Immunoelectron microscopic studies of type X collagen in endochondral ossification

The Role of Gene Expression Changes in ceRNA Network Underlying Ossification of Ligamentum Flavum Development

We aimed at exploring the role of gene expression changes regulated by non-coding RNAs in ossification of ligamentum flavum (OLF). Three microarray datasets, including long non-coding RNA (lncRNA)/mRNA expression profile (GSE106253), circular RNA (circRNA) expression profile (GSE106255), and microRNA (miRNA) expression profile (GSE106256), were downloaded from the public Gene Expression Omnibus repository. The differentially expressed (DE) mRNAs, lncRNAs, miRNAs, and circRNAs in OLF tissues were analyzed, compared with normal tissues. Two competing endogenous RNA (ceRNA) networks with lncRNA-miRNA-mRNA and circRNA-miRNA-mRNA interactions were constructed, separately. Random walk with the restart model was applied to calculate the correlations of mRNAs with the published OLF-related genes. The top 50 mRNAs were subjected to function enrichment analysis and active small-molecule prediction. Total 2323 DE mRNAs, 1168 lncRNAs, 336 circRNAs, and 29 miRNAs were identified based on the microarray datasets. The LncRNA-related ceRNA network was constructed with 614 lncRNA-miRNA, 494 miRNA-mRNA, and 2099 lncRNA-mRNA interaction pairs; the circRNA-related ceRNA network was constructed with 153 circRNA-miRNA, 190 miRNA-mRNA, and 210 circRNA-mRNA interaction pairs. There were 17 OLF-related genes retrieved from previous literature, such as NPPS, COL6A1, and COL11A2, among which COL6A1 was the overlapped gene with mRNAs in the ceRNA network. Subsequently, top 50 mRNAs that closely correlated with COL6A1 in the ceRNA network were captured and these genes were closely related with the collagen catabolic process, regulation of cell growth, and neuronal action potential. DRD1 and COL6A1 were predicted to be the targets by small active molecule drugs. The collagen catabolic process may be implicated in OLF development. COL6A1 and DRD1 may be the candidate targets for OLF. However, further validations were needed.

Respiratory chain inactivation links cartilage-mediated growth retardation to mitochondrial diseases

Children with mitochondrial diseases often present with slow growth and short stature, but the underlying mechanism remains unclear. In this study, Holzer et al. provide in vivo evidence that mitochondrial respiratory chain dysfunction induces cartilage degeneration coincident with altered metabolism, impaired extracellular matrix formation, and cell death at the cartilage–bone junction.

In childhood, skeletal growth is driven by transient expansion of cartilage in the growth plate. The common belief is that energy production in this hypoxic tissue mainly relies on anaerobic glycolysis and not on mitochondrial respiratory chain (RC) activity. However, children with mitochondrial diseases causing RC dysfunction often present with short stature, which indicates that RC activity may be essential for cartilage-mediated skeletal growth. To elucidate the role of the mitochondrial RC in cartilage growth and pathology, we generated mice with impaired RC function in cartilage. These mice develop normally until birth, but their later growth is retarded. A detailed molecular analysis revealed that metabolic signaling and extracellular matrix formation is disturbed and induces cell death at the cartilage–bone junction to cause a chondrodysplasia-like phenotype. Hence, the results demonstrate the overall importance of the metabolic switch from fetal glycolysis to postnatal RC activation in growth plate cartilage and explain why RC dysfunction can cause short stature in children with mitochondrial diseases.

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.

Suspension-based differentiation of adult mesenchymal stem cells toward chondrogenic lineage

Human mesenchymal stem cells (hMSCs) are derived from bone marrow and have the ability to differentiate into cartilage and other mesenchymal cell types found throughout the body. Traditionally, the differentiation of hMSCs toward chondrocytes occurs through a combination of pelleted static cell culture and chemical stimuli. As an alternative to these protocols, we developed an in vitro flow through microfluidic method to induce the differentiation of hMSCs into chondrocytes. Suspensions of unattached hMSCs were exposed to a constant shear flow over a period of 20 minutes, which promoted phenotypic and gene expression changes toward the chondrogenic lineage. These internal and external changes of chondrogenic differentiation were then observed over 3 weeks later in culture, as confirmed through fluorescent immunocytochemical staining and real-time quantitative reverse transcriptase polymerase chain reaction. The increased concentration of Type II collagen on the surface of shear stimulated hMSCs with the upregulation of MAPK1 and SOX9 demonstrated the capabilities of our approach to induce sustained differentiation. In conclusion, our shear stimulation method, in combination with chemical stimuli, illustrates enhanced differentiation of hMSCs toward the chondrogenic lineage.

Hypoxia-inducible factor-2α induces expression of type X collagen and matrix metalloproteinases 13 in osteoarthritic meniscal cells

OBJECTIVES

To evaluate whether Hypoxia-inducible factor-2α (HIF-2α) regulates expression of endochondral ossification-related molecules in human OA meniscus.

METHODS

Expressions of HIF-2α, type X collagen (COL10), matrix metalloproteinase (MMP)-13, and vascular endothelial growth factor (VEGF) in non-OA and OA menisci were analyzed by real-time RT-PCR and immunohistochemistry (IHC). Meniscal cells from OA patients were treated with interleukin-1β (IL-1β) and gene expression was analyzed. After knockdown of HIF-2α in OA meniscal cells, COL10 and MMP-13 expression were analyzed by RT-PCR, western blotting, immunofluorescence and ELISA.

RESULT

Histological analysis demonstrated weak staining of the superficial layer and large round cells in OA meniscus. RT-PCR analysis showed that HIF-2α, COL10, MMP-13, and VEGF mRNA expressions were higher in OA than non-OA meniscal cells. IHC showed a coordinated staining pattern of HIF-2α, COL10, and MMP-13 in OA meniscus. IL-1β treatment increased HIF-2α, COL10, and MMP-13 expressions in OA meniscal cells, and knockdown of HIF-2α suppressed IL-1β-mediated increase in COL10 and MMP-13 expression.

CONCLUSIONS

These results suggested that HIF-2α may cause meniscal matrix degradation by transactivation of MMP-13. HIF-2α may be a therapeutic target for modulating matrix degradation in both articular cartilage and meniscus during knee OA progression.

Mechanical loading regulates human MSC differentiation in a multi-layer hydrogel for osteochondral tissue engineering

A bioinspired multi-layer hydrogel was developed for the encapsulation of human mesenchymal stem cells (hMSCs) as a platform for osteochondral tissue engineering. The spatial presentation of biochemical cues, via incorporation of extracellular matrix analogs, and mechanical cues, via both hydrogel crosslink density and externally applied mechanical loads, were characterized in each layer. A simple sequential photopolymerization method was employed to form stable poly(ethylene glycol)-based hydrogels with a soft cartilage-like layer of chondroitin sulfate and low RGD concentrations, a stiff bone-like layer with high RGD concentrations, and an intermediate interfacial layer. Under a compressive load, the variation in hydrogel stiffness within each layer produced high strains in the soft cartilage-like layer, low strains in the stiff bone-like layer, and moderate strains in the interfacial layer. When hMSC-laden hydrogels were cultured statically in osteochondral differentiation media, the local biochemical and matrix stiffness cues were not sufficient to spatially guide hMSC differentiation after 21 days. However dynamic mechanical stimulation led to differentially high expression of collagens with collagen II in the cartilage-like layer, collagen X in the interfacial layer and collagen I in the bone-like layer and mineral deposits localized to the bone layer. Overall, these findings point to external mechanical stimulation as a potent regulator of hMSC differentiation toward osteochondral cellular phenotypes.

How best to preserve and reveal the structural intricacies of cartilaginous tissue

No single processing technique is capable of optimally preserving each and all of the structural entities of cartilaginous tissue. Hence, the choice of methodology must necessarily be governed by the nature of the component that is targeted for analysis, for example, fibrillar collagens or proteoglycans within the extracellular matrix, or the chondrocytes themselves. This article affords an insight into the pitfalls that are to be encountered when implementing the available techniques and how best to circumvent them. Adult articular cartilage is taken as a representative pars pro toto of the different bodily types. In mammals, this layer of tissue is a component of the synovial joints, wherein it fulfills crucial and diverse biomechanical functions. The biomechanical functions of articular cartilage have their structural and molecular correlates. During the natural course of postnatal development and after the onset of pathological disease processes, such as osteoarthritis, the tissue undergoes structural changes which are intimately reflected in biomechanical modulations. The fine structural intricacies that subserve the changes in tissue function can be accurately assessed only if they are faithfully preserved at the molecular level. For this reason, a careful consideration of the tissue-processing technique is indispensable. Since, as aforementioned, no single methodological tool is capable of optimally preserving all constituents, the approach must be pre-selected with a targeted structure in view. Guidance in this choice is offered.

Depletion of annexin A5, annexin A6, and collagen X causes no gross changes in matrix vesicle-mediated mineralization, but lack of collagen X affects hematopoiesis and the Th1/Th2 response

Numerous biochemical studies have pointed to an essential role of annexin A5 (AnxA5), annexin A6 (AnxA6), and collagen X in matrix vesicle-mediated biomineralization during endochondral ossification and in osteoarthritis. By binding to the extracellular matrix protein collagen X and matrix vesicles, annexins were proposed to anchor matrix vesicles in the extracellular space of hypertrophic chondrocytes to initiate the calcification of cartilage. However, mineralization appears to be normal in mice lacking AnxA5 and AnxA6, whereas collagen X-deficient mice show only subtle alterations in the growth plate organization. We hypothesized that the simultaneous lack of AnxA5, AnxA6, and collagen X in vivo induces more pronounced changes in the growth plate development and the initiation of mineralization. In this study, we generated and analyzed mice deficient for AnxA5, AnxA6, and collagen X. Surprisingly, mice were viable, fertile, and showed no obvious abnormalities. Assessment of growth plate development indicated that the hypertrophic zone was expanded in Col10a1(-/-) and AnxA5(-/-) AnxA6(-/-) Col10a1(-/-) newborns, whereas endochondral ossification and mineralization were not affected in 13-day- and 1-month-old mutants. In peripheral quantitative computed tomography, no changes in the degree of biomineralization were found in femora of 1-month- and 1-year-old mutants even though the diaphyseal circumference was reduced in Col10a1(-/-) and AnxA5(-/-) AnxA6(-/-) Col10a1(-/-) mice. The percentage of naive immature IgM(+) /IgM(+) B cells and peripheral T-helper cells were increased in Col10a1(-/-) and AnxA5(-/-) AnxA6(-/-) Col10a1(-/-) mutants, and activated splenic T cells isolated from Col10a1(-/-) mice secreted elevated levels of IL-4 and GM-CSF. Hence, collagen X is needed for hematopoiesis during endochondral ossification and for the immune response, but the interaction of annexin A5, annexin A6, and collagen X is not essential for physiological calcification of growth plate cartilage. Therefore, annexins and collagen X may rather fulfill functions in growth plate cartilage not directly linked to the mineralization process.

The constitutive expression of type x collagen in mesenchymal stem cells from osteoarthritis patients is reproduced in a rabbit model of osteoarthritis

The expression of type X collagen (COL X), a late-stage chondrocyte hypertrophy marker in human mesenchymal stem cells (MSCs) from osteoarthritis (OA) patients poses a major setback to current cartilage and intervertebral disc tissue engineering efforts. However, it is not yet clear whether COL X is expressed by all human bone marrow stem cells or if it is related to age, gender, site, disease status, or drug therapy. In the current study, we report that COL X expression is upregulated in MSCs from rabbits in a surgical instability model of OA (anterior cruciate ligament transection (ACLT)) when compared to control rabbit MSCs. Thus COL X expression in OA is a common phenomenon that is due to the disease process itself and not to other environmental factors. It is, therefore, critical to understand MSC phenotype in OA patients, as these cells are essential clinically for biological repair of cartilage lesions using autologous stem cells.

The Avian Eggshell as a Model of Biomineralization

The avian eggshell is one of the most rapidly mineralizing biological systems known. By understandi'ng the key components and steps in this process, we hope to provide relevant information for fabrication of ceramic composites. The calcification of the eggshell occurs in three main steps: 1) fabrication of an organic matrix, 2) nucleation of an inorganic phase on the organic matrix, and 3) space-filling growth of the calcite phase. The different layers of an eggshell can be separately isolated and studied. Three approaches have been used in our study of the eggshell: 1) characterization of the organization and chemical composition of the shell, 2) selective removal or blocking of particular components to improve the remineralization of demineralized shells, and 3) addition of new components to produce composite ceramics of different kinds. In this preliminary communication, the organization of the shell matrix and membranes and their association with the crystal phase, the immunohistochemical occurrence and distribution of types I and X collagen, and of different proteoglycans are reviewed. Also the preliminary findings of the remineralization of the intact or modified eggshell are presented. These experiments allow us to identify the essential steps in forming a natural composite ceramic.

Mutations of COL10A1 in Schmid metaphyseal chondrodysplasia

Schmid metaphyseal chondrodysplasia (SMCD) is a dominantly inherited cartilage disorder caused by mutations in the gene for the hypertrophic cartilage extracellular matrix structural protein, collagen X (COL10A1). Thirty heterozygous mutations have been described, about equally divided into two mutation types, missense mutations, and mutations that introduce premature termination signals. The COL10A1 mutations are clustered (33/36) in the 3' region of exon 3, which codes for the C-terminal NC1 trimerization domain. The effect of COL10A1 missense mutations have been examined by in vitro expression and assembly assays and cell transfection studies, which suggest that a common consequence is the disruption of collagen X trimerization and secretion, with consequent intracellular degradation. The effect of COL10A1 nonsense mutations in cartilage tissue has been examined in two patients, demonstrating that the mutant mRNA is completely removed by nonsense mediated mRNA decay. Thus for both classes of mutations, functional haploinsufficiency is the most probable cause of the clinical phenotype in SMCD.

Histochemical evidences on the chronological alterations of the hypertrophic zone of mandibular condylar cartilage

The hypertrophic chondrocytes lack the ability to proliferate, thus permitting matrix mineralization as well as vascular invasion from the bone in both the mandibular condyle and the epiphyseal cartilage. This study attempted to verify whether the histological appearance of the hypertrophic chondrocytes is in a steady state during postnatal development of the mouse mandibular condyle. Type X collagen immunohistochemistry apparently distinguished the fibrous layer described previously as the "articular zone," "articular layer," and "resting zone" from the hypertrophic zone. Interestingly, the ratio of the type X collagen-positive hypertrophic zone in the entire condyle seemed higher in the early stages but decreased in the later stages. Some apparently compacted cells in the hypertrophic zone showed proliferating cell nuclear antigen (PCNA) immunoreaction, indicating the potential for cell proliferation at the early stages. As the mice matured, in contrast, they further enlarged and assumed typical features of hypertrophic chondrocytes. Apoptotic cells were also discernible in the hypertrophic zone at the early but not later stages. Consistent with morphological configurations of hypertrophic chondrocytes, immunoreactions for alkaline phosphatase, osteopontin, and type I collagen were prominent at the later stage, but not the early stage. Cartilaginous matrices demonstrated scattered patches of mineralization at the early stage, but increased in their volume and connectivity at the later stage. Thus, the spatial and temporal occurrence of these immunoreactions as well as apoptosis likely reflect the prematurity of hypertrophying cells at the early stage, and imply a physiological relevance during the early development of the mandibular condyles.

DETAILED EXAMINATION OF CARTILAGE FORMATION and ENDOCHONDRAL OSSIFICATION USING HUMAN MESENCHYMAL STEM CELLS

1. Cartilage formation is one of the most complex processes in biology. The aim of the present study was to produce a simplified in vitro system to resolve its complexities. 2. Human mesenchymal stem cells (hMSC) were maintained in alginate beads with a chondrogenesis-induction medium containing 10 ng/mL transforming growth factor (TGF)-beta3. 3. At days 0, 2, 4, 8, 12, 16 and 19 of culture, we examined the cells using a light microscope and a transmission electron microscope. We also evaluated the cells using immunocryo-ultramicrotomy. 4. The present study demonstrated that hMSC produced numerous extracellular matrices containing abnormal collagen fibres following their exposure to a chondrogenesis-induction medium in alginate beads. At this time, calcification was detected by alizarin red staining and electron-dense particles, composed of hydroxyapatite, appeared in both the cytoplasm and the extracellular spaces. 5. In addition immunocryo-ultramicrotomy revealed that collagen type II, type X and proteoglycan were prominent and that osteocalcin was detectable at day 2. During 8-16 days of culture, collagen type X maintained its strong expression and the expression of osteocalcin increased markedly. In contrast, the expression of collagen type II and proteoglycan decreased with time. 6. These findings demonstrate that hMSC rapidly differentiate into chondrocytes expressing collagen type II and proteoglycan. 7. The expression of collagen type II and proteoglycan then dropped and the activity of collagen type X was the same as before (4-8 days). As a result, the cells developed into the next cell type, so-called hypertrophic chondrocytes. Finally, both osteocalcin activity and the calcification of cell bodies and extracellular matrices became evident, indicating endochondral ossification. Thus, we conclude that hMSC rapidly differentiate into chondrocytes, followed by the development of hypertrophic chondrocytes. Endochondral ossification is the final form in this culture. 8. The findings of the present study indicate that our three-dimensional culture is a convenient in vitro model for the investigation of the regulatory mechanisms of cartilage formation and endochondral ossification.

The metabolism of hyaluronan in cultured rabbit growth plate chondrocytes during differentiation

Hyaluronan (HA) is one of the major extracellular matrix components in cartilage. In addition to the biomechanical functions, HA has various important roles in the differentiation of chondrocytes. The purpose of this study was to clarify the nature of HA synthesis during chondrocyte differentiation. Growth plate chondrocytes were isolated from rabbit ribs and cultured in chondrocyte differentiation medium. The amount of HA and HA synthase (HAS) mRNA levels were analyzed for each stage of chondrocyte differentiation by means of high-performance liquid chromatography (HPLC) and real-time PCR, respectively. The distribution of HA in cultured chondrocytes was observed by histochemical staining. The amount of HA, ranging widely in size, was increased substantially during the hypertrophic stage. The expression levels of HAS2 and HAS3 mRNAs were low during the matrix-forming stage. HAS2 mRNA level was substantially enhanced at the pre-hypertrophic stage, whereas HAS3 mRNA level exhibited a slight increase. HAS1 mRNA was not detected. The intensity of HA staining was high around the hypertrophic chondrocytes. These results suggest that HA metabolism in chondrocyte differentiation is regulated by the selective expression of HASs, and HAS2 and the related large size-HA may have a certain association with the hypertrophic changes of chondrocytes.

The role of type X collagen in facilitating and regulating endochondral ossification of articular cartilage

AUTHOR: Shen G Objective -This review was compiled to explore the role of type X collagen in growth, development and remodeling of articular cartilage by elucidating the linkage between the synthesis of this protein and the phenotypic changes in chondrogenesis and the onset of endochondral ossification.

DESIGN

The current studies closely dedicated to elucidating the role of type X collagen incorporating into chondrogenesis and endochondral ossification of articular cartilage were assessed and analyzed to allow for obtaining the mainstream consensus on the bio-molecular mechanism with which type X collagen functions in articular cartilage.

RESULTS

There are spatial and temporal correlations between synthesis of type X collagen and occurrence of endochondral ossification. The expression of type X collagen is confined within hypertrophic condrocytes and precedes the embark of endochondral bone formation. Type X collagen facilitates endochondral ossification by regulating matrix mineralization and compartmentalizing matrix components.

CONCLUSION

Type X collagen is a reliable marker for new bone formation in articular cartilage. The future clinical application of this collagen in inducing or mediating endochondral ossification is perceived, e.g. the fracture healing of synovial joints and adaptive remodeling of madibular condyle.

Hyaluronidase expression in cultured growth plate chondrocytes during differentiation

Hyaluronan (HA) is a major component of the extracellular matrix of cartilage, contributes to its structural and functional integrity, and has various important roles in the differentiation of chondrocytes. HA metabolism is regulated by both anabolic and catabolic processes; however, the details have not yet been clarified. The purpose of this study was to clarify the expression patterns of hyaluronidase (HAase) mRNAs (from the relevant HAase genes: the HYALs) and HAase activity during chondrocyte differentiation. Cartilage tissue and growth plate chondrocytes were isolated from the ribs of 4-week-old male Japanese rabbits. The expression of HYAL mRNAs in cartilage was analyzed by in situ hybridization. The expression levels of HYAL mRNAs in the culture were analyzed for each of the chondrocyte differentiation stages by means of quantitative real-time polymerase chain reaction analysis. Enzymatic activity in the conditioned medium from the cultures was examined by using HA zymography and an enzyme-linked immunosorbent-like assay. The expression levels of HYAL1 and HYAL2 mRNAs were enhanced about 2.8-fold and 3.2-fold at the maximum during the early matrix forming stage, respectively, and by about 3.2-fold and 2.0-fold at the maximum in the hypertrophic stage, respectively. HYAL3 mRNA was not detected throughout the experimental period. HAase activity was enhanced at the early matrix forming and hypertrophic stages. These results suggest that selective expression of HYALs is essential for extracellular HA metabolism during chondrocyte differentiation.

Factors regulating mandibular condylar growth

Factors regulating condylar growth have not been identified before. This study was designed to identify a series of these factors, such as Sox 9 transcription factor and vascular endothelial growth factor (VEGF), and also to correlate the amount of type X collagen expressed during natural growth to the amount of bone newly formed. We used 115 Sprague-Dawley rats, 35 days old, in this study. The expression of these factors was identified on protein level by using immunostaining. Type X collagen was identified on mRNA and protein levels. Sox 9 was expressed by cells in the proliferative layer and by chondrocytes. Type X was expressed only by hypertrophic chondrocytes, and its expression precedes the onset of endochondral ossification. VEGF is expressed by hypertrophic chondrocytes, and its maximum level of expression precedes the maximum level of bone formation. Condylar growth involves a sequence of transitory stages uniquely defined by molecules that are intrinsically synthesized by cells in the condyles.

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.

The effects of dynamic axial loading on the rat growth plate

Longitudinal bone growth can be suppressed by compressive loading. In this study, we applied three different magnitudes (17, 8.5, and 4N) of compressive force to growing rat ulnas 10 minutes/day for 8 days and investigated the effects on the distal growth plate biology. Further, to investigate growth rate recovery after cessation of loading, we examined rats 7 days after the loading period. Longitudinal growth of the ulna was suppressed in a dose-dependent manner by applied compressive force. In the 17N group, the longitudinal mineralization rate (LMR) at the distal growth plate was suppressed completely by loading and did not recover. However, in the 8.5N and 4N groups, LMR suppression recovered in 1 week. In the 17N group, growth plate height and hypertrophic zone height were significantly greater than control; the number of hypertrophic chondrocytes was increased; and some traumatic changes such as cracks in the growth plate were found. In addition, 17N loading suppressed cartilage mineralization and capillary invasion beneath the growth plate, although the number of chondrocytes synthesizing vascular endothelial growth factor (VEGF) was increased. Our study shows longitudinal growth suppression caused by axial loading of the ulna, which is proportional to the magnitude of load. Only the largest load (17N) caused morphological changes in the distal growth plate cartilage. There was no association found between mineralization and type X collagen localization or capillary invasion and VEGF expression.

A dominant interference collagen X mutation disrupts hypertrophic chondrocyte pericellular matrix and glycosaminoglycan and proteoglycan distribution in transgenic mice

Collagen X transgenic (Tg) mice displayed skeleto-hematopoietic defects in tissues derived by endochondral skeletogenesis.(1) Here we demonstrate that co-expression of the transgene product containing truncated chicken collagen X with full-length mouse collagen X in a cell-free translation system yielded chicken-mouse hybrid trimers and truncated chicken homotrimers; this indicated that the mutant could assemble with endogenous collagen X and thus had potential for dominant interference. Moreover, species-specific collagen X antibodies co-localized the transgene product with endogenous collagen X to hypertrophic cartilage in growth plates and ossification centers; proliferative chondrocytes also stained diffusely. Electron microscopy revealed a disrupted hexagonal lattice network in the hypertrophic chondrocyte pericellular matrix in Tg growth plates, as well as altered mineral deposition. Ruthenium hexamine trichloride-positive aggregates, likely glycosaminoglycans (GAGs)/proteoglycans (PGs), were also dispersed throughout the chondro-osseous junction. These defects likely resulted from transgene co-localization and dominant interference with endogenous collagen X. Moreover, altered GAG/PG distribution in growth plates of both collagen X Tg and null mice was confirmed by a paucity of staining for hyaluronan and heparan sulfate PG. A provocative hypothesis links the disruption of the collagen X pericellular network and GAG/PG decompartmentalization to the potential locus for hematopoietic failure in the collagen X mice.

The roles of annexins and types II and X collagen in matrix vesicle-mediated mineralization of growth plate cartilage

Annexins II, V, and VI are major components of matrix vesicles (MV), i.e. particles that have the critical role of initiating the mineralization process in skeletal tissues. Furthermore, types II and X collagen are associated with MV, and these interactions mediated by annexin V stimulate Ca(2+) uptake and mineralization of MV. However, the exact roles of annexin II, V, and VI and the interaction between annexin V and types II and X collagen in MV function and initiation of mineralization are not well understood. In this study, we demonstrate that annexin II, V, or VI mediate Ca(2+) influx into phosphatidylserine (PS)-enriched liposomes, liposomes containing lipids extracted from authentic MV, and intact authentic MV. The annexin Ca(2+) channel blocker, K-201, not only inhibited Ca(2+) influx into fura-2-loaded PS-enriched liposomes mediated by annexin II, V, or VI, but also inhibited Ca(2+) uptake by authentic MV. Types II and X collagen only bound to liposomes in the presence of annexin V but not in the presence of annexin II or VI. Binding of these collagens to annexin V stimulated its Ca(2+) channel activities, leading to an increased Ca(2+) influx into the liposomes. These findings indicate that the formation of annexin II, V, and VI Ca(2+) channels in MV together with stimulation of annexin V channel activity by collagen (types II and X) binding can explain how MV are able to rapidly take up Ca(2+) and initiate the formation of the first crystal phase.

The alpha1(VIII) and alpha2(VIII) collagen chains form two distinct homotrimeric proteins in vivo

The short chain collagen variant, type VIII, is considered to be comprised of two distinct gene products, the alpha1 and alpha2 polypeptide chains. However, recent in vitro translation studies suggest that these chains can form homotrimers. We report here data from biochemical, immunohistochemical and molecular biological experiments, which together provide evidence that alpha1 and alpha2 polypeptides of type VIII collagen exist as homotrimers in cells and tissues. High-performance liquid chromatographic separation of type VIII collagen isolated from Descemet's membrane consistently demonstrated equimolar quantities of the two chains (alpha1:alpha2 1. 03+/-0.02 (S.E.M.); n=41). The availability of highly specific antibodies for the two polypeptides has assisted the in vivo characterisation of type VIII collagen. Immunoprecipitation of trimeric type VIII collagen from Descemet's membrane with purified anti-alpha1(VIII) and anti-alpha2(VIII) yielded fractions that contained only the alpha1(VIII) and alpha2(VIII) chains, respectively. Cultured human mesangial cells synthesised both polypeptides, but the alpha1(VIII) chain was found exclusively in the cell pellet, while the media contained only the alpha2(VIII) chain. The RNA from human mesangial cells and cornea showed message for both chains. However, in peritoneal fibroblast and mesothelial cell RNA, only alpha1(VIII) mRNA was detectable, demonstrating that the transcription of these two genes was not always co-ordinated. Immunohistochemistry showed that both polypeptides were present in cornea, optic nerve, aorta and umbilical cord but did not always co-localise. These results indicate the alpha1(VIII) and alpha2(VIII) chains preferentially form pepsin-resistant, homotrimeric molecules and so can exist as two distinct proteins.

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

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

Parathyroid receptor gene expression by epiphyseal growth plates in rickets and tibial dyschondroplasia

PTH/PTHrP receptor gene expression was evaluated in situ in avian epiphyseal growth plates taken from normal, rachitic and tibial dyschondroplasia (TD) afflicted chicks induced by thiram or by genetic selection. In the normal growth plates, PTH/PTHrP receptor gene expression was localized to the maturation zone as demonstrated by the expression of collagen type II (col II), osteopontin (OPN) genes and alkaline phosphatase activity (AP). In TD, either induced by thiram or by genetic selection, normal levels of PTH/PTHrP receptor gene expression were observed up to 21 days post-hatch. In rickets, on the other hand, no PTH/PTHrP receptor gene expression was observed in the growth plate from day 8 of a vitamin D-deficient diet. In cultured chondrocytes, PTH caused time-dependent down-regulation of its own receptor. These results suggest that alterations in the PTH/PTHrP receptor gene expression are associated with rickets but not with TD. The reduction in the PTH/PTHrP receptor gene expression in rickets may be due to the high plasma levels of PTH.

Restoration of normal bone development by human homologue of collagen type II (COL2A1) gene in Col2a1 null mice

Development of the vertebrate skeleton is a highly complex process in which collagen type II plays a vital role in the formation of long bones via endochondral ossification. Collagen type II, which is encoded by a single COL2A1/ Col2a1 gene, is the most abundant structural protein in the cartilage matrix, where it undergoes complex interactions with several other proteins. The sequence of mature collagen type II chains, each with about 1,100 amino acids, is conserved between different mammalian species. There are 37 amino acid positions that are different between mouse and human collagen type II. Previously, we have demonstrated that transgenic mice, in which Col2a1 gene is knocked out, exhibit a lethal phenotype due to the absence of endochondral bone formation. To investigate whether the biological role of collagen type II is conserved between the species, human COL2A1 gene was expressed in Col2a1 null mice by crossing with transgenic mice in which human COL2A1 gene was integrated. The collagen type II from human gene rescued the lethal phenotype in null mice, indicating that the biological function of collagen type II is conserved between human and mouse. The animals exhibited normal endochondral bone formation and a normal growth plate in tibio-tarsal joint. Chondrocytes isolated from the cartilage of these mice secreted human protein, suggesting that the animals incorporated heterologous protein to form cartilage which is essentially "humanized." The animals reached puberty and produced normal progeny. A completely normal phenotype in newborns indicates that human COL2A1 gene is expressed properly both temporally and spatially. These animals may be useful to generate models to study the effect of COL2A1 mutations on skeletal development in humans by introducing mutated gene constructs either into embryos or by crossing with transgenic animals with COL2A1 mutations.

Phenotypic and biochemical consequences of collagen X mutations in mice and humans

Skeletal biology has entered an exciting period with the technological advances in murine transgenesis and human genetics. This review focuses on how these two approaches are being used to address the role of collagen X, the major extracellular matrix component of the focal zone of endochondral ossification, the hypertrophic cartilage zone. The hypothesized role of this unique collagen in skeletal morphogenesis and the phenotypic and biochemical consequences resulting from the disruption of its function are discussed. Specifically, data from three murine models, including transgenic mice with a dominant interference phenotype for collagen X, and two sets of mice with an inactivated collagen X gene through gene targeting and homologous recombination, as well as the human disorder of Schmid metaphyseal chondrodysplasia resulting from mutations in collagen X, are summarized and compared. Several inconsistencies and unresolved issues regarding the murine and human phenotypes and the function of collagen X are discussed.

Inhibition of terminal differentiation and matrix calcification in cultured avian growth plate chondrocytes by Rous sarcoma virus transformation

Endochondral bone formation involves the progression of epiphyseal growth plate chondrocytes through a sequence of developmental stages which include proliferation, differentiation, hypertrophy, and matrix calcification. To study this highly coordinated process, we infected growth plate chondrocytes with Rous sarcoma virus (RSV) and studied the effects of RSV transformation on cell proliferation, differentiation, matrix synthesis, and mineralization. The RSV-transformed chondrocytes exhibited a distinct bipolar, fibroblast-like morphology, while the mock-infected chondrocytes had a typical polygonal morphology. The RSV-transformed chondrocytes actively synthesized extracellular matrix proteins consisting mainly of type I collagen and fibronectin. RSV-transformed cells produced much less type X collagen than was produced by mock-transformed cells. There also was a significant reduction of proteoglycan levels secreted in both the cell-matrix layer and culture media from RSV-transformed chondrocytes. RSV-transformed chondrocytes expressed two- to- threefold more matrix metalloproteinase, while expressing only one-half to one-third of the alkaline phosphatase activity of mock infected cells. Finally, RSV-transformed chondrocytes failed to calcify the extracellular matrix, while mock-transformed cells deposited high levels of calcium and phosphate into their extracellular matrix. These results collectively indicate that RSV transformation disrupts the preprogrammed differentiation pattern of growth plate chondrocytes and inhibit chondrocyte terminal differentiation and mineralization. They also suggest that the expression of extracellular matrix proteins, type II and type X collagens, and the cartilage proteoglycans are important for chondrocyte terminal differentiation and matrix calcification.

Multiple transcriptional elements in the avian type X collagen gene. Identification of Sp1 family proteins as regulators for high level expression in hypertrophic chondrocytes

During the cartilage-to-bone transition, participating chondrocytes eventually undergo hypertrophy and are replaced by bone and marrow. Type X collagen is synthesized by chondrocytes specifically when they become hypertrophic, and this specificity is primarily regulated at the level of transcription. Previously, we demonstrated that a proximal promoter region from nucleotide -562 to +86 contained cis-acting elements that directed high level expression of a reporter gene in a cell-specific manner (Long, F., and Linsenmayer, T. F. (1995) J. Biol. Chem. 270, 31310-31314). In the present study, we have further dissected this region by generating a series of constructs and examining their expression in hypertrophic versus nonhypertrophic chondrocytes. Several positive and negative elements have been delineated within the proximal promoter region to mediate the regulation of transcription in hypertrophic chondrocytes. Most notably, a sequence from nucleotide -139 to +5 was sufficient to direct high level expression in this cell type. Electrophoresis mobility shift assay and supershift experiments identified within this sequence two 10-base pair noncanonical binding sites for Sp1 proteins. Mutations within the Sp1 binding sites either diminished or abolished the expression driven by the sequence from -139 to +5. These results indicate that the Sp1 proteins mediate the cell-specific expression of type X collagen.

Generation of growth hormone binding protein by avian growth plate chondrocytes is dependent on cell differentiation

Growth hormone receptor (GH-R) gene expression was evaluated in avian growth-plates in situ and in cultured chondrocytes. In the epiphyseal growth-plate, chondrocytes at different stages of differentiation located at the proliferative and upper hypertrophic zones express the GH-R gene. In culture, addition of ascorbic acid facilitated chondrocyte differentiation as evaluated by decrease in collagen type II gene expression and increase in alkaline phosphatase activity and osteopontin gene expression. Both the ascorbic acid-treated and untreated chondrocytes expressed the gene coding for the chicken growth hormone receptor (cGH-R), but only the undifferentiated cells were capable of binding the hormone. This reduction in GH-binding resulted in alteration in GH-dependent regulation of the GH-R gene expression: only the undifferentiated chondrocytes responded to chicken GH (cGH) by down-regulation of the cGH-R gene expression. Chondrocyte differentiation induced by either ascorbic acid or retinoic acid was associated with the appearance of two growth hormone binding-proteins (GHBPs) in the culture medium with estimated MWs of 32 and 70 kDa, respectively. These GHBPs differ in their MW from the major GHBP found in chicken plasma. Chondrocyte GHBPs specifically bind [125I]cGH, which can be displaced by an excess of unlabeled cGH. The differentiation-dependent increase in the 70 kDa GHBP was observed also using specific chicken GHBP antiserum. Our data suggest that the reduction of the differentiated chondrocytes response to GH is due to differentiation-dependent loss of the extracellular domain of the GH-R, resulting in a lack of functional receptors on the cell surface and generation of GHBP.

Immunohistochemistry of collagen types II and X, and enzyme-histochemistry of alkaline phosphatase in the developing condylar cartilage of the fetal mouse mandible

We investigated the immunohistochemical localisation of types II and X collagen as well as the cytochemical localisation of alkaline phosphatase in the developing condylar cartilage of the fetal mouse mandible on d 14-16 of pregnancy. On d 14 of pregnancy, although no immunostaining for types II and X collagen was observed, alkaline phosphatase activity was detected in all cells in the anlage of the future condylar process. On d 15 of pregnancy, immunostaining for both collagen types was simultaneously detected in the primarily formed condylar cartilage. Alkaline phosphatase activity was also detected in chondrocytes at this stage. By d 16 of pregnancy, the hypertrophic cell zone rapidly increased in size. These findings strongly support a periosteal origin for the condylar cartilage of the fetal mouse mandible, and show that progenitor cells for condylar cartilage rapidly or directly differentiate into hypertrophic chondrocytes.

Role of type X collagen on experimental mineralization of eggshell membranes

Type X collagen is a transient and developmentally regulated collagen that has been postulated to be involved in controlling the later stages of endochondral bone formation. However, the role of this collagen in these events is not yet known. In order to understand the function of type X collagen, if any, in the process of biomineralization, the properties of type X collagen in eggshell membranes were further investigated. Specifically, calvaria-derived osteogenic cells were tested for their ability to mineralize eggshell membranes in vitro. Immunohistochemistry with specific monoclonal antibodies was used to correlate the presence or absence of type X collagen or its propeptide domains with the ability of shell membranes to be mineralized. The extent of mineralization was assessed by Von Kossa staining, scanning electron microscopy and energy-dispersive spectroscopy. The results indicate that the non-helical domains of type X collagen must be removed to facilitate the cell-mediated mineralization of eggshell membranes. In this tissue, intact type X collagen does not appear to stimulate or support cell-mediated mineralization. We postulate that the non-helical domains of type X collagen function in vivo to inhibit mineralization and thereby establish boundaries which are protected from mineral deposition.

Changes in the expression of type-X collagen in the fibrocartilage of rat Achilles tendon attachment during development

This study histologically and immunohistochemically demonstrated developmental changes in cell morphology and expression of type-X collagen in the attachment of the Achilles tendon to the calcaneus in the rat. Although the site of attachment in the mature rat showed a well organized, direct insertion that was composed of tendon, fibrocartilage, calcified fibrocartilage, and bone, this four-zone structure was not observed in the immature 1-week-old rat. Formation of fibrocartilage was observed at 2 weeks, together with the hypertrophy of chondrocytes and the appearance of the secondary center of ossification. Type-X collagen was not detected either in chondrocytes in the attachment area at 1 week or in hypertrophic chondrocytes at the attachment at 2 weeks. In the 3-week-old rat, the secondary center of ossification extended to the area of attachment and type-X collagen was detected both in cartilage spicules within the secondary center of ossification and in cells found at the attachment adjacent to the secondary center of ossification. A four-zone structure had been established by 6 weeks and remained through 20 weeks. After 6 weeks, type-X collagen was identified both in the attachment of the tendon and beneath the calcaneal fibrocartilage. Type-X collagen is produced by cells in transitional zones between calcified and noncalcified tissue, such as the interface between articular cartilage and subchondral bone. In these areas, the expression of this protein persists through maturity and is not transient.

Differential regulation of type-II and type-X collagen synthesis by parathyroid hormone-related protein in chick growth-plate chondrocytes

Parathyroid hormone-related protein is a critical autocrine regulator of endochondral ossification in the growth plate, as demonstrated by the severe disruption of growth-plate structure and function in parathyroid hormone-related protein-deficient transgenic mice. In the present study, the effects of parathyroid hormone-related protein on the synthesis of collagen mRNA and protein were studied in short-term cultures of isolated chick growth-plate chondrocytes. Parathyroid hormone-related protein selectively inhibits type-X collagen protein synthesis with no significant effect on type-II collagen protein synthesis. These effects were present in all maturationally distinct populations of chondrocytes separated by countercurrent centrifugal elutriation. In cultures of resting chondrocytes, the onset of type-X collagen expression was inhibited, while the synthesis of type-X collagen was decreased in cultures of hypertrophic chondrocytes. Synthesis of type-II and type-X collagen mRNA was examined by nonradioactive in situ hybridization with synthetic oligonucleotide cDNA probes, and the level of expression was quantified using digital image analysis. Dose-dependent suppression of type-X collagen gene expression by parathyroid hormone-related protein was observed, with no significant effect on type-II collagen mRNA detected. The results were confirmed by analysis of Northern blots of total chondrocyte mRNA. These experiments demonstrated differential transcriptional regulation of type-II and type-X collagen, with selective suppression of type-X collagen expression, by parathyroid hormone-related protein in growth-plate chondrocytes. In addition, excellent agreement was found between traditional protein and mRNA analyses and microscopic digital image analysis techniques, supporting the use of this convenient and sensitive assay method. Parathyroid hormone-related protein inhibits chondrocyte maturation and is known to stimulate proliferation, suggesting that this autocrine factor may function to regulate premature hypertrophy in the growth plate.

Abnormal compartmentalization of cartilage matrix components in mice lacking collagen X: implications for function

There are conflicting views on whether collagen X is a purely structural molecule, or regulates bone mineralization during endochondral ossification. Mutations in the human collagen alpha1 (X) gene (COL10A1) in Schmid metaphyseal chondrodysplasia (SMCD) suggest a supportive role. But mouse collagen alpha1 (X) gene (Col10a1) null mutants were previously reported to show no obvious phenotypic change. We have generated collagen X deficient mice, which shows that deficiency does have phenotypic consequences which partly resemble SMCD, such as abnormal trabecular bone architecture. In particular, the mutant mice develop coxa vara, a phenotypic change common in human SMCD. Other consequences of the mutation are reduction in thickness of growth plate resting zone and articular cartilage, altered bone content, and atypical distribution of matrix components within growth plate cartilage. We propose that collagen X plays a role in the normal distribution of matrix vesicles and proteoglycans within the growth plate matrix. Collagen X deficiency impacts on the supporting properties of the growth plate and the mineralization process, resulting in abnormal trabecular bone. This hypothesis would accommodate the previously conflicting views of the function of collagen X and of the molecular pathogenesis of SMCD.

Retarded skeletal development in transgenic mice with a type II collagen mutation

Transgenic mice harboring specific mutations provide a tool for systematic studies on the effects of dominant mutations on embryonic development and growth. In the present study, several different techniques were employed to follow the development of growth abnormalities and other phenotypic consequences in the mouse litters from matings of heterozygous Del1 mice carrying six copies of a mouse type II collagen transgene with a small engineered deletion mutation. Skeletal staining of complete litters with alcian blue/alizarin red S revealed that the onset of chondrogenesis and subsequent endochondral ossification were delayed in heterozygous and homozygous Del1 embryos. The lengths, widths, and shapes of long bones were all affected. Histological and in situ hybridization analyses revealed several types of abnormalities in the epiphyseal growth plates of homozygous Del1 embryos including disorganization of growth plate architecture, abnormal appositional growth activity, an increase in the number of hypertrophic chondrocytes, a deficiency in the formation of metaphyseal cancellous bone, and development of necrotic areas in the epiphyseal heads. Many of these findings parallel those seen in human chondrodysplasias. The basic information obtained on the effects of a specific deletion mutation in the type II collagen gene on the development and growth of the transgenic embryos provides a background for testing the efficacy of novel therapeutic strategies for prevention of such growth abnormalities.

Type X collagen is colocalized with a proteoglycan epitope to form distinct morphological structures in bovine growth cartilage

Using antibodies raised against type X collagen, isolated from deer antler, we have immunolocalized type X collagen in growth plate and epiphyseal cartilage from fetal and ambulatory calves. In ambulatory calf growth plate, type X collagen was demonstrated to be present in longitudinal septa that extend from the resting zone into the underlying trabecular bone. The much more restricted distribution of type X collagen seen in fetal growth plate, both here and previously, suggested that prominent localization of type X collagen to the longitudinal septa was stimulated by weight bearing. The location of type X collagen has been compared with that of an epitope (7D4), composed of specific sulfation patterns in chondroitin sulfate of some aggrecan molecules. Colocalization of type X collagen with the 7D4 epitope in longitudinal septa of growth plates from ambulatory calves suggested these molecules contribute to the formation and function of longitudinal septa. Immunolocalization of type X collagen and 7D4 in calf epiphyseal cartilage demonstrated colocalization at an interface that appears to represent the junction of articular cartilage and cartilage associated with formation of the secondary ossification center. Immunolocalization of type X collagen and the 7D4 proteoglycan epitope in these structures suggested that type X collagen and the 7D4 epitope form structures with important mechanical roles and capacity to influence the morphology of associated bony structures.

Chondrogenesis in the regenerating antler tip in red deer: expression of collagen types I, IIA, IIB, and X demonstrated by in situ nucleic acid hybridization and immunocytochemistry

The annual regrowth of antlers in male deer is a unique example of complete bone regeneration occurring in an adult animal. Growth is initiated at the distal antler tip, which is similar to the epiphyseal growth plate in some respects. However, there is some debate as to whether this process represents "true" endochondral ossification. As part of the characterization of the developmental process in pre-osseus antler tissue, we have studied, by in situ hybridization, the spatial expression of mRNAs for types I, II, and X collagen. Viewed in a coronal plane, type I procollagen mRNA was observed in skin, the fibrous perichondrium, and the densely cellular area immediately adjacent to the perichondrium. Below this area, as cells began to assume a columnar arrangement and coincident with the appearance of a vasculature and synthesis of a cartilaginous matrix, transcripts for types I, IIA, IIB procollagen and X collagen were detected. Further down in the cartilage zone, the pattern of type I procollagen mRNA expression was altered. Here, the signal was detected only in a morphologically distinct subpopulation of small, flattened cells within the intercellular matrix at the periphery of the columns of chondrocytes. The alternative splice form of type II procollagen mRNA (IIA), characteristic of chondroprogenitor cells (Sandell et al. [1991] J. Cell Biol. 114:1307-1319), was expressed by a subset of cells in the upper region of the columns, indicating that this zone contains a population of prechondrocytic cells. Positive hybridization to type IIA was most abundant in these cells. In contrast, transcripts for the other procollagen splice form (IIB) and type X collagen were expressed by chondrocytes throughout the whole of the cartilage region studied. The translation and export of type II collagen and type X collagen were confirmed by detecting specific immunoreactivity for each. The spatial distribution of immunoreactivity for collagen types II and X was consistent with that of corresponding mRNAs. These data demonstrate for the first time the distinct pattern of expression of genes for major cartilage matrix macromolecules, the expression of the differentially spliced form of type II procollagen mRNA (IIA), and specifically the co-localization of types II and X collagen in the developing antler tip. Taken together, they strongly indicate that antler growth involves an endochondral process.

Localization of collagens and alkaline phosphatase activity during mineralization and ossification of human first rib cartilage

The localization of type X collagen and alkaline phosphatase activity was examined in order to gain a better understanding of tissue remodelling during development of human first rib cartilage. First rib cartilages from children and adolescents showed no staining for type X collagen and alkaline phosphatase activity. After onset of mineralization in the late second decade, a peripheral ossification process preceded by mineralized fibrocartilage could be distinguished from a more central one preceded by mineralized hyaline cartilage. No immunostaining for type X collagen was found in either type of cartilage. However, strong staining for alkaline phosphatase activity was detected around chondrocyte-like cells within fibrocartilage adjacent to the peripheral mineralization front, while a weaker staining pattern was observed around chondrocytes of hyaline cartilage near the central mineralization front. In addition, the territorial matrix of some chondrocytes within the hyaline cartilage revealed staining for type I collagen, suggesting that these cells undergo a dedifferentiation process, which leads to a switch from type II to type I collagen synthesis. The study provides evidence that mineralization of the hyaline cartilage areas in human first rib cartilage occurs in the absence of type X collagen synthesis but in the presence of alkaline phosphatase. Thus, mineralization of first rib cartilage seems to follow a different pattern from endochondral ossification in epiphyseal discs.

In serum-free culture thyroid hormones can induce full expression of chondrocyte hypertrophy leading to matrix calcification

A serum-free culture system has been developed to examine the biologic factors involved in the regulation of cellular maturation, extracellular matrix assembly, and calcification in the physis of the bovine fetal growth plate. Isolated prehypertrophic chondrocytes in high density culture undergo a process of cellular maturation whereby full expression of the hypertrophic phenotype is characterized first by type X collagen synthesis followed by matrix calcification. Using this culture system, we compared the capacity of tri-iodothyronine (T3) with thyroxine (T4) to stimulate expression of the hypertrophic phenotype and matrix calcification in three (B, C, and D) maturationally distinct prehypertrophic chondrocyte subpopulations. The B cell subpopulation was the most mature followed by C and D subpopulations in order of decreasing maturity. Comparisons were made to cultures in fetal calf serum (FCS). In Dulbecco's modified Eagle's medium supplemented with insulin, transferrin, and selenium, both hormones (T3/T4) separately induced, in a dose-dependent manner, chondrocyte maturation to the hypertrophic phenotype characterized by increased type X collagen mRNA and induction of protein synthesis of this molecule, together with increased alkaline phosphatase activity, and eventually calcification of the extracellular matrix. Such cellular maturation to the hypertrophic phenotype was not observed in the absence of T3 or T4 with subpopulations C and D. Only in older fetuses (> 210 days) was this observed and then only in the B subpopulation. Furthermore, T3 was at least 50-fold more potent than T4. The effects of T3 were most pronounced with the most immature cells (subpopulations C and D) where, in the case of the subpopulation C, in contrast to 0.5 nM T3 50 nM T4 was unable to induce expression of the hypertrophic phenotype. Alkaline phosphatase activity was also increased in the C cell subpopulation treated with 1 nM T3 (35.5 U/micrograms of DNA) over that supplemented with 50 nM T4 (7.8 U/micrograms of DNA). Furthermore, matrix calcification, measured by the incorporation of 45Ca2+ into the cell layer, always occurred earlier in cells cultured with T3 compared with T4. Cellular maturation to the hypertrophic phenotype was not accompanied by significant changes in DNA content; this ordinarily increases during culture in the presence of serum. Compared with cells cultured in the presence of serum, either thyroid hormone more potently induced cellular maturation. This study demonstrates that the most immature chondrocytes at the prehypertrophic stage are direct targets for T3 and T4 and, to a much a lesser degree, that either hormone is able to induce full chondrocyte hypertrophy from an early maturational stage leading to matrix calcification. But T3 is much more potent than T4. These studies also offer a new serum-free chemically defined medium containing T3 or T4 for the culture of defined prehypertrophic chondrocytes that supports matrix assembly, hypertrophic expression, followed by matrix calcification.

Tissue-specific regulation of the type X collagen gene. Analyses by in vivo footprinting and transfection with a proximal promoter region

During endochondral bone formation, hypertrophic chondrocytes initiate synthesis of type X collagen. Previous studies have shown that regulation of this molecule is at the level of transcription. To further explore this regulation, we have studied a segment of the type X collagen gene extending from 562 base pairs (bp) upstream to 86 bp downstream of the transcriptional start site. We have studied this "proximal promoter region" by both structural analysis by DNase I in vivo footprinting and functional analysis by transient transfections. In type X collagen-expressing, hypertrophic chondrocytes, in vivo footprinting detected a fully protected TATA region flanked by hypersensitive sites but no other major protection. Type X collagen-negative cells (nonhypertrophic chondrocytes and tendon fibroblasts) showed major protection at a number of other sites, most notably an 8-bp region overlapping an AP2 site and a 9-bp region including the sequence CACACA. The importance of the proximal promoter region in restricting expression of type X collagen to hypertrophic chondrocytes was supported by transfection studies. A chloramphenicol acetyltransferase construct containing this region directed 5-10-fold higher chloramphenicol acetyltransferase expression in hypertrophic chondrocytes than in the other cell types. A 2.6-kilobase upstream fragment produced no additional effect. Thus, the proximal promoter region contains at least some regulatory elements for the cell-specific expression of type X collagen.

Vitamin E stimulates trabecular bone formation and alters epiphyseal cartilage morphometry

The effects of dietary vitamin E (VIT E) and lipids on tissue peroxidation and fatty acid composition, epiphyseal growth plate cartilage development, and trabecular bone formation were evaluated in chicks. A 2 x 2 factorial design was followed using two levels (30 and 90 IU/kg of diet) of dl-alpha-tocopheryl acetate and two different dietary lipids. The basal semipurified diet contained one of the following lipid treatments: anhydrous butter oil (40 g/kg) + soybean oil (60 g/kg), [BSO], or soybean oil (100 g/kg), [SBO]. After 14 days of feeding, the level of alpha tocopheryl in plasma was higher and thiobarbituric acid reactive substances (TBARS) were less in plasma and liver of chicks supplemented with 90 IU of VIT E compared with those given 30 IU of VIT E. Body weights and tibiotarsal bone lengths were not affected by the dietary treatments. Saturated fatty acids (14:0, 15:0, 16:0, 17:0, and 18:0) were increased in the tibiotarsal bone of chicks fed the BSO diet. In contrast, total polyunsaturated fatty acids and the ratio unsaturated fatty acids/saturated fatty acids were higher is plasma of chicks fed SBO compared with the values from chicks fed BSO. The thickness of the entire growth plate cartilage and the lower hypertrophic chondrocyte zone was significantly greater in chicks fed 90 IU/kg of VIT E. Kinetic parameters on bone histomorphometry indicated that mineral apposition rate was higher in chicks fed 90 IU of VIT E. The interaction effect between the VIT E and BSO treatments led to the highest trabecular bone formation rate among the groups. These data suggest that VIT E protects against cellular lipid peroxidation in cartilage to sustain normal bone growth and modeling.

Biosynthesis and characterization of type X collagen in human fetal epiphyseal growth plate cartilage

We examined in vitro collagen biosynthesis by organ cultures from human fetal epiphyseal growth plate cartilage. The biosynthetic products were characterized by NaCl fractional precipitation, limited proteolytic digestion, and sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis. Organ cultures of human fetal epiphyseal growth plate cartilage synthesized large amounts of type X collagen in addition to type II, type IX, and type XI collagens. The individual polypeptide chains of human type X collagen migrated with an apparent M(r) of 45 kDa after proteolytic digestion with pepsin. The migration pattern of these molecules did not change when examined under reducing and nonreducing conditions, indicating that they did not contain intrahelical disfulfide bonds. Comparison of the rates at type X collagen biosynthesis at weeks 20 and 24 of human fetal development showed a marked increase of 24 weeks. Northern hybridization analysis of total RNA from freshly isolated epiphyseal growth plate chondrocytes with a cDNA corresponding to the carboxyl terminus of human type X collagen indicated that the developmental increase of type X collagen production is determined by pre-translational mechanisms.

Areas of asbestoid (amianthoid) fibers in human thyroid cartilage characterized by immunolocalization of collagen types I, II, IX, XI and X

The distribution of type I, II, IX, XI and X collagens in and close to areas of asbestoid (amianthoid) fibers in thyroid cartilages of various ages was investigated in this study. Asbestoid fibers were first detected in thyroid cartilage from a 3-year-old male child. Areas of asbestoid fibers functionally appear to serve as guide rails for vascularization of thyroid cartilage. Alcian blue staining in the presence of 0.3 M MgCl2 revealed a loss of glycosaminoglycans in areas of asbestoid fibers. In addition, the fibers reacted positively with antibodies against collagen types II, IX and XI, but showed no staining with antibodies to collagen types I and X. Territorial matrix of adjacent chondrocytes showed the same staining pattern. In addition to staining for type II, IX and XI collagens, asbestoid fibers showed strong immunostaining for type I collagen after puberty but not for type X collagen. However, groups of chondrocytes within areas of asbestoid fibers reacted strongly with antibodies to type X collagen, suggesting that this collagen plays an important role in matrix of highly differentiated chondrocytes. The finding that these type X collagen-positive chondrocytes also revealed immunostaining for type I collagen confirms previous studies showing that hypertrophic chondrocytes can further differentiate into cells that are characterized by the synthesis of type X and I collagens.

Expression of bone morphogenetic protein-6 messenger RNA in bovine growth plate chondrocytes of different size

Bone morphogenetic protein-6 (BMP-6) is an osteoinductive factor that may have a regulatory role in the terminal differentiation of chondrocytes. We investigated the expression of BMP-6 messenger RNA in freshly isolated newborn calf rib chondrocytes separated by density gradient centrifugation into five highly enriched subpopulations at different stages of maturation as assessed by cell size and alkaline phosphatase activity. Expression of BMP-6 mRNA was compared with expression of type II collagen mRNA and type X collagen mRNA using Northern analysis. Type X collagen mRNA expression increased with increasing cell size whereas type II collagen mRNA varied little with cell size. BMP-6 mRNA expression was highest in small cells and lowest in the largest cells, which were maximally expressing type X collagen mRNA. This suggests that up-regulation of the BMP-6 gene may precede chondrocyte hypertrophy.

Chondrocyte differentiation

Data obtained while investigating growth plate chondrocyte differentiation during endochondral bone formation both in vivo and in vitro indicate that initial chondrogenesis depends on positional signaling mediated by selected homeobox-containing genes and soluble mediators. Continuation of the process strongly relies on interactions of the differentiating cells with the microenvironment, that is, other cells and extracellular matrix. Production of and response to different hormones and growth factors are observed at all times and autocrine and paracrine cell stimulations are key elements of the process. Particularly relevant is the role of the TGF-beta superfamily, and more specifically of the BMP subfamily. Other factors include retinoids, FGFs, GH, and IGFs, and perhaps transferrin. The influence of local microenvironment might also offer an acceptable settlement to the debate about whether hypertrophic chondrocytes convert to bone cells and live, or remain chondrocytes and die. We suggest that the ultimate fate of hypertrophic chondrocytes may be different at different microanatomical sites.

Fibre reinforcing by collagen in cartilage and soft connective tissues

Mechanical support in animals is performed by connective tissues. The soft tissues consist of collagen fibres embedded in a highly hydrated proteoglycan gel. By considering them as fibrous composite materials, a unifying theme can be found to explain their mechanical behaviour in terms of their structure and composition. Interactions between fibres and matrix are essential to their functioning in this way. Calculations are made of the maximum stress transfer per D-period required to enable collagen fibres of a given axial ratio to provide effective reinforcing. Weak non-specific interactions are shown to be sufficient. A mechanical function is proposed for type X collagen in the epiphyseal growth plate on the basis that it modifies and supplements the properties of the type II fibrils. This provides extra reinforcing and, hence, a greater stiffness to the cartilage to compensate for the reduced amount of extracellular matrix.