Estrogen and Alzheimer's disease: the apolipoprotein connection

Estrogen signalling and Alzheimer's disease: Decoding molecular mechanisms for therapeutic breakthrough

In females, Alzheimer's disease (AD) incidences increases as compared to males due to estrogen deficiency after menopause. Estrogen therapy is the mainstay therapy for menopause and associated complications. Estrogen, a hormone with multifaceted physiological functions, has been implicated in AD pathophysiology. Estrogen plays a crucial role in amyloid precursor protein (APP) processing and overall neuronal health by regulating various factors such as brain-derived neurotrophic factor (BDNF), intracellular calcium signalling, death domain-associated protein (Daxx) translocation, glutamatergic excitotoxicity, Voltage-Dependent Anion Channel, Insulin-Like Growth Factor 1 Receptor, estrogen-metabolising enzymes and apolipoprotein E (ApoE) protein polymorphisms. All these factors impact the physiology of postmenopausal women. Estrogen replacement therapies play an important treatment strategy to prevent AD after menopause. However, use of these therapies may lead to increased risks of breast cancer, venous thromboembolism and cardiovascular disease. Various therapeutic approaches have been used to mitigate the effects of estrogen on AD. These include hormone replacement therapy, Selective Estrogen Receptor Modulators (SERMs), Estrogen Receptor Beta (ERβ)-Selective Agonists, Transdermal Estrogen Delivery, Localised Estrogen Delivery, Combination Therapies, Estrogen Metabolism Modulation and Alternative Estrogenic Compounds like genistein from soy, a notable phytoestrogen from plant sources. However, mechanism via which these approaches modulate AD in postmenopausal women has not been explained earlier thoroughly. Present review will enlighten all the molecular mechanisms of estrogen and estrogen replacement therapies in AD. Along-with this, the association between estrogen, estrogen-metabolising enzymes and ApoE protein polymorphisms will also be discussed in postmenopausal AD.

Sex-specific differences in the association between APOE genotype and metabolic syndrome among middle-aged and older rural Indians

Background

Metabolic syndrome (MetS), characterized by elevated blood pressure, high blood glucose, excess abdominal fat, and abnormal cholesterol or triglyceride levels, significantly increases the risk of various non-communicable diseases. This study focuses on understanding the sex-specific association between Apolipoprotein E (APOE) polymorphism and MetS among middle-aged and older adults in rural southern India.

Methods

This cross-sectional study utilized data from the Centre for Brain Research-Srinivaspura Aging, Neuro Senescence, and COGnition (CBR-SANSCOG) study. Participants (n = 3741) underwent comprehensive clinical assessments and blood investigations, including APOE genotyping. MetS was defined using the National Cholesterol Education Program - Adult Treatment Panel III (NCEP ATP III) and the Consensus criteria. Statistical analyses, including chi-square tests, ANCOVA, and logistic regression, were conducted to explore the association of APOE genotype with MetS and its components, stratified by sex.

Results

Females carrying the APOE E4 allele had 1.31-fold increased odds of MetS (95 % CI: 1.02,1.69, p = 0.035) according to the NCEP ATP III criteria but not when the Consensus criteria were applied. The study also noted sex-specific differences in the association of APOE with various MetS components, including lipid levels and waist circumference.

Discussion

Our findings reveal a sex-specific association between the APOE E4 allele and MetS, with only females having an increased risk. This study contributes to the understanding of the genetic underpinnings of MetS and highlights the importance of considering sex-specific differences in MetS research and its prevention strategies. This study underscores the complexity of MetS etiology and emphasizes the need for further research to elucidate the role of genetic, environmental, and lifestyle factors in its progression, particularly in sex-specific contexts.

The phenotypic and transcriptomic effects of developmental exposure to nanomolar levels of estrone and bisphenol A in zebrafish

Estrone and BPA are two endocrine disrupting chemicals (EDCs) that are predicted to be less potent than estrogens such as 17β-estradiol and 17α-ethinylestradiol. Human exposure concentrations to estrone and BPA can be as low as nanomolar levels. However, very few toxicological studies have focused on the nanomolar-dose effects. Low level of EDCs can potentially cause non-monotonic responses. In addition, exposures at different developmental stages can lead to different health outcomes. To identify the nanomolar-dose effects of estrone and BPA, we used zebrafish modeling to study the phenotypic and transcriptomic responses after extended duration exposure from 0 to 5 days post-fertilization (dpf) and short-term exposure at days 4-5 post fertilization. We found that non-monotonic transcriptomic responses occurred after extended duration exposures at 1 nM of estrone or BPA. At this level, estrone also caused hypoactivity locomotive behavior in zebrafish. After both extended duration and short-term exposures, BPA led to more apparent phenotypic responses, i.e. skeletal abnormalities and locomotion changes, and more significant transcriptomic responses than estrone exposure. After short-term exposure, BPA at concentrations equal or above 100 nM affected locomotive behavior and changed the expression of both estrogenic and non-estrogenic genes that are linked to neurological diseases. These data provide gaps of mechanisms between neurological genes expression and associated phenotypic response due to estrone or BPA exposures. This study also provides insights for assessing the acceptable concentration of BPA and estrone in aquatic environments.

Neuroprotective effects of estrogens: the role of cholesterol

INTRODUCTION

Experimental and clinical evidence suggests that estrogens have protective effects in the brain. Nevertheless, their potential role against neurodegenerative diseases, in particular Alzheimer's disease (AD), is still a matter of debate. The identification of the seladin-1 gene (for SELective Alzheimer's Disease INdicator-1), which appeared to be significantly less expressed in brain region affected in AD, opened a new scenario in the field of neuroprotective mechanisms. Seladin-1 was found to have neuroprotective properties through its anti-apoptotic activity. In addition, it was subsequently demonstrated that seladin-1 also has enzymatic activity, because it catalyzes the conversion of desmosterol into cholesterol. Several studies have shown that an appropriate amount of membrane cholesterol plays a pivotal role to protect nerve cells against β-amyloid toxicity in AD and to counteract the synthesis of β-amyloid.

METHODS AND RESULTS

We demonstrated that the expression of seladin-1, as well as the synthesis of cell cholesterol, is stimulated by estrogens in human neuronal precursor cells. Cholesterol enriched cells became more resistant against oxidative stress and β-amyloid toxicity. We thus hypothesized that seladin-1 might be a mediator of the neuroprotective effects of estrogens. Indeed, in cells in which seladin-1 gene expression had been silenced by siRNA the protective effects of estrogens were lost. This finding indicates that seladin-1 is a crucial mediator of the neuroprotective effects of these hormones, at least in our cell model.

CONCLUSIONS

In summary, these results establish a new link between estrogens and cholesterol, which is represented by the neuroprotective factor seladin-1.

Hormonal modulation of cholesterol: experimental evidence and possible translational impact

Alzheimer's disease (AD) is still an incurable condition. There is in vitro evidence that estrogens exert neuroprotective effects; however, their role in the treatment of AD is still controversial. Approximately 10 years ago, a new gene, named seladin-1 (for selective AD indicator-1), was identified and found to be downregulated in brain regions affected by AD. Seladin-1 has neuroprotective properties, which have been associated, at least in part, with its anti-apoptotic activity. Estrogens stimulate the expression of the seladin-1 gene. Seladin-1 also has enzymatic activity (3-β-hydroxysterol Δ-24-reductase), which is involved in the synthesis of cholesterol from desmosterol. The amount of membrane cholesterol appears to play an important role in conferring protection to brain cells. This review focuses on the relationship between estrogens (and IGF-1, another hormone with neuroprotective properties), cholesterol and seladin-1.

Membrane cholesterol as a mediator of the neuroprotective effects of estrogens

Alzheimer's disease (AD), the most common neurodegenerative disease associated with aging, is still an incurable condition. Although in vitro evidence strongly indicates that estrogens exert neurotrophic and neuroprotective effects, the role of this class of hormones in the treatment of AD is still a debated issue. In 2000 a new gene, named seladin-1 (for SELective Alzheimer's Disease INdicator-1), was identified and found to be down regulated in vulnerable brain regions in AD. Seladin-1 was considered a novel neuroprotective factor, because of its anti-apoptotic activity. Subsequently, it was demonstrated that seladin-1 has also enzymatic activity [3-β-hydroxysterol delta-24-reductase, (DHCR24)], which catalyzes the synthesis of cholesterol from desmosterol. The amount of membrane cholesterol may play an important role both in protecting neuronal cells against toxic insults and in inhibiting the production of β-amyloid. We demonstrated that seladin-1 overexpression increases the amount of membrane cholesterol and induces resistance against β-amyloid aggregates in neuroblastoma cells, whereas a specific inhibitor of DHCR24 increased cell vulnerability. We also hypothesized that seladin-1 might be a mediator of the neuroprotective effects of estrogens. We first demonstrated that, in human fetal neuroepithelial cells (FNC), 17β-estradiol, raloxifene, and tamoxifen exert protective effects against β-amyloid toxicity and oxidative stress. In addition, these molecules significantly increased the expression of seladin-1 and the amount of cell cholesterol. Then, we showed that, upon seladin-1 silencing, the protective effects of estrogens were abolished, thus indicating this factor as a fundamental mediator of estrogen-mediated neuroprotection, at least in FNC cells. Furthermore, we detected the presence of functionally active half-palindromic estrogen responsive elements upstream the coding region of the seladin-1 gene. Overall, our results indicate that seladin-1 may be viewed as a multi-faceted protein, which conjugates both the neuroprotective properties of estrogens and the important functions of cholesterol in maintaining brain homeostasis. This article is part of a Special Issue entitled: Neuroactive Steroids: Focus on Human Brain.

Postmenopausal hormone therapy, timing of initiation, APOE and cognitive decline

Associations between postmenopausal hormone therapy (HT) and cognitive decline may depend on apolipoprotein E (APOE) status or timing of initiation. We included 16,514 Nurses' Health Study participants aged 70-81 years who were followed since 1976 and completed up to 3 telephone cognitive assessments (2 years apart), between 1995 and 2006. The tests assessed general cognition (Telephone Interview of Cognitive Status; TICS), verbal memory, and category fluency. We used longitudinal analyses to estimate differences in cognitive decline across hormone groups. APOE genotype was available in 3697 participants. Compared with never users, past or current HT users showed modest but statistically significant worse rates of decline in the TICS: the multivariable-adjusted difference in annual rate of decline in the TICS among current estrogen only users versus never users was -0.04 (95% confidence interval, -0.07 to -0.004); for current estrogen + progestin users, the mean difference was -0.05 (95% confidence interval, -0.10 to -0.002). These differences were equivalent to those observed in women who are 1-2 years apart in age. We observed no protective associations with early timing of hormone initiation. We found suggestive interactions with APOE e4 status (e.g., on TICS, p interaction, 0.10), where the fastest rate of decline was observed among APOE e4 carriers who were current HT users. Regardless of timing of initiation, HT may be associated with worse rates of decline in general cognition, especially among those with an APOE e4 allele.

Testosterone modifies the effect of APOE genotype on hippocampal volume in middle-aged men

BACKGROUND

The APOE epsilon4 allele is an established risk factor for Alzheimer disease (AD), yet findings are mixed for how early its effects are manifest. One reason for the mixed results could be the presence of interaction effects with other AD risk factors. Increasing evidence indicates that testosterone may play a significant role in the development of AD. The aim of the present study was to examine the potential interaction of testosterone and APOE genotype with respect to hippocampal volume in middle age.

METHODS

Participants were men from the Vietnam Era Twin Study of Aging (n = 375). The mean age was 55.9 years (range 51-59). Between-group comparisons were performed utilizing a hierarchical linear mixed model that adjusted for the nonindependence of twin data.

RESULTS

A significant interaction was observed between testosterone and APOE genotype (epsilon4-negative vs epsilon4-positive). Those with both low testosterone (> or =1 SD below the mean) and an epsilon4-positive status had the smallest hippocampal volumes, although comparisons with normal testosterone groups were not significant. However, individuals with low testosterone and epsilon4-negative status had significantly larger hippocampal volumes relative to all other groups. A main effect of APOE genotype on hippocampal volume was observed, but only when the APOE-by-testosterone interaction was present.

CONCLUSIONS

These findings demonstrate an interaction effect between testosterone and the APOE epsilon4 allele on hippocampal volume in middle-aged men, and they may suggest 2 low testosterone subgroups. Furthermore, these results allude to potential gene-gene interactions between APOE and either androgen receptor polymorphisms or genes associated with testosterone production.

[Cognitive function in menopausal women evaluated with the Mini-Mental State Examination and Word-List Memory Test]

There is little research in Brazil on cognition and menopause, despite the high frequency of neuropsychiatric complaints in this phase of women's life. The authors present a cross-sectional study aimed at describing the scores by 156 menopausal women on the Mini-Mental State Examination (MMSE) and the Word-List Memory Test (WLMT). The mean score on the MMSE was 25.86 points (SD = 2.67), similar to other studies, except for better performance by illiterate women; scores on the sub-items 'attention and calculation' and 'immediate recall' showed lower values. In the WLMT, the mean was also consistent with the literature (M = 18.83 words; SD = 3.82). The only significant associations with score were for schooling in both tests and arterial hypertension in the WLMT. The authors conclude that cognitive performance in these menopausal women is similar to that of other Brazilian samples, corroborating the wider range of scores among individuals with lower schooling. Complaints pertaining to memory in middle-aged women may be related to decreased attention.

New insights on the neuroprotective role of sterols and sex steroids: the seladin-1/DHCR24 paradigm

In 2000 a new gene, i.e. seladin-1 (for selective Alzheimer's disease indicator-1) was identified and found to be down regulated in vulnerable brain regions in Alzheimer's disease. Seladin-1 was considered a novel neuroprotective factor, because of its anti-apoptotic properties. Subsequently, it has been demonstrated that seladin-1 corresponds to the gene that encodes 3-beta-hydroxysterol delta-24-reductase (DHCR24), that catalyzes the synthesis of cholesterol from desmosterol. There is evidence that cholesterol plays a fundamental role in maintaining brain homeostasis. Because of its enzymatic activity, seladin-1/DHCR24 has been considered the human homolog of the plant protein DIMINUTO/DWARF1, that is involved in the synthesis of sterol plant hormones. We have recently demonstrated that seladin-1/DHCR24 is a fundamental mediator of the protective effects of estrogens in the brain. This review describes how this protein interacts with cholesterol and estrogens, thus generating a neuroprotective network, that might open new possibilities in the prevention/treatment of neurodegenerative diseases.

Estrogen receptor-mediated neuroprotection: The role of the Alzheimer's disease-related gene seladin-1

Experimental evidence supports a protective role of estrogen in the brain. According to the fact that Alzheimer's disease (AD) is more common in postmenopausal women, estrogen treatment has been proposed. However, there is no general consensus on the beneficial effect of estrogen or selective estrogen receptor modulators in preventing or treating AD. It has to be said that several factors may markedly affect the efficacy of the treatment. A few years ago, the seladin-1 gene (for selective Alzheimer's disease indicator-1) has been isolated and found to be down-regulated in brain regions affected by AD. Seladin-1 has been found to be identical to the gene encoding the enzyme 3-beta-hydroxysterol delta-24-reductase, involved in the cholesterol biosynthetic pathway, which confers protection against beta-amyloid-mediated toxicity and from oxidative stress, and is an effective inhibitor of caspase-3 activity, a key mediator of apoptosis. Interestingly, we found earlier that the expression of this gene is up-regulated by estrogen. Furthermore, our very recent data support the hypothesis that seladin-1 is a mediator of the neuroprotective effects of estrogen. This review will summarize the current knowledge regarding the neuroprotective effects of seladin-1 and the relationship between this protein and estrogen.

Regulation of soluble guanylyl cyclase activity by oestradiol and progesterone in the hypothalamus but not hippocampus of female rats

Oestradiol and progesterone act in the hypothalamus to coordinate the timing of lordosis and ovulation in female rats in part through regulation of nitric oxide (NO) and cyclic guanosine monophosphate (cyclic GMP) signalling pathways. Soluble guanylyl cyclase is an enzyme that produces cyclic GMP when stimulated by NO and plays a crucial role in the display of lordosis behaviour. We examined the effects of oestradiol and progesterone on the stimulation of cyclic GMP synthesis by NO-dependent and independent activators of soluble guanylyl cyclase in preoptic-hypothalamic and hippocampal slices. Ovariectomised Sprague-Dawley rats were injected with oestradiol (2 microg oestradiol benzoate, s.c.) or vehicle for 2 days. Progesterone (500 microg, s.c.) or vehicle was injected 44 h after the first dose of oestradiol. Rats were killed 48 h after the first oestradiol or vehicle injection, and hypothalamus and hippocampus were obtained. NO-dependent activation of soluble guanylyl cyclase was induced by NO donors, sodium nitroprusside or diethylamine NONOate; NO-independent activation of soluble guanylyl cyclase was induced with 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole and 5'-cyclopropyl-2-[1-2fluoro-benzyl)-1H-pyrazolo[3,4-b]pyridine-3-yl]pyridine-4-ylamine. The NO-dependent activators of soluble guanylyl cyclase produced a concentration-dependent increase in cyclic GMP accumulation and induced significantly greater cyclic GMP accumulation in preoptic-hypothalamic slices from animals treated with oestradiol and progesterone than in slices from rats injected with vehicle, oestradiol or progesterone alone. Hormones did not modify soluble guanylyl cyclase activation by NO-independent stimulators or influence NO content in preoptic-hypothalamic slices. Oestradiol and progesterone did not affect activation of soluble guanylyl cyclase in hippocampal slices by any pharmacological agent, indicating a strong regional selectivity for the hormone effect. Thus, oestradiol and progesterone, administered in vivo, enhance the ability of NO to activate soluble guanylyl cyclase in brain areas modulating female reproductive function without an effect on production of NO itself.

Astrocyte-derived estrogen enhances synapse formation and synaptic transmission between cultured neonatal rat cortical neurons

Recent in vitro studies have found that astrocytes exert powerful control over the number of neuronal synapses, leading us to consider why glia can exert this control and what the underlying mechanism(s) may be. To understand the potential possibility, we studied the formation of synapses and synaptic function in primary rat cortical neurons. We found that primary cultured neonatal rat cortical astrocytes modulate synaptogenesis and synaptic function through producing and secreting estradiol into culture medium. The concentration of estradiol produced by pure cultured astrocytes increased in correspondence with the days of culture and the number of proliferating astrocytes, which peaked at 266+/-22 ng/l around day 14 of culture. When astrocyte-conditioned medium (ACM) was added into pure cultured cortical neurons, the number of synapses formed between cortical neurons increased by nearly sixfold. The mean frequency and the amplitude of mini-postsynaptic currents (mPSCs) increased from 13+/-4 events/min and 20.5+/-2 pA to 73+/-16 events/min and 29.1+/-3 pA, respectively. In the meantime, the level of estrogen receptor-alpha (ER-alpha) expressed on neonatal rat cortical neurons was significantly up-regulated. Moreover, the effect of ACM on synaptic formation and transmission was blocked by tamoxifen (estrogen receptor antagonist) in culture. After the treatment of tamoxifen, the number of synapses on neurons decreased from 79+/-9 to 32+/-3. The mean amplitude and frequency of mPSCs were also dropped to 24.5+/-2 pA and 35+/-10/min, respectively. Unexpectedly, exogenic estradiol can mimic the effect of ACM on synaptic formation and transmission. Finally, to understand whether astrocyte-derived estradiol regulates the synaptic transmission via presynapse, the release of presynaptic vesicle from neuron was monitored by FM 4-64 assay. The results showed that when ACM or exogenic estradiol was added into neurons, the kinetics of vesicle release speed are similar to that of neuronal cultured with astrocytes, which were faster than that of just pure neuronal cultures. These observations suggest that estrogen synthesized and secreted by astrocytes can regulate synapse formation and synaptic transmission.

Suppression of beta-amyloid precursor protein signaling into the nucleus by estrogens mediated through complex formation between the estrogen receptor and Fe65

The C-terminal fragment of the beta-amyloid precursor protein produced after cleavage by gamma-secretase, namely, APPct or AICD, has been shown to form a multimeric complex with the adaptor protein Fe65 and to regulate transcription through the recruitment of the histone acetyltransferase Tip60. The present study shows that 17beta-estradiol inhibits the transcriptional and apoptotic activities of the APPct complex by a process involving the interaction of estrogen receptor alpha (ERalpha) with Fe65. ERalpha-Fe65 complexes were detected both in vitro and in the mouse brain, and recruitment of ERalpha to the promoter of an APPct target gene (KAI1) was demonstrated. Our studies reveal a novel mechanism of estrogen action, which may explain the well-known neuroprotective functions of estrogens as well as the complex role of this female hormone in the pathogenesis of neuronal degeneration diseases.

Polycystic ovary syndrome: etiology and pathogenesis

OBJECTIVE

To provide a review of the pathogenesis of polycystic ovary syndrome.

DESIGN

Literature survey.

RESULT(S)

Three major pathophysiologic hypotheses have been proposed to explain the clinical findings of polycystic ovary syndrome (PCOS) related to three major laboratory findings: the LH hypothesis, the insulin hypothesis and the ovarian hypothesis. Although the presence of many small follicles with a high androgen to estrogen ratio was first thought to represent a high rate of follicular atresia in polycystic ovaries, recent studies have demonstrated that the granulosa cells are viable and able to respond to FSH stimulation with normal increases in estradiol production. Thus, a new hypothesis has arisen that FSH activity is somehow blocked at the ovarian level.

CONCLUSION(S)

PCOS is a syndrome involving defects in primary cellular control mechanisms that result in the expression of chronic anovulation and hyperandrogenism. In this syndrome, the relation between the various parameters is of particular interest. These relations constitute the cornerstone of the pathogenesis of PCOS. The fact that the pathogenesis of PCOS has not yet been clarified, despite the plethora of relative information, may be the result of a general way of thinking in the interpretation of several scientific data, and especially those that refer to biochemical phenomena. The use of the various models of the theory of chaos, that permits a concrete approach for the interpretation of data, may constitute an optional procedure for the future understanding of the association of different parameters and their disturbances in the pathogenesis of the polycystic ovary syndrome.

Seladin-1 as a target of estrogen receptor activation in the brain: a new gene for a rather old story?

Experimental evidence indicates that estrogen exerts neuroprotective effects. According to the fact that Alzheimer's disease (AD) is more common in post-menopausal women, estrogen treatment has been proposed. However, the beneficial effect of estrogen or selective estrogen receptor modulators (SERMs) in preventing or treating AD is a controversial issue, which will be summarized in this review. Recently, a novel gene, named selective AD indicator-1 (seladin-1), has been isolated and found to be down-regulated in brain regions affected by AD. Seladin-1, which is considered the human homolog of the plant protein DIMINUTO/DWARF1, confers protection against beta-amyloid-mediated toxicity and from oxidative stress and is an effective inhibitor of caspase 3 activity, a key mediator of apoptosis. This review will present the up-to-date findings regarding seladin-1 and DIMINUTO/DWARF1. In addition, the possibility that seladin-1 may be a downstream effector of estrogen receptor activation in the brain, based on our recent experimental findings using a human fetal neuronal model, will be addressed.

Nuclear Hormone Receptors, Metabolism, and Aging: What Goes Around Comes Around

Previous studies have linked the mysterious and inevitable process of aging to essential processes such as metabolism, maturation, and fecundity. Each of these processes is controlled to a large extent by nuclear hormone receptors (NHRs). NHRs also play important roles in the control of periodical processes, the most recently implicated being circadian rhythm. This Review stresses the mounting evidence for tight relationships between each of these NHR-regulated processes and the processes of aging.