Sox9 is required for cartilage formation

Human immortalized chondrocytes carrying heterozygous FGFR3 mutations: an in vitro model to study chondrodysplasias

Achondroplasia and thanatophoric dysplasia are human chondrodysplasias caused by mutations in the fibroblast growth factor receptor 3 (FGFR3) gene. We have developed an immortalized human chondrocyte culture model to study the regulation of chondrocyte functions. One control and eight mutant chondrocytic lines expressing different FGFR3 heterozygous mutations were obtained. FGFR3 signaling pathways were modified in the mutant lines as revealed by the constitutive activation of the STAT pathway and an increased level of P21(WAF1/CIP1) protein. This model will be useful for the study of FGFR3 function in cartilage studies and future therapeutic approaches in chondrodysplasias.

Hif-1alpha regulates differentiation of limb bud mesenchyme and joint development

Recent evidence suggests that low oxygen tension (hypoxia) may control fetal development and differentiation. A crucial mediator of the adaptive response of cells to hypoxia is the transcription factor Hif-1α. In this study, we provide evidence that mesenchymal condensations that give origin to endochondral bones are hypoxic during fetal development, and we demonstrate that Hif-1α is expressed and transcriptionally active in limb bud mesenchyme and in mesenchymal condensations. To investigate the role of Hif-1α in mesenchymal condensations and in early chondrogenesis, we conditionally inactivated Hif-1α in limb bud mesenchyme using a Prx1 promoter-driven Cre transgenic mouse. Conditional knockout of Hif-1α in limb bud mesenchyme does not impair mesenchyme condensation, but alters the formation of the cartilaginous primordia. Late hypertrophic differentiation is also affected as a result of the delay in early chondrogenesis. In addition, mutant mice show a striking impairment of joint development. Our study demonstrates a crucial, and previously unrecognized, role of Hif-1α in early chondrogenesis and joint formation.

Enamel matrix derivative stimulates chondrogenic differentiation of ATDC5 cells

BACKGROUND AND OBJECTIVES

Although enamel matrix derivative can promote chondrogenic differentiation of pluripotent mesenchymal precursor cells, the molecular mechanism that underlies this phenomenon is unclear. The purpose of this study was to determine the effect of enamel matrix derivative on chondrogenic differentiation. ATDC5 cells, which undergo a reproducible multistep chondrogenic differentiation, were cultured with or without enamel matrix derivative for up to 35 d.

METHODS AND RESULTS

Cell proliferation and alkaline phosphatase activity increased markedly in cells cultured in the presence of enamel matrix derivative, compared with cells cultured in its absence. Deposition of Alcian blue-positive cartilage matrix and Alizarin red-positive mineralized nodules also increased significantly upon treatment with enamel matrix derivative. Expression of mRNAs encoding cartilage extracellular matrix proteins (type II collagen, type X collagen and aggrecan) and chondrogenic-related transcription factors (Sox9, Zfp60 and AJ18) were measured using the real-time polymerase chain reaction. Type II collagen, type X collagen and aggrecan mRNA expression increased markedly with enamel matrix derivative treatment. Transcription of Sox9, an important transcription factor that mediates chondrogenic differentiation, also increased with enamel matrix derivative treatment. The KRAB/C2H2 zinc-finger transcription factors, Zfp60 and AJ18, were transiently expressed in the prehypertrophic stage, and their expression increased with enamel matrix derivative treatment. In a western blot analysis with anti-insulin-like growth factor-I and anti-bone morphogenetic protein-6 immunoglobulin, bands corresponding to approximately 14, approximately 18 and approximately 60 kDa were found in enamel matrix derivative.

CONCLUSION

Our study provides clear evidence that enamel matrix derivative promotes chondrogenic differentiation of ATDC5 cells.

SOX9cre1, a cis-acting regulatory element located 1.1 Mb upstream of SOX9, mediates its enhancement through the SHH pathway

SOX9 is a temporal and tissue-specific transcription factor involved in male sexual development and bone formation. Haploinsufficiency of SOX9 is known to cause campomelic dysplasia (CD). CD cases without SOX9 coding region mutations have been described in association with translocations that have breakpoints mapping as far as 932 kb upstream from the gene. These rearrangements suggest that position effects acting from a great distance regulate SOX9 gene expression. Studies of one such case (900 kb upstream to SOX9) have led to the delineation of a potential 2.1 kb cis-acting regulatory element 1.1 Mb upstream of SOX9, termed SOX9cre1. We investigated the role of this putative regulator in SOX9 expression. SOX9cre1 increases the activity of a minimal SOX9 promoter in reporter constructs in a dose-dependent and tissue-specific manner, consistent with an enhancer role. In silico studies identify a putative binding site within SOX9cre1 for GLI1, a downstream mediator of sonic hedgehog (SHH). Furthermore, the stimulation of primary human chondrocyte cells in culture with SHH increases endogenous SOX9 expression 3-fold. Electrophoresis mobility shift assay (EMSA) studies that demonstrate physical interactions between the GLI1 transcription factor and a putative binding site within SOX9cre1, as well as experiments in which reporter constructs are co-transfected with GLI1, suggest a direct interaction between GLI1 and SOX9cre1. GLI1-SOX9cre1 interactions are verified in chromatin immunoprecipitation experiments. These data support a direct molecular link between the Hh signaling pathway and SOX9 regulation, wherein SHH stimulates SOX9 through its mediator GLI1, and are consistent with a mechanism of SOX9 regulation through distal chromatin interactions.

Osteogenic nodule formation from single embryonic stem cell-derived progenitors

The process of bone formation can be approximated in vitro in the form of a mineralized nodule. Osteoprogenitors and mesenchymal stem cells (MSCs), the immediate precursors of the osteoprogenitor, proliferate and differentiate into osteoblasts when placed into culture. These osteoblasts secrete and mineralize a matrix during a period of 3-4 weeks. The differentiation potential of embryonic stem (ES) cells suggests that ES cells should also have the ability to form osteogenic nodules in vitro. ES cells were allowed to form embryoid bodies (EBs) and were cultured in suspension for 2 days; EBs were disrupted and plated as single cells at concentrations as low as 25 cells/cm(2). We provide five lines of evidence for osteogenesis in these ES cell-derived cultures: (1) cell and colony morphology as revealed by phase-contrast microscopy, (2) mineralization of extracellular matrix as revealed by von Kossa staining, (3) quantitative real-time PCR (QRT-PCR) analysis of cDNA from entire plates and individual colonies revealing expression of genes characteristic of, and specific for, osteoblasts, (4) confocal microscopy of nodules from osteocalcin-green fluorescent protein (GFP) ES cell lines demonstrating the appropriate stage and position of osteoblasts expressing the reporter, and (5) immunostaining of nodules with a type I collagen antibody. Our method of initiating osteogenesis from ES cell-derived cultures is the only described method that allows for the observation and manipulation of the commitment stage of mesengenesis from single embryonic progenitors.

Activities of N-Myc in the developing limb link control of skeletal size with digit separation

The developing limb serves as a paradigm for studying pattern formation and morphogenetic cell death. Here, we show that conditional deletion of N-Myc (Mycn) in the developing mouse limb leads to uniformly small skeletal elements and profound soft-tissue syndactyly. The small skeletal elements are associated with decreased proliferation of limb bud mesenchyme and small cartilaginous condensations, and syndactyly is associated with a complete absence of interdigital cell death. Although Myc family proteins have pro-apoptotic activity, N-Myc is not expressed in interdigital cells undergoing programmed cell death. We provide evidence indicating that the lack of interdigital cell death and associated syndactyly is related to an absence of interdigital cells marked by expression of Fgfr2 and Msx2. Thus, instead of directly regulating interdigital cell death, we propose that N-Myc is required for the proper generation of undifferentiated mesenchymal cells that become localized to interdigital regions and trigger digit separation when eliminated by programmed cell death. Our results provide new insight into mechanisms that control limb development and suggest that defects in the formation of N-Myc-dependent interdigital tissue may be a root cause of common syndromic forms of syndactyly.

Sox9-dependent transcriptional regulation of the proprotein convertase furin

The proprotein convertase furin participates in the maturation/bioactivation of a variety of proproteins involved in chondrogenesis events. These include parathyroid hormone-related peptide (PTHrP), an autocrine/paracrine factor that is crucial to both normal cartilage development and cartilage-related pathological processes. Despite the known importance of furin activity in the bioactivation of the polypeptides, the mechanisms that control furin regulation in chondrogenesis remain unknown. To gain insight into the molecular regulation of furin, we used the mouse prechondrogenic ATDC5 cell line, an established in vitro model of cartilage differentiation. Peak expression of both furin mRNA and furin PTHrP maturation was observed during chondrocyte nodule formation stage, an event that correlated with increased mRNA levels of Sox9, a potent high-mobility-group (HMG) box-containing transcription factor required for cartilage formation. Inhibition of furin activity led to a diminution in maturation of PTHrP, suggesting a relationship between Sox9-induced regulation of furin and chondrogenesis events. Transient transfection of Sox9 in nonchondrogenic cells resulted in a marked increase in furin mRNA and in the transactivation of the furin P1A promoter. Direct Sox9 action on the P1A promoter was narrowed down to a critical paired site with Sox9 binding capability in vitro and in vivo. Sox9 transactivation effect was inhibited by L-Sox5 and Sox-6, two Sox9 homologs also expressed in ATDC5 cells. Sox6 inhibitory effect was reduced when using Sox6-HMG-box mutants, indicating a repressive effect through direct HMG-box/DNA binding. Our work suggests a mechanism by which furin is regulated during chondrogenesis. It also adds to the complexity of Sox molecule interaction during gene regulation.

Reconstitution of Runx2/Cbfa1-null cells identifies a requirement for BMP2 signaling through a Runx2 functional domain during osteoblast differentiation

The Runx2/Cbfa1 transcription factor is a scaffolding protein that promotes osteoblast differentiation; however, the specific Runx2-functional domains required for induction of the osteogenic lineage remain to be identified. We approached this question using a TERT-immortalized cell line derived from calvaria of Runx2-null mice by reconstituting the osteogenic activity with wild-type and deletion mutants of Runx2. The presence or absence of osteogenic media (beta-glycerol phosphate and ascorbic acid) and/or with BMP2 did not stimulate osteoblastic gene expression in the Runx2-null cells. However, cells infected with wild-type Runx2 adenovirus showed a robust temporal increase in the expression of osteoblast marker genes and were competent to respond to BMP2. Early markers (i.e., collagen type-1, alkaline phosphatase) were induced (four- to eightfold) at Days 4 and 8 of culture. Genes representing mature osteoblasts (e.g., Runx2, osteopontin, bone sialoprotein, osteocalcin) were temporally expressed and induced from 18- to 36-fold at Days 8 and 12. Interestingly, TGFbeta and Vitamin D-mediated transcription of osteoblast genes (except for osteopontin) required the presence of Runx2. Runx2 lacking the C-terminal 96 amino acids (Runx2 Delta432) showed a pattern of gene expression similar to wild-type protein, demonstrating the Groucho interaction and part of the activation domain are dispensable for Runx2 osteogenic activity. Upon further deletion of the Runx2 C-terminus containing the nuclear matrix targeting signal and Smad-interacting domain (Delta391), we find none of the osteoblast markers are expressed. Therefore, the Runx2 391-432 domain is essential for execution of the BMP2 osteogenic signal.

Histological development and dynamic expression of Bmp2-6 mRNAs in the embryonic and postnatal mouse cranial base

The cranial base is formed by endochondral ossification and is characterized by the presence of the synchondrosis growth centers. The aim of this study was to describe the histological development of the mouse midsagittal cranial base area from embryonic day 10 (E10) to the postnatal age of 2 months. The Bmp family of signaling molecules serves important functions in embryo and bone development and may therefore play a significant role in the early formation of the cranial base. To investigate this, we analyzed the mRNA pattern of expression of Bmp2-6 in the mouse cranial base from E10 to 5 days postnatally using radioactive in situ hybridization. We found that the formation of the mouse cranial base corresponds to that of rat and proceeds in a caudorostral sequence. Moreover, all Bmps studied showed distinct and overlapping developmentally regulated expression domains. Bmp2, Bmp5, and Bmp6 were expressed in the early mesenchymal condensations. Later, Bmp2, Bmp3, Bmp4, and Bmp5 were detected in the perichondrium and in the adjacent mesenchyme. Subsequently, Bmp2 and Bmp6 expressions were confined to hypertrophic chondrocytes, while Bmp3, Bmp4, and Bmp5 were expressed in the osteoblasts of the trabecular bone and bone collar. Interestingly, Bmp3 was uniquely expressed postnatally in the resting zone of the synchondrosis growth center, suggesting a role in the regulation of cranial base growth. These results suggest that Bmp signaling may serve specific and synergistic functions at different key stages of cranial base development and growth.

Restoration of the extracellular matrix in human osteoarthritic articular cartilage by overexpression of the transcription factor SOX9

OBJECTIVE

Human osteoarthritis (OA) is characterized by a pathologic shift in articular cartilage homeostasis toward the progressive loss of extracellular matrix (ECM). The purpose of this study was to investigate the ability of rAAV-mediated SOX9 overexpression to restore major ECM components in human OA articular cartilage.

METHODS

We monitored the synthesis and content of proteoglycans and type II collagen in 3-dimensional cultures of human normal and OA articular chondrocytes and in explant cultures of human normal and OA articular cartilage following direct application of a recombinant adeno-associated virus (rAAV) SOX9 vector in vitro and in situ. We also analyzed the effects of this treatment on cell proliferation in these systems.

RESULTS

Following SOX9 gene transfer, expression levels of proteoglycans and type II collagen increased over time in normal and OA articular chondrocytes in vitro. In situ, overexpression of SOX9 in normal and OA articular cartilage stimulated proteoglycan and type II collagen synthesis in a dose-dependent manner. These effects were not associated with changes in chondrocyte proliferation. Notably, expression of the 2 principal matrix components could be restored in OA articular cartilage to levels similar to those in normal cartilage.

CONCLUSION

These data support the concept of using direct, rAAV-mediated transfer of chondrogenic genes to articular cartilage for the treatment of OA in humans.

Connective tissue growth factor (CTGF) acts as a downstream mediator of TGF-beta1 to induce mesenchymal cell condensation

Mesenchymal cell (MC) condensation or the aggregation of MCs precedes chondrocyte differentiation and is required for subsequent cartilage formation during endochondral ossification. In this study, we used micromass cultures of C3H10T1/2 cells as an in vitro model system for studying MC condensation and the events important for this process. Transforming growth factor beta1 (TGF-beta1) served as the initiator of MC condensation in our model system and we were interested in determining whether CTGF functions as a downstream mediator of TGF-beta1. CTGF is a matricellular protein that has been found to be expressed in MC condensations and in the perichondrium. Micromass cultures of C3H10T1/2 cells condensed under TGF-beta1 stimulation concomitant with dramatic up-regulation of CTGF mRNA and protein levels. CTGF silencing by either CTGF siRNA or CTGF antisense oligonucleotide approaches showed that TGF-beta1-induced condensation was CTGF dependent. Furthermore, silencing of CTGF expression resulted in significant reductions in cell proliferation and migration, events that are crucial during MC condensation. In addition, up-regulation of Fibronectin (FN) and suppression of Sox9 expression by TGF-beta1 was also found to be mediated by CTGF. Immunofluorescence of developing mouse vertebrae showed that CTGF, TGF-beta1 and FN were co-expressed in condensations of MCs, while Sox9 expression was low at this stage. During subsequent chondrogenesis, Sox9 expression was high in chondrocytes while CTGF expression was limited to the perichondrium. Thus, CTGF is an essential downstream mediator of TGF-beta1-induced MC condensation through its effects on cell proliferation and migration. CTGF is also involved in up-regulating FN and suppressing Sox9 expression during TGF-beta1 induced MC condensation.

Sox9 is required for precursor cell expansion and extracellular matrix organization during mouse heart valve development

Heart valve structures derived from mesenchymal cells of the endocardial cushions (ECs) are composed of highly organized cell lineages and extracellular matrix. Sox9 is a transcription factor required for both early and late stages of cartilage formation that is also expressed in the developing valves of the heart. The requirements for Sox9 function during valvulogenesis and adult valve homeostasis in mice were examined by conditional inactivation of Sox9 using Tie2-cre and Col2a1-cre transgenes. Sox9(flox/flox);Tie2-cre mice die before E14.5 with hypoplastic ECs, reduced cell proliferation and altered extracellular matrix protein (ECM) deposition. Sox9(flox/flox);Col2a1-cre mice die at birth with thickened heart valve leaflets, reduced expression of cartilage-associated proteins and abnormal ECM patterning. Thickened valve leaflets and calcium deposits, characteristic of valve disease, are observed in heterozygous adult Sox9(flox/+);Col2a1-cre mice. Therefore, Sox9 is required early in valve development for expansion of the precursor cell population and later is required for normal expression and distribution of valvular ECM proteins. These data indicate that Sox9 is required for early and late stages of valvulogenesis and identify a potential role for Sox9 in valve disease mechanisms.

HC-gp39 contributes to chondrocyte differentiation by inducing SOX9 and type II collagen expressions

OBJECTIVE

The transcription factor SOX9 has been shown to be linked to chondrocyte differentiation and induction of type II collagen synthesis. Since the chitinase-like protein, human cartilage glycoprotein 39 (HC-gp39), can be expressed by articular chondrocytes and has been shown to enhance chondrocyte mitogenesis through MAP kinase and PI3 kinase-mediated signalling, we hypothesized that it may also promote synthesis of cartilage matrix components through induction of SOX9, utilizing similar signalling pathways.

METHODS

Primary chondrocytes from neonatal mouse rib cartilage were exposed to purified HC-gp39. The response of the cells was evaluated in terms of SOX9 induction and synthesis of type II collagen. Signalling pathways activated following HC-gp9 exposure were analyzed by Western blotting of cell lysates with phosphorylation-specific antibodies as well as by using selective inhibitors.

RESULTS

HC-gp39 induced both SOX9 and type II collagen synthesis. Similar results were observed for IGF-1. This process required signalling through both MAP kinase and PI3 kinase pathways resulting in rapid phosphorylation of ERK1/2 and AKT, respectively. Neither HC-gp39 nor IGF-1 induced activation of SAPK/JNK.

CONCLUSIONS

The effects of HC-gp39 on chondrocyte function suggest that this molecule may promote the maintenance or expression of a chondrocytic phenotype. Its expression in injured or degenerate cartilage could be related to the initial repair-response and increased matrix synthesis observed in osteoarthritic cartilage.

Small Round Cell Tumors of Bone

Context.—Primary small round cell tumors of the bone are a heterogeneous group of malignant neoplasms presenting predominantly in children and adolescents. They include Ewing sarcoma/peripheral neuroectodermal tumor or Ewing family tumors, lymphoma, mesenchymal chondrosarcoma, and small cell osteosarcoma. Even though they share many morphological similarities, their unique biological and genetic characteristics have provided substantial insights into the pathology of these diverse neoplasms.

Objective.—To provide an overview of the clinical, radiologic, pathologic, and genetic characteristics of these tumors along with a pertinent review of the literature.

Data Sources.—A literature search using PubMed and Ovid MEDLINE was performed, and data were obtained from various articles pertaining to clinicopathologic, biological, and genetic findings in these tumors. Additionally, findings from rare cases have been included from author's subspecialty experience.

Conclusion.—The diagnosis of small round cell tumors can be made accurately by applying clinicopathologic criteria, as well as a panel of immunohistochemical and genetic studies in appropriate cases. Molecular genetic studies may provide further insight into the biology, histogenesis, and prognosis of these tumors.

SOX9 is required for maintenance of the pancreatic progenitor cell pool

The factors necessary to maintain organ-specific progenitor cells are poorly understood and yet of extreme clinical importance. Here, we identify the transcription factor SOX9 as the first specific marker and maintenance factor of multipotential progenitors during pancreas organogenesis. In the developing pancreas, SOX9 expression is restricted to a mitotically active, Notch-responsive subset of PDX1(+) pluripotent progenitors and is absent from committed endocrine precursors or differentiated cells. Similar to Notch mutations, organ-specific Sox9 inactivation in mice causes severe pancreatic hypoplasia resulting from depletion of the progenitor cell pool. We show that Sox9 maintains pancreatic progenitors by stimulating their proliferation, survival, and persistence in an undifferentiated state. Our finding that SOX9 regulates the Notch-effector HES1 suggests a Notch-dependent mechanism and establishes a possible genetic link between SOX factors and Notch. These findings will be of major significance for the development of in vitro protocols for cell replacement therapies.

SOX9 Is Expressed in Normal Prostate Basal Cells and Regulates Androgen Receptor Expression in Prostate Cancer Cells

SOX9 is a member of the SOX [Sry-related high-mobility group (HMG) box] family of HMG DNA-binding domain transcription factors and is required for the development and differentiation of multiple cell lineages. This report shows that basal epithelial cells express SOX9 in normal prostate, with no detectable expression in luminal epithelial cells. In contrast, SOX9 is expressed in primary prostate cancers in vivo, at a higher frequency in recurrent prostate cancer and in prostate cancer cell lines (LNCaP, CWR22, PC3, and DU145). SOX9 message and protein levels in prostate cancer cells were increased by treatment with glycogen synthase kinase 3beta inhibitor (SB415286), and SOX9 was reduced when beta-catenin was down-regulated by small interfering RNA (siRNA), indicating that SOX9 expression in prostate cancer is regulated by Wnt/beta-catenin signaling. SOX9 bound specifically to androgen receptor (AR) DNA-binding domain glutathione S-transferase fusion proteins, and this interaction was dependent on a short peptide immediately COOH-terminal to the DNA-binding domain (the C-terminal extension), which is required for interactions between steroid hormone receptors and the architectural HMG proteins. Exogenous SOX9 expressed at high nonphysiologic levels decreased AR expression and activity; however, at lower levels, SOX9 increased AR protein expression. Significantly, down-regulation of SOX9 by siRNA in prostate cancer cells reduced endogenous AR protein levels, and cell growth indicating that SOX9 contributes to AR regulation and decreased cellular proliferation. These results indicate that SOX9 in prostate basal cells supports the development and maintenance of the luminal epithelium and that a subset of prostate cancer cells may escape basal cell requirements through SOX9 expression.

Preferential expression and frequent IgG responses of a tumor antigen, SOX5, in glioma patients

We previously reported to identify SOX5 as a glioma antigen by serological screening using a testis cDNA library. The present study was designed to analyze SOX5 expression, its immunoreactivity, and the correlation between SOX5 IgG responses and clinical features in glioma patients to evaluate the possibility of its use as a diagnostic marker. Quantitative RT-PCR and Western blot analysis revealed that SOX5 was expressed in glioma tissues, but not in normal adult tissues, except in the testis. An immunohistochemical analysis showed that SOX5 was expressed in glioma cells, but only a few SOX5-positive cells were detected in non-neoplastic tissues from the cerebral cortex. IgG antibodies against SOX5 were detected in sera from 8 of the 27 glioma patients (27.6%), 0 of the 14 patients with other brain diseases (0%), 1 of the 54 other cancer patients (1.9%) and 1 of the 37 healthy individuals (2.7%). Patients with glioblastoma (GBM) who showed IgG responses against SOX5 exhibited significantly better survival periods than GBM patients without SOX5 antibodies. In summary, SOX5 is aberrantly expressed in glioma and can be recognized as a glioma antigen using IgGs from the sera of glioma patients. Furthermore, there is a statistically significant correlation between the presence of SOX5 IgGs and survival in GBM patients, suggesting that the glioma antigen SOX5 may be useful not only as a diagnostic marker, but also as a prognostic marker in glioma patients.

Regulation of the human SOX9 promoter by Sp1 and CREB

The transcription factor SOX9 is essential for multiple steps during skeletal development, including mesenchymal cell chondrogenesis and endochondral bone formation. We recently reported that the human SOX9 proximal promoter region is regulated by the CCAAT-binding factor through two CCAAT boxes located within 100 bp of the transcriptional start site. Here we report that the human SOX9 proximal promoter is also regulated by the cyclic-AMP response element binding protein (CREB) and Sp1. We show by DNaseI protection and EMSA analysis that CREB and Sp1 interact with specific sites within the SOX9 proximal promoter region. By transient transfection analysis we also demonstrate that mutations of the CREB and Sp1 binding sites result in a profound reduction of SOX9 promoter activity. Chromatin immunoprecipitation (ChIP) assay demonstrated that both Sp1 and CREB interact with the SOX9 promoter in vivo. Finally, we demonstrate that IL-1beta treatment of chondrocytes isolated from human normal and osteoarthritic (OA) cartilage down-regulates SOX9 promoter activity, an effect accompanied by a reduction of Sp1 binding to the SOX9 proximal promoter.

Hypoxic conditions increase hypoxia-inducible transcription factor 2alpha and enhance chondrogenesis in stem cells from the infrapatellar fat pad of osteoarthritis patients

Stem cells derived from the infrapatellar fat pad (IPFP) are a potential source of stem cells for the repair of articular cartilage defects. Hypoxia has been shown to improve chondrogenesis in adult stem cells. In this study we investigated the effects of hypoxia on gene expression changes and chondrogenesis in stem cells from the IPFP removed from elderly patients with osteoarthritis at total knee replacement. Adherent colony-forming cells were isolated and cultured from the IPFP from total knee replacement. The cells at passage 2 were characterised for stem cell surface epitopes, and then cultured for 14 days as cell aggregates in chondrogenic medium under normoxic (20% oxygen) or hypoxic (5% oxygen) conditions. Gene expression analysis, DNA and glycosoaminoglycan assays and immunohistochemical staining were determined to assess chondrogenesis. IPFP-derived adherent colony-forming cells stained strongly for markers of adult mesenchymal stem cells, including CD44, CD90 and CD105, and they were negative for the haematopoietic cell marker CD34 and for the neural and myogenic cell marker CD56. Cell aggregates of IPFP cells showed a chondrogenic response. In hypoxic conditions there was increased matrix accumulation of proteoglycan but less cell proliferation, which resulted in 3.5-fold more glycosoaminoglycan per DNA after 14 days of culture. In hypoxia there was increased expression of hypoxia-inducible transcription factor (HIF)2alpha and not HIF1alpha, and the expression of key transcription factors SOX5, SOX6 and SOX9, and that of aggrecan, versican and collagens II, IX, X and XI, was also increased. These results show that cells with stem cell characteristics were isolated from the IPFP of elderly patients with osteoarthritis and that their response to chondrogenic culture was enhanced by lowered oxygen tension, which upregulated HIF2alpha and increased the synthesis and assembly of matrix during chondrogenesis. This has important implications for tissue engineering applications of cells derived from the IPFP.

Activating transcription factor 2 controls Bcl-2 promoter activity in growth plate chondrocytes

Activating transcription factor 2 (ATF-2) is expressed ubiquitously in mammals. Mice deficient in ATF-2 (ATF-2 m/m) are slightly smaller than their normal littermates at birth. Approximately 50% of mice born mutant in both alleles die within the first month. Those that survive develop a hypochondroplasia-like dwarfism, characterized by shortened growth plates and kyphosis. Expression of ATF-2 within the growth plate is limited to the resting and proliferating zones. We have previously shown that ATF-2 targets the cyclic AMP response element (CRE) in the promoters of cyclin A and cyclin D1 in growth plate chondrocytes to activate their expression. Here, we demonstrate that Bcl-2, a cell death inhibitor that regulates apoptosis, is expressed within the growth plate in proliferative and prehypertrophic chondrocytes. However, Bcl-2 expression declines in hypertrophic chondrocytes. The Bcl-2 promoter contains a CRE at -1,552 bp upstream of the translation start. Mutations within this CRE cause reduced Bcl-2 promoter activity. We show here that the absence of ATF-2 in growth plate chondrocytes corresponds to a decline in Bcl-2 promoter activity, as well as a reduction in Bcl-2 protein levels. In addition, we show that ATF-2 as well as CREB, a transcription factor that can heterodimerize with ATF-2, bind to the CRE within the Bcl-2 promoter. These data identify the Bcl-2 gene as a novel target of ATF-2 and CREB in growth plate chondrocytes.

Deconstructing digit chondrogenesis

Chondrogenesis is a key process in skeletogenesis since endochondral ossification requires the formation of a cartilaginous template. Knowledge of molecular mechanisms regulating chondrogenesis is extremely valuable not only to understand many human disorders but also in regenerative medicine. Embryonic skeletogenesis is an excellent model to study this mechanism. Most cartilages share the cellular basis underlying chondrogenesis but the high heterogeneity in morphologies of the different skeletal elements appears to be generated by differential participation of a variety of chondrogenic signals. Here we overview the regulatory factors responsible for chondrogenesis concluding that early chondrogenic signals for the digit cartilages differ from those implicated in the formation of other axial and appendicular skeletal components.

Integration of signaling pathways regulating chondrocyte differentiation during endochondral bone formation

During endochondral bone formation, chondrocytes undergo a process of terminal differentiation or maturation, during which the rate of proliferation decreases, cells become hypertrophic, and the extracellular matrix is altered by production of a unique protein, collagen X, as well as proteins that promote mineralization. The matrix surrounding the hypertrophic chondrocytes eventually becomes mineralized, and the mineralized matrix serves as a template for bone deposition. This process is responsible for most longitudinal bone growth, both during embryonic development and in the postnatal long bone growth plates. Chondrocyte maturation must be precisely controlled, balancing proliferation with terminal differentiation; changes in the rate of either proliferation or differentiation result in shortened bones. Numerous signaling molecules have been implicated in regulation of this process. These include the negative regulators Indian hedgehog (Ihh) and parathyroid hormone-related protein (PTHrP; Pthlh, PTH-like hormone), as well as a number of positive regulators. This review will focus on several positive regulators which exert profound effects on chondrocyte maturation: the thyroid hormones T3 and T4, retinoic acid (the major active metabolite of vitamin A) and bone morphogenetic proteins (BMPs), as well as the transcription factor Runx2. Each of these molecules is essential for endochondral bone formation and cannot compensate for the others; abrogation of any one of them prevents differentiation. The important features of each of these signaling pathways will be discussed as they relate to chondrocyte maturation, and a model will be proposed suggesting how these pathways may converge to regulate this process.

Misexpression of Sox9 in mouse limb bud mesenchyme induces polydactyly and rescues hypodactyly mice

Our previous studies have demonstrated the essential roles of the transcription factor Sox9 in the commitment of mesenchymal cells to a chondrogenic cell lineage and in overt chondrogenesis during limb bud development. However, it remains unknown if Sox9 induces chondrogenesis in mesenchyme ectopically in vivo as a master regulator of chondrogenesis. In this study, we first generated mutant mice in which Sox9 was misexpressed in the limb bud mesenchyme. The mutant mouse embryos exhibited polydactyly in limb buds in association with ectopic expression of Sox5 and Sox6 although markers for the different axes of limb bud development showed a normal pattern of expression. Misexpression of Sox9 stimulated cell proliferation in limb bud mesenchyme, suggesting that Sox9 has a role in recruiting mesenchymal cells to mesenchymal condensation. Second, despite the facts that misexpression of Sonic hedgehog (Shh) induces polydactyly in a number of mutant mice and Shh-null mutants have severely defective cartilage elements in limb buds, misexpression of Sox9 did not restore limb bud phenotypes in Shh-null mutants. Rather, there was no expression of Sox9 in digit I of Hoxa13Hd mutant embryos, and Sox9 partially rescued hypodactyly in Hoxa13Hd mutant embryos. These results provide evidence that Sox9 induces ectopic chondrogenesis in mesenchymal cells and strongly suggest that its expression may be regulated by Hox genes during limb bud development.

Zebrafish and medaka as models for bone research including implications regarding space-related issues

Teleost fish develop bones directly from mesenchymal condensations and from cartilage precursors. At the cellular level, the involved cell populations share many features with their mammalian counterparts. In addition, several genes are already described in fish showing high homology in amino acid sequence and expression with the corresponding genes of tetrapods that are involved in bone metabolism. Therefore, analysis of the underlying molecular mechanism in fish, in particular zebrafish and medaka, will increase the knowledge in teleosts. Furthermore, it will help to identify novel genes and regulatory pathways of bone homeostasis and skeletal disorders also in higher vertebrates, including disorders caused by altered gravity.

Large-scale gene expression profiling reveals major pathogenetic pathways of cartilage degeneration in osteoarthritis

OBJECTIVE

Despite many research efforts in recent decades, the major pathogenetic mechanisms of osteoarthritis (OA), including gene alterations occurring during OA cartilage degeneration, are poorly understood, and there is no disease-modifying treatment approach. The present study was therefore initiated in order to identify differentially expressed disease-related genes and potential therapeutic targets.

METHODS

This investigation consisted of a large gene expression profiling study performed based on 78 normal and disease samples, using a custom-made complementary DNA array covering >4,000 genes.

RESULTS

Many differentially expressed genes were identified, including the expected up-regulation of anabolic and catabolic matrix genes. In particular, the down-regulation of important oxidative defense genes, i.e., the genes for superoxide dismutases 2 and 3 and glutathione peroxidase 3, was prominent. This indicates that continuous oxidative stress to the cells and the matrix is one major underlying pathogenetic mechanism in OA. Also, genes that are involved in the phenotypic stability of cells, a feature that is greatly reduced in OA cartilage, appeared to be suppressed.

CONCLUSION

Our findings provide a reference data set on gene alterations in OA cartilage and, importantly, indicate major mechanisms underlying central cell biologic alterations that occur during the OA disease process. These results identify molecular targets that can be further investigated in the search for therapeutic interventions.

Inhibition of BMP signaling during zebrafish fin regeneration disrupts fin growth and scleroblast differentiation and function

The zebrafish caudal fin provides a simple model to study molecular mechanisms of dermal bone regeneration. We previously showed that misexpression of Bone morphogenetic protein 2b (Bmp2b) induces ectopic bone formation within the regenerate. Here we show that in addition to bmp2b and bmp4 another family member, bmp6, is involved in fin regeneration. We further investigated the function of BMP signaling by ectopically expressing the BMP signaling inhibitor Chordin which caused: (1) inhibition of regenerate outgrowth due to a decrease of blastema cell proliferation and downregulation of msxb and msxC expression and (2) reduced bone matrix deposition resulting from a defect in the maturation and function of bone-secreting cells. We then identified targets of BMP signaling involved in regeneration of the bone of the fin rays. runx2a/b and their target col10a1 were downregulated following BMP signaling inhibition. Unexpectedly, the sox9a/b transcription factors responsible for chondrocyte differentiation were detected in the non-cartilaginous fin rays, sox9a and sox9b were not only differentially expressed but also differentially regulated since sox9a, but not sox9b, was downregulated in the absence of BMP signaling. Finally, this analysis revealed the surprising finding of the expression, in the fin regenerate, of several factors which are normally the signatures of chondrogenic elements during endochondral bone formation although fin rays form through dermal ossification, without a cartilage intermediate.

Runx2 inhibits chondrocyte proliferation and hypertrophy through its expression in the perichondrium

The perichondrium, a structure made of undifferentiated mesenchymal cells surrounding growth plate cartilage, regulates chondrocyte maturation through poorly understood mechanisms. Analyses of loss- and gain-of-function models show that Twist-1, whose expression in cartilage is restricted to perichondrium, favors chondrocyte maturation in a Runx2-dependent manner. Runx2, in turn, enhances perichondrial expression of Fgf18, a regulator of chondrocyte maturation. Accordingly, compound heterozygous embryos for Runx2 and Fgf18 deletion display the same chondrocyte maturation phenotype as Fgf18-null embryos. This study identifies a transcriptional basis for the inhibition of chondrocyte maturation by perichondrium and reveals that Runx2 fulfills antagonistic functions during chondrogenesis.

Involvement of cyclic guanosine monophosphate-dependent protein kinase II in chondrocyte hypertrophy during endochondral ossification

During vertebrate skeletal development, the appendicular skeleton forms through endochondral ossification, which involves the intricately regulated multistep differentiation of mesenchymal cells. During this process, mesenchymal condensations initially differentiate into chondrocytes. Then chondrocytes in the center further differentiate into hypertrophic chondrocytes. Hypertrophic chondrocytes express a number of osteogenic factors and induce bone formation. Although numerous studies have provided novel insights into the regulation and function of cartilage development, little is known about the intracellular signaling pathways regulating chondrocyte hypertrophy. Recent study revealed that cyclic guanosine monophosphate (cGMP)-dependent protein kinase II (cGKII) coupled the stop of proliferation and the start of hypertrophic differentiation of chondrocytes. Herein, we review the molecular mechanism of regulation of chondrocyte hypertrophy by cGKII and the interaction between cGKII and other signaling pathways.

Effects of low-intensity pulsed ultrasound on the differentiation of C2C12 cells

Low-intensity pulsed ultrasound (LIPUS) is known to accelerate bone regeneration, but the precise cellular mechanism is still unclear. The purpose of this study was to determine the effect of LIPUS on the differentiation of pluripotent mesenchymal cell line C2C12. The cells were cultured in differentiation medium with or without the addition of LIPUS stimulation. The ultrasound signal consisted of 1.5 MHz at an intensity of 70 mW/cm2 for 20 min for all cultures. To verify the cell lineage after LIPUS stimulation, mRNA expression of cellular phenotype-specific markers characterizing osteoblasts (Runx2, Msx2, Dlx5, AJ18), chondroblasts (Sox9), myoblasts (MyoD), and adipocytes (C/EBP, PPARgamma) was studied using real-time polymerase chain reaction analysis. The protein expression of Runx2 and activated phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) and p38 mitogen-activated protein kinase (p38 MAPK) were performed using Western blotting. The mRNA expression of Runx2, Msx2, Dlx5, AJ18, and Sox9 was increased markedly by the LIPUS stimulation, whereas the expression of MyoD, C/EBP, and PPARgamma was drastically decreased. In the Western blot analysis, LIPUS stimulation increased Runx2 protein expression and phosphorylation of ERK1/2 and p38 MAPK. Our study demonstrated that LIPUS stimulation converts the differentiation pathway of C2C12 cells into the osteoblast and/or chondroblast lineage via activated phosphorylation of ERK1/2 and p38 MAPK.

Initiation of Mesenchymal Condensation in Alginate Hollow Spheres?A Useful Model for Understanding Cartilage Repair?

A promising strategy for the regeneration of degenerated cartilage tissue structure in osteoarthritic joints is the use of mesenchymal precursor cells. These cells can be triggered to undergo differentiation into functional active chondrocytes resulting in newly synthesized cartilage. Because chondrogenic differentiation is initiated by the step of mesenchymal condensation in vitro, it is of great interest to fully characterize the first lineage specific step in vitro. Therefore, a modified culture system was developed which mimics the process in vitro and may finally help to identify the key factors that are essential for the induction of chondrogenic differentiation in vivo. Compared to other established 3D culture systems like alginate beads and micromass cultures, the use of alginate hollow spheres bears the advantage to analyze different phases of cell aggregation starting from a single cell suspension of previously isolated and expanded human primary cells of mesenchymal origin.

Gene expression in human keloids is altered from dermal to chondrocytic and osteogenic lineage

Keloids are a dermal fibrotic disease whose etiology remains totally unknown and for which there is no successful treatment. Here, we employed cDNA microarray analysis to examine gene expression in keloid lesions and control skin. We found that 32 genes among the 9000 tested were strongly up-regulated in keloid lesions, of which 21 were confirmed by Northern blotting. These included at least seven chondrocyte/osteoblast marker genes, and RT-PCR analysis revealed that transcription factors specific for these genes, SOX9 and CBFA1, were induced. Immunostaining and in situ hybridization further supported that these markers are expressed in keloid lesions. Intriguingly, scleraxis, a transcription factor known as a marker of tendons and ligaments, was also induced in keloid fibroblasts. We propose that reprogramming of gene expression or disordered differentiation from a dermal pattern to that of a chondrocytic/osteogenic lineage, probably closer to that of tendon/ligament lineage, may be involved in the etiology of keloids.

Low-intensity Ultrasound Stimulation Enhances Chondrogenic Differentiation in Alginate Culture of Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) are regarded as a potential autologous source for cartilage repair, because they can differentiate into chondrocytes by transforming growth factor-beta (TGF-beta) treatment under the 3-dimensional (3-D) culture condition. However, more efficient and versatile methods for chondrogenic differentiation of MSCs are still in demand for its clinical application. Recently, low-intensity ultrasound (LIUS) was shown to enhance fracture healing in vitro and induce chondrogenesis of MSCs in vitro. In this study, we investigated the effects of LIUS on the chondrogenesis of rabbit MSCs (rMSCs) in a 3-D alginate culture and on the maintenance of chondrogenic phenotypes after replating them on a monolayer culture. The LIUS treatment of rMSCs increased: (i) the matrix formation; (ii) the expression of chondrogenic markers such as collagen type II, aggrecan, and Sox-9; (iii) the expression of tissue inhibitor of metalloprotease-2 implicated in the integrity of cartilage matrix; and (iv) the capacity to maintain the chondrogenic phenotypes in a monolayer culture. Notably, LIUS effects were clearly shown even without TGF-beta treatment. These results suggest that LIUS treatment could be an efficient and cost-effective method to induce chondrogenic differentiation of MSCs in vitro for cartilage tissue engineering.

Repetitive tensile stress to rat caudal vertebrae inducing cartilage formation in the spinal ligaments: a possible role of mechanical stress in the development of ossification of the spinal ligaments

Object

Mechanical stress has been considered one of the important factors in ossification of the spinal ligaments. According to previous clinical and in vitro studies, the accumulation of tensile stress to these ligaments may be responsible for ligament ossification. To elucidate the relationship between such mechanical stress and the development of ossification of the spinal ligaments, the authors established an animal experimental model in which the in vivo response of the spinal ligaments to direct repetitive tensile loading could be observed.

Methods

The caudal vertebrae of adult Wistar rats were studied. After creating a novel stimulating apparatus, cyclic tensile force was loaded to rat caudal spinal ligaments at 10 N in 600 to 1800 cycles per day for up to 2 weeks. The morphological responses were then evaluated histologically and immunohistochemically.

After the loadings, ectopic cartilaginous formations surrounded by proliferating round cells were observed near the insertion of the spinal ligaments. Several areas of the cartilaginous tissue were accompanied by woven bone. Bone morphogenetic protein–2 expression was clearly observed in the cytoplasm of the proliferating round cells. The histological features of the rat spinal ligaments induced by the tensile loadings resembled those of spinal ligament ossification observed in humans.

Conclusions

The findings obtained in the present study strongly suggest that repetitive tensile stress to the spinal ligaments is one of the important causes of ligament ossification in the spine.

Congenital cervical instability in a patient with camptomelic dysplasia

INTRODUCTION

Camptomelic dysplasia (CD) is a rare autosomal dominant skeletal dysplasia classically characterized by bent bones of the extremities, tracheobronchial narrowing, thoracic kyphoscoliosis, and various degrees of phenotypic sex reversal. Most patients die of complications in infancy, although long-term survivors have been reported.

CASE REPORT

We report a case of CD complicated by incomplete ossification of the cervical vertebral pedicles, resulting in congenital cervical instability and kyphosis. Closed reduction was performed, and the patient was fitted with a customized cervical orthosis.

FINDING

He subsequently developed a complete spinal cord injury at the kyphotic level. This underscores the grim prognosis associated with neonatal cervical spinal instability.

BMP action in skeletogenesis involves attenuation of retinoid signaling

The bone morphogenetic protein (BMP) and growth and differentiation factor (GDF) signaling pathways have well-established and essential roles within the developing skeleton in coordinating the formation of cartilaginous anlagen. However, the identification of bona fide targets that underlie the action of these signaling molecules in chondrogenesis has remained elusive. We have identified the gene for the retinoic acid (RA) synthesis enzyme Aldh1a2 as a principal target of BMP signaling; prochondrogenic BMPs or GDFs lead to attenuation of Aldh1a2 expression and, consequently, to reduced activation of the retinoid signaling pathway. Consistent with this, antagonism of retinoid signaling phenocopies BMP4 action, whereas RA inhibits the chondrogenic stimulatory activity of BMP4. BMP4 also down-regulates Aldh1a2 expression in organ culture and, consistent with this, Aldh1a2 is actively excluded from the developing cartilage anlagens. Collectively, these findings provide novel insights into BMP action and demonstrate that BMP signaling governs the fate of prechondrogenic mesenchyme, at least in part, through regulation of retinoid signaling.

Impact of transcription factor Sox8 on oligodendrocyte specification in the mouse embryonic spinal cord

The myelin-forming oligodendrocytes of the mouse embryonic spinal cord express the three group E Sox proteins Sox8, Sox9, and Sox10. They require Sox9 for their specification from neuroepithelial cells of the ventricular zone and Sox10 for their terminal differentiation and myelination. Here, we show that during oligodendrocyte development, Sox8 is expressed after Sox9, but before Sox10. Loss of Sox8 did not impair oligodendrocyte specification by itself, but enhanced the Sox9-dependent defect. Oligodendrocyte progenitors were still generated in the Sox9-deficient spinal cord, albeit at 20-fold lower rates than in the wildtype. Combined loss of Sox8 and Sox9, in contrast, led to a near complete loss of oligodendrocytes. Other cell types such as ventricular zone cells and radial glia remained unaffected in their numbers as well as their rates of proliferation and apoptosis. Oligodendrocyte development thus relies on the differential contribution of all three group E Sox proteins at various phases.

A promoter element of the CD-RAP gene is required for repression of gene expression in non-cartilage tissues in vitro and in vivo

The cartilage-derived retinoic acid-sensitive protein (CD-RAP) gene is expressed predominately in cartilage. Previous studies in transgenic mice have shown that the DNA promoter segment from -2,251 bp to -2,068 bp of the CD-RAP gene contains elements critical for gene expression. Subsequent studies revealed both positive and negative regulatory motifs in this 183 bp element. Here we show that this element demonstrates activation or repression of gene expression in vitro and in vivo based on cell type and content of transcription factors. The distribution of Sox (positive) and C/EBP (negative) transcription factors in cell lines and in mouse tissues is consistent with their positive and negative roles. In transgenic mice, when the 183-bp element was removed from a 3,345-bp cartilage-specific CD-RAP promoter, expression of the reporter gene became widespread, being observed in muscle, bone, lung, and liver in addition to cartilage. In vitro, mutation of the C/EBP site activated the inactive 3,345-bp CD-RAP gene promoter in myoblastic cells, suggesting that this site is responsible for (-2,079 bp) repression. These results indicate that the 183-bp element plays an important role in cartilage-specific gene expression by acting as a chondrocyte-regulatory module repressing transcription in non-chondrocytes and contributing to activation in chondrocytes. This is the first report of a functional DNA element necessary for repression in non-cartilage tissues in vivo.

An in situ hybridization study of Runx2, Osterix, and Sox9 at the onset of condylar cartilage formation in fetal mouse mandible

Mandibular condylar cartilage is the principal secondary cartilage, differing from primary cartilage in its rapid differentiation from progenitor cells (preosteoblasts/skeletoblasts) to hypertrophic chondrocytes. The expression of three transcription factors related to bone and cartilage formation, namely Runx2, Osterix and Sox9, was investigated at the onset of mouse mandibular condylar cartilage formation by in situ hybridization. Messenger RNAs for these three molecules were expressed in the condylar anlage, consisting of preosteoblasts/skeletoblasts, at embryonic day (E)14. Hypertrophic chondrocytes appeared at E15 as soon as cartilage tissue appeared. Runx2 mRNA was expressed in the embryonic zone at the posterior position of the newly formed cartilage, in the bone collar and in the newly formed cartilage, but expression intensity in the newly formed cartilage was slightly weaker. Osterix mRNA was also expressed in the embryonic zone and in the bone collar, but was at markedly lower levels in the newly formed cartilage. Sox9 mRNA was continuously expressed from the embryonic zone to the newly formed cartilage. At this stage, Sox5 mRNA was expressed only in the newly formed cartilage. These results suggest that reduced expression of Osterix in combination with Sox9-Sox5 expression is important for the onset of condylar (secondary) cartilage formation.

Zebrafish Trap230/Med12 is required as a coactivator for Sox9-dependent neural crest, cartilage and ear development

The vertebrate Sox9 transcription factor directs the development of neural crest, otic placodes, cartilage and bone. In zebrafish, there are two Sox9 orthologs, Sox9a and Sox9b, which together perform the functions of the single-copy tetrapod Sox9. In a large-scale genetic screen, we have identified a novel zebrafish mutant that strongly resembles the Sox9a/Sox9b double mutant phenotype. We show that this mutation disrupts the zebrafish Trap230/Med12 ortholog, a member of the Mediator complex. Mediator is a coactivator complex transducing the interaction of DNA-binding transcription factors with RNA polymerase II, and our results reveal a critical function of the Trap230 subunit as a coactivator for Sox9.

Gene therapy for cartilage defects

Focal defects of articular cartilage are an unsolved problem in clinical orthopaedics. These lesions do not heal spontaneously and no treatment leads to complete and durable cartilage regeneration. Although the concept of gene therapy for cartilage damage appears elegant and straightforward, current research indicates that an adaptation of gene transfer techniques to the problem of a circumscribed cartilage defect is required in order to successfully implement this approach. In particular, the localised delivery into the defect of therapeutic gene constructs is desirable. Current strategies aim at inducing chondrogenic pathways in the repair tissue that fills such defects. These include the stimulation of chondrocyte proliferation, maturation, and matrix synthesis via direct or cell transplantation-mediated approaches. Among the most studied candidates, polypeptide growth factors have shown promise to enhance the structural quality of the repair tissue. A better understanding of the basic scientific aspects of cartilage defect repair, together with the identification of additional molecular targets and the development of improved gene-delivery techniques, may allow a clinical translation of gene therapy for cartilage defects. The first experimental steps provide reason for cautious optimism.

Sox9 in testis determination

Sox9 is an Sry-box-containing gene that encodes a transcriptional activator. During mouse gonadogenesis, Sox9 is detected in the male gonad at 11.5 days postcoitus (dpc). At 12.5 dpc, testicular cords form, morphologically distinguishing the male gonad from the ovary. From this stage onwards, Sox9 expression is restricted to the Sertoli cell lineage and persists in the adult. Humans with heterozygous mutations in SOX9 develop a skeletal syndrome known as campomelic dysplasia. Furthermore, most XY SOX9 heterozygotes show variable male-to-female sex reversal, implicating SOX9 in testis development. Sox9 heterozygous knockout mice die at birth with a syndrome similar to that of human campomelic dysplasia. In contrast to humans, XY Sox9+/- mice form normal appearing testes. Germ-line knockout of Sox9 using a conditional null allele provides a tool for generating Sox9-/- mice by simple genetic crosses. However, Sox9-/- mice die soon after 11.5 dpc because of cardiovascular defects. In vitro culture of the urogenital ridges of XY Sox9-/- results in gonads lacking testicular cords and Sertoli cell marker expression, but with the expression of ovarian-specific markers. Therefore, Sox9 is essential for diverting an intrinsically ovarian program of organogenesis toward testis formation.

Stem cell marker expression in the Bergmann glia population of the adult mouse brain

Recent evidence suggests that the postnatal cerebellum contains cells with characteristics of neural stem cells, which had so far only been identified in the subventricular zone of the lateral ventricles and the subdentate gyrus of the hippocampus. In order to investigate the identity of these cells in the adult cerebellum, we have analyzed the expression of Sox1, a transcription factor from the SoxB1 subgroup and widely used marker of neural stem cells. In situ hybridization and the use of a transgenic mouse model show that, in the adult cerebellum, Sox 1 is only expressed in the Bergmann glia, a population of radial glia present in the Purkinje cell layer. Furthermore, another neural stem cell marker, Sox2 (also member of the SoxB1 subgroup), is also expressed in the Bergmann glia. We have previously shown that these same cells express Sox9, a member of the SoxE subgroup known for its role in glial development. Here we show that Sox9 is in fact also expressed in other regions harboring adult neural stem cells, suggesting that Sox9 represents a novel stem cell marker. Finally, using a Sox1-null mouse, we show that the formation of this Sox2/Sox9 positive Bergmann glia population does not require the presence of a functional Sox1. Our results identify these radial glia as a previously unreported Sox1/Sox2/Sox9 positive adult cell population, suggesting that these cells may represent the recently reported stem cells in the adult cerebellum.

SOX13 exhibits a distinct spatial and temporal expression pattern during chondrogenesis, neurogenesis, and limb development

SOX13 is a member of the SOX family of transcription factors. SOX proteins play essential roles in development, and some are associated with human genetic diseases. SOX13 maps to a multi-disease locus on chromosome 1q31-32, yet its function is unknown. Here we describe the temporal and spatial expression of SOX13 protein during mouse organogenesis. SOX13 is expressed in the three embryonic cell lineages, suggesting that it may direct various developmental processes. SOX13 is expressed in the developing central nervous system including the neural tube and the developing brain. Expression is also detected in the condensing mesenchyme and cartilage progenitor cells during endochondral bone formation in the limb as well as the somite sclerotome and its derivatives. SOX13 is also detected in the developing kidney, pancreas, and liver as well as in the visceral mesoderm of the extra-embryonic yolk sac and spongiotrophoblast layer of the placenta.

The Chondrocyte: Biology and Clinical Application

Chondrocyte is a unique cell type in articular cartilage tissue and is essential for cartilage formation and functionality. It arises from mesenchymal stem cells (MSCs) and is regulated by a series of cytokine and transcription factor interactions, including the transforming growth factor-beta super family, fibroblast growth factors, and insulin-like growth factor-1. To understand the biomechanisms of the chondrocyte differentiation process, various cellular model systems have been employed, such as primary chondrocyte culture, clonal normal cell lines (HCS-2/8, Ch-1, ATDC5, CFK-2, and RCJ3.1C5.18), and transformed clonal cell lines (T/C-28a2, T/C-28a4, C-28/I2, tsT/AC62, and HPV-16 E6/E7). Additionally, cell culture methods, including conventional monolayer culture, three-dimensional scaffold culture, bioreactor culture, pellet culture, and organ culture, have been established to create stable environments for the expansion, phenotypic maintenance, and subsequent biological study of chondrocytes for clinical application. Knowledge gained through these study systems has allowed for the use of chondrocytes in orthopedics for the treatment of cartilage injury and epiphyseal growth plate defects using tissue-engineering approaches. Furthermore, the potential of chondrocyte implantation for facial reconstruction, the treatment of long segmental tracheal defects, and urinary incontinence and vesicoureteral reflux are being investigated. This review summarizes the present study of chondrocyte biology and the potential uses of this cell in orthopedics and other disciplines.

A zebrafish screen for craniofacial mutants identifies wdr68 as a highly conserved gene required for endothelin-1 expression

BACKGROUND

Craniofacial birth defects result from defects in cranial neural crest (NC) patterning and morphogenesis. The vertebrate craniofacial skeleton is derived from cranial NC cells and the patterning of these cells occurs within the pharyngeal arches. Substantial efforts have led to the identification of several genes required for craniofacial skeletal development such as the endothelin-1 (edn1) signaling pathway that is required for lower jaw formation. However, many essential genes required for craniofacial development remain to be identified.

RESULTS

Through screening a collection of insertional zebrafish mutants containing approximately 25% of the genes essential for embryonic development, we present the identification of 15 essential genes that are required for craniofacial development. We identified 3 genes required for hyomandibular development. We also identified zebrafish models for Campomelic Dysplasia and Ehlers-Danlos syndrome. To further demonstrate the utility of this method, we include a characterization of the wdr68 gene. We show that wdr68 acts upstream of the edn1 pathway and is also required for formation of the upper jaw equivalent, the palatoquadrate. We also present evidence that the level of wdr68 activity required for edn1 pathway function differs between the 1st and 2nd arches. Wdr68 interacts with two minibrain-related kinases, Dyrk1a and Dyrk1b, required for embryonic growth and myotube differentiation, respectively. We show that a GFP-Wdr68 fusion protein localizes to the nucleus with Dyrk1a in contrast to an engineered loss of function mutation Wdr68-T284F that no longer accumulated in the cell nucleus and failed to rescue wdr68 mutant animals. Wdr68 homologs appear to exist in all eukaryotic genomes. Notably, we found that the Drosophila wdr68 homolog CG14614 could substitute for the vertebrate wdr68 gene even though insects lack the NC cell lineage.

CONCLUSION

This work represents a systematic identification of approximately 25% of the essential genes required for craniofacial development. The identification of zebrafish models for two human disease syndromes indicates that homologs to the other genes are likely to also be relevant for human craniofacial development. The initial characterization of wdr68 suggests an important role in craniofacial development for the highly conserved Wdr68-Dyrk1 protein complexes.

Sox9 is required for notochord maintenance in mice

Sox9 encodes a HMG-box transcription factor that has been implicated in numerous developmental processes including chondrogenesis, formation of cardiac valves, and neural crest, testis and spinal cord development. Here we show that Sox9 is expressed in the notochord and the sclerotome during mouse development suggesting that the gene may play additional roles in the development of the axial skeleton. We used ubiquitous mosaic inactivation of a conditional Sox9 allele by Cre/loxP-mediated recombination in the mouse to screen for novel functions of Sox9, and revealed that its absence results in severe malformations of the vertebral column. Besides its established role in chondrogenesis, Sox9 is required for maintaining the structural integrity of the notochord. Mutant embryos establish a normal notochord; however, starting from E9.5, the notochord disintegrates in a cranial to caudal manner. The late requirement in notochord development uncovered a function of notochord-derived signals in inducing segmentation of the ventral sclerotome and chondrogenesis. Thus, Sox9 is required for axial skeletogenesis by regulating notochord survival and chondrogenesis.

Interactions between PIAS Proteins and SOX9 Result in an Increase in the Cellular Concentrations of SOX9

We have identified PIAS1 (protein inhibitor of activated STAT-1), -3, -xalpha, and -xbeta as SOX9-associated polypeptides using the Gal4-based yeast two-hybrid system and a cDNA library derived from a chondrocytic cell line. These PIAS proteins were shown to interact directly with SOX9 in two-hybrid, co-immunoprecipitation, and electrophoretic mobility shift assays. SOX9 was sumoylated in cotransfection experiments with COS-7 cells using PIAS and SUMO-1 (small ubiquitin-like modifier-1) expression vectors. SOX9 was also sumoylated in vitro by PIAS proteins in the presence of SUMO-1, the SUMO-activating enzyme, and the SUMO-conjugating enzyme. In COS-7 cells, PIAS proteins stimulated the SOX9-dependent transcriptional activity of a Col2a1 promoter-enhancer reporter. This increase in reporter activity was paralleled by an increase in the cellular levels of SOX9. Cotransfection with a SUMO-expressing vector further enhanced the transcriptional activity of this SOX9-dependent Col2a1 reporter in COS-7 cells, and this additional activation was inhibited in the presence of either SUMO-1 mutants or PIAS RING domain mutants or by coexpression of a desumoylation enzyme. Immunofluorescence microscopy of SOX9-transfected COS-7 cells showed that the subnuclear distribution of SOX9 became more diffuse in the presence of PIAS1 and SUMO-1. Our results suggest that, by controlling the cellular concentrations of SOX9, PIAS proteins and sumoylation may be part of a major regulatory system of SOX9 functions.

Frontal nasal prominence expression driven by Tcfap2a relies on a conserved binding site for STAT proteins

The AP-2 transcription factor family is linked with development of the head and limbs in both vertebrate and invertebrate species. Recent evidence has also implicated this gene family in the evolution of the neural crest in chordates, a critical step that allowed the development and elaboration of the vertebrate craniofacial skeleton. In mice, the inappropriate embryonic expression of one particular AP-2 gene, Tcfap2a, encoding AP-2alpha, results in multiple developmental abnormalities, including craniofacial and limb defects. Thus, Tcfap2a provides a valuable genetic resource to analyze the regulatory hierarchy responsible for the evolution and development of the face and limbs. Previous studies have identified a 2-kilobase intronic region of both the mouse and human AP-2alpha locus that directs expression of a linked LacZ transgene to the facial processes and the distal mesenchyme of the limb bud in transgenic mice. Further analysis identified two highly conserved regions of approximately 200-400 bp within this tissue-specific enhancer. We have now initiated a transgenic and biochemical analysis of the most important of these highly conserved regions. Our analysis indicates that although the sequences regulating face and limb expression have been integrated into a single enhancer, different cis-acting sequences ultimately control these two expression domains. Moreover, these studies demonstrate that a conserved STAT binding site provides a major contribution to the expression of Tcfap2a in the facial prominences.

Hearts and bones: shared regulatory mechanisms in heart valve, cartilage, tendon, and bone development

The mature heart valves are dynamic structures composed of highly organized cell lineages and extracellular matrices. The discrete architecture of connective tissue within valve leaflets and supporting structures allows the valve to withstand life-long functional demands and changes in hemodynamic forces and load. The dysregulation of ECM organization is a common feature of heart valve disease and can often be linked to genetic defects in matrix protein structure or developmental regulation. Recent studies have identified specific regulatory pathways that are active in the developing valve structures and also control cartilage, tendon, and bone development. This review will focus on the regulatory hierarchies that control normal and abnormal heart valve development in parallel with other connective tissue cell types.

Mechanical compressive loading stimulates the activity of proximal region of human COL2A1 gene promoter in transfected chondrocytes

Previous studies have demonstrated that the mechanical compressive loading affects the biosynthesis of chondrocytes seeded in three dimensional scaffolds. In this study, the level of type II collagen mRNA expression was increased by a continuous dynamic compression at 10% compressive strain and 0.1 Hz in chondrocytes seeded in a biodegradable, elastomeric scaffold, poly(L-lactide-co-epsilon-caprolactone) (PLCL). To further examine this molecular mechanism, the promoter region of COL2A1 gene, which is encoding type II collagen, was analyzed using rabbit chondrocytes transfected with luciferase reporter vectors containing the 5'-flanking regions of human COL2A1 gene. A deletion mutant analysis revealed that the most active short promoter in response to continuous dynamic compression is in the region between -509 and -109 base pairs, where the transcription factor Sp1 is located. Additionally, an mRNA decay experiment using transcription inhibitor actinomycin D demonstrated that dynamic compression do not stabilize type II collagen mRNA. Our results indicate that mechanical compression increases the level of type II mRNA expression by transcriptional activation possibly through the Sp1 binding sites residing in the proximal region of the COL2A1 gene promoter.