Temporally-regulated retinoic acid depletion produces specific neural crest, ocular and nervous system defects

HSP110, caspase-3 and -9 expression in physiological apoptosis and apoptosis induced by in vivo embryonic exposition to all-trans retinoic acid or irradiation during early mouse eye development

Apoptosis is an essential physiological process in embryonic development. In the developing eye of vertebrates, three periods of developmental apoptosis can be distinguished: early, intermediate and later. Within the apoptosis pathway, caspases play a crucial role. It has also been shown that HSP110 may have a potential role in apoptosis. The aim of this research was to study the expression of HSP110, caspase-3 and -9 in physiological, retinoic- or irradiation-induced apoptosis during early eye development. Seven pregnant C57Bl/6J mice received 80 mg kg(-1) of all-trans retinoic acid mixed with sesame oil. Seven pregnant NMRI mice received 2 Gy irradiation at the same gestational day. Control mice of both strains (seven mice of each) were not submitted to any treatment. Embryos were harvested at 3, 6, 12 and 24 h after exposition, fixed, dehydrated and embedded. Coronal sections (5 microm) were made. Slide staining occurred alternatively using anti-caspase-3, anti-caspase-9 and anti-HSP110 immunohistochemistry. HSP110 and caspase-3 expression presented similar topographic and chronological patterns, whereas expression of HSP110 was more precocious in retinoic acid-treated embryos. After retinoic exposure, caspase-3- and HSP110-positive cells were increased in the region of the optic vesicle. By contrast, after irradiation, caspase-3- and HSP110-positive cells were noticeably increased in the optic vesicle, peri-optical mesoderm but less in lens placode. HSP110 was expressed before caspase-3. By contrast, caspase-9 was expressed by a very small number of cells in the optic vesicle either under physiological or under teratogenic conditions. Thus, it seems that activation of caspase-9 is dispensable in early eye developmental apoptosis.

Retinoic acid regulation of the somitogenesis clock

Retinoic acid (RA) is a signaling molecule synthesized from vitamin A that controls gene expression at the transcriptional level by functioning as a ligand for nuclear RA receptors. RA plays an essential role during embryonic development in higher animals by regulating key genes involved in pattern formation. RA is required for induction of several Hox genes involved in patterning of the hindbrain and spinal cord as neuroectoderm emerges from the primitive streak. Recent findings indicate that RA is also required to ensure bilaterally symmetrical generation of left and right somites as presomitic mesoderm emerges from the primitive streak. RA may control somitogenesis through its ability to repress posterior ectodermal expression of fibroblast growth factor-8 (Fgf8) for a short period of time during the late primitive streak stage when the somitogenesis clock initiates. During this tight temporal window, RA is required to limit Fgf8 expression to the most posterior ectoderm (epiblast), thus preventing ectopic Fgf8 expression in more anterior ectoderm including the node ectoderm and neuroectoderm. Although Fgf8 is required for the node to impart left-right asymmetry on specific tissues (heart, visceral organs, etc.), excess Fgf8 signaling following a loss of RA may stimulate the node to generate asymmetry also in presomitic mesoderm, leading to left-right asymmetry in the somitogenesis clock. These findings suggest that human vertebral birth defects such as scoliosis, an abnormal left-right bending of the vertebral column, may be caused by a defect in RA signaling during somitogenesis.

Vitamin A deficiency does not influence longitudinal growth in mice

OBJECTIVE

Growth hormone (GH) stimulates longitudinal growth, directly and indirectly, through the mediation of circulating and locally produced insulin-like growth factor 1 (IGF-1). The exact role of individual vitamins in modulating GH secretion or action, and somatic growth in general, is still not completely understood. It has been suggested that vitamin A (VA) and its physiologic active metabolite retinoic acid influence longitudinal growth by promoting the differentiation of pituitary cells toward GH-secreting cells and by stimulating secretion of GH. Accordingly, epidemiologic studies have shown that short children have lower VA intake than do children with normal stature.

METHODS

To determine whether VA deficiency causes impairment of GH secretion, we have investigated the effect of a severely VA-deficient diet on growth in mice. Ten male mice born from mothers fed with VA-deficient diet since conception were maintained on a VA-deficient diet until the end of week 8 of life. Ten male mice born from mothers fed with a VA-sufficient diet and receiving a normal diet after weaning served as the control group. At the end of the study, we measured animals' weight and length, body composition, tibia and femur lengths, liver retinol and retinyl esters storages, serum IGF-1, serum thyroxine, serum GH, and pituitary GH mRNA levels.

RESULTS

Despite evidence of significant VA deficiency, we observed no effect on longitudinal growth or changes in pituitary GH mRNA, serum thyroxine, serum GH, or serum IGF-1 levels.

CONCLUSION

VA deficiency does not negatively affect longitudinal growth and pituitary GH content and action in mice.

Fetal alcohol syndrome and developing craniofacial and dental structures--a review

OBJECTIVES

Fetal alcohol syndrome (FAS) is a collection of signs and symptoms seen in children exposed to alcohol in the prenatal period. It is characterized mainly by a distinct pattern of craniofacial malformations, physical and mental retardation. However, with the increased incidence of FAS, there is a great variation in the clinical features of FAS.

DESIGN

Narrative review.

RESULTS

This review describes data from clinical and experimental studies, and in vitro models. Experimental studies have shown that alcohol has a direct toxic effect on the ectodermal and mesodermal cells of the developing embryo, particularly in the cells destined to give rise to dentofacial structures (i.e. cranial neural crest cells). Other effects, such as, abnormal pattern of cranial and mandibular growth and altered odontogenesis are described in detail. The exact mechanism by which alcohol induces its teratogenic effects remains still unknown. The possible mechanisms are outlined here, with an emphasis on the developing face and tooth. Possible future research directions and treatment strategies are also discussed.

CONCLUSION

Early identification of children affected by prenatal alcohol exposure leads to interventions, services, and improved outcomes. FAS can be prevented with the elimination of alcohol consumption during pregnancy. We need to provide education, target high-risk groups, and make this issue a high priority in terms of public health.

Maternal vitamin A deficiency and neonatal microphthalmia: complications of biliopancreatic diversion?

Biliopancreatic diversion (BPD) for morbid obesity carries a serious risk of nutritional deficiencies that might impair embryogenesis. Consequently, attention should be given to the potential of risk to the fetus of BPD in women of childbearing age. We present the case of a pregnant woman who had undergone BPD 8 years previously, with documented vitamin A deficiency, who gave birth to a child with bilateral microphthalmia. Infectious and genetic causes of microphthalmia were excluded. A search of the literature revealed that vitamin A deficiency may cause a disruption of ocular development. We conclude that nutritional deficiencies may cause a spectrum of fetal malformations. As the effect of BPD relies on malabsorption, fetal risk should be considered before BPD is offered to morbid obese women of childbearing age.

Retinoic acid regulates morphogenesis and patterning of posterior foregut derivatives

Retinoic acid (RA) is an embryonic signaling molecule regulating a wide array of target genes, thereby being a master regulator of patterning and differentiation in a variety of organs. Here we show that mouse embryos deficient for the RA-synthesizing enzyme retinaldehyde dehydrogenase 2 (RALDH2), if rescued from early lethality by maternal RA supplementation between E7.5 and E8.5, lack active RA signaling in the foregut region. The resulting mutants completely fail to develop lungs. Development of more posterior foregut derivatives (stomach and duodenum), as well as liver growth, is also severely affected. A primary lung bud is specified in the RA-deficient embryos, which fails to outgrow due to defective FGF10 signaling and lack of activation of FGF-target genes, such as Pea3 and Bmp4 in the epithelium. Specific Hox and Tbx genes may mediate these RA regulatory effects. Development of foregut derivatives can be partly restored in mutants by extending the RA supplementation until at least E10.5, but lung growth and branching remain defective and a hypoplastic lung develops on the right side only. Such conditions poorly restore FGF10 signaling in the lung buds. Explant culture of RALDH2-deficient foreguts show a capacity to undergo lung budding and early branching in the presence of RA or FGF10. Our data implicate RA as a regulator of gene expression in the early embryonic lung and stomach region upstream of Hox, Tbx and FGF10 signaling.

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.

Distinct roles for retinoic acid receptors alpha and beta in early lung morphogenesis

Retinoic acid (RA) signaling is required for normal development of multiple organs. However, little is known about how RA influences the initial stages of lung development. Here, we used a combination of genetic, pharmacological and explant culture approaches to address this issue, and to investigate how signaling by different RA receptors (RAR) mediates the RA effects. We analyzed initiation of lung development in retinaldehyde dehydrogenase-2 (Raldh2) null mice, a model in which RA signaling is absent from the foregut from its earliest developmental stages. We provide evidence that RA is dispensable for specification of lung cell fate in the endoderm. By using synthetic retinoids to selectively activate RAR alpha or beta signaling in this model, we demonstrate novel and unique functions of these receptors in the early lung. We show that activation of RAR beta, but not alpha, induces expression of the fibroblast growth factor Fgf10 and bud morphogenesis in the lung field. Similar analysis of wild type foregut shows that endogenous RAR alpha activity is required to maintain overall RA signaling, and to refine the RAR beta effects in the lung field. Our data support the idea that balanced activation of RAR alpha and beta is critical for proper lung bud initiation and endodermal differentiation.

Growth factors in lung development

Organized and coordinated lung development follows transcriptional regulation of a complex set of cell-cell and cell-matrix interactions resulting in a blood-gas interface ready for physiologic gas exchange at birth. Transcription factors, growth factors, and various other signaling molecules regulate epithelial-mesenchymal interactions by paracrine and autocrine mechanisms. Transcriptional control at the earliest stages of lung development results in cell differentiation and cell commitment in the primitive lung bud, in essence setting up a framework for pattern formation and branching morphogenesis. Branching morphogenesis results in the formation of the conductive airway system, which is critical for alveolization. Lung development is influenced at all stages by spatial and temporal distribution of various signaling molecules and their receptors and also by the positive and negative control of signaling by paracrine, autocrine, and endocrine mechanisms. Lung bud formation, cell differentiation, and its interaction with the splanchnic mesoderm are regulated by HNF-3beta, Shh, Nkx2.1, HNF-3/Forkhead homolog-8 (HFH-8), Gli, and GATA transcription factors. HNF-3beta regulates Nkx2.1, a transcription factor critical to the formation of distal pulmonary structures. Nkx2.1 regulates surfactant protein genes that are important for the development of alveolar stability at birth. Shh, produced by the foregut endoderm, regulates lung morphogenesis signaling through Gli genes expressed in the mesenchyme. FGF10, produced by the mesoderm, regulates branching morphogenesis via its receptors on the lung epithelium. Alveolization and formation of the capillary network are influenced by various factors that include PDGF, vascular endothelial growth factor (VEGF), and retinoic acid. Epithelial-endothelial interactions during lung development are important in establishing a functional blood-gas interface. The effects of various growth factors on lung development have been demonstrated by gain- or loss-of-function studies in null mutant and transgenic mice models. Understanding the role of growth factors and various other signaling molecules and their cellular interactions in lung development will provide us with new insights into the pathogenesis of bronchopulmonary dysplasia and disorders of lung morphogenesis.

Apoptosis induced by vitamin A signaling is crucial for connecting the ureters to the bladder

Removal of toxic substances from the blood depends on patent connections between the kidney, ureters and bladder that are established when the ureter is transposed from its original insertion site in the male genital tract to the bladder. This transposition is thought to occur as the trigone forms from the common nephric duct and incorporates into the bladder. Here we re-examine this model in the context of normal and abnormal development. We show that the common nephric duct does not differentiate into the trigone but instead undergoes apoptosis, a crucial step for ureter transposition controlled by vitamin A-induced signals from the primitive bladder. Ureter abnormalities occur in 1-2% of the human population and can cause obstruction and end-stage renal disease. These studies provide an explanation for ureter defects underlying some forms of obstruction in humans and redefine the current model of ureter maturation.

Disruption of the lecithin:retinol acyltransferase gene makes mice more susceptible to vitamin A deficiency

Lecithin:retinol acyltransferase (LRAT) catalyzes the esterification of retinol (vitamin A) in the liver and in some extrahepatic tissues, including the lung. We produced an LRAT gene knock-out mouse strain and assessed whether LRAT-/- mice were more susceptible to vitamin A deficiency than wild type (WT) mice. After maintenance on a vitamin A-deficient diet for 6 weeks, the serum retinol level was 1.34 +/- 0.32 microM in WT mice versus 0.13 +/- 0.06 microM in LRAT-/- mice (p < 0.05). In liver, lung, eye, kidney, brain, tongue, adipose tissue, skeletal muscle, and pancreas, the retinol levels ranged from 0.05 pmol/mg (muscle and tongue) to 17.35 +/- 2.66 pmol/mg (liver) in WT mice. In contrast, retinol was not detectable (<0.007 pmol/mg) in most tissues from LRAT-/- mice after maintenance on a vitamin A-deficient diet for 6 weeks. Cyp26A1 mRNA was not detected in hepatic tissue samples from LRAT-/- mice but was detected in WT mice fed the vitamin A-deficient diet. These data indicate that LRAT-/- mice are much more susceptible to vitamin A deficiency and should be an excellent animal model of vitamin A deficiency. In addition, the retinol levels in serum rapidly increased in the LRAT-/- mice upon re-addition of vitamin A to the diet, indicating that serum retinol levels in LRAT-/- mice can be conveniently modulated by the quantitative manipulation of dietary retinol.

Retinoid absorption and storage is impaired in mice lacking lecithin:retinol acyltransferase (LRAT)

Lecithin:retinol acyltransferase (LRAT) is believed to be the predominant if not the sole enzyme in the body responsible for the physiologic esterification of retinol. We have studied Lrat-deficient (Lrat-/-) mice to gain a better understanding of how these mice take up and store dietary retinoids and to determine whether other enzymes may be responsible for retinol esterification in the body. Although the Lrat-/- mice possess only trace amounts of retinyl esters in liver, lung, and kidney, they possess elevated (by 2-3-fold) concentrations of retinyl esters in adipose tissue compared with wild type mice. These adipose retinyl ester depots are mobilized in times of dietary retinoid insufficiency. We further observed an up-regulation (3-4-fold) in the level of cytosolic retinol-binding protein type III (CRBPIII) in adipose tissue of Lrat-/- mice. Examination by electron microscopy reveals a striking total absence of large lipid-containing droplets that normally store hepatic retinoid within the hepatic stellate cells of Lrat-/- mice. Despite the absence of significant retinyl ester stores and stellate cell lipid droplets, the livers of Lrat-/- mice upon histologic analysis appear normal and show no histological signs of liver fibrosis. Lrat-/- mice absorb dietary retinol primarily as free retinol in chylomicrons; however, retinyl esters are also present within the chylomicron fraction obtained from Lrat-/- mice. The fatty acyl composition of these (chylomicron) retinyl esters suggests that they are synthesized via an acyl-CoA-dependent process suggesting the existence of a physiologically significant acyl-CoA:retinol acyltransferase.

Transcriptional activities of retinoic acid receptors

Vitamin A derivatives plays a crucial role in embryonic development, as demonstrated by the teratogenic effect of either an excess or a deficiency in vitamin A. Retinoid effects extend however beyond embryonic development, and tissue homeostasis, lipid metabolism, cellular differentiation and proliferation are in part controlled through the retinoid signaling pathway. Retinoids are also therapeutically effective in the treatment of skin diseases (acne, psoriasis and photoaging) and of some cancers. Most of these effects are the consequences of retinoic acid receptors activation, which triggers transcriptional events leading either to transcriptional activation or repression of retinoid-controlled genes. Synthetic molecules are able to mimic part of the biological effects of the natural retinoic acid receptors, all-trans retinoic acid. Therefore, retinoic acid receptors are considered as highly valuable therapeutic targets and limiting unwanted secondary effects due to retinoid treatment requires a molecular knowledge of retinoic acid receptors biology. In this review, we will examine experimental evidence which provide a molecular basis for the pleiotropic effects of retinoids, and emphasize the crucial roles of coregulators of retinoic acid receptors, providing a conceptual framework to identify novel therapeutic targets.

Identification and molecular regulation of neural stem cells in the olfactory epithelium

The sensory neurons that subserve olfaction, olfactory receptor neurons (ORNs), are regenerated throughout life, making the neuroepithelium in which they reside [the olfactory epithelium (OE)] an excellent model for studying how intrinsic and extrinsic factors regulate stem cell dynamics and neurogenesis during development and regeneration. Numerous studies indicate that transcription factors and signaling molecules together regulate generation of ORNs from stem and progenitor cells during development, and work on regenerative neurogenesis indicates that these same factors may operate at postnatal ages as well. This review describes our current knowledge of the identity of the OE neural stem cell; the different cell types that are thought to be the progeny (directly or indirectly) of this stem cell; and the factors that influence cell differentiation in the OE neuronal lineage. We review data suggesting that (1) the ORN lineage contains three distinct proliferating cell types--a stem cell and two populations of transit amplifying cells; (2) in established OE, these three cell types are present within the basal cell compartment of the epithelium; and (3) the stem cell that gives rise ultimately to ORNs may also generate two glial cell types of the primary olfactory pathway: sustentacular cells (SUS), which lie within OE proper; and olfactory ensheathing cells (OEC), which envelope the olfactory nerve. In addition, we describe factors that are both made by and found within the microenvironment of OE stem and progenitor cells, and which exert crucial growth regulatory effects on these cells. Thus, as with other regenerating tissues, the basis of regeneration in the OE appears be a population of stem cells, which resides within a microenvironment (niche) consisting of factors crucial for maintenance of its capacity for proliferation and differentiation.

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

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

Molecular signals regulating proliferation of stem and progenitor cells in mouse olfactory epithelium

To understand how signaling molecules regulate the generation of neurons from proliferating stem cells and neuronal progenitors in the developing and regenerating nervous system, we have studied neurogenesis in a model neurogenic epithelium, the olfactory epithelium (OE) of the mouse. Our studies have employed a candidate approach to test signaling molecules of potential importance in regulating neurogenesis and have utilized methods that include tissue culture, in situ hybridization and mouse genetics. Using these approaches, we have identified three distinct stages of stem and transit amplifying progenitor cells in the differentiation pathway of olfactory receptor neurons (ORNs) and have identified mechanisms by which the development of each of these progenitor cell types is regulated by signals produced both within the OE itself and by its underlying stroma. Our results indicate that regulation of olfactory neurogenesis is critically dependent on multiple signaling molecules from two different polypeptide growth factor superfamilies, the fibroblast growth factors and the transforming growth factor beta (TGF-beta) group. In addition, they indicate that these signaling molecules interact in at least two important ways: first, opposing signals converge on cells at specific developmental stages in the ORN pathway to regulate proliferation and differentiation; and second, these signaling molecules--particularly the TGF-betas and their antagonists--play key roles in feedback loops that regulate the size of progenitor cell pools and thereby neuron number, during development and regeneration.

Ethanol exposure affects gene expression in the embryonic organizer and reduces retinoic acid levels

Fetal Alcohol Spectrum Disorder (FASD) is a set of developmental malformations caused by alcohol consumption during pregnancy. Fetal Alcohol Syndrome (FAS), the strongest manifestation of FASD, results in short stature, microcephally and facial dysmorphogenesis including microphthalmia. Using Xenopus embryos as a model developmental system, we show that ethanol exposure recapitulates many aspects of FAS, including a shortened rostro-caudal axis, microcephally and microphthalmia. Temporal analysis revealed that Xenopus embryos are most sensitive to ethanol exposure between late blastula and early/mid gastrula stages. This window of sensitivity overlaps with the formation and early function of the embryonic organizer, Spemann's organizer. Molecular analysis revealed that ethanol exposure of embryos induces changes in the domains and levels of organizer-specific gene expression, identifying Spemann's organizer as an early target of ethanol. Ethanol also induces a defect in convergent extension movements that delays gastrulation movements and may affect the overall length. We show that mechanistically, ethanol is antagonistic to retinol (Vitamin A) and retinal conversion to retinoic acid, and that the organizer is active in retinoic acid signaling during early gastrulation. The model suggests that FASD is induced in part by an ethanol-dependent reduction in retinoic acid levels that are necessary for the normal function of Spemann's organizer.

Fetal lung and diaphragm development in congenital diaphragmatic hernia

Congenital Diaphragmatic Hernia (CDH) is a congenital disorder with an incidence of 1 in 2500 live births. Respiratory distress of newborns with CDH is the result of pulmonary hypoplasia and pulmonary hypertension. Hypoplastic lungs are characterized by a decreased number of airways with smaller airspaces, whereas the combination of a decreased number of vascular branches and an increased adventitia and medial thickness of the pulmonary arterial walls result in pulmonary hypertension. The appearance of the CDH lungs suggests that its complete formation is stalled during development. Understanding the basic mechanisms of lung development is mandatory to unravel the origin of CDH. Although the histological abnormalities in CDH lungs have been well described, less is known about the underlying molecular mechanisms. In this review we will discuss the current molecular and genetic background of lung formation, as well as a reflection of this knowledge towards CDH.

Stem cells and pulmonary metamorphosis: New concepts in repair and regeneration

Adult stem cells are likely to have much more versatile differentiation capabilities than once believed. Numerous studies have appeared over the past decade demonstrating the ability of adult stem cells to differentiate into a variety of cells from non-hematopoietic organs, including the lung. The goal of this review is to provide an overview of the growth factors which are thought to be involved in lung development and disease, describe the cells within the lung that are believed to replace cells that have been injured, review the studies that have demonstrated the transformation of bone marrow-derived stem cells into lung cells, and describe potential clinical applications with respect to human pulmonary disease.

Generating gradients of retinoic acid in the chick embryo: Cyp26C1 expression and a comparative analysis of the Cyp26 enzymes

We have cloned a novel retinoic acid (RA) catabolizing enzyme, Cyp26C1, in the chick and describe here its distribution during early stages of chick embryogenesis. It is expressed from stage 4 in the presumptive anterior (cephalic) mesoderm, in a subset of cephalic neural crest cells, the ventral otic vesicle, mesenchyme adjacent to the otic vesicle, the branchial pouches and grooves, a part of the neural retina, and the anterior telencephalon, and shows a dynamic expression in the hindbrain rhombomeres and neuronal populations within them. By examining the distribution of Cyp26C1 in the RA-free quail embryo, we can determine which of these expression domains is dependent on RA, and it is only the rhombomeric sites that do not appear, suggesting a role for RA in this location. The most striking domain of Cyp26C1 distribution is in the anterior cephalic mesoderm, which is adjacent to the domain of Raldh2 in the trunk mesoderm, but separated from it by a gap dorsal to which the posterior hindbrain will develop. We suggest that a gradient of RA within the mesoderm generated by Raldh2 and catabolized by Cyp26C1 could be responsible for patterning the hindbrain. We have compared this distribution of Cyp26C1 with that of Cyp26A1 and Cyp26B1 in the chick and shown that they generally occupy nonoverlapping sites of expression in the embryo, and as a result, we suggest individual roles for each of the Cyp enzymes in the developing embryo.

Microarray analysis of retinoid-dependent gene activity during rat embryogenesis: increased collagen fibril production in a model of retinoid insufficiency

Retinoic acid (RA) is an essential mediator of embryogenesis. Some, but not all, of its targets have been identified. We previously developed a rat model of gestational retinoid deficiency (RAD; Power et al. [1999] Dev. Dyn. 216:469-480) and generated embryos with developmental impairments that closely resemble genetic and dietary models of retinoid insufficiency. Here, we used microarray analysis and expression profiling to identify 88 transcripts whose abundance was altered under conditions of retinoid insufficiency, as compared with normal embryos. Among these, the induction by RAD of genes involved in collagen I synthesis (COL1A1, IA2 and VA2, prolyl-4-hydroxylase-alpha1) and protein galactosylation (galactokinase, ABO galactosyltransferase, UDP-galactose transporter-related protein) was especially noteworthy because extracellular matrix regulates many developmental events. We also identified several genes involved with stress responses (cathepsin H, UBC2E, IGFBP3, smoothelin). Real-time polymerase chain reaction analysis of selected candidates revealed excellent agreement with the array findings. Further validation came from the demonstration that these genes were similarly dysregulated in two genetic models of retinoid insufficiency, the retinol binding protein null-mutant embryo and the Raldh2 null-mutant embryo. In situ hybridization of RAD embryos found increased collagen IA1 and IGFBP3 mRNA within the connective mesenchyme and vasculature, respectively, and a failure to repress the growth factor midkine within the RAD neural tube. Many of the identified genes were not known previously to respond to retinoid status and will provide new insights to retinoid roles and to the consequences of retinoid insufficiency.

Retinoic acid selectively regulates Fgf10 expression and maintains cell identity in the prospective lung field of the developing foregut

Although respiratory tract defects that result from disruption of retinoic acid (RA) signaling have been widely reported, the mechanism by which endogenous RA regulates early lung morphogenesis is unknown. Here, we provide novel evidence that a major role for RA is to selectively maintain mesodermal proliferation and induce fibroblast growth factor 10 (Fgf10) expression in the foregut region where the lung forms. By using a pan-RAR antagonist (BMS493) in foregut explant cultures, we show that bud initiation is selectively blocked in the prospective respiratory region by failure to induce Fgf10 in the corresponding mesoderm. The RA regulation of Fgf10 expression occurs only in this region, within a defined developmental window, and is not seen in other foregut derivatives such as thyroid and pancreas where Fgf10 is also required for normal development. Furthermore, we show that RA activity is essential in the lung field to maintain lung cell identity in the endoderm; RAR antagonism disrupts expression of thyroid transcription factor 1 (Ttf1), an early marker of the respiratory region in the endoderm, and surfactant protein C (Sp-C) mRNAs. Our observations in mouse foregut cultures are corroborated by data from an in vivo model of vitamin A deficiency in rats. Our study supports RA as an essential regulator of gene expression and cellular activities during primary bud formation.

Beyond the neckless phenotype: influence of reduced retinoic acid signaling on motor neuron development in the zebrafish hindbrain

Retinoic acid (RA) has been identified as a key signal involved in the posteriorization of vertebrate neural ectoderm. The main biosynthetic enzyme responsible for RA signaling in the hindbrain and spinal cord is Raldh2. However, neckless/raldh2-mutant (nls) zebrafish exhibit only mild degrees of anteriorization in the neural ectoderm, compared to full vitamin A deficiency in amniotes and the Raldh2-/- mouse. Here we investigated the role of RA during neuronal development in the zebrafish hindbrain and anterior spinal cord using DEAB, an inhibitor of retinaldehyde dehydrogenases. We show that the nls hindbrain and spinal cord are not fully devoid of RA, since blocking Raldh-mediated RA signaling leads to a more severe hindbrain phenotype than in nls. The anteroposterior distribution of branchiomotor neurons in the facial and more posterior nuclei depends on full RA signaling throughout early and late gastrula stages. In contrast, inhibition of RA synthesis after gastrulation reduces the number of branchiomotor neurons in the vagal nucleus, but has no effect on anteroposterior cell fates. In addition, blockage of RA-mediated signaling not only interferes with the differentiation of branchiomotor neurons and their axons in the hindbrain, but also affects the development of the posterior lateral line nerve.

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.

Transcriptional control during mammalian anterior pituitary development

The mammalian anterior pituitary gland is a compound endocrine organ that regulates reproductive development and fitness, growth, metabolic homeostasis, the response to stress, and lactation, by actions on target organs such as the gonads, the liver, the thyroid, the adrenals, and the mammary gland. The protein and peptide hormones that control these physiological parameters are secreted by specialized pituitary cell types that derive from a common origin in the early ectoderm. Collectively, the broad physiological importance of the pituitary gland, its intriguing organogenesis, and the clinical and agricultural significance of its actions, have established pituitary development as an excellent model system for the study of the gene-regulatory cascades that guide vertebrate cell determination and differentiation. We review the transcriptional pathways that regulate the commitment of the individual pituitary cell lineages and that subsequently modulate trophic hormone gene activity in the differentiated cells of the mature gland.

Raldh2 expression in optic vesicle generates a retinoic acid signal needed for invagination of retina during optic cup formation

Three retinaldehyde dehydrogenase genes (Raldh1, Raldh2, and Raldh3) expressed in unique spatiotemporal patterns may control synthesis of retinoic acid (RA) needed for retina development. However, previous studies indicate that retina formation still proceeds normally in Raldh1-/- mouse embryos lacking RA synthesis in the dorsal neural retina at the optic cup stage. Here, we demonstrate that Raldh2-/- embryos lacking RA synthesis in the optic vesicle exhibit a failure in retina invagination needed to develop an optic cup. This was also observed in Raldh1-/-:Raldh2-/- double mutants, which develop similarly. Both mutants retain RA activity in the lens placode associated with Raldh3 expression, but this RA activity is insufficient to induce optic cup formation. Maternal RA administration at the optic vesicle stage rescues optic cup formation in Raldh2-/- and Raldh1-/-:Raldh2-/- embryos, demonstrating that Raldh1 is not required during rescue of optic cup development. The optic cup of rescued Raldh1-/-:Raldh2-/- embryos exhibits normal RA activity and this is associated with Raldh3 expression in the retina and lens. Thus, RA signaling initiates in the optic vesicle in response to Raldh2 but can be maintained during optic cup formation by a gene other than Raldh1, most likely Raldh3. Loss of optic vesicle RA signaling does not effect expression of early determinants of retina at the optic vesicle stage (Pax6, Six3, Rx, Mitf). Our findings suggest that RA functions as one of the signals needed for invagination of the retina to generate an optic cup.

Axial skeletal and hox expression domain alterations induced by retinoic acid, valproic acid, and bromoxynil during murine development

Retinoic acid (RA) alters the developmental fate of the axial skeletal anlagen. "Anteriorizations" or "posteriorizations," the assumption of characteristics of embryonic areas normally anterior or posterior to the affected tissues, are correlated with altered embryonal expression domains of Hox genes after in utero RA treatment. These "homeotic" changes have been hypothesized to result from alterations of a "Hox cod" which imparts positional identity in the axial skeleton. To investigate whether such developmental alterations were specific to RA, or were a more general response to xenobiotic exposure, CD-1 pregnant mice were exposed to RA, valproic acid (VA), or bromoxynil (Br) during organogenesis. Additionally, the expression domains of two Hox genes, Hoxa7 and Hoxa10, were examined in gestation day (GD) 12.5 embryos obtained from control, RA, VA, or Br, treated gravid dams exposed on GD 6, 7, or 8. The anterior expression boundary of Hoxa7 is at the level of the C7/T1 vertebrae and that of Hoxa10 is at L6/S1. Compound-induced changes in the incidence of skeletal variants were observed. These included supernumerary cervical ribs (CSNR) lateral to C7, 8 vertebrosternal ribs, supernumerary lumbar ribs (LSNR) lateral to L1, extra presacral vertebrae, and the induction of vertebral and/or rib malformations. RA and VA administration on GD 6 caused posteriorization in the cervico-thoracic region (CSNR) while GD 8 exposure to any of the three compounds resulted in anteriorizations in the thoraco-lumbar area (LSNR and an increase in the number of presacral vertebrae). These effects occurred across regions of the axial skeleton. Analysis of gene expression demonstrated changes in the anterior boundaries of Hoxa7 expression domains in embryos treated on GD 6 and 8 with RA. VA and Br did not induce any statistically significant alterations in Hoxa7 and none of the compounds caused alterations in Hoxa10 expression domains. The studies indicate that RA GD 6 treatment-induced Hoxa7 shifts were rostral (posteriorization) while the RA-induced GD 8 anterior expression boundary shift was caudal (anteriorization), correlating with the axial skeletal changes noted. These data suggest that xenobiotic compounds such as VA and Br may induce similar axial skeletal changes by affecting different components of the developmental processes involved in the patterning of the axial skeleton.

Down-regulation of retinoic acid receptor alpha signaling is required for sacculation and type I cell formation in the developing lung

Although retinoic acid (RA) has been shown to be critical for lung development, little is known about when RA is required and the role of individual RA receptors (RAR) in this process. Previously reported data from an RA responsive element RARE-lacZ reporter mouse show that when epithelial tubules are branching and differentiating RA signaling becomes markedly down-regulated in the epithelium. It is unclear why this down-regulation occurs and what role it might play in the developing lung. Here we analyze the effects of preventing potential progenitors of the distal lung from turning off RA signaling by locally expressing constitutively activated RARalpha or RARbeta chimeric receptors (RARVP16) in branching airways of transgenic mice. Continued RA activation resulted in lung immaturity in both cases, but the phenotypes were remarkably different. RARalphaVP16 lungs did not expand to form saccules or morphologically identifiable type I cells. High levels of surfactant protein C (Sp-C), thyroid transcription factor-1 (Ttf1), and Gata6, but not Sp-A or Sp-B in the epithelium at birth suggested that in these lungs differentiation was arrested at an early stage. These alterations were not observed in RARbetaVP16 lungs, which showed relatively less severe changes. Our data suggest a model in which activation of RAR signaling at the onset of lung development establishes an initial program that assigns distal cell fate to the prospective lung epithelium. Down-regulation of RA signaling, however, is required to allow completion of later steps of this differentiation program that ultimately form mature type I and II cells.

Retinoid receptors and their coregulators

Retinoids regulate gene transcription by binding to the nuclear receptors, the retinoic acid (RA) receptors (RARs), and the retinoid X receptors (RXRs). RARs and RXRs are ligand-activated transcription factors for the regulation of RA-responsive genes. The actions of RARs and RXRs on gene transcription require a highly coordinated interaction with a large number of coactivators and corepressors. This review focuses on our current understanding of these coregulators known to act in concert with RARs and RXRs. The mechanisms of action of these coregulators are beginning to be uncovered and include the modification of chromatin and the recruitment of basal transcription factors. Challenges remain to understand the specificity of action of RARs and RXRs and the formation of specific transcription complexes consisting of the receptors, coregulators, and other unknown factors.

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

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

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

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

Targeted disruption of Aldh1a1 (Raldh1) provides evidence for a complex mechanism of retinoic acid synthesis in the developing retina

Genetic studies have shown that retinoic acid (RA) signaling is required for mouse retina development, controlled in part by an RA-generating aldehyde dehydrogenase encoded by Aldh1a2 (Raldh2) expressed transiently in the optic vesicles. We examined the function of a related gene, Aldh1a1 (Raldh1), expressed throughout development in the dorsal retina. Raldh1(-/-) mice are viable and exhibit apparently normal retinal morphology despite a complete absence of Raldh1 protein in the dorsal neural retina. RA signaling in the optic cup, detected by using a RARE-lacZ transgene, is not significantly altered in Raldh1(-/-) embryos at embryonic day 10.5, possibly due to normal expression of Aldh1a3 (Raldh3) in dorsal retinal pigment epithelium and ventral neural retina. However, at E16.5 when Raldh3 is expressed ventrally but not dorsally, Raldh1(-/-) embryos lack RARE-lacZ expression in the dorsal retina and its retinocollicular axonal projections, whereas normal RARE-lacZ expression is detected in the ventral retina and its axonal projections. Retrograde labeling of adult Raldh1(-/-) retinal ganglion cells indicated that dorsal retinal axons project to the superior colliculus, and electroretinography revealed no defect of adult visual function, suggesting that dorsal RA signaling is unnecessary for retinal ganglion cell axonal outgrowth. We observed that RA synthesis in liver of Raldh1(-/-) mice was greatly reduced, thus showing that Raldh1 indeed participates in RA synthesis in vivo. Our findings suggest that RA signaling may be necessary only during early stages of retina development and that if RA synthesis is needed in dorsal retina, it is catalyzed by multiple enzymes, including Raldh1.

Frontonasal dysplasia in 3H1 Br/Br mice

The adult Brachyrrhine (3H1 Br/+) mouse displays severe midfacial retrognathia, with a "pugnose" external appearance, but information concerning craniofacial morphology of the homozygote (3H1 Br/Br) mutant is lacking. This study characterized craniofacial phenotype and genotypic features of the homozygous condition. Segregation analysis was performed by phenotypic scoring of offspring from 3H1 Br/+ reciprocal matings. Whole-mount staining was undertaken to determine the presence or absence of cranial base structures in newborn and adult mice, while features of cranial base chondrification were examined using light microscopy and type II collagen immunohistochemistry. Karyotype analysis was performed to determine whether gross chromosomal aberrations were present. Finally, microsatellite mapping analysis was undertaken to provide further resolution of the Br locus. Results showed that Br was inherited as an autosomal semidominant feature. 3H1 Br/Br mice consistently lacked a presphenoid (with its lateral projections, including a preoptic root, postoptic root, and lesser wing). Karyotyping did not reveal major gross aberrations; however, microsatellite analysis localized Br to distal mouse chromosome 17 in the vicinity of D17Mit155. These results indicated that 3H1 Br/Br mice show characteristic features of frontonasal dysplasia, including median facial clefting and bifid cranium, as well sphenoidal malformations. Furthermore, this mutant should serve as a useful model for examining mechanisms of frontonasal dysplasia.

Vitamin A deficiency leads to increased cell proliferation in olfactory epithelium of mature rats

We have shown previously that vitamin A deficiency (VAD) leads to the decreased expression of gene products that are specifically synthesized by mature neurons in the olfactory epithelium (OE) of adult rats. These results support the hypothesis that retinoic acid, a derivative of vitamin A, is required for neurogenesis and neuron replacement in vivo. VAD does not cause gross degeneration of the OE, raising the question: what types of cells continue to populate VAD OE? In this study, we compared the cell densities of VAD and VA-sufficient (VAS) OE and investigated whether cell proliferation is upregulated in VAD OE. The results show that (1) total cell number in VAD and VAS OE are comparable; (2) localized areas of hyperplasia are present in the basal regions of VAD, but not VAS, OE; (3) there is a substantial increase in the number of PCNA (proliferating cell nuclear antigen) positive cells in the basal region of VAD OE relative to VAS OE; and (4) there is a relative increase in the levels of mRNA encoding the transcription factor, MASH I, in VAD OE. We conclude that reduced availability of vitamin A derivatives, such as retinoic acid, leads to a loss of control over proliferation, hyperplasia, and increased numbers of pro-neural cells in vivo.

Cytosolic retinoid dehydrogenases govern ubiquitous metabolism of retinol to retinaldehyde followed by tissue-specific metabolism to retinoic acid

The ability of vitamin A (retinol) to control growth and development depends upon tissue-specific metabolism of retinol to retinoic acid (RA). RA then functions as a ligand for retinoid receptor signaling. Mouse genetic studies support a role for cytosolic alcohol dehydrogenases (ADH) in the first step (oxidation of retinol to retinaldehyde) and a role for cytosolic retinaldehyde dehydrogenases (RALDH) in the second step (oxidation of retinaldehyde to RA). Mice lacking ADH3 have reduced survival and a growth defect that can be rescued by dietary retinol supplementation, whereas the effect of a loss of ADH1 or ADH4 is noticed only in mice subjected to vitamin A excess or deficiency, respectively. Also, genetic deficiency of both ADH1 and ADH4 does not have additive effects, verifying separate roles for these enzymes in retinoid metabolism. As for the second step of RA synthesis, a null mutation of RALDH2 is embryonic lethal, eliminating most mesodermal RA synthesis, whereas loss of RALDH1 eliminates RA synthesis only in the embryonic dorsal retina with no obvious effect on development. Analysis of RA-rescued RALDH2 mutants has also revealed that RALDH3 and at least one additional enzyme produce RA tissue-specifically in embryos. Collectively, these genetic findings indicate that metabolism of retinol to retinaldehyde is not tissue-restricted as it is catalyzed by ubiquitously-expressed ADH3 (a low activity form) as well as by tissue-specifically expressed ADH1 and ADH4 (high activity forms). In contrast, further metabolism of retinaldehyde to RA is tissue-restricted as all enzymes identified are tissue-specific. An important concept to emerge is that selective expression of enzymes catalyzing the second step is what limits the tissues that can completely metabolize retinol to RA to initiate retinoid signaling.

Regulation of AP-2 and apoptosis in developing eye in a vitamin A-deficiency model

BACKGROUND

Eye malformations induced by vitamin A deficiency (VAD) during pregnancy is a major part of the VAD syndrome. But the signaling role of retinoic acid (RA) in ocular tissues is poorly understood. The goal of this study was to determine the role of retinoic acid receptor (RAR) in the development of eye and the possible signaling pathway.

METHODS

Time-pregnant mice were treated with 1 mg/kg dose of RAR antagonist AGN193109 (AGN) on 8 days postcoitum (dpc). Newborn mice and 18-dpc embryos were used for phenotype studies. Embryonic eyes of 18 dpc were sectioned for histological study. With immunohistochemistry and TUNEL method, we monitored the alternation of AP-2 expression and apoptotic cells in sections of 12- to 18-dpc embryos.

RESULTS

Treatment with AGN resulted in severe craniofacial and eye malformations in virtually all exposed fetuses. The ocular abnormalities included severe defects in anterior segments such as focal corneal thickening and eversion, absence of corneal endothelium and anterior chamber, differentiation defects of lens, as well as defects in posterior segment such as persistent hyperplastic primary vitreous and retinal eversions. The percentage of AP-2-positive cells in ocular tissues on 12, 14, and 18 dpc was significantly (P < 0.05) reduced in AGN-treated eyes compared to control ones. Additionally, the number of apoptotic cell was significantly (P < 0.05) increased in AGN-treated eyes.

CONCLUSIONS

The blocking of RAR function can lead to ocular abnormalities that depict partial phenocopies of vitamin A-deficiency syndrome. Both an inhibition of expression of AP-2 and an enhancement of cell death contribute to AGN-induced ocular defects.

Retinaldehyde dehydrogenase 2 (RALDH2)- independent patterns of retinoic acid synthesis in the mouse embryo

Knockout of the murine retinoic acid (RA)-synthesizing enzyme retinaldehyde dehydrogenase 2 (RALDH2) gene leads to early morphogenetic defects and embryonic lethality. Using a RA-responsive reporter transgene, we have looked for RA-generating activities in Raldh2-null mouse embryos and investigated whether these activities could be ascribed to the other known RALDH enzymes (RALDH1 and RALDH3). To this end, the early defects of Raldh2(-/-) embryos were rescued through maternal dietary RA supplementation under conditions that do not interfere with the activity of the reporter transgene in WT embryos. We show that RALDH2 is responsible for most of the patterns of reporter transgene activity in the spinal cord and trunk mesodermal derivatives. However, reporter transgene activity was selectively detected in Raldh2(-/-) embryos within the mesonephric area that expresses RALDH3 and in medial-ventral cells of the spinal cord and posterior hindbrain, up to the level of the fifth rhombomere. The craniofacial patterns of RA-reporter activity were unaltered in Raldh2(-/-) mutants. Although these patterns correlated with the presence of Raldh1 andor Raldh3 transcripts in eye, nasal, and inner ear epithelia, no such correlation was found within forebrain neuroepithelium. These data suggest the existence of additional RA-generating activities in the differentiating forebrain, hindbrain, and spinal cord, which, along with RALDH1 and RALDH3, may account for the development of Raldh2(-/-) mutants once these have been rescued for early lethality.

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.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

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.

Chimeric analysis of retinoic acid receptor function during cardiac looping

Retinoids (vitamin A and its derivatives) play essential roles during vertebrate development. Vitamin A deprivation leads to severe congenital malformations affecting many tissues, including diverse neural crest cell populations and the heart. The vitamin A signal is transduced by the retinoic acid receptors (RARalpha, RARbeta, and RARgamma). However, these receptors exhibit considerable functional redundancy, as judged by the mild phenotype of RAR single null mutants relative to the defects evoked by loss of multiple RARs. To circumvent this redundancy, the endogenous RARgamma2 allele was replaced with a ligand-binding RARgamma mutant (RARgammaE(305)) by gene targeting in mouse embryonic stem (ES) cells. Chimeric embryos derived from hemizygous RARgammaE(305) ES cells displayed several defects similar to those observed in certain RAR double null mutants, including hypoplasia or absence of the caudal pharyngeal arches and myocardial deficiencies. The latter defects were not due to abnormal cardiac specification as affected hearts still expressed chamber-specific markers in an appropriate manner. Chimeras also displayed cardiac looping anomalies, which were associated with a reduction of Pitx2. This work suggests a role for RAR signaling in late looping morphogenesis and illustrates the utility of using a dominant-negative gene substitution approach to circumvent the functional redundancy inherent to the RAR family.

Mechanisms involved in central nervous system dysfunctions induced by prenatal ethanol exposure

Clinical and experimental evidence has demonstrated that ethanol is a teratogen, and that its consumption during pregnancy induces harmful effects on the developing foetus that leads to foetal alcohol syndrome (FAS). Central nervous system dysfunctions are the most severe and permanent consequence of maternal alcohol intake and can occur in absence of gross morphological defects associated with FAS. Mental retardation and long-term cognitive and behavioural deficits are some of the problems commonly found in children of women who were moderate or heavy drinkers during pregnancy. Experimental evidence demonstrates that alcohol interferes with many molecular, neurochemical and cellular events occurring during the normal development of the brain. Some brain areas are more affected than others and, even within a given region, some cell populations are more vulnerable than others. The neocortex, hippocampus and cerebellum are especially susceptible to alcohol and have been associated with the behavioural deficits. For example, alcohol exposure during the development of neocortex increases natural apoptosis and induces cell necrosis. These effects may be associated with ethanol-induced alterations in both neurotrophic support, and the expression of cell adhesion molecules, which may affect cell-cell interactions and cell survival. Experimental evidence also shows that alcohol disrupts radial glial and astroglial development which may lead to alterations in cell migration and neuronal survival and differentiation. Impairment of several neurotransmitter systems and/or their receptors, as well as changes in the endocrine environment during brain development, are also important factors involved in the neurodevelopmental liabilities observed after in utero alcohol exposure.

Role and distribution of retinoic acid during CNS development

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

Roles of cytochrome p450 in development

Cytochrome P450 (CYP) forms are ubiquitous in nature, appearing in almost all phyla, with many forms appearing in any organism. About 50 different forms have been identified in man, and some of these are found in the embryo, some showing temporal dependence. Many of the forms of cytochrome P450 present in one species have homologues in other species. For example, CYP1A2 is present in many species, including man, rabbits, rodents, fish and fowl. The amino acid sequence identity of these homologues is often in excess of 70%. CYP26, too, has more than 61% identity in amino acid sequence between fish, fowl and mammals. In view of the high degree of conservation of sequence as well as of enzymatic activities, it is only reasonable to assume that such strong conservation of sequence also reflects a conservation of function. Since the 'xenobiotic metabolizing' enzymes predate the production of the many xenobiotics they are known to metabolize, perhaps it is reasonable to consider endobiotics as natural substrates for their metabolism. Of the identified forms of cytochrome P450 that are present in embryonic tissue, we consider the possibility that they serve the organism in support of morphogenesis of the embryonic tissue. These forms may either function to generate morphogenic molecules or to keep regions free of them, thereby creating temporal and spatial regions of morphogen action and supporting region-specific changes in cells. One known morphogen, retinoic acid, has the enzymes retinal dehydrogenase (RALDH) and CYP26 maintaining its actions, the former responsible for its generation and the latter for its elimination. Another form of cytochrome P450, CYP1B1 appears also to be involved in differentiation of tissue, with its absence resulting in primary congenital glaucoma. However, the nature of the morphogen it may maintain still remains to be elucidated.

Activation of epiblast gene expression by the hypoblast layer in the prestreak chick embryo

Axis formation is a highly regulated process in vertebrate embryos. In mammals, inductive interactions between an extra-embryonic layer, the visceral endoderm, and the embryonic layer before gastrulation are critical both for anterior neural patterning and normal primitive streak formation. The role(s) of the equivalent extra-embryonic endodermal layer in the chick, the hypoblast, is still less clear, and dramatic effects of hypoblast on embryonic gene expression have yet to be demonstrated. We present evidence that two genes later associated with the gastrula organizer (Gnot-1 and Gnot-2) are induced by hypoblast signals in prestreak embryos. The significance of this induction by hypoblast is discussed in terms of possible hypoblast functions and the regulation of axis formation in the early embryo. Several factors known to be expressed in hypoblast, and retinoic acid, synergistically induce Gnot-1 and Gnot-2 expression in blastoderm cell culture. The presence of retinoic acid in prestreak embryos has not yet been directly demonstrated, but exogenous retinoic acid appears to mimic the effects of hypoblast rotation on primitive streak extension, raising the possibility that retinoid signaling plays some role in the pregastrula embryo.

Molecular regulation of lung development

There is increasing evidence suggesting that formation of the tracheobronchial tree and alveoli results from heterogeneity of the epithelial-mesenchymal interactions along the developing respiratory tract. Recent genetic data support this idea and show that this heterogeneity is likely the result of activation of distinct networks of signaling molecules along the proximal-distal axis. Among these signals, fibroblast growth factors, retinoids, Sonic hedgehog, and transforming growth factors appear to play prominent roles. We discuss how these and other pattern regulators may be involved in initiation, branching, and differentiation of the respiratory system.