Cross-talk between membrane-initiated and nuclear-initiated oestrogen signalling in the hypothalamus

Abstract

It is increasingly evident that 17beta-oestradiol (E(2)), via a distinct membrane oestrogen receptor (Gq-mER), can rapidly activate kinase pathways to have multiple downstream actions in central nervous system (CNS) neurones. We have found that E(2) can rapidly reduce the potency of the GABA(B) receptor agonist baclofen and mu-opioid receptor agonist DAMGO to activate G-protein-coupled, inwardly rectifying K(+) (GIRK) channels in hypothalamic neurones, thereby increasing the excitability (firing activity) of pro-opiomelanocortin (POMC) and dopamine neurones. These effects are mimicked by the membrane impermeant E(2)-BSA and a new ligand (STX) that is selective for the Gq-mER that does not bind to ERalpha or ERbeta. Both E(2) and STX are fully efficacious in attenuating the GABA(B) response in ERalpha, ERbeta and GPR 30 knockout mice in an ICI 182 780 reversible manner. These findings are further proof that E(2) signals through a unique plasma membrane ER. We have characterised the coupling of this Gq-mER to a Gq-mediated activation of phospholipase C leading to the up-regulation of protein kinase Cdelta and protein kinase A activity in these neurones, which ultimately alters gene transcription. Finally, as proof of principle, we have found that STX, similar to E(2), reduces food intake and body weight gain in ovariectomised females. STX, presumably via the Gq-mER, also regulates gene expression of a number of relevant targets including cation channels and signalling molecules that are critical for regulating (as a prime example) POMC neuronal excitability. Therefore, E(2) can activate multiple receptor-mediated pathways to modulate excitability and gene transcription in CNS neurones that are critical for controlling homeostasis and motivated behaviors.

Figure 1. Activation of an oestrogen-responsive G-protein-coupled receptor (GPCR) and the associated signalling pathways in arcuate neurons

(1) The putative Gq-coupled membrane oestrogen receptor (mER) is a G-protein coupled receptor linked to Gα_q/11 protein. Oestrogen binding activates the Gα_q/11 (2) which in turn activates (+) phospholipase C (PLC) and initiates the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP₂). PLC hydrolyzes PIP₂ into diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP₃). (3) DAG activates protein kinase Cδ (PKCδ) which activates adenylate cyclase VII (AC VII). (4) AC VII increases cAMP production subsequently stimulating protein kinase A (PKA), which through phosphorylation uncouples the inhibitory GABA_B and μ-opioid receptors from activation of G-protein coupled inwardly rectifying K⁺ channels (GIRK) channels. (5) Activation of PKA will also phosphorylate cAMP-response element binding protein (pCREB) and control gene expression through the cAMP response element (CRE). (6) IP₃ produced from the hydrolysis of PIP₂ activates Ca²⁺ release from the endoplasmic reticulum that can activate calcium-dependent signalling. (7) Oestradiol will also bind to nuclear receptors and activate oestrogen response element (ERE)-dependent transcription. E₂, 17β-oestradiol.