The secreted glycoprotein lubricin protects cartilage surfaces and inhibits synovial cell overgrowth

Primary Cilia Direct Murine Articular Cartilage Tidemark Patterning Through Hedgehog Signaling and Ambulatory Load

Articular cartilage (AC) is essential for body movement but is highly susceptible to degenerative diseases and has poor self-repair capacity. To improve current subpar regenerative treatments, developmental mechanisms of AC should be clarified and, specifically, how its postnatal multizone organization is acquired. Primary cilia are cell surface organelles crucial for mammalian tissue morphogenesis. Although their importance for chondrocyte function is appreciated, their specific roles in postnatal AC morphogenesis remain unclear. To explore these mechanisms, we used a murine conditional loss-of-function approach (Ift88-flox) targeting joint-lineage progenitors (Gdf5Cre) and monitored postnatal knee AC development. Joint formation and growth up to juvenile stages were largely unaffected. However, mature AC (aged 2 months) exhibited disorganized extracellular matrix, decreased aggrecan and collagen II due to reduced gene expression (not increased catabolism), and marked reduction of AC modulus by 30%-50%. In addition, and unexpectedly, we discovered that tidemark patterning was severely disrupted, as was hedgehog signaling, and exhibited specificity based on regional load-bearing functions of AC. Interestingly, Prg4 expression was markedly increased in highly loaded sites in mutants. Together, our data provide evidence that primary cilia orchestrate postnatal AC morphogenesis including tidemark topography, zonal matrix composition, and ambulation load responses. © 2022 American Society for Bone and Mineral Research (ASBMR).

The superficial zone of articular cartilage

The superficial zone of articular cartilage contributes to smooth joint motion through the production of proteoglycan 4 (PRG4), also known as lubricin. Recent studies indicate novel effects of PRG4 as a signaling molecule, other than a simple extracellular matrix protein. Additionally, the accumulating evidence displays that various molecules and signaling pathways are involved in regulating the superficial zone and PRG4 expression. In addition, Prg4-expressing cells include a progenitor population of articular chondrocytes. Several non-clinical and clinical studies have shown that PRG4 and related molecules are promising candidates for disease-modifying drugs for treating osteoarthritis. Since PRG4 is also expressed in the synovium, tendons, and ligaments, further studies of PRG4-related pathways and PRG4-positive cells may elucidate the mechanisms underlying joint homeostasis.

Glycosaminoglycans and glycoproteins influence the elastic response of synovial fluid nanofilms on model oxide surfaces

Synovial fluid (SF) is the natural lubricant found in articulated joints, providing unique cartilage surface protecting films under confinement and relative motion. While it is known that the synergistic interactions of the macromolecular constituents provide its unique load-bearing and tribological performance, it is not fully understood how two of the main constituents, glycosaminoglycans (GAGs) and glycoproteins, regulate the formation and mechanics of robust load-bearing films. Here, we present evidence that the load-bearing capabilities, rather than the tribological performance, of the formed SF films depend strongly on its components' integrity. For this purpose, we used a combination of enzymatic treatments, quartz crystal microbalance with dissipation (QCM-D), and the surface forces apparatus (SFA) to characterize the formation and load-bearing capabilities of SF films on model oxide (i.e., silicates) surfaces. We find that, upon cleavage of proteins, the elasticity of the films is reduced and that cleaving GAGs results in irreversible (plastic) molecular re-arrangements of the film constituents when subjected to confinement. Understanding thin film mechanics of SF can provide insight into the progression of diseases, such as arthritis, but may also be applicable to the development of new implant surface treatments or new biomimetic lubricants.

A Mucin-Deficient Ocular Surface Mimetic Platform for Interrogating Drug Effects on Biolubrication, Antiadhesion Properties, and Barrier Functionality

Dry eye disease (DED) affects more than 100 million people worldwide, causing significant patient discomfort and imposing a multi-billion-dollar burden on global health care systems. In DED patients, the natural biolubrication process that facilitates pain-free blinking goes awry due to an imbalance of lipids, aqueous medium, and mucins in the tear film, resulting in ocular surface damage. Identifying strategies to reduce adhesion and shear stresses between the ocular surface and the conjunctival cells lining the inside of the eyelid during blink cycles is a promising approach to improve the signs and symptoms of DED. However, current preclinical models for screening ocular lubricants rely on scarce, heterogeneous tissue samples or model substrates that do not capture the complex biochemical and biophysical cues present at the ocular surface. To recapitulate the hierarchical architecture and phenotype of the ocular interface for preclinical drug screening, we developed an in vitro mucin-deficient DED model platform that mimics the complexity of the ocular interface and investigated its utility in biolubrication, antiadhesion, and barrier protection studies using recombinant human lubricin, a promising investigational therapy for DED. The biomimetic platform recapitulated the pathological changes in biolubrication, adhesion, and barrier functionality often observed in mucin-deficient DED patients and demonstrated that recombinant human lubricin can reverse the damage induced by mucin loss in a dose- and conformation-dependent manner. Taken together, these results highlight the potential of the platform─and recombinant human lubricin─in advancing the standard of care for mucin-deficient DED patients.

Synovial Macrophage and Fibroblast Heterogeneity in Joint Homeostasis and Inflammation

The synovial tissue is an immunologically challenging environment where, under homeostatic conditions, highly specialized subsets of immune-regulatory macrophages and fibroblasts constantly prevent synovial inflammation in response to cartilage- and synovial fluid-derived danger signals that accumulate in response to mechanical stress. During inflammatory joint diseases, this immune-regulatory environment becomes perturbed and activated synovial fibroblasts and infiltrating immune cells start to contribute to synovial inflammation and joint destruction. This review summarizes our current understanding of the phenotypic and molecular characteristics of resident synovial macrophages and fibroblasts and highlights their crosstalk during joint homeostasis and joint inflammation, which is increasingly appreciated as vital to understand the molecular basis of prevalent inflammatory joint diseases such as rheumatoid arthritis.

Regulatory Mechanisms of Prg4 and Gdf5 Expression in Articular Cartilage and Functions in Osteoarthritis

Owing to the rapid aging of society, the numbers of patients with joint disease continue to increase. Accordingly, a large number of patients require appropriate treatment for osteoarthritis (OA), the most frequent bone and joint disease. Thought to be caused by the degeneration and destruction of articular cartilage following persistent and excessive mechanical stimulation of the joints, OA can significantly impair patient quality of life with symptoms such as knee pain, lower limb muscle weakness, or difficulty walking. Because articular cartilage has a low self-repair ability and an extremely low proliferative capacity, healing of damaged articular cartilage has not been achieved to date. The current pharmaceutical treatment of OA is limited to the slight alleviation of symptoms (e.g., local injection of hyaluronic acid or non-steroidal anti-inflammatory drugs); hence, the development of effective drugs and regenerative therapies for OA is highly desirable. This review article summarizes findings indicating that proteoglycan 4 (Prg4)/lubricin, which is specifically expressed in the superficial zone of articular cartilage and synovium, functions in a protective manner against OA, and covers the transcriptional regulation of Prg4 in articular chondrocytes. We also focused on growth differentiation factor 5 (Gdf5), which is specifically expressed on the surface layer of articular cartilage, particularly in the developmental stage, describing its regulatory mechanisms and functions in joint formation and OA pathogenesis. Because several genetic studies in humans and mice indicate the involvement of these genes in the maintenance of articular cartilage homeostasis and the presentation of OA, molecular targeting of Prg4 and Gdf5 is expected to provide new insights into the aetiology, pathogenesis, and potential treatment of OA.

Quadruped Gait and Regulation of Apoptotic Factors in Tibiofemoral Joints following Intra-Articular rhPRG4 Injection in Prg4 Null Mice

Camptodactyly-arthropathy-coxa vara-pericarditis (CACP) syndrome leads to diarthrodial joint arthropathy and is caused by the absence of lubricin (proteoglycan 4—PRG4), a surface-active mucinous glycoprotein responsible for lubricating articular cartilage. In this study, mice lacking the orthologous gene Prg4 served as a model that recapitulates the destructive arthrosis that involves biofouling of cartilage by serum proteins in lieu of Prg4. This study hypothesized that Prg4-deficient mice would demonstrate a quadruped gait change and decreased markers of mitochondrial dyscrasia, following intra-articular injection of both hindlimbs with recombinant human PRG4 (rhPRG4). Prg4^−/− (N = 44) mice of both sexes were injected with rhPRG4 and gait alterations were studied at post-injection day 3 and 6, before joints were harvested for immunohistochemistry for caspase-3 activation. Increased stance and propulsion was shown at 3 days post-injection in male mice. There were significantly fewer caspase-3-positive chondrocytes in tibiofemoral cartilage from rhPRG4-injected mice. The mitochondrial gene Mt-tn, and myosin heavy (Myh7) and light chains (Myl2 and Myl3), known to play a cytoskeletal stabilizing role, were significantly upregulated in both sexes (RNA-Seq) following IA rhPRG4. Chondrocyte mitochondrial dyscrasias attributable to the arthrosis in CACP may be mitigated by IA rhPRG4. In a supporting in vitro crystal microbalance experiment, molecular fouling by albumin did not block the surface activity of rhPRG4.

Single-cell transcriptomics of popliteal lymphatic vessels and peripheral veins reveals altered lymphatic muscle and immune cell populations in the TNF-Tg arthritis model

Background

Lymphatic dysfunction exists in tumor necrosis factor transgenic (TNF-Tg) mice and rheumatoid arthritis (RA) patients. While joint-draining TNF-Tg popliteal lymphatic vessels (PLVs) have deficits in contractility during end-stage arthritis, the nature of lymphatic muscle cells (LMCs) and their TNF-altered transcriptome remain unknown. Thus, we performed single-cell RNA-sequencing (scRNAseq) on TNF-Tg LMCs in PLVs efferent to inflamed joints versus wild-type (WT) controls.

Methods

Single-cell suspensions of PLVs were sorted for smooth muscle cells (SMCs), which was validated by Cspg4-Cre;tdTomato reporter gene expression. Single-cell RNA-seq was performed on a 10x Genomics platform and analyzed using the Seurat R package. Uniform Manifold Approximation and Projections (UMAPs) and Ingenuity Pathway Analysis software were used to assess cell clusters and functional genomics in WT vs. TNF-Tg populations.

Results

Fluorescent imaging of Cspg4-Cre;tdTomato vessels demonstrated dim PLVs and strong reporter gene expression in the adjacent superficial saphenous vein, which was corroborated by flow cytometry of LMCs and vascular smooth muscle cells (VSMCs) from these vessels. Due to their unique morphology, these populations could also be readily detected by scatter analysis of cells from non-fluorescent mice. Bioinformatics analysis of flow sorted WT and TNF-Tg cells identified 20 unique cell clusters that together were 22.4% LMCs, 15.0% VSMCs, and 62.6% non-muscle cells of 8879 total cells. LMCs and M2-macrophages were decreased, while inflammatory monocytes were increased in TNF-Tg lower limb vasculature. SMC populations were defined by Cald1, Tpm1, and Pdgfrb expression and were enriched in myofibroblast-like gene expression. TNF-Tg LMCs exhibited enhanced functional genomics associated with cell death, phagocyte recruitment, and joint inflammation. Among the most prominent TNF-induced genes in SMCs were Mmp3, Cxcl12, and Ccl19, and the most downregulated genes were Zbtb16, Galnt15, and Apod.

Conclusions

Single-cell RNA-seq can be used to investigate functional genomics of lower limb vasculature in mice. Our findings confirm the inflammatory transcriptome of TNF-Tg vessels and altered gene expression in SMC populations. This study further supports a potential role of mesenchymal stromal cells in inflammatory-erosive arthritis pathogenesis, and warrants future studies to define the effects of this TNF-altered transcriptome on PLV function and joint homeostasis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13075-022-02730-z.

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.

Biotribological Tests of Osteochondral Grafts after Treatment with Pro-Inflammatory Cytokines

OBJECTIVE

During osteoarthritis progression, cartilage degrades in a manner that influences its biomechanical and biotribological properties, while chondrocytes reduce the synthesis of extracellular matrix components and become apoptotic. This study investigates the effects of inflammation on cartilage under biomechanical stress using biotribological tests.

METHODS

Bovine osteochondral grafts from five animals were punched out from the medial condyle and treated with or without pro-inflammatory cytokines (interleukin-1β [IL-1β], tumor necrosis factor-α [TNF-α], IL-6) for 2 weeks. After incubation, biotribological tests were performed for 2 hours (alternating 10 minutes test and pause respectively at 39°C, 180 N, 1 Hz, and 2 mm stroke). Before and after testing, the cartilage surface was imaged with a 3-dimensional microscope. During testing, the coefficient of friction (COF) was measured, while gene expression analysis and investigation of metabolic activity of chondrocytes were carried out after testing. Histological sections of the tissue and wear debris from the test fluid were also analyzed.

RESULTS

After biotribological tests, surface cracks were found in both treated and untreated osteochondral grafts. In treated grafts, the COF increased, and the proteoglycan content in the cartilage tissue decreased, leading to structural changes. Chondrocytes from treated grafts showed increased expression of genes encoding for degradative enzymes, while cartilage-specific gene expression and metabolic activity exhibited no significant differences between treated and untreated groups. No measurable difference in the wear debris in the test fluid was found.

CONCLUSIONS

Treatment of osteochondral grafts with cytokines results in a significantly increased COF, while also leading to significant changes in cartilage proteoglycan content and cartilage matrix compression during biotribological tests.

A Tribological Comparison of Facet Joint, Sacroiliac Joint, and Knee Cartilage in the Yucatan Minipig

OBJECTIVE

Pathology of the facet and sacroiliac (SI) joints contributes to 15% to 45% and 10% to 27% of lower back pain cases, respectively. Although tissue engineering may offer novel treatment options to patients suffering from cartilage degeneration in these joints, the tribological characteristics of the facet and SI joints have not been studied in either the human or relevant large animal models, which hinders the development of joint-specific cartilage implants.

DESIGN

Cartilage was isolated from the knee, cervical facet, thoracic facet, lumbar facet, and SI joints of 6 skeletally mature Yucatan minipigs (Sus scrofa). Tribological characteristics were assessed via coefficient of friction testing, interferometry, and immunohistochemistry for lubricin organization.

RESULTS

Compared with the knee, the coefficient of friction was higher by 43% in the cervical facet, 77% in the thoracic facet, 37% in the lumbar facet, and 28% in the SI joint. Likewise, topographical features of the facet and SI joints varied significantly, ranging from a 114% to 384% increase and a 48% to 107% increase in global and local surface roughness measures, respectively, compared with the knee. Additionally, the amount of lubricin in the SI joint was substantially greater than in the knee. Statistical correlations among the various tribological parameters revealed that there was a significant correlation between local roughness and coefficient of friction, but not global roughness or the presence of lubricin.

CONCLUSION

These location-specific tribological characteristics of the articular cartilages of the spine will need to be taken into consideration during the development of physiologically relevant, functional, and durable tissue-engineered replacements for these joints.

Fibroblast pathology in inflammatory diseases

Fibroblasts are important cells for the support of homeostatic tissue function. In inflammatory diseases such as rheumatoid arthritis and inflammatory bowel disease, fibroblasts take on different roles (a) as inflammatory cells themselves and (b) in recruiting leukocytes, driving angiogenesis, and enabling chronic inflammation in tissues. Recent advances in single-cell profiling techniques have transformed the ability to examine fibroblast states and populations in inflamed tissues, providing evidence of previously underappreciated heterogeneity and disease-associated fibroblast populations. These studies challenge the preconceived notion that fibroblasts are homogeneous and provide new insights into the role of fibroblasts in inflammatory pathology. In addition, new molecular insights into the mechanisms of fibroblast activation reveal powerful cell-intrinsic amplification loops that synergize with primary fibroblast stimuli to result in striking responses. In this Review, we focus on recent developments in our understanding of fibroblast heterogeneity and fibroblast pathology across tissues and diseases in rheumatoid arthritis and inflammatory bowel diseases. We highlight new approaches to, and applications of, single-cell profiling techniques and what they teach us about fibroblast biology. Finally, we address how these insights could lead to the development of novel therapeutic approaches to targeting fibroblasts in disease.

Non-clinical assessment of lubrication and free radical scavenging of an innovative non-animal carboxymethyl chitosan biomaterial for viscosupplementation: An in-vitro and ex-vivo study

OBJECTIVE

Lubrication and free radical scavenging are key features of biomaterials used for viscosupplementation (VS) of joints affected by osteoarthritis (OA). The objective of this study was to describe the non-clinical performance characterization of KiOmedine® CM-Chitosan, a non-animal carboxymethyl chitosan, in order to assess its intended action in VS and to compare it to existing viscosupplements based on crosslinked hyaluronan (HA) formulations.

METHOD

The lubrication capacity of the tested viscosupplements (VS) was evaluated in-vitro and ex-vivo. In-vitro, the coefficient of friction (COF) was measured using a novel tribological system. Meanwhile, an ex-vivo biomechanical model in ovine hindlimbs was developed to assess the recovery of join mobility after an intra-articular (IA) injection. Free radical scavenging capacity of HA and KiOmedine® CM-Chitosan formulations was evaluated using the Trolox Equivalent Antioxidant Capacity (TEAC) assay.

RESULTS

In the in-vitro tribological model, KiOmedine® CM-Chitosan showed high lubrication capacity with a significant COF reduction than crosslinked HA formulations. In the ex-vivo model, the lubrication effect of KiOmedine® CM-Chitosan following an IA injection in the injured knee was proven again by a COF reduction. The recovery of joint motion was optimal with an IA injection of 3 ml of KiOmedine® CM-Chitosan, which was significantly better than the crosslinked HA formulation at the same volume. In the in-vitro TEAC assay, KiOmedine® CM-Chitosan showed a significantly higher free radical scavenging capacity than HA formulations.

CONCLUSION

Overall, the results provide a first insight into the mechanism of action in terms of lubrication and free radical scavenging for the use of KiOmedine® CM-Chitosan as a VS treatment of OA. KiOmedine® CM-Chitosan demonstrated a higher capacity to scavenge free radicals, and it showed a higher recovery of mobility after a knee lesion than crosslinked HA formulations. This difference could be explained by the difference in chemical structure between KiOmedine® CM-Chitosan and HA and their formulations.

Role of the osteochondral unit in the pathogenesis of osteoarthritis: focus on the potential use of clodronate

Osteoarthritis (OA) is a chronic disease characterized by inflammation and progressive deterioration of the joint. The etiology of OA includes genetic, phlogistic, dismetabolic and mechanical factors. Historically, cartilage was considered the target of the disease and therapy was aimed at protecting and lubricating the articular cartilage. The osteochondral unit is composed of articular cartilage, calcified cartilage, and subchondral and trabecular bone, which work synergistically to support the functional loading of the joint. Numerous studies today show that OA involves the osteochondral unit, with the participation therefore of the bone in the starting and progression of the disease, which is associated with chondropathy. Cytokines involved in the process leading to cartilage damage are also mediators of subchondral bone edema. Therefore, OA therapy must be based on the use of painkillers and bisphosphonates for both the control of osteometabolic damage and its analgesic activity. Monitoring of the disease of the osteochondral unit must be extensive, since bone marrow edema can be considered as a marker of the evolution of OA. In the present review we discuss some of the pathogenetic mechanisms associated with osteoarthritis, with particular focus on the osteochondral unit and the use of clodronate.

Proteoglycan-4 is an essential regulator of synovial macrophage polarization and inflammatory macrophage joint infiltration

BACKGROUND

Synovial macrophages perform a multitude of functions that include clearance of cell debris and foreign bodies, tissue immune surveillance, and resolution of inflammation. The functional diversity of macrophages is enabled by distinct subpopulations that express unique surface markers. Proteoglycan-4 (PRG4) is an important regulator of synovial hyperplasia and fibrotic remodeling, and the involvement of macrophages in PRG4's synovial role is yet to be defined. Our objectives were to study the PRG4's importance to macrophage homeostatic regulation in the synovium and infiltration of pro-inflammatory macrophages in acute synovitis and investigate whether macrophages mediated synovial fibrosis in Prg4 gene-trap (Prg4GT/GT) murine knee joints.

METHODS

Macrophage phenotyping in Prg4GT/GT and Prg4+/+ joints was performed by flow cytometry using pan-macrophage markers, e.g., CD11b, F4/80, and surface markers of M1 macrophages (CD86) and M2 macrophages (CD206). Characterizations of the various macrophage subpopulations were performed in 2- and 6-month-old animals. The expression of inflammatory markers, IL-6, and iNOS in macrophages that are CD86+ and/or CD206+ was studied. The impact of Prg4 recombination on synovial macrophage populations of 2- and 6-month-old animals and infiltration of pro-inflammatory macrophages in response to a TLR2 agonist challenge was determined. Macrophages were depleted using liposomal clodronate and synovial membrane thickness, and the expression of fibrotic markers α-SMA, PLOD2, and collagen type I (COL-I) was assessed using immunohistochemistry.

RESULTS

Total macrophages in Prg4GT/GT joints were higher than Prg4+/+ joints (p<0.0001) at 2 and 6 months, and the percentages of CD86+/CD206- and CD86+/CD206+ macrophages increased in Prg4GT/GT joints at 6 months (p<0.0001), whereas the percentage of CD86-/CD206+ macrophages decreased (p<0.001). CD86+/CD206- and CD86+/CD206+ macrophages expressed iNOS and IL-6 compared to CD86-/CD206+ macrophages (p<0.0001). Prg4 re-expression limited the accumulation of CD86+ macrophages (p<0.05) and increased CD86-/CD206+ macrophages (p<0.001) at 6 months. Prg4 recombination attenuated synovial recruitment of pro-inflammatory macrophages in 2-month-old animals (p<0.001). Clodronate-mediated macrophage depletion reduced synovial hyperplasia, α-SMA, PLOD2, and COL-I expressions in the synovium (p<0.0001).

CONCLUSIONS

PRG4 regulates the accumulation and homeostatic balance of macrophages in the synovium. In its absence, the synovium becomes populated with M1 macrophages. Furthermore, macrophages exert an effector role in synovial fibrosis in Prg4GT/GT animals.

Critical Role of Synovial Tissue-Resident Macrophage and Fibroblast Subsets in the Persistence of Joint Inflammation

Rheumatoid arthritis (RA) is a chronic prototypic immune-mediated inflammatory disease which is characterized by persistent synovial inflammation, leading to progressive joint destruction. Whilst the introduction of targeted biological drugs has led to a step change in the management of RA, 30-40% of patients do not respond adequately to these treatments, regardless of the mechanism of action of the drug used (ceiling of therapeutic response). In addition, many patients who acheive clinical remission, quickly relapse following the withdrawal of treatment. These observations suggest the existence of additional pathways of disease persistence that remain to be identified and targeted therapeutically. A major barrier for the identification of therapeutic targets and successful clinical translation is the limited understanding of the cellular mechanisms that operate within the synovial microenvironment to sustain joint inflammation. Recent insights into the heterogeneity of tissue resident synovial cells, including macropahges and fibroblasts has revealed distinct subsets of these cells that differentially regulate specific aspects of inflammatory joint pathology, paving the way for targeted interventions to specifically modulate the behaviour of these cells. In this review, we will discuss the phenotypic and functional heterogeneity of tissue resident synovial cells and how this cellular diversity contributes to joint inflammation. We discuss how critical interactions between tissue resident cell types regulate the disease state by establishing critical cellular checkpoints within the synovium designed to suppress inflammation and restore joint homeostasis. We propose that failure of these cellular checkpoints leads to the emergence of imprinted pathogenic fibroblast cell states that drive the persistence of joint inflammation. Finally, we discuss therapeutic strategies that could be employed to specifically target pathogenic subsets of fibroblasts in RA.

"Lessons from Rare Forms of Osteoarthritis"

Osteoarthritis (OA) is one of the most prevalent conditions in the world, particularly in the developed world with a significant increase in cases and their predicted impact as we move through the twenty-first century and this will be exacerbated by the covid pandemic. The degeneration of cartilage and bone as part of this condition is becoming better understood but there are still significant challenges in painting a complete picture to recognise all aspects of the condition and what treatment(s) are most appropriate in individual causes. OA encompasses many different types and this causes some of the challenges in fully understanding the condition. There have been examples through history where much has been learnt about common disease(s) from the study of rare or extreme phenotypes, particularly where Mendelian disorders are involved. The often early onset of symptoms combined with the rapid and aggressive pathogenesis of these diseases and their predictable outcomes give an often-under-explored resource. It is these "rarer forms of disease" that William Harvey referred to that offer novel insights into more common conditions through their more extreme presentations. In the case of OA, GWAS analyses demonstrate the multiple genes that are implicated in OA in the general population. In some of these rarer forms, single defective genes are responsible. The extreme phenotypes seen in conditions such as Camptodactyly Arthropathy-Coxa Vara-pericarditis Syndrome, Chondrodysplasias and Alkaptonuria all present potential opportunities for greater understanding of disease pathogenesis, novel therapeutic interventions and diagnostic imaging. This review examines some of the rarer presenting forms of OA and linked conditions, some of the novel discoveries made whilst studying them, and findings on imaging and treatment strategies.

Cellular Interactions with Lubricin and Hyaluronic Acid-Lubricin Composite Coatings on Gold Electrodes in Passive and Electrically Stimulated Environments

In the field of bionics, the long-term effectiveness of implantable bionic interfaces depends upon maintaining a "clean" and unfouled electrical interface with biological tissues. Lubricin (LUB) is an innately biocompatible glycoprotein with impressive antifouling properties. Unlike traditional antiadhesive coatings, LUB coatings do not passivate electrode surfaces, giving LUB coatings great potential for controlling surface fouling of implantable electrode interfaces. This study characterizes the antifouling properties of bovine native LUB (N-LUB), recombinant human LUB (R-LUB), hyaluronic acid (HA), and composite coatings of HA and R-LUB (HA/R-LUB) on gold electrodes against human primary fibroblasts and chondrocytes in passive and electrically stimulated environments for up to 96 h. R-LUB coatings demonstrated highly effective antifouling properties, preventing nearly all adhesion and proliferation of fibroblasts and chondrocytes even under biphasic electrical stimulation. N-LUB coatings, while showing efficacy in the short term, failed to produce sustained antifouling properties against fibroblasts or chondrocytes over longer periods of time. HA/R-LUB composite films also demonstrated highly effective antifouling performance equal to the R-LUB coatings in both passive and electrically stimulated environments. The high electrochemical stability and long-lasting antifouling properties of R-LUB and HA/R-LUB coatings in electrically stimulating environments reveal the potential of these coatings for controlling unwanted cell adhesion in implantable bionic applications.

Involvement of Transient Receptor Potential Vanilloid Channel 2 in the Induction of Lubricin and Suppression of Ectopic Endochondral Ossification in Mouse Articular Cartilage

OBJECTIVE

Transient receptor potential vanilloid channel 2 (TRPV2) is a Ca²⁺ -permeable channel and plays a role in mediating intracellular Ca²⁺ current via mechanical stimuli. This study was undertaken to examine the expression and role of TRPV2 in adult articular cartilage and the development of osteoarthritis (OA).

METHODS

We examined TRPV2 expression in mouse and human articular cartilage. We analyzed the development of OA in Col2a1-Cre^ERt2 ;Trpv2^fl/fl mice and Trpv2^fl/fl littermates in the resection of the medial meniscus and medial collateral ligament model (n = 5 each), the destabilization of the medial meniscus model (n = 5 each), and the aging mouse model (n = 8-9 each). We examined marker protein expression in these joints, Ca²⁺ influx by mechanical stimuli, and downstream pathways in vitro.

RESULTS

TRPV2 was expressed in mouse and human articular cartilage and ectopic ossification lesions. In all mouse models of OA examined, Col2a1-Cre^ERt2 ;Trpv2^fl/fl mice were observed to have enhanced degradation of articular cartilage accompanied by decreased expression of lubricin/Prg4, and marked formation of periarticular ectopic ossification. Mechanical stress-induced Ca²⁺ influx was decreased by Trpv2 knockout (KO). Prg4 induction by fluid-flow shear stress was diminished in Trpv2-KO mouse chondrocytes, and this was mediated by the Ca²⁺ /calmodulin-dependent protein kinase kinase-cyclic AMP response element binding protein axis. Hypertrophic differentiation was enhanced in Trpv2-KO mouse chondrocytes. Increased activity of calcineurin and nuclear translocation of nuclear factor in activated T cells 1 induced by fluid-flow shear stress or TRP agonist treatment was reversed by Trpv2 knockout.

CONCLUSION

Our findings demonstrate regulation of articular cartilage by TRPV2 through Prg4 induction and suppression of ectopic ossification.

Fibroblast pathology in inflammatory joint disease

Rheumatoid arthritis is an immune-mediated inflammatory disease in which fibroblasts contribute to both joint damage and inflammation. Fibroblasts are a major cell constituent of the lining of the joint cavity called the synovial membrane. Under resting conditions, fibroblasts have an important role in maintaining joint homeostasis, producing extracellular matrix and joint lubricants. In contrast, during joint inflammation, fibroblasts contribute to disease pathology by producing pathogenic levels of inflammatory mediators that drive the recruitment and retention of inflammatory cells within the joint. Recent advances in single-cell profiling techniques have transformed our ability to examine fibroblast biology, leading to the identification of specific fibroblast subsets, defining a previously underappreciated heterogeneity of disease-associated fibroblast populations. These studies are challenging the previously held dogma that fibroblasts are homogeneous and are providing unique insights into their role in inflammatory joint pathology. In this review, we discuss the recent advances in our understanding of how fibroblast heterogeneity contributes to joint pathology in rheumatoid arthritis. Finally, we address how these insights could lead to the development of novel therapies that directly target selective populations of fibroblasts in the future.

Recent Progress in Cartilage Lubrication

Healthy articular cartilage, covering the ends of bones in major joints such as hips and knees, presents the most efficiently-lubricated surface known in nature, with friction coefficients as low as 0.001 up to physiologically high pressures. Such low friction is indeed essential for its well-being. It minimizes wear-and-tear and hence the cartilage degradation associated with osteoarthritis, the most common joint disease, and, by reducing shear stress on the mechanotransductive, cartilage-embedded chondrocytes (the only cell type in the cartilage), it regulates their function to maintain homeostasis. Understanding the origins of such low friction of the articular cartilage, therefore, is of major importance in order to alleviate disease symptoms, and slow or even reverse its breakdown. This progress report considers the relation between frictional behavior and the cellular mechanical environment in the cartilage, then reviews the mechanism of lubrication in the joints, in particular focusing on boundary lubrication. Following recent advances based on hydration lubrication, a proposed synergy between different molecular components of the synovial joints, acting together in enabling the low friction, has been proposed. Additionally, recent development of natural and bio-inspired lubricants is reviewed.

Autologous Adipose-Derived Mesenchymal Stem Cells Combined with Shockwave Therapy Synergistically Ameliorates the Osteoarthritic Pathological Factors in Knee Joint

Adipose-derived mesenchymal stem cells (ADSCs) and shockwave (SW) therapy have been shown to exert a chondroprotective effect for osteoarthritis (OA). The results of this study demonstrated that autologous ADSCs had dose-dependent and synergistic effects with SW therapy (0.25 mJ/mm2 with 800 impulses) in OA rat knee joint. Autologous, high-dose 2 × 106 ADSCs (ADSC2 group) combined with SW therapy significantly increased the bone volume, trabecular thickness, and trabecular number among in the treatment groups. ADSC2 combined with SW therapy significantly reduced the synovitis score and OARSI score in comparison with other treatments. In the analysis of inflammation-induced extracellular matrix factors of the articular cartilage in OA, the results displayed that ADSC2 combined with SW therapy had a greater than other treatments in terms of reducing tumor necrosis factor-inducible gene (TSG)-6 and proteoglycan (PRG)-4, in addition to increasing tissue inhibitor matrix metalloproteinase (TIMP)-1 and type II collagen. Furthermore, ADSC2 combined with SW therapy significantly reduced the expression of inflammation-induced bone morphogenetic protein (BMP)-2 and BMP-6. Therefore, the results demonstrated that ADSC2 combined with SW therapy had a synergistic effect to ameliorate osteoarthritic pathological factors in OA joints.

Lubricin Contributes to Homeostasis of Articular Cartilage by Modulating Differentiation of Superficial Zone Cells

Lubricin encoded by the proteoglycan 4 (Prg4) gene is produced from superficial zone (SFZ) cells of articular cartilage and synoviocytes, which is indispensable for lubrication of joint surfaces. Loss-of-function of human and mouse Prg4 results in early-onset arthropathy accompanied by lost SFZ cells and hyperplastic synovium. Here, we focused on increases in the thickness of articular cartilage in Prg4-knockout joints and analyzed the underlying mechanisms. In the late stage of articular cartilage development, the articular cartilage was thickened at 2 to 4 weeks and the SFZ disappeared at 8 weeks in Prg4-knockout mice. Similar changes were observed in cultured Prg4-knockout femoral heads. Cell tracking showed that Prg4-knockout SFZ cells at 1 week of age expanded to deep layers after 1 week. In in vitro experiments, overexpression of Prg4 lacking a mucin-like domain suppressed differentiation of ATDC5 cells markedly, whereas pellets of Prg4-knockout SFZ cells showed enhanced differentiation. RNA sequencing identified matrix metalloproteinase 9 (Mmp9) as the top upregulated gene by Prg4 knockout. Mmp9 expressed in the SFZ was further induced in Prg4-knockout mice. The increased expression of Mmp9 by Prg4 knockout was canceled by IκB kinase (IKK) inhibitor treatment. Phosphorylation of Smad2 was also enhanced in Prg4-knockout cell pellets, which was canceled by the IKK inhibitor. Expression of Mmp9 and phosphorylated Smad2 during articular cartilage development was enhanced in Prg4-knockout joints. Lubricin contributes to homeostasis of articular cartilage by suppressing differentiation of SFZ cells, and the nuclear factor-kappa B-Mmp9-TGF-β pathway is probably responsible for the downstream action of lubricin. © 2020 American Society for Bone and Mineral Research (ASBMR).

Lubricin as a tool for controlling adhesion in vivo and ex vivo

The ability to prevent or minimize the accumulation of unwanted biological materials on implantable medical devices is important in maintaining the long-term function of implants. To address this issue, there has been a focus on materials, both biological and synthetic, that have the potential to prevent device fouling. In this review, we introduce a glycoprotein called lubricin and report on its emergence as an effective antifouling coating material. We outline the versatility of lubricin coatings on different surfaces, describe the physical properties of its monolayer structures, and highlight its antifouling properties in improving implant compatibility as well as its use in treatment of ocular diseases and arthritis. This review further describes synthetic polymers mimicking the lubricin structure and function. We also discuss the potential future use of lubricin and its synthetic mimetics as antiadhesive biomaterials for therapeutic applications.

Creb5 establishes the competence for Prg4 expression in articular cartilage

A hallmark of cells comprising the superficial zone of articular cartilage is their expression of lubricin, encoded by the Prg4 gene, that lubricates the joint and protects against the development of arthritis. Here, we identify Creb5 as a transcription factor that is specifically expressed in superficial zone articular chondrocytes and is required for TGF-β and EGFR signaling to induce Prg4 expression. Notably, forced expression of Creb5 in chondrocytes derived from the deep zone of the articular cartilage confers the competence for TGF-β and EGFR signals to induce Prg4 expression. Chromatin-IP and ATAC-Seq analyses have revealed that Creb5 directly binds to two Prg4 promoter-proximal regulatory elements, that display an open chromatin conformation specifically in superficial zone articular chondrocytes; and which work in combination with a more distal regulatory element to drive induction of Prg4 by TGF-β. Our results indicate that Creb5 is a critical regulator of Prg4/lubricin expression in the articular cartilage.

Characteristics of MSCs in Synovial Fluid and Mode of Action of Intra-Articular Injections of Synovial MSCs in Knee Osteoarthritis

We have been studying mesenchymal stem cells (MSCs) in synovial fluid and the intra-articular injection of synovial MSCs in osteoarthritis (OA) knees. Here, mainly based on our own findings, we overview the characteristics of endogenous MSCs in the synovial fluid of OA knees and their mode of action when injected exogenously into OA knees. Many MSCs similar to synovial MSCs were detected in the synovial fluid of human OA knees, and their number correlated with the radiological OA grade. Our suspended synovium culture model demonstrated the release of MSCs from the synovium through a medium into a non-contacting culture dish. In OA knees, endogenous MSCs possibly mobilize in a similar manner from the synovium through the synovial fluid and act protectively. However, the number of mobilized MSCs is limited; therefore, OA progresses in its natural course. Synovial MSC injections inhibited the progression of cartilage degeneration in a rat OA model. Injected synovial MSCs migrated into the synovium, maintained their MSC properties, and increased the gene expressions of TSG-6, PRG-4, and BMP-2. Exogenous synovial MSCs can promote anti-inflammation, lubrication, and cartilage matrix synthesis in OA knees. Based on our findings, we have initiated a human clinical study of synovial MSC injections in OA knees.

Proteoglycan 4 Reduces Neuroinflammation and Protects the Blood-Brain Barrier after Traumatic Brain Injury

Neuroinflammation and dysfunction of the blood-brain barrier (BBB) are two prominent mechanisms of secondary injury in neurotrauma. It has been suggested that Toll-like receptors (TLRs) play important roles in initiating and propagating neuroinflammation resulting from traumatic brain injury (TBI), but potential beneficial effects of targeting these receptors in TBI have not been broadly studied. Here, we investigated the effect of targeting TLRs with proteoglycan 4 (PRG4) on post-traumatic neuroinflammation and BBB function. PRG4 is a mucinous glycoprotein with strong anti-inflammatory properties, exerting its biological effects by interfering with TLR2/4 signaling. In addition, PRG4 has the ability to inhibit activation of cluster of differentiation 44 (CD44), a cell-surface glycoprotein playing an important role in inflammation. Using the controlled cortical impact model of TBI in rats, we showed a rapid and prolonged upregulation of message for TLR2/4 and CD44 in the injured cortex. In the in vitro model of the BBB, recombinant human PRG4 (rhPRG4) crossed the endothelial monolayers through a high-capacity, saturable transport system. In rats sustaining TBI, PRG4 delivery to the brain was enhanced by post-traumatic increase in BBB permeability. rhPRG4 injected intravenously at 1 h post-TBI potently inhibited post-traumatic activation of nuclear factor kappa B and extracellular signal-regulated kinases 1/2, the two major signal transduction pathways associated with TLR2/4 and CD44, and curtailed the post-traumatic influx of monocytes. In addition, PRG4 restored normal BBB function after TBI by preventing the post-traumatic loss of tight junction protein claudin 5 and reduced neuronal death. Our observations provide support for therapeutic strategies targeting TLRs in TBI.

Latent developmental potential to form limb-like skeletal structures in zebrafish

Changes in appendage structure underlie key transitions in vertebrate evolution. Addition of skeletal elements along the proximal-distal axis facilitated critical transformations, including the fin-to-limb transition that permitted generation of diverse modes of locomotion. Here, we identify zebrafish mutants that form supernumerary long bones in their pectoral fins. These new bones integrate into musculature, form joints, and articulate with neighboring elements. This phenotype is caused by activating mutations in previously unrecognized regulators of appendage patterning, vav2 and waslb, that function in a common pathway. This pathway is required for appendage development across vertebrates, and loss of Wasl in mice causes defects similar to those seen in murine Hox mutants. Concordantly, formation of supernumerary bones requires Hox11 function, and mutations in the vav2/wasl pathway drive enhanced expression of hoxa11b, indicating developmental homology with the forearm. Our findings reveal a latent, limb-like pattern ability in fins that is activated by simple genetic perturbation.

Intracellular Staphylococcus aureus in bone and joint infections: A mechanism of disease recurrence, inflammation, and bone and cartilage destruction

Bone and joint infections are devastating afflictions. Although medical interventions and advents have improved their care, bone and joint infections still portend dismal outcomes. Indeed, bone and joint infections are associated with extremely high mortality and morbidity rates and, generally, occur secondary to the aggressive pathogen Staphylococcus aureus. The consequences of bone and joint infections are further compounded by the fact that although they are aggressively treated, they frequently recur and result in massive bone and articular cartilage loss. Here, we review the literature and chronicle the fact that the fundamental cellular components of the musculoskeletal system can be internally infected with Staphylococcus aureus, which explains the ready recurrence of bone and joint infections even after extensive administration of antibiotic therapy and debridement and offer potential treatment solutions for further study. Moreover, we review the ramifications of intracellular infection and expound that the massive bone and articular cartilage loss is caused by the sustained proinflammatory state induced by infection and offer potential combination therapies for further study to protect bone and cartilage.

Proteoglycan-4 is correlated with longer survival in HCC patients and enhances sorafenib and regorafenib effectiveness via CD44 in vitro

Sorafenib and regorafenib administration is among the preferential approaches to treat hepatocellular carcinoma (HCC), but does not provide satisfactory benefits. Intensive crosstalk occurring between cancer cells and other multiple non-cancerous cell subsets present in the surrounding microenvironment is assumed to affect tumor progression. This interplay is mediated by a number of soluble and structural extracellular matrix (ECM) proteins enriching the stromal milieu. Here we assess the HCC tumor expression of the ECM protein proteoglycan 4 (PRG4) and its potential pharmacologic activity either alone, or in combination with sorafenib and regorafenib. PRG4 mRNA levels resulted strongly correlated with increased survival rate of HCC patients (p = 0.000) in a prospective study involving 78 HCC subjects. We next showed that transforming growth factor beta stimulates PRG4 expression and secretion by primary human HCC cancer-associated fibroblasts, non-invasive HCC cell lines, and ex vivo specimens. By functional tests we found that recombinant human PRG4 (rhPRG4) impairs HCC cell migration. More importantly, the treatment of HCC cells expressing CD44 (the main PRG4 receptor) with rhPRG4 dramatically enhances the growth-limiting capacity of sorafenib and regorafenib, whereas not significantly affecting cell proliferation per se. Conversely, rhPRG4 only poorly potentiates drug effectiveness on low CD44-expressing or stably CD44-silenced HCC cells. Overall, these data suggest that the physiologically-produced compound PRG4 may function as a novel tumor-suppressive agent by strengthening sorafenib and regorafenib effects in the treatment of HCC.

Heparan sulfate deficiency leads to hypertrophic chondrocytes by increasing bone morphogenetic protein signaling

OBJECTIVE

Exostosin-1 (EXT1) and EXT2 are the major genetic etiologies of multiple hereditary exostoses and are essential for heparan sulfate (HS) biosynthesis. Previous studies investigating HS in several mouse models of multiple hereditary exostoses have reported that aberrant bone morphogenetic protein (BMP) signaling promotes osteochondroma formation in Ext1-deficient mice. This study examined the mechanism underlying the effects of HS deficiency on BMP/Smad signaling in articular cartilage in a cartilage-specific Ext^-/- mouse model.

METHOD

We generated mice with a conditional Ext1 knockout in cartilage tissue (Ext1-cKO mice) using Prg4-Cre transgenic mice. Structural cartilage alterations were histologically evaluated and phospho-Smad1/5/9 (pSmad1/5/9) expression in mouse chondrocytes was analyzed. The effect of pharmacological intervention of BMP signaling using a specific inhibitor was assessed in the articular cartilage of Ext1-cKO mice.

RESULTS

Hypertrophic chondrocytes were significantly more abundant (P = 0.021) and cartilage thickness was greater in Ext1-cKO mice at 3 months postnatal than in control littermates (P = 0.036 for femur; and P < 0.001 for tibia). However, osteoarthritis did not spontaneously occur before the 1-year follow-up. matrix metalloproteinase (MMP)-13 and adamalysin-like metalloproteinases with thrombospondin motifs(ADAMTS)-5 were upregulated in hypertrophic chondrocytes of transgenic mice. Immunostaining and western blotting revealed that pSmad1/5/9-positive chondrocytes were more abundant in the articular cartilage of Ext1-cKO mice than in control littermates. Furthermore, the BMP inhibitor significantly decreased the number of hypertrophic chondrocytes in Ext1-cKO mice (P = 0.007).

CONCLUSIONS

HS deficiency in articular chondrocytes causes chondrocyte hypertrophy, wherein upregulated BMP/Smad signaling partially contributes to this phenotype. HS might play an important role in maintaining the cartilaginous matrix by regulating BMP signaling.

Synovial fluid lubricin increases in spontaneous canine cruciate ligament rupture

Lubricin is an important boundary lubricant and chondroprotective glycoprotein in synovial fluid. Both increased and decreased synovial fluid lubricin concentrations have been reported in experimental post-traumatic osteoarthritis (PTOA) animal models and in naturally occurring joint injuries in humans and animals, with no consensus about how lubricin is altered in different species or injury types. Increased synovial fluid lubricin has been observed following intra-articular fracture in humans and horses and in human late-stage osteoarthritis; however, it is unknown how synovial lubricin is affected by knee-destabilizing injuries in large animals. Spontaneous rupture of cranial cruciate ligament (RCCL), the anterior cruciate ligament equivalent in quadrupeds, is a common injury in dogs often accompanied by OA. Here, clinical records, radiographs, and synovial fluid samples from 30 dogs that sustained RCCL and 9 clinically healthy dogs were analyzed. Synovial fluid lubricin concentrations were nearly 16-fold greater in RCCL joints as compared to control joints, while IL-2, IL-6, IL-8, and TNF-α concentrations did not differ between groups. Synovial fluid lubricin concentrations were correlated with the presence of radiographic OA and were elevated in three animals sustaining RCCL injury prior to the radiographic manifestation of OA, indicating that lubricin may be a potential biomarker for early joint injury.

Lubricin in experimental and naturally occurring osteoarthritis: a systematic review

OBJECTIVE

Lubricin is increasingly being evaluated as an outcome measure in studies investigating post-traumatic and naturally occurring osteoarthritis. However, there are discrepancies in results, making it unclear as to whether lubricin is increased, decreased or unchanged in osteoarthritis. The purpose of this study was to review all papers that measured lubricin in joint injury or osteoarthritis in order to draw conclusions about lubricin regulation in joint disease.

DESIGN

A systematic search of the Pubmed, Web of Knowledge, and EBSCOhost databases for papers was performed. Inclusion criteria were in vivo studies that measured lubricin in humans or animals with joint injury, that investigated lubricin supplementation in osteoarthritic joints, or that described the phenotype of a lubricin knock-out model. A methodological assessment was performed.

RESULTS

Sixty-two studies were included, of which thirty-eight measured endogenous lubricin in joint injury or osteoarthritis. Nineteen papers found an increase or no change in lubricin and nineteen reported a decrease. Papers that reported a decrease in lubricin were cited four times more often than those that reported an increase. Fifteen papers described lubricin supplementation, and all reported a beneficial effect. Eleven papers described lubricin knock-out models.

CONCLUSIONS

The human literature reveals similar distributions of papers reporting increased lubricin as compared to decreased lubricin in osteoarthritis. The animal literature is dominated by reports of decreased lubricin in the rat anterior cruciate ligament transection model, whereas studies in large animal models report increased lubricin. Intra-articular lubricin supplementation may be beneficial regardless of whether lubricin increases or decreases in OA.

NF-κB signaling in rheumatoid arthritis with focus on fibroblast-like synoviocytes

The nuclear factor-κB (NF-κB) signaling pathway regulates multiple processes in innate and adaptive immune cells. This pathway is involved in inflammation through the regulation of cytokines, chemokines, and adhesion molecules expression. The NF-κB transcription factor also participates in the survival, proliferation, and differentiation of cells. Therefore, deregulated NF-κB activation contributes to the pathogenesis of inflammatory diseases. Rheumatoid arthritis (RA) is classified as a heterogeneous and complex autoimmune inflammatory disease. Although different immune and non-immune cells contribute to the RA pathogenesis, fibroblast-like synoviocytes (FLSs) play a crucial role in disease progression. These cells are altered during the disease and produce inflammatory mediators, including inflammatory cytokines and matrix metalloproteinases, which result in joint and cartilage erosion. Among different cell signaling pathways, it seems that deregulated NF-κB activation is associated with the inflammatory picture of RA. NF-κB activation can also promote the proliferation of RA-FLSs as well as the inhibition of FLS apoptosis that results in hyperplasia in RA synovium. In this review, the role of NF-κB transcription factor in immune and non-immune cells (especially FLSs) that are involved in RA pathogenesis are discussed.

In slow pace towards the proteome of equine body fluids

Equine medicine represents a relevant field of veterinary science and the horse industry generates a significant economic impact. Horses can be involved in different sport disciplines, meat production, work and recreational purposes. Horses are also important for human health as they can be used as animal models for studying human diseases and in equine-assisted therapy. This review summarizes the data related to body fluids such as plasma/serum, urine, cerebrospinal fluid, synovial fluid, saliva, bronchoalveolar lavage fluid and peritoneal fluid obtained using proteomic analysis. Horse body fluid proteome analysis under various physiological and pathological conditions is a useful method for identifying new biomarkers for horse diseases which are still difficult to diagnose, but with serious consequences on equine health and welfare. The findings reported here reveal that further proteomic studies on equine body fluids collected from diseased animals are required. SIGNIFICANCE: Body fluids are sources of potential protein biomarkers for diagnosis and therapeutic target identification. Indeed, they contain proteins that play a crucial role in cell functions and whose presence or relative abundance are indicative of the health status of tissues/organs. The review reports the data on the equine body fluids obtained using proteomic analysis, including those which are commonly used to obtain a correct diagnosis and prognosis of horse diseases which still pose a significant challenge. For equine medicine, new biomarkers are needed to formulate early diagnosis and to distinguish among diseases with similar clinical signs.

Proteoglycan-4 regulates fibroblast to myofibroblast transition and expression of fibrotic genes in the synovium

Background

Synovial tissue fibrosis is common in advanced OA with features including the presence of stress fiber-positive myofibroblasts and deposition of cross-linked collagen type-I. Proteoglycan-4 (PRG4) is a mucinous glycoprotein secreted by synovial fibroblasts and is a major component of synovial fluid. PRG4 is a ligand of the CD44 receptor. Our objective was to examine the role of PRG4-CD44 interaction in regulating synovial tissue fibrosis in vitro and in vivo.

Methods

OA synoviocytes were treated with TGF-β ± PRG4 for 24 h and α-SMA content was determined using immunofluorescence. Rhodamine-labeled rhPRG4 was incubated with OA synoviocytes ± anti-CD44 or isotype control antibodies and cellular uptake of rhPRG4 was determined following a 30-min incubation and α-SMA expression following a 24-h incubation. HEK-TGF-β cells were treated with TGF-β ± rhPRG4 and Smad3 phosphorylation was determined using immunofluorescence and TGF-β/Smad pathway activation was determined colorimetrically. We probed for stress fibers and focal adhesions (FAs) in TGF-β-treated murine fibroblasts and fibroblast migration was quantified ± rhPRG4. Synovial expression of fibrotic markers: α-SMA, collagen type-I, and PLOD2 in Prg4 gene-trap (Prg4^GT) and recombined Prg4^GTR animals were studied at 2 and 9 months of age. Synovial expression of α-SMA and PLOD2 was determined in 2-month-old Prg4^GT/GT&Cd44^−/− and Prg4^GTR/GTR&Cd44^−/− animals.

Results

PRG4 reduced α-SMA content in OA synoviocytes (p < 0.001). rhPRG4 was internalized by OA synoviocytes via CD44 and CD44 neutralization attenuated rhPRG4’s antifibrotic effect (p < 0.05). rhPRG4 reduced pSmad3 signal in HEK-TGF-β cells (p < 0.001) and TGF-β/Smad pathway activation (p < 0.001). rhPRG4 reduced the number of stress fiber-positive myofibroblasts, FAs mean size, and cell migration in TGF-β-treated NIH3T3 fibroblasts (p < 0.05). rhPRG4 inhibited fibroblast migration in a macrophage and fibroblast co-culture model without altering active or total TGF-β levels. Synovial tissues of 9-month-old Prg4^GT/GT animals had higher α-SMA, collagen type-I, and PLOD2 (p < 0.001) content and Prg4 re-expression reduced these markers (p < 0.01). Prg4 re-expression also reduced α-SMA and PLOD2 staining in CD44-deficient mice.

Conclusion

PRG4 is an endogenous antifibrotic modulator in the joint and its effect on myofibroblast formation is partially mediated by CD44, but CD44 is not required to demonstrate an antifibrotic effect in vivo.

Notch signalling drives synovial fibroblast identity and arthritis pathology

The synovium is a mesenchymal tissue composed mainly of fibroblasts with a lining and sublining that surrounds the joints. In rheumatoid arthritis (RA), the synovial tissue undergoes marked hyperplasia, becomes inflamed and invasive and destroys the joint^1,2. Recently, we and others found that a subset of fibroblasts located in the sublining undergoes major expansion in RA and is linked to disease activity^3,4,5. However, the molecular mechanism by which these fibroblasts differentiate and expand in RA remains unknown. Here, we identified a critical role for NOTCH3 signaling in the differentiation of perivascular and sublining CD90(THY1)+ fibroblasts. Using single cell RNA-sequencing and synovial tissue organoids, we found that NOTCH3 signaling drives both transcriptional and spatial gradients in fibroblasts emanating from vascular endothelial cells outward. In active RA, NOTCH3 and NOTCH target genes are markedly upregulated in synovial fibroblasts. Importantly, genetic deletion of Notch3 or monoclonal antibody-blockade of NOTCH3 signaling attenuates inflammation and prevents joint damage in inflammatory arthritis. Our results indicate that synovial fibroblasts exhibit positional identity regulated by endothelium-derived Notch signaling and that this stromal crosstalk pathway underlies inflammation and pathology in inflammatory arthritis.

Monitoring tissue-level remodelling during inflammatory arthritis using a three-dimensional synovium-on-a-chip with non-invasive light scattering biosensing

Rheumatoid arthritis is a chronic, systemic joint disease in which an autoimmune response translates into an inflammatory attack resulting in joint damage, disability and decreased quality of life. Despite recent introduction of therapeutic agents such as anti-TNFα, even the best current therapies fail to achieve disease remission in most arthritis patients. Therefore, research into the mechanisms governing the destructive inflammatory process in rheumatoid arthritis is of great importance and may reveal novel strategies for the therapeutic interventions. To gain deeper insight into its pathogensis, we have developed for the first time a three-dimensional synovium-on-a-chip system in order to monitor the onset and progression of inflammatory synovial tissue responses. In our study, patient-derived primary synovial organoids are cultivated on a single chip platform containing embedded organic-photodetector arrays for over a week in the absence and presence of tumor-necrosis-factor. Using a label-free and non-invasive optical light-scatter biosensing strategy inflammation-induced 3D tissue-level architectural changes were already detected after two days. We demonstrate that the integration of complex human synovial organ cultures in a lab-on-a-chip provides reproducible and reliable information on how systemic stress factors affect synovial tissue architectures.

Inhibitory Effects of PRG4 on Migration and Proliferation of Human Venous Cells

BACKGROUND

Proteoglycan 4 (PRG4; lubricin) is a member of two gene co-expression network modules associated with human vein graft failure. However, little is known about PRG4 and the vascular system. Therefore, we have investigated the effects of recombinant human PRG4 (rhPRG4) on cell migration and proliferation in human veins.

METHODS

Effects of rhPRG4 on cell migration, proliferation, and neointima formation were determined in human venous tissue and cultured venous smooth muscle cells (SMCs), adventitial cells, and endothelial cells. Expression of PRG4 by cultured human saphenous veins, failed vein grafts, and varicose veins was determined by immunostaining or Western blotting.

RESULTS

Limited expression of PRG4 in fresh saphenous veins was dramatically increased around medial SMCs after culture ex vivo. rhPRG4 inhibited the migration of cultured SMCs, adventitial cells, and endothelial cells, as well as the proliferation of endothelial cells. rhPRG4 also inhibited the migration of SMCs and adventitial cells from tissue explants, but there was no effect on cell proliferation or neointima formation in ex vivo whole veins. Finally, PRG4 was largely absent in two examples of venous pathology, that is, failed human vein grafts and varicose veins.

CONCLUSIONS

Although rhPRG4 can inhibit the migration of venous SMCs, endothelial cells, and adventitial cells, and the proliferation of endothelial cells, PRG4 was only increased around medial SMCs in veins after ex vivo culture. PRG4 was not observed around medial SMCs in failed human vein grafts and varicose veins, suggesting the possibility that a failure of PRG4 upregulation may promote these pathologies.

Effects of Pomegranate Concentrate Powder: Eucommiae Cortex: Achyranthis Radix 5:4:1 (w/w) Mixed Formula on Monosodium Iodoacetate-Induced Osteoarthritis in Rats

We examined the antiosteoarthritis effect of a mixture of powdered pomegranate concentrate, eucommiae cortex, and achyranthis radix (5:4:1 w/w) (PEA-Mix). After the injection of monosodium iodoacetate (MIA), PEA-Mixs were orally administered. To assess pain-related behaviors, a von Frey filament test and open field test were performed. We also examined the knee thickness, maximum knee extension angle, bone mineral density (BMD), and compressive strength of the knee joint and performed a histopathologic analysis. The number of COX-2, tumor necrosis factor (TNF)-α, poly (ADP-ribose) polymerase (PARP), and 5-bromo-2′-deoxyuridine immunoreactive cells, the prostaglandin E2 (PGE2) and 5-lipoxygenase (LPO) levels, matrix metalloproteinase (MMP) activity, and mRNA levels of chondrogenesis-related genes were analyzed. PEA-Mix significantly inhibited the MIA-induced decrease in the paw-withdrawal threshold and total distance moved, and the MIA-induced increases in the maximum knee extension angle and knee thickness. Also, the MIA-induced loss of the knee joint articular surface region and decrease in the BMD were significantly suppressed by PEA-Mix. The MIA-induced increases in the 5-LPO, PGE2, MMP-2, MMP-9, COX-2, and TNF-α mRNA levels were reduced by PEA-Mix. PEA-Mix increased the MIA-mediated reduction in the SOX-9 and aggrecan mRNA levels. The number of PARP-positive cells was smaller in PEA-Mix-administered rats than in MIA-administered rats, while the number of 5-bromo-2′-deoxyuridine-positive cells was larger. Therefore, PEA-Mix relieved the MIA-induced pain-related behaviors, chondrocyte proliferation, and anti-inflammatory activity.

Interleukin-4 Gene Transfection and Spheroid Formation Potentiate Therapeutic Efficacy of Mesenchymal Stem Cells for Osteoarthritis

Osteoarthritis (OA) is a painful intractable disease that significantly affects patients' quality of life. However, current therapies, such as pain killers and joint replacement surgery, do not lead to cartilage protection. Mesenchymal stem cells (MSCs) have been proposed as an alternative strategy for OA therapy because MSCs can secrete chondroprotective and anti-inflammatory factors. However, interleukin-4 (IL-4), a potent anti-inflammatory cytokine, is barely produced by MSCs, and MSC therapy suffers from rapid MSC death following intra-articular implantation. MSCs in spheroids survive better than naïve MSCs in vitro and in vivo. IL-4-transfected MSCs in spheroids (IL-4 MSC spheroid) show increased chondroprotective and anti-inflammatory effects in an OA chondrocyte model in vitro. Following intra-articular implantation in OA rats, IL-4 MSC spheroids show better cartilage protection and pain relief than naïve MSCs. Thus, IL-4 MSC spheroid may potentiate the therapeutic efficacy of MSCs for OA.

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

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

Lubricin as a Therapeutic and Potential Biomarker in Sepsis

Proteoglycan 4 (or lubricin), a mucin-like glycoprotein, was originally classified as a lubricating substance within diarthrodial joints. More recently, lubricin has been found in other tissues and has been implicated in 2 inflammatory pathways within the cell, via the Toll-like receptors (TLRs) and CD44. Lubricin is an antagonist of TLR2 and TLR4, and appears to enter cells via the CD44 receptor. Because of lubricin's action on these receptors, downstream processes of inflammation are halted, thereby preventing release of cytokines (a hallmark of inflammation and sepsis) from the cell, indicating lubricin's role as a biomarker and possible therapeutic for sepsis.

The Roles of Indian Hedgehog Signaling in TMJ Formation

The temporomandibular joint (TMJ) is an intricate structure composed of the mandibular condyle, articular disc, and glenoid fossa in the temporal bone. Apical condylar cartilage is classified as a secondary cartilage, is fibrocartilaginous in nature, and is structurally distinct from growth plate and articular cartilage in long bones. Condylar cartilage is organized in distinct cellular layers that include a superficial layer that produces lubricants, a polymorphic/progenitor layer that contains stem/progenitor cells, and underlying layers of flattened and hypertrophic chondrocytes. Uniquely, progenitor cells reside near the articular surface, proliferate, undergo chondrogenesis, and mature into hypertrophic chondrocytes. During the past decades, there has been a growing interest in the molecular mechanisms by which the TMJ develops and acquires its unique structural and functional features. Indian hedgehog (Ihh), which regulates skeletal development including synovial joint formation, also plays pivotal roles in TMJ development and postnatal maintenance. This review provides a description of the many important recent advances in Hedgehog (Hh) signaling in TMJ biology. These include studies that used conventional approaches and those that analyzed the phenotype of tissue-specific mouse mutants lacking Ihh or associated molecules. The recent advances in understanding the molecular mechanism regulating TMJ development are impressive and these findings will have major implications for future translational medicine tools to repair and regenerate TMJ congenital anomalies and acquired diseases, such as degenerative damage in TMJ osteoarthritic conditions.

Wnt/β-catenin signaling contributes to articular cartilage homeostasis through lubricin induction in the superficial zone

Background

Both loss- and gain-of-function of Wnt/β-catenin signaling in chondrocytes result in exacerbation of osteoarthritis (OA). Here, we examined the activity and roles of Wnt/β-catenin signaling in the superficial zone (SFZ) of articular cartilage.

Methods

Wnt/β-catenin signaling activity was analyzed using TOPGAL mice. We generated Prg4-Cre^ERT2;Ctnnb1^fl/fl and Prg4-Cre^ERT2;Ctnnb1-ex3^fl/wt mice for loss- and gain-of-function, respectively, of Wnt/β-catenin signaling in the SFZ. Regulation of Prg4 expression by Wnt/β-catenin signaling was examined in vitro, as were upstream and downstream factors of Wnt/β-catenin signaling in SFZ cells.

Results

Wnt/β-catenin signaling activity, as determined by the TOPGAL reporter, was high specifically in the SFZ of mouse adult articular cartilage, where Prg4 is abundantly expressed. In SFZ-specific β-catenin-knockout mice, OA development was significantly accelerated, which was accompanied by decreased Prg4 expression and SFZ destruction. In contrast, Prg4 expression was enhanced and cartilage degeneration was suppressed in SFZ-specific β-catenin-stabilized mice. In primary SFZ cells, Prg4 expression was downregulated by β-catenin knockout, while it was upregulated by β-catenin stabilization by exon 3 deletion or treatment with CHIR99021. Among Wnt ligands, Wnt5a, Wnt5b, and Wnt9a were highly expressed in SFZ cells, and recombinant human WNT5A and WNT5B stimulated Prg4 expression. Mechanical loading upregulated expression of these ligands and further promoted Prg4 transcription. Moreover, mechanical loading and Wnt/β-catenin signaling activation increased mRNA levels of Creb1, a potent transcription factor for Prg4.

Conclusions

We demonstrated that Wnt/β-catenin signaling regulates Prg4 expression in the SFZ of mouse adult articular cartilage, which plays essential roles in the homeostasis of articular cartilage.

Lubricin expression in the lumbar endplate and its association with Modic changes

Objective

To explore the expression of lubricin in the lumbar endplate and its association with Modic changes (MCs).

Methods

Human endplate specimens harvested from patients undergoing surgery for thoracolumbar spine fractures ​or lumbar interbody fusion were divided into two groups: MCs group and normal group. Lubricin expression was examined by immunohistochemistry, and differences between the groups were analysed using quantitative polymerase chain reaction (qPCR). Lubricin expression and differences between endplates with MCs and normal endplates were confirmed using a rabbit model. In a final experiment, rabbit endplate chondrocytes were cocultured with Propionibacteria acnes (P. acnes) supernatant, and the expression of lubricin and endplate degeneration related genes were evaluated. In addition, the expression of matrix metalloproteinase 1(MMP-1), A disintegrin-like and metalloproteinase with thrombospondin type 5 motif (ADAMTS5) and inflammatory factors (Interleukin- 1β (IL-1β) and Interleukin-6 (IL-6)) were evaluated after lubricin overexpression.

Results

Lubricin was found in human lumbar endplates and its expression was lower in the MCs group compared to the normal group. In the rabbit model, lubricin was also found in the endplate. In rabbits injected with P. acnes (the MCs group), lubricin expression of endplate decreased compared to the normal group. In the culture of rabbit endplate chondrocytes with P. acnes supernatant, the expression of lubricin, aggrecan, sox9 and collagen type-II decreased significantly, while that of MMP-1 and ADAMTS5 increased significantly. Moreover, lubricin overexpression could downregulate the expression of MMP-1, ADAMTS5 and inflammatory factors (IL-1β and IL-6) compared to negative control.

Conclusion

Lubricin is present in the lumbar endplate where it may have an anti-inflammatory role. P. acnes infection inhibits lubricin expression by cartilage endplate cells and this may facilitate the progression of MCs and endplate degeneration.

The translational potential of this article

Lubricin may have an anti-inflammatory role. P. acnes infection inhibits lubricin expression by cartilage endplate cells and this may facilitate the progression of MCs and endplate degeneration.

Articular Cartilage Regeneration in Osteoarthritis

There has been considerable advancement over the last few years in the treatment of osteoarthritis, common chronic disease and a major cause of disability in older adults. In this pathology, the entire joint is involved and the regeneration of articular cartilage still remains one of the main challenges, particularly in an actively inflammatory environment. The recent strategies for osteoarthritis treatment are based on the use of different therapeutic solutions such as cell and gene therapies and tissue engineering. In this review, we provide an overview of current regenerative strategies highlighting the pros and cons, challenges and opportunities, and we try to identify areas where future work should be focused in order to advance this field.

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.

FoxO transcription factors modulate autophagy and proteoglycan 4 in cartilage homeostasis and osteoarthritis

Aging is a main risk factor for osteoarthritis (OA). FoxO transcription factors protect against cellular and organismal aging, and FoxO expression in cartilage is reduced with aging and in OA. To investigate the role of FoxO in cartilage, Col2Cre-FoxO1, 3, and 4 single knockout (KO) and triple KO mice (Col2Cre-TKO) were analyzed. Articular cartilage in Col2Cre-TKO and Col2Cre-FoxO1 KO mice was thicker than in control mice at 1 or 2 months of age. This was associated with increased proliferation of chondrocytes of Col2Cre-TKO mice in vivo and in vitro. OA-like changes developed in cartilage, synovium, and subchondral bone between 4 and 6 months of age in Col2Cre-TKO and Col2Cre-FoxO1 KO mice. Col2Cre-FoxO3 and FoxO4 KO mice showed no cartilage abnormalities until 18 months of age when Col2Cre-FoxO3 KO mice had more severe OA than control mice. Autophagy and antioxidant defense genes were reduced in Col2Cre-TKO mice. Deletion of FoxO1/3/4 in mature mice using Aggrecan(Acan)-CreERT2 (AcanCreERT-TKO) also led to spontaneous cartilage degradation and increased OA severity in a surgical model or treadmill running. The superficial zone of knee articular cartilage of Col2Cre-TKO and AcanCreERT-TKO mice exhibited reduced cell density and markedly decreased Prg4 In vitro, ectopic FoxO1 expression increased Prg4 and synergized with transforming growth factor-β stimulation. In OA chondrocytes, overexpression of FoxO1 reduced inflammatory mediators and cartilage-degrading enzymes, increased protective genes, and antagonized interleukin-1β effects. Our observations suggest that FoxO play a key role in postnatal cartilage development, maturation, and homeostasis and protect against OA-associated cartilage damage.