The Effects of G-Protein-Coupled Estrogen Receptor (GPER) on Cell Signaling, Dendritic Spines, and Memory Consolidation in the Female Mouse Hippocampus

Abstract

One of the most seminal findings in the literature on hormones and cognition is that the potent estrogen 17β-estradiol (E2) significantly increases the density of dendritic spines on pyramidal neurons in the CA1 region of the dorsal hippocampus (DH). However, the extent to which this E2-induced increase in hippocampal spinogenesis is necessary for memory formation remains unclear. The memory-enhancing effects of E2 in the DH can be mediated by intracellular estrogen receptors (ERs) or by the membrane-bound ER called G-protein coupled estrogen receptor (GPER). We previously reported that infusion of a GPER agonist, G-1, into the DH of ovariectomized female mice mimicked the beneficial effects of E2 on object recognition and spatial memory consolidation in a manner that depended on phosphorylation of the signaling kinase c-Jun N-terminal kinase (JNK). However, the role of CA1 dendritic spines in mediating GPER-induced memory consolidation, as well as the signaling mechanisms that might mediate effects of GPER activation on dendritic spine density, remain unclear. Thus, the present study examined in ovariectomized mice the effects of DH-infused G-1 on dendritic spine density and determined whether such effects are necessary for G-1-induced memory consolidation. We first examined whether object training itself might induce increased CA1 dendritic spine density, and showed that spine density is increased by object training. Next, we found that G-1 significantly increased the density of dendritic spines on apical dendrites of CA1 pyramidal neurons in the DH. We next examined cellular mechanisms regulating G-1 induced spinogenesis by measuring effects of DH G-1 infusion of the phosphorylation of the protein cofilin, which actively regulates actin reorganization. We found that G-1 significantly increased cofilin phosphorylation in the DH, suggesting that activation of GPER may increase dendritic spine morphogenesis through actin polymerization. As with memory consolidation in our previous study, we also found that the effects of G-1 on apical CA1 spine density and cofilin phosphorylation were dependent on JNK phosphorylation in the DH. To verify the importance of actin polymerization in GPER-mediated dendritic spine morphogenesis and hippocampal memory enhancement, we applied an actin polymerization inhibitor, latrunculin A, which prevents actin polymerization and promotes filament disassembly. DH infusion of latrunculin A prevented G-1 from inducing apical CA1 spinogenesis and enhancing both object recognition and spatial memory consolidation. Collectively, these data demonstrate that GPER-mediated hippocampal memory consolidation and spine density changes are dependent on modulating actin dynamics via JNK-Cofilin signaling, supporting a critical role of actin polymerization in the GPER-induced regulation of hippocampal function in female mice.