View metadata, citation and similar papers at core.ac.uk brought to you by CORE

provided by Elsevier - Publisher Connector

Dispatch R401

Mammalian development: New trick for an old dog Anthony Graham

A recent study has shown that the T-box gene differentiation [2]. If eomesodermin is eomesodermin, which was first identified in Xenopus, ectopically expressed in caps, then it induces the not only plays a conserved role in directing of early mesodermal genes. Moreover, Eome- , but interestingly in has acquired sodermin acts in a concentration-dependent manner, with a new function in the development of the . higher levels inducing more dorsal types. Correspond- ingly, if eomesodermin function is interfered with, then Address: Molecular Neurobiology Group, 4th Floor New Hunts House, Guys Campus, Kings College London, London SE1 9RT, UK. is arrested during gastrulation, E-mail: [email protected] and mesoderm formation is severely perturbed.

Current 2000, 10:R401–R403 The mouse eomesodermin gene shares many characteristics 0960-9822/00/$ – see front matter with its Xenopus orthologue, displaying a high degree of © 2000 Elsevier Science Ltd. All rights reserved. sequence relatedness — up to 95% amino acid sequence identity in the T-box region [3,5] — as well as prominent Mammals are viviparous, their developing and expression in the mesodermal cells of the gastrula stage being nurtured within the safe uterine environment. This . The mouse eomesodermin gene is notable, reproductive strategy has been very successful, as it greatly however, as it is also expressed at the very early increases the chances of survival of the neonate, and it has stages of development [3,6,7]. The blastocyst consists of involved major adaptations of early embryogenesis. In con- two groups of cells, the internally located trast to other classes, the early phase of mam- and the externally located trophectoderm (Figure 1), and malian development is not concerned with establishing it is this latter group of cells which first expresses eomeso- and organising the embryonic , but rather time is dermin. The specification of trophectoderm versus the spent generating the various extra-embryonic tissues which inner cell mass is the earliest cell-fate choice in mam- are essential for the embryo to implant into the uterine malian development, and the one which separates the wall [1]. Obviously, if one is to understand how the early cells of the trophoblast lineage, which form the foetal events in mammalian development are controlled, one portion of the , from the embryonic lineages. must examine mammalian embryos, but, as with many other aspects of , insights can come The function of eomesodermin in this peculiarly mammalian from unexpected places. Eomesodermin, a T-box gene first phase of development has recently been uncovered by characterised in Xenopus as being a key regulator of meso- mutational analysis in mice [3]. While heterozygous derm formation [2], has recently been functionally , which carry a normal and a mutated form of eome- analysed in mice [3]. This work has shown that, beside sodermin, are healthy and fertile, they never yield any playing a conserved role in mesoderm formation, this gene homozygous mutant offspring when crossed, suggesting has also acquired a novel function in regulating one of the that eomesodermin function is absolutely required for earliest events in mammalian development — the genera- normal development. Morphological analysis of mutant tion of the trophoblast lineage. embryos showed that, in the absence of eomesodermin, embryos arrest soon after implantation. Eomesodermin was first isolated in Xenopus as part of a screen for clones that were specifically expressed in the Eomesodermin mutant embryos reach the blastocyst stage, gastrula stage embryo [2]. An interesting feature of this generating trophectoderm and the inner cell mass, but the clone, from amongst the 120 that were initially isolated, trophectodermal cells are unable to progress and form the was that it was expressed in the equatorial region of the trophoblast [3]. If normal blastocyst embryos are grown early gastrula. In fact, further expression analysis demon- in vitro, they hatch from the , which has strated that this gene is expressed in all mesoderm cells, protected them since , and settle, and which in Xenopus lie as a band at the equator of the early trophoblast cells readily form and spread across the embryo, and it was from this characteristic that the name culture dish. But although cultured mutant embryos hatch of this gene arises — from the Greek Eoσ, for dawn, indi- normally, they maintain their blastocyst morphology and cating its early expression. The sequence of this clone trophoblast cells never form. It has been shown that tro- revealed that the gene encodes a member of the T-box phoblast stem cell lines can be derived from in family of transcription factors [4], which are defined by a culture, that these cells strongly express eomesodermin and shared DNA binding domain, and functional analysis has that their maintenance requires the presence of the suggested that this gene plays a key role in regulating fibroblast growth factor FGF-4 [8]. The lack of production R402 Current Biology Vol 10 No 11

Figure 1 One can create chimeras in which the extra-embryonic tissues are wild-type, but the embryonic tissues are mutant, (a) by introducing mutant embryonic stem cells, or inner cell masses, into a tetraploid host; tetraploid cells can only gen- Inner cell mass erate extra-embryonic tissues. When this is done, the chimeras show no obvious abnormalities at early gastrula- tion stages, indicating that the wild-type cells have formed the trophoblast [3]. At slightly later stages, however, defects become apparent. Although the forms, it does not elongate and the node, the organising center of the Trophectoderm mammalian embryo, is absent from its anterior end [3]. The fact that the primitive streak forms demonstrates that the necessary early mesoderm inducing signals are present, but (b) it seems that the mesoderm never emerges from the primi- Visceral Proximal tive streak. Analysis of the prospective neural tissue of the Mesoderm mutants also reveals that the anterior patterning of this Primitive embryonic layer is slightly perturbed [3]. streak This results mirrors what was observed in Xenopus, suggesting that eomesodermin has a conserved role in gastrulation and that the absence of its function results in gastrulation arrest. This phenotype could, however be due

Anterior to this gene being necessary for mesoderm differentiation Posterior or, more specifically, for the recruitment of prospective mesodermal cells into the primitive streak, or indeed both. While the experiments in Xenopus do not allow us to differ- entiate between these two scenarios, the analytical tools available in mice do. In chimeras made from diploid wild- Distal type cells and mutant cells, mesoderm formation is partially Current Biology rescued [3]. In this situation, wild-type and mutant cells intermingle in the epiblast, but only the wild-type cells (a) Schematic diagram of a blastocyst — a 3.5 day mouse embryo. The migrate through the primitive streak; the eomesodermin locations of the inner cell mass and the trophectoderm are highlighted. (b) Schematic diagram of a 6.5 day mouse embryo during the early mutant cells remain within the epiblast. In normal develop- period of gastrulation. As gastrulation proceeds, the cells of the ment, the differentiation of cells into mesodermal tissues presumptive mesodermal cells of epiblast move to and through the follows from their migration through the primitive streak, primitive streak, shown by blue arrows. The primitive streak itself will and so the block in early gastrulation observed in the eome- also extend anteriorly, indicated by the red arrow, and at its anterior end the node will be established. sodermin mutants does not tell us as whether or not this gene is also necessary for the differentiation of mesodermal cell types. But this issue can be addressed by producing of trophoblast cells from eomesodermin mutant embryos teratomas from the mutant cells, and analysing the range of cannot be overcome by the addition of FGF-4, however, differentiated cell types that are produced. When this is and it seems that eomesodermin must be required cell done, the eomesodermin mutant cells form muscle, autonomously for the initial transition from trophecto- and haemopoetic tissue. It thus seems that the function of derm to trophoblast [3]. eomesodermin during gastrulation is in directing the migra- tion of cells from the epiblast into the primitive streak. While the fact that eomesodermin is required for the development of the trophoblast is in itself of great inter- The results from the mutational analysis of the function of est, the early lethality that results in the absence of this eomesodermin in mice [3] are interesting for a number of gene means that one cannot quite so straightforwardly reasons. They confirm that this gene has a prominent role analyse its later roles in development. Of particular inter- in controlling gastrulation across the vertebrates. The est would be to probe the role of this gene in gastrulation results also, however, demonstrate that eomesodermin has in the mouse, as it is prominently expressed in the cells of acquired a novel role in mammals, being now also the primitive streak and mesodermal cells that have involved in controlling one of the very earliest develop- emerged from there (Figure 1). However, mouse embryol- mental events — the generation of the trophoblast. Yet, at ogists have devised the trickery necessary to circumvent the cellular level, eomesodermin may be playing a similar this problem. role in both trophoblast and mesodermal cells. Both of Dispatch R403

these cell types exhibit pronounced migratory behaviour, and in the absence of this gene trophoblast cells do not spread and migrate in culture and mesodermal cells do not If you found this dispatch interesting, you might also want to read the August 1999 issue of migrate into the primitive streak.

The most likely explanation for the emergence of eomeso- Current Opinion in dermin function in the trophoblast lineage must be that, in Genetics & Development the events leading to the of the mammals, this gene acquired novel regulatory elements that drove its which included the following reviews, edited expression in the trophectoderm of the blastocyst stage by Norbert Perrimon and Claudio Stern, on embryo. It would therefore be very exciting to probe the and developmental control elements that drive eomesodermin expression in the mechanisms: mouse, to determine whether one can identify trophecto- derm-specific regulatory sequences. It would also be inter- in the early embryo esting to compare the regulatory regions of eomesodermin Bruce Bowerman and Christopher A Shelton from representatives of different vertebrate classes, to try The polarisation of the anterior–posterior and and identify conserved elements that possibly control dorsal–ventral axes during Drosophila oogenesis expression of this gene during gastrulation, and further to Fredericus van Eeden and Daniel St Johnston get an insight into the evolutionary changes that allowed Wnt signaling and dorso-ventral axis specification eomesodermin to acquire a novel function. in vertebrates Sergei Y Sokol References Establishment of anterior–posterior polarity in 1. Cruz YP: Mammals. In : Constructing the organism Edited avian embryos by Gilbert SF and Raunio AM. Sauer Associates; 1997:459-489. Rosemary F Bachvarova 2. Ryan K, Garrett N, Mitchell A, Gurdon JB: Eomesodermin, a key early gene in Xenopus mesoderm differentiation. Cell 1996, Polarity in early mammalian development 87:989-1000. Richard L Gardner 3. Russ AP, Wattler S, Colledge WH, Aparicio SAJR, Carlton MBL, Pearce JJ, Barton SC, Surani MA, Ryan K, Nehls MC, et al.: Diverse initiation in a conserved left–right pathway? Eomesodermin is required for mouse trophoblast development H Joseph Yost and mesoderm formation. Nature 2000, 404:95-99. 4. Smith JC: T-box genes: what they do and how they do it. Trends Extracellular modulation of the Hedgehog, Wnt and TGF-β Genet 1999, 15:154-158. signalling pathways during embryonic development 5. Wattler S, Russ A, Evans M, Nehls M: A combined analysis of genomic and primary protein structure defines the phylogenetic Javier Capdevila and Juan Carlos Izpisúa Belmonte relationship of new members of the T-box family. Genomics 1998, Fringe, Notch, and making developmental boundaries 48:24-33. 6. Ciruna BG, Rossant J: Expression of the T-box gene Eomesodermin Kenneth D Irvine during early mouse development. Mech Dev 1999, 81:199-203. Polarity determination in the Drosophila eye 7. Hancock SN, Agulnik SI, Silver LM, Papaioannou VE: Mapping and expression analysis of the mouse ortholog of Helen Strutt and David Strutt Xenopus Eomesodermin. Mech Dev 1999, 81:205-208. Wnt signalling: pathway or network? 8. Tanaka S, Kunath T, Hadjantonakis AK, Nagy A, Rossant J: Promotion of trophoblast stem cell proliferation by FGF4. Science 1998, Alfonso Martinez Arias, Anthony MC Brown and Keith Brennan 282:2072-2075. Epithelial cell movements and interactions in limb, and vasculature and Muriel Altabef Cell movements in the embryo Charles A Ettensohn Roles of the JNK signaling pathway in Drosophila Stéphane Noselli and François Agnès in Drosophila Alexandria Forbes and Ruth Lehmann Cell migration and axon growth cone guidance in Caenorhabditis elegans Catherine S Branda and Michael J Stern The full text of Current Opinion in Genetics & Development is in the BioMedNet library at http://BioMedNet.com/cbiology/gen