New methods for the purification of vertebrate vitellogenin

Characterization, specificity and sensibility of produced anti-Rhamdia quelen vitellogenin in Brazilian fish species

Endocrine-disrupting chemicals (EDCs) are widespread used and can interfere on hormone regulation with adverse consequences for both biota and human. Vitellogenin (vtg) is a yolk precursor protein synthesized by the liver in response to estrogen. In order to characterize the vtg of tropical fish Rhamdia quelen and establish a molecular biomarker, adult male individuals were exposed to 17-β-estradiol (E₂) for vtg induction and anti-R. quelen vtg polyclonal antibodies production. Vitellogenic female fish were used as positive control group. E₂-induced vtg was characterized as a glycolipophosphoprotein of high molecular mass with peptide mass fingerprint very similar in E₂-exposed male and vitellogenic female fish. A polyclonal serum containing anti-R. quelen vtg antibodies was produced and showed high specificity and sensibility to detect the vtg of three fish species: R. quelen, Piaractus mesopotamicus and Prochilodus lineatus. Wildlife and laboratory studies reported that EDCs released into the environment may alter the levels of plasma vtg in male fish, making this protein a valuable biomarker of xenoestrogens exposure. Then, we propose the use of anti-R. quelen vtg as a tool for biomonitoring studies and water quality assessment in Brazil and South American countries where the three fish species occur.

Vitellogenin of the northern leopard frog (Rana pipiens): development of an ELISA assay and evaluation of induction after immersion in xenobiotic estrogens

An immunoassay for leopard frog (Rana pipiens) vitellogenin was developed for studying endocrine disruption. Male frogs were injected with estradiol-17β to stimulate vitellogenin for purification. SDS-PAGE revealed high amounts of a 170-180 kDa protein, which was confirmed to be vitellogenin by Western blotting. Vitellogenin was purified by DEAE chromatography and used to generate a polyclonal antibody. A competitive ELISA was developed for leopard frog vitellogenin with a detection limit of 6.0 ng mL(-1) and a working range of 20-1000 ng mL(-1). The intra-assay coefficient of variation averaged 5.47% for control sera and 9.71% for estrogen-treated sera. The inter-assay coefficient of variation averaged 8.21% for control sera and 9.93% for estrogen-treated sera. Recovery of purified vitellogenin averaged 95.2%. Vitellogenin was measured in male frogs immersed in the estrogenic compound diethylstilbestrol (DES) for various times and doses. Serum vitellogenin was detected within five days after immersion in 1.0 mg L(-1) DES and levels continued to increase through 20 d. In a 20-day dose-response experiment, serum vitellogenin was detected in frogs immersed in 0.01 mg L(-1) DES and vitellogenin concentration increased with dose. Immersion of frogs in one of several xenobiotic estrogens (nonylphenol, octylphenol, bisphenol-A) for 20 d did not increase vitellogenin for any treatment, suggesting that this frog may be less sensitive than fish to endocrine disruptors. Vitellogenin induction in R.pipiens may be a useful amphibian model system for field studies of endocrine disruption, due to its broad geographic range.

Endocrine disruption caused by oral administration of atrazine in European quail (Coturnix coturnix coturnix)

The widely used herbicide atrazine (ATZ) has been reported to exhibit reproductive toxicity in rats, fish and amphibians, with an avian LD(50) of 5000 mg/kg. In the present work, ATZ was administered as a single oral dose of 25 or 100 mg/kg to female European quail (Coturnix coturnix coturnix) at days 0, 5 and 10 of the experiment, being the animals sampled at days 15, 30 and 45. ATZ significantly increased the expression of hepatic estrogen receptor α (ERα) at both doses at day 30. An important increase was also observed in plasma 17β-estradiol (E2) concentrations. ATZ at 100 mg/kg increased the circulating concentration of vitellogenin (Vtg), but this effect was not related with an increase in hepatic Vtg mRNA levels. ATZ had no effect on the hepatic expression of both cytochrome P450 1A4 (CYP1A4) or the related biotransformation activity ethoxyresorufin-O-deethylase (EROD). These results led to the conclusion that ATZ provokes an estrogenic effect in sexually mature females of European quail. Further studies are necessary to establish the effect on sexual development or reproduction of female and male birds in the wild.

Full-grown oocytes from Xenopus laevis resume growth when placed in culture

When most full-grown, follicle cell-invested oocytes from Xenopus laevis are placed in an appropriate culture medium, they resume growth and remain physiologically healthy for at least 2-3 weeks. Rates of growth by full-grown oocytes in vitro generally approximate and can even exceed the most rapid growth rate achieved by vitellogenic oocytes in vivo. Resumption of oocyte growth can be correlated with the loss of investing follicle cells, which under normal conditions appear to interfere with vitellogenin and nutrient access to the oocyte. The final size reached by the oocyte within the ovary is thus not an intrinsic property of the oocyte but is extrinsically imposed by the somatic environment.

Development of an enzyme-linked immunosorbent assay for vitellogenin of Morelet's crocodile (Crocodylus moreletii)

The purpose of this study was to develop an immunoassay for vitellogenin in Morelet's crocodile (Crocodylus moreletii). Blood was collected from wild-caught crocodiles in Belize. Plasma samples from adult females taken during the breeding season were used for vitellogenin purification and samples from adult males were used for comparison. No differences were detected between males and females for plasma total protein concentration, as measured by Coomassie assay. However, denaturing polyacrylamide gel electrophoresis (SDS-PAGE) revealed that female plasma contained a 210-kDa protein, presumably the vitellogenin monomer, that was absent in adult male plasma. The identity of the putative vitellogenin was confirmed by its cross-reactivity in Western blots with a vitellogenin antiserum that was generated against a conserved vitellogenin peptide sequence. Crocodile vitellogenin was purified by two successive rounds of DEAE chromatography. The purified protein had an apparent molecular mass of 450 kDa, as determined by gel filtration chromatography, and 210 kDa on SDS-PAGE. An indirect enzyme-linked immunosorbent assay (ELISA) was then developed for C. moreletii vitellogenin. The detection limit of the assay was 20.0 ng/mL. The intra- and inter-assay coefficients of variation were 5.3% and 9.8%, respectively. The recovery of vitellogenin diluted into male plasma was 94.7%. The ELISA assay revealed that vitellogenin levels of adult female plasma during the breeding season ranged from 1.8 to 3.1 mg/mL with a mean of 2.5+/-0.25 mg/mL. No vitellogenin was detected in adult male plasma. Induction of vitellogenin in Morelet's crocodile may be a useful model system for field studies of crocodile reproduction and for investigations of endocrine disruption in this species.

Vitellogenin induction in the endangered goodeid fish Girardinichthys viviparus: vitellogenin characterization and estrogenic effects of polychlorinated biphenyls

Vitellogenin (VTG) is a widely used biomarker in studies of endocrine disruption induced by xenobiotics such as polychlorinated biphenyls (PCBs). This study evaluates the estrogenic effects of these compounds on the black-fin goodeid Girardinichthys viviparus, an endangered fish species in Mexico with a reduced range of distribution due to pollution of its natural environment. Adult fish born in the laboratory were exposed to half the LC(0) of Inerteen commercial PCB mixture. VTG was determined through an inhibition enzyme-linked immunosorbent assay (ELISA) using a homologous-heterologous system. Male and female fish were killed after 1, 2, 4, 8 and 16 days of exposure. The distal third of each specimen was used for analysis. VTG was obtained from cultured hepatocytes and blood serum of males previously exposed to 17beta-estradiol. VTG molecular mass was 348 kDa. PCBs were found to elicit greater estrogenic effects on VTG induction in males than in females (p<0.05) and sex differences were noted. Time-dependent VTG induction kinetics in males and a stationary phase in females were also observed.

Vitellogenin detection in Caiman latirostris (Crocodylia: Alligatoridae): a tool to assess environmental estrogen exposure in wildlife

Environmental pollution with endocrine disrupting compounds (EDCs) has adverse effects on the ecosystem's health. Caiman latirostris are widely distributed in South American aquatic ecosystems. Caimans have physiological and ecological characteristics that make them particularly vulnerable to EDCs exposure and suitable candidate as a sentinel species. Vitellogenin (Vtg) is a yolk pre-cursor protein synthesized by the liver of non-mammalian vertebrates and induced in response to estrogen. Purified plasma Vtg from caimans injected with estradiol-17beta (E2) was used to generate a polyclonal anti-body. Anti-body specificity was assessed using Western blot. The antiserum was also effective in detecting turtle Vtg, exhibiting high cross-reactivity with Vtg from Phrynops hilarii and Trachemys scripta dorbigni. We developed a specific and highly sensitive ELISA for caiman Vtg. This method has a detection limit of 0.1 ng/ml of plasma. The ELISA did not detect Vtg in plasma of non-induced male caimans. Induction of Vtg in male caimans was evaluated in response to one or two (7 days apart) doses of E2. Due to its high sensitivity, ELISA allows to measure the small increases in plasma Vtg after exposure to exogenous estrogen. A priming effect was observed following the second E2 dose, with a tenfold increase in circulating Vtg. Hepatic synthesis was confirmed by immunohistochemistry. The results presented herein suggest that detection of plasma Vtg in male caimans might become a valuable tool in biomonitoring xenoestrogen exposure in a polluted environment.

Purification of vitellogenin from the air breathing catfish, Clarias gariepinus

Vitellogenin (Vtg) is a female specific glycophospholipoprotein which can be induced both in male and female with estradiol and xeno-estrogens. The basic theme behind the purification of vitellogenin from the fish is to understand the evolutionary relationship and for the purification and characterization of the Vtg receptor. The male catfish, Clarias gariepinus was administrated with estradiol over a period of time for the synthesis of Vtg and the serum was collected. The Vtg was purified from the serum using a two step chromatographic technique. The serum was passed on to DEAE-ion exchange column and the protein was eluted using a salt gradient. The fractions containing the Vtg were pooled and passed onto a gel permeation chromatography column and the pure protein was obtained. The molecular weight is around 200 kDa on the SDS-PAGE and around 520 kDa on the native gel electrophoresis.

Vitellogenin exists as charge isomers in the Indian freshwater murrel, Channa punctatus (Bloch)

Three charge isomeric forms of vitellogenin are reported in the blood of estrogen-treated murrel, Channa punctatus on the basis of the results of native and SDS-PAGE, isoelectric focusing, and Ferguson plot analysis. Vitellogenin was induced in adult murrels by exogenous administration of estradiol-17beta and labelled with 32P in vivo. Labelled vitellogenin isolated from other plasma proteins on Ultrogel AcA 34 columns resolve into three bands on native-PAGE. Ferguson plot analysis reveals free electrophoretic mobilities of the three bands as 6.0, 4.5, and 3.7, which are very similar to the isoelectric point values of 5.9, 4.6, and 3.8, respectively. The apparent molecular weight of native murrel vitellogenin is 530 kDa. It consists of a single peptide with a molecular weight of 175 kDa. All the three bands on native PAGE resolves into a single peptide on SDS-PAGE. N-terminal amino acid sequence for murrel vitellogenin peptide is MKAVVLALLL. The protein phosphorus:lipid phosphorus ratio for murrel vitellogenin is 0.9. The total lipid content is 32.8%, consisting of 45% neutral lipids and 30% phospholipids. Phosphatidyl choline is the major phospholipid whereas triglycerides are the major neutral lipids. Results of these analytical analyses indicate that murrel native vitellogenin circulates as three charge isomers; poor in phosphorus but rich in lipids content.

Vitellogenin in tilapia male fishes exposed to organochlorine pesticides in Ouémé River in Republic of Benin

In many African countries, the economy largely depends on agriculture. Pesticides are therefore likely to represent an important source of xenoestrogens in contaminated rivers and lagoons. The largely uncontrolled use of diverse pesticides led us to hypothesize that these agents, and particularly organochlorine compounds, may pose a serious problem in the Republic of Benin. To verify our hypothesis, tilapia (Sarotherodon melanotheron) from five sites in the southern part of the main Ouémé River were analyzed. Ouémé River drains the southern region of the country. Vitellogenin (Vtg) was used as an indicator of contaminated sites. This approach has its limitations, because there are a wide variety of man-made chemicals present in the aquatic environment likely to induce Vtg in male fish. Therefore, in this study this approach allows us to define potential contaminated target sites. In order to determine whether the presence of Vtg could be attributable to pesticides, organochlorine pesticides in the flesh of tilapia were also analyzed. Significant amounts of Vtg in fish from contaminated sites were detected, and were correlated with organochlorine pesticide levels in tissue. These results indicate that organochlorine pesticides are present in the Ouémé River and that these compounds can act as endocrine modulators in this ecosystem. Eating fish from contaminated rivers, such as the Ouémé River, may contribute to the accumulation of high concentrations of these pesticides in the body, leading to exposure to their negative effects.

Comparison of reproductive parameters in male yellow-blotched map turtles (Graptemys flavimaculata) from a historically contaminated site and a reference site

From May to September of 1998, we collected monthly plasma samples from male yellow-blotched map turtles captured at two sites in the Pascagoula River drainage, Mississippi. One site (Vancleave) has a documented history of pollution from industrial sources (principally 2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD). Fish consumption advisories at the Vancleave site were lifted in 1996 and current impacts appear minimal. However, the yellow-blotched map turtle, a federally protected species, continues to decline in numbers. To determine if endocrine disruption could be a factor in the low reproductive rates observed in Vancleave turtles, we examined levels of plasma testosterone (T) and estradiol-17beta (E(2)) from males at this site and a second site (Leaksville), which has no known source of industrial pollution. Plasma was also tested for vitellogenin (VTG), which, in males, can be a biomarker of exposure to estrogenic contaminants. No males had detectable plasma VTG nor did mean monthly E(2) levels differ between sites. However, 10% of males from the historically polluted site were found to have high levels of E(2) (equivalent to levels found in females) and T was significantly lower for males captured at this site for 3 of 5 months. Our data suggest that the current impact of contaminants on reproduction in this population is limited. However, a portion of the population may have been affected developmentally, as represented by differences in reproductive parameters detected between sites.

Two-step purification method of vitellogenin from three teleost fish species: rainbow trout (Oncorhynchus mykiss), gudgeon (Gobio gobio) and chub (Leuciscus cephalus)

A two-step purification protocol was developed to purify rainbow trout (Oncorhynchus mykiss) vitellogenin (Vtg) and was successfully applied to Vtg of chub (Leuciscus cephalus) and gudgeon (Gobio gobio). Capture and intermediate purification were performed by anion-exchange chromatography on a Resource Q column and a polishing step was performed by gel permeation chromatography on Superdex 200 column. This method is a rapid two-step purification procedure that gave a pure solution of Vtg as assessed by silver staining electrophoresis and immunochemical characterisation.

Arctic charr (Salvelinus alpinus) vitellogenin: development and validation of an enzyme-linked immunosorbent assay

Vitellogenin (Vtg) was isolated from plasma of estradiol-17 beta-treated Arctic charr males by double precipitation with MgCl2-EDTA and distilled water, followed by ion-exchange chromatography. The monomeric form of Vtg, as revealed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, was 158 kDa. The purified Vtg was used to raise a polyclonal antibody for Vtg (AbVtg), and the specificity of the AbVtg was assessed by Western blot analysis. No cross-reactivity was observed with plasma from control males. Using this AbVtg, a competitive enzyme-linked immunosorbent assay was developed. The detection limit of the assay was 2 ng ml-1, and the intra- and inter-assay variations determined from plasma samples were 8.6 and 13.3%, respectively. The assay was validated by quantification of Vtg in plasma samples obtained during a reproductive cycle of Arctic charr. Vtg of females increased gradually from 3 mg ml-1 in early March to a peak value of 22 mg ml-1 in late August, followed by a rapid drop to 2 mg ml-1 at the time of spawning in mid-October. The temporal changes in plasma Vtg of females correlate well with the reproductive cycle. Vtg was undetectable in males, except on some sampling dates during July-September when minute amounts (3-13 micrograms ml-1) were detected in some individuals.

An ELISA for brown trout (Salmo trutta) vitellogenin and its use in bioassays for environmental estrogens

An enzyme linked immunosorbent assay (ELISA) was developed for the detection of the egg yolk precursor vitellogenin (Vg) in plasma of brown trout (Salmo trutta). Purified Vg from a 17 beta-estradiol-induced trout was used as the competing antigen in the ELISA which is based on polyclonal antibodies. The ELISA's performance was optimized and characterized. The assay's working range was (25-500 ng ml-1), its sensitivity was (10.5 ng ml-1), and it had an intra-assay coefficient of variation of less than 10% between 30 and 1000 ng ml-1. The ELISA was used in bioassays for the detection of environmental estrogens, including estrogen mimics, in whole and fractionated industrial waste waters. Those bioassays were based on intraperitoneal (i.p.) injection-, static renewal-, and flow through exposure systems. The response threshold of both bioassays is limited to 1-2 micrograms ml-1 Vg by a low level plasma interference that was regularly detected in plasma from non-induced male fish. The responsiveness of the bioassays was characterized using progressive doses of 17 beta-estradiol. The i.p.-based assay, which was responsive to at least 100 micrograms kg-1 of 17 beta-estradiol, was used to screen extracts of pulp mill effluent and black liquor for estrogenic effects. Neither extract induced Vg in our assay. The i.p. assay was also used to test 4-tert-octylphenol (OP) and the PAH derivative, retene, for estrogenic activity. OP induced Vg in the i.p.-exposed fish; no Vg induction was detected in the retene-exposed fish. The static renewal bioassay, which was responsive to at least 0.1 microgram ml-1 of 17 beta-estradiol over a 15-day exposure period, was used to screen whole pulp mill effluents for estrogenic effects. No Vg induction was detected in the effluent-treated fish.

Purification and partial characterization of English sole (Pleuronectes vetulus) vitellogenin

Vitellogenin (VTG) was purified by double-step chromatography from plasma of male English sole treated with 17 beta-estradiol. The intact protein appeared to exist as a dimer in two forms of approximately 300 and 320 kDa and had an isoelectric point of 6.63. In SDS-PAGE, it was reduced to a single monomer of approximately 130 kDa. In immunoblotting, the protein showed cross-reactivity with coho salmon VTG antiserum. Native PAGE (sample not treated with the reducing agent mercaptoethanol) and immunoblotting of plasma from control and estradiol-treated male sole and gravid female sole demonstrated that the putative English sole VTG was normally female specific and estradiol inducible in males. It was immunocytochemically localized in liver and ovary of English sole, rock sole and starry flounder, using polyclonal antiserum to the purified protein from the estradiol-treated male English sole. The protein was characterized as a phospholipoglycoprotein by native PAGE, staining the gels for phosphorus with methyl green, for lipid with Sudan black B and for carbohydrate by an improved periodic-acid Fuchsin sulfite method. The amino acid composition of the putative VTG was generally similar to that of VTGs from other teleosts, with the non-polar amino acids alanine, valine, leucine and isoleucine accounting for one-third of the total amino acids present. However, English sole vitellogenin contained a higher proportion of leucine and a lower proportion of glycine than most other teleost vitellogenins isolated to date.

Degradation of vitellogenins by 170 kDa trypsin-like protease in the plasma of the tilapia, Oreochromis niloticus

Proteolytic degradation of plasma vitellogenins during purification procedure has been noted in several teleost fishes. We have characterized here a trypsin-like serine protease in the plasma of the tilapia, Oreochromis niloticus, which degrades vitellogenins. The molecular mass of the protease was estimated as 230 kDa by gel filtration and as 170 kDa both by nondenaturing and by SDS-polyacrylamide gel electrophoresis. The protease efficiently hydrolyzed the synthetic peptide substrates for trypsin-like proteases but not the substrates for chymotrypsin-like proteases nor aminopeptidases. Hydrolysis of the peptide substrates was strongly inhibited by leupeptin, aprotinin and N-tosyl-L-lysine chloromethyl ketone and to certain extent by chymostatin, 3,4-dichloroisocoumarin, phenylmethanesulfonyl fluoride, and soybean trypsin inhibitor. Leupeptin and aprotinin also inhibited the degradation of a vitellogenin in the plasma. Although the physiological functions of the 170 kDa protease in vivo have not been elucidated, the results on exzymatic properties of this protease will be useful for the isolation and characterization of vitellogenin not only in tilapia but also in other organisms.

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.

Plasma vitellogenin of grouper (Epinephelus malabaricus): isolation and properties

Radiolabelled vitellogenin produced by juvenile grouper following injection of 3H-leucine, 32P-orthophosphate and estradiol-stimulation was purified by anion-exchange and gel filtration chromatography and characterized by electrophoresis and chemical analysis. The radiolabelled product was found to exist in two heterogeneous molecular weight forms by electrophoresis under nondenaturing conditions on polyacrylamide gel with M(r), of 260,000 and 525,000. It showed two protein monomers (M(r) 113,000 and 140,000) on electrophoresis under denaturing condition on a sodium dodecyl sulphate-polyacrylamide gel. The purified material contained 17.2% total lipid with 1.3% cholesterol and 2.1% triglycerides, 8-21 micrograms/mg protein total carbohydrate and 6.8 micrograms phosphate/mg protein; amino acid analysis showed a profile comparable to that found for vitellogenins isolated from the grouper, medaka, goldfish and rainbow trout. The study showed grouper vitellogenin to be a glycophospholipoprotein similar in composition to vitellogenins from other teleosts and demonstrated that vitellogenins can be induced in juveniles by injection with estradiol-17 beta.

Vitellogenesis-related ovary cathepsin D from Xenopus laevis: purification and properties in comparison with liver cathepsin D

Cathepsin D was purified from ovaries of Xenopus laevis by both QAE-cellulose and pepstatin-Sepharose chromatography and then characterized and compared with Xenopus liver cathepsin D. Ovary cathepsin D appeared predominantly as a 43-kilodalton (kDa) molecular mass, as revealed by SDS-polyacrylamide gel electrophoresis, whereas the liver enzyme was obtained exclusively as a 36-kDa protein. The purified 43-kDa ovary enzyme cleaved vitellogenin limitedly to produce yolk proteins at pH 5.6. The specific activity of ovary cathepsin D was five to six times lower than that of the liver enzyme, as measured by hemoglobin-hydrolysis at pH 3, but the ovary enzyme was shown to be superior to the liver enzyme in terms of vitellogenin-cleaving activity, as examined at pH 5.6. Ovarian enzyme preparations contained variable amounts of 36-kDa species; this form was considered to be an autolytic product of the 43-kDa form arising during purification, because it was not detected in oocyte extracts but was generated by incubation of the purified 43-kDa enzyme alone in an acid solution. The conversion of the 43-kDa form by hepatic factors was accompanied by a marked increase in hemoglobin-hydrolytic activity.

Protease activity appeared after trypsin treatment of the purified vitellogenin from eel Anguilla japonica

Density gradient ultracentrifugation and anion-exchange chromatography combination were effective for the purification of the eel vitellogenin from the plasma of estradiol-treated eels. The vitellogenin was very high density glycolipoprotein (P = 1.27 g/ml) and its apolipoprotein was M(r) 196 k in both reduced and non-reduced conditions by SDS-PAGE. The major lipid component was phospholipid. The N-terminal amino-acid sequence of the vitellogenin was as follows: (Ac)Thr-Pro-Ala-Leu/Ala-Asp-Tyr. Amino-acid composition of the eel vitellogenin was similar to those of other teleosts. The protease activity appeared in the trypsinized vitellogenin, but was not detected in the purified vitellogenin. The protease was separated from the used trypsin and the other cleaved vitellogenin by a dextran sulfate cellulose column. The molecular weight of the protease was determined by zymogram using SDS-polyacrylamide gel containing casein and was 50 k. It was concluded that the eel vitellogenin possesses the protease activity as a latent form.

Vitellogenin receptors: oocyte-specific members of the low-density lipoprotein receptor supergene family

Receptors that transport vitellogenin (VTG) into oocytes are of vital importance to egg-laying species, because they mediate a key step of oocyte maturation, a prerequisite to reproduction. Vitellogenins are lipophosphoglycoproteins that are produced under female hormonal control in large central organs (fat body in insects; liver in higher animals) and are transported in the circulation to the female gonads. VTG receptors localized in coated pits on the surface of growth-competent oocytes are able to accumulate in the yolk high concentrations of VTG and other ligands they recognize. The study of VTG receptors and their ligands has identified genes that specify related ligands, and a family of receptors. To date, all molecularly characterized VTG receptors belong to the low-density lipoprotein receptor supergene family, which ranges from a 600-kDa receptor in Caenorhabditis elegans to the 100-kDa so-called very-low-density lipoprotein receptors in mammals. These receptors, by and large, recognize ligands with similarities in structural elements first defined in the human apoplipoproteins B-100 and E. Recent studies on the receptor family have added VTG and lipoprotein lipase to the list of co-evolved ligands and have revealed that VTG receptors are able to interact with ligands other than VTG and also with some unrelated to lipoprotein metabolism. For example, the chicken VTG receptor also imports very-low-density lipoprotein, riboflavin-binding protein, and alpha-2-macroglobulin into growing oocytes. Such multifunctionality of receptors is likely the result of evolutionary pressure to provide the female germ cell with a highly economical machinery for vitellogenesis.

Atlantic halibut (Hippoglossus hippoglossus) vitellogenin: induction, isolation and partial characterization

Vitellogenin (VTG) synthesis was induced by repeated injections of estradiol-17β in juvenile Atlantic halibut (Hippoglossus hippoglossus). VTG eluted as a large, phosphoprotein containing peak on DEAE-Sephacel chromatography of plasma from estradiol-17β treated juvenile and mature female, but not mature male halibut. A purification procedure for Atlantic halibut VTG was developed, where VTG was precipitated with MgCl2, EDTA and distilled water, and the precipitated protein submitted to anion exchange chromatography on DEAE-Sephacel. Precipitated VTG eluted as a broad, partly dissociated peak on DEAE-Sephacel, when chromatography was run at 4°C, but the protein appeared intact when analysed both by SDS PAGE and native PAGE. DEAE-Sephacel chromatography at room temperature resulted in an irregular elution pattern and a dissociated protein fraction, as analysed by SDS PAGE. Biochemical characterization of VTG showed that the molecular mass of the monomer was ca 160 kDa, as estimated by SDS-PAGE. The total lipid content was 19.8% w/w, with 64%, or 12.7% of the total weight, as phospholipid. Protein bound phosphorus constituted 0.62% w/w of halibut VTG. Plasma dilution curves from mature and maturing female halibut were parallel with a dilution curve from halibut egg yolk homogenate in an homologous RIA. Plasma from mature male, but not juvenile halibut crossreacted with the VTG antiserum.

Yolk platelets in Xenopus oocytes maintain an acidic internal pH which may be essential for sodium accumulation

Yolk platelets constitute an embryonic endocytic compartment that stores maternally synthesized nutrients. The pH of Xenopus yolk platelets, measured by photometry on whole oocytes which had endocytosed FITC-vitellogenin, was found to be acidic (around pH 5.6). Experiments on digitonin-permeabilized oocytes showed that acidification was due to the activity of an NEM- and bafilomycin A1-sensitive vacuolar proton-ATPase. Proton pumping required chloride, but was not influenced by potassium or sodium. Passive proton leakage was slow, probably due to the buffer capacity of the yolk, and was dependent on the presence of cytoplasmic monovalent cations. In particular, sodium could drive proton efflux through an amiloride-sensitive Na+/H+ exchanger. 8-Bromo-cyclic-AMP was found to increase acidification, suggesting that pH can be regulated by intracellular second messengers. The moderately acidic pH does not promote degradation of the yolk platelets, which in oocytes are stable for weeks, but it is likely to be required to maintain the integrity of these organelles. Furthermore, the pH gradient created by the proton pump, when coupled with the Na+/H+ exchanger, is probably responsible for the accumulation and storage of sodium into the yolk platelets during oogenesis.

Hormonal control of in vitro vitellogenin synthesis in Rana esculenta liver: effects of mammalian and amphibian growth hormone

Estradiol 17-beta is known to induce hepatic synthesis and secretion of vitellogenin in all species studied and in Rana esculenta, previous experiments demonstrated the involvement of pituitary in these processes; indeed, in addition to estradiol 17-beta, homologous pituitary homogenate directly stimulated male and female liver to produce vitellogenin in tissue cultures. Therefore, the effect of ovine growth hormone (o-GH) and Rana catesbeiana growth hormone (f-GH) on hepatic vitellogenin synthesis was investigated. In the present in vitro experiments, both o-GH and f-GH positively stimulated vitellogenin synthesis, in female and male liver, in a dose-related fashion. No significant differences were found in VTG levels induced by o-GH and f-GH. The GH stimulatory effects, found during the different phases of the reproductive cycle, displayed different trends related to season and sex.

In vitro induction of vitellogenin synthesis in Rana esculenta: role of the pituitary

Vitellogenin (VTG), a complex protein, is a precursor of yolk protein (lipovitellin and phosvitin) in all oviparous vertebrates studied to date. In amphibians, as in other oviparous vertebrates, VTG synthesis is hormonally dependent; estradiol-17 beta (E2) is especially important, although in vivo in the frog Rana esculenta the pituitary may be involved in hepatic VTG synthesis and secretion. The present in vitro experiments carried out during the main phases of the annual reproductive cycle of this frog showed that homologous pituitary homogenate (HPH), as well as E2, stimulated VTG synthesis in male and female livers, although their patterns of VTG secretion showed some differences with respect to the induction time and the rate of VTG secretion. The hepatic response to HPH occurred after a longer time than that of E2 VTG induction, and no cooperation between HPH and E2 was found in the VTG synthesizing response. It should be emphasized that during the refractory period (July), hepatic VTG synthesis in males and females could only be induced by HPH. These data demonstrate, for the first time, a direct action of HPH on VTG synthesis in male and female frog livers in all the periods tested; hepatic responsiveness to HPH and E2 varies with season and sex.

Monoclonal antibody-based immunoassay of vitellogenin in the blood of male channel catfish (Ictalurus punctatus)

1. A monoclonal antibody to vitellogenin of channel catfish (Ictalurus punctatus) was made, and its specificity was demonstrated using Western blots of serum from female fish, estradiol-treated male fish, untreated male fish, vitellogenin purified by three different methods and egg extracts. 2. An enzyme-linked immunosorbent assay (ELISA), using this monoclonal antibody, detected vitellogenin in the plasma of 59 out of 60 untreated 17-24-month-old male channel catfish with a mean concentration of 338 micrograms/ml and a maximum concentration of 4240 micrograms/ml. 3. Vitellogenin levels in male channel catfish were unrelated to testicular stage, gonadosomatic index and month.

Plasma vitellogenin and 17 beta-estradiol levels during the annual reproductive cycle of Podarcis s. sicula Raf

Plasma vitellogenin and 17 beta-estradiol concentration were determined during the annual reproductive cycle of the female lizard Podarcis s. sicula Raf. living around Naples. Plasma vitellogenin was purified from estrogenized males for characterization and to raise specific immune serum. Using ELISA, plasma vitellogenin titers were determined in relation to ovary weight; plasma 17 beta-estradiol was measured by RIA method. Native vitellogenin was present as two polypeptide bands: alpha and beta. The electrophoretic patterns, studied in normal male and estrogenized male and female, showed vitellogenin to be a protein present in female and in estrogenized male plasma but not in normal males. Lizard monomeric VTG, determined by SDS-PAGE, was about 200 kDa. Correlations between seasonal ovarian weight variations and plasma vitellogenin and 17 beta-estradiol suggest that ovarian development in Podarcis depends on plasma vitellogenin synthesis, which in turn relies on plasma estradiol levels. The two ovulatory waves observed in this study coincided with the two peak values of plasma vitellogenin and 17 beta-estradiol.

Sex steroid profile and plasma vitellogenin during the annual reproductive cycle of the crested newt (Triturus carnifex Laur.)

The annual reproductive cycle of the crested newt, Triturus carnifex, has been studied in the field. Temperatures, rainfall, humidity, and photoperiod were recorded throughout the year. Adult male and female newts were sampled monthly; snout vent lengths, crest heights of males, and body ovarian, oviducal, testicular, and abdominal gland weights were recorded. Plasma samples were assayed for androgen, estradiol-17 beta, and progesterone by radioimmunoassay and for vitellogenin by enzyme-linked immunosorbent assay. Air, deep water, water surface, and soil temperatures were low from October to March, but increased in April and May without consistent summer variations. Ovarian and oviducal weights increased in October to reach maximum values between January and March (reproductive period). Crest height and abdominal gland weight in males mirrored the ovarian and oviducal pattern, while testicular weights were maximal in October and November. In females, plasma androgens were high during the reproductive period, and plasma estradiol peaked sharply in March, while plasma progesterone changed little. In the males plasma androgen and estradiol concentrations were similar to those of females, while plasma progesterone was significantly correlated with the cycle in testicular weight. In both sexes androgens showed a significantly negative correlation with air and water surface temperature. Plasma vitellogenin peaked in March but it did not correlate with either ovarian weight or plasma estradiol concentrations. These data support and confirm those previously reported for newts under laboratory conditions. The negative correlation between androgens and temperature suggests that this hormone may trigger the reproductive process. Moreover the correlations between plasma progesterone and testicular weight may indicate that this hormone is involved in male newts reproduction.(ABSTRACT TRUNCATED AT 250 WORDS)

Oogenesis in Fundulus heteroclitus. VI. Establishment and verification of conditions for vitellogenin incorporation by oocytes in vitro

A procedure was developed for studying vitellogenin (VTG) incorporation by vitellogenic oocytes of Fundulus heteroclitus in vitro. Since homologous VTG can be obtained from this animal only with great difficulty, the use of [32P]VTG from Xenopus laevis was explored as an alternative. Vitellogenic as well as maturational-stage oocytes were found to sequester X. laevis [32P]VTG from the medium, and incorporation was found to be linear with time for at least up to 12 hr. Once incorporated into the oocyte, [32P]VTG did not appear to undergo turnover. The effect of different [32P]VTG concentrations on incorporation indicated that the uptake mechanism was saturable. Unlabeled F. heteroclitus VTG and X. laevis VTG were also found to compete effectively with X. laevis [32P]VTG, whereas bovine serum albumin did not. These results represent the first documentation of a successful culture system for receptor-mediated VTG incorporation by teleost oocytes.

The purification and partial characterization of carp, Cyprinus carpio, vitellogenin

A procedure is described for the isolation of intact vitellogenin (c-VTG) from the carp, Cyprinus carpio. VTG was induced in juvenile females using oestradiol-17β and purified from the plasma using a combination of gel-filtration chromatography on Sepharose 6B and ion exchange chromatography on DEAE-cellulose. Purification procedures were conducted at low temperatures (below 9°C) in the presence of the proteolytic enzyme inhibitor aprotinin to prevent degradation. Intact c-VTG had an apparent molecular mass of 390,000 Daltons, but when extracted from plasma in the absence of aprotinin it underwent proteolysis into at least 2 protein fragments (apparent molecular masses of 230,000 and 96,000 Daltons), showing an instability of the native dimer. An amino acid analysis of c-VTG showed that its composition was almost identical to goldfish VTG, a species closely allied to the true carps and also similar to other oviparous vertebrate VTGs. Collectively, these data indicate that using these purification procedures VTG from carp, and probably other teleost species, can be isolated in an intact, highly purified form.

Exogenous vitellogenesis and micropinocytosis in the lizard, Podarcis sicula, treated with follicle-stimulating hormone

The regulation of oocyte growth and of exogenous vitellogenesis by micropinocytosis has been studied in lizard Podarcis sicula kept at 28 degrees during the winter stasis and stimulated by follicle-stimulating hormone (FSH). Under these experimental conditions, oocyte auxocytosis as well as vitellogenesis is stimulated, while the follicular hierarchy is preserved. At the ultrastructural level, the flow of exogenous yolk precursors toward the oocyte increases and the pathway taken by them through intercellular spaces and zona pellucida is the same as that taken by peroxidase (tracer). Yolk precursor endocytosis is found only in oocytes greater than 1500 microns in diameter and takes place through the formation of several coated pits and vesicles. It is suggested that membrane receptors necessary for micropinocytosis are available only in such oocytes. Last, a different permeability of the ovarian follicle to exogenous yolk precursors during the different stages of oocyte growth and endovarian control of vitellogenesis are suggested.

Influence of egg production on zinc, copper and iron metabolism in the turkey hen (Meleagris gallopavo)

1. Serum concentrations of zinc, copper, iron, calcium and protein increased significantly with the onset of egg production in turkey hens. 2. Hepatic concentrations of zinc, copper and iron declined significantly with the onset of egg production, whereas the total hepatic content of zinc and copper increased significantly. 3. When plasma from laying and nonlaying hens was subjected to gel-permeation column chromatography, a high molecular weight protein fraction was detected in laying but not in nonlaying plasma which bound significant amounts of zinc, copper and iron and which coeluted with purified vitellogenin. 4. Vitellogenin purified from the plasma of laying hens is a metalloprotein which upon analysis was found to contain 0.54, 0.09 and 0.72 micrograms of zinc, copper and iron, respectively, per mg of protein.

Vitellogenin induction by estradiol in channel catfish, Ictalurus punctatus

In oviparous vertebrates estrogens induce hepatic synthesis of vitellogenin (VG), a blood protein sequestered in vitellogenic oocytes and from which lipovitellin (LV) and phosvitin are derived. Our objective was to identify VG in the channel catfish, Ictalurus punctatus. An intraperitoneal injection of estradiol-17 beta into adult male fish induced a dose-dependent accumulation of a 150 kDa protein (EP) in the plasma. EP was detectable in Coomassie blue-stained polyacrylamide gels within 24 hr after injection of 2 mg hormone/100 g body weight. During the next 4 days, EP increased from 5 to about 25% of the total plasma protein. Electrophoretic mobility, peptide mapping, and immunological crossreactivity showed EP to be indistinguishable from a plasma protein in adult females with vitellogenic ovaries. Two major yolk polypeptides, YP1 (120 kDa) and YP2 (29.6 kDa), were precipitated by (NH4)2SO4 from a yolk protein extract. YP1 but not YP2 reacted with an anti-EP polyclonal antiserum in Western blots. Peptide mapping after proteolysis with trypsin showed YPs 1 and 2 to be unique and revealed structural homologies between YP1 and EP. Liver but not pancreatic explants from an estradiol-treated male synthesized and secreted a [35S]methionine-labeled, 150 kDa protein beginning about 2 hr after initial exposure to the label. We tentatively conclude that EP and YP1 represent VG and LV, respectively. YP2 remains unidentified.