Journal of Nutrition Animal Diets/Enrichment Products...

Home Help [Feedback] [For Subscribers] [Archive] [Search] [Contents]
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Purchase Article
Right arrow View Shopping Cart
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Dräger, U. C.
Right arrow Articles by McCaffery, P.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Dräger, U. C.
Right arrow Articles by McCaffery, P.

The Journal of Nutrition Vol. 128 No. 2 February 1998, pp. 463S-466S

Aldehyde Dehydrogenases in the Generation of Retinoic Acid in the Developing Vertebrate: A Central Role of the Eye1,2

Ursula C. Dräger3, Elisabeth Wagner, and Peter McCaffery

E. Kennedy Shriver Center, Waltham, MA 02254 and Department of Psychiatry, Harvard Medical School, Boston, MA 02115

    ABSTRACT
Abstract
References

In the developing vertebrate, retinoic acid is distributed in patterns that are highly regulated, both in the spatial and temporal domains. These patterns are generated by the localized expression of retinoic acid-synthesizing aldehyde dehydrogenases, which form the origins of retinoic acid-diffusion gradients in the surrounding tissues. The developing eye, known to be exceptionally vulnerable to vitamin A deficiency, is one of the retinoic acid-richest regions in the embryo. Several aldehyde dehydrogenases are expressed here, and they create a ventro-dorsal retinoic acid gradient in the embryonic retina. Aldehyde dehydrogenase expression persists in the mature eye and is stable, but the amount of retinoic acid synthesized is variable, depending on ambient light levels. This phenomenon is due to changing levels of the retinoic acid precursor retinaldehyde, which is released from illuminated rhodopsin, thus providing a mechanism by which light can directly influence gene expression. For arrestin mRNA, which is one of the factors known to be regulated by light, the light effect can be mimicked in the dark by injection of retinoic acid. The light-induced release of retinaldehyde from rhodopsin, which occurs only in vertebrate but not invertebrate photoreceptors, may have accelerated the rapid evolution of retinoic acid-mediated transcriptional regulation at the transition from invertebrates to vertebrates, and it may explain the prominent role of retinoic acid in the eye.

KEY WORDS: aldehyde dehydrogenases · retinaldehyde · retinoic acid · retina · rhodopsin

    PATTERNS OF ALDEHYDE DEHYDROGENASE EXPRESSION AND RETINOIC ACID TERATOGENICITY

Retinoic acid (RA), the most active form of vitamin A, is also one of the most potent teratogens. Among the list of known teratogens, exogenous RA is unusual in that it is identical to the endogenous compound and in that exogenous RA dosages that cause embryonic concentration levels well within the physiological range for transcriptional regulation can have teratogenic effects. The explanation for the teratogenicity of exogenous RA lies in the distribution of endogenous RA. In the developing embryo, RA concentrations are highly regulated, both spatially and temporally (Ang et al. 1996, Chen et al. 1992 and 1994, Creech Kraft et al. 1994, Durston et al. 1989, McCaffery and Dräger 1994): levels are very high (low micromolar range) at some locations and undetectably low at others, and levels change with developmental age, with high levels in the nervous system generally correlating with periods of neuronal differentiation (McCaffery and Dräger 1994, Yamamoto et al. 1996). The cellular responsiveness to RA, which is represented by the range of different RA receptors, is also present in developmentally regulated patterns, but its range exceeds the range of endogenous RA---practically every embryonic tissue expresses at least one, and usually several, RA receptors (Dollé et al. 1994 and 1990, Mangelsdorf et al. 1994). Uniform exposure of the embryo to micromolar RA levels will have little effect on the natural foci of high endogenous RA, but it will raise RA levels in the naturally low regions, causing spatially and temporally inappropriate RA responses.

A detailed knowledge of the distribution of endogenous RA is a necessary prerequisite for the analysis of RA teratogenicity. The conversion of retinol to retinaldehyde is reversible, and several members of the alcohol dehydrogenase family are able to catalyze it (Duester 1996). Most of the oxidation of retinaldehyde to RA is mediated by several functionally distinct isoforms of aldehyde dehydrogenases, which are expressed in spatially and temporally regulated patterns in the developing vertebrate (McCaffery et al. 1993). Comparisons of endogenous RA levels with aldehyde dehydrogenase levels in chunks of tissues dissected from developing mouse and zebrafish embryos have shown a good correlation between the two parameters: this applies to the trunk/spinal cord (Marsh-Armstrong et al. 1995, McCaffery and Dräger 1994), the cerebellar region (Yamamoto et al. 1996) and the eye, as long as the measurements are performed at low light levels (McCaffery et al. 1996) (for the effects of light, see below). Such a correlation implies that the distributions of aldehyde dehydrogenases, which can be much more easily visualized than the lipid RA, can be taken as coarse indicators for the RA distribution. When viewed at a higher resolution, however, the two parameters are related but cannot be distributed identically: because diffusion of the lipid RA through solid tissue is not impeded by cell membranes, the aldehyde dehydrogenase sites indicate the origins of RA diffusion gradients. These gradients are probably of morphogenetic significance, because different genes are known to differ in RA response thresholds. Among other effects, the RA gradients are likely to direct the spatially nested expression of some homeobox genes (Izpisua-Belmonte et al. 1991), which are known to respond sequentially to graded RA levels (Boncinelli et al. 1991, Simeone et al. 1990).

    RETINOIC ACID IN EYE FORMATION

In the first part of this century, when vitamin A was recognized as an essential nutrient, nightblindness was observed as an early manifestation of vitamin A deficiency. This symptom is explained by the essential role of vitamin A aldehyde in vision: retinaldehyde, bound covalently to rhodopsin, forms the light-receptive component on which visual perception is based (Wald 1968). Later it became obvious that the exceptional vitamin A dependence of the eye extends to developmental stages long before any opsin is present. Partial vitamin A deficiency in pregnant pigs (Hale 1937) and rats (Warkany and Schraffenberger 1946) was found to affect foremost the eyes in the developing embryos: the offspring might look more or less normal, except that the eyes would be too small or missing. Although the normal embryonic eye seems morphologically uniform, global vitamin A deprivation reduces mainly the size of the ventral half (Warkany and Schraffenberger 1946). The symptoms of vitamin A deficiency have recently been reproduced in RA receptor knock-out mice: when the eye is affected, preferentially the ventral half is reduced in size (Kastner et al. 1994). This selective effect on the ventral eye cannot be explained by the normal distribution of RA receptors, which seem to be symmetrically expressed throughout ocular tissues.

The existence of molecular asymmetries along the two retinal axes (dorso-ventral and antero-posterior) had been postulated in a different context, as the basis of positional information that is necessary for the formation of topographically ordered connections between the retina and brain (Sperry 1963). In a search for the determinants of positional information along the dorso-ventral axis of the embryonic retina, we found an axial asymmetry in RA-generating aldehyde dehydrogenases (McCaffery and Dräger 1993, McCaffery et al. 1991, 1992 and 1993). The retinas of all embryonic vertebrates express a class-1 aldehyde dehydrogenase in the dorsal part and a novel, more powerful aldehyde dehydrogenase ventrally; in mice, the dorsal enzyme is called AHD2 and the ventral enzyme V1. In all early embryos tested, the aldehyde dehydrogenase arrangement results in an RA gradient along the ventro-dorsal axis of the retina (Marsh-Armstrong et al. 1994, McCaffery et al. 1992 and 1993, Mey et al. 1997).

The tissue around the embryonic eye and the retinal pigment epithelium express a third aldehyde dehydrogenase, named V2 or RALDH2 (McCaffery and Dräger 1993, McCaffery et al. 1993, Niederreither et al. 1997, Zhao et al. 1996). RALDH2 is the earliest RA-generating enzyme detectable in the embryo, both in the eye region as well as the mesoderm surrounding Hensen's node (Dräger and McCaffery 1995, McCaffery et al. 1991 and 1993, Niederreither et al. 1997). In the eye region, RALDH2 is first detected biochemically around the time when the location of the future eyes becomes visible in the forebrain folds as optic pits. In situ hybridizations show an asymmetric expression restricted to one edge of the eye anlage (Niederreither et al. 1997), an arrangement that will create an RA gradient across the eye field.

Experimental manipulations of the early stages of eye development are performed much more easily in the freely accessible zebrafish embryo than in mice. When early zebrafish embryos, during the stage of optic-anlage formation, are briefly exposed to citral, a competitive inhibitor of RA-generating dehydrogenase, they develop into larvae that lack the ventral part of the eye (Marsh-Armstrong et al. 1994). The opposite effect is observed when early zebrafish are exposed uniformly to RA, in the form of 1 µM of RA dissolved in the tank water: they grow into larvae whose eyes appear duplicated along the dorso-ventral axis (Hyatt et al. 1992). In normal zebrafish, msh[c] and the dorsal aldehyde dehydrogenase represent molecular markers for the dorsal retina, and pax[b] and ventral aldehyde dehydrogenase are markers for ventral retina (Hyatt et al. 1996). Analyses of RA-treated and duplicated eyes for dorsal and ventral markers reveal that the applied RA has little effect on the ventral expression of normal ventral characteristics, but in the dorsal part, the exogenous RA suppresses normal dorsal markers and activates ventral markers (Hyatt et al. 1996). Moreover, when the early eye anlage is exposed to an artificial local source of RA through placement of an RA-soaked bead next to it, an ectopic fissure is induced in the developing eye at a location facing the bead (Hyatt et al. 1996); in the normal eye, the optic fissure marks the position of the ventral retinal pole.

All three of the effects on the shape of the zebrafish eye can be achieved only when RA levels are altered during a brief critical period of a few hours at the eye anlage stage, and they take several days to develop into morphologically apparent malformations (Hyatt et al. 1996). After the critical period, the dorso-ventral organization of the future eye becomes refractory to RA manipulations and remains irreversibly fixed. The responsiveness to RA manipulations during the critical period indicates that for a brief time the entire eye field is competent to respond to RA by activating ventral retinal characteristics; dorsal characteristics can be interpreted to represent the default state in the absence of RA or at low RA levels (Marsh-Armstrong et al. 1994). In the early mouse embryo, the site of RALDH2 expression, located eccentrically in the eye anlage (Niederreither et al. 1997), is likely to represent an RA-polarizing center for determination of the dorso-ventral axis, by activating ventral characteristics in the region closest to the RA origin. The brief critical period of RA responsiveness has the properties postulated by experimental embryologists for the hypothetical determination event of the dorso-ventral retinal axis (Jacobson 1991, Sperry 1963). This early polarization event induces directly the permanent, axially segregated expression of different aldehyde dehydrogenases (Hyatt et al. 1996). By propagating the axial orientation, the aldehyde dehydrogenase pattern represents the cellular memory for positional information, through which the early polarization event can be effective throughout development into adulthood (Dräger and McCaffery 1997).

    RETINOIC ACID SYNTHESIS IN THE POSTNATAL EYE

In the embryonic eye, levels of aldehyde dehydrogenases are a reliable indicator for levels of RA. Postnatally, however, a discrepancy appears that increases with maturation of vision: eyes of light-exposed mice generate relatively too much RA for the levels of aldehyde dehydrogenase present (McCaffery et al. 1996); eyes of dark-adapted mice contain less RA than light-exposed eyes. The light-induced RA increase is a direct corollary of the visual cycle in vertebrate photoreceptors: the visual chromophore 11-cis retinaldehyde, bound covalently via Schiff-base linkage to rhodopsin, is isomerized by light to all-trans retinaldehyde. This causes hydrolysis of the Schiff bond and release of free all-trans retinaldehyde, which diffuses away in order to be regenerated to 11-cis retinaldehyde. The re-isomerization process takes place in the retinal pigment epithelium, the tissue adjoining the photoreceptors (Saari 1994). Because the neural retina and the retinal pigment epithelium express high levels of cytosolic aldehyde dehydrogenases, and because the oxidation of retinaldehyde to RA is an irreversible reaction, conversion of some of the light-released all-trans retinaldehyde to RA is unavoidable. In addition to the electric visual signal, excitation of vertebrate rhodopsin by light thus generates a transcriptional signal (McCaffery et al. 1996); see Figure 1.


View larger version (18K):
[in this window]
[in a new window]
  		Fig 1. Schematic of vitamin A usage in the eye to illustrate the link between its two roles in vision and in transcriptional control. The conversion of retinol to retinaldehyde is reversible, but the conversion of retinaldehyde to retinoic acid (RA) is irreversible. Isomerization of 11-cis retinaldehyde to the all-trans form initiates the visual transduction cascade that eventually leads to visual perception. As the regeneration of 11-cis retinaldehyde takes place in the retinal pigment epithelium, some of the all-trans retinaldehyde, released from rhodopsin, is converted to the transcriptional activator RA.

Expression of several photoreceptor proteins is known to vary with the daily dark:light cycle, and a circadian clock that can be entrained and reset by light has been localized to photoreceptors (Bowes et al. 1988, Cahill and Besharse 1995, Farber et al. 1991, Korenbrot and Fernald 1989, McGinnis et al. 1994). In all cases where the mechanism of the light effect has been analyzed, the regulation was found to occur at the transcriptional level. To address the question of whether the light effect is mediated through RA, we tested arrestin, a protein that functions in the termination of visual excitation by capping illuminated and phosphorylated rhodopsin. We found that RA, injected in the dark into dark-adapted mice, exerts the same effect on arrestin mRNA as exposure of the mice to light (Wagner et al. 1997).

    A HYPOTHESIS LINKING THE TRANSCRIPTIONAL RETINOID USE TO ITS ROLE IN VISION

In vertebrates, expression of a very large number of proteins is known to be regulated by RA (Gudas et al. 1994). Invertebrates have close homologs to most of these proteins, but here their expression does not require RA. The receptors for all-trans RA, the RARs, have so far been found only in vertebrates (Mangelsdorf et al. 1995). In the ultraspiracle locus, Drosophila contains a gene that codes for a nuclear receptor that is homologous to the 9-cis RA receptors, the RXRs, but transcriptional regulation by ultraspiracle does not seem to be influenced by any retinoid (Mangelsdorf et al. 1994). Transcription of Drosophila opsin was reported to be upregulated by RA (Picking et al. 1996, Sun et al. 1993), but no receptor mediating this effect is yet known. Invertebrate and vertebrate photoreceptors differ in their visual cycle, i.e., the mechanism by which the photo-isomerization product all-trans retinaldehyde is regenerated to 11-cis retinaldehyde: in vertebrate photoreceptors, the all-trans retinaldehyde is released in order to be regenerated elsewhere, but in invertebrates, all-trans retinaldehyde remains covalently bound to the opsin moiety, to be regenerated in situ by light of a wavelength different from that of the excitation light (Saari 1994).

Aldehyde dehydrogenases are evolutionarily very ancient (Lindahl 1992), and isoforms able to convert retinaldehyde to RA can be detected in Drosophila, although it is not clear whether this conversion is part of their physiological role (Wagner, E., McCaffery, P. and Dräger, U. C., unpublished data). The general plan for eye development (Quiring et al. 1994), including a component of the transcriptional control system for dorso-ventral axis formation, seems to be conserved between the Drosophila and mammalian eye: like the genetic elimination of RXR in mice (Kastner et al. 1994), elimination of ultraspiracle function in the Drosophila eye causes a selective reduction in the size of the ventral eye (Oro et al. 1992). At the transition from the invertebrate to the vertebrate visual cycle, when the photo-isomerized retinaldehyde started to be released in response to light, aldehyde dehydrogenases present in the eye region probably served an essential protective role. By oxidizing the fraction of retinaldehyde that escapes the regeneration cycle, they protect the tissue from highly toxic free aldehyde; total bound retinaldehyde levels in the vertebrate retina are in the low millimolar range (Saari 1994). Because the stiff lipophilic nature of retinoids renders them ideal ligands for forcing conformations onto proteins (Wald 1968), some of the formed RA may have associated with previously existing orphan receptors, directing their rapid evolution into bona fide RA receptors. The functioning eye may thus represent the initial site of the genomic actions of vitamin A in evolution, with phototransduction proteins forming the earliest targets to be regulated by RA. Subsequently, RA-mediated control may have expanded to ocular development, as well as to a multitude of other processes. Rather than representing a new patterning system by itself, regulation by RA is likely to constitute a novel layer of control imparted over evolutionarily more ancient systems.

    FOOTNOTES
1   Presented as part of the symposium "Functional Metabolism of Vitamin A in Embryonic Development" given at the Experimental Biology 97 meeting, April 9, 1997, New Orleans, LA. This symposium was sponsored by the American Society for Nutritional Sciences and supported in part by Hoffman-LaRoche Inc. and Johnson & Johnson. Guest editor for the symposium publication was Maija H. Zile, Department of Food Science and Human Nutrition, Michigan State University, East Lansing, MI.
2   Supported by National Institutes of Health grants EY-01938 and HD-05515.
3   To whom correspondence should be addressed.

    LITERATURE CITED
Abstract
References
0022-3166/98 $3.00 ©1998 American Society for Nutritional Sciences
[Medline]



This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Purchase Article
Right arrow View Shopping Cart
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Dräger, U. C.
Right arrow Articles by McCaffery, P.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Dräger, U. C.
Right arrow Articles by McCaffery, P.

Home Help [Feedback] [For Subscribers] [Archive] [Search] [Contents]