Transthyretin deposition in articular cartilage: a novel mechanism in the pathogenesis of osteoarthritis

OBJECTIVE

Amyloid deposits are prevalent in osteoarthritic (OA) joints. We undertook this study to define the dominant precursor and to determine whether the deposits affect chondrocyte functions.

METHODS

Amyloid deposition in human normal and OA knee cartilage was determined by Congo red staining. Transthyretin (TTR) in cartilage and synovial fluid was analyzed by immunohistochemistry and Western blotting. The effects of recombinant amyloidogenic and nonamyloidogenic TTR variants were tested in human chondrocyte cultures.

RESULTS

Normal cartilage from young donors did not contain detectable amyloid deposits, but 7 of 12 aged normal cartilage samples (58%) and 12 of 12 OA cartilage samples (100%) had Congo red staining with green birefringence under polarized light. TTR, which is located predominantly at the cartilage surfaces, was detected in all OA cartilage samples and in a majority of aged normal cartilage samples, but not in normal cartilage samples from young donors. Chondrocytes and synoviocytes did not contain significant amounts of TTR messenger RNA. Synovial fluid TTR levels were similar in normal and OA knees. In cultured chondrocytes, only an amyloidogenic TTR variant induced cell death as well as the expression of proinflammatory cytokines and extracellular matrix-degrading enzymes. The effects of amyloidogenic TTR on gene expression were mediated in part by Toll-like receptor 4, receptor for advanced glycation end products, and p38 MAPK. TTR-induced cytotoxicity was inhibited by resveratrol, a plant polyphenol that stabilizes the native tetrameric structure of TTR.

CONCLUSION

These findings are the first to suggest that TTR amyloid deposition contributes to cell and extracellular matrix damage in articular cartilage in human OA and that therapies designed to reduce TTR amyloid formation might be useful.

The relationship of autophagy defects to cartilage damage during joint aging in a mouse model

OBJECTIVE

Aging is a main risk factor for osteo arthritis (OA), the most prevalent musculoskeletal disorder. Defects in autophagy, an essential cellular homeostasis mechanism, have recently been observed in OA articular cartilage. The objectives of this study were to establish the constitutive level of autophagy activation in normal cartilage and to monitor the temporal relationship between changes in autophagy and aging-related degradation of cartilage in a mouse model.

METHODS

In GFP-LC3-transgenic mice, green fluorescent protein (GFP)-light chain 3 (LC3) is ubiquitously expressed, and the accumulation of GFP puncta, representing autophagosomes, was quantified by confocal microscopy as a measure of autophagy activation. Expression of the autophagy proteins autophagy-related protein 5 (ATG-5) and microtubule-associated protein 1 light chain 3 (LC3) was analyzed by immunohistochemistry. Cartilage cellularity, apoptotic cell death, and cartilage structural damage and changes in synovium and bone were examined by histology and immunohistochemistry.

RESULTS

Basal autophagy activation was detected in liver and knee articular cartilage from young (6-month-old) mice, with higher levels in cartilage than in liver in the same animals. In 28-month-old mice, there was a statistically significant reduction in the total number of autophagic vesicles per cell (P < 0.01) and in the total area of vesicles per cell (P < 0.01) in the articular cartilage as compared to that from young 6-month-old mice. With increasing age, the expression of ATG-5 and LC3 decreased, and this was followed by a reduction in cartilage cellularity and an increase in the apoptosis marker poly(ADP-ribose) polymerase p85. Cartilage structural damage progressed in an age-dependent manner subsequent to the autophagy changes.

CONCLUSION

Autophagy is constitutively activated in normal cartilage. This is compromised with aging and precedes cartilage cell death and structural damage.

Autophagy activation and protection from mitochondrial dysfunction in human chondrocytes

OBJECTIVE

Autophagy is a key pathway of cellular homeostasis for removing damaged macromolecules and organelles, including mitochondria. Recent studies indicate that activation of autophagy is defective in aging and osteoarthritis (OA), contributing to cell death and tissue damage. In addition, there is increasing evidence that mitochondrial dysfunction plays an important role in OA pathogenesis. The objective of this study was to determine whether activation of autophagy protects against mitochondrial dysfunction in human chondrocytes.

METHODS

Human chondrocytes were treated with oligomycin, an inhibitor of mitochondrial respiratory chain complex V. Autophagy activation was analyzed by determination of light chain 3 membrane-bound form II (LC3-II), a marker of autophagosome formation. To investigate whether autophagy protects from mitochondrial dysfunction, autophagy was induced by rapamycin, the selective inhibitor of mammalian target of rapamycin complex 1 (mTORC-1), and by torin 1, the inhibitor of mTORC-1 and mTORC-2. Small interfering autophagy-related 5 was used to evaluate the role of autophagy in mitochondrial dysfunction.

RESULTS

Mitochondrial dysfunction was induced by treatment with oligomycin, which significantly decreased mitochondrial membrane potential (ΔΨm). This was associated with increased production of reactive oxygen species and cell death. Autophagy activation, as reflected by LC3-II, was decreased in a time-dependent manner. To evaluate whether autophagy regulates mitochondrial function, chondrocytes were pretreated with rapamycin and torin 1 before oligomycin. Autophagy activation significantly protected against mitochondrial dysfunction. Conversely, genetic inhibition of autophagy induced significant mitochondrial function defects.

CONCLUSION

Our data highlight the role of autophagy as a critical protective mechanism against mitochondrial dysfunction. Pharmacologic interventions that enhance autophagy may have chondroprotective activity in cartilage degenerative processes such as OA.

Chondrocyte clusters adjacent to sites of cartilage degeneration have characteristics of progenitor cells

The purpose of this study was to investigate the site-specific characteristics and roles of chondrocyte clusters in human knee osteoarthritis. Cartilage explants were obtained from 45 knees undergoing total knee replacement surgery. The explants were taken from 4 locations in the knee: the medial femoral condyle, the medial posterior femoral condyle (MPC), the lateral femoral condyle, and the lateral posterior femoral condyle (LPC). Cartilage degeneration, cell density, and cell arrangement were compared histologically. A live/dead cell viability assay and immunohistochemical analyses using antibodies against STRO-1, FGF2, and Ki-67 were performed. Cell proliferation and cartilaginous nodule production in MPC and LPC explants in monolayer culture were compared. Finally, MPC cartilage explants were cultured to observe histological changes. The cell density of the MPC explants was higher than that of the LPC because of clustering. MPC explants contained more live cells than the LPC did, and the expression of IHC markers in MPC explants was higher than that in LPC. Chondrocytes from MPC proliferated faster and produced more nodules in monolayer culture than those from the LPC and MPC explants were repaired during organ culture. In conclusion, chondrocyte clusters adjacent to severe cartilage degeneration have specific characteristics, with progenitor and proliferative potential.

FoxO transcription factors support oxidative stress resistance in human chondrocytes

OBJECTIVE

A major signaling pathway that regulates cellular aging is the insulin/insulin-like growth factor 1 (IGF-1)/phosphatidylinositol 3-kinase (PI3K)/Akt/FoxO transcription factor axis. We previously observed that FoxO transcription factors are dysregulated in aged and OA cartilage. The objective of this study was to investigate the impact of down-regulated FoxO transcription factors on chondrocytes.

METHODS

Small interfering RNAs (siRNAs) targeting FOXO1 (siFOXO1) and FOXO3 (siFOXO3) were transfected into human articular chondrocytes. Cell viability following treatment with the oxidant tert-butyl-hydroperoxide (tBHP) was measured by MTT assay. Caspase 3/7 activation and apoptotic cells were examined. Gene and protein expression of antioxidant proteins and autophagy-related proteins and changes in inflammatory mediators following treatment with interleukin-1β were assessed. Cells transfected with FOXO plasmids were also analyzed.

RESULTS

Cell viability was significantly reduced by siFOXO after treatment with tBHP. Apoptosis accompanied by caspase activation was significantly increased in siFOXO-transfected chondrocytes. Knockdown of FOXO1 and FOXO1+3 resulted in significant reductions in levels of glutathione peroxidase 1 (GPX-1), catalase, light chain 3 (LC3), Beclin1, and sirtuin 1 (SIRT-1) proteins following treatment with tBHP. In contrast, the constitutive active form of FOXO3 increased cell viability while inducing GPX-1, Beclin1, and LC3 in response to tBHP. Expression and production of ADAMTS-4 and chemerin were significantly increased in siFOXO-transfected chondrocytes.

CONCLUSION

Reduced expression of FoxO transcription factors in chondrocytes increased susceptibility to cell death induced by oxidative stress. This was associated with reduced levels of antioxidant proteins and autophagy-related proteins. Our data provide evidence for a key role of FoxO transcription factors as regulators of chondrocyte oxidative stress resistance and tissue homeostasis.

Palmitate has proapoptotic and proinflammatory effects on articular cartilage and synergizes with interleukin-1

OBJECTIVE

Obesity is a major risk factor for the development of osteoarthritis (OA) that is associated with a state of low-grade inflammation and increased circulating levels of adipokines and free fatty acids (FFAs). The aim of this study was to analyze the effects of saturated (palmitate) and monounsaturated (oleate) FFAs on articular chondrocytes, synoviocytes, and cartilage.

METHODS

Human articular chondrocytes and fibroblast-like synoviocytes obtained from young healthy donors and OA chondrocytes from patients undergoing total knee replacement surgery were treated with palmitate or oleate alone or in combination with interleukin-1β (IL-1β). Cell viability, caspase activation, and gene expression of proinflammatory factors, extracellular matrix (ECM) proteins, and proteases were studied. In addition, chondrocyte viability, IL-6 production, and matrix damage were assessed in bovine and human articular cartilage explants cultured with FFAs in the presence or absence of IL-1β.

RESULTS

Palmitate, but not oleate, induced caspase activation and cell death in IL-1β-stimulated normal chondrocytes, and up-regulated the expression of IL-6 and cyclooxygenase 2 in chondrocytes and fibroblast-like synoviocytes through Toll-like receptor 4 (TLR-4) signaling. In cartilage explants, palmitate induced chondrocyte death, IL-6 release, and ECM degradation. Palmitate synergized with IL-1β in stimulating proapoptotic and proinflammatory cellular responses. Pharmacologic inhibition of caspases or TLR-4 signaling reduced palmitate and IL-1β induced cartilage damage.

CONCLUSION

Palmitate acts as a proinflammatory and catabolic factor that, in synergy with IL-1β, induces chondrocyte apoptosis and articular cartilage breakdown. Collectively, our data suggest that elevated levels of saturated FFAs that are often found in patients who are obese may contribute to the pathogenesis of OA.

FoxO transcription factors support oxidative stress resistance in human chondrocytes

OBJECTIVE

A major signaling pathway that regulates cellular aging is the insulin/insulin-like growth factor 1 (IGF-1)/phosphatidylinositol 3-kinase (PI3K)/Akt/FoxO transcription factor axis. We previously observed that FoxO transcription factors are dysregulated in aged and OA cartilage. The objective of this study was to investigate the impact of down-regulated FoxO transcription factors on chondrocytes.

METHODS

Small interfering RNAs (siRNAs) targeting FOXO1 (siFOXO1) and FOXO3 (siFOXO3) were transfected into human articular chondrocytes. Cell viability following treatment with the oxidant tert-butyl-hydroperoxide (tBHP) was measured by MTT assay. Caspase 3/7 activation and apoptotic cells were examined. Gene and protein expression of antioxidant proteins and autophagy-related proteins and changes in inflammatory mediators following treatment with interleukin-1β were assessed. Cells transfected with FOXO plasmids were also analyzed.

RESULTS

Cell viability was significantly reduced by siFOXO after treatment with tBHP. Apoptosis accompanied by caspase activation was significantly increased in siFOXO-transfected chondrocytes. Knockdown of FOXO1 and FOXO1+3 resulted in significant reductions in levels of glutathione peroxidase 1 (GPX-1), catalase, light chain 3 (LC3), Beclin1, and sirtuin 1 (SIRT-1) proteins following treatment with tBHP. In contrast, the constitutive active form of FOXO3 increased cell viability while inducing GPX-1, Beclin1, and LC3 in response to tBHP. Expression and production of ADAMTS-4 and chemerin were significantly increased in siFOXO-transfected chondrocytes.

CONCLUSION

Reduced expression of FoxO transcription factors in chondrocytes increased susceptibility to cell death induced by oxidative stress. This was associated with reduced levels of antioxidant proteins and autophagy-related proteins. Our data provide evidence for a key role of FoxO transcription factors as regulators of chondrocyte oxidative stress resistance and tissue homeostasis.

Exosomes from IL-1β stimulated synovial fibroblasts induce osteoarthritic changes in articular chondrocytes

Introduction

Osteoarthritis (OA) is a whole joint disease, and characterized by progressive degradation of articular cartilage, synovial hyperplasia, bone remodeling and angiogenesis in various joint tissues. Exosomes are a type of microvesicles (MVs) that may play a role in tissue-tissue and cell-cell communication in homeostasis and diseases. We hypothesized that exosomes function in a novel regulatory network that contributes to OA pathogenesis and examined the function of exosomes in communication among joint tissue cells.

Methods

Human synovial fibroblasts (SFB) and articular chondrocytes were obtained from normal knee joints. Exosomes isolated from conditioned medium of SFB were analyzed for size, numbers, markers and function. Normal articular chondrocytes were treated with exosomes from SFB, and Interleukin-1β (IL-1β) stimulated SFB. OA-related genes expression was quantified using real-time PCR. To analyze exosome effects on cartilage tissue, we performed glycosaminoglycan release assay. Angiogenic activity of these exosomes was tested in migration and tube formation assays. Cytokines and miRNAs in exosomes were analyzed by Bio-Plex multiplex assay and NanoString analysis.

Results

Exosomes from IL-1β stimulated SFB significantly up-regulated MMP-13 and ADAMTS-5 expression in articular chondrocytes, and down-regulated COL2A1 and ACAN compared with SFB derived exosomes. Migration and tube formation activity were significantly higher in human umbilical vein endothelial cells (HUVECs) treated with the exosomes from IL-1β stimulated SFB, which also induced significantly more proteoglycan release from cartilage explants. Inflammatory cytokines, IL-6, MMP-3 and VEGF in exosomes were only detectable at low level. IL-1β, TNFα MMP-9 and MMP-13 were not detectable in exosomes. NanoString analysis showed that levels of 50 miRNAs were differentially expressed in exosomes from IL-1β stimulated SFB compared to non-stimulated SFB.

Conclusions

Exosomes from IL-1β stimulated SFB induce OA-like changes both in vitro and in ex vivo models. Exosomes represent a novel mechanism by which pathogenic signals are communicated among different cell types in OA-affected joints.

Repair of cartilage defects in arthritic tissue with differentiated human embryonic stem cells

Chondrocytes have been generated in vitro from a range of progenitor cell types and by a number of strategies. However, achieving reconstitution of actual physiologically relevant, appropriately-laminated cartilage in situ that would be applicable to conditions, such as arthritis and cartilage degeneration remains elusive. This lack of success is multifactorial and includes limited cell source, decreased proliferation rate of mature chondrocytes, lack of maintenance of phenotype, reduced matrix synthesis, and poor integration with host tissue. We report an efficient approach for deriving mesenchymal chondroprogenitor cells from human embryonic stem cells. These cells generated tissue containing cartilage-specific matrix proteins that integrated in situ in a partial-thickness defect in ex vivo articular cartilage harvested from human arthritic joints. Given that stem cells provide a virtually inexhaustible supply of starting material and that our technique is easily scalable, cartilaginous tissue primed and grafted in this manner could be suitable for clinical translation.

Transcription factor Mohawk and the pathogenesis of human anterior cruciate ligament degradation

OBJECTIVE

To investigate the expression and function of Mohawk (MKX) in human adult anterior cruciate ligament (ACL) tissue and ligament cells from normal and osteoarthritis (OA)-affected knees.

METHODS

Knee joints were obtained at autopsy (within 24-48 hours postmortem) from 13 donors with normal knees (mean ± SD age 36.9 ± 11.0 years), 16 donors with knee OA (age 79.7 ± 11.4 years), and 8 aging donors without knee OA (age 76.9 ± 12.9 years). All cartilage surfaces were graded macroscopically. MKX expression was analyzed by immunohistochemistry and quantitative polymerase chain reaction. ACL-derived cells were used to study regulation of MKX expression by interleukin-1β (IL-1β). MKX was knocked down with small interfering RNA (siRNA) to analyze the function of MKX in extracellular matrix (ECM) production and differentiation in ACL-derived cells.

RESULTS

The expression of MKX was significantly decreased in ACL-derived cells from OA knees compared with normal knees. Consistent with this finding, immunohistochemistry analysis showed that MKX-positive cells were significantly reduced in ACL tissue from OA donors, in particular in cells located in disorientated fibers. In ACL-derived cells, IL-1β strongly suppressed MKX expression and reduced expression of the ligament ECM genes COL1A1 and TNXB. In contrast, SOX9, a chondrocyte master transcription factor, was up-regulated by IL-1β treatment. Importantly, knockdown of MKX expression with siRNA up-regulated SOX9 expression in ACL-derived cells, whereas the expression of COL1A1 and TNXB was reduced.

CONCLUSION

Reduced expression of MKX is a feature of degenerated ACL in OA-affected joints, and this may be mediated in part by IL-1β. MKX appears necessary to maintain the tissue-specific cellular differentiation status and ECM production in adult human tendons and ligaments.

Influence of cartilage extracellular matrix molecules on cell phenotype and neocartilage formation

Interaction between chondrocytes and the cartilage extracellular matrix (ECM) is essential for maintaining the cartilage's role as a low-friction and load-bearing tissue. In this study, we examined the influence of cartilage zone-specific ECM on human articular chondrocytes (HAC) in two-dimensional and three-dimensional (3D) environments. Two culture systems were used. SYSTEM 1: HAC were cultured on cell-culture plates that had been precoated with the following ECM molecules for 7 days: decorin, biglycan, tenascin C (superficial zone), collagen type II, hyaluronan (HA) (middle and deep zones), and osteopontin (deep zone). Uncoated standard culture plates were used as controls. Expanded cells were examined for phenotypic changes using real-time polymerase chain reaction. In addition, expanded cells were placed into high-density pellet cultures for 14 days. Neocartilage formation was assessed via gene expression and histology evaluations. SYSTEM 2: HAC that were cultured on untreated plates and encapsulated in a 3D alginate scaffold were mixed with one of the zone-specific ECM molecules. Cell viability, gene expression, and histology assessments were conducted on 14-day-old tissues. In HAC monolayer culture, exposure to decorin, HA, and osteopontin increased COL2A1 and aggrecan messenger RNA (mRNA) levels compared with controls. Biglycan up-regulated aggrecan without a significant impact on COL2A1 expression; Tenascin C reduced COL2A1 expression. Neocartilage formed after preculture on tenascin C and collagen type II expressed higher COL2A1 mRNA compared with control pellets. Preculture of HAC on HA decreased both COL2A1 and aggrecan expression levels compared with controls, which was consistent with histology. Reduced proteoglycan 4 (PRG4) mRNA levels were observed in HAC pellets that had been precultured with biglycan and collagen type II. Exposing HAC to HA directly in 3D-alginate culture most effectively induced neocartilage formation, showing increased COL2A1 and aggrecan, and reduced COL1A1 compared with controls. Decorin treatments increased HAC COL2A1 mRNA levels. These data indicate that an appropriate exposure to cartilage-specific ECM proteins could be used to enhance cartilage formation and to even induce the formation of zone-specific phenotypes to improve cartilage regeneration.

Glucosamine activates autophagy in vitro and in vivo

OBJECTIVE

Aging-associated changes in articular cartilage represent a main risk factor for osteoarthritis (OA). Autophagy is an essential cellular homeostasis mechanism. Aging-associated or experimentally induced defects in autophagy contribute to organismal- and tissue-specific aging, while enhancement of autophagy may protect against certain aging-related pathologies such as OA. The objective of this study was to determine whether glucosamine can activate autophagy.

METHODS

Chondrocytes from normal human articular cartilage were treated with glucosamine (0.1- 10 mM). Autophagy activation and phosphorylation levels of Akt, FoxO3, and ribosomal protein S6 were determined by Western blotting. Autophagosome formation was analyzed by confocal microscopy. Reporter mice systemically expressing green fluorescent protein (GFP) fused to light chain 3 (LC3) (GFP-LC3-transgenic mice) were used to assess changes in autophagy in response to starvation and glucosamine treatment.

RESULTS

Glucosamine treatment of chondrocytes activated autophagy, as indicated by increased LC3-II levels, formation of LC3 puncta, and increased LC3 turnover. This was associated with glucosamine-mediated inhibition of the Akt/FoxO3/mammalian target of rapamycin pathway. Administration of glucosamine to GFP-LC3-transgenic mice markedly activated autophagy in articular cartilage.

CONCLUSION

Glucosamine modulates molecular targets of the autophagy pathway in vitro and in vivo, and the enhancement of autophagy is mainly dependent on the Akt/FoxO/mTOR pathway. These findings suggest that glucosamine is an effective autophagy activator and should motivate future studies on the efficacy of glucosamine in modifying aging-related cellular changes and supporting joint health.

Digital micromirror device projection printing system for meniscus tissue engineering

Meniscus degeneration due to age or injury can lead to osteoarthritis. Although promising, current cell-based approaches show limited success. Here we present three-dimensional methacrylated gelatin (GelMA) scaffolds patterned via projection stereolithography to emulate the circumferential alignment of cells in native meniscus tissue. Cultured human avascular zone meniscus cells from normal meniscus were seeded on the scaffolds. Cell viability was monitored, and new tissue formation was assessed by gene expression analysis and histology after 2weeks in serum-free culture with transforming growth factor β1 (10ngml(-1)). Light, confocal and scanning electron microscopy were used to observe cell-GelMA interactions. Tensile mechanical testing was performed on unseeded, fresh scaffolds and 2-week-old cell-seeded and unseeded scaffolds. 2-week-old cell-GelMA constructs were implanted into surgically created meniscus defects in an explant organ culture model. No cytotoxic effects were observed 3weeks after implantation, and cells grew and aligned to the patterned GelMA strands. Gene expression profiles and histology indicated promotion of a fibrocartilage-like meniscus phenotype, and scaffold integration with repair tissue was observed in the explant model. We show that micropatterned GelMA scaffolds are non-toxic, produce organized cellular alignment, and promote meniscus-like tissue formation. Prefabrication of GelMA scaffolds with architectures mimicking the meniscus collagen bundle organization shows promise for meniscal repair. Furthermore, the technique presented may be scaled up to repair larger defects.

Zone-specific gene expression patterns in articular cartilage

OBJECTIVE

To identify novel genes and pathways specific to the superficial zone (SZ), middle zone (MZ), and deep zone (DZ) of normal articular cartilage.

METHODS

Articular cartilage was obtained from the knees of 4 normal human donors. The cartilage zones were dissected on a microtome. RNA was analyzed on human genome arrays. The zone-specific DNA array data obtained from human tissue were compared to array data obtained from bovine cartilage. Genes differentially expressed between zones were evaluated using direct annotation for structural or functional features, and by enrichment analysis for integrated pathways or functions.

RESULTS

The greatest differences in genome-wide RNA expression data were between the SZ and DZ in both human and bovine cartilage. The MZ, being a transitional zone between the SZ and DZ, thereby shared some of the same pathways as well as structural/functional features of the adjacent zones. Cellular functions and biologic processes that were enriched in the SZ relative to the DZ included, most prominently, extracellular matrix-receptor interactions, cell adhesion molecule functions, regulation of actin cytoskeleton, ribosome-related functions, and signaling aspects such as the IFN, IL4, Cdc42/Rac, and JAK/STAT signaling pathways. Two pathways were enriched in the DZ relative to the SZ, including PPARG and EGFR/SMRTE.

CONCLUSION

These differences in cartilage zonal gene expression identify new markers and pathways that govern the unique differentiation status of chondrocyte subpopulations.

Cellular and extracellular matrix changes in anterior cruciate ligaments during human knee aging and osteoarthritis

INTRODUCTION

Anterior cruciate ligament (ACL) degeneration is observed in most osteoarthritis (OA)-affected knee joints. However, the specific spatial and temporal relations of these changes and their association with extracellular matrix (ECM) degeneration are not well understood. The objective of this study was to characterize the patterns and relations of aging-related and OA-associated changes in ACL cells and the ECM.

METHODS

Human knee joints from 80 donors (age 23 through 94) were obtained at autopsy. ACL degeneration was assessed histologically by using a quantitative scoring system. Tissue sections were analyzed for cell density, cell organization, ECM components, ECM-degrading enzymes and markers of differentiation, proliferation, and stem cells.

RESULTS

Total cell number in normal ACL decreased with aging but increased in degenerated ACL, because of the formation of perivascular cell aggregates and islands of chondrocyte-like cells. Matrix metalloproteinase (MMP)-1, -3, and -13 expression was reduced in aging ACL but increased in degenerated ACL, mainly in the chondrocyte-like cells. Collagen I was expressed throughout normal and degenerated ACL. Collagen II and X were detected only in the areas with chondroid metaplasia, which also expressed collagen III. Sox9, Runt-related transcription factor 2 (Runx2), and scleraxis expression was increased in the chondrocyte-like cells in degenerated ACL. Alpha-smooth muscle actin (α-SMA), a marker of myofibroblasts and the progenitor cell marker STRO-1, decreased with aging in normal ACL. In degenerated ACL, the new cell aggregates were positive for α-SMA and STRO-1.

CONCLUSIONS

ACL aging is characterized by reduced cell density and activation. In contrast, ACL degeneration is associated with cell recruitment or proliferation, including progenitor cells or myofibroblasts. Abnormally differentiated chondrocyte-like cell aggregates in degenerated ACL produce abnormal ECM and may predispose to mechanical failure.

MicroRNA-125b regulates the expression of aggrecanase-1 (ADAMTS-4) in human osteoarthritic chondrocytes

Introduction

Increased expression of aggrecanase-1 (ADAMTS-4) has emerged as an important factor in osteoarthritis (OA) and other joint diseases. This study aimed to determine whether the expression of ADAMTS-4 in human chondrocytes is regulated by miRNA.

Methods

MiRNA targets were identified using bioinformatics. Chondrocytes were isolated from knee cartilage and treated with interleukin-1 beta (IL-1β). Gene expression was quantified using TaqMan assays and protein production was determined by immunoblotting. Luciferase reporter assay was used to verify interaction between miRNA and target messenger RNA (mRNA).

Results

In silico analysis predicted putative target sequence of miR-125b on ADAMTS-4. MiR-125b was expressed in both normal and OA chondrocytes, with significantly lower expression in OA chondrocytes than in normal chondrocytes. Furthermore, IL-1β-induced upregulation of ADAMTS-4 was suppressed by overexpression of miR-125b in human OA chondrocytes. In the luciferase reporter assay, mutation of the putative miR-125b binding site in the ADAMTS-4 3'UTR abrogated the suppressive effect of miR125.

Conclusions

Our results indicate that miR-125b plays an important role in regulating the expression of ADAMTS-4 in human chondrocytes and this identifies miR-125b as a novel therapeutic target in OA.

Thermal inkjet printing in tissue engineering and regenerative medicine

With the advantages of high throughput, digital control, and highly accurate placement of cells and biomaterial scaffold to the desired 2D and 3D locations, bioprinting has great potential to develop promising approaches in translational medicine and organ replacement. The most recent advances in organ and tissue bioprinting based on the thermal inkjet printing technology are described in this review. Bioprinting has no or little side effect to the printed mammalian cells and it can conveniently combine with gene transfection or drug delivery to the ejected living systems during the precise placement for tissue construction. With layer-by-layer assembly, 3D tissues with complex structures can be printed using scanned CT or MRI images. Vascular or nerve systems can be enabled simultaneously during the organ construction with digital control. Therefore, bioprinting is the only solution to solve this critical issue in thick and complex tissues fabrication with vascular system. Collectively, bioprinting based on thermal inkjet has great potential and broad applications in tissue engineering and regenerative medicine. This review article introduces some important patents related to bioprinting of living systems and the applications of bioprinting in tissue engineering field.

Histopathological changes in the human posterior cruciate ligament during aging and osteoarthritis: correlations with anterior cruciate ligament and cartilage changes

OBJECTIVES

To determine the histological patterns of posterior cruciate ligament (PCL) degeneration during aging and in relation to changes in articular cartilage and anterior cruciate ligament (ACL) across the entire adult age spectrum.

METHODS

Human knee joints (n=120 from 65 donors) were processed within 72 h of postmortem. Articular cartilage surfaces were graded macroscopically. Each PCL was histologically evaluated for inflammation, mucinous changes, chondroid metaplasia, cystic changes and orientation of collagen fibres. The severity of PCL degeneration was classified as normal, mild, moderate or severe. PCL scores were compared to ACL and cartilage scores from the same knees.

RESULTS

All knees had intact PCL. Histologically, 6% were normal, 76% showed mild, 12% moderate and 9% severe degeneration. Fibre disorientation was the most prevalent and severe change. Histological grades of PCL and ACL correlated, but significantly fewer PCL than ACL showed severe changes. There was a weaker correlation between aging and total histological PCL scores (R=0.26) compared to aging and ACL scores (R=0.42). ACL scores correlated with cartilage scores (R=0.54) while PCL scores increased with the severity of osteoarthritis from grades 0 to III but not between osteoarthritis grades III-IV (R=0.32). In knees with ruptured ACL, the PCL scores correlated with cartilage scores of the lateral compartment.

CONCLUSIONS

PCL histopathological changes were less severe than in the ACL. PCL degeneration was associated with ACL and cartilage damage. The lack of correlation with age indicates independent pathways for PCL versus ACL degeneration.

Anterior cruciate ligament changes in the human knee joint in aging and osteoarthritis

OBJECTIVE

The development and patterns of spontaneous age-related changes in the anterior cruciate ligament (ACL) and their relationship to articular cartilage degeneration are not well characterized. This study was undertaken to investigate the types and temporal sequence of age-related ACL changes and to determine their correlation with cartilage lesion patterns at all stages of osteoarthritis (OA) development in human knee joints without prior joint trauma.

METHODS

Human knee joints (n = 120 from 65 donors ages 23-92) were obtained at autopsy, and ACLs and cartilage were graded macroscopically and histologically. Inflammation surrounding the ACL was assessed separately.

RESULTS

Histologic ACL substance scores and ligament sheath inflammation scores increased with age. Collagen fiber disorganization was the earliest and most prevalent change. The severity of mucoid degeneration and chondroid metaplasia in the ACL increased with the development of cartilage lesions. A correlation between ACL degeneration and cartilage degeneration was observed, especially in the medial compartment of the knee joint.

CONCLUSION

Our findings indicate that ACL degeneration is highly prevalent in knees with cartilage defects and may even precede cartilage changes. Hence, ACL deficiencies may not only be important in posttraumatic OA, but may also be a feature associated with knee OA pathogenesis in general.

Effects of perfusion and dynamic loading on human neocartilage formation in alginate hydrogels

Dynamic loading and perfusion culture environments alone are known to enhance cartilage extracellular matrix (ECM) production in dedifferentiated articular chondrocytes. In this study, we explored whether a combination of these factors would enhance these processes over a free-swelling (FS) condition using adult human articular chondrocytes embedded in 2% alginate. The alginate constructs were placed into a bioreactor for perfusion (P) only (100 μL/per minute) or perfusion and dynamic compressive loading (PL) culture (20% for 1 h, at 0.5 Hz), each day. Control FS alginate gels were maintained in six-well static culture. Gene expression analysis was conducted on days 7 and 14, while cell viability, immunostaining, and mechanical property testing were performed on day 14 only. Total glycosaminoglycan (GAG) content and GAG synthesis were assessed after 14 days. Col2a1 mRNA expression levels were significantly higher (at least threefold; p<0.05) in both bioreactor conditions compared with FS by days 7 and 14. For all gene studies, no significant differences were seen between P and PL treatments. Aggrecan mRNA levels were not significantly altered in any condition although both GAG/DNA and (35)S GAG incorporation studies indicated higher GAG retention and synthesis in the FS treatment. Collagen type II protein deposition was low in all samples, link protein distribution was more diffuse in FS condition, and aggrecan deposition was located in the outer regions of the alginate constructs in both bioreactor conditions, yet more uniformly in the FS condition. Catabolic gene expression (matrix metalloproteinase 3 [MMP3] and inducible nitric oxide synthase [iNOS]) was higher in bioreactor conditions compared with FS, although iNOS expression levels decreased to approximately fourfold less than the FS condition by day 14. Our data indicate that conditions created in the bioreactor enhanced both anabolic and catabolic responses, similar to other loading studies. Perfusion was sufficient alone to promote this dual response. PL increased the deposition of aggrecan surrounding cells compared with the other conditions; however, overall low GAG retention in the bioreactor system was likely due to both perfusion and catabolic conditions created. Optimal conditions, which permit appropriate anabolic and catabolic processes for accumulation of ECM and tissue remodeling for neocartilage development, specifically for humans, are needed.

Structural and functional maturation of distal femoral cartilage and bone during postnatal development and growth in humans and mice

The size and shape of joints markedly affect their biomechanical properties, but the macroscopic 3-dimensional (3-D) mechanism and extent of cartilage and joint maturation during normal growth are largely unknown. This study qualitatively illustrates the development of the bone-cartilage interface in the knee during postnatal growth in humans and C57BL/6 wild-type mice, quantitatively defines the 3-D shape using statistical shape modeling, and assesses growth strain rates in the mouse distal femur. Accurate quantification of the cartilage-bone interface geometry is imperative for furthering the understanding of the macroscopic mechanisms of cartilage maturation and overall joint development.

In situ tissue engineering using magnetically guided three-dimensional cell patterning

Manipulation of cell patterns in three dimensions in a manner that mimics natural tissue organization and function is critical for cell biological studies and likely essential for successfully regenerating tissues--especially cells with high physiological demands, such as those of the heart, liver, lungs, and articular cartilage.(1, 2) In the present study, we report on the feasibility of arranging iron oxide-labeled cells in three-dimensional hydrogels using magnetic fields. By manipulating the strength, shape, and orientation of the magnetic field and using crosslinking gradients in hydrogels, multi-directional cell arrangements can be produced in vitro and even directly in situ. We show that these ferromagnetic particles are nontoxic between 0.1 and 10 mg/mL; certain species of particles can permit or even enhance tissue formation, and these particles can be tracked using magnetic resonance imaging. Taken together, this approach can be adapted for studying basic biological processes in vitro, for general tissue engineering approaches, and for producing organized repair tissues directly in situ.

Vimentin contributes to changes in chondrocyte stiffness in osteoarthritis

Actin and tubulin cytoskeletal components are studied extensively in chondrocytes, but less is known about vimentin intermediate filaments. In other cell types, vimentin is a determinant of cell stiffness and disruption of vimentin networks weakens the mechanical integrity of cells. Changes in vimentin organization correlate with osteoarthritis progression, but the functional consequences of these changes remain undetermined in chondrocytes. The objective of this study was to compare the contribution of vimentin to the mechanical stiffness of primary human chondrocytes isolated from normal versus osteoarthritic cartilage. Chondrocytes were embedded in alginate and vimentin networks disrupted with acrylamide. Constructs were imaged while subjected to 20% nominal strain on a confocal microscope stage, and the aspect ratios of approximately 1,900 cells were measured. Cytosolic stiffness was estimated with a finite element model, and live-cell imaging of GFP-vimentin was used to further analyze the nature of vimentin disruption. Vimentin in normal chondrocytes formed an inner cage-like network that was substantially stiffer than the rest of the cytosol and contributed significantly to overall cellular stiffness. Disruption of vimentin reduced stiffness approximately 2.8-fold in normal chondrocytes. In contrast, osteoarthritic chondrocytes were less stiff and less affected by vimentin disruption. This 3D experimental system revealed contributions of vimentin to chondrocyte stiffness previously not apparent, and correlated changes in vimentin-based chondrocyte stiffness with osteoarthritis.

Glucosamine-6-sulfamate analogues of heparan sulfate as inhibitors of endosulfatases

Keeping Sulfate. The extracellular endosulfatases, which modulate signalling pathways by removing sulfate groups from heparan, can be inhibited by replacing the 6-sulfate destined for cleavage with an inhibitory sulfamate motif, as demonstrated by simple glucosamine-6-sulfamate analogs of heparan sulfate.

New developments in osteoarthritis. Posttraumatic osteoarthritis: pathogenesis and pharmacological treatment options

Joint trauma can lead to a spectrum of acute lesions, including osteochondral fractures, ligament or meniscus tears and damage to the articular cartilage. This is often associated with intraarticular bleeding and causes posttraumatic joint inflammation. Although the acute symptoms resolve and some of the lesions can be surgically repaired, joint injury triggers a chronic remodeling process in cartilage and other joint tissues that ultimately manifests as osteoarthritis in a majority of cases. The objective of the present review is to summarize information on pathogenetic mechanisms involved in the acute and chronic consequences of joint trauma and discuss potential pharmacological interventions. The focus of the review is on the early events that follow joint trauma since therapies for posttraumatic joint inflammation are not available and this represents a unique window of opportunity to limit chronic consequences.

Repression of chondrogenesis through binding of notch signaling proteins HES-1 and HEY-1 to N-box domains in the COL2A1 enhancer site

OBJECTIVE

Notch signaling is implicated in the repression of mesenchymal stem cell (MSC) chondrogenic differentiation. The purpose of this study was to examine the mechanism of this repression and how it is modulated to permit chondrogenesis.

METHODS

Notch intracellular domain (NICD) protein levels were monitored via Western blotting throughout chondrogenic differentiation of human MSCs in pellet cultures. Overexpression of Notch signaling components and their effect on chondrogenesis was achieved by transfecting plasmids coding for NICD, HES-1, and HERP-2/HEY-1. COL2A1 and AGGRECAN expression was monitored via quantitative polymerase chain reaction analysis. Chromatin immunoprecipitation (ChIP) was used to test whether HES-1 and HEY-1 bind putative N-box domains in intron 1 of COL2A1.

RESULTS

High levels of NICD proteins were reduced during chondrogenesis of human MSCs, and this was mediated by transforming growth factor beta3 (TGFbeta3). COL2A1 gene expression was repressed following overexpression of NICD (2-fold) and HES-1 (3-fold) and was markedly repressed by overexpression of HEY-1 (80-fold). HEY-1 repressed AGGRECAN expression 10-fold, while NICD and HES-1 had no effect. We identified 2 putative N-box domains adjacent to, and part of, the SOX9 enhancer binding site located in intron 1 of COL2A1. ChIP studies showed that endogenous HES-1 and HEY-1 bound to these sites. Transducin-like enhancer, the HES-1 corepressor protein, was displaced during chondrogenic differentiation and following TGFbeta3 treatment.

CONCLUSION

These results reveal novel mechanisms by which Notch signaling represses gene expression. Notch signaling proteins act on the SOX9 binding site in the COL2A1 enhancer and prevent SOX9-mediated transcriptional activation of COL2A1 and, thus, chondrogenic differentiation.

Rho kinase-dependent CCL20 induced by dynamic compression of human chondrocytes

OBJECTIVE

Mechanical stimulation of cartilage affects tissue homeostasis and chondrocyte function. The chondrocyte phenotype is dependent on cell shape, which is largely determined by the actin cytoskeleton. Reorganization of the actin cytoskeleton results from Rho GTPase activation. The purpose of this study was to examine the roles of both actin and Rho in mechanotransduction in chondrocytes.

METHODS

We embedded human articular chondrocytes in 2 x 6-mm agarose discs at 5 x 10(6) cells/ml and subjected the discs to unconfined dynamic compression at 0.5 Hz. By comparing samples with and without dynamic compression, we identified Rho activation according to the GTP-bound active RhoA measured in cell lysates. We identified rearrangements in filamentous actin structures using fluorescence-labeled phalloidin and confocal microscopy of fixed samples. We identified altered gene expression using TaqMan quantitative reverse transcription-polymerase chain reaction analysis. We tested for a requirement for Rho signaling by performing the dynamic compression in the presence of Rho kinase inhibitors.

RESULTS

RhoA activation occurred within 5-10 minutes of dynamic compression. Rho kinase-dependent actin reorganization occurred within 20 minutes after application of dynamic compression and was apparent as "punctate" F-actin structures that were visible under confocal microscopy. We identified early-phase mechanoresponsive genes (CCL20 and inducible nitric oxide synthase) that were highly up-regulated within 1 hour of dynamic compression in a Rho kinase-dependent and actin-dependent manner.

CONCLUSION

Together, these results are the first demonstration that the Rho-Rho kinase pathway and actin cytoskeletal reorganization are required for changes in the expression of genes involved in human chondrocyte mechanotransduction.

Rho kinase-dependent activation of SOX9 in chondrocytes

OBJECTIVE

The transcription factor SOX9 directly regulates the expression of the major proteoglycans and collagens comprising the cartilage extracellular matrix. The DNA binding activity and cellular localization of SOX9 is controlled through posttranslational modifications, including phosphorylation. The activity of Rho kinase (ROCK) has profound effects on the actin cytoskeleton, and these effects are instrumental in determining the phenotype and differentiation of chondrocytes. However, the mechanisms linking ROCK to altered chondrocyte gene expression remain unknown. The purpose of the present study was to test for a direct interaction between ROCK and SOX9.

METHODS

Human SW1353 chondrosarcoma cells were transfected with constructs coding for RhoA, ROCK, Lim kinase, and SOX9. The interaction between ROCK and SOX9 was tested on purified proteins, and was verified within a cellular context using induced overexpression and activation of the Rho pathway. The effects of SOX9 transcriptional activation were quantified with a luciferase reporter plasmid containing SOX9 binding sites from the COL2A1 enhancer element.

RESULTS

SOX9 was found to contain a consensus phosphorylation site for ROCK. In vitro, ROCK directly phosphorylated SOX9 at Ser(181), and the overexpression of ROCK or the activation of the RhoA pathway in SW1353 chondrosarcoma cells increased SOX9(Ser181) phosphorylation. ROCK caused a dose-dependent increase in the transcription of a SOX9-luciferase reporter construct, and increased phosphorylation and nuclear accumulation of SOX9 protein in response to transforming growth factor beta treatment and mechanical compression.

CONCLUSION

These results demonstrate a new interaction that directly links ROCK to increased cartilage matrix production via activation of SOX9 in response to mechanical and growth factor stimulation.

Tissue neogenesis and STRO-1 expression in immature and mature articular cartilage

This study determined the potential for neotissue formation and the role of STRO-1+ cells in immature versus mature articular cartilage. Cartilage explants from immature and mature bovine knee joints were cultured for up to 12 weeks and stained with safranin-O, for type II collagen and STRO-1. Bovine chondrocyte pellet cultures and murine knee joints at the age of 2 weeks and 3 months, and surgically injured cartilage, were analyzed for changes in STRO-1 expression patterns. Results show that immature explants contained more STRO-1+ cells than mature explants. After 8 weeks in culture, immature explants showed STRO-1+ cell proliferation and newly formed tissue, which contained glycosaminoglycan and type II collagen. Mature cartilage explants showed only minimal cell expansion and neotissue formation. Pellet cultures with chondrocytes from immature cartilage showed increased glycosaminoglycan synthesis and STRO-1+ staining, as compared to pellets with mature chondrocytes. The frequency of STRO-1+ cells in murine knee joints significantly declined with joint maturation. Following surgical injury, immature explants had higher potential for tissue repair than mature explants. In conclusion, these findings suggest that the high percentage of STRO-1+ cells in immature cartilage changes with joint maturation. STRO-1+ cells have the potential to form new cartilage spontaneously and after tissue injury.

Topographic Patterns of Cartilage Lesions in Knee Osteoarthritis

OBJECTIVE: Treatments for articular cartilage lesions could benefit from characterization of lesion patterns and their progression to end-stage osteoarthritis. The objective of this study was to identify, quantitatively, topographic patterns of cartilage lesions in the human knee. DESIGN: Photographs were taken of 127 unilateral distal femora (from 109 cadavers and 18 arthroplasty remnants) with full-thickness cartilage lesions. Using digital image analysis, the lesions were localized and normalized lesion size was determined for the patellofemoral groove (PFG) and the lateral and medial femoral condyles (LFC, MFC). Samples were classified into patterns using cluster analysis of the lesion size at each compartment. For each pattern, maps showing the extent and frequency of lesions were created. RESULTS: Four main patterns (a-d) were identified (each p<0.001), with the lesion size varying from small (a) to large in distinct regions (b-d). Pattern b had a predominant lesion (23% area) in the MFC, and smaller (<3%) lesions elsewhere. Pattern c had predominant lesions in the LFC (19%) and MFC (10%). Pattern d had a predominant lesion in the PFG (15%) and smaller lesions in the MFC (6%) and LFC (2%). The sub-patterns of a (a1, a2, a3) had relatively small lesions, with similarity between a2 and b, and a3 and d. CONCLUSION: The present methods facilitated quantitative identification of distinct topographic patterns of full-thickness cartilage lesions, based on lesion size and location. These results have implications for stratifying OA patients using precise quantitative methods and, with additional longitudinal data, targeting cartilage treatments.

MicroRNA-140 is expressed in differentiated human articular chondrocytes and modulates interleukin-1 responses

OBJECTIVE

MicroRNA (miRNA) are a class of noncoding small RNAs that act as negative regulators of gene expression. MiRNA exhibit tissue-specific expression patterns, and changes in their expression may contribute to pathogenesis. The objectives of this study were to identify miRNA expressed in articular chondrocytes, to determine changes in osteoarthritic (OA) cartilage, and to address the function of miRNA-140 (miR-140).

METHODS

To identify miRNA specifically expressed in chondrocytes, we performed gene expression profiling using miRNA microarrays and quantitative polymerase chain reaction with human articular chondrocytes compared with human mesenchymal stem cells (MSCs). The expression pattern of miR-140 was monitored during chondrogenic differentiation of human MSCs in pellet cultures and in human articular cartilage from normal and OA knee joints. We tested the effects of interleukin-1beta (IL-1beta) on miR-140 expression. Double-stranded miR-140 (ds-miR-140) was transfected into chondrocytes to analyze changes in the expression of genes associated with OA.

RESULTS

Microarray analysis showed that miR-140 had the largest difference in expression between chondrocytes and MSCs. During chondrogenesis, miR-140 expression in MSC cultures increased in parallel with the expression of SOX9 and COL2A1. Normal human articular cartilage expressed miR-140, and this expression was significantly reduced in OA tissue. In vitro treatment of chondrocytes with IL-1beta suppressed miR-140 expression. Transfection of chondrocytes with ds-miR-140 down-regulated IL-1beta-induced ADAMTS5 expression and rescued the IL-1beta-dependent repression of AGGRECAN gene expression.

CONCLUSION

This study shows that miR-140 has a chondrocyte differentiation-related expression pattern. The reduction in miR-140 expression in OA cartilage and in response to IL-1beta may contribute to the abnormal gene expression pattern characteristic of OA.

Characterization of the chondrocyte actin cytoskeleton in living three-dimensional culture: response to anabolic and catabolic stimuli

The actin cytoskeleton is a dynamic network required for intracellular transport, signal transduction, movement, attachment to the extracellular matrix, cellular stiffness and cell shape. Cell shape and the actin cytoskeletal configuration are linked to chondrocyte phenotype with regard to gene expression and matrix synthesis. Historically, the chondrocyte actin cytoskeleton has been studied after formaldehyde fixation--precluding real-time measurements of actin dynamics, or in monolayer cultured cells. Here we characterize the actin cytoskeleton of living low-passage human chondrocytes grown in three-dimensional culture using a stably expressed actin-GFP construct. GFP-actin expression does not substantially alter the production of endogenous actin at the protein level. GFP-actin incorporates into all actin structures stained by fluorescent phalloidin, and does not affect the actin cytoskeleton as seen by fluorescence microscopy. GFP-actin expression does not significantly change the chondrocyte cytosolic stiffness. GFP-actin does not alter the gene expression response to cytokines and growth factors such as IL-1beta and TGF-beta. Finally, GFP-actin does not alter production of extracellular matrix as measured by radiosulfate incorporation. Having established that GFP-actin does not measurably affect the chondrocyte phenotype, we tested the hypothesis that IL-1beta and TGF-beta differentially alter the actin cytoskeleton using time-lapse microscopy. TGF-beta increases actin extensions, and lamellar ruffling indicative of Rac/CDC42 activation, while IL-1beta causes cellular contraction indicative of RhoA activation. The ability to visualize GFP-actin in living chondrocytes in 3D culture without disrupting the organization or function of the cytoskeleton is an advance in chondrocyte cell biology and provides a powerful tool for future studies in actin-dependent chondrocyte differentiation and mechanotransduction pathways.

Differential metabolic effects of glucosamine and N-acetylglucosamine in human articular chondrocytes

OBJECTIVE

Aminosugars are commonly used to treat osteoarthritis; however, molecular mechanisms mediating their anti-arthritic activities are still poorly understood. This study analyzes facilitated transport and metabolic effects of glucosamine (GlcN) and N-acetylglucosamine (GlcNAc) in human articular chondrocytes.

METHODS

Human articular chondrocytes were isolated from knee cartilage. Facilitated transport of glucose, GlcN and GlcNAc was measured by uptake of [3H]2-deoxyglucose, [3H]GlcN and [3H]GlcNAc. Glucose transporter (GLUT) expression was analyzed by Western blotting. Production of sulfated glycosaminoglycans (SGAG) was measured using [(35)S]SO4. Hyaluronan was quantified using hyaluronan binding protein.

RESULTS

Chondrocytes actively import and metabolize GlcN but not GlcNAc and this represents a cell-type specific phenomenon. Similar to facilitated glucose transport, GlcN transport in chondrocytes is accelerated by cytokines and growth factors. GlcN non-competitively inhibits basal glucose transport, which in part depends on GlcN-mediated depletion of ATP stores. In IL-1beta-stimulated chondrocytes, GlcN inhibits membrane translocation of GLUT1 and 6, but does not affect the expression of GLUT3. In contrast to GlcN, GlcNAc accelerates facilitated glucose transport. In parallel with the opposing actions of these aminosugars on glucose transport, GlcN inhibits hyaluronan and SGAG synthesis while GlcNAc stimulates hyaluronan synthesis. GlcNAc-accelerated hyaluronan synthesis is associated with upregulation of hyaluronan synthase-2.

CONCLUSION

Differences in GlcN and GlcNAc uptake, and their subsequent effects on glucose transport, GLUT expression and SGAG and hyaluronan synthesis, indicate that these two aminosugars have distinct molecular mechanisms mediating their differential biological activities in chondrocytes.

Mesenchymal progenitor cell markers in human articular cartilage: normal distribution and changes in osteoarthritis

Introduction

Recent findings suggest that articular cartilage contains mesenchymal progenitor cells. The aim of this study was to examine the distribution of stem cell markers (Notch-1, Stro-1 and VCAM-1) and of molecules that modulate progenitor differentiation (Notch-1 and Sox9) in normal adult human articular cartilage and in osteoarthritis (OA) cartilage.

Methods

Expression of the markers was analyzed by immunohistochemistry (IHC) and flow cytometry. Hoechst 33342 dye was used to identify and sort the cartilage side population (SP). Multilineage differentiation assays including chondrogenesis, osteogenesis and adipogenesis were performed on SP and non-SP (NSP) cells.

Results

A surprisingly high number (>45%) of cells were positive for Notch-1, Stro-1 and VCAM-1 throughout normal cartilage. Expression of these markers was higher in the superficial zone (SZ) of normal cartilage as compared to the middle zone (MZ) and deep zone (DZ). Non-fibrillated OA cartilage SZ showed reduced Notch-1 and Sox9 staining frequency, while Notch-1, Stro-1 and VCAM-1 positive cells were increased in the MZ. Most cells in OA clusters were positive for each molecule tested. The frequency of SP cells in cartilage was 0.14 ± 0.05% and no difference was found between normal and OA. SP cells displayed chondrogenic and osteogenic but not adipogenic differentiation potential.

Conclusions

These results show a surprisingly high number of cells that express putative progenitor cell markers in human cartilage. In contrast, the percentage of SP cells is much lower and within the range of expected stem cell frequency. Thus, markers such as Notch-1, Stro-1 or VCAM-1 may not be useful to identify progenitors in cartilage. Instead, their increased expression in OA cartilage implicates involvement in the abnormal cell activation and differentiation process characteristic of OA.

Aging-related differences in chondrocyte viscoelastic properties

The biomechanical properties of articular cartilage change profoundly with aging. These changes have been linked with increased potential for cartilage degeneration and osteoarthritis. However, less is known about the change in biomechanical properties of chondrocytes with increasing age. Cell stiffness can affect mechanotransduction pathways and may alter cell function. We measured aging-related changes in the biomechanical properties of chondrocytes. Human chondrocytes were isolated from knee articular cartilage within 48 hours after death or from osteochondral specimens obtained from knee arthroplasty. Cells were divided into two age groups: between 18 and 35 years (18 - 35); and greater than 55 years (55+) of age. The 55+ group was further subdivided based on visual grade of osteoarthritis: normal (N) or osteoarthritic (OA). The viscoelastic properties of the cell were measured using the previously described micropipette cell aspiration technique. The equilibrium modulus, instantaneous modulus, and apparent viscosity were significantly higher in the 55+ year age group than in the 18 - 35 age group. On the other hand, no differences were found in the equilibrium modulus, instantaneous modulus, or apparent viscosity between the N and OA groups. The increase in cell stiffness can be attributed to altered mechanical properties of the cell membrane, the cytoplasm, or the cytoskeleton. Increased stiffness has been reported in osteoarthritic chondrocytes, which in turn has been attributed to the actin cytoskeleton. A similar mechanism may be responsible for our finding of increased stiffness in aging chondrocytes. With advancing age, changes in the biomechanical properties of the cell could alter molecular and biochemical responses.

Dynamic compression of chondrocytes induces a Rho kinase-dependent reorganization of the actin cytoskeleton

The signal transduction mechanisms in chondrocytes that recognize applied forces and elicit the appropriate biochemical cellular responses are not well characterized. A current theory is that the actin cytoskeleton provides an intracellular framework onto which mechanosensation mechanisms are assembled. The actin cytoskeleton is linked to the extracellular matrix at multi-protein complexes called focal adhesions, and evidence exists that focal adhesions mediate the conversion of external physical forces into appropriate biochemical signal transduction events. The Rho GTPases affect the arrangement of actin cytoskeletal structures, and enhance the formation of focal adhesions, which link the cytoskeleton to the extracellular matrix. A major effector pathway downstream of Rho is the activation of Rho kinase (ROCK), which phosphorylates and activates Lim kinase, which in turn phosphorylates and inhibits the actin-depolymerizing protein cofilin. The objectives of this study were threefold: first, to quantify the actin reorganization in response to dynamic compression of agarose-embedded chondrocytes. Second, to test whether Rho kinase is required for the actin cytoskeletal reorganization induced by dynamic compression. Third, to test whether dynamic compression alters the intracellular localization of Rho kinase and actin remodeling proteins in chondrocytes. Dynamic compression of agarose-embedded chondrocytes induced actin cytoskeletal remodeling causing a significant increase in punctate F-actin structures. Rho kinase activity was required for these cytoskeletal changes. Dynamic compression increased the amount of phosphorylated Rho kinase. The chemokine CCL20 and inducible nitric oxide synthase (iNOS) were the most highly upregulated genes by dynamic compression and this response was reduced by the Rho kinase inhibitors. In conclusion, we show that dynamic compression induces changes in the actin cytoskeleton of agarose-embedded chondrocytes, and we establish methodology to quantify these changes. Furthermore, we show that Rho kinase activity is required for this actin reorganization and gene expression induced by dynamic compression.

Human articular chondrocytes exhibit sexual dimorphism in their responses to 17beta-estradiol

OBJECTIVE

The higher incidence of osteoarthritis in females suggests that there may be intrinsic sex-specific differences in human articular chondrocytes. 17beta-Estradiol (E2) regulates rat growth plate chondrocytes through traditional nuclear receptor mechanisms, but only female cells exhibit rapid membrane-associated effects mediated through protein kinase C (PKC) alpha. Here we demonstrate sexual dimorphism in the physiological response of human articular chondrocytes to E2.

METHODS

Articular chondrocytes were obtained at the time of autopsy from three male and three female donors between 16 and 39 years of age. Second passage cultures were treated with E2 for 24 h to assess the effects of the hormone on [3H]-thymidine incorporation, [35S]-sulfate incorporation, and alkaline phosphatase specific activity. In addition, the chondrocytes were treated for 3, 9, 90 or 270 min and PKC specific activity was determined.

RESULTS

All chondrocytes were positive for aggrecan and estrogen receptor alpha mRNAs but were negative for type II collagen mRNA. Only cells from female donors responded to E2. DNA synthesis, sulfate incorporation and alkaline phosphatase activity were increased. E2 caused a rapid increase in PKC activity in the female cells within 9 min that was maximal at 90 min. Treatment with the PKC inhibitor chelerythrine blocked these effects.

CONCLUSIONS

These results provide the first definitive evidence that normal human cells exhibit an intrinsic sex-specific response to E2 and suggest that sexual dimorphism may be an important variable in assessing the pathways that modulate cell behavior.

Chondroprotective activity of N-acetylglucosamine in rabbits with experimental osteoarthritis

OBJECTIVE

To examine the therapeutic efficacy of N-acetylglucosamine (GlcNAc) in rabbits with experimental osteoarthritis (OA).

METHODS

Experimental OA was induced in rabbits by anterior cruciate ligament transection (ACLT). In the first study, rabbits (six in each group) received intramuscular injections of GlcNAc or normal saline three times a week starting 1 week postoperatively. In the second study, rabbits (eight in each group) were injected intra-articularly with GlcNAc (either once or twice a week) or normal saline. In the third study, rabbits (seven in each group) were injected intra-articularly twice a week with either GlcNAc, hyaluronan, or normal saline. Animals were killed 8 weeks after ACLT for macroscopic and histological assessment of the knee joints.

RESULTS

Intramuscular administration of GlcNAc in rabbits with experimental knee OA did not show chondroprotective effects but showed mild anti-inflammatory activity. In contrast, intra-articular administration of GlcNAc twice a week reduced cartilage degradation. Additionally, intra-articular GlcNAc also suppressed synovitis. Once a week intra-articular injections of GlcNAc did not demonstrate therapeutic efficacy. The chondroprotective efficacy of GlcNAc was better than that of viscosupplementation treatment with hyaluronan.

CONCLUSION

Intra-articular GlcNAc has chondroprotective and anti-inflammatory activity in experimental OA.

Detection of apoptosis in cartilage in situ and in isolated chondrocytes

Apoptosis is a form of programmed cell death conceptually opposed to necrosis. In view of the inherent difficulty in accurately detecting apoptosis in chondrocytes, this chapter describes complementary techniques that may be used in combination. During apoptotic death, protein and DNA breakdown is accomplished by caspases (cysteine proteases) in a highly regulated manner. Activation of caspases occurs in the initiation and/or the execution phase of certain apoptotic programs and represents an early physiologic marker of apoptosis. Here we present an immunoblotting technique that allows the detection of caspase-3 processing in cultured human chondrocytes. Apoptosis leads to plasma membrane asymmetry and to externalization of phosphatidylserine residues, which are bound with high affinity by annexin V. In the early stages of apoptosis, cells typically have an intact cell membrane. Apoptotic cells will not stain positive with propidium iodide, whereas externalization of phosphatidylserine will be detected by annexin V. Terminal deoxynucleotidyl transferase (TdT)-mediated nick-end labeling (TUNEL) works on the principle that DNA strand breaks (single or double) that occur during apoptosis can be identified by labeling free 3'-hydroxyl termini. Labeled nucleotides are polymerized to these termini in a reaction catalyzed by TdT. The tissue can then be examined histologically for identification of TUNEL-positive cells in situ.

Distinct pathways regulate facilitated glucose transport in human articular chondrocytes during anabolic and catabolic responses

Articular cartilage is an avascular, non-insulin-sensitive tissue that utilizes glucose as the main energy source, a precursor for glycosaminoglycan synthesis, and a regulator of gene expression. Facilitated glucose transport represents the first rate-limiting step in glucose metabolism. Previously, we demonstrated that glucose transport in chondrocytes is regulated by proinflammatory cytokines via upregulation of GLUT mRNA and protein expression. The objective of the present study was to determine differences in molecular mechanisms regulating glucose transport in chondrocytes stimulated with the anabolic transforming growth factor-beta1 (TGF-beta1) vs. the catabolic and proinflammatory cytokine IL-1beta. Both TGF-beta1 and IL-1beta accelerate glucose transport in chondrocytes. Although both IL-1beta and TGF-beta1 enhance glucose transport in chondrocytes to a similar magnitude, IL-1beta induces significantly higher levels of lactate. TGF-beta1-stimulated glucose transport is not associated with increased expression or membrane incorporation of GLUT1, -3, -6, -8, and -10 and depends on PKC and ERK activation. In contrast, IL-1beta-stimulated glucose transport is accompanied by increased expression and membrane incorporation of GLUT1 and -6 and depends upon activation of PKC and p38 MAP kinase. In conclusion, anabolic and catabolic stimuli regulate facilitated glucose transport in human articular chondrocytes via different effector and signaling mechanisms, and they have distinct effects on glycolysis.

WISP3-dependent regulation of type II collagen and aggrecan production in chondrocytes

OBJECTIVE

WISP3 (Wnt-1-inducible secreted protein 3) is a member of the CCN (connective tissue growth factor, cysteine-rich 61, nephroblastoma overexpressed) family of connective tissue growth factors. WISP3 mutations have been linked to progressive pseudorheumatoid dysplasia (PPRD). The present study was conducted to investigate whether WISP3 is responsible for the expression of cartilage-specific molecules.

METHODS

WISP3 expression in human cartilage was assessed by immunostaining with anti-WISP3 antibody. The effect of WISP3 on chondrocyte-specific gene regulation was determined by transfecting human chondrocyte lines C-28/I2 and T/C-28a2 with a WISP3 expression vector. Alterations in WISP3-mediated messenger RNA and protein expression of cartilage-specific molecules were assessed by reverse transcriptase-polymerase chain reaction and immunoblotting.

RESULTS

Immunohistochemistry experiments demonstrated that WISP3 protein is expressed in the midzone chondrocytes of normal adult articular cartilage, in chondrocyte clusters of osteoarthritic cartilage, and in the zone of proliferating chondrocytes of fetal growth cartilage. Human chondrocyte lines C-28/I2 and T/C-28a2 transfected with a WISP3 expression vector produced increased amounts of the cartilage-specific matrix molecules type II collagen and aggrecan, in part via activation of the sex-determining region Y-type high mobility group box (SOX) family of transcription factors. In contrast, a mutant WISP3, previously found to be associated with PPRD, had impaired effects on cartilage-specific gene expression.

CONCLUSION

Our experimental results suggest that WISP3 supports cartilage integrity by regulating the expression of type II collagen and aggrecan, and mutations linked with PPRD can compromise this function and produce cartilage loss.

Cytokine regulation of facilitated glucose transport in human articular chondrocytes

Glucose serves as the major energy substrate and the main precursor for the synthesis of glycosaminoglycans in chondrocytes. Facilitated glucose transport represents the first rate-limiting step in glucose metabolism. This study examines molecular regulation of facilitated glucose transport in normal human articular chondrocytes by proinflammatory cytokines. IL-1beta and TNF-alpha, and to a lesser degree IL-6, accelerate facilitated glucose transport as measured by [(3)H]2-deoxyglucose uptake. IL-1beta induces an increased expression of glucose transporter (GLUT) 1 mRNA and protein, and GLUT9 mRNA. GLUT3 and GLUT8 mRNA are constitutively expressed in chondrocytes and are not regulated by IL-1beta. GLUT2 and GLUT4 mRNA are not detected in chondrocytes. IL-1beta stimulates GLUT1 protein glycosylation and plasma membrane incorporation. IL-1beta regulation of glucose transport in chondrocytes depends on protein kinase C and p38 signal transduction pathways, and does not require phosphoinositide 3-kinase, extracellular signal-related kinase, or c-Jun N-terminal kinase activation. IL-1beta-accelerated glucose transport in chondrocytes is not mediated by endogenous NO or eicosanoids. These results demonstrate that stimulation of glucose transport represents a component of the chondrocyte response to IL-1beta. Two classes of GLUTs are identified in chondrocytes, constitutively expressed GLUT3 and GLUT8, and the inducible GLUT1 and GLUT9.

Human chondrocyte apoptosis in response to mechanical injury

OBJECTIVE

The effect of mechanical injury on chondrocyte viability and matrix degradation was studied. It was proposed that mechanical injury to human cartilage explants results in chondrocyte apoptosis with associated loss of glycosaminoglycans.

DESIGN

Full thickness human cartilage explants, 5 mm in diameter were subjected to a single static mechanical stress of 14 MPa for 500 ms under radially unconfined compression. Glycosaminoglycan (GAG) release and percentage of cells undergoing apoptosis were measured at 96 h after injury. To establish the time course of apoptosis, explants were subjected to 30% strain and cultured for varying intervals up to 7 days after injury. A group of loaded explants were also treated with the broad spectrum caspase inhibitor z-Vad.fmk after injury.

RESULTS

Internucleosomal DNA fragmentation as one indicator of apoptosis was observed in 34% (S.D.+/-11) of chondrocytes at 96 h in response to mechanical loading at 14 MPa, compared to 4% (S.D.+/-2) in the non-loaded explants. Evidence for cell death induction via apoptosis was also obtained by electron microscopy and caspase cleavage of cytokeratin. GAG release was also higher for the loaded explants, mean 1.9% (S.D.+/-0.14) of total GAG content, compared to control explants, mean 0.8% (S.D.+/-0.28). The percentage of apoptotic cells also correlated with the level of GAG release into the culture media. The percentage of apoptotic chondrocytes demonstrated a progressive increase from 6 h to 7 days post-injury. When loaded explants were cultured in z-Vad.fmk after injury, a 50% reduction in apoptosis rates was seen.

CONCLUSIONS

These results demonstrate that mechanical injury induces chondrocyte apoptosis and release of GAG from the matrix. The time course suggests that a therapeutic window may exist where apoptosis could be inhibited. This potentially identifies a new approach to chondroprotection.

Impact of mechanical trauma on matrix and cells

Posttraumatic arthritis is one of the most common causes of secondary osteoarthritis. The contribution of cell death to matrix degradation has not been characterized fully. The current study was designed to determine the effect of mechanical injury on chondrocyte viability and matrix degradation. Full-thickness bovine and human cartilage explants, 5 mm in diameter were subjected to mechanical loads representative of traumatic joint injury. Glycosaminoglycan release and percent apoptotic cells were measured. Unilateral patellas in eight anesthetized rabbits were subjected to an impact load. Rabbits were euthanized at 96 hours after injury and patellar cartilage was harvested for analysis. The effect of a pan-caspase inhibitor, z-VAD.fmk [benzyloxycarbonyl-Val-Ala-Asp (OMe) fluoromethylketone] in preventing chondrocyte apoptosis in human articular cartilage explants was determined. A significant increase in the number of apoptotic cells was observed in response to mechanical loading. The mean in vivo apoptotic rates were 1% in control rabbits and 15% in impacted patellas. Caspase inhibition reduced chondrocyte apoptosis from 34% to 25% after mechanical injury and was associated with reduction in glycosaminoglycan release. Mechanical injury induces chondrocyte apoptosis that is sensitive to pharmacologic inhibition. This identifies a new approach to limit traumatic cartilage injury and the subsequent development of secondary osteoarthritis.

Hexosaminidase inhibitors as new drug candidates for the therapy of osteoarthritis

BACKGROUND

Articular cartilage from patients with osteoarthritis is characterized by a decreased concentration and reduced size of glycosaminoglycans. Degeneration of the cartilage matrix is a multifactorial process, which is due in part to accelerated glycosaminoglycan catabolism. Recently, we have demonstrated that hexosaminidase represents the dominant glycosaminoglycan-degrading glycosidase released by chondrocytes into the extracellular compartment and is the dominant glycosidase in synovial fluid from patients with osteoarthritis. Inhibition of hexosaminidase activity may represent a novel approach to the prevention of cartilage matrix glycosaminoglycan degradation and a potentially new strategy to treat osteoarthritis.

RESULTS

We have synthesized and investigated a series of iminocyclitols designed as transition-state analog inhibitors of human hexosaminidase, and demonstrated that the five-membered iminocyclitol 4 expresses the strongest inhibitory activity with K(i)=24 nM. Inhibition of hexosaminidase activity in human cultured articular chondrocytes and human chondrosarcoma cells with iminocyclitol 4 resulted in accumulation of hyaluronic acid and sulfated glycosaminoglycans in the cell-associated fraction. Similarly, incubation of human cartilage tissue with iminocyclitol 4 resulted in an accumulation of glycosaminoglycans in the pericellular compartment.

CONCLUSIONS

Inhibition of hexosaminidase activity represents a new strategy for preventing or even reversing cartilage degradation in patients with osteoarthritis.