Neuroestrogen-Dependent Transcriptional Activity in the Brains of ERE-Luciferase Reporter Mice following Short- and Long-Term Ovariectomy

Chronic inflammation, neuroglial dysfunction, and plasmalogen deficiency as a new pathobiological hypothesis addressing the overlap between post-COVID-19 symptoms and myalgic encephalomyelitis/chronic fatigue syndrome

After five waves of coronavirus disease 2019 (COVID-19) outbreaks, it has been recognized that a significant portion of the affected individuals developed long-term debilitating symptoms marked by chronic fatigue, cognitive difficulties ("brain fog"), post-exertional malaise, and autonomic dysfunction. The onset, progression, and clinical presentation of this condition, generically named post-COVID-19 syndrome, overlap significantly with another enigmatic condition, referred to as myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). Several pathobiological mechanisms have been proposed for ME/CFS, including redox imbalance, systemic and central nervous system inflammation, and mitochondrial dysfunction. Chronic inflammation and glial pathological reactivity are common hallmarks of several neurodegenerative and neuropsychiatric disorders and have been consistently associated with reduced central and peripheral levels of plasmalogens, one of the major phospholipid components of cell membranes with several homeostatic functions. Of great interest, recent evidence revealed a significant reduction of plasmalogen contents, biosynthesis, and metabolism in ME/CFS and acute COVID-19, with a strong association to symptom severity and other relevant clinical outcomes. These bioactive lipids have increasingly attracted attention due to their reduced levels representing a common pathophysiological manifestation between several disorders associated with aging and chronic inflammation. However, alterations in plasmalogen levels or their lipidic metabolism have not yet been examined in individuals suffering from post-COVID-19 symptoms. Here, we proposed a pathobiological model for post-COVID-19 and ME/CFS based on their common inflammation and dysfunctional glial reactivity, and highlighted the emerging implications of plasmalogen deficiency in the underlying mechanisms. Along with the promising outcomes of plasmalogen replacement therapy (PRT) for various neurodegenerative/neuropsychiatric disorders, we sought to propose PRT as a simple, effective, and safe strategy for the potential relief of the debilitating symptoms associated with ME/CFS and post-COVID-19 syndrome.

Cardiometabolic health, menopausal estrogen therapy and the brain: How effects of estrogens diverge in healthy and unhealthy preclinical models of aging

Research in preclinical models indicates that estrogens are neuroprotective and positively impact cognitive aging. However, clinical data are equivocal as to the benefits of menopausal estrogen therapy to the brain and cognition. Pre-existing cardiometabolic disease may modulate mechanisms by which estrogens act, potentially reducing or reversing protections they provide against cognitive decline. In the current review we propose mechanisms by which cardiometabolic disease may alter estrogen effects, including both alterations in actions directly on brain memory systems and actions on cardiometabolic systems, which in turn impact brain memory systems. Consideration of mechanisms by which estrogen administration can exert differential effects dependent upon health phenotype is consistent with the move towards precision or personalized medicine, which aims to determine which treatment interventions will work for which individuals. Understanding effects of estrogens in both healthy and unhealthy models of aging is critical to optimizing the translational link between preclinical and clinical research.

Estrogen signaling via estrogen receptor alpha and its implications for neurodegeneration associated with Alzheimer's disease in aging women

Estrogen receptor alpha (ERα) is a transcription factor activated by estrogenic hormones to regulate gene expression in certain organs, including the brain. In the brain, estrogen signaling pathways are central for maintaining cognitive functions. Herein, we review the neuroprotective effects of estrogens mediated by ERα. The estrogen/ERα pathways are affected by the reduction of estrogens in menopause, and this event may be a risk factor for neurodegeneration associated with Alzheimer's disease in women. Thus, developing a better understanding of estrogen/ERα signaling may be critical for defining new biomarkers and potential therapeutic targets for Alzheimer's disease in women.

History of Previous Midlife Estradiol Treatment Permanently Alters Interactions of Brain Insulin-like Growth Factor-1 Signaling and Hippocampal Estrogen Synthesis to Enhance Cognitive Aging in a Rat Model of Menopause

Across species, including humans, elevated levels of brain estrogen receptor (ER) α are associated with enhanced cognitive aging, even in the absence of circulating estrogens. In rodents, short-term estrogen treatment, such as that commonly used in the menopausal transition, results in long-term increases in ERα levels in the hippocampus, leading to enhanced memory long after termination of estrogen treatment. However, mechanisms by which increased levels of brain ERα enhances cognitive aging remain unclear. Here we demonstrate in aging female rats that insulin-like growth factor-1 (IGF-1), which can activate ER via ligand-independent mechanisms, requires concomitant synthesis of brain-derived neuroestrogens to phosphorylate ERα via MAPK signaling, ultimately resulting in enhanced memory. In a rat model of menopause involving long-term ovarian hormone deprivation, hippocampal neuroestrogen activity decreases, altering IGF-1 activity and resulting in impaired memory. However, this process is reversed by short-term estradiol treatment. Forty days of estradiol exposure following ovariectomy results in maintenance of neuroestrogen levels that persist beyond the period of hormone treatment, allowing for continued interactions between IGF-1 and neuroestrogen signaling, elevated levels of hippocampal ERα, and ultimately enhanced memory. Collectively, results demonstrate that short-term estradiol use following loss of ovarian function has long-lasting effects on hippocampal function and memory by dynamically regulating cellular mechanisms that promote activity of ERα in the absence of circulating estrogens. Translational impacts of these findings suggest lasting cognitive benefits of short-term estrogen use near menopause and highlight the importance of hippocampal ERα, independent from the role of circulating estrogens, in regulating memory in aging females.SIGNIFICANCE STATEMENT Declines in ovarian hormones following menopause coincide with increased risk of cognitive decline. Because of potential health risks, current recommendations are that menopausal estrogen therapy be limited to a few years. Long-term consequences for the brain and memory of this short-term midlife estrogen therapy are unclear. Here, in a rodent model of menopause, we determined mechanisms by which short-term midlife estrogen exposure can enhance hippocampal function and memory with cognitive benefits and molecular changes enduring long after termination of estrogen exposure. Our model indicates long-lasting benefits of maintaining hippocampal estrogen receptor function in the absence of ongoing estrogen exposure and suggests potential strategies for combating age-related cognitive decline.

Ovarian status modulates endocrine and neuroinflammatory responses to a murine mammary tumor

Patients with breast cancer have increased circulating inflammatory markers and mammary tumors increase neuroinflammation in rodent models. Menopausal status is not only important in the context of breast cancer as circulating estrogen influences tumor progression, but also because estrogen is anti-inflammatory and an essential modulator of endocrine function in the brain and body. Here, we manipulated "menopause" status (ovary-intact and ovariectomized) in an estrogen receptor (ER)+ mouse mammary tumor model to determine the extent to which ovarian status modulates: 1) tumor effects on estrogen concentrations and signaling in the brain, 2) tumor effects on estrogen-associated neurobiology and inflammation, and 3) the ability for tumor resection to resolve the effects of a tumor. We hypothesized that reduced circulating estradiol (E2) after an ovariectomy exacerbates tumor-induced peripheral and central inflammation. Notably, we observed ovarian-dependent modulation on tumor-induced peripheral outcomes, including E2-dependent processes and, to a lesser degree, circulating inflammatory markers. In the brain, ovariectomy exacerbated neuroinflammatory markers in select brain regions and modulated E2-related neurobiology due to a tumor and/or resection. Overall, our data suggest that ovarian status has moderate implications for tumor-induced alterations in neuroendocrinology and neuroinflammation and mild effects on peripheral inflammatory outcomes in this murine mammary tumor model.

Aromatase Inhibition Eliminates Sexual Receptivity Without Enhancing Weight Gain in Ovariectomized Marmoset Monkeys

Context

Ovarian estradiol supports female sexual behavior and metabolic function. While ovariectomy (OVX) in rodents abolishes sexual behavior and enables obesity, OVX in nonhuman primates decreases, but does not abolish, sexual behavior, and inconsistently alters weight gain.

Objective

We hypothesize that extra-ovarian estradiol provides key support for both functions, and to test this idea, we employed aromatase inhibition to eliminate extra-ovarian estradiol biosynthesis and diet-induced obesity to enhance weight gain.

Methods

Thirteen adult female marmosets were OVX and received (1) estradiol-containing capsules and daily oral treatments of vehicle (E2; n = 5); empty capsules and daily oral treatments of either (2) vehicle (VEH, 1 mL/kg, n = 4), or (3) letrozole (LET, 1 mg/kg, n = 4).

Results

After 7 months, we observed robust sexual receptivity in E2, intermediate frequencies in VEH, and virtually none in LET females (P = .04). By contrast, few rejections of male mounts were observed in E2, intermediate frequencies in VEH, and high frequencies in LET females (P = .04). Receptive head turns were consistently observed in E2, but not in VEH and LET females. LET females, alone, exhibited robust aggressive rejection of males. VEH and LET females demonstrated increased % body weight gain (P = .01). Relative estradiol levels in peripheral serum were E2 >>> VEH > LET, while those in hypothalamus ranked E2 = VEH > LET, confirming inhibition of local hypothalamic estradiol synthesis by letrozole.

Conclusion

Our findings provide the first evidence for extra-ovarian estradiol contributing to female sexual behavior in a nonhuman primate, and prompt speculation that extra-ovarian estradiol, and in particular neuroestrogens, may similarly regulate sexual motivation in other primates, including humans.

Previous estradiol treatment during midlife maintains transcriptional regulation of memory-related proteins by ERα in the hippocampus in a rat model of menopause

Previous midlife estradiol treatment, like continuous treatment, improves memory and results in lasting increases in hippocampal levels of estrogen receptor (ER) α and ER-dependent transcription in ovariectomized rodents. We hypothesized that previous and continuous midlife estradiol act to specifically increase levels of nuclear ERα, resulting in transcriptional regulation of proteins that mediate estrogen effects on memory. Ovariectomized middle-aged rats received estradiol or vehicle capsule implants. After 40 days, rats initially receiving vehicle received another vehicle capsule (ovariectomized controls). Rats initially receiving estradiol received either another estradiol (continuous estradiol) or a vehicle (previous estradiol) capsule. One month later, hippocampi were dissected and processed. Continuous and previous estradiol increased levels of nuclear, but not membrane or cytosolic ERα and had no effect on Esr1. Continuous and previous estradiol impacted gene expression and/or protein levels of mediators of estrogenic action on memory including ChAT, BDNF, and PSD-95. Findings demonstrate a long-lasting role for hippocampal ERα as a transcriptional regulator of memory following termination of previous estradiol treatment in a rat model of menopause.

Expression of type one cannabinoid receptor in different subpopulation of kisspeptin neurons and kisspeptin afferents to GnRH neurons in female mice

The endocannabinoids have been shown to target the afferents of hypothalamic neurons via cannabinoid 1 receptor (CB1) and thereby to influence their excitability at various physiological and/or pathological processes. Kisspeptin (KP) neurons form afferents of multiple neuroendocrine cells and influence their activity via signaling through a variation of co-expressed classical neurotransmitters and neuropeptides. The differential potency of endocannabinoids to influence the release of classical transmitters or neuropeptides, and the ovarian cycle-dependent functioning of the endocannabinoid signaling in the gonadotropin-releasing hormone (GnRH) neurons initiated us to study whether (a) the different subpopulations of KP neurons express CB1 mRNAs, (b) the expression is influenced by estrogen, and (c) CB1-immunoreactivity is present in the KP afferents to GnRH neurons. The aim of the study was to investigate the site- and cell-specific expression of CB1 in female mice using multiple labeling in situ hybridization and immunofluorescent histochemical techniques. The results support that CB1 mRNAs are expressed by both the GABAergic and glutamatergic subpopulations of KP neurons, the receptor protein is detectable in two-thirds of the KP afferents to GnRH neurons, and the expression of CB1 mRNA shows an estrogen-dependency. The applied estrogen-treatment, known to induce proestrus, reduced the level of CB1 transcripts in the rostral periventricular area of the third ventricle and arcuate nucleus, and differently influenced its co-localization with vesicular GABA transporter or vesicular glutamate transporter-2 in KP neurons. This indicates a gonadal cycle-dependent role of endocannabinoid signaling in the neuronal circuits involving KP neurons.

Hippocampal Aromatase Knockdown Aggravates Ovariectomy-Induced Spatial Memory Impairment, Aβ Accumulation and Neural Plasticity Deficiency in Adult Female Mice

Ovarian estrogens (mainly 17β estradiol, E2) have been involved in the regulation of the structure of hippocampus, the center of spatial memory. In recent years, high levels of aromatase (AROM), the estrogen synthase, has been localized in hippocampus; and this hippocampus-derived E2 seems to be functional in synaptic plasticity and spatial memory as ovarian E2 does. However, the contribution of ovarian E2 and hippocampal E2 to spatial memory and neural plasticity remains unclear. In this study, AROM-specific RNA interference AAVs (shAROM) were constructed and injected into the hippocampus of control or ovariectomized (OVX) mice. Four weeks later the spatial learning and memory behavior was examined with Morris water maze, the expression of hippocampal Aβ related proteins, selected synaptic proteins and CA1 synapse density, actin polymerization related proteins and CA1 spine density were also examined. The results showed that while OVX and hippocampal shAROM contributed similarly to most of the parameters examined, shAROM induced more increase in BACE1 (amyloidogenic β-secretase), more decrease in neprilysin (Aβ remover) and Profilin-1 (actin polymerization inducer). More importantly, combined OVX and shAROM treatment displayed most significant impairment of spatial learning and memory as well as decrease in synaptic plasticity compared to OVX or shAROM alone. In conclusion, the above results clearly demonstrated the crucial role of hippocampal E2 in the regulation of the structure and function of hippocampus besides ovarian E2, indicating that hippocampal E2 content should also be taken into consideration during estrogenic replacement.

Estrogen receptor α: a critical role in successful female cognitive aging

Due to potential health risks, current recommendations are that individuals who wish to use hormone therapy to treat menopausal symptoms do so for the shortest period of time possible. In our investigation into how short-term use of estrogens in midlife following loss of ovarian function exerts long-term effects on female cognitive aging in rodents, we discovered a link between the ability of previous exposure to estradiol to enhance memory in the long term and its ability to increase estrogen receptor α (ERα) levels in the hippocampus, a brain area important for memory. Follow-up studies in model systems implicate a role for ERα in enhanced cognitive function independent of ovarian or exogenously administered estrogens. Results are consistent with clinical studies in which brain ERα levels in older women and men are related to cognitive functioning and risk of cognitive decline is associated with polymorphisms in the gene that transcribes ERα. Research in preclinical models reveals mechanisms through which ERα can be activated and affect cognition in the absence of ovarian estrogens, including ligand-independent activation via insulin-like growth factor-1 signaling and activation by brain-derived neuroestrogens. This report reviews preclinical and clinical data that collectively point to the importance of ERα in cognition and highlights the need to differentiate the role of estrogen receptors from their classical ligands as we seek approaches to facilitate successful cognitive aging.

Effects of Ovariectomy and Sex Hormone Replacement on Numbers of Kisspeptin-, Neurokinin B- and Dynorphin A-immunoreactive Neurons in the Arcuate Nucleus of the Hypothalamus in Obese and Diabetic Rats

KNDy neurons co-expressing kisspeptin (KP), neurokinin B (NKB) and dynorphin A (DYN A) in the arcuate nucleus of the hypothalamus (ARC) are key regulators of reproduction. Their activity is influenced by metabolic and hormonal signals. Previously, we have shown that orchidectomy alters the KP-, NKB-, and DYN A-immunoreactivity in the high-fat diet-induced (HFD) obesity and diabetes type 2 (DM2) models. Considering the potential sex difference in the response of KNDy neurons, we have hypothesized that ovariectomy (OVX) and post-ovariectomy replacement with estradiol (OVX+E₂) or estradiol and progesterone (OVX+E₂+P₄) will also affect these neurons in HFD and DM2 females. Thus, each of these treatment protocols were employed for control, HFD, and DM2 groups of rats leading to nine experimental conditions within which we have determined the number of KP-, NKB-, or DYN-immunoreactive (-ir) neurons and assessed the metabolic and hormonal profiles of the animals. Accordingly: (1) no effects of group and surgery were observed on the number of KP-ir neurons; (2) the overall number of NKB-ir neurons was higher in the OVX+E₂+P₄ and OVX+E₂ animals compared to OVX; (3) overall, the number of DYN A-ir neurons was higher in DM2 vs. control group, and surgery had an effect on the number of DYN A-ir neurons; (4) the metabolic and hormonal profiles were altered in HFD and DM2 animals compared to controls. Current data together with our previously published results indicate sex-specific differences in the response of KNDy neurons to DM2.