Immunological studies on collagen type transition in chondrogenesis

Cycloastragenol as an Exogenous Enhancer of Chondrogenic Differentiation of Human Adipose-Derived Mesenchymal Stem Cells. A Morphological Study

Stem cell therapy and tissue engineering represent a promising approach for cartilage regeneration. However, they present limits in terms of mechanical properties and premature de-differentiation of engineered cartilage. Cycloastragenol (CAG), a triterpenoid saponin compound and a hydrolysis product of the main ingredient in Astragalus membranaceous, has been explored for cartilage regeneration. The aim of this study was to investigate CAG’s ability to promote cell proliferation, maintain cells in their stable active phenotype, and support the production of cartilaginous extracellular matrix (ECM) in human adipose-derived mesenchymal stem cells (hAMSCs) in up to 28 days of three-dimensional (3D) chondrogenic culture. The hAMSC pellets were cultured in chondrogenic medium (CM) and in CM supplemented with CAG (CAG–CM) for 7, 14, 21, and 28 days. At each time-point, the pellets were harvested for histological (hematoxylin and eosin (H&E)), histochemical (Alcian-Blue) and immunohistochemical analysis (Type I, II, and X collagen, aggrecan, SOX9, lubricin). After excluding CAG’s cytotoxicity (MTT Assay), improved cell condensation, higher glycosaminoglycans (sGAG) content, and increased cell proliferation have been detected in CAG–CM pellets until 28 days of culture. Overall, CAG improved the chondrogenic differentiation of hAMSCs, maintaining stable the active chondrocyte phenotype in up to 28 days of 3D in vitro chondrogenic culture. It is proposed that CAG might have a beneficial impact on cartilage regeneration approaches.

Extracellular matrix components and culture regimen selectively regulate cartilage formation by self-assembling human mesenchymal stem cells in vitro and in vivo

BACKGROUND

Cartilage formation from self-assembling mesenchymal stem cells (MSCs) in vitro recapitulate important cellular events during mesenchymal condensation that precedes native cartilage development. The goal of this study was to investigate the effects of cartilaginous extracellular matrix (ECM) components and culture regimen on cartilage formation by self-assembling human MSCs in vitro and in vivo.

METHODS

Human bone marrow-derived MSCs (hMSCs) were seeded and compacted in 6.5-mm-diameter transwell inserts with coated (type I, type II collagen) or uncoated (vehicle) membranes, at different densities (0.5 × 10⁶, 1.0 × 10⁶, 1.5 × 10⁶ per insert). Pellets were formed by aggregating hMSCs (0.25 × 10⁶) in round-bottomed wells. All tissues were cultured for up to 6 weeks for in vitro analyses. Discs (cultured for 6, 8 or 10 weeks) and pellets (cultured for 10 weeks) were implanted subcutaneously in immunocompromised mice to evaluate the cartilage stability in vivo.

RESULTS

Type I and type II collagen coatings enabled cartilage disc formation from self-assembling hMSCs. Without ECM coating, hMSCs formed dome-shaped tissues resembling the pellets. Type I collagen, expressed in the prechondrogenic mesenchyme, improved early chondrogenesis versus type II collagen. High seeding density improved cartilage tissue properties but resulted in a lower yield of disc formation. Discs and pellets exhibited compositional and organizational differences in vitro and in vivo. Prolonged chondrogenic induction of the discs in vitro expedited endochondral ossification in vivo.

CONCLUSIONS

The outcomes of cartilage tissues formed from self-assembling MSCs in vitro and in vivo can be modulated by the control of culture parameters. These insights could motivate new directions for engineering cartilage and bone via a cartilage template from self-assembling MSCs.

Diethylstilbestrol-induced mouse hypospadias: "window of susceptibility"

This review presents published and novel results that define the programming window for diethylstilbestrol (DES)-induced abnormal development of the mouse penis. These data indicate that DES has its greatest effect during the period of most intense penile morphogenesis, namely postnatal days 0-15 (P0-P15). Pregnant mice and their neonatal pups were injected subcutaneously with 200 ng/gbw DES every other day from embryonic day 12-18 (DES E12-E18), postnatal day 0-10 (DES P0-P10), embryonic day 12 to postnatal day 10 (DES E12-P10), postnatal day 5-15 (DES P5-P15), and postnatal day 10-20 (DES P10-P20). Aged-matched controls received sesame oil vehicle. After euthanasia at 10, 15, 20 and 60 days, penises were analyzed by gross morphology, histology and morphometry. Penises of all 5 groups of DES-treated mice were reduced in size, which was confirmed by morphometric analysis of internal penile structures. The most profound effects were seen in the DES E12-P10, DES P0-P10, and DES P5-P15 groups, thus defining a DES "programming window". For all parameters, DES treatment from P10 to P20 showed the most mild of effects. Adverse effects of DES on the MUMP cartilage and erectile bodies observed shortly after the last DES injection reverted to normality in the DES P5-P15, but not in the E12-P10 and P0-P10 groups, in which MUMP cartilage and erectile body malformations persisted into adulthood, again emphasizing a "window of susceptibility" in the early neonatal period.

Chondrocyte and mesenchymal stem cell-based therapies for cartilage repair in osteoarthritis and related orthopaedic conditions

Osteoarthritis (OA) represents a final and common pathway for all major traumatic insults to synovial joints. OA is the most common form of degenerative joint disease and a major cause of pain and disability. Despite the global increase in the incidence of OA, there are no effective pharmacotherapies capable of restoring the original structure and function of damaged articular cartilage. Consequently cell-based and biological therapies for osteoarthritis (OA) and related orthopaedic disorders have become thriving areas of research and development. Autologous chondrocyte implantation (ACI) has been used for treatment of osteoarticular lesions for over two decades. Although chondrocyte-based therapy has the capacity to slow down the progression of OA and delay partial or total joint replacement surgery, currently used procedures are associated with the risk of serious adverse events. Complications of ACI include hypertrophy, disturbed fusion, delamination, and graft failure. Therefore there is significant interest in improving the success rate of ACI by improving surgical techniques and preserving the phenotype of the primary chondrocytes used in the procedure. Future tissue-engineering approaches for cartilage repair will also benefit from advances in chondrocyte-based repair strategies. This review article focuses on the structure and function of articular cartilage and the pathogenesis of OA in the context of the rising global burden of musculoskeletal disease. We explore the challenges associated with cartilage repair and regeneration using cell-based therapies that use chondrocytes and mesenchymal stem cells (MSCs). This paper also explores common misconceptions associated with cell-based therapy and highlights a few areas for future investigation.

Quantification of type II procollagen splice forms using alternative transcript-qPCR (AT-qPCR)

During skeletal development, the onset of chondrogenic differentiation is marked by expression of the α1(II) procollagen (Col2a1) gene. Exon 2 of Col2a1 codes for a cysteine-rich von Willebrand factor C-like domain. Chondroprogenitors express the exon 2-containing IIA and IID splice forms by utilizing adjacent 5' splice sites separated by 3 base pairs. There is a shift to expression of the shorter, exon 2-lacking IIB splice form with further differentiation. Alternative splicing analysis of Col2a1 splice forms has often relied upon semi-quantitative PCR, using a single set of PCR primers to amplify multiple splice forms. We show that this widely used method is inaccurate due to mismatched amplification efficiency of different-sized PCR products. We have developed the TaqMan®-based AT-qPCR (Alternative Transcript-qPCR) assay to more accurately quantify alternatively spliced mRNA, and demonstrate the measurement of Col2a1 splice form expression in differentiating ATDC5 cells in vitro, and in wild type mouse embryonic and postnatal cartilage in vivo. The AT-qPCR assay is based on the use of a multiple-amplicon standard (MAS) plasmid, containing a chemically synthesized cluster of splice site-spanning PCR amplicons, to quantify alternative splice forms by standard curve-based qPCR. The MAS plasmid designed for Col2a1 also contained an 18S rRNA amplicon for sample normalization, and an amplicon corresponding to a region spanning exon 52-53 to measure total Col2a1 mRNA. In mouse E12.5 to P70 cartilages, we observed the expected switch between the IIA and IIB splice forms; no IID or IIC splice products were observed. However, in the ATDC5 cultures, predominant expression of the IIA and IID splice forms was found at all times in culture. Additionally, we observed that the sum of the IIA, IIB and IID splice forms comprises only a small fraction of Col2a1 transcripts containing the constitutive exon 52-53 junction. We conclude from our results that the majority of ATDC5 cells in the assay described in this study remained as chondroprogenitors during culture in standard differentiation conditions, and that additional Col2a1 transcripts may be present. The validity of this novel AT-qPCR assay was confirmed by demonstrating the expected Col2a1 isoform expression patterns in vivo in developing mouse cartilage. The ability to measure true levels of procollagen type II splice forms will provide better monitoring of chondrocyte differentiation in other model systems. In addition, the AT-qPCR assay described here could be applied to any gene of interest to detect and quantify known and predicted alternative splice forms and can be scaled up for high throughput assays.

Wnt Influence on Chondrocyte Differentiation and Cartilage Function

The Wnt signaling network regulates chondrocyte differentiation, proliferation, and maturation during embryonic limb development. In this review, we summarize studies of Wnt signaling during the chondrocyte life cycle in avian and mammalian systems, both before and after birth. Recent reports that implicate abnormal Wnt signaling as a contributing factor to pathogenic joint conditions are also discussed. In addition, we show new data that suggests Wnt signaling is active in adult cartilage. Overall, it appears that the Wnt network has dual roles in cartilage, as has been described in other tissues: it is an important regulator of chondrocyte development, but deregulated signaling is detrimental to mature tissues and may lead to disease.

Sox9 neural crest determinant gene controls patterning and closure of the posterior frontal cranial suture

Cranial suture development involves a complex interaction of genes and tissues derived from neural crest cells (NCC) and paraxial mesoderm. In mice, the posterior frontal (PF) suture closes during the first month of life while other sutures remain patent throughout the life of the animal. Given the unique NCC origin of PF suture complex (analogous to metopic suture in humans), we performed quantitative real-time PCR and immunohistochemistry to study the expression pattern of the NCC determinant gene Sox9 and select markers of extracellular matrix. Our results indicated a unique up-regulated expression of Sox9, a regulator of chondrogenesis, during initiation of PF suture closure, along with the expression of specific cartilage markers (Type II Collagen and Type X Collagen), as well as cartilage tissue formation in the PF suture. This process was followed by expression of bone markers (Type I Collagen and Osteocalcin), suggesting endochondral ossification. Moreover, we studied the effect of haploinsufficiency of the NCC determinant gene Sox9 in the NCC derived PF suture complex. A decrease in dosage of Sox9 by haploinsufficiency in NCC-derived tissues resulted in delayed PF suture closure. These results demonstrate a unique development of the PF suture complex and the role of Sox9 as an important contributor to timely and proper closure of the PF suture through endochondral ossification.

Primary and secondary cartilages of the neonatal rat: the femoral head and the mandibular condyle

Primary and secondary cartilages differ in embryonic origin and in histological organization, and are generally considered to have a different mode of growth. However, few studies have directly compared the two types of cartilage of the same animal at the same age. Therefore, we analysed several histological and biochemical differences between secondary cartilage of the mandibular condyle and primary cartilage of the femoral head of 4-d-old rats. We evaluated the tissue organization, the level of DNA and glycosaminoglycan (GAG) synthesis, and the GAG and collagen content. The expression of collagen types I, II and III and of receptors for insulin-like growth factor (IGF)-I, fibroblast growth factor (FGF), and transforming growth factor (TGF)-beta were investigated by immunohistochemistry. The ex vivo DNA and GAG synthesis as well as the GAG content of femoral heads were much higher than that of mandibular condyles. Mandibular condyles expressed both collagen types I and II, while femoral heads expressed only type II collagen. In the mandibular condyles, receptors for IGF-I, FGF, and TGF-beta were observed mainly in the superficial layers, whereas they were found throughout the entire femoral head. In conclusion, major differences were found between both types of cartilage, which might be related to their specific functional demands.

Effect of oxygen tension and alginate encapsulation on restoration of the differentiated phenotype of passaged chondrocytes

The implantation of laboratory-grown tissue offers a valuable alternative approach to the treatment of cartilage defects. Procuring sufficient cell numbers for such tissue-engineered cartilage is a major problem since amplification of chondrocytes in culture typically leads to loss of normal cell phenotype yielding cartilage of inferior quality. In an effort to overcome this problem, we endeavored to regain the differentiated phenotype of chondrocytes after extensive proliferation in monolayer culture by modulating cell morphology and oxygen tension towards the in vivo state. Passaged cells were encapsulated in alginate hydrogel in an effort to regain the more rounded shape characteristic of differentiated chondrocytes. These cultures were exposed to reduced (5%-i.e., physiological), or control (20%) oxygen tensions. Both alginate encapsulation and reduced oxygen tension significantly upregulated collagen II and aggrecan core protein expression (differentiation markers). In fact, after 4 weeks in alginate at 5% oxygen, differentiated gene expression was comparable to primary chondrocytes. Collagen I expression (dedifferentiation marker) decreased dramatically after alginate entrapment, while reduced oxygen tension had no effect. It is concluded that alginate encapsulation and reduced oxygen tension help restore key differentiated phenotypic markers of passaged chondrocytes. These findings have important implications for cartilage tissue engineering, since they enable the increase in differentiated cell numbers needed for the in vitro development of functional cartilaginous tissue suitable for implantation.

Compressive force promotes chondrogenic differentiation and hypertrophy in midpalatal suture cartilage in growing rats

Midpalatal suture cartilage (MSC) is secondary cartilage located between the bilateral maxillary bones and has been utilized in the analysis of the biomechanical characteristics of secondary cartilage. The present study was designed to investigate the effects of compressive force on the differentiation of cartilage in midpalatal suture cartilage in rats. Forces of various magnitudes were applied to the midpalatal suture cartilage in 4-week-old male Wistar rats for 1, 2, 4, 7, or 14 days, mediated through the bilateral 1st molars using orthodontic wires. The differentiation pathways in the MSC cells were examined by immunohistochemistry for the differentiation markers type I, type II and type X collagen, and glycosaminoglycans (GAGs), chondroitin-4-sulfate, chondroitin-6-sulfate and keratan sulfate. Histologically and immunohistochemically, the midpalatal suture cartilage in control rats had the characteristic appearance of secondary cartilage. In the experimental groups, the center of the midpalatal suture cartilage that contained osteo-chondro progenitor cells seemed to become mature cartilage and its immuno-reaction to type II and X collagen and GAGs increased as the experiment progressed. This differentiation was dependent upon the magnitude and duration of the force applied to the midpalatal suture cartilage; i.e., cartilaginous differentiation progressed more rapidly as the applied force increased. The present results suggest that the differentiation of osteo-chondro progenitor cells into mature and hypertrophic chondrocytes in the precartilaginous cell layer is promoted by compressive force.

Col2-GFP reporter marks chondrocyte lineage and chondrogenesis during mouse skeletal development

Mice were generated in which a Col2-GFP transgene serves as a reporter for the chondrocyte lineage and for chondrogenesis in live embryos and newborn pups. Cells actively engaged in chondrogenesis were identified by confocal optical sectioning within their native environments in embryos and in thick tissue slices. Chondrocytes exhibiting GFP fluorescence were purified from rib cages by high-speed cell sorting of crude cell suspensions. Intensity of fluorescence correlated with biosynthesis of procollagen II in these cells. The use of these mice and their cells provides a novel approach for studying chondrocyte differentiation and chondrogenesis during skeletal development.

In situ hybridisation study of type I, II, X collagens and aggrecan mRNas in the developing condylar cartilage of fetal mouse mandible

The aim of this study was to investigate the developmental characteristics of the mandibular condyle in sequential phases at the gene level using in situ hybridisation. At d 14.5 of gestation, although no expression of type II collagen mRNA was observed, aggrecan mRNA was detected with type I collagen mRNA in the posterior region of the mesenchymal cell aggregation continuous with the ossifying mandibular bone anlage prior to chondrogenesis. At d 15.0 of gestation, the first cartilaginous tissue appeared at the posterior edge of the ossifying mandibular bone anlage. The primarily formed chondrocytes in the cartilage matrix had already shown the appearance of hypertrophy and expressed types I, II and X collagens and aggrecan mRNAs simultaneously. At d 16.0 of gestation, the condylar cartilage increased in size due to accumulation of hypertrophic chondrocytes characterised by the expression of type X collagen mRNA, whereas the expression of type I collagen mRNA had been reduced in the hypertrophic chondrocytes and was confined to the periosteal osteogenic cells surrounding the cartilaginous tissue. At d 18.0 of gestation before birth, cartilage-characteristic gene expression had been reduced in the chondrocytes of the lower half of the hypertrophic cell layer. The present findings demonstrate that the initial chondrogenesis for the mandibular condyle starts continuous with the posterior edge of the mandibular periosteum and that chondroprogenitor cells for the condylar cartilage rapidly differentiate into hypertrophic chondrocytes. Further, it is indicated that sequential rapid changes and reductions of each mRNA might be closely related to the construction of the temporal mandibular ramus in the fetal stage.

Collagen II is essential for the removal of the notochord and the formation of intervertebral discs

Collagen II is a fibril-forming collagen that is mainly expressed in cartilage. Collagen II-deficient mice produce structurally abnormal cartilage that lacks growth plates in long bones, and as a result these mice develop a skeleton without endochondral bone formation. Here, we report that Col2a1-null mice are unable to dismantle the notochord. This defect is associated with the inability to develop intervertebral discs (IVDs). During normal embryogenesis, the nucleus pulposus of future IVDs forms from regional expansion of the notochord, which is simultaneously dismantled in the region of the developing vertebral bodies. However, in Col2a1-null mice, the notochord is not removed in the vertebral bodies and persists as a rod-like structure until birth. It has been suggested that this regional notochordal degeneration results from changes in cell death and proliferation. Our experiments with wild-type mice showed that differential proliferation and apoptosis play no role in notochordal reorganization. An alternative hypothesis is that the cartilage matrix exerts mechanical forces that induce notochord removal. Several of our findings support this hypothesis. Immunohistological analyses, in situ hybridization, and biochemical analyses demonstrate that collagens I and III are ectopically expressed in Col2a1-null cartilage. Assembly of the abnormal collagens into a mature insoluble matrix is retarded and collagen fibrils are sparse, disorganized, and irregular. We propose that this disorganized abnormal cartilage collagen matrix is structurally weakened and is unable to constrain proteoglycan-induced osmotic swelling pressure. The accumulation of fluid leads to tissue enlargement and a reduction in the internal swelling pressure. These changes may be responsible for the abnormal notochord removal in Col2a1-null mice. Our studies also show that chondrocytes do not need a collagen II environment to express cartilage-specific matrix components and to hypertrophy. Furthermore, biochemical analysis of collagen XI in mutant cartilage showed that alpha1(XI) and alpha2 (XI) chains form unstable collagen XI molecules, demonstrating that the alpha3(XI) chain, which is an alternative, posttranslationally modified form of the Col2a1 gene, is essential for assembly and stability of triple helical collagen XI.

Immunohistochemical findings type I and type II collagen in prenatal mouse mandibular condylar cartilage compared with the tibial anlage

In growing animals the mandibular condylar cartilage serves not only as an articular but also as a growth cartilage, yet, condylar cartilage has some characteristic features that are not found in growth cartilage. For example, some reports suggest that type I collagen, which is not seen in the growth plate cartilage of long bones, is present in the extracellular matrix of condylar cartilage postnatally. Here, the condylar and limb bud cartilage of fetal mice was examined. The distribution of type I and type II collagen in condylar cartilage was already different from that in the limb bud at the first appearance of the cartilage. Type I collagen was demonstrated in the extracellular matrix of the condylar cartilage that first appeared on day 15 of gestation. However, the reaction for type II collagen was much weaker than that for type I collagen. On day 18 of gestation, type I collagen was still found throughout the cell layers but became gradually weaker with depth. Type II collagen was limited exclusively to the deeper layers at this stage. These findings are different from those in the limb bud cartilage, indicating a characteristic feature of the cells in the condylar cartilage present from the prenatal period.

Chondrocyte-specific enhancer elements in the Col11a2 gene resemble the Col2a1 tissue-specific enhancer

Type XI collagen and type II collagen are coexpressed in all cartilage, and both are essential for normal cartilage differentiation and skeletal morphogenesis. This laboratory has recently identified a 48-base pair (bp) enhancer element in the type II collagen gene Col2a1 that contains several HMG-type protein-binding sites and that can direct chondrocyte-specific expression in transient transfection and in transgenic mice. The present study has identified two short chondrocyte-specific enhancer elements within a region in the 5' portion of the type XI collagen gene Col11a2 that has previously been shown to influence chondrocyte-specific expression in transgenic mice. These Col11a2 enhancer elements, like the Col2a1 enhancer, contain several sites with homology to the high mobility group (HMG) protein-binding consensus sequence. In electrophoretic mobility shift assays, the Col11a2 elements formed a DNA-protein complex that was dependent on the presence of the HMG-like sites. It had the same mobility as the complex formed with the Col2a1 48-bp enhancer and appeared to contain the same or similar proteins, including SOX9. The Col11a2 elements directed gene expression in transient transfections of chondrocytes but not fibroblasts, and their activity was abolished by mutation of the HMG-like sites. Ectopically expressed SOX9 activated these enhancers in non-chondrocytic cells, as it also activates the Col2a1 enhancer. Finally, the Col11a2 enhancer elements both directed transgene expression to cartilage in developing mouse embryos. Overall, our results indicate that the two Col11a2 chondrocyte-specific enhancer elements share many similarities with the Col2a1 48-bp enhancer. These similarities suggest the existence of a genetic program designed to coordinately regulate the expression of these and perhaps other genes involved in the chondrocyte differentiation pathway.

Effects of titanium substratum and grooved surface topography on metalloproteinase-2 expression in human fibroblasts

The chemical and topographic effects of commercially pure titanium on cell morphology and the regulation of matrix metalloproteinase-2 (MMP-2) gene expression, synthesis, and activity were investigated in early passage human gingival fibroblasts. Scanning electron microscopy showed that on smooth titanium (Ti), fibroblasts remained well spread and randomly oriented throughout the culture period. In contrast, cells on V-shaped grooved titanium (VTi) were oriented along the grooves by 16 h and proliferated in this organization throughout the culture period. The effects of substratum surface chemistry on MMP-2 expression were found to be distinct from those of topography. Northern hybridization analysis of fibroblasts cultured on Ti revealed an MMP-2 mRNA time-course expression pattern parallel to that observed on the tissue culture plastic (TCP) dishes, but at significantly lower levels at each time-point. The Ti mRNA levels were decreased by 34% at 16 h, 55% at 40 h, and 45% at 90 h relative to TCP. In contrast, MMP-2 mRNA expression on VTi showed both an altered time-course expression pattern and altered levels compared to Ti and TCP. Relative to TCP, VTi MMP-2 mRNA levels were approximately 80% less at 16 h and approximately 50% less at 40 h, but not significantly different at 90 h. Relative to Ti, VTi MMP-2 mRNA levels were approximately 75% less at 16 h, but approximately 40% greater at 40 h and approximately 70% greater at 90 h. These differences may be explained in part by the observed changes in MMP-2 mRNA half-life which decreased by approximately 40% on Ti but increased by over fourfold on VTi relative to TCP. The smooth Ti also showed an approximate twofold increase of MMP-2 secretion in the late cultures over TCP controls. These results indicate that substratum surface chemistry and topography-induced changes in cell shape can alter MMP-2 expression in normal fibroblasts. The molecular approach to investigating the major molecules involved in tissue degradation may provide sensitive indicators of tissue remodeling at the tissue-biomaterial interface.

The region encoded by the alternatively spliced exon IIIA in mesenchymal fibronectin appears essential for chondrogenesis at the level of cellular condensation

Fibronectin in the extracellular matrix of tissues acts as a substrate for cell adhesion and migration during development. Heterogeneity in the structure of fibronectin is largely due to the alternative splicing of at least three exons (IIIB, IIIA, and V) during processing of a single primary transcript. Fibronectin mRNA alternative splicing patterns change from B+A+V+ to B+A-V+ during chondrogenesis. In this report, immunohistochemical analysis demonstrates that while fibronectin protein containing the region encoded by exon IIIB is present throughout the limb at all stages of development, fibronectin protein containing the region encoded by exon IIIA disappears from cartilaginous regions just after condensation in vivo and in high-density mesenchymal micromass cultures in vitro. Treatment of mesenchymal micromass cultures prior to condensation with an antibody specific for the region encoded by exon IIIA disrupts the formation of cellular condensations and inhibits subsequent chondrogenesis in a dose- and time-dependent manner. Furthermore, microinjection of the exon IIIA antibody into embryonic chick limb primordia in vivo results in malformations characterized by smaller limbs and loss of limb skeletal elements. These results strongly suggest that the presence of the region encoded by exon IIIA in mesenchymal fibronectin is necessary for the condensation event that occurs during chondrogenesis.

Age-related changes in the localization of glycosaminoglycans in condylar cartilage of the mandible in rats

There is little information available regarding the morphological and biomolecular characteristics of mandibular condylar cartilage. The purpose of this study was to determine the age-related changes in the morphology and immunolocalization of glycosaminoglycans (GAGs) in mandibular condyles. The mandibular condylar cartilages from 4-, 8-, 16-, 32-, and 64-week-old Wistar male rats were examined to verify the localization of chondroitin-4-sulfate (Ch-4S), chondroitin-6-sulfate (Ch-6S) and keratan sulfate (KS) using an indirect immunofluorescent technique with three monoclonal antibodies for glycosaminoglycans, 2-B-6, 3-B-3 and 5-D-4, respectively. Morphologically, the condylar cartilage was a growth cartilage during growing periods, began to differentiate into articular cartilage from the central area of 16-week-old condyles, and became mature articular cartilage at 32 weeks of age. A regional difference was found in the morphological features and distribution of GAGs between the anterior, central, postero-superior and posterior areas of the condyles at each age. The immunohistochemical localizations of these three glycosaminoglycans showed age-related, morphology-dependent changes, from growth cartilage to articular cartilage-like cartilage. Immunoreactions for all of the antibodies decreased progressively with age in the interterritorial matrix, while the pericellular and territorial matrix in the condylar cartilage of the mandible maintained relatively higher immunoreactivity. In conclusion, age-related and regional differences in the localization of glycosaminoglycans Ch-4S, Ch-6S, and KS were found in the mandibular condyles in rats, and these changes are believed to be related to functional and developmental requirements.

The chick alpha2(I) collagen gene contains two functional promoters, and its expression in chondrocytes is regulated at both transcriptional and post-transcriptional levels

Embryonic chick cartilages contain transcripts derived from the alpha2(I) collagen gene, although type I collagen is not normally found in these tissues; most of these RNAs are alternative transcripts initiating within intron 2. Use of the internal start site results in replacement of exons 1 and 2 with a previously undescribed exon and a change in the translational reading frame; thus, the alternative transcript cannot encode alpha2(I) collagen. We have demonstrated that production of the alternative transcript is due to activation of an internal promoter in chondrocytes and have identified a 179-base pair domain that is required for its activity. Furthermore, we have shown that the alternative transcript resulting from activation of the internal promoter turns over relatively rapidly; thus, the steady-state level of this transcript is less than predicted based on the transcription rate. The upstream promoter is only partially repressed in chondrocytes, suggesting that the lack of authentic alpha2(I) collagen mRNA may also be due in part to decreased mRNA stability. Thus, repression of alpha2(I) collagen synthesis in cartilage involves both transcriptional and post-transcriptional mechanisms. In contrast, repression of alpha1(I) collagen synthesis appears to be mediated primarily at the level of transcription.

Possible involvement of RGD (Arg-Gly-Asp)-containing extracellular matrix proteins in rat growth plate chondrocyte differentiation in culture

RGD (arg-gly-asp)-containing proteins have been shown to be components of cartilage matrix. In the present study, the role of interactions of cells with RGD-containing cartilage matrix proteins in rat costal epiphyseal chondrocyte differentiation was examined using a pelleted culture system as an in vitro model of endochondral ossification. Cell attachment assays showed the presence of integrins which mediated the binding of chondrocytes to fibronectin, a member of RGD-containing cartilage matrix proteins, in an RGD-dependent manner. In the early culture period, when chondrocytes had nonhypertrophic morphology with low levels of alkaline phosphatase, the exogenous addition of synthetic peptide GRGDSP (gly-arg-gly-asp-ser-pro) caused an increase in alkaline phosphatase levels and enlargement of chondrocytes in pelleted cultures. Treatment with GRGDSP from the early to late culture periods in association with the transition of chondrocytes from prehypertrophic to hypertrophic phenotypes followed by matrix mineralization resulted in suppression of mineral growth without significant effects on alkaline phosphatase levels or cellular morphology in the cultures. Similarly, addition of the synthetic peptide during the late culture period with the advance of cartilage mineralization suppressed mineral growth in pelleted cultures. These data indicate an important role of interactions of chondrocytes with RGD-containing cartilage matrix proteins through integrins in the regulation of epiphyseal chondrocyte differentiation in pelleted cultures.

End-organ resistance to growth hormone and IGF-I in epiphyseal chondrocytes of rats with chronic renal failure

We tested the hypothesis that there is direct end-organ resistance to growth hormone (GH) and IGF-I in chronic renal failure (CRF) independent of circulating inhibitors. Male Sprague-Dawley rats underwent 5/6 nephrectomy and were pair-fed with weight matched (100 g) sham operated controls for two weeks. Rats with CRF had significantly higher serum creatinine and blood urea nitrogen (P < 0.01 in both cases) and gained significantly less weight and length (P < 0.01 in both cases) compared with controls. Epiphyseal chondrocytes were grown in 10% fetal calf serum (FCS). Both CRF cells and control cells maintained chondrogenic phenotypes, and showed immunohistochemical staining with antibodies to collagen II and proteoglycan (aggrecan). Distribution of the cell subpopulations according to cell size (by flow cytometry) and alkaline phosphatase activity of CRF and control chondrocyte cultures were not different. Growth responses of CRF chondrocytes were reduced (P < 0.01) compared with control chondrocytes when grown in 10% FCS and 10% normal rat serum. Under serum free conditions, growth responses of CRF chondrocytes were reduced to GH and IGF-I at concentrations of 10, 30 and 100 ng/ml, and to insulin at 100, 300 and 1,000 ng/ml compared with controls cells (P < 0.01). To show that this resistance is specific for the GH/IGF system, growth responses to fibroblast growth factor and transforming growth factor beta 1 were studied and showed no difference between CRF and control cells. Thus, the present study provides direct evidence of specific end-organ resistance to GH, IGF-I in CRF chondrocytes in the absence of circulating factors.

Alternative transcript of the chick alpha 2(I) collagen gene is transiently expressed during endochondral bone formation and during development of the central nervous system

Endochondral bone formation is characterized by several transitions in the pattern of collagen gene expression, the best characterized of which occurs during chondrogenesis. Prechondrogenic mesenchymal cells synthesize predominantly type I collagen; during chondrogenesis, type I collagen synthesis ceases and production of cartilage-characteristic collagens is initiated. We previously identified the molecular mechanism that mediates cessation of alpha 2(I) collagen synthesis in chondrocytes (Bennett and Adams [1990] J. Biol. Chem. 265:2223-2230). This mechanism involves a change in the transcription initiation site, resulting in an alternative transcript that cannot encode alpha 2(I) collagen. In this report we demonstrate that the alternative transcript appears only transiently in cartilage. Its initial appearance is coincident with the onset of high levels of type II collagen synthesis in differentiated chondrocytes. However, it disappears in hypertrophic cartilage, and production of the authentic alpha 2(I) collagen mRNA is reinitiated, contributing to synthesis of a high level of type I collagen in hypertrophic chondrocytes at the chondro-osseous junction. We also show that the alternative transcript is not restricted to cartilage during embryonic development, since it initially appears in presomite embryos, well before the appearance of cartilage. At early stages of embryo-genesis the alternative transcript is restricted to tissues derived from neuroectoderm; its appearance in those tissues is also transient. These data suggest that production of the alternative transcript of the alpha 2(I) collagen gene may be required for cessation of alpha 2(I) collagen synthesis during chondrogenesis, but the alternative transcript may be involved in other important developmental programs as well.

Fibulin-1 and fibulin-2 expression during organogenesis in the developing mouse embryo

Fibulin-1 and fibulin-2 are extracellular matrix proteins with unique structural features. We used in situ hybridization and immunofluorescence staining to examine the expression of fibulin-1 and fibulin-2 during mouse embryogenesis. Both fibulins have previously been shown to be deposited at sites where polarized cells convert into mesenchyme, during early stages of development of endocardial cushion tissue and in neural crest cells. By Northern blots we confirm that expression of fibulin-2 is particularly high in the developing and newborn heart. We also show that fibulin-2 mRNA and protein remained highly expressed during organogenesis in tissues derived from neural crest mesenchyme. In addition, a locally restricted expression pattern of fibulin-1 and fibulin-2 mRNA and protein at sites of epithelial-mesenchymal interactions was detected in two tissues, the developing tooth and hair follicles. In other tissues where epithelial-mesenchymal interactions occur, fibulin-1 mRNA and its corresponding protein were detected rather uniformly around mesenchymal cells, and no expression of fibulin-2 was noted. Fibulin-1 protein was located in some embryonic epithelial basement membranes. Fibulin-1 mRNA was also expressed in the epidermal layer of brain and in the mesenchyme of choroid plexus and the meninges which surround the spinal cord. Overall, fibulin-2 expression was much more limited than fibulin-1 expression. A very prominent expression of fibulin-2 was seen during early stages of chondrogenesis in all cartilages analyzed. These studies show that the differential expression of the fibulin family contributes to the formation of molecularly distinct extracellular matrices already during early developmental stages of a large number of tissues.

Differential regulation of COL2A1 expression in developing and mature chondrocytes

To investigate the regulation of type II collagen gene expression in cells undergoing chondrogenic differentiation, we have employed a 5-kbp genomic fragment of the human type II collagen gene which contains 1.8kbp of upstream sequences, the transcription start site, the first exon and 3 kbp of intronic sequences, fused to either lac Z or chloramphenicol acetyl transferase-reporter gene. Transient expression studies revealed a parallel increase in transgene activity and endogenous type II collagen mRNA levels during the onset of the cartilage differentiation of limb mesenchymal cells in high-density micromass cultures. At later periods in culture, however, the transgene activity declines, although steady-state levels of type II collagen mRNA are reported to continue to increase (Kosher et al.: J. Cell. Biol. 102: 1151-1156, 1986; Kravis and Upholt. Dev. Biol. 108: 164-172, 1985). In addition, the activity of the transgene is seven-fold higher at the onset of chondrogenic differentiation in micromass cultures that in well differentiated sternal chondrocytes, although similar levels of type II collagen transcripts are found in these cells. Furthermore, deletions of intronic segments resulted in greater drop in activity of the constructs in differentiating chondrocytes in micromass cultures than in mature sternal chondrocytes. The expression of the construct in transgenic mice is higher at the onset of chondrogenic differentiation and in newly differentiated chondrocytes than in more mature differentiated chondrocytes. Based on these observations, it appears that the mechanisms involved in the regulation of the type II collagen gene at the onset of chondrocyte differentiation are different from those resulting in the maintenance of its expression in fully differentiated chondrocytes.

Developmental expression of a type II collagen/beta-galactosidase fusion gene in transgenic mice

The correct temporal and spatial expression of the type II collagen gene is believed to be important for normal development and growth of the skeleton and the eye, i.e., tissues where the protein product is predominantly found. To study transcriptional activation of type II collagen gene in skeletal and nonskeletal tissues we produced transgenic mice carrying murine proalpha1(II) collagen/beta-galactosidase fusion gene constructs. The expression of the fusion gene was found to depend on the presence of intron 1 deleted failed to reveal any beta-galactosidase activity confirming the important role of regulatory sequences within intron 1 of the gene. High-level expression of the functional construct was clearly confined to cartilaginous tissues but transient low-level expression was also observed in extraskeletal locations, such as the developing brain and the notochord. The results demonstrate that the regulatory elements in the proalpha1(II) collagen/beta-galactosidase fusion gene construct confer both temporal and spatial specificity indistinguishable from that of the endogenous proalpha1(II) collagen gene as determined by the presence of the corresponding mRNA by in situ hybridization. Furthermore the beta-galactosidase activity correlated well with the progression of chondrogenesis as seen by staining of whole mouse embryos with Alizarin red S and Alcian blue in the hybrid mouse strain used for microinjections. The transgenic mouse line produced should prove useful for studies on various aspects of chondrogenesis. Furthermore, the data shows that the regulatory elements present in the construct are sufficient for targetting the expression of other genes in cartilage.

Substratum surface topography alters cell shape and regulates fibronectin mRNA level, mRNA stability, secretion and assembly in human fibroblasts

The regulation of cell shape, fibronectin mRNA level, secretion and assembly by substratum surface topography was investigated in early passage human gingival fibroblasts cultured on titanium-coated smooth or V-shaped grooved substrata produced by micromachining. Cells on grooved surfaces were significantly elongated and orientated along the grooves of the substratum, while cell height, measured using confocal scanning laser microscopy, was approximately 1.5-fold greater than that of cells on smooth surfaces. Northern hybridization analysis revealed that on a per cell basis the grooved surface increased the amounts of fibronectin mRNA/cell approximately 3.5-fold at 16 hours, approximately 1.9-fold at 40 hours and approximately 2.2-fold at 90 hours, while the mRNA levels of the house-keeping gene glyceraldehyde-3-phosphate dehydrogenase (GAPD) were constant. The amounts of secreted fibronectin on the grooved surface were increased approximately 2-fold for all time points. The stability of fibronectin mRNA was also altered by substratum surface topography. The half-life of fibronectin mRNA on smooth surfaces was estimated to be approximately 5 hours, but on the grooved surfaces the half-life of fibronectin mRNA showed a two-phase response: a rapid 60% reduction in the first half-life (t1/2 approximately 2 hours) and a 2.4-fold increase in the second half-life (t1/2 approximately 12 hours) relative to that observed on the smooth surface. The GAPD mRNA half-lives were essentially unaffected by the surface topography of the substrata. The grooved surface was also found to alter the amount of fibronectin assembled into the extracellular matrix, producing a approximately 2-fold increase in the cultures at all time points. It thus appears that substratum surface topography alters cell shape and modulates fibronectin at the transcriptional and post-transcriptional levels, as well as the amount of fibronectin assembled into extracellular matrix. Micromachining, which has the ability to precisely control surface topography over a wide range of dimensions and shapes, appears to be a useful technique in investigating the relationship between cell shape and function.

Naturally occurring antisense transcripts are present in chick embryo chondrocytes simultaneously with the down-regulation of the alpha 1 (I) collagen gene

It has previously been shown that very low steady state levels of alpha 1 (I) collagen mRNA are present in chick embryo sternal chondrocytes (Askew, G. R., Wang, S., and Lukens, L. N. (1991) J. Biol. Chem. 266, 16834-16841), yet nuclear run-on experiments with double-stranded cDNA probes indicated a high transcription rate at this locus. These findings were investigated in this study using single-stranded probes, where nuclear run-on experiments showed that antisense transcription of the alpha 1 (I) collagen gene was occurring in chondrocytes, while sense strand transcription was down-regulated. Treatment of these chondrocytes with 5-bromo-2'-deoxyuridine (BrdU), which causes the cells to resemble their mesenchymal precursors, resulted in an antiparallel situation, where antisense transcription was lost, and instead, sense strand transcription was acquired, suggesting that the reverse switch from sense to antisense transcription occurs during chondrogenesis. Very large (> 10 kilobases) and heterogeneous antisense transcripts of moderate stability were shown to span both ends of the gene in chondrocytes, while their absence was shown in BrdU-treated chondrocytes, chick embryo fibroblasts, and a variety of other tissues. The function of these antisense transcripts is so far unknown, but their unusual chondrocyte-specific appearance, concurrent with little or no sense strand transcription, suggests a possible functional role in the down-regulation of the alpha 1 (I) collagen gene.

Cell-type specific promoters of the chick alpha 2(I) collagen gene in chondrocytes and fibroblasts

Two different promoter regions responsible for the cell-type specific transcripts of the collagen alpha 2(I) gene in chick embryonic fibroblasts and chondrocytes are described. A region including part of intron 1 of the fibroblast alpha 2(I) gene is able to promote transcription in chondrocytes. This promoter region is located 5' to the chondrocyte-specific transcript, from -2205 to +42, relative to the chondrocyte transcription start site. When it is placed upstream of a luciferase reporter gene in transiently transfected chondrocytes, luciferase activity is stimulated 78-fold. The region between -2205 and -1298 is most important for the activity of the chondrocyte promoter, since a truncated promoter from -1298 through +42 gives only 15% as much activity. A similar low level of activity is given by the sequence -96 through +42. In transfected fibroblasts, a region from -1600 through +46, relative to the fibroblast transcription start site, stimulates transcription of a luciferase reporter gene by more than 1,000-fold. Deletions from -1185 through -807 and from -707 through -90 reduce promoter activity to 22% and 32%, respectively, of that given by the intact promoter. The chondrocyte and fibroblast promoters are each cell-type specific and are relatively inactive in the other cell type. Subregions within the fibroblast promoter, from -1154 through -1055, from -712 through -85, and the sequence containing the inverted CCAAT motif from -93 through -67, differentially bind factors in fibroblast but not chondrocyte nuclear extracts.

Expression of collagen species in a cartilaginous tumor derived from a human osteogenic sarcoma

We have succeeded in transplanting human osteogenic sarcoma into nude mice. Morphologically, the transplanted tumor is chondrosarcoma and manifests calcification, but not ossification. This tumor is thought to be an excellent model for studying the process of morbid endochondral calcification. In this study, we have used in situ hybridization to examine expression of collagen type I, II, and III mRNAs in this tumor. In situ hybridization was carried out using biotinylated DNA probes. Hybridized probes were detected using a streptavidin-biotin-alkaline phosphatase reagent. The results showed that collagen type I and II mRNAs were produced by cells of the transplanted tumor. Collagen type I mRNA was chiefly localized in the marginal region of the tumor. Collagen type II mRNA, which was predominantly found in the premineralized region of the transplanted tumor, gradually decreased toward the mineralized region. Collagen type III mRNA was not expressed in the transplanted tumor. These results suggest that the character of progenitor chondrogenic cells might be transferred to the transplanted tumor, and that the tumor cells may change the expression of collagen genes with the differentiation or maturation.

Localization of collagen types I, II, and III mRNAs in human heterotopic ossification by non-radioactive in situ hybridization

Heterotopic ossification is a metabolically active process which shares several properties of orthotopic bone formation and, therefore, represents an excellent model for the investigation of matrix components. A novel tool for studying the ossifying process at the level of transcription is the technique of non-radioactive in situ hybridization. Using digoxigenin labeled cDNA probes we investigated the distribution patterns of types I, II and III collagen mRNAs in heterotopic ossification of pressure sores of paraplegic patients. The three collagen mRNAs exhibited substantially divergent distribution patterns. Type I (alpha 1) collagen mRNA was predominantly detectable in preosteoblasts, prechondroblasts and chondrocytes of the ossification zone. Type II (alpha 1) collagen mRNA was nearly exclusively found in cells of the chondrogenic lineage. Type III (alpha 1) collagen mRNA was detectable at low levels in soft tissue, but was strongly expressed by prechondroblasts and chondrocytes of heterotopic cartilage. Our in situ hybridization experiments provide evidence that chondrogenic cells in heterotopic ossification show a phenotypic alteration in collagen type expression. These results indicate that chondrocytes of heterotopic cartilage show a co-expression of types I (alpha 1), II (alpha 1) and III (alpha 1) collagen mRNAs.

Expression of collagens I, II, X, and XI and aggrecan mRNAs by bovine growth plate chondrocytes in situ

The cells responsible for skeletal growth are the chondrocytes of the cartilaginous growth plate. These cells differentiate through a series of maturational stages, establishing different zones in the growth plate. Among the major functions of these cells is the production of appropriate extracellular matrix, primarily composed of collagens and proteoglycans. To determine whether matrix synthesis varies with respect to maturational stage and in which cell populations different collagens are expressed, bovine growth plates were analyzed by in situ hybridization to mRNA and by Northern blot hybridization. The most abundant collagen mRNA in the growth plate was type-II collagen. This mRNA was present at relatively low levels in the most immature cells of the growth plate but increased several-fold as cells entered the proliferative stage and remained high through subsequent phases of maturation. Type-XI collagen mRNA and mRNA for the cartilage-characteristic proteoglycan, aggrecan, were codistributed with the type-II collagen mRNA; however, both were present in much smaller quantities. Type-X procollagen mRNA was localized to chondrocytes late in their maturation and was expressed at levels similar to the expression of type-II collagen. In situ hybridization of serial sections revealed that growth plate chondrocytes in their more mature stages contain both type-II and type-X collagen mRNA. Type-I collagen mRNA was not observed in growth plate chondrocytes at any maturational stage; rather, it was localized to a morphologically distinct population of cells attached to calcifying cartilage septa in the region of vascular invasion.(ABSTRACT TRUNCATED AT 250 WORDS)

Human osteogenic protein-1 induces both chondroblastic and osteoblastic differentiation of osteoprogenitor cells derived from newborn rat calvaria

Osteogenetic protein-1 (OP-1), a member of the TGF-beta superfamily, induces endochondrial bone formation at subcutaneous sites in vivo and stimulates osteoblastic phenotypic expression in vitro. Primary cultures of newborn rat calvarial cells contain a spectrum of osteogenic phenotypes ranging from undifferentiated mesenchymal osteoprogenitor cells to parathyroid hormone (PTH)-responsive osteoblasts. We examined whether treatment of this cell population with recombinant human osteogenic protein-1 could induce chondrogenesis in vitro. Markers of chondroblastic versus osteoblastic differentiation included alcian blue staining at pH 1, alkaline phosphatase-specific activity, osteocalcin radioimmunoassay, and expression of collagen mRNAs. 6 d of treatment (culture days 1-7) with 4-100 ng OP-1/ml caused dose-dependent increases in alcian blue staining intensity and alkaline phosphatase activity (4.7- and 3.4-fold, respectively, at 40 ng/ml), while osteocalcin production decreased twofold. Clusters of round, refractile, alcian blue-stained cells appeared by day 3, increased in number until day 7, and then became hypertrophic and gradually became less distinct. Histochemically, the day 7 clusters were associated with high alkaline phosphatase activity and became mineralized. mRNA transcripts for collagen types II and IX were increased by OP-1, peaking at day 4, while type X collagen mRNA was detectable only on day 7 in OP-1-treated cultures. Delay of OP-1 exposure until confluence (day 7) amplifies expression of the normal osteoblastic phenotype and accelerates its developmental maturation. In contrast, early OP-1 treatment commencing on day 1 strongly amplifies chondroblastic differentiation. In the same protocol, TGF-beta 1 alone at 0.01-40 ng/ml fails to induce any hypertrophic chondrocytes, and in combination with OP-1, TGF-beta 1 blocks OP-1-dependent chondroinduction. OP-1 is believed to act on a subpopulation of primitive osteoprogenitor cells to induce endochondrial ossification, but does not appear to reverse committed osteoblasts to the chondrocyte phenotype.

Altered cartilage phenotype expressed during intramembranous bone formation

The sequential phenotypic expression occurring during intramembranous bone formation was investigated using the tooth extraction socket created in rat alveolar bone in vivo. The differential expression of bone extracellular matrix genes, such as collagen I and osteocalcin, was confirmed by RNA transfer blot analysis and in situ hybridization during the active healing period of the bony socket. To clarify the possible involvement of the chondrogenic phenotype during the process of intramembranous bone formation, the expression of cartilage collagen II and IX was further examined in this model. It was found that both alpha 1(II) and alpha 1(IX) mRNAs were present, but the alpha 1(IX) mRNA was a transcript from the downstream start site/promoter, which is a different site in the alpha 1(IX) gene from that used in hyaline cartilage. In situ hybridization indicated that the alpha 1(IX) message was expressed by cells associated with bone matrix in the early formation stage. This finding led to the investigation of type IX collagen expression by osteogenic cells isolated from newborn rat calvariae, in which only the truncated form of alpha 1(IX) mRNA was indicated by RNA transfer analysis. The expression of collagen II and a truncated form of collagen IX may represent an early phenotypic feature of osteoblast differentiation.

Alpha 2(I) collagen gene expression is up-regulated in quail chondrocytes pretreated with retinoic acid

alpha 2(I) collagen gene expression is induced in quail embryo chondrocytes pretreated with retinoic acid (RA). The initial appearance of alpha 2(I) mRNA occurs around day 3 of culture in RA-free medium and rapidly progresses over the next 4 days. In transient transfection assays, expression of COL1A2-CAT, a chimeric gene bearing 3500 bp upstream the bone/tendon transcription start site from the human alpha 2(I) gene fused to the CAT gene, is stimulated severalfold in RA-treated chondrocytes. In contrast, enzyme activity is very low in untreated chondrocytes, suggesting that the sequences required for RA-induced transcription of the alpha 2(I) gene are present in this plasmid. Analysis of alpha 2(I) promoter sequences performed with deletion mutants gives overlapping results in collagen type I-producing fibroblasts and chondrocytes withdrawn from RA treatment. These experiments suggest that RA-induced transcription of the alpha 2(I) collagen gene in chondrocytes is regulated by the binding of transcription factors to the same regulatory sequences that control transcription in fibroblasts.

[Effect of synthetic hydroxyapatite ceramics on long-term cultures of isolated chondrocytes]

This study deals with the influence of hydroxyapatite-ceramic on long-term cultures of rabbit articular chondrocytes. By additioni of granules of this material into the culture wells improval of cell proliferation by factor 12 over a period of twelve weeks is found. More important is the maintaining of the status of differentiation of the chondrocytes, documented by immunoidentification of the expressed collagen I production over collagen II as marker for chondrocytes. In controls without addition of hydroxyapatite-ceramic there is a dynamic increase of collagen I production over the culture period and morphological alteration of the cell shape leading to fibroblast-like cells. As this dedifferentiation is the main problem concerning chondrocytes long-term cultures, the addition of HAC into the culture medium is a simple and effective method enhancing the productivity of the culture system. Moreover this experimental design with chondrocytes as cells familiar with mineralized matrix gives an important proof of the excellent cytocompatibility of the hydroxyapatite-ceramic used in this study.

Stable murine chondrogenic cell lines derived from c-fos-induced cartilage tumors

This study describes the detailed characterization of four murine chondrogenic cell lines (wT2-1, wT2-7, wT2-8, and wT2-9) that were isolated from a cartilage tumor induced by the protooncogene c-fos in chimeric mice. All cell lines are clonal and display a fibroblastic morphology with a doubling time of 1-2 days. Northern blot analysis demonstrated that in addition to expressing high levels of exogenous c-fos, all clones express varying levels of the cartilage marker gene type II collagen in addition to type I collagen. The clones also expressed high levels of the AP-1 genes c-jun and fra-1. The doubling times of these clones did not change over a period of 14 months in culture. Most importantly, however, expression of type II collagen was maintained in all cell lines for 8 months in culture, and two cell lines maintained type II collagen expression when analyzed after 14 months. Interestingly, type I collagen expression was lost after long-term culture. Following injection into syngeneic and nude mice, all cell lines formed tumors containing areas with the morphologic appearance of hyaline cartilage, indicating that these cell lines are chondrogenic. Thus, these stable murine chondrogenic cell lines provide a useful tool for studying the transcriptional control of cartilage-specific gene expression, as well as the growth control of chondrogenic cells.

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.

Distribution of cartilage proteoglycan (aggrecan) core protein and link protein gene expression during human skeletal development

The distribution of cartilage proteoglycan core protein (aggrecan) and cartilage proteoglycan link protein was investigated by in situ hybridization during different stages of human skeletal development. Aggrecan and link protein expression were confined to chondrocytes of the developing skeleton and other cartilaginous structures. Distribution and intensity of the signal was identical with aggrecan as compared to link protein probes. Parallel to the calcification of cartilaginous matrix, chondrocytes of this area lost the expression of aggrecan and link protein specific mRNA and stayed negative throughout the following stages of skeletal development. Highest expression was found in the lower proliferative and upper hypertrophic zone whereas the resting zone showed less expression. Aggrecan gene expression was additionally investigated in iliac crest biopsies of 3 patients with pseudoachondroplasia and compared to age-matched controls. Distribution and intensity of staining revealed no abnormalities. Thus, the phenotypic changes during chondrocyte maturation are accompanied by distinct changes in aggrecan and link protein gene expression. This pattern was maintained in the growth plate of patients with pseudoachondroplasia.

Association of a change in chromatin structure with a tissue-specific switch in transcription start sites in the alpha 2(I) collagen gene

Chick embryonic sternal chondrocytes do not synthesize alpha 2(I) collagen until they are shifted by treatment with 5-bromo-2'-deoxyuridine (BrdUrd) to a fibroblastic phenotype, yet they transcribe this gene as rapidly as BrdUrd-treated cells. To examine further this transcription, the DNase I hypersensitive sites were mapped in the 5' region of this gene in chondrocytes, BrdUrd-treated chondrocytes, fibroblasts and three types of non-transcribing cells. A DNase I hypersensitive site at -200 bp, previously shown to be associated with the active transcription of this gene in fibroblasts, is not present in chondrocyte chromatin. The chondrocyte alpha 2(I) gene contains, however, a novel major hypersensitive site in the DNA region corresponding to the fibroblast intron 2, near the chondrocyte-specific transcription initiation site of this gene. This novel hypersensitive site is associated with the use of this alternate start site by chondrocytes, since it is lost when BrdUrd treatment causes these chondrocytes to switch to the initiation of transcription at the fibroblast start site. The BrdUrd-treated chondrocytes contain the same alpha 2(I) hypersensitive sites as fibroblasts, except that fibroblasts have an additional, previously unreported, site at -1000 bp.

Developmental expression and hormonal regulation of the rat matrix Gla protein (MGP) gene in chondrogenesis and osteogenesis

Matrix Gla protein (MGP), a vitamin K dependent protein, has recently been identified in many tissues. However, it is accumulated only in bone and cartilage suggesting that the expression of MGP may be related to the development and/or maintenance of the phenotypic properties of these tissues. We systematically evaluated MGP mRNA expression as a function of bone and cartilage development and also as regulated by vitamin D during growth and cellular differentiation. Three experimental models of cartilage and bone development were employed: an in vivo model for endochondral bone formation, as well as in primary cells of normal diploid rat chondrocyte and osteoblast cultures. MGP was expressed at the highest level during cartilage formation and calcification in vivo during endochondral bone formation. In chondrocyte cultures, MGP mRNA was present throughout the culture period but increased only after 3 weeks concomitantly with type I collagen mRNA. In osteoblast cultures, MGP mRNA was expressed during the proliferative period and exhibited increased expression during the period of matrix development. In contrast to osteocalcin (bone Gla protein), this increase was not dependent on mineralization but was related to the extent of differentiation associated with and potentially induced by extracellular matrix formation. During the proliferative period, type I collagen mRNA peaked and thereafter declined, while type I collagen protein steadily accumulated in the extracellular matrix. Constant MGP levels were maintained in the mineralization period of osteoblast differentiation in vitro which is consistent with the constant levels found during the osteogenic period of the in vivo system. MGP mRNA levels in both osteoblasts and chondrocytes in culture were significantly elevated by 1,25-(OH)2D3 (10(-8) M, 48 h) throughout the time course of cellular growth and differentiation. Interestingly, when MGP mRNA transcripts from vitamin D treated and untreated chondrocytes and osteoblasts were analyzed by high resolution Northern blot analysis, we observed two distinct species of MGP mRNA in the vitamin D treated chondrocyte cultures while all other cultures examined exhibited only a single MGP mRNA transcript. Primer extension analysis indicated a single transcription start site in both osteoblasts and chondrocytes with or without vitamin D treatment, suggesting that the lower molecular weight MGP message in vitamin D treated chondrocytes may be related to a modification in post-transcriptional processing. In conclusion, these results show that the selective accumulation of MGP in bone and cartilage tissues in vitro may be related to the development and/or maintenance of a collagenous matrix as reflected by increases in MGP mRNA during these periods.(ABSTRACT TRUNCATED AT 400 WORDS)

Temporal and spatial distribution of type XII collagen in high cell density culture of periosteal-derived cells

Periosteal-derived cells of young chicks have been reported to possess the potential to undergo terminal differentiation into osteogenic or chondrogenic phenotypes under high cell density culture conditions. In this culture, the temporal and spatial distribution of type XII collagen was immunocytochemically assessed using a monoclonal antibody. These high-density plated cells first formed a multilayer of fibroblast-like cells, in which type I and XII collagen were evenly distributed throughout the full thickness of the culture. With time, the top portion of the culture differentiated into bone tissue, while cells below this top layer differentiated into hypertrophic chondrocytes. In this transition, type XII collagen was temporally and spatially colocalized primarily with type I collagen: the top portion of bone layer was positive for both type I and XII collagens, whereas their staining intensity in the bottom portion decreased with time in culture. Using this antibody, type XII collagen was also found in developing embryonic chick tibiotarsus. These observations, taken together, suggest that type XII collagen production is a characteristic property of bone-forming cells.