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.

Retinoid signaling regulates angiogenesis and blood-retinal barrier integrity in neonatal mouse retina.

OBJECTIVE: The neonatal mouse retina is a well-characterized experimental model for investigating factors impacting retinal angiogenesis and inner blood-retinal barrier (BRB) integrity. Retinoic acid (RA) is an essential signaling molecule. RA is needed for vasculogenic development in embryos and endothelial barrier integrity in zebrafish retina and adult mouse brain; however, the function of this signaling molecule in developing mammalian retinal vasculature remains unknown. This study aims to investigate the role of RA signaling in angiogenesis and inner BRB integrity in mouse neonatal retina. METHODS: RA distribution in the developing neurovascular retina was assessed in mice carrying an RA-responsive transgene. RA function in retinal angiogenesis was determined by treating C57BL/6 neonatal pups with a pharmacological inhibitor of RA signaling BMS493 or control vehicle. BRB integrity assessed by monitoring leakage of injected tracer into extravascular retinal tissue. RESULTS: RA signaling activity is present in peripheral astrocytes in domains corresponding to RA activity of the underlying neural retina. RA inhibition impaired retinal angiogenesis and reduced endothelial cell proliferation. RA inhibition also compromised BRB integrity. Vascular leakage was not associated with altered expression of CLDN5, PLVAP, LEF1, or VEcad. CONCLUSIONS: RA signaling is needed for angiogenesis and integrity of the BRB in the neonatal mouse retina.

Synergistic role of retinoic acid signaling and Gata3 during primitive choanae formation.

Developmental defects of primitive choanae, an anatomical path to connect the embryonic nasal and oral cavity, result in disorders called choanal atresia (CA), which are associated with many congenital diseases and require immediate clinical intervention after birth. Previous studies revealed that reduced retinoid signaling underlies the etiology of CA. In the present study, by using multiple mouse models which conditionally deleted Rdh10 and Gata3 during embryogenesis, we showed that Gata3 expression is regulated by retinoid signaling during embryonic craniofacial development and plays crucial roles for development of the primitive choanae. Interestingly, Gata3 loss of function is known to cause hypoparathyroidism, sensorineural deafness and renal disease (HDR) syndrome, which exhibits CA as one of the phenotypes in humans. Our model partially phenocopies HDR syndrome with CA, and is thus a useful tool for investigating the molecular and cellular mechanisms of HDR syndrome. We further uncovered critical synergy of Gata3 and retinoid signaling during embryonic development, which will shed light on novel molecular and cellular etiology of congenital defects in primitive choanae formation.

Branchiomeric Muscle Development Requires Proper Retinoic Acid Signaling.

The first and second branchiomeric (branchial arch) muscles are craniofacial muscles that derive from branchial arch mesoderm. In mammals, this set of muscles is indispensable for jaw movement and facial expression. Defects during embryonic development that result in congenital partial absence of these muscles can have significant impact on patients' quality of life. However, the detailed molecular and cellular mechanisms that regulate branchiomeric muscle development remains poorly understood. Herein we investigated the role of retinoic acid (RA) signaling in developing branchiomeric muscles using mice as a model. We administered all-trans RA (25 mg/kg body weight) to Institute of Cancer Research (ICR) pregnant mice by gastric intubation from E8.5 to E10.5. In their embryos at E13.5, we found that muscles derived from the first branchial arch (temporalis, masseter) and second branchial arch (frontalis, orbicularis oculi) were severely affected or undetectable, while other craniofacial muscles were hypoplastic. We detected elevated cell death in the branchial arch mesoderm cells in RA-treated embryos, suggesting that excessive RA signaling reduces the survival of precursor cells of branchiomeric muscles, resulting in the development of hypoplastic craniofacial muscles. In order to uncover the signaling pathway(s) underlying this etiology, we focused on Pitx2, Tbx1, and MyoD1, which are critical for cranial muscle development. Noticeably reduced expression of all these genes was detected in the first and second branchial arch of RA-treated embryos. Moreover, elevated RA signaling resulted in a reduction in Dlx5 and Dlx6 expression in cranial neural crest cells (CNCCs), which disturbed their interactions with branchiomeric mesoderm cells. Altogether, we discovered that embryonic craniofacial muscle defects caused by excessive RA signaling were associated with the downregulation of Pitx2, Tbx1, MyoD1, and Dlx5/6, and reduced survival of cranial myogenic precursor cells.

The Mediator Subunit, Med23 Is Required for Embryonic Survival and Regulation of Canonical WNT Signaling During Cranial Ganglia Development.

Development of the vertebrate head is a complex and dynamic process, which requires integration of all three germ layers and their derivatives. Of special importance are ectoderm-derived cells that form the cranial placodes, which then differentiate into the cranial ganglia and sensory organs. Critical to a fully functioning head, defects in cranial placode and sensory organ development can result in congenital craniofacial anomalies. In a forward genetic screen aimed at identifying novel regulators of craniofacial development, we discovered an embryonically lethal mouse mutant, snouty, which exhibits malformation of the facial prominences, cranial nerves and vasculature. The snouty mutation was mapped to a single nucleotide change in a ubiquitously expressed gene, Med23, which encodes a subunit of the global transcription co-factor complex, Mediator. Phenotypic analyses revealed that the craniofacial anomalies, particularly of the cranial ganglia, were caused by a failure in the proper specification of cranial placode neuronal precursors. Molecular analyses determined that defects in cranial placode neuronal differentiation in Med23 sn/sn mutants were associated with elevated WNT/ß-catenin signaling, which can be partially rescued through combined Lrp6 and Wise loss-of-function. Our work therefore reveals a surprisingly tissue specific role for the ubiquitously expressed mediator complex protein Med23 in placode differentiation during cranial ganglia development. This highlights the importance of coupling general transcription to the regulation of WNT signaling during embryogenesis.

Retinoic acid synthesis and autoregulation mediate zonal patterning of vestibular organs and inner ear morphogenesis.

Retinoic acid (RA), a vitamin A (retinol) derivative, has pleiotropic functions during embryonic development. The synthesis of RA requires two enzymatic reactions: oxidation of retinol into retinaldehyde by alcohol dehydrogenases (ADHs) or retinol dehydrogenases (RDHs); and oxidation of retinaldehyde into RA by aldehyde dehydrogenases family 1, subfamily A (ALDH1as), such as ALDH1a1, ALDH1a2 and ALDH1a3. Levels of RA in tissues are regulated by spatiotemporal expression patterns of genes encoding RA-synthesizing and -degrading enzymes, such as cytochrome P450 26 (Cyp26 genes). Here, we show that RDH10 is important for both sensory and non-sensory formation of the vestibule of the inner ear. Mice deficient in Rdh10 exhibit failure of utricle-saccule separation, otoconial formation and zonal patterning of vestibular sensory organs. These phenotypes are similar to those of Aldh1a3 knockouts, and the sensory phenotype is complementary to that of Cyp26b1 knockouts. Together, these results demonstrate that RDH10 and ALDH1a3 are the key RA-synthesis enzymes involved in vestibular development. Furthermore, we discovered that RA induces Cyp26b1 expression in the developing vestibular sensory organs, which generates the differential RA signaling required for zonal patterning.

RDH10 function is necessary for spontaneous fetal mouth movement that facilitates palate shelf elevation.

Cleft palate is a common birth defect, occurring in approximately 1 in 1000 live births worldwide. Known etiological mechanisms of cleft palate include defects within developing palate shelf tissues, defects in mandibular growth and defects in spontaneous fetal mouth movement. Until now, experimental studies directly documenting fetal mouth immobility as an underlying cause of cleft palate have been limited to models lacking neurotransmission. This study extends the range of anomalies directly demonstrated to have fetal mouth movement defects correlated with cleft palate. Here, we show that mouse embryos deficient in retinoic acid (RA) have mispatterned pharyngeal nerves and skeletal elements that block spontaneous fetal mouth movement in utero Using X-ray microtomography, in utero ultrasound video, ex vivo culture and tissue staining, we demonstrate that proper retinoid signaling and pharyngeal patterning are crucial for the fetal mouth movement needed for palate formation. Embryos with deficient retinoid signaling were generated by stage-specific inactivation of retinol dehydrogenase 10 (Rdh10), a gene crucial for the production of RA during embryogenesis. The finding that cleft palate in retinoid deficiency results from a lack of fetal mouth movement might help elucidate cleft palate etiology and improve early diagnosis in human disorders involving defects of pharyngeal development.

Foxc2 is required for proper cardiac neural crest cell migration, outflow tract septation, and ventricle expansion.

BACKGROUND: Proper development of the great vessels of the heart and septation of the cardiac outflow tract requires cardiac neural crest cells. These cells give rise to the parasympathetic cardiac ganglia, the smooth muscle layer of the great vessels, some cardiomyocytes, and the conotruncal cushions and aorticopulmonary septum of the outflow tract. Ablation of cardiac neural crest cells results in defective patterning of each of these structures. Previous studies have shown that targeted deletion of the forkhead transcription factor C2 (Foxc2), results in cardiac phenotypes similar to that derived from cardiac neural crest cell ablation. RESULTS: We report that Foxc2-/- embryos on the 129s6/SvEv inbred genetic background display persistent truncus arteriosus and hypoplastic ventricles before embryonic lethality. Foxc2 loss-of-function resulted in perturbed cardiac neural crest cell migration and their reduced contribution to the outflow tract as evidenced by lineage tracing analyses together with perturbed expression of the neural crest cell markers Sox10 and Crabp1. Foxc2 loss-of-function also resulted in alterations in PlexinD1, Twist1, PECAM1, and Hand1/2 expression in association with vascular and ventricular defects. CONCLUSIONS: Our data indicate Foxc2 is required for proper migration of cardiac neural crest cells, septation of the outflow tract, and development of the ventricles. Developmental Dynamics 247:1286-1296, 2018. © 2018 Wiley Periodicals, Inc.

RDH10-mediated retinol metabolism and RARα-mediated retinoic acid signaling are required for submandibular salivary gland initiation.

In mammals, the epithelial tissues of major salivary glands generate saliva and drain it into the oral cavity. For submandibular salivary glands (SMGs), the epithelial tissues arise during embryogenesis from naïve oral ectoderm adjacent to the base of the tongue, which begins to thicken, express SOX9 and invaginate into underlying mesenchyme. The developmental mechanisms initiating salivary gland development remain unexplored. In this study, we show that retinoic acid (RA) signaling activity at the site of gland initiation is colocalized with expression of retinol metabolic genes Rdh10 and Aldh1a2 in the underlying SMG mesenchyme. Utilizing a novel ex vivo assay for SMG initiation developed for this study, we show that RDH10 and RA are required for salivary gland initiation. Moreover, we show that the requirement for RA in gland initiation involves canonical signaling through retinoic acid receptors (RAR). Finally, we show that RA signaling essential for gland initiation is transduced specifically through RARα, with no contribution from other RAR isoforms. This is the first study to identify a molecular signal regulating mammalian salivary gland initiation.

Mllt10 knockout mouse model reveals critical role of Af10-dependent H3K79 methylation in midfacial development.

Epigenetic regulation is required to ensure the precise spatial and temporal pattern of gene expression that is necessary for embryonic development. Although the roles of some epigenetic modifications in embryonic development have been investigated in depth, the role of methylation at lysine 79 (H3K79me) is poorly understood. Dot1L, a unique methyltransferase for H3K79, forms complexes with distinct sets of co-factors. To further understand the role of H3K79me in embryogenesis, we generated a mouse knockout of Mllt10, the gene encoding Af10, one Dot1L complex co-factor. We find homozygous Mllt10 knockout mutants (Mllt10-KO) exhibit midline facial cleft. The midfacial defects of Mllt10-KO embryos correspond to hyperterolism and are associated with reduced proliferation of mesenchyme in developing nasal processes and adjacent tissue. We demonstrate that H3K79me level is significantly decreased in nasal processes of Mllt10-KO embryos. Importantly, we find that expression of AP2α, a gene critical for midfacial development, is directly regulated by Af10-dependent H3K79me, and expression AP2α is reduced specifically in nasal processes of Mllt10-KO embryos. Suppression of H3K79me completely mimicked the Mllt10-KO phenotype. Together these data are the first to demonstrate that Af10-dependent H3K79me is essential for development of nasal processes and adjacent tissues, and consequent midfacial formation.

The role of retinol dehydrogenase 10 in the cone visual cycle.

Pigment regeneration is critical for the function of cone photoreceptors in bright and rapidly-changing light conditions. This process is facilitated by the recently-characterized retina visual cycle, in which Müller cells recycle spent all-trans-retinol visual chromophore back to 11-cis-retinol. This 11-cis-retinol is oxidized selectively in cones to the 11-cis-retinal used for pigment regeneration. However, the enzyme responsible for the oxidation of 11-cis-retinol remains unknown. Here, we sought to determine whether retinol dehydrogenase 10 (RDH10), upregulated in rod/cone hybrid retinas and expressed abundantly in Müller cells, is the enzyme that drives this reaction. We created mice lacking RDH10 either in cone photoreceptors, Müller cells, or the entire retina. In vivo electroretinography and transretinal recordings revealed normal cone photoresponses in all RDH10-deficient mouse lines. Notably, their cone-driven dark adaptation both in vivo and in isolated retina was unaffected, indicating that RDH10 is not required for the function of the retina visual cycle. We also generated transgenic mice expressing RDH10 ectopically in rod cells. However, rod dark adaptation was unaffected by the expression of RDH10 and transgenic rods were unable to use cis-retinol for pigment regeneration. We conclude that RDH10 is not the dominant retina 11-cis-RDH, leaving its primary function in the retina unknown.

Retinoic acid signaling regulates Krt5 and Krt14 independently of stem cell markers in submandibular salivary gland epithelium.

BACKGROUND: Retinoic acid (RA), the active metabolite of vitamin A, has been demonstrated to be important for growth and branching morphogenesis of mammalian embryonic salivary gland epithelium. However, it is not known whether RA functions directly within epithelial cells or in associated tissues that influence morphogenesis of salivary epithelium. Moreover, downstream targets of RA regulation have not been identified. RESULTS: Here, we show that canonical RA signaling occurs in multiple tissues of embryonic mouse salivary glands, including epithelium, associated parasympathetic ganglion neurons, and nonneuronal mesenchyme. By culturing epithelium explants in isolation from other tissues, we demonstrate that RA influences epithelium morphogenesis by direct action in that tissue. Moreover, we demonstrate that inhibition of RA signaling represses cell proliferation and expression of FGF10 signaling targets, and upregulates expression of basal epithelial keratins Krt5 and Krt14. Importantly, we show that the stem cell gene Kit is regulated inversely from Krt5/Krt14 by RA signaling. CONCLUSIONS: RA regulates Krt5 and Krt14 expression independently of stem cell character in developing salivary epithelium. RA, or chemical inhibitors of RA signaling, could potentially be used for modulating growth and differentiation of epithelial stem cells for the purpose of re-populating damaged glands or generating bioengineered organs. Developmental Dynamics 246:135-147, 2017. © 2016 Wiley Periodicals, Inc.

Enzymatic Metabolism of Vitamin A in Developing Vertebrate Embryos.

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

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.

Retinoic Acid Activity in Undifferentiated Neural Progenitors Is Sufficient to Fulfill Its Role in Restricting Fgf8 Expression for Somitogenesis.

Bipotent axial stem cells residing in the caudal epiblast during late gastrulation generate neuroectodermal and presomitic mesodermal progeny that coordinate somitogenesis with neural tube formation, but the mechanism that controls these two fates is not fully understood. Retinoic acid (RA) restricts the anterior extent of caudal fibroblast growth factor 8 (Fgf8) expression in both mesoderm and neural plate to control somitogenesis and neurogenesis, however it remains unclear where RA acts to control the spatial expression of caudal Fgf8. Here, we found that mouse Raldh2-/- embryos, lacking RA synthesis and displaying a consistent small somite defect, exhibited abnormal expression of key markers of axial stem cell progeny, with decreased Sox2+ and Sox1+ neuroectodermal progeny and increased Tbx6+ presomitic mesodermal progeny. The Raldh2-/- small somite defect was rescued by treatment with an FGF receptor antagonist. Rdh10 mutants, with a less severe RA synthesis defect, were found to exhibit a small somite defect and anterior expansion of caudal Fgf8 expression only for somites 1-6, with normal somite size and Fgf8 expression thereafter. Rdh10 mutants were found to lack RA activity during the early phase when somites are small, but at the 6-somite stage RA activity was detected in neural plate although not in presomitic mesoderm. Expression of a dominant-negative RA receptor in mesoderm eliminated RA activity in presomitic mesoderm but did not affect somitogenesis. Thus, RA activity in the neural plate is sufficient to prevent anterior expansion of caudal Fgf8 expression associated with a small somite defect. Our studies provide evidence that RA restriction of Fgf8 expression in undifferentiated neural progenitors stimulates neurogenesis while also restricting the anterior extent of the mesodermal Fgf8 mRNA gradient that controls somite size, providing new insight into the mechanism that coordinates somitogenesis with neurogenesis.

Retinoic acid regulates embryonic development of mammalian submandibular salivary glands.

Organogenesis is orchestrated by cell and tissue interactions mediated by molecular signals. Identification of relevant signals, and the tissues that generate and receive them, are important goals of developmental research. Here, we demonstrate that Retinoic Acid (RA) is a critical signaling molecule important for morphogenesis of mammalian submandibular salivary glands (SMG). By examining late stage RA deficient embryos of Rdh10 mutant mice we show that SMG development requires RA in a dose-dependent manner. Additionally, we find that active RA signaling occurs in SMG tissues, arising earlier than any other known marker of SMG development and persisting throughout gland morphogenesis. At the initial bud stage of development, we find RA production occurs in SMG mesenchyme, while RA signaling occurs in epithelium. We also demonstrate active RA signaling occurs in glands cultured ex vivo, and treatment with an inhibitor of RA signaling blocks growth and branching. Together these data identify RA signaling as a direct regulator of SMG organogenesis.

Mef2c-F10N enhancer driven ß-galactosidase (LacZ) and Cre recombinase mice facilitate analyses of gene function and lineage fate in neural crest cells.

Neural crest cells (NCC) comprise a multipotent, migratory stem cell and progenitor population that gives rise to numerous cell and tissue types within a developing embryo, including craniofacial bone and cartilage, neurons and glia of the peripheral nervous system, and melanocytes within the skin. Here we describe two novel stable transgenic mouse lines suitable for lineage tracing and analysis of gene function in NCC. Firstly, using the F10N enhancer of the Mef2c gene (Mef2c-F10N) linked to LacZ, we generated transgenic mice (Mef2c-F10N-LacZ) that express LacZ in the majority, if not all migrating NCC that delaminate from the neural tube. Mef2c-F10N-LacZ then continues to be expressed primarily in neurogenic, gliogenic and melanocytic NCC and their derivatives, but not in ectomesenchymal derivatives. Secondly, we used the same Mef2c-F10N enhancer together with Cre recombinase to generate transgenic mice (Mef2c-F10N-Cre) that can be used to indelibly label, or alter gene function in, migrating NCC and their derivatives. At early stages of development, Mef2c-F10N-LacZ and Mef2c-F10N-Cre label NCC in a pattern similar to Wnt1-Cre mice, with the exception that Mef2c-F10N-LacZ and Mef2c-F10N-Cre specifically label NCC that have delaminated from the neural plate, while premigratory NCC are not labeled. Thus, our Mef2c-F10N-LacZ and Mef2c-F10N-Cre transgenic mice provide new resources for tracing migratory NCC and analyzing gene function in migrating and differentiating NCC independently of NCC formation.

Cochleovestibular nerve development is integrated with migratory neural crest cells.

The cochleovestibular (CV) nerve, which connects the inner ear to the brain, is the nerve that enables the senses of hearing and balance. The aim of this study was to document the morphological development of the mouse CV nerve with respect to the two embryonic cells types that produce it, specifically, the otic vesicle-derived progenitors that give rise to neurons, and the neural crest cell (NCC) progenitors that give rise to glia. Otic tissues of mouse embryos carrying NCC lineage reporter transgenes were whole mount immunostained to identify neurons and NCC. Serial optical sections were collected by confocal microscopy and were compiled to render the three dimensional (3D) structure of the developing CV nerve. Spatial organization of the NCC and developing neurons suggest that neuronal and glial populations of the CV nerve develop in tandem from early stages of nerve formation. NCC form a sheath surrounding the CV ganglia and central axons. NCC are also closely associated with neurites projecting peripherally during formation of the vestibular and cochlear nerves. Physical ablation of NCC in chick embryos demonstrates that survival or regeneration of even a few individual NCC from ectopic positions in the hindbrain results in central projection of axons precisely following ectopic pathways made by regenerating NCC.

Antagonism between retinoic acid and fibroblast growth factor signaling during limb development.

The vitamin A metabolite retinoic acid (RA) provides patterning information during vertebrate embryogenesis, but the mechanism through which RA influences limb development is unclear. During patterning of the limb proximodistal axis (upper limb to digits), avian studies suggest that a proximal RA signal generated in the trunk antagonizes a distal fibroblast growth factor (FGF) signal. However, mouse and zebrafish genetic studies suggest that loss of RA suppresses forelimb initiation. Here, using genetic and pharmacological approaches, we demonstrate that limb proximodistal patterning is not RA dependent, thus indicating that RA-FGF antagonism does not occur along the proximodistal axis of the limb. Instead, our studies show that RA-FGF antagonism acts prior to limb budding along the anteroposterior axis of the trunk lateral plate mesoderm to provide a patterning cue that guides formation of the forelimb field. These findings reconcile disparate ideas regarding RA-FGF antagonism and provide insight into how endogenous RA programs the early embryo.