Effects of Wnt3A and mechanical load on cartilage chondrocyte homeostasis

Sensory nerve regulation of bone homeostasis: Emerging therapeutic opportunities for bone-related diseases

Understanding the intricate interplay between sensory nerves and bone tissue cells is of paramount significance in the field of bone biology and clinical medicine. The regulatory role of sensory nerves in bone homeostasis offers a novel perspective for the development of targeted therapeutic interventions for a spectrum of bone-related diseases, including osteoarthritis, osteoporosis, and intervertebral disc degeneration. By elucidating the mechanisms through which sensory nerves and their neuropeptides influence the differentiation and function of bone tissue cells, this review aims to shed light on emerging therapeutic targets that harness the neuro-skeletal axis for the treatment and management of debilitating bone disorders. Moreover, a comprehensive understanding of sensory nerve-mediated bone regulation may pave the way for the development of innovative strategies to promote bone health and mitigate the burden of skeletal pathologies in clinical practice.

Size-Based Microfluidic-Enriched Mesenchymal Stem Cell Subpopulations Enhance Articular Cartilage Repair

BACKGROUND

The functional heterogeneity of culture-expanded mesenchymal stem cells (MSCs) has hindered the clinical application of MSCs. Previous studies have shown that MSC subpopulations with superior chondrogenic capacity can be isolated using a spiral microfluidic device based on the principle of inertial cell focusing.

HYPOTHESIS

The delivery of microfluidic-enriched chondrogenic MSCs that are consistent in size and function will overcome the challenge of the functional heterogeneity of expanded MSCs and will significantly improve MSC-based cartilage repair.

STUDY DESIGN

Controlled laboratory study.

METHODS

A next-generation, fully automated multidimensional double spiral microfluidic device was designed to provide more refined and efficient isolation of MSC subpopulations based on size. Analysis of in vitro chondrogenic potential and RNA sequencing was performed on size-sorted MSC subpopulations. In vivo cartilage repair efficacy was demonstrated in an osteochondral injury model in 12-week-old rats. Defects were implanted with MSC subpopulations (n = 6 per group) and compared with those implanted with unsegregated MSCs (n = 6). Osteochondral repair was assessed at 6 and 12 weeks after surgery by histological, micro-computed tomography, and mechanical analysis.

RESULTS

A chondrogenic MSC subpopulation was efficiently isolated using the multidimensional double spiral device. RNA sequencing revealed distinct transcriptomic profiles and identified differential gene expression between subpopulations. The delivery of a chondrogenic MSC subpopulation resulted in improved cartilage repair, as indicated by histological scoring, the compression modulus, and micro-computed tomography of the subchondral bone.

CONCLUSION

We have established a rapid, label-free, and reliable microfluidic protocol for more efficient size-based enrichment of a chondrogenic MSC subpopulation. Our proof-of-concept in vivo study demonstrates the enhanced cartilage repair efficacy of these enriched chondrogenic MSCs.

CLINICAL RELEVANCE

The delivery of microfluidic-enriched chondrogenic MSCs that are consistent in size and function can overcome the challenge of the functional heterogeneity of expanded MSCs, resulting in significant improvement in MSC-based cartilage repair. The availability of such rapid, label-free enriched chondrogenic MSCs can enable better cell therapy products for cartilage repair with improved treatment outcomes.

Plastrum testudinis Ameliorates Oxidative Stress in Nucleus Pulposus Cells via Downregulating the TNF-α Signaling Pathway

BackgroundPlastrum testudinis (PT), a widely used traditional Chinese medicine, exerts protective effects against bone diseases such as intervertebral disc degeneration (IDD). Despite its effectiveness, the molecular mechanisms underlying the effects of PT on IDD remain unclear. Methods In this study, we used a comprehensive strategy combining bioinformatic analysis with experimental verification to investigate the possible molecular mechanisms of PT against IDD. We retrieved targets for PT and IDD, and then used their overlapped targets for protein-protein interaction (PPI) analysis. In addition, we used Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses to investigate the anti-IDD mechanisms of PT. Moreover, in vivo and in vitro experiment validations including hematoxylin-eosin (HE) and safranine O-green staining, senescence-associated β-galactosidase (SA-β-gal) assay, cell immunofluorescence staining, intracellular ROS measurement and Western blot analysis were performed to verify bioinformatics findings. Results We identified 342 and 872 PT- and IDD-related targets (32 overlapping targets). GO enrichment analysis yielded 450 terms related to oxidative stress and inflammatory response regulation. KEGG analysis identified 48 signaling pathways, 10 of which were significant; the TNF-α signaling pathway had the highest p-value, and prostaglandin G/H synthase 2 (PTGS2), endothelin-1 (EDN1), TNF-α, JUN and FOS were enriched in this pathway. Histopathological results and safranin O/green staining demonstrated that PT attenuated IDD, and SA-β-gal assay showed that PT ameliorated nucleus pulposus cell (NPC) senescence. An ROS probe was adopted to confirm the protective effect of PT against oxidative stress. Western blot analyses confirmed that PT downregulated the protein expression of PTGS2, EDN1, TNF-α, JUN and FOS in the TNF-α signaling pathway as well as cellular senescence marker p16, proinflammatory cytokine interleukin-6 (IL6), while PT upregulated the expression of NPC-specific markers including COL2A1 and ACAN in a concentration-dependent manner. Conclusions To the best of our knowledge, this study is the first to report that PT alleviates IDD by downregulating the protein expression of PTGS2, EDN1, TNF-α, JUN and FOS in the TNF-α signaling pathway and upregulating that of COL2A1 and ACAN, thus suppressing inflammatory responses and oxidative stress in NPCs.

Role of Wnt signaling pathway in joint development and cartilage degeneration

Osteoarthritis (OA) is a prevalent musculoskeletal disease that affects approximately 500 million people worldwide. Unfortunately, there is currently no effective treatment available to stop or delay the degenerative progression of joint disease. Wnt signaling pathways play fundamental roles in the regulation of growth, development, and homeostasis of articular cartilage. This review aims to summarize the role of Wnt pathways in joint development during embryonic stages and in cartilage maintenance throughout adult life. Specifically, we focus on aberrant mechanical loading and inflammation as major players in OA progression. Excessive mechanical load activates Wnt pathway in chondrocytes, resulting in chondrocyte apoptosis, matrix destruction and other osteoarthritis-related changes. Additionally, we discuss emerging Wnt-related modulators and present an overview of emerging treatments of OA targeting Wnt signaling. Ultimately, this review provides valuable insights towards discovering new drugs or gene therapies targeting Wnt signaling pathway for diagnosing and treating osteoarthritis and other degenerative joint diseases.

An Integrated View of Stressors as Causative Agents in OA Pathogenesis

Cells in the body are exposed to dynamic external and internal environments, many of which cause cell damage. The cell's response to this damage, broadly called the stress response, is meant to promote survival and repair or remove damage. However, not all damage can be repaired, and sometimes, even worse, the stress response can overtax the system itself, further aggravating homeostasis and leading to its loss. Aging phenotypes are considered a manifestation of accumulated cellular damage and defective repair. This is particularly apparent in the primary cell type of the articular joint, the articular chondrocytes. Articular chondrocytes are constantly facing the challenge of stressors, including mechanical overloading, oxidation, DNA damage, proteostatic stress, and metabolic imbalance. The consequence of the accumulation of stress on articular chondrocytes is aberrant mitogenesis and differentiation, defective extracellular matrix production and turnover, cellular senescence, and cell death. The most severe form of stress-induced chondrocyte dysfunction in the joints is osteoarthritis (OA). Here, we summarize studies on the cellular effects of stressors on articular chondrocytes and demonstrate that the molecular effectors of the stress pathways connect to amplify articular joint dysfunction and OA development.

Mechanotransduction pathways in articular chondrocytes and the emerging role of estrogen receptor-α

In the synovial joint, mechanical force creates an important signal that influences chondrocyte behavior. The conversion of mechanical signals into biochemical cues relies on different elements in mechanotransduction pathways and culminates in changes in chondrocyte phenotype and extracellular matrix composition/structure. Recently, several mechanosensors, the first responders to mechanical force, have been discovered. However, we still have limited knowledge about the downstream molecules that enact alterations in the gene expression profile during mechanotransduction signaling. Recently, estrogen receptor α (ERα) has been shown to modulate the chondrocyte response to mechanical loading through a ligand-independent mechanism, in line with previous research showing that ERα exerts important mechanotransduction effects on other cell types, such as osteoblasts. In consideration of these recent discoveries, the goal of this review is to position ERα into the mechanotransduction pathways known to date. Specifically, we first summarize our most recent understanding of the mechanotransduction pathways in chondrocytes on the basis of three categories of actors, namely mechanosensors, mechanotransducers, and mechanoimpactors. Then, the specific roles played by ERα in mediating the chondrocyte response to mechanical loading are discussed, and the potential interactions of ERα with other molecules in mechanotransduction pathways are explored. Finally, we propose several future research directions that may advance our understanding of the roles played by ERα in mediating biomechanical cues under physiological and pathological conditions.

"Slow walk" mimetic tensile loading maintains human meniscus tissue resident progenitor cells homeostasis in photocrosslinked gelatin hydrogel

Meniscus, the cushion in knee joint, is a load-bearing tissue that transfers mechanical forces to extracellular matrix (ECM) and tissue resident cells. The mechanoresponse of human tissue resident stem/progenitor cells in meniscus (hMeSPCs) is significant to tissue homeostasis and regeneration but is not well understood. This study reports that a mild cyclic tensile loading regimen of ∼1800 loads/day on hMeSPCs seeded in 3-dimensional (3D) photocrosslinked gelatin methacryloyl (GelMA) hydrogel is critical in maintaining cellular homeostasis. Experimentally, a "slow walk" biomimetic cyclic loading regimen (10% tensile strain, 0.5 Hz, 1 h/day, up to 15 days) is applied to hMeSPCs encapsulated in GelMA hydrogel with a magnetic force-controlled loading actuator. The loading significantly increases cell differentiation and fibrocartilage-like ECM deposition without affecting cell viability. Transcriptomic analysis reveals 332 mechanoresponsive genes, clustered into cell senescence, mechanical sensitivity, and ECM dynamics, associated with interleukins, integrins, and collagens/matrix metalloproteinase pathways. The cell-GelMA constructs show active ECM remodeling, traced using a green fluorescence tagged (GFT)-GelMA hydrogel. Loading enhances nascent pericellular matrix production by the encapsulated hMeSPCs, which gradually compensates for the hydrogel loss in the cultures. These findings demonstrate the strong tissue-forming ability of hMeSPCs, and the importance of mechanical factors in maintaining meniscus homeostasis.

Spectral characterization of cell surface motion for mechanistic investigations of cellular mechanobiology

Understanding the specific mechanisms responsible for anabolic and catabolic responses to static or dynamic force are largely poorly understood. Because of this, most research groups studying mechanotransduction due to dynamic forces employ an empirical approach in deciding what frequencies to apply during experiments. While this has been shown to elucidate valuable information regarding how cells respond under controlled provocation, it is often difficult or impossible to determine a true optimal frequency for force application, as many intracellular complexes are involved in receiving, propagating, and responding to a given stimulus. Here we present a novel adaptation of an analytical technique from the fields of civil and mechanical engineering that may open the door to direct measurement of mechanobiological cellular frequencies which could be used to target specific cell signaling pathways leveraging synergy between outside-in and inside-out mechanotransduction approaches. This information could be useful in identifying how specific proteins are involved in the homeostatic balance, or disruption thereof, of cells and tissue, furthering the understanding of the pathogenesis and progression of many diseases across a wide variety of cell types, which may one day lead to the development of novel mechanobiological therapies for clinical use.

Mechanical Cues: Bidirectional Reciprocity in the Extracellular Matrix Drives Mechano-Signalling in Articular Cartilage

The composition and organisation of the extracellular matrix (ECM), particularly the pericellular matrix (PCM), in articular cartilage is critical to its biomechanical functionality; the presence of proteoglycans such as aggrecan, entrapped within a type II collagen fibrillar network, confers mechanical resilience underweight-bearing. Furthermore, components of the PCM including type VI collagen, perlecan, small leucine-rich proteoglycans—decorin and biglycan—and fibronectin facilitate the transduction of both biomechanical and biochemical signals to the residing chondrocytes, thereby regulating the process of mechanotransduction in cartilage. In this review, we summarise the literature reporting on the bidirectional reciprocity of the ECM in chondrocyte mechano-signalling and articular cartilage homeostasis. Specifically, we discuss studies that have characterised the response of articular cartilage to mechanical perturbations in the local tissue environment and how the magnitude or type of loading applied elicits cellular behaviours to effect change. In vivo, including transgenic approaches, and in vitro studies have illustrated how physiological loading maintains a homeostatic balance of anabolic and catabolic activities, involving the direct engagement of many PCM molecules in orchestrating this slow but consistent turnover of the cartilage matrix. Furthermore, we document studies characterising how abnormal, non-physiological loading including excessive loading or joint trauma negatively impacts matrix molecule biosynthesis and/or organisation, affecting PCM mechanical properties and reducing the tissue’s ability to withstand load. We present compelling evidence showing that reciprocal engagement of the cells with this altered ECM environment can thus impact tissue homeostasis and, if sustained, can result in cartilage degradation and onset of osteoarthritis pathology. Enhanced dysregulation of PCM/ECM turnover is partially driven by mechanically mediated proteolytic degradation of cartilage ECM components. This generates bioactive breakdown fragments such as fibronectin, biglycan and lumican fragments, which can subsequently activate or inhibit additional signalling pathways including those involved in inflammation. Finally, we discuss how bidirectionality within the ECM is critically important in enabling the chondrocytes to synthesise and release PCM/ECM molecules, growth factors, pro-inflammatory cytokines and proteolytic enzymes, under a specified load, to influence PCM/ECM composition and mechanical properties in cartilage health and disease.

Tumor-immune profiling of CT-26 and Colon 26 syngeneic mouse models reveals mechanism of anti-PD-1 response

BACKGROUND

Immune checkpoint blockade (ICB) therapies have changed the paradigm of cancer therapies. However, anti-tumor response of the ICB is insufficient for many patients and limited to specific tumor types. Despite many preclinical and clinical studies to understand the mechanism of anti-tumor efficacy of ICB, the mechanism is not completely understood. Harnessing preclinical tumor models is one way to understand the mechanism of treatment response.

METHODS

In order to delineate the mechanisms of anti-tumor activity of ICB in preclinical syngeneic tumor models, we selected two syngeneic murine colorectal cancer models based on in vivo screening for sensitivity with anti-PD-1 therapy. We performed tumor-immune profiling of the two models to identify the potential mechanism for anti-PD-1 response.

RESULTS

We performed in vivo screening for anti-PD-1 therapy across 23 syngeneic tumor models and found that CT-26 and Colon 26, which are murine colorectal carcinoma derived from BALB/c mice, showed different sensitivity to anti-PD-1. CT-26 tumor mice were more sensitive to the anti-PD-1 antibody than Colon 26, while both models show similarly sensitivity to anti-CTLA4 antibody. Immune-profiling showed that CT-26 tumor tissue was infiltrated with more immune cells than Colon 26. Genomic/transcriptomic analyses highlighted thatWnt pathway was one of the potential differences between CT-26 and Colon 26, showing Wnt activity was higher in Colon 26 than CT-26. .

CONCLUSIONS

CT-26 and Colon 26 syngeneic tumor models showed different sensitivity to anti-PD-1 therapy, although both tumor cells are murine colorectal carcinoma cell lines from BALB/c strain. By characterizing the mouse cells lines and tumor-immune context in the tumor tissues with comprehensive analysis approaches, we found that CT-26 showed "hot tumor" profile with more infiltrated immune cells than Colon 26. Further pathway analyses enable us to propose a hypothesis that Wnt pathway could be one of the major factors to differentiate CT-26 from Colon 26 model and link to anti-PD-1 response. Our approach to focus on preclinical tumor models with similar genetic background but different sensitivity to anti-PD-1 therapy would contribute to illustrating the potential mechanism of anti-PD-1 response and to generating a novel concept to synergize current anti-PD-1 therapies for cancer patients.

Transcriptome Profiling Reveals Important Transcription Factors and Biological Processes in Skin Regeneration Mediated by Mechanical Stretch

Background: Mechanical stretch is utilized to promote skin regeneration during tissue expansion for reconstructive surgery. Although mechanical stretch induces characteristic morphological changes in the skin, the biological processes and molecular mechanisms involved in mechanically induced skin regeneration are not well elucidated.

Methods: A male rat scalp expansion model was established and the important biological processes related to mechanical stretch-induced skin regeneration were identified using Gene Ontology (GO) analysis, Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, and gene set enrichment analysis (GSEA). Analysis was also conducted by constructing a protein–protein interaction (PPI) network, identifying key modules and hub genes, determining transcription factor (TF)-mRNA regulatory relationships, and confirming the expression pattern of the TFs and hub genes.

Results: We identified nine robust hub genes (CXCL1, NEB, ACTN3, MYOZ1, ACTA1, TNNT3, PYGM, AMPD1, and CKM) that may serve as key molecules in skin growth. These genes were determined to be involved in several important biological processes, including keratinocyte differentiation, cytoskeleton reorganization, chemokine signaling pathway, glycogen metabolism, and voltage-gated ion channel activity. The potentially significant pathways, including the glucagon signaling pathway, the Wnt signaling pathway, and cytokine–cytokine receptor interaction, were distinguished. In addition, we identified six TFs (LEF1, TCF7, HMGA1, TFAP2C, FOSL1, and ELF5) and constructed regulatory TF–mRNA interaction networks.

Conclusion: This study generated a comprehensive overview of the gene networks underlying mechanically induced skin regeneration. The functions of these key genes and the pathways in which they participate may reveal new aspects of skin regeneration under mechanical strain. Furthermore, the identified TF regulators can be used as potential candidates for clinical therapeutics for skin pretreatment before reconstructive surgery.

MicroRNAs and fracture healing: Pre-clinical studies

During the past several years, pre-clinical experiments have established that microRNAs (miRNAs), small non-coding RNAs, serve as key regulatory molecules of fracture healing. Their easy modulation with agonists and antagonists make them highly desirable targets for future therapeutic strategies, especially for pathophysiologic fractures that either do not heal (nonunions) or are delayed. It is now well documented that these problematic fractures lead to human suffering and impairment of life quality. Additionally, financial difficulties are also encountered as work productivity decreases and income is reduced. Moreover, targeting miRNAs may also be an avenue to enhancing normal physiological fracture healing. Herein we present the most current knowledge of the involvement of miRNAs during fracture healing in pre-clinical studies. Following a brief description on the nature of miRNAs and of the fracture healing process, we present data from studies focusing specifically, on miRNA regulation of osteoblast differentiation and osteogenesis (within the context of known signaling pathways), chondrocytes, angiogenesis, and apoptosis, all critical to successful bone repair. Further, we also discuss miRNAs and exosomes. We hope that this manuscript serves as a comprehensive review that will facilitate basic/translational scientists in the orthopaedic arena to realize and further decipher the biological and future therapeutic impact of these small regulatory RNA molecules, especially as they relate to the molecular events of each of the major phases of fracture healing.

Sub-toxic levels of cobalt ions impair chondrocyte mechanostranduction via HDAC6-dependent primary cilia shortening

Cobalt ions are the main wear particles associated with orthopaedic implants, causing adverse complications due to cytotoxicity and inflammatory mediators. Recent studies have shown that sub-toxic levels of cobalt ions regulate matrix synthesis and inflammation, but the influence of cobalt ions on mechanotransduction remains unclear. Previously, we reported that sub-toxic levels of cobalt ions modulated primary cilia, which are crucial for mechanotransduction. This study therefore aimed to investigate the effect of cobalt ions on chondrocyte mechanosensation in response to cyclic tensile strain and the association with primary cilia. Sub-toxic levels of cobalt ions impaired chondrocyte mechanosensation and affected the gene expression of aggrecan, collagen II and MMP-13. Moreover, cobalt ions induced HDAC6-dependent primary cilia disassembly, which was associated with either cytoplasmic or ciliary α-tubulin deacetylation. Pharmaceutical HDAC6 inhibition with tubacin restored primary cilia length and mechanotransduction, whereas chemical depletion of primary cilia by chloral hydrate prevented mechanosignalling. Thus, sub-toxic levels of cobalt ions impaired chondrocyte mechanotransduction via HDAC6 activation, which was associated with tubulin deacetylation and primary cilia shortening.

Inhibition of hypertrophy and improving chondrocyte differentiation by MMP-13 inhibitor small molecule encapsulated in alginate-chondroitin sulfate-platelet lysate hydrogel

BACKGROUND

Mesenchymal stem cells are a promising cell source for chondrogenic differentiation and have been widely used in several preclinical and clinical studies. However, they are prone to an unwanted differentiation process towards hypertrophy that limits their therapeutic efficacy. Matrix metallopeptidase 13 (MMP-13) is a well-known factor regulated during this undesirable event. MMP-13 is a collagen degrading enzyme, which is also highly expressed in the hypertrophic zone of the growth plate and in OA cartilage. Accordingly, we investigated the effect of MMP-13 inhibition on MSC hypertrophy.

METHODS

In this study, 5-bromoindole-2-carboxylic acid (BICA) was used as an inhibitory agent for MMP-13 expression. After identifying its optimal concentration, BICA was mixed into a hydrogel and the release rate was studied. To prepare the ideal hydrogel, chondroitin sulfate (CS) and platelet lysate (PL) were mixed with sodium alginate (Alg) at concentrations selected based on synergistic mechanical and rheometric properties. Then, four hydrogels were prepared by combining alginate (1.5%w/v) and/or CS (1%w/v) and/or PL (20%v/v). The chondrogenic potential and progression to hypertrophy of human bone marrow-derived mesenchymal stem cell (hBM-MSC)-loaded hydrogels were investigated under free swelling and mechanical loading conditions, in the presence and absence of BICA.

RESULTS

Viability of hBM-MSCs seeded in the four hydrogels was similar. qRT-PCR revealed that BICA could successfully inhibit MMP-13 expression, which led to an inhibition of Coll X and induction of Coll-II, in both free swelling and loading conditions. The GAG deposition was higher in the group combining BICA and mechanical stimulation.

CONCLUSIONS

It is concluded that BICA inhibition of MMP-13 reduces MSC hypertrophy during chondrogenesis.

Ubiquitin conjugating enzyme E2 M promotes apoptosis in osteoarthritis chondrocytes via Wnt/β-catenin signaling

In this study, the role of ubiquitin conjugating enzyme E2 M (UBE2M) and molecular mechanisms associated with osteoarthritis (OA) were explored. Cartilage tissues and corresponding healthy tissues from OA patients were isolated. Our data suggested that the expression level of UBE2M in OA patients was significantly higher compared to that in healthy individuals (P < 0.01). The apoptosis of human OA chondrocytes was inhibited when silencing UBE2M and increased when overexpressing UBE2M. XAV939, as a tankyrase 1 inhibitor, could block the signaling pathway of Wnt/β-catenin, which significantly reversed the change introduced by UBE2M. The expression level of cytoplasmic β-catenin in siUBE2M cells dramatically increased, and the expression levels of nuclear β-catenin, cleaved caspase-3 (C-caspase-3), and MMP13 remarkably downregulated. Moreover, the ubiquitination of Axin was enhanced by the overexpression of UBE2M. The expression level of Axin significantly decreased in OA chondrocytes with UBE2M overexpression and increased after MG132 treatment. Moreover, UBE2M enhanced the apoptosis of OA chondrocytes by activating the Axin-dependent Wnt/β-catenin pathway. In this process, UBE2M downregulated Axin in an ubiquitination-dependent degradation pathway and subsequently activated Wnt/β-catenin signaling.

Identification of a novel microRNA-141-3p/Forkhead box C1/β-catenin axis associated with rheumatoid arthritis synovial fibroblast function in vivo and in vitro

Rationale: Rheumatoid arthritis (RA) is a prototype of inflammatory arthritis in which synovial fibroblasts (SFs) play key roles in cartilage and bone destruction through tumor-like proliferation, migration, invasion and inflammation. This study aimed to research forkhead box protein C1 (FoxC1) and microRNA (miR)-141-3p, which modulate pathological changes in the synovial membrane, to find possible strategies for treating RA.

Methods: FoxC1, β-catenin and miR-141-3p gene expression in synovial tissues and SFs was quantified by real-time PCR; FoxC1 and β-catenin protein levels were evaluated by immunohistochemistry, immunofluorescence, and Western blotting. We transiently transfected human SFs with FoxC1 and β-catenin overexpression and silencing vectors and assessed proliferation, migration, invasion and inflammation by cell function and enzyme-linked immunosorbent assays. We also assessed downstream signaling activation using immunofluorescence, real-time PCR and Western blotting. Double luciferase, coimmunoprecipitation and chromatin immunoprecipitation assays were used to verify miR-141-3p, FoxC1 and β-catenin gene and protein combinations. Finally, the therapeutic effects of FoxC1 silencing and miR-141-3p overexpression were evaluated in type II collagen-induced arthritis (CIA) rats.

Results: We found that FoxC1 expression was significantly upregulated in synovium and SFs in both RA patients and rats with collagen-induced arthritis (CIA). FoxC1 overexpression increased β-catenin messenger RNA (mRNA) and protein levels and upregulated cyclin D1, c-Myc, fibronectin and matrix metalloproteinase 3 (MMP3) mRNA and protein expression in RA SFs (RASFs). In contrast, FoxC1 knockdown reduced β-catenin mRNA and protein levels as well as cyclin D1, c-Myc, and fibronectin mRNA and protein levels in RASFs. Furthermore, altering FoxC1 expression did not significantly change GSK3β and pGSK3β levels. FoxC1 overexpression promoted proliferation, migration, invasion and proinflammatory cytokine (interleukin (IL)-1β, IL-6, and tumor necrosis factor-α (TNF-α)) production and reduced anti-inflammatory cytokine (IL-10) levels in RASFs. FoxC1 bound to the β-catenin promoter, and β-catenin mediated the FoxC1-induced pathological changes. We also observed downregulated microRNA (miR)-141-3p expression in SFs from both RA patients and CIA rats and further found that miR-141-3p bound to the FoxC1 3′UTR and suppressed FoxC1 expression. Intra-ankle miR-141-3p agomir or FoxC1-specific siRNA injection hindered CIA development in rats.

Conclusions: FoxC1 and miR-141-3p participate in RA pathogenesis by mediating inflammation and SF proliferation, migration, and invasion and thus could be novel targets for RA therapy as a nonimmunosuppressive approach.

The CRD of Frizzled 7 exhibits chondroprotective effects in osteoarthritis via inhibition of the canonical Wnt3a/β-catenin signaling pathway

Osteoarthritis (OA) is a chronic inflammatory joint disease without effective drugs. Frizzled 7 (FzD7) binds its ligand Wnt3a through an extracellular cysteine-rich domain (CRD) to transduce the canonical Wnt/β-catenin signaling pathway, which has been strongly implicated in OA pathogenesis. Effects of recombinant protein of FzD7 CRD on Wnt/β-catenin signaling and chondral destruction was evaluated in this study. Firstly, increased protein levels of FzD7, Wnt3a and β-catenin were detected in human OA cartilage implying that the canonical Wnt/β-catenin signaling mediated by Wnt3a and FzD7 executes an essential role in OA. Then we showed that FzD7 CRD antagonized the Wnt3a/β-catenin signaling pathway in a dose-dependent manner by binding Wnt3a. In addition, FzD7 CRD increased the expression of glycosaminoglycans (GAGs), Collagen II, aggrecan and reduced the expression of matrix metalloproteinase (MMP)-1, MMP-13, a disintegrin and metalloproteinase with thrombospondin motifs 5 (ADAMTS-5) in Wnt3a-stimulated human chondrocytes. Furthermore, a single intra-articular injection of the FzD7 CRD was efficacious in destabilization of the medial meniscus (DMM) mouse OA model, significantly improving Osteoarthritis Research Society International (OARSI) histology scores compared to mice treated with PBS. The results indicate that the FzD7 CRD exhibits chondroprotective effects by binding Wnt3a to suppress the Wnt3a/β-catenin signaling. Targeting the FzD7 CRD may be a novel therapy for the treatment of OA.

Hyaluronan suppresses enhanced cathepsin K expression via activation of NF-κB with mechanical stress loading in a human chondrocytic HCS-2/8 cells

Cathepsin K is a protease known to be involved in not only bone remodeling and resorption, but also articular cartilage degradation that leads to osteoarthritis (OA). Hyaluronan (HA) plays a pivotal role in maintaining homeostasis within articular chondrocytes. Intra-articular supplementation of high molecular weight hyaluronan (HMW-HA) has been widely used in OA treatment. However, its prospective mechanism of action is still unclear. In this study, we examined the suppressive effect of HA on enhanced cathepsin K expression induced by mechanical stress loading. A human chondrocytic HCS-2/8 cells were cultured in silicon chambers and subjected to cyclic tensile stress (CTS) loading. CTS loading significantly increased messenger ribonucleic acid and protein expression of cathepsin K, which appeared to be suppressed by pre-treatment with HMW-HA. Activation of nuclear factor-kappa B (NF-κB) was induced by CTS loading, and suppressed by pre-treatment with HMW-HA. Helenalin, a chemical inhibitor of NF-κB, clearly suppressed the enhanced expression of cathepsin K, as well as NF-κB activation induced by CTS loading. The suppressive effect of HMW-HA on enhanced cathepsin K expression via NF-κB inhibition impacts the effectiveness of HMW-HA in OA treatment. Our findings provide new evidence supporting the biological effectiveness of intra-articular HMW-HA injections for treatment of OA.

Inhibition of CD44 intracellular domain production suppresses bovine articular chondrocyte de-differentiation induced by excessive mechanical stress loading

CD44 fragmentation is enhanced in chondrocytes of osteoarthritis (OA) patients. We hypothesized that mechanical stress-induced enhancement of CD44-intracellular domain (CD44-ICD) production plays an important role in the de-differentiation of chondrocytes and OA. This study aimed to assess the relationship between CD44-ICD and chondrocyte gene expression. Monolayer cultured primary bovine articular chondrocytes (BACs) were subjected to cyclic tensile strain (CTS) loading. ADAM10 inhibitor (GI254023X) and γ-secretase inhibitor (DAPT) were used to inhibit CD44 cleavage. In overexpression experiments, BACs were electroporated with a plasmid encoding CD44-ICD. CTS loading increased the expression of ADAM10 and subsequent CD44 cleavage, while decreasing the expression of SOX9, aggrecan, and type 2 collagen (COL2). Overexpression of CD44-ICD also resulted in decreased expression of these chondrocyte genes. Both GI254023X and DAPT reduced the production of CD44-ICD upon CTS loading, and significantly rescued the reduction of SOX9 expression by CTS loading. Chemical inhibition of CD44-ICD production also rescued aggrecan and COL2 expression following CTS loading. Our findings suggest that CD44-ICD is closely associated with the de-differentiation of chondrocytes. Excessive mechanical stress loading promoted the de-differentiation of BACs by enhancing CD44 cleavage and CD44-ICD production. Suppression of CD44 cleavage has potential as a novel treatment strategy for OA.

LRP receptors in chondrocytes are modulated by simulated microgravity and cyclic hydrostatic pressure

Mechanical loading is essential for the maintenance of musculoskeletal homeostasis. Cartilage has been demonstrated to be highly mechanoresponsive, but the mechanisms by which chondrocytes respond to mechanical stimuli are not clearly understood. The goal of the study was to determine how LRP4, LRP5, and LRP6 within canonical Wnt-signaling are regulated in simulated microgravity and cyclic hydrostatic pressure, and to investigate the potential role of LRP 4/5/6 in cartilage degeneration. Rat chondrosacroma cell (RCS) pellets were stimulated using either cyclic hydrostatic pressure (1Hz, 7.5 MPa, 4hr/day) or simulated microgravity in a rotating wall vessel (RWV) bioreactor (11RPM, 24hr/day). LRP4/5/6 mRNA expression was assessed by RT-qPCR and LRP5 protein expression was determined by fluorescent immunostaining. To further evaluate our in vitro findings in vivo, mice were subjected to hindlimb suspension for 14 days and the femoral heads stained for LRP5 expression. We found that, in vitro, LRP4/5/6 mRNA expression is modulated in a time-dependent manner by mechanical stimulation. Additionally, LRP5 protein expression is upregulated in response to both simulated microgravity and cyclic hydrostatic pressure. LRP5 is also upregulated in vivo in the articular cartilage of hindlimb suspended mice. This is the first study to examine how LRP4/5/6, critical receptors within musculoskeletal biology, respond to mechanical stimulation. Further elucidation of this mechanism could provide significant clinical benefit for the identification of pharmaceutical targets for the maintenance of cartilage health.

Wnt and RUNX2 mediate cartilage breakdown by osteoarthritis synovial fibroblast-derived ADAMTS-7 and -12

Failure of therapeutic approaches for the treatment of osteoarthritis (OA) based on the inhibition of metalloproteinases, might be because of their constitutive expression in homeostasis, together with their network complexity. The knowledge of this network would contribute to selective target pathological conditions. In this sense, blockade of mediators produced by neighbouring joint cells, such as synovial fibroblasts (SF), would prevent cartilage damage. Thus, we studied the contribution of ADAMTS-7 and -12 from SF to cartilage oligomeric matrix protein (COMP) degradation, and the signalling pathways involved in their expression. We report for the first time in SF, the involvement of ERK-Runx2 axis and Wnt/β-catenin signalling in ADAMTS-12 and ADAMTS-7 expressions, respectively, with the subsequent consequences in COMP degradation from cartilage extracellular matrix. After stimulation with IL-1β or fibronectin fragments, we showed that ERK inhibition decreased Runx2 activation and ADAMTS-12 expression in OA-SF, also reducing Fn-fs-induced COMP degradation. Blockage of Wnt signalling by DKK1 reduced ADAMTS-7 and COMP degradation in OA-SF as well. In addition, Wnt7B expression was induced by IL-1β and by itself, also increasing ADAMTS-7. Our results could contribute to the development of disease-modifying OA drugs targeting ADAMTS-7 and -12 for the prevention of extracellular matrix components degradation like COMP.

Hypergravity down-regulates c-fos gene expression via ROCK/Rho-GTP and the PI3K signaling pathway in murine ATDC5 chondroprogenitor cells

Chondrocytes are known to be physiologically loaded with diverse physical factors such as compressive stress, shear stress and hydrostatic pressure. Although the effects of those mechanical stimuli onto various cell models have been widely studied, those of hypergravity have not yet been revealed clearly. Hereby, we hypothesized that the hypergravity affects relative positions of intracellular elements including nucleus and cytoskeletons due to their density differences, triggering mechanotransduction in the cell. The aim of this study was to investigate the effect of hypergravity on c-fos expression in the murine ATDC5 chondroprogenitor cells, as c-fos is a well known key regulator of cell proliferation and differentiation, including in chondrocytes. We first found that hypergravity down-regulated c-fos expression transiently via ROCK/Rho-GTP and PI3K signaling, and the down-regulation was suppressed by inhibition of actin polymerization.

IFN-α Negatively Regulates the Expression of Endothelial Nitric Oxide Synthase and Nitric Oxide Production: Implications for Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a known risk factor for endothelial dysfunction. Murine and human lupus studies revealed a role for IFN-α in vascular abnormalities associated with impaired blood vessel dilation. However, the impact of IFN-α on mediators that induce vasodilation and modulate inflammation, including endothelial NO synthase (eNOS) and NO bioavailability, are unknown. The objectives of this study were to determine how IFN-α promotes endothelial dysfunction in SLE, focusing on its regulation of eNOS and NO production in endothelial cells. We demonstrate that IFN-α promotes an endothelial dysfunction signature in HUVECs that is characterized by transcription suppression and mRNA instability of eNOS complemented by upregulation of MCP1 and VCAM1 These changes are associated with IFN-inducible gene expression. IFN-α impairs insulin-mediated NO production, and altered gene expression resulted from eNOS instability, possibly due to enhanced miR-155 expression. IFN-α significantly impaired NO production in insulin-stimulated HUVECs. IFN-α treatment also led to enhanced neutrophil adhesion. Our study introduces a novel pathway by which IFN-α serves as a proatherogenic mediator through repression of eNOS-dependent pathways. This could promote the development of endothelial dysfunction and cardiovascular disease in SLE.

Emerging Roles of circRNA Related to the Mechanical Stress in Human Cartilage Degradation of Osteoarthritis

Circular RNAs (circRNAs) are involved in the development of various diseases; however, knowledge on circRNAs in osteoarthritis (OA) is limited. This study aims to identify circRNA expression in different regions affected by OA and to explore the function of mechanical stress-related circRNAs (circRNAs-MSR) in cartilage. Bioinformatics was employed to predict the interaction of circRNAs and mRNAs in the cartilage. Loss-of-function experiments for circRNAs-MSR were performed in vitro. A total of 104 circRNAs were differentially expressed in damaged versus intact cartilage. Of these circRNAs, 44 and 60 were upregulated and downregulated, respectively, in the damaged tissue. circRNA-MSR expression increased under mechanical stress in chondrocytes. circRNAs-MSR were silenced using small interfering RNA, and knockdown of circRNAs-MSR could suppress tumor necrosis factor alpha (TNF-α) expression and increase extracellular matrix (ECM) formation. Our results demonstrated that circRNAs-MSR regulated TNF-α expression and participated in the chondrocyte ECM degradation process. We propose that the inhibition of circRNAs-MSR could inhibit the degradation of chondrocyte ECM and knockdown of circRNAs-MSR could be a potential therapeutic target for OA.

Effects of mechanical stress on chondrocyte phenotype and chondrocyte extracellular matrix expression

Mechanical factors play a key role in regulating the development of cartilage degradation in osteoarthritis. This study aimed to identify the influence of mechanical stress in cartilage and chondrocytes. To explore the effects of mechanical stress on cartilage morphology, we observed cartilages in different regions by histological and microscopic examination. Nanoindentation was performed to assess cartilage biomechanics. To investigate the effects of mechanical stress on chondrocytes, cyclic tensile strain (CTS, 0.5 Hz, 10%) was applied to monolayer cultures of human articular chondrocytes by using Flexcell-5000. We quantified the mechanical properties of chondrocytes by atomic force microscopy. Chondrocytes were stained with Toluidine blue and Alcian blue after exposure to CTS. The expression of extracellular matrix (ECM) molecules was detected by qPCR and immunofluorescence analyses in chondrocytes after CTS. Our results demonstrated distinct morphologies and mechanical properties in different cartilage regions. In conclusion, mechanical stress can affect the chondrocyte phenotype, thereby altering the expression of chondrocyte ECM.

The TMSB4 Pseudogene LncRNA Functions as a Competing Endogenous RNA to Promote Cartilage Degradation in Human Osteoarthritis

Mechanical stress plays a key role in the development of cartilage degradation in osteoarthritis (OA). Nevertheless, the role of long noncoding RNAs in mechanical stress-induced regulation of chondrocytes remains unclear. The aim of this study was to explore the function of mechanical stress-related long noncoding RNAs in cartilage. Tissue samples were collected from 50 patients and chondrocytes were exposed to cyclic tensile strain (CTS). A total of 107 lncRNAs were differentially expressed in damaged cartilage versus intact cartilage. Of these lncRNAs, 51 were upregulated and 56 were downregulated in the damaged tissue. The TMSB4 pseudogene, lncRNA-MSR, was upregulated in the damaged cartilage and was activated in chondrocytes in response to mechanical stress. Furthermore, lncRNA-MSR regulated the expression of TMSB4 by competing with miRNA-152 in chondrocytes. Our results demonstrated that upregulation of lncRNA-MSR initiates pathological changes that lead to cartilage degradation, and the inhibition of lncRNA-MSR could represent a potential therapeutic target for OA.

Healthy and Osteoarthritic Synovial Fibroblasts Produce a Disintegrin and Metalloproteinase with Thrombospondin Motifs 4, 5, 7, and 12: Induction by IL-1β and Fibronectin and Contribution to Cartilage Damage

Current description of osteoarthritis includes the involvement of synovial inflammation. Studies contributing to understanding the mechanisms of cross-talk and feedback among the joint tissues could be relevant to the development of therapies that block disease progression. During osteoarthritis, synovial fibroblasts exposed to anomalous mechanical forces and an inflammatory microenvironment release factors such as a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) metalloproteinases that mediate tissue damage and perpetuate inflammation. We therefore studied the production of ADAMTS by synovial fibroblasts and their contribution to cartilage degradation. Moreover, we analyzed the implication of two mediators present in the osteoarthritis joint, IL-1β as proinflammatory cytokine, and 45-kDa fibronectin fragments as products of matrix degradation. We reported that synovial fibroblasts constitutively express and release ADAMTS 4, 5, 7, and 12. Despite the contribution of both mediators to the stimulation of Runx2 and Wnt/β-catenin signaling pathways, as well as to ADAMTS expression, promoting the degradation of aggrecan and cartilage oligomeric matrix protein from cartilage, fibronectin fragments rather than IL-1β played the major pathological role in osteoarthritis, contributing to the maintenance of the disease. Moreover, higher levels of ADAMTS 4 and 7 and a specific regulation of ADAMTS-12 were observed in osteoarthritis, suggesting them as new potential therapeutic targets. Therefore, synovial fibroblasts provide the biochemical tools to the chronicity and destruction of the osteoarthritic joints.

Importance of reference gene selection for articular cartilage mechanobiology studies

OBJECTIVE

Identification of genes differentially expressed in mechano-biological pathways in articular cartilage provides insight into the molecular mechanisms behind initiation and/or progression of osteoarthritis (OA). Quantitative PCR (qPCR) is commonly used to measure gene expression, and is reliant on the use of reference genes for normalisation. Appropriate validation of reference gene stability is imperative for accurate data analysis and interpretation. This study determined in vitro reference gene stability in articular cartilage explants and primary chondrocytes subjected to different compressive loads and tensile strain, respectively.

DESIGN

The expression of eight commonly used reference genes (18s, ACTB, GAPDH, HPRT1, PPIA, RPL4, SDHA and YWHAZ) was determined by qPCR and data compared using four software packages (comparative delta-Ct method, geNorm, NormFinder and BestKeeper). Calculation of geometric means of the ranked weightings was carried out using RefFinder.

RESULTS

Appropriate reference gene(s) for normalisation of mechanically-regulated transcript levels in articular cartilage tissue or isolated chondrocytes were dependent on experimental set-up. SDHA, YWHAZ and RPL4 were the most stable genes whilst glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and to a lesser extent Hypoxanthine-guanine phosphoribosyltransferase (HPRT), showed variable expression in response to load, demonstrating their unsuitability in such in vitro studies. The effect of using unstable reference genes to normalise the expression of aggrecan (ACAN) and matrix metalloproteinase 3 (MMP3) resulted in inaccurate quantification of these mechano-sensitive genes and erroneous interpretation/conclusions.

CONCLUSION

This study demonstrates that commonly used 'reference genes' may be unsuitable for in vitro cartilage chondrocyte mechanobiology studies, reinforcing the principle that careful validation of reference genes is essential prior to each experiment to obtain robust and reproducible qPCR data for analysis/interpretation.

Lentiviral vector-mediated over-expression of Sox9 protected chondrocytes from IL-1β induced degeneration and apoptosis

To explore whether the over-expression of Sry-related HMG box (Sox9) in degenerative chondrocytes is able to improve cell regeneration and protects cells from inflammation induced apoptosis, we generated a Sox9 over-expressing vector delivery system in which the Sox9 gene was inserted into a lentiviral vector. After infecting mouse chondrocytes with the Sox9-encoding vector, we observed a high level of gene transduction efficiency and achieved a high level of Sox9 expression in the infected chondrocytes. To explore whether over-expression of Sox9 is able to induce cell regeneration and improve cell survival, we induced Sox9 over-expression by lentiviral vector infection 48 hours before IL-1β treatment. The cells were infected with the reporter gene GFP-encoded lentiviral vector as a negative control or left uninfected. 48-hours after IL-1β treatment, the chrondrocytes treated with IL-1β alone, underwent a degenerative process, with elevated expression of MMP-3, MMP-13, ADAMTS-5 and ALP, but the cell specific anabolic proteins collagen II and aggrecan were significantly suppressed. The cells infected with the GFP reporter vector had no increased regeneration after IL-1β treatment. The results indicated that Sox9 is an important chondrocyte transcription factor, promoting chondrocyte regeneration and cell survival, which were mediated through affecting multiple cell differentiation as well as anti-apoptotic signaling pathways.

Effects of cyclic tensile strain on chondrocyte metabolism: a systematic review

Chondrocytes reorganize the extracellular matrix of articular cartilage in response to externally applied loads. Thereby, different loading characteristics lead to different biological responses. Despite of active research in this area, it is still unclear which parts of the extracellular matrix adapt in what ways, and how specific loading characteristics affect matrix changes. This review focuses on the influence of cyclic tensile strain on chondrocyte metabolism in vitro. It also aimed to identify anabolic or catabolic chondrocyte responses to different loading protocols. The key findings show that loading cells up to 3% strain, 0.17 Hz, and 2 h, resulted in weak or no biological responses. Loading between 3–10% strain, 0.17–0.5 Hz, and 2–12 h led to anabolic responses; and above 10% strain, 0.5 Hz, and 12 h catabolic events predominated. However, this review also discusses that various other factors are involved in the remodeling of the extracellular matrix in response to loading, and that parameters like an inflammatory environment might influence the biological response.

Gene expression differences between ruptured anterior cruciate ligaments in young male and female subjects

BACKGROUND

The incidence of anterior cruciate ligament (ACL) injuries is two to eightfold greater in female compared with male athletes. Anatomic, hormonal, and neuromuscular factors have been associated with this disparity. This study compared gene expression and structural features in ruptured but otherwise normal ACL tissue from young female and male athletes.

METHODS

A biopsy sample of ruptured ACL tissue (which would normally have been discarded) was obtained intraoperatively from seven female and seven male athletes (12.7 to 22.6 years old). Each sample was divided into portions for histological and gene expression analyses. Specimens for gene analysis were frozen and ground, and RNA was extracted and purified. Microarray analysis was performed on RNA isolated from four female and three male study participants (13.9 to 18.5 years old) who had a noncontact injury. Genes with an expression level that differed significantly between these female and male athletes were grouped into functionally associated networks with use of IPA software (Qiagen). Three genes of interest were chosen for further validation by RT-qPCR (reverse transcription-quantitative polymerase chain reaction) analysis of the samples from all fourteen patients. Several statistical methods were used to examine sex-related differences.

RESULTS

Microarray analysis of the RNA isolated from the ruptured ACL tissue from the female and male athletes identified thirty-two genes with significant differential expression. Fourteen of these genes were not linked to the X or Y chromosome. IPA analysis grouped these genes into pathways involving development and function of skeletal muscle and growth, maintenance, and proliferation of cells. RT-qPCR confirmed significant differences in expression of three selected genes: ACAN (aggrecan) and FMOD (fibromodulin) were upregulated in female compared with male study participants, and WISP2 (WNT1 inducible signaling pathway protein 2) was downregulated. No morphological differences among the ruptured tissue from the various participants were apparent on histological examination.

CONCLUSIONS

The genes identified in this study as differing distinctly according to sex produce major molecules in the ACL extracellular matrix. Significant upregulation of ACAN and FMOD (which regulate the matrix) and downregulation of WISP2 (which is involved in collagen turnover and production) may account for the weaker ACLs in female compared with male individuals.

Mechanical forces induce changes in VEGF and VEGFR-1/sFlt-1 expression in human chondrocytes

Expression of the pro-angiogenic vascular endothelial growth factor (VEGF) stimulates angiogenesis and correlates with the progression of osteoarthritis. Mechanical joint loading seems to contribute to this cartilage pathology. Cyclic equibiaxial strains of 1% to 16% for 12 h, respectively, induced expression of VEGF in human chondrocytes dose- and frequency-dependently. Stretch-mediated VEGF induction was more prominent in the human chondrocyte cell line C-28/I2 than in primary articular chondrocytes. Twelve hours of 8% stretch induced VEGF expression to 175% of unstrained controls for at least 24 h post stretching, in promoter reporter and enzyme-linked immunosorbent assay (ELISA) studies. High affinity soluble VEGF-receptor, sVEGFR-1/sFlt-1 was less stretch-inducible than its ligand, VEGF-A, in these cells. ELISA assays demonstrated, for the first time, a stretch-mediated suppression of sVEGFR-1 secretion 24 h after stretching. Overall, strained chondrocytes activate their VEGF expression, but in contrast, strain appears to suppress the secretion of the major VEGF decoy receptor (sVEGFR-1/sFlt-1). The latter may deplete a biologically relevant feedback regulation to inhibit destructive angiogenesis in articular cartilage. Our data suggest that mechanical stretch can induce morphological changes in human chondrocytes in vitro. More importantly, it induces disturbed VEGF signaling, providing a molecular mechanism for a stress-induced increase in angiogenesis in cartilage pathologies.

Protective role of frizzled-related protein B on matrix metalloproteinase induction in mouse chondrocytes

Introduction

Our objective was to investigate whether a lack of frizzled-related protein B (FrzB), an extracellular antagonist of the Wnt signaling pathways, could enhance cartilage degradation by facilitating the expression, release and activation of matrix metalloproteinases (MMPs) by chondrocytes in response to tissue-damaging stimuli.

Methods

Cartilage explants from FrzB^−/− and wild-type mice were challenged by excessive dynamic compression (0.5 Hz and 1 MPa for 6 hours). Load-induced glycosaminoglycan (GAG) release and MMP enzymatic activity were assessed. Interleukin-1β (IL-1β) (10, 100 and 1000 pg/mL for 24 hours) was used to stimulate primary cultures of articular chondrocytes from FrzB^−/− and wild-type mice. The expression and release of MMP-3 and −13 were determined by RT-PCR, western blot and ELISA. The accumulation of β-catenin was assessed by RT-PCR and western blot.

Results

Cartilage degradation, as revealed by a significant increase in GAG release (2.8-fold, P = 0.014) and MMP activity (4.5-fold, P = 0.014) by explants, was induced by an excessive load. Load-induced MMP activity appeared to be enhanced in FrzB^−/− cartilage explants compared to wild-type (P = 0.17). IL-1β dose-dependently induced Mmp-13 and −3 gene expression and protein release by cultured chondrocytes. IL-1β-mediated increase in MMP-13 and −3 was slightly enhanced in FrzB^−/− chondrocytes compared to wild-type (P = 0.05 and P = 0.10 at gene level, P = 0.17 and P = 0.10 at protein level, respectively). Analysis of Ctnn1b and Lef1 gene expression and β-catenin accumulation at protein level suggests that the enhanced catabolic response of FrzB^−/− chondrocytes to IL-1β and load may be associated with an over-stimulation of the canonical Wnt/β-catenin pathway.

Conclusions

Our results suggest that FrzB may have a protective role on cartilage degradation and MMP induction in mouse chondrocytes by attenuating deleterious effects of the activation of the canonical Wnt/β-catenin pathway.

Wnt/β-catenin signaling regulates the proliferation and differentiation of mesenchymal progenitor cells through the p53 pathway

Objective

Mesenchymal progenitor cells (MPCs) are found in articular cartilage from normal controls and patients with osteoarthritis (OA). Nevertheless, the molecular mechanisms of the proliferation and differentiation of these cells remain unclear. In this study, we aimed to determine the involvement of Wnt/β-catenin signaling in regulating the proliferation and differentiation of MPCs.

Methods

MPCs were isolated from the articular cartilage of normal and OA patients. Cells were sorted by immunomagnetic cell separation. Cell proliferation capacity was evaluated using the MTT assay. Toluidine blue staining and immunostaining with anti-collagen II or anti-aggrecan antibodies were used to determine the chondrogenic differentiation capabilities of MPCs. The mRNA and protein expression of target genes were examined by quantitative real-time polymerase chain reaction and Western blotting, respectively. Knock-down of p53 expression was achieved with RNA interference.

Results

Most cells isolated from the normal and OA patients were CD105⁺ and CD166⁺ positive (Normal subjects: CD105⁺/CD166⁺, 94.6%±1.1%; OA: CD105⁺/CD166⁺, 93.5%±1.1%). MPCs derived from OA subjects exhibited decreased differentiation capabilities and enhanced Wnt/β-catenin activity. Inhibition of Wnt/β-catenin signaling promoted proliferation and differentiation, whereas activation of this pathway by treatment with rWnt3a protein decreased the proliferation and differentiation of normal MPCs. Additionally, Wnt/β-catenin signaling positively regulated p53 expression, and silencing of p53 increased proliferation and differentiation of MPCs.

Conclusions

Wnt/β-catenin regulated the proliferation and differentiation of MPCs through the p53 pathway.

Verapamil protects against cartilage degradation in osteoarthritis by inhibiting Wnt/β-catenin signaling

In past years, the canonical Wnt/β-catenin signaling pathway has emerged as a critical regulator of cartilage development and homeostasis. FRZB, a soluble antagonist of Wnt signaling, has been studied in osteoarthritis (OA) animal models and OA patients as a modulator of Wnt signaling. We screened for FDA-approved drugs that induce FRZB expression and suppress Wnt/β-catenin signaling. We found that verapamil, a widely prescribed L-type calcium channel blocker, elevated FRZB expression and suppressed Wnt/β-catenin signaling in human OA chondrocytes. Expression and nuclear translocation of β-catenin was attenuated by verapamil in OA chondrocytes. Lack of the verapamil effects in LiCl-treated and FRZB-downregulated OA chondrocytes also suggested that verpamil suppressed Wnt signaling by inducing FRZB. Verapamil enhanced gene expressions of chondrogenic markers of ACAN encoding aggrecan, COL2A1 encoding collagen type II α1, and SOX9, and suppressed Wnt-responsive AXIN2 and MMP3 in human OA chondrocytes. Verapamil ameliorated Wnt3A-induced proteoglycan loss in chondrogenically differentiated ATDC5 cells. Verapamil inhibited hypertrophic differentiation of chondrocytes in the explant culture of mouse tibiae. Intraarticular injection of verapamil inhibited OA progression as well as nuclear localizations of β-catenin in a rat OA model. We propose that verapamil holds promise as a potent therapeutic agent for OA by upregulating FRZB and subsequently downregulating Wnt/β-catenin signaling.

GREM1, FRZB and DKK1 mRNA levels correlate with osteoarthritis and are regulated by osteoarthritis-associated factors

Introduction

Osteoarthritis is, at least in a subset of patients, associated with hypertrophic differentiation of articular chondrocytes. Recently, we identified the bone morphogenetic protein (BMP) and wingless-type MMTV integration site (WNT) signaling antagonists Gremlin 1 (GREM1), frizzled-related protein (FRZB) and dickkopf 1 homolog (Xenopus laevis) (DKK1) as articular cartilage’s natural brakes of hypertrophic differentiation. In this study, we investigated whether factors implicated in osteoarthritis or regulation of chondrocyte hypertrophy influence GREM1, FRZB and DKK1 expression levels.

Methods

GREM1, FRZB and DKK1 mRNA levels were studied in articular cartilage from healthy preadolescents and healthy adults as well as in preserved and degrading osteoarthritic cartilage from the same osteoarthritic joint by quantitative PCR. Subsequently, we exposed human articular chondrocytes to WNT, BMP, IL-1β, Indian hedgehog, parathyroid hormone-related peptide, mechanical loading, different medium tonicities or distinct oxygen levels and investigated GREM1, FRZB and DKK1 expression levels using a time-course analysis.

Results

GREM1, FRZB and DKK1 mRNA expression were strongly decreased in osteoarthritis. Moreover, this downregulation is stronger in degrading cartilage compared with macroscopically preserved cartilage from the same osteoarthritic joint. WNT, BMP, IL-1β signaling and mechanical loading regulated GREM1, FRZB and DKK1 mRNA levels. Indian hedgehog, parathyroid hormone-related peptide and tonicity influenced the mRNA levels of at least one antagonist, while oxygen levels did not demonstrate any statistically significant effect. Interestingly, BMP and WNT signaling upregulated the expression of each other’s antagonists.

Conclusions

Together, the current study demonstrates an inverse correlation between osteoarthritis and GREM1, FRZB and DKK1 gene expression in cartilage and provides insight into the underlying transcriptional regulation. Furthermore, we show that BMP and WNT signaling are linked in a negative feedback loop, which might prove essential in articular cartilage homeostasis by balancing BMP and WNT activity.

Rac1 mediates load-driven attenuation of mRNA expression of nerve growth factor beta in cartilage and chondrocytes

OBJECTIVES

To determine effect of gentle loads applied to the knee on mRNA expression of nerve growth factor, particularly, the active beta subunit (NGFβ) in cartilage and chondrocyte.

METHODS

Cyclic compressive loads in vivo and fluid flow in vitro were used to determine the mRNA levels. Alteration of Rac1 GTPase as well as effect of salubrinal, a specific inhibitor of eIF2α phosphatase was assessed using fluorescence resonance energy transfer (FRET)-based Rac1 biosensor.

RESULTS

Knee loading at 1 N reduced mRNA levels of NGFβ and its low affinity receptor, p75 in cartilage and subchondral bone. In cartilage, knee loading at 1 N reduced the phosphorylation level of p38 MAPK (p38-p) and activity of Rac1 GTPase. Consistent with in vivo results, fluid flow at 5 and 10 dyn/cm(2) reduced mRNA levels of NGFβ and p75 in C28/I2 human chondrocytes. SB203580, which decreases p38-p, reduced the mRNA levels of NGFβ and p75. Silencing Rac1 by siRNA decreased the levels of p38-p and NGFβ mRNA but not p75. Furthermore, administration of salubrinal reduced FRET-based activity of Rac1 as well as the mRNA levels of NGFβ and p75.

CONCLUSIONS

These results provide evidence that mechanical stimulation and salubrinal may attenuate pain perception-linked NGFβ signaling through Rac1-mediated p38 MAPK.

Transcriptomics of wild-type mice and mice lacking ADAMTS-5 activity identifies genes involved in osteoarthritis initiation and cartilage destruction

OBJECTIVE

To identify changes in gene expression in mice with osteoarthritis (OA) in order to explore the mechanisms of the disease.

METHODS

Gene expression profiling was performed in cartilage from mice with surgically induced OA. We used wild-type (WT) mice and Adamts5Δcat mice, in which ADAMTS-5 activity is lacking and aggrecan loss and cartilage erosion are inhibited, to distinguish gene expression changes that are independent of ADAMTS-5 activity and cartilage breakdown. Mechanical instability was introduced into the knee joints of 10-week-old male mice via surgical destabilization of the medial meniscus (DMM). Cartilage from the developing lesion in the destabilized medial meniscus and corresponding regions in sham-operated joints was harvested by microdissection at 1, 2, and 6 weeks postsurgery, and RNA was extracted, amplified, and hybridized to whole-genome microarrays.

RESULTS

Several previously identified OA-related genes, including Ptgs2, Crlf1, and Inhba, and novel genes, such as Phdla2 and Il11, were up-regulated in both WT mice and Adamts5Δcat mice, indicating that they are independent of ADAMTS-5 activity. The altered expression of other genes, including Col10a1, the sentinel marker of cartilage hypertrophy, and Wnt/β-catenin pathway genes, required ADAMTS-5 activity. Cell death pathway genes were dysregulated, and Tp53, Foxo4, and Xbp1 endoplasmic reticulum-stress transcriptional networks were activated. Analysis of degradome genes identified up-regulation of many proteases, including Mmp3, Capn2, and the novel cartilage proteases Prss46 and Klk8. Comparison with other studies identified 16 genes also dysregulated in rat and human OA as priorities for study.

CONCLUSION

We have identified, for the first time, several genes that have an ADAMTS-5-independent role in OA, identifying them as possible OA initiation candidates. This work provides new insights into the sequence of gene dysregulation and the molecular basis of cartilage destruction in OA.

Stimulation of superficial zone protein accumulation by hedgehog and Wnt signaling in surface zone bovine articular chondrocytes

OBJECTIVE

To determine the roles of the hedgehog and Wnt signaling pathways in accumulation of superficial zone protein (SZP) in surface zone articular chondrocytes.

METHODS

Explant cultures of disks of surface zone cartilage or isolated chondrocytes from the surface zone of articular cartilage of bovine stifle joints were cultured in serum-free chemically defined medium. Accumulation of SZP in the culture medium, in response to hedgehog proteins (sonic hedgehog [SHH] and Indian hedgehog [IHH]), Wnt proteins (Wnt-3a, Wnt-5a, and Wnt-11), agonists of the Wnt/β-catenin pathway (glycogen synthase kinase 3β [GSK-3β] inhibitors), and antagonists of the Wnt/β-catenin pathway, was investigated. The interaction between transforming growth factor β1 (TGFβ1) and hedgehog proteins or antagonists of the Wnt/β-catenin pathway was also investigated.

RESULTS

Hedgehog proteins stimulated SZP accumulation. Activation of the Wnt/β-catenin pathway by Wnt-3a and GSK-3β inhibitors led to inhibition of SZP accumulation; however, Wnt-5a and Wnt-11 had no influence on SZP accumulation. Conversely, antagonists of the Wnt/β-catenin pathway stimulated SZP accumulation. In addition, there were combinatorial effects of TGFβ1 and hedgehog proteins or antagonists of the Wnt/β-catenin pathway on SZP accumulation.

CONCLUSION

SHH and IHH signaling has a stimulatory effect on SZP accumulation in surface zone cartilage and isolated articular chondrocytes. These findings provide insight into the regulatory mechanisms of articular cartilage homeostasis and maintenance by morphogens.

Cartilage development and degeneration: a Wnt Wnt situation

The Wnt signaling pathway plays a crucial role in the development and homeostasis of a variety of adult tissues and, as such, is emerging as an important therapeutic target for numerous diseases. Factors involved in the Wnt pathway are expressed throughout limb development and chondrogenesis and have been shown to be critical in joint homeostasis and endochondral ossification. Therefore, in this review, we discuss Wnt regulation of chondrogenic differentiation, hypertrophy and cartilage function. Moreover, we detail the role of the Wnt signaling pathway in cartilage degeneration and its potential to act as a target for therapy in osteoarthritis.