Expression of zebrafish cyp11a1 as a maternal transcript and in yolk syncytial layer

Gonadal Development and Differentiation of Hybrid F1 Line of Ctenopharyngodon idella (♀) × Squaliobarbus curriculus (♂)

The hybrid F1 offspring of Ctenopharyngodon idella (♂) and Squaliobarbus curriculus (♀) exhibit heterosis in disease resistance and also show abnormal sex differentiation. To understand the mechanism behind gonadal differentiation in the hybrid F1, we analyzed the transcriptomes of C. idella, S. curriculus, and the hybrid F1; screened for genes related to gonad development in these samples; and measured their expression levels. Our results revealed that compared to either C. idella or S. curriculus, the gene expressions in most sub-pathways of the SNARE interactions in the vesicular transport pathway in the hypothalamus, pituitary, and gonadal tissues of their hybrid F1 offspring were significantly up-regulated. Furthermore, insufficient transcription of genes involved in oocyte meiosis may be the main reason for the insufficient reproductive ability of the hybrid F1 offspring. Through transcriptome screening, we identified key molecules involved in gonad development, including HSD3B7, HSD17B1, HSD17B3, HSD20B2, CYP17A2, CYP1B1, CYP2AA12, UGT2A1, UGT1A1, and FSHR, which showed significant differences in expression levels in the hypothalamus, pituitary, and gonads of these fish. Notably, the expression levels of UGT1A1 in the gonads of the hybrid F1 were significantly higher than those in C. idella and S. curriculus. These results provide a scientific basis for further research on the gonadal differentiation mechanism of hybrid F1 offspring.

The chromatin accessibility dynamics during cell fate specifications in zebrafish early embryogenesis

Chromatin accessibility plays a critical role in the regulation of cell fate decisions. Although gene expression changes have been extensively profiled at the single-cell level during early embryogenesis, the dynamics of chromatin accessibility at cis-regulatory elements remain poorly studied. Here, we used a plate-based single-cell ATAC-seq method to profile the chromatin accessibility dynamics of over 10 000 nuclei from zebrafish embryos. We investigated several important time points immediately after zygotic genome activation (ZGA), covering key developmental stages up to dome. The results revealed key chromatin signatures in the first cell fate specifications when cells start to differentiate into enveloping layer (EVL) and yolk syncytial layer (YSL) cells. Finally, we uncovered many potential cell-type specific enhancers and transcription factor motifs that are important for the cell fate specifications.

Direct male development in chromosomally ZZ zebrafish

The genetics of sex determination varies across taxa, sometimes even within a species. Major domesticated strains of zebrafish (Danio rerio), including AB and TU, lack a strong genetic sex determining locus, but strains more recently derived from nature, like Nadia (NA), possess a ZZ male/ZW female chromosomal sex-determination system. AB fish pass through a juvenile ovary stage, forming oocytes that survive in fish that become females but die in fish that become males. To understand mechanisms of gonad development in NA zebrafish, we studied histology and single cell transcriptomics in developing ZZ and ZW fish. ZW fish developed oocytes by 22 days post-fertilization (dpf) but ZZ fish directly formed testes, avoiding a juvenile ovary phase. Gonads of some ZW and WW fish, however, developed oocytes that died as the gonad became a testis, mimicking AB fish, suggesting that the gynogenetically derived AB strain is chromosomally WW. Single-cell RNA-seq of 19dpf gonads showed similar cell types in ZZ and ZW fish, including germ cells, precursors of gonadal support cells, steroidogenic cells, interstitial/stromal cells, and immune cells, consistent with a bipotential juvenile gonad. In contrast, scRNA-seq of 30dpf gonads revealed that cells in ZZ gonads had transcriptomes characteristic of testicular Sertoli, Leydig, and germ cells while ZW gonads had granulosa cells, theca cells, and developing oocytes. Hematopoietic and vascular cells were similar in both sex genotypes. These results show that juvenile NA zebrafish initially develop a bipotential gonad; that a factor on the NA W chromosome, or fewer than two Z chromosomes, is essential to initiate oocyte development; and without the W factor, or with two Z doses, NA gonads develop directly into testes without passing through the juvenile ovary stage. Sex determination in AB and TU strains mimics NA ZW and WW zebrafish, suggesting loss of the Z chromosome during domestication. Genetic analysis of the NA strain will facilitate our understanding of the evolution of sex determination mechanisms.

Direct Male Development in Chromosomally ZZ Zebrafish

The genetics of sex determination varies across taxa, sometimes even within a species. Major domesticated strains of zebrafish ( Danio rerio ), including AB and TU, lack a strong genetic sex determining locus, but strains more recently derived from nature, like Nadia (NA), possess a ZZ male/ZW female chromosomal sex-determination system. AB strain fish pass through a juvenile ovary stage, forming oocytes that survive in fish that become females but die in fish that become males. To understand mechanisms of gonad development in NA zebrafish, we studied histology and single cell transcriptomics in developing ZZ and ZW fish. ZW fish developed oocytes by 22 days post-fertilization (dpf) but ZZ fish directly formed testes, avoiding a juvenile ovary phase. Gonads of some ZW and WW fish, however, developed oocytes that died as the gonad became a testis, mimicking AB fish, suggesting that the gynogenetically derived AB strain is chromosomally WW. Single-cell RNA-seq of 19dpf gonads showed similar cell types in ZZ and ZW fish, including germ cells, precursors of gonadal support cells, steroidogenic cells, interstitial/stromal cells, and immune cells, consistent with a bipotential juvenile gonad. In contrast, scRNA-seq of 30dpf gonads revealed that cells in ZZ gonads had transcriptomes characteristic of testicular Sertoli, Leydig, and germ cells while ZW gonads had granulosa cells, theca cells, and developing oocytes. Hematopoietic and vascular cells were similar in both sex genotypes. These results show that juvenile NA zebrafish initially develop a bipotential gonad; that a factor on the NA W chromosome or fewer than two Z chromosomes is essential to initiate oocyte development; and without the W factor or with two Z doses, NA gonads develop directly into testes without passing through the juvenile ovary stage. Sex determination in AB and TU strains mimics NA ZW and WW zebrafish, suggesting loss of the Z chromosome during domestication. Genetic analysis of the NA strain will facilitate our understanding of the evolution of sex determination mechanisms.

The A'-helix of CYP11A1 remodels mitochondrial cristae

Background

CYP11A1 is a protein located in the inner membrane of mitochondria catalyzing the first step of steroid synthesis. As a marker gene for steroid-producing cells, the abundance of CYP11A1 characterizes the extent of steroidogenic cell differentiation. Besides, the mitochondria of fully differentiated steroidogenic cells are specialized with tubulovesicular cristae. The participation of CYP11A1 in the change of mitochondrial structure and the differentiation of steroid-producing cells, however, has not been investigated.

Methods

We engineered nonsteroidogenic monkey kidney COS1 cells to express CYP11A1 upon doxycycline induction and examined the mitochondrial structure of these cells. We also mapped the CYP11A1 domains that confer structural changes of mitochondria. We searched for CYP11A1-interacting proteins and investigated the role of this interacting protein in shaping mitochondrial structure. Finally, we examined the effect of CYP11A1 overexpression on the amount of mitochondrial contact site and cristae organizing system.

Results

We found that CYP11A1 overexpression led to the formation of tubulovesicular cristae in mitochondria. We also identified the A’-helix located at amino acid #57–68 to be sufficient for membrane insertion and crista remodeling. We identified heat shock protein 60 (Hsp60) as the CYP11A1-interacting protein and showed that Hsp60 is required for CYP11A1 accumulation and crista remodeling. Finally, we found that the small MIC10 subcomplex of the mitochondrial contact site and cristae organizing system was reduced when CYP11A1 was overexpressed.

Conclusions

CYP11A1 participates in the formation of tubulovesicular cristae in the mitochondria of steroidogenic cells. Its A’-helix is sufficient for the formation of tubulovesicular cristae and for protein integration into the membrane. CYP11A1 interacts with Hsp60, which is required for CYP11A1 accumulation. The accumulation of CYP11A1 leads to the reduction of MIC10 complex and changes mitochondrial structure.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12929-022-00846-7.

Cyp11a2 Is Essential for Oocyte Development and Spermatogonial Stem Cell Differentiation in Zebrafish

Cytochrome P45011A1, encoded by Cyp11a1, converts cholesterol to pregnenolone (P5), the first and rate-limiting step in steroidogenesis. In zebrafish, cyp11a1 is maternally expressed and cyp11a2 is considered the ortholog of Cyp11a1 in mammals. A recent study has shown that depletion of cyp11a2 resulted in steroidogenic deficiencies and the mutants developed into males with feminized secondary sexual characteristics. Here, we independently generated cyp11a2 mutants in zebrafish and showed that the mutants can develop into males and females in the juvenile stage, but finally into infertile males with defective mating behavior in the adult stage. In the developing ovaries, the cyp11a2 mutation led to stage I oocyte apoptosis and final sex reversal, which could be partially rescued by treatment with P5 but not estradiol. In the developing testes, depletion of cyp11a2 resulted in dysfunction of Sertoli cells and lack of functional Leydig cells. Spermatogonial stem cells (SSCs) in the mutant testes underwent active self-renewal but no differentiation, resulting in a high abundance of SSCs in the testis, as revealed by immunofluorescence staining with Nanos2 antibody. The high abundance and differentiation competence of SSCs in the mutant testes were verified by a novel testicular cell transplantation method developed in this study, by transplanting mutant testicular cells into germline-depleted wild-type (WT) fish. The transplanted mutant SSCs efficiently differentiated into functional spermatids in WT hosts. Overall, our study demonstrates the functional importance of cyp11a2 in early oogenesis and differentiation of SSCs.

Interrenal development and function in zebrafish

In this article we aim to provide an overview of the zebrafish interrenal development and function, as well as a review of its contribution to basic and translational research. A search of the PubMed database identified 41 relevant papers published over the last 20 years. Based on the common themes identified, we discuss the organogenesis of the interrenal gland and its functional development and we review what is known about the genes involved in zebrafish steroidogenesis. We also outline the consequences of specific defects in steroid biosynthesis, as revealed by evidence from genetically engineered zebrafish models, including cyp11a2, cyp21a2, hsd3b1, cyp11c1 and fdx1b deficiency. Finally, we summarise the impact of different chemicals and environmental factors on steroidogenesis. Our review highlights the utility of zebrafish as a research model for exploring important areas of basic science and human disease, especially in the current context of rapid technological progress in the field of Molecular Biology.

Characterization of CYP11A1 and its potential role in sex asynchronous gonadal development of viviparous black rockfish Sebastes schlegelii (Sebastidae)

Mitochondrial cytochrome P450 side-chain cleavage (P450scc), encoded by the cyp11a1 gene, initiates the first step of steroid biosynthesis. In this study, a 1554-bp open reading frame (ORF) of black rockfish (Sebastes schlegelii) cyp11a1 was cloned. The cyp11a1 gene is located on chromosome 5 and has 9 exons. The ORF encodes a putative precursor protein of 517 amino acids, and the predicted cleavable mitochondrial targeting peptide is located at amino acids 1-39. P450scc shares homology with other teleosts and tetrapods, which have relatively conserved binding regions with heme, cholesterol and adrenodoxin. Tissue distribution analysis revealed that the highest expression levels of cyp11a1 were detected in mature gonads and head kidney but that low levels were detected in gestational/regressed ovaries, regressed testes and other tissues. Immunostaining of P450scc was observed in testicular Leydig cells, ovarian theca cells, interrenal glands of head kidney, pituitary and multiple regions of brain. Particularly, two kinds of fish-specific P450scc-positive cells, including coronet cells of brain saccus vasculosus and hypophyseal somatolactin cells, were identified in black rockfish. Our results provide novel evidence for the potential role played by P450scc in reproduction behavior by mediating steroidogenesis in viviparous teleost.

The effects of norethindrone on the ontogeny of gene expression along the hypothalamic-pituitary-adrenal and hypothalamic-pituitary-gonadal axes in zebrafish (Danio rerio)

Little is known about the molecular effects of progestins on the hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes in fish prior to sexual differentiation. In this study, the effects of norethindrone (NET) on the ontogeny of HPG- and HPA-related genes in zebrafish embryo/early larvae prior to sexual differentiation were evaluated. Embryo/larvae were exposed to different concentrations (5, 50, 500 ng/L) of NET for 6 days. The levels of the transcripts of the genes closely related to the HPG and HPA axes were determined daily during 3 stages (embryo, embryo/larvae transition, and early larvae). The results showed that most genes were up-regulated and the ontogeny of genes in the HPA axis was earlier than that of HPG axis, especially for the upstream genes of both the HPG (gnrh2, gnrh3, fshb, lhb) and the HPA (crh, pomc, star) axes. In contrast, the transcriptional expressions of genes of the cortisol/stress pathway (cyp11b, mr) were inhibited and those of the progesterone pathway were not affected. More importantly, NET exposure induced the expressions of the genes (esr1, vtg1, hsd17b3, hsd11b2, ar) that are closely related to the steroid hormone pathways in the embryos/larvae stages, implying a precocious effects of NET in zebrafish. This study demonstrates that NET alters the expression of HPA- and HPG-axes related genes in zebrafish at early stages, pointing to the need for the same type of analysis during the zebrafish gonadal differentiation window.

Steroidogenesis and its regulation in teleost-a review

Steroid hormones modulate several important biological processes like metabolism, stress response, and reproduction. Steroidogenesis drives reproductive function wherein development and differentiation of undifferentiated gonads into testis or ovary, and their growth and maturation, are regulated. Steroidogenesis occurs in gonadal and non-gonadal tissues like head kidney, liver, intestine, and adipose tissue in teleosts. This process is regulated differently through multi-level modulation of promoter motif transcription factor regulation of steroidogenic enzyme genes to ultimately control enzyme activity and turnover. In view of this, understanding teleostean steroidogenesis provides major inputs for technological innovation of pisciculture. Unlike higher vertebrates, steroidal intermediates and shift in steroidogenesis is critical for gamete maturation in teleosts, more essentially oogenesis. Considering these characteristics, this review highlights the promoter regulation of steroidogenic enzyme genes by several transcription factors that are involved in teleostean steroidogenesis. It also addresses different methodologies involved in promoter regulation studies together with glucocorticoids and androgen relationship with reference to teleosts.

Roles of two cyp11 genes in sex hormone biosynthesis in Japanese flounder (Paralichthys olivaceus)

The P450 side-chain cleavage enzymes P450scc (Cyp11a) and 11β-hydroxylase (Cyp11b) play important roles in sex steroid and cortisol production. Here, two duplicates of cyp11 genes were identified in Japanese flounder (Paralichthys olivaceus): Pocyp11a and Pocyp11b, respectively. Phylogenetic analysis and amino acid sequence alignment revealed that Pocyp11a and Pocyp11b shared significant identity with sequences of other teleost fish species. The quantitative real-time polymerase chain reaction (qRT-PCR) results indicated that among the studied tissues, brain tissue showed the highest expression of Pocyp11a, followed by kidney and testis tissues, whereas Pocyp11b expression was highest in the testis. The expression patterns of these two genes showed sexual dimorphism, with both genes showing higher expression in the testis than in the ovary. In-situ hybridization analysis demonstrated that Pocyp11a and Pocyp11b mRNA were both detected in oocytes, spermatocytes, and Sertoli cells, indicating that they might be involved in hormone synthesis. The expression levels of Pocyp11a and Pocyp11b were significantly downregulated by treatment with 17α-methyltestosterone (17α-MT) in the testis and ovary in both in vivo and studies. In vivo studies showed that Pocyp11a and Pocyp11b transcripts were suppressed by 17β-estradiol (E₂ ) treatment in both the testis and ovary. In addition, in vitro studies showed that the expression level of Pocyp11b was decreased by treatment with E₂ , whereas that of Pocyp11a was largely unaffected. Moreover, the expression levels of Pocyp11a and Pocyp11b in the testis cell line were significantly upregulated after NR0b1 and NR5a2 (p < .05) treatment. These results indicate that Pocyp11a and Pocyp11b might play important roles in sex hormone biosynthesis. Our research can assist future studies of the mechanisms of steroid biosynthesis and functional differences between cyp11a and cyp11b in Japanese flounder.

Coupling Genome-wide Transcriptomics and Developmental Toxicity Profiles in Zebrafish to Characterize Polycyclic Aromatic Hydrocarbon (PAH) Hazard

Polycyclic Aromatic Hydrocarbons (PAHs) are diverse environmental pollutants associated with adverse human health effects. Many studies focus on the carcinogenic effects of a limited number of PAHs and there is an increasing need to understand mechanisms of developmental toxicity of more varied yet environmentally relevant PAHs. A previous study characterized the developmental toxicity of 123 PAHs in zebrafish. Based on phenotypic responses ranging from complete inactivity to acute mortality, we classified these PAHs into eight bins, selected 16 representative PAHs, and exposed developing zebrafish to the concentration of each PAH that induced 80% phenotypic effect. We conducted RNA sequencing at 48 h post fertilization to identify gene expression changes as a result of PAH exposure. Using the Context Likelihood of Relatedness algorithm, we inferred a network that links the PAHs based on coordinated gene responses to PAH exposure. The 16 PAHs formed two broad clusters: Cluster A was transcriptionally more similar to the controls, while Cluster B consisted of PAHs that were generally more developmentally toxic, significantly elevated cyp1a transcript levels, and induced Ahr2-dependent Cyp1a protein expression in the skin confirmed by gene-silencing studies. We found that cyp1a transcript levels were associated with transcriptomic response, but not with PAH developmental toxicity. While all cluster B PAHs predominantly activated Ahr2, they also each enriched unique pathways like ion transport signaling, which likely points to differing molecular events between the PAHs downstream of Ahr2. Thus, using a systems biology approach, we have begun to evaluate, classify, and define mechanisms of PAH toxicity.

Characteristics of Cyp11a during Gonad Differentiation of the Olive Flounder Paralichthys olivaceus

The P450 side-chain cleavage enzyme, P450scc (Cyp11a) catalyzes the first enzymatic step for the synthesis of all steroid hormones in fish. To study its roles in gonads of the olive flounder Paralichthys olivaceus, an important maricultured fish species, we isolated the cyp11a genomic DNA sequence of 1396 bp, which consists of 5 exons and 4 introns. Semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) results indicated that the flounder cyp11a was exclusively expressed in gonad and head kidney tissues. Its expression level in the testis was higher than that in the ovary. According to the in situ hybridization patterns, cyp11a was mainly expressed in the Leydig cells of the testis, and the thecal cells of the ovary. Immunofluorescence analysis showed that Cyp11a was located in the cytoplasm of the cultured flounder testis cells. Further quantitative real-time PCR results presented the cyp11a differential expression patterns during gonad differentiation. Among different sampling points of the 17β-estradiol (E2, 5 ppm) treatment group, cyp11a expression levels were relatively high in the differentiating ovary (30 and 40 mm total length, TL), and then significantly decreased in the differentiated ovary (80, 100 and 120 mm TL, p < 0.05). The pregnenolone level also dropped in the differentiated ovary. In the high temperature treatment group (HT group, 28 ± 0.5 °C), the cyp11a expression level fluctuated remarkably in the differentiating testis (60 mm TL), and then decreased in the differentiated testis (80, 100 mm TL, p < 0.05). In the testosterone (T, 5 ppm) treatment group, the cyp11a was expressed highly in undifferentiated gonads and the differentiating testis, and then dropped in the differentiated testis. Moreover, the levels of cholesterol and pregnenolone of the differentiating testis in the HT and T groups increased. The expression level of cyp11a was significantly down-regulated after the cultured flounder testis cells were treated with 75 and 150 μM cyclic adenosine monophosphate (cAMP), respectively (p < 0.05), and significantly up-regulated after treatment with 300 μM cAMP (p < 0.05). Both nuclear receptors NR5a2 and NR0b1 could significantly up-regulate the cyp11a gene expression in a dosage dependent way in the testis cells detected by cell transfection analysis (p < 0.05). The above data provides evidence that cyp11a would be involved in the flounder gonad differentiation and development.

Mechanistic Investigations Into the Developmental Toxicity of Nitrated and Heterocyclic PAHs

Nitrated polycyclic aromatic hydrocarbons (NPAHs) and heterocyclic PAHs (HPAHs) are recognized environmental pollutants. However, the health risks of NPAHs and HPAHs to humans and environmental systems are not well-studied. The developmental zebrafish (Danio rerio) model was used to evaluate the toxicity of a structurally diverse set of 27 NPAHs and 10 HPAHs. The individual activity of each compound towards the aryl hydrocarbon receptor (AHR), including the role of the AHR in observed toxicity, and genetic markers of oxidative stress and cardiac toxicity were evaluated. Zebrafish embryos were exposed from 6 to 120 hours post fertilization (hpf), to a broad concentration range of individual compounds, and evaluated for 22 developmental endpoints. The potential role of AHR was determined using the transgenic Tg(cyp1a:nls-egfp) reporter zebrafish line. All compounds were screened computationally through molecular docking using a previously developed AHR models of zebrafish isoforms 1A, 1B, and 2. Some compounds did not induce observable developmental toxic responses, whereas others produced statistically significant concentration-dependent toxicity. The tested compounds also exhibited a range of predicted AHR binding and cyp1a/GFP induction patterns, including cyp1a expression in the liver, vasculature, skin, and yolk, which we determined to be due to distinct isoforms of the AHR, using morpholino oligonucleotide knockdown. Furthermore, we investigated mRNA expression of oxidative and cardiac stress genes at 48 and 120 hpf, which indicated several potential mechanisms-of-action for NPAHs. Overall, we observed a range of developmental toxicities, cyp1a/GFP expression patterns, and gene expression profiles, suggestive of several potential mechanisms of action.

Expression and activity profiling of the steroidogenic enzymes of glucocorticoid biosynthesis and the fdx1 co-factors in zebrafish

The spatial and temporal expression of steroidogenic genes in zebrafish has not been fully characterised. Because zebrafish are increasingly employed in endocrine and stress research, a better characterisation of steroidogenic pathways is required to target specific steps in the biosynthetic pathways. In the present study, we have systematically defined the temporal and spatial expression of steroidogenic enzymes involved in glucocorticoid biosynthesis (cyp21a2, cyp11c1, cyp11a1, cyp11a2, cyp17a1, cyp17a2, hsd3b1, hsd3b2), as well as the mitochondrial electron-providing ferredoxin co-factors (fdx1, fdx1b), during zebrafish development. Our studies showed an early expression of all these genes during embryogenesis. In larvae, expression of cyp11a2, cyp11c1, cyp17a2, cyp21a2, hsd3b1 and fdx1b can be detected in the interrenal gland, which is the zebrafish counterpart of the mammalian adrenal gland, whereas the fdx1 transcript is mainly found in the digestive system. Gene expression studies using quantitative reverse transcriptase-PCR and whole-mount in situ hybridisation in the adult zebrafish brain revealed a wide expression of these genes throughout the encephalon, including neurogenic regions. Using ultra-high-performance liquid chromatography tandem mass spectrometry, we were able to demonstrate the presence of the glucocorticoid cortisol in the adult zebrafish brain. Moreover, we demonstrate de novo biosynthesis of cortisol and the neurosteroid tetrahydrodeoxycorticosterone in the adult zebrafish brain from radiolabelled pregnenolone. Taken together, the present study comprises a comprehensive characterisation of the steroidogenic genes and the fdx co-factors facilitating glucocorticoid biosynthesis in zebrafish. Furthermore, we provide additional evidence of de novo neurosteroid biosynthesising in the brain of adult zebrafish facilitated by enzymes involved in glucocorticoid biosynthesis. Our study provides a valuable source for establishing the zebrafish as a translational model with respect to understanding the roles of the genes for glucocorticoid biosynthesis and fdx co-factors during embryonic development and stress, as well as in brain homeostasis and function.

Comparative developmental toxicity of a comprehensive suite of polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are widespread environmental contaminants that occur in complex mixtures. Several PAHs are known or suspected mutagens and/or carcinogens, but developmental toxicity data is lacking for PAHs, particularly their oxygenated and nitrated derivatives. Such data are necessary to understand and predict the toxicity of environmental mixtures. 123 PAHs were assessed for morphological and neurobehavioral effects for a range of concentrations between 0.1 and 50 µM, using a high throughput early-life stage zebrafish assay, including 33 parent, 22 nitrated, 17 oxygenated, 19 hydroxylated, 14 methylated, 16 heterocyclic, and 2 aminated PAHs. Additionally, each PAH was evaluated for AHR activation, by assessing CYP1A protein expression using whole animal immunohistochemistry (IHC). Responses to PAHs varied in a structurally dependent manner. High-molecular weight PAHs were significantly more developmentally toxic than the low-molecular weight PAHs, and CYP1A expression was detected in five distinct tissues, including vasculature, liver, skin, neuromasts and yolk.

Function of CYP11A1 in the mitochondria

Steroids are synthesized from the adrenal glands and gonads by enzymes of the cytochromes P450 and hydroxysteroid dehydrogenase in nature. These enzymes are located in the membrane of endoplasmic reticulum and mitochondria to catalyze redox reactions using electrons transported from the membrane. In the mitochondria, steroidogenic enzymes are inserted into the inner membrane with the bulk of the protein facing the matrix. They are not only important for steroid biosynthesis, their presence also affects mitochondrial morphology. Mitochondria undergo constant fission and fusion; they play important roles in energy production, apoptosis, and metabolism. Their defects often lead to human diseases. Mitochondrial cristae are usually lamellar in shape, but can also assume different shapes. Cristae in the mitochondria of steroidogenic cells are tubular-vesicular in shape. This cristae shape is also related to the degree of steroidogenic cell differentiation. Steroidogenic enzymes in the mitochondria appear to have a dual role in shaping the morphology of mitochondria and in steroid production.

Updated perspectives on the cytosolic sulfotransferases (SULTs) and SULT-mediated sulfation

The cytosolic sulfotransferases (SULTs) are Phase II detoxifying enzymes that mediate the sulfate conjugation of numerous xenobiotic molecules. While the research on the SULTs has lagged behind the research on Phase I cytochrome P-450 enzymes and other Phase II conjugating enzymes, it has gained more momentum in recent years. This review aims to summarize information obtained in several fronts of the research on the SULTs, including the range of the SULTs in different life forms, concerted actions of the SULTs and other Phase II enzymes, insights into the structure-function relationships of the SULTs, regulation of SULT expression and activity, developmental expression of SULTs, as well as the use of a zebrafish model for studying the developmental pharmacology/toxicology.

Nongenomic actions of neurosteroid pregnenolone and its metabolites

Steroids have been widely used in the clinical setting. They bind and activate nuclear receptors to regulate gene expression. In addition to activating genomic transcription, steroids also exert nongenomic actions. The current article focuses on the nongenomic actions of neurosteroids, including pregnenolone (P5), 7α-hydroxypregnenolone, pregnenolone sulfate and allopregnanolone. Pregnenolone and its derivatives promote neuronal activity by enhancing learning and memory, relieving depression, enhancing locomotor activity, and promoting neuronal cell survival. They exert these effects by activating various target proteins located in the cytoplasm or cell membrane. Pregnenolone and its metabolites bind to receptors such as microtubule-associated proteins and neurotransmitter receptors to elicit a series of reactions including stabilization of microtubules, increase of ion flux into cells, and dopamine release. The wide actions of neurosteroids indicate that pregnenolone derivatives have great potential in future treatment of neurological diseases.

Cold-induced changes in stress hormone and steroidogenic transcript levels in cunner (Tautogolabrus adspersus), a fish capable of metabolic depression

The cunner (Tautogolabrus adspersus) is a fish with a wide latitudinal distribution that is capable of going into metabolic depression during the winter months, and thus, represents a unique model to investigate the impacts of cold temperatures on the stress response. In this study, we measured resting (pre-stress) plasma cortisol levels in 10 °C and 0 °C acclimated cunner from Newfoundland, and both catecholamine and cortisol levels after they were given a standardized handling stress (i.e. 1 min air exposure). In addition, we cloned and characterized cDNAs for several key genes of the cortisol-axis [cytochrome P450scc, steroidogenic acute regulatory protein (StAR) and a glucocorticoid receptor (GR) most likely to be an ortholog of the teleost GR2], determined the tissue distribution of their transcripts, and measured their constitutive (i.e. pre-stress) transcript levels in individuals acclimated to both temperatures. In cunner acclimated to 0 °C, post-stress epinephrine and norepinephrine levels were much lower (by approximately 9- and 5-fold, respectively) compared to 10 °C acclimated fish, and these fish had relatively low resting cortisol levels (~15 ngml(-1)) and showed a typical post-stress response. In contrast, those acclimated to 10 °C had quite high resting cortisol levels (~75 ngml(-1)) that actually decreased (to ~20 ngml(-1)) post-stress before returning to pre-stress levels. Finally, fish acclimated to 10 °C had higher P450scc transcript levels in the head kidney and lower levels of GR transcript in both the head kidney and liver. Taken together, these results suggest that: (1) temperature has a profound effect on the stress response of this species; and (2) although the ancestors of this species inhabited warm waters (i.e. they are members of the family Labridae), populations of cunner from colder regions may show signs of stress at temperatures as low as 10 °C.

Steroids in teleost fishes: A functional point of view

Steroid hormones are involved in the regulation of a variety of processes like embryonic development, sex differentiation, metabolism, immune responses, circadian rhythms, stress response, and reproduction in vertebrates. Teleost fishes and humans show a remarkable conservation in many developmental and physiological aspects, including the endocrine system in general and the steroid hormone related processes in particular. This review provides an overview of the current knowledge about steroid hormone biosynthesis and the steroid hormone receptors in teleost fishes and compares the findings to the human system. The impact of the duplicated genome in teleost fishes on steroid hormone biosynthesis and perception is addressed. Additionally, important processes in fish physiology regulated by steroid hormones, which are most dissimilar to humans, are described. We also give a short overview on the influence of anthropogenic endocrine disrupting compounds on steroid hormone signaling and the resulting adverse physiological effects for teleost fishes. By this approach, we show that the steroidogenesis, hormone receptors, and function of the steroid hormones are reasonably well understood when summarizing the available data of all teleost species analyzed to date. However, on the level of a single species or a certain fish-specific aspect of physiology, further research is needed.

Chemoprevention of Diethylnitrosamine-Initiated and Phenobarbital-Promoted Hepatocarcinogenesis in Rats by Sulfated Polysaccharides and Aqueous Extract of Ulva lactuca

Hepatocellular carcinoma (HCC) is one of the common cancers and lethal diseases worldwide. Both oxidative stress and chronic inflammation contribute to the pathogenesis of HCC. Because of limited treatment options and a grave prognosis of HCC, preventive management has been emphasized. The marine macroalgae Ulva lactuca (Ulvaceae) is consumed by humans and livestock because of its nutritional value. Recent studies showed that various extracts of U. lactuca possess antiviral, antiplasmodial, antinephrotoxic, antioxidant, and anti-inflammatory properties. However, very limited information is available on anticancer potential of U. lactuca with no reports on liver cancer chemopreventive efficacy of this marine algae. Accordingly, the present study was initiated to evaluate the possible antihepatocarcinogenic effects and antioxidant mechanisms of action of various U. lactuca extracts against a clinically relevant rodent model of HCC. Initiation of hepatocarcinogenesis was performed in Sprague-Dawley rats by a single injection of dietary carcinogen diethylnitrosamine (DENA, 200 mg/kg, intraperitoneally), followed by promotion with phenobarbital (0.05%) in drinking water. The rats were fed with daily oral dose (50 mg/kg) of polysaccharide sulfate or aqueous extract of U. lactuca for 2, 12, and 24 weeks. At these timepoints, blood samples were taken to measure hepatic injury markers, including alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, γ-glutamyl transferase, and bilirubin. The liver tissue was harvested for measurement of hepatic oxidative indices, including lipid peroxidation, reduced glutathione, nitric oxide, catalase, superoxide dismutase, glutathione reductase, and glutathione S-transferase. Hepatic histopathology, immunohistochemical analysis of cell proliferation and apoptosis by DNA fragmentation assay were performed. Our results clearly indicate that sulfated polysaccharides of U. lactuca exert a marked chemoprevention of DENA-initiated hepatocarcinogenesis through inhibition of abnormal cell proliferation and induction of apoptosis. A modest inhibition rat liver carcinogenesis was observed with the aqueous extract. The sulfated polysaccharides altered serum parameters of hepatic damage and modulated various components of the hepatic enzymatic and nonenzymatic antioxidant defense systems. The sulfated polysaccharides from U. lactuca may have unique properties of providing protection against DENA-induced oxidative stress which could contribute to chemoprevention of experimental hepatocarcinogenesis. U. lactuca sulfated polysaccharides could be developed as chemopreventive and therapeutic drug against human HCC.

A time-course transcriptional kinetics of the hypothalamic-pituitary-gonadal and hypothalamic-pituitary-adrenal axes in zebrafish eleutheroembryos after exposure to norgestrel

The objective of the present study was to investigate the effects of norgestrel on the hypothalamic-pituitary-gonadal (HPG) and hypothalamic-pituitary-adrenal (HPA) axes in zebrafish eleutheroembryos. Zebrafish embryos were exposed to different concentrations of norgestrel (0 ng L(-1) , 5 ng L(-1) , 50 ng L(-1) , and 100 ng L(-1) ) for 144 h post fertilization (hpf), and the transcriptional profiles of the HPG and HPA axes were examined every day. Norgestrel modulated the expression of Pgr and Vtg1 messenger (m)RNAs mainly at 96 hpf for all treatment groups. In addition, norgestrel strongly altered the expression of Cyp11a1 mRNA above 5 ng L(-1) (significant upregulation from 48 hpf to 120 hpf and significant downregulation for 144 hpf). Norgestrel treatment could significantly induce expression of Cyp19a1a, Cyp11b, Gnrh2, Gnrh3, and Lhb mRNAs but inhibit transcripts of Hsd11b2 and Crh genes above 5 ng L(-1) at different time points. The transcriptional expression levels of Esr1, Ar, Star, Hsd17b3, Fshb, and Pomc were also mediated by 5 ng L(-1) norgestrel or higher during different exposure periods. Taken together, the overall results imply that the transcriptional changes in zebrafish eleutheroembryos may pose a potential effect on embryonic development, in particular in the brain and gonadogenesis.

Dynamic regulation of the microtubule and actin cytoskeleton in zebrafish epiboly

Gastrulation is a key developmental stage with striking changes in morphology. Coordinated cell movements occur to bring cells to their correct positions in a timely manner. Cell movements and morphological changes are accomplished by precisely controlling dynamic changes in cytoskeletal proteins, microtubules, and actin filaments. Among those cellular movements, epiboly produces the first distinct morphological changes in teleosts. In this review, I describe epiboly and its mechanics, and the dynamic changes in microtubule networks and actin structures, mainly in zebrafish embryos. The factors regulating those cytoskeletal changes will also be discussed.

Genomic and functional analysis of the toxic effect of tachyplesin I on the embryonic development of zebrafish

Tachyplesin I (TP I) is an antimicrobial peptide isolated from the hemocytes of the horseshoe crab. With the developments of DNA microarray technology, the genetic analysis of the toxic effect of TP I on embryo was originally considered in our recent study. Based on our microarray data of the embryonic samples of zebrafish treated with the different doses of TP I, we performed a series of statistical data analyses to explore the toxic effect of TP I at the genomic level. In this paper, we first employed the hexaMplot to illustrate the continuous variation of the gene expressions of the embryonic cells treated with the different doses of TP I. The probabilistic model-based Hough transform was used to classify these differentially coexpressed genes of TP I on the zebrafish embryos. As a result, three line rays supported with the corresponding 174 genes were detected in our analysis. Some biological processes of the featured genes, such as antigen processing, nuclear chromatin, and structural constituent of eye lens, were significantly filtered with the smaller P values.

Zebrafish and steroids: what do we know and what do we need to know?

Zebrafish, Danio rerio, has long been used as a model organism in developmental biology. Nowadays, due to their advantages compared to other model animals, the fish gain popularity and are also increasingly used in endocrinology. This review focuses on an important aspect of endocrinology in zebrafish by summarizing the progress in steroid hormone related research. We present the state of the art of research on steroidogenesis, the action of steroid hormones, and steroid catabolism and cover the incremental usage of zebrafish as a test animal in endocrine disruption research. By this approach, we demonstrate that some aspects of steroid hormone research are well characterized (e.g., expression patterns of the genes involved), while other aspects such as functional analyses of enzymes, steroid hormone elimination, or the impact of steroid hormones on embryonic development or sex differentiation have not been extensively studied and are poorly understood. This article is part of a Special Issue entitled 'CSR 2013'.

Redefining the initiation and maintenance of zebrafish interrenal steroidogenesis by characterizing the key enzyme cyp11a2

Zebrafish are emerging as a model to study steroid hormone action and associated disease. However, steroidogenesis in zebrafish is not well characterized. Mammalian P450 side-chain cleavage enzyme (CYP11A1) catalyzes the first step of steroidogenesis, the conversion of cholesterol to pregnenolone. Previous studies describe an essential role for zebrafish Cyp11a1 during early development. Cyp11a1 has been suggested to be the functional equivalent of mammalian CYP11A1 in the zebrafish interrenal gland (equivalent to the mammalian adrenal), gonad, and brain. However, reported cyp11a1 expression is inconsistent in zebrafish larvae, after active cortisol synthesis commences. Recently a duplicated cyp11a gene, cyp11a2, has been described, which shares an 85% identity with cyp11a1. We aimed to elucidate the specific role of the two cyp11a paralogs. cyp11a1 was expressed from 0 to 48 hours post-fertilization (hpf), whereas cyp11a2 expression started after the development of the interrenal primordium (32 hpf) and was the only paralog in larvae. cyp11a2 is expressed in adult steroidogenic tissues, such as the interrenal, gonads, and brain. In contrast, cyp11a1 was mainly restricted to the gonads. Antisense morpholino knockdown studies confirmed abnormal gastrulation in cyp11a1 morphants. cyp11a2 morphants showed impaired steroidogenesis and a phenotype indicative of metabolic abnormalities. The phenotype was rescued by pregnenolone replacement in cyp11a2 morphants. Thus, we conclude that cyp11a1 is required for early development, whereas cyp11a2 is essential for the initiation and maintenance of zebrafish interrenal steroidogenesis. Importantly, this study highlights the need for a comprehensive characterization of steroidogenesis in zebrafish prior to its implementation as a model organism in translational research of adrenal disease.

Cytochrome P450 (CYP) in fish

Cytochrome P450 (CYP) enzymes are members of the hemoprotein superfamily, and are involved in the mono-oxygenation reactions of a wide range of endogenous and exogenous compounds in mammals and plants. Characterization of CYP genes in fish has been carried out intensively over the last 20 years. In Japanese pufferfish (Takifugu rubripes), 54 genes encoding P450s have been identified. Across all species of fish, 137 genes encoding P450s have been identified. These genes are classified into 18 CYP families: namely, CYP1, CYP2, CYP3, CYP4, CYP5, CYP7, CYP8, CYP11, CYP17, CYP19, CYP20, CYP21, CYP24, CYP26, CYP27, CYP39, CYP46 and CYP51.We pinpointed eight CYP families: namely, CYP1, CYP2, CYP3, CYP4, CYP11, CYP17, CYP19 and CYP26 in this review because these CYP families are studied in detail. Studies of fish P450s have provided insights into the regulation of P450 genes by environmental stresses including water pollution. In this review, we present an overview of the CYP families in fish.

The brain of teleost fish, a source, and a target of sexual steroids

Neurosteroids are defined as steroids de novo synthesized in the central nervous system. While the production of neurosteroids is well documented in mammals and amphibians, there is less information about teleosts, the largest group of fish. Teleosts have long been known for their high brain aromatase and 5α-reductase activities, but recent data now document the capacity of the fish brain to produce a large variety of sex steroids. This article aims at reviewing the available information regarding expression and/or activity of the main steroidogenic enzymes in the brain of fish. In addition, the distribution of estrogen, androgen, and progesterone nuclear receptors is documented in relation with the potential sites of production of neurosteroids. Interestingly, radial glial cells acting as neuronal progenitors, appear to be a potential source of neurosteroids, but also a target for centrally and/or peripherally produced steroids.

Requirement of Npc1 and availability of cholesterol for early embryonic cell movements in zebrafish

Niemann-Pick disease, type C (NP-C), often associated with Niemann-Pick disease, type C1 (NPC1) mutations, is a cholesterol-storage disorder characterized by cellular lipid accumulation, neurodegeneration, and reduced steroid production. To study NPC1 function in vivo, we cloned zebrafish npc1 and analyzed its gene expression and activity by reducing Npc1 protein with morpholino (MO)-oligonucleotides. Filipin staining in npc1-morphant cells was punctate, suggesting abnormal accumulation of cholesterol. Developmentally, reducing Npc1 did not disrupt early cell fate or survival; however, early morphogenetic movements were delayed, and the actin cytoskeleton network was abnormal. MO-induced defects were rescued with ectopic expression of mouse NPC1, demonstrating functional gene conservation, and by treatments with steroids pregnenolone or dexamethasone, suggesting that reduced steroidogenesis contributed to abnormal cell movements. Cell death was found in anterior tissues of npc1 morphants at later stages, consistent with findings in mammals. Collectively, these studies show that npc1 is required early for proper cell movement and cholesterol localization and later for cell survival.

Identification and developmental expression of the full complement of Cytochrome P450 genes in Zebrafish

BACKGROUND

Increasing use of zebrafish in drug discovery and mechanistic toxicology demands knowledge of cytochrome P450 (CYP) gene regulation and function. CYP enzymes catalyze oxidative transformation leading to activation or inactivation of many endogenous and exogenous chemicals, with consequences for normal physiology and disease processes. Many CYPs potentially have roles in developmental specification, and many chemicals that cause developmental abnormalities are substrates for CYPs. Here we identify and annotate the full suite of CYP genes in zebrafish, compare these to the human CYP gene complement, and determine the expression of CYP genes during normal development.

RESULTS

Zebrafish have a total of 94 CYP genes, distributed among 18 gene families found also in mammals. There are 32 genes in CYP families 5 to 51, most of which are direct orthologs of human CYPs that are involved in endogenous functions including synthesis or inactivation of regulatory molecules. The high degree of sequence similarity suggests conservation of enzyme activities for these CYPs, confirmed in reports for some steroidogenic enzymes (e.g. CYP19, aromatase; CYP11A, P450scc; CYP17, steroid 17a-hydroxylase), and the CYP26 retinoic acid hydroxylases. Complexity is much greater in gene families 1, 2, and 3, which include CYPs prominent in metabolism of drugs and pollutants, as well as of endogenous substrates. There are orthologous relationships for some CYP1 s and some CYP3 s between zebrafish and human. In contrast, zebrafish have 47 CYP2 genes, compared to 16 in human, with only two (CYP2R1 and CYP2U1) recognized as orthologous based on sequence. Analysis of shared synteny identified CYP2 gene clusters evolutionarily related to mammalian CYP2 s, as well as unique clusters.

CONCLUSIONS

Transcript profiling by microarray and quantitative PCR revealed that the majority of zebrafish CYP genes are expressed in embryos, with waves of expression of different sets of genes over the course of development. Transcripts of some CYP occur also in oocytes. The results provide a foundation for the use of zebrafish as a model in toxicological, pharmacological and chemical disease research.

Aromatase in the brain of teleost fish: expression, regulation and putative functions

Unlike that of mammals, the brain of teleost fish exhibits an intense aromatase activity due to the strong expression of one of two aromatase genes (aromatase A or cyp19a1a and aromatase B or cyp19a1b) that arose from a gene duplication event. In situ hybridization, immunohistochemistry and expression of GFP (green fluorescent protein) in transgenic tg(cyp19a1b-GFP) fish demonstrate that aromatase B is only expressed in radial glial cells (RGC) of adult fish. These cells persist throughout life and act as progenitors in the brain of both developing and adult fish. Although aromatase B-positive radial glial cells are most abundant in the preoptic area and the hypothalamus, they are observed throughout the entire central nervous system and spinal cord. In agreement with the fact that brain aromatase activity is correlated to sex steroid levels, the high expression of cyp19a1b is due to an auto-regulatory loop through which estrogens and aromatizable androgens up-regulate aromatase expression. This mechanism involves estrogen receptor binding on an estrogen response element located on the cyp19a1b promoter. Cell specificity is achieved by a mandatory cooperation between estrogen receptors and unidentified glial factors. Given the emerging roles of estrogens in neurogenesis, the unique feature of the adult fish brain suggests that, in addition to classical functions on brain sexual differentiation and sexual behaviour, aromatase expression in radial glial cells could be part of the mechanisms authorizing the maintenance of a high proliferative activity in the brain of fish.

Molecular characterization of cyp11a1 and cyp11b1 and their gene expression profile in pejerrey (Odontesthes bonariensis) during early gonadal development

Sex steroids are known to be involved in gonadal differentiation in fish, but whether androgens are early mediators of testis differentiation remains unclear. We studied the sex-related developmental variations in the gene expression of two key enzymes involved in steroids and androgen synthesis (cyp11a1 and cyp11b1) in trunks and isolated gonads of pejerrey (Odontesthes bonariensis) larvae during and after the sex determination period. Also, and in order to have a better characterization of this process we studied the expression of Sertoli (dmrt1, amh, sox9) and Leydig (nr5a1 or sf-1) cell markers as well as a gene with higher expression in females (cyp19a1a). No clear differences were observed in the expression of cyp11a1 and cyp11b1 during the temperature-sensitive window in the trunk of pejerrey larvae. Nevertheless, a clear increase of cyp11b1 was observed in isolated gonads taken from fish reared at the male producing temperature. In these gonads we also confirmed the trends of genes with higher expression in males (dmrt1, amh) and females (cyp19a1a) as previously described in larval trunks of pejerrey. Our results showed that the expression of cyp11b1 was positively associated with the morphological differentiation of the testis. Nevertheless the involvement of 11-oxygenated androgens during the temperature-sensitive window could not be clearly established.

Zebrafish cyp11a1 and hsd3b genes: structure, expression and steroidogenic development during embryogenesis

Zebrafish has been used increasingly as a good animal model for a number of studies. To facilitate the use of this zebrafish model, the current report put emphasis on the study of two steroidogenic genes: cyp11a1 and hsd3b. These two genes encode enzymes that catalyze the first two steps of the steroidogenic pathway, and both enzymes are important for the synthesis of all steroids. Zebrafish cyp11a1 and hsd3b genes are expressed in the same cells in the gonads and interrenal gland. The interrenal gland is the counterpart of mammalian adrenal; it is located inside the head kidney and is developed parallel to the development of the pronephros. In addition, cyp11a1 and hsd3b are also expressed in the blastomeres of the early embryos before gastrulation, and in the extra-embryonic yolk syncytial layer during gastrulation. This early expression implies a novel role of steroids at gastrulation.

Translocator protein (18 kDa) is involved in primitive erythropoiesis in zebrafish

The translocator protein (18 kDa) (TSPO), also known as peripheral-type benzodiazepine receptor, is directly or indirectly associated with many biological processes. Although extensively characterized, the specific function of TSPO during development remains unclear. It has been reported that TSPO is involved in a variety of mechanisms, including cell proliferation, apoptosis, regulation of mitochondrial functions, cholesterol transport and steroidogenesis, and porphyrin transport and heme synthesis. Although the literature has reported a murine knockout model, the experiment did not generate information because of early lethality. We then used the zebrafish model to address the function of tspo during development. Information about spatiotemporal expression showed that tspo has a maternal and a zygotic contribution which, during somatogenesis, seems to be erythroid restricted to the intermediate cell mass. Genetic and pharmacological approaches used to invalidate Tspo function resulted in embryos with specific erythropoietic cell depletion. Although unexpected, this lack of blood cells is independent of the Tspo cholesterol binding site and reveals a new in vivo key role for Tspo during erythropoiesis.

Development of the corticosteroid stress axis and receptor expression in zebrafish

Using zebrafish embryos and larvae, we examined the temporal patterns of cortisol and expression of genes involved in corticosteroid synthesis and signaling. Embryonic cortisol levels decreased approximately 70% from 1.5 h postfertilization (hpf) to hatch (approximately 42 hpf) and then increased 27-fold by 146 hpf. The mRNA abundances of steroidogenic acute regulatory protein, 11beta-hydroxylase and 11beta-hydroxysteroid dehydrogenase type 2, increased severalfold after hatch and preceded the rise of cortisol levels. In contrast to other teleosts that possess two glucocorticoid receptors (GRs) and one mineralocorticoid receptor (MR), only one GR and MR were identified in zebrafish, which were cloned and sequenced. GR mRNA abundance decreased from 1.5 to 25 hpf, rebounded, and then was stable from 49 to 146 hpf. MR transcripts increased continuously from 1.5 hpf and were 52-fold higher by 97 hpf. An acute cortisol response to a stressor was not detected until 97 hpf, whereas melanocortin type 2 receptor mRNA increased between 25 and 49 hpf. Collectively, the patterns of cortisol and the expression of cortisol biosynthetic genes and melanocortin type 2 receptor suggest that the corticoid stress axis in zebrafish is fully developed only after hatch. The temporal differences in GR, MR, and 11beta-hydroxysteroid dehydrogenase type 2 gene expression lead us to propose a key role for MR signaling by maternal cortisol during embryogenesis, whereas cortisol secretion after hatch may be regulating GR expression and signaling in zebrafish.

Tissue distribution and metabolism of benzo[a]pyrene in embryonic and larval medaka (Oryzias latipes)

The need to understand chemical uptake, distribution, and metabolism in embryonic and larval fish derives from the fact that these early life stages often exhibit greater sensitivity to xenobiotic compounds than do adult animals. In this study, a 6-h acute waterborne exposure immediately after fertilization was used to quickly load the egg with benzo[a]pyrene (BaP). This exposure was used to mimic the initial egg concentration of a persistent bioaccumulative toxicant that could result from maternal transfer. We used multiphoton laser scanning microscopy (MPLSM) in combination with conventional analytical chemistry methods to characterize the tissue distribution of BaP and its principal metabolites in medaka embryos and post-hatch larvae. Embryonic metabolism of BaP was evident by MPLSM prior to liver formation or heart development. A major product of this metabolism was identified by liquid chromatography/mass spectrometry as BaP-3-glucuronide. MPLSM showed that metabolites were sequestered within the yolk, biliary system, and gastrointestinal tract. When the gastrointestinal tract became patent a few days after hatch, the metabolites were rapidly eliminated. These findings indicate that some of the earliest embryonic tissues are metabolically competent and that redistribution of BaP and its metabolic products occurs throughout development. Rapid metabolism of BaP substantially reduces the body burden of parent chemical in the developing embryo, potentially reducing toxicity. It remains unclear whether metabolism of BaP in medaka embryos leads to the formation of DNA adducts associated with genotoxic effects or yields metabolites that later lead to other toxicity in juveniles or adults.

Expression and possible functional roles of cytochromes P450 2J1 (zfCyp 2J1) in zebrafish

Cytochrome P450 2J (Cyp2J) subfamilies are recognized as catalysts of arachidonic acid metabolism in extrahepatic tissues of many species. However, to date, no P450 2J have been identified in zebrafish. We describe here a zfCyp2J1 cDNA which encodes a putative protein of 496 amino acids and shares 51%, 51%, 50%, 51% and 50% identity with mouse Cyp2J6, rabbit Cyp2J1, human Cyp2J2, cow Cyp2J2, and rat Cyp2J4, respectively. Despite detectable levels of expression by RT-PCR, no expression was shown by in situ hybridization using whole-mount tissues of the embryos. Gene-specific knockdown by antisense morpholino oligonucleotide had no phenotypic effect on embryonic development. However, over-expression of zfCyp2J1 by injection of the embryos with the cDNA resulted in substantial dose-dependent morphological defects. With adult zebrafish, whole-mount in situ hybridization showed that zfCyp2J1 was expressed predominantly in the brain and gonads. A semi-quantitative RT-PCR analysis further revealed that the zfCyp2J1 transcript was also expressed in the ovary, testis, heart, liver, and kidney. High levels of zfCyp2J1 mRNA were evident in primary growth stage (stage I) oocytes and cortical alveolus stage (stage II) oocytes but nearly undetectable in stage III and matured oocytes. These results suggest that zfCyp2J1 may not be involved in zebrafish embryogenesis but may rather play an important role in the functioning of brain and gonads of the adults. In addition, zfCyp2J1 may play a particularly crucial role in early oocyte maturation.

Zebrafish dax1 is required for development of the interrenal organ, the adrenal cortex equivalent

Mutations in the human nuclear receptor, DAX1, cause X-linked adrenal hypoplasia congenita (AHC). We report the isolation and characterization of a DAX1 homolog, dax1, in zebrafish. The dax1 cDNA encodes a protein of 264 amino acids, including the conserved carboxy-terminal ligand binding-like motif; but the amino-terminal region lacks the unusual repeats of the DNA binding-like domain in mammals. Genomic sequence analysis indicates that the dax1 gene structure is conserved also. Whole-mount in situ hybridization revealed the onset of dax1 expression in the developing hypothalamus at approximately 26 h post fertilization (hpf). Later, at about 28 hpf, a novel expression domain for dax1 appeared in the trunk. This bilateral dax1-expressing structure was located immediately above the yolk sac, between the otic vesicle and the pronephros. Interestingly, weak and transient expression of dax1 was observed in the interrenal glands (adrenal cortical equivalents) at approximately 31 hpf. This gene was also expressed in the liver after 3 dpf in the zebrafish larvae. Disruption of dax1 function by morpholino oligonucleotides (MO) down-regulated expression of steroidogenic genes, cyp11a and star, and led to severe phenotypes similar to ff1b (SF1) MO-injected embryos. Injection of dax1 MO did not affect ff1b expression, whereas ff1b MO abolished dax1 expression in the interrenal organ. Based on these results, we propose that dax1 is the mammalian DAX1 ortholog, functions downstream of ff1b in the regulatory cascades, and is required for normal development and function of the zebrafish interrenal organ.

Cloning and characterization of a cDNA encoding cholesterol side-chain cleavage cytochrome P450 (CYP11A1): tissue-distribution and changes in the transcript abundance in ovarian tissue of Japanese eel, Anguilla japonica, during artificially induced sexual development

Cholesterol side-chain cleavage cytochrome P450 (CYP11A1: P450scc) is a crucial steroidogenic enzyme that catalyzes an initial step in the production of all classes of steroids. A cDNA encoding Japanese eel P450scc was cloned and characterized. The cDNA putatively encoded 521 amino acid residues with high homology to those of other vertebrate forms. The recombinant P450scc produced in COS-7 cells efficiently catalyzed the conversion of 25-hydroxycholesterol into pregnenolone. By northern blot, a single P450scc transcript of approximately 3.3 kb was detected in both ovary and head kidney. Transcript levels of this enzyme significantly increased throughout ovarian development artificially induced by salmon pituitary homogenate, which suggests that gonadotropic stimuli can induce ovarian expression of the P450scc gene in teleosts, as has been reported in mammals. Furthermore, RT-PCR analysis revealed that gene expression of three steroidogenic enzymes, P450scc, P450c17 and 3beta-hydroxysteroid dehydrogenase (3beta-HSD) show distinctly different tissue-specific patterns of expression in the Japanese eel. The P450scc gene was expressed in ovary and head kidney while the sole source of the P450c17 transcript was ovary. In contrast, 3beta-HSD transcript was detected in all tissues examined, brain, liver, spleen and trunk kidney, etc. These suggest that some steroidogenic enzymes are also expressed in non-endocrine tissues and could potentially regulate the local and/or circulating steroid levels in teleosts, as they do in mammals.

Steroidogenesis in zebrafish and mouse models

Steroid hormones regulate physiological homeostasis for salt, sugar, and sex differentiation. All steroids are synthesized from a common precursor, cholesterol, in a step that converts cholesterol to pregnenolone. The enzyme carrying out this first conversion step is CYP11A1. To further investigate the importance of steroid biosynthesis, animal models with defects in the Cyp11a1 gene are used. Mice with targeted disruption of the Cyp11a1 gene produce no steroids with severe adrenal defects. These mice survive during embryogenesis, but die after birth. Zebrafish with a block in cyp11a1 gene function has an earlier defect, presumably because it lacks adequate maternal steroid supply. When cyp11a1 activity was compensated by the injection of antisense morpholino oligos, the embryos have shortened axis and a defect of epibolic cell movement during early embryogenesis. The discovery of steroid function in cell movement is novel, and should provide new insights into our understanding of diverse functions of steroids.

Pregnenolone stabilizes microtubules and promotes zebrafish embryonic cell movement

Embryonic cell movement is essential for morphogenesis and the establishment of body shapes, but little is known about its mechanism. Here we report that pregnenolone, which is produced from cholesterol by the steroidogenic enzyme Cyp11a1 (cholesterol side-chain cleavage enzyme, P450scc), functions in promoting cell migration during epiboly. Epiboly is a process in which embryonic cells spread from the animal pole to cover the underlying yolk. During epiboly, cyp11a1 is expressed in an extra-embryonic yolk syncytial layer. Reducing cyp11a1 expression in zebrafish using antisense morpholino oligonucleotides did not perturb cell fates, but caused epibolic delay. This epibolic defect was partially rescued by the injection of cyp11a1 RNA or the supplementation of pregnenolone. We show that the epibolic delay is accompanied by a decrease in the level of polymerized microtubules, and that pregnenolone can rescue this microtubule defect. Our results indicate that pregnenolone preserves microtubule abundance and promotes cell movement during epiboly.

Gene duplication, gene loss and evolution of expression domains in the vertebrate nuclear receptor NR5A (Ftz-F1) family

Fushi tarazu factor 1 (Ftz-F1, NR5A) is a zinc-finger transcription factor that belongs to the nuclear receptor superfamily and regulates genes that are involved in sterol and steroid metabolism in gonads, adrenals, liver and other tissues. To understand the evolutionary origins and developmental genetic relationships of the Ftz-F1 genes, we have cloned four homologous Ftz-f1 genes in zebrafish, called ff1a, ff1b, ff1c and ff1d. These four genes have different temporal and spatial expression patterns during development, indicating that they have distinct mechanisms of genetic regulation. Among them, the ff1a expression pattern is similar to mammalian Nr5a2, while the ff1b pattern is similar to that of mammalian Nr5a1. Genetic mapping experiments show that these four ff1 genes are located on chromosome segments conserved between the zebrafish and human genomes, indicating a common ancestral origin. Phylogenetic and conserved synteny analysis show that ff1a is the orthologue of NR5A2, and that ff1b and ff1d genes are co-orthologues of NR5A1 that arose by a gene-duplication event, probably a whole-genome duplication, in the ray-fin lineage, and each gene is located next to an NR6A1 co-orthologue as in humans, showing that the tandem duplication occurred before the divergence of human and zebrafish lineages. ff1c does not have a mammalian counterpart. Thus we have characterized the phylogenetic relationships, expression patterns and chromosomal locations of these Ftz-F1 genes, and have demonstrated their identities as NR5A genes in relation to the orthologous genes in other species.