Protection against APOE4 -associated aging phenotypes with the longevity-promoting intervention 17α-estradiol in male mice.

The apolipoprotein ε4 allele ( APOE4 ) is associated with decreased longevity, increased vulnerability to age-related declines, and disorders across multiple systems. Interventions that promote healthspan and lifespan represent a promising strategy to attenuate the development of APOE4 -associated aging phenotypes. Here we studied the ability of the longevity-promoting intervention 17α-estradiol (17αE2) to protect against age-related impairments in APOE4 versus the predominant APOE3 genotype using early middle-aged mice with knock-in of human APOE alleles. Beginning at age 10 months, male APOE3 or APOE4 mice were treated for 20 weeks with 17αE2 or vehicle then compared for indices of aging phenotypes body-wide. Across peripheral and neural measures, APOE4 was associated with poorer outcomes. Notably, 17αE2 treatment improved outcomes in a genotype-dependent manner favoring APOE4 mice. These data demonstrate a positive APOE4 bias in 17αE2-mediated healthspan actions, suggesting that longevity-promoting interventions may be useful in mitigating deleterious age-related risks associated with APOE4 genotype.

Humanin variant P3S is associated with longevity in APOE4 carriers and resists APOE4-induced brain pathology.

The APOE4 allele is recognized as a significant genetic risk factor to Alzheimer's disease (AD) and influences longevity. Nonetheless, some APOE4 carriers exhibit resistance to AD even in advanced age. Humanin, a mitochondrial-derived peptide comprising 24 amino acids, has variants linked to cognitive resilience and longevity. Our research uncovered a unique humanin variant, P3S, specifically enriched in centenarians with the APOE4 allele. Through in silico analyses and subsequent experimental validation, we demonstrated a strong affinity between humanin P3S and APOE4. Utilizing an APOE4-centric mouse model of amyloidosis (APP/PS1/APOE4), we observed that humanin P3S significantly attenuated brain amyloid-beta accumulation compared to the wild-type humanin. Transcriptomic assessments of mice treated with humanin P3S highlighted its potential mechanism involving the enhancement of amyloid beta phagocytosis. Additionally, in vitro studies corroborated humanin P3S's efficacy in promoting amyloid-beta clearance. Notably, in the temporal cortex of APOE4 carriers, humanin expression is correlated with genes associated with phagocytosis. Our findings suggest a role of the rare humanin variant P3S, especially prevalent among individuals of Ashkenazi descent, in mitigating amyloid beta pathology and facilitating phagocytosis in APOE4-linked amyloidosis, underscoring its significance in longevity and cognitive health among APOE4 carriers.

Microglia/macrophage-specific deletion of TLR-4 protects against neural effects of diet-induced obesity.

Obesity is associated with numerous adverse neural effects, including reduced neurogenesis, cognitive impairment, and increased risks for developing Alzheimer's disease (AD) and vascular dementia. Obesity is also characterized by chronic, low-grade inflammation that is implicated in mediating negative consequences body-wide. Toll-like receptor 4 (TLR4) signaling from peripheral macrophages is implicated as an essential regulator of the systemic inflammatory effects of obesity. In the brain, obesity drives chronic neuroinflammation that involves microglial activation, however the contributions of microglia-derived TLR4 signaling to the consequences of obesity are poorly understood. To investigate this issue, we first generated mice that carry an inducible, microglia/macrophage-specific deletion of TLR4 that yields long-term TLR4 knockout only in brain indicating microglial specificity. Next, we analyzed the effects of microglial TLR4 deletion on systemic and neural effects of a 16-week of exposure to control versus obesogenic high-fat diets. In male mice, TLR4 deletion generally yielded limited effects on diet-induced systemic metabolic dysfunction but significantly reduced neuroinflammation and impairments in neurogenesis and cognitive performance. In female mice maintained on obesogenic diet, TLR4 deletion partially protected against weight gain, adiposity, and metabolic impairments. Compared to males, females showed milder diet-induced neural consequences, against which TLR4 deletion was protective. Collectively, these findings demonstrate a central role of microglial TLR4 signaling in mediating the neural effects of obesogenic diet and highlight sexual dimorphic responses to both diet and TLR4.

Microglial TLR4 Mediates White Matter Injury in a Combined Model of Diesel Exhaust Exposure and Cerebral Hypoperfusion.

BACKGROUND: Air pollution particulate matter exposure and chronic cerebral hypoperfusion (CCH) contribute to white matter toxicity through shared mechanisms of neuroinflammation, oxidative stress, and myelin breakdown. Prior studies showed that exposure of mice to joint particulate matter and CCH caused supra-additive injury to corpus callosum white matter. This study examines the role of TLR4 (toll-like receptor 4) signaling in mediating neurotoxicity and myelin damage observed in joint particulate matter and CCH exposures. METHODS: Experiments utilized a novel murine model of inducible monocyte/microglia-specific TLR4 knockout (i-mTLR4-ko). Bilateral carotid artery stenosis (BCAS) was induced surgically to model CCH. TLR4-intact (control) and i-mTLR4-ko mice were exposed to 8 weeks of either aerosolized diesel exhaust particulate (DEP) or filtered air (FA) in 8 experimental groups: (1) control/FA (n=10), (2) control/DEP (n=10), (3) control/FA+BCAS (n=9), (4) control/DEP+BCAS (n=10), (5) i-mTLR4-ko/FA (n=9), (6) i-mTLR4-ko/DEP (n=8), (7) i-mTLR4-ko/FA+BCAS (n=8), and (8) i-mTLR4-ko/DEP+BCAS (n=10). Corpus callosum levels of 4-hydroxynonenal, 8-Oxo-2'-deoxyguanosine, Iba-1 (ionized calcium-binding adapter molecule 1), and dMBP (degraded myelin basic protein) were assayed via immunofluorescence to measure oxidative stress, neuroinflammation, and myelin breakdown, respectively. RESULTS: Compared with control/FA mice, control/DEP+BCAS mice exhibited increased dMBP (41%; P<0.01), Iba-1 (51%; P<0.0001), 4-hydroxynonenal (100%; P<0.0001), and 8-Oxo-2'-deoxyguanosine (65%; P<0.05). I-mTLR4 knockout attenuated responses to DEP/BCAS for all markers. CONCLUSIONS: i-mTLR4-ko markedly reduced neuroinflammation and oxidative stress and attenuated white matter degradation following DEP and CCH exposures. This suggests a potential role for targeting TLR4 signaling in individuals with vascular cognitive impairment, particularly those exposed to substantial ambient air pollution.

Effects of APOE Genotype and Western Diet on Metabolic Phenotypes in Female Mice.

Western diets high in sugars and saturated fats have been reported to induce metabolic and inflammatory impairments that are associated with several age-related disorders, including Alzheimer's disease (AD) and type 2 diabetes (T2D). The apolipoprotein E (APOE) genotype is associated with metabolic and inflammatory outcomes that contribute to risks for AD and T2D, with the APOE4 genotype increasing risks relative to the more common APOE3 allele. In this study, we investigated the impacts of the APOE genotype on systemic and neural effects of the Western diet. Female mice with knock-in of human APOE3 or APOE4 were exposed to control or Western diet for 13 weeks. In the control diet, we observed that APOE4 mice presented with impaired metabolic phenotypes, exhibiting greater adiposity, higher plasma leptin and insulin levels, and poorer glucose clearance than APOE3 mice. Behaviorally, APOE4 mice exhibited worse performance in a hippocampal-dependent learning task. In visceral adipose tissue, APOE4 mice exhibited generally higher expression levels of macrophage- and inflammation-related genes. The cerebral cortex showed a similar pattern, with higher expression of macrophage- and inflammation-related genes in APOE4 than APOE3 mice. Exposure to the Western diet yielded modest, statistically non-significant effects on most metabolic, behavioral, and gene expression measures in both APOE genotypes. Interestingly, the Western diet resulted in reduced gene expression of a few macrophage markers, specifically in APOE4 mice. The observed relative resistance to the Western diet suggests protective roles of both female sex and young adult age. Further, the data demonstrate that APOE4 is associated with deleterious systemic and neural phenotypes and an altered response to a metabolic stressor, findings relevant to the understanding of interactions between the APOE genotype and risks for metabolic disorders.

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.

Fasting-mimicking diet cycles reduce neuroinflammation to attenuate cognitive decline in Alzheimer's models.

The effects of fasting-mimicking diet (FMD) cycles in reducing many aging and disease risk factors indicate it could affect Alzheimer's disease (AD). Here, we show that FMD cycles reduce cognitive decline and AD pathology in E4FAD and 3xTg AD mouse models, with effects superior to those caused by protein restriction cycles. In 3xTg mice, long-term FMD cycles reduce hippocampal Aß load and hyperphosphorylated tau, enhance genesis of neural stem cells, decrease microglia number, and reduce expression of neuroinflammatory genes, including superoxide-generating NADPH oxidase (Nox2). 3xTg mice lacking Nox2 or mice treated with the NADPH oxidase inhibitor apocynin also display improved cognition and reduced microglia activation compared with controls. Clinical data indicate that FMD cycles are feasible and generally safe in a small group of AD patients. These results indicate that FMD cycles delay cognitive decline in AD models in part by reducing neuroinflammation and/or superoxide production in the brain.

Androgens Regulate Tau Phosphorylation Through Phosphatidylinositol 3-Kinase-Protein Kinase B-Glycogen Synthase Kinase 3ß Signaling.

Age-related testosterone depletion in men is a risk factor for Alzheimer's disease (AD). How testosterone modulates AD risk remains to be fully elucidated, although regulation of tau phosphorylation has been suggested as a contributing protective action. To investigate the relationship between testosterone and tau phosphorylation, we first evaluated the effect of androgen status on tau phosphorylation in 3xTg-AD mice. Depletion of endogenous androgens via gonadectomy resulted in increased tau phosphorylation that was prevented by acute testosterone treatment. Parallel alterations in the phosphorylation of both glycogen synthase kinase 3ß (GSK3ß) and protein kinase B (Akt) suggest possible components of the underlying signaling pathway. To further explore mechanism, primary cultured neurons were treated with a physiological concentration of testosterone or its active metabolite dihydrotestosterone (DHT). Results showed that testosterone and DHT induced significant decreases in phosphorylated tau and significant increases in phosphorylation of Akt and GSK3ß. Pharmacological inhibition of phosphatidylinositol 3-kinase (PI3K) effectively inhibited androgen-induced increases in Akt and GSK3ß phosphorylation, and decreases in tau phosphorylation. In addition, androgen receptor (AR) knock-down by small interfering RNA prevented androgen-induced changes in the phosphorylation of Akt, GSK3ß and tau, suggesting an AR-dependent mechanism. Additional experiments demonstrated androgen-induced changes in Akt, GSK3ß and tau phosphorylation in AR-expressing PC12 cells but not in AR-negative PC12 cells. Together, these results suggest an AR-dependent pathway involving PI3K-Akt-GSK3ß signaling through which androgens can reduce tau phosphorylation. These findings identify an additional protective mechanism of androgens that can improve neural health and inhibit development of AD.

Microglial transcription profiles in mouse and human are driven by APOE4 and sex.

Apolipoprotein E4 (APOE4) is the strongest genetic risk factor for sporadic Alzheimer's disease (AD). APOE4 is known to affect the function of microglia, but to what extent this gene drives microglial gene expression has thus far not been examined. Using a transgenic mouse model of AD that expresses human APOE, we identify a unique transcriptional profile associated with APOE4 expression. We also show a sex and APOE interaction, such that both female sex and APOE4 drive expression of this gene profile. We confirm these findings in human cells, using microglia derived from induced pluripotent stem cells (iMGL). Moreover, we find that these interactions are driven in part by genes related to metal processing, and we show that zinc treatment has APOE genotype-dependent effects on iMGL. These data identify a sex- and APOE4-associated microglial transcription profile and highlight the importance of considering interactive risk factors such as sex and environmental exposures.

Second to fourth digit ratio (2D:4D) is associated with dementia in women.

Alzheimer's disease is characterized by sex differences that may involve sex hormone exposure during development. Finger length ratios, an indirect measure of prenatal androgen exposure, were found to significantly differ in women with and without dementia. This finding links a relatively feminine in utero development with vulnerability to dementia in women.

Dementia risk in women higher in same-sex than opposite-sex twins.

INTRODUCTION: Hormones may be one possible mechanism underlying sex differences in dementia incidence. We examined whether presumed differential prenatal hormone milieu is related to dementia risk by comparing dementia rates in same- and opposite-sex dizygotic twin pairs in male and female twins. METHODS: The sample comprised 43,254 individuals from dizygotic twin pairs aged 60 and older from the Swedish Twin Registry. Survival analyses were conducted separately for females and males. RESULTS: Female twins from opposite-sex pairs had significantly lower dementia risk than female twins from same-sex pairs, but the differences emerged only after age 70 (hazard ratio = 0.64, P = 0.004). Results were not explained by postnatal risk factors for dementia, and no interaction between twin type and apolipoprotein E (APOE) ε4 was found. Male twins from same-sex versus opposite-sex pairs did not differ significantly. DISCUSSION: The results suggest that relatively masculine prenatal hormone milieus correlate with lower dementia risk in females.

Aging Reduces Estradiol Protection Against Neural but Not Metabolic Effects of Obesity in Female 3xTg-AD Mice.

Vulnerability to Alzheimer's disease (AD) is increased by several risk factors, including midlife obesity, female sex, and the depletion of estrogens in women as a consequence of menopause. Conversely, estrogen-based hormone therapies have been linked with protection from age-related increases in adiposity and dementia risk, although treatment efficacy appears to be affected by the age of initiation. Potential interactions between obesity, AD, aging, and estrogen treatment are likely to have significant impact on optimizing the use of hormone therapies in postmenopausal women. In the current study, we compared how treatment with the primary estrogen, 17ß-estradiol (E2), affects levels of AD-like neuropathology, behavioral impairment, and other neural and systemic effects of preexisting diet-induced obesity in female 3xTg-AD mice. Importantly, experiments were conducted at chronological ages associated with both the early and late stages of reproductive senescence. We observed that E2 treatment was generally associated with significantly improved metabolic outcomes, including reductions in body weight, adiposity, and leptin, across both age groups. Conversely, neural benefits of E2 in obese mice, including decreased ß-amyloid burden, improved behavioral performance, and reduced microglial activation, were observed only in the early aging group. These results are consistent with the perspective that neural benefits of estrogen-based therapies require initiation of treatment during early rather than later phases of reproductive aging. Further, the discordance between E2 protection against systemic versus neural effects of obesity across age groups suggests that pathways other than general metabolic function, perhaps including reduced microglial activation, contribute to the mechanism(s) of the observed E2 actions. These findings reinforce the potential systemic and neural benefits of estrogen therapies against obesity, while also highlighting the critical role of aging as a mediator of estrogens' protective actions.

Staining and Quantification of ß-Amyloid Pathology in Transgenic Mouse Models of Alzheimer's Disease.

Studies of Alzheimer's disease (AD) using experimental systems most often involve transgenic mouse models that are characterized by neural accumulation of ß-amyloid protein (Aß), which is widely hypothesized to have a key role in AD pathogenesis. Quantification of Aß in transgenic mice typically is accomplished through both biochemical and histochemical approaches. In this chapter, we describe two techniques for the histological detection of Aß, immunostaining with Aß antibodies and staining with the amyloid dye thioflavin S, and its quantification using digital imaging.

APOE genotype affects metabolic and Alzheimer-related outcomes induced by Western diet in female EFAD mice.

Development of Alzheimer's disease (AD) is regulated by interactive effects of genetic and environmental risk factors. The most significant genetic risk factor for AD is the ε4 allele of apolipoprotein E ( APOE4), which has been shown to exert greater AD risk in women. An important modifiable AD risk factor is obesity and its associated metabolic dysfunctions. Whether APOE genotype might interact with obesity in females to regulate AD pathogenesis is unclear. To investigate this issue, we studied the effects of Western diet (WD) on female EFAD mice, a transgenic mouse model of AD that includes human APOE alleles ε3 (E3FAD) and ε4 (E4FAD). EFAD mice were fed either control (10% fat, 7% sugar) or WD (45% fat, 17% sugar), and both metabolic and neuropathologic outcomes were determined. Although E4FAD mice generally exhibited poorer metabolic status at baseline, E3FAD mice showed greater diet-induced metabolic impairments. Similarly, E4FAD mice exhibited higher levels of AD-related pathology overall, but only E3FAD showed significant increases on select measures of ß-amyloid pathology after exposure to WD. These data demonstrate a gene-environment interaction between APOE and obesogenic diets in females. Understanding how AD-promoting effects of obesity are modulated by genetic factors will foster the identification of at-risk populations and development of preventive interventions.-Christensen, A., Pike, C. J. APOE genotype affects metabolic and Alzheimer-related outcomes induced by Western diet in female EFAD mice.

Effects of aging, high-fat diet, and testosterone treatment on neural and metabolic outcomes in male brown Norway rats.

Risk for Alzheimer's disease (AD) is affected by multiple factors, including aging, obesity, and low testosterone. We previously showed that obesity and low testosterone independently and interactively exacerbate AD-related outcomes in young adult rodents. The goals of the present study are two-fold: to examine whether the effects of an obesogenic diet differ with increasing age and to determine if testosterone treatment in middle-aged and aged animals mitigates negative effects of the diet. Male brown Norway rats were maintained on control or high-fat diets for 12 weeks beginning in young adulthood, middle age, or advanced age. Separate cohorts of middle-aged and aged animals were treated with testosterone during dietary manipulations. Endpoints included metabolic indices, inflammation, cognitive performance, and neural health outcomes. Aging was associated with poorer outcomes that were generally exacerbated by high-fat diet, especially at middle age. Testosterone treatment was largely without benefit, exerting only subtle effects on a select number of measures. Understanding how the deleterious effects of obesity are affected by advancing age and the ability of protective strategies such as testosterone to reduce these effects may provide significant insight into both the development and prevention of age-related cognitive decline and AD.

TLR4 inhibitor TAK-242 attenuates the adverse neural effects of diet-induced obesity.

BACKGROUND: Obesity exerts negative effects on brain health, including decreased neurogenesis, impaired learning and memory, and increased risk for Alzheimer's disease and related dementias. Because obesity promotes glial activation, chronic neuroinflammation, and neural injury, microglia are implicated in the deleterious effects of obesity. One pathway that is particularly important in mediating the effects of obesity in peripheral tissues is toll-like receptor 4 (TLR4) signaling. The potential contribution of TLR4 pathways in mediating adverse neural outcomes of obesity has not been well addressed. To investigate this possibility, we examined how pharmacological inhibition of TLR4 affects the peripheral and neural outcomes of diet-induced obesity. METHODS: Male C57BL6/J mice were maintained on either a control or high-fat diet for 12 weeks in the presence or absence of the specific TLR4 signaling inhibitor TAK-242. Outcomes examined included metabolic indices, a range of behavioral assessments, microglial activation, systemic and neuroinflammation, and neural health endpoints. RESULTS: Peripherally, TAK-242 treatment was associated with partial inhibition of inflammation in the adipose tissue but exerted no significant effects on body weight, adiposity, and a range of metabolic measures. In the brain, obese mice treated with TAK-242 exhibited a significant reduction in microglial activation, improved levels of neurogenesis, and inhibition of Alzheimer-related amyloidogenic pathways. High-fat diet and TAK-242 were associated with only very modest effects on a range of behavioral measures. CONCLUSIONS: These results demonstrate a significant protective effect of TLR4 inhibition on neural consequences of obesity, findings that further define the role of microglia in obesity-mediated outcomes and identify a strategy for improving brain health in obese individuals.

Humanin Prevents Age-Related Cognitive Decline in Mice and is Associated with Improved Cognitive Age in Humans.

Advanced age is associated with a decline in cognitive function, likely caused by a combination of modifiable and non-modifiable factors such as genetics and lifestyle choices. Mounting evidence suggests that humanin and other mitochondrial derived peptides play a role in several age-related conditions including neurodegenerative disease. Here we demonstrate that humanin administration has neuroprotective effects in vitro in human cell culture models and is sufficient to improve cognition in vivo in aged mice. Furthermore, in a human cohort, using mitochondrial GWAS, we identified a specific SNP (rs2854128) in the humanin-coding region of the mitochondrial genome that is associated with a decrease in circulating humanin levels. In a large, independent cohort, consisting of a nationally-representative sample of older adults, we find that this SNP is associated with accelerated cognitive aging, supporting the concept that humanin is an important factor in cognitive aging.

TSPO ligand PK11195 improves Alzheimer-related outcomes in aged female 3xTg-AD mice.

Alzheimer's disease (AD) pathogenesis is a multifactorial process that involves numerous pathways within the central nervous system. Thus, interventions that interact with several disease-related pathways may offer an increased opportunity for successful prevention and treatment of AD. Translocator protein 18 kD (TSPO) is a mitochondrial protein that is associated with regulation of many cellular processes including inflammation, steroid synthesis, apoptosis, and mitochondrial respiration. Although TSPO ligands have been shown to be protective in several neurodegenerative paradigms, little work has been done to assess their potential as treatments for AD. Female 3xTg-AD mice were administered the TSPO ligand PK11195 once weekly for 5 weeks beginning at 16 months, an age characterized by extensive ß-amyloid pathology and behavioral impairments. Animals treated with PK11195 showed improvements in behavior and modest reductions of in both soluble and deposited ß-amyloid. The finding that short-term PK11195 treatment was effective in improving both behavioral and pathological outcomes in a model of late-stage AD supports further investigation of TSPO ligands as potential therapeutics for the treatment of AD.

The Oxygen Paradox, the French Paradox, and age-related diseases.

A paradox is a seemingly absurd or impossible concept, proposition, or theory that is often difficult to understand or explain, sometimes apparently self-contradictory, and yet ultimately correct or true. How is it possible, for example, that oxygen "a toxic environmental poison" could be also indispensable for life (Beckman and Ames Physiol Rev 78(2):547-81, 1998; Stadtman and Berlett Chem Res Toxicol 10(5):485-94, 1997)?: the so-called Oxygen Paradox (Davies and Ursini 1995; Davies Biochem Soc Symp 61:1-31, 1995). How can French people apparently disregard the rule that high dietary intakes of cholesterol and saturated fats (e.g., cheese and paté) will result in an early death from cardiovascular diseases (Renaud and de Lorgeril Lancet 339(8808):1523-6, 1992; Catalgol et al. Front Pharmacol 3:141, 2012; Eisenberg et al. Nat Med 22(12):1428-1438, 2016)?: the so-called, French Paradox. Doubtless, the truth is not a duality and epistemological bias probably generates apparently self-contradictory conclusions. Perhaps nowhere in biology are there so many apparently contradictory views, and even experimental results, affecting human physiology and pathology as in the fields of free radicals and oxidative stress, antioxidants, foods and drinks, and dietary recommendations; this is particularly true when issues such as disease-susceptibility or avoidance, "healthspan," "lifespan," and ageing are involved. Consider, for example, the apparently paradoxical observation that treatment with low doses of a substance that is toxic at high concentrations may actually induce transient adaptations that protect against a subsequent exposure to the same (or similar) toxin. This particular paradox is now mechanistically explained as "Adaptive Homeostasis" (Davies Mol Asp Med 49:1-7, 2016; Pomatto et al. 2017a; Lomeli et al. Clin Sci (Lond) 131(21):2573-2599, 2017; Pomatto and Davies 2017); the non-damaging process by which an apparent toxicant can activate biological signal transduction pathways to increase expression of protective genes, by mechanisms that are completely different from those by which the same agent induces toxicity at high concentrations. In this review, we explore the influences and effects of paradoxes such as the Oxygen Paradox and the French Paradox on the etiology, progression, and outcomes of many of the major human age-related diseases, as well as the basic biological phenomenon of ageing itself.

Age-dependent regulation of obesity and Alzheimer-related outcomes by hormone therapy in female 3xTg-AD mice.

Depletion of ovarian hormones at menopause is associated with increased Alzheimer's disease (AD) risk. Hormone loss also increases central adiposity, which promotes AD development. One strategy to improve health outcomes in postmenopausal women is estrogen-based hormone therapy (HT), though its efficacy is controversial. The window of opportunity hypothesis posits that HT is beneficial only if initiated near the onset of menopause. Here, we tested this hypothesis by assessing the efficacy of HT against diet-induced obesity and AD-related pathology in female 3xTg-AD mice at early versus late middle-age. HT protected against obesity and reduced ß-amyloid burden only at early middle-age. One mechanism that contributes to AD pathogenesis is microglial activation, which is increased by obesity and reduced by estrogens. In parallel to its effects on ß-amyloid accumulation, we observed that HT reduced morphological evidence of microglial activation in early but not late middle-age. These findings suggest that HT may be effective during human perimenopause in reducing indices of obesity and AD-related pathology, a conclusion consistent with the window of opportunity hypothesis.