Lupo G et al. (2005), Dorsoventral patterning of the Xenopus eye: a c...

XB-ART-2210

Development 2005 Apr 01;1327:1737-48. doi: 10.1242/dev.01726.

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Dorsoventral patterning of the Xenopus eye: a collaboration of Retinoid, Hedgehog and FGF receptor signaling.

Lupo G , Liu Y , Qiu R , Chandraratna RA , Barsacchi G , He RQ , Harris WA .

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In the developing spinal cord and telencephalon, ventral patterning involves the interplay of Hedgehog (Hh), Retinoic Acid (RA) and Fibroblast Growth Factor (FGF) signaling. In the eye, ventral specification involves Hh signaling, but the roles of RA and FGF signaling are less clear. By overexpression assays in Xenopus embryos, we found that both RA and FGF receptor (FGFR) signaling ventralize the eye, by expanding optic stalk and ventral retina, and repressing dorsal retina character. Co-overexpression experiments show that RA and FGFR can collaborate with Hh signaling and reinforce its ventralizing activity. In loss-of-function experiments, a strong eye dorsalization was observed after triple inhibition of Hh, RA and FGFR signaling, while weaker effects were obtained by inhibiting only one or two of these pathways. These results suggest that the ventral regionalization of the eye is specified by interactions of Hh, RA and FGFR signaling. We argue that similar mechanisms might control ventral neural patterning throughout the central nervous system.

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Species referenced: Xenopus
Genes referenced: aldh1a2 aldh1a3 atr fgf8 gal.2 ihh pax2 pax6 rax shh tbx2 tbx5 vax1 vax2 ventx2.2

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	Fig. 1. Effects of Hh signaling overexpression on eye DV polarity. (A,B) The red or light-blue Î²-gal staining identifies the injected side. The broken yellow circles highlight the eye region. (A) Unilateral injection of low doses of bhh mRNA (1 pg) upregulates Vax2 in the DR and reduces ET in stage 33 Xenopus embryos. Pax2, Vax1b and Raldh3 are partially upregulated, but not in the DR, and the Pax6 domain is slightly reduced in these embryos. On the uninjected side, Vax2 is normally expressed in the OS and the VR; ET is expressed in the DR; Pax2, Vax1b and Raldh3 are expressed in the OS region and Pax6 is expressed in the retina region. (B) High doses of bhh mRNA (250 pg) upregulate Vax2, Pax2, Vax1b and Raldh3 throughout the eye, and repress ET and Pax6 expression. (C) Schematic representation of the results shown in A and B. Low Hh levels partially expand the OS and ventralize in part the DR. Purple indicates the overlap of ventral and dorsal character in the most dorsal retina. High Hh levels transform the entire retina into OS.
	Fig. 2. Effects of RA signaling overexpression on eye DV polarity. (A) Low RA doses (0.1 Î¼M) expand Pax2, Vax1b and Raldh3 expression domains, but do not significantly affect Vax2- and ET-positive regions in stage 33 Xenopus embryos. Pax2-hybridized embryos are also shown in frontal view (right column). (B) High RA doses (10 Î¼M) upregulate Vax2 in the DR and repress Vent2, Pax2, Vax1b and Raldh3, but do not change Pax6 expression. (C) Schematic representation of the results shown in A and B. Low RA levels enlarge the OS. High RA levels ventralize the retina and repress OS formation.
	Fig. 3. Effects of FGFR signaling overexpression on eye DV polarity. (A) Unilateral injection of 2 pg iFGFR1 mRNA, followed by induction with AP20187 at stage 12.5/13, partially expands the expression domains of Vax2, Pax2, Vax1b and Raldh3 in the ventral eye, and weakly represses ET and Pax6 in stage 33 Xenopus embryos. (B) Injection of 4 pg iFGFR1 mRNA upregulates Vax2, Pax2, Vax1b and Raldh3 in the dorsal eye and strongly repress ET and Pax6. (C) Schematic representation of the results shown in A and B. Increasing FGFR levels progressively expands the OS at the expense of the retina.
	Fig. 4. Expression pattern of Raldh3 during Xenopus development. (A) Raldh3 expression as detected by whole-mount in situ hybridization of whole embryos (stage 10.25-31) or dissected neural retinas, without lens and pigmented epithelium (stage 59). cf, choroid fissure. Scale bar: 350 Î¼m. (B) Raldh3 expression in transverse sections of stage 33 embryos after whole-mount in situ hybridization. From left to right, sections show expression at progressively posterior levels. op, olfactory placode; os, optic stalk. (C) Comparison of Pax2, Raldh3 and Vax2 expression in the optic cup in transversal sections of stage 33 embryos after whole-mount in situ hybridization. (D) Double in situ hybridizations of Rx1 with Raldh2, Shh or FGF8 on mid- to late neurula embryos shown from anterior view. The inset shows a double in situ hybridization of Rx1 with FGF8 on an early neurula embryo.
	Fig. 5. Raldh3 overexpression in the presence of ATR reproduces the effects of RA treatments on eye DV polarity. (A) Embryos were bilaterally injected with a total of 1 ng Raldh3 mRNA and treated with 0.5 Î¼M ATR from stage 12.5/13, followed by molecular marker analysis at stage 33. The combination of Raldh3 and ATR expands Pax2 expression, while ATR alone has only a weak effect. A comparable expansion of Pax2 is obtained by a dose of 2 Î¼M ATR in the absence of exogenous Raldh3. (B) The combination of 4 ng Raldh3 mRNA and 2.5 Î¼M ATR can upregulate Vax2 in the DR, while ATR alone has only a weak effect. Strong Vax2 upregulation is also caused by a dose of 10 Î¼M ATR in the absence of exogenous Raldh3, as shown in both lateral (upper row) and anterior view (lower row).
	Fig. 6. RA, Hh and FGFR signaling pathways can partially ventralize the eye field at neurula stages. (A) In embryos treated with 10 Î¼M RA from stage 12.5/13 (right column), the lateral region of Vax2 domain (indicated by the yellow brackets) is expanded at late neurula stages with respect to control embryos, while Vent2 domain is reduced. (B) Mid-late neurula embryos unilaterally injected either with 500 pg bhh mRNA or with 2.5 pg iFGFR1 mRNA, followed by induction with AP20187 from stage 12.5/13. Compared with the uninjected side, Vax2 expression is expanded, while ET is repressed. The broken yellow lines indicate the embryo midline.
	Fig. 7. RA and Hh signaling can collaborate in eye ventralization. (A) Embryos were unilaterally injected with 1 pg bhh mRNA and treated with 2 Î¼M ATR from stage 12.5/13. Pax2 is strongly upregulated in the eye by the combination of bhh and ATR, but weakly by bhh or ATR alone in stage 33 Xenopus embryos. Vax2 expression can be activated in the DR by these doses of bhh in the absence of ATR. ET is reduced by bhh or ATR alone, but is strongly repressed by the combination of bhh and ATR. (B) Embryos were unilaterally injected with 0.2 pg bhh mRNA and treated with 5 Î¼M ATR from stage 12.5/13. Vax2 is upregulated in the DR by the combination of bhh and ATR, but not by bhh or ATR alone. These doses of bhh and ATR weakly activate Pax2 expression. ET expression in the DR is partially reduced by bhh or ATR alone, but is strongly repressed by the combination of bhh and ATR. The yellow arrow indicates ET expression domain in the DR. (C) Quantification of the effects of bhh and ATR on Pax2, Vax2 and ET expression. Between 21 and 25 embryos were analyzed for each sample in these experiments. (D) Schematic representation of the results shown in A and B. The combination of 1 pg bhh mRNA with 2-2.5 Î¼M ATR strongly expands the OS. The combination of 0.2 pg bhh with 5 Î¼M ATR ventralizes the DR.
	Fig. 8. FGFR and Hh signaling can collaborate in eye ventralization. (A) Embryos were unilaterally co-injected with 1 pg bhh mRNA and 0.5 pg iFGFR1 mRNA, and induced with AP20187 from stage 12.5/13, followed by molecular marker analysis at stage 33. Pax2 is mainly activated within the ventral half of the eye (class I eyes) by bhh alone, while in the presence of iFGFR1 and bhh Pax2 expression extends into the dorsal eye (class II and class III eyes). Co-injection of bhh and iFGFR1 mRNAs also slightly increases the frequency of eyes with ubiquitous Vax2 expression and more strongly represses Tbx5 when compared with bhh alone. Under these conditions, iFGFR1 mRNA alone has no obvious effect on Pax2, Vax2 and Tbx5 expression. (B) Quantification of the effects of bhh and iFGFR1 on Pax2, Vax2 and Tbx5 expression. Between 24 and 30 embryos were analyzed for each sample in these experiments. (C) Schematic representation of the results shown in A. In class II eyes, the expanded OS does not reach the most dorsal retina, which can acquire in part ventral identity. In class III eyes, the retina is mostly or completely transformed into OS.
	Fig. 9. RA, Hh and FGFR signaling pathways can crossregulate each other at the transcriptional level. (A) High dose RA treatments (10 Î¼M) strongly repress Shh expression in the anterior midline (indicated by the yellow bracket) both at neurula and stage 33. The same RA doses cause fusion of the two stripes of FGF8 expression in the ANR and the anteroventral ectoderm in neurula stage embryos, while at stage 33 FGF8 expression is strongly downregulated in the whole head region. Low dose RA treatments (0.1 Î¼M) have weaker effects on both Shh and FGF8. (B) Unilateral injection of 500 pg bhh mRNA laterally expands FGF8 expression in the anterior neural ridge but does not affect Raldh2 expression at neurula stages. The inset shows FGF8 expression in the ANR of an uninjected embryo at the same stage. The same dose of bhh upregulates both FGF8 and Raldh2 in the eye region at stage 33. (C) Unilateral injection of 4 pg iFGFR1 mRNA, followed by induction with AP20187 at stage 12.5/13, expands Shh expression in the prospective hypothalamic region at neurula, and activates Shh in the eye regions at stage 33. At doses of 4 and 2 pg, iFGFR-1 mRNA has no significant effect on Raldh2 expression at neurula stages and stage 33, respectively.
	Fig. 10. Loss-of-function effects of RA, Hh and FGFR signaling pathways on eye DV polarity. Embryos were treated from stage 10.5 with 10 Î¼M AGN194310 (AGN), 100 Î¼M cyclopamine (CPM) and 25 Î¼M SU5402 (SU) in different combinations, and analyzed for molecular marker expression at stage 30/31. (A) Effects of the single inhibition of any of the RA, Hh and FGFR signaling pathways, when compared with mock-treated embryos. (B) Effects of double and triple inhibition of RA, Hh and FGFR pathways. (C) Schematic representation of the results shown in A and B. Strong eye dorsalization is caused by triple inhibition of RA, Hh and FGFR signaling, while double inhibitions produce weaker effects.