Heads or tails? Retinoic acid will decide

Using an aquatic model, Xenopus laevis, to uncover the role of chromodomain 1 in craniofacial disorders

The chromodomain family member chromodomain 1 (CHD1) has been shown to have numerous critical molecular functions including transcriptional regulation, splicing, and DNA repair. Complete loss of function of this gene is not compatible with life. On the other hand, missense and copy number variants of CHD1 can result in intellectual disabilities and craniofacial malformations in human patients including cleft palate and Pilarowski-Bjornsson Syndrome. We have used the aquatic developmental model organism Xenopus laevis, to determine a specific role for Chd1 in such cranioafcial disorders. Protein and gene knockdown techniques in Xenopus, including antisense oligos and mosaic Crispr/Cas9-mediated mutagenesis, recapitulated the craniofacial defects observed in humans. Further analysis indicated that embryos deficient in Chd1 had defects in cranial neural crest development and jaw cartilage morphology. Additionally, flow cytometry and immunohistochemistry revealed that decreased Chd1 resulted in increased in apoptosis in the developing head. Together, these experiments demonstrate that Chd1 is critical for fundamental processes and cell survival in craniofacial development. We also presented evidence that Chd1 is regulated by retinoic acid signaling during craniofacial development. Expression levels of chd1 mRNA, specifically in the head, were increased by RAR agonist exposure and decreased upon antagonist treatment. Subphenotypic levels of an RAR antagonist and Chd1 morpholinos synergized to result in orofacial defects. Further, RAR DNA binding sequences (RAREs) were detected in chd1 regulatory regions by bioinformatic analysis. In summary, by combining human genetics and experiments in an aquatic model we now have a better understanding of the role of CHD1 in craniofacial disorders.

Toxic responses of Sox2 gene in the regeneration of the earthworm Eisenia foetida exposed to Retnoic acid

Exogenous retinoic acid delays and disturbs the regeneration of Eisenia foetida. The stem cell pluripotency factor, Sox2, can play a crucial role in cell reprogramming and dedifferentiation. In this study, we compared the regeneration of Eisenia foetida in different segments after amputation and the effects of retinoic acid on the regeneration of different segments. The results showed that the regeneration speed of the head and tail was slightly faster than the middle part, and retinoic acid disrupted and delayed the regeneration of the earthworm. The qRT-PCR and Western blot analysis showed that the expression of the Sox2 gene and Sox2 protein was highest on the seventh day in different segments (p<0.05). After treatment with retinoic acid, the expression level of the Sox2 gene and Sox2 protein was significantly reduced (p<0.05). The results indicated that the regeneration of earthworms and the formation of blastema are related to the expression of the Sox2 gene and protein. Retinoic acid delays and interferes with the regeneration of the earthworm by affecting the expression levels of the Sox2 gene and protein.

Aldehyde Dehydrogenases 1A2 Expression and Distribution are Potentially Associated with Neuron Death in Spinal Cord of Tg(SOD1*G93A)1Gur Mice

The pathogenesis of amyotrophic lateral sclerosis (ALS) has not been unclear yet, it might be associated with the abnormal expression and distribution of certain proteins. Aldehyde dehydrogenases 1A2 (ALDH1A2) was thought to be one of potential candidates. Therefore, in this study we observed and analyzed the alteration of the expression and distribution of ALDH1A2 in the spinal cord of wild-type (WT) and Tg(SOD1*G93A)1Gur mice. We compared the expression and distribution of ALDH1A2 in the different segments, anatomic regions and neural cells of spinal cord at the different stages of WT and Tg(SOD1*G93A)1Gur mice applied the methods of fluorescent immunohistochemistry and western blot. Results revealed that ALDH1A2 extensively expressed and distributed in the spinal cord of adult WT and Tg(SOD1*G93A)1Gur mice. The expression and distribution of ALDH1A2 in the white matter including the anterior, posterior and lateral funiculus were more than that in the gray matter including the central canal, the anterior and dorsal horn. ALDH1A2 majorly expressed and distributed in the astrocyte, microglial, oligodendrocyte and neuron cells. The ALDH1A2 expression significantly decreased and redistributed in some anatomic regions of spinal cord at the onset and progression stages of Tg(SOD1*G93A)1Gur mice. The expression decrease of ALDH1A2 followed with the increase of neuron cells death. This study suggested that the alteration of expression and distribution of ALDH1A2 was potentially associated with the pathogenesis of ALS.

Hard to swallow: Developmental biological insights into pediatric dysphagia

Pediatric dysphagia-feeding and swallowing difficulties that begin at birth, last throughout childhood, and continue into maturity--is one of the most common, least understood complications in children with developmental disorders. We argue that a major cause of pediatric dysphagia is altered hindbrain patterning during pre-natal development. Such changes can compromise craniofacial structures including oropharyngeal muscles and skeletal elements as well as motor and sensory circuits necessary for normal feeding and swallowing. Animal models of developmental disorders that include pediatric dysphagia in their phenotypic spectrum can provide mechanistic insight into pathogenesis of feeding and swallowing difficulties. A fairly common human genetic developmental disorder, DiGeorge/22q11.2 Deletion Syndrome (22q11DS) includes a substantial incidence of pediatric dysphagia in its phenotypic spectrum. Infant mice carrying a parallel deletion to 22q11DS patients have feeding and swallowing difficulties that approximate those seen in pediatric dysphagia. Altered hindbrain patterning, craniofacial malformations, and changes in cranial nerve growth prefigure these difficulties. Thus, in addition to craniofacial and pharyngeal anomalies that arise independently of altered neural development, pediatric dysphagia may result from disrupted hindbrain patterning and its impact on peripheral and central neural circuit development critical for feeding and swallowing. The mechanisms that disrupt hindbrain patterning and circuitry may provide a foundation to develop novel therapeutic approaches for improved clinical management of pediatric dysphagia.

The role of Zic transcription factors in regulating hindbrain retinoic acid signaling

Background

The reiterated architecture of cranial motor neurons aligns with the segmented structure of the embryonic vertebrate hindbrain. Anterior-posterior identity of cranial motor neurons depends, in part, on retinoic acid signaling levels. The early vertebrate embryo maintains a balance between retinoic acid synthetic and degradative zones on the basis of reciprocal expression domains of the retinoic acid synthesis gene aldhehyde dehydrogenase 1a2 (aldh1a2) posteriorly and the oxidative gene cytochrome p450 type 26a1 (cyp26a1) in the forebrain, midbrain, and anterior hindbrain.

Results

This manuscript investigates the role of zinc finger of the cerebellum (zic) transcription factors in regulating levels of retinoic acid and differentiation of cranial motor neurons. Depletion of zebrafish Zic2a and Zic2b results in a strong downregulation of aldh1a2 expression and a concomitant reduction in activity of a retinoid-dependent transgene. The vagal motor neuron phenotype caused by loss of Zic2a/2b mimics a depletion of Aldh1a2 and is rescued by exogenously supplied retinoic acid.

Conclusion

Zic transcription factors function in patterning hindbrain motor neurons through their regulation of embryonic retinoic acid signaling.

Developmental mechanisms directing early anterior forebrain specification in vertebrates

Research from the last 15 years has provided a working model for how the anterior forebrain is induced and specified during the early stages of embryogenesis. This model relies on three basic processes: (1) induction of the neural plate from naive ectoderm requires the inhibition of BMP/TGFβ signaling; (2) induced neural tissue initially acquires an anterior identity (i.e., anterior forebrain); (3) maintenance and expansion of the anterior forebrain depends on the antagonism of posteriorizing signals that would otherwise transform this tissue into posterior neural fates. In this review, we present a historical perspective examining some of the significant experiments that have helped to delineate this molecular model. In addition, we discuss the function of the relevant tissues that act prior to and during gastrulation to ensure proper anterior forebrain formation. Finally, we elaborate data, mainly obtained from the analyses of mouse mutants, supporting a role for transcriptional repressors in the regulation of cell competence within the anterior forebrain. The aim of this review is to provide the reader with a general overview of the signals as well as the signaling centers that control the development of the anterior neural plate.

Presence of Ribeiroia ondatrae in the developing anuran limb disrupts retinoic acid levels

The widespread reports of malformed frogs have sparked interest worldwide to try and determine the causes of such malformations. Ribeiroia ondatrae is a digenetic trematode, which has been implicated as one such cause, as this parasite encysts within the developing tadpole hind limb bud and inguinal region causing dramatic limb malformations. Currently, the mechanisms involved in parasite-induced limb deformities remain unclear. We sought to investigate whether the level of retinoic acid (RA), a morphogenetic factor known to play a critical role in limb bud formation, is altered by the presence of R. ondatrae within the infected tadpole. Alteration of RA levels within the limb bud caused by the presence of the parasite may be achieved in three ways. First, metacercariae are actively secreting RA; second, cercariae, upon entering the limb/inguinal region, may release a large amount of RA; finally, the metacercariae may induce either an increase in the synthesis or a decrease in the degradation of the host's endogenous retinoic acid levels. Here, we show through high performance liquid chromatography and mass spectrometry that limb bud tissue of Lithobates sylvaticus, which has been parasitised, contains 70% more RA compared to the unparasitised control. Furthermore, parasites that have encysted within the limb buds appear to contain substantially less RA (56%) than the free swimming cercariae (defined as the infectious stage of the parasite). Taken together, these data illustrate for the first time that encystment of R. ondatrae leads to an increase in RA levels in the tadpole limb bud and may offer insight into the mechanisms involved in parasite-induced limb deformities.

Positive association between ALDH1A2 and schizophrenia in the Chinese population

Vitamin A (retinol), in the biologically active form of retinoic acid (RA), has been proposed as involved in the pathogenesis of schizophrenia. We hypothesized that genetic basis of genes encoding RA metabolism enzymes, which control the cellular RA level, might be associated with this disease. This cascade genetic association model, using markers in genes of synthesis and degradation enzymes within the retinoid cascade, would better fit the biological character of the retinoid hypothesis than the single gene strategy. In the present study we chose to investigate 7 genes involved in the synthesis, degradation and transportation of RA, ALDH1A1, ALDH1A2, ALDH1A3, CYP26A1, CYP26B1, CYP26C1 and Transthyretin (TTR), for their roles in the development of schizophrenia. We genotyped 18 single nucleotide polymorphisms (SNPs) in the regulatory and coding regions of these 7 genes using LDR technology in the 617 Chinese Han subjects. Case-control analyses were performed to detect association of these 7 genes with schizophrenia. Association analyses using both allelic and genotypic single-locus tests revealed no significant association between the risk for each of investigated gene and schizophrenia. However, analyses of multiple-locus haplotypes indicated that the overall frequency of rs4646642-rs4646580 of ALDH1A2 gene showed significant difference between patients and control subjects (p=0.0055). We also employed multifactor dimensionality reduction method to detect multilocus effects. In summary, in this work we show multiple candidate genes involved in retinoid cascade in schizophrenics. In addition, our results suggest a positive association between ALDH1A2 and schizophrenics in the Chinese population and support the retinoid hypothesis of schizophrenia.

Retinoic acid regulates anterior-posterior patterning within the lateral plate mesoderm of Xenopus

The lateral plate mesoderm (LPM) lines the body cavities, gives rise to the heart and circulatory system and is responsible for patterning the underlying endoderm. We describe gene expression domains within the lateral plate mesoderm of the neurula stage Xenopus embryo that demonstrate a marked anterior posterior pattern in that tissue. FoxF1 and Nkx-2.5 are expressed in the anterior LPM, Hand1 in the middle and Xsal-1 in the posterior LPM. Since retinoic acid is known to pattern many tissues during development, and RALDH2, the enzyme primarily responsible for retinoic acid synthesis, is expressed in the anterior and dorsal LPM, we hypothesized that retinoic acid is necessary for correct patterning of the LPM. Exposure to exogenous retinoic acid during neurulation led to an expansion of the anterior and middle expression domains and a reduction of the posterior domain whereas exposure to a retinoic acid antagonist resulted in smaller anterior and middle expression domains. Furthermore, inhibition of RALDH2, which should decrease endogenous RA levels, caused a reduction of anterior domains indicating that endogenous RA is necessary for regulating their size. After altering retinoic acid signaling in a temporally restricted window, the displaced anterior-posterior pattern is maintained until gut looping, as demonstrated by permanently altered Hand1, FoxF1, xHoxC-10, and Pitx2 expression domains. We conclude that the broad expression domains of key transcription factors demonstrate a novel anterior-posterior pattern within the LPM and that retinoic acid can regulate the size of these domains in a coordinated manner.

Retinol dehydrogenase 10 is a feedback regulator of retinoic acid signalling during axis formation and patterning of the central nervous system

Retinoic acid (RA) is an important morphogen that regulates many biological processes, including the development of the central nervous system (CNS). Its synthesis from vitamin A (retinol) occurs in two steps, with the second reaction--catalyzed by retinal dehydrogenases (RALDHs)--long considered to be crucial for tissue-specific RA production in the embryo. We have recently identified the Xenopus homologue of retinol dehydrogenase 10 (XRDH10) that mediates the first step in RA synthesis from retinol to retinal. XRDH10 is specifically expressed in the dorsal blastopore lip and in other domains of the early embryo that partially overlap with XRALDH2 expression. We show that endogenous RA suppresses XRDH10 gene expression, suggesting negative-feedback regulation. In mRNA-injected Xenopus embryos, XRDH10 mimicked RA responses, influenced the gene expression of organizer markers, and synergized with XRALDH2 in posteriorizing the developing brain. Knockdown of XRDH10 and XRALDH2 by specific antisense morpholino oligonucleotides had the opposite effects on organizer gene expression, and caused a ventralized phenotype and anteriorization of the brain. These data indicate that the conversion of retinol into retinal is a developmentally controlled step involved in specification of the dorsoventral and anteroposterior body axes, as well as in pattern formation of the CNS. We suggest that the combinatorial gene expression and concerted action of XRDH10 and XRALDH2 constitute a ;biosynthetic enzyme code' for the establishment of a morphogen gradient in the embryo.

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.

Non-P450 aldehyde oxidizing enzymes: the aldehyde dehydrogenase superfamily

BACKGROUND

Aldehydes are highly reactive molecules. While several non-P450 enzyme systems participate in their metabolism, one of the most important is the aldehyde dehydrogenase (ALDH) superfamily, composed of NAD(P)+-dependent enzymes that catalyze aldehyde oxidation.

OBJECTIVE

This article presents a review of what is currently known about each member of the human ALDH superfamily including the pathophysiological significance of these enzymes.

METHODS

Relevant literature involving all members of the human ALDH family was extensively reviewed, with the primary focus on recent and novel findings.

CONCLUSION

To date, 19 ALDH genes have been identified in the human genome and mutations in these genes and subsequent inborn errors in aldehyde metabolism are the molecular basis of several diseases, including Sjögren-Larsson syndrome, type II hyperprolinemia, gamma-hydroxybutyric aciduria and pyridoxine-dependent seizures. ALDH enzymes also play important roles in embryogenesis and development, neurotransmission, oxidative stress and cancer. Finally, ALDH enzymes display multiple catalytic and non-catalytic functions including ester hydrolysis, antioxidant properties, xenobiotic bioactivation and UV light absorption.

Retinoic acid production by intestinal dendritic cells and its role in T-cell trafficking

Vitamin A deficiency causes a marked reduction in the number of T and B cells in the small intestinal tissues. The vitamin A metabolite retinoic acid imprints lymphocytes with gut-homing specificity upon antigenic stimulation. In the small intestinal lamina propria, Peyer's patches, and mesenteric lymph nodes, there are dendritic cells capable of producing retinoic acid. Their capacity depends on the expression of retinal dehydrogenases (RALDH). RALDH2, encoded by Aldh1a2, is a major isoform of RALDH in the intestinal dendritic cells under specific pathogen-free conditions, and can be induced by multiple factors constitutively present or induced in the small intestinal microenvironment.

Physiological Significance and Expression of P450s in the Developing Eye

Expression of 10 CYP orthologs (Families 1-3) in developing mouse conceptus is constitutive. These forms have specific temporal and spatial expression. Studies on CYP1B1 indicate its requirement for normal eye development, both in human and mouse. The distribution of the enzyme in the mouse eye is in three regions, which may reflect three different, perhaps equally important, functions in this organ. Its presence in the inner ciliary and lens epithelia appears to be necessary for normal development of the trabecular meshwork and its function in regulating intraocular pressure. Its expression in the retinal ganglion and inner nuclear layers may reflect a role in maintenance of the visual cycle. Its expression in the corneal epithelium may indicate a function in metabolism of environmental xenobiotics.

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.

Understanding the somitogenesis clock: what's missing?

The segmentation of vertebrate embryos depends on a complex genetic network that generates highly dynamic gene expression. Many of the elements of the network have been identified, but their interaction and their influence on segmentation remain poorly understood. A few mathematical models have been proposed to explain the dynamics of subsets of the network, but the mechanistic bases remain controversial. This review focuses on outstanding problems with the generation of somitogenesis clock oscillations, and the ways they could regulate segmentation. Proposals that oscillations are generated by a negative feedback loop formed by Lunatic fringe and Notch signaling are weighed against a model based on positive feedback, and the experimental basis for models of simple negative feedback involving Her/Hes genes or Wnt targets is evaluated. Differences are then made explicit between the many 'clock and wavefront' model variants that have been proposed to explain how the clock regulates segmentation. An understanding of the somitogenesis clock will require addressing experimentally the many questions that arise from the study of simple models.

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.

Evi1 is specifically expressed in the distal tubule and duct of the Xenopus pronephros and plays a role in its formation

The ecotropic viral integration site 1 (Evi1) and related MEL1 (MDS1/Evi1-like gene 1) genes are zinc finger oncogenic transcription factors involved in myeloid leukaemia. Here, we show that in Xenopus, Evi1 and MEL1 have partially overlapping restricted embryonic expression profiles. Within the pronephros, Evi1 and MEL1 are sequentially expressed within the distal tubule and duct compartments, Evi1 transcription being detected prior to any sign of pronephric morphogenesis. In the pronephros of zebrafish embryos, Evi1 expression is restricted to the posterior portion of the duct, the anterior portion having characteristics of proximal tubules. In the Xenopus pronephros, Evi1 expression is upregulated by retinoid signaling and repressed by overexpression of xWT1 and by Notch signaling. Overexpression of Evi1 from late neurula stage specifically inhibits the expression of proximal tubule and glomus pronephric markers. We show that the first zinc finger and CtBP interaction domains are required for this activity. Overexpression of a hormone-inducible Evi1-VP16 antimorphic fusion with activation at neurula stage disrupts distal tubule and duct formation and expands the expression of glomus markers. Although overexpression of this construct also causes in many embryos a reduction of proximal tubule markers, embryos with expanded and ectopic staining have been also observed. Together, these data indicate that Evi1 plays a role in the proximo-distal patterning of the pronephros and suggest that it may do so by functioning as a CtBP dependent repressor.

Analysis of ALDH1A2, CYP26A1, CYP26B1, CRABP1, and CRABP2 in human neural tube defects suggests a possible association with alleles in ALDH1A2

BACKGROUND

Vitamin A (retinol), in the form of retinoic acid (RA), is essential for normal development of the human embryo. Studies in the mouse and zebrafish have shown that retinol is metabolized in the developing spinal cord and must be maintained in a precise balance along the anteroposterior axis. Both excess and deficiency of RA can affect morphogenesis, including failures of neural tube closure.

METHODS

We chose to investigate 5 genes involved in the metabolism or synthesis of RA, ALDH1A2, CYP26A1, CYP26B1, CRABP1, and CRABP2, for their role in the development of human neural tube defects, such as spina bifida.

RESULTS

An association analysis using both allelic and genotypic single-locus tests revealed a significant association between the risk for spina bifida and 3 polymorphisms in the gene ALDH1A2; however, we found no evidence of a significant multilocus association.

CONCLUSIONS

These results may suggest that polymorphisms in ALDH1A2 may influence the risk for lumbosacral myelomeningocele in humans.

Global analysis of RAR-responsive genes in the Xenopus neurula using cDNA microarrays

Retinoid signaling is important for patterning the vertebrate hindbrain and midaxial regions. We recently showed that signaling through retinoic acid receptors (RARs) is essential for anteroposterior patterning along the entire body axis. To further investigate the mechanisms through which RARs act, we used microarray analysis to investigate the effects of modulating RAR activity on target gene expression. We identified 334 up-regulated genes (92% of which were validated), including known RA-responsive genes, known genes that have never been proposed as RA targets and many hypothetical and unidentified genes (n = 166). Sixty-seven validated down-regulated genes were identified, including known RA-responsive genes and anterior marker genes. The expression patterns of selected up-regulated genes (n = 45) were examined at neurula stages using whole-mount in situ hybridization. We found that most of these genes were expressed in the neural tube and many were expressed in anterior tissues such as neural crest, brain, eye anlagen, and cement gland. Some were expressed in tissues such as notochord, somites, pronephros, and blood islands, where retinoic acid (RA) plays established roles in organogenesis. Members of this set of newly identified RAR target genes are likely to play important roles in neural patterning and organogenesis under the control of RAR signaling pathways, and their further characterization will expand our understanding of RA signaling during development.

Molecular cloning and expression of a novel CYP26 gene (cyp26d1) during zebrafish early development

Proper restriction of retinoid signaling by Cyp26s is essential for development of vertebrate embryos while inappropriate retinoid signaling can cause teratogenesis. Here, we report cloning and expression analysis of a novel cyp26 gene (cyp26d1) isolated from zebrafish. The predicted protein encoded by cyp26d1 consists of 554 amino acids. It exhibits 54% amino acid identity with human Cyp26C1, 50% with zebrafish Cyp26B1 and 38% with zebrafish Cyp26A1. Whole-mount in situ hybridization shows that cyp26d1 is first expressed in sphere stage, then disappears at 50% epiboly and resumes its expression at 75% epiboly. During segmentation period, cyp26d1 message is found at presumptive hindbrain. Double in situ hybridization with krox20 and cyp26d1 reveals that cyp26d1 is expressed in presumptive rhombomere 2-4 (r2-r4) at 2-somite stage. At 3-somite stage, cyp26d1 gene is expressed in r6 and pharyngeal arch (pa) one in addition to its expression at r2 and r4. At 6-somite stage, cyp26d1 message is present in continuous bands at r2-r6 and in pa1. This expression pattern is maintained from 10-somite stage through 21-somite stage except that the expression level is greatly reduced at r2 and r4. At 21-somite stage, cyp26d1 is also found in a group of cells in telencephalon and diencephalons. At 25-31h post-fertilization (hpf), the zebrafish cyp26d1 expression domain is extended to eyes, otic vesicles and midbrain in addition to its expression in hindbrain, telencephalon, diencephalons, and pharyngeal arches. At 35-48hpf, the expression of cyp26d1 is mainly restricted to otic vesicles, pharyngeal arches and pectoral fins and the expression level is greatly reduced.

Expression patterns of mouse and human CYP orthologs (families 1-4) during development and in different adult tissues

The present study compared the relative expression pattern of 10 orthologous CYP forms from families 1-4 in cDNA panels of human and mouse fetal and adult tissues. Except for CYP1A2, all of these CYPs exhibited specific patterns of expression during mouse ontogeny, suggesting possible involvement in development. Cyp1a1 and Cyp2r1 were the only two of the orthologs to be expressed only in the E7 mouse; Cyp2s1 was expressed in all stages, including E7, while Cyp2e1 appeared only at E17. Highest expression of the individual CYPs in the different late term human fetal tissues was: thymus, CYP1B1 and CYP2U1; liver, CYP2E1; brain, CYP2R1, CYP1A1 and CYP4X1; and lung, CYP4B1 and CYP2W1. In general, the level of individual human CYP transcripts was lower in the fetal than the corresponding adult tissues. The pattern of expression in adult mouse and human tissues was fairly similar for CYP1A1, CYP1A2, CYP1B1, CYP2S1, and CYP2U1 orthologs.

Metabolism of retinoids and arachidonic acid by human and mouse cytochrome P450 1b1

The cytochrome P450 family 1 (CYP1) is considered to be one of the xenobiotic-metabolizing enzyme families and is responsible for oxidative metabolism of polycyclic aromatic hydrocarbons. For example, mouse Cyp1b1 was originally identified as the enzyme responsible for oxidative metabolism of 7,12-dimethylbenz(alpha)anthracene (DMBA). A comparison of the kinetics of this metabolism by mouse and human CYP1B1 orthologs revealed the mouse enzyme to have a more favorable metabolism of DMBA, with a catalytic efficiency ratio (CER) of 0.23. However, CYP1 enzymes are also capable of metabolism of endobiotics, and in the present study, the metabolism of retinoids and lipid endobiotics by human CYP1B1 and mouse Cyp1b1 orthologs was compared. Both hemoproteins oxidized retinol to retinal and retinal to retinoate, but did not oxidize retinoate. The CYP1B1 to Cyp1b1 CERs were 13 and 26 for the two steps, respectively; the Cyp1b1 K(m(app)) values for retinoids were 20-fold higher. Human family 1 cytochromes P450 had unique regional specificities for arachidonate oxidation: the major metabolites of CYP1A1, CYP1A2, and CYP1B1 were 75% terminal hydroxyeicosatetraenoic fatty acids (HETEs), 52% epoxyeicosatrienoic fatty acids (EETs), and 54% mid-chain HETEs, respectively. CYP1A1 and CYP1B1 K(m(app)) values for arachidonate were about 30 microM, whereas CYP1A2 K(m(app)) was 95 microM. The major metabolites of arachidonic acid by Cyp1b1 were EETs (50%) and midchain HETEs (37%). The mouse ortholog had a CER for metabolite production of 64 due to a K(m(app)) of 0.5 mM for arachidonate.

Identification of cDNAs associated with late dedifferentiation in adult newt forelimb regeneration

Epimorphic limb regeneration in the adult newt involves the dedifferentiation of differentiated cells to yield a pluripotent blastemal cell. These mesenchymal-like cells proliferate and subsequently respond to patterning and differentiation cues to form a new limb. Understanding the dedifferentiation process requires the selective identification of dedifferentiating cells within the heterogeneous population of cells in the regenerate. In this study, representational differences analysis was used to produce an enriched population of dedifferentiation-associated cDNA fragments. Fifty-nine unique cDNA fragments were identified, sequenced, and analyzed using bioinformatics tools and databases. Some of these clones demonstrate significant similarity to known genes in other species. Other clones can be linked by homology to pathways previously implicated in the dedifferentiation process. These data will form the basis for further analyses to elucidate the role of candidate genes in the dedifferentiation process during newt forelimb regeneration.

Generating gradients of retinoic acid in the chick embryo: Cyp26C1 expression and a comparative analysis of the Cyp26 enzymes

We have cloned a novel retinoic acid (RA) catabolizing enzyme, Cyp26C1, in the chick and describe here its distribution during early stages of chick embryogenesis. It is expressed from stage 4 in the presumptive anterior (cephalic) mesoderm, in a subset of cephalic neural crest cells, the ventral otic vesicle, mesenchyme adjacent to the otic vesicle, the branchial pouches and grooves, a part of the neural retina, and the anterior telencephalon, and shows a dynamic expression in the hindbrain rhombomeres and neuronal populations within them. By examining the distribution of Cyp26C1 in the RA-free quail embryo, we can determine which of these expression domains is dependent on RA, and it is only the rhombomeric sites that do not appear, suggesting a role for RA in this location. The most striking domain of Cyp26C1 distribution is in the anterior cephalic mesoderm, which is adjacent to the domain of Raldh2 in the trunk mesoderm, but separated from it by a gap dorsal to which the posterior hindbrain will develop. We suggest that a gradient of RA within the mesoderm generated by Raldh2 and catabolized by Cyp26C1 could be responsible for patterning the hindbrain. We have compared this distribution of Cyp26C1 with that of Cyp26A1 and Cyp26B1 in the chick and shown that they generally occupy nonoverlapping sites of expression in the embryo, and as a result, we suggest individual roles for each of the Cyp enzymes in the developing embryo.

Xenobiotic‐metabolizing Cytochromes P450 in Ontogeny: Evolving Perspective

While much is known about inducibility of the xenobiotic-metabolizing forms of cytochrome P450, the Family 1-3 enzymes, less well understood is the purpose for the presence of some of these forms in the developing conceptus. Many cytochrome P450 forms are present in the embryo and fetus, like the anabolic forms in Families 5 and higher, and are known to produce molecules with specific functions, e.g., cholesterol, steroids, and their metabolites necessary for normal physiological functions. As we gain greater understanding of the cell cycle and its regulation, and the roles of nuclear receptors in modulating transcriptional activities, a picture begins to emerge in which cytochrome P450 forms appear as molecule-altering enzymes producing and eliminating ligands associated with nuclear receptor activities. For these CYP enzymes to exert a developmental action, a controlled spatial and temporal expression pattern would be essential. Studies now indicate the existence of such temporal control on the appearance of a number of these enzymes and the necessary coenzyme, NADPH-cytochrome P450 reductase.

The tumor suppressor adenomatous polyposis coli and caudal related homeodomain protein regulate expression of retinol dehydrogenase L

Development of normal colon epithelial cells proceeds through a systematic differentiation of cells that emerge from stem cells within the base of colon crypts. Genetic mutations in the adenomatous polyposis coli (APC) gene are thought to cause colon adenoma and carcinoma formation by enhancing colonocyte proliferation and impairing differentiation. We currently have a limited understanding of the cellular mechanisms that promote colonocyte differentiation. Herein, we present evidence supporting a lack of retinoic acid biosynthesis as a mechanism contributing to the development of colon adenomas and carcinomas. Microarray and reverse transcriptase-PCR analyses revealed reduced expression of two retinoid biosynthesis genes: retinol dehydrogenase 5 (RDH5) and retinol dehydrogenase L (RDHL) in colon adenomas and carcinomas as compared with normal colon. Consistent with the adenoma and carcinomas samples, seven colon carcinoma cell lines also lacked expression of RDH5 and RDHL. Assessment of RDH enzymatic activity within these seven cell lines showed poor conversion of retinol into retinoic acid when compared with normal cells such as normal human mammary epithelial cells. Reintroduction of wild type APC into an APC-deficient colon carcinoma cell line (HT29) resulted in increased expression of RDHL without affecting RDH5. APC-mediated induction of RDHL was paralleled by increased production of retinoic acid. Investigations into the mechanism responsible for APC induction of RDHL indicated that beta-catenin fails to repress RDHL. The colon-specific transcription factor CDX2, however, activated an RDHL promoter construct and induced endogenous RDHL. Finally, the induction of RDHL by APC appears dependent on the presence of CDX2. We propose a novel role for APC and CDX2 in controlling retinoic acid biosynthesis and in promoting a retinoid-induced program of colonocyte differentiation.

Multiple points of interaction between retinoic acid and FGF signaling during embryonic axis formation

Anteroposterior (AP) patterning of the developing CNS is crucial for both regional specification and the timing of neurogenesis. Several important factors are involved in AP patterning, including members of the WNT and FGF growth factor families, retinoic acid receptors, and HOX genes. We have examined the interactions between FGF and retinoic signaling pathways. Blockade of FGF signaling downregulates the expression of members of the RAR signaling pathway, RARalpha, RALDH2 and CYP26. Overexpression of a constitutively active RARalpha2 rescues the effects of FGF blockade on the expression of XCAD3 and HOXB9. This suggests that RARalpha2 is required as a downstream target of FGF signaling for the posterior expression of XCAD3 and HOXB9. Surprisingly, we found that posterior expression of FGFR1 and FGFR4 was dependent on the expression of RARalpha2. Anterior expression was also altered with FGFR1 expression being lost, whereas FGFR4 expression was expanded beyond its normal expression domain. RARalpha2 is required for the expression of XCAD3 and HOXB9, and for the ability of XCAD3 to induce HOXB9 expression. We conclude that RARalpha2 is required at multiple points in the posteriorization pathway, suggesting that correct AP neural patterning depends on a series of mutually interactive feedback loops among FGFs, RARs and HOX genes.

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

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

Regulation of retinoic acid distribution is required for proximodistal patterning and outgrowth of the developing mouse limb

Exogenous retinoic acid (RA) induces marked effects on limb patterning, but the precise role of endogenous RA in this process has remained unknown. We have studied the role of RA in mouse limb development by focusing on CYP26B1, a cytochrome P450 enzyme that inactivates RA. Cyp26b1 was shown to be expressed in the distal region of the developing limb bud, and mice that lack CYP26B1 exhibited severe limb malformation (meromelia). The lack of CYP26B1 resulted in spreading of the RA signal toward the distal end of the developing limb and induced proximodistal patterning defects characterized by expansion of proximal identity and restriction of distal identity. CYP26B1 deficiency also induced pronounced apoptosis in the developing limb and delayed chondrocyte maturation. Wild-type embryos exposed to excess RA phenocopied the limb defects of Cyp26b1(-/-) mice. These observations suggest that RA acts as a morphogen to determine proximodistal identity, and that CYP26B1 prevents apoptosis and promotes chondrocyte maturation, in the developing limb.

Retinoic acid regulates a subset of Cdx1 function in vivo

Hox gene products are key players in establishing positional identity along the anteroposterior (AP) axis. In vertebrates, gain or loss of Hox expression along the AP axis often leads to inappropriate morphogenesis, typically manifesting as homeotic transformations that affect the vertebrae and/or hindbrain. Various signalling pathways are known to impact on Hox expression, including the retinoid signalling pathway. Exogenous retinoic acid (RA), disruption of enzymes involved in maintaining normal embryonic RA distribution or mutation of the retinoid receptors (RARs and RXRs) can all impact on Hox expression with concomitant effects on AP patterning. Several Hox loci have well characterized RA response elements (RAREs), which have been shown to regulate functionally relevant Hox expression in the neurectoderm. A similar crucial function for any RARE in mesodermal Hox expression has, however, not been documented. The means by which RA regulates mesodermal Hox expression could therefore be either through an undocumented direct mechanism or through an intermediary; these mechanisms are not necessarily exclusive. In this regard, we have found that Cdx1 may serve as such an intermediary. Cdx1 encodes a homeobox transcription factor that is crucial for normal somitic expression of several Hox genes, and is regulated by retinoid signalling in vivo and in vitro likely through an atypical RARE in the proximal promoter. In order to more fully understand the relationship between retinoid signalling, Cdx1 expression and AP patterning, we have derived mice in which the RARE has been functionally inactivated. These RARE-null mutants exhibit reduced expression of Cdx1 at all stages examined, vertebral homeotic transformations and altered Hox gene expression which correlates with certain of the defects seen in Cdx1-null offspring. These findings are consistent with a pivotal role for retinoid signalling in governing a subset of expression of Cdx1 crucial for normal vertebral patterning.

Feedback mechanisms regulate retinoic acid production and degradation in the zebrafish embryo

Retinoic acid (RA) signaling in vertebrate embryos occurs in a distinct physical and temporal pattern. Regulating this spatial distribution is crucial to the development of the embryo, as RA in excess or in inappropriate tissues is teratogenic. In order to understand how RA availability is determined in zebrafish we have investigated the expression of cyp26a1, an enzyme that inactivates RA, and its relationship to raldh2, one of the enzymes that produce RA from retinal. cyp26a1 expression follows three phases: in presumptive anterior neurectoderm and in a circumblastoporal ring during gastrulation, in the tailbud throughout somitogenesis, and in multiple specific tissue types beginning at mid-somitogenesis and continuing through 48 h postfertilization (hpf). This expression was either adjacent or opposite to those tissues expressing raldh2. We then investigated how RA production might regulate these relationships. Endogenous RA produced by raldhs did not play a role in setting cyp26a1 expression in most tissues. However, exogenous RA regulates expression of both enzymes. cyp26a1 is up regulated in the embryo in a time, concentration, and tissue-dependent manner. Conversely, raldh2 expression is reduced with RA treatment. Tests of the raldh2 promoter in cell transfections proved that RA directly represses its activity. These data demonstrate that the feedback mechanisms regulating production and degradation of RA must be considered in any experiments altering levels of RA in the developing vertebrate embryo.

A novel human cytochrome P450, CYP26C1, involved in metabolism of 9-cis and all-trans isomers of retinoic acid

Retinoids are potent regulators of cell proliferation, cell differentiation, and morphogenesis and are important therapeutic agents in oncology and dermatology. The gene regulatory activity of endogenous retinoids is effected primarily by retinoic acid isomers (all-trans and 9-cis) that are synthesized from retinaldehyde precursors in a broad range of tissues and act as ligands for nuclear retinoic acid receptors. The catabolism of all-trans-retinoic acid (atRA) is an important mechanism of controlling RA levels in cell and tissues. We have previously identified two cytochrome P450s, P450RAI-1 and P450RAI-2 (herein named CYP26A1 and CYP26B1), which were shown to be responsible for catabolism of atRA both in the embryo and the adult. In this report, we describe the identification, molecular cloning, and substrate characterization of a third member of the CYP26 family, named CYP26C1. Transiently transfected cells expressing CYP26C1 convert atRA to polar water-soluble metabolites similar to those generated by CYP26A1 and -B1. Competition studies with all-trans, 13-cis, and 9-cis isomers of retinoic acid demonstrated that atRA was the preferred substrate for CYP26C1. Although CYP26C1 shares extensive sequence similarity with CYP26A1 and CYP26B1, its catalytic activity appears distinct from those of other CYP26 family members. Specifically, CYP26C1 can also recognize and metabolize 9-cis-RA and is much less sensitive than the other CYP26 family members to the inhibitory effects of ketoconazole. CYP26C1 is not widely expressed in the adult but is inducible by RA in HPK1a, transformed human keratinocyte cell lines. This third CYP26 member may play a specific role in catabolizing both all-trans and 9-cis isomers of RA.

The meninges is a source of retinoic acid for the late-developing hindbrain

One general function for retinoic acid (RA) is pattern organization in the CNS. This regulatory factor has an essential role in spinal cord motor neuron and early posterior hindbrain development. In the anterior CNS, however, there is only a limited number of foci of RA synthesis, and less attention has been placed on regions such as the anterior hindbrain where RA synthesizing enzymes are absent. This study shows that a rich source of RA lies around the hindbrain from the RA synthetic enzyme retinaldehyde dehydrogenase-2 (RALDH2) present in the surrounding meninges and mesenchyme by embryonic day 13. RALDH2 is not distributed uniformly throughout the meninges but is restricted to territories over the developing hindbrain, suggesting that RA signaling may be localized to those regions. Further regulation of RA signaling is provided by the presence of a RA sink in the form of the CYP26B1 RA catabolic enzyme expressed in deeper regions of the brain. As a guide to the neural anatomy of hindbrain RA signaling, we used a mouse transgenic for a lacZ reporter gene driven by a RA response element (RAREhsplacZ) to identify regions of RA signaling. This reporter mouse provides evidence that RA signaling in the hindbrain after embryonic day 13 occurs in the regions of the cerebellum and precerebellar system adjacent to sources of RA, including the inferior olive and the pontine nuclei.

Signals from lateral plate mesoderm instruct endoderm toward a pancreatic fate

During embryonic development, organs arise along the gut tube as a series of buds in a stereotyped anterior-posterior (A-P) pattern. Using chick-quail chimeras and in vitro tissue recombination, we studied the interactions governing the induction and maintenance of endodermal organ identify focusing on the pancreas. Though several permissive signals in pancreatic development have been previously identified, here we provide evidence that lateral plate mesoderm sends instructive signals to the endoderm, signals that induce expression of the pancreatic genes Pdx1, p48, Nkx6.1, glucagon, and insulin. Moreover, this instructive signal directs cells to form ectopic insulin-positive islet-like clusters in endoderm that would otherwise form more rostral organs. Once generated, endocrine cells no longer require interaction with mesoderm, but nonendocrine cells continue to require permissive signals from the mesoderm. Stimulation of activin, BMP, or retinoic acid signaling is sufficient to induce Pdx1 expression in endoderm anterior to the pancreas. Lateral plate mesoderm appears to pattern the endoderm in a posterior-dominant fashion as first noted in the patterning of the neural tube at the same embryonic stage. These findings argue for a central role of the mesoderm in coordinating the A-P pattern of all three primary germ layers.

Somatic motoneurone specification in the hindbrain: the influence of somite-derived signals, retinoic acid and Hoxa3

We have investigated the mechanisms involved in generating hindbrain motoneurone subtypes, focusing on somatic motoneurones, which are confined to the caudal hindbrain within rhombomeres 5-8. Following heterotopic transplantation of rhombomeres along the rostrocaudal axis at various developmental stages, we have found that the capacity of rhombomeres to generate somatic motoneurones is labile at the neural plate stage but becomes fixed just after neural tube closure, at stage 10-11. Grafting of somites or retinoic acid-loaded beads beneath the rostral hindbrain induced the formation of somatic motoneurones in rhombomere 4 only, and Hox genes normally expressed more caudally (Hoxa3, Hoxd4) were induced in this region. Targeted overexpression of Hoxa3 in the rostral hindbrain led to the generation of ectopic somatic motoneurones in ventral rhombomeres 1-4, and was accompanied by the repression of the dorsoventral patterning gene Irx3. Taken together, these observations suggest that the somites, retinoic acid and Hox genes play a role in patterning somatic motoneurones in vivo.

A critical period for retinoic acid teratogenesis and loss of neurophilic migration of pontine nuclei neurons

Abnormalities in the pontine nuclei (PN) and inferior olive are hallmarks of human retinoic acid (RA) teratogenesis. This study shows that RA exposure of the mouse at a specific embryonic stage alters morphological structures that derive from the wall of the IVth ventricle to form components of the precerebellar system (the inferior olivary nucleus and the PN). The study employs both normal and a RAREhspLacZ transgenic RA reporter mouse. It is shown that abnormalities in the PN and inferior olive result from exposure at a critical period of embryonic day 9.5 and 10.5. The abnormalities in the PN are due to a failure in their usual neurophilic migration. The compact stream of cells that leads from the anterior rhombic lip to the ventral pons is instead scattered widely over the anterior medulla. Given that the RA exposure occurs after the resolution of rhombomere identity this suggests that teratogenic RA interferes with a regulatory event that overlays this original pattern.

Patterning of motor neurons by retinoic acid in the chick embryo hindbrain in vitro

Motor neurons are found throughout the developing chick hindbrain, while somatic motor (SM) neurons develop only in rhombomeres 5 to 8 (r5-8), and in r1. In r2-8 neuroepithelial explants from stage 7-10 embryos cultured in collagen gels, we found that motor neurons were generated throughout r2-8, while SM neuron differentiation was restricted to r5-8, as in vivo. Exposure of such explants to retinoic acid (RA) resulted in SM neuron differentiation throughout r2-8, while inclusion of the mesoderm and endoderm suppressed this effect. In explants with mesoderm/endoderm, RA-dependent SM neuron differentiation in rostral rhombomeres was restored by the application of an inhibitor of the RA-degrading enzyme CYP26. We found that the mesoderm/endoderm (either with or without RA) induced Cyp26 expression in the neuroepithelium in vitro, suggesting that the modulatory effect of CYP26 on RA-dependent patterning might be dependent on local signals.

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

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

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

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

Retinoic acid signaling is required for a critical early step in zebrafish pancreatic development

The mechanisms that subdivide the endoderm into the discrete primordia that give rise to organs such as the pancreas and liver are not well understood. However, it is known that retinoic acid (RA) signaling is critical for regionalization of the vertebrate embryo: when RA signaling is either prevented or augmented, anteroposterior (AP) patterning of the CNS and mesoderm is altered and major developmental defects occur. We have investigated the role of RA signaling in regionalization of the zebrafish endoderm. Using a mutant that prevents RA synthesis and an antagonist of the RA receptors, we show that specification of both the pancreas and liver requires RA signaling. By contrast, RA signaling is not required for the formation of the endodermal germ layer or for differentiation of other endodermal organs. Timed antagonist and RA treatments show that the RA-dependent step in pancreatic specification occurs at the end of gastrulation, significantly earlier than the expression of known markers of pancreatic progenitors. In addition to being required for pancreatic specification, RA has the capacity to transfate anterior endoderm to a pancreatic fate.

Progressive induction of caudal neural character by graded Wnt signaling

Early in differentiation, all neural cells have a rostral character. Only later do posteriorly positioned neural cells acquire characteristics of caudal forebrain, midbrain and hindbrain cells. Caudalization of neural tissue in the chick embryo apparently involves the convergent actions of (i) fibroblast growth factor (FGF) signaling and (ii) signaling from the caudal paraxial mesoderm, or 'PMC activity', which has not yet been defined molecularly. Here we report evidence that Wnt signaling underlies PMC activity, and show that Wnt signals act directly and in a graded manner on anterior neural cells to induce their progressive differentiation into caudal forebrain, midbrain and hindbrain cells.

Modeling a hox gene network in silico using a stochastic simulation algorithm

The amount of molecular information that has been gathered about Hox cis-regulatory mechanisms allows us to take the next important step: integrating the results and constructing a higher-level model for the interaction and regulation of the Hox genes. Here, we present the results of our investigation into a cis-regulatory network for the early Hox genes. Instead of using conventional differential equation approaches for analyzing the system, we have adopted the use of a stochastic simulation algorithm (SSA) to model the network. The model allows us to track in detail the behavior of each component of a biochemical pathway and to produce computerized movies of the time evolution of the system that is a result of the dynamic interplay of these various components. The simulation is able to reproduce key features of the wild-type pattern of gene expression, and in silico experiments yield results similar to their corresponding in vivo experiments. This analysis shows the utility of using stochastic methods to model biochemical networks. In addition, the model has suggested several intriguing new results that are currently being investigated in vivo.

Role and distribution of retinoic acid during CNS development

Retinoic acid (RA), the biologically active derivative of vitamin A, induces a variety of embryonal carcinoma and neuroblastoma cell lines to differentiate into neurons. The molecular events underlying this process are reviewed with a view to determining whether these data can lead to a better understanding of the normal process of neuronal differentiation during development. Several transcription factors, intracellular signaling molecules, cytoplasmic proteins, and extracellular molecules are shown to be necessary and sufficient for RA-induced differentiation. The evidence that RA is an endogenous component of the developing central nervous system (CNS) is then reviewed, data which include high-pressure liquid chromotography (HPLC) measurements, reporter systems and the distribution of the enzymes that synthesize RA. The latter is particularly relevant to whether RA signals in a paracrine fashion on adjacent tissues or whether it acts in an autocrine manner on cells that synthesize it. It seems that a paracrine system may operate to begin early patterning events within the developing CNS from adjacent somites and later within the CNS itself to induce subsets of neurons. The distribution of retinoid-binding proteins, retinoid receptors, and RA-synthesizing enzymes is described as well as the effects of knockouts of these genes. Finally, the effects of a deficiency and an excess of RA on the developing CNS are described from the point of view of patterning the CNS, where it seems that the hindbrain is the most susceptible part of the CNS to altered levels of RA or RA receptors and also from the point of view of neuronal differentiation where, as in the case of embryonal carcinoma (EC) cells, RA promotes neuronal differentiation. The crucial roles played by certain genes, particularly the Hox genes in RA-induced patterning processes, are also emphasized.