229Liu et al. 2003 ; Itasaki et al. 2003 ). The induced proximity of the intracellular domains
of Fzd and Lrp6 is necessary and sufficient to initiate downstream signaling and inhi-
bition of beta-catenin phosphorylation (Tolwinski et al. 2003 ).
The inhibition of beta-catenin degradation following Wnt receptor activation
remains incompletely understood. Recent observations together with extensive data
on biochemical interactions have suggested that Wnt-Fzd binding activates Dvl, pos-
sibly through GPCR activation, to recruit Axin/Gsk3b/Ck1a complexes to the Lrp6
intracellular domain, resulting in phosphorylation by Gsk3b and Ck1a (Mao et al.
2001 ; Tolwinski et al. 2003 ; Cliffe et al. 2003 ; Tamai et al. 2004 ; Davidson et al. 2005 ;
Zeng et al. 2005 , 2008 ; Egger-Adam and Katanaev 2010 ; Jernigan et al. 2010 ). Lrp6
phosphorylation occurs at PPPSPxS motifs, which serve as sites for additional Axin
complex recruitment and are thought to directly inhibit destruction complex Gsk3b
activity (Piao et al. 2008 ; Cselenyi et al. 2008 ; Wu et al. 2009 ). Furthermore, Axin
itself is a Gsk3b substrate (Yamamoto 1999 ) and Gsk3b inhibition results in Axin
dephosphorylation and its dissociation from phospho-Lrp6 and beta-catenin (Kim
et al. 2013 ). Axin is then free to be either phosphorylated again to reconstitute beta-
catenin destruction complexes or degraded. These data are consistent with a kinetic
analysis of beta-catenin regulation, which suggests that Wnt signaling results in par-
tial inhibition of both Gsk3b and Ck1a activities (Hernández et al. 2012 ). It is possible
that this effect could be explained by the inactivation of a subset of limiting and finite
destruction complexes through Axin dephosphorylation, which would then depress
overall beta-catenin phosphorylation at the population level in a distributed manner.
In a separate but not necessarily mutually exclusive model, Dvl recruitment
leads to multimerization of phospho-Lrp6-Fzd complexes, followed by accumula-
tion of Dvl aggregates, leading to positive feedback recruitment and inactivation of
destruction complexes (Metcalfe and Bienz 2011 ; Dobrowolski and De Robertis
2012 ). There is also evidence that these receptor complexes are incorporated into
signaling endosomes (Lrp6 signalosomes) to stabilize and amplify signaling (Bilic
et al. 2007 ). Other data suggest that these signalosomes are eventually sequestered
into multivesicular bodies, leading to the longer-term removal of Gsk3b activity and
the inability to phosphorylate newly synthesized beta-catenin (Taelman et al. 2010 ).
6.3.1.2 Transcriptional Regulation by Wnt/Beta-Catenin Signaling
Transcriptional responses in response to beta-catenin are mediated by binding to
Lymphoid Enhancer-binding Factor 1 (Lef1)/Transcription factor 7 (T-cell specific,
HMG-box; Tcf7) proteins. These proteins are constitutively nuclear and typically
repress target genes by recruiting Groucho family repressors (Roose et al. 1998 ;
Barker et al. 2000 ). Beta-catenin accumulation can lead to displacement of Groucho
and activation of target genes, through a combination of derepression and tran-
scriptional activation, mediated by distinct Lef1/Tcf7 proteins. Tcf7l1 (Tcf3 hereaf-
ter) likely exclusively acts as a transcriptional repressor during early development,
with Lef1 and Tcf1 proteins serving as activators, and Tcf7l2 (Tcf4) exhibiting
6 Vertebrate Axial Patterning: From Egg to Asymmetry