Estrogen protects against beta-amyloid-induced neurotoxicity in rat hippocampal neurons by activation of Akt

Steroidogenic enzymes in the hippocampus: Transcriptional regulation aspects

Neurosteroids are steroids synthesized de novo from cholesterol in brain regions, and regulate processes associated with the development and functioning of the nervous system. Enzymes and proteins involved in the synthesis of these steroids have been detected in several brain regions, including hippocampus, hypothalamus, and cerebral cortex. Hippocampus has long been associated with learning and memory functions, while the loss of its functionality has been linked to neurodegenerative pathologies. In this sense, neurosteroids are critical for the maintenance of hippocampal functions and neuroprotective effects. Moreover, several factors have been shown to deregulate expression of steroidogenic enzymes in the rodent brain, including aging, enrichment experiences, diet habits, drug/alcohol consumption, hormone fluctuations, neurodegenerative processes and other diseases. These transcriptional deregulations are mediated mainly by transcription factors and epigenetic mechanisms. An epigenetic modification of chromatin involves changes in bases and associated proteins in the absence of changes in the DNA sequence. One of the most well-studied mechanisms related to gene silencing is DNA methylation, which involves a reversible addition of methyl groups in a cytosine base. Importantly, these epigenetic marks could be maintained over time and could be transmitted transgenerationally. The aim of this chapter is to present the most relevant steroidogenic enzymes described in rodent hippocampus; to discuss about their transcriptional regulation under different conditions; to show the main gene control regions and to propose DNA methylation as an epigenetic mechanism through which the expression of these enzymes could be controlled.

The Molecular Basis of Depression: Implications of Sex-Related Differences in Epigenetic Regulation

Major depressive disorder (MDD) is a leading cause of disability worldwide. Although the etiology and pathophysiology of MDD remain poorly understood, aberrant neuroplasticity mediated by the epigenetic dysregulation of gene expression within the brain, which may occur due to genetic and environmental factors, may increase the risk of this disorder. Evidence has also been reported for sex-related differences in the pathophysiology of MDD, with female patients showing a greater severity of symptoms, higher degree of functional impairment, and more atypical depressive symptoms. Males and females also differ in their responsiveness to antidepressants. These clinical findings suggest that sex-dependent molecular and neural mechanisms may underlie the development of depression and the actions of antidepressant medications. This review discusses recent advances regarding the role of epigenetics in stress and depression. The first section presents a brief introduction of the basic mechanisms of epigenetic regulation, including histone modifications, DNA methylation, and non-coding RNAs. The second section reviews their contributions to neural plasticity, the risk of depression, and resilience against depression, with a particular focus on epigenetic modulators that have causal relationships with stress and depression in both clinical and animal studies. The third section highlights studies exploring sex-dependent epigenetic alterations associated with susceptibility to stress and depression. Finally, we discuss future directions to understand the etiology and pathophysiology of MDD, which would contribute to optimized and personalized therapy.

Estrogen Signaling in Alzheimer's Disease: Molecular Insights and Therapeutic Targets for Alzheimer's Dementia

Estrogens play a crucial physiological function in the brain; however, debates exist concerning the role of estrogens in Alzheimer's disease (AD). Women during pre-, peri-, or menopause periods are more susceptible for developing AD, suggesting the connection of sex factors and a decreased estrogen signaling in AD pathogenesis. Yet, the underlying mechanism of estrogen-mediated neuroprotection is unclarified and is complicated by the existence of estrogen-related factors. Consequently, a deeper analysis of estrogen receptor (ER) expression and estrogen-metabolizing enzymes could interpret the importance of estrogen in age-linked cognitive alterations. Previous studies propose that hormone replacement therapy may attenuate AD onset in postmenopausal women, demonstrating that estrogen signaling is important for the development and progression of AD. For example, ERα exerts neuroprotection against AD by maintaining intracellular signaling cascades and study reported reduced expression of ERα in hippocampal neurons of AD patients. Similarly, reduced expression of ERβ in female AD patients has been associated with abnormal function in mitochondria and improved markers of oxidative stress. In this review, we discuss the critical interaction between estrogen signaling and AD. Moreover, we highlight the potential of targeting estrogen-related signaling for therapeutic intervention in AD.

Sex differences and the neurobiology of affective disorders

Observations of the disproportionate incidence of depression in women compared with men have long preceded the recent explosion of interest in sex differences. Nonetheless, the source and implications of this epidemiologic sex difference remain unclear, as does the practical significance of the multitude of sex differences that have been reported in brain structure and function. In this article, we attempt to provide a framework for thinking about how sex and reproductive hormones (particularly estradiol as an example) might contribute to affective illness. After briefly reviewing some observed sex differences in depression, we discuss how sex might alter brain function through hormonal effects (both organizational (programmed) and activational (acute)), sex chromosome effects, and the interaction of sex with the environment. We next review sex differences in the brain at the structural, cellular, and network levels. We then focus on how sex and reproductive hormones regulate systems implicated in the pathophysiology of depression, including neuroplasticity, genetic and neural networks, the stress axis, and immune function. Finally, we suggest several models that might explain a sex-dependent differential regulation of affect and susceptibility to affective illness. As a disclaimer, the studies cited in this review are not intended to be comprehensive but rather serve as examples of the multitude of levels at which sex and reproductive hormones regulate brain structure and function. As such and despite our current ignorance regarding both the ontogeny of affective illness and the impact of sex on that ontogeny, sex differences may provide a lens through which we may better view the mechanisms underlying affective regulation and dysfunction.

Estradiol Activates PI3K/Akt/GSK3 Pathway Under Chronic Neurodegenerative Conditions Triggered by Perinatal Asphyxia

Perinatal asphyxia (PA) remains as one of the most important causes of short-term mortality, psychiatric and neurological disorders in children, without an effective treatment. In previous studies we have observed that the expression of different neurodegenerative markers increases in CA1 hippocampal area of 4-months-old male rats born by cesarean section and exposed for 19 min to PA. We have also shown that a late treatment with 17β estradiol (daily dose of 250 μg/kg for 3 days) was able to revert the brain alterations observed in those animals. Based on these previous results, the main aim of the present study was to explore the mechanism by which the estrogenic treatment is involved in the reversion of the chronic neurodegenerative conditions induced by PA. We demonstrated that estradiol treatment of adult PA exposed animals induced an increase in estrogen receptor (ER) α and insulin-like growth factor receptor (IGF-1R) protein levels, an activation of the phosphatidylinositol 3-kinase/Akt/glycogen synthase kinase 3 beta/β-catenin signaling pathway and an increase in Bcl-2/Bax ratio in the hippocampus in comparison to PA exposed animals treated with vehicle. Taking together, our data suggest that the interaction between ERα and IGF-IR, with the subsequent downstream activation, underlies the beneficial effects of estradiol observed in late treatment of PA.

Role of glycogen synthase kinase 3 in ischemia-induced blood-brain barrier disruption in aged female rats

Estrogen receptors have protective effects against ischemic brain injury. However, the molecular mechanisms underlying this phenomenon have yet to be well studied. Given that inhibition of glycogen synthase kinase (GSK3) can reduce cerebral ischemia/reperfusion injury, we hypothesized that estrogen receptors-mediated protective effects against ischemia-induced blood-brain barrier (BBB) disruption involve inhibition of GSK3. Thus, we evaluated GSK3 expression in the brain of ovariectomized female rats, and examined the effects of intracerebroventricular pre-treatments of SB216763, GSK3 inhibitor, on BBB permeability following middle cerebral artery occlusion (MCAO). We also examined the role of specific estrogen receptor subtype in regulation of GSK3 expression and BBB permeability after MCAO. We found that ovariectomized female rats exhibited increased mRNA levels of estrogen receptor α (ERα) and estrogen receptor β (ERβ), and increased protein levels of GSK3β but not GSK3α in brain cortical areas. Furthermore, intracerebroventricular pre-treatments of SB216763 dose-dependently attenuated brain infarction volume, brain water contents, neurological deficits, and BBB disruption, and increased tight junction protein ZO-1 and occludin expression at 24 h following MCAO. Finally, activation of ERβ but not ERα dose-dependently decreased GSK3β expression at 24 h following MCAO. This was associated with increased tight junction protein expression and improved neurological scores. Thus, our study suggested that activation of ERβ may protect against brain ischemia-induced BBB disruption by inhibiting GSK3β-mediated signaling.

Effects of estrogen receptor modulators on cytoskeletal proteins in the central nervous system

Estrogen receptor modulators are compounds of interest because of their estrogenic agonistic/antagonistic effects and tissue specificity. These compounds have many clinical applications, particularly for breast cancer treatment and osteoporosis in postmenopausal women, as well as for the treatment of climacteric symptoms. Similar to estrogens, neuroprotective effects of estrogen receptor modulators have been described in different models. However, the mechanisms of action of these compounds in the central nervous system have not been fully described. We conducted a systematic search to investigate the effects of estrogen receptor modulators in the central nervous system, focusing on the modulation of cytoskeletal proteins. We found that raloxifene, tamoxifen, and tibolone modulate some cytoskeletal proteins such as tau, microtuble-associated protein 1 (MAP1), MAP2, neurofilament 38 (NF38) by different mechanisms of action and at different levels: neuronal microfilaments, intermediate filaments, and microtubule-associated proteins. Finally, we emphasize the importance of the study of these compounds in the treatment of neurodegenerative diseases since they present the benefits of estrogens without their side effects.

17β-Estradiol Protects Rat Annulus Fibrosus Cells Against Apoptosis via α1 Integrin-Mediated Adhesion to Type I Collagen: An In-vitro Study

Background

17β-Estradiol (E2) has been reported to protect annulus fibrosus (AF) cells in vitro against interleukin-1β (IL-1β)-induced apoptosis in a concentration-dependent manner. However, its time-response effect remains unexplored. In addition, integrin α₂/collagen II interaction has been reported to influence the apoptosis of nucleus pulposus cells in vitro. Thus, we hypothesized that integrin α₁/collagen II might play a role in exerting the anti-apoptosis effect by E2. The aim of the current study was to further investigate the anti-apoptotic effect of E2 and determine the role of integrin α₁/collagen II interaction.

Material/Methods

Rat AF cells were primary cultured and used for the following experiments. AF cells were identified by immunocytochemistry of type I collagen. Cell apoptosis was detected by fluorescence-activated cell sorter (FACS) analysis. The activity of active caspase-3 was determined by use of a caspase-3 detection kit. AF cell adhesion to type I collagen was determined by cell adhesion assay. Protein level of integrin subunit α₁ was quantified by Western blot and mRNA expression was determined by real-time qPCR.

Results

The immunocytochemistry of type I collagen revealed that cell purity was eligible for the following experiments with 98% of purity. FACS analysis indicated time-dependent anti-apoptosis effect of E2 at time points of 6 h, 12 h, and 24 h, which was confirmed by Caspase-3 activity. Furthermore, cell adhesion assay showed that E2 significantly increased cell binding to 95% of control, and qPCR and Western blot analysis showed that E2 effectively upregulated integrin α₁. However, estrogen receptor antagonist ICI182780 prohibited the effect of E2.

Conclusions

This study shows that E2 protects against apoptosis in a time-dependent manner, and α₁ integrin-mediated adhesion to collagen II is essential for estrogen-dependent anti-apoptosis in rat annulus fibrosus cells in vitro.

Estrogens and progestins: molecular effects on brain cells

Sex steroids are known to regulate brain function and their role is so important that several diseases are strictly correlated with the onset of menopause when estrogen-progesterone deficiency makes neural cells much more vulnerable to toxic stimuli. Although in the past years several scientists have focused their studies on in vitro and in vivo effects of sex steroids on the brain, we are still far from complete knowledge. Indeed, contrasting results from large clinical trials have made the entire issue much more complicated. Currently we know that protective effects exerted by sex steroids depend on several factors among which the dose, the health of the cells and the type of molecule being used. In this review, we present an overview of the direct and indirect effects of estrogen and progesterone on the brain with specific focus on the molecular mechanisms by which these molecules act on neural cells.

Estrogen decreases anoikis of ovarian cancer cell line Caov-3 through reducing release of Bit1

Few studies have referred to the implication of anoikis processes following hormonal treatment. No data are available on the influence of estrogen in ovarian cancer anoikis. To gain insights into the effects and mechanism of estrogen in ovarian cancer cells, we have carried out studies on the anoikis of ovarian cancer cells treated with estrogen and on the pathways involved. We observed an anti-anoikis role of E2 in suspended Caov-3 cells, and this was mainly due to the decreasing of Bit1 level in cytosol. We also found that estrogen receptor α (ERα) was the main mediator involved in this process. To study the signaling pathways well, phosphatidylinositol 3-kinase (PI3K)/AKT were further investigated. Results demonstrated that the decreasing of the Bit1 level in cytosol mediated by E2 binding to ERα was mainly through PI3K/AKT pathways. Overall, these findings disclose a new perspective for estrogen on ovarian cancer therapy.

Mechanism of raloxifene-induced upregulation of glutamate transporters in rat primary astrocytes

Raloxifene (RX), a selective estrogen receptor modulator (SERM), exerts neuroprotection in multiple clinical and experimental settings. Astrocytic glutamate transporters GLT-1 (EAAT2) and GLAST (EAAT1) are the main glutamate transporters in the central nervous system, taking up most of excess glutamate from the synaptic cleft to prevent excitotoxic neuronal death. Since drugs targeting astrocytic glutamate transporters to enhance their expression and function represent potential therapeutics for neurodegenerative disorders associated with excitotoxicity, we tested if RX modulates the expression and function of GLT-1 and GLAST in rat primary astrocytes. The results showed that RX significantly increased glutamate uptake and expression of GLT-1 mRNA and protein levels. RX enhanced GLT-1 expression by the activation of multiple signaling pathways including ERK, EGFR, and CREB mediated by estrogen receptors (ERs) ER-α, ER-β, and GPR30. At the transcriptional level, NF-κB played a critical role in RX-induced GLT-1 expression as RX increased NF-κB reporter activity and induced binding of NF-κB p65 and p50 to the GLT-1 promoter. RX attenuated the reduction of GLT-1 expression and glutamate uptake induced by manganese (Mn) whose chronic high levels of exposure cause manganism. RX also upregulated GLAST by increasing its promoter activity and protein levels via the NF-κB pathway and ERs. Our findings provide new insight into the mechanism of RX-induced enhancement of GLT-1 and GLAST expression, as well as the attenuation of Mn-reduced expression of these transporters. These findings will be highly valuable for developing therapeutics of neurodegenerative diseases associated with impaired astrocytic glutamate transporters.

Oestrogens as modulators of neuronal signalosomes and brain lipid homeostasis related to protection against neurodegeneration

Oestrogens trigger several pathways at the plasma membrane that exert beneficial actions against neurodegenerative diseases, such as Alzheimer's disease and Parkinson's disease. Part of these actions takes place in lipid rafts, which are membrane domains with a singular protein and lipid composition. These microdomains also represent a preferential site for signalling protein complexes, or signalosomes. A plausible hypothesis is that the dynamic interaction of signalosomes with different extracellular ligands may be at the basis of neuronal maintenance against different neuropathologies. Oestrogen receptors are localised in neuronal lipid rafts, taking part of macromolecular complexes together with a voltage-dependent anion channel (VDAC), and other molecules. Oestradiol binding to its receptor at this level enhances neuroprotection against amyloid-β degeneration through the activation of different signal transduction pathways, including VDAC gating modulation. Moreover, part of the stability and functionality of signalling platforms lays on the distribution of lipid hallmarks in these microstructures, which modulate membrane physicochemical properties, thus favouring molecular interactions. Interestingly, recent findings indicate a potential role of oestrogens in the preservation of neuronal membrane physiology related to lipid homeostasis. Thus, oestrogens and docosahexaenoic acid may act synergistically to stabilise brain lipid structure by regulating neuronal lipid biosynthetic pathways, suggesting that part of the neuroprotective effects elicited by oestrogens occur through mechanisms aimed at preserving lipid homeostasis. Overall, oestrogen mechanisms of neuroprotection may occur not only by its interaction with neuronal protein targets through nongenomic and genomic mechanisms, but also through its participation in membrane architecture stabilisation via 'lipostatic' mechanisms.

Progesterone-estrogen interactions in synaptic plasticity and neuroprotection

17ß-Estradiol and progesterone exert a number of physiological effects throughout the brain due to interactions with several types of receptors belonging to the traditional family of intracellular hormonal receptors as well as to membrane-bound receptors. In particular, both hormones elicit rapid modifications of neuronal excitability that have been postulated to underlie their effects on synaptic plasticity and learning and memory. Likewise, both hormones have been shown to be neuroprotective under certain conditions, possibly due to the activation of pro-survival pathways and the inhibition of pro-apoptotic cascades. Because of the similarities in their cellular effects, there have been a number of questions raised by numerous observations that progesterone inhibits the effects of estrogen. In this manuscript, we first review the interactions between 17ß-estradiol (E2) and progesterone (P4) in synaptic plasticity, and conclude that, while E2 exerts a clear and important role in long-term potentiation of synaptic transmission in hippocampal neurons, the role of P4 is much less clear, and could be accounted by the direct or indirect regulation of GABAA receptors. We then discuss the neuroprotective roles of both hormones, in particular against excitotoxicity. In this case, the neuroprotective effects of these hormones are very similar to those of the neurotrophic factor BDNF. Interestingly, P4 antagonizes the effects of E2, possibly through the regulation of estrogen receptors or of proteins associated with the receptors or interactions with signaling pathways activated by E2. Overall, this review emphasizes the existence of common molecules and pathways that participate in the regulation of both synaptic plasticity and neurodegeneration.

Whole transcriptome analysis of the ERα synthetic fragment P295-T311 (ERα17p) identifies specific ERα-isoform (ERα, ERα36)-dependent and -independent actions in breast cancer cells

ERα17p is a peptide corresponding to the sequence P295LMIKRSKKNSLALSLT311 of the estrogen receptor alpha (ERα) and initially found to interfere with ERα-related calmodulin binding. ERα17p was subsequently found to elicit estrogenic responses in E2-deprived ERα-positive breast cancer cells, increasing proliferation and ERE-dependent gene transcription. Surprisingly, in E2-supplemented media, ERα17p-induced apoptosis and modified the actin network, influencing cell motility. Here, we report that ERα17p internalizes in breast cancer cells (T47D, MDA-MB-231, SKBR3) and induces a massive early (3 h) transcriptional activity. Remarkably, about 75% of significantly modified transcripts were also modified by E2, confirming the pro-estrogenic profile of ERα17p. The different ER spectra of the used cell lines allowed us to identify a specific ERα17p signature related to ERα as well as its variant ERα36. With respect to ERα, the peptide activates nuclear (cell cycle, cell proliferation, nucleic acid and protein synthesis) and extranuclear signaling pathways. In contrast, through ERα36, it mainly triggers inhibitory actions on inflammation. This is the first work reporting a detailed ERα36-specific transcriptional signature. In addition, we report that ERα17p-induced transcripts related to apoptosis and actin modifying effects of the peptide are independent from its estrogen receptor(s)-related actions. We discuss our findings in view of the potential use of ERα17p as a selective peptidomimetic estrogen receptor modulator (PERM).

Brain-derived neurotrophic factor-estrogen interactions in the hippocampal mossy fiber pathway: implications for normal brain function and disease

The neurotrophin brain-derived neurotrophic factor (BDNF) and the steroid hormone estrogen exhibit potent effects on hippocampal neurons during development and in adulthood. BDNF and estrogen have also been implicated in the etiology of diverse types of neurological disorders or psychiatric illnesses, or have been discussed as potentially important in treatment. Although both are typically studied independently, it has been suggested that BDNF mediates several of the effects of estrogen in the hippocampus, and that these interactions play a role in the normal brain as well as disease. Here we focus on the mossy fiber (MF) pathway of the hippocampus, a critical pathway in normal hippocampal function, and a prime example of a location where numerous studies support an interaction between BDNF and estrogen in the rodent brain. We first review the temporal and spatially regulated expression of BDNF and estrogen in the MFs, as well as their receptors. Then we consider the results of studies that suggest that 17β-estradiol alters hippocampal function by its influence on BDNF expression in the MF pathway. We also address the hypothesis that estrogen influences the hippocampus by mechanisms related not only to the mature form of BDNF, acting at trkB receptors, but also by regulating the precursor, proBDNF, acting at p75NTR. We suggest that the interactions between BDNF and 17β-estradiol in the MFs are potentially important in the normal function of the hippocampus, and have implications for sex differences in functions that depend on the MFs and in diseases where MF plasticity has been suggested to play an important role, Alzheimer's disease, epilepsy and addiction.

Amyloid β-protein suppressed nicotinic acetylcholine receptor-mediated currents in acutely isolated rat hippocampal CA1 pyramidal neurons

Amyloid β protein (Aβ) is responsible for the deficits of learning and memory in Alzheimer's disease (AD). The high affinity between Aβ and nicotinic acetylcholine receptors (nAChRs) suggests that the impairment of cognitive function in AD might be involved in the Aβ-induced damage of nAChRs. This study investigated the effects of Aβ fragments on nAChR-mediated membrane currents in acutely isolated rat hippocampal pyramidal neurons by using whole-cell patch clamp technique. The results showed that: (1) nonspecific nAChR agonist nicotine, selective α7 nAChR agonist choline, and α4β2 nAChR agonist epibatidine all effectively evoked inward currents in CA1 neurons at normal resting membrane potential, with different desensitization characteristics; (2) acute application of different concentrations (pM-μM) of Aβ25-35, Aβ31-35, or Aβ35-31 alone did not trigger any membrane current, but pretreatment with 1 μM Aβ25-35 and Aβ31-35 similarly and reversibly suppressed the nicotine-induced currents; (3) further, choline- and epibatidine-induced currents were also reversibly suppressed by the Aβ pretreatment, but more prominent for the choline-induced response. These results demonstrate that the functional activity of both α7 and α4β2 nAChRs in the membrane of acutely isolated hippocampal neurons was significantly downregulated by Aβ treatment, suggesting that nAChRs, especially α7 nAChRs, in the brain may be the important biological targets of neurotoxic Aβ in AD. In addition, the similar suppression of nAChR currents by Aβ25-35 and Aβ31-35 suggests that the sequence 31-35 in Aβ molecule may be a shorter active center responsible for the neurotoxicity of Aβ in AD.

Sex steroids and their receptors: molecular actions on brain cells

Sex steroids exert actions of paramount importance on brain cells. They contribute to shape the central nervous system during embryo development. They modulate the formation and the turnover of the interconnections between neurons. They control the function of glial cells. And they do it through a signaling machinery that is apparently simple, but that hides a level of complexity that has been unveiled only in part. Different receptor isoforms, different interactions between receptors and co-regulators, chains of events originating at the cell membrane and leading to effects in the nucleus (or the other way around) all interact to determine selective modulations of brain cells. All these actions end up in phenomenal effects on brain function that change through adolescence, pregnancy, adulthood, up to menopause and ageing. Many of these actions are relevant for degenerative processes and research may offer soon new strategies to counteract these diseases.

Sex hormones, aging, and Alzheimer's disease

A promising strategy to delay and perhaps prevent Alzheimer's disease (AD) is to identify the age-related changes that put the brain at risk for the disease. A significant normal age change known to result in tissue-specific dysfunction is the depletion of sex hormones. In women, menopause results in a relatively rapid loss of estradiol and progesterone. In men, aging is associated with a comparatively gradual yet significant decrease in testosterone. We review a broad literature that indicates age-related losses of estrogens in women and testosterone in men are risk factors for AD. Both estrogens and androgens exert a wide range of protective actions that improve multiple aspects of neural health, suggesting that hormone therapies have the potential to combat AD pathogenesis. However, translation of experimental findings into effective therapies has proven challenging. One emerging treatment option is the development of novel hormone mimetics termed selective estrogen and androgen receptor modulators. Continued research of sex hormones and their roles in the aging brain is expected to yield valuable approaches to reducing the risk of AD.

Sex hormones, aging, and Alzheimer's disease

A promising strategy to delay and perhaps prevent Alzheimer's disease (AD) is to identify the age-related changes that put the brain at risk for the disease. A significant normal age change known to result in tissue-specific dysfunction is the depletion of sex hormones. In women, menopause results in a relatively rapid loss of estradiol and progesterone. In men, aging is associated with a comparatively gradual yet significant decrease in testosterone. We review a broad literature that indicates age-related losses of estrogens in women and testosterone in men are risk factors for AD. Both estrogens and androgens exert a wide range of protective actions that improve multiple aspects of neural health, suggesting that hormone therapies have the potential to combat AD pathogenesis. However, translation of experimental findings into effective therapies has proven challenging. One emerging treatment option is the development of novel hormone mimetics termed selective estrogen and androgen receptor modulators. Continued research of sex hormones and their roles in the aging brain is expected to yield valuable approaches to reducing the risk of AD.

17β-Estradiol regulates insulin-degrading enzyme expression via an ERβ/PI3-K pathway in hippocampus: relevance to Alzheimer's prevention

Insulin-degrading enzyme (IDE), an enzyme that primarily degrades insulin, has recently been demonstrated to play a significant role in the catabolism of amyloid β (Aβ) protein in the brain. Reduced IDE expression and/or activity have been associated with the etiology and development of Alzheimer's disease (AD). Using three model systems, the present investigation provides the first documentation indicating that estrogen robustly regulates the expression of IDE in normal, menopausal and early-stage AD brains. In vitro analyses in primary cultures of rat hippocampal neurons revealed that 17β-estradiol (17β-E2) increased IDE in both mRNA and protein levels in a time-dependent manner. Further pharmacological analyses indicated that 17β-E2-induced IDE expression was dependent upon estrogen receptor (ER) β and required activation of phosphatidylinositol 3-kinase (PI3-K). In vivo analyses in adult female rats revealed a brain region-specific responsive profile. Ovariectomy (OVX) induced a significant decline in IDE expression in the hippocampus, which was prevented by 17β-E2. Neither OVX nor 17β-E2 affected IDE expression in the cerebellum. In vivo analyses in triple transgenic AD (3xTg-AD) female mice revealed an inverse correlation between the age-related increase in Aβ load and the decrease in IDE expression in the hippocampal formation. Treatment with 17β-E2 attenuated Aβ accumulation/plaque formation and elevated hippocampal IDE expression in 12-month-old 3xTg-AD OVX mice. Collectively, these findings indicate that 17β-E2 regulates IDE expression in a brain region-specific manner and such a regulatory role in the hippocampus, mediated by an ERβ/PI3-K pathway, could serve as a direct mechanism underlying estrogen-mediated preventative effect against AD when initiated at the onset of menopause.

Estrogen induces neurite outgrowth via Rho family GTPases in neuroblastoma cells

Estrogen (E2) has direct in vivo and in vitro effects, such as inducing neurite outgrowth, on neurons. We investigated the morphological changes and intracellular signaling pathway induced by E2 in neuroblastoma (SH-SY5Y) cells. The effect of medroxyprogesterone acetate (MPA) or progesterone (P4) on the E2-induced neurite outgrowth was also examined using SH-SY5Y cells. Neurite outgrowth was induced by E2 in association with the phosphorylation of Akt, and these effects of E2 were abolished by MPA but not by P4. Progesterone receptor antagonist RU486 blocked the inhibitory effects of MPA. Estrogen receptor antagonist ICI 182,780 and phosphatidylinositol 3-kinase inhibitor LY294002 inhibited the E2-induced neurite outgrowth. Because the Rho family of small GTPases has been shown to be involved in the regulation of neurite outgrowth, we examined the cross-talk among Rac1, Cdc42 and RhoA in the E2-induced neurite outgrowth. E2 immediately increased the Rac1 and Cdc42 activity and decreased the RhoA activity. E2-induced neurite outgrowth was attenuated in cells expressing dominant-negative mutants for Rac1 or Cdc42. These results suggest that regulation of Rho family GTPase activity by E2 is important for the neurite outgrowth in neuroblastoma cells, and that MPA may have an antagonistic effect against E2.

Estradiol and tamoxifen differentially regulate a plasmalemmal voltage-dependent anion channel involved in amyloid-beta induced neurotoxicity

There is a wealth of information indicating that estradiol exerts rapid actions involved in neuroprotection and cognitive-enhancing effects. Some of these effects appear to delay onset, or even ameliorate, the neuropathology of Alzheimer's disease (AD), although some controversy exists about the beneficial brain effects of estrogen therapies. Therefore, it is crucial to better understand the mechanisms developed by 17β-estradiol to signal in the brain. At the neuronal membrane, the hormone can rapidly interact with estrogen receptors (mERs) or activate other receptors, such as G protein-coupled and ionotropic receptors. And the list of membrane signalling molecules modulated by estradiol in neurons is increasing. VDAC is a voltage-dependent anion channel, known as a mitochondrial porin which is also found at the neuronal membrane, where it appears to be involved in redox regulation, extrinsic apoptosis and amyloid beta neurotoxicity. Moreover, VDAC is present in neuronal lipid rafts, where it is associated with estrogen receptor α-like (mER), forming part of a macromolecular complex together with caveolin-1 and other signalling proteins related to neuronal preservation. Interestingly, we have recently found that 17β-estradiol rapidly promotes VDAC phosphorylation through the activation of protein kinase A (PKA) and Src-kinase, which may be relevant to maintain this channel inactivated. On the contrary, tamoxifen, a selective estrogen receptor modulator (SERM), provokes the dephosphorylation of VDAC, and eventually its opening, by activating a cascade of phosphatases, including protein phosphatase 2 (PP2A). This review will focus on the relevance of these novel findings in the alternative estrogen mechanisms to achieve neuroprotection related to AD.

Estradiol rapidly rescues synaptic transmission from corticosterone-induced suppression via synaptic/extranuclear steroid receptors in the hippocampus

We investigated rapid protection effect by estradiol on corticosterone (CORT)-induced suppression of synaptic transmission. Rapid suppression by 1 μM CORT of long-term potentiation (LTP) at CA3-CA1 synapses was abolished via coperfusion of 1 nM estradiol. N-methyl-D-aspartate (NMDA) receptor-derived field excitatory postsynaptic potential (NMDA-R-fEPSP) was used to analyze the mechanisms of these events. Estradiol abolished CORT-induced suppression of NMDA-R-fEPSP slope. This CORT-induced suppression was abolished by calcineurin inhibitor, and the rescue effect by estradiol on the CORT-induced suppression was inhibited by mitogen-activated protein (MAP) kinase inhibitor. The CORT-induced suppressions of LTP and NMDA-R-fEPSP slope were abolished by glucocorticoid receptor (GR) antagonist, and the restorative effects by estradiol on these processes were mimicked by estrogen receptor α (ERα) and ERβ agonists. Taken together, estradiol rapidly rescued LTP and NMDA-R-fEPSP slope from CORT-induced suppressions. A GR→calcineurin pathway is involved in these suppressive effects. The rescue effects by estradiol are driven via ERα or ERβ→MAP kinase pathway. Synaptic/extranuclear GR, ERα, and ERβ probably participate in these rapid events. Mass-spectrometric analysis determined that acute hippocampal slices used for electrophysiological measurements contained 0.48 nM estradiol less than exogenously applied 1 nM. In vivo physiological level of 8 nM estradiol could protect the intact hippocampus against acute stress-induced neural suppression.

Neuroprotection in glaucoma - Is there a future role?

In glaucoma, the major cause of global irreversible blindness, there is an urgent need for treatment modalities that directly target the RGCs. The discovery of an alternative therapeutic approach, independent of IOP reduction, is highly sought after, due to the indirect nature and limited effectiveness of IOP lowering therapy in preventing RGC loss. Several mechanisms have been implicated in initiating the apoptotic cascade in glaucomatous retinopathy and numerous drugs have been shown to be neuroprotective in animal models of glaucoma. These mechanisms and their potential treatment include excitotoxicity, protein misfolding, mitochondrial dysfunction, oxidative stress, inflammation and neurotrophin deprivation. All of these mechanisms ultimately lead to programmed cell death with loss of RGCs. In this article we summarize the mechanisms involved in glaucomatous disease, highlight the rationale for neuroprotection in glaucoma management and review current potential neuroprotective strategies targeting RGCs from the laboratory to the clinic.

Estrogen and beta-amyloid toxicity: role of integrin and PI3-K

Beta-amyloid peptide (betaAP) induces apoptosis and down-regulation of alpha(1)beta(1) integrin in neuronal cells, indicating a relationship between betaAP neurotoxicity and modulation of integrin expression. Estrogen may play a role in protecting women from Alzheimer Disease (AD). It is here reported that both 17beta-estradiol (17betaE(2)) and its non-estrogenic stereoisomer 17alpha-estradiol (17alphaE(2)) rescue neuronal cells from betaAP-induced apoptosis. As cellular model, the human neuroblastoma cell line SK-N-BE was used, which responds to retinoic acid by growth arrest and differentiation toward the neuronal phenotype (RA-SK-N-BE). Estrogen receptor antagonist does not hinder estrogen protection. Inhibition of phosphatidylinositol 3-kinase (PI3-K), but not of tyrosine kinases or mitogen-activated protein kinases (MAPK) blocks 17betaE(2) protection against betaAP-induced apoptosis. 17betaE(2) up-regulates alpha(1)beta(1) integrin expression and completely abolishes betaAP-induced alpha(1)beta(1) down-regulation. Inadequate cell cycle control may contribute to neuronal death in AD. betaAP induces RA-SK-N-BE cells to enter cell cycle, which remains incomplete. 17betaE(2) induces betaAP-treated cells to complete cell cycle. Our data suggest that estrogen protects from betaAP neurotoxicity by restoring integrin expression and cell cycle control.

Neuroprotective effects of emodin in rat cortical neurons against beta-amyloid-induced neurotoxicity

Accumulation of beta-amyloid protein (Abeta) in the brain plays an important role in the pathogenesis of Alzheimer's disease (AD). In this study, the neuroprotective effect of emodin extracted from the traditional Chinese medicinal herb Polygonum cuspidatum Sieb. et Zucc against Abeta(25-35)-induced cell death in cultured cortical neurons was investigated. We found that pre-treatment with emodin prevented the cultured cortical neurons from beta-amyloid-induced toxicity. The preventive effect of emodin was blocked by pre-treatment with a phosphatidylinositol-3-kinase (PI3K) pathway inhibitor LY294002 or an estrogen receptor (ER) specific antagonist ICI182780, but not by pre-treatment with an extracellular signal-related kinases (ERK) inhibitor U0126. Furthermore, we found that emodin exposure induced the activation of the Akt serine/threonine kinase and increased the level of Bcl-2 expression. Moreover, the application of emodin for 24h was able to induce the activation of Abeta(25-35)-suppressed Akt and decrease the activation of the Jun-N-terminal kinases (JNK), but not of ERK. Interestingly, the up-regulation of Akt and Bcl-2 did not occur in the presence of LY294002 or ICI182780, suggesting that emodin-up-regulated Bcl-2 is mediated via the ER and PI3K/Akt pathway. Taken together, our results suggest that emodin is an effective neuroprotective drug and is a viable candidate for treating AD.

[Gly(14)]-humanin rescues long-term potentiation from amyloid beta protein-induced impairment in the rat hippocampal CA1 region in vivo

The novel neuroprotective action of Humanin (HN), especially its derivative [Gly(14)]-humanin (HNG), against Alzheimer's disease (AD)-related insults has been reported. However, it is still short of electrophysiological evidence for the protection of HN on synaptic plasticity, and the molecular mechanisms that underlie the neuroprotective function of HN remain largely unknown. The present study examined the effects of intracerebroventricular (i.c.v.) injection of HNG on amyloid beta (Abeta), a main constituent of senile plaques in the AD brain, induced suppression of long-term potentiation (LTP) in the rat hippocampal CA1 region in vivo and investigated the possible mechanism of HNG in LTP protection. We found that application of Abeta fragments 25-35 (Abeta25-35) and 31-35 (Abeta31-35) significantly inhibited high frequency stimulation-induced LTP, while HNG effectively prevented the suppression of LTP induced by Abeta fragments in a dose-dependent manner. After pretreatment with Genistein, a tyrosine kinase inhibitor, the protective action of HNG on LTP was nearly completely abolished. Therefore, the present study demonstrated for the first time that HNG could protect against the neurotoxic Abeta-induced hippocampal LTP impairment and the tyrosine kinase pathway was involved in the neuroprotective action of HNG, suggesting that HNG might be one of the promising candidates for the treatment of AD in the future.

Interactions of estradiol and insulin-like growth factor-I signalling in the nervous system: new advances

Estradiol and insulin-like growth factor-I (IGF-I) interact in the brain to regulate a variety of developmental and neuroplastic events. Some of these interactions are involved in the control of hormonal homeostasis and reproduction. However, the interactions may also potentially impact on affection and cognition by the regulation of adult neurogenesis in the hippocampus and by promoting neuroprotection under neurodegenerative conditions. Recent studies suggest that the interaction of estradiol and IGF-I is also relevant for the control of cholesterol homeostasis in neural cells. The molecular mechanisms involved in the interaction of estradiol and IGF-I include the cross-regulation of the expression of estrogen and IGF-I receptors, the regulation of estrogen receptor-mediated transcription by IGF-I and the regulation of IGF-I receptor signalling by estradiol. Current investigations are evidencing the role exerted by key signalling molecules, such as glycogen synthase kinase 3 and beta-catenin, in the cross-talk of estrogen receptors and IGF-I receptors in neural cells.

Effects of estrogen on AF64A-induced apoptosis in NG108-15 cells

In this study, we show that pretreatment with physiological concentrations (1-100 nM) of 17beta-estradiol decreased apoptosis induced by ethylcholine aziridinium (AF64A), a choline toxin, in the cholinergic neuronal cell line NG108-15. These protective effects were observed after short-term (30 min) pretreatment, and were blocked by treatment with an estrogen receptor antagonist and inhibitors of phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase kinase (MEK). The protective effects were, however, not reversed by a protein synthesis inhibitor. Furthermore, we examined the effects of 17beta-estradiol on choline uptake in NG108-15 cells. Although choline uptake was inhibited by a selective inhibitor of choline uptake, hemicholinium-3, it was not altered by treatment with 17beta-estradiol. These results indicated that the protective effect of 17beta-estradiol on AF64A-induced apoptosis could be nongenomic, and that this effect may be due to the activation of PI3K/Akt and/or MEK/extracellular signal-regulated kinase (ERK) pathways.

Role of protein phosphatases and mitochondria in the neuroprotective effects of estrogens

In the present treatise, we provide evidence that the neuroprotective and mito-protective effects of estrogens are inexorably linked and involve the ability of estrogens to maintain mitochondrial function during neurotoxic stress. This is achieved by the induction of nuclear and mitochondrial gene expression, the maintenance of protein phosphatases levels in a manner that likely involves modulation of the phosphorylation state of signaling kinases and mitochondrial pro- and anti-apoptotic proteins, and the potent redox/antioxidant activity of estrogens. These estrogen actions are mediated through a combination of estrogens receptor (ER)-mediated effects on nuclear and mitochondrial transcription of protein vital to mitochondrial function, ER-mediated, non-genomic signaling and non-ER-mediated effects of estrogens on signaling and oxidative stress. Collectively, these multifaceted, coordinated action of estrogens leads to their potency in protecting neurons from a wide variety of acute insults as well as chronic neurodegenerative processes.

Neuroprotective actions of sex steroids in Parkinson's disease

The sex difference in Parkinson's disease, with a higher susceptibility in men, suggests a modulatory effect of sex steroids in the brain. Numerous studies highlight that sex steroids have neuroprotective properties against various brain injuries. This paper reviews the protective effects of sex hormones, particularly estradiol, progesterone and androgens, in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) animal model of Parkinson's disease as compared to methamphetamine toxicity. The molecular mechanisms underlying beneficial actions of sex steroids on the brain have been investigated showing steroid, dose, timing and duration specificities and presently focus is on the dopamine signaling pathways, the next frontier. Both genomic and non-genomic actions of estrogen converge to promote survival factors and show sex differences. Neuroprotection by estrogen involves activation of signaling molecules such as the phosphatidylinositol-3 kinase/Akt and the mitogen-activated protein kinase pathways. Interaction with growth factors, such as insulin-like growth factor 1, also contributes to protective actions of estrogen.

Protective actions of sex steroid hormones in Alzheimer's disease

Risk for Alzheimer's disease (AD) is associated with age-related loss of sex steroid hormones in both women and men. In post-menopausal women, the precipitous depletion of estrogens and progestogens is hypothesized to increase susceptibility to AD pathogenesis, a concept largely supported by epidemiological evidence but refuted by some clinical findings. Experimental evidence suggests that estrogens have numerous neuroprotective actions relevant to prevention of AD, in particular promotion of neuron viability and reduction of beta-amyloid accumulation, a critical factor in the initiation and progression of AD. Recent findings suggest neural responsiveness to estrogen can diminish with age, reducing neuroprotective actions of estrogen and, consequently, potentially limiting the utility of hormone therapies in aged women. In addition, estrogen neuroprotective actions are also modulated by progestogens. Specifically, continuous progestogen exposure is associated with inhibition of estrogen actions whereas cyclic delivery of progestogens may enhance neural benefits of estrogen. In recent years, emerging literature has begun to elucidate a parallel relationship of sex steroid hormones and AD risk in men. Normal age-related testosterone loss in men is associated with increased risk to several diseases including AD. Like estrogen, testosterone has been established as an endogenous neuroprotective factor that not only increases neuronal resilience against AD-related insults, but also reduces beta-amyloid accumulation. Androgen neuroprotective effects are mediated both directly by activation of androgen pathways and indirectly by aromatization to estradiol and initiation of protective estrogen signaling mechanisms. The successful use of hormone therapies in aging men and women to delay, prevent, and or treat AD will require additional research to optimize key parameters of hormone therapy and may benefit from the continuing development of selective estrogen and androgen receptor modulators.

Mechanism of okadaic acid-induced neuronal death and the effect of estrogens

Serine/threonine protein phosphatases are important mediators of general cellular function as well as neurodegenerative processes. We have previously shown inhibition of protein phosphatases to be as neurotoxic as glutamate-induced neuronal death but resistant to neuroprotection by estrogens. In this study, the mechanism by which phosphatase inhibition via okadaic acid (OA) induced neurotoxicity is explored. Neurons were exposed to OA or glutamate in the presence or absence of various protein kinases inhibitors, and/or one of four estrogens. Both OA and glutamate induced cell death via increased reactive oxygen species, protein carbonylation, lipid peroxidation, caspase-3 activity, and mitochondrial dysfunction. All estrogens attenuated glutamate-mediated responses, but not OA-induced responses. In addition, inhibition of protein kinase C and mitogen-activated protein kinase pathway was neuroprotective against glutamate but not OA toxicity. Interestingly, inhibition of mitogen-activated protein kinase pathway with PD98096 or U0126 caused a decrease in reactive oxygen species production suggesting that activation of ERK1/2 could further exacerbate the oxidative stress caused by glutamate-induced toxicity; however, these inhibitors had no effect on OA-induced toxicity. Collectively, these results indicate that both glutamate and OA neurotoxicities are mediated by persistent activation of ERK1/2 and/or protein kinase C and a resulting oxidative stress, and that protein phosphatase activity is an important and necessary aspect of estrogen-mediated neuroprotection.

Alpha4beta2 nicotinic acetylcholine receptors are required for the amyloid beta protein-induced suppression of long-term potentiation in rat hippocampal CA1 region in vivo

Amyloid beta protein (Abeta) is thought to be responsible for the deficit of learning and memory in Alzheimer's disease (AD), possibly through interfering with synaptic plasticity such as hippocampal long-term potentiation (LTP). Nicotinic acetylcholine receptors (nAChRs) participate in various cognitive brain functions. However, it is unclear whether nAChRs, especially alpha4beta2 subtype nAChRs, are involved in Abeta-induced impairment of hippocampal LTP. The present study investigates a possible role of nAChRs during the impairment of LTP by Abeta. Our results showed that: (1) intracerebroventricular injection of Abeta(1-40), Abeta(25-35) or Abeta(31-35) significantly suppressed high-frequency stimulation-induced LTP, while Abeta(35-31), a reversed sequence of Abeta(31-35), have no effect on the LTP; (2) epibatidine, a specific agonist of alpha4beta2 subtype of nAChRs, dose-dependently suppressed the induction of LTP; (3) co-injection of epibatidine together with Abeta(31-35) did not further enhance the suppression of LTP induced by Abeta(31-35) or epibatidine alone; (4) dihydro-beta-erythroidine, a selective antagonist against alpha4beta2 subtype of nAChRs, showed no effect on the induction of LTP, but significantly reversed Abeta(31-35)-induced LTP impairment. These results indicate that: (1) sequence 31-35 in Abeta molecule might be a shorter active center responsible for the neurotoxicity of full length of Abeta; (2) alpha4beta2 subtype of nAChRs is required for the suppressive action of Abeta on the hippocampal LTP in vivo. Thus, the present study provides further insight into the mechanisms by which Abeta impairs synaptic plasticity and cognitive function in the AD brain.

TGF-beta 1 protects against Abeta-neurotoxicity via the phosphatidylinositol-3-kinase pathway

beta-Amyloid (A beta) injection into the rat dorsal hippocampus had a small neurotoxic effect that was amplified by i.c.v. injection of SB431542, a selective inhibitor of transforming growth factor-beta (TGF-beta) receptor. This suggested that TGF-beta acts as a factor limiting A beta toxicity. We examined the neuroprotective activity of TGF-beta1 in pure cultures of rat cortical neurons challenged with A beta. Neuronal death triggered by A beta is known to proceed along an aberrant re-activation of the cell cycle, and involves late beta-catenin degradation and tau hyperphosphorylation. TGF-beta1 was equally protective when added either in combination with, or 6 h after A beta. Co-added TGF-beta1 prevented A beta-induced cell cycle reactivation, whereas lately added TGF-beta1 had no effect on the cell cycle, but rescued the late beta-catenin degradation and tau hyperphosphorylation. The phosphatidylinositol-3-kinase (PI-3-K) inhibitor, LY294402, abrogated all effects. Thus, TGF-beta1 blocks the whole cascade of events leading to A beta neurotoxicity by activating the PI-3-K pathway.

Contribution of estrogen receptors alpha and beta to the effects of estradiol in the brain

Clinical and experimental studies show a modulatory role of estrogens in the brain and suggest their beneficial action in mental and neurodegenerative diseases. The estrogen receptors ERalpha and ERbeta are present in the brain and their targeting could bring selectivity and reduced risk of cancer. Implication of ERs in the effect of estradiol on dopamine, opiate and glutamate neurotransmission is reviewed. The ERalpha agonist, PPT, is shown as estradiol to modulate hippocampal NMDA receptors and AMPA receptors in cortex and striatum of ovariectomized rats whereas the ERbeta agonist DPN is inactive. Striatal DPN activity suggests implication of ERbeta in estradiol modulation of D2 receptors and transporters in ovariectomized rats and is supported by the lack of effect of estradiol in ERbeta knockout (ERKObeta) mice. Both ERalpha and ERbeta agonists modulate striatal preproenkephalin (PPE) gene expression in ovariectomized rats. In male mice PPT protects against MPTP toxicity to striatal dopamine; this implicates Akt/GSK3beta signaling and the apoptotic regulators Bcl2 and Bad. This suggests a role for ERalpha in striatal dopamine neuroprotection. ERKOalpha mice are more susceptible to MPTP toxicity and not protected by estradiol; differences in ERKObeta mice are subtler. These results suggest therapeutic potential for the brain of ER specific agonists.

17beta-estradiol attenuates glycogen synthase kinase-3beta activation and tau hyperphosphorylation in Akt-independent manner

Decline of estrogen is associated with high incidence of Alzheimer's disease (AD) characterized pathologically with tau hyperphosphorylation, and glycogen synthase kinase-3beta (GSK-3beta) is a major tau kinase. However, the role of estrogen on GSK3beta-induced tau hyperphosphorylation is elusive. Here, we treated N2a cells with wortmannin (Wort) and GF-109203X (GFX) or gene transfection to activate GSK-3beta and to induce tau hyperphosphorylation and then the effects of 17beta-estradiol (betaE2) on tau phosphorylation and GSK-3beta activity were studied. We found that betaE2 could attenuate tau hyperphosphorylation at multiple AD-related sites, including Ser396/404, Thr231, Thr205, and Ser199/202, induced by Wort/GFX or transient overexpression of GSK-3beta. Simultaneously, it increased the level of Ser9-phosphorylated (inactive) GSK-3beta. To study whether the protective effect of betaE2 on GSK-3beta and tau phosphorylation involves protein kinase B (Akt), an upstream effector of GSK-3, we transiently expressed the dominant negative Akt (dnAkt) in the cells. We found that betaE2 could attenuate Wort/GFX-induced GSK-3beta activation and tau hyperphosphorylation with Akt-independent manner. It suggests that betaE2 may arrest AD-like tau hyperphosphorylation by directly targeting GSK-3beta.

Estrogen receptor-independent neuroprotection via protein phosphatase preservation and attenuation of persistent extracellular signal-regulated kinase 1/2 activation

The mechanism of estrogen-mediated neuroprotection is not yet clear. Estrogens have a variety of modes of action, including transducing signaling events such as activation and/or suppression of the mitogen-activated protein kinase (MAPK) pathway. We have previously shown protein phosphatases to be involved in 17beta-estradiol-mediated neuroprotection. In the present study, we assessed the role of estrogen receptors (ERs) in estrogen-mediated neuroprotection from oxidative/excitotoxic stress and the consequential effects on MAPK signaling. Okadaic acid and calyculin A, nonspecific serine/threonine phosphatase inhibitors, were exposed to cells at various concentrations in the presence or absence of 17alpha-estradiol, the enantiomer of 17beta-estradiol, 2-(1-adamantyl)-3-hydroxyestra-1,3,5(10)-trien-17-one (ZYC3; non-ER-binding estrogen analog), and/or glutamate. All three compounds, which we have shown to have little or no binding to ERalpha and ERbeta, were protective against glutamate toxicity but not against okadaic acid and calyculin A toxicity. In addition, in the presence of effective concentrations of these inhibitors, the protective effects of these estrogen analogs were lost. Glutamate treatment caused a 50% decrease in levels of protein phosphatase 1 (PP1), protein phosphatase 2A (PP2A), and protein phosphatase 2B (calcineurin) (PP2B). Coadministration of ZYC3 with glutamate prevented the decreases in PP1, PP2A, and PP2B levels. Furthermore, glutamate treatment caused a persistent increase in phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 that corresponds with the decrease protein levels of serine/threonine phosphatases. ZYC3 blocked this persistent increase in ERK phosphorylation. These results suggest that estrogens protect cells against glutamate-induced oxidative stress through an ER-independent mediated mechanism that serves to preserve phosphatase activity in the face of oxidative insults, resulting in attenuation of the persistent phosphorylation of ERK associated with neuronal death.

Retinal ganglion cell protection by 17-beta-estradiol in a mouse model of inherited glaucoma

Glaucoma is the second leading cause of blindness in the world. The ultimate cause of vision loss due to glaucoma is thought to be retinal ganglion cell (RGC) apoptosis. Neuroprotection of RGC is becoming an important approach of glaucoma therapy. Several lines of evidence suggest that estrogen has neurotrophic and neuroprotective properties. In this study, we examine the role of estrogen in preventing RGC loss in DBA/2J mouse, an in vivo model of an inherited (pigmentary) glaucoma. Two-month-old female DBA/2J mice were anesthetized and ovariectomized with or without subcutaneous 17beta-estradiol (betaE2) pellet implantation. RGC survival was evaluated from flat-mounted whole retinas by counting retrograde-labeled cells. The loss of nerve fibers and RGC were also evaluated in paraffin-fixed retinal cross sections. Biochemical alterations in the retinas of DBA/2J mice in response to systemic injection of betaE2 were also examined. We have made several important observations showing that: (1) betaE2 treatment reduced the loss of RGC and neurofibers through inhibition of ganglion cell apoptosis, (2) betaE2 activated Akt and cAMP-responsive-element-binding-protein (CREB), (3) betaE2 up-regulated thioredoxin-1 (Trx-1) expression, (4) betaE2 reduced the increased activations of mitogen-activated protein kinases (MAPK) and NF-kappaB, (5) betaE2 inhibited the increased interleukin-18 (IL-18) expression, and (6) treatment with tamoxifen, an estrogen receptor antagonist, blocked betaE2-mediated activation of Akt and inhibition of MAPK phosphorylation in the retinas of DBA/2J mice. These findings suggest the possible involvement of multiple biochemical events, including estrogen receptor/Akt/CREB/thioredoxin-1, and estrogen receptor/MAPK/NF-kappaB, in estrogen-mediated retinal ganglion cell protection.

Neurotrophic and neuroprotective actions of estrogen: basic mechanisms and clinical implications

Estrogen is an important hormone signal that regulates multiple tissues and functions in the body. This review focuses on the neurotrophic and neuroprotective actions of estrogen in the brain, with particular emphasis on estrogen actions in the hippocampus, cerebral cortex and striatum. Sex differences in the risk, onset and severity of neurodegenerative disease such as Alzheimer's disease, Parkinson's disease and stroke are well known, and the potential role of estrogen as a neuroprotective factor is discussed in this context. The review assimilates a complex literature that spans research in humans, non-human primates and rodent animal models and attempts to contrast and compare the findings across species where possible. Current controversies regarding the Women's Health Initiative (WHI) study, its ramifications, concerns and the new studies needed to address these concerns are also addressed. Signaling mechanisms underlying estrogen-induced neuroprotection and synaptic plasticity are reviewed, including the important concepts of genomic versus nongenomic mechanisms, types of estrogen receptor involved and their subcellular targeting, and implicated downstream signaling pathways and mediators. Finally, a multicellular mode of estrogen action in the regulation of neuronal survival and neurotrophism is discussed, as are potential future directions for the field.

Cross-talk between estrogen receptors and insulin-like growth factor-I receptor in the brain: cellular and molecular mechanisms

Accumulating evidence suggests that insulin-like growth factor-I (IGF-I) and estradiol interact to regulate neural function. In this review, we focus on the cellular and molecular mechanisms involved in this interaction. The expression of estrogen receptors (ERs) and IGF-I receptor is cross-regulated in the central nervous system and many neurons and astrocytes coexpress both receptors. Furthermore, estradiol activates IGF-I receptor and its intracellular signaling. This effect may involve classical ERs since recent findings suggest that ERalpha may affect IGF-I actions in the brain by a direct interaction with some of the components of IGF-I signaling. In turn, IGF-I may regulate ER transcriptional activity in neuronal cells. In conclusion, ERs appear to be part of the signaling mechanism of IGF-I, and IGF-I receptor part of the mechanism of estradiol signaling in the nervous system.

Estrogen and brain-derived neurotrophic factor (BDNF) in hippocampus: complexity of steroid hormone-growth factor interactions in the adult CNS

In the CNS, there are widespread and diverse interactions between growth factors and estrogen. Here we examine the interactions of estrogen and brain-derived neurotrophic factor (BDNF), two molecules that have historically been studied separately, despite the fact that they seem to share common targets, effects, and mechanisms of action. The demonstration of an estrogen-sensitive response element on the BDNF gene provided an impetus to explore a direct relationship between estrogen and BDNF, and predicted that the effects of estrogen, at least in part, might be due to the induction of BDNF. This hypothesis is discussed with respect to the hippocampus, where substantial evidence has accumulated in favor of it, but alternate hypotheses are also raised. It is suggested that some of the interactions between estrogen and BDNF, as well as the controversies and implications associated with their respective actions, may be best appreciated in light of the ability of BDNF to induce neuropeptide Y (NPY) synthesis in hippocampal neurons. Taken together, this tri-molecular cascade, estrogen-BDNF-NPY, may be important in understanding the hormonal regulation of hippocampal function. It may also be relevant to other regions of the CNS where estrogen is known to exert profound effects, such as amygdala and hypothalamus; and may provide greater insight into neurological disorders and psychiatric illness, including Alzheimer's disease, depression and epilepsy.

Caspase-3- and calpain-mediated tau cleavage are differentially prevented by estrogen and testosterone in beta-amyloid-treated hippocampal neurons

A growing body of evidence suggests that the proteolytic cleavage of the microtubule-associated protein tau, the main component of neurofibrillary tangles, might play a role in the molecular mechanisms underlying beta-amyloid (Abeta) -induced neurotoxicity in central neurons. In the present study, we analyzed whether sex hormones could prevent such tau cleavage, and hence, protect rat hippocampal neurons against Abeta toxicity. Our results indicated that estrogen and testosterone prevented caspase-3- and calpain-mediated tau cleavage, respectively. Thus, estrogen decreased the levels of caspase-3-cleaved 50-kDa truncated tau, while testosterone prevented the generation of a calpain-cleaved 17-kDa tau fragment. In addition, our results showed that the decrease in the levels of these tau proteolytic forms was accompanied by an increased cell survival in Abeta-treated neurons. Furthermore, our findings indicated that testosterone was more effective than estrogen in protecting hippocampal neurons against Abeta-induced cell death. Collectively, our data suggest that preventing the decline of estrogen and testosterone associated with normal aging might reduce the susceptibility of central neurons to Abeta-induced toxicity.

Novel mechanisms for estrogen-induced neuroprotection

Estrogens are gonadal steroid hormones that are present in the circulation of both males and females and that can no longer be considered within the strict confines of reproductive function. In fact, the bone, the cardiovascular system, and extrahypothalamic regions of the brain are now well-established targets of estrogens. Among the numerous aspects of brain function regulated by estrogens are their effects on mood, cognitive function, and neuronal viability. Here, we review the supporting evidence for estrogens as neuroprotective agents and summarize the various mechanisms that may be involved in this effect, focusing particularly on the mitochondria as an important target. On the basis of this evidence, we discuss the clinical applicability of estrogens in treating various age-related disorders, including Alzheimer disease and stroke, and identify the caveats that must be considered.

The role of the phosphatidylinositide 3-kinase–protein kinase B pathway in schizophrenia

Neuroanatomical studies of brains from schizophrenic patients report evidence for neuronal dystrophy, while in genetic studies in schizophrenia there is evidence for mutations in growth factors and the downstream enzymes phosphatidylinositide 3-kinase (PI3K) and protein kinase B (PKB). Since the PI3K-PKB pathway is involved in cellular growth and proliferation, reduced activity of this cascade in schizophrenia could at least partly explain the neuronal dystrophy. Risk factors for schizophrenia, such as corticosteroids and cannabis, suppress the activity of the PI3K-PKB pathway. Conversely, estrogen and vitamin D, 2 factors with a moderate protective activity in schizophrenia, electroconvulsive shock therapy, and chronic treatment with antipsychotic compounds stimulate the pathway. Reduced activity of the PI3K-PKB pathway makes the brain more susceptible to virus infections, anoxia, and obstetric complications (recognized risk factors for schizophrenia), whereas a diminution of growth factor levels towards the end of puberty could contribute to an increase in schizophrenia symptoms observed around that time. On the other hand, constitutive (over)activation of the PI3K-PKB pathway increases cancer risk. Consequently, the presumed hypoactivity of the PI3K-PKB cascade might provide a partial explanation for the remarkable epidemiological finding of a reduced cancer rate in schizophrenic patients. Recognition of the role of a dysfunctional PI3K-PKB pathway in schizophrenia might help in the discovery of hitherto undetected causative gene mutations and could also lead to novel therapeutic approaches. However, a major challenge that remains to be solved is how the PI3K-PKB pathway can be activated without increasing the risk of cancer.

Role of protein phosphatases in estrogen-mediated neuroprotection

The signaling pathways that mediate neurodegeneration are complex and involve a balance between phosphorylation and dephosphorylation of signaling and structural proteins. We have shown previously that 17beta-estradiol and its analogs are potent neuroprotectants. The purpose of this study was to delineate the role of protein phosphatases (PPs) in estrogen neuroprotection against oxidative stress and excitotoxicity. HT-22 cells, C6-glioma cells, and primary rat cortical neurons were exposed to the nonspecific serine/threonine protein phosphatase inhibitors okadaic acid and calyculin A at various concentrations in the presence or absence of 17beta-estradiol and/or glutamate. Okadaic acid and calyculin A caused a dose-dependent decrease in cell viability in HT-22, C6-glioma, and primary rat cortical neurons. 17beta-Estradiol did not show protection against neurotoxic concentrations of either okadaic acid or calyculin A in these cells. In the absence of these serine/threonine protein phosphatase inhibitors, 17beta-estradiol attenuated glutamate toxicity. However, in the presence of effective concentrations of these protein phosphatase inhibitors, 17beta-estradiol protection against glutamate toxicity was lost. Furthermore, glutamate treatment in HT-22 cells and primary rat cortical neurons caused a 50% decrease in levels of PP1, PP2A, and PP2B protein, whereas coadministration of 17beta-estradiol with glutamate prevented the decrease in PP1, PP2A, and PP2B levels. These results suggest that 17beta-estradiol may protect cells against glutamate-induced oxidative stress and excitotoxicity by activating a combination of protein phosphatases.