Analysis of forkhead and snail expression reveals epithelial–mesenchymal transitions during embryonic and larval development of Nematostella vectensis

Abstract

The winged helix transcription factor Forkhead and the zinc finger transcription factor Snail are crucially involved in germ layer formation in Bilateria. Here, we isolated and characterized a homolog of forkhead/HNF3 (FoxA/group 1) and of snail from a diploblast, the sea anemone Nematostella vectensis. We show that Nematostella forkhead expression starts during late Blastula stage in a ring of cells that demarcate the blastopore margin during early gastrulation, thereby marking the boundary between ectodermal and endodermal tissue. snail, by contrast, is expressed in a complementary pattern in the center of forkhead-expressing cells marking the presumptive endodermal cells fated to ingress during gastrulation. In a significant portion of early gastrulating embryos, forkhead is expressed asymmetrically around the blastopore. While snail-expressing cells form the endodermal cell mass, forkhead marks the pharynx anlage throughout embryonic and larval development. In the primary polyp, forkhead remains expressed in the pharynx. The detailed analysis of forkhead and snail expression during Nematostella embryonic and larval development further suggests that endoderm formation results from epithelial invagination, mesenchymal immigration, and reorganization of the endodermal epithelial layer, that is, by epithelial-mesenchymal transitions (EMT) in combination with extensive morphogenetic movements. snail also governs EMT at different processes during embryonic development in Bilateria. Our data indicate that the function of snail in Diploblasts is to regulate motility and cell adhesion, supporting that the triggering of changes in cell behavior is the ancestral role of snail in Metazoa.

Molecular evidence for deep evolutionary roots of bilaterality in animal development

“…8) are expressed uniformly in the endoderm, and during gastrulation NvChordin is expressed topographically in an area separating ectoderm from endoderm (21). Arguments have been made that mesoderm evolved from a bifunctional mesendoderm, so it might be that regulation of TGF-␤ signaling [e.g., via nodal-related genes (22,23)] was involved with the evolution of a definitive mesodermal germ layer (50,51) and additional genes were recruited for mesodermal patterning at later stages of metazoan radiations. However, an alternative hypothesis would be that some of these genes played a role in D-V patterning in the common ancestor of cnidarians and bilaterians, and that this role has been reduced in some cnidarians.…”

The Wnt code: cnidarians signal the way

“…Does the stabilization of b-catenin require Wnt-independent mechanisms, that is, is it related to Cadherin and Protocadherin function? To fully understand the signalling function of Wnts along the oral-aboral axis it will be mandatory to identify the downstream target genes that are involved in the regulation of cell differentiation, for example the activation of mesodermal genes (Fritzenwanker et al, 2004;Martindale et al, 2004;Matus et al, 2006). On the other hand, it is important to unravel Wnt-inhibitory mechanisms that are a prerequisite for a number of differentiation processes, particularly for the activation of neuronal differentiation (Onai et al, 2004;Lie et al, 2005).…”

“…These data are confirmed by EST data from various Hydra labs, the recently released genome project in Nematostella (DOE Joint Genome Institute; http://www.jgi.doe.gov; Daniel Rokhsar, JGI Eukaryote Program Lead) and will soon receive further input from the Hydra genome project. While it is unclear at present, to what extent the genomes of Hydra and Nematostella differ among each other and bilaterians, the analysis of the cnidarian gene catalogue already revealed three major findings: (i) A large number of orthologous developmental genes known to be important for bilaterian development, including the genes for Wnt, TGF-b, and FGF signalling as well as Hox, Fox, Brachyury transcription factors are present in cnidarians (Technau and Bode, 1999;Spring et al, 2000Spring et al, , 2002Scholz and Technau, 2003;Technau and Scholz, 2003;Finnerty et al, 2004;Fritzenwanker et al, 2004;Hayward et al, 2004;Martindale et al, 2004;Torras et al, 2004;Extavour et al, 2005;Magie et al, 2005;Technau et al, 2005;Torras and Gonzalez-Crespo, 2005;Matus et al, 2006). (ii) Among these conserved factors, even 'mesodermal' proteins that are specific for mesoderm induction and differentiation in triploblastic bilaterian animals are present.…”

FGF signaling in gastrulation and neural development in Nematostella vectensis, an anthozoan cnidarian

“…Although there are reports of EMT occurring in N. vectensis (Kraus and Technau 2006), more recent work indicates that EMT does not occur during N. vectensis gastrulation (Magie et al, personal communication). The expression of "mesodermal" genes in the endoderm of N. vectensis (Fritzenwanker et al 2004;Martindale et al 2004;Technau 2001) suggests that the endoderm may be a precursor to bilaterian endoderm and mesoderm. The evolution of signaling systems that segregated receptors and ligands to endoderm and mesoderm, respectively, likely predated the evolution of mesoderm.…”