Membrane estrogen receptors (mERs) are a group of receptors which bind estrogen. [1] [2] Unlike nuclear estrogen receptors, which mediate their effects via slower genomic mechanisms, mERs are cell surface receptors that rapidly alter cell signaling via modulation of intracellular signaling cascades. [3] [4]
Nuclear estrogen receptors such as ERα and ERβ become mERs through palmitoylation, a post-translational modification that enhances ER association with caveolin-1 to enable trafficking of ERs to the membrane or membrane caveolae. [5] [6] Other putative mERs include GPER (GPR30), GPRC6A, ER-X, ERx and Gq-mER. [3] [7] [8] [9]
In mice and humans, ERβ localization in the plasma membrane occurs after palmitoylation on cysteine 418. [10] Dimerization of mERs appears necessary for their function in rapid cell signaling. [11]
Various electrophysiological studies support E2 signaling via GPCRs. [12] [13] [14] mERs are thought to activate G-protein coupled receptors to regulate L-type Ca2+ channels and activate protein kinase A (PKA), protein kinase C (PKC), and mitogen activated protein kinase (MAPK) signaling cascades. [15] [16]
Gq-coupled mERs (Gq-mERs) activation has been demonstrated to rapidly increase membrane excitability various neuronal cell types by desensitizing GABAB receptor coupling to G protein-coupled inwardly rectifying K+ channels (GIRKs). [17] [18] [19]
Localization of mERs in caveolae allows them to be held in close proximity to specific receptors such as mGluRs. [20] Various studies have demonstrated mER's ability to activate mGluR signaling, even in the absence of glutamate. [21] [22] [23] ER/mGluR signaling is thought to be highly relevant for female motivational behavior. Interestingly, modification of caveolin expression appears to alter the nature of ER-mGluR interactions. [24]
Membrane estrogen receptors have been implicated in reproductive, cardiovascular, neural, and immune function, including cancer, neurodegenerative disease, and cardiovascular disorders. [25] [26]
GPER1 pathways modify local inflammation and strengthen cellular immune responses in breast cancer and melanoma, making it a strong prognostic marker. [27] [28] [29]
mERs have a demonstrated neuroprotective effect against neurodegenerative disorders like Parkinson's disease, which is thought to underlie the lower incidence of the disorder in women compared to men. [30] [31]
mERβ has been demonstrated to mitigate cardiac cell pathology caused by angiotensin II. [10] [32] Activation of mER but not nuclear ER signaling in vascular epithelial cells promotes protection against vascular injury in mice. Striatin, a scaffolding protein that links mERs to membrane caveolae, is necessary for this effect. [33]
Propensity to addiction appears to be mediated by sex hormones such as estrogen. [34] In neural reward circuity, nuclear ERs are not commonly expressed, and mERs have been demonstrated to act on mGluR5 to facilitate psychostimulant-induced behavioral and neurochemical effects. [35] [36]
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