Sox9 is required for cartilage formation

Similar gene structure of two Sox9a genes and their expression patterns during gonadal differentiation in a teleost fish, rice field eel (Monopterus albus)

The Sox9 gene encodes a transcription factor that is critical for testis determination and chondrogenesis in vertebrates. Mutations in human SOX9 cause campomelic dysplasia, a dominant skeletal dysmorphology syndrome often associated with male to female sex reversal. Here we show that the Sox9a gene was duplicated during evolution of the rice field eel, Monopterus albus, a freshwater fish which undergoes natural sex reversal from female to male during its life, and has a haploid genome size (0.6-0.8 pg) that is among the smallest of the vertebrates. The duplicated copies of the gene (named Sox9al and Sox9a2) fit within the Sox9 clade of vertebrates, especially in the Sox9a subfamily, not in the Sox9b subfamily. They have similar structures as revealed by both genomic and cDNA analysis. Furthermore, both Sox9al and Sox9a2 are expressed in testis, ovary, and ovotestis; and specifically in the outer layer (mainly gonocytes) of gonadal epithelium with bipotential capacity to form testis or ovary, suggesting that they have similar roles in gonadal differentiation during sex reversal in this species. The closely related gene structure and expression patterns of the two sox9a genes in the rice field eel also suggest that they arose in recent gene duplication events during evolution of this fish lineage.

Potential role of Sox9 in patterning tracheal cartilage ring formation in an embryonic mouse model

OBJECTIVE

To identify genes expressed early in the formation of the mouse trachea that control patterning of tracheal cartilaginous rings.

DESIGN

The mouse larynx and trachea begin as an outpouching from the ventral foregut endoderm at embryonic day (E) 9. Digoxigenin-labeled RNA probes to putative tracheal patterning genes were generated by in vitro transcription. Embryos ranging in age from E9 to E16 were then subjected to whole-mount in situ hybridization using these labeled RNA probes. The RNA probes were then localized using antidigoxigenin antibodies tagged with a reporter molecule. In this manner, the 3-dimensional spatial and temporal expression of putative tracheal patterning genes was examined. Subjects F/VBN mice.

RESULTS

In the developing mouse trachea, the expression of Sox9 messenger RNA preceded cartilage ring formation. Sox9 was expressed as 2 distinct longitudinal stripes along the posterolateral aspect of the trachea as early as E9, when the developing trachea is first identified. Collagen 2A1, a cartilage-specific protein, was subsequently expressed in the same longitudinal pattern as Sox9, consistent with the early commitment of Sox9-expressing cells to the cartilage program. As cartilage rings formed, Sox9 and collagen 2A1 was expressed over the lateral and anterior aspects of the trachea.

CONCLUSIONS

We have developed a system to study the early expression of genes that may pattern the formation of the trachea. We have identified a gene (Sox9) with a known role in chondrocyte differentiation that is expressed in a highly specific temporal and spatial pattern in the developing upper respiratory tract.

Gene expression during redifferentiation of human articular chondrocytes

OBJECTIVE

The aim of the present study was to investigate gene expression during the in vitro redifferentiation process of human articular chondrocytes isolated from clinical samples from patient undergoing an autologous chondrocyte transplantation therapy (ACT).

METHOD

Monolayer (ML) expanded human articular chondrocytes from four donors were cultured in a 3D pellet model and the redifferentiation was investigated by biochemistry, histology, immunohistochemistry and microarray analysis.

RESULTS

The culture expanded chondrocytes redifferentiated in the pellet model as seen by an increase in collagen type II immunoreactivity between day 7 and 14. The gene expression from ML to pellet at day 7 included an increase in cartilage matrix proteins like collagen type XI, tenascin C, dermatopontin, COMP and fibronectin. The late phase consisted of a strong downregulation of extracellular signal-regulated protein kinase (ERK-1) and an upregulation of p38 kinase and SOX-9, suggesting that the late phase mimicked parts of the signaling processes involved in the early chondrogenesis in limb bud cells. Other genes, which indicated a transition from proliferation to tissue formation, were the downregulated cell cycle genes GSPT1 and the upregulated growth-arrest-specific protein (gas). The maturation of the pellets included no signs of hypertrophy or apoptosis as seen by downregulation of collagen type X, Matrix Gla protein and increased expression of caspase 3.

CONCLUSION

Our data show that human articular chondrocytes taken from surplus cells of patient undergoing ACT treatment and expanded in ML, redifferentiate and form cartilage like matrix in vitro and that this dynamic process involves genes known to be expressed in early chondrogenesis.

Runx2 induces osteoblast and chondrocyte differentiation and enhances their migration by coupling with PI3K-Akt signaling

Runx2 and phosphatidylinositol 3-kinase (PI3K)–Akt signaling play important roles in osteoblast and chondrocyte differentiation. We investigated the relationship between Runx2 and PI3K-Akt signaling. Forced expression of Runx2 enhanced osteoblastic differentiation of C3H10T1/2 and MC3T3-E1 cells and enhanced chondrogenic differentiation of ATDC5 cells, whereas these effects were blocked by treatment with IGF-I antibody or LY294002 or adenoviral introduction of dominant-negative (dn)–Akt. Forced expression of Runx2 or dn-Runx2 enhanced or inhibited cell migration, respectively, whereas the enhancement by Runx2 was abolished by treatment with LY294002 or adenoviral introduction of dn-Akt. Runx2 up-regulated PI3K subunits (p85 and p110β) and Akt, and their expression patterns were similar to that of Runx2 in growth plates. Treatment with LY294002 or introduction of dn-Akt severely diminished DNA binding of Runx2 and Runx2-dependent transcription, whereas forced expression of myrAkt enhanced them. These findings demonstrate that Runx2 and PI3K-Akt signaling are mutually dependent on each other in the regulation of osteoblast and chondrocyte differentiation and their migration.

Interactions between Sox9 and beta-catenin control chondrocyte differentiation

Chondrogenesis is a multistep process that is essential for endochondral bone formation. Previous results have indicated a role for beta-catenin and Wnt signaling in this pathway. Here we show the existence of physical and functional interactions between beta-catenin and Sox9, a transcription factor that is required in successive steps of chondrogenesis. In vivo, either overexpression of Sox9 or inactivation of beta-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 beta-catenin in chondrocytes also produces a similar phenotype of severe chondrodysplasia. Sox9 markedly inhibits activation of beta-catenin-dependent promoters and stimulates degradation of beta-catenin by the ubiquitination/proteasome pathway. Likewise, Sox9 inhibits beta-catenin-mediated secondary axis induction in Xenopus embryos. Beta-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 beta-catenin, followed by degradation of beta-catenin. Our results strongly suggest that chondrogenesis is controlled by interactions between Sox9 and the Wnt/beta-catenin signaling pathway.

Distinguishing the contributions of the perichondrium, cartilage, and vascular endothelium to skeletal development

During the initiation of endochondral ossification three events occur that are inextricably linked in time and space: chondrocytes undergo terminal differentiation and cell death, the skeletal vascular endothelium invades the hypertrophic cartilage matrix, and osteoblasts differentiate and begin to deposit a bony matrix. These developmental programs implicate three tissues, the cartilage, the perichondrium, and the vascular endothelium. Due to their intimate associations, the interactions among these three tissues are exceedingly difficult to distinguish and elucidate. We developed an ex vivo system to unlink the processes initiating endochondral ossification and establish more precisely the cellular and molecular contributions of the three tissues involved. In this ex vivo system, the renal capsule of adult mice was used as a host environment to grow skeletal elements. We first used a genetic strategy to follow the fate of cells derived from the perichondrium and from the vasculature. We found that the perichondrium, but not the host vasculature, is the source of both trabecular and cortical osteoblasts. Endothelial cells residing within the perichondrium are the first cells to participate in the invasion of the hypertrophic cartilage matrix, followed by endothelial cells derived from the host environment. We then combined these lineage analyses with a series of tissue manipulations to address how the absence of the perichondrium or the vascular endothelium affected skeletal development. We show that although the perichondrium influences the rate of chondrocytes maturation and hypertrophy, it is not essential for chondrocytes to undergo late hypertrophy. The perichondrium is crucial for the proper invasion of blood vessels into the hypertrophic cartilage and both the perichondrium and the vasculature are essential for endochondral ossification. Collectively, these studies clarify further the contributions of the cartilage, perichondrium, and vascular endothelium to long bone development.

Genetically modified animal models as tools for studying bone and mineral metabolism

Genetic modification of mice is a powerful tool for the study of bone development and metabolism. This review discusses the advantages and disadvantages of various approaches used in bone-related research and the contributions these studies have made to bone biology. Genetic modification of mice is a powerful tool for the study of bone development and metabolism. This review discusses the advantages and disadvantages of various approaches used in bone-related research and the contributions these studies have made to bone biology. The approaches to genetic modification included in this review are (1) overexpression of genes, (2) global gene knockouts, (3) tissue-specific gene deletion, and (4) gene knock-in models. This review also highlights issues that should be considered when using genetically modified animal models, including the rigorous control of genetic background, use of appropriate control lines, and confirmation of tissue specificity of gene expression where appropriate. This technology provides a unique and powerful way to probe the function of genes and is already revolutionizing our approach to understanding the physiology of bone development and metabolism.

Runx2 and Runx3 are essential for chondrocyte maturation, and Runx2 regulates limb growth through induction of Indian hedgehog

The differentiation of mesenchymal cells into chondrocytes and chondrocyte proliferation and maturation are fundamental steps in skeletal development. Runx2 is essential for osteoblast differentiation and is involved in chondrocyte maturation. Although chondrocyte maturation is delayed in Runx2-deficient (Runx2(-/-)) mice, terminal differentiation of chondrocytes does occur, indicating that additional factors are involved in chondrocyte maturation. We investigated the involvement of Runx3 in chondrocyte differentiation by generating Runx2-and-Runx3-deficient (Runx2(-/-)3(-/-)) mice. We found that chondrocyte differentiation was inhibited depending on the dosages of Runx2 and Runx3, and Runx2(-/-)3(-/-) mice showed a complete absence of chondrocyte maturation. Further, the length of the limbs was reduced depending on the dosages of Runx2 and Runx3, due to reduced and disorganized chondrocyte proliferation and reduced cell size in the diaphyses. Runx2(-/-)3(-/-) mice did not express Ihh, which regulates chondrocyte proliferation and maturation. Adenoviral introduction of Runx2 in Runx2(-/-) chondrocyte cultures strongly induced Ihh expression. Moreover, Runx2 directly bound to the promoter region of the Ihh gene and strongly induced expression of the reporter gene driven by the Ihh promoter. These findings demonstrate that Runx2 and Runx3 are essential for chondrocyte maturation and that Runx2 regulates limb growth by organizing chondrocyte maturation and proliferation through the induction of Ihh expression.

PTHrP, PTH, and the PTH/PTHrP receptor in endochondral bone development

Endochondral bone development is a fascinating story of proliferation, maturation, and death. An understanding of this process at the molecular level is emerging. In particular, significant advances have been made in understanding the role of parathyroid-hormone-related peptide (PTHrP), parathyroid hormone (PTH), and the PTH/PTHrP receptor in endochondral bone development. Mutations of the PTH/PTHrP receptor have been identified in Jansen metaphyseal chondrodysplasia, Blomstrand's lethal chondrodysplasia, and enchondromatosis. Furthermore, genetic manipulations of the PTHrP, PTH, and the PTH/PTHrP receptor genes, respectively, have demonstrated the critical role of these proteins in regulating both the switch between proliferation and differentiation of chondrocytes, and their replacement by bone cells. A future area of investigation will be the identification of downstream effectors of PTH, PTHrP, and PTH/PTHrP receptor activities. Furthermore, it will be of critical importance to study how these proteins cooperate and integrate with other molecules that are essential for growth plate development.

Primary murine limb bud mesenchymal cells in long-term culture complete chondrocyte differentiation: TGF-beta delays hypertrophy and PGE2 inhibits terminal differentiation

In vitro models of endochondral bone formation using both primary and immortalized cells have provided insight regarding factors and signaling pathways involved in chondrocyte maturation and endochondral bone formation. However, primary murine cell culture models of chondrocyte differentiation have not been established but have enormous potential due to the possible use of cells from transgenic and knockout animals. Here, we show that stage E11.5 embryonic murine limb bud mesenchymal stem cells in micromass cell culture progress through the stages of chondrogenesis, chondrocyte hypertrophy, terminal differentiation, and matrix calcification. This cell culture system recapitulated the sequential expression of genes that characterize chondrocyte differentiation, including Sox9, col2, colX, MMP13, VEGF, and osteocalcin. TGF-beta treatment for up to 21 days markedly delayed the rate of chondrocyte maturation and inhibited matrix calcification and its related gene expression. In TGF-beta-treated cultures, the hypertrophic and terminal differentiation markers colX, VEGF, MMP13, and osteocalcin were reduced or absent. PGE2 had minimal effects on chondrocyte hypertrophy but delayed terminal differentiation and matrix calcification. Thus, primary murine mesenchymal cells sequentially differentiate through the various stages of chondrocyte maturation and establish a model whereby the role of specific signaling molecules can be examined in cells derived from transgenic or knockout mice.

Transcription factors Sox8 and Sox10 perform non-equivalent roles during oligodendrocyte development despite functional redundancy

Development of myelin-forming oligodendrocytes in the central nervous system is dependent on at least two members of the Sox family of high-mobility-group-containing transcription factors. Sox9 is involved in oligodendrocyte specification, whereas Sox10 is required for terminal differentiation. We show that oligodendrocytes in the spinal cord additionally express the highly related Sox8. In Sox8-deficient mice, oligodendrocyte development proceeded normally until birth. However, terminal differentiation of oligodendrocytes was transiently delayed at early postnatal times. Sox8-deficient mice thus exhibited a similar, but less severe phenotype than did Sox10-deficient mice. Terminal oligodendrocyte differentiation was dramatically delayed in Sox8-deficient mice with only a single functional Sox10 allele hinting at redundancy between both Sox proteins. This redundancy was also evident from the fact that Sox8 bound to naturally occurring Sox10 response elements, was able to form DNA-dependent heterodimers with Sox10 and activated Sox10-specific oligodendrocytic target genes in a manner similar to Sox10. However, Sox8 expression levels were significantly lower than those for Sox10. Resulting differences in protein amounts might be a main reason for the weaker impact of Sox8 on oligodendrocyte development and for unidirectional compensation of the Sox8 loss by Sox10.

Essential role of Sox9 in the pathway that controls formation of cardiac valves and septa

Epithelial-mesenchymal transformation is a critical developmental process reiterated in multiple organs throughout embryogenesis. Formation of endocardial cushions, primordia of valves and septa, is a classic example of epithelial-mesenchymal transformation. Several gene mutations are known to affect cardiac valve formation. Sox9 is activated when endocardial endothelial cells undergo mesenchymal transformation and migrate into an extracellular matrix, called cardiac jelly, to form endocardial cushions. In Sox9-null mutants, endocardial cushions are markedly hypoplastic. In these mutants, Nfatc1 is ectopically expressed and no longer restricted to endothelial cells. Further, Sox9-deficient endocardial mesenchymal cells fail to express ErbB3, which is required for endocardial cushion cell differentiation and proliferation. Our results reveal a succession of molecular steps in the pathway of endocardial cushion development. We propose that loss of Sox9 inhibits epithelial-mesenchymal transformation after delamination and initial migration, but before definitive mesenchymal transformation.

Constitutive activation of MEK1 in chondrocytes causes Stat1-independent achondroplasia-like dwarfism and rescues the Fgfr3-deficient mouse phenotype

We generated transgenic mice that express a constitutively active mutant of MEK1 in chondrocytes. These mice showed a dwarf phenotype similar to achondroplasia, the most common human dwarfism, caused by activating mutations in FGFR3. These mice displayed incomplete hypertrophy of chondrocytes in the growth plates and a general delay in endochondral ossification, whereas chondrocyte proliferation was unaffected. Immunohistochemical analysis of the cranial base in transgenic embryos showed reduced staining for collagen type X and persistent expression of Sox9 in chondrocytes. These observations indicate that the MAPK pathway inhibits hypertrophic differentiation of chondrocytes and negatively regulates bone growth without inhibiting chondrocyte proliferation. Expression of a constitutively active mutant of MEK1 in chondrocytes of Fgfr3-deficient mice inhibited skeletal overgrowth, strongly suggesting that regulation of bone growth by FGFR3 is mediated at least in part by the MAPK pathway. Although loss of Stat1 restored the reduced chondrocyte proliferation in mice expressing an achondroplasia mutant of Fgfr3, it did not rescue the reduced hypertrophic zone, the delay in formation of secondary ossification centers, and the achondroplasia-like phenotype. These observations suggest a model in which Fgfr3 signaling inhibits bone growth by inhibiting chondrocyte differentiation through the MAPK pathway and by inhibiting chondrocyte proliferation through Stat1.

Specification of the otic placode depends on Sox9 function in Xenopus

The vertebrate inner ear develops from a thickening of the embryonic ectoderm, adjacent to the hindbrain, known as the otic placode. All components of the inner ear derive from the embryonic otic placode. Sox proteins form a large class of transcriptional regulators implicated in the control of a variety of developmental processes. One member of this family, Sox9, is expressed in the developing inner ear, but little is known about the early function of Sox9 in this tissue. We report the functional analysis of Sox9 during development of Xenopus inner ear. Sox9 otic expression is initiated shortly after gastrulation in the sensory layer of the ectoderm, in a bilateral patch of cells immediately adjacent to the cranial neural crest. In the otic placode, Sox9 colocalizes with Pax8 one of the earliest gene expressed in response to otic placode inducing signals. Depletion of Sox9 protein in whole embryos using morpholino antisense oligonucleotides causes a dramatic loss of the early otic placode markers Pax8 and Tbx2. Later in embryogenesis, Sox9 morpholino-injected embryos lack a morphologically recognizable otic vesicle and fail to express late otic markers (Tbx2, Bmp4, Otx2 and Wnt3a) that normally exhibit regionalized expression pattern throughout the otocyst. Using a hormone inducible inhibitory mutant of Sox9, we demonstrate that Sox9 function is required for otic placode specification but not for its subsequent patterning. We propose that Sox9 is one of the key regulators of inner ear specification in Xenopus.

Identification of a human glioma antigen, SOX6, recognized by patients' sera

To identify tumor antigens for glioma, a human testis cDNA library was screened by serological identification of antigens by recombinant expression cloning with sera from glioma patients. In this screening, the most frequently isolated antigen was SOX6, an Sry-related high-mobility group (HMG) box-containing gene. SOX6 is a transcriptional factor that is specifically expressed in the developing central nervous system and in the early stages of chondrogenesis in mouse embryos. IgG antibodies against SOX6 were detected in sera from 12 of 36 glioma patients (33.3%), 0 of 14 patients with other brain disease (0%), and one of 54 other cancer patients (1.9%). In sera from 37 healthy individuals, no IgG responses against SOX6 were detected, except in an elderly female. Furthermore, Western blot and ELISA analyses with sera from glioma patients revealed that the DNA-binding domain, the HMG box of SOX6, might be a dominant epitope of IgGs against SOX6. RT-PCR and Northern blot analysis revealed that the SOX6 gene was more highly expressed in glioma tissues than in normal adult tissues, except testis. Western blot analysis with an anti-SOX6 antibody demonstrated that the SOX6 protein was expressed in glioma tissues, but not in normal adult brain tissue. Immunohistochemical analysis with the anti-SOX6 antibody showed that all the glioma tissues analysed expressed SOX6 in tumor cells, but only a few SOX6-positive cells were detected in non-neoplastic tissues from the cerebral cortex. In summary, these results indicate that the developmentally regulated transcription factor SOX6 is aberrantly expressed in glioma and specifically recognized by IgGs from glioma patients' sera.

Molecular cloning and characterization of a novel gene, EMILIN-5, and its possible involvement in skeletal development

By analyzing expression profiles of human mesenchymal stem cells incubated in osteogenic supplements, we identified and characterized a novel human cDNA, elastin microfibril interface located protein-5 (EMILIN-5), that is likely to play a significant role in the process of osteogenesis. The deduced EMILIN-5 product consists of 766 amino acids with a cysteine-rich EMI domain at the NH(2) terminus. Western blotting detected EMILIN-5 expression in a variety of osteoblastic cell lines. Immunohistochemistry of mouse embryos 13.5 days post-coitus revealed relatively high levels of EMILIN-5 protein in perichondrium cells of developing limbs. Our findings suggest that the EMILIN-5 gene plays an important role in skeletal development.

Transduction of Passaged Human Articular Chondrocytes with Adenoviral, Retroviral, and Lentiviral Vectors and the Effects of Enhanced Expression of SOX9

Chondrocytes form and maintain the extracellular matrix of cartilage. The cells can be isolated from cartilage for applications such as tissue engineering, but their expansion in monolayer culture causes a progressive loss of chondrogenic phenotype. In this work, we have investigated the isolation of human articular chondrocytes from osteoarthritic (OA) cartilage at joint replacement, their expansion in monolayer culture, and their transduction with adenoviral, retroviral, and lentiviral vectors, using the gene encoding green fluorescent protein as a marker gene. The addition of growth factors (transforming growth factor beta(1), fibroblast growth factor 2, and platelet-derived growth factor BB) during cell culture was found to greatly increase cell proliferation and thereby to selectively enhance the efficiency of transduction with retrovirus. With adenoviral and lentiviral vectors the transduction efficiency achieved was 95 and 85%, respectively. Using growth factor-supplemented medium with a retroviral vector, efficiency in excess of 80% was achieved. The expression was stable for several months with both retrovirus and lentivirus when analyzed by fluorescence-activated cell-sorting flow analysis and immunoblotting. Transduction with SOX9 was investigated as a method to reinitiate cartilage matrix gene expression in passaged human OA chondrocytes. Endogenous collagen II expression (both mRNA and protein) was increased in monolayer culture using both adenoviral and retroviral vectors. Furthermore, collagen II gene expression in chondrocytes retrovirally transduced with SOX9 was stimulated by alginate bead culture, whereas in control chondrocytes it was not. These results demonstrated methods for rapid expansion and highly efficient transduction of human OA chondrocytes and the potential for the recovery of key features of chondrocyte phenotype by transduction with SOX9.

Potential use of Sox9 gene therapy for intervertebral degenerative disc disease

STUDY DESIGN

A new recombinant adenoviral vector expressing Sox9, a chondrocyte-specific transcription factor, was tested in a chondroblastic cell line and primary human intervertebral disc cells in vitro. Direct infection of intervertebral disc cells then was assessed in a rabbit model.

OBJECTIVES

To deliver a potentially therapeutic viral vector expressing Sox9 to degenerative human and rabbit intervertebral discs cells, and to assess the effect of Sox9 expression on Type 2 collagen production.

SUMMARY OF THE BACKGROUND DATA

The concentration of competent Type 2 collagen, an essential constituent of the healthy nucleus pulposus, declines with intervertebral disc degeneration. Recent studies suggest that Sox9 upregulates Type 2 collagen production. Interventions that augment Type 2 collagen production by intervertebral disc cells may represent a novel therapeutic method for patients with degenerative disc disease.

METHODS

Adenoviral delivery vectors expressing Sox9 and green fluorescent protein were constructed using the AdEasy system. The chondroblastic cell line, HTB-94, and cultured human degenerated intervertebral disc cells were infected with the vectors. Reverse transcriptase-polymerase chain reaction and immunohistochemical analyses were performed to document increased Type 2 collagen expression. The AdSox9 virus then was injected directly into the intervertebral discs of three rabbits. After 5 weeks, the injected discs were evaluated histologically.

RESULTS

The AdSox9 virus efficiently transduced HTB-94 cells and degenerated human disc cells. Western blot analysis confirmed increased Sox9 production. Increased Type 2 collagen production was demonstrated in infected HTB-94 and human disc cells using both reverse transcriptase-polymerase chain reaction and immunohistochemical staining. In the rabbit model, cells infected with AdSox9 maintained a chondrocytic phenotype, and the architecture of the nucleus pulposus was preserved over a 5-week study period compared to control discs.

CONCLUSIONS

A novel adenoviral vector efficiently increased Sox9 and Type 2 collagen synthesis in cultured chondroblastic cells and human degenerated disc cells. In a rabbit model, sustained Sox9 production preserved the histologic appearance of the nucleus pulposus cells in vivo. These findings suggest a potential role for Sox9 gene therapy in the treatment of human degenerative disc disease.

Negative regulation of chondrocyte differentiation by transcription factor AP-2alpha

This study investigated the role of transcription factor AP-2alpha in chondrocyte differentiation in vitro. AP-2alpha mRNA declined during differentiation, and overexpression of AP-2alpha inhibited differentiation. The results demonstrated that AP-2alpha plays a negative role in chondrocyte differentiation.

INTRODUCTION

Transcription factor AP-2alpha has been detected in growth plate and articular chondrocytes and has been shown to regulate cartilage matrix gene expression in vitro. However, the precise functional role of AP-2alpha in chondrocyte differentiation is not known. In this study, we assessed the expression and the function of AP-2alpha in chondrocyte differentiation of ATDC5 cells.

MATERIALS AND METHODS

Chondrocyte differentiation of ATDC5 cells was induced with insulin or transforming growth factor beta (TGF-beta). Proteoglycan production was assessed by alcian blue staining, and expression levels of chondrocyte marker genes and AP-2 gene family were determined by quantitative real time reverse transcriptase-polymerase chain reaction (RT-PCR). Overexpression of AP-2alpha in ATDC5 cells was accomplished by retroviral infection. Infected cells were selected for G418 resistance and pooled for further analysis.

RESULTS AND CONCLUSIONS

Quantitative real time RT-PCR analysis showed that among the four members of the AP-2 gene family, AP-2alpha mRNA was the most abundant. AP-2alpha mRNA levels progressively declined during the differentiation induced by either insulin or TGF-beta treatment. Retroviral expression of AP-2alpha in ATDC5 cells prevented the formation of cartilage nodules, suppressed the proteoglycan production, and inhibited the expression of type II collagen, aggrecan, and type X collagen. Expression profile analysis of key transcription factors involved in chondrogenesis showed that overexpression of AP-2alpha maintained the expression of Sox9 but suppressed the expression of SoxS and Sox6. Taken together, we provide, for the first time, molecular and cellular evidence suggesting that AP-2alpha is a negative regulator of chondrocyte differentiation.

Functional analysis of the regulatory regions of the matrilin-1 gene in transgenic mice reveals modular arrangement of tissue-specific control elements

Matrilin-1 is a non-collagenous protein, which functions in the organization of the extracellular matrix by forming collagen-dependent and -independent filamentous networks. It is secreted primarily by chondrocytes in a characteristic spatial, temporal and developmental stage-specific pattern during skeletogenesis. As a first step to define the tissue- and site-specific regulatory regions of the chicken matrilin-1 gene in vivo, we generated transgenic mice harboring various promoter and intronic fragments fused to the LacZ reporter gene. Histological analysis of the transgene expression pattern during ontogenic development revealed specific X-gal staining in most primordial elements of endochondral bones of transgenic mouse lines carrying either the long promoter between -2011 and +67 or the intronic fragment with a short promoter between -338 and +1819. The cartilage-specific activity of the latter transgene, however, was accompanied with variable ectopic expression pattern in neural and other tissues depending on the site of integration. The presence of both promoter upstream and intronic elements was necessary for the high level transgene activity in all chondrogenic tissues and for the extraskeletal transgene expression pattern resembling the most to that of the chicken matrilin-1 gene, e.g. expression in the eye, and lack of expression in the diminishing notochord and nucleus pulposus. The activity of the transgenes was restricted to the columnar proliferating and pre-hypertrophic chondrocytes visualized by BrdU incorporation and distribution of phosphorylated Sox9, respectively. DNA elements between -2011 and -338 also mediated ectopic LacZ expression in cells of neural crest origin. These results suggest that an interplay of modularly arranged cartilage- and neural crest-specific DNA elements control the expression of the matrilin-1 gene. The dispersal of cartilage-specific elements in the promoter upstream and intronic regions shows similarity to the transcriptional regulation of the Col11a2 gene.

The MEK-ERK Signaling Pathway Is a Negative Regulator of Cartilage-specific Gene Expression in Embryonic Limb Mesenchyme

The extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase pathway, also known as the MEK-ERK kinase cascade, has recently been implicated in the regulation of embryonic cartilage differentiation. However, its precise role in this complex process remains controversial. To more thoroughly examine the role of the MEK-ERK kinase cascade in chondrogenesis, we analyzed the effects of two structurally different pharmacological inhibitors of MEK, the upstream kinase activator of ERK, on chondrocyte differentiation in micromass cultures of embryonic chick limb mesenchyme cells. We found that the MEK inhibitors, U0126 and PD98059, promote increased accumulation of cartilage-characteristic mRNA transcripts for type II collagen, aggrecan, and the transcription factor, Sox9. PD98059 treatment stimulated increased deposition of sulfated glycosaminoglycan into both Alcian blue-stainable cartilage matrix and the surrounding culture medium, whereas U0126 elevated glycosaminoglycan secretion into the medium fraction alone. Both MEK inhibitors increased total type II collagen protein accumulation in micromass culture and elevated the activity of a transfected type II collagen enhancer-luciferase reporter gene. Thus, pharmacological MEK inhibition induced increased expression of multiple chondrocyte differentiation markers. Conversely, transfection of limb mesenchyme cells with a constitutively active MEK1 plasmid resulted in a prominent decrease in the activity of a co-transfected type II collagen enhancer-luciferase reporter gene. Collectively, these findings support the hypothesis that signaling through the MEK-ERK kinase cascade may function as an important inhibitory regulator of embryonic cartilage differentiation.

Identification of potential modifiers of Runx2/Cbfa1 activity in C2C12 cells in response to bone morphogenetic protein-7

Treatment with BMP-7 causes a shift in the differentiation pathway from myoblastic to osteoblastic in C2C12 mouse myoblast precursor cells in vitro. The underlying molecular mechanism is largely unknown. BMP-7 at 200 ng/ml completely inhibited myotube formation in C2C12 cells and dramatically induced alkaline phosphatase activity up to 20-fold when compared to untreated cells by day 12 in culture. The level of Runx2/Cbfa1 mRNA, a bone-specific transcription factor, was also stimulated up to 6-fold by BMP-7 with a peak at 24 h. In addition BMP-7 treatment stimulated a 55-fold increase in osteocalcin mRNA as early as 24 h after treatment. A novel finding was that the expression of the chondrocyte markers Sox9 and type II collagen was increased as well. Runx2/Cbfa1 is a molecular switch for osteoblast differentiation. To initiate the study of modulators of Runx2/Cbfa1, such as kinases and cofactors, during osteoblastic differentiation of C2C12 cells treated by BMP-7 in vitro, microarray analyses of gene expressions were performed. Microarray data suggested that a total of 882 transcripts were either up- or downregulated at least 2-fold. Cluster analyses revealed 76 genes (including ESTs) with expression patterns that paralleled Runx2/Cbfa1. Thirteen of these 76 genes were initially selected as potential transcription modulators for further study; including CCAAT/enhancer binding protein delta, distal- less homeobox 1, forkhead box F2, insulin-like growth factor binding protein 4, an ortholog of human osteoclast stimulating factor 1 and p300/CBP-associated factor. Some transcription modulators have been associated with osteoblastic differentiation or interacted with Runx2/Cbfa1. Most of them have not been extensively studied in osteoblastic differentiation and in relationship to Runx2/Cbfa1. Thus, these studies identify potential regulators for Runx2/Cbfa1 and osteoblast differentiation. In addition, our data revealed for the first time that BMP-7 not only induced the expression of osteoblastic differentiation markers but also stimulated the expression of chondroblastic markers in C2C12 cells.

Establishment of a bipotent cell line CL-1 which differentiates into chondrocytes and adipocytes from adult mouse

OBJECTIVE

Establishment of a clonal bipotent chondroprogenitor cell line from adult mouse to provide a new tool for the elucidation of chondrogenesis in adult animal.

DESIGN

A clonal cell line CL-1 was established from tibia of adult mouse. Differentiation of CL-1 was characterized in monolayer culture. Effects of growth factors (TGF-beta(1), IGF-I, bFGF) and hormones (all trans retinoic acid, 1 alpha.25(OH)(2)D(3), PTH (1-34)) on the growth and differentiation of CL-1 were examined. Bipotency of CL-1 in vivo was examined by transplantation into SCID mice.

RESULTS

CL-1 formed alcian blue (pH1.0) positive nodules spontaneously. The nodules were mineralized in the presence of ascorbic acid and beta-glycerophosphate. CL-1 differentiated also into oil red O positive adipocytes spontaneously. CL-1 cells expressed specific genes of chondrocytes (collagen type II, X, aggrecan) and adipocytes (PPAR-gamma(2), aP(2)). Hyaline cartilage and adipose tissue formation was observed also in subcutaneously transplanted CL-1 cells into SCID mice. These data demonstrate that CL-1 has bipotency either in vitro or in vivo. TGF-beta(1)suppressed growth of CL-1 and induced dominant chondrogenesis accompanied with marked suppression of adipogenesis in 10% FCS. IGF-I stimulated both growth (in 3% FCS) and differentiation of CL-1 into both lineages (in 10% FCS). 1 alpha.25(OH)(2)D(3)and all trans retinoic acid acted as negative regulators on proliferation and differentiation of CL-1 in 10% FCS.

CONCLUSIONS

CL-1 will be a useful tool for the understanding of chondrogenesis in adult animal. Furthermore, CL-1 can be also a powerful tool for screening of the chondrogenic agent.

Control of human articular chondrocyte differentiation by reduced oxygen tension

Cell number is often a limiting factor in studies of chondrocyte physiology, particularly for human investigations. Chondrocytes can be readily proliferated in monolayer culture, however, differentiated phenotype is soon lost. We therefore endeavored to restore normal phenotype to human chondrocytes after serial passage in monolayer culture by manipulating cell morphology and oxygen tension towards the in vivo state. Third passage cells were encapsulated in alginate and exposed to either 20% or more physiologic 5% oxygen tensions. To assess cell phenotype, gene expression was measured using TaqMan real-time PCR. Encapsulated, primary chondrocytes cultured in 20% oxygen were used as a positive reference. Passaged human chondrocytes were fibroblastic in appearance and had lost normal phenotype as evidenced by a decrease in expression of collagen II, aggrecan, and sox9 genes of 66, 6, and 14 fold, respectively; with concomitant high expression of type I collagen (22 fold increase). A partial regaining of the differentiated phenotype was observed by encapsulation in 20% oxygen; however, even after 4 weeks, collagen II gene expression was not fully restored. Collagen II and aggrecan expression were increased, on average, 3 fold, in 5% oxygen tension compared to 20% cultures. Furthermore, matrix glycosaminoglycan (GAG) levels were significantly increased in reduced oxygen. In fact, after 4 weeks in 5% oxygen, encapsulated third passage cells had collagen II expression fully regained and aggrecan and sox9 levels actually exceeding primary cell levels in 20% oxygen. Our results show that the phenotype of serially passaged human articular chondrocytes is more fully restored by combining encapsulation with culture in more physiological levels of oxygen. Sox9, an essential transcription factor for chondrocyte differentiation is strongly implicated in this process since its expression was upregulated almost 27 fold. These findings have implications for the optimal conditions for the in vitro culture of chondrocytes.

Conserved molecular program regulating cranial and appendicular skeletogenesis

The majority of in vivo studies on bone and cartilage differentiation are carried out using the appendicular skeleton as a model system, with the implicit assumption that skeletal formation is equivalent throughout the body. This assumption persists, despite differences in the cellular origins of the skeletogenic precursors. To test the hypothesis that a fundamental set of genes directs skeletal cell differentiation throughout the body, we analyzed cartilage and bone of the chick limb and head during mesenchymal condensation, and when the skeletal tissues had matured. First, we analyzed the expression patterns of transcription factors in early skeletogenic condensations, which revealed similarities among skeletal cell specification in the limb and head. For example, skeletogenic condensations that undergo endochondral ossification had equivalent expression patterns of skeletogenic transcription factors in both limb and head. In the head, many elements also differentiate through intramembranous ossification, or through persistent cartilage formation. Our analyses of these skeletogenic condensations revealed that a unique expression pattern of transcription factors distinguishes among three skeletal tissue fates. The vasculature was excluded from all three skeletogenic condensations, demonstrating that this is not a characteristic unique to endochondral ossification. Second, we compared three different types of more mature cartilage and bone tissue in both the limb and the head, by analyzing a variety of skeletal collagens and signaling molecules. Histological and molecular markers of cartilage and bone generally were conserved between the limb and head skeletons, although we uncovered subtle differences in signaling pathways that might influence cranial and appendicular skeletogenesis.

Paracrine and autocrine signals promoting full chondrogenic differentiation of a mesoblastic cell line

The pluripotent mesoblastic C1 cell line was used under serum-free culture conditions to investigate how paracrine and autocrine signals cooperate to drive chondrogenesis. Sequential addition of two systemic hormones, dexamethasone and triiodothyronine, permits full chondrogenic differentiation. The cell intrinsic activation of the BMP signaling pathway and Sox9 expression occurring on mesoblastic condensation is insufficient for recruitment of the progenitors. Dexamethasone-dependent Sox9 upregulation is essential for chondrogenesis.

INTRODUCTION

Differentiation of lineage stem cells relies on cell autonomous regulations modulated by external signals. We used the pluripotent mesoblastic C1 cell line under serum-free culture conditions to investigate how paracrine and autocrine signals cooperate to induce differentiation of a precursor clone along the chondrogenic lineage.

MATERIALS AND METHODS

C1 cells, cultured as aggregates, were induced toward chondrogenesis by addition of 10(-7) M dexamethasone in serum-free medium. After 30 days, dexamethasone was replaced by 10 nM triiodothyronine to promote final hypertrophic conversion. Mature and hypertrophic phenotypes were characterized by immunocytochemistry using specific antibodies against types II and X collagens, respectively. Type II collagen, bone morphogenetic proteins (BMPs), BMP receptors, Smads, and Sox9 expression were monitored by reverse transcriptase-polymerase chain reaction (RT-PCR), Northern blot, and/or Western blot analysis.

RESULTS AND CONCLUSIONS

Once C1 cells have formed nodules, sequential addition of two systemic hormones is sufficient to promote full chondrogenic differentiation. In response to dexamethasone, nearly 100% of the C1 precursors engage in chondrogenesis and convert within 30 days into mature chondrocytes, which triggers a typical cartilage matrix. On day 25, a switch in type II procollagen mRNA splicing acted as a limiting step in the acquisition of the mature chondrocyte phenotype. On day 30, substitution of dexamethasone with triiodothyronine triggers the final differentiation into hypertrophic chondrocytes within a further 15 days. The chondrogenic process is supported by intrinsic expression of Sox9 and BMP family genes. Similarly to the in vivo situation, activation of Sox9 expression and the BMP signaling pathway occurred on mesoblastic condensation. After induction, BMP-activated Smad nuclear translocation persisted throughout the process until the onset of hypertrophy. After dexamethasone addition, Sox9 expression was upregulated. Dexamethasone withdrawal reversed the increase in Sox9 expression and stopped differentiation. Thus, Sox9 seems to be a downstream mediator of dexamethasone action.

Origin and possible roles of the SOX8 transcription factor gene during sexual development

SOX8 is a member of the SOX family of developmental transcription factor genes and is closely related to SOX9, a critical gene involved in mammalian sex determination and differentiation. Both genes encode proteins with the ability to bind similar DNA target sequences, and to activate transcription in in vitro assays. Expression studies indicate that the two genes have largely overlapping patterns of activity during mammalian embryonic development. A knockout of SOX8 in mice has no obvious developmental phenotype, suggesting that the two genes are able to act redundantly in a variety of developmental contexts. In particular, both genes are expressed in the developing Sertoli cell lineage of the developing testes in mice, and both proteins are able to activate transcription of the gene encoding anti-Müllerian hormone (AMH), through synergistic action with steroidogenic factor 1 (SF1). We have hypothesized that SOX8 may substitute for SOX9 in species where SOX9 is expressed too late to be involved in sex determination or regulation of AMH expression. However, our studies involving the red-eared slider turtle indicate that SOX8 is expressed at similar levels in males and females throughout the sex-determining period, suggesting that SOX8 is neither a transcriptional regulator for AMH, nor responsible for sex determination or gonad differentiation in that species. Similarly, SOX8 is not expressed in a sexually dimorphic pattern during gonadogenesis in the chicken. Since a functional role(s) for SOX8 is implied by its conservation during evolution, the significance of SOX8 for sexual and other aspects of development will need to be uncovered through more directed lines of experimentation. Copyright 2003 S. Karger AG, Basel

Multiple roles of Hoxa11 and Hoxd11 in the formation of the mammalian forelimb zeugopod

Mutations in the 5' or posterior murine Hox genes (paralogous groups 9-13) markedly affect the formation of the stylopod, zeugopod and autopod of both forelimbs and hindlimbs. Targeted disruption of Hoxa11 and Hoxd11 or Hoxa10, Hoxc10 and Hoxd10 result in gross mispatterning of the radius and ulna or the femur, respectively. Similarly, in mice with disruptions of both Hoxa13 and Hoxd13, development of the forelimb and hindlimb autopod is severely curtailed. Although these examples clearly illustrate the major roles played by the posterior Hox genes, little is known regarding the stage or stages at which Hox transcription factors intersect with the limb development program to ensure proper patterning of the principle elements of the limb. Moreover, the cellular and/or molecular bases for the developmental defects observed in these mutant mice have not been described. In this study, we show that malformation of the forelimb zeugopod in Hoxa11/Hoxd11 double mutants is a consequence of interruption at multiple steps during the formation of the radius and ulna. In particular, reductions in the levels of Fgf8 and Fgf10 expression may be related to the observed delay in forelimb bud outgrowth that, in turn, leads to the formation of smaller mesenchymal condensations. However, the most significant defect appears to be the failure to form normal growth plates at the proximal and distal ends of the zeugopod bones. As a consequence, growth and maturation of these bones is highly disorganized, resulting in the creation of amorphous bony elements, rather than a normal radius and ulna.

Two alleles of the Sox9a2 in the rice field eel

A variety of strategies for sex determination mechanisms have been utilized in vertebrates. The Sox9 gene encodes a transcription factor that is critical for testis determination and chondrogenesis in vertebrates. We present here the polymorphisms of the Sox9a gene in population in the rice field eel, a fresh-water fish with naturally sex reversal characteristic from female via intersex into male during its life. Two alleles of the Sox9a2 were found in the population, which may potentially be associated with the dimorphic distribution of the male population of the rice field eel.

Expression profiles of two types of human knee-joint cartilage

We have performed a comprehensive analysis of gene-expression profiles in human articular cartilage (hyaline cartilage) and meniscus (fibrocartilage) by means of a cDNA microarray consisting of 23,040 human genes. Comparing the profiles of the two types of cartilage with those of 29 other normal human tissues identified 24 genes that were specifically expressed in both cartilaginous tissues; these genes might be involved in maintaining phenotypes common to cartilage. We also compared the cartilage profiles with gene expression in human mesenchymal stem cells (hMSC), and detected 22 genes that were differentially expressed in cells representing the two cartilaginous lineages, 11 specific to each type, which could serve as markers for predicting the direction of chondrocyte differentiation. Our data should also provide useful information about regeneration of cartilage, especially in support of efforts to identify cartilage-specific molecules as potential agents for therapeutic approaches to joint repair.

Smad-dependent recruitment of a histone deacetylase/Sin3A complex modulates the bone morphogenetic protein-dependent transcriptional repressor activity of Nkx3.2

We have previously shown that Nkx3.2, a transcriptional repressor that is expressed in the sclerotome and developing cartilage, can activate the chondrocyte differentiation program in somitic mesoderm in a bone morphogenetic protein (BMP)-dependent manner. In this work, we elucidate how BMP signaling modulates the transcriptional repressor activity of Nkx3.2. We have found that Nkx3.2 forms a complex, in vivo, with histone deacetylase 1 (HDAC1) and Smad1 and -4 in a BMP-dependent manner. The homeodomain and NK domain of Nkx3.2 support the interaction of this transcription factor with HDAC1 and Smad1, respectively, and both of these domains are required for the transcriptional repressor activity of Nkx3.2. Furthermore, the recruitment of an HDAC/Sin3A complex to Nkx3.2 requires that Nkx3.2 interact with Smad1 and -4. Indeed, Nkx3.2 both fails to associate with the HDAC/Sin3A complex and represses target gene transcription in a cell line lacking Smad4, but it performs these functions if exogenous Smad4 is added to these cells. While prior work has indicated that BMP-dependent Smads can support transcriptional activation, our findings indicate that BMP-dependent Smads can also potentiate transcriptional repression, depending upon the identity of the Smad-interacting transcription factor.

NF-kappaB mediates inhibition of mesenchymal cell differentiation through a posttranscriptional gene silencing mechanism

Cytokines, such as tumor necrosis factor-alpha (TNFalpha), potently inhibit the differentiation of mesenchymal cells and down-regulate the expression of Sox9 and MyoD, transcription factors required for chondrocyte and myocyte development. Previously, we demonstrated that NF-kappaB controls TNFalpha-mediated suppression of myogenesis through a mechanism involving MyoD mRNA down-regulation. Here, we show that NF-kappaB also suppresses chondrogenesis and destabilizes Sox9 mRNA levels. Multiple copies of an mRNA cis-regulatory motif (5'-ACUACAG-3') are necessary and sufficient for NF-kappaB-mediated Sox9 and MyoD down-regulation. Thus, in response to cytokine signaling, NF-kappaB modulates the differentiation of mesenchymal-derived cell lineages via RNA sequence-dependent, posttranscriptional down-regulation of key developmental regulators.

Interplays of Gli2 and Gli3 and their requirement in mediating Shh-dependent sclerotome induction

Sonic hedgehog (Shh) signaling is essential for sclerotome development in the mouse. Gli2 and Gli3 are thought to be the primary transcriptional mediators of Shh signaling; however, their roles in Shh induction of sclerotomal genes have not been investigated. Using a combination of mutant analysis and in vitro explant assays, we demonstrate that Gli2 and Gli3 are required for Shh-dependent sclerotome induction. Gli2(-/-)Gli3(-/-) embryos exhibit a severe loss of sclerotomal gene expression, and somitic mesoderm from these embryos cannot activate sclerotomal genes in response to exogenous Shh. We find that one copy of either Gli2 or Gli3 is required to mediate Shh induction of sclerotomal markers Pax1 and Pax9 in vivo and in vitro. Although Gli2 is generally considered an activator and Gli3 a repressor, our results also reveal a repressor function for Gli2 and an activator function for Gli3 in the developing somite. To further dissect the function of each Gli, we used adenovirus to overexpress Gli1, Gli2 and Gli3 in presomitic mesoderm explants. We find that each Gli preferentially activates a distinct set of Shh target genes, suggesting that the functions of Shh in patterning, growth and negative feedback are divided preferentially between different Gli proteins in the somite.

Mouse Snail family transcription repressors regulate chondrocyte, extracellular matrix, type II collagen, and aggrecan

Snail family genes are conserved among species during evolution and encode transcription factors expressed at different stages of development in different tissues. These genes are involved in a broad spectrum of biological functions: cell differentiation, cell motility, cell cycle regulation, and apoptosis. However, little is known about the target genes involved in these functions. Here we show that mouse Snail family members, Snail (Sna) and Slug (Slugh), are involved in chondrocyte differentiation by controlling the expression of type II collagen (Col2a1) and aggrecan. In situ hybridization analysis of developing mouse limb demonstrated that Snail and Slug mRNAs were highly expressed in hypertrophic chondrocytes. Inversely, the expression of collagen type II mRNA disappeared during hypertrophic differentiation. Snail and Slug mRNA expression was down-regulated during differentiation of the mouse chondrogenic cell line ATDC5 and overexpression of exogenous Snail or Slug in ATDC5 cells inhibited expression of collagen type II and aggrecan mRNA. Reporter analysis revealed Snail and Slug suppressed the promoter activity of Col2a1, and the E-boxes in the promoter region were the responsible element. Gel shift assay demonstrated the binding of Snail to the E-box. Because type II collagen and aggrecan are major functional components of extracellular matrix in cartilage, these results suggest an important role for Snail-related transcription repressors during chondrocyte differentiation.

Absence of transcription factor c-maf causes abnormal terminal differentiation of hypertrophic chondrocytes during endochondral bone development

In this study, we report that the transcription factor c-Maf is required for normal chondrocyte differentiation during endochondral bone development. c-maf is expressed in hypertrophic chondrocytes during fetal development (E14.5-E18.5), with maximal expression in the tibia occurring at E15.5 and E16.5, in terminally differentiated chondrocytes. In c-maf-null mice, fetal bone length is decreased approximately 10%, and hypertrophic chondrocyte differentiation is perturbed. There is an initial decrease in the number of mature hypertrophic chondrocytes at E15.5 in c-maf-null tibiae, with decreased expression domains of collagen X and osteopontin, markers of hypertrophic and terminal hypertrophic chondrocytes, respectively. By E16.5, there is an expanded domain of late hypertrophic, osteopontin-positive chondrocytes in the c-maf-/-. This accumulation of hypertrophic chondrocytes persists and is still observed at 4 weeks of age. These data suggest that c-Maf facilitates the initial chondrocyte terminal differentiation and influences the disappearance of hypertrophic chondrocytes. BrdU and TUNEL analyses show normal proliferation rate and apoptosis in the c-maf-null. There is a specific decrease in MMP-13 expression at E15.5 in the c-maf-null. MMP-13 is known to be regulated by AP-1 and may also be a target of c-Maf. Thus, cartilage is a novel system in which c-Maf acts during development, where c-Maf is required for normal chondrocyte differentiation.

Transforming growth factor-beta-mediated chondrogenesis of human mesenchymal progenitor cells involves N-cadherin and mitogen-activated protein kinase and Wnt signaling cross-talk

The multilineage differentiation potential of adult tissue-derived mesenchymal progenitor cells (MPCs), such as those from bone marrow and trabecular bone, makes them a useful model to investigate mechanisms regulating tissue development and regeneration, such as cartilage. Treatment with transforming growth factor-beta (TGF-beta) superfamily members is a key requirement for the in vitro chondrogenic differentiation of MPCs. Intracellular signaling cascades, particularly those involving the mitogen-activated protein (MAP) kinases, p38, ERK-1, and JNK, have been shown to be activated by TGF-betas in promoting cartilage-specific gene expression. MPC chondrogenesis in vitro also requires high cell seeding density, reminiscent of the cellular condensation requirements for embryonic mesenchymal chondrogenesis, suggesting common chondro-regulatory mechanisms. Prompted by recent findings of the crucial role of the cell adhesion protein, N-cadherin, and Wnt signaling in condensation and chondrogenesis, we have examined here their involvement, as well as MAP kinase signaling, in TGF-beta1-induced chondrogenesis of trabecular bone-derived MPCs. Our results showed that TGF-beta1 treatment initiates and maintains chondrogenesis of MPCs through the differential chondro-stimulatory activities of p38, ERK-1, and to a lesser extent, JNK. This regulation of MPC chondrogenic differentiation by the MAP kinases involves the modulation of N-cadherin expression levels, thereby likely controlling condensation-like cell-cell interaction and progression to chondrogenic differentiation, by the sequential up-regulation and progressive down-regulation of N-cadherin. TGF-beta1-mediated MAP kinase activation also controls WNT-7A gene expression and Wnt-mediated signaling through the intracellular beta-catenin-TCF pathway, which likely regulates N-cadherin expression and subsequent N-cadherin-mediated cell-adhesion complexes during the early steps of MPC chondrogenesis.

Secondary chondrocyte-derived Ihh stimulates proliferation of periosteal cells during chick development

The development of the skull is characterised by its dependence upon epigenetic influences. One of the most important of these is secondary chondrogenesis, which occurs following ossification within certain membrane bone periostea, as a result of biomechanical articulation. We have studied the genesis, character and function of the secondary chondrocytes of the quadratojugal of the chick between embryonic days 11 and 14. Analysis of gene expression revealed that secondary chondrocytes formed coincident with Sox9 upregulation from a precursor population expressing Cbfa1/Runx2: a reversal of the normal sequence. Such secondary chondrocytes rapidly acquired a phenotype that is a compound of prehypertrophic and hypertrophic chondrocytes, exited from the cell cycle and upregulated Ihh. Pulse and pulse/chase experiments with BrdU confirmed the germinal region as the highly proliferative source of the secondary chondrocytes, which formed by division of chondrocyte-committed precursors. By blocking Hh signalling in explant cultures we show that the enhanced proliferation of the germinal region surrounding the secondary chondrocytes derives from this Ihh source. Additionally, in vitro studies on membrane bone periosteal cells (non-germinal region) demonstrated that these cells can also respond to Ihh, and do so both by enhanced proliferation and precocious osteogenesis. Despite the pro-osteogenic effects of Ihh on periosteal cell differentiation, mechanical articulation of the quadratojugal/quadrate joint in explant culture revealed a negative role for articulation in the regulation of osteocalcin by germinal region descendants. Thus, the mechanical stimulus that is the spur to secondary chondrocyte formation appears able to override the osteogenic influence of Ihh on the periosteum, but does not interfere with the cell cycle-promoting component of Hh signalling.

Genetic disorders of the skeleton: a developmental approach

Although disorders of the skeleton are individually rare, they are of clinical relevance because of their overall frequency. Many attempts have been made in the past to identify disease groups in order to facilitate diagnosis and to draw conclusions about possible underlying pathomechanisms. Traditionally, skeletal disorders have been subdivided into dysostoses, defined as malformations of individual bones or groups of bones, and osteochondrodysplasias, defined as developmental disorders of chondro-osseous tissue. In light of the recent advances in molecular genetics, however, many phenotypically similar skeletal diseases comprising the classical categories turned out not to be based on defects in common genes or physiological pathways. In this article, we present a classification based on a combination of molecular pathology and embryology, taking into account the importance of development for the understanding of bone diseases.

The transcription factor CCAAT-binding factor CBF/NF-Y regulates the proximal promoter activity in the human alpha 1(XI) collagen gene (COL11A1)

We have characterized the proximal promoter region of the human COL11A1 gene. Transient transfection assays indicate that the segment from -199 to +1 is necessary for the activation of basal transcription. Electrophoretic mobility shift assays (EMSAs) demonstrated that the ATTGG sequence, within the -147 to -121 fragment, is critical to bind nuclear proteins in the proximal COL11A1 promoter. We demonstrated that the CCAAT binding factor (CBF/NF-Y) bound to this region using an interference assay with consensus oligonucleotides and a supershift assay with specific antibodies in an EMSA. In a chromatin immunoprecipitation assay and EMSA using DNA-affinity-purified proteins, CBF/NF-Y proteins directly bound this region in vitro and in vivo. We also showed that four tandem copies of the CBF/NF-Y-binding fragment produced higher transcriptional activity than one or two copies, whereas the absence of a CBF/NF-Y-binding fragment suppressed the COL11A1 promoter activity. Furthermore, overexpression of a dominant-negative CBF-B/NF-YA subunit significantly inhibited promoter activity in both transient and stable cells. These results indicate that the CBF/NF-Y proteins regulate the transcription of COL11A1 by directly binding to the ATTGG sequence in the proximal promoter region.

Membrane traffic fuses with cartilage development

The ability of cells to synthesize and secrete proteins is essential for numerous cellular functions. Therefore, when mutations in one component of the secretory pathway result in a tissue-specific defect, a unique opportunity arises to examine the molecular mechanisms at play. The recent finding that a defect in the protein sedlin, whose yeast counterpart is involved in the first step of the secretory pathway, leads to a cartilage-specific disorder in humans raises numerous questions and interesting possibilities for understanding both the pathobiology involved and the role of membrane traffic in normal cartilage development.

Skeletal development: insights from targeting the mouse genome

Manipulation of the mouse genome through mis-expressing, knocking out, and introducing mutations into genes of interest has provided important insights into the genetic pathways responsible for human skeletal development. These pathways contribute to the sequential phases of skeletal morphogenesis that include patterning, condensation, and overt organogenesis of the membranous and endochondral embryonic skeletons and to subsequent linear growth. Disturbances in these pathways account for many developmental syndromes and disorders of the human skeleton. Recurrent themes include establishment of interlocking regulatory circuits involving growth factors, receptors, signalling pathways, and transcription factors that control cellular programmes such as migration, adhesion, proliferation, differentiation, and apoptosis, and use of common molecules for different purposes. Technical advances suggest that genetic engineering in mice will continue to be highly instructive in the field of skeletal biology.

Sox8 is a specific marker for muscle satellite cells and inhibits myogenesis

Sox8 belongs to a family of transcription regulators characterized by a unique DNA-binding domain known as the high mobility group box. Many Sox proteins play fundamental roles in vertebrate development and differentiation processes. Expression of Sox8 is strong during embryonic muscle development and gradually declines postnatally. In this study, we report that in adult skeletal muscle Sox8 is confined to satellite cells. Down-regulation during myogenic differentiation was also detected in cell culture systems and occurred in parallel with down-regulation of the related Sox9. Overexpression of Sox8 or Sox9 on the other hand disrupted myoblasts in their ability to form myotubes. Concomitantly, expression of MyoD and myogenin decreased and basal as well as MyoD-induced activities of the myogenin promoter were strongly reduced in a Sox8-dependent manner. Our data suggest that Sox8 acts as a specific negative regulator of skeletal muscle differentiation, possibly by interfering with the function of myogenic basic helix-loop-helix proteins.

Sox9 is required for determination of the chondrogenic cell lineage in the cranial neural crest

Sox9 has essential roles in endochondral bone formation during axial and appendicular skeletogenesis. Sox9 is also expressed in neural crest cells, but its function in neural crest remains largely unknown. Because many craniofacial skeletal elements are derived from cranial neural crest (CNC) cells, we asked whether deletion of Sox9 in CNC cells by using the Cre recombinase/loxP recombination system would affect craniofacial development. Inactivation of Sox9 in neural crest resulted in a complete absence of cartilages and endochondral bones derived from the CNC. In contrast, all of the mesodermal skeletal elements and intramembranous bones were essentially conserved. The migration and the localization of Sox9-null mutant CNC cells were normal. Indeed, the size of branchial arches and the frontonasal mass of mutant embryos was comparable to that of WT embryos, and the pattern of expression of Ap2, a marker of migrating CNC cells, was normal. Moreover, in mouse embryo chimeras Sox9-null mutant cells migrated to their correct location in endochondral skeletal elements; however, Sox9-null CNC cells were unable to contribute chondrogenic mesenchymal condensations. In mutant embryos, ectopic expression of osteoblast marker genes, such as Runx2, Osterix, and Col1a1, was found in the locations where the nasal cartilages exist in WT embryos. These results indicate that inactivation of Sox9 causes CNC cells to lose their chondrogenic potential. We hypothesize that these cells change their cell fate and acquire the ability to differentiate into osteoblasts. We conclude that Sox9 is required for the determination of the chondrogenic lineage in CNC cells.

Runx1/AML1 hematopoietic transcription factor contributes to skeletal development in vivo

The requirement of Runx2 (Cbfal/AML3), a runt homology domain transcription factor essential for bone formation and osteoblast differentiation, is well established. Although Runx2 is expressed in the developing embryo prior to ossification, yet in the absence of Runx2 initial formation of the skeleton is normal, suggesting a potential redundancy in function of Runx family members. Here we addressed expression of the hematopoietic family member Runx1 (AML1/Cbfa2) in relation to skeletal development using a LacZ knock-in mouse model (Runx1(lz/+)). The resulting fusion protein reflects Runx1 promoter activity in its native context. Our studies show that Runx1 is expressed by prechondrocytic tissue forming the cartilaginous anlagen in the embryo, resting zone chondrocytes, suture lines of the calvarium, and in periosteal and perichondral membranes of all bone. Runx1 continues to be expressed in these tissues in adult mice, but is absent in mature cartilage or mineralized bone. However, hyaline cartilage outside the bone environment (trachea, xiphoid tissues), and epithelium of many soft tissues (trachea, thyroid, lung, skin) also express Runx1. The robust expression of Runx1 in vivo in chondroblasts at sites of cartilage growth and in osteoblasts at sites of new bone formation, suggests that Runx1 expression may be related to osteochondroprogenitor cell differentiation. This observation is further supported by high expression of Runx1 in ex vivo cultures of marrow stromal cells and calvarial derived osteoblasts from Runx1(lz/+) mice. These data indicate that Runx1 may contribute to the early stages of skeletogenesis and continues to function in the progenitor cells of tissues that support bone formation in the adult.

Sox10 is required for the early development of the prospective neural crest in Xenopus embryos

The Sox family of transcription factors has been implicated in the development of different tissues during embryogenesis. Several mutations in humans, mice, and zebrafish have shown that depletion of Sox10 activity produces defects in the development of neural crest derivatives, such as melanocytes, ganglia of the peripheral nervous system, and some specific cell types as glia. We have isolated the Xenopus homologue of the Sox10 gene. It is expressed in prospective neural crest and otic placode regions from the earliest stages of neural crest specification and in migrating cranial and trunk neural crest cells. Loss-of-function experiments using morpholino antisense oligos against Sox10 produce a loss of neural crest precursors and an enlargement of the surrounding neural plate and epidermis. This effect of Sox10 depletion is produced during some of the earliest steps of neural crest specification, as is shown by the inhibition in the expression of Slug and FoxD3, which are early markers of neural crest specification. In addition, we show that Sox10 depletion leads to an increase in apoptosis and a decrease in cell proliferation in the neural folds, suggesting that Sox10 could work as a survival as well as a specification factor in neural crest precursors during premigratory stages. Although some of the deficiencies found in the Waardenburg syndrome and in the Hirschprung disease could be associated with a failure of the development of crest derivatives during the late phase of its development, or even during adulthood, our results suggest that inhibition of Sox10 activity produces an earlier failure of neural crest precursors. In experiments where melanocytes and ganglia were induced in vivo and in vitro, we were able to block their development by inhibiting Sox10 activity. These results are compatible with an additional late role of Sox10 on development of neural crest derivatives, as it has been previously proposed. We show that Sox10 expression is dependent on FGF and Wnt activity, both in the neural crest and in the otic placode territories. Finally, in order to establish the position of Sox10 in the hierarchical cascade of gene activation required for neural crest specification, we used inducible forms of the wild type and dominant negatives for the Snail and Slug genes. Our results show that Snail is able to control Sox10 expression. However, the overexpression of Slug was not able to upregulate Sox10 expression. Taken together, these results indicate that Sox10 may lie between Snail and Slug in the genetic cascade that controls neural crest development.

The molecular action and regulation of the testis-determining factors, SRY (sex-determining region on the Y chromosome) and SOX9 [SRY-related high-mobility group (HMG) box 9]

Despite 12 yr since the discovery of SRY, little is known at the molecular level about how SRY and the SRY-related protein, SOX9 [SRY-related high-mobility group (HMG) box 9], initiate the program of gene expression required to commit the bipotential embryonic gonad to develop into a testis rather than an ovary. Analysis of SRY and SOX9 clinical mutant proteins and XX mice transgenic for testis-determining genes have provided some insight into their normal functions. SRY and SOX9 contain an HMG domain, a DNA-binding motif. The HMG domain plays a central role, being highly conserved between species and the site of nearly all missense mutations causing XY gonadal dysgenesis. SRY and SOX9 are architectural transcription factors; their HMG domain is capable of directing nuclear import and DNA bending. Whether SRY and SOX9 activate testis-forming genes, repress ovary-forming genes, or both remains speculative until downstream DNA target genes are identified. However, factors that control SRY and SOX9 gene expression have been identified, as have a dozen sex-determining genes, allowing some of the pieces in this molecular genetic puzzle to be connected. Many genes, however, remain unidentified, because in the majority of cases of XY females and in all cases of XX males lacking SRY, the mutated gene is unknown.

Tissue-specific gene expression in chondrocytes grown on three-dimensional hyaluronic acid scaffolds

The re-differentiation capacities of human articular and chick embryo sternal chondrocytes were evaluated by culture on HYAFF-11 and its sulphate derivative, HYAFF-11-S, polymers derived from the benzyl esterification of hyaluronate. Initial results showed that the HYAFF-11-S material promoted the highest rate of chondrocyte proliferation. RNA isolated from human and chick embryo chondrocytes cultured in Petri dishes, HYAFF-11 or HYAFF-11-S were subjected to semi-quantitative RT-PCR analyses. Human collagen types I, II, X, human Sox9 and aggrecan, chick collagen types I, II, IX and X were analysed. Results showed that human collagen type II mRNA expression was upregulated on HYAFF-11 biomaterials. In particular, a high level of collagen type IIB expression was associated with three-dimensional culture conditions, and the HYAFF-11 material was the most supportive for human collagen type X mRNA expression. Human Sox9 mRNA levels were constantly maintained in monolayer cell culture conditions over a period of 21 days, while these were upregulated when chondrocytes were cultured on HYAFF-11 and HYAFF-11S. Furthermore, chick collagen type IIA and IIB mRNA expression was detected after only 7 days of HYAFF-11 culture. Chick collagen type IX mRNA expression decreased in scaffold cultures over time. Histochemical staining performed in engineered cartilage revealed the presence of a de novo synthesized glycosaminoglycan-rich extracellular matrix; immunohistochemistry confirmed the deposition of collagen type II. This study showed that the three-dimensional HYAFF-11 culture system is both an effective chondrocyte delivery system for the treatment of articular cartilage defects, and an excellent in vitro model for studying cartilage differentiation.

Characterization of Nkx3.2 DNA binding specificity and its requirement for somitic chondrogenesis

We have previously shown that Nkx3.2, a member of the NK class of homeoproteins, functions as a transcriptional repressor to promote somitic chondrogenesis. However, it has not been addressed whether Nkx3.2 can bind to DNA in a sequence-specific manner and whether DNA binding by Nkx3.2 is required for its biological activity. In this work, we employed a DNA binding site selection assay, which identified TAAGTG as a high affinity Nkx3.2 binding sequence. Sequence-specific binding of Nkx3.2 to the TAAGTG motif in vitro was confirmed by electrophoretic mobility shift assays, and mutagenesis of this sequence revealed that HRAGTG (where H represents A, C, or T, and R represents A or G) comprises the consensus DNA binding site for Nkx3.2. Consistent with these findings, the expression of a reporter gene containing reiterated Nkx3.2 binding sites was repressed in vivo by Nkx3.2 co-expression. In addition, we have generated a DNA nonbinding point mutant of Nkx3.2 (Nkx3.2-N200Q), which contains an asparagine to glutamine missense mutation in the homeodomain. Interestingly, despite being defective in DNA binding, Nkx3.2-N200Q still retains its intrinsic transcriptional repressor function. Finally, we demonstrate that unlike wild-type Nkx3.2, Nkx3.2-N200Q is unable to activate the chondrocyte differentiation program in somitic mesoderm, indicating that DNA binding by Nkx3.2 is critical for this factor to induce somitic chondrogenesis.