Expression of the paired-box genes Pax-1 and Pax-9 in limb skeleton development

Engineering Musculoskeletal Grafts for Multi-Tissue Unit Repair: Lessons From Developmental Biology and Wound Healing

In the musculoskeletal system, bone, tendon, and skeletal muscle integrate and act coordinately as a single multi-tissue unit to facilitate body movement. The development, integration, and maturation of these essential components and their response to injury are vital for conferring efficient locomotion. The highly integrated nature of these components is evident under disease conditions, where rotator cuff tears at the bone-tendon interface have been reported to be associated with distal pathological alterations such as skeletal muscle degeneration and bone loss. To successfully treat musculoskeletal injuries and diseases, it is important to gain deep understanding of the development, integration and maturation of these musculoskeletal tissues along with their interfaces as well as the impact of inflammation on musculoskeletal healing and graft integration. This review highlights the current knowledge of developmental biology and wound healing in the bone-tendon-muscle multi-tissue unit and perspectives of what can be learnt from these biological and pathological processes within the context of musculoskeletal tissue engineering and regenerative medicine. Integrating these knowledge and perspectives can serve as guiding principles to inform the development and engineering of musculoskeletal grafts and other tissue engineering strategies to address challenging musculoskeletal injuries and diseases.

Overview of PAX gene family: analysis of human tissue-specific variant expression and involvement in human disease

Paired-box (PAX) genes encode a family of highly conserved transcription factors found in vertebrates and invertebrates. PAX proteins are defined by the presence of a paired domain that is evolutionarily conserved across phylogenies. Inclusion of a homeodomain and/or an octapeptide linker subdivides PAX proteins into four groups. Often termed "master regulators", PAX proteins orchestrate tissue and organ development throughout cell differentiation and lineage determination, and are essential for tissue structure and function through maintenance of cell identity. Mutations in PAX genes are associated with myriad human diseases (e.g., microphthalmia, anophthalmia, coloboma, hypothyroidism, acute lymphoblastic leukemia). Transcriptional regulation by PAX proteins is, in part, modulated by expression of alternatively spliced transcripts. Herein, we provide a genomics update on the nine human PAX family members and PAX homologs in 16 additional species. We also present a comprehensive summary of human tissue-specific PAX transcript variant expression and describe potential functional significance of PAX isoforms. While the functional roles of PAX proteins in developmental diseases and cancer are well characterized, much remains to be understood regarding the functional roles of PAX isoforms in human health. We anticipate the analysis of tissue-specific PAX transcript variant expression presented herein can serve as a starting point for such research endeavors.

The transcription factors regulating intervertebral disc development

Damage to the intervertebral discs (IVDs) occurs due to aging or excessive mechanical stress, causing a series of IVD-related degenerative diseases, such as spinal disc herniation and spondylosis. These IVD-related diseases are difficult to cure, partially because the regeneration ability of IVDs is not sufficient. As a novel strategy for treatment of IVD-related diseases, mesenchymal stem cell transplantation to the damaged discs has been reported in animal studies. To further develop and improve this approach, it is necessary to gain a better understanding of the molecular network regulating IVD development by critical transcription factors. Recent findings reveal that during IVD development, nucleus pulposus and annuls fibrosus differentiation is coordinated by a series of transcription factors, such as Mkx, Pax1, 9, Shh, Foxa1, 2, T-Brachyury, and Sox5, 6, 9. The combination of mesenchymal stem cell transplantation with the regulation of these molecules may provide a novel strategy for treatment of degenerative disc diseases.

Modeling appendicular skeletal cartilage development with modified high-density micromass cultures of adult human bone marrow-derived mesenchymal progenitor cells

Background

Animal cell-based systems have been critical tools in understanding tissue development and physiology, but they are less successful in more practical tasks, such as predicting human toxicity to pharmacological or environmental factors, in which the congruence between in vitro and clinical outcomes lies on average between 50 and 60%. Emblematic of this problem is the high-density micromass culture of embryonic limb bud mesenchymal cells, derived from chick, mouse, or rat. While estimated predictive value of this model system in toxicological studies is relatively high, important failures prevent its use by international regulatory agencies for toxicity testing and policy development. A likely underlying reason for the poor predictive capacity of animal-based culture models is the small but significant physiological differences between species. This deficiency has inspired investigators to develop more organotypic, 3-dimensional culture system using human cells to model normal tissue development and physiology and assess pharmacological and environmental toxicity.

Methods

We have developed a modified, miniaturized micromass culture model using adult human bone marrow-derived mesenchymal progenitor cells (hBM-MPCs) that is amenable to moderate throughput and high content analysis to study chondrogenesis. The number of cells per culture was reduced, and a methacrylated gelatin (gelMA) overlay was incorporated to normalize the morphology of the cultures.

Results

These modified human cell-based micromass cultures demonstrated robust chondrogenesis, indicated by increased Alcian blue staining and immunodetectable production of collagen type II and aggrecan, and stage-specific chondrogenic gene expression. In addition, in cultures of hBM-MPCs transduced with a lentiviral collagen type II promoter-driven GFP reporter construct, levels of GFP reporter activity correlated well with changes in endogenous collagen type II transcript levels, indicating the feasibility of non-invasive monitoring of chondrogenesis.

Conclusions

The modified hBM-MPC micromass culture system described here represents a reproducible and controlled model for analyzing mechanisms of human skeletal development that may later be applied to pharmacological and environmental toxicity studies.

The Roles of P53 and Its Family Proteins, P63 and P73, in the DNA Damage Stress Response in Organogenesis-Stage Mouse Embryos

Members of the P53 transcription factor family, P53, P63, and P73, play important roles in normal development and in regulating the expression of genes that control apoptosis and cell cycle progression in response to genotoxic stress. P53 is involved in the DNA damage response pathway that is activated by hydroxyurea in organogenesis-stage murine embryos. The extent to which P63 and P73 contribute to this stress response is not known. To address this question, we examined the roles of P53, P63, and P73 in mediating the response of Trp53-positive and Trp53-deficient murine embryos to a single dose of hydroxyurea (400 mg/kg) on gestational day 9. Hydroxyurea treatment downregulated the expression of Trp63 and upregulated Trp73 in the absence of effects on the levels of Trp53 transcripts; Trp73 upregulation was P53-dependent. At the protein level, hydroxyurea treatment increased the levels and phosphorylation of P53 in the absence of effects on P63 and P73. Upregulation of the expression of genes that regulate cell cycle arrest and apoptosis, Cdkn1a, Rb1, Fas, Trp53inp1, and Pmaip1, was P53-dependent in hydroxyurea-treated embryos. The increase in cleaved caspase-3 and cleaved mammalian sterile-20-like-1 kinase levels induced by hydroxyurea was also P53-dependent; in contrast, the increase in phosphorylated H2AX, a marker of DNA double-strand breaks, in response to hydroxyurea treatment was only partially P53-dependent. Together, our data show that P53 is the principal P53 family member that is activated in the embryonic DNA damage response.

Biological effects of apatite nanoparticle-constructed ceramic surfaces in regulating behaviours of mesenchymal stem cells

HCA nanoparticle-constructed nanotopography in vivo mediates bone marrow MSCs to condensate and spontaneously differentiate towards the osteogenic lineage.

Ancestral and novel roles of Pax family genes in mollusks

Background

Pax genes are transcription factors with significant roles in cell fate specification and tissue differentiation during animal ontogeny. Most information on their tempo-spatial mode of expression is available from well-studied model organisms where the Pax-subfamilies Pax2/5/8, Pax6, and Paxα/β are mainly involved in the development of the central nervous system (CNS), the eyes, and other sensory organs. In certain taxa, Pax2/5/8 seems to be additionally involved in the development of excretion organs. Data on expression patterns in lophotrochozoans, and in particular in mollusks, are very scarce for all the above-mentioned Pax-subfamilies, which hampers reconstruction of their putative ancestral roles in bilaterian animals. Thus, we studied the developmental expression of Pax2/5/8, Pax6, and the lophotrochozoan-specific Paxβ in the worm-shaped mollusk Wirenia argentea, a member of Aplacophora that together with Polyplacophora forms the Aculifera, the proposed sister taxon to all primarily single-shelled mollusks (Conchifera).

Results

All investigated Pax genes are expressed in the developing cerebral ganglia and in the ventral nerve cords, but not in the lateral nerve cords of the tetraneural nervous system. Additionally, Pax2/5/8 is expressed in epidermal spicule-secreting or associated cells of the larval trunk and in the region of the developing protonephridia. We found no indication for an involvement of the investigated Pax genes in the development of larval or adult sensory organs of Wirenia argentea.

Conclusions

Pax2/5/8 seems to have a conserved role in the development of the CNS, whereas expression in the spicule-secreting tissues of aplacophorans and polyplacophorans suggests co-option in aculiferan skeletogenesis. The Pax6 expression pattern in Aculifera largely resembles the common bilaterian expression during CNS development. All data available on Paxβ expression argue for a common role in lophotrochozoan neurogenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-0919-x) contains supplementary material, which is available to authorized users.

Transcriptional control of chondrocyte specification and differentiation

A milestone in the evolutionary emergence of vertebrates was the invention of cartilage, a tissue that has key roles in modeling, protecting and complementing the bony skeleton. Cartilage is elaborated and maintained by chondrocytes. These cells derive from multipotent skeletal progenitors and they perform highly specialized functions as they proceed through sequential lineage commitment and differentiation steps. They form cartilage primordia, the primary skeleton of the embryo. They then transform these primordia either into cartilage growth plates, temporary drivers of skeletal elongation and endochondral ossification, or into permanent tissues, namely articular cartilage. Chondrocyte fate decisions and differentiated activities are controlled by numerous extrinsic and intrinsic cues, and they are implemented at the gene expression level by transcription factors. The latter are the focus of this review. Meritorious efforts from many research groups have led over the last two decades to the identification of dozens of key chondrogenic transcription factors. These regulators belong to all types of transcription factor families. Some have master roles at one or several differentiation steps. They include SOX9 and RUNX2/3. Others decisively assist or antagonize the activities of these masters. They include TWIST1, SOX5/6, and MEF2C/D. Many more have tissue-patterning roles and regulate cell survival, proliferation and the pace of cell differentiation. They include, but are not limited to, homeodomain-containing proteins and growth factor signaling mediators. We here review current knowledge of all these factors, one superclass, class, and family at a time. We then compile all knowledge into transcriptional networks. We also identify remaining gaps in knowledge and directions for future research to fill these gaps and thereby provide novel insights into cartilage disease mechanisms and treatment options.

LNGFR+THY-1+ human pluripotent stem cell-derived neural crest-like cells have the potential to develop into mesenchymal stem cells

Mesenchymal stem cells (MSCs) are defined as non-hematopoietic, plastic-adherent, self-renewing cells that are capable of tri-lineage differentiation into bone, cartilage or fat in vitro. Thus, MSCs are promising candidates for cell-based medicine. However, classifications of MSCs have been defined retrospectively; moreover, this conventional criterion may be inaccurate due to contamination with other hematopoietic lineage cells. Human MSCs can be enriched by selection for LNGFR and THY-1, and this population may be analogous to murine PDGFRα⁺Sca-1⁺ cells, which are developmentally derived from neural crest cells (NCCs). Murine NCCs were labeled by fluorescence, which provided definitive proof of neural crest lineage, however, technical considerations prevent the use of a similar approach to determine the origin of human LNGFR⁺THY-1⁺ MSCs. To further clarify the origin of human MSCs, human embryonic stem cells (ESCs) and human induced pluripotent stem cells (iPSCs) were used in this study. Under culture conditions required for the induction of neural crest cells, human ESCs and iPSCs-derived cells highly expressed LNGFR and THY-1. These LNGFR⁺THY-1⁺ neural crest-like cells, designated as LT-NCLCs, showed a strong potential to differentiate into both mesenchymal and neural crest lineages. LT-NCLCs proliferated to form colonies and actively migrated in response to serum concentration. Furthermore, we transplanted LT-NCLCs into chick embryos, and traced their potential for survival, migration and differentiation in the host environment. These results suggest that LNGFR⁺THY-1⁺ cells identified following NCLC induction from ESCs/iPSCs shared similar potentials with multipotent MSCs.

Molecular mechanisms underlying the exceptional adaptations of batoid fins

Extreme novelties in the shape and size of paired fins are exemplified by extinct and extant cartilaginous and bony fishes. Pectoral fins of skates and rays, such as the little skate (Batoid, Leucoraja erinacea), show a strikingly unique morphology where the pectoral fin extends anteriorly to ultimately fuse with the head. This results in a morphology that essentially surrounds the body and is associated with the evolution of novel swimming mechanisms in the group. In an approach that extends from RNA sequencing to in situ hybridization to functional assays, we show that anterior and posterior portions of the pectoral fin have different genetic underpinnings: canonical genes of appendage development control posterior fin development via an apical ectodermal ridge (AER), whereas an alternative Homeobox (Hox)-Fibroblast growth factor (Fgf)-Wingless type MMTV integration site family (Wnt) genetic module in the anterior region creates an AER-like structure that drives anterior fin expansion. Finally, we show that GLI family zinc finger 3 (Gli3), which is an anterior repressor of tetrapod digits, is expressed in the posterior half of the pectoral fin of skate, shark, and zebrafish but in the anterior side of the pelvic fin. Taken together, these data point to both highly derived and deeply ancestral patterns of gene expression in skate pectoral fins, shedding light on the molecular mechanisms behind the evolution of novel fin morphologies.

From Skeletal Development to Tissue Engineering: Lessons from the Micromass Assay

Damage and degeneration of the skeletal elements due to disease, trauma, and aging lead to a significant health and economical burden. To reduce this burden, skeletal tissue engineering strategies aim to regenerate functional bone and cartilage in the adult body. However, challenges still exist. Such challenges involve the identification of the external cues that determine differentiation, how to control chondrocyte hypertrophy, and how to achieve specific tissue patterns and boundaries. To address these issues, it could be insightful to look at skeletal development, a robust morphogenetic process that takes place during embryonic development and is commonly modeled in vitro by the micromass assay. In this review, we investigate what the tissue engineering field can learn from this assay. By comparing embryonic skeletal precursor cells from different anatomic locations and developmental stages in micromass, the external cues that guide lineage commitment can be identified. The signaling pathways regulating chondrocyte hypertrophy, and the cues required for tissue patterning, can be elucidated by combining the micromass assay with genetic, molecular, and engineering tools. The lessons from the micromass assay are limited by two major differences between developmental and regenerative skeletogenesis: cell type and scale. We highlight an important difference between embryonic and adult skeletal progenitor cells, in that adult progenitors are not able to form mesenchymal condensations spontaneously. Also, the mechanisms of tissue patterning need to be adjusted to the larger tissue engineering constructs. In conclusion, mechanistic insights of skeletal tissue generation gained from the micromass model could lead to improved tissue engineering strategies and constructs.

Identification and characterization of the novel Col10a1 regulatory mechanism during chondrocyte hypertrophic differentiation

The majority of human skeleton develops through the endochondral pathway, in which cartilage-forming chondrocytes proliferate and enlarge into hypertrophic chondrocytes that eventually undergo apoptosis and are replaced by bone. Although at a terminal differentiation stage, hypertrophic chondrocytes have been implicated as the principal engine of bone growth. Abnormal chondrocyte hypertrophy has been seen in many skeletal dysplasia and osteoarthritis. Meanwhile, as a specific marker of hypertrophic chondrocytes, the type X collagen gene (COL10A1) is also critical for endochondral bone formation, as mutation and altered COL10A1 expression are often accompanied by abnormal chondrocyte hypertrophy in many skeletal diseases. However, how the type X collagen gene is regulated during chondrocyte hypertrophy has not been fully elucidated. We have recently demonstrated that Runx2 interaction with a 150-bp mouse Col10a1 cis-enhancer is required but not sufficient for its hypertrophic chondrocyte-specific reporter expression in transgenic mice, suggesting requirement of additional Col10a1 regulators. In this study, we report in silico sequence analysis of this 150-bp enhancer and identification of its multiple binding factors, including AP1, MEF2, NFAT, Runx1 and TBX5. Using this enhancer as bait, we performed yeast one-hybrid assay and identified multiple candidate Col10a1-interacting genes, including cyclooxygenase 1 (Cox-1) and Cox-2. We have also performed mass spectrometry analysis and detected EF1-alpha, Fus, GdF7 and Runx3 as components of the specific complex formed by the cis-enhancer and nuclear extracts from hypertrophic MCT (mouse chondrocytes immortalized with large T antigen) cells that express Col10a1 abundantly. Notably, some of the candidate genes are differentially expressed in hypertrophic MCT cells and have been associated with chondrocyte hypertrophy and Runx2, an indispensible Col10a1 regulator. Intriguingly, we detected high-level Cox-2 expression in hypertrophic MCT cells. Electrophoretic mobility shift assay and chromatin immunoprecipitation assays confirmed the interaction between Cox-2 and Col10a1 cis-enhancer, supporting its role as a candidate Col10a1 regulator. Together, our data support a Cox-2-containing, Runx2-centered Col10a1 regulatory mechanism, during chondrocyte hypertrophic differentiation.

The function of DrPax1b gene in the embryonic development of zebrafish

Vertebrate Pax1 gene is a member of Pax gene family and encodes a transcription factor associated with crucial roles in the development of pharyngeal pouch, scletrotome and limb bud. In zebrafish, the genome contains two Pax1 paralogs, DrPax1a and DrPax1b, which share high sequence similarity with other Pax1 genes. To elucidate the function of zebrafish DrPax1b gene, we first examined the gene expression pattern and found that it was mainly expressed in the endodermal pharyngeal pouch, caudal somites, notochord, and fin bud. Then, we performed knockdown experiments using antisense morpholino oligonucleotides, which lead to the defects in the vertebral column, tail, pharyngeal skeleton, and pectoral fin. Additionally, we also found that the mouse MmPax1 mRNA, but not the amphioxus AmphiPax1/9 mRNA, could rescue the MO-induced defects. Furthermore, sequence alignment revealed that the N-terminal region of vertebrate Pax1 and amphioxus Pax1/9 were highly conserved, whereas their C-terminal regions were relatively divergent. However, the chimeric Am(N)Dr(C)Pax1, Mm(N)Dr(C)Pax1 and Dr(N)Mm(C)Pax1 mRNA could partially rescue the defects, while the Dr(N)Am(C)Pax1 mRNA could not. In conclusion, our data demonstrate a conserved function of DrPax1b in the development of the vertebral column, pectoral fin and pharyngeal skeleton formation in zebrafish and also provide critical insight into the functional evolution of Pax1 gene by changing its C-terminal sequence.

Characterization of zebrafish Pax1b and Pax9 in fin bud development

Both Pax1 and Pax9 belong to the important paired box gene family (PAX), which mainly participates in animal development and sclerotome differentiation. To date, the precise molecular mechanism and related signaling pathway of Pax1 remain unclear. In our study, microinjection of morpholino- (MO-) modified antisense oligonucleotides against pax1b induced pectoral fin bud defects. Furthermore, we demonstrate that the phenotypes caused by the knockdown of Pax1b in zebrafish could not be phenocopied by pax9 MO and could not be rescued by either Pax1a or Pax9 overexpression. We further find that Pax1b affects the expression of col2a1, Uncx4.1, Noggin3, and aggrecan, confirming the role of Pax1b in chondrocyte differentiation and bone maturation. Moreover, we identify an interaction between PAX1 and FOXO1 and find that the interaction was enhanced under hypoxia stress. Together, this evidence for cell death caused by pax1b knockdown provides new insight into the role of the Pax protein family in cell fate determination and tissue specification.

Application of synthetic photostable retinoids induces novel limb and facial phenotypes during chick embryogenesis in vivo

We have recently developed a range of synthetic retinoid analogues which include the compounds EC23 and EC19. They are stable on exposure to light and are predicted to be resistant to the normal metabolic processes involved in the inactivation of retinoids in vivo. Based on the position of the terminal carboxylic acid groups in the compounds we suggest that EC23 is a structural analogue of all-trans retinoic acid (ATRA), and EC19 is an analogue of 13-cis retinoic acid. Their effects on the differentiation of pluripotent stem cells has been previously described in vitro and are consistent with this hypothesis. We present herein the first description of the effects of these molecules in vivo. Retinoids were applied to the anterior limb buds of chicken embryos in ovo via ion-exchange beads. We found that retinoid EC23 produces effects on the wing digits similar to ATRA, but does so at two orders of magnitude lower concentration. When larger quantities of EC23 are applied, a novel phenotype is obtained involving production of multiple digit 1s on the anterior limb. This corresponds to differential effects of ATRA and EC23 on sonic hedgehog (shh) expression in the developing limb bud. With EC23 application we also find digit 1 phenotypes similar to thumb duplications described in the clinical literature. EC23 and ATRA are shown to have effects on the entire proximal–distal axis of the limb, including hitherto undescribed effects on the scapula. This includes suppression of expression of the scapula marker Pax1. EC23 also produces effects similar to those of ATRA on the developing face, producing reductions of the upper beak at concentrations two orders of magnitude lower than ATRA. In contrast, EC19, which is structurally very similar to EC23, has novel, less severe effects on the face and rarely alters limb development. EC19 and ATRA are effective at similar concentrations. These results further demonstrate the ability of retinoids to influence embryonic development. Moreover, EC23 represents a useful new tool to investigate developmental processes and probe the mechanisms underlying congenital abnormalities in vertebrates including man.

Control of pelvic girdle development by genes of the Pbx family and Emx2

Genes expressed in the somatopleuric mesoderm, the embryonic domain giving rise to the vertebrate pelvis, appear important for pelvic girdle formation. Among such genes, Pbx family members and Emx2 were found to genetically interact in hindlimb and pectoral girdle formation. Here, we generated compound mutant embryos carrying combinations of mutated alleles for Pbx1, Pbx2, and Pbx3, as well as Pbx1 and Emx2, to examine potential genetic interactions during pelvic development. Indeed, Pbx genes share overlapping functions and Pbx1 and Emx2 genetically interact in pelvic formation. We show that, in compound Pbx1;Pbx2 and Pbx1;Emx2 mutants, pelvic mesenchymal condensation is markedly perturbed, indicative of an upstream control by these homeoproteins. We establish that expression of Tbx15, Prrx1, and Pax1, among other genes involved in the specification and development of select pelvic structures, is altered in our compound mutants. Lastly, we identify potential Pbx1-Emx2-regulated enhancers for Tbx15, Prrx1, and Pax1, using bioinformatics analyses.

Pbx homeodomain proteins: TALEnted regulators of limb patterning and outgrowth

Limb development has long provided an excellent model for understanding the genetic principles driving embryogenesis. Studies utilizing chick and mouse have led to new insights into limb patterning and morphogenesis. Recent research has centered on the regulatory networks underlying limb development. Here, we discuss the hierarchical, overlapping, and iterative roles of Pbx family members in appendicular development that have emerged from genetic analyses in the mouse. Pbx genes are essential in determining limb bud positioning, early bud formation, limb axes establishment and coordination, and patterning and morphogenesis of most elements of the limb and girdle. Pbx proteins directly regulate critical effectors of limb and girdle development, including morphogen-encoding genes like Shh in limb posterior mesoderm, and transcription factor-encoding genes like Alx1 in pre-scapular domains. Interestingly, at least in limb buds, Pbx appear to act not only as Hox cofactors, but also in the upstream control of 5' HoxA/D gene expression.

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.

The formation of the avian scapula blade takes place in the hypaxial domain of the somites and requires somatopleure-derived BMP signals

The avian scapula is a long bone located dorsally on the thorax. The cranial part that articulates with the upper limb is derived from the somatopleure of the forelimb field, while the caudal part, the scapula blade, originates from the dermomyotomes of brachial and thoracic somites. In previous studies, we have shown that scapula blade formation is intrinsically controlled by segment-specific information as well as extrinsically by ectoderm-derived signals. Here, we addressed the role of signals derived from the lateral plate mesoderm on scapula development. Chick-quail chimera experiments revealed that scapula precursor cells are located within the hypaxial domain of the dermomyotome adjacent to somatopleural cells. Barrier implantation between these two cell populations inhibited scapula blade formation. Furthermore, we identified BMPs as scapula-inducing signals from the somatopleure using injection of Noggin-producing cells into the hypaxial domain of scapula-forming dermomyotomes. We found that inhibition of BMP activity interfered with scapula-specific Pax1 expression and scapula blade formation. Taken together, we demonstrate that the scapula-forming cells located within the hypaxial somitic domain require BMP signals derived from the somatopleure for their specification and differentiation.

Expression and function of Bapx1 during chick limb development

The homeobox-containing transcription factor Bapx1 (also known as Nkx3.2) is crucial for development of the axial skeleton and parts of the chondrocranium. Here we describe the detailed expression of Bapx1 during chick limb development and show that in contrast to its expression in the axial skeleton, Bapx1 is expressed after the commitment to chondrogenesis. Bapx1 is initially expressed throughout the developing skeletal elements prior to the overt differentiation of the distinct chondrogenic layers. Once distinct layers (proliferating, prehypertrophic and hypertrophic) have formed, Bapx1 expression is restricted to the proliferating chondrocytes. Bapx1 transcripts are excluded from the articular cartilage. A second homeobox-containing transcription factor, Barx1, is expressed in a complementary fashion in the developing joint and articular cartilage. Interestingly, in vitro functional analyses showed that Bapx1 overexpression in micromass cultures increased both matrix production and nodule number suggesting that Bapx1 is sufficient to promote chondrogenesis in the limb. In contrast, Barx1 had the opposite effect on nodule number suggesting that it has an inhibitory effect on chondrogenic initiation consistent with its expression in the developing joint. A slight increase in matrix levels was also observed consistent with its expression in the articular chondrocytes. Finally, we show that Bapx1 is also expressed in the soft tissues such as the developing tendons, muscle sheaths and surrounding mesenchyme, and therefore may have additional as yet uncharacterized roles in limb morphogenesis.

Characterization and chondrocyte differentiation stage-specific expression of KRAB zinc-finger protein gene ZNF470

As part of a study to identify novel transcriptional regulators of chondrogenesis-related gene expression, we have cloned and characterized cDNA for zinc-finger protein 470 (ZNF470), the human ortholog of which encodes a 717 amino acid residue protein containing 17 Cys(2)His(2) zinc-finger domains, as well as KRAB-A and KRAB-B motifs. The cDNA library used to isolate the initial ZNF470 clone was prepared from human bone marrow-derived mesenchymal progenitor cells at an intermediate stage of chondrogenic differentiation. We have determined the intron-exon structure of the human ZNF470 gene, which has been mapped to a zinc-finger cluster in a known imprinted region of human chromosome 19q13.4. ZNF470 is expressed at high levels in human testis and is expressed at low or undetectible levels in other adult tissues. Human ZNF470 expressed in mammalian cells as an EGFP fusion protein localizes predominantly to the nucleus, consistent with a role in transcriptional regulation. ZNF470, analyzed by quantitative real time PCR, was transiently expressed before the maximal expression of COL2A1 during chondrogenic differentiation in vitro. We have also characterized the bovine ortholog of human ZNF470, which encodes a 508 amino acid residue protein having 10 zinc-finger domains. A bovine ZNF470 cDNA clone was used to examine expression of ZNF470 in bovine articular chondrocytes treated with retinoic acid to stimulate dedifferentiation. Bovine ZNF470 expression was undetectable in freshly isolated bovine articular chondrocytes, but was dramatically upregulated in dedifferentiated retinoic acid-treated chondrocytes. These results, in two model systems, suggest a possible role for ZNF470 in the regulation of chondrogenesis-specific gene expression.

The scapulocoracoid of the Queensland lungfish Neoceratodus forsteri (Dipnoi: Sarcopterygii): morphology, development and evolutionary implications for bony fishes (Osteichthyes)

Among bony fishes, the ontogenetic sequence by which the actinopterygian scapulocoracoid develops has been well described, but that of the sarcopterygian scapulocoracoid is poorly known, as the majority of taxa are only known from fossils. To rectify this, the cartilaginous scapulocoracoid of the extant lungfish Neoceratodus forsteri is examined. In initial stages of its development, the scapulocoracoid of Neoceratodus has a simple rounded shape, and supports the glenoid fossa. It appears nearly contemporaneously with the proximal endochondral element (humerus) of the pectoral fin. Pectoral fin elements develop by segmentation from a continuous field of cartilaginous precursor cells extending distally from the glenoid region of the scapulocoracoid. Subsequent scapulocoracoid development produces a ventromedial process, which is not associated with this field of precursor cells. A dorsal process also develops outside this field. Thus, the scapulocoracoid of Neoceratodus may consist of at least two developmentally distinct regions; (1) the ventromedial being homologous with the coracoid of actinopterygians, tetrapods and other jawed vertebrates and (2) a smaller dorsal process, homologous to the scapular region. The two, together with the glenoid region, give an overall triangular shape. The scapulocoracoids of fossil lungfish and other sarcopterygian fishes are also triangular and are composed of scapular and coracoid regions, rather than the 'buttresses' associated with scapulocoracoids of the Actinopterygii and Tetrapoda.

Carbon monoxide-induced axial skeletal dysmorphogenesis in the chick embryo

BACKGROUND

Congenital axial skeletal defects affect two to three individuals per 1,000 live births. Without strong evidence for heritability, the cause is assumed to be multi-factorial. Carbon monoxide (CO), an increasingly prevalent environmental toxicant, is a potential environmental component in the etiology of these defects. The chick embryo is a useful model for the characterization and assessment of the mechanism(s) of action of basic developmental mechanisms.

METHODS

We have determined a critical period and dose for CO teratogenicity and established a model of CO-induced axial skeletal dysmorphogenesis in the chick embryo. The resulting phenotypes reveal a spectrum of axial skeletal defects ranging from minor defects of the vertebral canal and inter-vertebral discs, to thoraco-lumbar scoliosis, to a tailless phenotype reminiscent of caudal dysgenesis syndrome. These axial skeletal defects have been related to earlier developmental defects in somitogenesis, including errors in segmentation and epithehalization and the expression of the somitic epithelialization factor, Paraxis. We have examined patterns of cell death and apoptosis in CO exposed chick embryos to assess the target tissue(s) involved in the teratogenicity of CO.

RESULTS

With respect to the embryonic axis, the neural tube was found to be the most sensitive to CO-induced apoptosis, followed by the somitic mesoderm and Hensen's node.

CONCLUSIONS

We hypothesize that the somitic defects and the resulting axial skeletal dysmorphogenesis are caused by disrupted neural tube or ectoderm functions related to somite formation and maintenance. We also hypothesize that CO-induced dysmorphogenesis at this critical period of somitogenesis is caused by the overabundance of CO acting endogenously as a cellular signal, while coincidentally exerting its influence as a toxicant of oxygen delivery or utilization.

Fibroblast growth factor signaling regulates Dach1 expression during skeletal development

Dach1 is a mouse homologue of the Drosophila dachshund gene, which is a key regulator of cell fate determination during eye, leg, and brain development in the fly. We have investigated the expression and growth factor regulation of Dach1 during pre- and postnatal skeletal development in the mouse limb to understand better the function of Dach1. Dach1 was expressed in the distal mesenchyme of the early embryonic mouse limb bud and subsequently became restricted to the tips of digital cartilages. Dach1 protein was localized to postmitotic, prehypertrophic, and early hypertrophic chondrocytes during the initiation of ossification centers, but Dach1 was not expressed in growth plates that exhibited extensive ossification. Dach1 colocalized with Runx2/Cbfa1 in chondrocytes but not in the forming bone collar or primary spongiosa. Dach1 also colocalized with cyclin-dependent kinase inhibitors p27 (Kip1) and p57 (Kip2) in chondrocytes of the growth plate and in the epiphysis before the formation of the secondary ossification center. Because fibroblast growth factors (FGF), bone morphogenetic proteins (BMP), and hedgehog molecules (Hh) regulate skeletal patterning of the limb bud and chondrocyte maturation in developing endochondral bones, we investigated the regulation of Dach1 by these growth and differentiation factors. Expression of Dach1 in 11 days postcoitus mouse limb buds in organ culture was up-regulated by implanting beads soaked in FGF1, 2, 8, or 9 but not FGF10. BMP4-soaked beads down-regulated Dach1 expression, whereas Shh and bovine serum albumin had no effect. Furthermore, FGF4 or 8 could substitute for the apical ectodermal ridge in maintaining Dach1 expression in the limb buds. Immunolocalization of FGFR2 and FGFR3 revealed overlap with Dach1 expression during skeletal patterning and chondrocyte maturation. We conclude that Dach1 is a target gene of FGF signaling during limb skeletal development, and Dach1 may function as an intermediary in the FGF signaling pathway regulating cell proliferation or differentiation.

Cooperation of Hoxa5 and Pax1 genes during formation of the pectoral girdle

Hox and Pax transcription factors are master regulators of skeletal and organ morphogenesis. Some skeletal malformations encountered in Hoxa5 mutants are shared by the undulated (un) mice, which bear a point mutation in the Pax1 gene. To investigate whether Hoxa5 and Pax1 act in common pathways during skeletal development, we analyzed Hoxa5;un compound mutants. Our genetic studies show that Hoxa5 and Pax1 cooperate in the vertebral patterning of the cervicothoracic transition region and in acromion morphogenesis. The dynamics of expression of Hoxa5 and Pax1 in the pectoral girdle region suggest that both genes function in a complementary fashion during acromion formation. Whereas Pax1 is required for the recruitment of acromion precursor cells, Hoxa5 may provide regional cues essential for the correct formation of the acromion by ensuring Pax1 expression at the proper time and position during morphogenesis of the pectoral girdle. Hoxa5 also has a distinctive role in specifying the fate of perichondrial and chondrogenic cell lineages in a Sox9-dependent way.

Frizzled-7 and limb mesenchymal chondrogenesis: effect of misexpression and involvement of N-cadherin

Products of the Frizzled family of tissue polarity genes have been identified as putative receptors for the Wnt family of signaling molecules. Wnt-signaling is implicated in the regulation of limb mesenchymal chondrogenesis, and our recent study indicates that N-cadherin and related activities are functionally involved in Wnt-7a-mediated inhibition of chondrogenesis. By using an in vitro high-density micromass culture system of chick limb mesenchymal cells, we have analyzed the spatiotemporal expression patterns and the effects on chondrogenesis of RCAS retroviral-mediated misexpression of Chfz-1 and Chfz-7, two Frizzled genes implicated in chondrogenic regulation. Chfz-1 expression was localized at areas surrounding the cartilaginous nodules at all time points examined, whereas Chfz-7 expression was limited to cellular aggregates during initial mesenchymal condensation, and subsequently was down-regulated from the centers toward the periphery of cartilage nodules at the time of chondrogenic differentiation, resembling the pattern of N-cadherin expression. Chondrogenesis in vitro was inhibited and limited to a smaller area of the culture upon misexpression of Chfz-7, but not affected by Chfz-1 misexpression. Analyses of cellular condensation and chondrogenic differentiation showed that the inhibitory action of Chfz-7 is unlikely to be at the chondrogenic differentiation step, but instead affects the earlier precartilage aggregate formation event. At 24 hr, expression of N-cadherin, a key component of the cellular condensation phase of chondrogenesis, was delayed/suppressed in Chfz-7 misexpressing cultures, and was limited to a significantly smaller cellular condensation area within the entire culture at 48 hr, when compared with control cultures. Chfz-1 misexpressing cultures appeared similar to control cultures at all time points. However, neither Chfz-1 nor Chfz-7 misexpression affected mesenchymal cell proliferation in vitro. These results suggest that Chfz-7 is active in regulating N-cadherin expression during the process of limb mesenchymal chondrogenesis and that Chfz-1 and Chfz-7 are involved in different Wnt-signaling pathways.

Dach1, a vertebrate homologue of Drosophila dachshund, is expressed in the developing eye and ear of both chick and mouse and is regulated independently of Pax and Eya genes

We have cloned a chick homologue of Drosophila dachshund (dac), termed Dach1. Dach1 is the orthologue of mouse and human Dac/Dach (hereafter referred to as Dach1). We show that chick Dach1 is expressed in a variety of sites during embryonic development, including the eye and ear. Previous work has demonstrated the existence of a functional network and genetic regulatory hierarchy in Drosophila in which eyeless (ey, the Pax6 orthologue), eyes absent (eya), and dac operate together to regulate Drosophila eye development, and that ey regulates the expression of eya and dac. We find that in the developing eye of both chick and mouse, expression domains of Dach1 overlap with those of Pax6, a gene required for normal eye development. Similarly, in the developing ear of both mouse and chick, Dach1 expression overlaps with the expression of another Pax gene, Pax2. In the mouse, Dach1 expression in the developing ear also overlaps with the expression of Eya1 (an eya homologue). Both Pax2 and Eya1 are required for normal ear development. Our expression studies suggest that the Drosophila Pax-eya-dac regulatory network may be evolutionarily conserved such that Pax genes, Eya1, and Dach1 may function together in vertebrates to regulate neural development. To address the further possibility that a regulatory hierarchy exists between Pax, Eya, and Dach genes, we have examined the expression of mouse Dach1 in Pax6, Pax2 and Eya1 mutant backgrounds. Our results indicate that Pax6, Pax2, and Eya1 do not regulate Dach1 expression through a simple linear hierarchy.

Liarozole markedly increases all trans-retinoic acid toxicity in mouse limb bud cell cultures: a model to explain the potency of the aromatic retinoid (E)-4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthylenyl)-1-propenyl] benzoic acid

The remarkable toxicity of (E)-4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthylenyl)-1-propenyl] benzoic acid (TTNPB) compared to all trans-retinoic acid (tRA) is due to multiple factors, including reduced affinities for cytosolic binding proteins (CRABPs), resistance to metabolism, and prolonged nuclear receptor activation. To further investigate the role of half-life in retinoid toxicity, experiments were performed to determine whether, and to what extent, inhibition of tRA metabolism by liarozole increased its toxicity comparable to that of TTNPB in the mouse limb bud system. Liarozole is a known inhibitor of tRA 4-hydroxylation (CYP26). In the absence of liarozole, the IC50 for inhibition of chondrogenesis by tRA was 140 nM compared to 0.3 nM for TTNPB, a 467-fold difference. Following the addition of liarozole (10(-6) M) to limb bud cultures, the potency of tRA to inhibit chondrogenesis was increased approximately 14-fold (IC50 of 9.8 nM). Although liarozole markedly increased toxicity of tRA in mouse limb bud micromass cultures, tRA metabolism was inhibited only about 10%. These results indicate that a relatively minor decrease in the metabolism of tRA in the mouse limb bud system is associated with a marked enhancement of toxicity that is likely related to the prolongation of tRA half-life during a critical period of development. Thus, the prolonged half-life of TTNPB is the most significant factor contributing to the remarkable teratogenicity of this synthetic aromatic retinoid.

The transcription factor SOX9 regulates cell cycle and differentiation genes in chondrocytic CFK2 cells

SOX9 is a transcription factor that is essential for chondrocyte differentiation and cartilage formation. We stably overexpressed SOX9 cDNA in the rat chondrocytic cell line CFK2. Compared with the vector control, a greater proportion of SOX9-transfected cells accumulated in the G0/G1 phase. This was associated with an increase in mRNA and protein expression of p21(cip1), an inhibitor of cyclin-dependent kinase activity. SOX9 enhanced p21(cip1) promoter activity in a luciferase reporter assay. CFK2 cells overexpressing SOX9 became more elongated and adhesive and demonstrated a shift in cytoplasmic F-actin distribution. N-cadherin mRNA levels were elevated in the SOX9-transfected cells, and SOX9 enhanced N-cadherin promoter activity. By electrophoretic mobility shift assay, nuclear extracts of SOX9-transfected CFK2 cells specifically bound an oligonucleotide comprising an N-cadherin promoter region containing a consensus SOX9-binding motif. The transcriptional activity of SOX9 depended upon nuclear localization signals required for SOX9 nuclear entry. Differentiation of transfected CFK2 cells was accelerated as evidenced by more rapid accumulation of alkaline phosphatase activity, increased production of proteoglycans, and increased calcium accumulation, and this was associated with decreased ERK1 expression. These studies demonstrate that SOX9 alters the rate of cell cycle progression of chondrocytes and their differentiation by enhancing or inhibiting the expression of selected genes, including p21(cip1) and ERK1, and that N-cadherin is an additional direct target of this transcriptional regulator.

Defining the breakpoints of proximal chromosome 14q rearrangements in nine patients using flow-sorted chromosomes

The breakpoints of deletions and translocations in the proximal chromosome 14q region were defined in nine patients, four of whom have not been reported previously. The aberrant chromosomes were isolated by flow cytometry and used to map the chromosome 14 deletion or translocation breakpoints. The parental origins of deletions were ascertained as paternal in five cases and maternal in one. With the draft genomic sequence for human chromosome 14 available, gene searches were performed on selected intervals of the 14q11.2-q21 region to identify candidate genes for the observed phenotype in some of those affected. Gain of function of the gene PAX9 on chromosome 14 is a possible candidate for a t(14;18) patient affected with mesomelic bone dysplasia. Furthermore, a compilation of other human chromosome 14q proximal deletion and translocation cases was obtained from a search on cytogenetic databases. These findings suggest a locus for myelofibrosis at chromosome 14q13. This study contributes to useful information for identifying disease genes in this region.

All for one and one for all: condensations and the initiation of skeletal development

Condensation is the pivotal stage in the development of skeletal and other mesenchymal tissues. It occurs when a previously dispersed population of cells gathers together to differentiate into a single cell/tissue type such as cartilage, bone, muscle, tendon, kidney, and lung and is the earliest stage during organ formation when tissue-specific genes are upregulated. We present a synopsis of our current understanding of how condensations are initiated and grown, how their boundaries and sizes are set, how condensation ceases, and how overt differentiation begins. Extracellular matrix molecules, cell surface receptors and cell adhesion molecules, such as fibronectin, tenascin, syndecan, and N-CAM, initiate condensation formation and set condensation boundaries. Hox genes (Hoxd-11-13) and other transcription factors (CFKH-1, MFH-1, osf-2), modulate the proliferation of cells within condensations. Cell adhesion is ensured indirectly through Hox genes (Hoxa-2, Hoxd-13), and directly via cell adhesion molecules (N-CAM and N-cadherin). Subsequent growth of condensations is regulated by BMPs, which activate Pax-2, Hoxa-2 and Hoxd-11 among other genes. Growth of a condensation ceases when Noggin inhibits BMP signalling, setting the stage for transition to the next stage of skeletal development, namely overt cell differentiation. BioEssays 22:138-147, 2000.