Retinoic acid synthesis in mouse embryos during gastrulation and craniofacial development linked to class IV alcohol dehydrogenase gene expression

Vitamin A and infancy. Biochemical, functional, and clinical aspects

Vitamin A is a very intriguing natural compound. The molecule not only has a complex array of physiological functions, but also represents the precursor of promising and powerful new pharmacological agents. Although several aspects of human retinol metabolism, including absorption and tissue delivery, have been clarified, the type and amounts of vitamin A derivatives that are intracellularly produced remain quite elusive. In addition, their precise function and targets still need to be identified. Retinoic acids, undoubtedly, play a major role in explaining activities of retinol, but, recently, a large number of physiological functions have been attributed to different retinoids and to vitamin A itself. One of the primary roles this vitamin plays is in embryogenesis. Almost all steps in organogenesis are controlled by retinoic acids, thus suggesting that retinol is necessary for proper development of embryonic tissues. These considerations point to the dramatic importance of a sufficient intake of vitamin A and explain the consequences if intake of retinol is deficient. However, hypervitaminosis A also has a number of remarkable negative consequences, which, in same cases, could be fatal. Thus, the use of large doses of retinol in the treatment of some human diseases and the use of megavitamin therapy for certain chronic disorders as well as the growing tendency toward vitamin faddism should alert physicians to the possibility of vitamin overdose.

Too much of a good thing: retinoic acid as an endogenous regulator of neural differentiation and exogenous teratogen

Retinoic acid (RA) is essential for both embryonic and adult growth, activating gene transcription via specific nuclear receptors. It is generated, via a retinaldehyde intermediate, from retinol (vitamin A). RA levels require precise regulation by controlled synthesis and catabolism, and when RA concentrations deviate from normal, in either direction, abnormal growth and development occurs. This review describes: (i) how the pattern of RA metabolic enzymes controls the actions of RA; and (ii) the type of abnormalities that result when this pattern breaks down. Examples are given of RA control of the anterior/posterior axis of the hindbrain, the dorsal/ventral axis of the spinal cord, as well as certain sex-specific segments of the spinal cord, using varied animal models including mouse, quail and mosquitofish. These functions are highly sensitive to abnormal changes in RA concentration. In rodents, the control of neural patterning and differentiation are disrupted when RA concentrations are lowered, whereas inappropriately high concentrations of RA result in abnormal development of cerebellum and hindbrain nuclei. The latter parallels the malformations seen in the human embryo exposed to RA due to treatment of the mother with the acne drug Accutane (13-cis RA) and, in cases where the child survives beyond birth, a particular set of behavioural anomalies can be described. Even the adult brain may be susceptible to an imbalance of RA, particularly the hippocampus. This report shows how the properties of RA as a neural induction agent and organizer of segmentation can explain the consequences of RA depletion and overexpression.

Retinoic acid-induced developmental defects are mediated by RARbeta/RXR heterodimers in the pharyngeal endoderm

Fusion and hypoplasia of the first two branchial arches, a defect typically observed in retinoic acid (RA) embryopathy, is generated in cultured mouse embryos upon treatment with BMS453, a synthetic compound that exhibits retinoic acid receptor beta (RARbeta) agonistic properties in transfected cells. By contrast, no branchial arch defects are observed following treatment with synthetic retinoids that exhibit RARalpha or RARgamma agonistic properties. The BMS453-induced branchial arch defects are mediated through RAR activation, as they are similar to those generated by a selective pan-RAR agonist, are prevented by a selective pan-RAR antagonist and cannot be mimicked by exposure to a pan-RXR agonist alone. They are enhanced in the presence of a pan-RXR agonist, and cannot be generated in Rarb-null embryos. Furthermore, they are accompanied, in the morphologically altered region, by ectopic expression of Rarb and of several other direct RA target genes. Therefore, craniofacial abnormalities characteristic of the RA embryopathy are mediated through ectopic activation of RARbeta/RXR heterodimers, in which the ligand-dependent activity of RXR is subordinated to that of RARbeta. Endodermal cells lining the first two branchial arches respond to treatment with the RARbeta agonist, in contrast to neural crest cells and ectoderm, which suggests that a faulty endodermal regionalization is directly responsible for RA-induced branchial arch dysmorphologies. Additionally, we provide the first in vivo evidence that the synthetic RARbeta agonist BMS453 exhibits an antagonistic activity on the two other RAR isotypes.

Distribution of class I, III and IV alcohol dehydrogenase mRNAs in the adult rat, mouse and human brain

The localization of different classes of alcohol dehydrogenases (ADH) in the brain is of great interest because of their role in both ethanol and retinoic acid metabolism. Conflicting data have been reported in the literature. By Northern blot and enzyme activity analyses only class III ADH has been detected in adult brain specimens, while results from riboprobe in situ hybridization indicate class I as well as class IV ADH expression in different regions of the rat brain. Here we have studied the expression patterns of three ADH classes in adult rat, mouse and human tissues using radioactive oligonucleotide in situ hybridization. Specificity of probes was tested on liver and stomach control tissue, as well as tissue from class IV ADH knock-out mice. Only class III ADH mRNA was found to be expressed in brain tissue of all three investigated species. Particularly high expression levels were found in neurons of the red nucleus in human tissue, while cortical neurons, pyramidal and granule cells of the hippocampus and dopamine neurons of substantia nigra showed moderate expression levels. Purkinje cells of cerebellum were positive for class III ADH mRNA in all species investigated, whereas granular layer neurons were positive only in rodents. The choroid plexus was highly positive for class III ADH, while no specific signal for class I or class IV ADH was detected. Our results thus support the notion that the only ADH expressed in adult mouse, rat and human brain is class III ADH.

Comparative expression analysis of Adh3 during arthropod, urochordate, cephalochordate, and vertebrate development challenges its predicted housekeeping role

Gene and genome duplications in the vertebrate lineage explain the complexity of extant gene families. Among these, the medium-chain alcohol dehydrogenase (ADH), which expanded by tandem duplications after the cephalochordate-vertebrate split, is a good model with which to analyze the evolution of gene function. Although the ancestral member of this family, ADH3, has been strictly conserved throughout animal evolution, its physiological role is still controversial. Previous evidence indicates that it contributes to formaldehyde cytoprotection, retinoic acid metabolism, and nitric oxide homeostasis. We performed in situ hybridization during Drosophila, ascidian (Ciona intestinalis), and zebrafish (Danio rerio) development. We showed that Adh3 expression was restricted to the fat body in Drosophila embryos at stage 17 and to the anterior endoderm in C. intestinalis tail bud, whereas in the zebrafish 2.5-day larvae the signal appeared widespread. A more comprehensive expression analysis including amphioxus and mice revealed that ancestral Adh3 was tissue specific, whereas a widespread expression was later attained in vertebrates. These variations occurred concomitantly with the expansion of the ADH family and the acquisition of new functions but were unlinked to the genomic changes that led to the transition from fractional to global methylation in vertebrates. Our data challenge the housekeeping role of ADH3 and question its involvement in the prevertebrate retinoic acid pathway.

Gene structure and minimal promoter of mouse rdh1

Mouse rdh1 encodes retinol dehydrogenase type 1 (RDH1), a short-chain dehydrogenase, which recognizes as substrates all-trans-retinol, 9-cis-retinol, 5alpha-androstan-3,17-diol and 5alpha-androstan-3-ol-17-one. RDH1 is the most efficient known mouse short-chain dehydrogenase that catalyzes dehydrogenation of all-trans-retinol, and contributes to a reconstituted path of all-trans-retinoic acid biosynthesis, when coexpressed in reporter cells with any one of three retinal dehydrogenases. Rdh1 shows widespread, if not ubiquitous, mRNA expression in the mouse beginning no later than embryo day 7. Here we report genomic organization, chromosomal localization and analysis of a minimum promoter of mouse rdh1. Rdh1 consists of four exons and three introns and spans approximately 14412 bp. Rdh1 is a single copy gene that maps to chromosome 10D3 with rdh5-9, but no known disorder maps precisely to rdh1. Rdh1 has three transcription start sites in kidney and one start site in liver. The rdh1 5'-region between -424 and +43 induces transcription maximally in COS7, mouse kidney RAG, and mouse liver NMu3Li cells. This section has no TATA box, but has a CCAAT box beginning 65 bp upstream of the major transcription start site, which is required for transcription of transfected reporter constructs. An AP1 binding site at -119 also activates transfected reporter constructs, and mediates 2-O-tetradecanoylphorbol-13-acetate (TPA) induced transcription. All-trans-retinoic acid antagonizes the TPA affect; however, no RARE or RXRE was found in the proximal promoter region, consistent with indirect regulation by all-trans-retinoic acid.

Human class IV alcohol dehydrogenase: kinetic mechanism, functional roles and medical relevance

Human alcohol dehydrogenase (ADH) constitutes a complex family. Class IV ADH (ADH4) is characteristic in its epithelial expression in the aerodigestive tract and high V(max) and K(m) for oxidation of ethanol. ADH4 exhibits the highest catalytic efficiency for retinol oxidation in human ADH family. Initial velocity, product inhibition, and dead-end inhibition studies indicate that ADH4, when functioning as ethanol dehydrogenase, conforms to an ordered sequential mechanism with coenzyme binding first and releasing last in catalytic cycle. When functioning as retinol dehydrogenase, the mechanism of ADH4 deduced from steady-state kinetic and equilibrium-binding studies is best described as a rapid equilibrium random mechanism with two dead-end ternary complex for retinol oxidation and a rapid equilibrium ordered mechanism with one dead-end ternary complex for retinal reduction, a unique mechanistic form for zinc-containing ADHs in the medium chain dehydrogenase/reductase superfamily. Kinetic and genetic studies support the proposal that ADH4 may play two important physiological roles, i.e., as a major contributor to first-pass metabolism of ethanol in stomach as well as involvement in the synthesis of retinoic acid, a hormonal ligand controlling a nuclear receptor signaling pathway that regulates growth, development, and epithelial maintenance. Quantitative simulation studies indicate that retinol metabolism through ADH pathway can be inhibited to a significant extent during alcohol consumption. The perturbation of retinoic acid synthesis by ethanol may underlie the pathogenesis of fetal alcohol syndrome and alcohol-related upper digestive tract cancer.

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.

The role of vitamin A in mammalian reproduction and embryonic development

Since the late 1980s, there has been an explosion of information on the molecular mechanisms and functions of vitamin A. This review focuses on the essential role of vitamin A in female reproduction and embryonic development and the metabolism of vitamin A (retinol) that results in these functions. Evidence strongly supports that in situ-generated all-trans retinoic acid (atRA) is the functional form of vitamin A in female reproduction and embryonic development. This is supported by the ability to reverse most reproductive and developmental blocks found in vitamin A deficiency with atRA, the block in embryonic development that occurs in retinaldehyde dehydrogenase type 2 null mutant mice, and the essential roles of the retinoic acid receptors, at least in embryogenesis. Early studies of embryos from marginally vitamin A-deficient (VAD) pregnant rats revealed a collection of defects called the vitamin A-deficiency syndrome. The manipulation of all-trans retinoic acid (atRA) levels in the diet of VAD female rats undergoing a reproduction cycle has proved to be an important new tool in deciphering the points of atRA function in early embryos and has provided a means to generate large numbers of embryos at later stages of development with the vitamin A-deficiency syndrome. The essentiality of the retinoid receptors in mediating the activity of atRA is exemplified by the many compound null mutant embryos that now recapitulate both the original vitamin A-deficiency syndrome and exhibit a host of new defects, many of which can also be observed in the VAD-atRA-supported rat embryo model and in retinaldehyde dehydrogenase type 2 (RALDH2) mutant mice. A major task for the future is to elucidate the atRA-dependent pathways that are normally operational in vitamin A-sufficient animals and that are perturbed in deficiency, thus leading to the characteristic VAD phenotypes described above.

Increased susceptibility to retinoid-induced teratogenesis in TGF-beta2 knockout mice

Transforming growth factor-beta (TGF-beta) and retinoic acid (RA) have been implicated in normal and abnormal embryonic development. The aim of this study was to investigate the effect of TGF-beta2 gene deletion on susceptibility to RA-induced teratogenesis in a mouse model. TGF-beta2 heterozygous or wild-type mice were mated and the dams dosed with a teratogenic dose of RA, or with control vehicle. The incidence of RA-induced cleft palate (CP) was 48% in wild-type embryos from wild-type dams, increasing to 71% in TGF-beta2 heterozygous littermates. Wild-type and TGF-beta2 heterozygous embryos from heterozygous dams exhibited a CP incidence of 74 and 77% respectively, following treatment with RA. Ninety-one percent of littermates nullizygous for TGF-beta2 were dead when examined; the remainder exhibited a CP. We conclude that the genotype of the dam and embryo with respect to TGF-beta2 affects the incidence of RA-induced teratogenesis.

Naso-maxillary deformity due to frontonasal expression of human transthyretin gene in transgenic mice

BACKGROUND

Retinoic acid, a metabolic product of retinol, is essential for craniofacial morphogenesis. Transthyretin (TTR) is a plasma protein delivering retinol to tissues. We produced several transgenic mouse lines using the human mutant TTR (hTTRMet30) gene to establish a mouse model of familial amyloidotic polyneuropathy. One of the lines showed an autosomal dominant inheritance of naso-maxillary deformity termed Nax.

RESULTS

The Nax malformation was characterized by a hypoplastic developmental defect of the frontonasal region. Homozygous mice with higher transgene expressions showed more severe phenotypes, but a subline, in which the copy number and expression of the transgene was reduced, showed a normal phenotype, indicating that the hTTRMet30 expression caused the malformation. Nax mice began to express the hTTRMet30 gene in the nasal placode from embryonic day 10.5 (E10.5), which was 2 days earlier than in the other transgenic lines with a normal phenotype. Excessive cell death was observed in the nasal placode of the E10.5 Nax embryos. In addition, the forced expression of hTTRMet30 in the nasal placode of transgenic mice resulted in similar phenotypes.

CONCLUSION

The expression of the hTTRMet30 gene in the nasal placode at E10.5 induced apoptotic cell death, leading to hypoplastic deformity in the frontonasal region.

Kinetic mechanism of human class IV alcohol dehydrogenase functioning as retinol dehydrogenase

Molecular genetic studies have indicated that alcohol dehydrogenase may be involved in the synthesis of retinoic acid, a hormonal molecule regulating diverse cellular functions at the transcriptional level. Class IV alcohol dehydrogenase (ADH) has been reported to be the most efficient enzyme catalyzing oxidation of retinol in human ADH family. Initial velocity, product inhibition, and dead-end inhibition experiments were performed with the recombinant human class IV ADH to elucidate kinetic mechanism with all-trans-retinol and all-trans-retinal as natural substrates. Fluorescence quenching was titrated in formation of the binary and abortive ternary enzyme complexes. The minimal mechanism deduced from steady-state kinetic and equilibrium binding studies is best described as an asymmetric rapid equilibrium random mechanism with two dead-end ternary complexes for retinol oxidation and a rapid equilibrium ordered mechanism with one dead-end ternary complex for retinal reduction, a unique mechanistic form for zinc-containing ADHs in the medium chain dehydrogenase/reductase superfamily. Dissociation constants for the binary complexes as well as the productive and abortive ternary complexes determined from different experimental approaches are in reasonable agreement. Kinetic isotope effect studies suggest rate-limiting isomerization of the central ternary complexes in both reaction directions. The potential interference of retinol metabolism by ethanol through the ADH pathway may play a significant role in the pathogenesis of fetal alcohol syndrome and alcohol-related upper digestive tract cancer.

Retinoic acid signalling in the zebrafish embryo is necessary during pre-segmentation stages to pattern the anterior-posterior axis of the CNS and to induce a pectoral fin bud

A number of studies have suggested that retinoic acid (RA) is an important signal for patterning the hindbrain, the branchial arches and the limb bud. Retinoic acid is thought to act on the posterior hindbrain and the limb buds at somitogenesis stages in chick and mouse embryos. Here we report a much earlier requirement for RA signalling during pre-segmentation stages for proper development of these structures in zebrafish. We present evidence that a RA signal is necessary during pre-segmentation stages for proper expression of the spinal cord markers hoxb5a and hoxb6b, suggesting an influence of RA on anteroposterior patterning of the neural plate posterior to the hindbrain. We report the identification and expression pattern of the zebrafish retinaldehyde dehydrogenase2 (raldh2/aldh1a2) gene. Raldh2 synthesises retinoic acid (RA) from its immediate precursor retinal. It is expressed in a highly ordered spatial and temporal fashion during gastrulation in the involuting mesoderm and during later embryogenesis in paraxial mesoderm, branchial arches, eyes and fin buds, suggesting the involvement of RA at different times of development in different functional contexts. Mapping of the raldh2 gene reveals close linkage to no-fin (nof), a newly discovered mutant lacking pectoral fins and cartilaginous gill arches. Cloning and functional tests of the wild-type and nof alleles of raldh2 reveal that nof is a raldh2 mutant. By treating nof mutants with RA during different time windows and by making use of a retinoic acid receptor antagonist, we show that RA signalling during pre-segmentation stages is necessary for anteroposterior patterning in the CNS and for fin induction to occur.

Novel retinoic acid generating activities in the neural tube and heart identified by conditional rescue of Raldh2 null mutant mice

Retinoid control of vertebrate development depends upon tissue-specific metabolism of retinol to retinoic acid (RA). The RA biosynthetic enzyme RALDH2 catalyzes much, but not all, RA production in mouse embryos, as revealed here with Raldh2 null mutants carrying an RA-responsive transgene. Targeted disruption of Raldh2 arrests development at midgestation and eliminates all RA synthesis except that associated with Raldh3 expression in the surface ectoderm of the eye field. Conditional rescue of Raldh2–/– embryos by limited maternal RA administration allows development to proceed and results in the establishment of additional sites of RA synthesis linked to Raldh1 expression in the dorsal retina and to Raldh3 expression in the ventral retina, olfactory pit and urinary tract. Unexpectedly, conditionally rescued Raldh2–/– embryos also possess novel sites of RA synthesis in the neural tube and heart that do not correspond to expression of Raldh1-3. RA synthesis in the mutant neural tube was localized in the spinal cord, posterior hindbrain and portions of the midbrain and forebrain, whereas activity in the mutant heart was localized in the conotruncus and sinus venosa. In the posterior hindbrain, this novel RA-generating activity was expressed during establishment of rhombomeric boundaries. In the spinal cord, the novel activity was localized in the floorplate plus in the intermediate region where retinoid-dependent interneurons develop. These novel RA-generating activities in the neural tube and heart fill gaps in our knowledge of how RA is generated spatiotemporally and may, along with Raldh1 and Raldh3, contribute to rescue of Raldh2–/– embryos by producing RA locally.

Expression of genes for alcohol and aldehyde metabolizing enzymes in mouse oocytes and preimplantation embryos

Alcohols and aldehydes are metabolized primarily by alcohol (ADH) and aldehyde (ALDH) dehydrogenase isozymes. Although significant progress has been made towards understanding the involvement of these isozymes in the oxidation of alcohol and aldehydes in the body, it is not known how these compounds are handled during fertilization and preimplantation embryogenesis. In this study, reverse transcription and the polymerase chain reaction (RT-PCR) was used to determine which ADH and ALDH isozymes are expressed at the oocyte, zygote, morula, and blastocyst stages of preimplantation development in the mouse. Transcripts of beta-actin and vimentin, assayed as controls, were detected at all stages, as well as Class III ADH (Adh-2) and Class 3 ALDH (Ahd-4), involved in the detoxification of formaldehyde and aromatic aldehydes, respectively. In contrast, transcripts for the major ethanol oxidizing isozyme, Class I ADH (Adh-1) was not detected during preimplantation development. Cytosolic retinol dehydrogenase (Adh-3) transcripts were marginally detected in oocytes and zygotes. The mRNA for cytosolic retinal dehydrogenase (Ahd-2), microsomal short-chain retinol dehydrogenases (RoDH Type I), and the mitochondrial low-Km acetaldehyde dehydrogenase (Ahd-5) only appeared as maternal transcripts. Microsomal ALDH (Ahd-3), which is induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), was not expressed until the blastocyst stage. ADH and ALDH enzyme systems may guard mouse preimplantation embryos against the toxic effects of industrial pollutants, such as formaldehyde and TCDD, as well as peroxidatic aldehydes generated during lipid peroxidation. The absence of enzymes to convert ethanol to acetaldehyde, coupled with oocyte expression of the acetaldehyde-degrading enzyme, Ahd-5, may be protective for the early embryo.

Stimulation of retinoic acid production and growth by ubiquitously expressed alcohol dehydrogenase Adh3

Influence of vitamin A (retinol) on growth depends on its sequential oxidation to retinal and then to retinoic acid (RA), producing a ligand for RA receptors essential in development of specific tissues. Genetic studies have revealed that aldehyde dehydrogenases function as tissue-specific catalysts for oxidation of retinal to RA. However, enzymes catalyzing the first step of RA synthesis, oxidation of retinol to retinal, remain unclear because none of the present candidate enzymes have expression patterns that fully overlap with those of aldehyde dehydrogenases during development. Here, we provide genetic evidence that alcohol dehydrogenase (ADH) performs this function by demonstrating a role for Adh3, a ubiquitously expressed form. Adh3 null mutant mice exhibit reduced RA generation in vivo, growth deficiency that can be rescued by retinol supplementation, and completely penetrant postnatal lethality during vitamin A deficiency. ADH3 was also shown to have in vitro retinol oxidation activity. Unlike the second step, the first step of RA synthesis is not tissue-restricted because it is catalyzed by ADH3, a ubiquitous enzyme having an ancient origin.

Role and distribution of retinoic acid during CNS development

Retinoic acid (RA), the biologically active derivative of vitamin A, induces a variety of embryonal carcinoma and neuroblastoma cell lines to differentiate into neurons. The molecular events underlying this process are reviewed with a view to determining whether these data can lead to a better understanding of the normal process of neuronal differentiation during development. Several transcription factors, intracellular signaling molecules, cytoplasmic proteins, and extracellular molecules are shown to be necessary and sufficient for RA-induced differentiation. The evidence that RA is an endogenous component of the developing central nervous system (CNS) is then reviewed, data which include high-pressure liquid chromotography (HPLC) measurements, reporter systems and the distribution of the enzymes that synthesize RA. The latter is particularly relevant to whether RA signals in a paracrine fashion on adjacent tissues or whether it acts in an autocrine manner on cells that synthesize it. It seems that a paracrine system may operate to begin early patterning events within the developing CNS from adjacent somites and later within the CNS itself to induce subsets of neurons. The distribution of retinoid-binding proteins, retinoid receptors, and RA-synthesizing enzymes is described as well as the effects of knockouts of these genes. Finally, the effects of a deficiency and an excess of RA on the developing CNS are described from the point of view of patterning the CNS, where it seems that the hindbrain is the most susceptible part of the CNS to altered levels of RA or RA receptors and also from the point of view of neuronal differentiation where, as in the case of embryonal carcinoma (EC) cells, RA promotes neuronal differentiation. The crucial roles played by certain genes, particularly the Hox genes in RA-induced patterning processes, are also emphasized.

Organization of six functional mouse alcohol dehydrogenase genes on two overlapping bacterial artificial chromosomes

Mammalian alcohol dehydrogenases (ADH) form a complex enzyme system based on amino-acid sequence, functional properties, and gene expression pattern. At least four mouse Adh genes are known to encode different enzyme classes that share less than 60% amino-acid sequence identity. Two ADH-containing and overlapping C57BL/6 bacterial artificial chromosome clones, RP23-393J8 and -463H24, were identified in a library screen, physically mapped, and sequenced. The gene order in the complex and two new mouse genes, Adh5a and Adh5b, and a pseudogene, Adh5ps, were obtained from the physical map and sequence. The mouse genes are all in the same transcriptional orientation in the order Adh4-Adh1-Adh5a-Adh5b-Adh5ps-Adh2-Adh3. A phylogenetic tree analysis shows that adjacent genes are most closely related suggesting a series of duplication events resulted in the gene complex. Although mouse and human ADH gene clusters contain at least one gene for ADH classes I-V, the human cluster contains 3 class I genes while the mouse cluster has two class V genes plus a class V pseudogene.

Molecular characterization of a mouse short chain dehydrogenase/reductase active with all-trans-retinol in intact cells, mRDH1

Metabolic activation of retinol (vitamin A) via sequential actions of retinol and retinal dehydrogenases produces the active metabolite all-trans-retinoic acid. This work reports cDNA cloning, enzymatic characterization, function in a reconstituted path of all-trans-retinoic acid biosynthesis in cell culture, and mRNA expression patterns in adult tissues and embryos of a mouse retinol dehydrogenase, RDH1. RDH1 represents a new member of the short chain dehydrogenase/reductase superfamily that differs from other mouse RDH in relative activity with all-trans and cis-retinols. RDH1 has a multifunctional catalytic nature, as do other short chain dehydrogenase/reductases. In addition to retinol dehydrogenase activity, RDH1 has strong 3alpha-hydroxy and weak 17beta-hydroxy steroid dehydrogenase activities. RDH1 has widespread and intense mRNA expression in tissues of embryonic and adult mice. The mouse embryo expresses RDH1 as early as 7.0 days post-coitus, and expression is especially intense within the neural tube, gut, and neural crest at embryo day 10.5. Cells cotransfected with RDH1 and any one of three retinal dehydrogenase isozymes synthesize all-trans-retinoic acid from retinol, demonstrating that RDH1contributes to a path of all-trans-retinoic acid biosynthesis in intact cells. These characteristics are consistent with RDH1 functioning in a path of all-trans-retinoic acid biosynthesis starting early during embryogenesis.

Roles of retinoic acid receptors in early embryonic morphogenesis and hindbrain patterning

Mutants mice carrying targeted inactivations of both retinoic acid receptor (RAR) alpha and RAR gamma (A alpha/A gamma mutants) were analyzed at different embryonic stages, in order to establish the timing of appearance of defects that we previously observed during the fetal period. We show that embryonic day (E)9.5 A alpha/A gamma embryos display severe malformations, similar to those already described in retinaldehyde dehydrogenase 2 null mutants. These malformations reflect early roles of retinoic acid signaling in axial rotation, segmentation and closure of the hindbrain; formation of otocysts, pharyngeal arches and forelimb buds; and in the closure of the primitive gut. The hindbrain of E8.5 A alpha/A gamma embryos shows a posterior expansion of rhombomere 3 and 4 (R3 and R4) markers, but fails to express kreisler, a normal marker of R5 and R6. This abnormal hindbrain phenotype is strikingly different from that of embryos lacking RAR alpha and RAR beta (A alpha/A beta mutants), in which we have previously shown that the territory corresponding to R5 and R6 is markedly enlarged. Administration of a pan-RAR antagonist at E8.0 to wild-type embryos cultured in vitro results in an A alpha/A beta-like hindbrain phenotype, whereas an earlier treatment at E7.0 yields an A alpha/A gamma-like phenotype. Altogether, our data suggest that RAR alpha and/or RAR gamma transduce the RA signal that is required first to specify the prospective R5/R6 territory, whereas RAR beta is subsequently involved in setting up the caudal boundary of this territory.

Molecular analysis of genetic differences among inbred mouse strains controlling tissue expression pattern of alcohol dehydrogenase 4

The ADH gene family in vertebrates is composed of at least seven distinct classes based upon sequence comparisons and enzyme properties. The Adh4 gene product may play an important role in differentiation and development because of its capacity to metabolize retinol to retinoic acid. Allelic gene differences exist among inbred mouse strains which control structure and tissue-specific regulation of Adh4. C57BL/6 mice are unique and have no detectable ADH4 enzyme activity in epididymis and low levels in seminal vesicle, ovary and uterus compared to other strains. C57BL/6 mice express Adh4 in stomach at levels similar to other strains. The goal of this research was to investigate this genetic variation at the molecular level. Northern analysis revealed that the content of ADH4 mRNA in tissues correlate with the enzyme expression pattern. Interestingly, C57BL/6 mice express an ADH4 mRNA in stomach which is smaller than expressed in C3H and other mice. An analysis of the 5'- and 3'-ends of the mRNA using RACE analysis determined that the ADH4 mRNA in C57BL/6 mice is truncated in the 3'-untranslated region. Sequence analysis of RACE products showed that the truncation is due to a single nucleotide mutation which produces an early polyadenylation signal. Additional RACE and Northern analysis revealed that at least five different polyadenylation sites are used in the Adh4 gene. Using 3'-end polymorphisms found between C57BL/6 and C3H strains and RT-PCR, it was shown that the lack of expression in epididymis in C57BL/6 mice is cis-acting in F(1) hybrid animals. The DNA sequence of the proximal promoter (-600/+42 nt) was determined in several mouse strains differing in tissue-specific expression patterns and did not reveal any nucleotide substitutions correlating with expression pattern suggesting further upstream or downstream sequences may be involved.

Genetic dissection of retinoid dehydrogenases

Biochemical studies indicate that alcohol dehydrogenase (ADH) metabolizes retinol to retinal, and that aldehyde dehydrogenase (ALDH) metabolizes retinal to retinoic acid, a molecule essential for growth and development. Summarized herein are several genetic studies supporting in vivo functions for ADH and ALDH in retinoic acid synthesis. Gene targeting was used to create knockout mice for either Adh1 or Adh4. Both knockout mice were viable and fertile without obvious defects. However, when wild-type and Adh4 knockout mice were subjected to vitamin A deficiency during gestation, the survival rate at birth was 3.3-fold lower for Adh4 knockout mice. When adult mice were examined for production of retinoic acid following retinol administration, Adh1 knockout mice exhibited 10-fold lower retinoic acid levels in liver compared with wild-type, whereas Adh4 knockout mice differed from wild-type by less than 2-fold. Thus, Adh1 plays a major role in the metabolism of a large dose of retinol to retinoic acid in adults, whereas Adh4 plays a role in maintaining sufficient retinol metabolism for development during retinol deficiency. ALDHs were examined by overexpression studies in frog embryos. Injection of mRNAs for either mouse Raldh1 or Raldh2 stimulated retinoic acid synthesis in frog embryos at the blastula stage when retinoic acid is normally undetectable. Overexpression of human ALDH2, human ALDH3, and mouse Aldh-pb did not stimulate retinoic acid production. In addition, Raldh2 knockout mice exhibit embryonic lethality with defects in retinoid-dependent tissues. Overall, these studies provide genetic evidence that Adh1, Adh4, Raldh1, and Raldh2 encode retinoid dehydrogenases involved in retinoic acid synthesis in vivo.

Amphioxus alcohol dehydrogenase is a class 3 form of single type and of structural conservation but with unique developmental expression

The coding region of amphioxus alcohol dehydrogenase class 3 (ADH3) has been characterized from two species, Branchiostoma lanceolatum and Branchiostoma floridae. The species variants have residue differences at positions that result in only marginal functional distinctions. Activity measurements show a class 3 glutathione-dependent formaldehyde dehydrogenase, with kcat/Km values about threefold those of the human class 3 ADH enzyme. Only a single ADH3 form is identified in each of the two amphioxus species, and no ethanol activity ascribed to other classes is detectable, supporting the conclusion that evolution of ethanol-active ADH classes by gene duplications occurred at early vertebrate radiation after the formation of the amphioxus lineage. Similarly, Southern blot analysis indicated that amphioxus ADH3 is encoded by a single gene present in the methylated fraction of the amphioxus genome and northern blots revealed a single 1.4-kb transcript. In situ experiments showed that amphioxus Adh3 expression is restricted to particular cell types in the embryos. Transcripts were first evident at the neurula stage and then located at the larval ventral region, in the intestinal epithelium. This tissue-specific pattern contrasts with the ubiquitous Adh3 expression in mammals.

Distribution of polarizing activity and potential for limb formation in mouse and chick embryos and possible relationships to polydactyly

A central feature of the tetrapod body plan is that two pairs of limbs develop at specific positions along the head-to-tail axis. However, the potential to form limbs in chick embryos is more widespread. This could have implications for understanding the basis of limb abnormalities. Here we extend the analysis to mouse embryos and examine systematically the potential of tissues in different regions outside the limbs to contribute to limb structures. We show that the ability of ectoderm to form an apical ridge in response to FGF4 in both mouse and chick embryos exists throughout the flank as does ability of mesenchyme to provide a polarizing region signal. In addition, neck tissue has weak polarizing activity. We show, in chick embryos, that polarizing activity of tissues correlates with the ability either to express Shh or to induce Shh expression. We also show that cells from chick tail can give rise to limb structures. Taken together these observations suggest that naturally occurring polydactyly could involve recruitment of cells from regions adjacent to the limb buds. We show that cells from neck, flank and tail can migrate into limb buds in response to FGF4, which mimics extension of the apical ectodermal ridge. Furthermore, when we apply simultaneously a polarizing signal and a limb induction signal to early chick flank, this leads to limb duplications.

Regulation of retinoic acid signaling in the embryonic nervous system: a master differentiation factor

This review describes some of the properties of retinoic acid (RA) in its functions as a locally synthesized differentiation factor for the developing nervous system. The emphasis is on the characterization of the metabolic enzymes that synthesize and inactivate RA, and which determine local RA concentrations. These enzymes create regions of autocrine and paracrine RA signaling in the embryo. One mechanism by which RA can act as a differentiation agent is through the induction of growth factors and their receptors. Induction of growth factor receptors in neural progenitor cells can lead to growth factor dependency, and the consequent developmental fate of the cell will depend on the local availability of growth factors. Because RA activates the early events of cell differentiation, which then induce context-specific differentiation programs, RA may be called a master differentiation factor.

Molecular basis for differential substrate specificity in class IV alcohol dehydrogenases: a conserved function in retinoid metabolism but not in ethanol oxidation

Mammalian class IV alcohol dehydrogenase enzymes are characteristic of epithelial tissues, exhibit moderate to high K(m) values for ethanol, and are very active in retinol oxidation. The human enzyme shows a K(m) value for ethanol which is 2 orders of magnitude lower than that of rat class IV. The uniquely significant difference in the substrate-binding pocket between the two enzymes appears to be at position 294, Val in the human enzyme and Ala in the rat enzyme. Moreover, a deletion at position 117 (Gly in class I) has been pointed out as probably responsible for class IV specificity toward retinoids. With the aim of establishing the role of these residues, we have studied the kinetics of the recombinant human and rat wild-type enzymes, the human G117ins and V294A mutants, and the rat A294V mutant toward aliphatic alcohols and retinoids. 9-cis-Retinol was the best retinoid substrate for both human and rat class IV, strongly supporting a role of class IV in the generation of 9-cis-retinoic acid. In contrast, 13-cis retinoids were not substrates. The G117ins mutant showed a decreased catalytic efficiency toward retinoids and toward three-carbon and longer primary aliphatic alcohols, a behavior that resembles that of the human class I enzyme, which has Gly(117). The K(m) values for ethanol dramatically changed in the 294 mutants, where the human V294A mutant showed a 280-fold increase, and the rat A294V mutant a 50-fold decrease, compared with those of the respective wild-type enzymes. This demonstrates that the Val/Ala exchange at position 294 is mostly responsible for the kinetic differences with ethanol between the human and rat class IV. In contrast, the kinetics toward retinoids was only slightly affected by the mutations at position 294, compatible with a more conserved function of mammalian class IV alcohol dehydrogenase in retinoid metabolism.

Families of retinoid dehydrogenases regulating vitamin A function: production of visual pigment and retinoic acid

Vitamin A (retinol) and provitamin A (beta-carotene) are metabolized to specific retinoid derivatives which function in either vision or growth and development. The metabolite 11-cis-retinal functions in light absorption for vision in chordate and nonchordate animals, whereas all-trans-retinoic acid and 9-cis-retinoic acid function as ligands for nuclear retinoic acid receptors that regulate gene expression only in chordate animals. Investigation of retinoid metabolic pathways has resulted in the identification of numerous retinoid dehydrogenases that potentially contribute to metabolism of various retinoid isomers to produce active forms. These enzymes fall into three major families. Dehydrogenases catalyzing the reversible oxidation/reduction of retinol and retinal are members of either the alcohol dehydrogenase (ADH) or short-chain dehydrogenase/reductase (SDR) enzyme families, whereas dehydrogenases catalyzing the oxidation of retinal to retinoic acid are members of the aldehyde dehydrogenase (ALDH) family. Compilation of the known retinoid dehydrogenases indicates the existence of 17 nonorthologous forms: five ADHs, eight SDRs, and four ALDHs, eight of which are conserved in both mouse and human. Genetic studies indicate in vivo roles for two ADHs (ADH1 and ADH4), one SDR (RDH5), and two ALDHs (ALDH1 and RALDH2) all of which are conserved between humans and rodents. For several SDRs (RoDH1, RoDH4, CRAD1, and CRAD2) androgens rather than retinoids are the predominant substrates suggesting a function in androgen metabolism as well as retinoid metabolism.

Retinoids in embryonal development

The key role of vitamin A in embryonal development is reviewed. Special emphasis is given to the physiological action of retinoids, as evident from the retinoid ligand knockout models. Retinoid metabolism in embryonic tissues and teratogenic consequences of retinoid administration at high doses are presented. Physiological and pharmacological actions of retinoids are outlined and explained on the basis of their interactions as ligands of the nuclear retinoid receptors. Immediate target genes and the retinoid response elements of their promoters are summarized. The fundamental role of homeobox genes in embryonal development and the actions of retinoids on their expression are discussed. The similarity of the effects of retinoid ligand knockouts to effects of compound retinoid receptor knockouts on embryogenesis is presented. Although much remains to be clarified, the emerging landscape offers exciting views for future research.

Gene structure and promoter for Crad2 encoding mouse cis-retinol/3alpha-hydroxysterol short-chain dehydrogenase isozyme

Cis-retinol/androgen dehydrogenase type 2 (CRAD2) has been shown to catalyze the dehydrogenation of retinols, including 9-cis retinol, and also to exhibit 3alpha- and 17beta- hydroxysteroid dehydrogenase activities. To examine the function of this enzyme and regulation of its gene, the Crad2 gene was cloned from a mouse genomic DNA library and characterized. The complete mouse CRAD2-coding region was found in four exons spanning an approximately 5kb region. The nucleotide sequences of the exons encoding 316 amino acids were identical to those of the previously reported mouse Crad2 cDNA. Primer extension analysis and RNase protection assay were used to map the major transcription initiation sites to the positions lying 87 and 89 base pairs upstream of the ATG translation start codon. The region proximal to the initiation sites exhibited the absence of both TATAA and CAAT boxes. This region had hepatocyte nuclear factor binding sites, consistent with its predominant expression in the liver. Computer analysis of an approximately 7.5kb 5'-flanking region also suggested the presence of binding sites for AP-1, SREBP1, HSF2, c-Rel, c-Myc, CREBP, GATA, Ets, E2F, and Oct-1, suggesting that various factors including retinoic acid, cholesterol, various kinds of stress, the cell cycle, and cyclic AMP may regulate the expression of this gene. Fluorescence in-situ hybridization analysis showed that Crad2 is located at the terminus of mouse chromosome 10, an area that corresponds to band 10D3, suggesting that RDH-related SDRs may be located together in the cluster locus. Northern blot hybridization and RT-PCR analysis demonstrated that CRAD2 was expressed not in early embryonic stages, and not in embryonic stem cells, but instead in the gastrointestinal tract during later embryonic development and adult stage. In conclusion, we have presented the first complete structural analysis, including that of the promoter and chromosomal location, of a member of the retinol/androgen dehydrogenase subfamily of the group of the short-chain dehydrogenase/reductase (SDR) isozymes. Our findings will provide the basis for in-vitro or in-vivo studies concerning the regulation of retinol and androgen metabolism and enable determination of the mechanism of diseases related to retinol, retinal, retinoic acid, and androgen.

Impaired retinol utilization in Adh4 alcohol dehydrogenase mutant mice

Adh4, a member of the mouse alcohol dehydrogenase (ADH) gene family, encodes an enzyme that functions in vitro as a retinol dehydrogenase in the conversion of retinol to retinoic acid, an important developmental signaling molecule. To explore the role of Adh4 in retinoid signaling in vivo, gene targeting was used to create a null mutation at the Adh4 locus. Homozygous Adh4 mutant mice were viable and fertile and demonstrated no obvious defects when maintained on a standard mouse diet. However, when subjected to vitamin A deficiency during gestation, Adh4 mutant mice demonstrated a higher number of stillbirths than did wild-type mice. The proportion of liveborn second generation vitamin A-deficient newborn mice was only 15% for Adh4 mutant mice but 49% for wild-type mice. After retinol administration to vitamin A-deficient dams in order to rescue embryonic development, Adh4 mutant mice demonstrated a higher resorption rate at stage E12.5 (69%), compared with wild-type mice (30%). The relative ability of Adh4 mutant and wild-type mice to metabolize retinol to retinoic acid was measured after administration of a 100-mg/kg dose of retinol. Whereas kidney retinoic acid levels were below the level of detection in all vehicle-treated mice (< 1 pmol/g), retinol treatment resulted in very high kidney retinoic acid levels in wild-type mice (273 pmol/g) but 8-fold lower levels in Adh4 mutant mice (32 pmol/g), indicating a defect in metabolism of retinol to retinoic acid. These findings demonstrate that another retinol dehydrogenase can compensate for a lack of Adh4 when vitamin A is sufficient, but that Adh4 helps optimize retinol utilization under conditions of both retinol deficiency and excess.

Identification of endogenous retinoids, enzymes, binding proteins, and receptors during early postimplantation development in mouse: important role of retinal dehydrogenase type 2 in synthesis of all-trans-retinoic acid

Specific combinations of nuclear retinoid receptors acting as ligand-inducible transcription factors mediate the essential role of retinoids in embryonic development. Whereas some data exist on the expression of these receptors during early postimplantation development in mouse, little is known about the enzymes controlling the production of active ligands for the retinoid receptors. Furthermore, at early stages of mouse development virtually no data are available on the presence of endogenous retinoids. In the present study we have used a recently developed high-performance liquid chromatographic (HPLC) technique to identify endogenous retinoids in mouse embryos down to the egg cylinder stage. All-trans-retinoic acid, a ligand for the retinoic acid receptors, was detected in embryos dissected as early as 7.5 dpc (i.e., a combination of midstreak until late allantoic bud stage embryos). At these stages, we detected mRNA coding for all the retinoid receptors, retinoid binding proteins, and two enzymes able to convert retinol to retinal (retinol dehydrogenase 5 (RDH5) and alcohol dehydrogenase 4 (ADH4)). We also detected retinal dehydrogenase type 2 (RALDH2), an enzyme capable of oxidising the final step in the all-trans-retinoic acid synthesis. In egg cylinder stage mouse embryos no all-trans-retinoic acid was detected. However, at this stage its precursor all-trans-retinal was present. In accordance with these HPLC observations, RDH5 and ADH4 were expressed, but no transcripts coding for enzymes that oxidise retinal to retinoic acid. Therefore, our results suggest that RALDH2 is a key regulator in initiating retinoic acid synthesis sometime between the mid-primitive streak stage and the late allantoic bud stage in mouse embryos.

Retinoid signaling is essential for patterning the endoderm of the third and fourth pharyngeal arches

The requirement of retinoic acid (RA) in the initial formation of the pharyngeal arches was investigated by treating headfold-stage mouse embryos with a pan-RAR antagonist in vitro and in vivo. This results in a complete absence of mesenchyme, arteries, nerves and epibranchial placodes of the 3rd and 4th pharyngeal arches, complete agenesis of the 3rd and 4th pouches and consistent lack of the 6th arch artery. Mesodermally derived endothelial cells are absent from the 3rd and 4th pharyngeal arch region and the distribution domain of EphA2 transcripts in mesodermal cells is shifted caudally. In situ hybridization with CRABPI, kreisler and EphA4 probes and the pattern of expression of a Wnt1-lacZ transgene show that neural crest cells (NCC) normally destined to the 3rd and 4th arches migrate ectopically. Most interestingly, the appearance of the 3rd and 4th arches is prevented by the antagonist only during a very narrow window of time, which does not correspond to the period of post-otic NCC migration. Both the timing of appearance and the nature of the defects in RAR antagonist-treated embryos indicate that migrating NCC and mesodermal cells destined to the caudal pharyngeal arches do not represent primary targets of RA action. Alterations in the endodermal expression pattern of Hoxa1, Hoxb1, Pax1, Pax9, Fgf3 and Fgf8 in response to the antagonist-induced block in RA signal transduction demonstrate for the first time that RA signaling is indispensable for the specification of the pharyngeal endoderm and suggest that this signaling is necessary to provide a permissive environment locally for the migration of NCC and mesodermal cells. Our study also indicates that the formation of the 2nd pharyngeal arch and that of the 3rd and 4th pharyngeal arches probably involve distinct RA-dependent developmental processes.

Regulation of the mammalian alcohol dehydrogenase genes

This review focuses on the regulation of the mammalian medium-chain alcohol dehydrogenase (ADH) genes. This family of genes encodes enzymes involved in the reversible oxidation of alcohols to aldehydes. Interest in these enzymes is increased because of their role in the metabolism of beverage alcohol as well as retinol, and their influence on the risk for alcoholism. There are six known classes ADH genes that evolved from a common ancestor. ADH genes differ in their patterns of expression: most are expressed in overlapping tissue-specific patterns, but class III ADH genes are expressed ubiquitously. All have proximal promoters with multiple cis-acting elements. These elements, and the transcription factors that can interact with them, are being defined. Subtle differences in sequence can affect affinity for these factors, and thereby influence the expression of the genes. This provides an interesting system in which to examine the evolution of tissue specificity. Among transcription factors that are important in multiple members of this gene family are the C/EBPs, Sp1,USF, and AP1, HNF-1, CTF/NF-1, glucocorticoid, and retinoic acid receptors, and several as-yet unidentified negative elements, are important in at least one of the genes. There is evidence that cis-acting elements located far from the proximal promoter are necessary for proper expression. Three of the genes have upstream AUGs in the 5' nontranslated regions of their mRNA, unusual for mammalian genes. The upstream AUGs have been shown to significantly affect expression of the human ADH5 gene.

Retinoic acid affects left-right patterning

Our understanding of the means by which the left-right axis is patterned is not fully understood, although a number of key intermediaries have been recently described. We report here that retinoic acid (RA) excess affects heart situs concomitant with alterations in the expression of genes implicated in the establishment of the left-right axis. Specifically, RA exposure during a specific developmental window evoked bilateral expression of lefty-1, lefty-2, nodal, and pitx-2 in the lateral plate mesoderm. Time course experiments, together with analysis of midline markers, suggest that nascent mesoderm constitutes a predominant RA target involved in this process. These events are likely to underlie the perturbations of heart looping provoked by excess RA and suggest a means by which retinoids influence the early steps in establishment of the left-right embryonic axis.

Retinoic acid: its biosynthesis and metabolism

This article presents a model that integrates the functions of retinoid-binding proteins with retinoid metabolism. One of these proteins, the widely expressed (throughout retinoid target tissues and in all vertebrates) and highly conserved cellular retinol-binding protein (CRBP), sequesters retinol in an internal binding pocket that segregates it from the intracellular milieu. The CRBP-retinol complex appears to be the quantitatively major form of retinol in vivo, and may protect the promiscuous substrate from nonenzymatic degradation and/or non-specific enzymes. For example, at least seven types of dehydrogenases catalyze retinal synthesis from unbound retinol in vitro (NAD+ vs. NADP+ dependent, cytosolic vs. microsomal, short-chain dehydrogenases/reductases vs. medium-chain alcohol dehydrogenases). But only a fraction of these (some of the short-chain de-hydrogenases/reductases) have the fascinating additional ability of catalyzing retinal synthesis from CRBP-bound retinol as well. Similarly, CRBP and/or other retinoid-binding proteins function in the synthesis of retinal esters, the reduction of retinal generated from intestinal beta-carotene metabolism, and retinoic acid metabolism. The discussion details the evidence supporting an integrated model of retinoid-binding protein/metabolism. Also addressed are retinoid-androgen interactions and evidence incompatible with ethanol causing fetal alcohol syndrome by competing directly with retinol dehydrogenation to impair retinoic acid biosynthesis.

Interactions of retinoid binding proteins and enzymes in retinoid metabolism

Naturally occurring retinoids (vitamin A or retinol and its active metabolites) are vital for vision, controlling the differentiation program of epithelial cells in the digestive tract and respiratory system, skin, bone, the nervous system, the immune system, and for hematopoiesis. Retinoids are essential for growth, reproduction (conception and embryonic development), and resistance to and recovery from infection. The functions of retinoids in the embryo begin soon after conception and continue throughout the lifespan of all vertebrates. Both naturally occurring and synthetic retinoids are used in the therapy of various skin diseases, especially acne, for augmenting the treatment of diabetes, and as cancer chemopreventive agents. Retinol metabolites serve as ligands that activate specific transcription factors in the superfamily of steroid/retinoid/thyroid/vitamin D/orphan receptors and thereby control gene expression. Additionally, retinoids may also function through non-genomic actions. Various retinoid binding proteins serve as partners in retinoid function. These binding proteins show high specificity and affinity for specific retinoids and seem to control retinoid metabolism in vivo qualitatively and quantitatively by reducing 'free' retinoid concentrations, protecting retinoids from non-specific interactions, and chaperoning access of metabolic enzymes to retinoids. Implementation of the physiological effects of retinoids depends on the spatial-temporal expressions of binding proteins, receptors and metabolic enzymes. This review will discuss current understanding of the enzymes that catalyze retinol and retinoic acid metabolism and their unique and integral relationship to retinoid binding proteins.

Structural and enzymatic properties of a gastric NADP(H)- dependent and retinal-active alcohol dehydrogenase

A class IV-type, gastric alcohol dehydrogenase (ADH) has been purified from frog (Rana perezi) tissues, meaning detection of this enzyme type also in nonmammalian vertebrates. However, the protein is unique among vertebrate ADHs thus far characterized in having preference for NADP(+) rather than NAD(+). Similarly, it deviates structurally from other class IV ADHs and has a phylogenetic tree position outside that of the conventional class IV cluster. The NADP(+) preference is structurally correlated with a replacement of Asp-223 of all other vertebrate ADHs with Gly-223, largely directing the coenzyme specificity. This residue replacement is expected metabolically to correlate with a change of the reaction direction catalyzed, from preferential alcohol oxidation to preferential aldehyde reduction. This is of importance in cellular growth regulation through retinoic acid formed from retinol/retinal precursors because the enzyme is highly efficient in retinal reduction (k(cat)/K(m) = 3.4.10(4) mM(-1) min(-1)). Remaining enzymatic details are also particular but resemble those of the human class I/class IV enzymes. However, overall structural relationships are distant (58-60% residue identity), and residues at substrate binding and coenzyme binding positions are fairly deviant, reflecting the formation of the new activity. The results are concluded to represent early events in the duplicatory origin of the class IV line or of a separate, class IV-type line. In both cases, the novel enzyme illustrates enzymogenesis of classes in the ADH system. The early origin (with tetrapods), the activity (with retinoids), and the specific location of this enzyme (gastric, like the gastric and epithelial location of the human class IV enzyme) suggest important functions of the class IV ADH type in vertebrates.

Recommended nomenclature for the vertebrate alcohol dehydrogenase gene family

The alcohol dehydrogenase (ADH) gene family encodes enzymes that metabolize a wide variety of substrates, including ethanol, retinol, other aliphatic alcohols, hydroxysteroids, and lipid peroxidation products. Studies on 19 vertebrate animals have identified ADH orthologs across several species, and this has now led to questions of how best to name ADH proteins and genes. Seven distinct classes of vertebrate ADH encoded by non-orthologous genes have been defined based upon sequence homology as well as unique catalytic properties or gene expression patterns. Each class of vertebrate ADH shares <70% sequence identity with other classes of ADH in the same species. Classes may be further divided into multiple closely related isoenzymes sharing >80% sequence identity such as the case for class I ADH where humans have three class I ADH genes, horses have two, and mice have only one. Presented here is a nomenclature that uses the widely accepted vertebrate ADH class system as its basis. It follows the guidelines of human and mouse gene nomenclature committees, which recommend coordinating names across species boundaries and eliminating Roman numerals and Greek symbols. We recommend that enzyme subunits be referred to by the symbol "ADH" (alcohol dehydrogenase) followed by an Arabic number denoting the class; i.e. ADH1 for class I ADH. For genes we recommend the italicized root symbol "ADH" for human and "Adh" for mouse, followed by the appropriate Arabic number for the class; i.e. ADH1 or Adh1 for class I ADH genes. For organisms where multiple species-specific isoenzymes exist within a class, we recommend adding a capital letter after the Arabic number; i.e. ADH1A, ADH1B, and ADH1C for human alpha, beta, and gamma class I ADHs, respectively. This nomenclature will accommodate newly discovered members of the vertebrate ADH family, and will facilitate functional and evolutionary studies.

Molecular docking studies on interaction of diverse retinol structures with human alcohol dehydrogenases predict a broad role in retinoid ligand synthesis

Some members of the human alcohol dehydrogenase (ADH) family possess retinol dehydrogenase activity and may thus function in production of the active nuclear receptor ligand retinoic acid. Many diverse natural forms of retinol exist including all-trans-retinol (vitamin A(1)), 9-cis-retinol, 3,4-didehydroretinol (vitamin A(2)), 4-oxo-retinol, and 4-hydroxy-retinol as well as their respective carboxylic acid derivatives which are active ligands for retinoid receptors. This raises the question of whether ADHs can accommodate all these different retinols and thus participate in the activation of several retinoid ligands. The crystal structures of human ADH1B and ADH4 provide the opportunity to examine their active sites for potential binding to many diverse retinol structures using molecular docking algorithms. The criteria used to score successful docking included achievement of distances of 1.9-2.4 A between the catalytic zinc and the hydroxyl oxygen of retinol and 3.2-3.6 A between C-4 of the coenzyme NAD and C-15 of retinol. These distances are sufficient to enable hydride transfer during the oxidation of an alcohol to an aldehyde. By these criteria, all-trans-retinol, 4-oxo-retinol, and 4-hydroxy-retinol were successfully docked to both ADH1B and ADH4. However, 9-cis-retinol and 3,4-didehydroretinol, which have more restrictive conformations, were successfully docked to only ADH4 which possesses a wider active site than ADH1B and more easily accommodates the C-19 methyl group. Furthermore, docking of all retinols was more favorable in the active site of ADH4 rather than ADH1B as measured by force field and contact scores. These findings suggest that ADH1B has a limited capacity to metabolize retinols, but that ADH4 is well suited to function in the metabolism of many diverse retinols and is predicted to participate in the synthesis of the active ligands all-trans-retinoic acid, 9-cis-retinoic acid, 3, 4-didehydroretinoic acid, 4-oxo-retinoic acid, and 4-hydroxy-retinoic acid.

Retinoic acid biosynthetic enzyme ALDH1 localizes in a subset of retinoid-dependent tissues during xenopus development

Control of retinoic acid synthesis in vertebrate organisms is undoubtedly important for regulating the numerous retinoid signaling events which occur during development. The mechanisms which accomplish this task involve enzymes such as class I aldehyde dehydrogenase (ALDH1), which has recently been found to be conserved from amphibians to mammals and which functions as a retinoic acid biosynthetic enzyme in vivo. Here we have found that Xenopus ALDH1 mRNA and protein is expressed in a subset of retinoid-dependent tissues which develop shortly after neurulation during the tail bud stages. ALDH1 mRNA was first clearly detectable by in situ hybridization in stage 28 tail bud embryos localized in the olfactory placode and pronephros, and at stage 35 mRNA was also detected in the pronephric duct. Antibodies were generated against Xenopus ALDH1, and immunohistochemistry was used to demonstrate that ALDH1 protein accumulates in the olfactory placode, pronephros, and dorsal retina at stage 28, and additionally in the lens placode and pronephric duct at stage 35. Neither ALDH1 mRNA nor protein was detected in the posterior region of Xenopus embryos during the tail bud stages. In contrast to neurula stage embryos in which retinoic acid is distributed in an anteroposterior gradient with the high end posteriorly, we found that tail bud stage embryos have retinoic acid present in significant levels in both the head and trunk regions, but with no detection in the posterior region. These findings are consistent with ALDH1 contributing to retinoic acid synthesis needed for development of certain head structures (olfactory placodes, dorsal retina, lens placode) and certain trunk structures (pronephros and pronephric duct). Dev Dyn 1999;215:264-272.

Molecular analysis of two closely related mouse aldehyde dehydrogenase genes: identification of a role for Aldh1, but not Aldh-pb, in the biosynthesis of retinoic acid

Mammalian class I aldehyde dehydrogenase (ALDH1) has been implicated as a retinal dehydrogenase in the biosynthesis of retinoic acid, a modulator of gene expression and cell differentiation. As the first step towards studying the regulation of ALDH1 and its physiological role in the biosynthesis of retinoic acid, mouse ALDH1 cDNA and genomic clones have been characterized. During the cloning process, an additional closely related gene was also isolated and named Aldh-pb, owing to its high amino acid sequence identity (92%) with the rat phenobarbitol-inducible ALDH protein (ALDH-PB). Aldh1 spans about 45 kb in length, whereas Aldh-pb spans about 35 kb. Both genes are composed of 13 exons, and the positions of all the exon/intron boundaries are conserved with those of human ALDH1. The promoter regions of Aldh1 and Aldh-pb demonstrate high sequence similarity with those of human ALDH1 and rat ALDH-PB. Expression of Aldh1 and Aldh-pb is tissue-specific, with mRNAs for both genes being found in the liver, lung and testis, but not in the heart, spleen or muscle. Expression of Aldh-pb, but not Aldh1, was also detected at high levels in the kidney. Aldh1 and Aldh-pb encode proteins of 501 amino acids with 90% positional identity. To examine the relative roles of these two enzymes in retinoic acid synthesis in vivo, Xenopus embryos were injected with mRNAs encoding these enzymes to assay the effect on conversion of endogenous retinal into retinoic acid. Injection of ALDH1, but not ALDH-PB, mRNA stimulated retinoic acid synthesis in Xenopus embryos at the blastula stage. Thus our results indicate that Aldh1 can function in retinoic acid synthesis under physiological conditions, but that the closely related Aldh-pb does not share this property.

Metabolic deficiencies in alcohol dehydrogenase Adh1, Adh3, and Adh4 null mutant mice. Overlapping roles of Adh1 and Adh4 in ethanol clearance and metabolism of retinol to retinoic acid

Targeting of mouse alcohol dehydrogenase genes Adh1, Adh3, and Adh4 resulted in null mutant mice that all developed and reproduced apparently normally but differed markedly in clearance of ethanol and formaldehyde plus metabolism of retinol to the signaling molecule retinoic acid. Following administration of an intoxicating dose of ethanol, Adh1 -/- mice, and to a lesser extent Adh4 -/- mice, but not Adh3 -/- mice, displayed significant reductions in blood ethanol clearance. Ethanol-induced sleep was significantly longer only in Adh1 -/- mice. The incidence of embryonic resorption following ethanol administration was increased 3-fold in Adh1 -/- mice and 1.5-fold in Adh4 -/- mice but was unchanged in Adh3 -/- mice. Formaldehyde toxicity studies revealed that only Adh3 -/- mice had a significantly reduced LD50 value. Retinoic acid production following retinol administration was reduced 4.8-fold in Adh1 -/- mice and 8.5-fold in Adh4 -/- mice. Thus, Adh1 and Adh4 demonstrate overlapping functions in ethanol and retinol metabolism in vivo, whereas Adh3 plays no role with these substrates but instead functions in formaldehyde metabolism. Redundant roles for Adh1 and Adh4 in retinoic acid production may explain the apparent normal development of mutant mice.

Embryonic retinoic acid synthesis is essential for early mouse post-implantation development

A number of studies have suggested that the active derivative of vitamin A, retinoic acid (RA), may be important for early development of mammalian embryos. Severe vitamin A deprivation in rodents results in maternal infertility, precluding a thorough investigation of the role of RA during embryogenesis. Here we show that production of RA by the retinaldehyde dehydrogenase-2 (Raldh2) enzyme is required for mouse embryo survival and early morphogenesis. Raldh2 is an NAD-dependent aldehyde dehydrogenase with high substrate specificity for retinaldehyde. Its pattern of expression during mouse development has suggested that it may be responsible for embryonic RA synthesis. We generated a targeted disruption of the mouse Raldh2 gene and found that Raldh2-/- embryos, which die at midgestation without undergoing axial rotation (body turning), exhibit shortening along the anterioposterior axis and do not form limb buds. Their heart consists of a single, medial, dilated cavity. Their frontonasal region is truncated and their otocysts are severely reduced. These defects result from a block in embryonic RA synthesis, as shown by the lack of activity of RA-responsive transgenes, the altered expression of an RA-target homeobox gene and the near full rescue of the mutant phenotype by maternal RA administration. Our data establish that RA synthesized by the post-implantation mammalian embryo is an essential developmental hormone whose lack leads to early embryo death.

Stimulation of premature retinoic acid synthesis in Xenopus embryos following premature expression of aldehyde dehydrogenase ALDH1

In order for nuclear retinoic acid receptors to mediate retinoid signaling, the ligand retinoic acid must first be produced from its vitamin A precursor retinal. Biochemical studies have shown that retinal can be metabolized in vitro to retinoic acid by members of the aldehyde dehydrogenase enzyme family, including ALDH1. Here we describe the first direct evidence that ALDH1 plays a physiological role in retinoic acid synthesis by analysis of retinoid signaling in Xenopus embryos, which have plentiful stores of maternally derived retinal. The Xenopus ALDH1 gene was cloned and shown to be highly conserved with chick and mammalian homologs. Xenopus ALDH1 was not expressed at blastula and gastrula stages, but was expressed at the neurula stage. We used a retinoic acid bioassay to demonstrate that retinoic acid is normally undetectable in embryos from fertilization to the initial gastrula stage, but that a tremendous increase in retinoic acid occurs during neurulation when ALDH1 is first expressed. Overexpression of ALDH1 by injection of Xenopus embryos with mRNAs encoding the mouse, chick or Xenopus ALDH1 homologs induced high levels of retinoic acid detection during the blastula stage. Thus, premature expression of ALDH1 stimulates premature synthesis of retinoic acid. These findings reveal an important conserved role for ALDH1 in retinoic acid synthesis in vivo, and demonstrate that conversion of retinoids from the aldehyde form to the carboxylic acid form is a crucial regulatory step in retinoid signaling.