227protein stability (beta-catenin protein hereafter), nuclear localization and transcriptional
activity, (2) the regulation of cytoskeletal organization and cell polarity, and (3) the
release of calcium from intracellular stores. With the caveats in mind that much of
Wnt signaling entails complex, context-dependent and networked interactions, it
remains useful to understand the basic features of Wnt pathways involved in early
axis development. There are numerous comprehensive reviews on different aspects
of Wnt signaling (MacDonald et al. 2009 ; Hikasa and Sokol 2013 ); here the key
evidence of Wnt signaling in axis formation and the core signal transduction mecha-
nisms most involved in axis formation and patterning will be briefly reviewed.
6.3.1.1 Wnt/Beta-Catenin Signal Transduction Mechanisms
In Wnt-unstimulated cells, beta-catenin protein is constitutively turned over by the
activity of a multiprotein “destruction” complex (Fig. 6.7). This complex contains
Axin1, Adenomatosis polyopsis coli (Apc), Glycogen synthase kinase 3 beta
(Gsk3b), and Casein kinase 1 alpha (Ck1a/Csnk1a1), and serves to regulate the
phosphorylation of cytoplasmic beta-catenin by Gsk3b and Ck1a (reviewed in
MacDonald et al. 2009 ; Clevers and Nusse 2012 ; Hikasa and Sokol 2013 ). These
phosphorylations in the beta-catenin N-terminus allow recognition by members of
the beta-transducin repeat containing E3 ubiquitin protein ligase family (Btrc, Jiang
and Struhl 1998 ; Liu et al. 1999a) and target phospho-beta-catenin for ubiquity-
lation, resulting in degradation by proteasomes (Aberle et al. 1997 ). Axin is thought
to be a key limiting component of this complex, regulating the assembly of the
destruction complex (Lee et al. 2003 ) and recruiting the beta-catenin kinases (Ck1a
and Gsk3b) for the priming and processive phosphorylation, respectively, of beta-
catenin (Liu et al. 2002 ; Amit et al. 2002 ).
Wnt stimulation inactivates the destruction complex through a little understood
mechanism involving recruitment of the cytoskeletal adaptor protein Dishevelled
(Dvl homologs 1–3). This blocks the activity of Gsk3b (Siegfried et al. 1992 , 1994 ;
Cook et al. 1996 ) and prevents beta-catenin phosphorylation, thereby allowing cyto-
plasmic accumulation and subsequent nuclear localization of beta-catenin
(MacDonald et al. 2009 ; Clevers and Nusse 2012 ; Hikasa and Sokol 2013 ). Nuclear
beta-catenin interacts with the LEF/TCF family of High Mobility Group (HMG)
domain transcription factors (Brunner et al. 1997 , see below) either to activate or to
“derepress” transcription of Wnt-responsive genes (Fig. 6.7).
Signaling is initiated by binding of Wnts to one of seven-transmembrane domain
Frizzled (Fzd) receptors plus a coreceptor, lipoprotein receptor-related protein Lrp5 or
Lrp6 (Lrp5/6; reviewed in He et al. 2004 ). Fzds are heterotrimeric G protein- coupled
receptors that are activated in response to Wnts (Slusarski et al. 1997b; Katanaev and
Buestorf 2009 ). Wnts bind to Fzd through the receptor’s extracellular cysteine-rich
domain (CRD), with key contacts being made between the Wnt lipid moiety and a
separate hydrophobic “index finger” interacting with grooves in the CRD (Janda et al.
2012 ). Wnts also interact with the Lrp6 extracellular domain, leading to clustering of
the receptors and coreceptors (Tamai et al. 2000 ; Mao et al. 2001 ; Kato et al. 2002 ;
6 Vertebrate Axial Patterning: From Egg to Asymmetry