Antisense inhibition of chondrocyte CD44 expression leading to cartilage chondrolysis

Glycosaminoglycan-Mediated Interactions in Articular, Auricular, Meniscal, and Nasal Cartilage

Glycosaminoglycans (GAGs) are ubiquitous components in the cartilage extracellular matrix (ECM). Ultrastructural arrangement of ECM and GAG-mediated interactions with collagen are known to govern the mechanics in articular cartilage, but these interactions are less clear in other cartilage types. Therefore, this article reviews the current literature on ultrastructure of articular, auricular, meniscal, and nasal septal cartilage, seeking insight into GAG-mediated interactions influencing mechanics. Ultrastructural features of these cartilages are discussed to highlight differences between them. GAG-mediated interactions are reviewed under two categories: interactions with chondrocytes and interactions with other fibrillar macromolecules of the ECM. Moreover, efforts to replicate GAG-mediated interactions to improve mechanical integrity of tissue-engineered cartilage constructs are discussed. In conclusion, studies exploring cartilage specific GAGs are poorly represented in the literature, and the ultrastructure of nasal septal and auricular cartilage is less studied compared with articular and meniscal cartilages. Understanding the contribution of GAGs in cartilage mechanics at the ultrastructural level and translating that knowledge to engineered cartilage will facilitate improvement of cartilage tissue engineering approaches.

Engineering Hyaluronic Acid for the Development of New Treatment Strategies for Osteoarthritis

Osteoarthritis (OA) is a degenerative joint disease that is characterized by inflammation of the joints, degradation of cartilage, and the remodeling of other joint tissues. Due to the absence of disease-modifying drugs for OA, current clinical treatment options are often only effective at slowing down disease progression and focus mainly on pain management. The field of tissue engineering has therefore been focusing on developing strategies that could be used not only to alleviate symptoms of OA but also to regenerate the damaged tissue. Hyaluronic acid (HA), an integral component of both the synovial fluid and articular cartilage, has gained widespread usage in developing hydrogels that deliver cells and biomolecules to the OA joint thanks to its biocompatibility and ability to support cell growth and the chondrogenic differentiation of encapsulated stem cells, providing binding sites for growth factors. Tissue-engineering strategies have further attempted to improve the role of HA as an OA therapeutic by developing diverse modified HA delivery platforms for enhanced joint retention and controlled drug release. This review summarizes recent advances in developing HA-based hydrogels for OA treatment and provides additional insights into how HA-based therapeutics could be further improved to maximize their potential as a viable treatment option for OA.

Synovial membrane mesenchymal stem cells: past life, current situation, and application in bone and joint diseases

Mesenchymal stem cells (MSCs) can be isolated from not only bone marrow, but also various adult mesenchymal tissues such as periosteum, skeletal muscle, and adipose tissue. MSCs from different tissue sources have different molecular phenotypes and differentiation potential. Synovial membrane (SM) is an important and highly specific component of synovial joints. Previous studies have suggested that the synovium is a structure with a few cell layers thick and consists mainly of fibroblast-like synoviocytes (FLS), which forms a layer that lining the synovial membrane on the joint cavity and synovial fluid through cell-cell contact. In recent years, studies have found that there are also mesenchymal stem cells in the synovium, and as an important part of the mesenchymal stem cell family, it has strong capabilities of cartilage forming and tissue repairing. This article reviews the sources, surface markers, subtypes, influencing factors, and applications in inflammatory joints of synovial membrane mesenchymal stem cells (SM-MSCs) in recent years, aiming to clarify the research status and existing problems of SM-MSCs.

Antisense oligonucleotide-based therapies for the treatment of osteoarthritis: Opportunities and roadblocks

Osteoarthritis (OA) is a debilitating disease with no approved disease-modifying therapies. Among the challenges for developing treatment is achieving targeted drug delivery to affected joints. This has contributed to the failure of several drug candidates for the treatment of OA. Over the past 20 years, significant advances have been made in antisense oligonucleotide (ASO) technology for achieving targeted delivery to tissues and cells both in vitro and in vivo. Since ASOs are able to bind specific gene regions and regulate protein translation, they are useful for correcting aberrant endogenous mechanisms associated with certain diseases. ASOs can be delivered locally through intra-articular injection, and can enter cells through natural cellular uptake mechanisms. Despite this, ASOs have yet to be successfully tested in clinical trials for the treatment of OA. Recent chemical modification to ASOs have further improved cellular uptake and reduced toxicity. Among these are locked nucleic acid (LNA)-based ASOs, which have shown promising results in clinical trials for diseases such as hepatitis and dyslipidemia. Recently, LNA-based ASOs have been tested both in vitro and in vivo for their therapeutic potential in OA, and some have shown promising joint-protective effects in preclinical OA animal models. In order to accelerate the testing of ASO therapies in a clinical trial setting for OA, further investigation into delivery mechanisms is required. In this review article, we discuss opportunities for viral-, particle-, biomaterial-, and chemical modification-based therapies, which are currently in preclinical testing. We also address potential roadblocks in the clinical translation of ASO-based therapies for the treatment of OA, such as the limitations associated with OA animal models and the challenges with drug toxicity. Taken together, we review what is known and what would be useful to accelerate translation of ASO-based therapies for the treatment of OA.

Hydrogel Containing Anti-CD44-Labeled Microparticles, Guide Bone Tissue Formation in Osteochondral Defects in Rabbits

Hydrogels are suitable for osteochondral defect regeneration as they mimic the viscoelastic environment of cartilage. However, their biomechanical properties are not sufficient to withstand high mechanical forces. Therefore, we have prepared electrospun poly-ε-caprolactone-chitosan (PCL-chit) and poly(ethylene oxide)-chitosan (PEO-chit) nanofibers, and FTIR analysis confirmed successful blending of chitosan with other polymers. The biocompatibility of PCL-chit and PEO-chit scaffolds was tested; fibrochondrocytes and chondrocytes seeded on PCL-chit showed superior metabolic activity. The PCL-chit nanofibers were cryogenically grinded into microparticles (mean size of about 500 µm) and further modified by polyethylene glycol-biotin in order to bind the anti-CD44 antibody, a glycoprotein interacting with hyaluronic acid (PCL-chit-PEGb-antiCD44). The PCL-chit or PCL-chit-PEGb-antiCD44 microparticles were mixed with a composite gel (collagen/fibrin/platelet rich plasma) to improve its biomechanical properties. The storage modulus was higher in the composite gel with microparticles compared to fibrin. The Eloss of the composite gel and fibrin was higher than that of the composite gel with microparticles. The composite gel either with or without microparticles was further tested in vivo in a model of osteochondral defects in rabbits. PCL-chit-PEGb-antiCD44 significantly enhanced osteogenic regeneration, mainly by desmogenous ossification, but decreased chondrogenic differentiation in the defects. PCL-chit-PEGb showed a more homogeneous distribution of hyaline cartilage and enhanced hyaline cartilage differentiation.

Anti-Inflammatory Effects of Intra-Articular Hyaluronic Acid: A Systematic Review

OBJECTIVE

Osteoarthritis (OA) is one of the leading causes of disability in the adult population. Common nonoperative treatment options include nonsteroidal anti-inflammatory drugs (NSAIDs), intra-articular corticosteroids, and intra-articular injections of hyaluronic acid (HA). HA is found intrinsically within the knee joint providing viscoelastic properties to the synovial fluid. HA therapy provides anti-inflammatory relief through a number of different pathways, including the suppression of pro-inflammatory cytokines and chemokines.

METHODS

We conducted a systematic review to summarize the published literature on the anti-inflammatory properties of hyaluronic acid in osteoarthritis. Included articles were categorized based on the primary anti-inflammatory responses described within them, by the immediate cell surface receptor protein assessed within the article, or based on the primary theme of the article. Key findings aimed to describe the macromolecules and inflammatory-mediated responses associated with the cell transmembrane receptors.

RESULTS

Forty-eight articles were included in this systematic review that focused on the general anti-inflammatory effects of HA in knee OA, mediated through receptor-binding relationships with cluster determinant 44 (CD44), toll-like receptor 2 (TLR-2) and 4 (TLR-4), intercellular adhesion molecule-1 (ICAM-1), and layilin (LAYN) cell surface receptors. Higher molecular weight HA (HMWHA) promotes anti-inflammatory responses, whereas short HA oligosaccharides produce inflammatory reactions.

CONCLUSIONS

Intra-articular HA is a viable therapeutic option in treating knee OA and suppressing inflammatory responses. HMWHA is effective in suppressing the key macromolecules that elicit the inflammatory response by short HA oligosaccharides.

Cartilage synthesis in hyaluronic acid-tyramine constructs

The objective of this study was to determine the potential for cartilage production within a hyaluronic acid-tyramine (HA-Tyr) hydrogel scaffold. Chondrocytes were encapsulated within HA-Tyr hydrogels and subcutaneously implanted in mice. The HA-Tyr hydrogels were formed by the oxidative coupling of Tyr moieties catalyzed by hydrogen peroxide (H₂O₂) and horseradish peroxidase (HRP). Harvested constructs were shown to achieve a glycosaminoglycan (GAG) content of 1.2 wt%, and they demonstrated 40% of the collagen content of normal articular cartilage, including the presence of Type II collagen, which is the characteristic of articular cartilage. Matrix production was found to be influenced by the initial cell density, scaffold degradation rate and Type II collagen concentration. Injectability was also imparted to the system by delivering HRP through thermoresponsive liposomes. The method of HRP delivery, either by simple addition or through thermoresponsive liposomes, was not shown to have an effect on matrix production.

Extracellular matrix recovery by human articular chondrocytes after treatment with hyaluronan hexasaccharides or Streptomyces hyaluronidase

Abstract The treatment of human articular chondrocytes with Streptomyces hyaluronidase (St-HA'ase) or hyaluronan hexasaccharides (HA6) provides two approaches to the selective depletion of specific components of the extracellular matrix, and an opportunity to follow the reparative responses initiated by these changes. In this study, changes in the relative expression of messenger RNA for hyaluronan synthase-2, CD44, and aggrecan were determined by competitive, quantitative reverse transcriptase-polymerase chain reaction. Changes in the size of the cell-associated matrix surrounding live chondrocytes were analyzed by the particle exclusion assay, and hyaluronan accumulation was characterized using a biotin-labeled hyaluronan-specific binding protein. Both Streptomyces hyaluronidase as well as hyaluronan hexasaccharide treatment of chondrocytes resulted in an approximately 2-fold increase in hyaluronan synthase-2 mRNA copy numbers, together with a 1.8-fold increase in the mRNA copy number for the proteoglycan aggrecan. However, although matrix biosynthesis was enhanced, the chondrocytes failed to retain these components. Both treatments resulted in a diminished accumulation of extracellular hyaluronan as well as a loss of the chondrocyte proteoglycan-rich cell-associated matrix. Thus, this model is similar to the early stages of osteoarthritis. Upon removal of the Streptomyces hyaluronidase or hyaluronan hexasaccharides, the normal, healthy, adult human chondrocytes used in this study regained their capacity to retain extracellular hyaluronan and to reassemble and retain a cell-associated matrix. This stimulation of hyaluronan synthase-2 (HAS-2) and aggrecan mRNA expression, and the subsequent capacity to retain the newly synthesized extracellular matrix, illustrate the events which are necessary for adult human articular chondrocytes to undergo effective repair.

P38 mitogen-activated protein kinase promotes dedifferentiation of primary articular chondrocytes in monolayer culture

Articular cartilage is an avascular tissue with poor regenerative capacity following injury, a contributing factor to joint degenerative disease. Cell-based therapies for cartilage tissue regeneration have rapidly advanced; however, expansion of autologous chondrocytes in vitro using standard methods causes 'dedifferentiation' into fibroblastic cells. Mitogen-activated protein kinase (MAPK) signalling is crucial for chondrocyte metabolism and matrix production, and changes in MAPK signals can affect the phenotype of cultured cells. We investigated the effects of inhibition of MAPK signalling on chondrocyte dedifferentiation during monolayer culture. Blockade of extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK) signalling caused a significant increase in cartilage gene expression, however, also caused up-regulation of fibrotic gene expression. Inhibition of p38 MAPK (p38) caused a significant up-regulation of collagen type II while suppressing collagen type I expression. P38 inhibition also resulted in consistently more organized secretion of collagen type II protein deposits on cell culture surfaces. Follow-on pellet culture of treated cells revealed that MAPK inhibition reduced cell migration from the pellet. ERK and JNK inhibition caused more collagen type I accumulation in pellets versus controls while p38 inhibition strongly promoted collagen type II accumulation with no effect on collagen type I. Blockade of all three MAPKs caused increased GAG content in pellets. These results indicate a role for MAPK signalling in chondrocyte phenotype loss during monolayer culture, with a strong contribution from p38 signalling. Thus, blockade of p38 enhances chondrocyte phenotype in monolayer culture and may promote more efficient cartilage tissue regeneration for cell-based therapies.

Expression of CD44 in articular cartilage is associated with disease severity in knee osteoarthritis

OBJECTIVES

To investigate CD44 levels in articular cartilage of knee osteoarthritis (OA) and the relationship between CD44 and severity of the disease.

METHODS

All 50 cartilage tissues included normal and OA cartilage, and were ascribed to the following four groups on the basis of modified Mankin score: normal, mild lesions, moderate lesions and severe lesions. CD44 levels in articular cartilage were assessed by immunohistochemical methods.

RESULTS

CD44 levels were detected in all four groups. The difference in average gray value of CD44 expression showed statistical significance when compared between each group (P < 0.05). In addition, CD44 expression in each group correlated with disease severity, according to the modified Mankin score (ρ = -0.848, P < 0.01).

CONCLUSIONS

CD44 in articular cartilage is associated with progressive knee OA joint damage.

Nuclear factor-κB activation by type II collagen peptide in articular chondrocytes: its inhibition by hyaluronan via the receptors

OBJECTIVE

This study aimed to examine nuclear factor-κB (NF-κB) activation by a synthetic peptide from type II collagen fragment (CB12-II) and its inhibition by hyaluronan (HA) via its receptors, CD44, and intercellular adhesion molecule-1 (ICAM-1) in chondrocytes.

METHODS

Osteoarthritic cartilage explants or chondrocytes in monolayer were cultured with CB12-II. Secreted levels of matrix metalloproteinase (MMP)-13 in conditioned media and NF-κB activation in chondrocytes were determined by immunoblotting and enzyme-linked immunosorbent assay (ELISA). Cultures were pretreated with HA to evaluate the inhibitory effect on CB12-II action, and the role of HA receptors in HA effect was investigated using antibodies to CD44 and ICAM-1.

RESULTS

CB12-II stimulated phosphorylation and nuclear translocation of NF-κB, leading to increased MMP-13 production. HA suppressed NF-κB activation and MMP-13 induction by CB12-II. The individual antibody to CD44 or ICAM-1 partially reversed HA effect on CB12-II action, and both antibodies in combination completely blocked the HA effect.

CONCLUSIONS

This study clearly demonstrates that CB12-II activates NF-κB for MMP-13 induction and that HA inhibits CB12-II action through interaction with CD44 and ICAM-1 in chondrocytes. HA administration into osteoarthritic joints could suppress the catabolic action of matrix degradation products such as CB12-II as a potent NF-κB inhibitor.

Culture of primary bovine chondrocytes on a continuously expanding surface inhibits dedifferentiation

Expansion of autologous chondrocytes in vitro is used to generate adequate populations for cell-based therapies. However, standard (SD) culture methods cause loss of chondrocyte phenotype and dedifferentiation to fibroblast-like cells. Here, we use a novel surface expansion culture system in an effort to inhibit chondrocyte dedifferentiation. A highly elastic silicone rubber culture surface was continuously stretched over a 13-day period to 600% of its initial surface area. This maintained cells at a high density while limiting contact inhibition and reducing the need for passaging. Gene expression analysis, biochemical assays, and immunofluorescence microscopy of follow-on pellet cultures were used to characterize the results of continuous expansion (CE) culture versus SD cultures on rigid polystyrene. CE culture yielded cells with a more chondrocyte-like morphology and higher RNA-level expression of the chondrogenic markers collagen type II, aggrecan, and cartilage oligomeric matrix protein. Furthermore, the expression of collagen type I RNA and α-smooth muscle actin protein were significantly reduced, indicating suppression of fibroblastic features. Pellet cultures from CE chondrocytes contained more sulphated glycosaminoglycan and collagen type II than pellets from SD culture. Additional control cultures on static (unexpanded) silicone (SS culture) indicated that benefits of CE culture were partially due to features of the culture surface itself and partially due to the reduced passaging which that surface enabled through CE. Chondrocytes grown in CE culture may, therefore, be a superior source for cell-based therapies.

Induction of CD44 cleavage in articular chondrocytes

OBJECTIVE

The hyaluronan receptor CD44 provides chondrocytes with a mechanism for sensing and responding to changes in the extracellular matrix. The purpose of this study was to document the fragmentation and loss of CD44 and to determine the likely mechanisms involved.

METHODS

A polyclonal anti-CD44 cytotail antibody was generated to detect CD44 fragmentation by Western blot analysis. Chondrocytes were isolated from human or bovine articular cartilage. Primary articular chondrocytes were treated with interleukin-1beta (IL-1beta), hyaluronan oligosaccharides, or phorbol myristate acetate or were passaged and subcultured in monolayer to induce dedifferentiation. Conditions that altered the capacity of CD44 to transit into lipid rafts, or pharmacologic inhibitors of metalloproteinase or gamma-secretase activity were used to define the mechanism of fragmentation of CD44.

RESULTS

Chondrocytes from osteoarthritic cartilage exhibited CD44 fragmentation as low molecular mass bands, corresponding to the CD44-EXT and CD44-ICD bands. Following dedifferentiation of chondrocytes or treatment of primary chondrocytes with hyaluronan oligosaccharides, IL-1beta, or phorbol myristate acetate, CD44 fragmentation was enhanced. Subsequent culture of the dedifferentiated chondrocytes in 3-dimensional alginate beads rescued the chondrocyte phenotype and diminished the fragmentation of CD44. Fragmentation of CD44 in chondrocytes was blocked in the presence of the metalloproteinase inhibitor GM6001 and the gamma-secretase inhibitor DAPT.

CONCLUSION

CD44 fragmentation, consistent with a signature pattern reported for sequential metalloproteinase/gamma-secretase cleavage of CD44, is a common metabolic feature of chondrocytes that have undergone dedifferentiation in vitro and osteoarthritic chondrocytes. Transit of CD44 into lipid rafts may be required for its fragmentation.

Transcriptional activation of human MMP-13 gene expression by c-Maf in osteoarthritic chondrocyte

Matrix metalloproteinase (MMP)-13 has pivotal roles in the pathogenesis of Osteoarthritis (OA) and it is necessary to understand the regulatory mechanisms of MMP-13 expression. MMP-13 gene expression is regulated primarily at the transcriptional level. In this study, we investigated the role of c-maf in regulating MMP-13 transcription. Using transient transfection system with an c-maf construct, and MMP-13 promoter-luciferase constructs with specific mutations in transcription factor binding sites, we found that c-maf can significantly enhance MMP-13 promoter activity via the AP-1 site, By gene suppression with RNAi technology, we could show that c-maf downregulation leads to a reduced expression of MMP13. Chromatin immunoprecipitation assays reveal that c-maf binds to the MMP-13 gene promoter to a region of the MMP-13 promoter containing the AP-1 site. Taken together, these studies demonstrate a new level of transcriptional regulation of MMP-13 expression by the c-maf.

Hyaluronan-CD44 interaction stimulates keratinocyte differentiation, lamellar body formation/secretion, and permeability barrier homeostasis

In this study we investigated whether hyaluronan (HA)-CD44 interaction influences epidermal structure and function. Our data show that CD44 deficiency is accompanied by reduction in HA staining in CD44 knockout (k/o) mouse skin leading to a marked thinning of epidermis versus wild-type mouse skin. A significant delay in the early barrier recovery (following acute barrier disruption) occurs in CD44 k/o versus wild-type mouse skin. To assess the basis for these alterations in CD44 k/o mouse epidermis, we determined that differentiation markers are greatly reduced in the epidermis of CD44 k/o versus wild-type mice, while conversely HA binding to CD44 triggers differentiation in cultured human keratinocytes. CD44 downregulation (using CD44 small interfering RNAs) also inhibits HA-mediated keratinocyte differentiation. Slower barrier recovery in CD44 k/o mice could be further attributed to reduced lamellar body formation, loss of apical polarization of LB secretion, and downregulation of cholesterol synthesis. Accordingly, HA-CD44 binding stimulates both LB formation and secretion. Together, these observations demonstrate new roles for HA-CD44 interaction in regulating both epidermal differentiation and lipid synthesis/secretion, which in turn influence permeability barrier homeostasis. HA-CD44 signaling could comprise a novel approach to treat skin disorders characterized by abnormalities in differentiation, lipid synthesis, and/or barrier function.

Hyaluronan oligosaccharides induce matrix metalloproteinase 13 via transcriptional activation of NFkappaB and p38 MAP kinase in articular chondrocytes

Hyaluronan exerts a variety of biological effects on cells including changes in cell migration, proliferation, and matrix metabolism. However, the signaling pathways associated with the action of hyaluronan on cells have not been clearly defined. In some cells, signaling is induced by the loss of cell-hyaluronan interactions. The goal of this study was to use hyaluronan oligosaccharides as a molecular tool to explore the effects of changes in cell-hyaluronan interactions and determine the underlying molecular events that become activated. In this study, hyaluronan oligosaccharides induced the loss of extracellular matrix proteoglycan and collagen from cultured slices of normal adult human articular cartilage. This loss was coincident with an increased expression of matrix metalloproteinase (MMP)-13. MMP-13 expression was also induced in articular chondrocytes by hyaluronan (HA) hexasaccharides but not by HA tetrasaccharides nor high molecular weight hyaluronan. MMP-13 promoter-reporter constructs in CD44-null COS-7 cells revealed that both CD44-dependent and CD44-independent events mediate the induction of MMP-13 by hyaluronan oligosaccharides. Electromobility gel shift assays demonstrated the activation of chondrocyte NFkappaB by hyaluronan oligosaccharides. NFkappaB activation was also documented in C-28/I2 immortalized human chondrocytes by luciferase promoter assays and phosphorylation of IKK-alpha/beta. The link between activation of NFkappaB and MMP-13 induction by HA oligosaccharides was further confirmed through the use of the NFkappaB inhibitor helenalin. Inhibition of MAP kinases also demonstrated the involvement of p38 MAP kinase in the hyaluronan oligosaccharide induction of MMP-13. Our findings suggest that hyaluronan-CD44 interactions affect matrix metabolism via activation of NFkappaB and p38 MAP kinase.

NF-kappaBp65-specific siRNA inhibits expression of genes of COX-2, NOS-2 and MMP-9 in rat IL-1beta-induced and TNF-alpha-induced chondrocytes

OBJECTIVE

Small interfering RNA (siRNA) triggers RNA interference in mammalian somatic cells. Nuclear factor kappaB (NF-kappaB) is a transcription factor that is implicated in inflammation and immune activation. This study was to use NF-kappaBp65-specific siRNA to inhibit the expression of genes of cyclooxygenase-2 (COX-2), nitric oxide synthase-2 (NOS-2) and matrix metalloproteinase-9 (MMP-9), which is paralleled with the initiation and progression of cartilage lesions in osteoarthritis (OA) model, in induced chondrocytes, and therefore to explore a new gene therapy for OA.

METHODS

Western blot and reverse transcriptase polymerase chain reaction (RT-PCR) were performed to optimize the silencing effects of NF-kappaBp65-specific siRNA in cultured rat chondrocytes, and then to determine the expression of COX-2, NOS-2 and MMP-9 in induced chondrocytes. The activation of NF-kappaB was determined by electrophoretic mobility shift assay (EMSA). Western blot and RT-PCR were subjected to densitometric analysis and then band intensities were also determined.

RESULTS

The NF-kappaBp65-specific siRNA inhibited the expression of NF-kappaBp65 and activation of NF-kappaB, reducing significantly the expression of COX-2, NOS-2 and MMP-9 induced by interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) in cultured chondrocytes.

CONCLUSIONS

NF-kappaBp65-specific siRNA can inhibit the expression of COX-2, NOS-2 and MMP-9 in IL-1beta-induced and TNF-alpha-induced chondrocytes. This suggests that NF-kappaBp65-specific siRNA has potential to be a useful, preventive and therapeutic agent for OA at early stages.

Hyaluronan fragments activate nitric oxide synthase and the production of nitric oxide by articular chondrocytes

Chondrocyte CD44 receptors anchor hyaluronan to the cell surface, enabling the assembly and retention of proteoglycan aggregates in the pericellular matrix. Hyaluronan-CD44 interactions also provide signaling important for maintaining cartilage homeostasis. Disruption of chondrocyte-hyaluronan contact alters CD44 occupancy, initiating alternative signaling cascades. Treatment with hyaluronan oligosaccharides is one approach to uncouple CD44 receptors from its native ligand, hyaluronan. In bovine articular chondrocytes, treatment with hyaluronan oligosaccharides or purified hyaluronan hexasaccharides induced the production of nitric oxide that mirrored nitric oxide production following interleukin-1 treatment. In contrast, 120 and 1,260 kDa hyaluronan did not induce production of nitric oxide. Human chondrocytes responded similarly to treatment with hyaluronan or hyaluronan oligosaccharides. Nitric oxide production from chondrocytes was mediated by activation of the inducible nitric oxide synthase, as confirmed by mRNA expression and inhibition of nitric oxide production by diphenyleneiodonium. Co-treatment of chondrocytes with hyaluronan oligosaccharides and interleukin-1 did not demonstrate additive effects. Blocking interleukin-1 receptors with an antagonist did not abolish the production of nitric oxide induced by treatment with hyaluronan oligosaccharides. Moreover, only COS-7 following transfection with a pCD44, not the CD44-null parental cells, responded to treatment with hyaluronan oligosaccharides by releasing nitric oxide. This study demonstrates a novel signaling potential by hyaluronan fragments, in lieu of endogenous hyaluronan-chondrocyte interactions, resulting in the activation of inducible nitric oxide synthase.

Hyaluronan oligosaccharide-induced activation of transcription factors in bovine articular chondrocytes

OBJECTIVE

To document the activity profile of transcription factors following chondrocyte stimulation with hyaluronan (HA) hexasaccharides (HA(6)) and to determine the expression of genes whose transcriptional activation is tightly associated with the transcription factors.

METHODS

Nuclear extracts from bovine articular chondrocytes treated with HA(6) were subjected to transcription factor protein-DNA array analysis. Electrophoretic mobility shift assay (EMSA) analyses were performed to confirm the results of protein-DNA array. The gene expressions of matrix metalloproteinase 3 (MMP-3), type II collagen, and cartilage oligomeric matrix protein (COMP) were examined by quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR), and protease activity was assessed by casein zymography.

RESULTS

In the protein-DNA array analysis, 12 transcription factors were up-regulated and 2 transcription factors were down-regulated in the chondrocytes treated with HA(6). The transcription factors retinoic acid receptor (RAR), retinoid X receptor (RXR), and Sp-1 exhibited >2-fold increased activity by HA(6) treatment, as confirmed by EMSA. RT-PCR analysis showed that the expression levels of MMP-3, type II collagen, and COMP messenger RNA, which are tightly associated with the activation of RAR, RXR, or Sp-1, were up-regulated by treatment with HA(6). Addition of high molecular mass HA after HA(6) treatment resulted in abrogation of the MMP-3 induction.

CONCLUSION

These results suggest that HA(6) increase the activity of multiple transcription factors in chondrocytes and signal the enhanced expression of key genes involved in cartilage-matrix remodeling and turnover. The data also demonstrate that high molecular mass HA has a potential to suppress the signaling activated by HA(6).

Effects of potassium and anion channel blockers on the cellular response of human osteoarthritic chondrocytes

The aim of this article is to determine to what extent the proliferation, CD44 expression, and apoptosis behavior of cells can be influenced by the modulation of ion channel activity on the cell membrane of human osteoarthritic chondrocytes. The potassium channel blocker 4-aminopyridine (4-AP) and the chloride and anion channel blocker 4-acetamido-4'-isothiocyano-2,2'-disulfonic acid stilbene (SITS) were used as ion channel modulators. Assessment of the proliferation was done by incorporation of (3)H-thymidine. The detection of apoptotic cells and expression of the hyaluronic acid binding CD44-receptor were determined by flow cytometry. The results showed that 4-AP and SITS lead to a temporary increase in (3)H-thymidine incorporation, followed by a suppression of proliferation after a 12-day incubation. 4-AP causes considerable cytotoxic effects. SITS leads to necrotic cell damage. CD44 expression is increased up to 43% after incubation with 4-AP for 24 or 48 h, whereas prolonged incubation under SITS influence leads to a clear inhibition of CD44 expression. In conclusion, proliferation, CD44 expression, and apoptosis behavior of human chondrocytes can be influenced by modulation of ion channel activity. These results serve as a basis for further investigations to extend the therapeutic possibilities in the treatment of arthritis.

Antisense inhibition of osteogenic protein 1 disturbs human articular cartilage integrity

OBJECTIVE

To delineate the role of endogenous osteogenic protein 1 (OP-1) in human articular cartilage homeostasis via the inhibition of OP-1 gene expression by antisense oligonucleotides.

METHODS

Human adult normal articular cartilage was obtained from the knee and ankle joints of 34 organ donors. Chondrocytes were cultured as tissue explants or isolated cells in alginate or high-density monolayers for 48 hours in the presence of OP-1 antisense or sense oligonucleotides. The effect of OP-1 antisense inhibition was evaluated by reverse transcription-polymerase chain reaction, (35)S incorporation, dimethylmethylene blue assay, histology with Safranin O staining, and immunohistochemistry with anti-proOP-1, anti-mature OP-1, and anti-aggrecan antibodies.

RESULTS

Antisense treatment inhibited OP-1 gene expression by a mean +/- SD of 34 +/- 12% (P < 0.01) in chondrocytes cultured in monolayers and by 77 +/- 27% (P < 0.03) in alginate beads. The inhibition of autocrine OP-1 caused a striking decrease in aggrecan gene expression, in total proteoglycan content accumulated in cartilage matrix, and in the ability of chondrocytes to newly synthesize proteoglycans. OP-1 antisense reduced aggrecan messenger RNA expression by 42 +/- 17% (P < 0.05) and proteoglycan synthesis by 48 +/- 23% (P < 0.01). Histology and immunohistochemistry revealed a dramatic decrease in Safranin O staining and reduced anti-aggrecan staining (primarily in the superficial and middle cartilage layers) with OP-1 antisense treatment.

CONCLUSION

Our results suggest that OP-1 is an important endogenous cartilage factor that regulates matrix integrity and possibly needs to be induced or up-regulated to maintain normal cartilage homeostasis. These findings confirm our hypothesis that a lack of autocrine OP-1 may lead to an elevated susceptibility of chondrocytes to the catabolic processes, thus contributing/promoting cartilage degeneration.

Inhibition of interleukin-1beta-stimulated production of matrix metalloproteinases by hyaluronan via CD44 in human articular cartilage

OBJECTIVE

To investigate the mechanism of the inhibitory action of hyaluronan (HA) on interleukin-1beta (IL-1beta)-stimulated production of matrix metalloproteinases (MMPs) in human articular cartilage.

METHODS

IL-1beta was added to normal and osteoarthritic (OA) human articular cartilage in explant culture to stimulate MMP production. Articular cartilage was incubated or preincubated with a clinically used form of 800-kd HA to assess its effect on IL-1beta-induced MMPs. Levels of secreted MMPs 1, 3, and 13 in conditioned media were detected by immunoblotting; intracellular MMP synthesis in chondrocytes was evaluated by immunofluorescence microscopy. Penetration of HA into cartilage tissue and its binding to CD44 were analyzed by fluorescence microscopy using fluoresceinated HA. Blocking experiments with anti-CD44 antibody were performed to investigate the mechanism of action of HA.

RESULTS

Treatment and pretreatment with 800-kd HA at 1 mg/ml resulted in significant suppression of IL-1beta-stimulated production of MMPs 1, 3, and 13 in normal and OA cartilage explant culture. Fluorescence histocytochemistry revealed that HA penetrated cartilage tissue and localized in the pericellular matrix around chondrocytes. HA-binding blocking experiments using anti-CD44 antibody demonstrated that the association of HA with chondrocytes was mediated by CD44. Preincubation with anti-CD44 antibody, which suppressed IL-1beta-stimulated MMPs, reversed the inhibitory effect of HA on MMP production that was induced by IL-1beta in normal and OA cartilage.

CONCLUSION

This study demonstrates that HA effectively inhibits IL-1beta-stimulated production of MMP-1, MMP-3, and MMP-13, which supports the clinical use of HA in the treatment of OA. The action of HA on IL-1beta may involve direct interaction between HA and CD44 on chondrocytes.

CD44H and the isoforms CD44v5 and CD44v6 in the synovial fluid of the osteoarthritic human knee joint

OBJECTIVE

To determine whether the concentrations of CD44H and v5 and v6 in the synovial fluid are correlated with the presence of accompanying synovitis in the osteoarthritic joint and with the grade of osteoarthritis.

DESIGN

Using antero-posterior and lateral X-rays of the knee joint and patellar view of 46 patients were graded with the Kellgren & Lawrence scale. Synovial fluid from these patients with different grades of primary osteoarthritis of the knee joint with and without synovial inflammation (synovitis) was collected during surgical procedures. A horseradish peroxidase conjugated anti-CD44H-, anti-sCD44v5- or anti-sCD44v6-antibody was added and labeled with tetramethylbenzidine. The absorbance was measured at wavelengths of 450/620 nm. Regression analysis was performed and the statistical significance was assessed, using the Student t-test for unequal variance.

RESULTS

CD44H and v5 and v6 were detected in the synovial fluid of all 46 patients. Osteoarthritic patients with synovial inflammation showed significantly higher levels of CD44H and v6, but not v5, than osteoarthritic patients without synovial inflammation. With progression of osteoarthritis from Kellgren grade II to III, CD44v5 decreased significantly. All other isoform concentrations remained statistically unchanged.

CONCLUSIONS

CD44H and the isoforms v5 and v6 were present in the synovial fluid of osteoarthritic patients. Their concentrations do not reflect the osteoarthritic stage in the Kellgren grading scale. CD44H and CD44v6, but not CD44v5, are significantly up-regulated in osteoarthritic synovial inflammation.

Hyaluronan and CD44: strategic players for cell-matrix interactions during chondrogenesis and matrix assembly

Embryonic induction, soluble and insoluble factors, receptors, and signal transduction are orchestrated for the morphogenesis of the cartilage elements. The interaction of cells with the extracellular matrix (ECM) may lead to altered cellular response to morphogens based on the formation of new adhesive contacts, or the uncoupling of cell-matrix interactions. Hyaluronan's influence on cell behavior, and its intimate association with cells are accomplished by a wide variety of specific binding proteins for hyaluronan. The temporal expression of the hyaluronan receptor CD44 (which is expressed as several alternatively spliced variants) may be strategic to many of these cell-matrix interactions during chondrogenesis. CD44 expression is temporally coincident with the reduction of intercellular spaces at the regions of future cartilage deposition. The spatial organization of CD44 at the cell surface may function to establish or regulate the structure of the pericellular matrix dependent on a hyaluronan scaffold. As the ECM is modified during embryogenesis, the cellular response to inductive signals may be altered. An uncoupling of chondrocyte-hyaluronan interaction leads to chondrocytic chondrolysis. Thus, consideration of cell-matrix interactions during chondrogenesis, in the light of our current understanding of the temporal and spatial expression of signaling morphogens, should become a promising focus of future research endeavors.

Intra-articular hyaluronan (hyaluronic acid) and hylans for the treatment of osteoarthritis: mechanisms of action

Although the predominant mechanism of intra-articular hyaluronan (hyaluronic acid) (HA) and hylans for the treatment of pain associated with knee osteoarthritis (OA) is unknown, in vivo, in vitro, and clinical studies demonstrate various physiological effects of exogenous HA. HA can reduce nerve impulses and nerve sensitivity associated with the pain of OA. In experimental OA, this glycosaminoglycan has protective effects on cartilage, which may be mediated by its molecular and cellular effects observed in vitro. Exogenous HA enhances chondrocyte HA and proteoglycan synthesis, reduces the production and activity of proinflammatory mediators and matrix metalloproteinases, and alters the behavior of immune cells. Many of the physiological effects of exogenous HA may be a function of its molecular weight. Several physiological effects probably contribute to the mechanisms by which HA and hylans exert their clinical effects in knee OA.

Chondrogenic differentiation of human mesenchymal stem cells within an alginate layer culture system

Human mesenchymal stem cells (hMSCs) derived from bone marrow have the capacity to differentiate along a number of connective tissue pathways and are an attractive source of chondrocyte precursor cells. When these cells are cultured in a three-dimensional format in the presence of transforming growth factor-beta, they undergo characteristic morphological changes concurrent with deposition of cartilaginous extracellular matrix (ECM). In this study, factors influencing hMSC chondrogenesis were investigated using an alginate layer culture system. Application of this system resulted in a more homogeneous and rapid synthesis of cartilaginous ECM than did micromass cultures and presented a more functional format than did alginate bead cultures. Differentiation was found to be dependent on initial cell seeding density and was interrelated to cellular proliferation. Maximal glycosaminoglycan (GAG) synthesis defined an optimal hMSC seeding density for chondrogenesis at 25 x 10(6) cells/ml. Inclusion of hyaluronan in the alginate layer at the initiation of cultures enhanced chondrogenic differentiation in a dose-dependent manner, with maximal effect seen at 100 microg/ml. Hyaluronan increased GAG synthesis at early time points, with greater effect seen at lower cell densities, signifying cell-cell contact involvement. This culture system offers additional opportunities for elucidating conditions influencing chondrogenesis and for modeling cartilage homeostasis or osteoarthritic changes.

A requirement for the CD44 cytoplasmic domain for hyaluronan binding, pericellular matrix assembly, and receptor-mediated endocytosis in COS-7 cells

CD44-negative COS-7 cells were transfected with expression constructs for CD44H (the predominant CD44 isoform), CD44E (epithelial isoform), or truncation mutant derivatives lacking the carboxyl-terminal 67 amino acids of the cytoplasmic domain, CD44HDelta67 and CD44EDelta67. The truncation mutant CD44HDelta67 is identical to a naturally occurring alternatively spliced "short tail" CD44 isoform (CD44st), which incorporates exon 19 in place of exon 20. CD44st lacks intracellular signaling motifs as well as protein domains necessary for interaction with cytoskeletal components. Transfection of COS-7 cells with each construct yielded equivalent levels of mRNA expression, whereas no CD44 expression was observed in parental, nontransfected COS-7 cells. Western analysis and immunostaining of COS-7 transfectants confirmed CD44 protein expression of the truncation mutant derivatives. COS-7 cells transfected with CD44H or CD44E gained the capacity to bind fluorescein-conjugated HA (fl-HA) and assemble HA-dependent pericellular matrices in the presence of exogenously added HA and proteoglycan. In addition, the CD44H- and CD44E-transfected cells were able to internalize surface-bound fl-HA. COS-7 cells transfected with the vector alone or with either of the mutant CD44 isoforms, CD44HDelta67 or CD44EDelta67, did not exhibit the capacity to assemble pericellular matrices or to bind and internalize the fl-HA. Cotransfection of CD44Delta67 mutants together with CD44H reduced the size of the HA-dependent pericellular matrices. Transfection of bovine articular chondrocytes with CD44Delta67 also inhibited pericellular matrix assembly. Collectively, these results indicate an obligatory requirement for the CD44 receptor cytoplasmic domain for ligand (HA) binding, formation and retention of the pericellular matrix, as well as CD44-mediated endocytosis of HA. In addition, the results suggest a potential regulatory role for the differentially expressed alternatively spliced short tail CD44 isoform.

Temporal expression of CD44 during embryonic chick limb development and modulation of its expression with retinoic acid

Hyaluronan-cell interactions are initiated co-ordinately with mesenchymal condensation during chondrogenic differentiation in the limb bud. Hyaluronan is responsible for the retention and organization of proteoglycan within the cartilage matrix. Hyaluronan-CD44 binding also retains proteoglycan aggregates to the chondrocyte plasma membrane. A sequence for CD44 protein in chick has recently been reported, but never evaluated in chick chondrocytes. Total RNA was isolated from embryonic chick limb buds, stages 18, 19, 24, 25 and 30. Using semi-quantitative RT-PCR, expression of aggrecan, this chick CD44 orthologue and GAPDH mRNA was analyzed. Aggrecan expression was detected at all stages, but was increased at stage 30. CD44 mRNA was detected at extremely low levels at stage 18 to higher levels in the latter stages. Thus, the temporal expression of CD44 mRNA correlated with the onset of pre-cartilage condensation. The full-length chick chondrocyte CD44 cDNA was obtained following RT-PCR using RNA derived from tibial chondrocytes from stage 37 chick embryos. The nucleotide sequence was used to generate an amino acid sequence and analyses revealed homologies of 44.4% with mouse, 47.8% with bovine and 46.3% with human CD44. Tibial chondrocytes were cultured in the presence or absence of retinoic acid for 36 or 72 h. By RT-PCR, expression of aggrecan and the CD44 mRNA by chick chondrocytes was decreased after retinoic acid treatment, while GAPDH expression showed no change. As expected, control chondrocytes exhibited a round morphology while retinoic acid-treated chondrocytes were elongated. The retinoic acid-treated chondrocytes also exhibited reduced hyaluronan binding. This functional assay indicates a role for a CD44 receptor in matrix retention by chick chondrocytes.

NG2/HMPG modulation of human articular chondrocyte adhesion to type VI collagen is lost in osteoarthritis

NG2/human melanoma proteoglycan (HMPG) is a chondroitin sulphate proteoglycan (CSPG), expressed by chondrocytes in fetal and in normal and osteoarthritic (OA) adult articular cartilage. NG2/HMPG is a receptor for extracellular matrix proteins, including type VI collagen, and regulates beta1 integrin binding to fibronectin. This study was undertaken to identify whether NG2/HMPG had similar activities in human articular chondrocytes (HACs). Normal and OA adult HAC adhesion to fibronectin, type II or type VI collagen was assessed using a methylene blue assay. The requirement for integrins, NG2/HMPG, and integrin-associated signalling molecules was investigated using anti-beta1 integrin and anti-HMPG antibodies and pharmacological inhibitors of signalling molecules. The adhesion of normal and OA HACs to fibronectin, type II and type VI collagen was beta1 integrin-dependent. Normal HAC adhesion to type VI collagen was stimulated by anti-HMPG antibodies. This effect was inhibited by pertussis toxin. Anti-HMPG antibodies had no effect on OA chondrocyte adhesion to type VI collagen, or on normal and OA cell adhesion to fibronectin and type II collagen. The results show that NG2/HMPG modulates integrin-mediated interactions of normal HACs with type VI collagen. Loss of this activity may be of importance in the progression of osteoarthritis.

Antisense inhibition of CD44 tailless splice variant in human articular chondrocytes promotes hyaluronan internalization

OBJECTIVE

To determine whether alternatively spliced variants of CD44, in particular a short, intracellular tail CD44 isoform, are used by articular chondrocytes to modulate the functions of this matrix receptor.

METHODS

Normal human articular chondrocytes were cultured with or without interleukin-1alpha (IL-1alpha), and the relative expression of CD44 exon 19 and CD44 exon 20, hyaluronan synthase 2, aggrecan, and GAPDH messenger RNA (mRNA) was determined using reverse transcriptase-polymerase chain reaction. Next, CD44 exon 19 mRNA was selectively inhibited by the use of antisense oligonucleotides. The effects of exon 19 loss were analyzed by matrix assembly and hyaluronan internalization assays.

RESULTS

Human articular chondrocytes express varying levels of exon 19 (short tail)- and exon 20 (long tail)-containing CD44 mRNA. Both CD44 mRNA are up-regulated by IL-1alpha. Selective inhibition of CD44 exon 19 results in enhanced hyaluronan internalization and smaller cell-associated matrices.

CONCLUSION

The expression of a natural CD44 decoy-like receptor by articular chondrocytes modulates the function of this matrix receptor.

Mechanisms of chondrocyte adhesion to cartilage: role of beta1-integrins, CD44, and annexin V

The initial adhesion of transplanted chondrocytes to surrounding host cartilage may be important in the repair of articular defects. Adhesion may position cells to secrete molecules that fill the defect and integrate repair tissue with host tissue. While chondrocytes are known to become increasingly adherent to cartilage with time, the molecular basis for this is unknown. The objective of this study was to investigate the role of beta1-integrin, CD44, and annexin V receptors in chondrocyte adhesion to cartilage. Chondrocytes were cultured in high density monolayer, released with trypsin, and allowed to recover in suspension for 2 h at 37 degrees C. Under these conditions, flow cytometry analysis showed that chondrocytes expressed beta1-integrins, CD44, and annexin V. In a rapid screening assay to assess chondrocyte adhesion to cartilage, cell detachment decreased from 79% at 10 min following transplantation to 10% at 320 min. Treatment of cells with a monoclonal antibody to block beta1-integrins significantly increased chondrocyte detachment from cartilage compared to untreated controls. Similarly, results from a parallel-plate shear flow adhesion assay showed that blocking beta1-integrins significantly increased chondrocyte detachment from cartilage compared to untreated controls at each level of applied shear (0-70 Pa). In both assays, treatment of cells with reagents that block CD44 (hyaluronan oligosaccharides or monoclonal Ab IM7) or annexin V (polyclonal Ab #8958) had no detectable effect on adhesion. With cartilage treated with chondroitinase ABC, blocking beta1-integrins also increased chondrocyte detachment, while blocking CD44 and annexin V also had no detectable effect. Under the conditions studied here, beta1-integrins appear to mediate chondrocyte adhesion to a cut cartilage surface. Delineation of the mechanisms of adhesion may have clinical implications by allowing cell manipulations or matrix treatments to enhance chondrocyte adhesion and retention at a defect site.

Composition and structure of articular cartilage: a template for tissue repair

The authors review the structure and composition of articular cartilage. This tissue is composed of an extensive extracellular matrix synthesized by chondrocytes. It contains different zones with respect to depth from the articular surface and has a regional organization around the chondrocytes. Its composition varies regionally and zonally in its collagen and proteoglycan contents and those of other matrix molecules. There is a macrofibrillar collagen network and a microfilamentous network about which other noncollagenous molecules are organized. Its structure and composition are reflective of its special mechanical properties that primarily reflect its tensile strength (collagens) and compressive stiffness (proteoglycan aggrecan) and cell-matrix interactions (noncollagenous proteins).

Anti-Fas-induced apoptosis in chondrocytes reduced by hyaluronan: evidence for CD44 and CD54 (intercellular adhesion molecule 1) invovement

OBJECTIVE

To investigate the in vitro effect of therapeutic hyaluronan (HA) of 500-730 kd on anti-Fas-induced apoptosis of chondrocytes from osteoarthritis (OA) patients, and to assess its mechanism of action by analyzing the role of the 2 HA receptors, CD44 and CD54 (intercellular adhesion molecule 1 [ICAM-1]).

METHODS

Chondrocytes isolated from human OA knee cartilage were cultured and the effect of HA on both spontaneous and anti-Fas-induced apoptosis was evaluated. Apoptosis was analyzed by JAM test (for quantitative analysis of fragmented DNA), cell death detection immunoassay (for quantitative analysis of oligonucleosome), TUNEL assay, and electron microscopy. Blocking experiments with anti-CD44 and anti-CD54 alone or in combination were performed to investigate the HA mechanism of action.

RESULTS

Both quantitative tests demonstrated that anti-Fas significantly induced apoptosis of isolated OA chondrocytes. HA at 1,000 microg/ml significantly reduced the anti-Fas-induced apoptosis of chondrocytes but did not affect spontaneous chondrocyte apoptosis. These data were also confirmed by TUNEL staining and by electron microscopy morphologic evaluation. The antiapoptotic effects of HA on anti-FAS-induced chondrocyte apoptosis were significantly decreased by both anti-CD44 (mean +/- SD 57 +/- 12% inhibition) and anti-ICAM-1 (31 +/- 22% inhibition). The mixture of the 2 antibodies had an additive effect, since the rate of inhibition increased to 87 +/- 13%.

CONCLUSION

These data demonstrate that 500-730-kd HA exerts an antiapoptotic effect on anti-FAS-induced chondrocyte apoptosis by binding its specific receptors (CD44 and ICAM-1). Furthermore, this HA fraction may be able to slow down chondrocyte apoptosis in OA by regulating the processes of cartilage matrix degradation.

Anti-Fas-induced apoptosis in chondrocytes reduced by hyaluronan: evidence for CD44 and CD54 (intercellular adhesion molecule 1) invovement

OBJECTIVE

To investigate the in vitro effect of therapeutic hyaluronan (HA) of 500-730 kd on anti-Fas-induced apoptosis of chondrocytes from osteoarthritis (OA) patients, and to assess its mechanism of action by analyzing the role of the 2 HA receptors, CD44 and CD54 (intercellular adhesion molecule 1 [ICAM-1]).

METHODS

Chondrocytes isolated from human OA knee cartilage were cultured and the effect of HA on both spontaneous and anti-Fas-induced apoptosis was evaluated. Apoptosis was analyzed by JAM test (for quantitative analysis of fragmented DNA), cell death detection immunoassay (for quantitative analysis of oligonucleosome), TUNEL assay, and electron microscopy. Blocking experiments with anti-CD44 and anti-CD54 alone or in combination were performed to investigate the HA mechanism of action.

RESULTS

Both quantitative tests demonstrated that anti-Fas significantly induced apoptosis of isolated OA chondrocytes. HA at 1,000 microg/ml significantly reduced the anti-Fas-induced apoptosis of chondrocytes but did not affect spontaneous chondrocyte apoptosis. These data were also confirmed by TUNEL staining and by electron microscopy morphologic evaluation. The antiapoptotic effects of HA on anti-FAS-induced chondrocyte apoptosis were significantly decreased by both anti-CD44 (mean +/- SD 57 +/- 12% inhibition) and anti-ICAM-1 (31 +/- 22% inhibition). The mixture of the 2 antibodies had an additive effect, since the rate of inhibition increased to 87 +/- 13%.

CONCLUSION

These data demonstrate that 500-730-kd HA exerts an antiapoptotic effect on anti-FAS-induced chondrocyte apoptosis by binding its specific receptors (CD44 and ICAM-1). Furthermore, this HA fraction may be able to slow down chondrocyte apoptosis in OA by regulating the processes of cartilage matrix degradation.

Cartilage proteoglycans

The predominant proteoglycan present in cartilage is the large chondroitin sulfate proteoglycan 'aggrecan'. Following its secretion, aggrecan self-assembles into a supramolecular structure with as many as 50 monomers bound to a filament of hyaluronan. Aggrecan serves a direct, primary role providing the osmotic resistance necessary for cartilage to resist compressive loads. Other proteoglycans expressed during chondrogenesis and in cartilage include the cell surface syndecans and glypican, the small leucine-rich proteoglycans decorin, biglycan, fibromodulin, lumican and epiphycan and the basement membrane proteoglycan, perlecan. The emerging functions of these proteoglycans in cartilage will enhance our understanding of chondrogenesis and cartilage degeneration.

CD44 enhances neuregulin signaling by Schwann cells

We describe a key role for the CD44 transmembrane glycoprotein in Schwann cell-neuron interactions. CD44 proteins have been implicated in cell adhesion and in the presentation of growth factors to high affinity receptors. We observed high CD44 expression in early rat neonatal nerves at times when Schwann cells proliferate but low expression in adult nerves, where CD44 was found in some nonmyelinating Schwann cells and to varying extents in some myelinating fibers. CD44 constitutively associated with erbB2 and erbB3, receptor tyrosine kinases that heterodimerize and signal in Schwann cells in response to neuregulins. Moreover, CD44 significantly enhanced neuregulin-induced erbB2 phosphorylation and erbB2-erbB3 heterodimerization. Reduction of CD44 expression in vitro resulted in loss of Schwann cell-neurite adhesion and Schwann cell apoptosis. CD44 is therefore crucial for maintaining neuron-Schwann cell interactions at least partly by facilitating neuregulin-induced erbB2-erbB3 activation.

Stimulation of hyaluronan metabolism by interleukin-1alpha in human articular cartilage

OBJECTIVE

To determine the effects of interleukin-1alpha (IL-1alpha) on the expression of hyaluronan synthase (HAS), CD44, and aggrecan in human articular chondrocytes, and to assess the net result of these metabolic changes on the accumulation of hyaluronan within articular cartilage.

METHODS

Normal human articular cartilage slices, as well as isolated chondrocytes, were treated with IL-1alpha. Changes in the relative expression of messenger RNA (mRNA) for HAS-2, CD44, and aggrecan were determined by competitive, quantitative reverse transcriptase-polymerase chain reaction. Hyaluronan accumulation was characterized by staining with a hyaluronan-specific binding protein and by fluorophore-assisted carbohydrate electrophoresis, while proteoglycan content was determined by alcian blue and Safranin O staining, CD44 protein expression by immunohistochemistry, and aggrecan biosynthesis by 35S-sulfate incorporation. Changes in cell-associated matrix sizes were visualized by a particle exclusion assay.

RESULTS

IL-1alpha stimulated the expression of HAS-2 and CD44 mRNA (3.5-fold and 3-fold, respectively), but inhibited the expression of aggrecan mRNA. In IL-1-treated chondrocytes, extracellular hyaluronan decreased, while intracellular accumulation of hyaluronan was enhanced. Together with the decrease in expression of aggrecan, a dramatic reduction in cell-associated matrix was observed. IL-1-treated cartilage slices displayed a prominent depletion of aggrecan as well as hyaluronan within the upper layers of the tissue. The regional loss of hyaluronan coincided with a regional up-regulation of CD44.

CONCLUSION

These data demonstrate that IL-1alpha stimulates HAS-2 at the same time as it inhibits the expression of aggrecan. Although hyaluronan biosynthesis is up-regulated, so too is the expression of CD44 and the internalization/catabolism of hyaluronan. The net result is a loss of hyaluronan in areas of the articular cartilage where increases in CD44 expression are most prominent. This depletion of hyaluronan in the upper layers of the tissue likely facilitates the prominent loss of aggrecan from the tissue.

Osteogenic protein-1 promotes the synthesis and retention of extracellular matrix within bovine articular cartilage and chondrocyte cultures

OBJECTIVE

We have used recombinant osteogenic protein-1 to investigate our hypothesis that proper repair and maintenance of cartilage requires not only enhanced biosynthesis and replenishment of the extracellular matrix but also the enhancement of components necessary for matrix retention.

DESIGN

The effects of osteogenic protein-1 were examined on bovine articular cartilage slices as well as isolated chondrocytes grown in alginate beads. Cartilage slices were examined for accumulation of proteoglycan by incorporation of 35S-sulfate and staining using Safranin O or, a biotinylated probe specific for hyaluronan. Bovine chondrocytes were characterized by use of a particle exclusion assay, in-situ hybridization, quantitative-competitive RT-PCR and a hyaluronan-binding assay.

RESULTS

Osteogenic protein-1 treatment substantially enhanced the accumulation of hyaluronan and proteoglycan within cartilage tissue slices. As with the tissue, osteogenic protein-1 enhanced the size of cell-associated matrices assembled and retained by chondrocytes in vitro. This enhanced matrix assembly was paralleled by an increased expression of mRNA for aggrecan, hyaluronan synthase-2 and CD44. Of the two hyaluronan synthase genes expressed by chondrocytes, only hyaluronan synthase-2 was upregulated by osteogenic protein-1. Coupled with the increase in the CD44 mRNA was an increase in functional hyaluronan binding activity present at the chondrocyte cell surface.

CONCLUSIONS

These results demonstrate that osteogenic protein-1 stimulates not only the synthesis of the major cartilage extracellular matrix component aggrecan, but also two associated molecules necessary for the retention of aggrecan, namely hyaluronan and CD44.

Osteogenic protein 1 stimulates cells-associated matrix assembly by normal human articular chondrocytes: up-regulation of hyaluronan synthase, CD44, and aggrecan

OBJECTIVE

To determine the effects of osteogenic protein 1 (OP-1) on hyaluronan (HA), CD44, and aggrecan biosynthesis as well as the contribution of these molecules in promoting matrix assembly by human articular chondrocytes.

METHODS

Normal human chondrocytes were cultured with or without OP-1 treatment. Changes in the relative expression of messenger RNA (mRNA) for HA synthases 2 and 3 (HAS-2 and HAS-3), CD44, and aggrecan were determined by competitive quantitative reverse transcriptase-polymerase chain reaction. Accumulation of HA was characterized by indirect staining, CD44 by flow cytometry, and aggrecan biosynthesis by 35SO4 incorporation.

RESULTS

OP-1 stimulated the expression of HAS-2, CD44, and aggrecan mRNA in a time-dependent manner, resulting in increased expression of HA, CD44, and aggrecan. Prominent increases in HA-rich cell-associated matrices were also observed.

CONCLUSION

OP-1 stimulates not only the synthesis of matrix macromolecules such as aggrecan, but also the synthesis of other molecules required for matrix retention, namely, HA and CD44.

The incidence of enlarged chondrons in normal and osteoarthritic human cartilage and their relative matrix density

OBJECTIVE

To quantitate changes in the pericellular matrix in osteoarthritic (OA) articular cartilage.

DESIGN

Chondrons were enzymatically isolated from normal and OA human cartilage. The cross-sectional area of the chondrons were measured. After immunolabeling for keratan sulfate, type VI collagen and type II collagen, the relative matrix density was determined for different size classes of chondrons with quantitative fluorescence microscopy.

RESULTS

For individual chondrons, the average cross-sectional area (344+/-28 microm(2), mean+/-SE) for the normal specimens was significantly smaller than the average area (439+/-30 microm(2)) for the OA specimens. Using 496 microm(2) (mean+2 SD of the normal area) as the cut-off for enlarged chondrons, 33% of individual OA chondrons were enlarged compared to 16% for the normal. Chondrons under 300 microm(2) had a significantly higher density of keratan sulfate and type VI collagen than larger chondrons, while chondrons over 400 microm(2) had similar matrix densities.

CONCLUSIONS

There is a higher incidence of enlarged chondrons in OA cartilage than in normal cartilage. The enlargement may initially be due to hydrodynamic swelling but further increases in size are due to increased matrix deposition.

Internalization of the hyaluronan receptor CD44 by chondrocytes

Chondrocytes express CD44 as a primary receptor for the matrix macromolecule hyaluronan. Hyaluronan is responsible for the retention and organization of proteoglycan within cartilage, and hyaluronan-chondrocyte interactions are important for the assembly and maintenance of the cartilage matrix. Bovine articular chondrocytes were used to study the endocytosis and turnover of CD44 and the effects of receptor occupancy on this turnover. Matrix-intact chondrocytes exhibit approximately a 6% internalization of cell surface CD44 by 4 h. Treatment with Streptomyces hyaluronidase to remove endogenous pericellular matrix increased internalization to approximately 20% of cell surface CD44 at 4 h. This turnover could be partially inhibited by the addition of exogenous hyaluronan to these matrix-depleted chondrocytes. Cell surface biotin-labeled CD44 was internalized by chondrocytes and this internalization was decreased in the presence of hyaluronan. Colocalization of internalized CD44 and fluorescein-labeled hyaluronan in intracellular vesicles correlates with the previous results of receptor-mediated endocytosis pathway for the degradation of hyaluronan by acid hydrolases. Taken together, our results indicate that CD44 is internalized by chondrocytes and that CD44 turnover is modulated by occupancy with hyaluronan.

Antisense inhibition of hyaluronan synthase-2 in human articular chondrocytes inhibits proteoglycan retention and matrix assembly

In order to define the role of cell-associated hyaluronan in cartilage matrix retention, human articular chondrocytes as well as cartilage slices were treated with phosphorothioate oligonucleotides comprised of sequence antisense to the mRNA of human HA synthase-2 (HAS-2). As a prerequisite for these studies, it was necessary to determine which HA synthase (HAS), of three separate human genes capable of synthesizing HA, designated HAS-1, HAS-2, or HAS-3, is primarily responsible for HA synthesis in human articular chondrocytes. The copy number of each HAS mRNA expressed in cultured human articular chondrocytes was determined using quantitative (competitive) reverse transcription-polymerase chain reaction (RT-PCR). Only HAS-2 and HAS-3 mRNA expression was detected. The level of HAS-2 mRNA expression was 40-fold higher than that of HAS-3. Cultures of human articular chondrocytes and cartilage tissue slices were then transfected with HAS-2-specific antisense oligonucleotides. This treatment resulted in time-dependent inhibition of HAS-2 mRNA expression, as measured by quantitative RT-PCR, and a significant loss of cell-associated HA staining. Sense and reverse HAS-2 oligonucleotides showed no effect. The consequences of reduced HA levels (due to HAS-2 antisense inhibition) were a decrease in the diameter of the cell-associated matrix and a decreased capacity to retain newly synthesized proteoglycan. These results suggest that HA synthesized by HAS-2 plays a crucial role in matrix assembly and retention by human articular chondrocytes.