The culture of chick embryo chondrocytes and the control of their differentiated functions in vitro. I. Characterization of the chondrocyte-specific phenotypes

Analyzing Biological Performance of 3D-Printed, Cell-Impregnated Hybrid Constructs for Cartilage Tissue Engineering

Three-dimensional (3D) bioprinting of hybrid constructs is a promising biofabrication method for cartilage tissue engineering because a synthetic polymer framework and cell-impregnated hydrogel provide structural and biological features of cartilage, respectively. During bioprinting, impregnated cells may be subjected to high temperatures (caused by the adjacent melted polymer) and process-induced mechanical forces, potentially compromising cell function. This study addresses these biofabrication issues, evaluating the heat distribution of printed polycaprolactone (PCL) strands and the rheological property and structural stability of alginate hydrogels at various temperatures and concentrations. The biocompatibility of parameters from these studies was tested by culturing 3D hybrid constructs bioprinted with primary cells from embryonic chick cartilage. During initial two-dimensional culture expansion of these primary cells, two morphologically and molecularly distinct cell populations ("rounded" and "fibroblastic") were isolated. The biological performance of each population was evaluated in 3D hybrid constructs separately. The cell viability, proliferation, and cartilage differentiation were observed at high levels in hybrid constructs of both cell populations, confirming the validity of these 3D bioprinting parameters for effective cartilage tissue engineering. Statistically significant performance variations were observed, however, between the rounded and fibroblastic cell populations. Molecular and morphological data support the notion that such performance differences may be attributed to the relative differentiation state of rounded versus fibroblastic cells (i.e., differentiated chondrocytes vs. chondroprogenitors, respectively), which is a relevant issue for cell-based tissue engineering strategies. Taken together, our study demonstrates that bioprinting 3D hybrid constructs of PCL and cell-impregnated alginate hydrogel is a promising approach for cartilage tissue engineering.

Tissue engineered cartilage using human articular chondrocytes immortalized by HPV-16 E6 and E7 genes

Chondrocytes are useful as a cell culture system for studying arthritic degeneration in tissue engineered cartilage. However, primary chondrocytes have short in vitro lifespan and rapid shift of collagen phenotype. In this study, we used a high dosage of retroviral vector LXSN16E6E7 to transduce human primary chondrocytes and obtained an actively proliferating cell line, designated hPi, which expresses HPV-16 E6/E7 mRNA in early passages. Parental primary chondrocytes cease to grow after five passages, whereas hPi could be propagated beyond 100 passages without requiring additional cell elements in defined medium. After 48 passages, hPi can also give many profiles similar to those of parental primary chondrocyte, including type II collagen in mRNA and protein level, aggrecan in mRNA level, lacunae in type I collagen matrices, and morphology with GAG-specific Alcian blue staining. hPi has shown neoplastic transformation, as examined by NOD-SCID mice tumorigenicity assays for 3 months. Our results indicated that human primary chondrocytes could be immortalized by transduction with HPV-16 E6/E7, preserving stable cartilage-specific differentiation markers. The established chondrocyte cell line could provide a novel model to engineer cartilage in vitro and in vivo for cartilage repair research and clinical application.

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.

Immunochemical and mechanical characterization of cartilage subtypes in rabbit

Cartilage is categorized into three general subgroups, hyaline, elastic, and fibrocartilage, based primarily on morphologic criteria and secondarily on collagen (Types I and II) and elastin content. To more precisely define the different cartilage subtypes, rabbit cartilage isolated from joint, nose, auricle, epiglottis, and meniscus was characterized by immunohistochemical (IHC) localization of elastin and of collagen Types I, II, V, VI, and X, by biochemical analysis of total glycosaminoglycan (GAG) content, and by biomechanical indentation assay. Toluidine blue staining and safranin-O staining were used for morphological assessment of the cartilage subtypes. IHC staining of the cartilage samples showed a characteristic pattern of staining for the collagen antibodies that varied in both location and intensity. Auricular cartilage is discriminated from other subtypes by interterritorial elastin staining and no staining for Type VI collagen. Epiglottal cartilage is characterized by positive elastin staining and intense staining for Type VI collagen. The unique pattern for nasal cartilage is intense staining for Type V collagen and collagen X, whereas articular cartilage is negative for elastin (interterritorially) and only weakly positive for collagen Types V and VI. Meniscal cartilage shows the greatest intensity of staining for Type I collagen, weak staining for collagens V and VI, and no staining with antibody to collagen Type X. Matching cartilage samples were categorized by total GAG content, which showed increasing total GAG content from elastic cartilage (auricle, epiglottis) to fibrocartilage (meniscus) to hyaline cartilage (nose, knee joint). Analysis of aggregate modulus showed nasal and auricular cartilage to have the greatest stiffness, epiglottal and meniscal tissue the lowest, and articular cartilage intermediate. This study illustrates the differences and identifies unique characteristics of the different cartilage subtypes in rabbits. The results provide a baseline of data for generating and evaluating engineered repair cartilage tissue synthesized in vitro or for post-implantation analysis.

Serum-free culture of rabbit meniscal fibrochondrocytes: proliferative response

We have formulated a serum-free medium capable of supporting DNA synthesis in rabbit meniscal fibrochondrocytes at a level equivalent to 10% fetal bovine serum (FBS). The medium consists of a 1:1 mixture of Dulbecco's Modified Eagle's Medium and Ham's F-12 medium supplemented with transferrin (1 microgram/ml), selenium (1 pg/ml), trace metal mix (1:100), dexamethasone (100 ng/ml), insulin-like growth factors I and II (50 ng/ml each), pituitary fibroblast growth factor (100 ng/ml), and lactalbumin hydrolysate (2 micrograms/ml). Endothelial cell growth supplement could be substituted for lactalbumin hydrolysate to obtain similar results. Ventrex PC-1, a commercially available, low-protein, serum-free medium, was found to support proliferation of fibrochondrocytes but not as well as 10% FBS or our medium formulation. Lipid supplements, which are known to support the serum-free growth of hyaline chondrocytes, were found to be either of no value or antagonistic for the culture of fibrochondrocytes. Likewise, vitamin E alone, progesterone, putrescine, and hydrocortisone were also without benefit in our culture system. The cells had a more chondrocytic morphology when grown in defined medium as opposed to 10% FBS. The results of this study should now make it possible to identify and quantitate those factors necessary to affect meniscal repair by utilizing further techniques in vitro.

Developmental expression of creatine kinase isoenzymes in chicken growth cartilage

We have shown previously that creatine kinase (CK) activity is required for normal development and mineralization of chicken growth cartilage and that expression of the cytosolic isoforms of CK is related to the biosynthetic and energy status of the chondrocyte. In this study, we have characterized changes in isoenzyme activity and mRNA levels of CK (muscle-specific CK, M-CK; brain-type CK, B-CK; and mitochondrial CK subunits, MiaCK and MibCK) in the growth plate in situ and in chondrocyte culture systems that model the development/maturation program of the cartilage. The in vitro culture systems analyzed were as follows: tibial chondrocytes, which undergo hypertrophy; embryonic cephalic and caudal sternal chondrocytes, which differ from each other in their mineralization response to retinoic acid; and long-term micromass cultures of embryonic limb mesenchymal cells, which recapitulate the chondrocyte differentiation program. In all systems analyzed, B-CK was found to be the predominant isoform. In the growth plate, B-CK expression was highest in the most calcified regions, and M-CK was less abundant than B-CK in all regions of the growth plate. In tibial chondrocytes, an increase in B-CK expression was seen when the cells became hypertrophic. Expression of B-CK increased slightly over 15 days in mineralizing, retinoic acid-treated cephalic chondrocytes, but it decreased in nonmineralizing caudal chondrocytes, while there was little expression of M-CK. Interestingly, in limb mesenchyme cultures, significant M-CK expression was detected during chondrogenesis (days 2-7), whereas hypertrophic cells expressed only B-CK. Finally, expression of MiaCK and MibCK was low both in situ and in vitro. These observations suggest that the CK genes are differentially regulated during cartilage development and maturation and that an increase in CK expression is important in initiating chondrocyte maturation.

Role of type X collagen on experimental mineralization of eggshell membranes

Type X collagen is a transient and developmentally regulated collagen that has been postulated to be involved in controlling the later stages of endochondral bone formation. However, the role of this collagen in these events is not yet known. In order to understand the function of type X collagen, if any, in the process of biomineralization, the properties of type X collagen in eggshell membranes were further investigated. Specifically, calvaria-derived osteogenic cells were tested for their ability to mineralize eggshell membranes in vitro. Immunohistochemistry with specific monoclonal antibodies was used to correlate the presence or absence of type X collagen or its propeptide domains with the ability of shell membranes to be mineralized. The extent of mineralization was assessed by Von Kossa staining, scanning electron microscopy and energy-dispersive spectroscopy. The results indicate that the non-helical domains of type X collagen must be removed to facilitate the cell-mediated mineralization of eggshell membranes. In this tissue, intact type X collagen does not appear to stimulate or support cell-mediated mineralization. We postulate that the non-helical domains of type X collagen function in vivo to inhibit mineralization and thereby establish boundaries which are protected from mineral deposition.

Involvement of alpha5beta1 integrin in matrix interactions and proliferation of chondrocytes

Integrins are cell surface receptors involved in cellular processes including adhesion, migration, and matrix assembly. In the present study, we analyzed the possible involvement of alpha 5 beta 1 integrin in the regulation of chondrocyte adhesion, spreading, and proliferation. We found that rabbit growth plate chondrocytes were able to attach to substrates coated with type I collagen, type II collagen, or fibronectin within 24 h of culture. During this time period, attachment to fibronectin appeared to be dependent on alpha 5 beta 1 integrin, whereas adhesion to collagens was not. By day 3 of culture, chondrocytes spread onto all the substrates tested. We found that regardless of the nature of the substrate, cell spreading was reversed by treatment with RGD peptide or antibodies against alpha 5 beta 1 or fibronectin, indicating that cell spreading involved alpha 5 beta 1 and fibronectin endogenously produced and deposited by the chondrocytes themselves. Colony formation by chondrocytes in soft agar was inhibited by treatment with RGD peptides or BIIG2, an antibody that interferes with alpha 5 beta 1 integrin-ligand interactions. Furthermore, DNA content was decreased by treatment with anti-fibronectin antibody in micromass culture of chondrocytes. Immunohistochemical analysis on tissue sections revealed that the alpha 5 subunit was particularly abundant in the proliferative and hypertrophic zones of growth plate. The results of the study indicate that alpha 5 beta 1 integrin plays multiple roles in chondrocyte behavior and function and appears to be involved in the regulation of both chondrocyte-matrix interactions and proliferation.

Partial characterization of the C-terminal non-collagenous domain (NC1) of collagen type X

Collagen type X is composed of three identical alpha 1(X) chains of 59 kDa, each containing a triple-helical region of 45 kDa flanked by a short N-terminal sequence and a larger non-collagenous C-terminal (NC1) domain of approx. 15 kDa. Collagen type X molecules can associate via their C-termini to form a regular hexagonal lattice in vitro, which in vivo may provide a modified extracellular matrix for the events of endochondral ossification. The NC1 domain of chick collagen type X was isolated and purified from a highly purified bacterial collagenase digest of hypertrophic chondrocyte medium proteins. The structure and aggregation properties of the NC1 domain of collagen X were investigated, independently of the triple helix. A trimer, a dimer and a monomer of the individual alpha-chain NC1 polypeptides were identified from a bacterial collagenase digest of cartilage collagens using [14C]tyrosine labelling, N-chlorosuccinimide peptide mapping and N-terminal sequencing. The trimer (50 kDa) remained intact in Laemmli sample buffer unless boiled, upon which it dissociated into the dimer (38 kDa) and the monomer (20 kDa). The dimer persisted even after prolonged periods of heating or reduction with beta-mercaptoethanol, and in preparations obtained from chondrocyte cultures treated with beta-aminoproprionitrile, indicating the presence of non-reducible, non-lysine-derived, covalent cross-links. Hexamers of the individual C-termini were observed in rotary-shadowed preparations of purified NC1 domain, reflecting the ability of collagen type X to self-assemble via its C-termini under appropriate conditions.

The developmentally regulated avian Ch21 lipocalin is an extracellular fatty acid-binding protein

Ch21, a developmentally regulated extracellular protein expressed in chick embryos and in cultured chondrocytes, was expressed in the baculovirus system, and the recombinant protein was purified to homogeneity by gel-filtration chromatography. Separation of two isoforms was achieved on an ion-exchange column. Previous work had shown that Ch21 belongs to the superfamily of lipocalins, which are transport proteins for small hydrophobic molecules. Studies were performed to identify the Ch21 ligand. By analysis of recombinant Ch21 on native polyacrylamide gel electrophoresis and by Lipidex assay, the binding of fatty acid to the protein was shown and a preferential binding of long-chain unsaturated fatty acids was observed. Both isoforms had the same behavior. The binding was saturable. Stoichiometry was about 0.7 mol of ligand/mol of protein. The protein binds the ligand in its monomeric form. Calculated dissociation constants were 2 X 10(-7) M for unsaturated fatty acids and 5 X 10(-7) M for stearic acid. The binding was specific; other hydrophobic molecules, as retinoic acid, progesterone, prostaglandins, and long-chain alcohols and aldehydes did not bind to the protein. Short-chain fatty acids did not bind to the protein. Ch21, also present in chicken serum, represents the first extracellular protein able to selectively bind and transport fatty acid in extracellular fluids and serum. We propose to rename the Ch21 protein as extracellular fatty acid-binding protein (Ex-FABP).

Expression of syndecan-3 and tenascin-C: possible involvement in periosteum development

The development of cartilaginous elements of long bone during embryogenesis and postnatal bone repair processes is a complex process that involves skeletal cells and surrounding mesenchymal periosteal cells. Relatively little is known of the mechanisms underlying these processes. Previous studies from this and other laboratories have suggested that the extracellular matrix protein tenascin-C is involved in skeletogenesis. Using in situ hybridization and immunofluorescence, we extended those studies by comparing the expression of tenascin-C with that of syndecan-3, which belongs to a family of cell surface receptors with which tenascins are known to interact. We found that syndecan-3 transcripts at first were very abundant in the presumptive periosteum surrounding the diaphysis of early chondrocytic skeletal elements in chick limb. As the elements developed further, syndecan-3 gene expression decreased in the diaphyseal periosteum, whereas it became stronger around the early epiphysis and within the forming articular cells. However, as the diaphyseal periosteum initiated osteogenesis and gave rise to the intramembranous bone collar, syndecan-3 gene expression increased again. At early stages of skeletogenesis: the tenascin-C gene exhibited patterns of expression that were similar to and temporally followed, those of the syndecan-3 gene. At later stages, however, tenascin-C gene expression was markedly reduced during intramembranous osteogenesis around the diaphysis. In addition, although syndecan-3 gene expression was low in osteoblasts and osteocytes located deep into trabecular bone, tenascin-C gene expression remained strong. Thus, tenascin-C and syndecan-3 display distinct temporal and spatial patterns of expression in periosteum and during the development of long bone. Given their multidomain structure and specific patterns of expression, these macromolecules may regulate site-specific skeletal processes, including interactions between developing periosteum and chondrocytes and delineation of the early cartilaginous skeletal elements.

Regulation of chondrocyte maturation by fibroblast growth factor-2 and parathyroid hormone

Fibroblast growth factor-2 and parathyroid hormone are strong modulators of the maturation process of chondrocytes during endochondral ossification. To clarify whether and how these agents may exert stage-specific effects during this process, we analyzed the responsiveness and phenotypic consequences of treatment with fibroblast growth factor-2 or parathyroid hormone on chondrocytes at different stages of maturation. Populations of immature lower sternal, maturing upper sternal, and hypertrophic tibial growth plate chondrocytes were isolated from day 18-20 chick embryos and were allowed to resume the maturation process by growth in standard monolayer cultures. Treatment of immature lower sternal cultures with as little as 0.1 ng/ml of fibroblast growth factor-2 or 10(-10) M parathyroid hormone prevented both the emergence of mature type-X collagen-synthesizing chondrocytes and the ensuing enlargement of cells that occurred in control (untreated) cultures. Similarly, the treatment of cultured early maturing upper sternal cells with these factors severely reduced the synthesis of type-X collagen and alkaline phosphatase activity and the levels of their respective mRNAs. In sharp contrast, when the cultured upper sternal cells were allowed to grow and mature further before treatment, the responsiveness to fibroblast growth factor-2 was markedly reduced and the responsiveness to parathyroid hormone remained strong and largely unchanged. Cultures of hypertrophic tibial growth plate cells displayed a similar reduced sensitivity to fibroblast growth factor-2, as also indicated by the lack of mitogenic effects, and strong sensitivity to parathyroid hormone. The phenotypic changes induced by treatment with either of these factors were fully reversible when cultures that had been treated were placed in control medium. The results demonstrate that fibroblast growth factor-2 and parathyroid hormone are equally potent in affecting the early stages of maturation but exert differential effects as the cells progress along the maturation pathway. The factors appear to be part of sequentially acting mechanisms to ensure normal progression of chondrocyte maturation during endochondral ossification.

Biosynthesis and characterization of type X collagen in human fetal epiphyseal growth plate cartilage

We examined in vitro collagen biosynthesis by organ cultures from human fetal epiphyseal growth plate cartilage. The biosynthetic products were characterized by NaCl fractional precipitation, limited proteolytic digestion, and sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis. Organ cultures of human fetal epiphyseal growth plate cartilage synthesized large amounts of type X collagen in addition to type II, type IX, and type XI collagens. The individual polypeptide chains of human type X collagen migrated with an apparent M(r) of 45 kDa after proteolytic digestion with pepsin. The migration pattern of these molecules did not change when examined under reducing and nonreducing conditions, indicating that they did not contain intrahelical disfulfide bonds. Comparison of the rates at type X collagen biosynthesis at weeks 20 and 24 of human fetal development showed a marked increase of 24 weeks. Northern hybridization analysis of total RNA from freshly isolated epiphyseal growth plate chondrocytes with a cDNA corresponding to the carboxyl terminus of human type X collagen indicated that the developmental increase of type X collagen production is determined by pre-translational mechanisms.

A rapid and ultrasensitive method for measurement of DNA, calcium and protein content, and alkaline phosphatase activity of chondrocyte cultures

Most investigators are cognizant of the problems inherent in counting cells embedded in a complex and abundant extracellular matrix. To overcome these obstacles, we developed a new method of isolating nucleic acids from chondrocytes which facilitates measurement of cell number by DNA analysis. Chondrocytes were isolated from chick embryo sterna and grown continuously without subculturing for 2-3 weeks in monolayer. The cells were treated with triton X-100 and the nucleic acid content of the extract was determined by measuring DNA fluorescence in the presence of Hoechst dye 33258. To minimize background fluorescence due to the triton, we precipitated the DNA with alcohol and then solubilized the nucleic acids in EDTA. This simple procedure removed the detergent and substantially increased the sensitivity of the method. Thus, we could measure with high precision and high recovery, the DNA content of cultures of 10,000-50,000 cells. In a single well containing 0.5-1.0 million cells, sufficient material remained for subsequent measurements of alkaline phosphatase activity and protein and calcium content. As the mineral present in the triton-treated samples was soluble in EDTA, we experienced no problems in measuring the calcium content of the culture. In addition, as triton X-100 is a nonionic detergent, we were able to measure cell and matrix proteins; moreover, the presence of the triton maintained the catalytic state of alkaline phosphatase. We conclude that this procedure provides a simple and rapid approach to measuring major indicators of chondrocyte maturation and function.

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.

Retinoic acid is a major regulator of chondrocyte maturation and matrix mineralization

During the process of endochondral bone formation, chondrocytes undergo a series of complex maturational changes. Our recent studies indicate that this maturational process is influenced by the vitamin A derivative retinoic acid (RA). To learn how this agent regulates chondrocyte development, we characterized matrix gene expression during maturation of cartilage cells in chick sternum. RNAs were isolated from the cephalic portion of day 13, 14, 16, 18, and 20 chick embryo sternum and analyzed via northern blots. Type II collagen RNA levels remained fairly constant during this developmental period. In contrast, expression of type X collagen and alkaline phosphatase (APase) genes was first detected at day 16, followed by that of osteonectin (ON) and osteopontin (OP). To explore the mechanisms triggering these changes, chondrocytes were isolated from the cephalic portion of day 17-18 sternum (US cells) and grown in monolayer in standard serum-containing medium. After 3 weeks in culture, most of the cells enlarged and became type X collagen-positive, but they exhibited low APase activity and contained only trace amounts of ON and OP mRNAs. Treatment of parallel 3-week-old cultures with RA (10-100 nM) rapidly increased expression of the APase, ON, and OP genes severalfold. In concert with a significant increase in APase activity, there was abundant calcium accumulation in the RA-treated cultures. Electron microscopy confirmed the formation of large matrix-associated mineral crystals and the presence of numerous matrix vesicles. The effects of RA were also studied in cultures of immature chondrocytes isolated from the caudal portion of sternum (LS cells).(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of primary cultures of chondrocytes from type II collagen/beta-galactosidase transgenic mice

Studies on the function of extracellular matrix components of cartilages and on chondrocyte-specific regulatory mechanisms will benefit from approaches in which transgenic mice and cell cultures will complement each other. We therefore established and extensively characterized primary cultures of mouse chondrocytes isolated from rib growth plates of newborn mice harboring a transgene in which type II collagen gene regulatory sequences were driving expression of an E. coli beta-galactosidase reporter gene. Primary chondrocytes expressed a fully differentiated phenotype in monolayer culture, producing mRNAs for the collagen types II, IX and X, and for the transgene. Transgenic cells also synthesized high levels of E. coli beta-galactosidase, easily quantifiable and also detectable in individual cells by X-gal staining. When chondrocytes were isolated from transgenic mice in which beta-galactosidase was fused to the product of the neomycin resistance gene, they displayed resistance to G418. After one to two weeks in culture, chondrocytes progressively lost expression of the transgenes, in parallel with that of cartilage-specific genes, and started expressing high levels of type I collagen RNA. The use of transgenic chondrocytes allowed us to easily score phenotypic changes by assaying beta-galactosidase activity and neomycin resistance. Cultures of mouse chondrocytes, such as those reported here, should also help characterize biochemically the phenotypes of other transgenic mice in studies of genetic diseases of cartilages and of mechanisms involved in chondrogenesis.

Retinoic acid induces a shift in the energetic state of hypertrophic chondrocytes

In the epiphyseal growth plate, chondrocyte maturation is accompanied by dramatic alterations in energy metabolism. To explore the relationship between these two events, we used retinoic acid (RA) to promote chondrocyte maturation in culture. The specific question that was addressed was, does RA treatment of cultured chondrocytes in vitro induce a change in energy status similar to that seen in hypertrophic chondrocytes in vivo. Maturing chondrocytes isolated from the cephalic region of day 18 chick embryo sterna were allowed to grow for 7-14 days in monolayer until confluent and then treated with 10-300 nM RA. Immature chondrocytes from the caudal region of sternum were grown in parallel and served as control cells for the study. We found that in maturing cephalic cell cultures, RA had a rapid and profound effect on oxidative metabolism. The retinoid caused a reduction in the energy charge ratio (ECR) and the ATP/ADP ratio and a sharp decrease in cell ATP levels. Maximum inhibition was observed when the RA concentration was 10-35 nM. Compared with the adenine nucleotides, creatine phosphate levels were decreased to a lesser extent by RA, although there was substantial inhibition of creatine kinase activity. We expected to find a compensatory elevation in glycolytic activities; however, the lactate levels in the medium of the treated cells indicated that anaerobic glycolysis was depressed. In contrast to the cephalic chondrocytes, when caudal cell cultures were treated with RA, lactate formation was stimulated and there were minimal effects on oxidative metabolism. To determine the mechanism of inhibition of glycolysis, we measured the activity of pyruvate kinase in RA-treated cephalic cells.(ABSTRACT TRUNCATED AT 250 WORDS)

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

We have infected primary embryonic mouse limb chondrocytes with a retrovirus carrying simian virus 40 early regions and have obtained a monoclonal mouse chondrocyte line, MC615, that was able to grow on culture dishes for at least 7 months and 20 passages. MC615 cells show expression of simian virus 40 large T (tumor) antigen and express markers characteristic of cartilage in vivo, such as types II, IX, and XI collagen, as well as cartilage aggrecan and link protein. These data show that cell growth induced by large T oncogene expression does not prevent the maintenance of the chondrocytic phenotype.

Localization of collagen X in human fetal and juvenile articular cartilage and bone

The tissue localization was analysed of collagen X during human fetal and juvenile articular cartilage-bone metamorphosis. This unique collagen type was found in the hypertrophic cartilage zone peri- and extracellularly and in cartilage residues within bone trabeculae. In addition, occasionally a slight intracellular staining reaction was found in prehypertrophic proliferating chondrocytes and in chondrocytes surrounding vascular channels. A slight staining was also seen in the zone of periosteal ossification and occasionally at the transition zone of the perichondrium to resting cartilage. Our data provide evidence that the appearance of collagen X is mainly associated with cartilage hypertrophy, analogous to the reported tissue distribution of this collagen type in animals. In addition, we observed an increased and often "spotty" distribution of collagen X with increasing cartilage "degeneration" associated with the closure of the growth plate. In basal hypertrophic cartilage areas, a co-distribution of collagens II and X was found with very little and "spotty" collagen III. In juvenile cartilage areas around single hypertrophic chondrocytes, co-localization of collagens X and I was also detected.

Type X collagen is transcriptionally activated and specifically localized during sternal cartilage maturation

Type X collagen is an extracellular matrix protein which is synthesized by chondrocytes when they undergo hypertrophy. We present evidence here that the expression of type X collagen in the developing chick sternum is controlled primarily by transcriptional mechanisms. Using chondrocyte nuclei isolated from 15-, 16-, 17- and 18-day chick embryonic sterna, nuclear run-off assays demonstrate that type X collagen gene transcription begins at day 16 in chondrocytes isolated from the cephalic portion. This occurs two days prior to mineralization of this tissue as observed by alizarin red staining. The rate of type X transcription increases dramatically through days 17 and 18. Western blot analyses of extracts of freshly isolated sternal chondrocytes from the same stages show that intracellular levels of the type X protein follow the same time course. Immunostaining with a monoclonal antibody specific for type X collagen demonstrates that the initial appearances of hypertrophic cells and pericellular type X collagen occur at embryonic day 16 in the cephalic portion of sterna. Observation of immunostained cephalic sternal sections from day 18 embryos by confocal microscopy reveals that type X collagen is localized in a capsule-like configuration around each hypertrophic chondrocyte.

Development of a serum-free system to study the effect of growth hormone and insulinlike growth factor-I on cultured postembryonic growth plate chondrocytes

We have developed a serum-free system to culture postembryonic growth plate chondrocytes while maintaining some important phenotypic characteristics of their tissue of origin. This serum-free medium was as effective as medium containing 10% newborn bovine serum (NBS) for recovering the cells from enzymatic isolation. Surface secretory activity of chondrocytes cultured in monolayer, assessed through scanning electron microscopy, was also comparable to cells grown in medium containing serum. The effects of growth hormone (GH) and insulinlike growth factor-I (IGF-I) were also studied using the serum-free medium. GH had no effect on cell density and morphology of the cells compared to the control without the hormone. In contrast, chondrocytes grown in medium containing IGF-I had a marked increase in cell density after 3 days and presented similar morphologic characteristics to cells grown in the presence of NBS. The growth factors required for proliferation of chondrocytes cultured in the serum-free medium are IGF-I and fibroblast growth factor (100 ng/ml, respectively). Addition of ascorbic acid to the serum-free medium (0 to 50 micrograms/ml) produced a dose-dependent decrease in cell proliferation. This medium should provide a useful tool for studying the effects of different growth factors/hormones in the regulation of longitudinal bone growth and their interactions.

SV40-immortalization of rabbit articular chondrocytes: alteration of differentiated functions

Cell lines were established from rabbit articular chondrocytes following transfection with a plasmid encoding SV40 early function genes. This resulted in cell immortalization (130 passages have been completed for the oldest cell line) with acquisition of characteristics of partial transformation such as reduced serum requirements for normal and clonal growth. The immortalized chondrocytes, called SVRAC, did not form multilayer foci when maintained in postconfluent culture. Their ability to form colonies in soft agar was not increased in comparison with normal chondrocytes, but they were weakly tumorigenic in nude mice. SVRAC lost the ability to synthesize type II collagen and Alcian blue-stainable matrix, which are markers of the differentiated chondrocyte phenotype, and synthesized predominantly type I collagen. Studies of collagen gene expression showed that pro alpha 1 (II) mRNA was undetectable, whereas pro alpha 1 (I) collagen mRNA was expressed even in late passage cultures. Unlike normal dedifferentiated chondrocytes, SVRAC were unable to re-express the differentiated phenotype in response to tridimensional culture or microfilament depolymerization. Cell lines obtained from chondrocytes transfected either in primary culture or just after release of cells from cartilage displayed the same behaviour. Thus SV40 early genes were able to immortalize rabbit articular chondrocytes, but the resulting cell lines displayed an apparently irreversibly dedifferentiated phenotype. These cell lines can be used as models to identify regulatory pathways that are required for the maintenance or reexpression of differentiated function in chondrocytes.

Fibronectin and keratan sulfate synthesis by canine articular chondrocytes in culture is modulated by dibutyryl cyclic adenosine monophosphate

The ability of cyclic adenosine monophosphate (cAMP) to maintain differentiated properties of canine articular chondrocytes in culture is reported. Treatment with 0.5 mM dibutyryl cAMP (DBcAMP) caused the cells to adopt a more rounded morphology. This change in morphology seems to have no effect on the overall biosynthetic rates of the cells. After a pulse with 35S-methionine, there was no difference in the concentration of labeled proteins between cultures treated with DBcAMP and control cultures. After 6 days, the amount of fibronectin (FN) in the media of DBcAMP-treated cultures detected by an enzyme-linked immunosorbent assay was specifically reduced by 30%. The amount of 35S-FN purified by gelatin-affinity chromatography decreased 33%. Moreover, the percentage of FN containing the extra domain A sequence was reduced from 19.4 +/- 8.7% in control cultures to 9.6 +/- 4.2%. Concomitant with the decrease in FN, there was an increase in the concentration of keratan sulfate in the media of DBcAMP-treated cultures. After 6 days, treated cultures had 47% more keratan sulfate than controls did. These changes appear not to be the result of a change in the deposition of FN or keratan sulfate, because the amount of these molecules that could be extracted from the cell layer was typically below the limit of detection of the assays. Instead, it seems there is a phenotypic change in the chondrocytes pertaining to the production of FN and keratan sulfate.

In vitro studies on the regulation of endochondral ossification by vitamin D

The research described in this article has focused on the complex autocrine, paracrine, and endocrine regulation of endochondral ossification using vitamin D metabolites and TGF-beta as models. By comparing results from a number of laboratories utilizing a diverse array of in vivo and in vitro systems, a coherent picture is beginning to emerge. Vitamin D metabolites influence cell differentiation and maturation and have direct effects on cell function. Differentiation of the mesenchymal cells into chondroblasts is regulated by both 1,25-(OH)2D3 and 24,25-(OH)2D3, as well as by TGF-beta. The resting zone chondrocytes respond primarily to 24,25-(OH)2D3 in terms of matrix synthesis and matrix vesicle biochemistry. They synthesize both metabolites and other factors that stabilize matrix vesicle enzymes like AHSG. In addition to the paracrine role these factors may play in regulating the matrix, it is possible that they may influence the cells in the growth plate itself. Growth zone chondrocytes also synthesize both metabolites, but respond primarily to 1,25-(OH)2D3 for the parameters measured in the studies described. These cells also synthesize TGF-beta which further increases alkaline phosphatase activity, perhaps via an autocrine stimulation of the cell. While cells from the calcified zone have not yet been studied directly in culture, it is likely that they respond to paracrine signals from the avascular cartilage as well as to serum-derived factors. How the signals are transferred among the cells is unknown. Certainly one can postulate information flow in both upward and downward directions. The signal transduction mechanisms for the factors at the cellular level are complex. While it is known that 1,25-(OH)2D3 stimulates gene transcription and stabilization of mRNA for proteins like alkaline phosphatase, its nongenomic effects are only beginning to emerge. Membrane effects of this metabolite have been shown in intestine and kidney in conjunction with studies on Ca flux. It is becoming increasingly evident that other steroid hormones may operate in similar ways. Studies with the rat costochondral chondrocytes are the first to show that there are specific membrane effects for at least two vitamin D metabolites and that membrane enzymes, including those involved in phospholipid metabolism, can be differentially regulated by them. Furthermore, these experiments have provided for the first time a clear hypothesis for how cells can regulate events in the extracellular matrix after the matrix vesicles are produced and incorporated into the matrix.

Retinoic acid treatment induces type X collagen gene expression in cultured chick chondrocytes

The vitamin A derivative retinoic acid (RA) is widely thought to be involved in cartilage development, but its precise roles and mechanisms of action in this complex process remain unclear. We have tested the hypothesis that RA is involved in chondrocyte maturation during endochondral ossification and, in particular, is an inducer of maturation-associated traits such as type X collagen and alkaline phosphatase. Immature chondrocytes isolated from the caudal region of Day 19 chick embryo sterna were seeded in secondary monolayer cultures and treated either with a high dose (100 nM) or with physiological doses (10-35 nM) of RA for up to 3 days. We found that after an initial lag of about 24 h, physiological doses of RA indeed induced type X collagen gene expression in the immature cells. This induction was not accompanied by obvious changes in expression of the type II collagen and large aggregating proteoglycan core protein genes. As revealed by immunocytochemistry, 30-35% of the cells in cultures treated with RA for 3 days were engaged in type X collagen production. Interestingly, these cells were relatively similar in size to chondrocytes in which no type X collagen was detected, suggesting that chondrocytes can initiate type X collagen production independent of cell hypertrophy. RA treatment also led to increased alkaline phosphatase activity occurring as early as 24 h after the start of treatment. The data in this study indicate that RA may have a role in endochondral ossification as an inducer/promoter of maturation-associated traits during chondrocyte maturation.

Expression of type X collagen is transiently stimulated in redifferentiating chondrocytes pretreated with retinoic acid

Growth of quail chondrocytes in the presence of retinoic acid (RA) results in the suppression of the differentiated phenotype. RA-treated chondrocytes recover their differentiated phenotype if they are cultured for an additional 15 days in the absence of RA. A few days after removal from RA, treated chondrocytes acquire the polygonal morphology characteristic of chondrocytes growing as attached cells; they also gradually resume collagen II expression and synthesize cultures. The levels of collagen X mRNA decrease during the second week of culture in the absence of RA. Finally, at the end of 15 days, the absolute levels of collagen II and collagen X mRNAs are very similar in control and recovering chondrocytes.

Spatiotemporal pattern of type X collagen gene expression and collagen deposition in embryonic chick vertebrae undergoing endochondral ossification

We examined the spatio-temporal pattern of type X collagen mRNA and its protein in the embryonic chick vertebrae undergoing ossification by in situ hybridization and immunohistochemistry. Hypertrophic chondrocytes, producing type X collagen, were developed as islands of cells in a few vertebral body segments of stage 36 embryos. These cells were increased in number at stages 37 and 38 and they expressed high levels of type X collagen mRNA and deposited its protein in the matrix. Blood vessels entered from the perichondrium at stage 37 and invaded deeply into hypertrophic cartilage at stage 38. As the vertebrae grew further at stage 40, the leading front of active hypertrophic chondrocytes with high levels of type X mRNA shifted from the midvertebral perivascular area towards intervertebral borders, while the perivascular area retained a number of inactive hypertrophic chondrocytes with low levels of type X mRNA. Type X collagen was found in large amounts throughout the matrix areas containing both active and inactive hypertrophic chondrocytes. Calcium was detected by von Kossa's technique in hypertrophic cartilage matrix in a small amount at stage 37, in parts of the matrix with type X collagen deposition in succeeding stages, and finally in almost the entire area of type X collagen deposition at stage 45. The vertebral segments of stage 45 embryos also showed a clearly reversed pattern of expression between type X collagen mRNA and types II and IX collagen mRNAs. The results demonstrate that the production of type X collagen by hypertrophic chondrocytes precedes both vascular invasion and mineralization of the matrix, suggesting that hypertrophic chondrocytes have an important role in regulating these events.

Rapid induction of type X collagen gene expression in cultured chick vertebral chondrocytes

During endochondral ossification, small rapidly proliferating chondrocytes mature into flattened disc-shaped cells and then into large round hypertrophic cells. These morphological changes are accompanied by a decrease in the rate of cell proliferation. Type X collagen synthesis is initiated during chondrocyte maturation and reaches very high levels in the hypertrophic cells. We have analyzed type X collagen gene expression in chick embryo vertebral chondrocytes that were allowed to mature in monolayer culture and were then switched to suspension culture. The resuspended chondrocytes changed in shape from flat to round and decreased the proliferation rate as they do in vivo. These events were accompanied by a rapid, dramatic increase in type X collagen gene expression at the levels of transcription, steady-state mRNA and protein synthesis, as well as an increase in the number of cells producing type X collagen. The amount of type X collagen gene expression in resuspended chondrocytes was comparable to that in mineralizing cartilage in vivo. These results indicate that events accompanying the switch from monolayer to suspension culture (for example, the change from a flat to a round shape and/or the decrease in proliferation rate) may play a role in the induction of type X collagen gene expression during chondrocyte maturation. Thus we have developed an in vitro system that appears to mimic the events occurring during in vivo chondrocyte maturation. This in vitro model may provide an ideal system for further examination of the parameters regulating chondrocyte maturation and type X collagen gene expression.

Cell hypertrophy and type X collagen synthesis in cultured articular chondrocytes

Articular cartilage is a permanent tissue whose cells do not normally take part in the endochondral ossification process. To determine whether articular chondrocytes possess the potential to express traits associated with this process such as cell hypertrophy and type X collagen, chondrocytes were isolated from adult chicken tibial articular cartilage and maintained in long-term suspension cultures. As a positive control in these experiments, we used parallel cultures of chondrocytes from the caudal portion of chick embryo sternum. Both articular and sternal chondrocytes readily proliferated and progressively increased in size with time in culture. Many had undergone hypertrophy by 4-5 weeks. Analysis of medium-released collagenous proteins revealed that both articular and sternal chondrocytes initiated type X collagen synthesis between 3 and 4 weeks of culture; synthesis of this macromolecule increased with further growth. Immunofluorescence analysis of 5-week-old cultures showed that about 15% of articular chondrocytes and 30% of sternal chondrocytes produced type X collagen; strikingly, there appeared to be no obvious relationship between type X collagen production and cell size. The results of this study show that articular chondrocytes from adult chicken tibia possess the ability to express traits associated with endochondral ossification when exposed to a permissive environment. They suggest also that the process of cell hypertrophy and initiation of type X collagen synthesis are independently regulated both in articular and sternal chondrocytes.

Type X collagen gene expression is transiently up-regulated by retinoic acid treatment in chick chondrocyte cultures

During endochondral ossification, resting and proliferating chondrocytes mature into hypertrophic chondrocytes that initiate synthesis of type X collagen. The mechanisms regulating the differential expression of type X collagen gene were examined in confluent Day 12 secondary cultures of chick vertebral chondrocytes in monolayer treated with the vitamin A analog retinoic acid (RA). Preliminary results showed that major effects of RA on chondrocyte gene expression occurred between 24 and 48 h of treatment. Thus in subsequent experiments cultures were treated for 24, 30, 36, 42, 48, 72, 96, and 120 h. Total RNAs were isolated and analyzed by hybridization with 32P-labeled plasmid probes coding for five matrix macromolecules including type X collagen. We found that the steady-state levels of mRNAs for the large keratan sulfate/chondroitin sulfate proteoglycan (KS:CS-PG) core protein and type II collagen decreased several fold between 24 and 48 h of treatment compared to untreated cells, and remained low with further treatment. In sharp contrast, the level of type X collagen mRNA increased threefold by 42 h of treatment; thereafter it began to decrease and reached minimal levels by 72-120 h of treatment. The changes in steady-state mRNA levels during RA regimen paralleled similar changes in relative rates of protein synthesis. The transient up-regulation of type X collagen gene expression at 42 h of treatment was preceded by a five-fold increase in fibronectin gene expression, was followed by a several fold increase in type I collagen gene expression, and was accompanied by cell flattening and loss of the pericellular proteoglycan matrix. Thus, RA treatment leads to a unique biphasic modulation of type X collagen gene expression in maturing chondrocyte cultures. The underlying, RA-sensitive mechanisms effecting this modulation may reflect those normally regulating the differential expression of this collagen gene during endochondral ossification.

In vitro development of hypertrophic chondrocytes starting from selected clones of dedifferentiated cells

Single cells from enzymatically dissociated chick embryo tibiae have been cloned and expanded in fresh or conditioned culture media. A cloning efficiency of approximately 13% was obtained using medium conditioned by dedifferentiated chondrocytes. A cloning efficiency of only 1.4% was obtained when conditioned medium from hypertrophic chondrocytes was used, and efficiencies of essentially 0 were found with fresh medium or medium conditioned by J2-3T3 mouse fibroblasts. Cell clones were selected by morphological criteria and clones showing a dedifferentiated phenotype (fibroblast-like) were further characterized. Out of 38 clones analyzed, 17 were able to differentiate to the hypertrophic chondrocyte stage and reconstitute hypertrophic cartilage when placed in the appropriate culture conditions. Cells from these clones expressed the typical markers of chondrocyte differentiation, i.e., type II and type X collagens. Clones not undergoing differentiation continued to express only type I collagen. Hypertrophic chondrocytes from differentiating clones were analyzed at the single cell level by immunofluorescence; all the cells were positive for type X collagen, while approximately 50% of them showed positivity for type II collagen.

Isolation of bovine type X collagen and immunolocalization in growth-plate cartilage

Type X collagen was extracted with 1 M-NaCl and 10 mM-dithiothreitol at neutral pH from fetal-bovine growth cartilage and purified to homogeneity by using f.p.l.c. gel filtration on a Superose 12 column, followed by ion-exchange chromatography on a Mono Q column. The purified protein migrates in SDS/polyacrylamide gels with an apparent Mr of 58,000 under reducing conditions and as a high-Mr oligomer in its unreduced form. The amino acid composition is similar to the published composition of chick type X collagen. Pepsin digestion at 4 degrees C decreases the Mr of the monomer to 43,000; purified bacterial collagenase digests most of the molecule, leaving a non-collagenous domain of apparent Mr 15,000, which probably represents the C-terminal globular domain. The IgG fraction from a rabbit antiserum raised against purified bovine type X collagen was specific for this collagen by the criteria of e.l.i.s.a. and immunoblotting after immunoabsorption with collagen types I, II, IX and XI. Immunofluorescence localization of type X collagen in sections of fetal-bovine and human cartilage was possible after acetone fixation of sections and hyaluronidase treatment. Type X collagen was restricted to the zone of hypertrophic and calcified cartilage inside the bone spicules of the growth plate.

Hypertrophy and calcification of rabbit permanent chondrocytes in pelleted cultures: synthesis of alkaline phosphatase and 1,25-dihydroxycholecalciferol receptor

Cartilage calcification at specific sites is a key event that leads to skeletal development and growth. To obtain insights into the control of cartilage calcification, we examined whether cells distributed in permanent cartilage regions might have the ability to express the calcification-related phenotype in a permissive environment. Chondrocytes were isolated from the permanent and growth plate cartilages of 4-week-old rabbit ribs. They were seeded as a pelleted mass in a centrifuge tube and cultured in Eagle's minimum essential medium supplemented with 10% fetal bovine serum. These cells proliferated for several generations, and then synthesized large amounts of proteoglycans, yielding a cartilage-like tissue in 16 days. Cultures from the permanent and growth plate cartilages showed similar time courses for increases in DNA synthesis and proteoglycan production that reached similar maximal levels. Thereafter, they initiated the syntheses of alkaline phosphatase and 1,25-dihydroxycholecalciferol receptor and induced matrix calcification without additional phosphate. The increases in alkaline phosphatase, 1,25-dihydroxycholecalciferol receptor, and calcium contents in cultures from the permanent cartilage were consistently delayed for 4-7 days relative to the growth plate-derived cells, but caught up by Day 28. The maximal levels of alkaline phosphatase and 1,25-dihydroxycholecalciferol receptor in the cultures from the permanent cartilage were 40- to 100-fold higher than that of the in vivo permanent cartilage. These results provide evidence that permanent cartilage cells in postnatal young rabbit ribs have the capacity to express alkaline phosphatase and 1,25-dihydroxycholecalciferol receptor and induce calcification in a permissive environment, although they never express these calcification-related phenotypes in vivo.

A continuous line of chicken embryo cells derived from a chondrocyte culture infected with RSV

A continuous line of chicken embryo cells was derived from a culture of chondrocytes infected with Rous sarcoma virus (RSV). These cells, designated as NA101, have reduced serum requirements and are able to grow in a semisolid medium. NA101 cells show the same phenotype as freshly RSV infected chondrocyte cultures, i.e. synthesis of fibronectin and type I collagen, elongated bipolar shape and absence of tumorigenicity following grafting onto the chorioallantoic membrane of embryonated duck eggs; thus they appear to be distinct from transformed fibroblasts. Neither NA101 cells nor freshly infected chondrocyte cultures synthesize type II or type X collagen, which are the differentiation markers of normal chicken chondrocytes in culture.

Expression of the human chondrocyte phenotype in vitro

We report a culture scheme in which human epiphyseal chondrocytes lose their differentiated phenotype in monolayer and subsequently reexpress the phenotype in an agarose gel. The scheme is based on a method using rabbit chondrocytes. Culture in monolayer allowed small quantities of cells to be amplified and provided a starting point to study expression of the differentiated human chondrocyte phenotype. The cells cultured in monolayer produced type I procollagen, fibronectin, and small noncartilaginous proteoglycans. Subsequent culture in agarose was associated with the acquisition of typical chondrocyte ultrastructural features and the synthesis of type II collagen and cartilage-specific proteoglycans. The switch from the nonchondrocyte to the differentiated chondrocyte phenotype occurred under these conditions between 1 and 2 wk of agarose culture and was not necessarily homogeneous throughout a culture. This culture technique will facilitate direct investigation of human disorders of cartilage that have been addressed in the past by alternative approaches.

Calcification of in vitro developed hypertrophic cartilage

We have recently reported that dedifferentiated cells derived from stage 28-30 chick embryo tibiae, when transferred in suspension culture in the presence of ascorbic acid, develop in a tissue closely resembling hypertrophic cartilage. Ultrastructural examination of this in vitro formed cartilage showed numerous matrix vesicles associated with the extracellular matrix (C. Tacchetti, R. Quarto, L. Nitsch, D. J. Hartmann, and R. Cancedda, 1987, J. Cell Biol. 105, 999-1006). In the present article we report that the in vitro developed hypertrophic cartilage undergoes calcification. We indicate a correlation between the levels of alkaline phosphatase activity and calcium deposition at different times of development. Following the transfer of cells into suspension culture and an initial lag phase, the level of alkaline phosphatase activity rapidly increased. In most experiments the maximum of activity was reached after 5 days of culture. When alkaline phosphatase activity and 45Ca deposition were measured in the same experiment, we observed that the increase in alkaline phosphatase preceded the deposition of nonwashable calcium deposits in the cartilage.

Chains of matrix-derived type X collagen: size and aggregation properties

Type X collagen was isolated from extracts of embryonic chick cartilages by immunoprecipitation and subsequently analyzed by SDS-PAGE. Most of the chains migrated with a molecular weight of 59 kDa, suggesting that the matrix form of type X collagen has not undergone post-secretory proteolytic processing. Minor amounts of material were also observed at 120 kDa, 70 kDa and 50 kDa. These were dimers or limited proteolytic products of type X chains.

Flow cytometric evaluation of cell cycle characteristics during in vitro differentiation of chick embryo chondrocytes

The cell cycle kinetic characteristics of chick endochondral chondrocytes differentiating in vitro were studied by flow cytometry. In addition, the synthesis of type I and type X collagens of the same cells was evaluated by immunoprecipitation. Dedifferentiated cells, derived from chick embryo tibiae and grown attached to a substratum, were characterized by type I collagen synthesis, a high growth fraction (GF = 0.94), minimal cell loss factor (phi = 0.02), and a total cell cycle time of the proliferating cells of about 17 h (tG1 = 8 h, tS = 5 h, and tG2 + M = 4 h). Transfer of dedifferentiated cells to suspension culture on agarose-coated dishes induced differentiation to hypertrophic chondrocytes. These were characterized by type X collagen synthesis, a low growth fraction (GF = 0.52), maximal cell loss factor (phi = 1.0), and a total cell cycle time of the proliferating cells of about 73 h (tG1 = 53 h, tS = 12 h, and tG2 + M = 8 h). The transition from dedifferentiated chondrocytes to hypertrophic chondrocytes was accompanied by large increases of the duration of all the cell cycle phases and of the number of quiescent and degenerating cells. Associated with these alterations in cell cycle kinetics was a switch from type I to type X collagen synthesis. Further preliminary data suggest that the population of differentiating chondrocytes (a state between dedifferentiated and hypertrophic chondrocytes) comprises a heterogeneous population of fast and slow growing cells.

Intrinsic and extrinsic controls of the hypertrophic program of chondrocytes in the avian columella

Immunohistochemical studies of the chick columella have shown that the extracellular matrix of this ossicular cartilage template is composed largely of type II collagen. As development proceeds, synthesis of type X collagen, a hypertrophic cartilage-specific molecule, is initiated by endochondral chondrocytes within the zone of cartilage cell hypertrophy. Subsequently, these cells and their surrounding extracellular matrix are removed, resulting in marrow cavity formation. We have examined which of these processes are programmed within the columella chondrocytes themselves, and which require involvement of exogenous factors. Prehypertrophic columella from 12-day chick embryos were grown either in organ culture on Nuclepore filters or as explants on the chorioallantoic membrane of host embryos. Chondrocytes from the same source were grown in monolayer cell cultures. In both organ culture and cell culture, chondrocytes developed to the stage at which some of them entered the hypertrophic program and initiated the production of type X collagen as determined by immunofluorescence histochemistry with a monoclonal antibody specific for that collagen type. The organ cultures, however, did not progress to the next stage, in which detectable removal of the type X collagen-containing matrix occurs. When identical columella were grown on the chorioallantoic membrane of host chicks, the type X collagen-containing matrix which formed was rapidly removed, resulting in the formation of a marrow cavity. Thus, progression of endochondral chondrocytes to the deposition of type X collagen-containing matrix seems to be programmed within the cells themselves. Subsequent removal of this matrix requires the involvement of exogenous factors.

Dimethyl sulfoxide interferes with in vitro differentiation of chick embryo endochondral chondrocytes

Dedifferentiated chondrocytes derived from 6-day-old chick embryo tibiae when transferred on agarose, revert to the chondrocytic phenotype and mature to hypertrophic, type X collagen-producing chondrocytes (Castagnola et al. (1986). J. Cell Biol. 102, 2310-2317). The continuous presence of 180 mM dimethyl sulfoxide (DMSO) during the culture specifically inhibited synthesis of type X collagen and accumulation of its mRNA. The synthesis of the cartilage-specific type II collagen and the level of its mRNA were essentially unchanged in treated and control untreated cells.

Effects of ascorbic acid on collagen mRNA levels in short term chondrocyte cultures

Chondrocytes isolated from 16 day chicken embryo sterna and adult (18 month) bovine metacarpalphalangeal joint cartilage were grown in monolayer culture for up to 5 days in the presence and absence of ascorbate (50 micrograms/ml). RNA was isolated from these cultures and the steady-state levels of alpha 1(I), alpha 2(I) and alpha 1(II) mRNAs were assayed using cloned DNA probes encoding the respective procollagen mRNAs. Both ascorbate-treated and control chicken chondrocytes maintained the characteristic morphology and phenotype synthesizing the same levels of type II procollagen mRNA observed for sternal chondrocytes. The chicken chondrocytes, with or without ascorbate, did not synthesize increased levels of alpha 1(I) or alpha 2(I) mRNA. In contrast, when bovine articular chondrocytes were cultured with ascorbate, an increase in type II procollagen mRNA and, more interestingly, an increase in type I procollagen mRNA was observed during the 5 day culture period. Low levels of type I procollagen mRNA were detected in untreated chicken and bovine cultured chondrocytes and chicken chondrocytes isolated from sterna. These experiments suggest that when cultured in the presence of ascorbate under the conditions examined, chicken embryo chondrocytes retain the differentiated phenotype unaffected by ascorbic acid while bovine articular chondrocytes begin to undergo a phenotypic change.

Fibroblast growth factor stimulates colony formation of differentiated chondrocytes in soft agar

The effect of fibroblast growth factor (FGF) on the growth of chondrocytes in soft agar was examined. FGF induced colony formation by chick embryo and rabbit chondrocytes. The colony-forming efficiency of FGF-exposed chondrocytes was similar to that of Rous sarcoma virus-transformed chondrocytes (15-20%). Other mitogenic agents tested, such as epidermal growth factor, insulin, insulin-like growth factor-l, and platelet-derived growth factor, induced very low levels of colony formation. The induction of growth in soft agar of chondrocytes by FGF was not due to cells' phenotypic transformation, because chondrocytes grown in soft agar with FGF retained the ability to synthesize cartilage-characteristic proteoglycan. FGF did not induce growth in soft agar of chondrocytes whose phenotypic expression was suppressed by retinoic acid or 5-bromodeoxyuridine. In addition, FGF did not induce growth in soft agar of primary fibroblasts and normal rat kidney (NRK) cells. These results suggest that FGF selectively stimulates growth of differentiated chondrocytes in soft agar.

Type X collagen synthesis by cultured chondrocytes derived from the permanent cartilaginous region of chick embryo sternum

In the developing chick embryo sternum, type X collagen is synthesized by chondrocytes from the cephalic region (presumptive mineralization zone) but not by chondrocytes from the caudal region (permanent cartilaginous zone) (Gibson et al., 1984, J. Cell Biol. 99, 208-216). To distinguish between two possibilities, the presence of a nonpermissive microenvironment in the permanent cartilage or the intrinsic inability of caudal chondrocytes to become hypertrophic, type X-producing cells, we have isolated chondrocytes from the caudal third of stage 44 chick embryo sterna and grown them in suspension on agarose-coated dishes. We have found that in these conditions chondrocytes from the caudal zone differentiate to hypertrophic chondrocytes and synthesize large amount of type X collagen, as revealed by the electrophoretic pattern of labeled proteins made in vitro and by slot blot analysis of mRNAs with specific cDNA probes.

Modulation of fibronectin gene expression in chondrocytes by viral transformation and substrate attachment

Chicken vertebral chondrocytes, which normally grow in suspension, synthesize large amounts of cartilage extracellular matrix proteins, but little fibronectin. We have analyzed the effects of both substrate attachment and transformation with a temperature-sensitive mutant of Rous sarcoma virus on fibronectin gene expression in these cells. Our experiments show that viral transformation increases fibronectin synthesis to a greater extent than substrate attachment. Furthermore, transformed chondrocytes have lost the ability to decrease fibronectin synthesis in response to suspension culture, suggesting that transformation alters the normal attachment-responsive control of fibronectin gene expression. Finally, infected substrate-attached chondrocytes shifted to the nonpermissive temperature for transformation use fibronectin RNA more efficiently in protein synthesis than cells grown under the other conditions, suggesting for the first time a role for translational control of fibronectin gene expression.

A comparison of the effects of different substrata on chondrocyte morphology and the synthesis of collagen types IX and X

Embryonic chick sternal chondrocytes were cultured either within three dimensional gels of type I collagen, type II collagen or agar, or as monolayers on plastic dishes coated with air-dried films of these matrix macromolecules. It was observed that cell shape and cell growth varied markedly between the different culture conditions. Flattened monolayers of cells on plastic or films of type I or type II collagen, proliferated more rapidly and reached a higher final cell density per culture than the more rounded cells found in the cultures on agar films or within three-dimensional gels. Biosynthetic studies demonstrated that in addition to the synthesis of type II collagen, all the cultures were producing collagen types IX and X. Chondrocytes cultured on plastic or films of the different matrix macromolecules all showed a similar expression of types IX and X collagen, independent of whether they displayed a flattened or round cell morphology. In contrast, marked variations in the proportions of the minor collagens, particularly type X collagen, were observed when the cells were cultured within three-dimensional gels. The data suggest that direct interaction of the cell surface with matrix constituents displaying a particular spatial array could be an important aspect in the control of type IX and X collagen expression by chondrocytes.