Abstract
Retinoic acid is a metabolic derivative of vitamin A that plays an essential function in cell-cell signaling by serving as a ligand for nuclear receptors that directly regulate gene expression. The final step in the conversion of retinol to retinoic acid is carried out by three retinaldehyde dehydrogenases encoded by Raldh1 (Aldh1a1), Raldh2 (Aldh1a2), and Raldh3 (Aldh1a3). Mouse Raldh gene knockout studies have been instrumental in understanding the mechanism of retinoic acid action during eye development. Retinoic acid signaling in the developing eye is particularly complex as all three Raldh genes contribute to retinoic acid synthesis in non-overlapping locations. During optic cup formation Raldh2 is first expressed transiently in perioptic mesenchyme, then later Raldh1 and Raldh3 expression begins in the dorsal and ventral retina, respectively, and these sources of retinoic acid are maintained in the fetus. Retinoic acid is not required for dorsoventral patterning of the retina as originally thought, but it is required for morphogenetic movements that form the optic cup, ventral retina, cornea, and eyelids. These findings will help guide future studies designed to identify retinoic acid target genes during eye organogenesis.
Keywords: Eye, Organogenesis, Aldehyde Dehydrogenase, RALDH, Retinoic Acid
1. Vitamin A and Eye Development
Eye development involves interactions among cells derived from forebrain neuroectoderm (forming the optic cup, retina, and optic nerve), surface ectoderm (forming the lens and corneal epithelium), and neural crest-derived perioptic mesenchyme (forming the corneal stroma, eyelid folds, anterior chamber, sclera, and vitreous body). Incomplete closure of the choroid fissure (located in the ventral eye) during the final stage of optic cup formation results in ocular coloboma, a congenital eye defect that represents an important cause of childhood blindness or vision impairment [1, 2]. Many cases of ocular coloboma are of unknown etiology and some may be caused by environmental influences such as vitamin A deficiency in humans [3] or animals [4]. During gestational vitamin A deficiency the eye is the most sensitive organ to malformations, thus demonstrating a major role for vitamin A in eye development [4]. Vitamin A (retinol) metabolism by alcohol- and aldehyde-metabolizing enzymes results in the production of retinoic acid (RA) which functions as a ligand for nuclear receptors that directly regulate gene expression [5]. A potential relationship may exist between impaired retinol metabolism and coloboma. For example, humans with missense mutations in the gene encoding serum retinol binding protein have been shown to exhibit ocular coloboma and retinal dystrophy [6]. These findings suggest that the availability of vitamin A in the diet and the ability to metabolize it to RA has an influence on human eye development, but the mechanism of vitamin A action is just beginning to be understood.
Two protein families are involved in RA signal transduction, i.e. the RA receptors (RARα, β, γ) and the retinoid X receptors (RXRα, β, γ) which form RAR/RXR heterodimers when bound to a retinoic acid response element of a target gene [7, 8]. RA is required as a ligand for only the RAR portion of RAR/RXR heterodimers, suggesting that RXR functions to facilitate proper DNA-binding of RAR [9]. RARα, β, γ are expressed in overlapping patterns in the eye during development resulting in significant functional redundancy [10]. Mice carrying single null mutations of each RAR exhibit very minor defects during embryogenesis and survive postnatally [11, 12]. RARβ mutant mice exhibit a minor eye defect, i.e. persistence of the primary vitreous body as a retrolenticular membrane [13]. When two RARs are knocked out together many embryonic defects are observed in the eye and other organs and postnatal lethality is observed soon after birth [14, 15]. Consistent defects of the eye in double RAR mutants include microphthalmia, coloboma of the retina and optic nerve, and abnormalities of the cornea, eyelids, and conjunctiva [14]. Thus, RARs mediate the functions of vitamin A as the defects observed are essentially the same as those seen during gestational vitamin A deficiency.
2. Role of Retinaldehyde Dehydrogenase in Retinoic Acid Synthesis
The first step of RA synthesis, oxidation of retinol to retinaldehyde, is catalyzed by either alcohol dehydrogenase (ADH) or short-chain dehydrogenase/reductase (SDR), the latter often referred to as retinol dehydrogenase (RDH) [5]. Genetic studies have identified several enzymes able to oxidize retinol in vivo including Adh1, Adh3, and Adh4 [16, 17] as well as Rdh1 [18] and Rdh10 [19]. Although oxidation of retinol to retinaldehyde may occur at higher levels in some tissues due to tissue-specific expression of Adh1, Adh4 and Rdh10, this reaction is not tissue-restricted as it is also stimulated by Adh3 and Rdh1 which are widely expressed during embryogenesis [16]. Also, it should be stressed that retinol oxidation is reversible, and that multiple enzymes (RDHs and aldoketo reductases) have been reported to participate in the reduction of retinaldehyde to retinol [20].
The second step of RA synthesis, oxidation of retinaldehyde to RA, is catalyzed by three members of the aldehyde dehydrogenase (ALDH) family also referred to as retinaldehyde dehydrogenase (RALDH) [5]. Raldh1 (Aldh1a1), Raldh2 (Aldh1a2), and Raldh3 (Aldh1a3) have unique non-overlapping expression patterns during development [21, 22]. As Raldh genes are expressed in unique dynamic spatiotemporal patterns, this step of RA synthesis is tissue-restricted and time-restricted.
Our understanding of what regulates synthesis of the ligand RA during embryogenesis is just beginning to emerge. The precursor retinol is made available to all cells via serum retinol-binding protein (RBP4) which can interact with a membrane receptor (STRA6) to stimulate retinol uptake [23, 24]. Cellular retinol-binding protein is an intracellular protein which facilitates uptake of retinol into cells and stimulates its reversible conversion to retinyl esters for storage [25]. Metabolism of retinol to RA takes place in specific tissues as observed in embryos using a sensitive RA-reporter assay in which a retinoic acid response element (RARE) is linked to a lacZ reporter gene. A transgenic RARE-lacZ mouse strain demonstrates that RA transcriptional activity can first be detected in embryos at 7.5 days of embryonic development (E7.5); at E7.5 RA activity is detected only posteriorly, but at later stages RA is also detected anteriorly in the head [26]. At E8.25, RA is detected only in the embryonic trunk [27], but at E8.5 RA is now also detected in the head including the optic vesicles which have just formed [26]. This suggests that endogenous RA synthesis initiates in the eye field near E8.5 (10-somite stage). Raldh2 is first expressed at E7.5 in the trunk mesoderm and by E8.5-E9.5 displays expression in the eye that appears similar to the pattern of RA localization using the RARE-lacZ RA-reporter mouse [21]. Studies on Raldh2−/− embryos carrying RARE-lacZ have shown that RALDH2 is responsible for all RA activity seen from E7.5-E8.5 [27], and for some but not all RA activity observed in the head at E9.5 [21]. Further genetic studies have demonstrated that Raldh1 and Raldh3 generate RA in the eye field at E9.5-E10.5 [28, 29]. RA generated posteriorly in the trunk by Raldh2 is necessary for development of the posterior portions of the central nervous system including the hindbrain [30, 31] and spinal cord [32, 33]. RA generated in the head from E8.5-E10.5 by all three Raldh genes is unnecessary for early forebrain development [29], but all three participate in development of the eye whose neural components are derived from an out-pocketing of the forebrain neuroectoderm that forms the optic vesicles. These findings suggest that RA synthesis and signaling during eye development is a complex process.
3. Raldh Gene Expression in the Developing Eye
Raldh1, Raldh2, and Raldh3 are expressed in unique non-overlapping tissues in the mouse embryonic eye field, and RA activity can be detected in those tissues plus surrounding tissues using embryos carrying the RARE-lacZ RA-reporter transgene (Table 1) [28]. Raldh2 is the first source of RA synthesis for the eye field. By E9.0 it is clear that Raldh2 is generating RA in the mesenchyme next to the temporal (lateral) side of the optic vesicle but not the nasal (medial) side prior to its invagination to form the optic cup. Also prior to invagination of the optic vesicle, Raldh3 begins to generate RA in the surface ectoderm over the eye field at E8.75 and later in the dorsal retinal pigment epithelium (RPE) starting at E9.5. Between E9.5-E10.5, invagination of the optic vesicle occurs resulting in an optic cup with separate layers for neural retina and RPE folded around the lens vesicle that developed from invagination of the surface ectoderm which occurs at the same time. Expression of Raldh2 and Raldh3 changes during optic cup formation. Raldh2 expression in the perioptic mesenchyme terminates at E10.0, and Raldh3 expression initiates in the ventral neural retina at E10.5. In addition, Raldh1 expression begins to generate RA in the dorsal neural retina at E10.5. From E10.5 onwards to birth (approximately E19.5), Raldh1 and Raldh3 continue to be expressed in the dorsal and ventral neural retina, respectively. Raldh1 and Raldh3 are the only sources of RA from E11.5–13.5 when the ventral folds of the optic cup fuse to form the choroid fissure at E13.5.
Table 1.
Roles for retinaldehyde dehydrogenase genes in eye development determined by defects observed in compound Raldh gene knockouts. These results support a paracrine function for RA signaling in eye development as the target of RA action is adjacent to the site of RA synthesis.
Stage | Gene | Tissue Expression | Function for Eye Development |
---|---|---|---|
E8.5–E9.0 | Raldh2 | Optic vesicle | None detected |
E8.75–E10.5 | Raldh3 | Surface ectoderm over eye field | Ventral invagination of optic cup |
E9.0–E10.0 | Raldh2 | Perioptic mesenchyme (temporal side of optic vesicle) | Ventral invagination of optic cup |
E9.5–E10.5 | Raldh3 | Dorsal retinal pigment epithelium | Ventral invagination of optic cup |
E10.5-birth | Raldh1 | Dorsal neural retina | Cornea and eyelid morphogenesis (apoptosis in perioptic mesenchyme) |
E10.5-birth | Raldh3 | Ventral neural retina | Cornea and eyelid morphogenesis (apoptosis in perioptic mesenchyme) |
4. Effects of Raldh Gene Knockouts on Embryonic Eye Development
Genetic studies have demonstrated that Raldh gene knockouts are quite useful for studying the function of RA signaling during embryogenesis as they produce embryos that completely lack RA activity in certain tissues [21, 27, 28, 34–36]. Raldh gene knockout mice have been used to sort out the individual contributions of each enzyme for eye development (Table 1). Raldh1−/− mice survive to adulthood and exhibit no noticeable defects in eye development [35]. Raldh1 mutants initially do not completely lose RA activity in the dorsal retina due to compensation by Raldh3, but even though they completely lose dorsal RA activity from E16.5 onwards, retinal lamination is normal in adult mice and retinal ganglion axons reach the brain both dorsally and ventrally [35]. Thus, RA generated by Raldh1 does not appear to be necessary for late stages of retina or optic nerve development. However, Raldh1/Raldh3 double mutants exhibit mesenchymal overgrowth in the cornea and eyelids that is associated with a defective apoptosis program in perioptic mesenchyme [28, 37]. Thus, Raldh1 and Raldh3 have completely redundant functions in generating RA that travels from the retina to the perioptic mesenchyme to control the amount of mesenchyme that reaches the anterior eye to form the cornea and eyelids.
Studies in this laboratory [28] and another [36] have demonstrated that Raldh3−/− embryos exhibit eye and nasal defects. Raldh3−/− mice die at birth due to a blockage of the nasal passages [36]. Raldh3−/− embryos begin the process of optic cup formation, but they exhibit a shortening of the ventral retina. These findings suggest that RALDH3 synthesizes RA required to complete ventral optic cup formation (including closure of the choroid fissure) and that RALDH1 cannot fulfill this function. As Raldh1/Raldh3 double mutants still undergo most of the morphogenetic movements needed for optic cup formation, further studies have indicated that RA generated by Raldh2 allows these movements to initiate but not proceed to completion [28].
Raldh2−/− embryos and Raldh1/Raldh2 double mutants do not develop optic cups, but optic cup formation can be rescued by maternal RA administration from E6.75–E8.25 which rescues a lethal developmental defect that stalls eye development at the optic vesicle stage [38]. Raldh3 expressed in rescued Raldh2 mutants may be responsible for allowing optic cup formation. RA administered to Raldh2−/− embryos does not itself induce RA signaling in the eye field, but this treatment does result in relatively normal Raldh3 expression in the eye field from E8.75–E10.5 which then results in relatively normal detection of RA signaling [28]. Examination of Raldh2/Raldh3 double mutants and Raldh1/Raldh2/Raldh3 triple mutants treated with RA to E8.25 (to rescue the early developmental functions of Raldh2) revealed that such embryos fail to produce RA in the eye field and do not form a complete optic cup; interestingly, the dorsal portion of the optic cup invaginates but the ventral portion does not invaginate resulting in an incomplete optic cup [28]. Thus, Raldh2 and Raldh3 both generate RA that stimulates the ventral morphogenetic movements of the optic vesicle that begin ventral optic cup formation. However, Raldh3 must be present in order to complete the ventral morphogenetic movements that culminate in closure of the choroid fissure at E13.5, presumably due to its continued expression in the eye after Raldh2 expression ends at E10.0.
The dorsal versus ventral expression patterns of Raldh1 and Raldh3 led to the hypothesis that RA may be involved in dorsoventral patterning of the retina and its axonal projections into the brain [39]. RA signaling in chick embryos has been reported to be necessary for expression of the retinal topographic guidance molecules ephrinB2 (dorsally) and EphB2 (ventrally) that function downstream of Tbx5 (dorsally) and Vax2 (ventrally) to provide dorsoventral patterning of the retina [40]. However, we found that ephrinB2 and EphB2 were still expressed in their correct locations in Raldh1−/−;Raldh3−/− mouse embryos that lack retinal RA activity [28]. Thus, the previous studies which relied upon addition of a dominant-negative RA receptor may have affected ephrinB2 and EphB2 expression by disturbing an RA-independent mechanism that controls their expression [40]. In addition, Raldh compound mutants retain dorsal expression of Tbx5 and ventral expression of Vax2 in the optic cup while losing all RA activity, indicating that RA signaling is not required for establishing dorsoventral polarity in the retina [28, 37].
Studies on gestational vitamin A deficiency have demonstrated that embryonic eye defects can be prevented or reduced by addition of RA to the maternal diet [41]. Similarly, eye defects in mouse Raldh mutants can be rescued by maternal dietary RA supplementation [28]. Thus, tools are now becoming available to manipulate both the enzymes controlling endogenous RA synthesis and the availability of dietary RA, making it possible to examine in more detail the mechanism of RA action in the eye.
4. Summary
The investigations reported here illustrate the usefulness of Raldh gene knockouts in revealing the mechanism of RA signaling in the developing eye. These findings indicate that RA signaling is initially required for ventral morphogenetic movements that allow a transition from optic vesicle to optic cup and closure of the choroid fissure. Immediately after optic cup formation RA is required to stimulate apoptosis in the perioptic mesenchyme needed to correctly generate the cornea and eyelids. In both of these processes Raldh genes generate RA that functions in a paracrine but not autocrine fashion (Table 1). For optic cup formation the bending of the ventral neural retina is controlled by RA generated outside the neural retina in the perioptic mesenchyme, dorsal retinal pigment epithelium, and surface ectoderm over the eye. Later, the perioptic mesenchyme becomes the RA target tissue but as Raldh2 is no longer expressed there, the source of RA is Raldh1 and Raldh3 expressed in the neural retina. Although the three Raldh genes do show distinct expression patterns, one must keep in mind that RA synthesized in one region by one RALDH may diffuse to another region, and the extent to which this occurs has been extensively documented [28]. Thus, Raldh2 and Raldh3 are redundant for generation of RA needed to initiate optic cup formation, and Raldh1 and Raldh3 are redundant for generation of RA needed to control perioptic mesenchyme movements during anterior eye formation. Further studies on compound Raldh mutants should provide insight into the target genes regulated by RA that are needed to control eye morphogenetic movements. This information will be useful in evaluating potential roles for RA signaling in eye development and congenital eye disease.
Acknowledgments
This work was supported by National Institutes of Health grant EY013969.
References
- 1.Gregory-Evans CY, Williams MJ, Halford S, Gregory-Evans K. Ocular coloboma: a reassessment in the age of molecular neuroscience. J Med Genet. 2004;41:881–891. doi: 10.1136/jmg.2004.025494. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Morrison D, FitzPatrick D, Hanson I, Williamson K, van Heyningen V, Fleck B, Jones I, Chalmers J, Campbell H. National study of microphthalmia, anophthalmia, and coloboma (MAC) in Scotland: investigation of genetic aetiology. J Med Genet. 2002;39:16–22. doi: 10.1136/jmg.39.1.16. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Hornby SJ, Ward SJ, Gilbert CE, Dandona L, Foster A, Jones RB. Environmental risk factors in congenital malformations of the eye. Ann Trop Paediatr. 2002;22:67–77. doi: 10.1179/027249302125000193. [DOI] [PubMed] [Google Scholar]
- 4.Wilson JG, Roth CB, Warkany J. An analysis of the syndrome of malformations induced by maternal vitamin A deficiency. Effects of restoration of vitamin A at various times during gestation. Amer J Anat. 1953;92:189–217. doi: 10.1002/aja.1000920202. [DOI] [PubMed] [Google Scholar]
- 5.Duester G. Families of retinoid dehydrogenases regulating vitamin A function: production of visual pigment and retinoic acid. FEBS J. 2000;267:4315–4324. doi: 10.1046/j.1432-1327.2000.01497.x. [DOI] [PubMed] [Google Scholar]
- 6.Seeliger MW, Biesalski HK, Wissinger B, Gollnick H, Gielen S, Frank J, Beck S, Zrenner E. Phenotype in retinol deficiency due to a hereditary defect in retinol binding protein synthesis. Invest Ophthalmol Vis Sci. 1999;40:3–11. [PubMed] [Google Scholar]
- 7.Chambon P. A decade of molecular biology of retinoic acid receptors. FASEB J. 1996;10:940–954. [PubMed] [Google Scholar]
- 8.Mangelsdorf DJ, Evans RM. The RXR heterodimers and orphan receptors. Cell. 1995;83:841–850. doi: 10.1016/0092-8674(95)90200-7. [DOI] [PubMed] [Google Scholar]
- 9.Mic FA, Molotkov A, Benbrook DM, Duester G. Retinoid activation of retinoic acid receptor but not retinoid X receptor is sufficient to rescue lethal defect in retinoic acid synthesis. Proc Natl Acad Sci USA. 2003;100:7135–7140. doi: 10.1073/pnas.1231422100. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Mori M, Ghyselinck NB, Chambon P, Mark M. Systematic immunolocalization of retinoid receptors in developing and adult mouse eyes. Invest Ophthalmol Vis Sci. 2001;42:1312–1318. [PubMed] [Google Scholar]
- 11.Lohnes D, Kastner P, Dierich A, Mark M, LeMeur M, Chambon P. Function of retinoic acid receptor gamma in the mouse. Cell. 1993;73:643–658. doi: 10.1016/0092-8674(93)90246-m. [DOI] [PubMed] [Google Scholar]
- 12.Lufkin T, Lohnes D, Mark M, Dierich A, Gorry P, Gaub MP, LeMeur M, Chambon P. High postnatal lethality and testis degeneration in retinoic acid receptor α mutant mice. Proc Natl Acad Sci USA. 1993;90:7225–7229. doi: 10.1073/pnas.90.15.7225. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 13.Ghyselinck NB, Dupé V, Dierich A, Messaddeq N, Garnier JM, Rochette-Egly C, Chambon P, Mark M. Role of the retinoic acid receptor beta (RARb) during mouse development. Int J Dev Biol. 1997;41:425–447. [PubMed] [Google Scholar]
- 14.Lohnes D, Mark M, Mendelsohn C, Dollé P, Dierich A, Gorry P, Gansmuller A, Chambon P. Function of the retinoic acid receptors (RARs) during development. (I) Craniofacial and skeletal abnormalities in RAR double mutants. Development. 1994;120:2723–2748. doi: 10.1242/dev.120.10.2723. [DOI] [PubMed] [Google Scholar]
- 15.Mendelsohn C, Lohnes D, Décimo D, Lufkin T, LeMeur M, Chambon P, Mark M. Function of the retinoic acid receptors (RARs) during development. (II) Multiple abnormalities at various stages of organogenesis in RAR double mutants. Development. 1994;120:2749–2771. doi: 10.1242/dev.120.10.2749. [DOI] [PubMed] [Google Scholar]
- 16.Molotkov A, Fan X, Deltour L, Foglio MH, Martras S, Farrés J, Parés X, Duester G. Stimulation of retinoic acid production and growth by ubiquitously-expressed alcohol dehydrogenase Adh3. Proc Natl Acad Sci USA. 2002;99:5337–5342. doi: 10.1073/pnas.082093299. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 17.Molotkov A, Deltour L, Foglio MH, Cuenca AE, Duester G. Distinct retinoid metabolic functions for alcohol dehydrogenase genes Adh1 and Adh4 in protection against vitamin A toxicity or deficiency revealed in double null mutant mice. J Biol Chem. 2002;277:13804–13811. doi: 10.1074/jbc.M112039200. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 18.Zhang M, Hu P, Krois CR, Kane MA, Napoli JL. Altered vitamin A homeostasis and increased size and adiposity in the rdh1-null mouse. FASEB J. 2007;21:2886–2896. doi: 10.1096/fj.06-7964com. [DOI] [PubMed] [Google Scholar]
- 19.Sandell LL, Sanderson BW, Moiseyev G, Johnson T, Mushegian A, Young K, Rey JP, Ma JX, Staehling-Hampton K, Trainor PA. RDH10 is essential for synthesis of embryonic retinoic acid and is required for limb, craniofacial, and organ development. Genes Dev. 2007;21:1113–1124. doi: 10.1101/gad.1533407. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 20.Gallego O, Belyaeva OV, Porte S, Ruiz FX, Stetsenko AV, Shabrova EV, Kostereva NV, Farres J, Pares X, Kedishvili NY. Comparative functional analysis of human medium-chain dehydrogenases, short-chain dehydrogenases/reductases and aldo-keto reductases with retinoids. Biochem J. 2006;399:101–109. doi: 10.1042/BJ20051988. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 21.Mic FA, Haselbeck RJ, Cuenca AE, Duester G. Novel retinoic acid generating activities in the neural tube and heart identified by conditional rescue of Raldh2 null mutant mice. Development. 2002;129:2271–2282. doi: 10.1242/dev.129.9.2271. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 22.Niederreither K, Fraulob V, Garnier JM, Chambon P, Dollé P. Differential expression of retinoic acid-synthesizing (RALDH) enzymes during fetal development and organ differentiation in the mouse. Mech Dev. 2002;110:165–171. doi: 10.1016/s0925-4773(01)00561-5. [DOI] [PubMed] [Google Scholar]
- 23.Quadro L, Blaner WS, Salchow DJ, Vogel S, Piantedosi R, Gouras P, Freeman S, Cosma MP, Colantuoni V, Gottesman ME. Impaired retinal function and vitamin A availability in mice lacking retinol-binding protein. EMBO Journal. 1999;18:4633–4644. doi: 10.1093/emboj/18.17.4633. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 24.Kawaguchi R, Yu J, Honda J, Hu J, Whitelegge J, Ping P, Wiita P, Bok D, Sun H. A membrane receptor for retinol binding protein mediates cellular uptake of vitamin A. Science. 2007;315:820–825. doi: 10.1126/science.1136244. [DOI] [PubMed] [Google Scholar]
- 25.Ghyselinck NB, Båvik C, Sapin V, Mark M, Bonnier D, Hindelang C, Dierich A, Nilsson CB, Håkansson H, Sauvant P, Azaïs-Braesco V, Frasson M, Picaud S, Chambon P. Cellular retinol-binding protein I is essential for vitamin A homeostasis. EMBO Journal. 1999;18:4903–4914. doi: 10.1093/emboj/18.18.4903. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 26.Rossant J, Zirngibl R, Cado D, Shago M, Giguère V. Expression of a retinoic acid response element-hsplacZ transgene defines specific domains of transcriptional activity during mouse embryogenesis. Genes Dev. 1991;5:1333–1344. doi: 10.1101/gad.5.8.1333. [DOI] [PubMed] [Google Scholar]
- 27.Sirbu IO, Duester G. Retinoic acid signaling in node ectoderm and posterior neural plate directs left-right patterning of somitic mesoderm. Nature Cell Biol. 2006;8:271–277. doi: 10.1038/ncb1374. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 28.Molotkov A, Molotkova N, Duester G. Retinoic acid guides eye morphogenetic movements via paracrine signaling but is unnecessary for retinal dorsoventral patterning. Development. 2006;133:1901–1910. doi: 10.1242/dev.02328. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 29.Molotkova N, Molotkov A, Duester G. Role of retinoic acid during forebrain development begins late when Raldh3 generates retinoic acid in the ventral subventricular zone. Dev Biol. 2007;303:601–610. doi: 10.1016/j.ydbio.2006.11.035. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 30.Niederreither K, Vermot J, Schuhbaur B, Chambon P, Dollé P. Retinoic acid synthesis and hindbrain patterning in the mouse embryo. Development. 2000;127:75–85. doi: 10.1242/dev.127.1.75. [DOI] [PubMed] [Google Scholar]
- 31.Sirbu IO, Gresh L, Barra J, Duester G. Shifting boundaries of retinoic acid activity control hindbrain segmental gene expression. Development. 2005;132:2611–2622. doi: 10.1242/dev.01845. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 32.Molotkova N, Molotkov A, Sirbu IO, Duester G. Requirement of mesodermal retinoic acid generated by Raldh2 for posterior neural transformation. Mech Dev. 2005;122:145–155. doi: 10.1016/j.mod.2004.10.008. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 33.Vermot J, Schuhbaur B, Le Mouellic H, McCaffery P, Garnier JM, Hentsch D, Brulet P, Niederreither K, Chambon P, Dollé P, Le Roux I. Retinaldehyde dehydrogenase 2 and Hoxc8 are required in the murine brachial spinal cord for the specification of Lim1+ motoneurons and the correct distribution of Islet1+ motoneurons. Development. 2005;132:1611–1621. doi: 10.1242/dev.01718. [DOI] [PubMed] [Google Scholar]
- 34.Niederreither K, Subbarayan V, Dollé P, Chambon P. Embryonic retinoic acid synthesis is essential for early mouse post-implantation development. Nature Genet. 1999;21:444–448. doi: 10.1038/7788. [DOI] [PubMed] [Google Scholar]
- 35.Fan X, Molotkov A, Manabe SI, Donmoyer CM, Deltour L, Foglio MH, Cuenca AE, Blaner WS, Lipton SA, Duester G. Targeted disruption of Aldh1a1 (Raldh1) provides evidence for a complex mechanism of retinoic acid synthesis in the developing retina. Mol Cell Biol. 2003;23:4637–4648. doi: 10.1128/MCB.23.13.4637-4648.2003. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 36.Dupé V, Matt N, Garnier JM, Chambon P, Mark M, Ghyselinck NB. A newborn lethal defect due to inactivation of retinaldehyde dehydrogenase type 3 is prevented by maternal retinoic acid treatment. Proc Natl Acad Sci USA. 2003;100:14036–14041. doi: 10.1073/pnas.2336223100. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 37.Matt N, Dupé V, Garnier JM, Dennefeld C, Chambon P, Mark M, Ghyselinck NB. Retinoic acid-dependent eye morphogenesis is orchestrated by neural crest cells. Development. 2005;132:4789–4800. doi: 10.1242/dev.02031. [DOI] [PubMed] [Google Scholar]
- 38.Mic FA, Molotkov A, Molotkova N, Duester G. Raldh2 expression in optic vesicle generates a retinoic acid signal needed for invagination of retina during optic cup formation. Dev Dyn. 2004;231:270–277. doi: 10.1002/dvdy.20128. [DOI] [PubMed] [Google Scholar]
- 39.Wagner E, McCaffery P, Dräger UC. Retinoic acid in the formation of the dorsoventral retina and its central projections. Dev Biol. 2000;222:460–470. doi: 10.1006/dbio.2000.9719. [DOI] [PubMed] [Google Scholar]
- 40.Sen J, Harpavat S, Peters MA, Cepko CL. Retinoic acid regulates the expression of dorsoventral topographic guidance molecules in the chick retina. Development. 2005;132:5147–5159. doi: 10.1242/dev.02100. [DOI] [PubMed] [Google Scholar]
- 41.Dickman ED, Thaller C, Smith SM. Temporally-regulated retinoic acid depletion produces specific neural crest, ocular and nervous system defects. Development. 1997;124:3111–3121. doi: 10.1242/dev.124.16.3111. [DOI] [PubMed] [Google Scholar]