Role Of GPER Research Articles

G protein-coupled estrogen receptor expression in postnatal developing mouse retina.

Estrogen has emerged as a multifaceted signaling molecule in the retina, playing an important role in neural development and providing neuroprotection in adults. It interacts with two receptor types: classical estrogen receptors (ERs) alpha and beta, and G protein-coupled estrogen receptor (Gper). Gper differs from classical ERs in structure, localization, and signaling. Here we provide the first report of the temporal and spatial properties of Gper transcript and protein expression in the developing and mature mouse retina. We applied qRT-PCR to determine Gper transcript expression in wild type mouse retina from P0-P21. Immunohistochemistry and Western blot were used to determine Gper protein expression and localization at the same time points. Gper expression showed a 6-fold increase during postnatal development, peaking at P14. Relative total Gper expression exhibited a significant decrease during retinal development, although variations emerged in the timing of changes among different forms of the protein. Gper immunoreactivity was seen in retinal ganglion cells (RGCs) throughout development and also in somas in the position of horizontal cells at early time points. Immunoreactivity was observed in the cytoplasm and Golgi at all time points, in the nucleus at early time points, and in RGC axons as the retina matured. In conclusion, our study illuminates the spatial and temporal expression patterns of Gper in the developing mouse retina and provides a vital foundation for further investigations into the role of Gper in retinal development and degeneration.

The Role of GPER in Sepsis-Induced Myocardial Cell Damage and 28-Day Mortality Risk

The Role of GPER in Sepsis-Induced Myocardial Cell Damage and 28-Day Mortality Risk

G protein-coupled estrogen receptor in GtoPdb v.2023.1

The G protein-coupled estrogen receptor (GPER, nomenclature as agreed by the NC-IUPHAR Subcommittee on the G protein-coupled estrogen receptor [26]) was identified following observations of estrogen-evoked cyclic AMP signalling in breast cancer cells [2], which mirrored the differential expression of an orphan 7-transmembrane receptor GPR30 [6]. There are observations of both cell-surface and intracellular expression of the GPER receptor [29, 34]. Selective agonist/ antagonists for GPER have been characterized [26]. Antagonists of the nuclear estrogen receptor, such as fulvestrant [11], tamoxifen [29, 34] and raloxifene [25], as well as the flavonoid 'phytoestrogens' genistein and quercetin [18], are agonists of GPER. Reviews of GPER pharmacology have been published [26]. The roles of GPER in (patho)physiological systems throughout the body (cardiovascular, metabolic, endocrine, immune, reproductive) and in cancer have also been reviewed [26, 27, 20, 17, 9]. The GPER-selective agonist G-1 is currently in Phase I/II clinical trials for cancer (NCT04130516).

Activation of GPER limits adverse changes to Ca2+ signalling and arrhythmogenic activity in cardiomyocytes of ovariectomised guinea pigs

Abstract Background Post-menopausal women have an enhanced risk of developing cardiovascular disease and disturbances of cardiac rhythm, generally attributed to declining oestrogen levels during menopause. In an animal model that mimics menopause, the long-term withdrawal of oestrogen dysregulated Ca2+ signalling and increased the formation of a pro-arrhythmic substrate. Selective targeting of the membrane bound G-protein coupled oestrogen receptor 1 (GPER) eliminated such arrhythmogenic activity associated with the loss of oestrogen. Purpose We aim to assess the “cardioprotective” role of GPER in response to oestrogen withdrawal. Methods Ovariectomy (OVx) or sham surgeries were conducted on female guinea pigs. Left ventricular cardiomyocytes were isolated 150-days post-operatively for experimental use. GPER expression was quantified by western blot. Myocytes were incubated in solutions containing GPER agonist G-1 for >2h before recording some electrophysiological and Ca2+ regulatory parameters. Results GPER expression was 32% higher in OVx. OVx cardiomyocytes had prolonged action potential duration (APD) compared with sham and in the presence of G-1, APD90 shortened by 12% and 22% in sham and OVx, respectively. G-1 reduced early after depolarisation (EAD) formation by >99% in OVx. OVx cells had larger sarcoplasmic reticulum (SR) Ca2+ content (by 13%), compared with sham. While G-1 had little effect on SR content, it reduced Ca2+ transient amplitude (by 40%), SR fractional release (by 11%) and sarcomere shortening (by 29%) in OVx cells. The frequency of occurrence of spontaneous Ca2+ waves evoked by periods of rapid stimulation reduced by 40% and wave-free survival time prolonged in OVx cells incubated with G-1. Conclusions In the hearts of an animal species whose electrophysiology and intracellular Ca2+ regulation is akin to humans, we show that following oestrogen deficiency, GPER expression increased and its activation induces negative inotropic responses in cardiomyocytes. It limits the adverse changes to Ca2+ signalling and induces anti-arrhythmogenic behaviours in OVx. Funding Acknowledgement Type of funding sources: Private grant(s) and/or Sponsorship. Main funding source(s): British Heart Foundation

G protein-coupled estrogen receptor in GtoPdb v.2021.3

The G protein-coupled estrogen receptor (GPER, nomenclature as agreed by the NC-IUPHAR Subcommittee on the G protein-coupled estrogen receptor [25]) was identified following observations of estrogen-evoked cyclic AMP signalling in breast cancer cells [2], which mirrored the differential expression of an orphan 7-transmembrane receptor GPR30 [6]. There are observations of both cell-surface and intracellular expression of the GPER receptor [28, 33]. Selective agonist/ antagonists for GPER have been characterized [25]. Antagonists of the nuclear estrogen receptor, such as fulvestrant [11], tamoxifen [28, 33] and raloxifene [24], as well as the flavonoid 'phytoestrogens' genistein and quercetin [17], are agonists of GPER. A complete review of GPER pharmacology has been published [25]. The roles of GPER in physiological systems throughout the body (cardiovascular, metabolic, endocrine, immune, reproductive) and in cancer have also been reviewed [25, 26, 19, 16, 9]. The GPER-selective agonist G-1 is currently in Phase I/II clinical trials for cancer (NCT04130516).

Effect of estrogen-active compounds on the expression of RACK1 and immunological implications.

We previously demonstrated the existence of a balance among steroid hormones, i.e. glucocorticoids and androgens, in RACK1 (receptor for activated C kinase 1) expression and innate immunity activation, which may offer the opportunity to use RACK1 expression as marker to evaluate immunotoxicity of hormone-active substances. Because of the existence of close interconnections between the different steroid hormone receptors with overlapping ligand specificities and signaling pathways, in this study, we wanted to investigate a possible effect of estrogenic active compounds, namely 17β-estradiol, diethylstilbestrol, and zearalenone, on RACK-1 expression and innate immune responses using THP-1 cells as experimental model. All compounds increased RACK1 transcriptional activity as evaluated by reporter luciferase activity, mRNA expression as assessed by real time-PCR and protein expression by western blot analysis, which paralleled an increase in LPS-induced IL-8, TNF-α production, and CD86 expression, which we previously demonstrated to be dependent on RACK1/PKCβ activation. As the induction of RACK1 expression can be blocked by the antagonist G15, induced by the agonist G1 and by the non-cell permeable 17β-estradiol conjugated with BSA, a role of GPER (previously named GPR30) activation in estrogen-induced RACK1 expression could be demonstrated. In addition, a role of androgen receptor (AR) in RACK1 transcription was also demonstrated by the ability of flutamide, a nonsteroidal antiandrogen, to completely prevent diethylstilbestrol-induced RACK1 transcriptional activity and protein expression. Altogether, our data suggest that RACK1 may represent an interesting target of steroid-active compounds, and its evaluation may offer the opportunity to screen the immunotoxic potential of hormone-active substances.

G Protein-Coupled Estrogen Receptor 1 (GPER) as a Novel Target for Schizophrenia Drug Treatment

AbstractThe observation that a person’s sex influences the onset age of schizophrenia, the course of the disease, and antipsychotic treatment response suggests a possible role for estrogen receptors in the pathophysiology of schizophrenia. Indeed, treatment with adjunctive estrogen or selective estrogen receptor modulators (SERMs) are known to reduce schizophrenia symptoms. While estrogen receptors (ER)α and ERβ have been studied, a third and more recently discovered estrogen receptor, the G protein-coupled estrogen receptor 1 (GPER), has been largely neglected. GPER is a membrane receptor that regulates non-genomic estrogen functions, such as the modulation of emotion and inflammatory response. This review discusses the possible role of GPER in brain impairments seen in schizophrenia and in its potential as a therapeutic target. We conducted a comprehensive literature search in the PubMed/MEDLINE database, using the following search terms: “Schizophrenia,” “Psychosis,” “GPER1 protein,” “Estrogen receptors,” “SERMS,” “GPER1 agonism, “Behavioral symptoms,” “Brain Inflammation.” Studies involving GPER in schizophrenia, whether preclinical or human studies, have been scarce, but the results are encouraging. Agonism of the GPER receptor could prove to be an essential mechanism of action for a new class of “anti-schizophrenia” drugs.

G protein-coupled estrogen receptor (version 2019.4) in the IUPHAR/BPS Guide to Pharmacology Database

The G protein-coupled estrogen receptor (GPER, nomenclature as agreed by the NC-IUPHAR Subcommittee on the G protein-coupled estrogen receptor [24]) was identified following observations of estrogen-evoked cyclic AMP signalling in breast cancer cells [2], which mirrored the differential expression of an orphan 7-transmembrane receptor GPR30 [5]. There are observations of both cell-surface and intracellular expression of the GPER receptor [27, 32]. Selective agonist/ antagonists for GPER have been characterized [24]. Antagonists of the nuclear estrogen receptor, such as fulvestrant [10], tamoxifen [27, 32] and raloxifene [23], as well as the flavonoid 'phytoestrogens' genistein and quercetin [16], are agonists of GPER. A complete review of GPER pharmacology has been recently published [24]. The roles of GPER in physiological systems throughout the body (cardiovascular, metabolic, endocrine, immune, reproductive) and in cancer have also been reviewed [24, 25, 18, 15, 8].

Evaluation of estrogen and G protein-coupled estrogen receptor 1 (GPER) levels in drug-naïve patients with attention deficit hyperactivity disorder (ADHD).

Estrogen has a crucial role in the regulation of reproductive and neuroendocrine function and exerts its effects through two classes of receptors, nuclear and membrane estrogen receptors (mERs). G protein-coupled estrogen receptor 1 (GPER) is a member of mERs, and despite limited research on the levels of GPER in patients with psychiatric diseases, a role of GPER in such conditions has been suggested. Here we evaluated serum estrogen and GPER levels in children with attention deficit hyperactivity disorder (ADHD) in relation to their age- and gender-matched healthy controls. A total of 82 children were included in the study, 47 drug- naïve patients with ADHD (age: 6-12 years; male/female: 34/13) and 35 healthy controls (age: 6-12 years; male/female: 19/16). The subgroups according to ADHD types were inattentive, hyperactive/impulsive, and combined. Serum estrogen was measured using an immunoassay system, while serum GPER was determined using a commercial sandwich enzyme-linked immunosorbent assay kit. Estrogen levels in children with ADHD were similar as in control group, while GPER levels were significantly lower in ADHD group compared to controls (p < 0.05). Logistic regression analysis showed a significant association between GPER levels and ADHD (p < 0.05), and no association between estrogen levels and ADHD (p > 0.05). No significant differences were found in GPER and estrogen levels between ADHD subgroups (p > 0.05). To the best of our knowledge, this study is the first to investigate estrogen and GPER levels in ADHD. Our preliminary findings suggest a relationship between serum GPER levels and ADHD, and this should be further investigated.

G-Protein-Coupled Estrogen Receptor (GPER) and Sex-Specific Metabolic Homeostasis.

Obesity and metabolic syndrome display disparate prevalence and regulation between males and females. Human, as well as rodent, females with regular menstrual/estrous cycles exhibit protection from weight gain and associated chronic diseases. These beneficial effects are predominantly attributed to the female hormone estrogen, specifically 17β-estradiol (E2). E2 exerts its actions via multiple receptors, nuclear and extranuclear estrogen receptor (ER) α and ERβ, and the G-protein-coupled estrogen receptor (GPER, previously termed GPR30). The roles of GPER in metabolic homeostasis are beginning to emerge but are complex and remain unclear. The discovery of GPER-selective pharmacological agents (agonists and antagonists) and the availability of GPER knockout mice have significantly enhanced our understanding of the functions of GPER in normal physiology and disease. GPER action manifests pleiotropic effects in metabolically active tissues such as the pancreas, adipose, liver, and skeletal muscle. Cellular and animal studies have established that GPER is involved in the regulation of body weight, feeding behavior, inflammation, as well as glucose and lipid homeostasis. GPER deficiency leads to increased adiposity, insulin resistance, and metabolic dysfunction in mice. In contrast, pharmacologic stimulation of GPER in vivo limits weight gain and improves metabolic output, revealing a promising novel therapeutic potential for the treatment of obesity and diabetes.

G protein-coupled estrogen receptor is involved in modulating colonic motor function via nitric oxide release in C57BL/6 female mice.

Estrogen may regulate gastrointestinal motor functions, but the mechanism(s) is not totally understood. Here, we investigated whether G protein-coupled estrogen receptor (GPER/GPR30) was involved in regulating colonic motor functions and explored the underlying physiological mechanisms. Adult female C57BL/6 mice were used. The expression and localization of GPER were examined by RT-PCR, western blot, and immuno-labeling. The role of GPER in modulating colonic motor functions was assessed by the bead propulsion test in vivo and organ bath experiments in vitro. GPER was expressed in colonic myenteric neurons. The colonic transit time (CTT) in proestrus and estrus was significantly longer than that in diestrus. In vivo treatment with the selective GPER blocker G15 significantly shortened CTT in proestrus and estrus. In ovariectomized mice, acute estrogen supplementation increased CTT, which could be abolished by G15 co-administration. The GPER agonist G-1 caused a concentration-dependent inhibition of carbachol -induced circular muscle strips contraction, which was abolished by tetrodotoxin and the neuronal nitric oxide synthase (nNOS) inhibitor N-propyl-l-arginine. G-1 stimulated NO production in isolated longitudinal muscle myenteric plexus and cultured myenteric neurons, which was dependent on nNOS. Immunofluorescence labeling showed co-localization of GPER with nNOS in the myenteric plexus. We suggest that activation of GPER exerts an inhibitory effect on colonic motility by promoting NO release from myenteric nitrergic nerves. These results raise a possibility that GPER may be involved in mediating the inhibitory effect of estrogen on colonic motor functions, via a non-genomic, neurogenic mechanism.

GPER-Targeted, 99mTc-Labeled, Nonsteroidal Ligands Demonstrate Selective Tumor Imaging and In Vivo Estrogen Binding

Our understanding of estrogen (17β-estradiol, E2) receptor biology has evolved in recent years with the discovery and characterization of a 7-transmembrane-spanning G protein-coupled estrogen receptor (GPER/GPR30) and the development of GPER-selective functional chemical probes. GPER is highly expressed in certain breast, endometrial, and ovarian cancers, establishing the importance of noninvasive methods to evaluate GPER expression in vivo. Here, we developed (99m)Tc-labeled GPER ligands to demonstrate the in vivo status of GPER as an estrogen receptor (ER) and for GPER visualization in whole animals. A series of (99m)Tc(I)-labeled nonsteroidal tetrahydro-3H-cyclopenta[c]quinolone derivatives was synthesized utilizing pyridin-2-yl hydrazine and picolylamine chelates. Radioligand receptor binding studies revealed binding affinities in the 10 to 30 nmol/L range. Cell signaling assays previously demonstrated that derivatives retaining a ketone functionality displayed agonist properties, whereas those lacking such a hydrogen bond acceptor were antagonists. In vivo biodistribution and imaging studies performed on mice bearing human endometrial and breast cancer cell xenografts yielded significant tumor uptake (0.4-1.1%ID/g). Blocking studies revealed specific uptake in multiple organs (adrenals, uterus, and mammary tissue), as well as tumor uptake with similar levels of competition by E2 and G-1, a GPER-selective agonist. In conclusion, we synthesized and evaluated a series of first-generation (99m)Tc-labeled GPER-specific radioligands, demonstrating GPER as an estrogen-binding receptor for the first time in vivo using competitive binding principles, and establishing the utility of such ligands as tumor imaging agents. These results warrant further investigation into the role of GPER in estrogen-mediated carcinogenesis and as a target for diagnostic/therapeutic/image-guided drug delivery. These studies provide a molecular basis to evaluate GPER expression and function as an ER through in vivo imaging.

The G-Protein Coupled Estrogen Receptor (GPER/GPR30) in ovarian granulosa cell tumors

Introduction: Ovarian granulosa cell tumors (GCTs) comprise a rare entity of ovarian sex cord stroma tumors. In line with their putative cells of origin GCTs possess the ability to synthesize estrogen and are supposed to be responsive to gonadotropins (FSH/LH). We recently identified the G-protein coupled estrogen receptor (GPER/GPR30) to be regulated by LH and FSH and to be differentially expressed throughout ovarian follicle development. Thus, supposing a possible role of GPER in GCTs, this study aimed to analyze GPER in GCTs and whether GPER might be correlated with clinicopathological parameters.

The G-protein-coupled estrogen receptor (GPER/GPR30) in ovarian granulosa cell tumors.

Ovarian granulosa cell tumors (GCTs) are thought to arise from cells of the ovarian follicle and comprise a rare entity of ovarian masses. We recently identified the G-protein-coupled estrogen receptor (GPER/GPR30) to be present in granulosa cells, to be regulated by gonadotropins in epithelial ovarian cancer and to be differentially expressed throughout folliculogenesis. Thus, supposing a possible role of GPER in GCTs, this study aimed to analyze GPER in GCTs. GPER immunoreactivity in GCTs (n = 26; n (primary diagnosis) = 15, n (recurrence) = 11) was studied and correlated with the main clinicopathological variables. Positive GPER staining was identified in 53.8% (14/26) of GCTs and there was no significant relation of GPER with tumor size or lymph node status. Those cases presenting with strong GPER intensity at primary diagnosis showed a significant reduced overall survival (p = 0.002). Due to the fact that GPER is regulated by estrogens, as well as gonadotropins, GPER may also be affected by endocrine therapies applied to GCT patients. Moreover, with our data supposing GPER to be associated with GCT prognosis, GPER might be considered as a possible confounder when assessing the efficacy of hormone-based therapeutic approaches in GCTs.

GPER-independent effects of estrogen in rat aortic vascular endothelial cells

GPER (aka GPR30) has been identified as an important mechanism by which estrogen mediates its effects. Previous studies from our laboratories and those of others have demonstrated that GPER activation mediates a range of vascular contractile and growth regulatory responses. However, the importance of GPER in mediating the actions of estradiol (E2) in rat aortic endothelial cells is unclear. Therefore, we sought to determine the importance of GPER vs. the “classical” estrogen receptor (ER) in mediating the endothelial growth regulatory effects of E2. To do this we assessed the effect of E2 in regulating phosphoERK content and apoptotic rates in rat aortic endothelial cells and the role of GPER in mediating these effects.E2 mediated a concentration-dependent inhibition of both ERK phosphorylation and serum deprivation-induced apoptosis with a maximal effect at a concentration of 10 nM. Pretreatment with the ER antagonist ICI 182780 abolished E2-mediated inhibition of both ERK phosphorylation and apoptosis. In contrast, pretreatment with GPER antagonist G15 had no significant effect on E2-mediated inhibition of ERK phosphorylation or on apoptosis. Further, downregulation of GPER expression with a GPER shRNA adenovirus did not block E2-mediated inhibitory effects on ERK phosphorylation and apoptosis. In fact, these inhibitory effects of E2 were further enhanced by GPER downregulation.Downregulation of ERα expression reversed the E2-mediated inhibitory effects to stimulatory effects. E2's phosphoERK and apoptosis stimulatory effects seen with ERα downregulation are attenuated by pretreatment with G15.In conclusion, in rat aortic endothelial cells, E2-mediated endothelial effects are predominantly driven by ER and not by GPER.

Abstract 43: The Hypofunctional GPER P16L Variant is Associated With a Gene Dosage-Related Increase in Plasma LDL Cholesterol

Introduction: Estrogen deficiency is linked with dyslipidemia, especially in postmenopausal women, through a poorly understood mechanism. GPER is a recently recognized GPCR which is activated by estrogens. However, the role of GPER in mediating estrogen’s effects on lipid metabolism is unknown. We recently identified a common hypofunctional missense variant of GPER, namely P16L (allele frequency ~ 20%). We studied association of this with plasma LDL cholesterol levels. Further, we studied the role of GPER in regulating expression of the LDL receptor. Methods: Our discovery cohort was a genetically isolated population of Northern European descent (n=415), and our validation cohort consisted of 505 normal, healthy subjects 18-56 years of age from London, Ontario. Genomic DNA was extracted from whole blood and genotyped for GPER using a dedicated TaqMan assay. Additionally we examined the role of GPER on the regulation of LDL receptor expression by treatment with the GPER agonist, G1. Results: In the discovery cohort, the GPER P16L genetic variant was associated with a significant gene-dosage related increase in LDL cholesterol (CC [homozygous wild type] =3.18±0.84 (mean+SD); CT [heterozygote] =3.25±0.80; and TT [homozygous variant] =4.25±0.87 mmol/L, p&lt;0.05). Total cholesterol concentrations followed a similar gradient across genotypes. In the validation cohort, the GPER P16L genetic variant was associated with a similar significant gene-dosage related increase in LDL cholesterol (CC =2.16±0.67; CT [heterozygote] =2.29±0.67; TT =2.40±0.84 mmol/L, p&lt;0.05). In HepG2 cells expressing GPER, G1 mediated a concentration-dependent increase in LDL receptor expression. Pre-treating the cells with the GPER antagonist G15 attenuated the effect of G1 on LDL receptor upregulation. Further, downregulation of GPER expression via infection with a shGPER construct also attenuated G1's effect on LDL receptor upregulation. Conclusion: GPER activation upregulates LDL receptor expression. Further, carrying the hypofunctional P16L genetic variant of GPER, increases plasma LDL cholesterol in humans. In aggregate these data suggest an important role of GPER in regulation of LDL receptor expression and consequently LDL metabolism.

“Role of GPER in Estrogen‐Mediated Inhibition of Cardiomyocyte Hypertrophy.”

Estrogen is known to attenuate cardiac hypertrophy and heart failure in females. To understand how estrogen mediates its effects on the heart, it is important to understand how the cell receives the estrogen signal. Estrogen activates two classes of receptors: Nuclear Hormone Receptors (Estrogen Receptor Alpha (ERa) and Estrogen Receptor Beta (ERβ) and a membrane bound G‐Protein Coupled Receptor for Estrogen (GPER or GPR30). It has not been established whether GPER plays a role in estrogen's ability to prevent cardiac hypertrophy or what that role may be. To determine if GPER plays a role in the antihypertrophic effects of estrogen, we have performed a series of in‐vitro experiments using primary rat cardiomyocyte cultures that have been serum starved for 24 hours and then exposed to the hypertrophic agent, Phenylephrine, in the presence or absence of a GPER specific inhibitor (G15) plus estrogen or the presence or absence of a GPER specific activator (G1) and compared to estrogen alone. The degree of hypertrophy was determined between the groups. Expression of known estrogen target genes were also assessed in cells exposed to the GPER specific activator by relative RT‐PCR. Taken together our results suggest that GPER does play a role in estrogen's antihypertrophic effects.

G-protein coupled estrogen receptor 1 expression in rat and human heart: Protective role during ischaemic stress

G-protein coupled estrogen receptor 1, GPER, formerly known as GPR30, is a seven transmembrane domain receptor that mediates rapid estrogen responses in a wide variety of cell types. To date, little is known about the role of GPER during ischaemia/reperfusion injury. In this study, we report both mRNA and protein expression of GPER in the rat and human heart. The role of GPER in estrogen protection against ischaemic stress in the rat heart was also assessed using the isolated Langendorff system. Pre-treatment with 17beta-estradiol (E2) significantly decreased infarct size, (61.48+/-2.2% to 27.92+/-2.9% (P<0.001). Similarly, treatment with the GPER agonist G1 prior to 30-min global ischaemia followed by 120-min reperfusion significantly reduced infarct size from 61.48+/-2.2% to 23.85+/-3.2% (P<0.001), whilst addition of GPR30 antibody, abolished the protective effect of G1 (infarct size: 55.42+/-1.3%). The results suggest that GPER under cardiac stress exerts direct protection in the heart and may serve as a potential therapeutic target for cardiac drug therapy.

Role of G protein-coupled estrogen receptor 1, GPER, in inhibition of oocyte maturation by endogenous estrogens in zebrafish

Estrogen inhibition of oocyte maturation (OM) and the role of GPER (formerly known as GPR30) were investigated in zebrafish. Estradiol-17β (E2) and G-1, a GPER-selective agonist, bound to zebrafish oocyte membranes suggesting the presence of GPER which was confirmed by immunocytochemistry using a specific GPER antibody. Incubation of follicle-enclosed oocytes with an aromatase inhibitor, ATD, and enzymatic and manual removal of the ovarian follicle cell layers significantly increased spontaneous OM which was partially reversed by co-treatment with either 100 nM E2 or G-1. Incubation of denuded oocytes with the GPER antibody blocked the inhibitory effects of estrogens on OM, whereas microinjection of estrogen receptor alpha (ERα) antisense oligonucleotides into the oocytes was ineffective. The results suggest that endogenous estrogens produced by the follicle cells inhibit or delay spontaneous maturation of zebrafish oocytes and that this estrogen action is mediated through GPER. Treatment with E2 and G-1 also attenuated the stimulatory effect of the teleost maturation-inducing steroid, 17,20β-dihyroxy-4-pregnen-3-one (DHP), on OM. Moreover, E2 and G-1 down-regulated the expression of membrane progestin receptor alpha (mPRα), the intermediary in DHP induction of OM. Conversely DHP treatment caused a > 50% decline in GPER mRNA levels. The results suggest that estrogens and GPER are critical components of the endocrine system controlling the onset of OM in zebrafish. A model is proposed for the dual control of the onset of oocyte maturation in teleosts by estrogens and progestins acting through GPER and mPRα, respectively, at different stages of oocyte development.