The role of vitamin A in the development of the central nervous system

Vitamin A deficiency and relational memory deficit in adult mice: relationships with changes in brain retinoid signalling

Vitamin A and its derivatives, the retinoids, have recently been reported to be implicated in the synaptic plasticity of the hippocampus and in cognitive functions. Acting via transcription factors, retinoids can regulate gene expression via their nuclear receptors [retinoic acid receptors (RARs) and retinoid X receptors (RXRs)]. We recently showed that a moderate (about 30%) hypoexpression of brain (and hippocampal) retinoid signalling, like that naturally occurring in the aged brain of mice, might be related to a selective relational memory deficit. To further assess this hypothesis, the present study investigated the effects of Vitamin A deprivation of varying duration both on the brain expression of retinoid receptors (RARbeta and RXRbeta/gamma) and two associated target genes [tissue-type transglutaminase (tTG) and neurogranin, (RC3)], and on radial maze discrimination learning using young adult mice as subjects. We observed that irrespective of its duration (i.e. 31 or 39 weeks), Vitamin A deprivation resulted in a significant reduction (25-30%) in the expression of brain RARbeta, RXRbeta/gamma and tTG mRNAs. Conversely, only the 39-week condition was found to induce a significant decrease in brain RC3 mRNAs contents and a selective relational memory impairment. Finally, daily administration of retinoic acid (RA) failed to reverse the 39-week Vitamin A deficiency (VAD)-related cognitive deficit and to fully normalise the associated brain retinoid hyposignalling. In particular, there was no evidence for an up-regulating effect of RA on whole brain (and hippocampal) RC3 mRNAs of the 39-week-depleted mice. The results show that post-natal VAD may induce a selective memory impairment and give further support to the hypothesis that the fine regulation of retinoid-mediated gene expression is important for optimal brain functioning and higher cognition.

Teratogenia da vitamina A

A vitamina A é essencial à preservação e ao funcionamento normal dos tecidos, assim como, ao crescimento e desenvolvimento. No humano há evidência indireta que a vitamina A em excesso, durante as primeiras semanas de gestação é teratogênica. Do contrário, não há dúvidas sobre os efeitos deletérios, de uma alimentação carente neste micronutriente e sobre a disponibilidade do conhecimento técnico para evitá-los. A preocupação com o fato de que a vitamina A conduziria a teratogenia em humanos, tem retardado a implementação de programas de combate a carência de vitamina A, atingindo principalmente os programas de enriquecimento de alimentos. A literatura é controvertida e dispõe de poucas informações sobre as doses para suplementação de gestantes. Como o retinol circulante materno é controlado homeostaticamente após o consumo de alimentos fonte de vitamina A, espera-se a mesma resposta metabólica após o consumo de alimentos fortificados, indicando que não há risco de teratogenia. Consequentemente, parece altamente improvável que o consumo de alimentos enriquecidos ou de suplementos de vitamina A pré-formada, nas doses unitárias habituais, tenha efeito teratogênico no homem.

Vitamin A and infancy. Biochemical, functional, and clinical aspects

Vitamin A is a very intriguing natural compound. The molecule not only has a complex array of physiological functions, but also represents the precursor of promising and powerful new pharmacological agents. Although several aspects of human retinol metabolism, including absorption and tissue delivery, have been clarified, the type and amounts of vitamin A derivatives that are intracellularly produced remain quite elusive. In addition, their precise function and targets still need to be identified. Retinoic acids, undoubtedly, play a major role in explaining activities of retinol, but, recently, a large number of physiological functions have been attributed to different retinoids and to vitamin A itself. One of the primary roles this vitamin plays is in embryogenesis. Almost all steps in organogenesis are controlled by retinoic acids, thus suggesting that retinol is necessary for proper development of embryonic tissues. These considerations point to the dramatic importance of a sufficient intake of vitamin A and explain the consequences if intake of retinol is deficient. However, hypervitaminosis A also has a number of remarkable negative consequences, which, in same cases, could be fatal. Thus, the use of large doses of retinol in the treatment of some human diseases and the use of megavitamin therapy for certain chronic disorders as well as the growing tendency toward vitamin faddism should alert physicians to the possibility of vitamin overdose.

Too much of a good thing: retinoic acid as an endogenous regulator of neural differentiation and exogenous teratogen

Retinoic acid (RA) is essential for both embryonic and adult growth, activating gene transcription via specific nuclear receptors. It is generated, via a retinaldehyde intermediate, from retinol (vitamin A). RA levels require precise regulation by controlled synthesis and catabolism, and when RA concentrations deviate from normal, in either direction, abnormal growth and development occurs. This review describes: (i) how the pattern of RA metabolic enzymes controls the actions of RA; and (ii) the type of abnormalities that result when this pattern breaks down. Examples are given of RA control of the anterior/posterior axis of the hindbrain, the dorsal/ventral axis of the spinal cord, as well as certain sex-specific segments of the spinal cord, using varied animal models including mouse, quail and mosquitofish. These functions are highly sensitive to abnormal changes in RA concentration. In rodents, the control of neural patterning and differentiation are disrupted when RA concentrations are lowered, whereas inappropriately high concentrations of RA result in abnormal development of cerebellum and hindbrain nuclei. The latter parallels the malformations seen in the human embryo exposed to RA due to treatment of the mother with the acne drug Accutane (13-cis RA) and, in cases where the child survives beyond birth, a particular set of behavioural anomalies can be described. Even the adult brain may be susceptible to an imbalance of RA, particularly the hippocampus. This report shows how the properties of RA as a neural induction agent and organizer of segmentation can explain the consequences of RA depletion and overexpression.

Triiodothyronine administration reverses vitamin A deficiency-related hypo-expression of retinoic acid and triiodothyronine nuclear receptors and of neurogranin in rat brain

Recent studies have revealed that retinoids play an important role in the adult central nervous system and cognitive functions. Previous investigations in mice have shown that vitamin A deficiency (VAD) generates a hypo-expression of retinoic acid (RA, the active metabolite of vitamin A) receptors and of neurogranin (RC3, a neuronal protein involved in synaptic plasticity) and a concomitant selective behavioural impairment. Knowing that RC3 is both a triiodothyronine (T3) and a RA target gene, and in consideration of the relationships between the signalling pathways of retinoids and thyroid hormones, the involvement of T3 on RA signalling functionality in VAD was investigated. Thus, the effects of vitamin A depletion and subsequent administration with RA and/or T3 on the expression of RA nuclear receptors (RAR, RXR), T3 nuclear receptor (TR) and on RC3 in the brain were examined. Rats fed a vitamin A-deficient diet for 10 weeks exhibited a decreased expression of RAR, RXR and TR mRNA and of RC3 mRNA and proteins. RA administration to these vitamin A-deficient rats reversed only the RA hypo-signalling in the brain. Interestingly, T3 is able to restore its own brain signalling simultaneously with that of vitamin A and the hypo-expression of RC3. These results obtained in vivo revealed that one of the consequences of VAD is a dysfunction in the thyroid signalling pathway in the brain. This seems of crucial importance since the down regulation of RC3 observed in the depleted rats was corrected only by T3.

The germ cell nuclear factor is required for retinoic acid signaling during Xenopus development

The germ cell nuclear factor (GCNF, NR6A1) is a nuclear orphan receptor that functions as a transcriptional repressor and is transiently expressed in mammalian carcinoma cells during retinoic acid (RA) induced neuronal differentiation. During Xenopus laevis development, the spatiotemporal expression pattern of embryonic GCNF (xEmGCNF) suggests a role in anteroposterior specification of the neuroectoderm. Here, we show that RA treatment of Xenopus embryos enhances xEmGCNF expression. Moreover, we present evidence for the relevance of this finding in the context of primary neurogenesis and hindbrain development. During early development of the central nervous system, RA signals promote posterior transformation of the neuroectoderm and increase the number of cells undergoing primary neurogenesis. Our loss-of-function analyses using a xEmGCNF-specific morpholino antisense oligonucleotide indicate that xEmGCNF is required for the effect of RA on primary neurogenesis. This may be caused by transcriptional regulation of the gene encoding the RA-degrading enzyme CYP26, since this gene is derepressed after depletion of xEmGCNF and an antimorph of xEmGCNF directly activates transcription of CYP26, also in absence of protein synthesis. The effect of xEmGCNF knockdown on hindbrain patterning is similar to conditions of reduced RA signaling, which may be caused by a reduction of RAR gamma expression specifically in the presumptive hindbrain.

Transient modulation of cytoplasmic and nuclear retinoid receptors expression in differentiating human teratocarcinoma NT2 cells

Human embryonal carcinoma Ntera2/D1 (NT2) cells treated with retinoic acid (RA) differentiate into several cell types including post-mitotic neurons. In this study we asked if RA-induced differentiation alters the expression of RA and retinol (ROL) binding proteins. The regulation of the intracellular carrier proteins for ROL and RA, cellular retinol binding protein I (CRBP-I), and cellular retinoic acid binding protein I and II (CRABP-I, CRABP-II) were studied along with the nuclear RA receptors RARalpha, RARbeta and RARgamma2. PCR analysis of total mRNA from RA-treated cells showed a biphasic early induction of CRBP-I, CRABP-II, and RARgamma2 genes. The immediate early gene Krox-24, a zinc finger transcription factor which is up-regulated during neuronal differentiation, was also induced, but after 1 week of treatment. The induction of CRBP-I protein synthesis in differentiating NT2 cells was confirmed by western blotting and immunofluorescence experiments. Conversely, the synthetic retinoid N-(4-hydroxyphenyl)retinamide, which induces cell death, but not differentiation in different tumour cell types, did not produce the same modulation on gene expression in NT2 cells. These data suggest that the RA-specific induction of CRBP-I and CRABP-II could be an early event in the process leading to neuronal differentiation of NT2 cells.

Development and validation of a fast and sensitive chromatographic assay for all-trans-retinol and tocopherols in human serum and plasma using liquid-liquid extraction

A sensitive HPLC assay for all-trans-retinol, alpha-tocopherol, and gamma-tocopherols in human serum and plasma is reported. Sample preparation is performed in one step and involves precipitation of proteins and extraction of lipids with two volumes of an ethanol-chloroform mixture (3:1, v/v) without I.S. addition. After removal of the precipitated protein, 20 microl aliquots of the supernatant (equivalent to 6.7 microl of serum or plasma) were injected into the HPLC system and analyzed using fluorometric detection. RP-HPLC was performed using a C(18) S3 ODS2 column with a methanol-water step gradient (97:3 to 100) at 1.0 ml/min. The quantification limit expressed as nanograms of analyte per milliliter of serum or plasma was approximately 30 ng for all-trans-retinol, 300 ng for alpha-tocopherol and 250 ng for gamma- and delta-tocopherol. The method was validated and applied to human serum and plasma from a total of 120 subjects. This procedure requires a small volume of serum or plasma and can therefore be a valuable tool for measuring low concentrations of these vitamins in preterm infants with sensitivity, precision and accuracy.

Retinoic acid receptor beta2 and neurite outgrowth in the adult mouse spinal cord in vitro

Retinoic acid, acting through the nuclear retinoic acid receptor beta2 (RARbeta2), stimulates neurite outgrowth from peripheral nervous system tissue that has the capacity to regenerate neurites, namely, embryonic and adult dorsal root ganglia. Similarly, in central nervous system tissue that can regenerate, namely, embryonic mouse spinal cord, retinoic acid also stimulates neurite outgrowth and RARbeta2 is upregulated. By contrast, in the adult mouse spinal cord, which cannot regenerate, no such upregulation of RARbeta2 by retinoic acid is observed and no neurites are extended in vitro. To test our hypothesis that the upregulation of RARbeta2 is crucial to neurite regeneration, we have transduced adult mouse or rat spinal cord in vitro with a minimal equine infectious anaemia virus vector expressing RARbeta2. After transduction, prolific neurite outgrowth occurs. Outgrowth does not occur when the cord is transduced with a different isoform of RARbeta nor does it occur following treatment with nerve growth factor. These data demonstrate that RARbeta2 is involved in neurite outgrowth, at least in vitro, and that this gene may in the future be of some therapeutic use.

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.

Retinoic acid is detected at relatively high levels in the CNS of adult rats

Retinoic acid (RA) is essential for cellular growth and differentiation in developing and adult animals. The central nervous system (CNS) suffers developmental defects if embryonic levels of RA are too high or too low. The production and function of RA in adult brain are unclear. We report that RA is present throughout the brain and spinal cord of adult, vitamin A-deficient (VAD) rats treated with a physiological amount of all-trans-retinol. The hippocampus/cortex contained the highest proportion of RA in the brain (27.2 +/- 2.9% of the organic phase radioactivity, and 23.5 +/- 0.8% of the organic phase radioactivity extracted from spinal cord was RA). RA comprises a higher proportion of the retinoid pool in the CNS compared with amounts reported in other target tissues (E Werner and HF DeLuca. Arch Biochem Biophys 393: 262-270, 2001). However, RA is not preferentially transported from the blood to the brain. There were 2.90 +/- 0.20 fmol RA/g tissue transported to the brain of VAD rats treated with 2.00 nmol [20-(3)H]all-trans-retinoic acid, but higher amounts of RA were delivered to the liver, testis, and spleen. Because RA is not transported preferentially to brain, this tissue likely synthesizes RA more efficiently than other target tissues.

Development of a retinoic acid receptor-binding assay with rainbow trout tissue: characterization of retinoic acid binding, receptor tissue distribution, and developmental changes

Retinoic acid (RA) regulates the transcription of various genes required for several essential functions in vertebrates through binding to two classes of nuclear receptors, the retinoic acid receptors (RAR) and retinoid X receptors (RXR). We investigated nuclear RA binding in tissues from rainbow trout using the radiolabeled all-trans and 9-cis isomers of RA. Specific binding (indicative of receptor binding) of both all-trans- and 9-cis-RA was found in all tissues tested, including the adult trout ovary, testis, gill, liver, kidney, blood, white muscle, and heart. The kinetics and absolute amount of RA binding were dependent on both the tissue and the isomer of RA used. All-trans-RA bound with high affinity (K(d) approximately 1.0-3.9 nM), and low capacity (B(max) approximately 75-484 fmol RA/mg protein), while 9-cis-RA bound with lower affinity (K(d) approximately 7-56 nM), but with a greater capacity (B(max) approximately 214-1076 fmol RA/mg protein). The B(max) results were used to estimate RAR and RXR levels and revealed that the gill possesses primarily RARs while the liver possesses primarily RXRs. The RAR-specific competitor TTNPB was able to effectively displace all-trans-[3H]RA in most tissues, and the RXR-specific competitor AGN 194204 was able to effectively displace 9-cis-[3H]RA. However, TTNPB and AGN 194204 could not displace all of the RA in the kidney and testis, suggesting the existence of another nuclear RA binding protein. Binding of all-trans- and 9-cis-RA was also found in developing trout embryos and fry. Kinetic analysis revealed that RAR levels predominated at the eyed-embryo stage, but decreased 87% by the swim-up fry stage, while RXR levels remained relatively constant over the same time period. These findings suggest that RA and its receptors may play a key role in early trout development. This study has provided a simple and rapid radioligand binding assay that can identify RAR and RXRs in trout tissues.

Alleviation of a selective age-related relational memory deficit in mice by pharmacologically induced normalization of brain retinoid signaling

Vitamin A and its derivatives, the retinoids, have been implicated recently in the synaptic plasticity of the hippocampus and might therefore play a role in associated cognitive functions. Acting via transcription factors, retinoids can regulate gene expression via their nuclear receptors [retinoic acid receptors (RARs) and retinoid X receptors]. In a series of experiments, the present study investigated the possible role of age-related downregulation of retinoid-mediated transcription events in the cognitive decline seen in aged mice. We observed that the brain (and hippocampal) levels of retinoid receptors and the expression of specific associated target genes were restored to presenescent (adult) levels in aged mice after acute administration (150 microg/kg, s.c.) of retinoic acid (RA). These effects of RA, however, could be abolished by the coadministration of an RAR antagonist. RA was also demonstrated to alleviate the age-related deficit in the CA1 long-term potentiation efficacy of aged mice in vivo. Moreover, RA was found to alleviate completely the performance deficit of aged mice to the control level in a two-stage spatial discrimination paradigm designed to assess relational memory. This promnesic effect of RA was again susceptible to abolition by RAR antagonist treatment. The parallel molecular, cellular, and behavioral correlates associated with the decrease of retinoid receptor expression and its normalization demonstrated here suggest that the fine regulation of retinoid-mediated gene expression is fundamentally important to optimal brain functioning and higher cognition. Specifically, a naturally occurring dysregulation of retinoid-mediated molecular events might be a potential etiological factor for cognitive deterioration during senescence.

Hindbrain patterning involves graded responses to retinoic acid signalling

Several recent studies have shown that retinoic acid signalling is required for correct patterning of the hindbrain. However, the data from these studies are disparate and the precise role of retinoic acid signalling in patterning the anteroposterior axis of the neural tube remains uncertain. To help clarify this issue, we have cultured a staged series of chick embryos in the presence of an antagonist to the all three retinoic acid receptors. Our data indicate that retinoic acid is the transforming signal involved in the expansion of posterior hindbrain structures. We find that the hindbrain region of the neural tube down to the level of the sixth somite acquires the identity of rhombomere 4 when retinoic acid signalling is blocked. Specification of future rhombomere boundaries has a retinoic acid dependency between stage 5 and stage 10(+) that is lost progressively in an anterior-to-posterior sequence. Furthermore, the application of various concentrations of antagonist shows that successively more posterior rhombomere boundaries require progressively higher concentration of endogenous retinoic acid for their correct positioning, a result that strengthens the hypothesis that a complex retinoid gradient acts to pattern the posterior hindbrain. Our dissection of early retinoic acid functions allows us to re-interpret the wide disparity of hindbrain phenotypes previously observed in various models of retinoic acid deficiency.

Regulation of mouse kappa opioid receptor gene expression by retinoids

The effect of retinoids on the expression of kappa opioid receptor (KOR) gene was examined in normal and transgenic animals. KOR-lacZ transgene expression was specifically elevated in KOR-positive areas of the developing CNS by depleting vitamin A from animal diets. The endogenous KOR mRNA species, including all three isoforms, were also upregulated by depleting vitamin A in developing animals. Change in the expression of isoforms a and b is similar in prenatal stages but differs during postnatal development. Interestingly, upregulation of isoform c is most significant postnatally. The regulation of KOR gene by vitamin A was substantiated in a mouse embryonal carcinoma P19 culture system in which retinoic acid (RA), the most potent ingredient of vitamin A, was able to suppress the expression of all the three KOR isoforms and KOR protein. The RA-mediated suppression was blocked by an RA receptor antagonist and a histone deacetylase (HDAC) inhibitor. By using a reporter transfection assay in P19 cells, the potential genetic element responsible for RA-mediated suppression of KOR gene expression was located to intron 1 of the mouse KOR gene, which could also be blocked by HDAC inhibitor. Furthermore, suppression of KOR gene expression by RA in P19 cells appeared to be an indirect event and required protein synthesis. A role of RA in KOR gene regulation during developmental stages was discussed.

Neural patterning in the vertebrate embryo

The embryonic central nervous system (CNS) is patterned along its antero-posterior, dorsal-ventral, and left-right axes. Along the dorsal-ventral axis, cell fate determination occurs during and following neural tube closure and involves the action of two opposing signaling pathways: SHH ventrally from the notochord and BMP/GDF dorsally from the boundary of neural and nonneural ectoderm and later from the roof plate. In addition, Wnt and retinoic acid signaling have been shown to act in dorsal-ventral patterning; however, their roles are understood in less detail. Along the antero-posterior axis, signals divide the neural tube into four major divisions: forebrain, midbrain, hindbrain, and spinal cord, and these differences can be detected soon after the formation of the neural plate. The FGF, Wnt, and retinoic acid signaling pathways have been implicated in the caudalization of neural tissue. Boundaries of Hox gene expression are observed along the anteroposterior axis and have been suggested to be involved in establishing different identities in the hindbrain and spinal cord. Complex gene expression patterns in the brain suggest the development of neuromeres dividing the brain into different regions that are elaborated further during development. Patterning along the left-right axis occurs concurrently with antero-posterior and dorsal-ventral patterning during gastrulation. A leading candidate for initiating asymmetry is activin, which acts through Nodal and Lefty before any morphological differences are observed. The big challenge will be understanding how these diverse signaling pathways interact both temporally and spatially to generate the complex adult nervous system.

Increased XRALDH2 activity has a posteriorizing effect on the central nervous system of Xenopus embryos

Retinoic acid (RA) metabolizing enzymes play important roles in RA signaling during vertebrate embryogenesis. We have previously reported on a RA degrading enzyme, XCYP26, which appears to be critical for the anteroposterior patterning of the central nervous system (EMBO J. 17 (1998) 7361). Here, we report on the sequence, expression and function of its counterpart, XRALDH2, a RA generating enzyme in Xenopus. During gastrulation and neurulation, XRALDH2 and XCYP26 show non-overlapping, complementary expression domains. Upon misexpression, XRALDH2 is found to reduce the forebrain territory and to posteriorize the molecular identity of midbrain and individual hindbrain rhombomeres in Xenopus embryos. Furthermore, ectopic XRALDH2, in combination with its substrate, all-trans-retinal (ATR), can mimic the RA phenotype to result in microcephalic embryos. Taken together, our data support the notion that XRALDH2 plays an important role in RA homeostasis by the creation of a critical RA concentration gradient along the anteroposterior axis of early embryos, which is essential for proper patterning of the central nervous system in Xenopus.

In vivo regulation of precursor cells in the subventricular zone of adult rat brain by thyroid hormone and retinoids

The mature central nervous system contains precursor cells in the subventricular zone of the lateral ventricle. In this study we examined the possibility to affect fate of precursor cells through exogenous manipulations. The results indicate that administration of thyroid hormone and retinoic acid increases the expression of Ki67, a nuclear antigen associated with cell proliferation, and of nestin, a marker protein for precursor cells in the subventricular zone of adult male rats. Moreover, retinoic acid increases polysialated-neural cell adhesion molecules (PSA-NCAM)-immunoreactivity. These data suggest that nuclear receptor ligands are potential candidates for fate determination of precursor cells in the subventricular zone also in the adult brain.

Retinoic acid, midkine, and defects of secondary neurulation

BACKGROUND

Retinoic acid (RA) is necessary for normal differentiation of the tail bud into the secondary neural tube. Excess RA, however, is teratogenic and causes neural tube defects (NTDs). The way in which RA modulates secondary neurulation is unclear but probably involves RA-regulated downstream genes such midkine (MK), which encodes a growth factor implicated in tail bud mesenchymal-neuroepithelial conversion. Our objective was to determine whether RA-deficiency would produce similar defects and if MK is involved.

METHODS

Citral, a drug that blocks endogenous RA formation, as well as a neutralizing antibody, were used to block RA activity in chick embryos. Immunohistochemistry and in situ hybridization were used to localize RA and MK in the tail bud. Competitive RT-PCR was used to examine the effects of excess RA and RA deficiency due to citral on the expression of MK mRNA.

RESULTS

Citral-induced NTDs displayed a morphological resemblance to those caused by excess RA. However, citral treatment did not significantly increase embryonic mortality, and RA rescue of citral-treated embryos proved unsuccessful. MK mRNA was detected in the differentiating tail bud by in situ hybridization. Competitive RT-PCR showed that excess RA decreased MK expression by 60%. Doses of citral that caused a comparable incidence of defects, however, caused only a 25% decrease.

CONCLUSIONS

The results show that excess RA and RA deficiency both cause defects of secondary neurulation. While excess RA decreased MK expression, RA deficiency had minimal effects. However, whether or not MK is an intermediary in the developmental phenomena regulated physiologically or pathologically by RA remains to be elucidated.

The role of retinoic acid receptors in neurite outgrowth from different populations of embryonic mouse dorsal root ganglia

Dorsal root ganglion (DRG) neurons can be categorised into at least three types, based upon their neurotrophin requirement for survival. We have analysed the expression of the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs) in NGF, NT-3 and BDNF dependent neurons isolated from embryonic day (E)13.5 mouse DRG. We show that each population of neurons expressed each of the three RXRs, (alpha), (beta) and (gamma); however, whilst the NGF and NT-3 dependent neurons expressed each of the RARs (alpha), (beta) and (gamma), the BDNF dependent neurons only expressed RAR(alpha) and (beta). When retinoic acid was added to each of the neuronal classes only the NGF and NT-3 dependent neurons responded by extending neurites, and this response involved the upregulation of RAR(beta)(2). This specificity was confirmed by the use of receptor-selective agonists as only a RAR(beta)-selective compound stimulated neurite outgrowth. These results suggest a role for RA acting via RAR(beta)(2) in the outgrowth of neurites.

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

Vitamin A-deficient (VAD) quail embryos have severe abnormalities, including a high incidence of reversed cardiac situs. Using this model we examined in vivo the physiological function of vitamin A in the left/right (L/R) cardiac asymmetry pathway. Molecular analysis reveals the expression of early asymmetry genes activin receptor IIa, sonic hedgehog, Caronte, Lefty-1, and Fgf8 to be unaffected by the lack of retinoids, while expression of the downstream genes nodal-related, snail-related (cSnR), and Pitx2 is altered. In VAD embryos nodal expression in left lateral plate mesoderm (LPM) is severely downregulated and the expression domain altered during neurulation. Similarly, the expression of cSnR in the right LPM and of Pitx2 in the left side posterior heart-forming region (HFR) is downregulated in the VAD embryos. The lack of retinoids does not cause randomization or ectopic expression of nodal, cSnR, or Pitx2. At the six- to eight-somite stage nodal is expressed transiently in the left posterior HFR of normal quail embryos; this expression is missing in VAD embryos and may be linked to the loss of Pitx2 expression in this region of VAD quail embryos. Administration of retinoids to VAD embryos prior to the six-somite stage rescues the expression of nodal, cSnR, and Pitx2 as well as the randomized VAD cardiac phenotype. There is an absolute requirement for retinoids at the four- to five-somite developmental window for cardiogenesis and cardiac L/R specification to proceed normally. We conclude that retinoids do not regulate the left/right-specific sidedness assignments for expression of genes on the vertebrate cardiac asymmetry pathway, but are required during neurulation for the maintenance of adequate levels of their expression and for the development of the posterior heart tube and a loopable heart. Cardiac asymmetry may be but one of several critical events regulated by retinoid signaling in the retinoid-sensitive developmental window.

Retinoids in embryonal development

The key role of vitamin A in embryonal development is reviewed. Special emphasis is given to the physiological action of retinoids, as evident from the retinoid ligand knockout models. Retinoid metabolism in embryonic tissues and teratogenic consequences of retinoid administration at high doses are presented. Physiological and pharmacological actions of retinoids are outlined and explained on the basis of their interactions as ligands of the nuclear retinoid receptors. Immediate target genes and the retinoid response elements of their promoters are summarized. The fundamental role of homeobox genes in embryonal development and the actions of retinoids on their expression are discussed. The similarity of the effects of retinoid ligand knockouts to effects of compound retinoid receptor knockouts on embryogenesis is presented. Although much remains to be clarified, the emerging landscape offers exciting views for future research.

Retinoic acid redifferentiation therapy for thyroid cancer

For the treatment of differentiated thyroid cancer, surgery, radioiodide therapy, and thyrotropin-suppressive thyroxine application represent established therapeutic measures of proven efficiency, affording a good prognosis for this disease. However, in up to 30% of the cases, dedifferentiation is observed, giving rise to tumors that are refractory to conventional treatment. Eventually, this may lead to the most malignant human tumor, anaplastic thyroid carcinoma, with a life expectancy of only a few months after diagnosis. Among novel approaches for the treatment of dedifferentiated thyroid carcinomas, retinoic acid redifferentiation therapy was evaluated in several in vitro and in vivo studies. Cell culture experiments in thyroid carcinoma lines show that RA treatment affects thyroid specific functions (type I 5'-deiodinase, sodium/iodide-symporter), cell-cell or cell-matrix interaction (intercellular adhesion molecule-1, E-cadherin), differentiation markers (alkaline phosphatase, CD97), growth, and tumorigenicity. The observed changes, which involve multiple parameters that characterize a mature, functional thyrocyte, may be interpreted as partial redifferentiation. In clinical pilot studies, about 40% of the patients responded to RA application with an increased radioiodide uptake. In an evaluation of 20 RA-treated patients with well-documented data sets, 8 exhibited a decrease (4) or stabilization (4) in tumor size and/or in serum thyroglobulin levels in addition to enhanced radioiodide transport. This indicates that these patients with a long history of unresponsiveness to other treatment may have experienced an actual therapeutic benefit. These data suggest that RA redifferentiation therapy, considering especially its comparatively mild side effects, may soon represent an alternative therapeutic approach to otherwise untreatable thyroid tumors.

Down-regulation of mu-opioid receptor expression in rat oligodendrocytes during their development in vitro

In the central nervous system, opioid receptors are found in neurons and also in glial cells. To gain more information on their presence and possibly on their function, we investigated the expression of mu-opioid receptors (MOR) during oligodendroglial cell development in two culture systems. In these models, during the first days, the cells are O-2A bipotential progenitor cells (also called OPCs; oligodendrocyte precursor cells), and then they differentiate into oligodendrocytes, which mature. In the first system, oligodendroglial cells, derived from newborn rat brain hemispheres, are grown in primary culture in the presence of a confluent layer of astrocytes, and they differentiate slowly. In the second, cells are specifically detached from the mixed cultures of the first system and are grown thereafter alone in secondary culture, a condition allowing a rapid cell differentiation. Under both conditions OPCs and immature oligodendrocytes were found to express a high level of MOR mRNA, whereas mature oligodendrocytes did not express it at all. The decrease of MOR expression during oligodendrocyte maturation was progressive, suggesting that it was not a primary effect of differentiation but an indirect secondary effect. Our study also shows that basic fibroblast growth factor (bFGF), which has been claimed by some authors to induce a dedifferentiation of the mature oligodendrocytes, and retinoic acid (RA), which had not been tested before, were not able to restore MOR expression in mature oligodendrocytes. These results indicate that bFGF and RA neither reverse the maturation process nor dedifferentiate the cells. However, RA was found to inhibit almost completely the expression of the myelin basic protein. The main result of this study is that MOR is expressed in progenitors and in immature oligodendrocytes, but not in mature oligodendrocytes. This suggests that MOR could be involved in some developmental process of the cells of the oligodendroglial lineage.

Developmental and tissue-specific expression of DEAD box protein p72

Retinoic acid (RA) is a vitamin A derivative that has been well documented to be involved in cell differentiation. Using RNA fingerprinting by arbitrarily primed PCR, we have previously identified a number of transcripts that are regulated during RA-induced neuronal differentiation of embryonal carcinoma NT2/D1 cells. DEAD box protein p72 is one of the clones found to be down-regulated following treatment with RA. To further investigate the regulation of p72, the mRNA expression of p72 in various neuronal cell lines and primary neuronal cultures was examined. Transcripts of p72 were reduced in differentiated PC12 and IMR-32 cells but not in SH-SYSY cells. Partial cDNA fragments of p72 were isolated from rat and chick for the systematic analysis of p72 expression in different adult tissues and developmental stages. While prominent expression of p72 was observed in brain and testis, the expression was down-regulated in brain, muscle and liver during development. Taken together, our findings provide the first demonstration on the spatial and temporal expression profile of p72 in rat and chick tissues which is consistent with a role of p72 during early development.

The role of retinoic acid in embryonic and post-embryonic development

Retinoic acid (RA) is the bioactive metabolite of vitamin A (retinol) which acts on cells to establish or change the pattern of gene activity. Retinol is converted to RA by the action of two types of enzyme, retinol dehydrogenases and retinal dehydrogenases. In the nucleus RA acts as a ligand to activate two families of transcription factors, the RA receptors (RAR) and the retinoid X receptors (RXR) which heterodimerize and bind to the upstream sequences of RA-responsive genes. Thus, in addition to the well-established experimental paradigm of depriving animals of vitamin A to determine the role of RA in embryonic and post-embryonic development, molecular biology has provided us with two additional methodologies: knockout the enzymes or the RAR and RXR in the mouse embryo. The distribution of the enzymes and receptors, and recent experiments to determine the endogenous distribution of RA in the embryo are described here, as well as the effects on the embryo of knocking out the enzymes and receptors. In addition, recent studies using the classical vitamin A-deprivation technique are described, as they have provided novel insights into the regions of the embryo which crucially require RA, and the gene pathways involved in their development. Finally, the post-embryonic or regenerating systems in which RA plays a part are described, i.e. the regenerating limb, lung regeneration, hair cell regeneration in the ear and spinal cord regeneration in the adult.

Heads or tails? Retinoic acid will decide

A recent study (Niederreither et al. Nat Genet 1999;21:444-448 [Ref. 1]) describes the phenotype of a gene knockout for an enzyme, retinaldehyde dehydrogenase 2 (RALDH-2), that synthesizes retinoic acid (RA) in the early embryo. The effects generated by this single enzyme mutation are remarkably similar to those previously described in vitamin A-deprivation studies and compound retinoic acid receptor knockouts, which involve multiple systems of the embryo. With other data on the distribution of RA, its role in axial specification of the early embryo is considerably clarified. Surprisingly, it seems that head development is unaffected in these RALDH-2 knockout embryos; thus, the anterior of the embryo does not require RA, despite the observations that the hindbrain seems exquisitely sensitive to RA perturbation. Head development may be realised by a cytochrome P450 enzyme (CYP26), which has been described recently. Between these two opposing forces, the hindbrain develops.

N-methyl-D-aspartate receptor agonists modulate homocysteine-induced developmental abnormalities

We showed previously that the induction of neural crest (NC) and neural tube (NT) defects is a general property of N-methyl-D-aspartate receptor (NMDAR) antagonists. Since homocysteine induces NC and NT defects and can also act as an NMDAR antagonist, we hypothesized that the mechanism of homocysteine-induced developmental defects is mediated by competitive inhibition of the NMDAR by homocysteine. If this hypothesis is correct, homocysteine-induced defects will be reduced by NMDAR agonists. To test the hypothesis, we treated chicken embryos during the process of neural tube closure with sufficient homocysteine thiolactone to induce NC and NT defects in approximately 40% of survivors or with homocysteine thiolactone in combination with each of a selected set of NMDAR agonists in 0. 05-5000 nmol doses. Glutamate site agonists selected were L-glutamate and N-methyl-D-aspartate. Glycine site agonists were glycine, D-cycloserine, and aminocyclopropane-carboxylic acid. Glycine was the most effective overall, reducing defects significantly at two different doses (each P>0.001). These results support the hypothesis that homocysteine may affect NC and NT development by its ability to inhibit the NMDAR. One potentially important consequence of this putative mechanism is that homocysteine may interact synergistically with other NMDAR antagonists to enhance its effect on development.

Abnormal anteroposterior and dorsoventral patterning of the limb bud in the absence of retinoids

We describe here how the early limb bud of the quail embryo develops in the absence of retinoids, including retinoic acid. Retinoid-deficient embryos develop to about stage 20/21, thus allowing patterns of early gene activity in the limb bud to be readily examined. Genes representing different aspects of limb polarity were analysed. Concerning the anteroposterior axis, Hoxb-8 was up-regulated and its border was shifted anteriorly whereas shh and the mesodermal expression of bmp-2 were down-regulated in the absence of retinoids. Concerning the apical ectodermal genes, fgf-4 was down-regulated whereas fgf-8 and the ectodermal domain of bmp-2 were unaffected. Genes involved in dorsoventral polarity were all disrupted. Wnt-7a, normally confined to the dorsal ectoderm, was ectopically expressed in the ventral ectoderm and the corresponding dorsal mesodermal gene Lmx-1 spread into the ventral mesoderm. En-1 was partially or completely absent from the ventral ectoderm. These dorsoventral patterns of expression resemble those seen in En-1 knockout mouse limb buds. Overall, the patterns of gene expression are also similar to the Japanese limbless mutant. These experiments demonstrate that the retinoid-deficient embryo is a valuable tool for dissecting pathways of gene activity in the limb bud and reveal for the first time a role for retinoic acid in the organisation of the dorsoventral axis.

Retinoid signaling required for normal heart development regulates GATA-4 in a pathway distinct from cardiomyocyte differentiation

Vitamin A is essential for normal embryonic cardiogenesis. The vitamin A-deficient phenotype in the avian embryo includes an abnormal heart tube closed at the sinus venosus and the absence of large vessels that normally connect the embryonic heart to the developing circulatory system. In vitamin A-deficient embryos the expression of cardiomyocyte differentiation genes, including atrial-specific myosin heavy chain, ventricular-specific myosin, and sarcomeric myosins as well as the putative cardiomyocyte specification gene Nkx-2.5, is not altered. However, the expression of transcription factor GATA-4 is severely decreased in the heart-forming regions of vitamin A-deficient stage 7-10 embryos. Significantly, GATA-4 transcripts are completely lacking in the lateral mesoderm posterior to the heart, in the area of the developing cardiac inflow tract that later displays prominent morphological defects, including a closed nonseptated heart lacking a sinus venosus. The administration of retinol to the vitamin A-deficient embryo restores GATA-4 expression and completely rescues the vitamin A-deficient phenotype. Our results indicate that GATA-4 is a component of the retinoid-mediated cardiogenic pathway unlinked to cardiomyocyte differentiation, but involved in the morphogenesis of the posterior heart tube and the development of the cardiac inflow tract.

Regionalized metabolic activity establishes boundaries of retinoic acid signalling

The competence of a cell to respond to the signalling molecule retinoic acid (RA) is thought to depend largely on its repertoire of cognate zinc finger nuclear receptors. XCYP26 is an RA hydroxylase that is expressed differentially during early Xenopus development. In Xenopus embryos, XCYP26 can rescue developmental defects induced by application of exogenous RA, suggesting that the enzymatic modifications introduced inhibit RA signalling activities in vivo. Alterations in the expression pattern of a number of different molecular markers for neural development induced upon ectopic expression of XCYP26 reflect a primary function of RA signalling in hindbrain development. Progressive inactivation of RA signalling results in a stepwise anteriorization of the molecular identity of individual rhombomeres. The expression pattern of XCYP26 during gastrulation appears to define areas within the prospective neural plate that develop in response to different concentrations of RA. Taken together, these observations appear to reflect an important regulatory function of XCYP26 for RA signalling; XCYP26-mediated modification of RA modulates its signalling activity and helps to establish boundaries of differentially responsive and non-responsive territories.

Initial retinoid requirement for early avian development coincides with retinoid receptor coexpression in the precardiac fields and induction of normal cardiovascular development

Vitamin A requirement for early embryonic development is clearly evident in the gross cardiovascular and central nervous system abnormalities and an early death of the vitamin A-deficient quail embryo. This retinoid knockout model system was used to examine the biological activity of various natural retinoids in early cardiovascular development. We demonstrate that all-trans-, 9-cis-, 4-oxo-, and didehydroretinoic acids, and didehydroretinol and all-trans-retinol induce and maintain normal cardiovascular development as well as induce expression of the retinoic acid receptor beta2 in the vitamin A-deficient quail embryo. The expression of RARbeta2 is at the same level and at the same sites where it is expressed in the normal embryo. Retinoids provided to the vitamin A-deficient embryo up to the 5-somite stage of development, but not later, completely rescue embryonic development, suggesting the 5-somite stage as a critical retinoid-sensitive time point during early avian embryogenesis. Retinoid receptors RARalpha, RARgamma, and RXRalpha are expressed in both the precardiac endoderm and mesoderm in the normal and the vitamin A-deficient quail embryo, while the expression of RXRgamma is restricted to precardiac endoderm. Vitamin A deficiency downregulates the expression of RARalpha and RARbeta. Our studies provide strong evidence for a narrow retinoid-requiring developmental window during early embryogenesis, in which the presence of bioactive retinoids and their receptors is essential for a subsequent normal embryonic development.

Vitamin A and embryonic development: an overview

Vitamin A is an essential micronutrient throughout the life cycle. Its active form, retinoic acid via retinoid receptors, is involved in signal transduction pathways regulating development. Both the lack and excess of vitamin A during embryonic development result in congenital malformations. Approaches to examine the function of vitamin A in embryonic development have included treatment with excess retinoids and the use of retinoid receptor knock-out mice, which have provided important insights into the complexity of the retinoid signaling system. A recently explored model is the retinoid ligand knock-out, i.e., the vitamin A-deficient embryo. Early development can be successfully examined in the vitamin A-deficient avian embryo, in which bioactive retinoids can rescue the deficient genotype as well as phenotype. In this model it has been possible to unequivocally link the physiological function of vitamin A to development of heart, embryonal circulatory and central nervous systems and the regulation of heart asymmetry. Several developmental genes regulated by endogenous vitamin A during early embryogenesis have been identified. Retinoid receptors and their endogenous ligands, the vitamin A-active forms, are present in the early embryo. It is the developmentally regulated biogeneration of the vitamin A-active forms via distinct spatio-temporal metabolic pathways that is critically linked to the initiation of retinoid signal transduction during embryonic development.

Retinoids and their receptors in vertebrate embryogenesis

Vitamin A and its derivatives, the retinoids, participate in formation of diverse embryonic structures, including face, heart, eye, limb and nervous system. Studies of retinoid-deficient and -treated embryos, and of receptor null mutants, provide evidence that this participation involves interactions between retinoids and their receptors. Targeted retinoid application and retinoid deficiency, using in ovo avian embryos, has identified early cardiogenic contributions, including cardiocyte gene expression and differentiation, heart tube fusion and laterality, and segmental identity. Also useful is a mammalian model, which targets retinoid deficiency to distinct gestational windows, circumventing limitations of traditional deficiency studies and current null mutant technologies. Rat embryos made deficient in retinoids during gestational d 11.5-13.5 exhibit specific cardiac, limb, ocular and nervous system deficits. That many of the anomalies previously reported in retinoid receptor null mutants are observed in deficiency confirms that ligand-receptor interactions are essential for embryonic development. Other defects are novel, reemphasizing the functional redundancy of retinoid receptors and that retinoid receptors have multiple and overlapping contributions to morphogenesis.