The chemokine receptor CCR5 plays a role in post-traumatic cartilage loss in mice, but does not affect synovium and bone

CCL4/CCR5 regulates chondrocyte biology and OA progression

BACKGROUND

Osteoarthritis (OA) is a common musculoskeletal disorder characterized by chondrocyte apoptosis and extracellular matrix degradation. This study aimed to investigate the role of CCL4/CCR5 in regulating chondrocyte apoptosis and reactive oxygen species (ROS) levels in OA progression.

METHODS

Bioinformatics analysis was employed to identify CCL4 as the target gene, following which primary chondrocytes were treated with varying concentrations of CCL4. Apoptosis rate of chondrocytes and ROS levels were assessed using flow cytometry. The mechanism by which CCL4 regulated the extracellular matrix was investigated through Western blot and Immunofluorescence analyses. Additionally, maraviroc, a CCR5 inhibitor, was administered to chondrocytes in order to explore the potential signaling pathway of CCL4/CCR5.

RESULTS

Our study found that CCL4 was predominantly up-regulated among the top 10 hub genes identified in RNA-sequencing analysis. Validation through quantitative polymerase chain reaction (qPCR) confirmed elevated CCL4 expression in patients with Hip joint osteoarthritis, knee joint osteoarthritis, and facet joint osteoarthritis. The upregulation of CCL4 was associated with an increase in chondrocyte apoptosis and ROS levels. Mechanistically, CCL4, upon binding to its receptor CCR5, triggered the downstream phosphorylation of P65 in the nuclear factor-κB (NF-κB) signaling pathway. In vitro experiments demonstrated that treatment with maraviroc mitigated chondrocyte apoptosis, reduced intracellular ROS levels, and attenuated extracellular matrix degradation.

CONCLUSION

The study highlights the critical role of CCL4/CCR5 in modulating chondrocyte apoptosis and ROS levels in OA progression. Targeting this pathway may offer promising therapeutic interventions for mitigating the pathogenic mechanisms associated with OA.

Uncovering the shared neuro-immune-related regulatory mechanisms between spinal cord injury and osteoarthritis

Adults with spinal cord injury (SCI), a destructive neurological injury, have a significantly higher incidence of osteoarthritis (OA), a highly prevalent chronic joint disorder. This study aimed to dissect the neuroimmune-related regulatory mechanisms of SCI and OA using bioinformatics analysis. Using microarray data from the Gene Expression Omnibus database, differentially expressed genes (DEGs) were screened between SCI and sham samples and between OA and control samples. Common DEGs were used to construct a protein-protein interaction (PPI) network. Weighted gene co-expression network analysis (WGCNA) was used to mine SCI- and OA-related modules. Shared miRNAs were identified, and target genes were predicted using the Human MicroRNA Disease Database (HMDD) database. A miRNA-gene-pathway regulatory network was constructed with overlapping genes, miRNAs, and significantly enriched pathways. Finally, the expression of the identified genes and miRNAs was verified using RT-qPCR. In both the SCI and OA groups, 185 common DEGs were identified, and three hub clusters were obtained from the PPI network. WGCNA revealed three SCI-related modules and two OA-related modules. There were 43 overlapping genes between the PPI network clusters and the WGCNA network modules. Seventeen miRNAs shared between patients with SCI and OA were identified. A regulatory network consisting of five genes, six miRNAs, and six signaling pathways was constructed. Upregulation of CD44, TGFBR1, CCR5, and IGF1, while lower levels of miR-125b-5p, miR-130a-3p, miR-16-5p, miR-204-5p, and miR-204-3p in both SCI and OA were successfully verified using RT-qPCR. Our study suggests that a miRNA-gene-pathway network is implicated in the neuroimmune-related regulatory mechanisms of SCI and OA. CD44, TGFBR1, CCR5, and IGF1, and their related miRNAs (miR-125b-5p, miR-130a-3p, miR-16-5p, miR-204-5p, and miR-204-3p) may serve as promising biomarkers and candidate therapeutic targets for SCI and OA.

CC chemokines and receptors in osteoarthritis: new insights and potential targets

Osteoarthritis (OA) is a prevalent degenerative disease accompanied by the activation of innate and adaptive immune systems-associated inflammatory responses. Due to the local inflammation, the expression of various cytokines was altered in affected joints, including CC motif chemokine ligands (CCLs) and their receptors (CCRs). As essential members of chemokines, CCLs and CCRs played an important role in the pathogenesis and treatment of OA. The bindings between CCLs and CCRs on the chondrocyte membrane promoted chondrocyte apoptosis and the release of multiple matrix-degrading enzymes, which resulted in cartilage degradation. In addition, CCLs and CCRs had chemoattractant functions to attract various immune cells to osteoarthritic joints, further leading to the aggravation of local inflammation. Furthermore, in the nerve endings of joints, CCLs and CCRs, along with several cellular factors, contributed to pain hypersensitivity by releasing neurotransmitters in the spinal cord. Given this family's diverse and complex functions, targeting the functional network of CCLs and CCRs is a promising strategy for the prognosis and treatment of OA in the future.

Harnessing cell-material interactions to control stem cell secretion for osteoarthritis treatment

Osteoarthritis (OA) is the most common debilitating joint disease, yet there is no curative treatment for OA to date. Delivering mesenchymal stromal cells (MSCs) as therapeutic cells to mitigate the inflammatory symptoms associated with OA is attracting increasing attention. In principle, MSCs could respond to the pro-inflammatory microenvironment of an OA joint by the secretion of anti-inflammatory, anti-apoptotic, immunomodulatory and pro-regenerative factors, therefore limiting pain, as well as the disease development. However, the microenvironment of MSCs is known to greatly affect their survival and bioactivity, and using tailored biomaterial scaffolds could be key to the success of intra-articular MSC-based therapies. The aim of this review is to identify and discuss essential characteristics of biomaterial scaffolds to best promote MSC secretory functions in the context of OA. First, a brief introduction to the OA physiopathology is provided, followed by an overview of the MSC secretory functions, as well as the current limitations of MSC-based therapy. Then, we review the current knowledge on the effects of cell-material interactions on MSC secretion. These considerations allow us to define rational guidelines for next-generation biomaterial design to improve the MSC-based therapy of OA.

Association between an Increased Serum CCL5 Level and Pathophysiology of Degenerative Joint Disease in the Temporomandibular Joint in Females

Degenerative joint disease of the temporomandibular joints (DJD-TMJ) clinically manifests with symptoms such as orofacial pain, joint sounds and limited jaw movements. Our research group previously reported the functional necessity of a chemokine-chemokine receptor axis of CCL5-CCR5 in osteoclasts. Accumulated studies reported that this axis was involved in the pathogenesis of bone and joint destructive diseases, suggesting CCL5 as a potent biomarker. This study investigated whether or not the serum level of CCL5 can be a biomarker of DJD-TMJ and concomitantly analyzed changes in the serum and urine levels of bone markers to see whether or not changes in the rate of bone metabolism were predisposing. We enrolled 17 female subjects with diagnosed DJD-TMJ and sexually and age-matched 17 controls. The serum CCL5 level in DJD-TMJ subjects was significantly higher than that in the control subjects. Multivariate analyses indicated an association between an augmented CCL5 level and the rate of bone metabolism, especially in relatively young DJD-TMJ subjects without other systemic symptoms. A principal component analysis of serum markers and our pharmacological experiment using a postmenopausal model of ovariectomized rats suggested that an augmented serum CCL5 level specifically reflected DJD-TMJ and that covert changes in the rate of bone metabolism predisposed individuals to DJD-TMJ.

An osteoarthritis subtype characterized by synovial lipid metabolism disorder and fibroblast-like synoviocyte dysfunction

Background

The heterogeneity of osteoarthritis (OA) significantly limits the effectiveness of pharmacological treatments in an unselected patient population. In this context, the identification of OA subtypes is meaningful for the development of therapies that target specific types of OA pathogenesis.

Methods

Expression array profiles of 70 OA and 36 control synovial samples were extracted from the GEO database. Unsupervised consensus clustering was performed based on the most variable genes to identify OA subclusters. Next, Joint samples from OA patients were obtained. We divided the OA patient into two subpopulations according to synovial ADCY7 levels. Synovium and cartilage samples from different OA subpopulations were evaluated. In addition, we established a high-fat diet (HFD)-induced rat OA model. We evaluated OA progression, lipid metabolism, synovitis and fibroblast-like synoviocytes (FLS) function in this HFD-induced OA model.

Results

70 OA patients were categorized into three distinct subclusters. We noted that one subcluster was characterized by synovial lipid metabolism disorder GO terms. We further identified the most noticeable KEGG pathway “Regulation of lipolysis in adipocytes” in this subcluster as well as the most significantly differentially expressed gene, ADCY7. We found that the ADCY7 high expressing group (32.6%) exhibited features of synovial inflammatory lipolysis epithelial-mesenchymal transition (EMT) tendency, as well as faster join space narrowing. The HFD induced OA-like degeneration in rat joints. We observed similar synovial inflammatory lipolysis and EMT in FLS, characterized by higher proliferative and invasive activity and elevated proinflammatory and procatabolic properties. ADCY7 was highly expressed in the synovium of the HFD-OA model rats and the inhibition of ADCY7 effectively attenuated these HFD-induced degenerative changes as well as synovial inflammatory lipolysis and FLS dysfunction. In HFD-FLSs, ADCY7 promoted the phosphorylation of PKA as well as its downstream lipid droplet-associated protein PLIN1 and hormone-sensitive lipase (HSL). The inhibition of PKA largely alleviated ADCY7-mediated HFD-FLS dysfunction.

Conclusions

We described a synovial EMT and lipid metabolism disorder in the pathogenesis of OA. This novel mechanism may represent a currently undefined OA subtype. ADCY7 is a potential molecular marker of this pathomechanism.

The Translational potential of this article

Utilizing synovial samples from OA patients, we identified a subpopulation with high ADCY7 expression. This may represent a currently undefined OA subtype and explain the clinical phenomenon of more severe synovial inflammation in obese OA patients. In addition, we established an HFD-induced OA rat model and found an upregulation of ADCY7 in the synovium. We confirmed that the inhibition of ADCY7 could effectively attenuate HFD-induced degenerative changes as well as the inflammatory lipolysis and FLS dysfunction observed in the rat model. This suggests that ADCY7 and its downstream pathways are potential pharmacological targets for treating this lipid-metabolism-disorder-related OA mechanism.

G Protein-Coupled Receptors in Osteoarthritis

Osteoarthritis (OA) is the most common chronic joint disease characterized, for which there are no available therapies being able to modify the progression of OA and prevent long-term disability. Critical roles of G-protein coupled receptors (GPCRs) have been established in OA cartilage degeneration, subchondral bone sclerosis and chronic pain. In this review, we describe the pathophysiological processes targeted by GPCRs in OA, along with related preclinical model and/or clinical trial data. We review examples of GPCRs which may offer attractive therapeutic strategies for OA, including receptors for cannabinoids, hormones, prostaglandins, fatty acids, adenosines, chemokines, and discuss the main challenges for developing these therapies.

Pulsed Electromagnetic Field Attenuates Osteoarthritis Progression in a Murine Destabilization-Induced Model through Inhibition of TNF-α and IL-6 Signaling

OBJECTIVE

To investigate the anti-inflammatory effects and mechanisms of pulsed electromagnetic field (PEMF) in the treatment of osteoarthritis (OA) in the destabilization of the medial meniscus (DMM) mice.

DESIGN

Ten-week-old male wild-type (WT), interleukin (IL)-6-/- and tumor necrosis factor (TNF)-α-/- mice undergoing DMM surgery were randomly divided into 2 groups (n = 10 each): mice with PEMF exposure and mice with sham PEMF exposure. PEMF (75 Hz, 3.8 mT, 1 h/day) or sham PEMF was applied for 4 weeks. Pain behavior of mice, histological assessment of cartilage and synovium, micro-CT (computed tomography) analysis of bone, real-time polymerase chain reaction, and immunohistochemical staining of cartilage were performed.

RESULTS

After DMM surgery, PEMF had a beneficial effect on pain, cartilage degeneration, synovitis, and trabecular bone microarchitecture in WT mice; these protective effects were reduced in IL-6-/- and TNF-α-/- mice. In addition, PEMF downregulated IL-6 and TNF-α expression in cartilage. PEMF also ameliorated cartilage matrix, chondrocyte apoptosis, and autophagy, while deletion of IL-6 or TNF-α suppressed the effects.

CONCLUSIONS

PEMF attenuates structural and functional progression of OA through inhibition of TNF-α and IL-6 signaling. The protective effects of PEMF on chondrocyte apoptosis and autophagy are regulated by TNF-α and IL-6 signaling.

Monocytes, Macrophages, and Their Potential Niches in Synovial Joints - Therapeutic Targets in Post-Traumatic Osteoarthritis?

Synovial joints are complex structures that enable normal locomotion. Following injury, they undergo a series of changes, including a prevalent inflammatory response. This increases the risk for development of osteoarthritis (OA), the most common joint disorder. In healthy joints, macrophages are the predominant immune cells. They regulate bone turnover, constantly scavenge debris from the joint cavity and, together with synovial fibroblasts, form a protective barrier. Macrophages thus work in concert with the non-hematopoietic stroma. In turn, the stroma provides a scaffold as well as molecular signals for macrophage survival and functional imprinting: “a macrophage niche”. These intricate cellular interactions are susceptible to perturbations like those induced by joint injury. With this review, we explore how the concepts of local tissue niches apply to synovial joints. We introduce the joint micro-anatomy and cellular players, and discuss their potential interactions in healthy joints, with an emphasis on molecular cues underlying their crosstalk and relevance to joint functionality. We then consider how these interactions are perturbed by joint injury and how they may contribute to OA pathogenesis. We conclude by discussing how understanding these changes might help identify novel therapeutic avenues with the potential of restoring joint function and reducing post-traumatic OA risk.

Cytokines and Chemokines Involved in Osteoarthritis Pathogenesis

Osteoarthritis is a common cause of disability worldwide. Although commonly referred to as a disease of the joint cartilage, osteoarthritis affects all joint tissues equally. The pathogenesis of this degenerative process is not completely understood; however, a low-grade inflammation leading to an imbalance between anabolic and katabolic processes is a well-established factor. The complex network of cytokines regulating these processes and cell communication has a central role in the development and progression of osteoarthritis. Concentrations of both proinflammatory and anti-inflammatory cytokines were found to be altered depending on the osteoarthritis stage and activity. In this review, we analyzed individual cytokines involved in the immune processes with an emphasis on their function in osteoarthritis.

Periostin loss-of-function protects mice from post-traumatic and age-related osteoarthritis

Background

Elevated levels of periostin (Postn) in the cartilage and bone are associated with osteoarthritis (OA). However, it remains unknown whether Postn loss-of-function can delay or prevent the development of OA. In this study, we sought to better understand the role of Postn in OA development and assessed the functional impact of Postn deficiency on post-traumatic and age-related OA in mice.

Methods

The effects of Postn deficiency were studied in two murine experimental OA models using Postn^−/− (n = 32) and littermate wild-type (wt) mice (n = 36). Post-traumatic OA was induced by destabilization of the medial meniscus (DMM) in 10-week-old mice (n = 20); age-related OA was analyzed in 24-month-old mice (n = 13). Cartilage degeneration was assessed histologically using the OARSI scoring system, and synovitis was evaluated by measuring the synovial lining cell layer and the cells density in the synovial stroma. Bone changes were measured by μCT analysis. Serum levels of Postn were determined by ELISA. Expression of Postn and collagenase-3 (MMP-13) was measured by immunostaining. RNA-seq was performed on chondrocytes isolated from 21-day old Postn^−/− (n = 3) and wt mice (n = 3) to discover genes and pathways altered by Postn knockout.

Results

Postn^−/− mice exhibited significantly reduced cartilage degeneration and OARSI score relative to wt mice in post-traumatic OA after 8 weeks (maximum: 2.37 ± 0.74 vs. 4.00 ± 1.20, P = 0.011; summed: 9.31 ± 2.52 vs. 21.44 ± 6.01, P = 0.0002) and spontaneous OA (maximum: 1.93 ± 0.45 vs. 3.58 ± 1.16, P = 0.014; summed: 6.14 ± 1.57 vs. 11.50 ± 3.02, P = 0.003). Synovitis was significantly lower in Postn^−/− mice than wt only in the DMM model (1.88 ± 1.01 vs. 3.17 ± 0.63; P = 0.039). Postn^−/− mice also showed lower trabecular bone parameters such as BV/TV, vBMD, Tb.Th, and Tb.N and high Tb. Sp in both models. Postn^−/− mice had negligible levels of serum Postn compared with wt. Immunofluorescent studies of cartilage indicated that Postn^−/− mice expressed lower MMP-13 levels than wt mice. RNA-seq revealed that cell-cell-adhesion and cell-differentiation processes were enriched in Postn^−/− mice, while those related to cell-cycle and DNA-repair were enriched in wt mice.

Conclusions

Postn deficiency protects against DMM-induced post-traumatic and age-related spontaneous OA. RNA-seq findings warrant further investigations to better understand the mechanistic role of Postn and its potential as a therapeutic target in OA.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13075-021-02477-z.

Of mice and men: converging on a common molecular understanding of osteoarthritis

Despite an increasing burden of osteoarthritis in developed societies, target discovery has been slow and there are currently no approved disease-modifying osteoarthritis drugs. This lack of progress is due in part to a series of misconceptions over the years: that osteoarthritis is an inevitable consequence of ageing, that damaged articular cartilage cannot heal itself, and that osteoarthritis is driven by synovial inflammation similar to that seen in rheumatoid arthritis. Molecular interrogation of disease through ex-vivo tissue analysis, in-vitro studies, and preclinical models have radically reshaped the knowledge landscape. Inflammation in osteoarthritis appears to be distinct from that seen in rheumatoid arthritis. Recent randomised controlled trials, using treatments repurposed from rheumatoid arthritis, have largely been unsuccessful. Genome-wide studies point to defects in repair pathways, which accords well with recent promise using growth factor therapies or Wnt pathway antagonism. Nerve growth factor has emerged as a robust target in osteoarthritis pain in phase 2-3 trials. These studies, both positive and negative, align well with those in preclinical surgical models of osteoarthritis, indicating that pathogenic mechanisms identified in mice can lead researchers to valid human targets. Several novel candidate pathways are emerging from preclinical studies that offer hope of future translational impact. Enhancing trust between industry, basic, and clinical scientists will optimise our collective chance of success.

Genetic correlations between cartilage regeneration and degeneration reveal an inverse relationship

OBJECTIVE

The etiology of osteoarthritis (OA) is unknown, however, there appears to be a significant contribution from genetics. We have identified recombinant inbred strains of mice derived from LG/J (large) and SM/J (small) strains that vary significantly in their ability to repair articular cartilage and susceptibility to post-traumatic OA due to their genetic composition. Here, we report cartilage repair phenotypes in the same strains of mice in which OA susceptibility was analyzed previously, and determine the genetic correlations between phenotypes.

DESIGN

We used 12 recombinant inbred strains, including the parental strains, to test three phenotypes: ear-wound healing (n = 263), knee articular cartilage repair (n = 131), and post-traumatic OA (n = 53) induced by the surgical destabilization of the medial meniscus (DMM). Genetic correlations between various traits were calculated as Pearson's correlation coefficients of strain means.

RESULTS

We found a significant positive correlation between ear-wound healing and articular cartilage regeneration (r = 0.71; P = 0.005). We observed a strong inverse correlation between articular cartilage regeneration and susceptibility to OA based on maximum (r = -0.54; P = 0.036) and summed Osteoarthritis Research Society International (OARSI) scores (r = -0.56; P = 0.028). Synovitis was not significantly correlated with articular cartilage regeneration but was significantly positively correlated with maximum (r = 0.63; P = 0.014) and summed (r = 0.70; P = 0.005) OARSI scores. Ectopic calcification was significantly positively correlated with articular cartilage regeneration (r = 0.59; P = 0.021).

CONCLUSIONS

Using recombinant inbred strains, our study allows, for the first time, the measurement of genetic correlations of regeneration phenotypes with degeneration phenotypes, characteristic of OA (cartilage degeneration, synovitis). We demonstrate that OA is positively correlated with synovitis and inversely correlated with the ability to repair cartilage. These results suggest an addition to the risk paradigm for OA from a focus on degeneration to regeneration.

Efficacy of arthrocentesis and lavage for treatment of post-traumatic arthritis in temporomandibular joints

Objectives

Joint injuries frequently lead to progressive joint degeneration that causes articular disc derangement, joint inflammation, and osteoarthritis. Such arthropathies that arise after trauma are defined as post-traumatic arthritis (PTA). Although PTA is well recognized in knee and elbow joints, PTA in the temporomandibular joint (TMJ) has not been clearly defined. Interestingly, patients experiencing head and neck trauma without direct jaw fracture have displayed TMJ disease symptoms; however, definitive diagnosis and treatment options are not available. This study will analyze clinical aspects of PTA in TMJ and their treatment outcomes after joint arthrocentesis and lavage.

Materials and Methods

Twenty patients with history of trauma to the head and neck especially without jaw fracture were retrospectively studied. Those patients developed TMJ disease symptoms and were diagnosed by computed tomography or magnetic resonance imaging. To decrease TMJ discomfort, arthrocentesis and lavage with or without conservative therapy were applied, and efficacy was evaluated by amount of mouth opening and pain scale. Statistical differences between pre- and post-treatment values were evaluated by Wilcoxon signed-rank test.

Results

Patient age varied widely between 20 and 80 years, and causes of trauma were diverse. Duration of disease onset was measured as 508 posttrauma days, and 85% of the patients sought clinic visit within 2 years after trauma. In addition, 85% of the patients showed TMJ disc derangement without reduction, and osteoarthritis was accompanied at the traumatized side or at both sides in 40% of the patients. After arthrocentesis or lavage, maximal mouth opening was significantly increased (28-44 mm on average, P<0.001) and pain scale was dramatically decreased (7.8-3.5 of 10, P<0.001); however, concomitant conservative therapy showed no difference in treatment outcome.

Conclusion

The results of this study clarify the disease identity of PTA in TMJ and suggest early diagnosis and treatment options to manage PTA in TMJ.

Identifying effector molecules, cells, and cytokines of innate immunity in OA

Inflammatory changes are observed in affected joints of osteoarthritis (OA) patients and are thought to be involved in the pathology that develops along OA progression. This narrative review provides an overview of the various cell types that are present in the joint during OA and which alarmins, cytokines, chemokines, growth factors, and other mediators they produce. Moreover, the involvement of more systemic processes like inflammaging and its associated cellular senescence in the context of OA are discussed.

The role of GPCRs in bone diseases and dysfunctions

The superfamily of G protein-coupled receptors (GPCRs) contains immense structural and functional diversity and mediates a myriad of biological processes upon activation by various extracellular signals. Critical roles of GPCRs have been established in bone development, remodeling, and disease. Multiple human GPCR mutations impair bone development or metabolism, resulting in osteopathologies. Here we summarize the disease phenotypes and dysfunctions caused by GPCR gene mutations in humans as well as by deletion in animals. To date, 92 receptors (5 glutamate family, 67 rhodopsin family, 5 adhesion, 4 frizzled/taste2 family, 5 secretin family, and 6 other 7TM receptors) have been associated with bone diseases and dysfunctions (36 in humans and 72 in animals). By analyzing data from these 92 GPCRs, we found that mutation or deletion of different individual GPCRs could induce similar bone diseases or dysfunctions, and the same individual GPCR mutation or deletion could induce different bone diseases or dysfunctions in different populations or animal models. Data from human diseases or dysfunctions identified 19 genes whose mutation was associated with human BMD: 9 genes each for human height and osteoporosis; 4 genes each for human osteoarthritis (OA) and fracture risk; and 2 genes each for adolescent idiopathic scoliosis (AIS), periodontitis, osteosarcoma growth, and tooth development. Reports from gene knockout animals found 40 GPCRs whose deficiency reduced bone mass, while deficiency of 22 GPCRs increased bone mass and BMD; deficiency of 8 GPCRs reduced body length, while 5 mice had reduced femur size upon GPCR deletion. Furthermore, deficiency in 6 GPCRs induced osteoporosis; 4 induced osteoarthritis; 3 delayed fracture healing; 3 reduced arthritis severity; and reduced bone strength, increased bone strength, and increased cortical thickness were each observed in 2 GPCR-deficiency models. The ever-expanding number of GPCR mutation-associated diseases warrants accelerated molecular analysis, population studies, and investigation of phenotype correlation with SNPs to elucidate GPCR function in human diseases.

Evidence for Genetic Contribution to Variation in Posttraumatic Osteoarthritis in Mice

OBJECTIVE

Recombinant inbred mouse strains generated from an LG/J and SM/J intercross offer a unique resource to study complex genetic traits such as osteoarthritis (OA). We undertook this study to determine the susceptibility of 14 strains to various phenotypes characteristic of posttraumatic OA. We hypothesized that phenotypic variability is associated with genetic variability.

METHODS

Ten-week-old male mice underwent surgical destabilization of the medial meniscus (DMM) to induce posttraumatic OA. Mice were killed 8 weeks after surgery, and knee joints were processed for histology to score cartilage degeneration and synovitis. Micro-computed tomography was used to analyze trabecular bone parameters including subchondral bone plate thickness and synovial ectopic calcifications. Gene expression in the knees was assessed using a QuantiGene Plex assay.

RESULTS

Broad-sense heritability ranged from 0.18 to 0.58, which suggested that the responses to surgery were moderately heritable. The LGXSM-33, LGXSM-5, LGXSM-46, and SM/J strains were highly susceptible to OA, while the LGXSM-131b, LGXSM-163, LGXSM-35, LGXSM-128a, LGXSM-6, and LG/J strains were relatively OA resistant. This study was the first to accomplish measurement of genetic correlations of phenotypes that are characteristic of posttraumatic OA. Cartilage degeneration was significantly positively associated with synovitis (r = 0.83-0.92), and subchondral bone plate thickness was negatively correlated with ectopic calcifications (r = -0.59). Moreover, we showed that 40 of the 78 genes tested were significantly correlated with various OA phenotypes. However, unlike the OA phenotypes, there was no evidence for genetic variation in differences in gene expression levels between DMM-operated and sham-operated knees.

CONCLUSION

For these mouse strains, various characteristics of posttraumatic OA varied with genetic composition, which demonstrated a genetic basis for susceptibility to posttraumatic OA. The heritability of posttraumatic OA was established. Phenotypes exhibited various degrees of correlations; cartilage degeneration was positively correlated with synovitis, but not with the formation of ectopic calcifications. Further investigation of the genome regions that contain genes implicated in OA, as well as further investigation of gene expression data, will be useful for studying mechanisms of OA and identifying therapeutic targets.

CCL25 chemokine-guided stem cell attraction: an assessment of possible benefits and risks

Due to its chemoattraction potential on mesenchymal stromal cells of the CCL25/CCR9 axis, local application of CCL25 to severely damaged tissues may be a promising approach for regenerative therapies. Analysis of the given data revealed that CCL25/CCR9 signaling has a crucial role in regulation of an adult immune homeostasis. CCR9 expression variations resulted in dysfunctional immune response in colitis, rheumatoid arthritis and endometriosis. Regarding oncology, different neoplastic tissues exploit CCL25-dependent CCR9 signaling for either local proliferation or migration processes. The CCR9 pathway likely can trigger crosstalk between the Akt and NOTCH pathway and thus participate in the regulation of the neoplastic behavior. In conclusion, the designated application-tissue requires precise molecular analysis of possible CCR9 expression due to its proto-oncogenic characteristics.

Knocking out or pharmaceutical inhibition of fatty acid binding protein 4 (FABP4) alleviates osteoarthritis induced by high-fat diet in mice

OBJECTIVES

Adipokines play roles in the pathogenesis of osteoarthritis (OA). Fatty acid binding protein 4 (FABP4) is a novel adipokine that is closely associated with obesity and metabolic diseases. The aim of this study was to discover the potential role of FABP4 in OA.

METHODS

Seventy-two FABP4 knockout mice (KO) in C57BL/6N background and wild-type littermates (WT) (male, 6-week-old) were fed with a high-fat diet (HFD, 60% calorie) or standard diet (STD, 11.6% calorie) for 3 months, 6 months and 9 months (n = 6 each). In the parallel study, forty-eight 6-week-old male WT mice were fed with HFD or STD, and simultaneously treated with daily oral gavage of selective FABP4 inhibitor BMS309403 (15 mg/kg/d) or vehicle for 4 months and 6 months (n = 6 each). Serum FABP4 and cartilage oligomeric matrix protein (COMP) concentration was quantified. Histological assessment of knee OA and micro-CT analysis of subchondral bone were performed.

RESULTS

HFD induced obesity in mice. After 3 months and 6 months of HFD, KO mice showed alleviated cartilage degradation and synovitis, with significantly lower COMP, modified Mankin OA score, and MMP-13/ADAMTS4 expression. After 6 months and 9 months of HFD, KO mice showed less osteophyte formation and subchondral bone sclerosis. Chronic treatment of BMS309403 for 4 months and 6 months significantly alleviated cartilage degradation, but had no effects on the subchondral bone. Knocking out or pharmaceutical inhibition of FABP4 did not have significant effects on lean mice fed with STD.

CONCLUSIONS

Knocking out or pharmaceutical inhibition of FABP4 alleviates OA induced by HFD in mice.

Molecular influence of anterior cruciate ligament tear remnants on chondrocytes: a biologic connection between injury and osteoarthritis

OBJECTIVE

Anterior cruciate ligament (ACL) injury initiates a cascade of events often leading to osteoarthritis (OA). ACL reconstruction does not alter the course of OA, suggesting that heightened OA risk is likely due to factors in addition to the joint instability. We showed that torn ACL remnants express periostin (POSTN) in the acute phase of injury. Considering that ACL injury predisposes to OA and that POSTN is associated with cartilage metabolism, we hypothesize that ACL injury affects chondrocytes via POSTN.

DESIGN

Cartilage was obtained from osteoarthritic patients and ACL remnants were collected from patients undergoing ACL reconstruction. Crosstalk between ACL remnants and chondrocytes was studied in a transwell co-culture system. Expression of POSTN and other anabolic and catabolic genes was assessed via real-time polymerase chain reaction (PCR). Immunostaining for periostin was performed in human and mouse cartilage. The impact of exogenous periostin and siRNA-mediated ablation of periostin on matrix metabolism and cell migration was examined. Furthermore, the effect of anabolic (transforming growth factor beta 1 [TGF-β1]) and catabolic (interleukin 1 beta [IL-1β]) factors on POSTN expression was investigated.

RESULTS

ACL remnants induced expression of POSTN, MMP13 and ADAMTS4. Periostin levels were significantly higher in osteoarthritic compared to normal cartilage. Exogenous periostin induced MMP13 expression and cell migration, and repressed COL1A1 expression while POSTN knockdown inhibited expression of both anabolic and catabolic genes and impeded cell migration. TGF-β1 and IL-1β treatment did not alter POSTN expression but influenced chondrocyte metabolism as determined by quantification of anabolic and catabolic genes via real-time PCR.

CONCLUSIONS

ACL remnants can exert paracrine effects on cartilage, altering cellular homeostasis. Over time, this metabolic imbalance could contribute to OA development.

Chemokine receptor-7 (CCR7) deficiency leads to delayed development of joint damage and functional deficits in a murine model of osteoarthritis

Elevated chemokine receptor Ccr7 is observed in knee osteoarthritis (OA) and associated with severity of symptoms. In this study, we confirmed that CCR7 protein expression is elevated in synovial tissue from OA patients by immunohistochemical staining. We then investigated whether Ccr7 deficiency impacted structural and functional joint degeneration utilizing a murine model of OA. OA-like disease was induced in male C57BL/6 and Ccr7-deficient (Ccr7^-/- ) mice by destabilization of the medial meniscus (DMM). Functional deficits were measured by computer integrated monitoring of spontaneous activity every 4 weeks after DMM surgery up 16 weeks. Joint degeneration was evaluated at 6 and 19 weeks post-surgery by histopathology, and subchondral bone changes analyzed by microCT. Results showed reduction in locomotor activities in DMM-operated C57BL/6 mice by 8 weeks, while activity decreases in Ccr7^-/- mice were delayed until 16 weeks. Histopathologic evaluation showed minimal protection from early cartilage degeneration (p = 0.06) and osteophytosis (p = 0.04) in Ccr7^-/- mice 6 weeks post-DMM compared to C57BL/6 controls, but not at 19 weeks. However, subchondral bone mineral density (p = 0.03) and histologic sclerosis (p = 0.02) increased in response to surgery in C57BL/6 mice at 6 weeks, while Ccr7^-/- mice were protected from these changes. Our results are the first to demonstrate a role for Ccr7 in early development of functional deficits and subchondral bone changes in the DMM model. Understanding the mechanism of Ccr7 receptor signaling in the initiation of joint pathology and disability will inform the development of innovative therapies to slow symptomatic OA development after injury. Published 2017. This article is a U.S. Government work and is in the public domain in the USA. J Orthop Res 36:864-875, 2018.

Comprehensive Expression Analysis of microRNAs and mRNAs in Synovial Tissue from a Mouse Model of Early Post-Traumatic Osteoarthritis

To better understand the molecular processes involved in driving osteoarthritis disease progression we characterized expression profiles of microRNAs (miRNA) and mRNAs in synovial tissue from a post-traumatic OA mouse model. OA was induced in 10–12 week old male C57BL6 mice by bilateral surgical destabilization of the medial meniscus (DMM). RNA isolated from the anterior synovium of mice at 1 and 6 weeks post-surgery was subject to expression profiling using Agilent microarrays and qPCR. OA severity was determined histologically. Anterior and posterior synovitis decreased with post-operative time after sham and DMM. No differences in synovitis parameters were evident between sham and DMM in the anterior synovium at either time. While expression profiling revealed 394 miRNAs were dysregulated between 1 and 6 week time-points in the anterior synovium, there were no significant changes in miRNA or mRNA expression between DMM and sham mice at both time-points. Bioinformatic analysis of the miRNAs and mRNAs differentially expressed in tandem with the resolution of anterior synovial inflammation revealed similar biological processes and functions, including organismal injury, connective tissue disorder and inflammatory responses. Our data demonstrates that early OA-specific patterns of synovial miRNAs or mRNAs dysregulation could not be identified in this model of post-traumatic OA.

Immune Contributions to Osteoarthritis

PURPOSE OF THE REVIEW

Mounting evidence supports a role of low-grade inflammation in the pathophysiology of osteoarthritis (OA). We review and discuss the role of synovitis, complement activation, cytokines, and immune cell population in OA.

RECENT FINDINGS

Using newer imaging modalities, synovitis is found in the majority of knees with OA. Complement activation and pro-inflammatory cytokines play a significant role in the development of cartilage destruction and synovitis. Immune cell infiltration of OA synovial tissue by sub-populations of T cells and activated macrophages correlates with OA disease progression and pain. The innate and acquired immune system plays a key role in the low-grade inflammation found associated with OA. Targets of these pathways my hold promise for future disease-modifying osteoarthritis drugs (DMOADs).

The Multifunctional Role of the Chemokine System in Arthritogenic Processes

PURPOSE OF REVIEW

The involvement of chemokines and their receptors in the genesis and perpetuation of rheumatoid arthritis, spondyloarthritis, and osteoarthritis has been clearly recognized for a long time. Nevertheless, the complexity of their contribution to these diseases is now becoming evident and this review focuses on published evidence on their mechanism of action.

RECENT FINDINGS

Studies performed on patients and in vivo models have identified a number of chemokine-mediated pathways involved in various aspects of arthritogenic processes. Chemokines promote leukocyte infiltration and activation, angiogenesis, osteoclast differentiation, and synoviocyte proliferation and activation and participate to the generation of pain by regulating the release of neurotransmitters. A number of chemokines are expressed in a timely controlled fashion in the joint during arthropathies, regulating all the aspects of inflammation as well as the equilibrium between damage and repair and between relief and pain. Thus, the targeting of specific chemokine/chemokine receptor interactions is considered a promising tool for therapeutic intervention.

Role of the C-C chemokine receptor-2 in a murine model of injury-induced osteoarthritis

OBJECTIVE

We previously found in our embryonic studies that proper regulation of the chemokine CCL12 through its sole receptor CCR2, is critical for joint and growth plate development. In the present study, we examined the role of CCR2 in injury-induced-osteoarthritis (OA).

METHOD

We used a murine model of injury-induced-OA (destabilization of medial meniscus, DMM), and systemically blocked CCR2 using a specific antagonist (RS504393) at different times during disease progression. We examined joint degeneration by assessing cartilage (cartilage loss, chondrocyte hypertrophy, MMP-13 expression) and bone lesions (bone sclerosis, osteophytes formation) with or without the CCR2 antagonist. We also performed pain behavioral studies by assessing the weight distribution between the normal and arthritic hind paws using the IITS incapacitance meter.

RESULTS

Testing early vs delayed administration of the CCR2 antagonist demonstrated differential effects on joint damage. We found that OA changes in articular cartilage and bone were ameliorated by pharmacological CCR2 blockade, if given early in OA development: specifically, pharmacological targeting of CCR2 during the first 4 weeks (wks) following injury, reduced OA cartilage and bone damage, with less effectiveness with later treatments. Importantly, our pain-related behavioral studies showed that blockade of CCR2 signaling during early, 1-4 wks post-surgery or moderate, 4-8 wks post-surgery, OA was sufficient to decrease pain measures, with sustained improvement at later stages, after treatment was stopped.

CONCLUSIONS

Our data highlight the potential efficacy of antagonizing CCR2 at early stages to slow the progression of post-injury OA and, in addition, improve pain symptoms.

Therapeutic efficacy of intra-articular hyaluronan derivative and platelet-rich plasma in mice following axial tibial loading

Objective

To investigate the therapeutic potential of intra-articular hyaluronan-derivative HYADD^® 4-G and/or platelet-rich plasma (PRP) in a mouse model of non-invasive joint injury.

Methods

Non-invasive axial tibial loading was used to induce joint injury in 10-week-old C57BL/6J mice (n = 86). Mice underwent a single loading of either 6 Newton (N) or 9N axial tibial compression. HYADD^® 4-G was injected intra-articularly at 8 mg/mL or 15 mg/mL either before or after loading with or without PRP. Phosphate-buffered-saline was injected as control. Knee joints were harvested at 5 or 56 days post-loading and prepared for micro-computed tomography scanning and subsequently processed for histology. Immunostaining was performed for aggrecan to monitor its distribution, for CD44 to monitor chondrocyte reactive changes and for COMP (cartilage oligomeric matrix protein) as an index for cartilage matrix changes related to loading and cartilage injury. TUNEL assay was performed to identify chondrocyte apoptosis.

Results

Loading initiated cartilage proteoglycan loss and chondrocyte apoptosis within 5 days with slowly progressive post-traumatic osteoarthritis (no cartilage degeneration, but increased synovitis and ectopic calcification after 9N loading) at 56 days. Mice treated with repeated HYADD^® 4-G (15 mg/mL) or HYADD^® 4-G (8 mg/mL) ± PRP or PRP alone exhibited no significant improvement in the short-term (5 days) and long-term (56 days) consequences of joint loading except for a trend for improved bone changes compared to non-loaded joints.

Conclusion

While we failed to show an overall effect of intra-articular delivery of hyaluronan-derivative and/or PRP in reversing/protecting the pathological events in cartilage and synovium following joint injury, some bone alterations were relatively less severe with hyaluronan-derivative at higher concentration or in association with PRP.

Chemokines and inflammation in osteoarthritis: Insights from patients and animal models

Evidence has been building that the pathologic drive for development of osteoarthritis (OA) involves more than simple mechanical "wear and tear." Inflammatory mechanisms play an important role in the tissue response to joint injury, and are involved in development of post-traumatic OA. Inflammation also appears integral to the progression of OA, whether post-traumatic or spontaneous, contributing to the evolution of joint tissue degradation and remodeling as well as joint pain. Both patient-based studies and in vivo models of disease have shed light on a number of inflammatory pathways and mediators that impact various aspects of this disease, both structurally and symptomatically. Recent work in this field has implicated inflammatory chemokines in osteoarthritis pathogenesis. Expression of multiple chemokines and their receptors is modulated during disease in both patients and animal models. Although best known for their effects on leukocyte migration and trafficking within the immune system, chemokines can have a wide variety of effects on both motile and non-motile cell types, impacting proliferation, differentiation, and activation of cellular responses. Their role in OA models has also demonstrated diverse effects on disease that exemplify their wide-ranging effects. Understanding how these important mediators of inflammation impact joint disease, and whether they can be targeted therapeutically, is actively being investigated by many groups in this field. This narrative review focuses on evidence published within the last 5 years highlighting chemokine-mediated pathways with mechanistic involvement in osteoarthritis and joint tissue repair. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:735-739, 2017.

Post-Traumatic Osteoarthritis in Mice Following Mechanical Injury to the Synovial Joint

We investigated the spectrum of lesions characteristic of post-traumatic osteoarthritis (PTOA) across the knee joint in response to mechanical injury. We hypothesized that alteration in knee joint stability in mice reproduces molecular and structural features of PTOA that would suggest potential therapeutic targets in humans. The right knees of eight-week old male mice from two recombinant inbred lines (LGXSM-6 and LGXSM-33) were subjected to axial tibial compression. Three separate loading magnitudes were applied: 6N, 9N, and 12N. Left knees served as non-loaded controls. Mice were sacrificed at 5, 9, 14, 28, and 56 days post-loading and whole knee joint changes were assessed by histology, immunostaining, micro-CT, and magnetic resonance imaging. We observed that tibial compression disrupted joint stability by rupturing the anterior cruciate ligament (except for 6N) and instigated a cascade of temporal and topographical features of PTOA. These features included cartilage extracellular matrix loss without proteoglycan replacement, chondrocyte apoptosis at day 5, synovitis present at day 14, osteophytes, ectopic calcification, and meniscus pathology. These findings provide a plausible model and a whole-joint approach for how joint injury in humans leads to PTOA. Chondrocyte apoptosis, synovitis, and ectopic calcification appear to be targets for potential therapeutic intervention.

Complementary models reveal cellular responses to contact stresses that contribute to post-traumatic osteoarthritis

Two categories of joint overloading cause post-traumatic osteoarthritis (PTOA): single acute traumatic loads/impactions and repetitive overloading due to incongruity/instability. We developed and refined three classes of complementary models to define relationships between joint overloading and progressive cartilage loss across the spectrum of acute injuries and chronic joint abnormalities: explant and whole joint models that allow probing of cellular responses to mechanical injury and contact stresses, animal models that enable study of PTOA pathways in living joints and pre-clinical testing of treatments, and patient-specific computational models that define the overloading that causes OA in humans. We coordinated methodologies across models so that results from each informed the others, maximizing the benefit of this complementary approach. We are incorporating results from these investigations into biomathematical models to provide predictions of PTOA risk and guide treatment. Each approach has limitations, but each provides opportunities to elucidate PTOA pathogenesis. Taken together, they help define levels of joint overloading that cause cartilage destruction, show that both forms of overloading can act through the same biologic pathways, and create a framework for initiating clinical interventions that decrease PTOA risk. Considered collectively, studies extending from explants to humans show that thresholds of joint overloading that cause cartilage loss can be defined, that to at least some extent both forms of joint overloading act through the same biologic pathways, and interventions that interrupt these pathways prevent cartilage damage. These observations suggest that treatments that decrease the risk of all forms of OA progression can be discovered. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:515-523, 2017.

CCL2 and CCR2 regulate pain-related behaviour and early gene expression in post-traumatic murine osteoarthritis but contribute little to chondropathy

OBJECTIVE

The role of inflammation in structural and symptomatic osteoarthritis (OA) remains unclear. One key mediator of inflammation is the chemokine CCL2, primarily responsible for attracting monocytes to sites of injury. We investigated the role of CCL2 and its receptor CCR2 in experimental OA.

DESIGN

OA was induced in 10 weeks old male wild type (WT), Ccl2^-/- and Ccr2^-/- mice, by destabilisation of the medial meniscus (DMM). RNA was extracted from whole joints at 6 h and 7 days post-surgery and examined by reverse transcription polymerase chain reaction (RT-PCR). Gene expression changes between naïve and DMM-operated mice were compared. Chondropathy scores, from mice at 8, 12, 16 and 20 weeks post DMM were calculated using modified Osteoarthritis Research Society International (OARSI) grading systems. Changes in hind paw weight distribution, as a measure of pain, were assessed by Linton incapacitance.

RESULTS

Absence of CCL2 strongly suppressed (>90%) selective inflammatory response genes in the joint 6 h post DMM, including arginase 1, prostaglandin synthase 2, nitric oxide synthase 2 and inhibin A. IL6, MMP3 and tissue inhibitor of metalloproteinase 1 were also significantly suppressed. Similar trends were also observed in the absence of CCR2. A lower average chondropathy score was observed in both Ccl2^-/- and Ccr2^-/- mice at 12, 16 and 20 weeks post DMM compared with WT mice, but this was only statistically significant at 20 weeks in Ccr2^-/- mice. Pain-related behaviour in Ccl2^-/- and Ccr2^-/- mice post DMM was delayed in onset.

CONCLUSION

The CCL2/CCR2 axis plays an important role in the development of pain in murine OA, but contributes little to cartilage damage.

Early changes in the knee of healer and non-healer mice following non-invasive mechanical injury

In this study, we examined early time-dependent changes in articular cartilage and synovium in response to tibial compression and sought the plausible origin of cells that respond to compression in the healer (LGXSM-6) and non-healer (LGXSM-33) recombinant inbred mouse strains. The right knee of 13-week old male mice was subjected to tibial compression using 9N axial loading. The contralateral left knee served as a control. Knees were harvested at 5, 9, and 14 days post-injury. Histological changes in cartilage and synovium, immunofluorescence pattern of CD44, aggrecan, type-II collagen, cartilage oligomeric matrix protein and the aggrecan neo-epitope NITEGE, and cell apoptosis (by TUNEL) were examined. We used a double nucleoside analog cell-labeling strategy to trace cells responsive to injury. We showed that tibial compression resulted in rupture of anterior cruciate ligament, cartilage matrix loss and chondrocyte apoptosis at the injury site. LGXSM-33 showed higher synovitis and ectopic synovial chondrogenesis than LGXSM-6 with no differences for articular cartilage lesions. With loading, an altered pattern of CD44 and NITEGE was observed: cells in the impacted area underwent apoptosis, cells closely surrounding the injured area expressed CD44, and cells in the intact area expressed NITEGE. Cells responding to injury were found in the synovium, subchondral bone marrow and the Groove of Ranvier. Taken together, we found no strain differences in chondrocytes in the early response to injury. However, the synovial response was greater in LGXSM-33 indicating that, at early time points, there is a genetic difference in synovial cell reaction to injury. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:524-536, 2017.

CCL2/CCR2, but not CCL5/CCR5, mediates monocyte recruitment, inflammation and cartilage destruction in osteoarthritis

OBJECTIVES

While various monocyte chemokine systems are increased in expression in osteoarthritis (OA), the hierarchy of chemokines and chemokine receptors in mediating monocyte/macrophage recruitment to the OA joint remains poorly defined. Here, we investigated the relative contributions of the CCL2/CCR2 versus CCL5/CCR5 chemokine axes in OA pathogenesis.

METHODS

Ccl2-, Ccr2-, Ccl5- and Ccr5-deficient and control mice were subjected to destabilisation of medial meniscus surgery to induce OA. The pharmacological utility of blocking CCL2/CCR2 signalling in mouse OA was investigated using bindarit, a CCL2 synthesis inhibitor, and RS-504393, a CCR2 antagonist. Levels of monocyte chemoattractants in synovial tissues and fluids from patients with joint injuries without OA and those with established OA were investigated using a combination of microarray analyses, multiplexed cytokine assays and immunostains.

RESULTS

Mice lacking CCL2 or CCR2, but not CCL5 or CCR5, were protected against OA with a concomitant reduction in local monocyte/macrophage numbers in their joints. In synovial fluids from patients with OA, levels of CCR2 ligands (CCL2, CCL7 and CCL8) but not CCR5 ligands (CCL3, CCL4 and CCL5) were elevated. We found that CCR2+ cells are abundant in human OA synovium and that CCR2+ macrophages line, invade and are associated with the erosion of OA cartilage. Further, blockade of CCL2/CCR2 signalling markedly attenuated macrophage accumulation, synovitis and cartilage damage in mouse OA.

CONCLUSIONS

Our findings demonstrate that monocytes recruited via CCL2/CCR2, rather than by CCL5/CCR5, propagate inflammation and tissue damage in OA. Selective targeting of the CCL2/CCR2 system represents a promising therapeutic approach for OA.

Osteoarthritis

Osteoarthritis (OA) is the most common joint disorder, is associated with an increasing socioeconomic impact owing to the ageing population and mainly affects the diarthrodial joints. Primary OA results from a combination of risk factors, with increasing age and obesity being the most prominent. The concept of the pathophysiology is still evolving, from being viewed as cartilage-limited to a multifactorial disease that affects the whole joint. An intricate relationship between local and systemic factors modulates its clinical and structural presentations, leading to a common final pathway of joint destruction. Pharmacological treatments are mostly related to relief of symptoms and there is no disease-modifying OA drug (that is, treatment that will reduce symptoms in addition to slowing or stopping the disease progression) yet approved by the regulatory agencies. Identifying phenotypes of patients will enable the detection of the disease in its early stages as well as distinguish individuals who are at higher risk of progression, which in turn could be used to guide clinical decision making and allow more effective and specific therapeutic interventions to be designed. This Primer is an update on the progress made in the field of OA epidemiology, quality of life, pathophysiological mechanisms, diagnosis, screening, prevention and disease management.

Inflammation in joint injury and post-traumatic osteoarthritis

Inflammation is a variable feature of osteoarthritis (OA), associated with joint symptoms and progression of disease. Signs of inflammation can be observed in joint fluids and tissues from patients with joint injuries at risk for development of post-traumatic osteoarthritis (PTOA). Furthermore, inflammatory mechanisms are hypothesized to contribute to the risk of OA development and progression after injury. Animal models of PTOA have been instrumental in understanding factors and mechanisms involved in chronic progressive cartilage degradation observed after a predisposing injury. Specific aspects of inflammation observed in humans, including cytokine and chemokine production, synovial reaction, cellular infiltration and inflammatory pathway activation, are also observed in models of PTOA. Many of these models are now being utilized to understand the impact of post-injury inflammatory response on PTOA development and progression, including risk of progressive cartilage degeneration and development of chronic symptoms post-injury. As evidenced from these models, a vigorous inflammatory response occurs very early after joint injury but is then sustained at a lower level at the later phases. This early inflammatory response contributes to the development of PTOA features including cartilage erosion and is potentially modifiable, but specific mediators may also play a role in tissue repair. Although the optimal approach and timing of anti-inflammatory interventions after joint injury are yet to be determined, this body of work should provide hope for the future of disease modification tin PTOA.

On the predictive utility of animal models of osteoarthritis

Animal models of osteoarthritis are extensively used for investigating disease pathways and for preclinical testing of novel therapies. Their predictive utility, however, has often been questioned, mainly because preclinical efficacy of novel therapeutics is poorly translated in clinical trials. In the current narrative review, we consider the preclinical models that were used to support undertaking clinical trials for disease-modifying osteoarthritis drugs, and compare outcomes between clinical and preclinical studies. We discuss this in light of the 1999 Food and Drug Administration draft guidelines for industry for use in the development of drugs, devices, and biological products intended for the treatment of osteoarthritis, which raised five considerations on the usefulness of osteoarthritis models. We systematically discuss what has been learnt regarding these five points since 1999, with emphasis on replicating distinct risk factors and subtypes of human osteoarthritis, and on comprehensive evaluation of the disease in animals, including pathology of all joint tissues, biomarker analysis, and assessment of pain and joint function. Finally, we discuss lessons learnt and propose some recommendations for how the evidence from preclinical research might be strengthened with a view to improving success in clinical translation.