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

RNA sequencing uncovers key players of cartilage calcification: potential implications for osteoarthritis pathogenesis

OBJECTIVE

OA is a joint disease linked with pathologic cartilage calcification, caused by the deposition of calcium-containing crystals by chondrocytes. Despite its clinical significance, the precise mechanisms driving calcification remain elusive. This study aimed to identify crucial players in cartilage calcification, offering insights for future targeted interventions against OA.

METHODS

Primary murine chondrocytes were stimulated with secondary calciprotein particles (CPP2) or left untreated (NT) for 6 h. Calcification was assessed by alizarin red staining. RNA was analysed by Bulk RNA sequencing. Differentially expressed (DE) genes were identified [cutoff: abs(LogFC)>1 and adjusted P-value < 0.05], and top 50 DE genes were cross-referenced with human OA datasets from previous studies (i.e. healthy vs. OA cartilage, or undamaged vs. damaged cartilage). RNA from NT and CPP2-stimulated primary human OA chondrocytes were used to validate genes by qPCR.

RESULTS

CPP2 induced crystal formation by chondrocytes and significantly modulated 1466 genes. Out of the top 50 DE genes in CPP2, 27 were confirmed in published OA cartilage datasets. Of those genes, some are described in calcification and/or OA (Errfi1, Ngf, Inhba, Col9a1). Two additional ones (Rcan1, Tnfrsf12a) appear novel and interesting in the context of calcification and OA. We validated modulation of these six genes in calcifying human chondrocytes from five patients. Ultimately, we unveiled two distinct gene families modulated by CPP2: the first comprised cytoskeletal genes (Actb, Tpm1, Cfl1, Tagln2, Lmna), while the second encompassed extracellular matrix genes (Fmod, Sparc, Col9a1, Cnmd).

CONCLUSION

CPP2 modulates genes in chondrocytes that could represent new targets for therapeutic interventions in OA.

Short-term overloading exercise attenuates articular chondrocyte features partly via synovium-cartilage interactions mediated by inhibin subunit beta A

Excessive mechanical loading leads to cartilage degeneration. However, short-term responses of the synovium and cartilage to overloading and interactions between these tissues remain poorly understood. We developed a mouse model to study excessive mechanical loading, combining treadmill exercise with weight attachment. Time-course RNA sequencing of the synovium and cartilage displayed transient upregulation of inflammation after single overloading, whereas it was prolonged by repeated overloading. Ingenuity pathway analysis identified Inhba, encoding inhibin subunit beta A, as an upstream molecule for the cartilage transcriptomic changes. Inhba was highly induced by single or repeated overloading in the synovium, and Inhba protein was detected in the superficial layer of the synovium. Supplementation with recombinant activin A, a homodimer of Inhba, exerted catabolic effects in mouse primary chondrocytes. Further insights into mechano-responses of the synovium and cartilage, including the role played by Inhba in the synovium-cartilage interaction, may contribute to the elucidation of osteoarthritis pathogenesis.

Identification of candidate genes and chemicals associated with osteonecrosis of femoral head by multiomics studies and chemical-gene interaction analysis

Objectives

In-depth understanding of osteonecrosis of femoral head (ONFH) has revealed that degeneration of the hip cartilage plays a crucial role in ONFH progression. However, the underlying molecular mechanisms and susceptibility to environmental factors in hip cartilage that contribute to ONFH progression remain elusive.

Methods

We conducted a multiomics study and chemical-gene interaction analysis of hip cartilage in ONFH. The differentially expressed genes (DEGs) involved in ONFH progression were identified in paired hip cartilage samples from 36 patients by combining genome-wide DNA methylation profiling, gene expression profiling, and quantitative proteomics. Gene functional enrichment and pathway analyses were performed via Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses. Functional links between proteins were discovered through protein-protein interaction (PPI) networks. The ONFH-associated chemicals were identified by integrating the DEGs with the chemical-gene interaction sets in the Comparative Toxicogenomics Database (CTD). Finally, the DEGs, including MMP13 and CHI3L1, were validated via quantitative real-time PCR (qRT-PCR) and immunohistochemistry (IHC).

Results

Twenty-two DEGs were identified across all three omics levels in ONFH cartilage, 16 of which were upregulated and six of which were downregulated. The collagen-containing extracellular matrix (ECM), ECM structural constituents, response to amino acids, the relaxin signaling pathway, and protein digestion and absorption were found to be primarily involved in cartilage degeneration in ONFH. Moreover, ten major ONFH-associated chemicals were identified, including, benzo(a)pyrene, valproic acid, and bisphenol A.

Conclusion

Overall, our study identified several candidate genes, pathways, and chemicals associated with cartilage degeneration in ONFH, providing novel clues into the etiology and biological processes of ONFH progression.

Exerkines and osteoarthritis

Osteoarthritis (OA) is the most prevalent chronic joint disease, with physical exercise being a widely endorsed strategy in its management guidelines. Exerkines, defined as cytokines secreted in response to acute and chronic exercise, function through endocrine, paracrine, and/or autocrine pathways. Various tissue-specific exerkines, encompassing exercise-induced myokines (muscle), cardiokines (heart), and adipokines (adipose tissue), have been linked to exercise therapy in OA. Exerkines are derived from these kines, but unlike them, only kines regulated by exercise can be called exerkines. Some of these exerkines serve a therapeutic role in OA, such as irisin, metrnl, lactate, secreted frizzled-related protein (SFRP), neuregulin, and adiponectin. While others may exacerbate the condition, such as IL-6, IL-7, IL-15, IL-33, myostatin, fractalkine, follistatin-like 1 (FSTL1), visfatin, activin A, migration inhibitory factor (MIF), apelin and growth differentiation factor (GDF)-15. They exerts anti-/pro-apoptosis/pyroptosis/inflammation, chondrogenic differentiation and cell senescence effect in chondrocyte, synoviocyte and mesenchymal stem cell. The modulation of adipokine effects on diverse cell types within the intra-articular joint emerges as a promising avenue for future OA interventions. This paper reviews recent findings that underscore the significant role of tissue-specific exerkines in OA, delving into the underlying cellular and molecular mechanisms involved.

Pro-Inflammatory Biomarkers Combined with Body Composition Display a Strong Association with Knee Osteoarthritis in a Community-Based Study

Knee osteoarthritis (KOA) is one of the most common progressive, age-dependent chronic degenerative joint diseases. KOA often develops as a result of a gradual articular cartilage loss caused by its wear and tear. Numerous studies suggest that the degradation of the knee joint involves inflammatory components. This process is also associated with body composition, particularly being overweight and muscle mass loss. The present study aimed to search for novel circulating KOA inflammatory biomarkers, taking into account body composition characteristics. To this aim, we recruited 98 patients diagnosed and radiologically confirmed with KOA and 519 healthy controls from the Arab community in Israel. A panel of soluble molecules, related to inflammatory, metabolic, and musculoskeletal disorders, was measured by ELISA in plasma samples, while several body composition parameters were assessed with bioimpedance analysis. Statistical analysis, including multivariable logistic regression, revealed a number of the factors significantly associated with KOA, independently of age and sex. The most significant independent associations [OR (95% CI)] were fat body mass/body weight index-1.56 (1.20-2.02), systemic immune-inflammation index-4.03 (2.23-7.27), circulating vaspin levels-1.39 (1.15-1.68), follistatin/FSTL1 ratio-1.32 (1.02-1.70), and activin A/FSTL1 ratio-1.33 (1.01-1.75). Further clinical studies are warranted to confirm the relevance of these KOA-associated biological factors. Hereafter, they could serve as reliable biomarkers for KOA in the general human population.

Purification and Isolation of Proteins from Hyaline Cartilage

Proteins from hyaline or articular cartilage can be isolated and purified using a series of chemical extraction steps and various identification techniques including mass spectrometry and immunoblotting. The isolation and purification of proteins from cartilage will facilitate the study of specific proteins and multimeric complexes of cartilage proteins to better understand their functions in normal healthy cartilage as well as pathological conditions of cartilage. Cartilage tissue engineering efforts rely on the comprehensive understanding of the composition of cartilage and the function of each of the protein constituents.

FGF receptor inhibitor BGJ398 partially rescues osteoarthritis-like phenotype in older high molecular weight FGF2 transgenic mice via multiple mechanisms

We have used Basic Fibroblast Growth Factor (FGF2) transgenic mice as experimental models for human X-linked hypophosphatemia (XLH)-related degenerative osteoarthritis (OA) to investigate the pathogenesis of the disease and to test potential pharmacotherapies for treatment. This study tested the efficacy of BJG398, a small molecule fibroblast growth factor receptor tyrosine kinase (FGFRTK) inhibitor, to rescue the knee joint osteoarthritis phenotype in High Molecular Weight fibroblast growth factor 2 transgenic (HMWTgFGF2) mice. BJG398 was administered in vivo to 8-month-old female HMWTgFGF2 mice for six weeks. Histomorphometry, immunohistochemistry and micro-CT were used to examine the knee joints in BGJ398-treated and control mice. We assessed: Fibroblast Growth Factor 23 (FGF23) expression and FGFR1 activity; Matrix metalloproteinase 13 (MMP13) and Aggrecanase2 (ADAMTS5) expression; then signaling by SMAD1/5/8-pSMAD6, pERK1/2 and Runt-related transcription factor 2 (RUNX2). Using PrimePCR arrays, we identified a contributing role for major target genes in the TGFB/BMP2 signaling pathway that were regulated by BGJ398. BGJ398 inhibited HMWFGF2/FGF23-induced increase in bone morphogenic protein receptor-1, bone morphogenic protein-2 and 4 and Serine peptidase inhibitor, clade E, member 1. The results from Micro-CT and histology show BGJ398 treatment rescued the OA changes in subchondral bone and knee articular cartilage of HMWTgFGF2 mice. The gene expression and signal transduction results provide convincing evidence that HMWFGF2 generates OA through FGFRTK with characteristic downstream signaling that defines OA, namely: increased FGF23-FGFR1 activity with BMP-BMPR, activation of pSMAD1/5/8-RUNX2 and pERK signaling pathways, then upregulation of MMP13 and ADAMTS5 to degrade matrix. BGJ398 treatment effectively reversed these OA molecular phenotypes, providing further evidence that the OA generated by HMWFGF2 in the transgenic mice is FGFR-mediated and phenocopies the OA found in the Hyp mouse homolog of XLH with a spontaneous mutation in the Phex (phosphate regulating endopeptidase on the X chromosome) gene and human XLH-OA. Overall, the results obtained here explain how the pleotropic effects of FGF2 emanate from the different functions of HMW protein isoforms for cartilage and bone homeostasis, and the pathogenesis of XLH-degenerative osteoarthropathy. BGJ398 inhibits HMWFGF2-induced osteoarthritis via multiple mechanisms. These results provided important scientific evidence for the potential application of BGJ398 as a therapeutic agent for osteoarthritis in XLH.

Pluripotent stem cells for skeletal tissue engineering

Here, we review the use of human pluripotent stem cells for skeletal tissue engineering. A number of approaches have been used for generating cartilage and bone from both human embryonic stem cells and induced pluripotent stem cells. These range from protocols relying on intrinsic cell interactions and signals from co-cultured cells to those attempting to recapitulate the series of steps occurring during mammalian skeletal development. The importance of generating authentic tissues rather than just differentiated cells is emphasized and enabling technologies for doing this are reported. We also review the different methods for characterization of skeletal cells and constructs at the tissue and single-cell level, and indicate newer resources not yet fully utilized in this field. There have been many challenges in this research area but the technologies to overcome these are beginning to appear, often adopted from related fields. This makes it more likely that cost-effective and efficacious human pluripotent stem cell-engineered constructs may become available for skeletal repair in the near future.

Bio-orthogonal Click Chemistry Methods to Evaluate the Metabolism of Inflammatory Challenged Cartilage after Traumatic Overloading

During traumatic joint injuries, impact overloading can cause mechanical damage to the cartilage. In the following inflammation phase, excessive inflammatory cytokines (e.g., interleukin-1β (IL-1β)) can act on chondrocytes, causing over-proliferation, apoptosis, and extracellular matrix (ECM) degradation that can lead to osteoarthritis. This study investigated the combined effects of traumatic overloading and IL-1β challenge on the metabolic activities of chondrocytes. Bovine cartilage explants underwent impact overloading followed by IL-1β exposure at a physiologically relevant dosage (1 ng/mL). New click chemistry-based methods were developed to visualize and quantify the proliferation of in situ chondrocytes in a nondestructive manner without the involvement of histological sectioning or antibodies. Click chemistry-based methods were also employed to measure the ECM synthesis and degradation in cartilage explants. As the click reactions are copper-free and bio-orthogonal, i.e., with negligible cellular toxicity, cartilage ECM was cultured and studied for 6 weeks. Traumatic overloading induced significant cell death, mainly in the superficial zone. The high number of dead cells reduced the overall proliferation of chondrocytes as well as the synthesis of glycosaminoglycan (GAG) and collagen contents, but overloading alone had no effects on ECM degradation. IL-1β challenge had little effect on cell viability, proliferation, or protein synthesis but induced over 40% GAG loss in 10 days and 61% collagen loss in 6 weeks. For the overloaded samples, IL-1β induced greater degrees of degradation, with 68% GAG loss in 10 days and 80% collagen loss in 6 weeks. The results imply a necessary immediate ease of inflammation after joint injuries when trauma damage on cartilage is present. The new click chemistry methods could benefit many cellular and tissue engineering studies, providing convenient and sensitive assays of metabolic activities of cells in native three-dimensional (3D) environments.

The regional turnover of cartilage collagen matrix in late-stage human knee osteoarthritis

OBJECTIVE

Cartilage collagen has very limited repair potential, though some turnover and incorporation has not been fully excluded. We aim to determine the regional turnover of human osteoarthritis cartilage.

DESIGN

Patients scheduled for knee joint replacement surgery due to osteoarthritis were recruited in this prospective study of four weeks duration. Deuterium oxide (D₂O) was administered orally by weekly boluses at 70% D₂O, initially 150 ml followed by three boluses of 50 ml. Cartilage from the medial tibia plateau was sampled centrally, under the meniscus, and from osteophytes and treated enzymatically with hyaluronidase and trypsin. Samples were analysed for deuterium incorporation in alanine using mass spectrometry and for gene expression by real-time reverse transcriptase polymerase chain reaction.

RESULTS

Twenty participants completed the study: mean (SD) age 64 ± 9.1 years, 45% female, BMI 29.5 ± 4.8 kg/m². Enzymatically treated cartilage from central and submeniscal regions showed similar enrichments at 0.063% APE, while osteophytes showed significantly greater enrichment at 0.072% APE (95% confidence interval of difference) [0.004-0.015]). Fractional synthesis rates were similar for central 0.027%/day and submeniscal cartilage 0.022%/day but 10-fold higher in osteophytes 0.22%/day [0.098-0.363]. When compared to central cartilage, submeniscal cartilage had increased gene expression of MMP-3 and decreased lubricin expression. Untreated cartilage had higher turnover (enrichments at 0.073% APE) than enzymatically treated cartilage (0.063% APE).

CONCLUSIONS

In OA, despite regional differences in gene expression, the turnover of the articular cartilage matrix across the entire joint surface is very limited, but higher turnover was observed in osteophyte cartilage.

Characterizing correlations among disease severity measures in osteochondral tissues from osteoarthritic knees

Osteoarthritis (OA) is a complex disease with biologic, biomechanical, and clinical heterogeneity among patients. Relationships among OA tissue metabolism, histopathology, and extracellular matrix (ECM) composition have not been well characterized. It was hypothesized that moderate (r = .4-.69) to strong (r > .7) correlations exist among these different measures of disease severity in osteochondral tissues from OA knees. Joint surfaces were obtained from patients (n = 6) undergoing total knee arthroplasty. Osteochondral explants (n = 136) were created and cultured for 3 days. Culture media were collected for biomarker analyses, and tissue was assessed for viability, histological scoring, and ECM composition. Correlations among media biomarker concentrations, histological scoring, ECM composition, and viability were determined using a Spearman correlation. GRO-α, IL-6, IL-8, and MCP-1 showed strong positive correlations to each other, and moderate positive correlations to NO, PGE2, and MMP-2. Total MMP activity, MMP-9, and MMP-13 had strong positive correlations to each other, and moderate positive correlations to MMP-1. MMP-2 had a moderate to strong positive correlations to histological scores (total and cartilage structure) and collagen content. MMP-2, IL-6, IL-8, and MCP-1 had moderate negative correlations, and MMP-9 had a moderate positive correlation, to viability. GRO-α, IL-6, IL-8, and MCP-1 had moderate positive correlations to collagen content. MMP-9, MMP-13, and total MMP activity had moderate negative correlations to tissue GAG. The data suggest links among proinflammatory and degradative pathways are present in OA osteochondral tissues. Further characterization of these links have the potential to delineate mechanisms of disease and diagnostic and therapeutic targets for knee OA.

A Microarray Study of Articular Cartilage in Relation to Obesity and Severity of Knee Osteoarthritis

OBJECTIVE

To query the transcript-level changes in the medial and lateral tibial plateau cartilage in tandem with obesity in patients with end-stage osteoarthritis (OA).

DESIGN

Cartilage was obtained from 23 patients (20 obese [body mass index > 30 kg/m²], 3 overweight [body mass index < 30 kg/m²]) at the time of total knee replacement. Cartilage integrity was assessed using Outerbridge scale, while radiographic changes were scored on preoperative X-rays using Kellgren-Lawrence (K-L) classification. RNA was probed for differentially expressed transcripts between medial and lateral compartments using Affymetrix Gene 2.0 ST Array and validated via real-time polymerase chain reaction. Gene ontology and pathway analyses were also queried.

RESULTS

Scoring of cartilage integrity by the Outerbridge scale indicated that the medial and lateral compartments were similar, while scoring by the K-L classification indicated that the medial compartment was more severely damaged than the lateral compartment. We observed a distinct transcript profile with >50% of transcripts unique between medial and lateral compartments. MMP13 and COL2A1 were more highly expressed in medial versus lateral compartment. Polymerase chain reaction confirmed expression of 4 differentially expressed transcripts. Numerous transcripts, biological processes, and pathways were significantly different between overweight and obese patients with a differential response of obesity on medial and lateral compartments.

CONCLUSIONS

Our findings support molecular differences between medial and lateral compartments reflective of the greater severity of OA in the medial compartment. The K-L system better reflected the molecular results than did the Outerbridge. Moreover, the molecular effect of obesity was different between the medial and lateral compartments of the same knee plausibly reflecting the molecular effects of differential biomechanical loading.

The involvement of cytokine-like 1 (Cytl1) in chondrogenesis and cartilage metabolism

The Cytokine-like 1 (Cytl1) is first identified in bone marrow cells and preferentially expressed in cartilaginous tissue, and showed chondrogenic effects in mesenchymal cells, not essential for cartilage or bone development as in Cytl1 knock-out mice but associated with cartilage inflammatory and destruction. Here, we show the involvement of Cytl1 in chondrogenesis. Using specified chondrogenic embryonic skeleton and adult cartilage, the Cytl1 gene expression was investigated with associated chondrogenic factors by quantitative RT-PCR. The effect of Cytl1 protein (rCytl1) on cultured chondrocytes to regulate expression of key factors and phenotypic markers was studied. The results revealed that Cytl1 was highly expressed in chondrogenic process in embryos and adult cartilage. The rCytl1 increased the expression of Sox9 and Col2α1 with stabilized Col1α1 in cultured chondrocytes (redifferentiation). The Cytl1 was expressed and involved at all stages of cartilage development. Furthermore, Cytl1 expression shared similar patterns as other chondrogenic factors, implying interactions with other factors in chondrogenic process. Cytl1 is involved in cartilage development and matrix homeostasis, which defines the dedifferentiation phenotype of chondrocytes, essential to forming of functional cartilage in both physiologic remodeling and pathologic regeneration.

The Effects of Age and Cell Isolation on Collagen II Synthesis by Articular Chondrocytes: Evidence for Transcriptional and Posttranscriptional Regulation

Adult articular cartilage synthesises very little type II collagen in comparison to young cartilage. The age-related difference in collagen II synthesis is poorly understood. This is the first systematic investigation of age-related differences in extracellular matrix synthesis in fresh articular cartilage and following isolation of chondrocytes. A histological comparison of 3-year-old skeletally mature and 6-month-old juvenile porcine cartilage was made. Differences in collagen II, aggrecan, and Sox5, 6, and 9 mRNA and protein expression and mRNA stability were measured. Adult cartilage was found to be thinner than juvenile cartilage but with similar chondrocyte density. Procollagen α1(II) and Sox9 mRNA levels were 10-fold and 3-fold reduced in adult cartilage. Sox9 protein was halved and collagen II protein synthesis was almost undetectable and calculated to be at least 30-fold reduced. Aggrecan expression did not differ. Isolation of chondrocytes caused a drop in procollagen α1(II) and Sox9 mRNA in both adult and juvenile cells along with a marked reduction in Sox9 mRNA stability. Interestingly, juvenile chondrocytes continued to synthesise collagen II protein with mRNA levels similar to those seen in adult articular cartilage. Age-related differences in collagen II protein synthesis are due to both transcriptional and posttranscription regulation. A better understanding of these regulatory mechanisms would be an important step in improving current cartilage regeneration techniques.

TGFβ/BMP Signaling Pathway in Cartilage Homeostasis

Cartilage homeostasis is governed by articular chondrocytes via their ability to modulate extracellular matrix production and degradation. In turn, chondrocyte activity is regulated by growth factors such as those of the transforming growth factor β (TGFβ) family. Members of this family include the TGFβs, bone morphogenetic proteins (BMPs), and growth and differentiation factors (GDFs). Signaling by this protein family uniquely activates SMAD-dependent signaling and transcription but also activates SMAD-independent signaling via MAPKs such as ERK and TAK1. This review will address the pivotal role of the TGFβ family in cartilage biology by listing several TGFβ family members and describing their signaling and importance for cartilage maintenance. In addition, it is discussed how (pathological) processes such as aging, mechanical stress, and inflammation contribute to altered TGFβ family signaling, leading to disturbed cartilage metabolism and disease.

The activin-follistatin anti-inflammatory cycle is deregulated in synovial fibroblasts

Background

Activin A and follistatin exhibit immunomodulatory functions, thus affecting autoinflammatory processes as found in rheumatoid arthritis (RA). The impact of both proteins on the behavior of synovial fibroblasts (SF) in RA as well as in osteoarthritis (OA) is unknown.

Methods

Immunohistochemical analyses of synovial tissue for expression of activin A and follistatin were performed. The influence of RASF overexpressing activin A on cartilage invasion in a SCID mouse model was examined. RASF and OASF were stimulated with either IL-1β or TNFα in combination with or solely with activin A, activin AB, or follistatin. Protein secretion was measured by ELISA and mRNA expression by RT-PCR. Smad signaling was confirmed by western blot.

Results

In human RA synovial tissue, the number of activin A-positive cells as well as its extracellular presence was higher than in the OA synovium. Single cells within the tissue expressed follistatin in RA and OA synovial tissue. In the SCID mouse model, activin A overexpression reduced RASF invasion. In human RASF, activin A was induced by IL-1β and TNFα. Activin A slightly increased IL-6 release by unstimulated RASF, but decreased protein and mRNA levels of follistatin.

Conclusion

The observed decrease of cartilage invasion by RASF overexpressing activin A in the SCID mouse model appears to be mediated by an interaction between activin/follistatin and other local cells indirectly affecting RASF because activin A displayed certain pro-inflammatory effects on RASF. Activin A even inhibits production and release of follistatin in RASF and therefore prevents itself from being blocked by its inhibitory binding protein follistatin in the local inflammatory joint environment.

Electronic supplementary material

The online version of this article (10.1186/s13075-019-1926-7) contains supplementary material, which is available to authorized users.

Aberrantly hydroxymethylated differentially expressed genes and the associated protein pathways in osteoarthritis

Background

The elderly population is at risk of osteoarthritis (OA), a common, multifactorial, degenerative joint disease. Environmental, genetic, and epigenetic (such as DNA hydroxymethylation) factors may be involved in the etiology, development, and pathogenesis of OA. Here, comprehensive bioinformatic analyses were used to identify aberrantly hydroxymethylated differentially expressed genes and pathways in osteoarthritis to determine the underlying molecular mechanisms of osteoarthritis and susceptibility-related genes for osteoarthritis inheritance.

Methods

Gene expression microarray data, mRNA expression profile data, and a whole genome 5hmC dataset were obtained from the Gene Expression Omnibus repository. Differentially expressed genes with abnormal hydroxymethylation were identified by MATCH function. Gene ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses of the genes differentially expressed in OA were performed using Metascape and the KOBAS online tool, respectively. The protein–protein interaction network was built using STRING and visualized in Cytoscape, and the modular analysis of the network was performed using the Molecular Complex Detection app.

Results

In total, 104 hyperhydroxymethylated highly expressed genes and 14 hypohydroxymethylated genes with low expression were identified. Gene ontology analyses indicated that the biological functions of hyperhydroxymethylated highly expressed genes included skeletal system development, ossification, and bone development; KEGG pathway analysis showed enrichment in protein digestion and absorption, extracellular matrix–receptor interaction, and focal adhesion. The top 10 hub genes in the protein–protein interaction network were COL1A1, COL1A2, COL2A1, COL3A1, COL5A1, COL5A2, COL6A1, COL8A1, COL11A1, and COL24A1. All the aforementioned results are consistent with changes observed in OA.

Conclusion

After comprehensive bioinformatics analysis, we found aberrantly hydroxymethylated differentially expressed genes and pathways in OA. The top 10 hub genes may be useful hydroxymethylation analysis biomarkers to provide more accurate OA diagnoses and target genes for treatment of OA.

CYTL1 inhibits tumor metastasis with decreasing STAT3 phosphorylation

CYTL1 is a novel cytokine that was first identified in CD34⁺ hematopoietic cells. We previously prepared recombinant CYTL1 and verified that it chemoattracted human monocytes via the CCR2/ERK pathway. It has been reported that CYTL1 plays contradictory roles in neuroblastoma and lung cancer. We found that the expression level of CYTL1 was notably decreased and it was hypermethylated in various tumors, including breast and lung cancer, by bioinformatics analyses. After validating the expression of CYTL1 in lung cancer, we identified that CYTL1 exerted no obvious effect on tumor cell proliferation but inhibited their migration and invasion, and these effects were accompanied by decreasing STAT3 phosphorylation, using recombinant CYTL1 and CYTL1-overexpressing tumor cell lines. Furthermore, we constructed experimental and spontaneous metastasis models of breast cancer in BALB/c mice and found that CYTL1 significantly inhibited tumor metastasis in vivo. In summary, CYTL1 is a cytokine with tumor-suppressing characteristics that inhibits tumor metastasis and STAT3 phosphorylation in multiple types of tumors.

Identification of prolargin expression in articular cartilage and its significance in rheumatoid arthritis pathology

Qualitative 2D gel-electrophoresis (2DE) protein profiling for osteoarthritis (OA) and rheumatoid arthritis (RA) is challenging because of selective protein loss due to discrepancies in protein precipitation methodologies. Thus, we aimed at developing qualitative protein representation from OA/RA articular cartilage without protein precipitation towards identification of clinically relevant proteins. Chondroitinase digested human articular cartilages from RA patients were subjected to protein extraction using guanidinium hydrochloride (GuHCl) or 8 M urea with 10 or 2% ASB-14-4 or 0.45 M urea with 2% ASB-14-4 with cetylpyridinium chloride (CPC). The GuHCl extract is further protein precipitated with acetone or ammonium acetate-methanol or centricon-fractionated using 100 kDa cut filters and protein precipitated using ethanol. Processed extracts were subjected to 2DE to identify protein profiles. Poor proteins representations were observed in 2D gels with protein precipitated samples compared to qualitative protein representations seen in 2D gels of 0.45 M urea and 2%ASB-14-4 extraction procedure reproducibly. The strategy circumventing protein precipitation generated qualitative 2D gels. RA vs OA gel comparison showed elevated prolargin levels in RA with biglycan levels remaining unaltered. Up regulation of prolargin in RA suggests the likelihood of an adaptive mechanism to control the increased osteoclastogenesis in RA and may have therapeutic value in controlling the disease.

Development of chondrocyte-seeded electrosprayed nanoparticles for repair of articular cartilage defects in rabbits

Due to limited self-healing capacity in cartilages, there is a rising demand for an innovative therapy that promotes chondrocyte proliferation while maintaining its biofunctionality for transplantation. Chondrocyte transplantation has received notable attention; however, the tendencies of cell de-differentiation and de-activation of biofunctionality have been major hurdles in its development, delaying this therapy from reaching the clinic. We believe it is due to the non-stimulative environment in the injured cartilage, which is unable to provide sustainable physical and biological supports to the newly grafted chondrocytes. Therefore, we evaluated whether providing an appropriate matrix to the transplanted chondrocytes could manipulate cell fate and recovery outcomes. Here, we proposed the development of electrosprayed nanoparticles composed of cartilage specific proteins, namely collagen type II and hyaluronic acid, for implantation with pre-seeded chondrocytes into articular cartilage defects. The fabricated nanoparticles were pre-cultured with chondrocytes before implantation into injured articular cartilage. The study revealed a significant potential for nanoparticles to support pre-seeded chondrocytes in cartilage repair, serving as a protein delivery system while improving the survival and biofunctionality of transplanted chondrocytes for prolonged period of time.

Chondrocyte secretome: a source of novel insights and exploratory biomarkers of osteoarthritis

The extracellular matrix (ECM) of articular cartilage is comprised of complex networks of proteins and glycoproteins, all of which are expressed by its resident cell, the chondrocyte. Cartilage is a unique tissue given its complexity and ability to resist repeated load and deformation. The mechanisms by which articular cartilage maintains its integrity throughout our lifetime is not fully understood, however there are numerous regulatory pathways known to govern ECM turnover in response to mechanical stimuli. To further our understanding of this field, we envision that proteomic analysis of the secretome will provide information on how the chondrocyte remodels the surrounding ECM in response to load, in addition to providing information on the metabolic state of the cell. In this review, we attempt to summarize the recent mass spectrometry-based proteomic discoveries in healthy and diseased cartilage and chondrocytes, to facilitate the discovery of novel biomarkers linked to degenerative pathologies, such as osteoarthritis (OA).

Biochemical and biophysical characterization of cytokine-like protein 1 (CYTL1)

Cytokine-like protein 1 (CYTL1) is a small widely expressed secreted protein lacking significant primary sequence homology to any other known protein. CYTL1 expression appears to be highest in the hematopoietic system and in chondrocytes; however, maintenance of cartilage in mouse models of arthritis is its only reported function in vivo. Despite lacking sequence homology to chemokines, CYTL1 is predicted by computational methods to fold like a chemokine, and has been reported to function as a chemotactic agonist at the chemokine receptor CCR2 in mouse monocyte/macrophages. Nevertheless, since chemokines are defined by structure and chemokine receptors are able to bind many non-chemokine ligands, direct determination of the CYTL1 tertiary structure will ultimately be required to know whether it actually folds as a chemokine and therefore is a chemokine. Towards this goal, we have developed a method for producing functional recombinant human CYTL1 in bacteria, and we provide new evidence about the biophysical and biochemical properties of recombinant CYTL1. Circular dichroism analysis showed that, like chemokines, CYTL1has a higher content of beta-sheet than alpha-helix secondary structure. Furthermore, recombinant CYTL1 promoted calcium flux in chondrocytes. Nevertheless, unlike chemokines, CYTL1 had limited affinity to proteoglycans. Together, these properties further support cytokine-like properties for CYTL1 with some overlap with the chemokines.

Quantitative proteomics analysis of cartilage response to mechanical injury and cytokine treatment

Mechanical damage at the time of joint injury and the ensuing inflammatory response associated with elevated levels of pro-inflammatory cytokines in the synovial fluid, are reported to contribute to the progression to osteoarthritis after injury. In this exploratory study, we used a targeted proteomics approach to follow the progression of matrix degradation in response to mechanical damage and cytokine treatment of human knee cartilage explants, and thereby to study potential molecular biomarkers. This proteomics approach allowed us to unambiguously identify and quantify multiple peptides and proteins in the cartilage medium and explants upon treatment with ±injurious compression ±cytokines, treatments that mimic the earliest events in post-traumatic OA. We followed degradation of different protein domains, e.g., G1/G2/G3 of aggrecan, by measuring representative peptides of matrix proteins released into the medium at 7 time points throughout the 21-day culture period. COMP neo-epitopes, which were previously identified in the synovial fluid of knee injury/OA patients, were also released by these human cartilage explants treated with cyt and cyt+inj. The absence of collagen pro-peptides and elevated levels of specific COMP and COL3A1 neo-epitopes after human knee trauma may be relevant as potential biomarkers for post-traumatic OA. This model system thereby enables study of the kinetics of cartilage degradation and the identification of biomarkers within cartilage explants and those released to culture medium. Discovery proteomics revealed that candidate proteases were identified after specific treatment conditions, including MMP1, MMP-3, MMP-10 and MMP-13.

Cytokine-Like 1 Regulates Cardiac Fibrosis via Modulation of TGF-β Signaling

Cytokine-like 1 (Cytl1) is a secreted protein that is involved in diverse biological processes. A comparative modeling study indicated that Cytl1 is structurally and functionally similar to monocyte chemoattractant protein 1 (MCP-1). As MCP-1 plays an important role in cardiac fibrosis (CF) and heart failure (HF), we investigated the role of Cytl1 in a mouse model of CF and HF. Cytl1 was upregulated in the failing mouse heart. Pressure overload-induced CF was significantly attenuated in cytl1 knock-out (KO) mice compared to that from wild-type (WT) mice. By contrast, adeno-associated virus (AAV)-mediated overexpression of cytl1 alone led to the development of CF in vivo. The endothelial-mesenchymal transition (EndMT) and the transdifferentiation of fibroblasts (FBs) to myofibroblasts (MFBs) have been suggested to contribute considerably to CF. Adenovirus-mediated overexpression of cytl1 was sufficient to induce these two critical CF-related processes in vitro, which were completely abrogated by co-treatment with SB-431542, an antagonist of TGF-β receptor 1. Cytl1 induced the expression of TGF-β2 both in vivo and in vitro. Antagonizing the receptor for MCP-1, C-C chemokine receptor type 2 (CCR2), with CAS 445479-97-0 did not block the pro-fibrotic activity of Cytl1 in vitro. Collectively, our data suggest that Cytl1 plays an essential role in CF likely through activating the TGF-β-SMAD signaling pathway. Although the receptor for Cyt1l remains to be identified, Cytl1 provides a novel platform for the development of anti-CF therapies.

Correlation of the osteoarthritis susceptibility variants that map to chromosome 20q13 with an expression quantitative trait locus operating on NCOA3 and with functional variation at the polymorphism rs116855380

Objective

To functionally characterize the osteoarthritis (OA) susceptibility variants that map to a region of high linkage disequilibrium (LD) on chromosome 20q13 marked by the single‐nucleotide polymorphism (SNP) rs6094710 and encompassing NCOA3 and SULF2.

Methods

Nucleic acids were extracted from the cartilage of OA patients. Overall and allelic expression of NCOA3 and SULF2 were measured by quantitative reverse transcription–polymerase chain reaction and pyrosequencing, respectively. The functional effect of SNPs within the 20q13 locus was assessed in vitro using luciferase reporter constructs and electrophoretic mobility shift assays (EMSAs). The in vivo effect of nuclear receptor coactivator 3 (NCOA3) protein depletion on primary human OA articular cartilage chondrocytes was assessed using RNA interference.

Results

Expression of NCOA3 correlated with the genotype at rs6094710 (P = 0.006), and the gene demonstrated allelic expression imbalance (AEI) in individuals heterozygous for the SNP (mean AEI 1.21; P < 0.0001). In both instances, expression of the OA‐associated allele was reduced. In addition, there was reduced enhancer activity of the OA‐associated allele of rs116855380, a SNP in perfect LD with rs6094710 in luciferase assays (P < 0.001). EMSAs demonstrated a protein complex binding with reduced affinity to this allele. Depletion of NCOA3 led to significant changes (all P < 0.05) in the expression of genes involved in cartilage homeostasis.

Conclusion

NCOA3 is subject to a cis‐acting expression quantitative trait locus in articular cartilage, which correlates with the OA association signal and with the OA‐associated allele of the functional SNP rs116855380, a SNP that is located only 10.3 kb upstream of NCOA3. These findings elucidate the effect of the association of the 20q13 region on OA cartilage and provide compelling evidence of a potentially causal candidate SNP.

In Vivo Identification and Induction of Articular Cartilage Stem Cells by Inhibiting NF-κB Signaling in Osteoarthritis

Osteoarthritis (OA) is a highly prevalent and debilitating joint disorder characterized by the degeneration of articular cartilage. However, no effective medical therapy has been found yet for such condition. In this study, we directly confirmed the existence of articular cartilage stem cells (ACSCs) in vivo and in situ for the first time both in normal and OA articular cartilage, and explored their chondrogenesis in Interleukin-1β (IL-1β) induced inflammation environment and disclose whether the inhibition of NF-κB signaling can induce ACSCs activation thus improve the progression of experimental OA. We found an interesting phenomenon that ACSCs were activated and exhibited a transient proliferative response in early OA as an initial attempt for self-repair. During the in vitro mechanism study, we discovered IL-1β can efficiently activate the NF-κB pathway and potently impair the responsiveness of ACSCs, whereas the NF-κB pathway inhibitor rescued the ACSCs chondrogenesis. The final in vivo experiments further confirmed ACSCs' activation were maintained by NF-κB pathway inhibitor, which induced cartilage regeneration, and protected articular cartilage from injury in an OA animal model. Our results provided in vivo evidence of the presence of ACSCs, and disclosed their action in the early OA stage and gradual quiet as OA process, presented a potential mechanism for both cartilage intrinsic repair and its final degradation, and demonstrated the feasibility of inducing endogenous adult tissue-specific mesenchymal stem cells for articular cartilage repair and OA therapy.

Lessons from the proteomic study of osteoarthritis

Osteoarthritis is the most common rheumatic pathology and one of the leading causes of disability worldwide. It is a very complex disease whose etiopathogenesis is not fully understood. Furthermore, there are serious limitations for its management, since it lacks specific and sensitive biomarkers for early diagnosis, prognosis and therapeutic monitoring. Proteomic approaches performed in the last few decades have contributed to the knowledge on the molecular mechanisms that participate in this pathology and they have also led to interesting panels of putative biomarker candidates. In the next few years, further efforts should be made for translating these findings into the clinical routines. It is expected that targeted proteomics strategies will be highly valuable for the verification and qualification of biomarkers of osteoarthritis.

Preserved Proteins from Extinct Bison latifrons Identified by Tandem Mass Spectrometry; Hydroxylysine Glycosides are a Common Feature of Ancient Collagen

Bone samples from several vertebrates were collected from the Ziegler Reservoir fossil site, in Snowmass Village, Colorado, and processed for proteomics analysis. The specimens come from Pleistocene megafauna Bison latifrons, dating back ∼ 120,000 years. Proteomics analysis using a simplified sample preparation procedure and tandem mass spectrometry (MS/MS) was applied to obtain protein identifications. Several bioinformatics resources were used to obtain peptide identifications based on sequence homology to extant species with annotated genomes. With the exception of soil sample controls, all samples resulted in confident peptide identifications that mapped to type I collagen. In addition, we analyzed a specimen from the extinct B. latifrons that yielded peptide identifications mapping to over 33 bovine proteins. Our analysis resulted in extensive fibrillar collagen sequence coverage, including the identification of posttranslational modifications. Hydroxylysine glucosylgalactosylation, a modification thought to be involved in collagen fiber formation and bone mineralization, was identified for the first time in an ancient protein dataset. Meta-analysis of data from other studies indicates that this modification may be common in well-preserved prehistoric samples. Additional peptide sequences from extracellular matrix (ECM) and non-ECM proteins have also been identified for the first time in ancient tissue samples. These data provide a framework for analyzing ancient protein signatures in well-preserved fossil specimens, while also contributing novel insights into the molecular basis of organic matter preservation. As such, this analysis has unearthed common posttranslational modifications of collagen that may assist in its preservation over time. The data are available via ProteomeXchange with identifier PXD001827.

Proteomic Analysis of Connexin 43 Reveals Novel Interactors Related to Osteoarthritis

We have previously reported that articular chondrocytes in tissue contain long cytoplasmic arms that physically connect two distant cells. Cell-to-cell communication occurs through connexin channels termed Gap Junction (GJ) channels, which achieve direct cellular communication by allowing the intercellular exchange of ions, small RNAs, nutrients, and second messengers. The Cx43 protein is overexpressed in several human diseases and inflammation processes and in articular cartilage from patients with osteoarthritis (OA). An increase in the level of Cx43 is known to alter gene expression, cell signaling, growth, and cell proliferation. The interaction of proteins with the C-terminal tail of connexin 43 (Cx43) directly modulates GJ-dependent and -independent functions. Here, we describe the isolation of Cx43 complexes using mild extraction conditions and immunoaffinity purification. Cx43 complexes were extracted from human primary articular chondrocytes isolated from healthy donors and patients with OA. The proteomic content of the native complexes was determined using LC-MS/MS, and protein associations with Cx43 were validated using Western blot and immunolocalization experiments. We identified >100 Cx43-associated proteins including previously uncharacterized proteins related to nucleolar functions, RNA transport, and translation. We also identified several proteins involved in human diseases, cartilage structure, and OA as novel functional Cx43 interactors, which emphasized the importance of Cx43 in the normal physiology and structural and functional integrity of chondrocytes and articular cartilage. Gene Ontology (GO) terms of the proteins identified in the OA samples showed an enrichment of Cx43-interactors related to cell adhesion, calmodulin binding, the nucleolus, and the cytoskeleton in OA samples compared with healthy samples. However, the mitochondrial proteins SOD2 and ATP5J2 were identified only in samples from healthy donors. The identification of Cx43 interactors will provide clues to the functions of Cx43 in human cells and its roles in the development of several diseases, including OA.

Proteomic analysis of rat cartilage: the identification of differentially expressed proteins in the early stages of osteoarthritis

Background

Osteoarthritis (OA) is a chronic degenerative disease of the articular cartilage, and its diagnosis is based on symptoms and radiological signs that are only present in the late stages of the disease. Due to the limitations in diagnosing OA before the onset of symptoms, such as pain, little is known about the molecular mechanisms involved in the pathogenesis of OA. Experimental OA models are often used to study the kinetics of the progression of this disease. In this report, we conducted a proteomic study of osteoarthritic cartilage during the early stages of OA using an experimental rat model.

Results

Ten proteins that are differentially expressed under early OA conditions were identified by 2-DE and MALDI-TOF/MS. These proteins mediated many processes, such as glycolysis and energy production (Nme2 and Pnp), cartilage matrix (Col2a1), transcription and protein synthesis (Eef1a1 and DJ-1), signal transduction (CaM and Pebp1), transport (Alb and Hba1), and latexin (Lxn). In addition, changes in Lxn expression in early OA were observed and validated by western blot and immunofluorescence analysis.

Conclusions

The proteins that we identified indicate that energy metabolism, cartilage matrix remodelling, and protective cellular mechanisms are associated with early OA. In addition, latexin expression during the early stages of OA could be implicated in cartilage repair.

Electronic supplementary material

The online version of this article (doi:10.1186/s12953-014-0055-0) contains supplementary material, which is available to authorized users.

Comparative serum proteome expression of the steroid-induced femoral head osteonecrosis in adults

Steroid-induced osteonecrosis of the femoral head (SONFH) is a disabling, aseptic and ischemic disease that develops following steroid therapy. The pathogenesis of SONFH is unclear, so the early diagnosis and treatment for this disease is yet to be established. The purpose of the present study was to identify potential biomarkers for SONFH. The differential expression of serum proteins from patients with SONFH and healthy volunteers was analyzed by the proteomics method. The protein samples were labeled and subjected to isoelectric focusing and two-dimensional gel electrophoresis. The resultant protein spots were matched and quantified by an imaging analysis system. The differentially-expressed protein spots were subjected to in-gel trypsin digestion followed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Significantly lower levels of complement component 3 (C3), C4, inter-α-trypsin inhibitor heavy chain H4 and α-2-macroglobulin were found in the serum of patients with SONFH. These proteins are reported to be actively involved in intravascular coagulation, apoptosis and reactive oxygen species imbalance, indicating that multiple pathological reactions occur in SONFH and these proteins may serve as potential biomarkers for the diagnosis of SONFH.

Carprofen inhibits the release of matrix metalloproteinases 1, 3, and 13 in the secretome of an explant model of articular cartilage stimulated with interleukin 1β

INTRODUCTION

Arthritic diseases are characterized by the degradation of collagenous and noncollagenous extracellular matrix (ECM) components in articular cartilage. The increased expression and activity of matrix metalloproteinases (MMPs) is partly responsible for cartilage degradation. This study used proteomics to identify inflammatory proteins and catabolic enzymes released in a serum-free explant model of articular cartilage stimulated with the pro-inflammatory cytokine interleukin 1β (IL-1β). Western blotting was used to quantify the release of selected proteins in the presence or absence of the cyclooxygenase-2 specific nonsteroidal pro-inflammatory drug carprofen.

METHODS

Cartilage explant cultures were established by using metacarpophalangeal joints from horses euthanized for purposes other than research. Samples were treated as follows: no treatment (control), IL-1β (10 ng/ml), carprofen (100 μg/ml), and carprofen (100 μg/ml) + IL-1β (10 ng/ml). Explants were incubated (37°C, 5% CO2) over twelve day time courses. High-throughput nano liquid chromatography/mass spectrometry/mass spectrometry uncovered candidate proteins for quantitative western blot analysis. Proteoglycan loss was assessed by using the dimethylmethylene blue (DMMB) assay, which measures the release of sulfated glycosaminoglycans (GAGs).

RESULTS

Mass spectrometry identified MMP-1, -3, -13, and the ECM constituents thrombospondin-1 (TSP-1) and fibronectin-1 (FN1). IL-1β stimulation increased the release of all three MMPs. IL-1β also stimulated the fragmentation of FN1 and increased chondrocyte cell death (as assessed by β-actin release). Addition of carprofen significantly decreased MMP release and the appearance of a 60 kDa fragment of FN1 without causing any detectable cytotoxicity to chondrocytes. DMMB assays suggested that carprofen initially inhibited IL-1β-induced GAG release, but this effect was transient. Overall, during the two time courses, GAG release was 58.67% ± 10.91% (SD) for IL-1β versus 52.91% ± 9.35% (SD) with carprofen + IL-1β.

CONCLUSIONS

Carprofen exhibits beneficial anti-inflammatory and anti-catabolic effects in vitro without causing any detectable cytotoxicity. Combining proteomics with this explant model provides a sensitive screening system for anti-inflammatory compounds.

Biomarkers and proteomic analysis of osteoarthritis

Our friend and colleague, Dr. Dick Heinegård, contributed greatly to the understanding of joint tissue biochemistry, the discovery and validation of arthritis-related biomarkers and the establishment of methodology for proteomic studies in osteoarthritis (OA). To date, discovery of OA-related biomarkers has focused on cartilage, synovial fluid and serum. Methods, such as affinity depletion and hyaluronidase treatment have facilitated proteomics discovery research from these sources. Osteoarthritis usually involves multiple joints; this characteristic makes it easier to detect OA with a systemic biomarker but makes it hard to delineate abnormalities of individual affected joints. Although the abundance of cartilage proteins in urine may generally be lower than other tissue/sample sources, the protein composition of urine is much less complex and its collection is non-invasive thereby facilitating the development of patient friendly biomarkers. To date however, relatively few proteomics studies have been conducted in OA urine. Proteomics strategies have identified many proteins that may relate to pathological mechanisms of OA. Further targeted approaches to validate the role of these proteins in OA are needed. Herein we summarize recent proteomic studies related to joint tissues and the cohorts used; a clear understanding of the cohorts is important for this work as we expect that the decisive discoveries of OA-related biomarkers rely on comprehensive phenotyping of healthy non-OA and OA subjects. Besides the common phenotyping criteria that include, gender, age, and body mass index (BMI), it is essential to collect data on symptoms and signs of OA outside the index joints and to bolster this with objective imaging data whenever possible to gain the most precise appreciation of the total burden of disease. Proteomic studies on systemic biospecimens, such as serum and urine, rely on comprehensive phenotyping data to unravel the true meaning of the proteomic results.

Human osteoarthritic cartilage shows reduced in vivo expression of IL-4, a chondroprotective cytokine that differentially modulates IL-1β-stimulated production of chemokines and matrix-degrading enzymes in vitro

Background

In osteoarthritis (OA), an inflammatory environment is responsible for the imbalance between the anabolic and catabolic activity of chondrocytes and, thus, for articular cartilage derangement. This study was aimed at providing further insight into the impairment of the anabolic cytokine IL-4 and its receptors in human OA cartilage, as well as the potential ability of IL-4 to antagonize the catabolic phenotype induced by IL-1β.

Methodology/Principal Findings

The in vivo expression of IL-4 and IL-4 receptor subunits (IL-4R, IL-2Rγ, IL-13Rα1) was investigated on full thickness OA or normal knee cartilage. IL-4 expression was found to be significantly lower in OA, both in terms of the percentage of positive cells and the amount of signal per cell. IL-4 receptor type I and II were mostly expressed in mid-deep cartilage layers. No significant difference for each IL-4 receptor subunit was noted. IL-4 anti-inflammatory and anti-catabolic activity was assessed in vitro in the presence of IL-1β and/or IL-4 for 24 hours using differentiated high density primary OA chondrocyte also exhibiting the three IL-4 R subunits found in vivo. Chemokines, extracellular matrix degrading enzymes and their inhibitors were evaluated at mRNA (real time PCR) and protein (ELISA or western blot) levels. IL-4 did not affect IL-1β-induced mRNA expression of GRO-α/CXCL1, IL-8/CXCL8, ADAMTS-5, TIMP-1 or TIMP-3. Conversely, IL-4 significantly inhibited RANTES/CCL5, MIP-1α/CCL3, MIP-1β/CCL4, MMP-13 and ADAMTS-4. These results were confirmed at protein level for RANTES/CCL5 and MMP-13.

Conclusions/Significance

Our results indicate for the first time that OA cartilage has a significantly lower expression of IL-4. Furthermore, we found differences in the spectrum of biological effects of IL-4. The findings that IL-4 has the ability to hamper the IL-1β-induced release of both MMP-13 and CCL5/RANTES, both markers of OA chondrocytes, strongly indicates IL-4 as a pivotal anabolic cytokine in cartilage whose impairment impacts on OA pathogenesis.

Fibroblast growth factor 2 drives changes in gene expression following injury to murine cartilage in vitro and in vivo

OBJECTIVE

The articular cartilage is known to be highly mechanosensitive, and a number of mechanosensing mechanisms have been proposed as mediators of the cellular responses to altered mechanical load. These pathways are likely to be important in tissue homeostasis as well as in the pathogenesis of osteoarthritis. One important injury-activated pathway involves the release of pericellular fibroblast growth factor 2 (FGF-2) from the articular cartilage. Using a novel model of murine cartilage injury and surgically destabilized joints in mice, we examined the extent to which FGF-2 contributes to the cellular gene response to injury.

METHODS

Femoral epiphyses from 5-week-old wild-type mice were avulsed and cultured in serum-free medium. Explant lysates were Western blotted for phospho-JNK, phospho-p38, and phospho-ERK or were fixed for immunohistochemical analysis of the nuclear translocation of p65 (indicative of NF-κB activation). RNA was extracted from injured explants, rested explants that had been stimulated with recombinant FGF-2 or FGF-18, or whole joints from either wild-type mice or FGF-2(-/-) mice. Reverse transcription-polymerase chain reaction was performed to examine a number of inflammatory response genes that had previously been identified in a microarray analysis.

RESULTS

Murine cartilage avulsion injury resulted in rapid activation of the 3 MAP kinase pathways as well as NF-κB. Almost all genes identified in murine joints following surgical destabilization were also regulated in cartilage explants upon injury. Many of these genes, including those for activin A (Inhba), tumor necrosis factor-stimulated gene 6 (Tnfaip6), matrix metalloproteinase 19 (Mmp19), tissue inhibitor of metalloproteinases 1 (Timp1), and podoplanin (Pdpn), were significantly FGF-2 dependent following injury to cartilage in vitro and to joint tissues in vivo.

CONCLUSION

FGF-2-dependent gene expression occurs in vitro and in vivo in response to cartilage/joint injury in mice.

Theoretical prediction of ultrasound elastography for detection of early osteoarthritis

Ultrasound elastography could be used as a new noninvasive technique for detecting early osteoarthritis. As the first critical step, this study theoretically predicted the excitation power and the measurement errors in detecting cartilage detect. A finite element model was used to simulate wave propagation of elastography in the cartilage. The wave was produced by a force F, and the wave speed C was calculated. The normal cartilage model was used to define the relationship between the wave speed and elastic modulus. Various stiffness values were simulated. F = 10 N with a duration of 0.5 ms was required for having measurable deformation (10 μm) at the distal site. The deformation had a significant rise when the wave crossed the defect. The relationship between the wave speed and elastic parameters was found as C = 1.57 × (E)/(2 × ρ(1+μ)))^1/2, where E was the elastic modulus, μ was Poisson's ratio, and ρ was the density. For the simulated defect with an elastic modulus of 7 MPa which was slightly stiffer than the normal cartilage, the measurement error was 0.1 MPa. The results suggested that, given the simulated conditions, this new technique could be used to detect the defect in early osteoarthritis.

Absolute quantification of selected proteins in the human osteoarthritic secretome

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

PTHrP-induced modifications of the sea bream (Sparus auratus) vertebral bone proteome

Endocrine factors play an essential role in the formation and turnover of the skeleton in vertebrates. In the present study sea bream vertebral bone transcripts for PTH1R and PTH3R were identified and the action of intermittent administration of parathyroid hormone related protein (PTHrP) on the proteome of vertebral bone was analysed. Treatment of immature sea bream (Sparus auratus, n=6) for 5days with homologous recombinant PTHrP(1-125; 150ng/g body weight) modified bone metabolism and caused a significant (p<0.05) reduction in both tartrate resistant acid phosphatase (TRACP) and alkaline phosphatase (ALP) in relation to control fish. However, the ratio of TRACP: ALP in PTHrP treated fish (1.3 to 2.2 cf. control) suggested it had an anabolic response. A sea bream vertebral bone proteome of 157 protein spots was generated and putative identity assigned to 118 (75.2%) proteins of which 72% had homology to proteins/transcripts from teleosts many of which have not previously been reported in teleost bone. Classification of bone proteins using gene ontology revealed those with protein or metal/ion (e.g., calcium, magnesium, zinc) binding (∼53%) activities were most abundant. The expression of eight proteins was significantly (p<0.05) modified in the vertebra of PTHrP treated compared to control fish; three were up-regulated, betainehomocystein S-methyltransferase, glial fibrillary acidic protein, parvalbumin beta and five were down-regulated, annexin A5, apolipoprotein A1, myosin light chain 2, fast skeletal myosin light chain 3, troponin C. In conclusion, intermittent administration of PTHrP to sea bream is associated with an anabolic response in vertebral bone metabolism and modifies calcium binding proteins in the proteome.

Transcriptomics of wild-type mice and mice lacking ADAMTS-5 activity identifies genes involved in osteoarthritis initiation and cartilage destruction

OBJECTIVE

To identify changes in gene expression in mice with osteoarthritis (OA) in order to explore the mechanisms of the disease.

METHODS

Gene expression profiling was performed in cartilage from mice with surgically induced OA. We used wild-type (WT) mice and Adamts5Δcat mice, in which ADAMTS-5 activity is lacking and aggrecan loss and cartilage erosion are inhibited, to distinguish gene expression changes that are independent of ADAMTS-5 activity and cartilage breakdown. Mechanical instability was introduced into the knee joints of 10-week-old male mice via surgical destabilization of the medial meniscus (DMM). Cartilage from the developing lesion in the destabilized medial meniscus and corresponding regions in sham-operated joints was harvested by microdissection at 1, 2, and 6 weeks postsurgery, and RNA was extracted, amplified, and hybridized to whole-genome microarrays.

RESULTS

Several previously identified OA-related genes, including Ptgs2, Crlf1, and Inhba, and novel genes, such as Phdla2 and Il11, were up-regulated in both WT mice and Adamts5Δcat mice, indicating that they are independent of ADAMTS-5 activity. The altered expression of other genes, including Col10a1, the sentinel marker of cartilage hypertrophy, and Wnt/β-catenin pathway genes, required ADAMTS-5 activity. Cell death pathway genes were dysregulated, and Tp53, Foxo4, and Xbp1 endoplasmic reticulum-stress transcriptional networks were activated. Analysis of degradome genes identified up-regulation of many proteases, including Mmp3, Capn2, and the novel cartilage proteases Prss46 and Klk8. Comparison with other studies identified 16 genes also dysregulated in rat and human OA as priorities for study.

CONCLUSION

We have identified, for the first time, several genes that have an ADAMTS-5-independent role in OA, identifying them as possible OA initiation candidates. This work provides new insights into the sequence of gene dysregulation and the molecular basis of cartilage destruction in OA.

Proteomic analysis of gingival tissue and alveolar bone during alveolar bone healing

Bone tissue regeneration is orchestrated by the surrounding supporting tissues and involves the build-up of osteogenic cells, which orchestrate remodeling/healing through the expression of numerous mediators and signaling molecules. Periodontal regeneration models have proven useful for studying the interaction and communication between alveolar bone and supporting soft tissue. We applied a quantitative proteomic approach to analyze and compare proteins with altered expression in gingival soft tissue and alveolar bone following tooth extraction. For target identification and validation, hard and soft tissue were extracted from mini-pigs at the indicated times after tooth extraction. From triplicate experiments, 56 proteins in soft tissue and 27 proteins in alveolar bone were found to be differentially expressed before and after tooth extraction. The expression of 21 of those proteins was altered in both soft tissue and bone. Comparison of the activated networks in soft tissue and alveolar bone highlighted their distinct responsibilities in bone and tissue healing. Moreover, we found that there is crosstalk between identified proteins in soft tissue and alveolar bone with respect to cellular assembly, organization, and communication. Among these proteins, we examined in detail the expression patterns and associated networks of ATP5B and fibronectin 1. ATP5B is involved in nucleic acid metabolism, small molecule biochemistry, and neurological disease, and fibronectin 1 is involved in cellular assembly, organization, and maintenance. Collectively, our findings indicate that bone regeneration is accompanied by a profound interaction among networks regulating cellular resources, and they provide novel insight into the molecular mechanisms involved in the healing of periodontal tissue after tooth extraction.

WNT signaling and cartilage: of mice and men

In adult articular cartilage, the extracellular matrix is maintained by a balance between the degradation and the synthesis of matrix components. Chondrocytes that sparsely reside in the matrix and rarely proliferate are the key cellular mediators for cartilage homeostasis. There are indications for the involvement of the WNT signaling pathway in maintaining articular cartilage. Various WNTs are involved in the subsequent stages of chondrocyte differentiation during development, and deregulation of WNT signaling was observed in cartilage degeneration. Even though gene expression and protein synthesis can be activated upon injury, articular cartilage has a limited ability of self-repair and efforts to regenerate articular cartilage have so far not been successful. Because WNT signaling was found to be involved in the development and maintenance of cartilage as well as in the degeneration of cartilage, interfering with this pathway might contribute to improving cartilage regeneration. However, most of the studies on elucidating the role of WNT signaling in these processes were conducted using in vitro or in vivo animal models. Discrepancies have been found in the role of WNT signaling between chondrocytes of mouse and human origin, and extrapolation of results from mouse models to the human situation remains a challenge. Elucidation of detailed WNT signaling functions will provide knowledge to improve cartilage regeneration.

Endoplasmic reticulum stress regulates rat mandibular cartilage thinning under compressive mechanical stress

Compressive mechanical stress-induced cartilage thinning has been characterized as a key step in the progression of temporomandibular joint diseases, such as osteoarthritis. However, the regulatory mechanisms underlying this loss have not been thoroughly studied. Here, we used an established animal model for loading compressive mechanical stress to induce cartilage thinning in vivo. The mechanically stressed mandibular chondrocytes were then isolated to screen potential candidates using a proteomics approach. A total of 28 proteins were identified that were directly or indirectly associated with endoplasmic reticulum stress, including protein disulfide-isomerase, calreticulin, translationally controlled tumor protein, and peptidyl-prolyl cis/trans-isomerase protein. The altered expression of these candidates was validated at both the mRNA and protein levels. The induction of endoplasmic reticulum stress by mechanical stress loading was confirmed by the activation of endoplasmic reticulum stress markers, the elevation of the cytoplasmic Ca(2+) level, and the expansion of endoplasmic reticulum membranes. More importantly, the use of a selective inhibitor to block endoplasmic reticulum stress in vivo reduced the apoptosis observed at the early stages of mechanical stress loading and inhibited the proliferation observed at the later stages of mechanical stress loading. Accordingly, the use of the inhibitor significantly restored cartilage thinning. Taken together, these results demonstrated that endoplasmic reticulum stress is significantly activated in mechanical stress-induced mandibular cartilage thinning and, more importantly, that endoplasmic reticulum stress inhibition alleviates this loss, suggesting a novel pharmaceutical strategy for the treatment of mechanical stress-induced temporomandibular joint diseases.

Src and fibroblast growth factor 2 independently regulate signaling and gene expression induced by experimental injury to intact articular cartilage

OBJECTIVE

To investigate whether cartilage injury activates protein tyrosine kinases distinct from fibroblast growth factor (FGF)-related signaling, and whether they contribute to injury-induced gene responses.

METHODS

Phosphokinases and protein tyrosine phosphorylation were assayed by Western blotting of cartilage lysates. Immunoprecipitation and Western blotting with 4G10 antibody and immunoprecipitation kinase assay were carried out. Tyrosine-phosphorylated proteins on silver-stained gels of injured cartilage lysates were identified by mass spectrometry. Messenger RNA induction in cartilage explants was assessed by quantitative reverse transcriptase-polymerase chain reaction.

RESULTS

Protein tyrosine phosphorylation occurred within seconds of injury to the surface of intact articular cartilage, as did activation of MAPKs and IKK. Activation did not reoccur upon reinjury of cultured explants. The prominent tyrosine-phosphorylated proteins focal adhesion kinase, paxillin, and cortactin were identified as substrates of Src family kinases. The Src family kinase inhibitor PP2 blocked injury-induced tyrosine phosphorylation. It did not prevent activation of the MAPKs and IKK but differentially inhibited 8 of 10 inflammatory response genes that were induced by injury. In contrast, FGF signaling blockade with PD173074 reduced all MAPK and IKK activation by ∼50% and inhibited a different subset of genes but had no effect on Src-like signaling.

CONCLUSION

Injury to the surface of intact articular cartilage activates Src-like kinases as well as MAPKs and IKK (implying NF-κB activation). FGF-2 contributes to MAPK/IKK activation but not to Src-like signaling, suggesting that the latter is a parallel pathway that also regulates the injury-induced inflammatory gene response.

Proteomic analysis of synovial fluid: insight into the pathogenesis of knee osteoarthritis

PURPOSE

We conducted a proteomic analysis of synovial fluid (SF) to identify differentially expressed proteins and analyse their correlation with osteoarthritis (OA) severity. Our primary purpose was to gain insight into the pathogenesis of OA.

METHODS

SF samples were acquired from 12 knee OA patients and 12 non-OA controls (ten had a meniscus injury, two had a discoid meniscus and all exhibited intact articular cartilage) and sequentially subjected to two-dimensional electrophoresis (2-DE). The radiographic grading of knee OA was performed using the Kellgren-Lawrence criteria. Differentially expressed proteins were identified by matrix-assisted laser desorption/ionisation time-of-flight/time-of-flight mass spectrometry (MALDI-TOF/TOF MS). Proteins of interest identified from SF were detected using an enzyme-linked immunosorbent assay (ELISA).

RESULTS

A total of 31 protein spots showed significant differences (p < 0.05) between the sample groups; 25 of the 31 spots (80.6 %) were identified as proteins of interest. Among them 20 corresponded to up-regulation and five to down-regulation in OA samples. HLA-DR was one of the proteins up-regulated, which was confirmed by ELISA.

CONCLUSIONS

These observations have implications in delineating the protein expression underlying the pathogenesis of OA and facilitate further elucidation of molecular mechanisms involved in disease progression. Substantial alterations of the protein profile in SF may be associated with OA severity.

Incorporation of an aggrecan mimic prevents proteolytic degradation of anisotropic cartilage analogs

Biomimetic scaffolds that promote regeneration and resist proteolysis are required as a tissue engineering solution to repair or replace a broad range of diseased tissues. Native corrosive environments, such as the richly enzymatic milieu of diseased articular cartilage, degrade the local extracellular matrix structure, so an implantable replacement must both replicate the healthy structure and demonstrate substantial proteolytic immunity, yet promote regeneration, if long-term functional success is to be achieved. Here, we combine magnetically aligned collagen with peptidoglycans, biosynthetic molecules that mimic proteoglycan activity but lack core proteins susceptible to proteases, to develop cartilage scaffold analogs with tailored functionality. With the incorporation of the aggrecan mimic, we demonstrate an ability to enhance bulk mechanical properties and prevent cytokine-induced degradation. Furthermore, fiber alignment in collagen scaffolds enhanced the gene expression of aggrecan, indicating cell responsiveness to anisotropy that also better replicates the natural environment of cartilage. Finally, the expression of type II collagen is enhanced with both alignment and incorporation of the aggrecan mimic, showing synergism between fiber alignment and incorporation of the aggrecan mimic. The work presented here identified a mechanistic synergy of matrix molecules and organization to prevent proteolysis while simultaneously upregulating protein expression.