1 Department of Anatomy, University of Cambridge, Downing Street, Cambridge CB2
3DY, UK
2 State Key Laboratory of Brain and Cognitive Science, Institute of Biophysics,
Chinese Academy of Sciences, 15 Datun Road, Beijing 100101, PR China
3 Retinoid Research, Departments of Chemistry and Biology, Allergan, Irvine, CA
92623, USA
4 Dipartimento di Fisiologia e Biochimica, Universita' degli Studi di Pisa, Via
Carducci 13, 56010 Ghezzano (Pisa), Italy
5 AMBISEN Center, High Technology Center for the Study of Environmental Damage
to the Endocrine and Nervous System, Universita' degli Studi di Pisa, Pisa,
Italy
Authors for correspondence (e-mail:
liuy{at}moon.ibp.ac.cn
and
harris{at}mole.bio.cam.ac.uk).
Accepted 18 January 2005
![]() |
SUMMARY |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Key words: Ventral retina, Dorsal retina, Optic stalk, Retinoic acid, Hedgehog, FGF receptor, Xenopus
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Before these morphogenetic events of DV specification, asymmetrically
expressed transcription factors regionalize the optic vesicle into three main
DV compartments. From ventral to dorsal, these are: the OS, which expresses
Pax2, Vax1 and Vax2, but not Pax6 and
Tbx5; the VR, which expresses Vax2 and Pax6, but
not Pax2, Vax1 or Tbx5; the DR, which expresses
Pax6 and Tbx5, but not Pax2, Vax1 and Vax2
(Barbieri et al., 2002;
Bertuzzi et al., 1999
;
Hallonet et al., 1999
;
Koshiba-Takeuchi et al., 2000
;
Liu et al., 2001
;
Mui et al., 2002
;
Schwarz et al., 2000
;
Torres et al., 1996
).
How is this transcriptional code established? In the developing spinal
cord, opposing ventralizing and dorsalizing activities of Hedgehog (Hh) and
Bone Morphogenetic Protein (BMP) signaling pathways have key roles in the
specification of DV polarity upstream of transcription factors
(Ruiz i Altaba et al., 2003).
Nonetheless, recent studies have indicated that ventral patterning cannot be
ascribed to Hh signaling alone, and Retinoic Acid (RA) and Fibroblast Growth
Factor (FGF) signaling are also crucial players in this process
(Appel and Eisen, 2003
;
Diez del Corral and Storey,
2004
; Harris,
2003
). Recent work has also shown that RA and FGF are required, in
addition to Hh signaling, to elicit full specification of the ventral
telencephalon (Lupo et al.,
2002
; Marklund et al.,
2004
; Shinya et al.,
2001
).
By analogy to the neural tube, it has been suggested that eye DV polarity
may also be established by Hh and BMP antagonistic activities
(Russell, 2003;
Wilson and Houart, 2004
;
Yang, 2004
). BMP
overexpression in the retina has a strong dorsalizing effect
(Koshiba-Takeuchi et al.,
2000
; Sasagawa et al.,
2002
), while BMP inhibition ventralizes the eye
(Murali et al., 2005
;
Sakuta et al., 2001
;
Sasagawa et al., 2002
). As to
Hh signaling, although its role in OS specification is well known on the basis
of both loss- and gain-of-function assays
(Russell, 2003
;
Yang, 2004
), its role in DV
patterning of the retina is less clear
(Perron et al., 2003
;
Zhang and Yang, 2001
). RA and
FGF signaling pathways may also play a role in the ventralization of the eye.
The VR is richer in RA than the DR (Drager
et al., 2001
), suggesting that high RA levels may specify ventral
character in the eye. RA treatments upregulate the OS marker Pax2 in
zebrafish embryos (Hyatt et al.,
1996
). By contrast, reduction of embryonic RA signaling by means
of vitamin A deficiency, knock out of RA receptors (RAR) or exposure to citral
caused morphological deficits in the VR
(Kastner et al., 1994
;
Marsh-Armstrong et al., 1994
;
Ross et al., 2000
). However, a
lack of appropriate molecular markers meant that it was not possible to
determine whether the observed morphological defects were due to impaired DV
specification rather than abnormal morphogenesis, growth or survival of the
VR. As to FGF signaling, although it may be responsible for maintaining some
ventral eye gene expression in the absence of all Nodal and Hh signaling in
zebrafish (Take-uchi et al.,
2003
), in the studies reported so far, inhibition of FGF signals
produced only weak effects on the expression of ventral eye genes
(Shanmugalingam et al., 2000
;
Walshe and Mason, 2003
), thus
leaving the role of FGF signaling during ventral eye specification not fully
resolved.
Here, we study the role of Hh, RA and FGF Receptor (FGFR) signaling in DV patterning of the Xenopus eye. Our results suggest that specification of the OS and VR depends on the levels of Hh, RA and FGFR signaling interacting on these territories during development. We argue that similar mechanisms may control ventral patterning throughout the central nervous system (CNS).
![]() |
Materials and methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Xenopus embryos and in situ hybridization
Embryos were obtained and staged as previously described
(Nieuwkoop and Faber, 1967).
Whole-mount in situ hybridization was performed as described by Harland
(Harland, 1991
). Whole-mount
in situ hybridization on dissected retinas, double in situ hybridizations, and
sectioning of whole-mount hybridized embryos were carried out as previously
described (Liu et al.,
2001
).
RNA methods and microinjections
Capped mRNAs were synthesized from linearized plasmid templates using
mMESSAGE mMACHINE kits (Ambion). Embryos were injected as previously described
(Vignali et al., 2000).
Injections were performed at the eight-cell stage in one or both dorsal-animal
blastomeres. The following template plasmids were used: banded
hedgehog, pT7TS-Xbhh (Ekker et al.,
1995
); iFGFR1, pCS2+-iFGFR1
(Pownall et al., 2003
); and
Raldh3, pCS2+-Xraldh3. iFGFR1 activity was induced by immersion of
embryos in 0.1 xMBS supplemented with 1.25 µM AP20187 (ARIAD
Pharmaceuticals;
http://www.ariad.com),
from 1 mM stock in 100% ethanol. When ß-galactosidase (ß-gal) was
used as a lineage tracer, embryos were co-injected with 100-500 pg of
ß-gal mRNA and stained as previously described
(Andreazzoli et al., 1999
).
Treatments with retinoids and receptor antagonists
For retinoid treatments, embryos were treated in the dark with
all-trans-Retinoic Acid (Sigma) or all-trans-Retinal (Sigma), diluted in 0.1
xMBS from 10-100 mM stocks in DMSO. Loss-of-function experiments were
performed with the following compounds: AGN194310 (Allergan;
http://www.allergan.com),
dissolved in 25 mM stock in DMSO; cyclopamine (a gift from W. Gaffield, and
Toronto Research Chemicals), dissolved in 20 mM stock in 95% ethanol; SU5402
(Calbiochem), dissolved in 25 mM stock in DMSO. Embryos were exposed in the
dark to appropriate concentrations of these inhibitors diluted in 0.1
xMBS. Control embryos were treated with identical concentrations of DMSO
and/or ethanol.
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
|
|
Localization of Hh, FGF and RA signaling components supports a role in patterning the Xenopus eye field
In all model systems examined so far, Shh is expressed in the
anterior midline at gastrula and neurula stages, then in the ventral forebrain
adjacent to the developing eye during later development
(Wilson and Houart, 2004). By
double in situ hybridization with the eye field marker Rx1
(Casarosa et al., 1997
), we
confirmed that the most anterior domain of Shh expression overlaps
with the medial part of the eye field at neurula stages
(Fig. 4D). FGF8 is
expressed in the anterior neural ridge (ANR) adjacent to the eye field from
neurula stages onwards. At later stages, FGF8 expression is
maintained in ventral forebrain regions close to the ventral eye and in the OS
(Wilson and Houart, 2004
).
Double in situ hybridization with Rx1 shows that, at early neurula
stages, FGF8 expression in the ANR is adjacent to the eye field.
However, during mid-late neurula stages, this FGF8 domain becomes
more medially restricted and overlaps with the medial part of the eye field
(Fig. 4D). Hh and FGF receptors
are also expressed in the prospective anterior neuroectoderm from early stages
of Xenopus development (Hongo et
al., 1999
; Koebernick et al.,
2001
).
|
Raldh3 is transcribed in the dorsal blastopore lip by early gastrula stages, and, later in gastrulation, in the involuting anterior mesendoderm underlying the anterior neural plate (Fig. 4A). This expression is quickly downregulated, but a new expression domain becomes evident by late neurula stages in the ventral part of the evaginating optic vesicle (data not shown). At early tailbud stages (stage 22/23), Raldh3 is expressed in the ventral optic vesicle, the midbrain-hindbrain boundary and the dorsal part of the otic vesicle; at later stages (stage 33) it is also expressed in the olfactory placode (Fig. 4A). Transverse sections of Raldh3-hybridized embryos at this stage confirmed these expression domains (Fig. 4B and data not shown). Comparative in situ hybridization analysis revealed that Pax2, Raldh3 and Vax2 show nested expression domains within the ventral optic cup (Fig. 4C). Raldh3 transcripts persist in the ventral part of the eye throughout development, including metamorphosis (stage 59/60), in a domain still contained within that of Vax2 (Fig. 4A; data not shown).
Raldh2 can mediate RA synthesis from all-trans Retinal (ATR) in
Xenopus embryos (Chen et al.,
2001). In order to determine whether Raldh3 could also efficiently
promote RA synthesis in vivo, we performed overexpression experiments by
bilateral microinjection of Raldh3 mRNA, and checked whether it could
mimic the ventralizing activity of RA on the developing eye. No significant
effects on DV specification in the eye were detected after injection of up to
5 ng Raldh3 mRNA (data not shown), possibly because of the complex
regulation of substrate availability in vivo
(Chen et al., 2001
). To test
this, we provided low doses of exogenous ATR. Injection of 1 ng
Raldh3 mRNA in combination with 0.5 µM ATR treatment from stage
12.5/13 induced a similar expansion of Pax2 expression to that
observed after treatments with low doses of RA
(Fig. 5A; compare with
Fig. 2A). By increasing the
doses of ATR to 2.5 µM ATR and Raldh3 mRNA to 4 ng, we could
phenocopy the strong Vax2 upregulation produced by treatments with
high RA doses (Fig. 5B; compare
with Fig. 2B). At the same
doses of ATR alone, Pax2 and Vax2 expression was only
slightly increased. Thus, Raldh3 can efficiently convert ATR to RA in vivo. We
also observed similar ventralizing effects after treatments with higher doses
of ATR in the absence of injected Raldh3 mRNA
(Fig. 5A,B), which were
prevented by the RAR antagonist AGN194310
(Hammond et al., 2001
) (see
Fig. S3 in the supplementary material).
|
|
|
We first unilaterally injected doses of 1 pg bhh mRNA, which partially expanded the expression domain of OS markers (Fig. 1A), and exposed injected embryos to different doses of ATR. In these experiments, we scored activation of the OS marker Pax2 by classifying Pax2-hybridized embryos in three classes: (1) class I embryos, where Pax2 expression was approximately confined to the ventral half of the eye; (2) class II embryos, where Pax2-positive domain spread to the dorsal half of the eye, without covering it completely; (3) class III embryos, in which Pax2 expression covered all or nearly all the eye region. Examples of class I, II and III eyes are shown in Fig. 8A. Although doses of ATR in the range of 2 µM can expand the expression domain of OS markers in the ventral eye, they never upregulate OS markers in dorsal eye regions (Fig. 5A). However, as shown in Fig. 7A, ATR treatments clearly reinforced induction of the OS marker Pax2 by low doses of Hh signaling. At doses of 2-2.5 µM ATR, the majority of the embryos had upregulated Pax2 in the dorsal eye, while only a minority of mock-treated bhh-injected embryos showed upregulation of Pax2 in the dorsal eye (Fig. 7C). On the uninjected side of ATR-treated embryos, Pax2 was upregulated only within the ventral eye region (Fig. 7A). Therefore, RA and Hh signaling can collaborate in OS specification. As described before, low doses of bhh mRNA in the range used for these experiments can ventralize the DR as shown by the upregulation of the VR marker Vax2 in nearly the whole of the eye region, and the downregulation of the DR marker ET (Fig. 7A). We found that the percentages of embryos showing Vax2 upregulation throughout the eye region, and nearly complete ET repression in the DR, were increased by ATR treatments, suggesting that RA and Hh signaling can also collaborate in VR specification (Fig. 7C). In order to clarify this issue, we used lower doses of bhh mRNA, which do not significantly affect DV specification in the eye, in combination with ATR treatments. As shown in Fig. 7B, doses of 0.2 pg bhh mRNA caused only a slight upregulation of Pax2 and Vax2 in the eye, whereas doses of 5 µM ATR locally expanded Pax2 in the ventral part of the eye, and only slightly increased Vax2 expression in the VR. Both bhh and ATR partially reduced ET expression at these doses. When embryos unilaterally injected with 0.2 pg bhh mRNA were also treated with 5 µM ATR, no strong increase was detected in the expression of Pax2 on the injected side with respect to the uninjected side and few class II and no class III embryos were detected. By contrast, the expression of Vax2 was upregulated throughout the eye region and the expression of ET was almost completely repressed in the DR in substantial fractions of injected eyes (Fig. 7B,C), indicating ventralization of the DR. Similar results were obtained with the DR marker Tbx5 (data not shown). Therefore, RA and Hh signaling can also cooperate in ventralizing the DR, in conditions where OS specification is weakly affected.
|
|
In conclusion, Hh, RA and FGFR signaling can collaborate in ventral eye specification. In particular, OS character is preferentially specified at higher Hh and FGFR signaling levels, and lower RA signaling levels, whereas VR character is preferentially specified at lower Hh and higher RA levels.
RA, Hh and FGF signaling pathways can cross-regulate each other at the transcriptional level
Eye ventralization by RA, Hh and FGF signals may involve crossregulatory
interactions among these three signaling pathways. We found that both
bhh and iFGFR1 overexpression upregulate Raldh3
expression in the eye at stage 33 (Fig.
1B, Fig. 3B).
Unilateral injection of 500 pg bhh mRNA did not have appreciable
effects on Raldh2 expression in the ANR at neurula stages
(Fig. 9B), but upregulated it
in stage 33 eyes (Fig. 9B); it
also caused the FGF8-positive domain in the ANR to expand laterally
and upregulated FGF8 in the eye region at stage 33
(Fig. 9B).
We also found that RA treatments downregulated Shh expression in the anterior midline (Fig. 9A). Treatments with high RA doses (10 µM) from stage 12.5/13 repressed the most anterior domain of Shh transcription at neurula stages, which overlaps with the medial part of the eye field (Fig. 4D). This effect was also evident at stage 33. Low RA doses (0.1 µM) partially downregulated Shh expression in the anterior midline at neurula stages, while no clear effect was evident at stage 33.
FGF8 expression was also affected in RA-treated embryos (Fig. 9A). At neurula stages, FGF8 mRNA is transcribed in two anterior stripes, one in the ANR, and the other in the anteroventral ectoderm outside the neural plate. These two stripes were closer to each other in low dose RA-treated embryos, and appeared to be merged in one broader stripe of expression in high dose RA-treated embryos. At tadpole stages, a strong general repression of FGF8 transcription was caused by high RA doses.
As shown in Fig. 9C, 4 pg iFGFR1 mRNA injections did not affect Raldh2 expression in the ANR at neurula after induction at stage 12.5/13, while Shh expression was expanded on the injected side at the level of the prospective hypothalamic region. Ectopic upregulation of Shh in the eye region at stage 33 was also detectable in these conditions. At lower doses (2 pg), only weak effects on Shh and Raldh2 expression were detected. Analysis of Pax2 and Vax2 expression showed that, in this experiment, 2 pg iFGFR1 mRNA induced eye ventralization similar to that reported in Fig. 3, while a 4 pg dose caused eye reductions (Fig. 9C and data not shown).
In conclusion, RA, Hh and FGF signals are able to crossregulate the expression of one another, although these effects are mainly mediated by doses of signal higher than those required to ventralize the eye. Shh downregulation in the anterior midline of RA-treated embryos suggested that the loss of the OS seen with high RA may be a secondary consequence of this effect. To address this issue, we analyzed whether Hh signaling could rescue OS formation in RA-treated embryos. Indeed, we found that expression of OS markers was recovered in embryos injected with 25 pg bhh mRNA followed by incubation in 10 µM RA from stage 12.5/13 (see Fig. S4 in the supplementary material). Therefore, RA treatments may repress OS formation indirectly by downregulating Shh expression in the anterior midline, although a direct inhibitory effect on OS gene expression cannot be ruled out.
Loss-of-function experiments suggest that ventral eye specification involves interactions among Hh, RA and FGFR signaling
To address whether interactions between Hh, RA and FGFR signaling pathways
play an important role in ventral eye specification, we inhibited them in all
possible combinations. Hh signaling was blocked with cyclopamine
(Incardona et al., 1998). RA
signaling was inhibited with the pan-RAR antagonist AGN194310, which
specifically antagonizes activity of all RAR receptors, but not RXR receptors
(Hammond et al., 2001
). FGFR
signaling was inhibited with SU5402
(Mohammadi et al., 1997
).
Xenopus embryos were treated with these antagonists from stage 10.5,
at conditions in which each inhibitor effectively reduced the levels of its
target signaling pathway, without significantly affecting the other two (see
Figs S5, S6 in the supplementary material). As shown in
Fig. 10, significant DV
organization was retained after any of the single inhibitions of Hh, RA or
FGFR signaling. Double inhibition of Hh, RA and FGFR signaling in different
combinations increased reduction of ventral eye territories when compared with
single inhibitions, especially after Hh and FGFR or RA and FGFR inhibitions.
The strongest effects were obtained with the simultaneous inhibition of all
three (Hh, RA and FGFR) signaling pathways, which caused a dorsalized eye
phenotype, with strong repression of ventral eye markers and upregulation of
the DR markers ET within the ventral eye. In conclusion, the results
of loss-of-function experiments support a model where ventral eye
specification involves interactions among Hh, RA and FGFR signaling
pathways.
|
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
RA has been suggested as an alternative signaling molecule that may control
DV specification within the retina (Drager
et al., 2001). RA treatments upregulated the OS marker
Pax2 in zebrafish (Hyatt et al.,
1996
), while reduced RA signaling caused abnormal development of
the VR in different animal models
(Marsh-Armstrong et al., 1994
;
Ross et al., 2000
), at least
at the morphological level. In this paper, we confirm in a different model
system that increasing RA levels can induce OS character in the eye
(Fig. 2A). More importantly, we
provide evidence that RA can act as a ventralizing factor within the retina.
This ventralizing effect requires higher levels of RA than those required to
induce OS expansion, and it is accompanied by a strong repression of OS genes,
probably owing to the downregulation of Shh in the anterior midline
(Fig. 9A). Upregulation of
Vax2 in the presence of Pax6, but in the absence of OS
markers, together with the strong downregulation of DR markers, indicates that
high RA levels cause the DR to acquire VR character
(Fig. 2B, see Fig. S1 in the
supplementary material). These ventralizing activities of RA can be mimicked,
albeit less efficiently, by the intermediate metabolic precursor ATR,
suggesting that correct localization of endogenous RA-generating enzymes is
important for DV patterning of the eye
(Fig. 5). These effects are
strongly inhibited by a RAR antagonist, indicating that they are specifically
mediated by activation of RAR receptors (see Fig. S3 in the supplementary
material).
Loss-of-function analyses of FGF signals in zebrafish have suggested that
this pathway may play a role in OS specification, although decreased FGF
signaling seems to have much more profound effects on the adjacent ventral
forebrain (Take-uchi et al.,
2003; Walshe and Mason,
2003
). There are no reports that we are aware of suggesting that
FGF signaling may have a role in DV patterning within the retina. We show that
overexpression of FGFR signaling has a strong ventralizing effect on the
developing Xenopus eye. At all doses analyzed, FGFR signaling expands
the expression of Vax2 and OS markers to a similar extent, while the
expression domain of Pax6 is proportionally reduced, suggesting that
FGFR signaling on its own can enhance specification of OS character, but it
cannot efficiently modify the DV character of the retina
(Fig. 3). FGFR and Hh signaling
can collaborate in the specification of OS character, and they may weakly
interact in specifying VR character (Fig.
8). Finally, strong effects on VR specification were observed when
FGFR signaling was inhibited together with RA and Hh signaling
(Fig. 10). Although further
work is needed to determine the precise role of FGFR signaling in DV
patterning of the retina, our results suggest that it may be involved in
controlling patterning throughout the eye DV axis; higher levels of FGFR
activation may promote OS fates, while lower levels of FGFR activation may
collaborate with other signals in the specification of VR fates.
RA, Hh and FGFR signaling interact in Xenopus ventral eye specification
The fact that RA, Hh and FGFR overexpression cause similar ventralizing
effects in the eye, and the observation that these signaling components are
expressed in adjacent or overlapping domains at early stages of eye
development, suggested that these pathways may interact during DV patterning
of the eye in Xenopus. Two lines of evidence in this work support
this idea. First, in co-overexpression experiments, Hh, RA and FGFR signaling
can collaborate in ventral eye specification (Figs
7,
8). Second, in loss-of-function
experiments, stronger effects on eye DV patterning were observed by inhibiting
more than one pathway compared with single inhibitions
(Fig. 10).
We propose a model of ventral eye specification that involves interactions
among RA, Hh and FGFR signaling pathways
(Fig. 11). According to this
model, high levels of Hh and FGFR signaling interact with low levels of RA
signaling to specify the OS by repressing retina-determination genes such as
Pax6, and promoting the expression of Vax1 and
Pax2. By contrast, high levels of RA act in concert with lower levels
of Hh and FGFR signaling to specify the VR by repressing DR-specific genes
such as ET and by inducing the expression of Vax2 in the
presence of Pax6, but not Vax1 and Pax2. As
previously proposed (Koshiba-Takeuchi et
al., 2000; Sasagawa et al.,
2002
), BMP signaling specifies DR regions by repressing
Vax2 and inducing ET and other members of the Tbx
gene family, such as Tbx5
(Koshiba-Takeuchi et al.,
2000
). In vivo, ventrodorsal (ventral high) gradients of Hh and
FGFR signaling may be created by diffusion of Hh and FGF signals from their
sources in the anteromedial neural plate
(Fig.4D). The regulation of RA
gradients is more complex and the localization of different anabolic and
catabolic enzymes needs to be considered. However, early expression of
Raldh2 and Raldh3 appears to be localized close to the
presumptive ventral eye (Fig.
4A,D), and the mediolateral gradient (lateral high) of
Raldh2 expression in the ANR (Fig.
4D) may contribute to create higher RA levels in the VR compared
with the OS region.
|
In the telencephalon, the medial ganglionic eminence (MGE) originates from
the ventral part of the telencephalic vesicle, while the lateral ganglionic
eminence (LGE) originates from a more intermediate region. Hh signaling is
involved in the specification of the MGE, while RA signaling appears to play a
crucial role in the specification of the LGE
(Gunhaga et al., 2000;
Marklund et al., 2004
). In
addition, FGF signaling is involved in the specification of ventral, but not
intermediate, telencephalic fates
(Marklund et al., 2004
;
Shinya et al., 2001
).
In the developing eye, cells located more ventrally in the anlage give rise to the OS, while the VR originates from a more intermediate region. As shown in Fig. 11, Hh and FGFR signaling play a crucial role in OS specification, although low levels of RA signaling may also be involved. Moreover, RA signaling could control specification of the VR in collaboration with low levels of Hh and possibly FGFR signaling.
In conclusion, similar mechanisms of ventral specification involving Hh, RA and FGFR signaling pathways appear to be at least partially conserved in different CNS regions. Several questions remain to be addressed concerning the precise role and the mechanism of action of these signaling systems. Clearly, DV patterning of the vertebrate CNS is a complex process, and the eye, because of its distinct regional composition, its finely graded topography and its experimental accessibility, is an exciting model with which to study how different signaling pathways interact to execute specific developmental programs.
![]() |
ACKNOWLEDGMENTS |
---|
![]() |
Footnotes |
---|
Supplementary material for this article is available at http://dev.biologists.org/cgi/content/full/132/7/1737/DC1
* These authors contributed equally to this work
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Andreazzoli, M., Gestri, G., Angeloni, D., Menna, E. and
Barsacchi, G. (1999). Role of Xrx1 in Xenopus eye and
anterior brain development. Development
126,2451
-2460.
Ang, H. L. and Duester, G. (1999a). Retinoic acid biosynthetic enzyme ALDH1 localizes in a subset of retinoid-dependent tissues during xenopus development. Dev. Dyn. 215,264 -272.[CrossRef][Medline]
Ang, H. L. and Duester, G. (1999b). Stimulation
of premature retinoic acid synthesis in Xenopus embryos following premature
expression of aldehyde dehydrogenase ALDH1. Eur. J.
Biochem. 260,227
-234.
Appel, B. and Eisen, J. S. (2003). Retinoids run rampant: multiple roles during spinal cord and motor neuron development. Neuron 40,461 -464.[CrossRef][Medline]
Barbieri, A. M., Broccoli, V., Bovolenta, P., Alfano, G.,
Marchitiello, A., Mocchetti, C., Crippa, L., Bulfone, A., Marigo, V.,
Ballabio, A. et al. (2002). Vax2 inactivation in mouse
determines alteration of the eye dorsal-ventral axis, misrouting of the optic
fibres and eye coloboma. Development
129,805
-813.
Bertuzzi, S., Hindges, R., Mui, S. H., O'Leary, D. D. and Lemke,
G. (1999). The homeodomain protein vax1 is required for axon
guidance and major tract formation in the developing forebrain.
Genes Dev. 13,3092
-3105.
Casarosa, S., Andreazzoli, M., Simeone, A. and Barsacchi, G. (1997). Xrx1, a novel Xenopus homeobox gene expressed during eye and pineal gland development. Mech. Dev. 61,187 -198.[CrossRef][Medline]
Chen, Y., Pollet, N., Niehrs, C. and Pieler, T. (2001). Increased XRALDH2 activity has a posteriorizing effect on the central nervous system of Xenopus embryos. Mech. Dev. 101,91 -103.[CrossRef][Medline]
Chow, R. L. and Lang, R. A. (2001). Early eye development in vertebrates. Annu. Rev. Cell Dev. Biol. 17,255 -296.[CrossRef][Medline]
Diez del Corral, R. and Storey, K. G. (2004). Opposing FGF and retinoid pathways: a signalling switch that controls differentiation and patterning onset in the extending vertebrate body axis. BioEssays 26,857 -869.[CrossRef][Medline]
Drager, U. C., Li, H., Wagner, E. and McCaffery, P. (2001). Retinoic acid synthesis and breakdown in the developing mouse retina. Prog. Brain Res. 131,579 -587.[Medline]
Durston, A. J., Timmermans, J. P., Hage, W. J., Hendriks, H. F., de Vries, N. J., Heideveld, M. and Nieuwkoop, P. D. (1989). Retinoic acid causes an anteroposterior transformation in the developing central nervous system. Nature 340,140 -144.[CrossRef][Medline]
Ekker, S. C., McGrew, L. L., Lai, C. J., Lee, J. J., von
Kessler, D. P., Moon, R. T. and Beachy, P. A. (1995).
Distinct expression and shared activities of members of the hedgehog gene
family of Xenopus laevis. Development
121,2337
-2347.
Gunhaga, L., Jessell, T. M. and Edlund, T.
(2000). Sonic hedgehog signaling at gastrula stages specifies
ventral telencephalic cells in the chick embryo.
Development 127,3283
-3293.
Hallonet, M., Hollemann, T., Pieler, T. and Gruss, P.
(1999). Vax1, a novel homeobox-containing gene, directs
development of the basal forebrain and visual system. Genes
Dev. 13,3106
-3114.
Hammond, L. A., van Krinks, C. H., Durham, J., Tomkins, S. E., Burnett, R. D., Jones, E. L., Chandraratna, R. A. and Brown, G. (2001). Antagonists of retinoic acid receptors (RARs) are potent growth inhibitors of prostate carcinoma cells. Br. J. Cancer 85,453 -462.[CrossRef][Medline]
Harland, R. M. (1991). In situ hybridization: an improved wholemount method for Xenopus embryos. Methods Cell Biol. 36,675 -685.[Medline]
Harris, W. A. (2003). Specifying motor neurons: up and down and back to front. Nat. Neurosci. 6,1247 -1249.[CrossRef][Medline]
Hongo, I., Kengaku, M. and Okamoto, H. (1999). FGF signaling and the anterior neural induction in Xenopus. Dev. Biol. 216,561 -581.[CrossRef][Medline]
Hyatt, G. A., Schmitt, E. A., Marsh-Armstrong, N., McCaffery,
P., Drager, U. C. and Dowling, J. E. (1996). Retinoic
acid establishes ventral retinal characteristics.
Development 122,195
-204.
Incardona, J. P., Gaffield, W., Kapur, R. P. and Roelink, H.
(1998). The teratogenic Veratrum alkaloid cyclopamine inhibits
sonic hedgehog signal transduction. Development
125,3553
-3562.
Kastner, P., Grondona, J. M., Mark, M., Gansmuller, A., LeMeur, M., Decimo, D., Vonesch, J. L., Dolle, P. and Chambon, P. (1994). Genetic analysis of RXR alpha developmental function: convergence of RXR and RAR signaling pathways in heart and eye morphogenesis. Cell 78,987 -1003.[CrossRef][Medline]
Koebernick, K., Hollemann, T. and Pieler, T. (2001). Molecular cloning and expression analysis of the Hedgehog receptors XPtc1 and XSmo in Xenopus laevis. Mech. Dev. 100,303 -308.[CrossRef][Medline]
Koshiba-Takeuchi, K., Takeuchi, J. K., Matsumoto, K., Momose,
T., Uno, K., Hoepker, V., Ogura, K., Takahashi, N., Nakamura, H.,
Yasuda, K. et al. (2000). Tbx5 and the retinotectum
projection. Science 287,134
-137.
Liu, Y., Lupo, G., Marchitiello, A., Gestri, G., He, R. Q., Banfi, S. and Barsacchi, G. (2001). Expression of the Xvax2 gene demarcates presumptive ventral telencephalon and specific visual structures in Xenopus laevis. Mech. Dev. 100,115 -118.[CrossRef][Medline]
Lupo, G., Harris, W. A., Barsacchi, G. and Vignali, R.
(2002). Induction and patterning of the telencephalon in Xenopus
laevis. Development 129,5421
-5436.
Marklund, M., Sjodal, M., Beehler, B. C., Jessell, T. M.,
Edlund, T. and Gunhaga, L. (2004). Retinoic acid
signalling specifies intermediate character in the developing telencephalon.
Development 131,4323
-4332.
Marsh-Armstrong, N., McCaffery, P., Gilbert, W., Dowling, J. E.
and Drager, U. C. (1994). Retinoic acid is necessary
for development of the ventral retina in zebrafish. Proc. Natl.
Acad. Sci. USA 91,7286
-7290.
McLaughlin, T., Hindges, R. and O'Leary, D. D. (2003). Regulation of axial patterning of the retina and its topographic mapping in the brain. Curr. Opin. Neurobiol. 13,57 -69.[CrossRef][Medline]
Mohammadi, M., McMahon, G., Sun, L., Tang, C., Hirth, P., Yeh,
B. K., Hubbard, S. R. and Schlessinger, J. (1997).
Structures of the tyrosine kinase domain of fibroblast growth factor receptor
in complex with inhibitors. Science
276,955
-960.
Mui, S. H., Hindges, R., O'Leary, D. D., Lemke, G. and Bertuzzi,
S. (2002). The homeodomain protein Vax2 patterns the
dorsoventral and nasotemporal axes of the eye.
Development 129,797
-804.
Murali, D., Yoshikawa, S., Corrigan, R. R., Plas, D. J., Crair,
M. C., Oliver, G., Lyons, K. M., Mishina, Y. and Furuta, Y.
(2005). Distinct developmental programs require different levels
of Bmp signaling during mouse retinal development.
Development 132,913
-923.
Nieuwkoop, P. D. and Faber, J. (1967).Normal Table of Xenopus laevis (Daudin) . Amsterdam, The Netherlands: North Holland.
Novitch, B. G., Wichterle, H., Jessell, T. M. and Sockanathan, S. (2003). A requirement for retinoic acid-mediated transcriptional activation in ventral neural patterning and motor neuron specification. Neuron 40, 81-95.[CrossRef][Medline]
Perron, M., Boy, S., Amato, M. A., Viczian, A., Koebernick, K.,
Pieler, T. and Harris, W. A. (2003). A novel function for
Hedgehog signalling in retinal pigment epithelium differentiation.
Development 130,1565
-1577.
Peters, M. A. (2002). Patterning the neural retina. Curr. Opin. Neurobiol. 12, 43-48.[CrossRef][Medline]
Pierani, A., Brenner-Morton, S., Chiang, C. and Jessell, T. M. (1999). A sonic hedgehog-independent, retinoid-activated pathway of neurogenesis in the ventral spinal cord. Cell 97,903 -915.[CrossRef][Medline]
Pownall, M. E., Welm, B. E., Freeman, K. W., Spencer, D. M., Rosen, J. M. and Isaacs, H. V. (2003). An inducible system for the study of FGF signalling in early amphibian development. Dev. Biol. 256,89 -99.[Medline]
Ross, S. A., McCaffery, P. J., Drager, U. C. and de Luca, L.
M. (2000). Retinoids in embryonal development.
Physiol. Rev. 80,1021
-1054.
Ruiz i Altaba, A., Nguyen, V. and Palma, V. (2003). The emergent design of the neural tube: prepattern, SHH morphogen and GLI code. Curr. Opin. Genet. Dev. 13,513 -521.[CrossRef][Medline]
Russell, C. (2003). The roles of Hedgehogs and Fibroblast Growth Factors in eye development and retinal cell rescue. Vision Res. 43,899 -912.[CrossRef][Medline]
Sakuta, H., Suzuki, R., Takahashi, H., Kato, A., Shintani, T.,
Iemura, S., Yamamoto, T. S., Ueno, N. and Noda, M.
(2001). Ventroptin: a BMP-4 antagonist expressed in a
double-gradient pattern in the retina. Science
293,111
-115.
Sasagawa, S., Takabatake, T., Takabatake, Y., Muramatsu, T. and Takeshima, K. (2002). Axes establishment during eye morphogenesis in Xenopus by coordinate and antagonistic actions of BMP4, Shh, and RA. Genesis 33,86 -96.[CrossRef][Medline]
Schwarz, M., Cecconi, F., Bernier, G., Andrejewski, N.,
Kammandel, B., Wagner, M. and Gruss, P. (2000).
Spatial specification of mammalian eye territories by reciprocal
transcriptional repression of Pax2 and Pax6.
Development 127,4325
-4334.
Shanmugalingam, S., Houart, C., Picker, A., Reifers, F.,
Macdonald, R., Barth, A., Griffin, K., Brand, M. and Wilson, S. W.
(2000). Ace/Fgf8 is required for forebrain commissure formation
and patterning of the telencephalon. Development
127,2549
-2561.
Shinya, M., Koshida, S., Sawada, A., Kuroiwa, A. and Takeda,
H. (2001). Fgf signalling through MAPK cascade is required
for development of the subpallial telencephalon in zebrafish embryos.
Development 128,4153
-4164.
Shiotsugu, J., Katsuyama, Y., Arima, K., Baxter, A., Koide, T.,
Song, J., Chandraratna, R. A. and Blumberg, B. (2004).
Multiple points of interaction between retinoic acid and FGF signaling during
embryonic axis formation. Development
131,2653
-2667.
Take-uchi, M., Clarke, J. D. and Wilson, S. W.
(2003). Hedgehog signalling maintains the optic stalk-retinal
interface through the regulation of Vax gene activity.
Development 130,955
-968.
Torres, M., Gomez-Pardo, E. and Gruss, P.
(1996). Pax2 contributes to inner ear patterning and optic nerve
trajectory. Development
122,3381
-3391.
Vignali, R., Colombetti, S., Lupo, G., Zhang, W., Stachel, S., Harland, R. M. and Barsacchi, G. (2000). Xotx5b, a new member of the Otx gene family, may be involved in anterior and eye development in Xenopus laevis. Mech. Dev. 96, 3-13.[CrossRef][Medline]
Walshe, J. and Mason, I. (2003). Unique and
combinatorial functions of Fgf3 and Fgf8 during zebrafish forebrain
development. Development
130,4337
-4349.
Wilson, S. W. and Houart, C. (2004). Early steps in the development of the forebrain. Dev. Cell 6, 167-181.[CrossRef][Medline]
Yang, X. J. (2004). Roles of cell-extrinsic growth factors in vertebrate eye pattern formation and retinogenesis. Semin. Cell Dev. Biol. 15, 91-103.[CrossRef][Medline]
Zhang, X. M. and Yang, X. J. (2001). Temporal and spatial effects of Sonic hedgehog signaling in chick eye morphogenesis. Dev. Biol. 233,271 -290.[CrossRef][Medline]