Collagen synthesis by articular in monolayer culture

Multimodal effects of an extracellular matrix on cellular morphology, dynamics and functionality

Articular cartilage defects can lead to pain and even disability in patients and have significant socioeconomic loss. Repairing articular cartilage defects remains a long-term challenge in medicine owing to the limited ability of cartilage to regenerate. At present, the treatment methods adopted in clinical practice have many limitations, thereby necessitating the rapid development of biomaterials. Among them, decellularized biomaterials have been particularly prominent, with numerous breakthroughs in research progress and translational applications. Although many studies show that decellularized cartilage biomaterials promote tissue regeneration, any differences in cellular morphology, dynamics, and functionality among various biomaterials upon comparison have not been reported. In this study, we prepared cartilage-derived extracellular matrix (cdECM) biomaterials with different bioactive contents and various physical properties to compare their effects on the morphology, dynamics and functionality of chondrocytes. This cellular multimodal analysis of the characteristics of cdECM biomaterials provided a theoretical basis for understanding the interactions between biomaterials and cells, thus laying an experimental foundation for the translation and application of decellularized cartilage biomaterials in the treatment of cartilage defects.

A high-resolution route map reveals distinct stages of chondrocyte dedifferentiation for cartilage regeneration

Articular cartilage damage is a universal health problem. Despite recent progress, chondrocyte dedifferentiation has severely compromised the clinical outcomes of cell-based cartilage regeneration. Loss-of-function changes are frequently observed in chondrocyte expansion and other pathological conditions, but the characteristics and intermediate molecular mechanisms remain unclear. In this study, we demonstrate a time-lapse atlas of chondrocyte dedifferentiation to provide molecular details and informative biomarkers associated with clinical chondrocyte evaluation. We performed various assays, such as single-cell RNA sequencing (scRNA-seq), live-cell metabolic assays, and assays for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq), to develop a biphasic dedifferentiation model consisting of early and late dedifferentiation stages. Early-stage chondrocytes exhibited a glycolytic phenotype with increased expression of genes involved in metabolism and antioxidation, whereas late-stage chondrocytes exhibited ultrastructural changes involving mitochondrial damage and stress-associated chromatin remodeling. Using the chemical inhibitor BTB06584, we revealed that early and late dedifferentiated chondrocytes possessed distinct recovery potentials from functional phenotype loss. Notably, this two-stage transition was also validated in human chondrocytes. An image-based approach was established for clinical use to efficiently predict chondrocyte plasticity using stage-specific biomarkers. Overall, this study lays a foundation to improve the quality of chondrocytes in clinical use and provides deep insights into chondrocyte dedifferentiation.

Anisotropic Chitosan Scaffolds Generated by Electrostatic Flocking Combined with Alginate Hydrogel Support Chondrogenic Differentiation

The replacement of damaged or degenerated articular cartilage tissue remains a challenge, as this non-vascularized tissue has a very limited self-healing capacity. Therefore, tissue engineering (TE) of cartilage is a promising treatment option. Although significant progress has been made in recent years, there is still a lack of scaffolds that ensure the formation of functional cartilage tissue while meeting the mechanical requirements for chondrogenic TE. In this article, we report the application of flock technology, a common process in the modern textile industry, to produce flock scaffolds made of chitosan (a biodegradable and biocompatible biopolymer) for chondrogenic TE. By combining an alginate hydrogel with a chitosan flock scaffold (CFS+ALG), a fiber-reinforced hydrogel with anisotropic properties was developed to support chondrogenic differentiation of embedded human chondrocytes. Pure alginate hydrogels (ALG) and pure chitosan flock scaffolds (CFS) were studied as controls. Morphology of primary human chondrocytes analyzed by cLSM and SEM showed a round, chondrogenic phenotype in CFS+ALG and ALG after 21 days of differentiation, whereas chondrocytes on CFS formed spheroids. The compressive strength of CFS+ALG was higher than the compressive strength of ALG and CFS alone. Chondrocytes embedded in CFS+ALG showed gene expression of chondrogenic markers (COL II, COMP, ACAN), the highest collagen II/I ratio, and production of the typical extracellular matrix such as sGAG and collagen II. The combination of alginate hydrogel with chitosan flock scaffolds resulted in a scaffold with anisotropic structure, good mechanical properties, elasticity, and porosity that supported chondrogenic differentiation of inserted human chondrocytes and expression of chondrogenic markers and typical extracellular matrix.

A-674563 increases chondrocyte marker expression in cultured chondrocytes by inhibiting Sox9 degradation

The implantation of autologous chondrocytes is a therapeutic treatment for articular cartilage damage. However, the benefits are limited due to the expansion of chondrocytes in monolayer culture, which causes loss of chondrocytic characters. Therefore, culture conditions that enhance chondrocytic characters are needed. We screened 5822 compounds and found that A-674563 enhanced the transcription of several chondrocyte marker genes, including Col2a1, Acan and Col11a2, in mouse primary chondrocytes. Experiments using cycloheximide, MG132 and bafilomycin A1 have revealed that Sox9 is degraded through the ubiquitin-proteasome pathway and that A-674563 inhibits this degradation, resulting in larger amount of Sox9 protein. RNA sequencing transcriptome analysis showed that A-674563 increases the expression of the gene that encodes ubiquitin-specific peptidase 29, which is known to induce the deubiquitination of proteins. Although the precise mechanism remains to be determined, our findings indicated that A-674563 could contribute to culture conditions that expand chondrocytes without losing chondrocytic characters.

iPS cell technologies and cartilage regeneration

Articular cartilage covers the ends of bone and provides shock absorption and lubrication to the diarthrodial joints. Cartilage has a limited capacity for repair when injured, and there is a need for cell sources for chondrocytes that can be transplanted as part of a regenerative medicine approach. Induced pluripotent stem cells (iPSCs) have pluripotency and the potential for self-renewal similar to embryonic stem cells (ESCs), but are not associated with the ethical issues that have plagued ESCs. Recent progress has made it possible to generate integration-free iPSCs and to differentiate iPSCs toward chondrocytes. An iPSC library prepared from donors homozygous for common HLA types is being developed, and will be able to provide allogeneic iPSC-derived chondrocytes at low cost that can cover the majority of the population. As an alternative approach, chondrocytic cells can be induced directly from dermal fibroblasts without going through the iPSC stage. Another important application of the iPSC technology is modeling cartilage diseases, such as skeletal dysplasia. Chondrogenically differentiated iPSCs generated from patients would recapitulate the pathology, and may serve as a useful platform both for exploring the disease mechanisms and for drug screening. This article is part of a Special Issue entitled "Stem Cells and Bone".

Time-lapse observation of the dedifferentiation process in mouse chondrocytes using chondrocyte-specific reporters

OBJECTIVE

When chondrocytes prepared from cartilage are expanded in monolayer culture, fibroblast-like cells gradually prevail. Although these prevailing fibroblast-like cells are believed to emerge because of the dedifferentiation of chondrocytes, the definite origin of the prevailing fibroblast-like cells has not been determined. We herein examined whether the prevailing non-chondrocytic cells observed after monolayer expansion culture arise from dedifferentiating chondrocytes or are the result of the overgrowth of fibroblasts that are present at the start of the culture. We also evaluated whether chondrocytes dedifferentiate because they proliferate or because they are cultured in monolayers.

METHODS

Chondrocytes were prepared from Col11a2-EGFP transgenic mice and Col11a2-Cre; R26-stop(flox)-EYFP transgenic mice, which respectively express enhanced green fluorescent protein (EGFP) and Cre specifically in chondrocytes under the control of Col11a2 promoter/enhancer sequences. Col11a2-Cre; R26-stop(flox)-EYFP mice express enhanced yellow fluorescent protein (EYFP) only in cells which express or used to express Cre. We performed a time-lapse observation of the chondrocytes during monolayer expansion culture, and also observed the chondrocytes after treatment with mitomycin C.

RESULTS

A time-lapse observation showed that Col11a2-EGFP chondrocytes underwent cell divisions, lost GFP fluorescence, increased cell numbers, and prevailed during the expansion culture. The observation of the Col11a2-Cre; R26-stop(flox)-EYFP chondrocytes confirmed that most of the cells after expansion in monolayer culture had been chondrocytes. Mitotically inactive chondrocytes generated by treatment with mitomycin C still underwent dedifferentiation, thus suggesting that chondrocyte dedifferentiation is not associated with cell division.

CONCLUSION

The non-chondrocytic cells that prevail after the monolayer expansion culture of chondrocytes originate from chondrocytes, and are not generated by the overgrowth of fibroblasts that are present at the start of the culture. Chondrocyte dedifferentiation does not appear to be associated with cell division.

Expression of collagen types I and II on articular cartilage in a rat knee contracture model

The purpose of our study was to clarify the expression patterns of collagen types I and II on articular cartilage after immobilization in a rat knee contracture model in 3 specific areas (noncontact area, transitional area, contact area). The unilateral knee joints of adult male rats were rigidly immobilized at 150 degrees of flexion using screws and a rigid plastic plate. Sham-operated animals had holes drilled in the femur and the tibia and screws inserted but were not plated. The expression patterns of collagen types I and II in each area were evaluated by in situ hybridization (ISH), immunohistochemistry (IHC), and quantitative real-time polymerase chain reaction (qPCR). The expression of collagen type II in the noncontact area was decreased by ISH but appeared unchanged when examined by IHC. In the transitional and contact areas, the expression of collagen type II was initially shown to have decreased and then increased at the hypertrophic chondrocytes by ISH but appeared decreased by IHC. Quantitative PCR revealed the decreased expression of type II collagen in the contact area. Immunostaining of collagen type I was increased at the noncontact area and transitional areas. Alterations of collagen types I and II expression may also affect the degeneration of articular cartilage after immobilization and the changes were different in the three areas.

Expression of collagen type I and type II in consecutive stages of human osteoarthritis

In normal hyaline cartilage the predominant collagen type is collagen type II along with its associated collagens, for example, types IX and XI, produced by normal chondrocytes. In contrast, investigations have demonstrated that in vitro a switch from collagen type II to collagen type I occurs. Some authors have detected collagen type I in osteoarthritic cartilage also in vivo, especially in late stages of osteoarthritis, while others have not. In the light of these diverging results, we have attempted to elucidate which type of collagen, type I and/or type II, is synthesized in the consecutive stages of human osteoarthritis. We performed in situ hybridization and immunohistochemistry with cartilage tissue samples from patients suffering from various stages of osteoarthritis. Furthermore, we quantitated our results on the gene expression of collagen type I and type II with the help of real-time PCR. We found that with the progression of the disease not only collagen type II, but also increasing amounts of collagen type I mRNA were produced. This supports the conclusion that collagen type I gradually becomes one of the factors involved in the pathogenesis of osteoarthritis.

Crosslinked type II collagen matrices: preparation, characterization, and potential for cartilage engineering

The limited intrinsic repair capacity of articular cartilage has stimulated continuing efforts to develop tissue engineered analogues. Matrices composed of type II collagen and chondroitin sulfate (CS), the major constituents of hyaline cartilage, may create an appropriate environment for the generation of cartilage-like tissue. In this study, we prepared, characterized, and evaluated type 11 collagen matrices with and without CS. Type II collagen matrices were prepared using purified, pepsin-treated, type II collagen. Techniques applied to prepare type I collagen matrices were found unsuitable for type II collagen. Crosslinking of collagen and covalent attachment of CS was performed using 1-ethyl-3-(3-dimethyl aminopropyl)carbodiimide. Porous matrices were prepared by freezing and lyophilization, and their physico-chemical characteristics (degree of crosslinking, denaturing temperature, collagenase-resistance, amount of CS incorporated) established. Matrices were evaluated for their capacity to sustain chondrocyte proliferation and differentiation in vitro. After 7 d of culture, chondrocytes were mainly located at the periphery of the matrices. In contrast to type I collagen, type II collagen supported the distribution of cells throughout the matrix. After 14 d of culture, matrices were surfaced with a cartilagenous-like layer, and occasionally clusters of chondrocytes were present inside the matrix. Chondrocytes proliferated and differentiated as indicated by biochemical analyses, ultrastructural observations, and reverse transcriptase PCR for collagen types I, II and X. No major differences were observed with respect to the presence or absence of CS in the matrices.

Some highlights in the emergence of modern concepts of osteoarthritis

The present-day concept that osteoarthritis may be amenable to biological modification rather than a hopeless expression of old age or injury has historical roots in the period of 1935 through the early 1970s. One root was the structural and chemical delineation of the connective tissues: discovery of the proteoglycans and multiple molecular species of collagen. Another was the recognition of the ability of mature articular chondrocytes to replicate themselves rather than being terminally differentiated. A third was the elucidation of the engineering physiology of the joint: the role of matrix hydrophilia to the material properties of articular cartilage and biolubrication. Each root has direct relevance to ongoing therapeutic approaches to degenerative joint disease. The early epidemiological studies of Kellgren and Lawrence evolved into new techniques for testing their validity in clinical practice. Along the way there was a rich 2-way interaction between scientists and clinicians in arriving at these ideas.

Canine chondrocytes seeded in type I and type II collagen implants investigated in vitro

Synthetic and natural absorbable polymers have been used as vehicles for implantation of cells into cartilage defects to promote regeneration of the articular joint surface. Implants should provide a pore structure that allows cell adhesion and growth, and not provoke inflammation or toxicity when implanted in vivo. The scaffold should be absorbable and the degradation should match the rate of tissue regeneration. To facilitate cartilage repair the chemical structure and pore architecture of the matrix should allow the seeded cells to maintain the chondrocytic phenotype, characterized by synthesis of cartilage-specific proteins. We investigated the behavior of canine chondrocytes in two spongelike matrices in vitro: a collagen-glycosaminoglycan (GAG) copolymer produced from bovine hide consisting of type I collagen and a porous scaffold made of type II collagen by extraction of porcine cartilage. Canine chondrocytes were seeded on both types of matrices and cultured for 3 h, 7 days, and 14 days. The histology of chondrocyte-seeded implants showed a significantly higher percentage of cells with spherical morphology, consistent with chondrocytic morphology, in the type II sponge at each time point. Pericellular matrix stained for proteoglycans and for type II collagen after 14 days. Biochemical analysis of the cell seeded sponges for GAG and DNA content showed increases with time. At day 14 there was a significantly higher amount of DNA and GAG in the type II matrix. This is the first study that directly compares the behavior of chondrocytes in type I and type II collagen matrices. The type II matrix may be of value as a vehicle for chondrocyte implantation on the basis of the higher percentage of chondrocytes retaining spherical morphology and greater biosynthetic activity that was reflected in the greater increase of GAG content.

An in vitro investigation of the PEMA/THFMA polymer system as a biomaterial for cartilage repair

A polymer system consisting of poly(ethyl methacrylate)/tetrahydrofurfuryl methacrylate (PEMA/THFMA) was investigated as a biomaterial for cartilage repair using chondrocyte culture. The PEMA/THFMA system and Thermanox control were shown to support chondrocytes seeded directly onto the surface for up to 28 days in culture. Differences were seen between the PEMA/THFMA system and Thermanox in DNA content, proliferation and glycosaminoglycon (GAG) synthesis. There was a significantly greater medium: cell GAG ratio for the PEMA/THFMA system compared to Thermanox. A greater number of chondrocytes isolated from the superficial zone of bovine cartilage attached to the PEMA/THFMA system compared to cells isolated from the deep zone, whereas the converse was seen for Thermanox. Matrix constituents including collagen type II were synthesised indicating that the differentiated phenotype was maintained for some of the chondrocytes, although the production of type I collagen indicated that dedifferentiation of some of the chondrocytes had occurred. In conclusion, this study has shown that the PEMA/THFMA system can support chondrocytes in vitro and together with further investigations could lead to the development of the polymer as an ideal candidate for articular cartilage repair.

Deep zone articular chondrocytes in vitro express genes that show specific changes with mineralization

We have developed a method to form reconstituted mineralized articular cartilagenous tissue in vitro from isolated deep zone chondrocytes. The aim of this study was to characterize further these cultures prior to and during mineralization. Histologic examination of the cells up to 8 days in culture showed that the chondrocytes had formed cartilagenous tissue. Similar to the in vivo cartilage, the chondrocytes expressed aggrecan, types II, I, and X collagens, osteopontin, and alkaline phosphatase (ALP). No osteocalcin mRNA expression was detected in either the in vivo cartilage or in vitro-generated tissue. Addition of beta-glycerophosphate (beta-GP) to the medium on day 5 induced mineralization and changes in gene expression. Expression of type X collagen, type II collagen, aggrecan core protein, and ALP were inhibited significantly between 2 h and 24 h after the addition of beta-GP. At 72 h, expression of these genes were still significantly depressed. These changes correlated with a decrease in collagen and proteoglycan synthesis, and ALP activity. Osteopontin expression increased within 8 h but returned to constitutive levels by 72 h. No change in type I collagen expression was detected. The changes in gene expression were not due to a direct effect of beta-GP itself, because similar gene changes occurred in the presence of phosphoethanolamine, another agent which induces mineralization. No changes in gene expression were seen in nonmineralizing cultures. In summary, articular chondrocytes grown on filter culture show expression of similar genes to the chondrocytes in the deep zone of articular cartilage and that changes in expression of specific genes were observed during tissue mineralization, suggesting that it is a suitable model to use to study the mechanism(s) regulating the localized mineralization of articular cartilage.

Immunocytochemical expression of type I and type II collagens by rat Meckel's chondrocytes in culture during phenotypic transformation

In culture, chondrocytes of Meckel's cartilage can differentiate further to become bone-type collagen-synthesizing cells. Here, the replacement of type II collagen by type I collagen, accompanying expression of the osteocytic phenotype, was analysed by double immunofluorescence staining, histochemistry and electron microscopy. After 1 week in culture, formation of a toluidine blue-positive matrix, demonstrating the synthesis of cartilaginous proteoglycans, and the expression of type II collagen were detected. After 2 weeks, immunoreactivity specific for type II collagen was detected along the cartilaginous areas of the nodules, and type I collagen appeared in association with the immunopositive extracellular matrix around spindle-shaped cells. Electron microscopy revealed that the extracellular matrix at this stage was composed of homogeneous fine fibrils of type II collagen and thick cross-banded bundles of type I collagen: there was also continuity between the type I and II collagens. Double immunofluorescence staining of 3 week-old cultures revealed that type II collagen had been replaced by type I which was synthesized by small round cells that appeared at the top of the nodules. With further passage of time in culture, the distribution of type I collagen expanded further towards the peripheral areas from the central areas of the nodules. The present combination of ultrastructural analysis and double immunofluorescence staining shows that the transition from synthesis of cartilage-specific type II collagen to expression of type I collagen occurred sequentially in spindle-shaped cells located at the top of nodules in conjunction with the further differentiation of Meckel's cartilage cells.

Effects of thapsigargin, an intracellular calcium-mobilizing agent, on synthesis and secretion of cartilage collagen and proteoglycan

The calcium-mobilizing agents thapsigargin and 2,5-di-(tert-butyl)-1,4- benzohydroquinone were shown to markedly elevate the intracellular calcium concentration of chick embryo chondrocytes in a dose-dependent manner. Under these conditions, the metabolism of macromolecules was variably affected. The synthesis and secretion of protein in general, and of collagen in particular, were significantly inhibited; in contrast, proteoglycan synthesis (but not glycosaminoglycan synthesis) was inhibited, whereas secretion was unaffected. Flunarizine, which prevented the thapsigargin-induced intracellular calcium elevation, and EGTA, which caused only a transient thapsigargin-induced intracellular calcium elevation, did not reverse these alterations. It was concluded, therefore, that the observed effects of thapsigargin and 2,5-di-(tert-butyl)-1,4-benzohydroquinone on chondrocyte macromolecule metabolism were not related to the ability of these drugs to increase the cytosolic free calcium concentration but may have been due to the specific depletion of the calcium sequestered in the endoplasmic reticulum. The differential effect of these drugs on protein and proteoglycan secretion suggests that the intracellular trafficking of these two classes of macromolecules may be controlled independently.

Prolonged exposure of human chondrocytes to ascorbic acid modifies cellular behavior in an agarose gel

Using an agarose gel culture system, the response of adult human chondrocytes to prolonged exposure of ascorbic acid was evaluated using histochemical, immunocytochemical and morphological techniques. The response of these cells to ascorbic acid was different from those previously reported in the literature. Many chondrocytes branched within the agarose gel with continued exposure to ascorbic acid while other chondrocytes maintained a round configuration typical of chondrocytes in vivo. Fibronectin and type I collagen were closely associated with the cell processes of the branching cells. Type II collagen and an alcian blue-staining matrix were associated with the rounded cells but not with the branched cells. These data suggest that the chondrocytes are able to express both dedifferentiated and redifferentiated phenotypes with ascorbic acid under these culture conditions. In addition, human chondrocytes were cultured in a collagen gel and began branching within 1 hour of culture. It is possible that an accumulation of type I collagen in the pericellular matrix of ascorbic acid treated cultures may enhance and explain the branching seen in these cultures. Studies by others have indicated that ascorbic acid may enhance, reduce, and/or modify the cartilage matrices produced by chondrocytes. These controversial reports in the literature are presumably due to variations between species and the culture methods employed.

Immunohistochemical analysis of interstitial collagens in cartilage of different stages of osteoarthrosis

The distribution of the interstitial collagens I, II and III was analyzed immunohistochemically in cartilage and bone samples from 32 patients with degenerative osteoarthrosis at various morphological stages. The alterations observed showed a very patchy, focal distribution demonstrating significant heterogeneity in the tissue reaction. In minor osteoarthrotic lesions a focal pericellular deposition of collagens III and I was seen, while the majority of the interterritorial matrix reacted exclusively with collagen II antibodies. These changes were first seen in the superficial cartilage layer. At the more advanced stages of osteoarthrosis, particularly when osteophytic bone spur formation was present, extensive changes in the expression of collagen types in the pericellular matrix was revealed with extensive and overlapping localization of collagens I, II and III in the whole cartilage. These observations support the suggestion that degenerative cartilage shows a collagen type "switch". In addition, it was demonstrated that the interterritorial cartilage matrix was still mainly composed of collagen II even in advanced lesions. These observations may explain some of the previous discrepancies reported.

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.

Chicken vigilin gene organization and expression pattern. The domain structure of the protein is reflected by the exon structure

Chicken vigilin was identified as a member of an evolutionary-conserved protein family with a unique repetitive domain structure. 14 tandemly repeated domains are found in chicken vigilin, all of which consist of a conserved sequence motif (subdomain A) and a potential alpha-helical region (subdomain B) [1]. We have established the physical structure of the chicken vigilin gene by restriction-fragment analysis and DNA sequencing of overlapping clones isolated from a phage lambda genomic DNA library. The chicken vigilin gene is a single-copy gene with a total of 27 exons which are distributed over a region of some 22 kbp. Exon 1 codes for a portion of the 5' untranslated region, exon 2 contains the translation start point and forms, along with exons 3 and 4, the N-terminal non-domain region. Exons 5-25 encode the vigilin domains 1-14 and the remaining exons 26 and 27 contain the non-domain C-terminal as well as the untranslated regions. The domain structure of the protein is reflected in the positioning of introns which demarcate individual domains. While domains 1-3 and 8-10 are each encoded by a single exon (5-7, 16-18); all other domains are contained in a set of two exons which are separated by introns interspersed at variable positions of the DNA segment coding for the conserved sequence motif. In conclusion, the data presented suggest that the chicken vigilin gene evolved by amplification of a primordial exon unit coding for the fundamental bipartite vigilin domain.

Complete cDNA sequence of chicken vigilin, a novel protein with amplified and evolutionary conserved domains

The complete cDNA (4375 bp), coding for a new protein called vigilin, was isolated from chicken chondrocytes. The cDNA shows an open reading frame of 1270 amino acids which are organized in 14 tandemly repeated homologous domains. Each domain consists of two subdomains, one with a conserved sequence motif of 35 amino acids (subdomain A) and another one with a presumptive alpha-helical structure of 21-33 amino acids (subdomain B). 149 amino acids at the N-terminus and 71 amino acids at the C-terminus of vigilin do not show the characteristic domain structure. No sequence characteristic of a signal peptide has been found, which argues for an intracellular localisation of vigilin. Vigilin is highly expressed in freshly isolated chicken chondrocytes but little in chondrocytes after prolonged time in culture. Vigilin mRNA exists in two size species, 4.4 kb and 6.5 kb in length due to the usage of different polyadenylation sites. Comparison of the vigilin sequence with data bases showed a remarkable similarity to protein HX from Saccharomyces cerevisiae [Delahodde, A., Becam, A. M., Perea, J. & Jacq, C. (1986) Nucleic Acids Res. 14, 9213-9214]. The yeast protein consists of eight homologous domains with 11 conserved amino acid residues within a set of 35 amino acids. The N-terminal and C-terminal regions of vigilin and protein HX do not reveal any sequence similarity. These results, together with the demonstration of the characteristic vigilin sequence motif in a human cDNA clone, suggest that the repeats represent evolutionary conserved autonomous domains within a family of proteins found in yeast, chicken and man.

Osteogenin promotes reexpression of cartilage phenotype by dedifferentiated articular chondrocytes in serum-free medium

Chondrocytes lose their phenotypic traits, including type II collagen, after serial passage in monolayer cultures. Osteogenin, a bone morphogenetic protein, induces cartilage and bone in nonskeletal sites. This investigation examined the ability of osteogenin to promote the reexpression of cartilage phenotype by dedifferentiated chondrocytes obtained from rabbit articular cartilage. The results revealed that osteogenin, in synergism with selected growth factors, promoted the reexpression of type II collagen and proteoglycans by dedifferentiated chondrocytes in agarose. Insulin, a constituent of the basal medium, appeared to be essential for the colony-forming aspect of this phenomenon, since when insulin was replaced by insulin-like growth factor-1 colony formation did not occur. Epidermal growth factor, platelet-derived growth factor (PDGF), and basic fibroblast growth factor appeared to be an optimal combination for the action of osteogenin. Neutralizing antibodies to transforming growth factor-beta did not influence the response to osteogenin. It is noteworthy that, compared to freshly passaged cells, those stored in liquid nitrogen were not as responsive to osteogenin and growth factors. A higher concentration of fibroblast growth factor in conjunction with osteogenin and PDGF, increased the responsiveness of frozen cells only in part, as the Alcian blue-positive proteoglycan matrix was not restored completely.

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.

Cryopreserved articular chondrocytes grow in culture, maintain cartilage phenotype, and synthesize matrix components

For osteochondral allograft transplantation to be successful, chondrocytes must survive preservation and retain their capacity to produce normal matrix components: proteoglycans and Type II collagen. Clinical success with osteochondral allograft transplantation has created an increased demand for supplies of suitable cartilage-bearing grafts. This demand has stimulated attempts to find successful methods for low temperature storage of cartilage for "banking" and heightened interest in cartilage cryobiology. In order to achieve the maximum viability of cryopreserved articular cartilage, previous comprehensive studies have focused on rates and temperatures of freezing, cryoprotective agents, and methods and influences of thawing. This study presents evidence that cryopreserved articular chondrocytes maintain their ability to grow in tissue culture following thawing and to produce normal matrix components. Chondrocytes isolated from Japanese white rabbits were divided into groups of fresh controls and experimental cryopreserved cells. Cells were incubated in dimethylsulfoxide, frozen at a rate of -1 degrees C/min, stored at -79 degrees C, rapidly thawed, and plated for culture. Growth rates were comparable in all groups. In all groups, typical chondroid characteristics were maintained throughout 14 days of culture. Typical cartilage phenotypic characteristics included maintenance of polygonal and rhomboidal cells, cell aggregation, proteoglycan production, and Type II collagen synthesis. This investigation strongly indicates that articular chondrocyte cryopreservation yields viable, functional cells and although these results cannot be directly extrapolated to intact adult articular cartilage, they do give further support for low temperature banking of cartilage-bearing allografts for transplantation.

Cathepsin B as a marker of the dedifferentiated chondrocyte phenotype

Rabbit articular cartilage does not secrete cathepsin B in organ culture. By established methods for modulating the chondrocyte phenotype in vitro, however, the synthesis, intracellular storage, and secretion of cathepsin B were followed up over a period of two months. With chondrocytes grown in monolayer cultures both the intracellular pool of the enzyme and its secretion were very small initially, but increased progressively to a factor of 110 after eight weeks. The secretion of cathepsin B was strongly depressed after transferring the cells from monolayer to collagen gel cultures. In contrast, collagenase was secreted in almost the same amounts during the whole period in both monolayer and collagen gel cultures. The cells cultured in collagen gels secreted more collagenase than those grown in monolayers. The reversible switch of cathepsin B secretion suggests that this enzyme, unlike collagenase, is a marker of the dedifferentiated chondrocyte phenotype. Cathepsin B was localised within cultured chondrocytes using antibodies raised against rabbit liver cathepsin B and shared with it many catalytic properties. Its Mr, however, was higher (34,000 compared with 27,000) and showed an unusual resistance to denaturation at neutral-alkaline pH, which may confer on this enzyme an important role in the degradation of cartilage matrix.

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.

Change in collagen synthesis of human chondrocyte culture. I. Development of a human model, demonstration of collagen type conversion by immunofluorescence

A research system constituted entirely of components of human origin was developed to study conversion of collagen synthesis by human chondrocytes. Type specificity of affinity chromatography-purified antibodies to human type II or type I collagen was proven by ELISA inhibition and immunofluorescence analysis. Human chondrocytes were isolated from articular cartilage and kept in monolayer cultures for eight subpassages. Conversion of type II to type I synthesis by chondrocytes was investigated by immunofluorescence. Staining with anti-type II collagen antibodies could be detected during primary cultures and in the first subpassage, whereas staining with anti-type I collagen antibodies occurred beginning from the end of primary cultures and was present up to the eighth subpassage. Results are compared to observations obtained in animal systems and their relevance to conditions in osteoarthritis is discussed.

In vitro cartilage formation on porous hydroxyapatite ceramic granules

Porous hydroxyapatite ceramic granules (diameter 0.5-1 mm) provide an excellent in vitro matrix for cell growth. Canine chondrocytes maintained their collagen (Type II) phenotype up to 11 mo when cultured on these granules. Chondrocytes proliferated throughout the 13 mo of culture. Cells divided and added on to form multilayers around each ceramic granule. By the end of 11 mo, some layers were thicker than the diameter of the granules. Cell number per culture dish increased 240-fold over the original number of cells seeded in 8 mo of culture. Morphologically, chondrocytes remained spherical and formed cohesive multilayers as early as 1 wk. Metachrometric extracellular matrix was evident by the first week and increased through the 13 mo period.

Synthesis of cartilage matrix by mammalian chondrocytes in vitro. III. Effects of ascorbate

Chondrocytes isolated from bovine articular cartilage were plated at high density and grown in the presence or absence of ascorbate. Collagen and proteoglycans, the major matrix macromolecules synthesized by these cells, were isolated at times during the course of the culture period and characterized. In both control and ascorbate-treated cultures, type II collagen and cartilage proteoglycans accumulated in the cell-associated matrix. Control cells secreted proteoglycans and type II collagen into the medium, whereas with time in culture, ascorbate-treated cells secreted an increasing proportion of types I and III collagens into the medium. The ascorbate-treated cells did not incorporate type I collagen into the cell-associated matrix, but continued to accumulate type II collagen in this compartment. Upon removal of ascorbate, the cells ceased to synthesize type I collagen. Morphological examination of ascorbate-treated and control chondrocyte culture revealed that both collagen and proteoglycans were deposited into the extracellular matrix. The ascorbate-treated cells accumulated a more extensive matrix that was rich in collagen fibrils and ruthenium red-positive proteoglycans. This study demonstrated that although ascorbate facilitates the formation of an extracellular matrix in chondrocyte cultures, it can also cause a reversible alteration in the phenotypic expression of those cells in vitro.

Synthesis of proteoglycans, collagen, and elastin by cultures of rabbit auricular chondrocytes--relation to age of the donor

Chondrocytes were isolated from the auricular cartilage of rabbits, aged 1 week to 30 months, and grown in short-term cell culture. The cells from the 1-week animals were small, polygonal, and mononucleated, while the chondrocytes from the older animals were larger, rounded, and frequently binucleated. The synthesis of proteoglycans, collagen, and elastin was determined by isotope incubation. Chemical characterization of the proteoglycans was also performed. The production of the matrix macromolecules showed a clear age dependence with peak synthesis occurring at different ages. Proteoglycans were actively synthesized by chondrocytes from all age groups with a broad maximum between 2 weeks and 5 months followed by a sharp decline to about 50% of the 1-week level at 12-30 months. Collagen synthesis peaked at 2 weeks, declining progressively thereafter to about 60% of the 1-week level at 30 months. Elastin synthesis was highest in the 1-week cultures and thereafter fell quickly to very low levels. In all age groups the chondrocytes synthesized predominantly cartilage-typic proteoglycans, i.e., large aggregate forming molecules containing chondroitin sulfate. Monomers and aggregates showed a size maximum at 2-8 weeks. The degree of sulfation of the chondroitin sulfate and the proportion of 6-sulfate increased with age. These findings support the concept of "age programs" for the biosynthesis and turnover of different matrix macromolecules.

DNA repair by articular chondrocytes. I. Unscheduled DNA synthesis following ultraviolet irradiation in monolayer culture

The hypothesis that aging of articular chondrocytes at a cellular level results from loss of DNA repair capability was studied by measuring unscheduled DNA synthesis (UDS). Cultured rabbit and human articular chondrocytes were irradiated with 254 nm ultraviolet light (20 J/m2) following treatment with 10 mM hydroxyurea. Neither the "in vitro senescence" nor spontaneous transformation that developed during serial passage of rabbit chondrocytes was accompanied by diminution of UDS. Synthesis of sulfated glycosaminoglycans declined more rapidly than the ability of the cells to divide. Levels of UDS by chondrocytes from old donors, rabbit or human, were comparable to those of younger individuals. UDS was greater in human than rabbit chondrocytes. Similar data have been reported previously for dermal fibroblasts but do not necessarily indicate that there is a direct or causative relationship between UDS capability and the longevity of mammalian species. X-Irradiation of rabbit chondrocytes or cartilage explants, in doses up to 40 000 rads, yielded no measurable UDS.

Synthesis of cartilage matrix by mammalian chondrocytes in vitro. II. Maintenance of collagen and proteoglycan phenotype

The in vitro phenotype of bovine articular chondrocytes is described. Chondrocytes plated at high density in roller-bottle and dish cultures were maintained in vitro. The major matrix macromolecules, collagen and proteoglycan, synthesized by these cells were characterized during the course of the culture period. The chondrocytes synthesized mainly Type II collagen, which was found predominantly in the cell-associated matrix. The media contained a mixture of Type II and Type III collagens. Type I collagen was detectable in neither the medium nor the cell-associated matrix. The proteoglycan monomers found in media and cell-associated matrix had the same hydrodynamic sizes as monomers synthesized by cartilage slices or those extracted from adult articular cartilage. The majority of proteoglycans synthesized by the cells were found in high molecular weight aggregates which were readily recovered from the media and were extractable from cell-associated matrix with low ionic strength buffers. The results demonstrate the long-term in vitro phenotypic stability of the bovine articular chondrocytes. The advantages of the in vitro system as a model for studying the effects of external agents, such as drugs and vitamins, are discussed.

Stimulation of DNA synthesis by ascorbate in cultures of articular chondrocytes

The addition of 0.2 mM Na L-ascorbate increased the incorporation of 3H-thymidine by rabbit articular chondrocytes in cell and organ culture. The stimulatory response of explants to ascorbate was potentiated by pretreatment of the cartilage with 0.2% clostridial collagenase (type 1) or trypsin for 15-30 minutes. In explants there was a latent period of 3 to 4 days before increased labeling of the nuclei could be detected. The effect was transient and declined after 8 days of culture. It was more evident in organ cultures of immature (3-month-old) than 2- to 3-year-old rabbits. Age differences were not detected in cell cultures. Explants of adult human articular cartilage were stimulated by ascorbate when the medium was supplemented with 10% fresh human serum but not by fetal bovine serum. The findings indicated that synthesis of DNA by articular chondrocytes in situ is regulated by responsiveness of the cells proper to compounds such as vitamin C, by properties of the extracellular matrix, and by factors in the serum. Ascorbate was cytotoxic at concentrations greater than 0.2 mM in the presence of certain batches of serum.

Change in the expression of collagen genes in dividing and nondividing chondrocytes

Chondrocytes were isolated from the sterna of 17-day-old chick embryos by enzyme digestion. Rapid proliferation of chondrocytes was achieved in the presence of chick serum (10%, v/v). Addition of either hydroxyurea (10(-3) M) or cytosine arabinoside (10 microgram/ml) to the culture medium was used to arrest growth of cells. Under both conditions, however, a rather fast switch of synthesis from type II to type I collagen was observed. This suggests that the loss of the differentiated state of chondrocytes in culture is not necessarily bound to mitosis.

Biosynthesis of collagen and other matrix proteins by articular cartilage in experimental osteoarthrosis

Osteoarthrosis was induced in one knee joint of dogs by an established surgical procedure. Changes in the articular cartilage in the biosynthesis of collagen and other proteins were sought by radiochemical labelling in vivo, with the following findings. (1) Collagen synthesis was stimulated in all cartilage surfaces of the experimental joints at 2, 8 and 24 weeks after surgery. Systemic labelling with [3H]proline showed that over 10 times more collagen was being deposited per dry weight of experimental cartilage compared with control cartilage in the unoperated knee. (2) Type-II collagen was the radiolabelled product in all samples of experimental cartilage ranging in quality from undamaged to overtly fibrillated, and was the only collagen detected chemically in the matrix of osteoarthrotic cartilage from either dog or human joints. (3) Hydroxylysine glycosylation was examined in the newly synthesized cartilage collagen by labelling dog joints in vivo with [3H]lysine. In experimental knees the new collagen was less glycosylated than in controls. However, no difference in glycosylation of the total collagen in the tissues was observed by chemical analysis. (4) Over half the protein-bound tritium was extracted by 4 M-guanidinium chloride from control cartilage labelled with [3H]proline, compared with one-quarter or less from experimental cartilage. Two-thirds of the extracted tritium separated in the upper fraction on density-gradient centrifugation in CsCl under associative conditions. Much of this ran with a single protein band on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis under reducing conditions. The identity of this protein was unknown, although it resembled serum albumin in mobility afte disulphide-bond cleavage.

Behaviour of epiphyseal mouse chondrocyte populations in monolayer culture. Morphological and immunohistochemical studies

Growth and dedifferentiation of a heterogeneous mouse chondrocyte population, prepared from epiphyses of mouse embryos (day 17 of gestation), were studied in primary monolayer culture. At different times of culture, light and electron microscopic investigations were carried out and the change of collagen types was shown by immunofluorescence microscopy. During the first four days in culture, chondrocytes express their typical phenotype. Round or polygonal cells are embedded in a metachromatically staining matrix and produce type II collagen. After four to eight days in vitro most of the chondrocytes lose their matrix capsule and alter to fibroblast-like cells. Simultaneously, a switch of collagen synthesis to type III and type I collagen occurs, whereas the type II collagen synthesis is stopped. Altered cells and transitional stages have intracellular glycogen like typical chondrocytes, but show phagocytosis and indications of cell migration like fibroblasts. It is proposed that these cells, originating from a subpopulation of epiphyseal cartilage, are able to differentiate and dedifferentiate in vitro.

Explant culture of human and rabbit articular chondrocytes

Copious outgrowth of chondrocytes was obtained by explantation from each of three rabbit and one surgically-resected human articular cartilages pretreated briefly with trypsin. In lapine explants, ascorbate (40 micrograms/ml) increased DNA three-fold over control values and resulted in deposition of a chondroid matrix. It doubled radiosulfate incorporation by the outgrowths. Up to 56% of the sulfated glycosaminoglycan synthesized was located in the trypsin-digestible pericellular coat compared with about 15% in previous monolayer cultures. The collagens synthesized were characterized partially. In rabbit cell cultures, the alpha 1:alpha 2 ratio varied from 2.9 to 3.8. In human cultures, an unusual post-alpha 2 peak was observed. The findings suggest an uncoupling of the phenotypic expression of the major cartilaginous macromolecules in the cultures. There were no distinctive differences between chondrocytes derived from normal and fibrillated human cartilage of the same individual.

The proliferation of chondrocytes and pannus in adjuvant arthritis

Cell proliferation in the pannus formation of adjuvant arthritis was studied by autoradiography. It was found that after day 9 an increased cell proliferation starts in the joint capsule recessus and synovial villi on the injected side. From these proliferating cells a pannus, which during the first phase frequently consists only of few cell layers, extends over the cartilage surface. With advancing disease the thickness of the pannus increases and further centripetal growth may cause the entire cartilage surface to be covered. This proliferating pannus tissue may invade the cartilage and destroy it. Since in this area of destruction labelled cells are frequently present, it may be assumed that proliferating cells with a high enzyme content are particularly responsible for the immediate degradation of cartilage. No involvement of chondrocytes in pannus formation was confirmed by the methods employed. There was neither increased proliferation of surface chondrocytes nor increased proliferation of chondrocytes in the depth of cartilage.