The Hedgehog gene family of the cnidarian, Nematostella vectensis, and implications for understanding metazoan Hedgehog pathway evolution

Abstract

Hedgehog signaling is an important component of cell-cell communication during bilaterian development, and abnormal Hedgehog signaling contributes to disease and birth defects. Hedgehog genes are composed of a ligand ("hedge") domain and an autocatalytic intein ("hog") domain. Hedgehog (hh) ligands bind to a conserved set of receptors and activate downstream signal transduction pathways terminating with Gli/Ci transcription factors. We have identified five intein-containing genes in the anthozoan cnidarian Nematostella vectensis, two of which (NvHh1 and NvHh2) contain definitive hedgehog ligand domains, suggesting that to date, cnidarians are the earliest branching metazoan phylum to possess definitive Hh orthologs. Expression analysis of NvHh1 and NvHh2, the receptor NvPatched, and a downstream transcription factor NvGli (a Gli3/Ci ortholog) indicate that these genes may have conserved roles in planar and trans-epithelial signaling during gut and germline development, while the three remaining intein-containing genes (NvHint1,2,3) are expressed in a cell-type-specific manner in putative neural precursors. Metazoan intein-containing genes that lack a hh ligand domain have previously only been identified within nematodes. However, we have identified intein-containing genes from both Nematostella and in two newly annotated lophotrochozoan genomes. Phylogenetic analyses suggest that while nematode inteins may be derived from an ancestral true hedgehog gene, the newly identified cnidarian and lophotrochozoan inteins may be orthologous, suggesting that both true hedgehog and hint genes may have been present in the cnidarian-bilaterian ancestor. Genomic surveys of N. vectensis suggest that most of the components of both protostome and deuterostome Hh signaling pathways are present in anthozoans and that some appear to have been lost in ecdysozoan lineages. Cnidarians possess many bilaterian cell-cell signaling pathways (Wnt, TGFbeta, FGF, and Hh) that appear to act in concert to pattern tissues along the oral-aboral axis of the polyp. Cnidarians represent a diverse group of animals with a predominantly epithelial body plan, and perhaps selective pressures to pattern epithelia resulted in the ontogeny of the hedgehog pathway in the common ancestor of the Cnidaria and Bilateria.

Genomic insights into the evolutionary origin of Myxozoa within Cnidaria

“…Specifically, the conserved transcriptional factors belonging to the Hox and Runx gene families, which have been shown to be important for cnidarian patterning and cellular differentiation, respectively (43,44), were found in neither the genomes nor transcriptomes of myxozoans, but were nevertheless present in P. hydriforme (Table 4 and Dataset S2). In addition, myxozoans appear to have lost the ligands, receptors, and most downstream elements of the Wnt-and Hedgehog-signaling pathways, which have been shown to be important for axial patterning (45) and cell signaling (46), respectively, whereas nearly all components of these pathways were recovered in P. hydriforme (Table 4 and Dataset S3). By contrast, myxozoans and P. hydriforme possess orthologs to the stem-cell markers FoxO (47) and Piwi (48), and P. hydriforme and K. iwatai appear to have the gene Hap2, which was shown to be involved in gamete fusion in Hydra (49) ( Table 4 and Dataset S2).…”

Developmental gene expression provides clues to relationships between sponge and eumetazoan body plans

“…Hedgehog signalling is involved in endomesoderm formation in bilaterians, and the key transcription factor of the pathway, Gli, is expressed in the presumptive endoderm of Nematostella 19,56 . SciGli is expressed in the oocytes and micromeres of postinversion stage embryos (Fig.…”

Phylogenetic evidence for the modular evolution of metazoan signalling pathways

“…the limb ZPA and neural floor plate) during embryogenesis [112]. Outside of Bilateria, cnidarians are the only metazoans to have true HH orthologues and their role in development has been studied only by whole mount in situ hybridization in the sea anemone N. vectensis [66]. The overlapping expression of the PTC receptor and GLI transcription factor in the endodermal mesenteries and their proximity to the expression of both HH genes suggests this pathway may be regulating the development of endodermal cell identity in cnidarians.…”