Developmental exposure to bisphenol A alters expression and DNA methylation of Fkbp5, an important regulator of the stress response

Bisphenol A (BPA), an abundant endocrine disruptor, affects stress-responsiveness and related behaviors in children. In rats, perinatal BPA exposure modifies stress response in pubertal offspring via unknown mechanisms. Here we examined possible epigenetic modifications in the glucocorticoid receptor gene and its regulator Fkbp5 in hypothalamus and hippocampus of exposed offspring. We found increased DNA methylation of Fkbp5 and reduced protein levels in the hippocampus of exposed male rats. Similar effects were obtained in a male hippocampal cell line when exposed to BPA during differentiation. The estrogen receptor (ER) antagonist ICI 182,780 or ERβ knock-down affected Fkbp5 expression and methylation similarly to BPA. Further, BPA's effect on Fkbp5 was abolished upon knock-down of ERβ, suggesting a role for this receptor in mediating BPA's effects on Fkbp5. These data demonstrate that developmental BPA exposure modifies Fkbp5 methylation and expression in male rats, which may be related to its impact on stress responsiveness.


Introduction
Bisphenol A (BPA) is a widely used component of plastics and resins with endocrine disruptive features, exhibiting agonistic properties on both estrogen receptor (ER) isoforms, ERa and ERb, and antagonistic properties on androgen receptor (Delfosse et al., 2014). Human exposure to this chemical is extensive since BPA is abundant in a vast number of consumer products, including toys, drinking bottles, food containers and dental sealants. Up to 95% of the human population has detectable BPA levels in their bodies and there is increasing concern for its higher bioaccumulation in developing organisms (Calafat et al., 2008). The reference dose of 50 mg BPA/kg of bw/day, previously determined as the safe daily human exposure (Vandenberg et al., 2012), has recently been reduced to 4 mg/kg of bw/day (http://www.efsa.europa.eu/en/ topics/topic/bisphenol.htm) due to increasing evidence for adverse effects at lower exposures, especially impacting brain and prostate development in fetuses and children (http://www.niehs. nih.gov/health/topics/agents/sya-bpa/).
Early life exposure to BPA affects a variety of developmental functions (Kundakovic and Champagne, 2011), including neuronal differentiation and migration (Wolstenholme et al., 2012). In the rodent brain, BPA modifies sexual and social behavior, impairs cognition and increases anxiety and depression-like behavior . In our previous studies, we showed that perinatal exposure of rats to a low BPA dose alters their basal and stress-induced Hippocampal-Hypothalamic Pituitary Adrenal (HHPA) axis activity and related behaviors at mid-puberty in a sexually dimorphic manner (Poimenova et al., 2010;Panagiotidou et al., 2014): BPA-exposed females exhibited increased basal corticosterone and reduced hypothalamic glucocorticoid receptors (GR) levels, as well as anxiety-like behavior but less efficient late stress responses. BPAexposed males, on the other hand, showed a heightened stress response compared to untreated counterparts. In humans, higher pre-and postnatal BPA levels have been associated with increased anxiety and depressive behavior in children, expressed differently between boys and girls (Braun et al., 2009;Braun et al., 2011;Harley et al., 2013;Evans et al., 2014). BPA crosses the placenta and although its metabolic clearance is high, its actions can be chronic and potentially engage epigenetic modifications. Indeed, BPA induces alterations in DNA methylation in various species, organs, and model systems upon different exposures (Kundakovic and Champagne, 2011).
The HHPA axis is an important regulator of the stress response and its dysfunction is correlated to several neuropsychiatric disorders including anxiety and depression. Glucocorticoids act as downstream effectors of the axis (Holsboer and Ising, 2010). By activating GR in the hippocampus and hypothalamus, glucocorticoids exert a negative feedback on the HHPA axis towards termination of stress response and resilience (Smith and Vale, 2006). GR function depends on a large complex of transcriptional coregulators, chaperones and co-chaperones. One of them, FKBP51 (the protein product of the Fkbp5 gene), reduces hormone binding affinity and nuclear translocation of GR (Riggs et al., 2003;Touma et al., 2011). Fkbp5 is itself a GR target and glucocorticoids induce its expression as part of an intracellular ultra-short negative feedback loop for GR activity (Vermeer et al., 2003). Recent evidence indicates the sensitivity of Fkbp5 to environmental factors and epigenetic changes, thus highlighting the importance of this co-regulator in stress related disorders (Schmidt et al., 2012). Chronic exposure to glucocorticoids persistently changes Fkbp5 expression by altering DNA methylation of Fkbp5 gene in the mouse hippocampus and hypothalamus (Lee et al., 2010;Yang et al., 2012;Wochnik et al., 2005). Interestingly, DNA methylation changes in the human FKBP5 gene are also found in patients with post-traumatic stress disorder (Klengel et al., 2013a) and bipolar disorder (Fries et al., 2014).
Based on the above, we herein examined whether developmental exposure to BPA may lead to epigenetic alterations in genes encoding important mediators of the stress response, such as the glucocorticoid receptor and its regulator Fkbp5. Therefore we first investigated DNA methylation changes in the regulatory regions of the aforementioned genes in the hypothalamus and hippocampus of BPA-exposed rats. The detected changes in Fkbp5 methylation in the hippocampus of male rats, which coincided with lower FKBP51 levels, led us to further examine the molecular basis of this BPA effect in a murine hippocampal cell line of male origin. Specifically, the involvement of estrogen receptors (ERs) in mediating BPA's effects in hippocampal neurons was analyzed by inhibiting ER using either ICI 182780 (ICI) or shRNA-mediated knock-down. Our results suggest an involvement of ERb in BPA's epigenetic effects on Fkbp5.

Chemicals
Bisphenol A and dexamethasone were purchased from Sigma-eAldrich (St. Louis, Missouri, USA), ICI 182780 from AstraZeneca (London, UK), cell culture reagents and Lipofectamine 2000 from Life Technologies (Carlsbad, CA, USA). Pre-designed shRNA against ERa and ERb and control shRNA were obtained from Sigma-eAldrich. The luciferase reporter construct 3 Â ERE-luc has been published (Legler et al., 1999). pRL-TK for normalisation of luciferase activity was purchased from Promega (Madison, WI, USA). Antibodies and primers used are listed in Supplemental material Tables S1 and S2.

Animals
Animal tissues used here were obtained in a previous study described elsewhere (Panagiotidou et al., 2014) and the protocol was approved by the Ethical Committee of the School of Health Sciences, National and Kapodistrian University of Athens, Greece. In brief, female Wistar rat breeders received BPA (40 mg BPA/kg bw/ day) or the vehicle (water, 1% in ethanol) orally via impregnated cornflakes throughout pregnancy and lactation. The offspring (BPAexposed or unexposed controls) were left to grow. At mid-puberty (postnatal day 46) the offspring were killed by decapitation either at basal conditions or two hours following a 15-min swimming stress.
For differentiation, HT22 cells were seeded into six-well plates (2 Â 10 5 cells/well) in phenol red-free DMEM containing 5% dextran coated charcoal (DCC)-treated FBS and allowed to settle. Medium was changed to phenol red-free Neurobasal medium containing N 2 supplement and 100 mM dibutyrylcAMP, and different concentrations of BPA and/or 10 nM E2 or ICI 182,780 (ICI). After two days, medium was changed to phenol red-free Neurobasal medium without dibutyrylcAMP, BPA, and ICI, and allowed to grow for another 3 days. 1 mM dexamethasone was added 16 h before For ER knock-down studies, HT22 cells were seeded into sixwell plates (2 Â 10 5 cells/well) in phenol red-free DMEM containing 5% DCC-treated FBS. The next day, mission shRNA was transfected using Lipofectamine 2000 according to the manufacturer's instructions using 2.5 mg plasmid DNA and 4 ml reagent per well.

List of abbreviations
After 24 h, medium was changed to phenol red-free Neurobasal medium containing N 2 supplement and 100 mM dibutyrylcAMP to induce differentiation.

Gene expression analyses
RNA was isolated using Trizol (LifeTechnologies, Carlsbad, CA, USA) according to the manufacturer's instructions. 1 mg of total RNA was treated with DNaseI and reverse transcribed using iScript (BioRad, California, USA). Real-time PCR was performed in a 15 ml reaction with 1 ml cDNA using SYBR® Select (LifeTechnologies, Carlsbad, CA, USA) and primers for Fkbp5, GR, ERs, and 36B4 (Supplemental material, Table S2) on an ABI 7500 Fast Real Time PCR System (Thermo Scientific, Waltham, Massachusetts, USA).
Relative gene expression was assessed using the DDCt method.
36B4 expression was chosen as most robust normalizer, compared to GAPDH, b-actin, and 18S RNA.

DNA methylation analyses
Genomic DNA was extracted from cells using GenElute™ (Sig-maeAldrich, St. Louis, Missouri, USA) and from rat brain tissues using the AllPrep DNA/RNA/Protein Mini Kit (Qiagen, Hilden, Germany) following manufacturer's instructions. Bisulfite conversion was performed using EZ Methylation Gold Kit (Zymo Research, Irvine, California, USA) and Nr3c1 promoter, Fkbp5 intron 5 and intron 1was amplified by nested PCR (for primer sequences (Supplemental material, Table S1). Pyrosequencing was performed with 25 ml of the resulting PCR product in a Pyromark Q96 ID using Pyromark Q96 Gold reagents (Qiagen, Hilden, Germany) according to the manufacturer's instructions.

Luciferase assays
HT22 cells were seeded into 24-well plates (5 Â 10 4 cells/well) in phenol red-free DMEM containing 5% DCC-treated FBS. The next day, pRL-TK and 3 Â ERE-luc were transfected using Lipofectamine 2000 according to the manufacturer's instructions using a total of 0.5 mg plasmid DNA and 1 ml reagent per well. 9 h after transfection, fresh medium was added containing 10 nM E2 and/or 1 mM BPA.
The next day, luciferase reporter assays were performed using the Dual-Luciferase ® Reporter Assay System (Promega, Madison, WI, USA) according to the manufacturer's protocol: cells were lysed in passive lysis buffer and firefly and renilla luciferase activity measured in 96-well plates using a Tecan infinite F200 luminometer (Tecan). Firefly luciferase activity was normalized to renilla luciferase activity.

Chromatin immunoprecipitation (ChIP)
ChIP assays were performed as described (Cortazar et al., 2011) 2e5 mg of the respective antibody was used (Supplemental material, Table S1). DNA was purified using the QIAquick PCR purification kit (Qiagen, Hilden, Germany) and the isolated DNA fragments were analysed by qPCR using Rotor-Gene SYBR Green PCR Kit on a Rotor-Gene RG-3000 (Qiagen, Hilden, Germany).

Statistical analyses
To evaluate Western blot data, the mean ratio of FKBP51 to GAPDH or b-actin for each analysed sample was used. Two-way analysis of variance (ANOVA) was used to examine the effect of BPA treatment and stress on hippocampal FKBP51 levels. The LSD posthoc test was used for group comparisons. Unpaired student's ttest was used for methylation, gene expression, and ChIP analyses unless stated otherwise. Significance was accepted for P < 0.05.

Perinatal BPA exposure affects Fkbp5 methylation and FKBP51 levels in male rats
We have previously shown that perinatal exposure to a dose of BPA close to the range of human exposure leads to prolonged sexspecific alterations in stress response and HHPA-axis function in rats, affecting among others GR levels (Poimenova et al., 2010;Panagiotidou et al., 2014). Here, we set out to investigate if perinatal BPA exposure leads to DNA methylation changes in the GR gene (Nr3c1) and/or in its regulator Fkbp5. We used the hippocampi and hypothalami from 46-day old rats whose mothers had been exposed to 40 mg BPA/kg bw/day during gestation and lactation, a dose belonging to the low-dose exposure range for BPA, repeatedly reported to exert adverse effects in experimental animals (Vandenberg et al., 2012), and investigated a region in the Nr3c1 promoter (Weaver et al., 2004) and the intron 5 of Fkbp5. No significant difference was found in the Nr3c1 promoter in either brain areas (Table 1), whereas in male hippocampi, we found a significant increase for methylation at Fkbp5 intron 5, an important regulatory region that is differentially methylated upon corticoid treatment (Lee et al., 2010) (Fig. 1a, Table 1). We also investigated methylation at Fkbp5 intron 1, another regulatory region of this gene (Lee et al., 2010). No significant differences were found in this region (Table 1).
Next, we examined if FKBP51 levels are affected in male hippocampi. At basal conditions FKBP51 protein levels were significantly lower in the hippocampus of BPA-exposed male rats compared to the controls (p ¼ 0.045) ( Fig. 1b and Supplemental material, Fig. S1a). Two hours following acute swimming stress, FKBP51 levels increased compared to baseline in BPA-treated animals (p ¼ 0.016), whereas they did not change significantly in untreated controls ( Fig. 1b and Supplemental material, Fig. S1a).
Thus, perinatal BPA exposure leads to sexually dimorphic changes in hippocampal Fkbp5 methylation approximately one month after exposure has finished. At basal conditions, methylation changes coincide with reduced FKBP51 levels in the hippocampus of male offspring. However, following stress, FKBP51 levels are increased to the levels of untreated controls.

BPA treatment of differentiating HT22 cells induces changes in Fkbp5 expression and methylation
To address the mechanisms underlying the expression and methylation changes of Fkbp5 in BPA-exposed male rats, we employed a cell model. We chose HT22 cells, a murine hippocampal cell line derived from male mouse primary hippocampal cells that can be differentiated from proliferating mitotic cells to cholinergic neurons displaying neurite outgrowth (Liu et al., 2009). To mimic developmental exposure in animals, these cells were treated with increasing amounts of BPA during the 2-days differentiation procedure and subsequently kept without BPA for another 3 days (Fig. 2a) before analysis. BPA treatment decreased Fkbp5 gene expression in a dose-dependent fashion with significant changes at doses as low as 10 nM (Fig. 2b). This was also reflected at the protein level showing a significant decrease upon BPA treatment ( Fig. 2c and Supplemental material, Fig. S1b).
We then analysed Fkbp5 methylation upon treatment with 10 and 1000 nM BPA at the region in intron 5 that is homologous to the regions investigated in the rats (Fig. 2d). We focused on the CpGs 5 and 6, corresponding to CpGs 6 and 7 in the rat, which are part of ã 200 bp region conserved between mouse and rat containing a GRE. BPA treatment led to an increase in methylation at these two CpGs, which reached significance at CpG 5 for both doses (Fig. 2e). Thus, BPA exposure of HT22 cells during differentiation leads to increased DNA methylation and decreased expression of Fkbp5 3 days after BPA treatment was stopped, similarly to the changes observed in vivo.

BPA treatment in HT22 cells changes Fkbp5 inducibility upon glucocorticoid treatment
As part of the negative feedback regulation of the HPA-axis, GR, activated by elevated glucocorticoid levels during stress, increases Fkbp5 expression by binding to GREs in regulatory regions of the gene, among others the one in intron 5 (Hubler and Scammell, 2004). We thus analyzed if Fkbp5 inducibility by glucocorticoids changes upon BPA treatment in the cell model. To this end, we differentiated HT22 cells in the absence and presence of 10 and 1000 nM BPA as described above. Additionally, cells were treated with a single dose of dexamethasone (dex), a stable synthetic glucocorticoid, 16e18 h before Fkbp5 expression was assessed on day 3 (Fig. 2a). We used 1 mM dex, a concentration commonly used in cell experiments, e.g. (Klengel et al., 2013b), that induces Fkbp5 expression (Nehme et al., 2009). As expected, Fkbp5 expression increased upon dex-treatment in the absence of BPA (Fig. 3, left grey vs. black bar). Similar inducibility with 10 and 100 nM dex were observed (data not shown). In cells exposed to BPA during differentiation, basal Fkbp5 was decreased significantly whereas upon dex-stimulation, the increase of Fkbp5 expression was even higher than in the absence of BPA (Fig. 3, middle and right grey vs. black bars). Thus, BPA treatment during HT22 differentiation changes inducibility of Fkbp5 in response to glucocorticoids, similarly to the rat model.   material, Fig. S2a,b). We tested if estradiol (E2) treatment during differentiation affects Fkbp5 expression, and if inhibition of ERs with ICI 182 780 (ICI) changes the effects of BPA. HT22 cells were differentiated as above in the absence or presence of 10 nM E2 or 1 mM ICI, which are commonly used doses of the hormone and antagonist (e.g. (Dauvois et al., 1992)). Gene expression was assessed 3 days after the differentiation was finished and the substances had been washed out (Fig. 2a). ICI treatment led to a decrease in Fkbp5 expression (Fig. 4a, left grey vs. black bar), similar to that observed upon BPA treatment (Fig. 4a, middle black bars). In accordance, FKBP51 protein levels were decreased by both BPA and ICI treatment and their combination (Fig. 4b, Supplemental  Fig. S1c). E2, on the other hand, did not affect Fkbp5 expression by itself, but reverted the decrease induced by ICI (Fig. 4a, bars to the right). We also tested if these treatments affect Fkbp5 expression through direct transcriptional activation. Acute treatment with BPA or E2 in the absence or presence of ICI for 6 h did not induce any differences in Fkbp5 expression (Fig. 4c). On the other hand, E2 readily activated an ERE-luciferase reporter construct transiently transfected into HT22 cells (Fig. 4d), demonstrating that ERE-driven transcription can be activated in these cells. BPA had no effect on luciferase activity by itself but reverted E2 induced transcription (Fig. 4d).
As ICI treatment had similar effects to BPA on Fkbp5 transcription, we also investigated its effects on DNA methylation at CpG 5 and 6. As shown in Fig. 4e, ICI increased Fkbp5 methylation to a similar extent as BPA. In the presence of both ICI and BPA during differentiation, Fkbp5 methylation increased even further, in particular for 1 mM BPA at CpG6 (Fig. 4e, middle and right grey vs. black bars). Thus, the ER inhibitor ICI affects Fkbp5 methylation and expression similarly to BPA, suggesting an involvement of the ERs in BPA's effects, however, not via a classical ERE-driven transcriptional mechanism.

ERb is involved in the effects of BPA on Fkbp5
To better understand the role of the ERs in regulating Fkbp5 transcription, we investigated if ERa and/or ERb is recruited to the regulatory region of Fkbp5 at intron 5. To this end, we conducted chromatin immunoprecipitation assays in HT22 cells, analyzing ERa and ERb binding to the differentially methylated region in intron 5 upon treatment with BPA and ICI. As shown in Fig. 5a, ERb was significantly enriched at this region, both in the absence and presence of E2. On the other hand, no significant enrichment could be found for ERa with two different antibodies (data not shown). Upon ICI treatment, ERb binding to intron 5 was lost (Fig. 5a), supporting a specific enrichment at this region. Following BPA treatment, ERb recruitment decreased significantly compared to control (Fig. 5a).
We then tested a direct involvement of ERb in mediating BPA's effect on Fkbp5. Using shRNA, we knocked-down ERb before induction of the differentiation process (Fig. 5b). Knock-down of ERb lead to a significant decrease in Fkbp5 expression ( Fig. 5c black bars, left panel) and to a significant increase in Fkbp5 methylation (Fig. 5c black bars, right panel). BPA treatment during differentiation decreased expression and increased methylation when cells were transfected with control shRNA. However, no further effect was observed when ERb was knocked-down ( Fig. 5c grey bars).
Together, these findings implicate that ERb is involved in the effects of BPA on Fkbp5 transcription and methylation.

Discussion
The HHPA axis is essential for the organism to cope with stress. Dysfunction of this system has been associated with a number of psychiatric illnesses such as major depression and post-traumatic stress disorder. While major research efforts have been made to identify genetic components involved in psychiatric diseases, much less is known about environmental factors contributing to these disorders and the underlying mechanisms. In this study, we demonstrate both in an in vivo and in an in vitro model that BPA, an abundant chemical that disrupts estrogen signaling, can induce lasting changes in the regulation of Fkbp5, an important regulator of the HHPA axis.
We show that perinatal exposure to a low, human-relevant, BPA dose (Vandenberg et al., 2012) leads to epigenetic changes in the Fkbp5 gene in the hippocampus of male rats about one month after BPA exposure had finished. Increased methylation coincided with reduced FKBP51 levels at basal conditions that imply an increased alertness towards threatening insults. Notably, upon stress, FKBP51 levels of these animals increase, suggesting that BPA-treated male rats become equally efficient as controls to terminate an acute stress response. This competency complies well with previous results where BPA-treated males exhibited similar behavioral coping and corticosterone responses to stress as the untreated animals (Panagiotidou et al., 2014). The finding that a short-term stress can modify FKBP51 levels despite the BPA-induced methylation changes, suggests an enhanced expression plasticity of this regulator that was also observed in the HT22 cell line deriving from male mouse hippocampus.
Our findings in male rats are in good agreement with the results obtained in HT22 cells and demonstrate that this cell line is a valid model to study the mechanisms underlying the effects of BPA exposure on Fkbp5 regulation. In the cell line, BPA exposure during differentiation lead to decreased Fkbp5 expression and increased methylation at the corresponding CpGs in intron 5. Of note, these changes were not due to acute effects of BPA as they were observed 3 days after BPA washout. Similarly to the rat model, BPA's effect on Fkbp5/FKBP51 levels was larger than on Fkbp5 methylation. This is at least partly due to the fact that the detection methods do not have the same sensitivity, which makes comparison of the results difficult. Further, other factors and/or genomic regions not investigated here, but involved in Fkbp5 regulation, might be affected by BPA.
Interestingly, dexamethasone treatment, simulating the stressful situation in the rat model, increased Fkbp5 expression to a greater extend in BPA-treated than in the untreated cells. The reason for decreased basal expression of Fkbp5 but increased inducibility of the gene by glucocorticoids despite an increase in DNA methylation is not solved. One possibility is that methylation changes at the responsive element under study affect mineralocorticoid receptor binding, responsible for mediating the effects of basal glucocorticoid levels, but not GR binding that follows the increase of hormone levels. Although sharing the DNA recognition sequence, the two receptors are modulated in their activity by diverse co-activators and co-repressors that might be affected differently by the methylation changes. Further studies are necessary to understand the detailed implications of the DNA methylation changes induced by BPA.
The ER inhibitor ICI affected Fkbp5 expression and methylation during HT22 differentiation similarly to BPA. Furthermore, ERb, but not ERa, bound to the differentially methylated region at intron 5, and this binding was disrupted by BPA and ICI. These results imply a function of ERb in mediating BPA's effects on Fkbp5, which was further supported by the fact that ERb knock-down abolished the effects of BPA on Fkbp5 expression and DNA methylation. Notably, however, the effects found here cannot be ascribed to BPA's estrogenic properties as i) they were observed three days after BPA had been washed out of the culture and ii) BPA did not display agonistic properties in the HT22 cells on an ERE-driven transcription, in contrast to E2 (Fig. 4c). Further, there is an agreement in the literature that BPA does not induce the same conformational changes as the natural ligands when binding to ERs (Wetherill et al., 2007;Delfosse et al., 2012), hence presumably attracting a different set of co-regulatory proteins than in the presence of natural ligand. Accordingly, we propose that BPA affects Fkbp5 transcriptional regulation by interfering with ERb binding to the regulatory region of intron 5, where ERb controls DNA methylation, a function of ERb that we have described previously (Ruegg et al., 2011). A tentative model is depicted in Supplemental material, Fig. S3. We could not see any effect of BPA on ER or GR protein expression in our cell model (Supplemental material, Fig. S2b and c). Further, ERb levels were not changed in the hippocampi of the treated rats (Supplemental material, Fig. S2d). However, in mice it was shown that BPA exposure leads to decreased ERb levels in brain areas other than the hippocampus (Cao et al., 2014). Thus in other regions BPA might not affect DNA binding of ERb but rather its protein levels. Ultimately, however, this will lead to the same result, a lack of ERb binding to intron 5 of Fkbp5 and thus an increase in DNA methylation. Interestingly, BPA seems to affect ERb expression in the rodent brain in a sexual dimorphic manner ( (Cao et al., 2014) and data not shown). This might explain why we could not detect any methylation changes in female rats at the investigated regions of Nr3c1 and Fkbp5. Sexually dimorphic effects of BPA in brain function have been reported in several previous in vivo studies (Cao et al., 2014;Cao et al., 2013;Chen et al., 2014;Gioiosa et al., 2013;Jasarevic et al., 2013;Wolstenholme et al., 2011;Xu et al., 2015). Furthermore, the few epidemiological studies linking BPA exposure to neuropsychiatric outcomes in children also show differences  between girls and boys (Braun et al., 2011;Harley et al., 2013;Evans et al., 2014). This demonstrates the intricate interaction between BPA and the endogenous sex hormones and consequently the importance to investigate its effects on both sexes.

Conclusion
We demonstrate here that perinatal exposure of rats to a low BPA dose alters Fkbp5 expression, methylation pattern and inducibility by stress in the hippocampus of male offspring. The observed alterations in Fkbp5 were also detected in differentiating hippocampal neurons of male origin. In the cell model, the mechanism implicates ERb in the regulation of the epigenetic impact, a finding that requires further studies in the in vivo setting. The BPA-induced changes in hippocampal Fkbp5 confer a link between environmental chemicals and stress-related disorders.

Financial disclosures
The authors have no competing financial interests. . ERb is involved in the effects of BPA on Fkbp5 expression and methylation. a: ERb binding after BPA, ICI, or E2-treatment in HT22 cells assessed by chromatin immunoprecipitation. ERb was precipitated using an anti-ERb antibody and the region at Fkbp5 intron 5 was amplified from co-precipitated DNA using qRT-PCR and normalised to input sample. *p < 0.05, **p < 0.005 enrichment ERb antibody vs. IgG, # p < 0.05 ERb enrichment in BPA-or ICI-treated vs. untreated cells. b: ERb gene expression in HT22 cells transfected with 2 different shRNA sequences against ERb or a control shRNA, assessed by real-time PCR. ### p < 0.001 shERb vs. shcontrol c: Fkbp5 gene expression (left panel) and methylation (right panel) upon BPA treatment during differentiation in HT22 cells transfected with 2 different shRNA sequences against ERb or a control shRNA. *p < 0.05 BPA-treated vs. untreated control, # p < 0.05 shERb vs. shcontrol. All bars represent the means and error bars the standard deviation.