Identification and initial characterization of 5000 expressed sequenced tags (ESTs) each from adult human normal and osteoarthritic cartilage cDNA libraries

PCPE-2 (procollagen C-proteinase enhancer-2): the NON-IDENTICAL twin of PCPE-1

PCPE-2 was discovered at the beginning of this century, and was soon identified as a close homolog of PCPE-1 (procollagen C-proteinase enhancer 1). After the demonstration that it could also stimulate the proteolytic maturation of fibrillar procollagens by BMP-1/tolloid-like proteinases (BTPs), PCPE-2 did not attract much attention as it was thought to fulfill the same functions as PCPE-1 which was already well-described. However, the tissue distribution of PCPE-2 shows both common points and significant differences with PCPE-1, suggesting that their activities are not fully overlapping. Also, the recently established connections between PCPE-2 (gene name PCOLCE2) and several important diseases such as atherosclerosis, inflammatory diseases and cancer have highlighted the need for a thorough reappraisal of the in vivo roles of this regulatory protein. In this context, the recent finding that, while retaining the ability to bind fibrillar procollagens and to activate their C-terminal maturation, PCPE-2 can also bind BTPs and inhibit their activity has substantially extended its potential functions. In this review, we describe the current knowledge about PCPE-2 with a focus on collagen fibrillogenesis, lipid metabolism and inflammation, and discuss how we could further advance our understanding of PCPE-2-dependent biological processes.

CTGF regulates mineralization in human mature chondrocyte by controlling Pit-1 and modulating ANK via the BMP/Smad signalling

OBJECTIVE

Connective tissue growth factor (CTGF) exhibits potent proliferative, differentiated, and mineralizing effects, and is believed to be contribute to cartilage mineralization in Osteoarthritis (OA). However, the underlying mechanism of chondrocyte mineralization induced by CTGF remains obscure. As a key regulator of mineral responses, type III phosphate transporter 1 (Pit-1) has been associated with the pathogenesis of articular mineralization. Therefore, the primary objective of this study was to investigate whether CTGF influences the development of mature chondrocyte mineralization and the underlying mechanisms governing such mineralization.

METHODS

The effect of Connective tissue growth factor (CTGF) on human C-28/I2 chondrocytes were investigated. The chondrocytes were treated with CTGF or related inhibitors, and transfected with Overexpression and siRNA transfection of Type III Phosphate Transporter 1(Pit-1). Subsequently, the cells were subjected to Alizarin red S staining, PiPer Phosphate Assay Kit, Alkaline Phosphatase Diethanolamine Activity Kit, ELISA, RT-PCR or Western blot analysis.

RESULTS

Stimulation with Connective tissue growth factor (CTGF) significantly upregulated the expression of the Type III Phosphate Transporter 1(Pit-1) and mineralization levels in chondrocytes through activation of α5β1 integrin and BMP/Samd1/5/8 signaling pathways. Furthermore, treatment with overexpressed Pit-1 markedly increased the expression of Multipass Transmembrane Ankylosis (ANK) transporter in the cells. The inhibitory effect of CTGF receptor blockade using α5β1 Integrin blocking antibody was demonstrated by significantly suppressed the expression of Pit-1 and ANK transporter, as well as chondrocyte mineralization.

CONCLUSIONS

Our data indicate that Connective tissue growth factor (CTGF) plays a critical role inchondrocyte mineralization, which is dependent on the expression of the Type III Phosphate Transporter 1(Pit-1) and Multipass Transmembrane Ankylosis (ANK) transporter. Consequently, inhibition of CTGF activity may represent a novel therapeutic approach for the management of Osteoarthritis (OA).

The Therapeutic and Prognostic Role of Clusterin in Diverse Musculoskeletal Diseases: A Mini Review

This mini-review aims to introduce the association between Secretory clusterin/apolipoprotein J (sCLU) and diverse musculoskeletal diseases. A comprehensive review of the literature was performed to identify basic science and clinical studies, which implied the therapeutic and prognostic role of sCLU in diverse musculoskeletal diseases. sCLU is a multifunctional glycoprotein that is ubiquitously expressed in various tissues and is implicated in many pathophysiological processes. Dysregulated expression of sCLU had been reported to be assocaited with proliferative or apoptotic molecular processes and inflammatory responses, which participated in many pathophysiological processes such as degenerative musculoskeletal diseases including ischemic osteonecrosis, osteoarthritis (OA) and degenerative cervical myelopathy (spinal cord injury), neoplastic musculoskeletal diseases, inflammatory and autoimmune musculoskeletal diseases including Rheumatoid arthritis (RA), joint damage induced by Brucella abortus, Sjogren's syndrome, idiopathic inflammatory myopathies, muscle glucose metabolism, insulin sensitivity and traumatic musculoskeletal diseases. Recent findings of sCLU in these musculoskeletal diseases provides insights on the therapeutic and prognostic role of sCLU in these musculoskeletal diseases. sCLU may serve as a promising therapeutic target for ischemic osteonecrosis, OA and spinal cord injury as well as a potential prognostic biomarker for OA and RA. Moreover, sCLU could act as a prognostic biomarker for osteosarcoma (OS) and a promising therapeutic target for OS resistance. Although many studies support the potential therapeutic and prognostic role of sCLU in some inflammatory and autoimmune-mediated musculoskeletal diseases, more future researches are needed to explore the molecular pathogenic mechanism mediated by sCLU implied in these musculoskeletal diseases.

Exploring the translational potential of clusterin as a biomarker of early osteoarthritis

BACKGROUND

Clusterin (CLU; also known as apolipoprotein J) is an ATP-independent holdase chaperone that prevents proteotoxicity as a consequence of protein aggregation. It is a ∼60 kDa disulfide-linked heterodimeric protein involved in the clearance of cellular debris and the regulation of apoptosis. CLU has been proposed to protect cells from cytolysis by complement components and has been implicated in Alzheimer's disease due to its ability to bind amyloid-β peptides and prevent aggregate formation in the brain. Recent studies suggest that CLU performs moonlighting functions. CLU exists in two major forms: an intracellular form and a secreted extracellular form. The intracellular form of CLU may suppress stress-induced apoptosis by forming complexes with misfolded proteins and facilitates their degradation. The secreted form of CLU functions as an extracellular chaperone that prevents protein aggregation.

METHODS

In this review, we discuss the published literature on the biology of CLU in cartilage, chondrocytes, and other synovial joint tissues. We also review clinical studies that have examined the potential for using this protein as a biomarker in synovial and systemic fluids of patients with rheumatoid arthritis (RA) or osteoarthritis (OA).

RESULTS

Since CLU functions as an extracellular chaperone, we propose that it may be involved in cytoprotective functions in osteoarticular tissues. The secreted form of CLU can be measured in synovial and systemic fluids and may have translational potential as a biomarker of early repair responses in OA.

CONCLUSION

There is significant potential for investigating synovial and systemic CLU as biomarkers of OA. Future translational and clinical orthopaedic studies should carefully consider the diverse roles of this protein and its involvement in other comorbidities. Therefore, future biomarker studies should not correlate circulating CLU levels exclusively to the process of OA pathogenesis and progression. Special attention should be paid to CLU levels in synovial fluid.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

There is significant potential for investigating synovial and systemic CLU as a predictive biomarker of osteoarthritis (OA) progression and response to novel treatments and interventions. Given that CLU plays diverse roles in other comorbidities such as rheumatoid arthritis (RA) and obesity, future translational and clinical orthopaedic biomarker studies should not directly correlate circulating CLU levels to the process of OA pathogenesis and progression. However, special attention should be paid to CLU levels in synovial fluid. The cytoprotective properties of CLU may support the implementation of regenerative strategies and new approaches for developing targeted therapeutics for OA.

Classification of four distinct osteoarthritis subtypes with a knee joint tissue transcriptome atlas

The limited molecular classifications and disease signatures of osteoarthritis (OA) impede the development of prediagnosis and targeted therapeutics for OA patients. To classify and understand the subtypes of OA, we collected three types of tissue including cartilage, subchondral bone, and synovium from multiple clinical centers and constructed an extensive transcriptome atlas of OA patients. By applying unsupervised clustering analysis to the cartilage transcriptome, OA patients were classified into four subtypes with distinct molecular signatures: a glycosaminoglycan metabolic disorder subtype (C1), a collagen metabolic disorder subtype (C2), an activated sensory neuron subtype (C3), and an inflammation subtype (C4). Through ligand-receptor crosstalk analysis of the three knee tissue types, we linked molecular functions with the clinical symptoms of different OA subtypes. For example, the Gene Ontology functional term of vasculature development was enriched in the subchondral bone-cartilage crosstalk of C2 and the cartilage-subchondral bone crosstalk of C4, which might lead to severe osteophytes in C2 patients and apparent joint space narrowing in C4 patients. Based on the marker genes of the four OA subtypes identified in this study, we modeled OA subtypes with two independent published RNA-seq datasets through random forest classification. The findings of this work contradicted traditional OA diagnosis by medical imaging and revealed distinct molecular subtypes in knee OA patients, which may allow for precise diagnosis and treatment of OA.

Gene Expression Profiling Studies Using Microarray in Osteoarthritis: Genes in Common and Different Conditions

Osteoarthritis (OA), which is characterized mainly by cartilage degradation, is the most prevalent joint disorder worldwide. Although OA is identified as a major cause of joint pain, disability, and socioeconomic burden, the etiology of OA is still not clearly known. Recently, gene microarray analysis has become an efficient method for the research of complex diseases and has been employed to determine what genes and pathways are involved in the pathological process of OA. In this review, OA study results over the last decade are summarized for gene expression profiling of various tissues, such as cartilage, subchondral bone, and synovium in human OA and mouse OA models. Many differentially expressed genes, which mainly involve matrix metabolism, bone turnover, and inflammation pathways, were identified in diseased compared with "normal" tissues. Nevertheless, rare common genes were reported from studies using different tissue sources, microarray chips, and research designs. Thus, future novel and carefully designed microarray studies are required to elucidate underlying genetic mechanisms in the pathogenesis of OA as well as new directions for potential OA-targeted pharmaceutical therapies.

Genome-wide DNA methylation analysis of articular chondrocytes identifies TRAF1, CTGF, and CX3CL1 genes as hypomethylated in osteoarthritis

The aim of this study is to identify osteoarthritis (OA)-associated differentially methylated genes in human articular chondrocytes from patients with OA. DNA methylation profiling of articular chondrocytes from OA patients, rheumatoid arthritis (RA) patients, and controls was performed, and candidate genes were chosen for validation of gene demethylation status. The mRNA expression levels of candidate genes in chondrocytes were detected by real-time quantitative PCR. Chondrocytes from OA and RA group were treated with 5-Aza-2-deoxycytidine (5-Aza), and then the mRNA expression levels were detected. Forty-five genes with significant methylation differences between OA and control group were identified. Tumor necrosis factor receptor-associated factor 1 (TRAF1), connective tissue growth factor (CTGF), and chemokine (C-X3-C motif) ligand 1(CX3CL1) genes were hypomethylated in chondrocytes of OA and RA patients, which verified by bisulfite sequencing analysis. The mRNA expression level of TRAF1 and CTGF was significantly increased in OA and RA group (p < 0.05), while the expression level of CX3CL1 was only increased in OA group (p < 0.05). For the chondrocytes from OA and RA treated with 5-Aza, the mRNA expression level of TRAF1 and CTGF was highly increased (p < 0.05). It is the first time to show that TRAF1, CTGF, and CX3CL1 genes were hypomethylated in OA chondrocytes and have a consistent correlation with mRNA expression, which suggests that epigenetic changes in the methylation status of TRAF1, CTGF, and CX3CL1 contribute to the pathology of OA.

The O-glycomap of lubricin, a novel mucin responsible for joint lubrication, identified by site-specific glycopeptide analysis

The lubricative, heavily glycosylated mucin-like synovial glycoprotein lubricin has previously been observed to contain glycosylation changes related to rheumatoid and osteoarthritis. Thus, a site-specific investigation of the glycosylation of lubricin was undertaken, in order to further understand the pathological mechanisms involved in these diseases. Lubricin contains an serine/threonine/proline (STP)-rich domain composed of imperfect tandem repeats (EPAPTTPK), the target for O-glycosylation. In this study, using a liquid chromatography-tandem mass spectrometry approach, employing both collision-induced and electron-transfer dissociation fragmentation methods, we identified 185 O-glycopeptides within the STP-rich domain of human synovial lubricin. This showed that adjacent threonine residues within the central STP-rich region could be simultaneously and/or individually glycosylated. In addition to core 1 structures responsible for biolubrication, core 2 O-glycopeptides were also identified, indicating that lubricin glycosylation may have other roles. Investigation of the expression of polypeptide N-acetylgalactosaminyltransferase genes was carried out using cultured primary fibroblast-like synoviocytes, a cell type that expresses lubricin in vivo. This analysis showed high mRNA expression levels of the less understood polypeptide N-acetylgalactosaminyltransferase 15 and 5 in addition to the ubiquitously expressed polypeptide N-acetylgalactosaminyltransferase 1 and 2 genes. This suggests that there is a unique combination of transferase genes important for the O-glycosylation of lubricin. The site-specific glycopeptide analysis covered 82% of the protein sequence and showed that lubricin glycosylation displays both micro- and macroheterogeneity. The density of glycosylation was shown to be high: 168 sites of O-glycosylation, predominately sialylated, were identified. These glycosylation sites were focused in the central STP-rich region, giving the domain a negative charge. The more positively charged lysine and arginine residues in the N and C termini suggest that synovial lubricin exists as an amphoteric molecule. The identification of these unique properties of lubricin may provide insight into the important low-friction lubricating functions of lubricin during natural joint movement.

Molecular regulation of CCN2 in the intervertebral disc: lessons learned from other connective tissues

Connective tissue growth factor (CCN2/CTGF) plays an important role in extracellular matrix synthesis, especially in skeletal tissues such as cartilage, bone, and the intervertebral disc. As a result there is a growing interest in examining the function and regulation of this important molecule in the disc. This review discusses the regulation of CCN2 by TGF-β and hypoxia, two critical determinants that characterize the disc microenvironment, and discusses known functions of CCN2 in the disc. The almost ubiquitous regulation of CCN2 by TGF-β, including that seen in the disc, emphasizes the importance of the TGF-β-CCN2 relationship, especially in terms of extracellular matrix synthesis. Likewise, the unique cross-talk between CCN2 and HIF-1 in the disc highlights the tissue and niche specific mode of regulation. Taken together the current literature supports an anabolic role for CCN2 in the disc and its involvement in the maintenance of tissue homeostasis during both health and disease. Further studies of CCN2 in this tissue may reveal valuable targets for the biological therapy of disc degeneration.

Changes in chondrocyte gene expression following in vitro impaction of porcine articular cartilage in an impact injury model

Our objective was to monitor chondrocyte gene expression at 0, 3, 7, and 14 days following in vitro impaction to the articular surface of porcine patellae. Patellar facets were either axially impacted with a cylindrical impactor (25 mm/s loading rate) to a load level of 2,000 N or not impacted to serve as controls. After being placed in organ culture for 0, 3, 7, or 14 days, total RNA was isolated from full thickness cartilage slices and gene expression measured for 17 genes by quantitative real-time RT-PCR. Targeted genes included those encoding proteins involved with biological stress, inflammation, or anabolism and catabolism of cartilage extracellular matrix. Some gene expression changes were detected on the day of impaction, but most significant changes occurred at 14 days in culture. At 14 days in culture, 10 of the 17 genes were differentially expressed with col1a1 most significantly up-regulated in the impacted samples, suggesting impacted chondrocytes may have reverted to a fibroblast-like phenotype.

Human articular chondrocytes express multiple gap junction proteins: differential expression of connexins in normal and osteoarthritic cartilage

Osteoarthritis (OA) is the most common joint disease and involves progressive degeneration of articular cartilage. The aim of this study was to investigate if chondrocytes from human articular cartilage express gap junction proteins called connexins (Cxs). We show that human chondrocytes in tissue express Cx43, Cx45, Cx32, and Cx46. We also find that primary chondrocytes from adults retain the capacity to form functional voltage-dependent gap junctions. Immunohistochemistry experiments in cartilage from OA patients revealed significantly elevated levels of Cx43 and Cx45 in the superficial zone and down through the next approximately 1000 μm of tissue. These zones corresponded with regions damaged in OA that also had high levels of proliferative cell nuclear antigen. An increased number of Cxs may help explain the increased proliferation of cells in clusters that finally lead to tissue homeostasis loss. Conversely, high levels of Cxs in OA cartilage reflect the increased number of adjacent cells in clusters that are able to interact directly by gap junctions as compared with hemichannels on single cells in normal cartilage. Our data provide strong evidence that OA patients have a loss of the usual ordered distribution of Cxs in the damaged zones and that the reductions in Cx43 levels are accompanied by the loss of correct Cx localization in the nondamaged areas.

Genome-wide study identifies the regulatory gene networks and signaling pathways from chondrocyte and peripheral blood monocyte of Kashin-Beck disease

This investigation was designed to unravel gene networks in Kashin-Beck disease (KBD) and better identify target genes of KBD for gene therapy development. RNA was isolated separately from cartilage and peripheral blood samples of patients with KBD and healthy controls. Agilent 44K human whole-genome oligonucleotide microarrays were used to detect differentially expressed genes. Three significant canonical pathways and nine chondrocyte networks from chondrocytic gene expression profiles were screened using ingenuity pathway analysis (IPA), but only one network and no canonical pathways from peripheral blood monocytic gene profile were identified. Bak1, APAF-1, CASP6, IGFBP2, Col5a2 and TGFBI extracted from significant genes that involved in chondrocytic canonical pathways and networks may have closer relationship with the etiopathogenesis of KBD. Those genes may be potential targets for gene diagnosis and treatment. Six physiological functions were predominant and unique to the chondrocytic genes, whereas two were unique to peripheral blood monocytic genes. The identified genes may represent a source of potentially novel molecular targets, which may provide a better understanding of the molecular details in KBD pathogenesis and also provide useful pathways and network maps for the future research in osteochondrosis.

Potential plasma biomarkers for progression of knee osteoarthritis using glycoproteomic analysis coupled with a 2D-LC-MALDI system

Background

Although osteoarthritis (OA) is a highly prevalent joint disease, to date, no reliable biomarkers have been found for the disease. In this study, we attempted to identify factors the amounts of which significantly change in association with the progression of knee OA.

Methods

A total of 68 subjects with primary knee OA were enrolled in the study. These subjects were followed up over an 18-month period, and plasma and serum samples were obtained together with knee radiographs every 6 months, i.e., 0, 6, 12 and 18 months after the enrollment. Progressors and non-progressors were determined from the changes on radiographs, and plasma samples from those subjects were subjected to N-glycoproteomic 2D-LC-MALDI analysis. MS peaks were identified, and intensities for respective peaks were compared between the progressors and non-progressors to find the peak intensities of which differed significantly between the two groups of subjects. Proteins represented by the chosen peaks were identified by MS/MS analysis. Expression of the identified proteins was evaluated in synovial tissues from 10 OA knee joints by in situ hybridization, western blotting analysis and ELISA.

Results

Among the subjects involved in the study, 3 subjects were determined to be progressors, and 6 plasma and serum samples from these subjects were subjected to the analysis together with another 6 samples from the non-progressors. More than 3000 MS peaks were identified by N-glycoproteomic 2D-LC-MALDI analysis. Among them, 4 peaks were found to have significantly different peak intensities between the progressors and non-progressors. MS/MS analysis revealed that these peaks represented clusterin, hemopexin, alpha-1 acid glycoprotein-2, and macrophage stimulating protein, respectively. The expression of these genes in OA synovium was confirmed by in situ hybridization, and for clusterin and hemopexin, by western blotting analysis and ELISA as well.

Conclusions

In this study, 4 potential biomarkers were identified as potential prognostic markers for knee OA through N-glycoproteomic analysis. To the best of our knowledge, this is the first report for the use of glycoproteomic technology in exploring potential biomarkers for knee OA.

Differences in the secretome of cartilage explants and cultured chondrocytes unveiled by SILAC technology

The main goal of our study was to analyze and compare the profiles of secreted proteins from adult human articular chondrocytes in monolayers, and cartilage explants in culture, using a de novo protein labeling approach. Stable isotope labeling of proteins in culture was used to differentiate between chondrocyte-derived proteins and other preexisting matrix-derived components, or proteins coming from serum or synovial fluids. Proteins in culture supernatants were resolved by one-dimensional SDS-PAGE electrophoresis, and analyzed in tandem with LC/MS-MS (liquid chromatography/double mass spectrometry). Results from stable isotope labeling with amino acids in cell culture (SILAC) were validated by specific immunoblotting of four relevant proteins identified in the secretion media. After 8-10 days of culture, over 90% of proteins secreted during monolayer growth contained (13)C(6)-Arg and (13)C(6)-Lys. Nonlabeled proteins corresponded mostly to plasma-associated proteins, indicating background contamination of medium with serum remnants. The majority of the secreted proteins in 2D cultures were extracellular matrix components and matrix regulators, along with some inflammatory agents and metabolic enzymes. In explants, only 25%-30% of proteins were labeled with heavy amino acids, corresponding to matrix regulators and carrier molecules. Nonlabeled proteins corresponded primarily to structural matrix components. In qualitative terms, all labeled proteins coming from cartilage explants were also found in chondrocytes supernatants. In summary, our results show differences in the labeling pattern of proteins found in supernatants from explants and monolayers. Most proteins found in the media of explants were subproducts of matrix turnover rather than newly synthesized. To our knowledge, this study is the first one so far applying SILAC technology in the context of cartilage and chondrocytes physiology.

Hypoxia-inducible factor-2alpha is a catabolic regulator of osteoarthritic cartilage destruction

Osteoarthritic cartilage destruction is caused by an imbalance between anabolic and catabolic factors. Here, we show that hypoxia-inducible factor-2alpha (HIF-2alpha, encoded by EPAS1) is a catabolic transcription factor in the osteoarthritic process. HIF-2alpha directly induces the expression in chondrocytes of genes encoding catabolic factors, including matrix metalloproteinases (MMP1, MMP3, MMP9, MMP12 and MMP13), aggrecanase-1 (ADAMTS4), nitric oxide synthase-2 (NOS2) and prostaglandin-endoperoxide synthase-2 (PTGS2). HIF-2alpha expression was markedly increased in human and mouse osteoarthritic cartilage, and its ectopic expression triggered articular cartilage destruction in mice and rabbits. Moreover, mice transgenic for Epas1 only in chondrocytes showed spontaneous cartilage destruction, whereas heterozygous genetic deletion of Epas1 in mice suppressed cartilage destruction caused by destabilization of the medial meniscus (DMM) or collagenase injection, with concomitant modulation of catabolic factors. Our results collectively demonstrate that HIF-2alpha causes cartilage destruction by regulating crucial catabolic genes.

Prostaglandin E2 regulates the expression of connective tissue growth factor (CTGF/CCN2) in human osteoarthritic chondrocytes via the EP4 receptor

Background

The regulatory mechanisms of the expression of connective tissue growth factor/CCN family member 2 (CTGF/CCN2) in human articular chondrocytes have not been clarified. We investigated the effect of prostaglandin E₂ (PGE₂) on CTGF/CCN2 expression in chondrocytes.

Findings

Articular cartilage samples were obtained from patients with osteoarthritis (OA) and chondrocytes were isolated and cultured in vitro. Chondrocytes were stimulated with PGE₂, PGE receptor (EP)-specific agonists, or interleukin (IL)-1. CTGF expression was analyzed using quantitative polymerase chain reaction, Western blot, and enzyme-linked immunosorbent assay. The inhibitory effects of EP receptor antagonists (for EP2 and EP4) against PGE₂ stimulation were also investigated. Stimulation of chondrocytes with PGE₂ or IL-1 significantly suppressed CTGF expression. The suppressive effect of PGE₂ was reproduced by EP2/EP4 receptor agonists but not by EP1/EP3 receptor agonists, and was partially blocked by an EP4 receptor antagonist, suggesting that the EP4 receptor has a dominant role.

Conclusions

PGE₂ may be involved in the regulation of CTGF/CCN2 expression in human articular chondrocytes via the EP4 receptor. Elucidation of EP4-mediated signaling in chondrocytes may contribute to a better understanding of the effects of PGE₂ in arthritis.

Chapter 8: Clusterin: A multifacet protein at the crossroad of inflammation and autoimmunity

For years, clusterin has been recognized as a secreted protein and a large number of works demonstrated that this ubiquitously expressed protein has multiple activities. Among the described activities several were related to inflammation and immunity such as its regulatory activity on complement. Then it became clear that a nuclear form of the protein with proapoptotic property existed and more recently that a cytoplasmic form could regulate NF-kappaB pathway. Again, these activities have a strong repercussion in inflammation and immunity. On the other hand, data available on the exact role of CLU in these processes and autoimmunity were quite scarce until recently. Indeed, in the last few years, a differential CLU expression in subtype of T cells, the regulation of CLU expression by proinflammatory cytokines and molecules, the regulation of expression and function of CLU depending on its subcellular localization, the interaction of CLU with nuclear and intracellular proteins were all reported. Adding these new roles of CLU to the already reported functions of this protein allows a better understanding of its role and potential involvement in several inflammatory and immunological processes and, in particular, autoimmunity. In this sense, rheumatoid arthritis appears to be a very attractive disease to build a new paradigm of the role and function of CLU because it makes the link between proliferation, inflammation, and autoimmunity. We will try to see in this review how to bring altogether the old and new knowledge on CLU with inflammation and autoimmunity. Nevertheless, it is clear that CLU has not yet revealed all its secrets in inflammation and autoimmunity.

Genetics in osteoarthritis

Osteoarthritis is a degenerative articular disease with complex pathogeny because diverse factors interact causing a process of deterioration of the cartilage. Despite the multifactorial nature of this pathology, from the 50's it s known that certain forms of osteoarthritis are related to a strong genetic component. The genetic bases of this disease do not follow the typical patterns of mendelian inheritance and probably they are related to alterations in multiple genes. The identification of a high number of candidate genes to confer susceptibility to the development of the osteoarthritis shows the complex nature of this disease. At the moment, the genetic mechanisms of this disease are not known, however, which seems clear is that expression levels of several genes are altered, and that the inheritance will become a substantial factor in future considerations of diagnosis and treatment of the osteoarthritis.

[Genetics in osteoarthritis]

Osteoarthritis is a degenerative articular pathology with complex pathogeny because diverse factors interact causing a process of deterioration of the cartilage. In spite of the multifactorial nature of this pathology, from years 50 one knows that certain forms of osteoarthritis are related to a strong genetic component. The genetic bases of this disease do not follow the typical patterns of mendelian inheritance and probably they are related to alterations in multiple genes. The identification of a high number of candidates genes to confer susceptibility to the development of the osteoarthritis shows the complex nature of this disease. At the moment, the genetic mechanisms of this pathology are not known, however, which seems clear is that levels of expression of several genes are altered, and that the inheritance will become a substantial factor in future considerations of diagnosis and treatment of the osteoarthitis.

Subtractive gene expression profiling of articular cartilage and mesenchymal stem cells: serpins as cartilage-relevant differentiation markers

OBJECTIVE

Mesenchymal stem cells (MSCs) are a population of cells broadly discussed to support cartilage repair. The differentiation of MSCs into articular chondrocytes is, however, still poorly understood on the molecular level. The aim of this study was to perform an almost genome-wide screen for genes differentially expressed between cartilage and MSCs and to extract new markers useful to define chondrocyte differentiation stages.

METHODS

Gene expression profiles of MSCs (n=8) and articular cartilage from OA patients (n=7) were compared on a 30,000 cDNA-fragment array and differentially expressed genes were extracted by subtraction. Expression of selected genes was assessed during in vitro chondrogenic differentiation of MSCs and during dedifferentiation of expanded chondrocytes using quantitative and semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR). Protein secretion was measured by enzyme-linked immunosorbent assay.

RESULTS

Eighty-seven genes were differentially expressed between MSCs and cartilage with a more than three-fold difference. Sixty-seven of them were higher expressed in cartilage and among them 15 genes were previously not detected in cartilage. Differential expression was confirmed for 69% of 26 reanalysed genes by RT-PCR. The profiles of three unknown transcripts and six protease-related molecules were characterised during differentiation. SERPINA1 and SERPINA3 mRNA expression correlated with chondrogenic differentiation of MSCs and dedifferentiation of chondrocytes, and SERPINA1 protein levels in culture supernatants could be correlated alike.

CONCLUSIONS

cDNA-array analysis identified SERPINA1 and A3 as new differentiation-relevant genes for cartilage. Since SERPINA1 secretion correlated with both chondrogenesis of MSCs and dedifferentiation during chondrocyte expansion, it represents an attractive marker for refinement of chondrocyte differentiation.

Cytokine-like 1 (CYTL1) Regulates the Chondrogenesis of Mesenchymal Cells

To identify novel molecules regulating chondrogenesis and cartilage development, we screened a cartilage-specific expressed sequence tag data base. Cytokine-like 1 (Cytl1), a possible cytokine candidate with unknown function that was originally identified in bone marrow-derived CD34-positive cells, was selected for functional characterization. In view of the initial observation that Cytl1 is predominantly expressed in chondrocytes and cartilage, we investigated its possible role in chondrogenesis and hypertrophic maturation of chondrocytes. Cytl1 expression was very low in mesenchymal cells, dramatically increased during chondrogenesis, and decreased during hypertrophic maturation, both in vivo and in vitro. The role of Cytl1 in chondrogenesis and hypertrophic maturation was examined by treating chondrifying mesenchymal cells with exogenous Cytl1 or ectopic expression of Cytl1. Notably, exogenous Cytl1 caused chondrogenic differentiation of mouse limb bud mesenchymal cells during micromass culture. Lentivirus-mediated overexpression of Cytl1 additionally induced chondrogenic differentiation of mesenchymal cells. However, Cytl1 did not affect the hypertrophic maturation of chondrocytes. Cytl1 exerted its chondrogenic effect via stimulation of Sox9 transcriptional activity. In addition, Cytl1 caused expression of insulin-like growth factor 1, which has a capacity to induce chondrogenesis. Thus, our results collectively suggest that chondrocyte-specific Cytl1 regulates chondrogenesis as a novel autocrine factor, but not hypertrophic maturation of chondrocytes during cartilage development.

Large-scale gene expression profiles, differentially represented in osteoarthritic synovium of the knee joint using cDNA microarray technology

Osteoarthritis (OA) is one of the most common age-related chronic disorders of articular cartilage, joints and bone tissue. Diagnosis of OA commonly depends on clinical and radiographic findings. However, changes in cartilage associated with the early stage of OA cannot be detected using radiographs, because significant cartilage degeneration must occur before radiographic findings show alterations of the appearance of cartilage. To identify new biomarkers of OA, we analysed gene expression profiles of synovium from 43 patients with OA, ten patients with rheumatoid arthritis (RA), and eight non-OA/non-RA patients using a novel cDNA microarray chip. We identified 21 genes with simultaneous significant differences in expression between OA and non-OA/non-RA groups and between OA and RA groups. Linear discriminant analysis showed that the three groups could be well separated using those 21 genes. Statistical analysis also revealed that several of the 21 genes were associated with disease progression and clinical presentation. The graphical modelling method indicated that some of the 21 genes are significantly associated with a particular clinical presentation, suggesting biological relationships among those genes. This is the first report of the use of cDNA microarray technology to create large-scale gene expression profiles differentially expressed in situ in OA synovium of the knee joint.

iCartiGD: the Integrated Cartilage Gene Database

Background

Diseases of cartilage, such as arthritis and degenerative disc disease, affect the majority of the general population, particularly with ageing. Discovery and understanding of the genes and pathways involved in cartilage biology will greatly assist research on the development, degeneration and disorders of cartilage.

Description

We have established the Integrated Cartilage Gene Database (iCartiGD) of genes that are known, based on results from high throughput experiments, to be expressed in cartilage. Information about these genes is extracted automatically from public databases and presented as a single page report via a web-browser. A variety of flexible search options are provided and the chromosomal distribution of cartilage associated genes can be presented.

Conclusion

iCartiGD provides a comprehensive source of information on genes known to be expressed in cartilage. It will remain current due to its automatic update capability and provide researchers with an easily accessible resource for studies involving cartilage. Genetic studies of the development and disorders of cartilage will benefit from this database.

Role of CCN2/CTGF/Hcs24 in bone growth

Our bones mostly develop through a process called endochondral ossification. This process is initiated in the cartilage prototype of each bone and continues through embryonic and postnatal development until the end of skeletal growth. Therefore, the central regulator of endochondral ossification is the director of body construction, which is, in other words, the determinant of skeletal size and shape. We suggest that CCN2/CTGF/Hcs24 (CCN2) is a molecule that conducts all of the procedures of endochondral ossification. CCN2, a member of the CCN family of novel modulator proteins, displays multiple functions by manipulating the local information network, using its conserved modules as an interface with a variety of other biomolecules. Under a precisely designed four-dimensional genetic program, CCN2 is produced from a limited population of chondrocytes and acts on all of the mesenchymal cells inside the bone callus to promote the integrated growth of the bone. Furthermore, the utility of CCN2 as regenerative therapeutics against connective tissue disorders, such as bone and cartilage defects and osteoarthritis, has been suggested. Over the years, the pathological action of CCN2 has been suggested. Nevertheless, it can also be regarded as another aspect of the physiological and regenerative function of CCN2, which is discussed as well.

Functional characterization of an orphan nuclear receptor, Rev-ErbAalpha, in chondrocytes and its potential role in osteoarthritis

OBJECTIVE

To evaluate the expression and function of the orphan nuclear receptor Rev-ErbAalpha in articular cartilage and to investigate its role in osteoarthritis (OA).

METHODS

Expression of Rev-ErbAalpha was analyzed at both the messenger RNA and protein levels in human and bovine articular cartilage and chondrocytes by real-time polymerase chain reaction (TaqMan) and immunocytochemical techniques. The effects of cartilage catabolic and anabolic agents on the expression of Rev-ErbAalpha were evaluated by TaqMan analysis. Overexpression was achieved by either adenoviral transduction or treatment with a peroxisome proliferator-activated receptor alpha agonist, whereas expression was suppressed by antisense oligonucleotides.

RESULTS

Among the 48 known nuclear receptors, Rev-ErbAalpha was found to be the most highly expressed in OA cartilage. It is known to function as a transcription repressor. Treatment of articular chondrocytes with known catabolic agents resulted in the induction of Rev-ErbAalpha, whereas stimulation with anabolic agents led to a decrease in expression. Overexpression of the nuclear receptor was associated with an increase in the expression of matrix-degrading enzymes such as matrix metalloproteinase 13 and aggrecanase. In contrast, a decrease in Rev-ErbAalpha expression led to a concomitant reduction in the activity of matrix-degrading enzymes.

CONCLUSION

This study is the first to demonstrate that Rev-ErbAalpha is highly expressed in OA articular chondrocytes and that its expression is modulated by known cartilage catabolic and anabolic stimuli. We also demonstrated that modulation of Rev-ErbAalpha expression in chondrocytes may be a novel means of regulating the expression and production of multiple matrix-degrading enzymes. These observations suggest that Rev-ErbAalpha may be a novel therapeutic target for OA.

Chondrocyte genomics: implications for disease modification in osteoarthritis

Advances in genomic technologies have made genome-wide-association and gene-expression studies a reality. Despite technical and analytical challenges, the application of genomic technologies to osteoarthritis research will lead to a better understanding of the disease at the molecular level. Functional genomics will identify genes involved in the chondrocyte response to cartilage injury and cartilage repair, and will help clarify the role of chondrocytes in arthritis onset, progression and outcome. Systems biology will enable researchers to develop a full portrait of osteoarthritis, a complex and multifactorial disease that involves not only articular cartilage but also synovium, synovial fluid, subchondral bone and peripheral blood. Ultimately such an approach will result in novel diagnostic and therapeutic targets and better disease management.

Assessment of the utility of biomarkers of osteoarthritis in the guinea pig

OBJECTIVE

To identify biochemical markers of osteoarthritis (OA) in the guinea pig, we characterized four biomarkers and 17 cytokines for age- and strain-related differences.

METHODS

Two guinea pig strains were examined in this study: (1) the Hartley (OA-prone) and (2) Strain 13 (OA-resistant). Levels of synovial fluid keratan sulfate (KS) and cartilage oligomeric matrix protein (COMP), as well as levels of serum C2C, CPII, and a panel of cytokines and chemokines were quantified in both guinea pig strains. These included: IL-1 alpha, IL-1 beta, IL-2, IL-3, IL-4, IL-5, IL-6, IL-10, IL-12p40, IL-12p70, IL-17, G-CSF, GM-CSF, IFN-gamma, KC, MIP-1 alpha, RANTES, and TNF-alpha.

RESULTS

Synovial fluid concentrations of KS and COMP increased coincident with histological OA and correlated positively with the severity of histological damage in both strains. Synovial fluid concentrations of these biomarkers were elevated in the knees of the Hartley compared to the Strain 13 animals, as early as 2 months of age. From as early as 4 months of age, the levels of serum C2C/CPII, representing the ratio of type II collagen degradation and synthesis, were elevated in the OA-prone Hartley compared with Strain 13 animals. Also, at 12 months of age, strain-related differences were apparent for 11 of the 16 cytokines and chemokines. Using multiple linear regression, serum IL-6 and TNF-alpha concentrations were each strongly associated with strain, weight, and their interaction (r2 = 0.80, P = 0.0002 for IL-6; r2 = 0.55, P = 0.02 for TNF-alpha).

CONCLUSIONS

Biomarkers derived from synovial fluid are reflective of histological severity in the spontaneous model of OA in the guinea pig. The synovial fluid biomarker profiles indicated accelerated cartilage matrix turnover in the Hartley strain as early as 2 months of age, prior to evidence of histological damage. The Hartley strain also exemplified an imbalance in type II collagen metabolism and a serum cytokine/chemokine profile indicative of a pro-inflammatory state. These findings elucidate additional disease-related features in the guinea pig that have relevance to OA in humans.

Expression profiling of human fetal growth plate cartilage by EST sequencing

The differentiation of mesenchymal stem cells into hypertrophic chondrocytes is an integral and multistep process important in pattern formation, endochondral ossification, and postnatal growth of the skeleton. In recent years, novel genes involved in these processes have been identified, but still only little is known about the large-scale gene expression profile during skeletal development. We initiated an expressed sequence tag (EST) project aiming at the identification of genes and pathways involved in this complex process. Candidate genes are expected to be of value for diagnosis and treatment of monogenic and multigenic heritable disorders of the skeleton. Here, we describe the sequences of 4,748 clones from a human growth plate cartilage cDNA library generated from 20 weeks prenatal-2 years postnatal specimens. In silico analysis of these sequences revealed 1,688 individual transcription units, corresponding to known (1,274) and to novel, yet uncharacterised potential genes (414). The tissue specificity of the library was reflected by its corresponding EST profile representing a total of approximately 10% proteins already shown to be involved in cartilage/bone development or homeostasis. The EST profile also reflects the developmental stage of the tissue with significant differences in the expression of matrix proteins compared to corresponding EST profiles from 8-12 and 12-20 week human fetal cartilage. Calculation of the relative frequency of transcripts in our cDNA library, as compared to their abundance in other EST datasets, revealed a set of approximately 200 genes, including 81 novel, yet uncharacterised genes, showing increased expression. These genes represent candidates for the large number of osteochondrodysplasias for which the causative gene defects have not yet been identified.

Gene expression in skeletal tissues: application of laser capture microdissection

Tissue differentiation is based on the expression of transcription factors, receptors for cytokines, and nuclear receptors that regulate a specific phenotype. The purpose of this study was to select cells from various skeletal tissues in order to analyse differential gene expression of cells in the native environment in vivo. It is a difficult task to obtain cells from skeletal tissues, such as cartilage, periost, bone and muscle, that are structured together and do not exist as individual organs. We used laser capture microdissection which permits the selection and isolation of individual cells from tissue sections. The RNA isolated from these tissues was used for reverse transcriptase-polymerase chain reactions for molecular analysis. We analysed the expression of transcription factors (cFOS, cbfa1, MyoD), receptors for cytokines, nuclear receptors, alkaline phosphatase and the structural proteins osteocalcin and collagen II. The results obtained demonstrate differential patterns of gene expression according to the tissue arrangement in their native in vivo environment, with reliable interpretation of the functions of the analysed genes in the context of intact skeletal tissue physiology.

Development of comprehensive functional genomic screens to identify novel mediators of osteoarthritis

OBJECTIVE

The aim of this study was to develop high-throughput assays for the analysis of major chondrocyte functions that are important in osteoarthritis (OA) pathogenesis and methods for high-level gene expression and analysis in primary human chondrocytes.

METHODS

In the first approach, complementary DNA (cDNA) libraries were constructed from OA cartilage RNA and full-length clones were selected. These cDNAs were transferred into a retroviral vector using Gateway Technology. Full-length clones were over-expressed in human articular chondrocytes (HAC) by retroviral-mediated gene transfer. The induction of OA-associated markers, including aggrecanase-1 (Agg-1), matrix metalloproteinase-13 (MMP-13), inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2), collagen IIA and collagen X was measured by quantitative real-time polymerase chain reaction (QPCR). Induction of a marker gene was verified by independent isolation of 2-3 clones per gene, re-transfection followed by QPCR as well as nucleotide sequencing. In the second approach, whole cDNA libraries were transduced into chondrocytes and screened for chondrocyte cluster formation in three-dimensional agarose cultures.

RESULTS

Using green fluorescent protein (eGFP) as a marker gene, it was shown that the retroviral method has a transduction efficiency of >90%. A total of 40 verified hits were identified in the QPCR screen. The first set of 19 hits coordinately induced iNOS, COX-2, Agg-1 and MMP-13. The most potent of these genes were the tyrosine kinases Axl and Tyro-3, receptor interacting kinase-2 (RIPK2), tumor necrosis factor receptor 1A (TNFR1A), fibroblast growth factor (FGF) and its receptor FGFR, MUS81 endonuclease and Sentrin/SUMO-specific protease 3. The second set of seven hits induced both Agg-1 and MMP-13 but none of the other markers. Five of these seven genes regulate the phosphoinositide-3-kinase pathway. The most potently induced OA marker was iNOS. This marker was induced 20-500 fold by seven genes. Collagen IIA was also induced by seven genes, the most potent being transforming growth factor beta (TGFbeta)-stimulated protein TSC22, vascular endothelial growth factor (VEGF) and splicing factor 3a. This screening assay did not identify inducers of collagen X. The second chondrocyte cluster formation screen identified 14 verified hits. Most of the genes inducing cluster formation were kinases. Additional genes had not been previously known to regulate chondrocyte cluster formation or any other chondrocyte function.

CONCLUSIONS

The methods developed in this study can be applied to screen for genes capable of inducing an OA-like phenotype in chondrocytes on a genome-wide scale and identify novel mediators of OA pathogenesis. Thus, coordinated functional genomic approaches can be used to delineate key genes and pathways activated in complex human diseases such as OA.

Treatment of cartilage with beta-aminopropionitrile accelerates subsequent collagen maturation and modulates integrative repair

Integrative repair of cartilage was previously found to depend on collagen synthesis and maturation. beta-aminopropionitrile (BAPN) treatment, which irreversibly blocks lysyl oxidase, inhibited the formation of collagen crosslinks, prevented development of adhesive strength, and caused a buildup of GuHCl-extractable collagen crosslink precursors. This buildup of crosslink precursor in the tissue may be useful for enhancing integrative repair. We tested in vitro the hypothesis that pre-treatment of cartilage with BAPN, followed by washout before implantation, could be a useful therapeutic strategy to accelerate subsequent collagen maturation. In individual cartilage disks, collagen processing was reversibly blocked by BAPN treatment (0.1 mM) as indicated by a BAPN-induced increase in the total and proportion of incorporated radiolabel that was extractable by 4M guanidine-HCl, followed by a decrease, within 3-4 days of BAPN washout, in the proportion of extractable radiolabel to control levels. With a similar pattern, integration between pairs of apposed cartilage blocks was reversibly blocked by BAPN treatment, and followed by an enhancement of integration after BAPN washout. The low and high levels of integration were associated with enrichment in [(3)H]proline in a form that was susceptible and resistant, respectively, to extraction. With increasing duration up to 7 days after BAPN pre-treatment, the levels of [(3)H]proline extraction decreased, and the development of adhesive strength increased. Thus, BAPN can be used to modulate integrative cartilage repair.

Expression and localization of connective tissue growth factor (CTGF/Hcs24/CCN2) in osteoarthritic cartilage

OBJECTIVE

The investigation of the expression and localization of connective tissue growth factor/hypertrophic chondrocyte-specific gene product 24/CCN family member 2 (CTGF/Hcs24/CCN2) in normal and osteoarthritic (OA) cartilage, and quantification of CTGF/Hcs24-positive cells.

METHODS

Cartilage samples of patients (n=20) with late stage OA were obtained at total joint replacement surgery. Morphologically normal cartilage was harvested for comparison purposes from the femoral heads of 6 other patients with femoral neck fracture. Paraffin-embedded sections were stained by Safranin O. The severity of the OA lesions was divided into four stages (normal, early, moderate, and severe). The localization of protein and mRNA for CTGF/Hcs24 was investigated by immunohistochemistry and in situ hybridization, respectively. The population of CTGF/Hcs24-positive chondrocytes in OA cartilage and chondro-osteophyte was quantified by counting the number of the cells under light microscopy.

RESULTS

Signals for CTGF/Hcs24 were detected in a small percentage of chondrocytes throughout the layers of normal cartilage. In early stage OA cartilage, the CTGF/Hcs24-positive chondrocytes were localized mainly in the superficial layer. In moderate to severe OA cartilage, intense staining for CTGF/Hcs24 was observed in proliferating chondrocytes forming cell clusters next to the cartilage surface. In chondro-osteophyte, strong signals were found in the chondrocytes of the proliferative and hypertrophic zones.

CONCLUSION

CTGF/Hcs24 expression was detected in both normal and OA chondrocytes of human samples. The results of the current study suggested that expression of CTGF/Hcs24 was concomitant with development of OA lesions and chondrocyte differentiation in chondro-osteophyte.

Proteomic Analysis of Articular Cartilage Shows Increased Type II Collagen Synthesis in Osteoarthritis and Expression of Inhibin βA (Activin A), a Regulatory Molecule for Chondrocytes

We show that proteomic analysis can be applied to study cartilage pathophysiology. Proteins secreted by articular cartilage were analyzed by two-dimensional SDS-PAGE and mass spectrometry. Cartilage explants were cultured in medium containing [35S]methionine/cysteine to radiolabel newly synthesized proteins. To resolve the cartilage proteins by two-dimensional electrophoresis, it was necessary to remove the proteoglycan aggrecan by precipitation with cetylpyridinium chloride. 50-100 radiolabeled protein spots were detected on two-dimensional gels of human cartilage cultures. Of 170 silver-stained proteins identified, 19 were radiolabeled, representing newly synthesized gene products. Most of these were known cartilage constituents. Several nonradiolabeled cartilage proteins were also detected. The secreted protein pattern of explants from 12 osteoarthritic joints (knee, hip, and shoulder) and 14 nonosteoarthritic adult joints were compared. The synthesis of type II collagen was strongly up-regulated in osteoarthritic cartilage. Normal adult cartilage synthesized little or no type II collagen in contrast to infant and juvenile cartilage. Potential regulatory molecules novel to cartilage were identified; pro-inhibin betaA and processed inhibin betaA (which dimerizes to activin A) were produced by all the osteoarthritic samples and half of the normals. Connective tissue growth factor and cytokine-like protein C17 (previously only identified as an mRNA) were also found. Activin induced the tissue inhibitor for metalloproteinases-1 in human chondrocytes. Its expression was induced in isolated chondrocytes by growth factors or interleukin-1. We conclude that type II collagen synthesis in articular cartilage is down-regulated at skeletal maturity and reactivated in osteoarthritis in attempted repair and that activin A may be an anabolic factor in cartilage.

The inflammatory side of human chondrocytes unveiled by antibody microarrays

Although being largely used for pathobiological models of cartilage diseases such as osteoarthritis (OA), human chondrocytes are still enigmatic cells, in as much as a large part of their secretome is unknown. We took advantage of the recent development of antibody-based microarrays to study multiple protein expression by human chondrocytes obtained from one healthy and five osteoarthritic joints, in unstimulated conditions or after stimulation by the proinflammatory cytokines interleukin-1 (IL-1) or tumour necrosis factor (TNF). The secretion media of chondrocytes were incubated with array membranes consisting of 79 antibodies directed against cytokines, chemokines, and angiogenic or growth factors. Several proteins were identified as new secretion products of chondrocytes, including the growth or angiogenic factors EGF, thrombopoietin, GDNF, NT-3 and -4, and PlGF, the chemokines ENA-78, MCP-2, IP-10, MIP-3alpha, NAP-2, PARC, and the cytokines MIF, IL-12, and IL-16. Most of the newly identified chemokines were increased intensely after stimulation by IL-1 or TNF, as for other proteins of the array, including GRO proteins, GM-CSF, IL-6, IL-8, MIP-1beta, GCP-2, and osteoprotegerin. The up-regulation by cytokines suggested that these proteins may participate in the destruction of cartilage and/or in the initiation of chemotactic events within the joint during OA. In conclusion, the microarray approach enabled to unveil part of an as yet unexplored chondrocyte secretome. Our findings demonstrated that chondrocytes were equipped with a proinflammatory arsenal of proteins which may play an important part in the pathogenesis of OA and/or its drift towards an inflammatory, rheumatoid phenotype.

A transcriptional profile of human fetal cartilage

Cartilage plays a central role in the patterning and growth of the skeletal elements, and mutations in genes expressed in cartilage are responsible for at least 250 distinct clinical conditions, the osteochondrodysplasias. While recent progress has been made in characterizing the genes that define cartilage biology, there are only limited data describing the gene expression profile of human cartilage. Here we describe the sequences and identities of 6266 clones from an 18-20-week human fetal cartilage cDNA library. Among the sequences, BLAST analysis identified 2404 individual transcripts. Of these, 1775 were defined as derived from characterized genes and the remaining 629 were classified as representing the products of uncharacterized genes. Analysis of the relative representation of each individual transcript showed that the 186 most abundant cDNAs in the library accounted for almost half (47.7%) of the clones. The most highly expressed gene was COL2A1, accounting for 4.15% of all cDNA clones. The cDNAs identified included clones derived from 27 genes which, when mutated, result in disorders of skeletal patterning, development and growth. There were cDNAs representing 22 genes encoding collagen subunits. The genes encoding the identified cDNAs represent candidates for the approximately 100 osteochondrodysplasias for which the causative gene has not yet been identified. Moreover, these data provide an extensive profile of human fetal cartilage gene expression at this developmental stage.

O-glycosylation in Toxoplasma gondii: identification and analysis of a family of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases

The initiation of mucin-type O-glycosylation is catalysed by a family of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (EC 2.4.1.41). These enzymes are responsible for the transfer of N-acetylgalactosamine from the nucleotide sugar donor, UDP-GalNAc, to the hydroxyl group on specific serine or threonine residues in acceptor proteins. By screening a Toxoplasma gondii cDNA library, three distinct isoforms of the ppGalNAc-T gene family were cloned. Two additional isoforms were identified and partially cloned following analysis of the T. gondii genome sequence database. All of the cloned and identified ppGalNAc-T's are type II membrane proteins that share up to 50% amino acid sequence identity within the conserved catalytic domain. They each contain an N-terminal cytoplasmic domain, a hydrophobic transmembrane domain, and a lumenal domain; the latter consists of stem, catalytic, and lectin-like domains. Moreover, each of this ppGalNAc-T's contains important sequence motifs that are typical for this class of glycosyltransferases. These include a glycosyltransferase 1 motif containing the DXH sequence, a Gal/GalNAc-T motif, and the CLD and QXW sequence motifs located in alpha-, beta-, and gamma-repeats present within the lectin-like domain. The coding regions of T. gondii ppGalNAc-T1, -T2, and -T3 reside in multiple exons ranging in number from 6 to 10. Our results demonstrate that mucin-type O-glycosylation in T. gondii is catalysed by a multimember gene family, which is evolutionarily conserved from single-celled eukaryotes through nematodes and insects up to mammals. Taken together, this information creates the basis for future studies of the function of the ppGalNAc-T gene family in the pathobiology of this apicomplexan parasite.

Functional Genomics Approaches in Arthritis

The post-genomic era of functional genomics and target validation will allow us to narrow the bridge between clinically correlative data and causative data for complex diseases, such as arthritis, for which the etiological agent remains elusive. The availability of human and other annotated genome sequences, and parallel developments of new technologies that allow analysis of minute amounts of human and animal cells (peripheral blood cells and infiltrating cells) and tissues (synovium and cartilage) under different pathophysiological conditions, has facilitated high-throughput gene mining approaches that can generate vast amounts of clinically correlative data. Characterizing some of the correlative/causative genes will require reverting to the hypothesis-driven, low throughput method of complementary experimental biology using genomic approaches as a tool. This will include in silico gene expression arrays, genome-wide scans, comparative genomics using various animal models (such as rodents and zebrafish), bioinformatics and a team of well trained translational scientists and physicians. For the first time, the "genomic tools" will allow us to analyze small amounts of surgical samples (such as needle biopsies) and clinical samples in the context of the whole genome. Preliminary genomic analysis in osteoarthritis has already resurrected the debate on the semantic issues in the definition of inflammation. Further analyses will not only facilitate the development of unbiased hypotheses at the molecular level, but also assist us in the identification and characterization of novel targets and disease markers for pharmacological intervention, gene therapy, and diagnosis.

Profiling genes expressed in human fetal cartilage using 13,155 expressed sequence tags

OBJECTIVE

To analyze the gene expression profile of human fetal cartilage by expressed sequence tags (ESTs).

METHODS

A human fetal cartilage (8-12 weeks) cDNA library was constructed using the lambda ZAP Express vector. ESTs were obtained by partial sequencing of cDNA clones. The basic local alignment search tool algorithm was used to compare all generated ESTs to known sequences.

RESULTS

A total of 13,155 ESTs were analyzed, of which 8696 ESTs (66.1%) matched known genes, 53 ESTs (0.4%) were putatively novel (with no match) and the rest matched other ESTs, genomic DNA and repetitive sequences. Importantly, we identified 2448 unique known genes through non-redundancy analysis of the known gene matches, which were then functionally categorized. The tissue specificity of this library was reflected by its EST profile of the extracellular matrix (ECM) proteins. Collagens were the major transcripts, representing 68.5% of the ECM proteins. Proteoglycans were the second most abundant, constituting 9.5%. Collagen type II was the most abundant gene of all. Glypican 3, decorin and aggrecan were the major transcripts of proteoglycans. Many genes involved in cartilage development were identified, such as insulin-like growth factor-II, its receptor and binding proteins, connective tissue growth factor and fibroblast growth factors. Proteases and their regulatory factors were also identified, including matrix metalloprotease 2 and tissue inhibitor of metalloproteinase 1.

CONCLUSIONS

The EST approach is an effective way of characterizing the genes expressed in cartilage. These data represent the most extensive molecular information on human fetal cartilage to date. The availability of this information will serve as a basis for further research to identify genes that are essential in cartilage development.

Monoclonal antibodies to human cartilage oligomeric matrix protein: epitope mapping and characterization of sandwich ELISA

BACKGROUND

Cartilage oligomeric matrix protein/thrombospondin 5 (COMP/TSP 5) is one of the most promising serologic markers with regard to an ability to prognose development of osteoarthritis (OA). Our aim was to map the epitopes of three monoclonal antibodies (mAb) to COMP and to develop and characterize a sandwich enzyme-linked immunosorbent assay (ELISA) for measuring COMP levels in human body fluids.

METHODS

COMP was digested with trypsin and the NH(2)-terminal sequence of the fragments recognized by each of the mAbs was determined. Steric competition among the mAbs was tested with an antibody capture assay. A sandwich ELISA was developed using unlabeled mAb 16-F12 as a capture antibody, and mAb 17-C10 labeled with biotin as the second antibody.

RESULTS

Epitopes of the three mAbs were mapped to three different domains within the COMP subunit (16-F12, NH(2)-terminal domain; 17-C10, EGF-like domain; 12-C4, COOH-terminal domain). These epitopes did not overlap. mAbs 17-C10 and 12-C4 yielded similar serum COMP results when used as the secondary antibodies. Serum COMP levels measured with the new sandwich ELISA using mAbs 16-F12 and 17-C10 correlated strongly with results based on an inhibition ELISA with mAb 17-C10 alone (r(2) = 0.836; P < 0.0001). We characterized the new sandwich ELISA with regards to inter- and intra-assay variability, the range of COMP levels that can be expected in human synovial fluids (SF) and sera (controls and OA and rheumatoid arthritis (RA) patients), and the day-to-day and diurnal variability of COMP levels in sera.

CONCLUSIONS

We have developed and characterized a sandwich ELISA for COMP that is sensitive and yields highly reproducible COMP results upon analysis of human sera and synovial fluids.

Microarray analysis reveals the involvement of beta-2 microglobulin (B2M) in human osteoarthritis

OBJECTIVE

To assess whether beta-2 microglobulin (B2M) has effects on articular chondrocytes that would implicate B2M involvement in osteoarthritis (OA) pathogenesis.

METHODS

The mRNA levels of B2M in fetal and osteoarthritic chondrocytes were detected by RT-PCR. B2M levels in synovial fluid and tissue cultured media from cartilage explants were tested using B2M ELISA kit. Primary cultured chondrocytes were used for proliferation and microarray experiments.

RESULTS

The average B2M level in OA synovial fluid is significantly higher than that found in normal synovial fluid. However, there was no significant difference in B2M synovial fluid levels amongst differing OA stages. The release of B2M by osteoarthritic cartilage was detectable after 24h in culture and continued to increase during the 72 h study period. B2M had an inhibitory effect on chondrocyte growth at 1.0 microg/ml, and became significantly inhibitory at 10.0 microg/ml. Genes regulated by B2M were detected through microarray technology. Twenty genes were found to be up-regulated by B2M, including collagen type III which is known to be up-regulated in OA. Eleven genes were found to be down-regulated at least two-fold by B2M.

CONCLUSION

These results indicate that B2M is highly expressed in OA cartilage and synovial fluid compared to normal, and suggest that B2M may have effects on chondrocyte function that could contribute to OA pathogenesis. Published by Elsevier Science Ltd.

Enhanced expression of the human chitinase 3-like 2 gene (YKL-39) but not chitinase 3-like 1 gene (YKL-40) in osteoarthritic cartilage

The knowledge of molecular alterations in osteoarthritic cartilage is important to identify novel therapeutic targets or to develop new diagnostic tools. We aimed to characterize the molecular response to cartilage degeneration by identification of differentially expressed genes in human osteoarthritic versus normal cartilage. Gene fragments selectively amplified in osteoarthritic cartilage by cDNA representational difference analysis included YKL-39 and the oesophageal-cancer-related-gene-4 (ECRG4). YKL-39 expression was significantly upregulated in cartilage from patients with osteoarthritis (n=14) versus normal subjects (n=8) according to real-time PCR (19-fold, p=0.009) and cDNA array analysis (mean 15-fold, p<0.001) and correlated with collagen 2 up-regulation. In contrast, the homologous cousin molecule YKL-40 (chitinase 3-like 1), which is elevated in serum and synovial fluid of patients with arthritis, showed no significant regulation in OA cartilage. Enhanced levels of YKL-40 may, therefore, be derived from synovial cells while modulation of YKL-39 and collagen 2 expression reflected the cartilage metabolism in response to degradation.

Matrix metalloproteinases in arthritic disease

The role of matrix metalloproteinases in the degradative events invoked in the cartilage and bone of arthritic joints has long been appreciated and attempts at the development of proteinase inhibitors as potential therapeutic agents have been made. However, the spectrum of these enzymes orchestrating connective tissue turnover and general biology is much larger than anticipated. Biochemical studies of the individual members of the matrix metalloproteinase family are now underway, ultimately leading to a more detailed understanding of the function of their domain structures and to defining their specific role in cellular systems and the way that they are regulated. Coupled with a more comprehensive and detailed study of proteinase expression in different cells of joint tissues during the progress of arthritic diseases, it will be possible for the future development and application of highly specific proteinase inhibitors to be directed at specific key cellular events.

Clusterin expression in adult human normal and osteoarthritic articular cartilage

OBJECTIVE

To characterize the expression pattern of clusterin in adult human normal and osteoarthritic cartilage.

METHODS

Clusterin mRNA expression in adult human normal and osteoarthritic cartilage was investigated by analysis of cDNA libraries, TaqMan quantitative RT-PCR, microarray and in situ hybridization.

RESULTS

Sequence analysis of ESTs from adult human normal and osteoarthritic cartilage cDNA libraries demonstrated that the abundance of clusterin in these libraries was equivalent to genes which have been more commonly associated with cartilage. To examine tissue distribution, TaqMan Quantitative PCR analysis was performed using RNA from a panel of individual normal tissues. Clusterin was expressed at significant levels in cartilage, brain, liver, and pancreas. The expression of clusterin mRNA was up-regulated in early osteoarthritic vs normal cartilage when analysed by microarray analysis. Using in situ hybridization, chondrocytes of normal cartilage expressed moderate levels of clusterin. Upper mid-zone chondrocytes in cartilage with early stages of osteoarthritic disease expressed high levels of clusterin mRNA. In advanced osteoarthritic cartilage, the overall expression of clusterin was reduced.

CONCLUSION

The induction of clusterin has been associated with a variety of disease states where it appears to provide a cytoprotective effect. The increased expression of clusterin mRNA in the early stages of osteoarthritis (OA) may reflect an attempt by the chondrocytes to protect and repair the tissue. In contrast, the decrease in clusterin mRNA in the advanced osteoarthritic cartilage accompanies the final degenerative stages of the disease. An understanding of the expression of clusterin in osteoarthritis may allow consideration of this protein as a marker for cartilage changes in this chronic degenerative condition.