Retinoic acid is required in the mouse embryo for left-right asymmetry determination and heart morphogenesis

Hyperglycaemia induces diet-dependent defects of the left-right axis by lowering intracellular pH

Pregestational diabetes is a risk factor for congenital anomalies, including heterotaxy syndrome, a rare birth defect characterized by the abnormal arrangement of organs relative to the left-right (L-R) body axis. To provide insight into the underlying mechanism by which diabetes induces heterotaxy, we here analyzed the L-R axis of mouse embryos of diabetic dams. Various Pitx2 expression patterns indicative of disruption of L-R axis formation were apparent in such embryos. Expression of Nodal at the node, which triggers a Nodal-Pitx2 expression cascade in lateral plate mesoderm, showed marked regression associated with L-R axis malformation. This regression was similar to that apparent in Wnt3a-/- embryos, and canonical Wnt signalling was indeed found to be downregulated in embryos of diabetic dams. RNA sequencing revealed dysregulation of glycolysis in embryos of diabetic dams, and high glucose lowered intracellular pH in the primitive streak, leading to the suppression of Wnt signalling and the regression of Nodal expression. Of note, maternal vitamin A intake increased the incidence of L-R axis defects in embryos of diabetic dams, with dysregulation of retinoic acid metabolism being apparent in these embryos and in Wnt3a-/- embryos. Our results shed light on the mechanisms underlying embryopathies associated with maternal diabetes and suggest the importance of diet for prevention of heterotaxy.

Transcriptional and epigenetic characterization of a new in vitro platform to model the formation of human pharyngeal endoderm

BACKGROUND

The Pharyngeal Endoderm (PE) is an extremely relevant developmental tissue, serving as the progenitor for the esophagus, parathyroids, thyroids, lungs, and thymus. While several studies have highlighted the importance of PE cells, a detailed transcriptional and epigenetic characterization of this important developmental stage is still missing, especially in humans, due to technical and ethical constraints pertaining to its early formation.

RESULTS

Here we fill this knowledge gap by developing an in vitro protocol for the derivation of PE-like cells from human Embryonic Stem Cells (hESCs) and by providing an integrated multi-omics characterization. Our PE-like cells robustly express PE markers and are transcriptionally homogenous and similar to in vivo mouse PE cells. In addition, we define their epigenetic landscape and dynamic changes in response to Retinoic Acid by combining ATAC-Seq and ChIP-Seq of histone modifications. The integration of multiple high-throughput datasets leads to the identification of new putative regulatory regions and to the inference of a Retinoic Acid-centered transcription factor network orchestrating the development of PE-like cells.

CONCLUSIONS

By combining hESCs differentiation with computational genomics, our work reveals the epigenetic dynamics that occur during human PE differentiation, providing a solid resource and foundation for research focused on the development of PE derivatives and the modeling of their developmental defects in genetic syndromes.

Understanding the Genetic and Non-genetic Interconnections in the Aetiology of Isolated Congenital Heart Disease: An Updated Review: Part 1

PURPOSE OF REVIEW

Congenital heart disease (CHD) is the most frequently occurring birth defect. Majority of the earlier reviews focussed on the association of genetic factors with CHD. A few epidemiological studies provide convincing evidence for environmental factors in the causation of CHD. Although the multifactorial theory of gene-environment interaction is the prevailing explanation, explicit understanding of the biological mechanism(s) involved, remains obscure. Nonetheless, integration of all the information into one platform would enable us to better understand the collective risk implicated in CHD development.

RECENT FINDINGS

Great strides in novel genomic technologies namely, massive parallel sequencing, whole exome sequencing, multiomics studies supported by system-biology have greatly improved our understanding of the aetiology of CHD. Molecular genetic studies reveal that cardiac specific gene variants in transcription factors or signalling molecules, or structural proteins could cause CHD. Additionally, non-hereditary contributors such as exposure to teratogens, maternal nutrition, parental age and lifestyle factors also contribute to induce CHD. Moreover, DNA methylation and non-coding RNA are also correlated with CHD. Here, we inform that a complex combination of genetic, environmental and epigenetic factors interact to interfere with morphogenetic processes of cardiac development leading to CHD. It is important, not only to identify individual genetic and non-inherited risk factors but also to recognize which factors interact mutually, causing cardiac defects.

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

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

Deciphering the role of retinoic acid in hepatic patterning and induction in the mouse

Retinoic acid (RA), a metabolite of vitamin A, is a small molecule and morphogen that is required for embryonic development. While normal RA signals are required for hepatic development in a variety of vertebrates, a role for RA during mammalian hepatic specification has yet to be defined. To examine the requirement for RA in murine liver induction, we performed whole embryo culture with the small molecule RA inhibitor, BMS493, to attenuate RA signaling immediately prior to hepatic induction and through liver bud formation. BMS493 treated embryos demonstrated a significant loss of hepatic specification that was confined to the prospective dorsal anterior liver bud. Examination of RA attenuated embryos demonstrates that while the liver bud displays normal expression of foregut endoderm markers and the hepato-pancreatobiliary domain marker, PROX1, the dorsal/anterior liver bud excludes the critical hepatic marker, HNF4α, indicating that RA signals are required for dorsal/anterior hepatic induction. These results were confirmed and extended by careful examination of Rdh10^trex/trex embryos, which carry a genetic perturbation in RA synthesis. At E9.5 Rdh10^trex/trex embryos display a similar yet more significant loss of the anterior/dorsal liver bud. Notably the anterior/dorsal liver bud loss correlates with the known dorsal-ventral gradient of the RA synthesis enzyme, Aldh1a2. In addition to altered hepatic specification, the mesoderm surrounding the liver bud is disorganized in RA abrogated embryos. Analysis of E10.5 Rdh10^trex/trex embryos reveals small livers that appear to lack the dorsal/caudal lobes. Finally, addition of exogenous RA prior to hepatic induction results in a liver bud that has failed to thicken and is largely unspecified. Taken together our ex vivo and in vivo evidence demonstrate that the generation of normal RA gradients is required for hepatic patterning, specification, and growth.

Cardiac Development: A Glimpse on Its Translational Contributions

Cardiac development is a complex developmental process that is initiated soon after gastrulation, as two sets of precardiac mesodermal precursors are symmetrically located and subsequently fused at the embryonic midline forming the cardiac straight tube. Thereafter, the cardiac straight tube invariably bends to the right, configuring the first sign of morphological left–right asymmetry and soon thereafter the atrial and ventricular chambers are formed, expanded and progressively septated. As a consequence of all these morphogenetic processes, the fetal heart acquired a four-chambered structure having distinct inlet and outlet connections and a specialized conduction system capable of directing the electrical impulse within the fully formed heart. Over the last decades, our understanding of the morphogenetic, cellular, and molecular pathways involved in cardiac development has exponentially grown. Multiples aspects of the initial discoveries during heart formation has served as guiding tools to understand the etiology of cardiac congenital anomalies and adult cardiac pathology, as well as to enlighten novels approaches to heal the damaged heart. In this review we provide an overview of the complex cellular and molecular pathways driving heart morphogenesis and how those discoveries have provided new roads into the genetic, clinical and therapeutic management of the diseased hearts.

Retinoic acid signaling in heart development

Retinoic acid (RA) is a vitamin A metabolite that acts as a morphogen and teratogen. Excess or defective RA signaling causes developmental defects including in the heart. The heart develops from the anterior lateral plate mesoderm. Cardiogenesis involves successive steps, including formation of the primitive heart tube, cardiac looping, septation, chamber development, coronary vascularization, and completion of the four-chambered heart. RA is dispensable for primitive heart tube formation. Before looping, RA is required to define the anterior/posterior boundaries of the heart-forming mesoderm as well as to form the atrium and sinus venosus. In outflow tract elongation and septation, RA signaling is required to maintain/differentiate cardiogenic progenitors in the second heart field at the posterior pharyngeal arches level. Epicardium-secreted insulin-like growth factor, the expression of which is regulated by hepatic mesoderm-derived erythropoietin under the control of RA, promotes myocardial proliferation of the ventricular wall. Epicardium-derived RA induces the expression of angiogenic factors in the myocardium to form the coronary vasculature. In cardiogenic events at different stages, properly controlled RA signaling is required to establish the functional heart.

Left-right asymmetry in heart development and disease: forming the right loop

Extensive studies have shown how bilateral symmetry of the vertebrate embryo is broken during early development, resulting in a molecular left-right bias in the mesoderm. However, how this early asymmetry drives the asymmetric morphogenesis of visceral organs remains poorly understood. The heart provides a striking model of left-right asymmetric morphogenesis, undergoing rightward looping to shape an initially linear heart tube and align cardiac chambers. Importantly, abnormal left-right patterning is associated with severe congenital heart defects, as exemplified in heterotaxy syndrome. Here, we compare the mechanisms underlying the rightward looping of the heart tube in fish, chick and mouse embryos. We propose that heart looping is not only a question of direction, but also one of fine-tuning shape. This is discussed in the context of evolutionary and clinical perspectives.

Transcription factor TLX1 controls retinoic acid signaling to ensure spleen development

The molecular mechanisms that underlie spleen development and congenital asplenia, a condition linked to increased risk of overwhelming infections, remain largely unknown. The transcription factor TLX1 controls cell fate specification and organ expansion during spleen development, and Tlx1 deletion causes asplenia in mice. Deregulation of TLX1 expression has recently been proposed in the pathogenesis of congenital asplenia in patients carrying mutations of the gene-encoding transcription factor SF-1. Herein, we have shown that TLX1-dependent regulation of retinoic acid (RA) metabolism is critical for spleen organogenesis. In a murine model, loss of Tlx1 during formation of the splenic anlage increased RA signaling by regulating several genes involved in RA metabolism. Uncontrolled RA activity resulted in premature differentiation of mesenchymal cells and reduced vasculogenesis of the splenic primordium. Pharmacological inhibition of RA signaling in Tlx1-deficient animals partially rescued the spleen defect. Finally, spleen growth was impaired in mice lacking either cytochrome P450 26B1 (Cyp26b1), which results in excess RA, or retinol dehydrogenase 10 (Rdh10), which results in RA deficiency. Together, these findings establish TLX1 as a critical regulator of RA metabolism and provide mechanistic insights into the molecular determinants of human congenital asplenia.

An in vitro gastrulation model recapitulates the morphogenetic impact of pharmacological inhibitors of developmental signaling pathways

Certain chemical agents act as teratogens, causing birth defects and fetal deaths when pregnant women are exposed to them. The establishment of in vitro models that recapitulate crucial embryonic events is therefore vital to facilitate screening of potential teratogens. Previously, we created a three-dimensional culture method for mouse P19C5 embryonal carcinoma stem cells that, when cultured as embryoid bodies, display elongation morphogenesis resembling gastrulation, which is the critical event resulting in the germ layers and major body axes. Determination of how well this in vitro morphogenesis represents in vivo gastrulation is essential to assess its applicability as well as to identify limitations of the model for detecting teratogenic agents. Here, we investigated the morphological and molecular characteristics of P19C5 morphogenesis using pharmacological agents that are known to cause abnormal patterning in the embryo in vivo by inhibiting major developmental signaling--e.g., involving Wnt, Nodal, Bone morphogenic protein (Bmp), Fibroblast growth factor (Fgf), Retinoic acid, Notch, and Hedgehog pathways. Inhibitors of Wnt, Nodal, Bmp, Fgf, and Retinoic acid signaling caused distinct changes in P19C5 morphogenesis that were quantifiable using morphometric parameters. These five inhibitors, plus the Notch inhibitor, also altered temporal expression profiles of developmental regulator genes in a manner consistent with the in vivo roles of the corresponding signaling pathways. In contrast, the Hedgehog inhibitor did not have any impact on the process, suggesting an absence of active Hedgehog signaling in these embryoid bodies. These results indicate that the P19C5 in vitro gastrulation model is a promising tool to screen for teratogenic agents that interfere with many of the key developmental signals.

Signaling through retinoic acid receptors in cardiac development: Doing the right things at the right times

Retinoic acid (RA) is a terpenoid that is synthesized from vitamin A/retinol (ROL) and binds to the nuclear receptors retinoic acid receptor (RAR)/retinoid X receptor (RXR) to control multiple developmental processes in vertebrates. The available clinical and experimental data provide uncontested evidence for the pleiotropic roles of RA signaling in development of multiple embryonic structures and organs such eyes, central nervous system, gonads, lungs and heart. The development of any of these above-mentioned embryonic organ systems can be effectively utilized to showcase the many strategies utilized by RA signaling. However, it is very likely that the strategies employed to transfer RA signals during cardiac development comprise the majority of the relevant and sophisticated ways through which retinoid signals can be conveyed in a complex biological system. Here, we provide the reader with arguments indicating that RA signaling is exquisitely regulated according to specific phases of cardiac development and that RA signaling itself is one of the major regulators of the timing of cardiac morphogenesis and differentiation. We will focus on the role of signaling by RA receptors (RARs) in early phases of heart development. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

Distinct mechanisms determine organ left-right asymmetry patterning in an uncoupled way

Disruption of Nodal in the lateral plate mesoderm (LPM) usually leads to left-right (LR) patterning defects in multiple organs. However, whether the LR patterning of organs is always regulated in a coupled way has largely not yet been elucidated. In addition, whether other crucial regulators exist in the LPM that coordinate with Nodal in regulating organ LR patterning is also undetermined. In this paper, after briefly summarizing the common process of LR patterning, the most puzzling question regarding the initiation of asymmetry is considered and the divergent mechanisms underlying the uncoupled LR patterning in different organs are discussed. On the basis of cases in which different organ LR patterning is determined in an uncoupled way via an independent mechanism or at a different time, we propose that there are other critical factors in the LPM that coordinate with Nodal to regulate heart LR asymmetry patterning during early LR patterning.

Retinoic acid is involved in the metamorphosis of the anal fin into an intromittent organ, the gonopodium, in the green swordtail (Xiphophorus hellerii)

In poeciliid fish the male anal fin has been transformed into a gonopodium, an intromittent organ required for internal fertilization. Elevated testosterone levels induce metamorphosis of a subset of anal fin rays to grow and form the specialized terminal structures of the gonopodium. The molecular mechanisms underlying these processes are largely unknown. Here, we investigated whether retinoic acid (RA) signaling is involved in gonopodium development in the swordtail Xiphophorus hellerii. We showed that aldh1a2, a RA synthesizing enzyme, and the RA receptors, rar-ga and rar-gb, are expressed in anal fins during metamorphosis. aldh1a2 expression is regulated by testosterone in a concentration-dependent manner and is up-regulated in both hormone-induced and naturally developing gonopodia. Androgen receptor (ar), a putative regulator of gonopodial development, is co-expressed with aldh1a2 and the RA receptors in gonopodial rays. Importantly, experimental increase of RA signaling promoted growth of the gonopodium and increased the number of new segments. Based on gene expression analyses and pharmacological manipulation of gonopodium development, we show that the RA signaling pathway is activated in response to androgen signaling and promotes fin ray growth and development during the metamorphosis of the anal fin into the gonopodium.

Cardiotoxicity of mycotoxin citrinin and involvement of microRNA-138 in zebrafish embryos

Citrinin (CTN) is a fungal secondary metabolite that contaminates various foodstuffs and animal feeds; it also exhibits organotoxicity in several animal models. In this study, the zebrafish was used to elucidate the mechanism of CTN cardiotoxicity in developing embryos. Following CTN administration, the gross morphology of the embryonic heart was apparently altered, including heart malformation, pericardial edema, and red blood accumulation. Whole-mount immunostaining and histological analysis of ventricle and atrium indicated incorrect heart looping and reduced size of heart chambers. From the perspective of cardiac function, the heartbeat and blood flow rate of embryos were significantly decreased in the presence of CTN. CTN also modulated the expression of tbx2a and jun B genes, but not that of bmp4 and nkx2.5. Furthermore, the heart areas of CTN-exposed embryos demonstrated an elevated levels of aldh1a2 and cspg2 messenger RNA; these 2 cardiac-related genes are known to be involved in retinoic acid (RA) pathway as well as downstream targets of microRNA-138 (miR-138) in zebrafish. CTN treatment also downregulated the expression of miR-138. Moreover, overexpression of miR-138 was able to rescue the heart defects generated by CTN. These results support the notion that CTN exposure has a severe impact on heart development, affecting heart morphogenesis through the dysregulation of miR-138, RA signaling, and tbx2a.

Common variants in left/right asymmetry genes and pathways are associated with relative hand skill

Humans display structural and functional asymmetries in brain organization, strikingly with respect to language and handedness. The molecular basis of these asymmetries is unknown. We report a genome-wide association study meta-analysis for a quantitative measure of relative hand skill in individuals with dyslexia [reading disability (RD)] (n = 728). The most strongly associated variant, rs7182874 (P = 8.68 × 10(-9)), is located in PCSK6, further supporting an association we previously reported. We also confirmed the specificity of this association in individuals with RD; the same locus was not associated with relative hand skill in a general population cohort (n = 2,666). As PCSK6 is known to regulate NODAL in the development of left/right (LR) asymmetry in mice, we developed a novel approach to GWAS pathway analysis, using gene-set enrichment to test for an over-representation of highly associated variants within the orthologs of genes whose disruption in mice yields LR asymmetry phenotypes. Four out of 15 LR asymmetry phenotypes showed an over-representation (FDR ≤ 5%). We replicated three of these phenotypes; situs inversus, heterotaxia, and double outlet right ventricle, in the general population cohort (FDR ≤ 5%). Our findings lead us to propose that handedness is a polygenic trait controlled in part by the molecular mechanisms that establish LR body asymmetry early in development.

Impaired cardiovascular function caused by different stressors elicits a common pathological and transcriptional response in zebrafish embryos

Zebrafish embryos have been widely used to study the genes and processes needed for normal vertebrate heart development. We recently observed that exposure to 2,3,7,8-tetra-chlorodibenzo-p-dioxin (TCDD) or retinoic acid (RA) produces very similar signs of heart failure in developing zebrafish via divergent molecular pathways. The fact that diverse stressors and mutations cause severe pericardial edema and circulatory collapse in developing zebrafish has been largely unexplored. We hypothesized that unrelated chemicals can trigger a common pathological response leading to the same end-stage heart failure. To test this hypothesis, we compared the effects of TCDD, RA, carbaryl, valproic acid, and morpholino oligonucleotide (MO) knockdown of TBX5 on the developing heart in zebrafish embryos. These model stressors have all been previously reported to affect zebrafish heart development, and elicited very similar signs of embryonic heart failure. Microarray analysis showed that one cluster of 92 transcripts affected by these different treatments was significantly downregulated by all treatments. This gene cluster is composed of transcripts required for chromosome assembly, DNA replication, and cell cycle progression. We refer to this cluster as the cell cycle gene cluster (CCGC). Immunohistochemistry revealed that downregulation of the CCGC precedes a halt in cardiomyocyte proliferation in the hearts of zebrafish exposed to any of the treatments. Previous work has shown that the initial response to TCDD is a decrease in cardiac output. Since this precedes the signs of edema, heart failure, and fall in CCGC expression, we postulated that any factor that decreases cardiac output will produce the same syndrome of heart failure responses. To test this, we used MO knockdown of cardiac troponin T2 (TNNT2) to specifically block contractility. The TNNT2-MO produced exactly the same signs of cardiotoxicity as the other treatments, including downregulation of the signature CCGC. Our results indicate that agents altering cardiac output can have amplified consequences during specific periods in development.

Developmental origins of a novel gut morphology in frogs

Phenotypic variation is a prerequisite for evolution by natural selection, yet the processes that give rise to the novel morphologies upon which selection acts are poorly understood. We employed a chemical genetic screen to identify developmental changes capable of generating ecologically relevant morphological variation as observed among extant species. Specifically, we assayed for exogenously applied small molecules capable of transforming the ancestral larval foregut of the herbivorous Xenopus laevis to resemble the derived larval foregut of the carnivorous Lepidobatrachus laevis. Appropriately, the small molecules that demonstrate this capacity modulate conserved morphogenetic pathways involved in gut development, including downregulation of retinoic acid (RA) signaling. Identical manipulation of RA signaling in a species that is more closely related to Lepidobatrachus, Ceratophrys cranwelli, yielded even more similar transformations, corroborating the relevance of RA signaling variation in interspecific morphological change. Finally, we were able to recover the ancestral gut phenotype in Lepidobatrachus by performing a reverse chemical manipulation to upregulate RA signaling, providing strong evidence that modifications to this specific pathway promoted the emergence of a lineage-specific phenotypic novelty. Interestingly, our screen also revealed pathways that have not yet been implicated in early gut morphogenesis, such as thyroid hormone signaling. In general, the chemical genetic screen may be a valuable tool for identifying developmental mechanisms that underlie ecologically and evolutionarily relevant phenotypic variation.

Rargb regulates organ laterality in a zebrafish model of right atrial isomerism

Developmental signals determine organ morphology and position during embryogenesis. To discover novel modifiers of liver development, we performed a chemical genetic screen in zebrafish and identified retinoic acid as a positive regulator of hepatogenesis. Knockdown of the four RA receptors revealed that all receptors affect liver formation, however specific receptors exert differential effects. Rargb knockdown results in bilateral livers but does not impact organ size, revealing a unique role for Rargb in conferring left-right positional information. Bilateral populations of hepatoblasts are detectable in rargb morphants, indicating Rargb acts during hepatic specification to position the liver, and primitive endoderm is competent to form liver on both sides. Hearts remain at the midline and gut looping is perturbed in rargb morphants, suggesting Rargb affects lateral plate mesoderm migration. Overexpression of Bmp during somitogenesis similarly results in bilateral livers and midline hearts, and inhibition of Bmp signaling rescues the rargb morphant phenotype, indicating Rargb functions upstream of Bmp to regulate organ sidedness. Loss of rargb causes biliary and organ laterality defects as well as asplenia, paralleling symptoms of the human condition right atrial isomerism. Our findings uncover a novel role for RA in regulating organ laterality and provide an animal model of one form of human heterotaxia.

Retinoic acid synthesis and functions in early embryonic development

Retinoic acid (RA) is a morphogen derived from retinol (vitamin A) that plays important roles in cell growth, differentiation, and organogenesis. The production of RA from retinol requires two consecutive enzymatic reactions catalyzed by different sets of dehydrogenases. The retinol is first oxidized into retinal, which is then oxidized into RA. The RA interacts with retinoic acid receptor (RAR) and retinoic acid X receptor (RXR) which then regulate the target gene expression. In this review, we have discussed the metabolism of RA and the important components of RA signaling pathway, and highlighted current understanding of the functions of RA during early embryonic development.

Echocardiographic evaluation of the single right ventricle in congenital heart disease: results of new techniques

Right ventricular (RV) function is increasingly recognized as having prognostic significance in various disease processes. The current gold standard for noninvasive measurement of RV function is cardiac magnetic resonance imaging; however, because of practical considerations, echocardiography remains the most often used modality for evaluating the RV. In the past, because of its complex morphology, echocardiographic assessment of the RV was usually qualitative in nature. Current advances in echocardiographic techniques have been able to overcome some of the previous limitations and thus quantification of RV function is increasingly being performed. In addition, recent echocardiographic guidelines for evaluating the RV have been published to aid in standardizing practice. The evaluation of RV function almost certainly has no greater importance than in the congenital heart population, especially in those patients that have a single RV acting as the systemic ventricle. As this complex population continues to increase in number, accurate and precise evaluation of RV function will be a major issue in determining clinical care.

Retinoic acid signaling sequentially controls visceral and heart laterality in zebrafish

During zebrafish development, the left-right (LR) asymmetric signals are first established around the Kupffer vesicle (KV), a ciliated organ generating directional fluid flow. Then, LR asymmetry is conveyed and stabilized in the lateral plate mesoderm. Although numerous molecules and signaling pathways are involved in controlling LR asymmetry, mechanistic difference and concordance between different organs during LR patterning are poorly understood. Here we show that RA signaling regulates laterality decisions at two stages in zebrafish. Before the 2-somite stage (2So), inhibition of RA signaling leads to randomized visceral laterality through bilateral expression of nodal/spaw in the lateral plate mesoderm, which is mediated by increases in cilia length and defective directional fluid flow in KV. Fgf8 is required for the regulation of cilia length by RA signaling. Blockage of RA signaling before 2So also leads to mild defects of heart laterality, which become much more severe through perturbation of cardiac bmp4 asymmetry when RA signaling is blocked after 2So. At this stage, visceral laterality and the left-sided Nodal remain unaffected. These findings suggest that RA signaling controls visceral laterality through the left-sided Nodal signal before 2So, and regulates heart laterality through cardiac bmp4 mainly after 2So, first identifying sequential control and concordance of visceral and heart laterality.

Vitamin A: a multifunctional tool for development

Extensive research carried out over the last 100 years has established that the fat-soluble organic compound vitamin A plays crucial roles in early development, organogenesis, cell proliferation, differentiation and apoptosis as well as in tissue homeostasis. Given its importance during development, the delivery of vitamin A to the embryo is very tightly regulated with perturbations leading to severe malformations. This review discusses the roles of vitamin A during human development and the molecular mechanisms controlling its biological effects, hence bridging the gap between human development and molecular genetic work carried out in animal models. Vitamin A delivery during pregnancy and its developmental teratology in humans are thus discussed alongside work on model organisms, such as chicken or mice, revealing the molecular layout and functions of vitamin A metabolism and signaling. We conclude that, during development, vitamin A-derived signals are very tightly controlled in time and space and that this complex regulation is achieved by elaborate autoregulatory loops and by sophisticated interactions with other signaling cascades.

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

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

Cooperative interaction of Nkx2.5 and Mef2c transcription factors during heart development

The interactions of diverse transcription factors mediate the molecular programs that regulate mammalian heart development. Among these, Nkx2.5 and the Mef2c regulate common downstream targets and exhibit striking phenotypic similarities when disrupted, suggesting a potential interaction during heart development. Co-immunoprecipitation and mammalian two-hybrid experiments revealed a direct molecular interaction between Nkx2.5 and Mef2c. Assessment of mRNA expression verified spatiotemporal cardiac coexpression. Finally, genetic interaction studies employing histological and molecular analyses showed that, although Nkx2.5(-/-) and Mef2c(-/-) individual mutants both have identifiable ventricles, Nkx2.5(-/-);Mef2c(-/-) double mutants do not, and that mutant cardiomyocytes express only atrial and second heart field markers. Molecular marker and cell death and proliferation analyses provide evidence that ventricular hypoplasia is the result of defective ventricular cell differentiation. Collectively, these data support a hypothesis where physical, functional, and genetic interactions between Nkx2.5 and Mef2c are necessary for ventricle formation.

The development and evolution of left-right asymmetry in invertebrates: lessons from Drosophila and snails

The unique nature of body handedness, which is distinct from the anteroposterior and dorsoventral polarities, has been attracting growing interest in diverse biological disciplines. Recent research progress on the left-right asymmetry of animal development has focused new attention on the mechanisms underlying the development and evolution of invertebrate handedness. This exploratory review of currently available information illuminates the prospective value of Drosophila and pulmonate snails for innovative new research aimed at elucidating these mechanisms. For example, findings in Drosophila and snails suggest that an actin filament-dependent mechanism may be evolutionarily conserved in protostomes. The polarity conservation of primary asymmetry across most metazoan phyla, which visceral handedness represents, indicates developmental constraint and purifying selection as possible but unexplored mechanisms. Comparative studies using Drosophila and snails, which have the great advantages of using genetic and evolutionary approaches, will accelerate our understanding of the mechanisms governing the conservation and diversity of animal handedness.

Retinoic acid and the heart

Retinoic acid (RA), the active derivative of vitamin A, by acting through retinoid receptors, is involved in signal transduction pathways regulating embryonic development, tissue homeostasis, and cellular differentiation and proliferation. RA is important for the development of the heart. The requirement of RA during early cardiovascular morphogenesis has been studied in targeted gene deletion of retinoic acid receptors and in the vitamin A-deficient avian embryo. The teratogenic effects of high doses of RA on cardiovascular morphogenesis have also been demonstrated in different animal models. Specific cardiovascular targets of retinoid action include effects on the specification of cardiovascular tissues during early development, anteroposterior patterning of the early heart, left/right decisions and cardiac situs, endocardial cushion formation, and in particular, the neural crest. In the postdevelopment period, RA has antigrowth activity in fully differentiated neonatal cardiomyocytes and cardiac fibroblasts. Recent studies have shown that RA has an important role in the cardiac remodeling process in rats with hypertension and following myocardial infarction. This chapter will focus on the role of RA in regulating cardiomyocyte growth and differentiation during embryonic and the postdevelopment period.

Identification of downstream genetic pathways of Tbx1 in the second heart field

Tbx1, a T-box transcription factor, and an important gene for velo-cardio-facial syndrome/DiGeorge syndrome (VCFS/DGS) in humans, causes outflow tract (OFT) heart defects when inactivated in the mouse. Tbx1 is expressed in the second heart field (SHF) and is required in this tissue for OFT development. To identify Tbx1 regulated genetic pathways in the SHF, we performed gene expression profiling of the caudal pharyngeal region in Tbx1(-/-) and wild type embryos. Isl1, a key marker for the SHF, as well as Hod and Nkx2-6, were downregulated in Tbx1(-/-) mutants, while genes required for cardiac morphogenesis, such as Raldh2, Gata4, and Tbx5, as well as a subset of muscle contractile genes, signifying myocardial differentiation, were ectopically expressed. Pan-mesodermal ablation of Tbx1 resulted in similar gene expression changes, suggesting cell-autonomous roles of Tbx1 in regulating these genes. Opposite expression changes concomitant with SHF-derived cardiac defects occurred in TBX1 gain-of-function mutants, indicating that appropriate levels of Tbx1 are required for heart development. When taken together, our studies show that Tbx1 acts upstream in a genetic network that positively regulates SHF cell proliferation and negatively regulates differentiation, cell-autonomously in the caudal pharyngeal region.

Left-right asymmetry in Drosophila

Seminal studies of left-right (L/R) patterning in vertebrate models have led to the discovery of roles for the nodal pathway, ion flows and cilia in this process. Although the molecular mechanisms underlying L/R asymmetries seen in protostomes are less well understood, recent work using Drosophila melanogaster as a novel genetic model system to study this process has identified a number of mutations affecting directional organ looping. The genetic analysis of this, the most evolutionary conserved feature of L/R patterning, revealed the existence of a L/R pathway that involves the actin cytoskeleton and an associated type I myosin. In this review, we describe this work in the context of Drosophila development, and discuss the implications of these results for our understanding of L/R patterning in general.

Rescue of morphogenetic defects and of retinoic acid signaling in retinaldehyde dehydrogenase 2 (Raldh2) mouse mutants by chimerism with wild-type cells

Retinoic acid (RA), the active vitamin A derivative, is an important developmental signaling molecule in vertebrates. In this study, we have assessed whether minimal numbers and/or specific distributions of RA-producing cells can support normal mouse embryonic development. Retinaldehyde dehydrogenase 2 (RALDH2) is the main RA-synthesizing enzyme acting during development. We have generated an embryonic stem (ES) cell line homozygous for an Raldh2 gene disruption, and have analyzed chimeric embryos with various contributions of wild-type cells. Whereas embryos almost completely derived from Raldh2(-/-) cells phenocopy the corresponding germline null mutants, the presence of even small numbers (<10%) of wild-type cells can rescue most of the morphogenetic defects, including embryonic turning and axial elongation, and left-right looping of the heart tube. No consistent bias in the distribution of wild-type cells was observed in the phenotypically rescued Raldh2(-/-) chimeras. Analysis of an RA-sensitive transgene indicates that RA can diffuse from wild-type cells and elicit a widespread transcriptional response in Raldh2-deficient cells. Our results show that few wild-type RA-producing cells, even when present in apparent random distributions, can support early morphogenesis of the mouse embryo. However, the Raldh2(-/-) chimeric fetuses display lung abnormalities, persistent truncus arteriosus, and abnormal myocardial differentiation, showing that subsequent RA-dependent events cannot be fully rescued by the mosaic presence of wild-type cells.

A multiple retinoic acid antagonist induces conotruncal anomalies, including transposition of the great arteries, in mice

BACKGROUND

The morphogenetic mechanisms that are responsible for the transposition of the great arteries are still largely unknown, mainly because this malformation is very difficult to experimentally reproduce. The aim of the present study was to test the effect of BMS-189453, a retinoic acid antagonist, on murine heart morphogenesis.

METHODS

We administered this drug at 5 mg/kg body weight (twice, at a 12-h interval) to pregnant mice on 6.25/6.75 days postcoitum (dpc) (Group A), 6.75/7.25 dpc (Group B), 7.25/7.75 dpc (Group C), 7.75/8.25 dpc (Group D), or 8.25/8.75 dpc (Group E). At birth, the anatomical features of fetuses were evaluated by stereomicroscopic examination.

RESULTS

In Group A (18 fetuses), cardiovascular anatomy was normal in 10 (56%) cases, and 8 (44%) fetuses presented with transposition of the great arteries. In Group B, no fetuses were obtained. In Group C (78 fetuses), cardiovascular anatomy was normal in 19 (24%) cases, while 59 (76%) mice presented with various types of cardiac defects, including 48 transpositions of the great arteries (61%). In Group D (80 fetuses), cardiac defects were seen in 22 (27%) mice: 14 of these (17%) were transpositions of the great arteries. In Group E (72 fetuses), cardiovascular anatomy was normal in all cases. Of 248 fetuses analyzed, 87% presented with thymic aplasia or hypoplasia, and 20% presented with meroanencephalia and/or rachischisis.

CONCLUSIONS

Transposition of the great arteries can be consistently reproduced in mice by administration of a retinoic acid competitive antagonist on 7.5 dpc.

Is the early left-right axis like a plant, a kidney, or a neuron? The integration of physiological signals in embryonic asymmetry

Embryonic morphogenesis occurs along three orthogonal axes. While the patterning of the anterior-posterior and dorsal-ventral axes has been increasingly well-characterized, the left-right (LR) axis has only relatively recently begun to be understood at the molecular level. The mechanisms that ensure invariant LR asymmetry of the heart, viscera, and brain involve fundamental aspects of cell biology, biophysics, and evolutionary biology, and are important not only for basic science but also for the biomedicine of a wide range of birth defects and human genetic syndromes. The LR axis links biomolecular chirality to embryonic development and ultimately to behavior and cognition, revealing feedback loops and conserved functional modules occurring as widely as plants and mammals. This review focuses on the unique and fascinating physiological aspects of LR patterning in a number of vertebrate and invertebrate species, discusses several profound mechanistic analogies between biological regulation in diverse systems (specifically proposing a nonciliary parallel between kidney cells and the LR axis based on subcellular regulation of ion transporter targeting), highlights the possible importance of early, highly-conserved intracellular events that are magnified to embryo-wide scales, and lays out the most important open questions about the function, evolutionary origin, and conservation of mechanisms underlying embryonic asymmetry.

Of minds and embryos: left-right asymmetry and the serotonergic controls of pre-neural morphogenesis

Serotonin is a clinically important neurotransmitter regulating diverse aspects of cognitive function, sleep, mood, and appetite. Increasingly, it is becoming appreciated that serotonin signaling among non-neuronal cells is a novel patterning mechanism existing throughout diverse phyla. Here, we review the evidence implicating serotonergic signaling in embryonic morphogenesis, including gastrulation, craniofacial and bone patterning, and the generation of left-right asymmetry. We propose two models suggesting movement of neurotransmitter molecules as a novel mechanism for how bioelectrical events may couple to downstream signaling cascades and gene activation networks. The discovery of serotonin-dependent patterning events occurring long before the development of the nervous system opens exciting new avenues for future research in evolutionary, developmental, and clinical biology.

Retinoic acid signaling is essential for formation of the heart tube in Xenopus

Retinoic acid is clearly important for the development of the heart. In this paper, we provide evidence that retinoic acid is essential for multiple aspects of cardiogenesis in Xenopus by examining embryos that have been exposed to retinoic acid receptor antagonists. Early in cardiogenesis, retinoic acid alters the expression of key genes in the lateral plate mesoderm including Nkx2.5 and HAND1, indicating that early patterning of the lateral plate mesoderm is, in part, controlled by retinoic acid. We found that, in Xenopus, the transition of the heart from a sheet of cells to a tube required retinoic acid signaling. The requirement for retinoic acid signaling was determined to take place during a narrow window of time between embryonic stages 14 and 18, well before heart tube closure. At the highest doses used, the lateral fields of myocardium fail to fuse, intermediate doses lead to a fusion of the two sides but failure to form a tube, and embryos exposed to lower concentrations of antagonist form a heart tube that failed to complete all the landmark changes that characterize looping. The myocardial phenotypes observed when exposed to the retinoic acid antagonist resemble the myocardium from earlier stages of cardiogenesis, although precocious expression of cardiac differentiation markers was not seen. The morphology of individual cells within the myocardium appeared immature, closely resembling the shape and size of cells at earlier stages of development. However, the failures in morphogenesis are not merely a slowing of development because, even when allowed to develop through stage 40, the heart tubes did not close when embryos were exposed to high levels of antagonist. Indeed, some aspects of left-right asymmetry also remained even in hearts that never formed a tube. These results demonstrate that components of the retinoic acid signaling pathway are necessary for the progression of cardiac morphogenesis in Xenopus.

Role for retinoid signaling in left–right asymmetric digestive organ morphogenesis

The looping events that establish left-right asymmetries in the vertebrate gut tube are poorly understood. Retinoic acid signaling is known to impact left-right development in multiple embryonic contexts, although its role in asymmetric digestive organ morphogenesis is unknown. Here, we show that the genes for retinaldehyde dehydrogenase (RALDH2) and a retinoic acid hydroxylase (CYP26A1) are expressed in complementary patterns in the Xenopus gut during looping. A late-stage chemical genetic assessment reveals that agonists and antagonists of retinoid signaling generate abnormal gut looping topologies, digestive organ heterotaxias, and intestinal malrotations. Accessory organ deformities commonly associated with intestinal malrotation in humans, such as annular pancreas, pancreas divisum, and extrahepatic biliary tree malformations, are also induced by distinct retinoid receptor agonists. Thus, late-stage retinoic acid signaling is likely to play a critical role in asymmetric gut tube morphogenesis and may underlie the etiology of several clinically relevant defects in the digestive system.

Retinoic acid signaling restricts the cardiac progenitor pool

Organogenesis begins with specification of a progenitor cell population, the size of which provides a foundation for the organ's final dimensions. Here, we present a new mechanism for regulating the number of progenitor cells by limiting their density within a competent region. We demonstrate that retinoic acid signaling restricts cardiac specification in the zebrafish embryo. Reduction of retinoic acid signaling causes formation of an excess of cardiomyocytes, via fate transformations that increase cardiac progenitor density within a multipotential zone. Thus, retinoic acid signaling creates a balance between cardiac and noncardiac identities, thereby refining the dimensions of the cardiac progenitor pool.

The evolutionary origin of cardiac chambers

Identification of cardiac mechanisms of retinoic acid (RA) signaling, description of homologous genetic circuits in Ciona intestinalis and consolidation of views on the secondary heart field have fundamental, but still unrecognized implications for vertebrate heart evolution. Utilizing concepts from evolution, development, zoology, and circulatory physiology, we evaluate the strengths of animal models and scenarios for the origin of vertebrate hearts. Analyzing chordates, lower and higher vertebrates, we propose a paradigm picturing vertebrate hearts as advanced circulatory pumps formed by segments, chambered or not, devoted to inflow or outflow. We suggest that chambers arose not as single units, but as components of a peristaltic pump divided by patterning events, contrasting with scenarios assuming that chambers developed one at a time. Recognizing RA signaling as a potential mechanism patterning cardiac segments, we propose to use it as a tool to scrutinize the phylogenetic origins of cardiac chambers within chordates. Finally, we integrate recent ideas on cardiac development such as the ballooning and secondary/anterior heart field paradigms, showing how inflow/outflow patterning may interact with developmental mechanisms suggested by these models.

Developmental mechanism and evolutionary origin of vertebrate left/right asymmetries

The systematically 'handed', or directionally asymmetrical way in which the major viscera are packed within the vertebrate body is known as situs. Other less obvious vertebrate lateralisations concern cognitive neural function, and include the human phenomena of hand-use preference and language-associated cognitive partitioning. An overview, rather than an exhaustive scholarly review, is given of recent advances in molecular understanding of the mechanism that ensures normal development of 'correct' situs. While the asymmetry itself and its left/right direction are clearly vertebrate-conserved characters, data available from various embryo types are compared in order to assess the likelihood that the developmental mechanism is evolutionarily conserved in its entirety. A conserved post-gastrular 'phylotypic' stage, with left- and right-specific cascades of key, orthologous gene expressions, clearly exists. It now seems probable that earlier steps, in which symmetry-breaking information is reliably transduced to trigger these cascades on the correct sides, are also conserved at depth although it remains unclear exactly how these steps operate. Earlier data indicated that the initiation of symmetry-breaking had been transformed, among the different vertebrate classes, as drastically as has the anatomy of pre-gastrular development itself, but it now seems more likely that this apparent diversity is deceptive. Ideas concerning the functional advantages to the vertebrate lifestyle of a systematically asymmetrical visceral packing arrangement, while untestable, are accepted because they form a plausible adaptationist 'just-so' story. Nevertheless, two contrasting beliefs are possible about the evolutionary origins of situs. Major recent advances in analysis of its developmental mechanism are largely due not to zoologists, comparative anatomists or evolutionary systematists, but to molecular geneticists, and these workers have generally assumed that the asymmetry is an evolutionary novelty imposed on a true bilateral symmetry, at or close to the origin of the vertebrate clade. A major purpose of this review is to advocate an alternative view, on the grounds of comparative anatomy and molecular systematics together with the comparative study of expressions of orthologous genes in different forms. This view is that situs represents a co-optation of a pre-existing, evolutionarily ancient non-bilaterality of the adult form in a vertebrate ancestor. Viewed this way, vertebrate or chordate origins are best understood as the novel imposition of an adaptively bilateral locomotory-skeletal-neural system, around a retained non-symmetrical 'visceral' animal. One component of neuro-anatomical asymmetry, the habenular/parapineal one that originates in the diencephalon, has recently been found (in teleosts) to be initiated from the same 'phylotypic' gene cascade that controls situs development. But the function of this particular diencephalic asymmetry is currently unclear. Other left-right partitionings of brain function, including the much more recently evolved, cerebral cortically located one associated with human language and hand-use, may be controlled entirely separately from situs even though their directionality has a particular relation to it in a majority of individuals. Finally, possible relationships are discussed between the vertebrate directional asymmetries and those that occur sporadically among protostome bilaterian forms. These may have very different evolutionary and molecular bases, such that there may have been constraints, in protostome evolution, upon any exploitation of left and right for complex organismic, and particularly cognitive neural function.

Teratogenicity of elevated egg incubation temperature and egg vitamin A status in Atlantic salmon, Salmo salar L

The present study was undertaken to investigate the possibility that high egg vitamin A (VA) status in combination with elevated egg incubation temperatures may cause deformities in Atlantic salmon, Salmo salar L. Egg batches selected for their total VA concentration were exposed to low (normal, 8 degrees C) or elevated (14 degrees C) egg incubation temperatures. Temperature was the main factor causing bone deformities such as warped gill opercula, fin and jaw deformities, but not for the development of spinal deformities where all groups displayed a 'baseline' occurrence of mild deformity (decreased vertebral size in the cephalic region) and no systematic variation in the occurrence of serious spinal deformities (fused vertebrae). A possible effect of egg incubation temperature fluctuation was found for the groups reared at low temperatures. An indication of a negative effect of elevated egg VA status for the development of organ deformities such as missing septum transversum and situs inversus was found in addition to temperature effects, however, no firm conclusions could be drawn from the present data. The phenotypes for temperature-induced deformities resembled the phenotype of VA-induced deformities, but no clear conclusions on the causality of the deformities found in the present study could be drawn. Egg incubation temperatures, both absolute temperature and temperature variations, should therefore be strictly controlled.

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.

Cardiac chamber formation: development, genes, and evolution

Concepts of cardiac development have greatly influenced the description of the formation of the four-chambered vertebrate heart. Traditionally, the embryonic tubular heart is considered to be a composite of serially arranged segments representing adult cardiac compartments. Conversion of such a serial arrangement into the parallel arrangement of the mammalian heart is difficult to understand. Logical integration of the development of the cardiac conduction system into the serial concept has remained puzzling as well. Therefore, the current description needed reconsideration, and we decided to evaluate the essentialities of cardiac design, its evolutionary and embryonic development, and the molecular pathways recruited to make the four-chambered mammalian heart. The three principal notions taken into consideration are as follows. 1) Both the ancestor chordate heart and the embryonic tubular heart of higher vertebrates consist of poorly developed and poorly coupled "pacemaker-like" cardiac muscle cells with the highest pacemaker activity at the venous pole, causing unidirectional peristaltic contraction waves. 2) From this heart tube, ventricular chambers differentiate ventrally and atrial chambers dorsally. The developing chambers display high proliferative activity and consist of structurally well-developed and well-coupled muscle cells with low pacemaker activity, which permits fast conduction of the impulse and efficacious contraction. The forming chambers remain flanked by slowly proliferating pacemaker-like myocardium that is temporally prevented from differentiating into chamber myocardium. 3) The trabecular myocardium proliferates slowly, consists of structurally poorly developed, but well-coupled, cells and contributes to the ventricular conduction system. The atrial and ventricular chambers of the formed heart are activated and interconnected by derivatives of embryonic myocardium. The topographical arrangement of the distinct cardiac muscle cells in the forming heart explains the embryonic electrocardiogram (ECG), does not require the invention of nodes, and allows a logical transition from a peristaltic tubular heart to a synchronously contracting four-chambered heart. This view on the development of cardiac design unfolds fascinating possibilities for future research.

Retinoids regulate survival and antigen presentation by immature dendritic cells

Maturation of dendritic cells (DCs) is a critical step for the induction of an immune response. We have examined the role of retinoid nuclear receptor pathways in this process. Retinoids induce DC apoptosis, in the absence of inflammatory signals, through retinoic acid receptor (RAR)alpha/retinoic X receptor (RXR) heterodimers. In contrast, via a cross talk with inflammatory cytokines, retinoids increase DNA binding activity of nuclear factor kappaB in DCs, trigger membrane major histocompatibility complex class II and costimulatory molecule expression, induce the differentiation of immature DCs into mature DCs, and enhance antigen-specific T cell response. This maturation of DCs is mediated via a RXR-dependent/RAR-independent pathway and via an RARalpha/RXR pathway distinct from the one responsible for apoptosis. Apoptosis and activation, mediated through distinct nuclear retinoid receptor pathways, can be dissociated from each other with selective synthetic retinoids. We identify a novel cellular function for retinoids and suggest that selective retinoids might be of interest for controlling antigen presentation.

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.

Chamber-specific differentiation of Nkx2.5-positive cardiac precursor cells from murine embryonic stem cells

Embryonic stem (ES) cells are a useful system to study cardiac differentiation in vitro. It has been difficult, however, to track the fates of chamber-specific cardiac lineages, since differentiation is induced within the embryoid body. We have established an in vitro culture system to track Nkx2.5(+) cell lineages during mouse ES cell differentiation by using green fluorescent protein (GFP) as a reporter. Nkx2.5/GFP(+) cardiomyocytes purified from embryoid bodies express sarcomeric tropomyosin and myosin heavy chain and heterogeneously express cardiac troponin I (cTnI), myosin light chain 2v (MLC2v) and atrial natriuretic peptide (ANP). After 4-week culture, GFP(+) cells exhibited electrophysiological characteristics specific to sinoatrial (SA) node, atrial, or ventricular type. Furthermore, we found that administration of 10(-7) M retinoic acid (RA) to embryoid bodies increased the percentage of MLC2v(-)ANP(+) cells; this also increased the expression of atrial-specific genes in the Nkx2.5/GFP(+) fraction, in a time- and dose-dependent fashion. These results suggest that Nkx2.5(+) lineage cells possess the potential to differentiate into various cardiomyocyte cell types and that RA can modify the differentiation potential of Nkx2.5(+) cardiomyocytes at an early stage.

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.

Patterning the vertebrate heart

The mammalian heart is crafted from a few progenitor cells that are subject to rapidly changing sets of instructions from their environment and from within. These instructions cause them to migrate, expand and diversify in lineage, and acquire form and function. Molecular information from various model systems, combined with increasingly detailed morphogenetic data, has provided insights into some of these key events. Many congenital heart abnormalities might arise from defects in the early stages of heart development, therefore it is important to understand the molecular pathways that underlie the lineage specification and patterning processes that shape this organ.

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.