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We have isolated one member of the frizzled family of wnt receptors from Xenopus (Xfz7) to study the role of cell-cell communication in the establishment of the vertebrate axis. We demonstrate that this maternally encoded protein specifically synergizes with wnt proteins in ectopic axis induction. Embryos derived from oocytes depleted of maternal Xfz7 RNA by antisense oligonucleotide injection are deficient in dorsoanterior structures. Xfz7-depleted embryos are deficient in dorsal but not ventral mesoderm due to the reduced expression of the wnt target genes siamois, Xnr3 and goosecoid. These signaling defects can be restored by the addition of beta-catenin but not Xwnt8b. Xfz7 thus functions upstream of the known GSK-3/axin/beta-catenin intracellular signaling complex in vertebrate dorsoventral mesoderm specification.
Fig. 1. Xenopus laevis frizzled 7 (Xfz7) sequence and expression in embryonic development. (A) Deduced Xfz7 protein sequence. The amino-terminal signal sequence is shown in purple, conserved cysteines are in bold, and the putative PDZ-binding motif is in red. Consensus N-linked glycosylation sites are in green, and the putative transmembrane domains are underlined. (B) Estimated evolutionary relationship of Xfz7 with other known frizzled proteins. Proteins displaying the greatest
sequence similarity cluster together. Using this analysis, three highly distinctive structural groups were identiï¬ed. (C) Northern analysis of
Xfz7 expression. O, 2, 7, 10, 11, 18, 27 denote oocyte, and stages 2-27, respectively. D, V lanes contain RNA from dorsal and ventral halves of
stage 10+ embryos. Only a single 4.6 kb band is observed in all stages examined. (D,E,G,J) dorsal is up, (F,H-J) anterior is to the left. (D-J) Xfz7
distribution as determined by in situ hybridization. The scale bars correspond to 1 mm. (D,E) Animal and vegetal views of stage 11 embryo. Note the preferential dorsal enrichment of Xfz7 transcripts at this stage of development that correlates with the northern data.
(F,G) Dorsal and rostral views of stage 14 embryo. Staining is apparent at, but not limited to, anterior neural folds. (H) Dorsal view of stage 17 embryo. Expression is apparent at neural folds and more weakly in the somites. (I) Dorsal view of stage 20 embryo. Xfz7 RNA is found in
the neural folds and forming pharyngeal arches. (J) Lateral view of stage 27 embryo. Black arrowhead marks the eye, white arrowhead labels the pharyngeal arches, and black arrow is the pronephros.
Fig. 2. Xfz7 synergizes with Wnts in axis induction. The scale bars correspond to 1 mm. All embryos shown are at tailbud stages except as
noted. (A,C) Control uninjected embryo. (A,B) Overexpression of Xfz7 interferes with neural fold closure at neurula stage.
(C,D) Overexpression of Xfz7 results in shortened, bent embryos. (E-H) Coinjection of Xfz7 with either Wnt8b, Wnt5A or Wnt11 results in
axis duplication. (E) Control uninjected embryo. (F) Wnt8b+Xfz7-injected embryo. (G) Wnt5A+Xfz7-injected embryo. (H) Wnt11+Xfz7-
injected embryo. (I-K) Secondary axes of Wnt5A+Xfz7 and Wnt11+Xfz7-injected embryos contain notochords, marked by arrowheads.
(I) Control uninjected embryo. (J) Wnt5A+Xfz7-injected embryo. (K) Wnt11+Xfz7-injected embryo. (L) Xfz7 synergizes with either Wnt8b,
Wnt5A or Wnt11 in the induction of a secondary axis. (M-Q) Coinjection of Wnt5A and Xfz7 does not block activin-induced elongation of
animal caps. (M) Control uninjected activin treated animal cap, (N) 1dose Wnt5A-injected animal cap. (O) 1dose Wnt5A+Xfz7-injected
animal cap. (P) 3dose Wnt5A-injected animal cap. Note that 3Wnt5A injection blocks elongation while 1Wnt5A+Xfz7 does not.
(Q) Xfz7 does not synergize with Wnt5A in blocking the elongation of activin-treated animal caps. (R-V) Coinjection of Wnt11 and Xfz7 does
not block activin-induced elongation of animal caps. (R) shows a control uninjected activin treated animal cap, (S) shows 1Wnt11-injected
animal cap, (T) shows 1Wnt11+Xfz7-injected animal cap, (U) shows 3Wnt11-injected animal cap. Note that 3Wnt11 injection blocks
elongation while 1Wnt11+Xfz7 does not. (V) Xfz7 does not synergize with Wnt11 in blocking the elongation of activin-treated animal caps.
