Hippocampal Dendritic Spines Are Segregated Depending on Their Actin Polymerization

Letrozole delays acquisition of water maze task in female BALB/c mice: Possible involvement of anxiety

Letrozole, an aromatase inhibitor preventing estrogen synthesis from testosterone, is used as an adjuvant therapy in estrogen receptor-positive breast cancer patients. However, like other aromatase inhibitors, it induces many side effects, including impaired cognition. Despite its negative effect in humans, results from animal models are inconsistent and suggest that letrozole can either impair or improve cognition. Here, we studied the effects of chronic letrozole treatment on cognitive behavior of adult female BALB/c mice, a relevant animal model for breast cancer studies, to develop an appropriate animal model aimed at testing therapies to mitigate side effects of letrozole. In Morris water maze, letrozole 0.1 mg/kg impaired reference learning and memory. Interestingly, most of the letrozole 0.1 mg/kg-treated mice were able to learn the new platform position in reversal training and performed similar to control mice in a reversal probe test. Results of the reversal test suggest that letrozole did not completely disrupt spatial navigation, but rather delayed acquisition of spatial information. The delay might be related to increased anxiety as suggested by increased thigmotactic behavior during the reference memory training. The learning impairment was water maze-specific since we did not observe impairment in other spatial tasks such as in Y-maze or object location test. In contrast, the dose of 0.3 mg/kg did not have effect on water maze learning and facilitated locomotor habituation and recognition in novel object recognition test. The current study shows that letrozole dose-dependently modulates behavioral response and that its effects are task-dependent.

Hippocampal Aromatase Knockdown Aggravates Ovariectomy-Induced Spatial Memory Impairment, Aβ Accumulation and Neural Plasticity Deficiency in Adult Female Mice

Ovarian estrogens (mainly 17β estradiol, E2) have been involved in the regulation of the structure of hippocampus, the center of spatial memory. In recent years, high levels of aromatase (AROM), the estrogen synthase, has been localized in hippocampus; and this hippocampus-derived E2 seems to be functional in synaptic plasticity and spatial memory as ovarian E2 does. However, the contribution of ovarian E2 and hippocampal E2 to spatial memory and neural plasticity remains unclear. In this study, AROM-specific RNA interference AAVs (shAROM) were constructed and injected into the hippocampus of control or ovariectomized (OVX) mice. Four weeks later the spatial learning and memory behavior was examined with Morris water maze, the expression of hippocampal Aβ related proteins, selected synaptic proteins and CA1 synapse density, actin polymerization related proteins and CA1 spine density were also examined. The results showed that while OVX and hippocampal shAROM contributed similarly to most of the parameters examined, shAROM induced more increase in BACE1 (amyloidogenic β-secretase), more decrease in neprilysin (Aβ remover) and Profilin-1 (actin polymerization inducer). More importantly, combined OVX and shAROM treatment displayed most significant impairment of spatial learning and memory as well as decrease in synaptic plasticity compared to OVX or shAROM alone. In conclusion, the above results clearly demonstrated the crucial role of hippocampal E2 in the regulation of the structure and function of hippocampus besides ovarian E2, indicating that hippocampal E2 content should also be taken into consideration during estrogenic replacement.

Letrozole induces worse hippocampal synaptic and dendritic changes and spatial memory impairment than ovariectomy in adult female mice

Estrogens (E2) derived from ovaries and/or local de novo synthesis in the hippocampus profoundly regulate hippocampal structure and function, but the importance of local E2 versus ovarian E2 on hippocampal synaptic plasticity and spatial memory has not been well elucidated. The present study used ovariectomy (OVX) and intraperitoneal injection of an E2 synthase inhibitor, letrozole (LET), in adult female mice to investigate changes in hippocampal steroid receptor coactivator-1 (SRC-1), postsynaptic proteins, and actin polymerization dynamics with these treatments. Changes in the CA1 spine density, synapse density and spatial learning and memory after OVX and LET were also investigated. As a result, OVX and LET showed similar regulation of the expression of GluR1, spinophilin and p-Cofilin, but LET tended to induce more significant changes in SRC-1, PSD95, Rictor, Cofilin and actin depolymerization. More significant decreases in F-actin/G-actin, CA1 spine density and synapse density were also observed after LET than after OVX. Notably, LET-treated mice showed worse learning and memory impairment than OVX mice. Taken together, these results demonstrated that circulating E2 played a limited role and that hippocampus-derived E2 played a more important role in the regulation of hippocampal synaptic plasticity and hippocampus-based spatial learning and memory.

GPR30-mediated estrogenic regulation of actin polymerization and spatial memory involves SRC-1 and PI3K-mTORC2 in the hippocampus of female mice

AIMS

The G-protein-coupled estrogen receptor GPR30 (also referred to as GPER) has been implicated in the estrogenic regulation of hippocampal plasticity and spatial memory; however, the molecular mechanisms are largely unclear.

METHODS

In this study, we initially examined the levels of GPR30 in the hippocampus of postnatal, ovariectomy (OVX)- and letrozole (LET)-treated female mice. Under G1, G15, and/or OVX treatment, the spatial memory, spine density, levels of ERα, ERβ, and SRC-1, selected synaptic proteins, mTORC2 signals (Rictor and p-AKT Ser473), and actin polymerization dynamics were subsequently evaluated. Furthermore, G1, G15, and/or E2 combined with SRC-1 and/or PI3K inhibitors, actin cytoskeleton polymerization modulator JPK, and CytoD treatments were used to address the mechanisms that underlie GPR30 regulation in vitro. Finally, mTORC2 activator A-443654 (A4) was used to explore the role of mTORC2 in GPR30 regulation of spatial memory.

RESULTS

The results showed that high levels of GPR30 were detected in the adult hippocampus and the levels were downregulated by OVX and LET. OVX induced an impairment of spatial memory, and changes in other parameters previously described were reversed by G1 and mimicked by G15. Furthermore, the E2 effects on SRC-1 and mTORC2 signals, synaptic proteins, and actin polymerization were inhibited by G15, whereas G1 effects on these parameters were inhibited by the blockade of SRC-1 or PI3K; the levels of synaptic proteins were regulated by JPK and CytoD. Importantly, G15-induced actin depolymerization and spatial memory impairment were rescued by mTORC2 activation with A4.

CONCLUSIONS

Taking together, these results demonstrated that decreased GPR30 induces actin depolymerization through SRC-1 and PI3K/mTORC2 pathways and ultimately impairs learning and memory, indicating its potential role as a therapeutic target against hippocampus-based, E2-related memory impairments.

Nuclear and membrane estrogen receptor antagonists induce similar mTORC2 activation-reversible changes in synaptic protein expression and actin polymerization in the mouse hippocampus

AIMS

Estrogens play pivotal roles in hippocampal synaptic plasticity through nuclear receptors (nERs; including ERα and ERβ) and the membrane receptor (mER; also called GPR30), but the underlying mechanism and the contributions of nERs and mER remain unclear. Mammalian target of rapamycin complex 2 (mTORC2) is involved in actin cytoskeleton polymerization and long-term memory, but whether mTORC2 is involved in the regulation of hippocampal synaptic plasticity by ERs is unclear.

METHODS

We treated animals with nER antagonists (MPP/PHTPP) or the mER antagonist (G15) alone or in combination with A-443654, an activator of mTORC2. Then, we examined the changes in hippocampal SRC-1 expression, mTORC2 signaling (rictor and phospho-AKTSer473), actin polymerization (phospho-cofilin and profilin-1), synaptic protein expression (GluR1, PSD95, spinophilin, and synaptophysin), CA1 spine density, and synapse density.

RESULTS

All of the examined parameters except synaptophysin expression were significantly decreased by MPP/PHTPP and G15 treatment. MPP/PHTPP and G15 induced a similar decrease in most parameters except p-cofilin, GluR1, and spinophilin expression. The ER antagonist-induced decreases in these parameters were significantly reversed by mTORC2 activation, except for the change in SRC-1, rictor, and synaptophysin expression.

CONCLUSIONS

nERs and mER contribute similarly to the changes in proteins and structures associated with synaptic plasticity, and mTORC2 may be a novel target of hippocampal-dependent dementia such as Alzheimer's disease as proposed by previous studies.

Estrogen receptor alpha and beta regulate actin polymerization and spatial memory through an SRC-1/mTORC2-dependent pathway in the hippocampus of female mice

Aging-related decline of estrogens, especially 17β-estradiol (E2), has been shown to play an important role in the impairment of learning and memory in dementias, such as Alzheimer's disease (AD), but the underlying molecular mechanisms are poorly understood. In this study, we first demonstrated decreases in E2 signaling (aromatase, classic estrogen receptor ERα and ERβ and their coactivator SRC-1), mTORC2 signaling (Rictor and phospho-AKTser473) and actin polymerization (phospho-Cofilin, Profilin-1 and the F-actin/G-actin ratio) in the hippocampus of old female mice compared with those levels detected in the adult hippocampus. We then showed that ERα and ERβ antagonists induced a significant decrease in SRC-1, mTORC2 signaling, actin polymerization, and CA1 spine density, as well as impairments of learning and memory; however, ovariectomy-induced changes of these parameters could be significantly reversed by treatment with ER agonists. We further showed that expression of SRC-1, mTORC2 signaling and actin polymerization could be upregulated by E2 treatment, and the effects of E2 were blocked by the ER antagonists but mimicked by the agonists. We also showed that the lentivirus-mediated SRC-1 knockdown significantly inhibited the agonist-activated mTORC2 signaling and actin polymerization, and the lentivirus-mediated Rictor knockdown also significantly inhibited the agonist-activated actin polymerization. Finally, we demonstrated that the ERα and ERβ antagonists induced a disruption in actin polymerization and an impairment of spatial memory, which were rescued by activation of mTORC2. Taken together, the above results clearly demonstrated an mTORC2-dependent regulation of actin polymerization that contributed to the effects of ERα and ERβ on spatial learning, which may provide a novel target for the prevention and treatment of E2-related dementia in the aged population.

Letrozole regulates actin cytoskeleton polymerization dynamics in a SRC-1 dependent manner in the hippocampus of mice

In the hippocampus, local estrogens (E₂) derived from testosterone that is catalyzed by aromatase play important roles in the regulation of hippocampal neural plasticity, but the underlying mechanisms remain unclear. The actin cytoskeleton contributes greatly to hippocampal synaptic plasticity; however, whether it is regulated by local E₂ and the related mechanisms remain to be elucidated. In this study, we first examined the postnatal developmental profiles of hippocampal aromatase and specific proteins responsible for actin cytoskeleton dynamics. Then we used aromatase inhibitor letrozole (LET) to block local E₂ synthesis and examined the changes of these proteins and steroid receptor coactivator-1 (SRC-1), the predominant coactivator for steroid nuclear receptors. Finally, SRC-1 specific RNA interference was used to examine the effects of SRC-1 on the expression of these actin remodeling proteins. The results showed a V-type profile for aromatase and increased profiles for actin cytoskeleton proteins in both male and female hippocampus without obvious sex differences. LET treatment dramatically decreased the F-actin/G-actin ratio, the expression of Rictor, phospho-AKT (ser473), Profilin-1, phospho-Cofilin (Ser3), and SRC-1 in a dose-dependent manner. In vitro studies demonstrated that LET induced downregulation of these proteins could be reversed by E₂, and E₂ induced increase of these proteins were significantly suppressed by SRC-1 shRNA interference. These results for the first time clearly demonstrated that local E₂ inhibition could induce aberrant actin polymerization; they also showed an important role of SRC-1 in the mediation of local E₂ action on hippocampal synaptic plasticity by regulation of actin cytoskeleton dynamics.