Cartilage macromolecules and the calcification of cartilage matrix

The extracellular matrix of cartilage in the growth plate before and during calcification: changes in composition and degradation of type II collagen

Calcification occurs in the extracellular matrix of the hypertrophic zone of the growth plate when the extracellular matrix volume is reduced to a minimum and alkaline phosphatase content is maximal. The present study shows that significant quantitative and qualitative changes occur in the composition and structure of macromolecules in the extracellular matrix before and during calcification in the proximal tibial growth plate of the bovine fetus. These were detected in part by using microchemical and microimmuno-chemical analyses of sequential transverse frozen sections at defined sites throughout the growth plate. Concentrations of matrix molecules in the extracellular matrix have not previously been determined biochemically. They were measured per unit matrix volume by using combined immunochemical/chemical-histomorphometric analyses. The concentrations within the extracellular matrix of the C-propeptide of type II collagen, aggregating proteoglycan (aggrecan), and hyaluronic acid all progressively increased in the maturing and hypertrophic zones, being maximal (or near maximal) at the time of initiation of mineralization. These results for proteoglycan are contrary to some earlier reports of a loss of proteoglycan are contrary to some earlier reports of a loss of proteoglycan prior to mineralization which measured the tissue content of proteoglycan rather than that present in the extracellular matrix, the volume of which is progressively reduced as the growth plate matures. The C-propeptide data provides a quantitative confirmation of previous immunohistochemical studies. Total collagen concentration (measured as hydroxyproline) in the extracellular matrix initially increased through the proliferating and maturing zones but then rapidly decreased in the hypertrophic zone.(ABSTRACT TRUNCATED AT 250 WORDS)

Matrix vesicles are enriched in metalloproteinases that degrade proteoglycans

This study examined the presence of extracellular matrix processing enzymes in matrix vesicles produced by rat costochondral resting zone and growth zone chondrocytes in culture. Optimum procedures for the extraction of each enzyme activity were determined. Enzyme activity associated with chondrocyte plasma membrane microsomes was used for comparison. There was a differential distribution of the enzyme activities related to the cartilage zone from which the cells were isolated. Acid and neutral metalloproteinase (TIMP), plasminogen activator, and beta-glucuronidase were highest in the growth zone chondrocyte (GC) membrane fractions when compared with matrix vesicles and plasma membranes isolated from resting zone chondrocyte (RC) cultures. There was a threefold enrichment of total and active acid metalloproteinase in GC matrix vesicles, whereas no enrichment in enzyme activity was observed in RC matrix vesicles. Total and active neutral metalloproteinase were similarly enriched twofold in GC matrix vesicles. TIMP, plasminogen activator, and beta-glucuronidase activities were highest in the plasma membranes of both cell types. No collagenase, lysozyme, or hyaluronidase activity was found in any of the membrane fractions. The data indicate that matrix vesicles are selectively enriched in enzymes which degrade proteoglycans. The highest concentrations of these enzymes are found in matrix vesicles produced by growth zone chondrocytes, suggesting that this may be a mechanism by which the more differentiated cell modulates the matrix for calcification.

Articular-cartilage matrix gamma-carboxyglutamic acid-containing protein. Characterization and immunolocalization

Matrix gamma-carboxyglutamic acid (Gla)-containing protein (MGP) was found to be present in articular cartilage by Western-blot analysis of guanidinium chloride extracts of human and bovine cartilage and was further localized by immunohistochemical studies on human and monkey specimens. In newborn articular cartilage MGP was present diffusely throughout the matrix, whereas in growth-plate cartilage it was seen mainly in late hypertrophic and calcifying-zone chondrocytes. In adult articular cartilage MGP was present primarily in chondrocytes and the pericellular matrix. Immunoelectron microscopy studies revealed an association between MGP and vesicular structures with an appearance consistent with matrix vesicles. MGP may be an important regulator of cartilage calcification because of its localization in cartilage and the known affinity of Gla-containing proteins for Ca2+ and hydroxyapatite.

Hyaluronan interactions with hydroxyapatite do not alter in vitro hydroxyapatite crystal proliferation and growth

The interaction of hyaluronan (Mr range 80-120 x 10(4)) with poorly crystalline hydroxyapatite, such as is found in calcified cartilage and bone, was studied to challenge the hypothesis that free hyaluronan found in proteoglycan aggregate preparations could affect in vitro mineralization. Using a Langmuir adsorption isotherm, based on uronic acid content, hyaluronan was found to bind to hydroxyapatite with an affinity K of 0.12 ml/microgram uronate and N = 6.8 micrograms uronate/m2 hydroxyapatite binding sites. This is contrasted with K = .047 ml/microgram uronate and N = 9.0 micrograms uronate/m2 for a bovine nasal proteoglycan monomer preparation. Although the proteoglycan monomer and aggregate preparations have been reported to inhibit hydroxyapatite growth at concentrations of 1 mg/ml, using solution concentrations of 0, 0.01, 0.1 and 1 mg/ml hyaluronan there were no detectable alterations in the rate of seeded hydroxyapatite growth and proliferation. These data indicate that although in vitro hyaluronan may bind with weak affinity to hydroxyapatite, this interaction does not affect mineral growth, and the presence of hyaluronan would not contribute to the increased inhibitory potential of cartilage proteoglycan aggregate relative to monomer preparations.

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

Type X collagen is a short, non-fibril-forming collagen restricted to the hypertrophic, calcifying zone of growth plate cartilage. It is developmentally regulated and found exclusively in hypertrophic cartilage. Here we report on the structure and distribution of human type X collagen based on the cloning of a PCR fragment covering 292 bp of the carboxy-terminal, non-triple-helical domain. Seventy-five percent of the sequence are identical to that of chicken type X collagen at nucleic acid level and 84% at amino acid level. This probe was used for in situ hybridization analyses of type X collagen expression in a human growth plate. Human fetal cartilage, which is different from the avian cartilage-bone transition zone, showed strong type X collagen expression confined to the lower hypertrophic zone of the growth plate. The upper zone of hypertrophic chondrocytes did not contain alpha 1(X) transcripts, indicating that type X collagen expression follows cellular hypertrophy. The distribution of type X collagen mRNA has been previously unreported in chondrocytes from zones of secondary ossification and in chondrocytes associated with endochondral bone trabecules containing calcified cartilage. In situ hybridization analyses with probes for type I and II collagen on consecutive sections indicated a spatial gradient in chondrocyte differentiation in the human epiphysis. Chondrocytes of low type II collagen expression in the resting zone are followed by proliferating columnar chondrocytes with strong type II collagen expression and a zone of hypertrophic chondrocytes synthesizing type X and type II collagen. In contrast to findings in avian growth cartilage in some of our samples of human epiphyseal cartilage hypertrophic chondrocytes continued to strongly express type II collagen down to the chondro-osseous junction. Transcripts of the alpha 2(I) collagen gene, however, were detected only in perichondrium, vascular cavities, and bone, but not in hypertrophic or any other chondrocytes. The above observations demonstrate that the isolation of the human type X collagen DNA will contribute to studies of pathways of chondrocyte differentiation in the mammalian growth plate.

Hydroxyapatite formation in the presence of proteoglycans of reduced sulfate content: studies in the brachymorphic mouse

Proteoglycans from the brachymorphic (bm/bm) mouse have a reduced sulfate content due to the impaired activity of adenosine phosphosulfate phosphokinase in these animals. X-ray diffraction and infrared analyses of the mineral from the calcified cartilage of the bm/bm mice demonstrate the presence of significantly larger and more perfect hydroxyapatite crystals of lower carbonate to phosphate content than crystals found in the control animals. No differences were seen in the mineral content, crystallite size, CO3:PO4 ratio, or infrared splitting factors measured in the diaphyseal bone from these animals. Electron microscopic examination similarly shows larger, more disorganized crystals in the bm/bm animals' calcified cartilage as contrasted with controls. In vitro, proteoglycan aggregates from these dwarf mice are shown in a collagen gel-growth system to be less effective inhibitors of hydroxyapatite formation and growth than similarly size sulfated proteoglycans from age-matched control animals. The proteoglycans from the control mice were comparable in inhibitory ability to proteoglycan aggregates extracted from fetal bovine epiphyses. The in vitro and in vivo mineral parameters suggest the importance of sulfate for the interaction between proteoglycans and mineral in growth plate calcification.

Elevated extracellular calcium concentrations induce type X collagen synthesis in chondrocyte cultures

Chondrocytes at different stages of cellular differentiation were isolated from the tarsal element (immature chondrocytes) and zones 2 and 3 (mature chondrocytes) of 12-d chick embryo tibiotarsus. The chondrocytes from the two sources differed in their cell morphologies, growth rate and production of type X collagen. In 24 h, zone 2 and 3 chondrocytes synthesized 800 times more type X collagen than tarsal chondrocytes. The effect of exogenous CaCl2 (5 and 10 mM) on the synthesis of type X collagen by both mature and immature chondrocytes was tested. After a 72-h incubation of zone 2 and 3 chondrocytes with CaCl2 type X collagen increased 8-fold with 5 mM and 10-fold with 10 mM Ca2+. [3H]Proline incorporation into culture medium and matrix macromolecules increased 11 and 32% with 5 and 10 mM CaCl2, respectively. Type II collagen synthesis was not affected by elevated extracellular Ca2+ during this 72-h period. Similar studies with tarsal chondrocytes demonstrated a time- and dose-dependent response to CaCl2 with type X collagen levels reaching a 4-fold and 15-fold increase over controls with 5 and 10 mM Ca2+, respectively, at 48 h. Elevated extracellular Ca2+ had no effect on cell proliferation. These observations offer the first direct evidence of the induction of type X collagen synthesis with elevated extracellular Ca2+.

The fabrication and collagenous substructure of the eggshell membrane in the isthmus of the hen oviduct

The eggshell of the chicken consists of a bi-layered shell membrane overlaid with a thick, calcified shell matrix. The shell membranes and matrix are deposited onto the egg as it passes through the oviduct. To assess the temporal and spatial aspects of the fabrication of type X collagen within the eggshell extracellular matrix, the immunohistochemical localization of type X collagen was studied in three regions of the hen oviduct (magnum, isthmus and uterus), in the membranes of uncalcified eggshells obtained from the oviduct prior to mineral deposition and in eggshell membrane and calcified eggshell matrix. Additionally, immunohistochemical localization of type I and III collagens was done in order to determine possible co-localization of collagen types or to define tissue compartments. None of the collagen epitopes assayed was found in the shell matrix. Type X collagen epitope was immunohistochemically localized only to the epithelial cell layer lining the isthmus region of the oviduct and in the shell membranes of both uncalcified and calcified eggshells. Antitype III collagen monoclonal antibody delineated the inter-tubular gland connective tissue of the oviduct and was negative in the shell layers under conditions which gave strong connective tissue reactivity. Type I collagen epitope was exposed after pepsin treatment of the tissue and co-localizes with the distribution of type III collagen. Type I collagen co-localized with type X collagen in the shell membranes of uncalcified shells. The type I collagen epitope was reactive in the shell membrane of the uncalcified shells, but could only be detected in calcified shells following pepsin digestion.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of the growth plate in longitudinal bone growth

The epiphyseal growth plate is the main site of longitudinal growth of the long bones. At this site, cartilage is formed by the proliferation and hypertrophy of cells and synthesis of the typical extracellular matrix. The formed cartilage is then calcified, degraded, and replaced by osseous tissue. Proliferation and differentiation of cartilage cells (i.e., chondrocytes) as studied mostly in culture, is regulated by various endocrine, paracrine, and autocrine agents such as growth hormone, insulin-like growth factor-I (IGF-I), transforming growth factor (TGE-beta), and vitamin D metabolites (1,25-dihydroxycholecalciferol and 24,25-dihydroxycholecalciferol). Avian chondrocyte proliferation is enhanced by agents which use adenosine 3':5'-cyclic monophosphate as a second messenger, such as parathyroid hormone or prostaglandin-E2, and is depressed by guanosine 3':5'-cyclic monophosphate agonists, such as atrial natriuretic peptide. Several of the regulating agents also affect synthesis of the main extracellular components (i.e., collagen and proteoglycans) and their transfer to the extracellular space. Cartilage calcification involves matrix vesicles secreted by the chondrocytes at a specific stage. Calcification probably involves some initial nucleation agent and participation of phosphatases. During sexual maturation, the growth plate closes by an unknown mechanism and longitudinal bone growth ceases. Disorders in the metabolism of the controlling agents or the cellular responses in growth plate may lead to several deformities classified as dysplasias. In poultry, this class of disorders is represented by chondrodystrophy and dyschondroplasia.

Matrix in cartilage and bone development: current views on the function and regulation of major organic components

Study of the growth and development of cartilage and bone has been difficult because the structure of the tissues makes biological experiments hard to conduct. Recent advances in molecular biology have offered new possibilities for studying these processes. Many cartilage and bone specific cDNAs have been cloned and characterized and consequently used to localize the corresponding mRNAs in tissue sections. Developing cartilage and bone serve as a model for the study of extracellular matrix gene regulation during the proliferation, growth and differentiation of connective tissue cells. Normal skeletal growth and development are regulated by both systemic and local factors. The effects of many systemic hormones on bone metabolism have been studied extensively, but the pathways triggered by these hormones in the target cells are less well known. Recent evidence suggests that some growth factors, such as TGF-beta, IGFs and PDGF, act as local regulators of cartilage and bone metabolism. The different extracellular matrix components, e.g. collagens, are expressed differently in distinct cell types and developmental stages during cartilage and bone development. This model, therefore, facilitates the study of relations between the production of the various extracellular matrix components and the growth factors and the proto-oncogenes which may regulate them. Existing knowledge of the expression of major cartilage and bone components and their regulation during growth, differentiation and development is reviewed. An understanding of the normal growth and development of cartilage and bone is fundamental for elucidating the mechanisms underlying the various diseases -- both hereditary and acquired -- affecting the human skeleton.

The role of extracellular matrix components in dentin mineralization

The extracellular matrix of dentin consists of mineral (hydroxyapatite), collagen, and several noncollagenous matrix proteins. These noncollagenous matrix proteins may be mediators of cell-matrix interactions, matrix maturation, and mineralization. This review describes the current knowledge of the chemistry of mineral crystal formation in dentin with special emphasis on the roles of the dentin matrix proteins. The functions of some of these matrix proteins in the mineralization process have been deduced based on in vitro studies. Functions for others have been postulated based on analogy with some of the bone matrix proteins. Evidence suggests that several of these matrix proteins may have multiple effects on nucleation, crystal growth, and orientation of dentin hydroxyapatite.

Gene expression and extracellular matrix ultrastructure of a mineralizing chondrocyte cell culture system

Conditions were defined for promoting cell growth, hypertrophy, and extracellular matrix mineralization of a culture system derived from embryonic chick vertebral chondrocytes. Ascorbic acid supplementation by itself led to the hypertrophic phenotype as assessed by respective 10- and 15-fold increases in alkaline phosphatase enzyme activity and type X synthesis. Maximal extracellular matrix mineralization was obtained, however, when cultures were grown in a nutrient-enriched medium supplemented with both ascorbic acid and 20 mM beta-glycerophosphate. Temporal studies over a 3-wk period showed a 3-4-fold increase in DNA accompanied by a nearly constant DNA to protein ratio. In this period, total collagen increased from 3 to 20% of the cell layer protein; total calcium and phosphorus contents increased 15-20-fold. Proteoglycan synthesis was maximal until day 12 but thereafter showed a fourfold decrease. In contrast, total collagen synthesis showed a greater than 10-fold increase until day 18, a result suggesting that collagen synthesis was replacing proteoglycan synthesis during cellular hypertrophy. Separate analysis of individual collagen types demonstrated a low level of type I collagen synthesis throughout the 21-d time course. Collagen types II and X synthesis increased during the first 2 wk of culture; thereafter, collagen type II synthesis decreased while collagen type X synthesis continued to rise. Type IX synthesis remained at undetectable levels throughout the time course. The levels of collagen types I, II, IX, and X mRNA and the large proteoglycan core protein mRNA paralleled their levels of synthesis, data indicating pretranslational control of synthesis. Ultrastructural examination revealed cellular and extracellular morphology similar to that for a developing hypertrophic phenotype in vivo. Chondrocytes in lacunae were surrounded by a well-formed extracellular matrix of randomly distributed collagen type II fibrils (approximately 20-nm diam) and extensive proteoglycan. Numerous vesicular structures could be detected. Cultures mineralized reproducibly and crystals were located in extracellular matrices, principally associated with collagen fibrils. There was no clear evidence of mineral association with extracellular vesicles. The mineral was composed of calcium and phosphorus on electron probe microanalysis and was identified as a very poorly crystalline hydroxyapatite on electron diffraction. In summary, these data suggest that this culture system consists of chondrocytes which undergo differentiation in vitro as assessed by their elevated levels of alkaline phosphatase and type X collagen and their ultrastructural appearance.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in cells's secretory organelles and extracellular matrix during endochondral ossification in the mandibular condyle of the growing rat

The mandibular condyle from 20-day-old rats was examined in the electron microscope with particular attention to intracellular secretory granules and extracellular matrix. Moreover, type II collagen was localized by an immunoperoxidase method. The condyle has been divided into five layers: (1) the most superficial, articular layer, (2) polymorphic cell layer, (3) flattened cell layer, (4) upper hypertrophic, and (5) lower hypertrophic cell layers. In the articular layer, the cells seldom divide, but in the polymorphic layer and upper part of the flattened cell layer, mitosis gives rise to new cells. In these layers, cells produce two types of secretory granules, usually in distinct stacks of the Golgi apparatus; type a, cylindrical granules, in which 300-nm-long threads are packed in bundles which appear "lucent" after formaldehyde fixation; and type b, spherical granules loaded with short, dotted filaments. The matrix is composed of thick banded "lucent" fibrils in a loose feltwork of short, dotted filaments. The cells arising from mitosis undergo endochondral differentiation, which begins in the lower part of the flattened cell layer and is completed in the upper hypertrophic cell layer; it is followed by gradual cell degeneration in the lower hypertrophic cell layer. The cells produce two main types of secretory granules: type b as above; and type c, ovoid granules containing 300-nm-long threads associated with short, dotted filaments. A possibly different secretory granule, type d, dense and cigar-shaped, is also produced. The matrix is composed of thin banded fibrils in a dense feltwork. In the matrix of the superficial layers, the "lucency" of the fibrils indicated that they were composed of collagen I, whereas the "lucency" of the cylindrical secretory granules suggested that they transported collagen I precursors to the matrix. Moreover, the use of ruthenium red indicated that the feltwork was composed of proteoglycan; the dotted filaments packed in spherical granules were similar to, and presumably the source of, the matrix feltwork. The superficial layers did not contain collagen II and were collectively referred to as perichondrium. In the deep layers, the ovoid secretory granules displayed collagen II antigenicity and were likely to transport precursors of this collagen to the matrix, where it appeared in the thin banded fibrils. That these granules also carried proteoglycan to the matrix was suggested by their content of short dotted filaments. Thus the deep layers contained collagen II and proteoglycan as in cartilage; they were collectively referred to as the hyaline cartilage region.

Immunohistochemical and biochemical demonstration of calcium-dependent cysteine proteinase (calpain) in calcifying cartilage of rats

Calpain is a Ca2(+)-dependent cysteine proteinase that has neutral pH optima. There are two classes of calpains that differ in their optimal calcium ion concentration for enzymatic activity. Calpain I requires a low concentration of Ca2+ for activation, and calpain II requires a much higher Ca2+ concentration. This report describes the immunohistochemical and biochemical demonstration of calpain II in calcifying cartilage in rats and also the degradation of the cartilage proteoglycan subunit by calpain II. Immunoperoxidase (peroxidase-antiperoxidase) staining of the frozen sections of the knee joint from 3-day-old and 6-day-old Wistar rats, using polyclonal antibodies against the respective heavy subunits of calpains I and II, showed positive staining only with the anti-calpain II antibody in the hypertrophic chondrocytes and surrounding cartilaginous matrix of the growth cartilage. Diethylaminoethyl-cellulose chromatography of the cartilaginous extract from 3-day-old rats showed a peak of caseinolytic activity attributable to calpain as well as an inhibitory peak of calpastatin, a specific inhibitor protein of calpains. Immunoblotting using the anti-calpain II antibody of the calpain peak demonstrated identity with the heavy subunit of calpain II (80 kDa). Proteoglycan-degrading activity of calpain was assessed using porcine kidney calpain II and the porcine articular cartilage proteoglycan subunit. After incubation in the presence of Ca2+, degradation of proteoglycan was demonstrated by the change of the elution position on Sepharose-2B chromatography. It is possible that calpain functions as one of the proteoglycan-degrading proteolytic enzymes of growth cartilage. Intracellular localization of calpain in hypertrophic chondrocytes also suggests a role in the hypertrophic process of the chondrocyte in growth cartilage.

Collagens of the chicken eggshell membranes

An immunohistochemical analysis of the eggshell membranes shows the occurrence of type X collagen while type I collagen was not detected by using an appropriate monoclonal antibody with untreated shell membranes. A positive immuno-reaction for type I collagen was obtained after digestion of the shell membranes with pepsin. These observations indicate the possibility that type I collagen epitope was masked by type X collagen and that type X collagen may serve as an inhibitory boundary for biomineralization.

Repair of intra-membranous bone fractures and defects in rats. Immunolocalization of bone and cartilage proteins and proteoglycans

Osseous healing of experimental fractures and defects in membranous bone was studied in an animal model and the appearance and localization of selected bone and cartilage proteins and proteoglycans determined by polyclonal antibodies. The bone lesions were made in the parietal bone of young rats and subsequently studied on days 3, 5, 8, 15, 30, and 100 after surgery. The bone matrix proteins investigated (62 kDa, bone sialoprotein I and II, and osteopontin) appeared early, adjacent to the periosteal surfaces (pericranium and dura mater) and the marginal bone. The staining reactions were maximal at days 8 or 15 after trauma. Similar patterns were discerned for some cartilage macromolecules studied (58 kDa, 59 kDa, and chondrocalcin), although others showed no labelling whatsoever (148 kDa, and 400 kDa proteins). The proteoglycans PG-S1, PG-S2, and PG-LA were not identified. No callus or cartilage formation were associated with the bone healing process, and the differences between the regenerative pattern of the fractures and defects were limited. The findings emphasize the importance of rigid fixation in craniomaxillofacial surgery.

Ultrastructural and immunocytochemical study of bone-derived cells cultured in three-dimensional matrices: influence of chondroitin-4 sulfate on mineralization

Bone-derived cells were cultured in three-dimensional reconstituted matrices made of type I collagen or type I collagen chondroitin-4-sulfate. As observed by microscope, their characteristics were as follows: The cells deposited a faint extracellular matrix mainly composed of type I collagen. In the collagen-chondroitin-sulfate sponge fibers, a calcification process, which involved the deposition of hydroxyapatite crystals, was demonstrated. Mineralization occurred only in collagen chondroitin sulfate sponge fibers when seeded with bone-derived cells and was not seen with nonosteogenic cells, such as gingival fibroblasts. Gla protein was intracellularly visualized in both types of sponges seeded with bone-derived cells while an extracellular secretion was seen only in the collagen chondroitin sulfate sponge fibers where calcification occurred. These results suggest that collagen chondroitin sulfate promotes in vitro mineralization of three-dimensional collagen matrices when seeded with bone-derived cells.

Immunohistochemical localization of type X collagen in the proximal tibiotarsi of broiler chickens and turkeys

Type X collagen is a prominent component of the extracellular matrix in cartilage destined to mineralize during endochondral ossification, yet its role is only now being determined. As a prelude to determining what, if any, alterations occur in the distribution of type X collagen in growth plates of poultry with rickets or tibial dyschondroplasia, our objective in the current study was to determine the distribution of type X collagen in the proximal tibiotarsi of broiler chickens and turkeys from 1 day of age through physeal closure. Proximal tibiotarsi from five male broiler chickens, five female broiler chickens and five male turkeys were collected at 1, 7, 14, 28, 56, and 98 days of age and processed for immunohistochemistry; a monoclonal antibody for type X collagen was used to demonstrate type X collagen distribution. Our findings indicate that type X collagen is produced in the prehypertrophic and early hypertrophic zones of the avian growth plate and is incorporated into the extracellular matrix in these zones. Furthermore, intracellular type X collagen is markedly decreased in more mature areas of the growth plate, although type X collagen remains a prominent component of the extracellular matrix until the matrix is completely resorbed. In addition, the distribution of type X collagen is similar in the proximal tibiotarsi of broiler chickens and turkeys at comparable stages of endochondral ossification and distribution of type X collagen in the secondary center of ossification parallels that in the physis.