GPER1 stimulation alters posttranslational modification of RGSz1 and induces desensitization of 5-HT1A receptor signaling in the rat hypothalamus

Hyperactivity of the hypothalamic-pituitary-adrenal axis is a consistent biological characteristic of depression, and response normalization coincides with clinical responsiveness to antidepressant medications. Desensitization of serotonin 1A receptor (5-HT1AR) signaling in the hypothalamic paraventricular nucleus of the hypothalamus (PVN) follows selective serotonin reuptake inhibitor (SSRI) antidepressant treatment and contributes to the antidepressant response. Estradiol alone produces a partial desensitization of 5-HT1AR signaling and synergizes with SSRIs to result in a complete and more rapid desensitization than with SSRIs alone as measured by a decrease in the oxytocin and adrenocorticotrophic hormone (ACTH) responses to 5-HT1AR stimulation. G protein-coupled estrogen receptor 1 (GPER1) is necessary for estradiol-induced desensitization of 5-HT1AR signaling, although the underlying mechanisms are still unclear. We now find that stimulation of GPER1 with the selective agonist G-1 and nonselective stimulation of estrogen receptors dramatically alter isoform expression of a key component of the 5-HT1AR signaling pathway, RGSz1, a GTPase-activating protein selective for Gαz, the Gα subunit necessary for 5-HT1AR-mediated hormone release. RGSz1 isoforms are differentially glycosylated, SUMOylated, and phosphorylated, and differentially distributed in subcellular organelles. High-molecular-weight RGSz1 is SUMOylated and glycosylated, localized to the detergent-resistant microdomain (DRM) of the cell membrane, and increased by estradiol and G-1 treatment. Because activated Gαz also localizes to the DRM, increased DRM-localized RGSz1 by estradiol and G-1 could reduce Gαz activity, functionally uncoupling 5-HT1AR signaling. Peripheral G-1 treatment produced a partial reduction in oxytocin and ACTH responses to 5-HT1AR stimulation similar to direct injections into the PVN. Together, these results identify GPER1 and RGSz1 as novel targets for the treatment of depression.

The G protein-coupled estrogen receptor (GPER) agonist G-1 expands the regulatory T-cell population under TH17-polarizing conditions

The transcription factor Foxp3 is critical to the suppressive phenotype of CD4+ regulatory T cells. Studies have clearly shown that numerous autoimmune diseases are marked by the presence of activated CD4+ T cells within the setting of chronic inflammation. Therefore, drugs capable of inducing Foxp3 expression in activated CD4+ T cells could be of great therapeutic interest. We have previously shown that the small molecule G-1, an agonist directed against the membrane-bound G protein-coupled estrogen receptor, can induce IL10 expression in naive CD4+ T cells. In addition, we and others have demonstrated that G-1 attenuates disease in an animal model of experimental autoimmune encephalomyelitis. Using ex vivo cultures of purified CD4+ T cells, we show that G-1 can elicit Foxp3 expression under TH17-polarizing conditions, which mimic the in situ inflammatory milieu of several autoimmune diseases. These findings build upon previous results demonstrating the immunosuppressive properties of the novel estrogenic small molecule G-1.

Activation of G protein-coupled estrogen receptor induces endothelium-independent relaxation of coronary artery smooth muscle

Estrogens can either relax or contract arteries via rapid, nongenomic mechanisms involving classic estrogen receptors (ER). In addition to ERα and ERβ, estrogen may also stimulate G protein-coupled estrogen receptor 1 (GPER) in nonvascular tissue; however, a potential role for GPER in coronary arteries is unclear. The purpose of this study was to determine how GPER activity influenced coronary artery reactivity. In vitro isometric force recordings were performed on endothelium-denuded porcine arteries. These studies were augmented by RT-PCR and single-cell patch-clamp experiments. RT-PCR and immunoblot studies confirmed expression of GPER mRNA and protein, respectively, in smooth muscle from either porcine or human coronary arteries. G-1, a selective GPER agonist, produced a concentration-dependent relaxation of endothelium-denuded porcine coronary arteries in vitro. This response was attenuated by G15, a GPER-selective antagonist, or by inhibiting large-conductance calcium-activated potassium (BK(Ca)) channels with iberiotoxin, but not by inhibiting NO signaling. Last, single-channel patch-clamp studies demonstrated that G-1 stimulates BK(Ca) channel activity in intact smooth muscle cells from either porcine or human coronary arteries but had no effect on channels isolated in excised membrane patches. In summary, GPER activation relaxes coronary artery smooth muscle by increasing potassium efflux via BK(Ca) channels and requires an intact cellular signaling mechanism. This novel action of estrogen-like compounds may help clarify some of the controversy surrounding the vascular effects of estrogens.

GPR30 co-localizes with cholinergic neurons in the basal forebrain and enhances potassium-stimulated acetylcholine release in the hippocampus

GPR30 is a novel, membrane-bound, G-protein coupled estrogen receptor (Filardo et al., 2002; Prossnitz et al., 2008). We hypothesize that GPR30 may mediate effects of estradiol (E2) on basal forebrain cholinergic neurons and cognitive performance. Recently we showed that G-1, a selective GPR30 agonist, enhances the rate of acquisition on a delayed matching-to-position (DMP) T-maze task (Hammond et al., 2009). In the present study, we examined the distribution of GPR30 in the rat forebrain, and the effects of G-1 on potassium-stimulated acetylcholine release in the hippocampus. GPR30-like immunoreactivity was detected in many regions of the forebrain including the hippocampus, frontal cortex, medial septum/diagonal band of Broca, nucleus basalis magnocellularis and striatum. GPR30 mRNA also was detected, with higher levels in the hippocampus and cortex than in the septum and striatum. Co-localization studies revealed that the majority (63-99%) of cholinergic neurons in the forebrain expressed GPR30-like immunoreactivity. A far lower percentage (0.4-42%) of GABAergic (parvalbumin-containing) cells also contained GPR30. Sustained administration of either G-1 or E2 (5 μg/day) to ovariectomized rats produced a nearly 3-fold increase in potassium-stimulated acetylcholine release in the hippocampus relative to vehicle-treated controls. These data demonstrate that GPR30 is expressed by cholinergic neurons in the basal forebrain, and suggest that activation of GPR30 enhances cholinergic function in the hippocampus similar to E2. This may account for the effects of G-1 on DMP acquisition previously reported.

Chronic treatment with estrogen receptor agonists restores acquisition of a spatial learning task in young ovariectomized rats

Previous work has shown that continuous estradiol replacement in young ovariectomized rats enhances acquisition of a delayed matching-to-position (DMP) T-maze task over that of ovariectomized controls. The mechanism by which estradiol confers this benefit has not been fully elucidated. This study examined the role of selective estrogen receptor agonists of ERalpha, ERbeta, and GPR30 in the enhancement of spatial learning on a DMP task by comparing continuous estradiol replacement with continuous administration of PPT (an agonist of ERalpha), DPN (an agonist of ERbeta), or G-1 (an agonist of GPR30) relative to gonadally intact and ovariectomized vehicle-treated controls. It was found that ovariectomy impaired acquisition on this task, whereas all ER selective agonists restored the rate of acquisition to that of gonadally intact controls. These data suggest that estradiol can work through any of several estrogen receptors to enhance the rate of acquisition on this task.