T-box genes in early embryogenesis

Revisiting chordoma with brachyury, a "new age" marker: analysis of a validation study on 51 cases

CONTEXT

Chordoma is a rare, notochordal tumor with a characteristic histomorphology and immunohistochemical profile. At times, it presents a diagnostic challenge, especially in small biopsies. Brachyury, a nuclear transcription factor, is a recently described immunohistochemical marker for diagnosing chordomas.

OBJECTIVE

To study the sensitivity and specificity of brachyury in diagnosing chordomas by comparing its expression in axial chordomas with nonchordomatous tumors.

DESIGN

Fifty-one axial chordomas, accessioned during a 10-year period, and 58 nonchordomatous tumors were subjected to brachyury staining by immunohistochemistry.

RESULTS

The 51 chordomas occurred in 36 men and 15 women. Sitewise, 34 cases (66.7%) occurred in the sacrococcyx, 9 (17.6%) in the spine, and 8 (15.7%) in the skull base. Histologically, 34 cases (66.7%) were classical chordomas, 13 cases (25.5%) had a dominant chondroid component, and 2 cases each (3.9%) were chondroid chordomas and dedifferentiated chordomas, respectively. Brachyury staining was positive in 46 of the 51 chordomas (90.2%) and negative in all 58 nonchordomatous tumors. The dedifferentiated area in 2 chordomas was negative for brachyury staining. Fourteen of 15 chordomas with chondroid component showed positive brachyury staining. Immunohistochemical expression of other markers, included cytokeratin (positive in 23 of 23 cases; 100%), epithelial membrane antigen (positive in 22 of 22 cases; 100%) and S100 protein (positive in 18 of 21 cases; 85.7%).

CONCLUSION

Exclusive brachyury expression in more than 90% of chordomas indicates its value as a unique, specific marker with other sensitive markers like cytokeratin, epithelial membrane antigen, and/or S100 protein in substantiating a diagnosis of chordoma, including on small biopsies.

The BMP pathway acts to directly regulate Tbx20 in the developing heart

TBX20 has been shown to be essential for vertebrate heart development. Mutations within the TBX20 coding region are associated with human congenital heart disease, and the loss of Tbx20 in a wide variety of model systems leads to cardiac defects and eventually heart failure. Despite the crucial role of TBX20 in a range of cardiac cellular processes, the signal transduction pathways that act upstream of Tbx20 remain unknown. Here, we have identified and characterized a conserved 334 bp Tbx20 cardiac regulatory element that is directly activated by the BMP/SMAD1 signaling pathway. We demonstrate that this element is both necessary and sufficient to drive cardiac-specific expression of Tbx20 in Xenopus, and that blocking SMAD1 signaling in vivo specifically abolishes transcription of Tbx20, but not that of other cardiac factors, such as Tbx5 and MHC, in the developing heart. We further demonstrate that activation of Tbx20 by SMAD1 is mediated by a set of novel, non-canonical, high-affinity SMAD-binding sites located within this regulatory element and that phospho-SMAD1 directly binds a non-canonical SMAD1 site in vivo. Finally, we show that these non-canonical sites are necessary and sufficient for Tbx20 expression in Xenopus, and that reporter constructs containing these sites are expressed in a cardiac-specific manner in zebrafish and mouse. Collectively, our findings define Tbx20 as a direct transcriptional target of the BMP/SMAD1 signaling pathway during cardiac maturation.

The T-box transcription factor Tbx2: its role in development and possible implication in cancer

Tbx2 is a member of the T-box family of transcription factors that are crucial in embryonic development. Recent studies suggest that T-box factors may also play a role in controlling cell cycle progression and in the genesis of cancer. Tbx2 has been implicated in several developmental processes such as coordinating cell fate, patterning and morphogenesis of a wide range of tissues and organs including limbs, kidneys, lungs, mammary glands, heart, and craniofacial structures. Importantly, Tbx2 is overexpressed in several cancers including melanoma, small cell lung carcinoma, breast, pancreatic, liver, and bladder cancers and can suppress senescence, a cellular process, which serves as a barrier to cancer development. This review presents a state of the art overview of the role and regulation of Tbx2 in early embryonic development and in cancer.

Additive and global functions of HoxA cluster genes in mesoderm derivatives

Hox genes encode transcription factors that play a central role in the specification of regional identities along the anterior to posterior body axis. In the developing mouse embryo, Hox genes from all four genomic clusters are involved in range of developmental processes, including the patterning of skeletal structures and the formation of several organs. However, the functional redundancy observed either between paralogous genes, or among neighboring genes from the same cluster, has hampered functional analyses, in particular when synergistic, cluster-specific functions are considered. Here, we report that mutant mice lacking the entire HoxA cluster in mesodermal lineages display the expected spectrum of postnatal respiratory, cardiac and urogenital defects, previously reported for single gene mutations. Likewise, mild phenotypes are observed in both appendicular and axial skeleton. However, a striking effect was uncovered in the hematopoietic system, much stronger than that seen for Hoxa9 inactivation alone, which involves stem cells (HSCs) as well as the erythroid lineage, indicating that several Hoxa genes are necessary for normal hematopoiesis to occur. Finally, the combined deletions of Hoxa and Hoxd genes reveal abnormalities in axial elongation as well as skin morphogenesis that are likely the results of defects in epithelial-mesenchymal interactions.

Second lineage of heart forming region provides new understanding of conotruncal heart defects

Abnormal heart development causes various congenital heart defects. Recent cardiovascular biology studies have elucidated the morphological mechanisms involved in normal and abnormal heart development. The primitive heart tube originates from the lateral-most part of the heart forming mesoderm and mainly gives rise to the left ventricle. Then, during the cardiac looping, the outflow tract is elongated by the addition of cardiogenic cells from the both pharyngeal and splanchnic mesoderm (corresponding to anterior and secondary heart field, respectively), which originate from the mediocaudal region of the heart forming mesoderm and are later located anteriorly (rostrally) to the dorsal region of the heart tube. Therefore, the heart progenitors that contribute to the outflow tract region are distinct from those that form the left ventricle. The knowledge that there are two different lineages of heart progenitors in the four-chambered heart provides new understanding of the morphological and molecular etiology of conotruncal heart defects.

Novel immunohistochemical markers in the diagnosis of nonglial tumors of nervous system

Immunohistochemistry (IHC) has become an important adjunct tool in diagnostic neuro-oncology practice enabling immunophenotypic characterization of tumor cells. There have been several recent publications regarding new IHC markers that are useful for diagnosis of brain tumors. To introduce the latest advances in IHC in this field, we review the features of novel IHC marker antibodies applicable to selected nonglial tumors in the nervous system, based on recently published reports and our own experiences. We discuss (1) aquaporin-1 and alpha-inhibin for hemangioblastoma, (2) beta-catenin for craniopharyngioma, (3) brachyury for chordoma, and (4) INI-1 for hereditary schwannomas. All the markers presented here are used primarily for supporting or confirming the histologic diagnosis, with the exception of (4), which may be of help in identification of inherited forms in schwannomas. As with other surgical pathology practices, the judicious use of a panel of IHC antibodies selected on the basis of the histologic findings is important for an accurate diagnosis of brain tumors. Of note is that IHC results should be always interpreted in the histopathologic context.

Analysis of the fibroblastic growth factor receptor-RAS/RAF/MEK/ERK-ETS2/brachyury signalling pathway in chordomas

Chordomas are rare primary malignant bone tumours that derive from notochord precursor cells and express brachyury, a molecule involved in notochord development. Little is known about the genetic events responsible for driving the growth of this tumour, but it is well established that brachyury is regulated through fibroblastic growth factor receptors (FGFRs) through RAS/RAF/MEK/ERK and ETS2 in ascidian, Xenopus and zebrafish, although little is known about its regulation in mammals. The aim of this study was to attempt to identify the molecular genetic events that are responsible for the pathogenesis of chordomas with particular focus on the FGFR signalling pathway on the basis of the evidence in the ascidian and Xenopus models that the expression of brachyury requires the activation of this pathway. Immunohistochemistry showed that 47 of 50 chordomas (94%) expressed at least one of the FGFRs, and western blotting showed phosphorylation of fibroblast growth factor receptor substrate 2 alpha (FRS2alpha), an adaptor signalling protein, that links FGFR to the RAS/RAF/MEK/ERK pathway. Screening for mutations in brachyury (all coding exons and promoter), FGFRs 1-4 (previously reported mutations), KRAS (codons 12, 13, 51, 61) and BRAF (exons 11 and 15) failed to show any genetic alterations in 23 chordomas. Fluorescent in situ hybridisation analysis on FGFR4, ETS2 and brachyury failed to show either amplification of these genes, although there was minor allelic gain in brachyury in three tumours, or translocation for ERG and ETS2 loci. The key genetic events responsible for the initiation and progression of chordomas remain to be discovered.

A microarray screen for direct targets of Zic1 identifies an aquaporin gene, aqp-3b, expressed in the neural folds

The Zic1 transcription factor plays multiple roles during early development, for example, in patterning the early neural plate and formation of the neural crest, somites, and cerebellum. To identify direct downstream target genes of Zic1, a microarray screen was conducted in Xenopus laevis that identified 85 genes upregulated twofold or more. These include transcription factors, receptors, enzymes, proteins involved in retinoic acid signaling, and an aquaglyceroporin (aqp-3b), but surprisingly no genes known to be involved in cell proliferation. We show that both aqp-3 and aqp-3b were expressed in adult tissues, while during early embryonic development, only aqp-3b was transcribed. During neurula stages, aqp-3b was expressed specifically in the neural folds. This pattern of aqp-3b expression closely resembled that of NF-protocadherin (NFPC), which is involved in cell adhesion and neural tube closure. Aqp-3b may also be involved in neural tube closure, since mammalian Aqp-3 promotes cell migration and proliferation.

Members of the miR-290 cluster modulate in vitro differentiation of mouse embryonic stem cells

We report the biological effects of miR-290 cluster via gain-of-function or loss-of-function experiments in mouse embryonic stem cells (ESCs) cultured under differentiation conditions. Under these conditions we found that overexpression of miR-290 cluster in ESCs cannot prevent downregulation of Oct-4, but inhibition results in earlier downregulation of Oct-4 compared with the negative control. In consistence with previous findings that report ectopic expression of Brachyury during gastrulation in Argonaute-2 KO mice due to impaired miRNA function, we show that miR-290 cluster regulates negatively differentiation of ESCs towards mesodermal and germ cell lineage. These results suggest that although incapable to maintain pluripotent state alone, miR-290 cluster inhibits ESC differentiation and it is involved in the pathways controlling mesoderm and primordial germ cell differentiation. Finally, we provide proofs that members of this cluster target Dkk-1 gene, a Wnt pathway inhibitor, and affect this pathway, which can partially explain why miR-290 cluster favours pluripotency against differentiation.

Smad6 inhibits the transcriptional activity of Tbx6 by mediating its degradation

Members of the bone morphogenetic protein (BMP) and T-box gene families play several critical roles in the early embryonic development and tissue homeostasis. Although BMP proteins are the upstream regulators of T-box genes, few studies have investigated the molecular mechanisms between these two protein families. Here, we report that Tbx6 interacts directly with Smad6, an inhibitory Smad that antagonizes the BMP signal. This interaction is mediated through the Mad homology 2 (MH2) domain of Smad6 and residues 90-180 of Tbx6. We demonstrate that Smad6 facilitates the degradation of Tbx6 protein through recruitment of Smurf1, a ubiquitin E3 ligase. Consequently, Smad6 reduces Tbx6-mediated Myf-5 gene activation. Furthermore, specific knockdown of endogenous Smad6 and Smurf1 by small interfering RNA increases the protein levels of Tbx6 and enhance the expression of Tbx6 target genes. Collectively, these findings reveal that Smad6 serves as a critical mediator of BMP signal via a functional interaction with Tbx6, thus regulating the activation of Tbx6 downstream genes during cell differentiation.

Overlapping functions of Cdx1, Cdx2, and Cdx4 in the development of the amphibian Xenopus tropicalis

Using Xenopus tropicalis, we present the first analysis of the developmental effects that result from knocking down the function of the three Cdx genes present in the typical vertebrate genome. Knockdowns of individual Cdx genes lead to a similar range of posterior defects; compound Cdx knockdowns result in increasingly severe posterior truncations, accompanied by posterior shifts and reduction of 5' Hox gene expression. We provide evidence that Cdx and Wnt3A genes are components of a positive feedback loop operating in the posterior axis. We show that Cdx function is required during later, but not early stages of development, for correct regional specification of the endoderm and morphogenesis of the gut. Our results support the hypothesis that during amphibian development the overall landscape of Cdx activity in the embryo is more important than the specific function of individual Cdx proteins.

Germ layer patterning in bichir and lamprey; an insight into its evolution in vertebrates

Amphibian holoblastic cleavage in which all blastomeres contribute to any one of the three primary germ layers has been widely thought to be a developmental pattern in the stem lineage of vertebrates, and meroblastic cleavage to have evolved independently in each vertebrate lineage. In extant primitive vertebrates, agnathan lamprey and basal bony fishes also undergo holoblastic cleavage, and their vegetal blastomeres have been generally thought to contribute to embryonic endoderm. However, the present marker analyses in basal ray-finned fish bichir and agnathan lamprey embryos indicated that their mesoderm and endoderm develop in the equatorial marginal zone, and their vegetal cell mass is extraembryonic nutritive yolk cells, having non-cell autonomous meso-endoderm inducing activity. Eomesodermin (eomes), but not VegT, orthologs are expressed maternally in these animals, suggesting that VegT is a maternal factor for endoderm differentiation only in amphibian. The study raises the viewpoint that the lamprey/bichir type holoblastic development would have been ancestral to extant vertebrates and retained in their stem lineage; amphibian-type holoblastic development would have been acquired secondarily, accompanied by the exploitation of new molecular machinery such as maternal VegT.

Conformational stability and DNA binding specificity of the cardiac T-box transcription factor Tbx20

The transcription factor Tbx20 acts within a hierarchy of T-box factors in lineage specification and morphogenesis in the mammalian heart and is mutated in congenital heart disease. T-box family members share a approximately 20-kDa DNA-binding domain termed the T-box. The question of how highly homologous T-box proteins achieve differential transcriptional control in heart development, while apparently binding to the same DNA sequence, remains unresolved. Here we show that the optimal DNA recognition sequence for the T-box of Tbx20 corresponds to a T-half-site. Furthermore, we demonstrate using purified recombinant domains that distinct T-boxes show significant differences in the affinity and kinetics of binding and in conformational stability, with the T-box of Tbx20 displaying molten globule character. Our data highlight unique features of Tbx20 and suggest mechanistic ways in which cardiac T-box factors might interact synergistically and/or competitively within the cardiac regulatory network.

Postnatal growth, differentiation, and aging of the mouse intervertebral disc

STUDY DESIGN

This study follows postnatal intervertebral disc (IVD) growth and differentiation in the mouse. OBJECTIVE.: To initiate use of the mouse as a model system for postnatal IVD differentiation and growth, and to serve as a basis for assaying changes caused by disease or genetic or experimental perturbation.

SUMMARY OF BACKGROUND DATA

Lower back pain caused by disc degeneration is one of the most common clinical conditions worldwide. There is currently no completely effective treatment, largely because of a lack of basic knowledge of the molecular and cellular controls of disc growth, differentiation, and maintenance after birth.

METHODS

Conventional histology of decalcified IVDs, differential interference contrast, polarizing optics, immunocytochemistry, laser capture microscopy followed by molecular analysis of the dissected cells by reverse transcriptase polymerase chain reaction.

RESULTS

There is a single postnatal growth spurt in the mouse IVD, between birth and 9 weeks of age. Cell proliferation was found in the nucleus pulposus (NP) and anulus fibrosus (AF) only until 3 weeks of age. Most of the postnatal growth of the IVD is due to accumulating extracellular matrix. NP cell numbers decline steadily after 2 weeks of age, because of apoptosis. Laser capture microscopy was used to dissect NP cells from the disc, and showed that these cells express markers of the embryonic notochord. The postnatal AF appears initially as a continuous structure surrounding the NP. This structure differentiates, during the first 2 postnatal weeks, to form the mineralized, but nonossified endplate over the surfaces of the vertebral growth plates, and the mature fibrous AF (fAF) passing between adjacent vertebrae. The fact that the mature fAF and the endplate form from an originally continuous layer of cells explains the anatomic relationship between these 2 structures, in which the fAF inserts into the vertebral endplate.

CONCLUSION

Growth of the IVD takes place during the first 9 postnatal weeks, although cell proliferation ceases after 3 weeks. After birth, the early postnatal IVD differentiates into 3 tissue types, the NP, the fAF between the vertebrae, and the mineralized endplates over the surfaces of the vertebrae.

Bmp inhibition is necessary for post-gastrulation patterning and morphogenesis of the zebrafish tailbud

Intricate interactions between the Wnt and Bmp signaling pathways pattern the gastrulating vertebrate embryo using a network of secreted protein ligands and inhibitors. While many of these proteins are expressed post-gastrula, their later roles have typically remained unclear, obscured by the effects of early perturbation. We find that Bmp signaling continues during somitogenesis in zebrafish embryos, with high activity in a small region of the mesodermal progenitor zone at the posterior end of the embryo. To test the hypothesis that Bmp inhibitors expressed just anterior to the tailbud are important to restrain Bmp signaling we produced a new zebrafish transgenic line, allowing temporal cell-autonomous activation of Bmp signaling and thereby bypassing the effects of the Bmp inhibitors. Ectopic activation of Bmp signaling during somitogenesis results in severe defects in the tailbud, including altered morphogenesis and gene expression. We show that these defects are due to non-autonomous effects on the tailbud, and present evidence that the tailbud defects are caused by alterations in Wnt signaling. We present a model in which the posteriorly expressed Bmp inhibitors function during somitogenesis to constrain Bmp signaling in the tailbud in order to allow normal expression of Wnt inhibitors in the presomitic mesoderm, which in turn constrain the levels of canonical and non-canonical Wnt signaling in the tailbud.

A gene regulatory network directed by zebrafish No tail accounts for its roles in mesoderm formation

Using chromatin immunoprecipitation combined with genomic microarrays we have identified targets of No tail (Ntl), a zebrafish Brachyury ortholog that plays a central role in mesoderm formation. We show that Ntl regulates a downstream network of other transcription factors and identify an in vivo Ntl binding site that resembles the consensus T-box binding site (TBS) previously identified by in vitro studies. We show that the notochord-expressed gene floating head (flh) is a direct transcriptional target of Ntl and that a combination of TBSs in the flh upstream region are required for Ntl-directed expression. Using our genome-scale data we have assembled a preliminary gene regulatory network that begins to describe mesoderm formation and patterning in the early zebrafish embryo.

PMesogenin1 and 2 function directly downstream of Xtbx6 in Xenopus somitogenesis and myogenesis

T-box transcription factor tbx6 and basic-helix-loop-helix transcription factor pMesogenin1 are reported to be involved in paraxial mesodermal differentiation. To clarify the relationship between these genes in Xenopus laevis, we isolated pMesogenin2, which showed high homology with pMesogenin1. Both pMesogenin1 and 2 appeared to be transcriptional activators and were induced by a hormone-inducible version of Xtbx6 without secondary protein synthesis in animal cap assays. The pMesogenin2 promoter contained three potential T-box binding sites with which Xtbx6 protein was shown to interact, and a reporter gene construct containing these sites was activated by Xtbx6. Xtbx6 knockdown reduced pMesogenin1 and 2 expressions, but not vice versa. Xtbx6 and pMesogenin1 and 2 knockdowns caused similar phenotypic abnormalities including somite malformation and ventral body wall muscle hypoplasia, suggesting that Xtbx6 is a direct regulator of pMesogenin1 and 2, which are both involved in somitogenesis and myogenesis including that of body wall muscle in Xenopus laevis.

Brachyury null mutant-induced defects in juvenile ascidian endodermal organs

We report the isolation of a recessive ENU-induced short-tailed mutant in the ascidian Ciona intestinalis that is the product of a premature stop in the brachyury gene. Notochord differentiation and morphogenesis are severely disrupted in the mutant line. At the larval stage, variable degrees of ectopic endoderm staining were observed in the homozygous mutants, indicating that loss of brachyury results in stochastic fate transformation. In post-metamorphosis mutants, a uniform defect in tail resorption was observed, together with variable defects in digestive tract development. Some cells misdirected from the notochord lineage were found to be incorporated into definitive endodermal structures, such as stomach and intestine.

Eomesodermin requires transforming growth factor-beta/activin signaling and binds Smad2 to activate mesodermal genes

The T-box gene Eomesodermin (Eomes) is required for early embryonic mesoderm differentiation in mouse, frog (Xenopus laevis), and zebrafish, is important in late cardiac development in Xenopus, and for CD8+ T effector cell function in mouse. Eomes can ectopically activate many mesodermal genes. However, the mechanism by which Eomes activates transcription of these genes is poorly understood. We report that Eomes protein interacts with Smad2 and is capable of working in a non-cell autonomous manner via transfer of Eomes protein between adjacent embryonic cells. Blocking of Eomes protein transfer using a farnesylated red fluorescent protein (CherryF) also prevents Eomes nuclear accumulation. Transfer of Eomes protein between cells is mediated by the Eomes carboxyl terminus (456-692). A carbohydrate binding domain within the Eomes carboxyl-terminal region is sufficient for transfer and important for gene activation. We propose a novel mechanism by which Eomes helps effect a cellular response to a morphogen gradient.

H2AZ is enriched at polycomb complex target genes in ES cells and is necessary for lineage commitment

Elucidating how chromatin influences gene expression patterns and ultimately cell fate is fundamental to understanding development and disease. The histone variant H2AZ has emerged as a key regulator of chromatin function and plays an essential but unknown role during mammalian development. Here, genome-wide analysis reveals that H2AZ occupies the promoters of developmentally important genes in a manner that is remarkably similar to that of the Polycomb group (PcG) protein Suz12. By using RNAi, we demonstrate a role for H2AZ in regulating target gene expression, find that H2AZ and PcG protein occupancy is interdependent at promoters, and further show that H2AZ is necessary for ES cell differentiation. Notably, H2AZ occupies a different subset of genes in lineage-committed cells, suggesting that its dynamic redistribution is necessary for cell fate transitions. Thus, H2AZ, together with PcG proteins, may establish specialized chromatin states in ES cells necessary for the proper execution of developmental gene expression programs.

Neuromancer1 and Neuromancer2 regulate cell fate specification in the developing embryonic CNS of Drosophila melanogaster

T-box genes encode a large family of transcription factors that regulate many developmental processes in vertebrates and invertebrates. In addition to their roles in regulating embryonic heart and epidermal development in Drosophila, we provide evidence that the T-box transcription factors neuromancer1 (nmr1) and neuromancer2 (nmr2) play key roles in embryonic CNS development. We verify that nmr1 and nmr2 function in a partially redundant manner to regulate neuronal cell fate by inhibiting even-skipped (eve) expression in specific cells in the CNS. Consistent with their redundant function, nmr1 and nmr2 exhibit overlapping yet distinct protein expression profiles within the CNS. Of note, nmr2 transcript and protein are expressed in identical patterns of segment polarity stripes, defined sets of neuroblasts, many ganglion mother cells and discrete populations of neurons. However, while we observe nmr1 transcripts in segment polarity stripes and specific neural precursors in early stages of CNS development, we first detect Nmr1 protein in later stages of CNS development where it is restricted to discrete subsets of Nmr2-positive neurons. Expression studies identify nearly all Nmr1/2 co-expressing neurons as interneurons, while a single Eve-positive U/CQ motor neuron weakly co-expresses Nmr2. Lineage studies map a subset of Nmr1/2-positive neurons to neuroblast lineages 2-2, 6-1, and 6-2 while genetic studies reveal that nmr2 collaborates with nkx6 to regulate eve expression in the CNS. Thus, nmr1 and nmr2 appear to act together as members of the combinatorial code of transcription factors that govern neuronal subtype identity in the CNS.

Chromatin remodeling during mouse and human embryonic stem cell differentiation

Embryonic stem cell (ESC) differentiation is an excellent model to study chromatin changes at developmentally regulated loci. Differentiating mouse and human ESCs increase genome-wide acetylation (euchromatic) and tri-methylation (heterochromatic) of lysine 9 on histone H3. The Oct4 locus is euchromatic when expressed in undifferentiated ESCs and heterochromatic after differentiation. Brachyury T, a mesoderm-specific transcription factor, is not yet expressed in undifferentiated cells, where its locus has "bivalent" tri-methyl lysine 4 and lysine 27 modifications. During directed differentiation to pre-cardiac mesoderm, the activated brachyury locus has high levels of tri-methyl lysine 4 (euchromatin), switching to heterochromatin after gene silencing. Thus, ESC differentiation is accompanied by genome-wide commitment to euchromatin or heterochromatin. Undifferentiated hESCs bivalently modify the brachyury locus, activate it to euchromatin during mesoderm induction, and subsequently repress it to heterochromatin, demonstrating, to our knowledge, the first analysis of chromatin dynamics at a locus essential for mesoderm and endoderm differentiation.

Teasing out T-box targets in early mesoderm

T-box transcription factor genes are widely conserved in metazoan development and widely involved in developmental processes. With the phase of T-box gene discovery winding down, the phase of transcriptional target discovery for T-box transcription factors is finally taking off and yielding rich rewards. Mutant phenotypes in mouse and zebrafish as well as morpholino studies in zebrafish have helped to link the T-box genes to a variety of signaling pathways through diverse target genes and feedback loops. Particularly in early mesoderm development, it is emerging that a network of T-box genes interacts with Wnt/beta-catenin and Notch/Delta signaling pathways, among others, to control the important processes of mesoderm specification, somite segmentation, and left/right body axis determination.

VACTERL/caudal regression/Currarino syndrome-like malformations in mice with mutation in the proprotein convertase Pcsk5

We have identified an ethylnitrosourea (ENU)-induced recessive mouse mutation (Vcc) with a pleiotropic phenotype that includes cardiac, tracheoesophageal, anorectal, anteroposterior patterning defects, exomphalos, hindlimb hypoplasia, a presacral mass, renal and palatal agenesis, and pulmonary hypoplasia. It results from a C470R mutation in the proprotein convertase PCSK5 (PC5/6). Compound mutants (Pcsk5(Vcc/null)) completely recapitulate the Pcsk5(Vcc/Vcc) phenotype, as does an epiblast-specific conditional deletion of Pcsk5. The C470R mutation ablates a disulfide bond in the P domain, and blocks export from the endoplasmic reticulum and proprotein convertase activity. We show that GDF11 is cleaved and activated by PCSK5A, but not by PCSK5A-C470R, and that Gdf11-deficient embryos, in addition to having anteroposterior patterning defects and renal and palatal agenesis, also have a presacral mass, anorectal malformation, and exomphalos. Pcsk5 mutation results in abnormal expression of several paralogous Hox genes (Hoxa, Hoxc, and Hoxd), and of Mnx1 (Hlxb9). These include known Gdf11 targets, and are necessary for caudal embryo development. We identified nonsynonymous mutations in PCSK5 in patients with VACTERL (vertebral, anorectal, cardiac, tracheoesophageal, renal, limb malformation OMIM 192350) and caudal regression syndrome, the phenotypic features of which resemble the mouse mutation. We propose that Pcsk5, at least in part via GDF11, coordinately regulates caudal Hox paralogs, to control anteroposterior patterning, nephrogenesis, skeletal, and anorectal development.

Regulation of canonical Wnt signaling by Brachyury is essential for posterior mesoderm formation

The T box transcription factor Brachyury is essential for the formation of the posterior body in all vertebrates, although its critical transcriptional targets have remained elusive. Loss-of-function studies of mouse Brachyury and the zebrafish Brachyury ortholog Ntl indicated that Brachyury plays a more significant role in higher vertebrates than lower vertebrates. We have identified a second zebrafish Brachyury ortholog (Bra), and show that a combined loss of Ntl and Bra recapitulates the mouse phenotype, demonstrating an ancient role for Brachyury in patterning all but the most anterior somites. Using cell transplantation, we show that the only essential role for Brachyury during somite formation is non-cell autonomous, and demonstrate that Ntl and Bra are required for and can induce expression of the canonical Wnts wnt8 and wnt3a. We propose that a positive autoregulatory loop between Ntl/Bra and canonical Wnt signaling maintains the mesodermal progenitors to facilitate posterior somite development in chordates.

Tint maps to mouse chromosome 6 and may interact with a notochordal enhancer of Brachyury

At the proximal part of mouse chromosome 17 there are three well-defined genes affecting the axis of the embryo and consequently tail length: Brachyury, Brachyury the second, and the t-complex tail interaction (T1, T2, and tct). The existence of T1 and tct in fact defines the classical "t-complex" that occupies approximately 40 cM of mouse chromosome 17. Their relationship to each other and various unlinked interacting genes has been enigmatic. The tint gene was the first of the latter to be identified. We report here its genetic mapping using a microsatellite scan together with outcrosses to Mus spretus and M. castaneous followed by a subsequent testcross to T, T1, and T2 mutants. Surprisingly, tint interacts with T2 but not with T1. The implications of our data suggest that T2 may be part of the T1 regulatory region through direct or indirect participation of tint.

Brachyury-downstream gene sets in a chordate, Ciona intestinalis: integrating notochord specification, morphogenesis and chordate evolution

In vertebrates, Brachyury, a T-box transcription factor gene, seems to have a dual role in the differentiation of axial midline mesoderm cells into notochord and gastrulation cell movements regulated by non-canonical Wnt/planar cell polarity (Wnt/PCP) signaling. To understand the function of Brachyury-downstream genes in chordate embryos, from a series of our survey on differential expression, including subtractive hybridization, dot-blot assays, EST sequences and the expression patterns in whole-mount in situ hybridization at embryonic stages, we developed a knowledge database called "CINOBI: CionaNotochord and Brachyury-downstream gene Index" to create comprehensive catalogues of Brachyury-downstream gene sets in Ciona intestinalis. Combining genome and large-scale cDNA data, we were able to characterize 450 non-redundant Brachyury-downstream genes: Twenty-four genes were newly annotated as notochord-expressed genes. Several genes are components of signaling pathways such as Wnt/PCP, Nf kappaB and TGF-beta signaling. We propose that Brachyury is linked to these pathways regulating the expression of each component, and such a regulatory mechanism might be conserved among chordates.

Tbx2b is required for the development of the parapineal organ

Structural differences between the left and right sides of the brain exist throughout the vertebrate lineage. By studying the zebrafish pineal complex, which exhibits notable asymmetries, both the genes and the cell movements that result in left-right differences can be characterized. The pineal complex consists of the midline pineal organ and the left-sided parapineal organ. The parapineal is responsible for instructing the asymmetric architecture of the bilateral habenulae, the brain nuclei that flank the pineal complex. Using in vivo time-lapse confocal microscopy, we find that the cells that form the parapineal organ migrate as a cluster of cells from the pineal complex anlage to the left side of the brain. In a screen for mutations that disrupted brain laterality, we identified a nonsense mutation in the T-box2b (tbx2b) gene, which encodes a transcription factor expressed in the pineal complex anlage. The tbx2b mutant makes fewer parapineal cells, and they remain as individuals near the midline rather than migrating leftward as a group. The reduced number and incorrect placement of parapineal cells result in symmetric development of the adjacent habenular nuclei. We conclude that tbx2b functions to specify the correct number of parapineal cells and to regulate their asymmetric migration.

Activator-to-repressor conversion of T-box transcription factors by the Ripply family of Groucho/TLE-associated mediators

The T-box family of transcription factors, defined by a conserved DNA binding domain called the T-box, regulate various aspects of embryogenesis by activating and/or repressing downstream genes. In spite of the biological significance of the T-box proteins, how they regulate transcription remains to be elucidated. Here we show that the Groucho/TLE-associated protein Ripply converts T-box proteins from activators to repressors. In cultured cells, zebrafish Ripply1, an essential component in somite segmentation, and its structural relatives, Ripply2 and -3, suppress the transcriptional activation mediated by the T-box protein Tbx24, which is coexpressed with ripply1 during segmentation. Ripply1 associates with Tbx24 and converts it to a repressor. Ripply1 also antagonizes the transcriptional activation of another T-box protein, No tail (Ntl), the zebrafish ortholog of Brachyury. Furthermore, injection of a high dosage of ripply1 mRNA into zebrafish eggs causes defective development of the posterior trunk, similar to the phenotype observed in homozygous mutants of ntl. A mutant form of Ripply1 defective in association with Tbx24 also lacks activity in zebrafish embryos. These results indicate that the intrinsic transcriptional property of T-box proteins is controlled by Ripply family proteins, which act as specific adaptors that recruit the global corepressor Groucho/TLE to T-box proteins.

Pivotal roles for eomesodermin during axis formation, epithelium-to-mesenchyme transition and endoderm specification in the mouse

The T-box transcription factor eomesodermin (Eomes) has been implicated as an important component in germ layer induction and patterning in vertebrate embryos. In the mouse, Eomes is essential for development of the trophectoderm lineage and Eomes loss-of-function mutants arrest at implantation. Here, we have used a novel Eomes conditional allele to test Eomes functions in the embryo proper. Eomes-deficient embryos express both Fgf8 and its downstream target Snail at normal levels but surprisingly fail to downregulate E-cadherin. Eomes functional loss thus efficiently and profoundly blocks EMT and concomitant mesoderm delamination. Marker analysis as well as fate-mapping and chimera studies demonstrate for the first time that Eomes is required for specification of the definitive endoderm lineage. We also describe developmental abnormalities in Eomes/Nodal double heterozygotes, and demonstrate that these phenotypes reflect Eomes and Nodal interactions in different tissue sites. Collectively, our experiments establish that Eomes is a key regulator of anteroposterior axis formation, EMT and definitive endoderm specification in the mouse.

UV-mediated regulation of the anti-senescence factor Tbx2

Several lines of evidence have implicated members of the developmentally important T-box gene family in cell cycle regulation and in cancer. Importantly, the highly related T-box factors Tbx2 and Tbx3 can suppress senescence through repressing the cyclin-dependent kinase inhibitors p19(ARF) and p21(WAF1/CIP1/SDII). Furthermore, Tbx2 is up-regulated in several cancers, including melanomas where it was shown to function as an anti-senescence factor, suggesting that this may be one of the mechanisms by which T-box proteins contribute to the oncogenic process. However, very little is known about whether Tbx2 is regulated by p21-mediated stress-induced senescence signaling pathways. In this study, using the MCF-7 breast cancer cell line known to overexpress Tbx2, we show that in response to stress induced by ultraviolet irradiation the Tbx2 protein is specifically phosphorylated by the p38 mitogen-activated protein kinase. Using site-directed mutagenesis and in vitro kinase assays, we have identified serine residues 336, 623, and 675 in the Tbx2 protein as the p38 target sites and show that these sites are phosphorylated in vivo. Importantly, we show by Western blotting, immunofluorescence, and reporter assays that this phosphorylation leads to increased Tbx2 protein levels, predominant nuclear localization of the protein, and an increase in the ability of Tbx2 to repress the p21(WAF1/CIP1/SDII) promoter. These results show for the first time that the ability of Tbx2 to repress the p21 gene is enhanced in response to a stress-induced senescence pathway, which leads to a better understanding of the regulation of the anti-senescence function of Tbx2.

Developmental expression patterns of Tbx1, Tbx2, Tbx5, and Tbx20 in Xenopus tropicalis

T-box genes have diverse functions during embryogenesis and are implicated in several human congenital disorders. Here, we report the identification, sequence analysis, and developmental expression patterns of four members of the T-box gene family in the diploid frog Xenopus tropicalis. These four genes-Tbx1, Tbx2, Tbx5, and Tbx20-have been shown to influence cardiac development in a variety of organisms, in addition to their individual roles in regulating other aspects of embryonic development. Our results highlight the high degree of evolutionary conservation between orthologs of these genes in X. tropicalis and other vertebrates, both at the molecular level and in their developmental expression patterns, and also identify novel features of their expression. Thus, X. tropicalis represents a potentially valuable vertebrate model in which to further investigate the functions of these genes through genetic approaches.

The T-box transcription factor eomesodermin controls CD8 T cell activity and lymph node metastasis in human colorectal cancer

BACKGROUND/AIMS

An efficient cytolytic T cell function is essential for immune mediated rejection of colorectal cancer. However, the molecular mechanisms driving T cell mediated cancer rejection are still poorly understood. Here, we assessed the relevance of the T-box transcription factor eomesodermin in colorectal cancer. METHODS/ RESULTS: By analysing tissue probes from 88 different colorectal tumours, a significant (p<0.02) inverse correlation between eomesodermin expression in colorectal cancers and the presence of lymph node metastases could be shown, whereas no such correlation was noted for the master transcription factor of regulatory T cells, FoxP3 and CD8 alpha expression. To evaluate whether this effect might be due to effects of eomesodermin on tumour infiltrating CD8 T cells, we subsequently analysed the regulated expression and function of this transcription factor in human T cells. Whereas overexpression of this factor induced perforin but not granzyme expression, siRNA mediated suppression of eomesodermin expression led to significantly reduced IFN-gamma production, perforin levels and cytolytic activity of CD8 T cells. Furthermore, TGF-beta and IL4 could be identified as important inducer of eomesodermin expression.

CONCLUSION

These data define for the first time a regulatory role of eomesodermin for CD8 T cell activity in humans. Our findings are consistent with a model in which eomesodermin expression in tumour infiltrating T cells regulates cytolytic functions of CD8 T cells via perforin expression. These data provide novel insights into control mechanisms governing the functional activity of human CD8 T lymphocytes via T-box transcription factors in cancer.

Parallel evolution of cortical areas involved in skilled hand use

Dexterous hands, used to manipulate food, tools, and other objects, are one of the hallmarks of primate evolution. However, the neural substrate of fine manual control necessary for these behaviors remains unclear. Here, we describe the functional organization of parietal cortical areas 2 and 5 in the cebus monkey. Whereas other New World monkeys can be quite dexterous, and possess a poorly developed area 5, cebus monkeys are the only New World primate known to use a precision grip, and thus have an extended repertoire of manual behaviors. Unlike other New World Monkeys, but much like the macaque monkey, cebus monkeys possess a proprioceptive cortical area 2 and a well developed area 5, which is associated with motor planning and the generation of internal body coordinates necessary for visually guided reaching, grasping, and manipulation. The similarity of these fields in cebus monkeys and distantly related macaque monkeys with similar manual abilities indicates that the range of cortical organizations that can emerge in primates is constrained, and those that emerge are the result of highly conserved developmental mechanisms that shape the boundaries and topographic organizations of cortical areas.

T-bet's ability to regulate individual target genes requires the conserved T-box domain to recruit histone methyltransferase activity and a separate family member-specific transactivation domain

Appropriate cellular differentiation and specification rely upon the ability of key developmental transcription factors to precisely establish gene expression patterns. These transcription factors often regulate epigenetic events. However, it has been unclear whether this is the only role that they play in functionally regulating developmental gene expression pathways or whether they also participate in downstream transactivation events at the same promoter. The T-box transcription factor family is important in cellular specification events in many developmental systems, and determining the molecular mechanisms by which this family regulates gene expression networks warrants attention. Here, we examine the mechanism by which T-bet, a critical T-box protein in the immune system, influences transcription. T-bet is both necessary and sufficient to induce permissive histone H3-K4 dimethyl modifications at the CXCR3 and IFN-gamma promoters. A T-bet structure-function analysis revealed that the conserved T-box domain, with a small C-terminal portion, is required for recruiting histone methyltransferase activity to promoters. Interestingly, this function is conserved in the T-box family and is necessary, but not sufficient, to induce transcription, with an independent transactivation activity also required. The requirement for two separable functional activities may ultimately contribute to the stringent role for T-box proteins in establishing specific developmental gene expression pathways.

The human T-box mesodermal transcription factor Brachyury is a candidate target for T-cell-mediated cancer immunotherapy

PURPOSE

Identification of tumor antigens is essential in advancing immune-based therapeutic interventions in cancer. Particularly attractive targets are those molecules that are selectively expressed by malignant cells and that are also essential for tumor progression.

EXPERIMENTAL DESIGN AND RESULTS

We have used a computer-based differential display analysis tool for mining of expressed sequence tag clusters in the human Unigene database and identified Brachyury as a novel tumor antigen. Brachyury, a member of the T-box transcription factor family, is a key player in mesoderm specification during embryonic development. Moreover, transcription factors that control mesoderm have been implicated in the epithelial-mesenchymal transition (EMT), which has been postulated to be a key step during tumor progression to metastasis. Reverse transcription-PCR analysis validated the in silico predictions and showed Brachyury expression in tumors of the small intestine, stomach, kidney, bladder, uterus, ovary, and testis, as well as in cell lines derived from lung, colon, and prostate carcinomas, but not in the vast majority of the normal tissues tested. An HLA-A0201 epitope of human Brachyury was identified that was able to expand T lymphocytes from blood of cancer patients and normal donors with the ability to lyse Brachyury-expressing tumor cells.

CONCLUSIONS

To our knowledge, this is the first demonstration that (a) a T-box transcription factor and (b) a molecule implicated in mesodermal development, i.e., EMT, can be a potential target for human T-cell-mediated cancer immunotherapy.

A study of inherited short tail and taillessness in Pembroke Welsh corgi

OBJECTIVES

To study whether natural short tail in adult Pembroke Welsh corgi is associated with congenital spinal defects. To report anatomical defects in two newborn tailless puppies from short-tailed parents, and to check whether they were homozygous for the dominant mutation in the T-gene (C295G).

METHODS

The vertebral column of 19 adult dogs with natural short tail, from short-tail x long-tail crossings, was radiographically examined. Two tailless puppies were radiographed and submitted for necropsy. Samples from the puppies, their parents and five siblings were analysed for the mutation of the T-gene.

RESULTS

No congenital spinal defects were diagnosed in any of the short-tailed dogs. The tailless puppies had anorectal atresia, had multiple spinal defects and were homozygous for the mutation in the T-gene.

CLINICAL SIGNIFICANCE

As tail docking is forbidden in many countries, breeding Pembroke Welsh corgis with natural short tail is becoming increasingly common. Previous studies indicated that the mutation in homozygotes is lethal in early fetal life. It is of clinical significance that natural short tail is probably not associated with congenital spinal defects, as is known from studies of other species, and that homozygotes for this mutation with serious anatomical defects may be born.

Surprisingly complex T-box gene complement in diploblastic metazoans

Ctenophores and cnidarians are two metazoan groups that evolved at least 600 Ma, predating the Cambrian explosion. Although both groups are commonly categorized as diploblastic animals without derivatives of the mesodermal germ layer, ctenophores possess definitive contractile "muscle" cells. T-box family transcription factors are an evolutionarily ancient gene family, arising in the common ancestor of metazoans, and have been divided into eight groups in five distinct subfamilies, many of which are involved in the specification of mesodermal as well as ectodermally and endodermally derived structures. Here, we report the cloning and expression of five T-box genes from a ctenophore, Mnemiopsis leidyi. Phylogenetic analyses demonstrated that ctenophores possess members of at least three of the five T-box subfamilies, and expression studies suggested distinct roles of each T-box genes during gastrulation and early organogenesis. Moreover, genome searches of the sea anemone, Nematostella vectensis (anthozoan cnidarian), showed at least 13 T-box genes in Nematostella, which are divided into at least six distinct groups in the same three subfamilies found in ctenophores. Our results from two diploblastic animals indicate that the common ancestor of eumetazoans had a complex set of T-box genes and that two distinct subfamilies might have appeared during triploblastic evolution.

Expression analysis of two Eomesodermin homologues in zebrafish lymphoid tissues and cells

Eomesodermin (Eomes) is a T-box transcription factor that is involved in mesoderm formation in most vertebrates. Eomes is also expressed in CD8+ T cells and NK cells. No information is available on the role of Eomes in the immune system of lower vertebrates to date, although developmental studies on Eomes (Eomes1) have been performed in zebrafish. Here we report the identification of a second Eomes (Eomes2) in zebrafish and compare expression of the two Eomes genes in the immune system. Zebrafish Eomes1 and Eomes2, composed of 661 and 534 amino acids, respectively, share 49.3% amino acid identity in their coding regions and 88.7% amino acid identity in their T-box regions. Conserved synteny between regions of the human and zebrafish genomes, gene organization and phylogenetic analysis all indicate that the zebrafish Eomes2 gene is a homologue of mammalian Eomes, as previously found for zebrafish Eomes1. Eomes1 mRNA was found to be expressed in the gonad, body kidney, spleen and gill, while Eomes2 mRNA was not detected in any of these tissues. However, strong expression of both Eomes mRNAs was detected in the leukocytes from the spleen, followed by those from body kidney and peripheral blood, with expression of Eomes1 always stronger than that of Eomes2. RT-PCR analysis of body kidney cells sorted by FACS revealed that Eomes1 was expressed strongly in lymphocytes, weakly in blast cells, and was not expressed in granulocytes, while Eomes2 was expressed weakly in lymphocytes. These results suggest that both Eomes genes are involved in the zebrafish immune response, particularly in lymphocyte function as has been found in mammals.

Maternal control of pattern formation inXenopus laevis

We review the essential role of maternal factors in pattern formation for Xenopus laevis, focusing on VegT, Vg1, and Wnt11. Results from loss of function experiments demonstrate a clear requirement for these genes in germ layer specification, dorsal-ventral axis formation, and convergence extension. We also discuss these genes in the broader context of metazoan development, exploring whether and how their functions in the X. laevis model organism may or may not be conserved in other species. Wnt11 signaling in particular provides a classic example where understanding context in development is crucial to understanding function. Genomic sequencing, gene expression, and functional screening data that are becoming available in more species are providing invaluable aid to decoding and modeling signaling pathways. More work is needed to develop a comprehensive catalog of the Wnt signaling, T-box, and TGF-beta genes in metazoans both near and far in evolutionary distance. We finally discuss some specific experimental and modeling efforts that will be needed to understand the behavior of these signaling networks in vivo so that we can interpret these critical pathways in an evolutionary framework.

Diagnosing an extra-axial chordoma of the proximal tibia with the help of brachyury, a molecule required for notochordal differentiation

Chordomas are rare malignant bone tumours considered to arise from notochordal remnants that persist in the axial skeleton. Although their morphology can resemble that of a carcinoma, chondrosarcoma or malignant melanoma, the axial location and their well-defined immunophenotype, including expression of cytokeratins (CK7/20/8/18/19) and S100, generally allow the diagnosis to be made with confidence once the possibility is considered. In contrast, making a robust diagnosis of an extra-axial chordoma has been difficult in the absence of specific markers for chordomas. We have recently shown in gene expression microarray and immunohistochemistry studies that brachyury, a transcription factor crucial for notochordal development, is a specific and sensitive maker for chordomas. We now present a case of an intracortical tibial tumour, with detailed report of the imaging, and morphological features consistent with a chordoma, where notochordal differentiation was demonstrated with an antibody to brachyury. The tumour cells were also positive for cytokeratins, including CK19, and S100, CEA, EMA and HMBE1, findings which support the diagnosis of chordoma. Brachyury can be employed as a marker of notochordal differentiation and help identify confidently, for the first time, extra-axial bone and soft tissue chordomas, and tumours which may show focal notochordal differentiation.

Polycomb silencers control cell fate, development and cancer

Polycomb group (PcG) proteins are epigenetic gene silencers that are implicated in neoplastic development. Their oncogenic function might be associated with their well-established role in the maintenance of embryonic and adult stem cells. In this review, we discuss new insights into the possible mechanisms by which PcGs regulate cellular identity, and speculate how these functions might be relevant during tumorigenesis.

Brachyury is required for elongation and vasculogenesis in the murine allantois

Mouse conceptuses homozygous for mutations in brachyury (T) exhibit a short, misshapen allantois that fails to fuse with the chorion. Ultimately, mutant embryos die during mid-gestation. In the 60 years since this discovery, the role of T in allantoic development has remained obscure. T protein was recently identified in several new sites during mouse gastrulation, including the core of the allantois, where its function is not known. Here, using molecular, genetic and classical techniques of embryology, we have investigated the role of T in allantoic development. Conceptuses homozygous for the T(Curtailed) (T(C)) mutation (T(C)/T(C)) exhibited allantoic dysmorphogenesis shortly after the allantoic bud formed. Diminution in allantoic cell number and proliferation was followed by cell death within the core. Fetal liver kinase (Flk1)-positive angioblasts were significantly decreased in T(C)/T(C) allantoises and did not coalesce into endothelial tubules, possibly as a result of the absence of platelet endothelial cell adhesion molecule 1 (Pecam1), whose spatiotemporal relationship to Flk1 suggested a role in patterning the umbilical vasculature. Remarkably, microsurgical perturbation of the wild-type allantoic core phenocopied the T(C)/T(C) vascularization defect, providing further support that an intact core is essential for vascularization. Last, abnormalities were observed in the T(C)/T(C) heart and yolk sac, recently reported sites of T localization. Our findings reveal that T is required to maintain the allantoic core, which is essential for allantoic elongation and vascular patterning. In addition, morphological defects in other extraembryonic and embryonic vascular organs suggest a global role for T in vascularization of the conceptus.