Elevations in the endogenous levels of the putative morphogen retinoic acid in embryonic mouse limb-buds associated with limb dysmorphogenesis

Retinoic Acid-Induced Limb Duplications

Retinoic acid (RA) and the family of molecules based on vitamin A known as retinoids have remarkable effects on limb regeneration in salamanders and newts and cause whole limb duplications in a concentration-dependent manner. They respecify all three axes of the limb-the proximodistal, the anteroposterior, and the dorsoventral axis. As a result, complete limbs can be induced to regenerate from distal amputation planes producing two limbs in tandem. Here, we describe the basic methods for undertaking these experiments as well as the use of new synthetic retinoids which have retinoic acid receptor-selective actions. These will be valuable tools in future studies on the molecular basis of limb duplications and thus our understanding of the nature of positional information in the regenerating salamander limb.

RA Signaling in Limb Development and Regeneration in Different Species

This chapter brings together data on the role of retinoic acid (RA) in the embryonic development of fins in zebrafish , limbs in amphibians , chicks , and mice, and regeneration of the amphibian limb . The intention is to determine whether there is a common set of principles by which we can understand the mode of action of RA in both embryos and adults. What emerges from this synthesis is that there are indeed commonalities in the involvement of RA in processes that ventralize, posteriorize, and proximalize the developing and regenerating limb . Different axes of the limb have historically been studied independently; as for example, the embryonic development of the anteroposterior (AP) axis of the chick limb bud versus the regeneration of the limb bud proximodistal (PD) axis . But when we take a broader view, a unifying principle emerges that explains why RA administration to embryos and regenerating limbs results in the development of multiple limbs in both cases. As might be expected, different molecular pathways govern the development of different systems and model organisms, but despite these differences, the pathways involve similar RA signaling genes, such as tbx5, meis, shh, fgfs and hox genes. Studies of developing and regenerating systems have highlighted that RA acts by being synthesized in one embryonic location while acting in another one, exactly as embryonic morphogens do, although there is no evidence for the presence of an RA gradient within the limb . What also emerges is that there is a paucity of information on the involvement of RA in development of the dorsoventral (DV) axis . A molecular explanation as to how RA establishes and alters positional information in all three axes is the most important area of study for the future.

Roles of retinoic acid signaling in normal and abnormal development of the palate and tongue

Palatogenesis involves various developmental events such as growth, elevation, elongation and fusion of opposing palatal shelves. Extrinsic factors such as mouth opening and subsequent tongue withdrawal are also needed for the horizontal elevation of palate shelves. Failure of any of these steps can lead to cleft palate, one of the most common birth defects in humans. It has been shown that retinoic acid (RA) plays important roles during palate development, but excess RA causes cleft palate in fetuses of both rodents and humans. Thus, the coordinated regulation of retinoid metabolism is essential for normal palatogenesis. The endogenous RA level is determined by the balance of RA-synthesizing (retinaldehyde dehydrogenases: RALDHs) and RA-degrading enzymes (CYP26s). Cyp26b1 is a key player in normal palatogenesis. In this review, we discuss recent progress in the study of the pathogenesis of RA-induced cleft palate, with special reference to the regulation of endogenous RA levels by RA-degrading enzymes.

Development of the marsupial shoulder girdle complex: a case study in Monodelphis domestica

During their embryogenesis, marsupials transiently develop a unique structure, the shoulder arch, which provides the structural support and muscle-attachments necessary for the newborn's crawl to the teat. One of the most pronounced and functionally important aspects of the shoulder arch is an enlarged coracoid. The goal of this study is to determine the molecular basis of shoulder arch formation in marsupials. To achieve this goal, this study investigates the relative expression of several genes with known roles in shoulder girdle morphogenesis in a marsupial-the opossum, Monodelphis domestica-and a placental, the mouse, Mus musculus. Results indicate that Hoxc6, a gene involved in coracoid patterning, is expressed for a longer period of time and at higher levels in opossum relative to mouse. Functional manipulation suggests that these differences in Hoxc6 expression are independent of documented differences in retinoic acid signaling in opossum and mouse forelimbs. Results also indicate that Emx2, a gene involved in scapular blade condensation, is upregulated in opossum relative to mouse. However, several other genes involved in shoulder girdle patterning (e.g., Gli3, Pax1, Pbx1, Tbx15) are comparably expressed in these species. These findings suggest that the upregulation of Hoxc6 and Emx2 occurs through independent genetic modifications in opossum relative to mouse. In summary, this study documents a correlation between gene expression and the divergent shoulder girdle morphogenesis of marsupial (i.e., opossum) and placental (i.e., mouse) mammals, and thereby provides a foundation for future research into the genetic basis of shoulder girdle morphogenesis in marsupials. Furthermore, this study supports the hypothesis that the mammalian shoulder girdle is a highly modular structure whose elements are relatively free to evolve independently.

Retinoic acid influences the embryoid body formation in mouse embryonic stem cells by induction of caspase and p38 MAPK/JNK-mediated apoptosis

Although all-trans retinoic acid (RA), the oxidative metabolite of vitamin A, is essential for normal development, high levels are teratogenic in many species. RA results in immediate effects on the preimplantation embryo and on blastocyst development in vitro and in vivo. To further elucidate the cellular mechanisms of early postimplantation embryo development induced by RA, we present an embryonic cell line, B5, as a candidate system for the investigation of these processes. We used undifferentiated ES cells as the model, which is from the undifferentiated status to differentiated status [embryoid body (EB) formation] mimicking postimplantation embryo development (egg-cylinder stage of embryo formation) to clarify the cellular mechanism of action of RA in the implanted blastocysts and cell apoptosis following the series of exposures to differing RA concentrations. Using an in vitro model, we identified the impact of RA on undifferentiated embryonic stem (ES) cells, including inhibition of cell proliferation and induction of cell apoptosis. JNK, P-38 and caspase activation were shown in the nature of RA-triggered apoptotic signaling in ES cells. The carry-on influences of RA on the ES cell were shown in the formation of EB from the pretreated ES cells. RA resulted in apparent impact on undifferentiated ES cells in vitro, with increased numbers of apoptotic cells initially and inhibited cell proliferation, which led to decreased size of EB. The process of EB formation (mimicking the early postimplantation embryo development) is regulated by RA-induced apoptosis through the activation of caspase and P38 MAPK/JNK pathway.

A paradoxical teratogenic mechanism for retinoic acid

Retinoic acid, an active metabolite of vitamin A, plays essential signaling roles in mammalian embryogenesis. Nevertheless, it has long been recognized that overexposure to vitamin A or retinoic acid causes widespread teratogenesis in rodents as well as humans. Although it has a short half-life, exposure to high levels of retinoic acid can disrupt development of yet-to-be formed organs, including the metanephros, the embryonic organ which normally differentiates into the mature kidney. Paradoxically, it is known that either an excess or a deficiency of retinoic acid results in similar malformations in some organs, including the mammalian kidney. Accordingly, we hypothesized that excess retinoic acid is teratogenic by inducing a longer lasting, local retinoic acid deficiency. This idea was tested in an established in vivo mouse model in which exposure to excess retinoic acid well before metanephric rudiments exist leads to failure of kidney formation several days later. Results showed that teratogen exposure was followed by decreased levels of Raldh transcripts encoding retinoic acid-synthesizing enzymes and increased levels of Cyp26a1 and Cyp26b1 mRNAs encoding enzymes that catabolize retinoic acid. Concomitantly, there was significant reduction in retinoic acid levels in whole embryos and kidney rudiments. Restoration of retinoic acid levels by maternal supplementation with low doses of retinoic acid following the teratogenic insult rescued metanephric kidney development and abrogated several extrarenal developmental defects. This previously undescribed and unsuspected mechanism provides insight into the molecular pathway of retinoic acid-induced teratogenesis.

Vitamin A in reproduction and development

The requirement for vitamin A in reproduction was first recognized in the early 1900's, and its importance in the eyes of developing embryos was realized shortly after. A greater understanding of the large number of developmental processes that require vitamin A emerged first from nutritional deficiency studies in rat embryos, and later from genetic studies in mice. It is now generally believed that all-trans retinoic acid (RA) is the form of vitamin A that supports both male and female reproduction as well as embryonic development. This conclusion is based on the ability to reverse most reproductive and developmental blocks found in vitamin A deficiency induced either by nutritional or genetic means with RA, and the ability to recapitulate the majority of embryonic defects in retinoic acid receptor compound null mutants. The activity of the catabolic CYP26 enzymes in determining what tissues have access to RA has emerged as a key regulatory mechanism, and helps to explain why exogenous RA can rescue many vitamin A deficiency defects. In severely vitamin A-deficient (VAD) female rats, reproduction fails prior to implantation, whereas in VAD pregnant rats given small amounts of carotene or supported on limiting quantities of RA early in organogenesis, embryos form but show a collection of defects called the vitamin A deficiency syndrome or late vitamin A deficiency. Vitamin A is also essential for the maintenance of the male genital tract and spermatogenesis. Recent studies show that vitamin A participates in a signaling mechanism to initiate meiosis in the female gonad during embryogenesis, and in the male gonad postnatally. Both nutritional and genetic approaches are being used to elucidate the vitamin A-dependent pathways upon which these processes depend.

Cephalometric Analysis of Craniofacial Malformations in Newborn Mice with Cleft Palate Induced by Retinoic Acid

Objective

To determine changes on craniofacial growth morphometrically in newborn mice with cleft palate induced by retinoic acid.

Design, Setting, Participants, Interventions

Gestation day 10 or 12 pregnant female C57BL/6N mice were given a single dose of all-trans retinoic acid (atRA) by gastric intubations via oral gavage. Sixty newborn mice with cleft palate (CP), 52 without CP from the experimental group, and 30 without CP from the control group were collected, and lateral cephalograms were taken of all of the mice.

Main Outcome Measures

Cephalometric analysis of the craniofacial skeleton was performed by means of a personal computer.

Results

Inhibition of craniofacial growth was found in the experimental groups but not in the control groups. In the maxillary bone and mandible, the amount of growth was significantly reduced.

Conclusions

These results suggest that craniofacial growth is inhibited in newborn mice with cleft palate induced by retinoic acid.

Retinoic acid induces prostatic bud formation

Formation of prostatic buds from the urogenital sinus (UGS) to initiate prostate development requires localized action of several morphogenetic factors. This report reveals all-trans-retinoic acid (RA) to be a powerful inducer of mouse prostatic budding that is associated with reciprocal changes in expression of two regulators of budding: sonic hedgehog (Shh) and bone morphogenetic protein 4 (Bmp4). Localization of retinoid signaling and expression of RA synthesis, metabolism, and receptor genes in the UGS on embryonic days 14.5-17.5 implicate RA in the mechanism of bud initiation. In UGS organ culture, RA increased prostatic budding, increased Shh expression, and decreased Bmp4. Prostatic budding was stimulated in the absence of RA by recombinant SHH, by blocking BMP4 signaling with NOGGIN, or by combined treatment with SHH and NOGGIN in UGS organ culture media. These observations suggest that reciprocal changes in hedgehog and BMP signaling by RA may regulate bud initiation.

Locust retinoid X receptors: 9-Cis-retinoic acid in embryos from a primitive insect

The retinoid X receptor (RXR) is activated by its often elusive cognate ligand, 9-cis-retinoic acid (9-cis-RA). In flies and moths, molting is mediated by a heterodimer ecdysone receptor consisting of the ecdysone monomer (EcR) and an RXR homolog, ultraspiracle (USP); the latter is believed to have diverged from its RXR origin. In the more primitive insect, Locusta migratoria (Lm), RXR is more similar to human RXRs than to USPs. LmRXR was detected in early embryos when EcR transcripts were absent, suggesting another role apart from ecdysone signaling. Recombinant LmRXRs bound 9-cis-RA and all-trans-RA with high affinity (IC(50) = 61.2-107.7 nM; K(d) = 3 nM), similar to human RXR. To determine whether specific binding had functional significance, the presence of endogenous retinoids was assessed. Embryos were extracted by using modified Bligh and Dyer and solid-phase protocols to avoid the oily precipitate that makes this material unsuitable for assay. These extracts contained retinoids (5.4 nM) as assessed by RA-inducible Cyp26A1-promoter luciferase reporter cell lines. Furthermore, the use of HPLC and MS confirmed the presence of retinoids and identified in any embryo, 9-cis-RA, in addition to all-trans-RA. We estimate that whole embryos contain 3 nM RA, including 9-cis-RA at a concentration of 1.6 nM. These findings strongly argue for a functional role for retinoids in primitive insects and favor a model where signaling through the binding of 9-cis-RA to its RXR is established relatively early in evolution and embryonic development.

Comparison of the expression patterns of newly identified zebrafish retinoic acid and retinoid X receptors

Retinoic acid (RA) signaling is important for multiple aspects of embryonic development and tissue homeostasis. Heterodimers of retinoic acid receptors (RARs) and retinoid X receptors (RXRs) transduce RA signaling. It is not yet clear how the diversity of receptor combinations relates to the diversity of functions for RA. The expression patterns of three zebrafish RARs and four RXRs were reported recently. Here, we identify an additional RAR, a zebrafish RARgamma paralog, and two additional RXRs, duplicates of the previously identified RXRalpha and RXRgamma. Thus, the zebrafish genome contains duplicates of each RAR and RXR gene. All zebrafish RAR and RXR paralogs have overlapping and distinct areas of expression, as might be expected for duplicate genes in the process of diverging in function. By representing what is potentially the complete set of zebrafish RARs and RXRs, this study provides a valuable reference for future functional studies of the individual zebrafish RARs and RXRs.

Metabolism of retinol during mammalian placental and embryonic development

Retinol (vitamin A) is a fat-soluble nutrient indispensable for a harmonious mammalian gestation. The absence or excess of retinol and its active derivatives [i.e., the retinoic acids (RAs)] can lead to abnormal development of embryonic and extraembryonic (placental) structures. The embryo is unable to synthesize the retinol and is strongly dependent on the maternal delivery of retinol itself or precursors: retinyl esters or carotenoids. Before reaching the embryonic tissue, the retinol or the precursors have to pass through the placental structures. During this placental step, a simple diffusion of retinol can occur between maternal and fetal compartments; but retinol can also be used in situ after its activation into RA(1) or stored as retinyl esters. Using retinol-binding protein knockout model, an alternative way of embryonic retinol supply was described using retinyl esters incorporated into maternal chylomicrons. In the embryo, the principal metabolic event occurring for retinol is its conversion into RAs, the active molecules implicated on the molecular control of embryonic morphogenesis and organogenesis. All these placental and embryonic events of retinol transport and metabolism are highly regulated. Nevertheless, some genetic and/or environmental abnormalities in the transport and/or metabolism of retinol can be related to developmental pathologies during mammalian development.

Identification and characterization of a novel gene, Mcpr1, and its possible function in the proliferation of embryonic palatal mesenchymal cells

We cloned a novel mouse cDNA, Mcpr1 (mouse cleft palate-related gene 1), between retinoic acid (RA)-treated murine embryonic palatal and control shelves by improved subtractive hybridization. Its transcript was identified by Northern blotting. The open reading frame encodes 132 amino acids and shows almost no identity to other genetic products. Mcpr1 expression could be detected extensively in adult mouse tissues and during murine embryonic development. It was identified to be significantly stimulated by RA in murine palatal shelves at embryonic day 12 and in palatal mesenchymal cells in vitro. We demonstrate that MCPR1 protein was localized primarily in the cytoplasm and could be synthesized and secreted by transfected COS-7 cells. Both the secretory and recombinant proteins of Mcpr1 inhibited proliferation of murine embryonic palatal mesenchymal cells and impeded the progression from the G1 to S phase in the cell cycle. The cells were prone to apoptosis after exposure to glutathione S-transferase-MCPR1. Furthermore, knockdown of MCPR1 protein levels by antisense oligodeoxynucleotides promoted progression of cells from the G1 to S phase and completely abolished the RA-induced block of the cell cycle from the G1 to S phase. These findings suggest that Mcpr1 might function as one of the RA-up-regulated genes involved in inhibiting cell proliferation during palatogenesis and RA-induced cleft palate by regulating proliferation and apoptosis of embryonic palatal mesenchymal cells and might even play a role in the development of many other organs.

All-trans retinoic acid inhibited chondrogenesis of mouse embryonic palate mesenchymal cells by down-regulation of TGF-beta/Smad signaling

Chondrogenesis is a critical step in palatogenesis. All-trans retinoic acid (atRA), a vitamin A derivative, is a known teratogenic effector of cleft palate. Here, we evaluated the effects of atRA on the osteo-/chondrogenic differentiation of mouse embryonic palate mesenchymal (MEPM) cells. MEPM cells, in a high-density micromass environment, undergo active chondrogenesis in a manner analogous to that of limb-derived mesenchymal cells, and served as a valid model system to investigate the mechanisms regulating chondrogenesis during palatogenesis. atRA-treated MEPM micromass expressed relatively higher levels of osteoblastic gene markers (alkaline phosphatase and collagen type I) and lower levels of chondrocytic gene markers (collagen type II and aggrecan). As transforming growth factor-beta3 (TGF-beta3) is an essential growth factor for chondrogenesis of embryonic mesenchymal cells both in in vivo and in vitro conditions, we thereby explored the effects of atRA on TGF-beta3 signaling pathway. atRA led to an increase in mRNA expression of TGF-beta3 and an instantaneous decrease in TGF-beta type II receptor (TbetaRII) as determined by real-time RT-PCR. Further study showed that atRA inhibited phosphorylation of Smad2 and Smad3 and increased Smad7 expression. Activation of the Smad pathways by transfection with Smad7deltaC mutant or constitutively active TbetaRII retroviral vector abolished atRA-induced inhibition of chondrogenesis as indicated by Alcian blue staining, indicating that Smad signaling is essential for this response. Taken together, these data for the first time demonstrated a role for RA-induced hypochondrogenesis through regulation of the TGF-beta3 pathway and suggested a role for TbetaRII /Smad in retinoid-induced cleft palate.

Adverse effects of retinoic acid on embryo development and the selective expression of retinoic acid receptors in mouse blastocysts

BACKGROUND

All-trans retinoic acid (RA), the oxidative metabolite of vitamin A, is essential for normal development. In addition, high levels of RA are teratogenic in many species. We have previously shown that excess RA results in immediate effects on the preimplantation embryo and on blastocyst development. This study was conducted to clarify the long-term survival of mouse blastocyst and the effect of RA on gene expression.

METHODS AND RESULTS

Using an in vitro model, we identified the immediate adverse impact of RA on mouse blastocyst development. This involved an inhibition of cell proliferation and growth retardation. Using an in vivo model, we also identified the resorption of postimplanted blastocysts that had been treated with excess RA. Analysis of RA-mediated gene induction was also included. The retinoic acid receptors RARalpha and RARgamma were constitutively expressed in the blastocyst and the inner cell mass, whereas RARbeta was induced upon RA treatment.

CONCLUSIONS

This is the first evidence to show the impacts of RA on mouse blastocysts in vitro and any carry-over effects in the uterus. There is a retardation of early postimplantation blastocyst development and then subsequent blastocyst death. Our findings also show that there is some degree of selective induction of retinoic acid receptors when excess RA is administered to the blastocysts.

Implication of Wt1 in the pathogenesis of nephrogenic failure in a mouse model of retinoic acid-induced caudal regression syndrome

Renal malformations are common human birth defects that sometimes occur in the context of the caudal regression syndrome. Here, we found that exposure of pregnant mice to all-trans retinoic acid, at a time when the metanephros has yet to form, causes a failure of kidney development along with caudal regression. Maternal treatment with Am580 (retinoic acid receptor alpha agonist) also induced similar patterns of kidney maldevelopment in the fetus. In metanephroi from retinoic acid-treated pregnancies, renal mesenchyme condensed around the ureteric bud but then failed to differentiate into nephrons, instead undergoing involution by fulminant apoptosis to produce a renal agenesis phenotype. Results of whole organ cultures in serum-free medium, and also tissue recombination experiments, showed that the nephrogenic defect was intrinsic to the kidney and that it resided in the metanephric mesenchyme and not the ureteric bud. Renal mesenchyme from control embryos expressed Wilms' tumor 1 (Wt1), but this transcription factor, which is indispensable for kidney development, failed to express in metanephroi of retinoic acid-exposed embryos. Wt1 expression and organogenesis were both restored, however, when metanephroi from retinoic acid-treated pregnancies were grown in serum-containing media. Our data illuminate the pathobiology of a severe, teratogen-induced kidney malformation.

Effects of retinoic acid on the inner cell mass in mouse blastocysts

This is the first evidence that excess retinoic acid has a direct cellular response from proliferation to cell death (apoptosis) and affects in vitro development in mouse inner cell mass.

Cellular anomalies underlying retinoid-induced phocomelia

The question of how alterations in cell behavior produced by retinoic acid (RA) influenced the development of skeletogenic mesenchyme of the limb bud was examined in this study. Our established model was employed, which involves treatment of pregnant mice with a teratogenic dose of RA (100 mg/kg) on 11 days postcoitum (dpc) resulting in a severe truncation of all long bones of the forelimbs in virtually every exposed fetus. It is shown that RA, administered at a stage to induce phocomelia in virtually all exposed embryos, resulted in immediate appearance of enhanced cell death within the mesenchyme in the central core of the limb bud, an area destined for chondrogenesis. The central core mesenchyme, which in the untreated limb buds experiences a sharp decline in cell proliferation heralding the onset of chondrogenesis, demonstrated a reversal of the process; this mesenchyme maintained a higher rate of cell proliferation upon RA exposure. These events resulted in a truncation and disorganization of the chondrogenic anlage, more pronounced in zeugopodal mesenchyme than in the autopod. We conclude that an inhibition of chondrogenesis was secondary to a disruption in cellular behavior caused by RA, a likely consequence of misregulation in the growth factor signaling cascade.

Chromosome 17: gene mapping studies of cleft lip with or without cleft palate in Chinese families

OBJECTIVE

Involvement of loci on chromosome 17, including retinoic acid receptor alpha (RARA) in nonsyndromic oral clefts has been reported in Caucasian populations, although never investigated in Asian populations. The purpose of the present study was to investigate several loci on chromosome 17, including RARA, in Chinese families.

PARTICIPANTS

Thirty-six multiplex families (310 individuals), ascertained through nonsyndromic cleft lip with or without cleft palate surgical probands from hospitals in Shanghai, China, participated in the present study. There were 23 families whose probands had cleft lip and cleft palate (CLP) and 13 with cleft lip alone (CL).

RESULTS

Seventeen markers, spanning chromosome 17 and about 10 cM apart were assessed. Logarithm of odds ratio (LOD) scores (two point and multipoint), model-free linkage analyses, and allelic association tests (transmission/disequilibrium, Fisher's exact tests, and chi-square) were performed on the total family sample, families with CLP probands (CLP subgroup), and families with CL probands (CL subgroup). LOD scores from the two-point analyses were inconclusive. Multipoint analyses rejected linkage except for a few regions in the CL subgroup. However, positive results were found using the model-free linkage and association methods (p < .05). The markers with positive results varied across the CL and CLP subgroups. However, the RARA region and loci nearby yielded consistently positive results.

CONCLUSION

Genetic variation within the RARA locus or nearby appears to be involved in the pathogenesis of nonsyndromic oral clefts in this population. Furthermore, based on the differing pattern of results in the CL versus CLP subgroups, it appears that the formation of CL and CLP is because of either differing alleles at the same genetic locus or different but related (and/or linked) genes that modify the severity and expression of oral clefting.

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.

Quantitative axial profiles of retinoic acid in the embryonic mouse spinal cord: 9-cis retinoic acid only detected after all-trans-retinoic acid levels are super-elevated experimentally

Studies using bioassays in normal mice and gene activation in transgenic reporter mice have demonstrated peaks of retinoic acid receptor (RAR) signaling in the brachial and lumbar regions of the spinal cord. Recently, Solomin et al. (Solomin et al. [1998] Nature 395:398-402) detected a retinoid X receptor (RXR) signal in the same region of the developing spinal cord at a slightly later stage than the RAR signal. This finding raises the question of which retinoid ligands underlie RAR and RXR signaling in this part of the embryo. Quantitative measurements of regional differences in retinoid profiles have not been reported previously due to limitation in the sensitivity and specificity of available retinoid detection methods. Here, by using a recently developed ultrasensitive HPLC technique (Sakhi et al. [1998] J. Chromatogr. A 828:451-460), we address this question in an attempt to identify definitively the endogenous retinoids present in different regions of the spinal cord at the stages when regional differences in RAR and RXR signaling have been reported. We find a bimodal distribution of all-trans retinoic acid (at-RA), the ligand for RARs, and relate this to the expression of several retinoid-synthesizing enzymes. However, we do not detect 9-cis-retinoic acid (9-cis-RA), the putative RXR ligand, in any region of the spinal cord unless retinoid levels are massively increased experimentally by gavage feeding pregnant mice with teratogenic doses of at-RA. This study provides for the first time quantitative profiles of endogenous retinoids along the axis of the developing spinal cord, thereby establishing a foundation for more definitive studies of retinoid function in the future. It sets definite limits on how much 9-cis-RA potentially is present and demonstrates that at-RA predominates over 9-cis-RA by at least 30- to 180-fold in different spinal cord regions.

Retinoic acid promotes differentiation of trophoblast stem cells to a giant cell fate

Trophoblast stem cell (TS cell) lines have the ability to differentiate into trophoblast subtypes in vitro and contribute to the formation of placenta in chimeras. In order to investigate the possible role of retinoic acid (RA) in placentation, we analyzed the effects of exogenous RA on TS cells in vitro and the developing ectoplacental cone in vivo. TS cells expressed all subtypes of the retinoid receptor family, with the exception of RARbeta, whose expression was stimulated in response to RA. TS cells treated with RA were compromised in their ability to proliferate and exhibited properties of differentiation into trophoblast giant cells. During TS cell differentiation into trophoblast subtypes induced by withdrawal of FGF4, RA treatment further illustrated its role in the specification of cell fate by the promotion of differentiation into giant cells and the suppression of spongiotrophoblast formation. Moreover, administration of RA during pregnancy resulted in the overabundance of giant cells at the expense of spongiotrophoblast cells. RA hereby acts as an extracellular signal whose potential function can be linked to specification events mediating trophoblast cell fate. Taken together with the spatial patterns of giant-cell formation and RA synthesis in vivo, these findings implicate a function for RA in giant-cell formation during placentation.

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.

Treatment with all-trans-retinoic acid decreases levels of endogenous TGF-beta(1) in the mesenchyme of the developing mouse inner ear

BACKGROUND

Previous studies have shown that in utero exposure of the mouse embryo to high doses of all-trans-retinoic acid (atRA) produces defects of the developing inner ear and its surrounding cartilaginous capsule, while exposure of cultured periotic mesenchyme plus otic epithelium to high doses of exogenous atRA results in an inhibition of otic capsule chondrogenesis.

METHODS

In this study, we examine the effects of atRA exposure on the endogenous expression of transforming growth factor-beta(1) (TGF-beta(1)), a signaling molecule that mediates the epithelial-mesenchymal interactions that guide the development of the capsule of the inner ear.

RESULTS

Our results demonstrate a marked reduction in immunostaining for TGF-beta(1) in the periotic mesenchyme of atRA-exposed embryos of age E10.5 and E12 days in comparison with control specimens. Consistent with these in vivo findings, high-density cultures of E10.5 periotic mesenchyme plus otic epithelium, treated with doses of atRA that suppress chondrogenesis, showed significantly decreased levels of TGF-beta(1), as compared with TGF-beta(1) levels in untreated control cultures. Furthermore, we demonstrate a rescue of cultured periotic mesenchyme plus otic epithelium from atRA-induced chondrogenic suppression by supplementation of cultures with excess TGF-beta(1).

CONCLUSIONS

Our results support the hypothesis that TGF-beta(1) plays a role in mechanisms of atRA teratogenicity during inner ear development.

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.

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.

Determination of the optimal time and dosage of all-trans retinoic acid for induction of murine exencephaly

Murine exencephaly corresponds to human anencephaly, and provides a model for studying the mechanism of development of the central nervous system. A system which induces exencephaly at an extremely high rate is required in order to examine embryos, before the stage of neural tube closure, as samples of future exencephaly. Herein, we report on a system which is close enough to the best conditions for induction of this malformation, involving ICR mice and all-trans retinoic acid. The intraperitoneal administration of 30 mg/kg of all-trans retinoic acid at 03:00 hr on day 8 (copulatory plug, day 0) induced exencephaly in 81.6% of live embryos, as evaluated on day 10, with a 41.7% embryonic death rate. Earlier administration (more than 3 hr before) greatly increased the rate of embryonic death, whereas later administration resulted in a reduction in the rate of exencephaly. These findings suggest that a specific time during early development is crucial for neural tube closure, and provide a system which may facilitate the study of neural development and the pathophysiology of human anencephaly.

Retinoid-induced limb defects 1: inhibition of cell proliferation in distal mesenchyme of limb buds in rats

The present study was undertaken to investigate the effects of all-trans-retinoic acid (RA) on cell death and limb bud growth in forelimb buds and also to examine whether these events are involved in limb bone defects induced by RA in rats. RA was given at doses of 50 and 100 mg/kg to pregnant rats on Day 12 of pregnancy. Although RA did not show teratogenecity in the 50 mg/kg group, micromelia was observed in the 100 mg/kg group in all live fetuses on Day 21 of gestation. Micromelia was characterized by high incidences of proximodistal reduction of forearm bones without reduction of the humerus. The incidence of cell death in prechondrogenic areas, which differentiate into humerus and forearm bone, significantly increased 24 h after RA treatment in not only the 100 mg/kg, but also the 50 mg/kg, group. There was no difference in the incidence of cell death in the prechondrogenic area between the two groups. These observations indicate that the bone-specific defects were not the result of cell death alone in the prechondrogenic area. We examined the effects of RA on early forelimb bud growth, which is indispensable for the morphogenesis of the forelimb. Proximodistal length and protein content were decreased significantly in the forelimb bud 24 h after RA treatment at a dose of 100 mg/kg, but not 50 mg/kg. The immunohistochemical detection of bromodeoxyuridine (BrdU) incorporated into cells showed that at a dose of 100 mg/kg, cell proliferation was reduced in the distal mesenchyme, but not in the forearm-bone prechondrocytes of the forelimb bud. As the distal margin provides the cells differentiating into the prechondrocytes of future bones in the limb bud, these observations suggested that RA-induced inhibition of cell proliferation in the distal margin resulted in a decrease of forearm-bone prechondrocytes localized at more distal sites. We conclude that RA may inhibit the chondrogenesis of forearm bones by reducing cell proliferation in the distal margin of the forelimb bud, not by increasing cell death, and that this results in reduction defects in forearm bones.

The distribution of endogenous retinoic acid in the chick embryo: implications for developmental mechanisms

The aim of these experiments was to determine the endogenous distribution of retinoic acid (RA) across a wide range of embryonic stages in the chick embryo. By high pressure liquid chromatography, it was revealed that didehydroRA is the most prevalent retinoic acid in the chick embryo and that the tissues of the stage 24 embryo differed widely in their total RA content (didehydroRA + all-trans-RA). Some tissues such as the heart had very little RA and some such as the neural tube had very high levels, the total variation between these two being 29-fold. We showed that these tissues also synthesised RA and released it into the medium, thus validating the use of the F9 reporter cell system for further analyses of younger staged embryos. With these F9 cells, we showed that, at stage 4, the posterior end of the embryo had barely detectably higher levels of RA than the anterior end, but that a significant level of RA generation was detected as soon as somitogenesis began. Then a sharp on/off boundary of RA was present at the level of the first somite. We could find no evidence for a posterior-to-anterior gradient of RA. Throughout further development, various consistent observations were made: the developing brain did not generate RA, but the spinal part of the neural tube generated it at very high levels so there must be a sharp on/off boundary in the region of the hindbrain/spinal cord junction; the mesenchyme surrounding the hindbrain generated RA whereas the hindbrain itself did not; there was a variation in RA levels from the midline outwards with the highest levels of RA in the spinal neural tube followed by lower levels in the somites followed by lower levels in the lateral plate; the posterior half of the limb bud generated higher levels than the anterior half. With these observations, we were able to draw maps of endogenous RA throughout these early stages of chick embryogenesis and the developmental implications of these results are discussed.

Expression of the murine Hoxa4 gene requires both autoregulation and a conserved retinoic acid response element

Analysis of the regulatory regions of the Hox genes has revealed a complex array of positive and negative cis-acting elements that control the spatial and temporal pattern of expression of these genes during embryogenesis. In this study we show that normal expression of the murine Hoxa4 gene during development requires both autoregulatory and retinoic acid-dependent modes of regulation. When introduced into a Hoxa4 null background, expression of a lacZ reporter gene driven by the Hoxa4 regulatory region (Hoxa4/lacZ) is either abolished or significantly reduced in all tissues at E10. 5-E12.5. Thus, the observed autoregulation of the Drosophila Deformed gene is conserved in a mouse homolog in vivo, and is reflected in a widespread requirement for positive feedback to maintain Hoxa4 expression. We also identify three potential retinoic acid response elements in the Hoxa4 5′ flanking region, one of which is identical to a well-characterized element flanking the Hoxd4 gene. Administration of retinoic acid to Hoxa4/lacZ transgenic embryos resulted in stage-dependent ectopic expression of the reporter gene in the neural tube and hindbrain. When administered to Hoxa4 null embryos, however, persistent ectopic expression was not observed, suggesting that autoregulation is required for maintenance of the retinoic acid-induced expression. Finally, mutation of the consensus retinoic acid response element eliminated the response of the reporter gene to exogenous retinoic acid, and abolished all embryonic expression in untreated embryos, with the exception of the neural tube and prevertebrae. These data add to the evidence that Hox gene expression is regulated, in part, by endogenous retinoids and autoregulatory loops.

Effect of retinoic acid on otic capsule chondrogenesis in high-density culture suggests disruption of epithelial-mesenchymal interactions

Previous studies have shown that in utero exposure of the mouse embryo to nonphysiological levels of all-trans retinoic acid (RA) produces malformations of the epithelial-derived auditory and vestibular receptors of the inner ear and its surrounding cartilaginous capsule. In this study, we demonstrate the effects of all-trans RA in high-density cultures of the periotic mesenchyme fated to form the otic capsule. Our results demonstrate an inhibition of chondrogenesis in cultured periotic mesenchyme + otic epithelium of embryonic age E10.5 days (E10.5) in response to all-trans RA exposure. However, at later stages of development (i.e., E12, E14), when epithelial-mesenchymal interactions are no longer required for initiation of chondrogenesis, exposure to this teratogen has no effect on the chondrogenic process. Two analogues of all-trans RA, i.e., cis-RA and trans-retinol, were investigated for their biological activity in chondrogenic cultures of inner ear mesenchyme + epithelium. Moreover, we tested the inductive capability and responsiveness of in utero RA-exposed inner ear tissues when cultured with inner ear tissues that were not exposed to this teratogen. Our results support the hypothesis that all-trans RA disrupts otic capsule formation by interfering with the tissue interactions required for its normal differentiation and development.

Correlation between loss of middle ear bones and altered goosecoid gene expression in the branchial region following retinoic acid treatment of mouse embryos in vivo

The homeobox gene goosecoid marks the Spemann organizer in vertebrate gastrula embryos, and is expressed in the craniofacial region, body wall and limbs during organogenesis. Mouse mutants of goosecoid displayed a variety of phenotypes related to the expression pattern at mid-embryogenesis. These defects included loss of the tympanic ring and malformation of the malleus, phenotypes which were reminiscent of the teratogenic effects of retinoic acid (RA). Here we investigated the correlation of goosecoid gene expression and RA-teratogenicity following treatment of mouse embryos in vivo at embryonic day (E) 8 + 5 h. We found that goosecoid was specifically affected at E10.5 in branchial arches I and II. Expression was either reduced to background levels or restricted to the branchial cleft region. This change in goosecoid gene expression correlated with a loss of middle ear ossicles and a partial or complete deletion of the tympanic ring, suggesting a role for goosecoid in executing the RA teratogenic effects.

Retinoic acid stimulates matrix calcification and initiates type I collagen synthesis in primary cultures of avian weight-bearing growth plate chondrocytes

The effect of retinoic acid (RA) on primary cultures of growth plate chondrocytes obtained from weight-bearing joints was examined, Chondrocytes were isolated from the tibial epiphysis of 6- to 8-week-old broiler-strain chickens and cultured in either serum-containing or serum-free media. RA was administered at low levels either transiently or continuously after the cells had become established in culture. Effects of RA on cellular protein levels, alkaline phosphatase (AP) activity, synthesis of proteoglycan (PG), matrix calcification, cellular morphology, synthesis of tissue-specific types of collagen, and level of matrix metalloproteinase (MMP) activity were explored. RA treatment generally increased AP activity and stimulated mineral deposition, especially if present continuously. RA also caused a shift in cell morphology from spherical/polygonal to spindle-like. This occurred in conjunction with a change in the type of collagen synthesized: type X and II collagens were decreased, while synthesis of type I collagen was increased. There was also a marked increase in the activity of MMP. Contrasting effects of continuous RA treatment on cellular protein levels were seen: they were enhanced in serum-containing media, but decreased in serum-free HL-1 media. Levels of RA as low as 10 nM significantly inhibited PG synthesis and caused depletion in the levels of PG in the medium and cell-matrix layer. Thus, in these appendicular chondrocytes, RA suppressed chondrocytic (PG, cartilage-specific collagens) and enhanced osteoblastic phenotype (cell morphology, type I collagen, alkaline phosphatase, and mineralization).

Tretinoin: a review of the nonclinical developmental toxicology experience

Tretinoin has been thoroughly evaluated for its potential as an embryofetal developmental toxicant. Oral tretinoin produces developmental anomalies in animal models; the minimal teratogenic dose is consistently 2.5 to 10 mg/kg. In contrast, topical application does not induce developmental malformations in laboratory animals. A structurally related compound, isotretinoin, is a potent toxicant in humans and animals; the lowest systemic dose that induces fetal anomalies varies more than 100-fold depending on the model. Oral isotretinoin is a more potent developmental toxicant than oral tretinoin in monkeys. Between-drug differences in the metabolism and transplacental transfer of the two retinoids account for the differences in toxicant potency. Pharmacokinetic studies reveal that absorption of tretinoin from the skin is poor and yields maternal plasma concentrations below the developmentally toxic threshold established after oral administration. Analysis of outcomes of developmental toxicology and pharmacokinetic studies suggests that the human risk of fetal anomalies is negligible after therapeutic application of topical tretinoin.

A developmental toxicity study of tretinoin emollient cream (Renova) applied topically to New Zealand white rabbits

BACKGROUND

Embryofetal developmental toxicity associated with oral administration of vitamin A analogs has led to concern about risks from topical application.

OBJECTIVE

This study was conducted to evaluate the potential developmental toxicity of tretinoin emollient cream when applied to the skin of pregnant New Zealand white rabbits during organogenesis (gestational days 7 through 19).

METHODS

Twenty rabbits each were randomly assigned to a control group (group I) or to receive vehicle (group II) or tretinoin emollient cream topically at dosages of 10 (0.05 mg/kg*, group III) or 100 (0.5 mg/kg*, group IV) times that used clinically in humans. Does and fetuses were examined for tretinoin-induced toxic effects, and maternal plasma tretinoin and metabolite levels were measured.

RESULTS

The rate of abortion was increased significantly in does in group IV (p < or = 0.01) compared with the control group. Dosage-dependent increases in incidence and severity of skin reactions occurred in groups administered the vehicle and the two dosages of tretinoin. Does in groups III and IV had clinical and necropsy observations that were considered direct or indirect effects of tretinoin administration, persistent weight loss, and reduced feed consumption. Maternal endogenous plasma tretinoin levels were below the lower limit of quantitation of 5 ng/ml and were not significantly altered with treatment. Group IV had significantly reduced mean fetal body weight (p < or = 0.01) and a greater frequency of resorptions compared with group I. Although external, visceral, or skeletal alterations occurred at significantly greater levels in group III, they were unrelated to tretinoin administration because the fetal incidences were not dosage dependent, and the litter incidence did not significantly differ from the control group values.

CONCLUSION

Maternally toxic dosages of tretinoin were associated with an increased incidence of abortions and resorptions and reduced fetal body weight, two end points of developmental toxicity. Consistent with the absence of detectable tretinoin plasma levels, however, no changes in fetal morphology were attributable to tretinoin administration. *The milligrams per kilogram dosage refers to the amount of active ingredient (tretinoin). The 0.05 mg/kg and 0.5 mg/kg groups were treated with 0.005% and 0.05% wt/wt tretinoin emollient cream formulation. The 0.05% tretinoin emollient cream is the Renova clinical formulation. The 10 and 100 times clinical multiples refer to Renova clinical multiples and are based on a 50 kg adult patient's applying 500 mg of 0.05% tretinoin emollient cream formulation daily to yield a clinical dosage of 0.005 mg/kg.

Molecular identification of a major retinoic-acid-synthesizing enzyme, a retinaldehyde-specific dehydrogenase

Retinoic acid, a developmental signal implicated in the formation of the neural axis, is present at high levels in the early embryonic trunk region, where it is synthesized by a novel dehydrogenase. Here we show that the same enzyme is inducible by retinoic acid in P19 teratocarcinoma cells, and we report the cloning from P19 cells of a cDNA encoding a novel dehydrogenase, named retinaldehyde dehydrogenase-2 (RALDH-2). Expression in COS cells shows RALDH-2 to be highly effective in oxidation of retinaldehyde, with no detectable activity on any other aldehyde tested. In situ hybridization histochemistry on the embryonic trunk reveals RALDH-2 mRNA both in mesoderm and neuroectoderm, with highest neuroectodermal expression in the ventral horn of the spinal cord at two restricted locations along the anteroposterior axis, presumably the subpopulation of motoneurons that innervate the limbs.

Retinoic acid stimulates pyrophosphate elaboration by cartilage and chondrocytes

Abnormal metabolism of extracellular inorganic pyrophosphate (PPi) by articular cartilage contributes to calcium pyrophosphate dihydrate (CPPD) crystal formation and the resultant arthritis known as CPPD deposition disease. The factors causing excess PPi elaboration in affected cartilage remain poorly defined. Retinoic acid (RA), a naturally occurring vitamin A metabolite, promotes cartilage degeneration and mineralization, two correlates of CPPD crystal deposition. RA was examined as a potential modifier of cartilage PPi elaboration. All-trans RA (200-1000 nM) increased PPi levels in culture medium of normal porcine cartilage and chondrocytes 2-3-fold over control values at 96 hours of incubation (P < 0.01). IGF1 and anti-EGF antibody diminished the effects of RA on PPi elaboration. RA modestly increased activity of the PPi-generating ectoenzyme NTPPPH in culture medium (P < 0.01). As some RA effects are mediated through increased activity of TGFbeta, a known PPi stimulant, we examined the effect of anti-TGFbeta antibody on RA-induced PPi elaboration. PPi levels in medium were reduced from 30 +/- 7 microM in cartilage cultures with 500 nM RA to 14 +/- 4 microM PPi in cartilage cultures with RA and anti-TGFbeta. Anti-TGFbeta antibody, however, had no significant effect on RA-induced PPi elaboration in chondrocyte cultures. Thus, RA, along with TGFbeta and ascorbate, can now be included in the list of known PPi stimulants. All three of these factors promote mineralization in growth plate cartilage. These data support a central role for TGFbeta in CPPD disease, and provide further evidence linking processes of normal and pathologic calcification in cartilage.

Cloning of a cDNA encoding an aldehyde dehydrogenase and its expression in Escherichia coli. Recognition of retinal as substrate

The biosynthesis of the hormone retinoic acid from retinol (vitamin A) involves two sequential steps, catalyzed by retinol dehydrogenases and retinal dehydrogenases, respectively. This report describes the cloning of a cDNA encoding a heretofore unknown aldehyde dehydrogenase from a rat testis library and its expression in Escherichia coli. This enzyme has been designated retinal dehydrogenase, type II, RalDH(II). The deduced amino acid sequence of RalDH(II) had the highest identity with mammalian aldehyde dehydrogenases that feature low Km values (microM) for retinal: human ALDH1 (72.2%), rat retinal dehydrogenase, type I (71.5%), bovine retina (72.7%), and mouse AHD-2 (71.5%). RalDH(II) expressed in E. coli recognizes as substrates free retinal, with a Km of approximately 0.7 microM, and cellular retinol-binding protein-bound retinal, with a Km of approximately 0.2 microM. RalDH(II) also can utilize as substrate retinal generated in situ by microsomal retinol dehydrogenases, from the physiologically most abundant substrate: retinol bound to cellular retinol-binding protein. Rat testis expresses RalDH(II) mRNA most abundantly, followed by (relative to testis): lung (6.7%), brain (6.3%), heart (5.2%), liver (4.4%), and kidney (2.7%). RalDH(II) does not recognize citral, benzaldehyde, acetaldehyde, and propanal efficiently as substrates, but does metabolize octanal and decanal efficiently. These data support a function for RalDH(II) in the pathway of retinoic acid biogenesis.

Retinoic acid-induced embryopathy of the mouse inner ear

Retinoic acid (RA) is an active metabolite of vitamin A that is teratogenic when present in excess during mammalian embryogenesis. We have investigated the effect of embryonic exposure to nonphysiological levels of all-trans RA on the development of the mouse inner ear. Dysmorphogenesis of both vestibular and auditory portions of the inner ear, and abnormal formation of the surrounding capsule are produced by exposure to teratogenic levels of RA at an embryonic age of 9 days (E9). There was no observable teratogenic effect of RA when administered at earlier (i.e., E7 or E8) or later (i.e., E10) stages of otic morphogenesis. We hypothesize that exposure to high levels of RA during a critical period of early otic morphogenesis interferes with the inductive tissue interactions required for inner ear development.

Pattern of retinoid-induced teratogenic effects: possible relationship with relative selectivity for nuclear retinoid receptors RAR alpha, RAR beta, and RAR gamma

Retinoic acid, an oxidative metabolite of vitamin A, is involved in the control of many biological processes including embryonic development. Excess as well as deficiency of retinoids were found to be teratogenic. The effects of retinoids in normal as well as abnormal development may be mediated by two members of retinoid receptors, the RAR's and RXR's, which exhibit a specific temporal and spatial expression during development. The significance of the retinoid receptors was investigated here by studying the teratogenic effects of retinoid ligands with relative selectivity for binding and transactivation of the retinoic acid receptors RAR alpha, RAR beta and RAR gamma. Pregnant NMRI mice were administered 5 or 15 mg/kg of CD 336 (Am 580) (alpha-ligand), CD 2019 (beta-ligand), CD 437 (gamma-ligand) or 37.5 mg/kg all-trans-retinoic acid in 25% Cremophor EL on day 8.25 or day 11 of gestation by gastric intubation. External, visceral and skeletal malformations were observed on day 18 of gestation. The order of teratogenic potency was: alpha-ligand > beta-ligand > gamma-ligand. In addition, these retinoids also produced a different spectrum of defects. The alpha-ligand induced the most varied defects including severe ear, mandible, and limb malformations. The beta-ligand induced defects of the urinary system and liver in greater frequency than expected from its relative potency. The gamma-ligand preferentially induced ossification deficiencies and defects of the sternebrae and vertebral body. Our results show that these three retinoids, which were previously demonstrated to exhibit retinoid-like activities in several systems, exert differing teratogenic activities, in regard to both potency and regioselectivity: we hypothesize that the relative selectivity for binding and transactivation of the three retinoic acid receptors could possibly be related to the differences of teratogenic effects observed in this study. The low potency of the gamma-ligand may lead the way to interesting new retinoids with improved therapeutic ratio.

Endogenous retinoids in the zebrafish embryo and adult

Retinoic acid and its isoforms are considered to be endogenous compounds which regulate embryonic development. In the work reported here we have determined which retinoids are present in zebrafish embryos and how their levels change throughout development and into adulthood. All-trans-RA is present and its level does not change significantly during embryogenesis. We failed to detect other retinoic acid isomers such as 9-cis-RA and 4-oxo-RA, but we did observe a rapid rise in the level of didehydroretinol after gastrulation. The most striking result is that the zebrafish embryo, like Xenopus and tunicates, contains a vast excess of t-retinal whereas the embryos of higher vertebrates have an excess of t-retinol. However, as the zebrafish grows, the levels of t-retinol rise so that by adulthood t-retinol and t-retinal concentrations are more equivalent, indicating a changing pattern of retinoid metabolism with growth. To examine the significance of the use of t-retinal as a precursor of t-RA we treated embryos with disulphiram, an inhibitor of retinaldehyde dehydrogenase. This resulted in embryos with an undulating notochord and correspondingly abnormal somites and ventral floor plate. In contrast to this effect, 4-methylpyrazole, which inhibits alcohol dehydrogenases, had no effect on development. This effect of disulphiram suggests that t-RA may be involved in the establishment of the anteroposterior axis of the embryo.

Distribution of all-trans-, 13-cis- and 9-cis-retinoic acid to whole rat embryos and maternal serum following oral administration of a teratogenic dose of all-trans-retinoic acid

Maternal administration of 200 mg/kg all-trans-retinoic acid to rat embryos at early limb stages of development (day 12 to day 13.5 post coitum) results in limb reduction defects. In order to determine the duration of exposure of the embryo to raised levels of all-trans-retinoic acid, we have used high performance liquid chromatography to measure retinoid levels at a series of time intervals following maternal administration on day 12.5 post coitum. Raised levels of all-trans-retinoic acid and 13-cis-retinoic acid were detectable in embryos after 30 min., reached a peak at 2 hr, and had fallen sharply by 4 hr. 13-cis-Retinoic acid levels were undetectable after 4 hr, and all-trans-retinoic acid levels after 8 hr. 9-cis-retinoic acid levels rose more slowly, were less elevated, and fell more gradually than the other two retinoids. The retinoid profiles in maternal serum were similar. The results indicate that induction of limb abnormalities by all-trans-retinoic acid in rat embryos is associated with a relatively short-term rise in embryonic retinoid levels.

Endogenous distribution of retinoids during normal development and teratogenesis in the mouse embryo

We have analysed the endogenous retinoids present in whole mouse embryos from day 9 to day 14 of development and in individual components of the embryo at two stages, day 10.5 and day 13, by HPLC. We can only detect two retinoids, all-trans-RA (tRA) and all-trans-retinol (t-retinol), and t-retinol is 5-10-fold in excess over tRA. We cannot detect 9-cis-RA or any didehydroretinoids; thus mammalian embryos seem to differ in their retinoid content from other embryos such as chick, Xenopus, and fish. The levels of tRA do not change significantly over the 6 days of development analysed, whereas t-retinol rises sharply as the liver develops. Within the embryo, tRA is present at high levels in the developing spinal cord and at very low levels in the forebrain; indeed there is a gradient of endogenous tRA from the forebrain to the spinal cord. Other parts of the embryo had intermediate levels of tRA. When a teratogenic dose of RA was administered to day 10.5 embryos, the levels of tRA present in individual tissues of the embryo rose dramatically--from 175-fold to 1,400-fold--and the levels rose in all tissues not in any exclusive areas. We then determined which areas of the embryo were malformed by such a teratogenic dose. The lower jaw, palate, vertebrae, tail, and limbs were consistently abnormal, and since these areas received a dose of tRA no higher than any other it was concluded that cell-specific factors must determine the teratogenic response of these tissues. We then considered whether cellular retinoic acid-binding protein I or II (CRABP I or II) played any role in this response by determining their relative levels in each of the tissues analysed. There was no correlation between the presence of CRABP I and II and the distribution of administered RA. Neither was there a clear correlation in detail between the presence of CRABP I and II and the sites of teratogenesis. We therefore conclude that other factors, for example, nuclear factors, must be responsible for the teratogenic response to RA.

Mice deficient in cellular retinoic acid binding protein II (CRABPII) or in both CRABPI and CRABPII are essentially normal

We have disrupted the CRABPII gene using homologous recombination in embryonic stem cells, and shown that this disruption results in a null mutation. CRABPII null mutant mice are essentially indistinguishable from wild-type mice as judged by their normal development, fertility, life span and general behaviour, with the exception of a minor limb malformation. Moreover, CRABPI−/−/CRABPII−/− double mutant mice also appear to be essentially normal, and both CRABPII−/− single mutant and CRABPI−/−/CRABPII−/− double mutant embryos are not more sensitive than wild-type embryos to retinoic acid excess treatment in utero. Thus, CRABPI and CRABPII are dispensable both during mouse development and adult life. Our present results demonstrate that CRABPs are not critically involved in the retinoic acid signaling pathway, and that none of the functions previously proposed for CRABPs are important enough to account for their evolutionary conservation.

Spatial distribution of endogenous retinoids in the murine embryonic mandible

Retinoids play an important part in pattern formation during embryonic development. Exogenous retinoids alter the pattern of skeletal, neural and odontogenic tissues. Endogenous retinoids have been demonstrated previously in the murine embryonic mandible, reaching a concentration peak during the initiation of odontogenesis. It was now found that endogenous retinoids are present in a concentration gradient in the embryonic mouse mandible at the time of the initiation of the dental lamina. All-trans-retinoic acid was more concentrated in the incisor region and retinol in the molar region. These results, and the fact that exogenous retinoids produce supernumerary incisors and missing molars, suggest that all-trans-retinoic acid may instruct incisor morphology.

Fetal alcohol syndrome: the vulnerability of the developing brain and possible mechanisms of damage

Fetal alcohol exposure has multiple deleterious effects on brain development, and represents a leading known cause of mental retardation. This review of the effects of alcohol exposure on the developing brain evaluates results from human, animal and in vitro studies, but focuses on key research issues, including possible mechanisms of damage. Factors that affect the risk and severity of fetal alcohol damage also are considered.

Distribution of C-CAM in developing oral tissues

C-CAM is a cell surface glycoprotein that is involved in cell adhesion and may play a role in histogenesis and organogenesis. It is a member of the carcinoembryonic antigen (CEA) gene family, which is a subfamily of the immunoglobulin gene superfamily. We have analyzed the expression of C-CAM during normal and disturbed craniofacial development in the mouse by immunohistochemistry and in situ hybridization. Developmental disturbances were induced by retinoic acid (RA) treatment of pregnant mice. Normal and malformed fetuses were examined on days 14, 15, 16, 17 and 18 of gestation. The expression of C-CAM was detected first at day 16. With age, the signal became gradually stronger. C-CAM was detected in the epithelia of both ectodermal and mesodermal origin, including oral and respiratory epithelia, epithelia of the developing vessels, glands and their ducts. In the RA-treated fetuses, the expression of C-CAM was higher in the epithelium of the oral cavity than in that of the nasal cavity, with a distinct borderline between differentiating nasal and oral epithelium of the palatal shelves. However, the submucosal nasal glands and ducts showed higher expression than oral glands in both normal and RA-treated mice. The expression of C-CAM did not differ significantly between control and RA-treated animals. The presence of C-CAM in all proliferating craniofacial epithelia indicates that this molecule may play an important role in development.