Retinoid signaling is required to complete the vertebrate cardiac left/right asymmetry pathway

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.

Retinoic acid signaling pathway perturbation impacts mesodermal-tissue development in the zebrafish embryo: Biomarker candidate identification using transcriptomics

The zebrafish embryo (ZE) model provides a developmental model well conserved throughout vertebrate embryogenesis, with relevance for early human embryo development. It was employed to search for gene expression biomarkers of compound-induced disruption of mesodermal development. We were particularly interested in the expression of genes related to the retinoic acid signaling pathway (RA-SP), as a major morphogenetic regulating mechanism. We exposed ZE to teratogenic concentrations of valproic acid (VPA) and all-trans retinoic acid (ATRA), using folic acid (FA) as a non-teratogenic control compound shortly after fertilization for 4 h, and performed gene expression analysis by RNA sequencing. We identified 248 genes specifically regulated by both teratogens but not by FA. Further analysis of this gene set revealed 54 GO-terms related to the development of mesodermal tissues, distributed along the paraxial, intermediate, and lateral plate sections of the mesoderm. Gene expression regulation was specific to tissues and was observed for somites, striated muscle, bone, kidney, circulatory system, and blood. Stitch analysis revealed 47 regulated genes related to the RA-SP, which were differentially expressed in the various mesodermal tissues. These genes provide potential molecular biomarkers of mesodermal tissue and organ (mal)formation in the early vertebrate embryo.

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.

Role of carotenoids and retinoids during heart development

The nutritional requirements of the developing embryo are complex. In the case of dietary vitamin A (retinol, retinyl esters and provitamin A carotenoids), maternal derived nutrients serve as precursors to signaling molecules such as retinoic acid, which is required for embryonic patterning and organogenesis. Despite variations in the composition and levels of maternal vitamin A, embryonic tissues need to generate a precise amount of retinoic acid to avoid congenital malformations. Here, we summarize recent findings regarding the role and metabolism of vitamin A during heart development and we survey the association of genes known to affect retinoid metabolism or signaling with various inherited disorders. A better understanding of the roles of vitamin A in the heart and of the factors that affect retinoid metabolism and signaling can help design strategies to meet nutritional needs and to prevent birth defects and disorders associated with altered retinoid metabolism. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

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.

Primary culture of avian embryonic heart forming region cells to study the regulation of vertebrate early heart morphogenesis by vitamin A

Background

Important knowledge about the role of vitamin A in vertebrate heart development has been obtained using the vitamin A-deficient avian in ovo model which enables the in vivo examination of very early stages of vertebrate heart morphogenesis. These studies have revealed the critical role of the vitamin A-active form, retinoic acid (RA) in the regulation of several developmental genes, including the important growth regulatory factor, transforming growth factor-beta2 (TGFβ2), involved in early events of heart morphogenesis. However, this in ovo model is not readily available for elucidating details of molecular mechanisms determining RA activity, thus limiting further examination of RA-regulated early heart morphogenesis. In order to obtain insights into RA-regulated gene expression during these early events, a reliable in vitro model is needed. Here we describe a cell culture that closely reproduces the in ovo observed regulatory effects of RA on TGFβ2 and on several developmental genes linked to TGFβ signaling during heart morphogenesis.

Results

We have developed an avian heart forming region (HFR) cell based in vitro model that displays the characteristics associated with vertebrate early heart morphogenesis, i.e. the expression of Nkx2.5 and GATA4, the cardiogenesis genes, of vascular endothelial growth factor (VEGF-A), the vasculogenesis gene and of fibronectin (FN1), an essential component in building the heart, and the expression of the multifunctional genes TGFβ2 and neogenin (NEO). Importantly, we established that the HFR cell culture is a valid model to study RA-regulated molecular events during heart morphogenesis and that the expression of TGFβ2 as well as the expression of several TGFβ2-linked developmental genes is regulated by RA.

Conclusions

Our findings reported here offer a biologically relevant experimental in vitro system for the elucidation of RA-regulated expression of TGFβ2 and other genes involved in vertebrate early cardiovascular morphogenesis.

Transforming growth factor beta2 is negatively regulated by endogenous retinoic acid during early heart morphogenesis

Vitamin A-deficient (VAD) quail embryos lack the vitamin A-active form, retinoic acid (RA) and are characterized by a phenotype that includes a grossly abnormal cardiovascular system that can be rescued by RA. Here we report that the transforming growth factor, TGFbeta2 is involved in RA-regulated cardiovascular development. In VAD embryos TGFbeta2 mRNA and protein expression are greatly elevated. The expression of TGFbeta receptor II is also elevated in VAD embryos but is normalized by treatment with TGFbeta2-specific antisense oligonucleotides (AS). Administration of this AS or an antibody specific for TGFbeta2 to VAD embryos normalizes posterior heart development and vascularization, while the administration of exogenous active TGFbeta2 protein to normal quail embryos mimics the excessive TGFbeta2 status of VAD embryos and induces VAD cardiovascular phenotype. In VAD embryos pSmad2/3 and pErk1 are not activated, while pErk2 and pcRaf are elevated and pSmad1/5/8 is diminished. We conclude that in the early avian embryo TGFbeta2 has a major role in the retinoic acid-regulated posterior heart morphogenesis for which it does not use Smad2/3 pathways, but may use other signaling pathways. Importantly, we conclude that retinoic acid is a critical negative physiological regulator of the magnitude of TGFbeta2 signals during vertebrate heart formation.

Vitamin A-not for your eyes only: requirement for heart formation begins early in embryogenesis

Vitamin A insufficiency has profound adverse effects on embryonic development. Major advances in understanding the role of vitamin A in vertebrate heart formation have been made since the discovery that the vitamin A active form, all-trans-retinoic acid, regulates many genes, including developmental genes. Among the experimental models used, the vitamin A-deficient avian embryo has been an important tool to study the function of vitamin A during early heart formation. A cluster of retinoic acid-regulated developmental genes have been identified that participate in building the heart. In the absence of retinoic acid the embryonic heart develops abnormally leading to embryolethality.

Retinoic acid is a negative physiological regulator of N-cadherin during early avian heart morphogenesis

The vitamin A-deficient (VAD) early avian embryo has a grossly abnormal cardiovascular system that is rescued by treating the embryo with the vitamin A-active form, retinoic acid (RA). Here we examine the role of N-cadherin (N-cad) in RA-regulated early cardiovascular morphogenesis. N-cad mRNA and protein are expressed globally in the presomite through HH14 normal and VAD quail embryos. The expression in VAD embryos prior to HH10 is significantly higher than that in normal embryos. Functional analyses of the N-cad overproducing VAD embryos reveal N-cad involvement in the RA-regulated cardiovascular development and suggest that N-cad expression may be mediated by Msx1. We provide evidence that in the early avian embryo, endogenous RA is a negative physiological regulator of N-cad. We hypothesize that a critical endogenous level of N-cad is needed for normal early cardiovascular morphogenesis to occur and that this level is ensured by stage-specific, developmentally regulated RA signaling.

Metabolism and regulation of gene expression by 4-oxoretinol versus all-trans retinoic acid in normal human mammary epithelial cells

We previously demonstrated that 4-oxoretinol (4-oxo-ROL) activated retinoic acid receptors (RARs) in F9 stem cells. We showed that 4-oxo-ROL inhibited the proliferation of normal human mammary epithelial cells (HMECs). To understand the mechanisms by which 4-oxo-ROL regulates HMEC growth we examined gene expression profiles following 4-oxo-ROL or all-trans retinoic acid (tRA). We also compared growth inhibition by tRA, 4-oxo-ROL, or 4-oxo-RA. All three retinoids inhibited HMEC proliferation. Gene expression analyses indicated that 4-oxo-ROL and tRA modulated gene expression in closely related pathways. The expression of many genes, e.g. ATP-binding cassette G1 (ABCG1); adrenergic receptorbeta2 (ADRB2); ras-related C3 botulinum toxin substrate (RAC2); and short-chain dehydrogenase/reductase 1 gene (SDR1) was changed after 4-oxo-ROL or tRA. Metabolism of these retinoids was analyzed by high-performance liquid chromatography (HPLC). In 1 microM tRA treated HMECs all of the tRA was found intracellularly, and tRA was the predominant intracellular retinoid. In 1 microM 4-oxo-ROL treated HMECs most 4-oxo-ROL was esterified to 4-oxoretinyl esters, no tRA was detected, and 4-oxo-ROL and 4-oxo-RA were observed intracellularly. In 1 microM 4-oxoretinoic acid (4-oxo-RA) treated HMECs little intracellular 4-oxo-RA was detected; most 4-oxo-RA was in the medium. Our results indicate that: (a) 4-oxo-ROL regulates gene expression and inhibits proliferation of HMECs; (b) 4-oxo-ROL and tRA regulate some of the same genes; (c) more tRA is found in cells, as compared to 4-oxoretinoic acid, when each drug is added at the same concentration in the medium; and (d) the mechanism by which 4-oxo-ROL exerts its biological activity does not involve intracellular tRA production.

Developmental expression of retinoic acid receptors (RARs)

Here, I review the developmental expression features of genes encoding the retinoic acid receptors (RARs) and the 'retinoid X' or rexinoid receptors (RXRs). The first detailed expression studies were performed in the mouse over two decades ago, following the cloning of the murine Rar genes. These studies revealed complex expression features at all stages of post-implantation development, one receptor gene (Rara) showing widespread expression, the two others (Rarb and Rarg) with highly regionalized and/or cell type-specific expression in both neural and non-neural tissues. Rxr genes also have either widespread (Rxra, Rxrb), or highly-restricted (Rxrg) expression patterns. Studies performed in zebrafish and Xenopus demonstrated expression of Rar and Rxr genes (both maternal and zygotic), at early pre-gastrulation stages. The eventual characterization of specific enzymes involved in the synthesis of retinoic acid (retinol/retinaldehyde dehydrogenases), or the triggering of its catabolism (CYP26 cytochrome P450s), all of them showing differential expression patterns, led to a clearer understanding of the phenomenons regulated by retinoic acid signaling during development. Functional studies involving targeted gene disruptions in the mouse, and additional approaches such as dominant negative receptor expression in other models, have pinpointed the specific, versus partly redundant, roles of the RARs and RXRs in many developing organ systems. These pleiotropic roles are summarized hereafter in relationship to the receptors' expression patterns.

Balancing segmentation and laterality during vertebrate development

Somites are the mesodermal segments of vertebrate embryos that become the vertebral column, skeletal muscle and dermis. Somites arise within the paraxial mesoderm by the periodic, bilaterally symmetric process of somitogenesis. However, specification of left-right asymmetry occurs in close spatial and temporal proximity to somitogenesis and involves some of the same cell signaling pathways that govern segmentation. Here, we review recent evidence that identifies cross-talk between these processes and that demonstrates a role for retinoic acid in maintaining symmetrical somitogenesis by preventing impingement of left-right patterning signals upon the paraxial mesoderm.

A ryanodine receptor-dependent Ca(i)(2+) asymmetry at Hensen's node mediates avian lateral identity

In mouse, the establishment of left-right (LR) asymmetry requires intracellular calcium (Ca(i)(2+)) enrichment on the left of the node. The use of Ca(i)(2+) asymmetry by other vertebrates, and its origins and relationship to other laterality effectors are largely unknown. Additionally, the architecture of Hensen's node raises doubts as to whether Ca(i)(2+) asymmetry is a broadly conserved mechanism to achieve laterality. We report here that the avian embryo uses a left-side enriched Ca(i)(2+) asymmetry across Hensen's node to govern its lateral identity. Elevated Ca(i)(2+) was first detected along the anterior node at early HH4, and its emergence and left-side enrichment by HH5 required both ryanodine receptor (RyR) activity and extracellular calcium, implicating calcium-induced calcium release (CICR) as the novel source of the Ca(i)(2+). Targeted manipulation of node Ca(i)(2+) randomized heart laterality and affected nodal expression. Bifurcation of the Ca(i)(2+) field by the emerging prechordal plate may permit the independent regulation of LR Ca(i)(2+) levels. To the left of the node, RyR/CICR and H(+)V-ATPase activity sustained elevated Ca(i)(2+). On the right, Ca(i)(2+) levels were actively repressed through the activities of H(+)K(+) ATPase and serotonin-dependent signaling, thus identifying a novel mechanism for the known effects of serotonin on laterality. Vitamin A-deficient quail have a high incidence of situs inversus hearts and had a reversed calcium asymmetry. Thus, Ca(i)(2+) asymmetry across the node represents a more broadly conserved mechanism for laterality among amniotes than had been previously believed.

Retinoic acid in development: towards an integrated view

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

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.

Left-right axis development: examples of similar and divergent strategies to generate asymmetric morphogenesis in chick and mouse embryos

Left-right asymmetry of internal organs is widely distributed in the animal kingdom. The chick and mouse embryos have served as important model organisms to analyze the mechanisms underlying the establishment of the left-right axis. In the chick embryo many genes have been found to be asymmetrically expressed in and around the node, while the same genes in the mouse show symmetric expression patterns. In the mouse there is strong evidence for an establishment of left-right asymmetry through nodal cilia. In contrast, in the chick and in many other organisms left-right asymmetry is probably generated by an early-acting event involving membrane depolarization. In both birds and mammals a conserved Nodal-Lefty-Pitx2 module exists that controls many aspects of asymmetric morphogenesis. This review also gives examples of divergent mechanisms of establishing asymmetric organ formation. Thus there is ample evidence for conserved and non-conserved strategies to generate asymmetry in birds and mammals.

Genetic and environmental influences on malformations of the cardiac outflow tract

Congenital cardiovascular malformations are the most common form of birth defect recorded. Specific malformations of the outflow portions of the heart are termed conotruncal malformations and arise from the septation of the common conotruncus of the heart. There are multiple lines of evidence that point towards genetic-environmental interactions in the genesis of conotruncal congenital cardiovascular malformations. In particular, environmental exposures that involve vitamin A, retinol, folic acid or retinol receptors are identified as cardiac teratogens. Other environmental agents for which there is evidence of cardiac teratogenicity for outflow tract malformations include nitrofen, ambient air pollution, chlorinated hydrocarbons and pesticides. Genetic polymorphisms of xenobiotic metabolism are clearly differentiating in the effect of potential teratogens. Work in this field is at a new cusp, with the ability to measure xenobiotic exposure, document xenobiotic metabolizing genetic polymorphisms and integrate these data into models of cardiac teratogenesis.

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.

Left-right asymmetry in the vertebrate embryo: from early information to higher-level integration

Although vertebrates seem to be essentially bilaterally symmetrical on the exterior, there are numerous interior left-right asymmetries in the disposition and placement of internal organs. These asymmetries are established during embryogenesis by complex epigenetic and genetic cascades. Recent studies in a range of model organisms have made important progress in understanding how this laterality information is generated and conveyed to large regions of the embryo. Both commonalities and divergences are emerging in the mechanisms that different vertebrates use in left-right axis specification. Recent evidence also provides intriguing links between the establishment of left-right asymmetries and the symmetrical elongation of the anterior-posterior axis.

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.

Distinct roles for hindbrain and paraxial mesoderm in the induction and patterning of the inner ear revealed by a study of vitamin-A-deficient quail

The hindbrain and cranial paraxial mesoderm have been implicated in the induction and patterning of the inner ear, but the precise role of the two tissues in these processes is still not clear. We have addressed these questions using the vitamin-A-deficient (VAD) quail model, in which VAD embryos lack the posterior half of the hindbrain that normally lies next to the inner ear. Using a battery of molecular markers, we show that the anlagen of the inner ear, the otic placode, is induced in VAD embryos in the absence of the posterior hindbrain. By performing grafting and ablation experiments in chick embryos, we also show that cranial paraxial mesoderm which normally lies beneath the presumptive otic placode is necessary for otic placode induction and that paraxial mesoderm from other locations cannot induce the otic placode. Two members of the fibroblast growth factor family, FGF3 and FGF19, continue to be expressed in this mesodermal population in VAD embryos, and these may be responsible for otic placode induction in the absence of the posterior hindbrain. Although the posterior hindbrain is not required for otic placode induction in VAD embryos, the subsequent patterning of the inner ear is severely disrupted. Several regional markers of the inner ear, such as Pax2, EphA4, SOHo1 and Wnt3a, are incorrectly expressed in VAD otocysts, and the sensory patches and vestibulo-acoustic ganglia are either greatly reduced or absent. Exogenous application of retinoic acid prior to 30 h of development is able rescue the VAD phenotype. By performing such rescue experiments before and after 30 h of development, we show that the inner ear defects of VAD embryos correlate with the absence of the posterior hindbrain. These results show that induction and patterning of the inner ear are governed by separate developmental processes that can be experimentally uncoupled from each other.

Unraveling signalling cascades for the Snail family of transcription factors

During development and carcinogenesis, the gradient of different molecular factors, the availability of corresponding receptors and the interplay between different signalling cascades combine to orchestrate the different stages. A good understanding of both developmental processes and oncogenesis leads to new insights into normal and aberrant regulation, processes that share some mutual key players. In this review, we will focus on the Snail family of transcription factors. These proteins, which share an evolutionarily conserved role in invertebrates and vertebrates, are implicated in several developmental processes, but are involved in carcinogenesis as well. We will highlight the different signalling cascades leading to the expression of Snail and Slug and how these factors are regulated on the transcriptional level. Then we will focus on how these factors execute their functions by repression of the numerous target genes that have been described to date.

Retinoic acid signalling links left–right asymmetric patterning and bilaterally symmetric somitogenesis in the zebrafish embryo

During embryogenesis, cells are spatially patterned as a result of highly coordinated and stereotyped morphogenetic events. In the vertebrate embryo, information on laterality is conveyed to the node, and subsequently to the lateral plate mesoderm, by a complex cascade of epigenetic and genetic events, eventually leading to a left-right asymmetric body plan. At the same time, the paraxial mesoderm is patterned along the anterior-posterior axis in metameric units, or somites, in a bilaterally symmetric fashion. Here we characterize a cascade of laterality information in the zebrafish embryo and show that blocking the early steps of this cascade (before it reaches the lateral plate mesoderm) results in random left-right asymmetric somitogenesis. We also uncover a mechanism mediated by retinoic acid signalling that is crucial in buffering the influence of the flow of laterality information on the left-right progression of somite formation, and thus in ensuring bilaterally symmetric somitogenesis.

Symmetry Breaking and the Evolution of Development

Because of its simplicity, the binary-switch nature of left-right asymmetry permits meaningful comparisons among many different organisms. Phylogenetic analyses of asymmetry variation, inheritance, and molecular mechanisms reveal unexpected insights into how development evolves. First, directional asymmetry, an evolutionary novelty, arose from nonheritable origins almost as often as from mutations, implying that genetic assimilation ("phenotype precedes genotype") is a common mode of evolution. Second, the molecular pathway directing hearts leftward-the nodal cascade-varies considerably among vertebrates (homology of form does not require homology of development) and was possibly co-opted from a preexisting asymmetrical chordate organ system. Finally, declining frequencies of spontaneous asymmetry reversal throughout vertebrate evolution suggest that heart development has become more canalized.

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.

Vitamin a requirement for early cardiovascular morphogenesis specification in the vertebrate embryo: insights from the avian embryo

Vitamin A is required throughout the life cycle, including crucial stages of embryonic and fetal development. With the identification of retinoic acid-specific nuclear transcription factors, the retinoid receptors, considerable advances have been made in understanding the molecular function of vitamin A. The requirement for vitamin A during early embryogenesis has successfully been examined in the vitamin A-deficient avian embryo during neurulation, when in the vertebrates crucial developmental decisions take place. These studies revealed that retinoic acid is essential during these early stages of embryogenesis for the initiation of organogenesis (i.e., formation of the heart). If retinoic acid is not present at this time, abnormal development ensues, leading to early embryonic death. Though the initial insult of the absence of vitamin A appears to be on the specification of cardiovascular tissues, subsequently all development is adversely affected and the embryo dies. Molecular and functional studies revealed that retinoic acid regulates the expression of the cardiogenic transcription factor GATA-4 and several heart asymmetry genes, which explains why the heart position is random in vitamin A-deficient quail embryos. During the crucial retinoic acid-requiring developmental window, retinoic acid transduces its signals to genes for heart morphogenesis via the receptors RARalpha2, RARgamma, and RXRalpha. Elucidation of the function of vitamin A during early embryonic development may lead to a better understanding of the cardiovascular birth defects prevalent in the Western world.

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.

A newborn lethal defect due to inactivation of retinaldehyde dehydrogenase type 3 is prevented by maternal retinoic acid treatment

The retinoic acid (RA) signal, produced locally from vitamin A by retinaldehyde dehydrogenase (Raldh) and transduced by the nuclear receptors for retinoids (RA receptor and 9-cis-RA receptor), is indispensable for ontogenesis and homeostasis of numerous tissues. We demonstrate that Raldh3 knockout in mouse suppresses RA synthesis and causes malformations restricted to ocular and nasal regions, which are similar to those observed in vitamin A-deficient fetuses and/or in retinoid receptor mutants. Raldh3 knockout notably causes choanal atresia (CA), which is responsible for respiratory distress and death of Raldh3-null mutants at birth. CA is due to persistence of nasal fins, whose rupture normally allows the communication between nasal and oral cavities. This malformation, which is similar to isolated congenital CA in humans and may result from impaired RA-controlled down-regulation of Fgf8 expression in nasal fins, can be prevented by a simple maternal treatment with RA.

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.

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

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

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.

Heart development: molecular insights into cardiac specification and early morphogenesis

The heart develops from two bilateral heart fields that are formed during early gastrulation. In recent years, signaling pathways that specify cardiac mesoderm have been extensively analyzed. In addition, a battery of transcription factors that regulate different aspects of cardiac morphogenesis and cytodifferentiation have been identified and characterized in model organisms. At the anterior pole, a secondary heart field is formed, which in its molecular make-up, appears to be similar to the primary heart field. The cardiac outflow tract and the right ventricle to a large extent are derivatives of this anterior heart field. Cardiac mesoderm receives positional information by which it is patterned along the three body axes. The molecular control of left-right axis development has received particular attention, and the underlying regulatory network begins to emerge. Cardiac chamber development involves the activation of a transcription program that is different from the one present in the primary heart field and regulates cardiac morphogenesis in a region-specific manner. This review also attempts to identify areas in which additional research is needed to fully understand early cardiac development.

The retinoic-like juvenile hormone controls the looping of left-right asymmetric organs in Drosophila

In vertebrate development, the establishment of left-right asymmetry is essential for sidedness and the directional looping of organs like the heart. Both the nodal pathway and retinoic acid play major and conserved regulatory roles in these processes. We carried out a novel screen in Drosophila to identify mutants that specifically affect the looping of left-right asymmetric organs. We report the isolation of spin, a novel mutant in which the looping of the genitalia and spermiduct are incomplete; under-rotation of the genitalia indicates that spin controls looping morphogenesis but not direction, thus uncoupling left-right asymmetry and looping morphogenesis. spin is a novel, rotation-specific allele of the fasciclin2 (Fas2) gene, which encodes a cell-adhesion protein involved in several aspects of neurogenesis. In spin mutants, the synapses connecting specific neurosecretory cells to the corpora allata are affected. The corpus allatum is part of the ring gland and is involved in the control of juvenile hormone titers during development. Our genetic and pharmacological results indicate that Fas2(spin) rotation defects are linked to an abnormal endocrine function and an elevated level of juvenile hormone. As juvenile hormone is an insect sesquiterpenoid related to retinoic acid, these results establish a new genetic model for studying organ looping and demonstrate an evolutionarily conserved role for terpenoids in this process.

Increased fibronectin deposition in embryonic hearts of retinol-binding protein-null mice

Precise regulation of retinoid levels is critical for normal heart development. Retinol-binding protein (RBP), an extracellular retinol transporter, is strongly secreted by cardiogenic endoderm. This study addresses whether RBP gene ablation affects heart development. Despite exhibiting an >85% decrease in serum retinol, adult RBP-null mice are viable, breed, and have normal vision when maintained on a vitamin A-sufficient diet. Comparison of RBP-null with wild-type (WT) hearts from embryos at day 9.0 (E9.0) through E12.5 revealed an RBP-null phenotype similar to that of other retinoid-deficient models. At an early stage, RBP-null hearts display retarded trabecular development, which recovers by E9.5. This is accompanied at E9.5 and E10.5 by precocious differentiation of subepicardial cardiac myocytes. Most remarkably, RBP-null hearts display augmented deposition of fibronectin protein in the cardiac jelly at E9.0 through E10.5 and in the outflow tract at E12.5. This phenomenon, which was detected by immunohistochemistry and Western blotting without increased fibronectin transcript levels, is accompanied by increased numbers of mesenchymal cells in the outflow tract but not in the atrioventricular canal. RBP-null cardiac myocytes, especially in the subepicardial layer, display increased cell proliferation. This phenotype may present a model of subclinical retinoid insufficiency characterized by alteration of an extracellular matrix component and altered cellular differentiation and proliferation, changes that may have functional consequences for adult cardiac function. This murine model may have relevance to fetal development in human populations with inadequate retinoid intake.

A novel role for retinoids in patterning the avian forebrain during presomite stages

Retinoids, and in particular retinoic acid (RA), are known to induce posterior fates in neural tissue. However, alterations in retinoid signalling dramatically affect anterior development. Previous reports have demonstrated a late role for retinoids in patterning craniofacial and forebrain structures, but an earlier role in anterior patterning is not well understood. We show that enzymes involved in synthesizing retinoids are expressed in the avian hypoblast and in tissues directly involved in head patterning, such as anterior definitive endoderm and prechordal mesendoderm. We found that in the vitamin A-deficient (VAD) quail model, which lacks biologically active RA from the first stages of development, anterior endodermal markers such as Bmp2, Bmp7, Hex and the Wnt antagonist crescent are affected during early gastrulation. Furthermore, prechordal mesendodermal and prospective ventral telencephalic markers are expanded posteriorly, Shh expression in the axial mesoderm is reduced, and Bmp2 and Bmp7 are abnormally expressed in the ventral midline of the neural tube. At early somite stages, VAD embryos have increased cell death in ventral neuroectoderm and foregut endoderm, but normal cranial neural crest production, whereas at later stages extensive apoptosis occurs in head mesenchyme and ventral neuroectoderm. As a result, VAD embryos end up with a single and reduced telencephalic vesicle and an abnormally patterned diencephalon. Therefore, we propose that retinoids have a dual role in patterning the anterior forebrain during development. During early gastrulation, RA acts in anterior endodermal cells to modulate the anteroposterior (AP) positional identity of prechordal mesendodermal inductive signals to the overlying neuroectoderm. Later on, at neural pore closure, RA is required for patterning of the mesenchyme of the frontonasal process and the forebrain by modulating signalling molecules involved in craniofacial morphogenesis.

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.

Regulation of heart morphology: current molecular and cellular perspectives on the coordinated emergence of cardiac form and function

BACKGROUND

During early heart development, in addition to cells being induced to differentiate into cardiomyocytes, pathways are activated that lead to cardiac morphogenesis or the development of form.

METHODS

Orchestration of organogenesis involves the incremental activation of regulatory pathways that lead to pivotal transition points, such as cardiac compartment delineation and looping. Each embryonic stage sets up the correct patterning of morphoregulatory molecules that will regulate the next process, until an organ is formed from the mesoderm layer after gastrulation. The current review provides an understanding of the morphoregulatory, cell adhesion and extracellular matrix-mediated, processes that coordinate development of heart form with that of function. The period reviewed encompasses the formation of a definitive cardiac compartment from the lateral plate mesoderm to the time-point in which the single, beating heart tube loops directionally to the right. Looping results in the correct spatial orientation for subsequent modeling of the four-chambered heart. Even subtle alterations in looping can form the basis upon which malformations of the inlet or the outlet regions of the heart, or both, are superimposed.

RESULTS

In the future, DNA microarray data sets may allow modeling the specific sequence of gene regulatory dynamics leading to these transition points to discover the regulatory "modes" that the cells adopt during heart organogenesis. The regulatory genes, however, can only specify the proteins that will be present.

CONCLUSIONS

To fully understand the timing and mechanisms underlying heart development, it is necessary to define the sequential synthesis, patterning, and interaction of the proteins, and of still other receptors, which eventually drive cells to organize into functioning organs.

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

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

The epigenesis of planum temporale asymmetry in twins

Variation in hemispheric asymmetry of the planum temporale (PT) has been related to verbal ability. The degree to which genetic and environmental factors mediate PT asymmetry is not known. This study examined the heritability for planar asymmetry in 12 dizygotic (DZ) and 27 monozygotic (MZ) male twin pairs who were between 6 and 16 years of age. There was weak but positive evidence for heritability of planar asymmetry. Co-twin similarity for planar asymmetry and Sylvian fissure morphology increased when excluding twins discordant for writing hand and when excluding twins exhibiting birth weight differences >20% from the analyses. Birth weight differences were also related to twin differences in total cerebral volume, but not central sulcus asymmetry. These results suggest that exogenous perinatal factors affect the epigenesis of planar asymmetry development.

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.

Left-right asymmetry determination in vertebrates

A distinctive and essential feature of the vertebrate body is a pronounced left-right asymmetry of internal organs and the central nervous system. Remarkably, the direction of left-right asymmetry is consistent among all normal individuals in a species and, for many organs, is also conserved across species, despite the normal health of individuals with mirror-image anatomy. The mechanisms that determine stereotypic left-right asymmetry have fascinated biologists for over a century. Only recently, however, has our understanding of the left-right patterning been pushed forward by links to specific genes and proteins. Here we examine the molecular biology of the three principal steps in left-right determination: breaking bilateral symmetry, propagation and reinforcement of pattern, and the translation of pattern into asymmetric organ morphogenesis.

Chick CFC controls Lefty1 expression in the embryonic midline and nodal expression in the lateral plate

Members of the EGF-CFC family of proteins have recently been implicated as essential cofactors for Nodal signaling. Here we report the isolation of chick CFC and describe its expression pattern, which appears to be similar to Cfc1 in mouse. During early gastrulation, chick CFC was asymmetrically expressed on the left side of Hensen's node as well as in the emerging notochord, prechordal plate, and lateral plate mesoderm. Subsequently, its expression became confined to the heart fields, notochord, and posterior mesoderm. Implantation experiments suggest that chick CFC expression in the lateral plate mesoderm is dependent on BMP signaling, while in the midline its expression depends on an Activin-like signal. The asymmetric expression domain within Hensen's node was not affected by application of FGF8, Noggin, or Shh antibody. Implantation of cells expressing human or mouse CFC2, or chick CFC on the right side of Hensen's node randomized heart looping without affecting expression of genes involved in left-right axis formation, including SnR, Nodal, Car, or Pitx2. Application of antisense oligodeoxynucleotides to the midline of Hamburger-Hamilton stage 4-5 embryos also randomized heart looping, but in contrast to the overexpression experiments, antisense oligodeoxynucleotide treatment resulted in bilateral expression of Nodal, Car, Pitx2, and NKX3.2, whereas Lefty1 expression in the midline was transiently lost. Application of the antisense oligodeoxynucleotides to the lateral plate mesoderm abolished Nodal expression. Thus, chick CFC seems to have a dual function in left-right axis formation by maintaining Nodal expression in the lateral plate mesoderm and controlling expression of Lefty1 expression in the midline territory.

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

Mutants mice carrying targeted inactivations of both retinoic acid receptor (RAR) alpha and RAR gamma (A alpha/A gamma mutants) were analyzed at different embryonic stages, in order to establish the timing of appearance of defects that we previously observed during the fetal period. We show that embryonic day (E)9.5 A alpha/A gamma embryos display severe malformations, similar to those already described in retinaldehyde dehydrogenase 2 null mutants. These malformations reflect early roles of retinoic acid signaling in axial rotation, segmentation and closure of the hindbrain; formation of otocysts, pharyngeal arches and forelimb buds; and in the closure of the primitive gut. The hindbrain of E8.5 A alpha/A gamma embryos shows a posterior expansion of rhombomere 3 and 4 (R3 and R4) markers, but fails to express kreisler, a normal marker of R5 and R6. This abnormal hindbrain phenotype is strikingly different from that of embryos lacking RAR alpha and RAR beta (A alpha/A beta mutants), in which we have previously shown that the territory corresponding to R5 and R6 is markedly enlarged. Administration of a pan-RAR antagonist at E8.0 to wild-type embryos cultured in vitro results in an A alpha/A beta-like hindbrain phenotype, whereas an earlier treatment at E7.0 yields an A alpha/A gamma-like phenotype. Altogether, our data suggest that RAR alpha and/or RAR gamma transduce the RA signal that is required first to specify the prospective R5/R6 territory, whereas RAR beta is subsequently involved in setting up the caudal boundary of this territory.

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.

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

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

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.

Differential expression and functional analysis of Pitx2 isoforms in regulation of heart looping in the chick

Pitx2, a bicoid-related homeobox gene, plays a crucial role in the left-right axis determination and dextral looping of the vertebrate developing heart. We have examined the differential expression and function of two Pitx2 isoforms (Pitx2a and Pitx2c) that differ in the region 5' to the homeodomain, in early chick embryogenesis. Northern blot and RT-PCR analyses indicated the existence of Pitx2a and Pitx2c but not Pitx2b in the developing chick embryos. In situ hybridization demonstrated a restricted expression of Pitx2c in the left lateral plate mesoderm (LPM), left half of heart tube and head mesoderm, but its absence in the extra-embryonic tissues where vasculogenesis occurs. RT-PCR experiments revealed that Pitx2a is absent in the left LPM, but is present in the head and extra-embryonic mesoderm. However, ectopic expression of either Pitx2c or Pitx2a via retroviral infection to the right LMP equally randomized heart looping direction. Mapping of the transcriptional activation function to the C terminus that is identical in both isoforms explained the similar results obtained by the gain-of-function approach. In contrast, elimination of Pitx2c expression from the left LMP by antisense oligonucleotide resulted in a randomization of heart looping, while treatment of embryos with antisense oligonucleotide specific to Pitx2a failed to generate similar effect. We further constructed RCAS retroviral vectors expressing dominant negative Pitx2 isoforms in which the C-terminal transcriptional activation domain was replaced by the repressor domain of the Drosophila Engrailed protein (En(r)). Ectopic expression of Pitx2c-En(r), but not Pitx2a-En(r), to the left LPM randomized the heart looping. The results thus demonstrate that Pitx2c plays a crucial role in the left-right axis determination and rightward heart looping during chick embryogenesis.

From the bottom of the heart: anteroposterior decisions in cardiac muscle differentiation

Recently, studies on specification of axes in the developing embryo have focused on the heart, which is the first functional organ to form and probably responds to common cues controlling positional information in surrounding tissues. The early differentiation of heart cells affords an opportunity to link the acquisition of regional identity with the signals underlying terminal differentiation. In the past year, a wealth of information on these signals has emerged, elucidating the general pathways controlling body axes in the context of the developing heart.