Expression and role of c-myc in chondrocytes undergoing endochondral ossification

Direct Reprogramming of Mouse Subchondral Bone Osteoblasts into Chondrocyte-like Cells

Treatment of full-thickness articular cartilage defects with exposure of subchondral bone often seen in osteoarthritic conditions has long been a great challenge, especially with a focus on the feasibility of in situ cartilage regeneration through minimally invasive procedures. Osteoblasts that situate in the subchondral bone plate may be considered a potentially vital endogenous source of cells for cartilage resurfacing through direct reprogramming into chondrocytes. Microarray-based gene expression profiles were generated to compare tissue-specific transcripts between subchondral bone and cartilage of mice and to assess age-dependent differences of chondrocytes as well. On osteoblast cell lines established from mouse proximal tibial subchondral bone, sequential screening by co-transduction of transcription factor (TF) genes that distinguish chondrocytes from osteoblasts reveals a shortlist of potential reprogramming factors exhibiting combined effects in inducing chondrogenesis of subchondral bone osteoblasts. A further combinatorial approach unexpectedly identified two 3-TF combinations containing Sox9 and Sox5 that exhibit differences in reprogramming propensity with the third TF c-Myc or Plagl1, which appeared to direct the converted chondrocytes toward either a superficial or a deeper zone phenotype. Thus, our approach demonstrates the possibility of converting osteoblasts into two major chondrocyte subpopulations with two combinations of three genes (Sox9, Sox5, and c-Myc or Plagl1). The findings may have important implications for developing novel in situ regeneration strategies for the reconstruction of full-thickness cartilage defects.

Mechanism of Action of Mesenchymal Stem Cell-Derived Exosomes in the Intervertebral Disc Degeneration Treatment and Bone Repair and Regeneration

Exosomes are extracellular vesicles formed by various donor cells that regulate gene expression and cellular function in recipient cells. Exosomes derived from mesenchymal stem cells (MSC-Exos) perform the regulatory function of stem cells by transporting proteins, nucleic acids, and lipids. Intervertebral disc degeneration (IDD) is one of the main causes of low back pain, and it is characterized by a decreased number of nucleus pulposus cells, extracellular matrix decomposition, aging of the annulus fibrosus, and cartilage endplate calcification. Besides, nutrient transport and structural repair of intervertebral discs depend on bone and cartilage and are closely related to the state of the bone. Trauma, disease and aging can all cause bone injury. However, there is a lack of effective drugs against IDD and bone injury. Recent MSC-Exos fine tuning has led to significant progress in the IDD treatment and bone repair and regeneration. In this review, we looked at the uniqueness of MSC-Exos, and the potential treatment mechanisms of MSC-Exos with respect to IDD, bone defects and injuries.

Epigenetic Mechanisms Mediating Cell State Transitions in Chondrocytes

Epigenetic modifications play critical roles in regulating cell lineage differentiation, but the epigenetic mechanisms guiding specific differentiation steps within a cell lineage have rarely been investigated. To decipher such mechanisms, we used the defined transition from proliferating (PC) into hypertrophic chondrocytes (HC) during endochondral ossification as a model. We established a map of activating and repressive histone modifications for each cell type. ChromHMM state transition analysis and Pareto-based integration of differential levels of mRNA and epigenetic marks revealed that differentiation-associated gene repression is initiated by the addition of H3K27me3 to promoters still carrying substantial levels of activating marks. Moreover, the integrative analysis identified genes specifically expressed in cells undergoing the transition into hypertrophy. Investigation of enhancer profiles detected surprising differences in enhancer number, location, and transcription factor binding sites between the two closely related cell types. Furthermore, cell type-specific upregulation of gene expression was associated with increased numbers of H3K27ac peaks. Pathway analysis identified PC-specific enhancers associated with chondrogenic genes, whereas HC-specific enhancers mainly control metabolic pathways linking epigenetic signature to biological functions. Since HC-specific enhancers show a higher conservation in postnatal tissues, the switch to metabolic pathways seems to be a hallmark of differentiated tissues. Surprisingly, the analysis of H3K27ac levels at super-enhancers revealed a rapid adaption of H3K27ac occupancy to changes in gene expression, supporting the importance of enhancer modulation for acute alterations in gene expression. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Kartogenin enhances the therapeutic effect of bone marrow mesenchymal stem cells derived exosomes in cartilage repair

The effectiveness of mesenchymal stem cells (MSC) in the treatment of cartilage diseases has been demonstrated to be attributed to the paracrine mechanisms, especially the mediation of exosomes. But the exosomes derived from unsynchronized MSCs may be nonhomogeneous and the therapeutic effect varies between samples. Aim: To produce homogeneous and more effective exosomes for the regeneration of cartilage. Materials & methods: In this study we produced specific exosomes from bone marrow MSCs (BMSC) through kartogenin (KGN) preconditioning and investigated their performance in either in vitro or in vivo experiments. Results & conclusion: The exosomes derived from KGN-preconditioned BMSCs (KGN-BMSC-Exos) performed more effectively than the exosomes derived from BMSCs (BMSC-Exos). KGN preconditioning endowed BMSC-Exos with stronger chondral matrix formation and less degradation.

Direct conversion of pig fibroblasts to chondrocyte-like cells by c-Myc

Unexpectedly, we found that c-Myc-expressing porcine embryonic fibroblasts (PEFs) subcutaneously implanted into nude mice formed cartilage-like tissues in vivo, while previous studies revealed the direct conversion of mouse and human somatic cells into chondrocytes by the combined use of several defined factors, including c-Myc, which prompted us to explore whether PEFs can be reprogrammed to become pig induced chondrocyte-like cells (piCLCs) via ectopic expression of c-Myc alone. In this study, c-Myc-expressing PEFs, designated piCLCs, which exhibited a significantly enhanced proliferation ability in vitro, displayed a chondrogenic phenotypes in vitro, as shown by the cell morphology, toluidine blue staining, alcian blue staining and chondrocyte marker gene expression. Additionally, piCLCs with a polygonal chondrocyte-like morphology were readily and efficiently converted from PEFs by enforced c-Myc expression within 10 days, while piCLCs maintained the chondrocytic phenotype and normal karyotype during long-term subculture. piCLC-derived single clones with a chondrogenic phenotype in vitro exhibited homogeneity in cell morphology and staining intensity compared with mixed piCLCs. Although the mixtures of cartilaginous tissues and tumorous tissues accounted for ~12% (6/51) of all xenografts (51), piCLCs generated stable, homogenous, hyaline cartilage-like tissues without tumour formation at 45 out of the 51 injected sites when subcutaneously injected into nude mice. The hyaline cartilage-like tissues remained for at least 16 weeks. Taken together, these findings demonstrate for the first time the direct induction of chondrocyte-like cells from PEFs with only c-Myc.

Drug-induced modulation of gp130 signalling prevents articular cartilage degeneration and promotes repair

OBJECTIVE

Human adult articular cartilage (AC) has little capacity for repair, and joint surface injuries often result in osteoarthritis (OA), characterised by loss of matrix, hypertrophy and chondrocyte apoptosis. Inflammation mediated by interleukin (IL)-6 family cytokines has been identified as a critical driver of proarthritic changes in mouse and human joints, resulting in a feed-forward process driving expression of matrix degrading enzymes and IL-6 itself. Here we show that signalling through glycoprotein 130 (gp130), the common receptor for IL-6 family cytokines, can have both context-specific and cytokine-specific effects on articular chondrocytes and that a small molecule gp130 modulator can bias signalling towards anti-inflammatory and antidegenerative outputs.

METHODS

High throughput screening of 170 000 compounds identified a small molecule gp130 modulator termed regulator of cartilage growth and differentiation (RCGD 423) that promotes atypical homodimeric signalling in the absence of cytokine ligands, driving transient increases in MYC and pSTAT3 while suppressing oncostatin M- and IL-6-mediated activation of ERK and NF-κB via direct competition for gp130 occupancy.

RESULTS

This small molecule increased proliferation while reducing apoptosis and hypertrophic responses in adult chondrocytes in vitro. In a rat partial meniscectomy model, RCGD 423 greatly reduced chondrocyte hypertrophy, loss and degeneration while increasing chondrocyte proliferation beyond that observed in response to injury. Moreover, RCGD 423 improved cartilage healing in a rat full-thickness osteochondral defect model, increasing proliferation of mesenchymal cells in the defect and also inhibiting breakdown of cartilage matrix in de novo generated cartilage.

CONCLUSION

These results identify a novel strategy for AC remediation via small molecule-mediated modulation of gp130 signalling.

Endogenous cannabinoid anandamide impairs cell growth and induces apoptosis in chondrocytes

Endocannabinoids has been described to be involved in articular degenerative disease by modulating nociception and immune system. However, the role of the endocannabinoid anandamide on chondrocyte cell viability is still unclear. Therefore, we decided to study anandamide's effects on chondrocytes viability and to evaluate its interactions with the catabolic factor TNF (tumor necrosis factor). Chondrocyte vitality was evaluated by MTT assay. We investigated LDH release, chromatin condensation, cleavage of focal adhesion kinase (FAK), and caspases-3, 8, and 9 activation. c-MYC mRNA levels were determined by RT-PCR. We studied by Western blot the activation patterns of AKT, AMPK, ERK, p38, and JNK kinases. Finally, we evaluate the effect of anandamide in TNF-induced caspase-3 cleavage. Anandamide decreased chondrocyte vitality independently of its receptors. It induced AMPK activation without LDH release. Anandamide induced chromatin condensation, activation of caspase-3, 8, and 9, and FAK cleavage. Surprisingly, despite anandamide inhibited cell proliferation, it increased c-MYC expression. Moreover anandamide inhibited AKT activation, whilst it induced a sustained activation of ERK, JNK, and p38. Finally, anandamide synergized with TNF-α in the cleavage of caspase-3. In conclusion, our findings suggest that anandamide, alone or in combination with TNF-α, may be a potential destructive agent in cartilage.

GSK-3α and GSK-3β proteins are involved in early stages of chondrocyte differentiation with functional redundancy through RelA protein phosphorylation

Here we examine the roles of two isoforms of glycogen synthase kinase-3 (GSK-3), GSK-3α and GSK-3β, in skeletal development. Both isoforms were unphosphorylated and active in chondrocyte differentiation stages during SOX9 and type II collagen (COL2A1) expression. Although knock-out of both alleles of Gsk3a (Gsk3a(-/-)) or a single allele of Gsk3b (Gsk3b(+/-)) in mice did not significantly affect skeletal development, compound knock-out (Gsk3a(-/-);Gsk3b(+/-)) caused dwarfism with impairment of chondrocyte differentiation. GSK-3α and GSK-3β induced differentiation of cultured chondrocytes with functional redundancy in a cell-autonomous fashion, independently of the Wnt/β-catenin signal. Computational predictions followed by SOX9 and COL2A1 transcriptional assays identified RelA (NF-κB p65) as a key phosphorylation target of GSK-3. Among several phosphorylation residues in RelA, Thr-254 was identified as the critical phosphorylation site for GSK-3 that modulated chondrocyte differentiation. In conclusion, redundant functions of GSK-3α and GSK-3β through phosphorylation of RelA at Thr-254 play a crucial role in early stages of chondrocyte differentiation.

Sequential and coordinated actions of c-Myc and N-Myc control appendicular skeletal development

Background

During limb development, chondrocytes and osteoblasts emerge from condensations of limb bud mesenchyme. These cells then proliferate and differentiate in separate but adjacent compartments and function cooperatively to promote bone growth through the process of endochondral ossification. While many aspects of limb skeletal formation are understood, little is known about the mechanisms that link the development of undifferentiated limb bud mesenchyme with formation of the precartilaginous condensation and subsequent proliferative expansion of chondrocyte and osteoblast lineages. The aim of this study was to gain insight into these processes by examining the roles of c-Myc and N-Myc in morphogenesis of the limb skeleton.

Methodology/Principal Findings

To investigate c-Myc function in skeletal development, we characterized mice in which floxed c-Myc alleles were deleted in undifferentiated limb bud mesenchyme with Prx1-Cre, in chondro-osteoprogenitors with Sox9-Cre and in osteoblasts with Osx1-Cre. We show that c-Myc promotes the proliferative expansion of both chondrocytes and osteoblasts and as a consequence controls the process of endochondral growth and ossification and determines bone size. The control of proliferation by c-Myc was related to its effects on global gene transcription, as phosphorylation of the C-Terminal Domain (pCTD) of RNA Polymerase II, a marker of general transcription initiation, was tightly coupled to cell proliferation of growth plate chondrocytes where c-Myc is expressed and severely downregulated in the absence of c-Myc. Finally, we show that combined deletion of N-Myc and c-Myc in early limb bud mesenchyme gives rise to a severely hypoplastic limb skeleton that exhibits features characteristic of individual c-Myc and N-Myc mutants.

Conclusions/Significance

Our results show that N-Myc and c-Myc act sequentially during limb development to coordinate the expansion of key progenitor populations responsible for forming the limb skeleton.

Transcription factor ERG and joint and articular cartilage formation during mouse limb and spine skeletogenesis

Articular cartilage and synovial joints are critical for skeletal function, but the mechanisms regulating their development are largely unknown. In previous studies we found that the ets transcription factor ERG and its alternatively-spliced variant C-1-1 have roles in joint formation in chick. Here, we extended our studies to mouse. We found that ERG is also expressed in developing mouse limb joints. To test regulation of ERG expression, beads coated with the joint master regulator protein GDF-5 were implanted close to incipient joints in mouse limb explants; this led to rapid and strong ectopic ERG expression. We cloned and characterized several mammalian ERG variants and expressed a human C-1-1 counterpart (hERG3Delta81) throughout the cartilaginous skeleton of transgenic mice, using Col2a1 gene promoter/enhancer sequences. The skeletal phenotype was severe and neonatal lethal, and the transgenic mice were smaller than wild type littermates and their skeletons were largely cartilaginous. Limb long bone anlagen were entirely composed of chondrocytes actively expressing collagen IX and aggrecan as well as articular markers such as tenascin-C. Typical growth plates were absent and there was very low expression of maturation and hypertrophy markers, including Indian hedgehog, collagen X and MMP-13. The results suggest that ERG is part of molecular mechanisms leading chondrocytes into a permanent developmental path and become joint forming cells, and may do so by acting downstream of GDF-5.

Proteoglycan production by immortalized human chondrocyte cell lines cultured under conditions that promote expression of the differentiated phenotype

Large and small proteoglycans are essential components of articular cartilage. How to induce chondrocytes to repair damaged cartilage with normal ratios of matrix components after their loss due to degenerative joint disease has been a major research focus. We have developed immortalized human chondrocyte cell lines for examining the regulation of cartilage-specific matrix gene expression. However, the decreased synthesis and deposition of cartilage matrix associated with a rapid rate of proliferation has presented difficulties for further examination at the protein level. In these studies, proteoglycan synthesis was characterized in two chondrocyte cell lines, T/C-28a2 and tsT/AC62, derived, respectively, from juvenile costal and adult articular cartilage, under culture conditions that either promoted or decreased cell proliferation. Analysis of proteo[36S]glycans by Sepharose CL-4B chromatography and SDS-PAGE showed that the large proteoglycan aggrecan and the small, leucine-rich proteoglycans, decorin and biglycan, were produced under every culture condition studied. In monolayer cultures, a high initial cell density and conditions that promoted proliferation (presence of serum for T/C-28a2 cells or permissive temperature for the temperature-sensitive tsT/AC62 cells) favored cell survival and ratios of proteoglycans expected for differentiated chondrocytes. However, the tsT/AC62 cells produced more proteoglycans at the nonpermissive temperature. Culture of cells suspended in alginate resulted in a significant decrease in proteoglycan production in all culture conditions. While the tsT/AC62 cells continued to produce a larger amount of aggrecan than small proteoglycans, the T/C-28a2 cells lost the ability to produce significant amounts of aggrecan in alginate culture. In addition, our data indicate that immortalized chondrocytes may alter their ability to retain pericellular matrix under changing culture conditions, although the production of the individual matrix components does not change. These findings provide critical information that will assist in the development of a reproducible chondrocyte culture model for the study of regulation of proteoglycan biosynthesis in cartilage.

Bone morphogenetic protein (BMP) localization in developing human and rat growth plate, metaphysis, epiphysis, and articular cartilage

We assessed the distribution and relative staining intensity of bone morphogenetic protein (BMP)-1-7 by immunohistochemistry in tibial growth plates, epiphyses, metaphyses, and articular cartilage in one 21-week and one 22-week human fetus and in five 10-week-old Sprague-Dawley rats. In the rats, articular cartilage was also examined. BMP proteins were mostly cytoplasmic, with negligible matrix staining. Highest BMP levels were seen in (a) hypertrophic and calcifying zone chondrocytes of growth plate (BMP-1-7), (b) osteoblasts and/or osteoprogenitor fibroblasts and vascular cells of the metaphyseal cortex and medulla (BMP-1-6), (c) osteoclasts of the metaphysis and epiphysis (BMP-1,-4,-5, and -6), and (d) mid to deep zone articular chondrocytes of weanling rats (BMP-1-7). BMP staining in osteoclasts, an unexpected finding, was consistently strong with BMP-4, -5, and -6 but was variable and dependent on osteoclast location with BMP-2,-3, and -7. BMP-1-7 were moderately to intensely stained in vascular canals of human fetal epiphyseal cartilage by endothelial cells and pericytes. BMP-1,-3,-5,-6, and -7 were localized in hypertrophic chondrocytes adjacent to cartilage canals. We conclude that BMP expression is associated with maturing chondrocytes of growth plate and articular cartilage, and may play a role in chondrocyte differentiation and/or apoptosis. BMP appears to be expressed by osteoclasts and might be involved in the intercellular "cross-talk" between osteoclasts and neighboring osteoprogenitor cells at sites of bone remodeling.

Transcription factor ERG variants and functional diversification of chondrocytes during limb long bone development

During limb development, chondrocytes located at the epiphyseal tip of long bone models give rise to articular tissue, whereas the more numerous chondrocytes in the shaft undergo maturation, hypertrophy, and mineralization and are replaced by bone cells. It is not understood how chondrocytes follow these alternative pathways to distinct fates and functions. In this study we describe the cloning of C-1-1, a novel variant of the ets transcription factor ch-ERG. C-1-1 lacks a short 27-amino acid segment located approximately 80 amino acids upstream of the ets DNA binding domain. We found that in chick embryo long bone anlagen, C-1-1 expression characterizes developing articular chondrocytes, whereas ch-ERG expression is particularly prominent in prehypertrophic chondrocytes in the growth plate. To analyze the function of C-1-1 and ch-ERG, viral vectors were used to constitutively express each factor in developing chick leg buds and cultured chondrocytes. We found that virally driven expression of C-1-1 maintained chondrocytes in a stable and immature phenotype, blocked their maturation into hypertrophic cells, and prevented the replacement of cartilage with bone. It also induced synthesis of tenascin-C, an extracellular matrix protein that is a unique product of developing articular chondrocytes. In contrast, virally driven expression of ch-ERG significantly stimulated chondrocyte maturation in culture, as indicated by increases in alkaline phosphatase activity and deposition of a mineralized matrix; however, it had modest effects in vivo. The data show that C-1-1 and ch-ERG have diverse biological properties and distinct expression patterns during skeletogenesis, and are part of molecular mechanisms by which limb chondrocytes follow alternative developmental pathways. C-1-1 is the first transcription factor identified to date that appears to be instrumental in the genesis and function of epiphyseal articular chondrocytes.

Cbfa1 is a positive regulatory factor in chondrocyte maturation

Cbfa1 is a transcription factor that belongs to the runt domain gene family. Cbfa1-deficient mice showed a complete lack of bone formation due to the maturational arrest of osteoblasts, demonstrating that Cbfa1 is an essential factor for osteoblast differentiation. Further, chondrocyte maturation was severely disturbed in Cbfa1-deficient mice. In this study, we examined the possibility that Cbfa1 is also involved in the regulation of chondrocyte differentiation. mRNAs for both Cbfa1 isotypes, type I Cbfa1 (Pebp2alphaA/Cbfa1) and type II Cbfa1 (Osf2/Cbfa1 or til-1), which are different in N-terminal domain, were expressed in terminal hypertrophic chondrocytes as well as osteoblasts. In addition, mRNA for type I Cbfa1 was expressed in other hypertrophic chondrocytes and prehypertrophic chondropcytes. In a chondrogenic cell line, ATDC5, the expression of type I Cbfa1 was elevated prior to differentiation to the hypertrophic phenotype, which is characterized by type X collagen expression. Treatment with antisense oligonucleotides for type I Cbfa1 severely reduced type X collagen expression in ATDC5 cells. Retrovirally forced expression of either type I or type II Cbfa1 in chick immature chondrocytes induced type X collagen and MMP13 expression, alkaline phosphatase activity, and extensive cartilage-matrix mineralization. These results indicate that Cbfa1 is an important regulatory factor in chondrocyte maturation.

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 transient increase in c-myc precedes the transdifferentiation of hepatic stellate cells to myofibroblast-like cells

AIMS/BACKGROUND

Liver stellate cells are transdifferentiated to collagen-producing myofibroblast-like cells in vivo during liver injury or when placed in culture. The purpose of this study was to determine the presence of retinoids and the expression of the immediate early genes as they relate to the transdifferentiation of liver stellate cells in culture.

METHODS

Rat liver stellate cells were studied immediately after isolation or sequentially after culture for varying periods of time. RNA was isolated and specific messages were determined by RT-PCR. Cells were also isolated for determination of retinoid autofluorescence and immunofluorescent staining with specific antibodies by laser confocal microscopy.

RESULTS

c-fos message and immunoprotein were high in the freshly isolated cells prior to culture, while c-myc expression increased markedly after one day of culture. Both c-fos and c-myc gene expression decreased prior to the transdifferentiation of the cells to myofibroblast-like cells and to the increase in alpha 1(I) and alpha 2(I) collagen messages and collagen production. The presence of retinoid autofluorescence and retinoic acid receptor (RAR-alpha and RAR-beta) messages and RAR-beta immunoprotein persisted during initial transdifferentiation of the stellate cells.

CONCLUSIONS

This study shows a high initial level of c-fos expression and a transient increase in c-myc expression followed by a decrease to lower levels prior to transdifferentiation and collagen production by stellate cells. A total loss of retinoid autofluorescence or a decrease in RAR-alpha or RAR-beta are not required for initial transdifferentiation of stellate cells or collagen production.

Cell cycle genes in chondrocyte proliferation and differentiation

Coordinated proliferation and differentiation of growth plate chondrocytes controls longitudinal growth of endochondral bones. While many extracellular factors regulating these processes have been identified, much less is known about the intracellular mechanisms transducing and integrating these extracellular signals. Recent evidence suggests that cell cycle proteins play an important role in the coordination of chondrocyte proliferation and differentiation. Our current knowledge of the function and regulation of cell cycle proteins in endochondral ossification is summarized.

Bone morphogenetic protein signaling is required for maintenance of differentiated phenotype, control of proliferation, and hypertrophy in chondrocytes

To examine the role of bone morphogenetic protein (BMP) signaling in chondrocytes during endochondral ossification, the dominant negative (DN) forms of BMP receptors were introduced into immature and mature chondrocytes isolated from lower and upper portions of chick embryo sternum, respectively. We found that control sternal chondrocyte populations expressed type IA, IB, and II BMP receptors as well as BMP-4 and -7. Expression of a DN-type II BMP receptor (termed DN-BMPR-II) in immature lower sternal (LS) chondrocytes led to a loss of differentiated functions; compared with control cells, the DN-BMPR- II-expressing LS chondrocytes proliferated more rapidly, acquired a fibroblastic morphology, showed little expression of type II collagen and aggrecan genes, and upregulated type I collagen gene expression. Expression of DN-BMPR-II in mature hypertrophic upper sternal (US) chondrocytes caused similar effects. In addition, the DN-BMPR-II-expressing US cells exhibited little alkaline phosphatase activity and type X collagen gene expression, while the control US cells produced both alkaline phosphatase and type X collagen. Both DN-BMPR-II-expressing US and LS chondrocytes failed to respond to treatment with BMP-2 . When we examined the effects of DN forms of types IA and IB BMP receptors, we found that DN-BMPR-IA had little effect, while DN-BMPR-IB had similar but weaker effects compared with those of DN-BMPR-II. We conclude that BMP signaling, particularly that mediated by the type II BMP receptor, is required for maintenance of the differentiated phenotype, control of cell proliferation, and expression of hypertrophic phenotype.

Chondrocytes are regulated by cellular adhesion through CD44 and hyaluronic acid pathway

The articular cartilage consists of resident chondrocytes embedded within the extracellular matrix which contains several components such as collagen and hyaluronic acids (HA). CD44 is a major cell surface receptor for HA and is homologous to cartilage-link proteins. Although CD44 is present in cartilage, it is not clear if chondrocytes adhere to HA through CD44 or whether such adhesion changes the function of chondrocytes. We studied the molecular mechanisms of CD44-related chondrocyte adhesion to HA and the effects of such adhesion on chondrocyte function. Experiments were performed using the human chondrosarcoma-derived chondrocyte-like cell line HCS-2/8. Our results showed that (a) HCS-2/8 cells highly expressed CD44; (b) HCS-2/8 cells efficiently adhered to HA without any stimuli; (c) monoclonal antibody (mAb)-blocking studies indicated that adhesion of HCS-2/8 cells to HA was mainly mediated by the CD44/HA pathway; (d) cellular adhesion to HA increased the proliferation of HCS-2/8 cells, independent of transforming growth factor-beta (TGF-beta), but this was inhibited by CD44 mAb; (e) the adhesion of chondrocytes to HA also induced c-myc mRNA expression and this was also inhibited by CD44 mAb; and (f) the adhesion of cells to HA augmented TGF-beta mRNA expression, a process also reduced by CD44 mAb. Thus, HCS-2/8 cells effectively adhered to HA through cell surface CD44. The adhesion was also involved in cellular signaling which induced cellular proliferation and expression of c-myc mRNA as well as TGF-beta mRNA expression within the cells. Our results indicate that CD44 on chondrocytes plays an important role in normal and abnormal functions of cartilage through its adhesion to HA, which induces a variety of stimulatory signals to regulate chondrocyte proliferation as well as matrix synthesis in cartilage microenvironment.

Rapid chondrocyte maturation by serum-free culture with BMP-2 and ascorbic acid

In serum-containing medium, ascorbic acid induces maturation of prehypertrophic chick embryo sternal chondrocytes. Recently, cultured chondrocytes have also been reported to undergo maturation in the presence of bone morphogenetic proteins or in serum-free medium supplemented with thyroxine. In the present study, we have examined the combined effect of ascorbic acid, BMP-2, and serum-free conditions on the induction of alkaline phosphatase and type X collagen in chick sternal chondrocytes. Addition of either ascorbate or rhBMP-2 to nonconfluent cephalic sternal chondrocytes produced elevated alkaline phosphatase levels within 24-72 h, and simultaneous exposure to both ascorbate and BMP yielded enzyme levels at least threefold those of either inducer alone. The effects of ascorbate and BMP were markedly potentiated by culture in serum-free medium, and alkaline phosphatase levels of preconfluent serum-free cultures treated for 48 h with BMP+ascorbate were equivalent to those reached in serum-containing medium only after confluence. While ascorbate addition was required for maximal alkaline phosphatase activity, it did not induce a rapid increase in type X collagen mRNA. In contrast, BMP added to serum-free medium induced a three- to fourfold increase in type X collagen mRNA within 24 h even in the presence of cyclohexamide, indicating that new protein synthesis was not required. Addition of thyroid hormone to serum-free medium was required for maximal ascorbate effects but not for BMP stimulation. Neither ascorbate nor BMP induced alkaline phosphatase activity in caudal sternal chondrocytes, which do not undergo hypertrophy during embryonic development. These results indicate that ascorbate+BMP in serum-free culture induces rapid chondrocyte maturation of prehypertrophic chondrocytes. The mechanisms for ascorbate and BMP action appear to be distinct, while BMP and thyroid hormone may share a similar mechanism for induction.

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

Under normal conditions, articular chondrocytes persist throughout postnatal life, whereas "transient" chondrocytes, which constitute the bulk of prenatal and early postnatal cartilaginous skeleton, undergo maturation, hypertrophy, and replacement by bone cells. The mechanisms regulating the markedly different behavior and fate of articular and transient chondrocytes are largely unclear. In the present study, we asked whether articular chondrocytes possess dominant antimaturation properties which may subtend their ability to persist throughout life. Adult chicken articular chondrocytes and transient maturing chondrocytes from the core region of day 17, chick embryo cephalic sternum were cultured or cocultured in serum-free agarose conditions. When the sternal cells were grown by themselves, they quickly developed into hypertrophic type X collagen-synthesizing cells; however, when they were cocultured with as few as 10% articular chondrocytes or fed with articular chondrocyte-conditioned medium, their maturation was markedly impaired, as revealed by a sharp drop in type X collagen synthesis. A similar, albeit less potent, antimaturation activity characterized resting and proliferating immature chondrocytes isolated from other regions of embryonic sternum. Transforming growth factor-beta 2 (TGF-beta 2) was previously suggested to be an inhibitor of chondrocyte maturation. We found, however, that treatment with a neutralizing antiserum to TGF-beta did not counteract the inhibition of maturation in cocultures of articular and maturing core sternal chondrocytes. Indeed, articular chondrocytes produced and accumulated relatively low levels of TGF-beta in their culture medium, about 15 ng/ml/48 h, of which over 90% was latent; surprisingly, maturing sternal core chondrocytes accumulated over 10-fold more TGF-beta in the medium, about 150 ng/ml/48 h, of which over 20% was endogenously active. These results indicate that articular chondrocytes do possess dominant antimaturation properties which appear to be TGF-beta independent. The TGF-beta s may thus have a more prominent role in the terminal phases of chondrocyte maturation, as indicated by their abundance and greater activity in hypertrophic chondrocytes.

Apoptosis of terminally differentiated chondrocytes in culture

During the process of endochondral ossification chondrocytes progress through stages of terminal differentiation culminating in apoptotic death. We have developed a serum-free suspension culture that allows terminal differentiation and facilitates the investigation of factors affecting chondrocyte apoptosis. We have found that chondrocytes not committed to terminal differentiation, i.e., those from the caudal region of chick embryo sterna, a region that remains cartilaginous for some months after the chick hatches, maintained high viability in serum-free suspension culture. A strong dependence of viability on culture density and sensitivity to induction of apoptosis with the protein kinase inhibitor, staurosporine, was consistent with the proposal that these chondrocytes, like nearly all cells, require intercellular communication for survival. Chondrocytes that were committed to terminal differentiation, i.e., those from the cephalic region of chick embryo sterna, a region that is replaced by bone before the chick hatches, expressed the hypertrophic phenotype but maintained their viability in culture for only approximately 6 days. Subsequent cell death was very consistent between cultures and shown to occur by an apoptotic process by analysis of DNA fragmentation and cell morphology. Short-term viability of hypertrophic chondrocytes was independent of culture density and relatively resistant to treatment with staurosporine. Induction of the hypertrophic phenotype in immature chondrocytes committed them to cell death and prevention of expression of the hypertrophic phenotype prevented cell death. We conclude that commitment of chondrocytes to terminal differentiation is associated with a commitment to apoptosis and apoptosis of hypertrophic chondrocytes in growth cartilage does not require initiation by external signals.

Protooncogene expression in osteogenesis induced by bone morphogenetic protein

In this study, changes in the expression of protooncogenes c-fos and c-myc messenger ribonucleic acid were investigated in mice after implantation of bone morphogenetic protein. The expression of c-fos showed a biphasic pattern. The first increase was observed on Day 1 with the aggregation of round cells. The second increase was observed on Day 7 with the appearance of chondroblasts. The amount of c-myc messenger ribonucleic acid showed the sustained high levels from Days 2 to 7. During this period, the proliferation of mesenchymal cells was histologically evident. After Day 11, the expression of c-fos and c-myc decreased and remained at low levels despite the progress in chondroosteogenesis. The protooncogenes c-fos and c-myc appear to increase before calcification in the process of bone morphogenetic protein induced bone and cartilage development.

Contiguous four-guanosine sequence in c-myc antisense phosphorothioate oligonucleotides inhibits cell growth on human lung cancer cells: possible involvement of cell adhesion inhibition

A contiguous four-guanosine (4G) sequence in c-myc antisense phosphorothioate oligonucleotides caused an antiproliferative effect in smooth muscle cells. To investigate the antiproliferative effect of c-myc antisense oligonucleotides on human lung cancer cell lines, we synthesized oligonucleotides of various lengths and sequences, focusing on the contiguous four-guanosine (4G) sequence. While a c-myc antisense oligonucleotide (20AS1 (4G)) targeted to the translation initiation codon of c-myc mRNA inhibited cell growth of A549 cells by 69% at 10 microM, a scrambled oligonucleotide (20SCR1 (4G)) containing the contiguous four-guanosine (4G) sequence also inhibited cell growth by 72% at the same dose. Although treatment with either 20AS1 (4G) or 20SCR1 (4G) inhibited cell adhesion by 70% at 10 microM, expression of c-myc protein was significantly suppressed only by 20AS1 (4G) (62%), and was only weakly inhibited by 20SCR1 (4G) (32%). Furthermore, a small cell lung carcinoma cell line, Lu65, which can grow in suspension form, was highly resistant to 20AS1 (4G) treatment (IC50>20 microM). These results suggest that the cell growth inhibition by c-myc antisense oligonucleotides containing the contiguous four-guanosine (4G) sequence was possibly correlated with inhibition of cell adhesion, but not with inhibition of c-myc protein expression, via a sequence-specific non-antisense mechanism.

Metallothionein induction in calcifying growth plate cartilage chondrocytes

Metallothionein (MT) induction was studied in mineralizing cultures of chicken growth plate chondrocytes and quantitated using a Cd-saturation assay. In serum free media, MT induction was observed for Cd concentrations of 0.1 microM and greater and at Zn concentrations of 100 microM and greater. Supplementation of culture media with cysteine and/or methionine resulted in higher levels of MT induction and reduced toxicity during Cd exposure. Maximum MT induction appeared to coincide with the earliest culture stages during which important enzymes and matrix components are being synthesized. Of non-metal MT inducers tested, sodium butyrate caused a low level induction of MT while interleukin-1 had no effect on basal MT levels. 1,25-dihydroxyvitamin D increased MT induction. The steroid hormone dexamethasone caused a reduction in basal and induced MT levels. These findings suggest that MT regulation in growth plate chondrocytes differs significantly from what is known in other cell types and that this difference may be related to the mineralization of this tissue.

The chondrocyte, architect of cartilage. Biomechanics, structure, function and molecular biology of cartilage matrix macromolecules

Chondrocytes are specialised cells which produce and maintain the extracellular matrix of cartilage, a tissue that is resilient and pliant. In vivo, it has to withstand very high compressive loads, and that is explicable in terms of the physico-chemical properties of cartilage-specific macromolecules and with the movement of water and ions within the matrix. The functions of the cartilage-specific collagens, aggrecan (a hydrophilic proteoglycan) and hyaluronan are discussed within this context. The structures of cartilage collagens and proteoglycans and their genes are known and a number of informative mutations have been identified. In particular, collagen fibrillogenesis is a complex process which can be altered by mutations whose effects fit what is known about collagen molecular structural functions. In other instances, mutations have indicated new functions for particular molecular domains. As cartilage provides the template for the developing skeleton, mutations in genes for cartilage-specific proteins often produce developmental abnormalities. The search for mutations amongst such genes in heritable disorders is being actively pursued by many groups, although mutation and phenotype are not always well correlated, probably because of compensatory mechanisms. The special nature of the chondrocyte is stressed in connection with its cell involvement in osteoarthritis, the most widespread disease of diarthrodial joints.

Defective expression of early activation genes in cartilage-hair hypoplasia (CHH) with severe combined immunodeficiency (SCID)

Cartilage-hair hypoplasia (CHH) is an autosomal recessive disease of unknown etiology characterized by metaphyseal dysostosis, unpigmented hair, and defective cellular immunity. We studied peripheral blood mononuclear cells (PBMC) of a boy with CHH and combined immunodeficiency in an attempt to characterize further the immune defect in this disease. Stimulation of his PBMC with mitogens was associated with severely depressed IL-2 and interferon-gamma (IFN-gamma) synthesis and IL-2 receptor alpha-chain (IL-2R alpha) expression and resulted in poor lymphocyte proliferation that was only modestly upregulated by the addition of recombinant IL-2 (rIL-2). The defective proliferation and lymphokine synthesis were not corrected by the addition of phorbol myristate acetate (PMA) and ionomycin, agents that bypass receptor-mediated signalling, indicative of a distal abnormality. Importantly, the levels of mRNA encoding c-myc, IL-2R alpha, IL-2 and IFN-gamma were markedly decreased in patient lymphocytes stimulated with PMA+ionomycin as compared to control lymphocytes. The defect in the expression of these early activation genes was selective in that induction by mitogens of mRNA encoding other early activation gene products such as c-fos and c-jun was not impaired. These results suggest that the underlying defect in this patient and perhaps others with CHH may be an abnormality in a component of intracellular signalling pathways or in a trans-acting factor which regulates the expression of a selected number of early activation genes.

Differentiation and mineralization in chick chondrocytes maintained in a high cell density culture: a model for endochondral ossification

Chondrocytes isolated from the proliferative and differentiating zones of 3-wk-old chick growth plates were cultured in the presence of 10% fetal bovine serum (FBS) and ascorbic acid for up to 21 d in a high cell density culture within Eppendorf tubes. The proliferative, differentiating, and calcification properties of the chondrocytes were examined by immunolocalization and by enzyme histochemical and biochemical methods. The cells maintained a chondrocyte phenotype throughout culture: they were round in shape and synthesized both collagen type II and proteoglycans. The expression of a hypertrophic phenotype was evident by Day 3 of culture and from this time onwards characteristics of terminal differentiation were observed. The cells were positive for both alkaline phosphatase (ALP) activity and c-myc protein and the surrounding matrix stained strongly for collagen type X. Small foci of mineralization associated with individual chondrocytes were first evident by Day 6 and more widespread areas of mineralization occupying large areas of matrix were present by Day 15. Mineralization occurred without the addition of exogenous phosphate to the medium. This culture system displays characteristics that are similar in both morphological and developmental terms to that of chick chondrocyte differentiation and calcification in vivo and therefore offers an excellent in vitro model for endochondral ossification.

Chondrocyte differentiation

Data obtained while investigating growth plate chondrocyte differentiation during endochondral bone formation both in vivo and in vitro indicate that initial chondrogenesis depends on positional signaling mediated by selected homeobox-containing genes and soluble mediators. Continuation of the process strongly relies on interactions of the differentiating cells with the microenvironment, that is, other cells and extracellular matrix. Production of and response to different hormones and growth factors are observed at all times and autocrine and paracrine cell stimulations are key elements of the process. Particularly relevant is the role of the TGF-beta superfamily, and more specifically of the BMP subfamily. Other factors include retinoids, FGFs, GH, and IGFs, and perhaps transferrin. The influence of local microenvironment might also offer an acceptable settlement to the debate about whether hypertrophic chondrocytes convert to bone cells and live, or remain chondrocytes and die. We suggest that the ultimate fate of hypertrophic chondrocytes may be different at different microanatomical sites.