The transcription factors L-Sox5 and Sox6 are essential for cartilage formation

Effect of chilling on sox2, sox3 and sox19a gene expression in zebrafish (Danio rerio) embryos

Zebrafish embryos have not been cryopreserved due to their structural limitations. Although embryo survival rates have been used as the measured outcome for most of the cryopreservation protocols studied, there are very limited data available at the molecular level. This study focused on the effect of chilling and subsequent warming on gene expression of sox2, sox3 and sox19a which play vital roles in the development of zebrafish embryos. A quantitative RT-PCR approach was used to investigate gene expression following chilling at 0°C for up to 180 min. The effect on gene expression was also studied during a 180 min warming period after chilling for 30 or 60 min. There were significant decreases in sox2 (up to 4-fold) and sox3 (up to 3-fold) expressions following chilling. Significant increases in gene expressions of sox2 (up to 2-fold), sox3 (up to 33-fold) and sox19a (up to 25-fold) were observed during warming in the embryos that had been chilled for 30 min. Similarly, significant increases were observed in sox2 (up to 3-fold) and sox3 (up to 2-fold) during warming in embryos that had been chilled for 60 min. These increases may be explained by compensation for the suppression observed during chilling and/or to activate repair mechanisms or maintain homeostasis.

A new mechanistic scenario for the origin and evolution of vertebrate cartilage

The appearance of cellular cartilage was a defining event in vertebrate evolution because it made possible the physical expansion of the vertebrate "new head". Despite its central role in vertebrate evolution, the origin of cellular cartilage has been difficult to understand. This is largely due to a lack of informative evolutionary intermediates linking vertebrate cellular cartilage to the acellular cartilage of invertebrate chordates. The basal jawless vertebrate, lamprey, has long been considered key to understanding the evolution of vertebrate cartilage. However, histological analyses of the lamprey head skeleton suggest it is composed of modern cellular cartilage and a putatively unrelated connective tissue called mucocartilage, with no obvious transitional tissue. Here we take a molecular approach to better understand the evolutionary relationships between lamprey cellular cartilage, gnathostome cellular cartilage, and lamprey mucocartilage. We find that despite overt histological similarity, lamprey and gnathostome cellular cartilage utilize divergent gene regulatory networks (GRNs). While the gnathostome cellular cartilage GRN broadly incorporates Runx, Barx, and Alx transcription factors, lamprey cellular cartilage does not express Runx or Barx, and only deploys Alx genes in certain regions. Furthermore, we find that lamprey mucocartilage, despite its distinctive mesenchymal morphology, deploys every component of the gnathostome cartilage GRN, albeit in different domains. Based on these findings, and previous work, we propose a stepwise model for the evolution of vertebrate cellular cartilage in which the appearance of a generic neural crest-derived skeletal tissue was followed by a phase of skeletal tissue diversification in early agnathans. In the gnathostome lineage, a single type of rigid cellular cartilage became dominant, replacing other skeletal tissues and evolving via gene cooption to become the definitive cellular cartilage of modern jawed vertebrates.

Endogenous tagging of the murine transcription factor Sox5 with hemaglutinin for functional studies

Gene expression is usually studied at the transcript level rather than at the protein level due to the lack of a specific and sensitive antibody. A way to overcome this is to fuse to the protein of interest an immunoreactive tag that has well-characterized antibodies. This epitope tagging approach is often used for in vitro experiments but for in vivo studies, the success rate of protein tagging has not been extensively analyzed and our study seeks to cover the void. A small epitope, hemaglutinin derived from the influenza virus was used to tag a transcription factor, Sox5 at the N-terminal via homologous recombination in the mouse. Sox5 is part of the Sry-related high-mobility-group box gene family and plays multiple roles in essential biological processes. Understanding of its molecular function in relation to its biological roles remains incomplete. In our study, we show that the longer isoform of Sox5 can be tagged endogenously with hemaglutinin without affecting its biological function in vivo. The tagged protein is easily and specifically detected with an anti-hemaglutinin antibody using immunohistochemistry with its expression matching the endogenous expression of Sox5. Immunoprecipitation of Sox5 was also carried out successfully using an anti-hemaglutinin antibody. The transgenic line generated from this study is predicted to be useful for future experiments such as co-immunoprecipitation and chromatin immunoprecipitation, allowing the further understanding of Sox5. Lastly, this approach can be easily employed for the investigation of other transcription factors and proteins in vivo to overcome technical limitations such as antibody cross-reactivity and to perform isoform-specific studies.

SOX trio decrease in the articular cartilage with the advancement of osteoarthritis

SOX trio (SOX-5, SOX-6, and SOX-9) maintain the chondrocytic phenotypes and are vital for chondrogenesis in embryonic development. The purpose of this study is to investigate the change in the expression of SOX trio with the advancement of osteoarthritis (OA) in human articular cartilage (AC). Human OA samples from eight patients were obtained from the distal femoral condyles during total knee arthroplasty. Minimally OA cartilage taken from areas with no obvious surface defects on lateral condyles was compared with advanced OA cartilage obtained from areas within 1 cm of overt lesion located on medial condyle surface. SOX-5, SOX-6, and SOX-9 gene expressions significantly decreased by 41% (p = 0.047), 46% (p = 0.047), and 56% (p = 0.029) in advanced OA area compared with the minimally OA area. There was a significant decrease in aggrecan and type II collagen (COL2A1) gene expressions by 73% (p = 0.029) and 65% (p = 0.029), respectively, in advanced OA area compared with the minimally OA area. From Western blotting and immunohistochemistry, SOX-5, SOX-6, SOX-9, type II collagen, and aggrecan protein expressions also significantly decreased in advanced OA cartilage compared with minimally OA cartilage. DNA methylation study of SOX-9 promoter regions revealed no difference in the epigenetic status between the two areas. It is concluded that SOX trio gene and protein decreased with advancement of OA in human articular cartilage.

Unraveling the transcriptional regulatory machinery in chondrogenesis

Since the discovery of SOX9 mutations in the severe human skeletal malformation syndrome campomelic dysplasia in 1994, Sox9 was shown to be both required and sufficient for chondrocyte specification and differentiation. At the same time, its distant relatives Sox5 and Sox6 were shown to act in redundancy with each other to robustly enhance its functions. The Sox trio is currently best known for its ability to activate the genes for cartilage-specific extracellular matrix components. Sox9 and Sox5/6 homodimerize through domains adjacent to their Sry-related high-mobility-group DNA-binding domain to increase the efficiency of their cooperative binding to chondrocyte-specific enhancers. Sox9 possesses a potent transactivation domain and thereby recruits diverse transcriptional co-activators, histone-modifying enzymes, subunits of the mediator complex, and components of the general transcriptional machinery, such as CBP/p300, Med12, Med25, and Wwp2. This information helps us begin to unravel the mechanisms responsible for Sox9-mediated transcription. We review here the discovery of this master chondrogenic trio and its roles in chondrogenesis in vivo and at the molecular level, and we discuss how these pioneering studies open the way for many additional studies that are needed to further increase our understanding of the transcriptional regulatory machinery operating in chondrogenesis.

Concerted regulation of myofiber-specific gene expression and muscle performance by the transcriptional repressor Sox6

In response to physiological stimuli, skeletal muscle alters its myofiber composition to significantly affect muscle performance and metabolism. This process requires concerted regulation of myofiber-specific isoforms of sarcomeric and calcium regulatory proteins that couple action potentials to the generation of contractile force. Here, we identify Sox6 as a fast myofiber-enriched repressor of slow muscle gene expression in vivo. Mice lacking Sox6 specifically in skeletal muscle have an increased number of slow myofibers, elevated mitochondrial activity, and exhibit down-regulation of the fast myofiber gene program, resulting in enhanced muscular endurance. In addition, microarray profiling of Sox6 knockout muscle revealed extensive muscle fiber-type remodeling, and identified numerous genes that display distinctive fiber-type enrichment. Sox6 directly represses the transcription of slow myofiber-enriched genes by binding to conserved cis-regulatory elements. These results identify Sox6 as a robust regulator of muscle contractile phenotype and metabolism, and elucidate a mechanism by which functionally related muscle fiber-type specific gene isoforms are collectively controlled.

Sox6, jack of all trades: a versatile regulatory protein in vertebrate development

Approximately 20,000 genes are encoded in our genome, one tenth of which are thought to be transcription factors. Considering the complexity and variety of cell types generated during development, many transcription factors likely play multiple roles. Uncovering the versatile roles of Sox6 in vertebrate development sheds some light on how an organism efficiently utilizes the limited resources of transcription factors. The structure of the Sox6 gene itself may dictate its functional versatility. First, Sox6 contains no known regulatory domains; instead, it utilizes various cofactors. Second, Sox6 has a long 3'-UTR that contains multiple microRNA targets, thus its protein level is duly adjusted by cell type-specific microRNAs. Just combining these two characteristics alone makes Sox6 extremely versatile. To date, Sox6 has been reported to regulate differentiation of tissues of mesoderm, ectoderm, and endoderm origins, making Sox6 a truly multifaceted transcription factor.

SOX5 is a candidate gene for chronic obstructive pulmonary disease susceptibility and is necessary for lung development

RATIONALE

Chromosome 12p has been linked to chronic obstructive pulmonary disease (COPD) in the Boston Early-Onset COPD Study (BEOCOPD), but a susceptibility gene in that region has not been identified.

OBJECTIVES

We used high-density single-nucleotide polymorphism (SNP) mapping to implicate a COPD susceptibility gene and an animal model to determine the potential role of SOX5 in lung development and COPD.

METHODS

On chromosome 12p, we genotyped 1,387 SNPs in 386 COPD cases from the National Emphysema Treatment Trial and 424 control smokers from the Normative Aging Study. SNPs with significant associations were then tested in the BEOCOPD study and the International COPD Genetics Network. Based on the human results, we assessed histology and gene expression in the lungs of Sox5(-/-) mice.

MEASUREMENTS AND MAIN RESULTS

In the case-control analysis, 27 SNPs were significant at P ≤ 0.01. The most significant SNP in the BEOCOPD replication was rs11046966 (National Emphysema Treatment Trial-Normative Aging Study P = 6.0 × 10(-4), BEOCOPD P = 1.5 × 10(-5), combined P = 1.7 × 10(-7)), located 3' to the gene SOX5. Association with rs11046966 was not replicated in the International COPD Genetics Network. Sox5(-/-) mice showed abnormal lung development, with a delay in maturation before the saccular stage, as early as E16.5. Lung pathology in Sox5(-/-) lungs was associated with a decrease in fibronectin expression, an extracellular matrix component critical for branching morphogenesis.

CONCLUSIONS

Genetic variation in the transcription factor SOX5 is associated with COPD susceptibility. A mouse model suggests that the effect may be due, in part, to its effects on lung development and/or repair processes.

Chondrocyte-specific regulatory activity of Runx2 is essential for survival and skeletal development

Coordinated activities of multiple mesenchymal cell types contribute to the development of the mammalian skeleton formed through endochondral ossification. Synthesis of a cartilage template by chondrocytes is an obligatory step for the generation of skeletal elements during endochondral ossification. Gene ablation studies have established that Runx2 is an essential transcription factor for bone formation and the differentiation of skeletal cells. However, global gene deletion has failed to discern the tissue- and cell type-specific roles of Runx2. We generated floxed mice to elucidate the Runx2 regulatory control distinctive to cartilage tissue during bone development. Exon 8 of the Runx2 gene was selectively deleted in developing chondrocytes by utilizing Col2a-Cre mice. Cell- and tissue-specific gene recombination was confirmed by β-gal activity in R26R mice. The chondrocyte-specific loss of Runx2 caused failure of endochondral ossification, impaired craniofacial development, dwarfism, and perinatal lethality. Radiographic imaging and histochemical approaches were used to characterize the skeletal phenotype. We conclude that regulatory control of Runx2 in chondrocytes is essential for endochondral ossification, and it is independent of the role of Runx2 in osteoblasts.

Electroporation-mediated gene transfer of SOX trio to enhance chondrogenesis in adipose stem cells

OBJECTIVE

The aim of the present study was to determine if the electroporation-mediated gene transfer of SOX trio enhances the chondrogenic potential of adipose stem cells (ASCs).

DESIGN

ASCs were transfected with SOX trio genes using an electroporation technique and cultured for 3 weeks. The pellets were analyzed for DNA and glycosaminoglycan (GAG) analysis, and the gene and protein expression of SOX-5, SOX-6, SOX-9, type 1 collagen (COL1Al), type 2 collagen (COL2Al) and type 10 collagen (COL10A1) using real-time PCR and Western blot analysis. Further in vivo studies were carried out by subcutaneous transplantation of pellets in severe combined immunodeficiency (SCID) mice for 3 weeks.

RESULTS

The gene transfer efficiency was high (approximately 70%). Transfected ASCs showed high expression of corresponding genes after 21 days, and each SOX protein was detected in ASCs transfected with the corresponding gene. The chondrogenic differentiation of ASCs, as demonstrated by GAG levels and Safranin-O staining, showed significant enhancement when SOX trio were co-transfected, while subsets with single gene transfer of SOX-5, -6, or -9 did not show significant elevation. SOX trio co-transfection enhanced COL2A1 mRNA, but did not increase COL1A1 and COL10A1 mRNA. Type II collagen protein dramatically increased, and type X collagen decreased with co-transfection of the SOX trio. When pellets were implanted in the subcutaneous pouch of SCID mice for 3 weeks, ASCs co-transfected with SOX trio demonstrated abundant proteoglycan, significantly reduced mineralization.

CONCLUSION

The electroporation-mediated transfection of SOX trio greatly enhances chondrogenesis from ASCs, while decreasing hypertrophy.

Sry-box (Sox) transcription factors in gastrointestinal physiology and disease

The genetic mechanisms underlying tissue maintenance of the gastrointestinal tract are critical for the proper function of the digestive system under normal physiological stress. The identification of transcription factors and related signal transduction pathways that regulate stem cell maintenance and lineage allocation is attractive from a clinical standpoint in that it may provide targets for novel cell- or drug-based therapies. Sox [sex-determining region Y (Sry) box-containing] factors are a family of transcription factors that are emerging as potent regulators of stem cell maintenance and cell fate decisions in multiple organ systems and might provide valuable insight toward the understanding of these processes in endodermally derived tissues of the gastrointestinal tract. In this review, we focus on the known genetic functions of Sox factors and their roles in epithelial tissues of the esophagus, stomach, intestine, colon, pancreas, and liver. Additionally, we discuss pathological conditions in the gastrointestinal tract that are associated with a dysregulation of Sox factors. Further study of Sox factors and their role in gastrointestinal physiology and pathophysiology may lead to advances that facilitate control of tissue maintenance and development of advanced clinical therapies.

Generation of hyaline cartilaginous tissue from mouse adult dermal fibroblast culture by defined factors

Repair of cartilage injury with hyaline cartilage continues to be a challenging clinical problem. Because of the limited number of chondrocytes in vivo, coupled with in vitro de-differentiation of chondrocytes into fibrochondrocytes, which secrete type I collagen and have an altered matrix architecture and mechanical function, there is a need for a novel cell source that produces hyaline cartilage. The generation of induced pluripotent stem (iPS) cells has provided a tool for reprogramming dermal fibroblasts to an undifferentiated state by ectopic expression of reprogramming factors. Here, we show that retroviral expression of two reprogramming factors (c-Myc and Klf4) and one chondrogenic factor (SOX9) induces polygonal chondrogenic cells directly from adult dermal fibroblast cultures. Induced cells expressed marker genes for chondrocytes but not fibroblasts, i.e., the promoters of type I collagen genes were extensively methylated. Although some induced cell lines formed tumors when subcutaneously injected into nude mice, other induced cell lines generated stable homogenous hyaline cartilage–like tissue. Further, the doxycycline-inducible induction system demonstrated that induced cells are able to respond to chondrogenic medium by expressing endogenous Sox9 and maintain chondrogenic potential after substantial reduction of transgene expression. Thus, this approach could lead to the preparation of hyaline cartilage directly from skin, without generating iPS cells.

Chondrogenesis of adipose stem cells in a porous PLGA scaffold impregnated with plasmid DNA containing SOX trio (SOX-5,-6 and -9) genes

We developed a chondrogenic scaffold system in which plasmid DNA (pDNA) containing SOX trio (SOX-5, -6, and -9) genes was incorporated into a PLGA scaffold and slowly released to transfect adipose stem cells (ASCs) seeded in the scaffold. The purpose of this study was to test the in vitro and in vivo efficacy of the system to induce chondrogenic differentiation of ASCs. The pDNA/PEI-PEG complex-incorporated PLGA/Pluronic F127 porous scaffolds were fabricated by a precipitation/particulate leaching method. The following five kinds of pDNA were incorporated into the scaffolds: 1) pECFP-C1 vector without an interposed gene (control group); 2) SOX-5 plasmids; 3) SOX-6 plasmids; 4) SOX-9 plasmids; and 5) one-third doses of each plasmid (SOX-5, -6, and -9). ASCs were seeded on pDNA-incorporated PLGA scaffolds and cultured in chondrogenic media for 21 days. ASCs were also isolated from rabbits, seeded in pDNA-incorporated PLGA scaffolds, and then implanted in the osteochondral defect created on the patellar groove. The rabbits were sacrificed and analyzed grossly and microscopically 8 weeks after implantation. The percentage of transfected cells was highest on day 14, around 70%. After 21 days, PLGA scaffolds incorporated with each gene showed markedly increased expression of the corresponding gene and protein. Glycosaminoglycan (GAG) assay and Safranin-O staining showed an increased proteoglycan production in SOX trio pDNA-incorporated scaffolds. The COL2A1 gene and protein were notably increased in SOX trio pDNA-incorporated scaffolds than in the control, while COL10A1 protein expression decreased. Gross and histological findings from the in vivo study showed enhanced cartilage regeneration in ASCs/SOX trio pDNA-incorporated PLGA scaffolds.

The promotion of cartilage defect repair using adenovirus mediated Sox9 gene transfer of rabbit bone marrow mesenchymal stem cells

Although Sox9 is essential for chondrogenic differentiation and matrix production, its application in cartilage tissue engineering has been rarely reported. In this study, the chondrogenic effect of Sox9 on bone marrow mesenchymal stem cells (BMSCs) in vitro and its application in articular cartilage repair in vivo were evaluated. Rabbit BMSCs were transduced with adenoviral vector containing Sox9. Toluidine blue, safranin O staining and real-time PCR were performed to check chondrogenic differentiation. The results showed that Sox9 could induce chondrogenesis of BMSCs both in monolayer and on PGA scaffold effectively. The rabbit model with full-thickness cartilage defects was established and then repaired by PGA scaffold and rabbit BMSCs with or without Sox9 transduction. HE, safranin O staining and immunohistochemistry were used to assess the repair of defects by the complex. Better repair, including more newly-formed cartilage tissue and hyaline cartilage-specific extracellular matrix and greater expression of several chondrogenesis marker genes were observed in PGA scaffold and BMSCs with Sox9 transduction, compared to that without transduction. Our findings defined the important role of Sox9 in the repair of cartilage defects in vivo and provided evidence that Sox9 had the potential and advantage in the application of tissue engineering.

Sox9 sustains chondrocyte survival and hypertrophy in part through Pik3ca-Akt pathways

During endochondral bone formation, Sox9 expression starts in mesenchymal progenitors, continues in the round and flat chondrocyte stages at high levels, and ceases just prior to the hypertrophic chondrocyte stage. Sox9 is important in mesenchymal progenitors for their differentiation into chondrocytes, but its functions post-differentiation have not been determined. To investigate Sox9 function in chondrocytes, we deleted mouse Sox9 at two different steps after chondrocyte differentiation. Sox9 inactivation in round chondrocytes resulted in a loss of Col2a1 expression and in apoptosis. Sox9 inactivation in flat chondrocytes caused immediate terminal maturation without hypertrophy and with excessive apoptosis. Inactivation of Sox9 in the last few cell layers resulted in the absence of Col10a1 expression, suggesting that continued expression of Sox9 just prior to hypertrophy is necessary for chondrocyte hypertrophy. SOX9 knockdown also caused apoptosis of human chondrosarcoma SW1353 cells. These phenotypes were associated with reduced Akt phosphorylation. Forced phosphorylation of Akt by Pten inactivation partially restored Col10a1 expression and cell survival in Sox9(floxdel/floxdel) mouse chondrocytes, suggesting that phosphorylated Akt mediates chondrocyte survival and hypertrophy induced by Sox9. When the molecular mechanism of Sox9-induced Akt phosphorylation was examined, we found that expression of the PI3K subunit Pik3ca (p110α) was decreased in Sox9(floxdel/floxdel) mouse chondrocytes. Sox9 binds to the promoter and enhances the transcriptional activities of Pik3ca. Thus, continued expression of Sox9 in differentiated chondrocytes is essential for subsequent hypertrophy and sustains chondrocyte-specific survival mechanisms by binding to the Pik3ca promoter, inducing Akt phosphorylation.

Arid5a cooperates with Sox9 to stimulate chondrocyte-specific transcription

SRY-box-containing gene 9 (Sox9) is an essential transcription factor in chondrocyte lineage determination and differentiation. Recent studies demonstrated that Sox9 controls the transcription of chondrocyte-specific genes in association with several other transcriptional regulators. To further understand the molecular mechanisms by which Sox9 influences transcriptional events during chondrocyte differentiation, we attempted to identify transcriptional partners of Sox9 and to examine their roles in chondrocyte differentiation. We isolated AT-rich interactive domain-containing protein 5a (Arid5a; also known as Mrf1) as an activator of the Col2a1 gene promoter from an ATDC5 cDNA library. Arid5a was highly expressed in cartilage and induced during chondrocyte differentiation. Furthermore, Arid5a physically interacted with Sox9 in nuclei and up-regulated the chondrocyte-specific action of Sox9. Overexpression of Arid5a stimulated chondrocyte differentiation in vitro and in an organ culture system. In contrast, Arid5a knockdown inhibited Col2a1 expression in chondrocytes. In addition, Arid5a binds directly to the promoter region of the Col2a1 gene and stimulates acetylation of histone 3 in the region. Our results suggest that Arid5a may directly interact with Sox9 and thereby enhance its chondrocyte-specific action.

Mechanisms of digit formation: Human malformation syndromes tell the story

Identifying the genetic basis of human limb malformation disorders has been instrumental in improving our understanding of limb development. Abnormalities of the hands and/or feet include defects affecting patterning, establishment, elongation, and segmentation of cartilaginous condensations, as well as growth of the individual skeletal elements. While the phenotype of such malformations is highly diverse, the mutations identified to date cluster in genes implicated in a limited number of molecular pathways, namely hedgehog, Wnt, and bone morphogenetic protein. The latter pathway appears to function as a key molecular network regulating different phases of digit and joint development. Studies in animal models not only extended our insight into the pathogenesis of these conditions, but have also contributed to our understanding of the in vivo functions and interactions of these key players. This review is aimed at integrating the current understanding of human digit malformations into the increasing knowledge of the molecular mechanisms of digit development. Developmental Dynamics 240:990-1004, 2011. © 2011 Wiley-Liss, Inc.

Evolutionarily conserved, growth plate zone-specific regulation of the matrilin-1 promoter: L-Sox5/Sox6 and Nfi factors bound near TATA finely tune activation by Sox9

To help uncover the mechanisms underlying the staggered expression of cartilage-specific genes in the growth plate, we dissected the transcriptional mechanisms driving expression of the matrilin-1 gene (Matn1). We show that a unique assembly of evolutionarily conserved cis-acting elements in the Matn1 proximal promoter restricts expression to the proliferative and prehypertrophic zones of the growth plate. These elements functionally interact with distal elements and likewise are capable of restricting the domain of activity of a pancartilaginous Col2a1 enhancer. The proximal elements include a Pe1 element binding the chondrogenic L-Sox5, Sox6, and Sox9 proteins, a SI element binding Nfi proteins, and an initiator Ine element binding the Sox trio and other factors. Sox9 binding to Pe1 is indispensable for functional interaction with the distal promoter. Binding of L-Sox5/Sox6 to Ine and Nfib to SI modulates Sox9 transactivation in a protein dose-dependent manner, possibly to enhance Sox9 activity in early stages of chondrogenesis and repress it at later stages. Hence, our data suggest a novel model whereby Sox and Nfi proteins bind to conserved Matn1 proximal elements and functionally interact with each other to finely tune gene expression in specific zones of the cartilage growth plate.

The E3 ubiquitin ligase Wwp2 regulates craniofacial development through mono-ubiquitylation of Goosecoid

Craniofacial anomalies (CFAs) are the most frequently occurring human congenital disease, and a major cause of infant mortality and childhood morbidity. Although CFAs seems to arise from a combination of genetic factors and environmental influences, the underlying gene defects and pathophysiological mechanisms for most CFAs are currently unknown. Here we reveal a role for the E3 ubiquitin ligase Wwp2 in regulating craniofacial patterning. Mice deficient in Wwp2 develop malformations of the craniofacial region. Wwp2 is present in cartilage where its expression is controlled by Sox9. Our studies demonstrate that Wwp2 influences craniofacial patterning through its interactions with Goosecoid (Gsc), a paired-like homeobox transcription factor that has an important role in craniofacial development. We show that Wwp2-associated Gsc is a transcriptional activator of the key cartilage regulatory protein Sox6. Wwp2 interacts with Gsc to facilitate its mono-ubiquitylation, a post-translational modification required for optimal transcriptional activation of Gsc. Our results identify for the first time a physiological pathway regulated by Wwp2 in vivo, and also a unique non-proteolytic mechanism through which Wwp2 controls craniofacial development.

Role of c-Maf in Chondrocyte Differentiation: A Review

Chondrocyte differentiation in the growth plate is an important process for the longitudinal growth of endochondral bones. Sox9 and Runx2 are the most often-studied transcriptional regulators of the chondrocyte differentiation process, but the importance of additional factors is also becoming apparent. Mafs are a subfamily of the basic ZIP (bZIP) transcription factor superfamily, which act as key regulators of tissue-specific gene expression and terminal differentiation in many tissues. There is increasing evidence that c-Maf and its splicing variant Lc-Maf play a role in chondrocyte differentiation in a temporal-spatial manner. This review summarizes the functions of c-Maf in chondrocyte differentiation and discusses the possible role of c-Maf in osteoarthritis progression.

Embryonic stem cells for osteo-degenerative diseases

Current orthopedic practice to treat osteo-degenerative diseases, such as osteoporosis, calls for antiresorptive therapies and anabolic bone medications. In some cases, surgery, in which metal rods are inserted into the bones, brings symptomatic relief. As these treatments may ameliorate the symptoms, but cannot cure the underlying dysregulation of the bone, the orthopedic field seems ripe for regenerative therapies using transplantation of stem cells. Stem cells bring with them the promise of completely curing a disease state, as these are the cells that normally regenerate tissues in a healthy organism. This chapter assembles reports that have successfully used stem cells to generate osteoblasts, osteoclasts, and chondrocytes - the cells that can be found in healthy bone tissue - in culture, and review and collate studies about animal models that were employed to test the function of these in vitro "made" cells. A particular emphasis is placed on embryonic stem cells, the most versatile of all stem cells. Due to their pluripotency, embryonic stem cells represent the probably most challenging stem cells to bring into the clinic, and therefore, the associated problems are discussed to put into perspective where the field currently is and what we can expect for the future.

Differentiation character of adult mesenchymal stem cells and transfection of MSCs with lentiviral vectors

This study examined the differentiation character and pluripotency of mesenchymal stem cells (MSCs) under different conditions. Adult MSCs were initially isolated from the bone marrow of rats, cultured in vitro and identified by flow cytometry. After MSCs were transferred to osteogenic and adipogenic medium respectively, the morphological characterization of induced cells was observed. The expression of marker genes was detected by RT-PCR analysis. Then MSCs were transfected with lentiviral vectors pGC-FU-Sox9-EGFP. Enhanced green fluorescence protein (EGFP) expression and transfection efficiency were determined by fluorescence microscopy. The results demonstrated that EGFP caused no effect on the multilineage potential of adult MSCs. Sox9 gene expression of high level was maintained stable in the transfected MSCs and induced MSCs to differentiate into chondrocytes. Aggracan was positive in chondrogenic lineages and the expression of aggracan and type collagen II was significantly increased during MSCs chondrogenic differentiation. It was concluded that Sox9 gene-modified adult MSCs may be promising candidate cells for further studies on tissue engineering. EGFP facilitates the research on MSCs physiological behavior and application in tissue engineering during differentiation both in vitro and in vivo.

Genomewide linkage and peakwide association analyses of carotid plaque in Caribbean Hispanics

BACKGROUND AND PURPOSE

Atherosclerosis is a complex subclinical cardiovascular disorder with a substantial genetic component. This study sought to identify genetic loci influencing carotid plaque in 2 independent samples.

METHODS

B-mode ultrasound was performed to determine the presence and area of carotid plaque. Variance components analysis was used to test for linkage using 383 autosomal microsatellite markers in 1308 subjects from 100 Dominican families. Multiple linear and logistic regression models were used to investigate the association between plaque traits and 18,904 single nucleotide polymorphisms under the 1-logarithm of odds unit down regions of linkage peaks in an independent community-based data set (N = 941, 41% Dominicans) from the Northern Manhattan Study.

RESULTS

After adjustment for age, hypertension, diabetes mellitus, cigarette pack-years, body mass index, and waist-to-hip ratio, significant heritability was detected for plaque presence (h² = 0.50 ± 0.14, P < 0.0001) and plaque area (h²=0.17 ± 0.04, P < 0.0001). Quantitative and dichotomous trait linkage analyses obtained similar results and identified 4 regions with multipoint logarithm of odds scores ≥ 2.00 on 7q36, 11p15, 14q32, and 15q23. In the association analysis of the 4 linkage peaks, several single nucleotide polymorphisms in or near SOX6, FSD2, AP3S2, EFTUD1, and MYOD1 were associated with carotid plaque traits with a nominal P ≤ 0.0005 in the Northern Manhattan Study data set and with a P ≤ 0.01 in Northern Manhattan Study Dominican subset.

CONCLUSIONS

Carotid plaque has considerable heritability and may be influenced by loci on chromosomes 11p15, 14q32, and 15q23. The SOX6 gene within the bone morphogenic protein pathway could be a candidate for carotid plaque. Larger independent studies are needed to validate these findings.

A genome-wide association analysis implicates SOX6 as a candidate gene for wrist bone mass

Osteoporosis is a highly heritable common bone disease leading to fractures that severely impair the life quality of patients. Wrist fractures caused by osteoporosis are largely due to the scarcity of wrist bone mass. Here we report the results of a genome-wide association study (GWAS) of wrist bone mineral density (BMD). We examined ∼500000 SNP markers in 1000 unrelated homogeneous Caucasian subjects and found a novel allelic association with wrist BMD at rs11023787 in the SOX6 (SRY (sex determining region Y)-box 6) gene (P=9.00×10(-5)). Subjects carrying the C allele of rs11023787 in SOX6 had significantly higher mean wrist BMD values than those with the T allele (0.485:0.462 g cm(-2) for C allele vs. T allele carriers). For validation, we performed SOX6 association for BMD in an independent Chinese sample and found that SNP rs11023787 was significantly associated with wrist BMD in the Chinese sample (P=6.41×10(-3)). Meta-analyses of the GWAS scan and the replication studies yielded P-values of 5.20×10(-6) for rs11023787. Results of this study, together with the functional relevance of SOX6 in cartilage formation, support the SOX6 gene as an important gene for BMD variation.

The transcription factor Znf219 regulates chondrocyte differentiation by assembling a transcription factory with Sox9

Sox9 is an essential transcription factor for chondrogenesis by regulating the expression of chondrogenic genes. However, its regulatory mechanism is not fully understood. To address this, we attempted to identify the transcriptional partners of Sox9 by screening the cDNA library of the chondrogenic cell line ATDC5 using the collagen 2α1 (Col2α1) gene promoter fused to a luciferase reporter gene. One of the positive clones encoded the Znf219 gene. Whole mount in situ hybridization experiments indicated that Znf219 mRNA was specifically expressed in the developing limb buds where Col2α1 and Sox9 were strongly expressed. Znf219 markedly enhanced the transcriptional activity of Sox9 on the Col2a1 gene promoter. In addition, Znf219 is physically associated with Sox9 and is colocalized with Sox9 in the nucleus. We also found that overexpression of Znf219 profoundly increased Sox9-induced mRNA expression of Col2a1, aggrecan and Col11a2. Consistently, knockdown of Znf219 decreased the Sox9-induced mRNA expression of these genes. Furthermore, a dominant-negative mutant Znf219 inhibited Bmp2-induced chondrocyte differentiation. Our results suggest that Znf219 plays an important role in the regulation of chondrocyte differentiation as a transcriptional partner of Sox9.

Common skeletal growth retardation disorders resulting from abnormalities within the mesenchymal stem cells reservoirs in the epiphyseal organs pertaining to the long bones

Among the objectives in writing the current chapter were the curiosity and the interest in allocating the sites and routes of migration of the reservoirs of the mesenchymal precartilaginous stem cells of the developing limbs in health and in disease. We chose to emphasize the events believed to initiate in these regions of stem cells, which may lead to growth retardation disorders. Thus, this narrow niche touches an enlarged scope of developmental biology angles and fields. The enclosed coverage sheds light on part of the musculoskeletal system, skeletogenesis, organogenesis of mobile structures and organs, the limbs, joints and digits (arthrology). It appears that the key role of the cartilage-bone regions is their responsibility to replenish the physis with committed chondrocytes, during the developmental, maturation and puberty periods. We shall start by outlining the framework of normal limb formation, the modalities, signals and the agents participating in this biological creation and regulation, illustrating potential sites that might deviate from normal development during the growth periods.

Assembling neural crest regulatory circuits into a gene regulatory network

The neural crest is a multipotent stem cell&#x2013;like population that gives rise to a wide range of derivatives in the vertebrate embryo including elements of the craniofacial skeleton and peripheral nervous system as well as melanocytes. The neural crest forms in a series of regulatory steps that include induction and specification of the prospective neural crest territory&#x2013;neural plate border, specification of bona fide neural crest progenitors, and differentiation into diverse derivatives. These individual processes during neural crest ontogeny are controlled by regulatory circuits that can be assembled into a hierarchical gene regulatory network (GRN). Here we present an overview of the GRN that orchestrates the formation of cranial neural crest cells. Formulation of this network relies on information largely inferred from gene perturbation studies performed in several vertebrate model organisms. Our representation of the cranial neural crest GRN also includes information about direct regulatory interactions obtained from the cis-regulatory analyses performed to date, which increases the resolution of the architectural circuitry within the network.

Coordination of chondrogenesis and osteogenesis by hypertrophic chondrocytes in endochondral bone development

Mammalian bones have three distinct origins (paraxial mesoderm, lateral plate mesoderm, and neural crest) and undergo two different modes of formation (intramembranous and endochondral). Bones derived from the paraxial mesoderm and lateral plate mesoderm mainly form through the endochondral process. During this process, hypertrophic chondrocytes play a vital role in inducing osteogenesis. So far, a number of published papers have provided evidence that chondrocyte hypertrophy and osteoblast differentiation are controlled by a variety of signaling pathways and factors; however, little is known about their hierarchy (which are upstream? which are most potent?). In this review, we discuss the signaling pathways and transcriptional factors regulating chondrocyte hypertrophy and osteoblast differentiation based on the evidence that has been reported and confirmed by multiple independent groups. We then discuss which factor would provide the most coherent evidence for its role in endochondral ossification.

Sox6 is necessary for efficient erythropoiesis in adult mice under physiological and anemia-induced stress conditions

BACKGROUND

Definitive erythropoiesis is a vital process throughout life. Both its basal activity under physiological conditions and its increased activity under anemia-induced stress conditions are highly stimulated by the hormone erythropoietin. The transcription factor Sox6 was previously shown to enhance fetal erythropoiesis together and beyond erythropoietin signaling, but its importance in adulthood and mechanisms of action remain unknown. We used here Sox6 conditional null mice and molecular assays to address these questions.

METHODOLOGY/PRINCIPAL FINDINGS

Sox6fl/flErGFPCre adult mice, which lacked Sox6 in erythroid cells, exhibited compensated anemia, erythroid cell developmental defects, and anisocytotic, short-lived red cells under physiological conditions, proving that Sox6 promotes basal erythropoiesis. Tamoxifen treatment of Sox6fl/flCaggCreER mice induced widespread inactivation of Sox6 in a timely controlled manner and resulted in erythroblast defects before reticulocytosis, demonstrating that impaired erythropoiesis is a primary cause rather than consequence of anemia in the absence of Sox6. Twenty five percent of Sox6fl/flErGFPCre mice died 4 or 5 days after induction of acute anemia with phenylhydrazine. The others recovered slowly. They promptly increased their erythropoietin level and amplified their erythroid progenitor pool, but then exhibited severe erythroblast and reticulocyte defects. Sox6 is thus essential in the maturation phase of stress erythropoiesis that follows the erythropoietin-dependent amplification phase. Sox6 inactivation resulted in upregulation of embryonic globin genes, but embryonic globin chains remained scarce and apparently inconsequential. Sox6 inactivation also resulted in downregulation of erythroid terminal markers, including the Bcl2l1 gene for the anti-apoptotic factor Bcl-xL, and in vitro assays indicated that Sox6 directly upregulates Bcl2l1 downstream of and beyond erythropoietin signaling.

CONCLUSIONS/SIGNIFICANCE

This study demonstrates that Sox6 is necessary for efficient erythropoiesis in adult mice under both basal and stress conditions. It is primarily involved in enhancing the survival rate and maturation process of erythroid cells and acts at least in part by upregulating Bcl2l1.

TAK1 regulates cartilage and joint development via the MAPK and BMP signaling pathways

The importance of canonical transforming growth factor beta (TGF-beta) and bone morphogenetic protein (BMP) signaling during cartilage and joint development is well established, but the necessity for noncanonical (SMAD-independent) signaling during these processes is largely unknown. TGF-beta activated kinase 1 (TAK1) is a MAP3K activated by TGF-beta, BMP, and other mitogen-activated protein kinase (MAPK) signaling components. We set out to define the potential role for noncanonical, TAK1-mediated signaling in cartilage and joint development via deletion of Tak1 in chondrocytes (Col2Cre;Tak1(f/f)) and the developing limb mesenchyme (Prx1Cre;Tak1(f/f)). Deletion of Tak1 in chondrocytes resulted in novel embryonic developmental cartilage defects including decreased chondrocyte proliferation, reduced proliferating chondrocyte survival, delayed onset of hypertrophy, reduced Mmp13 expression, and a failure to maintain interzone cells of the elbow joint, which were not observed previously in another Col2Cre;Tak1(f/f) model. Deletion of Tak1 in limb mesenchyme resulted in widespread joint fusions likely owing to the differentiation of interzone cells to the chondrocyte lineage. The Prx1Cre;Tak1(f/f) model also allowed us to identify novel columnar chondrocyte organization and terminal maturation defects owing to the interplay between chondrocytes and the surrounding mesenchyme. Furthermore, both our in vivo models and in vitro cell culture studies demonstrate that loss of Tak1 results in impaired activation of the downstream MAPK target p38, as well as diminished activation of the BMP/SMAD signaling pathway. Taken together, these data demonstrate that TAK1 is a critical regulator of both MAPK and BMP signaling and is necessary for proper cartilage and joint development.

Hypoxia, HIFs and bone development

Oxygen is not only an obviously important substrate, but it is also a regulatory signal that controls expression of a specific genetic program. Crucial mediator of the adaptive response of cells to hypoxia is the family of Hypoxia-Inducible Transcription Factors (HIFs).The fetal growth plate, which is an avascular structure of mesenchymal origin, has a unique out-in gradient of oxygenation. HIF-1alpha is necessary for chondrogenesis in vivo by controlling a complex homeostatic response that allows chondrocytes to survive and differentiate in a hypoxic environment. Moreover, HIFs are also essential in osteogenesis and joint development. This brief Perspective summarizes the critical role of HIFs in endochondral bone development.

Transcriptional regulation of an axonemal central apparatus gene, sperm-associated antigen 6, by a SRY-related high mobility group transcription factor, S-SOX5

SOX5 is a transcription factor with homology to the high mobility group box region of the testis-determining factor, SRY. Both the mouse and human SOX5 genes encode a 48-kDa SOX5 protein (S-SOX5) that is only present in tissues containing cells with motile cilia/flagella. The mammalian sperm-associated antigen 6 gene (SPAG6) encodes an axoneme central apparatus protein. Because human and mouse SPAG6 gene promoters contain multiple potential binding sites for SOX5, SPAG6 gene regulation by S-SOX5 was investigated in BEAS-2B cells, a line derived from human bronchial cells. Like FOXJ1, a transcription factor known to be essential for motile ciliogenesis, S-SOX5 stimulated mouse and human SPAG6 promoter function in BEAS-2B cells, but the effect was abrogated when the SOX5 binding sites were mutated or deleted. S-SOX5 and FOXJ1 functioned cooperatively in stimulating SPAG6 promoter activity. The SPAG6 message was up-regulated when S-SOX5 was overexpressed in BEAS-2B cells, and silencing of S-SOX5 by RNA interference down-regulated SPAG6 transcripts. Chromatin immunoprecipitation and EMSA experiments demonstrated that S-SOX5 associates with the SPAG6 promoter directly. The present study demonstrates that SPAG6 is a S-SOX5 target gene, indicating a key role for S-SOX5 in the formation and function of motile cilia.

Transcriptome analysis of early organogenesis in human embryos

Genome-wide expression analysis of embryonic development provides information that is useful in a variety of contexts. Here, we report transcriptome profiles of human early embryos covering development during the first third of organogenesis. We identified two major categories of genes, displaying gradually reduced or gradually increased expression patterns across this developmental window. The decreasing group appeared to include stemness-specific and differentiation-specific genes important for the initiation of organogenesis, whereas the increasing group appeared to be largely differentiation related and indicative of diverse organ formation. Based on these findings, we devised a putative molecular network that may provide a framework for the regulation of early human organogenesis. Our results represent a significant step in characterization of early human embryogenesis and provide a resource for understanding human development and for stem cell engineering.

Remodelling of human osteoarthritic cartilage by FGF-2, alone or combined with Sox9 via rAAV gene transfer

Compensating for the loss of extracellular cartilage matrix, as well as counteracting the alterations of the chondrocyte phenotype in osteoarthritis are of key importance to develop effective therapeutic strategies against this disorder. In the present study, we analysed the benefits of applying a potent gene combination to remodel human osteoarthritic (OA) cartilage. We employed the promising recombinant adeno-associated virus (rAAV) vector to deliver the mitogenic fibroblast growth factor 2 (FGF-2) factor, alone or simultaneously with the transcription factor Sox9 as a key activator of matrix synthesis, to human normal and OA articular chondrocytes. We evaluated the effects of single (FGF-2) or combined (FGF-2/SOX9) transgene expression upon the regenerative activities of chondrocytes in three dimensional cultures in vitro and in cartilage explants in situ. Single overexpression of FGF-2 enhanced the survival and proliferation of both normal and OA chondrocytes, without stimulating the matrix synthetic processes in the increased pools of cells. The mitogenic properties of FGF-2 were maintained when SOX9 was co-overexpressed and concomitant with an increase in the production of proteoglycans and type-II collagen, suggesting that the transcription factor was capable of counterbalancing the effects of FGF-2 on matrix accumulation. Also important, expression of type-X collagen, a marker of hypertrophy strongly decreased following treatment by the candidate vectors. Most remarkably, the levels of activities achieved in co-treated human OA cartilage were similar to or higher than those observed in normal cartilage. The present findings show that combined expression of candidate factors in OA cartilage can re-establish key features of normal cartilage and prevent the pathological shift of metabolic homeostasis. These data provide further motivation to develop coupled gene transfer approaches via rAAV for the treatment of human OA.

Inactivation of Patched1 in the mouse limb has novel inhibitory effects on the chondrogenic program

The bones of the vertebrate limb form by the process of endochondral ossification, whereby limb mesenchyme condenses to form an intermediate cartilage scaffold that is then replaced by bone. Although Indian hedgehog (IHH) is known to control hypertophic differentiation of chondrocytes during this process, the role of hedgehog signaling in the earlier stages of chondrogenesis is less clear. We have conditionally inactivated the hedgehog receptor Ptc1 in undifferentiated limb mesenchyme of the mouse limb using Prx1-Cre, thus inducing constitutively active ligand-independent hedgehog signaling. In addition to major patterning defects, we observed a marked disruption to the cartilage elements in the limbs of Prx1-Cre:Ptc1(c/c) embryos. Using an in vitro micromass culture system we show that this defect lies downstream of mesenchymal cell condensation and likely upstream of chondrocyte differentiation. Despite early increases in levels of chondrogenic genes, soon after mesenchymal condensation the stromal layer of Prx1-Cre:Ptc1(c/c)-derived micromass cultures is characterized by a loss of cell integrity, which is associated with increased cell death and a striking decrease in Alcian blue staining cartilage nodules. Furthermore, inhibition of the hedgehog pathway activation using cyclopamine was sufficient to essentially overcome this chondrogenic defect in both micromass and ex vivo explant assays of Prx1-Cre:Ptc1(c/c) limbs. These data demonstrate for the first time the inhibitory effect of cell autonomously activated hedgehog signaling on chondrogenesis, and stress the importance of PTC1 in maintaining strict control of signaling levels during this phase of skeletal development.

Clinical review 2: Genetic determinants of bone density and fracture risk--state of the art and future directions

CONTEXT

Osteoporosis is a common, highly heritable condition that causes substantial morbidity and mortality, the etiopathogenesis of which is poorly understood. Genetic studies are making increasingly rapid progress in identifying the genes involved.

EVIDENCE ACQUISITION AND SYNTHESIS

In this review, we will summarize the current understanding of the genetics of osteoporosis based on publications from PubMed from the year 1987 onward.

CONCLUSIONS

Most genes involved in osteoporosis identified to date encode components of known pathways involved in bone synthesis or resorption, but as the field progresses, new pathways are being identified. Only a small proportion of the total genetic variation involved in osteoporosis has been identified, and new approaches will be required to identify most of the remaining genes.

Synovial joint morphogenesis requires the chondrogenic action of Sox5 and Sox6 in growth plate and articular cartilage

The mechanisms underlying synovial joint development remain poorly understood. Here we use complete and cell-specific gene inactivation to identify the roles of the redundant chondrogenic transcription factors Sox5 and Sox6 in this process. We show that joint development aborts early in complete mutants (Sox5(-/-)6(-/-)). Gdf5 and Wnt9a expression is punctual in articular progenitor cells, but Sox9 downregulation and cell condensation in joint interzones are late. Joint cell differentiation is unsuccessful, regardless of lineage, and cavitation fails. Sox5 and Sox6 restricted expression to chondrocytes in wild-type embryos and continued Erg expression and weak Ihh expression in Sox5(-/-)6(-/-) growth plates suggest that growth plate failure contribute to this Sox5(-/-)6(-/-) joint morphogenesis block. Sox5/6 inactivation in specified joint cells and chondrocytes (Sox5(fl/fl)6(fl/fl)Col2Cre) also results in a joint morphogenesis block, whereas Sox5/6 inactivation in specified joint cells only (Sox5(fl/fl)6(fl/fl)Gdf5Cre) results in milder joint defects and normal growth plates. Sox5(fl/fl)6(fl/fl)Gdf5Cre articular chondrocytes remain undifferentiated, as shown by continued Gdf5 expression and pancartilaginous gene downregulation. Along with Prg4 downregulation, these defects likely account for joint tissue overgrowth and incomplete cavitation in adult mice. Together, these data suggest that synovial joint morphogenesis relies on essential roles for Sox5/6 in promoting both growth plate and articular chondrocyte differentiation.

NF-kappaB signaling: multiple angles to target OA

In the context of OA disease, NF-kappaB transcription factors can be triggered by a host of stress-related stimuli including pro-inflammatory cytokines, excessive mechanical stress and ECM degradation products. Activated NF-kappaB regulates the expression of many cytokines and chemokines, adhesion molecules, inflammatory mediators, and several matrix degrading enzymes. NF-kappaB also influences the regulated accumulation and remodeling of ECM proteins and has indirect positive effects on downstream regulators of terminal chondrocyte differentiation (including beta-catenin and Runx2). Although driven partly by pro-inflammatory and stress-related factors, OA pathogenesis also involves a "loss of maturational arrest" that inappropriately pushes chondrocytes towards a more differentiated, hypertrophic-like state. Growing evidence points to NF-kappaB signaling as not only playing a central role in the pro-inflammatory stress-related responses of chondrocytes to extra- and intra-cellular insults, but also in the control of their differentiation program. Thus unlike other signaling pathways the NF-kappaB activating kinases are potential therapeutic OA targets for multiple reasons. Targeted strategies to prevent unwanted NF-kappaB activation in this context, which do not cause side effects on other proteins or signaling pathways, need to be focused on the use of highly specific drug modalities, siRNAs or other biological inhibitors that are targeted to the activating NF-kappaB kinases IKKalpha or IKKbeta or specific activating canonical NF-kappaB subunits. However, work remains in its infancy to evaluate the effects of efficacious, targeted NF-kappaB inhibitors in animal models of OA disease in vivo and to also target these strategies only to affected cartilage and joints to avoid other undesirable systemic effects.

Evidence for pleiotropic factors in genetics of the musculoskeletal system

There are both theoretical and empirical underpinnings that provide evidence that the musculoskeletal system develops, functions, and ages as a whole. Thus, the risk of osteoporotic fracture can be viewed as a function of loading conditions and the ability of the bone to withstand the load. Both bone loss (osteoporosis) and muscle wasting (sarcopenia) are the two sides of the same coin, an involution of the musculoskeletal system. Skeletal loads are dominated by muscle action; both bone and muscle share environmental, endocrine and paracrine influences. Muscle also has an endocrine function by producing bioactive molecules, which can contribute to homeostatic regulation of both bone and muscle. It also becomes clear that bone and muscle share genetic determinants; therefore the consideration of pleiotropy is an important aspect in the study of the genetics of osteoporosis and sarcopenia. The aim of this review is to provide an additional evidence for existence of the tight genetic co-regulation of muscles and bones, starting early in development and still evident in aging. Recently, important papers were published, including those dealing with the cellular mechanisms and anatomic substrate of bone mechanosensitivity. Further evidence has emerged suggesting that the relationship between skeletal muscle and bone parameters extends beyond the general paradigm of bone responses to mechanical loading. We provide insights into several pathways and single genes, which apparently have a biologically plausible pleiotropic effect on both bones and muscles; the list is continuing to grow. Understanding the crosstalk between muscles and bones will translate into a conceptual framework aimed at studying the pleiotropic genetic relationships in the etiology of complex musculoskeletal disease. We believe that further progress in understanding the common genetic etiology of osteoporosis and sarcopenia will provide valuable insight into important biological underpinnings for both musculoskeletal conditions. This may translate into new approaches to reduce the burden of both conditions, which are prevalent in the elderly population.

Spatial and temporal regulation of gene expression in the mammalian growth plate

Growth plates are spatially polarized and structured into three histologically and functionally distinct layers-the resting zone (RZ), proliferative zone (PZ), and hypertrophic zone (HZ). With age, growth plates undergo functional and structural senescent changes including declines of growth rate, proliferation rate, growth plate height and cell number. To explore the mechanisms responsible for spatially-associated differentiation and temporally-associated senescence of growth plate in an unbiased manner, we used microdissection to collect individual growth plate zones from proximal tibiae of 1-week rats and the PZ and early hypertrophic zones of growth plates from 3-, 6-, 9-, and 12-week rats and analyzed gene expression using microarray. We then used bioinformatic approaches to identify significant changes in biological functions, molecular pathways, transcription factors and also to identify specific gene products that can be used as molecular markers for individual zones or for temporal development.

SOX10 structure-function analysis in the chicken neural tube reveals important insights into its role in human neurocristopathies

The HMG-domain containing transcription factor Sox10 is essential for neural crest (NC) development and for oligodendrocyte differentiation. Heterozygous SOX10 mutations in humans lead to corresponding defects in several NC-derived lineages and to leukodystrophies. Disease phenotypes range from Waardenburg syndrome and Waardenburg-Hirschsprung disease to Peripheral demyelinating neuropathy, Central dysmyelination, Waardenburg syndrome and Hirschsprung disease (PCWH). The phenotypic variability can partly be explained by the action of modifier genes, but is also influenced by the mutation that leads to haploinsufficiency in some and to mutant SOX10 proteins with altered properties in other cases. Here, we used in ovo electroporation in the developing neural tube of chicken to determine which regions and properties of SOX10 are required for early NC development. We found a strict reliance on the DNA-binding activity and the presence of the C-terminal transactivation domain and a lesser influence of the dimerization function and a conserved domain in the center of the protein. Intriguingly, dominant-negative effects on early NC development were mostly observed for truncated SOX10 proteins whose production in patients is probably prevented by nonsense-mediated decay. In contrast, mutant SOX10 proteins that occur in patients were usually inactive. Any dominant negative activity which some of these mutants undoubtedly possess must, therefore, be restricted to single NC-derived cell lineages or oligodendrocytes at later times. This contributes to the phenotypic variability of human SOX10 mutations.

Molecular profiling of the developing mouse axial skeleton: a role for Tgfbr2 in the development of the intervertebral disc

Background

Very little is known about how intervertebral disc (IVD) is formed or maintained. Members of the TGF-β superfamily are secreted signaling proteins that regulate many aspects of development including cellular differentiation. We recently showed that deletion of Tgfbr2 in Col2a expressing mouse tissue results in alterations in development of IVD annulus fibrosus. The results suggested TGF-β has an important role in regulating development of the axial skeleton, however, the mechanistic basis of TGF-β action in these specialized joints is not known. One of the hurdles to understanding development of IVD is a lack of known markers. To identify genes that are enriched in the developing mouse IVD and to begin to understand the mechanism of TGF-β action in IVD development, we undertook a global analysis of gene expression comparing gene expression profiles in developing mouse vertebrae and IVD. We also compared expression profiles in tissues from wild type and Tgfbr2 mutant mice as well as in sclerotome cultures treated with TGF-β or BMP4.

Results

Lists of IVD and vertebrae enriched genes were generated. Expression patterns for several genes were verified either through in situ hybridization or literature/database searches resulting in a list of genes that can be used as markers of IVD. Cluster analysis using genes listed under the Gene Ontology terms multicellular organism development and pattern specification indicated that mutant IVD more closely resembled vertebrae than wild type IVD. We also generated lists of genes regulated by TGF-β or BMP4 in cultured sclerotome. As expected, treatment with BMP4 resulted in up-regulation of cartilage marker genes including Acan, Sox 5, Sox6, and Sox9. In contrast, treatment with TGF-β1 did not regulate expression of cartilage markers but instead resulted in up-regulation of many IVD markers including Fmod and Adamtsl2.

Conclusions

We propose TGF-β has two functions in IVD development: 1) to prevent chondrocyte differentiation in the presumptive IVD and 2) to promote differentiation of annulus fibrosus from sclerotome. We have identified genes that are enriched in the IVD and regulated by TGF-β that warrant further investigation as regulators of IVD development.

Advances in the understanding of haemoglobin switching

The study of haemoglobin switching has represented a focus in haematology due in large part to the clinical relevance of the fetal to adult haemoglobin switch for developing targeted approaches to ameliorate the severity of the beta-haemoglobinopathies. Additionally, the process by which this switch occurs represents an important paradigm for developmental gene regulation. In this review, we provide an overview of both the embryonic primitive to definitive switch in haemoglobin expression, as well as the fetal to adult switch that is unique to humans and old world monkeys. We discuss the nature of these switches and models of their regulation. The factors that have been suggested to regulate this process are then discussed. With the increased understanding and discovery of molecular regulators of haemoglobin switching, such as BCL11A, new avenues of research may lead ultimately to novel therapeutic, mechanism-based approaches to fetal haemoglobin reactivation in patients.

The SoxD transcription factors--Sox5, Sox6, and Sox13--are key cell fate modulators

Sox5, Sox6, and Sox13 constitute the group D of sex-determining region (Sry)-related transcription factors. They are highly conserved in the family-specific high-mobility-group (HMG) box DNA-binding domain and in a group-specific coiled-coil domain. The latter mediates SoxD protein dimerization and thereby preferential binding to pairs of DNA recognition sites. The SoxD genes have overlapping expression and cell-autonomously control discrete lineages. Sox5 and Sox6 redundantly enhance chondrogenesis, but retard gliogenesis. Sox5 hinders melanogenesis, promotes neural crest generation, and controls the pace of neurogenesis. Sox6 promotes erythropoiesis, and Sox13 modulates T cell specification and is an autoimmune antigen. SoxD proteins enhance transactivation by Sox9 in chondrocytes, but antagonize Sox9 and other SoxE proteins in oligodendrocytes and melanocytes, and also repress transcription through various mechanisms in several other lineages. While their biological and molecular functions remain incompletely understood, the SoxD proteins have thus already proven that they critically modulate cell fate in major lineages.

Runx1 and Runx2 cooperate during sternal morphogenesis

Chondrocyte differentiation is strictly regulated by various transcription factors, including Runx2 and Runx3; however, the physiological role of Runx1 in chondrocyte differentiation remains unknown. To examine the role of Runx1, we generated mesenchymal-cell-specific and chondrocyte-specific Runx1-deficient mice [Prx1 Runx1(f/f) mice and alpha1(II) Runx1(f/f) mice, respectively] to circumvent the embryonic lethality of Runx1-deficient mice. We then mated these mice with Runx2 mutant mice to obtain mesenchymal-cell-specific or chondrocyte-specific Runx1; Runx2 double-mutant mice [Prx1 DKO mice and alpha1(II) DKO mice, respectively]. Prx1 Runx1(f/f) mice displayed a delay in sternal development and Prx1 DKO mice completely lacked a sternum. By contrast, alpha1(II) Runx1(f/f) mice and alpha1(II) DKO mice did not show any abnormal sternal morphogenesis or chondrocyte differentiation. Notably, Runx1, Runx2 and the Prx1-Cre transgene were co-expressed specifically in the sternum, which explains the observation that the abnormalities were limited to the sternum. Histologically, mesenchymal cells condensed normally in the prospective sternum of Prx1 DKO mice; however, commitment to the chondrocyte lineage, which follows mesenchymal condensation, was significantly impaired. In situ hybridization analyses demonstrated that the expression of alpha1(II) collagen (Col2a1 - Mouse Genome Informatics), Sox5 and Sox6 in the prospective sternum of Prx1 DKO mice was severely attenuated, whereas Sox9 expression was unchanged. Molecular analyses revealed that Runx1 and Runx2 induce the expression of Sox5 and Sox6, which leads to the induction of alpha1(II) collagen expression via the direct regulation of promoter activity. Collectively, these results show that Runx1 and Runx2 cooperatively regulate sternal morphogenesis and the commitment of mesenchymal cells to become chondrocytes through the induction of Sox5 and Sox6.

Regulation of the chondrogenic phenotype in culture

In recent years, there has been a great deal of interest in the development of regenerative approaches to produce hyaline cartilage ex vivo that can be utilized for the repair or replacement of damaged or diseased tissue. It is clinically imperative that cartilage engineered in vitro mimics the molecular composition and organization of and exhibits biomechanical properties similar to persistent hyaline cartilage in vivo. Experimentally, much of our current knowledge pertaining to the regulation of cartilage formation, or chondrogenesis, has been acquired in vitro utilizing high-density cultures of undifferentiated chondroprogenitor cells stimulated to differentiate into chondrocytes. In this review, we describe the extracellular matrix molecules, nuclear transcription factors, cytoplasmic protein kinases, cytoskeletal components, and plasma membrane receptors that characterize cells undergoing chondrogenesis in vitro and regulate the progression of these cells through the chondrogenic differentiation program. We also provide an extensive list of growth factors and other extracellular signaling molecules, as well as chromatin remodeling proteins such as histone deacetylases, known to regulate chondrogenic differentiation in culture. In addition, we selectively highlight experiments that demonstrate how an understanding of normal hyaline cartilage formation can lead to the development of novel cartilage tissue engineering strategies. Finally, we present directions for future studies that may yield information applicable to the in vitro generation of hyaline cartilage that more closely resembles native tissue.

Expression of cartilage developmental genes in Hoxc8- and Hoxd4-transgenic mice

Hox genes encode transcription factors, which regulate skeletal patterning and chondrocyte differentiation during the development of cartilage, the precursor to mature bone. Overexpression of the homeobox transcription factors Hoxc8 and Hoxd4 causes severe cartilage defects due to delay in cartilage maturation. Matrix metalloproteinases (MMPs), bone morphogenetic proteins (BMPs) and fibroblastic growth factors (FGFs) are known to play important roles in skeletal development and endochondral bone formation and remodeling. In order to investigate whether these molecules are aberrantly expressed in Hoxc8- and/or Hoxd4-transgenic cartilage, we performed quantitative RT-PCR on chondrocytes from Hox-transgenic mice. Gene expression levels of Bmp4, Fgf8, Fgf10, Mmp9, Mmp13, Nos3, Timp3, Wnt3a and Wnt5a were altered in Hoxc8-transgenic chondrocytes, and Fgfr3, Ihh, Mmp8, and Wnt3a expression levels were altered in Hoxd4-transgenic chondrocytes, respectively. Notably, Wnt3a expression was elevated in Hoxc8- and reduced in Hoxd4-transgenic cartilage. These results suggest that both transcription factors affect cartilage maturation through different molecular mechanisms, and provide the basis for future studies into the role of these genes and possible interactions in pathogenesis of cartilage defects in Hoxc8- and Hoxd4-transgenic mice.

Genome-wide association study of PR interval

The electrocardiographic PR interval reflects atrial and atrioventricular nodal conduction, disturbances of which increase risk of atrial fibrillation (AF). To identify underlying common genetic variation, we meta-analyzed genome-wide association results for PR interval from seven community-based studies of European-ancestry individuals in the CHARGE consortium: AGES, ARIC, CHS, FHS, KORA, Rotterdam Study, and SardiNIA (N=28,517). Statistically significant loci (P<5×10^-8) were tested for association with AF (N=5,741 cases). We identified nine loci associated with PR interval. At chromosome 3p22.2, we observed two independent associations in voltage gated sodium channel genes SCN10A and SCN5A, while six loci were near cardiac developmental genes CAV1/CAV2, NKX2-5 (CSX1), SOX5, WNT11, MEIS1, and TBX5/TBX3. Another signal was at ARHGAP24, a locus without known relevance to the heart. Five of the nine loci, SCN5A, SCN10A, NKX2-5, CAV1/CAV2, and SOX5, were also associated with AF (P<0.0056). Common genetic variation, particularly in ion channel and developmental genes, contributes significantly to atrial and atrioventricular conduction and to AF risk.