Estradiol protects against ischemic injury

Sex-specific responses to stroke

Stroke is a sexually dimorphic disease, with differences between males and females observed both clinically and in the laboratory. While males have a higher incidence of stroke throughout much of the lifespan, aged females have a higher burden of stroke. Sex differences in stroke result from a combination of factors, including elements intrinsic to the sex chromosomes as well as the effects of sex hormone exposure throughout the lifespan. Research investigating the sexual dimorphism of stroke is only in the beginning stages, but early findings suggest that different cell death pathways are activated in males and females after ischemic stroke. A greater understanding of the mechanisms underlying sex differences in stroke will lead to more appropriate treatment strategies for patients of both sexes.

Complexities of oestrogen in stroke

Evidence exists for the potential protective effects of circulating ovarian hormones in stroke, and oestrogen reduces brain damage in animal ischaemia models. However, a recent clinical trial indicated that HRT (hormone-replacement therapy) increased the incidence of stroke in post-menopausal women, and detrimental effects of oestrogen on stroke outcome have been identified in a meta-analysis of HRT trials and in pre-clinical research studies. Therefore oestrogen is not an agent that can be promoted as a potential stroke therapy. Many published reviews have reported the neuroprotective effects of oestrogen in stroke, but have failed to include information on the detrimental effects. This issue is addressed in the present review, along with potential mechanisms of action, and the translational capacity of pre-clinical research.

Mitochondrial mechanisms of estrogen neuroprotection

Mitochondria have become a primary focus in our search not only for the mechanism(s) of neuronal death but also for neuroprotective drugs and therapies that can delay or prevent Alzheimer's disease and other chronic neurodegenerative conditions. This is because mitochrondria play a central role in regulating viability and death of neurons, and mitochondrial dysfunction has been shown to contribute to neuronal death seen in neurodegenerative diseases. In this article, we review the evidence for the role of mitochondria in cell death and neurodegeneration and provide evidence that estrogens have multiple effects on mitochondria that enhance or preserve mitochondrial function during pathologic circumstances such as excitotoxicity, oxidative stress, and others. As such, estrogens and novel non-hormonal analogs have come to figure prominently in our efforts to protect neurons against both acute brain injury and chronic neurodegeneration.

Estradiol: a hormone with diverse and contradictory neuroprotective actions

The concept that estrogens exert important neuroprotective actions has gained considerable attention during the past decade. Numerous studies have provided a deep understanding of the seemingly contradictory actions of estrogens. We realize more than ever that the effects of estrogens (with and without simultaneous or sequential progestins) are diverse and sometimes opposite, depending on the use of different estrogenic and progestinic compounds, on different delivery routes, on different concentrations, on treatment sequence, and on the age and health status of the women who receive hormone therapy. During the past few years, we have gained an increasing appreciation of the impact of estrogens on the immune system and on inflammation. In addition, we have learned that estrogens cannot only protect against cell death, but can also stimulate the birth of new neurons. Here we posit the concept that estrogen's modulation of the immune status may be the basic mechanism that underlies its ability to protect against neurodegeneration and its powerful neuroregenerative actions. We hope that this update will encourage even richer dialogues between basic and clinical scientists to ensure that future clinical studies fully consider the information that can be derived from basic science studies. Only then will we have a better understanding of the impact of hormones on the menopausal and postmenopausal period in a woman's life.

Attenuation of subarachnoid hemorrhage-induced apoptotic cell death with 17 beta-estradiol. Laboratory investigation

OBJECT

Apoptosis is implicated in vasospasm and long-term sequelae of subarachnoid hemorrhage (SAH). The authors observed that 17beta-estradiol (E2) can attenuate cerebral vasospasm, lower endothelin-1 production, and preserve normal endothelial nitric oxide synthase expression by reduction of inducible NO synthase expression in experimental SAH. The authors investigated the potential antiapoptotic effects of E2 in an experimental rat model of SAH.

METHODS

The authors examined the antiapoptotic effects of E2 in a double-hemorrhage SAH model in male Sprague-Dawley rats. The rats underwent subcutaneous implantation of a Silastic tube containing corn oil either with or without E2, and some E2-treated animals also received ICI 182,780 (a nonselective estrogen receptor [ER] antagonist) for 7 days after SAH. The degree of vasospasm was determined by averaging the cross-sectional areas of the basilar artery 7 days after SAH. The expression of apoptotic indicators, including TNF-alpha, caspase 3, Bcl-2, Bax, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling (TUNEL), and cell death assays were used for detection of apoptosis.

RESULTS

Treatment with E2 significantly attenuated SAH-induced vasospasm. Seven days after the induction of SAH, positive TUNEL-staining was seen, and DNA fragmentation was increased in the dentate gyrus. Increased TNF-alpha and cleaved caspase-3 protein expression and decreased Bcl-2 protein expression in the dentate gyrus were also observed. These changes were reversed with E2-treatment but not in the presence of ICI 182,780. However, the expression of Bax did not change after SAH either with or without E2 treatment.

CONCLUSIONS

The authors found that E2 appears to confer an antiapoptotic effect that reduces secondary brain injury after SAH via estrogen receptor-dependent mechanisms. This finding provides support for possible future applications of E2 treatment for the reduction of secondary injury after SAH in patients.

Neuroprotective effects of estrogen therapy for cognitive and neurobiological profiles of monkey models of menopause

Many postmenopausal women question whether to start or continue hormone therapy because of recent clinical trial negative results. However, evidence from other studies of postmenopausal women, and from studies in menopausal monkeys, indicate that estrogen has neurocognitive protective effects, particularly when therapy is initiated close to the time of menopause before neural systems become increasingly compromised with age. In this review, we present studies of menopausal women and female monkeys that support the concept that estrogen therapies protect both cognitive function and neurobiological processes.

Dose-related neuroprotective versus neurodamaging effects of estrogens in rat cerebral ischemia: a systematic analysis

Numerous studies of the effects of estrogens for stroke prevention have yielded conflicting results in human and animal studies alike. We present a systematical analysis of study design and methodological differences between 66 studies where estrogens' impact on ischemic brain damage in rat models has been investigated, providing evidence that the differences in results may be explained by high estrogen doses produced by slow-release pellets. These pellets have been used in all studies showing increased neurologic damage because of estrogens. Our data indicate that the increased neurologic damage is related to the pellets' plasma concentration profile with an early, prolonged, supraphysiological peak. Neither the method of inducing the ischemic brain lesions, the choice of variables for measuring outcome, the measured plasma concentrations of estrogens at the time of ischemia nor rat population attributes (sex, strain, age, and diseases) are factors contributing to the discrepancies in results. This suggests that the effects of estrogens for stroke prevention are concentration related with a complex dose-response curve, and underscores the importance of carefully validating the experimental methods used. Future studies of hormone-replacement therapy in women may have to take dosage and administration regimens into account.

Amyloid precursor protein 17-mer peptide ameliorates hippocampal neurodegeneration in ovariectomized rats

Amyloid precursor protein 17-mer peptide (APP 17-mer peptide) is an active fragment of amyloid precursor protein (APP) in the nervous system that mediates various neuronal activities and functions. Estrogen deprivation during menopause disproportionately increases the risk of many neurodegenerative diseases, including Alzheimer's disease (AD). Currently, therapeutic approaches to treat Alzheimer's disease are less than effective. We have previously shown that APP 17-mer peptide participates in neuronal function in aged-hippocampal neurons. In this study, we investigate the effects of estrogen and APP 17-mer peptide on hippocampal neurodegeneration in ovariectomized rats. The results showed that decreases in learning and memory function in ovariectomized rats were associated with degenerative changes in hippocampal neurons. Estrogen deprivation also enhances apoptotic cell death and decreases expression of the anti-apoptotic protein Bcl-2. Administration of APP 17-mer peptide ameliorates changes associated with estrogen deprivation without affecting estrogen levels. These results indicate that APP 17-mer peptide may prevent neurodegeneration caused by estrogen deficiency. Our findings also suggest that estrogen deficiency-induced neurodegeneration is regulated by activation of an intracellular "cross talk" signaling pathway, connecting neurotrophins with APP 17-mer peptide.

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 effects of estrogens following ischemic stroke

Our laboratory has investigated whether and how 17beta-estradiol (E(2)) protects the brain against neurodegeneration associated with cerebrovascular stroke. We have discovered that low, physiological concentrations of E(2), which are strikingly similar to low-basal circulating levels found in cycling mice, dramatically protect the brain against stroke injury, and consequently revealed multiple signaling pathways and key genes that mediate protective action of E(2). Here we will review the discoveries comprising our current understanding of neuroprotective actions of estrogens against ischemic stroke. These findings may carry far reaching implications for improving the quality of life in aging populations.

Impact of sex steroids on neuroinflammatory processes and experimental multiple sclerosis

Synthetic and natural estrogens as well as progestins modulate neuronal development and activity. Neurons and glia are endowed with high-affinity steroid receptors. Besides regulating brain physiology, both steroids conciliate neuroprotection against toxicity and neurodegeneration. The majority of data derive from in vitro studies, although more recently, animal models have proven the efficaciousness of steroids as neuroprotective factors. Indications for a safeguarding role also emerge from first clinical trials. Gender-specific prevalence of degenerative disorders might be associated with the loss of hormonal activity or steroid malfunctions. Our studies and evidence from the literature support the view that steroids attenuate neuroinflammation by reducing the pro-inflammatory property of astrocytes. This effect appears variable depending on the brain region and toxic condition. Both hormones can individually mediate protection, but they are more effective in cooperation. A second research line, using an animal model for multiple sclerosis, provides evidence that steroids achieve remyelination after demyelination. The underlying cellular mechanisms involve interactions with astroglia, insulin-like growth factor-1 responses, and the recruitment of oligodendrocytes.

Cerebral microvascular endothelial cell Na/H exchange: evidence for the presence of NHE1 and NHE2 isoforms and regulation by arginine vasopressin

Blood-brain barrier (BBB) Na transporters are essential for brain water and electrolyte homeostasis. However, they also contribute to edema formation during the early hours of ischemic stroke by increased transport of Na from blood into brain across an intact BBB. We previously showed that a luminal BBB Na-K-Cl cotransporter is stimulated by hypoxia, aglycemia, and AVP and that inhibition of the cotransporter by intravenous bumetanide significantly reduces edema and infarct in the rat middle cerebral artery occlusion (MCAO) model of stroke. More recently, we found evidence that intravenous cariporide (HOE-642), a highly potent Na/H exchange inhibitor, also reduces brain edema after MCAO. The present study was conducted to investigate which Na/H exchange protein isoforms are present in BBB endothelial cells and to evaluate the effects of ischemic factors on BBB Na/H exchange activity. Western blot analysis of bovine cerebral microvascular endothelial cells (CMEC) and immunoelectron microscopy of perfusion-fixed rat brain revealed that Na/H exchanger isoforms 1 and 2 (NHE1 and NHE2) are present in BBB endothelial cells. Using microspectrofluorometry and the pH-sensitive dye BCECF, we found that hypoxia (2% O(2), 30 min), aglycemia (30 min), and AVP (1-200 nM, 5 min) significantly increased CMEC Na/H exchange activity, assessed as Na-dependent, HOE-642-sensitive H(+) flux. We found that AVP stimulation of CMEC Na/H exchange activity is dependent on intracellular Ca concentration and is blocked by V(1), but not V(2), vasopressin receptor antagonists. Our findings support the hypothesis that a BBB Na/H exchanger, possibly NHE1 and/or NHE2, is stimulated during ischemia to participate in cerebral edema formation.

Estradiol reverses a calcium-related biomarker of brain aging in female rats

An increase in L-type voltage-gated calcium channel (LTCC) current is a prominent biomarker of brain aging and is believed to contribute to cognitive decline and vulnerability to neuropathologies. Studies examining age-related changes in LTCCs have focused primarily on males, although estrogen (17beta-estradiol, E2) affects calcium-dependent activities associated with cognition. Therefore, to better understand brain aging in females, the effects of chronic E2 replacement on LTCC current activity in hippocampal neurons of young and aged ovariectomized rats were determined. The zipper slice preparation was used to expose cornu ammonis 1 (CA1) pyramidal neurons for recording LTCC currents using the cell-attached patch-clamp technique. We found that an age-related increase in LTCC current in neurons from control animals was prevented by E2 treatment. In addition, in situ hybridization revealed that within stratum pyramidale of the CA1 area, mRNA expression of the Ca(v)1.2 LTCC subunit, but not the Ca(v)1.3 subunit, was decreased in aged E2-treated rats. Thus, the reported benefits of E2 on cognition and neuronal health may be attributed, at least in part, to its age-related decrease in LTCC current.

Estradiol is a potent protective, restorative, and trophic factor after brain injury

Estrogens are a group of pleiotropic steroid hormones that exhibit diverse mechanisms of action in multiple physiologic systems. Over the past 30 years, biomedical science has begun to appreciate that endogenous estrogens and their receptors display important roles beyond the reproductive system. Our growing appreciation of novel, nonreproductive functions for estrogens has fundamentally contributed to our knowledge of their role in human health and disease. Recent findings from the Women's Health Initiative have caused clinicians and scientists to question whether estrogens are protective factors or risk factors. In light of the dichotomy between basic science and clinical studies, this review will attempt to reconcile differences between them. We will focus on studies from our laboratory and others highlighting the beneficial properties of the most abundant endogenous estrogen, 17beta-estradiol, using in vivo and in vitro models of cerebral ischemia and neuronal injury. These studies demonstrate that 17beta-estradiol powerfully protects the brain using multiple molecular mechanisms that promote: (1) decreased cell death, (2) increased neurogenesis, (3) an enhancement of neurotrophic support, and (4) the suppression of proinflammatory pathways.

Sex, sex steroids, and brain injury

Biologic sex and sex steroids are important factors in clinical and experimental stroke and traumatic brain injury (TBI). Laboratory data strongly show that progesterone treatment after TBI reduces edema, improves outcomes, and restores blood-brain barrier function. Clinical studies to date agree with these data, and there are ongoing human trials for progesterone treatment after TBI. Estrogen has accumulated an impressive reputation as a neuroprotectant when evaluated at physiologically relevant doses in laboratory studies of stroke, but translation to patients remains to be shown. The role of androgens in male stroke or TBI is understudied and important to pursue given the epidemiology of stroke and trauma in men. To date, male sex steroids remain largely evaluated at the bench rather than the bedside. This review evaluates key evidence and highlights the importance of the platform on which brain injury occurs (i.e., genetic sex and hormonal modulators).

Estrogen may contribute to ischemic tolerance through modulation of cellular stress-related proteins

Ischemic tolerance describes a phenomenon whereby subcritical stimuli evoke cellular protective mechanisms resulting in increased tolerance to subsequent ischemia. In the present study we propose that the cytoprotective effects attributed to 17beta-estradiol and tunicamycin in an in vivo rodent model of ischemia are reflected by changes in neuronal tissue levels of m-calpain, HSP70, GRP94 and GRP78. Rats pretreated with 17beta-estradiol, tunicamycin or both demonstrated dose-dependent reductions in infarct area following 4 h of permanent middle cerebral artery occlusion (MCAO). Western blot analysis revealed that 4 h of MCAO was associated with decreased cortical expression of HSP70 and m-calpain and increased expression of GRP78. Pretreatment with 12.5 microg/kg 17beta-estradiol did not change this pattern of protein expression following MCAO. While GRP94 expression was elevated in sham-operated rats pretreated with 17beta-estradiol, the ensuing ischemic tolerance did not appear to be mediated by changes in cellular stress proteins. Pretreatment with 50 microg/kg tunicamycin significantly reduced HSP70 in cortical tissue samples taken from sham-operated rats and appeared to attenuate the threshold for activation of m-calpain in rats undergoing 4 h of MCAO. Lastly, a combined treatment in which rats undergoing MCAO were pretreated with both tunicamycin (24 h prior) and 17beta-estradiol (30 min prior) was associated with an attenuated stress response as indicated by reduced expression of GRP78 and GRP94 when compared to saline-treated controls. The results of this study suggest that the ischemic tolerance observed following MCAO in rats pretreated with either 17beta-estradiol or tunicamycin is likely mediated in part through differential effects on cellular stress proteins.

Regulation of oestrogen receptor gene expression: new insights and novel mechanisms

17Beta-oestradiol is a pleiotropic hormone with organisational, activational and protective actions in both male and female mammals. It is responsible for numerous aspects of brain development, including sexual differentiation of the brain. The organisational effects of early oestrogen exposure are essential for long-lasting cognitive and behavioural functions. Oestradiol mediates its effects through several intracellular or membrane-associated receptor proteins. In the rodent cerebral cortex, oestrogen receptor (ER) expression, as demonstrated by hormone binding studies, is high early in postnatal life and declines precipitously as the animal approaches puberty. This decline is caused by a decreased expression of ERalpha mRNA. An understanding of the mechanisms involved in the regulation of ERalpha gene expression is critical for understanding the developmental as well as postpubertal expression of the ER. Despite recent data indicating the current hormone replacement therapies can be detrimental in older women, numerous animal studies have shown that the endogenous oestrogen, 17beta-oestradiol, is neuroprotective. Specifically, low levels of oestradiol protect the cortex from cell death caused by middle cerebral artery occlusion (MCAO). The attenuation of cell death by oestradiol in this model is mediated through an ERalpha-dependent mechanism. To this end, ERalpha expression is rapidly increased after MCAO, suggesting a return to the developmental programme of gene expression within neurones. One mechanism of suppressing gene expression is by the epigenetic modification of the promoter regions, which results in gene silencing. Of the epigenetic modifications utilised by cells, DNA methylation has been intensively studied as a mechanism by which genes are both permanently and reversibly silenced. Little is known about the mechanisms of ER gene regulation in the brain; however, in breast cancer cells, both ERalpha and ERbeta are down-regulated by promoter methylation, and subsequent binding of the methyl-CpG-binding protein, MeCP2. Data from our laboratory demonstrate that the promoters of the ERalpha gene are also methylated during development and after neuronal injury, suggesting a role of DNA methylation in regulating ER expression in the brain.

Maternal regulation of estrogen receptor alpha methylation

Advances in molecular biology have provided tools for studying the epigenetic factors that modulate gene expression. DNA methylation is an epigenetic modification that can have sustained effects on transcription and is associated with long-term gene silencing. In this review, we focus on the regulation of estrogen receptor alpha (ERalpha) expression by hormonal and environmental cues, the consequences of these cues for female maternal and sexual behavior, and recent studies that explore the role of DNA methylation in mediating these developmental effects, with particular focus on the mediating role of maternal care. The methylation status of ERalpha has implications for reproductive behavior, cancer susceptibility, and recovery from ischemic injury, suggesting an epigenetic basis for risk and resilience across the life span.

Progesterone blocks estrogen neuroprotection from kainate in middle-aged female rats

The neuroprotective effects of estrogen in young adult rodents are well established. Less well understood is how estrogen neuroprotection is affected by aging and interactions with progesterone. In this study, we investigated the effects of estrogen and continuous progesterone, both alone and in combination, on hippocampal neuron survival following kainate lesion in 14-month-old female rats entering reproductive senescence. Our results show that ovariectomy-induced hormone depletion did not significantly affect the extent of kainate-induced neuron loss. Treatment of ovariectomized rats with estrogen significantly reduced neuron loss, however this effect was blocked by co-administration of continuous progesterone. Treatment of ovariectomized rats with progesterone alone did not significantly affect kainate toxicity. These results provide new insight into factors that regulate estrogen neuroprotection, which has important implications for hormone therapy in postmenopausal women.

Focal cerebral ischemia in the TNFalpha-transgenic rat

BACKGROUND

To determine if chronic elevation of the inflammatory cytokine, tumor necrosis factor-alpha (TNFalpha), will affect infarct volume or cortical perfusion after focal cerebral ischemia.

METHODS

Transgenic (TNFalpha-Tg) rats overexpressing the murine TNFalpha gene in brain were prepared by injection of mouse DNA into rat oocytes. Brain levels of TNFalpha mRNA and protein were measured and compared between TNFalpha-Tg and non-transgenic (non-Tg) littermates. Mean infarct volume was calculated 24 hours or 7 days after one hour of reversible middle cerebral artery occlusion (MCAO). Cortical perfusion was monitored by laser-Doppler flowmetry (LDF) during MCAO. Cortical vascular density was quantified by stereology. Post-ischemic cell death was assessed by immunohistochemistry and regional measurement of caspase-3 activity or DNA fragmentation. Unpaired t tests or analysis of variance with post hoc tests were used for comparison of group means.

RESULTS

In TNFalpha-Tg rat brain, the aggregate mouse and rat TNFalpha mRNA level was fourfold higher than in non-Tg littermates and the corresponding TNFalpha protein level was increased fivefold (p

CONCLUSION

Chronic elevation of TNFalpha protein in brain increases susceptibility to ischemic injury but has no effect on vascular density. TNFalpha-Tg animals are more susceptible to apoptotic cell death after MCAO than are non-Tg animals. We conclude that the TNFalpha-Tg rat is a valuable new tool for the study of cytokine-mediated ischemic brain injury.

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MESH Headings

• Animals
• Apoptosis/genetics
• Apoptosis/immunology
• Brain/immunology
• Brain/pathology
• Brain/physiopathology
• Brain Infarction/immunology
• Brain Infarction/pathology
• Brain Infarction/physiopathology
• Brain Ischemia/immunology
• Brain Ischemia/physiopathology
• Caspase 3/metabolism
• Cerebral Arteries/immunology
• Cerebral Arteries/pathology
• Cerebral Arteries/physiopathology
• Disease Models, Animal
• Encephalitis/immunology
• Encephalitis/pathology
• Encephalitis/physiopathology
• Female
• Infarction, Middle Cerebral Artery/immunology
• Infarction, Middle Cerebral Artery/physiopathology
• Male
• Microcirculation/genetics
• Microcirculation/immunology
• Neurons/immunology
• Neurons/pathology
• RNA, Messenger/metabolism
• Rats
• Rats, Sprague-Dawley
• Rats, Transgenic
• Regional Blood Flow/genetics
• Regional Blood Flow/immunology
• Tumor Necrosis Factor-alpha/genetics
• Tumor Necrosis Factor-alpha/immunology
• Up-Regulation/genetics
• Up-Regulation/immunology

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Grants

• R01 NS039588 NINDS NIH HHS
• R01 NS047395 NINDS NIH HHS

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Affiliation(s)

L Creed Pettigrew Sanders-Brown Center on Aging, University of Kentucky, Lexington, Kentucky, USA Department of Neurology, University of Kentucky, Lexington, Kentucky, USA Veterans Administration (VA) Medical Center, Lexington, Kentucky, USA
Mark S Kindy Department of Neuroscience, Medical University of South Carolina, Charleston, South Carolina, USA Ralph H. Johnson VA Medical Center, Charleston, South Carolina, USA
Stephen Scheff Sanders-Brown Center on Aging, University of Kentucky, Lexington, Kentucky, USA
Joe E Springer Department of Physical Medicine & Rehabilitation, University of Kentucky, Lexington, Kentucky, USA
Richard J Kryscio Department of Statistics and School of Public Health, University of Kentucky, Lexington, Kentucky, USA
Yizhao Li Jinan Great Wall Hospital, Jinan, Shandong, PR China
David S Grass Xenogen Biosciences, Cranbury, New Jersey, USA

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Collaborators Collapse

Estradiol and neurodegenerative oxidative stress

Estradiol is a potent preventative against neurodegenerative disease, in part, by activating antioxidant defense systems scavenging reactive oxygen species, limiting mitochondrial protein damage, improving electron transport chain activity and reducing mitochondrial DNA damage. Estradiol also increases the activity of complex IV of the electron transport chain, improving mitochondrial respiration and ATP production under normal and stressful conditions. However, the high oxidative cellular environment present during neurodegeneration makes estradiol a poor agent for treatment of existing disease. Oxidative stress stimulates the production of the hydroperoxide-dependent hydroxylation of estradiol to the catecholestrogen metabolites, which can undergo reactive oxygen species producing redox cycling, setting up a self-generating toxic cascade offsetting any antioxidant/antiapoptotic effects generated by the parent estradiol. Additional disease-induced factors can further perpetuate this cycle. For example dysregulation of the catecholamine system could alter catechol-O-methyltransferase-catalyzed methylation, preventing removal of redox cycling catecholestrogens from the system enhancing pro-oxidant effects of estradiol.

Protein phosphatase 1, protein phosphatase 2A, and calcineurin play a role in estrogen-mediated neuroprotection

It is becoming increasingly clear that protein phosphatases are important modulators of cellular function and that disruption of these proteins are involved in neurodegenerative disease processes. Serine/threonine protein phosphatases (PP) such as protein phosphatase PP1, PP2A, and calcineurin are involved in hyperphosphorylation of tau- as well as beta-amyloid-induced cell death. We have previously shown serine/threonine protein phosphatases to be involved in estrogen-mediated neuroprotection. The purpose of this study was to delineate the role of PP1, PP2A, and calcineurin in the mechanism of estrogen mediated neuroprotection against oxidative stress and excitotoxicity. Treatment with protein phosphatases inhibitor II, endothall, or cyclosporin A, which are specific inhibitors of PP1, PP2A, and calcineurin, respectively, did not have an effect on cell viability. However, in combination, these inhibitors adversely affected cell survival, which suggests the importance of serine/threonine protein phosphatases in maintenance of cellular function. Inhibitors of PP1, PP2A, and calcineurin attenuated the protective effects of estrogen against glutamate-induced -neurotoxicity but did not completely abrogate the estrogen-mediated protection. The attenuation of estrogen-induced neuroprotection was achieved through decrease in the activity of theses serine/threonine phosphatases without the concomitant decrease in protein expression. In an animal model, transient middle cerebral artery occlusion caused a 50% decrease in levels of PP1, PP2A, and PP2B ipsilateral to the lesion in a manner that was prevented by estradiol pretreatment. Therefore, we conclude that in the face of cytotoxic challenges in vitro and in vivo, estrogens maintain the function of PP1, PP2A, and calcineurin.

Brain ischemia and ischemic blood-brain barrier as etiological factors in sporadic Alzheimer's disease

The development of neuronal death and amyloid plaques is a characteristic feature of ischemic- and Alzheimer-type dementia. An important aspect of neuronal loss and amyloid plaques are their topography and neuropathogenesis. This review was performed to present the hypothesis that different fragments of blood-borne amyloid precursor protein are able to enter the ischemic blood-brain barrier. Chronic disruption of the blood-brain barrier after ischemic injury was shown. As an effect of chronic ischemic blood-brain barrier injury, a visible connection of amyloid plaques with neurovasculature was observed. This neuropathology appears to have similar distribution and mechanisms to Alzheimer's disease. The usefulness of rival ischemic theory in elucidating the neuropathogenesis of amyloid plaques formation and neuronal death in Alzheimer's disorder is discussed.

Estradiol replacement enhances sleep deprivation-induced c-Fos immunoreactivity in forebrain arousal regions of ovariectomized rats

To understand how female sex hormones influence homeostatic mechanisms of sleep, we studied the effects of estradiol (E2) replacement on c-Fos immunoreactivity in sleep/wake-regulatory brain areas after sleep deprivation (SD) in ovariectomized rats. Adult rats were ovariectomized and implanted subcutaneously with capsules containing 17β-E2(10.5 μg; to mimic diestrous E2levels) or oil. After 2 wk, animals with E2capsules received a single subcutaneous injection of 17β-E2(10 μg/kg; to achieve proestrous E2levels) or oil; control animals with oil capsules received an oil injection. Twenty-four hours later, animals were either left undisturbed or sleep deprived by “gentle handling” for 6 h during the early light phase, and killed. E2treatment increased serum E2levels and uterus weights dose dependently, while attenuating body weight gain. Regardless of hormonal conditions, SD increased c-Fos immunoreactivity in all four arousal-promoting areas and four limbic and neuroendocrine nuclei studied, whereas it decreased c-Fos labeling in the sleep-promoting ventrolateral preoptic nucleus (VLPO). Low and high E2treatments enhanced the SD-induced c-Fos immunoreactivity in the laterodorsal subnucleus of the bed nucleus of stria terminalis and the tuberomammillary nucleus, and in orexin-containing hypothalamic neurons, with no effect on the basal forebrain and locus coeruleus. The high E2treatment decreased c-Fos labeling in the VLPO under nondeprived conditions. These results indicate that E2replacement modulates SD-induced or spontaneous c-Fos expression in sleep/wake-regulatory and limbic forebrain nuclei. These modulatory effects of E2replacement on neuronal activity may be, in part, responsible for E2's influence on sleep/wake behavior.

Sex differences in stroke

Increasing evidence suggests that sex differences exist in the etiology, presentation, treatment, and outcome from stroke. The reasons for these sex disparities are becoming increasingly explored, but large gaps still exist in our knowledge. Experimental studies over the past several years have demonstrated intrinsic sex differences both in vivo and in animal models which may have relevance to our understanding of stroke in clinical populations. A greater understanding of the differences and similarities between males and females with respect to the risk factors, pathophysiology, and response to stroke will facilitate the design of future clinical trials and enhance the development of treatment strategies to improve stroke care in both sexes. This article reviews the current literature on sex differences in stroke with an emphasis on the clinical data, incorporating an analysis of bench research as it pertains to the bedside.

Effects of phytoestrogen on mitochondrial structure and function of hippocampal CA1 region of ovariectomized rats

The present study was undertaken to evaluate whether estrogen deprivation might lead to mitochondrial alteration of hippocampal neurons of ovariectomized (OVX) rats, and to evaluate the protective effect of estrogen and phytoestrogen on the mitochondrial alteration. First, OVX rats were used to mimic the pathologic changes of neurodegeneration of postmenopausal female, and we looked into the alteration of the mitochondrial ultrastructure and ATP content of hippocampal CA1 region after ovariectomy on different phase by transmission electron microscope (TEM) and reversed-phase high-performance liquid chromatography (HPLC), and found the best phase points of the alteration of the mitochondrial ultrastructure and ATP content. Next, estrogen and phytoestrogen were administered to the OVX rats for the protective effects on the mitochondrial ultrastructure and ATP content. Meanwhile, the density, size, shape, and distribution parameters of mitochondrial ultrastructure were analyzed according to the morphometry principle. The experimental results presented that (1) The alteration of mitochondrial ultrastructure elicited by ovariectomy worsened with the days going on, and the changes were the most noteworthy in volume density (Vv), average surface area (S), specific surface area (delta), and particle dispersity (Clambdaz) on 12th day (P < 0.05 or P < 0.01). Moreover, there was no statistical significance of the numerical density (Nv) among the five groups in the first step experiment. (2) The treatment with estrogen, genistein (Gs), and ipriflavone (Ip) significantly reversed the effect elicited by ovariectomy on Vv, S, delta, Clambdaz, Nv, and particle average diameter (D) of mitochondria of hippocampal CA1 region (P < 0.05). (3) Furthermore, ATP content of hippocampal CA1 region after ovariectomy declined significantly on 7th day (P < 0.05), and estrogen and phytoestrogen could reverse the alteration (P < 0.05). Taken together, these results revealed that phytoestrogen may have a protective role against the neurodegeneration after menopause via protecting mitochondrial structure and functions. Phytoestrogen may be a good alternative as a novel therapeutic strategy for menopausal syndrome.

Estrogen actions on mitochondria--physiological and pathological implications

Estrogens are potent neuroprotective hormones and mitochondria are the site of cellular life-death decisions. As such, it is not surprising that we and others have shown that estrogens have remarkable effects on mitochondrial function. Herein we provide evidence for a primary effect of estrogens on mitochondrial function, achieved in part by the import of estrogen receptor beta (ERbeta) into the mitochondria where it mediates a number of estrogen actions on this vital organelle. ERbeta is imported into the mitochondria, through tethering to cytosolic chaperone protein and/or through direct interaction with mitochondrial import proteins. In the mitochondria, ERbeta can affect transcription of critical mitochondrial genes through the interaction with estrogen response elements (ERE) or through protein-protein interactions with mitochondrially imported transcription factors. The potent effects of estrogens on mitochondrial function, particularly during mitochondrial stress, argues for a role of estrogens in the treatment of mitochondrial defects in chronic neurodegenerative diseases like Alzheimer's disease (AD) and Parkinson's disease (PD) and more acute conditions of mitochondrial compromise, like cerebral ischemia and traumatic brain injury.

Estrogen receptor-alpha overexpression suppresses 17beta-estradiol-mediated vascular endothelial growth factor expression and activation of survival kinases

Estrogen and its receptors influence growth and differentiation by stimulating the production and secretion of growth factors. Our previous studies indicate an increased expression of estrogen receptor (ER)-alpha and decreased growth factor synthesis in the olfactory bulb of reproductive senescent female rats as compared with young animals. The present study tests the hypothesis that abnormal overexpression of ERalpha contributes to decreased growth factor synthesis. We developed the HeLa-Tet-On cell line stably transfected with ERalpha (HTERalpha) that expresses increasing amounts of ERalpha with increasing doses of doxycycline (Dox). Increasing doses of Dox had no effect on vascular endothelial growth factor (VEGF) secretion in HTERalpha cells. However, in the presence of 40 nm 17beta-estradiol, VEGF secretion increased in low-dose Dox-exposed HTERalpha cultures, which was attenuated by the ERalpha antagonist, 1,3-Bis(4-hydroxyphenyl)-4-methyl-5-[4-(2-piperidinylethoxy)phenol]1H-pyrazole dihydrochloride. However, at high-dose Dox and, consequently, high ERalpha levels, estradiol failed to increase VEGF. In the HeLa X6 cell line in which the Tet-On construct is upstream of an unrelated gene (Pitx2A), estradiol failed to induce VEGF at any Dox dose. Furthermore, in the HTERalpha cell line, estradiol selectively down-regulates phospho-ERK2 and phospho-Akt at high ERalpha expression. This study clearly demonstrates that the dose of receptor critically mediates estradiol's ability to regulate growth factors and survival kinases. The present data also support the hypothesis that 17beta-estradiol treatment to an ERalpha overexpressing system, such as the senescent brain, could reverse the normally observed beneficial effect of estrogen.

Dynamic regulation of estrogen receptor-alpha gene expression in the brain: a role for promoter methylation?

Estrogen has long been known to play an important role in coordinating the neuroendocrine events that control sexual development, sexual behavior and reproduction. Estrogen actions in other, non-reproductive areas of the brain have also been described. It is now known that estrogen can also influence learning, memory, and emotion and has neurotrophic and neuroprotective properties. The actions of estrogen are largely mediated through at least two intracellular estrogen receptors. Both estrogen receptor-alpha and estrogen receptor-beta are expressed in a wide variety of brain regions. Estrogen receptor-alpha (ERalpha), however, undergoes developmental and brain region-specific changes in expression. The precise molecular mechanisms that regulate its expression at the level of gene transcription are not well understood. Adding to the complexity of its regulation, the estrogen receptor gene contains multiple promoters that drive its expression. In the cortex in particular, the ERalpha mRNA expression is dynamically regulated during postnatal development and again following neuronal injury. Epigenetic modification of chromatin is increasingly being understood as a mechanism of neuronal gene regulation. This review examines the potential regulation of the ERalpha gene by such epigenetic mechanisms.

Non-genomic actions of estrogens and their interaction with genomic actions in the brain

Ligands for the nuclear receptor superfamily have at least two mechanisms of action: (a) classical transcriptional regulation of target genes (genomic mechanisms); and (b) non-genomic actions, which are initiated at the cell membrane, which could also impact transcription. Though transcriptional mechanisms are increasingly well understood, membrane-initiated actions of these ligands are incompletely understood. This has led to considerable debate over the physiological relevance of membrane-initiated actions of hormones versus genomic actions of hormones, with genomic actions predominating in the endocrine field. There is good evidence that the membrane-limited actions of hormones, particularly estrogens, involve the rapid activation of kinases and the release of calcium and that these are linked to physiologically relevant scenarios in the brain. We show evidence in this review, that membrane actions of estrogens, which activate these rapid signaling cascades, can also potentiate nuclear transcription in both the central nervous system and in non-neuronal cell lines. We present a theoretical scenario which can be used to understand this phenomenon. These signaling cascades may occur in parallel or in series but subsequently, converge at the modification of transcriptionally relevant molecules such as nuclear receptors and/or coactivators. In addition, other non-cognate hormones or neurotransmitters may also activate cascades to crosstalk with estrogen receptor-mediated transcription, though the relevance of this is less clear. The idea that coupling between membrane-initiated and genomic actions of hormones is a novel idea in neuroendocrinology and provides us with a unified view of hormone action in the central nervous system.

Epigenetic regulation of the estrogen receptor alpha promoter in the cerebral cortex following ischemia in male and female rats

Permanent middle cerebral artery occlusion (MCAO) causes neuronal cell death in the striatum and cortex. In rodents, estradiol treatment protects the cortex from cell death in an estrogen receptor alpha (ERalpha) dependent manner. ERalpha is only transiently expressed in the cortex during neonatal development and is very low in uninjured adult cortex. Following MCAO, ERalpha mRNA expression is upregulated in the cortex of female rats, but the mechanism of this increase is still unknown. It is also unknown whether a similar increase in ERalpha expression in seen in males. In the following studies, male and vehicle or estradiol-treated ovariectomized (OVX) female rats underwent MCAO to investigate the regulation of ERalpha expression after ischemia. Twenty-four hours after surgery, mRNA or genomic DNA was collected from 1 mm micropunches taken from 300 mum brain sections for quantitative reverse transcription-polymerase chain reaction (RT-PCR) or methylation-specific (MSP) PCR, respectively. Additionally, adjacent 20 mum sections were processed for ERalpha immunohistochemistry. In OVX females, ERalpha mRNA and protein were increased in the ischemic cortex, but unchanged in males. We hypothesized that this increase in ERalpha in females is due to a reversal of gene silencing by DNA methylation. Using MSP targeting of CpG islands within the 5' untranslated region (UTR) of the rat ERalpha gene, we found that ischemia decreased methylation in the ischemic cortex of both groups of females, but there was no change in methylation in males. Using chromatin immunoprecipitation, we found that MeCP2 associates with ERalpha 5'UTR corresponding with the methylation status of the promoter. These data are the first to demonstrate a difference in the regulation of ERalpha expression in response to MCAO between males and females and that methylation of the ERalpha gene corresponds with mRNA levels in the brain.

Estradiol attenuates the focal cerebral ischemic injury through mTOR/p70S6 kinase signaling pathway

We previously showed that estradiol prevents neuronal cell death through the activation of Akt and its downstream targets Bad and FKHR. This study investigated whether estradiol modulates the survival pathway through other downstream targets of Akt, including mammalian target of rapamycin (mTOR) and p70S6 kinase. It is known that mTOR is a downstream target of Akt and a central regulator of protein synthesis, cell growth, and cell cycle progression. Adult female rats were ovariectomied and treated with estradiol prior to middle cerebral artery occlusion (MCAO). Brains were collected 24h after MCAO and infarct volumes were analyzed. We confirmed that estradiol significantly reduces infarct volume and decreases the number of positive cells for TUNEL staining in the cerebral cortex. Brain injury-induced a decrease in phospho-mTOR and phospho-p70S6 kinase. Estradiol prevented the injury-induced decrease in Akt activation and phosphorylation of mTOR and p70S6 kinases, and the subsequent decrease in S6 phosphorylation. Our findings suggest that estradiol plays a potent protective role against brain injury by preventing the injury-induced decrease of mTOR and p70S6 kinase phosphorylation.

Hypoxia-induced apoptotic cell death is prevented by oestradiol via oestrogen receptors in the developing central nervous system

The neuroprotective effects of oestrogens have been demonstrated against a variety of insults, including excitotoxicity, oxidative stress and cerebral ischemia under certain conditions. However, the molecular mechanisms underlying oestrogen neuroprotection are still unclear. We aimed to determine whether 17beta-oestradiol (E(2)) administration post-hypoxia (p-hx) was neuroprotective and whether these actions were mediated through oestrogen receptors (ER). For this purpose, 12-embyonic day-old chickens were subjected to acute hypoxia [8% (O(2)), 60 min], followed by different reoxygenation periods. To test the neuroprotective effect of E(2) and its mechanism, embryos were injected 30 min after the end of hypoxia with E(2) alone or with ICI 182 780, a competitive antagonist of ER. Cytochrome c (cyt c) release, an indicator of mitochondrial apoptotic pathway, was measured by western blot in optic lobe cytosolic extracts. DNA fragmentation by TUNEL fluorescence and caspase-3 fragmentation by immunofluorescence were detected on optic lobe sections. Acute hypoxia produces a significant increase in cyt c release from mitochondria at 4 h p-hx, followed by an increase in TUNEL positive cells 2 h later (6 h p-hx). Administration of E(2) (0.5 mg/egg) produced a significant decrease in cytosolic cyt c levels at 4 h p-hx, in caspase-3 activation and in TUNEL positive cells at 6 h p-hx compared to vehicle treated embryos. In the E(2)-ICI 182 780 treated embryos, cyt c release, caspase-3 fragmentation and TUNEL positive cells were similar to the hypoxic embryos, thus suggesting the requirement of an E(2)-ER interaction for E(2) mediated neuroprotective effects. In conclusion, E(2) prevents hypoxia-induced cyt c release and posterior cell death and these effects are mediated by oestrogen receptors.

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.

Premenopausal Oophorectomy and the Risk for Dementia

Evaluation of: Rocca WA, Bower JH, Maraganore DM et al.: Increased risk of cognitive impairment or dementia in women who underwent oophorectomy before menopause. Neurology 69(11), 1074–1083 (2007). This study examines the incidence of dementia in a population of women who underwent unilateral or bilateral oophorectomy before menopause. Patients were drawn from the Mayo Clinic database and included women who had surgical removal of either one or both ovaries during a preceding 40-year period (1950–1987), as well as a reference group of women who did not undergo oophorectomy. Women who agreed to participate in the study were interviewed by phone and received a modified Telephone Interview for Cognition or a brief dementia questionnaire answered by a proxy if the subject was deceased or incapacitated. Women who had unilateral oophorectomy had a greater incidence of dementia as compared with surgical controls. In women with bilateral oophorectomies, the risk for dementia was increased in women who were younger at the time of surgery as well as in women who discontinued estrogen therapy before 50 years of age.

The selective oestrogen receptor modulator, LY362321, is not neuroprotective in a rat model of transient focal ischaemia

Selective oestrogen receptor modulators (SERMs) may offer improved alternatives to oestrogen as neuroprotectants in experimental stroke. The present study investigated the role of a novel SERM, LY362321, in a rat model of transient middle cerebral artery occlusion (MCAO). Female Sprague-Dawley rats were ovariectomised and began receiving daily s.c. injections of either 1 mg/kg (n = 13), 10 mg/kg (n = 14) of LY362321, or vehicle (n = 13). The left MCA was temporarily occluded (90 min), with cortical blood flow monitoring, at 12 days post ovariectomy. Sensorimotor function was assessed using a neurological score prior to the MCAO and daily for 3 days following the MCAO. Tissue was processed for infarct volume assessment using 2,3,5-triphenyltetra-zolium chloride staining. The results indicated that there were no significant differences amongst groups in cortical blood flow during the MCAO. Furthermore, there was no significant difference in infarct size amongst vehicle, 1, and 10 mg/kg treated animals: 22.9 +/- 5.0, 16.7 +/- 4.2, and 21.1 +/- 4.1, respectively, one-way anova [F(2,32) = 0.542, P = 0.587]. The MCAO induced a significant decline in neurological score in the vehicle group (from 14 to 7 at 24 h post-MCAO) but this was not significantly affected by LY362321 at either dose. In conclusion, pretreatment with a low or high dose of the novel SERM LY362321 did not significantly influence cerebral blood flow, infarct volume, or sensorimotor function in rats exposed to transient MCAO.

Tamoxifen neuroprotection in cerebral ischemia involves attenuation of kinase activation and superoxide production and potentiation of mitochondrial superoxide dismutase

The purpose of this study was to enhance our understanding of the mechanisms of neuronal death after focal cerebral ischemia and the neuroprotective effects of tamoxifen (TMX). The phosphorylation state of 31 protein kinases/signaling proteins and superoxide anion (O(2)(-)) production in the contralateral and ipsilateral cortex was measured after permanent middle cerebral artery occlusion (pMCAO) in ovariectomized rats treated with placebo or TMX. The study revealed that pMCAO modulated the phosphorylation of a number of kinases/proteins in the penumbra at 2 h after pMCAO. Of significant interest, phospho-ERK1/2 (pERK1/2) was elevated significantly after pMCAO. TMX attenuated the elevation of pERK1/2, an effect correlated with reduced infarct size. In situ detection of O(2)(-) production showed a significant elevation at 1-2 h after pMCAO in the ischemic cortex with enhanced oxidative damage detected at 24 h. ERK activation may be downstream of free radicals, a suggestion supported by the findings that cells positive for O(2)(-) had high pERK activation and that a superoxide dismutase (SOD) mimetic, tempol, significantly attenuated pERK activation after MCAO. TMX treatment significantly reduced the MCAO-induced elevation of O(2)(-) production, oxidative damage, and proapoptotic caspase-3 activation. Additionally, pMCAO induced a significant reduction in the levels of manganese SOD (MnSOD), which scavenge O(2)(-), an effect largely prevented by TMX treatment, thus providing a potential mechanistic basis for the antioxidant effects of TMX. As a whole, these studies suggest that TMX neuroprotection may be achieved via an antioxidant mechanism that involves enhancement of primarily MnSOD levels, with a corresponding reduction of O(2)(-) production, and downstream kinase and caspase-3 activation.

Estrogens and progesterone as neuroprotectants: what animal models teach us

Estradiol and progesterone are two steroid hormones that target a variety of organ systems, including the heart, the bone and the brain. With respect to the latter, a large volume of basic science studies support the neuroprotective role of estradiol and/or progesterone. In fact, the results of such studies prompted the assessment of these hormones as protective agents against such disorders as Alzheimer's disease, stroke and traumatic brain injury. Interestingly, results from the Women's Health Initiative (WHI) yielded results that appeared to be inconsistent with the data derived from in vitro and in vivo models. However, we argue that the results from the basic science studies were not inconsistent with the clinical trials, but rather, are consistent with, and may even have predicted, the results from the WHI. To illustrate this point, we review here certain in vivo paradigms that have been used to assess the protective effects of estrogens and progesterone, and describe how the results from these animal models point to the importance of the type of hormone, the age of the subjects and the method of hormone administration, in determining whether or not hormones are neuroprotective.

Shear stress and 17β-estradiol modulate cerebral microvascular endothelial Na-K-Cl cotransporter and Na/H exchanger protein levels

Ion transporters of blood-brain barrier (BBB) endothelial cells play an important role in regulating the movement of ions between the blood and brain. During ischemic stroke, reduction in cerebral blood flow is accompanied by transport of Na and Cl from the blood into the brain, with consequent brain edema formation. We have shown previously that a BBB Na-K-Cl cotransporter (NKCC) participates in ischemia-induced brain Na and water uptake and that a BBB Na/H exchanger (NHE) may also participate. While the abrupt reduction of blood flow is a prominent component of ischemia, the effects of flow on BBB NKCC and NHE are not known. In the present study, we examined the effects of changes in shear stress on NKCC and NHE protein levels in cerebral microvascular endothelial cells (CMECs). We have shown previously that estradiol attenuates both ischemia-induced cerebral edema and CMEC NKCC activity. Thus, in the present study, we also examined the effects of estradiol on NKCC and NHE protein levels in CMECs. Exposing CMECs to steady shear stress (19 dyn/cm2) increased the abundance of both NKCC and NHE. Estradiol abolished the shear stress-induced increase in NHE but not NKCC. Abrupt reduction of shear stress did not alter NKCC or NHE abundance in the absence of estradiol, but it decreased NKCC abundance in estradiol-treated cells. Our results indicate that changes in shear stress modulate BBB NKCC and NHE protein levels. They also support the hypothesis that estradiol attenuates edema formation in ischemic stroke in part by reducing the abundance of BBB NKCC protein.

More than a decade of estrogen neuroprotection

Considerable evidence has emerged through more than a decade of research supporting the neuroprotective and cognition-preserving effects of estrogens. Such basic research coupled with various epidemiological studies led quickly to the assessment of Premarin for the treatment of mild to moderate Alzheimer's disease (AD), initiated by the Alzheimer's Disease Cooperative Study Group and headed by Dr. Leon Thal. While this and subsequent trials with Premarin (Wyeth Research, Monmouth Junction, New Jersey) and PremPro (Wyeth Research), a conjugated equine estrogen preparation plus medoxyprogresterone acetate, have not supported the use of estrogens in treating advanced AD, considerable inferences have been made from these placebo controlled trials of estrogens. Here, we aimed to put these AD trials of estrogens in perspective by considering the potential mechanisms of these potent neuroprotective estrogens, the role of estrogens in other neurodegenerative conditions, such as cerebral ischemia, and based on our current understanding of estrogen neurobiology, offer insight into the design of future clinical trails of estrogens for neuronal protection.

Effects of estrogen on lifespan and motor functions in female hSOD1 G93A transgenic mice

Amyotrophic lateral sclerosis (ALS) is a progressive disease which is caused by degeneration of motor neurons in the central nervous system. The incidence of ALS is higher in men than women, but the female advantage disappears with increased age. Here, we report evidence that the female advantage is due to the protective role of estrogen. In an ALS mouse model carrying the human Cu/Zn superoxide dismutase (hSOD1) G93A transgene, ovariectomy did not alter the onset age of the disease while reducing the female lifespan by 7 days and making it comparable to that of the male transgenic mice. Treatment of ovariectomized females with 17beta-estradiol (E2) did not delay the onset of disease, but prevented progression of ALS motor dysfunctions as shown by extension reflex test for a limited time window. Importantly, E2 treatment rescued the lifespans in overiectomized females. These findings will provide important new insights to interpretation of disease progression in post-menopausal female ALS patients.