Retinoid requirements in the reproduction of zebrafish

Molecular characterization and potential function of Rxrγ in gonadal differentiation of Chinese soft-shelled turtle (Pelodiscus sinensis)

Retinoid X receptor (RXR) is a member of the ligand-dependent nuclear receptor family. Previous studies revealed that RXRs are involved in reproduction in vertebrates. However, information on the function of RXRs in turtles is scarce. In this study, the Rxrγ cDNA sequence of Pelodiscus sinensis was cloned and analyzed, and a polyclonal antibody was constructed. RXRγ protein showed a positive signal in both mature and differentiated gonads of the turtle. Subsequently, the function of the Rxrγ gene in gonadal differentiation was confirmed using short interfering RNA (RNAi). The full-length cDNA sequence of the Rxrγ gene in P. sinensis was 2152 bp, encoding 407 amino acids and containing typical nuclear receptor family domains, including the DNA-binding domain (DBD), ligand-binding domain (LBD), and activation function 1 (AF1). Moreover, gonadal Ps-Rxrγ showed sexual dimorphism expression patterns in differentiated gonads. Real-time quantitative PCR results revealed that the Rxrγ gene was highly expressed in the turtle ovary. RNAi treatment increased the number of Sertoli cells in ZZ embryonic gonads. Furthermore, RNA interference upregulated Dmrt1 and Sox9 in ZZ and ZW embryonic gonads. However, Foxl2, Cyp19a1, Stra8, and Cyp26b1 were downregulated in embryonic gonads. The results indicated that Rxrγ participated in gonadal differentiation and development in P. sinensis.

Transcriptome Analysis of Thiram-Treated Zebrafish (Danio rerio) Embryos Reveals Disruption of Reproduction Signaling Pathways

Thiram, a dithiocarbamate fungicide, is used for the treatment of various fungal infections affecting crops and ornamentals. However, thiram-associated toxicity has been reported in animals, including fish, and the underlying molecular mechanisms are unclear. Herein, we employed zebrafish (ZF) to gain further insights into thiram toxicity-associated molecular mechanisms. We studied developmental abnormalities and performed whole-transcriptome analysis of ZF embryos exposed to thiram for 96 h. Embryos exposed to 4.0 μg/L thiram exhibited several phenotypic abnormalities, including bradycardia, spinal curvature, hatching arrest, and growth retardation. Whole-transcriptome analysis revealed 1754 differentially expressed genes (DEGs), with 512 upregulated and 1242 downregulated DEGs. The majority of biological processes affected by thiram were metabolic. Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis yielded terms related to reproduction, such as steroid biosynthesis and steroid hormone biosynthesis. Quantitative real-time polymerase chain reaction validation results were in line with sequencing data for ten DEGs. The study results improve our current understanding of the effects of thiram exposure in ZF.

Insight into the differences in the toxicity mechanisms of dinotefuran enantiomers in zebrafish by UPLC-Q/TOF-MS

Dinotefuran is a chiral insecticide widely used to control Nilaparvata lugens in agriculture. However, little is known about the toxic effects of dinotefuran enantiomers on aquatic organisms. In this study, zebrafish were exposed to 1.00 and 10.00 mg/L dinotefuran enantiomers for 96 h, after which multivariate pattern recognition, metabolite identification, and pathway analysis were performed. Principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA) were then conducted to reveal the metabolic perturbations caused by dinotefuran enantiomers. Metabolic pathway analysis revealed the perturbation of five main pathways, including phenylalanine, tyrosine and tryptophan biosynthesis; phenylalanine metabolism; retinol metabolism; arginine and proline metabolism; and glycerophospholipid metabolism. These disturbed metabolic pathways were strongly correlated with energy, amino acid metabolism, and lipid metabolism. Pathway analysis also indicated that the metabolic pathway changes induced by the same level of R and S-dinotefuran were enantioselective. Our research may provide better insight into the risk of chiral dinotefuran in aquatic organisms in the environment.

Effects of vitamin A supplementation on ovarian development of Astyanax lacustris (Teleostei: Characidae) during the non-breeding season

Vitamin A (VA) is essential for fish reproduction, however, knowledge of VA requirement of broodstock fish is scarce. We investigated the physiological role of VA supplementation in the ovarian maturation of Astyanax lacustris throughout the non-breeding season. Adult females were distributed in five nutritional treatments, which differed by the amount of VA supplemented in a plant-based diet: T1 = 0, T2 = 1800, T3 = 3600, T4 = 7200, and T5 = 14,400 IU kg^-1 of VA for 60 days after a period of 16 days feeding on a diet poor in VA to reduce body VA content. Plasma total lipids (TL), lipoproteins, and 17β-estradiol (E₂) levels were measured after 30 and 60 days. Somatic indexes and morphophysiological ovarian parameters were calculated and VA concentration was analyzed in the ovaries and liver after 60 days. VA supplementation did not increase the concentration of this nutrient in the liver and ovaries. E₂ plasma level increased in animals fed on 3600 IU kg^-1 of VA (T3) after 60 days and females from T5 group presented a higher RF after 60 days. Ovarian development increased after 60 days compared to that after 30 days, confirmed by the higher gonadosomatic index (all groups). The supplementation with the highest VA level increased RF and decreased plasma TL (after 30 days) and low-density lipoprotein (LDL) (after 60 days) concentrations, showing a potential to improve reproduction even during the non-breeding season; however, its nutritional requirements of VA could be higher.

Metabolomics reveals the reproductive abnormality in female zebrafish exposed to environmentally relevant levels of climbazole

Climbazole (CBZ) ubiquitously detected in the aquatic environment may disrupt fish reproductive function. Thus far, the previous study has focused on its transcriptional impact of steroidogenesis-related genes on zebrafish, but the underlying toxic mechanism still needs further investigation at the metabolic level. In this study, adult zebrafish were chronically exposed to CBZ at concentrations of 0.1 (corresponding to the real concentration in surface water), 10, and 1000 μg/L and evaluated for reproductive function by egg production, with subsequent ovarian tissue samples taken for histology, metabolomics, and other biochemical analysis. After 28 days' exposure, fecundity was significantly decreased in all exposure groups, with the inhibition of oocytes in varying developmental stages to a certain degree. The decrease in retinoic acid and sex hormones, down-regulated genes important in steroidogenesis, and increase in oxidized/reduced glutathione ratio and occurrence of apoptotic cells were observed in zebrafish ovaries following exposure to CBZ even at environmentally realistic concentrations, suggesting that alternations in steroidogenesis and oxidative stress can play significant roles in CBZ-triggered reproductive toxicity. Besides, mass spectrometry imaging analysis validated the results from metabolomics analysis. Our findings provide novel perspectives for unveiling the mechanism of reproductive dysfunction by CBZ and highlight its risk to fish reproduction.

Biological markers to establish a relationship between the health status of the St. Lawrence River yellow perch (Perca flavescens) with a gradient of anthropogenic disturbances

Since the mid-1990s, the decline of the yellow perch population of Lake Saint-Pierre (hereinafter LSP) in Quebec, Canada has been the subject of several research programs. The combined effect of habitat deterioration, the presence of invasive species, and poor water quality negatively affected the yellow perch population in this lake. In 2013, we sampled yellow perch (larvae, juveniles and adults) at six sites along the St. Lawrence River representing a gradient of increasing human influences from upstream to downstream and measured several biomarkers including retinoid compounds (vitamin A). In the most contaminated sites (LSP, north and south shores), we found that retinoid stores were decreased in all three stages of development. To corroborate these results and to test other biomarkers, we once again sampled yellow perch (adults only) from the same sites. Results from our 2014 and 2015 samplings confirmed that LSP yellow perch appeared to be at a disadvantage compared to fish from upstream populations. Individuals from LSP have lower acetylcholinesterase (AChE) activity as well as lower retinoid levels in liver and plasma. These fish were also marked by lower levels of antioxidants such as lycopene and vitamin E. A discriminant analysis of this set of results confirmed that the yellow perch of the LSP could be easily discriminated from those of the other sites (2014 and 2015) on the basis of liver retinoid and, to a lesser extent, of the liver tocopherol and protein concentration of the muscle, as well as AChE activity and DROH (all-trans-3,4-dehydroretinol) measured in plasma.

Current basis and future directions of zebrafish nutrigenomics

This review investigates the current state of nutrigenomics in the zebrafish animal models. The zebrafish animal model has been used extensively in the study of disease onset and progression and associated molecular changes. In this review, we provide a synopsis of nutrigenomics using the zebrafish animal model. Obesity and dyslipidemia studies describe the genomics of dietary-induced obesity in relation to high-fat/high-calorie diets. Inflammation and cardiovascular studies describe dietary effects on the expression of acute inflammatory markers and resulting chronic inflammatory issues including atherosclerosis. We also evaluated the genomic response to bioactive dietary compounds associated with metabolic disorders. Carbohydrate metabolism and β-cell function studies describe the impacts of high-carbohydrate dietary challenges on nutritional programming. We also report tumorigenesis in relation to dietary carcinogen exposure studies that can result in permanent genomic changes. Vitamin and mineral deficiency studies demonstrate transgenerational genomic impacts of micronutrients in the diet and temporal expression changes. Circadian rhythm studies describe the relation between metabolism and natural temporal cycles of gene expression that impacts health. Bone formation studies describe the role of dietary composition that influences bone reabsorption regulation. Finally, this review provides future directions in the use of the zebrafish model for nutrigenomic and nutrigenetic research.

Impact of the partial replacement of fish meal with a chloroplast rich fraction on the growth and selected nutrient profile of zebrafish (Danio rerio)

Increasing global aquaculture production, is putting pressure on fishmeal and fish oil supply. There is therefore a growing search for more sustainable sources of proteins and polyunsaturated fatty acids as fish feed ingredients. Chloroplasts are the organelles in the leaves of plants where many of the valuable nutrients, fatty acids (FAs), amino acids, vitamins and pigments, are synthesised. Chloroplasts could be incorporated into fish diets either retained in, or liberated from, plant cells. In this study zebrafish were fed with seven different diets individually; fish were fed with diets reducing fishmeal levels (10, 20 or 50%) using either spinach leaf powder (SLP) or a chloroplast rich fraction (CRF) prepared by an established method to recover chloroplasts. Both SLP and CRF had a positive impact on the growth, taste response, whole fish FA composition, and carotenoid profile. Fish fed with CRF diets showed significantly (P ≤ 0.05) greater α-linolenic (C18:3 n-3) and hexadecatrienoic (C16:3) acid contents than those of SLP and the control. Hexadecanoic acid (C16:3) is a unique FA in the galactolipids of the chloroplast; its presence in zebrafish tissues proves that zebrafish digest and absorb chloroplast galactolipids. The lutein profile of eggs produced by zebrafish fed with the CRF diet was significantly (P ≤ 0.05) higher than those of SLP and the control. Alterations in egg colour were also noted, warranting further investigations of the diet impact on fish fecundity, embryo fertility, hatch rate and larval survival.

Comparative Metabonomic Investigations of Schistosoma japonicum From SCID Mice and BALB/c Mice: Clues to Developmental Abnormality of Schistosome in the Immunodeficient Host

The growth and development of schistosome has been affected in the immunodeficient hosts. But it remains unresolved about the molecular mechanisms involved in the development and reproduction regulation of schistosomes. This study tested and compared the metabolic profiles of the male and female Schistosoma japonicum worms collected from SCID mice and BALB/c mice at 5 weeks post infection using liquid chromatography tandem mass spectrometry (LC-MS/MS) platform, in which the worms from SCID mice were the investigated organisms and the worms from BALB/c mice were used as the controls. There were 1015 ion features in ESI+ mode and 342 ion features in ESI- mode were identified after filtration by false discovery rate. Distinct metabolic profiles were found to clearly differentiate both male and female worms in SCID mice from those in BALB/c mice using multivariate modeling methods including the Principal Component Analysis (PCA), Partial Least Squares Discriminant Analysis (PLS-DA), and Orthogonal Partial Least Squares Discriminant Analysis (OPLS-DA). There were more differential metabolites in female worms than in male worms between SCID mice and BALB/c mice. And common and uniquely perturbed metabolites and pathways were identified among male and female worms from SCID mice when compared with BALB/c mice. The enriched metabolite sets of the differential metabolites in male worms between SCID mice and BALB/c mice included bile acid biosynthesis, taurine and hypotaurine metabolism, sphingolipid metabolism, retinol metabolism, purine metabolism, etc. And the enriched metabolite sets of differential metabolites in female worms included retinol metabolism, alpha linolenic acid and linoleic acid metabolism, purine metabolism, sphingolipid metabolism, glutamate metabolism, etc. Further detection and comparison in transcript abundance of genes of the perturbed retinol metabolism and its associated meiosis process in worms identified clues suggesting accumulated retinyl ester and perturbed meiotic process. These findings suggested an association between the schistosome with retarded growth and development in SCID mice and their perturbed metabolites and metabolic pathways, and provided a new insight into the growth and development regulation of S. japonicum worms from the metabolic level, which indicated great clues for discovery of drugs or vaccines against the parasites and disease with more researches.

The Impact of Two Different Cold-Extruded Feeds and Feeding Regimens on Zebrafish Survival, Growth and Reproductive Performance

Zebrafish (Danio rerio) is one of the top model organisms used in biomedical research. Therefore, it is fundamental that zebrafish facilities continuously improve husbandry methods to provide fish with the best physiological and welfare conditions that suit each experimental purpose. Nutrition is a husbandry aspect that needs further optimization, as it greatly affects growth, reproduction, health and behaviour. Here, we have compared the impact of different feeding regimens on zebrafish survival, growth and reproductive performance. Mutant and wild-type zebrafish were raised using several combinations of two cold-extruded processed feeds—Skretting^®GemmaMicro and Sparos^®Zebrafeed—and one live feed (rotifers). Zebrafeed^® outperformed GemmaMicro^® in terms of survival rate, and embryo viability was also higher when the spawners were fed with Zebrafeed^® either from larval stage or upon sexual maturation. In contrast, GemmaMicro^® favoured growth, both in size and weight. The use of rotifers until 60 days post-fertilization improved survival of fish co-fed with GemmaMicro^®, while delaying their growth. Zebrafeed^® performance was not affected by co-feeding rotifers. Overall, we showed that different nutritional formulas affect physiological parameters, allowing for the establishment of feeding protocols adapted to the objectives of each facility. At the same time, we validated Skretting^®GemmaMicro and Sparos^®Zebrafeed as two commercially available feeds that are well suited for zebrafish nutrition in a laboratory environment.

The cycling gonad: retinoic-acid synthesis and degradation patterns during adult zebrafish Danio rerio oogenesis

The expression pattern of genes coding for enzymes of the retinoic acid (RA) synthetic and degradation pathways was characterized in adult female zebrafish Danio rerio. Females were conditioned until maturation and post-spawn expression dynamics were determined. A striking upregulation of cyp26b1, but not cyp26a1, was observed following egg deposition, decreasing to initial levels during recovery. A similar, yet lower, fluctuation was observed for aldh1a2 and rdh10a, the enzymes participating in the two-step RA biosynthesis cascade. The present work highlights the dynamics of the adult D. rerio oogenesis and uncovers novel, yet elusive, metabolic contributors. Possible compartmentalized roles for the different gene paralogue isoforms are discussed.

Reproductive toxicity of β-diketone antibiotic mixtures to zebrafish (Danio rerio)

So far, few data are available on the reproductive toxicological assessment of β-diketone antibiotics (DKAs), a class of ubiquitous pseudo-persistent pollutant, in zebrafish (Danio rerio). Herein, we reported the reproductive effects of DKAs by means of transcriptome analysis (F1-zebrafish), changes in a series of reproductive indices (F0-zebrafish) and histopathological observations. A total of 1170, 983 and 1399 genes were found to be differentially expressed when compared control vs. 6.25mg/L, control vs. 12.5mg/L and 6.25 vs. 12.5mg/L DKA-exposure treatments, respectively. Among three comparison groups, 670, 569 and 821 genes were respectively assigned for GO analyses based on matches with sequences of known functions. In 149 KEGG-noted metabolic pathways, the preferential one was the MAPK (mitogen-activated protein kinase) signaling pathway, followed by oxidative phosphorylation, neuroactive ligand-receptor interaction and so on. By qPCR verification, 6 genes (c6ast4, igfbp1b, mrpl42, tnnc2, emc4 and ddit4) showed consistent gene expression with those identified by transcriptome sequencing. Due to DKA-exposure, the concentrations of plasma estradiol and testosterone, and the gonado-somatic index were significantly dose-dependently declined. Also, DKA-exposure led to declining in zebrafish reproductive capacity, reflecting in fertilization, hatchability and egg production. Histopathological observations demonstrated that zebrafish ovary and testis suffered serious damage after DKA-exposure. The 4-oxo-TEMP signals increased obviously with increasing DKA-exposed concentrations, implying disruption of balance between generation and clearance of ¹O₂. In summary, DKAs not only produce reproductive toxicological effects on F0-zebrafish, but also result in adverse consequences for growth and development of F1-zebrafish.

Lipophilic antioxidants and lipid peroxidation in yellow perch subjected to various anthropogenic influences along the St. Lawrence River (QC, Canada)

In Lake Saint-Pierre (LSP), the last great widening of the St. Lawrence River (province of Québec, Canada), the yellow perch has been experiencing a significant decline since the mid-1990s. The combined effect of several stressors (deterioration of habitats appropriate for reproduction and growth, invasive species and poor water quality) seems to exert considerable influence on the yellow perch population in LSP, characterized by low recruitment. To better understand possible stressor effects at the biochemical level, LSP yellow perch were compared with other sites along a gradient of increasing human influences from upstream to downstream along the St. Lawrence River. Morphometry (size, weight, circumference and Fulton's condition factor) and biomarkers associated to the peroxidation of lipids, lipophilic antioxidants (α-tocopherol and carotenoids), along with retinoids (vitamins A1and A2) and proteins were compared between sites at the larval, juvenile and adult stages. Fulton's condition factor was similar between sites for juveniles but was significantly lower in LSP adults, suggesting a weakened physiological condition. In most contaminated sites as LSP, lipid peroxidation tended to be higher in juveniles and adults whereas the lipophilic antioxidant lycopene and proteins content were lower. Retinyl esters were significantly lower for LSP fish compared to other sites, not only in larvae but also in the livers of juveniles and adults. These results are consistent with possible altered metabolism in the retinoid system of LSP yellow perch. The overall results reflect the "pressure" gradient tested, where the yellow perch from the most affected sites located downstream had impaired physiological and biochemical conditions compared to the upstream sectors.

Dietary vitamin A supplementation ameliorates the effects of poly-aromatic hydrocarbons in Atlantic salmon (Salmo salar)

Several studies have reported on the interaction between vitamin A (VA) and aryl hydrocarbon receptor (AhR)-binding toxicants, including poly-aromatic hydrocarbons (PAHs). In aquaculture, the use of plant oils in novel aquafeeds can increase PAH levels while simultaneously lowering natural VA background levels, causing the need to supplement plant oil-based feeds with synthetic VA. To study dietary VA-PAH interactions, Atlantic salmon (initial weight 195±0.15g) were fed four identical plant-based diets that were supplemented with PAHs (100 and 10mgkg(-1) benzo[a]pyrene (BaP) and phenanthrene (Phe), respectively) or VA (retinyl acetate 8721IUkg(-1)) separately or combined for 2.5 months in a 2×2 factorial design, with triplicate net-pens per diet. Dietary PAH significantly reduced hepatic VA storage, and VA-enriched diets restored hepatic VA. There was a significant PAH-VA interaction effect on hepatic BaP, but not Phe, accumulation, with reduced hepatic BaP concentrations in fish fed VA+PAH compared to fish fed PAH alone. Concurrently, PAH and VA significantly interacted in their effects on CYP1A phase I biotransformation as observed from increased ethoxyresorufin-O-deethylase (EROD) activity, increased CYP1A protein concentration, and elevated transcription (cyp1a1 gene expression) in fish fed PAH+VA compared to PAH alone. Dietary VA supplementation alone had no significant effect on CYP1A phase I biotransformation. Metabolomic assessment showed that dietary VA caused a restoration of metabolic intermediates involved in energy metabolism that were affected by dietary PAH. Moreover, a PAH-induced growth inhibition was partially ameliorated by dietary VA supplementation. In conclusion, dietary VA interacted with PAH toxicity on the level of CYP1A-mediated detoxification, hepatic PAH accumulation, energy allocation, and growth.

Construction, De-Novo Assembly and Analysis of Transcriptome for Identification of Reproduction-Related Genes and Pathways from Rohu, Labeo rohita (Hamilton)

Rohu is a leading candidate species for freshwater aquaculture in South-East Asia. Unlike common carp the monsoon breeding habit of rohu restricts its seed production beyond season indicating strong genetic control over spawning. Genetic information is limited in this regard. The problem is exacerbated by the lack of genomic-resources. We identified 182 reproduction-related genes previously by Sanger-sequencing which were less to address the issue of seasonal spawning behaviour of this important carp. Therefore, the present work was taken up to generate transcriptome profile by mRNAseq. 16 GB, 72 bp paired end (PE) data was generated from the pooled-RNA of twelve-tissues from pre-spawning rohu using IlluminaGA-II-platform. There were 64.97 million high-quality reads producing 62,283 contigs and 88,612 numbers of transcripts using velvet and oases programs, respectively. Gene ontology annotation identified 940 reproduction-related genes consisting of 184 mainly associated with reproduction, 223 related to hormone-activity and receptor-binding, 178 receptor-activity and 355 embryonic-development related-proteins. The important reproduction-relevant pathways found in KEGG analysis were GnRH-signaling, oocyte-meiosis, steroid-biosynthesis, steroid-hormone biosynthesis, progesterone-mediated oocyte-maturation, retinol-metabolism, neuroactive-ligand-receptor interaction, neurotrophin-signaling and photo-transduction. Twenty nine simple sequence repeat containing sequences were also found out of which 12 repeat loci were polymorphic with mean expected-&-observed heterozygosity of 0.471 and 0.983 respectively. Quantitative RT-PCR analyses of 13-known and 6-unknown transcripts revealed differences in expression level between preparatory and post-spawning phase. These transcriptomic sequences have significantly increased the genetic-&-genomic resources for reproduction-research in Labeo rohita.

Inhibition of retinoic acid synthesis disrupts spermatogenesis and fecundity in zebrafish

Timing of germ cell entry into meiosis is sexually dimorphic in mammals. However it was recently shown that germ cells initiate meiosis at the same time in male and female zebrafish. Retinoic acid (RA) has been shown to be critical for mammalian spermatogenesis. Inhibition of RA synthesis by WIN 18,446 has been reported to inhibit spermatogenesis in a wide variety of animals including humans and was once used as a contraceptive in humans. In this study we explored the role of RA in zebrafish spermatogenesis. In silico analysis with Internal coordinate mechanics docking software showed that WIN 18,446 can bind to the rat, human and zebrafish Aldh1a2 catalytic domain with equivalent potency. RA exposure resulted in up-regulation of the RA metabolizing enzyme genes cyp26a1, cyp26b1 and cyp26c1 in vitro and in vivo. Exposure to WIN 18,446 resulted in down-regulation of Aldh1a2, cyp26a1 and cyp26b1 in vivo. WIN 18,446 was effective in disrupting spermatogenesis and fecundity in zebrafish but the reduction in sperm count and fecundity was only observed when zebrafish were maintained on a strict Artemia nauplii diet which is known to contain low levels of vitamin A. This study shows that RA is involved in spermatogenesis as well as oocyte development in zebrafish. As the zebrafish Aldh1a2 structure and function is similar to the mammalian counterpart, Aldh1a2 inhibitor screening using zebrafish as a model system may be beneficial in the discovery and development of new and safe contraceptives for humans.

Effects of Tributyltin and Other Retinoid Receptor Agonists in Reproductive-Related Endpoints in the Zebrafish (Danio rerio)

Both field and experimental data examined the influence of exposure to environmental contaminant tributyltin (TBT) on marine organisms. Although most attention focused on the imposex phenomenon in gastropods, adverse effects were also observed in other taxonomic groups. It has been shown that imposex induction involves modulation of retinoid signaling in gastropods. Whether TBT influences similar pathways in fish is yet to be addressed. In this study, larvae of the model teleost Danio rerio were exposed to natural retinoids, all-trans-retinoic acid, 9-cis-retinoic acid, and all-trans-retinol, as well as to the RXR synthetic pan-agonist methoprene acid (MA) and to TBT. Larvae were exposed to TBT from 5 days post fertilization (dpf) to adulthood, and reproductive capacity was assessed and correlated with mode of action. TBT significantly decreased fecundity at environmentally relevant levels at 1 μg TBT Sn/g in diet. Interestingly, in contrast to previous reports, TBT altered zebrafish sex ratio toward females, whereas MA exposure biased sex toward males. Since fecundity was significantly altered in the TBT-exposed group with up to 62% decrease, the potentially affected pathways were investigated. Significant downregulation was observed in brain mRNA levels of aromatase b (CYP19a1b) in females and peroxisome proliferator activated receptor gamma (PPARg) in both males and females, suggesting an involvement of these pathways in reproductive impairment associated with TBT.

Characterization of sea bass FSHβ 5' flanking region: transcriptional control by 17β-estradiol

The sea bass follicle-stimulating hormone 5' flanking region (sbFSHβ 5' FR) was cloned and characterized in order to study the molecular mechanisms underlying transcriptional regulation of the sbFSHβ gene. Analysis of the ~3.5 kb of this region revealed the presence of several putative cis-acting elements, including steroid hormone response elements, cAMP response elements, pituitary-specific transcription factor response elements, activator protein-1 response elements and TATA sequence. Deleted constructs containing ~3.5 kb of the sbFSHβ 5' FR fused to a luciferase reporter gene were transiently transfected into human embryonic kidney (HEK 293) and mouse mature gonadotrope (LβT2) cell lines. The sbFSHβ 5' FR was efficiently expressed under basal conditions in LβT2 but not in HEK 293, pointing to both positive and negative regulatory elements. In order to elucidate the estrogen-mediated sbFSHβ transcriptional activity, in vitro treatments with 17β-estradiol were carried out on primary cultures of pituitary cells and LβT2 cells transiently expressing luciferase under the control of sbFSHβ 5' FR. Overall, these results demonstrate that 17β-estradiol inhibits sbFSHβ gene expression directly at the level of the pituitary. However, it was also shown that estrogen did not induce changes of the sbFSH promoter-directed luciferase activity, suggesting that sbFSHβ 5'FR (~3.5 kb) activity is cell type dependent and its estrogen regulation could require cis-acting elements located upstream of the promoter region, which is characterized in this article.

Analysis of protein expression in zebrafish during gonad differentiation by targeted proteomics

The molecular mechanisms governing sex determination and differentiation in the zebrafish (Danio rerio) are not fully understood. To gain more insights into the function of specific genes in these complex processes, the expression of multiple candidates needs to be assessed, preferably on the protein level. Here, we developed a targeted proteomics method based on selected reaction monitoring (SRM) to study the candidate sex-related proteins in zebrafish which were selected based on a global proteomics analysis of adult gonads and representational difference analysis of male and female DNA, as well as on published information on zebrafish and other vertebrates. We employed the developed SRM protocols to acquire time-resolved protein expression profiles during the gonad differentiation period in vas::EGFP transgenic zebrafish. Evidence on protein expression was obtained for the first time for several candidate genes previously studied only on the mRNA level or suggested by bioinformatic predictions. Tuba1b (tubulin alpha 1b), initially included in the study as one of the potential housekeeping proteins, was found to be preferentially expressed in the adult testis with nearly absent expression in the ovary. The revealed changes in protein expression patterns associated with gonad differentiation suggest that several of the examined proteins, especially Ilf2 and Ilf3 (interleukin enhancer-binding factors 2 and 3), Raldh3 (retinaldehyde dehydrogenase type 3), Zgc:195027 (low density lipoprotein-related receptor protein 3) and Sept5a (septin 5a), may play a specific role in the sexual differentiation in zebrafish.

Retinoic acid metabolic genes, meiosis, and gonadal sex differentiation in zebrafish

To help understand the elusive mechanisms of zebrafish sex determination, we studied the genetic machinery regulating production and breakdown of retinoic acid (RA) during the onset of meiosis in gonadogenesis. Results uncovered unexpected mechanistic differences between zebrafish and mammals. Conserved synteny and expression analyses revealed that cyp26a1 in zebrafish and its paralog Cyp26b1 in tetrapods independently became the primary genes encoding enzymes available for gonadal RA-degradation, showing lineage-specific subfunctionalization of vertebrate genome duplication (VGD) paralogs. Experiments showed that zebrafish express aldh1a2, which encodes an RA-synthesizing enzyme, in the gonad rather than in the mesonephros as in mouse. Germ cells in bipotential gonads of all zebrafish analyzed were labeled by the early meiotic marker sycp3, suggesting that in zebrafish, the onset of meiosis is not sexually dimorphic as it is in mouse and is independent of Stra8, which is required in mouse but was lost in teleosts. Analysis of dead-end knockdown zebrafish depleted of germ cells revealed the germ cell-independent onset and maintenance of gonadal aldh1a2 and cyp26a1 expression. After meiosis initiated, somatic cell expression of cyp26a1 became sexually dimorphic: up-regulated in testes but not ovaries. Meiotic germ cells expressing the synaptonemal complex gene sycp3 occupied islands of somatic cells that lacked cyp26a1 expression, as predicted by the hypothesis that Cyp26a1 acts as a meiosis-inhibiting factor. Consistent with this hypothesis, females up-regulated cyp26a1 in oocytes that entered prophase-I meiotic arrest, and down-regulated cyp26a1 in oocytes resuming meiosis. Co-expression of cyp26a1 and the pluripotent germ cell stem cell marker pou5f1(oct4) in meiotically arrested oocytes was consistent with roles in mouse to promote germ cell survival and to prevent apoptosis, mechanisms that are central for tipping the sexual fate of gonads towards the female pathway in zebrafish.

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.

Tissue-specific distribution patterns of retinoids and didehydroretinoids in rainbow trout Oncorhynchus mykiss

Retinoids (vitamin A) are known to be involved in many key biological functions in mammals, such as embryonic development, reproduction or vision. Besides standard vitamin A forms, freshwater fish tissues contain high levels of didehydroretinoids or vitamin A(2) forms. However, the tissue distribution, metabolism and function of both standard and particularly the didehydroretinoids are still poorly known in fish. In this study, we have quantified the levels of retinoids, including retinol, retinaldehyde, retinyl palmitate and their corresponding didehydro forms, as well as the levels of the active polar retinoids all-trans-, 9-cis- and 13-cis-retinoic acid in distinct tissues of juvenile rainbow trout. Our results indicate that the liver is clearly the main retinoid storage tissue in juvenile rainbow trout. Didehydroretinoids were dominant over retinoids in all analyzed tissues with the exception of plasma. Additionally, significant differences among tissues were observed between retinoids and didehydroretinoids, such as differences in the ester profiles and the proportions between free and esterified forms, suggesting that mechanisms that favor the utilization or storage of one of the other groups of compounds might exist in fish. Our data also show the presence of polar retinoids in different tissues of fish at the fmol/g scale. Overall, this study clearly demonstrates the presence of tissue-specific patterns of accumulation of both polar and nonpolar retinoids in fish tissues. The biological relevance of these findings should be the focus of future studies.

Growth and body composition of zebrafish (Danio rerio) larvae fed a compound feed from first feeding onward: toward implications on nutrient requirements

Although zebrafish is used as a major model species for understanding a number of biological functions and mechanisms involved, there is practically no information on the nutritional requirements of this cyprinid. The rearing practices, especially with regard to feeds and feeding, also vary considerably. To obtain base line information on the nutrient requirements and to develop a standard diet, we initiated the present work of feeding zebrafish larvae with a formulated feed right from first feeding onward. Growth of zebrafish fed the compound feed was very good, reaching a total length of 23 ± 4 mm in 9 weeks with a survival rate of 89% ± 4%. We present the first ever published data on whole-body composition in terms of essential amino acids (ideal protein profile), total, neutral and polar fatty acid profiles, minerals, and trace elements. Results obtained here show clearly that zebrafish can be reared with formulated feed right from mouth opening without resorting to any live prey.

Advances in zebrafish husbandry and management

The zebrafish has emerged over the past several decades to become a mainstream animal model. While the growth of the use of the fish in basic biomedical research has been characterized by innovation, the methods and tools for fish husbandry, management, and care have been slow to evolve beyond those conceived during the initial establishment of the model system. While these approaches and technologies have certainly served the purposes of the field, they must now be improved to better match the widening scope and scale of research being done in fish. Such advances are made possible by applying new scientific information to the development of more sophisticated approaches for fish husbandry and management, and by considering the lessons learned during the establishment of the rodent model system. This review highlights the recent progress made in various areas of fish husbandry and management and points out new directions for further improvements.

Isolation of lipocalin-type protein from rainbow trout seminal plasma and its localisation in the reproductive system

The lipocalin protein family is a large and diverse group of small extracellular proteins characterised by their ability to bind hydrophobic molecules. In the present study, we describe the isolation procedure for rainbow trout seminal plasma protein, characterised by a moderate migration rate during polyacrylamide gel electrophoresis, providing information regarding its basic features and immunohistochemical localisation. This protein was identified as a lipocalin-type protein (LTP). The molecular mass of LTP was found to be 18 848 Da and it was found to lack any carbohydrate components. Only a few Salmoniformes contain LTP in their seminal plasma. The abundance of LTP in the Sertoli and Leydig cells of the testes of the rainbow trout, as well as in secretory cells of the efferent duct, suggests that this protein is specific for rainbow trout milt, where it acts as a lipophilic carrier protein. Moreover, the specific localisation of LTP in the flagella of the spermatozoa suggests a role for LTP in sperm motility. Further experiments are necessary to identify the endogenous ligands for LTP in rainbow trout seminal plasma and to characterise the binding properties of this protein.

Spermatogenesis in fish

Spermatogenesis is a developmental process during which a small number of diploid spermatogonial stem cells produce a large number of highly differentiated spermatozoa carrying a haploid, recombined genome. We characterise morphologically the different germ cell stages with particular attention for the spermatogonial generations, including the stem cells and their specific capacity to colonise a recipient's testis after transplantation. We propose a nomenclature for fish germ cells to improve the comparability among different teleost fish but also to higher vertebrates. Survival and development of germ cells depends on their continuous and close contact to Sertoli cells, and we review their multiple roles in the cystic mode of spermatogenesis seen in fish. We then discuss gene expression patterns associated with testis maturation. The endocrine system of vertebrates has evolved as master control system over spermatogenesis. In fish, both pituitary gonadotropins LH and FSH stimulate gonadal sex steroid hormone production directly by activating Leydig cells. Information is reviewed on the effects of progestin, androgens, and estrogens on global testicular gene expression patterns (microarray analysis), and on the molecular mechanisms by which steroids regulate specific candidate genes (identified by subtractive hybridization approaches) during early stages of testis maturation. Moreover, progestin and androgen effects on spermiation and milt hydration are discussed. Sex steroids mainly act via receptors expressed by Sertoli cells. One type of response is that Sertoli cells change growth factor expression, which subsequently modulates germ cell proliferation/differentiation via mechanisms yet to be characterised. Finally, we review data on germ cell autonomous processes, mainly derived from loss-of-function mutant fish lines, before identifying a number of focus areas for future research activities.