Two Oppositely Localised Frizzled RNAs as Axis Determinants in a Cnidarian Embryo

Abstract

In phylogenetically diverse animals, including the basally diverging cnidarians, “determinants” localised within the egg are responsible for directing development of the embryonic body plan. Many such determinants are known to regulate the Wnt signalling pathway, leading to regionalised stabilisation of the transcriptional coregulator β-catenin; however, the only strong molecular candidate for a Wnt-activating determinant identified to date is the ligand Wnt11 in Xenopus. We have identified embryonic “oral–aboral” axis determinants in the cnidarian Clytia hemisphaerica in the form of RNAs encoding two Frizzled family Wnt receptors, localised at opposite poles of the egg. Morpholino-mediated inhibition of translation showed that CheFz1, localised at the animal pole, activates the canonical Wnt pathway, promotes oral fates including gastrulation, and may also mediate global polarity in the ectoderm. CheFz3, whose RNA is localised at the egg vegetal cortex, was found to oppose CheFz1 function and to define an aboral territory. Active downregulation mechanisms maintained the reciprocal localisation domains of the two RNAs during early development. Importantly, ectopic expression of either CheFz1 or CheFz3 was able to redirect axis development. These findings identify Frizzled RNAs as axis determinants in Clytia, and have implications for the evolution of embryonic patterning mechanisms, notably that diverse Wnt pathway regulators have been adopted to initiate asymmetric Wnt pathway activation.

The Evolution of the Wnt Pathway

“…Dsh is necessary for this blastoporal activation (Lee et al 2007). In the hydrozoan Clytia, Fz1 and maternal Wnt3 are required for oral b-catenin activation (Momose and Houliston 2007;Momose et al 2008). In Nematostella gastrulation and planula larvae, Wnts are expressed in overlapping domains along the oral -aboral axis (Kusserow et al 2005).…”

Evolution of GPCR: Change and continuity

“…In contrast to rhodopsin-like GPCR, however, many of which appear to rely on internal hydrophobic sequences for plasma membrane targeting, most secretin-like GPCR possess an N-terminal signal peptide [48]. Members of the Frizzled-like receptor family are identified in sponges [49,50], cnidaria [51][52][53] and placozoa (Trichoplax adherens) [54] and have been regarded as an innovation of multicellular metazoan evolution [53]. There is no species specific expansion in number within this family.…”

A maternally localised Wnt ligand required for axial patterning in the cnidarianClytia hemisphaerica

“…Regionalised β-catenin stabilisation in cleavage-stage embryos has been visualised in two phylogenetically distant cnidarians, the anthozoan (sea anemone) Nematostella vectensis and the hydrozoan jellyfish Clytia hemisphaerica. In Clytia, analysis of regionally expressed genes showed that β-catenin stabilisation specifies the oral-aboral axis of the developing embryo by defining a presumptive oral territory (Momose and Houliston, 2007). In both these species, the animal pole of the egg marks the future gastrulation site from which endoderm forms, and later the oral pole of the larva (see Fig.…”

“…In both these species, the animal pole of the egg marks the future gastrulation site from which endoderm forms, and later the oral pole of the larva (see Fig. 1A): inhibition of the canonical Wnt pathway blocks gastrulation, whereas its forced activation favours endoderm formation (Wikramanayake et al, 2003;Momose and Houliston, 2007). Wnt signalling has also been shown to be involved in maintaining oralaboral polarity in cnidarian adult stages, notably in Hydra polyps, in which Wnt ligands, as well as the intracellular regulators TCF and β-catenin, are expressed around the hypostome (oral opening) and contribute to the 'head organiser' properties of this region along with other genes implicated in vertebrate organiser function (Bode, 2003;Broun et al, 2005;Hobmayer et al, 2000).…”