Targeted disruption of the mouse cis-retinol dehydrogenase gene: visual and nonvisual functions

A large animal model of RDH5-associated retinopathy recapitulates important features of the human phenotype

Pathogenic variants in retinol dehydrogenase 5 (RDH5) attenuate supply of 11-cis-retinal to photoreceptors leading to a range of clinical phenotypes including night blindness because of markedly slowed rod dark adaptation and in some patients, macular atrophy. Current animal models (such as Rdh5-/- mice) fail to recapitulate the functional or degenerative phenotype. Addressing this need for a relevant animal model we present a new domestic cat model with a loss-of-function missense mutation in RDH5 (c.542G > T; p.Gly181Val). As with patients, affected cats have a marked delay in recovery of dark adaptation. In addition, the cats develop a degeneration of the area centralis (equivalent to the human macula). This recapitulates the development of macular atrophy that is reported in a subset of patients with RDH5 mutations and is shown in this paper in seven patients with biallelic RDH5 mutations. There is notable variability in the age at onset of the area centralis changes in the cat, with most developing changes as juveniles but some not showing changes over the first few years of age. There is similar variability in development of macular atrophy in patients and while age is a risk factor, it is hypothesized that genetic modifying loci influence disease severity, and we suspect the same is true in the cat model. This novel cat model provides opportunities to improve molecular understanding of macular atrophy and test therapeutic interventions for RDH5-associated retinopathies.

Intracrine Regulation of Estrogen and Other Sex Steroid Levels in Endometrium and Non-gynecological Tissues; Pathology, Physiology, and Drug Discovery

Our understanding of the intracrine (or local) regulation of estrogen and other steroid synthesis and degradation expanded in the last decades, also thanks to recent technological advances in chromatography mass-spectrometry. Estrogen responsive tissues and organs are not passive receivers of the pool of steroids present in the blood but they can actively modify the intra-tissue steroid concentrations. This allows fine-tuning the exposure of responsive tissues and organs to estrogens and other steroids in order to best respond to the physiological needs of each specific organ. Deviations in such intracrine control can lead to unbalanced steroid hormone exposure and disturbances. Through a systematic bibliographic search on the expression of the intracrine enzymes in various tissues, this review gives an up-to-date view of the intracrine estrogen metabolisms, and to a lesser extent that of progestogens and androgens, in the lower female genital tract, including the physiological control of endometrial functions, receptivity, menopausal status and related pathological conditions. An overview of the intracrine regulation in extra gynecological tissues such as the lungs, gastrointestinal tract, brain, colon and bone is given. Current therapeutic approaches aimed at interfering with these metabolisms and future perspectives are discussed.

Pharmacological inhibition of ALDH1A enzymes suppresses weight gain in a mouse model of diet-induced obesity

BACKGROUND

Retinoic acid (RA) is known to play a role in weight regulation. Because mice without ALDH1A1, a major RA synthesizing enzyme, are resistant to diet-induced obesity, we tested a hypothesis that pharmacological inhibition of RA synthesis can suppress weight gain in a murine model of diet-induced obesity.

METHODS

C57BL/6J male mice were fed a high fat diet (HFD) for 8 weeks to induce obesity and then randomized to a HFD with or without WIN 18,446, an RA synthesis inhibitor, for an additional 9 weeks. Body weight, body composition, energy expenditure, activity, and food intake were measured. Levels of retinoids, lipids, and genes involved in the metabolism of retinoid and lipids were also determined.

RESULT

s Mice treated with WIN 18,446 gained significantly less weight and had decreased adipose tissue weight, adipocyte size, and macrophage infiltration in adipose tissue. In addition, we observed higher UCP1 expression in adipose tissues and decreased expression of RA responsive genes and genes involved in fatty acid synthesis in the livers and lungs of mice treated with WIN 18,446.

CONCLUSIONS

Pharmacological suppression of RA synthesis via inhibition of ALDH1A1 may be a potential target for treatment of obesity.

Lighting a candle in the dark: advances in genetics and gene therapy of recessive retinal dystrophies

Nonsyndromic recessive retinal dystrophies cause severe visual impairment due to the death of photoreceptor and retinal pigment epithelium cells. These diseases until recently have been considered to be incurable. Molecular genetic studies in the last two decades have revealed the underlying molecular causes in approximately two-thirds of patients. The mammalian eye has been at the forefront of therapeutic trials based on gene augmentation in humans with an early-onset nonsyndromic recessive retinal dystrophy due to mutations in the retinal pigment epithelium-specific protein 65kDa (RPE65) gene. Tremendous challenges still lie ahead to extrapolate these studies to other retinal disease-causing genes, as human gene augmentation studies require testing in animal models for each individual gene and sufficiently large patient cohorts for clinical trials remain to be identified through cost-effective mutation screening protocols.

Characterization of key residues and membrane association domains in retinol dehydrogenase 10

RDH10 (retinol dehydrogenase 10) was originally identified from the retinal pigment epithelium and retinal Müller cells. It has retinoid oxidoreductase activity and is thought to play a role in the retinoid visual cycle. A recent study showed that RDH10 is essential for generating retinoic acid at early embryonic stages. The present study demonstrated that wild-type RDH10 catalysed both oxidation of all-trans-retinol and reduction of all-trans-retinal in a cofactor-dependent manner In vitro. In cultured cells, however, oxidation is the favoured reaction catalysed by RDH10. Substitution of any of the predicted key residues in the catalytic centre conserved in the RDH family abolished the enzymatic activity of RDH10 without affecting its protein level. Unlike other RDH members, however, replacement of Ser(197), a key residue for stabilizing the substrate, by glycine and alanine did not abolish the enzymatic activity of RDH10, whereas RDH10 mutants S197C, S197T and S197V completely lost their enzymatic activity. These results suggest that the size of the residue at position 197 is critical for the activity of RDH10. Mutations of the three glycine residues (Gly(43), Gly(47) and Gly(49)) in the predicted cofactor-binding motif (Gly-Xaa(3)-Gly-Xaa-Gly) of RDH10 abolished its enzymatic activity, suggesting that the cofactor-binding motif is essential for its activity. Deletion of the two hydrophobic domains dissociated RDH10 from the membrane and abolished its activity. These studies identified the key residues for the activity of RDH10 and will contribute to the further elucidation of mechanism of this important enzyme.

The first bromodomain of Brdt, a testis-specific member of the BET sub-family of double-bromodomain-containing proteins, is essential for male germ cell differentiation

Brdt is a testis-specific member of the distinctive BET sub-family of bromodomain motif-containing proteins, a motif that binds acetylated lysines and is implicated in chromatin remodeling. Its expression is restricted to the germ line, specifically to pachytene and diplotene spermatocytes and early spermatids. Targeted mutagenesis was used to generate mice carrying a mutant allele of Brdt, Brdt(Delta)(BD1), which lacks only the first of the two bromodomains that uniquely characterize BET proteins. Homozygous Brdt(Delta)(BD1/)(Delta)(BD1) mice were viable but males were sterile, producing fewer and morphologically abnormal sperm. Aberrant morphogenesis was first detected in step 9 elongating spermatids, and those elongated spermatids that were formed lacked the distinctive foci of heterochromatin at the peri-nuclear envelope. Quantitative reverse transcription (RT)-PCR showed threefold increased levels of histone H1t (Hist1h1t) in Brdt(Delta)(BD1/)(Delta)(BD1) testes and chromatin immunoprecipitation revealed that Brdt protein, but not Brdt(DeltaBD1) protein, was associated with the promoter of H1t. Intracytoplasmic sperm injection suggested that the DNA in the Brdt(Delta)(BD1) mutant sperm could support early embryonic development and yield functional embryonic stem cells. This is the first demonstration that deletion of just one of the two bromodomains in members of the BET sub-family of bromodomain-containing proteins has profound effects on in vivo differentiation.

RDH10 is essential for synthesis of embryonic retinoic acid and is required for limb, craniofacial, and organ development

Regulation of patterning and morphogenesis during embryonic development depends on tissue-specific signaling by retinoic acid (RA), the active form of Vitamin A (retinol). The first enzymatic step in RA synthesis, the oxidation of retinol to retinal, is thought to be carried out by the ubiquitous or overlapping activities of redundant alcohol dehydrogenases. The second oxidation step, the conversion of retinal to RA, is performed by retinaldehyde dehydrogenases. Thus, the specific spatiotemporal distribution of retinoid synthesis is believed to be controlled exclusively at the level of the second oxidation reaction. In an N-ethyl-N-nitrosourea (ENU)-induced forward genetic screen we discovered a new midgestation lethal mouse mutant, called trex, which displays craniofacial, limb, and organ abnormalities. The trex phenotype is caused by a mutation in the short-chain dehydrogenase/reductase, RDH10. Using protein modeling, enzymatic assays, and mutant embryos, we determined that RDH10(trex) mutant protein lacks the ability to oxidize retinol to retinal, resulting in insufficient RA signaling. Thus, we show that the first oxidative step of Vitamin A metabolism, which is catalyzed in large part by the retinol dehydrogenase RDH10, is critical for the spatiotemporal synthesis of RA. Furthermore, these results identify a new nodal point in RA metabolism during embryogenesis.

Diseases caused by defects in the visual cycle: retinoids as potential therapeutic agents

Absorption of a photon by an opsin pigment causes isomerization of the chromophore from 11-cis-retinaldehyde to all-trans-retinaldehyde. Regeneration of visual chromophore following light exposure is dependent on an enzyme pathway called the retinoid or visual cycle. Our understanding of this pathway has been greatly facilitated by the identification of disease-causing mutations in the genes coding for visual cycle enzymes. Defects in nearly every step of this pathway are responsible for human-inherited retinal dystrophies. These retinal dystrophies can be divided into two etiologic groups. One involves the impaired synthesis of visual chromophore. The second involves accumulation of cytotoxic products derived from all-trans-retinaldehyde. Gene therapy has been successfully used in animal models of these diseases to rescue the function of enzymes involved in chromophore regeneration, restoring vision. Dystrophies resulting from impaired chromophore synthesis can also be treated by supplementation with a chromophore analog. Dystrophies resulting from the accumulation of toxic pigments can be treated pharmacologically by inhibiting the visual cycle, or limiting the supply of vitamin A to the eyes. Recent progress in both areas provides hope that multiple inherited retinal diseases will soon be treated by pharmaceutical intervention.

Aberrant metabolites in mouse models of congenital blinding diseases: formation and storage of retinyl esters

Regeneration of the visual chromophore, 11-cis-retinal, is a critical step in restoring photoreceptors to their dark-adapted conditions. This regeneration process, called the retinoid cycle, takes place in the photoreceptor outer segments and the retinal pigment epithelium (RPE). Disabling mutations in nearly all of the retinoid cycle genes are linked to human conditions that cause congenital or progressive defects in vision. Several mouse models with disrupted genes related to this cycle contain abnormal fatty acid retinyl ester levels in the RPE. To investigate the mechanisms of retinyl ester accumulation, we generated single or double knockout mice lacking retinoid cycle genes. All-trans-retinyl esters accumulated in mice lacking RPE65, but they are reduced in double knockout mice also lacking opsin, suggesting a connection between visual pigment regeneration and the retinoid cycle. Only Rdh5-deficient mice accumulate cis-retinyl esters, regardless of the simultaneous disruption of RPE65, opsin, and prRDH. 13-cis-Retinoids are produced at higher levels when the flow of retinoid through the cycle was increased, and these esters are stored in specific structures called retinosomes. Most importantly, retinylamine, a specific and effective inhibitor of the 11-cis-retinol formation, also inhibits the production of 13-cis-retinyl esters. The data presented here support the idea that 13-cis-retinyl esters are formed through an aberrant enzymatic isomerization process.

Functional Characterization of Mouse RDH11 as a Retinol Dehydrogenase Involved in Dark Adaptation in Vivo

We previously cloned mouse RDH11 (mRDH11) as a gene regulated by the transcription factor sterol regulatory element-binding proteins and showed that it is a retinol dehydrogenase expressed in non-ocular tissues such as the liver and testis and in the retina (Kasus-Jacobi, A., Ou, J., Bashmakov, Y. K., Shelton, J. M., Richardson, J. A., Goldstein, J. L., and Brown, M. S. (2003) J. Biol. Chem. 278, 32380-32389). It was proposed to function in the recycling of the visual chromophore 11-cis-retinal after photoisomerization by a bleaching light, a pathway referred to as the visual cycle. In this work, we describe our studies on the ocular function of mRDH11. We created a knockout mouse by replacing the mrdh11 coding sequence with the lacZ reporter gene for expression profiling. 5-Bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal) staining demonstrated active transcription of this gene in photoreceptor cells. We show by immunoblot analysis that mRDH11 is associated with retinal membranes purified from a non-outer segment fraction of the retina. No obvious retinal defect was found during development and aging of RDH11-deficient mice. The functional consequences of mRDH11 disruption were investigated by electroretinography. Dark adaptation was delayed by a factor of 2.5-3 compared with wild-type mice. However, the kinetics of 11-cis-retinal recycling during dark adaptation was not affected, suggesting that mRDH11 is not involved in the visual cycle. We propose that mRDH11 disruption affects retinoid metabolism in photoreceptor inner segments and delays the kinetics of dark adaptation through modulation of calcium homeostasis.

Opposing actions of cellular retinol-binding protein and alcohol dehydrogenase control the balance between retinol storage and degradation

Vitamin A homoeostasis requires the gene encoding cellular retinol-binding protein-1 (Crbp1) which stimulates conversion of retinol into retinyl esters that serve as a storage form of vitamin A. The gene encoding alcohol dehydrogenase-1 (Adh1) greatly facilitates degradative metabolism of excess retinol into retinoic acid to protect against toxic effects of high dietary vitamin A. Crbp1-/-/Adh1-/- double mutant mice were generated to explore whether the stimulatory effect of CRBP1 on retinyl ester formation is due to limitation of retinol oxidation by ADH1, and whether ADH1 limits retinyl ester formation by opposing CRBP1. Compared with wild-type mice, liver retinyl ester levels were greatly reduced in Crbp1-/- mice, but Adh1-/- mice exhibited a significant increase in liver retinyl esters. Importantly, relatively normal liver retinyl ester levels were restored in Crbp1-/-/Adh1-/- mice. During vitamin A deficiency, the additional loss of Adh1 completely prevented the excessive loss of liver retinyl esters observed in Crbp1-/- mice for the first 5 weeks of deficiency and greatly minimized this loss for up to 13 weeks. Crbp1-/- mice also exhibited increased metabolism of a dose of retinol into retinoic acid, and this increased metabolism was not observed in Crbp1-/-/Adh1-/- mice. Our findings suggest that opposing actions of CRBP1 and ADH1 enable a large fraction of liver retinol to remain esterified due to CRBP1 action, while continuously allowing some retinol to be oxidized to retinoic acid by ADH1 for degradative retinoid turnover under any dietary vitamin A conditions.

Delayed dark adaptation in 11-cis-retinol dehydrogenase-deficient mice: a role of RDH11 in visual processes in vivo

The oxidation of 11-cis-retinol to 11-cis-retinal in the retinal pigment epithelium (RPE) represents the final step in a metabolic cycle that culminates in visual pigment regeneration. Retinol dehydrogenase 5 (RDH5) is responsible for a majority of the 11-cis-RDH activity in the RPE, but the formation of 11-cis-retinal in rdh5-/- mice suggests another enzyme(s) is present. We have previously shown that RDH11 is also highly expressed in RPE cells and has dual specificity for both cis- and trans-retinoid substrates. To investigate the role of RDH11 in the retinoid cycle, we generated rdh11-/- and rdh5-/-rdh11-/- mice and examined their electrophysiological responses to various intensities of illumination and during dark adaptation. Retinoid profiles of darkadapted rdh11-/- mice did not show significant differences compared with wild-type mice, whereas an accumulation of cis-esters was detected in rdh5-/- and rdh5-/-rdh11-/- mice. Following light stimulation, 73% more cis-retinyl esters were stored in rdh5-/-rdh11-/- mice compared with rdh5-/- mice. Single-flash ERGs of rdh11-/- showed normal responses under dark- and light-adapted conditions, but exhibited delayed dark adaptation following high bleaching levels. Double knockout mice also had normal ERG responses in dark- and light-adapted conditions, but had a further delay in dark adaptation relative to either rdh11-/- or rdh5-/- mice. Taken together, these results suggest that RDH11 has a measurable role in regenerating the visual pigment by complementing RDH5 as an 11-cis-RDH in RPE cells, and indicate that an additional unidentified enzyme(s) oxidizes 11-cis-retinol or that an alternative pathway contributes to the retinoid cycle.

The specificity of alcohol dehydrogenase with cis-retinoids. Activity with 11-cis-retinol and localization in retina

Studies in knockout mice support the involvement of alcohol dehydrogenases ADH1 and ADH4 in retinoid metabolism, although kinetics with retinoids are not known for the mouse enzymes. Moreover, a role of alcohol dehydrogenase (ADH) in the eye retinoid interconversions cannot be ascertained due to the lack of information on the kinetics with 11-cis-retinoids. We report here the kinetics of human ADH1B1, ADH1B2, ADH4, and mouse ADH1 and ADH4 with all-trans-, 7-cis-, 9-cis-, 11-cis- and 13-cis-isomers of retinol and retinal. These retinoids are substrates for all enzymes tested, except the 13-cis isomers which are not used by ADH1. In general, human and mouse ADH4 exhibit similar activity, higher than that of ADH1, while mouse ADH1 is more efficient than the homologous human enzymes. All tested ADHs use 11-cis-retinoids efficiently. ADH4 shows much higher k(cat)/K(m) values for 11-cis-retinol oxidation than for 11-cis-retinal reduction, a unique property among mammalian ADHs for any alcohol/aldehyde substrate pair. Docking simulations and the kinetic properties of the human ADH4 M141L mutant demonstrated that residue 141, in the middle region of the active site, is essential for such ADH4 specificity. The distinct kinetics of ADH4 with 11-cis-retinol, its wide specificity with retinol isomers and its immunolocalization in several retinal cell layers, including pigment epithelium, support a role of this enzyme in the various retinol oxidations that occur in the retina. Cytosolic ADH4 activity may complement the isomer-specific microsomal enzymes involved in photopigment regeneration and retinoic acid synthesis.

Novel targeting strategy for generating mouse models with defects in the retinoid cycle

In addition to RDH5, other enzymes capable of oxidizing 11-cis-retinol are present within the retinal pigment epithelium, Müller cells and/or photoreceptors. Candidate proteins have meanwhile been identified. To study the physiological and pathological aspects of these enzymes, mice in which these genes are no longer functional are being generated. A fast-targeting strategy for the disruption of genes was developed. Generation of double and triple knockouts will aid in determining if these retinol dehydrogenases are responsible for the remaining 11-cis-retinol oxidation observed in RDH5 knockout animals.

Patterning of forelimb bud myogenic precursor cells requires retinoic acid signaling initiated by Raldh2

Limb skeletal muscle is derived from cells of the dermomyotome that detach and migrate into the limb buds to form separate dorsal and ventral myogenic precursor domains. Myogenic precursor cell migration is dependent on limb bud mesenchymal expression of hepatocyte growth factor/scatter factor (Hgf), which encodes a secreted ligand that signals to dermomyotome through the membrane receptor tyrosine kinase Met. Here, we find that correct patterning of Hgf expression in forelimb buds is dependent on retinoic acid (RA) synthesized by retinaldehyde dehydrogenase 2 (Raldh2) expressed proximally. Raldh2(-/-) forelimb buds lack RA and display an anteroproximal shift in expression of Hgf such that its normally separate dorsal and ventral expression domains are joined into a single anterior-proximal domain. Met and MyoD are expressed in this abnormal domain, indicating that myogenic cell migration and differentiation are occurring in the absence of RA, but in an abnormal location. An RA-reporter transgene revealed that RA signaling in the forelimb bud normally exists in a gradient across the proximodistal axis, but uniformly across the anteroposterior axis, with all proximal limb bud cells exhibiting activity. Expression of Bmp4, an inhibitor of Hgf expression, is increased and shifted anteroproximally in Raldh2(-/-) limb buds, thus encroaching into the normal expression domain of Hgf. Our studies suggest that RA signaling provides proximodistal information for limb buds that counterbalances Bmp signaling, which in turn helps mediate proximodistal and anteroposterior patterning of Hgf expression to correctly direct migration of Met-expressing myogenic precursor cells.

Genetic evidence that retinaldehyde dehydrogenase Raldh1 (Aldh1a1) functions downstream of alcohol dehydrogenase Adh1 in metabolism of retinol to retinoic acid

Vitamin A (retinol) is a nutrient that is essential for developmental regulation but toxic in large amounts. Previous genetic studies have revealed that alcohol dehydrogenase Adh1 is required for efficient clearance of excess retinol to prevent toxicity, thus demonstrating that the mechanism involves oxidation of excess retinol to retinoic acid (RA). Whereas Adh1 plays a dominant role in the first step of the clearance pathway (oxidation of retinol to retinaldehyde), it is unknown what controls the second step (oxidation of retinaldehyde to RA). We now present genetic evidence that aldehyde dehydrogenase Aldh1a1, also known as retinaldehyde dehydrogenase Raldh1, plays a dominant role in the second step of retinol clearance in adult mice. Serum RA levels following a 50 mg/kg dose of retinol were reduced 72% in Raldh1-/- mice and 82% in Adh1-/- mice. This represented reductions in RA synthesis of 77-78% for each mutant after corrections for altered RA degradation in each. After retinol dosing, serum retinaldehyde was increased 2.5-fold in Raldh1-/- mice (indicating defective retinaldehyde clearance) and decreased 3-fold in Adh1-/- mice (indicating defective retinaldehyde synthesis). Serum retinol clearance following retinol administration was decreased 7% in Raldh1-/- mice and 69% in Adh1-/- mice. LD50 studies indicated a small increase in retinol toxicity in Raldh1-/- mice and a large increase in Adh1-/- mice. These observations demonstrate that Raldh1 functions downstream of Adh1 in the oxidative metabolism of excess retinol and that toxicity correlates primarily with accumulating retinol rather than retinaldehyde.

Targeted disruption of Aldh1a1 (Raldh1) provides evidence for a complex mechanism of retinoic acid synthesis in the developing retina

Genetic studies have shown that retinoic acid (RA) signaling is required for mouse retina development, controlled in part by an RA-generating aldehyde dehydrogenase encoded by Aldh1a2 (Raldh2) expressed transiently in the optic vesicles. We examined the function of a related gene, Aldh1a1 (Raldh1), expressed throughout development in the dorsal retina. Raldh1(-/-) mice are viable and exhibit apparently normal retinal morphology despite a complete absence of Raldh1 protein in the dorsal neural retina. RA signaling in the optic cup, detected by using a RARE-lacZ transgene, is not significantly altered in Raldh1(-/-) embryos at embryonic day 10.5, possibly due to normal expression of Aldh1a3 (Raldh3) in dorsal retinal pigment epithelium and ventral neural retina. However, at E16.5 when Raldh3 is expressed ventrally but not dorsally, Raldh1(-/-) embryos lack RARE-lacZ expression in the dorsal retina and its retinocollicular axonal projections, whereas normal RARE-lacZ expression is detected in the ventral retina and its axonal projections. Retrograde labeling of adult Raldh1(-/-) retinal ganglion cells indicated that dorsal retinal axons project to the superior colliculus, and electroretinography revealed no defect of adult visual function, suggesting that dorsal RA signaling is unnecessary for retinal ganglion cell axonal outgrowth. We observed that RA synthesis in liver of Raldh1(-/-) mice was greatly reduced, thus showing that Raldh1 indeed participates in RA synthesis in vivo. Our findings suggest that RA signaling may be necessary only during early stages of retina development and that if RA synthesis is needed in dorsal retina, it is catalyzed by multiple enzymes, including Raldh1.

Cytosolic retinoid dehydrogenases govern ubiquitous metabolism of retinol to retinaldehyde followed by tissue-specific metabolism to retinoic acid

The ability of vitamin A (retinol) to control growth and development depends upon tissue-specific metabolism of retinol to retinoic acid (RA). RA then functions as a ligand for retinoid receptor signaling. Mouse genetic studies support a role for cytosolic alcohol dehydrogenases (ADH) in the first step (oxidation of retinol to retinaldehyde) and a role for cytosolic retinaldehyde dehydrogenases (RALDH) in the second step (oxidation of retinaldehyde to RA). Mice lacking ADH3 have reduced survival and a growth defect that can be rescued by dietary retinol supplementation, whereas the effect of a loss of ADH1 or ADH4 is noticed only in mice subjected to vitamin A excess or deficiency, respectively. Also, genetic deficiency of both ADH1 and ADH4 does not have additive effects, verifying separate roles for these enzymes in retinoid metabolism. As for the second step of RA synthesis, a null mutation of RALDH2 is embryonic lethal, eliminating most mesodermal RA synthesis, whereas loss of RALDH1 eliminates RA synthesis only in the embryonic dorsal retina with no obvious effect on development. Analysis of RA-rescued RALDH2 mutants has also revealed that RALDH3 and at least one additional enzyme produce RA tissue-specifically in embryos. Collectively, these genetic findings indicate that metabolism of retinol to retinaldehyde is not tissue-restricted as it is catalyzed by ubiquitously-expressed ADH3 (a low activity form) as well as by tissue-specifically expressed ADH1 and ADH4 (high activity forms). In contrast, further metabolism of retinaldehyde to RA is tissue-restricted as all enzymes identified are tissue-specific. An important concept to emerge is that selective expression of enzymes catalyzing the second step is what limits the tissues that can completely metabolize retinol to RA to initiate retinoid signaling.