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6.5.2.2 Self-Regulation of Nodal and Wnt Signaling During Anterior
Patterning
As is the case for BMP signaling/antagonism and the organizer, Nodal and Wnt
signaling are also precisely regulated through auto-regulation and negative feed-
back loops controlled by secreted antagonists. Nodal can directly auto-regulate its
own expression in the epiblast and visceral endoderm through a Smad2/Foxh1-
dependent enhancer in the first intron of the Nodal locus (Osada et al. 2000 ; Norris
et al. 2002 ). Also, Wnt3 is expressed in the proximal egg cylinder ectoderm and can
regulate Nodal expression through a Wnt/beta-catenin-responsive proximal ecto-
derm enhancer (Brennan et al. 2001 ; Ben-Haim et al. 2006 ). Wnt3 is in turn regu-
lated by secreted Bmp4 signals emanating from a population of extra-embryonic
ectoderm adjacent to the epiblast. This population is maintained by epiblast Nodal
signaling, forming the second feedback loop (Ben-Haim et al. 2006 ). Nodal signals
emanating primarily from the epiblast also induce the AVE at the distal end of the
egg cylinder in a Smad2-dependent manner, regulating (both directly and indirectly)
the expression of Nodal antagonists Cerl and Lefty1. These in turn feedback and
inhibit Nodal signaling in the underlying epiblast.
Wnt3 also activates its own expression in the posterior epiblast and visceral
endoderm directly through beta-catenin-dependent Wnt signaling (Tortelote et al.
2013 ) and Wnt signals (likely including Wnt3) directly activate Dkk1 expression in
a number of cell types (Chamorro et al. 2005 ). Interestingly, the autoregulation of
Wnt3 is evolutionarily ancient, occurring in sponges and in cnidarians (Holstein
2012 ). Both Wnt/Dkk1 and Nodal/Lefty1 interactions have been suggested to con-
stitute reaction-diffusion systems in vivo, with regard to hair follicle spacing in the
case of Wnt (Sick et al. 2006 ) and to left-right patterning in the case of Nodal
(Nakamura et al. 2006 ). Whether or not these match the strict mathematical assump-
tions of Turing reaction-diffusion systems (the case for Wnt has been disputed,
Meinhardt 2012 ), it is clear that interacting networks of self-propagating and
self- limiting loops of Nodal, Wnt and BMP signaling underlie much of axis forma-
tion in vertebrates and indeed likely all animals.
In addition to reciprocal regulation of Nodal and Wnt, there is extensive interaction
between Wnt/BMP and Nodal/BMP at the level of signaling integration during antero-
posterior patterning. Wnt signaling can potentiate BMP signaling through several pos-
sible mechanisms. In the frog, BMP-activated Smad1 can be inhibited by Gsk3b
phosphorylation of the linker region, and this in turn can be inhibited by Wnt leading
to the perdurance of active Smad1 (Fuentealba et al. 2007 ). In zebrafish embryos,
dorsoventral and anteroposterior patterning occur along a similar time course (Tucker
et al. 2008 ). Correspondingly, manipulation of Wnt signaling can alter the anteropos-
terior character of Chrd-induced tissues at any time during gastrulation (Hashiguchi
and Mullins 2013 ), suggesting that BMP and Wnt signaling mechanisms are active
together, along with FGF and retinoid signaling (Hashiguchi and Mullins 2013 ).
Additionally, in the chick epidermis Wnt signaling can block FGF inhibition of Bmp4
transcription during neural plate induction (Wilson et al. 2001 ), although it is unclear
whether this interaction is also critical for anteroposterior neural plate patterning.
D.W. Houston