Gene expression and extracellular matrix ultrastructure of a mineralizing chondrocyte cell culture system

Ascorbate independent differentiation of human chondrocytes in vitro: simultaneous expression of types I and X collagen and matrix mineralization

In this study we describe the collagen pattern synthesized by differentiating fetal human chondrocytes in vitro and correlate type X collagen synthesis with an intracellular increase of calcium and with matrix calcification. We show that type II collagen producing fetal human epiphyseal chondrocytes differentiate in suspension culture over agarose into hypertrophic cells in the absence of ascorbate, in contrast to chicken chondrocytes which have been shown to require ascorbate for hypertrophic differentiation. Analysis of the collagen synthesis by metabolic labeling and immunoprecipitation as well as by immunofluorescence double staining with anti type I, II or X collagen antibodies revealed that type X collagen synthesis was initiated during the third week. After 4 weeks culture over agarose we identified cells staining for both type I and X collagen, indicating further differentiation of chondrocytes to a new type of 'post-hypertrophic' cell. This cell type, descending from a type X collagen producing chondrocyte, is different from the previously described 'dedifferentiated' or 'modulated' types I and III collagen producing cell derived from a type II collagen producing chondrocyte. The appearance of type I collagen synthesis in agarose cultures was confirmed by metabolic labeling and immunoprecipitation and challenges the current view that the chondrocyte phenotype is stable in suspension cultures. An increase in the intracellular calcium concentration from 100 to 250 nM was measured about one week after onset of type X collagen synthesis. First calcium deposits were detected by alizarine red S staining in type X collagen positive cell nodules after 4 weeks, again in the absence of ascorbate. From these observations we conclude a sequence of events ultimately leading to matrix calcification in chondrocyte nodules in vitro that begins with chondrocyte hypertrophy and the initiation of type X collagen synthesis, followed by the increase of intracellular calcium, the deposition of calcium mineral, and finally by the onset of type I collagen synthesis.

High-resolution immunolocalization of osteopontin and osteocalcin in bone and cartilage during endochondral ossification in the chicken tibia

The ultrastructural distribution of two noncollagenous proteins, osteopontin (OPN) and osteocalcin (OC), originally extracted from bone matrix and proposed to play an important role in bone formation, was examined in the matrices of bone and cartilage from embryonic and postnatal chicken tibial growth plates by high-resolution immunocytochemistry using the colloidal gold technique. In bone, immunolabeling patterns using polyclonal antibodies against chicken OPN and OC were generally similar in that both showed an intense, but regionally variable, labeling of mineralized bone matrix and small mineralization loci dispersed throughout the osteoid and containing prominent condensed organic material. Unmineralized osteoid showed weak-to-moderate labeling. In the mineralized bone matrix proper, labeling was predominantly associated with amorphous, electron-dense patches of organic material among the collagen fibrils. In growth plate cartilage, both proteins first appeared related to calcified cartilage in the hypertrophic zone, although the labeling patterns were somewhat different. For OPN, gold particles were mostly associated with an organic lamina limitans-like density containing condensed, filamentous organic matrix at the periphery of small nodules and large masses of calcified cartilage, with additional moderate labeling throughout the interior of the calcified cartilage. For OC, labeling was observed over filamentous structures throughout the calcified cartilage matrix, with some, but less, labeling at the periphery. In the lowermost zones of the growth plate, the major reaction using both antibodies was found over a layer of dense, amorphous organic material at the periphery of the calcified cartilage at the future bone/calcified cartilage interface, a labeling pattern that persisted following bone deposition at these sites. OPN and to a lesser extent OC were also concentrated in cement (resting, reversal) lines. Throughout the bone and cartilage of the tibia, cells of both the osteoblastic and the osteoclastic lineages were found directly apposed to labeled surfaces and lamina limitans of organic matrix containing OPN and OC. In summary, it is concluded from the immunocytochemical data presented here that the association of OPN and OC with mineralized regions of the extracellular matrices of bone and cartilage and the accumulation of these proteins at tissue surfaces and interfaces are consistent with the hypotheses that they play a role in the extracellular mineralization process per se and/or that they may mediate cell adhesion and dynamics.

Remodelling of collagen types I, II and X and calcification of human fetal cartilage

Evidence from recent studies on type X collagen in hypertrophic chick cartilage suggests that it may be involved in cartilage calcification. Here we compare the distribution of type X collagen with that of calcium mineral deposition in fetal human growth plate cartilages of long bones and ribs. Using a specific antibody we demonstrate the presence of type X collagen in a narrow, sharply defined zone of hypertrophic chondrocytes. Type X collagen was also localized in the calcifying cartilage remaining within spongy bone trabecules. Calcium deposits were, however, detected by alizarine red S only in the lower hypertrophic zone and in bone, confirming the notion that type X collagen is deposited in the hypertrophic cartilage before mineral deposition. By immunofluorescence double staining we demonstrate codistribution of type II and X collagen in the hypertrophic zone, while type I collagen was absent from hypertrophic cartilage matrix; it was detected only in the perichondrium, in vascular cavities, and in osteoid and bone. From these observations we conclude that the sequence of events leading to cartilage mineralization begins with chondrocyte hypertrophy, followed by type X collagen synthesis and finally by deposition of calcium mineral.

Matrix mineralization in hypertrophic chondrocyte cultures. Beta glycerophosphate increases type X collagen messenger RNA and the specific activity of pp60c-src kinase

The phenomenon of chondrocyte hypertrophy is accompanied by the expression of type X collagen and the appearance of matrix mineralization. These events are also associated with changes in the phosphorylation of intracellular proteins. In this study the addition of 10 mM beta-glycerophosphate to hypertrophic chondrocytes resulted in stimulation of type X collagen synthesis up to 10 days in culture and an increase in the expression of type X collagen mRNA. This was followed by the onset of mineralization and the appearance of calcium hydroxyapatite. In contrast, the addition of beta-glycerophosphate to non-hypertrophic chondrocytes failed to induce expression of type X collagen or to produce changes in calcium and phosphate. The increased formation of type X collagen and of mineral in hypertrophic chondrocytes was accompanied by changes in the tyrosine kinase pp60c-src. While the level of c-src protein decreased approximately 2.5-fold in hypertrophic chondrocytes after 17 days of beta-glycerophosphate treatment, the specific activity of pp60c-src kinase increased approximately 3-fold in the cells that could be induced to mineralize but remained unchanged in cells that did not exhibit this property. Regulation of kinase activity may be an important event in endochondral ossification.

In vitro formation of mineralized nodules by periodontal ligament cells from the rat

The purposes of this study were to determine whether periodontal ligament (PDL) cells are capable of producing mineralized nodules in vitro and to analyze ultrastructural features of the nodules. Rat PDL cells were obtained from coagulum in the socket at 2 days after tooth extraction and cultured at confluence in standard medium containing Dulbecco's Modified Eagle's Medium supplemented with 10% FBS and antibiotics. To test mineralized nodule formation, cells were further cultured for an additional 3 weeks in the standard medium containing (1) ascorbic acid (50 micrograms/ml) and sodium beta-glycerophosphate (10 mM), (2) ascorbic acid, sodium beta-glycerophosphate, and dexamethasone (5 microM), or (3) ascorbic acid alone. Cells were then fixed in 2.5% glutaraldehyde, postfixed in 1% OsO4, and prepared for light and electron microscopy. Three-dimensional nodules containing mineralized matrices were formed only when the cells were cultured in the presence of ascorbic acid and dexamethasone. They were composed of multilayered fibroblasts (up to 13 layers), and highly organized collagen fibrils with 64 nm cross-banding patterns between the cell layers. The fibroblasts in the nodules exhibited an elongated shape with a high degree of cytoplasmic polarity throughout the nodule, and have the morphological features of PDL fibroblasts as seen in vivo. Mineral deposition with needle-like crystals was initiated on collagen fibrils located in intercellular spaces of the upper cell layers and became increasingly heavier towards the bottom half of the nodules. X-ray microanalysis and electron diffraction analysis confirmed that mineral deposition contained calcium and phosphate in the form of immature hydroxyapatite.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypertrophic chondrocytes undergo further differentiation in culture

Conditions have been defined for promoting growth and differentiation of hypertrophic chondrocytes obtained in culture starting from chick embryo tibiae. Hypertrophic chondrocytes, grown in suspension culture as described (Castagnola P., G. Moro, F. Descalzi Cancedda, and R. Cancedda. 1986. J. Cell Biol. 102:2310-2317), when they reached the stage of single cells, were transferred to substrate-dependent culture conditions in the presence of ascorbic acid. Cells showed a change in morphology, became more elongated and flattened, expressed alkaline phosphatase, and eventually mineralized. Type II and X collagen synthesis was halted and replaced by type I collagen synthesis. In addition the cells started to produce and to secrete in large amount a protein with an apparent molecular mass of 82 KD in reducing conditions and 63 KD in unreducing conditions. This protein is soluble in acidic solutions, does not contain collagenous domains, and is glycosylated. The Ch21 protein, a marker of hypertrophic chondrocytes and bone cells, was synthesized throughout the culture. We have defined this additional differentiation stage as an osteoblast-like stage. Calcium deposition in the extracellular matrix occurred regardless of the addition of beta glycerophosphate to the culture medium. Comparable results were obtained both when the cells were plated at low density and when they were already at confluence and maintained in culture without passaging up to 50 d. When retinoic acid was added to the hypertrophic chondrocyte culture between day 1 and day 5 the maturation of the cells to the osteoblast-like stage was highly accelerated. The switch in the collagen secretion was already observed after 2 d and the production of the 63-kD protein after 3 d. Mineralization was observed after 15-20 d.

The role of ascorbic acid in mesenchymal differentiation

Survival of all higher vertebrates requires that they either synthesize vitamin C (ascorbic acid) or obtain it from their diet. The role of ascorbic acid as a reductant for the iron prosthetic group of hydroxylase enzymes involved in collagen biosynthesis is well established. In contrast, the relationship between the biochemical functions of ascorbic acid and the broad defects in connective tissue formation associated with vitamin C deficiency is less obvious. This review will develop the hypothesis that vitamin C is required for the differentiation of mesenchyme-derived connective tissues such as muscle, cartilage, and bone. It is proposed that the collagen matrix produced by ascorbic acid-treated cells provides a permissive environment for tissue-specific gene expression.

Relationship between collagen synthesis and expression of the osteoblast phenotype in MC3T3-E1 cells

The MC3T3-E1 mouse calvaria-derived cell line has been used to study the role of collagen synthesis in osteoblast differentiation. MC3T3-E1 cells, like several previously characterized osteoblast culture systems, expressed osteoblast markers and formed a mineralized extracellular matrix only after exposure to ascorbic acid. Mineralization was stimulated further by beta-glycerol phosphate. Ultrastructural observations indicated that the extracellular matrix produced by ascorbic acid-treated cells was highly organized and contained well-banded collagen fibrils. Expression of osteoblast markers followed a clear temporal sequence. The earliest effects of ascorbic acid were to stimulate type I procollagen mRNA and collagen synthesis (24 h after ascorbate addition), followed by induction of alkaline phosphatase (48-72 h) and osteocalcin (96-144 h) mRNAs. Procollagen mRNA, which was expressed constitutively in the absence of ascorbate, increased only twofold after vitamin C addition. In contrast, alkaline phosphatase and osteocalcin mRNAs were undetectable in untreated cultures. Actions of ascorbic acid on osteoblast marker gene expression are mediated by increases in collagen synthesis and/or accumulation because (1) parallel dose-response relationships were obtained for ascorbic acid stimulation of collagen accumulation and alkaline phosphatase activity, and (2) the specific collagen synthesis inhibitors, 3,4-dehydroproline and cis-4-hydroxyproline, reversibly blocked ascorbic acid-dependent collagen synthesis and osteoblast marker gene expression.

Studies of mineralization in tissue culture: optimal conditions for cartilage calcification

The optimal conditions for obtaining a calcified cartilage matrix approximating that which exists in situ were established in a differentiating chick limb bud mesenchymal cell culture system. Using cells from stage 21-24 embryos in a micro-mass culture, at an optimal density of 0.5 million cells/20 microliters spot, the deposition of small crystals of hydroxyapatite on a collagenous matrix and matrix vesicles was detected by day 21 using X-ray diffraction, FT-IR microscopy, and electron microscopy. Optimal media, containing 1.1 mM Ca, 4 mM P, 25 micrograms/ml vitamin C, 0.3 mg/ml glutamine, no Hepes buffer, and 10% fetal bovine serum, produced matrix resembling the calcifying cartilage matrix of fetal chick long bones. Interestingly, higher concentrations of fetal bovine serum had an inhibitory effect on calcification. The cartilage phenotype was confirmed based on the cellular expression of cartilage collagen and proteoglycan mRNAs, the presence of type II and type X collagen, and cartilage type proteoglycan at the light microscopic level, and the presence of chondrocytes and matrix vesicles at the EM level. The system is proposed as a model for evaluating the events in cell mediated cartilage calcification.

Ca2+ binding properties of type X collagen

Type X collagen is a developmentally regulated collagen that is only synthesized by chondrocytes of the hypertrophic and calcifying zone in fetal cartilage. There is evidence in the literature that type X collagen may be involved in cartilage calcification. Here we show that type X collagen synthesis precedes calcium deposition in nodules of fetal human chondrocytes forming in cell culture and present evidence that type X collagen binds calcium in a specific and dose dependent manner. In an assay using bovine type X collagen coupled to beads and 45Ca2+ we determined a total of about 15 binding sites per alpha 1(X) chain with a dissociation of 32 microM.