Subchondral plate porosity colocalizes with the point of mechanical load during ambulation in a rat knee model of post-traumatic osteoarthritis

Network-based cytokine inference implicates Oncostatin M as a driver of an inflammation phenotype in knee osteoarthritis

Inflammatory cytokines released by synovium after trauma disturb the gene regulatory network and have been implicated in the pathophysiology of osteoarthritis. A mechanistic understanding of how aging perturbs this process can help identify novel interventions. Here, we introduced network paradigms to simulate cytokine-mediated pathological communication between the synovium and cartilage. Cartilage-specific network analysis of injured young and aged murine knees revealed aberrant matrix remodeling as a transcriptomic response unique to aged knees displaying accelerated cartilage degradation. Next, network-based cytokine inference with pharmacological manipulation uncovered IL6 family member, Oncostatin M (OSM), as a driver of the aberrant matrix remodeling. By implementing a phenotypic drug discovery approach, we identified that the activation of OSM recapitulated an "inflammatory" phenotype of knee osteoarthritis and highlighted high-value targets for drug development and repurposing. These findings offer translational opportunities targeting the inflammation-driven osteoarthritis phenotype.

Face and Predictive Validity of MI-RAT (Montreal Induction of Rat Arthritis Testing), a Surgical Model of Osteoarthritis Pain in Rodents Combined with Calibrated Exercise

Validating animal pain models is crucial to enhancing translational research and response to pharmacological treatment. This study investigated the effects of a calibrated slight exercise protocol alone or combined with multimodal analgesia on sensory sensitivity, neuroproteomics, and joint structural components in the MI-RAT model. Joint instability was induced surgically on day (D) 0 in female rats (N = 48) distributed into sedentary-placebo, exercise-placebo, sedentary-positive analgesic (PA), and exercise-PA groups. Daily analgesic treatment (D3-D56) included pregabalin and carprofen. Quantitative sensory testing was achieved temporally (D-1, D7, D21, D56), while cartilage alteration (modified Mankin's score (mMs)) and targeted spinal pain neuropeptide were quantified upon sacrifice. Compared with the sedentary-placebo (presenting allodynia from D7), the exercise-placebo group showed an increase in sensitivity threshold (p < 0.04 on D7, D21, and D56). PA treatment was efficient on D56 (p = 0.001) and presented a synergic anti-allodynic effect with exercise from D21 to D56 (p < 0.0001). Histological assessment demonstrated a detrimental influence of exercise (mMs = 33.3%) compared with sedentary counterparts (mMs = 12.0%; p < 0.001), with more mature transformations. Spinal neuropeptide concentration was correlated with sensory sensitization and modulation sites (inflammation and endogenous inhibitory control) of the forced mobility effect. The surgical MI-RAT OA model coupled with calibrated slight exercise demonstrated face and predictive validity, an assurance of higher clinical translatability.

One session of 20 ​N cyclic compression induces chronic knee osteoarthritis in rats: A long-term study

Objective

Mechanical stimulation is a risk factor for knee osteoarthritis. Non-surgical compression has been used to study the effects of mechanical stimulation in vivo. However, the long-term effects of low-force compression on knee joint had not been studied. Therefore, we sought to identify the long-term effects of low-force cyclic compression on the rat knee joint.

Design

In this study, we applied one session cyclic compression with a peak load of 20 ​N for 60 cycles to the rat knee joint in an approximately 140-degree flexion position (Wistar, male, 12 weeks old), followed by 1 year of observation (including data from 1 week, 2 weeks, 4 weeks, 8 weeks, 6 months, and 1 year after compression), and then performed a sub-regional analysis with hematoxylin-eosin, Safranin O and Fast Green, and MMP13 immunohistochemical staining.

Results

We observed osteoarthritis-like cartilage damage, synovial inflammation, and high expression of MMP13 within 1 year after compression. However, these changes progressed slowly, with obvious matrix cracks that did not appear until 1 year after compression. In the regional analysis, we found that low-force compression caused a much slower development of injury at the compression contact site, and no significant structural cartilage damage was observed after 1 year of compression. In contrast, the non-contact site during compression at tibial cartilage in the same joint was the first to show significant structural damage.

Conclusion

This study demonstrates that one session of 20 ​N cyclic compression induces a chronic osteoarthritis-like phenotype in the rat knee in the long term.

Chondrocyte apoptosis in temporomandibular joint osteoarthritis promotes bone resorption by enhancing chemotaxis of osteoclast precursors

OBJECTIVE

This study aimed to explore the effect and mechanism of chondrocyte apoptosis on the chemotaxis of osteoclast precursors (OCPs) during bone destruction.

DESIGN

The relationship between cartilage and bone destruction was verified with a rat temporomandibular joint osteoarthritis (TMJOA) model. The pan-caspase inhibitor Z-VAD-FMK (ZVAD) was applied to confirm the chemotactic effect of chondrocyte apoptosis on OCPs. Synthesis and release of the key chemokine CX3CL1 in apoptotic and non-apoptotic chondrocytes was assessed with IHC, IF, WB, and ELISA. The function of CX3CL1-CX3CR1 axis in the chemotaxis of OCPs was examined by CX3XR1 inhibitor AZD8797 (AZD) and si-CX3CL1. The regulatory effect of p38 MAPK on CX3CL1 release was verified by p38 inhibitor PH-797804.

RESULTS

A temporal and spatial association between cartilage degradation and bone resorption was found in the TMJOA model. The caspase-dependent chondrocyte apoptosis promoted chemotaxis of OCPs, which can be restrained by ZVAD. CX3CL1 was significantly upregulated when chondrocytes underwent apoptosis, and it played a critical role in the recruitment of OCPs, blockage of CX3CL1-CX3CR1 axis resulted in less bone resorption in TMJOA. P38 MAPK was activated in apoptotic chondrocytes, and had a regulatory effect on the synthesis and release of CX3CL1. After inhibition of p38 by PH-797804, the chemotactic effect of apoptotic chondrocytes on OCPs was limited.

CONCLUSIONS

This study indicates that apoptosis of chondrocytes in TMJOA enhances chemotaxis of OCPs toward osteoclast precursors through upregulation of the p38-CX3CL1 axis, thereby promoting the activation of local osteoclasts.

A musculoskeletal finite element model of rat knee joint for evaluating cartilage biomechanics during gait

Abnormal loading of the knee due to injuries or obesity is thought to contribute to the development of osteoarthritis (OA). Small animal models have been used for studying OA progression mechanisms. However, numerical models to study cartilage responses under dynamic loading in preclinical animal models have not been developed. Here we present a musculoskeletal finite element model of a rat knee joint to evaluate cartilage biomechanical responses during a gait cycle. The rat knee joint geometries were obtained from a 3-D MRI dataset and the boundary conditions regarding loading in the joint were extracted from a musculoskeletal model of the rat hindlimb. The fibril-reinforced poroelastic (FRPE) properties of the rat cartilage were derived from data of mechanical indentation tests. Our numerical results showed the relevance of simulating anatomical and locomotion characteristics in the rat knee joint for estimating tissue responses such as contact pressures, stresses, strains, and fluid pressures. We found that the contact pressure and maximum principal strain were virtually constant in the medial compartment whereas they showed the highest values at the beginning of the gait cycle in the lateral compartment. Furthermore, we found that the maximum principal stress increased during the stance phase of gait, with the greatest values at midstance. We anticipate that our approach serves as a first step towards investigating the effects of gait abnormalities on the adaptation and degeneration of rat knee joint tissues and could be used to evaluate biomechanically-driven mechanisms of the progression of OA as a consequence of joint injury or obesity.

Osteoarthritis is a disease of the musculoskeletal system which is characterized by the degradation of articular cartilage. Changes in the knee loading after injuries or obesity contribute to the development of cartilage degeneration. Since injured cartilage cannot be reversed back to intact conditions, small animal models have been widely used for investigating osteoarthritis progression mechanisms. Moreover, experimental studies have been complemented with numerical models to overcome inherent limitations such as cost, difficulties to obtain accurate measures and replicate degenerative situations in the knee joint. However, computational models to study articular cartilage responses under dynamic loading in small animal models have not been developed. Thus, here we present a musculoskeletal finite element model (MSFE) of a rat knee joint to evaluate cartilage biomechanical responses during gait. Our computational model considers both the anatomical and locomotion characteristics of the rat knee joint for estimating mechanical responses in the articular cartilage. We suggest that our approach can be used to investigate tissue adaptations based on the mechanobiological responses of the cartilage to prevent the progression of osteoarthritis.

Relationship between altered knee kinematics and subchondral bone remodeling in a clinically translational model of ACL injury

Abnormal joint kinematics are commonly reported in the acute and chronic stages of recovery after anterior cruciate ligament (ACL) injury and have long been mechanistically implicated as a primary driver in the development of posttraumatic osteoarthritis (PTOA). Though strongly theorized, it is unclear to what extent biomechanical adaptations after ACL injury culminate in the development of PTOA, as data that directly connects these factors does not exist. Using a preclinical, noninvasive ACL injury rodent model, our objective was to explore the direct effect of an isolated ACL injury on joint kinematics and the pathogenetic mechanisms involved in the development of PTOA. A total of 32, 16-week-old Long-Evans rats were exposed to a noninvasive ACL injury. Marker-less deep learning software (DeepLabCut) was used to track animal movement for sagittal-plane kinematic analyses and micro computed tomography was used to evaluate subchondral bone architecture at days 7, 14, 28, and 56 following injury. There was a significant decrease in peak knee flexion during walking (p < .05), which had a moderate-to-strong negative correlation (r = -.59 to -.71; p < .001) with subchondral bone plate porosity in all load bearing regions of the femur and tibia. Additional comprehensive analyses of knee flexion profiles revealed dramatic alterations throughout the step cycle. This occurred alongside considerable loss of epiphyseal trabecular bone and substantial changes in anatomical orientation. Knee flexion angle and subchondral bone microarchitecture are severely impacted after ACL injury. Reductions in peak knee flexion angle after ACL injury are directly associated with subchondral bone plate remodeling.

Dihydroartemisinin attenuates osteoclast formation and bone resorption via inhibiting the NF‑κB, MAPK and NFATc1 signaling pathways and alleviates osteoarthritis

Osteoarthritis (OA) is a chronic, progressive and degenerative disease, and its incidence is increasing on a yearly basis. However, the pathological mechanism of OA at each stage is still unclear. The present study aimed to explore the underlying mechanism of dihydroartemisinin (DHA) in terms of its ability to inhibit osteoclast activation, and to determine its effects on OA in rats. Bone marrow‑derived macrophages were isolated as osteoclast precursors. In the presence or absence of DHA, osteoclast formation was assessed by tartrate‑resistant acid phosphatase (TRAP) staining, cell viability was assessed by Cell Counting Kit‑8 assay, the presence of F‑actin rings was assessed by immunofluorescence, bone resorption was determined by bone slices, luciferase activities of NF‑κB and nuclear factor of activated T cell cytoplasmic 1 (NFATc1) were determined using luciferase assay kits, the protein levels of biomolecules associated with the NF‑κB, MAPK and NFATc1 signaling pathways were determined using western blotting, and the expression of genes involved in osteoclastogenesis were measured using reverse transcription‑quantitative PCR. A knee OA rat model was designed by destabilizing the medial meniscus (DMM). A total of 36 rats were assigned to three groups, namely the sham‑operated, DMM + vehicle and DMM + DHA groups, and the rats were administered DHA or DMSO. At 4 and 8 weeks postoperatively, the microarchitecture of the subchondral bone was analyzed using micro‑CT, the thickness of the cartilage layers was calculated using H&E staining, the extent of cartilage degeneration was scored using Safranin O‑Fast Green staining, TRAP‑stained osteoclasts were counted, and the levels of receptor activator of NF‑κB ligand (RANKL), C‑X‑C‑motif chemokine ligand 12 (CXCL12) and NFATc1 were measured using immunohistochemistry. DHA was found to inhibit osteoclast formation without cytotoxicity, and furthermore, it did not affect bone formation. In addition, DHA suppressed the expression levels of NF‑κB, MAPK, NFATc1 and genes involved in osteoclastogenesis. Progressive cartilage loss was observed at 8 weeks postoperatively. Subchondral bone remodeling was found to be dominated by bone resorption accompanied by increases in the levels of RANKL, CXCL12 and NFATc1 during the first 4 weeks. DHA was found to delay OA progression by inhibiting osteoclast formation and bone resorption during the early phase of OA. Taken together, the results of the present study demonstrated that the mechanism through which DHA could inhibit osteoclast activation may be associated with the NF‑κB, MAPK and NFATc1 signaling pathways, thereby indicating a potential novel strategy for OA treatment.

Time-dependently Appeared Microenvironmental Changes and Mechanism after Cartilage or Joint Damage and the Influences on Cartilage Regeneration

Cartilage and joint damage easily degenerates cartilage and turns into osteoarthritis (OA), which seriously affects human life and work, and has no cure currently. The temporal and spatial changes of multiple microenvironments upon the damage of cartilage and joint are noticed, including the emergences of inflammation, bone remodeling, blood vessels, and nerves, as well as alterations of extracellular and pericellular matrix, oxygen tension, biomechanics, underneath articular cartilage tissues, and pH value. This review summarizes the existing literatures on microenvironmental changes, mechanisms, and their negative effects on cartilage regeneration following cartilage and joint damage. We conclude that time-dependently rebuilding the multiple normal microenvironments of damaged cartilage is the key for cartilage regeneration after systematic studies for the timing and correlations of various microenvironment changes.

Macro, Micro, and Molecular. Changes of the Osteochondral Interface in Osteoarthritis Development

Osteoarthritis (OA) is a long-term condition that causes joint pain and reduced movement. Notably, the same pathways governing cell growth, death, and differentiation during the growth and development of the body are also common drivers of OA. The osteochondral interface is a vital structure located between hyaline cartilage and subchondral bone. It plays a critical role in maintaining the physical and biological function, conveying joint mechanical stress, maintaining chondral microenvironment, as well as crosstalk and substance exchange through the osteochondral unit. In this review, we summarized the progress in research concerning the area of osteochondral junction, including its pathophysiological changes, molecular interactions, and signaling pathways that are related to the ultrastructure change. Multiple potential treatment options were also discussed in this review. A thorough understanding of these biological changes and molecular mechanisms in the pathologic process will advance our understanding of OA progression, and inform the development of effective therapeutics targeting OA.

Acupotomy Contributes to Suppressing Subchondral Bone Resorption in KOA Rabbits by Regulating the OPG/RANKL Signaling Pathway

Subchondral bone lesions, as the crucial inducement for accelerating cartilage degeneration, have been considered as the initiating factor and the potential therapeutic target of knee osteoarthritis (KOA). Acupotomy, the biomechanical therapy guided by traditional Chinese meridians theory, alleviates cartilage deterioration by correcting abnormal mechanics. Whether this mechanical effect of acupotomy inhibits KOA subchondral bone lesions is indistinct. This study aimed to investigate the effects of acupotomy on inhibiting subchondral bone resorption and to define the possible mechanism in immobilization-induced KOA rabbits. After KOA modeling, 8 groups of rabbits (4w/6w acupotomy, 4w/6w electroacupuncture, 4w/6w model, and 4w/6w control groups) received the indicated intervention for 3 weeks. Histological and bone histomorphometry analyses revealed that acupotomy prevented both cartilage surface erosion and subchondral bone loss. Further, acupotomy suppressed osteoclast activity and enhanced osteoblast activity in KOA subchondral bone, showing a significantly decreased expression of tartrate-resistant acid phosphatase (TRAP), matrix metalloproteinases-9 (MMP-9), and cathepsin K (Ctsk) and a significantly increased expression of osteocalcin (OCN); this regulation may be mediated by blocking the decrease in osteoprotegerin (OPG) and the increase in NF-κB receptor activated protein ligand (RANKL). These findings indicated that acupotomy inhibited osteoclast activity and promoted osteoblast activity to ameliorate hyperactive subchondral bone resorption and cartilage degeneration in immobilization-induced KOA rabbits, which may be mediated by the OPG/RANKL signaling pathway. Taken together, our results indicate that acupotomy may have therapeutic potential in KOA by restoring the balance between bone formation and bone resorption to attenuate subchondral bone lesions.

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.

Effects of in vivo cyclic compressive loading on the distribution of local Col2 and superficial lubricin in rat knee cartilage

This study aimed to examine the effects of an episode of in vivo cyclic loading on rat knee articular cartilage (AC) under medium-term observation, while also investigating relevant factors associated with the progression of post-traumatic osteoarthritis (PTOA). Twelve-week-old Wistar rats underwent one episode comprising 60 cycles of 20 N or 50 N dynamic compression on the right knee joint. Spatiotemporal changes in the AC after loading were evaluated using histology and immunohistochemistry at 3 days and 1, 2, 4, and 8 weeks after loading (n = 6 for each condition). Chondrocyte vitality was assessed at 1, 3, 6, and 12 hours after loading (n = 2 for each condition). A localized AC lesion on the lateral femoral condyle was confirmed in all subjects. The surface and intermediate cartilage in the affected area degenerated after loading, but the calcified cartilage remained intact. Expression of type II collagen in the lesion cartilage was upregulated after loading, whereas the superficial lubricin layer was eroded in response to cyclic compression. However, the distribution of superficial lubricin gradually recovered to the normal level 4 weeks after loading-induced injury. We confirmed that 60 repetitions of cyclic loading exceeding 20 N could result in cartilage damage in the rat knee. Endogenous repairs in well-structured joints work well to rebuild protective layers on the lesion cartilage surface, which may be the latent factor delaying the progression of PTOA.

Gene Expression Signatures of Synovial Fluid Multipotent Stromal Cells in Advanced Knee Osteoarthritis and Following Knee Joint Distraction

Osteoarthritis (OA) is the most common musculoskeletal disorder. Although joint replacement remains the standard of care for knee OA patients, knee joint distraction (KJD), which works by temporarily off-loading the joint for 6–8 weeks, is becoming a novel joint-sparing alternative for younger OA sufferers. The biological mechanisms behind KJD structural improvements remain poorly understood but likely involve joint-resident regenerative cells including multipotent stromal cells (MSCs). In this study, we hypothesized that KJD leads to beneficial cartilage-anabolic and anti-catabolic changes in joint-resident MSCs and investigated gene expression profiles of synovial fluid (SF) MSCs following KJD as compared with baseline. To obtain further insights into the effects of local biomechanics on MSCs present in late OA joints, SF MSC gene expression was studied in a separate OA arthroplasty cohort and compared with subchondral bone (SB) MSCs from medial (more loaded) and lateral (less loaded) femoral condyles from the same joints. In OA arthroplasty cohort (n = 12 patients), SF MSCs expressed lower levels of ossification- and hypotrophy-related genes [bone sialoprotein (IBSP), parathyroid hormone 1 receptor (PTH1R), and runt-related transcription factor 2 (RUNX2)] than did SB MSCs. Interestingly, SF MSCs expressed 5- to 50-fold higher levels of transcripts for classical extracellular matrix turnover molecules matrix metalloproteinase 1 (MMP1), a disintegrin and metalloproteinase with thrombospondin motifs 5 (ADAMTS5), and tissue inhibitor of metalloproteinase-3 (TIMP3), all (p < 0.05) potentially indicating greater cartilage remodeling ability of OA SF MSCs, compared with SB MSCs. In KJD cohort (n = 9 patients), joint off-loading resulted in sustained, significant increase in SF MSC colonies’ sizes and densities and a notable transcript upregulation of key cartilage core protein aggrecan (ACAN) (weeks 3 and 6), as well as reduction in pro-inflammatory C–C motif chemokine ligand 2 (CCL2) expression (weeks 3 and 6). Additionally, early KJD changes (week 3) were marked by significant increases in MSC chondrogenic commitment markers gremlin 1 (GREM1) and growth differentiation factor 5 (GDF5). In combination, our results reveal distinct transcriptomes on joint-resident MSCs from different biomechanical environments and show that 6-week joint off-loading leads to transcriptional changes in SF MSCs that may be beneficial for cartilage regeneration. Biomechanical factors should be certainly considered in the development of novel MSC-based therapies for OA.

Specific inhibition of FAK signaling attenuates subchondral bone deterioration and articular cartilage degeneration during osteoarthritis pathogenesis

Osteoarthritis (OA), a disease of the entire joint, is characterized by abnormal bone remodeling and coalescent degradation of articular cartilage. We have previously found that elevated levels of H-type vessels in subchondral bone correlate with OA and that focal adhesion kinase (FAK) is critical for H-type vessel formation in osteoporosis. However, the potential role of FAK in OA remains unexplored. Here, we demonstrate that the p-FAK level was dramatically elevated in subchondral bone following anterior cruciate ligament transection (ACLT) in rats. Specific inhibition of FAK signaling with Y15 in subchondral bone resulted in the suppression of subchondral bone deterioration and this effect was mediated by H-type vessel-induced ectopic bone formation. Further, articular cartilage degeneration was also alleviated after Y15 treatment. In vitro, the p-FAK level was significantly elevated in mesenchymal stem cells (MSCs) from vehicle-treated ACLT rats as compared to that in MSCs from sham controls and Y15-treated ACLT rats. Elevated p-FAK level in MSCs promoted vascular endothelial growth factor (VEGF) expression, as demonstrated from the high VEGF level in the blood, subchondral bone, and conditioned medium (CM) of MSCs from vehicle-treated ACLT rats. The CM of MSCs from vehicle-treated ACLT rats might promote the angiogenesis of endothelial cells and the catabolic response of chondrocytes through the FAK-growth factor receptor-bound protein 2-mitogen-activated protein kinase-mediated expression of VEGF. The effect of the CM from MSCs of Y15-treated ACLT rats or that treated with a VEGF-neutralizing antibody on vessel formation and the catabolic response was lowered. Thus, the specific inhibition of FAK signaling may be a promising avenue for the prevention or early treatment of OA.

Biochemical Signals Mediate the Crosstalk between Cartilage and Bone in Osteoarthritis

Osteochondral junction is a functional unit comprising the articular cartilage, calcified cartilage, and subchondral bone. Alteration in any component of this composite unit can disrupt the joint integrity and function directly or indirectly. Biochemical signals mediate the crosstalk between tissues and play an essential role in the initiation and progression of osteoarthritis. As osteoarthritis progresses, abnormal subchondral bone remodelling leads to increased angiogenesis and porosity of the subchondral bone plate, which further triggers biochemical signals to mediate the crosstalk between cartilage and bone, contributing to the progression of osteoarthritis. Notably, common biochemical signals include the TGF-β/Smad, Wnt/β-catenin, RANK/RANKL/OPG, and MAPK pathways. This biomarker crosstalk network is the basis of osteoarthritis pathogenesis, and some of their key regulators may be potential therapeutic targets for osteoarthritis drug therapy. This review summarised the biochemical crosstalk between cartilage and bone in the pathogenesis of osteoarthritis, which may provide the basis for the discovery of osteoarthritis treatment targets.

Instability and excessive mechanical loading mediate subchondral bone changes to induce osteoarthritis

Background

To assess the diversified effects of mechanical instability, excessive mechanical loading on subchondral bone remodeling. And to investigate the underlying cartilage degeneration and osteoarthritis (OA) progression in ipsilateral and contralateral knees, given that OA progression always affects joints bilaterally.

Methods

Anterior cruciate ligament transection (ACLT) of the left knee was used to induce OA in C57/B6 mice for 1, 3 and 6 months. Both left (ipsilateral) and right (contralateral) knees underwent micro-computerized tomography (micro-CT) scan and morphological analysis. The subchondral bone metabolism analysis by immunostaining of tartrate-resistant acid phosphatase (TRAP) and Osterix. Behavioral analyses including von Frey test and CatWalk gait analysis were also performed. Western blot analysis was performed to assess the signaling pathways involved in OA progression.

Results

Analyses showed that various changes in ipsilateral and contralateral knees lead to OA progression. Articular cartilage was rapidly destroyed on the ipsilateral side but was only gradually destroyed on the contralateral side. Micro-CT data showed a rapid decrease with a subsequent partial recovery of bone volume in the late stage on the ipsilateral side, while a gradual condensation of bone density was seen on the contralateral side. Immunostaining showed increased osteoclastic and osteoblastic activity in the early stage on the ipsilateral side, but only slight osteoblastic changes on the contralateral side. Behavioral analyses including von Frey and gait analysis showed that contralateral knees compensate ipsilateral mechanical loading, but also that this mechanism failed to work in the late stage.

Conclusions

Diversified mechanical loading properties lead to OA progression through different mechanisms of subchondral bone remodeling. Acute ACLT led to OA through bone density reduction, while the contralateral side developed OA gradually due to subchondral bone sclerosis.

Knee loading repairs osteoporotic osteoarthritis by relieving abnormal remodeling of subchondral bone via Wnt/β-catenin signaling

Osteoporotic osteoarthritis (OPOA) is a common bone disease mostly in the elderly, but the relationship between Osteoporotic (OP) and osteoarthritis (OA) is complex. It has been shown that knee loading can mitigate OA symptoms. However, its effects on OPOA remain unclear. In this study, we characterized pathological linkage of OP to OA, and evaluated the effect of knee loading on OPOA. We employed two mouse models (OA and OPOA), and conducted histology, cytology, and molecular analyses. In the OA and OPOA groups, articular cartilage was degenerated and Osteoarthritis Research Society International score was increased. Subchondral bone underwent abnormal remodeling, the differentiation of bone marrow mesenchymal stem cells (BMSCs) to osteoblasts and chondrocytes was reduced, and migration and adhesion of pre-osteoclasts were enhanced. Compared to the OA group, the pathological changes of OA in the OPOA group were considerably aggravated. After knee loading, however, cartilage degradation was effectively prevented, and the abnormal remodeling of subchondral bone was significantly inhibited. The differentiation of BMSCs was also improved, and the expression of Wnt/β-catenin was elevated. Collectively, this study demonstrates that osteoporosis aggravates OA symptoms. Knee loading restores OPOA by regulating subchondral bone remodeling, and may provide an effective method for repairing OPOA.

Bone marrow lesions in knee osteoarthritis: regional differences in tibial subchondral bone microstructure and their association with cartilage degeneration

OBJECTIVE

The aim of this study was to investigate how bone microstructure within bone marrow lesions (BMLs) relates to the bone and cartilage across the whole human tibial plateau.

DESIGN

Thirty-two tibial plateaus from patients with osteoarthritis (OA) at total knee arthroplasty and eleven age-matched non-OA controls, were scanned ex vivo by MRI to identify BMLs and by micro CT to quantitate the subchondral (plate and trabecular) bone microstructure. For cartilage evaluation, specimens were processed histologically.

RESULTS

BMLs were detected in 75% of the OA samples (OA-BML), located predominantly in the anterior-medial (AM) region. In contrast to non-OA control and OA-no BML, in OA-BML differences in microstructure were significantly more evident between subregions. In OA-BML, the AM region contained the most prominent structural alterations. Between-group comparisons showed that the AM region of the OA-BML group had significantly higher histological degeneration (OARSI grade) (P < .0001, P < .05), thicker subchondral plate (P < .05, P < .05), trabeculae that are more anisotropic (P < .0001, P < .05), well connected (P < .05, P = n.s), and more plate-like (P < 0.05, P < 0.05), compared to controls and OA-no BML at this site. Compared to controls, OA-no BML had significantly higher OARSI grade (P < .0001), and lower trabecular number (P < .05).

CONCLUSION

In established knee OA, both the extent of cartilage damage and microstructural degeneration of the subchondral bone were dependent on the presence of a BML. In OA-no BML, bone microstructural alterations are consistent with a bone attrition phase of the disease. Thus, the use of BMLs as MRI image-based biomarkers appear to inform on the degenerative state within the osteochondral unit.

Regulatory Effects and Interactions of the Wnt and OPG-RANKL-RANK Signaling at the Bone-Cartilage Interface in Osteoarthritis

Cartilage and the bordering subchondral bone form a functionally active regulatory interface with a prominent role in osteoarthritis pathways. The Wnt and the OPG-RANKL-RANK signaling systems, as key mediators, interact in subchondral bone remodeling. Osteoarthritic osteoblasts polarize into two distinct phenotypes: a low secretory and an activated, pro-inflammatory and anti-resorptive subclass producing high quantities of IL-6, PGE2, and osteoprotegerin, but low levels of RANKL, thus acting as putative effectors of subchondral bone sclerosis. Wnt agonists, Wnt5a, Wisp-1 initiate excessive bone remodeling, while Wnt3a and 5a simultaneously cause loss of proteoglycans and phenotype shift in chondrocytes, with decreased expression of COL2A, aggrecan, and Sox-9. Sclerostin, a Wnt antagonist possesses a protective effect for the cartilage, while DKK-1 inhibits VEGF, suspending neoangiogenesis in the subchondral bone. Experimental conditions mimicking abnormal mechanical load, the pro-inflammatory milieu, but also a decreased OPG/RANKL ratio in the cartilage, trigger chondrocyte apoptosis and loss of the matrix via degradative matrix metalloproteinases, like MMP-13 or MMP-9. Hypoxia, an important cofactor exerts a dual role, promoting matrix synthesis via HIF-1α, a Wnt silencer, but turning on HIF-2α that enhances VEGF and MMP-13, along with aberrant collagen expression and extracellular matrix deterioration in the presence of pro-inflammatory cytokines.

SDF-1/CXCR4 axis coordinates crosstalk between subchondral bone and articular cartilage in osteoarthritis pathogenesis

Crosstalk between subchondral bone and articular cartilage is considered a central feature of osteoarthritis (OA) initiation and progression, but its underlying molecular mechanism remains elusive. Meanwhile, specific administration of drugs in subchondral bone is also a great challenge during investigation of the process. We here explore the role of stromal cell-derived factor 1 (SDF-1)/C-X-C chemokine receptor type 4 (CXCR4) axis in the crosstalk between subchondral bone and articular cartilage in OA pathogenesis, using osmotic infusion pumps implanted in tibial subchondral bone directly to ensure quantitative, continuous and steady drug delivery over the entire experiment. We found that increased SDF-1 in subchondral bone firstly induced subchondral bone deterioration by erroneous Mesenchymal Stem Cells (MSCs) recruitment and excessive bone resorption in anterior cruciate ligament transection (ACLT) mice. Deterioration of subchondral bone then led to the traverse of SDF-1 from subchondral bone to overlying cartilage. Finally, SDF-1 from underlying subchondral bone combined with CXCR4 in chondrocytes to induce articular cartilage degradation by promoting the shift of transforming growth factor-β receptor type I (TβRI) in chondrocytes from activin receptor-like kinase 5 (ALK5) to activin receptor-like kinase 1 (ALK1). More importantly, specific inhibition of SDF-1/CXCR4 axis in ACLT rats attenuated OA by stabilizing subchondral bone microarchitecture, reducing SDF-1 in cartilage and abrogating the shift of TβRI in chondrocytes. Our data demonstrate that the SDF-1/CXCR4 axis may coordinate the crosstalk between subchondral bone and articular cartilage in OA pathogenesis. Therefore, specific inhibition of SDF-1/CXCR4 axis in subchondral bone or intervention in SDF-1 traverse may be therapeutic targets for OA.

Bone marrow lesions in osteoarthritis: What lies beneath

Osteoarthritis (OA) is the most common joint disease in the United States, affecting more than 30 million people, and is characterized by cartilage degeneration in articulating joints. OA can be viewed as a group of overlapping disorders, which result in functional joint failure. However, the precise cellular and molecular events within which lead to these clinically observable changes are neither well understood nor easily measurable. It is now clear that multiple factors, in multiple joint tissues, contribute to degeneration. Changes in subchondral bone are recognized as a hallmark of OA, but are normally associated with late-stage disease when degeneration is well established. However, early changes such as Bone Marrow Lesions (BMLs) in OA are a relatively recent discovery. BMLs are patterns from magnetic resonance images (MRI) that have been linked with pain and cartilage degeneration. Their potential utility in predicting progression, or as a target for therapy, is not yet fully understood. Here, we will review the current state-of-the-art in this field under three broad headings: (i) BMLs in symptomatic OA: malalignment, joint pain, and disease progression; (ii) biological considerations for bone-cartilage crosstalk in joint disease; and (iii) mechanical factors that may underlie BMLs and drive their communication with other joint tissues. Thus, this review will provide insights on this topic from a clinical, biological, and mechanical perspective. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1818-1825, 2018.

Traumatic osteoarthritis-induced persistent mechanical hyperalgesia in a rat model of anterior cruciate ligament transection plus a medial meniscectomy

Background

Osteoarthritis (OA) is a degenerative joint disease characterized by progressive cartilage degeneration, subchondral bone changes, osteophyte formation, and synovitis. A major symptom is pain that is triggered by peripheral and central changes within the pain pathways. Some surgery-induced joint instability rat models of OA were described to mimic traumatic OA. Several behavioral tests were developed to access OA-induced pain. However, follow-up in most studies usually only occurred for about 4 weeks. Since traumatic OA is a chronic disease which gradually develops after trauma, the pattern of pain might differ between early and late stages after the trauma.

Purpose

To observe the time-dependent development of hypersensitivity after traumatic OA and to determine the best timing and methods to investigate traumatic OA-induced pain.

Methods

Anterior cruciate ligament transection plus medial meniscectomy was used to induce traumatic OA in Sprague-Dawley rats. Traumatic OA-induced pain was evaluated using four different behavioral tests for 15 weeks.

Results

A significant difference in mechanical hypersensitivity developed throughout the observational period. It was worst in the first 3 weeks after the operation, then became less significant after 5 weeks but persisted. There were no differences in thermal hyperalgesia or motor coordination.

Conclusion

Traumatic OA induced mechanical hyperalgesia but did not cause thermal hyperalgesia or influence motor coordination. Furthermore, to investigate chronic pain induced by OA, the observational period should be at least 5 weeks after the intervention. These findings may help in further research and improve our understanding of traumatic OA-induced pain mechanisms.

Ultrastructural change of the subchondral bone increases the severity of cartilage damage in osteoporotic osteoarthritis of the knee in rabbits

Osteoporotic osteoarthritis is a phenotype of osteoarthritis (OA) manifested as fragile and osteoporotic subchondral bone. However, the ultrastructural features of subchondral bone in osteoporosis OA have not been determined. The study was aimed to investigate the ultrastructural dynamic changes of subchondral bone in osteoporotic OA model and how the ultrastructural damage in the subchondral bone caused by osteoporosis deteriorated the cartilage damage in OA. Eighteen rabbits were equally randomized to three groups, including the control, the OA and the osteoporotic OA groups. The structural changes of cartilage were evaluated by HE and safranin-O fast green staining, the Mankin's grading system was used to assess the stage of OA progression. And microstructural or ultrastructural changes in subchondral bone were assessed by micro-computed tomography or by scanning electron microscopy. According to the changes of cartilage histopathology, the OA group was in the early pathological stage of OA while the osteoporotic OA group was in the middle stage of OA based on Mankin's grading system. In addition, the damage of cartilage surface, reduction in the number of chondrocytes and the matrix staining were more increased in the osteoporotic OA group compared to the OA group. Compared to the OA group, the subchondral bone in the microstructure and ultrastructure in the osteoporotic OA group showed more microfracture changes in trabecular bone with more destructions of the tree-like mesh. Moreover, the collagen fibers were random rough with a fewer amount of bone lacunae in subchondral cortical plate in the osteoporotic OA group compared to the OA group. These findings indicated that the subchondral bone ultrastructure in the osteoporotic OA model was characterized by the destruction of the network structure and collagen fibers. The subchondral bone ultrastructural damage caused by osteoporosis may change mechanical properties of the upper cartilage and aggravate OA cartilage. Therefore, early diagnosis and treatment of osteoporosis is of great significance to prevent early OA from further developing osteoporotic OA.

Double-stranded RNA released from damaged articular chondrocytes promotes cartilage degeneration via Toll-like receptor 3-interleukin-33 pathway

Pattern recognition receptors (PRRs), including Toll-like receptor 3 (TLR3), are involved in arthritic responses; however, whether interleukin-33 (IL-33) is involved in TLR3-mediated cartilage degeneration is unknown. Here, we found that IL-33 was abundantly increased in chondrocytes of osteoarthritis, especially the chondrocytes of weight-bearing cartilage. Furthermore, double-stranded RNA (dsRNA) released from damaged articular chondrocytes induced by mechanical stretching upregulated IL-33 expression to a greater degree than IL-1β and tumor necrosis factor-α. dsRNA induced IL-33 expression via the TLR3-p38 mitogen-activated protein kinase-nuclear factor-κB (NF-κB) pathway. In addition, formation of the p65 and peroxisome proliferator-activated receptor-γ transcriptional complex was required for dsRNA-induced IL-33 expression. IL-33, in turn, acted on chondrocytes to induce matrix metalloproteinase-1/13 and inhibit type II collagen expression. These findings reveal that dsRNA released from damaged articular chondrocytes promotes cartilage degeneration via the TLR3-IL-33 pathway.

Physiological exercise loading suppresses post-traumatic osteoarthritis progression via an increase in bone morphogenetic proteins expression in an experimental rat knee model

OBJECTIVE

To evaluate the dose-response relationship of exercise loading in the cartilage-subchondral bone (SB) unit in surgically-induced post-traumatic osteoarthritis (PTOA) of the knee.

DESIGN

Destabilized medial meniscus (DMM) surgery was performed on the right knee of 12-week-old male Wistar rats, and sham surgery was performed on the contralateral knee. Four weeks after the surgery, the animals were subjected to moderate (12 m/min) or intense (21 m/min) treadmill exercises for 30 min/day, 5 days/week for 4 weeks. PTOA development in articular cartilage and SB was examined using histological and immunohistochemical analyses, micro-computed tomography (micro-CT) analysis, and biomechanical testing at 8 weeks after surgery. Gremlin-1 was injected to determine the role of bone morphogenetic protein (BMP) signaling on PTOA development following moderate exercise.

RESULTS

Moderate exercise increased BMP-2, BMP-4, BMP-6, BMP receptor 2, pSmad-5, and inhibitor of DNA binding protein-1 expression in the superficial zone chondrocytes and suppressed cartilage degeneration, osteophyte growth, SB damage, and osteoclast-mediated SB resorption. However, intense exercise had little effect on BMP expression and even caused progression of these osteoarthritis (OA) changes. Gremlin-1 injection following moderate exercise caused progression of the PTOA development down to the level of the non-exercise DMM-operated knee.

CONCLUSIONS

Exercise regulated cartilage-SB PTOA development in DMM-operated knees in a dose-dependent manner. Our findings shed light on the important role of BMP expression in superficial zone chondrocytes in attenuation of PTOA development following physiological exercise loading. Further studies to support a mechanism by which BMPs would be beneficial in preventing PTOA progression are warranted.

Automated assessment of bone changes in cross-sectional micro-CT studies of murine experimental osteoarthritis

OBJECTIVE

The degradation of articular cartilage, which characterises osteoarthritis (OA), is usually paired with excessive bone remodelling, including subchondral bone sclerosis, cysts, and osteophyte formation. Experimental models of OA are widely used to investigate pathogenesis, yet few validated methodologies for assessing periarticular bone morphology exist and quantitative measurements are limited by manual segmentation of micro-CT scans. The aim of this work was to chart the temporal changes in periarticular bone in murine OA by novel, automated micro-CT methods.

METHODS

OA was induced by destabilisation of the medial meniscus (DMM) in 10-week old male mice and disease assessed cross-sectionally from 1- to 20-weeks post-surgery. A novel approach was developed to automatically segment subchondral bone compartments into plate and trabecular bone in micro-CT scans of tibial epiphyses. Osteophyte volume, as assessed by shape differences using 3D image registration, and by measuring total epiphyseal volume was performed.

RESULTS

Significant linear and volumetric structural modifications in subchondral bone compartments and osteophytes were measured from 4-weeks post-surgery and showed progressive changes at all time points; by 20 weeks, medial subchondral bone plate thickness increased by 160±19.5 μm and the medial osteophyte grew by 0.124±0.028 μm3. Excellent agreement was found when automated measurements were compared with manual assessments.

CONCLUSION

Our automated methods for assessing bone changes in murine periarticular bone are rapid, quantitative, and highly accurate, and promise to be a useful tool in future preclinical studies of OA progression and treatment. The current approaches were developed specifically for cross-sectional micro-CT studies but could be applied to longitudinal studies.

Bone-cartilage crosstalk: a conversation for understanding osteoarthritis

Although cartilage degradation is the characteristic feature of osteoarthritis (OA), it is now recognized that the whole joint is involved in the progression of OA. In particular, the interaction (crosstalk) between cartilage and subchondral bone is thought to be a central feature of this process. The interface between articular cartilage and bone of articulating long bones is a unique zone, which comprises articular cartilage, below which is the calcified cartilage sitting on and intercalated into the subchondral bone plate. Below the subchondral plate is the trabecular bone at the end of the respective long bones. In OA, there are well-described progressive destructive changes in the articular cartilage, which parallel characteristic changes in the underlying bone. This review examines the evidence that biochemical and biomechanical signaling between these tissue compartments is important in OA disease progression and asks whether such signaling might provide possibilities for therapeutic intervention to halt or slow disease development.