Ca2+ binding properties of type X collagen

3D Printing for Bone-Cartilage Interface Regeneration

Due to the vasculature defects and/or the avascular nature of cartilage, as well as the complex gradients for bone-cartilage interface regeneration and the layered zonal architecture, self-repair of cartilage and subchondral bone is challenging. Currently, the primary osteochondral defect treatment strategies, including artificial joint replacement and autologous and allogeneic bone graft, are limited by their ability to simply repair, rather than induce regeneration of tissues. Meanwhile, over the past two decades, three-dimension (3D) printing technology has achieved admirable advancements in bone and cartilage reconstruction, providing a new strategy for restoring joint function. The advantages of 3D printing hybrid materials include rapid and accurate molding, as well as personalized therapy. However, certain challenges also exist. For instance, 3D printing technology for osteochondral reconstruction must simulate the histological structure of cartilage and subchondral bone, thus, it is necessary to determine the optimal bioink concentrations to maintain mechanical strength and cell viability, while also identifying biomaterials with dual bioactivities capable of simultaneously regenerating cartilage. The study showed that the regeneration of bone-cartilage interface is crucial for the repair of osteochondral defect. In this review, we focus on the significant progress and application of 3D printing technology for bone-cartilage interface regeneration, while also expounding the potential prospects for 3D printing technology and highlighting some of the most significant challenges currently facing this field.

Inability of Low Oxygen Tension to Induce Chondrogenesis in Human Infrapatellar Fat Pad Mesenchymal Stem Cells

OBJECTIVE

Articular cartilage of the knee joint is avascular, exists under a low oxygen tension microenvironment, and does not self-heal when injured. Human infrapatellar fat pad-sourced mesenchymal stem cells (IFP-MSC) are an arthroscopically accessible source of mesenchymal stem cells (MSC) for the repair of articular cartilage defects. Human IFP-MSC exists physiologically under a low oxygen tension (i.e., 1-5%) microenvironment. Human bone marrow mesenchymal stem cells (BM-MSC) exist physiologically within a similar range of oxygen tension. A low oxygen tension of 2% spontaneously induced chondrogenesis in micromass pellets of human BM-MSC. However, this is yet to be demonstrated in human IFP-MSC or other adipose tissue-sourced MSC. In this study, we explored the potential of low oxygen tension at 2% to drive the in vitro chondrogenesis of IFP-MSC. We hypothesized that 2% O2 will induce stable chondrogenesis in human IFP-MSC without the risk of undergoing endochondral ossification at ectopic sites of implantation.

METHODS

Micromass pellets of human IFP-MSC were cultured under 2% O2 or 21% O2 (normal atmosphere O2) in the presence or absence of chondrogenic medium with transforming growth factor-β3 (TGFβ3) for 3 weeks. Following in vitro chondrogenesis, the resulting pellets were implanted in immunodeficient athymic nude mice for 3 weeks.

RESULTS

A low oxygen tension of 2% was unable to induce chondrogenesis in human IFP-MSC. In contrast, chondrogenic medium with TGFβ3 induced in vitro chondrogenesis. All pellets were devoid of any evidence of undergoing endochondral ossification after subcutaneous implantation in athymic mice.

The Thermodynamics of Medial Vascular Calcification

Medial vascular calcification (MVC) is a degenerative process that involves the deposition of calcium in the arteries, with a high prevalence in chronic kidney disease (CKD), diabetes, and aging. Calcification is the process of precipitation largely of calcium phosphate, governed by the laws of thermodynamics that should be acknowledged in studies of this disease. Amorphous calcium phosphate (ACP) is the key constituent of early calcifications, mainly composed of Ca2+ and PO4 3- ions, which over time transform into hydroxyapatite (HAP) crystals. The supersaturation of ACP related to Ca2+ and PO4 3- activities establishes the risk of MVC, which can be modulated by the presence of promoter and inhibitor biomolecules. According to the thermodynamic parameters, the process of MVC implies: (i) an increase in Ca2+ and PO4 3- activities (rather than concentrations) exceeding the solubility product at the precipitating sites in the media; (ii) focally impaired equilibrium between promoter and inhibitor biomolecules; and (iii) the progression of HAP crystallization associated with nominal irreversibility of the process, even when the levels of Ca2+ and PO4 3- ions return to normal. Thus, physical-chemical processes in the media are fundamental to understanding MVC and represent the most critical factor for treatments' considerations. Any pathogenetical proposal must therefore comply with the laws of thermodynamics and their expression within the medial layer.

Suppression of Hypertrophy During in vitro Chondrogenesis of Cocultures of Human Mesenchymal Stem Cells and Nasal Chondrocytes Correlates With Lack of in vivo Calcification and Vascular Invasion

Objective

Human nasal septal chondrocytes (NC) are a promising minimally invasive derivable chondrogenic cell source for cartilage repair. However, the quality of NC-derived cartilage is variable between donors. Coculture of NC with mesenchymal stem cells (MSCs) mitigates the variability but with undesirable markers of chondrocyte hypertrophy, such as type X collagen, and the formation of unstable calcifying cartilage at ectopic sites. In contrast, monoculture NC forms non-calcifying stable cartilage. Formation of a stable NC-MSC coculture cartilage is crucial for clinical application. The aim of this study was to explore the utility of parathyroid hormone-related peptide (PTHrP) hormone to suppress chondrocyte hypertrophy in NC-MSC cocultures and form stable non-calcifying cartilage at ectopic sites.

Methods

Human NC and bone marrow MSCs, and cocultures of NC and MSC (1:3 ratio) were aggregated in pellet form and subjected to in vitro chondrogenesis for 3 weeks in chondrogenic medium in the presence and absence of PTHrP. Following in vitro chondrogenesis, the resulting pellets were implanted in immunodeficient athymic nude mice for 3 weeks.

Results

Coculture of NC and MSC resulted in synergistic cartilage matrix production. PTHrP suppressed the expression of hypertrophy marker, type X collagen (COL10A1), in a dose-dependent fashion without affecting the synergism in cartilage matrix synthesis, and in vivo calcification was eradicated with PTHrP. In contrast, cocultured control (CC) pellets without PTHrP treatment expressed COL10A1, calcified, and became vascularized in vivo.

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.

Bone physiology as inspiration for tissue regenerative therapies

The development, maintenance of healthy bone and regeneration of injured tissue in the human body comprise a set of intricate and finely coordinated processes. However, an analysis of current bone regeneration strategies shows that only a small fraction of well-reported bone biology aspects has been used as inspiration and transposed into the development of therapeutic products. Specific topics that include inter-scale bone structural organization, developmental aspects of bone morphogenesis, bone repair mechanisms, role of specific cells and heterotypic cell contact in the bone niche (including vascularization networks and immune system cells), cell-cell direct and soluble-mediated contact, extracellular matrix composition (with particular focus on the non-soluble fraction of proteins), as well as mechanical aspects of native bone will be the main reviewed topics. In this Review we suggest a systematic parallelization of (i) fundamental well-established biology of bone, (ii) updated and recent advances on the understanding of biological phenomena occurring in native and injured tissue, and (iii) critical discussion of how those individual aspects have been translated into tissue regeneration strategies using biomaterials and other tissue engineering approaches. We aim at presenting a perspective on unexplored aspects of bone physiology and how they could be translated into innovative regeneration-driven concepts.

Stimulation of a calcified cartilage connecting zone by GDF-5-augmented fibrin hydrogel in a novel layered ectopic in vivo model

Tissue engineering approaches for reconstructing full-depth cartilage defects need to comprise a zone of calcified cartilage to tightly anchor cartilage constructs into the subchondral bone. Here, we investigated whether growth and differentiation factor-5-(GDF-5)-augmented fibrin hydrogel can induce a calcified cartilage-layer in vitro that seamlessly connects cartilage-relevant biomaterials with bone tissue. Human bone marrow stromal cells (BMSCs) were embedded in fibrin hydrogel and subjected to chondrogenesis with TGF-β with or without GDF-5 before constructs were implanted subcutaneously into SCID mice. A novel layered ectopic in vivo model was developed and GDF-5-augmented fibrin with BMSCs was used to glue hydrogel and collagen constructs onto bone disks to investigate formation of a calcified cartilage connecting zone. GDF-5 significantly enhanced ALP activity during in vitro chondrogenesis while ACAN and COL2A1 mRNA, proteoglycan-, collagen-type-II- and collagen-type-X-deposition remained similar to controls. Pellets pretreated with GDF-5 mineralized faster in vivo and formed more ectopic bone. In the novel layered ectopic model, GDF-5 strongly supported calcified cartilage formation that seamlessly connected with the bone. Pro-chondrogenic and pro-hypertrophic activity makes GDF-5-augmented fibrin an attractive bioactive hydrogel with high potential to stimulate a calcified cartilage connecting zone in situ that might promote integration of cartilage scaffolds with bone. Thus, GDF-5-augmented fibrin hydrogel promises to overcome poor fixation of biomaterials in cartilage defects facilitating their long-term regeneration. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2214-2224, 2018.

A micro-architecturally biomimetic collagen template for mesenchymal condensation based cartilage regeneration

The unique arcade-like orientation of collagen fibers enables cartilage to bear mechanical loads. In this study continuous-length aligned collagen threads were woven to emulate the interdigitated arcade structure of the cartilage. The weaving pattern provided a macropore network within which micromass cell pellets were seeded to take advantage of mesenchymal condensation driven chondrogenesis. Compression tests showed that the baseline scaffold had a modulus of 0.83±0.39MPa at a porosity of 80%. The modulus of pellet seeded scaffolds increased by 60% to 1.33±0.37MPa after 28days of culture, converging to the modulus of the native cartilage. The scaffolds displayed duress under displacement controlled low-cycle fatigue at 15% strain amplitude such that load reduction stabilized at 8% after 4500 cycles of loading. The woven structure demonstrated a substantial elastic recoil where 40% mechanical strain was close to completely recovered following unloading. A robust chondrogenesis was observed as evidenced by positive staining for GAGs and type II collagen and aggrecan. Dimethyl methylene blue and sircol assays showed GAGs and collagen productions to increase from 3.36±1.24 and 31.46±3.22 at day 3 to 56.61±12.12 and 136.70±12.29μg/μg of DNA at day 28 of culture. This woven collagen scaffold holds a significant potential for cartilage regeneration with shorter in vitro culture periods due to functionally sufficient mechanical robustness at the baseline. In conclusion, the mimicry of cartilage's arcade architecture resulted in substantial improvement of mechanical function while enabling one of the first pellet delivery platforms enabled by a macroporous network.

STATEMENT OF SIGNIFICANCE

Mesenchymal condensation is critical for driving chondrogenesis, making high density cell seeding a standard in cartilage tissue engineering. Efforts to date have utilized scaffold free delivery of MSCs in pellet form. This study developed a macroporous scaffold that is fabricated by weaving highly aligned collagen threads. The scaffold can deliver high density cell condensates while providing mechanical stiffness comparable to that of cartilage. The scaffold also mimicked the arcade-like orientation of collagen fibers in cartilage. A highly robust chondrogenesis was observed in this mesenchymal cell pellet delivery system. Baseline mechanical robustness of this scaffold system will enable delivery of cell pellets as early as three days.

TGF-ß1 enhances the BMP-2-induced chondrogenesis of bovine synovial explants and arrests downstream differentiation at an early stage of hypertrophy

Background

Synovial explants furnish an in-situ population of mesenchymal stem cells for the repair of articular cartilage. Although bone morphogenetic protein 2 (BMP-2) induces the chondrogenesis of bovine synovial explants, the cartilage formed is neither homogeneously distributed nor of an exclusively hyaline type. Furthermore, the downstream differentiation of chondrocytes proceeds to the stage of terminal hypertrophy, which is inextricably coupled with undesired matrix mineralization. With a view to optimizing BMP-2-induced chondrogenesis, the modulating influences of fibroblast growth factor 2 (FGF-2) and transforming growth factor beta 1 (TGF-ß1) were investigated.

Methodology/Principal Findings

Explants of bovine calf metacarpal synovium were exposed to BMP-2 (200 ng/ml) for 4 (or 6) weeks. FGF-2 (10 ng/ml) or TGF-ß1 (10 ng/ml) was introduced at the onset of incubation and was present either during the first week of culturing alone or throughout its entire course. FGF-2 enhanced the BMP-2-induced increase in metachromatic staining for glycosaminoglycans (GAGs) only when it was present during the first week of culturing alone. TGF-ß1 enhanced not only the BMP-2-induced increase in metachromasia (to a greater degree than FGF-2), but also the biochemically-assayed accumulation of GAGs, when it was present throughout the entire culturing period; in addition, it arrested the downstream differentiation of cells at an early stage of hypertrophy. These findings were corroborated by an analysis of the gene- and protein-expression levels of key cartilaginous markers and by an estimation of individual cell volume.

Conclusions/Significance

TGF-ß1 enhances the BMP-2-induced chondrogenesis of bovine synovial explants, improves the hyaline-like properties of the neocartilage, and arrests the downstream differentiation of cells at an early stage of hypertrophy. With the prospect of engineering a mature, truly articular type of cartilage in the context of clinical repair, our findings will be of importance in fine-tuning the stimulation protocol for the optimal chondrogenic differentiation of synovial explants.

Matrix recruitment and calcium sequestration for spatial specific otoconia development

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

Expression of cartilage-specific markers in calcified and non-calcified atherosclerotic lesions

Recently, molecular mechanisms resembling endochondral ossification were suggested to be important for atherosclerotic vessel calcification. The aim of this study was to investigate in a series of human atherosclerotic (non-diabetic) lesions of the crural arteries the distribution and expression of classical marker genes of the endochondral ossification pathway. Immunostaining for marker proteins S-100 protein and collagen types II and X were performed on atherosclerotic lesions of different grades (according to Stary). Quantitative real-time PCR for human COL1A1, COL2A1, COL10A1, SOX9, and BMP-2 was applied on RNA isolated from atherosclerotic arteries. In most samples, no expression of collagen type II and S-100 protein was found. Exceptionally, S-100 protein and type II collagen expression was observed very focally within advanced atherosclerotic plaques. Type X collagen was not detected in any of the lesions investigated. Overall, in our study we found no evidence that chondrogenic differentiation pathways are generally active in atherosclerotic plaque formation. In particular type X collagen, one important molecule in cartilage calcification, was not expressed in any of the investigated specimens. Occasionally, however, chondrocytic differentiation markers occur within atherosclerotic lesions. This most likely represents a metaplastic event associated, but not causative for atherosclerotic vessel degeneration and calcification.

Premature induction of hypertrophy during in vitro chondrogenesis of human mesenchymal stem cells correlates with calcification and vascular invasion after ectopic transplantation in SCID mice

OBJECTIVE

Functional suitability and phenotypic stability of ectopic transplants are crucial factors in the clinical application of mesenchymal stem cells (MSCs) for articular cartilage repair, and might require a stringent control of chondrogenic differentiation. This study evaluated whether human bone marrow-derived MSCs adopt natural differentiation stages during induction of chondrogenesis in vitro, and whether they can form ectopic stable cartilage that is resistant to vascular invasion and calcification in vivo.

METHODS

During in vitro chondrogenesis of MSCs, the expression of 44 cartilage-, stem cell-, and bone-related genes and the deposition of aggrecan and types II and X collagen were determined. Similarly treated, expanded articular chondrocytes served as controls. MSC pellets were allowed to differentiate in chondrogenic medium for 3-7 weeks, after which the chondrocytes were implanted subcutaneously into SCID mice; after 4 weeks in vivo, samples were evaluated by histology.

RESULTS

The 3-stage chondrogenic differentiation cascade initiated in MSCs was primarily characterized by sequential up-regulation of common cartilage genes. Premature induction of hypertrophy-related molecules (type X collagen and matrix metalloproteinase 13) occurred before production of type II collagen and was followed by up-regulation of alkaline phosphatase activity. In contrast, hypertrophy-associated genes were not induced in chondrocyte controls. Whereas control chondrocyte pellets resisted calcification and vascular invasion in vivo, most MSC pellets mineralized, in spite of persisting proteoglycan and type II collagen content.

CONCLUSION

An unnatural pathway of differentiation to chondrocyte-like cells was induced in MSCs by common in vitro protocols. MSC pellets transplanted to ectopic sites in SCID mice underwent alterations related to endochondral ossification rather than adopting a stable chondrogenic phenotype. Further studies are needed to evaluate whether a more stringent control of MSC differentiation to chondrocytes can be achieved during cartilage repair in a natural joint environment.

Determinants of pathological mineralization

PURPOSE OF REVIEW

Physiological mineralization is necessary for the formation of skeletal tissues and for their appropriate functions during adulthood. Pathological or ectopic mineralization of soft tissues, including articular cartilage and cardiovascular tissues, leads to morbidity and mortality. Recent findings suggest that the mechanisms and factors regulating physiological mineralization may be identical or similar to those regulating ectopic mineralization. Therefore, the purpose of this review is to describe the current knowledge of mechanisms and determinants that regulate physiological mineralization and how these determinants can be used to understand ectopic mineralization better.

RECENT FINDINGS

Recent findings have indicated that physiological and pathological mineralization are initiated by matrix vesicles, membrane-enclosed particles released from the plasma membrane of mineralization-competent cells. An understanding of how these vesicles initiate the physiological mineralization process may provide novel therapeutic strategies to prevent ectopic mineralization. In addition, other regulators (activators and inhibitors) of physiological mineralization have been identified and characterized, and evidence indicates that the same factors also contribute to the regulation of ectopic mineralization. Finally, programmed cell death (apoptosis) may be a contributor to physiological mineralization, and if occurring after tissue injury may induce ectopic mineralization and mineralization-related differentiation events in the injured area and surrounding areas.

SUMMARY

This review describes how the understanding of mechanisms and factors regulating physiological mineralization can be used to develop new therapeutic strategies to prevent pathological or ectopic mineralization events.

Assembly of the otoconia complex to the macular sensory epithelium of the vestibule

In the inner ear, specificity of stimulus perception is achieved by associating the sensory epithelia of the three mechanoreceptor organs, the utricle/saccule, cristae, and cochlea, with distinct types of acellular matrices. Only the utricle and saccule have an extremely dense matrix, the otoconial complex, which overlies the sensory epithelium (macula) and provides inertial mass to generate shearing forces essential for the mechanoreceptors to sense gravity and linear acceleration. Such sensation is necessary for spatial orientation and balance. The importance of otoconia is clearly demonstrated by the impact of balance disorders upon the elderly population that involve otoconia degeneration, as well as by canalithiasis and cupulolithiasis, in which otoconia are dislocated. This underscores the need to understand how otoconia are formed and maintained and how to prevent their degeneration. To date, a number of otoconia-related proteins have been identified mostly in mice and bony fish. Although most of these proteins are also present in other structures of the inner ear, a distinct collection of proteins in the macula plus the unique ionic microenvironment of the endolymph near its epithelium likely contribute to the site-specific calcification of otoconia. Based on the current literature and ongoing research, this mini-review postulates a working model of how the otoconia complex is assembled specifically above the macular sensory epithelium of the vestibule. The central hypothesis of this model is that proteins are critical in sequestering calcium for crystallization in the calcium-poor endolymph. The review also sets forth some issues that need to be resolved in the future.

Otolith matrix proteins OMP-1 and Otolin-1 are necessary for normal otolith growth and their correct anchoring onto the sensory maculae

Fish otoliths are highly calcified concretions deposited in the inner ear and serve as a part of the hearing and balance systems. They consist mainly of calcium carbonate and a small amount of organic matrix. The latter component is considered to play important roles in otolith formation. Previously, we identified two major otolith matrix proteins, OMP-1 (otolith matrix protein-1) and Otolin-1, from salmonid species. To assess the function of these proteins in otolith formation, we performed antisense morpholino oligonucleotide (MO)-mediated knockdown of omp-1 and otolin-1 in zebrafish embryos. We first identified zebrafish cDNA homologs of omp-1 (zomp-1) and otolin-1 (zotolin-1). Whole-mount in situ hybridization then revealed that the expression of both zomp-1 and zotolin-1 mRNAs is restricted to the otic vesicles. zomp-1 mRNA was expressed from the 14-somite stage in the otic placode, but the zOMP-1 protein was detected only from 26-somite stage onwards. In contrast, zotolin-1 mRNA expression became clear around 72 hpf. MOs designed to inhibit zomp-1 and zotolin-1 mRNA translation, respectively, were injected into 1-2 cell stage embryos. zomp-1 MO caused a reduction in otolith size and an absence of zOtolin-1 deposition, while zotolin-1 MO caused a fusion of the two otoliths, and an increased instability of otoliths after fixation. We conclude that zOMP-1 is required for normal otolith growth and deposition of zOtolin-1 in the otolith, while zOtolin-1, a collagenous protein, is involved in the correct arrangement of the otoliths onto the sensory epithelium of the inner ear and probably in stabilization of the otolith matrix.

Site-specific immunostaining for type X collagen in noncalcified articular cartilage of canine stifle knee joint

Type X collagen is a short-chain collagen that is strongly expressed in hypertrophic chondrocytes. In this study, we used an immunohistochemical technique exploiting a prolonged hyaluronidase unmasking of type X collagen epitopes to show that type X collagen is not restricted to calcified cartilage, but is also present in normal canine noncalcified articular cartilage. A 30 degrees valgus angulation procedure of the right tibia was performed in 15 dogs at the age of 3 months, whereas their nonoperated sister dogs served as controls. Samples were collected 7 and 18 months after the surgery and immunostained for type X collagen. The deposition of type X collagen increased during maturation from age 43 weeks to 91 weeks. In the patella, most of the noncalcified cartilage stained for type X collagen, whereas, in the patellar surface of the femur, it was present mainly in the femoral groove close to cartilage surface. In femoral condyles, the staining localized mostly in the superficial cartilage on the lateral and medial sides, but not in the central weight-bearing area. In tibial condyles, type X collagen was often observed close to the cartilage surface in medial parts of the condyles, although staining could also be seen in the deep zone of the cartilage. Staining for type X collagen appeared strongest at sites where the birefringence of polarized light was lowest, suggesting a colocalization of type X collagen with the collagen fibril arcades in the intermediate zone. No significant difference in type X collagen immunostaining was observed in lesion-free articular cartilage between controls and dogs that underwent a 30 degrees valgus osteotomy. In osteoarthritic lesions, however, there was strong immunostaining for both type X collagen and collagenase-induced collagen cleavage products. The presence of type X collagen in the transitional zone of cartilage in the patella, femoropatellar groove, and in tibial cartilage uncovered by menisci suggests that it may involve a modification of collagen fibril arrangement at the site of collagen fibril arcades, perhaps providing additional support to the collagen network.

Interaction properties of the procollagen C-proteinase enhancer protein shed light on the mechanism of stimulation of BMP-1

Procollagen C-proteinase enhancer (PCPE) is an extracellular matrix glycoprotein that binds to the C-propeptide of procollagen I and can enhance the activities of procollagen C-proteinases up to 20-fold. To determine the molecular mechanism of PCPE activity, the interactions of the recombinant protein with the procollagen molecule as well as with its isolated C-propeptide domain were studied using surface plasmon resonance (BIAcore) technology. Binding required the presence of divalent metal cations such as calcium and manganese. By ligand blotting, calcium was found to bind to the C-propeptide domains of procollagens I and III but not to PCPE. By chemical cross-linking, the stoichiometry of the PCPE/C-propeptide interaction was found to be 1:1 in accordance with enzyme kinetic data. The use of a monoclonal antibody directed against the N-terminal region of the C-propeptide suggested that this region is probably not involved in binding to PCPE. Association and dissociation kinetics of the C-propeptide domains of procollagens I and III on immobilized PCPE were rapid. Extrapolation to saturation equilibrium yielded apparent equilibrium dissociation constants in the range 150-400 nM. In contrast, the association/dissociation kinetics of intact procollagen molecules on immobilized PCPE were relatively slow, corresponding to a dissociation constant of 1 nM. Finally, pN-collagen (i.e. procollagen devoid of the C-terminal propeptide domain) was also found to bind to immobilized PCPE, suggesting that PCPE binds to sites on either side of the procollagen cleavage site, thereby facilitating the action of procollagen C-proteinases.

Insight into Schmid metaphyseal chondrodysplasia from the crystal structure of the collagen X NC1 domain trimer

Collagen X is expressed specifically in the growth plate of long bones. Its C1q-like C-terminal NC1 domain forms a stable homotrimer and is crucial for collagen X assembly. Mutations in the NC1 domain cause Schmid metaphyseal chondrodysplasia (SMCD). The crystal structure at 2.0 A resolution of the human collagen X NC1 domain reveals an intimate trimeric assembly strengthened by a buried cluster of calcium ions. Three strips of exposed aromatic residues on the surface of NC1 trimer are likely to be involved in the supramolecular assembly of collagen X. Most internal SMCD mutations probably prevent protein folding, whereas mutations of surface residues may affect the collagen X suprastructure in a dominant-negative manner.

Matrix gene expression analysis and cellular phenotyping in chordoma reveals focal differentiation pattern of neoplastic cells mimicking nucleus pulposus development

Chordoma is the fourth most common malignant primary neoplasm of the skeleton and almost the only one showing a real epithelial phenotype. Besides classic chordoma, so-called chondroid chordoma was described as a specific entity showing cartilage-like tissue within chordomatoid structures. However, since its first description, strongly conflicting results have been reported about the existence of chondroid chordoma and several studies suggested chondroid chordomas being in fact low-grade conventional chondrosarcomas. In the present study, we used cytoprotein expression profiling and molecular in situ localization techniques of marker gene products indicative of developmental phenotypes of chondrocytes to elucidate origin and biology of chondroid chordoma. We were able to demonstrate the chondrogenic potential of chordomas irrespectively of the appearance of overt cartilage formation by identifying the multifocal expression of type II collagen, the main marker of chondrocytic differentiation. Additionally, the cartilage-typical large aggregating proteoglycan aggrecan was present throughout all chordomas and, thus, a very characteristic gene product and marker of these neoplasms. Biochemical matrix composition and cell differentiation pattern analysis showed a high resemblance of classic chordomas and in chordoid areas of chondroid chordomas to the fetal chorda dorsalis, whereas chondroid areas of chondroid chordomas showed features similar to adult nucleus pulposus. This demonstrates on the cell function level the chondrocytic differentiation potential of neoplastic chordoid cells as a characteristic facet of chordomas, mimicking fetal vertebral development, ie, the transition of the chorda dorsalis to the nucleus pulposus. Our study firmly establishes a focal real chondrocytic phenotype of neoplastic cells in chordomas. Chondroid chordoma is neither a low-grade chondrosarcoma nor a misnomer as discussed previously.

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.

Plasma transglutaminase in hypertrophic chondrocytes: expression and cell-specific intracellular activation produce cell death and externalization

We previously used subtractive hybridization to isolate cDNAs for genes upregulated in chick hypertrophic chondrocytes (Nurminskaya, M. , and T.F. Linsenmayer. 1996. Dev. Dyn. 206:260-271). Certain of these showed homology with the "A" subunit of human plasma transglutaminase (factor XIIIA), a member of a family of enzymes that cross-link a variety of intracellular and matrix molecules. We now have isolated a full-length cDNA for this molecule, and confirmed that it is avian factor XIIIA. Northern and enzymatic analyses confirm that the molecule is upregulated in hypertrophic chondrocytes (as much as eightfold). The enzymatic analyses also show that appreciable transglutaminase activity in the hypertrophic zone becomes externalized into the extracellular matrix. This externalization most likely is effected by cell death and subsequent lysis-effected by the transglutaminase itself. When hypertrophic chondrocytes are transfected with a cDNA construct encoding the zymogen of factor XIIIA, the cells convert the translated protein to a lower molecular weight form, and they initiate cell death, become permeable to macromolecules and eventually undergo lysis. Non-hypertrophic cells transfected with the same construct do not show these degenerative changes. These results suggest that hypertrophic chondrocytes have a novel, tissue-specific cascade of mechanisms that upregulate the synthesis of plasma transglutaminase and activate its zymogen. This produces autocatalytic cell death, externalization of the enzyme, and presumably cross-linking of components within the hypertrophic matrix. These changes may in turn regulate the removal and/or calcification of this hypertrophic matrix, which are its ultimate fates.

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.

Type X collagen and other up-regulated components of the avian hypertrophic cartilage program

Elucidating the cellular and molecular processes involved in growth and remodeling of skeletal elements is important for our understanding of congenital limb deformities. These processes can be advantageously studied in the epiphyseal growth zone, the region in which all of the increase in length of a developing long bone is achieved. Here, young chondrocytes divide, mature, become hypertrophic, and ultimately are removed. During cartilage hypertrophy, a number of changes occur, including the acquisition of synthesis of new components, the most studied being type X collagen. In this review, which is based largely on our own work, we will first examine the structure and properties of the type X collagen molecule. We then will describe the supramolecular forms into which the molecule becomes assembled within tissues, and how this changes its physical properties, such as thermal stability. Certain of these studies involve a novel, immunohistochemical approach that utilizes an antitype X collagen monoclonal antibody that detects the native conformation of the molecule. We describe the developmental acquisition of the molecule, and its transcriptional regulation as deduced by in vivo footprinting, transient transfection, and gel-shift assays. We provide evidence that the molecule has unique diffusion and regulatory properties that combine to alter the hypertrophic cartilage matrix. These conclusions are derived from an in vitro system in which exogenously added type X collagen moves rapidly through the cartilage matrix and subsequently produces certain changes mimicking ones that have been shown normally to occur in vivo. These include altering the cartilage collagen fibrils and effecting changes in proteoglycans. Last, we describe the subtractive hybridization, isolation, and characterization of other genes up-regulated during cartilage hypertrophy, with specific emphasis on one of these--transglutaminase.

Characterization of human type X procollagen and its NC-1 domain expressed as recombinant proteins in HEK293 cells

Type X collagen is a short-chain, network-forming collagen found in hypertrophic cartilage in the growth zones of long bones, vertebrae, and ribs. To obtain information about the structure and assembly of mammalian type X collagen, we generated recombinant human type collagen X by stable expression of full-length human alpha1(X) cDNA in the human embryonal kidney cell line HEK293 and the fibrosarcoma cell line HT1080. Stable clones were obtained secreting recombinant human type X collagen (hrColX) in amounts of 50 microg/ml with alpha1(X)-chains of apparent molecular mass of 75 kDa. Pepsin digestion converted the native protein to a molecule migrating as one band at 65 kDa, while bands of 55 and 43 kDa were generated by trypsin digestion. Polyclonal antibodies prepared against purified hrColX reacted specifically with type X collagen in sections of human fetal growth cartilage. Circular dichroism spectra and trypsin/chymotrypsin digestion experiments of hrColX at increasing temperatures indicated triple helical molecules with a reduced melting temperature of 31 degrees C as a result of partial underhydroxylation. Ultrastructural analysis of hrColX by rotary shadowing demonstrated rodlike molecules with a length of 130 nm, assembling into aggregates via the globular noncollagenous (NC)-1 domains as reported for chick type X collagen. NC-1 domains generated by collagenase digestion of hrColX migrated as multimers of apparent mass of 40 kDa on SDS-polyacrylamide gel electrophoresis, even after reduction and heat denaturation, and gave rise to monomers of 18-20 kDa after treatment with trichloroacetic acid. The NC-1 domains prepared by collagenase digestion comigrated with NC-1 domains prepared as recombinant protein in HEK293 cells, both in the multimeric and monomeric form. These studies demonstrate the potential of the pCMVsis expression system to produce recombinant triple helical type X collagens in amounts sufficient for further studies on its structural and functional domains.

Deer antler does not represent a typical endochondral growth system: immunoidentification of collagen type X but little collagen type II in growing antler tissue

The collagen isotypes present at early (6 week) and late (5 month) stages of growing deer antler were isolated and identified. Pepsin-digested collagens were separated by differential salt fractionation, SDS-PAGE and Western blotting and subsequently identified by immunostaining. Cyanogen bromide digestion of antler tissue was used to establish a collagen type-specific pattern of peptides, and these were also identified by immunoblotting. Collagen type I was found to be the major collagen in both early- and late-stage antler. Collagen type II was present in the young antler in small amounts but was not confined to the soft "cartilaginous" tip of the antler. Collagen type XI was found in the pepsin digest of the young antler, but collagen type IX was not present at either stage of antler growth. Collagen type X was found in the young antler in all fractions studied. Microscopic study showed that the deer antler did not possess a discrete growth plate as found in endochondral bone growth. Unequivocal immunolocalization of the different collagen types in the antler were unsuccessful. These results show that, despite the presence in the antler of many cartilage collagens, growth does not occur through a simple endochondral process.

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.

Differential effects of parathyroid hormone fragments on collagen gene expression in chondrocytes

The effect of parathyroid hormone (PTH) in vivo after secretion by the parathyroid gland is mediated by bioactive fragments of the molecule. To elucidate their possible role in the regulation of cartilage matrix metabolism, the influence of the amino-terminal (NH2-terminal), the central, and the carboxyl-terminal (COOH-terminal) portion of the PTH on collagen gene expression was studied in a serum free cell culture system of fetal bovine and human chondrocytes. Expression of alpha1 (I), alpha1 (II), alpha1 (III), and alpha1 (X) mRNA was investigated by in situ hybridization and quantified by Northern blot analysis. NH2-terminal and mid-regional fragments containing a core sequence between amino acid residues 28-34 of PTH induced a significant rise in alpha1 (II) mRNA in proliferating chondrocytes. In addition, the COOH-terminal portion (aa 52-84) of the PTH molecule was shown to exert a stimulatory effect on alpha1 (II) and alpha1 (X) mRNA expression in chondrocytes from the hypertrophic zone of bovine epiphyseal cartilage. PTH peptides harboring either the functional domain in the central or COOH-terminal region of PTH can induce cAMP independent Ca2+ signaling in different subsets of chondrocytes as assessed by microfluorometry of Fura-2/AM loaded cells. These results support the hypothesis that different hormonal effects of PTH on cartilage matrix metabolism are exerted by distinct effector domains and depend on the differentiation stage of the target cell.

Doxycycline inhibits type X collagen synthesis in avian hypertrophic chondrocyte cultures

Doxycycline, a member of the tetracycline family, has been shown to reduce a type X collagen epitope as detected by immunohistochemistry with a monoclonal antibody in an avian explant culture system (). It was also shown to decrease collagenase and gelatinase activities and thus matrix degradation. This study investigates the effect of doxycycline on type X collagen synthesis in monolayer cultures of hypertrophic chondrocytes. Protein synthesis was evaluated by radioisotopic labeling during doxycycline, tetracycline, or minocycline treatment. Radiolabeled proteins were analyzed by gel electrophoresis, and total collagen was quantitated by hydroxyproline analysis. Additionally, the synthesis of type X collagen was measured by immunoprecipitation. Doxycycline was found to inhibit type X production more effectively than either of the other tetracyclines at comparable dose levels. Furthermore, type X collagen was inhibited more than other collagens, non-collagenous proteins and proteoglycans, with maximal inhibition at 80 microg/ml and an IC50 of 7 microg/ml. This inhibition by doxycycline was specific for type X collagen at 10 microg/ml, and the pattern was distinct from cycloheximide, a recognized inhibitor of protein translation. This suppression of type X collagen could not be overcome by excess extracellular calcium, conditions that have been demonstrated to induce synthesis of this protein (2).

Identification of lysine-derived crosslinks in porcine collagen type X from growth plate and newly mineralized bone

Intact collagen type X cannot readily be extracted from the growth plate. Both the use of pepsin to release this molecule from tissue and the relative solubility of collagen type X following treatment of chick embryos with beta-aminopropionitrile (Chen et al., 1992) suggest that the insolubility may by brought about by the formation of lysine-derived crosslinks. By immunocytochemical labelling using antibodies specific for collagen type X, we have shown that this collagen type persists in the cartilaginous spicules present in metaphyseal bone and appears to be colocalized with collagen type II. The combined concentration of the reducible bifunctional crosslinks, dihydroxylysinonorleucine and monohydroxylysinonorleucine, in collagen type X isolated from the premineralized and newly mineralized growth plate was about 0.6 residues/ molecule, a level which might explain the relative intractability of collagen type X. Pyridinoline and deoxypyridinoline were present in very small amounts in collagen type X; this suggests that, unlike the situation in other types of collagen, few of the bifunctional crosslinks undergo maturation to pyridinium compounds. Although it is clear that collagen type X contains lysinederived crosslinks, work is in progress to establish which molecule also participates in the formation of these crosslinks.

Matrix vesicles and focal proteoglycan aggregates are the nucleation sites revealed by the lanthanum incubation method: a correlated study on the hypertrophic zone of the rat epiphyseal cartilage

Correlated studies were performed with light and electron microscopy, and backscattered electron image in conjunction with X-ray microanalysis, of lanthanum-incubated epiphyseal cartilage of the young rat. The hall-mark of this procedure is the appearance of LaP electron-dense deposits (not present in control sections) in precise sites of the hypertrophic zone. The ultrastructural study revealed a dual nature of these sites: "dense matrix vesicles" and "focal filament aggregates". The dense matrix vesicles are a specific type of matrix vesicle with the intrinsic capacity of precipitating LaP mineral, as soon as they originate from the hypertrophic chondrocytes. Furthermore, the matrix vesicles were found to be heterogeneous because lanthanum-devoid, "light matrix vesicles" were also present. The focal filament aggregates, which were not recognized in unstained sections and in controls, are apparently focal concentrations of proteoglycans with high lanthanum binding capacity, although the presence in them of other components (e.g., type X collagen, C-propeptide of type II collagen) cannot be excluded. The were in close connection with the light matrix vesicles in the upper hypertrophic zone, and were loaded with a variable quantity of LaP irregular electron-dense deposits in the lower hypertrophic zone. These irregular deposits are similar to, but distinct from, calcification nodules. The lanthanum incubation method indirectly detects the matrix Ca-binding components (which bind La ions), and the calcification initiation sites (which precipitate a LaP-mineral phase). A sequence is proposed of successive steps of LaP nucleation within the focal filament aggregates, which possibly mimics calcium phosphate deposition. Such a sequence seems to require the participation not only of dense matrix vesicles, but also of the filamentous components of the focal aggregates, possibly together with the activity of alkaline phosphatase.

The extracellular matrix proteins fibulin-1 and fibulin-2 in the early human embryo

Fibulin-1 and fibulin-2, two recently identified extracellular matrix proteins with a homologous domain structure, are known to bind various extracellular ligands and calcium. In this study, they have been localized at the light microscopical level in human embryos of gestational weeks 4-10, using polyclonal antibodies. Identical localization patterns were observed for the two fibulins in most of the tissues. In the heart, the endocardial cushion tissue and endocardium, but not the myocardium, were stained, as were the basement membrane zones and adventitia of blood vessels. Staining was also observed in the perichondrium and calcifying regions of developing bones. Moreover, reactions occurred with the gut subepithelium and epithelial basement membranes of the skin. Differences in staining patterns, however, were observed in various neural structures. Fibulin-1 was prominent in the matrix of the leptomeningeal anlage, in basement membranes of the neuroepithelium and the perineurium of peripheral nerves. Fibulin-2 was detected primarily within the neuropithelium, spinal ganglia and peripheral nerves. The early embryonic expression of both fibulins indicates specific roles during organ development and, in particular, involvements in the differentiation of heart, skeletal and neuronal structures.

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.

Morphological and biochemical characterization of mineralizing primary cultures of avian growth plate chondrocytes: evidence for cellular processing of Ca2+ and Pi prior to matrix mineralization

Advances in the culture of mineralizing growth plate chondrocytes provided an opportunity to study endochondral calcification under controlled conditions. Here we report that these cultures synthesize large amounts of proteins characteristically associated with mineralization: type II and X collagens, sulfated proteoglycans, alkaline phosphatase, and the bone-related proteins, osteonectin and osteopontin. Certain chondrocytes appeared to accumulate large amounts of Ca2+ and Pi during the mineralization process: laser confocal imaging revealed high levels of intracellular Ca2+ in their periphery and X-ray microanalytical mapping revealed the presence of many Ca(2+)- and Pi-rich cell surface structures ranging from filamentous processes 0.14 +/- 0.02 microns by 0.5-2.0 microns, to spherical globules 0.70 +/- 0.27 microns in diameter. Removal of organic matter with alkaline sodium hypochlorite revealed numerous deposits of globular (0.77 +/- 0.19 micron) mineral (calcospherites) in the lacunae around these cells. The size and spatial distribution of these mineral deposits closely corresponded to the Ca(2+)-rich cell surface blebs. The globular mineral progressively transformed into clusters of crystallites. Taken with earlier studies, these findings indicate that cellular uptake of Ca2+ and Pi leads to formation of complexes of amorphous calcium phosphate, membrane lipids, and proteins that are released as cell surface blebs analogous to matrix vesicles. These structures initiate development of crystalline mineral. Thus, the current findings support the concept that the peripheral intracellular accumulation of Ca2+ and Pi is directly involved in endochondral calcification.

Beta-sheet secondary structure of the trimeric globular domain of C1q of complement and collagen types VIII and X by Fourier-transform infrared spectroscopy and averaged structure predictions

C1q plays a key role in the recognition of immune complexes, thereby initiating the classical pathway of complement activation. Although the triple-helix conformation of its N-terminal segment is well established, the secondary structure of the trimeric globular C-terminal domain is as yet unknown. The secondary structures of human C1q and C1q stalks and pepsin-extracted human collagen types I, III and IV (with no significant non-collagen-like structure) were studied by Fourier-transform i.r. spectroscopy in 2H2O buffers. After second-derivative calculation to resolve the fine structure of the broad amide I band, the Fourier-transform i.r. spectrum of C1q showed two major bands, one at 1637 cm-1, which is a characteristic frequency for beta-sheets, and one at 1661 cm-1. Both major bands were also detected for Clq in H2O buffers. Only the second major band was observed at 1655 cm-1 in pepsin-digested C1q which contains primarily the N-terminal triple-helix region. The Fourier-transform i.r. spectra of collagen in 2H2O also showed a major band at 1659 cm-1 (and minor bands at 1632 cm-1 and 1682 cm-1). It is concluded that the C1q globular heads contain primarily beta-sheet structure. The C-terminal domains of C1q show approximately 25% sequence identity with the non-collagen-like C-terminal regions of the short-chain collagen types VIII and X. To complement the Fourier-transform-i.r. spectroscopic data, averaged Robson and Chou-Fasman structure predictions on 15 similar sequences for the globular domains of C1q and collagen types VIII and X were performed. These showed a clear pattern of ten beta-strands interspersed by beta-turns and /or loops. Residues thought to be important for C1q-immune complex interactions with IgG and IgM were predicted to be at a surface-exposed loop. Sequence insertions and deletions, glycosylation sites, the free cysteine residue and RGD recognition sequences were also predicted to be at surface-exposed positions.

Immunolocalization of type X collagen before and after mineralization of human thyroid cartilage

In this study the distribution of type X collagen in thyroid cartilages of various ages is described. Fetal and juvenile thyroid cartilage was negative for type X collagen, but showed a strong staining reaction for type II collagen. Type X collagen and calcium deposition were first detected in thyroid cartilage of 18-to 21-year-old adults. Type X collagen was restricted to large chondrocytes near or in mineralized cartilage, confirming the notion that type X collagen precedes mineralization. From these observations it was concluded that chondrocytes in thyroid cartilage undergo differentiation steps that are similar, but much slower, compared to cells in growth plate and sternal cartilage. Some type X collagen-positive areas also showed staining for type I collagen, suggesting that there is a further differentiation of chondrocytes to cells which are characterized by the simultaneous synthesis of type X and I collagen. However, a dedifferentiation process during aging of thyroid cartilage where cells switch from synthesis of type II to type I collagen cannot be excluded.

Type X collagen expression in osteoarthritic and rheumatoid articular cartilage

Type X collagen is a short chain, non-fibril-forming collagen synthesized primarily by hypertrophic chondrocytes in the growth plate of fetal cartilage. Previously, we have also identified type X collagen in the extracellular matrix of fibrillated, osteoarthritic but not in normal articular cartilage using biochemical and immunohistochemical techniques (von der Mark et al. 1992a). Here we compare the expression of type X with types I and II collagen in normal and degenerate human articular cartilage by in situ hybridization. Signals for cytoplasmic alpha 1(X) collagen mRNA were not detectable in sections of healthy adult articular cartilage, but few specimens of osteoarthritic articular cartilage showed moderate expression of type X collagen in deep zones, but not in the upper fibrillated zone where type X collagen was detected by immunofluorescence. This apparent discrepancy may be explained by the relatively short phases of type X collagen gene activity in osteoarthritis and the short mRNA half-life compared with the longer half-life of the type X collagen protein. At sites of newly formed osteophytic and repair cartilage, alpha 1(X) mRNA was strongly expressed in hypertrophic cells, marking the areas of endochondral bone formation. As in hypertrophic chondrocytes in the proliferative zone of fetal cartilage, type X collagen expression was also associated with strong type II collagen expression.

Chicken tibial dyschondroplasia: a limb mutant with two growth plates and possible defects of collagen crosslinking

In the cartilaginous epiphyseal growth plate, extracellular matrix molecules such as collagens are believed to play important roles during both normal and abnormal development. One defect of the epiphyseal plate occurs in chickens with a condition termed tibial dyschondroplasia (TD). This abnormality occurs in certain strains of juvenile chickens and other rapidly developing animals. It is characterized by the presence of a mass of avascular, uncalcified cartilage which is retained in the proximal metaphysis of the tibiotarsus. To elucidate the developmental events which may be involved in this lesion, we have performed both immunohistochemistry and in situ hybridizations for collagen types II and X, known components of the extracellular matrix of the growth plate. By immunohistochemical analyses, the TD lesion contains both of these collagen types; therefore, the presence of these molecules per se is not sufficient for calcification of vascularization to occur. Since type X collagen is expressed exclusively in hypertrophic cartilage, the chondrocytes in the lesion must have undergone hypertrophy before their developmental arrest. The matrix of the lesion also reacted with a monoclonal antibody which is directed against an epitope in the NH2-terminal telopeptide of the alpha 1(II) chain. Our previous data suggest that this epitope is rendered unavailable in type II collagen which has undergone crosslink formation; its availability in the lesion suggests that crosslinking may be abnormal. Lastly, analyses by in situ hybridization failed to detect mRNA for either type II or type X collagen within the lesion, but chondrocytes distal to the lesion do contain mRNAs for these collagens in a spatial pattern suggesting the presence of a second growth plate.

Localization of collagen X in human fetal and juvenile articular cartilage and bone

The tissue localization was analysed of collagen X during human fetal and juvenile articular cartilage-bone metamorphosis. This unique collagen type was found in the hypertrophic cartilage zone peri- and extracellularly and in cartilage residues within bone trabeculae. In addition, occasionally a slight intracellular staining reaction was found in prehypertrophic proliferating chondrocytes and in chondrocytes surrounding vascular channels. A slight staining was also seen in the zone of periosteal ossification and occasionally at the transition zone of the perichondrium to resting cartilage. Our data provide evidence that the appearance of collagen X is mainly associated with cartilage hypertrophy, analogous to the reported tissue distribution of this collagen type in animals. In addition, we observed an increased and often "spotty" distribution of collagen X with increasing cartilage "degeneration" associated with the closure of the growth plate. In basal hypertrophic cartilage areas, a co-distribution of collagens II and X was found with very little and "spotty" collagen III. In juvenile cartilage areas around single hypertrophic chondrocytes, co-localization of collagens X and I was also detected.

Ascorbate independent differentiation of human chondrocytes in vitro: simultaneous expression of types I and X collagen and matrix mineralization

In this study we describe the collagen pattern synthesized by differentiating fetal human chondrocytes in vitro and correlate type X collagen synthesis with an intracellular increase of calcium and with matrix calcification. We show that type II collagen producing fetal human epiphyseal chondrocytes differentiate in suspension culture over agarose into hypertrophic cells in the absence of ascorbate, in contrast to chicken chondrocytes which have been shown to require ascorbate for hypertrophic differentiation. Analysis of the collagen synthesis by metabolic labeling and immunoprecipitation as well as by immunofluorescence double staining with anti type I, II or X collagen antibodies revealed that type X collagen synthesis was initiated during the third week. After 4 weeks culture over agarose we identified cells staining for both type I and X collagen, indicating further differentiation of chondrocytes to a new type of 'post-hypertrophic' cell. This cell type, descending from a type X collagen producing chondrocyte, is different from the previously described 'dedifferentiated' or 'modulated' types I and III collagen producing cell derived from a type II collagen producing chondrocyte. The appearance of type I collagen synthesis in agarose cultures was confirmed by metabolic labeling and immunoprecipitation and challenges the current view that the chondrocyte phenotype is stable in suspension cultures. An increase in the intracellular calcium concentration from 100 to 250 nM was measured about one week after onset of type X collagen synthesis. First calcium deposits were detected by alizarine red S staining in type X collagen positive cell nodules after 4 weeks, again in the absence of ascorbate. From these observations we conclude a sequence of events ultimately leading to matrix calcification in chondrocyte nodules in vitro that begins with chondrocyte hypertrophy and the initiation of type X collagen synthesis, followed by the increase of intracellular calcium, the deposition of calcium mineral, and finally by the onset of type I collagen synthesis.

Selective binding of anchorin CII (annexin V) to type II and X collagen and to chondrocalcin (C-propeptide of type II collagen). Implications for anchoring function between matrix vesicles and matrix proteins

Anchorin CII is a collagen binding protein of the annexin family associated with plasma membranes of chondrocytes, osteoblasts, and many other cells. As a major constituent of cartilage-derived matrix vesicles it has been shown to bind to native type II and X collagen. In accordance with this observation, here we show the localization of anchorin CII in the extracellular matrix of calcifying cartilage in the fetal human growth plate, and that it was restricted to the chondrocyte surface in proliferating and resting cartilage. Furthermore, we present evidence, using a slot blot assay, that anchorin CII not only binds to native type II and X collagen, but also to chondrocalcin, the carboxy-terminal extension of type II procollagen, in a calcium-independent manner. Pepsin digestion of type II collagen results in loss of anchorin CII binding, confirming our previous notion that the telopeptide region of type II collagen carries anchorin CII binding sites.

Remodelling of collagen types I, II and X and calcification of human fetal cartilage

Evidence from recent studies on type X collagen in hypertrophic chick cartilage suggests that it may be involved in cartilage calcification. Here we compare the distribution of type X collagen with that of calcium mineral deposition in fetal human growth plate cartilages of long bones and ribs. Using a specific antibody we demonstrate the presence of type X collagen in a narrow, sharply defined zone of hypertrophic chondrocytes. Type X collagen was also localized in the calcifying cartilage remaining within spongy bone trabecules. Calcium deposits were, however, detected by alizarine red S only in the lower hypertrophic zone and in bone, confirming the notion that type X collagen is deposited in the hypertrophic cartilage before mineral deposition. By immunofluorescence double staining we demonstrate codistribution of type II and X collagen in the hypertrophic zone, while type I collagen was absent from hypertrophic cartilage matrix; it was detected only in the perichondrium, in vascular cavities, and in osteoid and bone. From these observations we conclude that the sequence of events leading to cartilage mineralization begins with chondrocyte hypertrophy, followed by type X collagen synthesis and finally by deposition of calcium mineral.