Estrogen (E2) and glucocorticoid (Gc) effects on microglia and A beta clearance in vitro and in vivo

An Interaction between Brain-Derived Neurotrophic Factor and Stress-Related Glucocorticoids in the Pathophysiology of Alzheimer's Disease

Both the brain-derived neurotrophic factor (BDNF) and glucocorticoids (GCs) play multiple roles in various aspects of neurons, including cell survival and synaptic function. BDNF and its receptor TrkB are extensively expressed in neurons of the central nervous system (CNS), and the contribution of the BDNF/TrkB system to neuronal function is evident; thus, its downregulation has been considered to be involved in the pathogenesis of Alzheimer's disease (AD). GCs, stress-related molecules, and glucocorticoid receptors (GRs) are also considered to be associated with AD in addition to mental disorders such as depression. Importantly, a growing body of evidence suggests a close relationship between BDNF/TrkB-mediated signaling and the GCs/GR system in the CNS. Here, we introduce the current studies on the interaction between the neurotrophic system and stress in CNS neurons and discuss their involvement in the pathophysiology of AD.

Sex and Sleep Disruption as Contributing Factors in Alzheimer's Disease

Alzheimer's disease (AD) affects more women than men, with women throughout the menopausal transition potentially being the most under researched and at-risk group. Sleep disruptions, which are an established risk factor for AD, increase in prevalence with normal aging and are exacerbated in women during menopause. Sex differences showing more disrupted sleep patterns and increased AD pathology in women and female animal models have been established in literature, with much emphasis placed on loss of circulating gonadal hormones with age. Interestingly, increases in gonadotropins such as follicle stimulating hormone are emerging to be a major contributor to AD pathogenesis and may also play a role in sleep disruption, perhaps in combination with other lesser studied hormones. Several sleep influencing regions of the brain appear to be affected early in AD progression and some may exhibit sexual dimorphisms that may contribute to increased sleep disruptions in women with age. Additionally, some of the most common sleep disorders, as well as multiple health conditions that impair sleep quality, are more prevalent and more severe in women. These conditions are often comorbid with AD and have bi-directional relationships that contribute synergistically to cognitive decline and neuropathology. The association during aging of increased sleep disruption and sleep disorders, dramatic hormonal changes during and after menopause, and increased AD pathology may be interacting and contributing factors that lead to the increased number of women living with AD.

Targeting Nuclear Receptors in Neurodegeneration and Neuroinflammation

Nuclear receptors, also known as ligand-activated transcription factors, regulate gene expression upon ligand signals and present as attractive therapeutic targets especially in chronic diseases. Despite the therapeutic relevance of some nuclear receptors in various pathologies, their potential in neurodegeneration and neuroinflammation is insufficiently established. This perspective gathers preclinical and clinical data for a potential role of individual nuclear receptors as future targets in Alzheimer's disease, Parkinson's disease, and multiple sclerosis, and concomitantly evaluates the level of medicinal chemistry targeting these proteins. Considerable evidence suggests the high promise of ligand-activated transcription factors to counteract neurodegenerative diseases with a particularly high potential of several orphan nuclear receptors. However, potent tools are lacking for orphan receptors, and limited central nervous system exposure or insufficient selectivity also compromises the suitability of well-studied nuclear receptor ligands for functional studies. Medicinal chemistry efforts are needed to develop dedicated high-quality tool compounds for the therapeutic validation of nuclear receptors in neurodegenerative pathologies.

Trait anxiety, a personality risk factor associated with Alzheimer's Disease

Alzheimer's disease (AD) is the most common neurodegenerative disease in elderly population and the leading cause of dementia worldwide. While senile plaques and neurofibrillary tangles have been proposed as the principal histopathologic hallmarks of AD, the exact etiology of this disease is still far from being clearly understood. AD has been recognized as pathological consequences of complex interactions among genetic, aging, medical, life style and psychosocial factors. Recently, the roles of neuroticism personality traits in AD incidence and progression have come into focus. More specifically, increasing evidence has further shown that the trait anxiety, one major component of neuroticism predicting the individual vulnerability in response to stress, is a risk factor for AD and may correlated with various AD pathologies. In this review, we summarized recent literature on the association of trait anxiety with AD. We also discussed the possible neuroendocrinological and neurochemical mechanisms of this association, which may provide clinical implications for AD diagnosis and therapy.

Sleep quality and cortical amyloid-β deposition in postmenopausal women of the Kronos early estrogen prevention study

Hormone therapy improves sleep in menopausal women and recent data suggest that transdermal 17β-estradiol may reduce the accumulation of cortical amyloid-β. However, how menopausal hormone therapies modify the associations of amyloid-β accumulation with sleep quality is not known. In this study, associations of sleep quality with cortical amyloid-β deposition and cognitive function were assessed in a subset of women who had participated in the Kronos early estrogen prevention study. It was a randomized, placebo-controlled trial in which recently menopausal women (age, 42-58; 5-36 months past menopause) were randomized to (1) oral conjugated equine estrogen (n = 19); (2) transdermal 17β-estradiol (tE2, n = 21); (3) placebo pills and patch (n = 32) for 4 years. Global sleep quality score was calculated using Pittsburgh sleep quality index, cortical amyloid-β deposition was measured with Pittsburgh compound-B positron emission tomography standard uptake value ratio and cognitive function was assessed in four cognitive domains 3 years after completion of trial treatments. Lower global sleep quality score (i.e., better sleep quality) correlated with lower cortical Pittsburgh compound-B standard uptake value ratio only in the tE2 group (r = 0.45, P = 0.047). Better global sleep quality also correlated with higher visual attention and executive function scores in the tE2 group (r = -0.54, P = 0.02) and in the oral conjugated equine estrogen group (r = -0.65, P = 0.005). Menopausal hormone therapies may influence the effects of sleep on cognitive function, specifically, visual attention and executive function. There also appears to be a complex relationship between sleep, menopausal hormone therapies, cortical amyloid-β accumulation and cognitive function, and tE2 formulation may modify the relationship between sleep and amyloid-β accumulation.

Chronic Stress in Cognitive Processes: Cortisol Dynamic Range of Secretion Is Associated with Perception of Unsafety Environment in a Venezuelan Population

BACKGROUND

Understanding diurnal secretion of cortisol in association with behavioral attitudes as a result of perception of unsafety environment is a main interest in prospective studies establishing the impact of chronic stress in cognitive processes. Adaptive secretion of cortisol, a biomarker of the hypothalamic-hypophysis-adrenal (HPA) axis, has been correlated with perception of uncertainty in surroundings as a consequence of perseverative cognition and unconscious thoughts.

OBJECTIVE

To determine whether diurnal secretion pattern of cortisol was associated with behavioral attitudes indexes generated from answers to standardized questionnaires from Panamerican Health Organization/World Health Organization (PAHO/WHO) agencies.

METHODS

Saliva cortisol dynamic range was evaluated by immuno-essay. Cortisol awakening response (CAR) and total secreted cortisol was established in a cross-sectional study of four saliva samples per day from volunteers (n = 135) between 19 and 65 years old.

RESULTS

Saliva cortisol dynamic range followed a significant decay along the day. Reduction of social interaction and increase of defensive behavioral attitude was associated with older groups of age. In this study, two subgroups of subjects with a steeper cortisol secretion (slope significant non-zero), and flatter cortisol secretion (slope no significant non-zero) were detected. Noticeable, we determined an association between measurements of cortisol secretion from subjects with a flatter cortisol dynamic range and behavioral defensive and inhibition of social interaction indexes.

CONCLUSION

These findings suggested chronical dysregulation of HPA axis as a result of perseverative cognitive perception of unsafety environment which may be precedent to cognitive impairment in the population.

Association between chronic stress and Alzheimer's disease: Therapeutic effects of Saffron

Chronic stress and high levels of glucocorticoids produce functional and structural changes in brain and especially in the hippocampus, an important limbic system structure that plays a key role in cognitive functions including learning and memory. Alzheimer's disease (AD) is a chronic neurodegenerative disease that usually starts slowly and worsens over time. Indeed, cognitive dysfunction, neuronal atrophy, and synaptic loss are associated with both AD and chronic stress. Recent preclinical and clinical studies have highlighted a possible link between chronic stress, cognitive decline and the development of AD. It is suggested that Tau protein is an essential mediator of the neurodegenerative effects of stress and glucocorticoids towards the development of AD pathology. Recent findings from animal and humans studies demonstrated that saffron and its main constitutive crocin are effective against chronic stress-induced cognitive dysfunction and oxidative stress and slowed cognitive decline in AD. The inhibitory actions on acetylcholinesterase activity, aggregation of beta-amyloid protein into amyloid plaques and tau protein into neurofibrillary tangles, and also the antioxidant, anti-inflammatory, and the promotion of synaptic plasticity effects are among the possible mechanisms to explain the neuroprotective effects of saffron. New evidences demonstrate that saffron and its main component crocin might be a promising target for cognition improvement in AD and stress-related disorders.

The Role of Sex and Sex Hormones in Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are a wide class of disorders of the central nervous system (CNS) with unknown etiology. Several factors were hypothesized to be involved in the pathogenesis of these diseases, including genetic and environmental factors. Many of these diseases show a sex prevalence and sex steroids were shown to have a role in the progression of specific forms of neurodegeneration. Estrogens were reported to be neuroprotective through their action on cognate nuclear and membrane receptors, while adverse effects of male hormones have been described on neuronal cells, although some data also suggest neuroprotective activities. The response of the CNS to sex steroids is a complex and integrated process that depends on (i) the type and amount of the cognate steroid receptor and (ii) the target cell type-either neurons, glia, or microglia. Moreover, the levels of sex steroids in the CNS fluctuate due to gonadal activities and to local metabolism and synthesis. Importantly, biochemical processes involved in the pathogenesis of NDs are increasingly being recognized as different between the two sexes and as influenced by sex steroids. The aim of this review is to present current state-of-the-art understanding on the potential role of sex steroids and their receptors on the onset and progression of major neurodegenerative disorders, namely, Alzheimer's disease, Parkinson's diseases, amyotrophic lateral sclerosis, and the peculiar motoneuron disease spinal and bulbar muscular atrophy, in which hormonal therapy is potentially useful as disease modifier.

Stress and Alzheimer's disease

Alzheimer's disease (AD) is the leading cause of dementia worldwide. Due to demographic change in higher income countries and rising life expectancy in middle- and low-income countries, the prevalence of AD will increase significantly in the coming years. In the search for effective AD prevention, the role of stress in the development of AD has come into focus. There is increasing evidence that chronic exposure to stress is a risk factor for AD and may also adversely affect the course of the disease. In our review, we present the current literature on the association of specific personality traits and the risk of developing AD. We also report on findings on dementia risk in patients with posttraumatic stress disorder. Furthermore, we describe the role of anxiety symptoms in AD and give a brief overview over the biological mechanisms behind the association of stress and AD.

Androgen alleviates neurotoxicity of β-amyloid peptide (Aβ) by promoting microglial clearance of Aβ and inhibiting microglial inflammatory response to Aβ

AIMS

Lower androgen level in elderly men is a risk factor of Alzheimer's disease (AD). It has been reported that androgen reduces amyloid peptides (Aβ) production and increases Aβ degradation by neurons. Activated microglia are involved in AD by either clearing Aβ deposits through uptake of Aβ or releasing cytotoxic substances and pro-inflammatory cytokines. Here, we investigated the effect of androgen on Aβ uptake and clearance and Aβ-induced inflammatory response in microglia, on neuronal death induced by Aβ-activated microglia, and explored underlying mechanisms.

METHODS

Intracellular and extracellular Aβ were examined by immunofluorescence staining and Western blot. Amyloid peptides (Aβ) receptors, Aβ degrading enzymes, and pro-inflammatory cytokines were detected by RT-PCR, real-time PCR, and ELISA. Phosphorylation of MAP kinases and NF-κB was examined by Western blot.

RESULTS

We found that physiological concentrations of androgen enhanced Aβ₄₂ uptake and clearance, suppressed Aβ₄₂ -induced IL-1β and TNFα expression by murine microglia cell line N9 and primary microglia, and alleviated neuronal death induced by Aβ₄₂ -activated microglia. Androgen administration also reduced Aβ₄₂ -induced IL-1β expression and neuronal death in murine hippocampus. Mechanistic studies revealed that androgen promoted microglia to phagocytose and degrade Aβ₄₂ through upregulating formyl peptide receptor 2 and endothelin-converting enzyme 1c expression, and inhibited Aβ₄₂ -induced pro-inflammatory cytokines expression via suppressing MAPK p38 and NF-κB activation by Aβ₄₂ , in an androgen receptor independent manner.

CONCLUSION

Our study demonstrates that androgen promotes microglia to phagocytose and clear Aβ₄₂ and inhibits Aβ₄₂ -induced inflammatory response, which may play an important role in reducing the neurotoxicity of Aβ.

Depression as a risk factor for Alzheimer's disease: Genes, steroids, cytokines and neurogenesis - What do we need to know?

Depression (MDD) is prodromal to, and a component of, Alzheimer's disease (AD): it may also be a trigger for incipient AD. MDD is not a unitary disorder, so there may be particular subtypes of early life MDD that pose independent high risks for later AD, though the identification of these subtypes is problematical. There may either be a common pathological event underlying both MDD and AD, or MDD may sensitize the brain to a second event ('hit') that precipitates AD. MDD may also accelerate brain ageing, including altered DNA methylation, increased cortisol but decreasing DHEA and thus the risk for AD. So far, genes predicting AD (e.g. APOEε4) are not risk factors for MDD, and those implicated in MDD (e.g. SLC6A4) are not risks for AD, so a common genetic predisposition looks unlikely. There is as yet no strong indication that an epigenetic event occurs during some forms of MDD that predisposes to later AD, though the evidence is limited. Glucocorticoids (GCs) are disturbed in some cases of MDD and in AD. GCs have marked degenerative actions on the hippocampus, a site of early β-amyloid deposition, and rare genetic variants of GC-regulating enzymes (e.g. 11β-HSD) predispose to AD. GCs also inhibit hippocampal neurogenesis and plasticity, and thus episodic memory, a core symptom of AD. Disordered GCs in MDD may inhibit neurogenesis, but the contribution of diminished neurogenesis to the onset or progression of AD is still debated. GCs and cytokines also reduce BDNF, implicated in both MDD and AD and hippocampal neurogenesis, reinforcing the notion that those cases of MDD with disordered GCs may be a risk for AD. Cytokines, including IL1β, IL6 and TNFα, are increased in the blood in some cases of MDD. They also reduce hippocampal neurogenesis, and increased cytokines are a known risk for later AD. Inflammatory changes occur in both MDD and AD (e.g. raised CRP, TNFα). Both cytokines and GCs can have pro-inflammatory actions in the brain. Inflammation (e.g. microglial activation) may be a common link, but this has not been systematically investigated. We lack substantial, rigorous and comprehensive follow-up studies to better identify possible subtypes of MDD that may represent a major predictor for later AD. This would enable specific interventions during critical episodes of these subtypes of MDD that should reduce this substantial risk.

Potential therapeutic strategies for Alzheimer's disease targeting or beyond β-amyloid: insights from clinical trials

Alzheimer's disease (AD) is a progressive neurodegenerative disorder with two hallmarks: β-amyloid plagues and neurofibrillary tangles. It is one of the most alarming illnesses to elderly people. No effective drugs and therapies have been developed, while mechanism-based explorations of therapeutic approaches have been intensively investigated. Outcomes of clinical trials suggested several pitfalls in the choice of biomarkers, development of drug candidates, and interaction of drug-targeted molecules; however, they also aroused concerns on the potential deficiency in our understanding of pathogenesis of AD, and ultimately stimulated the advent of novel drug targets tests. The anticipated increase of AD patients in next few decades makes development of better therapy an urgent issue. Here we attempt to summarize and compare putative therapeutic strategies that have completed clinical trials or are currently being tested from various perspectives to provide insights for treatments of Alzheimer's disease.

Estradiol prevents olfactory dysfunction induced by A-β 25-35 injection in hippocampus

Background

Some neurodegenerative diseases, such as Alzheimer and Parkinson, present an olfactory impairment in early stages, and sometimes even before the clinical symptoms begin. In this study, we assess the role of CA1 hippocampus (structure highly affected in Alzheimer disease) subfield in the rats’ olfactory behavior, and the neuroprotective effect of 17 beta estradiol (E₂) against the oxidative stress produced by the injection of amyloid beta 25–35.

Results

162 Wistar rats were ovariectomized and two weeks after injected with 2 μl of amyloid beta 25–35 (A-β_25–35) in CA1 subfield. Olfactory behavior was evaluated with a social recognition test, odor discrimination, and search tests. Oxidative stress was evaluated with FOX assay and Western Blot against 4-HNE, Fluoro Jade staining was made to quantify degenerated neurons; all these evaluations were performed 24 h, 8 or 15 days after A-β_25–35 injection. Three additional groups treated with 17 beta estradiol (E₂) were also evaluated. The injection of A-β_25–35 produced an olfactory impairment 24 h and 8 days after, whereas a partial recovery of the olfactory behavior was observed at 15 days. A complete prevention of the olfactory impairment was observed with the administration of E₂ two weeks before the amyloid injection (A-β_25–35 24 h + E₂) and one or two weeks after (groups 8 A-β +E₂ and 15 A-β +E₂ days, respectively); a decrease of the oxidative stress and neurodegeneration were also observed.

Conclusions

Our finding shows that CA1 hippocampus subfield plays an important role in the olfactory behavior of the rat. The oxidative stress generated by the administration of A-β_25–35 is enough to produce an olfactory impairment. This can be prevented with the administration of E₂ before and after amyloid injection. This suggests a possible therapeutic use of estradiol in Alzheimer’s disease.

A review: inflammatory process in Alzheimer's disease, role of cytokines

Alzheimer's disease (AD) is the most common neurodegenerative disorder to date. Neuropathological hallmarks are β-amyloid (Aβ) plaques and neurofibrillary tangles, but the inflammatory process has a fundamental role in the pathogenesis of AD. Inflammatory components related to AD neuroinflammation include brain cells such as microglia and astrocytes, the complement system, as well as cytokines and chemokines. Cytokines play a key role in inflammatory and anti-inflammatory processes in AD. An important factor in the onset of inflammatory process is the overexpression of interleukin (IL)-1, which produces many reactions in a vicious circle that cause dysfunction and neuronal death. Other important cytokines in neuroinflammation are IL-6 and tumor necrosis factor (TNF)-α. By contrast, other cytokines such as IL-1 receptor antagonist (IL-1ra), IL-4, IL-10, and transforming growth factor (TGF)-β can suppress both proinflammatory cytokine production and their action, subsequently protecting the brain. It has been observed in epidemiological studies that treatment with nonsteroidal anti-inflammatory drugs (NSAIDs) decreases the risk for developing AD. Unfortunately, clinical trials of NSAIDs in AD patients have not been very fruitful. Proinflammatory responses may be countered through polyphenols. Supplementation of these natural compounds may provide a new therapeutic line of approach to this brain disorder.

Behavioural and cellular effects of exogenous amyloid-β peptides in rodents

A better understanding of Alzheimer's disease (AD) and the development of disease modifying therapies are some of the biggest challenges of the 21st century. One of the core features of AD are amyloid plaques composed of amyloid-beta (Aβ) peptides. The first hypothesis proposed that cognitive deficits are linked to plaque-development and transgenic mice have been generated to study this link, thereby providing a good model to develop new therapeutic approaches. Since later it was recognised that in AD patients the cognitive deficit is rather correlated to soluble amyloid levels, consequently, a new hypothesis appeared associating the earliest amyloid toxicity to these soluble species. The purpose of this review is to give a summary of behavioural and cellular data obtained after soluble Aβ peptide administration into rodents' brain, thereby showing that this model is a valid tool to investigate AD pathology when no plaques are present. Additionally, this method offers an excellent, efficient model to test compounds which could act at such early stages of the disease.

Inflammatory Animal Model for Parkinson's Disease: The Intranigral Injection of LPS Induced the Inflammatory Process along with the Selective Degeneration of Nigrostriatal Dopaminergic Neurons

We have developed an animal model of degeneration of the nigrostriatal dopaminergic neurons, the neuronal system involved in Parkinson's disease (PD). The implication of neuroinflammation on this disease was originally established in 1988, when the presence of activated microglia in the substantia nigra (SN) of parkinsonians was reported by McGeer et al. Neuroinflammation could be involved in the progression of the disease or even has more direct implications. We injected 2 μg of the potent proinflammatory compound lipopolysaccharide (LPS) in different areas of the CNS, finding that SN displayed the highest inflammatory response and that dopaminergic (body) neurons showed a special and specific sensitivity to this process with the induction of selective dopaminergic degeneration. Neurodegeneration is induced by inflammation since it is prevented by anti-inflammatory compounds. The special sensitivity of dopaminergic neurons seems to be related to the endogenous dopaminergic content, since it is overcome by dopamine depletion. Compounds that activate microglia or induce inflammation have similar effects to LPS. This model suggest that inflammation is an important component of the degeneration of the nigrostriatal dopaminergic system, probably also in PD. Anti-inflammatory treatments could be useful to prevent or slow down the rate of dopaminergic degeneration in this disease.

Effects of estrogens and endocrine-disrupting chemicals on cell differentiation-survival-proliferation in brain: contributions of neuronal cell lines

Estrogens and estrogen receptors (ER) are key actors in the control of differentiation and survival and act on extrareproductive tissues such as brain. Thus, estrogens may display neuritogenic effects during development and neuroprotective effects in the pathophysiological context of brain ischemia and neurodegenerative pathologies like Alzheimer's disease or Parkinson's disease. Some of these effects require classical transcriptional "genomic" mechanisms through ER, whereas other effects appear to rely clearly on "membrane-initiated mechanisms" through cytoplasmic signal transduction pathways. Disturbances of these mechanisms by endocrine-disrupting chemicals (EDC) may exert adverse effects on brain. Some EDC may act via ER-independent mechanisms but might cross-react with endogenous estrogen. Other EDC may act through ER-dependent mechanisms and display agonistic/antagonistic estrogenic properties. Because of these potential effects of EDC, it is necessary to establish sensitive cell-based assays to determine EDC effects on brain. In the present review, some effects of estrogens and EDC are described with focus on ER-mediated effects in neuronal cells. Particular attention is given to PC12 cells, an interesting model to study the mechanisms underlying ER-mediated differentiating and neuroprotective effects of estrogens.

Midlife psychological stress and risk of dementia: a 35-year longitudinal population study

The number of people with dementia has increased dramatically with global ageing. Nevertheless, the pathogeneses of these diseases are not sufficiently understood. The present study aims to analyse the relationship between psychological stress in midlife and the development of dementia in late-life. A representative sample of females (n = 1462) aged 38-60 years were examined in 1968-69 and re-examined in 1974-75, 1980-81, 1992-93 and 2000-03. Psychological stress was rated according to a standardized question in 1968, 1974 and 1980. Dementia was diagnosed according to Diagnostic and Statistical Manual of Mental Disorders criteria based on information from neuropsychiatric examinations, informant interviews, hospital records and registry data. During the 35-year follow-up, 161 females developed dementia (105 Alzheimer's disease, 40 vascular dementia and 16 other dementias). We found that the risk of dementia (hazard ratios, 95% confidence intervals) was increased in females reporting frequent/constant stress in 1968 (1.60, 1.10-2.34), in 1974 (1.65, 1.12-2.41) and in 1980 (1.60, 1.01-2.52). Frequent/constant stress reported in 1968 and 1974 was associated with Alzheimer's disease. Reporting stress at one, two or three examinations was related to a sequentially higher dementia risk. Compared to females reporting no stress, hazard ratios (95% confidence intervals) for incident dementia were 1.10 (0.71-1.71) for females reporting frequent/constant stress at one examination, 1.73 (1.01-2.95) for those reporting stress at two examinations and 2.51 (1.33-4.77) at three examinations. To conclude, we found an association between psychological stress in middle-aged women and development of dementia, especially Alzheimer's disease. More studies are needed to confirm our findings and to study potential neurobiological mechanisms of these associations.

Effects of stress and stress hormones on amyloid-beta protein and plaque deposition

Growing evidence indicates that physical and psychosocial stressors, in part acting through the hypothalamic-pituitary-adrenal (HPA) axis, may accelerate the process of Alzheimer's disease (AD). In this review, we summarize recent research related to the effects of stress and stress hormones on the various disease process elements associated with AD. Specifically, we focus on the relationships among chronic stressors, HPA axis activity, amyloid-beta protein, and amyloid-beta plaque deposition in mouse models of AD. The potential mechanisms by which stress and stress-related components, especially corticotrophin-releasing factor and its receptors, influence the pathogenesis of AD are discussed.

Protective actions of sex steroid hormones in Alzheimer's disease

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

Plasma cortisol and progression of dementia in subjects with Alzheimer-type dementia

OBJECTIVE

Studies of subjects with dementia of the Alzheimer type have reported correlations between increases in activity of the hypothalamic-pituitary-adrenal (HPA) axis and hippocampal degeneration. In this study, the authors sought to determine whether increases in plasma cortisol, a marker of HPA activity, were associated with clinical and cognitive measures of the rate of disease progression in subjects with Alzheimer-type dementia.

METHOD

Thirty-three subjects with very mild and mild Alzheimer-type dementia and 21 subjects without dementia were assessed annually for up to 4 years with the Clinical Dementia Rating scale and a battery of neuropsychological tests. Plasma was obtained at 8 a.m. on a single day and assayed for cortisol. Rates of change over time in the clinical and cognitive measures were derived from growth curve models.

RESULTS

In the subjects with dementia, but not in those without dementia, higher plasma cortisol levels were associated with more rapidly increasing symptoms of dementia and more rapidly decreasing performance on neuropsychological tests associated with temporal lobe function. No associations were observed between plasma cortisol levels and clinical and cognitive assessments obtained at the single assessment closest in time to the plasma collection.

CONCLUSIONS

Higher HPA activity, as reflected by increased plasma cortisol levels, is associated with more rapid disease progression in subjects with Alzheimer-type dementia.

Reproductive hormones modulate oxidative stress in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease characterized by gradual cognitive decline, impairments in speech and language, and dysfunction in the sensorimotor systems, culminating in complete reliance on nursing care. Oxidative stress, caused by an imbalance in the pro-oxidant/antioxidant mechanisms in the body, has been implicated in AD pathogenesis, as in many other age-associated diseases such as atherosclerosis, Parkinson disease, and amyotrophic lateral sclerosis. Although the hormones estrogen, progesterone, testosterone, and luteinizing hormone are best known for their roles in reproduction, many studies show these hormones have other roles, including neuroprotection. Changes in the levels of these hormones that occur in reproductive senescence are hypothesized to increase risk of AD, as a result of reduced protection against oxidative insults. The Abeta peptide, overproduction of which is thought to be a key pathogenic event in the development of AD, is neurotoxic, most likely due to its ability to promote oxidative stress. The reproductive hormones are known to influence Abeta metabolism, and this review discusses the beneficial and detrimental effects these hormones have on Abeta production and oxidative stress, and their relevance in potential AD therapies.

Glucocorticoids increase amyloid-beta and tau pathology in a mouse model of Alzheimer's disease

Various environmental and genetic factors influence the onset and progression of Alzheimer's disease (AD). Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis, which controls circulating levels of glucocorticoid hormones, occurs early in AD, resulting in increased cortisol levels. Disturbances of the HPA axis have been associated with memory impairments and may contribute to the cognitive decline that occurs in AD, although it is unknown whether such effects involve modulation of the amyloid beta-peptide (Abeta) and tau. Using in vitro and in vivo experiments, we report that stress-level glucocorticoid administration increases Abeta formation by increasing steady-state levels of amyloid precursor protein (APP) and beta-APP cleaving enzyme. Additionally, glucocorticoids augment tau accumulation, indicating that this hormone also accelerates the development of neurofibrillary tangles. These findings suggest that high levels of glucocorticoids, found in AD, are not merely a consequence of the disease process but rather play a central role in the development and progression of AD.

Sex steroids, APOE genotype and the innate immune system

Microglia are a primary cellular component of the CNS innate immune system. Their response to conserved pathogen motifs is inherent and leads to the release of cytoactive factors that impact surrounding neurons and glia. The microglial response is modified by the local tissue environment and by "global" factors such as gender. Exposure to estrogen and testosterone, in general, down regulate microglia and peripheral macrophage function, promoting an anti-inflammatory phenotype. Other global factors, however, can "override" the gender-based effects demonstrated by estrogen or testosterone. Apolipoprotein E (APOE) genotype and the expression of specific isoforms of apolipoprotein E differentially regulate microglial and peripheral macrophage function. Our studies have shown that the presence of the APOE4 gene, a known risk factor for AD and other neurodegenerative diseases, promotes a pro-inflammatory macrophage phenotype in neonatal microglia. However, in adult mice, the APOE genotype-specific effect depends on gender. Peritoneal macrophages from female adult APOE3 and APOE4 targeted replacement mice do not demonstrate an APOE genotype-specific response, whereas adult male APOE4 targeted replacement mice show enhanced macrophage responsiveness compared to adult male APOE3 mice. At least part of the altered macrophage response in APOE4 male mice may be due to differences in androgen receptor sensitivity to testosterone. These data re-enforce the concept that classical activation in macrophages has multiple levels of regulation, dictated by competing or synergistic factors and genotype.

The role of inflammation in Alzheimer's disease

Considerable evidence gained over the past decade has supported the conclusion that neuroinflammation is associated with Alzheimer's disease (AD) pathology. Inflammatory components related to AD neuroinflammation include brain cells such as microglia and astrocytes, the classic and alternate pathways of the complement system, the pentraxin acute-phase proteins, neuronal-type nicotinic acetylcholine receptors (AChRs), peroxisomal proliferators-activated receptors (PPARs), as well as cytokines and chemokines. Both the microglia and astrocytes have been shown to generate beta-amyloid protein (Abeta), one of the main pathologic features of AD. Abeta itself has been shown to act as a pro-inflammatory agent causing the activation of many of the inflammatory components. Further substantiation for the role of neuroinflammation in AD has come from studies that demonstrate patients who took non-steroidal anti-inflammatory drugs had a lower risk of AD than those who did not. These same results have led to increased interest in pursuing anti-inflammatory therapy for AD but with poor results. On the other hand, increasing amount of data suggest that AChRs and PPARs are involved in AD-induced neuroinflammation and in this regard, future therapy may focus on their specific targeting in the AD brain.

Estrogen's effects on central and circulating immune cells vary with reproductive age

Previous work from this lab has shown that estrogen attenuates inflammatory cytokine production following brain lesions in young adult female rats, but not in older, reproductive senescent females. The present study was designed to elucidate whether these effects result from estrogen's actions on brain-resident immune cells (microglia) or on circulating immune cells recruited to the brain from blood. Microglia, harvested from the olfactory bulbs of ovariectomized young adult and reproductive senescent animals, were pretreated with 17beta-estradiol and subsequently with the bacterial endotoxin LPS. LPS treatment significantly increased the pro-inflammatory cytokine IL-1beta in microglial cultures harvested from young and senescent females, but estrogen treatment had no effect on cytokine expression in either group. In young adult-derived microglia, LPS treatment also increased nitric oxide (NO), which was attenuated by estrogen, and MMP-9, which was not affected by estrogen. Reproductive senescent-derived microglia cultures had higher basal expression of NO and MMP-9 activity as compared to those from young adult microglial cultures, although LPS did not further stimulate these inflammatory markers. In blood cultures, LPS stimulated a dose-dependent increase in the inflammatory cytokine TNF-alpha expression in both young adult and reproductive senescent animals. Estrogen replacement significantly attenuated TNF-alpha induction by LPS in blood cultures derived from young adult females. Paradoxically, estrogen replacement increased LPS-induced TNF-alpha expression in blood cultures derived from reproductive senescent animals as compared to age-matched controls. The age and estrogen dependent effects on circulating immune cells found in whole blood cultures closely mimic the effects of estrogen on cytokine expression in the young and senescent animals that we reported in vivo, supporting the hypothesis that the immunosuppressive actions of estrogen replacement on neural injury may result from hormone-action on circulating immune cells.

Estrogen increases proteasome activity in murine microglial cells

During inflammation, microglial cells go through phenotypic and functional changes that include the production and release of large amounts of oxygen and nitrogen radicals. As such, activated microglia are subject to heightened oxidative stress. The multicatalytic proteasome clears oxidized and damaged proteins from cells, and has been shown to be an important aspect of the microglial compensatory response to activation. The female sex steroid estrogen is both cytoprotective and anti-inflammatory, and has been shown to affect microglial signaling in particular. To determine if estrogen might affect the proteasome in microglial cells, we examined the effects of 17 beta-estradiol treatment on proteasome activity in N9 microglial cells. Specifically, we measured ATP-dependent and ATP-independent chymotrypsin-like, trypsin-like, and peptidyl glutamyl peptide hydrolase (PGPH)-like activities in response to both 17 beta-estradiol and interferon gamma. Data indicate that estrogen, but not interferon gamma, significantly increases ATP-dependent chymotrypsin-like and PGPH-like activity. Furthermore, this effect was blocked by the p44/42 MAPK inhibitor PD98059. Hence, these data demonstrate that through the MAPK pathway, estrogen can upregulate proteasome activity, suggesting a possible mechanism for estrogen's cytoprotective effects.

Modulation of hippocampal cell proliferation, memory, and amyloid plaque deposition in APPsw (Tg2576) mutant mice by isolation stress

Tg2576 transgenic mice (mice overexpressing the "Swedish" mutation in the human amyloid precursor protein 695) demonstrated a decreased capacity for cell proliferation in the dentate gyrus of the hippocampus compared with non-transgenic littermates at 3 months, 6 months and 9 months of age. Isolation stress induced by individually housing each mouse from the time of weaning further decreased hippocampal cell proliferation in Tg2576 mice as well as in non-transgenic littermates at 6 months of age. Decreases in hippocampal cell proliferation in isolated Tg2576 mice were associated with impairments in contextual but not cued memory. Fluoxetine administration increased cell proliferation and improved contextual memory in isolated Tg2576 mice. Further, isolation stress accelerated the age-dependent deposition of beta-amyloid 42 plaques in Tg2576 mice. Numerous beta-amyloid plaques were found in isolated but not non-isolated Tg2576 mice at 6 months of age. These results suggest that Tg2576 mice, a mouse model of Alzheimer disease, have an impaired ability to generate new cells in the dentate gyrus of the hippocampus and that the magnitude of this impairment can be modulated by behavioral interventions and drugs known to have effects on hippocampal neurogenesis in normal rodents. Unexpectedly, isolation stress also appeared to accelerate the underlying process of beta-amyloid plaque deposition in Tg2576 mice. These results suggest that stress may have an impact on the underlying disease process associated with Alzheimer's disease.

Estrogen decreases zinc transporter 3 expression and synaptic vesicle zinc levels in mouse brain

Previous studies suggest that female sex hormones modulate synaptic zinc levels, which may influence amyloid plaque formation and Alzheimer's disease progression. We examined the effects of ovariectomy and estrogen supplement on the levels of synaptic zinc and zinc transporter protein Znt3 in the brain. Ovariectomy was performed on 5-month-old mice, and 2 weeks later, pellets containing vehicle, low (0.18 mg/pellet), or high dose (0.72 mg) 17beta-estradiol were implanted. After 4 weeks, animals were decapitated, and blood and brain were collected for analysis. Blood analysis indicated that estrogen implants altered plasma estrogen levels in a dose-dependent manner. Analysis of brain tissue showed that ovariectomy raised hippocampal synaptic vesicle zinc levels, whereas estrogen replacement lowered these zinc levels. Western blots revealed that Znt3 levels in the brain were modulated in parallel with synaptic zinc levels, whereas no change was detected in the levels of Znt3 mRNA, as determined by Northern blot and reverse transcriptase-PCR analysis. However, mRNA levels of the delta subunit of adaptor protein complex (AP)-3, which modulates the level of Znt3 levels, were altered by estrogen depletion or replacement. These data demonstrate that estrogen alters the levels of Znt3 and synaptic vesicle zinc in female mice, probably through changing AP-3 delta expression. Since synaptic zinc may play a key role in neuronal death in acute brain injury as well as in plaque formation in Alzheimer's disease, and since estrogen may be beneficial in both conditions, our results may provide new insights into the effects of estrogen on the brain.

Transient appearance of amyloid precursor protein plaques in the brain of thymectomized rats after human leptomeningeal cell grafts

Cells cultured from Alzheimer disease leptomeninges or skin were grafted into the cortex of adult thymectomized rats. At 3 days post-implant, plaque-like aggregates were found in the cortex, corpus callosum, septum and caudate nucleus. These structures were immunopositive for human amyloid precursor protein (APP), human amyloid beta peptide (Abeta), cathepsin D, apolipoprotein E and ubiquitin. Aberrant tau+ neurites, reactive astrocytes and microglia were associated with many aggregates. Although birefringent amyloid occupied the central area of most aggregates, these structures had disappeared by l month post-implant. Abeta and APP produced by grafted non-neural human cells can penetrate rat brain and form plaque-like structures, which can be effectively cleared by the rat.