Morphological and biochemical evidence for apoptosis in the terminal hypertrophic chondrocytes of the growth plate

Klf10 is involved in extracellular matrix calcification of chondrocytes alleviating chondrocyte senescence

Osteoarthritis (OA) is a chronic degenerative disease resulting joint disability and pain. Accumulating evidences suggest that chondrocyte extracellular matrix calcification plays an important role in the development of OA. Here, we showed that Krüppel-like factor 10 (Klf10) was involved in the regulation of chondrocyte extracellular matrix calcification by regulating the expression of Frizzled9. Knockdown of Klf10 attenuated TBHP induced calcification and reduced calcium content in chondrocytes. Restoring extracellular matrix calcification of chondrocytes could aggravate chondrocyte senescence. Destabilization of a medial meniscus (DMM) mouse model of OA, in vivo experiments revealed that knockdown Klf10 improved the calcification of articular cartilage and ameliorated articular cartilage degeneration. These findings suggested that knockdown Klf10 inhibited extracellular matrix calcification-related changes in chondrocytes and alleviated chondrocyte senescence.

Vascular injury of immature epiphyses impair stem cell engraftment in cartilage defects

The purpose of our study was to investigate if vascular injury in immature epiphyses affects cartilage repair outcomes of matrix-associated stem cell implants (MASI). Porcine bone marrow mesenchymal stromal stem cells (BMSCs) suspended in a fibrin glue scaffold were implanted into 24 full-thickness cartilage defects (5 mm ø) of the bilateral distal femur of six Göttingen minipigs (n = 12 defects in 6 knee joints of 3 immature pigs; age 3.5-4 months; n = 12 defects in 6 knee joints of 3 mature control pigs; age, 21-28 months). All pigs underwent magnetic resonance imaging (MRI) at 2, 4, 12 (n = 24 defects), and 24 weeks (n = 12 defects). After the last imaging study, pigs were sacrificed, joints explanted and evaluated with VEGF, H&E, van Gieson, Mallory, and Safranin O stains. Results of mature and immature cartilage groups were compared using the Wilcoxon signed-rank test. Quantitative scores for subchondral edema at 2 weeks were correlated with quantitative scores for cartilage repair (MOCART score and ICRS score) at 12 weeks as well as Pineda scores at end of the study, using linear regression analysis. On serial MRIs, mature joints demonstrated progressive healing of cartilage defects while immature joints demonstrated incomplete healing and damage of the subchondral bone. The MOCART score at 12 weeks was significantly higher for mature joints (79.583 ± 7.216) compared to immature joints (30.416 ± 10.543, p = 0.002). Immature cartilage demonstrated abundant microvessels while mature cartilage did not contain microvessels. Accordingly, cartilage defects in immature joints showed a significantly higher number of disrupted microvessels, subchondral edema, and angiogenesis compared to mature cartilage. Quantitative scores for subchondral edema at 2 weeks were negatively correlated with MOCART scores (r =  - 0.861) and ICRS scores (r =  - 0.901) at 12 weeks and positively correlated with Pineda scores at the end of the study (r = 0.782). Injury of epiphyseal blood vessels in immature joints leads to subchondral bone defects and limits cartilage repair after MASI.

Chlorogenic acid inhibits apoptosis in thiram-induced tibial dyschondroplasia via intrinsic pathway

Tibial dyschondroplasia (TD) is a common skeletal disease occurred in growth plate of fast-growing broilers. Thiram is a sort of chemical used for pesticide and fungicide. The excessive use of thiram increased the threat to animal and human health. In this study, we aimed to investigate the therapeutic mechanism of chlorogenic acid (CGA) on thiram-induced tibial dyschondroplasia. Broiler chickens were divided into three different groups, e.g., control, TD, and CGA. CGA was administrated after the induction of TD from 4^th day to 7^th day. Biochemical analysis was performed to detect the content of calcium (Ca) and phosphorus (P). Histological changes and degradation of extracellular matrix were observed through hematoxylin-eosin (H & E) and Masson staining. To further determine the mechanism, TUNEL staining and western blot were also performed to detect the apoptosis changes in growth plate of all groups. The results showed the disproportionation of Ca and P content and upregulation of apoptosis during the development of TD. But, after the administration of CGA, the ratio of Ca:P was upregulated, and the apoptosis was also downregulated. The current study shows the toxic effect of thiram on chickens and suggests that CGA is associated with a mechanism that plays a significant role in apoptosis induced by thiram in poultry industry.

Macro, Micro, and Molecular. Changes of the Osteochondral Interface in Osteoarthritis Development

Osteoarthritis (OA) is a long-term condition that causes joint pain and reduced movement. Notably, the same pathways governing cell growth, death, and differentiation during the growth and development of the body are also common drivers of OA. The osteochondral interface is a vital structure located between hyaline cartilage and subchondral bone. It plays a critical role in maintaining the physical and biological function, conveying joint mechanical stress, maintaining chondral microenvironment, as well as crosstalk and substance exchange through the osteochondral unit. In this review, we summarized the progress in research concerning the area of osteochondral junction, including its pathophysiological changes, molecular interactions, and signaling pathways that are related to the ultrastructure change. Multiple potential treatment options were also discussed in this review. A thorough understanding of these biological changes and molecular mechanisms in the pathologic process will advance our understanding of OA progression, and inform the development of effective therapeutics targeting OA.

Pathological calcification in osteoarthritis: an outcome or a disease initiator?

In the progression of osteoarthritis, pathological calcification in the affected joint is an important feature. The role of these crystallites in the pathogenesis and progression of osteoarthritis is controversial; it remains unclear whether they act as a disease initiator or are present as a result of joint damage. Recent studies reported that the molecular mechanisms regulating physiological calcification of skeletal tissues are similar to those regulating pathological or ectopic calcification of soft tissues. Pathological calcification takes place when the equilibrium is disrupted. Calcium phosphate crystallites are identified in most affected joints and the presence of these crystallites is closely correlated with the extent of joint destruction. These observations suggest that pathological calcification is most likely to be a disease initiator instead of an outcome of osteoarthritis progression. Inhibiting pathological crystallite deposition within joint tissues therefore represents a potential therapeutic target in the management of osteoarthritis.

Ontogeny of the anuran urostyle and the developmental context of evolutionary novelty

Fusion of caudal vertebrae has evolved multiple times independently: the pygostyle of birds, coccyx in apes and humans, ural plate of fish, and the urostyle of frogs. The anuran urostyle, however, is structurally and developmentally distinct because of the contribution of an ossifying hypochord. To date, the developmental mechanisms behind an ossifying hypochord have remained obscure. Here, we provide a detailed analysis of the development of this evolutionary innovative structure and of how neuromusculature, cell death, and proliferation paved their way to facilitate its formation. Finally, we propose that the ossifying hypochord plays a role in tail loss in anurans and reorganizing the dorsal aorta and thus is pivotal in the evolution of the anuran bauplan.

Developmental novelties often underlie the evolutionary origins of key metazoan features. The anuran urostyle, which evolved nearly 200 MYA, is one such structure. It forms as the tail regresses during metamorphosis, when locomotion changes from an axial-driven mode in larvae to a limb-driven one in adult frogs. The urostyle comprises of a coccyx and a hypochord. The coccyx forms by fusion of caudal vertebrae and has evolved repeatedly across vertebrates. However, the contribution of an ossifying hypochord to the coccyx in anurans is unique among vertebrates and remains a developmental enigma. Here, we focus on the developmental changes that lead to the anuran urostyle, with an emphasis on understanding the ossifying hypochord. We find that the coccyx and hypochord have two different developmental histories: First, the development of the coccyx initiates before metamorphic climax whereas the ossifying hypochord undergoes rapid ossification and hypertrophy; second, thyroid hormone directly affects hypochord formation and appears to have a secondary effect on the coccygeal portion of the urostyle. The embryonic hypochord is known to play a significant role in the positioning of the dorsal aorta (DA), but the reason for hypochordal ossification remains obscure. Our results suggest that the ossifying hypochord plays a role in remodeling the DA in the newly forming adult body by partially occluding the DA in the tail. We propose that the ossifying hypochord-induced loss of the tail during metamorphosis has enabled the evolution of the unique anuran bauplan.

The Chondro-Osseous Continuum: Is It Possible to Unlock the Potential Assigned Within?

Endochondral ossification (EO), by which long bones of the axial skeleton form, is a tightly regulated process involving chondrocyte maturation with successive stages of proliferation, maturation, and hypertrophy, accompanied by cartilage matrix synthesis, calcification, and angiogenesis, followed by osteoblast-mediated ossification. This developmental sequence reappears during fracture repair and in osteoarthritic etiopathology. These similarities suggest that EO, and the cells involved, are of great clinical importance for bone regeneration as it could provide novel targeted approaches to increase specific signaling to promote fracture healing, and if regulated appropriately in the treatment of osteoarthritis. The long-held accepted dogma states that hypertrophic chondrocytes are terminally differentiated and will eventually undergo apoptosis. In this mini review, we will explore recent evidence from experiments that revisit the idea that hypertrophic chondrocytes have pluripotent capacity and may instead transdifferentiate into a specific sub-population of osteoblast cells. There are multiple lines of evidence, including our own, showing that local, selective alterations in cartilage extracellular matrix (ECM) remodeling also indelibly alter bone quality. This would be consistent with the hypothesis that osteoblast behavior in long bones is regulated by a combination of their lineage origins and the epigenetic effects of chondrocyte-derived ECM which they encounter during their recruitment. Further exploration of these processes could help to unlock potential novel targets for bone repair and regeneration and in the treatment of osteoarthritis.

Osteogenic Potential of Caspases Related to Endochondral Ossification

Caspases have functions particularly in apoptosis and inflammation. Increasing evidence indicates novel roles of these proteases in cell differentiation, including those involved in osteogenesis. This investigation provides a complex screening of osteogenic markers affected by pan caspase inhibition in micromass cultures derived from mouse forelimbs. PCR Array analysis showed significant alterations in expression of 49 osteogenic genes after 7 days of inhibition. The largest change was a decrease in CD36 expression, which was confirmed at organ level by caspase inhibition in cultured mouse ulnae followed by CD36 immunohistochemical analysis. So far, available data point to osteogenic potential of pro-apoptotic caspases. Therefore, the expression of pro-apoptotic caspases (-3, -6, -7, -8, -9) within the growth plate of mouse forelimbs at the stage where the individual zones are clearly apparent was studied. Caspase-9 was reported in the growth plate for the first time as well as caspase-6 and -7 in the resting zone, caspase-7 in the proliferation, and caspase-6 and -8 in the ossification zone. For all caspases, there was a gradient increase in activation toward the ossification zone. The distribution of staining varied significantly from that of apoptotic cells, and thus, the results further support non-apoptotic participation of caspases in osteogenesis.

Insights on Molecular Mechanisms of Chondrocytes Death in Osteoarthritis

Osteoarthritis (OA) is a joint pathology characterized by progressive cartilage degradation. Medical care is mainly based on alleviating pain symptoms. Compelling studies report the presence of empty lacunae and hypocellularity in cartilage with aging and OA progression, suggesting that chondrocyte cell death occurs and participates to OA development. However, the relative contribution of apoptosis per se in OA pathogenesis appears complex to evaluate. Indeed, depending on technical approaches, OA stages, cartilage layers, animal models, as well as in vivo or in vitro experiments, the percentage of apoptosis and cell death types can vary. Apoptosis, chondroptosis, necrosis, and autophagic cell death are described in this review. The question of cell death causality in OA progression is also addressed, as well as the molecular pathways leading to cell death in response to the following inducers: Fas, Interleukin-1β (IL-1β), Tumor Necrosis factor-α (TNF-α), leptin, nitric oxide (NO) donors, and mechanical stresses. Furthermore, the protective role of autophagy in chondrocytes is highlighted, as well as its decline during OA progression, enhancing chondrocyte cell death; the transition being mainly controlled by HIF-1α/HIF-2α imbalance. Finally, we have considered whether interfering in chondrocyte apoptosis or promoting autophagy could constitute therapeutic strategies to impede OA progression.

Endogenous DKK1 and FRZB Regulate Chondrogenesis and Hypertrophy in Three-Dimensional Cultures of Human Chondrocytes and Human Mesenchymal Stem Cells

Hypertrophic differentiation occurs during in vitro chondrogenesis of mesenchymal stem cells (MSCs), decreasing the quality of the cartilage construct. Previously we identified WNT pathway antagonists Dickkopf 1 homolog (DKK1) and frizzled-related protein (FRZB) as key factors in blocking hypertrophic differentiation of human MSCs (hMSCs). In this study, we investigated the role of endogenously expressed DKK1 and FRZB in chondrogenesis of hMSC and chondrocyte redifferentiation and in preventing cell hypertrophy using three relevant human cell based systems, isolated hMSCs, isolated primary human chondrocytes (hChs), and cocultures of hMSCs with hChs for which we specifically designed neutralizing nano-antibodies. We selected and tested variable domain of single chain heavy chain only antibodies (VHH) for their ability to neutralize the function of DKK1 or FRZB. In the presence of DKK1 and FRZB neutralizing VHH, glycosaminoglycan and collagen type II staining were significantly reduced in monocultured hMSCs and monocultured chondrocytes. Furthermore, in cocultures, cells in pellets showed hypertrophic differentiation. In conclusion, endogenous expression of the WNT antagonists DKK1 and FRZB is necessary for multiple steps during chondrogenesis: first DKK1 and FRZB are indispensable for the initial steps of chondrogenic differentiation of hMSCs, second they are necessary for chondrocyte redifferentiation, and finally in preventing hypertrophic differentiation of articular chondrocytes.

Signaling pathways regulating cartilage growth plate formation and activity

The growth plate is a highly specialized and dynamic cartilage structure that serves many essential functions in skeleton patterning, growth and endochondral ossification in developing vertebrates. Major signaling pathways initiated by classical morphogens and by other systemic and tissue-specific factors are intimately involved in key aspects of growth plate development. As a corollary of these essential functions, disturbances in these pathways due to mutations or environmental factors lead to severe skeleton disorders. Here, we review these pathways and the most recent progress made in understanding their roles in chondrocyte differentiation in growth plate development and activity. Furthermore, we discuss newly uncovered pathways involved in growth plate formation, including mTOR, the circadian clock, and the COP9 signalosome.

2-Butoxyethanol Female-Rat Model of Hemolysis and Disseminated Thrombosis: X-Ray Characterization of Osteonecrosis and Growth-Plate Suppression

We recently proposed a chemically induced rat model for human hemolytic disorders associated with thrombosis. The objective of the present investigation was to apply a noninvasive, high-magnification X-ray analysis, the Faxitron radiography system, to characterize the protracted bone damage associated with this 2-butoxyethanol model and to validate it by histopathology. Groups of female Fischer 344 rats were given 0, 250, or 300 mg of 2-butoxyethanol/kg body weight daily for 4 consecutive days. Groups were then sacrificed 2 hours or 26 days after the final treatment. The treated animals displayed a darkened purple-red discoloration on the distal tail. Histopathological evaluation, including phosphotungstic acid-hematoxylin staining of animals sacrificed 2 hours after the final treatment, revealed disseminated thrombosis and infarction in multiple organs, including bones. The Faxitron MX-20 specimen radiography system was used to image selected bones of rats sacrificed 26 days posttreatment. Premature thinning of the growth plate occurred in the calcaneus, lumbar and coccygeal vertebrae, femur, and ilium of the treated animals. Areas of decreased radiographic densities were seen in the diaphysis of the femur of all treated animals. The bones were then examined histologically and showed a range of changes, including loss or damage to growth plates and necrosis of cortical bone. No thrombi were seen in the animals sacrificed at 30 days, but bone and growth plate changes consistent with prior ischemia were noted. The Faxitron proved to be an excellent noninvasive tool that can be used in future studies with this animal model to examine treatment modalities for the chronic effects of human thrombotic disorders.

Dual pathways to endochondral osteoblasts: a novel chondrocyte-derived osteoprogenitor cell identified in hypertrophic cartilage

According to the general understanding, the chondrocyte lineage terminates with the elimination of late hypertrophic cells by apoptosis in the growth plate. However, recent cell tracking studies have shown that murine hypertrophic chondrocytes can survive beyond “terminal” differentiation and give rise to a progeny of osteoblasts participating in endochondral bone formation. The question how chondrocytes convert into osteoblasts, however, remained open. Following the cell fate of hypertrophic chondrocytes by genetic lineage tracing using BACCol10;Cre induced YFP-reporter gene expression we show that a progeny of Col10Cre-reporter labelled osteoprogenitor cells and osteoblasts appears in the primary spongiosa and participates – depending on the developmental stage – substantially in trabecular, endosteal, and cortical bone formation. YFP⁺ trabecular and endosteal cells isolated by FACS expressed Col1a1, osteocalcin and runx2, thus confirming their osteogenic phenotype. In searching for transitory cells between hypertrophic chondrocytes and trabecular osteoblasts we identified by confocal microscopy a novel, small YFP⁺Osx⁺ cell type with mitotic activity in the lower hypertrophic zone at the chondro-osseous junction. When isolated from growth plates by fractional enzymatic digestion, these cells termed CDOP (chondrocyte-derived osteoprogenitor) cells expressed bone typical genes and differentiated into osteoblasts in vitro. We propose the Col10Cre-labeled CDOP cells mark the initiation point of a second pathway giving rise to endochondral osteoblasts, alternative to perichondrium derived osteoprogenitor cells. These findings add to current concepts of chondrocyte-osteocyte lineages and give new insight into the complex cartilage-bone transition process in the growth plate.

Fate of growth plate hypertrophic chondrocytes: death or lineage extension?

The vertebrate growth plate is an essential tissue that mediates and controls bone growth. It forms through a multistep differentiation process in which chondrocytes differentiate, proliferate, stop dividing and undergo hypertrophy, which entails a 20-fold increase in size. Hypertrophic chondrocytes are specialized cells considered to be the end state of the chondrocyte differentiation pathway, and are essential for bone growth. They are characterized by expression of type X collagen encoded by the Col10a1 gene, and synthesis of a calcified cartilage matrix. Whether hypertrophy marks a transition preceding osteogenesis, or it is the terminal differentiation stage of chondrocytes with cell death as the ultimate fate has been the subject of debate for over a century. In this review, we revisit this debate in the light of new findings arising from genetic-mediated lineage tracing studies showing that hypertrophic chondrocytes can survive at the chondro-osseous junction and further make the transition to become osteoblasts and osteocytes. The contribution of chondrocytes to the osteoblast lineage has important implications in bone development, disease and repair.

Growth cartilage expression of growth hormone/insulin-like growth factor I axis in spontaneous and growth hormone induced catch-up growth

INTRODUCTION

Catch-up growth following the cessation of a growth inhibiting cause occurs in humans and animals. Although its underlying regulatory mechanisms are not well understood, current hypothesis confer an increasing importance to local factors intrinsic to the long bones' growth plate (GP).

AIM

The present study was designed to analyze the growth-hormone (GH)-insulin-like growth factor I (IGF-I) axis in the epiphyseal cartilage of young rats exhibiting catch-up growth as well as to evaluate the effect of GH treatment on this process.

MATERIAL AND METHODS

Female Sprague-Dawley rats were randomly grouped: controls (group C), 50% diet restriction for 3 days+refeeding (group CR); 50% diet restriction for 3 days+refeeding & GH treatment (group CRGH). Analysis of GH receptor (GHR), IGF-I, IGF-I receptor (IGF-IR) and IGF binding protein 5 (IGFBP5) expressions by real-time PCR was performed in tibial growth plates extracted at the time of catch-up growth, identified by osseous front advance greater than that of C animals.

RESULTS

In the absence of GH treatment, catch-up growth was associated with increased IGF-I and IGFBP5 mRNA levels, without changes in GHR or IGF-IR. GH treatment maintained the overexpression of IGF-I mRNA and induced an important increase in IGF-IR expression.

CONCLUSIONS

Catch-up growth that happens after diet restriction might be related with a dual stimulating local effect of IGF-I in growth plate resulting from overexpression and increased bioavailability of IGF-I. GH treatment further enhanced expression of IGF-IR which likely resulted in a potentiation of local IGF-I actions. These findings point out to an important role of growth cartilage GH/IGF-I axis regulation in a rat model of catch-up growth.

Altered cellular kinetics in the growth plate of the femoral head of spontaneously hypertensive rats

PURPOSE

Pathologic changes in the growth plate remain unknown in Legg-Calvé-Perthes (LCP) disease. Spontaneously hypertensive rats have proven to be a good model for studying LCP disease. This study investigated the histopathologic changes and the expression of vascular endothelial growth factor in the growth plate of spontaneously hypertensive rats (SHR).

MATERIALS AND METHODS

Sixty SHR rats were divided into two groups: those showing osteonecrosis (SHR+n group: 32), and those showing normal ossification (SHR-n group: 28). Thirty Wister Kyoto rats served as a control. For histomorphological measurement, the length of each zone of the growth plate was measured. Cell kinetics was measured by 5-bromo-2'-deoxyuridin (BrdU) immunohistochemistry and transferase-mediated deoxyuridine triphosphate-biotin nick end labeling (TUNEL) assays. Vascular endothelial growth factor (VEGF) immunohistochemistry was used to identify of expression of VEGF.

RESULTS

The lengths of growth plates of the SHR+n group were significantly shorter in the initial growth period than those of the other groups. The lowest proliferative rate and the highest apoptosis rate were observed in the SHR+n group at the initial growth period. The expression of VEGF in the growth plate of the SHR group was lower than the control group, and it was lower in the SHR+n group than in the SHR-n group.

CONCLUSION

The growth plate of the SHR+n group was found to be affected by disease process of ischemic necrosis of the femoral head, and this might explain the relative overgrowth of the greater trochanter in the later stages of LCP disease.

Chondrocyte hypertrophy and osteoarthritis: role in initiation and progression of cartilage degeneration?

OBJECTIVE

To review the literature on the role and regulation of chondrocyte terminal differentiation (hypertrophy-like changes) in osteoarthritis (OA) and to integrate this in a conceptual model of primary OA development.

METHODS

Papers investigating chondrocyte terminal differentiation in human OA cartilage and experimental models of OA were recapitulated and discussed. Focus has been on the occurrence of hypertrophy-like changes in chondrocytes and the factors described to play a role in regulation of chondrocyte hypertrophy-like changes in OA.

RESULTS

Chondrocyte hypertrophy-like changes are reported in both human OA and experimental OA models by most investigators. These changes play a crucial part in the OA disease process by protease-mediated cartilage degradation. We propose that altered chondrocyte behavior and concomitant cartilage degradation result in a disease-amplifying loop, leading to a mixture of disease stages and cellular responses within an OA joint.

CONCLUSION

Chondrocyte hypertrophy-like changes play a role in early and late stage OA. Since not all cells in an OA joint are synchronized, inhibition of hypertrophy-like changes might be a therapeutic target to slow down further OA progression.

The role of apoptosis in mineralizing murine versus avian micromass culture systems

Chondrocyte apoptosis is thought to be an important step in the calcification of cartilage in vivo; however, there are conflicting reports as to whether or not this apoptosis is a necessary precursor to mineralization. The goal of this study was to determine whether or not apoptosis is necessary for mineralization in an in vitro murine micromass model of endochondral ossification. C3H10T1/2 murine mesenchymal stem cells were plated in micromass culture in the presence of 4 mM inorganic phosphate with the addition of the apoptogens, camptothecin, or staurosporine, to induce apoptosis. The rate and total accumulation of mineralization was measured with (45)Ca uptake. In these studies, both apoptogens increased the rate of mineralization, with staurosporine increasing (45)Ca accumulation by about 2.5 times that of controls and camptothecin increasing total amounts of mineralization about 1.5 times that of controls. Inhibiting cell apoptosis with the caspase inhibitor, ZVAD-fmk, to prevent apoptosis, caused slower rates of (45)Ca uptake; however, total amounts of (45)Ca accumulation reached the same values by day 30 of culture. FTIR data showed mineralization in all samples treated with 4 mM inorganic phosphate, with the highest mineral to matrix ratios in the camptothecin treated samples.

Localization of the cis-enhancer element for mouse type X collagen expression in hypertrophic chondrocytes in vivo

The type X collagen gene (Col10a1) is a specific molecular marker of hypertrophic chondrocytes during endochondral bone formation. Mutations in human COL10A1 and altered chondrocyte hypertrophy have been associated with multiple skeletal disorders. However, until recently, the cis-enhancer element that specifies Col10a1 expression in hypertrophic chondrocytes in vivo has remained unidentified. Previously, we and others have shown that the Col10a1 distal promoter (-4.4 to -3.8 kb) may harbor a critical enhancer that mediates its tissue specificity in transgenic mice studies. Here, we report further localization of the cis-enhancer element within this Col10a1 distal promoter by using a similar transgenic mouse approach. We identify a 150-bp Col10a1 promoter element (-4296 to -4147 bp) that is sufficient to direct its tissue-specific expression in vivo. In silico analysis identified several putative transcription factor binding sites including two potential activator protein-1 (AP-1) sites within its 5'- and 3'-ends (-4276 to -4243 and -4166 to -4152 bp), respectively. Interestingly, transgenic mice using a reporter construct deleted for these two AP-1 elements still showed tissue-specific reporter activity. EMSAs using oligonucleotide probes derived from this region and MCT cell nuclear extracts identified DNA/protein complexes that were enriched from cells stimulated to hypertrophy. Moreover, these elements mediated increased reporter activity on transfection into MCT cells. These data define a 90-bp cis-enhancer required for tissue-specific Col10a1 expression in vivo and putative DNA/protein complexes that contribute to the regulation of chondrocyte hypertrophy. This work will enable us to identify candidate transcription factors essential both for skeletal development and for the pathogenesis of skeletal disorders.

Dysplastic histogenesis of cartilage growth plate by alteration of sulphation pathway: a transgenic model

Mutations in the diastrophic dysplasia sulphate transporter (dtdst) gene causes different forms of chondrodysplasia in the human. The generation of a knock-in mouse strain with a mutation in dtdst gene provides the basis to study developmental dynamics in the epiphyseal growth plate and long bone growth after impairment of the sulphate pathway. Our microscopical and histochemical data demonstrate that dtdst gene impairment deeply affects tissue organization, matrix structure, and cell differentiation in the epiphyseal growth plate. In mutant animals, the height of the growth plate was significantly reduced, according to a concomitant decrease in cell density and proliferation. Although the pathway of chondrocyte differentiation seemed complete, alteration in cell morphology compared to normal counterparts was detected. In the extracellular matrix, it we observed a dramatic decrease in sulphated proteoglycans, alterations in the organization of type II and type X collagen fibers, and premature onset of mineralization. These data confirm the crucial role of sulphate pathway in proteoglycan biochemistry and suggest that a disarrangement of the extracellular matrix may be responsible for the development of dtdts cartilage dysplasia. Moreover, we corroborated the concept that proteoglycans not only are structural components of the cartilage architecture, but also play a dynamic role in the regulation of chondrocyte growth and differentiation.

Chondrocyte apoptosis enhanced at the growth plate: a physeal response to a diaphyseal fracture

Post-traumatic overgrowth of growing long bones is a common clinical phenomenon in paediatric traumatology and is the result of an enhanced stimulation of the nearby growth plate after fracture. To date, the exact post-fractural reactions of the growth plate are poorly understood. The aim of this study has been to determine the impact of fracture on the frequency of chondrocyte apoptosis of the growth plate. Rats sustained a mid-diaphyseal closed fracture of the left tibia or were left untreated. All animals were killed 3, 10, 14 or 29 days after trauma. The left and right tibiae were harvested and apoptotic chondrocytes of the proximal tibial growth plate were detected by TUNEL staining. The apoptosis percentage of physeal chondrocytes was statistically compared among fractured bones, intact contra-lateral bones and control bones. The physeal apoptosis rate of the fractured bone was significantly higher than that of the contra-lateral intact bone (valid for all evaluated days) and the control bone (valid from day 10 onwards). Contra-lateral intact tibiae never showed significantly higher apoptosis rates compared with control tibiae. Thus, mid-diaphyseal fracture influences the nearby growth plate by stimulating chondrocyte programmed cell death, which is associated with cartilage resorption and bone replacement. The lack of a significant difference between the intact contra-lateral and the intact control bone suggests that fracture only has a local effect that contributes to the greater apoptosis rate of the adjacent physis.

Induction of ES cell-derived cartilage formation

This unit describes the protocols used for cultivation of murine embryonic stem (ES) cells and their differentiation into chondrogenic cell types in vitro. ES cells cultivated as cellular aggregates, so-called embryoid bodies (EBs), differentiate spontaneously into chondrogenic cell types recapitulating cellular events of chondro- and osteogenesis. The undifferentiated ES cells differentiate into mesenchymal prechondrogenic cells in the EB outgrowths. These progenitor cells aggregate and form mesenchymal condensations. During further cultivation, these cells form cartilage nodules, show a phenotype typical for chondroblasts, and start to express marker molecules of cartilage tissue. Later, the chondrocytes become hypertrophic, and finally, marker molecules indicating bone formation can be detected in the nodules. This unit also contains protocols for characterization of the differentiated cells by immunostaining, mRNA-in situ hybridization, electron microscopy, and RT-PCR analysis.

Endochondral bone formation is involved in media calcification in rats and in men

Arterial media calcification is often considered a cell-regulated process resembling intramembranous bone formation, implying a conversion of vascular tissue into a bone-like structure without a cartilage intermediate. In this study, we examined the association of chondrocyte-specific marker expression with media calcification in arterial samples derived from rats with chronic renal failure (CRF) and from human transplant donors. CRF was induced in rats with a diet supplemented with adenine. Vascular calcification was evaluated histomorphometrically on Von Kossa-stained sections and the expression of the chondrocyte markers sox9 and collagen II with the osteogenic marker core-binding factor alpha1 (cbfa1) was determined immunohistochemically. Media calcification was detected in more than half of the rats with CRF. In over half of the rats with severe media calcification, a typical cartilage matrix was found by morphology. All of the animals with severe calcification showed the presence of chondrocyte-like cells expressing the markers sox9, collagen II, and cbfa1. Human aorta specimens showing mild to moderate media calcification also showed sox9, collagen II, and cbfa1 expression. The presence of chondrocytes in association with calcification of the media in aortas of rats with CRF mimics endochondral bone formation. The relevance of this association is further demonstrated by the chondrogenic conversion of medial smooth muscle cells in the human aorta.

Expression profiling of Dexamethasone-treated primary chondrocytes identifies targets of glucocorticoid signalling in endochondral bone development

Background

Glucocorticoids (GCs) are widely used anti-inflammatory drugs. While useful in clinical practice, patients taking GCs often suffer from skeletal side effects including growth retardation in children and adolescents, and decreased bone quality in adults. On a physiological level, GCs have been implicated in the regulation of chondrogenesis and osteoblast differentiation, as well as maintaining homeostasis in cartilage and bone. We identified the glucocorticoid receptor (GR) as a potential regulator of chondrocyte hypertrophy in a microarray screen of primary limb bud mesenchyme micromass cultures. Some targets of GC regulation in chondrogenesis are known, but the global effects of pharmacological GC doses on chondrocyte gene expression have not been comprehensively evaluated.

Results

This study systematically identifies a spectrum of GC target genes in embryonic growth plate chondrocytes treated with a synthetic GR agonist, dexamethasone (DEX), at 6 and 24 hrs. Conventional analysis of this data set and gene set enrichment analysis (GSEA) was performed. Transcripts associated with metabolism were enriched in the DEX condition along with extracellular matrix genes. In contrast, a subset of growth factors and cytokines were negatively correlated with DEX treatment. Comparing DEX-induced gene expression data to developmental changes in gene expression in micromass cultures revealed an additional layer of complexity in which DEX maintains the expression of certain chondrocyte marker genes while inhibiting factors that promote vascularization and ultimately ossification of the cartilaginous template.

Conclusion

Together, these results provide insight into the mechanisms and major molecular classes functioning downstream of DEX in primary chondrocytes. In addition, comparison of our data with microarray studies of DEX treatment in other cell types demonstrated that the majority of DEX effects are tissue-specific. This study provides novel insights into the effects of pharmacological GC on chondrocyte gene transcription and establishes the foundation for subsequent functional studies.

Decreased chondrocyte proliferation and dysregulated apoptosis in the cartilage growth plate are key features of a murine model of epiphyseal dysplasia caused by a matn3 mutation

Disruption to endochondral ossification leads to delayed and irregular bone formation and can result in a heterogeneous group of genetic disorders known as the chondrodysplasias. One such disorder, multiple epiphyseal dysplasia (MED), is characterized by mild dwarfism and early-onset osteoarthritis and can result from mutations in the gene encoding matrilin-3 (MATN3). To determine the disease mechanisms that underpin the pathophysiology of MED we generated a murine model of epiphyseal dysplasia by knocking-in a matn3 mutation. Mice that are homozygous for the mutation develop a progressive dysplasia and have short-limbed dwarfism that is consistent in severity with the relevant human phenotype. Mutant matrilin-3 is retained within the rough endoplasmic reticulum of chondrocytes and is associated with an unfolded protein response. Eventually, there is reduced proliferation and spatially dysregulated apoptosis of chondrocytes in the cartilage growth plate, which is likely to be the cause of disrupted linear bone growth and the resulting short-limbed dwarfism in the mutant mice.

Development of the terminally differentiated state sensitizes epiphyseal chondrocytes to apoptosis through caspase-3 activation

The maturation of epiphyseal chondrocytes is accompanied by dramatic changes in energy metabolism and shifts in proteins concerned with the induction of apoptosis. We evaluated the role of mitochondria in this process by evaluating the membrane potential (Delta psi m) of chondrocytes of embryonic tibia and the epiphyseal growth plate. We observed that there was a maturation-dependent change in fluorescence, indicating a fall in the Delta psi m. The level of mitochondrial Bcl-2 was decreased during maturation, while in the same time period there was an obvious increase in Bax levels in the mitochondrial fraction of the terminally differentiated chondrocytes. Bcl(xL), another anti-apoptotic protein, was also robustly expressed in the mitochondrial fraction, but its expression was not dependent on the maturation status of the chondrocytes. We found that caspase-3 was present throughout the growth plate and in hypertrophic cells in culture. We blocked caspase-3 activity and found that alkaline phosphatase staining and mineral formation was decreased, and the cells had lost their characteristic shape. Moreover, we noted that the undifferentiated cells were insensitive to elevated concentrations of inorganic phosphate (Pi). It is concluded that during hypertrophy, the change in membrane potential, the increased binding of a pro-apoptotic protein to mitochondria, and the activation of caspase-3 serve to prime cells for apoptosis. Only when the terminally differentiated chondrocytes are challenged with low levels of apoptogens there is activation of apoptosis.

Fate of the hypertrophic chondrocyte: microenvironmental perspectives on apoptosis and survival in the epiphyseal growth plate

The goal of this review is to examine the fate of the hypertrophic chondrocyte in the epiphyseal growth plate and consider the impact of the cartilage microenvironment on cell survival and apoptosis. Early investigations pointed to a direct role of the hypertrophic chondrocyte in osteogenesis. The terminally differentiated cells were considered to undergo a dramatic change in shape, size, and phenotype, and assume the characteristics of an osteoblast. While some studies have supported the notion of transdifferentiation, much of the evidence in favor of reprogramming epiphyseal chondrocytes is circumstantial and based on microscopic evaluation of cells that are present at the chondro-osseous junction. Although these investigations provided a novel perspective on endochondral bone formation, they were flawed by the failure to consider the importance of stem cells in osseous tissue formation. Subsequent studies indicated that many, if not all, of the cells of the cartilage plate die through the induction of apoptosis. With respect to agents that mediate apoptosis, at the chondro-osseous junction, solubilization of mineral and hydrolysis of organic matrix constituents by septoclasts generates high local concentrations of ions, peptides, and glycans, and secreted matrix metalloproteins. Individually, and in combination, a number of these agents serve as potent chondrocyte apoptogens. We present a new concept: hypertrophic cells die through the induction of autophagy. In the cartilage microenvironment, combinations of local factors cause chondrocytes to express an initial survival phenotype and oxidize their own structural macromolecules to generate ATP. While delaying death, autophagy leads to a state in which cells are further sensitized to changes in the local microenvironment. One such change is similar to ischemia reperfusion injury, a condition that leads to tissue damage and cell death. In the growth cartilage, an immediate effect of this type of injury is sensitization to local apoptogens. These two concepts (type II programmed cell death and ischemia reperfusion injury) emphasize the importance of the local microenvironment, in particular pO(2), in directing chondrocyte survival and apoptosis.

Increased chondrocyte apoptosis in growth plates from children with slipped capital femoral epiphysis

Ultrastructural studies of slipped capital femoral epiphysis (SCFE) growth plates have shown diminished cellularity and marked distortion of the architecture in the proliferative and hypertrophic zones. Chondrocyte degeneration and death were noted at all levels of the hypertrophic and proliferative zones, suggesting an accelerated disturbance in the life-to-death cycle of the chondrocytes. The current study examines the mechanism responsible for the diminished cell number and whether increased programmed cell death (apoptosis) or necrosis was operative. Proximal femoral growth plates from patients with SCFE (three patients) were prepared and sectioned for histochemistry, in situ detection of apoptosis, and immunohistochemistry. The results showed that the diminished cell number is due to an abnormal frequency and distribution of chondrocytes undergoing apoptosis. Although it is unclear whether the increased apoptosis is occurring early or late in the disease, it is highly likely that it is directly linked to pathogenesis.

Intrauterine Fetal Constraint Induces Chondrocyte Apoptosis and Premature Ossification of the Cranial Base

BACKGROUND

The spheno-occipital synchondrosis is an important growth center of the craniofacial skeleton and a primary site of malformation in syndromic forms of craniosynostosis. Clinical and laboratory investigations have demonstrated that premature closure of cranial vault sutures in nonsyndromic craniosynostosis is associated with characteristic alterations in cranial base morphology. However, a causal link between premature fusion of calvarial sutures and changes in the cranial base remains elusive. The purpose of these experiments was to test the hypothesis that intrauterine head constraint produces ultrastructural changes in the spheno-occipital synchondroses of fetal mice.

METHODS

Fetal constraint was induced through uterine cerclage of six pregnant C57Bl/6 mice on the eighteenth day of gestation. Fetuses were harvested after growing to 24, 48, and 72 hours beyond the normal 20-day gestational period. Between six and nine fetuses were harvested at all time points in both treatment and control groups. The morphology and cell biology of the spheno-occipital synchondroses, in constrained fetuses and unconstrained controls, were examined using hematoxylin and eosin-stained sections. Chondrocyte apoptosis was examined using terminal deoxynucleotidyl transferase-mediated dUDP end-labeling assays and electron microscopy.

RESULTS

In nonconstrained animals, the spheno-occipital synchondrosis demonstrated normal architecture and normal chondrocyte morphology at all time points. In contrast, intrauterine constraint resulted in a progressive disruption of the normal cellular architecture of the spheno-occipital synchondrosis over 72 hours, with premature ossification of the synchondrosis. Widespread chondrocyte apoptosis within the synchondrosial growth center was demonstrated by terminal deoxynucleotidyl transferase-mediated dUDP end-labeling assays and electron microscopy.

CONCLUSION

These experiments confirm the ability of intrauterine constraint to induce changes in the morphology and cell biology of the cranial base in synostotic fetuses.

The role of sex hormones in the kinetics of chondrocytes in the growth plate

Sex hormones play important roles in the regulation of the proliferation, maturation and death of chondrocytes in the epiphyseal growth plate. We have investigated the effects of male castration on the cell kinetics of chondrocytes as defined by the numbers of proliferating and dying cells. The growth plates of normal rabbits and animals castrated at eight weeks of age were obtained at 10, 15, 20 and 25 weeks of age.

Our study suggested that castration led to an increase in apoptosis and a decrease in the proliferation of chondrocytes in the growth plate. In addition, the number of chondrocytes in the castrated rabbits was less than that of normal animals of the same age.

Animal model of chondrocyte apoptosis in the epiphyseal cartilage of the neonatal bone

Apoptosis is considered to be the mechanism responsible for the death of chondrocytes during endochondral bone formation. It is also claimed that apoptosis of the chondrocytes is age related and that the apoptotic index increases with age. However, a detailed analysis of the apoptotic activity of the neonatal epiphyseal cartilage is lacking. A model that evaluates apoptosis in the femoral rat epiphyseal cartilage both quantitatively and qualitatively is reported. Apoptotic incidence in the epiphyseal cartilage reached a maximum at age 6 days, but the age in our study did not significantly affect the percentile rate of apoptotic chondrocytes (P > 0.05, Kruskal-Wallis test). Apoptosis in the zone of hypertrophic cartilage played the most important role in the growth plate's homeostasis. Morphologic evidence of apoptosis was necessary in addition to positive nick end labeling of cells. Electron microscopy studies revealed atypical modes of programmed death of the growth plate chondrocytes in addition to the classical apoptotic mode.

Matrix metalloproteinase-9 expression, tartrate-resistant acid phosphatase activity, and DNA fragmentation in vascular and cellular invasion into cartilage preceding primary endochondral ossification in long bones

Vascular and cellular invasion into cartilage are essential for endochondral ossification. Recently it has been shown that matrix metalloproteinase-9 (MMP-9)/gelatinase B is a key regulator of growth plate angiogenesis and apoptosis of hypertrophic chondrocytes. To study vascular and cellular invasion into cartilage preceding primary endochondral ossification in long bones, precursor femurs from 13- to 16-day-old murine embryos were sectioned. Tartrate-resistant acid phosphatase (TRAP) activity, in situ hybridization for matrix metalloproteinase-9 (MMP-9), immunostaining for CD31, and in situ detection of apoptosis (TUNEL) were studied. TRAP activity, MMP-9 mRNA, and CD31 expression were initially detected in the intertrabecular spaces of the perichondral collar, and then in cells migrating into the cartilage. The first cells involved in the primary invasion into cartilage were CD31-positive vascular endothelial cells and MMP-9-positive cells, followed by TRAP-positive cells. At the cartilage-marrow interface, CD31-positive vascular endothelial cells and MMP-9-positive cells were predominant. These results suggest that MMP-9-positive cells cooperate with vascular endothelial cells in cartilage angiogenesis. TUNEL-positive staining was detected on chondrocytes attached to the inner surface of the perichondral collar, and also detected in the area where cartilage was removed. These results suggest that chondrocytes separated from the cartilage matrix may undergo apoptosis.

Primary culture of rat growth plate chondrocytes: an in vitro model of growth plate histotype, matrix vesicle biogenesis and mineralization

During endochondral ossification (EO), cartilage is replaced by bone. Chondrocytes of growth plate undergo proliferation, maturation, hypertrophy, matrix vesicle (MV) biogenesis and programmed cell death (PCD, apoptosis). The in vitro system presented here provides a potential experimental model for studying in vitro differentiation and MV biogenesis in chondrocyte cultures. Chondrocytes were obtained from collagenase-digested tibial and femoral growth plate cartilage of 7-week-old rachitic rats. The isolated chondrocytes were plated as monolayers at a density of 0.5 x 10(6) cells per 35-mm plate and grown for 17 days in BGJ(b) medium supplemented with 10% fetal bovine serum, 50 microg/ml ascorbic acid. Light microscopy revealed Sirius red-positive, apparent bone matrix in layers at the surfaces of cartilaginous nodules that developed in the cultures. The central matrix was largely alcian blue staining thus resembling cartilage matrix. Electron microscopy revealed superficial areas of bone like matrix with large banded collagen fibrils, consistent with type I collagen. Most of the central matrix was cartilaginous, with small fibrils, randomly arranged consistent with type II collagen. The presence of peripheral type I and central type II and type X collagen was confirmed by immunohistochemical staining. Immunohistochemistry with anti-Bone morphogenetic proteins 2, 4 and 6 showed that BMP expression is associated with maturing hypertrophic central chondrocytes, many of which were TUNEL positive and undergoing cell death with plasma membrane breaks, hydropic swelling and cell fragmentation. During early mineralization, small radial clusters of hydroxyapatite-like mineral were associated with matrix vesicles. Collagenase digestion-released MVs from the cultures showed a high specific activity for alkaline phosphatase and demonstrated a pattern of AMP-stimulated nonradioactive (40)Calcium deposition comparable to that observed with native MVs. These studies confirm that primary cultures of rat growth plate chondrocytes are a reasonable in vitro model of growth plate histotype, MV biogenesis and programmed cell death.

Apoptosis of terminal hypertrophic chondrocytes in an in vitro model of endochondral ossification

It is widely accepted that growth plate chondrocytes undergo apoptosis when they reach the terminal hypertrophic stage of their differentiation during the process of endochondral ossification in vivo. In this report, an established chondrocyte cell culture model of mammalian endochondral ossification was utilized to investigate the fate of chondrocytes after they had entered hypertrophy in vitro. Fetal bovine epiphyseal chondrocytes were treated with the demethylating agent, 5-azacytidine, for 48 h and then cultured under azacytidine-depleted conditions. There was evidence for apoptosis in azacytidine-treated cells, as demonstrated by nuclear condensation and fragmentation (days 27 and 35) using transmission electron microscopy, and the detection of exposed phosphatidylserine on the plasma membrane surface of apoptotic chondrocytes (day 27) using fluorescence-labelled annexin V. Treated cultures on days 10 and 20 and untreated cultures at all corresponding time-points showed no morphological characteristics of apoptosis. In situ hybridization studies of treated cultures revealed that expression of the apoptotic suppressor, bcl-2, remained consistently high throughout the culture period, whilst the apoptotic inducer, bax, was not expressed until day 23. Quantification of these data showed a gradual shift in the ratio of the expression level of bcl-2 and bax in favour of bax with time in culture, particularly from day 23 onwards. Taken together, the results indicate that azacytidine-treated epiphyseal chondrocytes entered terminal hypertrophy from day 23 onwards in culture and died by apoptosis. This study confirms this culture system as a successful recapitulation of the entire mammalian chondrocyte differentiation pathway, including apoptosis. The culture model will prove valuable for studies of the apoptotic fate of terminally differentiated chondrocytes in the growth plate with a view to providing a better understanding of the underlying mechanisms of skeletal malformations and other pathological disorders such as osteoarthritis.

Short-term glucocorticoid treatment of piglets causes changes in growth plate morphology and angiogenesis

OBJECTIVE

Glucocorticoid treatment of children often leads to growth retardation, and the precise target(s) in the growth plate responsible for this effect are unknown. Angiogenesis is an important part of the endochondral ossification process, and VEGF expressed in the growth plate is essential for proper angiogenesis to occur. Since glucocorticoid treatment down-regulates VEGF expression in cultured chondrocytes, we hypothesized that in vivo glucocorticoid treatment could result in VEGF down-regulation in the growth plate and disturbed angiogenesis, thus contributing to the growth retardation.

DESIGN

We treated 6-week-old prepubertal piglets (10 kg) for 5 days with prednisolone (50 mg/day). Tibial growth plate sections were studied for apoptosis and the expression of VEGF protein and mRNA and MMP-9 protein. Capillaries in the metaphysis were visualized by CD31 immunostaining. Growth plate morphology (width of various zones) was determined by interactive measurements on hematoxylin/eosin stained sections and apoptotic cells were detected by TUNEL assay.

RESULTS

In the prednisolone-treated animals, the total width of the growth plate decreased to 81% of controls (P<0.02), which was explained by a decrease of the width of the proliferative zone to 73% (P<0.05). The treatment had no effect on the orderly organization of the chondrocyte columns. In the growth plates of control animals, apoptosis was shown in 5.8% of the hypertrophic chondrocytes and was limited to the terminal hypertrophic chondrocytes. In prednisolone-treated animals, 40.5% of the hypertrophic chondrocytes was apoptotic (P<0.02), with apoptotic chondrocytes also appearing higher in the hypertrophic zone. We observed fewer capillaries and loss of their parallel organization in the metaphysis in the prednisolone-treated animals. The capillaries were shorter and chaotic in appearance. In contrast to controls, in prednisolone-treated animals VEGF mRNA and protein could not be detected in the hypertrophic zone of the growth plate. Trabecular bone length in the primary spongiosa was also diminished by the treatment. No changes were observed in the expression pattern of MMP-9, a matrix metalloproteinase, which is also important for angiogenesis and bone formation.

CONCLUSIONS

These results indicate that short-term glucocorticoid treatment of growing piglets severely disturbs the width of the growth plate, apoptosis of chondrocytes, VEGF expression by hypertrophic chondrocytes, the normal invasion of blood vessels from the metaphysis to the growth plate and bone formation at the chondro-osseous junction. These effects could alter the dynamics of endochondral ossification and thus contribute to glucocorticoid-induced growth retardation.

Role of nitric oxide, reactive oxygen species, and p38 MAP kinase in the regulation of human chondrocyte apoptosis

This study addresses mechanisms by which interleukin-1beta (IL-1beta) regulates human chondrocyte apoptosis induced by a combination of the anti-CD95 antibody CH-11 and the proteasome inhibitor (PSI). The effect of IL-1beta on apoptosis varied among tissue samples. IL-1beta either enhanced (16/22 samples) or inhibited (6/22 samples) DNA fragmentation and caspase-3 processing. The protective effect of IL-1beta was abrogated by the nitric oxide (NO) synthesis inhibitor N-monomethyl-l-arginine (L-NMMA) while apoptosis stimulation was not affected. The NO-donors sodium nitroprusside (SNP) and S-nitroso-N-acetyl penicillamine (SNAP) blocked DNA fragmentation, and this was associated with partial inhibition of caspase-3 processing. Pyrrolidine dithiocarbamate (PDTC), a scavenger of reactive oxygen species (ROS) blocked apoptosis induction by CH-11/PSI as well as the enhancement by IL-1beta. The pro-apoptotic effects of IL-1beta were also abrogated by the p38 inhibitor SB 202190. In conclusion, IL-1beta augments CH-11/PSI induced apoptosis in the majority of chondrocyte samples. The pro-apoptotic effect of IL-1beta is not dependent on NO. In contrast, the anti-apoptotic effect of IL-1beta observed in a minority of samples is partially NO-dependent.

Normal proliferation and differentiation of Hoxc-8 transgenic chondrocytes in vitro

BACKGROUND

Hox genes encode transcription factors that are involved in pattern formation in the skeleton, and recent evidence suggests that they also play a role in the regulation of endochondral ossification. To analyze the role of Hoxc-8 in this process in more detail, we applied in vitro culture systems, using high density cultures of primary chondrocytes from neonatal mouse ribs.

RESULTS

Cultured cells were characterized on the basis of morphology (light microscopy) and production of cartilage-specific extracellular matrix (sulfated proteoglycans and type II Collagen). Hypertrophy was demonstrated by increase in cell size, alkaline phosphatase activity and type X Collagen immunohistochemistry. Proliferation was assessed by BrdU uptake and flow cytometry. Unexpectedly, chondrocytes from Hoxc-8 transgenic mice, which exhibit delayed cartilage maturation in vivo 1, were able to proliferate and differentiate normally in our culture systems. This was the case even though freshly isolated Hoxc-8 transgenic chondrocytes exhibited significant molecular differences as measured by real-time quantitative PCR.

CONCLUSIONS

The results demonstrate that primary rib chondrocytes behave similar to published reports for chondrocytes from other sources, validating in vitro approaches for studies of Hox genes in the regulation of endochondral ossification. Our analysis of cartilage-producing cells from Hoxc-8 transgenic mice provides evidence that the cellular phenotype induced by Hoxc-8 overexpression in vivo is reversible in vitro.

Coordinate down-regulation of cartilage matrix gene expression in Bcl-2 deficient chondrocytes is associated with decreased SOX9 expression and decreased mRNA stability

The anti-apoptotic protein Bcl-2 has been shown to function in roles unrelated to apoptosis in a variety of cell types. We have previously reported that loss of Bcl-2 expression alters chondrocyte morphology and modulates aggrecan expression via an apoptosis-independent pathway. Here we show that Bcl-2 is required for chondrocytes to maintain expression of a variety of cartilage-specific matrix proteins. Using quantitative, real-time PCR, we demonstrate that Bcl-2-deficient chondrocytes coordinately down-regulate genes coding for hyaline cartilage matrix proteins including collagen II, collagen IX, aggrecan, and link protein. The decrease in steady-state level of these mRNA transcripts results, in part, from decreased mRNA stability in Bcl-2-deficient chondrocytes. Transcriptional regulation is also likely involved because chondrocytes with decreased Bcl-2 levels show decreased expression of SOX9, a transcription factor necessary for expressing the major cartilage matrix proteins. In contrast, chondrocytes constitutively expressing Bcl-2 have a stable phenotype when subjected to loss of serum factor signaling. These cells maintain high levels of SOX9, as well as the SOX9 targets collagen II and aggrecan. These results suggest that Bcl-2 is involved in a pathway important for maintaining a stable chondrocyte phenotype.

Intracellular zinc fluxes associated with apoptosis in growth plate chondrocytes

Matrix vesicles released by epiphyseal growth plate chondrocytes are known to contain a significant quantity of labile Zn(2+). Zonal analysis of chicken metatarsal bones showed that the resting/proliferative region of the growth plate contained high levels of Zn(2+) with significantly lower levels in the hypertrophic cartilage suggesting a loss of cellular Zn(2+) as the chondrocytes mature. Intracellular labile Zn(2+) was measured in primary cultures of growth plate chondrocytes by assay with the fluorescent Zn-chelator toluenesulfonamidoquinoline (TSQ) and imaged by multi-photon laser scanning microscopy (MPLSM) with the TSQ derivative zinquin. Short-term exposure to Zn(2+), both in the presence and absence of pyrithione resulted in significant increases in cytosolic Zn(2+). Treatment with the membrane-permeant Zn(2+) chelator TPEN rapidly reduced the levels of labile Zn(2+) and triggered apoptosis. Cytosolic Zn(2+) levels were significantly reduced following 24-h incubations with known inducers of chondrocyte apoptosis. The loss of intracellular Zn(2+) was accompanied by a significant reduction in the cytosolic metal-binding protein metallothionein. Examination of Zn(2+)-treated cells with MPLSM showed uniformly higher zinquin fluorescence. Treatment of Zn(2+)-loaded cells with TPEN quenched zinquin fluorescence confirming that the observed fluorescence in chondrocytes is due to the presence of intracellular Zn(2+). A dose-dependent increase in zinquin fluorescence was observed in cells treated with a range of Zn(2+) concentrations. Short-term treatment of cultured chondrocytes with apoptosis-inducing chemicals resulted in transient increases in intracellular labile Zn(2+). These results indicate that Zn(2+) is mobilized from intracellular binding sites in the early stages of chondrocyte apoptosis and is subsequently lost from the cells. The early mobilization of Zn(2+) provides a mechanism for its movement to matrix vesicles and the extracellular matrix.

The fate of the terminally differentiated chondrocyte: evidence for microenvironmental regulation of chondrocyte apoptosis

Chondrocytes contained within the epiphyseal growth plate promote rapid bone growth. To achieve growth, cells activate a maturation program that results in an increase in chondrocyte number and volume and elaboration of a mineralized matrix; subsequently, the matrix is resorbed and the terminally differentiated cells are deleted from the bone. The major objective of this review is to examine the fate of the epiphyseal chondrocytes in the growing bone. Current studies strongly suggest that the terminally differentiated epiphyseal cells are deleted from the cartilage by apoptosis. Indeed, morphological, biochemical, and end-labeling techniques confirm that death is through the apoptotic pathway. Since the induction of apoptosis is spatially and temporally linked to the removal of the cartilage matrix, current studies have examined the apoptogenic activity of Ca(2+)-, Pi-, and RGD-containing peptides of extracellular matrix proteins. It is observed that all of these molecules are powerful apoptogens. With respect to the molecular mechanism of apoptosis, studies of cell death with Pi as an apoptogen indicate that the anion is transported into the cytosol via a Na(+/)Pi transporter. Subsequently, there is activation of caspases, generation of NO, and a decrease in the thiol reserve. Finally, we examine the notion that chondrocytes transdifferentiate into osteoblasts, and briefly review evidence for, and the rationale of, the transdifferentiation process. It is concluded that specific microenvironments exist in cartilage that can serve to direct chondrocyte apoptosis.

The fate of soft callus chondrocytes during long bone fracture repair

Following fracture, the cartilaginous tissue of the soft callus is eventually replaced by bone. Removal of the cartilage is a critical part of the bone healing process but information concerning the changes in chondrocytes during this process is sparse. The aim of the study was to investigate the fate of chondrocytes in the soft callus during the bone repair process using a rabbit tibial fracture model. Fracture tissue was processed for collagen I-III and keratan sulphate immunohistochemistry to study changes in matrix composition and the TUNEL technique (terminal deoxynucleotidyl transferase medicated dUTP nick-end labelling) to identify death of soft callus chondrocytes. Transmission electron microscopy (TEM) was also carried out to investigate the ultrastructure of chondrocytes within the soft callus. Results showed that the size of the cartilage area decreased over time and that cartilage matrix was replaced with new matrix rich in collagen I and III. Chondrocytes became engulfed in the new matrix and appeared to stop producing cartilage matrix. Chondrocyte cell death was seen at the border of the soft callus, just within the newly produced matrix. TEM revealed that these dying/dead cells were not typically apoptotic in appearance. In conclusion, results indicate that chondrocytes of the soft callus die as a result of the progressive production of bone matrix which eventually engulfs them and leads to the remodelling of the area and eventual bone repair.

Differentiation plasticity of chondrocytes derived from mouse embryonic stem cells

Evidence exists that cells of mesenchymal origin show a differentiation plasticity that depends on their differentiation state. We used in vitro differentiation of embryonic stem cells through embryoid bodies as a model to analyze chondrogenic and osteogenic differentiation because embryonic stem cells recapitulate early embryonic developmental phases during in vitro differentiation. Here, we show that embryonic stem cells differentiate into chondrocytes, which progressively develop into hypertrophic and calcifying cells. At a terminal differentiation stage, cells expressing an osteoblast-like phenotype appeared either by transdifferentiation from hypertrophic chondrocytes or directly from osteoblast precursor cells. Chondrocytes isolated from embryoid bodies initially dedifferentiated in culture but later re-expressed characteristics of mature chondrocytes. The process of redifferentiation was completely inhibited by transforming growth factor beta3. In clonal cultures of chondrocytes isolated from embryoid bodies, additional mesenchymal cell types expressing adipogenic properties were observed, which suggests that the subcultured chondrocytes indeed exhibit a certain differentiation plasticity. The clonal analysis confirmed that the chondrogenic cells change their developmental fate at least into the adipogenic lineage. In conclusion, we show that chondrocytic cells are able to transdifferentiate into other mesenchymal cells such as osteogenic and adipogenic cell types. These findings further strengthen the view that standardized selection strategies will be necessary to obtain defined cell populations for therapeutic applications.

Evidence of oncotic cell death and DNA fragmentation in human hypertrophic chondrocytes in chondro-osteophyte

OBJECTIVE

To investigate the population and morphology of in situ terminal deoxynucleotidyle transferase (TdT)-mediated dUTP nick end-labeling (TUNEL) stain positive non-apoptotic chondrocytes in hypertrophic zone of human chondro-osteophytes.

MATERIALS AND METHODS

Chondro-osteophytes from osteoarthritic patients were obtained at joint replacement surgery. Apoptosis was verified by light microscopic examination of Safranin O stained sections and TUNEL stain. TUNEL staining was also performed on hydrophilic resin embedded semi-thin and ultra-thin sections combined with the treatment with streptavidin-gold conjugates, observed by light microscopy with silver enhancement technique (TUNEL-LM with SE) and transmission electron microscopy (TUNEL-TEM) respectively for the simultaneous evaluation of cellular structure and DNA fragmentation.

RESULTS

In paraffin embedded sections (N=18), 31.5+/-6.1% of cells in the hypertrophic zone were TUNEL positive, but only 3.8+/-1.2% cells in this zone showed apoptotic appearances with cell shrinkage and nuclear condensation. Both in TUNEL-TEM and TUNEL-LM with SE, gold particles, which indicate DNA fragmentation, were observed within the nucleus of morphologically apoptotic chondrocytes, as well as of disintegrated, swollen chondrocytes.

CONCLUSIONS

In human chondro-osteophytes, hypertrophic chondrocytes might die by oncotic cell death with DNA fragmentation, as well as apoptosis.

Development and regulation of osteophyte formation during experimental osteoarthritis

OBJECTIVES

Osteophytes represent areas of new cartilage and bone formation in human and experimentally induced osteoarthritis (OA). The present study addressed the production of nitric oxide (NO), vascular endothelial growth factor (VEGF) and the occurrence of apoptosis during osteophyte formation.

DESIGN

Osteophytes in the knee joint of rabbits that developed OA-like lesions following anterior cruciate ligament transection (ACLT) were analysed by histology and immunohistochemistry for NO production, and the presence of VEGF. TUNEL was used to detect DNA fragmentation.

RESULTS

At the joint margins in the interface between cortical bone marrow and periosteal lining growth plate-like formations were detectable as early as 4 weeks after ACLT. By 12 weeks after ACLT osteophytes were visible in 100% of femoral condyles and tibial plateaus. Discrete areas with proliferating chondrocytes, hypertrophic chondrocytes, calcified matrix and vascular invasion were observed. VEGF immunoreactivity was most prominent in hypertrophic chondrocytes 9 weeks after ACLT. Nitrotyrosine immunoreactivity was detected in endothelial cells and in some hypertrophic chondrocytes in the calcified zone 4 weeks after ACLT. After 8 and 12 weeks, positive cells were detected in the hypertrophic and calcified zone. TUNEL-positive cells were seen in blood vessels, and among hypertrophic chondrocytes adjacent to the blood vessels 4 weeks after ACLT. The proliferative zone, pre-hypertrophic zone and hypertrophic zone showed only a few TUNEL positive cells. In contrast, 8 weeks and 12 weeks after ACLT, most hypertrophic chondrocytes, but few proliferative chondrocytes showed DNA fragmentation.

CONCLUSIONS

Hypertrophic chondrocytes in osteophytes express VEGF and this can promote vascular invasion of cartilage. The presence of TUNEL-positive cells shows a similar distribution as nitrotyrosine immunoreactivity during all phases of osteophyte development, suggesting that NO production and chondrocyte death are related events in osteophyte formation.

Induction of apoptosis in chondrocytes by tumor necrosis factor-alpha

Tumor necrosis factor alpha (TNF-alpha) induces apoptosis in a number of cell types and plays an essential role in bone remodeling, both stimulating the proliferation of osteoblasts and activating osteoclasts. During endochondral ossification, apoptosis of chondrocytes occurs concurrently with new bone formation and the resorption and replacement of mineralized cartilage with woven bone. In the present study, the role of TNF-alpha in promoting chondrocyte apoptosis was examined. Chondrocyte cell populations, enriched in either hypertrophic or non-hypertrophic cells, were isolated from the cephalic and caudal portions of 17-day chick embryo sterna, respectively, and treated in vitro with 0.1-10 nM recombinant human TNF-alpha. As a positive control, apoptosis was also induced by Fas receptor antibody binding. Dye exclusion assays of the live/dead ratios of cells showed that TNF-alpha caused a dose-dependent 1.5- and 2.0-fold increase in the number of dead cells in both hypertrophic and non-hypertrophic chondrocytes. Induction of apoptosis was independently assayed by measurement of interleukin-1beta-converting enzyme (ICE) activity, and analyzed by a semi-quantitative determination of DNA fragmentation. When compared to untreated cells, these analyses also showed dose-dependent increases in TNF-alpha induced apoptosis in both chondrocyte populations, with increases in the levels of ICE activity for all doses of TNF-alpha (from approximately 5 to approximately 20 fold). Osteoblasts, however, were not affected by treatment with TNF-alpha or by Fas antibody/protein G induction. Immunostaining of chondrocytes for Fas receptor and caspase-2 protein expression showed that most of the chondrocytes expressed these two markers of apoptosis after treatment with TNF-alpha. Although cell killing and ICE induction were higher in the more hypertrophic cells, TNF-alpha induced apoptosis in both hypertrophic and non-hypertrophic chondrocyte populations. These results demonstrate that apoptosis may be induced in both hypertrophic and non-hypertrophic chondrocytes through both Fas and TNF-alpha receptor mediated signaling, and suggest that chondrocytes are more sensitive to apoptotic effects of TNF-alpha within the skeletal lineage than are osteoblasts.

Cell proliferation and death of growth plate chondrocyte caused by ischemia and reperfusion

The aim of this study was to assess the short-term response of cell kinetics of growth plate chondrocytes under conditions of warm ischemia and reperfusion. To understand the time-course changes that occur after reperfusion, 0 and 6 h of warm ischemia was produced in the right hindlimb of 35-day-old Wistar rats by isolating the vascular pedicle occlusion. The animals were killed at 12, 24, 48, or 96 h postoperatively after reperfusion, and proximal tibia growth plates were investigated. To investigate the effect of the ischemia period on the kinetics of growth plate chondrocytes, 0, 2, 4, 6, and 8 h of ischemia was induced, and the animals were killed for evaluation 24 h after reperfusion. For evaluation of cell kinetics, BrdU was used to observe the changes in cell proliferation of growth plate chondrocytes, and TUNEL was used to estimate the changes in rate of cell death. In the time-course study, both 0 and 6 h of ischemia increased cell proliferation at 12 and 24 h after reperfusion; however, at 48 and 96 h, the proliferation rate was not further increased. At 12 and 24 h postoperatively, 6 h of ischemia increased chondrocyte proliferation more than 0 h of ischemia with significant differences; 6 h of ischemia led to an increased cell death rate at 12, 24, and 48 h postoperatively, whereas 0 h of ischemia did not affect the cell death rate. In the ischemia time-dependent study, the cell proliferation rate induced by 4 h of ischemia was highest in all controlled periods of ischemia. Cell death rate increased gradually with increases in ischemia time 24 h after reperfusion. This experiment showed that ischemic damage causes short-term postoperative changes in the kinetics of growth plate chondrocytes.