XB-ART-35685Dev Biol May 1, 2007; 305 (1): 103-19.
The competence of Xenopus blastomeres to produce neural and retinal progeny is repressed by two endo-mesoderm promoting pathways.
Only a subset of cleavage stage blastomeres in the Xenopus embryo is competent to contribute cells to the retina; ventral vegetal blastomeres do not form retina even when provided with neuralizing factors or transplanted to the most retinogenic position of the embryo. These results suggest that endogenous maternal factors in the vegetal region repress the ability of blastomeres to form retina. Herein we provide three lines of evidence that two vegetal-enriched maternal factors (VegT, Vg1), which are known to promote endo-mesodermal fates, negatively regulate which cells are competent to express anterior neural and retinal fates. First, both molecules can repress the ability of dorsal-animal retinogenic blastomeres to form retina, converting the lineage from neural/retinal to non-neural ectodermal and endo-mesodermal fates. Second, reducing the endogenous levels of either factor in dorsal-animal retinogenic blastomeres expands expression of neural/retinal genes and enlarges the retina. The dorsal-animal repression of neural/retinal fates by VegT and Vg1 is likely mediated by Sox17alpha and Derriere but not by XNr1. VegT and Vg1 likely exert their effects on neural/retinal fates through at least partially independent pathways because Notch1 can reverse the effects of VegT and Derriere but not those of Vg1 or XNr1. Third, reduction of endogenous VegT and/or Vg1 in ventral vegetal blastomeres can induce a neural fate, but only allows expression of a retinal fate when both BMP and Wnt signaling pathways are concomitantly repressed.
PubMed ID: 17428460
PMC ID: PMC1892348
Article link: Dev Biol
Species referenced: Xenopus
Genes referenced: a2m cer1 egr2 en2 foxd4l1.1 gal.2 gdf1 gdf3 irx1 krt70 nodal nodal1 nog notch1 otx2 rax sox17a sox3 tbxt vegt wnt8a
Morpholinos: vegt MO5
Article Images: [+] show captions
|Fig. 5. Reduction of VegT and Vg1 in the D1.1 lineage increases the size of the neural ectoderm. (A) Reduction of VegT by morpholino injection (VegTMO) and of Vg1 by expression of a dominant-negative construct (dnVg1) enlarged the retina (r) and forebrain (fb). Note the abundant D1.1 progeny (green) in both structures (cf. Fig. 2E). Reduction of both factors (VegTMO/dnVg1) resulted in a similar phenotype. (B) The mean volumes of retinas in embryos in which control morpholinos (cMO, cVegTMO) were injected are not different from GFP controls (Fig. 2B). Those from VegTMO-injected embryos are significantly larger on both sides, and those from dnVg1-injected embryos are significantly larger on the injected side. Those from embryos injected with both constructs (VegTMO/dnVg1) are larger than for dnVg1 alone and similar to those from VegTMO alone. * Indicates p < 0.01 compared to GFP controls. Numbers in parentheses indicate size of sample. (C) The expression domains of pan-neural plate (sox3, notch1; white bars indicate measurement of width of domain) and retinal (rx1; arrows) genes are expanded on the side (right) injected with VegTMO or dnVg1. The expansion of neural plate markers is somewhat enhanced in embryos co-injected with VegTMO and dnVg1 (see also Table 1). (D) The expression domains of mesodermal (Xbra) and endodermal (sox17α, edd) genes after injection into D1.1 of the constructs indicated on the left. The only notable effect is repression of Xbra by dnVg1 (arrow). (E) The expansion of rx1 expression (arrow) by VegTMO is not altered by co-expression of Vg1, whereas the expansion caused by dnVg1 is reversed by co-expression of VegT. (F) The small retinal volumes displayed by Vg1-injected embryos were significantly increased by co-injection of VegTMO (* indicates p < 0.01). The small retinal volumes displayed by VegTMO-injected embryos were not altered by co-injection of dnVg1 (p > 0.05).|
|Fig. 7. In order to convert the V2.1.1 lineage to a retinal fate endo-mesodermal factors must be blocked in combination with suppression of BMP and Wnt signaling. (A) Expression domains of mesodermal (Xbra) and endodermal (sox17α, edd) genes are reduced by VegTMO injection into the vegetal pole, whereas controls (β-gal, cMO, cVegTMO) have no effects. (B) The extent to which the hindgut (g) is populated by V2.1.1 progeny (green) is strikingly reduced in VegTMO embryos (*) compared to controls (GFP). a, archenteron; s, somite. (C) Ectopic expression of rx1 (blue, arrows) in the vegetal pole was monitored by in situ hybridization after the V2.1.1 blastomere was injected with the indicated constructs. Red cells indicate the V2.1.1 progeny expressing those constructs. Reduction of VegT alone (VegTMO) or in combination with Vg1 (VegTMO/dnVg1) was not sufficient to induce ectopic rx1 expression. The long form of Cerberus (Cer-L), which can inhibit BMP, Wnt and Nodal signaling, was effective whereas the short version of Cerberus (Cer-S), which can inhibit only Nodal signaling, was not. Although reduction of both BMP signaling (by expression of noggin [Nog] and a dominant-negative BMP receptor [tBR]) and Wnt signaling (by expression of a dominant-negative Wnt8 construct [dnWnt]) causes ectopic rx1 expression, the cells expressing rx1 are not derived from the V2.1.1 clone (red cells), demonstrating an indirect consequence of the induction of a secondary head (as reported in Moore and Moody, 1999). However, combining the reduction of VegT or VegT plus Vg1 with the inhibition of BMP and Wnt signaling caused the V2.1.1 clone to ectopically express rx1, equivalent to the Cer-L phenotype.|
|Fig. 3. VegT and Vg1 alter cell fates at gastrulation stages and these changes affect later neural patterning. (A) The expression domains of markers of the germ layers (top labels) during gastrulation after injection of one D1.1 blastomere with mRNAs indicated on the left. Early neural ectoderm is identified by foxD5 and otx2, non-neural ectoderm by keratin, mesoderm by Xbra and endoderm by sox17α and edd. Red cells indicate the D1.1 progeny expressing the injected mRNA, and arrows indicate regions of gene repression or ectopic expression. Frequencies of phenotypes are presented in Table 1. (B) The expression domains of pan-neural (sox3, notch1), eye field (rx1), forebrain (otx2), midbrain (en2) and hindbrain (krox20) genes during neural plate stages. White bars indicate the width of the expression domains on the injected (right) versus uninjected (left) side of the neural plate. Arrows are as above. Frequencies of phenotypes are presented in Table 1.|
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