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Figure 3.11 A model of the initiation of axis formation by an extracellular Wnt11 signal.
(A) The prevailing model of initiation of the canonical Wnt signaling pathway in Xenopus axis
formation by an intracellular signal. Vesicles (shown in blue) containing disheveled protein
are transferred from a vegetal to a dorsal location by cytoplasmic movements. They stabilize
β-catenin and activate the signaling cascade intracellularly. (B) A model of the initiation
of axis formation by an extracellular Wnt11 signal secreted by dorsal vegetal cells. Purple
represents cortically localized Wnt 11 mRNA. Speckled purple represents the dispersal of
Wnt 11 mRNA from the cortex during cleavage stages.
Source : From Tao et al. (2005) .
to ventralization or prevention of the development of dorsal structures ( Gore et al.,
2005 ).
Two models are presented to explain the formation of dorsal structures in Xenopus .
According to the prevailing model, an intracellular maternal protein, Dsh (disheveled),
involved in the Wnt dorsalizing pathway, moves along the egg cortex from the vegetal
to the dorsal side of the egg, thus determining the asymmetric dorsal localization of
this Dsh ( Figure 3.11A ). According to a newer model, the dorsalizing pathway is acti-
vated by the maternal transcripts of Wnt 11 with the involvement of the maternal FRL1
and HSPG (heparin/heparan sulfate proteoglycans) ( Tao et al., 2005 ) ( Figure 3.11B ).
The Wnt-β-catenin pathway has key roles in pattern formation, cell-fate determi-
nation, and cell polarity ( Hsieh, 2004 ). The components of the canonical Wnt path-
way are well conserved across species, and the key in the pathway is the regulation
of the β-catenin ( Willert and Nusse, 1998 ). The pathway is active during embryonic
development but is generally dormant in adult organisms ( Katanaev, 2010 ). Binding
of Wnts to the receptor Frizzled on the cell surface activates the intracellular pro-
tein Dsh, which in turn inactivates the trimeric GSK3beta/Axin/APC (adenomatous
polyposis coli) complex. Inactivation of the trimeric β-catenin degradation complex
by Wnt signaling allows free β-catenin to accumulate in the cytoplasm and enter the
nucleus (hence the term Wnt/β-catenin pathway ). Nuclear β-catenin in turn interacts
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