Severe disturbance of the distribution and expression of type VI collagen chains in osteoarthritic articular cartilage

Painful temporomandibular joint overloading induces structural remodeling in the pericellular matrix of that joint's chondrocytes

Mechanical stress to the temporomandibular joint (TMJ) is an important factor in cartilage degeneration, with both clinical and preclinical studies suggesting that repeated TMJ overloading could contribute to pain, inflammation, and/or structural damage in the joint. However, the relationship between pain severity and early signs of cartilage matrix microstructural dysregulation is not understood, limiting the advancement of diagnoses and treatments for temporomandibular joint-osteoarthritis (TMJ-OA). Changes in the pericellular matrix (PCM) surrounding chondrocytes may be early indicators of OA. A rat model of TMJ pain induced by repeated jaw loading (1 h/day for 7 days) was used to compare the extent of PCM modulation for different loading magnitudes with distinct pain profiles (3.5N-persistent pain, 2N-resolving pain, or unloaded controls-no pain) and macrostructural changes previously indicated by Mankin scoring. Expression of PCM structural molecules, collagen VI and aggrecan NITEGE neo-epitope, were evaluated at Day 15 by immunohistochemistry within TMJ fibrocartilage and compared between pain conditions. Pericellular collagen VI levels increased at Day 15 in both the 2N (p = 0.003) and 3.5N (p = 0.042) conditions compared to unloaded controls. PCM width expanded to a similar extent for both loading conditions at Day 15 (2N, p < 0.001; 3.5N, p = 0.002). Neo-epitope expression increased in the 3.5N group over levels in the 2N group (p = 0.041), indicating pericellular changes that were not identified in the same groups by Mankin scoring of the pericellular region. Although remodeling occurs in both pain conditions, the presence of pericellular catabolic neo-epitopes may be involved in the macrostructural changes and behavioral sensitivity observed in persistent TMJ pain.

High-resolution infrared microspectroscopic characterization of cartilage cell microenvironment

The lateral resolution of infrared spectroscopy has been inadequate for accurate biochemical characterization of the cell microenvironment, a region regulating biochemical and biomechanical signals to cells. In this study, we demonstrate the capacity of a high-resolution Fourier transform infrared microspectroscopy (HR-FTIR-MS) to characterize the collagen content of this region. Specifically, we focus on the collagen content in the cartilage cell (chondrocyte) microenvironment of healthy and osteoarthritic (OA) cartilage. Human tibial cartilage samples (N = 28) were harvested from 7 cadaveric donors and graded for OA severity (healthy, early OA, advanced OA). HR-FTIR-MS was used to analyze the collagen content of the chondrocyte microenvironment of five distinct zones across the tissue depth. HR-FTIR-MS successfully showed collagen content distribution across chondrocytes and their environment. In zones 2 and 3 (10 - 50% of the tissue thickness), we observed that collagen content was smaller (P < 0.05) in early OA compared to the healthy tissue in the vicinity of cells (pericellular region). The collagen content loss was extended to the extracellular matrix in advanced OA tissue. No significant differences in the collagen content of the chondrocyte microenvironment were observed between the groups in the most superficial (0-10%) and deep zones (50-100%). HR-FTIR-MS revealed collagen loss in the early OA cartilage pericellular region before detectable changes in the extracellular matrix in advanced OA. HR-FTIR-MS-based compositional assessment enables a better understanding of OA-related changes in tissues. This technique can be used to identify new disease mechanisms enabling better intervention strategies. STATEMENT OF SIGNIFICANCE: Osteoarthritis (OA) is the most common degenerative joint disease causing pain and disability. While significant progress has been made in OA research, OA pathogenesis is still poorly understood and current OA treatments are mainly palliative. This study demonstrates that high-resolution FTIR microspectroscopy (HR-FTIR-MS) can characterize OA-induced compositional changes in the cell microenvironment (pericellular matrix) during the early disease stages before tissue changes in the extracellular matrix become apparent. This technique may further enable the identification of new OA mechanisms and improve our current understanding of OA pathogenesis, thus, enabling the development of better treatment methods.

Perlecan in the Natural and Cell Therapy Repair of Human Adult Articular Cartilage: Can Modifications in This Proteoglycan Be a Novel Therapeutic Approach?

Articular cartilage is considered to have limited regenerative capacity, which has led to the search for therapies to limit or halt the progression of its destruction. Perlecan, a multifunctional heparan sulphate (HS) proteoglycan, promotes embryonic cartilage development and stabilises the mature tissue. We investigated the immunolocalisation of perlecan and collagen between donor-matched biopsies of human articular cartilage defects (n = 10 × 2) that were repaired either naturally or using autologous cell therapy, and with age-matched normal cartilage. We explored how the removal of HS from perlecan affects human chondrocytes in vitro. Immunohistochemistry showed both a pericellular and diffuse matrix staining pattern for perlecan in both natural and cell therapy repaired cartilage, which related to whether the morphology of the newly formed tissue was hyaline cartilage or fibrocartilage. Immunostaining for perlecan was significantly greater in both these repair tissues compared to normal age-matched controls. The immunolocalisation of collagens type III and VI was also dependent on tissue morphology. Heparanase treatment of chondrocytes in vitro resulted in significantly increased proliferation, while the expression of key chondrogenic surface and genetic markers was unaffected. Perlecan was more prominent in chondrocyte clusters than in individual cells after heparanase treatment. Heparanase treatment could be a means of increasing chondrocyte responsiveness to cartilage injury and perhaps to improve repair of defects.

Changes in stiffness and biochemical composition of the pericellular matrix as a function of spatial chondrocyte organisation in osteoarthritic cartilage

OBJECTIVE

During osteoarthritis (OA), chondrocytes seem to change their spatial arrangement from single to double strings, small and big clusters. Since the pericellular matrix (PCM) appears to degrade alongside this reorganisation, it has been suggested that spatial patterns act as an image-based biomarker for OA. The aim of this study was to establish the functional relevance of spatial organisation in articular cartilage.

METHOD

Cartilage samples were selected according to their predominant spatial cellular pattern. Young's modulus of their PCM was measured by atomic force microscopy (AFM) (∼500 measurements/pattern). The distribution of two major PCM components (collagen type VI and perlecan) was analysed by immunohistochemistry (8 patients) and protein content quantified by enzyme-linked immunosorbent assay (ELISA) (58 patients).

RESULTS

PCM stiffness significantly decreased with the development from single to double strings (p = 0.030), from double strings to small clusters (p = 0.015), and from small clusters to big clusters (p < 0.001). At the same time, the initially compact collagen type VI and perlecan staining progressively weakened and was less focalised. The earliest point with a significant reduction in protein content as shown by ELISA was the transition from single strings to small clusters for collagen type VI (p = 0.016) and from double strings to small clusters for perlecan (p = 0.008), with the lowest amounts for both proteins seen in big clusters.

CONCLUSIONS

This study demonstrates the functional relevance of spatial chondrocyte organisation as an image-based biomarker. At the transition from single to double strings PCM stiffness decreases, followed by protein degradation from double strings to small clusters.

In vitro method for 3D morphometry of human articular cartilage chondrons based on micro-computed tomography

OBJECTIVE

The aims of this study were: to 1) develop a novel sample processing protocol to visualize human articular cartilage (AC) chondrons using micro-computed tomography (μCT), 2) develop and validate an algorithm to quantify the chondron morphology in 3D, and 3) compare the differences in chondron morphology between intact and osteoarthritic AC.

METHOD

The developed protocol is based on the dehydration of samples with hexamethyldisilazane (HMDS), followed by imaging with a desktop μCT. Chondron density and depth, as well as volume and sphericity, were calculated in 3D with a custom-made and validated algorithm employing semi-automatic chondron selection and segmentation. The quantitative parameters were analyzed at three AC depth zones (zone 1: 0-10%; zone 2: 10-40%; zone 3: 40-100%) and grouped by the OARSI histological grades (OARSI grades 0-1.0, n = 6; OARSI grades 3.0-3.5, n = 6).

RESULTS

After semi-automatic chondron selection and segmentation, 1510 chondrons were approved for 3D morphometric analyses. The chondrons especially in the deeper tissue (zones 2 and 3) were significantly larger (P < 0.001) and less spherical (P < 0.001), respectively, in the OARSI grade 3-3.5 group compared to the OARSI grade 0-1.0 group. No statistically significant difference in chondron density between the OARSI grade groups was observed at different depths.

CONCLUSION

We have developed a novel sample processing protocol for chondron imaging in 3D, as well as a high-throughput algorithm to semi-automatically quantify chondron/chondrocyte 3D morphology in AC. Our results also suggest that 3D chondron morphology is affected by the progression of osteoarthritis (OA).

Osteoarthritis as a disease of the cartilage pericellular matrix

Osteoarthritis is a painful joint disease characterized by progressive degeneration of the articular cartilage as well as associated changes to the subchondral bone, synovium, and surrounding joint tissues. While the effects of osteoarthritis on the cartilage extracellular matrix (ECM) have been well recognized, it is now becoming apparent that in many cases, the onset of the disease may be initially reflected in the matrix region immediately surrounding the chondrocytes, termed the pericellular matrix (PCM). Growing evidence suggests that the PCM - which along with the enclosed chondrocytes are termed the "chondron" - acts as a critical transducer or "filter" of biochemical and biomechanical signals for the chondrocyte, serving to help regulate the homeostatic balance of chondrocyte metabolic activity in response to environmental signals. Indeed, it appears that alterations in PCM properties and cell-matrix interactions, secondary to genetic, epigenetic, metabolic, or biomechanical stimuli, could in fact serve as initiating or progressive factors for osteoarthritis. Here, we discuss recent advances in the understanding of the role of the PCM, with an emphasis on the reciprocity of changes that occur in this matrix region with disease, as well as how alterations in PCM properties could serve as a driver of ECM-based diseases such as osteoarthritis. Further study of the structure, function, and composition of the PCM in normal and diseased conditions may provide new insights into the understanding of the pathogenesis of osteoarthritis, and presumably new therapeutic approaches for this disease.

A preclinical evaluation of an autologous living hyaline-like cartilaginous graft for articular cartilage repair: a pilot study

In this pilot study, an autologous synthetic scaffold-free construct with hyaline quality, termed living hyaline cartilaginous graft (LhCG), was applied for treating cartilage lesions. Implantation of autologous LhCG was done at load-bearing regions of the knees in skeletally mature mini-pigs for 6 months. Over the course of this study, significant radiographical improvement in LhCG treated sites was observed via magnetic resonance imaging. Furthermore, macroscopic repair was effected by LhCG at endpoint. Microscopic inspection revealed that LhCG engraftment restored cartilage thickness, promoted integration with surrounding native cartilage, produced abundant cartilage-specific matrix molecules, and re-established an intact superficial tangential zone. Importantly, the repair efficacy of LhCG was quantitatively shown to be comparable to native, unaffected cartilage in terms of biochemical composition and biomechanical properties. There were no complications related to the donor site of cartilage biopsy. Collectively, these results imply that LhCG engraftment may be a viable approach for articular cartilage repair.

Comprehensive protein profiling of synovial fluid in osteoarthritis following protein equalization

OBJECTIVE

The aim of the study was to characterise the protein complement of synovial fluid (SF) in health and osteoarthritis (OA) using liquid chromatography mass spectrometry (LC-MS/MS) following peptide-based depletion of high abundance proteins.

DESIGN

SF was used from nine normal and nine OA Thoroughbred horses. Samples were analysed with LC-MS/MS using a NanoAcquity™ LC coupled to an LTQ Orbitrap Velos. In order to enrich the lower-abundance protein fractions protein equalisation was first undertaken using ProteoMiner™. Progenesis-QI™ LC-MS software was used for label-free quantification. In addition immunohistochemistry, western blotting and mRNA expression analysis was undertaken on selected joint tissues.

RESULTS

The number of protein identifications was increased by 33% in the ProteoMiner™ treated SF compared to undepleted SF. A total of 764 proteins (462 with≥2 significant peptides) were identified in SF. A subset of 10 proteins were identified which were differentially expressed in OA SF. S100-A10, a calcium binding protein was upregulated in OA and validated with western blotting and immunohistochemistry. Several new OA specific peptide fragments (neopeptides) were identified.

CONCLUSION

The protein equalisation method compressed the dynamic range of the synovial proteins identifying the most comprehensive SF proteome to date. A number of proteins were identified for the first time in SF which may be involved in the pathogenesis of OA. We identified a distinct set of proteins and neopeptides that may act as potential biomarkers to distinguish between normal and OA joints.

Chondropenia: current concept review

The term "chondropenia" indicates the early stage of degenerative cartilage disease, and it has been identified by carefully monitoring early-stage osteoarthritis (OA). Not only is it the loss of articular cartilage volume, but it is also a rearrangement of biomechanical, ultrastructural, biochemical and molecular properties typical of healthy cartilage tissue. Diagnosing OA at an early stage or an advanced stage is valuable in terms of clinical and therapeutic outcome. In fact degenerative phenomena are supported by a complex biochemical cascade which unbalances the extracellular matrix homeostasis, closely regulated by chondrocytes. In the first stage an intense inflammatory reaction is triggered: pro-catabolic cytokines such as IL-1β and TNF-α triggering matrix metalloproteases and aggrecanase (ADAMT-4 and 5), responsible for the early loss of ultrastructural components, such as type II collagen and aggrecan. In addition nitric oxide and reactive oxygen species modulate the physiopathology of the condral matrix inducing apoptosis of chondrocytes through a mitochondria-dependent pathway. In addition, "Lonely Death": chondrocytes, are confined within a dense, avascular extracellular matrix capsule, and can trigger a genetically induced apoptosis and necrosis. The degenerative process starts from a central point and then spreads in a centrifugal manner in depth and in adjacent areas, eventually covering the whole joint; chondropenia represents a journey from the first clinically detectable time-point until it can be characterized as frank osteoarthritis. Currently, there are no instruments sensitive enough which allow a timely diagnosis of chondropenia. Innovative magnetic resonance imaging techniques, such as T2 mapping, can be effective and a sensitive diagnostic instrument for quantifying cartilage volume and proteoglycan content. However, avant-garde biophysical techniques, such as mechanical indenters, ultrasound and biochemical markers (uCTX-II), are rational and scientific tools applicable to the clinical and therapeutic management of early degenerative cartilage disease. The objective of this review on chondropenia is to present a state of the art and innovative concepts.

Optical clearing in collagen- and proteoglycan-rich osteochondral tissues

OBJECTIVE

Recent developments in optical clearing and microscopy technology have enabled the imaging of intact tissues at the millimeter scale to characterize cells via fluorescence labeling. While these techniques have facilitated the three-dimensional (3D) cellular characterization within brain and heart, study of dense connective tissues of the musculoskeletal system have been largely unexplored. Here, we quantify how optical clearing impacted the cell and tissue morphology of collagen-, proteoglycan-, and mineral-rich cartilage and bone from the articulating knee joint.

METHODS

Water-based fructose solutions were used for optical clearing of bovine osteochondral tissues, followed by imaging with transmission and confocal microscopy. To confirm preservation of tissue structure during the clearing process, samples were mechanically tested in unconfined compression and visualized by cryo-SEM.

RESULTS

Optical clearing enhanced light transmission through cartilage, but not subchondral bone regions. Fluorescent staining and immunolabeling was preserved through sample preparations, enabling imaging to cartilage depths five times deeper than previously reported, limited only by the working distance of the microscope objective. Chondrocyte volume remained unchanged in response to, and upon the reversal, of clearing. Equilibrium modulus increased in cleared samples, and was attributed to exchange of interstitial fluid with the more viscous fructose solution, but returned to control levels upon unclearing. In addition, cryo-SEM-based analysis of cartilage showed no ultrastructural changes.

CONCLUSION

We anticipate large-scale microscopy of diverse connective tissues will enable the study of intact, 3D interfaces (e.g., osteochondral) and cellular connectivity as a function of development, disease, and regeneration, which have been previously hindered by specimen opacity.

Optical clearing in dense connective tissues to visualize cellular connectivity in situ

Visualizing the three-dimensional morphology and spatial patterning of cells embedded deep within dense connective tissues of the musculoskeletal system has been possible only by utilizing destructive techniques. Here we utilize fructose-based clearing solutions to image cell connectivity and deep tissue-scale patterning in situ by standard confocal microscopy. Optical clearing takes advantage of refractive index matching of tissue and the embedding medium to visualize light transmission through a broad range of bovine and whole mount murine tissues, including cartilage, bone, and ligament, of the head and hindlimb. Using non-destructive methods, we show for the first time intercellular chondrocyte connections throughout the bulk of cartilage, and we reveal in situ patterns of osteocyte processes and the lacunar-canalicular system deep within mineralized cortical bone. Optical clearing of connective tissues is expected to find broad application for the study of cell responses in normal physiology and disease pathology.

Effects of laser treatment on the expression of cytosolic proteins in the synovium of patients with osteoarthritis

BACKGROUND AND OBJECTIVE

Low level laser therapy (LLLT) has been developed for non-invasive treatment of joint diseases. We have previously shown that LLLT influenced synovial protein expression in rheumatoid arthritis (RA). The aim of this study was to assess the effects of laser irradiation on osteoarthritic (OA) synovial protein expression.

STUDY DESIGN/MATERIALS AND METHODS

The synovial membrane samples removed from the knees of 6 OA patients were irradiated ex vivo using near infrared diode laser (807-811 nm; 25 J/cm(2) ). An untreated sample taken from the same patient served as control. Synovial protein separation and identification were performed by two-dimensional differential gel electrophoresis and mass spectrometry, respectively.

RESULTS

Eleven proteins showing altered expression due to laser irradiation were identified. There were three patients whose tissue samples demonstrated a significant increase (P < 0.05) in mitochondrial heat shock 60 kD protein 1 variant 1. The expression of the other proteins (calpain small subunit 1, tubulin alpha-1C and beta 2, vimentin variant 3, annexin A1, annexin A5, cofilin 1, transgelin, and collagen type VI alpha 2 chain precursor) significantly decreased (P < 0.05) compared to the control samples.

CONCLUSIONS

A single diode laser irradiation of the synovial samples of patients with osteoarthritis can statistically significantly alter the expression of some proteins in vitro. These findings provide some more evidence for biological efficacy of LLLT treatment, used for osteoarthritis.

Micromechanical mapping of early osteoarthritic changes in the pericellular matrix of human articular cartilage

OBJECTIVE

Osteoarthritis (OA) is a degenerative joint disease characterized by the progressive loss of articular cartilage. While macroscale degradation of the cartilage extracellular matrix (ECM) has been extensively studied, microscale changes in the chondrocyte pericellular matrix (PCM) and immediate microenvironment with OA are not fully understood. The objective of this study was to quantify osteoarthritic changes in the micromechanical properties of the ECM and PCM of human articular cartilage in situ using atomic force microscopy (AFM).

METHOD

AFM elastic mapping was performed on cryosections of human cartilage harvested from both condyles of macroscopically normal and osteoarthritic knee joints. This method was used to test the hypotheses that both ECM and PCM regions exhibit a loss of mechanical properties with OA and that the size of the PCM is enlarged in OA cartilage as compared to normal tissue.

RESULTS

Significant decreases were observed in both ECM and PCM moduli of 45% and 30%, respectively, on the medial condyle of OA knee joints as compared to cartilage from macroscopically normal joints. Enlargement of the PCM, as measured biomechanically, was also observed in medial condyle OA cartilage, reflecting the underlying distribution of type VI collagen in the region. No significant differences were observed in elastic moduli or their spatial distribution on the lateral condyle between normal and OA joints.

CONCLUSION

Our findings provide new evidence of significant site-specific degenerative changes in the chondrocyte micromechanical environment with OA.

Effect of exercise on articular cartilage

This review primarily focuses on how the macromolecular composition and architecture of articular cartilage and its unique biomechanical properties play a pivotal role in the ability of articular cartilage to withstand mechanical loads several magnitudes higher than the weight of the individual. Current findings on short-term and long-term effects of exercise on human articular cartilage are reviewed, and the importance of appropriate exercises for individuals with normal and diseased or aberrated cartilage is discussed.

Role of c-Maf in Chondrocyte Differentiation: A Review

Chondrocyte differentiation in the growth plate is an important process for the longitudinal growth of endochondral bones. Sox9 and Runx2 are the most often-studied transcriptional regulators of the chondrocyte differentiation process, but the importance of additional factors is also becoming apparent. Mafs are a subfamily of the basic ZIP (bZIP) transcription factor superfamily, which act as key regulators of tissue-specific gene expression and terminal differentiation in many tissues. There is increasing evidence that c-Maf and its splicing variant Lc-Maf play a role in chondrocyte differentiation in a temporal-spatial manner. This review summarizes the functions of c-Maf in chondrocyte differentiation and discusses the possible role of c-Maf in osteoarthritis progression.

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.

Type II and VI collagen in nasal and articular cartilage and the effect of IL-1alpha on the distribution of these collagens

The distribution of type II and VI collagen was immunocytochemically investigated in bovine articular and nasal cartilage. Cartilage explants were used either fresh or cultured for up to 4 weeks with or without interleukin 1α (IL-1α). Sections of the explants were incubated with antibodies for both types of collagen. Microscopic analyses revealed that type II collagen was preferentially localized in the interchondron matrix whereas type VI collagen was primarily found in the direct vicinity of the chondrocytes. Treatment of the sections with hyaluronidase greatly enhanced the signal for both types of collagen. Also in sections of explants cultured with IL-1α a higher level of labeling of the collagens was found. This was apparent without any pre-treatment with hyaluronidase. Under the influence of IL-1α the area positive for type VI collagen that surrounded the chondrocytes broadened. Although the two collagens in both types of cartilage were distributed similarly, a remarkable difference was the higher degree of staining of type VI collagen in articular cartilage. Concomitantly we noted that digestion of this type of cartilage hardly occurred in the presence of IL-1α whereas nasal cartilage was almost completely degraded within 18 days of culture. Since type VI collagen is known to be relatively resistant to proteolysis we speculate that the higher level of type VI collagen in articular cartilage is important in protecting cartilage from digestion.

Phenotypic characterization of epiphycan-deficient and epiphycan/biglycan double-deficient mice

OBJECTIVE

To characterize the in vivo role epiphycan (Epn) has in cartilage development and/or maintenance.

METHODS

Epn-deficient mice were generated by disrupting the Epn gene in mouse embryonic stem cells. Epn/biglycan (Bgn) double-deficient mice were produced by crossing Epn-deficient mice with Bgn-deficient mice. Whole knee joint histological sections were stained using van Gieson or Fast green/Safranin-O to analyze collagen or proteoglycan content, respectively. Microarray analysis was performed to detect gene expression changes within knee joints.

RESULTS

Epn-deficient and Epn/Bgn double-deficient mice appeared normal at birth. No significant difference in body weight or femur length was detected in any animal at 1 month of age. However, 9-month Epn/Bgn double-deficient mice were significantly lighter and had shorter femurs than wild type mice, regardless of gender. Male Epn-deficient mice also had significantly shorter femurs than wild type mice at 9 months. Most of the deficient animals developed osteoarthritis (OA) with age; the onset of OA was observed earliest in Epn/Bgn double-deficient mice. Message RNA isolated from Epn/Bgn double-deficient knee joints displayed increased matrix protein expression compared with wild type mice, including other small leucine-rich proteoglycan (SLRP) members such as asporin, fibromodulin and lumican.

CONCLUSION

Similar to other previously studied SLRPs, EPN plays an important role in maintaining joint integrity. However, the severity of the OA phenotype in the Epn/Bgn double-deficient mouse suggests a synergy between these two proteins. These data are the first to show a genetic interaction involving class I and class III SLRPs in vivo.

Advanced osteoarthritis in humans is associated with altered collagen VI expression and upregulation of ER-stress markers Grp78 and bag-1

To test the hypothesis that a perturbation of endoplasmic reticulum (ER) function is involved in the pathogenesis of osteoarthritis (OA), articular cartilage was isolated from non-OA patients secondary to resection of osteo- or chondrosarcomas. Intra-joint samples of minimal and advanced osteoarthritic cartilage were isolated from patients undergoing total knee arthroplasty and scored for disease severity. Glucose-regulated protein-78 (grp78) and bcl-2-associated athanogene-1 (bag-1) were detected via immunofluorescence as markers of non-homeostatic ER function. Additionally, the expression of type VI collagen and its integrin receptor, NG2, was determined to examine cartilage matrix health and turnover. There was an upregulation of grp78 in advanced OA, and variable expression in minimal OA. Non-OA cartilage was consistently grp78 negative. The downstream regulator bag-1 was also upregulated in OA compared with normal cartilage. Collagen VI was mainly cell-associated in non-OA cartilage, with a more widespread distribution observed in OA cartilage along with increased intracellular staining intensity. The collagen VI integral membrane proteoglycan receptor NG2 was downregulated in advanced OA compared with its patient-matched minimally involved cartilage sample. These results suggest that chondrocytes exhibit ER stress during OA, in association with upregulation of a large secreted molecule, type VI collagen.

Over-expression of c-maf by chondrocytes in osteoarthritis

The c-maf gene expression profile was investigated in normal and osteoarthritic articular cartilage using in situ hybridization, qualitative reverse transcription-polymerase chain reaction (RT-PCR) and quantitative real-time RT-PCR. Osteoarthritic samples were obtained from 10 patients undergoing total knee replacement for severe osteoarthritis of the knee joints, and control samples from 10 trauma patients undergoing amputation. Expression of c-maf was significantly up-regulated in osteoarthritic cartilage compared with normal cartilage. Using in situ hybridization, distribution of a specific c-maf mRNA signal was found in the top zone and a decreased signal was found in the lower middle zone and the deep zone in osteoarthritic cartilage. A prominent c-maf mRNA signal was seen particularly in proliferating 'chondrocyte clusters'. In contrast, in normal cartilage almost no c-maf-positive cells were found. These findings suggest that c-maf may be important in chondrocyte hypertrophy and terminal differentiation, and may be involved in the pathogenesis of osteoarthritis.

Identification of calponin 3 as a novel Smad-binding modulator of BMP signaling expressed in cartilage

Bone morphogenetic proteins (BMP) play a prominent role in cartilage tissue homeostasis and perturbations of BMP signaling contribute to pathological processes like osteoarthritis. The response to BMP is determined by intracellular proteins interacting with the signal mediators Smads 1 and 5. Applying the yeast two-hybrid technique we could identify the actin-binding protein calponin 3 as a novel Smad-binding protein expressed in chondrocytes. It interacted with Smads 1 and 5 and overexpression led to an attenuation of BMP-dependent transcription. Calponin 3 mRNA and protein were expressed in cartilage tissue and isolated chondrocytes and a slight, but statistically significant reduction of mRNA expression levels could be detected in osteoarthritic cartilage. Our results suggest a role of calponin 3 in the regulation of BMP-dependent cellular responses. By interaction with the Smad proteins 1 and 5 and the inhibition of BMP-induced transcription, calponin 3 provides a negative regulatory mechanism for the BMP signaling pathway. This inhibitory effect likely depends on a sequestration of the Smads to the cytoskeleton due to the actin-binding properties of calponin 3. The down-regulation of calponin 3 expression in osteoarthritic joints could contribute to the increased responsiveness to BMPs described previously. Furthermore, our data provide a possible explanation for the effect of the related protein calponin 1 on bone and cartilage development.

Functional genomics, evo-devo and systems biology: a chance to overcome complexity?

PURPOSE OF REVIEW

This review addresses the key question of how to integrate a high complexity of processes and data to a unifying picture of disease processes and progression relevant for osteoarthritis.

RECENT FINDINGS

Many research efforts in the last few years have resulted in the accumulation of a huge amount of data. To date, however, these data have not led to a unifying concept of the pathogenesis and progression of the osteoarthritic disease process. Methods to integrate a lot of information are needed, therefore, in order to progress from experimental findings to practical knowledge. Several such strategies have been followed up in the past: in-vitro models, large-scale gene expression analysis/functional genomics, and an attempt to interpret gene expression patterns on the basis of developmental chondrocyte differentiation. A novel approach is systems biology, which promises to overcome issues of complexity using appropriate models and quantitative simulation.

SUMMARY

Efforts are required to integrate a continuously growing high complexity of experimental data into an understanding of the joint system and its derangement in osteoarthritis. Modelling of the 'whole' picture appears to be needed so that we do not get lost in the plethora of details.

Mechanisms of disease: role of chondrocytes in the pathogenesis of osteoarthritis--structure, chaos and senescence

The extracellular matrix of articular cartilage is the primary target of osteoarthritic cartilage degradation. However, cartilage cells have a pivotal role during osteoarthritis, as they are mainly responsible for the anabolic-catabolic balance required for matrix maintenance and tissue function. In addition to the severe changes in the extracellular matrix, the cells also display abnormalities during osteoarthritic cartilage degeneration, such as inappropriate activation of anabolic and catabolic activities, and alterations in cell number through processes like proliferation and (apoptotic) cell death. The cells are exposed to additional stimuli such as nonphysiologic loading conditions and byproducts of matrix destruction, as well as abnormal levels of cytokines and growth factors. This exposure can lead to a structured cellular response pattern that may be either beneficial or detrimental to the cartilage tissue. Potentially even more problematic for preserving tissue homeostasis, neighboring osteoarthritic chondrocytes display strong heterogeneity in their phenotype, gene expression patterns, and cellular responses. As the disease progresses, osteoarthritic chondrocytes can no longer maintain tissue integrity. Evidence suggests that cell aging is important in the pathogenesis of osteoarthritis. Thus, anti-aging strategies might complement existing therapeutic targets related to anabolism, catabolism, inflammation, and apoptosis-processes that are integral to the pathogenesis of osteoarthritis.

Activation of interleukin-1 signaling cascades in normal and osteoarthritic articular cartilage

Interleukin (IL)-1 is one of the most important catabolic cytokines in rheumatoid arthritis. In this study, we were interested in whether we could identify IL-1 expression and activity within normal and osteoarthritic cartilage. mRNA expression of IL-1beta and of one of its major target genes, IL-6, was observed at very low levels in normal cartilage, whereas only a minor up-regulation of these cytokines was noted in osteoarthritic cartilage, suggesting that IL-1 signaling is not a major event in osteoarthritis. However, immunolocalization of central mediators involved in IL-1 signaling pathways [38-kd protein kinases, phospho (P)-38-kd protein kinases, extracellular signal-regulated kinase 1/2, P-extracellular signal-regulated kinase 1/2, c-Jun NH(2)-terminal kinase 1/2, P-c-Jun NH(2)-terminal kinase 1/2, and nuclear factor kappaB] showed that the four IL-1 signaling cascades are functional in normal and osteoarthritic articular chondrocytes. In vivo, we found that IL-1 expression and signaling mechanisms were detectible in the upper zones of normal cartilage, whereas these observations were more pronounced in the upper portions of osteoarthritic cartilage. Given these expression and distribution patterns, our data support two roles for IL-1 in the pathophysiology of articular cartilage. First, chondrocytes in the upper zone of osteoarthritic articular cartilage seem to activate catabolic signaling pathways that may be in response to diffusion of external IL-1 from the synovial fluid. Second, IL-1 seems to be involved in normal cartilage tissue homeostasis as shown by identification of baseline expression patterns and signaling cascade activation.

Synergistic effect of IGF-1 and OP-1 on matrix formation by normal and OA chondrocytes cultured in alginate beads

OBJECTIVE

Growth factor therapy may be useful for stimulation of cartilage matrix synthesis and repair. Thus, the purpose of our study was to further understand the effect of combined insulin-like growth factor-1 (IGF-1) and osteogenic protein-1 (OP-1) treatment on the matrix synthesized by human adult normal and osteoarthritic (OA) chondrocytes.

DESIGN

Chondrocytes were isolated post-mortem from articular cartilage from tali of normal human donors and femoral condyles of OA patients undergoing knee replacement surgery. Cells were cultured in alginate beads for 21 days in four experimental groups: (1) "mini-ITS" control; (2) 100 ng/ml IGF-1; (3) 100 ng/ml OP-1; (4) IGF-1+OP-1, each at 100 ng/ml. Beads were processed for histological (Safranin O and fast green), morphometrical and immunohistochemical (aggrecan, decorin, type I, II, VI, and X collagens, and fibronectin accumulation) analyses.

RESULTS

Histology showed that IGF-1 alone did not induce substantial matrix production. OP-1 alone caused a considerable matrix formation, but the highest matrix accumulation by normal and OA chondrocytes was found when OP-1 and IGF-1 were added together. Morphometrical analysis indicated larger matrices produced by OA chondrocytes than by normal cells under the combined treatment. All tested matrix proteins were more abundant in the combination group. Type X collagen was detected only under the combined OP-1 and IGF-1 treatment and was present at very low levels. Type I collagen was found only in OA chondrocytes.

CONCLUSIONS

The results obtained in the current study suggest that combined therapy with IGF-1 and OP-1 may have a greater potential in treating cartilage defects seen in OA than use of either growth factor alone.

Large-scale gene expression profiling reveals major pathogenetic pathways of cartilage degeneration in osteoarthritis

OBJECTIVE

Despite many research efforts in recent decades, the major pathogenetic mechanisms of osteoarthritis (OA), including gene alterations occurring during OA cartilage degeneration, are poorly understood, and there is no disease-modifying treatment approach. The present study was therefore initiated in order to identify differentially expressed disease-related genes and potential therapeutic targets.

METHODS

This investigation consisted of a large gene expression profiling study performed based on 78 normal and disease samples, using a custom-made complementary DNA array covering >4,000 genes.

RESULTS

Many differentially expressed genes were identified, including the expected up-regulation of anabolic and catabolic matrix genes. In particular, the down-regulation of important oxidative defense genes, i.e., the genes for superoxide dismutases 2 and 3 and glutathione peroxidase 3, was prominent. This indicates that continuous oxidative stress to the cells and the matrix is one major underlying pathogenetic mechanism in OA. Also, genes that are involved in the phenotypic stability of cells, a feature that is greatly reduced in OA cartilage, appeared to be suppressed.

CONCLUSION

Our findings provide a reference data set on gene alterations in OA cartilage and, importantly, indicate major mechanisms underlying central cell biologic alterations that occur during the OA disease process. These results identify molecular targets that can be further investigated in the search for therapeutic interventions.

Zonal variations in the three-dimensional morphology of the chondron measured in situ using confocal microscopy

OBJECTIVE

Chondrocytes in articular cartilage are surrounded by a narrow pericellular matrix (PCM), which together with the enclosed cell(s) are termed the "chondron". Although the precise function of this tissue region is unknown, previous studies provide indirect evidence that the PCM plays an important role in governing the local mechanical environment of chondrocytes. In particular, theoretical models of the chondron under mechanical loading suggest that the shape, size, and biomechanical properties of the PCM significantly influence the stress-strain and fluid flow environment of the cell. The goal of this study was to quantify the three-dimensional morphology of chondron in situ using en bloc immunolabeling of type VI collagen coupled with fluorescence confocal microscopy.

METHODS

Three-dimensional reconstructions of intact, fluorescently labeled chondrons were made from stacks of confocal images recorded in situ from the superficial, middle, and deep zones of porcine articular cartilage of the medial femoral condyle.

RESULTS

Significant variations in the shape, size, and orientation of chondrocytes and chondrons were observed with depth from the tissue surface, revealing flattened discoidal chondrons in the superficial zone, rounded chondrons in the middle zone, and elongated, multicellular chondrons in the deep zone.

CONCLUSIONS

The shape and orientation of the chondron appear to reflect the local collagen architecture of the interterritorial matrix, which varies significantly with depth. Quantitative measurements of morphology of the chondron and its variation with site, disease, or aging may provide new insights into the influence of this structure on physiology and the pathology of articular cartilage.

Hgs physically interacts with Smad5 and attenuates BMP signaling

Signaling by members of the bone morphogenetic protein family plays a critical role in cartilage development and differentiation. Recently, the potential involvement of BMPs in the maintenance and repair of damaged adult articular cartilage has initiated an interest in the role of BMP signaling and the involved signaling pathways in the adult tissue. In this study, we identified Hgs as a novel Smad5 interactor using a cDNA expression library constructed from human adult cartilage. This interaction was confirmed by coimmunoprecipitation experiments in 293 EBNA cells and the chondrocytic cell line T/C-28a2. Overexpression of Hgs resulted in an attenuation of BMP-dependent transcriptional responses suggesting that Hgs acts as an inhibitor of BMP signaling. Of note, osteoarthritic chondrocytes which have been suggested previously to show increased reactivity to BMP-stimulation showed less expression of Hgs. Thus, it is tempting to speculate that both might be related to each other given the suppressive effect of BMP signaling on Hgs shown in this study.

Confocal laser scanning microscopy in orthopaedic research

Confocal laser scanning microscopy (CLSM) is a type of high-resolution fluorescence microscopy that overcomes the limitations of conventional widefield microscopy and facilitates the generation of high-resolution 3D images from relatively thick sections of tissue. As a comparatively non-destructive imaging technique, CLSM facilitates the in situ characterization of tissue microstructure. Images generated by CLSM have been utilized for the study of articular cartilage, bone, muscle, tendon, ligament and menisci by the foremost research groups in the field of orthopaedics including those teams headed by Bush, Errington, Guilak, Hall, Hunziker, Knight, Mow, Poole, Ratcliffe and White. Recent evolutions in techniques and technologies have facilitated a relatively widespread adoption of this imaging modality, with increased "user friendliness" and flexibility. Applications of CLSM also exist in the rapidly advancing field of orthopaedic implants and in the investigation of joint lubrication.

Phenotypic characterization of human chondrocyte cell line C-20/A4: a comparison between monolayer and alginate suspension culture

DNA microarray analysis was used to investigate the molecular phenotype of one of the first human chondrocyte cell lines, C-20/A4, derived from juvenile costal chondrocytes by immortalization with origin-defective simian virus 40 large T antigen. Clontech Human Cancer Arrays 1.2 and quantitative PCR were used to examine gene expression profiles of C-20/A4 cells cultured in the presence of serum in monolayer and alginate beads. In monolayer cultures, genes involved in cell proliferation were strongly upregulated compared to those expressed by human adult articular chondrocytes in primary culture. Of the cell cycle-regulated genes, only two, the CDK regulatory subunit and histone H4, were downregulated after culture in alginate beads, consistent with the ability of these cells to proliferate in suspension culture. In contrast, the expression of several genes that are involved in pericellular matrix formation, including MMP-14, COL6A1, fibronectin, biglycan and decorin, was upregulated when the C-20/A4 cells were transferred to suspension culture in alginate. Also, nexin-1, vimentin, and IGFBP-3, which are known to be expressed by primary chondrocytes, were differentially expressed in our study. Consistent with the proliferative phenotype of this cell line, few genes involved in matrix synthesis and turnover were highly expressed in the presence of serum. These results indicate that immortalized chondrocyte cell lines, rather than substituting for primary chondrocytes, may serve as models for extending findings on chondrocyte function not achievable by the use of primary chondrocytes.

Repression of anti-proliferative factor Tob1 in osteoarthritic cartilage

Osteoarthritis is the most common degenerative disorder of the modern world. However, many basic cellular features and molecular processes of the disease are poorly understood. In the present study we used oligonucleotide-based microarray analysis of genes of known or assumed relevance to the cellular phenotype to screen for relevant differences in gene expression between normal and osteoarthritic chondrocytes. Custom made oligonucleotide DNA arrays were used to screen for differentially expressed genes in normal (n = 9) and osteoarthritic (n = 10) cartilage samples. Real-time polymerase chain reaction (PCR) with gene-specific primers was used for quantification. Primary human adult articular chondrocytes and chondrosarcoma cell line HCS-2/8 were used to study changes in gene expression levels after stimulation with interleukin-1β and bone morphogenetic protein, as well as the dependence on cell differentiation. In situ hybridization with a gene-specific probe was applied to detect mRNA expression levels in fetal growth plate cartilage. Overall, more than 200 significantly regulated genes were detected between normal and osteoarthritic cartilage (P < 0.01). One of the significantly repressed genes, Tob1, encodes a protein belonging to a family involved in silencing cells in terms of proliferation and functional activity. The repression of Tob1 was confirmed by quantitative PCR and correlated to markers of chondrocyte activity and proliferation in vivo. Tob1 expression was also detected at a decreased level in isolated chondrocytes and in the chondrosarcoma cell line HCS-2/8. Again, in these cells it was negatively correlated with proliferative activity and positively with cellular differentiation. Altogether, the downregulation of the expression of Tob1 in osteoarthritic chondrocytes might be an important aspect of the cellular processes taking place during osteoarthritic cartilage degeneration. Activation, the reinitiation of proliferative activity and the loss of a stable phenotype are three major changes in osteoarthritic chondrocytes that are highly significantly correlated with the repression of Tob1 expression.

Aging theories of primary osteoarthritis: from epidemiology to molecular biology

Osteoarthritis is the most common disabling condition of humans in the western world. It has been known for a very long time that aging is the most prominent risk factor for the initiation and progression of the disease, but the explanations for this phenomenon have changed over time. The most longstanding theory is that osteoarthritis develops because of continuous mechanical wear and tear. However, osteoarthritis can also be the result of time/age-related modifications to cartilage matrix components. One of the simplest biological explanations for the initiation and progression of osteoarthritic cartilage degeneration is a mere loss of viable cells, due to apoptosis or other mechanisms. Overall, the most likely scenario is that the cells and the matrix of articular cartilage get older over time, and eventually the tissue enters a senescence-like state that makes it more prone to enter the osteoarthritic degeneration pathway. Thus, patients with osteoarthritis might progress more quickly to the senescence phenotype compared to others. Moreover, stressful conditions associated with the osteoarthritic disease process might further promote chondrocyte senescence. Primary osteoarthritis in this model would be a "premature" degeneration of the joint due to a premature chondrocyte senescence. By analogy to neurodegenerative disorders, one could refer to osteoarthritis as the "M. Alzheimer" of articular cartilage. One of the most important implications of this hypothesis is that it points to issues of cellular degeneration as the basis for understanding the initiation and progression of osteoarthritis. Equally important, it emphasizes that whatever treatment we envisage for osteoarthritis, we must take into account that we are dealing with aged/(pre)senescent cells that no longer have the ability of their juvenile counterparts to counteract the many mechanical, inflammatory, and/or other assaults to the tissue.

Distribution of type VI collagen in chondrocyte microenvironment: study of chondrons isolated from human normal and degenerative articular cartilage and cultured chondrocytes

The chondron is the microanatomical unit composed of a chondrocyte and its pericellular microenvironment (PCME), including the pericellular matrix and capsule. In the present study, we extracted chondrons from human articular cartilages and investigated the relationship between the distribution of the matrix molecules, including type VI collagen, and the degeneration of articular cartilage. We also investigated the effects of interleukin-1beta (IL-1beta) and transforming growth factor beta-1 (TGF-beta1) on the distribution of type VI collagen in cultured chondrocytes. Chondrons were extracted by low-speed homogenization from cartilage pieces obtained from forensic autopsies and from patients with knee osteoarthritis (OA) undergoing total knee arthroplasty. Cartilage sections were classified into three groups (normal, slight degeneration, and moderate degeneration) based on the degree of degeneration according to Mankin's score. Extracted chondrons were immunostained, and the distribution of the matrix molecules, including type VI collagen, was investigated using a confocal laser scanning microscope (CLSM). The chondrocytes isolated by enzymic treatment were subjected to three-dimensional culture in agarose gel and then treated with IL-1beta or TGF-beta1. The distribution of newly synthesized type VI collagen in agarose gel was also investigated using the CLSM. Type VI collagen was localized specifically within the PCME of chondrons. The volume ratio of PCME to chondrocyte (P/C ratio) was significantly higher in the moderate degeneration group than in the other two groups. The accumulation of type VI collagen around a chondrocyte was obviously increased by the addition of TGF-beta1. The P/C ratio significantly increased as the severity of the OA progressed, suggesting that type VI collagen distributed specifically in the PCME was playing a protective role for chondrocytes by maintaining the pericellular microenvironment in OA.

SOX9 expression does not correlate with type II collagen expression in adult articular chondrocytes

Anabolic activity is a crucial activity of articular chondrocytes and its failure is one major reason of osteoarthritic cartilage degeneration. The intracellular factors responsible for the increase or decrease of anabolic activity of articular chondrocytes remain largely unknown. A recent candidate, the transcription factor SOX9, has elicited much interest as it is suggested to be a central factor in chondrocytic differentiation during development, including collagen type II (COL2A1) expression, the major anabolic gene product of chondrocytes. Here we show that normal adult human articular chondrocytes in vivo contain high SOX9 mRNA levels, which are decreased in osteoarthritic cartilage. Surprisingly, no positive correlation between SOX9 and COL2A1 expression was observed--to the contrary, the expression of COL2A1 was significantly increased in the diseased cells. Immunolocalization confirmed the presence of SOX9 protein in normal and osteoarthritic chondrocytes without showing significant differences in both SOX9 quantity and subcellular localization in osteoarthritic compared to normal cartilage tissue. Interestingly, laser scanning confocal microscopy showed that the subcellular distribution of SOX9 in adult chondrocytes was not restricted to the nucleus as observed in fetal chondrocytes, but was also detected within the cytoplasm, with no differences in subcellular SOX9 distribution between normal and OA cartilage. This is consistent with the lack of positive correlation between SOX9 and COL2A1 expression in adult articular chondrocytes. Also, no positive correlation between SOX9 and COL2A1 expression was observed in vitro after challenge of chondrocytes with Il-1beta, which is a strong (negative) regulator of COL2A1 expression, or with IGF-I, which stimulates COL2A1 expression. These results suggest that SOX9 is not the key regulator of COL2A1 promoter activity in human adult articular chondrocytes. However, SOX9 might still be involved in maintaining the chondrocytic phenotype in normal and osteoarthritic cartilage.

Osteophyte development--molecular characterization of differentiation stages

OBJECTIVE

Osteophytes are non-neoplastic osteo-cartilaginous protrusions growing at the margins of osteoarthritic joints. They can not only be considered as in situ repair tissue, but also represent an excellent in vivo model for induced cartilage repair processes. Our focus was to identify different steps of osteophyte development via analysis of expression patterns of marker genes of chondrocytic differentiation.

DESIGN

We performed an extensive analysis of the presence and expression of matrix components using histochemical, immunohistochemical and in situ hybridization technology.

RESULTS

Four different stages of osteophyte formation could be identified based on histomorphological and cell biological parameters: starting from mesenchymal condensates, chondrogenic differentiation is indicated by the onset of Col2A and aggrecan expression (stage I). Stage II shows fibrocartilage with an admixture of cartilaginous and fibrous matrix components such as Col2 and aggrecan on the one hand and Col1 on the other hand. The proliferating osteophyte (stage III) shows a zonal organization similar to the fetal growth plate cartilage with extensive chondrocyte hypertrophy in the zones next to ongoing endochondral bone formation. 'Mature' osteophytes (stage IV) resembled largely articular hyaline cartilage with a predominance of Col2 and aggrecan and Col6 found mainly pericellularily.

CONCLUSIONS

The development of osteophytes is a good in vivo model to pursue chondrocyte differentiation from pluripotent mesenchymal cells to mature or hypertrophic chondrocytes in situ in the adult. The analysis of marker molecules of mesenchymal differentiation allows to identify different stages of repair tissue development and the transformation from fibrous tissue to neo-cartilage. Tissue architecture and matrix composition in mature osteophytes suggests that metaplastic neo-cartilagenous tissue might be one potential source of cartilage repair tissue in the adult joint.

Quantification of expression levels of cellular differentiation markers does not support a general shift in the cellular phenotype of osteoarthritic chondrocytes

Many studies have shown increased anabolic activity in osteoarthritic cartilage and have suggested changes in the cellular phenotypes of articular chondrocytes. Most of these studies relied on non-quantitative technologies, which did not allow the estimation of the relative importance of the different differentiation phenomena. In the present study, we developed and used quantitative PCR assays for collagen types I, II(total), IIA, III, and X as marker genes indicating cellular synthetic activity (collagen type II) as well as differentiation pattern of chondrocytes (collagen types I, IIA, III, and X) and quantified these genes in normal, early degenerative, and late stage osteoarthritic cartilage in parallel. At first sight, our results confirmed previously published data showing hardly any expression of collagen genes in normal and significantly enhanced expression in osteoarthritic cartilage. This included collagen types II, III, and IIA, but also collagen types I(alpha1) and X. However, if one considers the ratios of the various markers of chondrocytic differentiation in comparison to collagen type II, the main synthetic product of differentiated chondrocytes, no shift in the cellular phenotype was detectable. In fact, expression ratios remained constant or were even decreased in osteoarthritic cartilage. Our results confirm that normal adult human articular chondrocytes display hardly any expression activity of the collagen types investigated, whereas osteoarthritic chondrocytes show very increased synthetic activity. The largely unchanged ratios of collagen subtypes investigated indicate that no general shift in the cellular phenotype does occur in osteoarthritic cartilage as suggested by previous investigations.

Bone morphogenetic protein-mediating receptor-associated Smads as well as common Smad are expressed in human articular chondrocytes but not up-regulated or down-regulated in osteoarthritic cartilage

Bone morphogenetic proteins (BMPs) are supposed to be important for cartilage matrix anabolism. In this study, we investigated whether the intracellular mediators of BMP activity, Smads 1, 4, 5, and 8, are expressed in normal human articular chondrocytes in vivo and in vitro and whether alterations in expression and distribution pattern are found in osteoarthritic cartilage or in vitro after stimulation with interleukin (IL)-1, because down-regulation of these mediators could be responsible for the decrease of anabolic activity in osteoarthritic cartilage. RNA was isolated from normal and osteoarthritic human knee cartilage and analyzed by (quantitative) polymerase chain reaction (PCR) technology. Articular chondrocytes were cultured in alginate beads and short-term high-density monolayer cultures with and without stimulation by IL-1. In addition, immunolocalization of the receptor-associated Smads (R-Smads) was performed on sections of normal and diseased articular cartilage. Reverse-transcription (RT)-PCR analysis showed a moderate expression of all Smads investigated in normal, early degenerative, and late stage osteoarthritic cartilage. Immunolocalization detected the R-Smads in most chondrocytes on the protein level in all specimen groups investigated. In vitro, the Smads were also expressed and partly up-regulated by Il-1beta in alginate bead culture. Of note, for Smad 1, two truncated splice variants were expressed by articular chondrocytes missing exon 4 as well as exons 3 and 4. Our study showed that BMP-receptor Smads 1, 5, and 8 as well as common Smad (C-Smad) 4 are expressed and present in human normal and osteoarthritic articular chondrocytes corroborating the importance of BMPs and BMP signaling for articular cartilage. This study is the first to describe splicing variants for Smad 1. Smads 1, 4, and 5 are up-regulated in vitro by Il-1beta, suggesting a linkage of the Il-1 and BMP-signaling pathways within the chondrocytes. None of the Smads were grossly up- or down-regulated in osteoarthritic chondrocytes, suggesting that differences in overall expression levels of the investigated Smad proteins are not relevant for metabolic activity of articular chondrocytes in vivo.

Distribution of newly synthesized aggrecan in explant cultures of bovine cartilage treated with retinoic acid

This paper describes temporal changes in the metabolism and distribution of newly synthesized aggrecan and the organization of the extracellular matrix when explant cultures of articular cartilage maintained in the presence of fetal calf serum were exposed to retinoic acid for varying periods of time. Explant cultures of articular cartilage were incubated with radiolabeled sulfate prior to exposure to retinoic acid. The radiolabeled and chemical aggrecan present in the tissue and appearing in the culture medium was studied kinetically. Changes in the localization of radiolabeled aggrecan within the extracellular matrix were monitored by autoradiography in relation to type VI collagen distribution in the extracellular matrix. In control cultures where tissue levels of aggrecan remain constant the newly synthesized aggrecan remained closely associated with the territorial matrix surrounding the chondrocytes. Exposure of cultures to retinoic acid for the duration of the experiment, resulted in the extensive loss of aggrecan from the tissue and the redistribution of the remaining radiolabeled aggrecan from the chondron and territorial matrix into the inter-territorial matrix. These changes preceded alterations in the organization of type VI collagen in the extracellular matrix that involved the remodeling of the chondron and the appearance of type VI collagen in the inter-territorial matrix; there was also evidence of chondrocyte proliferation and clustering. In cartilage explant cultures exposed to retinoic acid for 24 h there was no loss of aggrecan from the matrix but there was an extensive redistribution of the radiolabeled aggrecan into the inter-territorial matrix. This work shows that maintenance of the structure and organization of the extracellular matrix that comprises the chondron and pericellular microenvironment of chondrocytes in articular cartilage is important for the regulation of the distribution of newly synthesized aggrecan monomers within the tissue.

Relative messenger RNA expression profiling of collagenases and aggrecanases in human articular chondrocytes in vivo and in vitro

OBJECTIVE

Osteoarthritic (OA) cartilage destruction depends on collagen- and aggrecan-degrading proteases such as collagenases (MMP-1 and MMP-13), stromelysin (MMP-3), MMP-14, as well as the so-called aggrecanases (ADAM-TS4 and ADAM-TS5). In this study, we tried to clarify whether these proteases are expressed in vivo in human normal and OA cartilage (and whether they are up-regulated or down-regulated during the disease process) and in interleukin-1beta (IL-1beta)-stimulated chondrocytes in vitro.

METHODS

Quantitative polymerase chain reaction assays were developed and performed on RNA isolated directly from normal and degenerative cartilage tissue as well as from primary human articular chondrocytes cultured with and without IL-1beta.

RESULTS

In vivo, MMP-1 was detectable only at very low levels in any condition. MMP-13 expression was low in normal and early degenerative cartilage but was strongly up-regulated in late-stage OA specimens. MMP-1 and MMP-13 were expressed much higher in vitro than in vivo and were up-regulated by IL-1beta. Among all proteases, MMP-3 was by far the most strongly expressed, although it was strongly down-regulated in late-stage OA specimens. Expression of MMP-3 was higher in vitro than in vivo and was up-regulated by IL-1beta. ADAM-TS5 and MMP-14 were expressed in all sample groups. Expression of ADAM-TS4 was very low in vivo and was induced in vitro after stimulation by IL-1beta.

CONCLUSION

Our expression data clearly support MMP-13 as the major collagenase in OA cartilage. The most strongly expressed aggrecanase was ADAM-TS5. ADAM-TS4 was expressed only at a very low level in normal cartilage and was only slightly up-regulated in OA cartilage, casting doubt on this enzyme being the relevant aggrecanase of articular cartilage. Results of our study show that expression of many enzymes is significantly different in vitro and in vivo and suggest that IL-1beta stimulation of articular chondrocytes might not be a good model for the matrix catabolism in OA cartilage.

Co-localization of insulin-like growth factor binding protein 3 and fibronectin in human articular cartilage

OBJECTIVE

The anabolic cytokine insulin-like growth factor I (IGF-I) stimulates chondrocyte synthesis of matrix macromolecules and several lines of evidence suggest that it has a major role in maintaining articular cartilage and possibly in cartilage repair. Despite the apparent importance of IGF-I in articular cartilage metabolism and its potential importance in joint diseases, little is known about the regulation of IGF-I activity within the tissue. Insulin-like growth factor binding proteins (IGFBPs) bind IGF-I and can modify its activity. At least three IGFBPs are expressed by chondrocytes: IGFBP-3, -4 and -5. Localization of IGFPBs in the articular cartilage extracellular matrix (ECM) could create reservoirs of IGF-I within the articular cartilage ECM and thereby regulate local IGF-I levels. We hypothesized that ECM molecules bind and concentrate IGFPBs in the pericellular/territorial matrix.

DESIGN

Semi-quantitative immunohistological measures of co-localization were used to compare the spatial distribution of IGFBP-3, -4, and -5 with the distributions of three peri-cellularly-enriched matrix molecules fibronectin, tenascin-C, and type VI collagen in osteoarthritic and non-osteoarthritic human articular cartilage. Purified proteins were used in an agarose diffusion assay to compare IGFBP-3 binding to the same three matrix proteins.

RESULTS

IGFBP-3 associated with fibronectin in the pericellular/territorial matrix (approximately 40% co-localization) but not with tenascin-C, or type VI collagen (approximately 6% and approximately 15% co-localization respectively, P< 0.05). Neither IGFBP-4, nor IGFBP-5 were associated with any of the three ECM proteins (P< 0.05). In agarose diffusion assays IGFBP-3 interacted with fibronectin and heparan sulfate proteoglycan but not with type VI collagen or tenascin-C.

CONCLUSIONS

Direct binding between purified IGFBP-3 and fibronectin and the strong co-localization the two proteins in the cartilage matrix support the hypothesis that IGFPB-3 and fibronectin help regulate local IGF-I levels.

Ultrastructural localization of type VI collagen in normal adult and osteoarthritic human articular cartilage

OBJECTIVE

Type VI collagen is a major component of the pericellular matrix compartment in articular cartilage and shows severe alterations in osteoarthritic cartilage degeneration. In this study, we analysed the exact localization of type VI collagen in its relationship to the chondrocyte and the (inter)territorial cartilage matrix. Additionally, we were interested in its ultrastructural appearance in normal and osteoarthritic cartilage.

DESIGN

Distribution and molecular appearance was investigated by conventional immunostaining, by multilabeling confocal scanning microscopy, conventional transmission, and immunoelectron microscopy.

RESULTS

Our analysis confirmed the pericellular concentration of type VI collagen in normal and degenerated cartilage. Type VI collagen formed an interface in between the cell surface and the type II collagen network. The type VI collagen and the type II collagen networks appeared to have a slight physical overlap in both normal and diseased cartilage. Additionally, some epitope staining was observed in the cell-associated interterritorial cartilage matrix, which did not appear to have an immediate relation to the type II collagen fibrillar network as evaluated by immunoelectron microscopy. In osteoarthritic cartilage, significant differences were found compared with normal articular cartilage: the overall dimension of the lacunar volume increased, and a significantly increased type VI collagen epitope staining was observed in the interterritorial cartilage matrix. Also, the banded isoform of type VI collagen was found around many chondrocytes.

CONCLUSIONS

Our study confirms the close association of type VI collagen with both, the chondrocyte cell surface and the territorial cartilage matrix. They show severe alterations in type VI collagen distribution and appearance in osteoarthritic cartilage. Our immunohistochemical and ultrastructural data are compatible with two ways of degradation of type VI collagen in osteoarthritic cartilage: (1) the pathologically increased physiological molecular degradation leading to the complete loss of type VI collagen filaments from the pericellular chondrocyte matrix and (2) the transformation of the fine filaments to the band-like form of type VI collagen. Both might implicate a significant loss of function of the pericellular microenvironment in osteoarthritic cartilage.

The C5 domain of Col6A3 is cleaved off from the Col6 fibrils immediately after secretion

In articular cartilage, type VI collagen is concentrated in the pericellular matrix compartment. During protein synthesis and processing at least the alpha3(VI) chain undergoes significant posttranslational modification and cleavage. In this study, we investigated the processing of type VI collagen in articular cartilage. Immunostaining with a specific polyclonal antiserum against the C5 domain of alpha3(VI) showed strong cellular staining seen in nearly all chondrocytes of articular cartilage. Confocal laser-scanning microscopy and immunoelectron microscopy allowed localization of this staining mainly to the cytoplasm and the immediate pericellular matrix. Double-labeling experiments showed a narrow overlap of the C5 domain and the pericellular mature type VI collagen. Our results suggest that at least in human adult articular cartilage the C5 domain of alpha3(VI) collagen is synthesized and initially incorporated into the newly formed type VI collagen fibrils, but immediately after secretion is cut off and is not present in the mature pericellular type VI matrix of articular cartilage.

Anabolic and catabolic gene expression pattern analysis in normal versus osteoarthritic cartilage using complementary DNA-array technology

OBJECTIVE

To understand changes in gene expression levels that occur during osteoarthritic (OA) cartilage degeneration, using complementary DNA (cDNA)-array technology.

METHODS

Nine normal, 6 early degenerated, and 6 late-stage OA cartilage samples of human knee joints were analyzed using the Human Cancer 1.2 cDNA array and TaqMan analysis.

RESULTS

In addition to a large variability of expression levels between different patients, significant expression patterns were detectable for many genes. Cartilage types II and VI collagen were strongly expressed in late-stage specimens, reflecting the high matrix-remodeling activity of advanced OA cartilage. The increase in fibronectin expression in early degeneration suggests that fibronectin is a crucial regulator of matrix turnover activity of chondrocytes during early disease development. Of the matrix metalloproteinases (MMPs), MMP-3 appeared to be strongly expressed in normal and early degenerative cartilage and down-regulated in the late stages of disease. This indicates that other degradation pathways might be more important in late stages of cartilage degeneration, involving other enzymes, such as MMP-2 and MMP-11, both of which were up-regulated in late-stage disease. MMP-11 was up-regulated in OA chondrocytes and, interestingly, also in the early-stage samples. Neither MMP-1 nor MMP-8 was detectable, and MMP-13 and MMP-2 were significantly detectable only in late-stage specimens, suggesting that early stages are characterized more by degradation of other matrix components, such as aggrecan and other noncollagenous molecules, than by degradation of type II collagen fibers.

CONCLUSION

This investigation allowed us to identify gene expression profiles of the disease process and to get new insights into disease mechanisms, for example, to develop a picture of matrix proteinases that are differentially involved in different phases of the disease process.