RANKL synthesized by articular chondrocytes contributes to juxta-articular bone loss in chronic arthritis

Evaluating the potential of Vitamin D and curcumin to alleviate inflammation and mitigate the progression of osteoarthritis through their effects on human chondrocytes: A proof-of-concept investigation

Osteoarthritis (OA) is a chronic degenerative joint disorder primarily affecting the elderly, characterized by a prominent inflammatory component. The long-term side effects associated with current therapeutic approaches necessitate the development of safer and more efficacious alternatives. Nutraceuticals, such as Vitamin D and curcumin, present promising therapeutic potentials due to their safety, efficacy, and cost-effectiveness. In this study, we utilized a proinflammatory human chondrocyte model of OA to assess the anti-inflammatory properties of Vitamin D and curcumin, with a particular focus on the Protease-Activated Receptor-2 (PAR-2) mediated inflammatory pathway. Employing a robust siRNA approach, we effectively modulated the expression of PAR-2 to understand its role in the inflammatory process. Our results reveal that both Vitamin D and curcumin attenuate the expression of PAR-2, leading to a reduction in the downstream proinflammatory cytokines, such as Tumor Necrosis Factor-alpha (TNF-α), Interleukin 6 (IL-6), and Interleukin 8 (IL-8), implicated in the OA pathogenesis. Concurrently, these compounds suppressed the expression of Receptor Activator of Nuclear Factor kappa-Β Ligand (RANKL) and its receptor RANK, which are associated with PAR-2 mediated TNF-α stimulation. Additionally, Vitamin D and curcumin downregulated the expression of Interferon gamma (IFN-γ), known to elevate RANKL levels, underscoring their potential therapeutic implications in OA. This study, for the first time, provides evidence of the mitigating effect of Vitamin D and curcumin on PAR-2 mediated inflammation, employing an siRNA approach in OA. Thus, our findings pave the way for future research and the development of novel, safer, and more effective therapeutic strategies for managing OA.

The dual pro-inflammatory and bone-protective role of calcitonin gene-related peptide alpha in age-related osteoarthritis

BACKGROUND

The vasoactive neuropeptide calcitonin gene-related peptide alpha (αCGRP) enhances nociception in primary knee osteoarthritis (OA) and has been shown to disrupt cartilage and joint integrity in experimental rheumatoid arthritis (RA). Little is known about how αCGRP may alter articular structures in primary OA. We investigated whether αCGRP modulates local inflammation and concomitant cartilage and bone changes in a murine model of age-dependent OA.

METHODS

Sixteen- to 18-month-old αCGRP-deficient mice (αCGRP-/-aged) were compared to, first, age-matched wild type (WTaged) and, second, young 4- to 5-month-old non-OA αCGRP-deficient (αCGRP-/-CTRL) and non-OA WT animals (WTCTRL). αCGRP levels were measured in serum. Knee and hip joint inflammation, cartilage degradation, and bone alterations were assessed by histology (OARSI histopathological grading score), gene expression analysis, and µ-computed tomography.

RESULTS

WTaged mice exhibited elevated αCGRP serum levels compared to young WTCTRL animals. Marked signs of OA-induced cartilage destruction were seen in WTaged animals, while αCGRP-/-aged mice were mostly protected from this effect. Age-dependent OA was accompanied by an increased gene expression of pro-inflammatory Tnfa, Il1b, and Il6 and catabolic Mmp13, Adamts5, Ctsk, Tnfs11 (Rankl), and Cxcl12/Cxcr4 in WTaged but not in αCGRP-/-aged mice. αCGRP-deficiency however further aggravated subchondral bone sclerosis of the medial tibial plateau and accelerated bone loss in the epi- and metaphyseal trabecular tibial bone in age-dependent OA.

CONCLUSIONS

Similar to its function in experimental RA, αCGRP exerts a dual pro-inflammatory and bone-protective function in murine primary OA. Although anti-CGRP treatment was previously not successful in reducing pain in OA clinically, these data underline a crucial pathophysiological role of αCGRP in age-related OA.

MSC therapy ameliorates experimental gouty arthritis hinting an early COX-2 induction

Objective

The specific effect of Adipose-Derived Mesenchymal Stem Cells (Ad-MSC) on acute joint inflammation, where the response mostly depends on innate immunity activation, remains elusive. The pathogenesis of gouty arthritis, characterized by the deposition of monosodium urate (MSU) crystals in the joints, associated to acute flares, has been associated to NLRP3 inflammasome activation and subsequent amplification of the inflammatory response. Our aim was to study the effect of human Ad-MSC administration in the clinical inflammatory response of rabbits after MSU injection, and the molecular mechanisms involved.

Methods

Ad-MSC were administered by intraarterial route shortly after intraarticular MSU crystal injections. Joint and systemic inflammation was sequentially studied, and the mechanisms involved in NLRP3 inflammasome activation, and the synthesis of inflammatory mediators were assessed in the synovial membranes 72h after insult. Ad-MSC and THP-1-derived macrophages stimulated with MSU were co-cultured in transwell system.

Results

A single systemic dose of Ad-MSC accelerated the resolution of local and systemic inflammatory response. In the synovial membrane, Ad-MSC promoted alternatively M2 macrophage presence, inhibiting NLRP3 inflammasome and inducing the production of anti-inflammatory cytokines, such as IL-10 or TGF-β, and decreasing nuclear factor-κB activity. Ad-MSC induced a net anti-inflammatory balance in MSU-stimulated THP-1 cells, with a higher increase in IL-10 and IDO expression than that observed for IL-1β and TNF.

Conclusion

Our in vivo and in vitro results showed that a single systemic dose of Ad-MSC decrease the intensity and duration of the inflammatory response by an early local COX-2 upregulation and PGE2 release. Ad-MSCs suppressed NF-kB activity, NLRP3 inflammasome, and promoted the presence of M2 alternative macrophages in the synovium. Therefore, this therapeutic approach could be considered as a pharmacological alternative in patients with comorbidities that preclude conventional treatment.

Bone marrow lesions in osteoarthritis: From basic science to clinical implications

Osteoarthritis (OA) is the most prevalent musculoskeletal disease characterized by multiple joint structure damages, including articular cartilage, subchondral bone and synovium, resulting in disability and economic burden. Bone marrow lesions (BMLs) are common and important magnetic resonance imaging (MRI) features in OA patients. Basic and clinical research on subchondral BMLs in the pathogenesis of OA has been a hotspot. New evidence shows that subchondral bone degeneration, including BML and angiogenesis, occurs not only at or after cartilage degeneration, but even earlier than cartilage degeneration. Although BMLs are recognized as important biomarkers for OA, their exact roles in the pathogenesis of OA are still unclear, and disputes about the clinical impact and treatment of BMLs remain. This review summarizes the current basic and clinical research progress of BMLs. We particularly focus on molecular pathways, cellular abnormalities and microenvironmental changes of subchondral bone that contributed to the formation of BMLs, and emphasize the crosstalk between subchondral bone and cartilage in OA development. Finally, potential therapeutic strategies targeting BMLs in OA are discussed, which provides novel strategies for OA treatment.

RANKL expression in chondrocytes and its promotion by lymphotoxin-α in the course of cartilage destruction during rheumatoid arthritis

We investigated the expression and localization of the receptor activator nuclear factor κB ligand (RANKL) in cartilage from patients with rheumatoid arthritis (RA) of relevance to cartilage degeneration. We also examined the role of exogenous lymphotoxin (LT)-α on RANKL expression in human chondrocytes and its effect on in vitro osteoclast differentiation. Cartilage and synovial fluid samples were obtained from 45 patients undergoing total joint replacement surgery or joint puncture, including 24 patients with osteoarthritis (OA) and 21 patients with RA. RANKL expression in articular cartilage was examined by immunohistochemistry. LT-α concentrations in synovial fluid were measured using an enzyme-linked immunosorbent assay (ELISA). Normal human chondrocytes were stimulated with LT-α, and the relative mRNA levels of RANKL, osteoprotegerin (OPG), matrix metalloproteinase-9, and vascular endothelial growth factor were examined by real-time polymerase chain reaction. Soluble RANKL protein in culture media was measured using ELISA, and membrane-bound RANKL protein in cells was examined by western blotting. Co-cultures of human chondrocytes with peripheral blood mononuclear cells (PBMCs) were stimulated with macrophage-colony stimulating factor and LT-α, and osteoclast differentiation was evaluated by staining for tartrate-resistant acid phosphatase. LT-α concentrations were higher in RA synovial fluid than in OA samples. The population of RANKL-positive chondrocytes of RA cartilage was higher than that of OA cartilage, and correlated with cartilage degeneration. Stimulation of cultured human chondrocytes by LT-α increased RANKL expression, the RANKL/OPG ratio, and angiogenic factors. Membrane-bound RANKL in chondrocytes was up-regulated after stimulation of LT-α, whereas soluble RANKL in culture medium did not increase. Co-cultures of human chondrocytes and PBMCs demonstrated that LT-α stimulated human chondrocytes to produce RANKL and induced osteoclastic differentiation of PBMCs. RANKL produced by chondrocytes may contribute to cartilage destruction during RA and LT-α could promote the expression of RANKL in human chondrocytes.

Subchondral bone microenvironment in osteoarthritis and pain

Osteoarthritis comprises several joint disorders characterized by articular cartilage degeneration and persistent pain, causing disability and economic burden. The incidence of osteoarthritis is rapidly increasing worldwide due to aging and obesity trends. Basic and clinical research on osteoarthritis has been carried out for decades, but many questions remain unanswered. The exact role of subchondral bone during the initiation and progression osteoarthritis remains unclear. Accumulating evidence shows that subchondral bone lesions, including bone marrow edema and angiogenesis, develop earlier than cartilage degeneration. Clinical interventions targeting subchondral bone have shown therapeutic potential, while others targeting cartilage have yielded disappointing results. Abnormal subchondral bone remodeling, angiogenesis and sensory nerve innervation contribute directly or indirectly to cartilage destruction and pain. This review is about bone-cartilage crosstalk, the subchondral microenvironment and the critical role of both in osteoarthritis progression. It also provides an update on the pathogenesis of and interventions for osteoarthritis and future research targeting subchondral bone.

Varying development of femoral and tibial subchondral bone tissue and their interaction with articular cartilage during progressing osteoarthritis

INTRODUCTION

Differences between tibial and femoral joint surfaces and knee compartments concerning coupled bone and cartilage turnover or bone-cartilage cross talk have not been previously examined, although the mechanical and biological interaction of the mineralized subchondral tissues with articular cartilage is of great importance for advancing osteoarthritis.

MATERIALS AND METHODS

Therefore, with the help of immunohistochemistry and real-time polymerase chain reaction (RT-PCR), human knee joint cartilage tissue was investigated for expression of key molecules of the extracellular matrix and cartilage composition (collagen type I and II, aggrecan) plus proteoglycan content (colorimetric analysis). Furthermore, we correlated the results with 3D microcomputed tomography of the underlying subchondral bone (high-resolution micro-CT system). Measurements were performed in dependence of the anatomical site (femoral vs. tibial, medial and lateral each) to identify regional differences during the osteoarthritic process. From an enduring series of 108 patients undergoing implantation of TKA, 34 osteochondral samples with lesions macroscopically classified as ICRS grade 1b (group A) and 34 samples with ICRS grade 3a/3b lesions (group B) were compared with 21 healthy controls.

RESULTS

Concerning 3D analysis, the medial femoral condyle and tibia showed the most significant increase in bone volume fraction and a decrease in the trabecular number in group B frequently accompanied by subchondral bone resorption pits and enchondral ossification. Under physiological conditions, tibia plateaus show lower bone volume fraction than the corresponding femoral site and this difference enlarges with advancing OA. Partially even contradictory behavior was observed such as trabecular separation at the lateral tibial and medial plateau in osteochondral OA samples of the same patients. Collagen type II expression levels show faster and varying changes than type I during the OA process, leading to a lower positive or negative correlation with bone microstructural analysis, especially on the tibia plateau.

CONCLUSIONS

Structural bone and cartilage parameter changes showed varying developments and correlations among each other in the different compartments of the knee. As a clinical conclusion, therapies to postpone or prevent cartilage degeneration by influencing the loss of mineralized bone could be site dependent.

Small Non-Coding RNAome of Ageing Chondrocytes

Ageing is a leading risk factor predisposing cartilage to osteoarthritis. However, little research has been conducted on the effect of ageing on the expression of small non-coding RNAs (sncRNAs). RNA from young and old chondrocytes from macroscopically normal equine metacarpophalangeal joints was extracted and subjected to small RNA sequencing (RNA-seq). Differential expression analysis was performed in R using package DESeq2. For transfer RNA (tRNA) fragment analysis, tRNA reads were aligned to horse tRNA sequences using Bowtie2 version 2.2.5. Selected microRNA (miRNAs or miRs) and small nucleolar RNA (snoRNA) findings were validated using real-time quantitative Polymerase Chain Reaction (qRT-PCR) in an extended cohort of equine chondrocytes. tRNA fragments were further investigated in low- and high-grade OA human cartilage tissue. In total, 83 sncRNAs were differentially expressed between young and old equine chondrocytes, including miRNAs, snoRNAs, small nuclear RNAs (snRNAs), and tRNAs. qRT-PCR analysis confirmed findings. tRNA fragment analysis revealed that tRNA halves (tiRNAs), tiRNA-5035-GluCTC and tiRNA-5031-GluCTC-1 were reduced in both high grade OA human cartilage and old equine chondrocytes. For the first time, we have measured the effect of ageing on the expression of sncRNAs in equine chondrocytes. Changes were detected in a number of different sncRNA species. This study supports a role for sncRNAs in ageing cartilage and their potential involvement in age-related cartilage diseases.

The role of indoleamine 2,3 dioxygenase 1 in the osteoarthritis

Osteoarthritis (OA) is a chronic degenerative joint disease and a leading cause of disability. It involves articular cartilage destruction and a whole joint inflammation. In spite of OA pathogenesis is still unclear, new studies on the OA pathophysiological aetiology and immunomodulation therapy continuously achieve significant advances with new concepts. Here, we focus on the indoleamine-2,3-dioxygenase1 (IDO1) activity in the osteoarthritis (OA), which is one of the noticeable enzymes in the synovial fluid of arthritis patients. It was recognized as an essential mediator of autoreactive B and T cell responses in rheumatoid arthritis (RA) and an interesting therapeutic target against RA. However, the role IDO1 plays in the OA pathogenesis hasn't been discussed. The new OA experimental analysis evidenced IDO1 overexpression in the synovial fluid of OA patients, and recent studies reported that IDO1 metabolites were found higher in the OA synovial fluid than RA and spondyloarthropathies (SpA) patients. Moreover, the positive relation of IDO1 metabolites with OA pain and joint stiffness has been confirmed. Thus, the IDO1 plays a pivotal role in the pathogenesis of OA. In this review, the role IDO1 plays in the OA pathogenesis has been deeply discussed. It could be a promising target in the immunotherapy of OA disease.

Dual Role of Chondrocytes in Rheumatoid Arthritis: The Chicken and the Egg

Rheumatoid arthritis (RA) is one of the inflammatory joint diseases that display features of articular cartilage destruction. The underlying disturbance results from immune dysregulation that directly and indirectly influence chondrocyte physiology. In the last years, significant evidence inferred from studies in vitro and in the animal model offered a more holistic vision of chondrocytes in RA. Chondrocytes, despite being one of injured cells in RA, also undergo molecular alterations to actively participate in inflammation and matrix destruction in the human rheumatoid joint. This review covers current knowledge about the specific cellular and biochemical mechanisms that account for the chondrocyte signatures of RA and its potential applications for diagnosis and prognosis in RA.

FOXO1 Deletion Reverses the Effect of Diabetic-Induced Impaired Fracture Healing

Type 1 diabetes impairs fracture healing. We tested the hypothesis that diabetes affects chondrocytes to impair fracture healing through a mechanism that involves the transcription factor FOXO1. Type 1 diabetes was induced by streptozotocin in mice with FOXO1 deletion in chondrocytes (Col2α1Cre+FOXO1L/L) or littermate controls (Col2α1Cre-FOXO1L/L) and closed femoral fractures induced. Diabetic mice had 77% less cartilage and 30% less bone than normoglycemics evaluated histologically and by micro-computed tomography. Both were reversed with lineage-specific FOXO1 ablation. Diabetic mice had a threefold increase in osteoclasts and a two- to threefold increase in RANKL mRNA or RANKL-expressing chondrocytes compared with normoglycemics. Both parameters were rescued by FOXO1 ablation in chondrocytes. Conditions present in diabetes, high glucose (HG), and increased advanced glycation end products (AGEs) stimulated FOXO1 association with the RANKL promoter in vitro, and overexpression of FOXO1 increased RANKL promoter activity in luciferase reporter assays. HG and AGE stimulated FOXO1 nuclear localization, which was reversed by insulin and inhibitors of TLR4, histone deacetylase, nitric oxide, and reactive oxygen species. The results indicate that chondrocytes play a prominent role in diabetes-impaired fracture healing and that high levels of glucose, AGEs, and tumor necrosis factor-α, which are elevated by diabetes, alter RANKL expression in chondrocytes via FOXO1.

Potential Role of Rebamipide in Osteoclast Differentiation and Mandibular Condylar Cartilage Homeostasis

Background:

Temporomandibular joint osteoarthritis (TMJ-OA) is a degenerative disease that involves changes in subchondral bone and progressive degradation of cartilage. Currently, rebamipide, a gastroprotective drug, is administered to protect gastric mucosa and accelerate ulcer healing.

Objectives:

Recent studies have shown that rebamipide also attenuates cartilage degeneration by suppressing oxidative damage and inducing homeostasis of the extracellular matrix of articular chondrocytes. Regarding the latter, reduced expression of cathepsin K, NFATc1, c-Src, and integrin β3, and increased expression of nuclear factor-kappa B, have been found to be mediated by the transcription factor, receptor activator of nuclear factor kappa-B ligand (RANKL).

Methods:

Treatment with rebamipide was also found to activate, mitogen-activated protein kinases such as p38, ERK, and JNK to reduce osteoclast differentiation. Taken together, these results strongly indicate that rebamipide mediates inhibitory effects on cartilage degradation and osteoclastogenesis in TMJ-OA.

Results and Conclusion:

Here, we highlight recent evidence regarding the potential for rebamipide to affect osteoclast differentiation and TMJ-OA pathogenesis. We also discuss the potential role of rebamipide to serve as a new strategy for the treatment of TMJ-OA.

Association of high titers of anti-carbamylated protein antibodies with decreased bone mineral density in early arthritis patients

Rheumatoid arthritis (RA) has a negative impact on bone that is partly mediated by anti-citrullinated proteins antibodies (ACPA). These antibodies are associated with erosions, and with juxta-articular and systemic bone loss. Other RA autoantibodies, the anti-carbamylated protein antibodies (anti-CarPA), are independently associated with erosions. However, we do not know if they are also associated with juxta-articular and systemic bone loss. Here, we have addressed this question with data from 548 early arthritis (EA) patients. Bone mineral density (BMD) was assessed by dual-energy x-ray absorptiometry at the lumbar spine (LS), total hip (TH) and metacarpophalangeal joints (MCP). The 25.9% anti-CarPA positive patients did not show significant differences in BMD Z-scores with the negative patients. Nevertheless, this result was due to the similarity between negative and low-positive (below the median of the positive) patients, whereas the high-positive patients showed significant decrease of BMD at LS (β = -0.39, p = 0.01) and TH (β = -0.30, p = 0.02); but not at the juxta-articular bone of MCP. Given the overlap between anti-CarPA and ACPA, we included the two autoantibodies in an analysis that showed significantly lower BMD Z-scores at LS and TH (p< 0.01) only in the ACPA positive/anti-CarPA high-positive subgroup. However, the similar coefficients of regression between the ACPA positive/anti-CarPA high-positive and the ACPA negative/anti-CarPA high-positive subgroups (β = -0.50 vs. -0.52 at LS, and β = -0.37 vs. -0.30 at TH) suggested an independent association. Overall, these results support a contribution of anti-CarPA to systemic bone loss in EA patients.

Increased synovial lipodystrophy induced by high fat diet aggravates synovitis in experimental osteoarthritis

BACKGROUND

Metabolic syndrome (MetS) may be associated with knee osteoarthritis (OA), but the association between the individual components and OA are not well-understood. We aimed to study the effect of hypercholesterolemia on synovial inflammation in knee OA.

METHODS

OA was surgically induced in rabbits fed with standard diet (OA group, n = 10) or in rabbits fed with high fat diet (OA-HFD, n = 10). Healthy rabbits receiving standard diet (Control, n = 10) or fed with HFD (HFD, n = 6) were also monitored. Twelve weeks after OA induction, synovial membranes were isolated and processed for studies.

RESULTS

Animals fed HFD showed higher levels of total serum cholesterol, triglycerides and C-reactive protein than control rabbits. Twelve weeks after OA induction, synovial membrane inflammation and macrophage infiltration were increased in rabbits with OA, particularly in the OA-HFD group. Extensive decrease of synovial adipose tissue area, adipocyte size and perilipin-1A synthesis were observed in the OA-HFD group in comparison to the OA and control groups. The HFD further increased the proinflammatory mediators IL-1β, IL-6 and TNF in the OA synovium. However, the synovial gene expression of adipokines, such as leptin and adiponectin, were markedly decreased in the rabbits with OA, especially in the OA-HFD group, in correlation with adipose tissue loss. However, circulating leptin was upregulated in the HFD and OA-HFD groups.

CONCLUSION

Our results indicate that a HFD is an aggravating factor worsening synovial membrane inflammation during OA, guided by increased infiltration of macrophages and removal of the adipose tissue, together with a remarkable presence of proinflammatory factors. Synovial adipocytes and dyslipemia could probably play pivotal roles in OA joint deterioration in patients with MetS, supporting that the link between obesity and OA transcends mechanical loading.

Mesenchymal Stem Cells for Cartilage Regeneration of TMJ Osteoarthritis

Temporomandibular joint osteoarthritis (TMJ OA) is a degenerative disease, characterized by progressive cartilage degradation, subchondral bone remodeling, synovitis, and chronic pain. Due to the limited self-healing capacity in condylar cartilage, traditional clinical treatments have limited symptom-modifying and structure-modifying effects to restore impaired cartilage as well as other TMJ tissues. In recent years, stem cell-based therapy has raised much attention as an alternative approach towards tissue repair and regeneration. Mesenchymal stem cells (MSCs), derived from the bone marrow, synovium, and even umbilical cord, play a role as seed cells for the cartilage regeneration of TMJ OA. MSCs possess multilineage differentiation potential, including chondrogenic differentiation as well as osteogenic differentiation. In addition, the trophic modulations of MSCs exert anti-inflammatory and immunomodulatory effects under aberrant conditions. Furthermore, MSCs combined with appropriate scaffolds can form cartilaginous or even osseous compartments to repair damaged tissue and impaired function of TMJ. In this review, we will briefly discuss the pathogenesis of cartilage degeneration in TMJ OA and emphasize the potential sources of MSCs and novel approaches for the cartilage regeneration of TMJ OA, particularly focusing on the MSC-based therapy and tissue engineering.

Unexpected Bone Formation Produced by RANKL Blockade

Denosumab (Dmab) is a humanized monoclonal antibody that blocks RANKL (receptor activator for nuclear factor κB ligand), thereby exerting a potent bone antiresorptive action. Dmab treatment leads to a dramatic and sustained increase in bone mass through mechanisms that are currently under debate. It is also a matter of controversy whether this potent action of Dmab could lead to intrabone dystrophic mineralization. Recent research has uncovered a possible anabolic role of Dmab involving RANKL-dependent reverse signaling in osteoblasts, and that bone marrow adipocytes can modulate osteoclastogenesis through the production of RANKL. We comment here on potential pathways which might account for the anabolic action of Dmab. The impact of this proposed mechanism needs to be addressed in further research.

Prolactin blocks the expression of receptor activator of nuclear factor κB ligand and reduces osteoclastogenesis and bone loss in murine inflammatory arthritis

Background

Prolactin (PRL) reduces joint inflammation, pannus formation, and bone destruction in rats with polyarticular adjuvant-induced arthritis (AIA). Here, we investigate the mechanism of PRL protection against bone loss in AIA and in monoarticular AIA (MAIA).

Methods

Joint inflammation, trabecular bone loss, and osteoclastogenesis were evaluated in rats with AIA treated with PRL (via osmotic minipumps) and in mice with MAIA that were null (Prlr-/-) or not (Prlr+/+) for the PRL receptor. To help define target cells, synovial fibroblasts from Prlr+/+ mice were treated or not with proinflammatory cytokines ((Cyt), including TNFα, IL-1β, and interferon (IFN)γ) with or without PRL, and these synovial cells were co-cultured or not with bone marrow osteoclast progenitors from Prlr+/+ or Prlr-/- mice.

Results

In AIA, PRL treatment reduced joint swelling, increased trabecular bone area, lowered osteoclast density, and reduced mRNA levels of osteoclast-associated genes (tartrate-resistant acid phosphatase (Trap)), cathepsin K (Ctsk), matrix metalloproteinase 9 (Mmp9), and receptor activator of nuclear factor κB or RANK (Tnfrsf11a)), of genes encoding cytokines with osteoclastogenic activity (Tnfa, Il1b, Il6, and receptor activator of nuclear factor κB ligand or RANKL (Tnfrsf11)), and of genes encoding for transcription factors and cytokines related to T helper (Th)17 cells (Rora, Rorc, Il17a, Il21, Il22) and to regulatory T cells (Foxp3, Ebi3, Il12a, Tgfb1, Il10). Prlr-/- mice with MAIA showed enhanced joint swelling, reduced trabecular bone area, increased osteoclast density, and elevated expression of Tnfa, Il1b, Il6, Trap, Tnfrsf11a, Tnfrsf11, Il17a, Il21, Il22, 1 l23, Foxp3, and Il10. The expression of the long PRL receptor form increased in arthritic joints, and in synovial membranes and cultured synovial fibroblasts treated with Cyt. PRL induced the phosphorylation/activation of signal transducer and activator of transcription-3 (STAT3) and inhibited the Cyt-induced expression of Il1b, Il6, and Tnfrsf11 in synovial fibroblast cultures. The STAT3 inhibitor S31-201 blocked inhibition of Tnfrsf11 by PRL. Finally, PRL acted on both synovial fibroblasts and osteoclast precursor cells to downregulate Cyt-induced osteoclast differentiation.

Conclusion

PRL protects against osteoclastogenesis and bone loss in inflammatory arthritis by inhibiting cytokine-induced expression of RANKL in joints and synovial fibroblasts via its canonical STAT3 signaling pathway.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-017-1290-4) contains supplementary material, which is available to authorized users.

Fine tuning of immunometabolism for the treatment of rheumatic diseases

All immune cells depend on specific and efficient metabolic pathways to mount an appropriate response. Over the past decade, the field of immunometabolism has expanded our understanding of the various means by which cells modulate metabolism to achieve the effector functions necessary to fight infection or maintain homeostasis. Harnessing these metabolic pathways to manipulate inappropriate immune responses as a therapeutic strategy in cancer and autoimmunity has received increasing scrutiny by the scientific community. Fine tuning immunometabolism to provide the desired response, or prevent a deleterious response, is an attractive alternative to chemotherapy or overt immunosuppression. The various metabolic pathways used by immune cells in rheumatoid arthritis, systemic lupus erythematosus and osteoarthritis offer numerous opportunities for selective targeting of specific immune cell subsets to manipulate cellular metabolism for therapeutic benefit in these rheumatologic diseases.

The combined therapy with chondroitin sulfate plus glucosamine sulfate or chondroitin sulfate plus glucosamine hydrochloride does not improve joint damage in an experimental model of knee osteoarthritis in rabbits

Osteoarthritis is the most common chronic joint disorder especially during aging. Although with controversies, glucosamine, both in its forms of sulfate and hydrochloride, and chondroitin sulfate are commonly employed to treat osteoarthritis. Due to the modest improve in the symptoms observed in patients treated with these drugs alone, a formulation combining both agents has been considered. The discrepant results achieved for pain control or structural improvement in osteoarthritis patients has been attributed to the quality of chemical formulations or different bias in clinical studies. The current study has been designed to test the effects of two different combined formulations with adequate pharmaceutical grade of these drugs in osteoarthritic joints, and to explore the underlying mechanisms modulated by both formulations in different osteoarthritis target tissues. Knee osteoarthritis was surgically induced in experimental rabbits. Some animals received the combined therapy (CT)1, (chondroitin sulfate 1200mg/day + glucosamine sulfate 1500mg/day), or the CT2 ((chondroitin sulfate 1200mg/day + glucosamine hydrochloride 1500mg/day). Neither CT1 nor CT2 significantly modified the cartilage damage or the synovial inflammation observed in osteoarthritic animals. Treatments were also unable to modify the presence of pro-inflammatory mediators, and the synthesis of metalloproteinases in the cartilage or in the synovium of osteoarthritic animals. Combined therapies did not modify the decrease in the subchondral bone mineral density observed in osteoarthritic rabbits. Therapies of chondroitin sulfate plus glucosamine sulfate or chondroitin sulfate plus glucosamine hydrochloride failed to improve structural damage or to ameliorate the inflammatory profile of joint tissues during experimental osteoarthritis.

Immune response and innervation signatures in aseptic hip implant loosening

Background

Aseptic loosening (AL) of hip prosthesis presents inflammation and pain as sign and symptom similarly to arthritis pathologies. Still, the immune and innervation profiles in hip AL remain unclear and their interplay is poorly explored. Herein, local tissue inflammatory response, sensory and sympathetic innervation as well as associated local mediators were assessed in hip joint microenvironment underlying AL and compared to osteoarthritis (OA).

Methods

Histopathological analysis, immune cells (macrophages, T, B cells and PMNs) as well as sensory and sympathetic nerve fibers (SP⁺, CGRP⁺, TH⁺) distribution and profiles were analyzed on tissues retrieved from patients with failed hip prostheses due to AL (n = 20) and hip OA (n = 15) by immunohistochemistry. Additionally, transcriptional levels of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6, IL-12a, iNOS), anti-inflammatory cytokine (IL-10), osteoclastic factor (RANKL) and bone remodeling factor (TGF-β1) were locally evaluated by qRT-PCR. Serum TGF-β1 levels were assessed preoperatively by ELISA.

Results

Histopathological analysis revealed that tissues, aseptic interface membranes of AL patients had distinct tissue architecture and immune cells profile when compared to OA synovial tissues. Macrophages, T cells and B cells showed significant differences in tissue distribution. In OA, inflammation is mostly confined to the vicinity of synovial membrane while in AL macrophages infiltrated throughout the tissue. This differential immune profile is also accompanied with a distinct pattern of sensory and sympathetic innervation. Importantly, in AL patients, a lack of sympathetic innervation aseptic interface membranes without compensation mechanisms at cellular levels was observed with simultaneous reorganization of sensorial innervation. Despite the different histopathological portrait, AL and OA patients exhibited similar transcriptional levels of genes encoding key proteins in local immune response. Nevertheless, in both pathologies, TGF-β1 expression was prominent in sites where the inflammation is occurring. However, at systemic level no differences were found.

Conclusion

These findings indicate that AL patients exhibit different local inflammatory response and innervation signatures from OA patients in hip joint. These insights shed the light on neuro-immune interplay in AL and highlight the need to better understand this crosstalk to unravel potential mechanisms for targeted-therapies to improve hip joint lifetime and treatment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-016-0950-5) contains supplementary material, which is available to authorized users.

The cast of clasts: catabolism and vascular invasion during bone growth, repair, and disease by osteoclasts, chondroclasts, and septoclasts

Three named cell types degrade and remove skeletal tissues during growth, repair, or disease: osteoclasts, chondroclasts, and septoclasts. A fourth type, unnamed and less understood, removes nonmineralized cartilage during development of secondary ossification centers. "Osteoclasts," best known and studied, are polykaryons formed by fusion of monocyte precursors under the influence of colony stimulating factor 1 (CSF)-1 (M-CSF) and RANKL. They resorb bone during growth, remodeling, repair, and disease. "Chondroclasts," originally described as highly similar in cytological detail to osteoclasts, reside on and degrade mineralized cartilage. They may be identical to osteoclasts since to date there are no distinguishing markers for them. Because osteoclasts also consume cartilage cores along with bone during growth, the term "chondroclast" might best be reserved for cells attached only to cartilage. "Septoclasts" are less studied and appreciated. They are mononuclear perivascular cells rich in cathepsin B. They extend a cytoplasmic projection with a ruffled membrane and degrade the last transverse septum of hypertrophic cartilage in the growth plate, permitting capillaries to bud into it. To do this, antiangiogenic signals in cartilage must give way to vascular trophic factors, mainly vascular endothelial growth factor (VEGF). The final cell type excavates cartilage canals for vascular invasion of articular cartilage during development of secondary ossification centers. The "clasts" are considered in the context of fracture repair and diseases such as arthritis and tumor metastasis. Many observations support an essential role for hypertrophic chondrocytes in recruiting septoclasts and osteoclasts/chondroclasts by supplying VEGF and RANKL. The intimate relationship between blood vessels and skeletal turnover and repair is also examined.

Influence of osteoarthritis grade on molecular signature of human cartilage

Articular chondrocytes maintain cartilage matrix turnover and have the capacity for anabolic and catabolic activities that can be influenced by injury and disease. This study tested the hypothesis that catabolic genes are upregulated with regional osteoarthritis (OA) disease severity within a joint. With IRB approval, specimens of knee cartilage obtained as discarded tissues from subjects undergoing arthroplasty were partitioned for each subject by OA disease severity and evaluated for gene expression by RT-PCR. There was regional OA grade-associated upregulation of expected inflammatory mediators TNF-α, TNF receptors, IFN-γ, and interleukins as well as genes encoding proteolytic enzymes, including Adamts-5 and MMPs. Osteoclast-related genes, cathepsin K, tartrate-resistant acid phosphatase (TRAP), RANKL, RANK, M-CSF, and c-fms, but not osteoprotegerin, were induced in advanced grades. In vitro treatment of normal human chondrocytes with interleukin-1β upregulated similar genes; this provides evidence that chondrocytes per se can be the source of osteoclast-related factors. Immunohistochemical staining showed that RANK- and RANKL-positive cells were abundant in advanced grades, especially in chondrocyte clusters. This suggests a possible autocrine mechanism by which an osteoclast phenotype is induced in articular chondrocytes. In sum, these studies identified gene expression signatures in human OA cartilage based upon regional disease severity within a joint. There was an effect of OA Grade on expression of osteoclastic lytic enzymes and regulatory factors in human articular chondrocytes. Induction of an osteoclast-like phenotype in chondrocytes may be part of OA progression and suggests specific therapeutic approaches.

Effects of short-term gentle treadmill walking on subchondral bone in a rat model of instability-induced osteoarthritis

OBJECTIVE

Subchondral bone cyst (SBC) growth, caused by osteoclast activity during early knee osteoarthritis (OA) pathogenesis, should be treated to prevent further progressions of OA. In the present study, we evaluated the effects of gentle treadmill walking on subchondral bone and cartilage changes in an experimental rat model of destabilized medial meniscus (DMM).

METHOD

Twelve-week-old Wistar rats underwent DMM surgery in their right knee and sham surgery in their left knee and were assigned to either the sedentary group or walking group (n = 42/group). Animals in the walking group were subjected to treadmill exercise 2 days after surgery, which included walking for 12 m/min, 30 min/day, 5 days/week for 1, 2, and 4 week(s). Subchondral bone and cartilage changes were evaluated by micro-CT analysis, histological analysis, and biomechanical analysis.

RESULTS

Treadmill walking had a tendency to suppress SBC growth, which was confirmed by micro-CT (P = 0.06) and positive staining for tartrate-resistant acid phosphatase (TRAP) activity for the osteoclast number per bone surface (P = 0.09) 4 weeks after surgery. These changes coincide with the prevention of cartilage degeneration as evaluated by the Osteoarthritis Research Society International (OARSI) score (P < 0.05) and biomechanically softening (P < 0.05). Furthermore, treadmill walking could suppressed increasing osteocyte deaths (P < 0.01), which was positively correlated with the OARSI score (r = 0.77; P < 0.01).

CONCLUSION

These results indicate biomechanical and biological links exist between cartilage and subchondral bone; preventive effects of treadmill walking on subchondral bone deterioration might be partly explained by the chondroprotective effects.

OsteoRheumatology: a new discipline?

This review summarises recent evidence about the interaction between bone, the immune system and cartilage in disabling conditions such as osteoarthritis, rheumatoid arthritis and spondyloarthritis. These topics have been recently discussed at the ‘OsteoRheumatology’ conference held in Genoa in October 2014. The meeting, at its 10th edition, has been conceived to bring together distinguished international experts in the fields of rheumatic and metabolic bone diseases with the aim of discussing emerging knowledge regarding the role of the bone tissue in rheumatic diseases. Moreover, this review focuses on new treatments based on underlying the pathophysiological processes in rheumatic diseases. Although, a number of issues still remain to be clarified, it seems quite clear that in clinical practice, as well as in basic and translational research, there is a need for more knowledge of the interactions between the cartilage, the immune system and the bone. In this context, ‘OsteoRheumatology’ represents a potential new discipline providing a greater insight into this interplay, in order to face the multifactorial and complex issues underlying common and disabling rheumatic diseases.

RANKL, OPG and CTR mRNA expression in the temporomandibular joint in rheumatoid arthritis

The calcitonin receptor (CTR) and receptor activator of nuclear factor κB ligand (RANKL) have been found to be involved in the differentiation of osteoclasts. The association between the RANKL:osteoprotegerin (OPG) expression ratio and the pathogenesis of bone-destructive rheumatoid arthritis (RA) has been described in several joints, but the available data for the temporomandibular joint (TMJ) are limited. The aim of the present study was to investigate the involvement of osteoclasts at sites of bone erosion by determining the CTR expression and the RANKL:OPG expression ratio in the TMJ in a collagen-induced arthritis (CIA) model. Forty-eight male Wistar rats were randomly divided into two groups: Control group, injected with saline solution for 6 weeks; and CIA group, injected with emulsion. The RANKL and OPG mRNA expression was significantly increased in immunized rats compared with that in non-immunized rats. The RANKL:OPG expression ratio on the trabecular bone surface was 9.0 and 6.4 in the CIA group at weeks 4 and 6, respectively, while the RANKL:OPG expression ratio in the controls was 1.0:2. CTR mRNA expression was significantly upregulated in immunized rats compared with that in non-immunized rats; the level of CTR mRNA in the CTR-positive osteoclasts on the trabecular bone surface was 10.9- and 7.8-fold higher in the CIA rats than that in the control rats at weeks 4 and 6, respectively. In conclusion, focal bone destruction in an experimental model of arthritis in the TMJ can be attributed to cells expressing CTR, a defining feature of osteoclasts. The expression of RANKL and OPG mRNA within the inflamed synovium provides an insight into the mechanism of osteoclast differentiation and function at the border of bone erosion in arthritis.

RANKL expressed on synovial fibroblasts is primarily responsible for bone erosions during joint inflammation

OBJECTIVE

RANKL is mainly expressed by synovial fibroblasts and T cells within the joints of rheumatoid arthritis patients. The relative importance of RANKL expression by these cell types for the formation of bone erosions is unclear. We therefore aimed to quantify the contribution of RANKL by each cell type to osteoclast differentiation and bone destruction during inflammatory arthritis.

METHODS

RANKL was specifically deleted in T cells (Tnfsf11(flox/Δ) Lck-Cre), in collagen VI expressing cells including synovial fibroblasts (Tnfsf11(flox/Δ) Col6a1-Cre) and in collagen II expressing cells including articular chondrocytes (Tnfsf11(flox/Δ) Col2a1-Cre). Erosive disease was induced using the collagen antibody-induced arthritis (CAIA) and collagen-induced arthritis (CIA) models. Osteoclasts and cartilage degradation were assessed by histology and bone erosions were assessed by micro-CT.

RESULTS

The inflammatory joint score during CAIA was equivalent in all mice regardless of cell-targeted deletion of RANKL. Significant increases in osteoclast numbers and bone erosions were observed in both the Tnfsf11(flox/Δ) and the Tnfsf11(flox/Δ) Lck-Cre groups during CAIA; however, the Tnfsf11(flox/Δ) Col6a1-Cre mice showed significant protection against osteoclast formation and bone erosions. Similar results on osteoclast formation and bone erosions were obtained in CIA mice. The deletion of RANKL on any cell type did not prevent articular cartilage loss in either model of arthritis used.

CONCLUSIONS

The expression of RANKL on synovial fibroblasts rather than T cells is predominantly responsible for the formation of osteoclasts and erosions during inflammatory arthritis. Synovial fibroblasts would be the best direct target in RANKL inhibition therapies.

Osteoclast activators are elevated in intervertebral disks with Modic changes among patients operated for herniated nucleus pulposus

PURPOSE

Modic changes (MC) are associated with low back pain (LBP). Inflammation is considered as a key factor that triggers symptoms in especially type I MC, but so far of the potential inflammatory candidates only TNFα has been linked to MC. The objective of the study was to analyze a set of inflammatory mediators in human surgical disk samples and quantify their association with MC in the adjacent vertebral bodies.

METHODS

The study sample consisted of 51 intervertebral disk tissue specimens; 20 'No MC' disks, 19 'Type I MC' disks, and 12 'Type II MC' disks. mRNA expression of 46 cytokines was quantified from isolated RNA. Tissue samples were stained using hematoxylin and eosin, toluidine blue, Herovici, CD68 and CD163.

RESULTS

No significant differences were found in the amount of macrophages or presence of chondrocyte conglomerates between the MC groups. Of the multiple genes tested, statistically significant associations were observed for M-CSF1 (p = 0.028), RANKL (p = 0.035), RUNX1 (p = 0.032), and RUNX2 (p = 0.047) that were increased in 'Type II MC,' while OSCAR (p = 0.042) was increased in 'Type I MC' group compared to 'No MC.'

CONCLUSIONS

Since these cytokines are related to differentiation and proliferation of osteoclasts, our data suggest that the stimulation of vertebral osteoclasts by factors secreted by disk tissue is involved in the pathophysiology of MC.

Deficiency of G-protein coupled receptor 40, a lipid-activated receptor, heightens in vitro- and in vivo-induced murine osteoarthritis

Osteoarthritis (OA) is an age-related degenerative joint disease. To date, its management is focused on symptoms (pain and inflammation). Studies suggest that fatty acids can reduce the expression of inflammatory and catalytic mediators, and improve in vivo joint function. Free fatty acid receptors (FFARs) such as G-protein coupled receptor 40 (GPR40) are proposed as attractive therapeutic targets to counteract inflammation and cartilage degradation observed in OA. This study aims to elucidate the involvement of GPR40 in OA. In this study, we used an in vitro model of OA, and surgically induced OA by ligament transection and partial meniscectomy in wild-type and GPR40 deficient mice. OA phenotype was investigated in vivo by histology and genes expression. We demonstrate that IL-1β-treated GPR40(-/-) chondrocytes secret more inflammatory mediators (nitric oxide, interleukin-6, prostaglandin E2) and active catabolic enzymes (metalloproteinase-2, -9 [MMP-2, MMP-9]), and show decreased anabolism (glycoaminoglycan) compared to GPR40(+/+) cells. In accordance with these results, we show that GPR40(-/-) mice exhibit an aggravated OA-induced phenotype characterized by higher tidemark exposure, frequency of osteophyte formation and subchondral bone sclerosis. Altogether our results demonstrate that GPR40 deficiency leads to an extended OA phenotype, providing evidence that increasing GPR40 activity, by natural or synthetic ligands, could be a new strategy in the management of OA.

Effects of pamidronate disodium on the loss of osteoarthritic subchondral bone and the expression of cartilaginous and subchondral osteoprotegerin and RANKL in rabbits

Background

Osteoarthritis (OA) is a major health problem in the increasingly elderly population. Therefore, it is crucial to prevent and treat OA at an early stage. The present study investigated whether pamidronate disodium (PAM), a bone-loss inhibitor, can significantly prevent or reverse the progression of early anterior cruciate ligament transection (ACLT)-induced OA. Whether therapeutic intervention is associated with regulation of the expression of osteoprotegerin (OPG), receptor activator of nuclear factor-κB ligand (RANKL), metalloproteinase-9 (MMP-9) or Toll-like receptor-4 (TLR-4) in cartilage and/or subchondral bone was also investigated.

Methods

60 New Zealand rabbits were randomized into four groups: Sham-operated (n = 20); ACLT (n = 20); short-term treatment with PAM (PAM-S, n = 10) and long-term treatment with PAM (PAM-L, n = 10). For cartilage and subchondral bone testing, rabbits from Sham and ACLT groups were harvested at 2, 4, 6, and 14 weeks. Rabbits were given PAM from the 4th week after ACLT operation in PAM-S and PAM-L group, and were harvested at 6 and 14 weeks, respectively. Trabecular characteristics and cartilage changes were detected using Micro-CT, safranin O and rapid green staining, respectively. Immunohistochemical staining for OPG and RANKL were also performed. OPG, RANKL, MMP-9 and TLR-4 expression was evaluated by western blot analysis.

Results

Micro-CT and histology analyses indicated that PAM treatment for 2 or 10 weeks could completely prevent or reverse osteoarthritic subchondral bone loss and cartilage surface erosion. Immunohistochemistry and western blot analysis indicated that expression of OPG and RANKL increased, although RANKL expression increased more significantly than that of OPG. Therefore the ratio of OPG to RANKL was lower in the ACLT group. However, the ratio of OPG to RANKL in the PAM group was significantly higher than that in the ACLT group. Additionally, expression of MMP-9 and TLR-4 were upregulated in the ACLT group and downregulated in the PAM treated groups.

Conclusions

PAM can significantly inhibit and even reverse early osteoarthritic subchondral bone loss, thus alleviating the process of cartilaginous degeneration. The mechanisms involved may be associated with the upregulation of OPG expression, and downregulation of RANKL, MMP-9 and TLR-4 expression.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2474-15-370) contains supplementary material, which is available to authorized users.

Key role of suppressor of cytokine signaling 3 in regulating gp130 cytokine-induced signaling and limiting chondrocyte responses during murine inflammatory arthritis

OBJECTIVE

To examine the impact of the gp130 cytokine family on murine articular cartilage and to explore a potential regulatory role of suppressor of cytokine signaling 3 (SOCS-3) in murine chondrocytes.

METHODS

In wild-type (WT) mouse chondrocytes, baseline receptor expression levels and gp130 cytokine-induced JAK/STAT signaling were determined by flow cytometry, and expression of SOCS-3 was assessed by quantitative polymerase chain reaction. The role of endogenous SOCS-3 was examined in cartilage explants and chondrocytes from mice with conditional deletion of Socs3 driven by the Col2a1 promoter in vitro (Socs3(Δ/Δcol2) ) and from mice during CD4+ T cell-dependent inflammatory monarthritis. Bone erosions in the murine joints were analyzed by micro-computed tomography.

RESULTS

On chondrocytes from WT mice, gp130 and the oncostatin M (OSM) receptor were strongly expressed, whereas the transmembrane interleukin-6 (IL-6) receptor was expressed at much lower levels. Compared to other gp130 cytokines, OSM was the most potent activator of the JAK/STAT pathway and of SOCS-3 induction. Treatment of Socs3(Δ/Δcol2) mouse cartilage explants and chondrocytes with gp130 cytokines prolonged JAK/STAT signaling, enhanced cartilage degradation, increased the expression of Adamts4, Adamts5, and RANKL, and elevated the production of IL-6, granulocyte colony-stimulating factor, CXCL1, and CCL2. Socs3(Δ/Δcol2) mice developed exacerbated inflammation and joint damage in response to gp130 cytokine injections, and these histopathologic features were also observed in mice with inflammatory monarthritis.

CONCLUSION

The results of this study highlight a key role for SOCS-3 in regulating chondrocyte responses during inflammatory arthritis. Within the gp130 cytokine family, OSM is a potent stimulus of chondrocyte responses, while IL-6 probably signals via trans-signaling. The gp130 cytokine-driven production of RANKL in chondrocytes may link chondrocyte activation and bone remodeling during inflammatory arthritis. Thus, these findings suggest that the inhibition of OSM might reduce the development and severity of structural joint damage during inflammatory arthritis.

Bone-cartilage interface crosstalk in osteoarthritis: potential pathways and future therapeutic strategies

Currently, osteoarthritis (OA) is considered a disease of the entire joint, which is not simply a process of wear and tear but rather abnormal remodelling and joint failure of an organ. The bone-cartilage interface is therefore a functioning synergistic unit, with a close physical association between subchondral bone and cartilage suggesting the existence of biochemical and molecular crosstalk across the OA interface. The crosstalk at the bone-cartilage interface may be elevated in OA in vivo and in vitro. Increased vascularisation and formation of microcracks associated with abnormal bone remodelling in joints during OA facilitate molecular transport from cartilage to bone and vice versa. Recent reports suggest that several critical signalling pathways and biological factors are key regulators and activate cellular and molecular processes in crosstalk among joint compartments. Therapeutic interventions including angiogenesis inhibitors, agonists/antagonists of molecules and drugs targeting bone remodelling are potential candidates for this interaction. This review summarised the premise for the presence of crosstalk in bone-cartilage interface as well as the current knowledge of the major signalling pathways and molecular interactions that regulate OA progression. A better understanding of crosstalk in bone-cartilage interface may lead to development of more effective strategies for treating OA patients.

A genetic variant in osteoprotegerin is associated with progression of joint destruction in rheumatoid arthritis

INTRODUCTION

Progression of joint destruction in rheumatoid arthritis (RA) is partly heritably; 45 to 58% of the variance in joint destruction is estimated to be explained by genetic factors. The binding of RANKL (Receptor Activator for Nuclear Factor κ B Ligand) to RANK results in the activation of TRAF6 (tumor necrosis factor (TNF) receptor associated factor-6), and osteoclast formation ultimately leading to enhanced bone resorption. This bone resorption is inhibited by osteoprotegerin (OPG) which prevents RANKL-RANK interactions. The OPG/RANK/RANKL/TRAF6 pathway plays an important role in bone remodeling. Therefore, we investigated whether genetic variants in OPG, RANK, RANKL and TRAF6 are associated with the rate of joint destruction in RA.

METHODS

1,418 patients with 4,885 X-rays of hands and feet derived from four independent data-sets were studied. In each data-set the relative increase of the progression rate per year in the presence of a genotype was assessed. First, explorative analyses were performed on 600 RA-patients from Leiden. 109 SNPs, tagging OPG, RANK, RANKL and TRAF6, were tested. Single nucleotide polymorphisms (SNPs) significantly associated in phase-1 were genotyped in data-sets from Groningen (Netherlands), Sheffield (United Kingdom) and Lund (Switzerland). Data were summarized in an inverse weighted variance meta-analysis. Bonferonni correction for multiple testing was applied.

RESULTS

We found that 33 SNPs were significantly associated with the rate of joint destruction in phase-1. In phase-2, six SNPs in OPG and four SNPs in RANK were associated with progression of joint destruction with P-value <0.05. In the meta-analyses of all four data-sets, RA-patients with the minor allele of OPG-rs1485305 expressed higher rates of joint destruction compared to patients without these risk variants (P = 2.35x10-4). This variant was also significant after Bonferroni correction.

CONCLUSIONS

These results indicate that a genetic variant in OPG is associated with a more severe rate of joint destruction in RA.

Mediolateral subchondral tibial bone mineral density difference does not predict osteoarthritis progression

Data on the relationship between osteoporosis and osteoarthritis are conflicting. Most studies report "snapshot" prevalences, and there are few studies evaluating localized knee bone mineral density (BMD) measurements with respect to subsequent osteoarthritis progression. The authors hypothesize that increased mediolateral difference of the proximal tibial BMD may predict progression of knee osteoarthritis. In this study, 246 female volunteers were followed up prospectively over 2 years. Baseline BMD measurements of bilateral proximal (subchondral) tibiae were performed, and the mediolateral BMD ratio was calculated. Precision studies were performed on healthy volunteers to validate the technique. The patients were divided into osteoarthritis progressors and non-progressors based on Kellgren-Lawrence radiographic criteria at 2-year follow-up. Patients who were taking bisphosphonates or who had a history of inflammatory, infectious, or metabolic bone disease and previous hip and knee surgery were excluded. Demographic data, calcium supplementation, physical activity, baseline knee radiographs, and radiographs at 2-year follow-up were obtained. The study was adequately powered to detect an effect size of 0.4. There were 121 progressors and 125 non-progressors. Mean mediolateral BMD ratio was 1.02 among the progressors and 1.01 among the non-progressors (t=0.632, P=.528). Potential confounders were equally distributed among both groups. The data suggest that there is no link between proximal tibial BMD ratio and progression of knee osteoarthritis.

Reducing dietary loading decreases mouse temporomandibular joint degradation induced by anterior crossbite prosthesis

OBJECTIVE

Dietary loading has been reported to have an effect on temporomandibular joint (TMJ) remodeling via periodontal-muscular reflex. We therefore examined whether reducing dietary loading decreased TMJ degradation induced by the unilateral anterior crossbite prosthesis as we recently reported.

METHODS

Forty 6-week-old female C57BL/6J mice were randomly divided into two experimental and two control groups. One experimental and one control group received small-size diet and the other two groups received large-size diet. Unilateral anterior crossbite prosthesis was created in the two experimental groups. The TMJ samples were collected 3 weeks after experimental operation. Histological changes in condylar cartilage and subchondral bone were assessed by Hematoxylin & Eosin, toluidine blue, Safranin O and tartrate-resistant acid phosphatase staining. Real-time polymerase chain reaction (PCR) and/or immunohistochemistry were performed to evaluate the expression levels of Collagen II, Aggrecan, a disintegrin and metalloproteinase with thrombospondin motifs 5 (ADAMTS-5) and RANKL/RANK/OPG in TMJ condylar cartilage and/or subchondral bone.

RESULTS

Thinner and degraded cartilage, reduced cartilage cellular density, decreased expression levels of Collagen II and Aggrecan, loss of subchondral bone and enhanced osteoclast activity were observed in TMJs of both experimental groups. However, the cartilage degradation phenotype was less severe and cartilage ADAMTS-5 mRNA was lower while OPG/RANKL ratio in cartilage and subchondral bone was higher in the small-size than large-size diet experimental group. No differences of histomorphology and the tested molecules were found between the two control groups.

CONCLUSIONS

The current findings suggest that a lower level of functional loading by providing small-size diet could reduce TMJ degradation induced by the biomechanical stimulation from abnormal occlusion.

Mice Deficient in NF-κB p50 and p52 or RANK Have Defective Growth Plate Formation and Post-natal Dwarfism

NF-κBp50/p52 double knockout (dKO) and RANK KO mice have no osteoclasts and develop severe osteopetrosis associated with dwarfism. In contrast, Op/Op mice, which form few osteoclasts, and Src KO mice, which have osteoclasts with defective resorptive function, are osteopetrotic, but they are not dwarfed. Here, we compared the morphologic features of long bones from p50/p52 dKO, RANK KO, Op/Op and Src KO mice to attempt to explain the differences in their long bone lengths. We found that growth plates in p50/p52 dKO and RANK KO mice are significantly thicker than those in WT mice due to a 2-3-fold increase in the hypertrophic chondrocyte zone associated with normal a proliferative chondrocyte zone. This growth plate abnormality disappears when animals become older, but their dwarfism persists. Op/Op or Src KO mice have relatively normal growth plate morphology. In-situ hybridization study of long bones from p50/p52 dKO mice showed marked thickening of the growth plate region containing type 10 collagen-expressing chondrocytes. Treatment of micro-mass chondrocyte cultures with RANKL did not affect expression levels of type 2 collagen and Sox9, markers for proliferative chondrocytes, but RANKL reduced the number of type 10 collagen-expressing hypertrophic chondrocytes. Thus, RANK/NF-κB signaling plays a regulatory role in post-natal endochondral ossification that maintains hypertrophic conversion and prevents dwarfism in normal mice.

Wnt inhibitory factor 1 deficiency uncouples cartilage and bone destruction in tumor necrosis factor α-mediated experimental arthritis

OBJECTIVE

Wnt signaling plays a pivotal role in skeletal development and in the control of cartilage and bone turnover. We have recently shown that the secreted Wnt antagonist Wnt inhibitory factor 1 (WIF-1) is mainly expressed in the upper layers of epiphyseal and articular cartilage and, to a lesser extent, in bone. Nevertheless, WIF-1(-/-) mice develop normally. In light of these findings, we undertook this study to analyze the role of WIF-1 in arthritis.

METHODS

Expression analyses for WIF-1 were performed by real-time reverse transcription-polymerase chain reaction (RT-PCR). WIF-1(-/-) and tumor necrosis factor (TNF)-transgenic mice were crossbred, and the progression of arthritis in TNF-transgenic WIF-1(-/-) mice and littermate controls was evaluated. Structural joint damage was analyzed by histologic staining, histomorphometry, and micro-computed tomography. Wnt/β-catenin signaling was investigated by real-time RT-PCR and immunofluorescence on primary chondrocytes.

RESULTS

WIF-1 expression was repressed by TNFα in chondrocytes and osteoblasts and down-regulated in experimental arthritis and in articular cartilage from patients with rheumatoid arthritis. WIF-1 deficiency partially protected TNF-transgenic mice against bone erosion and loss of trabecular bone, probably as a result of less osteoclast activity. In contrast, arthritis-related cartilage damage was aggravated by WIF-1 deficiency, while overexpression of WIF-1 attenuated cartilage degradation in TNF-transgenic mice. In chondrocytes, TNFα stimulated canonical Wnt signaling, which could be blocked by WIF-1, indicating a direct effect of TNFα and WIF-1 on Wnt signaling in this system.

CONCLUSION

These data suggest that WIF-1 may take part in the fine-tuning of cartilage and bone turnover, promoting the balance of cartilage versus bone anabolism.

Interplay between cartilage and subchondral bone contributing to pathogenesis of osteoarthritis

Osteoarthritis (OA) is a common debilitating joint disorder, affecting large sections of the population with significant disability and impaired quality of life. During OA, functional units of joints comprising cartilage and subchondral bone undergo uncontrolled catabolic and anabolic remodeling processes to adapt to local biochemical and biological signals. Changes in cartilage and subchondral bone are not merely secondary manifestations of OA but are active components of the disease, contributing to its severity. Increased vascularization and formation of microcracks in joints during OA have suggested the facilitation of molecules from cartilage to bone and vice versa. Observations from recent studies support the view that both cartilage and subchondral bone can communicate with each other through regulation of signaling pathways for joint homeostasis under pathological conditions. In this review we have tried to summarize the current knowledge on the major signaling pathways that could control the cartilage-bone biochemical unit in joints and participate in intercellular communication between cartilage and subchondral bone during the process of OA. An understanding of molecular communication that regulates the functional behavior of chondrocytes and osteoblasts in both physiological and pathological conditions may lead to development of more effective strategies for treating OA patients.

Kinetics and interplay of mediators of inflammation-induced bone damage in the course of adjuvant arthritis

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic inflammation, bone erosion, and cartilage destruction in the joints. It is increasingly being realized that inflammation might play an important role in inducing bone damage in arthritis. However, there is limited validation of this concept in vivo in well-controlled experimental conditions. We addressed this issue using the adjuvant arthritis (AA) model of RA. In AA, the draining lymph nodes are the main sites of activation of pathogenic leukocytes, which then migrate into the joints leading to the induction of arthritis. We tested the temporal kinetics of mediators of bone damage [e.g., receptor activator of nuclear factor kappa-B ligand (RANKL), osteoprotegerin (OPG) and osteopontin (OPN)] and inflammation (pro-inflammatory cytokines and chemokines) in the draining lymph node cells (LNC) at different phases of AA, and then examined their inter-relationships. Our study revealed that, together with cytokines/chemokines, some of the mediators of bone remodeling are also produced in LNC. Various cytokines/chemokines showed distinct kinetics of expression as well as patterns of correlation with mediators of bone remodeling at different phases of the disease. Pro-inflammatory cytokines such as TNF-alpha are known to play an important role in bone damage. Interestingly, there was a positive correlation between TNF-alpha and RANKL, between RANKL and each of the 3 chemokines tested (RANTES, MIP-1alpha, and GRO/KC), and between TNF-alpha and RANTES. Our results in the AA model lend support to the concept of osteo-immune crosstalk during the course of autoimmune arthritis.

Hypercholesterolemia boosts joint destruction in chronic arthritis. An experimental model aggravated by foam macrophage infiltration

Objective

The aim of this study was to determine whether hypercholesterolemia increases articular damage in a rabbit model of chronic arthritis.

Methods

Hypercholesterolemia was induced in 18 rabbits by administrating a high-fat diet (HFD). Fifteen rabbits were fed normal chow as controls. Chronic antigen-induced arthritis (AIA) was induced in half of the HFD and control rabbits, previously immunized, by intra-articular injections of ovalbumin. After sacrifice, lipid and systemic inflammation markers were analyzed in blood serum. Synovium was analyzed by Krenn score, multinucleated cell counting, immunohistochemistry of RAM11 and CD31, and TNF-α and macrophage chemoattractant protein-1 (MCP-1) gene expression. Active bone resorption was assessed by protein expression of receptor activator of nuclear factor kappa-B ligand (RANKL), osteoprotegerin (OPG) and quantification of cathepsin K, contact surface and the invasive area of pannus into bone.

Results

Rabbits receiving the HFD showed higher total serum cholesterol, HDL, triglycerides and CRP levels than rabbits fed a normal diet. Synovitis score was increased in HFD, and particularly in AIA and AIA + HFD groups. AIA + HFD synovium was characterized by a massive infiltration of RAM11+ cells, higher presence of multinucleated foam cells and bigger vascularization than AIA. Cathepsin K+ osteoclasts and the contact surface of bone resorbing pannus were also increased in rabbits with AIA + HFD compared with AIA alone. Synovial TNF-α and MCP-1 gene expression was increased in AIA and HFD rabbits compared with healthy animals. RANKL protein expression in AIA and AIA + HFD groups was higher compared with either HFD or normal groups.

Conclusions

This experimental model demonstrates that hypercholesterolemia increments joint tissue damage in chronic arthritis, with foam macrophages being key players in this process.

Total glucosides of paeony prevents juxta-articular bone loss in experimental arthritis

Background

Total glucosides of paeony (TGP) is a biologically active compound extracted from Paeony root. TGP has been used in rheumatoid arthritis therapy for many years. However, the mechanism by which TGP prevents bone loss has been less explored.

Methods

TGP was orally administered for 3 months to New Zealand rabbits with antigen-induced arthritis (AIA). Digital x-ray knee images and bone mineral density (BMD) measurements of the subchondral knee bone were performed before sacrifice. Chondrocytes were observed using transmission electron microscopy (TEM). Histological analysis and mRNA expression of receptor activator of nuclear factor-B ligand (RANKL) and osteoprotegerin (OPG) were evaluated in joint tissues.

Results

The BMD value in TGP rabbits was significantly higher compared with that seen in the AIA model rabbits. In addition, the subchondral bone plate was almost completely preserved by TGP treatment, while there was a decrease in bone plate integrity in AIA rabbits. There was less damage to the chondrocytes of the TGP treated group. Immunohistochemical examination of the TGP group showed that a higher percentage of TGP treated chondrocytes expressed OPG as compared to the chondrocytes isolated from AIA treated animals. In contrast, RANKL expression was significantly decreased in the TGP treated group compared to the AIA group. In support of the immunohistochemistry data, the expression of RANKL mRNA was decreased and OPG mRNA expression was enhanced in the TGP group when compared to that of the AIA model group.

Conclusion

These results reveal that TGP suppresses juxta-articular osteoporosis and prevents subchondral bone loss. The decreased RANKL and increased OPG expression seen in TGP treated animals could explain how administration of TGP maintains higher BMD.