Immunochemical detection of the major vitelline envelope proteins in the plasma and oocytes of the maturing female rainbow trout, Oncorhynchus mykiss

Detection of endocrine disrupting chemicals and evidence of their effects on the HPG axis of the European anchovy Engraulis encrasicolus

Natural/synthetic Endocrine Disrupting Chemicals (EDCs) may display estrogenic activity and a lower potency than 17β-estradiol. Nonetheless, their concentrations and additive effects can affect the endocrine system and reproductive processes related to the Hypothalamic-Pituitary-Gonadal (HPG) axis. Because of their persistence in both the environment and biological systems, they ultimately target multi-level predators, including humans. We detected presence and effects of xenobiotics on wild anchovy Engraulis encrasicolus in the Western Adriatic Sea. Twenty-one PCBs and five organochlorines were detected on the order of ng g^-1; vitellogenin, vitellogenin receptor and genes encoding for the zona radiata proteins were evaluated in gonad and/or liver and found transcribed in male specimens; in addition, intersex was histologically identified in the 13% of testis. Our results have developed the understanding of the European anchovy's reproductive toxicological risk and our approach could assist the comprehension of the complex dynamics of commercially relevant Teleost species.

Vitellogenin, zona radiata protein, cathepsin D and heat shock protein 70 as biomarkers of exposure to xenobiotics

The antagonistic and/or synergistic effects of different chemical compounds were examined in the marine teleost, Gobius niger, by testing a series of biomarkers involved in fish reproduction. Among the biomarkers analysed, vitellogenin (VTG) and zona radiata proteins (ZRP) are key molecules involved in reproduction, widely used to detect the presence of pollutants in the marine environment, while heat shock protein 70 (HSP70) and cathepsin D (CATD) have recently been introduced as bioindicators of endocrine disruption. The detection of VTG and ZRP in the plasma of wild male specimens is universally accepted as an early warning signal of environmental pollution. The evaluation of VTG, ZRP and CATD expression demonstrated the oestrogenic effect of nonylphenol on both male and female fish; on the contrary beta-naphthoflavone behaves mainly as an anti-oestrogen although, when co-injected with compounds with oestrogenic activity, it enhances ZRP gene expression. Regarding the chaperone, all treatments stressed the fish, inducing an increase in HSP70 gene transcription. The results obtained underlined the importance of testing the effects of compound mixtures: fish in the wild are subjected to a blend of chemicals and the effects observed derive from the synergic or antagonistic interactions of these compounds.

Molecular Characterization and Expression Pattern of Zona Pellucida Proteins in Gilthead Seabream (Sparus aurata)1

The developing oocyte is surrounded by an acellular envelope that is composed of 2-4 isoforms of zona pellucida (ZP) proteins. The ZP proteins comprise the ZP1, ZP2, ZP3, and ZPX isoforms. While ZP1 (ZPB) and ZP3 (ZPC) are present in all species, ZP2 (ZPA) is not found in teleost fish and ZPX is not found in mammals. In the present study, we identify and characterize the ZP1, ZP3 and ZPX isoforms of gilthead seabream. Furthermore, by analyzing the conserved domains, which include the external hydrophobic patch and the internal hydrophobic patch, we show that ZP2 and ZPX are closely related isoforms. ZP proteins are synthesized in either the liver or ovary of most teleosts. Only in rainbow trout has it been shown that zp3 has dual transcription sites. In gilthead seabream, all four mRNA isoforms are transcribed in both the liver and ovary, with zp1a, zp1b, and zp3 being highly expressed in the liver, and zpx being primarily expressed in the ovary. However, determination of the ZP proteins in plasma showed high levels of ZP1b, ZP3, and ZPX, with low or non-detectable levels of ZP1a. In similarity to other teleost ZPs, the hepatic transcription of all four ZP isoforms is under estrogenic control. Previously, we have shown that cortisol can potentiate estrogen-induced ZP synthesis in salmonids, and now we show that this is not the case in the gilthead seabream. The present study shows for the first time the endocrine regulation of a teleost ZPX isoform, and demonstrates the dual-organ transcriptional activities of all the ZP proteins in one species.

Mass spectrometric evidence that proteolytic processing of rainbow trout egg vitelline envelope proteins takes place on the egg

The rainbow trout egg vitelline envelope (VE) is constructed of three proteins, called VEalpha,VEbeta, and VEgamma, that are synthesized and secreted by the liver and transported in the bloodstream to the ovary, the site of VE assembly around eggs. All three proteins possess an N-terminal signal peptide, a zona pellucida domain, a consensus furin-like cleavage site (CFLCS) close to the C terminus, and a short propeptide downstream of the CFLCS. Proteolytic processing at the CFLCS results in loss of the short C-terminal propeptide from precursor proteins and enables incorporation of mature proteins into the VE. Here mass spectrometry (matrix-assisted laser desorption ionization time-of-flight-mass spectrometry and liquid chromatography-mass spectrometry with a micromass-quadrupole TOF hybrid mass and a QSTAR Pulsar i mass spectrometer) was employed with VE proteins isolated from rainbow trout eggs in a peptidomics-based approach to determine the following: 1) the C-terminal amino acid of mature, proteolytically processed VE proteins; 2) the cellular site of proteolytic processing at the CFLCS of VE precursor proteins; and 3) the relationship between proteolytic processing and limited covalent cross-linking of VE proteins. Peptides derived from the C-terminal region were found for all three VE proteins isolated from eggs, indicating that processing at the CFLCS occurs after the arrival of VE precursor proteins at the egg. Consistent with this conclusion, peptides containing an intact CFLCS were also found for all three VE proteins isolated from eggs. Furthermore, peptides derived from the C-terminal propeptides of VE protein heterodimers VEalpha-VEgamma and VEbeta-VEgamma were found, suggesting that a small amount of VE protein can be covalently cross-linked on eggs prior to proteolytic processing at the CFLCS. Collectively, these results provide important evidence about the process of VE formation in rainbow trout and other non-cyprinoid fish and allow comparisons to be made with the process of zona pellucida formation in mammals.

Structural characterization of fish egg vitelline envelope proteins by mass spectrometry

The extracellular coat, or vitelline envelope (VE), of rainbow trout (Oncorhynchus mykiss) eggs consists of three proteins, called VEalpha (M(r) approximately 52 kDa), VEbeta (M(r) approximately 48 kDa), and VEgamma (M(r) approximately 44 kDa). Each of these proteins is related to mammalian egg zona pellucida (ZP) glycoproteins ZP1-3 and possesses an N-terminal signal sequence, a ZP domain, and a protease cleavage site near the C-terminus. VEalpha and VEbeta also have a trefoil domain. All three proteins possess a relatively large number of cysteine residues (VEalpha, 18; VEbeta, 18; VEgamma, 12), of which 8 are present in the ZP domain and 6 are present in the trefoil domain of VEalpha and VEbeta. Here, several types of mass spectrometry were employed, together with gel electrophoresis of chemical and enzymatic digests, to identify intramolecular disulfide linkages, as well as the N- and C-terminal amino acids of VEalpha, VEbeta, and VEgamma. Additionally, these methods were used to characterize two high molecular weight proteins (HMWPs; M(r) > 110 kDa) of rainbow trout VEs that are heterodimers of individual VE proteins. These analyses have permitted assignment of disulfide linkages and identification of N- and C-terminal amino acids for the VE proteins and determination of the protein composition of two forms of HMWPs. These experiments provide important structural information about fish egg VE proteins and filaments and about structural relationships between extracellular coat proteins of mammalian and nonmammalian eggs.

A duplicated motif controls assembly of zona pellucida domain proteins

Many secreted eukaryotic glycoproteins that play fundamental roles in development, hearing, immunity, and cancer polymerize into filaments and extracellular matrices through zona pellucida (ZP) domains. ZP domain proteins are synthesized as precursors containing C-terminal propeptides that are cleaved at conserved sites. However, the consequences of this processing and the mechanism by which nascent proteins assemble are unclear. By microinjection of mutated DNA constructs into growing oocytes and mammalian cell transfection, we have identified a conserved duplicated motif [EHP (external hydrophobic patch)/IHP (internal hydrophobic patch)] regulating the assembly of mouse ZP proteins. Whereas the transmembrane domain (TMD) of ZP3 can be functionally replaced by an unrelated TMD, mutations in either EHP or IHP do not hinder secretion of full-length ZP3 but completely abolish its assembly. Because mutants truncated before the TMD are not processed, we conclude that the conserved TMD of mammalian ZP proteins does not engage them in specific interactions but is essential for C-terminal processing. Cleavage of ZP precursors results in loss of the EHP, thereby activating secreted polypeptides to assemble by using the IHP within the ZP domain. Taken together, these findings suggest a general mechanism for assembly of ZP domain proteins.

Regulation of Arctic char (Salvelinus alpinus) egg shell proteins and vitellogenin during reproduction and in response to 17beta-estradiol and cortisol

Estrogens induce both vitellogenin (Vtg) and egg shell (zona pellucida; ZP) protein synthesis in salmonids. However, while Vtg is strictly under estrogenic control, recent reports suggest that additional mechanisms are involved in ZP protein synthesis. During sexual maturation both estrogen and glucocorticoid levels increase in the circulation of female fish. As glucocorticoids have been shown to interfere with Vtg induction in fish we investigated whether cortisol (F) had similar effects on ZP regulation. In the present study we determined both the natural variation in Vtg and ZP during an annual reproductive cycle in female Arctic char (Salvelinus alpinus), and the effect of co-treatment of juvenile Arctic char with 17beta-estradiol (E2) and F. During sexual maturation the expression of Vtg and ZP correlated to plasma levels of E2 and F. Determination of Vtg and ZP protein levels following co-treatment with E2 and F showed that F antagonized E2 induction of Vtg. However, F was observed to potentiate the expression of ZP protein in the same fish. These results indicate that in Arctic char Vtg and ZP proteins are not regulated by the same mechanisms and suggest that ZP protein expression does not necessarily imply exposure to estrogenic compounds alone, and may thus not be ideally suited as a biomarker of exposure to estrogenic compounds.

Choriogenin mRNA induction in male medaka, Oryzias latipes as a biomarker of endocrine disruption

The fish egg envelope has been known to consist of two distinct layers. The outer layer is thin and is formed around the oocytes during the later stage of previtellogenic development. The inner layer, called zona radiata, is thicker than outer layer and occupies most of the egg envelope. Zona radiata proteins of medaka (Oryzias latipes) consist of two major subunit groups, ZI-1,2 and ZI-3. The precursors of ZI-1,2 and ZI-3 have been named choriogenin H and choriogenin L, respectively. They are synthesized in the liver in response to estrogen, and then released into the blood stream and incorporated into the zona radiata in sexually matured female medaka. However, choriogenin is also induced in male medaka when the fish are exposed to estrogenic chemicals. Choriogenin can be suggested as a possible biomarker of endocrine disruption in fish, however, relatively little has been known about chorogenin gene expression by estrogenic chemicals. In this study, full sequence of choriogenin L DNA was identified, and measurement of choriogenin mRNA induction was established in medaka by use of RT-PCR technique. Also estrogenic effects of industrial chemicals were evaluated by this tool. When mature male medaka was treated with bisphenol A, nonylphenol, and 17alpha-ethinylestradiol for 6 days, respectively, the expression levels of choriogenin L and H mRNA were elevated in treated groups. The induction of choriogenin subunits expression by these chemicals showed a dose-dependent pattern and choriogenin L was found to be more sensitive than choriogenin H.

Seasonal variations of vitelline envelope proteins, vitellogenin, and sex steroids in male and female eelpout (Zoarces viviparus)

The seasonal variations of vitelline envelope proteins, vitellogenin (VTG), and reproductive steroids were investigated in feral male and female eelpout, Zoarces viviparus. 17beta-Estradiol was present in both sexes with a peak in prespawning fish of 2.6 ng/ml in males and 2.7 ng/ml in females. 11-Ketotestosterone peaked in June at 4.2 and 0.47 ng/ml in males and females, respectively. A surge of testosterone was seen in both sexes in August, just prior to spawning. All steroid levels were low during early pregnancy. The vitelline envelope of the eelpout is composed of two major and one minor protein with molecular weights of 50, 55, and 44 kDa, respectively. An antiserum raised against solubilized vitelline envelope from turbot (Scophthalmus maximus) cross-reacted strongly with the 50-kDa protein from the isolated vitelline envelope and a similar-sized protein in female plasma and plasma from estrogenized males. Interestingly, the 50-kDa protein was also present at low levels in males as demonstrated by ELISA and Western blotting. In males, the 50-kDa protein did not follow the seasonal changes in 17beta-estradiol, but instead showed an almost perfect negative correlation with water temperature. VTG was present in female plasma as shown by Western blotting, but VTG was not detectable in male plasma despite relatively high endogenous estrogen levels. This suggests that the VTG induction by estradiol may be modulated by other factors in the eelpout.

Cloning of rainbow trout egg envelope proteins: members of a unique group of structural proteins

All vertebrate eggs are surrounded by an extracellular envelope that protects the egg and is vital for a successful fertilization. The terminology and functions of the egg envelope vary in different vertebrate groups, but the envelope itself is consistently composed of a few major proteins that are deposited around the oocyte during oocyte growth. Here, we describe the deduced amino acid sequences and tissue expression patterns of the three major egg envelope proteins for rainbow trout (Oncorhynchus mykiss). All three vitelline envelope proteins (VEPs) are expressed in the livers of both male and female fish, with higher expression in females. In addition, VEPgamma mRNA is also detected in the female gonads. To our knowledge, this is the first time that expression of a VEP protein gene has been demonstrated to occur in more than one organ. Sequence comparison reveals that all three VEP proteins share distinct homology with their amphibian, avian, and mammalian counterparts. Whereas mammalian zona pellucida protein 3 isoforms contain two conserved serines needed for sperm binding, these are not conserved in teleost species, in which sperm entry is restricted to the micropyle. Besides the difference in VEPgamma sperm-binding function, the high sequence homology suggests that the egg envelope proteins from these distinct vertebrate groups share a common ancestry and form a unique group of structural proteins.

Vitellogenin-induced pathology in male summer flounder (Paralichthys dentatus)

Male summer flounder (Paralichthys dentatus) were given two injections (initially and 2 weeks later) of 17beta-estradiol (E2) totaling 0.2 (2 x 0.1), 2.0 (2 x 1.0) or 20.0 (2 x 10.0) mg E2/kg body weight. Blood and tissue samples were collected 4, 6 and 8 weeks after the initial injection in the (2 x 0.1) mg/kg treatment, 4, 6, 8, and 15 weeks after the first injection in the (2 x 1.0) mg/kg treatment and at 4 weeks only in the (2 x 10.0) mg/kg treatment. Five of the 12 fish injected twice with 10.0 mg/kg were moribund before the first sampling period. Circulating levels of vitellogenin (VTG) in the blood of all E2-injected fish from all treatments were comparable with those concentrations found in the blood of wild male carp (Cyprinus carpio) and walleye (Stezostedion vitreum) previously collected near a sewage treatment plant (0.1-10.0 mg VTG/ml plasma). Excessive hyalin material accumulated in the livers, kidneys and testes of the treated fish. A portion of that material was identified as VTG by immunohistochemistry. The accumulation of VTG, and possibly other estrogen-inducible proteins, resulted in hepatocyte hypertrophy, disruption of spermatogenesis, and obstruction or rupture of renal glomeruli.

A monoclonal antibody against chorion proteins of the sea bass Dicentrarchus labrax (Linnaeus, 1758): studies of chorion precursors and applicability in immunoassays

The monoclonal antibody DLE7 was obtained against 44- to 50-kDa polypeptides solubilized from the vitelline envelope of the Mediterranean sea bass Dicentrarchus labrax. In Western blot analysis of chorion lysates it recognized cross-reactive bands at 44 kDa, 48 kDa, and 110 kDa. Previous affinity blotting with concanavalin-A showed that most of solubilized bands were glycosylated. Enzymatic deglycosylation of chorion proteins followed by Western blot analysis with DLE7 showed that the 48-kDa and 110-kDa antigens were differentially affected by endoglycosidase-F treatment. When DLE7 was employed in immunofluorescence analysis, isolated chorions and ovarian cryosections stained intensely. Positivity was also observed in liver cryosections of spawning females but not in liver of males and nonspawning females. To study the origin and delivery of chorion proteins, DLE7 was used in Western blot analysis of liver homogenates and blood serum of spawning females. Cross-reacting bands were detected in liver (90 kDa) and serum (180 kDa, 50 kDa). DLE7 was also used for the first time to set up an indirect ELISA assay to detect egg antigens in the blood of egg-producing females, raising the possibility of using DLE7 as a female-specific marker of spawning for sea bass.

Differential sensitivity of zonagenesis and vitellogenesis in Atlantic salmon (Salmo salar L) to DDT pesticides

In Atlantic salmon (Salmo salar L) female sexual maturation entails both zonagenesis and vitellogenesis, both of which are controlled by increasing levels of estradiol-17 beta (E2). Antibodies against salmon zona radiata proteins (eggshell zr-proteins) and vitellogenin were used to monitor induction of oogenesis in juvenile salmon. Molecular weights of zr-monomers were estimated to about 66, 61, and 55 kDa, and to about 180 kDa for vitellogenin. Xenobiotics such as the pesticide DDT impair biological reproduction. The o,p'-DDT (1,1,1-trichloro-2[2-chlorophenyl]-2-[4-chlorophenyl]ethane) isomer seems to be a xenoestrogen. Serum levels of zr-proteins and vitellogenin, and hepatocytic biosynthesis of these components, were determined after in vivo treatment of salmon with DDT (technical, p,p'-(1,1,1-trichloro-2,2-bis[4-chlorophenyl]ethane) or o,p'-DDT) or E2. Exposing fish to frequent doses of o,p'-DDT (25 mg/kg b.w. (body weight) twice a week, six times totally) resulted in induction of all three zr-protein monomers, but not of vitellogenin. In contrast, three weekly injection of 10 mg/kg b.w. of either of the three DDT preparations did not induce typical zr-proteins or vitellogenin in serum. In vivo studies with combined DDT + E2 injections showed that none of the DDT preparations influenced E2-induced biosynthesis of zr-proteins or vitellogenin. E2 induction of these oogenetic processes was not blocked even by a high concentration (125 mg/kg b.w.) of o,p'-DDT. Furthermore, pretreatment of salmon with o,p'-DDT for 2 weeks, followed by one injection of E2, did not antagonize biosynthesis of zr-proteins, but serum concentration of vitellogenin was decreased. The data indicate that in juvenile salmon o,p'-DDT may be xenoestrogenic with regard to zonagenesis, but weakly anti-(xeno)estrogenic with regard to vitellogenesis. These findings suggest new complexities in fish reproductive toxicology of xenoestrogens. Compared to vitellogenesis, zonagenesis is a more sensitive parameter for monitoring reproductive effects of xenoestrogens.

Cloning of cDNA and estrogen-induced hepatic gene expression for choriogenin H, a precursor protein of the fish egg envelope (chorion)

A cDNA for choriogenin H (Chg H; formerly high-molecular weight spawning female-specific substances, or H-SF), a precursor protein of the inner layer subunits of egg envelope (chorion) of the teleost fish, Oryzias latipes, was cloned and analyzed. The clone consisted of 1913 bp and contained an open reading frame encoding a signal peptide of 22 aa and Chg H protein of 569 aa. The Chg protein possessed three potential N-glycosylation sites and Pro-X-Y repeat sequences in the first two-fifths of the N terminus. There were amino acid sequence similarities between Chg H and a gene product expressed in the liver of female winter flounder during vitellogenesis. Moreover, the amino acid sequence of Chg H is similar to that of ZP2 rather than ZP3 of zona pellucida of some mammals. Northern blot analysis indicated that gene expression for Chg H occurred only in the livers of spawning female fish and 17beta-estradiol-treated male fish, but not in the ovary of the spawning female fish. Gene expression for Chg H and Chg L (formerly low-molecular weight spawning female-specific substance, or L-SF) was induced and increased in parallel in the male fish liver after 17beta-estradiol treatment.

Amino acid composition and endocrine control of vitelline envelope proteins in European sea bass (Dicentrarchus labrax) and gilthead sea bream (Sparus aurata)

The vitelline envelopes of European sea bass and gilthead sea bream are both composed of mainly four proteins with the molecular masses of 90, 52, 48, 45 kDa and 75, 50, 48, 44 kDa, respectively. Each protein has an amino acid composition that is characterized by a high content of proline and glutamic acid and a low content of cysteine, similar to the whole vitelline envelope of both species. The amino acid composition suggests that each protein is distinct but related to the other vitelline envelope proteins. The use of homologous antisera shows that both species have vitelline envelope proteins that are induced by estradiol-17 beta. As males of both species synthesize these proteins after treatment with estradiol-17 beta, the origin is not restricted to the ovaries. Vitellogenin of both European sea bass and gilthead sea bream has the apparent molecular mass of 170 kDa.

Formation of the vitelline envelope precedes the active uptake of vitellogenin during oocyte development in the rainbow trout, Oncorhynchus mykiss

In order to study the initial formation of the vitelline envelope and the appearance of vitellogenin in oocytes of rainbow trout, females were sampled monthly from 19 to 5 mo before ovulation. Immunohistochemistry revealed that the formation of the vitelline envelope starts when the oocytes reach a diameter of about 450 microns. Oocytes of this size were first found in females sampled a year before ovulation at the time when plasma levels of estradiol-17 beta increased from 0.2 to 0.6 ng/ml. An antiserum directed against vitellogenin crossreacted with small vesicles (around 2 microns) present just inside the oolemma, when the oocytes reached a diameter of 600 microns. This was interpreted as an active uptake of vitellogenin. Oocytes of this size were first found in females sampled 9 mo before ovulation at the time when estradiol-17 beta levels increased from 0.6 to 1.0 ng/ml and the gonadal somatic index was doubled. Oocytes with a diameter of 600 microns had an immunoreactive vitelline envelope with a thickness of about 3 microns. It is apparent that the initial formation of the vitelline envelope starts before the active uptake of vitellogenin and that the low previtellogenic plasma levels of estradiol-17 beta observed in females are of physiological significance.

Isolation of H-SF substances, the high-molecular-weight precursors of egg envelope proteins, from the ascites accumulated in the oestrogen-treated fish, Oryzias latipes

High- and low-molecular weight spawning-female-specific (H-SF and L-SF) substances are liver-derived putative precursors of the inner layer of egg envelope of the medaka Oryzias latipes, and are also found in the ascites which are accumulated in the male fish on prolonged treatment with oestrogen. In the present study, the H-SF substances purified from the ascites of the oestrogen-treated male medaka were found to consist of three proteins. Their molecular weights ranged from 74,000 to 76,000, which corresponds to those of ZI-1 and ZI-2, the major components of medaka egg envelope. The amino acid composition of the H-SF substances was characterised by high contents of proline (17.58 mol%) and glutamic acid/glutamine (13.34 mol%). The characteristics were almost identical to those of ZI-1 and ZI-2 but differed from those of L-SF substance, a precursor of ZI-3, another major component of the egg envelope. These results confirm our view that H-SF substances are the precursors of ZI-1 and ZI-2. The average amino acid compositions of H-SF substances and L-SF substance coincided with the amino acid composition of whole egg envelope. This result strongly suggests that an equal amount of H-SF substances and L-SF substance would construct the inner layer of egg envelope after a little modification.