- Research article
- Open Access
An amphioxus orthologue of the estrogen receptor that does not bind estradiol: Insights into estrogen receptor evolution
© Paris et al; licensee BioMed Central Ltd. 2008
Received: 03 January 2008
Accepted: 25 July 2008
Published: 25 July 2008
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.
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.
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.
Hormone signaling is a very important feature in metazoans, allowing communication between cells or organs within the organism. Two components of these signaling systems are of particular importance, the hormone and its receptor. The nuclear hormone receptor (NR) superfamily includes ligand dependent transcription factors that play a central role in various physiological processes as diverse as reproduction, development, and control of homeostasis [1, 2]. They share a common structural organization and exhibit a highly conserved DNA binding domain (DBD) and a moderately conserved ligand-binding domain (LBD). Some members of this superfamily are liganded receptors (24 among the 48 genes encoding NRs in the human genome) but many lack identified ligand and are therefore called "orphan" . Some orphan receptors are 'true' orphans in the sense that they do not possess a bona fide ligand-binding pocket (LBP), like the members of the NR4 subfamily (for instance, NURR1, DHR38 or NGFI-B. For review, see ), and are regulated by other mechanisms . Alternatively, the crystal structures of several orphan receptors such as HNF4 were found to have a phospholipid constitutively bound to a large ligand binding pocket [5, 6]. The functional and evolutionary implications of these constitutive ligands remain discussed. Other orphan nuclear receptors have a ligand binding pocket and thus have the potential to bind compounds. It is still not known whether those receptors have natural ligands, still to be discovered. Undoubtedly, the existence of such orphan receptors with physiological or developmental activities constitutes both a major challenge for understanding nuclear receptor evolution and a potential opportunity for pharmacology .
The existence of orphan and liganded members in the NR family raises the question of the evolution regarding their ligand binding ability. Whether the ancestral NR was liganded or orphan and more generally how NR ligand binding ability evolved has been recently debated [7–14]. In general, it is still unclear if there is a correlation between the evolution of the hormone repertoire and NRs. Moreover the mechanisms underlying this coevolution are of particular interest [7, 12, 15–19].
Among the scenarios of NR evolution that have been proposed, one suggests that the ancestral NR was a ligand-independent transcription factor which acquired the ability to be regulated by ligands several times during evolution [7, 18–20]. This hypothesis was based on the observation that compounds of similar chemical nature bind to divergent NRs and on the contrary compounds of very different nature bind to closely related receptors. For instance, orphan receptors are found in all families of NRs, and steroid receptors are not monophyletic but are located in two different subfamilies within the NR superfamily: the ecdysteroid as well as the sex steroid receptors. Interestingly, the evolution of sex steroid hormone receptors has also been used as an argument for an alternative hypothesis, the ligand exploitation model [8, 11] (for an alternative view, see [21, 22]). Phylogenetic trees show that sex steroid hormone receptors are grouped with ERRs as the NR3 subfamily, following the official nomenclature . They contain receptors that bind estradiol (ERs), that form the NR3A group as well as mineralocorticoids (MRs), glucocorticoids (GRs), progesterone (PRs), and androgen (ARs) that form the NR3C group. All known ligands in this subfamily can be seen as variations around the archetypical sterol skeleton. Consequently, Thornton et al. suggested in the ligand exploitation model that the ancestral steroid receptor was a high affinity estradiol receptor [8, 11] and the other steroid receptors that originated later on, experienced, following gene duplication, shifts in their binding affinities to eventually bind to their extant ligand. The model in fact suggests that the newly duplicated receptors (here NR3C) exploit as ligands chemical species that serve as intermediary compounds in the "ancestral ligand" synthesis pathway (here the estradiol synthesis pathway) . According to this view, orphan receptors, like ERRs, secondarily lost the ability to have their activity regulated by a ligand and became orphan. Interestingly, within the NR3 family, two receptor subfamilies, ERRs and ERs, appear to be ancient since they are found in a wide variety of metazoans including deuterostomes and protostomes, whereas, up to now, MRs, GRs, PRs and ARs have been found only in vertebrates. The only non-vertebrate ERs that have been described so far were from mollusks and were shown to be unable to bind estradiol [11, 12, 24–26]. Since the ligand exploitation model implies an ancestral estradiol-binding ER and since all liganded ER found so far come from vertebrates, and to improve taxonomic sampling, the ER orthologues from the basal vertebrate lamprey and the invertebrate chordate amphioxus were characterized here. Indeed, lamprey and amphioxus are located at key positions in the chordate phylum [27–30]. Moreover, amphioxus (Brachiostoma floridae) is much less derived than urochordates in its morphology as well as in its genome organization . Indeed, amphioxus and vertebrates share a similar general body plan whereas urochordate morphology is more derived. For instance, during metamorphosis of some urochordates, the tadpole-like larva transforms into an adult that looks so different that it was first considered as a mollusk . Moreover the urochordate genome is fast evolving , with for instance the loss of the clustering of the hox genes . There is no ER in the sequenced genome of Ciona intestinalis  or in the sea urchin , one ER was previously cloned in lamprey , only one ER was found in the amphioxus genome . These reasons make lamprey and amphioxus excellent models to study the evolution of estrogen signaling pathway at the origin of vertebrates. In this study, we cloned the unique ER from amphioxus (amphiER) and characterized it, as well as the previously cloned but uncharacterized lamprey ER (lampER). AmphiER is an orphan receptor, showing no affinity to the estrogen hormone estradiol, when in contrast, the lamprey ER behaves as a "classical" vertebrate ER. As no ER from invertebrates studied so far binds estradiol, we propose that the ancestral ER (and the ancestral steroid receptor) was not a receptor for estradiol and gained later on during evolution the ability to bind the hormone.
Cloning of the ER from amphioxus (amphiER)
Phylogenetic analysis of ERs
The orthology relationships of the amphioxus and lamprey ER sequences were studied in a phylogenetic analysis of the NR3 family using an exhaustive dataset comprising 69 members of the NR3 subfamily as well as sequences of RXRs as an outgroup. The dataset included the 6 currently known mollusk ER sequences (from Nucella lapillus, Crassostrea gigas, Marisa cornuarietis, Thais clavigera, Octopus vulgaris, Aplysia californica), as well as the 2 NR3 sequences previously known from amphioxus (1 ERR [GenBank: AAU88062] and 1 NR3C [JGI: 201600], retrieved from a previous work  or from the complete genome sequence ). In the resulting phylogenetic tree, the sequence of lamprey ER branches within the ER clade with a high bootstrap support (95%), at the expected position before the split of vertebrate ERα and ERβ (Figure 2B, and for a tree presenting all sequences, see Additional file 1A), as previously shown . The sequence of amphiER branches within the ERs and is located at the base of the vertebrate estrogen receptor group, before the split of ERα and ERβ but after the split of the mollusk ERs (bootstrap value of 81%, Figure 2B). However its precise position within the ER group is poorly supported (bootstrap value of 42%).
Such low bootstrap supports reveal either the weakness of the phylogenetic signal contained in ER proteins, or the presence of two incompatible signals in the data, one supporting the observed position of amphiER within ERs, and the other supporting another position. Whereas the weakness of the signal is not testable, the long branch leading to amphiER in the tree suggests that sites that have undergone a large number of substitutions may account for one of the two signals. Such sites may be saturated to the point that phylogenetic methods are not able to correctly recover their evolution, a situation leading to the long branch attraction artifact . It is therefore important to correctly characterize sites that support the ER position of amphiER: if only fast-evolving sites support this hypothesis, it is probably due to long branch attraction, and an alternative branching should be favored. Alternatively, if slowly-evolving sites support this position, one can confidently identify amphiER as a bona fide ER. To characterize sites with respect to their evolutionary rates and the amphiER position they favor, both site likelihoods and site evolutionary rates were computed for all possible positions of amphiER.
An additional test can be run to further confirm this hypothesis, and consists in reestimating the phylogeny using only slowly-evolving sites. For that purpose, the distribution of expected relative evolutionary rates across sites of the alignment was plotted, as found by phyml-aLRT [39, 40] (Figure 3B). Fastest-evolving sites were removed from the dataset based on three different rate thresholds (2.5, 2 or 1.5, Figure 3B and 3C), and trees were reconstructed based on the alignments containing only the remaining slowly-evolving sites. These operations did not impact the monophyly of ERs (Figure 3C) or the statistical support. This shows that the clustering of amphiER with vertebrate ERs does not come from saturated sites, which argues against long branch attraction being at the origin of this position . Accordingly, complementary phylogenetic analyses with different methods (bayesian, Neighbor-joining, parsimony) gave similar results (see Additional file 1). From these studies we conclude that amphiER does indeed belong to the ER subfamily, which is confirmed by the general conservation of the exon-intron structure of amphiER with human ERα, especially at two exon-intron splice sites in the DBD and in the LBD after helix 3 (Figure 2C and short red strokes in Figure 1) .
Chordate ERs, including amphioxus ER and lamprey ER, are able to bind estrogen specific response elements (ERE)
To test whether the lamprey ER and the amphioxus ER are able to bind DNA on specific estrogen response elements (EREs), electrophoresis mobility shift assays were performed using a radiolabeled consensus ERE sequence (see Additional file 2). These experiments show that, like vertebrate ERs, amphiER and lampER are able to bind DNA specifically on a consensus ERE. This binding is specific, since a 100-fold excess of non-specific DNA was not able to compete for binding, whereas a 100-fold excess of cold ERE completely suppressed it (see Additional file 2, compare lanes 15 and 17, as well as lanes 19 and 21). ERs contain two major conserved signatures in the DBD, the P-box (CEGCKA), responsible for the binding specificity to response elements, and the D-box, also involved in the DNA binding specificity of the ER dimers (Figure 2A). The P-box is highly conserved in all known ERs, including amphiER and lamprey ER and is different from other NR3 members. AmphiER and lamprey ER also have a well conserved D-box, amphiER D-box containing just a few conservative mutations, (e.g. a mutation of an alanine in glycine, Figure 2A). Since the three characterized mollusk ERs (from A. californica, O. vulgaris and Thais clavigera [11, 12, 24]) also bind EREs and since the P-box and D-box are well conserved in all known ERs, including those from mollusks, ERE binding appears to be a feature specific to all ERs.
Lamprey ER, but not amphioxus ER, is able to induce transactivation of a reporter gene in response to estradiol stimulation
LampER is an estradiol receptor whereas amphiER is not able to bind ER ligands except the synthetic compound Bisphenol A
In order to rule out the possibility that amphiER is activated by a compound related to E2, a large panel of 14 other steroids and cholesterol derivatives were tested for their ability to bind and activate amphiER. None of the tested compounds, even at high doses, had any effect on amphiER transcription activity (see Additional file 5A). Accordingly, no recruitment of the coactivator SRC1 by amphiER was detected in mammalian two hybrid assays (See Additional file 5B). The most probable explanation is thus the lack of binding by those compounds to amphiER (See Additional file 6).
Taken together these results show that the ER from lamprey behaves as a "classical" ER since it binds DNA on a classical ERE and is activated by binding E2. On the other hand, though the single ER from amphioxus is able to bind the ERE, it does not bind any tested ER ligand and cholesterol derivative, except bisphenol A. However, no transcriptional activity was detected upon stimulation by any of the tested ligands. Since none of the mollusk ERs sequenced up to now binds E2 either, our data suggest that E2-binding by ER is restricted to vertebrates, implying that vertebrates specifically gained the ability to be regulated by E2 (see Discussion).
Ancestral reconstruction of steroid receptors
The amphioxus ER does not bind estradiol
In this paper we cloned and functionally characterized the lamprey and amphioxus orthologues of the human estrogen receptors. Our results show that lampER binds estradiol whereas amphiER does not. We propose that 3 types of ERs can be distinguished, depending on their ligand binding properties: vertebrate ERs (including lamprey) are the only bona fide estradiol receptors, mollusk ERs do not bind estradiol and are constitutively active transcription factors and amphiER does not bind estradiol and is transcriptionally silent in mammalian cells. This is supported by two points: (i) the experimental approach developed here is biologically relevant since the binding of bisphenol A (BPA) to amphiER was observed using the same experimental conditions as for E2 suggesting that amphiER is correctly folded and that a ligand binding pocket is likely to be present. (ii) One of the three key amino acid positions within the LBP of amphiER (Cys 531, located between the helixes H5 and H6, in green in Figure 1) diverges from vertebrate ERs (Arg 394 in human ERα), whilst the two other key positions (Glu 490 and His 659 in amphiER, located in helix H3 and H10–H11 respectively, in green in Figure 1) are conserved with the vertebrate ERs (amino acids corresponding to Glu 353 and His 524 in human ERα), suggesting that potential contacts between amphiER and estradiol are impaired. Accordingly, a recent in silico study of amphiER ligand binding ability confirmed an "unusual ligand recognition in amphioxus ER" .
It was unexpected that no effect of the synthetic ER agonist BPA was detected in the transactivation assay of the receptor in mammalian cells since BPA induces a conformational change of amphiER. This apparent absence of coactivator recruitment (see Additional file 4B) resulting in no transcriptional activity in response to BPA can be interpreted in several ways: (i) because of the different geometry of the ligand-binding pocket in amphiER, BPA behaves as an antiestrogen (partial agonist or even a partial antagonist) and blocks the transcriptional activation properties of amphiER, for instance by inducing a conformational change that does not allow coactivator recruitment (like human ERα and 4-raloxifen, ) or by excluding amphiER from nucleus (like ICI-182,780 with human ERα, ). (ii) Alternatively, the coactivator interface of amphiER does not fit with mammalian coactivators, resulting in artifactual loss of activation. However, the conservation of the amino acids involved in co-activator interaction, compared to human ERα does not support this hypothesis (sites indicated with a star in Figure 1, as described in ). Among the divergent sites, at a position implicated in the charge clamp necessary for coactivator contact, amphiER contains an aspartate (D677) instead of a glutamate in humanERα (E542). Importantly, the divergence (E->D) is conservative and preserves the negative charge of the amino acid, which is important for interaction with the lysine from helix 3 (conserved in amphiER) to form this charge clamp . In addition, a unique orthologue of the p160 family of coactivators was found in the amphioxus genome  and its overall conservation with its 3 human ohnologues (genes that have been duplicated during the two rounds of whole genome duplications in the chordate lineage ) is good. (iii) Interaction between mammalian chaperones like HSP90 and amphiER is impaired, leading to improper binding to the hormone . Taking these results into account, it will be interesting to test the effect of BPA on the subcellular localization of amphiER and to study if other related compounds are able to bind and/or activate amphiER. In addition, it will be important, when cell cultures from amphioxus are available, to check the activity of amphiER in a monospecific transient transfection assay. It should be remembered that some orphan receptors such as ERRs are thought to have no natural ligands even if they are able to bind synthetic compounds . More generally, the precise status of amphiER in terms of ligand binding remains an open question. It is nevertheless clear, and this is an important issue for the current evolutionary debate, that amphiER is not able to bind estradiol.
Is there any receptor for estradiol in amphioxus?
The observation that amphiER does not bind E2 is indeed a surprising observation since E2 was detected in amphioxus by RIA, the hormonal production being correlated with breeding season . Several aspects of steroid metabolism were described in amphioxus  and the homologues of many enzymes necessary for estradiol synthesis in mammals were cloned from amphioxus ovaries [15, 61]. Of particular interest is the report of an aromatase gene (CYP19) in amphioxus, which suggests that the crucial step in estradiol synthesis is indeed possible in amphioxus. These experimental data were recently confirmed by the analysis of the complete amphioxus genome sequence . It may be that, in amphioxus, the active sex hormone is an E2-derivative  or another sex hormone, like in the case of androgens in lamprey , and this derivative is still to be discovered. In a similar way, we recently demonstrated that the amphioxus TR orthologue does not bind T3 or T4, the classical thyroid hormones, but deaminated derivatives TRIAC and TETRAC, which are able to induce amphioxus metamorphosis .
A second possibility is that E2 itself has a central role in sex maturation in amphioxus, and that the functional estrogen receptor in amphioxus is different from amphiER. Several candidates are possible. First, there is another steroid receptor in amphioxus (amphiNR3C in Figures 1 and 2)  that exhibits several ER-like features. Its P- and D-boxes are closer to ERs than to vertebrate NR3C (Figure 2A). The sequence identity of its LBD with human ERα (37%) and with NR3C members (35%) are similar. Moreover, most of the amino acids involved in ligand binding are more ER-like than AR-, PR- or MR-like (Figure 1). However it is the only NR3C receptor (orthologous to AR, PR, MR and GR) found in the amphioxus genome. Thus if amphiNR3C plays the role of an estradiol receptor, this suggests an absence of a "classical" steroid receptor able to bind testosterone, progesterone or corticoids. Alternatively, a non-nuclear receptor could mediate E2 action in amphioxus. Indeed, several non-genomic effects of estradiol were reported in mammals involving GPCRs (for reviews see [65–67]). For instance, very recently, a high affinity receptor for the steroid androstenedione linked to the membrane, was described in lamprey  and a GPCR with high affinity for progestines was isolated from sea trout .
Implications for the evolution of ERs
To describe the evolution of a protein, being able to study ancestral sequences at different nodes of a phylogeny would obviously provide historically relevant information that is not available otherwise . However such sequences have disappeared long ago and can only be statistically estimated. The accuracy and bias of these estimations therefore need to be investigated. Indeed, functional studies of ancestral sequences are of any value only if the ancestral reconstruction is reliable enough. The confidence associated with the previously published ancestral steroid receptor is quite low. Indeed, the overall accuracy of the reconstruction of the LBD (AncSR1) was only 62% . This is similar for the ancestor inferred here (AncSRa) on an enriched dataset, with an overall accuracy of the DBD+LBD of 70%. Moreover, amino acid uncertainty was high at many sites of AncSRa and AncSR1: more than 60 sites have more than 1 possible amino acid with a probability superior to 0.2. If one were to make an exhaustive study, one would need to reconstruct and test more than 1024 potential proteins (if all possible combinations of amino acids with probability > 0.2 were tested). In fact several of the sites involved in ligand binding have low probabilities. Examples of more reliable reconstructions of nuclear receptors have been published, e.g. the ancestor of MR and GR (mineralocortoid and glucocorticoid receptors) in which the overall accuracy of the LBD was above 99%, with no disrupting mutation at any site . The reconstructed ancestor of RARs also showed a high average confidence (99% ). This discrepancy between results obtained on the ancestor of all steroid receptors or merely of MR and GR for instance, can be explained by the higher sequence divergence observed among all SRs than simply among subfamilies MR and GR (see branch length in Figure 2B). Consequently, the uncertainty associated to the sequence of the ancestral steroid receptor as estimated with nowadays methods is probably too high to provide a firm basis for evolutionary conclusions. Moreover, the phylogenetic reconstruction of ancestral sequences has been shown to be biased towards the most frequent (and more stable) amino acids, resulting in an under-estimation of the less frequent amino acids (the stability of the ancestral protein is then over-estimated ). In reconstructions of ancient proteins, where the evolutionary signal has been lost due to a high number of substitutions, such biases might be problematic. Thus, current reconstruction methods do not seem powerful enough to infer a biologically meaningful ancestral steroid receptor given the amount of divergence between sequences.
Nonetheless, all these reservations put aside, it is surprising that the previously reconstructed ancestral SR, is vertebrate ER-like. As almost all the extant sequences used as matrix for the reconstruction came from vertebrates and led to the estimation of a "vertebrate-like" ancestral sequence, the same ancestral steroid receptor as previously published  was estimated, but adding more sequences from various taxa. This reconstruction was done using a phylogeny equivalent to the one previously published . Using this approach, the new AncSRa is more divergent from vertebrate ERs than AncSR1. Interestingly AncSR1 was shown to bind E2 with a very low affinity (250 times lower affinity than human ERα ), suggesting that AncSRa may be an even worse estradiol receptor.
The bias of AncSR1 towards vertebrate ERs is explained by a lack of non-vertebrate sequences used for the reconstruction. Indeed, removing some non-vertebrate sequences from our dataset leads to an estimation of an ancestral steroid receptor that is more "vertebrate ER"-like (AncSRb in Figure 7). The clustering of AncSRb with the ER clade and the exclusion of AncSRa from the ER clade were supported with good statistical values (minimum of Chi2-based and SH-like supports of 0.83 and 0.89, respectively). Those data show that AncSR1 reconstruction was probably sensitive to the vertebrate bias in the data set (Figure 7). Overall, we suggest that analysis based on ancestral reconstructions should be taken as tentative, especially in case of low statistical confidence and limited taxonomic sampling. In case of the ancestral steroid receptor, even if exhaustive taxonomic sampling is necessary, phylogenetic signal is weak and the resulting confidence is quite low. Thus we think that even if the ancestral sequence built here is biologically more relevant than previously calculated ones (because of better taxonomic sampling), it remains quite uncertain. Consequently conclusions regarding the ancestral steroid receptor should be based mostly on comparative characterization of extant receptors. In that case, all the data based on invertebrate ER receptors (from mollusks and amphioxus) support an ancestor of steroid receptors that was not able to bind estradiol. This conclusion will obviously require the functional characterization of ERs from other protostome phyla in order to carefully check if this observation is general. Thus, available data converge towards a re-evaluation of the ancestral status of estrogen receptors.
Sequence conservation reflects functional constraints: ligand binding ability is more recent than DNA-binding ability
From our and previous studies, only vertebrate ERs are able to bind and activate transcription under estradiol stimulation [11, 12, 24–26]. The LBD of amphiER is more divergent from its vertebrate counterparts (ca. 34% amino acid identities) than the LBD of other liganded amphioxus nuclear receptors such as amphiRAR (ca. 58%), which has been shown to bind the same ligand as its vertebrate homologue [7, 13]. This suggests that a conserved functional feature (e.g. binding to the same ligand) is reflected in the sequence conservation of the LBD.
The same observation can be done concerning the DBD since all ERs, including amphiER, have a highly conserved DBD and are able to bind EREs. Thus, for this domain also, a conservation of the function is reflected in sequence conservation.
Accordingly with this notion, the LBD of invertebrate ERs is highly divergent but their DNA binding domain, as well as other functionally important domains not directly linked to ligand binding such as the dimerization interface, or the amino acids responsible for interaction with the co-activators  are well conserved. This is true for amphiER as well as mollusk ERs. This strongly suggests that amphiER is a bona fide NR regulating ERE-containing genes in an E2-independent manner. Post-translational modifications such as phosphorylation or the presence/absence of other receptor-interacting proteins such as transcriptional coactivators have been shown to regulate unliganded nuclear receptors . Whether one of these mechanisms acts to regulate the activity of invertebrate ERs or if those receptors have unknown ligands still to be identified remains to be explored. Anyway our observations strongly suggest that for ERs, the DNA binding function of the receptor as well as its interaction with co-regulators have been conserved due to selective pressure. Interestingly, when studying the AncSRa, the P- and D-boxes in the DBD are ER/ERR-like (Figure 1), suggesting that ER/ERR DNA binding ability is ancestral, in accordance with the fact that these are the only receptors of the NR3 family found in invertebrates. This difference in the selection pressure between DBD and LBD has been proposed to be a general evolutionary pattern for the whole NR family . The plasticity of the ligand binding ability of NRs was recently illustrated in the case of RXR-USP where the ability of the receptor to be regulated by a ligand was suggested to have been subject to several successive episodes of gain and loss during evolution .
Evolution of endocrine systems: refinement of the ligand exploitation model
The ligand exploitation model hypothesizes how new hormones and new receptors appear during evolution. It suggests that the ancestral ligand is the last metabolite of a synthesis pathway . According to this model, the ancestral steroid ligand was estradiol (and the ancestral SR bound estradiol). During evolution, other steroid receptors appeared by duplication of the ancestral ER and gained the ability to bind other steroids, intermediate in the synthesis pathway (like testosterone or progesterone).
Our findings on the evolution of ERs do not support the ligand exploitation model, since our data strongly suggest that the ancestral ER did not bind estradiol. However, as estradiol has been detected in deuterostomes as well as protostomes (for instance in vertebrates, amphioxus, echinoderms, mollusks, for review, see ), steroid signaling may have been already present in bilaterian ancestor. However, up to now, the ancestral steroid molecule remains to be determined. If estradiol is an ancient hormone, it then probably bound another receptor and later on ER gained the ability to recognize it, as did other steroid receptors for their extant ligand. Thus the evolution of steroid system intermingles two distinct processes, the evolution of the receptor on one hand, and the evolution of the ligand on the other.
The receptor can evolve by point mutations and change its affinity for a ligand towards another. This idea was convincingly exemplified in the case of corticoid receptors (the ancestor of MR and GR) for which it was recently demonstrated that ability of the ancestral vertebrate corticoid receptor to bind gnathostome-specific hormone aldosterone (a MR ligand) was a by-product of its ability to bind the ancestral ligand 11-deoxycorticosterone (DOC) . GR gained the ability to bind cortisol only in the gnathostome lineage, in parallel to endogenous synthesis of the hormone [16, 71]. This detailed study shows that a receptor binding a given ligand can acquire affinity for compounds present in the cell that are structurally close to its natural ligand: this refines the ligand exploitation model, since new ligands are not necessarily precursors of ancient ligands, simply compounds present in the cell and structurally close to the ancestral ligand. Similar conclusions were drawn previously in the case of RAR evolution . It has to be emphasized that the pool of available compounds is also subject to evolutionary changes in parallel. For instance, the spatiotemporal production of estradiol is variable in the different vertebrate groups (reviewed in ). Glucocorticoids differ in mouse (cortisol) and in human (corticosterone), with both hormones being GR ligands . There are several androgens in teleost fishes, with 11-ketotestosterone being teleost-specific . As there are 2 androgen receptors (ARs) in teleost fishes, from a whole genome duplication , the study of the ligand-binding ability of those ARs is a potentially interesting case for the evolution of endocrine systems. As highlighted by Bridgham et al. (2006), lamprey does not produce cortisol . In accordance, their genomes do not contain the sequence corresponding to the enzyme responsible for cortisol synthesis (11b-hydroxylase) and in general classical steroids except estradiol are rarely found in lamprey. This suggests that the steroids actually found in lamprey are different from the ones found in mammals (reviewed in ). Those cases exemplify the largely underestimated diversity of endocrine systems: except for lamprey and some teleost fishes, the hormonal pool of animals remains largely unknown. As proposed for the study of steroid receptors, a comparative approach should be applied to determine the metabolism of steroids in poorly studied animals. Indeed, the hormonal pool of such animals is usually evaluated from the presence/absence of putative orthologues of mammalian enzymes. As the enzymatic machinery involved in hormonal metabolism has a very labile activity (reviewed in [81, 82]), equating orthology with functional identity might be unreliable.
The evolution of steroid receptors can be replaced in the more general context of ligand-nuclear receptor co-evolution. The evolution of the NR1H subfamily, that includes receptors for other steroidal compounds, like the major transcriptional regulator of bile salt synthesis farnesoid × receptor (FXR), the pregnane × receptor (PXR), the vitamin D receptor (VDR) or liver × receptor (LXR)/ecdysone receptor (Ecr), has been extensively studied and is not in line with the ligand exploitation model [83–85]. For instance, comparative functional studies of FXRs from various chordate species showed that the vertebrate FXRs bind "late" cholesterol derivatives (from a complex synthesis pathway) but are thought to have evolved from an ancestral FXR that bound early cholesterol derivatives (from a simpler synthesis pathway) .
In other cases, the evolution of ligand binding is more "chaotic" with close orthologs having a selective ligand binding ability that varies extensively (vertebrate VDRs are very well conserved when PXRs have the widest ligand repertoire of all NRs) .
These complex histories are probably linked to specific function of some of those NRs, considered as xenotoxic compounds "sensors". This tight relationship with the unstable environment probably makes receptors like FXR and especially PXR more prone to fast evolution . Yet they illustrate the impressive variety of scenarios of NR evolution.
In this article, we demonstrated that vertebrate ERs (including lamprey ER) are estradiol receptors whilst non-vertebrate ER (including amphioxus ER) are not. The most parsimonous scenario proposes that the ancestral ER was not able to bind estradiol and that it had another function. It later gained the ability to be regulated by estradiol, specifically in the vertebrate lineage. However, additional critical data remains to be discovered in poorly studied taxa . To fully understand the evolution of steroid signaling pathway, a larger number of taxa need to be targeted for detailed comparative studies. More precisely, ERs and other steroid receptors should be cloned from widely distributed taxa, especially in protostomes. Enzymes involved in steroidogenesis should also be cloned and characterized, to understand the evolution of steroid availability. In order to avoid the blinders of a "vertebrate-centered" view, it is of particular importance to establish the steroid hormone repertoire of an enlarged animal panel, including more protostomes. The description of various endocrine systems will certainly be relevant to the early evolution of hormone signaling.
Cloning of amphiER
An initial piece of amphiER was obtained by degenerate PCR on different RT reactions from total RNA extracted either from developing B. floridae embryos and larvae (at 13 h–15 h, 28 h, 36 h, 48 h or 3 d–4 d of development) or from B. floridae adults. The oligonucleotides used were as follows: forward primer 5'-TGYGARGGITGYAARGCITTYTT-3' and reverse primer 5'-GTRCAYTSRTTIGTIGCIGGRCA-3'.
The touchdown PCR program used was as follows:
5' 94 degrees
5× (30" 94 degrees, 1' 55 degrees, 1' 72 degrees)
5× the same cycle, but at 50 degrees annealing temperature
5× the same cycle, but at 45 degrees annealing temperature
5× the same cycle, but at 40 degrees annealing temperature
25× the same cycle, but at 37 degrees annealing temperature
7' 72 degrees
All degenerate PCRs irrespective of the RT reaction template used yielded a 83 bp fragment of amphiER. The fragment was sequenced on both strands and used for the design of oligonucleotides for 5' and 3' RACE experiments with the Invitrogen GeneRacer Kit. The template for the RACE experiments was pooled total RNA from 13 h–15 h B. floridae embryos and from B. floridae adults. In addition to the oligonucleotides provided by the kit, for the 3' RACE, the following primers were used:
3' RACE, 1st PCR: 5'-AACGGAGCATTCAGCAAGGTC-3'
3' RACE, 2nd PCR: 5'-GCATTCAGCAAGGTCAGACAG-3'
5' RACE, 1st strand cDNA synthesis: 5'-ATGTAATCTGTCTGACCTTGC-3'
5' RACE, 1st PCR: 5'-CTGTCTGACCTTGCTGAATGC-3'
5' RACE, 2nd PCR: 5'-TCTGACCTTGCTGAATGCTCC-3'
The protocols for the 1st and 2nd round of PCR experiments are given in the Invitrogen GeneRacer Kit. The 3' and 5' RACE products were subsequently sequenced on both strands and used for the design of oligonucleotides for the full-length cloning of amphiER: forward primer 5'-CGGCGAAGCGAAGAAGATCGAG-3' and reverse primer 5'-CTTAACCGATACTAACGGAACAG-3'. The full-length amphiER was obtained by PCR on pooled RT reactions from total RNA extracted from B. floridae 13 h–15 h embryos, 3 d–4 d larvae and B. floridae adults. The PCR protocol used was as follows:
10' 94 degrees
5× (30" 94 degrees, 30" 55 degrees, 2' 72 degrees)
35× the same cycle, but at 50 degrees annealing temperature
10' 72 degrees
The full-length amphiER clone resulting from this PCR is 2279 bp long, was cloned into the pCR2.1 vector (Invitrogen) and subsequently sequenced on both strands.
Plasmid constructs and reagents
Full length amphiER were amplified by polymerase chain reaction (PCR) and the obtained fragments were inserted into a pSG5 vector between EcoR1 sites. Lamprey ER was a generous gift from JW Thornton. Human pSG5-ERα and pSG5-ERβ and the 3xERE-Luc luciferase reporter construct have been described previously . The pS2-Luc reporter construct encompasses an 1100 bp estrogen-responsive region of the human pS2 promoter inserted into the pGL3 basic vector (Promega). Chimeras comprising the GAL4 DNA-binding domain fused with the LBD of the human ERα (residues 251 to 595), the LBD of amphiER (residues 364 to 705), the LBD of lampER (residues 234 to 554) have been cloned in the pG4MpolyII vector. 17β-estradiol, genistein, 3β-androstenediol, resveratrol, cholesterol, cholic acid, chenodeoxycholic acid, 22®-hydroxycholesterol, 20-Hydroxyecdysone, pregnenolone, trans-Dehydroandrosterone (DHEA), corticosterone, progesterone, 4-androstene-3,17-dione, estrone, testosterone, 5α-androstan-17β-ol-3-one and 1a,25-Dihydroxyvitamin D3 (calcitriol) were purchased from Sigma. Enterolactone was a generous gift from Dr Sari Mäkelä .
Phylogenetic analysis of NR3
Protein sequences of NR3 family members were obtained from GenBank by BLAST search using Homo sapiens ERα as a query. Eight additional sequences from the closely related RXR group were also obtained to serve as outgroup sequences. For accession numbers of the sequences used, see Additional file 8.
The retrieved sequences were aligned using the muscle 3.6 program  and the resulting alignment was manually corrected with SEAVIEW . Phylogenetic tree was calculated by maximum likelihood as implemented in PhyML version 2.4.3 under a JTT substitution matrix plus a eight-category gamma rate correction (α estimated) and with the proportion of invariant sites estimated. Both the DBD and the LBD were used. Robustness was assessed by bootstrap analysis (1,000 repetitions) .
The Bayesian inference was done using the program MrBayes 3.1.2 . Two simultaneous independent runs were performed. For each run, one chain was sampled every 100 generations for 1,000,000 generations after the burn-in cycles, until the average SD of split frequencies was <0.01; additionally, the potential scale reduction factors of the parameters were close to or equal to 1, which indicates that the runs had most probably converged. The neighbour-joining (Poisson correction) and maximum parsimony trees were done with Phylo_win .
Likelihood-based tests of alternative topologies placing amphiER at all possible positions in the tree
The 149 trees were built by reconnecting amphiER from the maximum likelihood tree, into the 149 possible positions. The branch length and the different parameters of the obtained trees were re-estimated using PhyML. Likelihood-based tests of the 149 alternative topologies were calculated using CONSEL: site-wise log-likelihood values, available as output of PhyML, were used to calculate the P-values of the different positions according to the AU test with the software R.
Ancestral sequence reconstruction
The aminoacid sequence of the ancestral AncSRa and AncSRb was inferred only for the most conserved part of the alignment, i.e. the DBD and LBD (defined as in Figure 1). The ancestral sequences were reconstructed by maximum likelihood as implemented in PAML , under the JTT substitution model and a gamma distribution with 8 categories of rates across sites, using the tree described in Additional file 1A for AncSRa and a the same topology truncated of the mollusk ER sequences and the amphioxus NR3 sequences, after reestimation of the branch lengths using phyml (JTT+γ) for AncSRb.
Electrophoretic Mobility Shift Assay (EMSA)
EMSAs were performed as previously described . Where indicated, a 10- and 100-fold molar excess of 30-bp unlabeled oligonucleotides (a consensus ERE and a non-related probe) were added as competitors. The sequence of the probe containing the consensus ERE is 5'-CGGGCCGAGGTCACAGTGACCTCGGCCCGT-3' and the sequence of the non-related probe is 5'-CTAGTCCTAGGTCTAGAGAATTCA-3'.
Cell culture and transfections
Human embryonic kidney 293 cell culture and transfections using Lipofectamine Plus reagent (Invitrogen) were done according to the manufacturer's recommendations and as previously described . Briefly, 200 ng of the chimeras comprising the GAL4 DNA-binding domain fused with the LBD of either human ERα, lampER or AmphiER (or LBD for the control) were co-transfected together with 100 ng of reporter plasmid and 10 ng of a β-galactosidase expression vector, included as a control for transfection efficiency. For the mammalian double hybrid assays, the GAL4-amphiER-LBD chimera was transfected with 200 ng of the coactivator SRC1 fused to the strong activation domain VP16. Three to five hours post-transfection, serum and hormones (as indicated in the figures) were added to the cells which were incubated for an additional 48 hours before harvest and luciferase and β-galactosidase activities were determined. Results show the mean ± s.e.m. (n = 3) of representative experiments. Human HeLa cervical cancer cells and CV-1 green monkey kidney cells were routinely maintained in Dulbecco's modified eagle's medium (Invitrogen), supplemented with 10% fetal bovine seum, 1% v/v L-glutamine and 1% v/v penicillin/streptomycin. Cells were seeded in 12 or 24 well plates one day prior to transfection. Transient transfections were carried out using the Lipofectamine Plus reagent according to instructions of the manufacturer (Invitrogen) in culture media devoid of serum, phenol-red and antibiotics. Briefly, 1 ng of ERα, ERβ or AmphiER expression vectors were co-transfected together with 100 ng 3xERE-Luc (or 200 ng pS2-Luc where indicated) and 20 ng of a β-galactosidase expression vector, included as a control for transfection efficiency. In the co-expression experiments, AmphiER was co-transfected together with ERα or ERβ in ratios of 0.5:1, 1:1 and 1:5, respectively. Three hours post-transfection, serum and hormones (as indicated in the figures) were added to the cells which were thereafter incubated for an additional 48 hours before harvest and luciferase and β-galactosidase activities were determined. Figures represent results from at least three independent experiments performed in duplicates. Data is presented as mean +/- SD of fold induction of relative luciferase values corrected against β-galactosidase activity, where activity obtained from transfected reporter plasmid alone and treated with vehicle, was arbitrarily set to 1.
Limited proteolytic digestion
These assays were done as previously described .
We thank Bastien Boussau for help with phylogenetic analysis and for comments on the manuscript. We are grateful to Joseph Thornton for the gift of the lamprey ER clone, Gerard Benoit and Gabriel Markov for critical reading of the manuscript. This work was supported by the EU funded CASCADE network of excellence, CNRS, UCB Lyon 1, ENS de Lyon and MENRT.
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