Articular chondrocytes produce factors that inhibit maturation of sternal chondrocytes in serum-free agarose cultures: a TGF-beta independent process

GDF5+ chondroprogenitors derived from human pluripotent stem cells preferentially form permanent chondrocytes

It has been established in the mouse model that during embryogenesis joint cartilage is generated from a specialized progenitor cell type, distinct from that responsible for the formation of growth plate cartilage. We recently found that mesodermal progeny of human pluripotent stem cells gave rise to two types of chondrogenic mesenchymal cells in culture: SOX9+ and GDF5+ cells. The fast-growing SOX9+ cells formed in vitro cartilage that expressed chondrocyte hypertrophy markers and readily underwent mineralization after ectopic transplantation. In contrast, the slowly growing GDF5+ cells derived from SOX9+ cells formed cartilage that tended to express low to undetectable levels of chondrocyte hypertrophy markers, but expressed PRG4, a marker of embryonic articular chondrocytes. The GDF5+-derived cartilage remained largely unmineralized in vivo. Interestingly, chondrocytes derived from the GDF5+ cells seemed to elicit these activities via non-cell-autonomous mechanisms. Genome-wide transcriptomic analyses suggested that GDF5+ cells might contain a teno/ligamento-genic potential, whereas SOX9+ cells resembled neural crest-like progeny-derived chondroprogenitors. Thus, human pluripotent stem cell-derived GDF5+ cells specified to generate permanent-like cartilage seem to emerge coincidentally with the commitment of the SOX9+ progeny to the tendon/ligament lineage.

Meniscus Repair and Regeneration: A Systematic Review from a Basic and Translational Science Perspective

Meniscus injuries are among the most common athletic injuries and result in functional impairment in the knee. Repair is crucial for pain relief and prevention of degenerative joint diseases like osteoarthritis. Current treatments, however, do not produce long-term improvements. Thus, recent research has been investigating new therapeutic options for regenerating injured meniscal tissue. This review comprehensively details the current methodologies being explored in the basic sciences to stimulate better meniscus injury repair. Furthermore, it describes how these preclinical strategies may improve current paradigms of how meniscal injuries are clinically treated through a unique and alternative perspective to traditional clinical methodology.

Identifying chondrogenesis strategies for tissue engineering of articular cartilage

A key step in the tissue engineering of articular cartilage is the chondrogenic differentiation of mesenchymal stem cells (MSCs) into chondrocytes (native cartilage cells). Chondrogenesis is regulated by transforming growth factor-β (TGF-β), a short-lived cytokine whose effect is prolonged by storage in the extracellular matrix. Tissue engineering applications aim to maximise the yield of differentiated MSCs. Recent experiments involve seeding a hydrogel construct with a layer of MSCs lying below a layer of chondrocytes, stimulating the seeded cells in the construct from above with exogenous TGF-β and then culturing it in vitro. To investigate the efficacy of this strategy, we develop a mathematical model to describe the interactions between MSCs, chondrocytes and TGF-β. Using this model, we investigate the effect of varying the initial concentration of TGF-β, the initial densities of the MSCs and chondrocytes, and the relative depths of the two layers on the long-time composition of the tissue construct.

Current Concepts in Meniscus Tissue Engineering and Repair

The meniscus is the most commonly injured structure in the human knee. Meniscus deficiency has been shown to lead to advanced osteoarthritis (OA) due to abnormal mechanical forces, and replacement strategies for this structure have lagged behind other tissue engineering endeavors. The challenges include the complex 3D structure with individualized size parameters, the significant compressive, tensile and shear loads encountered, and the poor blood supply. In this progress report, a review of the current clinical treatments for different types of meniscal injury is provided. The state-of-the-art research in cellular therapies and novel cell sources for these therapies is discussed. The clinically available cell-free biomaterial implants and the current progress on cell-free biomaterial implants are reviewed. Cell-based tissue engineering strategies for the repair and replacement of meniscus are presented, and the current challenges are identified. Tissue-engineered meniscal biocomposite implants may provide an alternative solution for the treatment of meniscal injury to prevent OA in the long run, because of the limitations of the existing therapies.

Measuring clinically relevant endpoints in a serum-free, three-dimensional, primary cell culture system of human osteoarthritic articular chondrocytes

Osteoarthritis (OA) is characterized by degeneration of articular cartilage within the joint, inflammation and pain. The purpose of this study was to develop a primary, serum free cell culture system of human osteoarthritic articular chondrocytes (HOACs) with which to study manifestations of the disease process. Joint tissues were obtained from OA patients undergoing total knee arthroplasty (TKA). HOACs isolated from the femoral condyles and tibial plateau of the same side were combined, plated in three-dimensional, alginate beads and cultured for five days in serum, hormone and protein free medium. More living cells were obtained from the femoral condyles than the tibial plateau. The optimal plating density was 2.5 × 10⁶ cells/ml of alginate. The amounts of DNA, RNA, proteoglycans and total collagen were similar in cultures prepared from the sides of least and greatest pathology. More type 1 than type 2 collagen was detected in the medium on days 2 and 5. A greater percentage of type 1 than type 2 collagen was degraded. The inflammatory cytokine interleukin-1 beta was present in the medium and alginate associated matrix. Although variation in the metabolic profiles between subjects was observed, HOACs from all patients continued to reflect the OA phenotype for five days in culture. This serum free, three-dimensional primary culture system of HOACs provides a platform with which to measure clinically relevant endpoints of OA and screen potential disease modifying OA therapeutics.

5-Aza-2'-deoxycytidine acts as a modulator of chondrocyte hypertrophy and maturation in chick caudal region chondrocytes in culture

This study was carried out to explore the effect of DNA hypomethylation on chondrocytes phenotype, in particular the effect on chondrocyte hypertrophy, maturation, and apoptosis. Chondrocytes derived from caudal region of day 17 embryonic chick sterna were pretreated with hypomethylating drug 5-aza-2'-deoxycytidine for 48 hours and then maintained in the normal culture medium for up to 14 days. Histological studies showed distinct morphological changes occurred in the pretreated cultures when compared to the control cultures. The pretreated chondrocytes after 7 days in culture became bigger in size and acquired more flattened fibroblastic phenotype as well as a loss of cartilage specific extracellular matrix. Scanning electron microscopy at day 7 showed chondrocytes to have increased in cell volume and at day 14 in culture the extracellular matrix of the pretreated cultures showed regular fibrillar structure heavily embedded with matrix vesicles, which is the characteristic feature of chondrocyte hypertrophy. Transmission electron microscopic studies indicated the terminal fate of the hypertrophic cells in culture. The pretreated chondrocytes grown for 14 days in culture showed two types of cells: dark cells which had condense chromatin in dark patches and dark cytoplasm. The other light chondrocytes appeared to be heavily loaded with endoplasmic reticulum indicative of very active protein and secretory activity; their cytoplasm had large vacuoles and disintegrating cytoplasm. The biosynthetic profile showed that the pretreated cultures were actively synthesizing and secreting type X collagen and alkaline phosphatase as a major biosynthetic product.

Combination of ADMSCs and chondrocytes reduces hypertrophy and improves the functional properties of osteoarthritic cartilage

OBJECTIVE

To evaluate the therapeutic efficacy of Adipose derived MSCs (ADMSCs) in combination with chondrocytes in counteracting oxidative stress in chondrocytes in vitro and in rat model of osteoarthritis (OA).

METHOD

Cultured chondrocytes were exposed to oxidative stress with 200 μM Hydrogen peroxide (H2O2), followed by co-culture with ADMSCs or chondrocytes or combination of both cell types in a transwell culture system for 36 h. The cytoprotective effect was assessed by immunocytochemistry and gene expression analysis. In vivo study evaluated therapeutic effect of the above mentioned three treatments after transplantation in OA rats.

RESULTS

The Combination of ADMSCs + Chondrocytes decreased the extent of oxidative stress-induced damage of chondrocytes. Enhanced expression level of Acan and Collagen type-II alpha (Col2a1) with a correspondingly decreased expression of Collagen type-I alpha (Col1a1) and Matrix metallopeptidase 13 (Mmp13) was maximally observed in this group. Moreover, reduced count of annexin-V positive cells, Caspase (Casp3) gene expression and Lactate dehydrogenase (LDH) release with concomitantly enhanced viability and expression of proliferating cell nuclear antigen (PCNA) gene was observed. In vivo study showed that homing of cells and proteoglycan contents of knee joints were significantly better in ADMSCs + Chondrocytes transplanted rats. Increased expression of Acan and Col2a1 along with decreased expression of Col1a1 and Mmp13 indicated formation of hyaline cartilage in this group. These rats also demonstrated significantly reduced expression of Casp3 while increased expression of PCNA genes than the other cell transplanted groups.

CONCLUSIONS

Our results demonstrated that a combination of ADMSCs and chondrocytes may be a more effective therapeutic strategy against OA than the use of ADMSCs or chondrocytes separately.

Temporal activation of β-catenin signaling in the chondrogenic process of mesenchymal stem cells affects the phenotype of the cartilage generated

Adult mesenchymal stem cells (MSCs) are an attractive cell source for cartilage tissue engineering. In vitro predifferentiation of MSCs has been explored as a means to enhance MSC-based articular cartilage repair. However, there remain challenges to control and prevent the premature progression of MSC-derived chondrocytes to the hypertrophy. This study investigated the temporal effect of transforming growth factor (TGF)-β and β-catenin signaling co-activation during MSC chondrogenic differentiation and evaluated the influence of these predifferentiation conditions to subsequent phenotypic development of the cartilage. MSCs were differentiated in chondrogenic medium that contained either TGFβ alone, TGFβ with transient β-catenin coactivation, or TGFβ with continuous β-catenin coactivation. After in vitro differentiation, the pellets were transplanted into SCID mice. Both coactivation protocols resulted in the enhancement of chondrogenic differentiation of MSCs. Compared with TGFβ activation, transient coactivation of TGFβ-induction with β-catenin activation resulted in heightened hypertrophy and formed highly ossified tissues with marrow-like hematopoietic tissue in vivo. The continuous coactivation of the 2 signaling pathways, however, resulted in inhibition of progression to hypertrophy, marked by the suppression of type X collagen, Runx2, and alkaline phosphatase expression, and did not result in ossified tissue in vivo. Chondrocytes of the continuous co-activation samples secreted significantly more parathyroid hormone-related protein (PTHrP) and expressed cyclin D1. Our results suggest that temporal co-activation of the TGFβ signaling pathway with β-catenin can yield cartilage of different phenotype, represents a potential MSC predifferentiation protocol before clinical implantation, and has potential applications for the engineering of cartilage tissue.

Coculture of human mesenchymal stem cells and articular chondrocytes reduces hypertrophy and enhances functional properties of engineered cartilage

Mesenchymal stem cells (MSCs) are being recognized as a viable cell source for cartilage repair; however, it still remains a challenge to recapitulate the functional properties of native articular cartilage using only MSCs. Additionally, MSCs may exhibit a hypertrophic phenotype under chondrogenic induction, resulting in calcification after ectopic transplantation. With this in mind, the objective of this study was to assess whether the addition of chondrocytes to MSC cultures influences the properties of tissue-engineered cartilage and MSC hypertrophy when cultured in hyaluronic acid hydrogels. Mixed cell populations (human MSCs and human chondrocytes at a ratio of 4:1) were encapsulated in the hydrogels and exhibited significantly higher Young's moduli, dynamic moduli, glycosaminoglycan levels, and collagen content than did constructs seeded with only MSCs or chondrocytes. Furthermore, the deposition of collagen X, a marker of MSC hypertrophy, was significantly lower in the coculture constructs than in the constructs seeded with MSCs alone. When MSCs and chondrocytes were cultured in distinct gels, but in the same wells, there was no improvement in biomechanical and biochemical properties of the engineered tissue, implying that a close proximity is essential. This approach can be used to improve the properties and prevent calcification of engineered cartilage formed from MSC-seeded hydrogels with the addition of lower fractions of chondrocytes, leading to improved clinical outcomes.

F-spondin regulates chondrocyte terminal differentiation and endochondral bone formation

This study examines the role of F-spondin, an extracellular matrix protein of osteoarthritic cartilage, during chondrocyte maturation in embryonic growth plate cartilage. In chick tibia, F-spondin expression localized to the hypertrophic and calcified zones of the growth plate. Functional studies using tibial organ cultures indicated that F-spondin inhibited (∼35%, p = 0.02), and antibodies to F-spondin increased (∼30%, p < 0.1) longitudinal limb growth relative to untreated controls. In cell cultures, induction of chondrocyte maturation, by retinoic acid (RA) or transforming growth factor (TGF)-β treatment led to a significant upregulation of F-spondin (p < 0.05). F-spondin transfection increased mineral deposition, alkaline phosphatase (AP) and matrix metalloproteinase (MMP)-13 mRNA levels (p < 0.05), and AP activity following RA stimulation, compared to mock transfected controls. Using AP as a differentiation marker we then investigated the mechanism of F-spondin promaturation effects. Blocking endogenous F-spondin via its thrombospondin (TSR) domain inhibited RA induced AP activity 40% compared to controls (p < 0.05). The stimulatory effect of F-spondin on AP expression was also inhibited following depletion of TGF-β from culture supernatants. Our findings indicate that F-spondin is expressed in embryonic cartilage, where it has the capacity to enhance chondrocyte terminal differentiation and mineralization via interactions in its TSR domain and TGF-β dependent pathways.

Derivation of chondrogenically-committed cells from human embryonic cells for cartilage tissue regeneration

Background

Heterogeneous and uncontrolled differentiation of human embryonic stem cells (hESCs) in embryoid bodies (EBs) limits the potential use of hESCs for cell-based therapies. More efficient strategies are needed for the commitment and differentiation of hESCs to produce a homogeneous population of specific cell types for tissue regeneration applications.

Methodology/Principal Findings

We report here that significant chondrocytic commitment of feeder-free cultured human embryonic stem cells (FF-hESCs), as determined by gene expression and immunostaining analysis, was induced by co-culture with primary chondrocytes. Furthermore, a dynamic expression profile of chondrocyte-specific genes was observed during monolayer expansion of the chondrogenically-committed cells. Chondrogenically-committed cells synergistically responded to transforming growth factor-β1 (TGF-β1) and β1-integrin activating antibody by increasing tissue mass in pellet culture. In addition, when encapsulated in hydrogels, these cells formed cartilage tissue both in vitro and in vivo. In contrast, the absence of chondrocyte co-culture did not result in an expandable cell population from FF-hESCs.

Conclusions/Significance

The direct chondrocytic commitment of FF-hESCs can be induced by morphogenetic factors from chondrocytes without EB formation and homogenous cartilage tissue can be formed in vitro and in vivo.

Conditional Kif3a ablation causes abnormal hedgehog signaling topography, growth plate dysfunction, and excessive bone and cartilage formation during mouse skeletogenesis

The motor protein Kif3a and primary cilia regulate important developmental processes, but their roles in skeletogenesis remain ill-defined. Here we created mice deficient in Kif3a in cartilage and focused on the cranial base and synchondroses. Kif3a deficiency caused cranial base growth retardation and dysmorphogenesis, which were evident in neonatal animals by anatomical and micro-computed tomography (microCT) inspection. Kif3a deficiency also changed synchondrosis growth plate organization and function, and the severity of these changes increased over time. By postnatal day (P)7, mutant growth plates lacked typical zones of chondrocyte proliferation and hypertrophy, and were instead composed of chondrocytes with an unusual phenotype characterized by strong collagen II (Col2a1) gene expression but barely detectable expression of Indian hedgehog (Ihh), collagen X (Col10a1), Vegf (Vegfa), MMP-13 (Mmp13) and osterix (Sp7). Concurrently, unexpected developmental events occurred in perichondrial tissues, including excessive intramembranous ossification all along the perichondrial border and the formation of ectopic cartilage masses. Looking for possible culprits for these latter processes, we analyzed hedgehog signalling topography and intensity by monitoring the expression of the hedgehog effectors Patched 1 and Gli1, and of the hedgehog-binding cell-surface component syndecan 3. Compared with controls, hedgehog signaling was quite feeble within mutant growth plates as early as P0, but was actually higher and was widespread all along mutant perichondrial tissues. Lastly, we studied postnatal mice deficient in Ihh in cartilage; their cranial base defects only minimally resembled those in Kif3a-deficient mice. In summary, Kif3a and primary cilia make unique contributions to cranial base development and synchondrosis growth plate function. Their deficiency causes abnormal topography of hedgehog signaling, growth plate dysfunction, and un-physiologic responses and processes in perichondrial tissues, including ectopic cartilage formation and excessive intramembranous ossification.

Retinoid signaling regulates CTGF expression in hypertrophic chondrocytes with differential involvement of MAP kinases

Retinoids are important for growth plate chondrocyte maturation, but their downstream effectors remain unclear. Recently, CTGF (CCN2) was found to regulate chondrocyte function, particularly in the hypertrophic zone. The goal of the study was to determine whether CTGF is a retinoid signaling effector molecule, how it is regulated, and how it acts.

INTRODUCTION

Using a combination of in vivo and in vitro approaches, we carried out a series of studies at the cellular, biochemical, and molecular level to determine whether and how retinoid signaling is related to expression and function of connective tissue growth factor (CTGF) in chondrocyte maturation and endochondral ossification.

MATERIALS AND METHODS

Limbs of chick embryos in ovo were implanted with retinoic pan-antagonist RO 41-5253-filled beads, and phenotypic changes were assessed by in situ hybridization. CTGF gene expression and roles were tested in primary cultures of immature and hypertrophic chondrocytes. Cross-talk between retinoid signaling and other pathways was tested by determining endogenous levels of active ERK1/2 and p38 MAP kinases and phenotypic modulations exerted by specific antagonists of mitogen-activated protein (MAP) kinases and BMP signaling (Noggin).

RESULTS

Interference with retinoid signaling blocked expression of CTGF and other posthypertrophic markers in long bone anlagen in vivo and hypertrophic chondrocyte cultures, whereas all-trans-retinoic acid (RA) boosted CTGF expression and even induced it in immature proliferating cultures. Exogenous recombinant CTGF stimulated chondrocyte maturation, but failed to do so in presence of retinoid antagonists. Immunoblots showed that hypertrophic chondrocytes contained sizable levels of phosphorylated ERK1/2 and p38 MAP kinases that were dose- and time-dependently increased by RA treatment. Experimental ERK1/2 inhibition led to a severe drop in baseline and RA-stimulated CTGF expression, whereas p38 inhibition increased it markedly. These responses were gene-specific, because the opposite was seen with other hypertrophic chondrocyte genes such as collagen X and RA receptor gamma (RARgamma). Tests with Noggin showed that RA induction of CTGF expression was negatively influenced by BMP signaling, whereas induction of collagen X expression was BMP-dependent.

CONCLUSIONS

Retinoids appear to have a preeminent role in controlling expression and function of CTGF in hypertrophic and posthypertrophic chondrocytes and do so with differential cooperation and intervention of MAP kinases and BMP signaling.

Chondrocytes isolated from tibial dyschondroplasia lesions and articular cartilage revert to a growth plate-like phenotype when cultured in vitro

We report here a comparative study of the development and behavior of chondrocytes isolated from normal growth plate tissue, tibial dyschondroplasic lesions, and from articular cartilage. The objective of these studies was to determine whether the properties exhibited by chondrocytes in dysplasic lesions or in articular cartilage were due to their cellular phenotype, their environment, or both. We had previously analyzed the electrolytes and amino acid levels in the extracellular fluid of avian growth plate chondrocytes. Using these data, we constructed a culture medium (DATP5) in which growth plate cells essentially recapitulate their normal behavior in vivo. Here, we used DATP5 to examine the behavior of chondrocytes isolated from lesions of tibial dyschondroplasia (TD). We found that once isolated from lesion and grown in this supportive medium, dysplasic chondrocytes behaved essentially like normal growth plate cells. These findings suggest that the cause of TD is local factors operating in vivo to prevent these cells from developing normally. With respect to articular chondrocytes, our data indicate that they more closely retain normal protein and proteoglycan synthesis when grown in serum-free media. These cells readily induced mineral formation in vitro, both in the presence and absence of serum. However, in serum-containing media, mineralization was significantly enhanced when the cells were exposed to retinoic acid (RA) or osteogenic protein-1 (OP-1). Our studies support previous work indicating the presence of autocrine factors produced by articular chondrocytes in vivo that prevent mineralization and preserve matrix integrity. The lack of inhibitory factors and the presence of supporting factors are likely reasons for the induction of mineralization by articular chondrocytes in vitro.

Short-term BMP-2 expression is sufficient for in vivo osteochondral differentiation of mesenchymal stem cells

Currently available murine models to evaluate mesenchymal stem cell (MSC) differentiation are based on cell injection at ectopic sites such as muscle or skin. Due to the importance of environmental factors on the differentiation capacities of stem cells in vivo, we investigated whether the peculiar synovial/cartilaginous environment may influence the lineage specificity of bone morphogenetic protein (BMP)-2-engineered MSCs. To this aim, we used the C3H10T1/2-derived C9 MSCs that express BMP-2 under control of the doxycycline (Dox)-repressible promoter, Tet-Off, and showed in vitro, using the micropellet culture system that C9 MSCs kept their potential to differentiate toward chondrocytes. Implantation of C9 cells, either into the tibialis anterior muscles or into the joints of CB17-severe combined immunodeficient bg mice led to the formation of cartilage and bone filled with bone marrow as soon as day 10. However, no differentiation was observed after injection of naïve MSCs or C9 cells that were repressed to secrete BMP-2 by Dox addition. The BMP-2-induced differentiation of adult MSCs is thus independent of soluble factors present in the local environment of the synovial/cartilaginous tissues. Importantly, we demonstrated that a short-term expression of the BMP-2 growth factor is necessary and sufficient to irreversibly induce bone formation, suggesting that a stable genetic modification of MSCs is not required for stem cell-based bone/cartilage engineering.

Pro-MMP-9 is a specific macrophage product and is activated by osteoarthritic chondrocytes via MMP-3 or a MT1-MMP/MMP-13 cascade

In joint diseases of both the inflammatory (rheumatoid arthritis, or RA) or the degenerative variety (osteoarthritis, or OA), matrix metalloproteinases (MMPs) are essential mediators of irreversible tissue destruction. MMP-9 is secreted as a stable, inactive zymogen and is proteolytically converted to the active enzyme. To understand the activation mechanism of MMP-9 in joint diseases, the process was investigated in serum-free cocultures of human articular chondrocytes and macrophages. Macrophages extensively expressed and secreted pro-MMP-9 whereas chondrocytes failed to produce the enzyme. However, efficient activation of pro-MMP-9 required soluble and membrane-associated chondrocyte proteinases. Two alternative activation pathways mainly involved MMPs and, marginally, serine or cysteine proteinases. MT1-MMP (MMP-14), the only MT-MMP expressed in chondrocytes, converted pro-MMP-13 which, in turn, cleaved pro-MMP-9. Alternatively, pro-MMP-9 was activated less efficiently by MMP-3, which was converted by autocatalysis or by serine or cysteine proteinases. Both pathways were triggered by chondrocytes from OA, but not normal joints. Therefore, articular chondrocytes are not innocent bystanders in joint diseases. They not only produce destructive enzymes guided by environmental cues but also they can instruct inflammatory cells or cells from surrounding tissues to do so by converting in several ways zymogens produced but not activated by these cells themselves.

Distinct phases of coordinated early and late gene expression in growth plate chondrocytes in relationship to cell proliferation, matrix assembly, remodeling, and cell differentiation

Although much has been learned about growth plate development and chondrocyte gene expression during cellular maturation and matrix remodeling in the mouse, there has been a limited study of the interrelationships of gene expression between proteinases, growth factors, and other regulatory molecules in the mouse and in other species. Here we use RT-PCR of sequential transverse sections to examine the expression profiles of genes involved in chondrocyte growth, differentiation, matrix assembly, remodeling, and mineralization in the bovine proximal tibial growth plate. Specifically, we studied the expression of genes encoding COL2A1 and COL10A1, the latter a marker of cellular hypertrophy, the matrix metalloproteinases (MMP), MMP-13 and MMP-9, as well as the transcriptional factors, Sox9 and Cbfa1, the growth factors basic fibroblast growth factor (bFGF), parathyroid hormone-related peptide (PTHrP), transforming growth factor (TGF)beta1, and beta2, Indian hedgehog (Ihh), and the matrix protein osteocalcin. These were analyzed in relationship to cell division defined by cyclin B2 expression. Two peaks of gene expression activity were observed. One was transient, limited, and located immediately before and at the onset of cyclin B2 expression in the early proliferative zone. The other was generally much more pronounced and was located in the early hypertrophic zone. The upregulation of expression of COL2A1, its transcriptional activator Sox9, osteocalcin, MMP-13, and TGFbeta2 was observed immediately before and at the onset of cyclin B2 expression and also in the hypertrophic zones. The upregulation of COL10A1, Cbfa1, MMP-9, TGFbeta-1, and Ihh gene expression was associated exclusively with the terminal differentiation of chondrocytes at the time of mineral formation in the extracellular matrix. In contrast, bFGF and PTHrP expression was observed in association with the onset of cyclin B2 expression and hypertrophy. This initial cluster of gene expression associated predominantly with matrix assembly and onset of cell proliferation is therefore characterized by expression of regulatory molecules distinct from those involved at hypertrophy. Together these results identify separate phases of coordinated gene expression associated with the development of the physis in endochondral bone formation.

A chondrogenesis-related lipocalin cluster includes a third new gene, CALgamma

We have previously reported the modulation, during chondrogenesis and/or inflammation, of two chicken genes laying in the same genomic locus and coding for two polypeptides of the lipocalin protein family, the extracellular fatty acid binding protein (ExFABP) and the chondrogenesis associated lipocalin beta (CALbeta). A third gene, located within the same cluster and coding for a new lipocalin, CALgamma, has been identified and is here characterized. Tissue distribution analyzed by real-time quantitative reverse transcriptase-polymerase chain reaction in chicken embryos shows a ubiquitous expression with predominant levels of mRNA transcripts in the liver and the brain. In the developing tibia, a high expression of CALgamma mRNA was evidenced by in situ hybridization within the pre-hypertrophic and the hypertrophic zones of the bone-forming cartilage. In agreement, dedifferentiated chondrocytes in vitro express the transcripts to the highest level when they re-differentiate reaching hypertrophy. Such peculiar developmental pattern of expression that is analogous to those already described for Ex-FABP and CALbeta suggests that all three proteins may act synergistically in the process of endochondral bone formation. Moreover, like Ex-FABP and CALbeta, CALgamma is also highly induced in dedifferentiated chondrocytes upon stimulation with lypopolysaccharides, indicating that the whole cluster quite possibly is transcriptionally activated not only in physiological morphogenic differentiation but also in pathological acute phase response.

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.

The release and activation of transforming growth factor beta2 associated with apoptosis of chick hypertrophic chondrocytes

The apoptosis of hypertrophic chondrocytes at the interface between growth cartilage and invading vessels is at the center of a series of critical events in endochondral formation. We have shown that the hypertrophy and apoptosis of chick chondrocytes in culture is associated with the release and activation of transforming growth factor beta2 (TGF-beta2). Supplementation of the culture medium with agents that influenced the maintenance of hypertrophic differentiation also influenced the release of TGF-beta2. A large proportion of the TGF-beta2 released from the cells was shown to be in an active form-particularly TGF-beta2 associated with the support matrix. Inhibition of apoptosis with a broad-spectrum caspase inhibitor inhibited activation of the matrix-associated TGF-beta2. However, inhibition of apoptosis did not diminish the release of TGF-beta2. Release of TGF-beta2 by chondrocytes at a late stage of their terminal differentiation and its activation in association with apoptosis may provide a mechanism controlling the processes of vascular invasion of growth cartilage and the deposition of bone matrix on nearby cartilage remnants.

Activation of transforming growth factor beta in chondrocytes undergoing endochondral ossification

Transforming growth factor beta (TGF-beta) has well-documented roles in chondrocyte maturation and endochondral ossification, but the mechanisms of TGF-beta activation during these processes remain unclear. In this study, we analyzed TGF-beta activation in chick embryo resting, proliferating, and hypertrophic chondrocytes in culture. We found that both levels and activation of TGF-beta increased substantially with maturation. The majority of TGF-beta produced by resting cells over culture time remained latent, but a larger portion produced by proliferating and hypertrophic cells was activated with increasing maturation. Zymography of gelatin gels revealed that matrix metalloprotease 2 (MMP-2) and MMP-9 were expressed by each population and that MMP-13 characterized hypertrophic chondrocytes and to a lesser extent proliferating chondrocytes in late cultures. Treatment with pharmacologic agents revealed that both MMPs and serine proteases are involved in activation. However, because inhibition of MMPs almost completely prevented TGF-beta activation, MMPs appear crucial for activation. During culture, inclusion of the tetracycline-derived, collagenase/gelatinase inhibitor chemically modified nonantimicrobial tetracycline (CMT-8) at concentrations specific for MMP-13 inhibition resulted in complete inhibition of TGF-beta activation by proliferating and hypertrophic chondrocytes. These results show that TGF-beta production, release, and activation are regulated developmentally in chondrocytes. Our findings point to a strict mode of regulation of this potent factor to elicit diverse and highly specific effects during chondrocyte maturation and ossification.

In vitro generation of scaffold independent neocartilage

A novel serum-free culture system was developed in an attempt to generate a three-dimensional hyalinelike neocartilage independent of polymer scaffolds. Neocartilage disks as much as 1.5 mm thick were produced, which were characterized by synthesis of the normal articular cartilage collagens and proteoglycans. In contrast to growth in serum-containing media, chondrocytes from juveniles maintained in static culture under defined serum-free conditions deposited an extracellular matrix that accumulated in the form of tissue disks. Electron microscopic evaluation of neocartilage disks revealed collagenous matrices characteristic of articular cartilage from human infants. The neocartilage did not show terminal chondrocyte differentiation as shown by the absence of Type X collagen production and lack of cellular hypertrophy. Although chondrocytes from preadolescent donor cartilage recapitulated embryonic development in the absence of exogenous factors, chondrocytes from articular cartilage from adults failed to produce neocartilage when grown under identical conditions. This is the first demonstration that autocrine morphogens are sufficient to guide production of hyaline cartilage in vitro. In addition to providing a unique model system to compare the healing response of mature and immature articular chondrocytes, this technology may be of clinical importance in the development of new biomaterials for repair of articular cartilage defects.

Development of articular cartilage: what do we know about it and how may it occur?

Articular cartilage has a fundamental role in joint function. While much is known about its structure, organization and biomechanical properties, there is a very poor understanding of how articular chondrocytes develop during embryogenesis and acquire the unique ability to organize and maintain the articular tissue. Given that articular cartilage forms in close juxtaposition with the joint, here we review past studies on limb joint determination and morphogenesis and more recent studies on a number of factors thought to have roles in joint and epiphysis development. These factors include: the homeobox gene Barx-1; the bone morphogenetic protein (BMP) family member GDF-5; the growth factors HGF and PTHrP; and the transcription factor ERG. We summarize current thinking on how these factors participate in joint development and how some of these factors may influence development and behavior of epiphyseal chondrocytes. We also describe pertinent recent studies from our laboratories on ERG and the newly-identified alternatively spliced variant C-1-1, and finally propose a sequela of events that may subtend the process of determination and emergence of articular chondrocytes during limb synovial joint development.

Authentic matrix vesicles contain active metalloproteases (MMP). a role for matrix vesicle-associated MMP-13 in activation of transforming growth factor-beta

Matrix vesicles (MV) play a key role in the initiation of cartilage mineralization. Although many components in these microstructures have been identified, the specific function of each component is still poorly understood. In this study, we show that metalloproteases (MMP), MMP-2, -9, and -13 are associated with MV isolated from growth plate cartilage. In addition, we provide evidence that MV contain transforming growth factor-beta (TGF-beta) and that MV-associated MMP-13 is capable of activating latent TGF-beta. To determine whether MMPs are associated directly with MV, vesicles isolated from growth plate cartilage were sequentially treated with hyaluronidase, NaCl, and bacterial collagenase to remove matrix proteins and other components attached to their outer surface. Finally, the vesicles were incubated with detergent to rupture the MV membrane and expose components that are inside the vesicles. Each treated MV fraction was subjected to substrate zymography, immunoblotting, and substrate activity assay. Whereas active MMP-13 was lost after combined treatment with hyaluronidase and NaCl, MMP-2 and -9 activities were still retained in the pellet fraction even after detergent treatment, suggesting that the gelatinases, MMP-2 and -9, are integral components of MV. In addition, MV contain TGF-beta in the small latent complex, and MMP-13 associated with the MV surface was responsible for activation of TGF-beta. Since the amount of TGF-beta activated by hypertrophic chondrocytes increased with mineral appearance in serum-free chondrocyte cultures, a role for active MV-associated MMPs is suggested in activation of TGF-beta seen during late chondrocyte hypertrophy and mineralization of growth plate cartilage.

Endochondral ossification of costal cartilage is arrested after chondrocytes have reached hypertrophic stage of late differentiation

Late cartilage differentiation during endochondral bone formation is a multistep process. Chondrocytes transit through a differentiation cascade under the direction of environmental signals that either stimulate or repress progression from one step to the next. In human costal cartilage, chondrocytes reach very advanced stages of late differentiation and express collagen X. However, remodeling of the tissue into bone is strongly repressed. The second hypertrophy marker, alkaline phosphatase, is not expressed before puberty. Upon sexual maturity, both alkaline phosphatase and collagen X activity levels are increased and slow ossification takes place. Thus, the expression of the two hypertrophy markers is widely separated in time in costal cartilage. Progression of endochondral ossification in this tissue beyond the stage of hypertrophic cartilage appears to be associated with the expression of alkaline phosphatase activity. Costal chondrocytes in culture are stimulated by parathyroid hormone in a PTH/PTHrP receptor-mediated manner to express the fully differentiated hypertrophic phenotype. In addition, the hormone stimulates hypertrophic development even more powerfully through its carboxyterminal domain, presumably by interaction with receptors distinct from PTH/PTHrP receptors. Therefore, PTH can support late cartilage differentiation at very advanced stages, whereas the same signal negatively controls the process at earlier stages.

Utilization of bone morphogenetic protein receptors during chondrocyte maturation

Cartilage from the upper, cephalic portion of embryonic chick sternums undergoes hypertrophy, while the lower, caudal portion of the sternum remains as cartilage. Bone morphogenetic proteins (BMPs) induce type X collagen (colX) in cultured upper but not lower sternal chondrocytes (LSCs). We have examined the utilization of BMP receptors (BMPRs) by upper sternal chondrocytes (USCs) and LSCs both by analyzing receptor expression and by overexpressing mutant BMPRs. Reverse-transcription polymerase chain reaction (RT-PCR) analyses indicate that both upper and lower chondrocytes produce messenger RNA (mRNA) for all three receptors: BMPR type IA (BMPR-IA), BMPR type IB (BMPR-IB), and BMPR type II (BMPR-II). Infection of USC with retroviral vectors expressing constitutively active (CA) BMPRs showed that CA-BMPR-IB, like exogenous BMP-4, induced both colX mRNA and elevated alkaline phosphatase (AP), while CA-BMPR-IA was markedly less potent. However, expression of activated receptors in LSC cultures resulted in only minimal induction of hypertrophic markers. Consistent with the results seen for CA receptors, dominant negative (DN) BMPR-IB blocked BMP-induced hypertrophy in USCs more effectively than DN-BMPR-IA. These results imply that the major BMPR required for BMP induction of chondrocyte hypertrophy is BMPR-IB, and that difference between permanent and prehypertrophic chondrocytes is not caused by absence of receptors required for BMP signaling.

Active role of chondrocyte apoptosis in endochondral ossification

Endochondral ossification involves an ordered progression from cell division through hypertrophic differentiation to cell death. The apoptotic nature of chondrocyte death was first suggested by characteristic changes in morphology; and more recently by the pattern of DNA fragmentation and other characteristic features of apoptosis. In situ detection, although controversial, suggests that DNA fragmentation probably does not occur until late in hypertrophic differentiation. From observations of key features of apoptosis, including activation of the caspase cascade, changes in mitochondrial function, and expression of apoptosis inhibitors, the commitment of chondrocytes to apoptosis, appears to occur very early in hypertrophic differentiation. It is proposed that these changes produce effects that go far beyond the process of cell death and exert a focal influence on the endochondral ossification processes occurring in the microenvironment of the growth cartilage vascular interface. Endochondral ossification processes mediated by chondrocyte apoptosis may include intracellular calcium accumulation and release; matrix calcification; matrix resorption; maintenance of growth plate homeostasis; attraction of blood vessels and osteoblast precursors; and stimulation of bone formation.

Terminal differentiation of chondrocytes is arrested at distinct stages identified by their expression repertoire of marker genes

During endochondral bone formation, cells in the emerging cartilaginous model transit through a cascade of several chondrocyte differentiation stages, each characterized by a specific expression repertoire of matrix macromolecules, until, as a final step, the hypertrophic cartilage is replaced by bone. In many permanent cartilage tissues, however, late differentiation of chondrocytes does not occur, due to negative regulation by the environment of the cells. Here, addressing the reason for the difference between chondrocyte fates in the chicken embryo sternum, cells from the caudal and cranial part were cultured separately in serum-free agarose gels with complements defined earlier that either permit or prevent hypertrophic development. Total RNA was extracted using a novel protocol adapted to agarose cultures, and the temporal changes in developmental stage-specific mRNA expression were monitored by Northern hybridization and phosphor image analysis. Kinetic studies of the mRNA accumulation not only showed significant differences between the expression patterns of cranial and caudal cultures after recovery, but also revealed two checkpoints of chondrocyte differentiation in keeping with cartilage development in vivo. Terminal differentiation of caudal chondrocytes is blocked at the late proliferative stage (stage Ib), while the cranial cells can undergo hypertrophic development spontaneously. The differentiation of cranial chondrocytes is reversible, since they can re-assume an early proliferative (stage Ia) phenotype under the influence of insulin, fibroblast growth factor-2 and transforming growth factor-beta in combination. Thus, the expression pattern in the latter culture resembles that of articular chondrocytes. We also provide evidence that the capacities of caudal and sternal chondrocytes to progress from the late proliferative (stage Ib) to hypertrophic stage (stage II) correlate with their differing abilities to express the Indian hedgehog gene.

Bone morphogenetic proteins and bFGF exert opposing regulatory effects on PTHrP expression and inorganic pyrophosphate elaboration in immortalized murine endochondral hypertrophic chondrocytes (MCT cells)

A fundamental question in endochondral development is why the expression of parathyroid hormone-related protein (PTHrP), which inhibits chondrocyte maturation and mineralization, becomes attenuated at the stage of chondrocyte hypertrophy. To address this question, we used clonal, phenotypically stable SV40-immortalized murine endochondral chondrocytes that express a growth-arrested hypertrophic phenotype in culture (MCT cells). Addition of individual cytokines to the medium of MCT cells revealed that bone morphogenetic protein (BMP)-6, which commits chondrocytes to hypertrophy, markedly inhibited PTHrP production. This activity was shared by three other osteogenic bone morphogenetic proteins (BMP-2, BMP-4, and BMP-7) and by transforming growth factor beta (TGF-beta), which all inhibited the level of PTHrP mRNA. In contrast, basic fibroblast growth factor (bFGF), an inhibitor of chondrocyte maturation to hypertrophy, induced PTHrP in MCT cells and antagonized the effects of BMP-2, BMP-4, BMP-6, and BMP-7 and TGF-beta on PTHrP expression. Opposing effects of bFGF and BMPs also were exerted on the elaboration of inorganic pyrophosphatase (PPi), which regulates the ability of hypertrophic chondrocytes to mineralize the matrix. Specifically, BMP-2 and BMP-4, but not BMP-6 and BMP-7, shared the ability of TGF-beta to induce PPi release, and this activity was inhibited by bFGF in MCT cells. Our results suggest that effects on PTHrP expression could contribute to the ability of BMP-6 to promote chondrocyte maturation. BMPs and bFGF exert opposing effects on more than one function in immortalized hypertrophic chondrocytes. Thus, the normal decrease in bFGF responsiveness that accompanies chondrocyte hypertrophy may function in part by removing the potential for bFGF to induce PTHrP expression and to oppose the effects of BMPs. MCT cells may be useful in further understanding the mechanisms regulating the differentiation and function of hypertrophic chondrocytes.

Regulation of growth region cartilage proliferation and differentiation by perichondrium

Endochondral bone formation in vertebrates requires precise coordination between proliferation and differentiation of the participating chondrocytes. We examined the role of perichondrium in this process using an organ culture system of chicken embryonic tibiotarsi. A monoclonal antibody against chicken collagen type X, specifically expressed by hypertrophic chondrocytes, was utilized to monitor the terminal differentiation of chondrocytes. Proliferation of chondrocytes was examined by a BrdU-labeling procedure. The absence of perichondrium is correlated with an extended zone of cartilage expressing collagen type X, suggesting that the perichondrium regulates chondrocyte hypertrophy in a negative manner. Removal of perichondrium, in addition, resulted in an extended zone of chondrocytes incorporating BrdU, indicating that the perichondrium also negatively regulates the proliferation of chondrocytes. Partial removal of perichondrium from one side of the tibiotarsus led to expansion of both the collagen type X-positive domain and the BrdU-positive zone at the site of removal but not where the perichondrium remained intact. This suggests that both types of regulation by the perichondrium are local effects. Furthermore, addition of bovine parathyroid hormone (PTH) to perichondrium-free cultures reversed the expansion of the collagen type X-positive domain but not that of the proliferative zone. This suggests that the regulation of differentiation is dependent upon the PTH/PTHrP receptor and that the regulation of proliferation is likely independent of it. Taken together, these results are consistent with a model where perichondrium regulates both the exit of chondrocytes from the cell cycle, and their subsequent differentiation.

Expression of type X collagen and matrix calcification in three-dimensional cultures of immortalized temperature-sensitive chondrocytes derived from adult human articular cartilage

Chondrocytes isolated from normal adult human articular cartilage were infected with a retroviral vector encoding a temperature-sensitive mutant of the simian virus 40 large tumor antigen and a linked geneticin (G418)-resistance marker. G418-resistant colonies were then isolated, ring-cloned, and expanded in serum-containing media. Several immortalized chondrocyte cell lines were established from the clones that survived, some of which have been maintained in continuous culture for over 2 years. Despite serial subcultures and maintenance as monolayers, these cells retain expression of markers specific for cells of the lineage, namely type II collagen and aggrecan, detected immunocytochemically. We also examined the phenotype of three of these immortalized cell lines (designated HAC [human articular chondrocyte]) using a pellet culture system, and in this report, we present evidence that a prototype of these lines (HAC-F cells) expresses markers normally associated with hypertrophic chondrocytes. When HAC-F cells were cultivated in centrifuge tubes, for periods of up to 63 days, at 39 degrees C with mild and intermittent centrifugation they continued to express both lineage markers; total type II collagen/pellet remained stable, whereas there was a temporal decrease in cartilage-specific glycosaminoglycans content. In addition, in the presence of ascorbate but in the absence of a phosphate donor or inorganic phosphate supplement, the cells also begin to express a hypertrophic phenotype characterized by type X collagen synthesis and extensive mineralization of the extracellular matrix in late stage cultures. The mRNA encoding type X collagen was detected in the cell pellets by reverse transcriptase polymerase chain reaction as early as day 2, and anti-type X collagen immunoreactivity was subsequently localized in the matrix. The mineral was characterized by energy-dispersive X-ray microanalysis as containing calcium (Ca) and phosphorus (P) with a Ca:P peak height ratio close to that of mineralized bone tissue. The unexpected phenotype of this human chondrocyte cell line provides an interesting opportunity for studying chondrocyte maturation in vitro.

Role of the subchondral vascular system in endochondral ossification: endothelial cells specifically derepress late differentiation in resting chondrocytes in vitro

Endochondral ossification in growth plates proceeds through several consecutive steps of late cartilage differentiation leading to chondrocyte hypertrophy, vascular invasion, and, eventually, to replacement of the tissue by bone. It is well established that the subchondral vascular system is pivotal in the regulation of this process. Cells of subchondral blood vessels act as a source of vascular invasion and, in addition, release factors influencing growth and differentiation of chondrocytes in the avascular growth plate. To elucidate the paracrine contribution of endothelial cells we studied the hypertrophic development of resting chondrocytes from the caudal third of chick embryo sterna in co-culture with endothelial cells. The design of the experiments prevented cell-to-cell contact but allowed paracrine communication between endothelial cells and chondrocytes. Under these conditions, chondrocytes rapidly became hypertrophied in vitro and expressed the stage-specific markers collagen X and alkaline phosphatase. This development also required signaling by thyroid hormone in synergy. Conditioned media could replace the endothelial cells, indicating that diffusible factors mediated this process. By contrast, smooth muscle cells, fibroblasts, or hypertrophic chondrocytes did not secrete this activity, suggesting that the factors were specific for endothelial cells. We conclude that endochondral ossification is under the control of a mutual communication between chondrocytes and endothelial cells. A finely tuned balance between chondrocyte-derived signals repressing cartilage maturation and endothelial signals promoting late differentiation of chondrocytes is essential for normal endochondral ossification during development, growth, and repair of bone. A dysregulation of this balance in permanent joint cartilage also may be responsible for the initiation of pathological cartilage degeneration in joint diseases.