Early asymmetric cues triggering the dorsal/ventral gene regulatory network of the sea urchin embryo.

Vincenzo Cavalieri,Giovanni Spinelli

doi:10.7554/elife.04664

Abstract

Dorsal/ventral (DV) patterning of the sea urchin embryo relies on a ventrally-localized organizer expressing Nodal, a pivotal regulator of the DV gene regulatory network. However, the inceptive mechanisms imposing the symmetry-breaking are incompletely understood. In Paracentrotus lividus, the Hbox12 homeodomain-containing repressor is expressed by prospective dorsal cells, spatially facing and preceding the onset of nodal transcription. We report that Hbox12 misexpression provokes DV abnormalities, attenuating nodal and nodal-dependent transcription. Reciprocally, impairing hbox12 function disrupts DV polarity by allowing ectopic expression of nodal. Clonal loss-of-function, inflicted by blastomere transplantation or gene-transfer assays, highlights that DV polarization requires Hbox12 action in dorsal cells. Remarkably, the localized knock-down of nodal restores DV polarity of embryos lacking hbox12 function. Finally, we show that hbox12 is a dorsal-specific negative modulator of the p38-MAPK activity, which is required for nodal expression. Altogether, our results suggest that Hbox12 function is essential for proper positioning of the DV organizer.

Highlights

Patterning of the embryonic ectoderm along the dorsal/ventral (DV) axis, known as oral/aboral axis, has been extensively studied in various species of sea urchins
The ectoderm of the pluteus larva is noticeably partitioned into four main domains: (1) the oral/ ventral ectoderm, a thickened epithelium surrounding the mouth, (2) the aboral/dorsal ectoderm, a squamous epithelium that covers most of the rest of the larval body, (3) the ciliary band, a belt of ciliated cells positioned at the border between oral and aboral ectoderm, and (4) the apical neurogenic domain
The genetic landmark of polarization along the secondary axis is the zygotic expression of the TGF-β superfamily member Nodal on the future oral side, which behaves as an organizing centre imposing DV polarity in all three germ layers of the embryo (Duboc et al, 2004; Flowers et al, 2004; Duboc et al, 2010; Materna et al, 2013)

Summary

Introduction

Patterning of the embryonic ectoderm along the dorsal/ventral (DV) axis, known as oral/aboral axis, has been extensively studied in various species of sea urchins. The observed pattern of expression is so far unique in sea urchin development, and suggests that the Hbox transcription factor could act as a precocious input within the gene regulatory network that directs DV patterning In agreement with this hypothesis, we have previously shown that disrupting the function of the Otx activator, a driver of hbox, downregulates hbox transcription and dramatically affects embryo polarization along the DV axis (Cavalieri et al, 2008). We extend these findings providing more direct evidence indicating that hbox is a key upstream gene in the symmetry-breaking sequence of events, functioning to prevent the ectopic activation of nodal transcription within the prospective dorsal side of the early sea urchin embryo

Results

Discussion

Materials and methods