Dishevelled: In Vivo Roles of a Multifunctional Gene Family During Development - Current Topics in Developmental Biology - page 227

noncanonical Wnt/PCP pathways were responsible for defects seen in Dvl1/
2 double mutants ( Wang, Hamblet, et al., 2006 ). As noted above, two of the
conserved domains in Dsh/Dvl, DIX and DEP, play different roles in the
Wnt pathway and the PCP/convergent extension (see Wallingford &Habas,
2005 ). In Xenopus , mediolateral intercalation in the mesoderm requires the
C-terminal DEP domain but not the N-terminal DIX domain of Xdsh. To
confirm that mammalian neurulation requires the same domain of Dvl2, we
generated mutant Dvl2 BAC transgenes identical to the D DEP mutant in

DEP - EGFP ). When crossed into Dvl1 / ; Dvl2 / background, Xenopus ( D

D DEP - EGFP completely failed to rescue the neurulation defects. In contrast, a

D DIX - EGFP transgene, deleting part of the DIX domain, fully rescue the neu-

rulation defect in Dvl1 / ; Dvl2 / mutants. As the

DIX - EGFP transgene
removed a VKEEIS motif in the N-terminal region essential for Dvl2 function
in the canonical Wnt pathway ( Capelluto et al., 2002 ), this result also confirmed
that the neural tube closure defect in Dvl1 / ; Dvl2 / mutants was not due to
the loss of function of Wnt signaling. When we investigated the distribution of
the mutant Dvl2 protein during neurulation, we found that while
D

D

DIX-EGFP was still primarily membrane localized, indistinguishable from
wild-type Dvl2-EGFP,

DEP-EGFP was evenly distributed in the cytoplasm.
Thus, the DEP domain was required for targeting Dvl2 to the plasma
membrane during mammalian neurulation. However, there were differ-
ences in Dvl2-EGFP localization in the neuroepithelium and in the organ
of Corti, where Dvl2-EGFP showed an asymmetric membrane distribution
that was disrupted in Lp/Lp mutants, reminiscent of fly PCP establishment
(see Wang et al., 2005 ). We tested whether the point mutation dsh1 iden-
tified in fly, which specifically abolished the PCP ( Axelrod et al., 1998 ),
might affect convergent extension in mammals. The dsh1 mutation results
in a K to M missense mutation in the C-terminal DEP domain ( Axelrod
et al., 1998 ). Using BAC recombineering, we introduced an identical mu-
tation into Dvl2 - EGFP BAC and produced transgenic mice. All Dvl1 / ;
Dvl2 / ; dsh1 - EGFP embryos recovered at E9.5 or later displayed
craniorachischisis, suggesting that the dsh1 mutation completely disrupted
the ability of Dvl2 to function during convergent extension. Also consistent
with the notion that convergent extension underlies cochlea elongation
( Wang et al., 2005 ), we observed a shorter and wider cochlea in Dvl1 / ;
Dvl2 / ; dsh1 - EGFP embryos ( Wang, Hamblet, et al., 2006 ). However,
in either wild-type or Dvl1 / ; Dvl2 / background, this mutation did
not appear to appreciably affect the membrane localization of Dvl2-EGFP
in neuroepithelium during neurulation (data not shown).

D