Olson DJ and Moon RT (1992), Distinct effects of ectopic expression of Wnt-1...

XB-ART-23771

Dev Biol 1992 May 01;1511:204-12. doi: 10.1016/0012-1606(92)90227-8.

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Distinct effects of ectopic expression of Wnt-1, activin B, and bFGF on gap junctional permeability in 32-cell Xenopus embryos.

Olson DJ , Moon RT .

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A polarity in gap junctional permeability normally exists in 32-cell stage Xenopus embryos, in that dorsal cells are relatively more coupled than ventral cells, as measured by transfer of Lucifer yellow dye. The current study extends our analysis of whether gap junctional permeability at this stage can be modulated by secreted factors, and whether the polarity in gap junctional permeability correlates with the effects of ectopic expression of these secreted factors on the subsequent phenotype of the developing embryo. Following ectopic expression of activin B or Wnt-1, but not bFGF, the transfer of Lucifer yellow between ventral animal pole cells is detected in a greater percentage of 32-cell stage embryos. This increased incidence of dye transfer between ventral cells correlates with axial duplications later in development. However, there are differences in the extent of Lucifer yellow transfer between animal and vegetal hemisphere blastomeres which is dependent on whether activin B or Wnt-1 RNA had previously been injected. These results suggest that enhanced gap junctional permeability between ventral cells of 32-cell Xenopus embryos correlates with subsequent defects in the dorsoventral axis, although there are at present no direct data demonstrating a role for gap junctions in establishment or maintenance of this axis. Moreover, while both activin B and bFGF are mesoderm-inducing growth factors, only activin B has effects on gap junctional permeability in 32-cell embryos following ectopic expression, demonstrating an interesting difference in physiological responses to expression of these factors.

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Species referenced: Xenopus laevis
Genes referenced: fgf2 inhbb wnt1

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	FIG. 1. Transfer of dye from a single ventral tier 1 animal pole cell injected with Lucifer yellow dye at the 32-cell stage. (A) Control embryo showing dye transfer only to a sister cell. Scale bar is 0.12 mm. (B) Embryo injected with activin B RNA prior to first cleavage, showing extensive dye transfer between ventral animal pole cells, including dye transfer to the vegetal hemisphere (direction of arrow). Scale bar is 0.09 mm. Cells injected with Lucifer yellow are denoted by a triangle in each panel, and intensity of fluorescence is decreased in animal pole cells due to pigmentation of this hemisphere.
	FIG. 2. Transfer of dye from a single ventral tier 3 vegetal pole cell injected with Lucifer yellow at the 32-cell stage in embryos previously injected with activin B or W&-l RNA prior to first cleavage. (A) Activin B RNA expression enhances ventral vegetal cell coupling but dye transfer in a vegetal to animal pole direction is less frequent. Scale bar is 0.17 mm. (B) Higher magnification view of a different embryo injected as in A, scale bar is 0.06 mm. (C) Writ-1 RNA expression enhances ventral vegetal cell coupling, which also includes coupling to the animal hemisphere. Scale bar is 0.17 mm. (D) Higher magnification view of a different embryo injected as in C, scale bar is 0.08 mm. In all panels, cells injected with dye are indicated with a triangle, and the direction of the animal pole is shown with an arrow.
	FIG. 3. Summary of effects of ectopic expression of W&l, activin B, and bFGF on gap junctional permeability in 32-cell Xewpus embryos. Cells injected with Lucifer yellow are denoted in red and cells accumulating transferred dye are denoted in yellow. It is important to note that this summary conveys trends of dye transfer in large numbers of embryos, as Tables 1-3 demonstrate that dye transfer is not an absolute in any of the conditions tested. The arrows indicate the direction of dye transfer scored in Tables 1-3. Abbreviations: ventral, V; dorsal, D; tier 1 blastomeres, Tl; tier 3 blastomeres, T3.