Department of Developmental and Regenerative Biology, Mount Sinai School of Medicine
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Itoh, K., Sokol, S. Y. Polarized Translocation of Fluorescent Proteins in Xenopus Ectoderm in Response to Wnt Signaling. J. Vis. Exp. (51), e2700, doi:10.3791/2700 (2011).
Cell polarity is a fundamental property of eukaryotic cells that is dynamically regulated by both intrinsic and extrinsic factors during embryonic development 1, 2. One of the signaling pathways involved in this regulation is the Wnt pathway, which is used many times during embryogenesis and critical for human disease3, 4, 5. Multiple molecular components of this pathway coordinately regulate signaling in a spatially-restricted manner, but the underlying mechanisms are not fully understood. Xenopus embryonic epithelial cells is an excellent system to study subcellular localization of various signaling proteins. Fluorescent fusion proteins are expressed in Xenopus embryos by RNA microinjection, ectodermal explants are prepared and protein localization is evaluated by epifluorescence. In this experimental protocol we describe how subcellular localization of Diversin, a cytoplasmic protein that has been implicated in signaling and cell polarity determination6, 7 is visualized in Xenopus ectodermal cells to study Wnt signal transduction8. Coexpression of a Wnt ligand or a Frizzled receptor alters the distribution of Diversin fused with red fluorescent protein, RFP, and recruits it to the cell membrane in a polarized fashion 8, 9. This ex vivo protocol should be a useful addition to in vitro studies of cultured mammalian cells, in which spatial control of signaling differs from that of the intact tissue and is much more difficult to analyze.
1. In Vitro Fertilization of Xenopus Eggs
2. RNA Microinjection
3. Preparing Ectodermal Explants
4. Imaging of Explants Under a Fluorescent Microscope
Cryosectioning is an alternative way to visualize the distribution of fluorescent proteins in the cell and more applicable for immunostaining. At stage 10, embryos are fixed for 1-2 hours with Dent's fixative (20% DMSO, 80% methanol), washed with PBS, and embedded in 15 % fish gelatin/15 % sucrose solution11. The embedded embryos are quickly frozen on dry ice and cryosections are generated on Leica Cryostat. Cross sections would include ectodermal cells that inherit injected RNAs and their translated protein products. The sections retain fluorescence and can be immunostained with specific antibodies and then labeled with secondary antibodies conjugated with fluorescence. Nuclei are stained with DAPI. The mounting media are the same as described above. Imaging can be performed as described above.
6. Representative Results:
Figure 1. Frizzled receptor recruits Diversin to the cell membrane. Ectoderm cells expressing Frizzled 8 (Xfz8) and Div-RFP RNAs reveal Div-RFP at the cell membrane, instead of the centrosome (as revealed by g-tubulin co-staining). The scheme of the experiment is at the top; a typical cross-section is shown.
We have used the above protocol to characterize the subcellular localization of Diversin. In animal pole explants, Diversin-RFP was detected next to the nucleus and colocalized with g-tubulin, a centrosomal marker, in cryosections (Figure 1). Once the subcellular localization of a protein is identified, deletion constructs can be generated to establish which protein domains are necessary and sufficient for the subcellular localization. Using this approach, the centrosomal localization domains of Diversin have been mapped to the middle and in the carboxy-terminal domains of the protein, both containing the coiled-coil motif 8.
The same protocol can be used in studies, in which protein localization is altered in response to signaling. We found that Wnt secreted proteins act to relocate Div-RFP to punctate structures adjacent to the cell membrane, whereas Fz8 recruits Div-RFP to cell membrane patches (Figure 1). We further discovered that the carboxy-terminal domain is not essential for membrane recruitment per se, but required for polarized membrane recruitment.
In summary, the above experimental protocol will help in diverse studies of protein-protein interactions and protein localization to different cellular compartments after stimulation of cells with specific growth factors or signaling proteins.
No conflicts of interest declared.
Research in the Sokol laboratory is sponsored by the National Institues of Health.