Evolution of Brachyury proteins: identification of a novel regulatory domain conserved within Bilateria

An ancestral Wnt-Brachyury feedback loop in axial patterning and recruitment of mesoderm-determining target genes

Transcription factors are crucial drivers of cellular differentiation during animal development and often share ancient evolutionary origins. The T-box transcription factor Brachyury plays a pivotal role as an early mesoderm determinant and neural repressor in vertebrates; yet, the ancestral function and key evolutionary transitions of the role of this transcription factor remain obscure. Here, we present a genome-wide target-gene screen using chromatin immunoprecipitation sequencing in the sea anemone Nematostella vectensis, an early branching non-bilaterian, and the sea urchin Strongylocentrotus purpuratus, a representative of the sister lineage of chordates. Our analysis reveals an ancestral gene regulatory feedback loop connecting Brachyury, FoxA and canonical Wnt signalling involved in axial patterning that predates the cnidarian-bilaterian split about 700 million years ago. Surprisingly, we also found that part of the gene regulatory network controlling the fate of neuromesodermal progenitors in vertebrates was already present in the common ancestor of cnidarians and bilaterians. However, while several endodermal and neuronal Brachyury target genes are ancestrally shared, hardly any of the key mesodermal downstream targets in vertebrates are found in the sea anemone or the sea urchin. Our study suggests that a limited number of target genes involved in mesoderm formation were newly acquired in the vertebrate lineage, leading to a dramatic shift in the function of this ancestral developmental regulator.

Eomes function is conserved between zebrafish and mouse and controls left-right organiser progenitor gene expression via interlocking feedforward loops

The T-box family transcription factor Eomesodermin (Eomes) is present in all vertebrates, with many key roles in the developing mammalian embryo and immune system. Homozygous Eomes mutant mouse embryos exhibit early lethality due to defects in both the embryonic mesendoderm and the extraembryonic trophoblast cell lineage. In contrast, zebrafish lacking the predominant Eomes homologue A (Eomesa) do not suffer complete lethality and can be maintained. This suggests fundamental differences in either the molecular function of Eomes orthologues or the molecular configuration of processes in which they participate. To explore these hypotheses we initially analysed the expression of distinct Eomes isoforms in various mouse cell types. Next we compared the functional capabilities of these murine isoforms to zebrafish Eomesa. These experiments provided no evidence for functional divergence. Next we examined the functions of zebrafish Eomesa and other T-box family members expressed in early development, as well as its paralogue Eomesb. Though Eomes is a member of the Tbr1 subfamily we found evidence for functional redundancy with the Tbx6 subfamily member Tbx16, known to be absent from eutherians. However, Tbx16 does not appear to synergise with Eomesa cofactors Mixl1 and Gata5. Finally, we analysed the ability of Eomesa and other T-box factors to induce zebrafish left-right organiser progenitors (known as dorsal forerunner cells) known to be positively regulated by vgll4l, a gene we had previously shown to be repressed by Eomesa. Here we demonstrate that Eomesa indirectly upregulates vgll4l expression via interlocking feedforward loops, suggesting a role in establishment of left-right asymmetry. Conversely, other T-box factors could not similarly induce left-right organiser progenitors. Overall these findings demonstrate conservation of Eomes molecular function and participation in similar processes, but differential requirements across evolution due to additional co-expressed T-box factors in teleosts, albeit with markedly different molecular capabilities. Our analyses also provide insights into the role of Eomesa in left-right organiser formation in zebrafish.

Establishment of the vertebrate body plan: Rethinking gastrulation through stem cell models of early embryogenesis

A striking property of vertebrate embryos is the emergence of a conserved body plan across a wide range of organisms through the process of gastrulation. As the body plan unfolds, gene regulatory networks (GRNs) and multicellular interactions (cell regulatory networks, CRNs) combine to generate a conserved set of morphogenetic events that lead to the phylotypic stage. Interrogation of these multilevel interactions requires manipulation of the mechanical environment, which is difficult in vivo. We review recent studies of stem cell models of early embryogenesis from different species showing that, independent of species origin, cells in culture form similar structures. The main difference between embryos and in vitro models is the boundary conditions of the multicellular ensembles. We discuss these observations and suggest that the mechanical and geometric boundary conditions of different embryos before gastrulation hide a morphogenetic ground state that is revealed in the stem-cell-based models of embryo development.

Towards understanding the origin of animal development

Almost all animals undergo embryonic development, going from a single-celled zygote to a complex multicellular adult. We know that the patterning and morphogenetic processes involved in development are deeply conserved within the animal kingdom. However, the origins of these developmental processes are just beginning to be unveiled. Here, we focus on how the protist lineages sister to animals are reshaping our view of animal development. Most intriguingly, many of these protistan lineages display transient multicellular structures, which are governed by similar morphogenetic and gene regulatory processes as animal development. We discuss here two potential alternative scenarios to explain the origin of animal embryonic development: either it originated concomitantly at the onset of animals or it evolved from morphogenetic processes already present in their unicellular ancestors. We propose that an integrative study of several unicellular taxa closely related to animals will allow a more refined picture of how the last common ancestor of animals underwent embryonic development.

Cardiac progenitors and paracrine mediators in cardiogenesis and heart regeneration

The mammalian hearts have the least regenerative capabilities among tissues and organs. As such, heart regeneration has been and continues to be the ultimate goal in the treatment against acquired and congenital heart diseases. Uncovering such a long-awaited therapy is still extremely challenging in the current settings. On the other hand, this desperate need for effective heart regeneration has developed various forms of modern biotechnologies in recent years. These involve the transplantation of pluripotent stem cell-derived cardiac progenitors or cardiomyocytes generated in vitro and novel biochemical molecules along with tissue engineering platforms. Such newly generated technologies and approaches have been shown to effectively proliferate cardiomyocytes and promote heart repair in the diseased settings, albeit mainly preclinically. These novel tools and medicines give somehow credence to breaking down the barriers associated with re-building heart muscle. However, in order to maximize efficacy and achieve better clinical outcomes through these cell-based and/or cell-free therapies, it is crucial to understand more deeply the developmental cellular hierarchies/paths and molecular mechanisms in normal or pathological cardiogenesis. Indeed, the morphogenetic process of mammalian cardiac development is highly complex and spatiotemporally regulated by various types of cardiac progenitors and their paracrine mediators. Here we discuss the most recent knowledge and findings in cardiac progenitor cell biology and the major cardiogenic paracrine mediators in the settings of cardiogenesis, congenital heart disease, and heart regeneration.

The Ciona myogenic regulatory factor functions as a typical MRF but possesses a novel N-terminus that is essential for activity

Electroporation-based assays were used to test whether the myogenic regulatory factor (MRF) of Ciona intestinalis (CiMRF) interferes with endogenous developmental programs, and to evaluate the importance of its unusual N-terminus for muscle development. We found that CiMRF suppresses both notochord and endoderm development when it is expressed in these tissues by a mechanism that may involve activation of muscle-specific microRNAs. Because these results add to a large body of evidence demonstrating the exceptionally high degree of functional conservation among MRFs, we were surprised to discover that non-ascidian MRFs were not myogenic in Ciona unless they formed part of a chimeric protein containing the CiMRF N-terminus. Equally surprising, we found that despite their widely differing primary sequences, the N-termini of MRFs of other ascidian species could form chimeric MRFs that were also myogenic in Ciona. This domain did not rescue the activity of a Brachyury protein whose transcriptional activation domain had been deleted, and so does not appear to constitute such a domain. Our results indicate that ascidians have previously unrecognized and potentially novel requirements for MRF-directed myogenesis. Moreover, they provide the first example of a domain that is essential to the core function of an important family of gene regulatory proteins, one that, to date, has been found in only a single branch of the family.

Penaeid shrimp brachyury: sequence analysis and expression during gastrulation

Gastrulation occurs by a variety of morphogenetic movements, often correlated with diverse expression of the T-box transcription factor Brachyury (Bra). Bra may be expressed in ectoderm, mesoderm, or endoderm, but its role in cell fate specification or regulation of gastrulation movements has not been studied in the development of crustaceans. Penaeid shrimp (Decapoda: Dendrobranchiata: Penaeidae) develop by complete cleavage and gastrulation by invagination to a free-swimming nauplius larva. Penaeid gastrulation diverges from other decapods and from insects, occurring early at a low cell number with the formation of a radial invagination. Toward a better understanding of gastrulation movements in penaeid shrimp, bra was identified from newly available penaeid shrimp genomes and transcriptomes of Litopenaeus vannamei, Marsupenaeus japonicus, and Penaeus monodon. Additional bra homologs were obtained from the outgroups Sicyonia ingentis (Decapoda: Dendrobranchiata: Sicyoniidae) and the caridean shrimp Caridina multidentata (Decapoda: Pleocymata). The genes encoded penaeid shrimp Bra proteins of 551-552 amino acids, containing the highly conserved T-box DNA-binding region. The N-terminal Smad1-binding domain, conserved in most animals, was absent in shrimp Bra. The R1 repressor domain was the best conserved of the C-terminal regulatory domains, which were widely divergent compared to other species. The penaeid shrimp bra gene consisted of six exons, with splice sites conserved with other phyla across the animal kingdom. Real-time qPCR and FPKM analysis showed that shrimp bra mRNA was strongly expressed during gastrulation. These findings begin to address the evolution of gastrulation in shrimp at the molecular level.

TGF-β Family Signaling in Early Vertebrate Development

TGF-β family ligands function in inducing and patterning many tissues of the early vertebrate embryonic body plan. Nodal signaling is essential for the specification of mesendodermal tissues and the concurrent cellular movements of gastrulation. Bone morphogenetic protein (BMP) signaling patterns tissues along the dorsal-ventral axis and simultaneously directs the cell movements of convergence and extension. After gastrulation, a second wave of Nodal signaling breaks the symmetry between the left and right sides of the embryo. During these processes, elaborate regulatory feedback between TGF-β ligands and their antagonists direct the proper specification and patterning of embryonic tissues. In this review, we summarize the current knowledge of the function and regulation of TGF-β family signaling in these processes. Although we cover principles that are involved in the development of all vertebrate embryos, we focus specifically on three popular model organisms: the mouse Mus musculus, the African clawed frog of the genus Xenopus, and the zebrafish Danio rerio, highlighting the similarities and differences between these species.

β-Catenin-dependent mechanotransduction dates back to the common ancestor of Cnidaria and Bilateria

Besides genetic regulation, mechanical forces have been identified as important cues in numerous developmental processes. Mechanical forces can activate biochemical cascades in a process called mechanotransduction. Recent studies in vertebrates and flies elucidated the role of mechanical forces for mesodermal gene expression. However, it remains unclear whether mechanotransduction is a universal regulatory mechanism throughout Metazoa. Here, we show in the sea anemone Nematostella vectensis that mechanical pressure can ectopically activate or restore brachyury expression. This mechanotransduction is dependent on β-catenin, similar to vertebrates. We propose that a regulatory feedback loop between genetic and mechanical gene activation exists during gastrulation and the β-catenin–dependent mechanotransduction is an ancient regulatory mechanism, which was present in the common ancestor of cnidarians and bilaterians.

Although the genetic regulation of cellular differentiation processes is well established, recent studies have revealed the role of mechanotransduction on a variety of biological processes, including regulation of gene expression. However, it remains unclear how universal and widespread mechanotransduction is in embryonic development of animals. Here, we investigate mechanosensitive gene expression during gastrulation of the starlet sea anemone Nematostella vectensis, a cnidarian model organism. We show that the blastoporal marker gene brachyury is down-regulated by blocking myosin II-dependent gastrulation movements. Brachyury expression can be restored by applying external mechanical force. Using CRISPR/Cas9 and morpholino antisense technology, we also show that mechanotransduction leading to brachyury expression is β-catenin dependent, similar to recent findings in fish and Drosophila [Brunet T, et al. (2013) Nat Commun 4:1–15]. Finally, we demonstrate that prolonged application of mechanical stress on the embryo leads to ectopic brachyury expression. Thus, our data indicate that β-catenin–dependent mechanotransduction is an ancient gene regulatory mechanism, which was present in the common ancestor of cnidarians and bilaterians, at least 600 million years ago.

A developmental framework for induced pluripotency

During development, cells transition from a pluripotent to a differentiated state, generating all the different types of cells in the body. Development is generally considered an irreversible process, meaning that a differentiated cell is thought to be unable to return to the pluripotent state. However, it is now possible to reprogram mature cells to pluripotency. It is generally thought that reprogramming is accomplished by reversing the natural developmental differentiation process, suggesting that the two mechanisms are closely related. Therefore, a detailed study of cell reprogramming has the potential to shed light on unexplained developmental mechanisms and, conversely, a better understanding of developmental differentiation can help improve cell reprogramming. However, fundamental differences between reprogramming processes and multi-lineage specification during early embryonic development have also been uncovered. In addition, there are multiple routes by which differentiated cells can re-enter the pluripotent state. In this Review, we discuss the connections and disparities between differentiation and reprogramming, and assess the degree to which reprogramming can be considered as a simple reversal of development.

Comparative analyses of developmental transcription factor repertoires in sponges reveal unexpected complexity of the earliest animals

Developmental transcription factors (DTFs) control development of animals by affecting expression of target genes, some of which are transcription factors themselves. In bilaterians and cnidarians, conserved DTFs are involved in homologous processes such as gastrulation or specification of neurons. The genome of Amphimedon queenslandica, the first sponge to be sequenced, revealed that only a fraction of these conserved DTF families are present in demosponges. This finding was in line with the view that morphological complexity in the animal lineage correlates with developmental toolkit complexity. However, as the phylum Porifera is very diverse, Amphimedon's genome may not be representative of all sponges. The recently sequenced genomes of calcareous sponges Sycon ciliatum and Leucosolenia complicata allowed investigations of DTFs in a sponge lineage evolutionarily distant from demosponges. Surprisingly, the phylogenetic analyses of identified DTFs revealed striking differences between the calcareous sponges and Amphimedon. As these differences appear to be a result of independent gene loss events in the two sponge lineages, the last common ancestor of sponges had to possess a much more diverse repertoire of DTFs than extant sponges. Developmental expression of sponge homologs of genes involved in specification of the Bilaterian endomesoderm and the neurosensory cells suggests that roles of many DTFs date back to the last common ancestor of all animals. Strikingly, even DTFs displaying apparent pan-metazoan conservation of sequence and function are not immune to being lost from individual species genomes. The quest for a comprehensive picture of the developmental toolkit in the last common metazoan ancestor is thus greatly benefitting from the increasing accessibility of sequencing, allowing comparisons of multiple genomes within each phylum.

Brachyury and SMAD signalling collaboratively orchestrate distinct mesoderm and endoderm gene regulatory networks in differentiating human embryonic stem cells

The transcription factor brachyury (T, BRA) is one of the first markers of gastrulation and lineage specification in vertebrates. Despite its wide use and importance in stem cell and developmental biology, its functional genomic targets in human cells are largely unknown. Here, we use differentiating human embryonic stem cells to study the role of BRA in activin A-induced endoderm and BMP4-induced mesoderm progenitors. We show that BRA has distinct genome-wide binding landscapes in these two cell populations, and that BRA interacts and collaborates with SMAD1 or SMAD2/3 signalling to regulate the expression of its target genes in a cell-specific manner. Importantly, by manipulating the levels of BRA in cells exposed to different signalling environments, we demonstrate that BRA is essential for mesoderm but not for endoderm formation. Together, our data illuminate the function of BRA in the context of human embryonic development and show that the regulatory role of BRA is context dependent. Our study reinforces the importance of analysing the functions of a transcription factor in different cellular and signalling environments.

A pipeline for the systematic identification of non-redundant full-ORF cDNAs for polymorphic and evolutionary divergent genomes: Application to the ascidian Ciona intestinalis

Genome-wide resources, such as collections of cDNA clones encoding for complete proteins (full-ORF clones), are crucial tools for studying the evolution of gene function and genetic interactions. Non-model organisms, in particular marine organisms, provide a rich source of functional diversity. Marine organism genomes are, however, frequently highly polymorphic and encode proteins that diverge significantly from those of well-annotated model genomes. The construction of full-ORF clone collections from non-model organisms is hindered by the difficulty of predicting accurately the N-terminal ends of proteins, and distinguishing recent paralogs from highly polymorphic alleles. We report a computational strategy that overcomes these difficulties, and allows for accurate gene level clustering of transcript data followed by the automated identification of full-ORFs with correct 5'- and 3'-ends. It is robust to polymorphism, includes paralog calling and does not require evolutionary proximity to well annotated model organisms. We developed this pipeline for the ascidian Ciona intestinalis, a highly polymorphic member of the divergent sister group of the vertebrates, emerging as a powerful model organism to study chordate gene function, Gene Regulatory Networks and molecular mechanisms underlying human pathologies. Using this pipeline we have generated the first full-ORF collection for a highly polymorphic marine invertebrate. It contains 19,163 full-ORF cDNA clones covering 60% of Ciona coding genes, and full-ORF orthologs for approximately half of curated human disease-associated genes.

Highly efficient targeted mutagenesis in axolotl using Cas9 RNA-guided nuclease

Among tetrapods, only urodele salamanders, such as the axolotl Ambystoma mexicanum, can completely regenerate limbs as adults. The mystery of why salamanders, but not other animals, possess this ability has for generations captivated scientists seeking to induce this phenomenon in other vertebrates. Although many recent advances in molecular biology have allowed limb regeneration and tissue repair in the axolotl to be investigated in increasing detail, the molecular toolkit for the study of this process has been limited. Here, we report that the CRISPR-Cas9 RNA-guided nuclease system can efficiently create mutations at targeted sites within the axolotl genome. We identify individual animals treated with RNA-guided nucleases that have mutation frequencies close to 100% at targeted sites. We employ this technique to completely functionally ablate EGFP expression in transgenic animals and recapitulate developmental phenotypes produced by loss of the conserved gene brachyury. Thus, this advance allows a reverse genetic approach in the axolotl and will undoubtedly provide invaluable insight into the mechanisms of salamanders' unique regenerative ability.

Early evolution of the T-box transcription factor family

Developmental transcription factors are key players in animal multicellularity, being members of the T-box family that are among the most important. Until recently, T-box transcription factors were thought to be exclusively present in metazoans. Here, we report the presence of T-box genes in several nonmetazoan lineages, including ichthyosporeans, filastereans, and fungi. Our data confirm that Brachyury is the most ancient member of the T-box family and establish that the T-box family diversified at the onset of Metazoa. Moreover, we demonstrate functional conservation of a homolog of Brachyury of the protist Capsaspora owczarzaki in Xenopus laevis. By comparing the molecular phenotype of C. owczarzaki Brachyury with that of homologs of early branching metazoans, we define a clear difference between unicellular holozoan and metazoan Brachyury homologs, suggesting that the specificity of Brachyury emerged at the origin of Metazoa. Experimental determination of the binding preferences of the C. owczarzaki Brachyury results in a similar motif to that of metazoan Brachyury and other T-box classes. This finding suggests that functional specificity between different T-box classes is likely achieved by interaction with alternative cofactors, as opposed to differences in binding specificity.

From notochord formation to hereditary chordoma: the many roles of Brachyury

Chordoma is a rare, but often malignant, bone cancer that preferentially affects the axial skeleton and the skull base. These tumors are both sporadic and hereditary and appear to occur more frequently after the fourth decade of life; however, modern technologies have increased the detection of pediatric chordomas. Chordomas originate from remnants of the notochord, the main embryonic axial structure that precedes the backbone, and share with notochord cells both histological features and the expression of characteristic genes. One such gene is Brachyury, which encodes for a sequence-specific transcription factor. Known for decades as a main regulator of notochord formation, Brachyury has recently gained interest as a biomarker and causative agent of chordoma, and therefore as a promising therapeutic target. Here, we review the main characteristics of chordoma, the molecular markers, and the clinical approaches currently available for the early detection and possible treatment of this cancer. In particular, we report on the current knowledge of the role of Brachyury and of its possible mechanisms of action in both notochord formation and chordoma etiogenesis.

Developmental diversity of amphibians

The current model amphibian, Xenopus laevis, develops rapidly in water to a tadpole which metamorphoses into a frog. Many amphibians deviate from the X. laevis developmental pattern. Among other adaptations, their embryos develop in foam nests on land or in pouches on their mother's back or on a leaf guarded by a parent. The diversity of developmental patterns includes multinucleated oogenesis, lack of RNA localization, huge non-pigmented eggs, and asynchronous, irregular early cleavages. Variations in patterns of gastrulation highlight the modularity of this critical developmental period. Many species have eliminated the larva or tadpole and directly develop to the adult. The wealth of developmental diversity among amphibians coupled with the wealth of mechanistic information from X. laevis permit comparisons that provide deeper insights into developmental processes.

In silico and in vivo identification of the intermediate filament vimentin that is downregulated downstream of Brachyury during Xenopus embryogenesis

Brachyury, a member of the T-box transcription family, has been suggested to be essential for morphogenetic movements in various processes of animal development. However, little is known about its critical transcriptional targets. In order to identify targets of Brachyury and understand the molecular mechanisms underlying morphogenetic movements, we first searched the genome sequence of Xenopus tropicalis, the only amphibian genomic sequence available, for Brachyury-binding sequences known as T-half sites, and then screened for the ones conserved between vertebrate genomes. We found three genes that have evolutionarily conserved T-half sites in the promoter regions and examined these genes experimentally to determine whether their expressions were regulated by Brachyury, using the animal cap system of Xenopus laevis embryos. Eventually, we obtained evidence that vimentin, encoding an intermediate filament protein, was a potential target of Brachyury. This is the first report to demonstrate that Brachyury might affect the cytoskeletal structure through regulating the expression of an intermediate filament protein, vimentin.

The conserved role and divergent regulation of foxa, a pan-eumetazoan developmental regulatory gene

Foxa is a forkhead transcription factor that is expressed in the endoderm lineage across metazoans. Orthologs of foxa are expressed in cells that intercalate, polarize, and form tight junctions in the digestive tracts of the mouse, the sea urchin, and the nematode and in the chordate notochord. The loss of foxa expression eliminates these morphogenetic processes. The remarkable similarity in foxa phenotypes in these diverse organisms raises the following questions: why is the developmental role of Foxa so highly conserved? Is foxa transcriptional regulation as conserved as its developmental role? Comparison of the regulation of foxa orthologs in sea urchin and in Caenorhabditis elegans shows that foxa transcriptional regulation has diverged significantly between these two organisms, particularly in the cells that contribute to the C. elegans pharynx formation. We suggest that the similarity of foxa phenotype is due to its role in an ancestral gene regulatory network that controlled intercalation followed by mesenchymal-to-epithelial transition. foxa transcriptional regulation had evolved to support the developmental program in each species so foxa would play its role controlling morphogenesis at the necessary embryonic address.

Evolutionary crossroads in developmental biology: Cnidaria

There is growing interest in the use of cnidarians (corals, sea anemones, jellyfish and hydroids) to investigate the evolution of key aspects of animal development, such as the formation of the third germ layer (mesoderm), the nervous system and the generation of bilaterality. The recent sequencing of the Nematostella and Hydra genomes, and the establishment of methods for manipulating gene expression, have inspired new research efforts using cnidarians. Here, we present the main features of cnidarian models and their advantages for research, and summarize key recent findings using these models that have informed our understanding of the evolution of the developmental processes underlying metazoan body plan formation.

Unexpected repertoire of metazoan transcription factors in the unicellular holozoan Capsaspora owczarzaki

How animals (metazoans) originated from their single-celled ancestors remains a major question in biology. As transcriptional regulation is crucial to animal development, deciphering the early evolution of associated transcription factors (TFs) is critical to understanding metazoan origins. In this study, we uncovered the repertoire of 17 metazoan TFs in the amoeboid holozoan Capsaspora owczarzaki, a representative of a unicellular lineage that is closely related to choanoflagellates and metazoans. Phylogenetic and comparative genomic analyses with the broadest possible taxonomic sampling allowed us to formulate new hypotheses regarding the origin and evolution of developmental metazoan TFs. We show that the complexity of the TF repertoire in C. owczarzaki is strikingly high, pushing back further the origin of some TFs formerly thought to be metazoan specific, such as T-box or Runx. Nonetheless, TF families whose beginnings antedate the origin of the animal kingdom, such as homeodomain or basic helix-loop-helix, underwent significant expansion and diversification along metazoan and eumetazoan stems.

A divergent Tbx6-related gene and Tbx6 are both required for neural crest and intermediate mesoderm development in Xenopus

T-box family transcription factors play many roles in Metazoan development. Here we characterise Tbx6r, a unique Tbx6 paralogue isolated from the amphibian Xenopus. The evolution and developmental integration of this divergent T-box gene within the vertebrates reveals an unexpected level of plasticity within this conserved family of developmental regulators. We show that despite their co-expression, Tbx6 and Tbx6r have dissimilar transcriptional responses to ligand treatment, and their ability to activate ligand expression is also very different. The two paralogues have distinct inductive properties: Tbx6 induces mesoderm whereas Tbx6r induces anterior neural markers. We use hybrid proteins in an effort to understand this difference, and implicate the C-terminal regions of the proteins in their inductive specificities. Through loss-of-function analyses using antisense morpholino oligonucleotides we show that both Tbx6 paralogues perform essential functions in the development of the paraxial and intermediate mesoderm and the neural crest in Xenopus. We demonstrate that Tbx6 and Tbx6r both induce FGF8 expression as well as that of pre-placodal markers, and that Tbx6 can also induce neural crest markers via a ligand-dependent mechanism involving FGF8 and Wnt8. Our data thus identify an important new function for this key developmental regulator.

Highly conserved functions of the Brachyury gene on morphogenetic movements: insight from the early-diverging phylum Ctenophora

Brachyury, a member of the T-box transcription family identified in a diverse array of metazoans, was initially recognized for its function in mesoderm formation and notochord differentiation in vertebrates; however, its ancestral role has been suggested to be in control of morphogenetic movements. Here, we show that morpholino oligonucleotide knockdown of Brachyury (MlBra) in embryos of a ctenophore, one of the most ancient groups of animals, prevents the invagination of MlBra expressing stomodeal cells and is rescued with corresponding RNA injections. Injection of RNA encoding a dominant-interfering construct of MlBra causes identical phenotypes to that of RNA encoding a dominant-interfering form of Xenopus Brachyury (Xbra) in Xenopus embryos. Both injected embryos down-regulate Xbra downstream genes, Xbra itself and Xwnt11 but not axial mesodermal markers, resulting in failure to complete gastrulation due to loss of convergent extension movements. Moreover, animal cap assay reveals that MlBra induces Xwnt11 like Xbra. Overall results using Xenopus embryos show that these two genes are functionally interchangeable. These functional experiments demonstrate for the first time in a basal metazoan that the primitive role of Brachyury is to regulate morphogenetic movements, rather than to specify endomesodermal fates, and the role is conserved between non-bilaterian metazoans and vertebrates.

Expression and mutation analysis of Tpit in the canine pituitary gland and corticotroph adenomas

Pituitary-dependent hyperadrenocorticism (PDH) in dogs is caused by a pituitary corticotroph adenoma. Although PDH is a common disorder in dogs, little is known about the underlying pathogenesis. In the pituitary glands of humans and mice, the pro-opiomelanocortin (POMC)-expressing cell lineages, the corticotrophs and melanotrophs, have a specific marker in common, the T-box transcription factor Tpit (Tbx19), which is obligate for POMC expression. Tpit also regulates the late differentiation of the corticotrophs and melanotrophs, and therefore may contribute to the pathogenesis of the corticotroph adenomas. The aim of this study was to perform an expression and mutation analysis of Tpit in the normal canine pituitary and in corticotroph adenomas. The distribution of the Tpit protein in the pituitary gland was studied with immunohistochemistry and the expression of the gene with RT-PCR. The coding region of Tpit cDNA from 14 dogs with PDH was screened for mutations. Tpit was expressed in corticotroph and melanotroph cells of the normal and adenomatous canine pituitary, and remained present in non-adenomatous corticotrophs of pituitaries from PDH dogs. No tumor-specific mutation in the Tpit cDNA from the corticotroph adenomas was found. However, a missense polymorphism in the highly conserved DNA-binding domain, the T-box, was discovered in one dog. It is concluded that Tpit can be used as a reliable marker for the corticotroph and melanotroph cells in the canine pituitary tissue and that mutations in the Tpit gene are unlikely to play a major role in the pathogenesis of canine corticotroph adenomas.

Divergent functions of two ancientHydra Brachyuryparalogues suggest specific roles for their C-terminal domains in tissue fate induction

Homologues of the T-box gene Brachyury play important roles in mesoderm differentiation and other aspects of early development in all bilaterians. In the diploblast Hydra, the Brachyuryhomologue HyBra1 acts early in the formation of the hypostome, the location of the organiser in adult Hydra. We now report the isolation and characterisation of a second Brachyury gene, HyBra2. Sequence analysis suggests that HyBra1 and HyBra2 are paralogues, resulting from an ancient lineage-specific gene duplication. We show that both paralogues acquired novel functions, both at the level of their cis-regulation as well as through significant divergence of the coding sequence. Both genes are expressed in the hypostome, but HyBra1 is predominantly endodermal, whereas HyBra2 transcripts are found primarily in the ectoderm. During bud formation, both genes are activated before any sign of evagination, suggesting an early role in head formation. During regeneration, HyBra1 is an immediate-early response gene and is insensitive to protein synthesis inhibition, whereas the onset of expression of HyBra2 is delayed and requires protein synthesis. The functional consequence of HyBra1/2 protein divergence on cell fate decisions was tested in Xenopus. HyBra1 induces mesoderm, like vertebrate Brachyury proteins. By contrast, HyBra2 shows a strong cement-gland and neural-inducing activity. Domain-swapping experiments show that the C-terminal domain of HyBra2 is responsible for this specific phenotype. Our data support the concept of sub- and neofunctionalisation upon gene duplication and show that divergence of cis-regulation and coding sequence in paralogues can lead to dramatic changes in structure and function.

A multicassette Gateway vector set for high throughput and comparative analyses in ciona and vertebrate embryos

Background

The past few years have seen a vast increase in the amount of genomic data available for a growing number of taxa, including sets of full length cDNA clones and cis-regulatory sequences. Large scale cross-species comparisons of protein function and cis-regulatory sequences may help to understand the emergence of specific traits during evolution.

Principal Findings

To facilitate such comparisons, we developed a Gateway compatible vector set, which can be used to systematically dissect cis-regulatory sequences, and overexpress wild type or tagged proteins in a variety of chordate systems. It was developed and first characterised in the embryos of the ascidian Ciona intestinalis, in which large scale analyses are easier to perform than in vertebrates, owing to the very efficient embryo electroporation protocol available in this organism. Its use was then extended to fish embryos and cultured mammalian cells.

Conclusion

This versatile vector set opens the way to the mid- to large-scale comparative analyses of protein function and cis-regulatory sequences across chordate evolution. A complete user manual is provided as supplemental material.

Why polyps regenerate and we don't: towards a cellular and molecular framework for Hydra regeneration

The basis for Hydra's enormous regeneration capacity is the "stem cellness" of its epithelium which continuously undergoes self-renewing mitotic divisions and also has the option to follow differentiation pathways. Now, emerging molecular tools have shed light on the molecular processes controlling these pathways. In this review I discuss how the modular tissue architecture may allow continuous replacement of cells in Hydra. I also describe the discovery and regulation of factors controlling the transition from self-renewing epithelial stem cells to differentiated cells.

When Brachyury meets Smad1: the evolution of bilateral symmetry during gastrulation

Understanding the events that led to the emergence of the bilaterians is a daunting task, impaired by the huge evolutionary gap separating us from the pre-Cambrian. During gastrulation, the expression of the transcription factor Brachyury is remarkably well conserved around the blastopore of bilaterians and cnidarians. Only the bilaterian Brachyury proteins, however, share a distinctive N-terminal sequence not found in outgroups such as cnidarians, sponges or placozoans. We now know that, in vertebrates, this N-terminal domain confers specific transcriptional activity, by recruiting Smad1, the first identified co-factor for Brachyury. Smad1 is an effector of the BMP pathway, and has been isolated in bilaterians and cnidarians. Here, I propose that the protein-protein interaction between Brachyury and Smad1 represents an evolutionary novelty of the Urbilateria. The gain of the N-terminal domain might have been selected to spatially modulate the activity of Brachyury, thereby facilitating the establishment of bilateral symmetry during gastrulation movements.

Evolution of the heart from bacteria to man

This review provides an overview of the evolutionary path to the mammalian heart from the beginnings of life (about four billion years ago ) to the present. Essential tools for cellular homeostasis and for extracting and burning energy are still in use and essentially unchanged since the appearance of the eukaryotes. The primitive coelom, characteristic of early multicellular organisms ( approximately 800 million years ago), is lined by endoderm and is a passive receptacle for gas exchange, feeding, and sexual reproduction. The cells around this structure express genes homologous to NKX2.5/tinman, and gradual specialization of this "gastroderm" results in the appearance of mesoderm in the phylum Bilateria, which will produce the first primitive cardiac myocytes. Investment of the coelom by these mesodermal cells forms a "gastrovascular" structure. Further evolution of this structure in the bilaterian branches Ecdysoa (Drosophila) and Deuterostoma (amphioxus) culminate in a peristaltic tubular heart, without valves, without blood vessels or blood, but featuring a single layer of contracting mesoderm. The appearance of Chordata and subsequently the vertebrates is accompanied by a rapid structural diversification of this primitive linear heart: looping, unidirectional circulation, an enclosed vasculature, and the conduction system. A later innovation is the parallel circulation to the lungs, followed by the appearance of septa and the four-chambered heart in reptiles, birds, and mammals. With differentiation of the cardiac chambers, regional specialization of the proteins in the cardiac myocyte can be detected in the teleost fish and amphibians. In mammals, growth constraints are placed on the heart, presumably to accommodate the constraints of the body plan and the thoracic cavity, and adult cardiac myocytes lose the ability to re-enter the cell cycle on demand. Mammalian cardiac myocyte innervation betrays the ancient link between the heart, the gut, and reproduction: the vagus nerve controlling heart rate emanates from centers in the central nervous system regulating feeding and affective behavior.

Expression pattern of the Brachyury and Tbx2 homologues from the sponge Suberites domuncula

BACKGROUND INFORMATION

T-box transcription factors are a large family of transcriptional regulators involved in many aspects of embryonic development. In a previous report, we described the isolation and genomic characterization of two T-box genes from the siliceous sponge Suberites domuncula: a Brachyury homologue, Sd-Bra, and a Tbx2 homologue, Sd-Tbx2. Elucidation of the genomic structure of Sd-Bra allowed us to demonstrate the existence of two different isoforms, resulting from alternative splicing. Moreover, we demonstrated that the shorter isoform exists in two different glycosylation states.

RESULTS

In the present study, we demonstrate a differential subcellular localization of the three Sd-Bra isoforms, suggesting that its differential nuclear import could be an important mechanism for its functional regulation. Furthermore, we demonstrate that Sd-Tbx2 exists only in one isoform, which is mainly localized in the nucleus. The pattern of expression of Sd-Bra and Sd-Tbx2 genes is analysed in sponge tissue, in gemmules and in cultured cells.

CONCLUSION

These results suggest a conserved role for Sd-Bra in the control of morphogenetic movements through the regulation of cell-adhesion properties and the involvement of Sd-Tbx2 in the determination of cell identity in the early stages of differentiation, reminiscent of the function of Tbx2-3-4-5 in vertebrates during limb specification. Also, the fact that a Brachyury and a Tbx2 homologue exist in S. domuncula suggests that the first divergence from the ancestral Brachyury-like gene might be a Tbx2-like gene and not a Tbrain-like gene as had been previously suggested.

Conservation and co-option in developmental programmes: the importance of homology relationships

One of the surprising insights gained from research in evolutionary developmental biology (evo-devo) is that increasing diversity in body plans and morphology in organisms across animal phyla are not reflected in similarly dramatic changes at the level of gene composition of their genomes. For instance, simplicity at the tissue level of organization often contrasts with a high degree of genetic complexity. Also intriguing is the observation that the coding regions of several genes of invertebrates show high sequence similarity to those in humans. This lack of change (conservation) indicates that evolutionary novelties may arise more frequently through combinatorial processes, such as changes in gene regulation and the recruitment of novel genes into existing regulatory gene networks (co-option), and less often through adaptive evolutionary processes in the coding portions of a gene. As a consequence, it is of great interest to examine whether the widespread conservation of the genetic machinery implies the same developmental function in a last common ancestor, or whether homologous genes acquired new developmental roles in structures of independent phylogenetic origin. To distinguish between these two possibilities one must refer to current concepts of phylogeny reconstruction and carefully investigate homology relationships. Particularly problematic in terms of homology decisions is the use of gene expression patterns of a given structure. In the future, research on more organisms other than the typical model systems will be required since these can provide insights that are not easily obtained from comparisons among only a few distantly related model species.

Control of foot differentiation in Hydra: Phylogenetic footprinting indicates interaction of head, bud and foot patterning systems

Homeodomain transcription factor CnNK-2 seems to play a major role in foot formation in Hydra. Recently, we reported in vitro evidence indicating that CnNK-2 has autoregulatory features and regulates expression of the morphogenetic peptide pedibin. We proposed that CnNK-2 and pedibin synergistically orchestrate foot differentiation processes. Here, we further analyzed the regulatory network controlling foot formation in Hydra. By phylogenetic footprinting we compared the CnNK-2 5'-flanking sequence from two closely related species, Hydra vulgaris and Hydra oligactis. Unexpectedly, we detected a highly conserved binding site for HNF-3beta, a vertebrate Forkhead transcription factor, in the CnNK-2 5'-flanking region. The Hydra HNF-3beta homolog budhead is predominantly expressed in the apical region of the body column and early during budding. Budhead is absent from tissue expressing CnNK-2 and thought to be involved in determining tissue for head differentiation. By electrophoretic mobility shift assays we demonstrate an in vitro interaction between recombinant budhead protein and the interspecific conserved HNF-3beta binding motif in the CnNK-2 5'-flanking region. Our results strengthen the view of CnNK-2 as an important regulator during foot patterning processes. Furtheron, they point to budhead as a candidate for a transcriptional regulator of CnNK-2 and to an interaction of foot and head patterning processes in Hydra on the molecular level.

Functional Specificity of the Xenopus T-Domain Protein Brachyury Is Conferred by Its Ability to Interact with Smad1

Members of the T-box gene family play important and diverse roles in development and disease. Here, we study the functional specificities of the Xenopus T-domain proteins Xbra and VegT, which differ in their abilities to induce gene expression in prospective ectodermal tissue. In particular, VegT induces strong expression of goosecoid whereas Xbra cannot. Our results indicate that Xbra is unable to induce goosecoid because it directly activates expression of Xom, a repressor of goosecoid that acts downstream of BMP signaling. We show that the inability of Xbra to induce goosecoid is imposed by an N-terminal domain that interacts with the C-terminal MH2 domain of Smad1, a component of the BMP signal transduction pathway. Interference with this interaction causes ectopic activation of goosecoid and anteriorization of the embryo. These findings suggest a mechanism by which individual T-domain proteins may interact with different partners to elicit a specific response.

Making very similar embryos with divergent genomes: conservation of regulatory mechanisms of Otx between the ascidians Halocynthia roretzi and Ciona intestinalis

Ascidian embryos develop with a fixed cell lineage into simple tadpoles. Their lineage is almost perfectly conserved, even between the evolutionarily distant species Halocynthia roretzi and Ciona intestinalis, which show no detectable sequence conservation in the non-coding regions of studied orthologous genes. To address how a common developmental program can be maintained without detectable cis-regulatory sequence conservation, we compared in both species the regulation of Otx, a gene with a shared complex expression pattern. We found that in Halocynthia, the regulatory logic is based on the use of very simple cell line-specific regulatory modules, the activities of which are conserved, in most cases, in the Ciona embryo. The activity of each of these enhancer modules relies on the conservation of a few repeated crucial binding sites for transcriptional activators, without obvious constraints on their precise number, order or orientation, or on the surrounding sequences. We propose that a combination of simplicity and degeneracy allows the conservation of the regulatory logic, despite drastic sequence divergence. The regulation of Otx in the anterior endoderm by Lhx and Fox factors may even be conserved with vertebrates.

Tbx5 and Tbx20 act synergistically to control vertebrate heart morphogenesis

Members of the T-box family of proteins play a fundamental role in patterning the developing vertebrate heart; however, the precise cellular requirements for any one family member and the mechanism by which individual T-box genes function remains largely unknown. In this study, we have investigated the cellular and molecular relationship between two T-box genes, Tbx5 and Tbx20. We demonstrate that blocking Tbx5 or Tbx20 produces phenotypes that display a high degree of similarity, as judged by overall gross morphology, molecular marker analysis and cardiac physiology, implying that the two genes are required for and have non-redundant functions in early heart development. In addition, we demonstrate that although co-expressed, Tbx5 and Tbx20 are not dependent on the expression of one another, but rather have a synergistic role during early heart development. Consistent with this proposal, we show that TBX5 and TBX20 can physically interact and map the interaction domains, and we show a cellular interaction for the two proteins in cardiac development, thus providing the first evidence for direct interaction between members of the T-box gene family.

The relative role of the T-domain and flanking sequences for developmental control and transcriptional regulation in protein chimeras of Drosophila OMB and ORG-1

optomotor-blind (omb) and optomotor-blind related-1 (org-1) encode T-domain DNA binding proteins in Drosophila. Members of this family of transcription factors play widely varying roles during early development and organogenesis in both vertebrates and invertebrates. Functional specificity differs in spite of similar DNA binding preferences of all family members. Using a series of domain swap chimeras, in which different parts of OMB and ORG-1 were mutually exchanged, we investigated the relevance of individual domains in vitro and in vivo. In cell culture transfection assays, ORG-1 was a strong transcriptional activator, whereas OMB appeared neutral. The main transcriptional activation function was identified in the C-terminal part of ORG-1. Also in vivo, OMB and ORG-1 showed qualitative differences when the proteins were ectopically expressed during development. Gain-of-function expression of OMB is known to counteract eye formation and resulted in the loss of the arista, whereas ORG-1 had little effect on eye development but caused antenna-to-leg transformations and shortened legs in the corresponding gain-of-function situations. The functional properties of OMB/ORG-1 chimeras in several developmental contexts was dominated by the origin of the C-terminal region, suggesting that the transcriptional activation potential can be one major determinant of developmental specificity. In late eye development, we observed, however, a strong influence of the T-domain on ommatidial differentiation. The specificity of chimeric omb/org-1transgenes, thus, depended on the cellular context in which they were expressed. This suggests that both transcriptional activation/repression properties as well as intrinsic DNA binding specificity can contribute to the functional characteristics of T-domain factors.

Using ascidian embryos to study the evolution of developmental gene regulatory networks

Ascidians are ideally positioned taxonomically at the base of the chordate tree to provide a point of comparison for developmental regulatory mechanisms that operate among protostomes, non-chordate deuterostomes, invertebrate chordates, and vertebrates. In this review, we propose a model for the gene regulatory network that gives rise to the ascidian notochord. The purpose of this model is not to clarify all of the interactions between molecules of this network, but to provide a working schematic of the regulatory architecture that leads to the specification of endoderm and the patterning of mesoderm in ascidian embryos. We describe a series of approaches, both computational and biological, that are currently being used, or are in development, for the study of ascidian embryo gene regulatory networks. It is our belief that the tools now available to ascidian biologists, in combination with a streamlined mode of development and small genome size, will allow for more rapid dissection of developmental gene regulatory networks than in more complex organisms such as vertebrates. It is our hope that the analysis of gene regulatory networks in ascidians can provide a basic template which will allow developmental biologists to superimpose the modifications and novelties that have arisen during deuterostome evolution.

Comparative analysis of Brachyury T-domains, with the characterization of two new sponge sequences, from a hexactinellid and a calcisponge

The Brachyury family of T-domain containing transcription factor has been recently the subject of a number of Evo-Devo studies, with expression data obtained from a wide sampling of eumetazans, pointing to a possible conserved role in the formation of the blastopore and the extremities of the digestive tract. Here we present a comparative analysis of Brachyury sequences at the metazoan scale, using published data and two new sponge Brachyury sequences. Alignment features, gene phylogeny, and the evolution of variable positions within the T-domain are discussed in the light of available data about functional constraints on the residues. Interestingly, the high sequence divergence observed in Brachyury T-domains from sponges appears to be mostly the consequence of autapomorphic changes within the sponge lineages, rather than the retention of primitive character states.

Molecular heterotopy in the expression of Brachyury orthologs in order Clypeasteroida (irregular sea urchins) and order Echinoida (regular sea urchins)

The expression patterns of Brachyury (Bra) orthologs in the development of four species of sand dollars (order: Clypeasteroida), including a direct-developing species, and of a sea urchin species (order: Echinoida) were investigated during the period from blastula to the pluteus stage, with special attention paid to the relationship between the expression pattern and the mode of gastrulation. The sand dollar species shared two expression domains of the Bra orthologs with the Echinoida species, in the vegetal ring (the first domain) and the oral ectoderm (the second domain). The following heterotopic changes in the expression of the Bra genes were found among the sand dollar species and between the sand dollars and the Echinoida species. (1) The vegetal ring expressing Bra in the sand dollars was much wider and was located at a higher position along the AV axis, compared with that in the Echinoida species. The characteristic Bra expression in the vegetal ring of the sand dollar embryos was thought to be involved in the mode of gastrulation, in which involution continues from the beginning of invagination until the end of gastrulation. (2) Two of the three indirect-developing sand dollar species that were examined exhibited a third domain, in which Bra was expressed on the oral side of the archenteron. (3) In the direct-developing sand dollar embryos, Bra was expressed with an oral-aboral asymmetry in the vegetal ring and with a left-right asymmetry in the oral ectoderm. In the Echinoida species, Bra was expressed in the vestibule at the six-armed pluteus stage.

A genetic regulatory network for Xenopus mesendoderm formation

We have constructed a genetic regulatory network (GRN) summarising the functional relationships between the transcription factors (TFs) and embryonic signals involved in Xenopus mesendoderm formation. It is supported by a relational database containing the experimental evidence and both are available in interactive form via the World Wide Web. This network highlights areas for further study and provides a framework for systematic interrogation of new data. Comparison with the equivalent network for the sea urchin identifies conserved features of the deuterostome ancestral pathway, including positive feedback loops, GATA factors, SoxB, Brachyury and a previously underemphasised role for beta-catenin. In contrast, some features central to one species have not yet been found in the other, for example, Krox and Otx in sea urchin, and Mix and Nodal in Xenopus. Such differences may represent evolved features or may eventually be resolved. For example, in Xenopus, Nodal-related genes are positively regulated by beta-catenin and at least one of them is repressed by Sox3, as is the uncharacterised early signal (ES) inducing endomesoderm in the sea urchin, suggesting that ES may be a Nodal-like TGF-beta. Wider comparisons of such networks will inform our understanding of developmental evolution.

T-box genes in early embryogenesis

The T-box gene family, encoding related DNA-binding transcriptional regulators, plays an essential role in controlling many aspects of embryogenesis in a wide variety of organisms. The T-box genes exhibit diverse patterns of spatial and temporal expression in the developing embryo, and both genetic and molecular embryological studies have demonstrated their importance in regulating cell fate decisions that establish the early body plan, and in later processes underlying organogenesis. Despite these studies, little is known of either the regulation of the T-box genes or the identities of their transcriptional targets. The aim of this review is to examine the diverse yet conserved roles of several T-box genes in regulating early patterning in chordates and to discuss possible mechanisms through which this functional diversity might arise. Developmental Dynamics 229:201-218, 2004.