Modern molecular analysis of a traditional disease: Progression in osteoarthritis

Prussian blue nanozymes coated with Pluronic attenuate inflammatory osteoarthritis by blocking c-Jun N-terminal kinase phosphorylation

Osteoarthritis (OA) is a degenerative joint disorder associated with inflammation, functional disability, and high socioeconomic costs. The development of effective therapies against inflammatory OA has been limited owing to its complex and multifactorial nature. The efficacy of Prussian blue nanozymes coated with Pluronic (PPBzymes), US Food and Drug Administration-approved components, and their mechanisms of action have been described in this study, and PPBzymes have been characterized as a new OA therapeutic. Spherical PPBzymes were developed via nucleation and stabilization of Prussian blue inside Pluronic micelles. A uniformly distributed diameter of approximately 204 nm was obtained, which was maintained after storage in an aqueous solution and biological buffer. This indicates that PPBzymes are stable and could have biomedical applications. In vitro data revealed that PPBzymes promote cartilage generation and reduce cartilage degradation. Moreover, intra-articular injections with PPBzymes into mouse joints revealed their long-term stability and effective uptake into the cartilage matrix. Furthermore, intra-articular PPBzymes injections attenuated cartilage degradation without exhibiting cytotoxicity toward the synovial membrane, lungs, and liver. Notably, based on proteome microarray data, PPBzymes specifically block the JNK phosphorylation, which modulates inflammatory OA pathogenesis. These findings indicate that PPBzymes might represent a biocompatible and effective nanotherapeutic for obstructing JNK phosphorylation.

WITHDRAWN: Prussian blue nanozymes coated with pluronic attenuate inflammatory osteoarthritis by blocking c-Jun N-terminal kinase phosphorylation

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Single-cell transcriptomics reveals variable trajectories of CSPCs in the progression of osteoarthritis

Osteoarthritis (OA) is characterised by cartilage destruction; however, there are no specific drugs available for its treatment. Cartilage-derived stem/progenitor cells (CSPCs) are multipotent cells that play an essential role in cartilage renewal and may provide critical insights into the medical needs for OA treatment. However, alterations in cell function and fate of CSPCs during OA progression have seldom been analysed, especially at the single-cell level. Additionally, it has been reported that CSPCs can migrate to the cartilage injury area, although the mechanism of migration remains elusive. Thus, understanding the changing patterns of CSPCs in the pathological process of OA is important in the effort to develop stem cell therapy for OA. Here, we downloaded single-cell transcriptomic data of patients with OA from the Gene Expression Omnibus (GEO) database and performed unbiased clustering of the cells based on gene expression patterns using the Seurat package. Using common stem cell markers and chondrogenic transcription factors, we traced CSPCs throughout all stages of OA. We further explored the dynamics of CSPCs in OA progression and validated the single-cell RNA sequencing data in vitro using qPCR, immunofluorescence, and western blotting. Specifically, we primarily explored the heterogeneity of CSPCs at the single-cell level and found that it was closely associated with OA progression. Our results indicate significantly reduced chondrogenic differentiation capacity in CSPCs during the late stage of OA, while their proliferation capacity tended to increase. We also found that genes implicated in fibrosis, cell motility, and extracellular matrix remodelling were upregulated in CSPCs during the progression of OA. Our study revealed the dynamics of stem cells in OA progression and may inform the development of stem cell therapy for OA.

SMURF1 and SMURF2 in Progenitor Cells from Articular Cartilage and Meniscus during Late-Stage Osteoarthritis

OBJECTIVE

The aim of this study was to investigate the roles of SMURF1 and SMURF2 in progenitor cells from the human knee in late-stage osteoarthritis (OA).

DESIGN

We applied immunohistochemistry, immunocytochemistry, RNAi, lentiviral transfection, and Western blot analysis. We obtained chondrogenic progenitor cells (CPCs) from the articular cartilage and meniscus progenitor cells (MPCs) from the nonvascularized part of the meniscus.

RESULTS

SMURF1 and SMURF2 appeared in both osteoarthritic tissues. CPCs and MPCs exhibited comparable amounts of these proteins, which influence the balance between RUNX2 and SOX9. The overexpression of SMURF1 reduced the levels of RUNX2, SOX9, and TGFBR1. The overexpression of SMURF2 also reduced the levels of RUNX2 and TGFBR1, while SOX9 levels were not affected. The knockdown of SMURF1 had no effect on RUNX2, SOX9, or TGFBR1. The knockdown of SMURF2 enhanced RUNX2 and SOX9 levels in CPCs. The respective protein levels in MPCs were not affected.

CONCLUSIONS

This study shows that SMURF1 and SMURF2 are regulatory players for the expression of the major regulator transcription factors RUNX2 and SOX9 in CPCs and MPCs. Our novel findings may help elucidate new treatment strategies for cartilage regeneration.

Distinct Pattern of Inflammation of Articular Cartilage and the Synovium in Early and Late Hip Femoroacetabular Impingement

BACKGROUND

The molecular mechanism of how femoroacetabular impingement (FAI) morphology leads to hip osteoarthritis (OA) is yet to be determined. The expression and location of inflammation-related molecules during early- and late-stage FAI have not been previously described. Moreover, the characterization of intra-articular inflammation away from the cam deformity as well as the nature of adjacent synovial tissue have also not been extensively reported.

HYPOTHESIS

Early-stage FAI has a similar expression of inflammation-related markers in the head-neck and acetabular cartilage but less synovitis than late-stage FAI.

STUDY DESIGN

Controlled laboratory study.

METHODS

Head-neck cartilage, acetabular cartilage, and synovial samples were obtained from patients undergoing hip preservation surgery for the treatment of symptomatic cam FAI (early FAI group; n = 15) and advanced OA secondary to cam FAI (late FAI group; n = 15). Samples procured from healthy young adult donors served as the control group (n = 7). Cartilage degeneration was assessed by histology, and the expression of inflammation-related proteins (interleukin-1 beta [IL-1β], matrix metalloproteinase-13 [MMP-13], a disintegrin and metalloproteinase with thrombospondin motifs-4 [ADAMTS-4], type II collagen [COL2], and aggrecan neoepitope [NITEGE]) was measured by immunostaining. Synovial samples in the early and late FAI groups were examined for synovitis and the expression of IL-1β.

RESULTS

Head-neck cartilage in the early FAI group showed significantly more degeneration than the control group and an increased expression of inflammation-related proteins (IL-1β: 69.7% ± 18.1% vs 20.2% ± 4.9%, respectively; MMP-13: 79.6% ± 12.6% vs 25.3% ± 9.5%; ADAMTS-4: 83.9% ± 12.2% vs 24.3% ± 11.1%; NITEGE: 89.7% ± 7.7% vs 39.8% ± 20.5%) (P < .001). Head-neck and acetabular cartilage in the early and late FAI groups showed a similar degree of degeneration. Moreover, a similar expression of inflammation-related proteins was observed between the early and late FAI groups for head-neck cartilage (IL-1β: 69.7% ± 18.1% vs 72.5% ± 13.2%; MMP-13: 79.6% ± 12.6% vs 71.4% ± 18.8%; ADAMTS-4: 83.9% ± 12.2% vs 82.6% ± 12.5%; COL2: 93.6% ± 3.9% vs 92.5% ± 5.8%; NITEGE: 89.7% ± 7.7% vs 95.7% ± 4.7%) and acetabular cartilage (IL-1β: 83.3% ± 24.8% vs 80.7% ± 15.6%; MMP-13: 94.3% ± 9.7% vs 85.2% ± 12.3%; ADAMTS-4: 98.5% ± 2.3% vs 98.4% ± 3.4%; COL2: 99.8% ± 0.7% vs 99.7% ± 1.1%; NITEGE: 96.7% ± 6.7% vs 99.2% ± 2.2%). In contrast, synovitis was minimal with a low expression of IL-1β in the early FAI group compared with the late FAI group.

CONCLUSION

Hip cartilage exhibited an OA phenotype in patients with early-stage FAI, similar to what was observed in hip OA secondary to FAI. Severe synovitis was only evident with late-stage FAI.

CLINICAL RELEVANCE

This study supports the concept that early hip impingement is associated with cartilage degeneration and catabolism.

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.

Viability and Tissue Quality of Cartilage Flaps From Patients With Femoroacetabular Hip Impingement: A Matched-Control Comparison

Background:

Chondrolabral damage is commonly observed in patients with cam-type femoroacetabular impingement (FAI). Chondral flap reattachment has recently been proposed as a possible preservation technique.

Purpose/Hypothesis:

The purpose of this study was to determine the viability and tissue quality of chondral flaps from patients with FAI at the time of arthroscopy. It was hypothesized that chondral flaps from patients with cam lesions of the hip would exhibit less viability and greater tissue degeneration than would those of a matched control group.

Study Design:

Cohort study; Level of evidence, 2.

Methods:

Patients with cam-type FAI who were treated with hip arthroscopy between 2014 and 2016 were asked to participate in this study. The cartilage lesions were localized and classified intraoperatively according to Beck classification. A chondral flap (study group) and a cartilage sample (control group) were obtained from each patient for histologic evaluation. Cellular viability and tissue quality were examined and compared in both groups. Cellular viability was determined with live/dead staining, and tissue quality was evaluated using safranin O/fast green, hematoxylin and eosin (H&E) staining, and immunohistochemistry for collagen II. Osteoarthritis Research Society International (OARSI) grading was used for quality assessment, and Image J software was used to calculate the percentage of tissue viability and Col II stain.

Results:

A total of 10 male patients with a mean age of 38.4 years (range, 30-55 years) were enrolled. All chondral flaps were classified as Beck grade 4. The mean cellular viability of the chondral flaps was reduced (54.6% ± 25.6%), and they were found to be degenerated (OARSI grade, 4 ± 1.27). Control samples also had reduced viability (38.8% ± 30.3%) and were degenerative (OARSI grade, 3.5 ± 1.38). There was no statistically significant intergroup difference for viability (P = .203) or OARSI grade (P = .645), nor was there an intragroup correlation between viability and OARSI grade (P > .05). A significant negative correlation (r = −0.9, P = .035) was found between OARSI grade and collagen II percentage scale in 5 selected samples.

Conclusion:

Despite appearing normal macroscopically, the chondral flaps from patients with cam-type FAI displayed loss of viability and tissue degeneration. In addition, control samples obtained away from the injury area also displayed cartilage damage and degeneration. Careful consideration should be taken when attempting to reattach the chondral flap.

Inflammation and Degeneration in Cartilage Samples from Patients with Femoroacetabular Impingement

BACKGROUND

Femoroacetabular impingement (FAI) has been reported as a cause of hip pain in young patients and is suggested as the trigger for hip osteoarthritis (OA). The goal of this study was to quantify the metabolic profiles of articular tissues (cartilage, synovium, and labrum) harvested from patients with FAI and with end-stage OA. In addition, we sought to investigate the development of secondary OA in hips with FAI.

METHODS

Tissue samples were obtained from thirty hips undergoing arthroscopic surgery for FAI with or without labral tear and thirty hips undergoing total hip arthroplasty for OA. Quantitative real-time polymerase chain reaction (PCR) was performed to determine the gene expression of inflammatory cytokines and metabolic (anabolic and catabolic) enzymes. The differences in gene expression in articular tissues from the patients with FAI were also evaluated on the basis of clinical parameters (age range and alpha angle).

RESULTS

The messenger RNA (mRNA) expression of the inflammatory cytokines interleukin-1 beta (IL-1β) and IL-8 and of matrix metalloproteinase (MMP)-3 (a catabolic gene) in both the synovium and the labrum, and the expression of collagen type I alpha 1 (an anabolic gene) in the labrum, was higher in the samples from hips with OA than in those from hips with FAI (p < 0.05). In cartilage, however, the mRNA expression of the inflammatory cytokines and the catabolic genes MMP-13 and ADAMTS-4 (a disintegrin and metalloproteinase with thrombospondin motifs-4) was higher in the FAI samples compared with the OA samples (p < 0.01). When the expression of inflammatory cytokines was evaluated among the three types of tissues within each disease group, the expression levels were the highest in cartilage within the FAI samples (p < 0.01). In FAI cartilage, we found higher gene expression of aggrecan (ACAN) and ADAMTS-4 in the samples from patients with larger alpha angles (≥60°) (p < 0.01).

CONCLUSIONS

Our results indicate that the metabolic conditions of articular cartilage in FAI and OA are different and that the expression of genes associated with inflammation is greater in the articular cartilage of patients with FAI compared with the synovium and the labrum. The metabolic changes were heightened by mechanical impingement.

CLINICAL RELEVANCE

The articular cartilage from the impingement lesion in patients with FAI showed biologically higher inflammation and degeneration, supporting the concept that FAI may be a trigger for joint degeneration.

Laminins and Nidogens in the Pericellular Matrix of Chondrocytes: Their Role in Osteoarthritis and Chondrogenic Differentiation

The aim of this study was to investigate the role of laminins and nidogen-2 in osteoarthritis (OA) and their potential to support chondrogenic differentiation. We applied immunohistochemistry, electron microscopy, siRNA, quantitative RT-PCR, Western blot, and proteome analysis for the investigation of cartilage tissue and isolated chondrocytes in three-dimensional culture obtained from patients with late-stage knee OA and nidogen-2 knockout mice. We demonstrate that subunits of laminins appear in OA cartilage and that nidogen-2-null mice exhibit typical osteoarthritic features. Chondrogenic progenitor cells (CPCs) produced high levels of laminin-α1, laminin-α5, and nidogen-2 in their pericellular matrix, and laminin-α1 enhanced collagen type II and reduced collagen type I expression by cultured CPCs. Nidogen-2 increased SOX9 gene expression. Knockdown of nidogen-2 reduced SOX9 expression, whereas it up-regulated RUNX2 expression. This study reveals that the influence of the pericellular matrix on CPCs is important for the expression of the major regulator transcription factors, SOX9 and RUNX2. Our novel findings that laminins and nidogen-2 drive CPCs toward chondrogenesis may help in the elucidation of new treatment strategies for cartilage tissue regeneration.

Elevated expression of periostin in human osteoarthritic cartilage and its potential role in matrix degradation via matrix metalloproteinase-13

We investigated the role of periostin, an extracellular matrix protein, in the pathophysiology of osteoarthritis (OA). In OA, dysregulated gene expression and phenotypic changes in articular chondrocytes culminate in progressive loss of cartilage from the joint surface. The molecular mechanisms underlying this process are poorly understood. We examined periostin expression by immunohistochemical analysis of lesional and nonlesional cartilage from human and rodent OA knee cartilage. In addition, we used small interfering (si)RNA and adenovirus transduction of chondrocytes to knock down and up-regulate periostin levels, respectively, and analyzed its effect on matrix metalloproteinase (MMP)-13, a disintegrin and MMP with thrombospondin motifs (ADAMTS)-4, and type II collagen expression. We found high periostin levels in human and rodent OA cartilage. Periostin increased MMP-13 expression dose [1-10 µg/ml (EC50 0.5-1 μg/ml)] and time (24-72 h) dependently, significantly enhanced expression of ADAMTS4 mRNA, and promoted cartilage degeneration through collagen and proteoglycan degradation. Periostin induction of MMP-13 expression was inhibited by CCT031374 hydrobromide, an inhibitor of the canonical Wnt/β-catenin signaling pathway. In addition, siRNA-mediated knockdown of endogenous periostin blocked constitutive MMP-13 expression. These findings implicate periostin as a catabolic protein that promotes cartilage degeneration in OA by up-regulating MMP-13 through canonical Wnt signaling.

A discoidin domain receptor 1 knock-out mouse as a novel model for osteoarthritis of the temporomandibular joint

Discoidin domain receptor 1 (DDR-1)-deficient mice exhibited a high incidence of osteoarthritis (OA) in the temporomandibular joint (TMJ) as early as 9 weeks of age. They showed typical histological signs of OA, including surface fissures, loss of proteoglycans, chondrocyte cluster formation, collagen type I upregulation, and atypical collagen fibril arrangements. Chondrocytes isolated from the TMJs of DDR-1-deficient mice maintained their osteoarthritic characteristics when placed in culture. They expressed high levels of runx-2 and collagen type I, as well as low levels of sox-9 and aggrecan. The expression of DDR-2, a key factor in OA, was increased. DDR-1-deficient chondrocytes from the TMJ were positively influenced towards chondrogenesis by a three-dimensional matrix combined with a runx-2 knockdown or stimulation with extracellular matrix components, such as nidogen-2. Therefore, the DDR-1 knock-out mouse can serve as a novel model for temporomandibular disorders, such as OA of the TMJ, and will help to develop new treatment options, particularly those involving tissue regeneration.

Molecular characterization of articular cartilage from young adults with femoroacetabular impingement

BACKGROUND

Femoroacetabular impingement is a frequent cause of hip pain and may lead to secondary osteoarthritis, yet little is known about the molecular events linking mechanical hip impingement and articular cartilage degeneration. The first goal of this study was to quantify the expression of inflammatory cytokine and chemokine, matrix-degrading, and extracellular matrix genes in articular cartilage harvested from control hips and hips with femoroacetabular impingement and end-stage osteoarthritis. The second goal was to analyze the relative expression of these genes in articular cartilage harvested at various stages of osteoarthritis.

METHODS

Cartilage samples were obtained from thirty-two hips undergoing hip preservation surgery for femoroacetabular impingement or hip arthroplasty. Three control cartilage samples were also analyzed. Specimens were graded intraoperatively with regard to the severity of cartilage damage, the radiographic osteoarthritis grade was recorded, and quantitative RT-PCR (real-time polymerase chain reaction) was performed to determine relative gene expression.

RESULTS

Except for interleukin-1β (IL-1β) and CXCL2, the mRNA (messenger RNA) expression of all other chemokine (IL-8, CXCL1, CXCL3, CXCL6, CCL3, and CCL3L1), matrix-degrading (matrix metalloproteinase [MMP]-13 and ADAMTS-4), and structural matrix (COL2A1 [collagen, type II, alpha] and ACAN [aggregan]) genes was higher overall in cartilage from hips with femoroacetabular impingement compared with hips with osteoarthritis and normal controls. The differences reached significance (p ≤ 0.05) for seven of these ten quantified genes, with CXCL3, CXCL6, and COL2A1 being elevated in the femoroacetabular impingement group compared with only the control group and IL-8, CCL3L1, ADAMTS-4, and ACAN being elevated compared with both the osteoarthritis and control groups. When samples were grouped according to the stage of the degenerative cascade, mRNA expression was relatively higher in one of the two middle stages of femoroacetabular impingement (chondromalacia or cleavage/thinning), with the difference reaching significance for IL-8, CXCL2, CXCL3, CCL3L1, and ACAN. ACAN expression was diminished in hips with osteoarthritis compared with femoroacetabular impingement but elevated compared with the control articular cartilage.

CONCLUSIONS

Articular cartilage from the impingement zone of hips with femoroacetabular impingement (and particularly those hips in the cleavage/thinning stage) expressed higher levels of certain inflammatory, anabolic, and catabolic genes, representing a heightened metabolic state.

CLINICAL RELEVANCE

The articular cartilage from the impingement zone of hips with femoroacetabular impingement was metabolically hyperactive, supporting the concept that such impingement is a structural precursor to hip osteoarthritis.

Non-invasive imaging of cartilage in early osteoarthritis

Treatment for osteoarthritis (OA) has traditionally focused on joint replacement for end-stage disease. An increasing number of surgical and pharmaceutical strategies for disease prevention have now been proposed. However, these require the ability to identify OA at a stage when it is potentially reversible, and detect small changes in cartilage structure and function to enable treatment efficacy to be evaluated within an acceptable timeframe. This has not been possible using conventional imaging techniques but recent advances in musculoskeletal imaging have been significant. In this review we discuss the role of different imaging modalities in the diagnosis of the earliest changes of OA. The increasing number of MRI sequences that are able to non-invasively detect biochemical changes in cartilage that precede structural damage may offer a great advance in the diagnosis and treatment of this debilitating condition.

Time-series transcriptional profiling yields new perspectives on susceptibility to murine osteoarthritis

OBJECTIVE

Chronological age is a powerful epidemiologic risk factor for osteoarthritis (OA), a multifactorial disease that is characterized by articular cartilage (AC) degradation. It is unclear from a molecular perspective how aging interacts with OA to produce this risk to AC integrity. To address this key question, we used in vivo time-course analysis of OA development and murine interstrain variability in natural susceptibility to OA to examine changes in non-OA-prone CBA mice versus OA-prone STR/Ort mice, which develop disease that bears significant histologic resemblance to human OA. Through global transcriptome profiling, we attempted to discover the molecular signature linked with both OA vulnerability and progression.

METHODS

Affymetrix Mouse Gene 1.0 ST Array profiles were generated from AC samples derived from CBA and STR/Ort mice at 3 different ages, corresponding to the stages prior to, at, and late after the natural onset of OA in the STR/Ort mice.

RESULTS

We found that the OA in STR/Ort mice exhibited a molecular phenotype resembling human OA, and we pinpointed a central role of NF-κB signaling and the emergence of an immune-related signature in OA cartilage over time. We discovered that, strikingly, young healthy AC has a highly expressed skeletal muscle gene expression program, which is switched off during maturation, but is intriguingly retained in AC during OA development in STR/Ort mice.

CONCLUSION

This study is the first to show that AC chondrocytes share a high-abundance gene-expression program with skeletal muscle. We show that failure to switch this program off, as well as the restoration of this program, is associated with inappropriate expression of NF-κB signaling pathways, skeletal muscle-related genes, and induction and/or progression of OA.

Current concepts in stem cell therapy for articular cartilage repair

INTRODUCTION

Hyaline articular cartilage is the connective tissue responsible for frictionless joint movement. Its degeneration ultimately results in complete loss of joint function in the late stages of osteoarthritis. Intrinsic repair is compromised, and cartilage tissue regeneration is difficult. However, new options are available to repair cartilage tissue by applying ESCs, MSCs and CPCs.

AREAS COVERED

In this review, the authors shed light on the different concepts currently under investigation for cartilage repair.

EXPERT OPINION

So far, there is no way to derive a chondrogenic lineage from stem cells that forms functional hyaline cartilage tissue in vivo. One alternative might be to enhance the chondrogenic potential of repair cells, which are already present in diseased cartilage tissue. CPCs found in diseased cartilage tissue in situ are biologically driven toward the osteochondrogenic lineage and can be directed toward chondrogenesis at least in vitro.

Sequential alterations in catabolic and anabolic gene expression parallel pathological changes during progression of monoiodoacetate-induced arthritis

Chronic inflammation is one of the major causes of cartilage destruction in osteoarthritis. Here, we systematically analyzed the changes in gene expression associated with the progression of cartilage destruction in monoiodoacetate-induced arthritis (MIA) of the rat knee. Sprague Dawley female rats were given intra-articular injection of monoiodoacetate in the knee. The progression of MIA was monitored macroscopically, microscopically and by micro-computed tomography. Grade 1 damage was observed by day 5 post-monoiodoacetate injection, progressively increasing to Grade 2 by day 9, and to Grade 3-3.5 by day 21. Affymetrix GeneChip was utilized to analyze the transcriptome-wide changes in gene expression, and the expression of salient genes was confirmed by real-time-PCR. Functional networks generated by Ingenuity Pathways Analysis (IPA) from the microarray data correlated the macroscopic/histologic findings with molecular interactions of genes/gene products. Temporal changes in gene expression during the progression of MIA were categorized into five major gene clusters. IPA revealed that Grade 1 damage was associated with upregulation of acute/innate inflammatory responsive genes (Cluster I) and suppression of genes associated with musculoskeletal development and function (Cluster IV). Grade 2 damage was associated with upregulation of chronic inflammatory and immune trafficking genes (Cluster II) and downregulation of genes associated with musculoskeletal disorders (Cluster IV). The Grade 3 to 3.5 cartilage damage was associated with chronic inflammatory and immune adaptation genes (Cluster III). These findings suggest that temporal regulation of discrete gene clusters involving inflammatory mediators, receptors, and proteases may control the progression of cartilage destruction. In this process, IL-1β, TNF-α, IL-15, IL-12, chemokines, and NF-κB act as central nodes of the inflammatory networks, regulating catabolic processes. Simultaneously, upregulation of asporin, and downregulation of TGF-β complex, SOX-9, IGF and CTGF may be central to suppress matrix synthesis and chondrocytic anabolic activities, collectively contributing to the progression of cartilage destruction in MIA.

Migratory chondrogenic progenitor cells from repair tissue during the later stages of human osteoarthritis

The regeneration of diseased hyaline cartilage continues to be a great challenge, mainly because degeneration--caused either by major injury or by age-related processes--can overextend the tissue's self-renewal capacity. We show that repair tissue from human articular cartilage during the late stages of osteoarthritis harbors a unique progenitor cell population, termed chondrogenic progenitor cells (CPCs). These exhibit stem cell characteristics such as clonogenicity, multipotency, and migratory activity. The isolated CPCs, which exhibit a high chondrogenic potential, were shown to populate diseased tissue ex vivo. Moreover, downregulation of the osteogenic transcription factor runx-2 enhanced the expression of the chondrogenic transcription factor sox-9. This, in turn, increased the matrix synthesis potential of the CPCs without altering their migratory capacity. Our results offer new insights into the biology of progenitor cells in the context of diseased cartilage tissue. Our work may be relevant in the development of novel therapeutics for the later stages of osteoarthritis.

Prostaglandin E2 exerts catabolic effects in osteoarthritis cartilage: evidence for signaling via the EP4 receptor

Elevated levels of PGE(2) have been reported in synovial fluid and cartilage from patients with osteoarthritis (OA). However, the functions of PGE(2) in cartilage metabolism have not previously been studied in detail. To do so, we cultured cartilage explants, obtained from patients undergoing knee replacement surgery for advanced OA, with PGE(2) (0.1-10 muM). PGE(2) inhibited proteoglycan synthesis in a dose-dependent manner (maximum 25% inhibition (p < 0.01)). PGE(2) also induced collagen degradation, in a manner inhibitable by the matrix metalloproteinase (MMP) inhibitor ilomastat. PGE(2) inhibited spontaneous MMP-1, but augmented MMP-13 secretion by OA cartilage explant cultures. PCR analysis of OA chondrocytes treated with PGE(2) with or without IL-1 revealed that IL-1-induced MMP-13 expression was augmented by PGE(2) and significantly inhibited by the cycolooygenase 2 selective inhibitor celecoxib. Conversely, MMP-1 expression was inhibited by PGE(2), while celecoxib enhanced both spontaneous and IL-1-induced expression. IL-1 induction of aggrecanase 5 (ADAMTS-5), but not ADAMTS-4, was also enhanced by PGE(2) (10 muM) and reversed by celecoxib (2 muM). Quantitative PCR screening of nondiseased and end-stage human knee OA articular cartilage specimens revealed that the PGE(2) receptor EP4 was up-regulated in OA cartilage. Moreover, blocking the EP4 receptor (EP4 antagonist, AH23848) mimicked celecoxib by inhibiting MMP-13, ADAMST-5 expression, and proteoglycan degradation. These results suggest that PGE(2) inhibits proteoglycan synthesis and stimulates matrix degradation in OA chondrocytes via the EP4 receptor. Targeting EP4, rather than cyclooxygenase 2, could represent a future strategy for OA disease modification.

The assessment of early osteoarthritis

Treatment strategies for osteoarthritis most commonly involve the removal or replacement of damaged joint tissue. Relatively few treatments attempt to arrest, slow down or reverse the disease process. Such options include peri-articular osteotomy around the hip or knee, and treatment of femoro-acetabular impingement, where early intervention may potentially alter the natural history of the disease. A relatively small proportion of patients with osteoarthritis have a clear predisposing factor that is both suitable for modification and who present early enough for intervention to be deemed worthwhile. This paper reviews recent advances in our understanding of the pathology, imaging and progression of early osteoarthritis.