Endogenous retinoids in the zebrafish embryo and adult

Molecular signatures of angiogenesis inhibitors: a single-embryo untargeted metabolomics approach in zebrafish

Angiogenesis is a key process in embryonic development, a disruption of this process can lead to severe developmental defects, such as limb malformations. The identification of molecular perturbations representative of antiangiogenesis in zebrafish embryo (ZFE) may guide the assessment of developmental toxicity from an endpoint- to a mechanism-based approach, thereby improving the extrapolation of findings to humans. Thus, the aim of the study was to discover molecular changes characteristic of antiangiogenesis and developmental toxicity. We exposed ZFEs to two antiangiogenic drugs (SU4312, sorafenib) and two developmental toxicants (methotrexate, rotenone) with putative antiangiogenic action. Molecular changes were measured by performing untargeted metabolomics in single embryos. The metabolome response was accompanied by the occurrence of morphological alterations. Two distinct metabolic effect patterns were observed. The first pattern comprised common effects of two specific angiogenesis inhibitors and the known teratogen methotrexate, strongly suggesting a shared mode of action of antiangiogenesis and developmental toxicity. The second pattern involved joint effects of methotrexate and rotenone, likely related to disturbances in energy metabolism. The metabolites of the first pattern, such as phosphatidylserines, pterines, retinol, or coenzyme Q precursors, represented potential links to antiangiogenesis and related developmental toxicity. The metabolic effect pattern can contribute to biomarker identification for a mechanism-based toxicological testing.

The crosstalk effects of polybrominated diphenyl ethers on the retinoic acid and thyroid hormone signaling pathway

The toxicological and pathological influences of polybrominated diphenyl ethers (PBDEs) on the animal central nervous system have attracted worldwide attention. However, their mechanism of action has not been completely elucidated. Given that retinoic acid (RA) and thyroid hormone (TH) signaling pathway are closely related to neurodevelopment, the crosstalk between the two signaling pathways at the levels of metabolite conversion, gene expression and ligand-receptor interaction after exposure to two representative PBDE congeners (BDE-47 and BDE-209) using zebrafish larvae, dual reporter gene assay, and docking simulation was studied. Our results clarified that BDE-47 could disrupt the transport and metabolism of retinoids, induce changes in expression of key genes, bind with the seven nuclear receptors, and activate RA signaling pathway. BDE-47 exhibited more effects on the indicators of the two signaling pathways than BDE-209. Furthermore, BDE-47 may disrupt TH signaling pathway by disrupting RA signaling pathway, indicating that RA signal is priorly influenced than TH signal. This work offered a new perspective to elucidate TH signal disruption mechanism induced by PBDEs from RA signaling pathway, which is of great significance to elucidate the health effects of PBDEs.

Zebrafish as a Model to Study Retinoic Acid Signaling in Development and Disease

Retinoic acid (RA) is a metabolite of vitamin A (retinol) that plays various roles in development to influence differentiation, patterning, and organogenesis. RA also serves as a crucial homeostatic regulator in adult tissues. The role of RA and its associated pathways are well conserved from zebrafish to humans in both development and disease. This makes the zebrafish a natural model for further interrogation into the functions of RA and RA-associated maladies for the sake of basic research, as well as human health. In this review, we explore both foundational and recent studies using zebrafish as a translational model for investigating RA from the molecular to the organismal scale.

Involvement of Oxidative and Endoplasmic Reticulum Stress in RDH12-Related Retinopathies

Retinol dehydrogenase 12 (RDH12) is expressed in photoreceptor inner segments and catalyses the reduction of all-trans retinal (atRAL) to all-trans retinol (atROL), as part of the visual cycle. Mutations in RDH12 are primarily associated with autosomal recessive Leber congenital amaurosis. To further our understanding of the disease mechanisms, HEK-293 cell lines expressing wildtype (WT) and mutant RDH12 were created. The WT cells afforded protection from atRAL-induced toxicity and oxidative stress. Mutant RDH12 cells displayed reduced protein expression and activity, with an inability to protect cells from atRAL toxicity, inducing oxidative and endoplasmic reticulum (ER) stress, with upregulation of sXBP1, CHOP, and ATF4. Pregabalin, a retinal scavenger, attenuated atRAL-induced ER stress in the mutant RDH12 cell lines. A zebrafish rdh12 mutant model (rdh12^u533 c.17_23del; p.(Val6AlafsTer5)) was generated through CRISPR-Cas9 gene editing. Mutant fish showed disrupted phagocytosis through transmission electron microscopy, with increased phagosome size at 12 months post-fertilisation. Rhodopsin mislocalisation and reduced expression of atg12 and sod2 indicated early signs of a rod-predominant degeneration. A lack of functional RDH12 results in ER and oxidative stress representing key pathways to be targeted for potential therapeutics.

Identifying vitamin A signaling by visualizing gene and protein activity, and by quantification of vitamin A metabolites

Vitamin A (retinol) is an essential nutrient for embryonic development and adult homeostasis. Signaling by vitamin A is carried out by its active metabolite, retinoic acid (RA), following a two-step conversion. RA is a small, lipophilic molecule that can diffuse from its site of synthesis to neighboring RA-responsive cells where it binds retinoic acid receptors within RA response elements of target genes. It is critical that both vitamin A and RA are maintained within a tight physiological range to protect against developmental disorders and disease. Therefore, a series of compensatory mechanisms exist to ensure appropriate levels of each. This strict regulation is provided by a number synthesizing and metabolizing enzymes that facilitate the precise spatiotemporal control of vitamin A metabolism, and RA synthesis and signaling. In this chapter we describe protocols that (1) biochemically isolate and quantify vitamin A and its metabolites and (2) visualize the spatiotemporal activity of genes and proteins involved in the signaling pathway.

Dysregulatory effects of retinoic acid isomers in late zebrafish embryos

All-trans retinoic acid (atRA) and 9-cis retinoic acid (9cRA) are two natural derivatives of vitamin A that contribute to the normal vertebrate development by affecting gene expression through the retinoic acid signalling pathway. We show transcriptomic effects of the ectopic addition of atRA or 9cRA to zebrafish embryos at the posthatching embryonic stage. Exposure for 24 or 72 h to sublethal concentrations of both isomers resulted in characteristic transcriptome changes, in which many proliferation and development-related genes became underexpressed, whereas genes related to retinoid metabolism and some metabolic functions became overrepresented. While short and long exposures elicit essentially the same set of genes, atRA specifically induced expression of a specific subset of proteases, likely acting at the extracellular level, and of elements of the response to xenobiotics. These results reflect the well-known antiproliferative activity of retinoids, and they suggest a dysregulation of the developmental process at final stages of embryogenesis. They also indicate a potential role of endopeptidases as markers of developmental alterations, as well as their possible control by the retinoic signalling pathway. We propose to monitor mRNA levels of cyp16a, cyp16b, and cyp16c in zebrafish embryos as a bioassay for retinoid disruption.

RNA-sequencing and pathway analysis reveal alteration of hepatic steroid biosynthesis and retinol metabolism by tributyltin exposure in male rare minnow (Gobiocypris rarus)

Tributyltin (TBT) is widely spread in aquatic ecosystems. Although adverse effects of TBT on reproduction and lipogenesis are observed in fishes, the underlying mechanisms, especially in livers, are still scarce and inconclusive. Thus, RNA-sequencing runs were performed on the hepatic libraries of adult male rare minnow (Gobiocypris rarus) after TBT exposure for 60d. After differentially expressed genes were identified, enrichment analysis and validation by quantitative real-time PCR were conducted. The results showed that TBT up-regulated the profile of hepatic genes in the steroid biosynthesis pathway and down-regulated the profile of hepatic genes in the retinol metabolism pathway. In the hepatic steroid biosynthesis pathway, TBT might induce biosynthesis of cholesterol, which could affect the bioavailability of steroid hormones. More important, 3beta-hydroxysteroid 3-dehydrogenase, a key enzyme in the biosynthesis of all active steroid hormones, was up-regulated by TBT exposure. In the hepatic retinol metabolism pathway, TBT impaired retinoic acid homeostasis which plays essential roles in both reproduction and lipogenesis. The results of two pathways offered new mechanisms underlying the toxicology of TBT and represented a starting point from which detailed mechanistic links should be explored.

Insights into electrosensory organ development, physiology and evolution from a lateral line-enriched transcriptome

The anamniote lateral line system, comprising mechanosensory neuromasts and electrosensory ampullary organs, is a useful model for investigating the developmental and evolutionary diversification of different organs and cell types. Zebrafish neuromast development is increasingly well understood, but neither zebrafish nor Xenopus is electroreceptive and our molecular understanding of ampullary organ development is rudimentary. We have used RNA-seq to generate a lateral line-enriched gene-set from late-larval paddlefish (Polyodon spathula). Validation of a subset reveals expression in developing ampullary organs of transcription factor genes critical for hair cell development, and genes essential for glutamate release at hair cell ribbon synapses, suggesting close developmental, physiological and evolutionary links between non-teleost electroreceptors and hair cells. We identify an ampullary organ-specific proneural transcription factor, and candidates for the voltage-sensing L-type Ca_v channel and rectifying K_v channel predicted from skate (cartilaginous fish) ampullary organ electrophysiology. Overall, our results illuminate ampullary organ development, physiology and evolution.

Enzymatic Metabolism of Vitamin A in Developing Vertebrate Embryos

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

An Endogenous Mammalian Retinoid X Receptor Ligand, At Last!

9-cis-Retinoic acid was identified and claimed to be the endogenous ligand of the retinoid X receptors (RXRs) in 1992. Since then, the endogenous presence of this compound has never been rigorously confirmed. Instead, concerns have been raised by other groups that have reported that 9-cis-retinoic acid is undetectable or that its presence occurs at very low levels. Furthermore, these low levels could not satisfactorily explain the physiological activation of RXR. Alternative ligands, among them various lipids, have also been identified, but also did not fulfill criteria for rigorous endogenous relevance, and their consideration as bona fide endogenous mammalian RXR ligand has likewise been questioned. Recently, novel studies claim that the saturated analogue 9-cis-13,14-dihydroretinoic acid functions as an endogenous physiologically relevant mammalian RXR ligand.

Rdh10a Provides a Conserved Critical Step in the Synthesis of Retinoic Acid during Zebrafish Embryogenesis

The first step in the conversion of vitamin A into retinoic acid (RA) in embryos requires retinol dehydrogenases (RDHs). Recent studies have demonstrated that RDH10 is a critical core component of the machinery that produces RA in mouse and Xenopus embryos. If the conservation of Rdh10 function in the production of RA extends to teleost embryos has not been investigated. Here, we report that zebrafish Rdh10a deficient embryos have defects consistent with loss of RA signaling, including anteriorization of the nervous system and enlarged hearts with increased cardiomyocyte number. While knockdown of Rdh10a alone produces relatively mild RA deficient phenotypes, Rdh10a can sensitize embryos to RA deficiency and enhance phenotypes observed when Aldh1a2 function is perturbed. Moreover, excess Rdh10a enhances embryonic sensitivity to retinol, which has relatively mild teratogenic effects compared to retinal and RA treatment. Performing Rdh10a regulatory expression analysis, we also demonstrate that a conserved teleost rdh10a enhancer requires Pax2 sites to drive expression in the eyes of transgenic embryos. Altogether, our results demonstrate that Rdh10a has a conserved requirement in the first step of RA production within vertebrate embryos.

Plasma retinoids concentration in Leptodactylus chaquensis (Amphibia: Leptodactylidae) from rice agroecosystems, Santa Fe province, Argentina

Retinoids are known to regulate important processes such as differentiation, development, and embryogenesis of vertebrates: Alteration in endogenous retinoids concentration is linked with teratogenic effects. Retinol (ROH), retinoid acid (RA), and isoform 13-Cis-retinoic acid (13-Cis-RA), in plasma of a native adults frog, Leptodactylus chaquensis from a rice field (RF) and a forest (reference site; RS) were measured. ROH did not vary between treatment sites. RA and 13-Cis-RA activities were higher (93.7±8.6 μg mL(-1) and 131.7±11.4 μg mL(-1), respectively) in individuals collected from RF than in those from RS (65.5±8.6 μg mL(-1) and 92.2±10.2 μg mL(-1), respectively). The ratios retinoic acid-retinol (RA/ROH) and 13-Cis-RA/ROH revealed significantly higher values in RF than in RS. RA and 13-Cis-RA concentrations in plasma on wild amphibian's species such as L. chaquensis would be suitable biomarkers of pesticide exposure in field monitoring. Finally, the mechanism of alteration in retinoid metabolites alteration should be further explored both in larvae and adult, considering that the potential exposition and uptake contaminants vary between the double lives of these vertebrates.

STRA6: role in cellular retinol uptake and efflux

Distribution of vitamin A throughout the body is important to maintain retinoid function in peripheral tissues and to ensure optimal vision. A critical step of this process is the transport of vitamin A across cell membranes. Increasing evidence indicates that this process is mediated by a multidomian membrane protein that is encoded by the stimulated by retinoic acid 6 (STRA6) gene. Biochemical studies revealed that STRA6 is a transmembrane pore which transports vitamin A bidirectionally between extra- and intracellular retinoid binding proteins. Vitamin A accumulation in cells is driven by coupling of transport with vitamin A esterification. Loss-of-function studies in zebrafish and mouse models have unraveled the critical importance of STRA6 for vitamin A homeostasis of peripheral tissues. Impairment in vitamin A transport and uptake homeostasis are associated with diseases including type 2 diabetes and a microphthalmic syndrome known as Matthew Wood Syndrome. This review will discuss the advanced state of knowledge about STRA6's biochemistry, biology and association with disease.

Retinoic acid receptors' expression and function during zebrafish early development

Retinoic acid (RA) regulates many developmental processes through its binding to two types of nuclear receptors, the retinoic acid receptor (RAR), and the retinoid-X receptor (RXR), which preferentially binds to the 9-cis isomer. Here we analyzed the RAR/RXR regulatory system during the first 5 days of development of zebrafish. Analysis of the relative transcript abundances for the four RAR and the six RXR zebrafish genes present in the zebrafish genome indicates a transition from maternal to embryonic transcripts during the first 24h post fertilization. These changes did not affect the response to exogenous RA of the known RAR-responsive genes cyp26a1, dhrs3a, hoxb1b, hoxb5a, and hoxb5b. At the transcriptomic level, RA treatment elicited a negative feedback of genes involved in the endogenous RA synthesis and reduced levels of transcripts related to organ and anatomic development. These effects occurred at concentrations at which no morphological changes were observed. Data analysis suggests that exposure to exogenous RA results in an advance of the developing program, activating genes that should remain silent until later developmental stages and inhibiting expression of development-related genes. We conclude that zebrafish embryos are particularly sensitive to potential disruptors of the RAR/RXR regulatory system.

The retinaldehyde reductase DHRS3 is essential for preventing the formation of excess retinoic acid during embryonic development

Oxidation of retinol via retinaldehyde results in the formation of the essential morphogen all-trans-retinoic acid (ATRA). Previous studies have identified critical roles in the regulation of embryonic ATRA levels for retinol, retinaldehyde, and ATRA-oxidizing enzymes; however, the contribution of retinaldehyde reductases to ATRA metabolism is not completely understood. Herein, we investigate the role of the retinaldehyde reductase Dhrs3 in embryonic retinoid metabolism using a Dhrs3-deficient mouse. Lack of DHRS3 leads to a 40% increase in the levels of ATRA and a 60% and 55% decrease in the levels of retinol and retinyl esters, respectively, in Dhrs3(-/-) embryos compared to wild-type littermates. Furthermore, accumulation of excess ATRA is accompanied by a compensatory 30-50% reduction in the expression of ATRA synthetic genes and a 120% increase in the expression of the ATRA catabolic enzyme Cyp26a1 in Dhrs3(-/-) embryos vs. controls. Excess ATRA also leads to alterations (40-80%) in the expression of several developmentally important ATRA target genes. Consequently, Dhrs3(-/-) embryos die late in gestation and display defects in cardiac outflow tract formation, atrial and ventricular septation, skeletal development, and palatogenesis. These data demonstrate that the reduction of retinaldehyde by DHRS3 is critical for preventing formation of excess ATRA during embryonic development.

Effects of acute exposure to polybrominated diphenyl ethers on retinoid signaling in zebrafish larvae

The objectives of the present study were to investigate the effects of acute exposure to PBDEs on retinoid signaling in fish. Zebrafish embryos (2h post-fertilization, hpf) were exposed to DE-71 (0, 31.0, 68.7, and 227.6μg/L) until 120hpf. Retinoid profiles showed the content of retinal and retinoic acid was reduced significantly. While a significant up-regulation was observed in the transcription of retinal dehydrogenase (raldh2), the transcription of retinol binding protein (rbp1a), retinol dehydrogenase (rdh1), cellular retinoic acid binding protein (crabp1a and crabp2a) and retinoic acid receptor subunit (raraa) were down-regulated significantly, indicating disruption of retinoid signaling. However, the transcriptions of five opsin genes (zfrho, zfuv, zfred, zfblue, and zfgr1) were up-regulated. Furthermore, whole mount immunostaining and western blotting demonstrated increased rhodopsin protein expression in the exposure groups. Overall, the results indicated that acute exposure to PBDEs could disturb retinoid signaling and may impact on eye development of zebrafish larvae.

Spatiotemporal manipulation of retinoic acid activity in zebrafish hindbrain development via photo-isomerization

All-trans retinoic acid (RA) is a key player in many developmental pathways. Most methods used to study its effects in development involve continuous all-trans RA activation by incubation in a solution of all-trans RA or by implanting all-trans RA-soaked beads at desired locations in the embryo. Here we show that the UV-driven photo-isomerization of 13-cis RA to the trans-isomer (and vice versa) can be used to non-invasively and quantitatively control the concentration of all-trans RA in a developing embryo in time and space. This facilitates the global or local perturbation of developmental pathways with a pulse of all-trans RA of known concentration or its inactivation by UV illumination. In zebrafish embryos in which endogenous synthesis of all-trans RA is impaired, incubation for as little as 5 minutes in 1 nM all-trans RA (a pulse) or 5 nM 13-cis RA followed by 1-minute UV illumination is sufficient to rescue the development of the hindbrain if performed no later than bud stage. However, if subsequent to this all-trans RA pulse the embryo is illuminated (no later than bud stage) for 1 minute with UV light (to isomerize, i.e. deactivate, all-trans RA), the rescue of hindbrain development is impaired. This suggests that all-trans RA is sequestered in embryos that have been transiently exposed to it. Using 13-cis RA isomerization with UV light, we further show that local illumination at bud stage of the head region (but not the tail) is sufficient to rescue hindbrain formation in embryos whose all-trans RA synthetic pathway has been impaired.

Alterations in retinoid status after long-term exposure to PBDEs in zebrafish (Danio rerio)

This study examined the disruptive effect of exposure to polybrominated diphenyl ethers (PBDEs) on retinoid content in zebrafish (Danio rerio). Adult zebrafish were exposed to an environmentally relevant concentration (0.45 μg/L) and a higher concentration (9.6 μg/L) of DE-71 for 60 days. Retinoid content and gene transcription levels were examined in female zebrafish. PBDE exposure caused a significant decrease of retinyl ester content in the intestine and a downregulation of intestinal cellular retinol binding protein gene transcription (CRBP1a). In the liver, retinyl ester content was significantly decreased, while retinol content was increased. An upregulation of liver CRBP2a and retinol binding protein (RBP) gene transcription and an increased level of RBP protein were observed. In the eyes, both the retinal and retinyl ester content were increased and CRBP1a gene transcription was upregulated. However, the gene encoding for retinal dehydrogenase (RALDH2), responsible for retinoic acid synthesis, was downregulated in the eyes. CYP26a, the gene responsible for retinoic acid degradation, was upregulated, which indicated an increased level of retinoic acid. In the ovaries, the increased deposition of retinoids was also observed, while gene transcription levels of both CRBPs (CRBP1a and CRBP1b) were upregulated. An increased deposition of retinal was measured in the eggs. Overall, this study demonstrated that long-term exposure of zebrafish to environmentally relevant concentrations of DE-71 disrupted the transport, storage and metabolism of retinoid in various tissues. This study also indicated that retinoid levels in zebrafish are sensitive to PBDE exposure and highlighted the importance of liver storage, which appears to support important functions in reproduction and vision.

Insight into molecular pathways of retinal metabolism, associated with vitellogenesis in zebrafish

Retinal is the main retinoid stored in oviparous eggs of fish, amphibians, and reptiles, reaching the oocytes in association with vitellogenins, the yolk precursor proteins. During early presegmentation stages of zebrafish embryos, retinal is metabolized to retinoic acid (RA), which regulates genes involved in cell proliferation, differentiation, and tissue function and is therefore essential for normal embryonic development. While synthesis of vitellogenin and its regulation by 17β-estradiol (E(2)) were extensively investigated, pathways for retinal synthesis remain obscure. We determined the expression pattern of 46 candidate genes, aiming at identifying enzymes associated with retinal synthesis, ascertaining whether they were regulated by E(2), and finding pathways that could fulfill the demand for retinoids during vitellogenesis. Genes associated with retinal synthesis were upregulated in liver (rdh10, rdh13, sdr) and surprisingly also in intestine (rdh13) and ovary (rdh1, sdr), concomitantly with higher gene expression and synthesis of vitellogenins in liver but also in extrahepatic tissues, shown here for the first time. Vitellogenin synthesis in the ovary was regulated by E(2). Gene expression studies suggest that elevated retinal synthesis in liver, intestine, and ovary also depends on cleavage of carotenoids (by Bcdo2 or Bmco1), but in the ovary it may also be contingent on higher uptake of retinol from the circulatory system (via Stra6) and retinol synthesis from retinyl esters (by Lpl). Decrease in oxidation (by Raldh2 or Raldh3) of retinal to RA and/or degradation of RA (by Cyp26a1) may also facilitate higher hepatic retinal levels. Together, these processes enable meeting the putative demands of retinal for binding to vitellogenins. Bioinformatic tools reveal multiple hormone response elements in the studied genes, suggesting complex and intricate regulation of these processes.

Alcohol and aldehyde dehydrogenases: retinoid metabolic effects in mouse knockout models

Retinoic acid (RA) is the active metabolite of vitamin A (retinol) that controls growth and development. The first step of RA synthesis is controlled by enzymes of the alcohol dehydrogenase (ADH) and retinol dehydrogenase (RDH) families that catalyze oxidation of retinol to retinaldehyde. The second step of RA synthesis is controlled by members of the aldehyde dehydrogenase (ALDH) family also known as retinaldehyde dehydrogenase (RALDH) that further oxidize retinaldehyde to produce RA. RA functions as a ligand for DNA-binding RA receptors that directly regulate transcription of specific target genes. Elucidation of the vitamin A metabolic pathway and investigation of the endogenous function of vitamin A metabolites has been greatly improved by development of mouse ADH, RDH, and RALDH loss-of-function models. ADH knockouts have demonstrated a postnatal role for this enzyme family in clearance of excess retinol to prevent vitamin A toxicity and in generation of RA for postnatal survival during vitamin A deficiency. A point mutation in Rdh10 generated by ethylnitrosourea has demonstrated that RDH10 generates much of the retinaldehyde needed for RA synthesis during embryonic development. Raldh1, Raldh2, and Raldh3 knockouts have demonstrated that RALDH1, RALDH2, and RALDH3 generate most of the RA needed during embryogenesis. These mouse models serve as instrumental tools for providing new insight into retinoid function. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Detection of vitellogenin incorporation into zebrafish oocytes by FITC fluorescence

BACKGROUND

Large volumes of lymph can be collected from the eye-sacs of bubble-eye goldfish. We attempted to induce vitellogenin (Vtg) in the eye-sac lymph of bubble-eye goldfish and develop a method for visualizing Vtg incorporation by zebrafish oocytes using FITC-labeling.

METHODS

Estrogen efficiently induced Vtg in the eye-sac lymph of goldfish. After FITC-labeled Vtg was prepared, it was injected into mature female zebrafish.

RESULTS

Incorporation of FITC-labeled Vtg by zebrafish oocytes was detected in in vivo and in vitro experiments. The embryos obtained from zebrafish females injected with FITC-labeled Vtg emitted FITC fluorescence from the yolk sac and developed normally.

CONCLUSION

This method for achieving Vtg incorporation by zebrafish oocytes could be useful in experiments related to the development and endocrinology of zebrafish oocytes.

prep1.2 and aldh1a2 participate to a positive loop required for branchial arches development in zebrafish

Segmentation is a key step in embryonic development. Acting in all germ layers, it is responsible for the generation of antero-posterior asymmetries. Hox genes, with their diverse expression in individual segments, are fundamental players in the determination of different segmental fates. In vertebrates, Hox gene products gain specificity for DNA sequences by interacting with Pbx, Prep and Meis homeodomain transcription factors. In this work we cloned and analysed prep1.2 in zebrafish. In-situ hybridization experiments show that prep1.2 is maternally and ubiquitously expressed up to early somitogenesis when its expression pattern becomes more restricted to the head and trunk mesenchyme. Experiments of loss of function with prep1.2 morpholinos change the shape of the hyoid and third pharyngeal cartilages while arches 4-7 and pectoral fins are absent, a phenotype strikingly similar to that caused by loss of retinoic acid (RA). In fact, we show that prep1.2 is positively regulated by RA and required for the normal expression of aldh1a2 at later stages, particularly in tissues involved in the development of the branchial arches and pectoral fins. Thus, prep1.2 and aldh1a2 are members of an indirect positive feedback loop required for pharyngeal endoderm and posterior branchial arches development. As the paralogue gene prep1.1 is more important in hindbrain patterning and neural crest chondrogenesis, we provide evidence of a functional specialization of prep genes in zebrafish head segmentation and morphogenesis.

Maternal and zygotic aldh1a2 activity is required for pancreas development in zebrafish

We have isolated and characterized a novel zebrafish pancreas mutant. Mutant embryos lack expression of isl1 and sst in the endocrine pancreas, but retain isl1 expression in the CNS. Non-endocrine endodermal gene expression is less affected in the mutant, with varying degrees of residual expression observed for pdx1, carbA, hhex, prox1, sid4, transferrin and ifabp. In addition, mutant embryos display a swollen pericardium and lack fin buds. Genetic mapping revealed a mutation resulting in a glycine to arginine change in the catalytic domain of the aldh1a2 gene, which is required for the production of retinoic acid from vitamin A. Comparison of our mutant (aldh1a2^um22) to neckless (aldh1a2ⁱ²⁶), a previously identified aldh1a2 mutant, revealed similarities in residual endodermal gene expression. In contrast, treatment with DEAB (diethylaminobenzaldehyde), a competitive reversible inhibitor of Aldh enzymes, produces a more severe phenotype with complete loss of endodermal gene expression, indicating that a source of Aldh activity persists in both mutants. We find that mRNA from the aldh1a2^um22 mutant allele is inactive, indicating that it represents a null allele. Instead, the residual Aldh activity is likely due to maternal aldh1a2, since we find that translation-blocking, but not splice-blocking, aldh1a2 morpholinos produce a phenotype similar to DEAB treatment. We conclude that Aldh1a2 is the primary Aldh acting during pancreas development and that maternal Aldh1a2 activity persists in aldh1a2^um22 and aldh1a2ⁱ²⁶ mutant embryos.

Cyp26 enzymes function in endoderm to regulate pancreatic field size

The control of organ size and position relies, at least in part, upon appropriate regulation of the signals that specify organ progenitor fields. Pancreatic cell fates are specified by retinoic acid (RA), and proper size and localization of the pancreatic field are dependent on tight control of RA signaling. Here we show that the RA-degrading Cyp26 enzymes play a critical role in defining the normal anterior limit of the pancreatic field. Disruption of Cyp26 function causes a dramatic expansion of pancreatic cell types toward the anterior of the embryo. The cyp26a1 gene is expressed in the anterior trunk endoderm at developmental stages when RA is signaling to specify pancreas, and analysis of cyp26a1/giraffe (gir) mutant zebrafish embryos confirms that cyp26a1 plays the primary role in setting the anterior limit of the pancreas. Analysis of the gir mutants further reveals that cyp26b1 and cyp26c1 function redundantly to partially compensate for loss of Cyp26a1 function. We used cell transplantation to determine that Cyp26a1 functions directly in endoderm to modulate RA signaling and limit the pancreatic field. Taken together with our finding that endodermal expression of cyp26 genes is subject to positive regulation by RA, our data reveal a feedback loop within the endoderm. Such feedback can maintain consistent levels of RA signaling, despite environmental fluctuations in RA concentration, thus ensuring a consistent size and location of the pancreatic field.

Ethanol induces embryonic malformations by competing for retinaldehyde dehydrogenase activity during vertebrate gastrulation

Human embryos exposed to alcohol (ethanol) develop a complex developmental phenotype known as fetal alcohol spectrum disorder (FASD). In Xenopus embryos, ethanol reduces the levels of retinoic acid (RA) signaling during gastrulation. RA, a metabolite of vitamin A (retinol), is required for vertebrate embryogenesis, and deviation from its normal levels results in developmental malformations. Retinaldehyde dehydrogenase 2 (RALDH2) is required to activate RA signaling at the onset of gastrulation. We studied the effect of alcohol on embryogenesis by manipulating retinaldehyde dehydrogenase activity in ethanol-treated embryos. In alcohol-treated embryos, we analyzed RA signaling levels, phenotypes induced and changes in gene expression. Developmental defects that were characteristic of high ethanol concentrations were phenocopied by a low ethanol concentration combined with partial RALDH inhibition, whereas Raldh2 overexpression rescued the developmental malformations induced by high ethanol. RALDH2 knockdown resulted in similar RA signaling levels when carried out alone or in combination with ethanol treatment, suggesting that RALDH2 is the main target of ethanol. The biochemical evidence that we present shows that, at the onset of RA signaling during early gastrulation, the ethanol effect centers on the competition for the available retinaldehyde dehydrogenase activity. In light of the multiple regulatory roles of RA, continued embryogenesis in the presence of abnormally low RA levels provides an etiological explanation for the malformations observed in individuals with FASD.

Locust retinoid X receptors: 9-Cis-retinoic acid in embryos from a primitive insect

The retinoid X receptor (RXR) is activated by its often elusive cognate ligand, 9-cis-retinoic acid (9-cis-RA). In flies and moths, molting is mediated by a heterodimer ecdysone receptor consisting of the ecdysone monomer (EcR) and an RXR homolog, ultraspiracle (USP); the latter is believed to have diverged from its RXR origin. In the more primitive insect, Locusta migratoria (Lm), RXR is more similar to human RXRs than to USPs. LmRXR was detected in early embryos when EcR transcripts were absent, suggesting another role apart from ecdysone signaling. Recombinant LmRXRs bound 9-cis-RA and all-trans-RA with high affinity (IC(50) = 61.2-107.7 nM; K(d) = 3 nM), similar to human RXR. To determine whether specific binding had functional significance, the presence of endogenous retinoids was assessed. Embryos were extracted by using modified Bligh and Dyer and solid-phase protocols to avoid the oily precipitate that makes this material unsuitable for assay. These extracts contained retinoids (5.4 nM) as assessed by RA-inducible Cyp26A1-promoter luciferase reporter cell lines. Furthermore, the use of HPLC and MS confirmed the presence of retinoids and identified in any embryo, 9-cis-RA, in addition to all-trans-RA. We estimate that whole embryos contain 3 nM RA, including 9-cis-RA at a concentration of 1.6 nM. These findings strongly argue for a functional role for retinoids in primitive insects and favor a model where signaling through the binding of 9-cis-RA to its RXR is established relatively early in evolution and embryonic development.

The search for non-chordate retinoic acid signaling: lessons from chordates

Signaling by retinoic acid (RA) is an important pathway in the development and homeostasis of vertebrate and invertebrate chordates, with a critical role in mesoderm patterning. Classical studies on the distribution of nuclear receptors of animals suggested that the family of RA receptors (RARs/NR1B) was restricted to chordates, while the family of RA X receptors (RXR/NR2B) was distributed from cnidarians to chordates. However, the accumulation of data from genome projects and studies in non-model species is questioning this traditional view. Here we discuss the evidence for non-chordate RA signaling systems in the light of recent advances in our understanding of carotene (pro-Vitamin A) metabolism and of the identification of potential RARs and members of the NR1 family in echinoderms and lophotrochozoan trematodes, respectively. We conclude, as have others before (Bertrand et al., 2004. Mol Biol Evol 21(10):1923-1937), that signaling by RA is more likely an ancestral feature of bilaterians than a chordate innovation.

Retinoid requirements in the reproduction of zebrafish

This study examines whether retinoids are essential in the reproduction of zebrafish. Using RT-PCR, it was shown that the ovaries and testes express enzymes that synthesize and metabolize the hormone retinoic acid (RA) (raldh2 and cyp26a, respectively), and RA receptors (raraa, rarga, rxrba, rxrbb, rxrga but not rxrab). Three new isoforms of rxrba were also observed in a variety of tissues. In other experiments, zebrafish were exposed for 11 d to diethylaminobenzaldehyde (DEAB), an inhibitor of RA synthesis, or fed a retinoid deficient diet for 130 d in order to evaluate the functional requirements of retinoids in reproduction. DEAB altered cyp26a transcript numbers in the gonads, suggesting an impact on RA, and decreased the number of spawned eggs by 95%. The retinoid deficient diet decreased whole body retinoids (retinol and retinal) by 68% in females and 33% in males. Females fed the retinoid deficient diet also produced 73% fewer eggs that contained 78% less retinal than controls. Fertilization rates were not affected. These studies have shown that the RA receptors are expressed in zebrafish gonads, and RA is required for the spawning of eggs. Dietary retinoid content influences reproduction, while retinyl ester storage levels appear to be of little significance. Females were more susceptible to retinoid perturbation than males, likely due to the cost of retinal deposition in the eggs. Overall, these studies have shown retinoids play a fundamental role in the reproduction of zebrafish, and the lack of retinyl ester stores in controls that successfully spawned illustrates that we have only a limited understanding of the retinoid physiology and requirements of fish.

The cdx genes and retinoic acid control the positioning and segmentation of the zebrafish pronephros

Kidney function depends on the nephron, which comprises a blood filter, a tubule that is subdivided into functionally distinct segments, and a collecting duct. How these regions arise during development is poorly understood. The zebrafish pronephros consists of two linear nephrons that develop from the intermediate mesoderm along the length of the trunk. Here we show that, contrary to current dogma, these nephrons possess multiple proximal and distal tubule domains that resemble the organization of the mammalian nephron. We examined whether pronephric segmentation is mediated by retinoic acid (RA) and the caudal (cdx) transcription factors, which are known regulators of segmental identity during development. Inhibition of RA signaling resulted in a loss of the proximal segments and an expansion of the distal segments, while exogenous RA treatment induced proximal segment fates at the expense of distal fates. Loss of cdx function caused abrogation of distal segments, a posterior shift in the position of the pronephros, and alterations in the expression boundaries of raldh2 and cyp26a1, which encode enzymes that synthesize and degrade RA, respectively. These results suggest that the cdx genes act to localize the activity of RA along the axis, thereby determining where the pronephros forms. Consistent with this, the pronephric-positioning defect and the loss of distal tubule fate were rescued in embryos doubly-deficient for cdx and RA. These findings reveal a novel link between the RA and cdx pathways and provide a model for how pronephric nephrons are segmented and positioned along the embryonic axis.

Cyp26 enzymes generate the retinoic acid response pattern necessary for hindbrain development

Retinoic acid (RA) is essential for normal vertebrate development, including the patterning of the central nervous system. During early embryogenesis, RA is produced in the trunk mesoderm through the metabolism of vitamin A derived from the maternal diet and behaves as a morphogen in the developing hindbrain where it specifies nested domains of Hox gene expression. The loss of endogenous sources of RA can be rescued by treatment with a uniform concentration of exogenous RA, indicating that domains of RA responsiveness can be shaped by mechanisms other than the simple diffusion of RA from a localized posterior source. Here, we show that the cytochrome p450 enzymes of the Cyp26 class, which metabolize RA into polar derivatives, function redundantly to shape RA-dependent gene-expression domains during hindbrain development. In zebrafish embryos depleted of the orthologs of the three mammalian CYP26 genes CYP26A1, CYP26B1 and CYP26C1, the entire hindbrain expresses RA-responsive genes that are normally restricted to nested domains in the posterior hindbrain. Furthermore, we show that Cyp26 enzymes are essential for exogenous RA to rescue hindbrain patterning in RA-depleted embryos. We present a ;gradient-free' model for hindbrain patterning in which differential RA responsiveness along the hindbrain anterior-posterior axis is shaped primarily by the dynamic expression of RA-degrading enzymes.

Is 9-cis-retinoic acid the endogenous ligand for the retinoic acid-X receptor?

Specific proteins in the nucleus act as transcription factors upon activation through binding of small molecules (all-trans-retinoic acid, thyroid hormone, vitamin D, and others). The activated (liganded) receptors bind to specific DNA elements as heterodimers, each in combination with the retinoic acid-X receptor (RXR). 9-Cis-retinoic acid binds to RXR with high affinity and activates it. Though 9-cis-retinoic acid was initially found in animal tissues, in later work 9-cis-retinoic acid could not be detected. A search for a ligand for RXR in tissues showed that unsaturated fatty acids, particularly linoleic, linolenic, and docosahexaenoic acids, bound to and activated RXR as specific ligands, although with low affinity. A critical experiment demonstrated that, at least in developing mouse skin, 9-cis-retinoic acid is not the ligand for RXR.

Specificity of zebrafish retinol saturase: formation of all-trans-13,14-dihydroretinol and all-trans-7,8- dihydroretinol

Metabolism of vitamin A, all-trans-retinol, leads to the formation of 11-cis-retinaldehyde, the visual chromophore, and all-trans-retinoic acid, which is involved in the regulation of gene expression through the retinoic acid receptor. Enzymes and binding proteins involved in retinoid metabolism are highly conserved across species. We previously described a novel mammalian enzyme that saturates the 13-14 double bond of all-trans-retinol to produce all-trans-13,14-dihydroretinol, which then follows the same metabolic fate as that of all-trans-retinol. Specifically, all-trans-13,14-dihydroretinol is transiently oxidized to all-trans-13,14-dihydroretinoic acid before being oxidized further by Cyp26 enzymes. Here, we report the identification of two putative RetSat homologues in zebrafish, one of which, zebrafish RetSat A (zRetSat A), also had retinol saturase activity, whereas zebrafish RetSat B (zRetSat B) was inactive under similar conditions. Unlike mouse RetSat (mRetSat), zRetSat A had an altered bond specificity saturating either the 13-14 or 7-8 double bonds of all-trans-retinol to produce either all-trans-13,14-dihydroretinol or all-trans-7,8-dihydroretinol, respectively. zRetSat A also saturated the 13-14 or 7-8 double bonds of all-trans-3,4-didehydroretinol (vitamin A2), a second endogenous form of vitamin A in zebrafish. The dual enzymatic activity of zRetSat A displays a newly acquired specificity for the 13-14 double bond retained in higher vertebrates and also the evolutionarily preserved activity of bacterial phytoene desaturases and plant carotenoid isomerases. Expression of zRetSat A was restricted to the liver and intestine of hatchlings and adult zebrafish, whereas zRetSat B was expressed in the same tissues but at earlier developmental stages. Exogenous all-trans-retinol, all-trans-13,14-dihydroretinol, or all-trans-7,8-dihydroretinol led to the strong induction of the expression of the retinoic acid-metabolizing enzyme, Cyp26A1, arguing for an active signaling function of dihydroretinoid metabolites in zebrafish. These findings point to a conserved function but altered specificity of RetSat in vertebrates, leading to the generation of various dihydroretinoid compounds, some of which could have signaling functions.

Merging protein, gene and genomic data: the evolution of the MDR-ADH family

Multiple members of the MDR-ADH (MDR: Medium-chain dehydrogenases/reductases; ADH: alcohol dehydrogenase) family are found in vertebrates, although the enzymes that belong to this family have also been isolated from bacteria, yeast, plant and animal sources. Initial understanding of the physiological roles and evolution of the family relied on biochemical studies, protein alignments and protein structure comparisons. Subsequently, studies at the genetic level yielded new information: the expression pattern, exon-intron distribution, in silico-derived protein sequences and murine knockout phenotypes. More recently, genomic and EST databases have revealed new family members and the chromosomal location and position in the cluster of both the first and new forms. The data now available provide a comprehensive scenario, from which a reliable picture of the evolutionary history of this family can be made.

Retinoic acid activates myogenesis in vivo through Fgf8 signalling

Retinoic acid (RA) has been shown to regulate muscle differentiation in vitro. Here, we have investigated the role of RA signalling during embryonic myogenesis in zebrafish. We have altered RA signalling from gastrulation stages onwards by either inhibiting endogenous RA synthesis using an inhibitor of retinaldehyde dehydrogenases (DEAB) or by addition of exogenous RA. DEAB reduces expression of the myogenic markers myoD and myogenin in somites, whereas RA induces increased expression of these genes and strongly induces premature myoD expression in the presomitic mesoderm (psm). The expression dynamics of myf5 in presomitic and somitic mesoderm suggest that RA promotes muscle differentiation, a role supported by the fact that RA activates expression of fast myosin, while DEAB represses it. We identify Fgf8 as a major relay factor in RA-mediated activation of myogenesis. We show that fgf8 expression in somites and anterior psm is regulated by RA, and find that in the absence of Fgf8 signalling in the acerebellar mutant RA fails to promote myoD expression. We propose that, in the developing embryo, localised synthesis of RA by Raldh2 in the anterior psm and in somites activates fgf8 expression which in turn induces the expression of myogenic genes and fast muscle differentiation.

Retinoic acid and hindbrain patterning

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

Retinoic acid-metabolizing enzyme Cyp26a1 is essential for determining territories of hindbrain and spinal cord in zebrafish

Retinoic acid (RA) plays a critical role in neural patterning and organogenesis in the vertebrate embryo. Here we characterize a mutant of the zebrafish named giraffe (gir) in which the gene for the RA-degrading enzyme Cyp26a1 is mutated. The gir mutant displayed patterning defects in multiple organs including the common cardinal vein, pectoral fin, tail, hindbrain, and spinal cord. Analyses of molecular markers suggested that the lateral plate mesoderm is posteriorized in the gir mutant, which is likely to cause the defects of the common cardinal vein and pectoral fin. The cyp26a1 expression in the rostral spinal cord was strongly upregulated in the gir mutant, suggesting a strong feedback control of its expression by RA signaling. We also found that the rostral spinal cord territory was expanded at the expense of the hindbrain territory in the gir mutant. Such a phenotype is the opposite of that of the mutant for Raldh2, an enzyme that synthesizes RA. We propose a model in which Cyp26a1 attenuates RA signaling in the prospective rostral spinal cord to limit the expression of hox genes and to determine the hindbrain-spinal cord boundary.

Studies on retinol-binding protein during vitellogenesis in the Rainbow Trout (Oncorhynchus mykiss)

Retinoids are important regulatory signaling molecules during embryonic development and therefore, should be present in the eggs of oviparous animals that develop independently of the maternal organism. Studies were initiated in Rainbow Trout to elucidate the role of retinol-binding protein (RBP), the specific retinol carrier protein in vertebrate plasma, during vitellogenesis. Plasma levels of RBP in pre-vitellogenic, vitellogenic, and post-vitellogenic females were compared to plasma of male trout, using a rabbit polyclonal antiserum that was generated to His-tagged RBP recombinant protein. Western-blot analyses showed that there were no differences in the relative plasma levels of RBP between pre-vitellogenic, vitellogenic or post-vitellogenic females and similar levels were also found in males. In contrast, strong elevation of vitellogenin (VTG) was observed in the plasma from vitellogenic females. Northern-blot analysis of hepatic mRNA revealed that there were no dramatic changes in the abundance of RBP transcripts in the liver of females during vitellogenesis, but showed a significant increase in the expression of VTG in the livers of vitellogenic females. These results indicate differences in the regulation of RBP and VTG during vitellogenesis, suggesting that RBP may not be the main transporter protein for retinoids to fish egg. Recent publications on the association of retinal with VTG in fish and the occurrence of RBP transcripts in ovarian tissues raise the need for reevaluation of the role of RBP during vitellogenesis in oviparous non-mammalian vertebrate species.

Retinal is the essential form of retinoid for storage and transport in the adult of the ascidian Halocynthia roretzi

Retinoids in the organs (gonad [GND], body wall muscle [BWM], hepatopancreas [HP], gill, hemolymph cells and hemolymph plasma) of the adult ascidian Halocynthia roretzi were analyzed by high performance liquid chromatography. Retinal (RAL) occurred in every organ examined, and most of RAL (>/=99%) was localized in the GND and BWM. None of the organs contained significant amounts of retinol (ROL) or retinyl ester (RE). Lipid droplets, which are characteristic of stellate cells (RE-storing cells of vertebrates), could not be found in the GND, BWM and HP by microscopic observations. These results indicate that this ascidian lacks the RE-storing mechanism, which is ubiquitous in adult vertebrates. The amount and localization of RAL showed the annual change in relation to the reproductive cycle. During summer, the growing season, RAL was present in both GND and BWM at a ratio of about 3:2. From summer to winter, RAL in the GND gradually increased, concomitant with the decrease of RAL in the BWM. In winter, the spawning season, most of RAL was present in the GND (ca. 98%). RAL appears to be accumulated first in the BWM and transported to oocytes accompanying yolk accumulation. ROL and RE were not implicated in the storage and transport of retinoids. The results in the present research strongly suggest that retinoic acid (RA) is produced by the two-step enzymatic reaction: carotenoid cleavage to RAL followed by RAL oxidation to RA and that the prevertebrate chordate lacks ROL-metabolizing systems.

The cellular retinol-binding protein genes are duplicated and differentially transcribed in the developing and adult zebrafish (Danio rerio)

There are single copies of the genes encoding the cellular retinol-binding protein type I and II (CRBPI and CRBPII) in the human and rodent genomes. We have identified duplicate genes for both CRBPI and CRBPII in the zebrafish (Danio rerio) genome (rbp1b and rbp2b). The zebrafish rbp1b and rbp2b have conserved gene structures, amino acid sequence similarities, gene phylogenies, and syntenic relationships with their mammalian orthologs and zebrafish paralogs, rbp1a and rbp2a. Like the mammalian genes for CRBPI and CRBPII, the zebrafish rbp1b and rbp2b genes are closely linked on a single linkage group. Comparative analysis suggests that the duplicate genes of rbp1 and rbp2 in the zebrafish genome may have arisen by chromosomal or whole-genome duplication. During embryonic development, rbp1b transcripts were detected in the gall bladder of 5-day postfertilization (5 dpf) larvae. The rbp2b mRNA was abundant in the developing liver through 48 hours postfertilization (48 hpf) to 5 dpf. Using reverse transcription-polymerase chain reaction (RT-PCR), rbp1b transcripts were detected in the ovary, and rbp2b mRNA was observed predominantly in the adult liver. Tissue section in situ hybridization and emulsion autoradiography localized rbp1b mRNA to primary oocytes within the zebrafish ovary. The differential mRNA distribution patterns of the rbp1a, rbp1b, rbp2a, and rbp2b genes in the developing and adult zebrafish suggest that shuffling of subfunctions among duplicate copies of paralogous genes may be a mechanism for the retention of duplicated genes in vertebrates.

Isolation and characterization of unsaturated fatty acids as natural ligands for the retinoid-X receptor

The retinoid-X receptor (RXR) is a ligand activated nuclear receptor that is the heterodimer partner for many class II nuclear receptors. Previously identified natural ligands for this receptor include 9-cis retinoic acid (9cRA), docosahexaenoic acid, and phytanic acid. Our studies were performed to determine if there are any unidentified, physiologically important RXR ligands. Agonists for RXR were purified from rat heart and testes lipid extracts with the use of a cell-based reporter assay to monitor RXR activation. Purified active fractions contained a variety of unsaturated fatty acids and components were quantified by gas-liquid chromatography of derivatized samples. The corresponding fatty acid standards elicited a similar response in the reporter cell assay. Competition binding analysis revealed that the active fatty acids compete with [3H]9cRA for binding to RXR. Non-esterified fatty acids were analyzed from lipid extracts of isolated heart and testes nuclei and endogenous concentrations were found to be within the range of their determined binding affinities. Our studies reveal tissue dependent profiles of RXR agonists and support the idea of unsaturated fatty acids as physiological ligands of RXR.

Crystal structure of the vertebrate NADP(H)-dependent alcohol dehydrogenase (ADH8)

The amphibian enzyme ADH8, previously named class IV-like, is the only known vertebrate alcohol dehydrogenase (ADH) with specificity towards NADP(H). The three-dimensional structures of ADH8 and of the binary complex ADH8-NADP(+) have been now determined and refined to resolutions of 2.2A and 1.8A, respectively. The coenzyme and substrate specificity of ADH8, that has 50-65% sequence identity with vertebrate NAD(H)-dependent ADHs, suggest a role in aldehyde reduction probably as a retinal reductase. The large volume of the substrate-binding pocket can explain both the high catalytic efficiency of ADH8 with retinoids and the high K(m) value for ethanol. Preference of NADP(H) appears to be achieved by the presence in ADH8 of the triad Gly223-Thr224-His225 and the recruitment of conserved Lys228, which define a binding pocket for the terminal phosphate group of the cofactor. NADP(H) binds to ADH8 in an extended conformation that superimposes well with the NAD(H) molecules found in NAD(H)-dependent ADH complexes. No additional reshaping of the dinucleotide-binding site is observed which explains why NAD(H) can also be used as a cofactor by ADH8. The structural features support the classification of ADH8 as an independent ADH class.

Provitamin A conversion to retinal via the beta,beta-carotene-15,15'-oxygenase (bcox) is essential for pattern formation and differentiation during zebrafish embryogenesis

The egg yolk of vertebrates contains carotenoids, which account for its characteristic yellow color in some species. Such plant-derived compounds, e.g. beta-carotene, serve as the natural precursors (provitamins) of vitamin A, which is indispensable for chordate development. As egg yolk also contains stored vitamin A, carotenoids have so far been solely discussed as pigments for the coloration of the offspring. Based on our recent molecular identification of the enzyme catalyzing provitamin A conversion to vitamin A, we address a possible role of provitamin A during zebrafish (Danio rerio) development. We cloned the zebrafish gene encoding the vitamin A-forming enzyme, a beta,beta-carotene-15,15'-oxygenase. Analysis of its mRNA expression revealed that it is under complex spatial and temporal control during development. Targeted gene knockdown using the morpholino antisense oligonucleotide technique indicated a vital role of the provitamin A-converting enzyme. Morpholino-injected embryos developed a morphological phenotype that included severe malformation of the eyes, the craniofacial skeleton and pectoral fins, as well as reduced pigmentation. Analyses of gene expression changes in the morphants revealed that distinct retinoic acid-dependent developmental processes are impaired, such as patterning of the hindbrain and differentiation of hindbrain neurons, differentiation of neural crest derivatives (including the craniofacial skeleton), and the establishment of the ventral retina. Our data provide strong evidence that, for several developmental processes, retinoic acid generation depends on local de novo formation of retinal from provitamin A via the carotene oxygenase, revealing an unexpected, essential role for carotenoids in embryonic development.

Storage of retinal in the eggs of the ascidian, Halocynthia roretzi

Retinoids in the eggs of the solitary ascidian, Halocynthia roretzi, were analyzed by high performance liquid chromatography. Retinal was the almost exclusive retinoid (>>99%), and the concentration of retinal was 25.9-40.1 (30.6 on average) ng/mg of protein. The egg retinal consisted of four isomers: all-trans (50.9%), 9-cis (6.8%), 11-cis (20.4%) and 13-cis (21.9%). The presence of retinal in the eggs of this ascidian is a characteristic shared with the wide range of oviparous vertebrates, although the isomer composition differs between ascidian eggs and vertebrate eggs; in vertebrate eggs, almost all the retinal is in the all-trans form. The egg retinal was bound to a protein complex via a Schiff base linkage. The electrophoretic characteristics of the protein complex were similar to that of egg yolk proteins of oviparous vertebrates. The results presented in this study strongly suggest that, as is found with oviparous vertebrates, retinal in the ascidian eggs is the essential mode of retinoid storage, and is the precursor of photoreceptive pigment chromophores and retinoic acid during development.

The state of the art of the zebrafish model for toxicology and toxicologic pathology research--advantages and current limitations

The zebrafish (Danio rerio) is now the pre-eminent vertebrate model system for clarification of the roles of specific genes and signaling pathways in development. The zebrafish genome will be completely sequenced within the next 1-2 years. Together with the substantial historical database regarding basic developmental biology, toxicology, and gene transfer, the rich foundation of molecular genetic and genomic data makes zebrafish a powerful model system for clarifying mechanisms in toxicity. In contrast to the highly advanced knowledge base on molecular developmental genetics in zebrafish, our database regarding infectious and noninfectious diseases and pathologic lesions in zebrafish lags far behind the information available on most other domestic mammalian and avian species, particularly rodents. Currently, minimal data are available regarding spontaneous neoplasm rates or spontaneous aging lesions in any of the commonly used wild-type or mutant lines of zebrafish. Therefore, to fully utilize the potential of zebrafish as an animal model for understanding human development, disease, and toxicology we must greatly advance our knowledge on zebrafish diseases and pathology.