Development of an Enzyme-linked Immunosorbent Assay Using Vitellin for Vitellogenin Measurement in the Pale Chub, Zacco platypus

OBJECTIVES

Fish vitellogenin (VTG) is produced in the female liver during oogenesis through the estradiol cycle and produced in the male liver by endocrine disrupting chemicals (EDCs) such as alkylphenols. In this study, we propose that the VTG concentration in the pale chub could be detected using monoclonal antibodies and polyclonal antibodies against vitellin (Vn) in a VTG enzyme-linked immunosorbent assay (ELISA) system.

METHODS

Monoclonal antibodies and polyclonal antibodies were produced using the Vn extracted from the matured ovum of the ovary. The VTG was extracted from the plasma of the male pale chub. The Vn and VTG were confirmed by measuring the molecular weight of their proteins using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and the specificity of the antibodies was checked through western blotting methods. The assay system was validated with respect to optimal assay concentrations, specificity, recovery, and intra- and inter-assay variations.

RESULTS

The Vn consisted of two protein bands with apparent molecular weights of 64 and 37 kDa. The SDS-PAGE indicated protein weights of 146 and 77 kDa in the VTG. The assay range was 15.6 ng/mL to 2,000 ng/mL, and the value of the intra- and inter-assay variations were within 10.0% and 14.7%, respectively. The recovery rate was 99.5±5.5%.

CONCLUSIONS

A sandwich ELISA was developed that could be used to qualify the VTG of pale chub in screening for EDCs. Pale chub is an ideal species for observing estrogen activity in the environment because of its extensive habitat and extensive food chain. The ELISA developed here would be more favorable than those for other species for determining the effect of long-term food chain accumulation of EDCs in aquatic environments.

Vitellogenin genes in fish: differential expression on exposure to estradiol

Three types of vitellogenins (Vgs) namely vitellogenin A (VgA), vitellogenin B (VgB) and vitellogenin C (VgC) have been identified in fishes. The existence of VgA and VgB is reported in the Indian freshwater murrel Channa punctatus. Gene-specific primers were designed using available nucleotide sequences in National Centre for Biotechnology Information (NCBI), for amplification of VgA and VgB cDNA. Differential processing of Vgs is evident in many fishes. Adult male murrel expressed both the VgA and VgB genes when estradiol-17β (E(2)) is injected in vivo and Vg levels in blood quantified by Enzyme linked immunosorbent assay (ELISA) showed a dose-related response in such treatments. Cultured hepatocytes on treatment with E(2), however, expressed only VgB as detected by RT-PCR, suggesting different regulatory mechanism for the VgA and VgB genes.

Electrochemical Immunoassay for Vitellogenin Based on Sequential Injection Using Antigen-immobilized Magnetic Microbeads

A rapid and sensitive immunoassay for the determination of vitellogenin (Vg) is described. The method involves a sequential injection analysis (SIA) system equipped with an amperometric detector and a neodymium magnet. Magnetic beads, onto which an antigen (Vg) was immobilized, were used as a solid support in an immunoassay. The introduction, trapping and release of magnetic beads in an immunoreaction cell were controlled by means of the neodymium magnet and by adjusting the flow of the carrier solution. The immunoassay was based on an indirect competitive immunoreaction of an alkaline phosphatase (ALP) labeled anti-Vg monoclonal antibody between the fraction of Vg immobilized on the magnetic beads and Vg in the sample solution. The immobilization of Vg on the beads involved coupling an amino group moiety of Vg with the magnetic beads after activation of a carboxylate moiety on the surface of magnetic beads that had been coated with a polylactate film. The Vg-immobilized magnetic beads were introduced and trapped in the immunoreaction cell equipped with the neodymium magnet; a Vg sample solution containing an ALP labeled anti-Vg antibody at a constant concentration and a p-aminophenyl phosphate (PAPP) solution were sequentially introduced into the immunoreaction cell. The product of the enzyme reaction of PAPP with ALP on the antibody, paminophenol, was transported to an amperometric detector, the applied voltage of which was set at +0.2 V vs. an Ag/AgCl reference electrode. A sigmoid calibration curve was obtained when the logarithm of the concentration of Vg was plotted against the peak current of the amperometric detector using various concentrations of standard Vg sample solutions (0-500 ppb). The time required for the analysis is less than 15 min.

Induction of vitellogenesis in 17alpha-ethinylestradiol-exposed rainbow trout (Oncorhynchus mykiss): a method comparison

Juvenile rainbow trout, Oncorhynchus mykiss, were exposed to the synthetic estrogen 17alpha-ethinylestradiol (EE(2)) through injection (1, 10, 25 and 50 microg EE(2)/g fish/week) and via water exposure (1, 10 and 100 ng EE(2)/l). After seven (injection and water exposure) and 14 days (only for water exposure), blood and plasma vitellogenin concentrations were quantified using indirect endpoints, i.e. plasma alkaline-labile phosphorus (ALP), plasma protein and plasma calcium. In addition, the relative gonad (GSI) and liver weight (HSI) were recorded. Actual plasma vitellogenin concentrations were measured with an enzyme immunoassay. Only fish injected with 50 microg EE(2)/g fish had a significantly higher gonad weight. No concentration-dependent changes in the HSI were detected in fish exposed via the water, but a significant dose-dependent increase of the HSI was observed in fish injected with EE(2). Exposure of rainbow trout to EE(2) had a significant effect on all tested plasma parameters. Plasma protein, phosphoprotein and calcium concentrations were significantly higher after two weeks exposure to 100 ng EE(2)/l. Fish injected with 10, 25 and 50 microg EE(2)/g fish exhibited increased plasma protein concentrations after 1 week. Compared to the controls, plasma ALP and calcium levels were significantly higher in all injected fish. A significant and positive correlation was observed between all three plasma parameters and between these indirect parameters and the actual plasma vitellogenin concentrations. These findings indicate that both the plasma ALP and the plasma calcium assay have a similar sensitivity as that of available antibody-based assays (EIA), at least in EE(2) exposure studies, and thus these assays can provide a rapid, simple and cost-effective alternative to available immunoassays.

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.

Transition of serum vitellogenin cycle in Sakhalin taimen (Hucho perryi)

A specific and sensitive enzyme-linked immunosorbent assay (ELISA) and a single radial immunodiffusion (SRID) were developed for measurement of serum vitellogenin (Vg) levels in Sakhalin taimen (Hucho perryi). Regarding specificity for serum Vg, an antiserum raised against lipovitellin of taimen (a-Lv) was adequate for both assays. ELISA and SRID could detect Vg in serum at concentrations as low as 10 ng/ml and 25 micrograms/ml, respectively. In estrogen administration experiments, the level of serum Vg began clearly increasing within 12 to 24 hr after injection of immature females with estradiol-17 beta (E2). The appearance and levels of Vg in males treated with E2 were delayed and smaller, respectively, than for females. Vg levels varied throughout natural vitellogenesis from 0-4 micrograms/ml (3 years old) to approximately 30 mg/ml (5-6 years old). We observed an early transitory peak of serum Vg levels (primary reaction) at the time of early vitellogenesis and chronic high Vg levels (for 6-7 months) in winter period before ovulation. Changes of serum E2 levels were correlated with Vg levels. However, E2 levels decreased a month earlier than Vg levels near ovulation. It appears that the duration of vitellogenesis in taimen is considerably longer than that in other salmonids, lasting more than 2 years.

Characterization of a heat-stable fraction of lipovitellin and development of an immunoassay for vitellogenin and yolk protein in winter flounder (Pleuronectes americanus)

An enzyme-linked immunoabsorbent assay was developed for detection and quantification of the yolk protein lipovitellin (Lv) and its plasma precursor, vitellogenin (Vg), in winter flounder (Pleuronectes americanus). Native Lv was found to be a mixture of heat-stable and heat-labile molecules in mature, ovulated eggs. A heat-stable Lv fraction was purified from extracts of unfertilized eggs by brief heat treatment and gel permeation chromatography on Bio-Gel A-1.5. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) of heat-stable Lv revealed a single polypeptide of 94 kD, while native Lv also possessed several smaller polypeptides, suggesting that heat-labile Lv contains proteolytic cleavages of the 94-kD polypeptide which destabilize its structure. The Stokes radius of the native protein on Bio-Gel A-1.5 was estimated at 4.50 nm, while the Stokes radii of heat-stable and heat-labile Lv were 4.26 nm and 5.17 nm, respectively. Heat-stable Lv was used to produce a rabbit polyclonal antiserum which reacted with a single 175-kD polypeptide in Western blots of vitellogenic female winter flounder serum, but did not react with any component of male serum. Ouchterlony double diffusion using this antiserum demonstrated immunological identity of Lv, heat-stable Lv, and Vg. The anti-Lv anti-serum was used to construct an homologous ELISA with a linear response between 25 and 300 ng/ml. This assay was used to characterize a Bio-Gel A-1.5 column profile of serum from an estradiol-treated male winter flounder, and a single peak, with Stokes radius of 6.70 nm, was identified as Vg. Winter flounder Vg was confirmed to be a dimer, while Lv from mature eggs was found to be a monomer of a lower molecular weight polypeptide.

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.

Enzyme-linked immunosorbent assay (ELISA) for rainbow trout (Oncorhynchus mykiss) vitellogenin

1. A specific and simple enzyme-linked immunoassay for rainbow trout (Onchorynchus mykiss) vitellogenin (Vtg) is described. This assay is performed using a rabbit antiserum for Vtg purified from trout plasma. 2. This assay is based upon the competition between soluble Vtg and Vtg adsorbed on microtiter plates, for the rabbit anti-Vtg antibody binding sites. 3. The adsorbed Vtg-antibody complexes are revealed through the peroxidase-antiperoxidase antibody, which is colored by o-phenylendiamin. This assay can be performed in a day and a night. 4. Under our conditions, 90-20% of binding gave a sensibility range of 33-1473 ng/ml. With almost a 50% binding yield (335 ng/ml) the intra-assay coefficient of variation (CV) was 5.2% (n = 26) and the inter-assay CV was 12.5% (n = 5). 5. There was low immunological cross-reactivity with sera from other salmonids and with ovary extracts. Extracts of liver from oestrogenized male rainbow trout yielded displacements parallel to the vitellogenin standard and to mature female serum or oestrogenized male serum. 6. This enzyme immunoassay is simple and easy to use. Its great specificity allows its use only for the rainbow trout species.

Induction by beta-estradiol of vitellogenin in striped bass (Morone saxatilis): characterization and quantification in plasma and mucus

Striped bass (Morone saxatilis) were implanted with beta-estradiol to induce the production of vitellogenin, the egg yolk precursor produced by the liver. Electrophoretic analysis revealed that beta-estradiol caused marked production of a plasma protein of apparent molecular mass 170 kDa. Size exclusion chromatography suggested that the estradiol-induced protein circulated as a dimer. This protein was purified from the plasma of estradiol-treated fish by DEAE-agarose column chromatography and used to induce antibodies in rabbits and goats. Western blots revealed that the antiserum bound to the putative vitellogenin in plasma from estradiol-treated fish and adult females, but not with any proteins in male plasma. Western blot of ovarian extract revealed several smaller immunoreactive protein bands and supported the identity of the purified protein as vitellogenin. A competitive ELISA was developed with sensitivity in a range from 8 to 1000 ng/ml. Plasma concentrations of adult females during their spawning migration ranged from 100 to 600 micrograms/ml. Western blot of mucus extract revealed the presence of a 170-kDa protein in vitellogenic female fish along with several minor bands ranging from 50 to 110 kDa. Positive immunoreactivity was present in the surface mucus of all females and in none of the males collected during a spawning migration in the Hudson River.

The development of a radioimmunoassay for carp, Cyprinus carpio, vitellogenin

The development of an easily - performed and robust radioimmunoassay (RIA) to carp, Cyprinus carpio, vitellogenin (c-VTG) is described. Purified c-VTG was iodinated using Iodogen. The resulting c-VTG label was useful for up to 60 days. High titre antibodies were raised in rabbits to the purified c-VTG. The practical operating range of the c-VTG RIA was between 2 and 200 ng. ml(-1). VTG was detected in plasma from all female mirror carp investigated, and all plasmas diluted parallel to the standard. The plasma VTG level in female carp increased concomitantly with the GSI; levels increasing from the ng. ml(-1) level in juveniles to a maximum of 1 mg. ml(-1) in fully mature females. VTG level was a far more sensitive index of the degree of sexual development than was GSI. In males, blood levels of VTG were always undetectable. Vitellogenic plasma from all subspecies of C. carpio (e.g. mirror, common, Koi) diluted parallel to the standard, as did blood from most other female cyprinids, such as the roach (Rutilis rutilis), bream (Abramis brama), and dace (Leuciscus leuciscus), but not all. These results suggest that the structure of VTG is highly conserved within this family. However, plasma from vitellogenic salmonids did not cross-react in the RIA. The relationship between plasma VTG and calcium levels was studied in both carp and rainbow trout. In rainbow trout it was found that plasma calcium levels do not rise above basal levels until the VTG level exceeds about 1 mg. ml(-1), and therefore in this species it is only useful as an indicator of the degree of ovarian development in the later stages of the reproductive cycle. In the carp, however, and probably other cyprinids, blood VTG levels do not appear to naturally exceed about 1 mg.ml(-1), and plasma calcium levels are not suitable as an indirect measure of the VTG level.

An homologous radioimmunoassay for coho salmon (Oncorhynchus kisutch) vitellogenin, with general applicability to other Pacific salmonids

This paper describes an homologous radioimmunoassay for coho salmon vitellogenin that demonstrates parallel cross-reactivity for plasma vitellogenin of all Pacific salmonids tested (chinook, chum, coho, pink, and sockeye salmon, and cutthroat and rainbow trout), but not for Atlantic salmon or two nonsalmonids: common carp and sablefish. Plasma vitellogenin levels were high in ovulatory female Pacific salmonids (micrograms/ml to mg/ml range), but were mostly nondetectable in spermiating males of the same species.

An homologous radioimmunoassay for brown trout (Salmo trutta) vitellogenin

An homologous radioimmunoassay for brown trout vitellogenin (VTG) was developed. Intact VTG, isolated from juvenile brown trout by selective precipitation and anion exchange chromatography was labelled with Na(125)I, with 1,3,4,6-tetrachloro-3α,6α-diphenylglycoluril (Iodogen) as the oxidizing agent. Incorporation of Na(125)I into VTG was higher than 75% and there was little degradation of the labelled protein. Labelled VTG eluted at the same position as unlabelled, purified brown trout VTG when analyzed by gel filtration on Sepharose 6B. Antisera with high titers, i.e. 1∶250 000, against brown trout VTG were raised in rabbits. The sensitivity of the assay was 5 ng VTG/ml and the standard curve was linear between 10 and 100 ng VTG/ml. Plasma from maturing female brown trout, as well as estradiol-treated and untreated juvenile brown trout diluted parallel to the standard curve, while plasma from maturing female rainbow trout and estradiol-treated arctic charr diluted non-parallel to the standard curve for brown trout VTG. Purified rainbow trout VTG and plasma from maturing female rainbow trout diluted parallel to each other, but with lower sensitivity than for brown trout VTG. Determinations of protein-bound phosphorus in the plasma of estradiol-treated juvenile brown trout correlated well with the RIA determinations of VTG. Repeated freezing and thawing of plasma samples yielded up to a hundred-fold increase in the apparent VTG level, while storage of a plasma sample for one year at -20°C did not affect the VTG level as measured by RIA.

The measurement of plasma vitellogenin levels in a marine teleost, the spotted seatrout (Cynoscion nebulosus) by homologous radioimmunoassay

1. Vitellogenin was purified from plasma of estrogen-treated spotted seatrout (Cynoscion nebulosus) and was shown to be a protein of approximately 200,000 mol. wt. 2. Antisera were raised against the isolated protein and were used to develop a radioimmunoassay for spotted seatrout vitellogenin. 3. The substance measured was shown to be female-specific, hepatically-derived, estrogen-inducible and immunologically related to oocyte yolk proteins. 4. Vitellogenin levels in the plasma of female spotted seatrout were shown to rise dramatically during the period of ovarian recrudescence.

Vitellogenin levels in male and female rainbow trout (Salmo gairdneri Richardson) at various stages of the reproductive cycle

The immunoassayable vitellogenin (VTG) in plasma from male rainbow trout had the same molecular weight as authentic VTG from female fish. The VTG level in male trout was low (usually nanograms, occasionally up to a few micrograms, per ml) and did not correlate with the stage of sexual maturity. The plasma VTG level of female trout that were two years from first spawning was 200-fold higher than males of the same strain and age. The plasma VTG level of female rainbow trout rose approximately a million-fold during the two or three years required to attain sexual maturity.

Hepatic estrogen receptors and plasma estrogen-binding activity in the Atlantic salmon

Livers of male and female immature Atlantic Salmon (Salmo salar) contain specific high-affinity [3H]estradiol binding sites in cytosol (Kd 2-4 nM, concentration about 0.6 pmol/g liver). Low levels of high-affinity binding are detectable in salt extracts of nuclei of untreated fish, but injections of estradiol result in transient depletion of the cytosol binder and in accumulation of high levels of binding sites in nuclear salt extracts (Kd 5-6 nM; concentration about 6 pmol/g liver). Both the cytosol and nuclear binding sites are temperature sensitive and are optimally assayed by incubation at 2 degrees. Both are specific for estradiol and diethylstilbestrol (DES) and no significant competition by dihydrotestosterone (DHT), progesterone, or hydrocortisone is seen. The triphenylethylene nonsteroidal antiestrogen, 4-hydroxytamoxifen, exhibits an affinity comparable to that of estradiol. The nuclear binding activity sediments with a coefficient of 3.6 S in salt-containing sucrose density gradients, and is stable on storage at -20 degrees for several months. The cytosol binder on the other hand is not stable on sucrose density gradients or on prolonged storage. Salmon plasma contains two [3H]estradiol binding components, one with a relatively high affinity for [3H]estradiol (kd 13 nM) and the other having a much lower affinity but present in high concentrations. The high-affinity plasma binder exhibits distinctive specificity with no affinity for DES or 4-hydroxytamoxifen but some affinity for DHT and progesterone. These properties serve to distinguish the plasma activity from the intrahepatic estrogen binders. The salmon liver estrogen receptor system has many features in common with typical estradiol receptors from other vertebrates. Immature salmon liver appears to be the richest source of hepatic estrogen receptor so far found for any vitellogenic species.

Induction, isolation and a characterization of the lipid content of plasma vitellogenin from two Salmo species: rainbow trout (Salmo gairdneri) and sea trout (Salmo trutta)

Vitellogenin synthesis is induced in juvenile rainbow trout (Salmo gairdneri) and juvenile sea trout (Salmo trutta) by estradiol-17 beta. A purification procedure for vitellogenin from trout plasma by precipitation with MgCl2-EDTA and subsequent anion exchange chromatography on DEAE-Sephacel is described. The total lipid contents of purified rainbow trout and sea trout vitellogenins are 18 and 19%, respectively. Approximately 2/3 of the lipids are phospholipids, while the remainder consists of triglycerides and cholesterol. Phosphorus determinations on delipidated vitellogenin yield a phosphorus content of 0.63% in rainbow trout and 0.58% in sea trout vitellogenin. Native (dimeric) vitellogenins from rainbow trout and sea trout both have an apparent molecular weight of 440,000, when estimated by gel filtration on Sepharose 6B.

Plasma vitellogenin in landlocked Atlantic salmon (Salmo salar Ouananiche): isolation, homologous radioimmunoassay and immunological cross-reactivity with vitellogenin from other teleosts

Vitellogenin was isolated by affinity chromatography and gel filtration from landlocked Atlantic salmon plasma. Vitellogenin was labelled with iodine-131 using iodogen and an homologous radioimmunoassay was developed. There was poor immunological cross-reactivity with vitellogenin or plasma from other teleosts. Parallelism of the vitellogenin standard to the displacement by plasma of vitellogenic salmon allowed the assay to be used to evaluate the seasonal concentration profile of vitellogenin in female adult salmon. Extracts of liver or ovary from female Atlantic salmon also yielded displacements parallel to the vitellogenin standard in the assay.

Failure of antibody to carbohydrate-rich gonadotropin to inhibit rapid ovarian growth in landlocked Atlantic salmon

The discovery of a carbohydrate-poor gonadotropin has resulted in a reassessment of the role of the classical, carbohydrate-rich gonadotropin (Con A II GtH) in teleost ovarian development. Prolonged treatment of landlocked Atlantic salmon with antibody to Con A II GtH failed to inhibit rapid ovarian growth or accumulation of triglyceride or vitellogenin derivative. Sera from antibody-treated fish showed substantial capacity to bind Con A II GtH indicating that antibody treatment successfully removed endogenous Con A II GtH from the circulation. These results are consistent with previous reports that a factor other than Con A II GtH is responsible for stimulating yolk accumulation.

Impairment of early ovarian growth in landlocked Atlantic salmon by an antibody to carbohydrate-rich gonadotropin

The role of the carbohydrate-rich gonadotropin (Con AII GtH) in an early phase of ovarian growth in landlocked Atlantic salmon (Salmo salar) was investigated by treating in vivo with an antibody to chum salmon (Oncorhynchus keta) Con AII GtH (anti-Con AII). Anti-Con AII treatment significantly inhibited ovarian growth but did not consistently lower serum levels of vitellogenin. This suggests that Con AII GtH is necessary for early ovarian growth and, has one or more functions in early ovarian growth in addition to stimulation, via estrogen, of vitellogenin production.