Expression of simian virus 40 large T (tumor) oncogene in mouse chondrocytes induces cell proliferation without loss of the differentiated phenotype

Radiosensitizing Effect of PARP Inhibition on Chondrosarcoma and Chondrocyte Cells Is Dependent on Radiation LET

Chondrosarcoma is a rare malignant tumor that forms in bone and cartilage. The primary treatment involves surgical removal of the tumor with a margin of healthy tissue. Especially if complete surgical removal is not possible, radiation therapy and chemotherapy are used in conjunction with surgery, but with a generally low efficiency. Ongoing researches are focused on understanding the genetic and molecular basis of chondrosarcoma following high linear energy transfer (LET) irradiation, which may lead to treatments that are more effective. The goal of this study is to evaluate the differential effects of DNA damage repair inhibitors and high LET irradiation on chondrosarcoma versus chondrocyte cells and the LET-dependency of the effects. Two chondrosarcoma cell lines with different IDH mutation status and one chondrocyte cell line were exposed to low LET (X-ray) and high LET (carbon ion) irradiation in combination with an Olaparib PARP inhibitor. Cell survival and DNA repair mechanisms were investigated. High LET irradiation drastically reduced cell survival, with a biological efficiency three times that of low LET. Olaparib significantly inhibited PARylation in all the tested cells. A significant reduction in cell survival of both chondrosarcoma and chondrocyte cells was observed following the treatment combining Olaparib and X-ray. PARP inhibition induced an increase in PARP-1 expression and a reduced effect on the cell survival of WT IDH chondrosarcoma cells. No radiosensitizing effect was observed in cells exposed to Olaparib paired with high LET irradiation. NHEJ was activated in response to high LET irradiation, neutralizing the PARP inhibition effect in both chondrosarcoma cell lines. When high LET irradiation is not available, PARP inhibition could be used in combination with low LET irradiation, with significant radiosensitizing effects on chondrosarcoma cells. Chondrocytes may be affected by the treatment combination too, showing the need to preserve normal tissues from radiation fields when this kind of treatment is suggested.

Phenotypic Characterization of Immortalized Chondrocytes from a Desbuquois Dysplasia Type 1 Mouse Model: A Tool for Studying Defects in Glycosaminoglycan Biosynthesis

The complexity of skeletal pathologies makes use of in vivo models essential to elucidate the pathogenesis of the diseases; nevertheless, chondrocyte and osteoblast cell lines provide relevant information on the underlying disease mechanisms. Due to the limitations of primary chondrocytes, immortalized cells represent a unique tool to overcome this problem since they grow very easily for several passages. However, in the immortalization procedure the cells might lose the original phenotype; thus, these cell lines should be deeply characterized before their use. We immortalized primary chondrocytes from a Cant1 knock-out mouse, an animal model of Desbuquois dysplasia type 1, with a plasmid expressing the SV40 large and small T antigen. This cell line, based on morphological and biochemical parameters, showed preservation of the chondrocyte phenotype. In addition reduced proteoglycan synthesis and oversulfation of glycosaminoglycan chains were demonstrated, as already observed in primary chondrocytes from the Cant1 knock-out mouse. In conclusion, immortalized Cant1 knock-out chondrocytes maintained the disease phenotype observed in primary cells validating the in vitro model and providing an additional tool to further study the proteoglycan biosynthesis defect. The same approach might be extended to other cartilage disorders.

[Reconstruction of nasal cartilage defects using a tissue engineering technique based on combination of high-density polyethylene and hydrogel]

AIM OF THE STUDY

Nasal reconstruction remains a challenge for any surgeon. The surgical indications for nasal reconstruction after oncologic resection, trauma or as part of cosmetic rhinoplasty, are steadily increasing. The current attitude for reconstruction is the use of autologous cartilage grafts of various origins (septal, ear or rib) trying to restore a physiological anatomy but their quantity is limited. Thus, in order to produce an implantable cartilaginous model, we developed a study protocol involving human nasal chondrocytes, growth factors and a composite biomaterial and studied at the molecular, cellular and tissue level the phenotype of the chondrocytes cultured in this model.

MATERIALS AND METHODS

After extraction of chondrocytes and their amplification on plastic, the cells were cultured for 15 days either in monolayer or within an agarose hydrogel or a composite biomaterial (agarose/high density polyethylene: Medpor(®)) in the presence or not of a cocktail of soluble factors (BIT): bone morphogenetic protein-2 (BMP-2), insulin and triiodothyronine (T3). The quality of the chondrocyte phenotype was analyzed by PCR, western blotting and immunohistochemistry.

RESULTS

During their amplification in monolayer, chondrocytes dedifferentiate. However, our results show that the BIT cocktail induces redifferentiation of chondrocytes cultured in agarose/Medpor with synthesis of mature chondrogenic markers. Thereby, chondrocytes associated with the agarose hydrogel will colonize Medpor and synthesize an extracellular matrix characteristic of nasal cartilage.

CONCLUSION

This nasal cartilage tissue engineering protocol provides the first interesting results for nasal reconstruction.

Disruption of the developmentally-regulated Col2a1 pre-mRNA alternative splicing switch in a transgenic knock-in mouse model

The present study describes the generation of a knock-in mouse model to address the role of type II procollagen (Col2a1) alternative splicing in skeletal development and maintenance. Alternative splicing of Col2a1 precursor mRNA is a developmentally-regulated event that only occurs in chondrogenic tissue. Normally, chondroprogenitor cells synthesize predominantly exon 2-containing mRNA isoforms (type IIA and IID) while Col2a1 mRNA devoid of exon 2 (type IIB) is the major isoform produced by differentiated chondrocytes. Another isoform, IIC, has also been identified that contains a truncated exon 2 and is not translated into protein. The biological significance of this IIA/IID to IIB splicing switch is not known. Utilizing a splice site targeting knock-in approach, a 4 nucleotide mutation was created to convert the 5' splice site of Col2a1 exon 2 from a weak, non-consensus sequence to a strong, consensus splice site. This resulted in apparent expression of only the IIA mRNA isoform, as confirmed in vitro by splicing of a type II procollagen mini-gene containing the 5' splice site mutation. To test the splice site targeting approach in vivo, homozygote mice engineered to retain IIA exon 2 (Col2a1(+ex2)) were generated. Chondrocytes from hindlimb epiphyseal cartilage of homozygote mice were shown to express only IIA mRNA and protein at all pre- and post-natal developmental stages analyzed (E12.5, E16.5, P0, P3, P7, P14, P28 and P70). As expected, type IIB procollagen was the major isoform produced in wild type cartilage at all post-natal time points. Col2a1(+ex2) homozygote mice are viable, appear healthy and display no overt phenotype to date. However, research is currently underway to investigate the biological consequence of persistent expression of the exon 2-encoded conserved cysteine-rich domain in post-natal skeletal tissues.

Stable subclones of the chondrogenic murine cell line MC615 mimic distinct stages of chondrocyte differentiation

Fourteen stable subclones derived from the murine chondrogenic cell line MC615 were established and characterised regarding their differentiation stages and responsivity to BMP2. Based on their gene expression profiles which revealed remarkable variances in Col2a1 and Col10a1 expression, subclones could be grouped into at least three distinct categories. Three representative subclones (4C3, 4C6 and 4H4) were further characterised with respect to gene expression pattern and differentiation capacity. These subclones resembled (i) weakly differentiated chondrogenic precursors, strongly responding to BMP2 stimulation (4C3), (ii) collagen II expressing chondrocytes which could be induced to undergo maturation (4C6) and (iii) mature chondrocytes expressing Col10a1 and other markers of hypertrophy (4H4). Interestingly, BMP2 administration caused Smad protein phosphorylation and stimulated Col10a1 expression in all clones, but induced Col2a1 expression only in precursor-like cells. Most remarkably, these clones maintained a stable gene expression profile at least until the 30th passage of subconfluent culture, but revealed reproducible changes in gene expression and differentiation pattern in long term high density cultures. Thus, the newly established MC615 subclones may serve as a potent new tool for investigations on the regulation of chondrocyte differentiation and function.

Primary culture and phenotyping of murine chondrocytes

The culture of chondrocytes is one of the most powerful tools for exploring the intracellular and molecular features of chondrocyte differentiation and activation. However, chondrocytes tend to dedifferentiate into fibroblasts when they are subcultured, which is a major problem. This protocol, involving primary cultures to limit dedifferentiation, describes two different methods for culturing chondrocytes of different anatomical origins (articular and costal chondrocytes, both of which represent hyaline cartilage) from mice. Mice are of particular interest for cellular and molecular studies, as many tools suitable for use in mice are available. In addition, rapid development of transgenic and gene-targeted mice provides powerful instruments for biological studies. The protocol can be divided into four stages: isolation of cartilage (15 min per animal), isolation of chondrocytes (2 h extended overnight), seeding of chondrocytes (1 h 30 min) and growth in culture (6 d). To obtain confluency of chondrocytes using this protocol takes 7 d. Methods for phenotyping chondrocytes are also provided.

Immortalized mouse articular cartilage cell lines retain chondrocyte phenotype and respond to both anabolic factor BMP-2 and pro-inflammatory factor IL-1

Articular cartilage chondrocytes help in the maintenance of tissue homeostasis and function of the articular joint. Study of primary chondrocytes in culture provides information closely related to in vivo functions of these cells. Limitations in the primary culture of chondrocytes have lead to the development of cells lines that serve as good surrogate models for the study of chondrocyte biology. In this study, we report the establishment and characterization of chondrocyte cell lines, MM-Sv/HP and MM-Sv/HP-2 from mouse articular cartilage. Cells were isolated from mouse femoral head articular cartilage, immortalized and maintained in culture through numerous passages. The morphology of the cells was from fibroblastic to polygonal in nature. Gene expression studies using quantitative PCR (Q-PCR) were performed on cells in monolayer culture and cells embedded in a three-dimensional alginate matrix. Stimulation of cells in monolayer culture with anabolic factor, BMP-2, resulted in increased gene expression of the extracellular matrix molecules, aggrecan and type II collagen and their regulator transcription factor, Sox9. Treatment by pro-inflammatory IL-1 resulted in increased gene expression of catabolic effectors including Aggrecanases (ADAMTS4, ADAMTS5), MMP-13 and nitric oxide synthase (Nos2). Cells in alginate treated with BMP-2 resulted in increased synthesis of proteoglycan which was released into the conditioned media on IL-1 stimulation. Western analysis of conditioned media showed the presence of Aggrecanase-cleaved aggrecan fragments. In summary, MM-Sv/HP and MM-Sv/HP-2 show preservation of important characteristics of articular chondrocytes as examined under multiple culture conditions and would provide a useful reagent in the study of chondrocyte biology.

Ucma, a novel secreted cartilage-specific protein with implications in osteogenesis

Here we report on the structure, expression, and function of a novel cartilage-specific gene coding for a 17-kDa small, highly charged, and secreted protein that we termed Ucma (unique cartilage matrix-associated protein). The protein is processed by a furin-like protease into an N-terminal peptide of 37 amino acids and a C-terminal fragment (Ucma-C) of 74 amino acids. Ucma is highly conserved between mouse, rat, human, dog, clawed frog, and zebrafish, but has no homology to other known proteins. Remarkable are 1-2 tyrosine sulfate residues/molecule and dense clusters of acidic and basic residues in the C-terminal part. In the developing mouse skeleton Ucma mRNA is expressed in resting chondrocytes in the distal and peripheral zones of epiphyseal and vertebral cartilage. Ucma is secreted into the extracellular matrix as an uncleaved precursor and shows the same restricted distribution pattern in cartilage as Ucma mRNA. In contrast, antibodies prepared against the processed C-terminal fragment located Ucma-C in the entire cartilage matrix, indicating that it either diffuses or is retained until chondrocytes reach hypertrophy. During differentiation of an MC615 chondrocyte subclone in vitro, Ucma expression parallels largely the expression of collagen II and decreases with maturation toward hypertrophic cells. Recombinant Ucma-C does not affect expression of chondrocyte-specific genes or proliferation of chondrocytes, but interferes with osteogenic differentiation of primary osteoblasts, mesenchymal stem cells, and MC3T3-E1 pre-osteoblasts. These findings suggest that Ucma may be involved in the negative control of osteogenic differentiation of osteochondrogenic precursor cells in peripheral zones of fetal cartilage and at the cartilage-bone interface.

Knockdown of the intraflagellar transport protein IFT46 stimulates selective gene expression in mouse chondrocytes and affects early development in zebrafish

Bone morphogenetic proteins (BMPs) act as multifunctional regulators in morphogenesis during development. In particular they play a determinant role in the formation of cartilage molds and their replacement by bone during endochondral ossification. In cell culture, BMP-2 favors chondrogenic expression and promotes hypertrophic maturation of chondrocytes. In mouse chondrocytes we have identified a BMP-2-sensitive gene encoding a protein of 301 amino acids. This protein, named mIFT46, is the mouse ortholog of recently identified Caenorhabditis elegans and Chlamydomonas reinhardtii intraflagellar transport (IFT) proteins. After generation of a polyclonal antibody against mIFT46, we showed for the first time that the endogenous protein is located in the primary cilium of chondrocytes. We also found that mIFT46 is preferentially expressed in early hypertrophic chondrocytes located in the growth plate. Additionally, mIFT46 knockdown by small interfering RNA oligonucleotides in cultured chondrocytes specifically stimulated the expression of several genes related to skeletogenesis. Furthermore, Northern blotting analysis indicated that mIFT46 is also expressed before chondrogenesis in embryonic mouse development, suggesting that the role of mIFT46 might not be restricted to cartilage. To explore the role of IFT46 during early development, we injected antisense morpholino oligonucleotides in Danio rerio embryos to reduce zebrafish IFT46 protein (zIFT46) synthesis. Dramatic defects in embryonic development such as a dorsalization and a tail duplication were observed. Thus our results taken together indicate that the ciliary protein IFT46 has a specific function in chondrocytes and is also essential for normal development of vertebrates.

Human immortalized chondrocytes carrying heterozygous FGFR3 mutations: an in vitro model to study chondrodysplasias

Achondroplasia and thanatophoric dysplasia are human chondrodysplasias caused by mutations in the fibroblast growth factor receptor 3 (FGFR3) gene. We have developed an immortalized human chondrocyte culture model to study the regulation of chondrocyte functions. One control and eight mutant chondrocytic lines expressing different FGFR3 heterozygous mutations were obtained. FGFR3 signaling pathways were modified in the mutant lines as revealed by the constitutive activation of the STAT pathway and an increased level of P21(WAF1/CIP1) protein. This model will be useful for the study of FGFR3 function in cartilage studies and future therapeutic approaches in chondrodysplasias.

Proliferative arrest and activation of apoptosis related genes in Rolly Protein-silenced cells

Here we describe a novel small polypeptide expressed in chick embryo and mouse adult tissues referred to as Rolly Protein (Rolp), expressed at the highest levels in tibial cartilage and lung respectively. Investigating its putative role in cartilage differentiation we found that its expression is restricted to proliferative stages consistently with a decreased proliferation rate observed in Rolp-silenced cells. Additional functional studies demonstrate that inhibition of Rolp expression causes a transcription modulation of genes involved in apoptosis. The results here provided strongly suggest an active role of Rolp in the control of cell proliferation and apoptosis.

CRTAP Is Required for Prolyl 3- Hydroxylation and Mutations Cause Recessive Osteogenesis Imperfecta

Prolyl hydroxylation is a critical posttranslational modification that affects structure, function, and turnover of target proteins. Prolyl 3-hydroxylation occurs at only one position in the triple-helical domain of fibrillar collagen chains, and its biological significance is unknown. CRTAP shares homology with a family of putative prolyl 3-hydroxylases (P3Hs), but it does not contain their common dioxygenase domain. Loss of Crtap in mice causes an osteochondrodysplasia characterized by severe osteoporosis and decreased osteoid production. CRTAP can form a complex with P3H1 and cyclophilin B (CYPB), and Crtap-/- bone and cartilage collagens show decreased prolyl 3-hydroxylation. Moreover, mutant collagen shows evidence of overmodification, and collagen fibrils in mutant skin have increased diameter consistent with altered fibrillogenesis. In humans, CRTAP mutations are associated with the clinical spectrum of recessive osteogenesis imperfecta, including the type II and VII forms. Hence, dysregulation of prolyl 3-hydroxylation is a mechanism for connective tissue disease.

Twisted Gastrulation Modulates Bone Morphogenetic Protein-induced Collagen II and X Expression in Chondrocytesin Vitroandin Vivo

Twisted gastrulation (TSG) is an extracellular modulator of bone morphogenetic protein (BMP) activity and regulates dorsoventral axis formation in early Drosophila and Xenopus development. Studies on tsg-deficient mice also indicated a role of this protein in skeletal growth, but the mechanism of TSG activity in this process has not yet been investigated. Here we show for the first time by in situ hybridization and immunohistochemistry that TSG is strongly expressed in bovine and mouse growth plate cartilage as well as in fetal ribs, vertebral cartilage, and cartilage anlagen of the skull. Furthermore we provide evidence that TSG is directly involved in BMP-regulated chondrocyte differentiation and maturation. In vitro, TSG impaired the dose-dependent BMP-2 stimulation of collagen II and X expression in cultures of MC615 chondrocytes and primary mouse chondrocytes. In the presence of chordin, a BMP antagonist, the inhibitory effect of TSG was further enhanced. TSG also inhibited BMP-2-stimulated phosphorylation of Smad factors in chondrocytes, confirming the role of TSG as a modulator of BMP signaling. For analysis of TSG functions in cartilage development in vivo, the gene was overexpressed in transgenic mice under the control of the cartilage-specific Col2a1 promoter. As a result, Col10a1 expression was significantly reduced in the growth plates of transgenic embryos and newborns in comparison with wild type littermates as shown by in situ hybridization and by real time PCR analysis. The data suggest that TSG is an important modulator of BMP-regulated cartilage development and chondrocyte differentiation.

The role of adenosine in chondrocyte death in murine osteoarthritis and in a murine chondrocyte cell line

OBJECTIVE

To investigate the role of adenosine in chondrocyte death in murine osteoarthritis (OA).

METHODS

5'-Nucleotidase (5'NT) generates adenosine. Enzyme activity was measured histochemically in normal murine and osteoarthritic STR/ort strain tibial cartilage. Adenosine-mediated cell death was investigated in MC615 chondrocyte cultures. Adenosine receptors (ARs) were assessed by reverse transcriptase polymerase chain reaction (RT-PCR). Cellular uptake of [(3)H] adenosine was measured with or without the inhibitor, nitrobenzylthioinosine (NBTI). Cell death was assessed by cell counting and DNA laddering following selective receptor stimulation, or after modulating adenosine metabolism with adenosine deaminase (ADA) or adenosine kinase (AK) inhibitors [erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) and Iodotubericidin (Itub)], or with homocysteine (HC). Markers of apoptosis were assessed by Western blotting. Cell studies were validated by incubating normal murine knee joints in a medium containing adenosine and metabolic inhibitors. Apoptotic chondrocytes were identified with the TUNEL reaction.

RESULTS

5'NT activity in STR/ort tibial cartilage increased with development of OA, especially close to OA lesions. Adenosine induced MC615 cell death in the presence of ADA inhibition (100 microM EHNA), or 1mM HC, or both. Adenosine uptake, mediated by NBTI-sensitive adenosine transporters, was required for cell death. ARs were expressed (A2b>A2a>A1) but were not involved in mediating cell death. Cell death involved the activation of caspase-3 and DNA fragmentation and was prevented by inhibiting caspase activity. However, neither caspase-8 nor caspase-9 was detected. Adenosine+EHNA induced chondrocyte apoptosis in normal murine knee joints.

CONCLUSION

Increased adenosine production may induce chondrocyte apoptosis and play a role in OA in STR/ort mice.

Expression of Semaphorin-3A and its receptors in endochondral ossification: potential role in skeletal development and innervation

Bone tissue is densely innervated, and there is increasing evidence for a neural control of bone metabolism. Semaphorin-3A is a very important regulator of neuronal targeting in the peripheral nervous system as well as in angiogenesis, and knockout of the Semaphorin-3A gene induces abnormal bone and cartilage development. We analyzed the spatial and temporal expression patterns of Semaphorin-3A signaling molecules during endochondral ossification, in parallel with the establishment of innervation. We show that osteoblasts and chondrocytes differentiated in vitro express most members of the Semaphorin-3A signaling system (Semaphorin-3A, Neuropilin-1, and Plexins-A1 and -A2). In vitro, osteoclasts express most receptor chains but not the ligand. In situ, these molecules are all expressed in the periosteum and by resting, prehypertrophic and hypertrophic chondrocytes in ossification centers before the onset of neurovascular invasion. They are detected later in osteoblasts and also osteoclasts, with differences in intensity and regional distribution. Semaphorin-3A and Neuropilin-1 are also expressed in the bone marrow. Plexin-A3 is not expressed by bone cell lineages in vitro. It is detected early in the periosteum and hypertrophic chondrocytes. After the onset of ossification, this chain is restricted to a network of cell processes in close vicinity to the cells lining the trabeculae, similar to the pattern observed for neural markers at the same stages. After birth, while the density of innervation decreases, Plexin-A3 is strongly expressed by blood vessels on the ossification front. In conclusion, Semaphorin-3A signaling is present in bone and seems to precede or coincide at the temporal but also spatial level with the invasion of bone by blood vessels and nerve fibers. Expression patterns suggest Plexin-A3/Neuropilin-1 as a candidate receptor in target cells for the regulation of bone innervation by Semaphorin-3A.

Immortalized cell lines from mouse xiphisternum preserve chondrocyte phenotype

Chondrocytes are unique to cartilage and the study of these cells in vitro is important for advancing our understanding of the role of these cells in normal homeostasis and disease including osteoarthritis (OA). As there are limitations to the culture of primary chondrocytes, cell lines have been developed to overcome some of these obstacles. In this study, we developed a procedure to immortalize and characterize chondrocyte cell lines from mouse xiphisternum. The cells displayed a polygonal to fibroblastic morphology in monolayer culture. Gene expression studies using quantitative PCR showed that the cell lines responded to bone morphogenetic protein 2 (BMP-2) by increased expression of matrix molecules, aggrecan, and type II collagen together with transcriptional factor, Sox9. Stimulation by IL-1 results in the increased expression of catabolic effectors including MMP-13, nitric oxide synthase, ADAMTS4, and ADAMTS5. Cells cultured in alginate responded to BMP-2 by increased synthesis of proteoglycan (PG), a major matrix molecule of cartilage. IL-1 treatment of cells in alginate results in increased release of PG into the conditioned media. Further analysis of the media showed the presence of Aggrecanase-cleaved aggrecan fragments, a signature of matrix degradation. These results show that the xiphisternum chondrocyte cell lines preserve their chondrocyte phenotype cultured in either monolayer or 3-dimensional alginate bead culture systems. In summary, this study describes the establishment of chondrocyte cell lines from the mouse xiphisternum that may be useful as a surrogate model system to understand chondrocyte biology and to shed light on the underlying mechanism of pathogenesis in OA.

Integrin alpha1beta1 mediates collagen induction of MMP-13 expression in MC615 chondrocytes

During endochondral ossification, type I collagen is synthesized by osteoblasts together with some hypertrophic chondrocytes. Type I collagen has also been reported to be progressively synthesized in degenerative joints. Because Matrix Metalloproteinase-13 (MMP-13) plays an active role in remodeling cartilage in fetal development and osteoarthritic cartilage, we investigated whether type I collagen could activate MMP-13 expression in chondrocytes. We used a well-established chondrocytic cell line (MC615) and we found that MMP-13 expression was induced in MC615 cells cultured in type I collagen gel. We also found that alpha1beta1 integrin, a major collagen receptor, was expressed by MC615 cells and we further assessed the role of alpha1beta1 integrin in conducting MMP-13 expression. Induction of MMP-13 expression by collagen was potently and synergistically inhibited by blocking antibodies against alpha1 and beta1 integrin subunits, indicating that alpha1beta1 integrin mediates the MMP-13-inducing cellular signal generated by three-dimensional type I collagen. We also determined that activities of tyrosine kinase and ERK and JNK MAP kinases were required for this collagen-induced MMP-13 expression. Interestingly, bone morphogenetic protein (BMP)-2 opposed this induction, an effect that may be related to a role of BMP-2 in the maintenance of cartilage matrix.

Species variability in the differentiation potential of in vitro-expanded articular chondrocytes restricts predictive studies on cartilage repair using animal models

Autologous chondrocyte implantation is currently applied in clinics as an innovative tool for articular cartilage repair. Animal models have been and still are being used to validate and further improve the technique. However, in various species, the outcome varies from hyaline-like cartilage to fibrocartilage. This may be due partly to the spontaneous dedifferentiation of chondrocytes once cultured in vitro. Here we assessed whether the extent of dedifferentiation varies between species and we hypothesized that the level of chondrocyte phenotype stability during expansion may contribute to the maintenance of their chondrogenic commitment and redifferentiation potential. Condyle chondrocytes were harvested from sheep, dog, and human, and expanded for 1, 6, or 12 cell duplications. At each interval, cell phenotype was monitored (morphology and biosynthesis of cartilage markers) and redifferentiation was assessed by an in vitro assay of chondrogenesis in micromass pellet and an in vivo assay of ectopic cartilage formation in immunodeficient mice. Results indicate that, during culture, the sheep chondrocyte phenotype is maintained better than that of human chondrocytes, which in turn dedifferentiate to a lesser extent than dog chondrocytes Accordingly, after expansion, sheep chondrocytes spontaneously reform hyaline-like cartilage; human chondrocytes redifferentiate only under stimulation with chondrogenic inducers whereas, after a few passages, dog chondrocytes lose any capacity to redifferentiate regardless of the presence of inducers. Thus, conditions allowing cartilage formation in one species are not necessarily transposable to other species. Therefore, results with animal models should be cautiously applied to humans. In addition, for tissue-engineering purposes, the number of cell duplications must be, for each species, carefully monitored to remain in the range of amplification allowing redifferentiation and chondrogenesis.

Analysis of Regulatory Regions of Emilin1 Gene and Their Combinatorial Contribution to Tissue-specific Transcription

The location of regions that regulate transcription of the murine Emilin1 gene was investigated in a DNA fragment of 16.8 kb, including the entire gene and about 8.7 and 0.6 kb of 5'- and 3'-flanking sequences, respectively. The 8.7-kb segment contains the 5'-end of the putative 2310015E02Rik gene and the sequence that separates it from Emilin1, whereas the 0.6-kb fragment covers the region between Emilin1 and Ketohexokinase genes. Sequence comparison between species identified several conserved regions in the 5'-flanking sequence. Most of them contained chromatin DNase I-hypersensitive sites, which were located at about -950 (HS1), -3100 (HS2), -4750 (HS3), and -5150 (HS4) in cells expressing Emilin1 mRNA. Emilin1 transcription initiates at multiple sites, the major of which correspond to two Initiator sequences. Promoter assays suggest that core promoter activity was mainly dependent on Initiator1 and on Sp1-binding sites close to the Initiators. Moreover, one important regulatory region was contained between -1 and -169 bp and a second one between -630 bp and -1.1 kb. The latter harbors a putative binding site for transcription factor AP1 matching the location of HS1. The function of different regions was studied by expressing lacZ constructs in transgenic mice. The results show that the 16.8-kb segment contains regulatory sequences driving high level transcription in all the tissues where Emilin1 is expressed. Moreover, the data suggest that transcription in different tissues is achieved through combinatorial cooperation between various regions, rather than being dependent on a single cis-activating region specific for each tissue.

A common pathway in differentiation and inflammation: p38 mediates expression of the acute phase SIP24 iron binding lipocalin in chondrocytes

SIP24 is an acute phase iron binding lipocalin physiologically expressed in vivo in developing cartilage by prehypertrophic/hypertrophic chondrocytes. Taking advantage of the chondrocytic cell line MC615 and using SIP24 as a marker we investigated the pathways active in cartilage differentiation and inflammation. MC615 cells were cultured as: (i) proliferating prechondrogenic cells expressing type I collagen (ii) differentiated hyperconfluent cells expressing Sox9 and type II collagen. In proliferating cells the pathway PKC/ERK1, ERK2 was activated and SIP24 was not expressed while in differentiated cells the pathway p38/NF-kappaB was activated and SIP24 was expressed. Proliferating cells treated with inflammatory agents expressed a large amount of SIP24 and showed activation of p38/NF-kappaB pathway and inhibition of PKC/ERK1, ERK2 pathway indicating that in inflammation and differentiation the same factors are activated (p38, NF-kappaB) or inactivated (PKC, ERKs). Treatment of proliferating cells with the p38 specific inhibitor SB203580 inhibited the inflammation induced activation of p38 and the synthesis of SIP24. PMA treatment induced activation of PKC, inactivation of p38 and suppression of SIP24 synthesis, suggesting that PKC activation inhibits p38 activation. In differentiated hyperconfluent cells the same factors (p38/NF-kappaB/SIP24) are constitutively activated: treatment with inflammatory agents does not increase synthesis of SIP24 while treatment with SB203580 and with PMA does not repress activation of p38 nor synthesis of SIP24. We propose that the SIP24 stress related protein is expressed via p38 activation/NF-kappaB recruitment both in chondrocyte differentiation and inflammation and that a signaling pathway active in the acute phase response is physiologically activated in differentiation.

BMP-2 and CDMP-2: stimulation of chondrocyte production of proteoglycan

A hallmark of intervertebral disc degeneration is loss of proteoglycans. Cytokines may be used to stimulate proteoglycan production in the disc to reverse or prevent disc degeneration. The effects of bone morphogenetic protein 2 (BMP-2) and cartilage-derived morphogenetic protein 2 (CDMP-2) (singly and jointly) on proteoglycan synthesis by a chondrocytic cell line (MC615) were studied. MC615 cells were dosed with BMP-2, CDMP-2, or both, cultured for 6 days, and then assayed as follows: (1). The proteoglycan content of the medium and extracellular matrix were determined by dimethyl-methylene blue staining; (2). cell numbers were determined after 6 days of culture using the Hoechst dye DNA assay; (3). aggrecan mRNA was measured with the reverse transcription-polymerase chain reaction and Northern blotting assays. Both BMP-2 and CDMP-2 significantly enhanced proteoglycan production and aggrecan mRNA expression in a dose-dependent manner, although BMP-2 is more effective than CDMP-2 in increasing sulfated proteoglycan production. But BMP-2 and CDMP-2 have only a slight, nonsignificant effect on cell proliferation. There is no evidence of antagonism or synergy between the two growth factors in the aggrecan gene expression.

Distribution of Endo180 receptor and ligand in developing articular cartilage

OBJECTIVE

To investigate the expression of a novel member of the mannose receptor family, Endo180 (also known as uPARAP), and the distribution of Endo180 ligand(s) in the articular cartilage and growth plate of normal CBA mice and STR/ort mice, a well characterized model of spontaneous osteoarthritis.

DESIGN

A polyclonal anti-Endo180 antibody was used to determine receptor expression. The Endo180 extracellular domain fused to a human immunoglobulin Fc tail was used to detect ligand.

RESULTS

Endo180 receptor was strongly expressed in chondrocytes both in vitro and throughout the articular cartilage of young CBA and STR/ort mice. Expression decreased in older animals. In STR/ort mice with osteoarthritic lesions, no upregulation of Endo180 was detected. In the developing growth plate, Endo180 was expressed strongly by the proliferating chondrocytes. In contrast, Endo180 ligand was detected most strongly in hypertrophic zone of the growth plate and only at low levels in articular cartilage. In cultured chondrocytes, Endo180 was localized on the cell surface and in intracellular vesicles.

CONCLUSION

Constitutively recycling endocytic receptors function to internalize ligand from the extracellular milieu and the ability of Endo180 to bind both glycosylated ligands and collagens suggests a role in extracellular matrix remodeling. Expression of Endo180 in articular cartilage chondrocytes of young, but not old, mice and the reciprocal expression of Endo180 and its ligands in the growth plate suggest that this receptor is involved in cartilage development but not in cartilage homeostasis. In addition, our data indicates that Endo180 does not appear to play a role in the development or progression of murine osteoarthritis.

Effects of [3H]glucosamine concentration on [3H]chondroitin sulphate formation by cultured chondrocytes

GlcN (glucosamine) is now promoted over the counter for implied treatment of osteoarthritis, ostensibly by stimulating biosynthesis of cartilage chondroitin sulphate. In order to evaluate whether exogenous GlcN has any stimulatory effect, we have incubated mouse chondrocytes with [(35)S]sulphate and various amounts of GlcN, to determine whether any increment in chondroitin [(35)S]sulphate formation occurs. Similarly we have used varying concentrations of [(3)H]GlcN to determine the dilution of incorporation into [(3)H]chondroitin sulphate due to provision of endogenous GlcN by metabolism from glucose at two different glucose concentrations. The incorporation of both (35)S and (3)H was essentially linear over a 5 h time period. We found no stimulation of chondroitin [(35)S]sulphate synthesis at lower concentrations of GlcN, and a significant reduction at higher concentrations. Even at concentrations of [(3)H]GlcN that were greater than could be achieved with standard doses of oral GlcN, there was significant dilution of exogenous GlcN. Furthermore, an artificial acceptor for glycosaminoglycan synthesis in cell culture, 4-methylumbelliferyl beta-D-xyloside, did not modify the provision of GlcN from endogenous sources, even though it stimulated chondroitin sulphate synthesis 4 -5-fold at each GlcN concentration. We conclude that the cells have excess capacity to form maximal amounts of GlcN from glucose so that exogenous GlcN does not stimulate chondroitin sulphate synthesis.

NF-kappaB mediates inhibition of mesenchymal cell differentiation through a posttranscriptional gene silencing mechanism

Cytokines, such as tumor necrosis factor-alpha (TNFalpha), potently inhibit the differentiation of mesenchymal cells and down-regulate the expression of Sox9 and MyoD, transcription factors required for chondrocyte and myocyte development. Previously, we demonstrated that NF-kappaB controls TNFalpha-mediated suppression of myogenesis through a mechanism involving MyoD mRNA down-regulation. Here, we show that NF-kappaB also suppresses chondrogenesis and destabilizes Sox9 mRNA levels. Multiple copies of an mRNA cis-regulatory motif (5'-ACUACAG-3') are necessary and sufficient for NF-kappaB-mediated Sox9 and MyoD down-regulation. Thus, in response to cytokine signaling, NF-kappaB modulates the differentiation of mesenchymal-derived cell lineages via RNA sequence-dependent, posttranscriptional down-regulation of key developmental regulators.

Smad signaling in mesenchymal and chondroprogenitor cells

BACKGROUND

Bone morphogenetic proteins (BMPs) are pleiotropic differentiation factors that regulate cell fate determination by orchestrating the activities of downstream signal transducers. Although BMP ligands can elicit signal transduction from heterodimeric combinations of several type-I and type-II receptors, cytoplasmic transducers of the BMP signal include only three known BMP-specific regulatory Smad proteins: Smad1, 5, and 8. In order to determine the combination of signals that regulate chondrogenesis by BMPs, we analyzed the functions of BMP Smad subtypes.

METHODS

Multipotential mesenchymal C3H10T1/2 cells and monopotential chondroprogenitor MC615 cells were placed in micromass culture in the presence or absence of BMP4. Chondrogenic differentiation was assayed by measuring Sox9 and type-II collagen gene expression and by alcian blue staining. Transactivation of type-II collagen by regulatory Smads singly, or in combination with Smad4, which partners with regulatory Smads, was assayed by luciferase activity.

RESULTS

In the absence of BMP4, mesenchymal cells did not exhibit chondrogenic differentiation, whereas chondroprogenitor cells showed increased cartilage marker expression. In the presence of BMP4, the rate and extent of chondrogenesis increased in a dose-dependent manner for both cell types. We further determined that Smad1 or Smad5, but not Smad8, synergized with Smad4 in the transactivation of the type-II collagen promoter in chondroprogenitor cells. In contrast, Smad8 and Smad4 presented modest synergy in mesenchymal cells.

CONCLUSIONS

Taken together, our data suggest that uncommitted mesenchymal cells do not have the cellular competence to respond to the rate-limiting chondroinductive factor BMP. However, in chondroprogenitor cells, BMP stimulates differentiation through mechanisms mediated by Smad1 or Smad5 in combination with Smad4.

In vitro demonstration of cell-to-cell interaction in growth plate cartilage using chondrocytes established from p53-/- mice

Three clonal cell lines (MMR14, MMR17, and MMR32) were established from the costal cartilage derived from p53-/- mice. Expression profiles of cartilage-related molecules in MMR14 and MMR17 were compatible with those in cells of the hypertrophic zone. Prolonged in vitro culture induced the expression of calcification-related genes in both cell lines, but calcified nodules were observed only in MMR14. The expression profile of cartilage-related molecules in MMR32 was compatible with that of cells in the perichondrium, with high expression levels of decorin, bone morphogenetic protein-3, and parathyroid hormone-related peptide (PTHrP). When MMR14 was co-cultured with an equal amount of MMR32 without direct contact, the nodule formation was completely inhibited, whereas no such inhibition was observed when MMR14 was co-cultured with MMR17, indicating that soluble factors produced by MMR32 were responsible for the inhibition. Blocking the effects of PTHrP by either antagonizing peptide or neutralizing antibody against PTHrP failed to rescue the inhibitory effects of MMR32, and no increase of the cyclic adenosine monophosphate production in MMR14 was observed when co-cultured with MMR32, suggesting that soluble factors other than PTHrP produced by MMR32 were responsible for the inhibition of terminal differentiation of hypertrophic chondrocytes. This report is the first to show cell-to-cell interaction in the growth plate using cell lines, which will be useful material to investigate the regulatory mechanism of chondrocyte differentiation.

Phenotypic differences in murine chondrocyte cell lines derived from mature articular cartilage

OBJECTIVE

To obtain well characterized immortalized murine chondrocyte cell lines. The cell lines were obtained from mature articular chondrocytes, instead of embryonal cells which are used in most other studies.

METHODS

Pieces of articular cartilage were cut from murine patellae and femoral heads. Chondrocytes were isolated by digestion with collagenase. These cells were cultured in monolayer and immortalized by transfection of the SV40 large T antigen gene. To preserve the differentiated phenotype, the resulting clones were cultured in three-dimensional carriers, alginate beads. The phenotypes of the cells were characterized using the following parameters: Cell morphology (light microscopy), messenger RNA (RT-PCR) and protein (immunohistochemistry) levels of extracellular matrix molecules. Moreover, responsiveness to interleukin-1(IL-1) was determined by measuring production of proteoglycans ((35)S-sulfate incorporation) and of nitric oxide (Griess reaction).

RESULTS

Sixteen clones were obtained, ten (P1 to P10) derived from patellar cartilage, and six (H1 to H6) from femoral head cartilage. In seven cell lines (P2, P5, H1, H3, H4, H5, H6) high production of type II collagen corresponded with high levels of mRNA of type II collagen (and prevalence of the IIB type) and with high IL-1-induced suppression of proteoglycan synthesis. Like intact murine articular cartilage, all cell lines produced type I and type X collagens, but mRNA levels of both types of collagen were never higher in the cell lines as compared with intact cartilage.

CONCLUSION

Our results demonstrate that it is possible to immortalize mature murine articular chondrocytes. Each of the obtained chondrocyte cell lines appeared to have a stable phenotype. Both relatively differentiated and relatively dedifferentiated chondrocyte cell lines could be identified.

Human articular chondrocytes immortalized by HPV-16 E6 and E7 genes: Maintenance of differentiated phenotype under defined culture conditions

OBJECTIVE

To establish an immortalized normal human articular chondrocyte line which could be useful for a better understanding of cell molecular mechanisms relevant for the development of new therapeutic approaches in rheumatic diseases.

DESIGN

Chondrocytes from human adult articular healthy cartilage were transfected in primary culture with a plasmid containing two human papilloma virus type 16 (HPV-16) early function genes: E6 and E7, using the highly efficient cationic liposome-mediated (lipofection) procedure. The transfection was verified by reverse transcriptase-polymerase chain reaction analysis of E7 mRNA and by immunofluorence localization of the E7 protein in the cell cytoplasm. The established chondrocyte cell line was examined in monolayer and in two culture conditions that were described to re-induce differentiated characteristics: culturing in a serum-free defined medium supplemented with an insulin-containing serum substitute and seeding on a hyaluronan-based non-woven structured biomaterial. The expression of markers characteristic of cartilage was shown in the mRNA by reverse transcriptase-polymerase chain reaction. Immunohistological staining and Western blotting analysis were performed to evaluate type II collagen synthesis. Proteoglycans deposition was detected by Alcian Blue staining. A Field Emission In Lens Scanning Microscopy was used to look at the morphology of the immortalized cells at very high magnification.

RESULTS

Normal human articular chondrocytes were efficiently transfected leading to the establishment of an immortalized cell line as confirmed by HPV-16 E7 mRNA and protein detection. These cells were able to re-express type II collagen both at mRNA and protein levels under the two defined cultured conditions we used, still maintaining type I collagen expression. Collagen IX mRNA was present only in early primary culture while collagen type X and aggrecan transcripts were always detected. Alcian Blue staining showed a proteoglycan-rich matrix production. The ultrastructural analysis of the immortalized cells revealed that their morphology strictly resembled that of normal chondrocytes.

CONCLUSIONS

The cell line that we obtained may be a useful tool for increasing our knowledge of the genetic and biochemical events involved in the processes of cartilage growth and differentiation. Moreover, it appears to be a suitable model for pharmacological and toxicological studies related to rheumatic diseases relevant to humans.

Establishment of a novel chondrocytic cell line N1511 derived from p53-null mice

We established a clonal chondrocytic cell line N1511 derived from rib cartilage of a p53-null mouse. N1511 cells proliferated in polygonal shape and elicited differentiation at confluence when treated with combination of bone morphogenetic protein (BMP) 2 and insulin or parathyroid hormone (PTH) and dexamethasone. BMP-2/insulin-treated cells became refractile without forming cartilaginous nodules and reached terminal differentiation, became positive for alizarin red staining, and developed considerable ALP activity. In contrast, PTH/dexamethasone-treated cells formed Alcian blue-positive nodules but remained negative for alizarin red staining and ALP activity. Northern blot analysis revealed that BMP-2/insulin-treated cells sequentially expressed type II, IX, and X collagens, whereas PTH/dexamethasone-treated cells slowly expressed type II collagen and then type IX, and they did not exhibit type X collagen expression. These results show that BMP-2/insulin treatment induces full differentiation toward hypertrophy, whereas treatment with PTH/dexamethasone slows and limits differentiation. Recovery of p53 expression in N1511 cells by transient transfection inhibited cell proliferation, suggesting that cell proliferation could be regulated with p53 in this cell line. These results indicate that N1511 is the only cell line with known genetic mutation, which undergoes multiple steps of chondrocyte differentiation toward hypertrophy, and because proliferation could be regulated by expression of p53, N1511 could be an excellent model for studies of chondrogenesis, the function of p53, and genetic engineering of cartilage tissue.

Functions of transforming growth factor-beta family type I receptors and Smad proteins in the hypertrophic maturation and osteoblastic differentiation of chondrocytes

We investigated the effects of bone morphogenetic protein (BMP)-2, a member of the transforming growth factor-beta superfamily, on the regulation of the chondrocyte phenotype, and we identified signaling molecules involved in this regulation. BMP-2 triggers three concomitant responses in mouse primary chondrocytes and chondrocytic MC615 cells. First, BMP-2 stimulates expression or synthesis of type II collagen. Second, BMP-2 induces expression of molecular markers characteristic of pre- and hypertrophic chondrocytes, such as Indian hedgehog, parathyroid hormone/parathyroid hormone-related peptide receptor, type X collagen, and alkaline phosphatase. Third, BMP-2 induces osteocalcin expression, a specific trait of osteoblasts. Constitutively active forms of transforming growth factor-beta family type I receptors and Smad proteins were overexpressed to address their role in this process. Activin receptor-like kinase (ALK)-1, ALK-2, ALK-3, and ALK-6 were able to reproduce the hypertrophic maturation of chondrocytes induced by BMP-2. In addition, ALK-2 mimicked further the osteoblastic differentiation of chondrocytes induced by BMP-2. In the presence of BMP-2, Smad1, Smad5, and Smad8 potentiated the hypertrophic maturation of chondrocytes, but failed to induce osteocalcin expression. Smad6 and Smad7 impaired chondrocytic expression and osteoblastic differentiation induced by BMP-2. Thus, our results indicate that Smad-mediated pathways are essential for the regulation of the different steps of chondrocyte and osteoblast differentiation and suggest that additional Smad-independent pathways might be activated by ALK-2.

Differential effects of osteogenic protein-1 (BMP-7) on gene expression of BMP and GDF family members during differentiation of the mouse MC615 chondrocyte cells

The mRNA expression patterns of several bone morphogenetic proteins (BMPs) and growth differentiation factors (GDFs) in long-term cultures of the clonal mouse chondrocyte cell line MC615 were examined. Distinct spatial and temporal patterns of expression of BMPs and GDFs were observed. The temporal orders of expression were correlated with those of several biochemical markers characteristic of chondrocytic cell differentiation. BMP-1, -2, -5, and -6 mRNA expression increased throughout the chondrogenic process and BMP-4 mRNA expression was not changed. GDF-1 and -3 mRNA expression increased throughout the chondrogenic process, and GDF-5, -6, -8, and -9 mRNA expressions were not changed. Effects of osteogenic protein-1 (OP-1, BMP-7) on the expression patterns of several other members of the BMP family and of the GDF family were also examined. OP-1 downregulated the BMP-1, -4, -5, and -6 mRNA expression by a maximal 3-, 5-, 2.5-, and 3-fold, respectively. The BMP-2 mRNA expression was not changed significantly by a low concentration of OP-1, but was increased at 200 ng/ml at day 7 of treatment. In contrast to the BMPs, OP-1 upregulated significantly the six GDF members examined (GDF-1, -3, -5, -6, -8, and -9) by three- to four-fold. Our findings demonstrate that OP-1 differentially regulates the mRNA expression of several related members of the BMP family and upregulates the mRNA expression of several members of the GDF family. The observations suggest that OP-1 action on cartilage differentiation involves a complex regulation of gene expression of several members of the BMP and the GDF family.

Chondrocyte phenotype and cell survival are regulated by culture conditions and by specific cytokines through the expression of Sox-9 transcription factor

OBJECTIVE

To investigate the effects of culture conditions, serum and specific cytokines such as insulin-like growth factor (IGF) 1 and interleukin (IL) 1alpha on phenotype and cell survival in cultures of Syrian hamster embryonic chondrocyte-like cells (DES4(+).2).

METHODS

Proteins and RNA extracted from subconfluent and confluent early- and late-passage DES4(+).2 cells cultured in the presence or absence of serum and IL-1alpha or IGF-1 or both cytokines together were analysed for the expression of chondrocyte-specific genes and for the chondrogenic transcription factor Sox-9 by Western and Northern blotting. Apoptosis was assessed by agarose gel electrophoresis of labelled low-molecular weight DNA extracted from DES4(+).2 cells and another Syrian hamster embryonic chondrocyte-like cell line, 10W(+).1, cultured under the different conditions and treatments.

RESULTS

Early passage DES4(+).2 cells expressed chondrocyte-specific molecules such as collagen types alpha1(II) and alpha1(IX), aggrecan, biglycan and link protein and collagen types alpha1(I) and alpha1(X) mRNAs, suggesting a prehypertrophic chondrocyte-like phenotype. The expression of all genes investigated was cell density- and serum-dependent and was low to undetectable in cell populations from later passages. Early-passage DES4(+).2 and 10W(+).1 cells survived when cultured at low cell density, but died by apoptosis when cultured at high cell density in the absence of serum or IGF-1. IGF-1 and IL-1alpha had opposite and antagonistic effects on the chondrocyte phenotype and survival. Whereas IL-1alpha acting alone suppressed cartilage-specific gene expression without significantly affecting cell survival, IGF-1 increased the steady-state mRNA levels and relieved the IL-1alpha-induced suppression of all the chondrocyte-specific genes investigated; it also enhanced chondrocyte survival. Suppression of the chondrocyte phenotype by the inflammatory cytokine IL-1alpha correlated with marked down-regulation of the transcription factor Sox-9, which was relieved by IGF-1. The expression of the Sox9 gene was closely correlated with the expression of the chondrocyte-specific genes under all conditions and treatments.

CONCLUSIONS

The results suggest that the effects of cartilage anabolic and catabolic cytokines IGF-1 and IL-1alpha on the expression of the chondrocyte phenotype are mediated by Sox-9. As Sox-9 appears to be essential for matrix production, the potent effect of IL-1alpha in suppressing Sox-9 expression may limit the ability of cartilage to repair during inflammatory joint diseases.

Establishment and characterization of chondrocyte cell lines from the costal cartilage of SV40 large T antigen transgenic mice

Complete understanding of the physiology and pathology of the cartilage is essential to establish treatments for a variety of cartilage disorders and defects such as rheumatoid arthritis, congenital malformations, and tumors of cartilage. Although synthetic materials have been used in many cases, they possess inherent problems including wear of the materials and low mechanical strength. Autograft has been considered very effective to overcome these problems. However, the limitation of the transplant volume is a major problem in autograft to be overcome. The costal cartilage is the most serious candidate for donor site transplantation, since it is the largest permanent hyaline cartilage in the body. To investigate the possibility using the costal cartilage as a transplant source, we have established and characterized three mouse chondrocyte cell lines (MCC-2, MCC-5, and MCC-35) derived from the costal cartilage of 8-week-old male SV40 large T-antigen transgenic mice. At confluence, all the cell lines formed nodules that could be positively stained with alcian blue (pH 2.5). The size of nodules gradually increased during culturing time. After 2 and 6 weeks of culture, RT-PCR analysis demonstrated that all three cell lines expressed mRNA from the cartilage-specific genes for type II collagen, type XI collagen, aggrecan, and link protein. Furthermore, type X collagen expression was detected in MCC-5 and MCC-35 but not in MCC-2. Any phenotypic changes were not observed over 31 cell divisions. Immunocytochemistry showed further that MCC-2, MCC-5, and MCC-35 produced cartilage-specific proteins type II collagen and type XI collagen, while in addition MCC-5 and MCC-35 produced type X collagen. Treatment with 1alpha, 25-dihydroxyvitamin D(3) inhibited cell proliferation and differentiation of the three cell lines in a dose-dependent manner. These phenotypic characteristics have been found consistent with chondrocyte cell lines established from cartilage tissues other than costal cartilage. In conclusion, costal cartilage shows phenotypic similarities to other cartilages, i.e., articular cartilage and embryonic limbs, suggesting that costal cartilage may be very useful as the donor transplantation site for the treatment of cartilage disorders. Furthermore, the cell lines established in this study are also beneficial in basic research of cartilage physiology and pathology.

Matrix metalloproteinase-1, -3, -13 and aggrecanase-1 and -2 are differentially expressed in experimental osteoarthritis

The aim of this study was to characterize the cellular phenotypes of articular cartilage and meniscus in rabbits with experimentally induced osteoarthritis (OA), by histological and molecular biological techniques. OA was induced by severing the anterior cruciate ligament of the knee and rabbits were killed 2, 4 or 9 weeks following surgery. Our histological observations show a progressive destruction of extracellular matrix in both tissues. To determine whether these morphological changes could be related to alterations in the regulation of gene expression for a subset of relevant molecules, levels of mRNA for proteinases and one inhibitor (MMP-1, -3 and -13, aggrecanase-1 and -2 and TIMP-1), matrix molecules and one chaperone (type II and X collagens, aggrecan, osteonectin, betaig-h3 and BiP) were assessed by reverse transcription-polymerase chain reaction. Our results indicate that for most markers expression profiles were similar in both tissues. In particular, matrix protein gene expression remained stable or varied little during progression of OA, suggesting a poor repair capacity of the tissues. MMP gene expression increased rapidly whereas aggrecanase gene expression remained stable. These findings suggest that differential regulation of mRNA levels of MMP-1, -3 and -13 on the one hand and aggrecanase-1 and -2 on the other, occurs during OA.

Immortalized human adult articular chondrocytes maintain cartilage-specific phenotype and responses to interleukin-1beta

OBJECTIVE

To develop a reproducible immortalized human chondrocyte culture model for studying the regulation of chondrocyte functions relevant to arthritic diseases in adult humans.

METHODS

Primary adult articular chondrocytes were immortalized with a retrovirus expressing a temperature-sensitive mutant of SV40-large T antigen (tsTAg). The established tsT/AC62 chondrocyte cell line was examined in monolayer and alginate culture systems. The levels of messenger RNA (mRNA) encoding cartilage matrix proteins and interleukin-1beta (IL-1beta)-inducible mRNA were analyzed by reverse transcriptase-polymerase chain reaction. Matrix protein synthesis was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of 35S-sulfate-labeled proteoglycans and Western blotting of type II collagen and aggrecan. Type II collagen (COL2A1)-luciferase reporter gene expression was analyzed by transient transfection. Phosphorylated stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK), p38 mitogen-activated protein kinase (p38 MAPK), and activating transcription factor 2 (ATF-2) were detected by Western blotting.

RESULTS

The tsT/AC62 cells expressed TAg at the permissive temperature (32degrees C), and the loss of TAg at 37 degrees C and 39 degrees C correlated with decreased cell proliferation. Cells in alginate culture deposited abundant alcian blue-stainable matrix and continued to proliferate at 32 degrees C. Preferential retention of aggrecan was observed in the cell-associated matrix, while biglycan and decorin were secreted into the medium of monolayer and alginate cultures. The levels of COL2A1 and aggrecan mRNA were increased after transfer from monolayer to alginate culture at 32 degrees C. Treatment with IL-1beta decreased COL2A1 and aggrecan mRNA levels and increased the levels of matrix metalloproteinases 1, 3, and 13 mRNA, as well as those of cyclooxygenase 2, type I collagen, and secretory phospholipase A2 type IIA mRNA, but not those of inducible nitric oxide synthase mRNA. IL-1beta also stimulated phosphorylation of p38 MAPK, SAPK/JNK, and ATF-2. The p38 MAPK-selective inhibitor, SB203580, partially reversed IL-1beta-induced inhibition of COL2A1 mRNA levels and COL2A1-luciferase reporter gene expression.

CONCLUSION

The tsT/AC62 cells provide a reproducible model that mimics the adult articular chondrocyte phenotype, particularly in alginate culture, and demonstrates characteristic responses to IL-1beta. These studies also show, for the first time, that p38 MAPK is one of the signals required for IL-1beta-induced inhibition of COL2A1 gene expression. Availability of this model will permit identification of signals that regulate cytokine responses, and will also provide rational strategies for targeting these pathways.

Analysis of transcription of the Col6a1 gene in a specific set of tissues suggests a new variant of enhancer region

The region extending from -5.4 to -3.9 kilobase pairs from the transcription start site of the Col6a1 gene has been previously shown to contain sequences activating tissue-specific transcription in articular cartilage, intervertebral disks, subepidermal, and vibrissae mesenchyme and peripheral nervous system (Braghetta, P., Fabbro, C., Piccolo, S., Marvulli, D., Bonaldo, P., Volpin, D., and Bressan, G. M. (1996) J. Cell Biol. 135, 1163-1177). Analysis of expression of deletions of this region in transgenic mice has identified the 383-base pair fragment E-L as the most active sequence of the region. Linker-scanning mutagenesis analysis of segment E-J, which spans the 5' 245 base pairs of E-L and is sufficient for high frequency expression in articular cartilage, showed that all the mutations reduced transcription considerably, suggesting that the integrity of the entire cluster of elements is necessary for enhancer activity. Electrophoretic mobility shift assays with nuclear extracts derived from various sources showed that fragment E-J binds numerous transcription factors (at least 22). These factors are present in most cells, expressing and nonexpressing alpha1(VI) collagen mRNA, but in different relative proportions, and none of them appears to be cell type-specific. Several lines of evidence indicate that sequence elements of the enhancer may have different functional roles in various cells. The data configure the -5.4/-3.9 region of the Col6a1 gene as a new type of tissue-specific enhancer, characterized by a variety of tissues supporting its activation and by the dependence of its function only on ubiquitous transcription factors. This type of enhancer is postulated to be particularly important for genes such as those of the extracellular matrix, which are often expressed with broad tissue specificity.

Potent inhibition of the master chondrogenic factor Sox9 gene by interleukin-1 and tumor necrosis factor-alpha

The inflammatory cytokines interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha) strongly inhibit the expression of genes for cartilage extracellular matrix proteins. We have recently obtained genetic evidence indicating that the high mobility group domain containing transcription factor Sox9 is required for cartilage formation and for expression of chondrocyte-specific genes including the gene for type II collagen (Col2a1). We show here that IL-1 and TNF-alpha cause a marked and rapid decrease in the levels of Sox9 mRNA and/or protein in chondrocytes. A role for the transcription factor NFkappaB in Sox9 down-regulation was suggested by the ability of pyrrolidine dithiocarbamate, an inhibitor of the NFkappaB pathway, to block the effects of IL-1 and TNF-alpha. This role was further supported by the ability of a dominant-negative mutant of IkappaBalpha to block the IL-1 and TNF-alpha inhibition of Sox9-dependent Col2a1 enhancer elements. Furthermore, forced expression of the NFkappaB subunits p65 or p50 also inhibited Sox9-dependent Col2a1 enhancer. Because Sox9 is essential for chondrogenesis, the marked down-regulation of the Sox9 gene by IL-1 and TNF-alpha in chondrocytes is sufficient to account for the inhibition of the chondrocyte phenotype by these cytokines. The down-regulation of Sox9 may have a crucial role in inhibiting expression of the cartilage phenotype in inflammatory joint diseases.

Lineage-dependent collagen expression and assembly during osteogenic or chondrogenic differentiation of a mesoblastic cell line

The mesoblastic clone, C1, behaves as a tripotential progenitor able to self-renew and to differentiate toward osteogenesis, chondrogenesis, or adipogenesis in response to specific inducers. In this study, expression and deposition by the C1 cells of essential components of the extracellular matrix, collagens type I, II, III, V, XI, VI, IX, and X were followed along the osteogenic and chondrogenic pathways, through biochemical, immunochemical, and electron microscopy analyses. Implementation of each program involves profiles of collagen synthesis and matrix assembly close to those documented in vivo. Depending on the applied inducers, cells adopt a defined identity and, controls acting at transcriptional and posttranslational levels adapt the set of deposited collagens to one particular cell fate. Osteogenic C1 cells selectively build a type I collagen matrix also containing type III, V, and XI collagens but selectively exclude type II collagen. Chondrogenic C1 cells first elaborate a type II collagen network and then acquire hypertrophic chondrocyte properties while assembling a type X collagen matrix as in the growth plate. This study provides an example of how a mesoblastic cell line can develop, in vitro, each of its genetic programs up to terminal differentiation. Intrinsic factors and time-dependent cell-matrix interactions might, as in vivo, underline the implementation of an entire differentiation program.

Isolation, cloning, and expression of a new murine zinc finger encoding gene

With the aim of identifying genes involved in cartilage differentiation, we have used a subtractive hybridization strategy with cDNAs from a chondrocytic cell line (MC615) and mRNAs from a mesenchymal precursor cell line (10T1/2). We have isolated a cDNA clone representing a novel mouse gene. The predicted 368-amino acid protein, designated ZF-12, contains four C(2)H(2)-type zinc finger motifs and one region homologous to the LeR domain, a finger-associated structural domain. ZF-12 mRNAs are expressed during embryonic development and in different organs in adult, including rib cartilage. These data suggest that ZF-12 might play an important role not only in cartilage differentiation, but also in basic cellular processes.

Different effects of bone morphogenetic proteins 2, 4, 12, and 13 on the expression of cartilage and bone markers in the MC615 chondrocyte cell line

In order to study the lineage leading to chondrocyte and osteoblast phenotype in vertebrate development, we examined the effect of recombinant human bone morphogenetic protein (BMP)-2, BMP-4, BMP-12 [or growth and differentiation factor (GDF)-7], and BMP-13 (or GDF-6) on the phenotypic expression of the mouse chondrocyte cell line MC615, grown for 1 or 2 weeks in monolayer. Protein synthesis rates were monitored after incubation with [(14)C]proline. BMP-2 and BMP-4 increased protein synthesis, in agreement with our observation by phase-contrast microscopy of a highly refractile matrix around MC615 cells treated with BMP-2 and -4. Markers of the chondrocytic and osteoblastic differentiation were analyzed at mRNA level. Expression of the type II collagen gene, a marker of the cartilage phenotype, was up-regulated in the presence of low concentration of BMP-2 or -4 (50 ng/ml) and down-regulated at higher concentrations (100-400 ng/ml). In parallel, this expression was stable in the presence of BMP-12 or -13 in the dose range tested (50-400 ng/ml). Expression of the matrix Gla protein (MGP) gene, another marker of cartilage, was also reduced in the presence of 100 ng/ml BMP-2 or -4, while it remained stable in the presence of BMP-12 or -13 at the same concentration. In contrast, expression of the bone Gla protein (BGP) gene, or osteocalcin, a marker of the bone phenotype, was induced when the cells were treated with BMP-2 or -4 but was not detected when the cells were treated with BMP-12 or -13. At the same time, BMP-2 or -4 markedly up-regulated expression of type X collagen mRNA, indicating that MC615 cells possess the ability to express traits associated with endochondral ossification, when exposed to specific BMPs. Furthermore, detailed analysis of type II collagen expression showed that the alternatively spliced transcript collagen IIB, specific for cartilage, is expressed concomitantly with BGP. Therefore, MC615 chondrocytes can simultaneously express chondrocytic and osteoblastic markers, in response to BMP-2 or -4, but show minimal response to BMP-12 (or GDF-7) or to BMP-13 (or GDF-6). These results raise the possibility that chondrocytes in vivo can express osteoblastic properties, provided they are induced by BMP-2 or -4.

SV40 large T antigen expression driven by col2a1 regulatory sequences immortalizes articular chondrocytes but does not allow stabilization of type II collagen expression

Immortalization of chondrocytes by SV40 T Ag has often been reported to trigger the loss of expression of type II collagen, one of the main differentiation markers, although some immortalized chondrocyte lines maintaining a differentiated phenotype have also been described. Here, we show using transient cotransfections in differentiated chondrocytes that, in contrast to c-src, neither SV40 T Ag, nor c-myc, decreases col2a1 transcriptional activity. Then, we report the possibility of immortalizing rabbit articular chondrocytes by expression of SV40 T Ag controlled by the col2a1 promoter and enhancer (pCol2SV). This strategy allows one to select within a population of differentiated chondrocytes those which are able to maintain functional regulation of the col2a1 gene through long-term culture. In precrisis pCol2SV-transfected chondrocytes, all-trans-retinoic acid, a down-regulator of col2a1 expression, induced apoptosis, strongly suggesting the strict control of T Ag expression by col2a1 regulatory sequences. Some pCol2SV-transfected chondrocytes were definitively immortalized, after a short crisis period. However, type II collagen synthesis was restricted to a small proportion of cells, which went on to decrease with subculture, while the proportion of cells expressing T Ag was not affected. In these postcrisis cells, T Ag remained at least partially under the control of functional col2a1 regulatory elements as assessed by all-trans-retinoic acid down-regulation.

A new long form of Sox5 (L-Sox5), Sox6 and Sox9 are coexpressed in chondrogenesis and cooperatively activate the type II collagen gene

Transcripts for a new form of Sox5, called L-Sox5, and Sox6 are coexpressed with Sox9 in all chondrogenic sites of mouse embryos. A coiled-coil domain located in the N-terminal part of L-Sox5, and absent in Sox5, showed >90% identity with a similar domain in Sox6 and mediated homodimerization and heterodimerization with Sox6. Dimerization of L-Sox5/Sox6 greatly increased efficiency of binding of the two Sox proteins to DNA containing adjacent HMG sites. L-Sox5, Sox6 and Sox9 cooperatively activated expression of the chondrocyte differentiation marker Col2a1 in 10T1/2 and MC615 cells. A 48 bp chondrocyte-specific enhancer in this gene, which contains several HMG-like sites that are necessary for enhancer activity, bound the three Sox proteins and was cooperatively activated by the three Sox proteins in non-chondrogenic cells. Our data suggest that L-Sox5/Sox6 and Sox9, which belong to two different classes of Sox transcription factors, cooperate with each other in expression of Col2a1 and possibly other genes of the chondrocytic program.

Dedifferentiated chondrocytes cultured in alginate beads: restoration of the differentiated phenotype and of the metabolic responses to interleukin-1beta

Chondrocytes cultivated in monolayer rapidly divide and lose their morphological and biochemical characteristics, whereas they maintain their phenotype for long periods of time when they are cultivated in alginate beads. Because cartilage has a low cellularity and is difficult to obtain in large quantities, the number of available cells often becomes a limiting factor in studies of chondrocyte biology. Therefore, we explored the possibility of restoring the differentiated properties of chondrocytes by cultivating them in alginate beads after two multiplication passages in monolayer. This resulted in the reexpression of the two main markers of differentiated chondrocytes: Aggrecan and type II collagen gene expression was strongly reinduced from day 4 after alginate inclusion and paralleled protein expression. However, 2 weeks were necessary for total suppression of type I and III collagen synthesis, indicators of a modulated phenotype. Interleukin-1beta, a cytokine that is present in the synovial fluid of rheumatoid arthritis patients, induces many metabolic changes on the chondrocyte biology. Compared with cells in primary culture, the production of nitric oxide and 92-kDa gelatinase in response to interleukin-1beta was impaired in cells at passage 2 in monolayer but was fully recovered after their culture in alginate beads for 2 weeks. This suggests that the effects of interleukin-1beta on cartilage depend on the differentiation state of chondrocytes. This makes the culture in alginate beads a relevant model for the study of chondrocyte biology in the presence of interleukin-1beta and other mediators of cartilage destruction in rheumatoid arthritis and osteoarthrosis.

Cytokine inducible matrix metalloproteinase expression in immortalized rat chondrocytes is independent of nitric oxide stimulation

The objective of this study was to determine if an immortalized mammalian chondrocyte cell line had a profile of matrix metalloproteinase (MMP) expression that was consistent with what has been reported for primary chondrocytes in vitro and in vivo. A combination of zymography, Western, and Northern analysis was used to examine the expression of MMPs that are relevant to cartilage degradation. Both interleukin-1beta and tumor necrosis factor alpha induced a 4- to 9-fold increase in the level of MMP-9 expression in conditioned media, and a 17- to 24-fold increase in MMP-3 mRNA. Other compounds such as basic fibroblast growth factor and staurosporine each increased MMP-9 expression individually and potentiated the effects of the two cytokines. Transforming growth factor beta had no positive or inhibitory effects. N-methyl arginine blocked the increase in nitric oxide observed following treatment with the cytokines but did not prevent the increased expression of MMPs. The pattern of metalloproteinase expression observed in IRC cells and the response to cytokines is very similar to what has been reported during the pathogenesis of osteoarthritis. The IRC cells should be useful as a model system to study basic mechanisms controlling chondrocyte MMP expression and to identify pharmacological modulators of this process.

Immortalized, cloned mouse chondrocytic cells (MC615) produce three different matrix proteoglycans with core-protein-specific chondroitin/dermatan sulphate structures

Cloned immortalized MC615 mouse chondrocytic cells were used to examine their capability to produce multiple types of matrix proteoglycans. Immunofluorescence staining indicated a uniform expression of aggrecan, biglycan and decorin by all cells. After culture with [35S]sulphate, proteo[35S]glycans secreted by the cells were found to elute in two peaks from a Sepharose CL-4B column. The first peak, at the void volume of the column, contained a large proteoglycan with an estimated average hydrodynamic mass of 10(3) kDa. The glycosaminoglycan chains of this proteoglycan had an average hydrodynamic size of 17 kDa, estimated by Sepharose CL-6B chromatography, indicating the presence of 30-70 glycosaminoglycan chains per core protein, which was consistent with the characteristics of aggrecan. Biglycan and decorin were immunoisolated from the second Sepharose CL-4B peak, and had average glycosaminoglycan hydrodynamic sizes of approx. 25 kDa and 32 kDa respectively. Glycosaminoglycan chains of the aggrecan, biglycan and decorin were treated with chondroitin ABC lyase, chondroitin AC lyase and chondroitin B lyase to determine the positions of sulphation and the degree of uronic acid epimerization. The aggrecan glycosaminoglycan chains were found to contain a 4-sulphate/6-sulphate ratio of 7:3, with no epimerization of glucuronic acid to iduronic acid. The biglycan glycosaminoglycan chains were found to contain a similar ratio of 4-sulphate/6-sulphate, but with approx. 40-45% of the glucuronic acid epimerized to iduronic acid. The decorin glycosaminoglycan chains were found to contain 4-sulphate but no detectable 6-sulphate, and approx. 30-35% epimerization of the glucuronic acid to iduronic acid. The results, using these cloned cells, indicated that a single MC615 cell is able to make all three proteoglycans with distinctive differences between the glycosaminoglycans of aggrecan, biglycan and decorin. These data indicate that a mechanism must exist for a single MC615 cell to regulate the sizes and fine structures of glycosaminoglycans on simultaneously produced, different proteoglycans in a core-protein-specific manner.