How Does Joint Remodeling Work?: New Insights in the Molecular Regulation of the Architecture of Joints

Remodeling of joints is a key feature of inflammatory and degenerative joint disease. Bone erosion, cartilage degeneration and growth of bony spurs termed osteophytes are key features of structural joint pathology in the course of arthritis, which lead to impairment of joint function. Understanding their molecular mechanisms is essential to tailor targeted therapeutic approaches to protect joint architecture from inflammatory and mechanical stress. This addendum summarizes the new insights in the molecular regulation of bone formation in the joint and its relation to bone resorption. It describes how inflammatory cytokines impair bone formation and block the repair response of joints towards inflammatory stimuli. It particularly points out the key role of Dickkopf-1 protein, a regulator of the Wingless signaling and inhibitor of bone formation. This new link between inflammation and bone formation is also crucial for explaining the generation of osteophytes, bony spurs along joints, which are characterized by new bone and cartilage formation. This mechanism is largely dependent on an activation of wingless protein signaling and can lead to complete joint fusion. This addendum summarized the current concepts of joint remodeling in the limelight of these new findings.Key words: joint remodeling, arthritis, bone formation, bone erosion, osteoblasts, osteoclasts, Dickkopf, wingless proteinsJoints face profound remodeling in the course of arthritis. In humans, pathologic joint remodeling manifests as (i) destruction of joints due to bone erosion (rheumatoid arthritis), (ii) fusion of joints due to formation of bony spurs such as osteophytes, spondylophytes and syndesmophytes (ankylosing spondylitis) or (iii) a mixture of both changes (psoriatic arthritis). The molecular mechanisms determining these different forms of joint remodeling are not fully clarified, Insights in these mechanisms however are a clue to a deeper understanding of the architectural changes of human joints.Similar to systemic bone turnover, which most is most prominent in the trabecular bone compartment of the spine and long bones, joints are hot spots of bone remodeling during inflammatory disease. Cytokines expressed by inflammatory cells in the synovial membrane regulate local bone homeostasis and enable to remodel joints during disease—a process which can either lead to crippling and functional loss or to fusion and stabilization of the affected joint. Rheumatoid arthritis is characterized by bone erosions, which are the result of an enhanced bone resorption. In rheumatoid arthritis osteoclasts, the primary bone resorbing cells, accumulate and degrade the periarticular bone as well as the mineralized cartilage.¹ Molecularly increased osteoclast formation is based on the expression of macrophage colony-stimulating factor (MCSF) and receptor-antagonist of NFκB ligand (RANKL) in the synovial tissue, which both drive the differentiation of osteoclasts from monocytic precursors.^2–4 Osteoclasts are specialized cells to resorb bone and their local accumulation in the joint leads to a catabolic state, which by far outweighs bone formation resulting in a negative net effect of bone remodeling. Inflammatory cytokines, such as TNF, IL-1, IL-6 and IL-17 induce osteoclast formation by enhancing the expression of RANKL and promoting differentiation of osteoclast precursor cells to mature osteoclasts.^5–8 Abundance of proinflammatory cytokines in the synovial membrane of patients with RA, their induction of molecules involved in osteoclast formation and the influx of monocytes/macrophages serving as osteoclast precursor cells represent ideal prerequisites for osteoclast formation in joints.⁹The fact that appropriate repair strategies are virtually absent in patients with RA and that bone is hardly rebuilt when bone erosions have emerged, suggests activation of molecular signals, which blunt bone formation. Bone formation itself is regulated by growth factors and hormones, which stimulate differentiation and activity of osteoblasts. Typical regulators of bone formation constitute parathyroid hormone, prostaglandins, bone morphogenic proteins (BMPs) and wingless proteins (Wnt). Particularly the role of Wnt proteins in bone formation have achieved growing interest during the past few years, leading to identification of the LRP5/6 receptor as a key molecule for anabolic skeletal responses. Wnt proteins bind to the LRP5/6 receptor and lead to activation of a signal pathway involving GSK3 and β-catenin, which drive differentiation of mesenchymal cells into osteoblastogenesis.¹⁰ Regulators of Wnt- induced bone formation are Dickkopf (DKK) proteins, which competitively bind to LRP5/6 and prevent signaling activation by additionally engaging a negative coreceptor termed Kremen-1.^11,12 DKK proteins thus regulate bone homeostasis by interference with Wnt signaling.¹³We recently showed that inflammatory cytokines such as TNF induce DKK-1, a member of the DKK- family, which inhibits Wnt signaling. DKK-1 is highly expressed in inflammatory lesions of experimental arthritis and human rheumatoid arthritis.¹⁴ Moreover, increased levels can be detected in the serum of patients with RA, which depend on TNF. This is supported by the normalization of elevated DKK-1 levels in RA patients upon initiation of systemic TNF- blockade. Inhibition of DKK-1 in mice completely abolishes bone erosions in different models of experimental arthritis and leads to increased bone growth, which manifests as osteophyte formation in the joint.DKK-1 links the inflammation with bone formation as RANKL links inflammation with bone resorption. The fact that TNF and presumably also other inflammatory mediators induce both proteins explains the profound negative effect of inflammation on bone. Inflammation uncouples the balance between bone resorption and formation, enhancing the former by inducing RANKL and by repressing the latter by DKK-1. Also appears to be a tight cross talk between the Wnt- and RANKL-pathways.¹⁵ Inhibition of DKK-1 in arthritic mice lead to protection from bone erosions and osteoclasts did not appropriately form. This effect is based on the induction of osteoprotegerin (OPG) a natural decoy receptor for RANKL, which blocks RANKL and thus osteoclast formation. OPG is induced by Wnt proteins and shifts the balance from bone resorption to bone formation.In contrast to rheumatoid arthritis joints in ankylosing spondylitis and also in degenerative joint disease (osteoarthritis) show an attempt towards joint fusion rather than joint destruction. These bony spurs are the result of endochondral bone formation starting from the periosteum close to the joints, where osteoblasts differentiate build up bone matrix. We could demonstrate that Wnt proteins are crucially involved in this process since inhibition of DKK-1 lead to emergence of osteophytes and even complete fusion of joints. Taken together these data suggest that the balance of the Wnt/DKK system determines the remodeling of joints by governing bone destruction as well as osteophyte formation in joints (Fig. 1).Open in a separate windowFigure 1Patterns of joint remodeling.     

Dickkopf-1 is a master regulator of joint remodeling

Degenerative and inflammatory joint diseases lead to a destruction of the joint architecture. Whereas degenerative osteoarthritis results in the formation of new bone, rheumatoid arthritis leads to bone resorption. The molecular basis of these different patterns of joint disease is unknown. By inhibiting Dickkopf-1 (DKK-1), a regulatory molecule of the Wnt pathway, we were able to reverse the bone-destructive pattern of a mouse model of rheumatoid arthritis to the bone-forming pattern of osteoarthritis. In this way, no overall bone erosion resulted, although bony nodules, so-called osteophytes, did form. We identified tumor necrosis factor-alpha (TNF) as a key inducer of DKK-1 in the mouse inflammatory arthritis model and in human rheumatoid arthritis. These results suggest that the Wnt pathway is a key regulator of joint remodeling.     

Bone morphogenetic proteins in destructive and remodeling arthritis

Joint destruction and tissue responses determine the outcome of chronic arthritis. Joint inflammation and damage are often the dominant clinical presentation. However, in some arthritic diseases, in particular the spondyloarthritides, joint remodeling is a prominent feature, with new cartilage and bone formation leading to ankylosis and contributing to loss of function. A role for bone morphogenetic proteins in joint remodeling has been demonstrated in the formation of both enthesophytes and osteophytes. Data from genetic models support a role for bone morphogenetic protein signaling in cartilage homeostasis. Finally, this signaling pathway is likely to play a steering role in the synovium.     

Three-Dimensional Quantitative Morphometric Analysis (QMA) for In Situ Joint and Tissue Assessment of Osteoarthritis in a Preclinical Rabbit Disease Model

This work utilises advances in multi-tissue imaging, and incorporates new metrics which define in situ joint changes and individual tissue changes in osteoarthritis (OA). The aims are to (1) demonstrate a protocol for processing intact animal joints for microCT to visualise relevant joint, bone and cartilage structures for understanding OA in a preclinical rabbit model, and (2) introduce a comprehensive three-dimensional (3D) quantitative morphometric analysis (QMA), including an assessment of reproducibility. Sixteen rabbit joints with and without transection of the anterior cruciate ligament were scanned with microCT and contrast agents, and processed for histology. Semi-quantitative evaluation was performed on matching two-dimensional (2D) histology and microCT images. Subsequently, 3D QMA was performed; including measures of cartilage, subchondral cortical and epiphyseal bone, and novel tibio-femoral joint metrics. Reproducibility of the QMA was tested on seven additional joints. A significant correlation was observed in cartilage thickness from matching histology-microCT pairs. The lateral compartment of operated joints had larger joint space width, thicker femoral cartilage and reduced bone volume, while osteophytes could be detected quantitatively. Measures between the in situ tibia and femur indicated an altered loading scenario. High measurement reproducibility was observed for all new parameters; with ICC ranging from 0.754 to 0.998. In conclusion, this study provides a novel 3D QMA to quantify macro and micro tissue measures in the joint of a rabbit OA model. New metrics were established consisting of: an angle to quantitatively measure osteophytes (σ), an angle to indicate erosion between the lateral and medial femoral condyles (ρ), a vector defining altered angulation (λ, α, β, γ) and a twist angle (τ) measuring instability and tissue degeneration between the femur and tibia, a length measure of joint space width (JSW), and a slope and intercept (m, Χ) of joint contact to demonstrate altered loading with disease progression, as well as traditional bone and cartilage and histo-morphometry measures. We demonstrate correlation of microCT and histology, sensitive discrimination of OA change and robust reproducibility.     

Biomechanical implications of degenerative joint disease in the apophyseal joints of human thoracic and lumbar vertebrae

An experimental technique for quantifying load-sharing in cadaveric spines is used to test the hypothesis that degenerative changes in human apophyseal joints are directly related to high levels of compressive load-bearing by these joints. About 36 cadaveric thoraco-lumbar motion segments aged 64-92 years were subjected to a compressive load of 1.5 kN. The distribution of compressive stress was measured in the intervertebral discs using a miniature pressure transducer, and stress measurements were summed over area to give the compressive force resisted by the disc. This was subtracted from the applied 1.5 kN to indicate compressive load-bearing by the apophyseal joints. The cartilage of each apophyseal joint surface was then graded for degree of degeneration. After maceration, each joint surface was scored for degenerative joint disease (DJD) affecting the bone. Results demonstrated that the apophyseal joints resisted 5-96% (mean 45%) of the applied compressive force. A significant positive correlation was demonstrated between age and cartilage degeneration, age and DJD bone score, apophyseal joint load-bearing and bone score, and cartilage score and load-bearing. The latter correlation was strongest for load-bearing above 50%. Ordinal regression showed that the variables describing bone DJD (marginal osteophytes, pitting, bony contour change, and eburnation) were significantly correlated with degree of cartilage degeneration. It is concluded that in elderly individuals apophyseal joint load-bearing above a threshold of 50% is associated with severe degenerative changes in cartilage and bone, and that markers of DJD observed palaeopathologically may be used as predictors of such loadingin life.     

Biomechanical factors and physical examination findings in osteoarthritis of the knee: associations with tissue abnormalities assessed by conventional radiography and high-resolution 3.0 Tesla magnetic resonance imaging

Introduction

We aimed to explore the associations between knee osteoarthritis (OA)-related tissue abnormalities assessed by conventional radiography (CR) and by high-resolution 3.0 Tesla magnetic resonance imaging (MRI), as well as biomechanical factors and findings from physical examination in patients with knee OA.

Methods

This was an explorative cross-sectional study of 105 patients with knee OA. Index knees were imaged using CR and MRI. Multiple features from CR and MRI (cartilage, osteophytes, bone marrow lesions, effusion and synovitis) were related to biomechanical factors (quadriceps and hamstrings muscle strength, proprioceptive accuracy and varus-valgus laxity) and physical examination findings (bony tenderness, crepitus, bony enlargement and palpable warmth), using multivariable regression analyses.

Results

Quadriceps weakness was associated with cartilage integrity, effusion, synovitis (all detected by MRI) and CR-detected joint space narrowing. Knee joint laxity was associated with MRI-detected cartilage integrity, CR-detected joint space narrowing and osteophyte formation. Multiple tissue abnormalities including cartilage integrity, osteophytes and effusion, but only those detected by MRI, were found to be associated with physical examination findings such as crepitus.

Conclusion

We observed clinically relevant findings, including a significant association between quadriceps weakness and both effusion and synovitis, detected by MRI. Inflammation was detected in over one-third of the participants, emphasizing the inflammatory component of OA and a possible important role for anti-inflammatory therapies in knee OA. In general, OA-related tissue abnormalities of the knee, even those detected by MRI, were found to be discordant with biomechanical and physical examination features.     

Inhibition of endogenous TGF-beta during experimental osteoarthritis prevents osteophyte formation and impairs cartilage repair

Osteoarthritis has as main characteristics the degradation of articular cartilage and the formation of new bone at the joint edges, so-called osteophytes. In this study enhanced expression of TGF-beta1 and -beta3 was detected in developing osteophytes and articular cartilage during murine experimental osteoarthritis. To determine the role of endogenous TGF-beta on osteophyte formation and articular cartilage, TGF-beta activity was blocked via a scavenging soluble TGF-beta-RII. Our results clearly show that inhibition of endogenous TGF-beta nearly completely prevented osteophyte formation. In contrast, treatment with recombinant soluble TGF-beta-RII markedly enhanced articular cartilage proteoglycan loss and reduced the thickness of articular cartilage. In conclusion, we show for the first time that endogenous TGF-beta is a crucial factor in the process of osteophyte formation and has an important function in protection against cartilage loss.     

强直性脊柱炎新骨形成机制研究进展

强直性脊柱炎（Ankylosing spondylitis, AS）是以骶髂关节和脊柱病变为主的炎性疾病。其特征性病理表现为炎症和新骨形 成。近年来使用肿瘤坏死因子（Tumor necrosis factor,TNF）抑制剂控制AS 炎症已卓有成效,却无法阻止影像学进程,其病理性新 骨形成可致残,严重影响患者健康生活,但其机制尚不完全清楚。目前研究认为复杂的新骨形成机制与Wnt/beta-catenin信号通路及 其调控因子、炎症介质密切相关。本文结合当前国内外的研究就AS 新骨形成机制进展展开综述,为深入研究新骨形成机制提供 新思想。     

Radiographic features of experimentalCandida arthritis in rats

Sprague-Dawley rats were inoculated intravenously (i.v.) withCandida albicans, and limb joints showing signs ofCandida-induced arthritis were subjected to radiographic and histologic examination. New bone formation and bone resorption were morbidly enhanced in bones sampled from the arthritic joints. Sparsely distributed needle-shaped calcified deposits began to be formed on bony surfaces in parallel with the onset of joint swelling. The calcified deposits gradually became denser and then covered the bony surfaces almost entirely, giving rise to an exostosis-like profile. In addition to the new bone formation, bone resorption was also observed in regions adjacent to the sites of new bone formation, and punched-out bone lesions were produced. Eventually, severe deformation of joint bones due to new bone formation and bone resorption was evident. Reflecting these unusual radiographic changes, abundant osteoblasts and osteoclasts were demonstrated histologically in the bones. On the basis of these results, possible mechanisms for the induction of arthritis byCandida infection are discussed.     

Osteophyte formation and matrix mineralization in a TMJ osteoarthritis mouse model are associated with ectopic hedgehog signaling

The temporomandibular joint (TMJ) is a diarthrodial joint that relies on lubricants for frictionless movement and long-term function. It remains unclear what temporal and causal relationships may exist between compromised lubrication and onset and progression of TMJ disease. Here we report that Proteoglycan 4 (Prg4)-null TMJs exhibit irreversible osteoarthritis-like changes over time and are linked to formation of ectopic mineralized tissues and osteophytes in articular disc, mandibular condyle and glenoid fossa. In the presumptive layer of mutant glenoid fossa's articulating surface, numerous chondrogenic cells and/or chondrocytes emerged ectopically within the type I collagen-expressing cell population, underwent endochondral bone formation accompanied by enhanced Ihh expression, became entrapped into temporal bone mineralized matrix, and thereby elicited excessive chondroid bone formation. As the osteophytes grew, the roof of the glenoid fossa/eminence became significantly thicker and flatter, resulting in loss of its characteristic concave shape for accommodation of condyle and disc. Concurrently, the condyles became flatter and larger and exhibited ectopic bone along their neck, likely supporting the enlarged condylar heads. Articular discs lost their concave configuration, and ectopic cartilage developed and articulated with osteophytes. In glenoid fossa cells in culture, hedgehog signaling stimulated chondrocyte maturation and mineralization including alkaline phosphatase, while treatment with hedgehog inhibitor HhAntag prevented such maturation process. In sum, our data indicate that Prg4 is needed for TMJ integrity and long-term postnatal function. In its absence, progenitor cells near presumptive articular layer and disc undergo ectopic chondrogenesis and generate ectopic cartilage, possibly driven by aberrant activation of Hh signaling. The data suggest also that the Prg4-null mice represent a useful model to study TMJ osteoarthritis-like degeneration and clarify its pathogenesis.     

Wnt signaling pathway in rheumatoid arthritis,with special emphasis on the different roles in synovial inflammation and bone remodeling

Rheumatoid arthritis (RA) is a chronic symmetrical autoimmune disease of unknown etiology that affects primarily the diarthrodial joints. Characteristic features of RA pathogenesis are synovial inflammation and proliferation accompanied by cartilage erosion and bone loss. Fibroblast-like synoviocytes (FLS) display an important role in the pathogenesis of RA. Several lines of evidence show that the Wnt signaling pathway significantly participates in the RA pathogenesis. The Wnt proteins are glycoproteins that bind to the Fz receptors on the cell surface, which leads to several important biological functions, such as cell differentiation, embryonic development, limb development and joint formation. Accumulated evidence has suggested that this signaling pathway plays a key role in the FLS activation, bone resorption and joint destruction during RA development. Greater knowledge of the role of the Wnt signaling pathway in RA could improve understanding of the RA pathogenesis and the differences in RA clinical presentation and prognosis. In this review, new advances of the Wnt signaling pathway in RA pathogenesis are discussed, with special emphasis on its different roles in synovial inflammation and bone remodeling. Further studies are needed to reveal the important role of the members of the Wnt signaling pathway in the RA pathogenesis and treatment.     

Multiple joint and skeletal patterning defects caused by single and double mutations in the mouse Gdf6 and Gdf5 genes

Growth/differentiation factors 5, 6, and 7 (GDF5/6/7) represent a distinct subgroup within the bone morphogenetic protein (BMP) family of secreted signaling molecules. Previous studies have shown that the Gdf5 gene is expressed in transverse stripes across developing skeletal elements and is one of the earliest known markers of joint formation during embryonic development. Although null mutations in this gene disrupt formation of some bones and joints in the skeleton, many sites are unaffected. Here, we show that the closely related family members Gdf6 and Gdf7 are expressed in different subsets of developing joints. Inactivation of the Gdf6 gene causes defects in joint, ligament, and cartilage formation at sites distinct from those seen in Gdf5 mutants, including the wrist and ankle, the middle ear, and the coronal suture between bones in the skull. Mice lacking both Gdf5 and Gdf6 show additional defects, including severe reduction or loss of some skeletal elements in the limb, additional fusions between skeletal structures, scoliosis, and altered cartilage in the intervertebral joints of the spinal column. These results show that members of the GDF5/6/7 subgroup are required for normal formation of bones and joints in the limbs, skull, and axial skeleton. The diverse effects on joint development and the different types of joints affected in the mutants suggest that members of the GDF family play a key role in establishing boundaries between many different skeletal elements during normal development. Some of the skeletal defects seen in single or double mutant mice resemble defects seen in human skeletal diseases, which suggests that these genes may be candidates that underlie some forms of carpal/tarsal coalition, conductive deafness, scoliosis, and craniosynostosis.     

TGF-beta/Smad3 signals repress chondrocyte hypertrophic differentiation and are required for maintaining articular cartilage

Endochondral ossification begins from the condensation and differentiation of mesenchymal cells into cartilage. The cartilage then goes through a program of cell proliferation, hypertrophic differentiation, calcification, apoptosis, and eventually is replaced by bone. Unlike most cartilage, articular cartilage is arrested before terminal hypertrophic differentiation. In this study, we showed that TGF-beta/Smad3 signals inhibit terminal hypertrophic differentiation of chondrocyte and are essential for maintaining articular cartilage. Mutant mice homozygous for a targeted disruption of Smad3 exon 8 (Smad3(ex8/ex8)) developed degenerative joint disease resembling human osteoarthritis, as characterized by progressive loss of articular cartilage, formation of large osteophytes, decreased production of proteoglycans, and abnormally increased number of type X collagen-expressing chondrocytes in synovial joints. Enhanced terminal differentiation of epiphyseal growth plate chondrocytes was also observed in mutant mice shortly after weaning. In an in vitro embryonic metatarsal rudiment culture system, we found that TGF-beta1 significantly inhibits chondrocyte differentiation of wild-type metatarsal rudiments. However, this inhibition is diminished in metatarsal bones isolated from Smad3(ex8/ex8) mice. These data suggest that TGF-beta/Smad3 signals are essential for repressing articular chondrocyte differentiation. Without these inhibition signals, chondrocytes break quiescent state and undergo abnormal terminal differentiation, ultimately leading to osteoarthritis.     

Angiogenesis in rheumatoid arthritis

The expansion of the synovial lining of joints in rheumatoid arthritis (RA) and the subsequent invasion by the pannus of underlying cartilage and bone necessitate an increase in the vascular supply to the synovium, to cope with the increased requirement for oxygen and nutrients. The formation of new blood vessels - termed 'angiogenesis' - is now recognised as a key event in the formation and maintenance of the pannus in RA. This pannus is highly vascularised, suggesting that targeting blood vessels in RA may be an effective future therapeutic strategy. Disruption of the formation of new blood vessels would not only prevent delivery of nutrients to the inflammatory site, but could also lead to vessel regression and possibly reversal of disease. Although many proangiogenic factors are expressed in the synovium in RA, the potent proangiogenic cytokine vascular endothelial growth factor (VEGF) has been shown to a have a central involvement in the angiogenic process in RA. The additional activity of VEGF as a vascular permeability factor may also increase oedema and hence joint swelling in RA. Several studies have shown that targeting angiogenesis in animal models of arthritis ameliorates disease. Our own study showed that inhibition of VEGF activity in murine collagen-induced arthritis, using a soluble VEGF receptor, reduced disease severity, paw swelling, and joint destruction. Although no clinical trials of anti-angiogenic therapy in RA have been reported to date, the blockade of angiogenesis - and especially of VEGF - appears to be a promising avenue for the future treatment of RA.     

GDF5 coordinates bone and joint formation during digit development

A functional skeletal system requires the coordinated development of many different tissue types, including cartilage, bones, joints, and tendons. Members of the Bone morphogenetic protein (BMP) family of secreted signaling molecules have been implicated as endogenous regulators of skeletal development. This is based on their expression during bone and joint formation, their ability to induce ectopic bone and cartilage, and the skeletal abnormalities present in animals with mutations in BMP family members. One member of this family, Growth/differentiation factor 5 (GDF5), is encoded by the mouse brachypodism locus. Mice with mutations in this gene show reductions in the length of bones in the limbs, altered formation of bones and joints in the sternum, and a reduction in the number of bones in the digits. The expression pattern of Gdf5 during normal development and the phenotypes seen in mice with single or double mutations in Gdf5 and Bmp5 suggested that Gdf5 has multiple functions in skeletogenesis, including roles in joint and cartilage development. To further understand the function of GDF5 in skeletal development, we assayed the response of developing chick and mouse limbs to recombinant GDF5 protein. The results from these assays, coupled with an analysis of the development of brachypodism digits, indicate that GDF5 is necessary and sufficient for both cartilage development and the restriction of joint formation to the appropriate location. Thus, GDF5 function in the digits demonstrates a link between cartilage development and joint development and is an important determinant of the pattern of bones and articulations in the digits.     

Toll-like receptor 2 pathway drives streptococcal cell wall-induced joint inflammation: critical role of myeloid differentiation factor 88

The IL-1R/Toll-like receptor (TLR) superfamily of receptors has a key role in innate immunity and inflammation. In this study, we report that streptococcal cell wall (SCW)-induced joint inflammation is predominantly dependent on TLR-2 signaling, since TLR-2-deficient mice were unable to develop either joint swelling or inhibition of cartilage matrix synthesis. Myeloid differentiation factor 88 (MyD88) is a Toll/IL-1R domain containing adaptor molecule known to have a central role in both IL-1R/IL-18R and TLR signaling. Mice deficient for MyD88 did not develop SCW-induced arthritis; both joint swelling and disturbance of cartilage chondrocyte anabolic function was completely abolished. Local levels of proinflammatory cytokines and chemokines in synovial tissue washouts were strongly reduced in MyD88-deficient mice. Histology confirmed the pivotal role of MyD88 in acute joint inflammation. TLR-2-deficient mice still allow influx of inflammatory cells into the joint cavity, although the number of cells was markedly reduced. No influx of inflammatory cells was seen in joints of MyD88-deficient mice. In addition, cartilage matrix proteoglycan loss was completely absent in MyD88 knockout mice. These findings clearly demonstrated that MyD88 is a key component in SCW-induced joint inflammation. Since agonists of the Toll-like pathway are abundantly involved in both septic and rheumatoid arthritis, targeting of MyD88 may be a novel therapy in inflammatory joint diseases.     

Are bone losers distinguishable from bone formers in a skeletal series? Implications for adult age at death assessment methods.

Clinical studies indicate that genetic factors play a crucial role in primary osteoarthritis and osteoporosis. In addition, it has been suggested that these two diseases are inversely related. Within a population, one can find two sub-groups: the "bone formers" and the "bone losers". The changes to the joint surfaces used to assess adult age at death are related to the loss of bone substance and to bone formation (osteophytes). The modification of these indicators with age differs between bone formers and bone losers. Therefore, age-at-death assessment methods should make use of two standards, one for each sub-group. A preliminary study examining the possibility of distinguishing those who lose cortical bone from those who show signs of bony formation was conducted on a series of skeletons from Portugal, dating to the end of 19th century and the beginning of the 20th. Bone loss was evaluated using the cortical index (CI) of the second metacarpal on X-rays. The presence of osteophytes on dry bones was assessed macroscopically. Our study indicates that females' CI decreases with age, whereas the presence of osteophytes is strongly related to age in both sexes. But we have failed to find the inverse relationship between osteophytes and bone loss. Our study, however, shows that within a population, some individuals are not likely to develop osteophytes.     

Excessive bone formation in a mouse model of ankylosing spondylitis is associated with decreases in Wnt pathway inhibitors

Introduction

Ankylosing spondylitis (AS) is unique in its pathology where inflammation commences at the entheses before progressing to an osteoproliferative phenotype generating excessive bone formation that can result in joint fusion. The underlying mechanisms of this progression are poorly understood. Recent work has suggested that changes in Wnt signalling, a key bone regulatory pathway, may contribute to joint ankylosis in AS. Using the proteoglycan-induced spondylitis (PGISp) mouse model which displays spondylitis and eventual joint fusion following an initial inflammatory stimulus, we have characterised the structural and molecular changes that underlie disease progression.

Methods

PGISp mice were characterised 12 weeks after initiation of inflammation using histology, immunohistochemistry (IHC) and expression profiling.

Results

Inflammation initiated at the periphery of the intervertebral discs progressing to disc destruction followed by massively excessive cartilage and bone matrix formation, as demonstrated by toluidine blue staining and IHC for collagen type I and osteocalcin, leading to syndesmophyte formation. Expression levels of DKK1 and SOST, Wnt signalling inhibitors highly expressed in joints, were reduced by 49% and 63% respectively in the spine PGISp compared with control mice (P < 0.05) with SOST inhibition confirmed by IHC. Microarray profiling showed genes involved in inflammation and immune-regulation were altered. Further, a number of genes specifically involved in bone regulation including other members of the Wnt pathway were also dysregulated.

Conclusions

This study implicates the Wnt pathway as a likely mediator of the mechanism by which inflammation induces bony ankylosis in spondyloarthritis, raising the potential that therapies targeting this pathway may be effective in preventing this process.     

Development of a traumatic anterior cruciate ligament and meniscal rupture model with a pilot in vivo study

The current study describes the development of a small animal, closed-joint model of traumatic anterior cruciate ligament (ACL) and meniscal rupture. This model can be used in future studies to investigate the roles of these acute damages on the long-term health of an injured knee joint. Forty-two Flemish Giant rabbits received an insult to the left tibiofemoral joint ex vivo in order to document optimal energy and joint orientation needed to generate ACL and meniscal rupture, without gross fracture of bone. Impact energies ranged from 10 J to 22 J, and joint flexion angle ranged from 60 deg to 90 deg. Three in vivo animals were impacted at 13 J with the knee flexed at 90 deg, as this was determined to be the optimal load and joint orientation for ACL and meniscal ruptures, and sacrificed at 12 weeks. Impact data from the ex vivo group revealed that 13 J of dropped-mass energy, generating approximately 1100 N of load on the knee, would cause ACL and meniscal ruptures, without gross bone fracture. Acute damage to the lateral and medial menisci was documented in numerous ex vivo specimens, with isolated lateral meniscal tears being more frequent than isolated medial tears in other cases. The in vivo animals showed no signs of ill health or other physical complications. At 12 week post-trauma these animals displayed marked degeneration of the traumatized joint including synovitis, cartilage erosion, and the formation of peripheral osteophytes. Histological microcracks at the calcified cartilage-subchondral bone interface were also evident in histological sections of these animals. A closed-joint model of traumatic ACL and meniscal rupture was produced, without gross bone fracture, and a pilot, in vivo study showed progressive joint degeneration without any other noticeable physical impairments of the animals over 12 weeks. This closed-joint, traumatic injury model may be useful in future experimental studies of joint disease and various intervention strategies.