The distribution of cartilage oligomeric matrix protein (COMP) in tendon and its variation with tendon site, age and load

Novel In Vitro Platform for Studying the Cell Response to Healthy and Diseased Tendon Matrices

Current in vitro models poorly represent the healthy or diseased tendon microenvironment, limiting the translation of the findings to clinics. The present work aims to establish a physiologically relevant in vitro tendon platform that mimics biophysical aspects of a healthy and tendinopathic tendon matrix using a decellularized bovine tendon and to characterize tendon cells cultured using this platform. Bovine tendons were subjected to various decellularization techniques, with the efficacy of decellularization determined histologically. The biomechanical and architectural properties of the decellularized tendons were characterized using an atomic force microscope. Tendinopathy-mimicking matrices were prepared by treating the decellularized tendons with collagenase for 3 h or collagenase-chondroitinase (CC) for 1 h. The tendon tissue collected from healthy and tendinopathic patients was characterized using an atomic force microscope and compared to that of decellularized matrices. Healthy human tendon-derived cells (hTDCs) from the hamstring tendon were cultured on the decellularized matrices for 24 or 48 h, with cell morphology characterized using f-actin staining and gene expression characterized using real-time PCR. Tendon matrices prepared by freeze-thawing and 48 h nuclease treatment were fully decellularized, and the aligned structure and tendon stiffness (1.46 MPa) were maintained. Collagenase treatment prepared matrices with a disorganized architecture and reduced stiffness (0.75 MPa), mimicking chronic tendinopathy. Treatment with CC prepared matrices with a disorganized architecture without altering stiffness, mimicking early tendinopathy (1.52 MPa). hTDCs on a healthy tendon matrix were elongated, and the scleraxis (SCX) expression was maintained. On tendinopathic matrices, hTDCs had altered morphological characteristics and lower SCX expression. The expression of genes related to actin polymerization, matrix degradation and remodeling, and immune cell invasion were higher in hTDCs on tendinopathic matrices. Overall, the present study developed a physiological in vitro system to mimic healthy tendons and early and late tendinopathy, and it can be used to better understand tendon cell characteristics in healthy and diseased states.

Spatial gene expression in the adult rat patellar tendon

Tendons are dense connective tissues with relatively few cells which makes studying the molecular profile of the tissue challenging. There is not a consensus on the spatial location of various cell types within a tendon, nor the accompanying transcriptional profile. In the present study, we used two male rat patellar tendon samples for sequencing-based spatial transcriptomics to determine the gene expression profile. We integrated our data with a mouse Achilles single cell dataset to predict the cell type composition of the patellar tendon as a function of location within the tissue. The spatial location of the predicated cell types suggested that there were two populations of tendon fibroblasts, one located in the tendon midsubstance, while the other localized with red blood cells, pericytes, and immune cells to the tendon peripheral connective tissue. Of the highest expressed spatially variable genes, there were multiple genes with known function in tendon: Col1a1, Col1a2, Dcn, Fmod, Sparc, and Comp. Further, a novel spatially regulated gene (AABR07000398.1) with no known function was identified. The spatial gene expression of tendon associated genes (Scx, Thbs4, Tnmd, Can, Bgn, Lum, Adamts2, Lox, Ppib, Col2a1, Col3a1, Col6a2) was also visualized. Both patellar tendon samples had similar expression patterns for all these genes. This dataset provides new spatial insights into gene expression in a healthy tendon.

Tendon tissue engineering: An overview of biologics to promote tendon healing and repair

Tendons are dense connective tissues with a hierarchical polarized structure that respond to and adapt to the transmission of muscle contraction forces to the skeleton, enabling motion and maintaining posture. Tendon injuries, also known as tendinopathies, are becoming more common as populations age and participation in sports/leisure activities increases. The tendon has a poor ability to self-heal and regenerate given its intrinsic, constrained vascular supply and exposure to frequent, severe loading. There is a lack of understanding of the underlying pathophysiology, and it is not surprising that disorder-targeted medicines have only been partially effective at best. Recent tissue engineering approaches have emerged as a potential tool to drive tendon regeneration and healing. In this review, we investigated the physiochemical factors involved in tendon ontogeny and discussed their potential application in vitro to reproduce functional and self-renewing tendon tissue. We sought to understand whether stem cells are capable of forming tendons, how they can be directed towards the tenogenic lineage, and how their growth is regulated and monitored during the entire differentiation path. Finally, we showed recent developments in tendon tissue engineering, specifically the use of mesenchymal stem cells (MSCs), which can differentiate into tendon cells, as well as the potential role of extracellular vesicles (EVs) in tendon regeneration and their potential for use in accelerating the healing response after injury.

The synergistic effect of physicochemical in vitro microenvironment modulators in human bone marrow stem cell cultures

Modern bioengineering utilises biomimetic cell culture approaches to control cell fate during in vitro expansion. In this spirit, herein we assessed the influence of bidirectional surface topography, substrate rigidity, collagen type I coating and macromolecular crowding (MMC) in human bone marrow stem cell cultures. In the absence of MMC, surface topography was a strong modulator of cell morphology. MMC significantly increased extracellular matrix deposition, albeit in a globular manner, independently of the surface topography, substrate rigidity and collagen type I coating. Collagen type I coating significantly increased cell metabolic activity and none of the assessed parameters affected cell viability. At day 14, in the absence of MMC, none of the assessed genes was affected by surface topography, substrate rigidity and collagen type I coating, whilst in the presence of MMC, in general, collagen type I α1 chain, tenascin C, osteonectin, bone sialoprotein, aggrecan, cartilage oligomeric protein and runt-related transcription factor were downregulated. Interestingly, in the presence of the MMC, the 1000 kPa grooved substrate without collagen type I coating upregulated aggrecan, cartilage oligomeric protein, scleraxis homolog A, tenomodulin and thrombospondin 4, indicative of tenogenic differentiation. This study further supports the notion for multifactorial bioengineering to control cell fate in culture.

The Role of the Non-Collagenous Extracellular Matrix in Tendon and Ligament Mechanical Behavior: A Review

Tendon is a connective tissue that transmits loads from muscle to bone, while ligament is a similar tissue that stabilizes joint articulation by connecting bone to bone. The 70-90% of tendon and ligament's extracellular matrix (ECM) is composed of a hierarchical collagen structure that provides resistance to deformation primarily in the fiber direction, and the remaining fraction consists of a variety of non-collagenous proteins, proteoglycans, and glycosaminoglycans (GAGs) whose mechanical roles are not well characterized. ECM constituents such as elastin, the proteoglycans decorin, biglycan, lumican, fibromodulin, lubricin, and aggrecan and their associated GAGs, and cartilage oligomeric matrix protein (COMP) have been suggested to contribute to tendon and ligament's characteristic quasi-static and viscoelastic mechanical behavior in tension, shear, and compression. The purpose of this review is to summarize existing literature regarding the contribution of the non-collagenous ECM to tendon and ligament mechanics, and to highlight key gaps in knowledge that future studies may address. Using insights from theoretical mechanics and biology, we discuss the role of the non-collagenous ECM in quasi-static and viscoelastic tensile, compressive, and shear behavior in the fiber direction and orthogonal to the fiber direction. We also address the efficacy of tools that are commonly used to assess these relationships, including enzymatic degradation, mouse knockout models, and computational models. Further work in this field will foster a better understanding of tendon and ligament damage and healing as well as inform strategies for tissue repair and regeneration.

In Vitro Cellular Strain Models of Tendon Biology and Tenogenic Differentiation

Research has shown that the surrounding biomechanical environment plays a significant role in the development, differentiation, repair, and degradation of tendon, but the interactions between tendon cells and the forces they experience are complex. In vitro mechanical stimulation models attempt to understand the effects of mechanical load on tendon and connective tissue progenitor cells. This article reviews multiple mechanical stimulation models used to study tendon mechanobiology and provides an overview of the current progress in modelling the complex native biomechanical environment of tendon. Though great strides have been made in advancing the understanding of the role of mechanical stimulation in tendon development, damage, and repair, there exists no ideal in vitro model. Further comparative studies and careful consideration of loading parameters, cell populations, and biochemical additives may further offer new insight into an ideal model for the support of tendon regeneration studies.

Antibodies to cartilage oligomeric matrix protein are pathogenic in mice and may be clinically relevant in rheumatoid arthritis

Objective

Cartilage oligomeric matrix protein (COMP) is an autoantigen in rheumatoid arthritis (RA) and experimental models of arthritis. This study was undertaken to investigate the structure, function, and relevance of anti‐COMP antibodies.

Methods

We investigated the pathogenicity of monoclonal anti‐COMP antibodies in mice using passive transfer experiments, and we explored the interaction of anti‐COMP antibodies with cartilage using immunohistochemical staining. The interaction of the monoclonal antibody 15A11 in complex with its specific COMP epitope P6 was determined by x‐ray crystallography. An enzyme‐linked immunosorbent assay and a surface plasma resonance technique were used to study the modulation of calcium ion binding to 15A11. The clinical relevance and value of serum IgG specific to the COMP P6 epitope and its citrullinated variants were evaluated in a large Swedish cohort of RA patients.

Results

The murine monoclonal anti‐COMP antibody 15A11 induced arthritis in naive mice. The crystal structure of the 15A11–P6 complex explained how the antibody could bind to COMP, which can be modulated by calcium ions. Moreover, serum IgG specific to the COMP P6 peptide and its citrullinated variants was detectable at significantly higher levels in RA patients compared to healthy controls and correlated with a higher disease activity score.

Conclusion

Our findings provide the structural basis for binding a pathogenic anti‐COMP antibody to cartilage. The recognized epitope can be citrullinated, and levels of antibodies to this epitope are elevated in RA patients and correlate with higher disease activity, implicating a pathogenic role of anti‐COMP antibodies in a subset of RA patients.

Basic Components of Connective Tissues and Extracellular Matrix: Fibronectin, Fibrinogen, Laminin, Elastin, Fibrillins, Fibulins, Matrilins, Tenascins and Thrombospondins

Collagens are the most abundant components of the extracellular matrix (ECM) and many types of soft tissues. Elastin is another major component of certain soft tissues, such as arterial walls and ligaments. It is an insoluble polymer of the monomeric soluble precursor tropoelastin, and the main component of elastic fibers in matrix tissue where it provides elastic recoil and resilience to a variety of connective tissues, e.g., aorta and ligaments. Elastic fibers regulate activity of transforming growth factors β (TGFβ) through their association with fibrillin microfibrils. Elastin also plays a role in cell adhesion, cell migration, and has the ability to participate in cell signaling. Mutations in the elastin gene lead to cutis laxa. Many other molecules, though lower in quantity, function as essential, structural and/or functional components of the extracellular matrix in soft tissues. Some of these are reviewed in this chapter. Besides their basic structure, biochemistry and physiology, their roles in disorders of soft tissues are discussed only briefly as most chapters in this volume deal with relevant individual compounds. Fibronectin with its multidomain structure plays a role of "master organizer" in matrix assembly as it forms a bridge between cell surface receptors, e.g., integrins, and compounds such collagen, proteoglycans and other focal adhesion molecules. It also plays an essential role in the assembly of fibrillin-1 into a structured network. Though the primary role of fibrinogen is in clot formation, after conversion to fibrin by thrombin it also binds to a variety of compounds, particularly to various growth factors, and as such, fibrinogen is a player in cardiovascular and extracellular matrix physiology. Laminins contribute to the structure of the ECM and modulate cellular functions such as adhesion, differentiation, migration, stability of phenotype, and resistance towards apoptosis. Fibrillins represent the predominant core of microfibrils in elastic as well as non-elastic extracellular matrixes, and interact closely with tropoelastin and integrins. Not only do microfibrils provide structural integrity of specific organ systems, but they also provide basis for elastogenesis in elastic tissues. Fibrillin is important for the assembly of elastin into elastic fibers. Mutations in the fibrillin-1 gene are closely associated with Marfan syndrome. Latent TGFβ binding proteins (LTBPs) are included here as their structure is similar to fibrillins. Several categories of ECM components described after fibrillins are sub-classified as matricellular proteins, i.e., they are secreted into ECM, but do not provide structure. Rather they interact with cell membrane receptors, collagens, proteases, hormones and growth factors, communicating and directing cell-ECM traffic. Fibulins are tightly connected with basement membranes, elastic fibers and other components of extracellular matrix and participate in formation of elastic fibers. Matrilins have been emerging as a new group of supporting actors, and their role in connective tissue physiology and pathophysiology has not been fully characterized. Tenascins are ECM polymorphic glycoproteins found in many connective tissues in the body. Their expression is regulated by mechanical stress both during development and in adulthood. Tenascins mediate both inflammatory and fibrotic processes to enable effective tissue repair and play roles in pathogenesis of Ehlers-Danlos, heart disease, and regeneration and recovery of musculo-tendinous tissue. One of the roles of thrombospondin 1 is activation of TGFβ. Increased expression of thrombospondin and TGFβ activity was observed in fibrotic skin disorders such as keloids and scleroderma. Cartilage oligomeric matrix protein (COMP) or thrombospondin-5 is primarily present in the cartilage. High levels of COMP are present in fibrotic scars and systemic sclerosis of the skin, and in tendon, especially with physical activity, loading and post-injury. It plays a role in vascular wall remodeling and has been found in atherosclerotic plaques as well.

Hierarchical ultrastructure: An overview of what is known about tendons and future perspective for tendon engineering

Abnormal tendons are rarely ever repaired to the natural structure and morphology of normal tendons. To better guide the repair and regeneration of injured tendons through a tissue engineering method, it is necessary to have insights into the internal morphology, organization, and composition of natural tendons. This review summarized recent researches on the structure and function of the extracellular matrix (ECM) components of tendons and highlight the application of multiple detection methodologies concerning the structure of ECMs. In addition, we look forward to the future of multi-dimensional biomaterial design methods and the potential of structural repair for tendon ECM components. In addition, focus is placed on the macro to micro detection methods for tendons, and current techniques for evaluating the extracellular matrix of tendons at the micro level are introduced in detail. Finally, emphasis is given to future extracellular matrix detection methods, as well as to how future efforts could concentrate on fabricating the biomimetic tendons.

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Summarize recent research on the structure and function of the extracellular matrix (ECM) components of tendons.

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Comments on current research methods concerning the structure of ECMs.

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Perspective on the future of multi-dimensional detection techniques and structural repair of tendon ECM components.

Innovative Strategies in Tendon Tissue Engineering

The tendon is a highly aligned connective tissue that transmits force from muscle to bone. Each year, more than 32 million tendon injuries have been reported, in fact, tendinopathies represent at least 50% of all sports injuries, and their incidence rates have increased in recent decades due to the aging population. Current clinical grafts used in tendon treatment are subject to several restrictions and there is a significant demand for alternative engineered tissue. For this reason, innovative strategies need to be explored. Tendon replacement and regeneration are complex since scaffolds need to guarantee an adequate hierarchical structured morphology and mechanical properties to stand the load. Moreover, to guide cell proliferation and growth, scaffolds should provide a fibrous network that mimics the collagen arrangement of the extracellular matrix in the tendons. This review focuses on tendon repair and regeneration. Particular attention has been devoted to the innovative approaches in tissue engineering. Advanced manufacturing techniques, such as electrospinning, soft lithography, and three-dimensional (3D) printing, have been described. Furthermore, biological augmentation has been considered, as an emerging strategy with great therapeutic potential.

Tendon Extracellular Matrix Assembly, Maintenance and Dysregulation Throughout Life

In his Lissner Award medal lecture in 2000, Stephen Cowin asked the question: "How is a tissue built?" It is not a new question, but it remains as relevant today as it did when it was asked 20 years ago. In fact, research on the organization and development of tissue structure has been a primary focus of tendon and ligament research for over two centuries. The tendon extracellular matrix (ECM) is critical to overall tissue function; it gives the tissue its unique mechanical properties, exhibiting complex non-linear responses, viscoelasticity and flow mechanisms, excellent energy storage and fatigue resistance. This matrix also creates a unique microenvironment for resident cells, allowing cells to maintain their phenotype and translate mechanical and chemical signals into biological responses. Importantly, this architecture is constantly remodeled by local cell populations in response to changing biochemical (systemic and local disease or injury) and mechanical (exercise, disuse, and overuse) stimuli. Here, we review the current understanding of matrix remodeling throughout life, focusing on formation and assembly during the postnatal period, maintenance and homeostasis during adulthood, and changes to homeostasis in natural aging. We also discuss advances in model systems and novel tools for studying collagen and non-collagenous matrix remodeling throughout life, and finally conclude by identifying key questions that have yet to be answered.

Development of a Cartilage Oligomeric Matrix Protein Neo-Epitope Assay for the Detection of Intra-Thecal Tendon Disease

The diagnosis of tendon injury relies on clinical signs and diagnostic imaging but imaging is subjective and does not always correlate with clinical signs. A molecular marker would potentially offer a sensitive and specific diagnostic tool that could also provide objective assessment of healing for the comparison of different treatments. Cartilage Oligomeric Matrix Protein (COMP) has been used as a molecular marker for osteoarthritis in humans and horses but assays for the protein in tendon sheath synovial fluids have shown overlap between horses affected by tendinopathy and controls. We hypothesized that quantifying a COMP neoepitope would be more discriminatory of injury. COMP fragments were purified from synovial fluids of horses with intra-thecal tendon injuries and media from equine tendon explants, and mass spectrometry of a consistent and abundant fragment revealed a ~100 kDa COMP fragment with a new N-terminus at the 78th amino-acid (NH₂-TPRVSVRP) located just outside the junctional region of the protein. A competitive inhibition ELISA based on a polyclonal antibody raised to this sequence yielded more than a 10-fold rise in the mean neoepitope levels for tendinopathy cases compared to controls (5.3 ± 1.3 µg/mL (n = 7) versus 58.8 ± 64.3 µg/mL (n = 13); p = 0.002). However, there was some cross-reactivity of the neoepitope polyclonal antiserum with intact COMP, which could be blocked by a peptide spanning the neoepitope. The modified assay demonstrated a lower concentration but a significant > 500-fold average rise with tendon injury (2.5 ± 2.2 ng/mL (n = 6) versus 1029.8 ± 2188.8 ng/ml (n = 14); p = 0.013). This neo-epitope assay therefore offers a potentially useful marker for clinical use.

The "other" 15-40%: The Role of Non-Collagenous Extracellular Matrix Proteins and Minor Collagens in Tendon

Extracellular matrix (ECM) determines the physiological function of all tissues, including musculoskeletal tissues. In tendon, ECM provides overall tissue architecture, which is tailored to match the biomechanical requirements of their physiological function, that is, force transmission from muscle to bone. Tendon ECM also constitutes the microenvironment that allows tendon-resident cells to maintain their phenotype and that transmits biomechanical forces from the macro-level to the micro-level. The structure and function of adult tendons is largely determined by the hierarchical organization of collagen type I fibrils. However, non-collagenous ECM proteins such as small leucine-rich proteoglycans (SLRPs), ADAMTS proteases, and cross-linking enzymes play critical roles in collagen fibrillogenesis and guide the hierarchical bundling of collagen fibrils into tendon fascicles. Other non-collagenous ECM proteins such as the less abundant collagens, fibrillins, or elastin, contribute to tendon formation or determine some of their biomechanical properties. The interfascicular matrix or endotenon and the outer layer of tendons, the epi- and paratenon, includes collagens and non-collagenous ECM proteins, but their function is less well understood. The ECM proteins in the epi- and paratenon may provide the appropriate microenvironment to maintain the identity of distinct tendon cell populations that are thought to play a role during repair processes after injury. The aim of this review is to provide an overview of the role of non-collagenous ECM proteins and less abundant collagens in tendon development and homeostasis. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:23-35, 2020.

Age-related changes of tendon fibril micro-morphology and gene expression

Aging is hypothesized to be associated with changes in tendon matrix composition which may lead to alteration of tendon material properties and hence propensity to injury. Altered gene expression may offer insights into disease pathophysiology and thus open new perspectives toward designing pathophysiology‐driven therapeutics. Therefore, the current study aimed at identifying naturally occurring differences in tendon micro‐morphology and gene expression of newborn, young and old horses. Age‐related differences in the distribution pattern of tendon fibril thickness and in the expression of the tendon relevant genes collagen type 1 (Col1), Col3, Col5, tenascin‐C, decorin, tenomodulin, versican, scleraxis and cartilage oligomeric matrix protein were investigated. A qualitative and quantitative gene expression and collagen fibril diameter analysis was performed for the most frequently injured equine tendon, the superficial digital flexor tendon, in comparison with the deep digital flexor tendon. Most analyzed genes (Col1, Col3, Col5, tenascin‐C, tenomodulin, scleraxis) were expressed at a higher level in foals (age ≤ 6 months) than in horses of 2.75 years (age at which flexor tendons become mature in structure) and older, decorin expression increased with age. Decorin was previously reported to inhibit the lateral fusion of collagen fibrils, causing a thinner fibril diameter with increased decorin concentration. The results of this study suggested that reduction of tendon fibril diameters commonly seen in equine tendons with increasing age might be a natural age‐related phenomenon leading to greater fibril surface areas with increased fibrillar interaction and reduced sliding at the fascicular/fibrillar interface and hence a stiffer interfascicular/interfibrillar matrix. This may be a potential reason for the higher propensity to tendinopathies with increasing age.

Cellular, matrix, and mechano-biological differences in load-bearing versus positional tendons throughout development and aging: a narrative review

PURPOSE

Summarise available evidence comparing the cellular, biochemical, structural and biomechanical properties, and the changes that occur in these parameters in response to stimuli, in differentially loaded tendons across different stages of life.

METHODS

The PubMed database was searched for literature pertaining to differences between tendons using the term "tendon" or "tendinopathy", plus one or more of the following descriptors: "loading", "positional", "weight- or load-bearing", and "energy-storing". The abstracts were reviewed and relevant full-length articles retrieved and used to assemble a narrative review.

RESULTS

The incidence and prevalence of tendon disorders ("tendinopathies") is increasing in Western societies, with limited evidence that currently available treatments have any significant long-term effect on the disease course. A key emerging hypothesis is that disease in different tendons and even different regions within a tendon may be distinct. The available literature indicates that there are phenotypic differences, not only in the constitutive compositional and material properties but also in resident cells of positional compared with load-bearing tendons. Evident during early tendon growth, such differences have become well established by adulthood.

CONCLUSIONS

The pheno-endotype of tendinopathy may be distinct between load-bearing tendons compared to positional tendons, which has translational implications with regard to preventing and managing tendinopathy. Better understanding of the molecular, cellular, and biomechanical pathophysiology underlying disease phenotypes, will allow more targeted/personalised treatment and therefore improve outcomes.

Understanding Tendons: Lessons from Transgenic Mouse Models

Tendons and ligaments are connective tissues that have been comparatively less studied than muscle and cartilage/bone, even though they are crucial for proper function of the musculoskeletal system. In tendon biology, considerable progress has been made in identifying tendon-specific genes (Scleraxis, Mohawk, and Tenomodulin) in the past decade. However, besides tendon function and the knowledge of a small number of important players in tendon biology, neither the ontogeny of the tenogenic lineage nor signaling cascades have been fully understood. This results in major drawbacks in treatment and repair options following tendon degeneration. In this review, we have systematically evaluated publications describing tendon-related genes, which were studied in depth and characterized by using knockout technologies and the subsequently generated transgenic mouse models (Tg) (knockout mice, KO). We report in a tabular manner, that from a total of 24 tendon-related genes, in 22 of the respective knockout mouse models, phenotypic changes were detected. Additionally, in some of the models it was described at which developmental stages these changes appeared and progressed. To summarize, only loss of Scleraxis and TGFβ signaling led to severe tendon developmental phenotypes, while mice deficient for various proteoglycans, Mohawk, EGR1 and 2, and Tenomodulin presented mild phenotypes. These data suggest that the tendon developmental system is well organized, orchestrated, and backed up; this is even more evident among the members of the proteoglycan family, where the compensatory effects are much clearer. In future, it will be of great importance to discover additional master tendon transcription factors and the genes that play crucial roles in tendon development. This would improve our understanding of the genetic makeup of tendons, and will increase the chances of generating tendon-specific drugs to advance overall treatment strategies.

Effect of increased mechanical knee joint loading during running on the serum concentration of cartilage oligomeric matrix protein (COMP)

The purpose of the study was to investigate the effect of an increase in mechanical knee joint loading during running on the serum COMP level. On two different test days, 20 healthy men ran with knee orthoses for 30 min on a treadmill (v = 2.2 m/s). On day 1, the orthoses were passive, whereas on day 2 they were pneumatically driven (active) and thus increased the external knee flexion moments (+30.9 Nm) during stance phase. Lower-limb mechanics and serum COMP levels (baseline; 0, 0.5, 1, 2 h post running) were analyzed. COMP levels increased immediately after running with passive (+35%; pre: 7.5 U/l, 95%CI: 6.4, 8.7, post: 9.8 U/l, 95%CI: 8.8, 10.8, p < 0.001) and active orthoses (+45%; pre: 7.6 U/l; 95%CI: 6.4, 8.8, post: 10.3 U/l, 95%CI: 9.2, 11.5, p < 0.001), but they did not differ between interventions. While running with active orthoses, greater ankle dorsiflexion angles, knee flexion angles, and moments occurred (p < 0.05). Comparing both interventions, the Δ COMP pre-post, meaning the difference (Δ) between running with active and passive orthoses in pre to post COMP level change (=level after (post) running minus level before (pre) running), correlated negatively with Δ COMP baseline (difference between the baseline COMP level before running with active and passive orthoses, r = -0.616; p = 0.004), and with a positive tendence with the Δ maximum knee flexion (r = 0.388; p = 0.091). Therefore, changes in COMP concentration after physical activity seem to be highly influenced by the COMP baseline level. In addition, correlation analysis indicates that modifications in knee joint kinematics have a greater effect on cartilage metabolism than an increase in joint moments. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1937-1946, 2018.

The distribution patterns of COMP and matrilin-3 in septal, alar and triangular cartilages of the human nose

The biomechanical characteristics of septal cartilage depend strongly on the distinct extracellular matrix of cartilage tissue; therefore, it is essential that the components of this matrix are identified and understood. Cartilage oligomeric matrix protein (COMP) and matrilin-3 are localised in articular cartilage. This study was the first to examine all subtypes of mature human nasal cartilages (alar, triangular and septal) with specific attention to the distribution of COMP and matrilin-3. Three whole fresh-frozen noses from human donors were dissected, and exemplary biopsies were examined using histochemical staining (haematoxylin and eosin and Alcian blue) and immunohistochemistry (collagen II, COMP and matrilin-3). The following three zones within the nasal cartilage were identified: superficial, intermediate and central. COMP was detected as highest in the intermediate zones in all three subtypes of nasal cartilage, whereas matrilin-3 was detected with pericellular deposition mainly within septal cartilage predominantly in the superficial zones. The distinct staining patterns of COMP and matrilin-3 underscore the different functional roles of both proteins in nasal cartilage. According to the literature, COMP might be involved with collagen II in the formation of networks, whereas matrilin-3 is reported to prevent ossification or regulate mechanosensitivity. The predominant staining observed in septal cartilage suggests matrilin-3's modulatory role because of its presence in the osteochondral junctional zone and given that the biomechanical load in septal cartilage is different from that in alar or triangular cartilage. In conclusion, COMP and matrilin-3 were detected in mature human nasal cartilage but displayed different staining patterns that might be explained by the functional roles of the respective matrix protein; however, further research is necessary to identify and define the functional aspects of this morphological difference.

Lubricin binds cartilage proteins, cartilage oligomeric matrix protein, fibronectin and collagen II at the cartilage surface

Lubricin, a heavily O-glycosylated protein, is essential for boundary lubrication of articular cartilage. Strong surface adherence of lubricin is required given the extreme force it must withstand. Disulfide bound complexes of lubricin and cartilage oligomeric matrix protein (COMP) have recently been identified in arthritic synovial fluid suggesting they may be lost from the cartilage surface in osteoarthritis and inflammatory arthritis. This investigation was undertaken to localise COMP-lubricin complexes within cartilage and investigate if other cartilage proteins are involved in anchoring lubricin to the joint. Immunohistochemical analysis of human cartilage biopsies showed lubricin and COMP co-localise to the cartilage surface. COMP knockout mice, however, presented with a lubricin layer on the articular cartilage leading to the further investigation of additional lubricin binding mechanisms. Proximity ligation assays (PLA) on human cartilage biopsies was used to localise additional lubricin binding partners and demonstrated that lubricin bound COMP, but also fibronectin and collagen II on the cartilage surface. Fibronectin and collagen II binding to lubricin was confirmed and characterised by solid phase binding assays with recombinant lubricin fragments. Overall, COMP, fibronectin and collagen II bind lubricin, exposed on the articular cartilage surface suggesting they may be involved in maintaining essential boundary lubrication.

Human adipose tissue-derived tenomodulin positive subpopulation of stem cells: A promising source of tendon progenitor cells

Cell-based therapies are of particular interest for tendon and ligament regeneration given the low regenerative potential of these tissues. Adipose tissue is an abundant source of stem cells, which may be employed for the healing of tendon lesions. However, human adult multipotent adipose-derived stem cells (hASCs) isolated from the stromal vascular fraction of adipose tissue originate highly heterogeneous cell populations that hinder their use in specific tissue-oriented applications. In this study, distinct subpopulations of hASCs were immunomagnetic separated and their tenogenic differentiation capacity evaluated in the presence of several growth factors (GFs), namely endothelial GF, basic-fibroblast GF, transforming GF-β1 and platelet-derived GF-BB, which are well-known regulators of tendon development, growth and healing. Among the screened hASCs subpopulations, tenomodulin-positive cells were shown to be more promising for tenogenic applications and therefore this subpopulation was further studied, assessing tendon-related markers (scleraxis, tenomodulin, tenascin C and decorin) both at gene and protein level. Additionally, the ability for depositing collagen type I and III forming extracellular matrix structures were weekly assessed up to 28 days. The results obtained indicated that tenomodulin-positive cells exhibit phenotypical features of tendon progenitor cells and can be biochemically induced towards tenogenic lineage, demonstrating that this subset of hASCs can provide a reliable source of progenitor cells for therapies targeting tendon regeneration.

Quantification of cartilage oligomeric matrix protein (COMP) and a COMP neoepitope in synovial fluid of patients with different joint disorders by novel automated assays

OBJECTIVE

To develop automated immunoassays for the quantification of Cartilage Oligomeric Matrix Protein (COMP) and a COMP neoepitope in synovial fluid and to investigate their diagnostic potential in different joint conditions.

METHODS

Two sandwich immunoassays were developed for the quantification of COMP and a COMP neoepitope, using an automated analyser (IDS-iSYS, Immunodiagnostic Systems, Boldon, UK). Assay performance was evaluated in terms of sensitivity, recovery, linearity, and intra- and inter-assay precision. Clinical performance was evaluated by analysing synovial fluid from patients diagnosed with rheumatoid arthritis (RA), reactive arthritis (ReA), osteoarthritis (OA) or acute trauma (AT).

RESULTS

Both automated assays showed good performance for all parameters tested. Quantification of these biomarkers showed the highest median values for Total COMP in the OA group, followed by the AT group, the ReA group, and the RA group. For the COMP neoepitope the AT group showed the highest median value, followed by the ReA group, the OA group, and the RA group. The ratio COMP neoepitope/Total COMP showed distinct differences between the patients groups, as well as between RA patients with slow or rapid progression of joint damage.

CONCLUSIONS

The newly developed automated assays have a good technical performance, can reliably quantify different epitopes on the COMP molecule and show different levels of the two biomarkers in synovial fluid in patients with different joint diseases. The combination of these two assays, measuring their ratio, shows promise for early detection of patients with RA with different prognosis regarding progression of joint damage.

Effects of substrate stiffness on the tenoinduction of human mesenchymal stem cells

Extracellular matrix modulus plays an important role in regulating cell morphology, proliferation and differentiation during regular and diseased states. Although the effects of substrate topography and modulus on MSC differentiation are well known with respect to osteogenesis and adipogenesis, there has been relatively little investigation on the effects of this phenomenon on tenogenesis. Furthermore, relative roles of topographical factors (matrix alignment vs. matrix modulus) in inducing tenogenic differentiation is not well understood. In this study we investigated the effects of modulus and topographical alignment of type I collagen substrate on tendon differentiation. Type I collagen sheet substrates with random topographical alignment were fabricated with their moduli tuned in the range of 0.1, 1, 10 and 100MPa by using electrocompaction and controlled crosslinking. In one of the groups, topographical alignment was introduced at 10MPa stiffness, by controlled unidirectional stretching of the sheet. RT-PCR, immunohistochemistry and immunofluorescence results showed that mimicking the tendon topography, i.e. increasing the substrate modulus as well as alignment increased the tenogenic differentiation. Higher substrate modulus increased the expression of COLI, COLIII, COMP and TSP-4 about 2-3-fold and increased the production of COLI, COLIII and TSP-4 about 2-4-fold. Substrate alignment up regulated COLIII and COMP expression by 2-fold. Therefore, the tenoinductive collagen material model developed in this study can be used in the research and development of tissue engineering tendon repair constructs in future.

STATEMENT OF SIGNIFICANCE

Although the effects of substrate topography and modulus on MSC differentiation are well known with respect to osteogenesis and adipogenesis, there has been relatively little investigation on the effects of this phenomenon on tenogenesis. Furthermore, a relative role of topographical factors (matrix alignment vs. matrix modulus) in inducing tenogenic differentiation is not well understood. We investigated the effects of modulus and topographical alignment of type I collagen substrate on tendon differentiation. This study showed mimicking the tendon topography, i.e. increasing the substrate modulus as well as alignment increased the tenogenic differentiation. Therefore, the tenoinductive collagen material model developed in this study can be used in the research and development of tissue engineering tendon repair constructs in future.

A quantitative label-free analysis of the extracellular proteome of human supraspinatus tendon reveals damage to the pericellular and elastic fibre niches in torn and aged tissue

Tears of the human supraspinatus tendon are common and often cause painful and debilitating loss of function. Progressive failure of the tendon leading to structural abnormality and tearing is accompanied by numerous cellular and extra-cellular matrix (ECM) changes in the tendon tissue. This proteomics study aimed to compare torn and aged rotator cuff tissue to young and healthy tissue, and provide the first ECM inventory of human supraspinatus tendon generated using label-free quantitative LC-MS/MS. Employing two digestion protocols (trypsin and elastase), we analysed grain-sized tendon supraspinatus biopsies from older patients with torn tendons and from healthy, young controls. Our findings confirm measurable degradation of collagen fibrils and associated proteins in old and torn tendons, suggesting a significant loss of tissue organisation. A particularly marked reduction of cartilage oligomeric matrix protein (COMP) raises the possibility of using changes in levels of this glycoprotein as a marker of abnormal tissue, as previously suggested in horse models. Surprisingly, and despite using an elastase digestion for validation, elastin was not detected, suggesting that it is not highly abundant in human supraspinatus tendon as previously thought. Finally, we identified marked changes to the elastic fibre, fibrillin-rich niche and the pericellular matrix. Further investigation of these regions may yield other potential biomarkers and help to explain detrimental cellular processes associated with tendon ageing and tendinopathy.

A high-glucose diet affects Achilles tendon healing in rats

Chronic and acute tendinopathies are difficult to treat and tendon healing is generally a very slow and incomplete process and our general understanding of tendon biology and regeneration lags behind that of muscle or bone. Although still largely unexplored, several studies suggest a positive effect of nutritional interventions on tendon health and repair. With this study, we aim to reveal effects of a high-glucose diet on tendon neoformation in a non-diabetic rat model of Achilles tenotomy. After surgery animals received either a high-glucose diet or a control diet for 2 and 4 weeks, respectively. Compared to the control group, tendon repair tissue thickness and stiffness were increased in the high-glucose group after 2 weeks and gait pattern was altered after 1 and 2 weeks. Cell proliferation was up to 3-fold higher and the expression of the chondrogenic marker genes Sox9, Col2a1, Acan and Comp was significantly increased 2 and 4 weeks post-surgery. Further, a moderate increase in cartilage-like areas within the repair tissue was evident after 4 weeks of a high-glucose diet regimen. In summary, we propose that a high-glucose diet significantly affects tendon healing after injury in non-diabetic rats, potentially driving chondrogenic degeneration.

Painful locking of the wrist in a patient with pseudoachondroplasia confirmed by COMP mutation

We report the case of a 40-year-old woman with pseudoachondroplasia (PSACH), with a heterozygous mutation (c.806A > G, p.Asp269Gly) located in the Type 3 repeats domain of the cartilage oligomeric matrix protein gene, who complained of the unusual symptom of painful locking of the wrist. Her condition was caused by a non-traumatic enlargement of the extensor carpi radialis longus (ECRL) and brevis (ECRB) tendons along with bulbous swelling of the synoviums around them. Surgical treatment resolved these unusual tendon-related symptoms. Repetitive mechanical loading of the wrist in daily activities, including distal intersection tenosynovitis between the extensor pollicis longus tendon and ECRL and ECRB tendons, may have contributed to changes in the structural integrity of the tendons. We should pay more attention to tendon-related symptoms in patients with PSACH.

Differential expression of extracellular matrix genes in glenohumeral capsule of shoulder instability patients

Anterior shoulder instability is a common orthopedic problem. After a traumatic shoulder dislocation, patients present a plastic deformation of the capsule. The shoulder instability biology remains poorly understood. We evaluated the expression of genes that encode the cartilage oligomeric matrix protein (COMP), fibronectin 1 (FN1), tenascin C (TNC) and tenascin XB (TNXB) in the glenohumeral capsule of anterior shoulder instability patients and controls. Moreover, we investigated the associations between gene expression and clinical parameters. The gene expression was evaluated by quantitative reverse transcription-polymerase chain reaction in the antero-inferior (macroscopically injured region), antero-superior and posterior regions of the capsule of 29 patients with shoulder instability and 8 controls. COMP expression was reduced and FN1 and TNC expression was increased in the antero-inferior capsule region of cases compared to controls (p < 0.05). TNC expression was increased in the posterior capsule portion of shoulder instability patients (p = 0.022). COMP expression was reduced in the antero-inferior region compared to the posterior region of shoulder instability patients (p = 0.007). In the antero-inferior region, FN1 expression was increased in the capsule of patients with more than one year of symptoms (p = 0.003) and with recurrent dislocations (p = 0.004) compared with controls. FN1 and TNXB expression was correlated with the duration of symptoms in the posterior region (p < 0.05). Thus, COMP, FN1, TNC and TNXB expression was altered across the capsule of shoulder instability patients. Dislocation episodes modify FN1, TNC and TNXB expression in the injured tissue. COMP altered expression may be associated with capsule integrity after shoulder dislocation, particularly in the macroscopically injured portion.

Cartilage Oligomeric Matrix Protein Increases in Photodamaged Skin

Cartilage oligomeric matrix protein (COMP) is a structural component of cartilage. Recent studies have described COMP as a pathogenic factor that promotes collagen deposition in fibrotic skin disorders such as scleroderma and keloid skin. Although collagen, a major dermis component, is thought to decrease in photoaged skin, recent reports have demonstrated the presence of tightly packed collagen fibrils with a structural resemblance to fibrosis in the papillary dermis of photoaged skin. Here we examined how photoaging damage relates to COMP expression and localization in photoaged skin. In situ hybridization revealed an increase in COMP-mRNA-positive cells with the progress of photoaging in preauricular skin (sun-exposed skin). The signal intensity of immunostaining for COMP increased with photoaging in not only the papillary dermis but also the reticular dermis affected by advancing solar elastosis. Immunoelectron microscopy detected the colocalization of COMP with both elastotic materials and collagen fibrils in photoaged skin. Ultraviolet light A irradiation of human dermal fibroblasts induced COMP expression at both the mRNA and protein levels. Ultraviolet light A-induced COMP expression was inhibited by an anti-transforming growth factor-β antibody or SB431542, an activin receptor-like kinase 5 inhibitor. These results suggest that the transforming growth factor-β-mediated upregulation of COMP expression may contribute to the modulation of dermal extracellular matrix in the photoaging process.

Anatomical heterogeneity of tendon: Fascicular and interfascicular tendon compartments have distinct proteomic composition

Tendon is a simple aligned fibre composite, consisting of collagen-rich fascicles surrounded by a softer interfascicular matrix (IFM). The composition and interactions between these material phases are fundamental in ensuring tissue mechanics meet functional requirements. However the IFM is poorly defined, therefore tendon structure-function relationships are incompletely understood. We hypothesised that the IFM has a more complex proteome, with faster turnover than the fascicular matrix (FM). Using laser-capture microdissection and mass spectrometry, we demonstrate that the IFM contains more proteins, and that many proteins show differential abundance between matrix phases. The IFM contained more protein fragments (neopeptides), indicating greater matrix degradation in this compartment, which may act to maintain healthy tendon structure. Protein abundance did not alter with ageing, but neopeptide numbers decreased in the aged IFM, indicating decreased turnover which may contribute to age-related tendon injury. These data provide important insights into how differences in tendon composition and turnover contribute to tendon structure-function relationships and the effects of ageing.

Changes in synovial fluid biomarker concentrations following arthroscopic surgery in horses with osteochondritis dissecans of the distal intermediate ridge of the tibia

OBJECTIVE

To quantify concentrations of cartilage oligomeric matrix protein (COMP) and fibromodulin in synovial fluid from the tarsocrural joints (TCJs) of horses with osteochondritis dissecans (OCD) of the distal intermediate ridge of the tibia and determine whether concentrations would change following arthroscopic removal of osteochondral fragments.

ANIMALS

115 client-owned horses with OCD of the TCJ and 29 control horses euthanized for unrelated reasons.

PROCEDURES

COMP and fibromodulin concentrations were measured in synovial fluid from the TCJs of the affected horses before and after osteochondral fragments were removed arthroscopically and in synovial fluid from the TCJs of the control horses after euthanasia. Synovial biopsy specimens from the TCJs of affected and control horses were examined histologically for evidence of inflammation.

RESULTS

Synovial fluid COMP and fibromodulin concentrations prior to surgery in horses with OCD were not significantly different from concentrations in control horses. Fibromodulin, but not COMP, concentration in horses with OCD was significantly decreased after surgery, compared with the concentration before surgery. Fibromodulin concentration was significantly correlated with joint effusion score but not with lameness score or results of a flexion test and was correlated with histologic score for number of synoviocytes on the surface of the synovium but not with score for degree of infiltration of inflammatory cells in the synovium. Synovial fluid COMP concentration was not significantly correlated with clinical or histologic findings.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that fibromodulin, but not COMP, could potentially be a biomarker of joint inflammation in horses with OCD of the TCJ.

The Effects of a Crosslinked, Modified Hyaluronic Acid (xCMHA-S) Gel on Equine Tendon Healing

OBJECTIVE

To assess the effects of a crosslinked, modified hyaluronic acid (xCMHA-S) gel on equine tendon healing using an in vivo surgical model.

STUDY DESIGN

In vivo experimental study.

ANIMALS

Adult horses (n = 5).

METHODS

Full thickness bilateral forelimb window tenectomies were surgically created in both forelimb superficial digital flexor tendons and xCMHA-S gel was implanted intraoperatively into the right forelimb lesion of each horse whereas the left forelimb served as the untreated control. Healing was monitored by serial ultrasound examinations every 14 days over the course of the 84 day study. In addition, gross pathology, scanning electron microscopy for fiber diameter, and histological scoring were completed on tendon samples harvested after euthanasia at 84 days.

RESULTS

Ultrasound assessment demonstrated a significant decrease in mean lesion size of treated (0%) compared to control (30%) tendons at 84 days. Mean (±SD) cumulative histologic tendon scores for control tendons (17.7 ± 2.7) were significantly higher than treated tendons (13.6 ± 1.9), indicating less advanced healing in the control group. Tendon cell density was increased and neovascularization, intensity of inflammation, and uniformity of fiber diameter were increased in control compared to treated tendons. There were no differences in fibroblast shape, levels of intralesional hemorrhage, linearity of collagen fibers, or collagen fiber diameter or distribution between treated and control tendons.

CONCLUSION

Tendons treated with xCMHA-S gel at the time of model induction had superior histologic healing scores and sonographically smaller lesions compared to controls, suggesting that xCMHA-S gel may aid the natural healing process.

Tendon Structure and Composition

Tendons are soft, fibrous tissues that connect muscle to bone. Their main function is to transfer muscle generated force to the bony skeleton, facilitating movement around a joint, and as such they are relatively passive, inelastic structures, able to resist high forces. Tendons are predominantly composed of collagen, which is arranged in a hierarchical manner parallel to the long axis of the tendon, resulting in high tensile strength. Tendon also contains a range of non-collagenous proteins, present in low amounts, which nevertheless have important functional roles. In this chapter, we describe general tendon composition and structure, and discuss how variations in composition and structure at different levels of the tendon hierarchy confer specific mechanical properties, which are related to tendon function.

Proteomic analysis reveals age-related changes in tendon matrix composition, with age- and injury-specific matrix fragmentation

Energy storing tendons, such as the human Achilles and equine superficial digital flexor tendon (SDFT), are highly prone to injury, the incidence of which increases with aging. The cellular and molecular mechanisms that result in increased injury in aged tendons are not well established but are thought to result in altered matrix turnover. However, little attempt has been made to fully characterize the tendon proteome nor determine how the abundance of specific tendon proteins changes with aging and/or injury. The aim of this study was, therefore, to assess the protein profile of normal SDFTs from young and old horses using label-free relative quantification to identify differentially abundant proteins and peptide fragments between age groups. The protein profile of injured SDFTs from young and old horses was also assessed. The results demonstrate distinct proteomic profiles in young and old tendon, with alterations in the levels of proteins involved in matrix organization and regulation of cell tension. Furthermore, we identified several new peptide fragments (neopeptides) present in aged tendons, suggesting that there are age-specific cleavage patterns within the SDFT. Proteomic profile also differed between young and old injured tendon, with a greater number of neopeptides identified in young injured tendon. This study has increased the knowledge of molecular events associated with tendon aging and injury, suggesting that maintenance and repair of tendon tissue may be reduced in aged individuals and may help to explain why the risk of injury increases with aging.

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.

Proteomic analysis of tendon extracellular matrix reveals disease stage-specific fragmentation and differential cleavage of COMP (cartilage oligomeric matrix protein)

During inflammatory processes the extracellular matrix (ECM) is extensively remodeled, and many of the constituent components are released as proteolytically cleaved fragments. These degradative processes are better documented for inflammatory joint diseases than tendinopathy even though the pathogenesis has many similarities. The aims of this study were to investigate the proteomic composition of injured tendons during early and late disease stages to identify disease-specific cleavage patterns of the ECM protein cartilage oligomeric matrix protein (COMP). In addition to characterizing fragments released in naturally occurring disease, we hypothesized that stimulation of tendon explants with proinflammatory mediators in vitro would induce fragments of COMP analogous to natural disease. Therefore, normal tendon explants were stimulated with IL-1β and prostaglandin E₂, and their effects on the release of COMP and its cleavage patterns were characterized. Analyses of injured tendons identified an altered proteomic composition of the ECM at all stages post injury, showing protein fragments that were specific to disease stage. IL-1β enhanced the proteolytic cleavage and release of COMP from tendon explants, whereas PGE₂ had no catabolic effect. Of the cleavage fragments identified in early stage tendon disease, two fragments were generated by an IL-1-mediated mechanism. These fragments provide a platform for the development of neo-epitope assays specific to injury stage for tendon disease.

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.

Tenascin-C Expression in Equine Tendon-derived Cells During Proliferation and Migration

In vitro cell studies might be a useful tool for studying tendon pathology, but no suitable in vitro models exist for tendon disorders. The purpose of this study was to confirm whether cell scratch culture using tendon-derived fibroblasts can provide a suitable in vitro tendon disorder model. Extracellular matrix components were examined immunohistochemically in tendon tissue, and then their related gene expression levels were analyzed by conventional reverse transcription polymerase chain reaction (RT-PCR) and/or quantitative real-time RT-PCR in tissues and cells. Collagen type I (Col I), collagen type III (Col III), tenascin-C (TN-C) and cartilage oligomeric matrix protein (COMP) were detected in tendon tissue sections, and RT-PCR confirmed their expression in tendon tissue and cells. Cells that had been cultured from explanted tendon tissue maintained the characteristics of in vivo tendon cells. The combination of TN-C and COMP might be a useful marker of tendon cells because they display more tendon-specific expression than Col I and III. In particular, the significant increase of TN-C mRNA expression in the scratch wound assay, at 12 hr after scratching, concomitant with the regeneration of the cell sheet, indicates its crucial role in tendon cell proliferation and migration. Thus, TN-C appears to be a key factor in tendon wound healing. In vitro cell scratch assays using tendon cells appear to mimic the repair of tendon tissue after injury.

The role of the non-collagenous matrix in tendon function

Tendon consists of highly ordered type I collagen molecules that are grouped together to form subunits of increasing diameter. At each hierarchical level, the type I collagen is interspersed with a predominantly non-collagenous matrix (NCM) (Connect. Tissue Res., 6, 1978, 11). Whilst many studies have investigated the structure, organization and function of the collagenous matrix within tendon, relatively few have studied the non-collagenous components. However, there is a growing body of research suggesting the NCM plays an important role within tendon; adaptations to this matrix may confer the specific properties required by tendons with different functions. Furthermore, age-related alterations to non-collagenous proteins have been identified, which may affect tendon resistance to injury. This review focuses on the NCM within the tensional region of developing and mature tendon, discussing the current knowledge and identifying areas that require further study to fully understand structure-function relationships within tendon. This information will aid in the development of appropriate techniques for tendon injury prevention and treatment.

Transforming growth factor beta3 promotes tendon differentiation of equine embryo-derived stem cells

Tendon injuries occur frequently in horses and have a poor capacity to regenerate, which leads to high re-injury rates. Equine embryo-derived stem cells (ESCs) survive in high numbers in the injured horse tendon and we hypothesized that they differentiate into tenocytes in vivo. Immunocytochemistry revealed that in the injured horse tendon ESCs express the tendon progenitor marker scleraxis and that there is a local upregulation of the transforming growth factor-β (TGF-β) at the injury site. The aim of this study was to determine if TGF-β signaling was able to drive tenocyte differentiation by ESCs. Exposure of differentiating ESCs to TGF-β in vitro produced an upregulation of scleraxis at the gene and protein level with the greatest effect being produced in the presence of TGF-β3. TGF-β3 treatment of differentiating ESCs also promotes a significant upregulation of other tendon-associated genes and proteins suggesting it can promote ESC differentiation into tenocytes. Our results demonstrate that equine ESCs can differentiate into a therapeutically relevant cell type and that TGF-β driven differentiation of ESCs may provide a model to study tendon development and better understand the transcriptional networks that are involved in equine tendon cell differentiation from the early embryonic stages.

Immunolocalization of collagens (I and III) and cartilage oligomeric matrix protein in the normal and injured equine superficial digital flexor tendon

This is a descriptive study of tendon pathology with different structural appearances of repair tissue correlated to immunolocalization of cartilage oligomeric matrix protein (COMP) and type I and III collagens and expression of COMP mRNA. The material consists of nine tendons from seven horses (5-25 years old; mean age of 10 years) with clinical tendinopathy and three normal tendons from horses (3, 3, and 13 years old) euthanized for non-orthopedic reasons. The injured tendons displayed different repair-tissue appearances with organized and disorganized fibroblastic regions as well as areas of necrosis. The normal tendons presented distinct immunoreactivity for COMP and expression of COMP mRNA and type I collagen in the normal aligned fiber structures, but no immunolabeling of type III collagen. However, immunoreactivity for type III collagen was present in the endotenon surrounding the fiber bundles, where no expression of COMP could be seen. Immunostaining for type I and III collagens was present in all of the pathologic regions indicating repair tissue. Interestingly, the granulation tissues showed immunostaining for COMP and expression of COMP mRNA, indicating a role for COMP in repair and remodeling of the tendon after fiber degeneration and rupture. The present results suggest that not only type III collagen but also COMP is involved in the repair and remodeling processes of the tendon.

Bone Response to Loading in Mice With Targeted Disruption of the Cartilage Oligomeric Matrix Protein Gene

Exercise induced bone response although established, little is known about the molecular components that mediate bone response to mechanical loading (ML). In our recent QTL study, we identified one such possible molecular component responding to ML: cartilage oligomeric matrix protein (COMP). To address the COMP role in mediating ML effects on bone formation, COMP expression was evaluated as a function of duration and age in response to ML in female B6 mice. A 9N load was applied using a four-point bending device at 2Hz frequency for 36 cycles, once per day for 2-, 4- and 12-days on the right tibia. The left tibia was used as an internal control. Loading caused an increase in COMP expression by 1.3-, 2- and 4-fold respectively after 2-, 4- and 12-days of loading. This increase was also seen in 16 and 36-week old mice. Based on these findings, we next used COMP knockout (KO) mice to evaluate the cause and effect relationship. Quantitative analysis revealed 2 weeks of ML induced changes in vBMD and bone size in the KO mice (5.9 % and 21 % vs. unloaded bones) was not significantly different from control mice (7 % and 24 % vs. unloaded bones). Our results imply that COMP is not a key upstream mediator of the anabolic effects of ML on the skeleton.

The pentameric channel of COMPcc in complex with different fatty acids

BACKGROUND

COMPcc forms a pentameric left-handed coiled coil that is known to bind hydrophilic signaling molecules such as vitamin D(3), and vitamin A.

PRINCIPAL FINDINGS

In an integrated approach we reveal the unique binding properties of COMPcc for saturated and unsaturated fatty acids. Our observations suggest that residues Met33 (gating pore), Thr40/Asn41 (water chamber) and Gln54 (electrostatic trap) are key elements for the binding of fatty acids by COMPcc. In addition, this work characterizes the binding of various fatty acids to COMPcc using fluorescence spectroscopy. Our findings reveal a binding trend within the hydrophobic channel of COMPcc, namely, that is driven by length of the methylene tail and incorporation of unsaturation.

CONCLUSION/SIGNIFICANCE

The unique binding properties imply that COMPcc may be involved in signalling functions in which hydrophilic ligands are involved. The pentameric channel is a unique carrier for lipophilic compounds. This opens the exciting possibility that COMPcc could be developed as a targeted drug delivery system.

Optimization of leukocyte concentration in platelet-rich plasma for the treatment of tendinopathy

BACKGROUND

Numerous methods are available for platelet-rich plasma (PRP) generation, but evidence defining the optimum composition is lacking. We hypothesized that leukocyte-reduced PRP would result in lower inflammatory cytokine expression compared with concentrated-leukocyte PRP and that maintaining the platelet:white blood cell (WBC) ratio would compensate for the effect of increased WBC concentration.

METHODS

Blood and flexor digitorum superficialis tendons were collected from young adult horses. Three PRP groups were generated with the same platelet concentration but different WBC concentrations: intermediate-concentration standard PRP, leukocyte-reduced PRP, and concentrated-leukocyte PRP. An additional high-concentration PRP group was generated with the same WBC concentration as the concentrated-leukocyte PRP group and the same platelet:WBC ratio as the standard PRP group. The PRP groups were used as media for flexor digitorum superficialis tendon explants in culture for seventy-two hours with 10% plasma in Dulbecco modified Eagle medium (DMEM) serving as control. Tendon gene expression for collagen types I (COL1A1) and III (COL3A1), cartilage oligomeric matrix protein (COMP), matrix metalloproteinase (MMP-13), interleukin-1β (IL-1β), and tumor necrosis factor-α (TNF-α) was performed.

RESULTS

The desired PRP groups were successfully generated. The expression of COMP, the COL1A1:COL3A1 ratio, and the expression of MMP-13 in flexor digitorum superficialis tendon explants was not different between PRP groups. The expression of COMP (p = 0.0027) and the COL1A1:COL3A1 ratio (p &lt; 0.0001) were increased in the PRP groups as compared with the control group, and the expression of MMP-13 was decreased in the PRP groups as compared with the control group (p &lt; 0.0001). The expression of IL-1β was lowest in leukocyte-reduced PRP and highest in concentrated-leukocyte PRP (p = 0.0001). The leukocyte-reduced PRP group and the control group had the lowest TNF-α expression, whereas the high-concentration PRP and concentrated-leukocyte PRP groups had the highest expression (p = 0.0224).

CONCLUSIONS

A high absolute WBC concentration in PRP contributes to the expression of inflammatory cytokines in flexor digitorum superficialis tendon explants, and maintenance of the platelet:WBC ratio is not able to counteract this effect.

CLINICAL RELEVANCE

The optimum composition of PRP for the treatment of tendinopathy has not been directly investigated. Persistent inflammation results in inferior repair with scar tissue. The present study indicates that in an animal model, WBC in PRP contributes to inflammatory cytokine production. Therefore, leukocyte-reduced PRP may be the optimum preparation to stimulate superior healing without scar tissue formation.

Cartilage oligomeric matrix protein enhances matrix assembly during chondrogenesis of human mesenchymal stem cells

Cartilage oligomeric matrix protein/thrombospondin-5 (COMP/TSP5) is an abundant cartilage extracellular matrix (ECM) protein that interacts with major cartilage ECM components, including aggrecan and collagens. To test our hypothesis that COMP/TSP5 functions in the assembly of the ECM during cartilage morphogenesis, we have employed mesenchymal stem cell (MSC) chondrogenesis in vitro as a model to examine the effects of COMP over-expression on neo-cartilage formation. Human bone marrow-derived MSCs were transfected with either full-length COMP cDNA or control plasmid, followed by chondrogenic induction in three-dimensional pellet or alginate hydrogel culture. MSC chondrogenesis and ECM production was estimated based on quantitation of sulfated glycosaminoglycan (sGAG) accumulation, immunohistochemistry of the presence and distribution of cartilage ECM proteins, and real-time RT-PCR analyis of mRNA expression of cartilage markers. Our results showed that COMP over-expression resulted in increased total sGAG content during the early phase of MSC chondrogenesis, and increased immuno-detectable levels of aggrecan and collagen type II in the ECM of COMP-transfected pellet and alginate cultures, indicating more abundant cartilaginous matrix. COMP transfection did not significantly increase the transcript levels of the early chondrogenic marker, Sox9, or aggrecan, suggesting that enhancement of MSC cartilage ECM was effected at post-transcriptional levels. These findings strongly suggest that COMP functions in mesenchymal chondrogenesis by enhancing cartilage ECM organization and assembly. The action of COMP is most likely mediated not via direct changes in cartilage matrix gene expression but via interactions of COMP with other cartilage ECM proteins, such as aggrecan and collagens, that result in enhanced assembly and retention.

Stem cell-based tissue engineering in veterinary orthopaedics

Regenerative medicine is one of the most intensively researched medical branches, with enormous progress every year. When it comes to translating research from bench to bedside, many of the pioneering innovations are achieved by cooperating teams of human and veterinary medical scientists. The veterinary profession has an important role to play in this new and evolving technology, holding a great scientific potential, because animals serve widely as models for human medicine and results obtained from animals may serve as preclinical results for human medicine. Regenerative veterinary medicine utilizing mesenchymal stromal cells (MSC) for the treatment of acute injuries as well as chronic disorders is gradually turning into clinical routine. As orthopaedic disorders represent a major part of all cases in veterinary clinical practice, it is not surprising that they are currently taking a leading role in MSC therapies. Therefore, the purpose of this paper is to give an overview on past and current achievements as well as future perspectives in stem cell-based tissue engineering in veterinary orthopaedics.