239graded fashion in the late blastula/early gastrula. These genes depend on vegetally
localized maternal vegt activity, showing high dorsovegetal expression at the onset
of gastrulation, followed by a shift ventrolaterally by late gastrulation, mirrored by
Smad2 activity (Agius et al. 2000 ; Faure et al. 2000 ; Lee et al. 2001 ). Maternal beta-
catenin directly patterns this activity first by contributing to early nodal paralogue
expression (nodal5, nodal6) in dorsovegetal blastomeres prior to and immediately
after the onset of zygotic transcription at the mid-blastula transition (MBT; see Chap.
9 ) (Takahashi et al. 2000 ; Rex et al. 2002 ; Xanthos et al. 2002 ; Hilton et al. 2003 ;
Blythe et al. 2010 ). This role of beta-catenin may be related to its function in recruit-
ing Prmt2 to prime gene expression during the cleavage stages (Blythe et al. 2010 ).
Beta-catenin also synergizes with Nodal activity to generate higher nodal1 expres-
sion dorsally in the early gastrula, although mid-late gastrula expression becomes
uniform and is independent of beta-catenin (Agius et al. 2000 ; Lee et al. 2001 ).
In addition to overlapping with Vegt, beta-catenin might also functionally inter-
act with Gdf1 (alias Vg1), which is encoded by another maternally localized mRNA
(Rebagliati et al. 1985 ; Melton 1987 ; Weeks and Melton 1987 ). Gdf1 is a Nodal-
related Tgfb family ligand and activates Smad2 signaling through Nodal receptors/
coreceptors. Wnt/beta-catenin activity synergizes with gdf1 overexpression to
induce axial structures in Xenopus (Cui et al. 1996 ). Maternally depleted gdf1
embryos lack anterior structures and are deficient in the expression of certain orga-
nizer markers, including secreted antagonists nog, chrd, dkk1, and cer (Birsoy et al.
2006 ). Early Smad2 activation is also compromised, indicating that Gdf1 contrib-
utes to overall Nodal signaling on the dorsal side of the late blastula (Birsoy et al.
2006 ). Thus, beta-catenin activity can enrich Nodal expression and activity dorsally
at multiple regulatory levels.
This temporal control of Nodal activity is functionally significant. In Nieuwkoop
conjugate experiments, late blastula vegetal explants from ctnnb1-depleted Xenopus
embryos fail induce dorsal fates in early equatorial or animal cap tissue (Xanthos et al.
2002 ; Wylie et al. 1996 ). However, if ctnnb1-depleted vegetal explants from late gas-
trula embryos are used, which now express nodal genes, dorsal mesoderm is induced
(Xanthos et al. 2002 ). Additionally, pre-MBT Nodal activity regulated by beta-catenin
is essential for normal axis formation, possibly for perpetuating nodal expression
through autoinduction (Skirkanich et al. 2011 ). Beta-catenin also likely contributes to
Nodal ongoing autoregulation, both directly and indirectly. Dorsal beta-catenin activ-
ity is required to repress microRNAs 15 and 16 (miR15/16) through an unknown
mechanism (Martello et al. 2007 ). These miRs target and downregulate an essential
component of the Nodal receptor complex, activin A receptor, type IIA (acrv2a)
(Martello et al. 2007 ), which contributes to the early dorsal bias in Nodal activity.
In zebrafish, maternal beta-catenin signaling similarly regulates early nodal
homolog gene expression. Two paralogues are expressed in the early dYSL (nodal1/
squint and nodal2/cyclops); nodal1 is likely a direct beta-catenin target (Kelly et al.
2000 ) and nodal2 is autoregulated by Nodal signaling itself. Genetic and other loss-
of- function experiments support a role for the early Nieuwkoop center nodal genes
in axis formation. Single and double mutants for nodal1, nodal2 (squint, cyclops)
exhibit axial patterning and mesendoderm defects (Rebagliati et al. 1998 ; Feldman
6 Vertebrate Axial Patterning: From Egg to Asymmetry