α (TNFα) in rat chondrosarcoma cells increased Mid1, oxidative stress did not, establishing that ER stress- or TNFα-driven inflammation alone is sufficient to elevate MID1 expression. Since MID1 ubiquitinates protein phosphatase 2A (PP2A), a negative regulator of mTORC1, PP2A was evaluated in MT-COMP growth plate chondrocytes. PP2A was decreased, indicating de-repression of mTORC1 signaling. Rapamycin treatment in MT-COMP mice reduced mTORC1 signaling and intracellular retention of COMP, and increased proliferation, but did not change inflammatory markers IL-16 and eosinophil peroxidase. Lastly, mRNA from tuberous sclerosis-1/2-null mice brain tissue exhibiting ER stress had increased Mid1 expression, confirming the relationship between ER stress and MID1/mTORC1 signaling. These findings suggest a mechanistic link between ER stress and MID1/mTORC1 signaling that has implications extending to other conditions involving ER stress.

Muscle transcriptome analysis reveals genes and metabolic pathways related to mineral concentration in Bos indicus

Mineral content affects the biological processes underlying beef quality. Muscle mineral concentration depends not only on intake-outtake balance and muscle type, but also on age, environment, breed, and genetic factors. To unveil the genetic factors involved in muscle mineral concentration, we applied a pairwise differential gene expression analysis in groups of Nelore steers genetically divergent for nine different mineral concentrations. Here, based on significant expression differences between contrasting groups, we presented candidate genes for the genetic regulation of mineral concentration in muscle. Functional enrichment and protein-protein interaction network analyses were carried out to search for gene regulatory processes concerning each mineral. The core genetic regulation for all minerals studied, except Zn, seems to rest on interactions between components of the extracellular matrix. Regulation of adipogenesis-related pathways was also significant in our results. Antagonistic patterns of gene expression for fatty acid metabolism-related genes may explain the Cu and Zn antagonistic effect on fatty acid accumulation. Our results shed light on the role of these minerals on cell function.

Articular fibrocartilage - Why does hyaline cartilage fail to repair?

Once damaged, articular cartilage has a limited potential to repair. Clinically, a repair tissue is formed, yet, it is often mechanically inferior fibrocartilage. The use of monolayer expanded versus naïve cells may explain one of the biggest discrepancies in mesenchymal stromal/stem cell (MSC) based cartilage regeneration. Namely, studies utilizing monolayer expanded MSCs, as indicated by numerous in vitro studies, report as a main limitation the induction of type X collagen and hypertrophy, a phenotype associated with endochondral bone formation. However, marrow stimulation and transfer studies report a mechanically inferior collagen I/II fibrocartilage as the main outcome. Therefore, this review will highlight the collagen species produced during the different therapeutic approaches. New developments in scaffold design and delivery of therapeutic molecules will be described. Potential future directions towards clinical translation will be discussed. New delivery mechanisms are being developed and they offer new hope in targeted therapeutic delivery.

The effects of laser acupuncture on the modulation of cartilage extracellular matrix macromolecules in rats with adjuvant-induced arthritis

Objectives

Articular cartilage damage related to irreversible physical disability affects most patients with chronic rheumatoid arthritis (RA). Strategies targeting the preservation of cartilage function are needed. Laser acupuncture (LA) can be an emerging alternative therapy for RA; however, its molecular mechanism underlying the beneficial effect on cartilage has not been elucidated. This study aimed to examine the potential chondroprotective effects of LA on extracellular matrix (ECM) macromolecules and proinflammatory cytokines in the articular cartilage of adjuvant-induced arthritis (AIA) rats and explore its related mechanisms.

Design

Monoarthritis was induced in adult male Sprague–Dawley rats (250–300 g) via intraarticular injection of complete Freund’s adjuvant (CFA) into the tibiotarsal joint. Animals were treated with LA at BL60 and KI3 acupoints three days after CFA administration with a 780 nm GaAlAs laser at 15 J/cm² daily for ten days. The main outcome measures including paw circumference, paw withdrawal threshold, histopathology and immunoassays of tumor necrosis factor-α (TNF-α), collagen type II (CoII), cartilage oligomeric matrix protein (COMP) were analyzed.

Results

LA significantly reduced ankle edema and inflammation-induced hyperalgesia in AIA rats (P < 0.05). Moreover, the TNF-α levels were significantly decreased while CoII, COMP and proteoglycans proteins were significantly enhanced following LA stimulation of the AIA cartilage compared to those treated with sham-LA (P < 0.05).

Conclusions

LA attenuates cartilage degradation in AIA rat by suppressing TNF-α activation and up-regulating ECM macromolecules, suggesting LA might be of potential clinical interest in RA treatment.

Proteome analysis of human CD56neg NK cells reveals a homogeneous phenotype surprisingly similar to CD56dim NK cells

NK cells lacking CD56 (CD56^neg ) were first identified in chronic HIV-1 infection. However, CD56^neg NK cells also exist in healthy individuals, albeit in significantly lower numbers. Here, we provide an extensive proteomic characterisation of human CD56^neg peripheral blood NK cells of healthy donors and compare them to their CD56^dim and CD56^bright counterparts. Unbiased large-scale surface receptor profiling clustered CD56^neg cells as part of the main NK cell compartment and indicated an overall CD56^dim -like phenotype. Total proteome analyses of CD56^neg NK cells further confirmed their similarity with CD56^dim NK cells, and revealed a complete cytolytic inventory with high levels of perforin and granzyme H and M. In the present study, twelve proteins discriminated CD56^neg NK cells from CD56^dim NK cells with nine up-regulated and three down-regulated proteins in the CD56^neg NK cell population. Those proteins were functionally related to lytic granule composition and transport, interaction with the extracellular matrix, DNA transcription or repair, and proliferation. Corroborating these results, CD56^neg NK cells showed modest cytotoxicity, degranulation, and IFN-ɣ secretion as compared to CD56^dim NK cells. In conclusion, CD56^neg NK cells constitute functionally competent cells sharing many features of bona fide CD56^dim NK cells in healthy individuals, but with some distinct characteristics.

Cartilage oligomeric matrix protein promotes prostate cancer progression by enhancing invasion and disrupting intracellular calcium homeostasis

Cartilage oligomeric matrix protein (COMP) was recently implicated in the progression of breast cancer. Immunostaining of 342 prostate cancer specimens in tissue microarrays showed that COMP expression is not breast cancer-specific but also occurs in prostate cancer. The expression of COMP in prostate cancer cells correlated with a more aggressive disease with faster recurrence. Subcutaneous xenografts in immunodeficient mice showed that the prostate cancer cell line DU145 overexpressing COMP formed larger tumors in vivo as compared to mock-transfected cells. Purified COMP bound to and enhanced the invasion of DU145 cells in vitro in an integrin-dependent manner. In addition, intracellular COMP expression interfered with cellular metabolism by causing a decreased level of oxidative phosphorylation with a concurrent upregulation of lactate production (Warburg effect). Further, expression of COMP protected cells from induction of apoptosis via several pathways. The effect of COMP on metabolism and apoptosis induction was dependent on the ability of COMP to disrupt intracellular Ca²⁺ signalling by preventing Ca²⁺ release from the endoplasmic reticulum. In conclusion, COMP is a potent driver of the progression of prostate cancer, acting in an anti-apoptotic fashion by interfering with the Ca²⁺ homeostasis of cancer cells.

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.

Hypoxia decrease expression of cartilage oligomeric matrix protein to promote phenotype switching of pulmonary arterial smooth muscle cells

Extracellular matrix proteins play important roles in the development of pulmonary hypertension(pH). However, the role of Cartilage oligomeric matrix protein (COMP) in the development of hypoxia-induced pH is largely unknown. We tested the hypothesis that COMP deficiency induced by hypoxia leads to the phenotype switching of pulmonary arterial smooth muscle cells (PASMCs). The expression of COMP decreased in a chronic hypoxia rat pH model (P<0.05) and in PASMCs under hypoxia (3%O₂) (P<0.05). The expressions of differentiated marker proteins reduced in the pulmonary arteries from 5 month old COMP^-/- mice and in PASMCs under hypoxia or with the siRNA of COMP treatment under normoxia, but increased in PASMCs with adenovirus-increased COMP under hypoxia. The absorbance of cell counting kit-8 at 450nm and the expressions of proliferating cell nuclear antigen (PCNA) and osteopontin increased in PASMCs with the siRNA of COMP under normoxia (P<0.05). PCNA and osteopontin decreased in PASMCs with adenovirus-increased COMP under hypoxia (P<0.05). Additionally, the expression of bone morphogenetic protein receptor 2 (BMPR2) was reduced in COMP^-/- mice (P<0.01). Both mRNA and protein levels of bone morphogenetic protein 2 (BMP2) were lower in PASMCs with the siRNA of COMP (P<0.05). The protein level of BMP2 could be reversed by adenovirus-increased COMP under hypoxia (P<0.05). These data suggest that COMP could normally have a protective role against PASMC phenotype switching and maintain BMP2/BMPR2 signaling, and these protective actions could be lost as a result of hypoxia promoting a depletion of COMP.

Cartilage oligomeric matrix protein neoepitope in the synovial fluid of horses with acute lameness: A new biomarker for the early stages of osteoarthritis

Background

Clinical tools to diagnose the early changes of osteoarthritis (OA) that occur in the articular cartilage are lacking.

Objectives

We sought to identify and quantify a novel cartilage oligomeric matrix protein (COMP) neoepitope in the synovial fluid from the joints of healthy horses and those with different stages of OA.

Study design

In vitro quantitative proteomics and assay development with application in synovial fluids samples obtained from biobanks of well‐characterised horses.

Methods

Articular cartilage explants were incubated with or without interleukin‐1β for 25 days. Media were analysed via quantitative proteomics. Synovial fluid was obtained from either normal joints (n = 15) or joints causing lameness (n = 17) or with structural OA lesions (n = 7) and analysed for concentrations of the COMP neoepitope using a custom‐developed inhibition enzyme‐linked immunosorbent assay (ELISA). Explants were immunostained with polyclonal antibodies against COMP and the COMP neoepitopes.

Results

Semitryptic COMP peptides were identified and quantified in cell culture media from cartilage explants. A rabbit polyclonal antibody was raised against the neoepitope of the N‐terminal portion of one COMP fragment (sequence SGPTHEGVC). An inhibition ELISA was developed to quantify the COMP neoepitope in synovial fluid. The mean concentration of the COMP neoepitope significantly increased in the synovial fluid from the joints responsible for acute lameness compared with normal joints and the joints of chronically lame horses and in joints with chronic structural OA. Immunolabelling for the COMP neoepitope revealed a pericellular staining in the interleukin‐1β‐stimulated explants.

Main limitations

The ELISA is based on polyclonal antisera rather than a monoclonal antibody.

Conclusions

The increase in the COMP neoepitope in the synovial fluid from horses with acute lameness suggests that this neoepitope has the potential to be a unique candidate biomarker for the early molecular changes in articular cartilage associated with OA.

Shift of Macrophage Phenotype Due to Cartilage Oligomeric Matrix Protein Deficiency Drives Atherosclerotic Calcification

Rationale:

Intimal calcification is highly correlated with atherosclerotic plaque burden, but the underlying mechanism is poorly understood. We recently reported that cartilage oligomeric matrix protein (COMP), a component of vascular extracellular matrix, is an endogenous inhibitor of vascular smooth muscle cell calcification.

Objective:

To investigate whether COMP affects atherosclerotic calcification.

Methods and Results:

ApoE −/− COMP −/− mice fed with chow diet for 12 months manifested more extensive atherosclerotic calcification in the innominate arteries than did ApoE −/− mice. To investigate which origins of COMP contributed to atherosclerotic calcification, bone marrow transplantation was performed between ApoE −/− and ApoE −/− COMP −/− mice. Enhanced calcification was observed in mice transplanted with ApoE −/− COMP −/− bone marrow compared with mice transplanted with ApoE −/− bone marrow, indicating that bone marrow–derived COMP may play a critical role in atherosclerotic calcification. Furthermore, microarray profiling of wild-type and COMP −/− macrophages revealed that COMP-deficient macrophages exerted atherogenic and osteogenic characters. Integrin β3 protein was attenuated in COMP −/− macrophages, and overexpression of integrin β3 inhibited the shift of macrophage phenotypes by COMP deficiency. Furthermore, adeno-associated virus 2–integrin β3 infection attenuated atherosclerotic calcification in ApoE −/− COMP −/− mice. Mechanistically, COMP bound directly to β-tail domain of integrin β3 via its C-terminus, and blocking of the COMP–integrin β3 association by β-tail domain mimicked the COMP deficiency–induced shift in macrophage phenotypes. Similar to COMP deficiency in mice, transduction of adeno-associated virus 2–β-tail domain enhanced atherosclerotic calcification in ApoE −/− mice.

Conclusions:

These results reveal that COMP deficiency acted via integrin β3 to drive macrophages toward the atherogenic and osteogenic phenotype and thereby aggravate atherosclerotic calcification.

Deficiency of Thrombospondin-4 in Mice Does Not Affect Skeletal Growth or Bone Mass Acquisition, but Causes a Transient Reduction of Articular Cartilage Thickness

Although articular cartilage degeneration represents a major public health problem, the underlying molecular mechanisms are still poorly characterized. We have previously utilized genome-wide expression analysis to identify specific markers of porcine articular cartilage, one of them being Thrombospondin-4 (Thbs4). In the present study we analyzed Thbs4 expression in mice, thereby confirming its predominant expression in articular cartilage, but also identifying expression in other tissues, including bone. To study the role of Thbs4 in skeletal development and integrity we took advantage of a Thbs4-deficient mouse model that was analyzed by undecalcified bone histology. We found that Thbs4-deficient mice do not display phenotypic differences towards wildtype littermates in terms of skeletal growth or bone mass acquisition. Since Thbs4 has previously been found over-expressed in bones of Phex-deficient Hyp mice, we additionally generated Thbs4-deficient Hyp mice, but failed to detect phenotypic differences towards Hyp littermates. With respect to articular cartilage we found that Thbs4-deficient mice display transient thinning of articular cartilage, suggesting a protective role of Thbs4 for joint integrity. Gene expression analysis using porcine primary cells revealed that Thbs4 is not expressed by synovial fibroblasts and that it represents the only member of the Thbs gene family with specific expression in articular, but not in growth plate chondrocytes. In an attempt to identify specific molecular effects of Thbs4 we treated porcine articular chondrocytes with human THBS4 in the absence or presence of conditioned medium from porcine synovial fibroblasts. Here we did not observe a significant influence of THBS4 on proliferation, metabolic activity, apoptosis or gene expression, suggesting that it does not act as a signaling molecule. Taken together, our data demonstrate that Thbs4 is highly expressed in articular chondrocytes, where its presence in the extracellular matrix is required for articular cartilage integrity.

COMP: an endogenous thrombin inhibitor

In this issue of Blood, Liang and colleagues demonstrate that cartilage oligomeric matrix protein (COMP) acts as a major endogenous plasma- and platelet-derived inhibitor of thrombin activity in vitro and in vivo.

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.

Pilot investigation of the molecular discrimination of human osteoblasts from different bone entities

In oral and maxillofacial surgery, autologous grafts from the iliac crest remain the 'gold standard' for alveolar ridge reconstruction, whereas intraoral bone grafts are considered in smaller defects. To date, a comparison of the osteogenic potential of osteoblasts with regard to their tissue origin is missing. Primary osteoblasts have proven useful for the investigation of the tissue-specific osteogenic properties. The present study compares primary human alveolar (aHOBs) and iliac osteoblasts (iHOBs) derived from three female patients undergoing routine intraoral bone grafting. Proliferation potential of the osteoblasts was evaluated using real-time impedance monitoring. Relative gene expression of bone specific biomarkers was analyzed and quantified using quantitative polymerase chain reactions (qPCR). Immunohistochemistry and phase contrast microscopy were performed, as well as alkaline phosphatase assay and alizarin red staining to visualize morphology and mineralization capacity. A twofold faster proliferation rate of aHOBs compared with iHOBs (130 h vs. 80 h) was observed. Alkaline phosphatase activity and alizarin red staining in both HOBs indicated similar mineralization capacity. Gene expression of seven genes (BMP1, CSF-1, TGFBR1, ICAM1, VCAM1, SPP1 and DLX5) was significantly higher in iHOB than in aHOB samples. These data suggest a higher osteogenic potential of osteoblasts derived from the iliac crest compared with primary osteoblasts from the alveolar bone and may lead to a better understanding of the molecular impact of bone cells from different bone entities on bone regeneration in alveolar ridge reconstructions.

A 3D Porous Gelatin-Alginate-Based-IPN Acts as an Efficient Promoter of Chondrogenesis from Human Adipose-Derived Stem Cells

Cartilage has limited regeneration potential. Thus, there is an imperative need to develop new strategies for cartilage tissue engineering (CTE) amenable for clinical use. Recent CTE approaches rely on optimal cell-scaffold interactions, which require a great deal of optimization. In this study we attempt to build a novel gelatin- (G-) alginate- (A-) polyacrylamide (PAA) 3D interpenetrating network (IPN) with superior performance in promoting chondrogenesis from human adipose-derived stem cells (hADSCs). We show that our G-A-PAA scaffold is capable of supporting hADSCs proliferation and survival, with no apparent cytotoxic effect. Moreover, we find that after exposure to prochondrogenic conditions a key transcription factor known to induce chondrogenesis, namely, Sox9, is highly expressed in our hADSCs/G-A-PAA bioconstruct, along with cartilage specific markers such as collagen type II, CEP68, and COMP extracellular matrix (ECM) components. These data suggest that our G-A-PAA structural properties and formulation might enable hADSCs conversion towards functional chondrocytes. We conclude that our novel G-A-PAA biomatrix is a good candidate for prospective in vivo CTE applications.

Antioxidant and anti-inflammatory agents mitigate pathology in a mouse model of pseudoachondroplasia

Pseudoachondroplasia (PSACH), a severe short-limb dwarfing condition, results from mutations that cause misfolding of the cartilage oligomeric matrix protein (COMP). Accumulated COMP in growth plate chondrocytes activates endoplasmic reticulum stress, leading to inflammation and chondrocyte death. Using a MT-COMP mouse model of PSACH that recapitulates the molecular and clinical PSACH phenotype, we previously reported that oxidative stress and inflammation play important and unappreciated roles in PSACH pathology. In this study, we assessed the ability of antioxidant and anti-inflammatory agents to affect skeletal and cellular pathology in our mouse model of PSACH. Treatment of MT-COMP mice with aspirin or resveratrol from birth to P28 decreased mutant COMP intracellular retention and chondrocyte cell death, and restored chondrocyte proliferation. Inflammatory markers associated with cartilage degradation and eosinophils were present in the joints of untreated juvenile MT-COMP mice, but were undetectable in treated mice. Most importantly, these treatments resulted in significantly increased femur length. This is the first and only therapeutic approach shown to mitigate both the chondrocyte and long-bone pathology of PSACH in a mouse model and suggests that reducing inflammation and oxidative stress early in the disease process may be a novel approach to treat this disorder.

In vitro 30 nm silver nanoparticles promote chondrogenesis of human mesenchymal stem cells

Silver nanoparticles positively influence chondrogenesis of human mesenchymal stem cells through promoting expression of chondrogenic markers while reducing hypertrophy.

COMP does not directly modify the expression of genes involved in cartilage homeostasis in contrast to several other cartilage matrix proteins

OBJECTIVE

We investigated whether COMP may modify cartilage metabolism and play a role as an endogenous disease aggravating factor in OA.

MATERIALS AND METHODS

Full-length and momomeric COMP was recombinantly expressed in human embryonic kidney cells and purified it via affinity chromatography. Purified COMP was used to stimulate either primary human chondrocytes or cartilage explants. Changes in the expression profiles of inflammatory genes, differentiation markers and growth factors were examined by immunoassay and by quantitative real-time reverse-transcription polymerase chain reaction.

RESULTS

Incubation of primary human chondrocytes or cartilage explants in the presence of COMP did not induce statistically significant changes in the expression of IL-6, MMP1, MMP13, collagen I, collagen II, collagen X, TGF-β1 and BMP-2.

CONCLUSIONS

In contrast to collagen II and matrilin-3, COMP lacks the ability to trigger a proinflammatory response in chondrocytes, although it carries an RGD motif and can bind to integrins. COMP is a well-accepted biomarker for osteoarthritis but increased COMP levels do not necessarily correlate with inflammation.

Integrins and chondrocyte-matrix interactions in articular cartilage

The integrin family of cell adhesion receptors plays a major role in mediating interactions between cells and the extracellular matrix. Normal adult articular chondrocytes express α1β1, α3β1, α5β1, α10β1, αVβ1, αVβ3, and αVβ5 integrins, while chondrocytes from osteoarthritic tissue also express α2β1, α4β1, α6β1. These integrins bind a host of cartilage extracellular matrix (ECM) proteins, most notably fibronectin and collagen types II and VI, which provide signals that regulate cell proliferation, survival, differentiation, and matrix remodeling. By initiating signals in response to mechanical forces, chondrocyte integrins also serve as mechanotransducers. When the cartilage matrix is damaged in osteoarthritis, fragments of fibronectin are generated that signal through the α5β1 integrin to activate a pro-inflammatory and pro-catabolic response which, if left unchecked, could contribute to progressive matrix degradation. The cell signaling pathways activated in response to excessive mechanical signals and to fibronectin fragments are being unraveled and may represent useful therapeutic targets for slowing or stopping progressive matrix destruction in arthritis.

Cartilage oligomeric matrix protein and its binding partners in the cartilage extracellular matrix: interaction, regulation and role in chondrogenesis

Thrombospondins (TSPs) are widely known as a family of five calcium-binding matricellular proteins. While these proteins belong to the same family, they are encoded by different genes, regulate different cellular functions and are localized to specific regions of the body. TSP-5 or Cartilage Oligomeric Matrix Protein (COMP) is the only TSP that has been associated with skeletal disorders in humans, including pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED). The pentameric structure of COMP, the evidence that it interacts with multiple cellular proteins, and the recent reports of COMP acting as a 'lattice' to present growth factors to cells, inspired this review of COMP and its interacting partners. In our review, we have compiled the interactions of COMP with other proteins in the cartilage extracellular matrix and summarized their importance in maintaining the structural integrity of cartilage as well as in regulating cellular functions.

Revisiting the matricellular concept

The concept of a matricellular protein was first proposed by Paul Bornstein in the mid-1990s to account for the non-lethal phenotypes of mice with inactivated genes encoding thrombospondin-1, tenascin-C, or SPARC. It was also recognized that these extracellular matrix proteins were primarily counter or de-adhesive. This review reappraises the matricellular concept after nearly two decades of continuous investigation. The expanded matricellular family as well as the diverse and often unexpected functions, cellular location, and interacting partners/receptors of matricellular proteins are considered. Development of therapeutic strategies that target matricellular proteins are discussed in the context of pathology and regenerative medicine.

Pseudoachondroplasia/COMP - translating from the bench to the bedside

Pseudoachondroplasia (PSACH) is a skeletal dysplasia characterized by disproportionate short stature, small hands and feet, abnormal joints and early onset osteoarthritis. PSACH is caused by mutations in thrombospondin-5 (TSP-5, also known as cartilage oligomeric matrix protein or COMP), a pentameric extracellular matrix protein primarily expressed in chondrocytes and musculoskeletal tissues. The thrombospondin gene family is composed of matricellular proteins that associate with the extracellular matrix (ECM) and regulate processes in the matrix. Mutations in COMP interfere with calcium-binding, protein conformation and export to the extracellular matrix, resulting in inappropriate intracellular COMP retention. This accumulation of misfolded protein is cytotoxic and triggers premature death of chondrocytes during linear bone growth, leading to shortened long bones. Both in vitro and in vivo models have been employed to study the molecular processes underlying development of the PSACH pathology. Here, we compare the strengths and weaknesses of current mouse models of PSACH and discuss how the resulting phenotypes may be translated to clinical therapies.

Abnormal chondrocyte apoptosis in the cartilage growth plate is influenced by genetic background and deletion of CHOP in a targeted mouse model of pseudoachondroplasia

Pseudoachondroplasia (PSACH) is an autosomal dominant skeletal dysplasia caused by mutations in cartilage oligomeric matrix protein (COMP) and characterised by short limbed dwarfism and early onset osteoarthritis. Mouse models of PSACH show variable retention of mutant COMP in the ER of chondrocytes, however, in each case a different stress pathway is activated and the underlying disease mechanisms remain largely unknown. T585M COMP mutant mice are a model of moderate PSACH and demonstrate a mild ER stress response. Although mutant COMP is not retained in significant quantities within the ER of chondrocytes, both BiP and the pro-apoptotic ER stress-related transcription factor CHOP are mildly elevated, whilst bcl-2 levels are decreased, resulting in increased and spatially dysregulated chondrocyte apoptosis. To determine whether the abnormal chondrocyte apoptosis observed in the growth plate of mutant mice is CHOP-mediated, we bred T585M COMP mutant mice with CHOP-null mice to homozygosity, and analysed the resulting phenotype. Although abnormal apoptosis was alleviated in the resting zone following CHOP deletion, the mutant growth plates were generally more disorganised. Furthermore, the bone lengths of COMP mutant CHOP null mice were significantly shorter at 9 weeks of age when compared to the COMP mutant mice, including a significant difference in the skull length. Overall, these data demonstrate that CHOP-mediated apoptosis is an early event in the pathobiology of PSACH and suggest that the lack of CHOP, in conjunction with a COMP mutation, may lead to aggravation of the skeletal phenotype via a potentially synergistic effect on endochondral ossification.

Growth factor priming differentially modulates components of the extracellular matrix proteome in chondrocytes and synovium-derived stem cells

To make progress in cartilage repair it is essential to optimize protocols for two-dimensional cell expansion. Chondrocytes and SDSCs are promising cell sources for cartilage repair. We previously observed that priming with a specific growth factor cocktail (1 ng/mL transforming growth factor-β1, 5 ng/mL basic fibroblast growth factor, and 10 ng/mL platelet-derived growth factor-BB) in two-dimensional culture, led to significant improvement in mechanical and biochemical properties of synovium-derived stem cell (SDSC)-seeded constructs. The current study assessed the effect of growth factor priming on the proteome of canine chondrocytes and SDSCs. In particular, growth factor priming modulated the proteins associated with the extracellular matrix in two-dimensional cultures of chondrocytes and SDSCs, inducing a partial dedifferentiation of chondrocytes (most proteins associated with cartilage were down-regulated in primed chondrocytes) and a partial differentiation of SDSCs (some collagen-related proteins were up-regulated in primed SDSCs). However, when chondrocytes and SDSCs were grown in pellet culture, growth factor-primed cells maintained their chondrogenic potential with respect to glycosaminoglycan and collagen production. In conclusion, the strength of the label-free proteomics technique is that it allows for the determination of changes in components of the extracellular matrix proteome in chondrocytes and SDSCs in response to growth factor priming, which could help in future tissue engineering strategies.

Cartilage oligomeric matrix protein in idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive and life threatening disease with median survival of 2.5-3 years. The IPF lung is characterized by abnormal lung remodeling, epithelial cell hyperplasia, myofibroblast foci formation, and extracellular matrix deposition. Analysis of gene expression microarray data revealed that cartilage oligomeric matrix protein (COMP), a non-collagenous extracellular matrix protein is among the most significantly up-regulated genes (Fold change 13, p-value <0.05) in IPF lungs. This finding was confirmed at the mRNA level by nCounter® expression analysis in additional 115 IPF lungs and 154 control lungs as well as at the protein level by western blot analysis. Immunohistochemical analysis revealed that COMP was expressed in dense fibrotic regions of IPF lungs and co-localized with vimentin and around pSMAD3 expressing cells. Stimulation of normal human lung fibroblasts with TGF-β1 induced an increase in COMP mRNA and protein expression. Silencing COMP in normal human lung fibroblasts significantly inhibited cell proliferation and negatively impacted the effects of TGF-β1 on COL1A1 and PAI1. COMP protein concentration measured by ELISA assay was significantly increased in serum of IPF patients compared to controls. Analysis of serum COMP concentrations in 23 patients who had prospective blood draws revealed that COMP levels increased in a time dependent fashion and correlated with declines in force vital capacity (FVC). Taken together, our results should encourage more research into the potential use of COMP as a biomarker for disease activity and TGF-β1 activity in patients with IPF. Hence, studies that explore modalities that affect COMP expression, alleviate extracellular matrix rigidity and lung restriction in IPF and interfere with the amplification of TGF-β1 signaling should be persuaded.

Mild myopathy is associated with COMP but not MATN3 mutations in mouse models of genetic skeletal diseases

Pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED) are skeletal disorders resulting from mutations in COMP, matrilin-3 or collagen IX and are characterised by short-limbed dwarfism and premature osteoarthritis. Interestingly, recent reports suggest patients can also manifest with muscle weakness. Here we present a detailed analysis of two mouse models of the PSACH/MED disease spectrum; ΔD469 T3-COMP (PSACH) and V194D matrilin-3 (MED). In grip test experiments T3-COMP mice were weaker than wild-type littermates, whereas V194D mice behaved as controls, confirming that short-limbed dwarfism alone does not contribute to PSACH/MED-related muscle weakness. Muscles from T3-COMP mice showed an increase in centronuclear fibers at the myotendinous junction. T3-COMP tendons became more lax in cyclic testing and showed thicker collagen fibers when compared with wild-type tissue; matrilin-3 mutant tissues were indistinguishable from controls. This comprehensive study of the myopathy associated with PSACH/MED mutations enables a better understanding of the disease progression, confirms that it is genotype specific and that the limb weakness originates from muscle and tendon pathology rather than short-limbed dwarfism itself. Since some patients are primarily diagnosed with neuromuscular symptoms, this study will facilitate better awareness of the differential diagnoses that might be associated with the PSACH/MED spectrum and subsequent care of PSACH/MED patients.

Absolute quantification of selected proteins in the human osteoarthritic secretome

Osteoarthritis (OA) is characterized by a loss of extracellular matrix which is driven by catabolic cytokines. Proteomic analysis of the OA cartilage secretome enables the global study of secreted proteins. These are an important class of molecules with roles in numerous pathological mechanisms. Although cartilage studies have identified profiles of secreted proteins, quantitative proteomics techniques have been implemented that would enable further biological questions to be addressed. To overcome this limitation, we used the secretome from human OA cartilage explants stimulated with IL-1β and compared proteins released into the media using a label-free LC-MS/MS-based strategy. We employed QconCAT technology to quantify specific proteins using selected reaction monitoring. A total of 252 proteins were identified, nine were differentially expressed by IL-1 β stimulation. Selected protein candidates were quantified in absolute amounts using QconCAT. These findings confirmed a significant reduction in TIMP-1 in the secretome following IL-1β stimulation. Label-free and QconCAT analysis produced equivocal results indicating no effect of cytokine stimulation on aggrecan, cartilage oligomeric matrix protein, fibromodulin, matrix metalloproteinases 1 and 3 or plasminogen release. This study enabled comparative protein profiling and absolute quantification of proteins involved in molecular pathways pertinent to understanding the pathogenesis of OA.

ADAMTS-7 forms a positive feedback loop with TNF-α in the pathogenesis of osteoarthritis

OBJECTIVE

To examine the expression of ADAMTS-7 during the progression of osteoarthritis (OA), defining its role in the pathogenesis of OA, and elucidating the molecular events involved.

METHODS

ADAMTS-7 expression in cartilage of a rat OA model was assayed using immunohistochemistry. Cartilage-specific ADAMTS-7 transgenic mice and ADAMTS-7 small interfering (si)RNA knockdown mice were generated and used to analyse OA progression in both spontaneous and surgically induced OA models. Cartilage degradation and OA was evaluated using Safranin-O staining, immunohistochemistry, ELISA and western blotting. In addition, mRNA expression of tumour necrosis factor (TNF)-α and metalloproteinases known to be involved in cartilage degeneration in OA was analysed. Furthermore, the transactivation of ADAMTS-7 by TNF-α and its downstream NF-κB signalling was measured using reporter gene assay.

RESULTS

ADAMTS-7 expression was elevated during disease progression in the surgically induced rat OA model. Targeted overexpression of ADAMTS-7 in chondrocytes led to chondrodysplasia characterised by short-limbed dwarfism and a delay in endochondral ossification in 'young mice' and a spontaneous OA-like phenotype in 'aged' mice. In addition, overexpression of ADAMTS-7 led to exaggerated breakdown of cartilage and accelerated OA progression, while knockdown of ADAMTS-7 attenuated degradation of cartilage matrix and protected against OA development, in surgically induced OA models. ADAMTS-7 upregulated TNF-α and metalloproteinases associated with OA; in addition, TNF-α induced ADAMTS-7 through NF-κB signalling.

CONCLUSIONS

ADAMTS-7 and TNF-α form a positive feedback loop in the regulation of cartilage degradation and OA progression, making them potential molecular targets for prevention and treatment of joint degenerative diseases, including OA.

Cartilage oligomeric matrix protein enhances osteogenesis by directly binding and activating bone morphogenetic protein-2

Bone morphogenetic proteins (BMPs) are effective for bone regeneration, and are used clinically. However, supraphysiological doses are required, which limits their use. Cartilage oligomeric matrix protein is an extracellular matrix protein, which we have previously shown can bind to growth factors of the TGFs family, suggesting that COMP may also bind to BMP-2. Rather than being a passive component of the matrix, COMP may serve as an "instructive matrix" component capable of increasing local growth factor concentration, slowing the diffusion of growth factors, and promoting their biological activity. The purpose of this investigation was to determine whether COMP binds to BMP-2, and whether it promotes the biological activity of BMP-2 with respect to osteogenesis. We found that COMP binds BMP-2, and characterized the biochemical nature of the binding interaction. COMP binding enhanced BMP-2-induced intracellular signaling through Smad proteins, increased the levels of BMP receptors, and up-regulated the luciferase activity from a BMP-2-responsive reporter construct. COMP binding enhanced BMP-2-dependent osteogenesis in vitro, in the C2C12 cell line and in primary human bone mesenchymal stem cells, as measured by alkaline phosphatase activity, matrix mineralization, and gene expression. Finally, we found that COMP enhanced BMP-2-dependent ectopic bone formation in a rat model assessed histologically, by alkaline phosphatase activity, gene expression, and micro-CT. In summary, this study demonstrates that COMP enhances the osteogenic activity of BMP-2, both in-vitro and in-vivo.