We have identified a novel zinc finger-containing transcription factor, called Osterix (Osx), that is specifically expressed in all developing bones. In Osx null mice, no bone formation occurs. In endochondral skeletal elements of Osx null mice, mesenchymal cells, together with osteoclasts and blood vessels, invade the mineralized cartilage matrix. However, the mesenchymal cells do not deposit bone matrix. Similarly, cells in the periosteum and in the condensed mesenchyme of membranous skeletal elements cannot differentiate into osteoblasts. These cells do, however, express Runx2/Cbfa1, another transcription factor required for bone formation. In contrast, Osx is not expressed in Runx2/Cbfa1 null mice. Thus, Osx acts downstream of Runx2/Cbfa1. Because Osx null preosteoblasts express typical chondrocyte marker genes, we propose that Runx2/Cbfa1-expressing preosteoblasts are still bipotential cells.

To examine whether the transcription factor Sox9 has an essential role during the sequential steps of chondrocyte differentiation, we have used the Cre/loxP recombination system to generate mouse embryos in which either Sox9 is missing from undifferentiated mesenchymal cells of limb buds or the Sox9 gene is inactivated after chondrogenic mesenchymal condensations. Inactivation of Sox9 in limb buds before mesenchymal condensations resulted in a complete absence of both cartilage and bone, but markers for the different axes of limb development showed a normal pattern of expression. Apoptotic domains within the developing limbs were expanded, suggesting that Sox9 suppresses apoptosis. Expression of Sox5 and Sox6, two other Sox genes involved in chondrogenesis, was no longer detected. Moreover, expression of Runx2, a transcription factor needed for osteoblast differentiation, was also abolished. Embryos, in which Sox9 was deleted after mesenchymal condensations, exhibited a severe generalized chondrodysplasia, similar to that in Sox5; Sox6 double-null mutant mice. Most cells were arrested as condensed mesenchymal cells and did not undergo overt differentiation into chondrocytes. Furthermore, chondrocyte proliferation was severely inhibited and joint formation was defective. Although Indian hedgehog, Patched1, parathyroid hormone-related peptide (Pthrp), and Pth/Pthrp receptor were expressed, their expression was down-regulated. Our experiments further suggested that Sox9 is also needed to prevent conversion of proliferating chondrocytes into hypertrophic chondrocytes. We conclude that Sox9 is required during sequential steps of the chondrocyte differentiation pathway. At the onset of endochondral bone formation, a number of patterning molecules target the mesenchyme by outlining the three-dimensional coordinates that determine the shape, number, and size of the primordia of skeletal elements. In parallel to these patterning effects, the multistep process of endochondral bone formation is initially marked by the acquisition in undifferentiated mesenchymal cells of chondrogenic potency. These committed mesenchymal cells first undergo condensation, by which cells become closely packed, followed by the overt differentiation of cells within these condensations into chondrocytes. These differentiated chondrocytes then sustain a series of sequential changes that include unidirectional proliferation, conversion to hypertrophic chondrocytes, ability to calcify the extracellular matrix, cell death, and replacement by bone. A number of secreted polypeptides are known to cooperatively regulate the rates of proliferation of chondrocytes and their transition to hypertrophy (Karaplis et al. 1994;Lanske et al. 1996;Vortkamp et al. 1996;St-Jacques et al. 1999;Hartmann and Tabin 2000;Vortkamp 2001). However, until recently, much less was known about intracellular events, especially about the transcription factors that control the onset of chondrogenesis in undifferentiated mesenchymal cells as well as the progression of chondrocytic ...

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.

The identification of mutations in the SRY-related SOX9 gene in patients with campomelic dysplasia, a severe skeletal malformation syndrome, and the abundant expression of Sox9 in mouse chondroprogenitor cells and fully differentiated chondrocytes during embryonic development have suggested the hypothesis that SOX9 might play a role in chondrogenesis. Our previous experiments with the gene (Col2a1) for collagen II, an early and abundant marker of chondrocyte differentiation, identified a minimal DNA element in intron 1 which directs chondrocyte-specific expression in transgenic mice. This element is also a strong chondrocyte-specific enhancer in transient transfection experiments. We show here that Col2a1 expression is closely correlated with high levels of SOX9 RNA and protein in chondrocytes. Our experiments indicate that the minimal Col2a1 enhancer is a direct target for Sox9. Indeed, SOX9 binds to a sequence of the minimal Col2a1 enhancer that is essential for activity in chondrocytes, and SOX9 acts as a potent activator of this enhancer in cotransfection experiments in nonchondrocytic cells. Mutations in the enhancer that prevent binding of SOX9 abolish enhancer activity in chondrocytes and suppress enhancer activation by SOX9 in nonchondrocytic cells. Other SOX family members are ineffective. Expression of a truncated SOX9 protein lacking the transactivation domain but retaining DNA-binding activity interferes with enhancer activation by full-length SOX9 in fibroblasts and inhibits enhancer activity in chondrocytes. Our results strongly suggest a model whereby SOX9 is involved in the control of the cell-specific activation of COL2A1 in chondrocytes, an essential component of the differentiation program of these cells. We speculate that in campomelic dysplasia a decrease in SOX9 activity would inhibit production of collagen II, and eventually other cartilage matrix proteins, leading to major skeletal anomalies.Acquisition of the chondrocyte phenotype is one of the major pathways of mesenchymal cell differentiation. During and after condensation of mesenchymal cells, cartilage-specific genes are switched on (2, 12). The products of these genes, which include collagen types II, IX, and XI, the link protein, and aggrecan, form the characteristic extracellular matrix of cartilages. In recent years, differentiation of mesenchymal cells into myocytes and adipocytes has been shown to be controlled by cell-specific transcription factors belonging to different protein families (10,28,39,41,46). By analogy, we speculate that specific transcription factors could also control the differentiation of mesenchymal cells into chondrocytes and activate cartilage-specific genes.In order to identify transcription factors which control chondrocyte differentiation, we have used the gene for type II collagen (Col2a1), an early and abundant marker of chondrocytes (3), and have delineated a minimal sequence in this gene that is sufficient to direct chondrocyte-specific expression both in transgenic mice and in transient transfectio...

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Chondrogenesis is a multistep process that is essential for endochondral bone formation. Previous results have indicated a role for ␤-catenin and Wnt signaling in this pathway. Here we show the existence of physical and functional interactions between ␤-catenin and Sox9, a transcription factor that is required in successive steps of chondrogenesis. In vivo, either overexpression of Sox9 or inactivation of ␤-catenin in chondrocytes of mouse embryos produces a similar phenotype of dwarfism with decreased chondrocyte proliferation, delayed hypertrophic chondrocyte differentiation, and endochondral bone formation. Furthermore, either inactivation of Sox9 or stabilization of ␤-catenin in chondrocytes also produces a similar phenotype of severe chondrodysplasia. Sox9 markedly inhibits activation of ␤-catenin-dependent promoters and stimulates degradation of ␤-catenin by the ubiquitination/proteasome pathway. Likewise, Sox9 inhibits ␤-catenin-mediated secondary axis induction in Xenopus embryos. ␤-Catenin physically interacts through its Armadillo repeats with the C-terminal transactivation domain of Sox9. We hypothesize that the inhibitory activity of Sox9 is caused by its ability to compete with Tcf/Lef for binding to ␤-catenin, followed by degradation of ␤-catenin. Our results strongly suggest that chondrogenesis is controlled by interactions between Sox9 and the Wnt/␤-catenin signaling pathway. Chondrogenesis, an obligatory process in endochondral bone formation, starts with the recruitment of chondrogenic mesenchymal cells into condensations. This is followed by the differentiation of these cells into chondrocytes, which produce cartilage-specific extracellular matrix (ECM) proteins including type II collagen and the proteoglycan aggrecan. Chondrocytes then undergo a unidirectional proliferation to form orderly parallel columns, exit the cell cycle, become prehypertrophic, and then hypertrophic. Sox9, a high-mobility-group (HMGbox) transcription factor, is required at sequential steps in this pathway (Bi et al. 1999(Bi et al. , 2001Akiyama et al. 2002).Both the human disease campomelic dysplasia, which is caused by heterozygous mutations in the Sox9 gene and is due to Sox9 haploinsufficiency, as well as Sox9 heterozygous mutant mice are characterized by a general hypoplasia of endochondral bones (Foster et al. 1994;Wagner et al. 1994). Inactivation of Sox9 in limb buds using the Cre recombinase/loxP recombination system before chondrogenic mesenchymal condensations results in the complete absence of mesenchymal condensations and of subsequent cartilage and bone formation, indicating that Sox9 is needed for an early step in chondrogenesis, that of mesenchymal condensations (Akiyama et al. 2002). A similar conclusion was also reached by analysis of mouse embryo chimeras derived from homozygous Sox9 mutant embryonic stem (ES) cells (Bi et al. 1999). That Sox9 is needed at sequential steps is shown by the severe generalized chondrodysplasia of mouse embryos in which Sox9 is deleted after chondrogenic mesenchymal conde...

L-Sox5 and Sox6 are highly identical Sry-related transcription factors coexpressed in cartilage. Whereas Sox5 and Sox6 single null mice are born with mild skeletal abnormalities, Sox5; Sox6 double null fetuses die with a severe, generalized chondrodysplasia. In these double mutants, chondroblasts poorly differentiate. They express the genes for all essential cartilage extracellular matrix components at low or undetectable levels and initiate proliferation after a long delay. All cartilages are thus extracellular matrix deficient and remain rudimentary. While chondroblasts in the center of cartilages ultimately activate prehypertrophic chondrocyte markers, epiphyseal chondroblasts ectopically activate hypertrophic chondrocyte markers. Thick intramembranous bone collars develop, but the formation of cartilage growth plates and endochondral bones is disrupted. L-Sox5 and Sox6 are thus redundant, potent enhancers of chondroblast functions, thereby essential for endochondral skeleton formation.

One of the crucial steps in endochondral bone formation is the replacement of a cartilage matrix produced by chondrocytes with bone trabeculae made by osteoblasts. However, the precise sources of osteoblasts responsible for trabecular bone formation have not been fully defined. To investigate whether cells derived from hypertrophic chondrocytes contribute to the osteoblast pool in trabecular bones, we genetically labeled either hypertrophic chondrocytes by Col10a1-Cre or chondrocytes by tamoxifen-induced Agc1-CreERT2 using EGFP, LacZ or Tomato expression. Both Cre drivers were specifically active in chondrocytic cells and not in perichondrium, in periosteum or in any of the osteoblast lineage cells. These in vivo experiments allowed us to follow the fate of cells labeled in Col10a1-Cre or Agc1-CreERT2 -expressing chondrocytes. After the labeling of chondrocytes, both during prenatal development and after birth, abundant labeled non-chondrocytic cells were present in the primary spongiosa. These cells were distributed throughout trabeculae surfaces and later were present in the endosteum, and embedded within the bone matrix. Co-expression studies using osteoblast markers indicated that a proportion of the non-chondrocytic cells derived from chondrocytes labeled by Col10a1-Cre or by Agc1-CreERT2 were functional osteoblasts. Hence, our results show that both chondrocytes prior to initial ossification and growth plate chondrocytes before or after birth have the capacity to undergo transdifferentiation to become osteoblasts. The osteoblasts derived from Col10a1-expressing hypertrophic chondrocytes represent about sixty percent of all mature osteoblasts in endochondral bones of one month old mice. A similar process of chondrocyte to osteoblast transdifferentiation was involved during bone fracture healing in adult mice. Thus, in addition to cells in the periosteum chondrocytes represent a major source of osteoblasts contributing to endochondral bone formation in vivo.