Abnormal anteroposterior and dorsoventral patterning of the limb bud in the absence of retinoids

Retinoic acid signaling is directly activated in cardiomyocytes and protects mouse hearts from apoptosis after myocardial infarction

Retinoic acid (RA) is an essential signaling molecule for cardiac development and plays a protective role in the heart after myocardial infarction (MI). In both cases, the effect of RA signaling on cardiomyocytes, the principle cell type of the heart, has been reported to be indirect. Here we have developed an inducible murine transgenic RA-reporter line using CreER^T2 technology that permits lineage tracing of RA-responsive cells and faithfully recapitulates endogenous RA activity in multiple organs during embryonic development. Strikingly, we have observed a direct RA response in cardiomyocytes during mid-late gestation and after MI. Ablation of RA signaling through deletion of the Aldh1a1/a2/a3 genes encoding RA-synthesizing enzymes leads to increased cardiomyocyte apoptosis in adults subjected to MI. RNA sequencing analysis reveals Tgm2 and Ace1, two genes with well-established links to cardiac repair, as potential targets of RA signaling in primary cardiomyocytes, thereby providing novel links between the RA pathway and heart disease.

Regulating Retinoic Acid Availability during Development and Regeneration: The Role of the CYP26 Enzymes

This review focuses on the role of the Cytochrome p450 subfamily 26 (CYP26) retinoic acid (RA) degrading enzymes during development and regeneration. Cyp26 enzymes, along with retinoic acid synthesising enzymes, are absolutely required for RA homeostasis in these processes by regulating availability of RA for receptor binding and signalling. Cyp26 enzymes are necessary to generate RA gradients and to protect specific tissues from RA signalling. Disruption of RA homeostasis leads to a wide variety of embryonic defects affecting many tissues. Here, the function of CYP26 enzymes is discussed in the context of the RA signalling pathway, enzymatic structure and biochemistry, human genetic disease, and function in development and regeneration as elucidated from animal model studies.

RA Signaling in Limb Development and Regeneration in Different Species

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

Sonic Hedgehog Signaling in Limb Development

The gene encoding the secreted protein Sonic hedgehog (Shh) is expressed in the polarizing region (or zone of polarizing activity), a small group of mesenchyme cells at the posterior margin of the vertebrate limb bud. Detailed analyses have revealed that Shh has the properties of the long sought after polarizing region morphogen that specifies positional values across the antero-posterior axis (e.g., thumb to little finger axis) of the limb. Shh has also been shown to control the width of the limb bud by stimulating mesenchyme cell proliferation and by regulating the antero-posterior length of the apical ectodermal ridge, the signaling region required for limb bud outgrowth and the laying down of structures along the proximo-distal axis (e.g., shoulder to digits axis) of the limb. It has been shown that Shh signaling can specify antero-posterior positional values in limb buds in both a concentration- (paracrine) and time-dependent (autocrine) fashion. Currently there are several models for how Shh specifies positional values over time in the limb buds of chick and mouse embryos and how this is integrated with growth. Extensive work has elucidated downstream transcriptional targets of Shh signaling. Nevertheless, it remains unclear how antero-posterior positional values are encoded and then interpreted to give the particular structure appropriate to that position, for example, the type of digit. A distant cis-regulatory enhancer controls limb-bud-specific expression of Shh and the discovery of increasing numbers of interacting transcription factors indicate complex spatiotemporal regulation. Altered Shh signaling is implicated in clinical conditions with congenital limb defects and in the evolution of the morphological diversity of vertebrate limbs.

Dietary vitamin A regulates wingless-related MMTV integration site signaling to alter the hair cycle

Alopecia areata (AA) is an autoimmune hair loss disease caused by a cell-mediated immune attack of the lower portion of the cycling hair follicle. Feeding mice 3-7 times the recommended level of dietary vitamin A accelerated the progression of AA in the graft-induced C3H/HeJ mouse model of AA. In this study, we also found that dietary vitamin A, in a dose dependent manner, activated the hair follicle stem cells (SCs) to induce the development and growth phase of the hair cycle (anagen), which may have made the hair follicle more susceptible to autoimmune attack. Our purpose here is to determine the mechanism by which dietary vitamin A regulates the hair cycle. We found that vitamin A in a dose-dependent manner increased nuclear localized beta-catenin (CTNNB1; a marker of canonical wingless-type Mouse Mammary Tumor Virus integration site family (WNT) signaling) and levels of WNT7A within the hair follicle bulge in these C3H/HeJ mice. These findings suggest that feeding mice high levels of dietary vitamin A increases WNT signaling to activate hair follicle SCs.

RIPPLY3 is a retinoic acid-inducible repressor required for setting the borders of the pre-placodal ectoderm

Retinoic acid signaling is a major component of the neural posteriorizing process in vertebrate development. Here, we identify a new role for the retinoic acid receptor (RAR) in the anterior of the embryo, where RAR regulates Fgf8 expression and formation of the pre-placodal ectoderm (PPE). RARα2 signaling induces key pre-placodal genes and establishes the posterolateral borders of the PPE. RAR signaling upregulates two important genes, Tbx1 and Ripply3, during early PPE development. In the absence of RIPPLY3, TBX1 is required for the expression of Fgf8 and hence, PPE formation. In the presence of RIPPLY3, TBX1 acts as a transcriptional repressor, and functions to restrict the positional expression of Fgf8, a key regulator of PPE gene expression. These results establish a novel role for RAR as a regulator of spatial patterning of the PPE through Tbx1 and RIPPLY3. Moreover, we demonstrate that Ripply3, acting downstream of RAR signaling, is a key player in establishing boundaries in the PPE.

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

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

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.

Comparative gene expression analysis of genital tubercle development reveals a putative appendicular Wnt7 network for the epidermal differentiation

Here we describe the first detailed catalog of gene expression in the developing lower urinary tract (LUT), including epithelial and mesenchymal portions of the developing bladder, urogenital sinus, urethra, and genital tubercle (GT) at E13 and E14. Top compartment-specific genes implicated by the microarray data were validated using whole-mount in situ hybridization (ISH) over the entire LUT. To demonstrate the potential of this resource to implicate developmentally critical features, we focused on gene expression patterns and pathways in the sexually indeterminate, androgen-independent GT. GT expression patterns reinforced the proposed similarities between development of GT, limb, and craniofacial prominences. Comparison of spatial expression patterns predicted a network of Wnt7a-associated GT-enriched epithelial genes, including Gjb2, Dsc3, Krt5, and Sostdc1. Known from other contexts, these genes are associated with normal epidermal differentiation, with disruptions in Dsc3 and Gjb2 showing palmo-plantar keratoderma in the limb. We propose that this gene network contributes to normal foreskin, scrotum, and labial development. As several of these genes are known to be regulated by, or contain cis elements responsive to retinoic acid, estrogen, or androgen, this implicates this pathway in the later androgen-dependent development of the GT.

Retinoic acid promotes limb induction through effects on body axis extension but is unnecessary for limb patterning

Retinoic acid (RA) is thought to be a key signaling molecule involved in limb bud patterning along the proximodistal or anteroposterior axes functioning through induction of Meis2 and Shh, respectively. Here, we utilize Raldh2-/- and Raldh3-/- mouse embryos lacking RA synthesis to demonstrate that RA signaling is not required for limb expression of Shh and Meis2. We demonstrate that RA action is required outside of the limb field in the body axis during forelimb induction but that RA is unnecessary at later stages when hindlimb budding and patterning occur. We provide evidence for a model of trunk mesodermal RA action in which forelimb induction requires RA repression of Fgf8 in the developing trunk similar to how RA controls somitogenesis and heart development. We demonstrate that pectoral fin development in RA-deficient zebrafish embryos can be rescued by an FGF receptor antagonist SU5402. In addition, embryo ChIP assays demonstrate that RA receptors bind the Fgf8 promoter in vivo. Our findings suggest that RA signaling is not required for limb proximodistal or anteroposterior patterning but that RA inhibition of FGF8 signaling during the early stages of body axis extension provides an environment permissive for induction of forelimb buds.

Shared developmental mechanisms pattern the vertebrate gill arch and paired fin skeletons

Here, we describe the molecular patterning of chondrichthyan branchial rays (gill rays) and reveal profound developmental similarities between gill rays and vertebrate appendages. Sonic hedgehog (Shh) and fibroblast growth factor 8 (Fgf8) regulate the outgrowth and patterning of the chondrichthyan gill arch skeleton, in an interdependent manner similar to their roles in gnathostome paired appendages. Additionally, we demonstrate that paired appendages and branchial rays share other conserved developmental features, including Shh-mediated mirror-image duplications of the endoskeleton after exposure to retinoic acid, and Fgf8 expression by a pseudostratified distal epithelial ridge directing endoskeletal outgrowth. These data suggest that the skeletal patterning role of the retinoic acid/Shh/Fgf8 regulatory circuit has a deep evolutionary origin predating vertebrate paired appendages and may have functioned initially in patterning pharyngeal structures in a deuterostome ancestor of vertebrates.

Retinoid signaling is involved in governing the waiting period for axons in chick hindlimb

During embryonic development in chick, axons pause in a plexus region for approximately 1 day prior to invading the limb. We have previously shown that this "waiting period" is governed by maturational changes in the limb. Here we provide a detailed description of the spatiotemporal pattern of Raldh2 expression in lumbosacral motoneurons and in the limb, and show that retinoid signaling in the limb contributes significantly to terminating the waiting period. Raldh2, indicative of retinoid signaling, first appears in hindlimb mesenchyme near the end of the waiting period. Transcripts are more abundant in connective tissue associated with predominantly fast muscles than predominantly slow muscles, but are not expressed in muscle cells themselves. The tips of ingrowing axons are always found in association with domains of Raldh2, but development of Raldh2 expression is not regulated by the axons. Instead, retinoid signaling appears to regulate axon entry into the limb. Supplying exogenous retinoic acid to proximal limb during the waiting period caused both motor and sensory axons to invade the limb prematurely and altered the normal stereotyped pattern of axon ingrowth without obvious effects on limb morphogenesis or motoneuron specification. Conversely, locally decreasing retinoid synthesis reduced axon growth into the limb. Retinoic acid significantly enhanced motor axon growth in vitro, suggesting that retinoic acid may directly promote axon growth into the limb in vivo. In addition, retinoid signaling may indirectly affect the waiting period by regulating the maturation of other gate keeping or guidance molecules in the limb. Together these findings reveal a novel function of retinoid signaling in governing the timing and patterning of axon growth into the limb.

Cyp26 genes a1, b1 and c1 are down-regulated in Tbx1 null mice and inhibition of Cyp26 enzyme function produces a phenocopy of DiGeorge Syndrome in the chick

Cyp26a1, a gene required for retinoic acid (RA) inactivation during embryogenesis, was previously identified as a potential Tbx1 target from a microarray screen comparing wild-type and null Tbx1 mouse embryo pharyngeal arches (pa) at E9.5. Using real-time PCR and in situ hybridization analysis of Cyp26a1 and its two functionally related family members Cyp26b1 and c1, we demonstrate reduced and/or altered expression for all three genes in pharyngeal tissues of Tbx1 null embryos. Blockade of Cyp26 function in the chick embryo using R115866, a specific inhibitor of Cyp26 enzyme function, resulted in a dose-dependent phenocopy of the Tbx1 null mouse including loss of caudal pa and pharyngeal arch arteries (paa), small otic vesicles, loss of head mesenchyme and, at later stages, DiGeorge Syndrome-like heart defects, including common arterial trunk and perimembranous ventricular septal defects. Molecular markers revealed a serious disruption of pharyngeal pouch endoderm (ppe) morphogenesis and reduced staining for smooth muscle cells in paa. Expression of the RA synthesizing enzyme Raldh2 was also up-regulated and altered Hoxb1 expression indicated that RA levels are raised in R115866-treated embryos as reported for Tbx1 null mice. Down-regulation of Tbx1 itself was observed, in accordance with previous observations that RA represses Tbx1 expression. Thus, by specifically blocking the action of the Cyp26 enzymes we can recapitulate many elements of the Tbx1 mutant mouse, supporting the hypothesis that the dysregulation of RA-controlled morphogenesis contributes to the Tbx1 loss of function phenotype.

The microRNA miR-196 acts upstream of Hoxb8 and Shh in limb development

MicroRNAs (miRNAs) are an abundant class of gene regulatory molecules (reviewed in refs 1, 2). Although computational work indicates that miRNAs repress more than a third of human genes, their roles in vertebrate development are only now beginning to be determined. Here we show that miR-196 acts upstream of Hoxb8 and Sonic hedgehog (Shh) in vivo in the context of limb development, thereby identifying a previously observed but uncharacterized inhibitory activity that operates specifically in the hindlimb. Our data indicate that miR-196 functions in a fail-safe mechanism to assure the fidelity of expression domains that are primarily regulated at the transcriptional level, supporting the idea that many vertebrate miRNAs may function as a secondary level of gene regulation.

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.

Retinoic Acid Synthesis Controlled by Raldh2 Is Required Early for Limb Bud Initiation and Then Later as a Proximodistal Signal during Apical Ectodermal Ridge Formation

We present evidence for the existence of two phases of retinoic acid (RA) signaling required for vertebrate limb development. Limb RA synthesis is under the control of retinaldehyde dehydrogenase-2 (Raldh2) expressed in the lateral plate mesoderm, which generates a proximodistal RA signal during limb outgrowth. We report that Raldh2(-/-) embryos lack trunk mesodermal RA activity and fail to initiate forelimb development. This is associated with deficient expression of important limb determinants Tbx5, Meis2, and dHand needed to establish forelimb bud initiation, proximal identity, and the zone of polarizing activity (ZPA), respectively. Limb expression of these genes can be rescued by maternal RA treatment limited to embryonic day 8 (E8) during limb field establishment, but the mutant forelimbs obtained at E10 display a significant growth defect associated with a smaller apical ectodermal ridge (AER), referred to here as an apical ectodermal mound (AEM). In these RA-deficient forelimbs, a ZPA expressing Shh forms, but it is located distally adjacent to the Fgf8 expression domain in the AEM rather than posteriorly as is normal. AER formation in Raldh2(-/-) forelimbs is rescued by continuous RA treatment through E10, which restores RA to distal ectoderm fated to become the AER. Our findings indicate the existence of an early phase of RA signaling acting upstream of Tbx5, Meis2, and dHand, followed by a late phase of RA signaling needed to expand AER structure fully along the distal ectoderm. During ZPA formation, RA acts early to activate expression of dHand, but it is not required later for Shh activation.

Retinoic acid and the control of dorsoventral patterning in the avian spinal cord

The development of neural subtypes in the dorsoventral (DV) axis of the vertebrate central nervous system (CNS) involves the integration of signalling pathways coupled with the combinatorial expression of homeodomain transcription factors. Previous studies have implicated a role for retinoic acid in the specification of a subtype of motor neurons (MN) and in the patterning of a group of interneurons within the ventral spinal cord. In this study, we use the vitamin A-deficient (VAD) quail model to further investigate the role of retinoids in the patterning of the neural tube. Using genetic markers specific to neuronal cell populations, we demonstrate that in the absence of retinoic acid, there is a disruption to the molecular mechanisms associated with the dorsoventral patterning of the spinal cord. In particular, we observe an uneven dorsal expansion of ventral-specific genes, accompanied by a reduction in the domain of roof plate and dorsal patterning genes, both of which are rescued upon addition of retinoids during development. In addition, there is a loss of V1 interneuron-specific gene expression and a decrease in the ventricular zone expression of motor neuron patterning genes. Interestingly, these effects are localised to the rostral half of the spinal cord, indicating that RA is integrated in both anteroposterior (AP) and dorsoventral patterning processes. Using differential display techniques, we have isolated 27 retinoic acid-regulated genes within the spinal cord that together reveal several interesting potential biological functions for retinoids within the avian neural tube. In summary, we propose that retinoids have an essential role in the patterning of the dorsoventral axis of the spinal cord, and are also required for the correct integration of anteroposterior patterning signals with dorsoventral determinants in the rostral spinal cord.

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

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

Spatio-temporal distribution of cellular retinol-binding protein gene transcripts (CRBPI and CRBPII) in the developing and adult zebrafish (Danio rerio)

We have cloned and determined the nucleotide sequence of the cDNA coding for a cellular retinol-binding protein type I (CRBPI) from zebrafish. The deduced amino acid sequence of the zebrafish CRBPI showed highest sequence identity ( approximately 59%) to the mammalian CRBPIs of the intracellular lipid-binding protein (iLBP) multigene family. Phylogenetic analysis clustered the zebrafish CRBPI to the CRBPI clade. The zebrafish CRBPI gene (rbp1) and CRBPII gene (rbp2) both consist of four exons separated by three introns, identical to all other iLBP genes in vertebrates. Two transcription start sites were identified in the rbp1 promoter and a single transcription start site was identified for rbp2. Radiation hybrid mapping assigned the zebrafish rbp1 gene to linkage group 16 and conserved syntenic genes were found by comparative analysis of mammalian orthologous rbp1 genes. RT-PCR detected mRNA transcripts in the adult intestine, liver, brain, ovary and testis for rbp1 gene and in the intestine and liver for rbp2 gene. Whole mount in situ hybridization of zebrafish embryos revealed rbp1 mRNA expression in the developing zebrafish central nervous system at specific sites that are known to have abundant retinoic acid distribution and significant retinoic acid action. Whole mount in situ hybridization also showed that the zebrafish rbp2 mRNA was localized specifically in the embryonic intestinal bulb and the developing intestine during the larval stage, implying a novel function for the rbp2 gene product during organogenesis and development of the zebrafish intestine.

Function of RARgamma and RARalpha2 at the initiation of retinoid signaling is essential for avian embryo survival and for distinct events in cardiac morphogenesis

Avian embryogenesis requires retinoid receptor activation by the vitamin A active form, retinoic acid (RA), during neurulation. We conducted loss-of-function analysis in quail embryos by nutritional deprivation of RA and by blocking generation of retinoid receptors. Here we identify a distinct role for RARalpha2 in cardiac inflow tract morphogenesis and for RARgamma in cardiac left/right orientation and looping morphogenesis. Blocking normal embryos with antisense oligonucleotides to RARalpha2 or RXRalpha diminishes GATA-4 transcripts, while blocking RARgamma or RXRalpha diminishes nodal and Pitx2 transcripts; the expression of these genes in the heart forming region resembles that of the vitamin A-deficient embryo. Blocking the function of RARgamma, RARalpha2, and RXRalpha recapitulates the complete vitamin A-deficient phenotype. RARgamma is the most potent mediator of the retinoid signal at this time of development. Our studies provide strong evidence that critical RA-requiring developmental events in the early avian embryo are regulated by means of distinct retinoid receptor signaling pathways.

Expression of the retinoic acid catabolising enzyme CYP26B1 in the chick embryo and its regulation by retinoic acid

We have cloned a fragment of Cyp26B1, a novel retinoic acid (RA) catabolising enzyme, and examined its expression pattern during early stages of chick embryogenesis. It is expressed from stage 7 in the tail bud, an anterior patch of mesenchyme, the heart, the endothelium of the vasculature, the eye, the limb bud, the hindgut and in a complex pattern in the rhombomeres of the hindbrain. As such it has a non-overlapping expression with chick Cyp26A1, the other RA catabolising enzyme, but shows a combination of features of mouse Cyp26A1 and Cyp26B1. We have also examined its expression in the quail embryo and in the RA-free quail embryo. In the absence of RA, Cyp26B1 is only expressed in the hindbrain and fails to be expressed in all the other regions of the embryo, most dramatically in the trunk. Adding back RA rescues Cyp26B1 expression.

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.

Retinoic acid signalling centres in the avian embryo identified by sites of expression of synthesising and catabolising enzymes

Retinoic acid is an important signalling molecule in the developing embryo, but its precise distribution throughout development is very difficult to determine by available techniques. Examining the distribution of the enzymes by which it is synthesised by using in situ hybridisation is an alternative strategy. Here, we describe the distribution of three retinoic acid synthesising enzymes and one retinoic acid catabolic enzyme during the early stages of chick embryogenesis with the intention of identifying localized retinoic acid signalling regions. The enzymes involved are Raldh1, Raldh2, Raldh3, and Cyp26A1. Although some of these distributions have been described before, here we assemble them all in one species and several novel sites of enzyme expression are identified, including Hensen's node, the cardiac endoderm, the presumptive pancreatic endoderm, and the dorsal lens. This study emphasizes the dynamic pattern of expression of the enzymes that control the availability of retinoic acid as well as the role that retinoic acid plays in the development of many regions of the embryo throughout embryogenesis. This strategy provides a basis for understanding the phenotypes of retinoic acid teratology and retinoic acid-deficiency syndromes.

The roles of Hedgehogs and Fibroblast Growth Factors in eye development and retinal cell rescue

Knowledge of normal eye development is crucial for the development of retinal rescue strategies. I shall focus on two signalling pathways that affect retinal development. Fibroblast growth factors function in retinal cell proliferation, retinal ganglion cell axon guidance and target recognition, craniofacial patterning and lens induction. Hedgehog proteins are required for progression of the neurogenic wave, cell proliferation, photoreceptor differentiation, retinal ganglion cell axon growth and craniofacial patterning. These signalling pathways have pleiotropic effects, can interact and have the potential to be used therapeutically. The zebrafish model organism may be well suited to studying how signalling pathways interact.

The chick oligozeugodactyly (ozd) mutant lacks sonic hedgehog function in the limb

We have analyzed a new limb mutant in the chicken that we name oligozeugodactyly (ozd). The limbs of this mutant have a longitudinal postaxial defect, lacking the posterior element in the zeugopod (ulna/fibula) and all digits except digit 1 in the leg. Classical recombination experiments show that the limb mesoderm is the defective tissue layer in ozd limb buds. Molecular analysis revealed that the ozd limbs develop in the absence of Shh expression, while all other organs express Shh and develop normally. Neither Ptc1 nor Gli1 are detectable in mutant limb buds. However, Bmp2 and dHAND are expressed in the posterior wing and leg bud mesoderm, although at lower levels than in normal embryos. Activation of Hoxd11-13 occurs normally in ozd limbs but progressively declines with time. Phase III of expression is more affected than phase II, and expression is more severely affected in the more 5' genes. Interestingly, re-expression of Hoxd13 occurs at late stages in the distal mesoderm of ozd leg buds, correlating with formation of digit 1. Fgf8 and Fgf4 expression are initiated normally in the mutant AER but their expression is progressively downregulated in the anterior AER. Recombinant Shh protein or ZPA grafts restore normal pattern to ozd limbs; however, retinoic acid fails to induce Shh in ozd limb mesoderm. We conclude that Shh function is required for limb development distal to the elbow/knee joints, similar to the Shh(-/-) mouse. Accordingly we classify the limb skeletal elements as Shh dependent or independent, with the ulna/fibula and digits other than digit 1 in the leg being Shh dependent. Finally we propose that the ozd mutation is most likely a defect in a regulatory element that controls limb-specific expression of Shh.

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 Tbx3 expression by anteroposterior signalling in vertebrate limb development

Tbx3, a T-box gene family member related to the Drosophila gene optomotor blind (omb) and encoding a transcription factor, is expressed in anterior and posterior stripes in developing chick limb buds. Tbx3 haploinsufficiency has been linked with the human condition ulnar-mammary syndrome, in which predominantly posterior defects occur in the upper limb. Omb is expressed in Drosophila wing development in response to a signalling cascade involving Hedgehog and Dpp. Homologous vertebrate signals Sonic hedgehog (Shh) and bone morphogenetic protein 2 (Bmp2) are associated in chick limbs with signalling of the polarising region which controls anteroposterior pattern. Here we carried out tissue transplantations, grafted beads soaked in Shh, Bmps, and Noggin in chick limb buds, and analysed Tbx3 expression. We also investigated Tbx3 expression in limb buds of chicken and mouse mutants and retinoid-deficient quail in which anteroposterior patterning is abnormal. We show that Tbx3 expression in anterior and posterior stripes is regulated differently. Posterior Tbx3 expression is stable and depends on the signalling cascade centred on the polarising region involving Shh and Bmps, while anterior Tbx3 expression is labile and depends on the balance between positive Bmp signals, produced anteriorly, and negative Shh signals, produced posteriorly. Our results are consistent with the idea that posterior Tbx3 expression is involved in specifying digit pattern and thus provides an explanation for the posterior defects in human patients. Anterior Tbx3 expression appears to be related to the width of limb bud, which determines digit number.

Embryonic retinoic acid synthesis is required for forelimb growth and anteroposterior patterning in the mouse

Numerous studies, often performed on avian embryos, have implicated retinoic acid (RA) in the control of limb bud growth and patterning. Here we have investigated whether the lack of endogenous RA synthesis affects limb morphogenesis in mutant mouse embryos deficient for the retinaldehyde dehydrogenase 2 (Raldh2/Aldh1a2). These mutants, which have no detectable embryonic RA except in the developing retina, die at E9.5-E10 without any evidence of limb bud formation, but maternal RA supplementation through oral gavage from E7.5 can extend their survival. Such survivors exhibit highly reduced forelimb rudiments, but apparently normal hindlimbs. By providing RA within maternal food, we found both a stage- and dose-dependency for rescue of forelimb growth and patterning. Following RA supplementation from E7.5 to 8.5, mutant forelimbs are markedly hypoplastic and lack anteroposterior (AP) patterning, with a single medial cartilage and 1-2 digit rudiments. RA provided until E9.5 significantly rescues forelimb growth, but cannot restore normal AP patterning. Increasing the RA dose rescues the hypodactyly, but leads to lack of asymmetry of the digit pattern, with abnormally long first digit or symmetrical polydactyly. Mutant forelimb buds are characterized by lack of expression or abnormal distal distribution of Sonic hedgehog (Shh) transcripts, sometimes with highest expression anteriorly. Downregulation or ectopic anterior expression of Fgf4 is also seen. As a result, genes such as Bmp2 or Hoxd genes are expressed symmetrically along the AP axis of the forelimb buds, and/or later, of the autopod. We suggest that RA signaling cooperates with a posteriorly restricted factor such as dHand, to generate a functional zone of polarizing activity (ZPA).

Novel retinoic acid generating activities in the neural tube and heart identified by conditional rescue of Raldh2 null mutant mice

Retinoid control of vertebrate development depends upon tissue-specific metabolism of retinol to retinoic acid (RA). The RA biosynthetic enzyme RALDH2 catalyzes much, but not all, RA production in mouse embryos, as revealed here with Raldh2 null mutants carrying an RA-responsive transgene. Targeted disruption of Raldh2 arrests development at midgestation and eliminates all RA synthesis except that associated with Raldh3 expression in the surface ectoderm of the eye field. Conditional rescue of Raldh2–/– embryos by limited maternal RA administration allows development to proceed and results in the establishment of additional sites of RA synthesis linked to Raldh1 expression in the dorsal retina and to Raldh3 expression in the ventral retina, olfactory pit and urinary tract. Unexpectedly, conditionally rescued Raldh2–/– embryos also possess novel sites of RA synthesis in the neural tube and heart that do not correspond to expression of Raldh1-3. RA synthesis in the mutant neural tube was localized in the spinal cord, posterior hindbrain and portions of the midbrain and forebrain, whereas activity in the mutant heart was localized in the conotruncus and sinus venosa. In the posterior hindbrain, this novel RA-generating activity was expressed during establishment of rhombomeric boundaries. In the spinal cord, the novel activity was localized in the floorplate plus in the intermediate region where retinoid-dependent interneurons develop. These novel RA-generating activities in the neural tube and heart fill gaps in our knowledge of how RA is generated spatiotemporally and may, along with Raldh1 and Raldh3, contribute to rescue of Raldh2–/– embryos by producing RA locally.

Patterning mechanisms controlling vertebrate limb development

Vertebrate limb buds are embryonic structures for which much molecular and cellular data are known regarding the mechanisms that control pattern formation during development. Specialized regions of the developing limb bud, such as the zone of polarizing activity (ZPA), the apical ectodermal ridge (AER), and the non-ridge ectoderm, direct and coordinate the development of the limb bud along the anterior-posterior (AP), dorsal-ventral (DV), and proximal-distal (PD) axes, giving rise to a stereotyped pattern of elements well conserved among tetrapods. In recent years, specific gene functions have been shown to mediate the organizing and patterning activities of the ZPA, the AER, and the non-ridge ectoderm. The analysis of these gene functions has revealed the existence of complex interactions between signaling pathways operated by secreted factors of the HH, TGF-beta/BMP, WNT, and FGF superfamilies, which interact with many other genetic networks to control limb positioning, outgrowth, and patterning. The study of limb development has helped to establish paradigms for the analysis of pattern formation in many other embryonic structures and organs.

Leaping lopsided: a review of the current hypotheses regarding etiologies of limb malformations in frogs

Recent progress in the investigation of limb malformations in free-living frogs has underlined the wide range in the types of limb malformations and the apparent spatiotemporal clustering of their occurrence. Here, we review the current understanding of normal and abnormal vertebrate limb development and regeneration and discuss some of the molecular events that may bring about limb malformation. Consideration of the differences between limb development and regeneration in amphibians has led us to the hypothesis that some of the observed limb malformations come about through misdirected regeneration. We report the results of a pilot study that supports this hypothesis. In this study, the distal aspect of the right hindlimb buds of X. laevis tadpoles was amputated at the pre-foot paddle stage. The tadpoles were raised in water from a pond in Minnesota at which 7% of surveyed newly metamorphosed feral frogs had malformations. Six percent (6 of 100) of the right limbs of the tadpoles raised in pond water developed abnormally. One truncated right limb was the only malformation in the control group, which was raised in dechlorinated municipal water. All unamputated limbs developed normally in both groups. Three major factors under consideration for effecting the limb malformations are discussed. These factors include environmental chemicals (primarily agrichemicals), encysted larvae (metacercariae) of trematode parasites, and increased levels of ultraviolet light. Emphasis is placed on the necessary intersection of environmental stressors and developmental events to bring about the specific malformations that are observed in free-living frog populations.

Expression and regulation of the retinoic acid synthetic enzyme RALDH-2 in the embryonic chicken wing

Retinaldehyde dehydrogenase type 2 (RALDH-2) is a major retinoic acid (RA) generating enzyme in the embryo. Here, we report immunolocalization of this enzyme (RALDH-2-IR) in the developing wings of stage 17-30 chicken embryos. RALDH-2-IR is located in the area of the presumptive muscle masses, although it is not colocalized with developing muscle cells. RALDH-2-IR is located in tendon precursor cells and may be present in muscular connective tissue. We show that motor neurons and blood vessels, tissues showing RALDH-2-IR as they enter the limb, are capable of synthesizing and releasing RA in culture. RALDH-2-IR in the limb mesenchyme is under the control of both the vasculature and the motor innervation; it is decreased with denervation and increased with hypervascularization. RALDH-2-IR is present in the motor neuron pool of the brachial spinal cord, but this expression pattern is apparently not under the control of limb target tissues, RA in the periphery, or somitic factors. RA is known to be a potent inducer of cellular differentiation; we propose that locally synthesized RA may be involved in aspects of wing tissue specification, including cartilage condensation and outgrowth, skeletal muscle differentiation, and recruitment of smooth muscle cells to the vasculature.

Local retinoid signaling coordinates forebrain and facial morphogenesis by maintaining FGF8 and SHH

Correlations between facial anomalies and brain defects are well characterized throughout the clinical literature, yet a developmental basis for this association has not been identified. We demonstrate that the frontonasal process, which gives rise to the mid- and upper face, and the forebrain are linked early in their morphogenesis by a local retinoid signaling event that maintains the expression of key regulatory molecules. First, we show that aldehyde dehydrogenase 6, which synthesizes the ligand, retinoic acid, is localized to the ventral epithelium of the presumptive frontonasal process of chick embryos. At least two retinoid receptors are expressed in adjacent populations of mesenchyme. Second, using synthetic pan-specific retinoid antagonists, we transiently inhibit the ability of retinoid receptors to bind retinoic acid in the rostral head and we generate embryos with a hypoplastic forebrain, fused eyes, and no frontonasal process-derived structures such as the upper beak. These defects are not due to eliminating mesenchymal progenitors, as neural crest cells still migrate into the frontonasal process, despite disruptions to retinoid signaling. Rather, these malformations result from loss of fibroblast growth factor 8 and sonic hedgehog expression, which leads to increased programmed cell death and decreased proliferation in the forebrain and frontonasal process. Most significantly, we can rescue the morphological defects by re-introducing retinoic acid, or fibroblast growth factor and sonic hedgehog proteins into antagonist-treated embryos. We propose that the local source of retinoic acid in the rostral head initiates a regulatory cascade that coordinates forebrain and frontonasal process morphogenesis.

Identification and characterization of Lbh, a novel conserved nuclear protein expressed during early limb and heart development

We report the cloning, protein characterization, and expression of a novel vertebrate gene, termed Lbh (Limb-bud-and-heart), with a spatiotemporal expression pattern that marks embryologically significant domains in the developing limbs and heart. Lbh encodes a highly conserved nuclear protein, which in tissue culture cells possesses a transcriptional activator function. During limb development, expression of Lbh initiates in the ectoderm of the presumptive limb territory in the lateral body wall. As the limb buds appear, Lbh expression is restricted primarily to the distal ventral limb ectoderm and the apical ectodermal ridge, and overlaps in these ectodermal compartments with En1 and Fgf8 expression. During heart formation, Lbh is expressed as early as Nkx2.5 and dHand in the bilateral heart primordia, with the highest levels in the anterior promyocardium. After heart tube fusion and looping, Lbh expression is confined to the ventricular myocardium, with the highest intensity in the right ventricle and atrioventricular canal, as well as in the sinus venosus. Based on the molecular characteristics and the domain-specific expression pattern, it is possible that Lbh functions in synergy with other genes known to be required for heart and limb development.

Function of vitamin A in vertebrate embryonic development

Advances in molecular biology and retinoic acid receptor research have significantly contributed to the understanding of the role of vitamin A during vertebrate development. Examination of the function of this vitamin during very early developmental stages using the completely vitamin A-depleted avian embryo has revealed that the vitamin A requirement begins at the time of formation of the primitive heart, circulation and specification of hindbrain. The lack of vitamin A at this critical time results in gross abnormalities and early embryonic death. In rodent models, vitamin A deficiency can be targeted to later gestational windows and documents the need for vitamin A for more advanced stages of development. Major target tissues of vitamin A deficiency include the heart, central nervous system and structures derived from it, the circulatory, urogenital and respiratory systems, and the development of skull, skeleton and limbs. These abnormalities are also evident in mice mutants from retinoid receptor knockouts; they have revealed both morphological and molecular aspects of vitamin A function during development. Retinoic acid receptors (RAR) in partnership with retinoid X receptor (RXR)alpha appear to be the important retinoid receptor transcription factors regulating vitamin A function at the gene level during development via the physiologic ligand all-trans-retinoic acid. Homeostasis of retinoic acid is maintained by developmentally regulated vitamin A metabolism enzyme systems. Inadequate vitamin A nutrition during early pregnancy may account for some pediatric congenital abnormalities.

Retinoid signalling and hindbrain patterning

Retinoid signalling has been implicated in regulating a wide variety of processes in vertebrate development. Recent advances from analyses on the synthesis, degradation and distribution of retinoids in combination with functional analysis of signalling components have provided important insights into the regulation of patterning the nervous system and the hindbrain in particular.

Retinoids in embryonal development

The key role of vitamin A in embryonal development is reviewed. Special emphasis is given to the physiological action of retinoids, as evident from the retinoid ligand knockout models. Retinoid metabolism in embryonic tissues and teratogenic consequences of retinoid administration at high doses are presented. Physiological and pharmacological actions of retinoids are outlined and explained on the basis of their interactions as ligands of the nuclear retinoid receptors. Immediate target genes and the retinoid response elements of their promoters are summarized. The fundamental role of homeobox genes in embryonal development and the actions of retinoids on their expression are discussed. The similarity of the effects of retinoid ligand knockouts to effects of compound retinoid receptor knockouts on embryogenesis is presented. Although much remains to be clarified, the emerging landscape offers exciting views for future research.

Retinoic acid is essential for Shh/Hoxd signaling during rat limb outgrowth but not for limb initiation

Retinoids long have been implicated in limb development and their endogenous contributions to this process are finally being elucidated. Here we use an established model of retinoid depletion during specific gestational windows to investigate the role of endogenous retinoic acid (RA) in supporting limb outgrowth. Rat embryos were deprived of RA starting at days-postcoitum (dpc) 3.0, 5.5, or 7.0 and harvested at the 35-somite stage (dpc 12-12.5). Although embryos from all these windows possessed many characteristics of gestational retinoid deficiency (frontonasal hypoplasia, straight tail, reduced CRBPI and RAR beta), their limb buds emerged with only modest size reductions. Molecular analysis of RA-deficient limb buds revealed enhanced gli-3 and reduced hoxd-12, hoxd-13, shh, and fgf-4, while fgf-8, en-1, and wnt-7a expression remained unaltered. Occasional posterior truncations were observed at low incidence in the longest deficiency window; otherwise, the deficiency window length had no discernable impact on the severity of these changes. At the 45-somite stage, RA-deficient limbs had additional losses of hoxd-13 and fgf-8, accompanied by a flattened AER, suggestive of an ultimate failure in limb bud outgrowth. Results could not confirm a function for endogenous retinoids in limb initiation, but show they are required to maintain the signaling loops between the developing mesenchyme and AER that govern limb outgrowth after the initial emergence of limb bud.

Citral, an inhibitor of retinoic acid synthesis, modifies pattern formation during limb regeneration in the axolotl Ambystoma mexicanum

While the effects of exogenous retinoids on amphibian limb regeneration have been studied extensively, the role of endogenous retinoids is not clear. Hence, I wished to investigate the role of endogenous retinoic acid during axolotl limb regeneration. Citral is a known inhibitor of retinoic acid synthesis. Thus, I treated regenerating limbs of the larval axolotl Ambystoma mexicanum with citral. The result of this inhibition of retinoic acid synthesis was that limb regeneration became extremely irregular and hypomorphic, with serious pattern defects, or was inhibited altogether. I conclude that endogenous retinoic acid plays an important role in pattern formation during limb regeneration.

Heads or tails? Retinoic acid will decide

A recent study (Niederreither et al. Nat Genet 1999;21:444-448 [Ref. 1]) describes the phenotype of a gene knockout for an enzyme, retinaldehyde dehydrogenase 2 (RALDH-2), that synthesizes retinoic acid (RA) in the early embryo. The effects generated by this single enzyme mutation are remarkably similar to those previously described in vitamin A-deprivation studies and compound retinoic acid receptor knockouts, which involve multiple systems of the embryo. With other data on the distribution of RA, its role in axial specification of the early embryo is considerably clarified. Surprisingly, it seems that head development is unaffected in these RALDH-2 knockout embryos; thus, the anterior of the embryo does not require RA, despite the observations that the hindbrain seems exquisitely sensitive to RA perturbation. Head development may be realised by a cytochrome P450 enzyme (CYP26), which has been described recently. Between these two opposing forces, the hindbrain develops.