Oyster estrogen receptor: cDNA cloning and immunolocalization

Biological transformations of steroidal compounds: a review

Microbial transformation is an important tool for structural modification of organic compounds, especially natural products with complex structures like steroids. It can be used to synthesize chemical structures that are difficult to obtain by ordinary methods and as a model of mammalian metabolism due to similarity between mammalian and microbial enzyme systems. During recent years research has been focused on the structural modifications of bioactive steroids by using various microorganisms, in order to obtain biologically potent compounds with diverse structures. Steroidal compounds are responsible for important biological functions in the cells and manifest a variety of activities. This article covers the microbial transformation of sterols, steroidal hormones and some new types of steroids known as bufadienolides. Emphasis has placed on reporting metabolites that may be of general interest and on the practical aspects of work in the field of microbial transformations. The review covers the literature from 1994 to 2011.

Molecular cloning and characterization of estrogen receptor gene in the scallop Chlamys farreri: expression profiles in response to endocrine disrupting chemicals

In order to gain insights into the mechanism of sex steroid signaling in molluscs, the full-length cDNA of estrogen receptor (ER) was isolated and characterized from Chlamys farreri for the first time. The positions of cysteine residues and other residues around them that constitute the two zinc finger motifs and the P-box are conserved. Phylogenetic analysis revealed that the CfER is an ortholog of the other mollusk ERs. Tissue distribution analysis of the CfER mRNA revealed that the expression of ER mRNA was observed in various tissues, and highest in the gonad of males and females. C. farreri were exposed for 10 days to endocrine disrupting chemicals including Benzo(a)pyrene (B(a)p) and polybrominated diphenyl ethers (BDE-47). B(a)p exposure at 0.4 and 2 μg/L caused significant increase in mRNA expression of ER and VTG, but B(a)p at 10 μg/L down-regulated CfER and VTG mRNA expression compared to control. Varying increase of ER and VTG mRNA transcripts was resulted in by BDE-47 at 0.1, 1 and 10 μg/L. These results elucidate potential roles of CfER induced by xenobiotics in C. farreri and can be helpful for investigating the mechanism of sex steroid signaling in bivalve mollusks.

Conservative nature of oestradiol signalling pathways in the brain lobes of octopus vulgaris involved in reproduction, learning and motor coordination

Oestradiol plays crucial roles in the mammalian brain by modulating reproductive behaviour, neural plasticity and pain perception. The cephalopod Octopus vulgaris is considered, along with its relatives, to be the most behaviourally advanced invertebrate, although the neurophysiological basis of its behaviours, including pain perception, remain largely unknown. In the present study, using a combination of molecular and imaging techniques, we found that oestradiol up-regulated O. vulgaris gonadotrophin-releasing hormone (Oct-GnRH) and O. vulgaris oestrogen receptor (Oct-ER) mRNA levels in the olfactory lobes; in turn, Oct-ER mRNA was regulated by NMDA in lobes involved in learning and motor coordination. Fluorescence resonance energy transfer analysis revealed that oestradiol binds Oct-ER causing conformational modifications and nuclear translocation consistent with the classical genomic mechanism of the oestrogen receptor. Moreover, oestradiol triggered a calcium influx and cyclic AMP response element binding protein phosphorylation via membrane receptors, providing evidence for a rapid nongenomic action of oestradiol in O. vulgaris. In the present study, we demonstrate, for the first time, the physiological role of oestradiol in the brain lobes of O. vulgaris involved in reproduction, learning and motor coordination.

Immunolocalization of estrogen receptor α in Neomysis japonica oocytes and follicle cells during ovarian development

Estrogen induces oocytes development and vitellogenesis in crustacean by interacting with estrogen receptor (ER) subtypes. In the present study, we detect for the first time the ERα in oocytes and follicle cells and hepatopancreas cells of mysis by immunohistochemistry using a specific ERα antibody. ERα was mainly localized in the nuclei of oocytes and follicle cells, while mainly detected in nuclei of oogonia (OG), previtellogenic oocyte (PR) and endogenous vitellogenic oocyte (EN) at previtellogenic and early vitellogenic stage (I-early III). Follicle cells in all stages of ovary (all vitellogenic stages) showed strong ERα positive reaction, and they were able to gradually move to oocytes during the development of oocytes. In addition, ERα was also localized in the nuclei and cytoplasm of four hepatopancreas cells (including E-, R-, F- and B-cell) in all ovary stages. These findings suggest, for the first time to our knowledge, that there could be a close link between oogenesis, follicle cells, hepatopancreas cells and endocrine regulation, and estrogens might be involved in the regulation of oocytes at early ovarian stage in mysis.

Freshwater mudsnail (Potamopyrgus antipodarum) estrogen receptor: identification and expression analysis under exposure to (xeno-)hormones

Molluscs are raising attention as ecotoxicological test organisms due to their high diversity and ecological importance. The ovoviviparous prosobranch gastropod Potamopyrgus antipodarum (freshwater mudsnail) responds very sensitively to xenobiotics and has therefore been proposed as OECD standard test organism. Endocrine disrupting chemicals influence the reproduction of P. antipodarum, which can be assessed by embryo numbers in the brood pouch. However, the knowledge about the endocrine system of P. antipodarum is rather limited. The aim of this study was to identify an estrogen receptor in the endocrine system of P. antipodarum and to investigate if this receptor is differentially expressed under exposure to (xeno-)hormones (17α-ethinylestradiol, bisphenol A and 17α-methyltestosterone). The DNA-binding domain of the identified ER-like transcript has an amino acid identity of 92 percent compared to the ER of the gastropod Nucella lapillus (84 percent to human ERα) and 83 percent in the ligand binding domain (38 percent to human ERα). Furthermore, the P. antipodarum ER is transcriptionally regulated as shown by quantitative real-time PCRs of (xeno-)hormone exposed snails. 17α-ethinylestradiol and bisphenol A exposure resulted in a transitory ER-mRNA increase while17α-methyltestosterone caused a transitory reduction of ER-mRNA. In addition the solvent dimethyl sulfoxide had also a modulating effect on the receptor.

Evolutionary origins of the estrogen signaling system: insights from amphioxus

Classically, the estrogen signaling system has two core components: cytochrome P450 aromatase (CYP19), the enzyme complex that catalyzes the rate limiting step in estrogen biosynthesis; and estrogen receptors (ERs), ligand activated transcription factors that interact with the regulatory region of target genes to mediate the biological effects of estrogen. While the importance of estrogens for regulation of reproduction, development and physiology has been well-documented in gnathostome vertebrates, the evolutionary origins of estrogen as a hormone are still unclear. As invertebrates within the phylum Chordata, cephalochordates (e.g., the amphioxus of the genus Branchiostoma) are among the closest invertebrate relatives of the vertebrates and can provide critical insight into the evolution of vertebrate-specific molecules and pathways. To address this question, this paper briefly reviews relevant earlier studies that help to illuminate the history of the aromatase and ER genes, with a particular emphasis on insights from amphioxus and other invertebrates. We then present new analyses of amphioxus aromatase and ER sequence and function, including an in silico model of the amphioxus aromatase protein, and CYP19 gene analysis. CYP19 shares a conserved gene structure with vertebrates (9 coding exons) and moderate sequence conservation (40% amino acid identity with human CYP19). Modeling of the amphioxus aromatase substrate binding site and simulated docking of androstenedione in comparison to the human aromatase shows that the substrate binding site is conserved and predicts that androstenedione could be a substrate for amphioxus CYP19. The amphioxus ER is structurally similar to vertebrate ERs, but differs in sequence and key residues of the ligand binding domain. Consistent with results from other laboratories, amphioxus ER did not bind radiolabeled estradiol, nor did it modulate gene expression on an estrogen-responsive element (ERE) in the presence of estradiol, 4-hydroxytamoxifen, diethylstilbestrol, bisphenol A or genistein. Interestingly, it has been shown that a related gene, the amphioxus "steroid receptor" (SR), can be activated by estrogens and that amphioxus ER can repress this activation. CYP19, ER and SR are all primarily expressed in gonadal tissue, suggesting an ancient paracrine/autocrine signaling role, but it is not yet known how their expression is regulated and, if estrogen is actually synthesized in amphioxus, whether it has a role in mediating any biological effects. Functional studies are clearly needed to link emerging bioinformatics and in vitro molecular biology results with organismal physiology to develop an understanding of the evolution of estrogen signaling. This article is part of a Special Issue entitled 'Marine organisms'.

Biosynthesis and metabolism of steroids in molluscs

Molluscs are the second most diverse animal group, they are ecologically important and they are considered excellent indicators of ecosystem health. Some species have been widely used in pollution biomonitoring programs; however, their endocrinology is still poorly known. Despite some studies reporting the presence of (vertebrate-type) steroids in molluscs, information regarding enzymatic pathways involved in steroid synthesis and further catabolism of those steroids is still fragmentary. Regarding steroidogenesis, a number of excellent studies were performed in the 70s using different radio-labelled steroid precursors and detecting the formation of different metabolites. But, since then a long gap of research exist until the late 90s when the 'endocrine disruption' issue raised the need of a better knowledge of mollusc (and invertebrate) endocrinology in order to assess alterations caused by pollutants. Here we summarize past and recent studies dealing with steroid biosynthesis and metabolism in different mollusc species. Most of these studies suggest the involvement of steroids in mollusc reproduction. However, the knowledge is still fragmentary and many questions remain to be answered.

Identification of reproduction-specific genes associated with maturation and estrogen exposure in a marine bivalve Mytilus edulis

Background

While it is established that vertebrate-like steroids, particularly estrogens (estradiol, estrone) and androgens (testosterone), are present in various tissues of molluscs, it is still unclear what role these play in reproductive endocrinology in such organisms. This is despite the significant commercial shellfishery interest in several bivalve species and their decline.

Methodology/Principal Findings

Using suppression subtraction hybridisation of mussel gonad samples at two stages (early and mature) of gametogenesis and (in parallel) following controlled laboratory estrogen exposure, we isolate several differentially regulated genes including testis-specific kinases, vitelline lysin and envelope sequences.

Conclusions

The differentially expressed mRNAs isolated provide evidence that mussels may be impacted by exogenous estrogen exposure.

Exposure to 17α-ethynylestradiol causes dose and temporally dependent changes in intersex, females and vitellogenin production in the Sydney rock oyster

Although mounting evidence suggests exposure to estrogenic contaminants increases vitellogenin production in molluscs, demonstration of dose-response relationships and knowledge of the temporal nature of the vitellogenin response with continual exposure is currently lacking for biomarker utility. To address this knowledge gap, adult Sydney rock oysters, Saccostrea glomerata, were exposed to a range of environmentally relevant concentrations of 17α-ethynylestradiol (EE2) (0, 6.25, 12.5, 25 or 50 ng/l) in seawater under laboratory conditions. Vitellogenin induction and gonadal development was assessed following 4, 21 and 49 days exposure to EE2. Vitellogenin was found to increase in a dose dependent manner with EE2 exposure for females (4 and 49 days) and males (4 and 21 days). Histological examination of gonads revealed a number of individuals exhibited intersex (ovotestis) in 50 ng/l EE2 (after 21 days) and in 6.25 and 12.5 ng/l EE2 (after 49 days). Furthermore, a significant shift towards females was observed following 49 days exposure at 50 ng/l EE2 suggesting estrogenic exposure is capable of facilitating a progression for protandric males from male-intersex-female gametal status. Increases in female vitellogenin (4 days) were predictive of later increases in female developmental stages at 21 days and increases in oocyte area following 49 days. Male vitellogenin (4 days) was predictive of decreased male percentages and lower male developmental stages at 49 days. Vitellogenin in S. glomerata is a predictive biomarker of estrogenic exposure and effect if sampled soon after exposure and at the commencement of a gonadal development cycle.

Effects of estrogen exposure in mussels, Mytilus edulis, at different stages of gametogenesis

Mytilus edulis were exposed to 17beta-estradiol (E2) and the synthetic estrogens ethinyl estradiol (EE2) and estradiol benzoate (EB) for 10 days. Two exposures were performed to determine their effect on vitellogenin (VTG) and estrogen receptor 2 (ER2) mRNA expression at different stages of the reproductive cycle. Significant natural variation was not observed in VTG mRNA expression, though ER2 mRNA expression displayed significantly lower values during January, February and July compared with other times of the year. A significant increase in VTG and ER2 mRNA expression was observed in mussels exposed to estrogens at the early stage of gametogenesis. In contrast, mature mussels displayed no statistically significant change in the VTG or ER2 mRNA expression. The data presented suggests that the reproductive physiology of molluscs, in terms of VTG and ER2 mRNA expression, may be susceptible to damage by environmental estrogens at certain points in their gametogenesis process.

Estrogen-dependent transactivation of amphioxus steroid hormone receptor via both estrogen and androgen response elements

Estrogens are necessary for ovarian differentiation during critical developmental windows in most vertebrates and promote the growth and differentiation of the adult female reproductive system. Estrogen actions are largely mediated through the estrogen receptors (ERs), which are ligand-activated transcription factors. To understand the molecular evolution of sex steroid hormone receptors, we isolated cDNAs encoding two steroid receptors from Japanese amphioxus, Branchiostoma belcheri: an ER ortholog and a ketosteroid receptor (SR) ortholog. Reporter gene assays revealed that the SR ortholog has molecular functions similar to those of the vertebrate ER. Surprisingly, the ER ortholog is an estrogen-insensitive repressor of SR-mediated transcription. Furthermore, we found that the SR ortholog can bind to both estrogen-responsive elements (EREs) and androgen-responsive elements (AREs) and mediates transcriptional activation by estrogens through both types of elements. Our findings suggest that the ancestral SR, but not ER, could bind estrone and induce the ERE- and ARE-dependent transactivation and that it gained the ability to be regulated by ketosteroid and recognize ARE specifically before jawless vertebrates split. These results highlight the importance of comparative experimental approaches for the evolutionary study of endocrine systems.

Hormone-activated estrogen receptors in annelid invertebrates: implications for evolution and endocrine disruption

As the primary mediators of estrogen signaling in vertebrates, estrogen receptors (ERs) play crucial roles in reproduction, development, and behavior. They are also the major mediators of endocrine disruption by xenobiotic pollutants that mimic or block estrogen action. ERs that are sensitive to estrogen and endocrine disrupters have long been thought to be restricted to vertebrates: although there is evidence for estrogen signaling in invertebrates, the only ERs studied to date, from mollusks and cephalochordates, have been insensitive to estrogen and therefore incapable of mediating estrogen signaling or disruption. To determine whether estrogen sensitivity is ancestral or a unique characteristic of vertebrate ERs, we isolated and characterized ERs from two annelids, Platynereis dumerilii and Capitella capitata, because annelids are the sister phylum to mollusks and have been shown to produce and respond to estrogens. Functional assays show that annelid ERs specifically activate transcription in response to low estrogen concentrations and bind estrogen with high affinity. Furthermore, numerous known endocrine-disrupting chemicals activate or antagonize the annelid ER. This is the first report of a hormone-activated invertebrate ER. Our results indicate that estrogen signaling via the ER is as ancient as the ancestral bilaterian animal and corroborate the estrogen sensitivity of the ancestral steroid receptor. They suggest that the taxonomic scope of endocrine disruption by xenoestrogens may be very broad and reveal how functional diversity evolved in a gene family central to animal endocrinology.

Analysis of genes isolated from plated hemocytes of the Pacific oyster, Crassostreas gigas

A complementary deoxyribonucleic acid library was constructed from hemocytes of Crassostrea gigas that had been plated on poly-lysine plates for 24 h. From this library, 2,198 expressed sequence tags (ESTs) of greater than or equal to 100 bp were generated and analyzed. A large number of genes that potentially could be involved in the physiology of the oyster hemocyte were uncovered. They included proteins involved in cytoskeleton rearrangement, proteases and antiproteases, regulators of transcription and translation, cell death regulators, receptors and their associated protein factors, lectins, signal transduction proteins, and enzymes involved in eicosanoid and steroid synthesis and xenobiotic metabolism. Based on their relationship with innate immunity, the expression of selected genes was analyzed by quantitative polymerase chain reaction in gills from bacterial-challenged oysters. Several genes observed in the library were significantly upregulated by bacterial challenge including interleukin 17, astacin, cystatin B, the EP4 receptor for prostaglandin E, the ectodysplasin receptor, c-jun, and the p100 subunit of nuclear factor-kB. Using a similar approach, we have been analyzing the genes expressed in trout macrophages. While there are significant differences between the types of genes present in vertebrate macrophages compared with oyster hemocytes, there are some striking similarities including proteins involved in cytoskeletal rearrangement, proteases and antiproteases, and genes involved in certain signal transduction pathways underlying immune processes such as phagocytosis. Finally, C. virginica homologs of some of the C. gigas genes uncovered in the ESTs were obtained by aligning the ESTs reported here, against the assembled C. virginica ESTs at the National Center for Biotechnology Information.

The evolution of the ligand/receptor couple: a long road from comparative endocrinology to comparative genomics

Comparative endocrinology considers the evolution of bioregulatory systems and the anatomical structures and molecules that constitute the neuroendocrine and endocrine systems. One aim of comparative endocrinology is to trace the origins of the main endocrine systems. The understanding of the evolution of the ligand/receptor couple is central to this objective. One classical approach to tackle this question is the characterization of receptors and ligands in various types of non-model organisms using as a starting point the knowledge accumulated on classical models such as mammals (mainly human and mouse) and arthropods (with Drosophila among other insects). In this review we discuss the potential caveats associated to this two-by-two comparison between a classical model and non-model organisms. We suggest that the use of an evolutionary approach involving comparisons of several organisms in a coherent framework permits reconstruction of the most probable scenarios. The use of the vast amount of genomic data now available, coupled to functional experiments, offers unprecedented possibilities to trace back the origins of the main ligand/receptor couples.

Trichoplax, the simplest known animal, contains an estrogen-related receptor but no estrogen receptor: Implications for estrogen receptor evolution

Although, as their names imply, estrogen receptors [ERs] and estrogen-related receptors [ERRs] are related transcription factors, their evolutionary relationships to each other are not fully understood. To elucidate the origins and evolution of ERs and ERRs, we searched for their orthologs in the recently sequenced genome of Trichoplax, the simplest known animal, and in the genomes of three lophotrochozoans: Capitella, an annelid worm, Helobdella robusta, a leech, and Lottia gigantea, a snail. BLAST searches found an ERR in Trichoplax, but no ER. BLAST searches also found ERRs in all three lophotrochozoans and invertebrate-like ERs in Capitella and Lottia, but not in Helobdella. Unexpectedly we find that the Capitella ER sequence is closest to ERbeta, unlike the other invertebrate ER sequences, which are closest to ERalpha. Our database searches and phylogenetic analysis indicate that invertebrate ERs evolved in a lophotrochozoan and steroid-binding ERs evolved in a deuterostome.

An amphioxus orthologue of the estrogen receptor that does not bind estradiol: insights into estrogen receptor evolution

Background

The origin of nuclear receptors (NRs) and the question whether the ancestral NR was a liganded or an unliganded transcription factor has been recently debated. To obtain insight into the evolution of the ligand binding ability of estrogen receptors (ER), we comparatively characterized the ER from the protochordate amphioxus (Branchiostoma floridae), and the ER from lamprey (Petromyzon marinus), a basal vertebrate.

Results

Extensive phylogenetic studies as well as signature analysis allowed us to confirm that the amphioxus ER (amphiER) and the lamprey ER (lampER) belong to the ER group. LampER behaves as a "classical" vertebrate ER, as it binds to specific DNA Estrogen Responsive Elements (EREs), and is activated by estradiol (E₂), the classical ER natural ligand. In contrast, we found that although amphiER binds EREs, it is unable to bind E₂ and to activate transcription in response to E₂. Among the 7 natural and synthetic ER ligands tested as well as a large repertoire of 14 cholesterol derivatives, only Bisphenol A (an endocrine disruptor with estrogenic activity) bound to amphiER, suggesting that a ligand binding pocket exists within the receptor. Parsimony analysis considering all available ER sequences suggest that the ancestral ER was not able to bind E₂ and that this ability evolved specifically in the vertebrate lineage. This result does not support a previous analysis based on ancestral sequence reconstruction that proposed the ancestral steroid receptor to bind estradiol. We show that biased taxonomic sampling can alter the calculation of ancestral sequence and that the previous result might stem from a high proportion of vertebrate ERs in the dataset used to compute the ancestral sequence.

Conclusion

Taken together, our results highlight the importance of comparative experimental approaches vs ancestral reconstructions for the evolutionary study of endocrine systems: comparative analysis of extant ERs suggests that the ancestral ER did not bind estradiol and that it gained the ability to be regulated by estradiol specifically in the vertebrate lineage, before lamprey split.

Effects of 4-nonylphenol and 17alpha-ethynylestradiol exposure in the Sydney rock oyster, Saccostrea glomerata: Vitellogenin induction and gonadal development

Adult Saccostrea glomerata were exposed to environmentally relevant concentrations of 4-nonylphenol (1microg/L and 100microg/L) and 17alpha-ethynylestradiol (5ng/L and 50ng/L) in seawater over 8 weeks. Exposures were performed to assess effects on vitellogenin induction and gonadal development during reproductive conditioning. Chronic direct estrogenicity within gonadal tissue was assessed via an estrogen receptor-mediated, chemical-activated luciferase reporter gene-expression assay (ER-CALUX). Estradiol equivalents (EEQ) were greatest in the 100microg/L 4-nonylphenol exposure (28.7+/-2.3ng/g tissue EEQ) while 17alpha-ethynylestradiol at concentrations of 50ng/L were 2.2+/-1.5ng/g tissue EEQ. Results suggest 4-nonylphenol may be accumulated in tissue and is partly resistant to biotransformation; maintaining its potential for chronic estrogenic action, while 17alpha-ethynylestradiol, although exhibiting greater estrogenic potency on biological endpoints possibly exerts its estrogenic action before being rapidly metabolised and/or excreted. A novel methodology was developed to assess vitellogenin using high-performance liquid chromatography (HPLC). Exposure to both 17alpha-ethynylestradiol (50ng/L) and 4-nonylphenol (100microg/L) produced increases in vitellogenin for females, whereas males exhibited increases in vitellogenin when exposed to 50ng/L 17alpha-ethynylestradiol only. Females exhibited greater vitellogenin responses than males at 50ng/L 17alpha-ethynylestradiol only. Histological examination of gonads revealed a number of individuals exhibiting intersex (ovotestis) in 50ng/L 17alpha-ethynylestradiol exposures. Male individuals in 1microg/L and 100microg/L 4-nonylphenol exposures and 5ng/L 17alpha-ethynylestradiol were at earlier stages of spermatogenic development than corresponding controls.

Vitellogenin as a biomarker of exposure to estrogenic compounds in aquatic invertebrates: a review

Vitellogenins (Vg) are the major precursor of the egg-yolk proteins, vitellins (Vn), which provide energy reserves for embryonic development in oviparous organisms. In mature females, Vg are generally synthesised in response to endogenous estrogens, such as 17beta-estradiol (E2), released into the bloodstream and then stored in developing oocytes. In males, the Vg gene, although present, is normally silent. However, it may be activated by (xeno)-estrogens. These substances belong to a large and heterogeneous group of environmental contaminants capable of altering endocrine functions in organisms. For this reason, they are named endocrine disrupting chemicals (EDCs). Natural steroidal estrogens [E2, estrone (E1), estriol (E3)] and synthetic chemicals, such as pesticides, polychlorinated biphenyls (PCBs) and household products (surfactants) are estrogenic compounds widespread in aquatic ecosystems. The main sources of contamination mostly stem (or originate) from municipal and industrial effluents, agricultural practices, livestock wastes and sewage treatment plants (STP). Recently, Vg induction, mainly in males and immature females, has been proposed as a useful biomarker to assess estrogenic contamination in aquatic environments. Indeed, estrogenicity of individual chemicals and mixtures has extensively been evaluated, in both laboratory and field studies, albeit mostly in fish. In contrast, limited attention has centred on evaluating xenoestrogen effects in aquatic invertebrates, probably owing to limited knowledge of their endocrinology. This review focuses on a brief description of xenoestrogens and their occurrence in aquatic environments, and on Vg induction in aquatic invertebrates in response to both experimental and environmental exposure to estrogenic compounds. Results of recent field and laboratory studies are presented. In addition, new perspectives about the use of Vg induction as a biomarker of exposure to estrogenic compounds are also discussed.

Endocrine-related reproductive effects in molluscs

Research on endocrine disruption has been a major topic of the past decade. Although most studies concentrated on vertebrate species, invertebrates are now gaining more attention. In particular, data on molluscs is increasing. One of the best-documented and more relevant examples of endocrine disruption is the imposex phenomenon affecting some gastropod species. But the increasing interest is also due to the fact that molluscs, especially bivalves, are good bioindicators used for decades in environmental studies and that progress have been made in the understanding of the physiology and endocrinology of some mollusc species. Recent results suggest that molluscs can be adversely affected by compounds that alter their reproduction and that vertebrate-type sex-steroids metabolism or mechanism of action could be involved in these effects. Nevertheless, the endocrine system of molluscs appears to be dissimilar in many aspects to those of vertebrates and sex-steroids might not have the same importance in all mollusc species. This diversity constitutes an important opportunity to examine and understand new and alternative mechanisms for endocrine disruption.

Analysis of 3D models of octopus estrogen receptor with estradiol: Evidence for steric clashes that prevent estrogen binding

Relatives of the vertebrate estrogen receptor (ER) are found in Aplysia californica, Octopus vulgaris, Thais clavigera, and Marisa cornuarietis. Unlike vertebrate ERs, invertebrate ERs are constitutively active and do not bind estradiol. To investigate the molecular basis of the absence of estrogen binding, we constructed a 3D model of the putative steroid-binding domain on octopus ER. Our 3D model indicates that binding of estradiol to octopus ER is prevented by steric clashes between estradiol and amino acids in the steroid-binding pocket. In this respect, octopus ER resembles vertebrate estrogen-related receptors (ERR), which have a ligand-binding pocket that cannot accommodate estradiol. Like ERR, octopus ER also may have the activation function 2 domain (AF2) in a configuration that can bind to coactivators in the absence of estrogens, which would explain constitutive activity of octopus ER.