Benzothiophene Selective Estrogen Receptor Modulators Provide Neuroprotection by a novel GPR30-dependent Mechanism

Neuroprotective effect of estrogen: role of nonsynaptic NR2B-containing NMDA receptors

Excessive activation of N-methyl-D-aspartate receptors (NMDARs) has been implicated in the pathophysiology of chronic neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, and Huntington's disease. Some studies reported that NR2A and NR2B play different roles in the central nervous system (CNS). The NR2A subunit is primarily found in the synapses and is required for glutamate-mediated neuronal survival. On the other hand, the NR2B subunit is primarily found in the extrasynaptic sites and is required for glutamate-mediated neuronal death in both in vitro and in vivo experiments. Estrogen is a steroid hormone well known for its widespread effects such as neuroprotection in the brain. Classically, estrogen can bind to two kinds of nuclear receptors, namely, estrogen receptor α (ERα) and estrogen receptor β (ERβ), and produce physiological and neuroprotective effects. Aside from nuclear receptors, estrogen has one membrane receptor, which can either be G-protein-coupled receptor 30 (GPR30), Gq-mER, or ER-X. NMDA exposure clearly promotes NR2B subunit phosphorylation at Ser-1303 and causes neuronal cell death. GPR30 mediates rapid non-genomic effects to protect neurons against injury by inhibiting p-DAPK1 dephosphorylation, which inhibits NR2B subunit phosphorylation at Ser-1303. In addition, NMDA exposure and global ischemia activate the autophagy pathway and induce cell death, which are markedly blocked by the NR2B antagonist Ro 25-6981. Thus, NR2B signaling, autophagy induction and cell death may be closely related. Ro 25-6981 inhibits the dissociation of the NR2B-Beclin-1 signaling complex and delays autophagy in vivo, thus confirming the link between NR2B signaling and autophagy. In short, ERα, ERβ, and GPR30 are involved in the neuroprotection of estrogen in the CNS. Additional research must be conducted to reveal the mechanism of estrogen action fully and to identify better targets for the development of more effective drugs. This article is part of a Special Issue entitled 'Extrasynaptic ionotropic receptors'.

Neuroprotection and estrogen receptors

This review is intended to assess the state of current knowledge on the role of estrogen receptors (ERs) in the neuroprotective effects of estrogens in models for acute neuronal injury and death. We evaluate the overall evidence that estrogens are neuroprotective in acute injury and critically assess the role of ERα, ERβ, GPR 30, and nonreceptor-mediated mechanisms in these robust neuroprotective effects of this ovarian steroid hormone. We conclude that all three receptors, as well as nonreceptor-mediated mechanisms can be involved in neuroprotection, depending on the model used, the level of estrogen administrated, and the mode of administration of the steroid. Also, the signaling pathways used by both ER-dependent and ER-independent mechanisms to exert neuroprotection are considered. Finally, further studies that are needed to parse out the relative contribution of receptor versus nonreceptor-mediated signaling are discussed.

GPER1/GPR30 activation improves neuronal survival following global cerebral ischemia induced by cardiac arrest in mice

Female sex steroids, particularly estrogens, contribute to the sexually dimorphic response observed in cerebral ischemic outcome, with females being relatively protected compared to males. Using a mouse model of cardiac arrest and cardiopulmonary resuscitation (CA/CPR), we previously demonstrated that estrogen neuroprotection is mediated in part by the estrogen receptor β, with no involvement of estrogen receptor α. In this study we examined the neuroprotective effect of the novel estrogen receptor, G-protein coupled estrogen receptor 1 (GPER1/GPR30). Male mice administered the GPR30 agonist G1 exhibited significantly reduced neuronal injury in the hippocampal CA1 region and striatum. The magnitude of neuroprotection observed in G1 treated mice was indistinguishable from estrogen treated mice, implicating GPR30 in estrogen neuroprotection. Real-time quantitative RT-PCR indicates that G1 treatment increases expression of the neuroprotective ion channel, small conductance calcium-activated potassium channel 2. We conclude that GPR30 agonists show promise in reducing brain injury following global cerebral ischemia.

Position paper: The membrane estrogen receptor GPER--Clues and questions

Rapid signaling of estrogen involves membrane estrogen receptors (ERs), including membrane subpopulations of ERα and ERβ. In the mid-1990s, several laboratories independently reported the cloning of an orphan G protein-coupled receptor from vascular and cancer cells that was named GPR30. Research published between 2000 and 2005 provided evidence that GPR30 binds and signals via estrogen indicating that this intracellular receptor is involved in rapid, non-genomic estrogen signaling. The receptor has since been designated as the G protein-coupled estrogen receptor (GPER) by the International Union of Pharmacology. The availability of genetic tools such as different lines of GPER knock-out mice, as well as GPER-selective agonists and antagonists has advanced our understanding, but also added some confusion about the new function of this receptor. GPER not only binds estrogens but also other substances, including SERMs, SERDs, and environmental ER activators (endocrine disruptors; xenoestrogens) and also interacts with other proteins. This article represents a summary of a lecture given at the 7(th) International Meeting on Rapid Responses to Steroid Hormones in September 2011 in Axos, Crete, and reviews the current knowledge and questions about GPER-dependent signaling and function. Controversies that have complicated our understanding of GPER, including interactions with human ERα-36 and aldosterone as a potential ligand, will also be discussed.

Design and synthesis of neuroprotective methylthiazoles and modification as NO-chimeras for neurodegenerative therapy

Learning and memory deficits in Alzheimer's disease (AD) result from synaptic failure and neuronal loss, the latter caused in part by excitotoxicity and oxidative stress. A therapeutic approach is described that uses NO-chimeras directed at restoration of both synaptic function and neuroprotection. 4-Methylthiazole (MZ) derivatives were synthesized, based upon a lead neuroprotective pharmacophore acting in part by GABA(A) receptor potentiation. MZ derivatives were assayed for protection of primary neurons against oxygen-glucose deprivation and excitotoxicity. Selected neuroprotective derivatives were incorporated into NO-chimera prodrugs, coined nomethiazoles. To provide proof of concept for the nomethiazole drug class, selected examples were assayed for restoration of synaptic function in hippocampal slices from AD-transgenic mice, reversal of cognitive deficits, and brain bioavailability of the prodrug and its neuroprotective MZ metabolite. Taken together, the assay data suggest that these chimeric nomethiazoles may be of use in treatment of multiple components of neurodegenerative disorders, such as AD.

GPR30 regulates glutamate transporter GLT-1 expression in rat primary astrocytes

The G protein-coupled estrogen receptor GPR30 contributes to the neuroprotective effects of 17β-estradiol (E2); however, the mechanisms associated with this protection have yet to be elucidated. Given that E2 increases astrocytic expression of glutamate transporter-1 (GLT-1), which would prevent excitotoxic-induced neuronal death, we proposed that GPR30 mediates E2 action on GLT-1 expression. To investigate this hypothesis, we examined the effects of G1, a selective agonist of GPR30, and GPR30 siRNA on astrocytic GLT-1 expression, as well as glutamate uptake in rat primary astrocytes, and explored potential signaling pathways linking GPR30 to GLT-1. G1 increased GLT-1 protein and mRNA levels, subject to regulation by both MAPK and PI3K signaling. Inhibition of TGF-α receptor suppressed the G1-induced increase in GLT-1 expression. Silencing GPR30 reduced the expression of both GLT-1 and TGF-α and abrogated the G1-induced increase in GLT-1 expression. Moreover, the G1-induced increase in GLT-1 protein expression was abolished by a protein kinase A inhibitor and an NF-κB inhibitor. G1 also enhanced cAMP response element-binding protein (CREB), as well as both NF-κB p50 and NF-κB p65 binding to the GLT-1 promoter. Finally, to model dysfunction of glutamate transporters, manganese was used, and G1 was found to attenuate manganese-induced impairment in GLT-1 protein expression and glutamate uptake. Taken together, the present data demonstrate that activation of GPR30 increases GLT-1 expression via multiple pathways, suggesting that GPR30 is worthwhile as a potential target to be explored for developing therapeutics of excitotoxic neuronal injury.

Signaling pathways mediating the neuroprotective effects of sex steroids and SERMs in Parkinson's disease

Studies with the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) animal model of Parkinson's disease have shown the ability of 17β-estradiol to protect the nigrostriatal dopaminergic system. This paper reviews the signaling pathways mediating the neuroprotective effect of 17β-estradiol against MPTP-induced toxicity. The mechanisms of 17β-estradiol action implicate activation of signaling pathways such as the phosphatidylinositol-3 kinase/Akt and the mitogen-activated protein kinase pathways. 17β-estradiol signaling is complex and integrates multiple interactions with signaling molecules that act to potentiate a protective effect. 17β-estradiol signaling is mediated via estrogen receptors, including GPER1, but others receptors, such as the IGF-1 receptor, are implicated in the neuroprotective effect. Glial and neuronal crosstalk is a critical factor in the maintenance of dopamine neuronal survival and in the neuroprotective action of 17β-estradiol. Compounds that stimulate GPER1 such as selective estrogen receptor modulators and phytoestrogens show neuroprotective activity and are alternatives to 17β-estradiol.

GPER/GPR30 and Regulation of Vascular Tone and Blood Pressure

Natural estrogens such as 17β-estradiol are endogenous vasodilators and have been implicated in the gender differences of hypertension. These hormones activate estrogen receptors ERα and ERβ, which mediate part of estrogen-dependent vasodilation. In addition, a novel G protein-coupled estrogen-binding receptor termed GPER/GPR30 has been identified that is expressed in the cardiovascular system. Using knock-out animals or drugs selectively targeting GPER/GPR30, a significant role for this receptor as a mediator of acute estrogen-dependent vasodilation involving nitric oxide (NO) and blood pressure-lowering activity has been demonstrated. The accumulating evidence that GPER/GPR30 is responsible for control of vascular tone indicates that this receptor may represent a novel drug target for pharmacologic treatment of hypertension in postmenopausal women and possibly also men.

The G protein-coupled estrogen receptor GPER/GPR30 as a regulator of cardiovascular function

Endogenous estrogens are important regulators of cardiovascular homeostasis in premenopausal women and delay the development of hypertension and coronary artery disease. These hormones act via three different estrogen receptors affecting both gene transcription and rapid signaling pathways in a complex interplay. In addition to the classical estrogen receptors ERα and ERβ, which are known mediators of estrogen-dependent vascular effects, a G protein-coupled estrogen receptor termed GPER that is expressed in the cardiovascular system has recently been identified. Endogenous human 17β-estradiol, selective estrogen receptor modulators (SERMs) including tamoxifen and raloxifene, and selective estrogen receptor downregulators (SERDs) such as ICI 182,780 are all agonists of GPER, which has been implicated in the regulation of vasomotor tone and protection from myocardial ischemia/reperfusion injury. As a result, understanding the individual role of ERα, ERβ, and GPER in cardiovascular function has become increasingly complex. With accumulating evidence that GPER is responsible for a variety of beneficial cardiovascular effects of estrogens, this receptor may represent a novel target to develop effective strategies for the treatment of cardiovascular diseases by tissue-specific, selective activation of estrogen-dependent molecular pathways devoid of side effects seen with conventional hormone therapy.