Retinoic acid synthesis and hindbrain patterning in the mouse embryo

Dhrs3a regulates retinoic acid biosynthesis through a feedback inhibition mechanism

Retinoic acid (RA) is an important developmental signaling molecule responsible for the patterning of multiple vertebrate tissues. RA is also a potent teratogen, causing multi-organ birth defects in humans. Endogenous RA levels must therefore be tightly controlled in the developing embryo. We used a microarray approach to identify genes that function as negative feedback regulators of retinoic acid signaling. We screened for genes expressed in early somite-stage embryos that respond oppositely to treatment with RA versus RA antagonists and validated them by RNA in situ hybridization. Focusing on genes known to be involved in RA metabolism, we determined that dhrs3a, which encodes a member of the short-chain dehydrogenase/reductase protein family, is both RA dependent and strongly RA inducible. Dhrs3a is known to catalyze the reduction of the RA precursor all-trans retinaldehyde to vitamin A; however, a developmental function has not been demonstrated. Using morpholino knockdown and mRNA over-expression, we demonstrate that Dhrs3a is required to limit RA levels in the embryo, primarily within the central nervous system. Dhrs3a is thus an RA-induced feedback inhibitor of RA biosynthesis. We conclude that retinaldehyde availability is an important level at which RA biosynthesis is regulated in vertebrate embryos.

Effect of retinoic acid signaling on Wnt/beta-catenin and FGF signaling during body axis extension

Cell-cell signaling regulated by retinoic acid (RA), Wnt/beta-catenin, and fibroblast growth factor (FGF) is important during body axis extension, and interactions between these pathways have been suggested. At early somite stages, Wnt/beta-catenin and FGF signaling domains exist both anterior and posterior to the developing trunk, whereas RA signaling occurs in between in the trunk under the control of the RA-synthesizing enzyme retinaldehyde dehydrogenase-2 (Raldh2). Previous studies demonstrated that vitamin A deficient quail embryos and Raldh2(-/-) mouse embryos lacking RA synthesis exhibit ectopic expression of Fgf8 and Wnt8a in the developing trunk. Here, we demonstrate that Raldh2(-/-) mouse embryos display an expansion of FGF signaling into the trunk monitored by Sprouty2 and Pea3 expression, and an expansion of Wnt/beta-catenin signaling detected by expression of Axin2, Tbx6, Cdx2, and Cdx4. Following loss of RA signaling, the caudal expression domains of Fgf8, Wnt8a, and Wnt3a expand anteriorly into the trunk, but no change is observed in caudal expression of Fgf4 or Fgf17 plus caudal expression of Fgf18 and Cdx1 is reduced. These findings suggest that RA repression of Fgf8, Wnt8a, and Wnt3a in the developing trunk functions to down-regulate FGF signaling and Wnt/beta-catenin signaling as the body axis extends.

Function of retinoic acid receptors during embryonic development

Retinoids, the active metabolites of vitamin A, regulate complex gene networks involved in vertebrate morphogenesis, growth, cellular differentiation and homeostasis. Studies performed in vitro, using either acellular systems or transfected cells, have shown that retinoid actions are mediated through heterodimers between the RAR and RXR nuclear receptors. However, in vitro studies indicate what is possible, but not necessarily what is actually occurring in vivo, because they are performed under non-physiological conditions. Therefore, genetic approaches in the animal have been be used to determine the physiological functions of retinoid receptors. Homologous recombination in embryonic stem cells has been used to generate germline null mutations of the RAR- and RXR-coding genes in the mouse. As reviewed here, the generation of such germline mutations, combined with pharmacological approaches to block the RA signalling pathway, has provided genetic evidence that RAR/RXR heterodimers are indeed the functional units transducing the RA signal during prenatal development. However, due to (i) the complexity in “hormonal” signalling through transduction by the multiple RARs and RXRs, (ii) the functional redundancies (possibly artefactually generated by the mutations) within receptor isotypes belonging to a given family, and (iii) in utero or early postnatal lethality of certain germline null mutations, these genetic studies have failed to reveal all the physiological functions of RARs and RXRs, notably in adults. Spatio-temporally-controlled somatic mutations generated in given cell types/tissues and at chosen times during postnatal life, will be required to reveal all the functions of RAR and RXR throughout the lifetime of the mouse.

Keeping an eye on retinoic acid signaling during eye development

Retinoic acid is a metabolic derivative of vitamin A that plays an essential function in cell-cell signaling by serving as a ligand for nuclear receptors that directly regulate gene expression. The final step in the conversion of retinol to retinoic acid is carried out by three retinaldehyde dehydrogenases encoded by Raldh1 (Aldh1a1), Raldh2 (Aldh1a2), and Raldh3 (Aldh1a3). Mouse Raldh gene knockout studies have been instrumental in understanding the mechanism of retinoic acid action during eye development. Retinoic acid signaling in the developing eye is particularly complex as all three Raldh genes contribute to retinoic acid synthesis in non-overlapping locations. During optic cup formation Raldh2 is first expressed transiently in perioptic mesenchyme, then later Raldh1 and Raldh3 expression begins in the dorsal and ventral retina, respectively, and these sources of retinoic acid are maintained in the fetus. Retinoic acid is not required for dorsoventral patterning of the retina as originally thought, but it is required for morphogenetic movements that form the optic cup, ventral retina, cornea, and eyelids. These findings will help guide future studies designed to identify retinoic acid target genes during eye organogenesis.

Transcription factor TBX1 overexpression induces downregulation of proteins involved in retinoic acid metabolism: a comparative proteomic analysis

TBX1 haploinsufficiency is considered a major contributor to the del22q11.2/DiGeorge syndrome (DGS) phenotype. We have used proteomic tools to look at all the major proteins involved in the TBX1-mediated pathways in an attempt to better understand the molecular interactions instrumental to its cellular functions. We found more than 90 proteins that could be targeted by TBX1 through different mechanisms. The most interesting observation is that overexpression of TBX1 results in down-regulation of two proteins involved in retinoic acid metabolism.

Regulation of Hoxb4 induction after neurulation by somite signal and neural competence

BACKGROUND

While the body axis is largely patterned along the anterior-posterior (A-P) axis during gastrulation, the central nervous system (CNS) shows dynamic changes in the expression pattern of Hox genes during neurulation, suggesting that the CNS refines the A-P pattern continuously after neural tube formation. This study aims at clarifying the role of somites in up-regulating Hoxb4 expression to eventually establish its final pattern and how the neural tube develops a competence to respond to extrinsic signals.

RESULTS

We show that somites are required for the up-regulation of Hoxb4 in the neural tube at the level of somites 1 to 5, the anterior-most domain of expression. However, each somite immediately adjacent to the neural tube is not sufficient at each level; planar signaling is additionally required particularly at the anterior-most segments of the expression domain. We also show that the dorsal side of the neural tube has a greater susceptibility to expressing Hoxb4 than the ventral region, a feature associated with dorsalization of the neural tube by BMP signals. BMP4 is additionally able to up-regulate Hoxb4 ventrally, but the effect is restricted to the axial levels at which Hoxb4 is normally expressed, and only in the presence of retinoic acid (RA) or somites, suggesting a role for BMP in rendering the neural tube competent to express Hoxb4 in response to RA or somite signals.

CONCLUSION

In identifying the collaboration between somites and neural tube competence in the induction of Hoxb4, this study demonstrates interplay between A-P and dorsal-ventral (D-V) patterning systems, whereby a specific feature of D-V polarity may be a prerequisite for proper A-P patterning by Hox genes.

Effects of retinoic acid on Dominant hemimelia expression in mice

BACKGROUND

Dominant hemimelia (Dh) is an autosomal dominant mutation that arose spontaneously in mice. Dh animals are asplenic and they exhibit asymmetric hindlimb defects in association with reduced numbers of lumbar vertebrae. These defects suggest that Dh acts early in embryonic development to affect patterning of the anterior-posterior (A-P) and left-right axes. This study was undertaken to determine whether retinoic acid (RA), which is involved in A-P patterning and coordination of bilaterally synchronized somitogenesis, affects phenotypic expression of the Dh gene.

METHODS

Thirty-four pregnant females were given, by oral intubation, a single dose of 50 or 75 mg all-trans RA per kilogram body weight at GD 9, 10, or 11. The pregnant females were then euthanized at GD 18 and fetuses removed by cesarean section. A total of 326 fetuses were identified by phenotype and linked DNA and their skeletons were analyzed.

RESULTS

There was a differential effect of RA on the axial skeleton and hindlimb of Dh/+ mice as compared to their wild-type littermates. Dose- and stage-specific effects on sternebrae and vertebrae were observed.

CONCLUSIONS

The effects of RA dosing on numbers of sternebrae and vertebrae suggest that Dh embryos have a primary defect in retinoid-mediated A-P patterning. Dosing with RA may produce the observed effects on phenotypic expression of Dh/+ by indirectly or directly modifying an already existing altered Hox expression pattern. As the relationship between axial patterning and the asymmetric limb is unknown, Dh is an important model for studying this relationship.

Combinatorial roles for zebrafish retinoic acid receptors in the hindbrain, limbs and pharyngeal arches

Retinoic acid (RA) signaling regulates multiple aspects of vertebrate embryonic development and tissue patterning, in part through the local availability of nuclear hormone receptors called retinoic acid receptors (RARs) and retinoid receptors (RXRs). RAR/RXR heterodimers transduce the RA signal, and loss-of-function studies in mice have demonstrated requirements for distinct receptor combinations at different stages of embryogenesis. However, the tissue-specific functions of each receptor and their individual contributions to RA signaling in vivo are only partially understood. Here we use morpholino oligonucleotides to deplete the four known zebrafish RARs (raraa, rarab, rarga, and rargb). We show that while all four are required for anterior-posterior patterning of rhombomeres in the hindbrain, there are unique requirements for rarga in the cranial mesoderm for hindbrain patterning, and rarab in lateral plate mesoderm for specification of the pectoral fins. In addition, the alpha subclass (raraa, rarab) is RA inducible, and of these only raraa expression is RA-dependent, suggesting that these receptors establish a region of particularly high RA signaling through positive-feedback. These studies reveal novel tissue-specific roles for RARs in controlling the competence and sensitivity of cells to respond to RA.

How degrading: Cyp26s in hindbrain development

The vitamin A derivative retinoic acid performs many functions in vertebrate development and is thought to act as a diffusible morphogen that patterns the anterior-posterior axis of the hindbrain. Recent work in several systems has led to insights into how the spatial distribution of retinoic acid is regulated. These have shown local control of synthesis and degradation, and computational models suggest that degradation by the Cyp26 enzymes plays a critical role in the formation of a morphogen gradient as well as its ability to compensate for fluctuations in RA levels.

Retinoic acid synthesis and signaling during early organogenesis

Retinoic acid, a derivative of vitamin A, is an essential component of cell-cell signaling during vertebrate organogenesis. In early development, retinoic acid organizes the trunk by providing an instructive signal for posterior neuroectoderm and foregut endoderm and a permissive signal for trunk mesoderm differentiation. At later stages, retinoic acid contributes to the development of the eye and other organs. Recent studies suggest that retinoic acid may act primarily in a paracrine manner and provide insight into the cell-cell signaling networks that control differentiation of pluripotent cells.

Opposing effects of retinoid signaling on astrogliogenesis in embryonic day 13 and 17 cortical progenitor cells

All-trans retinoic acid (RA) is a differentiation factor in many tissues. However, its role in astrogliogenesis has not been extensively studied. Here, we investigated the effect of RA on the regulation of astrogliogenesis at different cortical developmental stages. We prepared rat cortical progenitor cells from embryonic day (E) 13 and E17, which correspond to the beginning of neurogenic and astrogliogenic periods, respectively. Surprisingly, RA promoted astrogliogenesis at E17 but inhibited astrogliogenesis induced by ciliary neurotrophic factor (CNTF) at E13. The inhibitory effect of RA on astrogliogenesis at E13 was not due to premature commitment of progenitors to a neuronal or oligodendroglial lineage. Rather, RA retained more progenitors in a proliferative state. Furthermore, RA inhibition of astrogliogenesis at E13 was independent of STAT3 signaling and required the function of the alpha and beta isoforms of the RA receptors (RAR). Moreover, the differential response of E13 and E17 progenitors to RA was due to differences in the intrinsic properties of these cells that are preserved in vitro. The inhibitory effect of RA on cytokine-induced astrogliogenesis at E13 may contribute to silencing of any potential precocious astrogliogenesis during the neurogenic period.

Dynamic expression of the retinoic acid-synthesizing enzyme retinol dehydrogenase 10 (rdh10) in the developing mouse brain and sensory organs

Organs develop through many tissue interactions during embryogenesis, involving numerous signaling cascades and gene products. One of these signaling molecules is retinoic acid (RA), an active vitamin A derivative, which in mammalian embryos is synthesized from maternal retinol by two oxidative reactions involving alcohol/retinol dehydrogenases (ADH/RDHs) and retinaldehyde dehydrogenases (RALDHs), respectively. The activity of RALDHs is known to be crucial for RA synthesis; however, recently a retinol dehydrogenase (RDH10) has been shown to represent a new limiting factor in this synthesis. We investigated the spatiotemporal distribution of Rdh10 gene transcripts by in situ hybridization and quantitative polymerase chain reaction (PCR) during development of the brain and sensory organs. Although Rdh10 relative mRNA levels decline throughout brain development, we show a strong and lasting expression in the meninges and choroid plexuses. Rdh10 expression is also specifically seen in the striatum, a known site of retinoid signaling. In the eye, regional expression is observed both in the prospective pigmented epithelium and neural retina. In the inner ear Rdh10 expression is specific to the endolymphatic system and later the stria vascularis, both organs being involved in endolymph homeostasis. Furthermore, in the peripheral olfactory system and the vibrissae follicles, expression is present from early stages in regions where sensory receptors appear and mesenchymal/epithelial interactions take place. The distribution of Rdh10 transcripts during brain and sensory organ development is consistent with a role of this enzyme in generating region-specific pools of retinaldehyde that will be used by the various RALDHs to refine the patterns of RA synthesis.

pCREB is involved in neural induction of mouse embryonic stem cells by RA

Mouse embryonic stem (ES) cells can be induced by various chemicals to differentiate into a variety of cell types in vitro. In our study, retinoic acid (RA), one of the most important inducers, used at a concentration of 5 microM, was found to induce the differentiation of ES cells into neural progenitor cells (NPCs). During embryoid body (EB) differentiation, the level of active cyclic AMP response element-binding protein (CREB) was relatively high when 5 microM RA treatment was performed. Inhibition of CREB activity committed EBs to becoming other germ layers, whereas increased expression of CREB enhanced NPC differentiation. Moreover, RA increased the expression of active CREB by enhancing the activity of JNK. Our research suggests that CREB plays a role in RA-induced NPC differentiation by increasing the expression of active JNK.

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.

Complex regulation of cyp26a1 creates a robust retinoic acid gradient in the zebrafish embryo

Positional identities along the anterior–posterior axis of the vertebrate nervous system are assigned during gastrulation by multiple posteriorizing signals, including retinoic acid (RA), fibroblast growth factors (Fgfs), and Wnts. Experimental evidence has suggested that RA, which is produced in paraxial mesoderm posterior to the hindbrain by aldehyde dehydrogenase 1a2 (aldh1a2/raldh2), forms a posterior-to-anterior gradient across the hindbrain field, and provides the positional information that specifies the locations and fates of rhombomeres. Recently, alternative models have been proposed in which RA plays only a permissive role, signaling wherever it is not degraded. Here we use a combination of experimental and modeling tools to address the role of RA in providing long-range positional cues in the zebrafish hindbrain. Using cell transplantation and implantation of RA-coated beads into RA-deficient zebrafish embryos, we demonstrate that RA can directly convey graded positional information over long distances. We also show that expression of Cyp26a1, the major RA-degrading enzyme during gastrulation, is under complex feedback and feedforward control by RA and Fgf signaling. The predicted consequence of such control is that RA gradients will be both robust to fluctuations in RA synthesis and adaptive to changes in embryo length during gastrulation. Such control also provides an explanation for the fact that loss of an endogenous RA gradient can be compensated for by RA that is provided in a spatially uniform manner.

The formation of gradients of morphogens, signaling molecules that determine cell fates in a concentration-dependent manner, is a fundamental process in developmental biology. Several morphogens pattern the anterior–posterior (head to tail) axis of the vertebrate nervous system, including the vitamin A derivative, retinoic acid (RA) and fibroblast growth factors (Fgfs). However, it remains unclear how the activities of such morphogen gradients are coordinated. We have addressed this question by combining genetic experiments in zebrafish and computational analyses. We show that RA acts as a graded signal over long distances and that its gradient is shaped, to a large extent, by local control of RA degradation. In particular, RA promotes and Fgf suppresses RA degradation, thereby linking the shapes of RA and Fgf gradients. Computational models suggest that this linkage helps make RA-mediated patterning robust to changes in the rate at which RA is synthesized (which may vary with levels of dietary vitamin A) as well as in the size and shape of the embryo during development. Analogous regulatory loops may be used for similar purposes in other tissues in which RA and Fgfs interact, as well as in other morphogen systems.

Experimental and computational studies in zebrafish reveal a complex system regulating degradation of the vitamin A derivative retinoic acid along the anterior-posterior axis, which helps explain how morphogen gradients are established and maintained.

Retinoic acid is a potential dorsalising signal in the late embryonic chick hindbrain

BACKGROUND

Human retinoic acid teratogenesis results in malformations of dorsally derived hindbrain structures such as the cerebellum, noradrenergic hindbrain neurons and the precerebellar system. These structures originate from the rhombic lip and adjacent dorsal precursor pools that border the fourth ventricle roofplate. While retinoic acid synthesis is known to occur in the meninges that blanket the hindbrain, the particular sensitivity of only dorsal structures to disruptions in retinoid signalling is puzzling. We therefore looked for evidence within the neural tube for more spatiotemporally specific signalling pathways using an in situ hybridisation screen of known retinoic acid pathway transcripts.

RESULTS

We find that there are highly restricted domains of retinoic acid synthesis and breakdown within specific hindbrain nuclei as well as the ventricular layer and roofplate. Intriguingly, transcripts of cellular retinoic acid binding protein 1 are always found at the interface between dividing and post-mitotic cells. By contrast to earlier stages of development, domains of synthesis and breakdown in post-mitotic neurons are co-localised. At the rhombic lip, expression of the mRNA for retinoic acid synthesising and catabolising enzymes is spatially highly organised with respect to the Cath1-positive precursors of migratory precerebellar neurons.

CONCLUSION

The late developing hindbrain shows patterns of retinoic acid synthesis and use that are distinct from the well characterised phase of rostrocaudal patterning. Selected post-mitotic populations, such as the locus coeruleus, appear to both make and break down retinoic acid suggesting that a requirement for an autocrine, or at least a highly localised paracrine signalling network, might explain its acute sensitivity to retinoic acid disruption. At the rhombic lip, retinoic acid is likely to act as a dorsalising factor in parallel with other roofplate signalling pathways. While its precise role is unclear, retinoic acid is potentially well placed to regulate temporally determined cell fate decisions within the rhombic lip precursor pool.

Patterning and axon guidance of cranial motor neurons

The cranial motor nerves control muscles involved in eye, head and neck movements, feeding, speech and facial expression. The generic and specific properties of cranial motor neurons depend on a matrix of rostrocaudal and dorsoventral patterning information. Repertoires of transcription factors, including Hox genes, confer generic and specific properties on motor neurons, and endow subpopulations at various axial levels with the ability to navigate to their targets. Cranial motor axon projections are guided by diffusible cues and aided by guideposts, such as nerve exit points, glial cells and muscle primordia. The recent identification of genes that are mutated in human cranial dysinnervation disorders is now shedding light on the functional consequences of perturbations of cranial motor neuron development.

Direct regulation of vHnf1 by retinoic acid signaling and MAF-related factors in the neural tube

The homeodomain transcription factor vHNF1 plays an essential role in the patterning of the caudal segmented hindbrain, where it participates in the definition of the boundary between rhombomeres (r) 4 and 5 and in the specification of the identity of r5 and r6. Understanding the molecular basis of vHnf1 own expression therefore constitutes an important issue to decipher the regulatory network governing hindbrain patterning. We have identified a highly conserved 800-bp enhancer element located in the fourth intron of vHnf1 and whose activity recapitulates vHnf1 neural expression in transgenic mice. Functional analysis of this enhancer revealed that it contains two types of essential motifs, a retinoic acid response element and two half T-MARE sites, indicating that it integrates direct inputs from the retinoic acid signaling cascade and MAF-related factors. Our data suggest that MAFB, which is itself regulated by vHNF1, acts as a positive modulator of vHnf1 in r5 and r6, whereas another MAF-related factor is absolutely required for the expression of vHnf1 in both the hindbrain and the spinal cord. We propose a model accounting for the initiation and maintenance phases of vHnf1 expression and for the establishment of the r4/r5 boundary, based on cooperative contributions of Maf factors and retinoic acid signaling.

Retinoic acid regulation of the somitogenesis clock

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

Platelet-derived growth factor C plays a role in the branchial arch malformations induced by retinoic acid

BACKGROUND

All-trans-retinoic acid (RA) can produce branchial arch abnormalities in postimplantation rodent embryos cultured in vitro. Platelet-derived growth factor C (PDGF-C) was recently identified as a member of the PDGF ligand family. Many members of the PDGF family are essential for branchial arch morphogenesis and can be regulated by RA. The roles of PDGF-C in branchial arch malformations induced by RA and possible mechanisms were investigated.

METHODS

In whole embryo culture (WEC), mouse embryos were exposed to RA at 0, 0.1, 0.4, 1.0, or 10.0 microM, PDGF-C at 25, 50, or 75 ng/mL, or PDGF-C at 25, 50, or 75 ng/mL containing 0.4 microM RA. After 48 h of culture, mouse embryos were examined for dysmorphogenesis, and whole-mount immunohistochemistry was applied to PDGF-C. In explant cultures, explants were exposed to the same doses of RA and PDGF-C as WEC. Semiquantitative RT-PCR, zymography, and reverse zymography were used to evaluate the expressions and activities of matrix metalloproteinase (MMP)-2, MMP-14, and tissue inhibitor of metalloproteinase (TIMP)-2.

RESULTS

PDGF-C was reduced by RA, and exogenous PDGF-C rescued the branchial arch malformations induced by RA. Moreover, PDGF-C prevented RA-induced inhibition of the migratory ability of mesenchymal cells in the first branchial arch, by regulating the expressions of MMP-2, MMP-14, and TIPM-2.

CONCLUSIONS

Our results suggest that RA exposure reduces the expression of PDGF-C. The branchial arch malformations resulting from fetal RA exposure are caused at least partially by loss of PDGF-C and subsequent misregulations of the expressions of MMP-2, MMP-14, and TIMP-2.

Role of retinoic acid during forebrain development begins late when Raldh3 generates retinoic acid in the ventral subventricular zone

Retinoic acid (RA) synthesized by Raldh3 in the frontonasal surface ectoderm of chick embryos has been suggested to function in early forebrain patterning by regulating Fgf8, Shh, and Meis2 expression. Similar expression of Raldh3 exists in E8.75 mouse embryos, but Raldh2 is also expressed in the optic vesicle at this stage suggesting that both genes may play a role in early forebrain patterning. Furthermore, Raldh3 is expressed later in the forebrain itself (lateral ganglionic eminence; LGE) starting at E12.5, suggesting a later role in forebrain neurogenesis. Here we have analyzed mouse embryos carrying single or double null mutations in Raldh2 and Raldh3 for defects in forebrain development. Raldh2(-/-);Raldh3(-/-) embryos completely lacked RA signaling activity in the early forebrain, but exhibited relatively normal expression of Fgf8, Shh, and Meis2 in the forebrain. Thus, we find no clear requirement for RA in controlling expression of these important forebrain patterning genes, but Raldh3 expression in the frontonasal surface ectoderm was found to be needed for normal Fgf8 expression in the olfactory pit. Our studies revealed that later expression of Raldh3 in the subventricular zone of the LGE is required for RA signaling activity in the ventral forebrain. Importantly, expression of dopamine receptor D2 in E18.5 Raldh3(-/-) embryos was essentially eliminated in the developing nucleus accumbens, a tissue lying close to the source of RA provided by Raldh3. Our results suggest that the role of RA during forebrain development begins late when Raldh3 expression initiates in the ventral subventricular zone.

A review of the functional role and of the expression profile of retinoid signaling and of nuclear receptors in human spinal cord

Spinal cord degenerative pathologies in humans cause extensive disability and require a broad range of specialist and palliative medical interventions. In amyotrophic lateral sclerosis (ALS), motor cell loss leads to extensive paralysis and to death from respiratory failure in 3-5 years form disease onset. A wide range of molecular changes forms the basis of spinal cord involvement in ALS, including the reactivation of molecular pathways with potentially neurorestorative properties. Central to this tissue repair mechanism is the differential regulation of components of the retinoid signaling (ReS), a molecular pathway encompassing a variety of proteins functioning as transporters, signaling factors and metabolizing enzymes for retinoic acid. In this paper, we review the strong body of experimental evidence supporting retinoid signaling's primary role in spinal cord embryonic differentiation and its likely survival-promoting function in ALS. We discuss the potential involvement in ALS pathogenesis of a subgroup of nuclear receptors (NRs) that act as functional partners of retinoid receptors in human spinal cord. We also provide a review of the expression profile of 25 ReS and NRs genes in human adult spinal cord and in motor neurons of healthy and ALS individuals, using data retrieved from independent datasets obtained through serial analysis of gene expression and array investigations. Based on published expression data, we outline a tentative expression profile of ReS and functionally synergic NR genes in human spinal cord that could guide further experiments to clarify the role of these molecules in mature nervous tissue and suggest potential treatment strategies that could have therapeutic potentials in ALS.

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

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

Krox20 is down-regulated following triazole in vitro embryonic exposure: a polycompetitor-based assay

This study was conducted in order to analyse gene-expression alterations in rat embryos following exposure to triazoles, using an easy-handling approach. Triazole derivatives have been shown to alter the morphology of cranio-facial structures and to induce abnormalities in hindbrain patterning and neural crest cell migration. Specification of hindbrain segments is regulated by retinoic acid and the hox code. Krox20 was chosen as molecular marker for its specific distribution in the anterior neural tube. In fact, this zinc-finger protein is expressed in rhombomere 3 and 5. Mis-regulation of Krox20 levels have shown to induce severe alterations in the correct patterning of the rhomboencephalon and the derived structures. In order to analyse Krox20 mRNA levels in rat embryos exposed in vitro to the triazole derivative triadimefon, a semi-quantitative approach utilising the competitive RT-PCR was chosen. A lambda phage-based plasmid construct that could compete with target and internal standard gene at the same time during enzymatic reaction was generated. Results were confirmed by real-time RT-PCR analysis on the same samples. Our data show a down-regulation of Krox20 transcript levels after exposure to the triazole derivative, implying a key role of this molecule in the pathogenic pathway induced by triazole exposure.

The oxidizing enzyme CYP26a1 tightly regulates the availability of retinoic acid in the gastrulating mouse embryo to ensure proper head development and vasculogenesis

Retinoic acid (RA) has been implicated as one of the signals providing a posterior character to the developing vertebrate central nervous system. Embryonic RA first appears in the posterior region of the gastrulating embryo up to the node level, where it may signal within the adjacent epiblast and/or newly induced neural plate to induce a hindbrain and spinal cord fate. Conversely, rostral head development requires forebrain-inducing signals produced by the anterior visceral endoderm and/or prechordal mesoderm, and there is evidence that RA receptors must be in an unliganded state to ensure proper head development. As RA is a diffusible lipophilic molecule, some mechanism(s) must therefore have evolved to prevent activation of RA targets in anterior regions of the embryo. This might result from RA catabolism mediated by the CYP26A1 oxidizing enzyme, which is transiently expressed in anteriormost embryonic tissues; however, previous analysis of Cyp26a1(-/-) mouse mutants did not clearly support this hypothesis. Here we show that Cyp26a1(-/-) null mutants undergo head truncations when exposed to maternally-derived RA, at doses that do not affect wild-type head development. These anomalies are linked to a widespread ectopic RA signaling activity in rostral head tissues of CYP26A1-deficient embryos. Thus, CYP26A1 is required in the anterior region of the gastrulating mouse embryo to prevent teratological effects that may result from RA signaling. We also report a novel role of CYP26A1 during early development of the intra- and extra-embryonic vascular networks.

21st century neontology and the comparative development of the vertebrate skull

Classic neontology (comparative embryology and anatomy), through the application of the concept of homology, has demonstrated that the development of the gnathostome (jawed vertebrate) skull is characterized both by a fidelity to the gnathostome bauplan and the exquisite elaboration of final structural design. Just as homology is an old concept amended for modern purposes, so are many of the questions regarding the development of the skull. With due deference to Geoffroy-St. Hilaire, Cuvier, Owen, Lankester et al., we are still asking: How are bauplan fidelity and elaboration of design maintained, coordinated, and modified to generate the amazing diversity seen in cranial morphologies? What establishes and maintains pattern in the skull? Are there universal developmental mechanisms underlying gnathostome autapomorphic structural traits? Can we detect and identify the etiologies of heterotopic (change in the topology of a developmental event), heterochronic (change in the timing of a developmental event), and heterofacient (change in the active capacetence, or the elaboration of capacity, of a developmental event) changes in craniofacial development within and between taxa? To address whether jaws are all made in a like manner (and if not, then how not), one needs a starting point for the sake of comparison. To this end, we present here a "hinge and caps" model that places the articulation, and subsequently the polarity and modularity, of the upper and lower jaws in the context of cranial neural crest competence to respond to positionally located epithelial signals. This model expands on an evolving model of polarity within the mandibular arch and seeks to explain a developmental patterning system that apparently keeps gnathostome jaws in functional registration yet tractable to potential changes in functional demands over time. It relies upon a system for the establishment of positional information where pattern and placement of the "hinge" is driven by factors common to the junction of the maxillary and mandibular branches of the first arch and of the "caps" by the signals emanating from the distal-most first arch midline and the lamboidal junction (where the maxillary branch meets the frontonasal processes). In this particular model, the functional registration of jaws is achieved by the integration of "hinge" and "caps" signaling, with the "caps" sharing at some critical level a developmental history that potentiates their own coordination. We examine the evidential foundation for this model in mice, examine the robustness with which it can be applied to other taxa, and examine potential proximate sources of the signaling centers. Lastly, as developmental biologists have long held that the anterior-most mesendoderm (anterior archenteron roof or prechordal plate) is in some way integral to the normal formation of the head, including the cranial skeletal midlines, we review evidence that the seminal patterning influences on the early anterior ectoderm extend well beyond the neural plate and are just as important to establishing pattern within the cephalic ectoderm, in particular for the "caps" that will yield medial signaling centers known to coordinate jaw development.

An early role for WNT signaling in specifying neural patterns of Cdx and Hox gene expression and motor neuron subtype identity

The link between extrinsic signaling, progenitor cell specification and neuronal subtype identity is central to the developmental organization of the vertebrate central nervous system. In the hindbrain and spinal cord, distinctions in the rostrocaudal identity of progenitor cells are associated with the generation of different motor neuron subtypes. Two fundamental classes of motor neurons, those with dorsal (dMN) and ventral (vMN) exit points, are generated over largely non-overlapping rostrocaudal domains of the caudal neural tube. Cdx and Hox genes are important determinants of the rostrocaudal identity of neural progenitor cells, but the link between early patterning signals, neural Cdx and Hox gene expression, and the generation of dMN and vMN subtypes, is unclear. Using an in vitro assay of neural differentiation, we provide evidence that an early Wnt-based program is required to interact with a later retinoic acid- and fibroblast growth factor-mediated mechanism to generate a pattern of Cdx and Hox profiles characteristic of hindbrain and spinal cord progenitor cells that prefigure the generation of vMNs and dMNs.

Rescue of cytochrome P450 oxidoreductase (Por) mouse mutants reveals functions in vasculogenesis, brain and limb patterning linked to retinoic acid homeostasis

Cytochrome P450 oxidoreductase (POR) acts as an electron donor for all cytochrome P450 enzymes. Knockout mouse Por(-/-) mutants, which are early embryonic (E9.5) lethal, have been found to have overall elevated retinoic acid (RA) levels, leading to the idea that POR early developmental function is mainly linked to the activity of the CYP26 RA-metabolizing enzymes (Otto et al., Mol. Cell. Biol. 23, 6103-6116). By crossing Por mutants with a RA-reporter lacZ transgene, we show that Por(-/-) embryos exhibit both elevated and ectopic RA signaling activity e.g. in cephalic and caudal tissues. Two strategies were used to functionally demonstrate that decreasing retinoid levels can reverse Por(-/-) phenotypic defects, (i) by culturing Por(-/-) embryos in defined serum-free medium, and (ii) by generating compound mutants defective in RA synthesis due to haploinsufficiency of the retinaldehyde dehydrogenase 2 (Raldh2) gene. Both approaches clearly improved the Por(-/-) early phenotype, the latter allowing mutants to be recovered up until E13.5. Abnormal brain patterning, with posteriorization of hindbrain cell fates and defective mid- and forebrain development and vascular defects were rescued in E9.5 Por(-/-) embryos. E13.5 Por(-/-); Raldh2(+/-) embryos exhibited abdominal/caudal and limb defects that strikingly phenocopy those of Cyp26a1(-/-) and Cyp26b1(-/-) mutants, respectively. Por(-/-); Raldh2(+/-) limb buds were truncated and proximalized and the anterior-posterior patterning system was not established. Thus, POR function is indispensable for the proper regulation of RA levels and tissue distribution not only during early embryonic development but also in later morphogenesis and molecular patterning of the brain, abdominal/caudal region and limbs.

PDGF-C participates in branchial arch morphogenesis and is down-regulated by retinoic acid

Retinoic acid (RA) is a teratogen that induces a variety of craniofacial abnormalities, including branchial arch deformities and cleft palate. Platelet-derived growth factor C (PDGF-C) is a recently identified member of the PDGF family. PDGF-C contributes to normal development of the heart, central nervous system, kidney and palatogenesis. But the roles of PDGF-C in branchial arches development and the relationship between PDGF-C and RA-induced branchial arches abnormalities are poorly understood. We examined the effects of RA on PDGF-C and its receptor PDGFR-alpha expressions. We demonstrated that administration of RA to mouse embryos resulted in dramatic losses of PDGF-C and its receptor PDGFR-alpha. Furthermore, we confirmed that blocking PDGF-C signaling by anti-PDGF-C neutralization antibody led to branchial arch malformations similar to that of RA induced, both hypoplastic branchial arches and FBA. These findings suggest the down-regulation of PDGF-C may be one of mechanisms of branchial arch abnormalities induced by RA and PDGF-C signaling is required for branchial arch morphogenesis.

CYP26A1 and CYP26C1 cooperatively regulate anterior-posterior patterning of the developing brain and the production of migratory cranial neural crest cells in the mouse

The appropriate regulation of retinoic acid signaling is indispensable for patterning of the vertebrate central nervous system along the anteroposterior (A-P) axis. Although both CYP26A1 and CYP26C1, retinoic acid-degrading enzymes that are expressed at the anterior end of the gastrulating mouse embryo, have been thought to play an important role in central nervous system patterning, the detailed mechanism of their contribution has remained largely unknown. We have now analyzed CYP26A1 and CYP26C1 function by generating knockout mice. Loss of CYP26C1 did not appear to affect embryonic development, suggesting that CYP26A1 and CYP26C1 are functionally redundant. In contrast, mice lacking both CYP26A1 and CYP26C1 were found to manifest a pronounced anterior truncation of the brain associated with A-P patterning defects that reflect expansion of posterior identity at the expense of anterior identity. Furthermore, Cyp26a1-/-Cyp26c1-/- mice fail to produce migratory cranial neural crest cells in the forebrain and midbrain. These observations, together with a reevaluation of Cyp26a1 mutant mice, suggest that the activity of CYP26A1 and CYP26C1 is required for correct A-P patterning and production of migratory cranial neural crest cells in the developing mammalian brain.

Retinoic acid signalling is required for specification of pronephric cell fate

The mechanisms by which a subset of mesodermal cells are committed to a nephrogenic fate are largely unknown. In this study, we have investigated the role of retinoic acid (RA) signalling in this process using Xenopus laevis as a model system and Raldh2 knockout mice. Pronephros formation in Xenopus embryo is severely impaired when RA signalling is inhibited either through expression of a dominant-negative RA receptor, or by expressing the RA-catabolizing enzyme XCyp26 or through treatment with chemical inhibitors. Conversely, ectopic RA signalling expands the size of the pronephros. Using a transplantation assay that inhibits RA signalling specifically in pronephric precursors, we demonstrate that this signalling is required within this cell population. Timed antagonist treatments show that RA signalling is required during gastrulation for expression of Xlim-1 and XPax-8 in pronephric precursors. Moreover, experiments conducted with a protein synthesis inhibitor indicate that RA may directly regulate Xlim-1. Raldh2 knockout mouse embryos fail to initiate the expression of early kidney-specific genes, suggesting that implication of RA signalling in the early steps of kidney formation is evolutionary conserved in vertebrates.

Postulated pathogenic pathway in triazole fungicide induced dysmorphogenic effects

Triazole fungicides are used in medicine as well as in agricultural treatment of mycoses. The pharmacological mechanism is related to the inhibition of CYP enzymes involved in the formation of the fungal walls. A similar inhibition of human CYP enzymes has been suggested as the cause of triazole side effects in humans. An important role of some CYP isoforms (CYP26 isoforms) expressed during mammalian development is the catabolism of retinoic acid, a known morphogen in vertebrates and invertebrates. The adverse effects on morphogenesis, observed after exposure of mammalian, amphibian and ascidiacea, are compared to the reported effects of triazole in humans. The possible pathogenic pathway in triazole-related teratogenesis is discussed on the basis of different experimental approaches.

Dynamic expression of retinoic acid-synthesizing and -metabolizing enzymes in the developing mouse inner ear

Retinoic acid signaling plays essential roles in morphogenesis and neural development through transcriptional regulation of downstream target genes. It is believed that the balance between the activities of synthesizing and metabolizing enzymes determines the amount of active retinoic acid to which a developing tissue is exposed. In this study, we investigated spatiotemporal expression patterns of four synthesizing enzymes, the retinaldehyde dehydrogenases 1, 2, 3, and 4 (Raldh1, Raldh2, Raldh3, and Raldh4) and two metabolizing enzymes (Cyp26A1 and Cyp26B1) in the embryonic and postnatal mouse inner ear by using quantitative reverse transcriptase polymerase chain reaction (RT-PCR), in situ hybridization, and Western blot analysis. Quantitative RT-PCR analysis and Western blot data revealed that the expression of CYP26s was much higher than that of Raldhs at early embryonic ages but that Cyp26 expression was downregulated during embryonic development. Conversely, the expression levels of Raldh2 and -3 increased during development and were significantly higher than the Cyp26 levels at postnatal day 20. At this age, Raldh3 was expressed predominantly in the cochlea, whereas Raldh2 was present in the vestibular end organ. At early embryonic stages, as observed by in situ hybridization, the synthesizing enzymes were expressed only in the dorsoventral epithelium of the otocyst, whereas the metabolizing enzymes were present mainly in mesenchymal cells surrounding the otic epithelium. At later stages, Raldh2, Raldh3, and Cyp26B1 were confined to the stria vascularis, spiral ganglion, and supporting cells in the cochlear and vestibular epithelia, respectively. The downregulation of Cyp26s and the upregulation of Raldhs after birth during inner ear maturation suggest tissue changes in the sensitivity to retinoic acid concentrations.

Retinoic acid regulates morphogenesis and patterning of posterior foregut derivatives

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

Dynamic and sequential patterning of the zebrafish posterior hindbrain by retinoic acid

A prominent region of the vertebrate hindbrain is subdivided along the anterior-posterior axis into a series of seven segments, or rhombomeres. The identity of each rhombomere is specified by the expression of conserved transcription factors, including Krox-20, vHnf1, Val (Kreisler, Mafb) and several Hox proteins. Previous work has shown that retinoic acid (RA) signaling plays a critical role in regulating the expression of these factors and that more posterior rhombomeres require higher levels of RA than more anterior rhombomeres. Models to account for RA concentration dependency have proposed either a static RA gradient or increasing time periods of RA exposure. Here, we provide evidence against both of these models. We show that early zebrafish rhombomere-specification genes, including vhnf1 in r5-r6 and hoxd4a in r7, initiate expression sequentially in the hindbrain, each adjacent to the source of RA synthesis in paraxial mesoderm. By knocking down RA signaling, we show that progressively more posterior rhombomeres require increasingly higher levels of RA signaling, and vhnf1 and hoxd4a expression are particularly RA-dependent. RA synthesis is required just at the time of initiation, but not for maintenance, of vhnf1 and hoxd4a expression. Furthermore, a premature RA increase causes premature activation of vhnf1 and hoxd4a expression. Our results support a new model of dynamic RA action in the hindbrain, in which a temporally increasing source of RA is required to sequentially initiate progressively more posterior rhombomere identities.

Function of retinoid nuclear receptors: lessons from genetic and pharmacological dissections of the retinoic acid signaling pathway during mouse embryogenesis

Retinoic acid (RA) is involved in vertebrate morphogenesis, growth, cellular differentiation, and tissue homeostasis. The use of in vitro systems initially led to the identification of nuclear receptor RXR/RAR heterodimers as possible transducers of the RA signal. To unveil the physiological functions of RARs and RXRs, genetic and pharmacological studies have been performed in the mouse. Together, their results demonstrate that (a) RXR/RAR heterodimers in which RXR is either transcriptionally active or silent are involved in the transduction of the RA signal during prenatal development, (b) specific RXRalpha/RAR heterodimers are required at many distinct stages during early embryogenesis and organogenesis, (c) the physiological role of RA and its receptors cannot be extrapolated from teratogenesis studies using retinoids in excess. Additional cell type-restricted and temporally controlled somatic mutagenesis is required to determine the functions of RARs and RXRs during postnatal life.

Loss of orphan nuclear receptor GCNF function disrupts forebrain development and the establishment of the isthmic organizer

The isthmic organizer, which is located at the midbrain-hindbrain boundary, is important for midbrain development. The mechanism by which the development of the organizer is initiated and maintained is not well understood. Inactivation of the gene encoding the orphan nuclear receptor, GCNF, diminishes the expression of secreted signaling molecules, Fgf8 and Wnt1, the paired box genes Pax2/5, En1/2, and homeodomain transcription factor Gbx2; all of which are essential for isthmic organizer function. In addition, full neuronal differentiation is not observed in the midbrain region of GCNF-/- embryos. Increased cell death may contribute to the loss of midbrain structure in GCNF-/- embryos. These results indicate that GCNF is required for establishment of the isthmic organizer, thereby regulating the midbrain development.

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

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

Retinoid signaling in inner ear development

The inner ear originates from an embryonic ectodermal placode and rapidly develops into a three-dimensional structure (the otocyst) through complex molecular and cellular interactions. Many genes and their products are involved in inner ear induction, organogenesis, and cell differentiation. Retinoic acid (RA) is an endogenous signaling molecule that may play a role during different phases of inner ear development, as shown from pathological observations. To gain insight into the function of RA during inner ear development, we have investigated the spatio-temporal expression patterns of major components of RA signaling pathway, including cellular retinoic acid binding proteins (CRABPs), cellular retinoid binding proteins (CRBPs), retinaldehyde dehydrogenases (RALDHs), catabolic enzymes (CYP26s), and nuclear receptors (RARs). Although the CrbpI, CrabpI, and -II genes are specifically expressed in the inner ear throughout development, loss-of-function studies have revealed that these proteins are dispensable for inner development and function. Several Raldh and Cyp26 gene transcripts are expressed at embryological day (E) 9.0-9.5 in the otocyst and show mainly complementary distributions in the otic epithelium and mesenchyme during following stages. From Western blot, RT-PCR, and in situ hybridization analysis, there is a low expression of Raldhs in the early otocyst at E9, while Cyp26s are strongly expressed. During the following days, there is an up-regulation of Raldhs and a down-regulation for Cyp26s. Specific RA receptor (Rar and Rxr) genes are expressed in the otocyst and during further development of the inner ear. At the otocyst stage, most of the components of the retinoid pathway are present, suggesting that the embryonic inner ear might act as an autocrine system, which is able to synthesize and metabolize RA necessary for its development. We propose a model in which two RA-dependent pathways may control inner ear ontogenesis: one indirect with RA from somitic mesoderm acting to regulate gene expression within the hindbrain neuroepithelium, and another with RA acting directly on the otocyst. Current evidence suggests that RA may regulate several genes involved in mesenchyme-epithelial interactions, thereby controlling inner ear morphogenesis. Our investigations suggest that RA signaling is a critical component not only of embryonic development, but also of postnatal maintenance of the inner ear.

Retinoic acid and hindbrain patterning

Retinoid signaling plays an important role in the developmental patterning of the hindbrain. Studies of the teratogenic effects of retinoids showed early on that the hindbrain suffered patterning defects in cases of retinoid excess or deficiency. Closer examination of these effects in animal models suggested that retinoids might play a physiological role in specifying the antero-posterior axis of the hindbrain. This idea was supported by the localization of retinoid synthetic and degradative enzymes, binding proteins, and receptors to the hindbrain and neighboring regions of the neuroepithelium and the mesoderm. In parallel, it became clear that the molecular patterning of the hindbrain, in terms of the regionalized expression of Hox genes and other developmental regulatory genes, is profoundly influenced by retinoid signaling.

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.