Hyaluronan secretion by synoviocytes is mechanosensitive

Rethinking Osteoarthritis Management: Synergistic Effects of Chronoexercise, Circadian Rhythm, and Chondroprotective Agents

Osteoarthritis (OA) is a chronic and debilitating joint disease characterized by progressive cartilage degeneration for which no definitive cure exists. Conventional management approaches often rely on fragmented and poorly coordinated pharmacological and non-pharmacological interventions that are inconsistently applied throughout the disease course. Persistent controversies regarding the clinical efficacy of chondroprotective agents, frequently highlighted by pharmacovigilance agencies, underscore the need for a structured evidence-based approach. Emerging evidence suggests that synchronizing pharmacotherapy and exercise regimens with circadian biology may optimize therapeutic outcomes by addressing early pathological processes, including low-grade inflammation, oxidative stress, and matrix degradation. Recognizing the influence of the chondrocyte clock on these processes, this study proposes a 'prototype' for a novel framework that leverages the circadian rhythm-aligned administration of traditional chondroprotective agents along with tailored, accessible exercise protocols to mitigate cartilage breakdown and support joint function. In addition, this model-based framework emphasizes the interdependence between cartilage chronobiology and time-of-day-dependent responses to exercise, where strategically timed joint activity enhances nutrient and waste exchange, mitigates mitochondrial dysfunction, supports cellular metabolism, and promotes tissue maintenance, whereas nighttime rest promotes cartilage rehydration and repair. This time-sensitive, comprehensive approach aims to slow OA progression, reduce structural damage, and delay invasive procedures, particularly in weight-bearing joints such as the knee and hip. However, significant challenges remain, including inter-individual variability in circadian rhythms, a lack of reliable biomarkers for pharmacotherapeutic monitoring, and limited clinical evidence supporting chronoexercise protocols. Future large-scale, longitudinal trials are critical to evaluate the efficacy and scalability of this rational integrative strategy, paving the way for a new era in OA management.

Synovial joints were present in the common ancestor of jawed fish but lacking in jawless fish

Synovial joints, characterized by reciprocally congruent and lubricated articular surfaces separated by a cavity, can simultaneously provide mobility and load bearing. Here, we study the early evolution of synovial joints by examining the morphological, genetic, and molecular features required for the development and function of the joints in elasmobranchs and cyclostomes. We show the presence of cavitated and articulated joints in the skeleton of elasmobranchs, such as the little skate (Leucoraja erinacea) and bamboo shark (Chiloscyllium plagiosum). However, our results do not support the presence of articular cavities between cartilaginous elements in cyclostomes such as sea lampreys (Petromyozon marinus) and hagfish (Myxine glutinosa). Immunostaining reveals the expression of lubrication-related proteoglycans like aggrecan and glycoproteins such as hyaluronic acid receptor (CD44) at the articular surfaces in little skates. Analysis of joint development in little skate embryos shows the expression of growth differentiation factor-5 (Gdf5) and β-catenin at the joint interzones like tetrapods. Muscle paralysis in little skate embryos leads to joint fusion, suggesting that muscle activity is necessary for the formation of synovial cavity and development of normal articular surfaces, in a manner similar to zebrafish and tetrapods. Together, these data suggest that synovial joints originated in the common ancestor of extant gnathostomes. A review of fossils from the extinct clades along the gnathostome stem suggests that joints with reciprocally articulating surfaces arose in the dermal skeleton of the common ancestor of all jawed vertebrates. Synovial joints in cartilaginous tissue were a subsequent gnathostome innovation.

Synovium friction properties are influenced by proteoglycan content

The synovium plays a crucial role in diarthrodial joint health, and its study has garnered appreciation as synovitis has been linked to osteoarthritis symptoms and progression. Quantitative synovium structure-function data, however, remain sparse. In the present study, we hypothesized that tissue glycosaminoglycan (GAG) content contributes to the low friction properties of the synovium. Bovine and human synovium tribological properties were evaluated using a custom friction testing device in two different cases: (1) proteoglycan depletion to isolate the influence of tissue GAGs in the synovium friction response and (2) interleukin-1 (IL) treatment to observe inflammation-induced structural and functional changes. Following proteoglycan depletion, synovium friction coefficients increased while GAG content decreased. Conversely, synovium explants treated with the proinflammatory cytokine IL exhibited elevated GAG concentrations and decreased friction coefficients. For the first time, a relationship between synovium friction coefficient and GAG concentration is demonstrated. The study of synovium tribology is necessary to fully understand the mechanical environment of the healthy and diseased joint.

Mechanobiology of Hyaluronan: Connecting Biomechanics and Bioactivity in Musculoskeletal Tissues

Hyaluronan (HA) plays well-recognized mechanical and biological roles in articular cartilage and synovial fluid, where it contributes to tissue structure and lubrication. An understanding of how HA contributes to the structure of other musculoskeletal tissues, including muscle, bone, tendon, and intervertebral discs, is growing. In addition, the use of HA-based therapies to restore damaged tissue is becoming more prevalent. Nevertheless, the relationship between biomechanical stimuli and HA synthesis, degradation, and signaling in musculoskeletal tissues remains understudied, limiting the utility of HA in regenerative medicine. In this review, we discuss the various roles and significance of endogenous HA in musculoskeletal tissues. We use what is known and unknown to motivate new lines of inquiry into HA biology within musculoskeletal tissues and in the mechanobiology governing HA metabolism by suggesting questions that remain regarding the relationship and interaction between biological and mechanical roles of HA in musculoskeletal health and disease.

Lovastatin impairs cellular proliferation and enhances hyaluronic acid production in fibroblast-like synoviocytes

INTRODUCTION

Statins have demonstrated chondroprotective effects by reducing inflammation and mitigating extracellular matrix degradation. However, statins are also reported to be cytotoxic to several types of cells. Early-onset osteoarthritis (OA) is characterized by synovial inflammation, which adversely affects hyaluronan (HA) production in fibroblast-like synoviocytes (FLSs). Nevertheless, the precise effects of statins on the synovium remain unclear.

METHODS

This study investigated the impact of lovastatin on human FLSs, and HA secretion-related genes, signaling pathways, and production were evaluated.

RESULTS

The findings revealed that high doses of lovastatin (20 or 40 μM) decreased FLS viability and increased cell death. FLS proliferation ceased when cultured in a medium containing 5 or 10 μM lovastatin. mRNA expression analysis demonstrated that lovastatin (5 and 10 μM) upregulated the gene level of hyaluronan synthase 1 (HAS1), HAS2, and proteoglycan 4 (PRG4), but not HAS3. While the expression of multidrug resistance-associated protein 5 transporter gene remained unaffected, both inward-rectifying potassium channel and acid-sensing ion channel 3 were upregulated. Western blot further confirmed that lovastatin increased the production of HAS1 and PRG4, and activated the PKC-α, ERK1/2, and p38-MAPK signaling pathways. Additionally, lovastatin elevated intracellular cAMP levels and HA production in FLSs.

CONCLUSION

Lovastatin impairs cellular proliferation but enhances HA production in human FLSs.

Human vascularised synovium-on-a-chip: a mechanically stimulated, microfluidic model to investigate synovial inflammation and monocyte recruitment

Healthy synovium is critical for joint homeostasis. Synovial inflammation (synovitis) is implicated in the onset, progression and symptomatic presentation of arthritic joint diseases such as rheumatoid arthritis and osteoarthritis. Thus, the synovium is a promising target for the development of novel, disease-modifying therapeutics. However, target exploration is hampered by a lack of good pre-clinical models that accurately replicate human physiology and that are developed in a way that allows for widespread uptake. The current study presents a multi-channel, microfluidic, organ-on-a-chip (OOAC) model, comprising a 3D configuration of the human synovium and its associated vasculature, with biomechanical and inflammatory stimulation, built upon a commercially available OOAC platform. Healthy human fibroblast-like synoviocytes (hFLS) were co-cultured with human umbilical vein endothelial cells (HUVECs) with appropriate matrix proteins, separated by a flexible, porous membrane. The model was developed within the Emulate organ-chip platform enabling the application of physiological biomechanical stimulation in the form of fluid shear and cyclic tensile strain. The hFLS exhibited characteristic morphology, cytoskeletal architecture and matrix protein deposition. Synovial inflammation was initiated through the addition of interleukin-1β(IL-1β) into the synovium channel resulting in the increased secretion of inflammatory and catabolic mediators, interleukin-6 (IL-6), prostaglandin E2 (PGE2), matrix metalloproteinase 1 (MMP-1), as well as the synovial fluid constituent protein, hyaluronan. Enhanced expression of the inflammatory marker, intercellular adhesion molecule-1 (ICAM-1), was observed in HUVECs in the vascular channel, accompanied by increased attachment of circulating monocytes. This vascularised human synovium-on-a-chip model recapitulates a number of the functional characteristics of both healthy and inflamed human synovium. Thus, this model offers the first human synovium organ-chip suitable for widespread adoption to understand synovial joint disease mechanisms, permit the identification of novel therapeutic targets and support pre-clinical testing of therapies.

Adipose Tissue-Derived Products May Present Inflammatory Properties That Affect Chondrocytes and Synoviocytes from Patients with Knee Osteoarthritis

Adipose tissue-derived cell-based injectable therapies have been demonstrated to have disease-modifying effects on joint tissues in preclinical studies on animal osteoarthritis (OA) models, but clinical results are heterogeneous and not always satisfactory. The aim of this study was to investigate the influence of adipose tissue properties on the therapeutic effects of the adipose-derived product in an in vitro OA setting. Micro-fragmented adipose tissue (MF-AT) samples were obtained from 21 OA patients (mean age 51.7 ± 11.8 years, mean BMI 25.7 ± 4.1 kg/m2). The analysis of the MF-AT supernatant was performed to analyze the release of inflammatory factors. The effects of MF-AT inflammatory factors were investigated on chondrocytes and synoviocytes gene expression levels. Patients' characteristics were analyzed to explore their influence on MF-AT inflammatory molecules and on the MF-AT effects on the gene expression of chondrocytes and synoviocytes. The study results demonstrated that adipose tissue-derived products may present inflammatory properties that influence the therapeutic potential for OA treatment, with products with a higher pro-inflammatory profile stimulating a higher expression of genes related to a more inflamed and catabolic phenotype. A higher pro-inflammatory cytokine pattern and a higher pro-inflammatory effect were found in adipose tissue-derived products obtained from OA patients with higher BMI.

Endogenous production of hyaluronan, PRG4, and cytokines is sensitive to cyclic loading in synoviocytes

Synovial fluid is composed of hyaluronan and proteoglycan-4 (PRG4 or lubricin), which work synergistically to maintain joint lubrication. In diseases like osteoarthritis, hyaluronan and PRG4 concentrations can be altered, resulting in lowered synovial fluid viscosity, and pro-inflammatory cytokine concentrations within the synovial fluid increase. Synovial fibroblasts within the synovium are responsible for contributing to synovial fluid and can be targeted to improve endogenous production of hyaluronan and PRG4 and to alter the cytokine profile. We cyclically loaded SW982 synoviocytes to 0%, 5%, 10%, or 20% strain for three hours at 1 Hz. To assess the impact of substrate stiffness, we compared the 0% strain group to cells grown on tissue culture plastic. We measured the expression of hyaluronan turnover genes, hyaluronan localization within the cell layer, hyaluronan concentration, PRG4 concentration, and the cytokine profile within the media. Our results show that the addition of cyclic loading increased HAS3 expression, but not in a magnitude-dependent response. Hyaluronidase expression was impacted by strain magnitude, which is exemplified by the decrease in hyaluronan concentration due to cyclic loading. We also show that PRG4 concentration is increased at 5% strain, while higher strain magnitude decreases overall PRG4 concentration. Finally, 10% and 20% strain show a distinct, more pro-inflammatory cytokine profile when compared to the unloaded group. Multivariate analysis showed distinct separation between certain strain groups in being able to predict strain group, hyaluronan concentration, and PRG4 concentration from gene expression or cytokine concentration data, highlighting the complexity of the system. Overall, this study shows that cyclic loading can be used tool to modulate the endogenous production of hyaluronan, PRG4, and cytokines from synovial fibroblasts.

UDP-glucose dehydrogenase (UGDH) activity is suppressed by peroxide and promoted by PDGF in fibroblast-like synoviocytes: Evidence of a redox control mechanism

UDP-glucose dehydrogenase (UGDH) generates essential precursors of hyaluronic acid (HA) synthesis, however mechanisms regulating its activity are unclear. We used enzyme histostaining and quantitative image analysis to test whether cytokines that stimulate HA synthesis upregulate UGDH activity. Fibroblast-like synoviocytes (FLS, from N = 6 human donors with knee pain) were cultured, freeze-thawed, and incubated for 1 hour with UDP-glucose, NAD+ and nitroblue tetrazolium (NBT) which allows UGDH to generate NADH, and NADH to reduce NBT to a blue stain. Compared to serum-free medium, FLS treated with PDGF showed 3-fold higher UGDH activity and 6-fold higher HA release, but IL-1beta/TGF-beta1 induced 27-fold higher HA release without enhancing UGDH activity. In selected proliferating cells, UGDH activity was lost in the cytosol, but preserved in the nucleus. Cell-free assays led us to discover that diaphorase, a cytosolic enzyme, or glutathione reductase, a nuclear enzyme, was necessary and sufficient for NADH to reduce NBT to a blue formazan dye in a 1-hour timeframe. Primary synovial fibroblasts and transformed A549 fibroblasts showed constitutive diaphorase/GR staining activity that varied according to supplied NADH levels, with relatively stronger UGDH and diaphorase activity in A549 cells. Unilateral knee injury in New Zealand White rabbits (N = 3) stimulated a coordinated increase in synovial membrane UGDH and diaphorase activity, but higher synovial fluid HA in only 2 out of 3 injured joints. UGDH activity (but not diaphorase) was abolished by N-ethyl maleimide, and inhibited by peroxide or UDP-xylose. Our results do not support the hypothesis that UGDH is a rate-liming enzyme for HA synthesis under catabolic inflammatory conditions that can oxidize and inactivate the UGDH active site cysteine. Our novel data suggest a model where UGDH activity is controlled by a redox switch, where intracellular peroxide inactivates, and high glutathione and diaphorase promote UGDH activity by maintaining the active site cysteine in a reduced state, and by recycling NAD+ from NADH.

Mechanosensory and mechanotransductive processes mediated by ion channels in articular chondrocytes: Potential therapeutic targets for osteoarthritis

Articular cartilage consists of an extracellular matrix including many proteins as well as embedded chondrocytes. Articular cartilage formation and function are influenced by mechanical forces. Hind limb unloading or simulated microgravity causes articular cartilage loss, suggesting the importance of the healthy mechanical environment in articular cartilage homeostasis and implying a significant role of appropriate mechanical stimulation in articular cartilage degeneration. Mechanosensitive ion channels participate in regulating the metabolism of articular chondrocytes, including matrix protein production and extracellular matrix synthesis. Mechanical stimuli, including fluid shear stress, stretch, compression and cell swelling and decreased mechanical conditions (such as simulated microgravity) can alter the membrane potential and regulate the metabolism of articular chondrocytes via transmembrane ion channel-induced ionic fluxes. This process includes Ca2+ influx and the resulting mobilization of Ca2+ that is due to massive released Ca2+ from stores, intracellular cation efflux and extracellular cation influx. This review brings together published information on mechanosensitive ion channels, such as stretch-activated channels (SACs), voltage-gated Ca2+ channels (VGCCs), large conductance Ca2+-activated K+ channels (BKCa channels), Ca2+-activated K+ channels (SKCa channels), voltage-activated H+ channels (VAHCs), acid sensing ion channels (ASICs), transient receptor potential (TRP) family channels, and piezo1/2 channels. Data based on epithelial sodium channels (ENaCs), purinergic receptors and N-methyl-d-aspartate (NMDA) receptors are also included. These channels mediate mechanoelectrical physiological processes essential for converting physical force signals into biological signals. The primary channel-mediated effects and signaling pathways regulated by these mechanosensitive ion channels can influence the progression of osteoarthritis during the mechanosensory and mechanoadaptive process of articular chondrocytes.

Attachment of cartilage wear particles to the synovium negatively impacts friction properties

Cartilage wear particles are released into the synovial fluid by mechanical and chemical degradation of the articular surfaces during osteoarthritis and attach to the synovial membrane. Accumulation of wear particles could alter key tissue-level mechanical properties of the synovium, hindering its characteristically low-friction interactions with underlying articular surfaces in the synovial joint. The present study employs a custom loading device to further the characterization of native synovium friction properties, while investigating the hypothesis that attachment of cartilage wear particles increases friction coefficient. Juvenile bovine synovium demonstrated characteristically low friction coefficients in sliding contact with glass, in agreement with historical measurements. Friction coefficient increased with higher normal load in saline, while lubrication with native synovial fluid maintained low friction coefficients at higher loads. Cartilage wear particles generated from juvenile bovine cartilage attached directly to synovium explants in static culture, with incorporation onto the tissue denoted by cell migration onto the particle surface. In dilute synovial fluid mimicking the decreased lubricating properties during osteoarthritis, wear particle attachment significantly increased friction coefficient against glass, and native cartilage and synovium. In addition to providing a novel characterization of synovial joint tribology this work highlights a potential mechanism for cartilage wear particles to perpetuate the degradative environment of osteoarthritis by modulating tissue-level properties of the synovium that could impact macroscopic wear as well as mechanical stimuli transmitted to resident cells.

Mechanical adaptation of synoviocytes A and B to immobilization and remobilization: a study in the rat knee flexion model

The objective of this study was to quantify the in vivo response of synoviocytes type A and B in the posterior joint capsule to knee immobilization and remobilization. Also, to correlate the immunohistochemical data with selected mRNA expression in the posterior joint capsule. Forty-two adult male Sprague-Dawley rats had one knee joint immobilized in flexion for durations of 1-4 weeks. Fifteen were harvested after immobilization and 15 were remobilized for 4 weeks. They were analyzed immunohistochemically with CD68 and CD55 antibodies as markers for synoviocytes type A and type B, respectively. Controls were 15 age-matched rats. The remaining 12 rats had their posterior capsule harvested and synoviocyte-specific CD68, CD55, and uridine diphosphoglucose dehydrogenase (UDPGD) mRNA expression was measured. Controls were 12 sham-operated knees. Knee immobilization for 2 weeks significantly increased synoviocytes A:B staining ratio compared to controls (3.88 ± 1.39 vs. 1.83 ± 0.76; p < 0.05). Remobilization for 4 weeks abolished the increase. Remobilization of knees that were immobilized for 1 week also significantly lowered the synoviocytes A:B staining ratios compared to immobilized-only knees (0.66 ± 0.23 vs. 2.19 ± 0.54; p < 0.05) and to controls (0.66 ± 0.23 vs. 1.32 ± 0.29; p < 0.05). Consistent with the immunohistochemistry, mRNA expression of synoviocyte type B-specific CD55 and UDPGD genes were significantly lower in the capsules immobilized for 2 weeks (both p < 0.05). Knee immobilization and remobilization significantly modulated synoviocytes in vivo, stressing their mechanosensitive nature and possible contribution to immobility-induced changes of the joint capsule.

Modulation of the mechanical responses of synovial fibroblasts by osteoarthritis-associated inflammatory stressors

OBJECTIVES

To compare mechanobiological response of synovial fibroblasts (SFb) from OA patient cohorts under mechanical load and inflammatory stressors for better understanding of SFb homeostatic functions.

METHODS

Primary SFb isolated from knee synovium of OA obese (OA-ob:SFb), OA-pre-obese (OA-Pob:SFb), non-OA arthroscopic (scope:SFb), and non-OA arthroscopic with cartilage damage (scope-CD:SFb) were exposed to OA-conditioned media (OACM), derived from OA obese (OA-ob:CM), OA-pre-obese (OA-Pob:CM), and mechanical stretch at either 0 %, 6 % or 10 % for 24 h. Differences in the mRNA levels of genes involved in extracellular matrix production, inflammation and secretory activity were measured.

RESULTS

Despite the significant BMI differences between the OA-ob and OA-Pob groups, OA-Pob has more patients with underlying dyslipidaemia, and low-grade synovitis with higher levels of secreted proteins, CXCL8, COL4A1, CCL4, SPARC and FGF2 in OA-Pob:CM. All primary SFb exhibited anti-proliferative activity with both OA-CM. Mechanical stretch stimulated lubricin production in scope:SFb, higher TGFβ1 and COL1A1 expressions in scope-CD:SFb. OA-Pob:CM stimulated greater detrimental effects than the OA-ob:CM, with higher pro-inflammatory cytokines, IL1β, IL6, COX2 and proteases such as aggrecanases, ADAMTS4 and ADAMTS5, and lower ECM matrix, COL1A1 expressions in all SFb. OA-ob:SFb were unresponsive but expressed higher pro-inflammatory cytokines under OA-Pob:CM treatment.

CONCLUSION

Both mechanical and inflammatory stressors regulate SFb molecular functions with heterogeneity in responses that are dependent on their pathological tissue of origins. While mechanical stretch promotes a favorable effect with enhanced lubricin production in scope:SFb and TGFβ1 and COL1A1 in scope-CD:SFb, the presence of excessively high OA-associated inflammatory mediators in OA-Pob:CM, predominantly SPARC, CXCL8 and FGF2 drive all SFb regardless of pathology, towards greater pro-inflammatory activities.

[Synovial fluid cytodiagnosis]

Synovial fluid samples represent only a very small percentage of routine work in a cytology laboratory. However, its microscopic examination allows us to observe different types of cells, particles and structures that, due to their morphological characteristics, may provide relevant data for cytodiagnosis. We present certain aspects related to arthrocentesis, the relationship between the gross appearance of synovial fluid and certain pathological processes, as well as the different techniques for processing and staining the smears. Furthermore, we describe the main cytological findings in various pathological conditions of the synovial joints, such as infections (bacterial and fungal), non-infectious inflammatory type (osteoarthrosis, rheumatoid arthritis, connective tissue diseases) and tumoral, distinguishing between primary and metastatic, both solid and haematological neoplasms.

Olive leaf extract prevents cartilage degeneration in osteoarthritis of STR/ort mice

The chondroprotective effect of olive leaf extract (OLE) on knee osteoarthritis (OA) was studied with STR/ort mice (n = 5). OLE was administrated with a dosage of 100 mg/kg for 8 weeks and the OA severity score of hind limb knee joints was then measured. The Mankin scores of the knee joints of the non-OA control group, OA control group and OLE-treated group were 3.50, 11.13 and 7.20, respectively. This suggests that oral OLE supplements help prevent cartilage degeneration in STR/ort mice. In vitro, the synthesis of high molecular weight hyaluronan in synovial cells (HIG-82) was increased by OLE stimulation. This suggests that OLE modulates hyaluronan metabolism in synovial cells and improves OA symptoms. Our findings indicate that OLE intake inhibits cartilage destruction by increasing high molecular weight hyaluronan and thus preventing OA progress.

Dysregulation of hyaluronan homeostasis during aortic valve disease

Aortic valve disease (AVD) is one of the leading causes of cardiovascular mortality. Abnormal expression of hyaluronan (HA) and its synthesizing/degrading enzymes have been observed during latent AVD however, the mechanism of impaired HA homeostasis prior to and after the onset of AVD remains unexplored. Transforming growth factor beta (TGFβ) pathway defects and biomechanical dysfunction are hallmarks of AVD, however their association with altered HA regulation is understudied. Expression of HA homeostatic markers was evaluated in diseased human aortic valves and TGFβ1-cultured porcine aortic valve tissues using histology, immunohistochemistry and Western blotting. Further, porcine valve interstitial cell cultures were stretched (using Flexcell) and simultaneously treated with exogenous TGFβ1±inhibitors for activated Smad2/3 (SB431542) and ERK1/2 (U0126) pathways, and differential HA regulation was assessed using qRT-PCR. Pathological heavy chain HA together with abnormal regional expression of the enzymes HAS2, HYAL1, KIAA1199, TSG6 and IαI was demonstrated in calcified valve tissues identifying the collapse of HA homeostatic machinery during human AVD. Heightened TSG6 activity likely preceded the end-stage of disease, with the existence of a transitional, pre-calcific phase characterized by HA dysregulation. TGFβ1 elicited a fibrotic remodeling response in porcine aortic valves similar to human disease pathology, with increased collagen and HYAL to HAS ratio, and site-specific abnormalities in the expression of CD44 and RHAMM receptors. Further in these porcine valves, expression of HAS2 and HYAL1 was found to be differentially regulated by the Smad2/3 and ERK1/2 pathways, and CD44 expression was highly responsive to biomechanical strain. Leveraging the regulatory pathways that control both HA maintenance in normal valves and early postnatal dysregulation of HA homeostasis during disease may identify new mechanistic insight into AVD pathogenesis.

Adenosine monophosphate-activated protein kinase activation and suppression of inflammatory response by cell stretching in rabbit synovial fibroblasts

Joint mobilization is known to be beneficial in osteoarthritis (OA) patients. This study aimed to investigate the effect of stretching on adenosine monophosphate-activated protein kinase (AMPK) activity and its role in modulating inflammation in rabbit synovial fibroblasts. Uniaxial stretching of isolated rabbit synovial fibroblasts for ten min was performed. Stretching-induced AMPK activation, its underlying mechanism, and its anti-inflammatory effect were investigated using Western blot. Static stretching at 20 % of initial length resulted in AMPK activation characterized by expression of phosphorylated AMPK and phosphorylated acetyl-Co A carboxylase. AMP-activated protein kinase phosphorylation peaked 1 h after stretching and declined toward resting activity. Using cell viability assays, static stretching did not appear to cause cellular damage. Activation of AMPK involves Ca²⁺ influx via a mechanosensitive L-type Ca²⁺ channel, which subsequently raises intracellular Ca²⁺ and activates AMPK via Ca²⁺/calmodulin-dependent protein kinase kinase β (CaMKKβ). Interestingly, stretching suppressed TNFα-induced expression of COX-2, iNOS, and phosphorylated NF-κB. These effects were prevented by pretreatment with compound C, an AMPK inhibitor. These results suggest that mechanical stretching suppressed inflammatory responses in synovial fibroblasts via a L-type Ca²⁺-channel-CaMKKβ-AMPK-dependent pathway which may underlie joint mobilization's ability to alleviate OA symptoms.

Chitosan oligosaccharide suppresses synovial inflammation via AMPK activation: An in vitro and in vivo study

Synovial inflammation plays an important role in the early pathogenesis of osteoarthritis (OA). Chitosan oligosaccharide (COS) has been shown to activate AMPK and suppress inflammatory responses in intestinal epithelial cells. This study aimed to investigate the effect of COS on AMPK activation and synovial inflammation using both primary cultures of synoviocytes and a rabbit model of anterior cruciate ligament (ACL) transection-induced OA. COS induced AMPK activation in both rabbit and human synoviocytes. The mechanism of COS-induced AMPK activation involves an increase in the ADP/ATP ratio but not calcium/calmodulin-dependent protein kinase kinase beta (CaMKKβ). Interestingly, COS suppressed the TNFα-induced iNOS and COX-2 expression via an AMPK-dependent mechanism in both rabbit and human synoviocytes. Importantly, oral administration of COS (10mg/kg/day) induced AMPK activation and alleviated signs of inflammation including COX-2 expression in the synovium of a rabbit ACL transection model. Taken together, our results indicate that COS suppresses synovial inflammation in vitro and in vivo via AMPK activation. COS may be useful in the prevention of OA.

Comparison of meniscal fibrochondrocyte and synoviocyte bioscaffolds toward meniscal tissue engineering in the dog

Tissue engineering is a promising field of study toward curing the meniscal deficient stifle; however the ideal cell type for this task is not known. We describe here the extraction of synoviocytes and meniscal fibrochondrocytes from arthroscopic debris from six dogs, which were cultured as tensioned bioscaffolds to synthesize meniscal-like fibrocartilage sheets. Despite the diseased status of the original tissues, synoviocytes and meniscal fibrochondrocytes had high viability at the time of removal from the joint. Glycosaminoglycan and collagen content of bioscaffolds did not differ. Meniscal fibrochondrocyte bioscaffolds contained more type II collagen, but collagen deposition was disorganized, with only 30-40% of cells viable. The collagen of synoviocyte bioscaffolds was organized into sheets and bands and 80-90% of cells were viable. Autologous, diseased meniscal fibrochondrocytes and synoviocytes are plausible cell sources for future meniscal tissue engineering research, however cell viability of meniscal fibrochondrocytes in the tensioned bioscaffolds was low.

In vitro synthesis of tensioned synoviocyte bioscaffolds for meniscal fibrocartilage tissue engineering

Background

Meniscal injury is a common cause of lameness in the dog. Tissue engineered bioscaffolds may be a treatment option for meniscal incompetency, and ideally would possess meniscus- like extracellular matrix (ECM) and withstand meniscal tensile hoop strains. Synovium may be a useful cell source for meniscal tissue engineering because of its natural role in meniscal deficiency and its in vitro chondrogenic potential. The objective of this study is to compare meniscal -like extracellular matrix content of hyperconfluent synoviocyte cell sheets (“HCS”) and hyperconfluent synoviocyte sheets which have been tensioned over wire hoops (tensioned synoviocyte bioscaffolds, “TSB”) and cultured for 1 month.

Results

Long term culture with tension resulted in higher GAG concentration, higher chondrogenic index, higher collagen concentration, and type II collagen immunoreactivity in TSB versus HCS. Both HCS and TSB were immunoreactive for type I collagen, however, HCS had mild, patchy intracellular immunoreactivity while TSB had diffuse moderate immunoreactivity over the entire bisocaffold. The tissue architecture was markedly different between TSB and HCS, with TSB containing collagen organized in bands and sheets. Both HCS and TSB expressed alpha smooth muscle actin and displayed active contractile behavior. Double stranded DNA content was not different between TSB and HCS, while cell viability decreased in TSB.

Conclusions

Long term culture of synoviocytes with tension improved meniscal- like extra cellular matrix components, specifically, the total collagen content, including type I and II collagen, and increased GAG content relative to HCS. Future research is warranted to investigate the potential of TSB for meniscal tissue engineering.

Hyaluronic acid secretion by synoviocytes alters under cyclic compressive load in contracted collagen gels

Knee osteoarthritis is a degenerative disease of diarthrodial joints. Biomechanical factors are considered as risk factors for the disease, the knee joint being normally subject to pressure. Some studies have examined the biomechanical environment of the knee joint in vitro. The aim of this study was to establish a culture model to mimic the knee joint environment. As a first step, synoviocytes induced contraction of three-dimensional collagen gels. Next, contracted collagen gels containing synoviocytes underwent cyclical compression ranging from 0 to 40 kPa at a frequency of 1.0 Hz for 1.5, 3, 6 and 12 h using the FX-4000C™ Flexercell(®) Compression Plus™ System. RNA in collagen gels was extracted immediately after compression and mRNA expression levels of HAS genes were analyzed by quantitative RT-PCR. Culture medium was collected 48 h after compression and analyzed by agarose gel electrophoresis and cellulose acetate electrophoresis. Synoviocytes in contracted collagen gels were stimulated by cyclic compressive load. Long-term compressive stimulation led to the production of higher molecular weight hyaluronic acid, whereas, short-term, compressive stimulation increased the total amount of hyaluronic acid. Furthermore, mRNA expression levels of both HAS-1 and HAS-2 were significantly higher than without compression. Taken together, using this gel culture system, synoviocytes synthesized higher molecular weight hyaluronic acid and produced large quantities of hyaluronic acid through up-regulation of HAS gene expression. Therefore, the contracted collagen gel model will be a useful in vitro three-dimensional model of the knee joint.

Hyaluronan and synovial joint: function, distribution and healing

Synovial fluid is a viscous solution found in the cavities of synovial joints. The principal role of synovial fluid is to reduce friction between the articular cartilages of synovial joints during movement. The presence of high molar mass hyaluronan (HA) in this fluid gives it the required viscosity for its function as lubricant solution. Inflammation oxidation stress enhances normal degradation of hyaluronan causing several diseases related to joints. This review describes hyaluronan properties and distribution, applications and its function in synovial joints, with short review for using thiol compounds as antioxidants preventing HA degradations under inflammation conditions.

Cyclic stretch augments production of neutrophil chemokines and matrix metalloproteinases-1 (MMP-1) from human decidual cells, and the production was reduced by progesterone

PROBLEM

The purpose of this study was to evaluate the impact of mechanical stretch caused by uterine contraction and progesterone (P₄) on decidual cells (DC), neutrophil chemokines, and MMP-1 expression.

METHOD OF STUDY

DC were cultured, and cyclic stretch was applied using a computer-operated cell stretch system. Interleukin (IL)-8, growth-regulated oncogene (GRO) α, matrix metalloproteinase (MMP)-1, and mRNA and/or protein expression/activity was measured using RT-PCR and ELISA. Neutrophil chemotactic activity in conditioned media was evaluated using migration assays. The effect of P₄ was also studied.

RESULTS

Cyclic mechanical stretch increased IL-8, GROα, mRNA and protein, and MMP-1 production and activity level. Supernatant from stretched cells induced neutrophil chemotactic activity significantly. P₄ suppressed the effect of stretch.

CONCLUSION

The current study demonstrates that cyclic mechanical stretch stimulates the production of neutrophil chemokines and MMP-1 from human decidual cells, and the production was reduced by progesterone. These findings suggest that decidual cells are responding to mechanical and endocrine signals and induce biochemical factors and thereby contribute to the regulation of human labor.

Stretch-induced actin remodeling requires targeting of zyxin to stress fibers and recruitment of actin regulators

Mechanical stimulation induces zyxin-dependent actin cytoskeletal reinforcement. Stretch induces MAPK activation, zyxin phosphorylation, and recruitment to actin stress fibers, independent of p130Cas. Zyxin's C-terminal LIM domains are required for stretch-induced targeting to stress fibers, and zyxin's N-terminus is necessary for actin remodeling.

Reinforcement of actin stress fibers in response to mechanical stimulation depends on a posttranslational mechanism that requires the LIM protein zyxin. The C-terminal LIM region of zyxin directs the force-sensitive accumulation of zyxin on actin stress fibers. The N-terminal region of zyxin promotes actin reinforcement even when Rho kinase is inhibited. The mechanosensitive integrin effector p130Cas binds zyxin but is not required for mitogen-activated protein kinase–dependent zyxin phosphorylation or stress fiber remodeling in cells exposed to uniaxial cyclic stretch. α-Actinin and Ena/VASP proteins bind to the stress fiber reinforcement domain of zyxin. Mutation of their docking sites reveals that zyxin is required for recruitment of both groups of proteins to regions of stress fiber remodeling. Zyxin-null cells reconstituted with zyxin variants that lack either α-actinin or Ena/VASP-binding capacity display compromised response to mechanical stimulation. Our findings define a bipartite mechanism for stretch-induced actin remodeling that involves mechanosensitive targeting of zyxin to actin stress fibers and localized recruitment of actin regulatory machinery.

Fluid movement and joint capsule strains due to flexion in rabbit knees

Diarthrodial joints are freely moveable joints containing synovial fluid (SF) within a connective tissue joint capsule that allows for low-friction and low-wear articulation of the cartilaginous ends of long bones. Biomechanical cues from joint articulation regulate synoviocyte and cartilage biology via joint capsule strain, in turn altering the composition of SF. Joint flexion is clinically associated with pain in knees with arthritis and effusion, with the nociception possibly originating from joint capsule strain. The hypothesis of this study was that knee fluid volume distribution and joint capsule strain are altered with passive flexion in the rabbit model. The aims were to (a) determine the volume distribution of fluid in the joint at different total volumes and with flexion of rabbit knees ex vivo, (b) correlate the volume distribution for the ex vivo model to in vivo data, and (c) determine the strains at different locations in the joint capsule with flexion. During knee flexion, ∼20% of anteriorly located joint fluid moved posteriorly, correlating well with the fluid motion observed in in vivo joints. Planar joint capsule principal strains were ∼100% (tension) in the proximal-distal direction and ∼-40% (shortening) in the circumferential direction, relative to the femur axis and 30° strain state. The joint capsule strains with flexion are consistent with the mechanics of the tendons and ligaments from which the capsule tissue is derived. The movement and mixing of SF volume with flexion determine the mechanical and biological fluid environment within the knee joint. Joint fluid movement and capsular strains affect synovial cell biology and likely modulate trans-synovial transport.

A systems biology approach to synovial joint lubrication in health, injury, and disease

The synovial joint contains synovial fluid (SF) within a cavity bounded by articular cartilage and synovium. SF is a viscous fluid that has lubrication, metabolic, and regulatory functions within synovial joints. SF contains lubricant molecules, including proteoglycan-4 and hyaluronan. SF is an ultrafiltrate of plasma with secreted contributions from cell populations lining and within the synovial joint space, including chondrocytes and synoviocytes. Maintenance of normal SF lubricant composition and function are important for joint homeostasis. In osteoarthritis, rheumatoid arthritis, and joint injury, changes in lubricant composition and function accompany alterations in the cytokine and growth factor environment and increased fluid and molecular transport through joint tissues. Thus, understanding the synovial joint lubrication system requires a multifaceted study of the various parts of the synovial joint and their interactions. Systems biology approaches at multiple scales are being used to describe the molecular, cellular, and tissue components and their interactions that comprise the functioning synovial joint. Analyses of the transcriptome and proteome of SF, cartilage, and synovium suggest that particular molecules and pathways play important roles in joint homeostasis and disease. Such information may be integrated with physicochemical tissue descriptions to construct integrative models of the synovial joint that ultimately may explain maintenance of health, recovery from injury, or development and progression of arthritis.

Ionic currents in intimal cultured synoviocytes from the rabbit

Hyaluronan, a joint lubricant and regulator of synovial fluid content, is secreted by fibroblast-like synoviocytes lining the joint cavity, and secretion is greatly stimulated by Ca²⁺-dependent protein kinase C. This study aimed to define synoviocyte membrane currents and channels that may influence synoviocyte Ca²⁺ dynamics. Resting membrane potential ranged from −30 mV to −66 mV (mean −45 ± 8.60 mV, n = 40). Input resistance ranged from 0.54 GΩ to 2.6 GΩ (mean 1.28 ± 0.57 GΩ; ν = 33). Cell capacitance averaged 97.97 ± 5.93 pF. Voltage clamp using C^s+ pipette solution yielded a transient inward current that disappeared in Ca²⁺-free solutions and was blocked by 1 μM nifedipine, indicating an L-type calcium current. The current was increased fourfold by the calcium channel activator FPL 64176 (300 nM). Using K⁺ pipette solution, depolarizing steps positive to −40 mV evoked an outward current that showed kinetics and voltage dependence of activation and inactivation typical of the delayed rectifier potassium current. This was blocked by the nonspecific delayed rectifier blocker 4-aminopyridine. The synoviocytes expressed mRNA for four Kv1 subtypes (Kv1.1, Kv1.4, Kv1.5, and Kv1.6). Correolide (1 μM), margatoxin (100 nM), and α-dendrotoxin block these Kv1 subtypes, and all of these drugs significantly reduced synoviocyte outward current. The current was blocked most effectively by 50 nM κ-dendrotoxin, which is specific for channels containing a Kv1.1 subunit, indicating that Kv1.1 is critical, either as a homomultimeric channel or as a component of a heteromultimeric Kv1 channel. When 50 nM κ-dendrotoxin was added to current-clamped synoviocytes, the cells depolarized by >20 mV and this was accompanied by an increase in intracellular calcium concentration. Similarly, depolarization of the cells with high external potassium solution caused an increase in intracellular calcium, and this effect was greatly reduced by 1 μM nifedipine. In conclusion, fibroblast-like synoviocytes cultured from the inner synovium of the rabbit exhibit voltage-dependent inward and outward currents, including Ca²⁺ currents. They thus express ion channels regulating membrane Ca²⁺ permeability and electrochemical gradient. Since Ca²⁺-dependent kinases are major regulators of synovial hyaluronan secretion, the synoviocyte ion channels are likely to be important in the regulation of hyaluronan secretion.

Effects of low-intensity pulsed ultrasound on the expression and activity of hyaluronan synthase and hyaluronidase in IL-1β-stimulated synovial cells

The purpose of this study is to examine effects of low-intensity pulsed ultrasound (LIPUS) on metabolism of hyaluronan (HA) in synovial membrane cells stimulated by IL-1β. Rabbit knee synovial membrane cell line, HIG-82, was cultured in medium with the presence or absence of 1 ng/mL IL-1β, and after 4 h the cell was exposed to LIPUS for 15 min. The mRNA levels of HA synthase (HAS) 2,3, hyaluronidase (HYAL) 2, and cyclooxygenase (COX)-2 were examined by real-time PCR analysis. Concentrations of HA and PGE₂ were quantified by use of enzyme linked immunosorbent assay (ELISA). The COX-2 level was analyzed by western blotting. Gene levels of HAS2 and HAS3 in IL-1β-stimulated cells were up-regulated significantly (p < 0.01) by LIPUS. HYAL2 mRNA was up-regulated by the treatment with IL-1β, whereas down-regulated significantly (p < 0.01) by the following LIPUS exposure. Furthermore, IL-1β stimulation enhanced COX-2 and PGE₂ expression as compared to the untreated control, and IL-1β-induced COX-2 and PGE₂ expression was inhibited by LIPUS. These results suggest that LIPUS enhanced HA synthesis and inhibited HYAL2 expression, leading to the accumulation of high-molecular weight HA. Therefore, LIPUS stimulation may be a better candidate as medical remedy to treat inflammatory joint diseases accompanied with HA degradation in synovial fluid.

Acid-sensing ion channel 3 expressed in type B synoviocytes and chondrocytes modulates hyaluronan expression and release

BACKGROUND

Rheumatoid arthritis is an inflammatory disease marked by intra-articular decreases in pH, aberrant hyaluronan regulation and destruction of bone and cartilage. Acid-sensing ion channels (ASICs) are the primary acid sensors in the nervous system, particularly in sensory neurons and are important in nociception. ASIC3 was recently discovered in synoviocytes, non-neuronal joint cells critical to the inflammatory process.

OBJECTIVES

To investigate the role of ASIC3 in joint tissue, specifically the relationship between ASIC3 and hyaluronan and the response to decreased pH.

METHODS

Histochemical methods were used to compare morphology, hyaluronan expression and ASIC3 expression in ASIC3+/+ and ASIC3-/- mouse knee joints. Isolated fibroblast-like synoviocytes (FLS) were used to examine hyaluronan release and intracellular calcium in response to decreases in pH.

RESULTS

In tissue sections from ASIC3+/+ mice, ASIC3 localised to articular cartilage, growth plate, meniscus and type B synoviocytes. In cultured FLS, ASIC3 mRNA and protein was also expressed. In FLS cultures, pH 5.5 increased hyaluronan release in ASIC3+/+ FLS, but not ASIC3-/- FLS. In FLS from ASIC3+/+ mice, approximately 50% of cells (25/53) increased intracellular calcium while only 24% (14/59) showed an increase in ASIC3-/- FLS. Of the cells that responded to pH 5.5, there was significantly less intracellular calcium increases in ASIC3-/- FLS compared to ASIC3+/+ FLS.

CONCLUSION

ASIC3 may serve as a pH sensor in synoviocytes and be important for modulation of expression of hyaluronan within joint tissue.

Differential expression of matrix metalloproteinases and tissue inhibitors of metalloproteinases in anterior cruciate ligament and medial collateral ligament fibroblasts after a mechanical injury: involvement of the p65 subunit of NF-kappaB

The anterior cruciate ligament (ACL) is known to have a poor healing ability, especially in comparison with the medial collateral ligament, which can heal relatively well. In this study, we detected significant increases in the mRNA levels of multiple matrix metalloproteinases (MMPs) (MMP-1, -2, -7, -9, -11, -14, -17, -21, -23A, -24, -25, -27, and -28) and tissue inhibitors of metalloproteinases (TIMPs) (TIMP-1, -2, -3, and -4) in ACL fibroblasts after an in vitro injury with an equi-biaxial stretch chamber. However, only some MMPs (MMP-7, -9, -14, -21, and -24) showed increases in injured medial collateral ligament fibroblasts, and to a much lesser degree than that observed in the injured ACL fibroblasts. Zymography revealed a 6.3-fold increase of MMP-2 activity in injured ACL but not medial collateral ligament fibroblasts, which agrees with the global MMP activities assay. Bay-11 and curcumin can significantly decrease MMP-2 activities to 13% and 29% in injured ACL fibroblasts, respectively, which implies the involvement of p65 subunits of nuclear factor kappaB and AP-1 pathways. Furthermore, Bay-11 can decrease the global MMP activity released from injured ACL fibroblasts in a dose-dependent manner. In summary, the differential expression and activities of MMPs might help to explain the poor healing ability of ACL, and the p65 subunit of nuclear factor kappaB might be a potential target to facilitate the ACL repair.

Signal pathways regulating hyaluronan secretion into static and cycled synovial joints of rabbits

Joint lubrication, synovial fluid conservation and many pathophysiological processes depend on hyaluronan (HA). Intra-articular HA injection and exercise, which stimulates articular HA production, ameliorate osteoarthritis. We therefore investigated the pathways regulating movement-stimulated articular HA secretion rate ( ) in vivo. Endogenous HA was removed from the knee joint cavity of anaesthetised rabbits by washout. Joints were then cycled passively or remained static for 5 h, with/without intra-articular agonist/inhibitor, after which newly secreted HA was harvested for analysis. Movement almost doubled . Similar or larger increases were elicited in static joints by the intra-articular Ca(2+) ionophore ionomycin, prostaglandin E(2), cAMP-raising agents, serine/threonine phosphatase inhibitor and activation of protein kinase C (PKC). PKC-stimulated secretion was inhibited by the PKC inhibitor bisindolylmaleimide I and inhibitors of the downstream kinases MEK-ERK (U0126, PD98059). These agents inhibited movement-stimulated secretion of HA (MSHA) only when the parallel p38 kinase path was simultaneously inhibited by SB203580 (ineffective alone). The phospholipase C inhibitor U73122 almost fully blocked MSHA (P = 0.001, n = 10), without affecting static . The ENaC channel blocker amiloride inhibited MSHA, whereas other inhibitors of stretch-activated channels (Gd(3+), ruthenium red, SKF96365) did not. It is proposed that MSHA may be mediated by PLC activation, leading to activation of parallel PKC-MEK-ERK and p38 kinase pathways.

Does running cause metatarsophalangeal joint effusions? A comparison of synovial fluid volumes on MRI in athletes before and after running

OBJECTIVE

The metatarsophalangeal joints (MTPJ) are the only joints that bear weight directly through synovium. The purpose of this study was to determine whether there is an association between synovial stresses during running and increases in volume of joint fluid.

MATERIALS AND METHODS

This was a prospective case controlled study (nine healthy athlete volunteers acting as own controls). High-resolution coronal 3D T2W magnetic resonance imaging of the MTPJs were obtained following 24 h rest and after a 30-min run. The volume of joint fluid in each MTPJ (n = 90) was measured by two independent observers using an automated propagating segmentation tool.

RESULTS

The median volume of synovial fluid in the MTPJs at rest was 0.018 ml (inter-quartile range (IQ) range 0.005-0.04) and after running 0.019 ml (IQ range 0.005-0.04, p = 0.34, 99% confidence interval (CI), 0.330.35). The volume of fluid in the MTPJs of the great toes was substantially larger than other toes (0.152 ml at rest, 0.154 ml after exercise, p = 0.903). Median volumes decrease from second to fifth MTPJs (0.032-0.007 ml at rest and 0.035-0.004 ml after exercise). Subset analysis for each toe revealed no significant difference in volumes before and after running (p = 0.39 to p = 0.9). The inter-rater reliability for observer measurements was good with an intra-class correlation of 0.70 (95% CI, 0.60 to 0.78).

CONCLUSION

It appears to be normal to find synovial fluid, particularly in the MTPJs of the great toes, of athletes at rest and after running. There does not appear to be an association between moderate distance running and an increase in the volume of synovial fluid.

Mechanosensitive hyaluronan secretion: stimulus-response curves and role of transcription-translation-translocation in rabbit joints

Joint movement was recently shown to stimulate the secretion of the lubricant hyaluronan (HA); also, exercise therapy and intra-articular hyaluronan injections are used to treat moderate osteoarthritis. The present study quantifies the stimulus-response curves for HA secretion in vivo and reports a role of transcription-translation-translocation in the secretory response. After washing out endogenous HA from anaesthetized, cannulated rabbit knees, the joints were cycled passively at various frequencies and durations, with or without intra-articular inhibitors of protein synthesis and Golgi processing. Newly secreted HA was harvested for analysis after 5 h. Joints displayed graded, non-linear stimulus-response curves to both duration and frequency of movement; 1 min duration per 15 min or a frequency of 0.17 Hz raised HA secretion by 42-54%, while rapid (1.5 Hz) or prolonged cycling (9 min per 15 min) raised it by 110-130%. Movement-stimulated secretion and phorbol ester-stimulated secretion were partly inhibited by the translation inhibitor cycloheximide, by the transcription-translation inhibitors actinomycin D and puromycin and by the Golgi translocation inhibitor brefeldin A. There is thus a graded coupling between HA secretion and cyclic joint movement that depends partly on new protein synthesis. This is likely to be important for joint homeostasis, providing protection during repetitive cycling and potentially contributing to exercise therapy for osteoarthritis.

Role of ASIC3 in the primary and secondary hyperalgesia produced by joint inflammation in mice

The acid sensing ion channel 3 (ASIC3) is critical for the development of secondary hyperalgesia as measured by mechanical stimulation of the paw following muscle insult. We designed experiments to test whether ASIC3 was necessary for the development of both primary and secondary mechanical hyperalgesia that develops after joint inflammation. We used ASIC3 -/- mice and examined the primary (response to tweezers) and secondary hyperalgesia (von-Frey filaments) that develops after joint inflammation comparing to ASIC3 +/+ mice. We also examined the localization of ASIC3 to the knee joint afferents innervating the synovium using immunohistochemical techniques before and after joint inflammation. We show that secondary mechanical hyperalgesia does not develop in ASIC3 -/- mice. However, the primary mechanical hyperalgesia of the inflamed knee joint still develops in ASIC3 -/- mice and is similar to ASIC3 +/+ mice. In knee joint synovium from ASIC3 +/+ mice without joint inflammation, ASIC3 was not localized to joint afferents that were stained with an antibody to protein gene product (PGP) 9.5 or calcitonin gene-related peptide (CGRP). ASIC3 was found, however, in synoviocytes of the knee joint of uninflamed mice. In ASIC3 +/+ mice with joint inflammation, ASIC3 co-localized with PGP 9.5 or CGRP in joint afferents innervating the synovium. We conclude that the decreased pH that occurs after inflammation would activate ASIC3 on primary afferent fibers innervating the knee joint, increasing the input to the spinal cord resulting in central sensitization manifested behaviorally as secondary hyperalgesia of the paw.

Endogenous overexpression of hyaluronan synthases within dynamically cultured collagen gels: Implications for vascular and valvular disease

Hyaluronan is a ubiquitous component of the extracellular matrix with important roles in cell and tissue functions. Hyaluronan content is often elevated in cardiovascular diseases, such as mitral valve disease and atherosclerosis. The objective of this study was to determine the impact of endogenously produced hyaluronan dynamically cultured three-dimensional model of collagenous tissues. Collagen gels containing excess HA and hyaluronan synthase (has) overexpressing cells were grown in a cyclic strain environment to simulate cell-mediated matrix organization. Cyclic strain caused a significant elevation in the collagen fibril density, cell number, and hyaluronan content of the resulting collagen gels compared to those grown under a static strain regimen. The material behavior of collagen gels containing has overexpressing cells was also notably weakened compared to controls. Transmission electron microscopy and immunohistochemistry showed that proteoglycan distribution was influenced by both strain and has overexpression. The results were also dependent on the specific has isozyme overexpressed. This investigation helps to identify the mechanism by which hyaluronan acts in vivo to alter tissue material behavior in cardiovascular diseases such as myxomatous mitral valve disease and atherosclerosis.

Cyclic movement stimulates hyaluronan secretion into the synovial cavity of rabbit joints

The novel hypothesis that the secretion of the joint lubricant hyaluronan (HA) is coupled to movement has implications for normal function and osteoarthritis, and was tested in the knee joints of anaesthetized rabbits. After washing out the endogenous synovial fluid HA (miscibility coefficient 0.4), secretion into the joint cavity was measured over 5 h in static joints and in passively cycled joints. The net static secretion rate (11.2 +/- 0.7 microg h(-1), mean +/- s.e.m., n = 90) correlated with the variable endogenous HA mass (mean 367 +/- 8 microg), with a normalized value of 3.4 +/- 0.2 microg h(-1) (100 microg)(-1) . Cyclic joint movement approximately doubled the net HA secretion rate to 22.6 +/- 1.2 microg h(-1) (n = 77) and raised the normalized percentage to 5.9 +/- 0.3 microg h(-1) (100 microg)(-1). Secretion was inhibited by 2-deoxyglucose and iodoacetate, confirming active secretion. The net accumulation rate underestimated true secretion rate due to some trans-synovial loss. HA turnover time (endogenous mass/secretion rate) was 17-30 h (static) to 8-15 h (moved) The results demonstrate for the first time that the active secretion of HA is coupled to joint usage. Movement-secretion coupling may protect joints against the damaging effects of repetitive joint use, replace HA lost during periods of immobility (overnight), and contribute to the clinical benefit of exercise therapy in moderate osteoarthritis.

The development of synovial joints

During vertebrate evolution, successful adaptation of animal limbs to a variety of ecological niches depended largely on the formation and positioning of synovial joints. The function of a joint is to allow smooth articulation between opposing skeletal elements and to transmit biomechanical loads through the structure, and this is achieved through covering the ends of bones with articular cartilage, lubricating the joint with synovial fluid, using ligaments to bind the skeletal elements together, and encapsulating the joint in a protective fibrous layer of tissue. The diversity of limb generation has been proposed to occur through sequential branching and segmentation of precartilaginous skeletal elements along the proximodistal axis of the limb. The position of future joints is first delimited by areas of higher cell density called interzones initially through an as yet unidentified inductive signal, subsequently specification of these regions is controlled hierarchically by wnt14 and gdf5, respectively. Joint-forming cell fate although specified is not fixed, and joints will fuse if growth factor signaling is perturbed. Cavitation, the separation of the two opposing skeletal elements, and joint morphogenesis, the process whereby the joint cells organize and mature to establish a functional interlocking and reciprocally shaped joint, are slowly being unraveled through studying the plethora of molecules that make up the unique extracellular matrix of the forming structure. The joint lining tissue, articular cartilage, is avascular, and this limits its reparative capacity such that arthritis and associated joint pathologies are the single largest cause of disability in the adult population. Recent discoveries of adult stem cells and more specifically the isolation of chondroprogenitor cells from articular cartilage are extending available therapeutic options, though only with a more complete understanding of synovial joint development can such options have greater chances of success.

Interstitial matrix proteins determine hyaluronan reflection and fluid retention in rabbit joints: effect of protease

Hyaluronan (HA) retention inside the synovial cavity of joints serves diverse protective roles. We tested the hypothesis that HA retention is mediated by the network of extracellular matrix proteins in the synovial lining. Cannulated rabbit knee joints were infused with HA solution with or without pretreatment by chymopapain, a collagen-sparing protease. Trans-synovial fluid escape rate was measured and, after a period of trans-synovial filtration, samples of intra-articular fluid and subsynovial fluid were analysed for HA to assess its trans-synovial ultrafiltration. In control joints, HA ultrafiltration was confirmed by postfiltration increases in intra-articular HA concentration (259 +/- 17% of infused concentration) and reduced subsynovial concentration (30 +/- 8%; n = 11). The proportion of HA molecules reflected by the synovium was 57-75%. Chymopapain treatment increased the hydraulic permeability of the synovial lining approximately 13-fold, almost abolished the trans-synovial difference in HA concentration and reduced the HA reflected fraction to 3-7% (n = 6; P < 0.001, ANOVA). Structural studies confirmed that chymopapain treatment depleted the matrix of proteoglycans but preserved its collagen. The findings thus demonstrate that HA ultrafiltration and synovial hydraulic permeability are determined by the network of non-collagen, extracellular matrix proteins. This may be important clinically, since protease activity is raised in rheumatoid arthritis, as are HA and fluid escape.

Mechanosensitive synoviocytes: A Ca2+–PKCα–MAP kinase pathway contributes to stretch-induced hyaluronan synthesis in vitro

Hyaluronan (HA) is central to joint function, contributing to synovial fluid retention, lubrication, matrix organisation and joint embryogenesis. HA synthesis by intimal synoviocytes is stimulated by stretch (SSHA), linking HA production to joint usage; but the signal transduction paths are unknown. Low passage rabbit synoviocytes (RS), cultured from micro dissected synovial intima, were subjected to 10min of 10% static stretch followed by 170-min relaxation, or to sustained stretch for 180min in a Flexcell 2000 apparatus. Medium HA content was analysed by a HA-binding assay. The roles of protein kinase C (PKC) isoforms, extracellular signal-regulated kinases (ERK1/2) and Ca(2+) signalling in SSHA were tested using kinase inhibitors, Ca(2+) chelators and Ca(2+) channel activators combined with Western blots for activated kinases. Stretch increased HA secretion by 57%, independently of stretch duration. PKCalpha translocated from cytosol to membrane and triggered the phosphorylation of ERK1/2. The PKC inhibitor bisindolylmaleimide (BIM) blocked both SSHA and ERK phosphorylation, as did Gö 6976, a specific inhibitor of Ca(2+)-dependent PKC. The Ca(2+) channel activator Bay K stimulated HA secretion and ERK phosphorylation. Extra- and intra-cellular Ca(2+) chelation by EGTA and BAPTA-AM (respectively) inhibited SSHA. SSHA was also blocked by the partially selective protein kinase A inhibitor, H-89. Connective tissue growth factor, CTGF, was not involved in SSHA. Thus, stimulation of synoviocyte HA secretion by static stretch is due at least in part the o activation of a Ca(2+) influx-dependent activation of the PKCalpha-MEK-ERK1/2 cascade. This is functionally important because it links joint lubrication to joint use.

Hyaluronan molecular reflection by synovial lining is concentration dependent and reduced in dilute effusions in a rabbit model

OBJECTIVE

Hyaluronan (HA) has a major role in regulating synovial fluid volume. This role depends on the synovium functioning as an ultrafilter that reflects HA during trans-synovial fluid drainage. Reflection boosts the HA concentration on the membrane surface, leading to osmotic retention of synovial fluid ("buffering"). In arthritic effusions, however, HA concentration and osmotic buffering are greatly reduced. We tested the hypothesis that reflection is reduced (escape increased) when the HA concentration falls below the molecular entanglement concentration (C*).

METHODS

HA at 0.2 mg/ml (<C*) or 1.5 mg/ml (>C*) was infused continuously into rabbit knee joints to set up a steady trans-synovial filtration. Joint-derived lymph was sampled over 3 hours, and subsynovial fluid was sampled at the end of the 3-hour period. HA was quantified by high-performance liquid chromatography to evaluate the reflected fraction. C* was determined by viscometry.

RESULTS

Viscometry showed that 0.2 mg/ml HA was below C* and 1.5 mg/ml was above it. At 0.2 mg/ml, the mean +/- SEM HA reflected fraction was 0.66 +/- 0.04 (n = 7). At 1.5 mg/ml the reflection increased to 0.88 +/- 0.04 (n = 5) (P < 0.005). HA permeation increased almost 3-fold, from 12% to 34%, at the lower concentration.

CONCLUSION

Chain-chain interaction at >C* increases effective molecular domain size and hence HA reflection, promoting effective conservation of synovial fluid in normal joints. HA can fall below C* (approximately 1 mg/ml) in arthritic effusions, promoting loss of HA. The attendant failure of outflow buffering facilitates fluid escape and periarticular edema.