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

Abstract

The fibroblast growth factor (FGF) signal transduction pathway serves as one of the key regulators of early metazoan development, displaying conserved roles in the specification of endodermal, mesodermal, and neural fates during vertebrate development. FGF signals also regulate gastrulation, in part, by triggering epithelial to mesenchymal transitions in embryos of both vertebrates and invertebrates. Thus, FGF signals coordinate gastrulation movements across many different phyla. To help understand the breadth of FGF signaling deployment across the animal kingdom, we have examined the presence and expression of genes encoding FGF pathway components in the anthozoan cnidarian Nematostella vectensis. We isolated three FGF ligands (NvFGF8A, NvFGF8B, and NvFGF1A), two FGF receptors (NvFGFRa and NvFGFRb), and two orthologs of vertebrate FGF responsive genes, Sprouty (NvSprouty), an inhibitor of FGF signaling, and Churchill (NvChurchill), a Zn finger transcription factor. We found these FGF ligands, receptors, and response gene expressed asymmetrically along the oral/aboral axis during gastrulation and in a developing chemosensory structure of planula stages known as the apical tuft. These results suggest a conserved role for FGF signaling molecules in coordinating both gastrulation and neural induction that predates the Cambrian explosion and the origins of the Bilateria.

Fig. 1

Molecular phylogeny FGF signaling pathway members. Previous studies have identified eight classes of FGFs within the Metazoa (Popovici et al. 2005). We identified 13 putative genes that possessed FGF core domains within the N. vectensis genome. Of the 13, a Bayesian analysis confirms the orthology for four FGF ligands (blue arrows) all within the FGF-D class (FGF8/17/18). The remaining nine ligands (black arrows) in the N. vectensis genome appear to cluster together and may either represent cnidarian-specific FGF groups or belong to one of the established classes, but the phyloge netic relationship has been obscured. N. vectensis sequences are shown in bold with arrows. Boxes demark those FGF ligands where expression patterns have been determined. Numbers above branches indicate posterior probabilities, whereas numbers below branches indicate bootstrap support from a maximum likelihood analysis. Clades have been condensed down for illustrative purposes. See supplementary information for the complete tree

Fig. 2

Ancient genomic linkage for FGF and Sprouty genes. a Gene loci maps for human FGF and Sprouty show linkage in the human genome. FGF1/2 (FGF-A class) genes are linked to Sprouty orthologs in the human (a) and mouse genomes (data not shown; Itoh and Ornitz 2004). Members of FGF-B (FGF3), FGF-C (FGF4 and FGF6) and FGF-G (FGF19 and FGF23) classes also show linkage in mammalian genomes. The approximate distances separating linked genes in kilobases (kb) is shown below each linkage group. b In the FGF-D class, three of four genes (NvFGF8A, 204532, and NvFGF8B) are all linked in the Nematostella genome, with NvFGF8A also closely linked to the Sprouty ortholog, NvSprouty, suggesting that this association between FGF and Sprouty genes is an ancient one, predating the cnidarian–bilaterian split (∼600 MYA). There is also evidence of linkage between other FGF genes in Nematostella, with two pairs of FGFs closely associated on two different genomic scaffolds

Fig. 3

NvFGF8A is expressed during gastrulation and during apical tuft formation. In situ hybridization of an antisense RNA probe towards NvFGF8A shows that NvFGF8A is not expressed during cleavage (a) or blastula (b) stages. Transcripts first appear at the onset of gastrulation, with staining visible in the blastopore (c–e). Expression remains restricted orally, to the developing pharynx in pharyngeal ectoderm (f–h). During planula and polyp development, transcripts are also detectable at the base of the apical tuft, in the ectoderm (g–j). During tentacle bud formation, expression is also detected in a few cells at the tips of the growing mesenteries and in the accompanied body wall endoderm (h–j). All embryo views are lateral, with the asterisk denoting the blastopore and future oral side, except (a) and (b), which are cleavage and blastula stage embryos, and (e), which is an oral view

Fig. 4

Co-expression of an FGF ligand, NvFGF1A, and a receptor, FGFRa at the aboral pole during development. NvFGF1A is first expressed during gastrulation where transcripts accumulate at the aboral pole in a broad domain (a). This domain of NvFGF1A expression becomes restricted to fewer cells as development proceeds (b–d). During planula and polyp development, expression can only be found in the ectodermal cells that give rise to the apical tuft (c–d). e–h NvFGFRa is expressed in a pattern nearly identical to that of NvFGF1A, except the initial expression domain of NvFGF1A is broader during gastrulation and early planula development (e, f). All embryo views are lateral, with the asterisk denoting the blastopore and future oral side

Fig. 5

Expression of the receptor, FGFRb at the oral and aboral poles. NvFGFRb is not expressed during gastrulation (a), but transcripts are first visible in the developing pharynx of the planula (b). During later planula development, expression can also be visualized in a few cells in the endoderm below the apical tuft (arrow). Later, during polyp formation, NvFGFRb is expressed in the endoderm of the growing tentacles (d, f). Expression is also visible in discrete cells forming a symmetric pattern in the intra-tentacle endoderm (e, f). The asterisk denotes the blastopore and future oral pole (ph pharynx; at apical tuft; ten tentacle; int inter-tentacular endoderm). All embryo views are lateral except (f), which is an oral view

Fig. 6

Expression of two potential targets of FGF signaling, NvSprouty and NvChurchill. The expression profile of NvSprouty largely follows that of NvFGF8A and NvFGF1A during embryogenesis, with a few notable exceptions. NvSprouty is first expressed in presumptive endoderm during blastula-stages (a). During gastrulation, NvSprouty is expressed broadly in the blastopore and invaginating endoderm as well as at the apical tuft (b, c). In planula development, the oral/aboral expression pattern remains, although it expands orally such that expression is seen in the ectoderm and endoderm of the apical tuft (d, e) in the ectoderm and endoderm of growing tentacles (e, f) and in a broad domain in the pharynx (d–f). Transcripts to the zinc finger transcription factor NvChurchill are not detectable during cleavage (data not shown) and gastrulation (g). Expression begins during planula stages (h) in body wall endoderm near the oral pole. During polyp formation, expression is restricted to a ring of cells in the pharyngeal endoderm (i–l), where it remains expressed throughout juvenile stages (k, l). The asterisk denotes the blastopore and future oral pole. (ph pharynx; at apical tuft; ten tentacle; t. ect tentacle ectoderm; t. end tentacle endoderm, b. end body-wall endoderm) All embryo views are lateral, except (c), (j), and (l), which are oral views

Fig. 7

Summary of expression of FGF signaling molecules during gastrulation and planula development in N. vectensis. During gastrulation (a), FGF ligands are expressed at the blastopore and in invaginating endoderm (NvFGFA8) and at the aboral pole (NvFGF1A). An FGF receptor NvFGFRa is also expressed at the aboral pole in a slightly broader domain than that of the ligand NvFGF1A. NvSprouty, a potential downstream target and known inhibitor of the pathway, is expressed in oral and aboral domains coincident with ligand and receptor localization. During planula development (b), FGF ligands, receptors, and an inhibitor continue to show restricted expression orally in the pharynx (NvFGF8A, NvFGFRb, NvChurchill and NvSprouty) and at the base of the apical tuft in endoderm (NvFGFRb, NvFGF8A, NvFGF8B, and NvSprouty) and ectoderm (NvFGF1A, NvFGFRa, NvFGF8A, and NvSprouty). The expression profiles of FGF pathway genes suggest a role in both gastrulation and the induction of a known neural structure, the planula's apical tuft