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Department of Cell and Developmental Biology, Graduate School of Biostudies, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
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| Introduction |
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| Results |
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Since it has been reported that Dishevelled translocates to the plasma membrane region from the cytoplasm during vertebrate gastrulation (Wallingford et al. 2000), we examined the subcellular localization of Dishevelled in more detail. To this end, we expressed GFP-tagged Xenopus Dishevelled (GFP-Xdsh) in Xenopus embryos. In sections of the embryo at stage 11, GFP-Xdsh was found to accumulate at the apical region of cell–cell contacts in involuting mesodermal cells (Fig. 1A). This polarized localization of Dishevelled has not been reported before. Since Wnt11, a non-canonical Wnt ligand, is expressed in involuting dorsal mesodermal cells and has been shown to be involved in the gastrulation cell movements (Heisenberg et al. 2000; Saka et al. 2000; Tada & Smith 2000; Ulrich et al. 2003), we ectopically expressed Wnt11 with GFP-Xdsh in Xenopus ectodermal explants to examine whether the accumulation of GFP-Xdsh was induced by Wnt11. Explants were stained with either anti-ß-catenin antibody or anti-ZO1 antibody, and sectioned. While GFP-Xdsh localized punctatedly in the cytoplasm in the absence of Wnt11, it was translocated and accumulated at the apical region of cell–cell contacts in the presence of Wnt11 (Fig. 1B,C). The accumulated GFP-Xdsh co-localized with ß-catenin (Fig. 1B), but not with ZO1 (Fig. 1C), which is located more apically than ß-catenin or GFP-Xdsh. These results indicate that the accumulated Dishevelled localizes at apical adherens junctions. To examine whether the translocation of Xdsh is induced by secreted Wnt11, we attached an explant expressing GFP-Xdsh to an explant expressing Wnt11 (Fig. 1D). GFP-Xdsh translocated to the cell–cell contacts when the attached with the explant expressing Wnt11 (Fig. 1E), indicating that GFP-Xdsh is able to translocate to cell–cell contacts in response to extracellular Wnt11.
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The Wnt receptors, Frizzled proteins, transmit signals to Dishevelled (Miller et al. 1999), and Frizzled7 has been shown to mediate Wnt11 signaling (Djiane et al. 2000; Penzo-Mendez et al. 2003; Witzel et al. 2006). To examine whether Frizzled7 is necessary for Wnt11 to localize Xdsh, we injected a morpholino antisense oligonucleotide (Fz7-MO) that has been shown to deplete endogenous Frizzled7 (Winklbauer et al. 2001). Depletion of Frizzled7 specifically suppressed the Wnt11-induced accumulation of GFP-Xdsh (Fig. 2A). A 5 mismatch MO (Fz7-5misMO) had little effect on the localization of GFP-Xdsh. These results indicate a requirement of Frizzled7 for Wnt11-induced translocation of Xdsh. We next examined the subcellular localization of Frizzled7 itself. We expressed GFP-tagged Frizzled7 (Fz7-GFP) in the explants. In the absence of Wnt11, Fz7-GFP localized uniformly at the plasma membrane (Fig. 2B, upper). When co-expressed with Wnt11, Fz7-GFP translocated and accumulated at discrete spots at cell–cell contacts (Fig. 2B, lower). This accumulation pattern is similar to the recently reported pattern of Frizzled7 accumulation in zebrafish embryos (Witzel et al. 2006). In sections, while Fz7-GFP mainly localized at the basolateral plasma membrane in the absence of Wnt11, it accumulated at the apical region in the presence of Wnt11 (Fig. 2C). The accumulated Fz7-GFP co-localized with ß-catenin (Fig. 2C). We also made GFP-tagged Frizzled3 (Fz3-GFP) and expressed it with Wnt11. Fz3-GFP did not translocate or accumulate at all (data not shown). The accumulated pattern of Fz7-GFP was very similar to the Wnt11-induced accumulated pattern of GFP-Xdsh. In fact, when CFP-tagged Xdsh (CFP-Xdsh) and YFP-tagged Frizzled7 (Fz7-YFP) were co-expressed, CFP-Xdsh and Fz7-YFP completely co-localized at the cell–cell contacts (Fig. 2D). Our results here are consistent with a recent report showing Wnt11-induced accumulation of Frizzled7 in zebrafish embryos (Witzel et al. 2006), and further reveal that both Dishevelled and Frizzled7 translocate to the apical region and accumulate at apical adherens junctions in response to Wnt11 stimulation.
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The DIX domain is necessary and sufficient to accumulate in response to Wnt11
Dishevelled consists of three domains, the DIX, PDZ and DEP domains. To examine which domain is important for Wnt11-induced polarized accumulation, we expressed GFP-tagged deletion mutants of Xdsh with and without Wnt11 in Xenopus ectodermal explants (Fig. 3A). The explants were observed apically or stained with anti-ß-catenin antibody and sectioned. In the absence of Wnt11, GFP-Xdsh
DIX localized to the plasma membrane, GFP-Xdsh
PDZ was broadly distributed in the cytoplasm and partially localized to the plasma membrane, and GFP-Xdsh
DEP localized punctatedly within the cell (Fig. 3B, –Wnt11). When co-expressed with Wnt11, both GFP-Xdsh
PDZ and GFP-Xdsh
DEP translocated and accumulated at the apical region of the cell–cell contacts, like GFP-Xdsh WT. In contrast, GFP-Xdsh
DIX did not respond to Wnt11 stimulation and remained unchanged (Fig. 3B, +Wnt11). These results indicate that the DIX domain is required for the polarized accumulation of Dishevelled in response to Wnt11. Since there are an actin binding motif and a lipid binding motif in the DIX domain (Capelluto et al. 2002), we then examined whether these motifs are involved in the Wnt11-induced change in Dishevelled localization. Xdsh K47A and Xdsh K57AE58A, which are defective in actin binding and lipid binding, respectively (Capelluto et al. 2002), were expressed with and without Wnt11. Without Wnt11, GFP-Xdsh K47A localized in the cytoplasm, like GFP-Xdsh WT, and GFP-Xdsh K57AE58A localized in the cytoplasm diffusely. Both mutants translocated and accumulated at the apical region of cell–cell contacts, like GFP-Xdsh WT (Fig. 3C, +Wnt11). Thus, these motifs are dispensable for the Wnt11-dependent change in the subcellular localization of Dishevelled. Finally, we expressed a GFP-tagged version of the DIX domain alone (GFP-Xdsh DIX) in explants. Surprisingly, GFP-Xdsh DIX translocated from the cytoplasm to the plasma membrane region and accumulated at apical adherens junctions in response to Wnt11 stimulation (Fig. 3D). This behavior of GFP-Xdsh is indistinguishable from that of GFP-Xdsh WT. Thus, the DIX domain is not only necessary but also sufficient for Wnt11-induced polarized accumulation of Dishevelled.
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| Discussion |
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| Experimental procedures |
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The Xdsh cDNA was kindly given by Dr S. Y. Sokol. GFP-Xdsh, YFP-Xdsh and CFP-Xdsh were constructed by inserting the coding sequence of Xdsh into pCS2-GFP, pCS2-YFP and pCS2-CFP. GFP-tagged deletion mutants of Xdsh were constructed as follows. GFP-
DIX (bp 1–246 deleted); A PCR fragment (EcoRI-247-2211-EcoRI) was inserted into the EcoRI site of pCS2-GFP. GFP-Xdsh
PDZ (bp 787–1026 deleted); PCR fragments (EcoRI-1-786-BamHI and BglII-1027-2211-AGGCCTGCGCCCT-StuI) were combined and inserted into the EcoRI-StuI site of pCS2-GFP. GFP-Xdsh
DEP (bp1288-2211 deleted); A PCR fragment (EcoRI-1-1287-TGAAGGCCTGCGCCCT-StuI) was inserted into the EcoRI-StuI site of pCS2-GFP. GFP-Xdsh DIX (bp 247–2211 deleted); A PCR fragment (EcoRI-1-246-TGA-BglII) was inserted into the EcoRI-BamHI site of pCS2-GFP. GFP-Xdsh K47A was constructed by replacing Lys47 with alanine. GFP-Xdsh K57AE58A was constructed by replacing Lys57 and Glu58 with alanines. These mutations were made by site-directed mutagenesis (Quick ChangeTM, Stratagene) and confirmed by DNA sequencing. The Wnt11 cDNA was isolated by PCR amplification, and inserted into pCS2-HA. The Fz7 cDNA was isolated by PCR amplification, and inserted into pCS2-GFP and pCS2-YFP. Fz7ex-Myc and Fz7ex-GFP were constructed by inserting a PCR fragment of Fz7ex (BamHI-1-66-EcoRI) into the BamHI-EcoRI site of pCS2-Myc and pCS2-GFP.
Embryonic manipulation and microinjection
Embryos were in vitro fertilized, dejellied and cultured in 0.1x MBS. In vitro synthesis of capped mRNA was performed using the Ambion mMESSAGE mMACHINE kit. The RNAs were injected into 4-cell stage embryos. Ectodermal explants were excised from stage 8 embryos and cultured in 0.5x Sater's modified blastocoel buffer until stage 14. Morpholino oligonucleotides were mixed with RNAs and injected into embryos. Fz7-MO and Fz7-5misMO had following sequences: (Fz7-MO; 5'-CCAACAAGTGATCTCTGGACAGCAG-3') and (Fz7-5misMO; 5'-CCAAGAACTGATCTGTGGAGAGGAG-3') (Winklbauer et al. 2001).
Immunofluorescence
The ectodermal explants were fixed in MEMFA and incubated in methanol at –20 °C for 6 h. Explants were washed and blocked in TBS containing 2% skim milk and 2.5% DMSO, and were incubated with rabbit anti-ß-catenin antibody (H-102, Santa Cruz Biotechnology, Inc.) or mouse anti-ZO1 antibody (kindly given by Dr S. Tsukita) overnight at 4 °C in the same buffer. The explants were then washed in TBS, were incubated with Alexa Fluor 594 goat anti-rabbit IgG or anti-mouse IgG, and were cryosectioned.
Co-immunoprecipitation assay
RNAs of Fz7ex-Myc (250 pg) and Fz7ex-GFP (250 pg) were injected into the animal pole of 4-cell stage embryos. Fifteen ectodermal explants were harvested at stage 14 and lysed in buffer (20 mM HEPES pH 7.5, 100 mM NaCl, 1.5 mM MgCl2, 1 mM EGTA, 10 mM sodium pyrophosphate, 10% glycerol, 10 mM NaF, 1 mM vanadate, 1 mM PMSF, 0.5% aprotinin, 5 µg/mL leupeptin and 1% NP-40). Mouse anti-myc antibody (9E10, Santa Cruz Biotechnology, Inc.) was used for immunoprecipitation. The immunoprecipitates were washed 3 times with the same buffer containing no detergent, and were subjected to immunoblotting with mouse anti-GFP antibody (JL-8, Clontech Laboratories, Inc.).
| Acknowledgements |
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| Footnotes |
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* Correspondence: E-mail: L50174{at}sakura.kudpc.kyoto-u.ac.jp
| References |
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Received: 2 May 2007
Accepted: 21 May 2007
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