Elsevier

Developmental Biology

Volume 276, Issue 2, 15 December 2004, Pages 552-562
Developmental Biology

Genomes & Developmental Control
Regulation of hhex expression in the yolk syncytial layer, the potential Nieuwkoop center homolog in zebrafish

https://doi.org/10.1016/j.ydbio.2004.09.035Get rights and content
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Abstract

The Nieuwkoop center is the earliest signaling center during dorsal–ventral pattern formation in amphibian embryos and has been implied to function in induction of the Spemann–Mangold organizer. In zebrafish, Nieuwkoop-center-like activity resides in the dorsal yolk syncytial layer (YSL) at the interface of the vegetal yolk cell and the blastoderm. hex homologs are expressed in the anterior endomesoderm in frogs (Xhex), the anterior visceral endoderm in mice, and the dorsal YSL in zebrafish (hhex). Here, we investigate the control of hhex expression in the YSL. We demonstrate that bozozok (boz) is absolutely required for early hhex expression, while overexpression of boz causes ectopic hhex expression. Activation of Wnt/β-catenin signaling by LiCl induces hhex expression in wild-type YSL but not in boz mutant embryos, revealing that boz activity is required downstream of Wnt/β-catenin signaling for hhex expression. Further, we show that the boz-mediated induction of hhex is independent of the Boz-mediated repression of bmp2b. Our data reveal that repressive effects of both Vega1 and Vega2 may be responsible for the exclusion of hhex expression from the ventral and lateral parts of the YSL. In summary, zebrafish hhex appears to be activated by Wnt/β-catenin in the dorsal YSL, where Boz acts in a permissive way to limit repression of hhex by Vega1 and Vega2.

Keywords

Gastrula organizer
Nieuwkoop center
Yolk syncytial layer
hhex
bozozok
wnt8
vega1
vega2
Nodal
Zebrafish

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Present address: Institut für Molekularbiologie, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland.