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

Effects of Bisphenol A and Retinoic Acid Exposure on Neuron and Brain Formation: A Study in Human Induced Pluripotent Stem Cells and Zebrafish Embryos

BACKGROUND

Developing human fetuses may be exposed to the chemical compound bisphenol A (BPA), and retinoic acid (RA) has been detected at low levels in water sources. RA signaling regulates key developmental genes and is essential for organ development, including the brain. We previously reported that RA/BPA coexposure of mouse embryonic stem cells potentiates RA signaling, which warrants further investigation.

OBJECTIVE

This study was undertaken in human induced pluripotent stem cells (iPSCs) and zebrafish embryos to investigate whether coexposure to BPA and exogenous RA could potentiate HOX gene expression and exert pleiotropic effects on RA signaling.

METHODS

Human iPSCs and zebrafish embryos were exposed to exogenous RA (0, 7.5, 10, 12.5, 100, 200 or 500  nM ) or BPA (20 μ M ) alone or coexposed to BPA (2  nM - 20 μ M ) and exogenous RA (7.5 - 100  nM ). Postexposure changes in HOX genes were assessed by quantitative polymerase chain reaction and/or transcriptome analyses. RA receptor antagonists were used to identify the receptor responsible for signaling. In zebrafish, spatial expression of fgf8a and hoxb1a was evaluated by whole-mount in situ hybridization. Mauthner cell and craniofacial cartilage anomalies were studied by immunostaining and Alcian blue staining, respectively. Transcriptome was compared between iPSCs and zebrafish to identify alterations of common biological processes. Gradient curves of RA signal were calculated to simulate the effects of exogenous RA and BPA in zebrafish.

RESULTS

In both iPSCs and zebrafish, RA/BPA coexposure had higher expression of 3' HOX genes in comparison with RA alone; BPA alone had no effect. Addition of RA receptor antagonists abolished these changes. In zebrafish, RA/BPA coexposure, in comparison with RA alone, resulted in a significant rostral shift in hoxb1a expression and increased rate of anomalies in Mauthner cells and craniofacial cartilage. Transcriptome comparison and correlations between the experimental results and gradient curve simulations strengthened these observations.

CONCLUSION

Our findings suggest a mechanistic link between chemical exposure and neurodevelopmental impairments and demonstrate involvement of exogenous RA signaling in endocrine disruption. Further investigation is needed to explore why BPA alone did not affect endogenous RA signaling, whereas exogenous RA signaling was potentiated with RA/BPA coexposure. https://doi.org/10.1289/EHP15574.

Nr5a2 is dispensable for zygotic genome activation but essential for morula development

Early embryogenesis is driven by transcription factors (TFs) that first activate the zygotic genome and then specify the lineages constituting the blastocyst. Although the TFs specifying the blastocyst's lineages are well characterized, those playing earlier roles remain poorly defined. Using mouse models of the TF Nr5a2, we show that Nr5a2-/- embryos arrest at the early morula stage and exhibit altered lineage specification, frequent mitotic failure, and substantial chromosome segregation defects. Although NR5A2 plays a minor but measurable role during zygotic genome activation, it predominantly acts as a master regulator at the eight-cell stage, controlling expression of lineage-specifying TFs and genes involved in mitosis, telomere maintenance, and DNA repair. We conclude that NR5A2 coordinates proliferation, genome stability, and lineage specification to ensure correct morula development.

Whole-genome resequencing reveals genomic variation and dynamics in Ethiopian indigenous goats

Ethiopia has about 52 million indigenous goats with marked phenotypic variability, which is the outcome of natural and artificial selection. Here, we obtained whole-genome sequence data of three Ethiopian indigenous goat populations (Arab, Fellata, and Oromo) from northwestern Ethiopia and analyzed their genome-wide genetic diversity, population structure, and signatures of selection. We included genotype data from four other Ethiopian goat populations (Abergelle, Keffa, Gumuz, and Woyto-Guji) and goats from Asia; Europe; and eastern, southern, western, and northern Africa to investigate the genetic predisposition of the three Ethiopian populations and performed comparative genomic analysis. Genetic diversity analysis showed that Fellata goats exhibited the lowest heterozygosity values (Ho = 0.288 ± 0.005 and He = 0.334 ± 0.0001). The highest values were observed in Arab goats (Ho = 0.310 ± 0.010 and He = 0.347 ± 4.35e-05). A higher inbreeding coefficient (FROH = 0.137 ± 0.016) was recorded for Fellata goats than the 0.105 ± 0.030 recorded for Arab and the 0.112 ± 0.034 recorded for Oromo goats. This indicates that the Fellata goat population should be prioritized in future conservation activities. The three goat populations showed the majority (∼63%) of runs of homozygosity in the shorter (100-150 Kb) length category, illustrating ancient inbreeding and/or small founder effects. Population relationship and structure analysis separated the Ethiopian indigenous goats into two distinct genetic clusters lacking phylogeographic structure. Arab, Fellata, Oromo, Abergelle, and Keffa represented one genetic cluster. Gumuz and Woyto-Guji formed a separate cluster and shared a common genetic background with the Kenyan Boran goat. Genome-wide selection signature analysis identified nine strongest regions spanning 163 genes influencing adaptation to arid and semi-arid environments (HOXC12, HOXC13, HOXC4, HOXC6, and HOXC9, MAPK8IP2), immune response (IL18, TYK2, ICAM3, ADGRG1, and ADGRG3), and production and reproduction (RARG and DNMT1). Our results provide insights into a thorough understanding of genetic architecture underlying selection signatures in Ethiopian indigenous goats in a semi-arid tropical environment and deliver valuable information for goat genetic improvement, conservation strategy, genome-wide association study, and marker-assisted breeding.

Effects of all-trans and 9-cis retinoic acid on differentiating human neural stem cells in vitro

Cyanobacterial blooms are known sources of environmentally-occurring retinoid compounds, including all-trans and 9-cis retinoic acids (RAs). The developmental hazard for aquatic organisms has been described, while the implications for human health hazard assessment are not yet sufficiently characterized. Here, we employ a human neural stem cell model that can differentiate in vitro into a mixed culture of neurons and glia. Cells were exposed to non-cytotoxic 8-1000 nM all-trans or 9-cis RA for 9-18 days (DIV13 and DIV22, respectively). Impact on biomarkers was analyzed on gene expression (RT-qPCR) and protein level (western blot and proteomics) at both time points; network patterning (immunofluorescence) on DIV22. RA exposure significantly concentration-dependently increased gene expression of retinoic acid receptors and the metabolizing enzyme CYP26A1, confirming the chemical-specific response of the model. Expression of thyroid hormone signaling-related genes remained mostly unchanged. Markers of neural progenitors/stem cells (PAX6, SOX1, SOX2, NESTIN) were decreased with increasing RA concentrations, though a basal population remained. Neural markers (DCX, TUJ1, MAP2, NeuN, SYP) remained unchanged or were decreased at high concentrations (200-1000 nM). Conversely, (astro-)glial marker S100β was increased concentration-dependently on DIV22. Together, the biomarker analysis indicates an RA-dependent promotion of glial cell fates over neural differentiation, despite the increased abundance of neural protein biomarkers during differentiation. Interestingly, RA exposure induced substantial changes to the cell culture morphology: while low concentrations resulted in a network-like differentiation pattern, high concentrations (200-1000 nM RA) almost completely prevented such network patterning. After functional confirmation for implications in network function, such morphological features could present a proxy for network formation assessment, an apical key event in (neuro-)developmental Adverse Outcome Pathways. The described application of a human in vitro model for (developmental) neurotoxicity to emerging environmentally-relevant retinoids contributes to the evidence-base for the use of differentiating human in vitro models for human health hazard and risk assessment.

Timeline of Developmental Defects Generated upon Genetic Inhibition of the Retinoic Acid Receptor Signaling Pathway

It has been established for almost 30 years that the retinoic acid receptor (RAR) signalling pathway plays essential roles in the morphogenesis of a large variety of organs and systems. Here, we used a temporally controlled genetic ablation procedure to precisely determine the time windows requiring RAR functions. Our results indicate that from E8.5 to E9.5, RAR functions are critical for the axial rotation of the embryo, the appearance of the sinus venosus, the modelling of blood vessels, and the formation of forelimb buds, lung buds, dorsal pancreatic bud, lens, and otocyst. They also reveal that E9.5 to E10.5 spans a critical developmental period during which the RARs are required for trachea formation, lung branching morphogenesis, patterning of great arteries derived from aortic arches, closure of the optic fissure, and growth of inner ear structures and of facial processes. Comparing the phenotypes of mutants lacking the 3 RARs with that of mutants deprived of all-trans retinoic acid (ATRA) synthesising enzymes establishes that cardiac looping is the earliest known morphogenetic event requiring a functional ATRA-activated RAR signalling pathway.

Mechanisms of Feedback Regulation of Vitamin A Metabolism

Vitamin A is an essential nutrient required throughout life. Through its various metabolites, vitamin A sustains fetal development, immunity, vision, and the maintenance, regulation, and repair of adult tissues. Abnormal tissue levels of the vitamin A metabolite, retinoic acid, can result in detrimental effects which can include congenital defects, immune deficiencies, proliferative defects, and toxicity. For this reason, intricate feedback mechanisms have evolved to allow tissues to generate appropriate levels of active retinoid metabolites despite variations in the level and format, or in the absorption and conversion efficiency of dietary vitamin A precursors. Here, we review basic mechanisms that govern vitamin A signaling and metabolism, and we focus on retinoic acid-controlled feedback mechanisms that contribute to vitamin A homeostasis. Several approaches to investigate mechanistic details of the vitamin A homeostatic regulation using genomic, gene editing, and chromatin capture technologies are also discussed.

Impact of endocrine disrupting chemicals on neurodevelopment: the need for better testing strategies for endocrine disruption-induced developmental neurotoxicity

INTRODUCTION

Brain development is highly dependent on hormonal regulation. Exposure to chemicals disrupting endocrine signaling has been associated with neurodevelopmental impairment. This raises concern about exposure to the suspected thousands of endocrine disruptors, and has resulted in efforts to improve regulation of these chemicals. Yet, the causal links between endocrine disruption and developmental neurotoxicity, which would be required for regulatory action, are still largely missing.

AREAS COVERED

In this review, we illustrate the importance of two endocrine systems, thyroid hormone and retinoic acid pathways, for neurodevelopment. We place special emphasis on TH and RA synthesis, metabolism, and how endocrine disrupting chemicals known or suspected to affect these systems are associated with developmental neurotoxicity.

EXPERT OPINION

While it is clear that neurodevelopment is dependent on proper hormonal functioning, and evidence is increasing for developmental neurotoxicity induced by endocrine disrupting chemicals, this is not grasped by current chemical testing. Thus, there is an urgent need to develop test methods detecting endocrine disruption in the context of neurodevelopment. Key to this development is further mechanistic insights on the involvement of endocrine signaling in neurodevelopment as well as increased support to develop and validate new test methods for the regulatory context.

Pathogenesis of Anorectal Malformations in Retinoic Acid Receptor Knockout Mice Studied by HREM

Anorectal malformations (ARMs) are relatively common congenital abnormalities, but their pathogenesis is poorly understood. Previous gene knockout studies indicated that the signalling pathway mediated by the retinoic acid receptors (RAR) is instrumental to the formation of the anorectal canal and of various urogenital structures. Here, we show that simultaneous ablation of the three RARs in the mouse embryo results in a spectrum of malformations of the pelvic organs in which anorectal and urinary bladder ageneses are consistently associated. We found that these ageneses could be accounted for by defects in the processes of growth and migration of the cloaca, the embryonic structure from which the anorectal canal and urinary bladder originate. We further show that these defects are preceded by a failure of the lateral shift of the umbilical arteries and propose vascular abnormalities as a possible cause of ARM. Through the comparisons of these phenotypes with those of other mutant mice and of human patients, we would like to suggest that morphological data may provide a solid base to test molecular as well as clinical hypotheses.

Application of hiPSCs in tooth regeneration via cellular modulation

BACKGROUND

Induced pluripotent stem cell (iPSC)-based technology provides limitless resources for customized development of organs without any ethical concerns. In theory, iPSCs generated from terminally differentiated cells can be induced to further differentiate into all types of organs that are derived from the embryonic germ layers. Since iPSC reprogramming technology is relatively new, extensive efforts by the researchers have been put together to optimize the protocols to establish in vitro differentiation of human iPSCs (hiPSCs) into various desirable cell types/organs.

HIGHLIGHTS

In the present study, we review the potential application of iPSCs as an efficient alternative to primary cells for modulating signal molecules. Furthermore, an efficient culture system that promotes the differentiation of cell lineages and tissue formation has been reviewed. We also summarize the recent studies wherein tissue engineering of the three germ layers has been explored. Particularly, we focus on the current research strategies for iPSC-based tooth regeneration via molecular modulation.

CONCLUSION

In recent decades, robust knowledge regarding the hiPSC-based regenerative therapy has been accumulated, especially focusing on cellular modulation. This review provides the optimization of the procedures designed to regenerate specific organs.

Meiosis occurs normally in the fetal ovary of mice lacking all retinoic acid receptors

Gametes are generated through a specialized cell differentiation process, meiosis, which, in ovaries of most mammals, is initiated during fetal life. All-trans retinoic acid (ATRA) is considered as the molecular signal triggering meiosis initiation. In the present study, we analyzed female fetuses ubiquitously lacking all ATRA nuclear receptors (RAR), obtained through a tamoxifen-inducible cre recombinase-mediated gene targeting approach. Unexpectedly, mutant oocytes robustly expressed meiotic genes, including the meiotic gatekeeper STRA8. In addition, ovaries from mutant fetuses grafted into adult recipient females yielded offspring bearing null alleles for all Rar genes. Thus, our results show that RAR are fully dispensable for meiotic initiation, as well as for the production of functional oocytes. Assuming that the effects of ATRA all rely on RAR, our study goes against the current model according to which meiosis is triggered by endogenous ATRA in the developing ovary. It therefore revives the search for the meiosis-inducing substance.

F-box protein FBXO30 mediates retinoic acid receptor γ ubiquitination and regulates BMP signaling in neural tube defects

Retinoic acid (RA), an active derivative of vitamin A, is critical for the neural system development. During the neural development, the RA/RA receptor (RAR) pathway suppresses BMP signaling-mediated proliferation and differentiation of neural progenitor cells. However, how the stability of RAR is regulated during neural system development and how BMP pathway genes expression in neural tissue from human fetuses affected with neural tube defects (NTDs) remain elusive. Here, we report that FBXO30 acts as an E3 ubiquitin ligase and targets RARγ for ubiquitination and proteasomal degradation. In this way, FBXO30 positively regulates BMP signaling in mammalian cells. Moreover, RA treatment leads to suppression of BMP signaling by reducing the level of FBXO30 in mammalian cells and in mouse embryos with NTDs. In samples from human NTDs with high levels of retinol, downregulation of BMP target genes was observed, along with aberrant FBXO30 levels. Collectively, our results demonstrate that RARγ levels are controlled by FBXO30-mediated ubiquitination and that FBXO30 is a key regulator of BMP signaling. Furthermore, we suggest a novel mechanism by which high-retinol levels affect the level of FBXO30, which antagonizes BMP signaling during early stage development.

What's retinoic acid got to do with it? Retinoic acid regulation of the neural crest in craniofacial and ocular development

Retinoic acid (RA), the active derivative of vitamin A (retinol), is an essential morphogen signaling molecule and major regulator of embryonic development. The dysregulation of RA levels during embryogenesis has been associated with numerous congenital anomalies, including craniofacial, auditory, and ocular defects. These anomalies result from disruptions in the cranial neural crest, a vertebrate-specific transient population of stem cells that contribute to the formation of diverse cell lineages and embryonic structures during development. In this review, we summarize our current knowledge of the RA-mediated regulation of cranial neural crest induction at the edge of the neural tube and the migration of these cells into the craniofacial region. Further, we discuss the role of RA in the regulation of cranial neural crest cells found within the frontonasal process, periocular mesenchyme, and pharyngeal arches, which eventually form the bones and connective tissues of the head and neck and contribute to structures in the anterior segment of the eye. We then review our understanding of the mechanisms underlying congenital craniofacial and ocular diseases caused by either the genetic or toxic disruption of RA signaling. Finally, we discuss the role of RA in maintaining neural crest-derived structures in postembryonic tissues and the implications of these studies in creating new treatments for degenerative craniofacial and ocular diseases.

Hindbrain induction and patterning during early vertebrate development

The hindbrain is a key relay hub of the central nervous system (CNS), linking the bilaterally symmetric half-sides of lower and upper CNS centers via an extensive network of neural pathways. Dedicated neural assemblies within the hindbrain control many physiological processes, including respiration, blood pressure, motor coordination and different sensations. During early development, the hindbrain forms metameric segmented units known as rhombomeres along the antero-posterior (AP) axis of the nervous system. These compartmentalized units are highly conserved during vertebrate evolution and act as the template for adult brainstem structure and function. TALE and HOX homeodomain family transcription factors play a key role in the initial induction of the hindbrain and its specification into rhombomeric cell fate identities along the AP axis. Signaling pathways, such as canonical-Wnt, FGF and retinoic acid, play multiple roles to initially induce the hindbrain and regulate Hox gene-family expression to control rhombomeric identity. Additional transcription factors including Krox20, Kreisler and others act both upstream and downstream to Hox genes, modulating their expression and protein activity. In this review, we will examine the earliest embryonic signaling pathways that induce the hindbrain and subsequent rhombomeric segmentation via Hox and other gene expression. We will examine how these signaling pathways and transcription factors interact to activate downstream targets that organize the segmented AP pattern of the embryonic vertebrate hindbrain.

Vitamin A Deficiency and the Lung

Vitamin A (all-trans-retinol) is a fat-soluble micronutrient which together with its natural derivatives and synthetic analogues constitutes the group of retinoids. They are involved in a wide range of physiological processes such as embryonic development, vision, immunity and cellular differentiation and proliferation. Retinoic acid (RA) is the main active form of vitamin A and multiple genes respond to RA signalling through transcriptional and non-transcriptional mechanisms. Vitamin A deficiency (VAD) is a remarkable public health problem. An adequate vitamin A intake is required in early lung development, alveolar formation, tissue maintenance and regeneration. In fact, chronic VAD has been associated with histopathological changes in the pulmonary epithelial lining that disrupt the normal lung physiology predisposing to severe tissue dysfunction and respiratory diseases. In addition, there are important alterations of the structure and composition of extracellular matrix with thickening of the alveolar basement membrane and ectopic deposition of collagen I. In this review, we show our recent findings on the modification of cell-junction proteins in VAD lungs, summarize up-to-date information related to the effects of chronic VAD in the impairment of lung physiology and pulmonary disease which represent a major global health problem and provide an overview of possible pathways involved.

Biocompatible Gold Nanoparticles Ameliorate Retinoic Acid-Induced Cell Death and Induce Differentiation in F9 Teratocarcinoma Stem Cells

The unique properties of gold nanoparticles (AuNPs) have attracted much interest for a range of applications, including biomedical applications in the cosmetic industry. The current study assessed the anti-oxidative effect of AuNPs against retinoic acid (RA)-induced loss of cell viability; cell proliferation; expression of oxidative and anti-oxidative stress markers, pro- and anti-apoptotic genes, and differentiation markers; and mitochondrial dysfunction in F9 teratocarcinoma stem cells. AuNPs were prepared by reduction of gold salts using luteolin as a reducing and stabilizing agent. The prepared AuNPs were spherical in shape with an average diameter of 18 nm. F9 cells exposed to various concentrations of these AuNPs were not harmed, whereas cells exposed to RA exhibited a dose-dependent change in cell viability and cell proliferation. The RA-mediated toxicity was associated with increased leakage of lactate dehydrogenase, reactive oxygen species, increased levels of malondialdehyde and nitric oxide, loss of mitochondrial membrane potential, and a reduced level of ATP. Finally, RA increased the level of pro-apoptotic gene expression and decreased the expression of anti-apoptotic genes. Interestingly, the toxic effect of RA appeared to be decreased in cells treated with RA in the presence of AuNPs, which was coincident with the increased levels of anti-oxidant markers including thioredoxin, glutathione peroxidases, glutathione, glutathione disulfide, catalase, and superoxide dismutase. Concomitantly, AuNPs ameliorated the apoptotic response by decreasing the mRNA expression of p53, p21, Bax, Bak, caspase-3, caspase-9, and increasing the expressions of Bcl-2 and Bcl-Xl. Interestingly, AuNPs not only ameliorated oxidative stress but also induced differentiation in F9 cells by increasing the expression of differentiation markers including retinoic acid binding protein, laminin 1, collagen type IV, and Gata 6 and decreasing the expressions of markers of stem cell pluripotency including Nanog, Rex1, octamer-binding transcription factor 4, and Sox-2. These consistent cellular and biochemical data suggest that AuNPs could ameliorate RA-induced cell death and facilitate F9 cell differentiation. AuNPs could be suitable therapeutic agents for the treatment of oxidative stress-related diseases such as atherosclerosis, cancer, diabetes, rheumatoid arthritis, and neurodegenerative diseases.

Pokemon decreases the transcriptional activity of RARα in the absence of ligand

Pokemon is a transcriptional repressor that belongs to the POZ and Krüppel (POK) protein family. In this study, we investigated the potential interaction between Pokemon and retinoic acid receptor alpha (RARα) and determined the role of Pokemon in regulation of RARα transcriptional activity in the absence of ligand. We found that Pokemon could directly interact with RARα. Moreover, we demonstrated that Pokemon could decrease the transcriptional activity of RARα in the absence of ligand. Furthermore, we showed that Pokemon could repress the transcriptional activity of RARα by increasing the recruitment of nuclear receptor co-repressor (NCoR) and silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) to the retinoic acid response element (RARE) element. Taken together, these data suggest that Pokemon is a novel partner of RARα that acts as a co-repressor to regulate RARα transcriptional activity in the absence of ligand.

Enzymatic Metabolism of Vitamin A in Developing Vertebrate Embryos

Embryonic development is orchestrated by a small number of signaling pathways, one of which is the retinoic acid (RA) signaling pathway. Vitamin A is essential for vertebrate embryonic development because it is the molecular precursor of the essential signaling molecule RA. The level and distribution of RA signaling within a developing embryo must be tightly regulated; too much, or too little, or abnormal distribution, all disrupt embryonic development. Precise regulation of RA signaling during embryogenesis is achieved by proteins involved in vitamin A metabolism, retinoid transport, nuclear signaling, and RA catabolism. The reversible first step in conversion of the precursor vitamin A to the active retinoid RA is mediated by retinol dehydrogenase 10 (RDH10) and dehydrogenase/reductase (SDR family) member 3 (DHRS3), two related membrane-bound proteins that functionally activate each other to mediate the interconversion of retinol and retinal. Alcohol dehydrogenase (ADH) enzymes do not contribute to RA production under normal conditions during embryogenesis. Genes involved in vitamin A metabolism and RA catabolism are expressed in tissue-specific patterns and are subject to feedback regulation. Mutations in genes encoding these proteins disrupt morphogenesis of many systems in a developing embryo. Together these observations demonstrate the importance of vitamin A metabolism in regulating RA signaling during embryonic development in vertebrates.

Retinoic Acid Induces Embryonic Stem Cell Differentiation by Altering Both Encoding RNA and microRNA Expression

Retinoic acid (RA) is a vitamin A metabolite that is essential for early embryonic development and promotes stem cell neural lineage specification; however, little is known regarding the impact of RA on mRNA transcription and microRNA levels on embryonic stem cell differentiation. Here, we present mRNA microarray and microRNA high-output sequencing to clarify how RA regulates gene expression. Using mRNA microarray analysis, we showed that RA repressed pluripotency-associated genes while activating ectoderm markers in mouse embryonic stem cells (mESCs). Moreover, RA modulated the DNA methylation of mESCs by altering the expression of epigenetic-associated genes such as Dnmt3b and Dnmt3l. Furthermore, H3K4me2, a pluripotent histone modification, was repressed by RA stimulation. From microRNA sequence data, we identified two downregulated microRNAs, namely, miR-200b and miR-200c, which regulated the pluripotency of stem cells. We found that miR-200b or miR-200c deficiency suppressed the expression of pluripotent genes, including Oct4 and Nanog, and activated the expression of the ectodermal marker gene Nestin. These results demonstrate that retinoid induces mESCs to differentiate by regulating miR-200b/200c. Our findings provide the landscapes of mRNA and microRNA gene networks and indicate the crucial role of miR-200b/200c in the RA-induced differentiation of mESCs.

Triphenyl phosphate-induced developmental toxicity in zebrafish: potential role of the retinoic acid receptor

Using zebrafish as a model, we previously reported that developmental exposure to triphenyl phosphate (TPP) - a high-production volume organophosphate-based flame retardant - results in dioxin-like cardiac looping impairments that are independent of the aryl hydrocarbon receptor. Using a pharmacologic approach, the objective of this study was to investigate the potential role of retinoic acid receptor (RAR) - a nuclear receptor that regulates vertebrate heart morphogenesis - in mediating TPP-induced developmental toxicity in zebrafish. We first revealed that static exposure of zebrafish from 5-72h post-fertilization (hpf) to TPP in the presence of non-toxic concentrations of an RAR antagonist (BMS493) significantly enhanced TPP-induced toxicity (relative to TPP alone), even though identical non-toxic BMS493 concentrations mitigated retinoic acid (RA)-induced toxicity. BMS493-mediated enhancement of TPP toxicity was not a result of differential TPP uptake or metabolism, as internal embryonic doses of TPP and diphenyl phosphate (DPP) - a primary TPP metabolite - were not different in the presence or absence of BMS493. Using real-time PCR, we then quantified the relative change in expression of cytochrome P450 26a1 (cyp26a1) - a major target gene for RA-induced RAR activation in zebrafish - and found that RA and TPP exposure resulted in a ∼5-fold increase and decrease in cyp26a1 expression, respectively, relative to vehicle-exposed embryos. To address whether TPP may interact with human RARs, we then exposed Chinese hamster ovary cells stably transfected with chimeric human RARα-, RARβ-, or RARγ to TPP in the presence of RA, and found that TPP significantly inhibited RA-induced luciferase activity in a concentration-dependent manner. Overall, our findings suggest that zebrafish RARs may be involved in mediating TPP-induced developmental toxicity, a mechanism of action that may have relevance to humans.

The gene regulatory networks underlying formation of the auditory hindbrain

Development and evolution of auditory hindbrain nuclei are two major unsolved issues in hearing research. Recent characterization of transgenic mice identified the rhombomeric origins of mammalian auditory nuclei and unraveled genes involved in their formation. Here, we provide an overview on these data by assembling them into rhombomere-specific gene regulatory networks (GRNs), as they underlie developmental and evolutionary processes. To explore evolutionary mechanisms, we compare the GRNs operating in the mammalian auditory hindbrain with data available from the inner ear and other vertebrate groups. Finally, we propose that the availability of genomic sequences from all major vertebrate taxa and novel genetic techniques for non-model organisms provide an unprecedented opportunity to investigate development and evolution of the auditory hindbrain by comparative molecular approaches. The dissection of the molecular mechanisms leading to auditory structures will also provide an important framework for auditory processing disorders, a clinical problem difficult to tackle so far. These data will, therefore, foster basic and clinical hearing research alike.

Retinoic acid receptors: from molecular mechanisms to cancer therapy

Retinoic acid (RA), the major bioactive metabolite of retinol or vitamin A, induces a spectrum of pleiotropic effects in cell growth and differentiation that are relevant for embryonic development and adult physiology. The RA activity is mediated primarily by members of the retinoic acid receptor (RAR) subfamily, namely RARα, RARβ and RARγ, which belong to the nuclear receptor (NR) superfamily of transcription factors. RARs form heterodimers with members of the retinoid X receptor (RXR) subfamily and act as ligand-regulated transcription factors through binding specific RA response elements (RAREs) located in target genes promoters. RARs also have non-genomic effects and activate kinase signaling pathways, which fine-tune the transcription of the RA target genes. The disruption of RA signaling pathways is thought to underlie the etiology of a number of hematological and non-hematological malignancies, including leukemias, skin cancer, head/neck cancer, lung cancer, breast cancer, ovarian cancer, prostate cancer, renal cell carcinoma, pancreatic cancer, liver cancer, glioblastoma and neuroblastoma. Of note, RA and its derivatives (retinoids) are employed as potential chemotherapeutic or chemopreventive agents because of their differentiation, anti-proliferative, pro-apoptotic, and anti-oxidant effects. In humans, retinoids reverse premalignant epithelial lesions, induce the differentiation of myeloid normal and leukemic cells, and prevent lung, liver, and breast cancer. Here, we provide an overview of the biochemical and molecular mechanisms that regulate the RA and retinoid signaling pathways. Moreover, mechanisms through which deregulation of RA signaling pathways ultimately impact on cancer are examined. Finally, the therapeutic effects of retinoids are reported.

RARγ is essential for retinoic acid induced chromatin remodeling and transcriptional activation in embryonic stem cells

We have utilized retinoic acid receptor γ (gamma) knockout (RARγ(-/-)) embryonic stem (ES) cells as a model system to analyze RARγ mediated transcriptional regulation of stem cell differentiation. Most of the transcripts regulated by all-trans retinoic acid (RA) in ES cells are dependent upon functional RARγ signaling. Notably, many of these RA-RARγ target genes are implicated in retinoid uptake and metabolism. For instance, Lrat (lecithin:retinol acyltransferase), Stra6 (stimulated by retinoic acid 6), Crabp2 (cellular retinoic acid binding protein 2), and Cyp26a1 (cytochrome p450 26a1) transcripts are induced in wild type (WT), but not in RARγ(-/-) cells. Transcripts for the transcription factors Pbx1 (pre-B cell leukemia homeobox-1), Wt1 (Wilm's tumor gene-1), and Meis1 (myeloid ecotropic viral integration site-1) increase upon RA treatment of WT, but not RARγ(-/-) cells. In contrast, Stra8, Dleu7, Leftb, Pitx2, and Cdx1 mRNAs are induced by RA even in the absence of RARγ. Mapping of the epigenetic signature of Meis1 revealed that RA induces a rapid increase in the H3K9/K14ac epigenetic mark at the proximal promoter and at two sites downstream of the transcription start site in WT, but not in RARγ(-/-) cells. Thus, RA-associated increases in H3K9/K14ac epigenetic marks require RARγ and are associated with increased Meis1 transcript levels, whereas H3K4me3 is present at the Meis1 proximal promoter even in the absence of RARγ. In contrast, at the Lrat proximal promoter primarily the H3K4me3 mark, and not the H3K9/K14ac mark, increases in response to RA, independently of the presence of RARγ. Our data show major epigenetic changes associated with addition of the RARγ agonist RA in ES cells.

Retinoic acid signalling during development

Retinoic acid (RA) is a vitamin A-derived, non-peptidic, small lipophilic molecule that acts as ligand for nuclear RA receptors (RARs), converting them from transcriptional repressors to activators. The distribution and levels of RA in embryonic tissues are tightly controlled by regulated synthesis through the action of specific retinol and retinaldehyde dehydrogenases and by degradation via specific cytochrome P450s (CYP26s). Recent studies indicate that RA action involves an interplay between diffusion (morphogen-like) gradients and the establishment of signalling boundaries due to RA metabolism, thereby allowing RA to finely control the differentiation and patterning of various stem/progenitor cell populations. Here, we provide an overview of the RA biosynthesis, degradation and signalling pathways and review the main functions of this molecule during embryogenesis.

Retinoic acid drives aryl hydrocarbon receptor expression and is instrumental to dioxin-induced toxicity during palate development

BACKGROUND

Palate development depends on complex events and is very sensitive to disruption. Accordingly, clefts are the most common congenital malformations worldwide, and a connection is proposed with fetal exposure to toxic factors or environmental contaminants, such as dioxins. There is increasing evidence that dioxin interferes with all-trans-retinoic acid (atRA), a hormone-like signal derived from vitamin A, which plays an essential role during embryonic development. Although similarities have been described between dioxin-induced toxicity and the outcome of altered atRA signaling during palate development, their relationship needs to be clarified.

OBJECTIVES

We used a genetic approach to understand the interaction between atRA and dioxin and to identify the cell type targeted by dioxin toxicity during secondary palate formation in mice.

METHODS

We analyzed the phenotype of mouse embryos harboring an atRA-sensitive reporter transgene or bearing null mutations for atRA-synthesizing enzymes (RALDH) or atRA receptors (RAR) and maternally exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) at gestation day 10.5.

RESULTS

We found that an intact atRA signal was required to enable TCDD to induce cleft palate. This mandatory atRA signal was generated through the activity of RALDH3 in the nasal epithelium and was transduced by RARγ (RARG) in the nasal mesenchyme, where it notably controlled aryl hydrocarbon receptor (Ahr) transcript levels. TCDD also did not alter the developmental pattern of atRA signaling during palate formation.

CONCLUSIONS

TCDD-induced alteration of secondary palate development in the mouse appears to depend on atRA signaling, which controls AHR expression. This mechanism is likely conserved throughout vertebrate evolution and may therefore be relevant in humans.

Mesodermal retinoic acid signaling regulates endothelial cell coalescence in caudal pharyngeal arch artery vasculogenesis

Disruption of retinoic acid signaling causes a variety of pharyngeal arch artery and great vessel defects, as well as malformations in many other tissues, including those derived from the pharyngeal endoderm. Previous studies implied that arch artery defects in the context of defective RA signaling occur secondary to pharyngeal pouch segmentation defects, although this model has never been experimentally verified. In this study, we examined arch artery morphogenesis during mouse development, and the role of RA in this process. We show in normal embryos that the arch arteries form by vasculogenic differentiation of pharyngeal mesoderm. Using various genetic backgrounds and tissue-specific mutation approaches, we segregate pharyngeal arch artery and pharyngeal pouch defects in RA receptor mutants, and show that RA signal transduction only in pharyngeal mesoderm is required for arch artery formation. RA does not control pharyngeal mesodermal differentiation to endothelium, but instead promotes the aggregation of endothelial cells into nascent vessels. Expression of VE-cadherin was substantially reduced in RAR mutants, and this deficiency may underlie the arch artery defects. The consequences of disrupted mesodermal and endodermal RA signaling were restricted to the 4th and 6th arch arteries and to the 4th pharyngeal pouch, respectively, suggesting that different regulatory mechanisms control the formation of the more anterior arch arteries and pouches.

Involvement of retinol dehydrogenase 10 in embryonic patterning and rescue of its loss of function by maternal retinaldehyde treatment

Retinoic acid (RA), an active vitamin A metabolite, is a key signaling molecule in vertebrate embryos. Morphogenetic RA gradients are thought to be set up by tissue-specific actions of retinaldehyde dehydrogenases (RALDHs) and catabolizing enzymes. According to the species, two enzymatic pathways (β-carotene cleavage and retinol oxidation) generate retinaldehyde, the substrate of RALDHs. Placental species depend on maternal retinol transferred to the embryo. The retinol-to-retinaldehyde conversion was thought to be achieved by several redundant enzymes; however, a random mutagenesis screen identified retinol dehydrogenase 10 [Rdh10(Trex) allele; Sandell LL, et al. (2007) Genes Dev 21:1113-1124] as responsible for a homozygous lethal phenotype with features of RA deficiency. We report here the production and characterization of unique murine Rdh10 loss-of-function alleles generated by gene targeting. We show that although Rdh10(-/-) mutants die at an earlier stage than Rdh10(Trex) mutants, their molecular patterning defects do not reflect a complete state of RA deficiency. Furthermore, we were able to correct most developmental abnormalities by administering retinaldehyde to pregnant mothers, thereby obtaining viable Rdh10(-/-) mutants. This demonstrates the rescue of an embryonic lethal phenotype by simple maternal administration of the missing retinoid compound. These results underscore the importance of maternal retinoids in preventing congenital birth defects, and lead to a revised model of the importance of RDH10 and RALDHs in controlling embryonic RA distribution.

miR-196 regulates axial patterning and pectoral appendage initiation

Vertebrate Hox clusters contain protein-coding genes that regulate body axis development and microRNA (miRNA) genes whose functions are not yet well understood. We overexpressed the Hox cluster microRNA miR-196 in zebrafish embryos and found four specific, viable phenotypes: failure of pectoral fin bud initiation, deletion of the 6th pharyngeal arch, homeotic aberration and loss of rostral vertebrae, and reduced number of ribs and somites. Reciprocally, miR-196 knockdown evoked an extra pharyngeal arch, extra ribs, and extra somites, confirming endogenous roles of miR-196. miR-196 injection altered expression of hox genes and the signaling of retinoic acid through the retinoic acid receptor gene rarab. Knocking down rarab mimicked the pectoral fin phenotype of miR-196 overexpression, and reporter constructs tested in tissue culture and in embryos showed that the rarab 3'UTR is a miR-196 target for pectoral fin bud initiation. These results show that a Hox cluster microRNA modulates development of axial patterning similar to nearby protein-coding Hox genes, and acts on appendicular patterning at least in part by modulating retinoic acid signaling.

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.

Cdx1 refines positional identity of the vertebrate hindbrain by directly repressing Mafb expression

An interplay of transcription factors interprets signalling pathways to define anteroposterior positions along the vertebrate axis. In the hindbrain, these transcription factors prompt the position-appropriate appearance of seven to eight segmental structures, known as rhombomeres (r1-r8). The evolutionarily conserved Cdx caudal-type homeodomain transcription factors help specify the vertebrate trunk and tail but have not been shown to directly regulate hindbrain patterning genes. Mafb (Kreisler, Krml1, valentino), a basic domain leucine zipper transcription factor, is required for development of r5 and r6 and is the first gene to show restricted expression within these two segments. The homeodomain protein vHnf1 (Hnf1b) directly activates Mafb expression. vHnf1 and Mafb share an anterior expression limit at the r4/r5 boundary but vHnf1 expression extends beyond the posterior limit of Mafb and, therefore, cannot establish the posterior Mafb expression boundary. Upon identifying regulatory sequences responsible for posterior Mafb repression, we have used in situ hybridization, immunofluorescence and chromatin immunoprecipitation (ChIP) analyses to determine that Cdx1 directly inhibits early Mafb expression in the neural tube posterior of the r6/r7 boundary, which is the anteriormost boundary of Cdx1 expression in the hindbrain. Cdx1 dependent repression of Mafb is transient. After the 10-somite stage, another mechanism acts to restrict Mafb expression in its normal r5 and r6 domain, even in the absence of Cdx1. Our findings identify Mafb as one of the earliest direct targets of Cdx1 and show that Cdx1 plays a direct role in early hindbrain patterning. Thus, just as Cdx2 and Cdx4 govern the trunk-to-tail transition, Cdx1 may regulate the hindbrain-to-spinal cord transition.

Vitamin A facilitates enteric nervous system precursor migration by reducing Pten accumulation

Hirschsprung disease is a serious disorder of enteric nervous system (ENS) development caused by the failure of ENS precursor migration into the distal bowel. We now demonstrate that retinoic acid (RA) is crucial for GDNF-induced ENS precursor migration, cell polarization and lamellipodia formation, and that vitamin A depletion causes distal bowel aganglionosis in serum retinol-binding-protein-deficient (Rbp4(-/-)) mice. Ret heterozygosity increases the incidence and severity of distal bowel aganglionosis induced by vitamin A deficiency in Rbp4(-/-) animals. Furthermore, RA reduces phosphatase and tensin homolog (Pten) accumulation in migrating cells, whereas Pten overexpression slows ENS precursor migration. Collectively, these data support the hypothesis that vitamin A deficiency is a non-genetic risk factor that increases Hirschsprung disease penetrance and expressivity, suggesting that some cases of Hirschsprung disease might be preventable by optimizing maternal nutrition.

Retinoic acid receptors exhibit cell-autonomous functions in cranial neural crest cells

Previous work has emphasized the crucial role of retinoic acid (RA) in the ontogenesis of the vast majority of mesenchymal structures derived from the neural crest cells (NCC), which migrate through, or populate, the frontonasal process and branchial arches. Using somatic mutagenesis in the mouse, we have selectively ablated two or three retinoic acid receptors (i.e., RARalpha/RARbeta, RARalpha/RARgamma and RARalpha/RARbeta/RARgamma) in NCC. By rigorously analyzing these mutant mice, we found that survival and migration of NCC is normal until gestational day 10.5, suggesting that RAR-dependent signaling is not intrinsically required for the early steps of NCC development. However, ablation of Rara and Rarg genes in NCC yields an agenesis of the median portion of the face, demonstrating that RARalpha and RARgamma act cell-autonomously in postmigratory NCC to control the development of structures derived from the frontonasal process. In contrast, ablation of the three Rar genes in NCC leads to less severe defects of the branchial arches derived structures compared with Rar compound null mutants. Therefore, RARs exert a function in the NCC as well as in a separated cell population. This work demonstrates that RARs use distinct mechanisms to pattern cranial NCC.

Model organisms inform the search for the genes and developmental pathology underlying malformations of the human hindbrain

Congenital malformations of the human hindbrain, including the cerebellum, are poorly understood largely because their recognition is a relatively recent advance for imaging diagnostics. Cerebellar malformations are the most obvious and best characterized hindbrain malformations due to their relative ease of viewing by magnetic resonance imaging and the recent identification of several causative genes (Millen et al. Curr Opin Neurobiol 18:12-19, 2008). Malformations of the pons and medulla have also been described both in isolation and in association with cerebellar malformations (Barkovich et al. Ann Neurol 62:625-639, 2007). Although little is understood regarding the specific developmental pathologies underlying hindbrain malformations in humans, much is known regarding the mechanisms and genes driving hindbrain development in vertebrate model organisms. Thus, studies in vertebrate models provide a developmental framework in which to categorize human hindbrain malformations and serve to provide information regarding disrupted developmental processes and candidate genes. Here, we survey the basic principles of vertebrate hindbrain development and integrate our current knowledge of human hindbrain malformations into this framework.

Function of retinoic acid receptors during embryonic development

Retinoids, the active metabolites of vitamin A, regulate complex gene networks involved in vertebrate morphogenesis, growth, cellular differentiation and homeostasis. Studies performed in vitro, using either acellular systems or transfected cells, have shown that retinoid actions are mediated through heterodimers between the RAR and RXR nuclear receptors. However, in vitro studies indicate what is possible, but not necessarily what is actually occurring in vivo, because they are performed under non-physiological conditions. Therefore, genetic approaches in the animal have been be used to determine the physiological functions of retinoid receptors. Homologous recombination in embryonic stem cells has been used to generate germline null mutations of the RAR- and RXR-coding genes in the mouse. As reviewed here, the generation of such germline mutations, combined with pharmacological approaches to block the RA signalling pathway, has provided genetic evidence that RAR/RXR heterodimers are indeed the functional units transducing the RA signal during prenatal development. However, due to (i) the complexity in “hormonal” signalling through transduction by the multiple RARs and RXRs, (ii) the functional redundancies (possibly artefactually generated by the mutations) within receptor isotypes belonging to a given family, and (iii) in utero or early postnatal lethality of certain germline null mutations, these genetic studies have failed to reveal all the physiological functions of RARs and RXRs, notably in adults. Spatio-temporally-controlled somatic mutations generated in given cell types/tissues and at chosen times during postnatal life, will be required to reveal all the functions of RAR and RXR throughout the lifetime of the mouse.

A transcriptionally silent RXRalpha supports early embryonic morphogenesis and heart development

Retinoic acid (RA) receptors (RARs) alpha, beta, and gamma heterodimerized with rexinoid receptors (RXRs) alpha, beta, and gamma mediate the RA signal. To analyze the contribution of the transcriptional activity of RXRalpha, the main RXR during embryogenesis, we have engineered a mouse line harboring a transcriptionally silent RXRalpha mutant that lacks the activation functions AF1 and AF2. All homozygous mutants (Rxra(afo)) display the ocular defects previously observed in compound Rar-null and Rxra/Rar-null mutants, thus demonstrating that a transcriptionally active RXRalpha is required during eye development. In contrast, the vast majority of Rxra(afo) fetuses do not display the Rxra-null mutant hypoplasia of the myocardium, thus demonstrating that RXRalpha can act as a transcriptionally silent heterodimerization partner. Similarly, a transcriptionally silent RXRalpha mutant can support early embryogenesis, as Rxra(afo)/Rxrb-null embryos display a normal morphology, contrasting with the severe malformations exhibited by compound Rxra/Rxrb-null embryos. Along the same line, we show that a silent RXRalpha mutant is sufficient to allow the initial formation of the placental labyrinth, whereas later steps of trophoblast cell differentiation critically requires the AF2, but not the AF1, function of RXRalpha.

Combinatorial roles for zebrafish retinoic acid receptors in the hindbrain, limbs and pharyngeal arches

Retinoic acid (RA) signaling regulates multiple aspects of vertebrate embryonic development and tissue patterning, in part through the local availability of nuclear hormone receptors called retinoic acid receptors (RARs) and retinoid receptors (RXRs). RAR/RXR heterodimers transduce the RA signal, and loss-of-function studies in mice have demonstrated requirements for distinct receptor combinations at different stages of embryogenesis. However, the tissue-specific functions of each receptor and their individual contributions to RA signaling in vivo are only partially understood. Here we use morpholino oligonucleotides to deplete the four known zebrafish RARs (raraa, rarab, rarga, and rargb). We show that while all four are required for anterior-posterior patterning of rhombomeres in the hindbrain, there are unique requirements for rarga in the cranial mesoderm for hindbrain patterning, and rarab in lateral plate mesoderm for specification of the pectoral fins. In addition, the alpha subclass (raraa, rarab) is RA inducible, and of these only raraa expression is RA-dependent, suggesting that these receptors establish a region of particularly high RA signaling through positive-feedback. These studies reveal novel tissue-specific roles for RARs in controlling the competence and sensitivity of cells to respond to RA.

How degrading: Cyp26s in hindbrain development

The vitamin A derivative retinoic acid performs many functions in vertebrate development and is thought to act as a diffusible morphogen that patterns the anterior-posterior axis of the hindbrain. Recent work in several systems has led to insights into how the spatial distribution of retinoic acid is regulated. These have shown local control of synthesis and degradation, and computational models suggest that degradation by the Cyp26 enzymes plays a critical role in the formation of a morphogen gradient as well as its ability to compensate for fluctuations in RA levels.

Opposing effects of retinoid signaling on astrogliogenesis in embryonic day 13 and 17 cortical progenitor cells

All-trans retinoic acid (RA) is a differentiation factor in many tissues. However, its role in astrogliogenesis has not been extensively studied. Here, we investigated the effect of RA on the regulation of astrogliogenesis at different cortical developmental stages. We prepared rat cortical progenitor cells from embryonic day (E) 13 and E17, which correspond to the beginning of neurogenic and astrogliogenic periods, respectively. Surprisingly, RA promoted astrogliogenesis at E17 but inhibited astrogliogenesis induced by ciliary neurotrophic factor (CNTF) at E13. The inhibitory effect of RA on astrogliogenesis at E13 was not due to premature commitment of progenitors to a neuronal or oligodendroglial lineage. Rather, RA retained more progenitors in a proliferative state. Furthermore, RA inhibition of astrogliogenesis at E13 was independent of STAT3 signaling and required the function of the alpha and beta isoforms of the RA receptors (RAR). Moreover, the differential response of E13 and E17 progenitors to RA was due to differences in the intrinsic properties of these cells that are preserved in vitro. The inhibitory effect of RA on cytokine-induced astrogliogenesis at E13 may contribute to silencing of any potential precocious astrogliogenesis during the neurogenic period.

Effect of 9-cis retinoic acid on dopamine D2 receptor expression in pituitary adenoma cells

The dopamine receptor subtype 2 (D2R) promoter contains a functional retinoic acid response element involved in the control of D2R expression. The aim of the study was to evaluate the effect of 9-cis retinoic acid (9-cis RA) on D2R protein expression in human pituitary adenomas and GH3 cell line. Treatment with 9-cis RA (100 nM for 48 hrs) caused a 109 +/- 32% increase of basal D2R levels in five of eight growth hormone (GH)-secreting adenomas (GH-omas), a 129 +/- 28% increase in 7 of 11 nonfunctioning adenomas, and no effect in two resistant prolactinomas by Western blotting. The lack of D2R induction in some tumors was not associated with a different pattern of retinoid x receptor (RXR) and retinoic acid receptor (RAR) isoform expression that was similar in all tumors by immunohistochemistry. While the induction of D2R did not affect the slight but significant inhibitory effect exerted by dopamine (10 nM) on in vitro GH release by GH-oma cultured cells, in pituitary GH3 cell lines cis-9 RA enhanced the dopamine-induced inhibition of in vitro GH release (% inhibition: 16 +/- 2 versus 26 +/- 5, P < 0.05), cell proliferation (25 +/- 2% versus 44 +/- 5%, P < 0.05) and cell viability (16 +/- 0.8% versus 29 +/- 1%, P < 0.05), likely by activating caspase-3 (28 +/- 3% versus basal, P < 0.05). In conclusion, this study provides novel evidence for a permissive role of retinoids on the expression of D2R in a good proportion of pituitary tumors and on the generation of pro-apoptotic signals in GH3 cell line.

Retinoic acid in development: towards an integrated view

Retinoic acid (RA) has complex and pleiotropic functions during vertebrate development. Recent work in several species has increased our understanding of the roles of RA as a signalling molecule. These functions rely on a tight control of RA distribution within embryonic tissues through the combined action of synthesizing and metabolizing enzymes, possibly leading to diffusion gradients. Also important is the switching of nuclear receptors from a transcriptionally repressing state to an activating state. In addition, cross-talk with other key embryonic signals, especially fibroblast growth factors (FGFs) and sonic hedgehog (SHH), is being uncovered. Some of these functions could be maintained throughout the life of an organism to regulate cell-lineage decisions and/or the differentiation of stem cell populations, highlighting possibilities for regenerative medicine.

Gene expression profiling elucidates a specific role for RARgamma in the retinoic acid-induced differentiation of F9 teratocarcinoma stem cells

The biological effects of all-trans-retinoic acid (RA), a major active metabolite of retinol, are mainly mediated through its interactions with retinoic acid receptor (RARs alpha, beta, gamma) and retinoid X receptor (RXRs alpha, beta, gamma) heterodimers. RAR/RXR heterodimers activate transcription by binding to RA-response elements (RAREs or RXREs) in the promoters of primary target genes. Murine F9 teratocarcinoma stem cells have been widely used as a model for cellular differentiation and RA signaling during embryonic development. We identified and characterized genes that are differentially expressed in F9 wild type (Wt) and F9 RARgamma-/- cells, with and without RA treatment, through the use of oligonucleotide-based microarrays. Our data indicate that RARgamma, in the absence of exogenous RA, modulates gene expression. Genes such as Sfrp2, Tie1, Fbp2, Emp1, and Emp3 exhibited higher transcript levels in RA-treated Wt, RARalpha-/- and RARbeta2-/- lines than in RA-treated RARgamma-/- cells, and represent specific RARgamma targets. Other genes, such as Runx1, were expressed at lower levels in both F9 RARbeta2-/- and RARgamma-/- cell lines than in F9 Wt and RARalpha-/-. Genes specifically induced by RA at 6h with the protein synthesis inhibitor cycloheximide in F9 Wt, but not in RARgamma-/- cells, included Hoxa3, Hoxa5, Gas1, Cyp26a1, Sfrp2, Fbp2, and Emp1. These genes represent specific primary RARgamma targets in F9 cells. Several genes in the Wnt signaling pathway were regulated by RARgamma. Delineation of the receptor-specific actions of RA with respect to cell proliferation and differentiation should result in more effective therapies with this drug.

Inhibition of histone deacetylase as a new mechanism of teratogenesis

Histone deacetylases (HDACs) are nuclear and cytoplasmic enzymes that deacetylate a number of substrates, of which histones are the best known and described in the literature. HDACs are present in eukaryotic and bacteria cells, and are fundamental for a number of cellular functions, including correct gene expression. Surprisingly, only up to 20% of the whole genome is controlled by HDACs, but key processes for survival, proliferation, and differentiation have been strictly linked to HDAC enzyme functioning. The use of HDAC inhibitors (HDACi) has been proposed for the treatment of neoplastic diseases. Their effectiveness has been suggested for a number of liquid and solid tumors, particularly acute promyelocytic leukemia (APL). The role of HDACs in embryo development is currently under investigation. Published data indicate knockout phenotype analysis to be of particular interest, in which a number of HDACs play a key role during development. Little data have been published on the effects of HDACi on embryonic development, although for valproic acid (VPA), literature from the 1980s described its teratogenic effects in experimental animals and humans. To date, all tested HDACi have shown teratogenic effects similar to those described for VPA when tested in zebrafish, Xenopus laevis, and mice. HDACs were also able to alter embryo development in invertebrates and plants. A model, similar to that proposed in APL, involving retinoic acid receptors (RAR) and tissue specific Hox gene expression, is suggested to explain the HDAC effects on embryo development.

Dynamic and sequential patterning of the zebrafish posterior hindbrain by retinoic acid

A prominent region of the vertebrate hindbrain is subdivided along the anterior-posterior axis into a series of seven segments, or rhombomeres. The identity of each rhombomere is specified by the expression of conserved transcription factors, including Krox-20, vHnf1, Val (Kreisler, Mafb) and several Hox proteins. Previous work has shown that retinoic acid (RA) signaling plays a critical role in regulating the expression of these factors and that more posterior rhombomeres require higher levels of RA than more anterior rhombomeres. Models to account for RA concentration dependency have proposed either a static RA gradient or increasing time periods of RA exposure. Here, we provide evidence against both of these models. We show that early zebrafish rhombomere-specification genes, including vhnf1 in r5-r6 and hoxd4a in r7, initiate expression sequentially in the hindbrain, each adjacent to the source of RA synthesis in paraxial mesoderm. By knocking down RA signaling, we show that progressively more posterior rhombomeres require increasingly higher levels of RA signaling, and vhnf1 and hoxd4a expression are particularly RA-dependent. RA synthesis is required just at the time of initiation, but not for maintenance, of vhnf1 and hoxd4a expression. Furthermore, a premature RA increase causes premature activation of vhnf1 and hoxd4a expression. Our results support a new model of dynamic RA action in the hindbrain, in which a temporally increasing source of RA is required to sequentially initiate progressively more posterior rhombomere identities.

Function of retinoid nuclear receptors: lessons from genetic and pharmacological dissections of the retinoic acid signaling pathway during mouse embryogenesis

Retinoic acid (RA) is involved in vertebrate morphogenesis, growth, cellular differentiation, and tissue homeostasis. The use of in vitro systems initially led to the identification of nuclear receptor RXR/RAR heterodimers as possible transducers of the RA signal. To unveil the physiological functions of RARs and RXRs, genetic and pharmacological studies have been performed in the mouse. Together, their results demonstrate that (a) RXR/RAR heterodimers in which RXR is either transcriptionally active or silent are involved in the transduction of the RA signal during prenatal development, (b) specific RXRalpha/RAR heterodimers are required at many distinct stages during early embryogenesis and organogenesis, (c) the physiological role of RA and its receptors cannot be extrapolated from teratogenesis studies using retinoids in excess. Additional cell type-restricted and temporally controlled somatic mutagenesis is required to determine the functions of RARs and RXRs during postnatal life.

Retinoid signaling in inner ear development

The inner ear originates from an embryonic ectodermal placode and rapidly develops into a three-dimensional structure (the otocyst) through complex molecular and cellular interactions. Many genes and their products are involved in inner ear induction, organogenesis, and cell differentiation. Retinoic acid (RA) is an endogenous signaling molecule that may play a role during different phases of inner ear development, as shown from pathological observations. To gain insight into the function of RA during inner ear development, we have investigated the spatio-temporal expression patterns of major components of RA signaling pathway, including cellular retinoic acid binding proteins (CRABPs), cellular retinoid binding proteins (CRBPs), retinaldehyde dehydrogenases (RALDHs), catabolic enzymes (CYP26s), and nuclear receptors (RARs). Although the CrbpI, CrabpI, and -II genes are specifically expressed in the inner ear throughout development, loss-of-function studies have revealed that these proteins are dispensable for inner development and function. Several Raldh and Cyp26 gene transcripts are expressed at embryological day (E) 9.0-9.5 in the otocyst and show mainly complementary distributions in the otic epithelium and mesenchyme during following stages. From Western blot, RT-PCR, and in situ hybridization analysis, there is a low expression of Raldhs in the early otocyst at E9, while Cyp26s are strongly expressed. During the following days, there is an up-regulation of Raldhs and a down-regulation for Cyp26s. Specific RA receptor (Rar and Rxr) genes are expressed in the otocyst and during further development of the inner ear. At the otocyst stage, most of the components of the retinoid pathway are present, suggesting that the embryonic inner ear might act as an autocrine system, which is able to synthesize and metabolize RA necessary for its development. We propose a model in which two RA-dependent pathways may control inner ear ontogenesis: one indirect with RA from somitic mesoderm acting to regulate gene expression within the hindbrain neuroepithelium, and another with RA acting directly on the otocyst. Current evidence suggests that RA may regulate several genes involved in mesenchyme-epithelial interactions, thereby controlling inner ear morphogenesis. Our investigations suggest that RA signaling is a critical component not only of embryonic development, but also of postnatal maintenance of the inner ear.

Retinoic acid and hindbrain patterning

Retinoid signaling plays an important role in the developmental patterning of the hindbrain. Studies of the teratogenic effects of retinoids showed early on that the hindbrain suffered patterning defects in cases of retinoid excess or deficiency. Closer examination of these effects in animal models suggested that retinoids might play a physiological role in specifying the antero-posterior axis of the hindbrain. This idea was supported by the localization of retinoid synthetic and degradative enzymes, binding proteins, and receptors to the hindbrain and neighboring regions of the neuroepithelium and the mesoderm. In parallel, it became clear that the molecular patterning of the hindbrain, in terms of the regionalized expression of Hox genes and other developmental regulatory genes, is profoundly influenced by retinoid signaling.

Role of morphogens in neural crest cell determination

The neural crest is a transient, migratory cell population found in all vertebrate embryos that generate a diverse range of cell and tissue derivatives including, but not limited, to the neurons and glia of the peripheral nervous system, smooth muscle, connective tissue, melanocytes, craniofacial cartilage, and bone. Over the past few years, many studies have provided tremendous insights into understanding the mechanisms regulating the induction and migration of neural crest cell development. This review highlights the surprising and perhaps unexpected roles for morphogens in these distinct processes. A comparison of studies performed in several different vertebrates emphasizes the requirement for coordination between multiple signaling pathways in the induction and migration of neural crest cells in the developing embryo.