234
Secreted Frizzled-related proteins (Sfrps), which bind directly to Wnts and antagonize
different Wnt ligands, and Dickkopf1 (Dkk1), which acts at the level of the Wnt/Lrp6
receptor complex. In addition, the secreted Notum pectinacetylesterase homolog was
identified as a Wnt antagonist in Drosophila (Giraldez et al. 2002 ; Gerlitz and Basler
2002 ) and is thought to act by promoting membrane shedding of Glypican Wnt core-
ceptors (Kreuger et al. 2004 ). Recent data from flies and vertebrates also suggests
that Notum acts as a Wnt deacylase, cleaving the Wnt palmitoleate moiety, resulting
in Wnt ligand oxidation and inactivation (Kakugawa et al. 2015 ; Zhang et al. 2015 ).
Notum is conserved and is involved in feedback regulation of Wnt signaling body
axis patterning in Planaria (Petersen and Reddien 2011 ), and recent data suggest a
role in dorsoventral neural tube pattering in zebrafish (Flowers et al. 2012 ).
Transmembrane antagonists have recently been identified as well. Those with roles
in axis formation include the leucine-rich repeat protein Trophoblast glycoprotein
(Tbgp/Waif1) and Tiki1 (Trabd2a). Tbgp is thought to act as a feedback Wnt inhibitor,
acting in Wnt-receiving cells to alter Lrp6 subcellular localization (Kagermeier-
Schenk et al. 2011 ). Trabd2a/Tiki1 is a transmembrane metalloproteinase enriched in
the organizer that can cleave a subset of Wnt ligands, causing their abnormal oxida-
tion and oligomerization and reduced receptor binding (Zhang et al. 2012 ).
6.3.2 Wnt/Beta-Catenin Signaling in Early Axis Formation
The central role of Wnt/beta-catenin signaling in axis formation was initially dem-
onstrated largely through simple overexpression experiments in Xenopus and zebraf-
ish embryos. The first of these was the induction of axis duplications in Xenopus by
injected mouse Wnt1 mRNA (McMahon and Moon 1989 ). Xenopus wnt8a (Xwnt-8;
Christian et al. 1991 ; Sokol et al. 1991 , Smith and Harland 1991 ), and several other
Wnt ligands (Wolda et al. 1993 ; Du et al. 1995 ; Kelly et al. 1995a) can also induce
secondary axes and rescue UV ventralization. Importantly, beta-catenin also exhibits
axis inducing activity (Funayama et al. 1995 ; Guger and Gumbiner 1995 ), and both
Wnt and beta-catenin can induce axial structures non- cell autonomously when
expressed in vegetal blastomeres, suggesting that this activity acts analogously to a
Nieuwkoop center (Smith and Harland 1991 ; Guger and Gumbiner 1995 ).
Interestingly, later overexpression of wnt8a during gastrulation (from injected
plasmid DNA as opposed to mRNA) causes a loss of anterior structures, indicating
roles for Wnts in patterning of the axis as well as its induction (Christian and Moon
1993 ). Other Wnts, including Wnts 4, 5a and 11b, do not elicit axis duplications but
disrupt gastrulation movements and cell adhesion when overexpressed (Moon et al.
1993 ; Ku and Melton 1993 ; Du et al. 1995 ). Additionally, these Wnts can be antago-
nistic to the axis-inducing Wnts in some cases (Torres et al. 1996 ). The same ligands
were also shown to trigger intracellular calcium release when expressed in the early
zebrafish embryo (Slusarski et al. 1997a, b).
Loss-of-function experiments have established that Wnt/beta-catenin signaling is
essential for axis formation in vertebrates. The first evidence for this came from
D.W. Houston