Induced superficial chondrocyte death reduces catabolic cartilage damage in murine posttraumatic osteoarthritis

Advances in regenerative rehabilitation in the rehabilitation of musculoskeletal injuries

In the rapidly advancing field of regenerative medicine, relying solely on cell transplantation alone may be insufficient for achieving functional recovery, and rehabilitation before and after transplantation is crucial. Regenerative rehabilitation functions by synergizing the therapeutic effects of regeneration and rehabilitation to maximize tissue regeneration and patient function. We used the keywords "regenerative rehabilitation" to search across the database for published works; this review discusses the development of regenerative rehabilitation for the treatment of musculoskeletal injuries. Rehabilitation has become a crucial component of regenerative medicine because it can enhance patients' functional activity and facilitate their early return to society. Experimental data increasingly demonstrates that rehabilitation interventions support the regeneration of transplanted tissues.

Regenerative capacity of human pluripotent stem cell-derived articular chondrocytes in vitro

The ideal cell source for articular cartilage repair remains elusive. Using developmentally inspired differentiation protocols, we induced human pluripotent stem cells (hPSCs) toward articular chondrocytes capable of joint cartilage repair in rodent models, which were distinct from growth plate chondrocytes, fated to be replaced by bone in vivo. Working toward clinical translation, we demonstrated controlled differentiation into chondrocytes by comprehensive gene expression analysis at each step of the differentiation. Articular chondrocytes derived from hPSCs could be expanded several passages in vitro without losing chondrogenic potential. Furthermore, chondrocytes isolated from these articular cartilage tissues had the potential to serially regenerate new articular and growth plate cartilage tissues. Finally, the ability to cryopreserve articular chondrocytes with the desired phenotype is critical for clinical translation and here we report no loss in cell viability or regenerative potential following cryopreservation. These results support the immense potential of hPSC-derived articular chondrocytes as a cell-based therapy for cartilage repair.

The Actin Cytoskeleton as a Regulator of Proteoglycan 4

OBJECTIVE

The superficial zone (SZ) of articular cartilage is responsible for distributing shear forces for optimal cartilage loading and contributes to joint lubrication through the production of proteoglycan 4 (PRG4). PRG4 plays a critical role in joint homeostasis and is chondroprotective. Normal PRG4 production is affected by inflammation and irregular mechanical loading in post-traumatic osteoarthritis (PTOA). THe SZ chondrocyte (SZC) phenotype, including PRG4 expression, is regulated by the actin cytoskeleton in vitro. There remains a limited understanding of the regulation of PRG4 by the actin cytoskeleton in native articular chondrocytes. The filamentous (F)-actin cytoskeleton is a potential node in crosstalk between mechanical stimulation and cytokine activation and the regulation of PRG4 in SZCs, therefore developing insights in the regulation of PRG4 by actin may identify molecular targets for novel PTOA therapies.

MATERIALS AND METHODS

A comprehensive literature search on PRG4 and the regulation of the SZC phenotype by actin organization was performed.

RESULTS

PRG4 is strongly regulated by the actin cytoskeleton in isolated SZCs in vitro. Biochemical and mechanical stimuli have been characterized to regulate PRG4 and may converge upon actin cytoskeleton signaling.

CONCLUSION

Actin-based regulation of PRG4 in native SZCs is not fully understood and requires further elucidation. Understanding the regulation of PRG4 by actin in SZCs requires an in vivo context to further potential of leveraging actin arrangement to arthritic therapeutics.

Role of Apoptosis in the Pathogenesis of Osteoarthritis: An Explicative Review

Apoptosis is a complex regulatory, active cell death process that plays a role in cell development, homeostasis, and ageing. Cancer, developmental defects, and degenerative diseases are all pathogenic disorders caused by apoptosis dysregulation. Osteoarthritis (OA) is by far the most frequently diagnosed joint disease in the aged, and it is characterized by the ongoing breakdown of articular cartilage, which causes severe disability. Multiple variables regulate the anabolic and catabolic pathways of the cartilage matrix, which either directly or indirectly contribute to cartilage degeneration in osteoarthritis. Articular cartilage is a highly specialized tissue made up of an extracellular matrix of cells that are tightly packed together. As a result, chondrocyte survival is crucial for the preservation of an optimal cartilage matrix, and chondrocyte characteristics and survival compromise may result in articular cartilage failure. Inflammatory cytokines can either promote or inhibit apoptosis, the process of programmed cell death. Pro-apoptotic cytokines like TNF-α can induce cell death, while anti-apoptotic cytokines like IL-4 and IL-10 protect against apoptosis. The balance between these cytokines plays a critical role in determining cell fate and has implications for tissue damage and disease progression. Similarly, they contribute to the progression of OA by disrupting the metabolic balance in joint tissues by promoting catabolic and anabolic pathways. Their impact on cell joints, as well as the impacts of cell signalling pathways on cytokines and inflammatory substances, determines their function in osteoarthritis development. Apoptosis is evident in osteoarthritic cartilage; however, determining the relative role of chondrocyte apoptosis in the aetiology of OA is difficult, and the rate of apoptotic chondrocytes in osteoarthritic cartilage is inconsistent. The current study summarises the role of apoptosis in the development of osteoarthritis, the mediators, and signalling pathways that trigger the cascade of events, and the other inflammatory features involved.

P53: A Key Target in the Development of Osteoarthritis

Osteoarthritis (OA), a chronic degenerative disease characterized mainly by damage to the articular cartilage, is increasingly relevant to the pathological processes of senescence, apoptosis, autophagy, proliferation, and differentiation of chondrocytes. Clinical strategies for osteoarthritis can only improve symptoms and even along with side effects due to age, sex, disease, and other factors. Therefore, there is an urgent need to identify new ideas and targets for current clinical treatment. The tumor suppressor gene p53, which has been identified as a potential target for tumor therapeutic intervention, is responsible for the direct induction of the pathological processes involved in OA modulation. Consequently, deciphering the characteristics of p53 in chondrocytes is essential for investigating OA pathogenesis due to p53 regulation in an array of signaling pathways. This review highlights the effects of p53 on senescence, apoptosis, and autophagy of chondrocytes and its role in the development of OA. It also elucidates the underlying mechanism of p53 regulation in OA, which may help provide a novel strategies for the clinical treatment of OA.

Cross-talk of inflammation and chondrocyte intracellular metabolism in osteoarthritis

Osteoarthritis is a disease that impacts millions around the world, leading to significant financial and medical burden for patients and the healthcare system. However, no effective biomarkers or disease modifying therapeutics exist for the early identification and management of the disease. Inflammation drives chondrocytes to express extracellular matrix (ECM) degrading enzymes and interruption of this pathway is a viable target to prevent degradation of cartilage. It has been demonstrated that inflammation can alter the intracellular metabolism of chondrocytes, a process known as metabolic reprogramming. This metabolic reprogramming is critical for cartilage breakdown by shifting chondrocytes to an ECM-catabolic state and likely as a potential therapeutic target for osteoarthritis. Metabolic modulators hold the potential to reduce chondrocyte inflammatory responses and protect cartilage. In this narrative review, we explore some of the existing examples of interactions between metabolism and inflammatory pathways in chondrocytes. We summarize the impact of inflammatory stimulation on various metabolic pathways and describe several examples by which targeting of metabolism is able to modulate ECM-degrading activity of chondrocytes to protect against cartilage damage.

CAMKK2 is upregulated in primary human osteoarthritis and its inhibition protects against chondrocyte apoptosis

OBJECTIVE

To investigate the role of calcium/calmodulin-dependent protein kinase kinase 2 (CAMKK2) in human osteoarthritis.

MATERIALS AND METHODS

Paired osteochondral plugs and articular chondrocytes were isolated from the relatively healthier (intact) and damaged portions of human femoral heads collected from patients undergoing total hip arthroplasty for primary osteoarthritis (OA). Cartilage from femoral plugs were either flash frozen for gene expression analysis or histology and immunohistochemistry. Chondrocyte apoptosis in the presence or absence of CAMKK2 inhibition was measured using flow cytometry. CAMKK2 overexpression and knockdown in articular chondrocytes were achieved via Lentivirus- and siRNA-mediated approaches respectively, and their effect on pro-apoptotic and cartilage catabolic mechanisms was assessed by immunoblotting.

RESULTS

CAMKK2 mRNA and protein levels were elevated in articular chondrocytes from human OA cartilage compared to paired healthier intact samples. This increase was associated with elevated catabolic marker matrix metalloproteinase 13 (MMP-13), and diminished anabolic markers aggrecan (ACAN) and type II collagen (COL2A1) levels. OA chondrocytes displayed enhanced apoptosis, which was suppressed following pharmacological inhibition of CAMKK2. Levels of MMP13, pSTAT3, and the pro-apoptotic marker BAX became elevated when CAMKK2, but not its kinase-defective mutant was overexpressed, whereas knockdown of the kinase decreased the levels of these proteins.

CONCLUSIONS

CAMKK2 is upregulated in human OA cartilage and is associated with elevated levels of pro-apoptotic and catabolic proteins. Inhibition or knockdown of CAMKK2 led to decreased chondrocyte apoptosis and catabolic protein levels, whereas its overexpression elevated them. CAMKK2 may be a therapeutic target to prevent or mitigate human OA.

SPTLC2 ameliorates chondrocyte dysfunction and extracellular matrix metabolism disturbance in vitro and in vivo in osteoarthritis

Disturbances in chondrocyte extracellular matrix (ECM) metabolism in osteoarthritis (OA) are a major cause of OA and potentially lead to personal disability, placing a huge burden on society. Chondrocyte apoptosis and ECM catabolism have a major role in the OA process. Firstly, bioinformatics analysis was performed to screen differentially expressed genes (DEGs) in OA, and serine palmitoyltransferase subunit 2 (SPTLC2) was chosen, which had high-level expression in the OA cartilage tissues and OA chondrocytes. Overexpression and knockdown of SPTLC2 were achieved in OA chondrocytes and normal chondrocytes respectively to study the effect of SPTLC2 upon ECM metabolism of chondrocytes. Cell viability and apoptosis were measured using MTT and flow cytometry analyses; SPTLC2 overexpression enhanced the OA chondrocyte viability and decreased apoptotic rate. In addition, Western blot detection of ECM-related factors (Collagen I, Collage II, MMP-1, MMP-3, and MMP-13) revealed that SPTLC2 overexpression promoted the expression of collagens (Collagen I and Collage II) and suppressed matrix metalloproteinase (MMP-1, MMP-3, and MMP-13) level. In contrast, SPTLC2 knockdown in normal chondrocytes showed opposite effects on cell viability, apoptosis, and ECM degeneration. The articular cartilage of OA rats was transfected with lentivirus overexpressing SPTLC2; HE and Safranin-O fast green demonstrated that SPTLC2 overexpression could alleviate chondrocyte injuries and slow down the development of OA. In conclusion, SPTLC2 plays a role in OA and may be a potential target gene for the treatment of OA.

β-Hydroxybutyrate alleviates cartilage senescence through hnRNP A1-mediated up-regulation of PTEN

Senescence chondrocytes play an important role in Osteoarthritis (OA) progression. However, alleviating OA progression through senescent chondrocyte intervention still faces great challenges. β-Hydroxybutyrate (BHB) exhibits anti-senescence effects in a variety of age-related dis-eases, but its role in osteoarthritis remains poorly understood. To explore the molecular mechanisms, gene sequencing was used to identify critical genes and potential cellular signaling pathways and male SD rats were used to generate an osteoarthritis model. Results showed that BHB attenuated the senescence of Osteoarthritis chondrocytes (OA-Chos) and alleviated OA progression. Gene ontology (GO) enrichment analysis revealed significant changes in cell cycle genes, with PTEN being the most significant differentially expressed gene. BHB up-regulated the expression of PTEN in OA-Chos, thereby alleviating chondrocyte senescence. Furthermore, BHB facilitated the expression of PTEN by binding to hnRNP A1 and inhibiting the phosphorylation of Akt. This study provided evidence that BHB mitigated chondrocyte senescence and delayed OA, and could thus be used as a novel therapeutic approach for osteoarthritis treatment.

CircZSWIM6 mediates dysregulation of ECM and energy homeostasis in ageing chondrocytes through RPS14 post-translational modification

BACKGROUND

Circular RNAs (CircRNAs) are important and have different roles in disease progression. Herein, we aim to elucidate the roles of a novel CircRNA (CircZSWIM6) which is upregulated in ageing chondrocytes.

METHODS

We verified the roles of CircZSWIM6 in senescent and osteoarthritis (OA) development in vitro through CircZSWIM6 knockdown and overexpression. RNA pulldown assay and RNA binding protein immunoprecipitation were performed to identify the interaction between CircZSWIM6 and Ribosomal protein S14 (RPS14). The roles of CircZSWIM6 in ageing-related OA were also confirmed in non-traumatic and traumatic model respectively.

RESULTS

CircZSWIM6 regulates extracellular matrix (ECM) and energy metabolism in ageing chondrocyte. Mechanistically, CircZSWIM6 competitively bound to the E3 ligase STUB1 binding site on RPS14 (K125) to inhibit proteasomal degradation of RPS14 to maintain RPS14 function. CircZSWIM6-RPS14 axis is highly associated with AMPK signaling transduction, which keeps energy metabolism in chondrocyte. Furthermore, CircZSWIM6 AAV infection leads to senescent and OA phenotypes in a non-traumatic model and accelerates OA progression in a traumatic model.

CONCLUSION

Our results revealed a significant role of CircZSWIM6 in age-related OA by regulating ECM metabolism and AMPK-associated energy metabolism. We highlight the CircZSWIM6-RPS14-PCK1-AMPK axis is a potential biomarker for OA.

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.

The synovial microenvironment suppresses chondrocyte hypertrophy and promotes articular chondrocyte differentiation

During the development of the appendicular skeleton, the cartilaginous templates undergo hypertrophic differentiation and remodels into bone, except for the cartilage most adjacent to joint cavities where hypertrophic differentiation and endochondral bone formation are prevented, and chondrocytes instead form articular cartilage. The mechanisms that prevent hypertrophic differentiation and endochondral bone formation of the articular cartilage have not been elucidated. To explore the role of the synovial microenvironment in chondrocyte differentiation, osteochondral allografts consisting of articular cartilage, epiphyseal bone, and growth plate cartilage from distal femoral epiphyses of inbred Lewis rats expressing enhanced green fluorescent protein from a ubiquitous promoter were transplanted either in inverted or original (control) orientation to matching sites in wildtype littermates, thereby allowing for tracing of transplanted cells and their progenies. We found that no hypertrophic differentiation occurred in the growth plate cartilage ectopically placed at the joint surface. Instead, the transplanted growth plate cartilage, with time, remodeled into articular cartilage. This finding suggests that the microenvironment at the articular surface inhibits hypertrophic differentiation and supports articular cartilage formation. To explore this hypothesis, rat chondrocyte pellets were cultured with and without synoviocyte-conditioned media. Consistent with the hypothesis, hypertrophic differentiation was inhibited and expression of the articular surface marker lubricin (Prg4) was dramatically induced when chondrocyte pellets were exposed to synovium- or synoviocyte-conditioned media, but not to chondrocyte- or osteoblast-conditioned media. Taken together, we present evidence for a novel mechanism by which synoviocytes, through the secretion of a factor or factors, act directly on chondrocytes to inhibit hypertrophic differentiation and endochondral bone formation and promote articular cartilage formation. This mechanism may have important implications for articular cartilage development, maintenance, and regeneration.

Roles of Cartilage-Resident Stem/Progenitor Cells in Cartilage Physiology, Development, Repair and Osteoarthritis

Osteoarthritis (OA) is a degenerative disease that causes irreversible destruction of articular cartilage for which there is no effective treatment at present. Although articular cartilage lacks intrinsic reparative capacity, numerous studies have confirmed the existence of cartilage-resident stem/progenitor cells (CSPCs) in the superficial zone (SFZ) of articular cartilage. CSPCs are characterized by the expression of mesenchymal stromal cell (MSC)-related surface markers, multilineage differentiation ability, colony formation ability, and migration ability in response to injury. In contrast to MSCs and chondrocytes, CSPCs exhibit extensive proliferative and chondrogenic potential with no signs of hypertrophic differentiation, highlighting them as suitable cell sources for cartilage repair. In this review, we focus on the organizational distribution, markers, cytological features and roles of CSPCs in cartilage development, homeostasis and repair, and the application potential of CSPCs in cartilage repair and OA therapies.

Chondrocyte death involvement in osteoarthritis

Chondrocyte apoptosis is known to contribute to articular cartilage damage in osteoarthritis and is correlated to a number of cartilage disorders. Micromass cultures represent a convenient means for studying chondrocyte biology, and, in particular, their death. In this review, we focused the different kinds of chondrocyte death through a comparison between data reported in the literature. Chondrocytes show necrotic features and, occasionally, also apoptotic features, but usually undergo a new form of cell death called Chondroptosis, which occurs in a non-classical manner. Chondroptosis has some features in common with classical apoptosis, such as cell shrinkage, chromatin condensation, and involvement, not always, of caspases. The most crucial peculiarity of chondroptosis relates to the ultimate elimination of cellular remnants. Independent of phagocytosis, chondroptosis may serve to eliminate cells without inflammation in situations in which phagocytosis would be difficult. This particular death mechanism is probably due to the unusual condition chondrocytes both in vivo and in micromass culture. This review highlights on the morpho-fuctional alterations of articular cartilage and focus attention on various types of chondrocyte death involved in this degeneration. The death features have been detailed and discussed through in vitro studies based on tridimensional chondrocyte culture (micromasses culture). The study of this particular mechanism of cartilage death and the characterization of different biological and biochemical underlying mechanisms can lead to the identification of new potentially therapeutic targets in various joint diseases.

Intra-articular injection of rapamycin microparticles prevent senescence and effectively treat osteoarthritis

Trauma to the knee joint is associated with significant cartilage degeneration and erosion of subchondral bone, which eventually leads to osteoarthritis (OA), resulting in substantial morbidity and healthcare burden. With no disease-modifying drugs in clinics, the current standard of care focuses on symptomatic relief and viscosupplementation. Modulation of autophagy and targeting senescence pathways are emerging as potential treatment strategies. Rapamycin has shown promise in OA disease amelioration by autophagy upregulation, yet its clinical use is hindered by difficulties in achieving therapeutic concentrations, necessitating multiple weekly injections. Rapamycin-loaded in poly(lactic-co-glycolic acid) microparticles (RMPs) induced autophagy, prevented senescence, and sustained sulphated glycosaminoglycans production in primary human articular chondrocytes from OA patients. RMPs were potent, nontoxic, and exhibited high retention time (up to 35 days) in mice joints. Intra-articular delivery of RMPs effectively mitigated cartilage damage and inflammation in surgery-induced OA when administered as a prophylactic or therapeutic regimen. Together, the study demonstrates the feasibility of using RMPs as a potential clinically translatable therapy to prevent the progression of post-traumatic OA.

Targeting cellular senescence as a novel treatment for osteoarthritis

Cellular senescence is associated with normal development and wound healing, but has also been implicated in the pathogenesis of numerous aging-related diseases including osteoarthritis (OA). Treatment strategies for OA are being developed that target senescent cells and the paracrine and autocrine secretions of the senescence-associated secretory phenotype (SASP). The field of potential therapies continues to expand as new mechanistic targets of cell senescence and the SASP are identified. Ongoing pre-clinical and clinical studies of drugs targeting cellular senescence yield significant promise, but have yet to demonstrate long-term efficacy. Therapeutic targeting of senescence is challenged by the diverse phenotypes of senescent cells, which can vary depending on age, species, tissue source, and type of physiologic stressor. Accordingly, there remains considerable demand for more studies to further develop and assess senotherapeutics as disease-modifying treatments for OA.

Toll-like receptor 3 activation promotes joint degeneration in osteoarthritis

Osteoarthritis (OA) is characterized by cartilage degradation that is induced by inflammation. Sterile inflammation can be caused by damage-associated molecular patterns that are released by chondrocytes and activate pattern recognition receptors. We evaluate the role of toll-like receptor-3-activating RNA in the pathogenesis of OA. Toll-like receptor 3 (TLR3) was detected by semiquantitative reverse transcriptase PCR, western blotting and microscopy. Rhodamine-labelled poly(I:C) was used to image uptake in chondrocytes and full-thickness cartilage. The production of IFNβ in chondrocytes after stimulation with poly(I:C) as well as in the synovial fluid of OA patients was measured using ELISA. Chondrocyte apoptosis was chemically induced using staurosporine. Immunohistochemistry was performed to examine TLR3 expression and apoptosis in human and murine OA cartilage. RNA in synovial fluid was quantified by RiboGreen assay. Destabilisation of the medial meniscus was performed in TLR3^−/− and wildtype mice. OA was assessed after eight weeks using OARSI score. TLR3 expression was confirmed by western blot and RT-PCR. Poly(I:C) was internalised by chondrocytes as well as cartilage and caused an increase of IFNβ production in murine (11.46 ± 11.63 (wo) to 108.7 ± 25.53 pg/ml; N = 6) and human chondrocytes (1.88 ± 0.32 (wo) to 737.6 ± 130.5 pg/ml; N = 3; p < 0.001). OA cartilage showed significantly more TLR3-positive (KL0 = 0.22 ± 0.24; KL4 = 6.02 ± 6.75; N ≥ 15) and apoptotic chondrocytes (KL0 = 0.6 ± 1.02; KL4 = 9.78 ± 7.79; N ≥ 12) than healthy cartilage (p < 0.001). Staurosporine-induced chondrocyte apoptosis causes a dose-dependent RNA release (0 ng/ml = 1090 ± 39.1 ng/ml; 1000 ng/ml=2014 ± 160 ng/ml; N = 4; p < 0.001). Human OA synovial fluid contained increased concentrations of RNA (KL0-2 = 3408 ± 1129 ng/ml; KL4 = 4870 ± 1612ng/ml; N ≥ 7; p < 0.05) and IFNβ (KL0-2 = 41.95 ± 92.94 ng/ml; KL3 = 1181 ± 1865ng/ml; N ≥ 8; p < 0.05). TLR3^−/− mice showed reduced cartilage degradation eight weeks after OA induction (OARSI WT = 5.5 ± 0.04; TLR3^−/− = 3.75 ± 1.04; N ≥ 6) which was accompanied by gradually decreasing levels of TUNEL-positive cells (WT = 34.87 ± 24.10; TLR3^−/ = 19.64 ± 7.89) resulting in decreased IFNβ expression (WT = 12.57 ± 5.43; TLR3^−/− = 6.09 ± 2.07) in cartilage (p < 0.05). The release of RNA by apoptotic chondrocytes thus activating TLR3 signalling is one possible way of perpetuating inflammatory cartilage changes. The inhibition of TLR3 could be a possible therapeutic target for OA treatment.

Fxyd5 activates the NF‑κB pathway and is involved in chondrocytes inflammation and extracellular matrix degradation

It is known that increased inflammation and extracellular matrix (ECM) degradation in chondrocytes can promote the development of osteoarthritis (OA). The FXYD domain containing ion transport regulator 5 (Fxyd5) has been found to promote chronic inflammatory responses. The present study aimed to investigate the role of Fxyd5 in OA. Murine ATDC5 chondrocytes were transfected with short hairpin RNAs specifically targeting Fxyd5 to silence its expression. Subsequently, cells were induced with lipopolysaccharide (LPS). The protein expression levels of Fxyd5, MMPs and proteins related to ECM, apoptosis and NF-κB signaling were detected using western blot analysis. In addition, cell viability was assessed using a Cell Counting Kit-8 assay, while the secretion of the proinflammatory factors and those of the oxidative stress-related markers were measured using the corresponding kits. Finally, cells were treated with the NF-κB activator, betulinic acid (BA) and the above experiments were repeated. The results demonstrated that Fxyd5 was significantly upregulated in ATDC5 cells treated with LPS. Additionally, Fxyd5 knockdown increased cell viability, enhanced the protein expression of Bcl-2, Aggrecan and collagen II, while reduced the expression of Bax, cleaved caspase-3/caspase-3, MMP3 and MMP13 in LPS-induced ATDC5 cells. The production of IL-1β, IL-6 and IL-18 as well as reactive oxygen species and malondialdehyde, and the reduction of superoxide dismutase caused by LPS in ATDC5 cells, were also reversed by Fxyd5 silencing. Fxyd5 silencing inhibited the phosphorylation of p65 and IκBα induced by LPS. Finally, BA reversed the protective effect of Fxyd5 silencing on LPS induced chondrocytes injury. In conclusion, Fxyd5 could enhance chondrocyte inflammation and ECM degradation via activating the NF-κB signaling.

Mechanosignalling in cartilage: an emerging target for the treatment of osteoarthritis

Mechanical stimuli have fundamental roles in articular cartilage during health and disease. Chondrocytes respond to the physical properties of the cartilage extracellular matrix (ECM) and the mechanical forces exerted on them during joint loading. In osteoarthritis (OA), catabolic processes degrade the functional ECM and the composition and viscoelastic properties of the ECM produced by chondrocytes are altered. The abnormal loading environment created by these alterations propagates cell dysfunction and inflammation. Chondrocytes sense their physical environment via an array of mechanosensitive receptors and channels that activate a complex network of downstream signalling pathways to regulate several cell processes central to OA pathology. Advances in understanding the complex roles of specific mechanosignalling mechanisms in healthy and OA cartilage have highlighted molecular processes that can be therapeutically targeted to interrupt pathological feedback loops. The potential for combining these mechanosignalling targets with the rapidly expanding field of smart mechanoresponsive biomaterials and delivery systems is an emerging paradigm in OA treatment. The continued advances in this field have the potential to enable restoration of healthy mechanical microenvironments and signalling through the development of precision therapeutics, mechanoregulated biomaterials and drug systems in the near future.

Disease-modifying therapeutic strategies in osteoarthritis: current status and future directions

Osteoarthritis (OA) is the most common form of arthritis. It is characterized by progressive destruction of articular cartilage and the development of chronic pain and constitutes a considerable socioeconomic burden. Currently, pharmacological treatments mostly aim to relieve the OA symptoms associated with inflammation and pain. However, with increasing understanding of OA pathology, several potential therapeutic targets have been identified, enabling the development of disease-modifying OA drugs (DMOADs). By targeting inflammatory cytokines, matrix-degrading enzymes, the Wnt pathway, and OA-associated pain, DMOADs successfully modulate the degenerative changes in osteoarthritic cartilage. Moreover, regenerative approaches aim to counterbalance the loss of cartilage matrix by stimulating chondrogenesis in endogenous stem cells and matrix anabolism in chondrocytes. Emerging strategies include the development of senolytic drugs or RNA therapeutics to eliminate the cellular or molecular sources of factors driving OA. This review describes the current developmental status of DMOADs and the corresponding results from preclinical and clinical trials and discusses the potential of emerging therapeutic approaches to treat OA.

Cell depleted areas do not repopulate after diphtheria toxin-induced killing of mandibular cartilage chondrocytes

OBJECTIVE

Growth of mandibular condylar cartilage (MCC) is associated with cell proliferation within the polymorphic cell layer and subsequent differentiation into chondrocytes that reside along the condylar surface and along the cartilage/subchondral bone interface. We examined whether cells in the polymorphic layer would proliferate and repopulate toxin-induced cell-depleted areas in MCCs of adult mice.

METHOD

We induced diphtheria toxin (DTA) expression (ROSA26l-s-lDTA) to cell-autonomously kill large fractions of MCC chondrocytes throughout the cartilage or along the articular cartilage surface with Aggrecan-CreERt2 (AcanCreERt2) or Lubricin-CreERt2 (Prg4CreERt2) Cre-recombinase-inducible mice, respectively. We examined MCCs from these mice shortly after cell killing or several months later with histology and confocal microscopy for evidence of chondrocyte proliferation and repopulation.

RESULTS

AcanCreERt2-induced DTA expression killed an average of 53% MCC chondrocytes in adult mice after 1 week (39-66%, 95% confidence interval (CI)). Twelve weeks later, surviving chondrocytes had proliferated but not migrated to cell depleted areas. Prg4CreERt2-induced DTA expression killed an average of 24% surface chondrocytes in mice after 5 weeks (14-34% CI). After thirteen weeks there was 34% fewer surface chondrocytes (4-63% CI) in Prg4CreERt2 DTA-induced mice compared to controls.

CONCLUSION

In adult mice, after diphtheria toxin-mediated chondrocyte killing, cell depleted areas within MCC cartilage are not repopulated by new cells.

Targeting Dysregulation of Metalloproteinase Activity in Osteoarthritis

Metalloproteinases were first identified as collagen cleaving enzymes and are now appreciated to play important roles in a wide variety of biological processes. The aberrant activity and dysregulation of the metalloproteinase family are linked to numerous diseases including cardiovascular and pulmonary diseases, chronic wounds, cancer, fibrosis and arthritis. Osteoarthritis (OA) is the most prevalent age-related joint disorder that causes pain and disability, but there are no disease-modifying drugs available. The hallmark of OA is loss of articular cartilage and elevated activities of matrix-degrading metalloproteinases are responsible. These enzymes do not exist in isolation and their activity is tightly regulated by a number of processes, such as transcription, proteolytic activation, interaction with their inhibitors, cell surface and extracellular matrix molecules, and endocytic clearance from the extracellular milieu. Here, we describe the functions and roles of metalloproteinase family in OA pathogenesis. We highlight recent studies that have illustrated novel mechanisms regulating their extracellular activity and impairment of such regulations that lead to the development of OA. We also discuss how to stop or slow down the degenerative processes by targeting aberrant metalloproteinase activity, which may in future become therapeutic interventions for the disease.

Inhibition of sphingosine 1-phosphate protects mice against chondrocyte catabolism and osteoarthritis

OBJECTIVE

Cartilage loss observed in osteoarthritis (OA) is prevented when osteoclasts in the subchondral bone are inhibited in mice. Here, we investigated the role of the osteoclast secretome and of the lipid mediator sphingosine 1-phosphate (S1P) in chondrocyte metabolism and OA.

MATERIALS AND METHODS

We used SphK1^LysMCre and wild type mice to assess the effect of murine osteoclast secretome in chondrocyte metabolism. Gene and protein expressions of matrix metalloproteinase (Mmp) were quantified in chondrocytes and explants by RT-qPCR and Western blots. SphK1^LysMCre mice or wild type mice treated with S1P₂ receptor inhibitor JTE013 or anti-S1P neutralizing antibody sphingomab are analyzed by OA score and immunohistochemistry.

RESULTS

The osteoclast secretome increased the expression of Mmp3 and Mmp13 in murine chondrocytes and cartilage explants and activated the JNK signaling pathway, which led to matrix degradation. JTE013 reversed the osteoclast-mediated chondrocyte catabolism and protected mice against OA, suggesting that osteoclastic S1P contributes to cartilage damage in OA via S1P/S1P₂ signaling. The activity of sphingosine kinase 1 (SphK1) increased with osteoclast differentiation, and its expression was enhanced in subchondral bone of mice with OA. The expression of Mmp3 and Mmp13 in chondrocytes was low upon stimulation with the secretome of Sphk1-lacking osteoclasts. Cartilage damage was significantly reduced in SphK1^LysMCre mice, but not the synovial inflammation. Finally, intra-articular administration of sphingomab inhibited the cartilage damage and synovial inflammation.

CONCLUSIONS

Lack of S1P in myeloid cells and local S1P neutralization alleviates from osteoarthritis in mice. These data identify S1P as a therapeutic target in OA.

Icariin alleviates osteoarthritis by regulating autophagy of chondrocytes by mediating PI3K/AKT/mTOR signaling

Osteoarthritis (OA) is a chronic degenerative disease that significantly impacts the quality of life of the elderly population. Recently, the pathogenesis of OA has been reported to involve autophagy in chondrocytes. Intriguingly, icariin, one of the main components of epimedium, exerts multiple pharmacological effects, including a protective effect against chondrocyte damage. Thus, we aimed to investigate the therapeutic effect of icariin on OA and its potential underlying mechanism by using a rat model of OA. After treatment with icariin or an autophagy activator (rapamycin) or inhibitor (3-methyladenine), OA chondrocyte viability was measured using the CCK-8 assay, apoptosis in the chondrocytes was evaluated using the acridine orange-propidium iodide assay and flow cytometry, and OA tissue pathological state was assessed using micro-CT scanning and safranin O staining. Furthermore, immunohistochemical staining was used to measure the expression level of Beclin-1 and immunofluorescence labeling was used to visualize LC3 expression, and western blotting was used to determine the expression levels of autophagy proteins and key proteins in the PI3K signaling pathway. The apoptotic rate of OA chondrocytes was markedly elevated by 3-methyladenine and suppressed by rapamycin and icariin; autophagy genes were drastically downregulated in the 3-methyladenine group and upregulated in the rapamycin and icariin groups; and the PI3K/AKT/mTOR signaling pathway was activated by 3-methyladenine and inhibited by rapamycin and icariin. Notably, following treatment with rapamycin and icariin, the severe pathological state in OA cartilage tissues was substantially alleviated, and this was accompanied by activated autophagy and inhibited PI3K signaling in the cartilage tissues. Taken together, these findings indicate that icariin alleviates OA by regulating the autophagy of chondrocytes by mediating PI3K/AKT/mTOR signaling.

Clinical use of mass spectrometry (imaging) for hard tissue analysis in abnormal fracture healing

Common traumas to the skeletal system are bone fractures and injury-related articular cartilage damage. The healing process can be impaired resulting in non-unions in 5-10% of the bone fractures and in post-traumatic osteoarthritis (PTOA) in up to 75% of the cases of cartilage damage. Despite the amount of research performed in the areas of fracture healing and cartilage repair as well as non-unions and PTOA, still, the outcome of a bone fracture or articular cartilage damage cannot be predicted. Here, we discuss known risk factors and key molecules involved in the repair process, together with the main challenges associated with the prediction of outcome of these injuries. Furthermore, we review and discuss the opportunities for mass spectrometry (MS) - an analytical tool capable of detecting a wide variety of molecules in tissues - to contribute to extending molecular understanding of impaired healing and the discovery of predictive biomarkers. Therefore, the current knowledge and challenges concerning MS imaging of bone and cartilage tissue as well as in vivo MS are discussed. Finally, we explore the possibilities of in situ, real-time MS for the prediction of outcome during surgery of bone fractures and injury-related articular cartilage damage.

Overloading stress-induced progressive degeneration and self-repair in condylar cartilage

Overloading stress-induced condylar cartilage degeneration acts as the main pathologic change in temporomandibular joint osteoarthritis (TMJ-OA). However, the progression of degeneration and the ability for self-repair remain poorly understood. Here, we explored the progression of cartilage degeneration by dividing pathological stages using a steady mouth-opening mouse model. Then, we observed changes of cartilage by removing the loading at different stages to test the potential self-repair after degeneration induced. Three-dimensional confocal microscopy combined with histology and micro-CT scanning was applied to examine TMJ at different stages of degeneration before and after self-repair. We found the cartilage underwent progressive and thorough degeneration as the overloading stress developed. During the initial adaptation stage, robust proliferation of posteromedial cartilage began at the area of direct loading. Subsequently, widespread chondrocyte apoptosis was found, followed by new chondrocyte proliferation in aggregates with matrix degradation and subchondral bone catabolism. Finally, with cartilage surface damage, the degeneration reached a point where the lesion could not be reversed by self-repair. While the cartilage nearly returned to normal when the interference was removed within 5 days. These results suggested overloading force induces a pathological process of successive degeneration in TMJ cartilage, which can be reversed by self-repair at early stages.

Rejuvenated Stem/Progenitor Cells for Cartilage Repair Using the Pluripotent Stem Cell Technology

It is widely accepted that chondral defects in articular cartilage of adult joints are never repaired spontaneously, which is considered to be one of the major causes of age-related degenerative joint disorders, such as osteoarthritis. Since mobilization of subchondral bone (marrow) cells and addition of chondrocytes or mesenchymal stromal cells into full-thickness defects show some degrees of repair, the lack of self-repair activity in adult articular cartilage can be attributed to lack of reparative cells in adult joints. In contrast, during a fetal or embryonic stage, joint articular cartilage has a scar-less repair activity, suggesting that embryonic joints may contain cells responsible for such activity, which can be chondrocytes, chondroprogenitors, or other cell types such as skeletal stem cells. In this respect, the tendency of pluripotent stem cells (PSCs) to give rise to cells of embryonic characteristics will provide opportunity, especially for humans, to obtain cells carrying similar cartilage self-repair activity. Making use of PSC-derived cells for cartilage repair is still in a basic or preclinical research phase. This review will provide brief overviews on how human PSCs have been used for cartilage repair studies.

Down-regulation of protease-activated receptor 2 ameliorated osteoarthritis in rats through regulation of MAPK/NF-κB signaling pathway in vivo and in vitro

Recently, protease-activated receptor 2 (PAR2) has been proved to be involved in the inflammatory response including osteoarthritis (OA). In the present study, we found that PAR2 antagonist could remarkably improve the pathological condition of OA rats in vivo. In addition, we also found that PAR2 antagonist could suppress the production of inflammatory factors (TNF-α and Cox-2), decrease the levels of MMP-1 and MMP-13, and restrain the levels of P62 proteins and aggravate the expression of LC3-II both in vivo and in vitro. Besides, in vitro, PAR2 antagonist could increase the proliferation and colony formation of chondrocytes induced with IL-1β. Moreover, PAR2 antagonist could decrease the expression of expressions of p-p38, p-IκBα and p-NF-κB in vitro. However, PAR2 agonist exhibited the opposite effects. Furthermore, SB203580, a p38 MAPK inhibitor, could remarkably promote the proliferation of chondrocytes induced with IL-1β, could alleviate the production of TNF-α and Cox-2, could down-regulate the protein expressions of MMP-1 and MMP-13, and could decrease the expression of P62 and increase the expressions of LC3-II of chondrocytes induced with IL-1β. Importantly, SB203580 could reverse the effects of PAR2 agonist on the functions of chondrocytes induced with IL-1β. Taken together, the present data suggest that down-regulation of PAR2 can ameliorate OA through inducing autophagy via regulation of MAPK/NF-κB signaling pathway in vivo and in vitro, and PAR2 can be considered as a potential candidate to treat OA.

NF-κB-mediated effects on behavior and cartilage pathology in a non-invasive loading model of post-traumatic osteoarthritis

OBJECTIVE

This study aimed to examine the temporal activation of NF-κB and its relationship to the development of pain-related sensitivity and behavioral changes in a non-invasive murine knee loading model of PTOA.

METHOD

Following knee injury NF-κB activity was assessed longitudinally via in vivo imaging in FVB. Cg-Tg (HIV-EGFP,luc)8Tsb/J mice. Measures of pain-related sensitivity and behavior were also assessed longitudinally for 16 weeks. Additionally, we antagonized NF-κB signaling via intra-articular delivery of an IκB kinase two antagonist to understand how local NF-κB inhibition might alter disease progression.

RESULTS

Following joint injury NF-κB signaling within the knee joint was transiently increased and peaked on day 3 with an estimated 1.35 p/s/cm2/sr (95% CI 0.913.1.792 p/s/cm2/sr) fold increase in signaling when compared to control joints. Furthermore, injury resulted in the long-term development of hindpaw allodynia. Hyperalgesia withdrawal thresholds were reduced at injured knee joints, with the largest reduction occurring 2 days following injury (estimate of between group difference 129.1 g with 95% CI 60.9,197.4 g), static weight bearing on injured limbs was also reduced. Local delivery of an NF-κB inhibitor following joint injury reduced chondrocyte death and influenced the development of pain-related sensitivity but did not reduce long-term cartilage degeneration.

CONCLUSION

These findings underscore the development of behavioral changes in this non-invasive loading model of PTOA and their relationships to NF-κB activation and pathology. They also highlight the potential chondroprotective effects of NF-κB inhibition shortly following joint injury despite limitations in preventing the long-term development of joint degeneration in this model of PTOA.

Accelerating functional gene discovery in osteoarthritis

Osteoarthritis causes debilitating pain and disability, resulting in a considerable socioeconomic burden, yet no drugs are available that prevent disease onset or progression. Here, we develop, validate and use rapid-throughput imaging techniques to identify abnormal joint phenotypes in randomly selected mutant mice generated by the International Knockout Mouse Consortium. We identify 14 genes with functional involvement in osteoarthritis pathogenesis, including the homeobox gene Pitx1, and functionally characterize 6 candidate human osteoarthritis genes in mouse models. We demonstrate sensitivity of the methods by identifying age-related degenerative joint damage in wild-type mice. Finally, we phenotype previously generated mutant mice with an osteoarthritis-associated polymorphism in the Dio2 gene by CRISPR/Cas9 genome editing and demonstrate a protective role in disease onset with public health implications. We hope this expanding resource of mutant mice will accelerate functional gene discovery in osteoarthritis and offer drug discovery opportunities for this common, incapacitating chronic disease.

TGF-β/Alk5 signaling prevents osteoarthritis initiation via regulating the senescence of articular cartilage stem cells

Osteoarthritis (OA) is the most common joint disease. The surface of joint cartilage is a defensive and first affected structure of articular cartilage (AC) during the pathogenesis of OA. Alk5 signaling is critical for maintaining AC homeostasis, however, the role and underlying mechanism for the involvement of Alk5 signaling in the phenotypes of articular cartilage stem cells (ACSCs) at the surface of AC is still unclear. The role of Alk5 in OA development was explored using an ACSCs-specific Alk5-deficient (cKO) mouse model. Alterations in cartilage structure were evaluated histologically. Senescence was detected by SA-β-gal, while reactive oxygen species (ROS), MitoTracker, and LysoTracker staining were used to detect changes related to senescence. In addition, mice were injected intra-articularly with ganciclovir to limit the detrimental roles of senescent cells (SnCs). Alk5 cKO mice showed a decreased number of the slow-cell cycle cells and less lubricant secretion at the surface accompanied with drastically accelerated cartilage degeneration under ageing and surgically induced OA conditions. Further studies showed that Alk5 deficient ACSCs exhibited senescence-like manifestations including decreased proliferation and differentiation, more SA-β-gal-positive cells and ROS production, as well as significantly swollen mitochondria and lysosome breakdown. We further found that local limitation of the detrimental roles of SnCs can attenuate the development of posttraumatic OA. Taken together, our findings suggest that Alk5 signaling acts as an important regulator of the SnCs in the superficial layer during AC maintenance and OA initiation.

Mechanisms and therapeutic implications of cellular senescence in osteoarthritis

The development of osteoarthritis (OA) correlates with a rise in the number of senescent cells in joint tissues, and the senescence-associated secretory phenotype (SASP) has been implicated in cartilage degradation and OA. Age-related mitochondrial dysfunction and associated oxidative stress might induce senescence in joint tissue cells. However, senescence is not the only driver of OA, and the mechanisms by which senescent cells contribute to disease progression are not fully understood. Furthermore, it remains uncertain which joint cells and SASP-factors contribute to the OA phenotype. Research in the field has looked at developing therapeutics (namely senolytics and senomorphics) that eliminate or alter senescent cells to stop disease progression and pathogenesis. A better understanding of how senescence contributes to joint dysfunction may enhance the effectiveness of these approaches and provide relief for patients with OA.

LDHA-mediated ROS generation in chondrocytes is a potential therapeutic target for osteoarthritis

The contribution of inflammation to the chronic joint disease osteoarthritis (OA) is unclear, and this lack of clarity is detrimental to efforts to identify therapeutic targets. Here we show that chondrocytes under inflammatory conditions undergo a metabolic shift that is regulated by NF-κB activation, leading to reprogramming of cell metabolism towards glycolysis and lactate dehydrogenase A (LDHA). Inflammation and metabolism can reciprocally modulate each other to regulate cartilage degradation. LDHA binds to NADH and promotes reactive oxygen species (ROS) to induce catabolic changes through stabilization of IκB-ζ, a critical pro-inflammatory mediator in chondrocytes. IκB-ζ is regulated bi-modally at the stages of transcription and protein degradation. Overall, this work highlights the function of NF-κB activity in the OA joint as well as a ROS promoting function for LDHA and identifies LDHA as a potential therapeutic target for OA treatment.

Navitoclax (ABT263) reduces inflammation and promotes chondrogenic phenotype by clearing senescent osteoarthritic chondrocytes in osteoarthritis

Cell senescence is a chronic process associated with age-related degenerative diseases such as osteoarthritis (OA). Senescent cells (SnCs) accumulate in the articular cartilage and synovium, leading to OA pathologies. The accumulation of SnCs in the cartilage results in a senescence-associated secretory phenotype (SASP) and age-related inflammation and dysfunction. Selective removal of SnCs by senolytic agent as a therapeutic strategy has been developed recently. In this study, we examined the ability of the senolytic drug ABT263 (navitoclax) to clear SnCs and further evaluated the therapeutic effect of ABT263 on post-traumatic OA. Monolayer and 3D pellet cultured osteoarthritic chondrocytes were used to evaluate the effect of ABT263 in vitro and a DMM rat model was established for in vivo experiments. We found that ABT263 reduced the expression of inflammatory cytokines and promoted cartilage matrix aggregation in OA chondrocyte pellet culture by inducing SnC apoptosis. Moreover, OA pathological changes in the cartilage and subchondral bone in post-traumatic OA rat were alleviated by ABT263 intra-articular injection. These results demonstrated that ABT263 not only improves inflammatory microenvironment but also promotes cartilage phenotype maintenance in vitro. Furthermore, ABT263 might play a protective role against post-traumatic OA development. Therefore, strategies targeting SnC elimination might be promising for the clinical therapy of OA.

Mechanoflammation in osteoarthritis pathogenesis

Mechanical injury is the most important risk factor in osteoarthritis (OA) development. Although once considered a passive disease of mechanical attrition, injury drives active mechanosensitive intracellular signalling which affects the structural and symptomatic course of disease. Mechanosensitive signalling in cartilage has been elucidated over the years and two principal responses emerge: those that cause the release of growth factors from the matrix and which stimulate repair, and those that drive inflammatory signalling, a process that we have termed "mechanoflammation". The up-stream activator of mechanoflammation remains unknown, but it results in rapid activation of NFkB and the inflammatory mitogen activated protein (MAP) kinases and this controls the bioavailability of aggrecanase and regulation of nerve growth factor (NGF), causing pain. The precise relationship between mechanoflammation and cartilage repair is currently unclear but it is likely that chronic mechanoflammation will contribute to disease by also suppressing intrinisic tissue repair.

Rapamycin-PLGA microparticles prevent senescence, sustain cartilage matrix production under stress and exhibit prolonged retention in mouse joints

Osteoarthritis (OA) is a joint disease characterized by progressive damage of articular cartilage and the adjoining subchondral bone. Chondrocytes, the primary cells of the cartilage, have limited regenerative capacity and when they undergo stress due to trauma or with aging, they senesce or become apoptotic. Rapamycin, a potent immunomodulator, has shown promise in OA treatment. It activates autophagy and is known to prevent senescence. However, its clinical translation for OA is hampered due to systemic toxicity as high and frequent doses are required. Here, we have fabricated rapamycin encapsulated poly(lactic-co-glycolic acid) (PLGA) based carriers that induced autophagy and prevented cellular senescence in human chondrocytes. The microparticle (MP) delivery system showed sustained release of the drug for several weeks. Rapamycin microparticles protected in vitro cartilage mimics (micromass cultures) from degradation, allowing sustained production of sGAG, and demonstrated a prolonged senescence preventive effect under oxidative and genomic stress conditions. These microparticles also exhibited a residence time of ∼30 days after intra-articular injections in murine knee joints. Such particulate systems are promising candidates for intra-articular delivery of rapamycin for the treatment of osteoarthritis.

Cartilage Trauma Induces Necroptotic Chondrocyte Death and Expulsion of Cellular Contents

Necroptotic cell death is characterized by an activation of RIPK3 and MLKL that leads to plasma membrane permeabilization and the release of immunostimulatory cellular contents. High levels of chondrocyte death occur following intra-articular trauma, which frequently leads to post-traumatic osteoarthritis development. The aim of this study is to assess necroptosis levels in cartilage post-trauma and to examine whether chondrocyte necroptotic mechanisms may be investigated and modified in vitro. Fractured human and murine cartilage, analysed immunohistochemically for necroptosis marker expression, demonstrated significantly higher levels of RIPK3 and phospho-MLKL than uninjured controls. Primary murine chondrocytes stimulated in vitro with the TNFα and AKT-inhibitor alongside the pan-caspase inhibitor Z-VAD-fmk exhibited a significant loss of metabolic activity and viability, accompanied by an increase in MLKL phosphorylation, which was rescued by further treatment of chondrocytes with necrostatin-1. Transmission electron microscopy demonstrated morphological features of necroptosis in chondrocytes following TNFα and Z-VAD-fmk treatment. Release of dsDNA from necroptotic chondrocytes was found to be significantly increased compared to controls. This study demonstrates that cartilage trauma leads to a high prevalence of necroptotic chondrocyte death, which can be induced and inhibited in vitro, indicating that both necroptosis and its consequential release of immunostimulatory cellular contents are potential therapeutic targets in post-traumatic arthritis treatment.

Eliminating senescent chondrogenic progenitor cells enhances chondrogenesis under intermittent hydrostatic pressure for the treatment of OA

Background

Osteoarthritis (OA) is a major cause of limb dysfunction, and distraction arthroplasty which generates intermittent hydrostatic pressure (IHP) is an effective approach for OA treatment. However, the result was not always satisfactory and the reasons remained unresolved. Because aging is recognized as an important risk factor for OA and chondrogenic progenitor cells (CPCs) could acquire senescent phenotype, we made a hypothesis that CPCs senescence could have harmful effect on chondrogenesis and the outcome of distraction arthroplasty could be improved by eliminating senescent CPCs pharmacologically.

Methods

The role of senescent CPCs on distraction arthroplasty was first determined by comparing the cartilage samples from the failure and non-failure patients. Next, the biological behaviors of senescent CPCs were observed in the in vitro cell culture and IHP model. Finally, the beneficial effect of senescent CPCs clearance by senolytic dasatinib and quercetin (DQ) on cartilage regeneration was observed in the in vitro and in vivo IHP model.

Results

Larger quantities of senescent CPCs along with increased IL-1 β secretion were demonstrated in the failure patients of distraction arthroplasty. Senescent CPCs revealed impaired proliferation and chondrogenic capability and also had increased IL-1 β synthesis, typical of senescence-associated secretory phenotype (SASP). CPCs senescence and SASP formation were mutually dependent in vitro. Greater amounts of senescent CPCs were negatively correlated with IHP-induced chondrogenesis. In contrast, chondrogenesis could be significantly improved by DQ pretreatment which selectively induced senescent CPCs into apoptosis in the in vitro and in vivo IHP model. Mechanistically, senescent CPCs elimination could decrease SASP formation and therefore promote the proliferation and chondrogenic regeneration capacity of the surrounding survived CPCs under IHP stimulation.

Conclusions

Eliminating senescent CPCs by senolytics could decrease SASP formation and improve the result of joint distraction arthroplasty effectively. Our study provided a novel CPCs senescence-based therapeutic target for improving the outcome of OA treatment.

Stress-activated miR-204 governs senescent phenotypes of chondrocytes to promote osteoarthritis development

A progressive loss of cartilage matrix leads to the development of osteoarthritis (OA). Matrix homeostasis is disturbed in OA cartilage as the result of reduced production of cartilage-specific matrix and increased secretion of catabolic mediators by chondrocytes. Chondrocyte senescence is a crucial cellular event contributing to such imbalance in matrix metabolism during OA development. Here, we identify miR-204 as a markedly up-regulated microRNA in OA cartilage. miR-204 is induced by transcription factors GATA4 and NF-κB in response to senescence signals. Up-regulated miR-204 simultaneously targets multiple components of the sulfated proteoglycan (PG) biosynthesis pathway, effectively shutting down PG anabolism. Ectopic expression of miR-204 in joints triggers spontaneous cartilage loss and OA development, whereas miR-204 inhibition ameliorates experimental OA, with concomitant recovery of PG synthesis and suppression of inflammatory senescence-associated secretory phenotype (SASP) factors in cartilage. Collectively, we unravel a stress-activated senescence pathway that underlies disrupted matrix homeostasis in OA cartilage.

Hypoxia-inducible factor-1 alpha maintains mouse articular cartilage through suppression of NF-κB signaling

HIF-1α, an essential transcription factor under hypoxic condition, is indispensable for chondrocytes during skeletal development but its expression and roles in articular chondrocytes are yet to be revealed. We examined HIF-1α protein expression and the hypoxic condition during mouse osteoarthritis (OA) development using state of the art hypoxic probes and found that its expression decreased as OA progressed, coinciding with the change in hypoxic conditions in articular cartilage. Gain- and loss-of-function of HIF-1α in cell culture experiments showed that HIF-1α suppressed catabolic genes such as Mmp13 and Hif2a. We confirmed these anticatabolic effects by measuring glycosaminoglycan release from wild type and conditional knock-out mice femoral heads cultured ex vivo. We went on to surgically induce OA in mice with chondrocyte-specific deletion of Hif1a and found that the development of OA was exacerbated. Increased expression of catabolic factors and activation of NF-κB signalling was clearly evident in the knock-out mice. By microarray analysis, C1qtnf3 was identified as a downstream molecule of HIF-1α, and experiments showed it exerted anti-catabolic effects through suppression of NF-κB. We conclude that HIF-1α has an anti-catabolic function in the maintenance of articular cartilage through suppression of NF-κB signalling.

Pathomechanisms of Posttraumatic Osteoarthritis: Chondrocyte Behavior and Fate in a Precarious Environment

Traumatic injuries of the knee joint result in a wide variety of pathomechanisms, which contribute to the development of so-called posttraumatic osteoarthritis (PTOA). These pathogenetic processes include oxidative stress, excessive expression of catabolic enzymes, release of damage-associated molecular patterns (DAMPs), and synovial inflammation. The present review focuses on the underlying pathomechanisms of PTOA and in particular the behavior and fate of the surviving chondrocytes, comprising chondrocyte metabolism, regulated cell death, and phenotypical changes comprising hypertrophy and senescence. Moreover, possible therapeutic strategies, such as chondroanabolic stimulation, anti-oxidative and anti-inflammatory treatment, as well as novel therapeutic targets are discussed.

Adenosinergic signalling in chondrogenesis and cartilage homeostasis: Friend or foe?

Chondrocytes and their mesenchymal cell progenitors secrete a variety of bioactive molecules, including adenine nucleotides and nucleosides, but these molecules are not usually highlighted in review papers about the secretome of these cells. Ageing and inflammatory insults compromise chondrocytes ability to keep ATP/adenosine synthesis, release and turnover. Cartilage homeostasis depends on extracellular adenosine levels, which acting via four P1 purinoceptor subtypes modulates the release of pro-inflammatory mediators, including NO, PGE₂ and several cytokines. Native articular cartilage is challenged by synovial fluid flow during normal joint motion transiently increasing ATP release and adenosine formation in the joint microenvironment. Excessive joint motion and shockwave trauma are deleterious to cartilage homeostasis due to HIF-1α overexpression, resulting in disproportionate ecto-5'-nucleotidase/CD73 production, adenosine accumulation and superfluous A_2B receptors activation. Scarcity of data however exists on the putative interplay between coexistent high affinity (A_2A and A₃) and low affinity (A_2B) adenosine receptors activation affecting stem cells fate towards preferential chondrogenic or osteogenic lineages in the human cartilage. Hints gathered in this commentary result mainly from studies using human immortalized cell lines and animal (e.g. rodent, equine, bovine) tissue samples. The available data point towards adenosine A_2A and A₃ receptors having cartilage protective roles, while excessive adenosine accumulation may be detrimental via low affinity A_2B receptors activation, with little reference to the putative role of the adenosine forming enzyme ecto-5'-nucleotidase/CD73. Thus, emphasizing the multiple pathways responsible for controlling adenosine signalling in cartilage will certainly impact on the search for novel therapeutic targets for highly disabling articular disorders.

Angiopoietin-like 2 upregulation promotes human chondrocyte injury via NF-κB and p38/MAPK signaling pathway

Several cellular and molecular processes participate in the pathologic changes of osteoarthritis (OA). However, the core molecular regulators of these processes are unclear, and no effective treatment for OA disease has been developed so far. ANGPTL2 is well known for its tissue remolding and pro-inflammation properties. However, the role of ANGPTL2 in osteoarthritis (OA) still remains unclear. To explore the expression level of ANGPTL2 in human OA cartilage and investigate the function of ANGPTL2 in human chondrocytes injury, qRT-PCR, western blot and immunohistochemistry were employed to investigate the expression of ANGPTL2 between human OA and normal cartilage samples. Next, human primary chondrocytes were treated with IL-1β to mimic OA progress in vitro, and the expression of ANGPTL2 were tested by qRT-PCR and western blot. Furthermore, the effect of ANGPTL2 in the expression of pro-inflammation cytokines (IL-1β, IL-6), proteolytic enzymes (MMP-1, MMP-13) and component of the cartilage matrix (COL2A1 and aggrecan) in human primary chondrocyte were explored by gain-of-function and loss-of-function methods. Finally, the nuclear factor kappa B (NF-κB) and p38/MAPK signaling pathways were also tested by western blot analysis. In this study, firstly, the expression level of ANGPTL2 was elevated both in human OA cartilage samples and IL-1β stimulated human chondrocytes. Secondly, ANGPTL2 upregulation promotes extracellular matrix (ECM) degradation and inflammation mediator production in human chondrocytes. Finally, ANGPTL2 activated the NF-κB and p38/MAPK signaling pathways via integrin α5β1. This study, for the first time, highlights that ANGPTL2 secreted by human chondrocytes plays a negative role in the pathogenesis of osteoarthritis, and it may be a potential therapeutic target in OA.

Absence of Proteoglycan 4 (Prg4) Leads to Increased Subchondral Bone Porosity Which Can Be Mitigated Through Intra-Articular Injection of PRG4

Proteoglycan 4 (PRG4) is a mucin-like glycoprotein important for joint health. Mice lacking Prg4 demonstrate degeneration of the cartilage and altered skeletal morphology. The purpose of this study was to examine if Prg4 deficiency leads to subchondral bone defects and if these defects could be mitigated through intra-articular injection of recombinant human PRG4 (rhPRG4). Mice deficient in Prg4 expression demonstrated increased cartilage thickness and increased subchondral bone porosity compared with C57BL/6 controls. While the porosity of the subchondral bone of Prg4^-/- mice decreased over time with maturation, intra-articular injection of rhPRG4 was able to forestall the increase in porosity. In contrast, neither hyaluronan (HA) nor methylprednisolone injections had beneficial effects on the subchondral bone porosity in the Prg4 knockout mice. Bone marrow progenitor cells from Prg4^-/- mice demonstrated reduced osteogenic differentiation capacity at 4 weeks of age, but not at 16 weeks of age. While most studies on PRG4/lubricin focus on the health of the cartilage, this study demonstrates that PRG4 plays a role in the maturation of the subchondral bone. Furthermore, increasing joint lubrication/viscosupplementation through injection of HA or controlling joint inflammation through injection of methylprednisolone may help maintain the cartilage surface, but had no positive effect on the subchondral bone in animals lacking Prg4. Therefore, alterations in the subchondral bone in models with absent or diminished Prg4 expression should not be overlooked when investigating changes within the articular cartilage regarding the pathogenesis of osteoarthritis/arthrosis. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2077-2088, 2019.

The microRNAs miR-204 and miR-211 maintain joint homeostasis and protect against osteoarthritis progression

Osteoarthritis (OA) is a common, painful disease. Currently OA is incurable, and its etiology largely unknown, partly due to limited understanding of OA as a whole-joint disease. Here we report that two homologous microRNAs, miR-204 and miR-211, maintain joint homeostasis to suppress OA pathogenesis. Specific knockout of miR-204/-211 in mesenchymal progenitor cells (MPCs) results in Runx2 accumulation in multi-type joint cells, causing whole-joint degeneration. Specifically, miR-204/-211 loss-of-function induces matrix-degrading proteases in articular chondrocytes and synoviocytes, stimulating articular cartilage destruction. Moreover, miR-204/-211 ablation enhances NGF expression in a Runx2-dependent manner, and thus hyper-activates Akt signaling and MPC proliferation, underlying multiplex non-cartilaginous OA conditions including synovial hyperplasia, osteophyte outgrowth and subchondral sclerosis. Importantly, miR-204/-211-deficiency-induced OA is largely rescued by Runx2 insufficiency, confirming the miR-204/-211-Runx2 axis. Further, intraarticular administration of miR-204-expressing adeno-associated virus significantly decelerates OA progression. Collectively, miR-204/-211 are essential in maintaining healthy homeostasis of mesenchymal joint cells to counteract OA pathogenesis.

Osteoarthritis involves whole-joint tissue degeneration. Here, the authors show that miR-204 and miR-211 in mesenchymal joint cells regulate their proliferation, catabolic and osteogenic responses, and that disease progression is ameliorated by intra-articular miR-204 delivery in mice.

Adenosine-Functionalized Biodegradable PLA-b-PEG Nanoparticles Ameliorate Osteoarthritis in Rats

Short biologic half-lives limit the therapeutic utility of many small molecules. One approach to extending the half-life of pharmacologically active small molecules is conjugation to less degradable nanoparticles; here we report the synthesis and activity of six targeted polymeric (PEG-b-PLA) nanoparticles for use as adenosine receptor agonists. Using click chemistry, PLA-b-PEG400-N₃ and PLA-b-PEG2000 block copolymers were bound to adenosine at the 3′,4′-OH, 5′-OH, and 6-NH₂ positions with an acetylene group. Activity of the conjugates as adenosine receptor ligands was tested by their capacity to stimulate cAMP increases in RAW264.7 murine macrophage cells. Only adenosine-conjugated nanoparticles (A-3′,4′-OH-TPN2), in which PEG2000 was bound to adenosine on the 3′,4′ hydroxyl groups, stimulated cAMP increases and these increases were blocked by selective antagonists of both adenosine A2A and A2B receptors, consistent with ligation of these receptors. Adenosine nanoparticles were tested in vivo in a rat model of post-traumatic osteoarthritis; intra-articular injection of adenosine nanoparticles prevented the development of osteoarthritis in this model. These studies suggest that attachment of adenosine to biodegradable nanoparticles provides a novel approach to achieving prolonged therapeutic effects.

Discerning the spatio-temporal disease patterns of surgically induced OA mouse models

Osteoarthritis (OA) is the most common cause of disability in ageing societies, with no effective therapies available to date. Two preclinical models are widely used to validate novel OA interventions (MCL-MM and DMM). Our aim is to discern disease dynamics in these models to provide a clear timeline in which various pathological changes occur. OA was surgically induced in mice by destabilisation of the medial meniscus. Analysis of OA progression revealed that the intensity and duration of chondrocyte loss and cartilage lesion formation were significantly different in MCL-MM vs DMM. Firstly, apoptosis was seen prior to week two and was narrowly restricted to the weight bearing area. Four weeks post injury the magnitude of apoptosis led to a 40-60% reduction of chondrocytes in the non-calcified zone. Secondly, the progression of cell loss preceded the structural changes of the cartilage spatio-temporally. Lastly, while proteoglycan loss was similar in both models, collagen type II degradation only occurred more prominently in MCL-MM. Dynamics of chondrocyte loss and lesion formation in preclinical models has important implications for validating new therapeutic strategies. Our work could be helpful in assessing the feasibility and expected response of the DMM- and the MCL-MM models to chondrocyte mediated therapies.

Bax Targeted by miR-29a Regulates Chondrocyte Apoptosis in Osteoarthritis

Osteoarthritis (OA) is a chronic degenerative joint disease, where chondrocyte apoptosis is responsible for cartilage degeneration. Bax is a well-known proapoptotic protein of the Bcl-2 family, involved in a large number of physiological and pathological processes. However, the regulation mechanisms of Bax underlying chondrocyte apoptosis in OA remain unknown. In the present study, we determined the role of Bax in human OA and chondrocyte apoptosis. The results showed that Bax was upregulated in chondrocytes from the articular cartilage of OA patients and in cultured chondrocyte-like ATDC5 cells treated by IL-1β. Bax was identified to be the direct target of miR-29a by luciferase reporter assay and by western blotting. Inhibition of miR-29a by the mimics protested and overexpression by miR-29a inhibitors aggravated ATDC5 apoptosis induced by IL-1β. These data reveal that miR-29a/Bax axis plays an important role in regulating chondrocyte apoptosis and suggest that targeting the proapoptotic protein Bax and increasing expression levels of miR-29a emerge as potential approach for protection against the development of OA.

Reactive oxygen species, aging and articular cartilage homeostasis

Chondrocytes are responsible for the maintenance of the articular cartilage. A loss of homeostasis in cartilage contributes to the development of osteoarthritis (OA) when the synthetic capacity of chondrocytes is overwhelmed by processes that promote matrix degradation. There is evidence for an age-related imbalance in reactive oxygen species (ROS) production relative to the anti-oxidant capacity of chondrocytes that plays a role in cartilage degradation as well as chondrocyte cell death. The ROS produced by chondrocytes that have received the most attention include superoxide, hydrogen peroxide, the reactive nitrogen species nitric oxide, and the nitric oxide derived product peroxynitrite. Excess levels of these ROS not only cause oxidative-damage but, perhaps more importantly, cause a disruption in cell signaling pathways that are redox-regulated, including Akt and MAP kinase signaling. Age-related mitochondrial dysfunction and reduced activity of the mitochondrial superoxide dismutase (SOD2) are associated with an increase in mitochondrial-derived ROS and are in part responsible for the increase in chondrocyte ROS with age. Peroxiredoxins (Prxs) are a key family of peroxidases responsible for removal of H₂O₂, as well as for regulating redox-signaling events. Prxs are inactivated by hyperoxidation. An age-related increase in chondrocyte Prx hyperoxidation and an increase in OA cartilage has been noted. The finding in mice that deletion of SOD2 or the anti-oxidant gene transcriptional regulator nuclear factor-erythroid 2- related factor (Nrf2) result in more severe OA, while overexpression or treatment with mitochondrial targeted anti-oxidants reduces OA, further support a role for excessive ROS in the pathogenesis of OA. Therefore, new therapeutic strategies targeting specific anti-oxidant systems including mitochondrial ROS may be of value in reducing the progression of age-related OA.