Elisabeth Wagner

A retinoic acid synthesizing enzyme in ventral retina and telencephalon of the embryonic mouse

Most retinoic acid (RA) in the embryonic mouse is generated by three retinaldehyde dehydrogenases (RALDHs). RALDH1 (also called E1, AHD2 or ALDH1) is expressed in the dorsal retina, and RALDH2 (V2, ALDH11) generates most RA in the embryonic trunk. The third one, RALDH3 (V1), synthesizes the bulk of RA in the head of the early embryo. We show here that RALDH3 is a mouse homologue to ALDH6, an aldehyde dehydrogenase cloned from adult human salivary gland (Hsu, L.C., Chang, W.-C., Hiraoka, L., Hsien, C.-L., 1994. Molecular cloning, genomic organization, and chromosomal localization of an additional human aldehyde dehydrogenase gene, ALDH6. Genomics 24, 333-341), which was recently reported to act as a RALDH (Yoshida, A., Rzhetsky, A., Hsu, L.C., Chang, C., 1998. Human aldehyde dehydrogenase gene family. Eur. J. Biochem. 251, 549-557). RALDH3 expression begins in the surface ectoderm over the optic recess. In rapidly changing expression patterns it labels the appearance of several ectodermal structures: it marks the formation of the lens and the olfactory organ from ectodermal placodes, and it delineates the beginning eyelid field. Within the optic vesicle, RALDH3 is expressed in the ventral retina and the dorsal pigment epithelium. In the telencephalon, RALDH3 is expressed at high levels in the lateral part of the ganglionic eminence. From here it extends via the piriform cortex into the lower part of the septum. Of the three RALDHs, RALDH3 shows the strongest predilection for epithelia.

DORSAL AND VENTRAL RETINAL TERRITORIES DEFINED BY RETINOIC ACID SYNTHESIS,BREAK-DOWN AND NUCLEAR RECEPTOR EXPRESSION

Determination of the dorso-ventral dimension of the vertebrate retina is known to involve retinoic acid (RA), in that high RA activates expression of a ventral retinaldehyde dehydrogenase and low RA of a dorsal dehydrogenase. Here we show that in the early eye vesicle of the mouse embryo, expression of the dorsal dehydrogenase is preceded by, and transiently overlaps with, the RA-degrading oxidase CYP26. Subsequently in the embryonic retina, CYP26 forms a narrow horizontal boundary between the dorsal and ventral dehydrogenases, creating a trough between very high ventral and moderately high dorsal RA levels. Most of the RA receptors are expressed uniformly throughout the retina except for the RA-sensitive RARbeta, which is down-regulated in the CYP26 stripe. The orphan receptor COUP-TFII, which modulates RA responses, colocalizes with the dorsal dehydrogenase. The organization of the embryonic vertebrate retina into dorsal and ventral territories divided by a horizontal boundary has parallels to the division of the Drosophila eye disc into dorsal, equatorial and ventral zones, indicating that the similarities in eye morphogenesis extend beyond single molecules to topographical patterns.

Retinoic acid signaling in the brain marks formation of optic projections, maturation of the dorsal telencephalon, and function of limbic sites.

As retinoic acid (RA) is known to regulate the expression of many neuronal proteins, it is likely to influence overall development and function of the brain; few particulars, however, are available about its role in neurobiological contexts due mainly to problems in RA detection. To ask whether the function of RA in the rostral brain is concentrated in particular neurobiological systems, we compared sites of RA synthesis and actions, as detected by RA signaling in reporter mice, for embryonic and adult ages. We found that most sites of RA actions in the forebrain do not colocalize with RA synthesis, consistent with a dominant RA supply by diffusion and the circulation. The changing RA patterns distinguish preferentially two complex functional schemes. (1) Within the visual system when the first optic axons grow toward their targets, RA signaling delineates the topographical adjustment of the retinal map, which is encoded in the coordinates of the visual world, to central visual maps, which are formed in the segmental brain coordinates. (2) The second scheme begins early in forebrain morphogenesis as a distinction of the dorsal telencephalon. With progressing development, and in the adult, the RA patterns then focus on widely distributed structures, most of which belong to the limbic system. These are sites in which emotional perception is combined with higher cognitive processes and in which normal function requires ongoing remodeling of synaptic connections, indicating that the developmental role of RA in promotion of neuronal differentiation programs continues in the adult brain for highly flexible neural circuits. J. Comp. Neurol. 470:297–316, 2004.

Aldehyde Dehydrogenases in the Generation of Retinoic Acid in the Developing Vertebrate: A Central Role of the Eye

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.

Retinoic acid delineates the topography of neuronal plasticity in postnatal cerebral cortex

Retinoic acid is well recognized to promote neuronal differentiation in the embryonic nervous system, but how it influences the postnatal cerebral cortex remains largely unknown. The domain of highest retinoic acid actions in the cortex of the mouse constricts postnatally to a narrow band that includes the dorsal visual stream and the attentional and executive networks. This band of cortex, which is distinguished by the retinoic acid‐synthesizing enzyme RALDH3, exhibits signs of delayed maturation and enhanced plasticity compared to the surrounding cortex, as indicated by suppression of parvalbumin, neurofilament, cytochrome oxidase and perineuronal net maturation, and persistence of the embryonic, polysialated form of the neural cell‐adhesion molecule PSA‐NCAM. During the first postnatal week, the RALDH3‐expressing territory translocates in the caudal cortex from the medial limbic lobe to the adjacent neocortex. This topographical shift requires the neurotrophin NT‐3 because in mice lacking neuronal NT‐3 the RALDH3 enzyme maintains its early postnatal pattern up to adulthood. In the NT‐3‐null mutants, expression of the markers, whose topography colocalizes with RALDH3 in the normal cortex, matches the abnormal RALDH3 pattern. This indicates that the uneven retinoic acid distribution serves a role in patterning the maturation and to some extent function of the normal postnatal cerebral cortex.

The role and evolutionary development of retinoic-acid signalling in the eye

From the earliest time of its discovery as an essential nutrient, vitamin A has been known to be vital for the eye, both for its development and for its adult function. In the mature organism, the earliest sign of vitamin A deficiency is night-blindness, and in the developing embryo vitamin A deficiency causes the ventral eye to be defective, leading to micro- or anophthalmia [1, 2]. Because the oxidation of retinaldehyde to retinoic acid (RA) is an irreversible reaction, it was possible to assay their biological roles separately [3]. These assays revealed that when adult rats are fed a diet completely lacking in vitamin A (retinol and β-carotene) and are given RA, they survive but turn blind due to photoreceptor degeneration. This illustrates the two distinct functions of the retinoids: 11-cis retinaldehyde forms the visual chromophore of rhodopsin, and RA regulates gene transcription throughout the body. In this brief review we will summarize evidence that these two functions, which are commonly studied in different fields of science, are connected in the eye. We discovered highly intricate expression patterns of different RA-generating aldehyde dehydrogenases in the developing and mature eye, and in the functioning eye we find that light causes an increase in RA. These observations indicate that the transcriptional role of RA may have its origin in vision.