Retinals and retinols induced by estrogen in the blood plasma of Xenopus laevis

Comparative transcriptome sequencing analysis of female and male Decapterus macrosoma

Sexual growth dimorphism is a common phenomenon in teleost fish and has led to many reproductive strategies. Growth- and sex-related gene research in teleost fish would broaden our understanding of the process. In this study, transcriptome sequencing of shortfin scad Decapterus macrosoma was performed for the first time, and a high-quality reference transcriptome was constructed. After identification and assembly, a total of 58,475 nonredundant unigenes were obtained with an N50 length of 2,266 bp, and 28,174 unigenes were successfully annotated with multiple public databases. BUSCO analysis determined a level of 92.9% completeness for the assembled transcriptome. Gene expression analysis revealed 2,345 differentially expressed genes (DEGs) in the female and male D. macrosoma, 1,150 of which were female-biased DEGs, and 1,195 unigenes were male-biased DEGs. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses showed that the DEGs were mainly involved in biological processes including protein synthesis, growth, rhythmic processes, immune defense, and vitellogenesis. Then, we identified many growth- and sex-related genes, including Igf, Fabps, EF-hand family genes, Zp3, Zp4 and Vg. In addition, a total of 19,573 simple sequence repeats (SSRs) were screened and identified from the transcriptome sequences. The results of this study can provide valuable information on growth- and sex-related genes and facilitate further exploration of the molecular mechanism of sexual growth dimorphism.

Insight of vitellogenesis patterns: A comparative analysis of the differences between the primary and secondary vitellogenesis period in the ovary, hepatopancreas, and muscle of mud crab, scylla paramamosain

The mud crab, Scylla paramamosain, has abundant nutrients in its edible parts, ovary, hepatopancreas, and muscle during the ovarian maturation stage. The ovary of S. paramamosain can re-mature after spawning during the secondary ovarian maturation period. We aimed to analyze the characteristics of the first vitellogenesis period (FVP)¹ and second vitellogenesis period (SVP)² of S. paramamosain during ovarian maturation to understand the differences in vitellogenesis patterns between the first and second ovarian maturation periods. Accordingly, the gonadosomatic index (GSI) and hepatopancreatic index (HSI), the external and histological characteristics of the ovary and hepatopancreas, the Sp-Vg (vitellogenin, Vg) expression levels in the hepatopancreas and ovary, and the dynamics of the biochemical components in the ovary, hepatopancreas, and muscle were determined. Based on the results, the GSI was significantly positively correlated with HSI during the FVP and significantly negatively correlated with HSI from stage Ⅳ to stage Ⅴ of the SVP. A significant difference was found between the FVP and SVP in the hepatopancreas. Notably, the hepatopancreas displayed a gradual degeneration trend during the SVP. The expression level of Sp-Vg was significantly higher in the hepatopancreas than that in the ovary during the FVP and SVP. Seventeen amino acids were detected in the hepatopancreas, ovary, and muscle during the FVP and SVP, with glutamate as the predominant amino acid. During the FVP and SVP, the C16:0 and C18:1n9c were the dominant fatty acids in the hepatopancreas and ovary, the MUFA gradually increased in the ovary and hepatopancreas, and a significant difference was found in the dynamic trend of the HUFA and SFA contents from stage Ⅳ to stage Ⅴ between the FVP and SVP. These findings indicate that the ovary can re-mature after spawning in S. paramamosain and can maintain the status of the first ovarian maturation; however, the hepatopancreas gradually degenerate during the SVP.

Molecular cloning, characterisation and expression analysis of the vitellogenin genes vtgAo1 and vtgC during ovarian development in Chinese hook snout carp Opsariichthys bidens

Vitellogenesis is essential for oocyte maturation. Vitellogenin (Vtg), a yolk precursor protein, plays an important role in oogenesis and vitellogenesis. Chinese hook snout carp Opsariichthys bidens is an economically important freshwater fish in China whose reproductive and developmental biology are not well understood. In this study, we undertook histological analysis to examine ovary development and oogenesis in O. bidens. The ovaries were divided into Stages II-V and oocytes were divided into perinuclear oocytes, cortical alveoli oocytes, vitellogenic oocytes and mature oocytes. Full-length cDNA sequences were cloned of two vtg genes from the liver of O. bidens, namely Ob-vtgAo1 and Ob-vtgC. Ob-vtgAo1 and Ob-vtgC cDNA are made up of 4136 and 4392 bases respectively and encode proteins containing 1335 and 1250 amino acids respectively. Ob-vtgAo1 contains three yolk protein domains: lipovitellin heavy chain (LvH), phosvitin (Pv) and lipovitellin light chain (LvL), whereas Ob-VtgC contains LvH and LvL, which are incomplete Vtgs. Ob-vtgAo1 and Ob-vtgC mRNA expression was significantly higher in the liver of O. bidens than in all other tissues. In oocytes of Stage II-III ovaries, yolk granules are almost absent and ovarian and hepatic Ob-vtgAo1 and Ob-vtgC expression is low. At Stage IV, the oocyte is filled with yolk granules and ovarian and hepatic Ob-vtgAo1 and Ob-vtgC expression is significantly increased. Collectively, these findings help us better understand vitellogenesis in O. bidens.

Vitamin A (retinoid) metabolism and actions: What we know and what we need to know about amphibians

Vitamin A status is an important consideration in the health of both wild and captive amphibians. Data concerning whole body vitamin A homeostasis in amphibians are scarce, although these animals have been used as experimental models to study the actions of vitamin A in vision, limb regeneration and embryogenesis. The available data suggest that many aspects of vitamin A biology in amphibians are similar to the canonical characteristics of vitamin A metabolism and actions established in mammals. This is consistent with the evolutionary conservation of these important biological processes. Amphibians must obtain vitamin A in their diet, with captive animals being prone to vitamin A deficiency. There is still much to be learned about vitamin A biology in amphibians that can only be achieved through rigorous scientific research. Improved understanding of amphibian vitamin A biology will aid the conservation of endangered amphibians in the wild, as well as the successful maintenance of ex situ populations.

Molecular cloning and mRNA expression of the vitellogenin and nuclear receptor gene induced by 17β-estradiol in the mud carp, Cirrhinus molitorella

Vitellogenins (VTGs), the precursors of yolk proteins, are crucial for the embryonic development of teleosts, and have also been studied extensively as biomarkers for environmental estrogenic mimics. The cDNA of a VTG was isolated from the liver of male mud carp (Cirrhinus molitorella) following induction by 17β-estradiol (E(2)) using 3'- and 5'-RACE methods. It was 4,216 bp in length, and encoded a putative protein of 1,342 amino acids. This putative VTG contained complete portions of lipovitellin I (LVI) and phosvitin, but lacked the C-terminal half of LVII and was thus classed as a type I vitellogenin. In addition, the partial cDNA of estrogen receptors (ERα and ERβ) and androgen receptors (AR) were cloned. The mRNA expression of VTG, ERα, ERβ and AR were examined in the liver of juvenile mud carp exposed to E(2) (0, 5, 50 and 500 ng/l) in the water for 2, 4, 6 and 8 days. Quantitative reverse transcription PCR (RT-PCR) revealed a significant increase in transcript levels of VTG and ERα but not of ERβ and AR. VTG and ERα were expressed in a time-dose-dependent manner. Our results imply that ERα may be the principal subtype that regulates VTG, and that VTG mRNA of mud carp is an ideal biomarker to detect environmental estrogens.

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.

Retinoid storage in the egg of reptiles and birds

Storage of retinal has been confirmed in eggs from a range of anamniotic vertebrates (teleosts and amphibians) and an ascidian, but the retinoid-storage state in eggs of oviparous amniotic vertebrates (reptiles and birds) has yet to be clarified in detail. We studied four reptilian and five avian species and found that retinal was commonly stored in their egg yolk. Furthermore, retinal was the major retinoid in reptilian eggs, with only low levels of retinol, whereas significant amounts of retinol as well as retinal were stored in avian eggs. In both reptilian and avian eggs, retinal was commonly bound to proteins, which were assumed to be homologous to the proteins that bind retinal in the eggs of anamniotic vertebrates. Despite the common storage state of retinal, retinol would be bound to different proteins. In the reptilian eggs, retinol was found in the yolk-granule fraction, which also contained retinal. However, retinol in avian eggs was found largely in the yolk-plasma fraction, separate from retinal. These results suggest that retinol storage in avian eggs acquired after the divergence of birds from the reptiles, while retinal storage was acquired before the appearance of the vertebrates, and has subsequently been conserved during evolution of oviparous vertebrates.

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

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

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

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

Dietary retinoic acid induces hindlimb and eye deformities in Xenopus laevis

This study investigated the effects of dietary retinoic acid (RA) on frog hindlimb development. Xenopus laevis (African clawed frog) tadpoles were fed a diet supplemented with 0, 1, 10, or 100 microg of RA/g of food for 2 or 5 d at different stages of metamorphosis. Hindlimb deformities were induced in the group fed 100 microg of RA/g of diet for 5 d. Exposures beginning at mid-hindlimb bud development induced bilaterally bent tibiafibula (bony triangles), while exposures later in hindlimb development induced deformities of the feet, including fusion of the 1st and 2nd clawed digits and reduced length of the 4th and 5th digits (due to reduced, missing, or misplaced phalanges). There were also cases of extra phalanges in the 5th digit. The eye was another target of RA exposure. In one experiment, 58% of the tadpoles fed 10 microg of RA/g had a smaller or absent right eye. Additionally, 11% of the tadpoles fed 100 microg of RA/g of diet developed a smaller or absent left eye. Waterborne heavy metals (Zn or Cu) modified RA effects on the hindlimb and eye. Co-exposure to metals and RA resulted in cases of unilateral bony triangles and reduced rates of smaller eyes. There were also cases of extra hindlimb digits in Zn-exposed animals. Dietary RA exposure in tadpoles can cause some deformities that differ from waterborne RA exposures in previous studies. RA also induced deformities that resemble those in affected wild frog populations (bony triangles), although the patterns of other deformities and missing segments (phalanges and metatarsals) are not similar to those documented in the wild.

Carotenoid and retinoid transport to fish oocytes and eggs: what is the role of retinol binding protein?

Fish eggs contain carotenoids, retinals (retinal and dehydroretinal) and retinols (retinol, dehydroretinol and retinyl-esters) that are utilized during embryonic development, after fertilization. The carotenoids (mainly astaxanthins) are transported in the plasma by the low density lipoproteins, high density lipoproteins, and very high density lipoproteins (VHDL) and were found to be associated also with serum albumin. Retinals were found to be associated vitellogenin (VTG), a component of the plasma VHDL fraction that is internalized by oocytes during vitellogenesis. However, the transport of retinols and retinyl-esters that were located in the oil droplet fraction of homogenized eggs, has yet to be elucidated. Retinols are more abundant in freshwater fish eggs than in eggs of marine fish species. Since retinol is transported in the plasma of vertebrates in association with retinol binding protein (RBP), recent studies on the molecular characterization and expression sites of RBP, could contribute to determining the involvement of RBP in transporting retinol to developing oocytes in vertebrates.Recently, results from our laboratory show that RBP mRNA levels in the liver and RBP plasma levels did not significantly change with the onset and during vitellogenesis in the Rainbow trout. These results were in contrast with a dramatic elevation in the mRNA levels of VTG in the liver and an increase in VTG plasma levels that was observed in the same females. Moreover, 17beta-estradiol treatment of immature fish, resulted in relatively lower mRNA levels of RBP in the liver, concomitantly with an increase in the level of VTG transcripts and the appearance of VTG in the plasma of treated fish. In addition, RBP was localized in the cytosol of ovulated oocytes. These results for Rainbow trout are similar to those reported for the chicken but differ from those of Xenopus, where an increase in RBP mRNA was reported in the liver and higher levels of retinal and retinol were found in the plasma of 17beta-estradiol treated animals. The results, reported here for the first time in Rainbow trout, showing RBP transcripts in the ovary, oviduct (the ovarian tissue adjacent to the gonopore) and oocytes, suggest a modulating role for RBP in follicular development, as has been suggested for the bovine ovary.

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

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

Retinoid composition and retinal localization in the eggs of teleost fishes

Retinoids in the eggs of four teleosts, chum salmon (Oncorhynchus keta), black porgy (Acanthopagrus schlegeli), marbled flounder (Pleuronectes yokohamae), and stingfish (Inimicus japonicus), were analyzed by high performance liquid chromatography. Retinal (RAL1) or both RAL1 and 3,4-didehydroretinal (RAL2) were major or exclusive retinoids in the eggs of every species examined. In O. keta eggs, both RAL1 and RAL2 were present at the ratio of approximately 3:4, whereas RAL1 was the only retinal in the eggs of the other three marine species. RAL1 was the exclusive retinoid in the eggs of P. yokohamae and I. japonicus, whose eggs lack lipid bodies. In the eggs of O. keta and A. schlegeli, which have lipid bodies, retinylesters were also detected, and retinals composed 69% and 93%, respectively, of total retinoids. In O. keta eggs, retinals were present mostly in the aqueous part and were bound to a protein homologous to lipovitellin 1, an amphibian yolk protein, and retinylesters were located in lipids. These results indicate that retinals are the essential mode of retinoid storage in eggs of teleosts and they are the precursors of functional retinoids, such as retinoic acid and visual pigment chromophores. Retinylesters are additional retinoids that accompany lipid accumulation.

Purification, partial characterization and peptide sequences of vitellogenin from a reptile, the tuatara (Sphenodon punctatus)

Vitellogenin (Vg), a major precursor to egg yolk proteins, was purified from plasma of an estradiol-treated female tuatara (Sphenodon punctatus) by MgCl2-EDTA precipitation and DEAE-cellulose chromatography. The amino acid composition of tuatara Vg is similar to that of other vertebtate Vgs and contains a large proportion of serine (13.7 mol/100 mol of total amino acid). The amino acid sequences of the N-terminus of mature Vg (33 residues) and of several trypsin- and CNBr-generated peptides were determined. Six peptide sequences obtained from tuatara Vg could be aligned with Vg sequences from other vertebrates. Reduced and non-reduced forms of tuatara Vg have the same apparent molecular mass (approximately 218 kDa) when resolved by SDS-PAGE, indicating that inter-chain disulfide bonds are not a feature of the molecule in this species. Western blot analysis with anti-tuatara Vg antiserum indicated that at least some epitopes are shared among Vgs of turtle, alligator and tuatara.