Genomic disturbance of vitellogenin 2 (vtg2) leads to vitellin membrane deficiencies and significant mortalities at early stages of embryonic development in zebrafish (Danio rerio)

The specific functions and essentiality of type II vitellogenin (Vtg2) in early zebrafish development were investigated in this study. A vtg2-mutant zebrafish line was produced and effects of genomic disturbance were observed in F2 females and F3 offspring. No change in vtg2 transcript has been detected, however, Vtg2 abundance in F2 female liver was 5×, and in 1 hpf F3 vtg2-mutant embryos was 3.8× less than Wt (p < 0.05). Fecundity was unaffected while fertilization rate was more than halved in F2 vtg2-mutant females (p < 0.05). Hatching rate was significantly higher in F3 vtg2-mutant embryos in comparison to Wt embryos. Survival rate declined drastically to 29% and 18% at 24 hpf and 20 dpf, respectively, in F3 vtg2-mutant embryos. The introduced mutation caused vitelline membrane deficiencies, significant mortalities at early embryonic stages, and morphological abnormalities in the surviving F3 vtg2-mutant larvae. Overrepresentation of histones, zona pellucida proteins, lectins, and protein degradation related proteins in F3 vtg2-mutant embryos provide evidence to impaired mechanisms involved in vitellin membrane formation. Overall findings imply a potential function of Vtg2 in acquisition of vitellin membrane integrity, among other reproductive functions, and therefore, its essentiality in early zebrafish embryo development.

Developmental RNA-Seq transcriptomics of haploid germ cells and spermatozoa uncovers novel pathways associated with teleost spermiogenesis

In non-mammalian vertebrates, the molecular mechanisms involved in the transformation of haploid germ cells (HGCs) into spermatozoa (spermiogenesis) are largely unknown. Here, we investigated this process in the marine teleost gilthead seabream (Sparus aurata) through the examination of the changes in the transcriptome between cell-sorted HGCs and ejaculated sperm (SPZEJ). Samples were collected under strict quality controls employing immunofluorescence microscopy as well as by determining the sperm motion kinematic parameters by computer-assisted sperm analysis. Deep sequencing by RNA-seq identified a total of 7286 differentially expressed genes (DEGs) (p-value < 0.01) between both cell types, of which nearly half were upregulated in SPZEJ compared to HCGs. In addition, approximately 9000 long non-coding RNAs (lncRNAs) were found, of which 56% were accumulated or emerged de novo in SPZEJ. The upregulated transcripts are involved in transcriptional and translational regulation, chromatin and cytoskeleton organization, metabolic processes such as glycolysis and oxidative phosphorylation, and also include a number of ion and water channels, exchangers, transporters and receptors. Pathway analysis conducted on DEGs identified 37 different signaling pathways enriched in SPZEJ, including 13 receptor pathways, from which the most predominant correspond to the chemokine and cytokine, gonadotropin-releasing hormone receptor and platelet derived growth factor signaling pathways. Our data provide new insight into the mRNA and lncRNA cargos of teleost spermatozoa and uncover the possible involvement of novel endocrine mechanisms during the differentiation and maturation of spermatozoa.

Medaka, Oryzias latipes, egg envelopes are created by ovarian-expressed ZP proteins and liver-expressed choriogenins

The medaka (Oryzias latipes) egg envelope (chorion) is composed of three major glycoproteins, Zona Interna (ZI)-1, -2, and -3, that originate in the spawning female liver as the precursor proteins Choriogenin (Chg.)H, Chg.Hm, and Chg.L, respectively. These ZI and Chg. proteins contain a structural ZP protein domain that is conserved among the egg envelope proteins of all animals. While ovarian expression of ZP proteins (e.g., ZPCs and ZPB) has been reported in medakas, the functions of these proteins remain unknown. Thus, the present study aimed to determine whether the ovary-expressed medaka ZP protein, mZPC5, is involved in forming the chorion matrix.The mZPC5 gene (mzpc5) was expressed in the ovaries but not the livers of mature female medakas, as shown by reverse transcription-polymerase chain reaction assays with mzpc5-specific primers. In situ hybridization analysis revealed that ovarian mzpc5 expression was restricted to the ooplasm of early (stage I-III) previtellogenic oocytes, and its expression signal weakened with oocyte growth. Following sodium-dodecyl-sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting analysis with anti-mZPC5 antibodies, two immunoreactive proteins were detected in the ovary and chorion extracts. These proteins were approximately 50 and 74 kDa in size, like ZI-3 and ZI-2, respectively.Immunohistochemical assays using anti-mZPC5 and anti-Chg.H antibodies localized the mZPC5 protein in the ooplasm of early previtellogenic oocytes. With oocyte growth, mZPC5 tended to accumulate in the chorion, co-localizing with Chg.H.We previously showed that ovary-expressed ZP proteins could not compensate for Chg.L function loss in gene knock-out (chg.l -/-) medakas. As in our previous study, the chg.l-/- females produced oocytes with thin chorions, resulting in infertile soft eggs. However, in the present study, mZPC5 and Chg.H were co-localized in the chg.l-/- chorions. These results suggested that in the medaka previtellogenic oocyte, 1) mZPC5 is secreted from the ooplasm and deposited on the outer surface of its plasma membrane, creating the thin chorion layer; and 2) following the accumulation of liver-derived Chgs., the 3D structure of the chorion matrix is formed cooperatively with mZPC5 and Chgs. during oogenesis. More research is needed to confirm the functions of mZPC5 in chorion structure and physiology.

Cellular and molecular modification of egg envelope hardening in fertilization

Fertilization is an essential process that fundamentally impacts fitness. An egg changes dramatically after fertilization mediating the beginning of life, which mainly includes the transformation of the egg envelope via hardening, which is thought to be due to complex reactions involved in the conversion of cellular and molecular. This review highlights the mechanisms of egg envelope hardening in teleost fish. We conclude that the egg envelope hardening might be carried out in two steps. (a) A metalloprotease (alveolin) hydrolyzes the N-terminal proline-glutamine (Pro-Gln) region of zona pellucida (ZP) 1 and (b) triggers intermolecular cross-linking to ZP3 catalyzed by transglutaminase (TGase). The post-fertilization hardening of the egg envelope is an evolutionarily conserved phenomenon across species. We discuss the biochemical function and interaction of some factors reported to be essential to egg envelope hardening in mammalian and nonmammalian species, including metalloprotease, TGase, peroxidase/ovoperoxidase, and other factors (carbohydrate moieties, zinc and Larp6 proteins), and the relevant data suggest that egg envelope hardening is crucial to block polyspermy in internal fertilization, in addition to protecting the developing embryo from mechanical shock and preventing bacterial infection in external fertilization. Increased knowledge of the processes of egg envelope hardening and fertilization is likely to make a remarkable contribution to reproduction and aquaculture.

Delineation of metals and radionuclides bioconcentration in eggs of seabream Sparus aurata and effect of environmental pCO2

Considered as the most vulnerable ontogenic stages to environmental stressors, the early-life stages of fish paid a peculiar attention with respect to their vulnerability to metal and radionuclides contamination. Concomitantly, the increasing anthropogenic CO₂ release in the atmosphere will cause major change of the seawater chemistry that could affect the trace elements and radionuclides bioconcentration efficiencies by marine organisms. The aim of this work was to 1) delineate the uptake behaviours of Ag, Am, Cd, Co and Zn in seabream eggs during 65 h of development and retention by newly hatched and 7 h-old larvae maintained in clean seawater, respectively, and 2) investigate the effects of elevated pCO₂ on the bioconcentration efficiencies of these elements in eggs. Besides differing in terms of maximal concentration factors values, the uptake kinetics showed element-specific patterns with Am being linearly bioconcentrated and Co and Zn showing a saturation state equilibrium. The ^110mAg and ¹⁰⁹Cd uptake kinetics shared a two-phases pattern being best described by a saturation equation during the first 24 h of development, and then an exponential loss of accumulated elements although the radiotracer concentrations in the surrounding water remained constant. At hatching time, the radioactivity of ^110mAg was the highest among radiotracers detected in the larvae. After 7 h in depuration conditions, 60% of this metal was still detected whereas ²⁴¹Am, ⁶⁰Co and ⁶⁵Zn were almost totally lost, suggesting an efficient incorporation of Ag in the embryo during the egg development. Finally, this study brought first qualitative data on the effect of pCO₂/pH on metal bioconcentration in eggs, raising the need to unravel chemical and biological processes to predict a potential shift of the toxicity of environmental contamination of fish early life stages with future ocean change.

Bioinformatic analyses of zona pellucida genes in vertebrates and their expression in Nile tilapia

Zona pellucida (ZP) genes encode ZP glycoproteins which constitute the coat surrounding oocytes and early embryos. Genome-wide identification of ZP genes is still lacking in vertebrates, especially in fish species. Herein, we conducted bioinformatic analyses of the ZP genes of the Nile tilapia and other vertebrates. Totally 16, 9, 17, 27, 21, 20, 26, 19, 14,11, 24, 17, 9, 18, 8, 11, 9, 8, 5, and 4 ZP genes belonging to 5 subfamilies (ZPA, ZPB, ZPC, ZPD, and ZPAX) were found in the sea lamprey, elephant shark, coelacanth, spotted gar, zebrafish, medaka, stickleback, Nile tilapia, Amazon molly, platyfish, seahorse, Northern snakehead, cavefish, tetraodon, clawed frog, turtle, chicken, platypus, kangaroo rat, and human genomes, respectively. The expansion of ZP genes in basal vertebrates was mainly achieved by gene duplication of ZPB, ZPC, and ZPAX subfamilies, while the shrink of ZP gene number in viviparous mammals was achieved by keeping only one copy of the ZP genes in each subfamily or even secondary loss of some subfamilies. The number of ZP gene is related to the environment where the eggs are fertilized and the embryos develop in vertebrates. Transcriptomic analysis showed that 14 ZP genes were expressed in the ovary of Nile tilapia, while two (ZPB2b and ZPC2) were highly expressed in the liver. On the other hand, ZPB1a and ZPB2c were not found to be expressed in any tissue or at any developmental stage of the gonads examined. In the ovary, the expression of ZP genes started from 30 dah (days after hatching), significantly upregulated at 90 dah and maintained this level at 180 dah. The expression of ZPC2 in the liver and ZPC5-2 and ZPAX1 in the ovary was confirmed by in situ hybridization. The ovary- and liver-expressed ZP genes are expressed coordinately with oocyte growth in tilapia.

Four glycoproteins are expressed in the cat zona pellucida

The mammalian oocyte is surrounded by a matrix called the zona pellucida (ZP). This envelope participates in processes such as acrosome reaction induction, sperm binding and may be involved in speciation. In cat (Felis catus), this matrix is composed of at least three glycoproteins called ZP2, ZP3, and ZP4. However, recent studies have pointed to the presence of a fourth protein in several mammals (rat, human, hamster or rabbit), meaning that a reevaluation of cat ZP is needed. For this reason, the objective of this research was to analyze the protein composition of cat ZP by means of proteomic analysis. Using ZP from ovaries and oocytes, several peptides corresponding to four proteins were detected, yielding a coverage of 33.17%, 71.50%, 50.23%, and 49.64% for ZP1, ZP2, ZP3, and ZP4, respectively. Moreover, the expression of four genes was confirmed by molecular analysis. Using total RNA isolated from cat ovaries, the complementary deoxyribonucleic acids encoding cat ZP were partially amplified by reverse-transcribed polymerase chain reaction. Furthermore, ZP1 was totally amplified for the first time in this species. As far as we are aware, this is the first study that confirms the presence of four proteins in cat ZP.

A developmental hepatotoxicity study of dietary bisphenol A in Sparus aurata juveniles

Previous studies in rats have indicated that a diet enriched with Bisphenol A adversely effects metabolism and reproductive success. In rats exposed to BPA by maternal gavage, alteration in the developmental programming, higher obesity rates and reproductive anomalies were induced. Starting with this evidence, the aim of this study was to provide important insights on the effects induced by a BPA enriched diet, on the reproductive physiology and metabolism of juvenile fish, simulating the scenario occurring when wild fish fed on prey contaminated with environmental BPA. Seabream was chosen as model, as it is one of the primary commercial species valued by consumers and these results could provide important findings on adverse effects that could be passed on to humans by eating contaminated fish. A novel method for measuring BPA in the food and water by affinity chromatography was developed. Analysis of signals involved in reproduction uncovered altered levels of vtg and Zp, clearly indicating the estrogenic effect of BPA. Similarly, BPA up-regulated catd and era gene expression. A noteworthy outcome from this study was the full length cloning of two vtg encoding proteins, namely vtgA and vtgB, which are differently modulated by BPA. Cyp1a1 and EROD activity were significantly downregulated, confirming the ability of estrogenic compounds to inhibit the detoxification process. GST activity was unaffected by BPA contamination, while CAT activity was down regulated. These results collectively confirm the estrogenic effect of BPA and provide additional characterization of novel vtg genes in Sparus aurata.

Expression of zona pellucida B proteins in juvenile rare minnow (Gobiocypris rarus) exposed to 17α-ethinylestradiol, 4-nonylphenol and bisphenol A

Zona pellucida (ZP) containing proteins are glycoproteins in teleost chorion and are encoded by several gene subfamilies, mainly including ZPA, ZPB, ZPC and ZPX genes. In teleost species, ZP genes are expressed either in liver under regulation of estrogen or in ovary. In the present study, five ZP gene isoforms were isolated and characterized in Gobiocypris rarus. The putative amino acid sequences of these ZP gene isoforms contain the typical trefoil motif and a ZP domain. These five G. rarus ZP gene isoforms were named as grZPB.1, grZPB.2, grZPB.3, grZPB.4 and grZPB.5. Real-time quantitative reverse transcription PCR (RT-qPCR) analysis indicated that all these ZP mRNA isoforms were exclusively expressed in ovary. G. rarus juveniles at the age of 21 days postfertilization were exposed to 17α-ethinylestradiol (EE2; 0.01, 0.1 and 1 nM), 4-nonylphenol (4-NP; 10, 100 and 1000 nM) or bisphenol A (BPA; 0.1, 1 and 10nM) for 3 days. mRNA expressions of ZPB isoforms following the exposure to xenoestrogen were detected by RT-qPCR. Data were analyzed by the 2(-△△Cq) method. The results indicate that induction by 0.1-1nM EE2 on mRNA expression of the grZPB isoforms is weaker than for vitellogenin. 4-NP exposures at three concentrations had differential effects on the grZPBs. BPA at three concentrations weakly induced mRNA expression of the grZPB isoforms.

Evolution of the teleostean zona pellucida gene inferred from the egg envelope protein genes of the Japanese eel, Anguilla japonica

A fish egg envelope is composed of several glycoproteins, called zona pellucida (ZP) proteins, which are conserved among vertebrate species. Euteleost fishes synthesize ZP proteins in the liver, while otocephalans synthesize them in the growing oocyte. We investigated ZP proteins of the Japanese eel, Anguilla japonica, belonging to Elopomorpha, which diverged earlier than Euteleostei and Otocephala. Five major components of the egg envelope were purified and their partial amino acid sequences were determined by sequencing. cDNA cloning revealed that the eel egg envelope was composed of four ZPC homologues and one ZPB homologue. Four of the five eel ZP (eZP) proteins possessed a transmembrane domain, which is not found in the ZP proteins of Euteleostei and Otocephala that diverged later, but is found in most other vertebrate ZP proteins. This result suggests that fish ZP proteins originally possessed a transmembrane domain and lost it during evolution. Northern blotting and RT-PCR revealed that all of the eZP transcripts were present in the ovary, but not in the liver. Phylogenetic analyses of fish zp genes showed that ezps formed a group with other fish zp genes that are expressed in the ovary, and which are distinct from the group of genes expressed in the liver. Our results support the hypothesis that fish ZP proteins were originally synthesized in the ovary, and then the site of synthesis was switched to the liver during the evolutionary pathway to Euteleostei.