Chondrocyte differentiation

Collagen metabolism is markedly altered in the hypertrophic cartilage of growth plates from rats with growth impairment secondary to chronic renal failure

Skeletal growth depends on growth plate cartilage activity, in which matrix synthesis by chondrocytes is one of the major processes contributing to the final length of a bone. On this basis, the present work was undertaken to ascertain if growth impairment secondary to chronic renal insufficiency is associated with disturbances of the extracellular matrix (ECM) of the growth plate. By combining stereological and in situ hybridization techniques, we examined the expression patterns of types II and X collagens and collagenase-3 in tibial growth plates of rats made uremic by subtotal nephrectomy (NX) in comparison with those of sham-operated rats fed ad libitum (SAL) and sham-operated rats pair-fed with NX (SPF). NX rats were severely uremic, as shown by markedly elevated serum concentrations of urea nitrogen, and growth retarded, as shown by significantly decreased longitudinal bone growth rates. NX rats showed disturbances in the normal pattern of chondrocyte differentiation and in the rates and degree of substitution of hypertrophic cartilage with bone, which resulted in accumulation of cartilage at the hypertrophic zone. These changes were associated with an overall decrease in the expression of types II and X collagens, which was especially marked in the abnormally extended zone of the hypertrophic cartilage. Unlike collagen, the expression of collagenase-3 was not disturbed severely. Electron microscopic analysis proved that changes in gene expression were coupled to alterations in the mineralization as well as in the collagen fibril architecture at the hypertrophic cartilage. Because the composition and structure of the ECM have a critical role in regulating the behavior of the growth plate chondrocytes, results obtained are consistent with the hypothesis that alteration of collagen metabolism in these cells could be a key process underlying growth retardation in uremia.

Phosphate stimulates differentiation and mineralization of the chondroprogenitor clone ATDC5

ATDC5 cells were employed to examine how inorganic phosphate (Pi) influences chondrocytic bone formation. 1) Pi (3 - 30 mM) plus ascorbic acid (50 microg/ml) dose-dependently accelerated proliferative differentiation and mineralization of ATDC5. 2) Northern blot analysis revealed that 10 mM Pi suppressed expression of type II collagen and PTH (parathyroid hormone) / PTH-related peptide (PTHrP) receptor, while it accelerated type X collagen expression. 3) Pi (3 - 30 mM) dose-dependently increased luciferase activity in the cells transfected with 3000 bp type X collagen promoter fused to the luciferase gene. The results suggest a regulatory role of Pi in endochondral osteogenesis.

Continuous expression of Cbfa1 in nonhypertrophic chondrocytes uncovers its ability to induce hypertrophic chondrocyte differentiation and partially rescues Cbfa1-deficient mice

Chondrocyte hypertrophy is a mandatory step during endochondral ossification. Cbfa1-deficient mice lack hypertrophic chondrocytes in some skeletal elements, indicating that Cbfa1 may control hypertrophic chondrocyte differentiation. To address this question we generated transgenic mice expressing Cbfa1 in nonhypertrophic chondrocytes (alpha1(II) Cbfa1). This continuous expression of Cbfa1 in nonhypertrophic chondrocytes induced chondrocyte hypertrophy and endochondral ossification in locations where it normally never occurs. To determine if this was caused by transdifferentiation of chondrocytes into osteoblasts or by a specific hypertrophic chondrocyte differentiation ability of Cbfa1, we used the alpha1(II) Cbfa1 transgene to restore Cbfa1 expression in mesenchymal condensations of the Cbfa1-deficient mice. The transgene restored chondrocyte hypertrophy and vascular invasion in the bones of the mutant mice but did not induce osteoblast differentiation. This rescue occurred cell-autonomously, as skeletal elements not expressing the transgene were not affected. Despite the absence of osteoblasts in the rescued animals there were multinucleated, TRAP-positive cells resorbing the hypertrophic cartilage matrix. These results identify Cbfa1 as a hypertrophic chondrocyte differentiation factor and provide a genetic argument for a common regulation of osteoblast and chondrocyte differentiation mediated by Cbfa1.

Articular cartilage and changes in arthritis. An introduction: cell biology of osteoarthritis

The reaction patterns of chondrocytes in osteoarthritis can be summarized in five categories: (1) proliferation and cell death (apoptosis); changes in (2) synthetic activity and (3) degradation; (4) phenotypic modulation of the articular chondrocytes; and (5) formation of osteophytes. In osteoarthritis, the primary responses are reinitiation of synthesis of cartilage macromolecules, the initiation of synthesis of types IIA and III procollagens as markers of a more primitive phenotype, and synthesis of active proteolytic enzymes. Reversion to a fibroblast-like phenotype, known as "dedifferentiation", does not appear to be an important component. Proliferation plays a role in forming characteristic chondrocyte clusters near the surface, while apoptosis probably occurs primarily in the calcified cartilage.

Recombinant human extracellular matrix protein 1 inhibits alkaline phosphatase activity and mineralization of mouse embryonic metatarsals in vitro

Two mRNAs are transcribed from the extracellular matrix protein 1 gene (Ecm1): Ecm1a and an alternatively spliced Ecm1b. We studied Ecm1 mRNA expression and localization during endochondral bone formation and investigated the effect of recombinant human (rh) Ecm1a protein on organ cultures of embryonic mouse metatarsals. Of the two transcripts, Ecm1a mRNA was predominantly expressed in fetal metacarpals from day 16 to 19 after gestation. Ecm1 expression was not found in 16- and 17-day-old metatarsals of which the perichondrium was removed. In situ hybridization and immunohistochemistry demonstrated Ecm1 expression in the connective tissues surrounding the developing bones, but not in the cartilage. Biological effects of rhEcm1a protein on fetal metatarsal cultures were biphasic: at low concentrations, Ecm1a stimulated alkaline phosphatase activity and had no effect on mineralization, whereas at higher concentrations, Ecm1a dose dependently inhibited alkaline phosphatase activity and mineralization. These results suggest that Ecm1a acts as a novel negative regulator of endochondral bone formation.

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.

Roles of growth hormone and insulin-like growth factor 1 in mouse postnatal growth

To examine the relationship between growth hormone (GH) and insulin-like growth factor 1 (IGF1) in controlling postnatal growth, we performed a comparative analysis of dwarfing phenotypes manifested in mouse mutants lacking GH receptor, IGF1, or both. This genetic study has provided conclusive evidence demonstrating that GH and IGF1 promote postnatal growth by both independent and common functions, as the growth retardation of double Ghr/Igf1 nullizygotes is more severe than that observed with either class of single mutant. In fact, the body weight of these double-mutant mice is only approximately 17% of normal and, in absolute magnitude ( approximately 5 g), only twice that of the smallest known mammal. Thus, the growth control pathway in which the components of the GH/IGF1 signaling systems participate constitutes the major determinant of body size. To complement this conclusion mainly based on extensive growth curve analyses, we also present details concerning the involvement of the GH/IGF1 axis in linear growth derived by a developmental study of long bone ossification in the mutants.

Effective isolation of high-quality total RNA from human adult articular cartilage

The isolation of large quantities of good-quality RNA from human articular cartilage has been a long-standing problem for researchers working with human articular cartilage. In this paper we report a protocol which we have developed based on the Qiagen RNeasy procedure to produce high yields of purified, DNA-free RNA from normal and osteosteoarthritic human articular cartilage. The average yield of RNA was 8.39 microg/g (n = 59) for normal and 6.69 microg/g (n = 58) for osteoarthritic cartilage (average ratio OD 260/280 = 1.8-1.9). Quantitative PCR, cDNA array technology, and Northern blot analysis were used to verify the quality of the RNA.

Ex-FABP: a fatty acid binding lipocalin developmentally regulated in chicken endochondral bone formation and myogenesis

Extracellular fatty acid binding protein (Ex-FABP) is a 21 kDa lipocalin specifically binding fatty acids, expressed during chicken embryo development in hypertrophic cartilage, in muscle fibers and in blood granulocytes. In chondrocyte and myoblast cultures Ex-FABP expression is increased by inflammatory agents and repressed by anti-inflammatory agents. In adult cartilage Ex-FABP is expressed only in pathological conditions such as in dyschondroplastic and osteoarthritic chickens. The possible mammalian counterpart is the Neu-related lipocalin (NRL), a lipocalin overexpressed in rat mammary cancer; NRL is homologous to the human neutrophil gelatinase associated lipocalin (NGAL) expressed in granulocytes and in epithelial cells in inflammation and malignancy and to the Sip24 (super-inducible protein 24), an acute phase lipocalin expressed in mouse after turpentine injection. Immunolocalization and in situ hybridization showed that NRL/NGAL is expressed in hypertrophic cartilage, in forming skeletal muscle fibers and in developing heart. In adult cartilage NRL/NGAL was expressed in articular cartilage from osteoarthritic patients and in chondrosarcoma. Moreover, NRL was induced in chondrocyte and myoblast cultures by an inflammatory agent. We propose that these lipocalins (Ex-FABP, NRL/NGAL, Sip24) represent stress proteins physiologically expressed in tissues where active remodeling is taking place during development and also present in tissues characterized by an acute phase response due to pathological conditions.

Targeted deletion of Minpp1 provides new insight into the activity of multiple inositol polyphosphate phosphatase in vivo

Multiple inositol polyphosphate phosphatase (Minpp1) metabolizes inositol 1,3,4,5,6-pentakisphosphate (InsP(5)) and inositol hexakisphosphate (InsP(6)) with high affinity in vitro. However, Minpp1 is compartmentalized in the endoplasmic reticulum (ER) lumen, where access of enzyme to these predominantly cytosolic substrates in vivo has not previously been demonstrated. To gain insight into the physiological activity of Minpp1, Minpp1-deficient mice were generated by homologous recombination. Tissue extracts from Minpp1-deficient mice lacked detectable Minpp1 mRNA expression and Minpp1 enzyme activity. Unexpectedly, Minpp1-deficient mice were viable, fertile, and without obvious defects. Although Minpp1 expression is upregulated during chondrocyte hypertrophy, normal chondrocyte differentiation and bone development were observed in Minpp1-deficient mice. Biochemical analyses demonstrate that InsP(5) and InsP(6) are in vivo substrates for ER-based Minpp1, as levels of these polyphosphates in Minpp1-deficient embryonic fibroblasts were 30 to 45% higher than in wild-type cells. This increase was reversed by reintroducing exogenous Minpp1 into the ER. Thus, ER-based Minpp1 plays a significant role in the maintenance of steady-state levels of InsP(5) and InsP(6). These polyphosphates could be reduced below their natural levels by aberrant expression in the cytosol of a truncated Minpp1 lacking its ER-targeting N terminus. This was accompanied by slowed cellular proliferation, indicating that maintenance of cellular InsP(5) and InsP(6) is essential to normal cell growth. Yet, depletion of cellular inositol polyphosphates during erythropoiesis emerges as an additional physiological activity of Minpp1; loss of this enzyme activity in erythrocytes from Minpp1-deficient mice was accompanied by upregulation of a novel, substitutive inositol polyphosphate phosphatase.

Transcriptional mechanisms of chondrocyte differentiation

With the goal of identifying master transcription factors that control the genetic program of differentiation of mesenchymal cells into chondrocytes, we first delineated a 48-bp chondrocyte-specific enhancer element in the gene for proalpha1(II) collagen (Col2a1), an early and abundant marker of chondrocytes. Our experiments have demonstrated that the HMG-box-containing transcription factor, Sox9 which binds and activates this enhancer element, is required for chondrocyte differentiation and for expression of a series of chondrocyte-specific marker genes including Col2a1, Col9a2, Col11a2 and Aggrecan. In the absence of Sox9 the block in differentiation occurs at the stage of mesenchymal condensation, suggesting the hypothesis that Sox9 might also control expression of cell surface proteins needed for mesenchymal condensation. Since Sox9 also contains a potent transcription activation domain, it is a typical transcription factor. Two other members of the Sox family, L-Sox5 and Sox6, also bind to the 48-bp Col2a1 enhancer and together with Sox9 activate this enhancer as well as the endogenous Col2a1 and aggrecan genes. L-Sox5 and Sox6 have a high degree of sequence identity to each other and are likely to have redundant functions. Except for the HMG-box, L-Sox5 and Sox6 have no similarity to Sox9 and, hence, are likely to have a complementary function to that of Sox9. Our experiments suggest the hypothesis that, like Sox9, Sox5 and Sox6 might also be needed for chondrocyte differentiation. Other experiments, have provided evidence that the Sox9 polypeptide and the Sox9 gene are targets of signaling molecules that are known to control discrete steps of chondrogenesis in the growth plate of endochondral bones. Protein kinase A (PKA) phosphorylation of Sox9 increases its DNA binding and transcriptional activity. Since PKA-phosphorylated-Sox9 is found in the prehypertrophic zone of the growth plate, the same location where the gene for the receptor of the parathyroid hormone-related peptide (PTHrP) is expressed and since PTHrP signaling is mediated by cyclic AMP, we have hypothesized that Sox9 is a target for PTHrP signaling. Other experiments have also shown that fibroblast growth factors (FGFs) increase the expression of Sox9 in chondrocytes in culture and that this activation is mediated by the mitogen-activated protein kinase pathway. These results favor the hypothesis that in achondroplasia, a disease caused by activating mutations in FGF receptor 3, there might also be an abnormally high Sox9 expression.

Utilization of bone morphogenetic protein receptors during chondrocyte maturation

Cartilage from the upper, cephalic portion of embryonic chick sternums undergoes hypertrophy, while the lower, caudal portion of the sternum remains as cartilage. Bone morphogenetic proteins (BMPs) induce type X collagen (colX) in cultured upper but not lower sternal chondrocytes (LSCs). We have examined the utilization of BMP receptors (BMPRs) by upper sternal chondrocytes (USCs) and LSCs both by analyzing receptor expression and by overexpressing mutant BMPRs. Reverse-transcription polymerase chain reaction (RT-PCR) analyses indicate that both upper and lower chondrocytes produce messenger RNA (mRNA) for all three receptors: BMPR type IA (BMPR-IA), BMPR type IB (BMPR-IB), and BMPR type II (BMPR-II). Infection of USC with retroviral vectors expressing constitutively active (CA) BMPRs showed that CA-BMPR-IB, like exogenous BMP-4, induced both colX mRNA and elevated alkaline phosphatase (AP), while CA-BMPR-IA was markedly less potent. However, expression of activated receptors in LSC cultures resulted in only minimal induction of hypertrophic markers. Consistent with the results seen for CA receptors, dominant negative (DN) BMPR-IB blocked BMP-induced hypertrophy in USCs more effectively than DN-BMPR-IA. These results imply that the major BMPR required for BMP induction of chondrocyte hypertrophy is BMPR-IB, and that difference between permanent and prehypertrophic chondrocytes is not caused by absence of receptors required for BMP signaling.

Chondrocytes and longitudinal bone growth: the development of tibial dyschondroplasia

Growth plate cartilage is central to the process of bone elongation. Chondrocytes originating within the resting zone of the growth plate proceed through a series of intermediate phenotypes: proliferating, prehypertrophic and hypertrophic, before reaching a terminally differentiated state. Disruption of this chondrocyte maturational sequence causes many skeletal abnormalities in poultry such as tibial dyschondroplasia (TD), which is a common cause of deformity and lameness in the broiler chicken. Cell and matrix components of the growth plate have been studied in order to determine the cause(s) of the premature arrest of chondrocyte differentiation and retention of prehypertrophic chondrocytes observed in TD. Chondrocyte proliferation proceeds normally in TD, but markers of the differentiated phenotype, local growth factors, and the vitamin D receptor are abnormally expressed within the prehypertrophic chondrocytes above, and within, the lesion. Tibial dyschondroplasia is also associated with a reduced incidence of apoptosis, suggesting that the lesion contains an accumulation of immature cells that have outlived their normal life span. Immunolocalization studies of matrix components suggest an abnormal distribution within the TD growth plate that is consistent with a failure of the chondrocytes to fully hypertrophy. In addition, the collagen matrix of the TD lesion is highly crosslinked, which may make the formed lesion more impervious to vascular invasion and osteoclastic resorption. Recent studies have applied the techniques of differential display and semiquantitative reverse transcriptase-polymerase chain reaction to RNA obtained from discrete populations of growth plate chondrocytes of different maturational phenotypes. This strategy has allowed us to compare phenotypically identical cell fractions from normal and TD growth plates in an attempt to identify possible candidate genes for TD.

A novel in vitro culture system for analysis of functional role of phosphate transport in endochondral ossification

In vivo expression of the type III sodium-dependent phosphate transporter (NaPiT) Glvr-1 during endochondral ossification, suggests a functional role for inorganic phosphate (Pi) transport in cartilage calcification. For further analysis of this relationship, an in vitro model of endochondral ossification is required. In this context, we investigated the characteristics of Pi transport in the new chondrogenic cell line ATDC5 in relation to extracellular matrix (ECM) formation and mineralization. Pi uptake in ATDC-5 cells and in isolated matrix vesicles (MVs) is mediated by an Na-dependent Pi transporter with a pH dependency characteristic of a type III Pi carrier (lower activity at alkaline pH). Northern blot analysis indicated that ATDC-5 cells express Glvr-1 transcripts during the various stages of their maturation with a maximal level during the proliferating stage. In isolated MVs, Pi transport activity was maximal at day 21, concomitant with the beginning of type X collagen messenger RNA expression. These events preceded the initiation of matrix mineralization, which was apparent at day 25, and then gradually increased until day 47. This temporal relationship between maximal Pi transport activity in MVs and the expression of a marker of mineralizing chondrocytes is compatible with the possible involvement of Pi transport in the ECM calcification observed in ATDC-5 cell cultures. In conclusion, these observations suggest that ATCD-5 cells in culture represent a promising model for the analysis of a functional role of Pi transport in the initial events of endochondral ossification.

Disrupted expression of matrix genes in the growth plate of the mouse cartilage matrix deficiency (cmd) mutant

Chondrodysplasia in the autosomal recessive cartilage matrix deficiency (cmd) mutant is caused by lack of the proteoglycan aggrecan arising from a mutation in the gene. Homozygous cmd/cmd mice are characterized by disorganisation of chondrocytes in the growth plate, disproportionate dwarfism, cleft palate, and perinatal lethality. We have studied the impact of the aggrecan deficiency on the expression of other matrix genes during the differentiation of chondrocytes in the growth plate of cmd/cmd 18.5 day fetuses. Compared with the wild-type, there are significant differences in the growth plates of cmd mutants in the combinations of co-expression of genes encoding the glycoprotein link protein, proteoglycan syndecan 3, collagens alpha 1 (X) [Col10a1], alpha 2(XI) [Col11a2], and the alternative transcripts of alpha 1 (II) [Col2a1 type IIA form], and alpha 1 (IX) [Col9a1 long and short forms]. The discordance of gene expression in cmd chondrocytes may be additional factors contributing to the disrupted cellular architecture of the growth plate resulting from the primary absence of aggrecan.

Expression of Indian hedgehog, parathyroid hormone-related protein, and their receptors in the postnatal growth plate of the rat: evidence for a locally acting growth restraining feedback loop after birth

A locally acting growth restraining feedback loop has been identified in the murine embryonic growth plate in which the level of parathyroid hormone-related peptide (PTHrP) expression regulates the pace of chondrocyte differentiation. To date, it is largely unknown whether this feedback loop also regulates the pace of chondrocyte differentiation in the growth plate after birth. We therefore characterized the spatio-temporal expression of Indian hedgehog (IHH), PTHrP, and their receptors in the postnatal growth plate from female and male rats of 1, 4, 7, and 12 weeks of age. These stages are representative for early life and puberty in rats. Using semiquantitative reverse-transcription polymerase chain reaction (RT-PCR) on growth plate tissue, IHH and components of its receptor complex, patched (PTC) and smoothened (SMO), PTHrP and the type I PTH/PTHrP receptor messenger RNA (mRNA) were shown at all ages studied irrespective of gender. Using in situ hybridization, IHH, PTHrP, and PTH/PTHrP receptor mRNA were detected in prehypertrophic and hypertrophic chondrocytes in both sexes during development. In addition, especially in the younger age groups, faint expression of PTH/PTHrP receptor mRNA also was shown in stem cells and proliferative chondrocytes. Immunohistochemistry confirmed the observations made with in situ hybridization, by showing the presence of IHH, PTC, PTHrP, and PTH/PTHrP receptor protein in prehypertrophic and hypertrophic chondrocytes. In addition, staining for hedgehog, PTC, and PTHrP also was observed in growth plate stem cells. No differences in staining patterns were observed between the sexes. Furthermore, no mRNA or protein expression of the mentioned factors was detected in the perichondrium. Our data suggest that in contrast to the proposed feedback loop in the early embryonic growth plate, which requires the presence of the perichondrium, a feedback loop in the postnatal growth plate can be confined to the growth plate itself. In fact, two loops might exist: (1) a loop confined to the transition zone and early hypertrophic chondrocytes, which might in part be autocrine and (2) a loop involving the growth plate stem cells.

The ribosomal protein QM is expressed differentially during vertebrate endochondral bone development

Endochondral ossification is a carefully coordinated developmental process that converts the cartilaginous model of the embryonic skeleton to bone with accompanying long bone growth. To identify genes that regulate this process we performed a complementary DNA (cDNA) subtractive hybridization of fetal bovine proliferative chondrocyte cDNA from epiphyseal cartilage cDNA. The subtracted product was used to screen a fetal bovine cartilage cDNA library. Ten percent of the clones identified encoded the bovine orthologue of the human ribosomal protein "QM." Northern and western blot analysis confirmed that QM was highly expressed by cells isolated from epiphyseal cartilage as opposed to proliferative chondrocytes. In contrast, no detectable difference in the expression of mRNA for the ribosomal protein S11 was detected. Immunohistochemical analysis of fetal bovine limb sections revealed that QM was not expressed by the majority of the epiphyseal chondrocytes but only by chondrocytes in close proximity to capillaries that had invaded the epiphyseal cartilage. Strongest QM expression was seen in osteoblasts in the diaphyseal region of the bone adjoining the growth plate, within the periosteum covering the growth plate and within secondary centers of ossification. Hypertrophic chondrocytes within the growth plate adjoining the periosteum also were positive for QM as were chondrocytes in the perichondrium adjoining the periosteum. In vitro investigation of the expression of QM revealed higher QM expression in nonmineralizing osteoblast and pericyte cultures as compared with mineralizing cultures. The in vivo and in vitro expression pattern of QM suggests that this protein may have a role in cell differentiation before mineralization.

Activating transcription factor 2 is necessary for maximal activity and serum induction of the cyclin A promoter in chondrocytes

Endochondral bone growth is regulated through the proliferation and differentiation of growth plate chondrocytes. Mice deficient for the activating transcription factor 2 (ATF-2) gene show reduced proliferation of chondrocytes. Here we demonstrate that the cyclin A gene is a target of ATF-2 in chondrocytes. Serum stimulation of chondrogenic rat chondrosarcoma cells induces cyclin A expression. A cyclic AMP response element (CRE) is necessary for optimal activity and serum inducibility of the cyclin A promoter and confers regulation by ATF-2. Phosphorylation and activity of ATF-2 are enhanced dramatically upon serum stimulation of rat chondrosarcoma cells. Mutation of the CRE or overexpression of dominant-negative ATF-2 inhibits serum induction of the cyclin A promoter. Chondrocytes from ATF-2-deficient mice display reduced and delayed induction of cyclin A upon serum stimulation. The ATF-2-related transcription factor CRE-binding protein contributes to the activity of the cyclin A CRE in chondrocytes, whereas c-Jun and c-Fos regulate the promoter independently of the CRE. Our data suggest that the reduction in cyclin A levels in chondrocytes from ATF-2-deficient mice contributes to their phenotype of reduced chondrocyte proliferation and dwarfism.

Cell differentiation and matrix gene expression in mesenchymal chondrosarcomas

Mesenchymal chondrosarcomas are small-cell malignancies named as chondrosarcomas due to the focal appearance of cartilage islands. In this study, the use of in situ detection techniques on a large series of mesenchymal chondrosarcoma specimens allowed the identification of tumor-cell differentiation pathways in these neoplasms. We were able to trace all steps of chondrogenesis within mesenchymal chondrosarcoma by using characteristic marker genes of chondrocytic development. Starting from undifferentiated cells, which were negative for vimentin and any other mesenchymal marker, a substantial portion of the cellular (undifferentiated) tumor areas showed a chondroprogenitor phenotype with an onset of expression of vimentin and collagen type IIA. Cells in the chondroid areas showed the full expression panel of mature chondrocytes including type X collagen indicating focal hypertrophic differentiation of the neoplastic chondrocytes. Finally, evidence was found for transdifferentiation of the neoplastic chondrocytes to osteoblast-like cells in areas of neoplastic bone formation. These results establish mesenchymal chondrosarcoma as the very neoplasm of differentiating premesenchymal chondroprogenitor cells. The potential of neoplastic bone formation in mesenchymal chondrosarcoma introduces a new concept of neoplastic (chondrocytic) osteogenesis in musculoskeletal malignant neoplasms, which qualifies the old dogma that neoplastic bone/osteoid formation automatically implies the diagnosis of osteosarcoma.

Microscopic and histochemical manifestations of hyaline cartilage dynamics

Structure and function of hyaline cartilages has been the focus of many correlative studies for over a hundred years. Much of what is known regarding dynamics and function of cartilage constituents has been derived or inferred from biochemical and electron microscopic investigations. Here we show that in conjunction with ultrastructural, and high-magnification transmission light and polarization microscopy, the well-developed histochemical methods are indispensable for the analysis of cartilage dynamics. Microscopically demonstrable aspects of cartilage dynamics include, but are not limited to, formation of the intracellular liquid crystals, phase transitions of the extracellular matrix and tubular connections between chondrocytes. The role of the interchondrocytic liquid crystals is considered in terms of the tensegrity hypothesis and non-apoptotic cell death. Phase transitions of the extracellular matrix are discussed in terms of self-alignment of chondrons, matrix guidance pathways and cartilage growth in the absence of mitosis. The possible role of nonenzymatic glycation reactions in cartilage dynamics is also reviewed.

Mapping and developmental expression analysis of the WD-repeat gene Preb

We have isolated from mouse a novel WD-motif-containing gene designated Preb. This gene encodes a predicted protein of 416 amino acids and has significant homology with other members of the WD-motif gene superfamily that play a role in cell fate determination. Preb maps to the proximal end of chromosome 5 in mouse, near the Hmx1 homeobox gene. Preb is detectable in early stage embryos in the peripheral nervous system, developing liver, and surface ectoderm. Later, Preb is expressed in the anterior portion of Rathke's pouch, which gives rise to the anterior pituitary, the organ responsible for the production of prolactin and other hormones. In midgestation embryos, the most extensive expression of Preb is observed in the perichondrium of the craniofacial, axial, and appendicular skeleton. The expression pattern of Preb in murine embryos suggests a potential role in the specification of multiple cell types, in particular, the fetal skeleton.

Different bone growth rates are associated with changes in the expression pattern of types II and X collagens and collagenase 3 in proximal growth plates of the rat tibia

Skeletal growth depends on endochondral ossification in growth plate cartilage, where proliferation of chondrocytes, matrix synthesis, and increases in chondrocyte size all contribute to the final length of a bone. To learn more about the potential role of matrix synthesis/degradation dynamics in the determination of bone growth rate, we investigated the expression of matrix collagens and collagenase 3 in tibial growth plates in three age groups of rats (21, 35, and 80 days after birth), each characterized by specific growth rates. By combining stereological and in situ hybridization techniques, it was found that the expression of matrix collagens and collagenase 3 was specifically turned on or off at specific stages of the chondrocyte-differentiation cycle, and these changes occurred as a temporal sequence that varied depending of animal growth rate. Furthermore, the expression of these matrix proteins by a growth plate chondrocyte was found to be sped up or slowed down depending of the growth rate. In addition to expression of types II and X collagen, collagenase-3 expression was found to constitute a constant event in the series of changes in gene expression that takes place during the chondrocyte-differentiation process. Collagenase-3 expression was found to show a biphasic pattern: it was intermittently expressed at the proliferative phase and uniformly expressed at the hypertrophic stage. An intimate relationship between morphological and kinetic changes associated with chondrocyte hypertrophy and changes in the expression pattern of matrix collagens and collagenase 3 was observed. Present data prove that the matrix synthesis/degradation dynamics of the growth plate cartilage varied depending on growth rate; these results support the hypothesis that changes in matrix degradation and synthesis are a critical link in the sequence of tightly regulated events that lead to chondrocytic differentiation.

The transmembrane mutation G380R in fibroblast growth factor receptor 3 uncouples ligand-mediated receptor activation from down-regulation

A point mutation, Gly380Arg, in the transmembrane domain of fibroblast growth factor receptor 3 (FGFR3) leads to achondroplasia, the most common form of genetic dwarfism in humans. This substitution was suggested to enhance mutant receptor dimerization, leading to constitutive, ligand-independent activation. We found that dimerization and activation of the G380R mutant receptor are predominantly ligand dependent. However, using both transient and stable transfections, we found significant overexpression only of the mutant receptor protein. Metabolic pulse-chase experiments, cell surface labeling, and kinetics of uptake of radiolabeled ligand demonstrated a selective delay in the down-regulation of the mutant receptor. Moreover, this receptor was now resistant to ligand-mediated internalization, even at saturating ligand concentrations. Finally, transgenic mice expressing the human G380R mutant receptor under the mouse receptor transcriptional control demonstrated a markedly expanded area of FGFR3 immunoreactivity within their epiphyseal growth plates, compatible with an in vivo defect in receptor down-regulation. We propose that the achondroplasia mutation G380R uncouples ligand-mediated receptor activation from down-regulation at a site where the levels and kinetics of FGFR3 signals are crucial for chondrocyte maturation and bone formation.

The murine Bapx1 homeobox gene plays a critical role in embryonic development of the axial skeleton and spleen

Our previous studies in both mouse and human identified the Bapx1 homeobox gene, a member of the NK gene family, as one of the earliest markers for prechondrogenic cells that will subsequently undergo mesenchymal condensation, cartilage production and, finally, endochondral bone formation. In addition, Bapx1 is an early developmental marker for splanchnic mesoderm, consistent with a role in visceral mesoderm specification, a function performed by its homologue bagpipe, in Drosophila. The human homologue of Bapx1 has been identified and mapped to 4p16.1, a region containing loci for several skeletal diseases. Bapx1 null mice are affected by a perinatal lethal skeletal dysplasia and asplenia, with severe malformation or absence of specific bones of the vertebral column and cranial bones of mesodermal origin, with the most severely affected skeletal elements corresponding to ventral structures associated with the notochord. We provide evidence that the failure of the formation of skeletal elements in Bapx1 null embryos is a consequence of a failure of cartilage development, as demonstrated by downregulation of several molecular markers required for normal chondroblast differentiation (α 1(II) collagen, Fgfr3, Osf2, Indian hedgehog, Sox9), as well as a chondrocyte-specific alpha1 (II) collagen-lacZ transgene. The cartilage defects are correlated with failed differentiation of the sclerotome at the time when these cells are normally initiating chondrogenesis. Loss of Bapx1 is accompanied by an increase in apoptotic cell death in affected tissues, although cell cycling rates are unaltered.

Lineage-dependent collagen expression and assembly during osteogenic or chondrogenic differentiation of a mesoblastic cell line

The mesoblastic clone, C1, behaves as a tripotential progenitor able to self-renew and to differentiate toward osteogenesis, chondrogenesis, or adipogenesis in response to specific inducers. In this study, expression and deposition by the C1 cells of essential components of the extracellular matrix, collagens type I, II, III, V, XI, VI, IX, and X were followed along the osteogenic and chondrogenic pathways, through biochemical, immunochemical, and electron microscopy analyses. Implementation of each program involves profiles of collagen synthesis and matrix assembly close to those documented in vivo. Depending on the applied inducers, cells adopt a defined identity and, controls acting at transcriptional and posttranslational levels adapt the set of deposited collagens to one particular cell fate. Osteogenic C1 cells selectively build a type I collagen matrix also containing type III, V, and XI collagens but selectively exclude type II collagen. Chondrogenic C1 cells first elaborate a type II collagen network and then acquire hypertrophic chondrocyte properties while assembling a type X collagen matrix as in the growth plate. This study provides an example of how a mesoblastic cell line can develop, in vitro, each of its genetic programs up to terminal differentiation. Intrinsic factors and time-dependent cell-matrix interactions might, as in vivo, underline the implementation of an entire differentiation program.

A chondral modeling theory revisited

The mechanical environment of limb joints constantly changes during growth due to growth-related changes in muscle and tendon lengths, long bone dimensions, and body mass. The size and shape of limb joint surfaces must therefore also change throughout post-natal development in order to maintain normal joint function. Frost's (1979, 1999) chondral modeling theory proposed that joint congruence is maintained in mammalian limbs throughout postnatal ontogeny because cartilage growth in articular regions is regulated in part by mechanical load. This paper incorporates recent findings concerning the distribution of stress in developing articular units, the response of chondrocytes to mechanically induced deformation, and the development of articular cartilage in order to expand upon Frost's chondral modeling theory. The theory presented here assumes that muscular contraction during post-natal locomotor development produces regional fluctuating, intermittent hydrostatic pressure within the articular cartilage of limb joints. The model also predicts that peak levels of hydrostatic pressure in articular cartilage increase between birth and adulthood. Finally, the chondral modeling theory proposes that the cell-cell and cell-extracellular matrix interactions within immature articular cartilage resulting from mechanically induced changes in hydrostatic pressure regulate the metabolic activity of chondrocytes. Site-specific rates of articular cartilage growth are therefore regulated in part by the magnitude, frequency, and orientation of prevailing loading vectors. The chondral modeling response maintains a normal kinematic pathway as the magnitude and direction of joint loads change throughout ontogeny. The chondral modeling theory also explains ontogenetic scaling patterns of limb joint curvature observed in mammals. The chondral modeling response is therefore an important physiological mechanism that maintains the match between skeletal structure, function, and locomotor performance throughout mammalian ontogeny and phylogeny.

Alkaline phosphatase knock-out mice recapitulate the metabolic and skeletal defects of infantile hypophosphatasia

Hypophosphatasia is an inborn error of metabolism characterized by deficient activity of the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP) and skeletal disease due to impaired mineralization of cartilage and bone matrix. We investigated two independently generated TNSALP gene knock-out mouse strains as potential models for hypophosphatasia. Homozygous mice (-/-) had < 1% of wild-type plasma TNSALP activity; heterozygotes had the predicted mean of approximately 50%. Phosphoethanolamine, inorganic pyrophosphate, and pyridoxal 5'-phosphate are putative natural substrates for TNSALP and all were increased endogenously in the knock-out mice. Skeletal disease first appeared radiographically at approximately 10 days of age and featured worsening rachitic changes, osteopenia, and fracture. Histologic studies revealed developmental arrest of chondrocyte differentiation in epiphyses and in growth plates with diminished or absent hypertrophic zones. Progressive osteoidosis from defective skeletal matrix mineralization was noted but not associated with features of secondary hyperparathyroidism. Plasma and urine calcium and phosphate levels were unremarkable. Our findings demonstrate that TNSALP knock-out mice are a good model for the infantile form of hypophosphatasia and provide compelling evidence for an important role for TNSALP in postnatal development and mineralization of the murine skeleton.

Human chondrocyte cell lines from articular cartilage of metatarsal phalangeal joints

Chondrocytes can be isolated from human adult cartilage from metatarsal phalangeal joints. After enzymatic digestion to isolate viable cells, confluent monolayers were obtained 2-4 weeks after the start of cell division. Chondrocytes cultures, initiated and maintained in HAM's F12 with bovine fetal serum without the addition of other growth factors, produced in vitro a matrix rich in collagen and proteoglycans. Although several studies reported phenotypic instability, our results showed that the cell retain for more than 5 months in culture their differentiated characteristics, including the ability to produce cartilage-specific molecules. Chondrocyte cell lines should be useful in studying the functions of these cells from normal and abnormal tissue and for pharmacological studies in vitro.

Gly369Cys mutation in mouse FGFR3 causes achondroplasia by affecting both chondrogenesis and osteogenesis

Missense mutations in fibroblast growth factor receptor 3 (FGFR3) result in several human skeletal dysplasias, including the most common form of dwarfism, achondroplasia. Here we show that a glycine-to-cysteine substitution at position 375 (Gly375Cys) in human FGFR3 causes ligand-independent dimerization and phosphorylation of FGFR3 and that the equivalent substitution at position 369 (Gly369Cys) in mouse FGFR3 causes dwarfism with features mimicking human achondroplasia. Accordingly, homozygous mice were more severely affected than heterozygotes. The resulting mutant mice exhibited macrocephaly and shortened limbs due to retarded endochondral bone growth and premature closure of cranial base synchondroses. Compared with their wild-type littermates, mutant mice growth plates shared an expanded resting zone and narrowed proliferating and hypertrophic zones, which is correlated with the activation of Stat proteins and upregulation of cell-cycle inhibitors. Reduced bone density is accompanied by increased activity of osteoclasts and upregulation of genes that are related to osteoblast differentiation, including osteopontin, osteonectin, and osteocalcin. These data reveal an essential role for FGF/FGFR3 signals in both chondrogenesis and osteogenesis during endochondral ossification.

Regulation of growth, protein synthesis, and maturation of fetal bovine epiphyseal chondrocytes grown in high-density culture in the presence of ascorbic acid, retinoic acid, and dihydrocytochalasin B

Phenotypic expression of chondrocytes can be modulated in vitro by changing the culture technique and by agents such vitamins and growth factors. We studied the effects of ascorbic acid, retinoic acid (0.5 and 10 microM), and dihydrocytochalasin B (3, 10, 20 microM DHCB), separately or in combination (ascorbic acid + retinoic acid or ascorbic acid + DHCB), on the induction of maturation of fetal bovine epiphyseal chondrocytes grown for up to 4 weeks at high density in medium containing 10% fetal calf serum and the various agents. In the absence of any agent or with retinoic acid or DHCB alone, the metabolic activity of the cells remained very low after day 6, with no induction of type I or X collagen synthesis nor increase in alkaline phosphatase activity. Chondrocytes treated with fresh ascorbic acid showed active protein synthesis associated with expression of types I and X after 6 and 13 days, respectively. This maturation was not accompanied by obvious hypertrophy of the cells or high alkaline phosphatase activity. Addition of retinoic acid to the ascorbic acid-treated cultures decreased the level of type II collagen synthesis and delayed the induction of types I and X collagen, which were present only after 30 days. A striking increase in alkaline phosphatase activity (15-20-fold) was observed in the presence of both ascorbic acid and the highest dose of retinoic acid (10 microM). DHCB was also a potent inhibitor of the maturation induced by treatment with ascorbic acid, as the chondrocytes maintained their rounded shape and synthesized type II collagen without induction of type I or X collagen. The pattern of protein secretion was compared under all culture conditions by two-dimensional gel electrophoresis. The different regulations of chondrocyte differentiation by ascorbic acid, retinoic acid, and DHCB were confirmed by the important qualitative and quantitative changes in the pattern of secreted proteins observed by two-dimensional gel electrophoresis along the study.

Spatio-temporal distribution of chondromodulin-I mRNA in the chicken embryo: expression during cartilage development and formation of the heart and eye

To define genes specifically expressed in cartilage and during chondrogenesis, we compared by differential display-polymerase chain reaction (DD-PCR) the mRNA populations of differentiated sternal chondrocytes from chicken embryos with mRNA species modulated in vitro by retinoic acid (RA). Chondrocyte-specific gene expression is downregulated by RA, and PCR-amplified cDNAs from both untreated and RA-modulated cells were differentially displayed. Amplification products only from RNA of untreated chondrocytes were further analyzed, and a cDNA-fragment of the chondromodulin-I (ChM-I) mRNA was isolated. After obtaining full length cDNA clones, we have analyzed the mRNA expression patterns at different developmental stages by RNase protection assay and in situ hybridization. Analysis of different tissues and cartilage from 17-day-old chicken embryos showed ChM-I mRNA only in chondrocytes. During somitogenesis of the chicken embryo, ChM-I transcripts were detected in the notochord, the floor and the roof plate of the neural tube, and in cartilage precursor tissues such as the sclerotomes of the somites, the developing limbs, the pharyngeal arches, the otic vesicle, and the sclera. ChM-I continued to be expressed in differentiated cartilages derived from these tissues and also in noncartilaginous domains of the developing heart and retina. Thus, in the chicken, the expression of ChM-I is not restricted to mature cartilage but is already present during early development in precartilaginous tissues as well as in heart and eye.

The Raf-1/MEK/ERK pathway regulates the expression of the p21(Cip1/Waf1) gene in chondrocytes

The gene encoding the cyclin-dependent kinase inhibitor p21(Cip1/Waf1) is up-regulated in many differentiating cells, including maturing chondrocytes. Since strict control of chondrocyte proliferation is essential for proper bone formation and since p21 is likely involved in this control, we initiated analyses of the mechanisms regulating expression of p21 in chondrocytes. p21 expression and promoter activity was strongly increased during the differentiation of chondrogenic MCT cells. We have identified a 68-base pair fragment conferring transcriptional up-regulation of the p21 gene in chondrocytes. The activity of this fragment required active Raf-1 in MCT cells as well as in primary mouse chondrocytes. Inhibition of downstream factors of Raf-1 (MEK1/2, ERK1/2, and Ets2) also repressed the activity of the 68-base pair fragment in MCT cells. The chemical MEK1/2 inhibitor PD98059 reduced protein levels of p21 in MCTs and primary mouse chondrocytes. These data suggest that signaling through the Raf-1 pathway is necessary for the optimal expression of p21 in chondrocytes and may play an important role in the control of bone formation.

Distribution of cartilage molecules in the developing mouse joint

This study describes the precise spatial and temporal patterns of protein distribution for aggrecan, fibromodulin, cartilage oligomeric matrix protein (COMP) and cartilage matrix protein (CMP) in the developing mouse limb with particular attention to those cells destined to form articular chondrocytes in comparison to those cells destined to form a mineralized tissue and become replaced by bone. Mouse glenohumeral joints from fetal mice (12-18 days post coitus (dpc) to the young adult (37 days after birth) were immunostained with antibodies specific for these molecules. Aggrecan staining defined the general chondrocytic phenotype, whether articular or transient. Fibromodulin was associated with prechondrocytic mesenchymal cells in the interzone prior to joint cavitation and with the mesenchymal cells of the perichondrium or the periosteum encapsulating the joint elements of the maturing and young adult limb. Staining was most intense around developing articular chondrocytes and much less abundant or absent in those differentiating cells along the anlage. CMP showed an almost reciprocal staining pattern to fibromodulin and was not detected in the matrix surrounding articular chondrocytes. COMP was not detected in the cells at the articular surface prior to cavitation but by 18 dpc, as coordinated movement of the mouse forelimb intensifies, staining for COMP was most intense around the maturing articular chondrocytes. These results show that the cells that differentiate into articular chondrocytes elaborate an extracellular matrix distinct from those cells that are destined to form bone. Fibromodulin may function in the early genesis of articular cartilage and COMP may be associated with elaboration of a weight-bearing chondrocyte matrix.

Different effects of bone morphogenetic proteins 2, 4, 12, and 13 on the expression of cartilage and bone markers in the MC615 chondrocyte cell line

In order to study the lineage leading to chondrocyte and osteoblast phenotype in vertebrate development, we examined the effect of recombinant human bone morphogenetic protein (BMP)-2, BMP-4, BMP-12 [or growth and differentiation factor (GDF)-7], and BMP-13 (or GDF-6) on the phenotypic expression of the mouse chondrocyte cell line MC615, grown for 1 or 2 weeks in monolayer. Protein synthesis rates were monitored after incubation with [(14)C]proline. BMP-2 and BMP-4 increased protein synthesis, in agreement with our observation by phase-contrast microscopy of a highly refractile matrix around MC615 cells treated with BMP-2 and -4. Markers of the chondrocytic and osteoblastic differentiation were analyzed at mRNA level. Expression of the type II collagen gene, a marker of the cartilage phenotype, was up-regulated in the presence of low concentration of BMP-2 or -4 (50 ng/ml) and down-regulated at higher concentrations (100-400 ng/ml). In parallel, this expression was stable in the presence of BMP-12 or -13 in the dose range tested (50-400 ng/ml). Expression of the matrix Gla protein (MGP) gene, another marker of cartilage, was also reduced in the presence of 100 ng/ml BMP-2 or -4, while it remained stable in the presence of BMP-12 or -13 at the same concentration. In contrast, expression of the bone Gla protein (BGP) gene, or osteocalcin, a marker of the bone phenotype, was induced when the cells were treated with BMP-2 or -4 but was not detected when the cells were treated with BMP-12 or -13. At the same time, BMP-2 or -4 markedly up-regulated expression of type X collagen mRNA, indicating that MC615 cells possess the ability to express traits associated with endochondral ossification, when exposed to specific BMPs. Furthermore, detailed analysis of type II collagen expression showed that the alternatively spliced transcript collagen IIB, specific for cartilage, is expressed concomitantly with BGP. Therefore, MC615 chondrocytes can simultaneously express chondrocytic and osteoblastic markers, in response to BMP-2 or -4, but show minimal response to BMP-12 (or GDF-7) or to BMP-13 (or GDF-6). These results raise the possibility that chondrocytes in vivo can express osteoblastic properties, provided they are induced by BMP-2 or -4.

Serum induction of the collagen X promoter requires the Raf/MEK/ERK and p38 pathways

The collagen X gene is expressed exclusively by differentiated, hypertrophic chondrocytes. The mechanisms controlling collagen X expression remain largely unknown. Here we show that collagen X promoter activity can be induced by serum stimulation of chondrogenic MCT cells. The serum response is conferred by a 462 nucleotide promoter fragment. Both the c-Raf/MEK/ERK and p38 MAP kinase pathways are required for this effect, whereas phosphatidylinositol-3-kinase and protein kinase A repress promoter activation. These data are the first to demonstrate serum inducibility of the collagen X promoter and to identify signal transduction pathways involved.

Extracellular phosphate ions cause apoptosis of terminally differentiated epiphyseal chondrocytes

Epiphyseal chondrocytes end their life cycle through apoptosis. While this event provides a mechanism for the removal of terminally differentiated cells from cartilage, agents that promote this physiological process have not been defined. To address this issue, using a cell culture technique that models events that take place in the growth plate, we asked the following questions: Can agents that promote chondrocyte maturation and cartilage mineralization serve as specific triggers for cell death? Are chondrocytes susceptible to apoptogens at a singular developmental stage? Treatment of embryonic tibial chondrocytes with inorganic phosphate (Pi) induced death in a dose- and time-dependent manner. Within 48 hr, 3 mM Pi increased chondrocyte death by 30%; lower concentrations of Pi induced death after 48 hr. To ascertain if death was due to apoptosis, we evaluated Pi-induced death by a number of different methods and compared the results to those induced by the apoptogen, staurosporine. Analysis of the death process indicated that cartilage cells shared many of the common biological features of the apoptotic process. Thus, there was DNA fragmentation, terminal deoxynucleotidyl transferase (TUNEL) labeling, an increase in cells in the sub-G1 fraction of the cell cycle, and morphological evidence of apoptosis. To explore the specificity of the Pi effect, the experiment was repeated using embryonic sternal cephalic and caudal chondrocytes, cells that are at an earlier developmental stage than the terminally differentiated tibial cells. We noted that these cells remained vital despite a major increase in the medium Pi content. Results of this study suggest that Pi is a stage-specific inducer of apoptosis in maturing chondrocytes and that this role may be linked to chondrocyte maturation and mineralization of the extracellular matrix.

The epiphyseal cartilage and growth of long bones in Rana catesbeiana

The structure of the epiphyseal cartilage of the bullfrog Rana catesbeiana and its role in the growth of long bones were examined. The epiphyseal cartilage was inserted into the end of a tubular bone shaft, defining three regions: articular cartilage, lateral articular cartilage and growth cartilage. Joining the lateral cartilage to the bone was a fibrous layer of periosteum, rich in blood vessels. Osteoblasts with alkaline phosphatase activity were found on the surface of the periosteal bone, which presented a fibrous non-mineralised tip. The growth cartilage was inside the bone. The proliferative chondrocytes presented perpendicular separation of daughter cells and there was no columnar arrangement of the cells. Furthermore, chondrocyte hypertrophy was not associated with either calcification or endochondral ossification, in apparent contrast to the avian and mammalian models. Finally, there was no reinforcement system capable of directing cell volume increase into longitudinal growth. Since bone extension depends on the intramembranous ossification of the periosteum, the growth cartilage is inside and not at the end of the bone and the cells in the growth cartilage show no columnar arrangement and separate in a direction perpendicular to the long bone axis, we conclude that the growth cartilage mainly contributes to the radial expansion of the bone.

Parathyroid hormone-related peptide (PTHrP)-dependent and -independent effects of transforming growth factor beta (TGF-beta) on endochondral bone formation

Previously, we showed that expression of a dominant-negative form of the transforming growth factor beta (TGF-beta) type II receptor in skeletal tissue resulted in increased hypertrophic differentiation in growth plate and articular chondrocytes, suggesting a role for TGF-beta in limiting terminal differentiation in vivo. Parathyroid hormone-related peptide (PTHrP) has also been demonstrated to regulate chondrocyte differentiation in vivo. Mice with targeted deletion of the PTHrP gene demonstrate increased endochondral bone formation, and misexpression of PTHrP in cartilage results in delayed bone formation due to slowed conversion of proliferative chondrocytes into hypertrophic chondrocytes. Since the development of skeletal elements requires the coordination of signals from several sources, this report tests the hypothesis that TGF-beta and PTHrP act in a common signal cascade to regulate endochondral bone formation. Mouse embryonic metatarsal bone rudiments grown in organ culture were used to demonstrate that TGF-beta inhibits several stages of endochondral bone formation, including chondrocyte proliferation, hypertrophic differentiation, and matrix mineralization. Treatment with TGF-beta1 also stimulated the expression of PTHrP mRNA. PTHrP added to cultures inhibited hypertrophic differentiation and matrix mineralization but did not affect cell proliferation. Furthermore, terminal differentiation was not inhibited by TGF-beta in metatarsal rudiments from PTHrP-null embryos; however, growth and matrix mineralization were still inhibited. The data support the model that TGF-beta acts upstream of PTHrP to regulate the rate of hypertrophic differentiation and suggest that TGF-beta has both PTHrP-dependent and PTHrP-independent effects on endochondral bone formation.

Trans-activation of the mouse cartilage-derived retinoic acid-sensitive protein gene by Sox9

The transcription factor Sox9 is capable of enhancing type II collagen gene expression and may play a crucial role in chondrogenesis. To determine whether Sox9 is an inducer of the chondrocyte phenotype, we investigated the role of Sox9 in transcription of another cartilage gene encoding the cartilage-derived retinoic acid-sensitive protein (CD-RAP). CD-RAP is specifically expressed during chondrogenesis. We show here that Sox9 protein is able to bind to a SOX consensus sequence in the CD-RAP promoter. Mutation of the SOX motif led to decreased transcription of a CD-RAP promoter construct in chondrocytes. Overexpression of SOX9 resulted in a dose-dependent increased activity of CD-RAP promoter-driven reporter gene in both chondrocytes and nonchondrogenic cells. A truncated SOX9, which contains a binding domain but no trans-activation function, inhibited CD-RAP promoter activity. Overexpression of SOX9 increased the level of endogenous CD-RAP mRNA in chondrocytes, but was unable to induce endogenous gene expression in 10T1/2 mesenchymal cells or BALB/c-3T3 fibroblasts. These results suggest that Sox9 is a general transcriptional regulator of cartilage-specific genes. However, Sox9 does not appear to be able to induce the chondrocyte phenotype in nonchondrogenic cells, implying that other factors are involved in chondrogenesis.

GDF5 coordinates bone and joint formation during digit development

A functional skeletal system requires the coordinated development of many different tissue types, including cartilage, bones, joints, and tendons. Members of the Bone morphogenetic protein (BMP) family of secreted signaling molecules have been implicated as endogenous regulators of skeletal development. This is based on their expression during bone and joint formation, their ability to induce ectopic bone and cartilage, and the skeletal abnormalities present in animals with mutations in BMP family members. One member of this family, Growth/differentiation factor 5 (GDF5), is encoded by the mouse brachypodism locus. Mice with mutations in this gene show reductions in the length of bones in the limbs, altered formation of bones and joints in the sternum, and a reduction in the number of bones in the digits. The expression pattern of Gdf5 during normal development and the phenotypes seen in mice with single or double mutations in Gdf5 and Bmp5 suggested that Gdf5 has multiple functions in skeletogenesis, including roles in joint and cartilage development. To further understand the function of GDF5 in skeletal development, we assayed the response of developing chick and mouse limbs to recombinant GDF5 protein. The results from these assays, coupled with an analysis of the development of brachypodism digits, indicate that GDF5 is necessary and sufficient for both cartilage development and the restriction of joint formation to the appropriate location. Thus, GDF5 function in the digits demonstrates a link between cartilage development and joint development and is an important determinant of the pattern of bones and articulations in the digits.

Sox9 is required for cartilage formation

Chondrogenesis results in the formation of cartilages, initial skeletal elements that can serve as templates for endochondral bone formation. Cartilage formation begins with the condensation of mesenchyme cells followed by their differentiation into chondrocytes. Although much is known about the terminal differentiation products that are expressed by chondrocytes, little is known about the factors that specify the chondrocyte lineage. SOX9 is a high-mobility-group (HMG) domain transcription factor that is expressed in chondrocytes and other tissues. In humans, SOX9 haploinsufficiency results in campomelic dysplasia, a lethal skeletal malformation syndrome, and XY sex reversal. During embryogenesis, Sox9 is expressed in all cartilage primordia and cartilages, coincident with the expression of the collagen alpha1(II) gene (Col2a1) . Sox9 is also expressed in other tissues, including the central nervous and urogenital systems. Sox9 binds to essential sequences in the Col2a1 and collagen alpha2(XI) gene (Col11a2) chondrocyte-specific enhancers and can activate these enhancers in non-chondrocytic cells. Here, Sox9 is identified as a regulator of the chondrocyte lineage. In mouse chimaeras, Sox9-/- cells are excluded from all cartilages but are present as a juxtaposed mesenchyme that does not express the chondrocyte-specific markers Col2a1, Col9a2, Col11a2 and Agc. This exclusion occurred cell autonomously at the condensing mesenchyme stage of chondrogenesis. Moreover, no cartilage developed in teratomas derived from Sox9-/- embryonic stem (ES) cells. Our results identify Sox9 as the first transcription factor that is essential for chondrocyte differentiation and cartilage formation.

Cell cycle genes in chondrocyte proliferation and differentiation

Coordinated proliferation and differentiation of growth plate chondrocytes controls longitudinal growth of endochondral bones. While many extracellular factors regulating these processes have been identified, much less is known about the intracellular mechanisms transducing and integrating these extracellular signals. Recent evidence suggests that cell cycle proteins play an important role in the coordination of chondrocyte proliferation and differentiation. Our current knowledge of the function and regulation of cell cycle proteins in endochondral ossification is summarized.

Raf signaling stimulates and represses the human collagen X promoter through distinguishable elements

Endochondral bone growth is regulated through the rates of proliferation and differentiation of growth plate chondrocytes. While little is known about the intracellular events controlling these processes, the protein kinase c-Raf, a central component of the cellular signal transduction machinery, has recently been shown to be expressed only by differentiated, hypertrophic chondrocytes. The involvement of c-Raf in the transcriptional regulation of the hypertrophic chondrocyte-specific collagen X gene was investigated using cotransfections of collagen X reporter plasmids and expression vectors for mutant c-Raf proteins. Both activated and dominant-negative forms of c-Raf reduced the activity of the collagen X promoter to approximately 30%. The element mediating the repressing effect of activated c-Raf was located between nucleotides -2864 and -2410 of the promoter, whereas the effect of the dominant-negative form of c-Raf was conferred by the 462 nucleotides immediately upstream of the transcription start site. Inhibition of MEK1/2 and ERK1/2, downstream components of Raf-signaling, also caused repression of basal collagen X promoter activity. These data suggest that c-Raf regulates collagen X promoter activity positively and negatively through different cis-acting elements and represent the first evidence of c-Raf activity described in hypertrophic chondrocytes.

Multiple inositol polyphosphate phosphatase: evolution as a distinct group within the histidine phosphatase family and chromosomal localization of the human and mouse genes to chromosomes 10q23 and 19

Multiple inositol polyphosphate phosphatase is the only enzyme known to hydrolyze the abundant metabolites inositol pentakisphosphate and inositol hexakisphosphate. We have previously demonstrated that the chick homolog of multiple inositol polyphosphate phosphatase, designated HiPER1, has a role in growth plate chondrocyte differentiation. The relationship of these enzymes to intracellular signaling is obscure, and as part of our investigation we have examined the murine ((MMU)Minpp1) and human ((HSA)MINPP1) homologs. Northern blot analysis demonstrated expression of ((MMU)Minpp1 in a variety of mouse tissues, comparable to the expression of other mammalian homologs, but less restricted than the expression of HiPER1 in chick. A purified (MMU)Minpp1 fusion protein cleaved phosphate from inositol (1,3,4,5)-tetrakisphosphate and para-nitrophenyl phosphate. When the presumptive active site histidine was altered to alanine by site-directed mutagenesis, enzyme activity was abolished, confirming the classification of (MMU)Minpp1 as a histidine phosphatase. The amino acid sequences of the murine and human MINPP proteins share >80% identity with the rat enzyme and >56% identity with HiPER1, with conservation of the C-terminal consensus sequence that retains proteins in the endoplasmic reticulum. The intron/exon structure of the mammalian (MMU)Minpp1 and (HSA)MINPP1 genes is also conserved compared to the chick HiPER1 gene. Sequence analysis of plant and fruit fly MINPP homologs supports the hypothesis that the MINPP enzymes constitute a distinct evolutionary group within the histidine phosphatase family. We have mapped (HSA)MINPP1 to human chromosome 10q23 by fluorescence in situ hybridization, YAC screening, and radiation hybrid mapping. This assignment places (HSA)MINPP1 in a region of chromosome 10 that is frequently mutated in human cancers and places (HSA)MINPP1 proximal to the tumor suppressor PTEN, which maps to 10q23.3. Using a radiation hybrid panel, we localized (MMU)Minpp1 to a region of mouse chromosome 19 that includes the murine homolog of Pten. The evolutionary conservation of this novel enzyme within the inositol polyphosphate pathway suggests a significant role for multiple inositol polyphosphate phosphatase throughout higher eukaryotes.

BMP-5 deficiency alters chondrocytic activity in the mouse proximal tibial growth plate

The role of bone morphogenetic protein-5 (BMP-5) in regulating chondrocytic activity during endochondral ossification was examined in the mouse proximal tibial growth plate. Short ear mice homozygous for the SEA/Gn point mutation in the coding region for BMP-5 (King, J. A. et al. Dev Biol 166:112122; 1994) and heterozygous long ear littermates were examined at 5 and 9 weeks of age (n = 9/group, four groups). Animals were injected with oxytetracycline to estimate the rate of growth and with bromodeoxyuridine to identify proliferative chondrocytes. Age-related changes in chondrocytic stereological and kinetic parameters were compared by image analysis of 1-microm-thick growth plate sections. The number of proliferative chondrocytes did not vary with age in either genotype, but proliferative phase duration increased significantly (approximately 67%) with age in the long ear mice, whereas no change was detected in the short ear mice. The number of hypertrophic chondrocytes increased significantly (approximately 27%) in the short ears, whereas this number decreased significantly (approximately 40%) in the long ears. There was a small, but significant, increase in hypertrophic phase duration (approximately 45%) in short ear mice, but no change was detected in the long ears. These results indicate that BMP-5 deficiency prevents age-related decelerations in chondrocytic proliferation and initiation of hypertrophic differentiation, suggesting a role of BMP-5 in inhibiting these processes.

Mice lacking link protein develop dwarfism and craniofacial abnormalities

Link protein (LP), an extracellular matrix protein in cartilage, stabilizes aggregates of aggrecan and hyaluronan, giving cartilage its tensile strength and elasticity. Cartilage provides the template for endochondral ossification and is crucial for determining the length and width of the skeleton. During endochondral bone formation, hypertrophic chondrocytes die and the cartilage is replaced with bone matrix. Here, we have generated targeted mutations in mice in the gene encoding LP (Crtl1). Homozygotes showed defects in cartilage development and delayed bone formation with short limbs and craniofacial anomalies. Most Crtl1(tm1Nid/tm1Nid) mice died shortly after birth due to respiratory failure, but some survived and developed progressive dwarfism and lordosis of the cervical spine. They showed small epiphysis, slightly flared metaphysis of long bones and flattened vertebrae, characteristic of spondyloepiphyseal dysplasias. The cartilage contained significantly reduced aggrecan depositions in the hypertrophic zone, and decreased numbers of prehypertrophic and hypertrophic chondrocytes. Reduced Indian hedgehog (Ihh) expression was observed in prehypertrophic chondrocytes, and apoptosis was inhibited in hypertrophic chondrocytes. These results indicate that LP is important for the formation of proteoglycan aggregates and normal organization of hypertrophic chondrocytes, and suggest that cartilage matrix has a role in chondrocyte differentiation and maturation.

Abnormal collagen assembly, though normal phenotype, in alginate bead cultures of chick embryo chondrocytes

The collagens produced by chick embryo chondrocytes cultured in alginate beads were investigated both biochemically and ultrastructurally. The cartilage phenotype is maintained for at least 14 days, as indicated by the production of the cartilage-specific collagens II, IX, and XI and the absence of collagen I. There were differences in the distributions of collagens among the three different compartments analyzed (cells and their associated matrix, further-removed matrix (released by alginate solubilization), and culture medium), with large amounts of collagen IX (mainly in proteoglycan form) in the culture medium. Inhibition of lysyl oxidase activity by beta-aminopropionitrile led to an overall decrease in collagen production. In contrast to the biochemical observations, collagen ultrastructure in the extracellular matrix of alginate cultures was not in the form of the expected 64-nm banded fibrils, but rather in the form of segment-long-spacing-like crystallites. This abnormal structure is likely to be a result of alginate disrupting normal assembly. We conclude that, in this system, the native fibrillar structure of the collagenous matrix is not essential for the maintenance of the differentiated phenotype of chondrocytes.