Regionally specific modulation of brain apolipoprotein E in the mouse during the estrous cycle and by exogenous 17β estradiol

Effects of DHA (omega-3 fatty acid) and estradiol on amyloid β-peptide regulation in the brain

In the early stages of sporadic Alzheimer's disease (SAD), there is a strong correlation between memory impairment and cortical levels of soluble amyloid-β peptide oligomers (Aβ). It has become clear that Aβ disrupt glutamatergic synaptic function, which can in turn lead to the characteristic cognitive deficits of SAD, but the actual pathways are still not well understood. This opinion article describes the pathogenic mechanisms underlying cerebral amyloidosis. These mechanisms are dependent on the amyloid precursor protein and concern the synthesis of Aβ peptides with competition between the non-amyloidogenic pathway and the amyloidogenic pathway (i.e. a competition between the ADAM10 and BACE1 enzymes), on the one hand, and the various processes of Aβ residue clearance, on the other hand. This clearance mobilizes both endopeptidases (NEP, and IDE) and removal transporters across the blood-brain barrier (LRP1, ABCB1, and RAGE). Lipidated ApoE also plays a major role in all processes. The disturbance of these pathways induces an accumulation of Aβ. The description of the mechanisms reveals two key molecules in particular: (i) free estradiol, which has genomic and non-genomic action, and (ii) free DHA as a preferential ligand of PPARα-RXRα and PPARɣ-RXRα heterodimers. DHA and free estradiol are also self-regulating, and act in synergy. When a certain level of chronic DHA and free estradiol deficiency is reached, a permanent imbalance is established in the central nervous system. The consequences of these deficits are revealed in particular by the presence of Aβ peptide deposits, as well as other markers of the etiology of SAD.

Alzheimer’s Disease and Premature Ovarian Insufficiency

Estradiol promotes neuronal growth, transmission, survival, myelinization, plasticity, synaptogenesis, and dendritic branching and it improves cognitive function. Alzheimer’s disease (AD) is characterized by amyloid plaques, neurofibrillary tangles, and the loss of neuronal connection in the brain. Genomic analysis has concluded that hypoestrogenism influences the APOE gene and increases the risk of AD. Premature ovarian insufficiency (POI) is defined as oligo/amenorrhea in women below 40 years of age, low estradiol, and high-gonadotropin levels. Early symptoms and signs of POI must be detected in time in order to prevent subsequent complications, such as Alzheimer’s disease. Meta-analysis has shown favorable effects of estrogen in preventing Alzheimer’s. We measured some of the typical markers of AD in women with POI such as interleukin 6 (IL-6), interleukin 8 (IL-8), tissue necrosis factor α (TNFα), TAU1, TREM2, and amyloid precursor proteins (APP). While FSH, LH, and IL-8 were significantly higher in POI group, compared to controls, testosterone and DHEAS were lower. A significant decrease in IL-6 was found in the POI group during a 6-month therapy, as well as an increase in amyloid precursor proteins. CONCLUSION: Neurological complications of POI, such as declining short-term memory, cognitive function, and dementia, have to be promptly stopped by initiating estro-progestogen therapy in POI. A long-term continuation of the therapy would be strongly advised.

Role of estrogen in women's Alzheimer's disease risk as modified by APOE

Alzheimer's disease (AD) is characterized by numerous sexual dimorphisms that impact the development, progression, and probably the strategies to prevent and treat the most common form of dementia. In this review, we consider this topic from a female perspective with a specific focus on how women's vulnerability to the disease is affected by the individual and interactive effects of estrogens and apolipoprotein E (APOE) genotype. Importantly, APOE appears to modulate systemic and neural outcomes of both menopause and estrogen-based hormone therapy. In the brain, dementia risk is greater in APOE4 carriers, and the impacts of hormone therapy on cognitive decline and dementia risk vary according to both outcome measure and APOE genotype. Beyond the CNS, estrogen and APOE genotype affect vulnerability to menopause-associated bone loss, dyslipidemia and cardiovascular disease risk. An emerging concept that may link these relationships is the possibility that the effects of APOE in women interact with estrogen status by mechanisms that may include modulation of estrogen responsiveness. This review highlights the need to consider the key AD risk factors of advancing age in a sex-specific manner to optimize development of therapeutic approaches for AD, a view aligned with the principle of personalized medicine.

Use of Hormone Therapy in Postmenopausal Women with Alzheimer's Disease: A Systematic Review

BACKGROUND

Around two-thirds of patients with Alzheimer's disease (AD) are women, which could be related to the depletion of female sexual hormones at menopause. The replacement of these hormones with hormone therapy (HT) to possibly decrease AD risk or treat AD patients has generated conflicting results in the literature.

OBJECTIVE

Our aim was to systematically review the relationship between HT use in postmenopausal women with AD and the risk of developing or treating AD symptoms.

DATA SOURCES

The PubMed, LILACS, Scopus, Scielo, and Web of Science databases were searched from January 1994 to December 2020 using the descriptors 'Alzheimer Disease OR Alzheimer's Disease' and 'Hormone Replacement Therapy OR Estrogen Replacement Therapy'.

STUDY SELECTION

Observational and controlled clinical trials including postmenopausal women diagnosed with AD and evaluating HT efficacy were eligible for inclusion.

DATA EXTRACTION

Extracted data comprise study design, covariates, inclusion criteria for sample selection, AD diagnosis criteria, biases, HT regimen, and cognitive measurement tools used.

RESULTS

Overall, 25 studies were selected. Among the 14 observational studies, 8 reported an improvement in cognitive function and a decrease in AD risk, especially in younger postmenopausal women. Five observational studies did not demonstrate any association between HT and AD, and one study reported an increase in AD risk, regardless of time of HT initiation. Of the 11 controlled clinical trials included, 7 showed an amelioration in cognitive function after HT. The remaining 4 trials saw no difference between HT and control.

CONCLUSION

Both observational and controlled clinical trials had methodological issues and discrepancies in inclusion criteria and HT protocols. These inconsistencies made it difficult to establish an association between HT and AD.

The Effect of Estrogen Replacement Therapy on Alzheimer's Disease and Parkinson's Disease in Postmenopausal Women: A Meta-Analysis

Background: Estrogen replacement therapy (ERT) is a common treatment method for menopausal syndrome; however, its therapeutic value for the treatment of neurological diseases is still unclear. Epidemiological studies were performed, and the effect of postmenopausal ERT on treating neurodegenerative diseases, including Alzheimer's disease (AD) and Parkinson's disease (PD), was summarized through a meta-analysis.

Methods: Twenty-one articles were selected using a systematic searching of the contents listed on PubMed and Web of Science before June 1, 2019. Epidemiological studies were extracted, and relevant research data were obtained from the original articles based on the predefined inclusion criteria and data screening principles. The Comprehensive Meta-Analysis Version 2 software was used to pool effective size, test heterogeneity, conduct meta-regression and subgroup analysis, and to calculate publication bias.

Results: Our results showed that ERT significantly decreased the risk of onset and/or development of AD [odds ratio (OR): 0.672; 95% CI: 0.581–0.779; P < 0.001] and PD (OR: 0.470; 95% CI: 0.368–0.600; P < 0.001) compared with the control group. A subgroup and meta-regression analysis showed that study design and measure of effect were the source of heterogeneity. Age, sample size, hormone therapy ascertainment, duration of the treatment, or route of administration did not play a significant role in affecting the outcome of the meta-analysis.

Conclusion: We presented evidence here to support the use of estrogen therapy for the treatment of AD and PD.

Sex and Gender Driven Modifiers of Alzheimer's: The Role for Estrogenic Control Across Age, Race, Medical, and Lifestyle Risks

Research indicates that after advanced age, the major risk factor for late-onset Alzheimer’s disease (AD) is female sex. Out of every three AD patients, two are females with postmenopausal women contributing to over 60% of all those affected. Sex- and gender-related differences in AD have been widely researched and several emerging lines of evidence point to different vulnerabilities that contribute to dementia risk. Among those being considered, it is becoming widely accepted that gonadal steroids contribute to the gender disparity in AD, as evidenced by the “estrogen hypothesis.” This posits that sex hormones, 17β-estradiol in particular, exert a neuroprotective effect by shielding females’ brains from disease development. This theory is further supported by recent findings that the onset of menopause is associated with the emergence of AD-related brain changes in women in contrast to men of the same age. In this review, we discuss genetic, medical, societal, and lifestyle risk factors known to increase AD risk differently between the genders, with a focus on the role of hormonal changes, particularly declines in 17β-estradiol during the menopause transition (MT) as key underlying mechanisms.

A Quarter Century of APOE and Alzheimer's Disease: Progress to Date and the Path Forward

Alzheimer's disease (AD) is considered a polygenic disorder. This view is clouded, however, by lingering uncertainty over how to treat the quasi "monogenic" role of apolipoprotein E (APOE). The APOE4 allele is not only the strongest genetic risk factor for AD, it also affects risk for cardiovascular disease, stroke, and other neurodegenerative disorders. This review, based mostly on data from human studies, ranges across a variety of APOE-related pathologies, touching on evolutionary genetics and risk mitigation by ethnicity and sex. The authors also address one of the most fundamental question pertaining to APOE4 and AD: does APOE4 increase AD risk via a loss or gain of function? The answer will be of the utmost importance in guiding future research in AD.

Estrogen receptor β in Alzheimer's disease: From mechanisms to therapeutics

Alzheimer's disease (AD) disproportionally affects women and men. The female susceptibility for AD has been largely associated with the loss of ovarian sex hormones during menopause. This review examines the current understanding of the role of estrogen receptor β (ERβ) in the regulation of neurological health and its implication in the development and intervention of AD. Since its discovery in 1996, research conducted over the last 15-20 years has documented a great deal of evidence indicating that ERβ plays a pivotal role in a broad spectrum of brain activities from development to aging. ERβ genetic polymorphisms have been associated with cognitive impairment and increased risk for AD predominantly in women. The role of ERβ in the intervention of AD has been demonstrated by the alteration of AD pathology in response to treatment with ERβ-selective modulators in transgenic models that display pronounced plaque and tangle histopathological presentations as well as learning and memory deficits. Future studies that explore the potential interactions between ERβ signaling and the genetic isoforms of human apolipoprotein E (APOE) in brain aging and development of AD-risk phenotype are critically needed. The current trend of lost-in-translation in AD drug development that has primarily been based on early-onset familial AD (FAD) models underscores the urgent need for novel models that recapitulate the etiology of late-onset sporadic AD (SAD), the most common form of AD representing more than 95% of the current human AD population. Combining the use of FAD-related models that generally have excellent face validity with SAD-related models that hold more reliable construct validity would together increase the predictive validity of preclinical findings for successful translation into humans.

Hormonal modulators of glial ABCA1 and apoE levels

Apolipoprotein E (apoE) is the major lipid carrier in the central nervous system. As apoE plays a major role in the pathogenesis of Alzheimer disease (AD) and also mediates repair pathways after several forms of acute brain injury, modulating the expression, secretion, or function of apoE may provide potential therapeutic approaches for several neurological disorders. Here we show that progesterone and a synthetic progestin, lynestrenol, significantly induce apoE secretion from human CCF-STTG1 astrocytoma cells, whereas estrogens and the progesterone metabolite allopregnanolone have negligible effects. Intriguingly, lynestrenol also increases expression of the cholesterol transporter ABCA1 in CCF-STTG1 astrocytoma cells, primary murine glia, and immortalized murine astrocytes that express human apoE3. The progesterone receptor inhibitor RU486 attenuates the effect of progestins on apoE expression in CCF-STTG1 astrocytoma cells but has no effect on ABCA1 expression in all glial cell models tested, suggesting that the progesterone receptor (PR) may participate in apoE but does not affect ABCA1 regulation. These results suggest that selective reproductive steroid hormones have the potential to influence glial lipid homeostasis through liver X receptor-dependent and progesterone receptor-dependent pathways.

Estrogen and the prefrontal cortex: towards a new understanding of estrogen's effects on executive functions in the menopause transition

Midlife decline in cognition, specifically in areas of executive functioning, is a frequent concern for which menopausal women seek clinical intervention. The dependence of executive processes on prefrontal cortex function suggests estrogen effects on this brain region may be key in identifying the sources of this decline. Recent evidence from rodent, nonhuman primate, and human subject studies indicates the importance of considering interactions of estrogen with neurotransmitter systems, stress, genotype, and individual life events when determining the cognitive effects of menopause and estrogen therapy.

Long-term effects of estradiol replacement in the olfactory system

Olfactory dysfunction often precedes other clinical symptoms in chronic neurodegenerative diseases like Alzheimer's disease and Parkinson's disease. Estrogen deficiency and apoE genotype are known risk factors in these diseases and these factors also affect olfaction. Therefore we examined the effects of estradiol replacement following ovariectomy on expression of apoE and markers of cell proliferation, neuronal maturation, synaptogenesis and reactive gliosis in the primary olfactory pathway of wild-type (WT) and apoE knockout (KO) mice. Estradiol replacement increased apoE staining in the olfactory nerve and glomerular layers. Estradiol increased astrocyte density and olfactory epithelium (OE) thickness regardless of the genotype. In addition estradiol treatment increased the number of mature neurons in the OE and glomerular synaptophysin in both genotypes, but the magnitude of increase was greater in the WT than in the KO mice. These data suggest that estrogen and apoE act synergistically to minimize the loss of mature sensory neurons and synapses following ovariectomy.

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

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

Gonadal steroids do not affect apolipoprotein E expression in aging mouse cerebral cortex

The allelic variant of apolipoprotein (Apo) E4 is a known risk factor for the development of most common late onset form of Alzheimer's disease (AD). As aging is associated with reduced circulating level of gonadal steroid hormones, hormone replacement therapies have been used for the possible treatment of AD. Both estrogen and testosterone have beneficial effects on brain due to interaction with apoE, but the underlying mechanism is still not clear. In this article, we report the effects of gonadectomy and hormone supplementation on apoE protein level in male and female mouse cerebral cortex during normal aging. We could not get any effect of gonadectomy and estradiol or testosterone treatment in adult and old mice of either sex. This suggests that during normal aging apoE protein level is not affected due to steroid hormone withdrawal or supplementation in the mouse cerebral cortex.

A review of estrogen receptor alpha gene (ESR1) polymorphisms, mood, and cognition

OBJECTIVE

There are significant individual differences in the extent to which mood and cognition change as a function of reproductive stage, menstrual phase, postpartum, and hormone therapy use. This review explores the extent to which variations or polymorphisms in the estrogen receptor alpha gene (ESR1) predict cognitive and mood outcomes.

METHODS

A literature search was conducted from 1995 to November 2009 through PubMed, Embase, and PsychINFO. Twenty-five manuscripts that summarize investigations of ESR1 in mental health were reviewed.

RESULTS

Among studies investigating ESR1 in relation to cognition, 11 of 14 case-control studies reported an association between ESR1 polymorphisms and risk for developing dementia. Three of four prospective cohort studies reported an association between ESR1 polymorphisms and significant cognitive decline. There are inconsistencies between case-control and cohort studies regarding whether specific ESR1 alleles increase or decrease the risk for cognitive dysfunction. The relationships between ESR1 and cognitive impairment tend to be specific to or driven by women and restricted to risk for Alzheimer disease rather than other dementia causes. Three of five studies examining ESR1 polymorphisms in relation to anxiety or depressive symptoms found significant associations. Significant associations have also been reported between ESR1 polymorphisms and childhood-onset mood disorder and premenstrual dysphoric disorder.

CONCLUSIONS

A strong relationship between ESR1 variants and cognitive outcomes is evident, and preliminary evidence suggests a role of the ESR1 gene in certain mood outcomes. Insights into the discordant results will come from future studies that include haplotype analyses, analyses within specific ethnic/racial populations, and sex-stratified analyses.

Acute responses to estradiol replacement in the olfactory system of apoE-deficient and wild-type mice

Epidemiological studies suggest that estrogen therapy protects against clinical expression of chronic neurological diseases. These beneficial effects of estrogen therapy are highly modified by apolipoprotein E (apoE) through an unknown mechanism. We examined the short-term effects of estradiol replacement in ovariectomized mice on apoE expression and markers for cell proliferation, reactive gliosis, neuronal maturation, and synaptogenesis in the primary olfactory pathway of wild-type (WT) and apoE knockout (KO) mice. Three days of estradiol replacement increased apoE expression in the olfactory nerve and in the glomerular layer. Estradiol treatment also increased cell proliferation, total cell numbers, number of mature neurons in the olfactory epithelium, and reactive astrocyte numbers in the olfactory bulb (OB) in both WT and KO mice. We also found that estradiol increased glomerular synaptophysin (Syn), but the magnitude of increase was potentiated by the presence of apoE. These data suggest that apoE may be necessary to elicit the complete effect of estradiol on Syn upregulation.

The role of apolipoprotein E in Guillain-Barré syndrome and experimental autoimmune neuritis

Apolipoprotein E (apoE) is a 34.2 kDa glycosylated protein characterized by its wide tissue distribution and multiple functions. ApoE has been widely studied in lipid metabolism, cardiocerebrovascular diseases, and neurodegenerative diseases like Alzheimer's disease and mild cognitive impairment, and so forth. Recently, a growing body of evidence has pointed to nonlipid related properties of apoE, including suppression of T cell proliferation, regulation of macrophage function, facilitation of lipid antigen presentation by CD1 molecules to natural killer T (NKT) cells, and modulation of inflammation and oxidation. By these properties, apoE impacts physiology and pathophysiology at multiple levels. The present paper summarizes updated studies on the immunoregulatory function of apoE, with special focus on isoform-specific effects of apoE on Guillain-Barré syndrome (GBS) and its animal model experimental autoimmune neuritis (EAN).

Facilitated acquisition of the classically conditioned eyeblink response in women taking oral contraceptives

Although anecdotal reports suggest that associative learning processes are affected by menstrual phase, empirical evidence has been equivocal. Moreover, there is a dearth of research concerning fluctuations of artificial or exogenous female hormones on learning and memory. Therefore, in this preliminary study we assessed learning in women who take oral contraceptives and those who do not during the three phases of the menstrual cycle: early, middle, and later cycle. The behavioral assessment included short-trace eyeblink conditioning, acoustic startle reactivity, and a fine motor coordination task (grooved pegboard). Oral contraceptive users generally acquired the conditioned eyeblink response better than non-users. Similar enhancements were observed for fine motor coordination and startle responsiveness. Further research will need to distinguish whether the hormone influence is upon the associative processes or the sensory-motor pathways involved in nonassociative learning.

The effect of hormone therapy on olfactory sensitivity is dependent on apolipoprotein E genotype

Patients with Alzheimer's disease (AD) show a deficit in olfactory threshold sensitivity. The Apolipoprotein E (ApoE) epsilon4 allele is associated with increased risk of AD and earlier symptom onset. Hormone therapy (HT) may exert neuroprotective effects on brain areas affected by AD. The current study investigated the effect of HT on performance on an olfactory threshold test in epsilon4 positive and epsilon4 negative non-hysterectomized, non-demented, elderly females and AD patients. Among the non-demented participants, epsilon4 positive females who had received HT performed 1) significantly better than those without HT, and 2) at levels similar to those of epsilon4 negative females. In contrast, those without HT who were epsilon4 positive performed significantly worse than those who were epsilon4 negative. HT had no effect on performance in AD patients regardless of epsilon4 status. These results suggest that HT may offer protection against loss of olfactory function in epsilon4 positive individuals in preclinical stages of AD. Future research is warranted in order to investigate further the neuroprotective role of HT on sensory and cognitive functions in non-demented aging individuals.

Estradiol-17beta-induced human neural progenitor cell proliferation is mediated by an estrogen receptor beta-phosphorylated extracellularly regulated kinase pathway

Estradiol-17beta (E(2)) induces rodent hippocampal neural progenitor cell (NPC) proliferation in vitro, in vivo, and after brain injury. The purpose of the present investigation was to determine whether E(2)-induced proliferation observed in rodent model systems generalized to cells of human neural origin and the signaling pathway by which E(2) promotes mitosis of human NPCs (hNPCs). Results of these analyses indicate that E(2) induced a significant increase in hNPC proliferation in a time- and dose-dependent manner. E(2)-induced hNPC DNA replication was paralleled by elevated cell cycle protein expression and centrosome amplification, which was associated with augmentation of total cell number. To determine whether estrogen receptor (ER) and which ER subtype were required for E(2)-induced hNPC proliferation, ER expression was first determined by real-time RT-PCR, followed by Western blot analysis, and subsequently verified pharmacologically using ERalpha or beta-selective ligands. Results of these analyses indicated that ERbeta expression was predominant relative to ERalpha, which was barely detectable in hNPCs. Activation of ERbeta by the ERbeta-selective ligand, diarylpropionitrile, led to an increase in phosphorylated extracellular signal-regulated kinase, and subsequent centrosome amplification and hNPC proliferation, which were blocked by the MEKK antagonist, UO126, but not its inactive analog, UO124. These findings, for the first time, demonstrate the molecular cascade and related cell biology events involved in E(2)-induced hNPC proliferation in vitro. Therapeutic implications of these findings relevant to hormone therapy and prevention of neurodegenerative disease are discussed.

Estradiol replacement increases the low-density lipoprotein receptor related protein (LRP) in the mouse brain

Numerous epidemiology studies have shown protective effects of hormone therapy (HT) on chronic neurological diseases. We have proposed that some of the neuroprotective effects of estrogen are mediated by apolipoprotein E (apoE). Polymorphisms of receptors for apoE modify the risk for dementia. To our knowledge, no reports exist showing CNS effects of estrogen replacement on members of the low-density lipoprotein receptor family. The current study focused on the effect of estradiol-17beta (E2) replacement on protein expression of two members of the receptor family, the low-density lipoprotein receptor (LDL-r) and low-density lipoprotein receptor related protein (LRP) in ovariectomized mice. Five days of E2 replacement significantly increased LRP expression in the hippocampus, olfactory bulb and neocortex but not in cerebellum. In contrast, E2 treatment decreased LDL-r protein expression in olfactory bulb. HT modification of both apoE and LRP could have wide-spread effects on cellular function given LRP's manifold signaling functions.

Activation of estrogen receptor alpha increases and estrogen receptor beta decreases apolipoprotein E expression in hippocampus in vitro and in vivo

Previous evidence indicates that, in carriers of apolipoprotein E4 (ApoE4), estrogen therapy increased the risk of late-onset Alzheimer's disease (AD), whereas in individuals carrying ApoE2/3, estrogen therapy reduced the risk of AD [Cauley JA, Zmuda JM, Yaffe K, Kuller LH, Ferrell RE, Wisniewski SR, Cummings SR (1999) J Bone Miner Res 14:1175-1181; Yaffe K, Haan M, Byers A, Tangen C, Kuller L (2000) Neurology 54:1949-1954]. Estrogen mechanisms of action are mediated by two estrogen receptors (ERs), ERalpha and ERbeta. In this study, we determined the relationship between ER subtype and estrogen regulation of ApoE expression in HT-22 cells ectopically transfected with ERalpha or ERbeta, in primary cultured rat hippocampal neurons in vitro and in rat hippocampus in vivo by both molecular biological and pharmacological analyses. Results of these analyses demonstrated that activation of ERalpha either by 17beta-estradiol or a specific-agonist, propylpyrazole triol, up-regulated ApoE mRNA and protein expression. In contrast, the ERbeta-selective agonist, diarylpropionitrile, down-regulated ApoE mRNA and protein expression. These results demonstrate that, in vitro and in vivo, ApoE expression can be differentially regulated depending on activation of ER subtypes. These data suggest that use of ER-selective ligands could provide therapeutic benefit to reduce the risk of AD by increasing ApoE expression in ApoE2/3 allele carriers and decreasing ApoE expression in ApoE4 allele carriers.

Estradiol regulation of astroglia and apolipoprotein E: an important role in neuronal regeneration

The effects of ovarian hormone on neuronal growth and function are well known. However, equally important, but often neglected, are ovarian hormone effects on glia. Our in vivo and in vitro studies show that estradiol modifies both neuronal growth and glial activity and these effects are tightly linked. Estradiol stimulates neurite growth and the release of the glial apolipoprotein E (apoE) in culture studies. Estradiol-stimulated neurite growth in these cultures requires apoE. Estradiol replacement in ovariectomized mice transiently increases the expression of apoE, the low density lipoprotein receptor related protein (LRP) and synaptophysin throughout the brain. Continuous estradiol replacement over two months loses effect on apoE, LRP, and synaptophysin and suppresses reactive gliosis. Estrous cycle variation of glial activation (GFAP) and apoE are not identical. We propose that estradiol (and other ovarian hormones) functions as a zeitgeber to co-ordinate neuronal-glial interactions. Co-ordination assures temporally appropriate excitatory and inhibitory interactions between glia and neurons. With aging and the loss of ovarian cyclicity, some of this co-ordination must be diminished. These observations present significant clinical implications. Approaches to hormone therapy (HT), for diminishing the risk of chronic neurological diseases, need to consider the temporal nature of ovarian hormones in brain repair and plasticity. Moreover, approaches must consider apoE genotype. The neuroprotective effects of HT in numerous chronic age-related diseases may represent effective co-ordination of repair processes rather than direct disease-specific actions. Moreover, the role of glial-derived proteins in neuroprotection should not be ignored.

Neocortical and hippocampal glial fibrillary acidic protein immunoreactivity shows region-specific variation during the mouse estrous cycle

Ovarian hormones modulate both neuronal and glial activation during the estrous cycle. These effects are particularly well characterized in the hypothalamus. Ovarian hormones also affect brain regions not directly related to reproductive function. In this study we used glial fibrillary acidic protein (GFAP) immunocytochemistry to quantify astroglial cells and process density in both the neocortex and hippocampus during the estrous cycle. Our data show that the density of GFAP immunoreactive processes in the hippocampus peaks on proestrus although cell density does not change. In contrast, both GFAP immunoreactive cell and process densities are elevated on diestrus and proestrus in the supragranular layer of the somatosensory cortex and reach a nadir on estrus and metestrus. This activation pattern is not apparent in the motor or cingulate cortex. Neocortical GFAP immunoreactivity appears to follow the distribution of estrogen receptor-alpha-like immunoreactivity. Our data show that ovarian hormones have regionally specific effects on glial activation within the neocortex. Characterizing glial activation by ovarian hormones is important since astroglia are the source of numerous trophic factors and play an important, although often unrecognized, role in neuronal metabolism and function.

Time course of response to estradiol replacement in ovariectomized mice: brain apolipoprotein E and synaptophysin transiently increase and glial fibrillary acidic protein is suppressed

The current study examined the effect of long-term estradiol replacement in ovariectomized mice. Estradiol-17beta (E2) pellets or vehicle pellets were implanted at the time of ovariectomy (OVX) in young adult female mice. Five mice from each group were sacrificed at 5, 14, 28 and 49 days after OVX and pellet replacement. Western blotting of homogenates from somatosensory cortex, hippocampus, olfactory bulb and cerebellum was performed to obtain concentrations of glial fibrillary acidic protein (GFAP), apolipoprotein E (apoE) and synaptophysin (SYN). At 5 days after OVX, GFAP levels were not affected by E2 replacement. In contrast to GFAP, synaptophysin and apoE concentrations were significantly elevated by 15% and 25%, respectively, in the E2-replaced group compared to the vehicle-replaced group at 5 days but by 14 days concentrations were equivalent. Late in the time course of this study, at 49 days, GFAP concentrations were higher in the E2-deprived mice but did not increase in the E2-replaced group. Immunocytochemistry for GFAP confirmed this observation. Of note was that these effects occurred in all four brain regions measured. These observations suggest that estradiol is able to suppress reactive gliosis. In addition, E2 replacement in OVX mice is associated with transiently higher levels of apoE and synaptophysin.

From clinical evidence to molecular mechanisms underlying neuroprotection afforded by estrogens

Recent studies have highlighted that female sex hormones represent potential neuroprotective agents against damage produced by acute and chronic injuries in the adult brain. Clinical reports have documented the effectiveness of estrogens to attenuate symptoms associated with Parkinson's disease, and to reduce the risk of Alzheimer's disease and cerebrovascular stroke. This evidence is corroborated by numerous experimental studies documenting the protective role of female sex hormones both in vitro and in vivo. Accordingly, estrogens have been shown to promote survival and differentiation of several neuronal populations maintained in culture, and to reduce cell death associated with excitotoxicity, oxidative stress, serum deprivation or exposure to beta-amyloid. The neuroprotective effects of estrogens have been widely documented in animal models of neurological disorders, such as Alzheimer's and Parkinson's diseases, as well as cerebral ischemia. Although estrogens are known to exert several direct effects on neurones, the cellular and molecular mechanisms implicated in their protective actions on the brain are not completely understood. Thus, on the basis of clinical and experimental evidence, in this review, we discuss recent findings concerning the neuronal effects of estrogens that may contribute to their neuroprotective actions. Both estrogen receptor-dependent and -independent mechanisms will be described. These include modulation of cell death regulators, such as Bcl-2, Akt and calpain, as well as interaction with growth factors, such as BDNF, NGF, IGF-I and their receptors. The anti-inflammatory effects of estrogens will also be described, namely their ability to reduce brain levels of inflammatory mediators, cytokines and chemokines. Finally, a brief overview about receptor-independent mechanisms of neuroprotection will aim at describing the antioxidant effects of estrogens, as well as their ability to modulate neurotransmission.

Effect of estrogen receptor agonists treatment in MPTP mice: evidence of neuroprotection by an ER alpha agonist

Beneficial effects of 17 beta-estradiol (17 beta-E(2)) on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced striatal dopamine (DA) depletion are well documented but the mechanisms implicated are poorly understood. The present experiments investigated the effect of estrogen receptor (ER) agonists treatment in MPTP mice as compared to 17 beta-E(2). The agonists specific for each subtype were 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl)tris-phenol (PPT) (ER alpha agonist), 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN) and Delta 3-diol (5-androsten-3 beta, 17 beta-diol, also known as 5-androstenediol, androstenediol or hermaphrodiol) (ER beta agonists). Biogenic amines were assayed by HPLC with electrochemical detection. 8 mg/kg of MPTP was administered to give a moderate depletion of striatal DA and its metabolite dihydroxyphenylacetic acid (DOPAC). Protection against MPTP-induced striatal DA and DOPAC depletion was obtained with PPT and 17 beta-E(2) but not with DPN or Delta 3-diol. The striatal dopamine transporter (DAT) was assayed by autoradiography with [(125)I]RTI-121-specific binding. A positive and significant correlation was observed between striatal DA concentrations and [(125)I]RTI-121-specific binding, suggesting that estrogenic treatment that prevented the MPTP-induced DA depletion also prevented loss of DAT. The effect of PPT suggests the implication of an ER alpha in the estrogenic neuroprotection against MPTP. Pointing out which ER is implicated in neuroprotection becomes helpful in designing more specific estrogenic drugs for protection of the aging brain.

Variations in estrogen receptor alpha gene and risk of dementia, and brain volumes on MRI

The role of estrogens in Alzheimer's disease (AD) is controversial. We investigated the association between well-recognized, and potentially functional, polymorphisms in the estrogen receptor (ER) alpha gene and the risk of AD in a prospective study of 6056 Caucasian older men and women aged 55 years and over. In a subset of 468 participants, we assessed volumes of the hippocampus and amygdala, which have a high density of ER alpha, with brain magnetic resonance imaging (MRI) (1.5 T MR unit). During a total of 35 405 person-years of follow-up (mean per persons 5.8 years), 312 new cases of dementia were detected, of whom 230 were diagnosed with AD. Neither the PvuII nor the XbaI polymorphism or haplotypes thereof were associated with the risk of all-cause dementia or AD. In contrast, we found that nondemented women who carried the PvuII p allele or haplotype 'px' had smaller amygdalar volumes on MRI in an allele-dose-dependent fashion. Total amygdalar volume was 4.50 (SE 0.10) in PP genotype, 4.45 (SE 0.06) in Pp genotype, and 4.18 ml (SE 0.08) in pp genotype (P trend=0.008). Further studies are required to investigate whether this smaller amygdalar volume has functional significance.

Estrogen facilitates neurite extension via apolipoprotein E in cultured adult mouse cortical neurons

Literature review suggests a close relationship between estrogen and apolipoprotein E (ApoE) in the central nervous system. Epidemiology studies show that estrogen replacement therapy (ERT) decreases the morbidity from several chronic neurological diseases. Alleles of ApoE modify the risk for and progression of the same diseases. ApoE levels in the rodent brain vary during the estrous cycle and increase after 17beta-estradiol administration. Both estradiol and ApoE3, the most common isoform of human ApoE, increase the extent of neurite outgrowth in culture. Combined, these observations suggest a common mechanism whereby estrogen may increase ApoE levels to facilitate neurite growth. We tested this hypothesis by characterizing the effects of estradiol and ApoE isoforms on neurite outgrowth in cultured adult mouse cortical neurons. Estradiol increased ApoE levels and neurite outgrowth. ApoE2 increased neurite length more so than ApoE3 in the presence of estradiol. Estradiol had no effect on neurite outgrowth from mice lacking the ApoE gene or when only ApoE4, the isoform of ApoE that is associated with increased risk of neurological disease, was exogenously supplied. Cultures from mice transgenic for human ApoE3 or ApoE4 showed the same isoform-specific effect. Neuronal internalization of recombinant human ApoE3 was greater than ApoE4, and ApoE3 was more effective than ApoE4 in facilitating neuronal uptake of a fatty acid. We conclude that estradiol facilitates neurite growth through an ApoE-dependent mechanism. The effects of ERT on chronic neurological diseases may vary with ApoE genotype. The clinical use of ERT may require ApoE genotyping for optimal efficacy.