Differential contributions of Mesp1 and Mesp2 to the epithelialization and rostro-caudal patterning of somites

Blastocyst complementation-based rat-derived heart generation reveals cardiac anomaly barriers to interspecies chimera development

The use of pluripotent stem cells (PSCs) to generate functional organs via blastocyst complementation is a cutting-edge strategy in regenerative medicine. However, existing models that use this method for heart generation do not meet expectations owing to the complexity of heart development. Here, we investigated a Mesp1/2 deficient mouse model, which is characterized by abnormalities in the cardiac mesodermal cells. The injection of either mouse or rat PSCs into Mesp1/2 deficient mouse blastocysts led to successful heart generation. In chimeras, the resulting hearts were predominantly composed of rat cells; however, their functionality was limited to the embryonic developmental stage on day 12.5. These results present the functional limitation of the xenogeneic heart, which poses a significant challenge to the development in mouse-rat chimeras.

Deletion of Pax1 scoliosis-associated regulatory elements leads to a female-biased tail abnormality

Adolescent idiopathic scoliosis (AIS), a sideways curvature of the spine, is sexually dimorphic, with increased incidence in females. A genome-wide association study identified a female-specific AIS susceptibility locus near the PAX1 gene. Here, we use mouse enhancer assays, three mouse enhancer knockouts, and subsequent phenotypic analyses to characterize this region. Using mouse enhancer assays, we characterize a sequence, PEC7, which overlaps the AIS-associated variant, and find it to be active in the tail tip and intervertebral disc. Removal of PEC7 or Xe1, a known sclerotome enhancer nearby, or deletion of both sequences lead to a kinky tail phenotype only in the Xe1 and combined (Xe1+PEC7) knockouts, with only the latter showing a female sex dimorphic phenotype. Extensive phenotypic characterization of these mouse lines implicates several differentially expressed genes and estrogen signaling in the sex dimorphic bias. In summary, our work functionally characterizes an AIS-associated locus and dissects the mechanism for its sexual dimorphism.

Building a vertebra: Development of the amniote sclerotome

In embryonic development, the vertebral column arises from the sclerotomal compartment of the somites. The sclerotome is a mesenchymal cell mass which can be subdivided into several subpopulations specified by different regulatory mechanisms and giving rise to different parts of the vertebrae like vertebral body, vertebral arch, ribs, and vertebral joints. This review gives a short overview on the molecular and cellular basis of the formation of sclerotomal subdomains and the morphogenesis of their vertebral derivatives.

Deletion of Pax1 scoliosis-associated regulatory elements leads to a female-biased tail abnormality

Adolescent idiopathic scoliosis (AIS), a sideways curvature of the spine, is sexually dimorphic, with increased incidence in females. A GWAS identified a female-specific AIS susceptibility locus near the PAX1 gene. Here, we used mouse enhancer assays, three mouse enhancer knockouts and subsequent phenotypic analyses to characterize this region. Using mouse enhancer assays, we characterized a sequence, PEC7, that overlaps the AIS-associated variant, and found it to be active in the tail tip and intervertebral disc. Removal of PEC7 or Xe1, a known sclerotome enhancer nearby, and deletion of both sequences led to a kinky phenotype only in the Xe1 and combined (Xe1+PEC7) knockouts, with only the latter showing a female sex dimorphic phenotype. Extensive phenotypic characterization of these mouse lines implicated several differentially expressed genes and estrogen signaling in the sex dimorphic bias. In summary, our work functionally characterizes an AIS-associated locus and dissects the mechanism for its sexual dimorphism.

Formal proof of the requirement of MESP1 and MESP2 in mesoderm specification and their transcriptional control via specific enhancers in mice

MESP1 and MESP2 are transcriptional factors involved in mesoderm specification, somite boundary formation and somite polarity regulation. However, Mesp quadruple mutant zebrafish displayed only abnormal somite polarity without mesoderm specification defects. In order to re-evaluate Mesp1/Mesp2 mutants in mice, Mesp1 and Mesp2 single knockouts (KOs), and a Mesp1/Mesp2 double KO were established using genome-editing techniques without introducing selection markers commonly used before. The Mesp1/Mesp2 double KO embryos exhibited markedly severe mesoderm formation defects that were similar to the previously reported Mesp1/Mesp2 double KO embryos, indicating species differences in the function of MESP family proteins. However, the Mesp1 KO did not display any phenotype, including heart formation defects, which have been reported previously. We noted upregulation of Mesp2 in the Mesp1 KO embryos, suggesting that MESP2 rescues the loss of MESP1 in mesoderm specification. We also found that Mesp1 and Mesp2 expression in the early mesoderm is regulated by the cooperation of two independent enhancers containing T-box- and TCF/Lef-binding sites. Deletion of both enhancers caused the downregulation of both genes, resulting in heart formation defects. This study suggests dose-dependent roles of MESP1 and MESP2 in early mesoderm formation.

Construction of a mammalian embryo model from stem cells organized by a morphogen signalling centre

Generating properly differentiated embryonic structures in vitro from pluripotent stem cells remains a challenge. Here we show that instruction of aggregates of mouse embryonic stem cells with an experimentally engineered morphogen signalling centre, that functions as an organizer, results in the development of embryo-like entities (embryoids). In situ hybridization, immunolabelling, cell tracking and transcriptomic analyses show that these embryoids form the three germ layers through a gastrulation process and that they exhibit a wide range of developmental structures, highly similar to neurula-stage mouse embryos. Embryoids are organized around an axial chordamesoderm, with a dorsal neural plate that displays histological properties similar to the murine embryo neuroepithelium and that folds into a neural tube patterned antero-posteriorly from the posterior midbrain to the tip of the tail. Lateral to the chordamesoderm, embryoids display somitic and intermediate mesoderm, with beating cardiac tissue anteriorly and formation of a vasculature network. Ventrally, embryoids differentiate a primitive gut tube, which is patterned both antero-posteriorly and dorso-ventrally. Altogether, embryoids provide an in vitro model of mammalian embryo that displays extensive development of germ layer derivatives and that promises to be a powerful tool for in vitro studies and disease modelling.

A Single-Cell Atlas of the Human Healthy Airways

Rationale: The respiratory tract constitutes an elaborate line of defense that is based on a unique cellular ecosystem.Objectives: We aimed to investigate cell population distributions and transcriptional changes along the airways by using single-cell RNA profiling.Methods: We have explored the cellular heterogeneity of the human airway epithelium in 10 healthy living volunteers by single-cell RNA profiling. A total of 77,969 cells were collected at 35 distinct locations, from the nose to the 12th division of the airway tree.Measurements and Main Results: The resulting atlas is composed of a high percentage of epithelial cells (89.1%) but also immune (6.2%) and stromal (4.7%) cells with distinct cellular proportions in different regions of the airways. It reveals differential gene expression between identical cell types (suprabasal, secretory, and multiciliated cells) from the nose (MUC4, PI3, SIX3) and tracheobronchial (SCGB1A1, TFF3) airways. By contrast, cell-type-specific gene expression is stable across all tracheobronchial samples. Our atlas improves the description of ionocytes, pulmonary neuroendocrine cells, and brush cells and identifies a related population of NREP-positive cells. We also report the association of KRT13 with dividing cells that are reminiscent of previously described mouse "hillock" cells and with squamous cells expressing SCEL and SPRR1A/B.Conclusions: Robust characterization of a single-cell cohort in healthy airways establishes a valuable resource for future investigations. The precise description of the continuum existing from the nasal epithelium to successive divisions of the airways and the stable gene expression profile of these regions better defines conditions under which relevant tracheobronchial proxies of human respiratory diseases can be developed.

The roles of Mesp family proteins: functional diversity and redundancy in differentiation of pluripotent stem cells and mammalian mesodermal development

Mesp family proteins comprise two members named mesodermal posterior 1 (Mesp1) and mesodermal posterior 2 (Mesp2). Both Mesp1 and Mesp2 are transcription factors and they share an almost identical basic helix-loop-helix motif. They have been shown to play critical regulating roles in mammalian heart and somite development. Mesp1 sits in the core of the complicated regulatory network for generation of cardiovascular progenitors while Mesp2 is central for somitogenesis. Here we summarize the similarities and differences in their molecular functions during mammalian early mesodermal development and discuss possible future research directions for further study of the functions of Mesp1 and Mesp2. A comprehensive knowledge of molecular functions of Mesp family proteins will eventually help us better understand mammalian heart development and somitogenesis as well as improve the production of specific cell types from pluripotent stem cells for future regenerative therapies.

Tbx6, Mesp-b and Ripply1 regulate the onset of skeletal myogenesis in zebrafish

During embryonic development, the paraxial mesoderm becomes segmented into somites, within which proliferative muscle progenitors and muscle fibers establish the skeletal musculature. Here, we demonstrate that a gene network previously implicated in somite boundary formation, involving the transcriptional regulators Tbx6, Mesp-b and Ripply1, also confers spatial and temporal regulation to skeletal myogenesis in zebrafish. We show that Tbx6 directly regulates mesp-b and ripply1 expression in vivo, and that the interactions within the regulatory network are largely conserved among vertebrates. Mesp-b is necessary and sufficient for the specification of a subpopulation of muscle progenitors, the central proportion of the Pax3(+)/Pax7(+) dermomyotome. Conditional ubiquitous expression indicates that Mesp-b acts by inhibiting myogenic differentiation and by inducing the dermomyotome marker meox1. By contrast, Ripply1 induces a negative-feedback loop by promoting Tbx6 protein degradation. Persistent Tbx6 expression in Ripply1 knockdown embryos correlates with a deficit in dermomyotome and myotome marker gene expression, suggesting that Ripply1 promotes myogenesis by terminating Tbx6-dependent inhibition of myogenic maturation. Together, our data suggest that Mesp-b is an intrinsic upstream regulator of skeletal muscle progenitors and that, in zebrafish, the genes regulating somite boundary formation also regulate the development of the dermomyotome in the anterior somite compartment.

Metameric pattern of intervertebral disc/vertebral body is generated independently of Mesp2/Ripply-mediated rostro-caudal patterning of somites in the mouse embryo

The vertebrae are derived from the sclerotome of somites. Formation of the vertebral body involves a process called resegmentation, by which the caudal half of a sclerotome is combined with the rostral half of the next sclerotome. To elucidate the relationship between resegmentation and rostro-caudal patterning of somite, we used the Uncx4.1-LacZ transgene to characterize the resegmentation process. Our observations suggested that in the thoracic and lumbar vertebrae, the Uncx4.1-expressing caudal sclerotome gave rise to the intervertebral disc (IVD) and rostral portion of the vertebral body (VB). In the cervical vertebrae, the Uncx4.1-expressing caudal sclerotome appeared to contribute to the IVD and both caudal and rostral ends of the VB. This finding suggests that the rostro-caudal gene expression boundary does not necessarily coincide with the resegmentation boundary. This conclusion was supported by analyses of Mesp2 KO and Ripply1/2 double KO embryos lacking rostral and caudal properties, respectively. Resegmentation was not observed in Mesp2 KO embryos, but both the IVD and whole VB were formed from the caudalized sclerotome. Expression analysis of IVD marker genes including Pax1 in the wild-type, Mesp2 KO, and Ripply1/2 DKO embryos also supported the idea that a metameric pattern of IVD/VB is generated independently of Mesp2/Ripply-mediated rostro-caudal patterning of somite. However, in the lumbar region, IVD differentiation appeared to be stimulated by the caudal property and suppressed by the rostral property. Therefore, we propose that rostro-caudal patterning of somites is not a prerequisite for metameric patterning of the IVD and VB, but instead required to stimulate IVD differentiation in the caudal half of the sclerotome.

Spatiotemporal disorder in the axial skeleton development of the Mesp2-null mouse: a model of spondylocostal dysostosis and spondylothoracic dysostosis

Spondylocostal dysostosis (SCDO) is a genetic disorder characterized by severe malformation of the axial skeleton. Mesp2 encodes a basic helix-loop-helix type transcription factor that is required for somite formation. Its human homologue, Mesp2, is a gene affected in patients with SCDO and a related vertebral disorder, spondylothoracic dysostosis (STDO). This work investigated how the loss of Mesp2 affects axial skeleton development and causes the clinical features of SCDO and STDO. We first confirmed, by three-dimensional computed tomography scanning, that Mesp2-null mice exhibited mineralized tissue patterning resembling the radiological features of SCDO and STDO. Histological observations and in situ hybridization probing for extracellular matrix molecules demonstrated that the developing vertebral bodies in Mesp2-null mice were extensively fused with rare insertions of intervertebral tissue. Unexpectedly, the intervertebral tissues were mostly fused longitudinally in the vertebral column, instead of exhibiting extended formation, as was expected based on the caudalized properties of Mesp2-null somite derivatives. Furthermore, the differentiation of vertebral body chondrocytes in Mesp2-null mice was spatially disordered and largely delayed, with an increased cell proliferation rate. The quantitative three-dimensional immunofluorescence image analyses of phospho-Smad2 and -Smad1/5/8 revealed that these chondrogenic phenotypes were associated with spatially disordered inputs of TGF-β and BMP signaling in the Mesp2-null chondrocytes, and also demonstrated an amorphous arrangement of cells with distinct properties. Furthermore, a significant delay in ossification in Mesp2-null vertebrae was observed by peripheral quantitative computed tomography. The current observations of the spatiotemporal disorder of vertebral organogenesis in the Mesp2-null mice provide further insight into the pathogenesis of SCDO and STDO, and the physiological development of the axial skeleton.

A fluorescence spotlight on the clockwork development and metabolism of bone

Biological phenomena that exhibit periodic activity are often referred as biorhythms or biological clocks. Among these, circadian rhythms, cyclic patterns reflecting a 24-h cycle, are the most obvious in many physiological activities including bone growth and metabolism. In the late 1990s, several clock genes were isolated and their primary structures and functions were identified. The feedback loop model of transcriptional factors was proposed to work as a circadian core oscillator not only in the suprachiasmatic nuclei of the anterior hypothalamus, which is recognized as the mammalian central clock, but also in various peripheral tissues including cartilage and bone. Looking back to embryonic development, the fundamental architecture of skeletal patterning is regulated by ultradian clocks that are defined as biorhythms that cycle more than once every 24 h. As post-genomic approaches, transcriptome analysis by micro-array and bioimaging assays to detect luminescent and fluorescent signals have been exploited to uncover a more comprehensive set of genes and spatio-temporal regulation of the clockwork machinery in animal models. In this review paper, we provide an overview of topics related to these molecular clocks in skeletal biology and medicine, and discuss how fluorescence imaging approaches can contribute to widening our views of this realm of biomedical science.

The repression of Notch signaling occurs via the destabilization of mastermind-like 1 by Mesp2 and is essential for somitogenesis

The rostro-caudal polarity within a somite is primarily determined by the on/off state of Notch signaling, but the mechanism by which Notch is repressed has remained elusive. Here, we present genetic and biochemical evidence that the suppression of Notch signaling is essential for the establishment of rostro-caudal polarity within a somite and that Mesp2 acts as a novel negative regulator of the Notch signaling pathway. We generated a knock-in mouse in which a dominant-negative form of Rbpj is introduced into the Mesp2 locus. Intriguingly, this resulted in an almost complete rescue of the segmental defects in the Mesp2-null mouse. Furthermore, we demonstrate that Mesp2 potently represses Notch signaling by inducing the destabilization of mastermind-like 1, a core regulator of this pathway. Surprisingly, this function of Mesp2 is found to be independent of its function as a transcription factor. Together, these data demonstrate that Mesp2 is a novel component involved in the suppression of Notch target genes.

Cardiac regeneration: still a 21st century challenge in search for cardiac progenitors from stem cells and embryos

Regeneration of the heart after a stroke would be the best biologic response to restore its function. However, although this phenomenon occurs in primitive organisms, the regenerative potential is lost in mammals. Thus, the search for an appropriate cardiac progenitor with the potential to differentiate into a functional cardiomyocyte in vitro and in vivo has been the subject of intensive investigation. We summarize the cardiogenic transcriptional pathway that constitutes the molecular scaffold to drive pluripotent stem cells toward a cardiac progenitor fate. Then we overview the literature on derivation of cardiac progenitors from both embryos and stem cells.

Transitions between epithelial and mesenchymal states and the morphogenesis of the early mouse embryo

Multicellular organisms arise from the generation of different cell types and the organization of cells into tissues and organs. Cells of metazoa display two main phenotypes, the ancestral epithelial state and the recent mesenchymal derivative. Epithelial cells are usually stationary and reside in two-dimensional sheets. By contrast mesenchymal cells are loosely packed and can move to new positions, thereby providing a vehicle for cell rearrangement, dispersal and novel cell-cell interactions. Transitions between epithelial and mesenchymal states drive key morphogenetic events in the early vertebrate embryo, including gastrulation, germ layer formation and somitogenesis. The cell behaviors and molecular mechanisms promoting transitions between these two states in the early mouse embryo are discussed in this review.

Extensive molecular differences between anterior- and posterior-half-sclerotomes underlie somite polarity and spinal nerve segmentation

Background

The polarization of somite-derived sclerotomes into anterior and posterior halves underlies vertebral morphogenesis and spinal nerve segmentation. To characterize the full extent of molecular differences that underlie this polarity, we have undertaken a systematic comparison of gene expression between the two sclerotome halves in the mouse embryo.

Results

Several hundred genes are differentially-expressed between the two sclerotome halves, showing that a marked degree of molecular heterogeneity underpins the development of somite polarity.

Conclusion

We have identified a set of genes that warrant further investigation as regulators of somite polarity and vertebral morphogenesis, as well as repellents of spinal axon growth. Moreover the results indicate that, unlike the posterior half-sclerotome, the central region of the anterior-half-sclerotome does not contribute bone and cartilage to the vertebral column, being associated instead with the development of the segmented spinal nerves.

EphrinB2 coordinates the formation of a morphological boundary and cell epithelialization during somite segmentation

During early morphogenesis, tissue segregation is often accompanied by changes in cell shape. To understand how such coordination is regulated, somitogenesis was used as a model. When a somite forms in the anterior end of the presomitic mesoderm, an intersomitic boundary (gap) emerges, and it is rapidly followed by cell epithelialization at this border. It has been known that the gap formation is regulated by intercellular signals. We here demonstrate that cMeso-1, the chicken homolog of mouse Mesp2, up-regulates EphA4 in the cells located posteriorly to a forming boundary. This in turn activates EphrinB2-reverse signals in the anteriorly juxtaposed cells, where the EphrinB2 signal is sufficient to cause a gap formation and cell epithelialization cell-autonomously. During these processes, Cdc42 needs to be repressed via tyrosine phosphorylation of EphrinB2. This is the first demonstration that Ephrin-reverse signal acts as a platform that couples distinct morphogenetic changes in cell polarity and tissue shape.

Association of matrix metalloproteinase-3 with cardiogenic activity during Noggin-induced differentiation of mouse embryonic stem cells

BACKGROUND

Despite the pluripotency of embryonic stem (ES) cells, their clinical applications have been hindered due to the lack of reliable differentiation methods. Recently, it was shown that Noggin could effectively induce cardiomyocyte differentiation by transient treatment of ES cells.

METHODS

To determine how Noggin may induce cardiac differentiation, we compared differentially expressed genes during Noggin-induced differentiation of ES cells using microarray analysis. We found Matrix metalloproteinase-3 (Mmp-3) expression was highly up-regulated by Noggin treatment. To understand the role of Mmp-3 in the cardiac differentiation of ES cells, we inhibited Mmp-3 activity by treating with a specific Mmp-3 inhibitor during Noggin-induced cardiac differentiation of ES cells. We also analyzed the expression levels of cardiac markers and the ratio of spontaneously beating embryoid bodies (EBs) in the presence of the Mmp-3 inhibitor.

RESULTS

We analyzed EB samples from zero, two, and four days with or without Noggin treatment, and found that the expression levels of 2 (0 day), 56 (2 days), and 805 (4 days) genes were altered with Noggin treatment. Up-regulation of Mmp-3 was closely associated with relative increases of cardiogenic, vasculogenic, and hematopoietic genes in EB treated with Noggin. By inhibiting Mmp-3 activity, we verified that Mmp-3 activation is partly responsible for both the expression of cardiac markers and the elevated ratio of spontaneously beating to non-beating EBs.

CONCLUSIONS

The concurrent expression of Mmp-3 with many cardiogenic genes and the specific inhibition of Mmp-3 revealed a critical role for Mmp-3 in Noggin-induced cardiac differentiation of ES cells.

Mesp1 acts as a master regulator of multipotent cardiovascular progenitor specification

During embryonic development, multipotent cardiovascular progenitor cells are specified from early mesoderm. Using mouse ESCs in which gene expression can be temporally regulated, we have found that transient expression of Mesp1 dramatically accelerates and enhances multipotent cardiovascular progenitor specification through an intrinsic and cell autonomous mechanism. Genome-wide transcriptional analysis indicates that Mesp1 rapidly activates and represses a discrete set of genes, and chromatin immunoprecipitation shows that Mesp1 directly binds to regulatory DNA sequences located in the promoter of many key genes in the core cardiac transcriptional machinery, resulting in their rapid upregulation. Mesp1 also directly represses the expression of key genes regulating other early mesoderm and endoderm cell fates. Our results demonstrate that Mesp1 acts as a key regulatory switch during cardiovascular specification, residing at the top of the hierarchy of the gene network responsible for cardiovascular cell-fate determination.

Mesp1 coordinately regulates cardiovascular fate restriction and epithelial-mesenchymal transition in differentiating ESCs

Wnt signaling is required for development of mesoderm-derived lineages and expression of transcription factors associated with the primitive streak. In a functional screen, we examined the mesoderm-inducing capacity of transcription factors whose expression was Wnt-dependent in differentiating ESCs. In contrast to many inactive factors, we found that mesoderm posterior 1 (Mesp1) promoted mesoderm development independently of Wnt signaling. Transient Mesp1 expression in ESCs promotes changes associated with epithelial-mesenchymal transition (EMT) and induction of Snai1, consistent with a role in gastrulation. Mesp1 expression also restricted the potential fates derived from ESCs, generating mesoderm progenitors with cardiovascular, but not hematopoietic, potential. Thus, in addition to its effects on EMT, Mesp1 may be capable of generating the recently identified multipotent cardiovascular progenitor from ESCs in vitro.

Transcriptomic and phenotypic analysis of murine embryonic stem cell derived BMP2+ lineage cells: an insight into mesodermal patterning

Transcriptome analysis of BMP2⁺ cells in comparison to the undifferentiated BMP2 ES cells and the control population from 7-day old embryoid bodies led to the identification of 479 specifically upregulated and 193 downregulated transcripts.

Background

Bone morphogenetic protein (BMP)2 is a late mesodermal marker expressed during vertebrate development and plays a crucial role in early embryonic development. The nature of the BMP2-expressing cells during the early stages of embryonic development, their transcriptome and cell phenotypes developed from these cells have not yet been characterized.

Results

We generated a transgenic BMP2 embryonic stem (ES) cell lineage expressing both puromycin acetyltransferase and enhanced green fluorescent protein (EGFP) driven by the BMP2 promoter. Puromycin resistant and EGFP positive BMP2⁺ cells with a purity of over 93% were isolated. Complete transcriptome analysis of BMP2⁺ cells in comparison to the undifferentiated ES cells and the control population from seven-day-old embryoid bodies (EBs; intersection of genes differentially expressed between undifferentiated ES cells and BMP2⁺ EBs as well as differentially expressed between seven-day-old control EBs and BMP2⁺ EBs by t-test, p < 0.01, fold change >2) by microarray analysis led to identification of 479 specifically upregulated and 193 downregulated transcripts. Transcription factors, apoptosis promoting factors and other signaling molecules involved in early embryonic development are mainly upregulated in BMP2⁺ cells. Long-term differentiation of the BMP2⁺ cells resulted in neural crest stem cells (NCSCs), smooth muscle cells, epithelial-like cells, neuronal-like cells, osteoblasts and monocytes. Interestingly, development of cardiomyocytes from the BMP2⁺ cells requires secondary EB formation.

Conclusion

This is the first study to identify the complete transcriptome of BMP2⁺ cells and cell phenotypes from a mesodermal origin, thus offering an insight into the role of BMP2⁺ cells during embryonic developmental processes in vivo.

Identification of presomitic mesoderm (PSM)-specific Mesp1 enhancer and generation of a PSM-specific Mesp1/Mesp2-null mouse using BAC-based rescue technology

Bacterial artificial chromosome (BAC) modification technology is a powerful method for the identification of enhancer sequences and genetic modifications. Using this method, we have analyzed the Mesp1 and/or Mesp2 enhancers and identified P1-PSME, a PSM-specific enhancer of Mesp1, which contains a T-box binding site similar to the previously identified P2-PSME. Hence, Mesp1 and Mesp2 use different enhancers for their PSM-specific expression. In addition, we find that these two genes also use distinct enhancers for their early mesodermal expression. Based on these results, we generated a PSM-specific Mesp1/Mesp2-null mouse by introducing a BAC clone, from which only early mesodermal Mesp1 expression is possible, into the Mesp1/Mesp2 double knockout (dKO) genetic background. This successfully rescued gastrulation defects due to the lack of the early mesoderm in the dKO mouse and we thereby obtained a PSM-specific Mesp1/Mesp2-null mouse showing a lack of segmented somites.

Retinoic acid regulation of the Mesp-Ripply feedback loop during vertebrate segmental patterning

The Mesp bHLH genes play a conserved role during segmental patterning of the mesoderm in the vertebrate embryo by specifying segmental boundaries and anteroposterior (A-P) segmental polarity. Here we use a xenotransgenic approach to compare the transcriptional enhancers that drive expression of the Mesp genes within segments of the presomitic mesoderm (PSM) of different vertebrate species. We find that the genomic sequences upstream of the mespb gene in the pufferfish Takifugu rubripes (Tr-mespb) are able to drive segmental expression in transgenic Xenopus embryos while those from the Xenopus laevis mespb (Xl-mespb) gene drive segmental expression in transgenic zebrafish. In both cases, the anterior segmental boundary of transgene expression closely matches the expression of the endogenous Mesp genes, indicating that many inputs into segmental gene expression are highly conserved. By contrast, we find that direct retinoic acid (RA) regulation of endogenous Mesp gene expression is variable among vertebrate species. Both Tr-mespb and Xl-mespb are directly upregulated by RA, through a complex, distal element. By contrast, RA represses the zebrafish Mesp genes. We show that this repression is mediated, in part, by RA-mediated activation of the Ripply genes, which together with Mesp genes form an RA-responsive negative feedback loop. These observations suggest that variations in a direct response to RA input may allow for changes in A-P patterning of the segments in different vertebrate species.

Genetic analysis of somite formation in laboratory fish models

The repeated appearance of somites is one of the most fascinating aspects of vertebrate embryogenesis. Recent studies identified complex regulatory circuits that provide the molecular basis for the "clock and wave front" model, postulated almost 30 years ago by Cooke and Zeeman. The highly coordinated process of somite formation involves several networks of molecular cascades including the Delta/Notch, Wnt, FGF and retinoid signalling pathways. Studies in mouse, Xenopus and especially chicken over the last decade have helped to understand the role and interactions of these pathways in somitogenesis. More recently, this has been supplemented by experiments in zebrafish. This animal model offers the possibility of performing large scale mutagenesis screens to identify novel factors and pathways involved in somitogenesis. Molecular cloning of zebrafish somite mutants mainly resulted in genes that belong to the Delta/Notch pathway and therefore underlined the importance of this pathway during somitogenesis. The fact that other pathways have not yet been identified by genetic screening in this species was assumed to be caused by functional redundancy of duplicated genes in zebrafish. In 2000, a large-scale mutagenesis screen has been initiated in Kyoto, Japan using the related teleost medaka (Oryzias latipes). In this screen, mutants with unique phenotypes have been identified, which have not been described in zebrafish or mouse. In this chapter, we will review the progress that has been made in understanding the molecular control of somite formation in zebrafish and will discuss recent efforts to screen for novel phenotypes using medaka somitogenesis mutants.

BHLH proteins and their role in somitogenesis

The most obvious manifestation of the existence of a segmented, or metameric, body plan in vertebrate embryos is seen during the formation of the somites. Somites are transient embryonic structures formed in a progressive manner from a nonsegmented mesoderm in a highly regulated process called somitogenesis. As development proceeds different compartments are formed within each somite and these progressively follow a variety of differentiation programs to form segmented organs, such as the different bones that make the axial skeleton, body skeletal muscles and part of the dermis. Transcription factors from the basic helix-loop-helix (bHLH) protein family have been described to be implicated in each of the processes involved in somite formation. bHLH proteins are a family of transcription factors characterized by the presence of a DNA binding domain and a dimerization motif that consists of a basic region adjacent to an amphipathic helix, a loop and a second amphipathic helix. In this chapter we will review a number of bHLH proteins known to play a role in somitogenesis.

Mathematical models for somite formation

Somitogenesis is the process of division of the anterior-posterior vertebrate embryonic axis into similar morphological units known as somites. These segments generate the prepattern which guides formation of the vertebrae, ribs and other associated features of the body trunk. In this work, we review and discuss a series of mathematical models which account for different stages of somite formation. We begin by presenting current experimental information and mechanisms explaining somite formation, highlighting features which will be included in the models. For each model we outline the mathematical basis, show results of numerical simulations, discuss their successes and shortcomings and avenues for future exploration. We conclude with a brief discussion of the state of modeling in the field and current challenges which need to be overcome in order to further our understanding in this area.

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.

Mesenchymal to epithelial transition in development and disease

Cellular plasticity is fundamental to embryonic development. The importance of cellular transitions in development is first apparent during gastrulation when the process of epithelial to mesenchymal transition transforms polarized epithelial cells into migratory mesenchymal cells that constitute the embryonic and extraembryonic mesoderm. It is now widely accepted that this developmental pathway is exploited in various disease states, including cancer progression. The loss of epithelial characteristics and the acquisition of a mesenchymal-like migratory phenotype are crucial to the development of invasive carcinoma and metastasis. However, given the morphological similarities between primary tumour and metastatic lesions, it is likely that tumour cells re-activate certain epithelial properties through a mesenchymal to epithelial transition (MET) at the secondary site, although this is yet to be proven. MET is also an essential developmental process and has been extensively studied in kidney organogenesis and somitogenesis. In this review we describe the process of MET, highlight important mediators, and discuss their implication in the context of cancer progression.

From segment to somite: segmentation to epithelialization analyzed within quantitative frameworks

One of the most visually striking patterns in the early developing embryo is somite segmentation. Somites form as repeated, periodic structures in pairs along nearly the entire caudal vertebrate axis. The morphological process involves short- and long-range signals that drive cell rearrangements and cell shaping to create discrete, epithelialized segments. Key to developing novel strategies to prevent somite birth defects that involve axial bone and skeletal muscle development is understanding how the molecular choreography is coordinated across multiple spatial scales and in a repeating temporal manner. Mathematical models have emerged as useful tools to integrate spatiotemporal data and simulate model mechanisms to provide unique insights into somite pattern formation. In this short review, we present two quantitative frameworks that address the morphogenesis from segment to somite and discuss recent data of segmentation and epithelialization.

Appropriate suppression of Notch signaling by Mesp factors is essential for stripe pattern formation leading to segment boundary formation

Mesp1 and Mesp2 are homologous transcription factors that are co-expressed in the anterior presomitic mesoderm (PSM) during mouse somitogenesis. The loss of Mesp2 alone in our conventional Mesp2-null mice results in the complete disruption of somitogenesis, including segment border formation, rostro-caudal patterning and epithelialization of somitic mesoderm. This has led us to interpret that Mesp2 is solely responsible for somitogenesis. Our novel Mesp2 knock-in alleles, however, exhibit a remarkable upregulation of Mesp1. Removal of the pgk-neo cassette from the new allele leads to localization of Mesp1 and several gene expression, and somite formation in the tail region. Moreover, a reduction in the gene dosage of Mesp1 by one copy disrupts somite formation, confirming the involvement of Mesp1 in the rescue events. Furthermore, we find that activated Notch1 knock-in significantly upregulates Mesp1 expression, even in the absence of a Notch signal mediator, Psen1. This indicates that the Psen1-independent effects of activated Notch1 are mostly attributable to the induction of Mesp1. However, we have also confirmed that Mesp2 enhances the expression of the Notch1 receptor in the anterior PSM. The activation and subsequent suppression of Notch signaling might thus be a crucial event for both stripe pattern formation and boundary formation.

Disruption of the somitic molecular clock causes abnormal vertebral segmentation

Somites are the precursors of the vertebral column. They segment from the presomitic mesoderm (PSM) that is caudally located and newly generated from the tailbud. Somites form in synchrony on either side of the embryonic midline in a reiterative manner. A molecular clock that operates in the PSM drives this reiterative process. Genetic manipulation in mouse, chick and zebrafish has revealed that the molecular clock controls the activity of the Notch and WNT signaling pathways in the PSM. Disruption of the molecular clock impacts on somite formation causing abnormal vertebral segmentation (AVS). A number of dysmorphic syndromes manifest AVS defects. Interaction between developmental biologists and clinicians has lead to groundbreaking research in this area with the identification that spondylocostal dysostosis (SCD) is caused by mutation in Delta-like 3 (DLL3), Mesoderm posterior 2 (MESP2), and Lunatic fringe (LFNG); three genes that are components of the Notch signaling pathway. This review describes our current understanding of the somitic molecular clock and highlights how key findings in developmental biology can impact on clinical practice.

Cooperative Mesp activity is required for normal somitogenesis along the anterior–posterior axis

Mesp2 is a bHLH-type transcription factor that plays a key role during somitogenesis. Mesp2 is transiently expressed and is quickly degraded once translated. In our current study, we find that Mesp2 contains a degradation domain, which acts as a target of proteasome-mediated proteolysis and appears to play this role in vivo. We have also defined the nuclear localization signals (NLS) and constructed a minimum Mesp2 protein (P2-HD) composed of the NLS, bHLH and the degradation domains. The ability of the P2-HD as a transcription factor in vivo was examined. Some of the defects that had been previously observed in the Mesp2-null mice were rescued in the knock-in mice but only in the posterior half of the body, indicating differential effects of P2-HD along the anterior-posterior (AP) axis. In addition, quantitative analysis of the expression along the AP axis revealed that the relative levels of Mesp2 increased, whereas Mesp1 is down-regulated in the later stages of development by the activities of Mesp2 in the wild-type embryo. Moreover, we have found that somitogenesis in the early stages is more susceptible to changes in the Mesp gene dosage, indicating that a threshold level of Mesp activity must be required for the progression of normal somitogenesis.

The amphioxus T-box gene, AmphiTbx15/18/22, illuminates the origins of chordate segmentation

Amphioxus and vertebrates are the only deuterostomes to exhibit unequivocal somitic segmentation. The relative simplicity of the amphioxus genome makes it a favorable organism for elucidating the basic genetic network required for chordate somite development. Here we describe the developmental expression of the somite marker, AmphiTbx15/18/22, which is first expressed at the mid-gastrula stage in dorsolateral mesendoderm. At the early neurula stage, expression is detected in the first three pairs of developing somites. By the mid-neurula stage, expression is downregulated in anterior somites, and only detected in the penultimate somite primordia. In early larvae, the gene is expressed in nascent somites before they pinch off from the posterior archenteron (tail bud). Integrating functional, phylogenetic and expression data from a variety of triploblast organisms, we have reconstructed the evolutionary history of the Tbx15/18/22 subfamily. This analysis suggests that the Tbx15/18/22 gene may have played a role in patterning somites in the last common ancestor of all chordates, a role that was later conserved by its descendents following gene duplications within the vertebrate lineage. Furthermore, the comparison of expression domains within this gene subfamily reveals similarities in the genetic bases of trunk and cranial mesoderm segmentation. This lends support to the hypothesis that the vertebrate head evolved from an ancestor possessing segmented cranial mesoderm.

Expression of Notch signaling pathway genes in mouse embryos lacking beta4galactosyltransferase-1

A requirement for beta4galactosyltransferase-1 (beta4GalT-1) activity in the modulation of Notch signaling by the glycosyltransferase Fringe was previously identified in a mammalian co-culture assay. Notch signaling is necessary for the formation of somites in mammals. We therefore investigated the expression of eleven Notch pathway and somitogenic genes in E9.5 mouse embryos lacking beta4GalT-1. Four of these genes were altered in expression pattern or expression level. The Notch target genes Hes5 and Mesp2 were affected to some degree in all mutant embryos. The Notch ligand genes Dll1 and Dll3 were reduced or altered in expression in a significant proportion of mutants. While there were no differences in the number or morphology of somites in E9.5 B4galt1 null embryos, the number of lumbar vertebrae in mutant embryos differed from control littermates (P < or = 0.01). The subtlety of the in vivo phenotype may be due to redundancy since several B4galt genes related to B4galt1 are expressed during embryogenesis.

Surfing with the tunicates into the post-genome era

This year is the centenary of Edward G. Conklin's signal findings in embryology: the elucidation of complete cell lineages and the discovery of localized maternal determinants. Conklin used ascidian embryos to elucidate universal principles in embryology. A century later, ascidians, or sea squirts, have not only entered the post-genome era, but in many ways are leading the way to the promise of a "systems-level" understanding of complex processes such as notochord formation, neurogenesis, and even behavior.

Uncoupling heart cell specification and migration in the simple chordate Ciona intestinalis

The bHLH transcription factor Mesp has an essential but ambiguous role in early chordate heart development. Here, we employ the genetic and morphological simplicity of the basal chordate Ciona intestinalis to elucidate Mesp regulation and function. Characterization of a minimal cardiac enhancer for the Ciona Mesp gene demonstrated direct activation by the T-box transcription factor Tbx6c. The Mespenhancer was fused to GFP, permitting high-resolution visualization of heart cells as they migrate and divide. The enhancer was also used to drive targeted expression of an activator form of Mesp, which induces heart formation without migration. We discuss the implications of Tbx6-Mespinteractions for the evolution of cardiac mesoderm in invertebrates and vertebrates.