Stage and tissue-specific expression of the alcohol dehydrogenase 1 (Adh-1) gene during mouse development

Functions of Intracellular Retinoid Binding-Proteins

Multiple binding and transport proteins facilitate many aspects of retinoid biology through effects on retinoid transport, cellular uptake, metabolism, and nuclear delivery. These include the serum retinol binding protein sRBP (aka Rbp4), the plasma membrane sRBP receptor Stra6, and the intracellular retinoid binding-proteins such as cellular retinol-binding proteins (CRBP) and cellular retinoic acid binding-proteins (CRABP). sRBP transports the highly lipophilic retinol through an aqueous medium. The major intracellular retinol-binding protein, CRBP1, likely enhances efficient retinoid use by providing a sink to facilitate retinol uptake from sRBP through the plasma membrane or via Stra6, delivering retinol or retinal to select enzymes that generate retinyl esters or retinoic acid, and protecting retinol/retinal from excess catabolism or opportunistic metabolism. Intracellular retinoic acid binding-proteins (CRABP1 and 2, and FABP5) seem to have more diverse functions distinctive to each, such as directing retinoic acid to catabolism, delivering retinoic acid to specific nuclear receptors, and generating non-canonical actions. Gene ablation of intracellular retinoid binding-proteins does not cause embryonic lethality or gross morphological defects. Metabolic and functional defects manifested in knockouts of CRBP1, CRBP2 and CRBP3, however, illustrate their essentiality to health, and in the case of CRBP2, to survival during limited dietary vitamin A. Future studies should continue to address the specific molecular interactions that occur between retinoid binding-proteins and their targets and their precise physiologic contributions to retinoid homeostasis and function.

Physiological insights into all-trans-retinoic acid biosynthesis

All-trans-retinoic acid (atRA) provides essential support to diverse biological systems and physiological processes. Epithelial differentiation and its relationship to cancer, and embryogenesis have typified intense areas of interest into atRA function. Recently, however, interest in atRA action in the nervous system, the immune system, energy balance and obesity has increased considerably, especially concerning postnatal function. atRA action depends on atRA biosynthesis: defects in retinoid-dependent processes increasingly relate to defects in atRA biogenesis. Considerable evidence indicates that physiological atRA biosynthesis occurs via a regulated process, consisting of a complex interaction of retinoid binding-proteins and retinoid recognizing enzymes. An accrual of biochemical, physiological and genetic data have identified specific functional outcomes for the retinol dehydrogenases, RDH1, RDH10, and DHRS9, as physiological catalysts of the first step in atRA biosynthesis, and for the retinal dehydrogenases RALDH1, RALDH2, and RALDH3, as catalysts of the second and irreversible step. Each of these enzymes associates with explicit biological processes mediated by atRA. Redundancy occurs, but seems limited. Cumulative data support a model of interactions among these enzymes with retinoid binding-proteins, with feedback regulation and/or control by atRA via modulating gene expression of multiple participants. The ratio apo-CRBP1/holo-CRBP1 participates by influencing retinol flux into and out of storage as retinyl esters, thereby modulating substrate to support atRA biosynthesis. atRA biosynthesis requires the presence of both an RDH and an RALDH: conversely, absence of one isozyme of either step does not indicate lack of atRA biosynthesis at the site. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Changes in gene expression in somatic cells of rat testes resulting from hormonal modulation and radiation-induced germ cell depletion

Although gonadotropins and androgen are required for normal spermatogenesis and both testosterone and follicle-stimulating hormone (FSH) are responsible for the inhibition of spermatogonial differentiation that occurs in irradiated rats, it has been difficult to identify the specific genes involved. To study specific hormonally regulated changes in somatic cell gene expression in the testis that may be involved in these processes, without the complication of changing populations of germ cells, we used irradiated LBNF(1) rats, the testes of which contain almost exclusively somatic cells except for a few type A spermatogonia. Three different groups of these rats were treated with various combinations of gonadotropin-releasing hormone antagonist, an androgen receptor antagonist (flutamide), testosterone, and FSH, and we compared the gene expression levels 2 wk later to those of irradiated-only rats by microarray analysis. By dividing the gene expression patterns into three major patterns and 11 subpatterns, we successfully distinguished, in a single study, the genes that were specifically regulated by testosterone, by luteinizing hormone (LH), and by FSH from the large number of genes that were not hormonally regulated in the testis. We found that hormones produced more dramatic upregulation than downregulation of gene expression: Testosterone had the strongest upregulatory effect, LH had a modest but appreciable upregulatory effect, and FSH had a minor upregulatory effect. We also separately identified the somatic cell genes that were chronically upregulated by irradiation. Thus, the present study identified gene expression changes that may be responsible for hormonal action on somatic cells to support normal spermatogenesis and the hormone-mediated block in spermatogonial development after irradiation.

Evidence for the expression of alcohol dehydrogenase class I gene in rat uterus and its up-regulation by progesterone

The endometrium is one of the target tissues of the ovarian steroid hormones, estrogen and progesterone. In order to elucidate the mechanism of gene regulation in the endometrium, suppressive subtraction hybridization was performed to isolate the candidate genes controlled by progesterone in rat uterus. Alcohol dehydrogenase (ADH) class I gene was one of the candidate genes. Here we investigated the expression and regulation of ADH class I gene in rat uterus. The mRNA of ADH class I was detected in uterus by RT-PCR using specific primers. Using specific probe for ADH class I, in situ hybridization was performed to investigate localization in rat uterus. Positive signals were detected in the endometrial stromal cells of rat uterus by in situ hybridization and were not detected in endometrial epithelial cells and myometrium in rat uterus. Ovariectomized rats were treated with 17-beta estradiol and progesterone and the uteri of these rats were used for Northern blot analysis and assay of the ADH activity. Northern blot analysis revealed that the expression of ADH class I mRNA in rat uteri was up-regulated approximately two-fold after progesterone treatment, but not estrogen. Likewise, ADH activity was approximately two-fold higher in progesterone-treated rat uteri compared with controls. This study demonstrated that ADH class I gene is progesterone-responsive in the uterus. This implies that progesterone might be involved with retinoic acid synthesis in the uterus, since ADH is the key enzyme for retinoic acid synthesis.

Expression of the murine retinol dehydrogenase 10 (Rdh10) gene correlates with many sites of retinoid signalling during embryogenesis and organ differentiation

Retinoic acid acts as a signalling molecule regulating many developmental events in vertebrates. As this molecule directly influences gene expression by activating nuclear receptors, its patterns of synthesis have to be tightly regulated, and it is well established that at least three retinaldehyde dehydrogenases (RALDHs) are involved in such tissue-specific synthesis. Whereas embryos from oviparous species can obtain retinaldehyde by metabolizing carotenoids stored in the yolk, placental embryos rely on retinol transferred from the maternal circulation. Here, we show that the gene encoding one of the murine retinol dehydrogenases, Rdh10, is expressed according to complex profiles both during early embryogenesis and organ differentiation. Many of its expression sites correlate with regions of active retinoid signalling and Raldh gene expression, especially with Raldh2 in the early presomitic and somitic mesoderm, retrocardiac and posterior branchial arch region, or later in the pleural mesothelium and kidney cortical region. Rdh10 also shows cell-type and/or regional specificity during development of the palate, teeth, and olfactory system. During limb bud development, it may participate in retinoic acid production in proximal/posterior cells, and eventually in interdigital mesenchyme. These data implicate the retinol to retinaldehyde conversion as the first step in the tissue-specific regulation of retinoic acid synthesis, at least in mammalian embryos.

Disease gene candidates revealed by expression profiling of retinal ganglion cell development

To what extent do postmitotic neurons regulate gene expression during development or after injury? We took advantage of our ability to highly purify retinal ganglion cells (RGCs) to profile their pattern of gene expression at 13 ages from embryonic day 17 through postnatal day 21. We found that a large proportion of RGC genes are regulated dramatically throughout their postmitotic development, although the genes regulated through development in vivo generally are not regulated similarly by RGCs allowed to age in vitro. Interestingly, we found that genes regulated by developing RGCs are not generally correlated with genes regulated in RGCs stimulated to regenerate their axons. We unexpectedly found three genes associated with glaucoma, optineurin, cochlin, and CYP1B1 (cytochrome P450, family 1, subfamily B, polypeptide 1), previously thought to be primarily expressed in the trabecular meshwork, which are highly expressed by RGCs and regulated through their development. We also identified several other RGC genes that are encoded by loci linked to glaucoma. The expression of glaucoma-linked genes by RGCs suggests that, at least in some cases, RGCs may be directly involved in glaucoma pathogenesis rather than indirectly involved in response to increased intraocular pressure. Consistent with this hypothesis, we found that CYP1B1 overexpression potentiates RGC survival.

Retinoic acid down-regulates Tbx1 expression in vivo and in vitro

Both Tbx1 and retinoic acid (RA) are key players in embryonic pharyngeal development; loss of Tbx1 produces DiGeorge syndrome-like phenotypes in mouse models as does disruption of retinoic acid homeostasis. We have demonstrated that perturbation of retinoic acid levels in the avian embryo produces altered Tbx1 expression. In vitamin A-deficient quails, which lack endogenous retinoic acid, Tbx1 expression patterns were disrupted early in development and expression was subsequently lost in all tissues. "Gain-of-function" experiments where RA-soaked beads were grafted into the pharyngeal region produced localized down-regulation of Tbx1 expression. In these embryos, analysis of Shh and Foxa2, upstream control factors for Tbx1, suggested that the effect of RA was independent of this regulatory pathway. Real-time polymerase chain reaction analysis of retinoic acid-treated P19 cells showed a dose-dependent repression of Tbx1 by retinoic acid. Repression of Tbx1 transcript levels was first evident after 8-12 hr in culture in the presence of retinoic acid, and to achieve the highest levels of repression, de novo protein synthesis was required.

Contribution of cellular retinol-binding protein type 1 to retinol metabolism during mouse development

Within cells, retinol (ROL) is bound to cytoplasmic proteins (cellular retinol-binding proteins [CRBPs]), whose proposed function is to protect it from unspecific enzymes through channeling to retinoid-metabolizing pathways. We show that, during development, ROL and retinyl ester levels are decreased in CRBP type 1 (CRBP1) -deficient embryos and fetuses by 50% and 80%, respectively. The steady state level of retinoic acid (RA) is also decreased but to a lesser extent. However, CRBP1-null fetuses do not exhibit the abnormalities characteristic of a vitamin A-deficiency syndrome. Neither CRBP1 deficiency alters the expression patterns of RA-responding genes during development, nor does CRBP1 availability modify the expression of an RA-dependent gene in primary embryonic fibroblasts treated with ROL. Therefore, CRBP1 is required in prenatal life to maintain normal amounts of ROL and to ensure its efficient storage but seems of secondary importance for RA synthesis, at least under conditions of maternal vitamin A sufficiency.

Tissue- and species-specific expression patterns of class I, III, and IV Adh and Aldh1 mRNAs in rodent embryos

Alcohol and aldehyde dehydrogenases (ADHs and ALDHs) may be of interest in the pathology of Parkinson's disease (PD) because of their role in protection against toxins and in retinoid metabolism, which is required for growth and development of the mesencephalic dopamine system. In the present study, the spatial and temporal expression patterns of Adh 1, Adh 3, Adh 4, and Aldh 1 mRNAs in embryonic C57BL/6 mice (E 9.5-E19.5) and Sprague-Dawley rats (E12.5-P0) have been investigated by using radioactive oligonucleotide in situ hybridization. High expression of Aldh 1 mRNA was found in the developing mesencephalic dopamine neurons of both mice and rats. Expression of Adh 1 and Adh 4 mRNAs was observed in adrenal cortex and olfactory epithelium in mice. Additionally, Adh 1 was expressed in epidermis, liver, conjunctival, and intestinal epithelium. In rat embryos, expression was less extensive, with Adh 1 mRNA being found in liver and intestines. Adh 3 expression was ubiquitous in both mouse and rat embryos, suggesting a housekeeping function of the gene. Consistent with previous studies in adult rats and mice, our data suggest that Adh 3 is the only ADH class present in rodent brain. Adh and Aldh gene activity in mouse and rat embryos indicate the possible involvement of the respective enzymes in retinoid metabolism and participation in defense against toxic insults, including those that may be involved in the pathogenesis of PD.

Alcohol dehydrogenases and aldehyde dehydrogenases among inbred strains of mice: multiplicity, development, genetic studies and metabolic roles

Alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) are the major enzymes responsible for the metabolism of alcohols and aldehydes in the body. Both exist as a family of isozymes in mammals, and have been extensively studied in animal models, particularly among inbred strains of mice. Mouse ADH exists as at least three major classes, which are predominantly localized in liver (classes I and III), and in stomach/cornea (class IV). Mouse ALDH exhibits extensive multiplicity, several forms of which have been characterized, including ALDH1 (liver cytoplasmic/class 1 isozyme); ALDH2 (liver mitochondrial/class 2.); ALDH3 (stomach cytosolic/class 3); ALDH4 (liver microsomal/class 3); and ALDH5 (testis cytosolic/class 3). Biochemical, genetic and molecular genetic analyses have been performed on several of these enzymes, including studies on variant forms of ADH and ALDH. Distinct metabolic roles are proposed, based upon their tissue and subcellular distribution characteristics and the biochemical properties for these enzymes.

Differential gene-expression patterns in genital fibroblasts of normal males and 46,XY females with androgen insensitivity syndrome: evidence for early programming involving the androgen receptor

BACKGROUND

Androgen insensitivity syndrome (AIS) comprises a range of phenotypes from male infertility to complete feminization. Most individuals with AIS carry germline mutations of the androgen receptor (AR) that interfere with or ablate its function. As genital fibroblasts retain expression of the AR in vitro, we used genital skin fibroblasts from normal males and 46,XY females with complete AIS due to known AR mutations to gain insights into the role of the AR in human genital differentiation.

RESULTS

Using DNA microarrays representing 32,968 different genes, we identified 404 transcripts with significant differences in transcription levels between genital skin fibroblasts cultured from normal and AIS-affected individuals. Gene-cluster analyses uncovered coordinated expression of genes involved in key processes of morphogenesis. On the basis of animal studies and human genetic syndromes, several of these genes are known to have specific roles in genital differentiation. Remarkably, genital fibroblasts from both normal and AIS-affected individuals showed no transcriptional response to dihydrotestosterone treatment despite expression of the AR.

CONCLUSIONS

The results suggest that in addition to differences in the anatomic origin of the cells, androgen signaling during prenatal development contributes to setting long-lasting, androgen-independent transcriptional programs in genital fibroblasts. Our findings have broad implications in understanding the establishment and the stability of sexual dimorphism in human genital development.

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.

Embryonic liver cells and permanent lines as models for hepatocyte and bile duct cell differentiation

Analysis of liver cells during development is facilitated by the possibility of complementing in vivo analysis with experiments on cultured cells. In this review, we discuss results from several laboratories concerning bipotential hepatic stem cells from mouse (HBC-3, H-CFU-C, MMH and BMEL), rat (rhe14321) and primate (IPFLS) embryos. Several groups have used fluorescence-activated cell sorting to identify clonogenic bipotential cells; others have derived bipotential cell lines by plating liver cell suspensions and cloning. The bipotential cells, which probably originate from hepatoblasts, can differentiate as hepatocytes or bile duct cells, and undergo morphogenesis in culture. Disparities in differentiation can be explained by distinct medium compositions, extracellular matrix coated culture surfaces, and gene expression detection methods. Potential applications of these cell lines are discussed.

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.

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.

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.

Acetaldehyde in vitro exposure and apoptosis: a possible mechanism of teratogenesis

Alcohol abuse by pregnant women can result in fetal alcohol effects (FAE) and fetal alcohol syndrome (FAS). Both ethanol itself and its main metabolite, acetaldehyde (Ach), are able to produce specific FAS-related malformations. In previous in vitro studies, we documented that 10-day-old rat embryos exposed to Ach show a characteristic embryonic Ach syndrome, histologically characterized by marked cellular death. As both necrosis and pathological apoptosis are teratological mechanisms, the aim of this work was to evaluate if cellular death, observed in Ach-exposed embryos, can be related to necrotic or apoptotic events. Ten-day-old rat embryos were cultured in the presence of Ach 30-60 microg/ml and stained with the vital dye acridine orange to visualize apoptotic areas. After fixation, the TUNEL [3' terminal deoxynucleotide transferase (TdT)-mediated dUTP-biotin nick end labeling] method was used to histologically identify apoptosis. Both acridine orange and TUNEL staining showed signs of physiological apoptosis in controls and abnormal apoptotic regions in Ach-exposed embryos. Our results show a clear correlation between malformed organs and apoptotic embryonic districts, suggesting the role of apoptosis in Ach-induced abnormalities.

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.

Regulation of retinoic acid signaling during lung morphogenesis

Little is known about how retinoic acid (RA) synthesis, utilization and metabolism are regulated in the embryonic lung and how these activities relate to lung pattern formation. Here we report that early lung bud formation and subsequent branching morphogenesis are characterized by distinct stages of RA signaling. At the onset of lung development RA signaling is ubiquitously activated in primary buds, as shown by expression of the major RA-synthesizing enzyme, RALDH-2 and activation of a RARE-lacZ transgene. Nevertheless, further airway branching appears to require downregulation of RA pathways by decreased synthesis, increased RA degradation in the epithelium via P450RAI-mediated metabolism, and inhibition of RA signaling in the mesenchyme by COUPTF-II expression. These mechanisms controlling local RA signaling may be critical for normal branching, since we show that manipulating RA levels in vitro to maintain RA signaling activated as in the initial stage, leads to an immature lung phenotype characterized by failure to form typical distal buds. We show that this phenotype likely results from RA interfering with the establishment of a distal signaling center, altering levels and distribution of Fgf10 and Bmp4, genes that are essential for distal lung formation. Furthermore, RA upregulates P450RAI expression, suggesting the presence of feedback mechanisms controlling RA availability. Our study illustrates the importance of regional mechanisms that control RA availability and utilization for correct expression of pattern regulators and normal morphogenesis during lung development.

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.

The role of retinoic acid in embryonic and post-embryonic development

Retinoic acid (RA) is the bioactive metabolite of vitamin A (retinol) which acts on cells to establish or change the pattern of gene activity. Retinol is converted to RA by the action of two types of enzyme, retinol dehydrogenases and retinal dehydrogenases. In the nucleus RA acts as a ligand to activate two families of transcription factors, the RA receptors (RAR) and the retinoid X receptors (RXR) which heterodimerize and bind to the upstream sequences of RA-responsive genes. Thus, in addition to the well-established experimental paradigm of depriving animals of vitamin A to determine the role of RA in embryonic and post-embryonic development, molecular biology has provided us with two additional methodologies: knockout the enzymes or the RAR and RXR in the mouse embryo. The distribution of the enzymes and receptors, and recent experiments to determine the endogenous distribution of RA in the embryo are described here, as well as the effects on the embryo of knocking out the enzymes and receptors. In addition, recent studies using the classical vitamin A-deprivation technique are described, as they have provided novel insights into the regions of the embryo which crucially require RA, and the gene pathways involved in their development. Finally, the post-embryonic or regenerating systems in which RA plays a part are described, i.e. the regenerating limb, lung regeneration, hair cell regeneration in the ear and spinal cord regeneration in the adult.

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.

Changing patterns of renal retinal dehydrogenase expression parallel nephron development in the rat

We have recently characterized a cytosolic aldehyde dehydrogenase from rat kidney that functions as a retinal dehydrogenase (RALDH) and have cloned the corresponding gene. RALDH catalyzes the oxidation of retinal to retinoic acid, which regulates cell growth and differentiation by activating retinoic acid receptors. In situ hybridization demonstrates that RALDH mRNA expression is prominent in kidney in 2-day-old rats, is detected in lung and in epithelia of several tissues, but is not found in liver tissue. Retinal dehydrogenase activity peaks in kidney at Day 2 after birth and decreases gradually until adulthood, correlating well with RALDH expression. Weaker activity is also detectable in lungs but not in liver. Notably, distribution patterns of RALDH in kidney tissues are dramatically altered during postnatal development (P). From P0 to P6, hybridization is essentially concentrated within the marginal nephrogenic zone of the cortex. Expression progresses to deeper cortical layers from P12 to P16 and is intense in the medulla at P42, and focal expression is still detectable in the cortex. Immunocytochemical localization of RALDH in neonatal kidney shows staining mostly in cortical zone convoluted tubules and in adult rat shows staining in segments of distal and proximal tubules. These data suggest an important role for RALDH in modulating retinoic acid levels in different cell types during rat kidney development. The changing patterns of RALDH expression mirror stages of nephron formation in the developing rat kidney, strongly suggesting a central role for RALDH and thus for retinoids in controlling kidney development.

Restricted expression and retinoic acid-induced downregulation of the retinaldehyde dehydrogenase type 2 (RALDH-2) gene during mouse development

Retinaldehyde dehydrogenase type 2 (RALDH-2) was identified as a major retinoic acid generating enzyme in the early embryo. Here we report the expression domains of the RALDH-2 gene during mouse embryogenesis, which are likely to indicate regions of endogenous retinoic acid (RA) synthesis. During early gastrulation, RALDH-2 is expressed in the mesoderm adjacent to the node and primitive streak. At the headfold stage, mesodermal expression is restricted to posterior regions up to the base of the headfolds. Later, RALDH-2 is transiently expressed in the undifferentiated somites and the optic vesicles, and more persistently along the lateral walls of the intraembryonic coelom and around the hindgut diverticulum. The RALDH-2 expression domains in differentiating limbs, which include presumptive interdigital regions, coincide with, but slightly precede, those of the RA-inducible RAR beta gene. The RALDH-2 gene is also expressed in specific regions of the developing head, including the tooth buds, inner ear, meninges and pituitary gland, and in several viscera. Administration of a teratogenic dose of RA at embryonic day 8.5 results in downregulation of RALDH-2 transcript levels in caudal regions of the embryo, and may reflect a mechanism of negative feedback regulation of RA synthesis.

Erythropoietin in mouse avascular yolk sacs is increased by retinoic acid

Erythropoiesis begins first in the visceral yolk sac (VYS) of the embryo; however, the involvement of erythropoietin (EPO) in yolk-sac erythropoiesis has not been studied adequately. This study reports the expression of EPO in normal and hypoxic VYSs and alterations in yolk sac components induced by retinoic acid (RA) in mice. Gravid mice (plug day = day 0 of gestation) were given one oral dose of 60 mg/kg of RA in olive oil on days 6, 6.5, 7, 7.5, or 8 of gestation and were sacrificed 2.5, 3, or 3.5 days later. Control mice received olive oil without RA. None of the dams developed anemia, but more than 80% of the embryos of the dams that received RA on day 6, 6.5, or 7 of gestation had avascular yolk sacs (AVYs) and anemia. In these AVYs, the adenosine triphosphate (ATP) level was as low as 18-59% of that in the control VYSs. Reverse transcription-polymerase chain reaction and Southern analysis of products demonstrated that mRNA for EPO receptor (EPR) was expressed in both VYSs and AVYs on days 9-11 of gestation, and EPO mRNA was present in VYSs and AVYs on days 9 and 10 of gestation and in vehicle-exposed VYSs on day 11 of gestation. Furthermore, enzyme immunoassay of EPO indicated that AVYs contained more EPO protein than control VYSs. Light microscopy revealed that, in AVYs, in addition to the defective hemopoietic cells, the endodermal layer was exclusively altered: The presence of focal proliferated regions and the separation from the mesenchyme led to a single layer from which some immature cells seemed to be migrating. Immunolocalization of EPO showed its presence in all components of VYSs with a characteristic distribution pattern: In the endodermal layer, cells with positive EPO staining decreased as gestation advanced, and erythroid precursor cells showed positive staining. In AVYs, the proliferated endodermal cells had EPO in abundance; in the separated regions, the distinction between positive and negative EPO staining became clearer than that in the control VYSs, and the immature cells in the lumens also had EPO. EPR was seen on the cell surface of the corresponding cells that reacted to EPO. These findings suggest that VYSs not only produce EPO temporarily but also respond to the oxygen content in situ. EPO and EPR appear to be synthesized in the endodermal cells of the VYSs that are likely to respond to the circumstances induced by RA.

Expression patterns of class I and class IV alcohol dehydrogenase genes in developing epithelia suggest a role for alcohol dehydrogenase in local retinoic acid synthesis

Vitamin A (retinol) regulates embryonic development and adult epithelial function via metabolism to retinoic acid, a pleiotrophic regulator of gene expression. Retinoic acid is synthesized locally and functions in an autocrine or paracrine fashion, but the enzymes involved remain obscure. Alcohol dehydrogenase (ADH) isozymes capable of metabolizing retinol include class I and class IV ADHs, with class III ADH unable to perform this function. ADHs also metabolize ethanol, and high levels of ethanol inhibit retinol metabolism, suggesting a possible mode of action for some of the medical complications of alcoholism. To explore whether any ADH isozymes are linked to retinoic acid synthesis, herein we have examined the expression patterns of all known classes of ADH in mouse embryonic and adult tissues, and also measured retinoic acid levels. Using in situ hybridization, class I ADH mRNA was localized in the embryo to the epithelia of the genitourinary tract, intestinal tract, adrenal gland, liver, conjunctival sac, epidermis, nasal epithelium, and lung, plus in the adult to epithelia within the testis, epididymis, uterus, kidney, intestine, adrenal cortex, and liver. Class IV ADH mRNA was localized in the embryo to the adrenal gland and nasal epithelium, plus in the adult to the epithelia of the esophagus, stomach, testis, epididymis, epidermis, and adrenal cortex. Class III ADH mRNA, in contrast, was present at low levels and not highly localized in the embryonic and adult tissues examined. We detected significant retinoic acid levels in the fetal kidney, fetal/adult intestine and adrenal gland, as well as the adult liver, lung, testis, epididymis, and uterus--all sites of class I and/or class IV ADH gene expression. These findings indicate that the expression patterns of class I ADH and class IV ADH, but not class III ADH, are consistent with a function in local retinoic acid synthesis needed for the development and maintenance of many specialized epithelial tissues.

Abnormal spermatogenesis in RXR beta mutant mice

We have generated mouse lines in which the RXR beta gene was disrupted by homologous recombination. Approximately 50% of the RXR beta homozygous mutants died before or at birth, but those that survived appeared normal except that the males were sterile, owing to oligo-astheno-teratozoospermia. Failure of spermatid release occurred within the germinal epithelium, and the epididymis contained very few spermatozoa that, in addition, exhibited abnormal acrosomes and tails. There was a progressive accumulation of lipids within the mutant Sertoli cells, which were histochemically characterized as unsaturated triglycerides. In old mutant males, progressive degeneration of the germinal epithelium occurred, ending with the formation of acellular lipid-filled tubules. The selective expression of RXR beta in Sertoli cells, together with the timing of appearance of the histological abnormalities, suggests that the primary defect resulting from the mutation resides in these cells.

The class I alcohol dehydrogenase gene is glucocorticoid-responsive in the rat hepatoma microcell hybrid cell line, 11-3

Expression of the class I alcohol dehydrogenase (ADH) gene in the rat hepatoma microcell hybrid cell line, 11-3, was examined. The steady-state level of ADH mRNA in 11-3 was approximately 2-fold higher than that or rat liver and Fao, the parental cell line of 11-3. Removal of steroid hormones by activated charcoal from the serum in which 11-3 cells were maintained resulted in a significant decrease in the level of ADH transcript. Dexamethasone at a concentration of 1 muM increased the ADH mRNA content in 11-3 in a time-dependent fashion, up to 48 hr after its addition to cells that had first been deprived of steroid hormones. In addition, levels of ADH transcript in cells treated with dexamethasone increased in a dose-dependent manner, and the concentration of dexamethasone required to achieve half-maximal activation was 5 nM. By using the techniques of reverse transcription and polymerase chain reaction, and by taking advantage of a restriction polymorphism present between the rat and mouse ADH cDNA, we found that 11-3 contained both the rat and mouse class I ADH transcripts, although the rat sequence accounted for the great majority. Moreover, levels of both rat and mouse class I ADH transcripts increased in a similarly time-dependent manner in cells treated with dexamethasone. These results indicate that expression of class I ADH gene in 11-3 is high and is regulated by glucocorticoids, making the cell line an excellent model for the in vitro study of ADH expression.

Ethanol-induced teratogenesis: free radical damage as a possible mechanism

To investigate the possibility of a free radical mechanism for ethanol-induced teratogenesis, gestational day 8 mouse embryos were exposed for 6 hr in whole embryo culture to a teratogenic dosage of ethanol alone (500 mg%) or in conjunction with an antioxidant, superoxide dismutase (SOD; 300 U/ml). For subsequent analysis, some embryos were examined at the end of this 6-hr period, while others were removed to control medium and cultured for an additional time period. Ethanol exposure resulted in increased superoxide anion generation and increased lipid peroxidation (as noted 6 hr after initial ethanol exposure) and in excessive cell death (as noted 12 hr after initial exposure) in the embryos. Following a total of 36 hr in culture, a high incidence of malformation, including failure of the anterior neural tube to close in 63% of the ethanol-exposed embryos, was noted. The ethanol-induced superoxide anion generation, lipid peroxidation, excessive cell death, and dysmorphogenesis were diminished in embryos co-treated with SOD, suggesting that the teratogenicity of ethanol is mediated, at least in part, by free radical damage.

Triopia: craniofacial malformation with prosencephalic duplication

A case of ocular duplication with complex craniofacial and central nervous system anomalies is described. The anomaly is termed triopia because the child's most overt and distinguishing feature was three eyes: the left orbit contained two globes with independent ocular adnexa; the right orbit contained one normal appearing and functioning globe. Computer assisted medical imaging was used to define, in vivo, the intra- and extracranial soft and hard tissue anomalies: the cerebral hemisphere ipsilateral to the ocular duplication was also duplicated. Possible bases for this anomaly include duplication of primordia for the eye and secondary prosencephalon.

Retinoic acid regulates the development of oligodendrocyte precursor cells in vitro

Cultures of oligodendrocyte precursor cells can be grown from brain hemispheres of newborn rats. These cells, also called O-2A progenitor cells, can differentiate in vitro into oligodendrocytes or type 2 astrocytes. Basic FGF and PDGF are known to stimulate their proliferation and delay their differentiation. Lack or excess of retinoic acid (RA) has been known for a long time to alter brain development suggesting that this compound is involved in normal brain development. Here we report that RA partially inhibits both the proliferation and the differentiation of oligodendrocyte precursor cells. It also down-regulates the mitogenic effect of bFGF on these cells while keeping them in an immature stage. RA is more effective than bFGF in inhibiting myelin basic protein mRNA expression in these cells, and like bFGF, it preserves their bipotential character. RA nuclear receptors RAR-alpha and their transcripts are expressed in oligodendrocyte precursor cells as seen by Western blot, Northern blot and in situ hybridization. The expression of RAR-alpha transcripts is stimulated transiently by RA alone or associated to bFGF. The expression of RAR-beta transcripts is not constitutive and is induced by RA alone or associated to bFGF and to a lesser extent by bFGF alone. These results suggest that retinoids participate in the control of the development of glial cells of the oligodendrocyte lineage.