Differential expression of transcripts encoding retinoid binding proteins and retinoic acid receptors during placentation of the mouse

Potential therapeutic uses of rexinoids

The discovery of nuclear receptors, particularly retinoid X receptors (RXR), and their involvement in numerous pathways related to development sparked interest in their immunomodulatory properties. Genetic models using deletion or overexpression of RXR and the subsequent development of several small molecules that are agonists or antagonists of this receptor support a promising therapeutic role for these receptors in immunology. Bexarotene was approved in 1999 for the treatment of cutaneous T cell lymphoma. Several other small molecule RXR agonists have since been synthesized with limited preclinical development, but none have yet achieved FDA approval. Cancer treatment has recently been revolutionized with the introduction of immune checkpoint inhibitors, but their success has been restricted to a minority of patients. This review showcases the emerging immunomodulatory effects of RXR and the potential of small molecules that target this receptor as therapies for cancer and other diseases. Here we describe the essential roles that RXR and partner receptors play in T cells, dendritic cells, macrophages and epithelial cells, especially within the tumor microenvironment. Most of these effects are site and cancer type dependent but skew immune cells toward an anti-inflammatory and anti-tumor effect. This beneficial effect on immune cells supports the promise of combining rexinoids with approved checkpoint blockade therapies in order to enhance efficacy of the latter and to delay or potentially eliminate drug resistance. The data compiled in this review strongly suggest that targeting RXR nuclear receptors is a promising new avenue in immunomodulation for cancer and other chronic inflammatory diseases.

Retinol-binding protein 2 (RBP2): biology and pathobiology

Retinol-binding protein 2 (RBP2; originally cellular retinol-binding protein, type II (CRBPII)) is a 16 kDa cytosolic protein that in the adult is localized predominantly to absorptive cells of the proximal small intestine. It is well established that RBP2 plays a central role in facilitating uptake of dietary retinoid, retinoid metabolism in enterocytes, and retinoid actions locally within the intestine. Studies of mice lacking Rbp2 establish that Rbp2 is not required in times of dietary retinoid-sufficiency. However, in times of dietary retinoid-insufficiency, the complete lack of Rbp2 gives rise to perinatal lethality owing to RBP2 absence in both placental (maternal) and neonatal tissues. Moreover, when maintained on a high-fat diet, Rbp2-knockout mice develop obesity, glucose intolerance and a fatty liver. Unexpectedly, recent investigations have demonstrated that RBP2 binds long-chain 2-monoacylglycerols (2-MAGs), including the canonical endocannabinoid 2-arachidonoylglycerol, with very high affinity, equivalent to that of retinol binding. Crystallographic studies establish that 2-MAGs bind to a site within RBP2 that fully overlaps with the retinol binding site. When challenged orally with fat, mucosal levels of 2-MAGs in Rbp2 null mice are significantly greater than those of matched controls establishing that RBP2 is a physiologically relevant MAG-binding protein. The rise in MAG levels is accompanied by elevations in circulating levels of the hormone glucose-dependent insulinotropic polypeptide (GIP). It is not understood how retinoid and/or MAG binding to RBP2 affects the functions of this protein, nor is it presently understood how these contribute to the metabolic and hormonal phenotypes observed for Rbp2-deficient mice.

Near to One's Heart: The Intimate Relationship Between the Placenta and Fetal Heart

The development of the fetal heart is exquisitely controlled by a multitude of factors, ranging from humoral to mechanical forces. The gatekeeper regulating many of these factors is the placenta, an external fetal organ. As such, resistance within the placental vascular bed has a direct influence on the fetal circulation and therefore, the developing heart. In addition, the placenta serves as the interface between the mother and fetus, controlling substrate exchange and release of hormones into both circulations. The intricate relationship between the placenta and fetal heart is appreciated in instances of clinical placental pathology. Abnormal umbilical cord insertion is associated with congenital heart defects. Likewise, twin-to-twin transfusion syndrome, where monochorionic twins have unequal sharing of their placenta due to inter-twin vascular anastomoses, can result in cardiac remodeling and dysfunction in both fetuses. Moreover, epidemiological studies have suggested a link between placental phenotypic traits and increased risk of cardiovascular disease in adult life. To date, the mechanistic basis of the relationships between the placenta, fetal heart development and later risk of cardiac dysfunction have not been fully elucidated. However, studies using environmental exposures and gene manipulations in experimental animals are providing insights into the pathways involved. Likewise, surgical instrumentation of the maternal and fetal circulations in large animal species has enabled the manipulation of specific humoral and mechanical factors to investigate their roles in fetal cardiac development. This review will focus on such studies and what is known to date about the link between the placenta and heart development.

Nuclear retinoid receptors and pregnancy: placental transfer, functions, and pharmacological aspects

Animal models of vitamin A (retinol) deficiency have highlighted its crucial role in reproduction and placentation, whereas an excess of retinoids (structurally or functionally related entities) can cause toxic and teratogenic effects in the embryo and foetus, especially in the first trimester of human pregnancy. Knock-out experimental strategies-targeting retinoid nuclear receptors RARs and RXRs have confirmed that the effects of vitamin A are mediated by retinoic acid (especially all-trans retinoic acid) and that this vitamin is essential for the developmental process. All these data show that the vitamin A pathway and metabolism are as important for the well-being of the foetus, as they are for that of the adult. Accordingly, during this last decade, extensive research on retinoid metabolism has yielded detailed knowledge on all the actors in this pathway, spurring the development of antagonists and agonists for therapeutic and research applications. Natural and synthetic retinoids are currently used in clinical practice, most often on the skin for the treatment of acne, and as anti-oncogenic agents in acute promyelocytic leukaemia. However, because of the toxicity and teratogenicity of retinoids during pregnancy, their pharmacological use needs a sound knowledge of their metabolism, molecular aspects, placental transfer, and action.

PPARγ controls pregnancy outcome through activation of EG-VEGF: new insights into the mechanism of placental development

PPARγ-deficient mice die at E9.5 due to placental abnormalities. The mechanism by which this occurs is unknown. We demonstrated that the new endocrine factor EG-VEGF controls the same processes as those described for PPARγ, suggesting potential regulation of EG-VEGF by PPARγ. EG-VEGF exerts its functions via prokineticin receptor 1 (PROKR1) and 2 (PROKR2). This study sought to investigate whether EG-VEGF mediates part of PPARγ effects on placental development. Three approaches were used: 1) in vitro, using human primary isolated cytotrophoblasts and the extravillous trophoblast cell line (HTR-8/SVneo); 2) ex vivo, using human placental explants (n = 46 placentas); and 3) in vivo, using gravid wild-type PPARγ(+/-) and PPARγ(-/-) mice. Major processes of placental development that are known to be controlled by PPARγ, such as trophoblast proliferation, migration, and invasion, were assessed in the absence or presence of PROKR1 and PROKR2 antagonists. In both human trophoblast cell and placental explants, we demonstrated that rosiglitazone, a PPARγ agonist, 1) increased EG-VEGF secretion, 2) increased EG-VEGF and its receptors mRNA and protein expression, 3) increased placental vascularization via PROKR1 and PROKR2, and 4) inhibited trophoblast migration and invasion via PROKR2. In the PPARγ(-/-) mouse placentas, EG-VEGF levels were significantly decreased, supporting an in vivo control of EG-VEGF/PROKRs system during pregnancy. The present data reveal EG-VEGF as a new mediator of PPARγ effects during pregnancy and bring new insights into the fine mechanism of trophoblast invasion.

Biochemical Basis for Dominant Inheritance, Variable Penetrance, and Maternal Effects in RBP4 Congenital Eye Disease

Gestational vitamin A (retinol) deficiency poses a risk for ocular birth defects and blindness. We identified missense mutations in RBP4, encoding serum retinol binding protein, in three families with eye malformations of differing severity, including bilateral anophthalmia. The mutant phenotypes exhibit dominant inheritance, but incomplete penetrance. Maternal transmission significantly increases the probability of phenotypic expression. RBP normally delivers retinol from hepatic stores to peripheral tissues, including the placenta and fetal eye. The disease mutations greatly reduce retinol binding to RBP, yet paradoxically increase the affinity of RBP for its cell surface receptor, STRA6. By occupying STRA6 nonproductively, the dominant-negative proteins disrupt vitamin A delivery from wild-type proteins within the fetus, but also, in the case of maternal transmission, at the placenta. These findings establish a previously uncharacterized mode of maternal inheritance, distinct from imprinting and oocyte-derived mRNA, and define a group of hereditary disorders plausibly modulated by dietary vitamin A.

Expression of aldehyde dehydrogenase family 1, member A3 in glycogen trophoblast cells of the murine placenta

INTRODUCTION

Retinoic acid (RA) signaling is a well known regulator of trophoblast differentiation and placental development, and maternal decidual cells are recognized as the source of much of this RA. We explored possible trophoblast-derived sources of RA by examining the expression of RA synthesis enzymes in the developing mouse placenta, as well as addressed potential sites of RA action by examining the ontogeny of gene expression for other RA metabolizing and receptor genes. Furthermore, we investigated the effects of endogenous RA production on trophoblast differentiation.

METHODS

Placental tissues were examined by in situ hybridization and assayed for RARE-LacZ transgene activity to locate sites of RAR signaling. Trophoblast stem cell cultures were differentiated in the presence of ALDH1 inhibitors (DEAB and citral), and expression of labyrinth (Syna, Ctsq) and junctional zone (Tpbpa, Prl7b1, Prl7a2) marker genes were analyzed by qRT-PCR.

RESULTS

We show Aldh1a3 is strongly expressed in a subset of ectoplacental cone cells and in glycogen trophoblast cells of the definitive murine placenta. Most trophoblast subtypes of the placenta express RA receptor combinations that would enable them to respond to RA signaling. Furthermore, expression of junctional zone markers decrease in differentiating trophoblast cultures when endogenous ALDH1 enzymes are inhibited.

DISCUSSION

Aldh1a3 is a novel marker for glycogen trophoblast cells and their precursors and may play a role in the differentiation of junctional zone cell types via production of a local source of RA.

Nitrofen increases total retinol levels in placenta during lung morphogenesis in the nitrofen model of congenital diaphragmatic hernia

BACKGROUND

It has been shown that pulmonary retinol level is decreased during lung morphogenesis in the nitrofen-induced PH in congenital diaphragmatic hernia (CDH). Placenta has a major role in the retinol homeostasis in fetal life. Since there is no fetal retinol synthesis, maternal retinol has to cross the placenta. Placenta is the main fetal retinol store where retinol is stored in retinyl-ester formation. Trophoblasts have to produce its own retinol-binding protein (RBP) for retinol transport from placenta to fetus. Recently, we demonstrated that trophoblastic RBP expression is decreased in the nitrofen model of CDH. The aim of this study was to investigate the retinol transfer from mother to the placenta in nitrofen model of CDH.

METHODS

Pregnant rats were exposed to either olive oil or nitrofen on day 9 of gestation (D9). Fetal placenta harvested on D21 and divided into two groups: control (n = 11) and nitrofen with CDH (n = 11). Retinoid levels in placenta were measured using HPLC. Immunohistochemistry was performed to evaluate trophoblastic expression of main RSP genes.

RESULTS

Total retinol levels in the placenta were significantly increased in CDH placenta compared to control placenta. The retinyl-ester levels were significantly increased in CDH placenta compared to control placenta. Markedly, decreased immunoreactivity of retinoid signaling pathway was observed in trophoblast cells in CDH compared to control placenta.

CONCLUSIONS

Increased placental retinol levels show that retinol is transferred from mother to placenta and stored in the placenta in nitrofen model of CDH during lung morphogenesis. Nitrofen may disturb the mobilization of retinol from placenta to fetal circulation causing PH in CDH.

Increased uptake of dietary retinoids at the maternal-fetal barrier in the nitrofen model of congenital diaphragmatic hernia

BACKGROUND/PURPOSE

The retinol signaling pathway is disrupted in congenital diaphragmatic hernia (CDH). Since there is no fetal retinol synthesis, maternal retinol has to cross the placenta. Nitrofen interferes with the retinol-binding protein (RBP) transfer pathway in CDH. However, in RBP knockout mice, retinol has been shown to be present. In this model, increased uptake of maternal dietary retinyl ester (RE) bounded in low-dense-lipoprotein (LDL) through low-density-lipoprotein-receptor 1 (LRP1) and increased activity of RE hydrolysis by lipoprotein-lipase (LPL) have been found. The aim of this study was to investigate the RE transfer pathway in the nitrofen CDH model.

METHODS

Pregnant rats were treated with nitrofen or vehicle on gestational day (D9) and sacrificed on D21. Immunohistochemistry was performed to evaluate LRP1 and LPL protein expression. Serum LDL levels were measured by ELISA. Pulmonary and serum retinoid levels were measured using HPLC.

RESULTS

Markedly increased trophoblastic and pulmonary LRP1 and LPL immunoreactivity were observed in CDH compared to controls. Significantly increased serum LDL and RE levels were observed in CDH compared to controls.

CONCLUSIONS

The increased uptake of dietary retinoids at the maternal-fetal barrier in the nitrofen CDH model suggests that the RE transfer pathway may be the main source of retinol in this model.

Identifying and mapping cell-type-specific chromatin programming of gene expression

A problem of substantial interest is to systematically map variation in chromatin structure to gene-expression regulation across conditions, environments, or differentiated cell types. We developed and applied a quantitative framework for determining the existence, strength, and type of relationship between high-resolution chromatin structure in terms of DNaseI hypersensitivity and genome-wide gene-expression levels in 20 diverse human cell types. We show that ∼25% of genes show cell-type-specific expression explained by alterations in chromatin structure. We find that distal regions of chromatin structure (e.g., ±200 kb) capture more genes with this relationship than local regions (e.g., ±2.5 kb), yet the local regions show a more pronounced effect. By exploiting variation across cell types, we were capable of pinpointing the most likely hypersensitive sites related to cell-type-specific expression, which we show have a range of contextual uses. This quantitative framework is likely applicable to other settings aimed at relating continuous genomic measurements to gene-expression variation.

T antigen transformation reveals Tp53/RB-dependent route to PLAC1 transcription activation in primary fibroblasts

PLAC1 (placenta-specific 1) is a gene that is placenta specific and transcribed very little, if at all, in any somatic tissue. It is nevertheless expressed in many cancer cell lines. To understand how cancer cells may activate the gene in nonexpressing cells, we found that a model is provided by classical transformation of normal fibroblasts by SV40 T antigen. T antigen derepressed the PLAC1 P1 promoter, with Tp53 and RB exerting critical and opposing actions and nuclear receptors, retinoid X receptor and  liver X receptor, sharply increasing the level of expression.

Role of nuclear receptors in blastocyst implantation

The regulation of blastocyst implantation in the uterus is orchestrated by the ovarian hormones estrogen and progesterone. These hormones act via their nuclear receptors to direct the transcriptional activity of the endometrial compartments and create a defined period in which the uterus is permissive to embryo implantation termed the "window of receptivity". Additional members of the nuclear receptor family have also been described to have a potential role in endometrial function. Much of what we know about the function of these nuclear receptors during implantation we have learned from the use of mouse models. Transgenic murine models with targeted gene ablation have allowed us to identify a complex network of paracrine signaling between the endometrial epithelium and stroma. While some of the critical molecules have been identified, the mechanism underlying the intricate communication between endometrial compartments during the implantation window has not been fully elucidated. Defining this mechanism will help identify markers of a receptive uterine environment, ultimately providing a useful tool to help improve the fertility outlook for reproductively challenged couples. The aim of this review is to outline our current understanding of how nuclear receptors and their effector molecules regulate blastocyst implantation in the endometrium.

Transcriptomic analysis of murine embryos lacking endogenous retinoic acid signaling

Retinoic acid (RA), an active derivative of the liposoluble vitamin A (retinol), acts as an important signaling molecule during embryonic development, regulating phenomenons as diverse as anterior-posterior axial patterning, forebrain and optic vesicle development, specification of hindbrain rhombomeres, pharyngeal arches and second heart field, somitogenesis, and differentiation of spinal cord neurons. This small molecule directly triggers gene activation by binding to nuclear receptors (RARs), switching them from potential repressors to transcriptional activators. The repertoire of RA-regulated genes in embryonic tissues is poorly characterized. We performed a comparative analysis of the transcriptomes of murine wild-type and Retinaldehyde Dehydrogenase 2 null-mutant (Raldh2^−/−) embryos — unable to synthesize RA from maternally-derived retinol — using Affymetrix DNA microarrays. Transcriptomic changes were analyzed in two embryonic regions: anterior tissues including forebrain and optic vesicle, and posterior (trunk) tissues, at early stages preceding the appearance of overt phenotypic abnormalities. Several genes expected to be downregulated under RA deficiency appeared in the transcriptome data (e.g. Emx2, Foxg1 anteriorly, Cdx1, Hoxa1, Rarb posteriorly), whereas reverse-transcriptase-PCR and in situ hybridization performed for additional selected genes validated the changes identified through microarray analysis. Altogether, the affected genes belonged to numerous molecular pathways and cellular/organismal functions, demonstrating the pleiotropic nature of RA-dependent events. In both tissue samples, genes upregulated were more numerous than those downregulated, probably due to feedback regulatory loops. Bioinformatic analyses highlighted groups (clusters) of genes displaying similar behaviors in mutant tissues, and biological functions most significantly affected (e.g. mTOR, VEGF, ILK signaling in forebrain tissues; pyrimidine and purine metabolism, calcium signaling, one carbon metabolism in posterior tissues). Overall, these data give an overview of the gene expression changes resulting from embryonic RA deficiency, and provide new candidate genes and pathways that may help understanding retinoid-dependent molecular events.

Increased trophoblastic apoptosis mediated by neutrophil gelatinase-associated lipocalin (NGAL) activation in the nitrofen model of congenital diaphragmatic hernia

BACKGROUND

Retinoids play a key role in fetal lung development. It has been suggested that the maternal-fetal retinol transport is disrupted by trophoblastic apoptosis. The mechanism underlying nitrofen-induced apoptosis in placenta is not fully understood. Neutrophil gelatinase-associated lipocalin (NGAL) is expressed in the fetal part of the maternal-fetal interface. NGAL is part of the immune barrier and serves primarily as a transport protein transferring biologically hazardous molecules in a safe and controlled way. It has been shown that over-activation of NGAL induces apoptosis. We hypothesized that increased placental NGAL expression induces trophoblastic apoptosis in the nitrofen model of CDH.

METHODS

Pregnant rats were exposed to either olive oil or nitrofen on day 9 of gestation (D9). Placenta harvested on D21 and divided into two groups: control and nitrofen with CDH. Immunohistochemistry was performed to evaluate trophoblasts (by cytokeratin expression), NGAL expression, and apoptotic trophoblastic cells (using TUNEL assay). Total RNA was extracted from each placenta and the relative mRNA expression levels of NGAL were analyzed using RT-PCR.

RESULTS

Immunohistochemistry showed NGAL immunoreactivity both in control and CDH in the fetal part of the fetal-maternal interface of placenta. Markedly increased NGAL expression was detected in CDH group compared to controls. Relative mRNA expression levels of NGAL gene were significantly increased in the CDH group compared to control in the placenta (5.924 ± 0.93 vs. 1.895 ± 0.54, p < 0.001). Markedly increased numbers of apoptotic trophoblastic cells were seen in the maternal-fetal interface in the CDH group compared to controls.

CONCLUSIONS

NGAL activation may lead to increased trophoblastic apoptosis in the maternal-fetal interface in the nitrofen model of CDH. These changes may therefore cause disturbance in maternal-fetal retinol transport affecting fetal lung morphogenesis.

Cellular retinoic acid-binding proteins are essential for hindbrain patterning and signal robustness in zebrafish

The vitamin A derivative retinoic acid (RA) is a morphogen that patterns the anterior-posterior axis of the vertebrate hindbrain. Cellular retinoic acid-binding proteins (Crabps) transport RA within cells to both its nuclear receptors (RARs) and degrading enzymes (Cyp26s). However, mice lacking Crabps are viable, suggesting that Crabp functions are redundant with those of other fatty acid-binding proteins. Here we show that Crabps in zebrafish are essential for posterior patterning of the hindbrain and that they provide a key feedback mechanism that makes signaling robust as they are able to compensate for changes in RA production. Of the four zebrafish Crabps, Crabp2a is uniquely RA inducible and depletion or overexpression of Crabp2a makes embryos hypersensitive to exogenous RA. Computational models confirm that Crabp2a improves robustness within a narrow concentration range that optimizes a 'robustness index', integrating spatial information along the RA morphogen gradient. Exploration of signaling parameters in our models suggests that the ability of Crabp2a to transport RA to Cyp26 enzymes for degradation is a major factor in promoting robustness. These results demonstrate a previously unrecognized requirement for Crabps in RA signaling and hindbrain development, as well as a novel mechanism for stabilizing morphogen gradients despite genetic or environmental fluctuations in morphogen availability.

Nitrofen interferes with trophoblastic expression of retinol-binding protein and transthyretin during lung morphogenesis in the nitrofen-induced congenital diaphragmatic hernia model

BACKGROUND

Retinoids play a key role in lung development. Retinoid signaling pathway has been shown to be disrupted in the nitrofen model of congenital diaphragmatic hernia (CDH) but the exact mechanism is not clearly understood. Retinol-binding protein (RBP) and transthyretin (TTR) are transport proteins for delivery of retinol to the tissues via circulation. Previous studies have shown that pulmonary retinol levels are decreased during lung morphogenesis in the nitrofen CDH model. In human newborns with CDH, both retinol and RBP levels are decreased. It has been reported that maternal RBP does not cross the placenta and the fetus produces its own RBP by trophoblast. RBP and TTR synthesized in the fetus are essential for retinol transport to the developing organs including lung morphogenesis. We hypothesized that nitrofen interferes with the trophoblastic expression of RBP and TTR during lung morphogenesis and designed this study to examine the trophoblastic expression of RBP and TTR, and the total level of RBP and TTR in the lung in the nitrofen model of CDH.

METHODS

Pregnant rats were exposed to either olive oil or nitrofen on day 9 of gestation (D9). Fetal lungs and placenta harvested on D21 and divided into two groups: control (n = 8) and nitrofen with CDH (n = 8). Total lung RBP and TTR levels using protein extraction were compared with enzyme linked immunoassay (ELISA). Immunohistochemistry was performed to evaluate trophoblastic RBP and TTR expression.

RESULTS

Total protein levels of lung RBP and TTR were significantly lower in CDH (0.26 ± 0.003 and 6.4 ± 0.5 μg/mL) compared with controls (0.4 ± 0.001 and 9.9 ± 1.6 μg/mL, p < 0.05). In the control group, immunohistochemical staining showed strong immunoreactivity of RBP and TTR in the trophoblast compared to CDH group.

CONCLUSIONS

Decreased trophoblast expression of retinol transport proteins suggest that nitrofen may interfere with the fetal retinol transport resulting in reduced pulmonary RBP and TTR levels and causing pulmonary hypoplasia in CDH.

Maternal-fetal transfer and metabolism of vitamin A and its precursor β-carotene in the developing tissues

The requirement of the developing mammalian embryo for retinoic acid is well established. Retinoic acid, the active form of vitamin A, can be generated from retinol and retinyl ester obtained from food of animal origin, and from carotenoids, mainly β-carotene, from vegetables and fruits. The mammalian embryo relies on retinol, retinyl ester and β-carotene circulating in the maternal bloodstream for its supply of vitamin A. The maternal-fetal transfer of retinoids and carotenoids, as well as the metabolism of these compounds in the developing tissues are still poorly understood. The existing knowledge in this field has been summarized in this review in reference to our basic understanding of the transport and metabolism of retinoids and carotenoids in adult tissues. The need for future research on the metabolism of these essential lipophilic nutrients during development is highlighted. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Three cases of cerebellar hypoplasia and vitamin a deficiency: a case report and a possible pathophysiology

Studies in animal models have established that intra-uterine vitamin A deficiency can hinder hindbrain formation; however, reports of such a phenomenon in humans had not been published until recently, when our group presented the case of an infant diagnosed with pontocerebellar hypoplasia and vitamin A deficiency. We currently report the cases of 3 infants with cerebellar hypoplasia and hypovitaminosis A, whose vitamin A consumption was determined to be adequate, and whose mothers had no such deficiency. We suggest a possible pathophysiology whereby a mutation in the gene coding for cytoplasmic retinol-binding protein II, which is expressed both in the placenta and the yolk sac (during fetal development) and in the absorptive intestinal cells, can cause vitamin A deficiency, forming hindbrain anomalies. Validation of our hypothesis will require further research, including fetal vitamin A measurements and hindbrain examination in cytoplasmic retinol-binding protein II knockout animals.

Epigenetic regulation by RARα maintains ligand-independent transcriptional activity

Retinoic acid receptors (RARs) α, β and γ are key regulators of embryonic development. Hematopoietic differentiation is regulated by RARα, and several types of leukemia show aberrant RARα activity. Through microarray expression analysis, we identified transcripts differentially expressed between F9 wild-type (Wt) and RARα knockout cells cultured in the absence or presence of the RAR-specific ligand all trans retinoic acid (RA). We validated the decreased Mest, Tex13, Gab1, Bcl11a, Tcfap2a and HMGcs1 transcript levels, and increased Slc38a4, Stmn2, RpL39l, Ref2L, Mobp and Rlf1 transcript levels in the RARa knockout cells. The decreased Mest and Tex13 transcript levels were associated with increased promoter CpG-island methylation and increased repressive histone modifications (H3K9me3) in RARα knockout cells. Increased Slc38a4 and Stmn2 transcript levels were associated with decreased promoter CpG-island methylation and increased permissive histone modifications (H3K9/K14ac, H3K4me3) in RARα knockout cells. We demonstrated specific association of RARα and RXRα with the Mest promoter. Importantly, stable expression of a dominant negative, oncogenic PML–RARα fusion protein in F9 Wt cells recapitulated the decreased Mest transcript levels observed in RARα knockout cells. We propose that RARα plays an important role in cellular memory and imprinting by regulating the CpG methylation status of specific promoter regions.

Uptake of dietary retinoids at the maternal-fetal barrier: in vivo evidence for the role of lipoprotein lipase and alternative pathways

Dietary retinoids (vitamin A and its derivatives) contribute to normal embryonic development. However, the mechanism(s) involved in the transfer of recently ingested vitamin A from mother to embryo is not fully understood. We investigated in vivo whether lipoprotein lipase (LPL) facilitates the placental uptake of dietary retinyl ester incorporated in chylomicrons and their remnants and its transfer to the embryo. We examined the effects of both genetic ablation (MCK-L0 mice) and pharmacological inhibition (P-407) of LPL by maintaining wild type and MCK-L0 mice on diets with different vitamin A content or administering them an oral gavage dose of [(3)H]retinol with or without P-407 treatment. We showed that LPL expressed in placenta facilitates uptake of retinoids by this organ and their transfer to the embryo, mainly through its catalytic activity. In addition, through its "bridging function," LPL can mediate the acquisition of nascent chylomicrons by the placenta, although less efficiently. Quantitative real-time PCR and Western blot analysis showed that placental LPL acts in concert with LDL receptor and LRP1. Finally, by knocking out the retinol-binding protein (RBP) gene in the MCK-L0 background (MCK-L0-RBP(-/-) mice) we demonstrated that the placenta acquires dietary retinoids also via the maternal circulating RBP-retinol complex. RBP expressed in the placenta facilitate the transfer of postprandial retinoids across the placental layers toward the embryo.

PPAR Signaling in Placental Development and Function

With the major attention to the pivotal roles of PPARs in diverse aspects of energy metabolism, the essential functions of PPARγ and PPARβ/δ in placental development came as a surprise and were often considered a nuisance en route to their genetic analysis. However, these findings provided an opportune entrée into placental biology. Genetic and pharmacological studies, primarily of knockout animal models and cell culture, uncovered networks of PPARγ and PPARδ, their heterodimeric RXR partners, associated transcriptional coactivators, and target genes, that regulate various aspects of placental development and function. These studies furnish both specific information about trophoblasts and the placenta and potential hints about the functions of PPARs in other tissues and cell types. They reveal that the remarkable versatility of PPARs extends beyond the orchestration of metabolism to the regulation of cellular differentiation, tissue development, and trophoblast-specific functions. This information and its implications are the subject of this review.

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.

Microarray assessment of the influence of the conceptus on gene expression in the mouse uterus during decidualization

During pregnancy in several species including humans and rodents, the endometrium undergoes decidualization. This process of differentiation from endometrial to decidual tissue occurs only after the onset of implantation in mice. It can also be artificially induced causing the formation of deciduomal tissue. The purpose of this study was to compare the gene expression profile of the developing decidua in pregnant mice with the deciduoma formed after artificial induction in an effort to identify conceptus-influenced changes in uterine gene expression during decidualization. We induced decidualization artificially by transferring blastocyst-sized ConA-coated agarose beads into the uterus on day 2.5 of pseudopregnancy. Recently published work has found this model to be more 'physiological' than other methods. Total RNA was isolated from blastocyst and bead-induced 'implantation' sites of the uteri of day 7.5 pregnant (decidua) and pseudopregnant (deciduoma) mice respectively. This RNA was then used for microarray analysis using Mouse Illumina BeadArray chips. This analysis revealed potential differential mRNA levels of only 45 genes between the decidua and bead-induced deciduoma tissues. We confirmed the differential mRNA levels of 31 of these genes using quantitative RT-PCR. Finally, the level and localization of some of the mRNAs for select genes (Aldh3a1, Bcmo1, Guca2b, and Inhbb) identified by our microarray analysis were examined in more detail. This study provides the identity of a small set of genes whose expression in the uterus during decidualization may be influenced by molecular signals from the conceptus.

Coordinated gene expression during gilthead sea bream skeletogenesis and its disruption by nutritional hypervitaminosis A

BACKGROUND

Vitamin A (VA) has a key role in vertebrate morphogenesis, determining body patterning and growth through the control of cell proliferation and differentiation processes. VA regulates primary molecular pathways of those processes by the binding of its active metabolite (retinoic acid) to two types of specific nuclear receptors: retinoic acid receptors (RARs) and retinoid X receptors (RXRs), which promote transcription of downstream target genes. This process is well known in most of higher vertebrates; however, scarce information is available regarding fishes. Therefore, in order to gain further knowledge of fish larval development and its disruption by nutritional VA imbalance, the relative expression of some RARs and RXRs, as well as several genes involved in morpho- and skeletogenesis such as peroxisome proliferator-activated receptors (PPARA, PPARB and PPARG); retinol-binding protein (RBP); insulin-like growth factors I and II (IGF1 and IGF2, respectively); bone morphogenetic protein 2 (Bmp2); transforming growth factor β-1 (TGFB1); and genes encoding different extracellular matrix (ECM) proteins such as matrix Gla protein (mgp), osteocalcin (bglap), osteopontin (SPP1), secreted protein acidic and rich in cysteine (SPARC) and type I collagen α1 chain (COL1A1) have been studied in gilthead sea bream.

RESULTS

During gilthead sea bream larval development, specific expression profiles for each gene were tightly regulated during fish morphogenesis and correlated with specific morphogenetic events and tissue development. Dietary hypervitaminosis A during early larval development disrupted the normal gene expression profile for genes involved in RA signalling (RARA), VA homeostasis (RBP) and several genes encoding ECM proteins that are linked to skeletogenesis, such as bglap and mgp.

CONCLUSIONS

Present data reflects the specific gene expression patterns of several genes involved in larval fish RA signalling and skeletogenesis; and how specific gene disruption induced by a nutritional VA imbalance underlie the skeletal deformities. Our results are of basic interest for fish VA signalling and point out some of the potential molecular players involved in fish skeletogenesis. Increased incidences of skeletal deformities in gilthead sea bream fed with hypervitaminosis A were the likely ultimate consequence of specific gene expression disruption at critical development stages.

Vitamin A deficiency (VAD), teratogenic, and surgical models of congenital diaphragmatic hernia (CDH)

Congenital diaphragmatic hernia (CDH) is a congenital malformation that occurs with a frequency of 0.08 to 0.45 per 1,000 births. Children with CDH are born with the abdominal contents herniated through the diaphragm and exhibit an associated pulmonary hypoplasia which is frequently accompanied by severe morbidity and mortality. Although the etiology of CDH is largely unknown, considerable progress has been made in understanding its molecular mechanisms through the usage of genetic, teratogenic, and surgical models. The following review focuses on the teratogenic and surgical models of CDH and the possible molecular mechanisms of nitrofen (a diphenyl ether, formerly used as an herbicide) in both induction of CDH and pulmonary hypoplasia. In addition, the mechanisms of other compounds including several anti-inflammatory agents that have been linked to CDH will be discussed. Furthermore, this review will also explore the importance of vitamin A in lung and diaphragm development and the possible mechanisms of teratogen interference in vitamin A homeostasis. Continued exploration of these models will bring forth a clearer understanding of CDH and its molecular underpinnings, which will ultimately facilitate development of therapeutic strategies.

RXRalpha regulates the pregnancy-specific glycoprotein 5 gene transcription through a functional retinoic acid responsive element

Human pregnancy-specific glycoproteins (PSG) are major placental polypeptides encoded by eleven highly conserved genes expressed by the syncytiotrophoblast. The minimal promoter region of all PSG genes contains a putative Retinoic Acid Responsive Element (RARE) though the ability of retinoids to regulate PSG gene expression has not been established. Retinoid signaling pathway plays a key role for overall placenta biology and is essential for trophoblast differentiation. In this work, we investigated the participation of the RARE motif in the regulation of PSG5 gene transcription by retinoic acid and its receptors. The minimal promoter region of PSG5 gene was activated by RXRalpha but not by RARalpha, in a ligand-dependent manner. The RARE sequence of PSG5 gene promoter was recognized by endogenous RXRalpha present in placental nuclear extracts as well as by RXRalpha either over expressed in cultured non-placental cells or in vitro translated. Mutations at specific nucleotides within the RARE motif abrogated both RXRalpha DNA binding and transcriptional activation of PSG5 promoter mediated by RXRalpha. Moreover, endogenous PSG expression was significantly induced in trophoblast-derived Jeg-3 cells upon 9-cis retinoic acid treatment. Interestingly, the induction level was higher following methotrexate-induced differentiation of Jeg-3 cells to syncytiotrophoblast-like structures. Altogether, these data provide the first evidences demonstrating that transcriptional activity of PSG5 gene is responsive to an external signal involving the retinoids-RXRalpha axis through a conserved RARE motif shared by all PSG gene family members.

Epidermal transient down-regulation of retinol-binding protein 4 and mirror expression of apolipoprotein Eb and estrogen receptor 2a during zebrafish fin and scale development

Very little is known about the molecular control of skin patterning and scale morphogenesis in teleost fish. We have found radially symmetrical epidermal placodes with down-regulation of retinol-binding protein 4 (rbp4) expression during the initial paired fin and scale morphogenesis in zebrafish (Danio rerio). This finding may be related to changes in keratinocyte cytodifferentiation and/or the integument retinoid metabolism. rbp4 transcripts are expressed afterward in the central epidermis of the scale papilla and gradually extend to the epidermis, covering the growing scale, whereas no transcripts were detected in posterior margin epidermis. In contrast, induction of apolipoprotein Eb (apoeb) and up-regulation of estrogen receptor 2a (esr2a) transcripts were observed in the epidermis at initiator sites of zebrafish ectodermal/dermal appendage morphogenesis. This expression was maintained in the posterior margin epidermis of the formed scales. esr2a was also strongly expressed in neuromasts, whereas no rbp4 and apoeb transcripts were detected in these mechanosensory structures. The observed epidermal molecular events suggest that epidermis patterning is due to an activator-inhibitor mechanism operational at epidermal-dermal interaction sites. rbp4 transcript expression was also strongly down-regulated by 1-phenyl-2-thio-urea (PTU). As this inhibitor is commonly used to block obscuring pigmentation during in situ hybridization studies, this finding suggests that PTU should be used with caution, particularly in studying skin development.

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.

Changes in Expression and Localization of GPRC5B and RAR.ALPHA. in the Placenta and Yolk Sac During Middle to Late Gestation in Mice

The mRNA expression of GPRC5B, an orphan G protein-coupled receptor, is induced by retinoic acid (RA). Because RA plays critical roles in embryonic development, reproductive functions, metabolism and homeostasis, GPRC5B is also considered crucial in these physiological events. We investigated the changes in expression of GPRC5B and RA receptor (RAR) alpha mRNAs and immunohistochemical localization of their proteins in the murine placenta and yolk sac at 13.5, 15.5 and 17.5 days post coitus. Stable levels of GPRC5B and RARalpha mRNAs were detected in the placenta and yolk sac. In the placenta, GPRC5B was present in maternal and fetal vascular endothelial cells, stromal cells, fibroblast-like cells and glycogen cells. A strong reaction to RARalpha was detected in maternal and fetal vascular endothelial cells and stromal cells. The levels of GPRC5B and RARalpha proteins in maternal and fetal vascular endothelial cells decreased with gestation. In the yolk sac, GPRC5B and RARalpha proteins were detected in vascular endothelial cells, but their levels did not change during the gestation period. These findings indicate that GPRC5B is involved in RA-dependent morphogenesis/angiogenesis and regulation of extracellular matrix synthesis in the murine placenta and yolk sac.

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.

Identification of murine uterine genes regulated in a ligand-dependent manner by the progesterone receptor

Progesterone (P4) acting through its cognate receptor, the progesterone receptor (PR), plays an important role in uterine physiology. The PR knockout (PRKO) mouse has demonstrated the importance of the P4-PR axis in the regulation of uterine function. To define the molecular pathways regulated by P4-PR in the mouse uterus, Affymetrix MG U74Av2 oligonucleotide arrays were used to identify alterations in gene expression after acute and chronic P4 treatments. PRKO and wild-type mice were ovariectomized and then treated with vehicle or 1 mg P4 every 12 h. Mice were killed either 4 h after the first injection (acute P4 treatment) or after the fourth injection of P4 (chronic P4 treatment). At the genomic level, the major change in gene expression after acute P4 treatment was an increase in the expression of 55 genes. Conversely, the major change in gene expression after chronic P4 treatment was an overall reduction in the expression of 102 genes. In the analysis, retinoic acid metabolic genes, cytochrome P 450 26a1 (Cyp26a1), alcohol dehydrogenase 5, and aldehyde dehydrogenase 1a1 (Aldh1a1); kallikrein genes, Klk5 and Klk6; and specific transcription factors, GATA-2 and Cited2 [cAMP-corticosterone-binding protein/p300-interacting transactivator with glutamic acid (E) and aspartic acid (D)-rich tail], were validated as regulated by the P4-PR axis. Identification and analysis of these responsive genes will help define the role of PR in regulating uterine biology.

Retinoid X receptors: X-ploring their (patho)physiological functions

Retinoid X receptor (RXR) belongs to a family of ligand-activated transcription factors that regulate many aspects of metazoan life. A class of nuclear receptors requires RXR as heterodimerization partner for their function. This places RXR in the crossroad of multiple distinct biological pathways. This and the fact that the debate on the endogenous ligand requirement for RXR is not yet settled make RXR still an enigmatic transcription factor. Here, we review some of the biology of RXR. We place RXR into the evolution of nuclear receptors, review structural details and ligands of the receptor. Then processes regulated by RXR are discussed focusing on the developmental roles deduced from studies on knockout animals and metabolic roles in diseases such as diabetes and atherosclerosis deduced from pharmacological studies. Finally, aspects of RXR's involvement in myeloid differentiation and apoptosis are summarized along with issues on RXR's suitability as a therapeutic target.

Trialkyltin compounds bind retinoid X receptor to alter human placental endocrine functions

Retinoid X receptor (RXR) is a nuclear receptor that plays important and multiple roles in mammalian development and homeostasis. We previously reported that, in human choriocarcinoma cells, tributyltin chloride and triphenyltin hydroxide, which are typical environmental contaminants and cause masculinization in female mollusks, are potent stimulators of human chorionic gonadotropin production and aromatase activity, which play key endocrine functions in maintaining pregnancy and fetal development. However, the molecular mechanism through which these compounds stimulate these endocrine functions remains unclear. Our current study shows that trialkyltin compounds, including tributyltin chloride and triphenyltin hydroxide, function as RXR agonists. Trialkyltins directly bind to the ligand-binding domain of RXR with high affinity and function as transcriptional activators. Unlike the natural RXR ligand, 9-cis-retinoic acid, the activity of trialkyltins is RXR specific and does not activate the retinoic acid receptor pathway. In addition, trialkyltins activate RXR to stimulate the expression of a luciferase reporter gene containing the human placental promoter I.1 sequence of aromatase, suggesting that trialkyltins stimulate human placental endocrine functions through RXR-dependent signaling pathways. Therefore, our results suggest that activation of RXR may be a novel mechanism by which trialkyltins alter human endocrine functions.

Effect of methylmercury on glutamate-cysteine ligase expression in the placenta and yolk sac during mouse development

The placenta and the yolk sac play critical roles in fetal development, including protection from oxidative stress through the presence of detoxifying enzymes. Glutathione (GSH; gamma-glutamylcysteinylglycine), a crucial molecule in the maintenance of cellular redox status, plays a critical role in development, and it is also protective against methylmercury toxicity. Glutamate-cysteine ligase (GCL), the enzyme that catalyzes the rate-limiting step in GSH synthesis, is widely expressed in the mouse embryo and extraembryonic membranes throughout development. The aim of this study was to investigate the effect of low-level subchronic methylmercury exposure on GCL expression in the mouse placenta and yolk sac, after describing the basal developmental expression of the enzyme in these tissues. We found that basal mRNA expression levels increased dramatically in the placenta and the yolk sac at gd 18, whereas protein levels did not increase in parallel with the mRNA. We also found that methylmercury induced GCLc mRNA expression in the placenta at gd 18 in a dose-dependent manner, suggesting an important role for this enzyme in the response of the placenta to toxicants. These changes in expression may be useful as a biomarker of MeHg exposure during development.

Placental expression of the nuclear receptors for oxysterols LXR? and LXR? during mouse and human development

Ontogenesis and localization expression of genes encoding for the nuclear receptors for oxysterols (lxralpha and lxrbeta) were investigated in human and mouse placenta during gestation. Both mRNAs were detectable in the placentation throughout the gestation (from 7 days postcoitum and 6th week of gestation in mouse and human, respectively) by Northern blots or RT-PCR experiments. Lxralpha showed a higher accumulation in the mouse yolk sac. In situ hybridization pointed a specific expression of lxralpha in the human amniotic membranes.

Signaling hierarchy downstream of retinoic acid that independently regulates vascular remodeling and endothelial cell proliferation

We previously demonstrated that during vascular morphogenesis, retinoic acid (RA) is required for the control of endothelial cell proliferation and capillary plexus remodeling. Herein, we investigate the mechanisms by which RA regulates these processes in the yolk sac. We found that although the enzyme required for RA production during early embryogenesis, retinaldehyde dehydrogenase-2 (Raldh2), was expressed in the visceral endoderm, RA receptors alpha1 and alpha2 were expressed in endothelial cells in the mesoderm, indicating that they are direct targets of RA. In Raldh2(-/-) embryos, there was down-regulation of TGF-beta1, fibronectin (Fn) and integrin alpha5, which was associated with decreased visceral endoderm survival and production of VEGF-A, Indian hedgehog (IHH), and bFGF. Exogenous provision of RA or Fn to Raldh2(-/-) explants in whole mouse embryo culture restored vascular remodeling, visceral endoderm survival, as well as integrin alpha5 expression and its downstream signaling that controls endothelial growth. Exogenous provision of visceral endoderm-derived factors (VEGF-A, IHH, and bFGF) failed to rescue endothelial cell proliferative control but collectively promoted vascular remodeling, suggesting that these processes are independently regulated via a signaling hierarchy downstream of RA.

Transplacental delivery of retinoid: the role of retinol-binding protein and lipoprotein retinyl ester

Retinoids are required for normal embryonic development. Both embryonic retinoid deficiency and excess result in congenital malformations. There is little understanding of the physiology underlying retinoid transfer from the maternal circulation to the embryo. We now report studies that explore this process using retinol-binding protein-deficient (RBP-/-) mice and mice that express human RBP on the RBP-/-) background. Our studies establish that dietary retinoid, bound to lipoproteins, can serve as an important source for meeting tissue retinoid requirements during embryogenesis. Indeed, retinyl ester concentrations in the circulations of pregnant RBP-/- mice are significantly elevated over those observed in wild-type mice, suggesting that lipoprotein retinyl esters may compensate for the absence of retinol-RBP during pregnancy. We also demonstrate, contrary to earlier proposals, that maternal RBP does not cross the placenta and cannot enter the fetal circulation. Overall, our data indicate that both retinol-RBP and retinyl esters bound to lipoproteins are able to provide sufficient retinoid to the embryo to allow for normal embryonic development.

Hyperglycaemia potentiates the teratogenicity of retinoic acid in diabetic pregnancy in mice

AIMS/HYPOTHESIS

We recently showed in mice that maternal diabetes increases embryonic susceptibility to caudal regression induced by vitamin A metabolite retinoic acid. Here we tested whether in the maternal diabetic milieu hyperglycaemia is the critical factor responsible for mediating this increased susceptibility.

METHODS

Non-diabetic pregnant mice were made hyperglycaemic by subcutaneous injections of glucose at regular intervals. Conversely, diabetic pregnant mice were treated with phlorizin to induce renal glucosuria and thus reduce blood glucose concentrations. Pregnant mice were treated with retinoic acid and the extent of caudal regression in mouse embryos, measured in terms of the ratio of tail length to crown-rump length was assessed. Embryos were also examined for Wnt-3a expression and cell death.

RESULTS

Embryos of mice treated with glucose had a greater extent of caudal regression induced by retinoic acid than saline-treated controls, with enhanced down-regulation of Wnt-3a expression and exacerbated cell death specifically at the caudal end of the embryo. Embryos of diabetic mice treated with phlorizin had a similar extent of caudal regression to embryos of non-diabetic mice after treatment with retinoic acid.

CONCLUSIONS/INTERPRETATION

Hyperglycaemia increases embryonic susceptibility to caudal regression induced by retinoic acid, with the underlying cellular and molecular changes closely mimicking those that occur in maternal diabetes. Reduction of blood glucose concentrations in diabetic mice completely abolishes this increased susceptibility to retinoic acid. These results suggest that in maternal diabetes hyperglycaemia is the critical factor responsible for potentiating the teratogenic effect of retinoic acid.

Retinoid receptors and vitamin A deficiency: differential patterns of transcription during early avian development and the rapid induction of RARs by retinoic acid

The functional links of specific retinoid receptors to early developmental events in the avian embryo are not known. Before such studies are undertaken, knowledge is required of the spatiotemporal expression patterns of the receptor genes and their regulation by endogenous retinoic acid levels during the early stages of development. Here, we report the expression patterns of mRNAs for RARalpha, RARalpha2, RARbeta2, RARgamma, RARgamma2, RXRalpha, and RARgamma from neurulation to HH10 in the normal and vitamin A-deficient (VAD) quail embryo. The transcripts for all retinoid receptors are detectable at HH5, except for RXRgamma, which is detected at the beginning of HH6. At the 4/5 somite stage of HH8, when retinoid signaling is initiated in the avian embryo, mRNAs of all receptors are present, with very strong and ubiquitous expression patterns for RARalpha, RARalpha2, RARgamma, RARgamma2, and RXRalpha, a persistent expression of RARgamma in the neural tissues, a strong expression of RARbeta2 in lateral plate mesoderm and somites, and an anterior expression of RXRgamma. All retinoid receptors are expressed in the heart primordia. In the VAD quail embryo, the general pattern of retinoid receptor transcript localization is similar to that of the normal, except that the expression of RARalpha2 and RARbeta2 is severely diminished. Administration of retinol or retinoic acid to VAD embryos at or before the 4/5 somite stage rescues the expression of RARalpha2 and RARbeta2 within approximately 45 min and restores normal development. RARbeta2 expression requires the expression of RARalpha2. After neurulation, the expression of all retinoid receptors in the VAD quail embryo becomes independent of vitamin A status and is similar to that of the normal. The mRNA levels and sites of expression of the key enzyme for retinoic acid biosynthesis, Raldh-2, are not affected by vitamin A status; the expression pattern is restricted and does not correspond to that of retinoid receptors at all sites. The general patterns and intensity of retinoid receptor gene expression during early quail development are comparable to those of the mammalian and thus validate the application of results from retinoid-regulated avian development studies to those of the mammalian.

The role of vitamin A in mammalian reproduction and embryonic development

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

Increased neonatal mortality in mice lacking cellular retinol-binding protein II

Cellular retinol-binding protein II (CRBP II) is a member of the cellular retinol-binding protein family, which is expressed primarily in the small intestine. To investigate the physiological role of CRBP II, the gene encoding CRBP II was inactivated. The saturable component of intestinal retinol uptake is impaired in CRBP II(-/-) mice. The knockout mice, while maintained on a vitamin A-enriched diet, have reduced (40%) hepatic vitamin A stores but grow and reproduce normally. However, reducing maternal dietary vitamin A to marginal levels during the latter half of gestation results in 100% mortality/litter within 24 h after birth in the CRBP II(-/-) line but no mortality in the wild type line. The neonatal mortality in heterozygote offspring of CRBP II(-/-) dams (79 +/- 21% deaths/litter) was increased as compared with the neonatal mortality in heterozygote offspring of wild type dams (29 +/- 25% deaths per litter, p < 0.05). Maternal CRBP II was localized by immunostaining in the placenta at 18 days postcoitum as well as in the small intestine. These studies suggest that both fetal as well as maternal CRBP II are required to ensure adequate delivery of vitamin A to the developing fetus when dietary vitamin A is limiting.

Expression patterns of retinoid X receptors, retinaldehyde dehydrogenase, and peroxisome proliferator activated receptor gamma in bovine preattachment embryos

In cattle, administration of retinol at the time of superovulation has been indirectly associated with enhanced developmental potential of the embryo. Vitamin A and its metabolites influence several developmental processes by interacting with 2 different types of nuclear receptors, retinoic acid receptors and retinoid X receptors (RXRs). Given the limited information available concerning the RXR-mediated retinoid signaling system, particularly in species other than rodents, this study was performed to gain insight into the potential role of retinoid signaling during preattachment embryo development in the cow. Bovine embryos were produced in vitro from oocytes harvested from abattoir ovaries and frozen in liquid nitrogen at the oocyte, 2-, 4-, 8-, and 16- to 20-cell, morula, blastocyst, and hatched blastocyst stages. Reverse transcription polymerase chain reaction (PCR) and whole mount in situ hybridization were utilized to investigate mRNA expression for RXR alpha, RXR beta, RXR gamma, alcohol dehydrogenase I (ADH-I), retinaldehyde dehydrogenase 2 (RALDH2), peroxisome proliferator activated receptor gamma (PPAR gamma), and glyceraldehyde-3-phosphate dehydrogenase. Transcripts for RXR alpha, RXR beta, RALDH2, and PPAR gamma were detected in all stages beginning from the oocyte through to the hatched blastocyst. Whole mount in situ hybridization performed using digoxigenin-labeled antisense probes detected all 4 transcripts in both the inner cell mass and the trophectoderm of hatched blastocysts. PCR products obtained for ADH-I exhibited very low homology to known human and mouse sequences. Immunohistochemistry was performed using polyclonal anti-rabbit antibodies against RXR beta and PPAR gamma to investigate whether these embryonic mRNAs were translated to the mature protein. Strong immunostaining was observed for both RXR beta and PPAR gamma in the trophectoderm and inner cell mass cells of intact and hatched blastocysts. Messenger RNA was not detected at any stage for RXR gamma. Expression of mRNA for RXR alpha, RXR beta, RALDH2, and PPAR gamma suggests that the early embryo may be competent to synthesize retinoic acid and regulate gene expression during preattachment development in vitro.

Comparative developmental anatomy of the murine and human definitive placentae

The placenta of eutherian mammals is a remarkable biological structure. It is composed of both zygote-derived and maternal cells, and mediates the complex interactions between the mother and the fetus that are necessary for fetal growth and survival. While the genetic basis of human placental development and function is largely unknown, its understanding is of immense clinical importance because placentopathies of unknown genetic aetiology are thought to be the cause of many types of pregnancy complications including unexplained miscarriage and intrauterine growth retardation. The mouse is the best-studied mammalian experimental genetic model system and research is not restricted by the inherent ethical and practical limitations associated with the human. As a result, knowledge about the genetic control of mouse placental development has expanded greatly in recent years. In order for this to be of benefit to medical practice, extrapolations from murine to human placentation have to be made. However, comprehensive comparisons of the placentae of these two species are rare. This review therefore compares the developmental anatomy of the placenta between humans and mice with emphasis on structures and cell types that might be analogous between the two species. This could be of particular benefit to mouse developmental geneticists who study placental development and have an interest in the possible clinical implications of their work.

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.

Embryonic retinoic acid synthesis is essential for heart morphogenesis in the mouse

Retinoic acid (RA), the active derivative of vitamin A, has been implicated in various steps of cardiovascular development, but its contribution to early heart morphogenesis has not been clearly established in a mammalian system. To block endogenous RA synthesis, we have disrupted the gene encoding RALDH2, the first retinaldehyde dehydrogenase whose expression has been detected during early mouse post-implantation development. We describe here the heart abnormalities of the RA-deficient Raldh2 mutants that die in utero at gestational day 10.5. The embryonic heart tube forms properly, but fails to undergo rightward looping and, instead, forms a medial distended cavity. Expression of early heart determination factors is not altered in mutants, and the defect in heart looping does not appear to involve the Nodal/Lefty/Pitx2 pathway. Histological and molecular analysis reveal distinct anteroposterior components in the mutant heart tube, although posterior chamber (atria and sinus venosus) development is severely impaired. Instead of forming trabeculae, the developing ventricular myocardium consists of a thick layer of loosely attached cells. Ultrastructural analysis shows that most of the ventricular wall consists of prematurely differentiated cardiomyocytes, whereas undifferentiated cells remain clustered rostrally. We conclude that embryonic RA synthesis is required for realization of heart looping, development of posterior chambers and proper differentiation of ventricular cardiomyocytes. Nevertheless, the precise location of this synthesis may not be crucial, as these defects can mostly be rescued by systemic (maternal) RA administration. However, cardiac neural crest cells cannot be properly rescued in Raldh2(−/−)embryos, leading to outflow tract septation defects.

Molecular characterization and developmental expression of NORPEG, a novel gene induced by retinoic acid

We have characterized NORPEG, a novel gene from human retinal pigment epithelial cells (ARPE-19), in which its expression is induced by all-trans-retinoic acid. Two transcripts ( approximately 3 and approximately 5 kilobases in size) have been detected for this gene, which is localized to chromosome band 5p13.2-13.3. Placenta and testis showed the highest level of expression among various human tissues tested. Six ankyrin repeats and a long coiled-coil domain are present in the predicted sequence of the NORPEG protein, which contains 980 amino acid residues. This approximately 110-kDa protein was transiently expressed in COS-7 cells as a FLAG fusion protein and immunolocalized to the cytoplasm. Confocal microscopic analysis of the NORPEG protein in ARPE-19 cells showed threadlike projections in the cytoplasm reminiscent of the cytoskeleton. Consistent with this localization, the expressed NORPEG protein showed resistance to solubilization by Triton X-100 and KCl. An ortholog of NORPEG characterized from mouse encoded a protein that showed 91% sequence similarity to the human NORPEG protein. The expression of Norpeg mRNA was detected in mouse embryo at embryonic day 9.5 by in situ hybridization, and the expression appears to be developmentally regulated. In adult mouse, the highest level of expression was detected in the seminiferous tubules of testis.

Immunohistochemical localization of retinoid binding proteins at the materno-fetal interface of the porcine epitheliochorial placenta

Retinol and retinoic acid that are potent modulators of gene expression are vital for development and growth of the conceptus. Apart from being transported across the placenta, retinol and retinoic acid may also be active in the placenta per se. Three proteins involved in 1) serum transport of retinol (retinol binding protein [RBP]), 2) cellular transport and metabolism of retinol (cellular RBP [CRBP] I), and 3) retinoic acid (cellular retinoic acid binding protein [CRABP] I), respectively, have been located by immunohistochemistry during gestation in the porcine placenta. This is a diffuse epitheliochorial placenta composed of areolar-gland subunits, where transport of larger molecules takes place, and interareolar regions, where gas-exchange and trophoblast absorption of hemotroph occur. Immunoreactive-RBP (ir-RBP) as well as CRBP I (ir-CRBP) was detected in uterine glands and in areolar trophoblasts, suggesting that RBP-retinol is secreted by the glands and absorbed by the trophoblasts. Both proteins were present also at the interareolar regions, with ir-CRBP in both the uterine epithelium and the apposing trophoblasts, but ir-RBP only in the former. The localization of ir-CRABP was, in contrast, strictly limited to interareolar trophoblasts. Together these findings suggest that 1) the areolar gland subunits are important for transport of retinol and retinol-RBP, and 2) retinoid binding proteins are involved in the development and growth of the porcine placenta.

Functional heterogeneity in the zinc fingers of metalloregulatory protein metal response element-binding transcription factor-1

Metal response element-binding transcription factor-1 (MTF-1) is a unique, zinc-inducible transcription factor that binds to metal response elements in the metallothionein promoter and activates transcription in response to metals and oxidative stress. MTF-1 contains six zinc fingers of the Cys(2)-His(2) type. It was previously shown that MTF-1 is reversibly activated to bind DNA in response to changes in zinc status, unlike other zinc finger transcription factors, which do not appear to be reversibly activated by zinc in the cellular environment. Here we show that zinc fingers 2-4 constitute the core DNA-binding domain, whereas fingers 5 and 6 appear to be unnecessary for DNA binding in vitro. Deletion of finger 1 resulted in a protein that bound DNA constitutively in vitro. Furthermore, transfer of MTF-1 finger 1 to a position immediately preceding the three zinc fingers of Sp1 resulted in a chimeric protein that required exogenous zinc to activate DNA binding in vitro, unlike native Sp1, which binds DNA constitutively. Transient transfection experiments demonstrated that intact MTF-1 activated a reporter 2.5-4-fold above basal levels after metal treatment in mouse MTF-1 knockout cells, Drosophila SL2 cells, and yeast. However, the metal response was lost in all three systems when finger 1 was deleted, but was unaffected by deletion of fingers 5 and 6. These data suggest that finger 1 of MTF-1 constitutes a unique metal-sensing domain that, in cooperation with the transactivation domains, produces a zinc-sensing metalloregulatory transcription factor.