Estradiol modulates translocator protein (TSPO) and steroid acute regulatory protein (StAR) via protein kinase A (PKA) signaling in hypothalamic astrocytes

TSPO deficiency exacerbates acute brain damage after intracerebral hemorrhage in male mice

Intracerebral hemorrhage (ICH) is a stroke subtype with no effective treatment despite high morbidity and mortality rates. The delineation of the mechanisms of brain damage after ICH is critical to identifying novel molecular targets for therapeutic intervention. Apart from the augmented expression of 18 kDa translocator protein (TSPO) in microglia/macrophages post-ICH and its potential to track neuroinflammation, the precise function of TSPO after brain damage remains largely enigmatic. In the present study, we employed transgenic animal models, such as global and myeloid-specific conditional knockouts, to elucidate the functional role of TSPO in ICH-induced acute brain damage. Neurological deficits, neurodegeneration, and neuroinflammation were assessed at 3-days post-ICH in male and female mice. Male TSPO global knockout and conditional knockout exhibited enhanced neurobehavioral deficits with a concomitant increase in neurodegeneration and neuroinflammation compared to their respective controls. Interestingly, their female counterparts did not exhibit augmented brain damage compared to the respective controls. Mechanistically, studies employing RNA-Seq and subsequent functional validation demonstrate that TSPO could regulate brain cholesterol efflux, which could partly be responsible for enhanced brain damage in TSPO KO male mice after ICH, warranting further investigation.

Steroidogenic acute regulatory protein in fish

The importance of steroidogenesis is underscored by its vital and conserved functions from higher to lower vertebrate species, such as stress, immune and inflammatory responses, sexual development and reproduction, osmoregulation and even the ability to adapt to the environment and environmental changes. Correspondingly, the rate-limiting step of steroidogenesis mediated by the steroidogenic acute regulatory protein is an ongoing target for scientific investigation. An expanding collection of studies has now reported key similarities, as well as some differences, in the transcriptional and translational regulation of steroidogenic acute regulatory protein across species. This review will discuss the current understanding of steroidogenic acute regulatory protein in fish, as these lower vertebrate models uniquely rely on steroid hormones for osmotic balance, reproductive functions, responses to environmental stimuli and much more.

Integrated Approach for Testing and Assessment for Developmental Neurotoxicity (DNT) to Prioritize Aromatic Organophosphorus Flame Retardants

Organophosphorus flame retardants (OPFRs) are abundant and persistent in the environment but have limited toxicity information. Their similarity in structure to organophosphate pesticides presents great concern for developmental neurotoxicity (DNT). However, current in vivo testing is not suitable to provide DNT information on the amount of OPFRs that lack data. Over the past decade, an in vitro battery was developed to enhance DNT assessment, consisting of assays that evaluate cellular processes in neurodevelopment and function. In this study, behavioral data of small model organisms were also included. To assess if these assays provide sufficient mechanistic coverage to prioritize chemicals for further testing and/or identify hazards, an integrated approach to testing and assessment (IATA) was developed with additional information from the Integrated Chemical Environment (ICE) and the literature. Human biomonitoring and exposure data were identified and physiologically-based toxicokinetic models were applied to relate in vitro toxicity data to human exposure based on maximum plasma concentration. Eight OPFRs were evaluated, including aromatic OPFRs (triphenyl phosphate (TPHP), isopropylated phenyl phosphate (IPP), 2-ethylhexyl diphenyl phosphate (EHDP), tricresyl phosphate (TMPP), isodecyl diphenyl phosphate (IDDP), tert-butylphenyl diphenyl phosphate (BPDP)) and halogenated FRs ((Tris(1,3-dichloro-2-propyl) phosphate (TDCIPP), tris(2-chloroethyl) phosphate (TCEP)). Two representative brominated flame retardants (BFRs) (2,2'4,4'-tetrabromodiphenyl ether (BDE-47) and 3,3',5,5'-tetrabromobisphenol A (TBBPA)) with known DNT potential were selected for toxicity benchmarking. Data from the DNT battery indicate that the aromatic OPFRs have activity at similar concentrations as the BFRs and should therefore be evaluated further. However, these assays provide limited information on the mechanism of the compounds. By integrating information from ICE and the literature, endocrine disruption was identified as a potential mechanism. This IATA case study indicates that human exposure to some OPFRs could lead to a plasma concentration similar to those exerting in vitro activities, indicating potential concern for human health.

Identification of a mechanism promoting mitochondrial sterol accumulation during myocardial ischemia-reperfusion: role of TSPO and STAR

Hypercholesterolemia is a major risk factor for coronary artery diseases and cardiac ischemic events. Cholesterol per se could also have negative effects on the myocardium, independently from hypercholesterolemia. Previously, we reported that myocardial ischemia-reperfusion induces a deleterious build-up of mitochondrial cholesterol and oxysterols, which is potentiated by hypercholesterolemia and prevented by translocator protein (TSPO) ligands. Here, we studied the mechanism by which sterols accumulate in cardiac mitochondria and promote mitochondrial dysfunction. We performed myocardial ischemia-reperfusion in rats to evaluate mitochondrial function, TSPO, and steroidogenic acute regulatory protein (STAR) levels and the related mitochondrial concentrations of sterols. Rats were treated with the cholesterol synthesis inhibitor pravastatin or the TSPO ligand 4'-chlorodiazepam. We used Tspo deleted rats, which were phenotypically characterized. Inhibition of cholesterol synthesis reduced mitochondrial sterol accumulation and protected mitochondria during myocardial ischemia-reperfusion. We found that cardiac mitochondrial sterol accumulation is the consequence of enhanced influx of cholesterol and not of the inhibition of its mitochondrial metabolism during ischemia-reperfusion. Mitochondrial cholesterol accumulation at reperfusion was related to an increase in mitochondrial STAR but not to changes in TSPO levels. 4'-Chlorodiazepam inhibited this mechanism and prevented mitochondrial sterol accumulation and mitochondrial ischemia-reperfusion injury, underlying the close cooperation between STAR and TSPO. Conversely, Tspo deletion, which did not alter cardiac phenotype, abolished the effects of 4'-chlorodiazepam. This study reveals a novel mitochondrial interaction between TSPO and STAR to promote cholesterol and deleterious sterol mitochondrial accumulation during myocardial ischemia-reperfusion. This interaction regulates mitochondrial homeostasis and plays a key role during mitochondrial injury.

Evidence for crosstalk between the aryl hydrocarbon receptor and the translocator protein in mouse lung epithelial cells

Cellular homeostasis requires the use of multiple environmental sensors that can respond to a variety of endogenous and exogenous compounds. The aryl hydrocarbon receptor (AHR) is classically known as a transcription factor that induces genes that encode drug metabolizing enzymes when bound to toxicants such as 2,3,7,8-tetrachlorodibenzo-ρ-dioxin (TCDD). The receptor has a growing number of putative endogenous ligands, such as tryptophan, cholesterol, and heme metabolites. Many of these compounds are also linked to the translocator protein (TSPO), an outer mitochondrial membrane protein. Given a portion of the cellular pool of the AHR has also been localized to mitochondria and the overlap in putative ligands, we tested the hypothesis that crosstalk exists between the two proteins. CRISPR/Cas9 was used to create knockouts for AHR and TSPO in a mouse lung epithelial cell line (MLE-12). WT, AHR-/-, and TSPO-/- cells were then exposed to AHR ligand (TCDD), TSPO ligand (PK11195), or both and RNA-seq was performed. More mitochondrial-related genes were altered by loss of both AHR and TSPO than would have been expected just by chance. Some of the genes altered included those that encode for components of the electron transport system and the mitochondrial calcium uniporter. Both proteins altered the activity of the other as AHR loss caused the increase of TSPO at both the mRNA and protein level and loss of TSPO significantly increased the expression of classic AHR battery genes after TCDD treatment. This research provides evidence that AHR and TSPO participate in similar pathways that contribute to mitochondrial homeostasis.

Anxiety-like behavior and GABAergic system in ovariectomized rats exposed to chronic mild stress

Stress or low level of estrogen could promote anxiety and depression; thus, it is of interest to investigate the combined effect of mild stress and the lack of estrogen on mental disorders by utilizing an animal model. This study was conducted to assess anxiety- and depressive- like behaviors in ovariectomized (Ovx) rats exposed to chronic mild stress (CMS) and determine the alteration in gamma-aminobutyric acid (GABA)-related transmission. Ovx rats were randomly assigned into four groups: (1) estrogen replacement (E2-NoCMS), (2) estrogen replacement and exposure to CMS (E2-CMS), (3) vehicle (VEH-NoCMS), and (4) vehicle and exposure to CMS (VEH-CMS). Following 4-week CMS, VEH groups (VEH-NoCMS and VEH-CMS) showed a similar level of anxiety-like behavior in elevated T-maze, whereas E2-CMS, VEH-NoCMS and VEH-CMS showed anxiety-like behavior in open field. The depressive-like behavior in the force swimming test tended to be affected by estrogen deprivation than CMS. The alteration of the GABAergic system as determined from the GABA level and mRNA expression of GABA-related transmission (i.e., glutamic acid decarboxylase, GABA transporter and GABA_A subunits) showed that the GABA level in the amygdala and frontal cortex was affected by CMS. For mRNA expression, the mRNA profile in the amygdala and hippocampus of VEH-NoCMS and E2-CMS was the same but different from those of VEH-NoCMS and E2-CMS. In addition, compared with E2-NoCMS, the mRNA profile in the frontal cortex was similar in VEH-NoCMS, E2-CMS, and VEH-CMS. These findings indicated that the underlying mechanism of the GABAergic system was differently modified, although VEH-NoCMS and VEH-CMS showed anxiety-like behavior. The findings of this study may provide a comprehensive understanding of the modulation of the GABAergic system during estrogen deprivation under CMS, as observed in menopausal women who were daily exposed to stress.

The interaction of membrane estradiol receptors and metabotropic glutamate receptors in adaptive and maladaptive estradiol-mediated motivated behaviors in females

Estrogen receptors were initially identified as intracellular, ligand-regulated transcription factors that result in genomic change upon ligand binding. However, rapid estrogen receptor signaling initiated outside of the nucleus was also known to occur via mechanisms that were less clear. Recent studies indicate that these traditional receptors, estrogen receptor α and estrogen receptor β, can also be trafficked to act at the surface membrane. Signaling cascades from these membrane-bound estrogen receptors (mERs) can rapidly alter cellular excitability and gene expression, particularly through the phosphorylation of CREB. A principal mechanism of neuronal mER action has been shown to occur through glutamate-independent transactivation of metabotropic glutamate receptors (mGlu), which elicits multiple signaling outcomes. The interaction of mERs with mGlu has been shown to be important in many diverse functions in females, including driving motivated behaviors. Experimental evidence suggests that a large part of estradiol-induced neuroplasticity and motivated behaviors, both adaptive and maladaptive, occurs through estradiol-dependent mER activation of mGlu. Herein we will review signaling through estrogen receptors, both "classical" nuclear receptors and membrane-bound receptors, as well as estradiol signaling through mGlu. We will focus on how the interactions of these receptors and their downstream signaling cascades are involved in driving motivated behaviors in females, discussing a representative adaptive motivated behavior (reproduction) and maladaptive motivated behavior (addiction).

Oral contraceptives and stroke: Foes or friends

Incidents of strokes are increased in young women relative to young men, suggesting that oral contraceptive (OC) use is one of the causes of stroke among young women. Long-term exposures to the varying combinations of estrogen and progestogen found in OCs affect blood clotting, lipid and lipoprotein metabolism, endothelial function, and de novo synthesis of neurosteroids, especially brain-derived 17β-estradiol. The latter is essential for neuroprotection, memory, sexual differentiation, synaptic transmission, and behavior. Deleterious effects of OCs may be exacerbated due to comorbidities like polycystic ovary syndrome, sickle cell anemia, COVID-19, exposures to endocrine disrupting chemicals, and conventional or electronic cigarette smoking. The goal of the current review is to revisit the available literature regarding the impact of OC use on stroke, to explain possible underlying mechanisms, and to identify gaps in our understanding to promote future research to reduce and cure stroke in OC users.

Positron Emission Tomography reveals age-associated hypothalamic microglial activation in women

In rodents, hypothalamic inflammation plays a critical role in aging and age-related diseases. Hypothalamic inflammation has not previously been assessed in vivo in humans. We used Positron Emission Tomography (PET) with a radiotracer sensitive to the translocator protein (TSPO) expressed by activated microglia, to assess correlations between age and regional brain TSPO in a group of healthy subjects (n = 43, 19 female, aged 23-78), focusing on hypothalamus. We found robust age-correlated TSPO expression in thalamus but not hypothalamus in the combined group of women and men. This pattern differs from what has been described in rodents. Prominent age-correlated TSPO expression in thalamus in humans, but in hypothalamus in rodents, could reflect evolutionary changes in size and function of thalamus versus hypothalamus, and may be relevant to the appropriateness of using rodents to model human aging. When examining TSPO PET results in women and men separately, we found that only women showed age-correlated hypothalamic TSPO expression. We suggest this novel result is relevant to understanding a stark sex difference in human aging: that only women undergo loss of fertility-menopause-at mid-life. Our finding of age-correlated hypothalamic inflammation in women could have implications for understanding and perhaps altering reproductive aging in women.

Neuroendocrine mechanisms underlying estrogen positive feedback and the LH surge

A fundamental principle in reproductive neuroendocrinology is sex steroid feedback: steroid hormones secreted by the gonads circulate back to the brain to regulate the neural circuits governing the reproductive neuroendocrine axis. These regulatory feedback loops ultimately act to modulate gonadotropin-releasing hormone (GnRH) secretion, thereby affecting gonadotropin secretion from the anterior pituitary. In females, rising estradiol (E₂) during the middle of the menstrual (or estrous) cycle paradoxically "switch" from being inhibitory on GnRH secretion ("negative feedback") to stimulating GnRH release ("positive feedback"), resulting in a surge in GnRH secretion and a downstream LH surge that triggers ovulation. While upstream neural afferents of GnRH neurons, including kisspeptin neurons in the rostral hypothalamus, are proposed as critical loci of E₂ feedback action, the underlying mechanisms governing the shift between E₂ negative and positive feedback are still poorly understood. Indeed, the precise cell targets, neural signaling factors and receptors, hormonal pathways, and molecular mechanisms by which ovarian-derived E₂ indirectly stimulates GnRH surge secretion remain incompletely known. In many species, there is also a circadian component to the LH surge, restricting its occurrence to specific times of day, but how the circadian clock interacts with endocrine signals to ultimately time LH surge generation also remains a major gap in knowledge. Here, we focus on classic and recent data from rodent models and discuss the consensus knowledge of the neural players, including kisspeptin, the suprachiasmatic nucleus, and glia, as well as endocrine players, including estradiol and progesterone, in the complex regulation and generation of E₂-induced LH surges in females.

Puberty enables oestradiol-induced progesterone synthesis in female mouse hypothalamic astrocytes

The development of oestrogen positive feedback is a hallmark of female puberty. Both oestrogen and progesterone signalling are required for the functioning of this neuroendocrine feedback loop but the physiological changes that underlie the emergence of positive feedback remain unknown. Only after puberty does oestradiol (E2) facilitate progesterone synthesis in the rat female hypothalamus (neuroP), an event critical for positive feedback and the LH surge. We hypothesize that prior to puberty, these astrocytes have low levels of membrane oestrogen receptor alpha (ERα), which is needed for facilitation of neuroP synthesis. Thus, we hypothesized that prepubertal astrocytes are unable to respond to E2 with increased neuroP synthesis due a lack of membrane ERα. To test this, hypothalamic tissues and enriched primary hypothalamic astrocyte cultures were acquired from prepubertal (postnatal week 3) and post-pubertal (week 8) female mice. E2-facilitated neuroP was measured in the hypothalamus pre- and post-puberty, and hypothalamic astrocyte responses were measured after treatment with E2. Prior to puberty, E2-facilitated neuroP synthesis did not occur in the hypothalamus, and mERα expression was low in hypothalamic astrocytes, but E2-facilitated neuroP synthesis in the rostral hypothalamus and mERα expression increased post-puberty. The increase in mERα expression in hypothalamic astrocytes corresponded with a post-pubertal increase in caveolin-1 protein, PKA phosphorylation, and a more rapid [Ca²⁺ ]_i flux in response to E2. Together, results from the present study indicate that E2-facilitated neuroP synthesis occurs in the rostral hypothalamus, develops during puberty, and corresponds to a post-pubertal increase in mERα levels in hypothalamic astrocytes.

Membrane estrogen signaling in female reproduction and motivation

Estrogen receptors were initially identified in the uterus, and later throughout the brain and body as intracellular, ligand-regulated transcription factors that affect genomic change upon ligand binding. However, rapid estrogen receptor signaling initiated outside of the nucleus was also known to occur via mechanisms that were less clear. Recent studies indicate that these traditional receptors, estrogen receptor-α and estrogen receptor-β, can also be trafficked to act at the surface membrane. Signaling cascades from these membrane-bound estrogen receptors (mERs) not only rapidly effect cellular excitability, but can and do ultimately affect gene expression, as seen through the phosphorylation of CREB. A principal mechanism of neuronal mER action is through glutamate-independent transactivation of metabotropic glutamate receptors (mGluRs), which elicits multiple signaling outcomes. The interaction of mERs with mGluRs has been shown to be important in many diverse functions in females, including, but not limited to, reproduction and motivation. Here we review membrane-initiated estrogen receptor signaling in females, with a focus on the interactions between these mERs and mGluRs.

Norepinephrine Regulation of Adrenergic Receptor Expression, 5' AMP-Activated Protein Kinase Activity, and Glycogen Metabolism and Mass in Male Versus Female Hypothalamic Primary Astrocyte Cultures

Norepinephrine (NE) control of hypothalamic gluco-regulation involves astrocyte-derived energy fuel supply. In male rats, exogenous NE regulates astrocyte glycogen metabolic enzyme expression in vivo through 5’-AMP-activated protein kinase (AMPK)-dependent mechanisms. Current research utilized a rat hypothalamic astrocyte primary culture model to investigate the premise that NE imposes sex-specific direct control of AMPK activity and glycogen mass and metabolism in these glia. In male rats, NE down-regulation of pAMPK correlates with decreased CaMMKB and increased PP1 expression, whereas noradrenergic augmentation of female astrocyte pAMPK may not involve these upstream regulators. NE concentration is a critical determinant of control of hypothalamic astrocyte glycogen enzyme expression, but efficacy varies between sexes. Data show sex variations in glycogen synthase expression and glycogen phosphorylase-brain and –muscle type dose-responsiveness to NE. Narrow dose-dependent NE augmentation of astrocyte glycogen content during energy homeostasis infers that NE maintains, over a broad exposure range, constancy of glycogen content despite possible changes in turnover. In male rats, beta₁- and beta₂-adrenergic receptor (AR) profiles displayed bi-directional responses to increasing NE doses; female astrocytes exhibited diminished beta₁-AR content at low dose exposure, but enhanced beta₂-AR expression at high NE dosages. Thus, graded variations in noradrenergic stimulation may modulate astrocyte receptivity to NE in vivo. Sex dimorphic NE regulation of hypothalamic astrocyte AMPK activation and glycogen metabolism may be mediated, in part, by one or more ARs characterized here by divergent sensitivity to this transmitter.

TSPO protein binding partners in bacteria, animals, and plants

The ancient membrane protein TSPO is phylogenetically widespread from archaea and bacteria to insects, vertebrates, plants, and fungi. TSPO’s primary amino acid sequence is only modestly conserved between diverse species, although its five transmembrane helical structure appears mainly conserved. Its cellular location and orientation in membranes have been reported to vary between species and tissues, with implications for potential diverse binding partners and function. Most TSPO functions relate to stress-induced changes in metabolism, but in many cases it is unclear how TSPO itself functions—whether as a receptor, a sensor, a transporter, or a translocator. Much evidence suggests that TSPO acts indirectly by association with various protein binding partners or with endogenous or exogenous ligands. In this review, we focus on proteins that have most commonly been invoked as TSPO binding partners. We suggest that TSPO was originally a bacterial receptor/stress sensor associated with porphyrin binding as its most ancestral function and that it later developed additional stress-related roles in eukaryotes as its ability to bind new partners evolved.

Regulation and role of Acyl-CoA synthetase 4 in glial cells

Acyl-CoA synthetase 4 (Acsl4), an enzyme involved in arachidonic acid (AA) metabolism, participates in physiological and pathological processes such as steroidogenesis and cancer. The role of Acsl4 in neurons and in nervous system development has also been documented but little is known regarding its functionality in glial cells. In turn, several processes in glial cells, including neurosteroidogenesis, stellation and AA uptake, are regulated by cyclic adenosine monophosphate (cAMP) signal. In this context, the aim of this work was to analyze the expression and functional role of Acsl4 in primary rat astrocyte cultures and in the C6 glioma cell line by chemical inhibition and stable silencing, respectively. Results show that Acsl4 expression was regulated by cAMP in both models and that cAMP stimulation of steroidogenic acute regulatory protein mRNA levels was reduced by Acsl4 inhibition or silencing. Also, Acsl4 inhibition reduced progesterone synthesis stimulated by cAMP and also affected cAMP-induced astrocyte stellation, decreasing process elongation and increasing branching complexity. Similar effects were observed for Acsl4 silencing on cAMP-induced C6 cell morphological shift. Moreover, Acsl4 inhibition and silencing reduced proliferation and migration of both cell types. Acsl4 silencing in C6 cells reduced the capacity for colony proliferation and neurosphere formation, the latter ability also being abolished by Acsl4 inhibition. In sum, this work presents novel evidence of Acsl4 involvement in neurosteroidogenesis and the morphological changes of glial cells promoted by cAMP. Furthermore, Acsl4 participates in migration and proliferation, also affecting cell self-renewal. Altogether, these findings provide insights into Acsl4 functions in glial cells.

Role of glial cells in the generation of sex differences in neurodegenerative diseases and brain aging

Diseases and aging-associated alterations of the nervous system often show sex-specific characteristics. Glial cells play a major role in the endogenous homeostatic response of neural tissue, and sex differences in the glial transcriptome and function have been described. Therefore, the possible role of these cells in the generation of sex differences in pathological alterations of the nervous system is reviewed here. Studies have shown that glia react to pathological insults with sex-specific neuroprotective and regenerative effects. At least three factors determine this sex-specific response of glia: sex chromosome genes, gonadal hormones and neuroactive steroid hormone metabolites. The sex chromosome complement determines differences in the transcriptional responses in glia after brain injury, while gonadal hormones and their metabolites activate sex-specific neuroprotective mechanisms in these cells. Since the sex-specific neuroprotective and regenerative activity of glial cells causes sex differences in the pathological alterations of the nervous system, glia may represent a relevant target for sex-specific therapeutic interventions.

The antidepressant and anxiolytic effect of GPER on translocator protein (TSPO) via protein kinase a (PKA) signaling in menopausal female rats

Postmenopausal depression is mainly caused by the deprivation of ovarian hormones during menopausal transition, it is of great importance to study on the treatment that could effectively relieve symptoms of menopausal depression with fewer side effects. Activation of G-protein-coupled estrogen receptor (GPER) has long been reported to facilitate neuronal plasticity and improve cognition in animals. Meanwhile, it could participate in regulation of intracellular signaling pathways through the characteristic of GPER, ameliorate intracellular mitochondrial function and oxidative stress. However, the impact of GPER on regulating estrogen deprived-depressant and anxious behaviors is still largely unknown. Here we used the ovariectomized female rats to imitate the condition of menopause. Owing to the lateral ventricle administration of G-1 which specifically react with GPER receptor intracerebrally, Ovariectomized (OVX) female rats showed depressive- or anxiety-like phenotypes with attenuated mitochondrial function. In addition, G-1 facilitated PKA activation, which further accelerated TSPO phosphorylation and alleviated menopausal depression- and anxiety-like behaviors. Moreover, PKA inhibitor PKI could partially antagonized the anti-anxiety and anti-depression effects of G-1. Taken together, we concluded that GPER activation might exhibit antidepressant and anxiolytic effect by elevating TSPO phosphorylation via protein kinase A signaling and rescuing the redox status in menopausal female rats.

Biosynthesis and signalling functions of central and peripheral nervous system neurosteroids in health and disease

Neurosteroids are steroid hormones synthesised de novo in the brain and peripheral nervous tissues. In contrast to adrenal steroid hormones that act on intracellular nuclear receptors, neurosteroids directly modulate plasma membrane ion channels and regulate intracellular signalling. This review provides an overview of the work that led to the discovery of neurosteroids, our current understanding of their intracellular biosynthetic machinery, and their roles in regulating the development and function of nervous tissue. Neurosteroids mediate signalling in the brain via multiple mechanisms. Here, we describe in detail their effects on GABA (inhibitory) and NMDA (excitatory) receptors, two signalling pathways of opposing function. Furthermore, emerging evidence points to altered neurosteroid function and signalling in neurological disease. This review focuses on neurodegenerative diseases associated with altered neurosteroid metabolism, mainly Niemann-Pick type C, multiple sclerosis and Alzheimer disease. Finally, we summarise the use of natural and synthetic neurosteroids as current and emerging therapeutics alongside their potential use as disease biomarkers.

HPLC-electrospray ionization-mass spectrometry optimization by high-performance design of experiments for astrocyte glutamine measurement

The amino acid glutamine (Gln) is a likely source of energy in the brain during neuroglucopenia. Effects of glucose deficiency on astrocyte Gln homeostasis remain unclear, as analytical tools of requisite sensitivity for quantification of intracellular levels of this molecule are not currently available. Here, a primary hypothalamic astrocyte culture model was used in conjunction with design of experiments (DOE)-refined high-performance liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) methodology to investigate the hypothesis that glucoprivation alters astrocyte Gln content in a sex-specific manner. Critical mass spectrometric parameters for Gln derivative chromatographic response were identified by comparing the performance of central composite design, Box-Behnken design, and Optimal Design (OD)-A, -D, -I, -Distance, and -Modified Distance DOE models. The outcomes showed that the OD-A-generated response was superior relative to other design outcomes. Forecasted surface plot critical mass spectrometric parameters were maximized by OD-A, OD-Distance, and OD-Modified Distance designs. OD-A produced a high-performance method that yielded experimental run and forecasted surface plot maximal responses. Optimized mass spectrometric analysis of male versus female astrocyte Gln content provides novel evidence that glucoprivation significantly depletes this amino acid in female, but not in male, and that this sex-specific response may involve differential sensitivity to estrogen receptor signaling. This technological advance will facilitate efforts to ascertain how distinctive physiological and pathophysiological stimuli impact astrocyte Gln metabolism in each sex.

Sex differences in glucoprivic regulation of glycogen metabolism in hypothalamic primary astrocyte cultures: Role of estrogen receptor signaling

Hypoglycemia causes sex-reliant changes in hypothalamic astrocyte glycogen metabolism in vivo. The role of nuclear versus membrane astrocyte estrogen receptors (ER) in glucoprivic regulation of glycogen is unclear. Here, primary hypothalamic astrocyte cultures were treated with selective ER antagonists during glucoprivation to investigate the hypothesis that ER mediate sex-specific glycogen responses to glucoprivation. Results show that glucoprivic down-regulation of glycogen synthase expression is mediated by transmembrane G protein-coupled ER-1 (GPER) signaling in each sex and estrogen receptor (ER)-beta (ERβ) activity in females. Glucoprivic inhibition of glycogen phosphorylase involves GPER and ERβ in females, but ER-independent mechanisms in males. GPER, ERβ, and ER-alpha (ERα) inhibit or stimulate AMPK protein expression in male versus female astrocytes, respectively. Glucoprivic augmentation of phospho-AMPK profiles in male glia was opposed by GPER activation, whereas GPER and ERβ suppress this protein in females. Astrocyte ERα and GPER content was down-regulated in each sex during glucose deficiency, whereas ERβ levels was unaltered (males) or increased (females). Glucoprivation correspondingly elevated or diminished male versus female astrocyte glycogen content; ER antagonism reversed this response in males, but not females. Results identify distinctive ER variants involved in sex-similar versus sex-specific astrocyte protein responses to withdrawal of this substrate fuel. Notably, glucoprivation elicits a directional switch or gain-of-effect of GPER and ERβ on specific glial protein profiles. Outcomes infer that ERs are crucial for glucoprivic regulation of astrocyte glycogen accumulation in males. Alternatively, estradiol may act independently of ER signaling to disassemble this reserve in females.

Hypothalamic Astrocyte Development and Physiology for Neuroprogesterone Induction of the Luteinizing Hormone Surge

Neural circuits in female rats sequentially exposed to estradiol and progesterone underlie so-called estrogen positive feedback that induce the surge release of pituitary luteinizing hormone (LH) leading to ovulation and luteinization of the corpus hemorrhagicum. It is now well-established that gonadotropin releasing hormone (GnRH) neurons express neither the reproductively critical estrogen receptor-α (ERα) nor classical progesterone receptor (PGR). Estradiol from developing ovarian follicles acts on ERα-expressing kisspeptin neurons in the rostral periventricular region of the third ventricle (RP3V) to induce PGR expression, and kisspeptin release. Circulating estradiol levels that induce positive feedback also induce neuroprogesterone (neuroP) synthesis in hypothalamic astrocytes. This local neuroP acts on kisspeptin neurons that express PGR to augment kisspeptin expression and release needed to stimulate GnRH release, triggering the LH surge. In vitro and in vivo studies demonstrate that neuroP signaling in kisspeptin neurons occurs through membrane PGR activation of Src family kinase (Src). This signaling cascade has been also implicated in PGR signaling in the arcuate nucleus of the hypothalamus, suggesting that Src may be a common mode of membrane PGR signaling. Sexual maturation requires that signaling between neuroP synthesizing astrocytes, kisspeptin and GnRH neurons be established. Prior to puberty, estradiol does not facilitate the synthesis of neuroP in hypothalamic astrocytes. During pubertal development, levels of membrane ERα increase in astrocytes coincident with an increase of PKA phosphorylation needed for neuroP synthesis. Currently, it is not clear whether these developmental changes occur in existing astrocytes or are due to a new population of astrocytes born during puberty. However, strong evidence suggests that it is the former. Blocking new cell addition during puberty attenuates the LH surge. Together these results demonstrate the importance of pubertal maturation involving hypothalamic astrocytes, estradiol-induced neuroP synthesis and membrane-initiated progesterone signaling for the CNS control of ovulation and reproduction.

PTSD is associated with neuroimmune suppression: evidence from PET imaging and postmortem transcriptomic studies

Despite well-known peripheral immune activation in posttraumatic stress disorder (PTSD), there are no studies of brain immunologic regulation in individuals with PTSD. [11C]PBR28 Positron Emission Tomography brain imaging of the 18-kDa translocator protein (TSPO), a microglial biomarker, was conducted in 23 individuals with PTSD and 26 healthy individuals-with or without trauma exposure. Prefrontal-limbic TSPO availability in the PTSD group was negatively associated with PTSD symptom severity and was significantly lower than in controls. Higher C-reactive protein levels were also associated with lower prefrontal-limbic TSPO availability and PTSD severity. An independent postmortem study found no differential gene expression in 22 PTSD vs. 22 controls, but showed lower relative expression of TSPO and microglia-associated genes TNFRSF14 and TSPOAP1 in a female PTSD subgroup. These findings suggest that peripheral immune activation in PTSD is associated with deficient brain microglial activation, challenging prevailing hypotheses positing neuroimmune activation as central to stress-related pathophysiology.

18-kDa translocator protein association complexes in the brain: From structure to function

The outer mitochondrial membrane 18-kDa translocator protein (TSPO) is highly conserved in organisms of different species and ubiquitously expressed throughout tissues, including the nervous system. In the healthy adult brain, TSPO expression levels are low and promptly modulated under different pathological conditions, such as cancer, inflammatory states, and neurological and psychiatric disorders. Not surprisingly, several endogenous and synthetic molecules capable of binding TSPO have been proposed as drugs or diagnostic tools for brain diseases. The most studied biochemical function of TSPO is cholesterol translocation into mitochondria, which in turn affects the synthesis of steroids in the periphery and neurosteroids in the brain. In the last 30 years, roles for TSPO have also been suggested in other cellular processes, such as heme synthesis, apoptosis, autophagy, calcium signalling and reactive oxygen species production. Herein, we provide an overview of TSPO associations with different proteins, focusing particular attention on their related functions. Furthermore, recent TSPO-targeted therapeutic interventions are explored and discussed as prospect for innovative treatments in mental and brain diseases.

Estradiol-induced senescence of hypothalamic astrocytes contributes to aging-related reproductive function declines in female mice

Hypothalamic astrocytes are important contributors that activate gonadotropin-releasing hormone (GnRH) neurons and promote GnRH/LH (luteinizing hormone) surge. However, the potential roles and mechanisms of astrocytes during the early reproductive decline remain obscure. The current study reported that, in intact middle-aged female mice, astrocytes within the hypothalamic RP3V accumulated senescence-related markers with increasing age. It employed an ovariectomized animal model and a cell model receiving estrogen intervention to confirm the estrogen-induced senescence of hypothalamic astrocytes. It found that estrogen metabolites may be an important factor for the estrogen-induced astrocyte senescence. In vitro molecular analysis revealed that ovarian estradiol activated PKA and up-regulated CYPs expression, metabolizing estradiol into 2-OHE₂ and 4-OHE₂. Of note, in middle-aged mice, the progesterone synthesis and the ability to promote GnRH release were significantly reduced. Besides, the expression of growth factors decreased and the mRNA levels of proinflammatory cytokines significantly increased in the aging astrocytes. The findings confirm that ovarian estradiol induces the senescence of hypothalamic astrocytes and that the senescent astrocytes compromise the regulation of progesterone synthesis and GnRH secretion, which may contribute to the aging-related declines in female reproductive function.

TSPO ligands prevent the proliferation of vascular smooth muscle cells and attenuate neointima formation through AMPK activation

Abnormal growth of the intimal layer of blood vessels (neointima formation) contributes to the progression of atherosclerosis and in-stent restenosis. Recent evidence shows that the 18-kDa translocator protein (TSPO), a mitochondrial membrane protein, is involved in diverse cardiovascular diseases. In this study we investigated the role of endogenous TSPO in neointima formation after angioplasty in vitro and in vivo. We established a vascular injury model in vitro by using platelet-derived growth factor-BB (PDGF-BB) to stimulate rat thoracic aortic smooth muscle cells (A10 cells). We found that treatment with PDGF-BB (1–20 ng/mL) dose-dependently increased TSPO expression in A10 cells, which was blocked in the presence of PKC inhibitor or MAPK inhibitor. Overexpression of TSPO significantly promoted the proliferation and migration in A10 cells, whereas downregulation of TSPO expression by siRNA or treatment with TSPO ligands PK11195 or Ro5-4864 (10⁴ nM) produced the opposite effects. Furthermore, we found that PK11195 (10−10⁴ nM) dose-dependently activated AMPK in A10 cells. PK11195-induced inhibition on the proliferation and migration of PDGF-BB-treated A10 cells were abolished by compound C (an AMPK-specific inhibitor, 10³ nM). In rats with balloon-injured carotid arteries, TSPO expression was markedly upregulated in the carotid arteries. Administration of PK11195 (3 mg/kg every 3 days, ip), starting from the initial balloon injury and lasting for 2 weeks, greatly attenuated carotid neointima formation by suppressing balloon injury-induced phenotype switching of VSMCs (increased α-SMA expression). These results suggest that TSPO is a vascular injury-response molecule that promotes VSMC proliferation and migration and is responsible for the neointima formation after vascular injury, which provides a novel therapeutic target for various cardiovascular diseases including atherosclerosis and restenosis.

Protein kinase inhibitors infused intraventricularly or into the ventromedial hypothalamus block short latency facilitation of lordosis by oestradiol

An injection of unesterified oestradiol (E₂ ) facilitates receptive behaviour in E₂ benzoate (EB)-primed, ovariectomised female rats when it is administered i.c.v. or systemically. The present study tested the hypothesis that inhibitors of protein kinase A (PKA), protein kinase G (PKG) or the Src/mitogen-activated protein kinase (MAPK) complex interfere with E₂ facilitation of receptive behaviour. In Experiment 1, lordosis induced by i.c.v. infusion of E₂ was significantly reduced by i.c.v. administration of Rp-cAMPS, a PKA inhibitor, KT5823, a PKG inhibitor, and PP2 and PD98059, Src and MAPK inhibitors, respectively, between 30 and 240 minutes after infusion. In Experiment 2, we determined whether the ventromedial hypothalamus (VMH) is one of the neural sites at which those intracellular pathways participate in lordosis behaviour induced by E₂ . Administration of each of the four protein kinase inhibitors into the VMH blocked facilitation of lordosis induced by infusion of E₂ also into the VMH. These data support the hypothesis that activation of several protein kinase pathways is involved in the facilitation of lordosis by E₂ in EB-primed rats.

ERαΔ4, an ERα splice variant missing exon4, interacts with caveolin-3 and mGluR2/3

The two isoforms of the nuclear estrogen receptor, ERα and ERβ are widely expressed in the central nervous system. Although they were first described as nuclear receptors, both isoforms have also been found at the cell membrane where they mediate cell signaling. Surface biotinylation studies using neuronal and glial primary cultures label an alternatively spliced form of ERα. The 52 kDa protein, ERαΔ4, is missing exon 4 and is highly expressed in membrane fractions derived from cultured cells. In vivo, both full-length (66 kDa) ERα and ERαΔ4 are present in membrane fractions. In response to estradiol, full-length ERα and ERαΔ4 are initially trafficked to the membrane, and then internalized in parallel. Previous studies determined that only the full-length ERα associates with metabotropic glutamate receptor-1a (mGluR1a), initiating cellular signaling. The role of ERαΔ4, remained to be elucidated. Here, we report ERαΔ4 trafficking, association with mGluR2/3, and downstream signaling in female rat arcuate nucleus (ARH). Caveolin (CAV) proteins are needed for ER transport to the cell membrane, and using co-immunoprecipitation CAV-3 was shown to associate with ERαΔ4. CAV-3 was necessary for ERαΔ4 trafficking to the membrane: in the ARH, microinjection of CAV-3 siRNA reduced CAV-3 and ERαΔ4a in membrane fractions by 50%, and 60%, respectively. Moreover, co-immunoprecipitation revealed that ERαΔ4 associated with inhibitory mGluRs, mGluR2/3. Estrogen benzoate (EB) treatment (5 μg; s.c.; every 4 days; three cycles) reduced levels of cAMP, an effect attenuated by antagonizing mGluR2/3. Following EB treatment, membrane levels of ERαΔ4 and mGluR2/3 were reduced implying ligand-induced internalization. These results implicate ERαΔ4 in an estradiol-induced inhibitory cell signaling in the ARH.

History of Estrogen: Its Purification, Structure, Synthesis, Biologic Actions, and Clinical Implications

This mini-review summarizes key points from the Clark Sawin Memorial Lecture on the History of Estrogen delivered at Endo 2018 and focuses on the rationales and motivation leading to various discoveries and their clinical applications. During the classical period of antiquity, incisive clinical observations uncovered important findings; however, extensive anatomical dissections to solidify proof were generally lacking. Initiation of the experimental approach followed later, influenced by Claude Bernard's treatise "An Introduction to the Study of Experimental Medicine." With this approach, investigators began to explore the function of the ovaries and their "internal secretions" and, after intensive investigations for several years, purified various estrogens. Clinical therapies for hot flashes, osteoporosis, and dysmenorrhea were quickly developed and, later, methods of hormonal contraception. Sophisticated biochemical methods revealed the mechanisms of estrogen synthesis through the enzyme aromatase and, after discovery of the estrogen receptors, their specific biologic actions. Molecular techniques facilitated understanding of the specific transcriptional and translational events requiring estrogen. This body of knowledge led to methods to prevent and treat hormone-dependent neoplasms as well as a variety of other estrogen-related conditions. More recently, the role of estrogen in men was uncovered by prismatic examples of estrogen deficiency in male patients and by knockout of the estrogen receptor and aromatase in animals. As studies became more extensive, the effects of estrogen on nearly every organ were described. We conclude that the history of estrogen illustrates the role of intellectual reasoning, motivation, and serendipity in advancing knowledge about this important sex steroid.

Seasonal regulation of steroidogenic enzyme expression within the green anole lizard (Anolis carolinensis) brain and gonad

Steroid hormones, such as testosterone and estradiol, are necessary for reproductive behavior. Seasonally breeding animals have increased sex steroid hormone levels during the breeding compared to non-breeding season, with increased reproductive behaviors and altered brain morphology in breeding individuals. Similar to other seasonally breeding animals, green anole lizards (Anolis carolinensis) have high sex steroid hormone levels and increased reproductive behaviors in the breeding season. Relatively less is known regarding the regulation of steroidogenesis in reptiles and this experiment examined whether enzymes involved in sex steroid hormone synthesis vary seasonally within the brain and gonads in wild-caught anole lizards. Specifically, we examined mRNA expression of steroidogenic acute regulatory protein (StAR), P450 17α-hydroxylase/C17-20lyase (Cyp17α1), 17 beta-hydroxysteroid dehydrogenase type 3 (17βHSD 3), and aromatase (Cyp19α1). We found that the mRNA for each of these genes was expressed in the lizard brain. Interestingly, Cyp19α1 mRNA expression in the brain was increased during the non-breeding season, potentially revealing a role for aromatase expression in the non-breeding brain. In the anole gonads, StAR mRNA expression levels were increased in both males and females during the breeding season, while the mRNA expression levels of CYP17α1 and 17βHSD 3 are increased when StAR mRNA expression was decreased, suggesting that the enzymes in the steroidogenic pathway are potentially regulated independently of StAR. This work reveals the seasonal regulation of steroidogenesis in the reptilian brain and gonad, although more work is necessary to determine the regulatory mechanisms that control these expression patterns.

An intracrine view of sex steroids, immunity, and metabolic regulation

Background

Over the past two decades, parallel recognition has grown of the importance of both sex steroids and immune activity in metabolic regulation. More recently, these discrete areas have been integrated in studies examining the metabolic effects of sex steroid immunomodulation. Implicit in these studies has been a traditional, endocrine model of sex steroid delivery from the gonads to target cells, including immune cells. Thus, research to date has focused on the metabolic effects of sex steroid receptor signaling in immune cells. This endocrine model, however, overlooks the extensive capacity of immune cells to generate and metabolize sex steroids, enabling the production of sex steroids for intracrine signaling – that is, sex steroid production for signaling within the cell of origin. Intracrine function allows highly cell-autonomous regulation of sex steroid exposure, and sex steroid secretion by immune cells could confer paracrine signaling effects in neighboring cells within metabolic tissues. In this review, immune cell intracrinology will denote sex steroid production within immune cells for either intracrine or paracrine signaling. This intracrine capacity of immune cells has been well established, and prior work has supported its importance in autoimmune disorders, trauma, and cancer. The potential relevance of immune cell intracrine function to the regulation of energy balance, body weight, body composition, and insulin sensitivity has yet to be explored.

Scope of review

The following review will detail findings to date regarding the steroidogenic and steroid metabolizing capacity of immune cells, the regulation of immune cell intracrine function, and the biological effects of immune-derived sex steroids, including the clinical relevance of immune cell intracrinology in fields other than metabolism. These findings will serve as the basis for a proposed model of immune cell intracrinology constituting a new frontier in metabolism research.

Major conclusions

The development of highly sensitive mass spectrometric methods for sex steroid measurement and quantitation of metabolic flux now allows unprecedented ability to interrogate sex steroid production, metabolism and secretion by immune cells. Immune cell intracrinology could reveal key mechanisms underlying immune cell-mediated metabolic regulation.

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Sex steroids exert immunomodulatory effects that may influence metabolic health.

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Immune cells can synthesize, modify, and metabolize sex steroids.

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Immune cell-derived sex steroids may play intracrine, autocrine, paracrine, and possibly even endocrine roles.

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Immune cell steroidogenesis is a largely unexplored area of metabolism research.

Steroid Transport, Local Synthesis, and Signaling within the Brain: Roles in Neurogenesis, Neuroprotection, and Sexual Behaviors

Sex steroid hormones are synthesized from cholesterol and exert pleiotropic effects notably in the central nervous system. Pioneering studies from Baulieu and colleagues have suggested that steroids are also locally-synthesized in the brain. Such steroids, called neurosteroids, can rapidly modulate neuronal excitability and functions, brain plasticity, and behavior. Accumulating data obtained on a wide variety of species demonstrate that neurosteroidogenesis is an evolutionary conserved feature across fish, birds, and mammals. In this review, we will first document neurosteroidogenesis and steroid signaling for estrogens, progestagens, and androgens in the brain of teleost fish, birds, and mammals. We will next consider the effects of sex steroids in homeostatic and regenerative neurogenesis, in neuroprotection, and in sexual behaviors. In a last part, we will discuss the transport of steroids and lipoproteins from the periphery within the brain (and vice-versa) and document their effects on the blood-brain barrier (BBB) permeability and on neuroprotection. We will emphasize the potential interaction between lipoproteins and sex steroids, addressing the beneficial effects of steroids and lipoproteins, particularly HDL-cholesterol, against the breakdown of the BBB reported to occur during brain ischemic stroke. We will consequently highlight the potential anti-inflammatory, anti-oxidant, and neuroprotective properties of sex steroid and lipoproteins, these latest improving cholesterol and steroid ester transport within the brain after insults.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Non-Neuronal Cells in the Hypothalamic Adaptation to Metabolic Signals

Although the brain is composed of numerous cell types, neurons have received the vast majority of attention in the attempt to understand how this organ functions. Neurons are indeed fundamental but, in order for them to function correctly, they rely on the surrounding "non-neuronal" cells. These different cell types, which include glia, epithelial cells, pericytes, and endothelia, supply essential substances to neurons, in addition to protecting them from dangerous substances and situations. Moreover, it is now clear that non-neuronal cells can also actively participate in determining neuronal signaling outcomes. Due to the increasing problem of obesity in industrialized countries, investigation of the central control of energy balance has greatly increased in attempts to identify new therapeutic targets. This has led to interesting advances in our understanding of how appetite and systemic metabolism are modulated by non-neuronal cells. For example, not only are nutrients and hormones transported into the brain by non-neuronal cells, but these cells can also metabolize these metabolic factors, thus modifying the signals reaching the neurons. The hypothalamus is the main integrating center of incoming metabolic and hormonal signals and interprets this information in order to control appetite and systemic metabolism. Hence, the factors transported and released from surrounding non-neuronal cells will undoubtedly influence metabolic homeostasis. This review focuses on what is known to date regarding the involvement of different cell types in the transport and metabolism of nutrients and hormones in the hypothalamus. The possible involvement of non-neuronal cells, in particular glial cells, in physiopathological outcomes of poor dietary habits and excess weight gain are also discussed.

Rodent Models of Non-classical Progesterone Action Regulating Ovulation

It is becoming clear that steroid hormones act not only by binding to nuclear receptors that associate with specific response elements in the nucleus but also by binding to receptors on the cell membrane. In this newly discovered manner, steroid hormones can initiate intracellular signaling cascades which elicit rapid effects such as release of internal calcium stores and activation of kinases. We have learned much about the translocation and signaling of steroid hormone receptors from investigations into estrogen receptor α, which can be trafficked to, and signal from, the cell membrane. It is now clear that progesterone (P4) can also elicit effects that cannot be exclusively explained by transcriptional changes. Similar to E2 and its receptors, P4 can initiate signaling at the cell membrane, both through progesterone receptor and via a host of newly discovered membrane receptors (e.g., membrane progesterone receptors, progesterone receptor membrane components). This review discusses the parallels between neurotransmitter-like E2 action and the more recently investigated non-classical P4 signaling, in the context of reproductive behaviors in the rodent.

Regulation and the Mechanism of Estrogen on Cav1.2 Gene in Rat-Cultured Cortical Astrocytes

L-type calcium channel (LTCC) gene Cav1.2 is believed to play an important role in the alteration of Ca(2+) homeostasis in brain astrocytes. Increasing evidence shows that alteration of intracellular Ca(2+) concentration is related to the effect of 17β-estradiol (E2) in a variety of neurophysiological and neuropathological conditions. In this study, we measured immunoreactivity of Cav1.2 protein expression in rat primary cortical astrocytes by using Western blots. We demonstrated that E2 upregulated Cav1.2 expression in a dose- and time-dependent manner and the effect of E2 on Cav1.2 expression were blocked by an estrogen receptor (ER) antagonist, ICI-182,780. The ER subtype-selective ERα agonists propylpyrazole triole (PPT) and ERβ agonist diarylpropionitrile (DPN) both increase the expression of Cav1.2 in a dose-dependent manner. Also, the PPT most closely mimicked the upregulation of Cav1.2 protein expression by E2. Similar experiments of 10 nM E2-treated ERα- or ERβ-knockdown astrocytes have also shown that the E2 regulation of Cav1.2 protein expression is mediated through an ERα-dependent pathway. Furthermore, we established that E2 did not change the level of Cav1.2 mRNA. The induction of E2-mediated Cav1.2 expression was inhibited by cycloheximide (CHX) but not by actinomycin D (Act-D), suggesting that E2 regulation of Cav1.2 expression occurred at a posttranscriptional level. We also found that E2 may increase Cav1.2 levels by decreasing its ubiquitination and degradation rate. These findings provide new information about the effect of E2 on Cav1.2 in astrocytes, particularly necessary for the treatment of neurological disease.

Gonadal hormone modulation of intracellular calcium as a mechanism of neuroprotection

Hormones have wide-ranging effects throughout the nervous system, including the ability interact with and modulate many aspects of intracellular calcium regulation and calcium signaling. Indeed, these interactions specifically may help to explain the often opposing or paradoxical effects of hormones, such as their ability to both promote and prevent neuronal cell death during development, as well as reduce or exacerbate damage following an insult or injury in adulthood. Here, we review the basic mechanisms underlying intracellular calcium regulation-perhaps the most dynamic and flexible of all signaling molecules-and discuss how gonadal hormones might manipulate these mechanisms to coordinate diverse cellular responses and achieve disparate outcomes. Additional future research that specifically addresses questions of sex and hormone effects on calcium signaling at different ages will be critical to understanding hormone-mediated neuroprotection.

Cooperation of Genomic and Rapid Nongenomic Actions of Estrogens in Synaptic Plasticity

Neuroplasticity refers to the changes in the molecular and cellular processes of neural circuits that occur in response to environmental experiences. Clinical and experimental studies have increasingly shown that estrogens participate in the neuroplasticity involved in cognition, behavior, and memory. It is generally accepted that estrogens exert their effects through genomic actions that occur over a period of hours to days. However, emerging evidence indicates that estrogens also rapidly influence the neural circuitry through nongenomic actions. In this review, we provide an overview of the genomic and nongenomic actions of estrogens and discuss how these actions may cooperate in synaptic plasticity. We then summarize the role of epigenetic modifications, synaptic protein synthesis, and posttranslational modifications, and the splice variants of estrogen receptors in the complicated network of estrogens. The combination of genomic and nongenomic mechanisms endows estrogens with considerable diversity in modulating neural functions including synaptic plasticity.

TSPO: kaleidoscopic 18-kDa amid biochemical pharmacology, control and targeting of mitochondria

The 18-kDa translocator protein (TSPO) localizes in the outer mitochondrial membrane (OMM) of cells and is readily up-regulated under various pathological conditions such as cancer, inflammation, mechanical lesions and neurological diseases. Able to bind with high affinity synthetic and endogenous ligands, its core biochemical function resides in the translocation of cholesterol into the mitochondria influencing the subsequent steps of (neuro-)steroid synthesis and systemic endocrine regulation. Over the years, however, TSPO has also been linked to core cellular processes such as apoptosis and autophagy. It interacts and forms complexes with other mitochondrial proteins such as the voltage-dependent anion channel (VDAC) via which signalling and regulatory transduction of these core cellular events may be influenced. Despite nearly 40 years of study, the precise functional role of TSPO beyond cholesterol trafficking remains elusive even though the recent breakthroughs on its high-resolution crystal structure and contribution to quality-control signalling of mitochondria. All this along with a captivating pharmacological profile provides novel opportunities to investigate and understand the significance of this highly conserved protein as well as contribute the development of specific therapeutics as presented and discussed in the present review.

TSPO-ligands prevent oxidative damage and inflammatory response in C6 glioma cells by neurosteroid synthesis

Translocator protein 18kDa (TSPO) is predominantly located in the mitochondrial outer membrane, playing an important role in steroidogenesis, inflammation, cell survival and proliferation. Its expression in central nervous system, mainly in glial cells, has been found to be upregulated in neuropathology, and brain injury. In this study, we investigated the anti-oxidative and anti-inflammatory effects of a group of TSPO ligands from the N,N-dialkyl-2-phenylindol-3-ylglyoxylamide class (PIGAs), highlighting the involvement of neurosteroids in their pharmacological effects. To this aim we used a well-known in vitro model of neurosteroidogenesis: the astrocytic C6 glioma cell line, where TSPO expression and localization, as well as cell response to TSPO ligand treatment, have been established. All PIGAs reduced l-buthionine-(S,R)-sulfoximine (BSO)-driven cell cytotoxicity and lipid peroxidation. Moreover, an anti-inflammatory effect was observed due to the reduction of inducible nitric oxide synthase and cyclooxygenase-2 induction in LPS/IFNγ challenged cells. Both effects were blunted by aminoglutethimide (AMG), an inhibitor of pregnenolone synthesis, suggesting neurosteroids' involvement in PIGA protective mechanism. Finally, pregnenolone evaluation in PIGA exposed cells revealed an increase in its synthesis, which was prevented by AMG pre-treatment. These findings indicate that these TSPO ligands reduce oxidative stress and pro-inflammatory enzymes in glial cells through the de novo synthesis of neurosteroids, suggesting that these compounds could be potential new therapeutic tools for the treatment of inflammatory-based neuropathologies with beneficial effects possibly comparable to steroids, but potentially avoiding the negative side effects of long-term therapies with steroid hormones.

Estradiol Membrane-Initiated Signaling and Female Reproduction

The discoveries of rapid, membrane-initiated steroid actions and central nervous system steroidogenesis have changed our understanding of the neuroendocrinology of reproduction. Classical nuclear actions of estradiol and progesterone steroids affecting transcription are essential. However, with the discoveries of membrane-associated steroid receptors, it is becoming clear that estradiol and progesterone have neurotransmitter-like actions activating intracellular events. Ultimately, membrane-initiated actions can influence transcription. Estradiol membrane-initiated signaling (EMS) modulates female sexual receptivity and estrogen feedback regulating the luteinizing hormone (LH) surge. In the arcuate nucleus, EMS activates a lordosis-regulating circuit that extends to the medial preoptic nucleus and subsequently to the ventromedial nucleus (VMH)--the output from the limbic and hypothalamic regions. Here, we discuss how EMS leads to an active inhibition of lordosis behavior. To stimulate ovulation, EMS facilitates astrocyte synthesis of progesterone (neuroP) in the hypothalamus. Regulation of GnRH release driving the LH surge is dependent on estradiol-sensitive kisspeptin (Kiss1) expression in the rostral periventricular nucleus of the third ventricle (RP3V). NeuroP activation of the LH surge depends on Kiss1, but the specifics of signaling have not been well elucidated. RP3V Kiss1 neurons appear to integrate estradiol and progesterone information which feeds back onto GnRH neurons to stimulate the LH surge. In a second population of Kiss1 neurons, estradiol suppresses the surge but maintains tonic LH release, another critical component of the estrous cycle. Together, evidence suggests that regulation of reproduction involves membrane action of steroids, some of which are synthesized in the brain.

Developmental abnormalities and differential expression of genes induced in oil and dispersant exposed Menidia beryllina embryos

Exposure of fish embryos to relatively low concentrations of oil has been implicated in sub-lethal toxicity. The objective of this study was to determine the effects of the exposure of Menidia beryllina embryos at 30-48h post-fertilization to the water accommodated fractions of oil (WAF, 200ppm, v/v), dispersants (20ppm, v/v, Corexit 9500 or 9527), and mixtures of oil and each of the dispersants to produce chemically enhanced water accommodated fractions (CEWAFs) over a 72-hour period. The polyaromatic hydrocarbon (PAH) and benzene, toluene, ethylene and xylene (BTEX) constituents of the 5X concentrated exposure solutions (control, WAF, dispersants and CEWAFs) were determined and those of the 1× exposures were derived using a dilution factor. PAH, BTEX and low molecular weight PAH constituents greater than 1ppb were observed in WAF and the dispersants, but at much higher levels in CEWAFs. The WAF and CEWAFs post-weathering were diluted at 1:5 (200ml WAF/CEWAF: 800ml 25ppt saltwater) for embryo exposures. Mortality, heartbeat, embryo normalcy, abnormality types and severities were recorded. The qPCR assay was used to quantify abundances of transcripts of target genes for sexual differentiation and sex determination (StAR, dmrt-1, amh, cyp19b, vtg and chg-L,), growth regulation (ghr) and stress response (cyp1a and Hsp90); and gapdh served as the housekeeping gene. Temperature was 21±1.5°C throughout the experimental period, while mortality was low and not significantly different (p=0.68) among treatments. Heartbeat was significantly different (0.0034) with the lowest heartbeats recorded in Corexit 9500 (67.5beats/min) and 9527 (67.1beats/min) exposed embryos compared with controls (82.7beats/min). Significantly more treated embryos were in a state of deterioration, with significantly more embryos presenting arrested tissue differentiation compared with controls (p=0.021). Exposure to WAF, dispersants and CEWAF induced aberrant expression of all the genes, with star, dmrt-1, ghr and hsp90 being significantly down-regulated in CEWAF and cyp19b in Corexit 9527. The cyp1a and cyp19b were significantly up-regulated in CEWAFs and WAF, respectively. The molecular endpoints were most sensitive, especially the expression of star, cyp19b, cyp1a, hsp90 and could therefore be used as early indicators of long term effects of Corexit 9500 and 9527 usage in oil spill management on M. beryllina, a valid sentinel for oil pollution events.

Therapeutic actions of translocator protein (18 kDa) ligands in experimental models of psychiatric disorders and neurodegenerative diseases

Translocator protein (TSPO) is an 18kDa protein located at contact sites between the outer and the inner mitochondrial membrane. Numerous studies have associated TSPO with the translocation of cholesterol across the aqueous mitochondrial intermembrane space and the regulation of steroidogenesis, as well as with the control of some other mitochondrial functions, such as mitochondrial respiration, mitochondrial permeability transition pore opening, apoptosis and cell proliferation. In the brain, changes in TSPO expression occur in several neuropathological conditions including neurodegenerative diseases and psychiatric disorders. Furthermore, TSPO ligands have been shown to promote neuroprotection in animal models of brain pathology. At least in some cases, the mechanisms of neuroprotection are associated with modifications in brain steroidogenesis. In addition, regulation of neuroinflammation seems to be a common mechanism in the neuroprotective actions of TSPO ligands in different animal models of brain pathology.

The estrogenic retina: The potential contribution to healthy aging and age-related neurodegenerative diseases of the retina

These last two decades have seen an explosion of clinical and epidemiological research, and basic research devoted to envisage the influence of gender and hormonal fluctuations in the retina/ocular diseases. Particular attention has been paid to age-related disorders because of the overlap of endocrine and neuronal dysfunction with aging. Hormonal withdrawal has been considered among risk factors for diseases such as glaucoma, diabetic retinopathy and age-related macular disease (AMD), as well as, for Alzheimer's disease, Parkinson's disease, or other neurodegenerative disorders. Sex hormones and aging have been also suggested to drive the incidence of ocular surface diseases such as dry eye and cataract. Hormone therapy has been approached in several clinical trials. The discovery that the retina is another CNS tissue synthesizing neurosteroids, among which neuroactive steroids, has favored these studies. However, the puzzling data emerged from clinical, epidemiological and experimental studies have added several dimensions of complexity; the current landscape is inherently limited to the weak information on the influence and interdependence of endocrine, paracrine and autocrine regulation in the retina, but also in the brain. Focusing on the estrogenic retina, we here review our knowledge on local 17β-oestradiol (E2) synthesis from cholesterol-based neurosteroidogenic path and testosterone aromatization, and presence of estrogen receptors (ERα and ERβ). The first cholesterol-limiting step and the final aromatase-limiting step are discussed as possible check-points of retinal functional/dysfunctional E2. Possible E2 neuroprotection is commented as a group of experimental evidence on excitotoxic and oxidative retinal paradigms, and models of retinal neurodegenerative diseases, such as glaucoma, diabetic retinopathy and AMD. These findings may provide a framework to support clinical studies, although further basic research is needed.

New steps forward in the neuroactive steroid field

Evidence accumulated in recent years suggests that the systemic treatment with neuroactive steroids, or the pharmacological modulation of its production by brain cells, represent therapeutic options to promote neuroprotection. However, new findings, which are reviewed in this paper, suggest that the factors to be considered for the design of possible therapies based on neuroactive steroids are more complex than previously thought. Thus, although as recently reported, the nervous system regulates neuroactive steroid synthesis and metabolism in adaptation to modifications in peripheral steroidogenesis, the neuroactive steroid levels in the brain do not fully reflect its levels in plasma. Even, in some cases, neuroactive steroid level modifications occurring in the nervous tissues, under physiological and pathological conditions, are in the opposite direction than in the periphery. This suggests that the systemic treatment with these molecules may have unexpected outcomes on neural steroid levels. In addition, the multiple metabolic pathways and signaling mechanisms of neuroactive steroids, which may change from one brain region to another, together with the existence of regional and sex differences in its neural levels are additional sources of complexity that should be clarified. This complexity in the levels and actions of these molecules may explain why in some cases these molecules have detrimental rather than beneficial actions for the nervous system. This article is part of a Special Issue entitled 'Steroid Perspectives'.

The changing landscape in translocator protein (TSPO) function

Translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor (PBR), is an outer mitochondrial membrane protein. TSPO has been shown to cooperate with steroidogenic acute regulatory protein (StAR) and function in the transport of cholesterol into mitochondria. TSPO has also been considered as a structural component of the mitochondrial permeability transition pore (MPTP). However, recent advances have changed these views of TSPO's functions and have prompted a re-evaluation of established concepts. This review summarizes the history of TSPO, key elements of the debate, and functional experiments that have changed our understanding. Moving forward, we examine how this fundamental change impacts our understanding of TSPO and affects the future of TSPO as a therapeutic and diagnostic target.

Role of neurosteroids in hepatic encephalopathy

Hepatic encephalopathy (HE) is a neuropsychiatric manifestation of chronic or acute liver disease. Neurosteroids are synthesized from cholesterol and its precursors by glial cells, oligodendrocytes and neurons in the brain. The mechanisms by which neurosteroids affect brain function may involve both genetic and non-genetic effects. On one hand, neurosteroids bind and modulate different types of neuronal membrane receptors, including gamma-amino butyric acid-A receptor (GABA-A), N-methyl-D-aspartic acid receptor (NMDA), 5-hydroxytryptamine 3 (5-HT3) and opioid receptors which have been showed to be involved in HE. On the other hand, some neurosteroids bind to intracellular receptors through which they also regulate gene expression. Of note, neurosteroids play a role in the pathogenesis of HE through inhibiting long-term potentiation. Neurosteroids might provide a new avenue for HE treatment.