Estrogen receptor α and β differentially regulate intracellular Ca²⁺ dynamics leading to ERK phosphorylation and estrogen neuroprotection in hippocampal neurons

Our previous analyses indicated that both estrogen receptor (ER) subtypes, ERα and ERβ, contribute to estrogen neuroprotection [Zhao, L., Wu, T.-W., Brinton, R.D., 2004. Estrogen receptor subtypes alpha and beta contribute to neuroprotection and increased Bcl-2 expression in primary hippocampal neurons. Brain Res. 1010, 22-34]. In the present study, we sought to determine the underlying mechanisms by which ERα and ERβ promote neuronal function, with a focus on neuroprotection, and whether these mechanisms are consistent with a classical nuclear or membrane ER-mediated response. Results of these analyses demonstrated that both the ERα-selective agonist, PPT (100 pM), and the ERβ-selective agonist, DPN (100 pM), were effective in dynamically but differentially regulating intracellular calcium (Ca²⁺) signaling in hippocampal neurons. Consistent with the direct measurement of neuroprotective outcomes [Zhao, L., Wu, T.-W., Brinton, R.D., 2004. Estrogen receptor subtypes alpha and beta contribute to neuroprotection and increased Bcl-2 expression in primary hippocampal neurons. Brain Res. 1010, 22-34], PPT and DPN exerted comparable efficacy in attenuating excitotoxic glutamate (200 μM)-induced intracellular Ca²⁺ rise. In contrast, DPN was more efficacious than PPT in potentiating a physiological concentration of glutamate (25 μM)-induced intracellular Ca²⁺ rise in these neurons. Further analyses revealed that both PPT and DPN increased ERK phosphorylation, however, the temporal profile and magnitude of response were unique to each molecule. The presence of the L-type Ca²⁺ channel inhibitor, nifedipine (10 μM), partially inhibited 17β-estradiol- and PPT-induced increase in phosphorylated ERK expression, whereas it induced a complete inhibition of DPN-induced increase in ERK phosphorylation. Additional neuroprotective experiments demonstrated that the MAPK inhibitor, PD 98059 (5 μM), partially blocked 17β-estradiol-induced promotion of neuronal survival against excitotoxic glutamate (200 μM)-induced neurotoxicity, whereas it completely blocked both PPT- and DPN-induced neuroprotection. The presence of the nuclear ER antagonist, ICI 182,780 (1 μM), not only failed to block all 3 molecule-induced neuroprotection, but coadministration of ICI 182,780 and each single molecule exerted a comparable or even greater neuroprotection. Taken together, as an expansion of our previous analyses, these data indicate that both ERα and ERβ contribute to neuronal mechanisms leading to estrogen promotion of neuronal function but with unique signaling profiles. Activation of ERβ and induction of intracellular Ca²⁺ influx via the L-type channels appears to be more closely associated with estrogen promotion of memory mechanisms. However, ERα and ERβ play an equivalently important role in mediating estrogen neuroprotection, and, which is dependent upon the activation of the MAPK signaling. Further, the present analyses suggest that separate from a classical nuclear ER-mediated response, estrogen promotes neuronal survival likely through a non-nuclear cytoplasm or membrane-associated ER-mediated rapid signaling cascade.