Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 138

Figure 3.15 Molecular pathway leading to endoderm
in the zebrafish and Xenopus . In contrast, Xnrs
are Xnr1, Xnr2, Xnr4, Xnr5, and Xnr6. Mix-type
homeobox proteins are Mixer, Mix.1, Mix.2, Bix.1,
Bix.2, Bix.3, and Bix.4.
Source : From Fukuda and Kikuchi (2005) .

Xenopus

VegT

Sox7 Sox3

Xnrs (Xnr5)

Mix-type
homeobox
proteins

Gata 4, 5, 6

XSox17

Formation of Endoderm
The maternal transcription factor VegT controls the formation of endoderm in
Xenopus and zebrafish ( Figure 3.15 ) and is also necessary for mesoderm formation
( Fukuda and Kikuchi, 2005; Zhang et al., 1998 ). Two maternal factors for Sox 7 and
Sox 3 are translated downstream of the maternal VegT. Sox 7 is necessary for inducing
Xnrs ( Xenopus Nodal-related genes) and the downstream gene regulatory network,
which induces other zygotic genes to converge to the expression of XSox 17 ( Fukuda
and Kikuchi, 2005; Zhang et al., 2005 ). Certainly, the diagram in the figure is very
simplified and it is estimated that only about 10% of more than 300 genes involved
in endoderm formation are regulated by the transcription factors presented in the fig-
ure ( Sinner et al., 2006 ). Another maternal protein necessary for proper endoderm
and mesoderm formation is Vg1, localized in the vegetal pole of the Xenopus egg
( Birsoy et al., 2006 ).

Formation of Mesoderm
Mesoderm formation is controlled and regulated epigenetically by maternal factors.
Key in mesoderm induction is the group of transcription factors VegT, transcribed
from maternal mRNAs in the vegetal pole. Diffusing dorsally, the transcription fac-
tor VegT, along the maternal β-catenin, activates Xenopus Nodal-related genes ( Xnr ),
which then initiate the formation of the mesoderm by activating fibroblast growth
factor (FGF)-, Wnt-, and BMP pathways. The two first pathways are responsible
for maintaining the mesoderm state and the latter one is responsible for patterning it