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Biology of Reproduction 65, 1548-1557 (2001)
© 2001 Society for the Study of Reproduction, Inc.

Regular Article

Tissue-Specific Expression of Two Structurally Different Estrogen Receptor Alpha Isoforms along the Female Reproductive Axis of an Oviparous Species, the Rainbow Trout1

Arnaud Menueta, Isabelle Angladea, Gilles Flouriota, Farzad Pakdela, and Olivier Kah2,a

a Endocrinologie Moléculaire de la Reproduction, UMR CNRS 6026, Campus de Beaulieu, 35042 Rennes Cedex, France

ABSTRACT

In oviparous species, in addition to a full-length estrogen receptor alpha (ER{alpha}), another ER{alpha} isoform lacking the A domain and exhibiting a ligand-independent transactivation function has been consistently reported. Although both isoforms are expressed in the liver, their respective sites of expression in other potential target tissues are unknown. In contrast to the situation in Xenopus and chicken, the two isoforms of rainbow trout (Oncorhynchus mykiss) are generated from two classes of transcripts with different 5' untranslated sequences issued from the same gene by alternative splicing and promoter usage. The aim of this study was to take advantage of the unique organization of the rainbow trout ER{alpha} gene to investigate the tissue distribution of these two messenger species along the reproductive axis of female trout. The S1 nuclease assay and in situ hybridization were used, with probes specific for each of the transcripts. Reverse transcription polymerase chain reaction (RT-PCR) with primers specific for each of the isoforms also was performed. The data indicated that the full-length ER{alpha} is expressed in liver, brain, pituitary, and ovary, whereas expression of the isoform with the truncated A domain is restricted to the liver, demonstrating a tissue-specific expression of these two ER{alpha} isoforms. The presence of a short liver-specific isoform in oviparous species suggests its role in the development and/or maintenance of the unique function of the liver in the vitellogenesis process.

estradiol receptor, hypothalamus, oocyte development, ovary, pituitary

INTRODUCTION

In vertebrates, estradiol (E2) exerts a wide range of effects on many physiologic processes such as growth, development, or cell differentiation, but it is best known for being a key actor at all levels of the brain-pituitary-gonad axis in both males and females. These effects are mediated by specific nuclear receptors, the estrogen receptors (ER) [1]. These proteins, which belong to the nuclear receptor superfamily, are able to regulate the transcription of target genes and possess a modular organization in six domains from A to F. In the C-terminal region, the E domain is clearly involved in hormone binding and plays a key role in the ligand-dependent transactivation function (AF2). The C domain permits specific interactions with DNA by binding to specific cis-elements called estrogen-responsive elements (ERE) and possesses a dimerization capacity. In the N-terminal region, the A/B domain has a ligand-independent transactivation function (AF1) [2, 3].

In mammals, two subtypes of ER have been characterized by molecular cloning, ER{alpha} and ERß, which are generated by two different genes [4–6], and for each subtype, different isoforms have been reported [7, 8]. These isoforms are generated from either an alternative exon splicing or usage of different promoters of a single gene [9, 10], and their tissue-specific expression [1] could explain the pleiotropic roles of estrogens on different target tissues.

In addition to these functions, in oviparous species E2 plays a crucial role in the liver by stimulating the synthesis of vitellogenin, the main precursor of the oocyte reserves [11]. In teleosts, an ER{alpha} subtype was first characterized in rainbow trout (Oncorhynchus mykiss) [12] and then in other species such as the tilapia [13], the channel catfish [14], the Japanese eel [15], the red sea bream [16], and the gilthead sea bream [17]. More recently, the presence of an ERß has been reported in the gilthead sea bream [18], the channel catfish [19], the rainbow trout (GenBank AJ289883), the goldfish [20], and the zebrafish (GenBank AF349414). In several species (e.g., the Atlantic croaker [21], goldfish [22], and zebrafish (GenBank AF349413), a third form, either ER{gamma} or ERß2, has been described.

A characteristic shared by all fish ER{alpha} cloned until now is the lack of an A domain, but very recently the existence of two ER{alpha} isoforms having different estrogen dependencies and issuing from the same gene was reported in rainbow trout [23]. In the rainbow trout, the short isoform (rtER{alpha}S, 65 kDa) corresponds to the cDNA previously cloned from the liver [24], whereas the long isoform (rtER{alpha}L, 71 kDa) obtained from rainbow trout ovary possesses an extra 45 residues in the N-terminal region [23]. Very recent data in channel catfish also indicate the presence of several N-terminal ER{alpha} variants [25]. Similarly, two isoforms of ER{alpha}, including one lacking an A domain, have been reported in amphibians [26] and chicken [27], suggesting that the existence of the short ER{alpha} isoform could be a common feature of oviparous species.

Functional analysis of both ER{alpha} isoforms in chicken and rainbow trout showed that the short form exhibits basal estrogen-independent transactivation activity that can be further increased in the presence of E2, whereas the long form is characterized by strictly estrogen-dependent transcriptional activity [23, 27]. In a recent comparative study of the human and rainbow trout ER{alpha}, a conserved {alpha}-helicoidal structure that plays a key role in AF1 activity was identified at the beginning of the B domain [28]. The same authors showed that in the absence of ligand the presence of the A domain could repress this AF1 hormone-independent transactivation function [28].

The identification of this new isoform of rtER raises the question of the respective sites of expression of the two forms. All studies aiming at localizing ER mRNAs or proteins were based on riboprobes or antibodies corresponding to the hormone-binding domain, which is common to both rtER{alpha}S and rtER{alpha}L [29–33], and thus did not allow discrimination between the two mRNAs or proteins. Considering the difference in the structural characteristics of the two rtER{alpha} proteins, it is of importance to investigate the tissue specificity, if any, in the expression of these two forms.

The truncated form and the full-length form are encoded by two classes of transcripts derived from a single ER{alpha} gene. In chicken, the truncated receptor is encoded by a class of mRNA whose initiation site is in the open reading frame of the second class [27], whereas in trout an alternative usage of two promoters generates two classes of transcripts, rtER{alpha}L mRNA and rtER{alpha}S mRNA1,2, which differ significantly in their 5' untranslated region (Fig. 1) [23]. This unique organization of the rainbow trout ER{alpha} gene thus permits generation of specific probes of each class of mRNA encoding two structurally different proteins.



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  		FIG. 1. Schematic representation of the rtER{alpha} gene, transcripts, and resulting proteins (adapted from [23]). The positions of the transcription start points of rtER{alpha}S (+1) and rtER{alpha}L (+1') transcripts are noted. S1 probes A and B were used for the S1 nuclease mapping. Riboprobes C, L, and S were used for in situ hybridization. Riboprobes S and L are specific for exon 1 and exon 2a, respectively, whereas riboprobe C hybridizes with all rtER{alpha} mRNAs.

Here, we describe in an oviparous species the tissue distribution of the mRNAs corresponding to two different isoforms in different E2 target tissues. We used nuclease S1 mapping to identify which transcripts are expressed in the different reproductive organs, and these results were confirmed by in situ hybridization and reverse transcription polymerase chain reaction (RT-PCR) techniques, demonstrating the tissue-specific expression of the two mRNA species.

MATERIALS AND METHODS

Animals

Rainbow trout were supplied by the INRA fish farm (Le Drennec, Finistère, France) and kept in the laboratory in a recirculating water system at 12–15°C under an artificial light regime mimicking the natural photoperiod (46°N). The fish were fed a trout diet ad libitum and were treated in agreement with the European Union regulation concerning the protection of experimental animals. For RNA isolation, fish were killed by decapitation, and for in situ hybridization they were perfused intracardially under anesthesia. Immature (100 g), previtellogenic (650 g; gonadosomatic index [GSI]: 0.2%), early vitellogenic (800 g; GSI: 0.3%–0.4%), vitellogenic (1200 g; GSI: 5%), and mature (1500 g; GSI: 12%) females were used. An i.p. injection of E2 (1.5 mg/kg in ethanol/saline, v/v) was given to three immature, three previtellogenic, and early vitellogenic females 48 h before death. Control fish were given the vehicle alone.

RNA Isolation

Total RNA from liver, brain, pituitary, and ovary was extracted with TRIzol (Gibco-BRL, Eggenstein, Germany) as described by the manufacturer. Forebrain, pituitary, liver, and ovary total RNA corresponded to a pool of two to nine fish. The quality of total RNA was checked by agarose gel electrophoresis, and quantification was performed by spectrophotometry at 260 nm.

S1 Nuclease Assay

A modified S1 nuclease protection assay was performed as previously described [34]. The method involves the use of biotinylated single-stranded DNA template to prepare highly labeled single-stranded DNA probes by extension from a specific primer with the T7 DNA polymerase in the presence of [{alpha}-32P]dCTP (3000 Ci/mmol). These probes were hybridized with an appropriate RNA sample and digested by S1 nuclease (Boehringer Mannheim, Mannheim, Germany). The vectors containing complete rtER{alpha}S or rtER{alpha}L cDNAs were used to obtain PCR products corresponding to the region from +80 base pairs (bp) in exon 1 to +510 bp in exon 2 or from +120 bp in exon 2a to +595 bp in exon 2, respectively (Fig. 1) [23]. These DNA fragments were subcloned in the PCRTM2.1 vector (Invitrogen, San Diego, CA) downstream of T7 and upstream of the M13 reverse primer. To generate the probes, a PCR was performed with biotinylated T7 primer and the M13 reverse primer. The biotinylated PCR products were purified with the High Pure PCR product purification kit (Boehringer Manheim). Products were then bound to streptavidin-coated magnetic beads (Dynal, Great Neck, NY), and nonbiotinylated DNA strands were eluted in 0.1 M NaOH. The labeled probes were obtained in the presence of [{alpha}-32P]dCTP (3000 Ci/mmol) by extending primer 5 (5'-TCTCCAGGTAGTATGACTGGCTGG-3'), complementary to the 3' end of exon 2 and annealed to the biotinylated single-stranded template. The probes were then eluted by an alkaline solution and purified on a 4% denaturing polyacrylamide/urea gel. An amount corresponding to 2 x 105 cpm was coprecipitated with different quantities of RNA (30 µg for liver mRNA samples; 100 µg for ovaries, forebrain, and pituitary mRNA and yeast tRNA samples) and resuspended in 20 µl of hybridization buffer. The templates were then denaturated at 80°C for 10 min and incubated at 53°C overnight. Following S1 digestion, the samples were electrophoresed through a denaturing polyacrylamide/urea gel and exposed to a hyperfilm-ßMax (Amersham International, Uppsala, Sweden) plate for 24 h (liver) or 15 days (brain, pituitary, ovary, and yeast).

Qualitative RT-PCR

Complementary DNA was obtained from 1 µg of total RNA following reverse transcription with 50 U of Expand reverse transcriptase (Boehringer Mannheim) and random hexamers. To amplify rtER{alpha}L cDNA, PCR was performed with forward primer 6 located in exon 2a (5'-GTGAGCCAGTCTAAACCAAGCTG-3') and reverse primer 7 in exon 3 (5'-GTACCTCGTCTCGTTGGCCAT-3'). These primers generated a product 628 bp in length. A forward primer 8 located in exon 1 (5'-TCAACACAACAATGCTCATGAT-3') and reverse primer 7 were used to obtain a PCR product of 578 bp corresponding to rtER{alpha}S cDNA. Thirty-five cycles of amplification were carried out under the following conditions: denaturation at 94°C for 30 sec, annealing at 59°C for 30 sec, and extension at 72°C for 1 min. The amplification products were analyzed on a 1.5% agarose gel stained with ethidium bromide.

Riboprobe Synthesis

For in situ hybridization, three riboprobes were used (Fig. 1) [23]. To generate riboprobe C, a fragment of 776 bp corresponding to the E/F domain of the rtER{alpha} was inserted in the site ApaI of the plasmid pBS (Bluescribe; Stratagene, La Jolla, CA). The plasmid was linearized with EcoRI to give the antisense riboprobe using RNA polymerase T3. For the sense riboprobe synthesis by RNA polymerase T7, after linearization by KpnI the plasmid was pretreated with Klenow for blunt-end formation of recessed ends. For riboprobe L, a genomic PCR product corresponding to the region of -56 bp in intron 1 to +172 bp in exon 2a was obtained with primer 1 (5'-ACGAGAAAAGAGAGAGAGAGA-3') and primer 2 (5'-CTGTCTGACCAGCATCACATT-3') and was subcloned into PCR 2.1. Likewise, for riboprobe S, a PCR product corresponding to the region of -58 bp in proximal promoter rtER{alpha}S to +140 bp in exon 1 was obtained with primer 3 (5'-AGATACACTACTATCAATATCGAT-3') and primer 4 (5'-TTAATGACGTTATGGATCAAT-3') and inserted into PCR 2.1. After determination by PCR of the orientation of each inserted fragment, plasmids were linearized by BamHI. Sense and antisense riboprobes L and S were synthesized by RNA polymerase T7, using the RNA transcription kit (Boehringer Mannheim). The linearized DNA template (1 µg) was incubated for 1 h at 37°C in a solution containing transcription buffer (1x), rATP (0.4 mM), rCTP (0.4 mM), rGTP (0.4 mM), 35S-UTP (5 µCi/µl), RNase inhibitor (1.6 U/µl), and the adapted RNA polymerase and adjusted to 20 µl with sterile water. The DNA template was digested with 1 µl RQ-1 DNase (1 U/µl) for 15 min at 37°C. After incubation, 10 µg yeast tRNA dissolved in 8% formamide was added. Fragments were separated on a Sephadex G50 column as previously described [35].

For in situ hybridization, anesthetized rainbow trout (phenoxy ethanol, 4 ml/10 L fresh water) were perfused intracardially with 0.65% NaCl and 4% paraformaldehyde in PBS (pH 7.4). Brain, pituitary, ovary, and liver were removed and maintained in the same fixative overnight at 4°C. After dehydration in ethanol, tissues were embedded in Paraplast and transversally sectioned at 6 µm. Adjacent sections were mounted on parallel TESPA-treated slides (Sigma) and rehydrated prior to hybridization.

In Situ Hybridization

Radioactive in situ hybridization was performed as previously described [35]. Tissues were prepared for hybridization by rehydration, treatment with proteinase K (2 µg/ml for brain and pituitary, 20 µg/ml for liver and ovary), acetylation, and dehydration. After being air dried, tissue sections were hybridized under coverslips with the radioactive probe diluted in hybridization buffer (2 x 104 cpm riboprobe/µl; 70 µl on each slide) and incubated for 12–14 h at 55°C in a humid chamber. After hybridization, coverslips were removed by bathing in 5x standard saline citrate (SSC; 1x: 15 mM sodium citrate, 150 mM NaCl, pH 7) with 10 mM dithiothreitol (DTT) at 55°C for 30 min. The slides were washed twice in 2x SSC, 50% formamide, and 10 mM DTT at 65°C. After a rinse in NTE buffer (10 mM Tris-HCl, 0.5 M NaCl, 85 mM EDTA, pH 8), sections were treated with RNase A (20 µg/ml NTE; 30 min at 37°C) and then rinsed again with NTE buffer before another wash in 2x SSC, 50% formamide, and 10 mM DTT at 65°C for 30 min. Before autoradiography, the tissue was rinsed in 2x SSC (15 min) and 0.1x SSC (15 min) at room temperature and dehydrated in an ethanol series containing 0.3 M ammonium acetate. Autoradiography was performed by apposing the slides to hyperfilm-ßMax to produce film autoradiographs for analysis or by dipping the slides into Ilford K5 nuclear track emulsion. After exposure in the dark for 5–18 days at 4°C, slides were developed, counterstained with toluidine blue (0.01%), and coversliped in Depex (Gurr).

The slides were observed and photographed with a Provis photomicroscope (Olympus France, Rungis, France) using Ilford 400 ASA or Agfapan 25 ASA for bright-field and dark-field photographs, respectively. The specificity of the hybridization signal was systematically checked by hybridizing sense probes on parallel sections.

RESULTS

S1 Nuclease Assay

In the liver, the rtER{alpha} gene generates two classes of transcripts, rtER{alpha}S mRNA1,2 and rtER{alpha}L mRNA, by alternative splicing and by promoter usage (Fig. 1) [23]. The mRNA of rtER{alpha}L encodes a protein of 622 amino acids, whereas mRNA1,2 rtER{alpha}S codes for an N-terminus truncated protein of 577 amino acids that is 100% identical to the C-terminal portion of the rtER{alpha}L protein.

To determine the pattern of distribution of these two classes of transcripts in a panel of E2 target tissues (forebrain, pituitary, liver, and ovary) and to investigate whether any other splicing events occur, S1 nuclease assays were performed. Two single-stranded DNA S1 probes, probe A and probe B specific for rtER{alpha}L and rtER{alpha}S, respectively, were used.

Because rtER{alpha} mRNA expression levels strongly differ between tissues, films were exposed for either 24 h (liver) or 15 days (brain, pituitary, and ovary). Yeast tRNA was used as a negative control. After 24 h of exposure, hybridization of probe A with total RNA from liver of E2-treated and untreated previtellogenic female trout and S1 digestion showed three protected fragments, whereas no signal was detected with yeast tRNA (Fig. 2A). The signal obtained at 475 bp corresponds in size to a total protection of rtER{alpha}-specific sequences of probe A, confirming the existence of rtER{alpha}L mRNA in the liver. In addition to this band, two specific products of 408 and 366 bp were also detected, resulting from a partial protection of probe A by, respectively, rtER{alpha}S mRNA2 and rtER{alpha}S mRNA1, which remains homologous to probe A as far as the two acceptor splice sites of exon 2a. Hybridization with probe B followed by S1 digestion and a 24-h exposure, resulted in two protected fragments in liver mRNA samples (treated or not treated with E2; Fig. 2B). The fragment of 430 bp indicates full protection of probe B by mRNA1 rtER{alpha}S, corresponding to exons 1 and 2, and the shortest band of 366 bp indicates partial protection by mRNA rtER{alpha}L. The same pattern was obtained with RNA from mature females and E2-injected males (data not shown).



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  		FIG. 2. Trout tissue specificity of each class of transcripts, rtER{alpha}S and rtER{alpha}L, as shown by S1 nuclease assay. Total RNA from pituitary, forebrain, ovary (100 µg), and liver (30 µg) were incubated with S1 probe A (A) or probe B (B), submitted to S1 digestion, and separated on a 4% polyacrylamide gel. All samples are from females: brain and pituitary correspond to mature fish, ovary (+E2, -E2) to immature fish, and liver (+E2, -E2) to previtellogenic fish. Films were exposed for 15 days for pituitary, forebrain, and ovary and for 24 h for liver. Yeast tRNA was used as a negative control. The origin of the different protected fragments is schematically represented on the left of each panel. A) Partially protected fragments of 408 (*) and 366 (**) bp corresponding to mRNA2 and mRNA1, respectively, of rtER{alpha}S detected only in the liver. A fully protected fragment of 475 bp, corresponding to mRNA of rtER{alpha}L, is observed in all tissues. > indicates the undigested probe. B) Partially protected fragment of 366 bp, corresponding to mRNA of rtER{alpha}L, in all tissues. A fully protected fragment of 430 bp and corresponding to mRNA of rtER{alpha}S was detected only in the liver. > indicates the undigested probe

After 15 days of autoradiography, only one protected mRNA fragment was detectable with both probes in the brain and pituitary of mature females or in the ovary of immature females. With probe A, this fragment results from the protection of the full-length probe (exons 2a and 2; Fig. 2A), whereas with probe B, only the fragment corresponding to exon 2 is protected (Fig. 2B). These results indicate that only rtER{alpha}L mRNAs are expressed in the brain, pituitary, and ovary. Clear protected fragments were obtained in the ovary and pituitary, but in the forebrain the band intensity was lower, suggesting that rtER{alpha}L mRNA expression is low (Fig. 2, A and B). The absence of other protected fragments with probe A suggests that no other 5' splicing mechanisms are at work in these tissues. Whereas E2 treatment caused a clear increase in the amount of both mRNAs in the liver, the difference was not obvious in the ovary. No signal was observed in the ovary of previtellogenic females (GSI: 0.2%; oocyte diameter < 1 mm; data not shown).

In Situ Hybridization

The results obtained by S1 nuclease assay were in part confirmed by in situ hybridization on paraffin-embedded sections using three riboprobes. Riboprobe C (776 bp) corresponds to the C-terminal domain of rtER{alpha} and hybridizes with both short and long mRNA species. Riboprobe S (198 bp) corresponds to exon 1 and hybridizes only with rtER{alpha}S mRNAs, and riboprobe L (238 bp) corresponds to exon 2a and thus is specific of rtER{alpha}L mRNAs (Fig. 1) [23].

The liver of E2-treated and untreated previtellogenic females was used as a positive control because both mRNA populations are expressed in this organ [23]. With riboprobe C, a strong signal was detected in both E2-treated and untreated samples (Fig. 3A). Riboprobes L and S also resulted in a positive hybridization signal as compared with the respective controls (Fig. 3, B and C). A lower signal was obtained with riboprobes L and S probably because these probes are three times shorter than riboprobe C or because riboprobe C recognizes both short and long transcripts. In all cases, no signal was detected following hybridization of adjacent sections with the corresponding sense riboprobes (Fig. 3). These results established that the three probes were suitable for the detection of the corresponding mRNAs by in situ hybridization.



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  		FIG. 3. X-ray film images of in situ hybridization in trout liver using riboprobes C (A), L (B), and S (C). The upper panels (-E2) correspond to control previtellogenic females (650 g; GSI: 0.2%), and the lower panels correspond to E2-treated previtellogenic females. AS, Antisense probe; S, sense probe. Bar = 2 mm

As expected, the distribution of total rtER{alpha} mRNA as revealed by riboprobe C in the brain and the pituitary of mature females was similar to that previously reported in rainbow trout [29]. A clear signal was detected in the preoptic area at the level of the anterior parvocellular preoptic nucleus (Fig. 4) and in the mediobasal hypothalamus at the level of the posterior nucleus lateralis tuberis (Fig. 5). Hybridization of adjacent sections with riboprobe L resulted in exactly the same pattern of distribution of the long rtER{alpha}L mRNA, although the signal was lower, probably because of the shorter probe size. However, absolutely no signal was obtained with riboprobe S, indicating the absence or very low expression of rtER{alpha}S mRNA in these regions (compare Fig. 4, A–C; Fig. 4, D–F; and Fig. 5, A–C). Adjacent sections treated with the corresponding sense probes did not exhibit any signal (data not shown).



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  		FIG. 4. In situ hybridization of parallel sections in the preoptic region of mature female trout hybridized with riboprobe C (A and D), riboprobe L (B and E), and riboprobe S (C and F). Panels A, B, and C correspond to the anterior preoptic recess (rpo), and D, E, and F correspond to the level of maximal extension of the preoptic recess. A specific signal was obtained in the anterior nucleus preopticus parvicellularis (Ppa) with riboprobes C and L, but no signal was detected with riboprobe S. Bar = 100 µm



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  		FIG. 5. In situ hybridization of parallel sections in the trout mediobasal hypothalamus (A–C) and pituitary (D–F) of mature females hybridized with riboprobe C (A and D), riboprobe L (B and E), and riboprobe S (C and F). A specific signal was obtained in the nucleus lateralis tuberis (nlt) and in cells of the proximal pars distalis (PPD) with riboprobes C and L, but no signal was detected with riboprobe S. III, Third ventricle. Bars = 200 µm (A–C) and 15 µm (D–F)

Similar data were obtained in the pituitary gland, where a hybridization signal was obtained in the proximal pars distalis with riboprobes C (Fig. 5D) and L (Fig. 5E), but no signal was detected on adjacent sections with riboprobe S (Fig. 5F). No hybridization was detected when using the corresponding sense probes (data not shown).

The presence of rtER{alpha} mRNAs was also investigated in the ovary of E2-treated and untreated previtellogenic and early vitellogenic females. Using riboprobe C, a weak but consistent signal was detected in the follicular layer surrounding the oocytes (Fig. 6A) only in previtellogenic animals. This signal seemed to be increased by E2 treatment (Fig. 6B). At the histologic level, this labeling clearly corresponds primarily to the granulosa cells (Fig. 6D); the thecal and interstitial cells have fewer silver grains. Hybridization with the sense probe on adjacent sections showed no signal (Fig. 6, C and E). No labeling could be detected with riboprobe L or riboprobe S, again probably because of their lower specific activities (data not shown). As already observed with the S1 nuclease assay, no in situ hybridization signal was detected in the ovary of previtellogenic females.



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  		FIG. 6. In situ hybridization of ovaries from a control (A) and E2-treated (B and C) early vitellogenic female trout using riboprobe C. A signal was detected in the follicular layers with the antisense probe (A, B, and E) but not with the sense probe (C and F). Gr, Granulosa cells; oo, oocyte; th, thecal cells. Bars = 200 µm (A–C) and 35 µm (E and F)

Qualitative RT-PCR

To confirm expression of rtER{alpha}L mRNA in the ovary during the reproductive cycle, qualitative RT-PCR was used. Single-stranded cDNAs were synthesized by random priming of total RNA from ovaries from immature, previtellogenic, vitellogenic, and mature fish. The liver from previtellogenic E2-treated and untreated females was used as a positive control. As expected, short and long rtER{alpha} amplification products of 578 bp and 628 bp, respectively, were evident in the liver (Fig. 7). Whatever the stage, only amplification products corresponding to rtER{alpha}L were visible in the ovary.



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  		FIG. 7. Qualitative RT-PCR analysis of the rtER{alpha} long (lanes 1–7) and rtER{alpha} short (lanes 8–14) isoforms in the trout ovary at different stages of maturation. M, markers, bp number is indicated on the left. Lanes 1 and 8: liver, positive control (E2 treated); lanes 2 and 9: liver, positive control (untreated); lanes 3 and 10: immature; lanes 4 and 11: previtellogenic; lanes 5 and 12: vitellogenic; lanes 6 and 13: mature; lanes 7 and 14: negative control (water)

DISCUSSION

The existence of two rtER{alpha} mRNA isoforms differing in their N-terminus has been reported in Xenopus [26], chicken [27, 36], rainbow trout [23], and channel catfish [25] and thus seems to be a common feature of oviparous species. However, although in these species the two isoforms are expressed in the liver, information concerning their tissue expression is limited because in Xenopus and chicken the structure of the gene does not allow manufacture of probes specific for each of the two isoforms. However, the unique structure of the rtER{alpha} gene, in which the 5' untranslated regions of the two mRNAs are transcribed from two different exons [23], permits study of the tissue specificity of expression of the two mRNA populations using probes without overlapping sequences. Using a combination of S1 nuclease assay, in situ hybridization, and RT-PCR, tissue-specific expression of two rtER{alpha} mRNA isoforms was demonstrated along the female reproductive axis: the rtER{alpha}L mRNA species was detected in the liver, brain, pituitary, and ovary, and the rtER{alpha}S transcripts were restricted to the liver. With the exception of the liver, expression of rtER{alpha} in other tissues was low, as shown by in situ hybridization, but despite the dilution of the rtER{alpha} transcripts in a complex mRNA mixture, S1 nuclease assays still allowed detection of a specific signal, highlighting the usefulness of this method. The results indicate that in the different tissues investigated no other splicing mechanisms between exon 1 and exon 2 occur upstream of the gene, but we cannot exclude the possible downstream existence of truncated mRNAs, as suggested by the fact that a 1.4-kilobase band was detected in the rainbow trout pituitary by Northern blot [24]. Very short N-terminal-deleted transcripts have been reported in both fishes [25] and mammals [37], but neither these isoforms nor the rtERß would be recognized by our probes.

In the brain and pituitary, the present results confirm data obtained previously by in situ hybridization and showing that rtER{alpha} mRNAs are localized in portions of the ventral telencephalon, the anterior ventral preoptic region, the mediobasal hypothalamus, and the proximal pars distalis of the pituitary [29]. However, both nuclease S1 and in situ hybridization, using probes specific of the two mRNAs species, indicate that only the long isoform, rtER{alpha}L mRNA corresponding to the full-length receptor, is active in these regions. The long mRNAs are likely to be translated into a functional full-length receptor because immunohistochemical studies using antibodies against the hormone-binding domain resulted in the same brain and pituitary distribution as that of the rtER{alpha}L mRNAs in the present study [30–33]. Therefore, all these data indicate that a full-length rtER{alpha} is expressed in the brain and pituitary of the rainbow trout. At present, there is little information on the expression of ERß in the brain and the pituitary of fishes, but central expression of ERß and ER{gamma} has been reported in the Atlantic croaker [21]. In this species, preliminary in situ hybridization data indicated that ER{alpha}, ERß, and ER{gamma} mRNAs have a similar but slightly different distribution in the preoptic region and mediobasal hypothalamus [21]. Further studies will be necessary to firmly establish the expression patterns of the different ERs in the brain of teleosts. In mammals, ERß mRNAs have also been detected in the brain and pituitary [38].

The presence of ER{alpha} in fish ovary has been revealed by RT-PCR or Northern blot in several species, notably the rainbow trout [39, 40], the gilthead sea bream [18], and the channel catfish [41], but the precise localization and the potential functions of ovarian ERs remain unknown. Full-length rtER{alpha} has been cloned from an ovarian cDNA library [23], and our nuclease S1 assays showed a clear band, corresponding to rtER{alpha}L, in the ovary of immature (oocyte diameter < 100 µm) but not in previtellogenic females (oocyte diameter < 1 mm). However, by in situ hybridization, a weak positive signal could be obtained only with riboprobe C in early vitellogenic ovaries (mean oocyte diameter: 1.3 mm) but not in previtellogenic females. These data agree with those of a recent study in rainbow trout, based on quantitative RT-PCR, showing that ovarian ER{alpha} mRNAs are found during two distinct periods of ovarian development: in previtellogenic fish with ovarian follicle diameter of <=100 µm and in midvitellogenic females with ovarian follicle diameter >1000 µm [39]. However, the primers used in that study did not discriminate between short and long rtER{alpha} mRNA species. Our qualitative RT-PCR data indicate that at all stages only rtER{alpha}L mRNA is expressed in the ovary. These data together suggest that only the full-length rtER{alpha} is present in the ovary, but the low expression level probably explains why riboprobe L did not reveal an in situ hybridization signal. Nevertheless, our in situ hybridization results indicate that these transcripts are localized within the follicular layers surrounding the oocytes, more specifically in the granulosa cells. This finding is the first evidence of ER{alpha} mRNA in the ovary of a fish. A role of rtER{alpha} in sex determination is possible because E2 treatment of male fish causes intersex [42] or sex reversal [40]. The pattern of expression of rtER{alpha} in the ovary is opposite that of the vitellogenin receptor, whose maximum expression decreases when oocytes reach a diameter of 1000 µm [43]. Thus, ER{alpha} in the fish ovary may be involved in the regulation of vitellogenin receptor expression. The question remains whether, as shown in the liver [12, 44, 45], rtER{alpha} in the ovary is upregulated by E2 itself.

In addition to rtER{alpha}, there is recent evidence in fishes for ovarian expression of rtERß, as reported in tilapia [46], channel catfish [14, 41], gilthead sea bream [18], and goldfish [19, 20]. At present, there is no detailed information regarding the precise ovarian distribution of ERß in teleosts. In mammals, there appears to be a higher level of ovarian expression of ERß as compared with ER{alpha}, but there are discrepancies in the literature concerning the cellular distribution of ER{alpha} and ERß [38, 47, 48].

The two rtER{alpha} isoforms differ strikingly with respect to their transcriptional activity. Similar to ER{alpha} from other vertebrates, the full-length receptor exhibits a strictly estrogen-dependent transcriptional activity, whereas the short isoform has a hormone-independent transactivation capacity, representing 15%–25% of the total receptor activity depending on the cell and promoter context [49]. Steroid receptors possess two transactivation functions, AF1 and AF2, located in the B and E domains, respectively [50]. Recent data have demonstrated that AF1 relies on the presence in the B domain of an alpha helicoidal structure, which is conserved from fishes to mammals, and that AF1 is repressed in the presence of the A domain [28], which explains the hormone-independent activity of the A-truncated rtER{alpha}. Until now, there has been no precise information regarding the respective physiologic functions of those two isoforms, whose existence seems to be a characteristic of oviparous species. However, given that the short ER{alpha} isoform appears to be mainly expressed in the liver, such receptors could be somehow involved in driving the liver toward its vitellogenic function in the adult and/or during development in oviparous species, according to estrogen-independent mechanisms [23]. Experiments are in progress to study the expression and function of these isoforms during development.

In mammals, the existence of several ER{alpha} truncated isoforms also has been reported, but these isoforms have been mainly studied in cancer tissues and cell lines. The human ER{alpha} gene generates several mRNA variants (A–F mRNA), which differ in their 5' untranslated region but code for the same ER{alpha} protein of 66 kDa [51]. Recently, a novel isoform of 46 kDa encoded by the same gene has been described. This isoform is characterized by the lack of an A/B domain and could play a key role in cell proliferation [7]. In the rat, ER{alpha} isoforms lacking the second zinc finger of domain C (isoform {Sigma}3), part of the hormone-binding domain ({Sigma}4), or both ({Sigma}3-4) are transiently expressed in the pituitary during development 4 days earlier than are the full-length ER{alpha} isoforms. These {Sigma}3 and {Sigma}4 isoforms could block estrogen responsiveness and/or modulate gene transcription in an estrogen-independent fashion [52]. In the adult female rat, TERP-1 (a truncated ER{alpha} lacking the A–C domains) is specifically expressed in the pituitary on the day of estrus [37]. The ability of TERP-1 to both enhance and inhibit ER-dependent promoter activity suggests that TERP-1 may play a physiologic role in estrogen feedback in the rat pituitary. The existence of various tissue-specific ER isoforms could partly explain the pleiotropic effects of E2 in a variety of target organs.

Two structurally different rtER{alpha} isoforms with different E2 dependencies are differentially expressed along the reproductive axis of the rainbow trout. The liver specifically expresses an A domain-truncated rtER{alpha}, which might be involved in the establishment and/or maintenance of the liver vitellogenic function in oviparous species. Because most ER{alpha} cDNA sequences from fishes have been obtained from the liver, a full-length ER{alpha} may exist in other teleost species, and the differential distribution of the two ER{alpha} isoforms reported in rainbow trout may be a common characteristic of all fishes and other oviparous vertebrates.

FOOTNOTES

First decision: 25 January 2001.

1 This work was supported by the CNRS, the INRA, the French Ministry for Education, Research and Technology, and the Foundation Langlois. Back

2 Correspondence. FAX: 33 2 99 28 67 94;olivier.kah{at}univ-rennes1.fr Back

Accepted: June 27, 2001.

Received: January 8, 2001.

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