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Fig. 4. Inducible expression of Dkk1 in the surface epithelium and
inhibition of TOPGAL activity. (A) Scheme for doxycycline-inducible expression
of Dkk1. Mice carrying the K5-rtTA transgene (yellow) are
mated to mice carrying the tetO-Dkk1 transgene (blue). In double
transgenic offspring (green), expression of Dkk1 can be induced by
doxycycline in cells where the K5 promoter is active. (B) Phenotype
of newborn double transgenic (DT) pup (right) compared with control single
transgenic (ST) littermate (left) after doxycycline treatment from E0.5. The
DT pup has severe limb defects (arrows), open eyes and lacks vibrissa
follicles. (C-F) A K5-rtTA, tetO-Dkk1, TOPGAL
(Dkk1-expressing/TOPGAL) embryo (D,F) and tetO-Dkk1, TOPGAL
(Control/TOPGAL) control littermate (C,E) were doxycycline-treated throughout
gestation, harvested at E11.5 and stained with X-gal to reveal sites of TOPGAL
WNT reporter expression. (E,F) Higher-magnification photographs of the mammary
regions of the embryos shown in C,D, respectively. Strong staining for
ß-galactosidase is visible in the mammary region of the control at sites
of mammary placode development (C,E, red arrows) and other sites, including
the apical ectodermal ridge of each limb bud. TOPGAL activity was severely
reduced in the mammary region of the K5-rtTA, tetO-Dkk1, TOPGAL
embryo (D,F). Deformities of the limb buds correlate with loss of
ß-galactosidase expression from the limb edge (yellow arrows in D),
consistent with the known requirement for canonical WNT signaling in
maintenance of the apical ectodermal ridge
(Barrow et al., 2003).
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