In situ hybridization studies on the expression of type X collagen in fetal human cartilage

Effects of Modified Glucosamine on the Chondrogenic Potential of Circulating Stem Cells under Experimental Inflammation

Glucosamine (GlcN) is a glycosaminoglycan (GAGs) constituent in connective tissues. It is naturally produced by our body or consumed from diets. In the last decade, in vitro and in vivo trials have demonstrated that the administration of GlcN or its derivates has a protective effect on cartilage when the balance between catabolic and anabolic processes is disrupted and cells are no longer able to fully compensate for the loss of collagen and proteoglycans. To date, these benefits are still controversial because the mechanism of action of GlcN is not yet well clarified. In this study, we have characterized the biological activities of an amino acid (AA) derivate of GlcN, called DCF001, in the growth and chondrogenic induction of circulating multipotent stem cells (CMCs) after priming with tumor necrosis factor-alpha (TNFα), a pleiotropic cytokine commonly expressed in chronic inflammatory joint diseases. In the present work, stem cells were isolated from the human peripheral blood of healthy donors. After priming with TNFα (10 ng/mL) for 3 h, cultures were treated for 24 h with DCF001 (1 μg/mL) dissolved in a proliferative (PM) or chondrogenic (CM) medium. Cell proliferation was analyzed using a Corning^® Cell Counter and trypan blue exclusion technique. To evaluate the potentialities of DCF001 in counteracting the inflammatory response to TNFα, we measured the amount of extracellular ATP (eATP) and the expression of adenosine-generating enzymes CD39/CD73, TNFα receptors, and NF-κB inhibitor IκBα using flow cytometry. Finally, total RNA was extracted to perform a gene expression study of some chondrogenic differentiation markers (COL2A1, RUNX2, and MMP13). Our analysis has shed light on the ability of DCF001 to (a) regulate the expression of CD39, CD73, and TNF receptors; (b) modulate eATP under differentiative induction; (c) enhance the inhibitory activity of IκBα, reducing its phosphorylation after TNFα stimulation; and (d) preserve the chondrogenic potentialities of stem cells. Although preliminary, these results suggest that DCF001 could be a valuable supplement for ameliorating the outcome of cartilage repair interventions, enhancing the efficacy of endogenous stem cells under inflammatory stimuli.

CEMIP (KIAA1199) induces a fibrosis-like process in osteoarthritic chondrocytes

CEMIP (for "Cell migration-inducing protein" also called KIAA1199 and Hybid for "Hyaluronan-binding protein") expression is increased in cancers and described as a regulator of cell survival, growth and invasion. In rheumatoid arthritis, CEMIP is referred to as an angiogenic marker and participates in hyaluronic acid degradation. In this study, CEMIP expression is investigated in healthy and osteoarthritis (OA) cartilage from human and mouse. Its role in OA physiopathology is deciphered, specifically in chondrocytes proliferation and dedifferentiation and in the extracellular matrix remodeling. To this end, CEMIP, αSMA and types I and III collagen expressions were assessed in human OA and non-OA cartilage. CEMIP expression was also investigated in a mouse OA model. CEMIP expression was studied in vitro using a chondrocyte dedifferentiation model. High-throughput RNA sequencing was performed on chondrocytes after CEMIP silencing. Results showed that CEMIP was overexpressed in human and murine OA cartilage and along chondrocytes dedifferentiation. Most of genes deregulated in CEMIP-depleted cells were involved in cartilage turnover (e.g., collagens), mesenchymal transition and fibrosis. CEMIP regulated β-catenin protein level. Moreover, CEMIP was essential for chondrocytes proliferation and promoted αSMA expression, a fibrosis marker, and TGFβ signaling towards the p-Smad2/3 (Alk5/PAI-1) pathway. Interestingly, CEMIP was induced by the pSmad1/5 (Alk1) pathway. αSMA and type III collagen expressions were overexpressed in human OA cartilage and along chondrocytes dedifferentiation. Finally, CEMIP was co-expressed in situ with αSMA in all OA cartilage layers. In conclusion, CEMIP was sharply overexpressed in human and mouse OA cartilage and along chondrocytes dedifferentiation. CEMIP-regulated transdifferentiation of chondrocytes into "chondro-myo-fibroblasts" expressing α-SMA and type III collagen, two fibrosis markers. Moreover, these "chondro-myo-fibroblasts" were found in OA cartilage but not in healthy cartilage.

Thermal challenge alters the transcriptional profile of the breast muscle in turkey poults

Extremes in temperature represent environmental stressors that impact the well-being and economic value of poultry. As homeotherms, young poultry with immature thermoregulatory systems are especially susceptible to thermal extremes. Genetic variation and differences in gene expression resulting from selection for production traits, likely contribute to thermal stress response. This study was designed to investigate in vivo transcriptional changes in the breast muscle of young turkey poults from an unselected randombred line and one selected for 16 wk body weight under hot and cold thermal challenge. Newly hatched turkey poults were brooded for 3 d at one of 3 temperatures: control (35°C), cold (31°C), or hot (39°C). Samples of the pectoralis major were harvested and subjected to deep RNA sequencing. Significant differential gene expression was observed in both growth-selected and randombred birds at both temperature extremes when compared to control-brooded poults. Growth-selected birds responded to thermal stress through changes in genes predicted to have downstream transcriptional effects and that would result in reduced muscle growth. Slower growing randombred birds responded to thermal stress through modulation of lipid-related genes, suggesting reduction in lipid storage, transport, and synthesis, consistent with changes in energy metabolism required to maintain body temperature.

Homogeneous hydroxyapatite/alginate composite hydrogel promotes calcified cartilage matrix deposition with potential for three-dimensional bioprinting

Calcified cartilage regeneration plays an important role in successful osteochondral repair, since it provides a biological and mechanical transition from the unmineralized cartilage at the articulating surface to the underlying mineralized bone. To biomimic native calcified cartilage in engineered constructs, here we test the hypothesis that hydroxyapatite (HAP) stimulates chondrocytes to secrete the characteristic matrix of calcified cartilage. Sodium citrate (SC) was added as a dispersant of HAP within alginate (ALG), and homogeneous dispersal of HAP within ALG hydrogel was confirmed using sedimentation tests, electron microscopy, and energy dispersive spectroscopy. To examine the biological performance of ALG/HAP composites, chondrocyte survival and proliferation, extracellular matrix production, and mineralization potential were evaluated in the presence or absence of the HAP phase. Chondrocytes in ALG/HAP constructs survived well and proliferated, but also expressed higher levels of calcified cartilage markers compared to controls, including Collagen type X secretion, alkaline phosphatase (ALP) activity, and mineral deposition. Compared to controls, ALG/HAP constructs also showed an elevated level of mineralized matrix in vivo when implanted subcutaneously in mice. The printability of ALG/HAP composite hydrogel precursors was verified by 3D printing of ALG/HAP hydrogel scaffolds with a porous structure. In summary, these results confirm the hypothesis that HAP in ALG hydrogel stimulates chondrocytes to secrete calcified matrix in vitro and in vivo and reveal that ALG/HAP composites have the potential for 3D bioprinting and osteochondral regeneration.

Calcified cartilage or bone? Collagens in the tessellated endoskeletons of cartilaginous fish (sharks and rays)

The primary skeletal tissue in elasmobranchs -sharks, rays and relatives- is cartilage, forming both embryonic and adult endoskeletons. Only the skeletal surface calcifies, exhibiting mineralized tiles (tesserae) sandwiched between a cartilage core and overlying fibrous perichondrium. These two tissues are based on different collagens (Coll II and I, respectively), fueling a long-standing debate as to whether tesserae are more like calcified cartilage or bone (Coll 1-based) in their matrix composition. We demonstrate that stingray (Urobatis halleri) tesserae are bipartite, having an upper Coll I-based 'cap' that merges into a lower Coll II-based 'body' zone, although tesserae are surrounded by cartilage. We identify a 'supratesseral' unmineralized cartilage layer, between tesserae and perichondrium, distinguished from the cartilage core in containing Coll I and X (a common marker for mammalian mineralization), in addition to Coll II. Chondrocytes within tesserae appear intact and sit in lacunae filled with Coll II-based matrix, suggesting tesserae originate in cartilage, despite comprising a diversity of collagens. Intertesseral joints are also complex in their collagenous composition, being similar to supratesseral cartilage closer to the perichondrium, but containing unidentified fibrils nearer the cartilage core. Our results indicate a unique potential for tessellated cartilage in skeletal biology research, since it lacks features believed diagnostic for vertebrate cartilage mineralization (e.g. hypertrophic and apoptotic chondrocytes), while offering morphologies amenable for investigating the regulation of complex mineralized ultrastructure and tissues patterned on multiple collagens.

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

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

Activating enhancer binding protein 2 epsilon (AP-2ε)-deficient mice exhibit increased matrix metalloproteinase 13 expression and progressive osteoarthritis development

Introduction

The transcription factor activating enhancer binding protein 2 epsilon (AP-2ε) was recently shown to be expressed during chondrogenesis as well as in articular chondrocytes of humans and mice. Furthermore, expression of AP-2ε was found to be upregulated in affected cartilage of patients with osteoarthritis (OA). Despite these findings, adult mice deficient for AP-2ε (Tfap2e^−/−) do not exhibit an obviously abnormal cartilaginous phenotype. We therefore analyzed embryogenesis of Tfap2e^−/− mice to elucidate potential transient abnormalities that provide information on the influence of AP-2ε on skeletal development. In a second part, we aimed to define potential influences of AP-2ε on articular cartilage function and gene expression, as well as on OA progression, in adult mice.

Methods

Murine embryonic development was accessed via in situ hybridization, measurement of skeletal parameters and micromass differentiation of mesenchymal cells. To reveal discrepancies in articular cartilage of adult wild-type (WT) and Tfap2e^−/− mice, light and electron microscopy, in vitro culture of cartilage explants, and quantification of gene expression via real-time PCR were performed. OA was induced via surgical destabilization of the medial meniscus in both genotypes, and disease progression was monitored on histological and molecular levels.

Results

Only minor differences between WT and embryos deficient for AP-2ε were observed, suggesting that redundancy mechanisms effectively compensate for the loss of AP-2ε during skeletal development. Surprisingly, though, we found matrix metalloproteinase 13 (Mmp13), a major mediator of cartilage destruction, to be significantly upregulated in articular cartilage of adult Tfap2e^−/− mice. This finding was further confirmed by increased Mmp13 activity and extracellular matrix degradation in Tfap2e^−/− cartilage explants. OA progression was significantly enhanced in the Tfap2e^−/− mice, which provided evidence for in vivo relevance. This finding is most likely attributable to the increased basal Mmp13 expression level in Tfap2e^−/− articular chondrocytes that results in a significantly higher total Mmp13 expression rate during OA as compared with the WT.

Conclusions

We reveal a novel role of AP-2ε in the regulation of gene expression in articular chondrocytes, as well as in OA development, through modulation of Mmp13 expression and activity.

Formin 1 and filamin B physically interact to coordinate chondrocyte proliferation and differentiation in the growth plate

Filamin B (FlnB) is an actin-binding protein thought to transduce signals from various membrane receptors and intracellular proteins onto the actin cytoskeleton. Formin1 (Fmn1) is an actin-nucleating protein, implicated in actin assembly and intracellular signaling. Human mutations in FLNB cause several skeletal disorders associated with dwarfism and early bone fusion. Mouse mutations in Fmn1 cause aberrant fusion of carpal digits. We report here that FlnB and Fmn1 physically interact, are co-expressed in chondrocytes in the growth plate and share overlapping expression in the cell cytoplasm and nucleus. Loss of FlnB leads to a dramatic decrease in Fmn1 expression at the hypertrophic-to-ossification border. Loss of Fmn1-FlnB in mice leads to a more severe reduction in body size, weight and growth plate length, than observed in mice following knockout of either gene alone. Shortening of the long bone is associated with a decrease in chondrocyte proliferation and an overall delay in ossification in the double-knockout mice. In contrast to FlnB null, Fmn1 loss results in a decrease in the width of the prehypertrophic zone. Loss of both proteins, however, causes an overall decrease in the width of the proliferation zone and an increase in the differentiated hypertrophic zone. The current findings suggest that Fmn1 and FlnB have shared and independent functions. FlnB loss promotes prehypertrophic differentiation whereas Fmn1 leads to a delay. Both proteins, however, regulate chondrocyte proliferation, and FlnB may regulate Fmn1 function at the hypertrophic-to-ossification border, thereby explaining the overall delay in ossification.

Filamin B regulates chondrocyte proliferation and differentiation through Cdk1 signaling

Humans who harbor loss of function mutations in the actin-associated filamin B (FLNB) gene develop spondylocarpotarsal syndrome (SCT), a disorder characterized by dwarfism (delayed bone formation) and premature fusion of the vertebral, carpal and tarsal bones (premature differentiation). To better understand the cellular and molecular mechanisms governing these seemingly divergent processes, we generated and characterized FlnB knockdown ATDC5 cell lines. We found that FlnB knockdown led to reduced proliferation and enhanced differentiation in chondrocytes. Within the shortened growth plate of postnatal FlnB(-/-) mice long bone, we observed a similarly progressive decline in the number of rapidly proliferating chondrocytes and premature differentiation characterized by an enlarged prehypertrophic zone, a widened Col2a1(+)/Col10a1(+) overlapping region, but relatively reduced hypertrophic zone length. The reduced chondrocyte proliferation and premature differentiation were, in part, attributable to enhanced G2/M phase progression, where fewer FlnB deficient ATDC5 chondrocytes resided in the G2/M phase of the cell cycle. FlnB loss reduced Cdk1 phosphorylation (an inhibitor of G2/M phase progression) and Cdk1 inhibition in chondrocytes mimicked the null FlnB, premature differentiation phenotype, through a β1-integrin receptor- Pi3k/Akt (a key regulator of chondrocyte differentiation) mediated pathway. In this context, the early prehypertrophic differentiation provides an explanation for the premature differentiation seen in this disorder, whereas the progressive decline in proliferating chondrocytes would ultimately lead to reduced chondrocyte production and shortened bone length. These findings begin to define a role for filamin proteins in directing both cell proliferation and differentiation through indirect regulation of cell cycle associated proteins.

Towards a better understanding of bone bridge formation in the growth plate - an immunohistochemical approach

The growth plate at the end of long bones is the cartilaginous organ responsible for longitudinal bone growth in children. Trauma to the growth plate, i.e. fractures, can severely impair longitudinal bone growth, leading to growth disorders due to destruction of the epiphyseal circulation and formation of a bone bridge. From the clinical experience it is known that in some patients this bone bridge eventually disappears during the growth process. However, the molecular mechanisms involved in bone bridge formation and dissolution have not been clarified yet. The aim of this study was to investigate the spatial and temporal protein level of molecules potentially involved in these processes, i.e. RANKL, OPG, DKK-1, Coll 10, BMP-2 and IL-6, in an experimental rat model using an immunohistochemical approach. The results from our study suggest that bone bridge formation might be an early event starting immediately after growth plate injury and involving several pro-osteoblastic molecules, i.e. IL-6, BMP-2 as well as OPG and Coll X. In the late studied time points 3- and 9-month post-injury expression of anti-osteoblastic proteins, i.e. DKK1 and RANKL, was increased. This indicates that bone bridge dissolution might be a late event and potentially linked to Wnt signaling inhibition and RANK/RANKL signaling activation.

3β-Hydroxysterol-Delta24 reductase plays an important role in long bone growth by protecting chondrocytes from reactive oxygen species

Desmosterolosis is an autosomal recessive disease caused by mutations in the 3β-hydroxysterol-Delta24 reductase (DHCR24) gene, with severe developmental anomalies including short limbs. We utilized DHCR24 knockout (KO) mice to study the underlying bone pathology. Because the KO mice died within a few hours after birth, we cultured metatarsal bones from newborn mice. The growth of bones from KO mice was significantly retarded after 1 week of culture. Absence of proliferating chondrocytes in the growth plate and abnormal hypertrophy of prehypertrophic chondrocytes were observed in the bones from KO mice. Hypertrophic differentiation was evidenced by higher expression of Indian hedgehog, alkaline phosphatase, and matrix metalloproteinase 13. Since elevated levels of reactive oxygen species (ROS) during chondrogenesis are known to inhibit proliferation and to initiate chondrocyte hypertrophy in the growth plate, and since DHCR24 acts as a potent ROS scavenger, we hypothesized that the abnormal chondrocyte proliferation and differentiation in KO mice were due to decreased ROS scavenging activity. Treatment with an antioxidant, N-acetyl cysteine, could correct the abnormalities observed in the bones from KO mice. Treatment of bones from wild-type mice with U18666A, a chemical inhibitor of DHCR24, resulted in short broad bones with a disrupted proliferating zone. Treatment of ATDC cells with hydrogen peroxide (H(2)O(2)) induced hypertrophic changes as evidenced by the expression of the marker genes specific for hypertrophic chondrocyte differentiation. H(2)O(2)-induced hypertrophic change was prevented by adenoviral delivery of DHCR24. Induction of chondrocyte differentiation in ATDC cells by insulin was associated with increased ROS production that was markedly enhanced by treatment of ATDC5 cells with DHCR24 siRNA. This is the first demonstration that DHCR24 plays an important role in long bone growth by protecting chondrocytes from ROS.

Matrix recruitment and calcium sequestration for spatial specific otoconia development

Otoconia are bio-crystals anchored to the macular sensory epithelium of the utricle and saccule in the inner ear for motion sensing and bodily balance. Otoconia dislocation, degeneration and ectopic calcification can have detrimental effects on balance and vertigo/dizziness, yet the mechanism underlying otoconia formation is not fully understood. In this study, we show that selected matrix components are recruited to form the crystal matrix and sequester Ca(2+) for spatial specific formation of otoconia. Specifically, otoconin-90 (Oc90) binds otolin through both domains (TH and C1q) of otolin, but full-length otolin shows the strongest interaction. These proteins have much higher expression levels in the utricle and saccule than other inner ear epithelial tissues in mice. In vivo, the presence of Oc90 in wildtype (wt) mice leads to an enrichment of Ca(2+) in the luminal matrices of the utricle and saccule, whereas absence of Oc90 in the null mice leads to drastically reduced matrix-Ca(2+). In vitro, either Oc90 or otolin can increase the propensity of extracellular matrix to calcify in cell culture, and co-expression has a synergistic effect on calcification. Molecular modeling and sequence analysis predict structural features that may underlie the interaction and Ca(2+)-sequestering ability of these proteins. Together, the data provide a mechanism for the otoconial matrix assembly and the role of this matrix in accumulating micro-environmental Ca(2+) for efficient CaCO(3) crystallization, thus uncover a critical process governing spatial specific otoconia formation.

Discoidin domain receptor-1 deficiency attenuates atherosclerotic calcification and smooth muscle cell-mediated mineralization

Intimal calcification is a feature of advanced atherosclerotic disease that predicts a two- to eightfold increase in the risk of coronary events. Type I collagen promotes vascular smooth muscle cell-mediated calcification, although the mechanism by which this occurs is unknown. The discoidin domain receptor 1 (DDR1) is a collagen receptor that is emerging as a critical mediator of atherosclerosis. To determine whether DDR1 is involved in intimal calcification, we fed male Ddr1(-/-);Ldlr(-/-) and Ddr1(+/+);Ldlr(-/-) mice an atherogenic diet for 6, 12, or 24 weeks. DDR1 deficiency significantly reduced the calcium content of the aortic arch, and microcomputed tomography demonstrated a significant decrease in hydroxyapatite deposition after 24 weeks of atherogenic diet. Reduced calcification was correlated with decreases in macrophage accumulation and tumor necrosis factor alpha staining, suggesting that the reduction in calcification was in part due to decreased inflammation. The chondrogenic markers type II collagen, type X collagen, and Sox-9 were expressed within the mineralized foci. An in vitro assay performed with vascular smooth muscle cells revealed that DDR1 was required for cell-mediated calcification of the matrix, and Ddr1(+/+) smooth muscle cells expressed more alkaline phosphatase activity, whereas Ddr1(-/-) smooth muscle cells expressed elevated levels of mRNA for nucleotide pyrophosphatase phosphodiesterase 1, an inhibitor of tissue mineralization. Taken together, our results demonstrate that DDR1 mediates an important mechanism for atherosclerotic calcification.

Molecular basis for skeletal variation: insights from developmental genetic studies in mice

Skeletal variations are common in humans, and potentially are caused by genetic as well as environmental factors. We here review molecular principles in skeletal development to develop a knowledge base of possible alterations that could explain variations in skeletal element number, shape or size. Environmental agents that induce variations, such as teratogens, likely interact with the molecular pathways that regulate skeletal development.

Regulation of chondrocyte differentiation by the actin cytoskeleton and adhesive interactions

Chondrocyte differentiation is a multi-step process characterized by successive changes in cell morphology and gene expression. In addition to tight regulation by numerous soluble factors, these processes are controlled by adhesive events. During the early phase of the chondrocyte life cycle, cell-cell adhesion through molecules such as N-cadherin and neural cell adhesion molecule (N-CAM) is required for differentiation of mesenchymal precursor cells to chondrocytes. At later stages, for example in growth plate chondrocytes, adhesion signaling from extracellular matrix (ECM) proteins through integrins and other ECM receptors such as the discoidin domain receptor (DDR) 2 (a collagen receptor) and Annexin V is necessary for normal chondrocyte proliferation and hypertrophy. Cell-matrix interactions are also important for chondrogenesis, for example through the activity of CD44, a receptor for Hyaluronan and collagens. The roles of several signaling molecules involved in adhesive signaling, such as integrin-linked kinase (ILK) and Rho GTPases, during chondrocyte differentiation are beginning to be understood, and the actin cytoskeleton has been identified as a common target of these adhesive pathways. Complete elucidation of the pathways connecting adhesion receptors to downstream effectors and the mechanisms integrating adhesion signaling with growth factor- and hormone-induced pathways is required for a better understanding of physiological and pathological skeletal development.

Functional genomics, evo-devo and systems biology: a chance to overcome complexity?

PURPOSE OF REVIEW

This review addresses the key question of how to integrate a high complexity of processes and data to a unifying picture of disease processes and progression relevant for osteoarthritis.

RECENT FINDINGS

Many research efforts in the last few years have resulted in the accumulation of a huge amount of data. To date, however, these data have not led to a unifying concept of the pathogenesis and progression of the osteoarthritic disease process. Methods to integrate a lot of information are needed, therefore, in order to progress from experimental findings to practical knowledge. Several such strategies have been followed up in the past: in-vitro models, large-scale gene expression analysis/functional genomics, and an attempt to interpret gene expression patterns on the basis of developmental chondrocyte differentiation. A novel approach is systems biology, which promises to overcome issues of complexity using appropriate models and quantitative simulation.

SUMMARY

Efforts are required to integrate a continuously growing high complexity of experimental data into an understanding of the joint system and its derangement in osteoarthritis. Modelling of the 'whole' picture appears to be needed so that we do not get lost in the plethora of details.

Global analyses of gene expression in early experimental osteoarthritis

OBJECTIVE

To analyze genome-wide changes in chondrocyte gene expression in a surgically induced model of early osteoarthritis (OA) in rats, to assess the similarity of this model to human OA, and to identify genes and mechanisms leading to OA pathogenesis.

METHODS

OA was surgically induced in 5 rats by anterior cruciate ligament transection and partial medial meniscectomy. Sham surgery was performed in 5 additional animals, which were used as controls. Both groups underwent 4 weeks of forced mobilization, 3 times per week. RNA was extracted directly from articular chondrocytes in the OA (operated), contralateral, and sham-operated knees. Affymetrix GeneChip expression arrays were used to assess genome-wide changes in gene expression. Expression patterns of selected dysregulated genes, including Col2a1, Mmp13, Adamts5, Ctsc, Ptges, and Cxcr4, were validated by real-time polymerase chain reaction, immunofluorescence, or immunohistochemistry 2, 4, and 8 weeks after surgery.

RESULTS

After normalization, comparison of OA and sham-operated samples showed 1,619 differentially expressed probe sets with changes in their levels of expression > or = 1.5-fold, 722 with changes > or = 2-fold, 135 with changes > or = 4-fold, and 20 with changes of 8-fold. Dysregulated genes known to be involved in human OA included Mmp13, Adamts5, and Ptgs2, among others. Several dysregulated genes (e.g., Reln, Phex, and Ltbp2) had been identified in our earlier microarray study of hypertrophic chondrocyte differentiation. Other genes involved in cytokine and chemokine signaling, including Cxcr4 and Ccl2, were identified. Changes in gene expression were also observed in the contralateral knee, validating the sham operation as the appropriate control.

CONCLUSION

Our results demonstrate that the animal model mimics gene expression changes seen in human OA, supporting the relevance of newly identified genes and pathways to early human OA. We propose new avenues for OA pathogenesis research and potential targets for novel OA treatments, including cathepsins and cytokine, chemokine, and growth factor signaling pathways, in addition to factors controlling the progression of chondrocyte differentiation.

Identification of a promoter element within the zebrafish colXalpha1 gene responsive to runx2 isoforms Osf2/Cbfa1 and til-1 but not to pebp2alphaA2

Type X collagen is a short chain collagen specifically expressed by hypertrophic chondrocytes during endochondral ossification. We report here the functional analysis of the zebrafish (Danio rerio) collagen Xalpha1 gene (colXalpha1) promoter with the identification of a region responsive to two isoforms of the runt domain transcription factor runx2. Furthermore, we provide evidence for the presence of dual promoter usage in zebrafish, a finding that should be important to further understanding of the regulation of its restricted tissue distribution and spatial-temporal expression during early development. The zebrafish colXalpha1 gene structure is comparable to that recently identified by comparative genomics in takifugu and shows homology with corresponding mammalian genes, indicating that its general architecture has been maintained throughout vertebrate evolution. Our data suggest that, as in mammals, runx2 plays a role in the development of the osteogenic lineage, supporting zebrafish as a model for studies of bone and cartilage development.

Rac1/Cdc42 and RhoA GTPases antagonistically regulate chondrocyte proliferation, hypertrophy, and apoptosis

The intracellular signaling pathways controlling chondrocyte physiology are largely unknown. Here we show that the small GTPases, Rac1 and Cdc42, accelerate the rate of chondrocyte differentiation and apoptosis, thereby antagonizing the activity of RhoA. These results identify Rac1 and Cdc42 pathways as novel regulators of cartilage development.

INTRODUCTION

Proliferation, hypertrophic differentiation, and ultimate apoptosis of chondrocytes regulate endochondral bone growth and development, but the intracellular signaling pathways controlling chondrocyte biology are incompletely understood. In this study, we investigated the role of the small GTPases Rac1 and Cdc42 in chondrocytes.

MATERIALS AND METHODS

Rac1 and Cdc42 expression during chondrogenic differentiation was assessed by RT-PCR and Western blotting. Effects of Rac1 and Cdc42 on parameters of chondrocyte biology were studied using transient transfections into primary mouse chondrocytes and stable transfections of the chondrogenic cell line ATDC5. Luciferase assays, RT-PCR, cell proliferation, alkaline phosphatases assays, staining procedures, TUNEL assays, and caspase activity assays were performed to study the chondrocyte response to overexpression of Rac1 and Cdc42 proteins. Activation of the p38 pathway was analyzed using Western blotting with phospho-specific antibodies, and mitogen-activated protein (MAP) kinase pathways were inhibited using pharmacological approaches.

RESULTS AND CONCLUSIONS

Rac1 and Cdc42 activities are required for maximal activity of the collagen X promoter, a hypertrophic marker, in primary chondrocytes, suggesting essential roles of these GTPases in chondrocyte hypertrophy. Overexpression of Rac1 or Cdc42 in chondrogenic ATDC5 cells results in reductions in cell numbers and marked acceleration of hypertrophic differentiation, thus opposing the effects of the related GTPase RhoA. Rac1 and Cdc42 also induce accelerated chondrocyte apoptosis, as shown by TUNEL and caspase activity assays and changes in cell morphology and actin organization. Rac1 and Cdc42 overexpression results in activation of the p38 MAP kinase pathway in ATDC5 cells, and pharmacological inhibition of p38 signaling blocks the effects of Rac1 and Cdc42 overexpression on hypertrophy and apoptosis. Our results therefore suggest that Rac1 and Cdc42 signaling accelerates progression through the chondrocyte life cycle in a p38-dependent fashion and antagonizes RhoA signaling pathways in chondrocyte proliferation, hypertrophy, and apoptosis.

Tissue engineering of human cartilage in bioreactors using single and composite cell-seeded scaffolds

Chondrocytes isolated from human fetal epiphyseal cartilage were seeded under mixed conditions into 15-mm-diameter polyglycolic acid (PGA) scaffolds and cultured in recirculation column bioreactors to generate cartilage constructs. After seeding, the cell distributions in thick (4.75 mm) and thin (2.15 mm) PGA disks were nonuniform, with higher cell densities accumulating near the top surfaces. Composite scaffolds were developed by suturing together two thin PGA disks after seeding to manipulate the initial cell distribution before bioreactor culture. The effect of medium flow direction in the bioreactors, including periodic reversal of medium flow, was also investigated. The quality of the tissue-engineered cartilage was assessed after 5 weeks of culture in terms of the tissue wet weight, glycosaminoglycan (GAG), total collagen and collagen type II contents, histological analysis of cell, GAG and collagen distributions, and immunohistochemical analysis of collagen types I and II. Significant enhancement in construct quality was achieved using composite scaffolds compared with single PGA disks. Operation of the bioreactors with periodic medium flow reversal instead of unidirectional flow yielded further improvements in tissue weight and GAG and collagen contents with the composite scaffolds. At harvest, the constructs contained GAG concentrations similar to those measured in ex vivo human adult articular cartilage; however, total collagen and collagen type II levels were substantially lower than those in adult tissue. This study demonstrates that the location of regions of high cell density in the scaffold coupled with application of dynamic bioreactor operating conditions has a significant influence on the quality of tissue-engineered cartilage.

Deer antlers as a model of Mammalian regeneration

Deer antlers are cranial appendages that develop after birth as extensions of a permanent protuberance (pedicle) on the frontal bone. Pedicles and antlers originate from a specialized region of the frontal bone; the 'antlerogeneic periosteum' and the systemic cue which triggers their development in the fawn is an increase in circulating androgen. These primary antlers are then shed and regenerated the following year in a larger, more complex form. Antler growth is extremely rapid-an adult red deer can produce a pair of antlers weighing approximately 30kg in three months, and involves both endochondral and intramembranous ossification. Since antlers are sexual secondary characteristics, their annual cycles of growth have evolved to be closely coordinated to the reproductive cycle which, in temperate species, is linked to the photoperiod. Cessation of antler growth and death of the overlying skin (velvet) coincides with a rise in circulating testosterone as the autumn breeding season approaches. The 'dead' antlers remain attached to the pedicle until they are shed (cast) the following spring when circulating testosterone levels fall. In red deer, the species that we study, casting of the old set of antlers is followed immediately by growth of the new set. Although the anatomy of antler growth and the endocrine changes associated with it have been well documented, the molecular mechanisms involved remain poorly understood. The case for continuing to decipher them remains compelling, despite the obvious limitations of using deer as an experimental model, because this research will help provide insight into why humans and other mammals have lost the ability to regenerate organs. From the information so far available, it would appear that the signaling pathways that control the development of skeletal elements are recapitulated in regenerating antlers. This apparent lack of any specific 'antlerogenic molecular machinery' suggests that the secret of deers' ability to regenerate antlers lies in the particular cues to which multipotential progenitor/stem cells in an antler's 'regeneration territory' are exposed. This in turn suggests that with appropriate manipulation of the environment, pluripotential cells in other adult mammalian tissues could be stimulated to increase the healing capacity of organs, even if not to regenerate them completely. The need for replacement organs in humans is substantial. The benefits of increasing individuals' own capacity for regeneration and repair are self evident.

Mesenchymal chondrosarcoma: an immunohistochemical study of 10 cases examining prognostic significance of proliferative activity and cellular differentiation

AIMS

Mesenchymal chondrosarcoma is a rare malignant chondrogenic neoplasm that tends to affect young adults and teenagers. The prognosis is unpredictable, and the identification of prognostic markers that could aid in determining the behaviour of this tumour would be helpful. There are few studies in the literature that have attempted to address this issue.

METHODS AND RESULTS

In this study, we explored the prognostic significance of three different parameters: (1) tissue morphology of small cell areas, (2) the expression of tumour differentiation marker genes, and (3) the proliferation rate. Our results did not show a correlation of prognosis with the histological features of the neoplastic small cell areas or the expression of tumour differentiation genes. However, the proliferative activity of the tumour cells appeared to have some prognostic significance as related to patient survival.

CONCLUSION

Mesenchymal chondrosarcoma is a rare tumour with a wide clinical range of behaviour. Therefore, it is difficult to obtain reliable prognostic parameters. Nevertheless, our study suggests that proliferative activity may be a useful prognostic parameter for mesenchymal chondrosarcomas.

p38 MAP kinase signalling is required for hypertrophic chondrocyte differentiation

Longitudinal growth of endochondral bones is accomplished through the co-ordinated proliferation and hypertrophic differentiation of growth plate chondrocytes. The molecular mechanisms and signalling cascades controlling these processes are not well understood. To analyse the expression and roles of p38 mitogen-activated protein kinases in this process, we have established a micromass system for the reproducible hypertrophic differentiation of mouse mesenchymal limb bud cells. Our results show that all four mammalian p38 kinase genes are expressed during the chondrogenic programme, as well as their upstream regulators MKK3 (mitogen-activated protein kinase kinase 3) and MKK6. Treatment of micromass cultures with pharmacological inhibitors of p38 results in a marked delay in hypertrophic differentiation in micromass cultures, indicating a requirement for p38 signalling in chondrocyte differentiation. Inhibition of p38 kinase activity leads to reduced and delayed induction of alkaline phosphatase activity and matrix mineralization. In addition, p38 inhibition causes reduced expression of hypertrophic marker genes such as collagen X, matrix metalloproteinase 13 and bone sialoprotein. The function of p38 in hypertrophic differentiation appears to be mediated, at least in part, by the transcription factor myocyte enhancer factor 2C. In summary, we have demonstrated a novel requirement for p38 signalling in hypertrophic differentiation of chondrocytes and identified myocyte enhancer factor 2C as an important regulator of chondrocyte gene expression.

RhoA/ROCK Signaling Suppresses Hypertrophic Chondrocyte Differentiation

Coordinated proliferation and differentiation of growth plate chondrocytes is required for normal growth and development of the endochondral skeleton, but little is known about the intracellular signal transduction pathways regulating these processes. We have investigated the roles of the GTPase RhoA and its effector kinases ROCK1/2 in hypertrophic chondrocyte differentiation. RhoA, ROCK1, and ROCK2 are expressed throughout chondrogenic differentiation. RhoA overexpression in chondrogenic ATDC5 cells results in increased proliferation and a marked delay of hypertrophic differentiation, as shown by decreased induction of alkaline phosphatase activity, mineralization, and expression of the hypertrophic markers collagen X, bone sialoprotein, and matrix metalloproteinase 13. These effects are accompanied by activation of cyclin D1 transcription and repression of the collagen X promoter by RhoA. In contrast, inhibition of Rho/ROCK signaling by the pharmacological inhibitor Y27632 inhibits chondrocyte proliferation and accelerates hypertrophic differentiation. Dominant-negative RhoA also inhibits induction of the cyclin D1 promoter by parathyroid hormone-related peptide. Finally, Y27632 treatment partially rescues the effects of RhoA overexpression. In summary, we identify the RhoA/ROCK signaling pathway as a novel and important regulator of chondrocyte proliferation and differentiation.

Extraskeletal myxoid chondrosarcomas do not show a chondrocytic phenotype

Extraskeletal myxoid chondrosarcoma is a rare mesenchymal soft-tissue malignancy of putative chondrocytic differentiation. Occasional overt cartilage formation, positivity for S-100 protein, and ultrastructural analysis have supported this view. However, most extraskeletal myxoid chondrosarcomas do not show chondroid tissue formation, and S-100 protein is found much less commonly than has been reported. Both these observations cast doubt on the histogenetic classification of extraskeletal myxoid chondrosarcoma as a chondroblastic entity. Mostly using matrix proteins as markers of mesenchymal cell differentiation, we investigated the biochemical matrix composition and cellular phenotype of the tumor cells in representative specimens from 14 extraskeletal myxoid chondrosarcomas. In all but one tumor specimen, which showed histomorphologically overt cartilage formation, only occasional staining for the proteoglycan aggrecan was found. Specimens from two tumors showed presence of collagen type II, and none was positive for collagen type X. Instead, collagen types I, III, and VI were diffusely positive. Also, S-100 protein was largely absent. Our results suggest that the basic cellular phenotype of extraskeletal myxoid chondrosarcoma is not chondrocytic or prechondrocytic and that extraskeletal myxoid chondrosarcoma is not a chondrosarcomatous entity. Extraskeletal myxoid chondrosarcoma consists most likely of primitive mesenchymal cells with focal, multidirectional differentiation. Chondrocytic differentiation is an unusual facet in the spectrum of differentiation patterns exhibited by these lesions.

MAP kinases in chondrocyte differentiation

The majority of the vertebrate skeleton develops through the process of endochondral ossification and involves successive steps of chondrogenesis, chondrocyte proliferation, and hypertrophic chondrocyte differentiation. Interruption of this program through gene mutations and hormonal or environmental factors contributes to numerous diseases, including growth disorders and chondrodysplasias. While a large number of growth factors and hormones have been implicated in the regulation of chondrocyte biology, relatively little is known about the intracellular signaling pathways involved. Recent data provide novel insights into the mechanisms governing acquisition of new phenotypes within the chondrogenic program and suggest multiple pivotal roles for members of the mitogen-activated protein kinase family and their downstream targets in cartilage development. These data are summarized and discussed here.

Expression of NG2 proteoglycan during endochondral and intramembranous ossification

We have used immunohistochemistry to study the distribution of the NG2 proteoglycan during bone development in the mouse. At embryonic day 15.5, NG2 was strongly detected in the immature cartilage of developing limbs. After transient down-regulation in mature chondrocytes, NG2 was up-regulated during primary ossification, colocalizing with alkaline phosphatase and tenascin C. In the epiphyseal growth plates of newborn mouse tibia, NG2 and alkaline phosphatase exhibited overlapping patterns of expression by hypertrophic chondrocytes and by osteoblasts surrounding newly formed bone trabeculae. NG2 was down-regulated after puberty, being only faintly detectable in the tibial growth plates of 3-month-old mice. In cranial sutures, NG2 was strongly labeled in osteogenic bone fronts and in the suture matrix. Our results indicate that NG2 expression is up-regulated during both endochondral and intramembranous ossification, but is down-regulated as ossification is completed.

Skeletal dysplasias caused by a disruption of skeletal patterning and endochondral ossification

Identification of a number of the genes that cause skeletal dysplasias has helped clinicians to provide accurate diagnoses, genetic counseling, and pre-natal diagnosis for this complex group of disorders. This review considers how some of the recent advances in human and murine genetics have led to an increased understanding of normal bone development and, in particular, the processes of skeletal patterning and endochondral ossification.

Osteophyte development--molecular characterization of differentiation stages

OBJECTIVE

Osteophytes are non-neoplastic osteo-cartilaginous protrusions growing at the margins of osteoarthritic joints. They can not only be considered as in situ repair tissue, but also represent an excellent in vivo model for induced cartilage repair processes. Our focus was to identify different steps of osteophyte development via analysis of expression patterns of marker genes of chondrocytic differentiation.

DESIGN

We performed an extensive analysis of the presence and expression of matrix components using histochemical, immunohistochemical and in situ hybridization technology.

RESULTS

Four different stages of osteophyte formation could be identified based on histomorphological and cell biological parameters: starting from mesenchymal condensates, chondrogenic differentiation is indicated by the onset of Col2A and aggrecan expression (stage I). Stage II shows fibrocartilage with an admixture of cartilaginous and fibrous matrix components such as Col2 and aggrecan on the one hand and Col1 on the other hand. The proliferating osteophyte (stage III) shows a zonal organization similar to the fetal growth plate cartilage with extensive chondrocyte hypertrophy in the zones next to ongoing endochondral bone formation. 'Mature' osteophytes (stage IV) resembled largely articular hyaline cartilage with a predominance of Col2 and aggrecan and Col6 found mainly pericellularily.

CONCLUSIONS

The development of osteophytes is a good in vivo model to pursue chondrocyte differentiation from pluripotent mesenchymal cells to mature or hypertrophic chondrocytes in situ in the adult. The analysis of marker molecules of mesenchymal differentiation allows to identify different stages of repair tissue development and the transformation from fibrous tissue to neo-cartilage. Tissue architecture and matrix composition in mature osteophytes suggests that metaplastic neo-cartilagenous tissue might be one potential source of cartilage repair tissue in the adult joint.

Quantification of expression levels of cellular differentiation markers does not support a general shift in the cellular phenotype of osteoarthritic chondrocytes

Many studies have shown increased anabolic activity in osteoarthritic cartilage and have suggested changes in the cellular phenotypes of articular chondrocytes. Most of these studies relied on non-quantitative technologies, which did not allow the estimation of the relative importance of the different differentiation phenomena. In the present study, we developed and used quantitative PCR assays for collagen types I, II(total), IIA, III, and X as marker genes indicating cellular synthetic activity (collagen type II) as well as differentiation pattern of chondrocytes (collagen types I, IIA, III, and X) and quantified these genes in normal, early degenerative, and late stage osteoarthritic cartilage in parallel. At first sight, our results confirmed previously published data showing hardly any expression of collagen genes in normal and significantly enhanced expression in osteoarthritic cartilage. This included collagen types II, III, and IIA, but also collagen types I(alpha1) and X. However, if one considers the ratios of the various markers of chondrocytic differentiation in comparison to collagen type II, the main synthetic product of differentiated chondrocytes, no shift in the cellular phenotype was detectable. In fact, expression ratios remained constant or were even decreased in osteoarthritic cartilage. Our results confirm that normal adult human articular chondrocytes display hardly any expression activity of the collagen types investigated, whereas osteoarthritic chondrocytes show very increased synthetic activity. The largely unchanged ratios of collagen subtypes investigated indicate that no general shift in the cellular phenotype does occur in osteoarthritic cartilage as suggested by previous investigations.

Collagen X chains harboring Schmid metaphyseal chondrodysplasia NC1 domain mutations are selectively retained and degraded in stably transfected cells

Collagen X is a short chain, homotrimeric collagen expressed specifically by hypertrophic chondrocytes during endochondral bone formation and growth. Although the exact role of collagen X remains unresolved, mutations in the COL10A1 gene disrupt growth plate function and result in Schmid metaphyseal chondrodysplasia (SMCD). With the exception of two mutations that impair signal peptide cleavage during alpha1(X) chain biosynthesis, SMCD mutations are clustered within the carboxyl-terminal NC1 domain. The formation of stable NC1 domain trimers is a critical stage in collagen X assembly, suggesting that mutations within this domain may result in subunit mis-folding or reduce trimer stability. When expressed in transiently transfected cells, alpha1(X) chains containing SMCD mutations were unstable and presumed to be degraded intracellularly. More recently, in vitro studies have shown that certain missense mutations may exert a dominant negative effect on alpha1(X) chain assembly by formation of mutant homotrimers and normal-mutant heterotrimers. In contrast, analysis of cartilage tissue from two SMCD patients revealed that the truncated mutant message was fully degraded, resulting in 50% reduction of functional collagen X within the growth plate. Therefore, in the absence of data that conclusively demonstrates the full cellular response to mutant collagen X chains, the molecular mechanisms underlying SMCD remain controversial. To address this, we closely examined the effect of two NC1 domain mutations, one frameshift mutation (1963del10) and one missense mutation (Y598D), using both semi-permeabilized cell and stable cell transfection expression systems. Although able to assemble to a limited extent in both systems, we show that, in intact cells, collagen X chains harboring both SMCD mutations did not evade quality control mechanisms within the secretory pathway and were degraded intracellularly. Furthermore, co-expression of wild-type and mutant chains in stable transfected cells demonstrated that, although wild-type chains were secreted, mutant chains were largely excluded from hetero-trimer formation. Our data indicate, therefore, that the predominant effect of the NC1 mutations Y598D and 1963del10 is a reduction in the amount of functional collagen X within the growth cartilage extracellular matrix.

Gene expression by human articular chondrocytes cultured in alginate beads

Culture of articular chondrocytes in alginate beads offers several advantages over culture in monolayer; cells retain their phenotype for 8 months or longer. Earlier studies of chondrocytes cultured in alginate concentrated on collagen and proteoglycan synthesis. However, gene expression by in situ hybridization (ISH) has not been investigated. The purposes of the present study on human chondrocytes were (a) to modify the ISH procedure for the alginate beads to examine the mRNA expression of alpha1 (II) procollagen, aggrecan, and two matrix metalloproteinases (MMP-3 and MMP-8) thought to be involved in cartilage matrix degradation, and (b) to compare expression in cultured chondrocytes with that in chondrocytes of intact human cartilage. The modifications made for ISH include the presence of CaCl2 and BaCl2 in the fixation and washing steps and exclusion of cetyl pyridinium chloride. By ISH we show that aggrecan, MMP-3, and MMP-8 are continuously expressed during 8 months of culture. The alpha1 (II) procollagen gene is expressed only during the first 2 months of culture and after 3 months its expression is undetectable, which is consistent with its absence in adult articular cartilage. By Western blotting, Type II collagen protein had been synthesized and deposited in both the cell-associated and further-removed matrix compartments at 7 and 14 days of culture. These data indicate that chondrocytes cultured in alginate beads could be preserved for immunohistochemistry and ISH and that culture of human chondrocytes in alginate beads may serve as a good model for studying cartilage-specific phenotype as well as factors that influence cartilage matrix turnover.

Coordinated expression of matrix Gla protein is required during endochondral ossification for chondrocyte survival

Matrix Gla protein (MGP) is a 14-kD extracellular matrix protein of the mineral-binding Gla protein family. Studies of MGP-deficient mice suggest that MGP is an inhibitor of extracellular matrix calcification in arteries and the epiphyseal growth plate. In the mammalian growth plate, MGP is expressed by proliferative and late hypertrophic chondrocytes, but not by the intervening chondrocytes. To investigate the functional significance of this biphasic expression pattern, we used the ATDC5 mouse chondrogenic cell line. We found that after induction of the cell line with insulin, the differentiating chondrocytes express MGP in a stage-specific biphasic manner as in vivo. Treatment of the ATDC5 cultures with MGP antiserum during the proliferative phase leads to their apoptosis before maturation, whereas treatment during the hypertrophic phase has no effect on chondrocyte viability or mineralization. After stable transfection of ATDC5 cells with inducible sense or antisense MGP cDNA constructs, we found that overexpression of MGP in maturing chondrocytes and underexpression of MGP in proliferative and hypertrophic chondrocytes induced apoptosis. However, overexpression of MGP during the hypertrophic phase has no effect on chondrocyte viability, but it does reduce mineralization. This work suggests that coordinated levels of MGP are required for chondrocyte differentiation and matrix mineralization.

Type II collagen degradation in articular cartilage fibrillation after anterior cruciate ligament transection in rats

OBJECTIVE

To investigate the kinetics of early cartilage changes in mechanically induced osteoarthritis (OA) and the association of these changes with damage to the type II collagen network.

METHODS

Experimental OA was induced by anterior cruciate ligament transsection in the rat knee joint (ACLT-OA). Animals were sacrificed after 2, 7, 14, 28 and 70 days. Knee joints were evaluated using routine histology and immunohistochemistry for denatured (unwound) type II collagen to detect collagen damage. An antibody recognizing the collagenase cleavage site in type II collagen was used to study the role of collagenase in this process.

RESULTS

The first changes of the articular cartilage after anterior cruciate ligament transection occurred in the superficial zone. These changes included loss of superficial chondrocytes, swelling of the remaining chondrocytes and superficial fibrillation. The swelling of the chondrocytes did not result from a change towards the hypertrophic phenotype, since these cells did not stain for type X collagen. A marked increase in denatured type II collagen staining was present in the fibrillated areas. Staining of the collagenase cleavage site showed the same distribution as denatured collagen but was clearly less intense. Collagen damage could never be detected before fibrillation occurred and was not present in non-fibrillated areas.

CONCLUSIONS

These results indicate that in this model cartilage degeneration starts at the articular surface and that this degeneration is associated with a localized expression of type II collagen degradation products.

Expression of type X collagen in young and old C57Bl/6 and Balb/c mice. Relation with articular cartilage degeneration

OBJECTIVE

To investigate whether the development of osteoarthritic lesions in the knee joints of mice is associated with increased immunostaining of type X collagen.

METHODS

Sections of total knee joints in combination with immunohistochemistry were used to study the distribution of type X collagen in the cartilage of young and old mice of two mouse strains, Balb/c and C57Bl/6, known to develop osteoarthritic lesions at different locations. Expression of type X collagen and PTH/PTHrP-receptor mRNA were studied by RT-PCR.

RESULTS

Young adult Balb/c and C57Bl/6 mice both expressed type X collagen in the non-calcified cartilage of the tibia-femoral joint. Old mice of both strains had a strongly increased deposition of type X collagen in the patella-femoral but not in the tibia-femoral joint. The locations in the murine knee joints prone to develop osteoarthritis (OA) did not preferentially express increased amounts of type X collagen. Thus, whereas increased type X was observed in both strains in the patella-femoral joints, only Balb/c mice preferentially developed osteoarthritic lesions in these joints. Also cartilage degeneration was usually seen only in the lateral compartment of the knee joints of C57Bl/6 mice but this was not accompanied by increased type X collagen immunostaining. Increased deposition of type X collagen was not associated with elevated levels of type X collagen mRNA or with decreased levels of PTH/PTHrP-receptor mRNA.

CONCLUSION

Type X collagen expression and spontaneous OA in mice are not necessarily related since OA prone locations in the murine knee joint do not preferentially express type X collagen.

Articular cartilage and changes in arthritis. An introduction: cell biology of osteoarthritis

The reaction patterns of chondrocytes in osteoarthritis can be summarized in five categories: (1) proliferation and cell death (apoptosis); changes in (2) synthetic activity and (3) degradation; (4) phenotypic modulation of the articular chondrocytes; and (5) formation of osteophytes. In osteoarthritis, the primary responses are reinitiation of synthesis of cartilage macromolecules, the initiation of synthesis of types IIA and III procollagens as markers of a more primitive phenotype, and synthesis of active proteolytic enzymes. Reversion to a fibroblast-like phenotype, known as "dedifferentiation", does not appear to be an important component. Proliferation plays a role in forming characteristic chondrocyte clusters near the surface, while apoptosis probably occurs primarily in the calcified cartilage.

Type I collagen influences cartilage calcification: an immunoblocking study in differentiating chick limb-bud mesenchymal cell cultures

Chick limb-bud mesenchymal cells, plated in high-density micro-mass culture, differentiate and form a matrix resembling chick epiphyseal cartilage. In the presence of 4 mM inorganic phosphate or 2.5 mM beta-glycerophosphate mineral deposits upon this matrix forming a mineralized tissue that, based on electron microscopy, x-ray diffraction and Fourier Transform Infrared microspectoscopy, is like that of chick calcified cartilage. In this culture system the initial mineral deposits are found on the periphery of the chondrocyte nodules. During differentiation of the cells in the high-density micro-mass cultures there is a switch from expression of type I collagen to type II, and then to type X collagen. However, type I collagen persists in the matrix. Because there is some debate about whether type I collagen influences cartilage calcification, an immunoblocking technique was used to determine the importance of type I collagen on the mineralization process in this system. Studies using nonspecific goat anti-chick IgG demonstrated that 1-100 ng/ml antibody added with the media after the cartilage nodules had developed (day 7) had no effect on the accumulation of mineral in the cultures. Nonspecific antibody added before day 7 blocked development of the cultures. Parallel solution based cell-free studies showed that IgG did not have a strong affinity for apatite crystals, and had no significant effect on apatite crystal growth. Type I collagen antibodies (1-200 ng/ml) added to cultures one time on day 9 (before mineralization started), or on day 11 (at the start of mineralization), slightly inhibited the accumulation of mineral. There was a statistically significant decrease in mineral accretion with 100 or 200 ng/ml collagen antibody addition continuously after these times. Fab' fragments of nonspecific and type I collagen antibodies had effects parallel to those of the intact antibodies, indicating that the decreased mineralization was not attributable to the presence of the larger, bulkier antibodies. The altered accumulation of mineral was not associated with cell death in the presence of antibody (demonstrated by fluorescent labeling of DNA) or with increased apoptosis (TUNEL-stain). In the immunoblocked cultures, EM analysis demonstrated that mineral continued to deposit on collagen fibrils, but there appeared to be fewer deposits. The data demonstrate that type I collagen is important for the mineralization of these cultures.

Cell differentiation and matrix gene expression in mesenchymal chondrosarcomas

Mesenchymal chondrosarcomas are small-cell malignancies named as chondrosarcomas due to the focal appearance of cartilage islands. In this study, the use of in situ detection techniques on a large series of mesenchymal chondrosarcoma specimens allowed the identification of tumor-cell differentiation pathways in these neoplasms. We were able to trace all steps of chondrogenesis within mesenchymal chondrosarcoma by using characteristic marker genes of chondrocytic development. Starting from undifferentiated cells, which were negative for vimentin and any other mesenchymal marker, a substantial portion of the cellular (undifferentiated) tumor areas showed a chondroprogenitor phenotype with an onset of expression of vimentin and collagen type IIA. Cells in the chondroid areas showed the full expression panel of mature chondrocytes including type X collagen indicating focal hypertrophic differentiation of the neoplastic chondrocytes. Finally, evidence was found for transdifferentiation of the neoplastic chondrocytes to osteoblast-like cells in areas of neoplastic bone formation. These results establish mesenchymal chondrosarcoma as the very neoplasm of differentiating premesenchymal chondroprogenitor cells. The potential of neoplastic bone formation in mesenchymal chondrosarcoma introduces a new concept of neoplastic (chondrocytic) osteogenesis in musculoskeletal malignant neoplasms, which qualifies the old dogma that neoplastic bone/osteoid formation automatically implies the diagnosis of osteosarcoma.

Altered postnatal expression of insulin-like growth factor-I (IGF-I) and type X collagen preceding the Perthes' disease-like lesion of a rat model

The spontaneously hypertensive rat (SHR) is a widely used animal model for the study of hypertension. It also exhibits an osteonecrosis of the femoral epiphysis that resembles the clinical features of Perthes' disease in humans. In this rat model, occlusion of the epiphyseal vessels occurs as a result of a breakdown of the mechanically vulnerable epiphysis. The postnatal development of the epiphysis recapitulates the serial events of the endochondral ossification (i.e., cartilage formation), chondrocyte hypertrophy, cartilage mineralization, vascularization, and introduction of osteoblasts that form the secondary ossification center within the epiphysis. In the present study, a detailed radiographic and histological analysis demonstrates that the osteonecrosis is preceded by a disturbance of the cartilage mineralization and a disturbance of the ossification, despite a normal hypertrophy of the epiphyseal cartilage. These observations suggest that abnormal development of the femoral epiphysis occurs much earlier than manifestation of the osteonecrosis. They lead us to a hypothesis that yet-unclarified transitional events between the cartilage hypertrophy and the cartilage mineralization may be affected in SHRs. Type X collagen is a developmentally regulated matrix molecule that is implicated in the mineralization of the hypertrophied chondrocytes. We show that the expression of type X collagen during epiphyseal ossification is delayed in SHRs (vs. normal controls), suggesting disturbed growth and/or differentiation of the epiphyseal chondrocytes. Postnatal growth and differentiation of the chondrocytes at least partly depend on insulin-like growth factor-I (IGF-I), which is produced by the chondrocytes in response to the pituitary growth hormone and stimulates cartilage growth in situ. The present study demonstrates an altered IGF-I expression during early postnatal life in SHRs and suggests that the altered IGF-I expression as well as the following delay in upregulation of type X collagen may cause the mechanical vulnerability of the femoral epiphysis in SHRs.

Lineage-dependent collagen expression and assembly during osteogenic or chondrogenic differentiation of a mesoblastic cell line

The mesoblastic clone, C1, behaves as a tripotential progenitor able to self-renew and to differentiate toward osteogenesis, chondrogenesis, or adipogenesis in response to specific inducers. In this study, expression and deposition by the C1 cells of essential components of the extracellular matrix, collagens type I, II, III, V, XI, VI, IX, and X were followed along the osteogenic and chondrogenic pathways, through biochemical, immunochemical, and electron microscopy analyses. Implementation of each program involves profiles of collagen synthesis and matrix assembly close to those documented in vivo. Depending on the applied inducers, cells adopt a defined identity and, controls acting at transcriptional and posttranslational levels adapt the set of deposited collagens to one particular cell fate. Osteogenic C1 cells selectively build a type I collagen matrix also containing type III, V, and XI collagens but selectively exclude type II collagen. Chondrogenic C1 cells first elaborate a type II collagen network and then acquire hypertrophic chondrocyte properties while assembling a type X collagen matrix as in the growth plate. This study provides an example of how a mesoblastic cell line can develop, in vitro, each of its genetic programs up to terminal differentiation. Intrinsic factors and time-dependent cell-matrix interactions might, as in vivo, underline the implementation of an entire differentiation program.

Expression of the components of the insulin-like growth factor axis across the growth-plate

Linear bone growth occurs as the result of proliferation and differentiation of growth-plate chondrocytes. These two phases of chondrocyte growth are regulated separately, with insulin-like growth factor I (IGF-I) being the primary stimulator of proliferation. We studied the expression of the components of the growth hormone GH/IGF system to learn if this proliferative signal is altered as chondrocytes undergo differentiation. Growth-plate chondrocytes were isolated from fetal cows and fractionated on discontinuous Percoll gradients. Five populations were recovered, ranging from high density cells (proliferative chondrocytes) to low density cells (hypertrophic chondrocytes). Messenger RNAs (mRNAs) were analyzed by a reverse transcriptase/quantitative polymerase chain reaction (RT/qPCR) technique. Results showed that mRNA of IGF-I and IGF-II in proliferative chondrocytes was 32 and five fold more abundant, respectively, than in hypertrophic chondrocytes. Of the four major IGF-I mRNA transcripts, the class 1-Ea transcript was predominant. Messenger RNA levels for IGFBP-3, -4, and -5 were also reduced in hypertrophic chondrocytes. Levels of GH receptor, the type 1 IGF receptor, and IGF binding protein-2 (IGFBP-2) mRNAs were unchanged across the growth-plate. Since IGF-I and -II are potent stimulators of proliferation, the down-regulation of these genes may be necessary in order for hypertrophy to proceed.

Serum induction of the collagen X promoter requires the Raf/MEK/ERK and p38 pathways

The collagen X gene is expressed exclusively by differentiated, hypertrophic chondrocytes. The mechanisms controlling collagen X expression remain largely unknown. Here we show that collagen X promoter activity can be induced by serum stimulation of chondrogenic MCT cells. The serum response is conferred by a 462 nucleotide promoter fragment. Both the c-Raf/MEK/ERK and p38 MAP kinase pathways are required for this effect, whereas phosphatidylinositol-3-kinase and protein kinase A repress promoter activation. These data are the first to demonstrate serum inducibility of the collagen X promoter and to identify signal transduction pathways involved.

The noncollagenous domain 1 of type X collagen. A novel motif for trimer and higher order multimer formation without a triple helix

In this study, we test the hypothesis that the carboxyl noncollagenous (NC1) domain of collagen X is sufficient to direct multimer formation without a triple helix. Two peptides containing the NC1 domain of avian collagen X have been synthesized using a bacterial expression system and their properties characterized. One peptide consists only of the NC1 domain, and the other is a chimeric molecule with a noncollagenous A domain of matrilin-1 fused to the N terminus of NC1. The NC1 peptide alone forms a 45-kDa trimer under native conditions, suggesting that NC1 contains all the information for trimerization without any triple helical residues. This trimeric association is highly thermostable without intermolecular disulfide bonds. This indicates that the NC1 domain contributes to the remarkable structural stability of collagen X. Chemical cross-linking of the NC1 trimer results in a series of varying sized multimers, the smallest of which is a trimer. Therefore the NC1 trimer is sufficient to form higher order multimers. The chimeric A-NC1 peptide forms a homotrimer by itself, and a series of heterotrimers with the NC1 peptide via the NC1 domain. Thus the NC1(X) domain directs multimer formation, even in a noncollagenous molecule.

Raf signaling stimulates and represses the human collagen X promoter through distinguishable elements

Endochondral bone growth is regulated through the rates of proliferation and differentiation of growth plate chondrocytes. While little is known about the intracellular events controlling these processes, the protein kinase c-Raf, a central component of the cellular signal transduction machinery, has recently been shown to be expressed only by differentiated, hypertrophic chondrocytes. The involvement of c-Raf in the transcriptional regulation of the hypertrophic chondrocyte-specific collagen X gene was investigated using cotransfections of collagen X reporter plasmids and expression vectors for mutant c-Raf proteins. Both activated and dominant-negative forms of c-Raf reduced the activity of the collagen X promoter to approximately 30%. The element mediating the repressing effect of activated c-Raf was located between nucleotides -2864 and -2410 of the promoter, whereas the effect of the dominant-negative form of c-Raf was conferred by the 462 nucleotides immediately upstream of the transcription start site. Inhibition of MEK1/2 and ERK1/2, downstream components of Raf-signaling, also caused repression of basal collagen X promoter activity. These data suggest that c-Raf regulates collagen X promoter activity positively and negatively through different cis-acting elements and represent the first evidence of c-Raf activity described in hypertrophic chondrocytes.