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

Influence of DNA-methylation at multiple stages of limb chondrogenesis

Precise regulation of gene expression is of utmost importance during cell fate specification. DNA methylation is a key epigenetic mechanism that plays a significant role in the regulation of cell fate by recruiting repression proteins or inhibiting the binding of transcription factors to DNA to regulate gene expression. Limb development is a well-established model for understanding cell fate decisions, and the formation of skeletal elements is coordinated through a sequence of events that control chondrogenesis spatiotemporally. It has been established that epigenetic control participates in cartilage maturation. However, further investigation is required to determine its role in the earliest stages of chondrocyte differentiation. This study investigates how the DNA methylation environment affects cell fate divergence during the early chondrogenic events. Our research has shown for the first time that inhibiting DNA methylation in interdigital tissue with 5-azacytidine results in the formation of an ectopic digit. This discovery suggested that DNA methylation dynamics could regulate the fate of cells between chondrogenesis and cell death during autopod development. Our in vitro findings indicate that DNA methylation at the early stages of chondrogenesis is integral in regulating condensation by controlling cell adhesion and proapoptotic genes. As a result, the dynamics of methylation and demethylation are crucial in governing chondrogenesis and cell death during different stages of limb chondrogenesis.

The Regulation of Growth in Developing, Homeostatic, and Regenerating Tetrapod Limbs: A Minireview

The size and shape of the tetrapod limb play central roles in their functionality and the overall physiology of the organism. In this minireview we will discuss observations on mutant animal models and humans, which show that the growth and final size of the limb is most impacted by factors that regulate either limb bud patterning or the elongation of the long bones. We will also apply the lessons that have been learned from embryos to how growth could be regulated in regenerating limb structures and outline the challenges that are unique to regenerating animals.

Transforming growth factor beta signaling: The master sculptor of fingers

Transforming growth factor beta (TGFβ) constitutes a large and evolutionarily conserved superfamily of secreted factors that play essential roles in embryonic development, cancer, tissue regeneration, and human degenerative pathology. Studies of this signaling cascade in the regulation of cellular and tissue changes in the three-dimensional context of a developing embryo have notably advanced in the understanding of the action mechanism of these growth factors. In this review, we address the role of TGFβ signaling in the developing limb, focusing on its essential function in the morphogenesis of the autopod. As we discuss in this work, modern mouse genetic experiments together with more classical embryological approaches in chick embryos, provided very valuable information concerning the role of TGFβ and Activin family members in the morphogenesis of the digits of tetrapods, including the formation of phalanxes, digital tendons, and interphalangeal joints. We emphasize the importance of the Activin and TGFβ proteins as digit inducing factors and their critical interaction with the BMP signaling to sculpt the hand and foot morphology.

Diverse Regulation but Conserved Function: SOX9 in Vertebrate Sex Determination

Sex determination occurs early during embryogenesis among vertebrates. It involves the differentiation of the bipotential gonad to ovaries or testes by a fascinating diversity of molecular switches. In most mammals, the switch is SRY (sex determining region Y); in other vertebrates it could be one of a variety of genes including Dmrt1 or dmy. Downstream of the switch gene, SOX9 upregulation is a central event in testes development, controlled by gonad-specific enhancers across the 2 Mb SOX9 locus. SOX9 is a ‘hub’ gene of gonadal development, regulated positively in males and negatively in females. Despite this diversity, SOX9 protein sequence and function among vertebrates remains highly conserved. This article explores the cellular, morphological, and genetic mechanisms initiated by SOX9 for male gonad differentiation.

Light-induced local gene expression in primary chick cell culture system

The ability to manipulate gene expression at a specific region in a tissue or cell culture system is critical for analysis of target gene function. For chick embryos/cells, several gene introduction/induction methods have been established such as those involving retrovirus, electroporation, sonoporation, and lipofection. However, these methods have limitations in the accurate induction of localized gene expression. Here we demonstrate the effective application of a recently developed light‐dependent gene expression induction system (LightOn system) using the Neurospora crassa photoreceptor Vivid fused with a Gal4 DNA binding domain and p65 activation domain (GAVPO) that alters its activity in response to light stimulus in a primary chicken cell culture system. We show that the gene expression level and induction specificity in this system are strongly dependent on the light irradiation conditions. Especially, the irradiation interval is an important parameter for modulating gene expression; for shorter time intervals, higher induction specificity can be achieved. Further, by adjusting light irradiation conditions, the expression level in primary chicken cells can be regulated in a multiple step manner, in contrast to the binary expression seen for gene disruption or introduction (i.e., null or overexpression). This result indicates that the light‐dependent expression control method can be a useful technique in chick models to examine how gene function is affected by gradual changes in gene expression levels. We applied this light induction system to regulate Sox9 expression in cultures of chick limb mesenchyme cells and showed that induced SOX9 protein could modulate expression of downstream genes.

Transcriptional, epigenetic and microRNA regulation of growth plate

Endochondral ossification is a critical event in bone formation, particularly in long shaft bones. Many cellular differentiation processes work in concert to facilitate the generation of cartilage primordium to formation of trabecular structures, all of which occur within the growth plate. Previous studies have revealed that the growth plate is tightly regulated by various transcription factors, epigenetic systems, and microRNAs. Hence, understanding these mechanisms that regulate the growth plate is crucial to furthering the current understanding on skeletal diseases, and in formulating effective treatment strategies. In this review, we focus on describing the function and mechanisms of the transcription factors, epigenetic systems, and microRNAs known to regulate the growth plate.

Regulation and function of SOX9 during cartilage development and regeneration

Chondrogenesis is a highly coordinated event in embryo development, adult homeostasis, and repair of the vertebrate cartilage. Fate decisions and differentiation of chondrocytes accompany differential expression of genes critical for each step of chondrogenesis. SOX9 is a master transcription factor that participates in sequential events in chondrogenesis by regulating a series of downstream factors in a stage-specific manner. SOX9 either works alone or in combination with downstream SOX transcription factors, SOX5 and SOX6 as chondrogenic SOX Trio. SOX9 is reduced in the articular cartilage of patients with osteoarthritis while highly maintained during tumorigenesis of cartilage and bone. Gene therapy using viral and non-viral vectors accompanied by tissue engineering (scaffolds) is a promising tool to regenerate impaired cartilage. Delivery of SOX9 or chondrogenic SOX Trio into cells produces efficient therapeutic effects on chondrogenesis and this event is facilitated by scaffolds. Non-viral vector-guided delivery systems encapsulated or loaded in mechanically stable solid scaffolds are useful for the regeneration of articular cartilage. Here we review major milestones and most recent studies focusing on regulation and function of chondrogenic SOX Trio, during chondrogenesis and cartilage regeneration, and on the development of advanced technologies in gene delivery with tissue engineering to improve efficiency of cartilage repair process.

Understanding the Cellular and Molecular Mechanisms That Control Early Cell Fate Decisions During Appendicular Skeletogenesis

The formation of the vertebrate skeleton is orchestrated in time and space by a number of gene regulatory networks that specify and position all skeletal tissues. During embryonic development, bones have two distinct origins: bone tissue differentiates directly from mesenchymal progenitors, whereas most long bones arise from cartilaginous templates through a process known as endochondral ossification. Before endochondral bone development takes place, chondrocytes form a cartilage analgen that will be sequentially segmented to form joints; thus, in the cartilage template, either the cartilage maturation programme or the joint formation programme is activated. Once the cartilage differentiation programme starts, the growth plate begins to form. In contrast, when the joint formation programme is activated, a capsule begins to form that contains special articular cartilage and synovium to generate a functional joint. In this review, we will discuss the mechanisms controlling the earliest molecular events that regulate cell fate during skeletogenesis in long bones. We will explore the initial processes that lead to the recruitment of mesenchymal stem/progenitor cells, the commitment of chondrocyte lineages, and the formation of skeletal elements during morphogenesis. Thereafter, we will review the process of joint specification and joint morphogenesis. We will discuss the links between transcription factor activity, cell–cell interactions, cell–extracellular matrix interactions, growth factor signalling, and other molecular interactions that control mesenchymal stem/progenitor cell fate during embryonic skeletogenesis.

Minimal Sustainability of Dedifferentiation by ROCK Inhibitor on Rat Nucleus Pulposus Cells In Vitro

Introduction

Intervertebral disc degeneration is strongly associated with low back pain. Cell transplantation has been extensively studied as a treatment option for intervertebral disc degeneration. It is often necessary to perform cell culture prior to cell transplantation; however, during cell expansion, the cells tend to dedifferentiate and lose their potency. Although the ability to suppress dedifferentiation by ROCK inhibitor (ROCKi) has recently been reported for chondrocytes, its effects on nucleus pulposus cells are still largely unknown.

Methods

Rat nucleus pulposus cells were cultured with or without the addition of ROCKi (Y-27632), and cell proliferation; CD24 positivity; expression of SOX9, COL2A1, Aggrecan, and COL1A1; and cell redifferentiation ability in pellet culture were evaluated.

Results

Although the addition of ROCKi tended to slightly increase the cell proliferative capacity, no significant differences were observed between treated and untreated conditions. The addition of ROCKi showed a trend of minimally increased COL2A1, ACAN, and SOX9 expression. Increases in COL1A1 expression was slightly suppressed by ROCKi. In pellet culture, strong increase in type II collagen deposition was observed by the addition of ROCKi. The addition of ROCKi did not significantly change the levels of CD24 positivity. The supplementation of ROCKi did not significantly enhance nucleus pulposus cell marker expression during monolayer expansion. However, ROCKi addition did result in an increased type II collagen deposition in 3D pellet culture.

Conclusions

Taken together, the results suggest a minimal effect by ROCKi on nucleus pulposus cell phenotype maintenance.

Development, repair, and regeneration of the limb musculoskeletal system

The limb musculoskeletal system provides a primary means for locomotion, manipulation of objects and protection for most vertebrate organisms. Intricate integration of the bone, tendon and muscle tissues are required for function. These three tissues arise largely independent of one another, but the connections formed during later development are maintained throughout life and are re-established following injury. Each of these tissues also have mesenchymal stem/progenitor cells that function in maintenance and repair. Here in, we will review the major events in the development of limb skeleton, tendon, and muscle tissues, their response to injury, and discuss current knowledge regarding resident progenitor/stem cells within each tissue that participate in development, repair, and regeneration in vivo.

The Loss of ATRX Increases Susceptibility to Pancreatic Injury and Oncogenic KRAS in Female But Not Male Mice

Background

Pancreatic ductal adenocarcinoma (PDAC) is the third leading cause of cancer death in North America, accounting for >30,000 deaths annually. Although somatic activating mutations in KRAS appear in 97% of PDAC patients, additional factors are required to initiate PDAC. Because mutations in genes encoding chromatin remodelling proteins have been implicated in KRAS-mediated PDAC, we investigated whether loss of chromatin remodeler ɑ-thalassemia, mental-retardation, X-linked (ATRX) affects oncogenic KRAS's ability to promote PDAC. ATRX affects DNA replication, repair, and gene expression and is implicated in other cancers including glioblastomas and pancreatic neuroendocrine tumors. The hypothesis was that deletion of Atrx in pancreatic acinar cells will increase susceptibility to injury and oncogenic KRAS.

Methods

Mice allowing conditional loss of Atrx within pancreatic acinar cells were examined after induction of recurrent cerulein-induced pancreatitis or oncogenic KRAS (KRASG12D ). Histologic, biochemical, and molecular analysis examined pancreatic pathologies up to 2 months after induction of Atrx deletion.

Results

Mice lacking Atrx showed more progressive damage, inflammation, and acinar-to-duct cell metaplasia in response to injury relative to wild-type mice. In combination with KRASG12D, Atrx-deficient acinar cells showed increased fibrosis, inflammation, progression to acinar-to-duct cell metaplasia, and pre-cancerous lesions relative to mice expressing only KRASG12D. This sensitivity appears only in female mice, mimicking a significant prevalence of ATRX mutations in human female PDAC patients.

Conclusions

Our results indicate the absence of ATRX increases sensitivity to injury and oncogenic KRAS only in female mice. This is an instance of a sex-specific mutation that enhances oncogenic KRAS's ability to promote pancreatic intraepithelial lesion formation.

Combinatorial CRISPR/Cas9 Approach to Elucidate a Far-Upstream Enhancer Complex for Tissue-Specific Sox9 Expression

SRY-box 9 (SOX9) is a master transcription factor that regulates cartilage development. SOX9 haploinsufficiency resulting from breakpoints in a ∼1-Mb region upstream of SOX9 was reported in acampomelic campomelic dysplasia (ACD) patients, suggesting that essential enhancer regions of SOX9 for cartilage development are located in this long non-coding sequence. However, the cis-acting enhancer region regulating cartilage-specific SOX9 expression remains to be identified. To identify distant cartilage Sox9 enhancers, we utilized the combination of multiple CRISPR/Cas9 technologies including enrichment of the promoter-enhancer complex followed by next-generation sequencing and mass spectrometry (MS), SIN3A-dCas9-mediated epigenetic silencing, and generation of enhancer deletion mice. As a result, we could identify a critical far-upstream cis-element and Stat3 as a trans-acting factor, regulating cartilage-specific Sox9 expression and subsequent skeletal development. Our strategy could facilitate definitive ACD diagnosis and should be useful to reveal the detailed chromatin conformation and regulation.

Smad4 deficiency impairs chondrocyte hypertrophy via the Runx2 transcription factor in mouse skeletal development

Chondrocyte hypertrophy is the terminal step in chondrocyte differentiation and is crucial for endochondral bone formation. How signaling pathways regulate chondrocyte hypertrophic differentiation remains incompletely understood. In this study, using a Tbx18:Cre (Tbx18^Cre/+) gene-deletion approach, we selectively deleted the gene for the signaling protein SMAD family member 4 (Smad4^f/f ) in the limbs of mice. We found that the Smad4-deficient mice develop a prominent shortened limb, with decreased expression of chondrocyte differentiation markers, including Col2a1 and Acan, in the humerus at mid-to-late gestation. The most striking defects in these mice were the absence of stylopod elements and failure of chondrocyte hypertrophy in the humerus. Moreover, expression levels of the chondrocyte hypertrophy-related markers Col10a1 and Panx3 were significantly decreased. Of note, we also observed that the expression of runt-related transcription factor 2 (Runx2), a critical mediator of chondrocyte hypertrophy, was also down-regulated in Smad4-deficient limbs. To determine how the skeletal defects arose in the mouse mutants, we performed RNA-Seq with ChIP-Seq analyses and found that Smad4 directly binds to regulatory elements in the Runx2 promoter. Our results suggest a new mechanism whereby Smad4 controls chondrocyte hypertrophy by up-regulating Runx2 expression during skeletal development. The regulatory mechanism involving Smad4-mediated Runx2 activation uncovered here provides critical insights into bone development and pathogenesis of chondrodysplasia.

Transcriptional Network Controlling Endochondral Ossification

Endochondral ossification is the fundamental process of skeletal development in vertebrates. Chondrocytes undergo sequential steps of differentiation, including mesenchymal condensation, proliferation, hypertrophy, and mineralization. These steps, which are required for the morphological and functional changes in differentiating chondrocytes, are strictly regulated by a complex transcriptional network. Biochemical and mice genetic studies identified chondrogenic transcription factors critical for endochondral ossification. The transcription factor sex-determining region Y (SRY)-box 9 (Sox9) is essential for early chondrogenesis, and impaired Sox9 function causes severe chondrodysplasia in humans and mice. In addition, recent genome-wide chromatin immunoprecipitation-sequencing studies revealed the precise regulatory mechanism of Sox9 during early chondrogenesis. Runt-related transcription factor 2 promotes chondrocyte hypertrophy and terminal differentiation. Interestingly, endoplasmic reticulum (ER) stress-related transcription factors have recently emerged as novel regulators of chondrocyte differentiation. Here we review the transcriptional mechanisms that regulate endochondral ossification, with a focus on Sox9.

The molecular pathogenesis of morphoea: from genetics to future treatment targets

A number of immunoinflammatory and profibrotic mechanisms are recognized in the pathogenesis of broad sclerotic skin processes and, more specifically, morphoea. However, the precise aetiopathogenesis is complex and remains unclear. Morphoea is clinically heterogeneous, with variable anatomical patterning, depth of tissue involvement and sclerotic, inflammatory, atrophic and dyspigmented morphology. Underlying mechanisms determining these reproducible clinical subsets are poorly understood but of great clinical and therapeutic relevance. Regional susceptibility mechanisms (e.g. environmental triggers, mosaicism and positional identity) together with distinct pathogenic determinants (including innate, adaptive and imbalanced pro- and antifibrotic signalling pathways) are likely implicated. In the age of genetic profiling and personalized medicine, improved characterization of the environmental, systemic, local, genetic and immunopathological factors underpinning morphoea pathogenesis may open the door to novel targeted therapeutic approaches.

An Integrative Developmental Genomics and Systems Biology Approach to Identify an In Vivo Sox Trio-Mediated Gene Regulatory Network in Murine Embryos

Embryogenesis is an intricate process involving multiple genes and pathways. Some of the key transcription factors controlling specific cell types are the Sox trio, namely, Sox5, Sox6, and Sox9, which play crucial roles in organogenesis working in a concerted manner. Much however still needs to be learned about their combinatorial roles during this process. A developmental genomics and systems biology approach offers to complement the reductionist methodology of current developmental biology and provide a more comprehensive and integrated view of the interrelationships of complex regulatory networks that occur during organogenesis. By combining cell type-specific transcriptome analysis and in vivo ChIP-Seq of the Sox trio using mouse embryos, we provide evidence for the direct control of Sox5 and Sox6 by the transcriptional trio in the murine model and by Morpholino knockdown in zebrafish and demonstrate the novel role of Tgfb2, Fbxl18, and Tle3 in formation of Sox5, Sox6, and Sox9 dependent tissues. Concurrently, a complete embryonic gene regulatory network has been generated, identifying a wide repertoire of genes involved and controlled by the Sox trio in the intricate process of normal embryogenesis.

Sox9 Expression in Amniotes: Species-Specific Differences in the Formation of Digits

In tetrapods the digit pattern has evolved to adapt to distinct locomotive strategies. The number of digits varies between species or even between hindlimb and forelimb within the same species. These facts illustrate the plasticity of embryonic limb autopods. Sox9 is a precocious marker of skeletal differentiation of limb mesenchymal cells. Its pattern of expression in the developing limb has been widely studied and reflects the activity of signaling cascades responsible for skeletogenesis. In this assay we stress previously overlooked differences in the pattern of expression of Sox9 in limbs of avian, mouse and turtle embryos which may reflect signaling differences associated with distinct limb skeletal morphologies observed in these species. Furthermore, we show that Sox9 gene expression is higher and maintained in the interdigital region in species with webbed digits in comparison with free digit animals.

Correlation Analysis of SOX9, -5, and -6 as well as COL2A1 and Aggrecan Gene Expression of Collagen I Implant-Derived and Osteoarthritic Chondrocytes

OBJECTIVE

Matrix-assisted autologous chondrocyte implantation is frequently applied to replace damaged cartilage in order to support tissue regeneration or repair and to prevent progressive cartilage degradation and osteoarthritis. Its application, however, is limited to primary defects and contraindicated in the case of osteoarthritis that is partially ascribed to dedifferentiation and phenotype alterations of chondrocytes obtainable from patients' biopsies. The differentiation state of chondrocytes is reflected at the level of structural gene (COL2A1, ACAN, COL1A1) and transcription factor (SOX9, 5, 6) expression.

METHODS/DESIGN

We determined the mRNA abundances of COL2A1, ACAN, and COL1A1as well as SOX9, -5, and -6 of freshly isolated and passaged collagen I implant-derived and osteoarthritic chondrocytes via reverse transcription-polymerase chain reaction. Moreover, we analyzed the correlation of structural and transcription factor gene expression. Thus, we were able to evaluate the impact of the mRNA levels of transcription factors on the expression of cartilage-specific structural genes.

RESULTS

Significant differences were obtained (1) for freshly isolated osteoarthritic versus collagen I implant-derived chondrocytes, (2) due to passaging of the respective cell sources, (3) for osteoarthritic versus nonosteoarthritic chondrocytes, and (4) for COL2A1 versus ACAN expression with respect to the coherence with SOX9, -5, and -6 transcript levels.

CONCLUSION

Our results might contribute to a better understanding of the transcriptional regulation of structural gene expression of chondrocytes with implications for their use in matrix-assisted autologous chondrocyte implantation.

Key pathways regulated by HoxA9,10,11/HoxD9,10,11 during limb development

Background

The 39 mammalian Hox genes show problematic patterns of functional overlap. In order to more fully define the developmental roles of Hox genes it is necessary to remove multiple combinations of paralogous and flanking genes. In addition, the downstream molecular pathways regulated by Hox genes during limb development remain incompletely delineated.

Results

In this report we examine limb development in mice with frameshift mutations in six Hox genes, Hoxa9,10,11 and Hoxd9,10,11. The mice were made with a novel recombineering method that allows the simultaneous targeting of frameshift mutations into multiple flanking genes. The Hoxa9,10,11^−/−/Hoxd9,10,11^−/− mutant mice show a reduced ulna and radius that is more severe than seen in Hoxa11^−/−/Hoxd11^−/− mice, indicating a minor role for the flanking Hox9,10 genes in zeugopod development, as well as their primary function in stylopod development. The mutant mice also show severe reduction of Shh expression in the zone of polarizing activity, and decreased Fgf8 expression in the apical ectodermal ridge, thereby better defining the roles of these specific Hox genes in the regulation of critical signaling centers during limb development. Importantly, we also used laser capture microdissection coupled with RNA-Seq to characterize the gene expression programs in wild type and mutant limbs. Resting, proliferative and hypertrophic compartments of E15.5 forelimb zeugopods were examined. The results provide an RNA-Seq characterization of the progression of gene expression patterns during normal endochondral bone formation. In addition of the Hox mutants showed strongly altered expression of Pknox2, Zfp467, Gdf5, Bmpr1b, Dkk3, Igf1, Hand2, Shox2, Runx3, Bmp7 and Lef1, all of which have been previously shown to play important roles in bone formation.

Conclusions

The recombineering based frameshift mutation of the six flanking and paralogous Hoxa9,10,11 and Hoxd9,10,11 genes provides a resource for the analysis of their overlapping functions. Analysis of the Hoxa9,10,11^−/−/Hoxd9,10,11^−/− mutant limbs confirms and extends the results of previous studies using mice with Hox mutations in single paralogous groups or with entire Hox cluster deletions. The RNA-Seq analysis of specific compartments of the normal and mutant limbs defines the multiple key perturbed pathways downstream of these Hox genes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-015-0078-5) contains supplementary material, which is available to authorized users.

Fate of growth plate hypertrophic chondrocytes: death or lineage extension?

The vertebrate growth plate is an essential tissue that mediates and controls bone growth. It forms through a multistep differentiation process in which chondrocytes differentiate, proliferate, stop dividing and undergo hypertrophy, which entails a 20-fold increase in size. Hypertrophic chondrocytes are specialized cells considered to be the end state of the chondrocyte differentiation pathway, and are essential for bone growth. They are characterized by expression of type X collagen encoded by the Col10a1 gene, and synthesis of a calcified cartilage matrix. Whether hypertrophy marks a transition preceding osteogenesis, or it is the terminal differentiation stage of chondrocytes with cell death as the ultimate fate has been the subject of debate for over a century. In this review, we revisit this debate in the light of new findings arising from genetic-mediated lineage tracing studies showing that hypertrophic chondrocytes can survive at the chondro-osseous junction and further make the transition to become osteoblasts and osteocytes. The contribution of chondrocytes to the osteoblast lineage has important implications in bone development, disease and repair.

The versatile functions of Sox9 in development, stem cells, and human diseases

The transcription factor Sox9 was first discovered in patients with campomelic dysplasia, a haploinsufficiency disorder with skeletal deformities caused by dysregulation of Sox9 expression during chondrogenesis. Since then, its role as a cell fate determiner during embryonic development has been well characterized; Sox9 expression differentiates cells derived from all three germ layers into a large variety of specialized tissues and organs. However, recent data has shown that ectoderm- and endoderm-derived tissues continue to express Sox9 in mature organs and stem cell pools, suggesting its role in cell maintenance and specification during adult life. The versatility of Sox9 may be explained by a combination of post-transcriptional modifications, binding partners, and the tissue type in which it is expressed. Considering its importance during both development and adult life, it follows that dysregulation of Sox9 has been implicated in various congenital and acquired diseases, including fibrosis and cancer. This review provides a summary of the various roles of Sox9 in cell fate specification, stem cell biology, and related human diseases. Ultimately, understanding the mechanisms that regulate Sox9 will be crucial for developing effective therapies to treat disease caused by stem cell dysregulation or even reverse organ damage.

Modeling digits. Digit patterning is controlled by a Bmp-Sox9-Wnt Turing network modulated by morphogen gradients

During limb development, digits emerge from the undifferentiated mesenchymal tissue that constitutes the limb bud. It has been proposed that this process is controlled by a self-organizing Turing mechanism, whereby diffusible molecules interact to produce a periodic pattern of digital and interdigital fates. However, the identities of the molecules remain unknown. By combining experiments and modeling, we reveal evidence that a Turing network implemented by Bmp, Sox9, and Wnt drives digit specification. We develop a realistic two-dimensional simulation of digit patterning and show that this network, when modulated by morphogen gradients, recapitulates the expression patterns of Sox9 in the wild type and in perturbation experiments. Our systems biology approach reveals how a combination of growth, morphogen gradients, and a self-organizing Turing network can achieve robust and reproducible pattern formation.

Odd-skipped related-1 controls neural crest chondrogenesis during tongue development

The tongue is a critical element of the feeding system in tetrapod animals for their successful adaptation to terrestrial life. Whereas the oral part of the mammalian tongue contains soft tissues only, the avian tongue has an internal skeleton extending to the anterior tip. The mechanisms underlying the evolutionary divergence in tongue skeleton formation are completely unknown. We show here that the odd-skipped related-1 (Osr1) transcription factor is expressed throughout the neural crest-derived tongue mesenchyme in mouse, but not in chick, embryos during early tongue morphogenesis. Neural crest-specific inactivation of Osr1 resulted in formation of an ectopic cartilage in the mouse tongue, reminiscent in shape and developmental ontogeny of the anterior tongue cartilage in chick. SRY-box containing gene-9 (Sox9), the master regulator of chondrogenesis, is widely expressed in the nascent tongue mesenchyme at the onset of tongue morphogenesis but its expression is dramatically down-regulated concomitant with activation of Osr1 expression in the developing mouse tongue. In Osr1 mutant mouse embryos, expression of Sox9 persisted in the developing tongue mesenchyme where chondrogenesis is subsequently activated to form the ectopic cartilage. Furthermore, we show that Osr1 binds to the Sox9 gene promoter and that overexpression of Osr1 suppressed expression of endogenous Sox9 mRNAs and Sox9 promoter-driven reporter. These data indicate that Osr1 normally prevents chondrogenesis in the mammalian tongue through repression of Sox9 expression and suggest that changes in regulation of Osr1 expression in the neural crest-derived tongue mesenchyme underlie the evolutionary divergence of birds from other vertebrates in tongue morphogenesis.

Sost and its paralog Sostdc1 coordinate digit number in a Gli3-dependent manner

WNT signaling is critical in most aspects of skeletal development and homeostasis, and antagonists of WNT signaling are emerging as key regulatory proteins with great promise as therapeutic agents for bone disorders. Here we show that Sost and its paralog Sostdc1 emerged through ancestral genome duplication and their expression patterns have diverged to delineate non-overlapping domains in most organ systems including musculoskeletal, cardiovascular, nervous, digestive, reproductive and respiratory. In the developing limb, Sost and Sostdc1 display dynamic expression patterns with Sost being restricted to the distal ectoderm and Sostdc1 to the proximal ectoderm and the mesenchyme. While Sostdc1^–/– mice lack any obvious limb or skeletal defects, Sost^–/– mice recapitulate the hand defects described for Sclerosteosis patients. However, elevated WNT signaling in Sost^–/–; Sostdc1^–/– mice causes misregulation of SHH signaling, ectopic activation of Sox9 in the digit 1 field and preaxial polydactyly in a Gli1- and Gli3-dependent manner. In addition, we show that the syndactyly documented in Sclerosteosis is present in both Sost^–/– and Sost^–/–; Sostdc1^–/– mice, and is driven by misregulation of Fgf8 in the AER, a region lacking Sost and Sostdc1 expression. This study highlights the complexity of WNT signaling in skeletal biology and disease and emphasizes how redundant mechanism and non-cell autonomous effects can synergize to unveil new intricate phenotypes caused by elevated WNT signaling.

Attenuation of bone morphogenetic protein signaling during amphibian limb development results in the generation of stage-specific defects

The vertebrate limb is one of the most intensively studied organs in the field of developmental biology. Limb development in tetrapod vertebrates is highly conserved and dependent on the interaction of several important molecular pathways. The bone morphogenetic protein (BMP) signaling cascade is one of these pathways and has been shown to be crucial for several aspects of limb development. Here, we have used a Xenopus laevis transgenic line, in which expression of the inhibitor Noggin is under the control of the heat-shock promoter hsp70 to examine the effects of attenuation of BMP signaling at different stages of limb development. Remarkably different phenotypes were produced at different stages, illustrating the varied roles of BMP in development of the limb. Very early limb buds appeared to be refractory to the effects of BMP attenuation, developing normally in most cases. Ectopic limbs were produced by overexpression of Noggin corresponding to a brief window of limb development at about stage 49/50, as recently described by Christen et al. (2012). Attenuation of BMP signaling in stage 51 or 52 tadpoles lead to a reduction in the number of digits formed, resulting in hypodactyly or ectrodactyly, as well as occasional defects in the more proximal tibia-fibula. Finally, inhibition at stage 54 (paddle stage) led to the formation of dramatically shortened digits resulting from loss of distal phalanges. Transcriptome analysis has revealed the possibility that more Noggin-sensitive members of the BMP family could be involved in limb development than previously suspected. Our analysis demonstrates the usefulness of heat-shock-driven gene expression as an effective method for inhibiting a developmental pathway at different times during limb development.

SOX9 governs differentiation stage-specific gene expression in growth plate chondrocytes via direct concomitant transactivation and repression

Cartilage and endochondral bone development require SOX9 activity to regulate chondrogenesis, chondrocyte proliferation, and transition to a non-mitotic hypertrophic state. The restricted and reciprocal expression of the collagen X gene, Col10a1, in hypertrophic chondrocytes and Sox9 in immature chondrocytes epitomise the precise spatiotemporal control of gene expression as chondrocytes progress through phases of differentiation, but how this is achieved is not clear. Here, we have identified a regulatory element upstream of Col10a1 that enhances its expression in hypertrophic chondrocytes in vivo. In immature chondrocytes, where Col10a1 is not expressed, SOX9 interacts with a conserved sequence within this element that is analogous to that within the intronic enhancer of the collagen II gene Col2a1, the known transactivation target of SOX9. By analysing a series of Col10a1 reporter genes in transgenic mice, we show that the SOX9 binding consensus in this element is required to repress expression of the transgene in non-hypertrophic chondrocytes. Forced ectopic Sox9 expression in hypertrophic chondrocytes in vitro and in mice resulted in down-regulation of Col10a1. Mutation of a binding consensus motif for GLI transcription factors, which are the effectors of Indian hedgehog signaling, close to the SOX9 site in the Col10a1 regulatory element, also derepressed transgene expression in non-hypertrophic chondrocytes. GLI2 and GLI3 bound to the Col10a1 regulatory element but not to the enhancer of Col2a1. In addition to Col10a1, paired SOX9-GLI binding motifs are present in the conserved non-coding regions of several genes that are preferentially expressed in hypertrophic chondrocytes and the occurrence of pairing is unlikely to be by chance. We propose a regulatory paradigm whereby direct concomitant positive and negative transcriptional control by SOX9 ensures differentiation phase-specific gene expression in chondrocytes. Discrimination between these opposing modes of transcriptional control by SOX9 may be mediated by cooperation with different partners such as GLI factors.

Molecular control of cell differentiation and programmed cell death during digit development

During the hand plate development, the processes of cell differentiation and control of cell death are relevant to ensure a correct shape of the limb. The progenitor cell pool that later will differentiate into cartilage to form the digits arises from undifferentiated mesenchymal cells beneath the apical ectodermal ridge (AER). Once these cells abandon the area of influence of signals from AER and ectoderm, some cells are committed to chondrocyte lineage forming the digital rays. However, if the cells are not committed to chondrocyte lineage, they will form the prospective interdigits that in species with free digits will subsequently die. In this work, we provide the overview of the molecular interactions between different signaling pathways responsible for the formation of digit and interdigit regions. In addition, we briefly describe some experiments concerning the most important signals responsible for promoting cell death. Finally, on the basis that the interdigital tissue has chondrogenic potential, we discuss the hypothesis that apoptotic-promoting signals might also act as antichondrogenic factors and chondrogenic factors might operate as anti-apoptotic factors.

Notch1 signaling regulates chondrogenic lineage determination through Sox9 activation

Notch signaling is involved in several cell lineage determination processes during embryonic development. Recently, we have shown that Sox9 is most likely a primary target gene of Notch1 signaling in embryonic stem cells (ESCs). By using our in vitro differentiation protocol for chondrogenesis from ESCs through embryoid bodies (EBs) together with our tamoxifen-inducible system to activate Notch1, we analyzed the function of Notch signaling and its induction of Sox9 during EB differentiation towards the chondrogenic lineage. Temporary activation of Notch1 during early stages of EB, when lineage determination occurs, was accompanied by rapid and transient Sox9 upregulation and resulted in induction of chondrogenic differentiation during later stages of EB cultivation. Using siRNA targeting Sox9, we knocked down and adjusted this early Notch1-induced Sox9 expression peak to non-induced levels, which led to reversion of Notch1-induced chondrogenic differentiation. In contrast, continuous Notch1 activation during EB cultivation resulted in complete inhibition of chondrogenic differentiation. Furthermore, a reduction and delay of cardiac differentiation observed in EBs after early Notch1 activation was not reversed by siRNA-mediated Sox9 knockdown. Our data indicate that Notch1 signaling has an important role during early stages of chondrogenic lineage determination by regulation of Sox9 expression.

Identification of two novel mutations in Shh long-range regulator associated with familial pre-axial polydactyly

Pre-axial polydactyly type II (PPDII, MIM #174500), Werner mesomelic syndrome (MIM %188770) and Haas polysyndactyly (MIM #186200) are a group of closely related conditions caused by mutations in a long-range Sonic hedgehog (SHH, MIM *600725) regulator called ZRS. To date, 19 point mutations, 10 duplications and 1 triplication of the ZRS associated with those pre-axial polydactylies have been reported in humans, mice, cats and chickens. Some of these have been shown to cause ectopic expression of Shh in the limb bud in mice, leading to a polydactylous phenotype, but the precise mechanism by which ZRS mutations generate this phenotype remains unknown. We present two PPDII families with fully penetrant point mutations in ultra-conserved predicted binding sites for transcription factors SOX9 and PAX3, two possible candidates for regulating SHH expression. Screening for point mutations or copy-number variation of the ZRS, high-resolution array-CGH, and screening of other conserved non-coding sequences (CNS) surrounding SHH in a third family are negative. This is the sixth PPDII pedigree with possible linkage to 7q36 that presents with no detectable ZRS mutation. We hypothesize that another nearby regulatory sequence, or an undetected position effect between ZRS and SHH, could be responsible for negative familial cases linked to 7q36.

MicroRNA-145 regulates chondrogenic differentiation of mesenchymal stem cells by targeting Sox9

Chondrogenic differentiation of mesenchymal stem cells (MSCs) is accurately regulated by essential transcription factors and signaling cascades. However, the precise mechanisms involved in this process still remain to be defined. MicroRNAs (miRNAs) regulate various biological processes by binding target mRNA to attenuate protein synthesis. To investigate the mechanisms for miRNAs-mediated regulation of chondrogenic differentiation, we identified that miR-145 was decreased during transforming growth factor beta 3 (TGF-β3)-induced chondrogenic differentiation of murine MSCs. Subsequently, dual-luciferase reporter gene assay data demonstrated that miR-145 targets a putative binding site in the 3'-UTR of SRY-related high mobility group-Box gene 9 (Sox9) gene, the key transcription factor for chondrogenesis. In addition, over-expression of miR-145 decreased expression of Sox9 only at protein levels and miR-145 inhibition significantly elevated Sox9 protein levels. Furthermore, over-expression of miR-145 decreased mRNA levels for three chondrogenic marker genes, type II collagen (Col2a1), aggrecan (Agc1), cartilage oligomeric matrix protein (COMP), type IX collagen (Col9a2) and type XI collagen (Col11a1) in C3H10T1/2 cells induced by TGF-β3, whereas anti-miR-145 inhibitor increased the expression of these chondrogenic marker genes. Thus, our studies demonstrated that miR-145 is a key negative regulator of chondrogenic differentiation by directly targeting Sox9 at early stage of chondrogenic differentiation.

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.

cis-regulatory mutations are a genetic cause of human limb malformations

The underlying mutations that cause human limb malformations are often difficult to determine, particularly for limb malformations that occur as isolated traits. Evidence from a variety of studies shows that cis-regulatory mutations, specifically in enhancers, can lead to some of these isolated limb malformations. Here, we provide a review of human limb malformations that have been shown to be caused by enhancer mutations and propose that cis-regulatory mutations will continue to be identified as the cause of additional human malformations as our understanding of regulatory sequences improves.

Generation of transgenic mice for conditional overexpression of Sox9

Sox9 belongs to the family of Sry-related high-mobility group box transcription factors controlling cell fate, cell proliferation and differentiation in various tissues, including cartilage, testis, the central nervous system, kidney, and gastrointestine. Mice conditionally lacking Sox9 have revealed essential roles for Sox9 in these tissues. To gain further insight into the role of Sox9 in different tissues and at different stages of development, we have generated a transgenic mouse line to express Sox9 in a Cre recombinase-dependent manner. The construct contained the human cytomegalovirus enhancer and chicken β-actin promoter, and a floxed monomeric red fluorescence protein 1 (mRFP1) cassette to direct ubiquitous expression of mRFP1. Cre expression removed the mRFP1 gene, allowing expression of Sox9 and enhanced green fluorescent protein. Expectedly, overexpression of Sox9 in chondrocytes using Col2a1-Cre mice suppressed chondrocyte hypertrophy, and delayed terminal differentiation and subsequent ossification. Misexpression of Sox9 in hypertrophic chondrocytes using Col10a1-Cre knock-in mice also delayed terminal differentiation. This Sox9 conditional transgenic mouse line will be a valuable tool to uncover tissue-specific and developmental stage-specific functions of Sox9.

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.

BMP-2 functions independently of SHH signaling and triggers cell condensation and apoptosis in regenerating axolotl limbs

Background

Axolotls have the unique ability, among vertebrates, to perfectly regenerate complex body parts, such as limbs, after amputation. In addition, axolotls pattern developing and regenerating autopods from the anterior to posterior axis instead of posterior to anterior like all tetrapods studied to date. Sonic hedgehog is important in establishing this anterior-posterior axis of limbs in all tetrapods including axolotls. Interestingly, its expression is conserved (to the posterior side of limb buds and blastemas) in axolotl limbs as in other tetrapods. It has been suggested that BMP-2 may be the secondary mediator of sonic hedgehog, although there is mounting evidence to the contrary in mice. Since BMP-2 expression is on the anterior portion of developing and regenerating limbs prior to digit patterning, opposite to the expression of sonic hedgehog, we examined whether BMP-2 expression was dependent on sonic hedgehog signaling and whether it affects patterning of the autopod during regeneration.

Results

The expression of BMP-2 and SOX-9 in developing and regenerating axolotl limbs corresponded to the first digits forming in the anterior portion of the autopods. The inhibition of sonic hedgehog signaling with cyclopamine caused hypomorphic limbs (during development and regeneration) but did not affect the expression of BMP-2 and SOX-9. Overexpression of BMP-2 in regenerating limbs caused a loss of digits. Overexpression of Noggin (BMP inhibitor) in regenerating limbs also resulted in a loss of digits. Histological analysis indicated that the loss due to BMP-2 overexpression was the result of increased cell condensation and apoptosis while the loss caused by Noggin was due to a decrease in cell division.

Conclusion

The expression of BMP-2 and its target SOX-9 was independent of sonic hedgehog signaling in developing and regenerating limbs. Their expression correlated with chondrogenesis and the appearance of skeletal elements has described in other tetrapods. Overexpression of BMP-2 did not cause the formation of extra digits, which is consistent with the hypothesis that it is not the secondary signal of sonic hedgehog. However, it did cause the formation of hypomorphic limbs as a result of increased cellular condensation and apoptosis. Taken together, these results suggest that BMP-2 does not have a direct role in patterning regenerating limbs but may be important to trigger condensation prior to ossification and to mediate apoptosis.

Sox9 family members negatively regulate maturation and calcification of chondrocytes through up-regulation of parathyroid hormone-related protein

Sox9 is a transcription factor that plays an essential role in chondrogenesis and has been proposed to inhibit the late stages of endochondral ossification. However, the molecular mechanisms underlying the regulation of chondrocyte maturation and calcification by Sox9 remain unknown. In this study, we attempted to clarify roles of Sox9 in the late stages of chondrocyte differentiation. We found that overexpression of Sox9 alone or Sox9 together with Sox5 and Sox6 (Sox5/6/9) inhibited the maturation and calcification of murine primary chondrocytes and up-regulated parathyroid hormone-related protein (PTHrP) expression in primary chondrocytes and the mesenchymal cell line C3H10T1/2. Sox5/6/9 stimulated the early stages of chondrocyte proliferation and development. In contrast, Sox5/6/9 inhibited maturation and calcification of chondrocytes in organ culture. The inhibitory effects of Sox5/6/9 were rescued by treating with anti-PTHrP antibody. Moreover, Sox5/6/9 bound to the promoter region of the PTHrP gene and up-regulated PTHrP gene promoter activity. Interestingly, we also found that the Sox9 family members functionally collaborated with Ihh/Gli2 signaling to regulate PTHrP expression and chondrocyte differentiation. Our results provide novel evidence that Sox9 family members mediate endochondral ossification by up-regulating PTHrP expression in association with Ihh/Gli2 signaling.

Transforming growth factors beta coordinate cartilage and tendon differentiation in the developing limb mesenchyme

Transforming growth factor beta (TGFbeta) signaling has an increasing interest in regenerative medicine as a potential tool to repair cartilages, however the chondrogenic effect of this pathway in developing systems is controversial. Here we have analyzed the function of TGFbeta signaling in the differentiation of the developing limb mesoderm in vivo and in high density micromass cultures. In these systems highest signaling activity corresponded with cells at stages preceding overt chondrocyte differentiation. Interestingly treatments with TGFbetas shifted the differentiation outcome of the cultures from chondrogenesis to fibrogenesis. This phenotypic reprogramming involved down-regulation of Sox9 and Aggrecan and up-regulation of Scleraxis, and Tenomodulin through the Smad pathway. We further show that TGFbeta signaling up-regulates Sox9 in the in vivo experimental model system in which TGFbeta treatments induce ectopic chondrogenesis. Looking for clues explaining the dual role of TGFbeta signaling, we found that TGFbetas appear to be direct inducers of the chondrogenic gene Sox9, but the existence of transcriptional repressors of TGFbeta signaling modulates this role. We identified TGF-interacting factor Tgif1 and SKI-like oncogene SnoN as potential candidates for this inhibitory function. Tgif1 gene regulation by TGFbeta signaling correlated with the differential chondrogenic and fibrogenic effects of this pathway, and its expression pattern in the limb marks the developing tendons. In functional experiments we found that Tgif1 reproduces the profibrogenic effect of TGFbeta treatments.

Msx genes are important apoptosis effectors downstream of the Shh/Gli3 pathway in the limb

In tetrapods, the anteroposterior (AP) patterning of the limb is under the control of the antagonistic activities of the secreted factor Sonic hedgehog (Shh) and Gli3R, the truncated repressor form of the transcription factor Gli3. In this report, we show that Msx1 and Msx2 are targets and downstream effectors of Gli3R. Consequently, in Shh null mutants, Msx genes are overexpressed and, furthermore, partially responsible for the limb phenotype. This is exemplified by the fact that reducing Msx activity in Shh mutants partially restores a normal limb development. Finally, we show that the main action of the Msx genes, in both normal and Shh(-/-) limb development, is to control cell death in the mesenchyme. We propose that, in the limb, Msx genes act downstream of the Shh/Gli3 pathway by transducing BMP signaling and that, in the absence of Shh signaling, their deregulation contributes to the extensive apoptosis that impairs limb development.

The forming limb skeleton serves as a signaling center for limb vasculature patterning via regulation of Vegf

Limb development constitutes a central model for the study of tissue and organ patterning; yet, the mechanisms that regulate the patterning of limb vasculature have been left understudied. Vascular patterning in the forming limb is tightly regulated in order to ensure sufficient gas exchange and nutrient supply to the developing organ. Once skeletogenesis is initiated,limb vasculature undergoes two seemingly opposing processes: vessel regression from regions that undergo mesenchymal condensation; and vessel morphogenesis. During the latter, vessels that surround the condensations undergo an extensive rearrangement, forming a stereotypical enriched network that is segregated from the skeleton. In this study, we provide evidence for the centrality of the condensing mesenchyme of the forming skeleton in regulating limb vascular patterning. Both Vegf loss- and gain-of-function experiments in limb bud mesenchyme firmly established VEGF as the signal by which the condensing mesenchyme regulates the vasculature. Normal vasculature observed in limbs where VEGF receptors Flt1, Flk1, Nrp1 and Nrp2 were blocked in limb bud mesenchyme suggested that VEGF, which is secreted by the condensing mesenchyme, regulates limb vasculature via a direct long-range mechanism. Finally, we provide evidence for the involvement of SOX9 in the regulation of Vegf expression in the condensing mesenchyme. This study establishes Vegf expression in the condensing mesenchyme as the mechanism by which the skeleton patterns limb vasculature.

Global comparative transcriptome analysis of cartilage formation in vivo

Background

During vertebrate embryogenesis the initial stages of bone formation by endochondral ossification involve the aggregation and proliferation of mesenchymal cells into condensations. Continued growth of the condensations and differentiation of the mesenchymal cells into chondrocytes results in the formation of cartilage templates, or anlagen, which prefigure the shape of the future bones. The chondrocytes in the anlagen further differentiate by undergoing a complex sequence of maturation and hypertrophy, and are eventually replaced by mineralized bone. Regulation of the onset of chondrogenesis is incompletely understood, and would be informed by comprehensive analyses of in vivo gene expression.

Results

Tibial and fibular pre-condensed mesenchyme was microdissected from mouse hind limbs at 11.5 dpc, and the corresponding condensations at 12.5 dpc and cartilage anlagen at 13.5 dpc. Total RNA was isolated, and cRNA generated by linear amplification was interrogated using mouse whole genome microarrays. Differential expression was validated by quantitative PCR for Agc1, Bmp8a, Col2a1, Fgfr4, Foxa3, Gdf5, Klf2, Klf4, Lepre1, Ncad, Sox11, and Trpv4. Further, independent validation of the microarray data was achieved by in situ hybridization to analyse the expression of Lepre1, Pcdh8, Sox11, and Trpv4 from 11.5 dpc to 13.5 dpc during mouse hind limb development. We found significant differential expression of 931 genes during these early stages of chondrogenesis. Of these, 380 genes were down-regulated and 551 up-regulated. Our studies characterized the expression pattern of gene families previously associated with chondrogenesis, such as adhesion molecules, secreted signalling molecules, transcription factors, and extracellular matrix components. Gene ontology approaches identified 892 differentially expressed genes not previously identified during the initiation of chondrogenesis. These included several Bmp, Gdf, Wnt, Sox and Fox family members.

Conclusion

These data represent the first global gene expression profiling analysis of chondrogenic tissues during in vivo development. They identify genes for further study on their functional roles in chondrogenesis, and provide a comprehensive and important resource for future studies on cartilage development and disease.

Hoxd and Gli3 interactions modulate digit number in the amniote limb

During limb development, Sonic hedgehog (SHH) and HOX proteins are considered among the most important factors regulating digit number and identity. SHH signaling prevents the processing of GLI3 into a short form that functions as a strong transcriptional repressor. Gli3 mutant limbs are characterized by a severe polydactyly and associated ectopic anterior expression of 5'Hoxd genes. To genetically determine the involvement of 5'Hoxd genes in the polydactyly of Gli3 mutants, we have generated a compound mutant that simultaneously removes the three most 5'-located Hoxd genes and Gli3. Remarkably, the limbs that form in the absence of all four of these genes show the most severe polydactyly so far reported in the mouse. The analysis of gene expression performed in compound mutants allows us to propose that the increase in the number of digits is mediated by the gain in function of Hoxd10 and Hoxd9. Our results also support the notion that an adequate balance between positive and negative effects of different Hoxd genes is required for pentadactyly.

The role of Hox genes during vertebrate limb development

The potential role of Hox genes during vertebrate limb development was brought into focus by gene expression analyses in mice (P Dolle, JC Izpisua-Belmonte, H Falkenstein, A Renucci, D Duboule, Nature 1989, 342:767-772), at a time when limb growth and patterning were thought to depend upon two distinct and rather independent systems of coordinates; one for the anterior-to-posterior axis and the other for the proximal-to-distal axis (see D Duboule, P Dolle, EMBO J 1989, 8:1497-1505). Over the past years, the function and regulation of these genes have been addressed using both gain-of-function and loss-of-function approaches in chick and mice. The use of multiple mutations either in cis-configuration in trans-configuration or in cis/trans configurations, has confirmed that Hox genes are essential for proper limb development, where they participate in both the growth and organization of the structures. Even though their molecular mechanisms of action remain somewhat elusive, the results of these extensive genetic analyses confirm that, during the development of the limbs, the various axes cannot be considered in isolation from each other and that a more holistic view of limb development should prevail over a simple cartesian, chess grid-like approach of these complex structures. With this in mind, the functional input of Hox genes during limb growth and development can now be re-assessed.