Type X collagen: covalent crosslinking to hypertrophic cartilage-collagen fibrils

A Collagen10a1 mutation disrupts cell polarity in a medaka model for metaphyseal chondrodysplasia type Schmid

Heterozygous mutations in COL10A1 lead to metaphyseal chondrodysplasia type Schmid (MCDS), a skeletal disorder characterized by epiphyseal abnormalities. Prior analysis revealed impaired trimerization and intracellular retention of mutant collagen type X alpha 1 chains as cause for elevated endoplasmic reticulum (ER) stress. However, how ER stress translates into structural defects remained unclear. We generated a medaka (Oryzias latipes) MCDS model harboring a 5 base pair deletion in col10a1, which led to a frameshift and disruption of 11 amino acids in the conserved trimerization domain. col10a1Δ633a heterozygotes recapitulated key features of MCDS and revealed early cell polarity defects as cause for dysregulated matrix secretion and deformed skeletal structures. Carbamazepine, an ER stress-reducing drug, rescued this polarity impairment and alleviated skeletal defects in col10a1Δ633a heterozygotes. Our data imply cell polarity dysregulation as a potential contributor to MCDS and suggest the col10a1Δ633a medaka mutant as an attractive MCDS animal model for drug screening.

Spontaneous differentiating primary chondrocytic tissue culture: a model for endochondral ossification

Primary cartilage-derived cell cultures tend to undergo dedifferentiation, acquire fibroblastic features, and lose most of the characteristics of mature chondrocytes. This phenomenon is due mainly to the close matrix-cell interrelationship typical of cartilage tissue, which is vital for the preservation of the cartilaginous features. In this study we present a model for spontaneous redifferentiation of primary chondrocytic culture. Mandibular condyles excised from 3-day-old mice, thoroughly cleaned of all soft tissue, were digested with 0.1% collagenase. These mandibular condyle-derived chondrocytes (MCDC) were cultured under chondrogenesis-supporting conditions; that is, 5 x 10(5) cells/mL were incubated in Dulbecco's modified Eagle medium supplemented with 100 microg/mL ascorbic acid, 1 mmol/L calcium chloride, 10 mmol/L beta-glycerophosphate, 10% fetal calf serum, and antibiotics. Development and growth rates of these cartilage-derived cultures were determined by following morphological and functional changes. MCDC proliferated intensively during the first 24-48 h following plating, showing fibroblast-like (long spindle-shaped) morphology and producing mainly type I collagen. The proliferation rate gradually declined, and the cells developed polygonal shapes and started to produce type II collagen. In the 10-14-day-old cultures, cells began to aggregate in cartilaginous nodules and exhibited positive staining for acidic Alcian blue, type X collagen, and von Kossa. Expression of core-binding factor alpha(1) increased between 3 and 5 days and declined gradually thereafter. The condylar-derived tissue culture presented here depicts a spontaneous redifferentiation chondrocytic tissue culture that exhibits features of mature chondrocytes typically found in skeletal growth centers. The present study offers a model for primary chondrocytic tissue culture, which might serve as a model for in vitro endochondral ossification.

Endochondral ossification of costal cartilage is arrested after chondrocytes have reached hypertrophic stage of late differentiation

Late cartilage differentiation during endochondral bone formation is a multistep process. Chondrocytes transit through a differentiation cascade under the direction of environmental signals that either stimulate or repress progression from one step to the next. In human costal cartilage, chondrocytes reach very advanced stages of late differentiation and express collagen X. However, remodeling of the tissue into bone is strongly repressed. The second hypertrophy marker, alkaline phosphatase, is not expressed before puberty. Upon sexual maturity, both alkaline phosphatase and collagen X activity levels are increased and slow ossification takes place. Thus, the expression of the two hypertrophy markers is widely separated in time in costal cartilage. Progression of endochondral ossification in this tissue beyond the stage of hypertrophic cartilage appears to be associated with the expression of alkaline phosphatase activity. Costal chondrocytes in culture are stimulated by parathyroid hormone in a PTH/PTHrP receptor-mediated manner to express the fully differentiated hypertrophic phenotype. In addition, the hormone stimulates hypertrophic development even more powerfully through its carboxyterminal domain, presumably by interaction with receptors distinct from PTH/PTHrP receptors. Therefore, PTH can support late cartilage differentiation at very advanced stages, whereas the same signal negatively controls the process at earlier stages.

Mechanisms of maturation and ageing of collagen

The deleterious age-related changes in collagen that manifest in the stiffening of the joints, the vascular system and the renal and retinal capillaries are primarily due to the intermolecular cross-linking of the collagen molecules within the tissues. The formation of cross-links was elegantly demonstrated by Verzar over 40 years ago but the nature and mechanisms are only now being unravelled. Cross-linking involves two different mechanisms, one a precise enzymically controlled cross-linking during development and maturation and the other an adventitious non-enzymic mechanism following maturation of the tissue. It is this additional non-enzymic cross-linking, known as glycation, involving reaction with glucose and subsequent oxidation products of the complex, that is the major cause of dysfunction of collagenous tissues in old age. The process is accelerated in diabetic subjects due to the higher levels of glucose. The effect of glycation on cell-matrix interactions is now being studied and may be shown to be an equally important aspect of ageing of collagen. An understanding of these mechanisms is now leading to the development of inhibitors of glycation and compounds capable of cleaving the cross-links, thus alleviating the devastating effects of ageing.

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.

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.

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.

Distribution and quantification of pyridinium cross-links of collagen within the different maturational zones of the chick growth plate

In order to assess alterations in the collagen network during endochondral ossification the pyridinium cross-links of collagen were quantified in sequential transverse sections through the chick growth plate. This was accomplished using both morphological (alkaline phosphatase (ALP) histochemistry and collagen type X immunostaining) and analytical (HPLC) analyses. In articular cartilage, pyridinoline concentrations were maximal in the deep mature zones. In contrast, the proliferating chondrocyte zone of the growth plate had approximately a 10-fold greater pyridinoline cross-link concentration than the mature hypertrophic zone. Deoxypyridinoline was first found in the prehypertrophic zone of the growth plate cartilage that reacted positively for ALP activity but before collagen type X was detected. However, deoxypyridinoline concentrations were highest in the most differentiated regions of the growth plate where it was the principal pyridinium cross-link. In tibial dyschondroplasia, where chondrocyte differentiation is arrested in the prehypertrophic zone, higher concentrations of both cross-links were found with increasing distance down the lesion. We conclude that the decrease in pyridinoline cross-link concentration down the growth plate may be an essential adaptation (via increased collagenase activity and collagen turnover) of the matrix for vascular invasion and osteoclastic resorption to occur.

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.

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.

Avian tibial dyschondroplasia: a morphological and biochemical review of the growth plate lesion and its causes

Avian tibial dyschondroplasia is a disease found in fast growing strains of chickens, ducks, and turkeys worldwide in which growth plate cartilage accumulates in the metaphyseal region of the tibiotarsus; it is similar to mammalian osteochondrosis. Several biochemical and pathologic studies have shown that the growth plate chondrocytes do not reach their expected size in the hypertrophic zone and necroses prematurely. The chondrocytes also produce decreased amounts of extracellular proteins, such as collagen X and fibroblast growth factor-beta, that are necessary for cartilage maturation. This immature cartilage becomes highly cross-linked in the collagen molecules and apparently resistant to resorption and vascularization by the metaphyseal vessels. The dyschondroplastic cartilage remains in the metaphysis for several weeks. Not until the growth rate of the birds slows down is the cartilage able to be resorbed and replaced by trabecular bone. Many conditions have been found to induce tibial dyschondroplasia, including copper deficiency; fusarochromanone, thiram, and antabuse intoxication; excessive dietary levels of cysteine and homocysteine; metabolic acidosis; and bird rearing environment. However, the mechanism(s) by which these various methods induce tibial dyschondroplasia is presently not known. Current research is focusing on understanding the development of the disease and whether or not all these methods work by the same physiological chain of events. Recent biochemical evidence suggests that a copper deficiency might be caused by a different mechanism than genetically and thiram-induced tibial dyschondroplasia.

The presence of lysylpyridinoline in the hypertrophic cartilage of newly hatched chicks

The presence of lysylpyridinoline (LP) as a nonreducible cross-link in appreciable quantities has primarily been limited to the mineralized tissues, bone and dentin. However, the results reported here show that LP is not only present in the hypertrophic cartilage of the tibiotarsus isolated from newly hatched broiler chicks, but it is approx. 4-fold as concentrated as hydroxylysylpyridinoline (HP). Bone and articular cartilage surrounding the hypertrophic cartilage do not contain measurable quantities of LP. Purified LP has a fluorescent scan similar to purified HP and literature values, confirming that we indeed were measuring LP. Also, the cartilage lesion produced by immature chondrocytes from birds with tibial dyschondroplasia had LP but the HP:LP ratio was > 1. Thus, the low HP:LP ratio could be a marker for hypertrophic cartilage in avians.

Type II collagen during cartilage and corneal development: immunohistochemical analysis with an anti-telopeptide antibody

We have examined the pattern of immunoreactivity of a monoclonal antibody, II-5B2, with specificity for an epitope which resides within the NH2-terminal extension peptide (telopeptide) of the avian type II collagen molecule. This epitope is available in regions of matrix where de novo synthesis of the molecule is ongoing, but not where synthesis has ceased and maturation and crosslink formation have occurred. Within the cartilaginous growth plate, the epitope disappears from the matrix soon after the chondrocytes become hypertrophic; within the cornea, the epitope disappears subjacent to the epithelium. The II-5B2 epitope is not made available by a variety of procedures shown to remove potentially masking substances and to disrupt fibrillar organization. It is rendered available, however, when covalent crosslink formation between collagen molecules is blocked through administration of beta-aminopropionitrile or penicillamine. In contrast, the epitope of another monoclonal antibody against type II collagen, II-II6B3, which resides in the triple-helical domain of the molecule, in cartilage is present throughout the growth plate including the hypertrophic zone, and in cornea extends for a considerable distance into the stroma. Thus, it is available for antibody binding regardless of fibril maturation and crosslinking. These data suggest that the II-5B2 epitope becomes unavailable when the telopeptide becomes crosslinked. By using these two monoclonal antibodies in serial sections, one can establish the crosslinking pattern of type II collagen in the tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

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.