Estrogen preconditioning protects the hippocampal CA1 against ischemia

Adaptation to oxidative stress at cellular and tissue level

Several in vitro and in vivo investigations have already proved that cells and tissues, when pre-exposed to low oxidative stress by different stimuli such as chemical, physical agents and environmental factors, display more resistance against subsequent stronger ischaemic injuries, resulting in an adaptive response known as ischaemic preconditioning (IPC). The aim of this review is to report the most recent knowledge about the complex adaptive mechanisms, including signalling transduction pathways, antioxidant systems, apoptotic and inflammation pathways, underlying cell protection against oxidative damage. In addition, an update about in vivo adaptation strategies in response to ischaemic/reperfusion episodes and brain trauma is also given.

COVID-19: Oxidative Preconditioning as a Potential Therapeutic Approach

This Article summarizes the likely benefits of central nervous system oxidative preconditioning in the reduction of COVID-19 based on its putative pathogenesis. The current COVID-19 outbreak caused a pandemic with millions of infected patients and death cases worldwide. The clinical features of severe acute respiratory syndrome coronavirus (SARS-CoV) was initially linked with respiratory disorders, but recent studies have reported alterations of neurological and cerebrovascular functions in COVID-19 patients. The main viral infection features are related to cell death, inflammation, and cytokine generation, which can be associated with the dysregulation of redox systems or oxidative stress. However, until now, there is no available and effective therapeutic approach. Thus, it is necessary to search for care and adequate protection against the disease, especially for susceptible and vulnerable groups. Preconditioning, a well-known antioxidative stress and anti-inflammatory approach, is protective against many neurological age-related disorders. COVID-19 severity and morbidity have been observed in elderly patients. The aim of the present study is to elucidate the possible protective role of oxidative preconditioning in aged patients at high risk of developing severe COVID-19 complications.

Review Cerebral Ischemic Tolerance and Preconditioning: Methods, Mechanisms, Clinical Applications, and Challenges

Stroke is one of the leading causes of morbidity and mortality worldwide, and it is increasing in prevalence. The limited therapeutic window and potential severe side effects prevent the widespread clinical application of the venous injection of thrombolytic tissue plasminogen activator and thrombectomy, which are regarded as the only approved treatments for acute ischemic stroke. Triggered by various types of mild stressors or stimuli, ischemic preconditioning (IPreC) induces adaptive endogenous tolerance to ischemia/reperfusion (I/R) injury by activating a multitude cascade of biomolecules, for example, proteins, enzymes, receptors, transcription factors, and others, which eventually lead to transcriptional regulation and epigenetic and genomic reprogramming. During the past 30 years, IPreC has been widely studied to confirm its neuroprotection against subsequent I/R injury, mainly including local ischemic preconditioning (LIPreC), remote ischemic preconditioning (RIPreC), and cross preconditioning. Although LIPreC has a strong neuroprotective effect, the clinical application of IPreC for subsequent cerebral ischemia is difficult. There are two main reasons for the above result: Cerebral ischemia is unpredictable, and LIPreC is also capable of inducing unexpected injury with only minor differences to durations or intensity. RIPreC and pharmacological preconditioning, an easy-to-use and non-invasive therapy, can be performed in a variety of clinical settings and appear to be more suitable for the clinical management of ischemic stroke. Hoping to advance our understanding of IPreC, this review mainly focuses on recent advances in IPreC in stroke management, its challenges, and the potential study directions.

Sex differences in Hippocampal Memory and Learning following Neonatal Brain Injury: Is There a Role for Estrogen Receptor-α?

Neonatal encephalopathy due to hypoxia-ischemia (HI) leads to severe, life-long morbidities in thousands of neonates born in the USA and worldwide each year. Varying capacities of long-term episodic memory, verbal working memory, and learning can present without cerebral palsy and have been associated with the severity of neonatal encephalopathy sustained at birth. Among children who sustain a moderate degree of HI at birth, girls have larger hippocampal volumes compared to boys. Clinical studies indicate that female neonatal brains are more resistant to the effects of neonatal HI, resulting in better long-term cognitive outcomes compared to males with comparable brain injury. Our most recent mechanistic studies have addressed the origins and cellular basis of sex differences in hippocampal neuroprotection following neonatal HI-related brain injury and implicate estrogen receptor-α (ERα) in the neurotrophin receptor-mediated hippocampal neuroprotection in female mice. This review summarizes the recent findings on ERα-dependent, neurotrophin-mediated hippocampal neuroprotection and weighs the evidence that this mechanism plays an important role in preservation of long-term memory and learning following HI in females.

Optimized Model of Cerebral Ischemia In situ for the Long-Lasting Assessment of Hippocampal Cell Death

Among all the brain, the hippocampus is the most susceptible region to ischemic lesion, with the highest vulnerability of CA1 pyramidal neurons to ischemic damage. This damage may cause either prompt neuronal death (within hours) or with a delayed appearance (over days), providing a window for applying potential therapies to reduce or prevent ischemic impairments. However, the time course when ischemic damage turns to neuronal death strictly depends on experimental modeling of cerebral ischemia and, up to now, studies were predominantly focused on a short time-window—from hours to up to a few days post-lesion. Using different schemes of oxygen-glucose deprivation (OGD), the conditions taking place upon cerebral ischemia, we optimized a model of mimicking ischemic conditions in organotypical hippocampal slices for the long-lasting assessment of CA1 neuronal death (at least 3 weeks). By combining morphology and electrophysiology, we show that prolonged (30-min duration) OGD results in a massive neuronal death and overwhelmed astrogliosis within a week post-OGD whereas OGD of a shorter duration (10-min) triggered programmed CA1 neuronal death with a significant delay—within 2 weeks—accompanied with drastically impaired CA1 neuron functions. Our results provide a rationale toward optimized modeling of cerebral ischemia for reliable examination of potential treatments for brain neuroprotection, neuro-regeneration, or testing neuroprotective compounds in situ.

Non-coding RNAs and neuroprotection after acute CNS injuries

Accumulating evidence indicates that various classes of non-coding RNAs (ncRNAs) including microRNAs (miRNAs), PIWI-interacting RNAs (piRNAs) and long non-coding RNAs (lncRNAs) play important roles in normal state as well as the diseases of the CNS. Interestingly, ncRNAs have been shown to interact with messenger RNA, DNA and proteins, and these interactions could induce epigenetic modifications and control transcription and translation, thereby adding a new layer of genomic regulation. The ncRNA expression profiles are known to be altered after acute CNS injuries including stroke, traumatic brain injury and spinal cord injury that are major contributors of morbidity and mortality worldwide. Hence, a better understanding of the functional significance of ncRNAs following CNS injuries could help in developing potential therapeutic strategies to minimize the neuronal damage in those conditions. The potential of ncRNAs in blood and CSF as biomarkers for diagnosis and/or prognosis of acute CNS injuries has also gained importance in the recent years. This review highlighted the current progress in the understanding of the role of ncRNAs in initiation and progression of secondary neuronal damage and their application as biomarkers after acute CNS injuries.

Effects of sex steroid hormones and their metabolites on neuronal injury caused by oxygen-glucose deprivation/reoxygenation in organotypic hippocampal slice cultures

In this study, protective actions of the sex steroid hormones, progesterone, testosterone, and 17β-estradiol, against oxygen-glucose deprivation (OGD)/reoxygenation-induced neuronal cell death were examined using rat organotypic hippocampal slice cultures. Progesterone, testosterone, and 17β-estradiol significantly attenuated neuronal cell death elicited by OGD/reoxygenation. While the neuroprotection conferred by progesterone was not affected by SU-10603, an inhibitor of cytochrome P45017α, finasteride, a 5α-reductase inhibitor that blocks the conversion of progesterone to allopregnanolone, partially reversed the neuroprotection induced by progesterone. The progesterone metabolite, allopregnanolone attenuated neuronal injury induced by OGD/reoxygenation. Pretreatment with letrozole, a cytochrome P450 aromatase inhibitor or 4-hydroxyphenyl-1-naphthol, a 17β-hydroxysteroid dehydrogenase 2 inhibitor showed no effect on testosterone-mediated neuroprotection, while finasteride completely abolished the protective action of testosterone. Treatment with 5α-dihydrotestosterone significantly suppressed neuronal injury. Pretreatment with mifepristone, a progesterone receptor antagonist and hydroxyflutamid, an androgen receptor antagonist significantly diminished the neuroprotective effects of progesterone and testosterone, respectively. ICI182,780, an estrogen receptor antagonist, showed no effect on neuroprotection mediated by 17β-estradiol. Pretreatment with actinomycin D or cycloheximide clearly abolished the neuroprotective effects of progesterone and testosterone, while actinomycin D and cycloheximide did not show any effect on neuroprotection mediated by 17β-estradiol. Taken together, progesterone protects neurons via progesterone receptor-dependent genomic pathway, and allopregnanolone is involved in progesterone-mediated neuroprotection. Testosterone and its metabolite 5α-dihydrotestosterone protect neurons via the genomic pathway of the androgen receptor. Metabolism of sex steroid hormones in the brain might complicate their protective actions in the brain.

Inhibition of p38 pathway-dependent MPTP-induced dopaminergic neurodegeneration in estrogen receptor alpha knockout mice

Approximately, 7-10 million people in the world suffer from Parkinson's disease (PD). Recently, increasing evidence has suggested the protective effect of estrogens against nigrostriatal dopaminergic damage in PD. In this study, we investigated whether estrogen affects 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced behavioral impairment in estrogen receptor alpha (ERα)-deficient mice. MPTP (15mg/kg, four times with 1.5-h interval)-induced dopaminergic neurodegeneration was evaluated in ERα wild-type (WT) and knockout (KO) mice. Larger dopamine depletion, behavioral impairments (Rotarod test, Pole test, and Gait test), activation of microglia and astrocytes, and neuroinflammation after MPTP injection were observed in ERα KO mice compared to those in WT mice. Immunostaining for tyrosine hydroxylase (TH) after MPTP injection showed fewer TH-positive neurons in ERα KO mice than WT mice. Levels of dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC, metabolite of dopamine) were also lowered in ERα KO mice after MPTP injection. Interestingly, a higher immunoreactivity for monoamine oxidase (MAO) B was found in the substantia nigra and striatum of ERα KO mice after MPTP injection. We also found an increased activation of p38 kinase (which positively regulates MAO B expression) in ERα KO mice. In vitro estrogen treatment inhibited neuroinflammation in 1-methyl-4-phenyl pyridium (MPP+)-treated cultured astrocyte cells; however, these inhibitory effects were removed by p38 inhibitor. These results indicate that ERα might be important for dopaminergic neuronal survival through inhibition of p38 pathway.

Sex differences in cell genesis, hippocampal volume and behavioral outcomes in a rat model of neonatal HI

Hypoxia-ischemia (HI) of the brain in near-term and term infants is a leading cause of infant mortality and lifelong disability but current therapeutic approaches remain limited. Males consistently display greater vulnerability to the deleterious consequences of HI in both humans and animal models. Neurogenesis increases after neonatal HI and offers a potential therapeutic target for recovery. The steroid hormone estradiol has been extensively explored as a neuroprotectant in adult models of stroke but with mixed results. Less consideration has been afforded to this naturally occurring agent in the developing brain, which has unique challenges from the adult. Using a model of term HI in the rat we have explored the impact of this insult on cell genesis in the hippocampus of males and females and the ability of estradiol treatment immediately after insult to restore function. Both short-term (3 days) and long-term (7 days) post-injury were assessed and revealed that only females had markedly increased cell genesis on the short-term but both sexes were increased long-term. A battery of behavioral tests revealed motor impairment in males and compromised episodic memory while both sexes were modestly impaired in spatial memory. Juvenile social play was also depressed in both sexes after HI. Estradiol therapy improved behavioral performance in both sexes but did not reverse a deficit in hippocampal volume ipsilateral to the insult. Thus the effects of estradiol do not appear to be via cell death or proliferation but rather involve other components of neural functioning.

Cerebral Ischemic Preconditioning: the Road So Far…

Cerebral preconditioning constitutes the brain's adaptation to lethal ischemia when first exposed to mild doses of a subtoxic stressor. The phenomenon of preconditioning has been largely studied in the heart, and data from in vivo and in vitro models from past 2-3 decades have provided sufficient evidence that similar machinery exists in the brain as well. Since preconditioning results in a transient protective phenotype labeled as ischemic tolerance, it can open many doors in the medical warfare against stroke, a debilitating cerebrovascular disorder that kills or cripples thousands of people worldwide every year. Preconditioning can be induced by a variety of stimuli from hypoxia to pharmacological anesthetics, and each, in turn, induces tolerance by activating a multitude of proteins, enzymes, receptors, transcription factors, and other biomolecules eventually leading to genomic reprogramming. The intracellular signaling pathways and molecular cascades behind preconditioning are extensively being investigated, and several first-rate papers have come out in the last few years centered on the topic of cerebral ischemic tolerance. However, translating the experimental knowledge into the clinical scaffold still evades practicality and faces several challenges. Of the various preconditioning strategies, remote ischemic preconditioning and pharmacological preconditioning appears to be more clinically relevant for the management of ischemic stroke. In this review, we discuss current developments in the field of cerebral preconditioning and then examine the potential of various preconditioning agents to confer neuroprotection in the brain.

17β-estradiol modulates gene expression in the female mouse cerebral cortex

17β-estradiol (E2) plays critical roles in a number of target tissues including the mammary gland, reproductive tract, bone, and brain. Although it is clear that E2 reduces inflammation and ischemia-induced damage in the cerebral cortex, the molecular mechanisms mediating the effects of E2 in this brain region are lacking. Thus, we examined the cortical transcriptome using a mouse model system. Female adult mice were ovariectomized and implanted with silastic tubing containing oil or E2. After 7 days, the cerebral cortices were dissected and RNA was isolated and analyzed using RNA-sequencing. Analysis of the transcriptomes of control and E2-treated animals revealed that E2 treatment significantly altered the transcript levels of 88 genes. These genes were associated with long term synaptic potentiation, myelination, phosphoprotein phosphatase activity, mitogen activated protein kinase, and phosphatidylinositol 3-kinase signaling. E2 also altered the expression of genes linked to lipid synthesis and metabolism, vasoconstriction and vasodilation, cell-cell communication, and histone modification. These results demonstrate the far-reaching and diverse effects of E2 in the cerebral cortex and provide valuable insight to begin to understand cortical processes that may fluctuate in a dynamic hormonal environment.

Low dose Hsp90 inhibitor 17AAG protects neural progenitor cells from ischemia induced death

Stress adaptation effect provides cell protection against ischemia induced apoptosis. Whether this mechanism prevents other types of cell death in stroke is not well studied. This is an important question for regenerative medicine to treat stroke since other types of cell death such as necrosis are also prominent in the stroke brain apart from apoptosis. We report here that treatment with 17-N-Allylamino-17-demethoxygeldanamycin (17AAG), an Hsp90 inhibitor, protected neural progenitor cells (NPCs) against oxygen glucose deprivation (OGD) induced cell death in a dose dependent fashion. Cell death assays indicated that 17AAG not only ameliorated apoptosis, but also necrosis mediated by OGD. This NPC protection was confirmed by exposing cells to oxidative stress, a major stress signal prevalent in the stroke brain. Mechanistic studies demonstrated that 17AAG activated PI3K/Akt and MAPK cell protective pathways. More interestingly, these two pathways were activated in vivo by 17AAG and 17AAG treatment reduced infarct volume in a middle cerebral artery occlusion (MCAO) stroke model. These data suggest that 17AAG protects cells against major cell death pathways and thus might be used as a pharmacological conditioning agent for regenerative medicine for stroke.

Periodic Estrogen Receptor-Beta Activation: A Novel Approach to Prevent Ischemic Brain Damage

In women, the risk for cerebral ischemia climbs rapidly after menopause. At menopause, production of ovarian hormones; i.e., progesterone and estrogen, slowly diminishes. Estrogen has been suggested to confer natural protection to premenopausal women from ischemic stroke and some of its debilitating consequences. This notion is also strongly supported by laboratory studies showing that a continuous chronic 17β-estradiol (E2; a potent estrogen) regimen protects brain from ischemic injury. However, concerns regarding the safety of the continuous intake of E2 were raised by the failed translation to the clinic. Recent studies demonstrated that repetitive periodic E2 pretreatments, in contrast to continuous E2 treatment, provided neuroprotection against cerebral ischemia in ovariectomized rats. Periodic E2 pretreatment protects hippocampal neurons through activation of estrogen receptor subtype beta (ER-β). Apart from neuroprotection, periodic activation of ER-β in ovariectomized rats significantly improves hippocampus-dependent learning and memory. Difficulties in learning and memory loss are the major consequence of ischemic brain damage. Periodic ER-β agonist pretreatment may provide pharmacological access to a protective state against ischemic stroke and its debilitating consequences. The use of ER-β-selective agonists constitutes a safer target for future research than ER-α agonist or E2, inasmuch as it lacks the ability to stimulate the proliferation of breast or endometrial tissue. In this review, we highlight ER-β signaling as a guide for future translational research to reduce cognitive decline and cerebral ischemia incidents/impact in post-menopausal women, while avoiding the side effects produced by chronic E2 treatment.

Pharmacologic preconditioning: translating the promise

A transient, ischemia-resistant phenotype known as "ischemic tolerance" can be established in brain in a rapid or delayed fashion by a preceding noninjurious "preconditioning" stimulus. Initial preclinical studies of this phenomenon relied primarily on brief periods of ischemia or hypoxia as preconditioning stimuli, but it was later realized that many other stressors, including pharmacologic ones, are also effective. This review highlights the surprisingly wide variety of drugs now known to promote ischemic tolerance, documented and to some extent mechanistically characterized in preclinical animal models of stroke. Although considerably more experimentation is needed to thoroughly validate the ability of any currently identified preconditioning agent to protect ischemic brain, the fact that some of these drugs are already clinically approved for other indications implies that the growing enthusiasm for translational success in the field of pharmacologic preconditioning may be well justified.

17β-estradiol increases expression of the oxidative stress response and DNA repair protein apurinic endonuclease (Ape1) in the cerebral cortex of female mice following hypoxia

While it is well established that 17β-estradiol (E2) protects the rodent brain from ischemia-induced damage, it has been unclear how this neuroprotective effect is mediated. Interestingly, convincing evidence has also demonstrated that maintaining or increasing the expression of the oxidative stress response and DNA repair protein apurinic endonuclease 1 (Ape1) is instrumental in reducing ischemia-induced damage in the brain. Since E2 increases expression of the oxidative stress response proteins Cu/Zn superoxide dismutase and thioredoxin in the brain, we hypothesized that E2 may also increase Ape1 expression and that this E2-induced expression of Ape1 may help to mediate the neuroprotective effects of E2 in the brain. To test this hypothesis, we utilized three model systems including primary cortical neurons, brain slice cultures, and whole animals. Although estrogen receptor α and Ape1 were expressed in primary cortical neurons, E2 did not alter Ape1 expression in these cells. However, immunofluorescent staining and quantitative Western blot analysis demonstrated that estrogen receptor α and Ape1 were expressed in the nuclei of cortical neurons in brain slice cultures and that E2 increased Ape1 expression in the cerebral cortex of these cultures. Furthermore, Ape1 expression was increased and oxidative DNA damage was decreased in the cerebral cortices of ovariectomized female C57Bl/6J mice that had been treated with E2 and exposed to hypoxia. Taken together, our studies demonstrate that the neuronal microenvironment may be required for increased Ape1 expression and that E2 enhances expression of Ape1 and reduces oxidative DNA damage, which may in turn help to reduce ischemia-induced damage in the cerebral cortex and mediate the neuroprotective effects of E2.

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.

Periodic 17β-estradiol pretreatment protects rat brain from cerebral ischemic damage via estrogen receptor-β

Although chronic 17β-estradiol (E2) has been shown to be a cognition-preserving and neuroprotective agent in animal brain injury models, concern regarding its safety was raised by the failed translation of this phenomenon to the clinic. Previously, we demonstrated that a single bolus of E2 48 hr prior to ischemia protected the hippocampus from damage in ovariectomized rats via phosphorylation of cyclic-AMP response element binding protein, which requires activation of estrogen receptor subtype beta (ER-β). The current study tests the hypothesis that long-term periodic E2-treatment improves cognition and reduces post-ischemic hippocampal injury by means of ER-β activation. Ovariectomized rats were given ten injections of E2 at 48 hr intervals for 21 days. Hippocampal-dependent learning, memory and ischemic neuronal loss were monitored. Results demonstrated that periodic E2 treatments improved spatial learning, memory and ischemic neuronal survival in ovariectomized rats. Additionally, periodic ER-β agonist treatments every 48 hr improved post-ischemic cognition. Silencing of hippocampal ER-β attenuated E2-mediated ischemic protection suggesting that ER-β plays a key role in mediating the beneficial effects of periodic E2 treatments. This study emphasizes the need to investigate a periodic estrogen replacement regimen to reduce cognitive decline and cerebral ischemia incidents/impact in post-menopausal women.

Sex differences in white matter alterations accompanying obstructive sleep apnea

STUDY OBJECTIVES

Females with obstructive sleep apnea (OSA) show different psychological and physiological symptoms from males, which may be associated with sex-related variations in neural injury occurring with the disorder. To determine whether male- or female-specific brain injury is present in OSA, we assessed influences of sex on white matter changes in the condition.

DESIGN

Two-group factorial.

SETTING

University medical center.

PATIENTS OR PARTICIPANTS

80 subjects total, with newly diagnosed, untreated OSA groups of 10 female (age mean ± SE: 52.6 ± 2.4 years, AHI 22.5 ± 4.1 events/h) and 20 male (age 48.9 ± 1.7, AHI 25.5 ± 2.9) patients, and 20 female (age 50.3 ± 1.7) and 30 male (age 49.2 ± 1.4) healthy control subjects.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Brain fiber integrity was assessed with fractional anisotropy (FA), a diffusion tensor imaging-derived measure. Sleep quality, daytime sleepiness, depression, and anxiety were assessed with questionnaires. We identified regions of differing injury in male versus female OSA patients by assessing brain regions with significant interaction effects of OSA and sex on FA. Areas of sex-specific, OSA-related FA reductions appeared in females relative to males, including in the bilateral cingulum bundle adjacent to the mid hippocampus, right stria terminalis near the amygdala, prefrontal and posterior-parietal white matter, corpus callosum, and left superior cerebellar peduncle. Females with OSA showed higher daytime sleepiness, anxiety and depression levels, and reduced sleep quality.

CONCLUSIONS

Sex differences in white matter structural integrity appeared in OSA patients, with females more affected than males. These female-specific structural changes may contribute to or derive from neuropsychological and physiological symptom differences between sexes.

From rapid to delayed and remote postconditioning: the evolving concept of ischemic postconditioning in brain ischemia

Ischemic postconditioning is a concept originally defined to contrast with that of ischemic preconditioning. While both preconditioning and postconditioning confer a neuroprotective effect on brain ischemia, preconditioning is a sublethal insult performed in advance of brain ischemia, and postconditioning, which conventionally refers to a series of brief occlusions and reperfusions of the blood vessels, is conducted after ischemia/reperfusion. In this article, we first briefly review the history of preconditioning, including the experimentation that initially uncovered its neuroprotective effects and later revealed its underlying mechanisms-of-action. We then discuss how preconditioning research evolved into that of postconditioning--a concept that now represents a broad range of stimuli or triggers, including delayed postconditioning, pharmacological postconditioning, remote postconditioning--and its underlying protective mechanisms involving the Akt, MAPK, PKC and K(ATP) channel cell-signaling pathways. Because the concept of postconditioning is so closely associated with that of preconditioning, and both share some common protective mechanisms, we also discuss whether a combination of preconditioning and postconditioning offers greater protection than preconditioning or postconditioning alone.

Neuroprotective role of estradiol against neuronal death induced by glucose deprivation in cultured rat hippocampal neurons

Studies have reported the protective effect of estradiol (E(2)) against neuronal death induced by several insults including oxygen deprivation, mitochondrial toxins and activation of glutamate receptors. Glucose deprivation (GD) is associated with ischemia and hypoglycemia, and to date there is no effective therapeutic agent able to prevent neuronal damage induced by these conditions. In this study, we have investigated the effects of 17β-E(2) and the selective agonists of the alpha (ERα) and beta (ERβ) estrogen receptors, propyl pyrazole triol (PPT) and diarylpropionitrile (DPN), respectively, on neuronal death induced by GD in cultured rat hippocampal neurons. We have also analyzed the expression of both ER isoforms after GD. Results show that GD for 2 and 4 h reduces cell survival by 42 and 55%, respectively. Treatment with 17β-E(2) (10 nM to 10 µM) induces a dose-dependent protective effect that is blocked by ICI 182,780, an ER antagonist, and by 1,3-bis(4-hydroxyphenyl)-4-methyl-5-[4-(-piperidinylethoxy)phenol]-1H'pyrazole dihydrochloride (MPP) and 4-[2-phenyl-5,7-bis(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-3-yl]phenol (PHTPP), selective ERα and ERβ antagonists, respectively. The ERα and ERβ agonists PPT and DPN show a similar neuroprotective effect to that of 17β-E(2), but DPN is more efficient. In addition, hippocampal neurons under normal conditions show a higher expression of the ERβ isoform. When exposed to GD during 4 h, the expression of both ER isoforms is increased, while only that of the ERβ isoform significantly increases after 2 h of GD. Results demonstrate that E(2) prevents neuronal death induced by GD through its interaction with ER, although the ERβ isoform might have a predominant role. Results also suggest that GD differentially alters the expression of ERα and ERβ in hippocampal neurons.

Isoflurane preconditioning protects neurons from male and female mice against oxygen and glucose deprivation and is modulated by estradiol only in neurons from female mice

The volatile anesthetic, isoflurane, can protect the brain if administered before an insult such as an ischemic stroke. However, this protective "preconditioning" response to isoflurane is specific to males, with females showing an increase in brain damage following isoflurane preconditioning and subsequent focal cerebral ischemia. Innate cell sex is emerging as an important player in neuronal cell death, but its role in the sexually dimorphic response to isoflurane preconditioning has not been investigated. We used an in vitro model of isoflurane preconditioning and ischemia (oxygen and glucose deprivation, OGD) to test the hypotheses that innate cell sex dictates the response to isoflurane preconditioning and that 17β-estradiol attenuates any protective effect from isoflurane preconditioning in neurons via nuclear estrogen receptors. Sex-segregated neuron cultures derived from postnatal day 0-1 mice were exposed to either 0% or 3% isoflurane preconditioning for 1 h. In separate experiments, 17β-estradiol and the non-selective estrogen receptor antagonist ICI 182,780 were added 24 h before preconditioning and then removed at the end of the preconditioning period. Twenty-three hours after preconditioning, all cultures underwent 2 h of OGD. Twenty-four hours following OGD, cell viability was quantified using calcein-AM fluorescence. We observed that isoflurane preconditioning increased cell survival following subsequent OGD regardless of innate cell sex, but that the presence of 17β-estradiol before and during isoflurane preconditioning attenuated this protection only in female neurons independent of nuclear estrogen receptors. We also found that independent of preconditioning treatment, female neurons were less sensitive to OGD compared with male neurons and that transient treatment with 17β-estradiol protected both male and female neurons from subsequent OGD. More studies are needed to determine how cell type, cell sex, and sex steroids like 17β-estradiol may impact on anesthetic preconditioning and subsequent ischemic outcomes in the brain.

Nicotine and estrogen synergistically exacerbate cerebral ischemic injury

The greater incidence of myocardial infarction, cardiac arrest, and ischemic stroke among women who smoke and use oral contraception (OC) compared to women who do not smoke and who do or do not use OC may be due in part to how nicotine influences endocrine function in women. For example, we recently demonstrated that chronic exposure to nicotine, the addictive agent in tobacco smoke responsible for the elevated risk of cardiac arrest, abolishes the endogenous or exogenous 17β-estradiol-conferred protection of the hippocampus against global cerebral ischemia (a potential outcome of cardiac arrest) in naive or ovariectomized female rats. In the current study we examined the hypotheses that (1) a synergistic deleterious effect of nicotine plus oral contraceptives exacerbates post-ischemic hippocampal damage in female rats, and (2) nicotine directly inhibits estrogen-mediated intracellular signaling in the hippocampus. To test first hypothesis and to simulate smoking behavior-induced nicotine levels in the human body, we implanted osmotic pumps containing nicotine in the female rats for 16 days. Furthermore, we mimicked the use of oral contraceptives in females by administering oral contraceptives orally to the rat. Rats exposed to either nicotine alone or in combination with oral contraceptives were subjected to an episode of cerebral ischemia and the resultant brain damage was quantified. These results showed for the first time that nicotine with oral contraceptives did indeed exacerbate post-ischemic CA1 damage as compared to nicotine alone in naive female rats. In ex vivo hippocampal slice cultures, we found that nicotine alone or with 17β-estradiol directly hinders estrogen receptors-mediated phosphorylation of cyclic-AMP element binding protein, a process required for neuronal survival and also exacerbates ischemic damage. Thus, nicotine can affect the outcome of cerebral ischemia by influencing brain endocrine function directly rather than through indirect systemic effects.

S100b counteracts neurodegeneration of rat cholinergic neurons in brain slices after oxygen-glucose deprivation

Alzheimer's disease is a severe chronic neurodegenerative disorder characterized by beta-amyloid plaques, tau pathology, cerebrovascular damage, inflammation, reactive gliosis, and cell death of cholinergic neurons. The aim of the present study is to test whether the glia-derived molecule S100b can counteract neurodegeneration of cholinergic neurons after oxygen-glucose deprivation (OGD) in organotypic brain slices of basal nucleus of Meynert. Our data showed that 3 days of OGD induced a marked decrease of cholinergic neurons (60% of control), which could be counteracted by 50 μg/mL recombinant S100b. The effect was dose and time dependent. Application of nerve growth factor or fibroblast growth factor-2 was less protective. C-fos-like immunoreactivity was enhanced 3 hours after OGD indicating metabolic stress. We conclude that S100b is a potent neuroprotective factor for cholinergic neurons during ischemic events.

Estrogen-receptor-mediated protection of cerebral endothelial cell viability and mitochondrial function after ischemic insult in vitro

Protective effects of estrogen against experimental stroke and neuronal ischemic insult are well-documented, but it is not known whether estrogen prevents ischemic injury to brain endothelium, a key component of the neurovascular unit. Increasing evidence indicates that estrogen exerts protective effects through mitochondrial mechanisms. We previously found 17beta-estradiol (E2) to improve mitochondrial efficiency and reduce mitochondrial superoxide in brain blood vessels and endothelial cells. Thus we hypothesized E2 will preserve mitochondrial function and protect brain endothelial cells against ischemic damage. To test this, an in vitro ischemic model, oxygen-glucose deprivation (OGD)/reperfusion, was applied to immortalized mouse brain endothelial cells (bEnd.3). OGD/reperfusion-induced cell death was prevented by long-term (24, 48 h), but not short-term (0.5, 12 h), pretreatment with 10 nmol/L E2. Protective effects of E2 on endothelial cell viability were mimicked by an estrogen-receptor (ER) agonist selective for ERalpha (PPT), but not by one selective for ERbeta (DPN). In addition, E2 significantly decreased mitochondrial superoxide and preserved mitochondrial membrane potential and ATP levels in early stages of OGD/reperfusion. All of the E2 effects were blocked by the ER antagonist, ICI-182,780. These findings indicate that E2 can preserve endothelial mitochondrial function and provide protection against ischemic injury through ER-mediated mechanisms.

Preconditioning-induced ischemic tolerance: a window into endogenous gearing for cerebroprotection

Ischemic tolerance defines transient resistance to lethal ischemia gained by a prior sublethal noxious stimulus (i.e., preconditioning). This adaptive response is thought to be an evolutionarily conserved defense mechanism, observed in a wide variety of species. Preconditioning confers ischemic tolerance if not in all, in most organ systems, including the heart, kidney, liver, and small intestine. Since the first landmark experimental demonstration of ischemic tolerance in the gerbil brain in early 1990's, basic scientific knowledge on the mechanisms of cerebral ischemic tolerance increased substantially. Various noxious stimuli can precondition the brain, presumably through a common mechanism, genomic reprogramming. Ischemic tolerance occurs in two temporally distinct windows. Early tolerance can be achieved within minutes, but wanes also rapidly, within hours. Delayed tolerance develops in hours and lasts for days. The main mechanism involved in early tolerance is adaptation of membrane receptors, whereas gene activation with subsequent de novo protein synthesis dominates delayed tolerance. Ischemic preconditioning is associated with robust cerebroprotection in animals. In humans, transient ischemic attacks may be the clinical correlate of preconditioning leading to ischemic tolerance. Mimicking the mechanisms of this unique endogenous protection process is therefore a potential strategy for stroke prevention. Perhaps new remedies for stroke are very close, right in our cells.

Resveratrol preconditioning induces cellular stress proteins and is mediated via NMDA and estrogen receptors

Resveratrol pretreatment has been shown to provide neuroprotection in models of cerebral ischemia. This phenomenon, commonly termed preconditioning, promotes ischemic tolerance and may involve mild activation of endoplasmic reticulum stress pathways in the affected tissue. Systemic injection of resveratrol (2 x 10(-3), 2 x 10(-4), 1 x 10(-4) mg/kg) 30 min prior to a 4 h period of right middle cerebral artery occlusion significantly reduced infarct area in the insular region of rat prefrontal cortex. This affect was blocked when resveratrol treatment was combined with a non-selective estrogen receptor antagonist, or preceded by intracortical injection of an NMDA receptor antagonist. The neuroprotective effect of resveratrol was associated with reduced renal sympathetic nerve activity as well as induction of resident endoplasmic reticulum chaperone proteins, glucose-regulated proteins 78 and 94. The calcium-sensitive chaperone heat shock protein 70 and the cysteine protease m calpain did not respond to resveratrol pretreatment. However, a significant induction of heat shock protein 70 was observed in the contralateral cortex of resveratrol pretreated rats following 4 h of right middle cerebral artery occlusion. These data suggest that resveratrol preconditioning promotes ischemic tolerance in the short term, in part via effects mediated through activation of estrogen and NMDA receptors, as well as through mild activation of cellular stress proteins.

Pretreatment with a single estradiol-17beta bolus activates cyclic-AMP response element binding protein and protects CA1 neurons against global cerebral ischemia

Estradiol-17beta is released from the ovaries in a cyclic manner during the normal estrous cycle in rats. During the transition from the diestrous to proestrous stage, the 17beta-estradiol increases in blood circulation. We hypothesized that a higher serum level of endogenous 17beta-estradiol would protect hippocampal pyramidal neurons against global cerebral ischemia via activation of the cyclic-AMP response element binding protein (CREB)-mediated signaling cascade. Furthermore, we asked if a single 17beta-estradiol bolus provides protection against ischemia in the absence of endogenous estradiol. To test these hypotheses, rats were subjected to global cerebral ischemia at different stages of the estrous cycle. Ischemia was produced by bilateral carotid occlusion and systemic hypotension. Brains were examined for histopathology at 7 days of reperfusion. Higher serum levels of 17beta-estradiol (at proestrus and estrus stages) correlated with increased immunoreactivity of pCREB in hippocampus and ischemic tolerance. At diestrus, when circulating gonadal hormone concentrations were lowest, the pCREB protein content of hippocampus was reduced and showed the least number of normal neurons after ischemia compared to other stages of the estrous cycle. A similar phosphorylation pattern was also observed for mitogen-activated protein kinase (MAPK) and calcium-calmodulin-dependent protein kinase (CaMKII) in hippocampus. The cyclic variation in ovarian hormones did not reflect phosphorylation of protein kinase B (Akt). To test the efficacy of a single bolus of 17beta-estradiol before ischemia, ovariectomized rats were treated with 17beta-estradiol (5/10/50 microg/kg) or vehicle (oil) and 48/72/96 h later rats were exposed to cerebral ischemia. A single 17beta-estradiol bolus treatment in ovariectomized rats significantly increased CREB mRNA activation and protected CA1 pyramidal neurons against ischemia. These results suggest that an exogenous bolus of 17beta-estradiol to ovariectomized rats protects hippocampus against ischemia via activation of the CREB pathway in a manner similar to the endogenous estrous cycle.

17beta-estradiol-mediated neuroprotection and ERK activation require a pertussis toxin-sensitive mechanism involving GRK2 and beta-arrestin-1

17-beta-Estradiol (E2) is a steroid hormone involved in numerous bodily functions, including several brain functions. In particular, E2 is neuroprotective against excitotoxicity and other forms of brain injuries, a property that requires the extracellular signal-regulated kinase (ERK) pathway and possibly that of other signaling molecules. The mechanism and identity of the receptor(s) involved remain unclear, although it has been suggested that E2 receptor alpha (ERalpha) and G proteins are involved. We, therefore, investigated whether E2-mediated neuroprotection and ERK activation were linked to pertussis toxin (PTX)-sensitive G-protein-coupled effector systems. Biochemical and image analysis of organotypic hippocampal slices and cortical neuronal cultures showed that E2-mediated neuroprotection as well as E2-induced ERK activation were sensitive to PTX. The sensitivity to PTX suggested a possible role of G-protein- and beta-arrestin-mediated mechanisms. Western immunoblots from E2-treated cortical neuronal cultures revealed an increase in phosphorylation of both G-protein-coupled receptor-kinase 2 and beta-arrestin-1, a G-protein-coupled receptor adaptor protein. Transfection of neurons with beta-arrestin-1 small interfering RNA prevented E2-induced ERK activation. Coimmunoprecipitation experiments indicated that E2 increased the recruitment of beta-arrestin-1 and c-Src to ERalpha. These findings suggested that ERalpha is regulated by a mechanism associated with receptor desensitization and downregulation. In support of this idea, we found that E2 treatment of cortical synaptoneurosomes resulted in internalization of ERalpha, whereas treatment of cortical neurons with the ER agonists E-6-BSA-FITC [beta-estradiol-6-(O-carboxymethyl)oxime-bovine serum albumin conjugated with fluorescein isothiocyanate] and E-6-biotin [1,3,5(10)-estratrien-3,17beta-diol-6-one-6-carboxymethloxime-NH-propyl-biotin] resulted in agonist internalization. These results demonstrate that E2-mediated neuroprotection and ERK activation involve ERalpha activation of G-protein- and beta-arrestin-mediated mechanisms.

Effects of global cerebral ischemia in the pregnant rat

BACKGROUND AND PURPOSE

Stroke during pregnancy is an emerging concern. Although females undergo many physiological, endocrine, and neurological alterations during pregnancy, the consequences of such changes on outcome after stroke are unclear. It is predicted that increases in steroid hormones observed during pregnancy may confer protective effects against the neurological and pathological sequelae of stroke.

METHODS

We therefore investigated behavioral and histological consequences of a global cerebral ischemia (2-vessel occlusion; 2VO), and how these outcomes correlated with pregnancy-related changes in hormones in Sprague-Dawley rats.

RESULTS

After the 2VO, pregnant rats exhibited poorer memory in a contextual fear conditioning test of learning and memory than sham-treated controls, whereas nonpregnant rats did not. They also showed enhanced CA1 hippocampal neuronal injury. This susceptibility to damage is despite significant pregnancy-associated hypothermia and is probably not associated with alterations in 17beta-estradiol or corticosterone levels.

CONCLUSIONS

These findings are the first to show enhanced neuronal damage in pregnant animals after global cerebral ischemia. They also suggest that the mechanism may be independent of changes in estrogen, corticosterone, and body temperature.

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.

Molecular physiology of preconditioning-induced brain tolerance to ischemia

Ischemic tolerance describes the adaptive biological response of cells and organs that is initiated by preconditioning (i.e., exposure to stressor of mild severity) and the associated period during which their resistance to ischemia is markedly increased. This topic is attracting much attention because preconditioning-induced ischemic tolerance is an effective experimental probe to understand how the brain protects itself. This review is focused on the molecular and related functional changes that are associated with, and may contribute to, brain ischemic tolerance. When the tolerant brain is subjected to ischemia, the resulting insult severity (i.e., residual blood flow, disruption of cellular transmembrane gradients) appears to be the same as in the naive brain, but the ensuing lesion is substantially reduced. This suggests that the adaptive changes in the tolerant brain may be primarily directed against postischemic and delayed processes that contribute to ischemic damage, but adaptive changes that are beneficial during the subsequent test insult cannot be ruled out. It has become clear that multiple effectors contribute to ischemic tolerance, including: 1) activation of fundamental cellular defense mechanisms such as antioxidant systems, heat shock proteins, and cell death/survival determinants; 2) responses at tissue level, especially reduced inflammatory responsiveness; and 3) a shift of the neuronal excitatory/inhibitory balance toward inhibition. Accordingly, an improved knowledge of preconditioning/ischemic tolerance should help us to identify neuroprotective strategies that are similar in nature to combination therapy, hence potentially capable of suppressing the multiple, parallel pathophysiological events that cause ischemic brain damage.

Estrogen-mediated neuroprotection in the cortex may require NMDA receptor activation

Several studies have suggested that a potential mechanism for estrogen-mediated neuroprotection following experimental stroke is a result of modulating glutamate-mediated excitotoxicity. Our laboratory has shown that in male rats, estrogen injection (systemic or direct intracortical injection) resulted in an immediate depolarization of cortical neurons. Therefore, the present study was designed to investigate whether the estrogen-induced depolarization of cortical neurons was required in mediating the early events associated with this neuroprotection. We tested this hypothesis by co-injecting selective antagonists of the NMDA (MK-801) or AMPA (DNQX) glutamatergic receptors with estrogen. Systemic injection of estrogen significantly attenuated the MK-801-induced decrease in infarct volume following middle cerebral artery occlusion (MCAO). Similarly, when estrogen and MK-801 were co-injected directly into the cortex, no neuroprotection was observed. However, when estrogen or MK-801 was injected centrally 10 min prior to the injection of the other drug, significant neuroprotection was observed. This led us to hypothesize that estrogen-mediated neuroprotection required an initial activation of NMDA receptors. Furthermore, our results suggest that this estrogen-mediated neuroprotection was also associated with a significant increase in m-calpain and activation of an endoplasmic reticulum (ER) specific caspase-12. Finally, the results of current clamp experiments showed that estrogen significantly depolarized cortical neurons as well as enhanced NMDA-induced depolarization. Taken together, these results suggest that estrogen pretreatment may activate NMDA receptors resulting in modification of ER-associated molecular mechanisms involved in neuroprotection following MCAO.