Bapx1: an evolutionary conserved homologue of the Drosophila bagpipe homeobox gene is expressed in splanchnic mesoderm and the embryonic skeleton

In vitro induction of patterned branchial arch-like aggregate from human pluripotent stem cells

Early patterning of neural crest cells (NCCs) in the craniofacial primordium is important for subsequent development of proper craniofacial structures. However, because of the complexity of the environment of developing tissues, surveying the early specification and patterning of NCCs is difficult. In this study, we develop a simplified in vitro 3D model using human pluripotent stem cells to analyze the early stages of facial development. In this model, cranial NCC-like cells spontaneously differentiate from neural plate border-like cells into maxillary arch-like mesenchyme after a long-term culture. Upon the addition of EDN1 and BMP4, these aggregates are converted into a mandibular arch-like state. Furthermore, temporary treatment with EDN1 and BMP4 induces the formation of spatially separated domains expressing mandibular and maxillary arch markers within a single aggregate. These results suggest that this in vitro model is useful for determining the mechanisms underlying cell fate specification and patterning during early facial development.

The broad role of Nkx3.2 in the development of the zebrafish axial skeleton

The transcription factor Nkx3.2 (Bapx1) is an important chondrocyte maturation inhibitor. Previous Nkx3.2 knockdown and overexpression studies in non-mammalian gnathostomes have focused on its role in primary jaw joint development, while the function of this gene in broader skeletal development is not fully described. We generated a mutant allele of nkx3.2 in zebrafish with CRISPR/Cas9 and applied a range of techniques to characterize skeletal phenotypes at developmental stages from larva to adult, revealing loss of the jaw joint, fusions in bones of the occiput, morphological changes in the Weberian apparatus, and the loss or deformation of bony elements derived from basiventral cartilages of the vertebrae. Axial phenotypes are reminiscent of Nkx3.2 knockout in mammals, suggesting that the function of this gene in axial skeletal development is ancestral to osteichthyans. Our results highlight the broad role of nkx3.2 in zebrafish skeletal development and its context-specific functions in different skeletal elements.

Functional Duality of Chondrocyte Hypertrophy and Biomedical Application Trends in Osteoarthritis

Chondrocyte hypertrophy is one of the key indicators in the progression of osteoarthritis (OA). However, compared with other OA indications, such as cartilage collapse, sclerosis, inflammation, and protease activation, the mechanisms by which chondrocyte hypertrophy contributes to OA remain elusive. As the pathological processes in the OA cartilage microenvironment, such as the alterations in the extracellular matrix, are initiated and dictated by the physiological state of the chondrocytes, in-depth knowledge of chondrocyte hypertrophy is necessary to enhance our understanding of the disease pathology and develop therapeutic agents. Chondrocyte hypertrophy is a factor that induces OA progression; it is also a crucial factor in the endochondral ossification. This review elaborates on this dual functionality of chondrocyte hypertrophy in OA progression and endochondral ossification through a description of the characteristics of various genes and signaling, their mechanism, and their distinguishable physiological effects. Chondrocyte hypertrophy in OA progression leads to a decrease in chondrogenic genes and destruction of cartilage tissue. However, in endochondral ossification, it represents an intermediate stage at the process of differentiation of chondrocytes into osteogenic cells. In addition, this review describes the current therapeutic strategies and their mechanisms, involving genes, proteins, cytokines, small molecules, three-dimensional environments, or exosomes, against the OA induced by chondrocyte hypertrophy. Finally, this review proposes that the contrasting roles of chondrocyte hypertrophy are essential for both OA progression and endochondral ossification, and that this cellular process may be targeted to develop OA therapeutics.

Zebrafish model for spondylo-megaepiphyseal-metaphyseal dysplasia reveals post-embryonic roles of Nkx3.2 in the skeleton

The regulated expansion of chondrocytes within growth plates and joints ensures proper skeletal development through adulthood. Mutations in the transcription factor NKX3.2 underlie spondylo-megaepiphyseal-metaphyseal dysplasia (SMMD), which is characterized by skeletal defects including scoliosis, large epiphyses, wide growth plates and supernumerary distal limb joints. Whereas nkx3.2 knockdown zebrafish and mouse Nkx3.2 mutants display embryonic lethal jaw joint fusions and skeletal reductions, respectively, they lack the skeletal overgrowth seen in SMMD patients. Here, we report adult viable nkx3.2 mutant zebrafish displaying cartilage overgrowth in place of a missing jaw joint, as well as severe dysmorphologies of the facial skeleton, skullcap and spine. In contrast, cartilage overgrowth and scoliosis are absent in rare viable nkx3.2 knockdown animals that lack jaw joints, supporting post-embryonic roles for Nkx3.2. Single-cell RNA-sequencing and in vivo validation reveal increased proliferation and upregulation of stress-induced pathways, including prostaglandin synthases, in mutant chondrocytes. By generating a zebrafish model for the skeletal overgrowth defects of SMMD, we reveal post-embryonic roles for Nkx3.2 in dampening proliferation and buffering the stress response in joint-associated chondrocytes.

Uncovering the secreted signals and transcription factors regulating the development of mammalian middle ear ossicles

The mammalian middle ear comprises a chain of ossicles, the malleus, incus, and stapes that act as an impedance matching device during the transmission of sound from the tympanic membrane to the inner ear. These ossicles are derived from cranial neural crest cells that undergo endochondral ossification and subsequently differentiate into their final functional forms. Defects that occur during middle ear development can result in conductive hearing loss. In this review, we summarize studies describing the crucial roles played by signaling molecules such as sonic hedgehog, bone morphogenetic proteins, fibroblast growth factors, notch ligands, and chemokines during the differentiation of neural crest into the middle ear ossicles. In addition to these cell-extrinsic signals, we also discuss studies on the function of transcription factor genes such as Foxi3, Tbx1, Bapx1, Pou3f4, and Gsc in regulating the development and morphology of the middle ear ossicles.

The tissues and regulatory pattern of limb chondrogenesis

Initial limb chondrogenesis offers the first differentiated tissues that resemble the mature skeletal anatomy. It is a developmental progression of three tissues. The limb begins with undifferentiated mesenchyme-1, some of which differentiates into condensations-2, and this tissue then transforms into cartilage-3. Each tissue is identified by physical characteristics of cell density, shape, and extracellular matrix composition. Tissue specific regimes of gene regulation underlie the diagnostic physical and chemical properties of these three tissues. These three tissue based regimes co-exist amid a background of other gene regulatory regimes within the same tissues and time-frame of limb development. The bio-molecular indicators of gene regulation reveal six identifiable patterns. Three of these patterns describe the unique bio-molecular indicators of each of the three tissues. A fourth pattern shares bio-molecular indicators between condensation and cartilage. Finally, a fifth pattern is composed of bio-molecular indicators that are found in undifferentiated mesenchyme prior to any condensation differentiation, then these bio-molecular indicators are upregulated in condensations and downregulated in undifferentiated mesenchyme. The undifferentiated mesenchyme that remains in between the condensations and cartilage, the interdigit, contains a unique set of bio-molecular indicators that exhibit dynamic behaviour during chondrogenesis and therefore argue for its own inclusion as a tissue in its own right and for more study into this process of differentiation.

A Renewable Source of Human Beige Adipocytes for Development of Therapies to Treat Metabolic Syndrome

Molecular- and cellular-based therapies have the potential to reduce obesity-associated disease. In response to cold, beige adipocytes form in subcutaneous white adipose tissue and convert energy stored in metabolic substrates to heat, making them an attractive therapeutic target. We developed a robust method to generate a renewable source of human beige adipocytes from induced pluripotent stem cells (iPSCs). Developmentally, these cells are derived from FOXF1⁺ mesoderm and progress through an expandable mural-like mesenchymal stem cell (MSC) to form mature beige adipocytes that display a thermogenically active profile. This includes expression of uncoupling protein 1 (UCP1) concomitant with increased uncoupled respiration. With this method, dysfunctional adipogenic precursors can be reprogrammed and differentiated into beige adipocytes with increased thermogenic function and anti-diabetic secretion potential. This resource can be used to (1) elucidate mechanisms that underlie the control of beige adipogenesis and (2) generate material for cellular-based therapies that target metabolic syndrome in humans.

Su et al. demonstrate a method for producing beige adipocytes from human induced pluripotent stem cells in a stepwise manner through defined precursor lineages. This renewable resource provides a developmental framework to study human beige adipogenesis and can be used to develop treatments for obesity-related disorders.

Differential transactivation of the upstream aggrecan enhancer regulated by PAX1/9 depends on SOX9-driven transactivation

A previously identified enhancer 10 kb upstream of the Aggrecan (Acan) gene (UE) can drive cartilage specific reporter expression in vivo. Here, we report that the paralogous transcription factors PAX1 and PAX9 differentially drive UE, depending on the presence or absence of SOX9-driven transactivation. In the developing vertebral column, PAX1/9 expression was inversely correlated with Acan expression. Moreover, PAX1/9 was co-expressed with SOX9/5/6 in the intervertebral mesenchyme and the inner annulus fibrosus (AF), and with SOX9 in the outer AF. Significant Acan upregulation was observed during chondrification of Pax1-silenced AF cells, while, Acan was significantly downregulated by persistent expression of Pax1 in cartilage. Deletion of UE using CRISPR/Cas9 resulted in ~30% and ~40% reduction of Acan expression in cartilage and the AF, respectively. In the UE, PAX1/9 acts as weak transactivators through a PAX1/9-binding site partially overlapped with a SOX9-binding site. In the presence of SOX9, which otherwise drives robust Acan expression along with SOX5/6, PAX1/9 competes with SOX9 for occupancy of the binding site, resulting in reduced transactivation of Acan. Coimmunoprecipitation revealed the physical interaction of Pax1 with SOX9. Thus, transactivation of the UE is differentially regulated by concerted action of PAX1/9, SOX9, and SOX5/6 in a context-dependent manner.

Bapx1 upregulation is associated with ectopic mandibular cartilage development in amphibians

BACKGROUND

The emergence of novel structures during evolution is crucial for creating variation among organisms, but the underlying processes which lead to the emergence of evolutionary novelties are poorly understood. The gnathostome jaw joint is such a novelty, and the incorporation of bapx1 expression into the intermediate first pharyngeal arch may have played a major role in the evolution of this joint. Knockdown experiments revealed that loss of bapx1 function leads to the loss of the jaw joint, because Meckel's cartilage and the palatoquadrate fuse during development. We used Xenopus laevis and Ambystoma mexicanum to further investigate the function of bapx1 in amphibians. Bapx1 expression levels were upregulated through the use of Ly-294,002 and we investigated the potential consequences of the enhanced bapx1 expression in amphibians to test the hypothesized joint inducing function of bapx1.

RESULTS

We show that Ly-294,002 upregulates bapx1 expression in vivo. Additionally, ectopic mandibular arch derived cartilages develop after Ly-294,002 treatment. These ectopic cartilages are dorsoventrally oriented rods situated lateral to the palatoquadrate. The development of these additional cartilages did not change the muscular arrangement of mandibular arch-derived muscles.

CONCLUSIONS

Development of additional mandibular cartilages is not unusual in larval anurans. Therefore, changes in the bapx1 expression during evolution may have been the reason for the development of several additional cartilages in the larval anuran jaw. Furthermore, our observations imply a joint-promoting function of bapx1, which further substantiates its hypothetical role in the evolution of the gnathostome jaw joint.

Neonatal pancreatic pericytes support β-cell proliferation

OBJECTIVE

The maintenance and expansion of β-cell mass rely on their proliferation, which reaches its peak in the neonatal stage. β-cell proliferation was found to rely on cells of the islet microenvironment. We hypothesized that pericytes, which are components of the islet vasculature, support neonatal β-cell proliferation.

METHODS

To test our hypothesis, we combined in vivo and in vitro approaches. Briefly, we used a Diphtheria toxin-based transgenic mouse system to specifically deplete neonatal pancreatic pericytes in vivo. We further cultured neonatal pericytes isolated from the neonatal pancreas and combined the use of a β-cell line and primary cultured mouse β-cells.

RESULTS

Our findings indicate that neonatal pancreatic pericytes are required and sufficient for β-cell proliferation. We observed impaired proliferation of neonatal β-cells upon in vivo depletion of pancreatic pericytes. Furthermore, exposure to pericyte-conditioned medium stimulated proliferation in cultured β-cells.

CONCLUSIONS

This study introduces pancreatic pericytes as regulators of neonatal β-cell proliferation. In addition to advancing current understanding of the physiological β-cell replication process, these findings could facilitate the development of protocols aimed at expending these cells as a potential cure for diabetes.

Isolating and Analyzing Cells of the Pancreas Mesenchyme by Flow Cytometry

The pancreas is comprised of epithelial cells that are required for food digestion and blood glucose regulation. Cells of the pancreas microenvironment, including endothelial, neuronal, and mesenchymal cells were shown to regulate cell differentiation and proliferation in the embryonic pancreas. In the adult, the function and mass of insulin-producing cells were shown to depend on cells in their microenvironment, including pericyte, immune, endothelial, and neuronal cells. Lastly, changes in the pancreas microenvironment were shown to regulate pancreas tumorigenesis. However, the cues underlying these processes are not fully defined. Therefore, characterizing the different cell types that comprise the pancreas microenvironment and profiling their gene expression are crucial to delineate the tissue development and function under normal and diseased states. Here, we describe a method that allows for the isolation of mesenchymal cells from the pancreas of embryonic, neonatal, and adult mice. This method utilizes the enzymatic digestion of mouse pancreatic tissue and the subsequent fluorescence-activated cell sorting (FACS) or flow-cytometric analysis of labeled cells. Cells can be labeled by either immunostaining for surface markers or by the expression of fluorescent proteins. Cell isolation can facilitate the characterization of genes and proteins expressed in cells of the pancreas mesenchyme. This protocol was successful in isolating and culturing highly enriched mesenchymal cell populations from the embryonic, neonatal, and adult mouse pancreas.

Metamorphosis of the Drosophila visceral musculature and its role in intestinal morphogenesis and stem cell formation

The visceral musculature of the Drosophila intestine plays important roles in digestion as well as development. Detailed studies investigating the embryonic development of the visceral muscle exist; comparatively little is known about postembryonic development and metamorphosis of this tissue. In this study we have combined the use of specific markers with electron microscopy to follow the formation of the adult visceral musculature and its involvement in gut development during metamorphosis. Unlike the adult somatic musculature, which is derived from a pool of undifferentiated myoblasts, the visceral musculature of the adult is a direct descendant of the larval fibers, as shown by activating a lineage tracing construct in the larval muscle and obtaining labeled visceral fibers in the adult. However, visceral muscles undergo a phase of remodeling that coincides with the metamorphosis of the intestinal epithelium. During the first day following puparium formation, both circular and longitudinal syncytial fibers dedifferentiate, losing their myofibrils and extracellular matrix, and dissociating into mononuclear cells ("secondary myoblasts"). Towards the end of the second day, this process is reversed, and between 48 and 72h after puparium formation, a structurally fully differentiated adult muscle layer has formed. We could not obtain evidence that cells apart from the dedifferentiated larval visceral muscle contributed to the adult muscle, nor does it appear that the number of adult fibers (or nuclei per fiber) is increased over that of the larva by proliferation. In contrast to the musculature, the intestinal epithelium is completely renewed during metamorphosis. The adult midgut epithelium rapidly expands over the larval layer during the first few hours after puparium formation; in case of the hindgut, replacement takes longer, and proceeds by the gradual caudad extension of a proliferating growth zone, the hindgut proliferation zone (HPZ). The subsequent elongation of the hindgut and midgut, as well as the establishment of a population of intestinal stem cells active in the adult midgut and hindgut, requires the presence of the visceral muscle layer, based on the finding that ablation of this layer causes a severe disruption of both processes.

In vivo genome-wide analysis of multiple tissues identifies gene regulatory networks, novel functions and downstream regulatory genes for Bapx1 and its co-regulation with Sox9 in the mammalian vertebral column

Background

Vertebrate organogenesis is a highly complex process involving sequential cascades of transcription factor activation or repression. Interestingly a single developmental control gene can occasionally be essential for the morphogenesis and differentiation of tissues and organs arising from vastly disparate embryological lineages.

Results

Here we elucidated the role of the mammalian homeobox gene Bapx1 during the embryogenesis of five distinct organs at E12.5 - vertebral column, spleen, gut, forelimb and hindlimb - using expression profiling of sorted wildtype and mutant cells combined with genome wide binding site analysis. Furthermore we analyzed the development of the vertebral column at the molecular level by combining transcriptional profiling and genome wide binding data for Bapx1 with similarly generated data sets for Sox9 to assemble a detailed gene regulatory network revealing genes previously not reported to be controlled by either of these two transcription factors.

Conclusions

The gene regulatory network appears to control cell fate decisions and morphogenesis in the vertebral column along with the prevention of premature chondrocyte differentiation thus providing a detailed molecular view of vertebral column development.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1072) contains supplementary material, which is available to authorized users.

Small molecule-directed specification of sclerotome-like chondroprogenitors and induction of a somitic chondrogenesis program from embryonic stem cells

Pluripotent embryonic stem cells (ESCs) generate rostral paraxial mesoderm-like progeny in 5-6 days of differentiation induced by Wnt3a and Noggin (Nog). We report that canonical Wnt signaling introduced either by forced expression of activated β-catenin, or the small-molecule inhibitor of Gsk3, CHIR99021, satisfied the need for Wnt3a signaling, and that the small-molecule inhibitor of BMP type I receptors, LDN193189, was able to replace Nog. Mesodermal progeny generated using such small molecules were chondrogenic in vitro, and expressed trunk paraxial mesoderm markers such as Tcf15 and Meox1, and somite markers such as Uncx, but failed to express sclerotome markers such as Pax1. Induction of the osteochondrogenically committed sclerotome from somite requires sonic hedgehog and Nog. Consistently, Pax1 and Bapx1 expression was induced when the isolated paraxial mesodermal progeny were treated with SAG1 (a hedgehog receptor agonist) and LDN193189, then Sox9 expression was induced, leading to cartilaginous nodules and particles in the presence of BMP, indicative of chondrogenesis via sclerotome specification. By contrast, treatment with TGFβ also supported chondrogenesis and stimulated Sox9 expression, but failed to induce the expression of Pax1 and Bapx1. On ectopic transplantation to immunocompromised mice, the cartilage particles developed under either condition became similarly mineralized and formed pieces of bone with marrow. Thus, the use of small molecules led to the effective generation from ESCs of paraxial mesodermal progeny, and to their further differentiation in vitro through sclerotome specification into growth plate-like chondrocytes, a mechanism resembling in vivo somitic chondrogenesis that is not recapitulated with TGFβ.

Noncoding origins of anthropoid traits and a new null model of transposon functionalization

Little is known about novel genetic elements that drove the emergence of anthropoid primates. We exploited the sequencing of the marmoset genome to identify 23,849 anthropoid-specific constrained (ASC) regions and confirmed their robust functional signatures. Of the ASC base pairs, 99.7% were noncoding, suggesting that novel anthropoid functional elements were overwhelmingly cis-regulatory. ASCs were highly enriched in loci associated with fetal brain development, motor coordination, neurotransmission, and vision, thus providing a large set of candidate elements for exploring the molecular basis of hallmark primate traits. We validated ASC192 as a primate-specific enhancer in proliferative zones of the developing brain. Unexpectedly, transposable elements (TEs) contributed to >56% of ASCs, and almost all TE families showed functional potential similar to that of nonrepetitive DNA. Three L1PA repeat-derived ASCs displayed coherent eye-enhancer function, thus demonstrating that the "gene-battery" model of TE functionalization applies to enhancers in vivo. Our study provides fundamental insights into genome evolution and the origins of anthropoid phenotypes and supports an elegantly simple new null model of TE exaptation.

The role of Nkx3.2 in chondrogenesis

Transcription factor, Nkx3.2, is a member of the NK family of developmental genes and is expressed during embryogenesis in a variety of mammalian model organisms, including chicken and mouse. It was first identified in Drosophila as the Bagpipe (bap) gene, where it has been demonstrated to be essential during formation of the midgut musculature. However, mammalian homolog Nkx3.2 has been shown to play a significant role in axial and limb skeletogenesis; in particular, the human skeletal disease, spondylo-megaepiphyseal-metaphyseal dysplasia (SMMD), is associated with mutations of the Nkx3.2 gene. In this review, we highlight the role of Nkx3.2 during musculoskeletal development, with an emphasis on the factor's role in determining chondrogenic cell fate and its subsequent role in endochondral ossification and chondrocyte survival.

Genome-wide linkage study suggests a susceptibility locus for isolated bilateral microtia on 4p15.32-4p16.2

Microtia is a congenital deformity where the external ear is underdeveloped. Genetic investigations have identified many susceptibility genes of microtia-related syndromes. However, no causal genes were reported for isolated microtia, the main form of microtia. We conducted a genome-wide linkage analysis on a 5-generation Chinese pedigree with isolated bilateral microtia. We identified a suggestive linkage locus on 4p15.32-4p16.2 with parametric LOD score of 2.70 and nonparametric linkage score (Zmean) of 12.28 (simulated occurrence per genome scan equal to 0.46 and 0.47, respectively). Haplotype reconstruction analysis of the 4p15.32-4p16.2 region further confined the linkage signal to a 10-Mb segment located between rs12505562 and rs12649803 (9.65-30.24 cM; 5.54-15.58 Mb). Various human organ developmental genes reside in this 10-Mb susceptibility region, such as EVC, EVC2, SLC2A9, NKX3-2, and HMX1. The coding regions of three genes, EVC known for cartilage development and NKX3-2, HMX1 involved in microtia, were selected for sequencing with 5 individuals from the pedigree. Of the 38 identified sequence variants, none segregates along with the disease phenotype. Other genes or DNA sequences of the 10-Mb region warrant for further investigation. In conclusion, we report a susceptibility locus of isolated microtia, and this finding will encourage future studies on the genetic basis of ear deformity.

Pax1 acts as a negative regulator of chondrocyte maturation

Paired box gene 1 (Pax1) indirectly promotes the early stages of chondrogenic differentiation through induction and transactivation of Nk3 homeobox 2 (Nkx3.2), a transcriptional repressor. Later in chondrogenic differentiation, Nkx3.2 blocks chondrocyte hypertrophy by repressing Runt-related transcription factor 2 (Runx2). Here we report the inhibitory action of Pax1 on chondrocyte maturation, independently of Nkx3.2. Upon cartilage formation, Pax1 expression in the ventral sclerotome was gradually decreased except for the perichondrial region of the vertebral bodies and the intervertebral region, both of which express SRY-box containing gene 9 (Sox9). Forced expression of Pax1 in the chick forelimb resulted in the formation of shortened skeletal elements with a significant reduction of proteoglycans (PGs) accumulation in cartilage as well as a lack of the cortical bone formation and vascular invasion into the primary ossification center. Pax1-misexpressing chondrocytes exhibited aberrant cell morphology with a marked downregulation of Aggrecan (Agc1). Pax1-misexpressing cultured chondrocytes failed to accumulate cartilaginous PGs and became fibroblastic, in association with downregulation of the expression of Sox9, Nkx3.2, Indian hedgehog (Ihh), type II collagen (Col2a1), Chondromodulin-1 (Chm1), and Agc1. Accumulation of cartilaginous PGs in chondrocytes was also reduced by forced expression of Pax1 and Sox9. Thus, chondrocyte maturation driven by Sox9 is antagonized by Pax1 that is downregulated during chondrogenic differentiation.

Ectopic activation of the canonical wnt signaling pathway affects ectodermal patterning along the primary axis during larval development in the anthozoan Nematostella vectensis

The primary axis of cnidarians runs from the oral pole to the apical tuft and defines the major body axis of both the planula larva and adult polyp. In the anthozoan cnidarian Nematostella vectensis, the primary oral-aboral (O-Ab) axis first develops during the early embryonic stage. Here, we present evidence that pharmaceutical activators of canonical wnt signaling affect molecular patterning along the primary axis of Nematostella. Although not overtly morphologically complex, molecular investigations in Nematostella reveal that the O-Ab axis is demarcated by the expression of differentially localized signaling molecules and transcription factors that may serve roles in establishing distinct ectodermal domains. We have further characterized the larval epithelium by determining the position of a nested set of molecular boundaries, utilizing several newly characterized as well as previously reported epithelial markers along the primary axis. We have assayed shifts in their position in control embryos and in embryos treated with the pharmacological agents alsterpaullone and azakenpaullone, Gsk3β inhibitors that act as canonical wnt agonists, and the Wnt antagonist iCRT14, following gastrulation. Agonist drug treatments result in an absence of aboral markers, a shift in the expression boundaries of oral markers toward the aboral pole, and changes in the position of differentially localized populations of neurons in a dose-dependent manner, while antagonist treatment had the opposite effect. These experiments are consistent with canonical wnt signaling playing a role in an orally localized wnt signaling center. These findings suggest that in Nematostella, wnt signaling mediates O-Ab ectodermal patterning across a surprisingly complex epithelium in planula stages following gastrulation in addition to previously described roles for the wnt signaling pathway in endomesoderm specification during gastrulation and overall animal-vegetal patterning at earlier stages of anthozoan development.

Distinct spatiotemporal roles of hedgehog signalling during chick and mouse cranial base and axial skeleton development

The cranial base exerts a supportive role for the brain and includes the occipital, sphenoid and ethmoid bones that arise from cartilaginous precursors in the early embryo. As the occipital bone and the posterior part of the sphenoid are mesoderm derivatives that arise in close proximity to the notochord and floor plate, it has been assumed that their development, like the axial skeleton, is dependent on Sonic hedgehog (Shh) and modulation of bone morphogenetic protein (Bmp) signalling. Here we examined the development of the cranial base in chick and mouse embryos to compare the molecular signals that are required for chondrogenic induction in the trunk and head. We found that Shh signalling is required but the molecular network controlling cranial base development is distinct from that in the trunk. In the absence of Shh, the presumptive cranial base did not undergo chondrogenic commitment as determined by the loss of Sox9 expression and there was a decrease in cell survival. In contrast, induction of the otic capsule occurred normally demonstrating that induction of the cranial base is uncoupled from formation of the sensory capsules. Lastly, we found that the early cranial mesoderm is refractory to Shh signalling, likely accounting for why development of the cranial base occurs after the axial skeleton. Our data reveal that cranial and axial skeletal induction is controlled by conserved, yet spatiotemporally distinct mechanisms that co-ordinate development of the cranial base with that of the cranial musculature and the pharyngeal arches.

New Bapx1(Cre-EGFP) mouse lines for lineage tracing and conditional knockout studies

To gain insight into the roles of various genes in development and to circumvent embryonic lethality that hinders genetic studies, lineage tracing and conditional knockout techniques have been widely performed on mice using the increasing numbers of gene-targeted Cre mouse lines. Employing the internal ribosome entry site (IRES) and the 2A peptide multicistronic expression strategies, we report two new Bapx1 mouse lines with functional Bapx1 whereby Cre and enhanced green fluorescence protein (EGFP) are expressed discretely under the control of the Bapx1 promoter. These mouse lines, when mated with the Rosa26R-lacZ reporter line, can be used to trace the lineage of Bapx1-expressing cells whereas stage-specific, spatial expression of Bapx1 can be visualized by the EGFP fluorescence. In addition, both of our Bapx1(Cre-EGFP) mouse lines can be used to enrich for Bapx1-specific cells and also serve as effective conditional knockout tools to investigate gene functions in the skeleton and/or visceral organs.

Comparison of IRES and F2A-based locus-specific multicistronic expression in stable mouse lines

Efficient and stoichiometric expression of genes concatenated by bi- or multi-cistronic vectors has become an invaluable tool not only in basic biology to track and visualize proteins in vivo, but also for vaccine development and in the clinics for gene therapy. To adequately compare, in vivo, the effectiveness of two of the currently popular co-expression strategies - the internal ribosome entry site (IRES) derived from the picornavirus and the 2A peptide from the foot-and-mouth disease virus (FDMV) (F2A), we analyzed two locus-specific knock-in mouse lines co-expressing SRY-box containing gene 9 (Sox9) and enhanced green fluorescent protein (EGFP) linked by the IRES (Sox9^IRES-EGFP) or the F2A (Sox9^F2A-EGFP) sequence. Both the constructs expressed Sox9 and EGFP proteins in the appropriate Sox9 expression domains, with the IRES construct expressing reduced levels of EGFP compared to that of the F2A. The latter, on the other hand, produced about 42.2% Sox9-EGFP fusion protein, reflecting an inefficient ribosome ‘skipping’ mechanism. To investigate if the discrepancy in the ‘skipping’ process was locus-dependent, we further analyzed the FLAG₃-Bapx1^F2A-EGFP mouse line and found similar levels of fusion protein being produced. To assess if EGFP was hindering the ‘skipping’ mechanism, we examined another mouse line co-expressing Bagpipe homeobox gene 1 homolog (Bapx1), Cre recombinase and EGFP (Bapx1^{F2A-Cre-F2A-EGFP}). While the ‘skipping’ was highly efficient between Bapx1 and Cre, the ‘skipping’ between Cre and EGFP was highly inefficient. We have thus demonstrated in our comparison study that the efficient and close to equivalent expression of genes linked by F2A is achievable in stable mouse lines, but the EGFP reporter may cause undesirable inhibition of the ‘skipping’ at the F2A sequence. Hence, the use of other reporter genes should be explored when utilizing F2A peptides.

Pancreatic mesenchyme regulates epithelial organogenesis throughout development

Genetic disruption of the pancreatic mesenchyme reveals that it is critical for the expansion of epithelial progenitors and for the proliferation of insulin-producing beta cells.

The developing pancreatic epithelium gives rise to all endocrine and exocrine cells of the mature organ. During organogenesis, the epithelial cells receive essential signals from the overlying mesenchyme. Previous studies, focusing on ex vivo tissue explants or complete knockout mice, have identified an important role for the mesenchyme in regulating the expansion of progenitor cells in the early pancreas epithelium. However, due to the lack of genetic tools directing expression specifically to the mesenchyme, the potential roles of this supporting tissue in vivo, especially in guiding later stages of pancreas organogenesis, have not been elucidated. We employed transgenic tools and fetal surgical techniques to ablate mesenchyme via Cre-mediated mesenchymal expression of Diphtheria Toxin (DT) at the onset of pancreas formation, and at later developmental stages via in utero injection of DT into transgenic mice expressing the Diphtheria Toxin receptor (DTR) in this tissue. Our results demonstrate that mesenchymal cells regulate pancreatic growth and branching at both early and late developmental stages by supporting proliferation of precursors and differentiated cells, respectively. Interestingly, while cell differentiation was not affected, the expansion of both the endocrine and exocrine compartments was equally impaired. To further elucidate signals required for mesenchymal cell function, we eliminated β-catenin signaling and determined that it is a critical pathway in regulating mesenchyme survival and growth. Our study presents the first in vivo evidence that the embryonic mesenchyme provides critical signals to the epithelium throughout pancreas organogenesis. The findings are novel and relevant as they indicate a critical role for the mesenchyme during late expansion of endocrine and exocrine compartments. In addition, our results provide a molecular mechanism for mesenchymal expansion and survival by identifying β-catenin signaling as an essential mediator of this process. These results have implications for developing strategies to expand pancreas progenitors and β-cells for clinical transplantation.

Embryonic development is a highly complex process that requires tight orchestration of cellular proliferation, differentiation, and migration as cells grow within loosely aggregated mesenchyme and more organized epithelial sheets to form organs and tissues. In addition to intrinsic cell-autonomous signals, these events are further regulated by environmental cues provided by neighboring cells. Prior work demonstrated a critical role for the surrounding mesenchyme in guiding epithelial growth during the early stages of pancreas development. However, it remained unclear whether the mesenchyme also guided the later stages of pancreas organogenesis when the functional exocrine and endocrine cells are formed. Here, we show that specific genetic ablation of the mesenchyme at distinct developmental stages in vivo results in the formation of a smaller, misshapen pancreas. Loss of the mesenchyme profoundly impairs the expansion of both endocrine and exocrine pancreatic progenitors, as well as the proliferative capacity of maturing cells, including insulin-producing beta-cells. Thus, our studies reveal unappreciated roles for the mesenchyme in guiding the formation of the epithelial pancreas throughout development. The results suggest that identifying the specific mesenchymal signals might help to optimize cell culture protocols that aim to achieve the differentiation of stem cells into insulin-producing beta cells.

A role for FoxN3 in the development of cranial cartilages and muscles in Xenopus laevis (Amphibia: Anura: Pipidae) with special emphasis on the novel rostral cartilages

The origin of morphological novelties is a controversial topic in evolutionary developmental biology. The heads of anuran larvae have several unique structures, including the supra- and infrarostral cartilages, the specialised structure of the gill basket (used for filtration), and novel cranial muscle arrangements. FoxN3, a member of the forkhead/winged helix family of transcription factors, has been implicated as important for normal craniofacial development in the pipid anuran Xenopus laevis. We have investigated the effects of functional knockdown of FoxN3 (using antisense oligonucleotide morpholino) on the development of the larval head skeleton and the associated cranial muscles in X. laevis. Our data complement earlier studies and provide a more complete account of the requirement of FoxN3 in chondrocranium development. In addition, we analyse the effects of FoxN3 knockdown on cranial muscle development. We show that FoxN3 knockdown primarily affects the novel skeletal structures unique to anuran larvae, i.e. the rostralia or the fine structure of the gill apparatus. The articulation between the infrarostral and Meckel's cartilage is malformed and the filigreed processes of the gill basket do not develop. Because these features do not develop after FoxN3 knockdown, the head morphology resembles that in the less specialised larvae of salamanders. Furthermore, the development of all cartilages derived from the neural crest is delayed and cranial muscle fibre development incomplete. The cartilage precursors initially condense in their proper position but later differentiate incompletely; several visceral arch muscles start to differentiate at their origin but fail to extend toward their insertion. Our findings indicate that FoxN3 is essential for the development of novel cartilages such as the infrarostral and other cranial tissues derived from the neural crest and, indirectly, also for muscle morphogenesis.

Serotonin 2B receptor signaling is required for craniofacial morphogenesis and jaw joint formation in Xenopus

Serotonin (5-HT) is a neuromodulator that plays many different roles in adult and embryonic life. Among the 5-HT receptors, 5-HT2B is one of the key mediators of 5-HT functions during development. We used Xenopus laevis as a model system to further investigate the role of 5-HT2B in embryogenesis, focusing on craniofacial development. By means of gene gain- and loss-of-function approaches and tissue transplantation assays, we demonstrated that 5-HT2B modulates, in a cell-autonomous manner, postmigratory skeletogenic cranial neural crest cell (NCC) behavior without altering early steps of cranial NCC development and migration. 5-HT2B overexpression induced the formation of an ectopic visceral skeletal element and altered the dorsoventral patterning of the branchial arches. Loss-of-function experiments revealed that 5-HT2B signaling is necessary for jaw joint formation and for shaping the mandibular arch skeletal elements. In particular, 5-HT2B signaling is required to define and sustain the Xbap expression necessary for jaw joint formation. To shed light on the molecular identity of the transduction pathway acting downstream of 5-HT2B, we analyzed the function of phospholipase C beta 3 (PLC) in Xenopus development and showed that PLC is the effector of 5-HT2B during craniofacial development. Our results unveiled an unsuspected role of 5-HT2B in craniofacial development and contribute to our understanding of the interactive network of patterning signals that is involved in the development and evolution of the vertebrate mandibular arch.

Regulation of the chondrogenic phenotype in culture

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

Left-right asymmetry in gut development: what happens next?

The gastrointestinal tract is an asymmetrically patterned organ system. The signals which initiate left-right asymmetry in the developing embryo have been extensively studied, but the downstream steps required to confer asymmetric morphogenesis on the gut organ primordia are less well understood. In this paper we outline key findings on the tissue mechanics underlying gut asymmetry, across a range of species, and use these to synthesise a conserved model for asymmetric gut morphogenesis. We also discuss the importance of correct establishment of left-right asymmetry for gut development and the consequences of perturbations in this process.

Role of the homeodomain transcription factor Bapx1 in mouse distal stomach development

BACKGROUND & AIMS

Expansion and patterning of the endoderm generate a highly ordered, multiorgan digestive system in vertebrate animals. Among distal foregut derivatives, the gastric corpus, antrum, pylorus, and duodenum are distinct structures with sharp boundaries. Some homeodomain transcription factors expressed in gut mesenchyme convey positional information required for anterior-posterior patterning of the digestive tract. Barx1, in particular, controls stomach differentiation and morphogenesis. The Nirenberg and Kim homeobox gene Bapx1 (Nkx3-2) has an established role in skeletal development, but its function in the mammalian gut is less clear.

METHODS

We generated a Bapx1(Cre) knock-in allele to fate map Bapx1-expressing cells and evaluate its function in gastrointestinal development.

RESULTS

Bapx1-expressing cells populate the gut mesenchyme with a rostral boundary in the hindstomach near the junction of the gastric corpus and antrum. Smooth muscle differentiation and distribution of early regional markers are ostensibly normal in Bapx1(Cre/Cre) gut, but there are distinctive morphologic abnormalities near this rostral Bapx1 domain: the antral segment of the stomach is markedly shortened, and the pyloric constriction is lost. Comparison of expression domains and examination of stomach phenotypes in single and compound Barx1 and Bapx1 mutant mice suggests a hierarchy between these 2 factors; Bapx1 expression is lost in the absence of Barx1.

CONCLUSIONS

This study reveals the nonredundant requirement for Bapx1 in distal stomach development, places it within a Barx1-dependent pathway, and illustrates the pervasive influence of gut mesenchyme homeobox genes on endoderm differentiation and digestive organogenesis.

Sox9 directly promotes Bapx1 gene expression to repress Runx2 in chondrocytes

The transcription factor, Sry-related High Mobility Group (HMG) box containing gene 9 (Sox9), plays a critical role in cartilage development by initiating chondrogenesis and preventing the subsequent maturation process called chondrocyte hypertrophy. This suppression mechanism by Sox9 on late-stage chondrogenesis partially results from the inhibition of Runt-related transcription factor 2 (Runx2), the main activator of hypertrophic chondrocyte differentiation. However, the precise mechanism by which Sox9 regulates late chondrogenesis is poorly understood. In the present study, the transcriptional repressor vertebrate homolog of Drosophila bagpipe (Bapx1) was found to be a direct target of Sox9 for repression of Runx2 expression in chondrocytes. We identified a critical Sox9 responsive region in the Bapx1 promoter via a luciferase reporter assay. Analysis by chromatin immunoprecipitation and electrophoretic mobility shift assays indicated that Sox9 physically bound to this region of the Bapx1 promoter. Consistent with the notion that Bapx1 and Sox9 act as negative regulators of chondrocyte hypertrophy by regulating Runx2 expression, transient knockdown of Sox9 or Bapx1 expression by shRNA in chondrocytes increased Runx2 expression, as well as expression of the late chondrogenesis marker, Col10a1. Furthermore, while over-expression of Sox9 decreased Runx2 and Col10a1 expressions, simultaneous transient knockdown of Bapx1 diminished that Sox9 over-expressing effect. Our findings reveal that the molecular pathway modulated by Bapx1 links two major regulators in chondrogenesis, Sox9 and Runx2, to coordinate skeletal formation.

Integrating differentiation and cancer: the Nkx3.1 homeobox gene in prostate organogenesis and carcinogenesis

Several tissue-specific regulatory genes have been found to play essential roles in both organogenesis and carcinogenesis. In the prostate, the Nkx3.1 homeobox gene plays an important role in normal differentiation of the prostatic epithelium while its loss of function is an initiating event in prostate carcinogenesis in both mouse models and human patients. Thus, the Nkx3.1 homeobox gene provides a paradigm for understanding the relationship between normal differentiation and cancer, as well as studying the roles of homeobox genes in these processes. Here, we review recent findings concerning the roles of Nkx3.1 in development and discuss how its normal function is disrupted in processes of early prostate carcinogenesis.

Gene expression patterns underlying proximal-distal skeletal segmentation in late-stage zebrafish, Danio rerio

Timing and pattern of expression of ten candidate segmentation genes or gene pairs were reviewed or examined in developing median fins of late-stage zebrafish, Danio rerio. We found a general correspondence in timing and pattern of expression between zebrafish fin radial segmentation and tetrapod joint development, suggesting that molecular mechanisms underlying radial segmentation have been conserved over 400 million years of evolution. Gene co-expression during segmentation (5.5-6.5 mm SL) is similar between tetrapods and zebrafish: bmp2b, bmp4, chordin, and gdf5 in interradial mesenchyme and ZS; bapx1, col2a1, noggin3, and sox9a in chondrocytes. Surprisingly, wnt9a is not expressed in the developing median fins, though wnt9b is detected. In contrast to all other candidate segmentation genes we examined, bapx1 is not expressed in the caudal fin, which does not segment. Together, these data suggest a scenario of gene interactions underlying radial segmentation based on the patterns and timing of candidate gene expression.

Complementary striped expression patterns of NK homeobox genes during segment formation in the annelid Platynereis

NK genes are related pan-metazoan homeobox genes. In the fruitfly, NK genes are clustered and involved in patterning various mesodermal derivatives during embryogenesis. It was therefore suggested that the NK cluster emerged in evolution as an ancestral mesodermal patterning cluster. To test this hypothesis, we cloned and analysed the expression patterns of the homologues of NK cluster genes Msx, NK4, NK3, Lbx, Tlx, NK1 and NK5 in the marine annelid Platynereis dumerilii, a representative of trochozoans, the third great branch of bilaterian animals alongside deuterostomes and ecdysozoans. We found that most of these genes are involved, as they are in the fly, in the specification of distinct mesodermal derivatives, notably subsets of muscle precursors. The expression of the homologue of NK4/tinman in the pulsatile dorsal vessel of Platynereis strongly supports the hypothesis that the vertebrate heart derived from a dorsal vessel relocated to a ventral position by D/V axis inversion in a chordate ancestor. Additionally and more surprisingly, NK4, Lbx, Msx, Tlx and NK1 orthologues are expressed in complementary sets of stripes in the ectoderm and/or mesoderm of forming segments, suggesting an involvement in the segment formation process. A potentially ancient role of the NK cluster genes in segment formation, unsuspected from vertebrate and fruitfly studies so far, now deserves to be investigated in other bilaterian species, especially non-insect arthropods and onychophorans.

Transcription factors Mesp2 and Paraxis have critical roles in axial musculoskeletal formation

Mesp2 and Paraxis are basic helix-loop-helix (bHLH) -type transcription factors coexpressed in the presomitic mesoderm (PSM) and are required for normal somite formation. Here, we show that Mesp2/Paraxis double-null mice exhibit a distinct phenotype unexpected from either Mesp2 or Paraxis single-null mice. In the posterior region of the body, most of the skeletal components of both the vertebral body and neural arches are severely reduced and only a rudimental lamina and ribs remain, indicating a strong genetic interaction in the sclerotomal cell lineage. However, yeast two-hybrid analyses revealed no direct interaction between Mesp2 and Paraxis. The Mesp2/Paraxis double-null embryo has caudalized somites, revealed by expanded Uncx4.1 expression pattern observed in the Mesp2-null embryo, but the expression level of Uncx4.1 was significantly decreased in mature somites, indicative of hypoplasia of lateral sclerotome derivatives. By focusing on vertebral column formation, we found that expressions of Pax1, Nkx3.1, and Bapx1 are regulated by Paraxis and that Pax9 expression was severely affected in the Mesp2/Paraxis double-null embryo. Furthermore, the expression of Pax3, a crucial factor for hypaxial muscle differentiation, is regulated by both Mesp2 and Paraxis in the anteriormost PSM and nascent somite region. The present data strongly suggest that patterning events by bHLH-type transcription factors have deep impacts on regional chondrogenic and myogenic differentiation of somitic cells, mainly by means of control of Pax genes.

Development and function of the mammalian spleen

The vertebrate spleen has important functions in immunity and haematopoiesis, many of which have been well studied. In contrast, we know much less about the mechanisms governing its early embryonic development. However, as a result of work over the past decade-mostly using knockout mice--significant progress has been made in unravelling the genetic processes governing the spleen's early development. Key genetic regulators, such as Tlx1 and Pbx1, have been identified, and we know some of the early transcriptional hierarchies that control the early patterning and proliferation of the splenic primordium. In mouse and humans, asplenia can arise as a result of laterality defects, or the spleen can be absent with no other discernible abnormalities. Surprisingly, given the spleen's diverse functions, asplenic individuals suffer no major haematopoietic or immune defects apart from a susceptibility to infection with encapsulated bacteria. Recent evidence has shed light on a previously unknown role of the spleen in the development and maintenance of specific B cell populations that are involved in the initial response to infection caused by encapsulated bacteria. The lack of these populations in asplenic mice and humans may go some way to explaining this susceptibility.

Expression of an Nkx3.1-CRE gene using ROSA26 reporter mice

The genetic locus of Nkx3.1, an early murine marker of sclerotome and prostate development, was disrupted by a knock in of CRE recombinase via homologous recombination in embryonic stem cells. Cell fate mapping revealed previously unidentified cell lineages expanded from Nkx3.1-expressing cell populations and recapitulated reported Nkx3.1 expression patterns. In lineage trace experiments of E18.5 Nkx3.1-CRE; R26R embryos novel staining was observed in areas of the lungs, portions of the duodenum, and vertebral elements of the skeleton. beta-galactosidase activity measured in Nkx3.1-CRE; R26R and Nkx3.2-CRE; R26R embryos was observed in overlapping regions of the sclerotome but no apparent change in Nkx3.1 expression was seen in the Nkx3.2 mutants by in situ hybridization.

Bapx1 homeobox gene gain-of-function mice show preaxial polydactyly and activated Shh signaling in the developing limb

To explore Bapx1 homeobox gene function in embryonic control of development, we employed a gain-of-function approach to complement our previous loss-of-function mutant analysis. We show that transgenic mice overexpressing Bapx1 are affected by skeletal defects including hindlimb preaxial polydactyly and tibial hypoplasia. Bapx1 overexpression generates limb anteroposterior patterning defects including induction of Shh signaling and ectopic activation of functions downstream of Shh signaling into the anterior region of the autopod. Moreover, Bapx1 overexpression stimulates formation of limb prechondrogenic condensations. We also show that Shh is reciprocally able to activate Bapx1 expression in mouse embryos as the orthologous hedgehog (hh) does with the bagpipe/Bapx1 gene in Drosophila. Our results indicate that Bapx1 can modulate appendicular skeletal formation, that the genetic hierarchy between Shh/hh and Bapx1/bagpipe has been conserved during evolution, and that in mouse embryos these two genes can influence one another in a genetically reciprocal manner. We conclude that it is reasonable to expect overexpression of Bapx1 in certain forms of polydactyly.

Histone acetylation dependent allelic expression imbalance of BAPX1 in patients with the oculo-auriculo-vertebral spectrum

The oculo-auriculo-vertebral spectrum (OAVS) (OMIM % 164210) is a common developmental disorder characterized by hemifacial microsomia, epibulbar tumours, ear malformation and vertebral anomalies. Although rare familial cases suggest that OAVS has a genetic basis, no genetic defect has been identified so far. In a patient with OAVS and a chromosomal translocation t(4;8) we have found that the chromosome 4 breakpoint is 76.4 kb distal to the BAPX1 gene, which plays an essential role in craniofacial development. We did not detect any BAPX1 mutation in 105 patients, but observed a strong allelic expression imbalance (sAEI) in fibroblasts from five of 12 patients, but not in nine normal controls (Fisher's exact test, P=0.038). sAEI was de novo in one patient and inherited in two other patients. Prolonged cell culture or treatment with the histone deacetylase inhibitor Trichostatin A led to reactivation of the downregulated allele. We propose that epigenetic dysregulation of BAPX1 plays an important role in OAVS.

A Pbx1-dependent genetic and transcriptional network regulates spleen ontogeny

The genetic control of cell fate specification, morphogenesis and expansion of the spleen, a crucial lymphoid organ, is poorly understood. Recent studies of mutant mice implicate various transcription factors in spleen development, but the hierarchical relationships between these factors have not been explored. In this report, we establish a genetic network that regulates spleen ontogeny, by analyzing asplenic mice mutant for the transcription factors Pbx1, Hox11 (Tlx1), Nkx3.2 (Bapx1) and Pod1 (capsulin, Tcf21). We show that Hox11 and Nkx2.5, among the earliest known markers for splenic progenitor cells, are absent in the splenic anlage of Pbx1 homozygous mutant (-/-) embryos, implicating the TALE homeoprotein Pbx1 in splenic cell specification. Pbx1 and Hox11 genetically interact in spleen formation and loss of either is associated with a similar reduction of progenitor cell proliferation and failed expansion of the splenic anlage. Chromatin immunoprecipitation assays show that Pbx1 binds to the Hox11 promoter in spleen mesenchymal cells, which co-express Pbx1 and Hox11. Furthermore, Hox11 binds its own promoter in vivo and acts synergistically with TALE proteins to activate transcription, supporting its role in an auto-regulatory circuit. These studies establish a Pbx1-Hox11-dependent genetic and transcriptional pathway in spleen ontogeny. Additionally, we demonstrate that while Nkx3.2 and Pod1 control spleen development via separate pathways, Pbx1 genetically regulates key players in both pathways, and thus emerges as a central hierarchical co-regulator in spleen genesis.

The molecular mechanisms of stomach development in vertebrates

The tissue interactions between endodermal epithelium and mesenchyme originated from splanchnic mesoderm are essential during the formation of digestive tract. In this review, we introduce a series of works to elucidate the molecular mechanisms of the epithelial-mesenchymal interaction of stomach development in mainly the chicken embryo. We also describe some molecular studies in mouse stomach development.

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.

Transcriptional control of chondrocyte fate and differentiation

Chondrogenesis is an essential process in vertebrates. It leads to the formation of cartilage growth plates, which drive body growth and have primary roles in endochondral ossification. It also leads to the formation of permanent cartilaginous tissues that provide major structural support in the articular joints and respiratory and auditory tracts throughout life. Defects in chondrogenesis cause chondrodysostoses and chondrodysplasias. These skeletal malformation diseases account for a significant proportion of birth defects in humans and can dramatically affect a person's expectancy and quality of life. Chondrogenesis occurs when pluripotent mesenchymal cells commit to the chondrocyte lineage, and through a series of differentiation steps build and eventually remodel cartilage. This review summarizes and discusses our current knowledge and lack of knowledge about the chondrocyte differentiation pathway, from mesenchymal cells to growth plate and articular chondrocytes, with a main focus on how it is controlled by tissue patterning and cell differentiation transcription factors, such as, but not limited to, Pax1 and Pax9, Nkx3.1 and Nkx3.2, Sox9, Sox5 and Sox6, Runx2 and Runx3, and c-Maf.

The splanchnic mesodermal plate directs spleen and pancreatic laterality, and is regulated by Bapx1/Nkx3.2

The mechanism by which left-right (LR) information is interpreted by organ primordia during asymmetric morphogenesis is largely unknown. We show that spleen and pancreatic laterality is dependent on a specialised, columnar mesodermal-derived cell layer referred to here as the splanchnic mesodermal plate (SMP). At early embryonic stages, the SMP is bilateral, surrounding the midline-located stomach and dorsal pancreatic bud. Under control of the LR asymmetry pathway, the left SMP is maintained and grows laterally. Mice carrying the dominant hemimelia (Dh) mutation lack the SMP. Significantly, the mice are asplenic and the pancreas remains positioned along the embryonic midline. In the absence of Fgf10 expression, the spleno-pancreatic mesenchyme and surrounding SMP grow laterally but contain no endodermal component, showing that leftward growth is autonomous and independent of endoderm. In the Bapx1(-/-) mutants, the SMP is defective. Normally, the SMP is a source for both Fgf9 and Fgf10; however, in the Bapx1 mutant, Fgf10 expression is downregulated and the dorsal pancreas remains at the midline. We conclude that the SMP is an organiser responsible for the leftward growth of the spleno-pancreatic region and that Bapx1 regulates SMP functions required for pancreatic laterality.

Roles of the Nkx3.1 homeobox gene in prostate organogenesis and carcinogenesis

Although it is often presumed that the molecular pathways that underlie normal organogenesis are similar to those perturbed during carcinogenesis, few examples exist of tissue-specific regulatory genes that play central roles in both processes. In the case of the prostate gland, molecular genetic analyses have demonstrated that the Nkx3.1 homeobox gene plays an important role in normal differentiation of the prostatic epithelium and that its loss of function is an initiating event in prostate carcinogenesis. Thus, the Nkx3.1 homeobox gene provides a paradigm for understanding the relationship between normal differentiation and cancer, as well as a model for studying the roles of homeobox genes in these processes. Here, we review recent findings concerning the biological as well as biochemical function of this central regulator of prostate development and carcinogenesis.

Meox homeodomain proteins are required for Bapx1 expression in the sclerotome and activate its transcription by direct binding to its promoter

The axial skeleton of vertebrates derives from the sclerotomal compartment of the somites. Genetic analysis has demonstrated that the transcription factors Pax1, Pax9, Meox1, Meox2, and Bapx1 are all required for sclerotomal differentiation. Their hierarchical relationship is, however, poorly understood. Because Bapx1 expression in the somites starts slightly later than that of the Meox genes, we asked whether Bapx1 is one of their downstream targets. Our analysis of Meox1; Meox2 mutant mice supports this hypothesis, as Bapx1 expression in the sclerotome is lost in the absence of both Meox proteins. Using transient-transfection assays, we show that Meox1 activates the Bapx1 promoter in a dose-dependent manner and that this activity is enhanced in the presence of Pax1 and/or Pax9. Furthermore, by electrophoretic mobility shift and chromatin immunoprecipitation experiments, we demonstrate that Meox1 can bind the Bapx1 promoter. The palindromic sequence TAATTA, present in the Bapx1 promoter, binds the Meox1 protein in vitro and is necessary for Meox1-induced transactivation of the Bapx1 promoter. Our data demonstrate that the Meox genes are required for Bapx1 expression in the sclerotome and suggest that the mechanism by which the Meox proteins exert this function is through direct activation of the Bapx1 gene.

Fgf and Bmp signals repress the expression of Bapx1 in the mandibular mesenchyme and control the position of the developing jaw joint

The development of the jaw joint between the palatoquadrate and proximal part Meckel's cartilage (articular) has recently been shown to involve the gene Bapx1. Bapx1 is expressed in the developing mandibular arch in two distinct caudal, proximal patches, one on either side of the head. These domains coincide later with the position of the developing jaw joint. The mechanisms that result in the restricted expression of Bapx1 in the mandibular arch were investigated, and two signaling factors that act as repressors were identified. Fibroblast growth factors (Fgfs) expressed in the oral epithelium restrict expression of Bapx1 to the caudal half of the mandibular arch, while bone morphogenetic proteins (Bmps) expressed in the distal mandibular arch restrict expression of Bapx1 to the proximal part of the mandible. Application of Fgf8 and Bmp4 beads to the proximal mesenchyme led to loss of Bapx1 expression and later fusion of the quadrate and articular as the jaw joint failed to form. In addition to fusion of the jaw joint, loss of Bapx1 lead to loss of the retroarticular process (RAP), phenocopying the defects seen after Bapx1 function was reduced in the zebrafish. By manipulating these signals, we were able to alter the expression domain of Bapx1, resulting in a new position of the jaw joint.

Bapx1 regulates patterning in the middle ear: altered regulatory role in the transition from the proximal jaw during vertebrate evolution

The middle ear apparatus is composed of three endochondrial ossicles (the stapes, incus and malleus) and two membranous bones, the tympanic ring and the gonium, which act as structural components to anchor the ossicles to the skull. Except for the stapes, these skeletal elements are unique to mammals and are derived from the first and second branchial arches. We show that, in combination with goosecoid (Gsc), the Bapx1 gene defines the structural components of the murine middle ear.

During embryogenesis, Bapx1 is expressed in a discrete domain within the mandibular component of the first branchial arch and later in the primordia of middle ear-associated bones, the gonium and tympanic ring. Consistent with the expression pattern of Bapx1, mouse embryos deficient for Bapx1 lack a gonium and display hypoplasia of the anterior end of the tympanic ring. At E10.5, expression of Bapx1partially overlaps that of Gsc and although Gsc is required for development of the entire tympanic ring, the role of Bapx1 is restricted to the specification of the gonium and the anterior tympanic ring. Thus, simple overlapping expression of these two genes appears to account for the patterning of the elements that compose the structural components of the middle ear and suggests that they act in concert.

In addition, Bapx1 is expressed both within and surrounding the incus and the malleus. Examination of the malleus shows that the width, but not the length, of this ossicle is decreased in the mutant mice. In non-mammalian jawed vertebrates, the bones homologous to the mammalian middle ear ossicles compose the proximal jaw bones that form the jaw articulation(primary jaw joint). In fish, Bapx1 is responsible for the formation of the joint between the quadrate and articular (homologues of the malleus and incus, respectively) enabling an evolutionary comparison of the role of a regulatory gene in the transition of the proximal jawbones to middle ear ossicles. Contrary to expectations, murine Bapx1 does not affect the articulation of the malleus and incus. We show that this change in role of Bapx1 following the transition to the mammalian ossicle configuration is not due to a change in expression pattern but results from an inability to regulate Gdf5 and Gdf6, two genes predicted to be essential in joint formation.