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

Abstract

Regionalised activation of canonical Wnt signalling via β-catenin stabilisation is a key early step in embryonic patterning in many metazoans, including the basally diverging cnidarians, but the upstream maternal cues appear surprisingly variable. In Clytia, regionalised β-catenin stabilisation defining a presumptive 'oral' territory is determined by two maternally coded Frizzled family Wnt receptors of opposite localisation and function. We have identified a maternally coded ligand, CheWnt3, the RNA of which is localised to the animal cortex (future oral side) of the egg. Antisense morpholino oligonucleotide experiments showed that CheWnt3 is required maternally for regionalised oral β-catenin stabilisation in the early embryo, being only the second clear example of a maternally required Wnt ligand after Xenopus Xwnt11. In line with the determinant role of the maternally localised Frizzleds, CheWnt3 overexpression by RNA injection initially had little effect on establishing the oral domain. Subsequently, however, overexpression had dramatic consequences for axis development, causing progressive expansion of β-catenin stabilisation to yield spherical 'oralised' larvae. Upregulation of both CheFz1 and CheFz3 RNAs in CheWnt3 morpholino embryos indicated that CheWnt3 participates in an active axial patterning system involving reciprocal downregulation of the receptors to maintain oral and aboral territories. Localised introduction of CheWnt3 RNA induced ectopic oral poles in CheWnt3 morpholino embryos, demonstrating its importance in directing oral fate. These findings suggest that the complete ligand-dependent Wnt signalling cascade is involved in axial patterning in ancestral eumetazoans. In Clytia, two variant Frizzled receptors and one Wnt ligand produced from localised RNAs cooperate to initiate regionalised Wnt pathway activation.

Hydractinia, a pioneering model for stem cell biology and reprogramming somatic cells to pluripotency

“…Concerning Wnt in embryonic development, however, Hydractinia and also Clytia appear to be different from Hydra as colonial hydroids direct primary axis formation by local deposits of Wnt mRNA in the egg Momose et al, 2008). In contrast, Hydra activates Wnt only in late embryogenesis when the axis has been already fixed (Fröbius et al, 2003).…”

“…Wnt and Tcf transcripts are deposited in the oocyte and early embryo at the oral pole . In Hydractinia, and likewise in the thecate hydrozoan Clytia (Momose et al, 2008) and the anthozoan Nematostella (Fritzenwanker et al, 2007), this pole is the pole marked by the polar bodies, whereas in sea urchin embryos the transcripts are located at the opposite pole. Common denominator is that b-catenin stabilization co-localizes with cleavage initiation and gastrulation.…”

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).…”

Convergent origins and rapid evolution of spliced leader trans-splicing in Metazoa: Insights from the Ctenophora and Hydrozoa

“…5B), indicating that the A-rich area is more likely to be linked to SL transsplicing than to translation initiation. In contrast, no particular nucleoside enrichment was detected in the 59 UTRs of 88 non-trans-spliced cDNAs of C. hemisphaerica chosen from conserved and well studied proteins, e.g., ribosomal proteins (Philippe et al 2009) and developmental regulator genes (Chevalier et al 2006;Momose et al 2008;Amiel et al 2009), except in the Kozac environment (first 3 base pairs [bp] upstream of the AUG initiator codon) (Fig. 5B,C).…”