Kevin J. P. Griffin

Vertebrate mesendoderm induction and patterning.

Many of the key molecular events underlying the induction and patterning of the vertebrate mesoderm and endoderm have recently been elucidated. T-box transcription factors and TGF-beta and Wnt signaling pathways play crucial roles in the initial induction of the mesendoderm and the subdivision of the posterior mesoderm into rostral and caudal domains.

One-Eyed Pinhead and Spadetail are essential for heart and somite formation.

Mutant analysis in the zebrafish Danio rerio has demonstrated distinct developmental roles for the T-box transcription factor Spadetail (Spt) and the Nodal-receptor cofactor One-Eyed Pinhead (Oep) in the formation of mesoderm and endoderm. Here, we show that spt and oep genetically interact and are together essential for the formation of cardiac and somitic mesoderm. These two mesodermal defects are dependent on different effectors of Nodal signalling; cardiac mesoderm formation involves the mix-like transcription factor Bonnie and Clyde (Bon), whereas somitogenesis is dependent on a different pathway. Analysis of the somite defect in Zoep;spt embryos has provided insights into the control of somitic mesoderm formation by Spt, which was previously implicated in the regulation of cell adhesion and motility. We show that the failure to form somites in Zoep;spt embryos is independent of this and that Spt must have an additional function. We propose that the major role of Spt in somitogenesis is to promote the differentiation of presomitic mesoderm from tailbud progenitors by antagonizing progenitor-type gene expression and behaviour.

Nodal and Fgf pathways interact through a positive regulatory loop and synergize to maintain mesodermal cell populations

Interactions between Nodal/Activin and Fibroblast growth factor (Fgf) signalling pathways have long been thought to play an important role in mesoderm formation. However, the molecular and cellular processes underlying these interactions have remained elusive. Here, we address the epistatic relationships between Nodal and Fgf pathways during early embryogenesis in zebrafish. First, we find that Fgf signalling is required downstream of Nodal signals for inducing the Nodal co-factor One-eyed-pinhead (Oep). Thus, Fgf is likely to be involved in the amplification and propagation of Nodal signalling during early embryonic stages. This could account for the previously described ability of Fgf to render cells competent to respond to Nodal/Activin signals. In addition, overexpression data shows that Fgf8 and Fgf3 can take part in this process. Second, combining zygotic mutations in ace/fgf8 and oep disrupts mesoderm formation, a phenotype that is not produced by either mutation alone and is consistent with our model of an interdependence of Fgf8 and Nodal pathways through the genetic regulation of the Nodal co-factor Oep and the cell propagation of Nodal signalling. Moreover, mesodermal cell populations are affected differentially by double loss-of-function of Zoep;ace. Most of the dorsal mesoderm undergoes massive cell death by the end of gastrulation, in contrast to either single-mutant phenotype. However, some mesoderm cells are still able to undergo myogenic differentiation in the anterior trunk of Zoep;ace embryos, revealing a morphological transition at the level of somites 6-8. Further decreasing Oep levels by removing maternal oep products aggravates the mesodermal defects in double mutants by disrupting the fate of the entire mesoderm. Together, these results demonstrate synergy between oep and fgf8 that operates with regional differences and is involved in the induction, maintenance, movement and survival of mesodermal cell populations.

Overlapping expression of zebrafish T-brain-1 and eomesodermin during forebrain development

T-box transcription factors are important determinants of embryonic cell fate and behaviour. Two T-box genes are expressed in the developing telencephalon of several vertebrate species, including amphibia, birds and mammals. Here we report the cloning of zebrafish T-brain-1 (tbr1) and eomesodermin (eom). As a prelude to genetic studies of neuro-ectodermal fate determination we studied their expression pattern during embryogenesis and early larval development. Eom is expressed in the presumptive telencephalon from around the 4-5 somite stage in bilaterally symmetric groups of cells; the number of positive cells increases dramatically with time and encompasses the entire dorsal telencephalon by the 22 somite stage. Tbr1 is expressed from the 18 somite stage in a subset of eom-expressing cells. By 24 hpf eom and tbr1 are expressed in largely overlapping domains in the dorsal telencephalon, tbr1 is expressed in postmitotic cells whereas eomes is also expressed in proliferative ventricular zone cells. Both genes are also found in a small domain of the diencephalon bordering the telencephalon. A detailed analysis of the expression of tbr1 and eom in the brain of 4 day old larvae shows that the two T-box genes are differentially expressed in various cell populations of the developing brain.

Mesoderm Induction: A Postmodern View

and that the active form is present at undetectable levels in vivo. In support of a role for Vg1, a recent study using a dominant-negative mutant indicates that Vg1 is David Kimelman and Kevin J. P. Griffin Department of Biochemistry University of Washington Seattle, Washington 98195-7350 required for the development of dorsal mesoderm and dorsal endoderm (Joseph and Melton, 1998). However, the dominant-negative Vg1 had no effect on ventral and lateral mesoderm (ventral and lateral endoderm were The origin of all tissues in the adult animal can be traced back to one of three primary germ layers: endoderm not examined), arguing that a different TGF-b family member is involved in mesoderm, and possibly endo(gut), mesoderm (muscle, bone, and connective tissues), and ectoderm (epidermis and neural tissue). Germ layer derm, induction. Several authors have presented evidence opposing and favoring a role for activin in early formation is one of the first subdivisions that occurs in embryonic development, and its regulation has engaged Xenopus development, and it still remains possible that the early embryo contains a yet undiscovered member developmental biologists for over a century. The seminal work of Nieuwkoop (1969) demonstrated that a signal of the TGF-b family. And the Mesoderm-Inducing Signal Is ... released by the most vegetal cells, the prospective endoderm, converts the overlying prospective ectoderm a Transcription Factor? The work of Zhang et al. (1998) in this issue of Cell toward a mesodermal fate, creating the three germ layers of the amphibian embryo (Figure 1A; reviewed by seriously challenges the orthodox view of mesoderm induction. Their work shows that a crucial component Harland and Gerhart, 1997). Later experiments showed that the endogenous mesoderm-inducing signal is presof the vegetal maternal mesoderm-inducing signal is not a secreted factor, but a member of the intriguing T-box ent as early as the 32-cell stage (Jones and Woodland, 1987), many hours before transcription occurs in the transcription factor family. Since transcription factors cannot act until the start of zygotic transcription at the embryo, and is mimicked in vitro by members of the TGF-b and FGF families of secreted growth factors. It mid-blastula (4000-cell) stage, a clear implication of this work is that mesoderm (and endoderm) induction occurs is now clear that mesoderm induction requires a TGF-b signal operating in concert with an FGF signal. (TGF-b much later than originally supposed. Over the last two years, four groups have described is used throughout this review to connote any member of the TGF-b superfamily.) Endoderm development also a novel Xenopus transcription factor containing a T-box DNA-binding motif and gave it a variety of names includrequires TGF-b signaling, suggesting that mesoderm and endoderm may be induced by a common pathway ing VegT, Xombi, Antipodean, and Brat (citations can be found in Zhang et al., 1998). VegT is first observed (Henry et al., 1996). However, it is not at all clear how mesoderm and endoderm induction are spatially sepaas a maternal transcript localized to the vegetal hemisphere of eggs and embryos, which corresponds prirated in the embryo. The widely accepted synthesis of these data has been marily to the prospective endoderm and possibly some of the mesoderm (Figure 1B), in a pattern quite similar that the endogenous mesoderm-inducing signal must be present as a maternal mRNA or protein encoding a to that of Vg1 (Weeks and Melton, 1987). Just before gastrulation, zygotic VegT transcripts are found throughsecreted factor, most likely a TGF-b family member, that is localized to the vegetal cytoplasm during oogenesis. out the mesoderm (Figure 1C). Ectopic expression experiments by all of the groups indicated an important Fortunately, Melton and colleagues identified a maternal, vegetally localized mRNA encoding the TGF-b family role for VegT in regulating mesoderm and endoderm specification and morphogenesis, but it was not possimember Vg1 (Weeks and Melton, 1987). The active form of Vg1 protein, however, has never been detected in ble to distinguish between the maternal and zygotic roles of VegT. vivo, and ectopically expressed wild-type Vg1 does not induce mesoderm or endoderm. Advocates for Vg1 Zhang et al. (1998) used antisense oligonucleotides to specifically deplete the maternal VegT mRNA and speculate that its processing must be tightly regulated,