Transcriptome-wide analysis of messenger RNA decay in normal and osteoarthritic human articular chondrocytes

Gene expression and immune infiltration analysis comparing lesioned and preserved subchondral bone in osteoarthritis

Background

Osteoarthritis (OA) is a degenerative disease requiring additional research. This study compared gene expression and immune infiltration between lesioned and preserved subchondral bone. The results were validated using multiple tissue datasets and experiments.

Methods

Differentially expressed genes (DEGs) between the lesioned and preserved tibial plateaus of OA patients were identified in the GSE51588 dataset. Moreover, functional annotation and protein-protein interaction (PPI) network analyses were performed on the lesioned and preserved sides to explore potential therapeutic targets in OA subchondral bones. In addition, multiple tissues were used to screen coexpressed genes, and the expression levels of identified candidate DEGs in OA were measured by quantitative real-time polymerase chain reaction. Finally, an immune infiltration analysis was conducted.

Results

A total of 1,010 DEGs were identified, 423 upregulated and 587 downregulated. The biological process (BP) terms enriched in the upregulated genes included "skeletal system development", "sister chromatid cohesion", and "ossification". Pathways were enriched in "Wnt signaling pathway" and "proteoglycans in cancer". The BP terms enriched in the downregulated genes included "inflammatory response", "xenobiotic metabolic process", and "positive regulation of inflammatory response". The enriched pathways included "neuroactive ligand-receptor interaction" and "AMP-activated protein kinase signaling". JUN, tumor necrosis factor α, and interleukin-1β were the hub genes in the PPI network. Collagen XI A1 and leucine-rich repeat-containing 15 were screened from multiple datasets and experimentally validated. Immune infiltration analyses showed fewer infiltrating adipocytes and endothelial cells in the lesioned versus preserved samples.

Conclusion

Our findings provide valuable information for future studies on the pathogenic mechanism of OA and potential therapeutic and diagnostic targets.

Analysis of RNA Polyadenylation in Healthy and Osteoarthritic Human Articular Cartilage

Polyadenylation (polyA) defines the 3' boundary of a transcript's genetic information. Its position can vary and alternative polyadenylation (APA) transcripts can exist for a gene. This causes variance in 3' regulatory domains and can affect coding sequence if intronic events occur. The distribution of polyA sites on articular chondrocyte transcripts has not been studied so we aimed to define their transcriptome-wide location in age-matched healthy and osteoarthritic knee articular cartilage. Total RNA was isolated from frozen tissue samples and analysed using the QuantSeq-Reverse 3' RNA sequencing approach, where each read runs 3' to 5' from within the polyA tail into the transcript and contains a distinct polyA site. Differential expression of transcripts was significant altered between healthy and osteoarthritic samples with enrichment for functionalities that were strongly associated with joint pathology. Subsequent examination of polyA site data allowed us to define the extent of site usage across all the samples. When comparing healthy and osteoarthritic samples, we found that differential use of polyadenylation sites was modest. However, in the genes affected, there was potential for the APA to have functional relevance. We have characterised the polyadenylation landscape of human knee articular chondrocytes and conclude that osteoarthritis does not elicit a widespread change in their polyadenylation site usage. This finding differentiates knee osteoarthritis from pathologies such as cancer where APA is more commonly observed.

Maclurin Promotes the Chondrogenic Differentiation of Bone Marrow Mesenchymal Stem Cells by Regulating miR-203a-3p/Smad1

Bone marrow mesenchymal stem cells (BMSCs) differentiate into chondrocytes under appropriate conditions, providing a method for the treatment of bone- and joint-related diseases. Previously, we found that mulberry (Morus nigra) promoted the chondrogenic differentiation of BMSCs. Although the mechanism of action and active ingredients remain unknown, several studies describe the involvement of micro-RNAs. We obtained BMSCs from the bone marrow of Sprague Dawley rats. Cell Counting Kit-8 assays showed that maclurin (25 μg/mL) treatment was not toxic to BMSCs, and compared with untreated controls, maclurin upregulated Sox9 and Col2a expression. Quantitative-PCR revealed that miR-203a-3p levels decreased significantly during chondrogenic differentiation of BMSCs promoted by maclurin. Compared with treatment with an miR-203a-3p inhibitor, miR-203a-3p mimic inhibited expression of Sox9 and Col2a as evidenced by immunofluorescence staining and Western blotting. Smad1 was identified as a key target gene of miR-203a-3p according to biological-prediction software, and miR-203a-3p negatively regulated its transcription and translation in the dual-luciferase reporter gene assay and Western blotting. Sox9 and Col2a expression was downregulated following transfection of short interfering Smad1 (siSmad1) plasmids into BMSCs. We elucidated how maclurin promotes the chondrogenic differentiation of BMSCs by regulating miR-203a-3p/Smad1, which provides a strategy for future exploration of osteoarthritis therapy through cell transplantation.

Apolipoprotein D as a Potential Biomarker and Construction of a Transcriptional Regulatory-Immune Network Associated with Osteoarthritis by Weighted Gene Coexpression Network Analysis

OBJECTIVE

Synovial inflammation influences the progression of osteoarthritis (OA). Herein, we aimed to identify potential biomarkers and analyze transcriptional regulatory-immune mechanism of synovitis in OA using weighted gene coexpression network analysis (WGCNA).

DESIGN

A data set of OA synovium samples (GSE55235) was analyzed based on WGCNA. The most significant module with OA was identified and function annotation of the module was performed, following which the hub genes of the module were identified using Pearson correlation and a protein-protein interaction network was constructed. A transcriptional regulatory network of hub genes was constructed using the TRRUST database. The immune cell infiltration of OA samples was evaluated using the single-sample Gene Set Enrichment Analysis (ssGSEA) method. The hub genes coexpressed in multiple tissues were then screened out using data sets of synovium, cartilage, chondrocyte, subchondral bone, and synovial fluid samples. Finally, transcriptional factors and coexpressed hub genes were validated via experiments.

RESULTS

The turquoise module of GSE55235 was identified via WGCNA. Functional annotation analysis showed that "mineral absorption" and "FoxO signaling pathway" were mostly enriched in the module. JUN, EGR1, FOSB, and KLF4 acted as central nodes in protein-protein interaction network and transcription factors to connect several target genes. "Activated B cell," "activated CD4T cell," "eosinophil," "neutrophil," and "type 17 T helper cell" showed high immune infiltration, while FOSB, KLF6, and MYBL2 showed significant negative correlation with type 17 T helper cell.

CONCLUSIONS

Our results suggest that the expression level of apolipoprotein D (APOD) was correlated with OA. Furthermore, transcriptional regulatory-immune network was constructed, which may contribute to OA therapy.

Identification of exosomal mRNA, lncRNA and circRNA signatures in an osteoarthritis synovial fluid-exosomal study

AIMS

Osteoarthritis (OA) is a common degenerative disease that is pathologically characterized by destruction of the joint matrix and reduction of articular chondrocytes, resulting in joint deformity and motor dysfunction. However, the molecular mechanisms governing this pathology have not been elucidated to date.

METHODS

In this study, we determined the expression levels of lncRNAs, circRNAs, and mRNAs extracted from synovial exosomes of OA and control patients. A network of circRNA/lncRNA-miRNA-mRNA interactions was established using MiRanda and TargetScan software to explore OA pathogenesis. The exosomal lncRNA, circRNA and mRNA expression profiles of the OA and control groups were analysed using LC human competing endogenous RNA (ceRNA) microarrays. The differentially expressed genes were analysed to determine their potential roles in the pathogenesis of OA by bioinformatic analysis.

RESULTS

There were 52 mRNAs, 196 lncRNAs and 98 circRNAs differentially expressed in synovial exosomes between osteoarthritis synovial and the control group. The final ceRNA network of lncRNAs and circRNAs exhibited a complex interaction between ncRNA and mRNA related to OA pathological mechanisms. An intersection analysis of the ceRNA network showed that 22 miRNAs, 45 lncRNAs, and 34 circRNAs enriched in the PI3K/Akt and autophagy pathways correlated with 7 mRNAs and may play important roles in OA pathological mechanisms.

CONCLUSION

Our work analysed mRNA/lncRNA/circRNA expression and displayed the ceRNA network of lncRNAs and circRNAs to profile the pathogenesis of OA in synovial exosomes. The results of this study may help to elucidate the pathogenesis of OA and may provide important references for further research attempting to identify more effective targets for the diagnosis and therapy of OA.

The first human induced pluripotent stem cell line of Kashin-Beck disease reveals involvement of heparan sulfate proteoglycan biosynthesis and PPAR pathway

Kashin-Beck disease (KBD) is an endemic osteochondropathy. Due to a lack of suitable animal or cellular disease models, the research progress on KBD has been limited. Our goal was to establish the first disease-specific human induced pluripotent stem cell (hiPSC) cellular disease model of KBD, and to explore its etiology and pathogenesis exploiting transcriptome sequencing. HiPSCs were reprogrammed from dermal fibroblasts of two KBD and one healthy control donor via integration-free vectors. Subsequently, hiPSCs were differentiated into chondrocytes through three-week culture. Gene expression profiles in KBD, normal primary chondrocytes, and hiPSC-derived chondrocytes were defined by RNA sequencing. A Venn diagram was constructed to show the number of shared differentially expressed genes (DEGs) between KBD and normal. Gene oncology and Kyoto Encyclopedia of Genes and Genomes annotations were performed, and six DEGs were further validated in other individuals by RT-qPCR. KBD cellular disease models were successfully established by generation of hiPSC lines. Seventeen consistent and significant DEGs present in all compared groups (KBD and normal) were identified. RT-qPCR validation gave consistent results with the sequencing data. Glycosaminoglycan biosynthesis-heparan sulfate/heparin; PPAR signaling pathway; and cell adhesion molecules (CAMs) were identified to be significantly altered in KBD. Differentiated chondrocytes derived from KBD-origin hiPSCs provide the first cellular disease model for etiological studies of KBD. This study also provides new sights into the pathogenesis and etiology of KBD and is likely to inform the development of targeted therapeutics for its treatment.

Effects of miR-146a-5p on chondrocyte interleukin-1-induced inflammation and apoptosis involving thioredoxin interacting protein regulation

Objective

Osteoarthritis (OA) is a chronic degenerative arthropathy characterized by articular cartilage degeneration, subchondral osteosclerosis, and hyperosteogeny. MicroRNAs (miRNAs) play an important regulatory role in its pathological development, so this study explored the effect and potential mechanism of miR-146a-5p in interleukin (IL)-1β-induced OA cartilage injury.

Methods

The human chondrosarcoma cell line SW1353 and normal human chondrocytes C28/I2 were stimulated by IL-1β to construct the OA chondrocyte model. miR-146a-5p and thioredoxin interacting protein (TXNIP) expression levels were detected by quantitative real-time (qRT)-PCR and western blot. Their expression was modified by transfecting an miR-146a-5p inhibitor, mimic, and pcDNA-TXNIP. The relationship between miR-146a-5p and TXNIP was analyzed by the dual-luciferase assay, while cell viability, apoptosis, and inflammatory expression were determined by cell counting, TUNEL staining, and ELISA, respectively.

Results

miR-146a-5p expression was upregulated in SW1353 and C28/I2 cells stimulated by IL-1β. miR-146a-5p knockdown significantly enhanced cell activity, inhibited inflammatory factor expression, and reduced cell apoptosis. The dual-luciferase assay revealed TXNIP as a target gene of miR-146a-5p and suggested that miR-146a-5p promotion of OA damage could be reversed by upregulating TXNIP.

Conclusion

These results suggest that miR-146a-5p inhibits cell proliferation and promotes apoptosis and the inflammatory response in OA cartilage injury by modulating TXNIP.

Targeting downstream subcellular YAP activity as a function of matrix stiffness with Verteporfin-encapsulated chitosan microsphere attenuates osteoarthritis

Changes in the stiffness of chondrocyte extracellular matrix (ECM) are involved in the pathological progression of osteoarthritis (OA). However, the downstream responses of cartilage ECM stiffness are still unclear. YAP (Yes-associated protein) has been extensively studied as a mechanotransducer, we thus hypothesized that by targeting the downstream molecule activity of ECM stiffness could maintain chondrocyte phenotype and prevent cartilage degeneration in OA. Here, we showed that human cartilage matrix stiffened during pathological progression of OA, and the chondrocyte YAP activity was associated with ECM stiffness. We then mimicked the physiological and pathological stiffness of human cartilage by using PDMS-based substrates, and found that YAP was activated in chondrocytes seeded on stiff substrate, gradually losing their phenotype. In addition, it was observed that YAP was also significantly activated in mice OA development, and conditional knockout (cKO) of YAP in mice preserved collagen II expression and protected cartilage from degeneration in the OA model. Furthermore, intra-articular injection of YAP-selective inhibitor, Verteporfin, significantly maintained cartilage homeostasis in mice OA model. This study indicates that the application of mechanotransducer-targeted drugs could be a potential therapeutic approach for cartilage repair in OA.

MicroRNA miR-1002 Enhances NMNAT-Mediated Stress Response by Modulating Alternative Splicing

Understanding endogenous regulation of stress resistance and homeostasis maintenance is critical to developing neuroprotective therapies. Nicotinamide mononucleotide adenylyltransferase (NMNAT) is a conserved essential enzyme that confers extraordinary protection and stress resistance in many neurodegenerative disease models. Drosophila Nmnat is alternatively spliced to two mRNA variants, RA and RB. RB translates to protein isoform PD with robust protective activity and is upregulated upon stress to confer enhanced neuroprotection. The mechanisms regulating the alternative splicing and stress response of NMNAT remain unclear. We have discovered a Drosophila microRNA, dme-miR-1002, which promotes the splicing of NMNAT pre-mRNA to RB by disrupting a pre-mRNA stem-loop structure. NMNAT pre-mRNA is preferentially spliced to RA in basal conditions, whereas miR-1002 enhances NMNAT PD-mediated stress protection by binding via RISC component Argonaute1 to the pre-mRNA, facilitating the splicing switch to RB. These results outline a new process for microRNAs in regulating alternative splicing and modulating stress resistance.

Translational regulation contributes to the secretory response of chondrocytic cells following exposure to interleukin-1β

Osteoarthritis is a chronic disease characterized by the loss of articular cartilage in synovial joints through a process of extracellular matrix destruction that is strongly associated with inflammatory stimuli. Chondrocytes undergo changes to their protein translational capacity during osteoarthritis, but a study of how disease-relevant signals affect chondrocyte protein translation at the transcriptomic level has not previously been performed. In this study, we describe how the inflammatory cytokine interleukin 1-β (IL-1β) rapidly affects protein translation in the chondrocytic cell line SW1353. Using ribosome profiling we demonstrate that IL-1β induced altered translation of inflammatory-associated transcripts such as NFKB1, TNFAIP2, MMP13, CCL2, and CCL7, as well as a number of ribosome-associated transcripts, through differential translation and the use of multiple open reading frames. Proteomic analysis of the cellular layer and the conditioned media of these cells identified changes in a number of the proteins that were differentially translated. Translationally regulated secreted proteins included a number of chemokines and cytokines, underlining the rapid, translationally mediated inflammatory cascade that is initiated by IL-1β. Although fewer cellular proteins were found to be regulated in both ribosome profiling and proteomic data sets, we did find increased levels of SOD2, indicative of redox changes within SW1353 cells being modulated at the translational level. In conclusion, we have produced combined ribosome profiling and proteomic data sets that provide a valuable resource in understanding the processes that occur during cytokine stimulation of chondrocytic cells.

Identification of TGFβ-related genes regulated in murine osteoarthritis and chondrocyte hypertrophy by comparison of multiple microarray datasets

OBJECTIVE

Osteoarthritis (OA) is a joint disease characterized by progressive degeneration of articular cartilage. Some features of OA, including chondrocyte hypertrophy and focal calcification of articular cartilage, resemble the endochondral ossification processes. Alterations in transforming growth factor β (TGFβ) signaling have been associated with OA as well as with chondrocyte hypertrophy. Our aim was to identify novel candidate genes implicated in chondrocyte hypertrophy during OA pathogenesis by determining which TGFβ-related genes are regulated during murine OA and endochondral ossification.

METHODS

A list of 580 TGFβ-related genes, including TGFβ signaling pathway components and TGFβ-target genes, was generated. Regulation of these TGFβ-related genes was assessed in a microarray of murine OA cartilage: 1, 2 and 6 weeks after destabilization of the medial meniscus (DMM). Subsequently, genes regulated in the DMM model were studied in two independent murine microarray datasets on endochondral ossification: the growth plate and transient embryonic cartilage (joint development).

RESULTS

A total of 106 TGFβ-related genes were differentially expressed in articular cartilage of DMM-operated mice compared to sham-control. From these genes, 43 were similarly regulated during chondrocyte hypertrophy in the growth plate or embryonic joint development. Among these 43 genes, 18 genes have already been associated with OA. The remaining 25 genes were considered as novel candidate genes involved in OA pathogenesis and endochondral ossification. In supplementary data of published human OA microarrays we found indications that 15 of the 25 novel genes are indeed regulated in articular cartilage of human OA patients.

CONCLUSION

By focusing on TGFβ-related genes during OA and chondrocyte hypertrophy in mice, we identified 18 known and 25 new candidate genes potentially implicated in phenotypical changes in chondrocytes leading to OA. We propose that 15 of these candidates warrant further investigation as therapeutic target for OA as they are also regulated in articular cartilage of OA patients.

Pathways to understanding the genomic aetiology of osteoarthritis

Osteoarthritis is a common, complex disease with no curative therapy. In this review, we summarize current knowledge on disease aetiopathogenesis and outline genetics and genomics approaches that are helping catalyse a much-needed improved understanding of the biological underpinning of disease development and progression.

Integrative epigenomics, transcriptomics and proteomics of patient chondrocytes reveal genes and pathways involved in osteoarthritis

Osteoarthritis (OA) is a common disease characterized by cartilage degeneration and joint remodeling. The underlying molecular changes underpinning disease progression are incompletely understood. We investigated genes and pathways that mark OA progression in isolated primary chondrocytes taken from paired intact versus degraded articular cartilage samples across 38 patients undergoing joint replacement surgery (discovery cohort: 12 knee OA, replication cohorts: 17 knee OA, 9 hip OA patients). We combined genome-wide DNA methylation, RNA sequencing, and quantitative proteomics data. We identified 49 genes differentially regulated between intact and degraded cartilage in at least two -omics levels, 16 of which have not previously been implicated in OA progression. Integrated pathway analysis implicated the involvement of extracellular matrix degradation, collagen catabolism and angiogenesis in disease progression. Using independent replication datasets, we showed that the direction of change is consistent for over 90% of differentially expressed genes and differentially methylated CpG probes. AQP1, COL1A1 and CLEC3B were significantly differentially regulated across all three -omics levels, confirming their differential expression in human disease. Through integration of genome-wide methylation, gene and protein expression data in human primary chondrocytes, we identified consistent molecular players in OA progression that replicated across independent datasets and that have translational potential.

MicroRNA Profiling in Cartilage Ageing

Osteoarthritis (OA) is the most common age-related joint disorder in man. MicroRNAs (miRNA), a class of small noncoding RNAs, are potential therapeutic targets for regulating molecular mechanisms in both disease and ageing. Whilst there is an increasing amount of research on the roles of miRNAs in ageing, there has been scant research on age-related changes in miRNA in a cartilage. We undertook a microarray study on young and old human cartilages. Findings were validated in an independent cohort. Contrasts between these samples identified twenty differentially expressed miRNAs in a cartilage from old donors, derived from an OA environment which clustered based on OA severity. We identified a number of recognised and novel miRNAs changing in cartilage ageing and OA including miR-126: a potential new candidate with a role in OA pathogenesis. These analyses represent important candidates that have the potential as cartilage ageing and OA biomarkers and therapeutic targets.

Systems approaches in osteoarthritis: Identifying routes to novel diagnostic and therapeutic strategies

Systems orientated research offers the possibility of identifying novel therapeutic targets and relevant diagnostic markers for complex diseases such as osteoarthritis. This review demonstrates that the osteoarthritis research community has been slow to incorporate systems orientated approaches into research studies, although a number of key studies reveal novel insights into the regulatory mechanisms that contribute both to joint tissue homeostasis and its dysfunction. The review introduces both top-down and bottom-up approaches employed in the study of osteoarthritis. A holistic and multiscale approach, where clinical measurements may predict dysregulation and progression of joint degeneration, should be a key objective in future research. The review concludes with suggestions for further research and emerging trends not least of which is the coupled development of diagnostic tests and therapeutics as part of a concerted effort by the osteoarthritis research community to meet clinical needs. © 2017 The Authors. Journal of Orthopaedic Research Published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 35:1573-1588, 2017.

Increased Activity of the Chondrocyte Translational Apparatus Accompanies Osteoarthritic Changes in Human and Rodent Knee Cartilage

OBJECTIVE

Degeneration of articular cartilage is central to the pathology of osteoarthritis (OA). However, the molecular mechanisms leading to these irreversible changes are still poorly understood. This study was undertaken to investigate how changes in the chondrocyte translational apparatus may contribute to the development and progression of knee OA.

METHODS

Articular cartilage from the knees of normal healthy subjects and patients with OA was used to analyze the activity of different components of the translational machinery. Chondrocytes isolated from lesional and nonlesional areas of the human OA cartilage were used to estimate the relative rate of protein synthesis by metabolic labeling. Experimental OA was induced by transection of the anterior cruciate ligament of rats to investigate changes in the translational apparatus associated with OA. The role of interleukin-1β (IL-1β) signaling was assessed in vitro using rat articular chondrocytes. In human or rodent knee cartilage, messenger RNA expression was analyzed by quantitative polymerase chain reaction, and protein levels were determined by immunohistochemistry and Western blotting.

RESULTS

Several novel traits of OA chondrocytes were identified, including up-regulation of the serine/threonine kinases Akt-2 and Akt-3 at the posttranscriptional level and an increased rate of total protein synthesis, likely attributable to inactivation of eukaryotic initiation factor 4E binding protein 1 (4E-BP1), a known repressor of cap-dependent translation. Inactivation of 4E-BP1 was dependent on the activity of mechanistic target of rapamycin and was crucial for the up-regulation of protein synthesis in general and expression of matrix metalloproteinase 13 and ADAMTS-5 in particular. In addition, treatment of articular chondrocytes with IL-1β led to inactivation of 4E-BP1 and up-regulation of protein synthesis.

CONCLUSION

Precise control of protein synthesis is vital for cartilage homeostasis, and its dysregulation contributes to the molecular pathology of OA. The results of this study therefore identify a novel set of potential therapeutic targets to ameliorate the effects of knee OA.

Gene expression changes in damaged osteoarthritic cartilage identify a signature of non-chondrogenic and mechanical responses

OBJECTIVES

Joint degeneration in osteoarthritis (OA) is characterised by damage and loss of articular cartilage. The pattern of loss is consistent with damage occurring only where the mechanical loading is high. We have investigated using RNA-sequencing (RNA-seq) and systems analyses the changes that occur in damaged OA cartilage by comparing it with intact cartilage from the same joint.

METHODS

Cartilage was obtained from eight OA patients undergoing total knee replacement. RNA was extracted from cartilage on the damaged distal medial condyle (DMC) and the intact posterior lateral condyle (PLC). RNA-seq was performed to identify differentially expressed genes (DEGs) and systems analyses applied to identify dysregulated pathways.

RESULTS

In the damaged OA cartilage, there was decreased expression of chondrogenic genes SOX9, SOX6, COL11A2, COL9A1/2/3, ACAN and HAPLN1; increases in non-chondrogenic genes COL1A1, COMP and FN1; an altered pattern of secreted proteinase expression; but no expression of major inflammatory cytokines. Systems analyses by PhenomeExpress revealed significant sub-networks of DEGs including mitotic cell cycle, Wnt signalling, apoptosis and matrix organisation that were influenced by a core of altered transcription factors (TFs), FOSL1, AHR, E2F1 and FOXM1.

CONCLUSIONS

Gene expression changes in damaged cartilage suggested a signature non-chondrogenic response of altered matrix protein and secreted proteinase expression. There was evidence of a damage response in this late OA cartilage, which surprisingly showed features detected experimentally in the early response of cartilage to mechanical overload. PhenomeExpress analysis identified a hub of DEGs linked by a core of four differentially regulated TFs.

Functional genomics in osteoarthritis: Past, present, and future

Osteoarthritis (OA) is a common complex disease of high public health burden. OA is characterized by the degeneration of affected joints leading to pain and reduced mobility. Over the last few years, several studies have focused on the genomic changes underpinning OA. Here, we provide a comprehensive overview of genome-wide, non-hypothesis-driven functional genomics (methylation, gene, and protein expression) studies of knee and hip OA in humans. Individual studies have generally been limited in sample size and hence power, and have differed in their approaches; nonetheless, some common themes have started to emerge, notably the role played by biological processes related to the extracellular matrix, immune response, the WNT pathway, angiogenesis, and skeletal development. Larger-scale studies and streamlined, robust methodologies will be needed to further elucidate the biological etiology of OA going forward. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1105-1110, 2016.

4-Methylumbelliferone Diminishes Catabolically Activated Articular Chondrocytes and Cartilage Explants via a Mechanism Independent of Hyaluronan Inhibition

Depletion of the cartilage proteoglycan aggrecan is one of the earliest events that occurs in association with osteoarthritis. This loss is often accompanied by a coordinate loss in another glycosaminoglycan, hyaluronan. Chondrocytes experimentally depleted of cell-associated hyaluronan respond by switching to a pro-catabolic metabolism that includes enhanced production of endogenous inflammatory mediators and increased synthesis of matrix metalloproteinases. Hyaluronan turnover is also increased. Together, such a response provides for possible establishment of a self-perpetuating spiral of events that maintains or prolongs the pro-catabolic state. Chondrocytes or cartilage can also be activated by treatment with pro-inflammatory cytokines and mediators such as IL-1β, TNFα, LPS, fibronectin fragments, and hyaluronan oligosaccharides. To determine the mechanism of chondrocyte activation due to hyaluronan loss, a depletion method was required that did not include degrading the hyaluronan. In recent years, several laboratories have used the coumarin derivative, 4-methylumbelliferone, as a potent inhibitor of hyaluronan biosynthesis, due in part to its ability to sequester intracellular UDP-glucuronic acid and inhibition of hyaluronan synthase transcription. However, contrary to our expectation, although 4-methylumbelliferone was indeed an inhibitor of hyaluronan biosynthesis, this depletion did not give rise to an activation of chondrocytes or cartilage. Rather, 4-methylumbelliferone directly and selectively blocked gene products associated with the pro-catabolic metabolic state of chondrocytes and did so through a mechanism preceding and independent of hyaluronan inhibition. These data suggest that 4-methylumbelliferone has additional useful applications to block pro-inflammatory cell activation events but complicates how it is used for defining functions related to hyaluronan.

Osteoarthritis year in review 2015: biology

This review highlights a selection of recently published literature in the area of osteoarthritis biology. Major themes transpiring from a PubMed search covering the year between the 2014 and the 2015 Osteoarthritis Research Society International (OARSI) World Congress are explored. Inflammation emerged as a significant theme, revealing complex pathways that drive dramatic changes in cartilage homeostasis and in the synovium. Highlights include a homeostatic role for CXC chemokines in cartilage, identification of the zinc-ZIP8-MTF1 axis as an essential regulator of cartilage catabolism, and the discovery that a small aggrecan fragment can have catabolic and pro-inflammatory effects through Toll-like receptor 2. Synovitis can promote joint damage, partly through alarmins such as S100A8. Synovitis and synovial expression of the pro-algesic neurotrophin, Nerve Growth Factor, are associated with pain. Increasingly, researchers are considering specific pathogenic pathways that may operate in distinct subsets of osteoarthritis associated with distinct risk factors, including obesity, age, and joint injury. In obesity, the contribution of metabolic factors and diet is under intense investigation. The role of autophagy and oxidative stress in age-related osteoarthritis has been further explored. This approach may open avenues for targeted treatment of distinct phenotypes of osteoarthritis. Finally, a small selection of novel analgesic targets in the periphery is briefly discussed, including calcitonin gene-related peptide and the neuronal sodium voltage-gated channels, Nav1.7 and Nav1.8.

Human genome-wide expression analysis reorients the study of inflammatory mediators and biomechanics in osteoarthritis

A major objective of this article is to examine the research implications of recently available genome-wide expression profiles of cartilage from human osteoarthritis (OA) joints. We propose that, when viewed in the light of extensive earlier work, this novel data provides a unique opportunity to reorient the design of experimental systems toward clinical relevance. Specifically, in the area of cartilage explant biology, this will require a fresh evaluation of existing paradigms, so as to optimize the choices of tissue source, cytokine/growth factor/nutrient addition, and biomechanical environment for discovery. Within this context, we firstly discuss the literature on the nature and role of potential catabolic mediators in OA pathology, including data from human OA cartilage, animal models of OA, and ex vivo studies. Secondly, due to the number and breadth of studies on IL-1β in this area, a major focus of the article is a critical analysis of the design and interpretation of cartilage studies where IL-1β has been used as a model cytokine. Thirdly, the article provides a data-driven perspective (including genome-wide analysis of clinical samples, studies on mutant mice, and clinical trials), which concludes that IL-1β should be replaced by soluble mediators such as IL-17 or TGF-β1, which are much more likely to mimic the disease in OA model systems. We also discuss the evidence that changes in early OA can be attributed to the activity of such soluble mediators, whereas late-stage disease results more from a chronic biomechanical effect on the matrix and cells of the remaining cartilage and on other local mediator-secreting cells. Lastly, an updated protocol for in vitro studies with cartilage explants and chondrocytes (including the use of specific gene expression arrays) is provided to motivate more disease-relevant studies on the interplay of cytokines, growth factors, and biomechanics on cellular behavior.

Progression of Gene Expression Changes following a Mechanical Injury to Articular Cartilage as a Model of Early Stage Osteoarthritis

An impact injury model of early stage osteoarthritis (OA) progression was developed using a mechanical insult to an articular cartilage surface to evaluate differential gene expression changes over time and treatment. Porcine patellae with intact cartilage surfaces were randomized to one of three treatments: nonimpacted control, axial impaction (2000 N), or a shear impaction (500 N axial, with tangential displacement to induce shear forces). After impact, the patellae were returned to culture for 0, 3, 7, or 14 days. At the appropriate time point, RNA was extracted from full-thickness cartilage slices at the impact site. Quantitative real-time PCR was used to evaluate differential gene expression for 18 OA related genes from four categories: cartilage matrix, degradative enzymes and inhibitors, inflammatory response and signaling, and cell apoptosis. The shear impacted specimens were compared to the axial impacted specimens and showed that shear specimens more highly expressed type I collagen (Col1a1) at the early time points. In addition, there was generally elevated expression of degradative enzymes, inflammatory response genes, and apoptosis markers at the early time points. These changes suggest that the more physiologically relevant shear loading may initially be more damaging to the cartilage and induces more repair efforts after loading.