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We isolated a Xenopus homolog of Frzb, a newly described protein containing an amino-terminal Frizzled motif. It dorsalized Xenopus embryos and was expressed in the Spemann organizer during early gastrulation. Unlike Frizzled proteins, endogenous Frzb was soluble. Frzb was secretable and could act across cell boundaries. In several functional assays, Frzb antagonized Xwnt-8, a proposed ventralizing factor with an expression pattern complementary to that of Frzb. Furthermore, Frzb blocked induction of MyoD, an action reported recently for a dominant-negative Xwnt-8. Frzb coimmunoprecipitated with Wnt proteins, providing direct biochemical evidence for Frzb-Wnt interactions. These observations implicate Frzb in axial patterning and support the concept that Frzb binds and inactivates Xwnt-8 during gastrulation, preventing inappropriate ventral signaling in developing dorsal tissues.
Partial Axis Duplication and Rescue by frzb
Control embryos are shown in (A) and (B). Of the embryos injected with 1 ng of Bfrzb mRNA into single ventral blastomeres at the 4–8 cell stage, 15% developed partial secondary axes (C and D), as indicated by arrows. Nearly all of the UV-irradiated embryos (E and F) were ventralized completely. 56% of the UV-irradiated embryos injected with 1 ng of frzb mRNA showed partial rescue of a dorsal axis as shown (G and H); the other embryos remained completely ventralized. (B), (D), (F), and (H) are frontal sections. The partial secondary axis in (C) and (D) and the rescued axis in (G) and (H) contain muscle and neural tissue, but no notochord. Nevertheless, frzb overexpression in animal cap explants did not induce mesoderm or dorsal markers (see text for details).
Expression of Frzb during Xenopus Development
(A) Localization of Xfrzb mRNA during early development by whole-mount hybridization in situ. Numbers indicate embryonic stages. (V), vegetal view; (D), dorsal view; (SS), sagittal section; (L), lateral view. All embryos except the dorsal views (11D and 13D) are shown with dorsal on top; the stage 13 and 21 embryos are presented with anterior to the right. High levels of Xfrzb expression are identified by dark purple-blue staining. In the late blastula (stage 9) and early gastrula (stage 10), expression is most prominent in the region of the Spemann organizer. The red arrowhead (stage 11SS) indicates the dorsal lip of the blastopore; this section demonstrates hybridization in the involuted dorsal mesoderm. Black arrowheads identify the yolk plug; the cleared early neurula (stage 13D) and corresponding section (13SS) both show prominent staining in anterior mesoderm. By the neural tube stage (21L, 21SS), Frzb is expressed primarily in anterior and posterior mesoderm.
(B) Expression levels of Xfrzb mRNA. RT–PCR with total RNA isolated from the indicated stages was performed for Xfrzb; cSrc was used to confirm similar amounts of input cDNA between samples.
(C) Immunoblot analysis of endogenous Frzb protein. Each lane represents 0.5 embryo. Specific staining is indicated by the arrow. These analyses were performed twice with similar results, and they confirm the presence of endogenous Frzb protein in developing embryos.
(D) Induction of Xfrzb by activin but not bFGF. Animal caps were explanted at stage 8 and cultured until sibling embryos reached stage 11. RT–PCR analysis of explants incubated with BSA, bFGF (20 ng/mL), or activin (5 U/mL) indicated that Xfrzb behaves as a dorsal marker. cDNA from whole embryos (Embryo) was used as a positive control template. Brachyury (Xbra), a general mesodermal marker known to be induced by both growth factors, is used as a positive control for mesoderm induction. Histone H4 was used to confirm similar amounts of input cDNA between samples. For both RT–PCR assays, reactions were done with template from which reverse transcriptase was omitted (no RT) to control contamination.
Frzb Blocks Wnt-8 Signaling In Vivo
(A–C) Xfrzb blocks induction of secondary axes by Xwnt-8. Embryos injected with 25 pg preprolactin mRNA (A), 5 pg Xwnt-8myc mRNA, and 25 pg preprolactin mRNA (B), or 5 pg Xwnt-8myc mRNA and 25 pg Xfrzb mRNA (C). Dorsal views are shown, with anterior to the left. All injections were into a single ventral vegetal blastomere at the eight cell stage. This experiment was done twice with Xfrzb and twice with Bfrzb with identical results. In the experiment shown, the group corresponding to panel (A) showed 0/56 secondary axes; to panel (B), 27/38 secondary axes; and to panel (C), 0/32 secondary axes.
(D) Frzb blocks ventralization of animal cap explants by CSKA-Xwnt-8 plasmid. Embryos were injected with the indicated combinations of prolactin or Xfrzb mRNA (1 ng) and CSKA-Xwnt-8 expression plasmid (100 pg) into both blastomeres at the two cell stage. Caps were explanted at stage 8 and incubated in the absence and presence of activin (5 U/mL) until stage 11. The dorsal marker ADMP but not the ventral marker Xpo was induced in animal caps cultured in the presence of activin. Post-MBT overexpression of Xwnt-8 in cap explants reversed this pattern; when Xfrzb was coexpressed with Xwnt-8, these effects were blocked.
(E) Frzb prevents induction of Xwnt-8 response genes. Animal cap assays were as in (D), but no activin was used. Coinjection of either Xenopus or bovine frzb mRNA (100 pg) blocked induction of Siamois and Xnr3 by Xwnt-8 (10 pg mRNA).
Frzb Is a Soluble, Secreted Protein That Can Act across Cell Boundaries
(A) Immunoblot analysis of normal stage 30 embryos (lanes 1–3) and metabolic labeling pattern of oocytes injected with Xfrzb mRNA (lanes 4–8). Lane 1, 20,000 × g supernatant; lane 2, 105,000 × g supernatant; lane 3, 105,000 × g pellet. Frzb was recovered in the soluble fraction. Lane 4, culture supernatant from uninjected oocytes; lane 5, supernatant from oocytes injected with Xfrzb mRNA; lane 6, lysate from oocytes injected with Xfrzb mRNA; lane 7, lysate from uninjected oocytes; lane 8, bovine Frzb expressed in E. coli. The band in lane 5 corresponds to secreted protein that has undergone proteolytic processing. The somewhat larger band in lane 6 corresponds to unprocessed protein.
(B) Frzb can block Xwnt-8 signaling across cell boundaries. Ventral blastomeres were injected with either prolactin (P) or Xfrzb (F) mRNA (50–100 pg per blastomere), as shown at the early 16 cell stage. At the late 16 cell stage, single blastomeres surrounded by those injected previously were injected with Xwnt-8 (W) mRNA (10 pg). They were then scored for secondary axes. This experiment was performed three times with similar results; the data were pooled for the graph presented.
Frzb Binds to Wnt Proteins Directly
(A) In vitro translation of Xwnt-8myc, Xfrzb, β-lactamase, or the indicated combinations of these mRNAs. Each of these proteins contains a signal sequence; the observed patterns are consistent with a mixture of processed and unprocessed translation products.
(B) Immunoprecipitation of in vitro translation products. The antiserum 374-PEP was used to immunoprecipitate Frzb; a commercial monoclonal was used to precipitate myc-tagged Xwnt-8. β-lactamase was used as a control for nonspecific interactions. When Frzb and Xwnt-8 were cotranslated, either antibody precipitated both proteins. These analyses were done at least twice with both Xenopus and bovine Frzb.
(C) Coimmunoprecipitation of Frzb and Wnt-1 from COS7 cells. Cells were transfected with expression plasmids encoding Frzb, an HA-tagged Wnt-1, or both together. They were then lysed, immunoprecipitated with an anti-HA antibody, and immunoblotted with the 374-PEP antiserum to detect Frzb. The specific band is indicated by the arrow. Frzb was detected in the immunoprecipitate only if cotransfected with Wnt-1. This experiment was performed at least four times with identical results.
Frzb Blocks MyoD Expression
Embryos were injected at the four cell stage with 500 pg prolactin (A and C) or Xfrzb (B and D) mRNA into the marginal zone of each blastomere (instead of a single ventral blastomere as in Figure 1). Though radial injection of Xfrzb mRNA produced severe shortening of the trunk, anterior structures (cement gland, eyes) were present. MyoD expression in control (prolactin-injected) stage 11 embryos is shown in (C). Ubiquitous overexpression of Xfrzb completely blocked expression of MyoD (D). (E) shows RT–PCR analysis of animal cap explants as in Figure 3D. Xfrzb blocked induction of MyoD by activin.
Proposed Interaction of Frzb and Wnt-8 in Dorsoventral Mesoderm Patterning
During gastrulation, Xfrzb (A) is concentrated in the Spemann organizer, which is associated with specification of dorsal structures. Xwnt-8 (B) is excluded from this region, but is expressed in lateral and ventral mesoderm. In (C), the stylized (F) denotes the Frizzled-like amino-terminal sequence conserved between Frzb and Frizzled proteins (indicated by [Fz]), which are proposed as receptors for Wnt proteins. In the dorsal marginal zone, Xfrzb could compete with a Frizzled protein for Wnt binding and prevent signaling. In the ventral regions of the embryo, where Frzb is not present, Wnt-8 binding to the cognate receptor will not be affected. Question marks denote possible alternative actions of Frzb or Frzb–Wnt complexes.