Developmental and osteoarthritic changes in Col6a1-knockout mice: biomechanics of type VI collagen in the cartilage pericellular matrix

High resistance of the mechanical properties of the chondrocyte pericellular matrix to proteoglycan digestion by chondroitinase, aggrecanase, or hyaluronidase

In articular cartilage, the extracellular matrix (ECM) and chondrocyte-associated pericellular matrix (PCM) are characterized by a high concentration of proteoglycans (PGs) and their associated glycosaminoglycans (GAGs). These molecules serve important biochemical, structural, and biomechanical roles in the tissue and differences in their regional distributions suggest that different GAG/PG species contribute to the specific biomechanical properties of the ECM and PCM. The objective of this study was to investigate region-specific contributions of aggrecan, chondroitin and dermatan sulfate, and hyaluronan to the micromechanical properties of articular cartilage PCM and ECM in situ. Cryosections of porcine cartilage underwent digestion with ADAMTS-4, chondroitinase ABC, bacterial hyaluronidase or human leukocyte elastase. Guided by immunofluorescence for type VI collagen, AFM stiffness mapping was used to evaluate the elastic properties of matched PCM and ECM regions in paired control and digested cartilage sections. These methods were used to test the hypotheses that specific enzymatic digestion of GAGs or PGs would reduce both PCM and ECM elastic moduli. Elastase, which digests a number of PGs, some types of collagen, and non-collagenous proteins, was used as a positive control. ECM elastic moduli were significantly reduced by all enzyme treatments. However, PCM micromechanical properties were unaffected by enzymatic digestion of aggrecan, chondroitin/dermatan sulfate, and hyaluronan but were significantly reduced by 24% following elastase digestion. Our results provide new evidence for high resistance of PCM micromechanical properties to PG digestion and suggest a potential role for elastase in the degradation of the ECM and PCM.

A commentary on modelling osteoarthritis pain in small animals

OBJECTIVE

To describe the currently used animal models for the study of osteoarthritis (OA) pain, with an emphasis on small animals (predominantly mice and rats).

OUTLINE

Narrative review summarizing the opportunities and limitations of the most commonly used small animal models for the study of pain and pain pathways associated with OA, and discussing currently used methods for pain assessment. Involvement of neural degeneration in OA is briefly discussed. A list of considerations when studying pain-related behaviours and pathways in animal models of OA is proposed.

CONCLUSIONS

Animal models offer great potential to unravel the complex pathophysiology of OA pain, its molecular and temporal regulation. They constitute a critical pathway for developing and testing disease-specific symptom-modifying therapeutic interventions. However, a number of issues remain to be resolved in order to standardize pre-clinical OA pain research and to optimize translation to clinical trials and patient therapies.

Post-traumatic osteoarthritis: from mouse models to clinical trials

Osteoarthritis (OA), the most common of all arthropathies, is a leading cause of disability and has a large (and growing) worldwide socioeconomic cost. Despite its burgeoning importance, translation of disease-modifying OA therapies from the laboratory into clinical practice has slowed. Differences between the OA models studied preclinically and the disease evaluated in human clinical trials contribute to this failure. Most animal models of OA induce disease through surgical or mechanical disruption of joint biomechanics in young individuals rather than the spontaneous development of age-associated disease. This instability-induced joint disease in animals best models the arthritis that develops in humans after an injurious event, known as post-traumatic OA (PTOA). Studies in genetically modified mice suggest that PTOA has a distinct molecular pathophysiology compared with that of spontaneous OA, which might explain the poor translation from preclinical to clinical OA therapeutic trials. This Review summarizes the latest data on potential molecular targets for PTOA prevention and modification derived from studies in genetically modified mice, and describes their validation in preclinical therapeutic trials. This article focuses on how these findings might best be translated to humans, and identifies the potential challenges to successful implementation of clinical trials of disease-modifying drugs for PTOA.

Spontaneous osteoarthritis in Str/ort mice is unlikely due to greater vulnerability to mechanical trauma

OBJECTIVE

Relative contributions of genetic and mechanical factors to osteoarthritis (OA) remain ill-defined. We have used a joint loading model found to produce focal articular cartilage (AC) lesions, to address whether genetic susceptibility to OA in Str/ort mice is related to AC vulnerability to mechanical trauma and whether joint loading influences spontaneous OA development. We also develop finite element (FE) models to examine whether AC thickness may explain any differential vulnerability to load-induced lesions.

METHODS

Right knees of 8-week-old Str/ort mice were loaded, AC integrity scored and thickness compared to CBA mice. Mechanical forces engendered in this model and the impact of AC thickness were simulated in C57Bl/6 mice using quasi-static FE modelling.

RESULTS

Unlike joints in non-OA prone CBA mice, Str/ort knees did not exhibit lateral femur (LF) lesions in response to applied loading; but exhibited thicker AC. FE modeling showed increased contact pressure and shear on the lateral femoral surface in loaded joints, and these diminished in joints containing thicker AC. Histological analysis of natural lesions in the tibia of Str/ort joints revealed that applied loading increased OA severity, proteoglycan loss and collagen type II degradation.

CONCLUSION

Genetic OA susceptibility in Str/ort mice is not apparently related to greater AC vulnerability to trauma, but joint loading modifies severity of natural OA lesions in the medial tibia. FE modelling suggests that thicker AC in Str/ort mice diminishes tissue stresses and protects against load-induced AC lesions in the LF but that this is unrelated to their genetic susceptibility to OA.

Collagen VI regulates satellite cell self-renewal and muscle regeneration

Adult muscle stem cells, or satellite cells have essential roles in homeostasis and regeneration of skeletal muscles. Satellite cells are located within a niche that includes myofibers and extracellular matrix. The function of specific extracellular matrix molecules in regulating SCs is poorly understood. Here, we show that the extracellular matrix protein collagen VI is a key component of the satellite cell niche. Lack of collagen VI in Col6a1(-/-) mice causes impaired muscle regeneration and reduced satellite cell self-renewal capability after injury. Collagen VI null muscles display significant decrease of stiffness, which is able to compromise the in vitro and in vivo activity of wild-type satellite cells. When collagen VI is reinstated in vivo by grafting wild-type fibroblasts, the biomechanical properties of Col6a1(-/-) muscles are ameliorated and satellite cell defects rescued. Our findings establish a critical role for an extracellular matrix molecule in satellite cell self-renewal and open new venues for therapies of collagen VI-related muscle diseases.

WARP interacts with collagen VI-containing microfibrils in the pericellular matrix of human chondrocytes

Collagen VI and WARP are extracellular structural macromolecules present in cartilage and associated with BM suprastructures in non-skeletal tissues. We have previously shown that in WARP-deficient mice, collagen VI is specifically reduced in regions of the peripheral nerve ECM where WARP is expressed, suggesting that both macromolecules are part of the same suprastructure. The object of this study was to conduct a detailed analysis of WARP-collagen VI interactions in vitro in cartilage, a tissue rich in WARP and collagen VI. Immunohistochemical analysis of mouse and human articular cartilage showed that WARP and collagen VI co-localize in the pericellular matrix of superficial zone articular chondrocytes. EM analysis on extracts of human articular cartilage showed that WARP associates closely with collagen VI-containing suprastructures. Additional evidence of an interaction is provided by immunogold EM and immunoblot analysis showing that WARP was present in collagen VI-containing networks isolated from cartilage. Further characterization were done by solid phase binding studies and reconstitution experiments using purified recombinant WARP and isolated collagen VI. Collagen VI binds to WARP with an apparent K(d) of approximately 22 nM and the binding site(s) for WARP resides within the triple helical domain since WARP binds to both intact collagen VI tetramers and pepsinized collagen VI. Together, these data confirm and extend our previous findings by demonstrating that WARP and collagen VI form high affinity associations in vivo in cartilage. We conclude that WARP is ideally placed to function as an adapter protein in the cartilage pericellular matrix.

Cartilage tissue engineering using differentiated and purified induced pluripotent stem cells

The development of regenerative therapies for cartilage injury has been greatly aided by recent advances in stem cell biology. Induced pluripotent stem cells (iPSCs) have the potential to provide an abundant cell source for tissue engineering, as well as generating patient-matched in vitro models to study genetic and environmental factors in cartilage repair and osteoarthritis. However, both cell therapy and modeling approaches require a purified and uniformly differentiated cell population to predictably recapitulate the physiological characteristics of cartilage. Here, iPSCs derived from adult mouse fibroblasts were chondrogenically differentiated and purified by type II collagen (Col2)-driven green fluorescent protein (GFP) expression. Col2 and aggrecan gene expression levels were significantly up-regulated in GFP+ cells compared with GFP- cells and decreased with monolayer expansion. An in vitro cartilage defect model was used to demonstrate integrative repair by GFP+ cells seeded in agarose, supporting their potential use in cartilage therapies. In chondrogenic pellet culture, cells synthesized cartilage-specific matrix as indicated by high levels of glycosaminoglycans and type II collagen and low levels of type I and type X collagen. The feasibility of cell expansion after initial differentiation was illustrated by homogenous matrix deposition in pellets from twice-passaged GFP+ cells. Finally, atomic force microscopy analysis showed increased microscale elastic moduli associated with collagen alignment at the periphery of pellets, mimicking zonal variation in native cartilage. This study demonstrates the potential use of iPSCs for cartilage defect repair and for creating tissue models of cartilage that can be matched to specific genetic backgrounds.

Depth-dependent anisotropy of the micromechanical properties of the extracellular and pericellular matrices of articular cartilage evaluated via atomic force microscopy

The extracellular matrix (ECM) of articular cartilage is structurally and mechanically inhomogeneous and anisotropic, exhibiting variations in composition, collagen fiber architecture, and pericellular matrix (PCM) morphology among the different zones (superficial, middle, and deep). Joint loading exposes chondrocytes to a complex biomechanical environment, as the microscale mechanical environment of the chondrocyte depends on the relative properties of its PCM and local ECM. ECM anisotropy and chondrocyte deformation are influenced by the split-line direction, the preferred collagen fiber orientation parallel to the articular surface. While previous studies have demonstrated that cartilage macroscale properties vary with depth and the direction of loading relative to the split-line direction, the potential anisotropic behavior of the ECM and PCM at the microscale has yet to be examined. The goal of this study was to characterize the depth and directional dependence of the microscale biomechanical properties of porcine cartilage ECM and PCM in situ. Cartilage was cryosectioned to generate samples oriented parallel and perpendicular to the split-line direction and normal to the articular surface. Atomic force microscopy (AFM)-based stiffness mapping was utilized to measure ECM and PCM microscale elastic properties in all three directions within each zone. Distinct anisotropy in ECM elastic moduli was observed in the superficial and deep zones, while the middle zone exhibited subtle anisotropy. PCM elastic moduli exhibited zonal uniformity with depth and directional dependence when pooled across the zones. These findings provide new evidence for mechanical inhomogeneity and anisotropy at the microscale in articular cartilage.

Regional variations in the distribution and colocalization of extracellular matrix proteins in the juvenile bovine meniscus

A deeper understanding of the composition and organization of extracellular matrix molecules in native, healthy meniscus tissue is required to fully appreciate the degeneration that occurs in joint disease and the intricate environment in which an engineered meniscal graft would need to function. In this study, regional variations in the tissue-level and pericellular distributions of collagen types I, II and VI and the proteoglycans aggrecan, biglycan and decorin were examined in the juvenile bovine meniscus. The collagen networks were extensively, but not completely, colocalized, with tissue-level organization that varied with radial position across the meniscus. Type VI collagen exhibited close association with large bundles composed of type I and II collagen and, in contrast to type I and II collagen, was further concentrated in the pericellular matrix. Aggrecan was detected throughout the inner region of the meniscus but was restricted to the pericellular matrix and sheaths of collagen bundles in the middle and outer regions. The small proteoglycans biglycan and decorin exhibited regional variations in staining intensity but were consistently localized in the intra- and/or peri-cellular compartments. These results provide insight into the complex hierarchy of extracellular matrix organization in the meniscus and provide a framework for better understanding meniscal degeneration and disease progression and evaluating potential repair and regeneration strategies.

Immunofluorescence-guided atomic force microscopy to measure the micromechanical properties of the pericellular matrix of porcine articular cartilage

The pericellular matrix (PCM) is a narrow region that is rich in type VI collagen that surrounds each chondrocyte within the extracellular matrix (ECM) of articular cartilage. Previous studies have demonstrated that the chondrocyte micromechanical environment depends on the relative properties of the chondrocyte, its PCM and the ECM. The objective of this study was to measure the influence of type VI collagen on site-specific micromechanical properties of cartilage in situ by combining atomic force microscopy stiffness mapping with immunofluorescence imaging of PCM and ECM regions in cryo-sectioned tissue samples. This method was used to test the hypotheses that PCM biomechanical properties correlate with the presence of type VI collagen and are uniform with depth from the articular surface. Control experiments verified that immunolabelling did not affect the properties of the ECM or PCM. PCM biomechanical properties correlated with the presence of type VI collagen, and matrix regions lacking type VI collagen immediately adjacent to the PCM exhibited higher elastic moduli than regions positive for type VI collagen. PCM elastic moduli were similar in all three zones. Our findings provide further support for type VI collagen in defining the chondrocyte PCM and contributing to its biological and biomechanical properties.

A biomechanical role for perlecan in the pericellular matrix of articular cartilage

Chondrocytes are surrounded by a narrow pericellular matrix (PCM) that is biochemically, structurally, and biomechanically distinct from the bulk extracellular matrix (ECM) of articular cartilage. While the PCM is often defined by the presence of type VI collagen, other macromolecules such as perlecan, a heparan sulfate (HS) proteoglycan, are also exclusively localized to the PCM in normal cartilage and likely contribute to PCM structural integrity and biomechanical properties. Though perlecan is essential for normal cartilage development, its exact role in the PCM is unknown. The objective of this study was to determine the biomechanical role of perlecan in the articular cartilage PCM in situ and its potential as a defining factor of the PCM. To this end, atomic force microscopy (AFM) stiffness mapping was combined with dual immunofluorescence labeling of cryosectioned porcine cartilage samples for type VI collagen and perlecan. While there was no difference in overall PCM mechanical properties between type VI collagen- and perlecan-based definitions of the PCM, within the PCM, interior regions containing both type VI collagen and perlecan exhibited lower elastic moduli than more peripheral regions rich in type VI collagen alone. Enzymatic removal of HS chains from perlecan with heparinase III increased PCM elastic moduli both overall and locally in interior regions rich in both perlecan and type VI collagen. Heparinase III digestion had no effect on ECM elastic moduli. Our findings provide new evidence for perlecan as a defining factor in both the biochemical and biomechanical properties of the PCM.

Human mesenchymal stem cell-derived matrices for enhanced osteoregeneration

The methodology for the repair of critical-sized or non-union bone lesions has unpredictable efficacy due in part to our incomplete knowledge of bone repair and the biocompatibility of bone substitutes. Although human mesenchymal stem cells (hMSCs) differentiate into osteoblasts, which promote bone growth, their ability to repair bone in vivo has been variable. We hypothesized that given the multistage process of osteogenesis, hMSC-mediated repair might be maximal at a specific time point of healing. Using a mouse model of calvarial healing, we demonstrate that the osteo-repair capacity of hMSCs can be substantially augmented by treatment with an inhibitor of peroxisome proliferator-activated receptor γ, but efficacy is confined to the rapid osteogenic phase. Upon entry into the bone-remodeling phase, hMSC retention signals are lost, resulting in truncation of healing. To solve this limitation, we prepared a scaffold consisting of hMSC-derived extracellular matrix (ECM) containing the necessary biomolecules for extended site-specific hMSC retention. When inhibitor-treated hMSCs were coadministered with ECM, they remained at the injury, well into the remodeling phase of healing, which resulted in reproducible and complete repair of critical-sized bone defects in mice in 3 weeks. These data suggest that hMSC-derived ECM and inhibitor-treated hMSCs could be used at optimal times to substantially and reproducibly improve bone repair.

Altered trabecular bone structure and delayed cartilage degeneration in the knees of collagen VI null mice

Mutation or loss of collagen VI has been linked to a variety of musculoskeletal abnormalities, particularly muscular dystrophies, tissue ossification and/or fibrosis, and hip osteoarthritis. However, the role of collagen VI in bone and cartilage structure and function in the knee is unknown. In this study, we examined the role of collagen VI in the morphology and physical properties of bone and cartilage in the knee joint of Col6a1(-/-) mice by micro-computed tomography (microCT), histology, atomic force microscopy (AFM), and scanning microphotolysis (SCAMP). Col6a1(-/-) mice showed significant differences in trabecular bone structure, with lower bone volume, connectivity density, trabecular number, and trabecular thickness but higher structure model index and trabecular separation compared to Col6a1(+/+) mice. Subchondral bone thickness and mineral content increased significantly with age in Col6a1(+/+) mice, but not in Col6a1(-/-) mice. Col6a1(-/-) mice had lower cartilage degradation scores, but developed early, severe osteophytes compared to Col6a1(+/+) mice. In both groups, cartilage roughness increased with age, but neither the frictional coefficient nor compressive modulus of the cartilage changed with age or genotype, as measured by AFM. Cartilage diffusivity, measured via SCAMP, varied minimally with age or genotype. The absence of type VI collagen has profound effects on knee joint structure and morphometry, yet minimal influences on the physical properties of the cartilage. Together with previous studies showing accelerated hip osteoarthritis in Col6a1(-/-) mice, these findings suggest different roles for collagen VI at different sites in the body, consistent with clinical data.

Altered responsiveness to TGF-β results in reduced Papss2 expression and alterations in the biomechanical properties of mouse articular cartilage

INTRODUCTION

Previous studies have indicated that transforming growth factor β (TGF-β) signaling has a critical role in cartilage homeostasis and repair, yet the mechanisms of TGF-β's chondroprotective effects are not known. Our objective in this study was to identify downstream targets of TGF-β that could act to maintain biochemical and biomechanical properties of cartilage.

METHODS

Tibial joints from 20-week-old mice that express a dominant-negative mutation of the TGF-β type II receptor (DNIIR) were graded histologically for osteoarthritic changes and tested by indentation to evaluate their mechanical properties. To identify gene targets of TGF-β, microarray analysis was performed using bovine articular chondrocytes grown in micromass culture that were either treated with TGF-β or left untreated. Phosphoadenosine phosphosynthetase 2 (PAPSS2) was identified as a TGF-β-responsive gene. Papss2 expression is crucial for proper sulfation of cartilage matrix, and its deficiency causes skeletal defects in mice and humans that overlap with those seen in mice with mutations in TGF-β-signaling genes. Regulation of Papss2 was verified by real time RT-PCR and Western blot analyses. Alterations in sulfation of glycosaminoglycans were analyzed by critical electrolyte concentration and Alcian blue staining and immunofluorescence for chondroitin-4-sulfate, unsulfated chondroitin and the aggrecan core protein.

RESULTS

DNIIR mutants showed reduced mechanical properties and osteoarthritis-like changes when compared to wild-type control mice. Microarray analysis identified a group of genes encoding matrix-modifying enzymes that were regulated by TGF-β. Papss2 was upregulated in bovine articular chondrocytes after treatment with TGF-β and downregulated in cartilage from DNIIR mice. Articular cartilage in DNIIR mice demonstrated reduced Alcian blue staining at critical electrolyte concentrations and reduced chondroitin-4-sulfate staining. Staining for unsulfated chondroitin sulfate was increased, whereas staining for the aggrecan core protein was comparable in DNIIR and wild-type mice.

CONCLUSION

TGF-β maintains biomechanical properties and regulates expression of Papss2 and sulfation of glycosaminoglycans in mouse articular cartilage.

Absence of type VI collagen paradoxically improves cardiac function, structure, and remodeling after myocardial infarction

RATIONALE

We previously reported that type VI collagen deposition increases in the infarcted myocardium in vivo. To date, a specific role for this nonfibrillar collagen has not been explored in the setting of myocardial infarction (MI).

OBJECTIVE

To determine whether deletion of type VI collagen in an in vivo model of post-MI wound healing would alter cardiac function and remodeling in the days to weeks after injury.

METHODS AND RESULTS

Wild-type and Col6a1(-/-) mice were subjected to MI, followed by serial echocardiographic and histological assessments. At 8 weeks after MI, infarct size was significantly reduced, ejection fraction was significantly preserved (43.9% ± 3.3% versus 29.1% ± 4.3% for wild-type), and left ventricular chamber dilation was attenuated in the Col6a1(-/-) MI group (25.8% ± 7.9% increase versus 62.6% ± 16.5% for wild-type). The improvement in cardiac remodeling was evident as early as 10 days after MI in the Col6a1(-/-) mice. Myocyte apoptosis within the infarcted zones was initially greater in the Col6a1(-/-) group 3 days after MI, but by day 14 this was significantly reduced. Collagen deposition also was reduced in the infarcted and remote areas of the Col6a1(-/-) hearts. The reductions in chronic myocyte apoptosis and fibrosis are critical events leading to improved long-term remodeling and functional outcomes.

CONCLUSIONS

These unexpected results demonstrate for the first time that deletion of type VI collagen in this knockout model plays a critical protective role after MI by limiting infarct size, chronic apoptosis, aberrant remodeling, and fibrosis, leading to preservation of cardiac function.

Depletion of primary cilia in articular chondrocytes results in reduced Gli3 repressor to activator ratio, increased Hedgehog signaling, and symptoms of early osteoarthritis

OBJECTIVE

Primary cilia are present in almost every cell type including chondrocytes. Studies have shown that defects in primary cilia result in skeletal dysplasia. The purpose of this study was to understand how loss of primary cilia affects articular cartilage.

DESIGN

Ift88 encodes a protein that is required for intraflagellar transport and formation of primary cilia. In this study, we used Col2aCre;Ift88(fl/fl) transgenic mice in which primary cilia were deleted in chondrocytes. Col2aCre;Ift88(fl/fl) articular cartilage was characterized by histological staining, real time RT-PCR, and microindentation. Hedgehog (Hh) signaling was measured by expression of Ptch1 and Gli1 mRNA. The levels of Gli3 proteins were determined by western blot.

RESULTS

Col2aCre;Ift88(fl/fl) articular cartilage was thicker and had increased cell density, likely due to decreased apoptosis during cartilage remodeling. Mutant articular cartilage also showed increased expression of osteoarthritis (OA) markers including Mmp13, Adamts5, ColX, and Runx2. OA was also evident by reduced stiffness in mutant cartilage as measured by microindentation. Up-regulation of Hh signaling, which has been associated with OA, was present in mutant articular cartilage as measured by expression of Ptch1 and Gli1. Col2aCre;Ift88(fl/fl) cartilage also demonstrated reduced Gli3 repressor to activator ratio.

CONCLUSION

Our results indicate that primary cilia are required for normal development and maintenance of articular cartilage. It was shown that primary cilia are required for processing full length Gli3 to the truncated repressor form. We propose that OA symptoms in Col2aCre;Ift88(fl/fl) cartilage are due to reduced Hh signal repression by Gli3.

Intact pericellular matrix of articular cartilage is required for unactivated discoidin domain receptor 2 in the mouse model

Increased expression of the discoidin domain receptor 2 (DDR2) results from its interaction with collagen type II. This induces expression of matrix metalloproteinase (MMP)-13, leading to osteoarthritis (OA). To investigate the impact of the pericellular matrix of chondrocytes on DDR2, we generated a mouse model with inducible overexpression of DDR2 in cartilage. Conditional overexpression of DDR2 in mature mouse articular cartilage was controlled via the cartilage oligomeric matrix protein promoter using the Tet-Off-inducible system. Doxycycline was withdrawn at 1 month of age, and knee joints were examined at 2, 3, and 4 months of age. Microsurgery was performed on 3-month-old transgenic mice overexpressing DDR2 to destabilize the medial meniscus, and serial paraffin sections were examined at 2, 4, 8, and 12 weeks after surgery. DDR2 expression increased in the knee joints of transgenic mice. However, the increased DDR2 did not induce MMP-13 expression. No OA-like changes were observed in the transgenic mice at the age of 4 months. When transgenic mice were subjected to destabilizing of the medial meniscus, we observed accelerated progression to OA, which was associated with DDR2 activation. Therefore, conditionally overexpressing DDR2 in the mature articular cartilage of mouse knee joints requires activation to induce OA, and altered biomechanical stress can accelerate the onset of cartilage loss and progression to OA in transgenic mice.

Type VI collagen deficiency induces osteopenia with distortion of osteoblastic cell morphology

Bone consists of type I collagen as a major protein with minor various matrix proteins. Type VI collagen is one of bone matrix proteins but its function is not known. We therefore examined the effects of type VI collagen deficiency on bone. 3D-μCT analysis revealed that type VI collagen deficiency reduced cancellous bone mass. Cortical bone mass was not affected. Type VI collagen deficiency distorted the shape of osteoblasts both in the cancellous bone and in the cambium layer of periosteal region. Furthermore, type VI collagen deficiency disorganized collagen arrangement. These data indicate that type VI collagen contributes to maintain bone mass.

Tissue engineering for intervertebral disk degeneration

Many challenges confront intervertebral disk engineering owing to complexity and the presence of extraordinary stresses. Rebuilding a disk of native function could be useful for removal of the symptoms and correction of altered spine kinematics. Improvement in understanding of disk properties and techniques for disk engineering brings promise to the fabrication of a functional motion segment for the treatment of disk degeneration. Increasing sophistication of techniques available in biomedical sciences will bring its application into clinics. This review provides an account of current progress and challenges of intervertebral disk bioengineering and discusses means to move forward and toward bedside translation.

Metabolic labeling of chondrocytes for the quantitative analysis of the interleukin-1-beta-mediated modulation of their intracellular and extracellular proteomes

Chondrocytes are widely used as an in vitro model of cartilage diseases such as osteoarthritis (OA). As the unique residents of mature cartilage, they are responsible of the synthesis and release of proteins essential for a proper tissue turnover. In this work, the stable isotope labeling with amino acids in cell culture (SILAC) technique has been standardized in primary human articular chondrocytes (HACs) for quantitative proteomic analyses. Then, it has been employed to study those protein modifications caused by the proinflammatory cytokine Interleukin-1beta (IL-1β), a well-known OA mediator, in these cells. Quantitative analysis of the IL-1β-treated HACs proteome revealed a global increase in cellular chaperones concurrent with a down-regulation of the actin cytoskeleton. HACs secretome analysis led to the identification and quantification of 115 proteins and unveiled the effects of the cytokine on the cartilage extracellular matrix metabolism. Among those modulated proteins, three protein clusters were found to be remarkably increased by IL-1β: proinflammatory mediators and proteases, type VI collagen and proteins known to bind this molecule, and proteins related with the TGF-beta pathway. On the other hand, secretion of aggrecan, two vitamin K-dependent proteins, and thrombospondin, among others, was strongly reduced. Altogether, these data demonstrate the usefulness of metabolic labeling for quantitative proteomics studies in HACs, show the complementarity of intracellular proteome and secretome analyses, and provide a comprehensive study of the IL-1β-mediated effects on these cells. Proteins identified in the secretome approach have a potential use as biomarkers or therapeutic targets for OA.

A proteomic approach for identification and localization of the pericellular components of chondrocytes

Although the pericellular matrix (PCM) plays a central role in the communication between chondrocytes and extracellular matrix, its composition is largely unknown. In this study, the PCM was investigated with a proteomic approach using chondrons, which are enzymatically isolated constructs including the chondrocyte and its surrounding PCM. Chondrons and chondrocytes alone were isolated from human articular cartilage. Proteins extracted from chondrons and chondrocytes were used for two-dimensional electrophoresis. Protein spots were quantitatively compared between chondron and chondrocyte gels. Cellular proteins, which had similar density between chondron and chondrocyte gels, did not proceed for analysis. Since chondrons only differ from chondrocytes in association of the PCM, protein spots in the chondron gels that had higher quantity than that in the chondrocyte gels were selected as candidates of the PCM components and processed for mass spectrometry. Among 15 identified peptides, several were fragments of the three type VI collagen chains (α-1, α-2, and α-3). Other identified PCM proteins included triosephosphate isomerase, transforming growth factor-β induced protein, peroxiredoxin-4, ADAM (A disintegrin and metalloproteinases) 28, and latent-transforming growth factor beta-binding protein-2. These PCM components were verified with immunohisto(cyto)chemistry for localization in the PCM region of articular cartilage. The abundance of type VI collagen in the PCM emphasizes its importance to the microenvironment of chondrocytes. Several proteins were localized in the PCM of chondrocytes for the first time and that warrants further investigation for their functions in cartilage biology.

The collagen VI-related myopathies: muscle meets its matrix

The collagen VI-related myopathy known as Ullrich congenital muscular dystrophy is an early-onset disease that combines substantial muscle weakness with striking joint laxity and progressive contractures. Patients might learn to walk in early childhood; however, this ability is subsequently lost, concomitant with the development of frequent nocturnal respiratory failure. Patients with intermediate phenotypes of collagen VI-related myopathy display a lesser degree of weakness and a longer period of ambulation than do individuals with Ullrich congenital muscular dystrophy, and the spectrum of disease finally encompasses mild Bethlem myopathy, in which ambulation persists into adulthood. Dominant and recessive autosomal mutations in the three major collagen VI genes-COL6A1, COL6A2, and COL6A3-can underlie this entire clinical spectrum, and result in deficient or dysfunctional microfibrillar collagen VI in the extracellular matrix of muscle and other connective tissues, such as skin and tendons. The potential effects on muscle include progressive dystrophic changes, fibrosis and evidence for increased apoptosis, which potentially open avenues for pharmacological intervention. Optimized respiratory management, including noninvasive nocturnal ventilation together with careful orthopedic management, are the current mainstays of treatment and have already led to a considerable improvement in life expectancy for children with Ullrich congenital muscular dystrophy.

The protective role of the pericellular matrix in chondrocyte apoptosis

INTRODUCTION

This study was designed to quantify the role of the pericellular matrix (PCM) in chondrocyte apoptosis using chondrons, which are a cartilage functional unit including a chondrocyte and its associated PCM.

METHODS

Chondrocytes and chondrons were enzymatically isolated from human articular cartilage and exposed to monosodium iodoacetate (MIA) and staurosporine for apoptosis induction. Chondrons were defined by the presence of type VI collagen, a basic component of the PCM. Apoptosis of chondrocytes and chondrons was measured with annexin V binding by flow cytometry and verified with terminal dUTP nick end-labeling staining. In a separate experiment, isolated chondrocytes were treated with soluble type VI collagen, before or after apoptosis induction with MIA, and cell death was measured by the activity of LDH and terminal dUTP nick end-labeling staining.

RESULTS

Chondrocytes treated with MIA incurred 27% cell death, compared with 12% in chondrons. On treating with MIA, 9% of chondrocytes underwent apoptosis, compared with only 1.6% of chondrons. Similarly, staurosporine induced 13% apoptosis in chondrocytes, whereas it was 3% in chondrons. Preincubation of type VI collagen effectively prevented chondrocytes from MIA-induced cell death. After apoptosis was induced with MIA, however, treatment with type VI collagen failed to rescue chondrocytes from death.

CONCLUSION

The PCM, a native microenvironment of chondrocytes, protects chondrocytes from apoptosis. Type VI collagen is a functional component of the PCM that contributes to the survival of chondrocytes.

Differential and restricted expression of novel collagen VI chains in mouse

Recently, three novel collagen VI chains, α4, α5 and α6, were identified. These are thought to substitute for the collagen VI α3 chain, probably forming α1α2α4, α1α2α5 or α1α2α6 heterotrimers. The expression pattern of the novel chains is so far largely unknown. In the present study, we compared the tissue distribution of the novel collagen VI chains in mouse with that of the α3 chain by immunohistochemistry, immunoelectron microscopy and immunoblots. In contrast to the widely expressed α3 chain, the novel chains show a highly differential, restricted and often complementary expression. The α4 chain is strongly expressed in the intestinal smooth muscle, surrounding the follicles in ovary, and in testis. The α5 chain is present in perimysium and at the neuromuscular junctions in skeletal muscle, in skin, in the kidney glomerulus, in the interfollicular stroma in ovary and in the tunica albuginea of testis. The α6 chain is most abundant in the endomysium and perimysium of skeletal muscle and in myocard. Immunoelectron microscopy of skeletal muscle localized the α6 chain to the reticular lamina of muscle fibers. The highly differential and restricted expression points to the possibility of tissue-specific roles of the novel chains in collagen VI assembly and function.

Regulation of UDP-glucose dehydrogenase is sufficient to modulate hyaluronan production and release, control sulfated GAG synthesis, and promote chondrogenesis

Glycosaminoglycans (GAGs) are critical for extracellular matrix (ECM) integrity in cartilage but mechanisms regulating their synthesis are not defined. UDP-glucose dehydrogenase (UGDH) catalyses UDP-glucose oxidation to UDP-glucuronic acid, an essential monosaccharide in many GAGs. Our previous studies in articular surface (AS) cells from embryonic joints have established pivotal roles for mitogen-activated protein kinases (MAPK) in synthesis of the unsulfated GAG, hyaluronan (HA). We investigated the functional significance of UGDH in GAG production and chondrogenesis, and determined roles for MEK-ERK and p38MAPK pathways in regulating UGDH expression and function. Inhibitors of MEK and p38MAPK reduced UGDH protein in AS cells. Treatment with TGF-β (archetypal growth factor) increased UGDH expression, sulfated (s)-GAG/HA release and pericellular matrix formation in a p38MAPK-dependent manner. Retroviral overexpression of UGDH augmented HA/sGAG release and pericellular matrix elaboration, which were blocked by inhibiting MEK but not p38MAPK. UGDH overexpression increased cartilage nodule size in bone marrow culture, promoted chondrogenesis in limb bud micromass culture and selectively suppressed medium HA levels and modified GAG sulfation, as assessed by FACE analysis. Our data provide evidence that: (i) TGF-β regulates UGDH expression via p38MAPK to modulate sGAG/HA secretion, (ii) MEK-ERK, but not p38MAPK facilitates UGDH-induced HA and sGAG release, and (iii) increased UGDH expression promotes chondrogenesis directly and differential modifies GAG levels and sulfation. These results indicate a more diverse role for UGDH in the support of selective GAG production than previously described. Factors regulating UGDH may provide novel candidates for restoring ECM integrity in degenerative cartilage diseases, such as osteoarthritis.Arthritis Research Campaign.

Attenuation of osteoarthritis progression by reduction of discoidin domain receptor 2 in mice

OBJECTIVE

To investigate whether the reduction of discoidin domain receptor 2 (DDR-2), a cell membrane tyrosine kinase receptor for native type II collagen, attenuates the progression of articular cartilage degeneration in mouse models of osteoarthritis (OA).

METHODS

Double-heterozygous (type XI collagen-deficient [Col11a1(+/-)] and Ddr2-deficient [Ddr2(+/-)]) mutant mice were generated. Knee joints of Ddr2(+/-) mice were subjected to microsurgical destabilization of the medial meniscus. Conditions of the articular cartilage from the knee joints of the double-heterozygous mutant and surgically treated mice were examined by histology, evaluated using a modified Mankin scoring system, and characterized by immunohistochemistry.

RESULTS

The rate of progressive degeneration in knee joints was dramatically reduced in the double-heterozygous mutant mice compared with that in the type XI collagen-deficient mice. The progression in the double-heterozygous mutant mice was delayed by ∼6 months. Following surgical destabilization of the medial meniscus, the progressive degeneration toward OA was dramatically delayed in the Ddr2(+/-) mice compared with that in their wild-type littermates. The articular cartilage damage present in the knee joints of the mice was directly correlated with the expression profiles of DDR-2 and matrix metalloproteinase 13.

CONCLUSION

Reduction of DDR-2 expression attenuates the articular cartilage degeneration of knee joints induced either by type XI collagen deficiency or by surgical destabilization of the medial meniscus.

Abnormal human chondrocyte morphology is related to increased levels of cell-associated IL-1β and disruption to pericellular collagen type VI

Early osteoarthritis (OA) is poorly understood, but abnormal chondrocyte morphology might be important. We studied IL-1β and pericellular collagen type VI in morphologically normal and abnormal chondrocytes. In situ chondrocytes within explants from nondegenerate (grade 0/1) areas of human tibial plateaus (n = 21) were fluorescently labeled and visualized [2-photon laser scanning microscopy (2PLSM)]. Normal chondrocytes exhibited a "smooth" membrane surface, whereas abnormal cells were defined as demonstrating ≥1 cytoplasmic process. Abnormal chondrocytes were further classified by number and average length of cytoplasmic processes/cell. IL-1β or collagen type VI associated with single chondrocytes were visualized by fluorescence immuno-histochemistry and confocal laser scanning microscopy (CLSM). Fluorescence was quantified as the number of positive voxels (i.e., 3D pixels with fluorescence above baseline)/cell. IL-1β-associated fluorescence increased between normal and all abnormal cells in the superficial (99.7 ± 29.8 [11 (72)] vs. 784 ± 382 [15 (132)]; p = 0.04, positive voxels/cell) and deep zones (66.5 ± 29.4 [9 (64)] vs. 795 ± 224 [9 (56)]; p = 0.006). There was a correlation (r(2) = 0.988) between the number of processes/cell (0-5) and IL-1β, and an increase particularly with short processes (≤5 µm; p = 0.022). Collagen type VI coverage and thickness decreased (p < 0.001 and p = 0.005, respectively) with development of processes. Abnormal chondrocytes in macroscopically nondegenerate cartilage demonstrated a marked increase in IL-1β and loss of pericellular type VI collagen, changes that could lead to cartilage degeneration.

Dysfunctional tendon collagen fibrillogenesis in collagen VI null mice

Tendons are composed of fibroblasts and collagen fibrils. The fibrils are organized uniaxially and grouped together into fibers. Collagen VI is a non-fibrillar collagen expressed in developing and adult tendons. Human collagen VI mutations result in muscular dystrophy, joint hyperlaxity and contractures. The purpose of this study is to determine the functional roles of collagen VI in tendon matrix assembly. During tendon development, collagen VI was expressed throughout the extracellular matrix, but enriched around fibroblasts and their processes. To analyze the functional roles of collagen VI a mouse model with a targeted inactivation of Col6a1 gene was utilized. Ultrastructural analysis of Col6a1-/- versus wild type tendons demonstrated disorganized extracellular micro-domains and associated collagen fibers in the Col6a1-/- tendon. In Col6a1-/- tendons, fibril structure and diameter distribution were abnormal compared to wild type controls. The diameter distributions were shifted significantly toward the smaller diameters in Col6a1-/- tendons compared to controls. An analysis of fibril density (number/μm(2)) demonstrated a ~2.5 fold increase in the Col6a1-/- versus wild type tendons. In addition, the fibril arrangement and structure were aberrant in the peri-cellular regions of Col6a1-/- tendons with frequent very large fibrils and twisted fibrils observed restricted to this region. The biomechanical properties were analyzed in mature tendons. A significant decrease in cross-sectional area was observed. The percent relaxation, maximum load, maximum stress, stiffness and modulus were analyzed and Col6a1-/- tendons demonstrated a significant reduction in maximum load and stiffness compared to wild type tendons. An increase in matrix metalloproteinase activity was suggested in the absence of collagen VI. This suggests alterations in tenocyte expression due to disruption of cell-matrix interactions. The changes in expression may result in alterations in the peri-cellular environment. In addition, the absence of collagen VI may alter the sequestering of regulatory molecules such as leucine rich proteoglycans. These changes would result in dysfunctional regulation of tendon fibrillogenesis indirectly mediated by collagen VI.

Spatial mapping of the biomechanical properties of the pericellular matrix of articular cartilage measured in situ via atomic force microscopy

In articular cartilage, chondrocytes are surrounded by a narrow region called the pericellular matrix (PCM), which is biochemically, structurally, and mechanically distinct from the bulk extracellular matrix (ECM). Although multiple techniques have been used to measure the mechanical properties of the PCM using isolated chondrons (the PCM with enclosed cells), few studies have measured the biomechanical properties of the PCM in situ. The objective of this study was to quantify the in situ mechanical properties of the PCM and ECM of human, porcine, and murine articular cartilage using atomic force microscopy (AFM). Microscale elastic moduli were quantitatively measured for a region of interest using stiffness mapping, or force-volume mapping, via AFM. This technique was first validated by means of elastomeric models (polyacrylamide or polydimethylsiloxane) of a soft inclusion surrounded by a stiff medium. The elastic properties of the PCM were evaluated for regions surrounding cell voids in the middle/deep zone of sectioned articular cartilage samples. ECM elastic properties were evaluated in regions visually devoid of PCM. Stiffness mapping successfully depicted the spatial arrangement of moduli in both model and cartilage surfaces. The modulus of the PCM was significantly lower than that of the ECM in human, porcine, and murine articular cartilage, with a ratio of PCM to ECM properties of approximately 0.35 for all species. These findings are consistent with previous studies of mechanically isolated chondrons, and suggest that stiffness mapping via AFM can provide a means of determining microscale inhomogeneities in the mechanical properties of articular cartilage in situ.

An axisymmetric boundary element model for determination of articular cartilage pericellular matrix properties in situ via inverse analysis of chondron deformation

The pericellular matrix (PCM) is the narrow tissue region surrounding all chondrocytes in articular cartilage and, together, the chondrocyte(s) and surrounding PCM have been termed the chondron. Previous theoretical and experimental studies suggest that the structure and properties of the PCM significantly influence the biomechanical environment at the microscopic scale of the chondrocytes within cartilage. In the present study, an axisymmetric boundary element method (BEM) was developed for linear elastic domains with internal interfaces. The new BEM was employed in a multiscale continuum model to determine linear elastic properties of the PCM in situ, via inverse analysis of previously reported experimental data for the three-dimensional morphological changes of chondrons within a cartilage explant in equilibrium unconfined compression (Choi, et al., 2007, "Zonal Changes in the Three-Dimensional Morphology of the Chondron Under Compression: The Relationship Among Cellular, Pericellular, and Extracellular Deformation in Articular Cartilage," J. Biomech., 40, pp. 2596-2603). The microscale geometry of the chondron (cell and PCM) within the cartilage extracellular matrix (ECM) was represented as a three-zone equilibrated biphasic region comprised of an ellipsoidal chondrocyte with encapsulating PCM that was embedded within a spherical ECM subjected to boundary conditions for unconfined compression at its outer boundary. Accuracy of the three-zone BEM model was evaluated and compared with analytical finite element solutions. The model was then integrated with a nonlinear optimization technique (Nelder-Mead) to determine PCM elastic properties within the cartilage explant by solving an inverse problem associated with the in situ experimental data for chondron deformation. Depending on the assumed material properties of the ECM and the choice of cost function in the optimization, estimates of the PCM Young's modulus ranged from approximately 24 kPa to 59 kPa, consistent with previous measurements of PCM properties on extracted chondrons using micropipette aspiration. Taken together with previous experimental and theoretical studies of cell-matrix interactions in cartilage, these findings suggest an important role for the PCM in modulating the mechanical environment of the chondrocyte.

Molecular genetic studies of gene identification for osteoporosis: the 2009 update

Osteoporosis is a complex human disease that results in increased susceptibility to fragility fractures. It can be phenotypically characterized using several traits, including bone mineral density, bone size, bone strength, and bone turnover markers. The identification of gene variants that contribute to osteoporosis phenotypes, or responses to therapy, can eventually help individualize the prognosis, treatment, and prevention of fractures and their adverse outcomes. Our previously published reviews have comprehensively summarized the progress of molecular genetic studies of gene identification for osteoporosis and have covered the data available to the end of September 2007. This review represents our continuing efforts to summarize the important and representative findings published between October 2007 and November 2009. The topics covered include genetic association and linkage studies in humans, transgenic and knockout mouse models, as well as gene-expression microarray and proteomics studies. Major results are tabulated for comparison and ease of reference. Comments are made on the notable findings and representative studies for their potential influence and implications on our present understanding of the genetics of osteoporosis.

Pharmacoproteomic study of the effects of chondroitin and glucosamine sulfate on human articular chondrocytes

INTRODUCTION

Chondroitin sulfate (CS) and glucosamine sulfate (GS) are symptomatic slow-acting drugs for osteoarthritis (OA) widely used in clinic. Despite their widespread use, knowledge of the specific molecular mechanisms of their action is limited. The aim of this work is to explore the utility of a pharmacoproteomic approach for the identification of specific molecules involved in the pharmacological effect of GS and CS.

METHODS

Chondrocytes obtained from three healthy donors were treated with GS 10 mM and/or CS 200 μg/mL, and then stimulated with interleukin-1β (IL-1β) 10 ng/mL. Whole cell proteins were isolated 24 hours later and resolved by two-dimensional electrophoresis. The gels were stained with SYPRORuby. Modulated proteins were identified by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF/TOF) mass spectrometry. Real-time PCR and Western blot analyses were performed to validate our results.

RESULTS

A total of 31 different proteins were altered by GS or/and CS treatment when compared to control. Regarding their predicted biological function, 35% of the proteins modulated by GS are involved in signal transduction pathways, 15% in redox and stress response, and 25% in protein synthesis and folding processes. Interestingly, CS affects mainly energy production (31%) and metabolic pathways (13%), decreasing the expression levels of ten proteins. The chaperone GRP78 was found to be remarkably increased by GS alone and in combination with CS, a fact that unveils a putative mechanism for the reported anti-inflammatory effect of GS in OA. On the other hand, the antioxidant enzyme superoxide dismutase 2 (SOD2) was significantly decreased by both drugs and synergistically by their combination, thus suggesting a drug-induced decrease of the oxidative stress caused by IL-1β in chondrocytes.

CONCLUSIONS

CS and GS differentially modulate the proteomic profile of human chondrocytes. This pharmacoproteomic approach unravels the complex intracellular mechanisms that are modulated by these drugs on IL1β-stimulated human articular chondrocytes.

Role of HTRA1, a serine protease, in the progression of articular cartilage degeneration

This study is to investigate the possible role of high temperature requirement A 1 (HtrA1) in the articular cartilage degeneration. Paraffin sections were prepared from the knee and temporomandibular (TM) joints of four mouse OA models; two of the models had a genetic mutation (type IX collagen-deficient and type XI collagen-haploinsufficient) and two were surgically induced (destabilization of the medial meniscus of knee joint and discectomy of TM joint). The HtrA1 protein expression profiles of the prepared sections were examined by immunohistostaining. The level of HtrA1 mRNA in the articular cartilage taken from the knee joints of one of the genetically mutated OA models was determined by real-time PCR. Double immunohistostaining was used to examine the expression of co-localization of HtrA1 with type VI collagen and HtrA1 with discoidin domain receptor 2 (Ddr2) in the articular cartilage of knee joints from the genetically mutated OA model. The expression of HtrA1 was found to be increased in the knee and TM joints of these four models at early stages of the disease. An examination of the knee joint of a mutant mouse indicated an 8-fold increase in the level of HtrA1 mRNA, when compared to the levels observed in the knee joints of its wild-type littermates. Pericellular type VI collagen was not present in chondrocytes expressing HtrA1. Meanwhile, the expression of HtrA1 was associated with the expression of Ddr2 in the chondrocytes. Results indicate that HtrA1 may disrupt the pericellular matrix network, resulting in alteration of chondrocyte metabolisms. This eventually leads to OA.

The extracellular matrix in development and morphogenesis: a dynamic view

The extracellular matrix (ECM) is synthesized and secreted by embryonic cells beginning at the earliest stages of development. Our understanding of ECM composition, structure and function has grown considerably in the last several decades and this knowledge has revealed that the extracellular microenvironment is critically important for cell growth, survival, differentiation and morphogenesis. ECM and the cellular receptors that interact with it mediate both physical linkages with the cytoskeleton and the bidirectional flow of information between the extracellular and intracellular compartments. This review considers the range of cell and tissue functions attributed to ECM molecules and summarizes recent findings specific to key developmental processes. The importance of ECM as a dynamic repository for growth factors is highlighted along with more recent studies implicating the 3-dimensional organization and physical properties of the ECM as it relates to cell signaling and the regulation of morphogenetic cell behaviors. Embryonic cell and tissue generated forces and mechanical signals arising from ECM adhesion represent emerging areas of interest in this field.

Biglycan and fibromodulin have essential roles in regulating chondrogenesis and extracellular matrix turnover in temporomandibular joint osteoarthritis

The temporomandibular joint is critical for jaw movements and allows for mastication, digestion of food, and speech. Temporomandibular joint osteoarthritis is a degenerative disease that is marked by permanent cartilage destruction and loss of extracellular matrix (ECM). To understand how the ECM regulates mandibular condylar chondrocyte (MCC) differentiation and function, we used a genetic mouse model of temporomandibular joint osteoarthritis that is deficient in two ECM proteins, biglycan and fibromodulin (Bgn(-/0)Fmod(-/-)). Given the unavailability of cell lines, we first isolated primary MCCs and found that they were phenotypically unique from hyaline articular chondrocytes isolated from the knee joint. Using Bgn(-/0) Fmod(-/-) MCCs, we discovered the early basis for temporomandibular joint osteoarthritis arises from abnormal and accelerated chondrogenesis. Transforming growth factor (TGF)-beta1 is a growth factor that is critical for chondrogenesis and binds to both biglycan and fibromodulin. Our studies revealed the sequestration of TGF-beta1 was decreased within the ECM of Bgn(-/0) Fmod(-/-) MCCs, leading to overactive TGF-beta1 signal transduction. Using an explant culture system, we found that overactive TGF-beta1 signals induced chondrogenesis and ECM turnover in this model. We demonstrated for the first time a comprehensive study revealing the importance of the ECM in maintaining the mandibular condylar cartilage integrity and identified biglycan and fibromodulin as novel key players in regulating chondrogenesis and ECM turnover during temoporomandibular joint osteoarthritis pathology.

Transcriptome analysis reveals an unexpected role of a collagen tyrosine kinase receptor gene, Ddr2, as a regulator of ovarian function

Mice homozygous for the smallie (slie) mutation lack a collagen receptor, discoidin domain receptor 2 (DDR2), and are dwarfed and infertile due to peripheral dysregulation of the endocrine system of unknown etiology. We used a systems biology approach to identify biological networks affected by Ddr2(slie/slie) mutation in ovaries using microarray analysis and validate findings using molecular, cellular, and functional biological assays. Transcriptome analysis indicated several altered gene categories in Ddr2(slie/slie) mutants, including gonadal development, ovulation, antiapoptosis, and steroid hormones. Subsequent biological experiments confirmed the transcriptome analysis predictions. For instance, a significant increase of TUNEL-positive follicles was found in Ddr2(slie/slie) mutants vs. wild type, which confirm the transcriptome prediction for decreased chromatin maintenance and antiapoptosis. Decreases in gene expression were confirmed by RT-PCR and/or qPCR; luteinizing hormone receptor and prostaglandin type E and F receptors in Ddr2(slie/slie) mutants, compared with wild type, confirm hormonal signaling pathways involved in ovulation. Furthermore, deficiencies in immunohistochemistry for DDR2 and luteinizing hormone receptor in the somatic cells, but not the oocytes, of Ddr2(slie/slie) mutant ovaries suggest against an intrinsic defect in germ cells. Indeed, Ddr2(slie/slie) mutants ovulated significantly fewer oocytes; their oocytes were competent to complete meiosis and fertilization in vitro. Taken together, our convergent data signify DDR2 as a novel critical player in ovarian function, which acts upon classical endocrine pathways in somatic, rather than germline, cells.