Aromatase expression in the human temporal cortex

The multiple roles of estrogens and the enzyme aromatase

Aromatase is the enzyme that catalyzes the last step of estrogen biosynthesis. It is expressed in many tissues such as the gonads, brain and adipose tissue. The regulation of the level and activity of aromatase determines the levels of estrogens that have endocrine, paracrine and autocrine effects on tissues. Estrogens play many roles in the body, regulating reproduction, metabolism and behavior. In the brain, cell survival and the activity of neurons are affected by estrogens and hence aromatase.

Aromatase expression in the normal and epileptic human hippocampus

Aromatase is a key enzyme in estrogen biosynthesis that is involved in neuronal plasticity in the rodent hippocampus. Although aromatase mRNA expression has been detected in the human hippocampus, its cellular distribution has yet to be determined. Here, we have examined the immunohistochemical distribution of aromatase in the normal and the epileptic and sclerotic human hippocampus. In both the normal and epileptic hippocampus, aromatase was detected in numerous CA1-CA3 pyramidal neurons, in granule cells of the dentate gyrus and in interneurons that co-expressed the calcium-binding proteins calbindin, calretinin or parvalbumin. However, only a small subpopulation of astrocytes was immunoreactive for aromatase in either the normal and epileptic hippocampus. The widespread expression of aromatase in a large population of neurons in the normal and damaged hippocampus suggests that local estrogen formation may play an important role in human hippocampal function.

Neurosteroid biosynthesis: enzymatic pathways and neuroendocrine regulation by neurotransmitters and neuropeptides

Neuroactive steroids synthesized in neuronal tissue, referred to as neurosteroids, are implicated in proliferation, differentiation, activity and survival of nerve cells. Neurosteroids are also involved in the control of a number of behavioral, neuroendocrine and metabolic processes such as regulation of food intake, locomotor activity, sexual activity, aggressiveness, anxiety, depression, body temperature and blood pressure. In this article, we summarize the current knowledge regarding the existence, neuroanatomical distribution and biological activity of the enzymes responsible for the biosynthesis of neurosteroids in the brain of vertebrates, and we review the neuronal mechanisms that control the activity of these enzymes. The observation that the activity of key steroidogenic enzymes is finely tuned by various neurotransmitters and neuropeptides strongly suggests that some of the central effects of these neuromodulators may be mediated via the regulation of neurosteroid production.

Interactions between neuroactive steroids and reelin haploinsufficiency in Purkinje cell survival

We determined total Purkinje cell (PC) numbers in cerebella of wild-type (+/+) and heterozygous (rl/+) reeler mice of either sex during early postnatal development; in parallel, we quantified levels of neuroactive steroids in the cerebellum with mass spectrometry. We also quantified reelin mRNA and protein expression with RT-PCR and Western blotting. PC numbers are selectively reduced at postnatal day 15 (P15) in rl/+ males in comparison to +/+ males, +/+ females, and rl/+ females. Administration of 17beta-estradiol (17beta-E) into the cisterna magna at P5 increases PC numbers in rl/+ males, but not in the other groups; conversely, estrogen antagonists 4-OH-tamoxifen or ICI 182,780 reduce PC numbers in +/+ and rl/+ females, but have no effect in males. Testosterone (T) levels at P5 are much higher in males than in females, reflecting the perinatal testosterone surge in males. In addition, rl/+ male cerebella at P5 show a peculiar hormonal profile in comparison with the other groups, consisting of increased levels of T and 17beta-E, and decreased levels of dihydrotestosterone. RT-PCR analysis indicated that heterozygosity leads to a 50% reduction of reelin mRNA in the cerebellum in both sexes, as expected, and that 17beta-E upregulates reelin mRNA, particularly in rl/+ males; reelin mRNA upregulation is associated with an increase of all major reelin isoforms. These effects may represent a novel model of how reelin deficiency interacts with variable perinatal levels of neuroactive steroids, leading to gender-dependent differences in genetic vulnerability.

Neuroprotective actions of brain aromatase

The steroidal regulation of vertebrate neuroanatomy and neurophysiology includes a seemingly unending list of brain areas, cellular structures and behaviors modulated by these hormones. Estrogens, in particular have emerged as potent neuromodulators, exerting a range of effects including neuroprotection and perhaps neural repair. In songbirds and mammals, the brain itself appears to be the site of injury-induced estrogen synthesis via the rapid transcription and translation of aromatase (estrogen synthase) in astroglia. This induction seems to occur regardless of the nature and location of primary brain damage. The induced expression of aromatase apparently elevates local estrogen levels enough to interfere with apoptotic pathways, thereby decreasing secondary degeneration and ultimately lessening the extent of damage. There is even evidence suggesting that aromatization may affect injury-induced cytogenesis. Thus, aromatization in the brain appears to confer neuroprotection by an array of mechanisms that involve the deceleration and acceleration of degeneration and repair, respectively. We are only beginning to understand the factors responsible for the injury-induced transcription of aromatase in astroglia. In contrast, much of the manner in which local and circulating estrogens may achieve their neuroprotective effects has been elucidated. However, gaps in our knowledge include issues about the cell-specific regulation of aromatase expression, steroidal influences of aromatization distinct from estrogen formation, and questions about the role of constitutive aromatase in neuroprotection. Here we describe the considerable consensus and some interesting differences in knowledge gained from studies conducted on diverse animal models, experimental paradigms and preparations towards understanding the neuroprotective actions of brain aromatase.

Brain aromatization: classic roles and new perspectives

Aromatization of testosterone to estradiol by neural tissue has classically been associated with the regulation of sexual differentiation, gonadotropin secretion, and copulatory behavior. However, new data indicate that the capacity for aromatization is not restricted to the endocrine brain and demonstrate roles for locally formed estrogens in neurogenesis and in responses of brain tissue to injury. This article summaries our current understanding of the distribution and regulation of aromatase in the brain and describes the classic and novel roles it plays. A better understanding of brain aromatization could shed new light on its physiologic and pathologic functions and someday lead to new, centrally acting drug therapies.

Selective transcriptional regulation of aromatase gene by vitamin D, dexamethasone, and mifepristone in human glioma cells

The human aromatase gene (CYP19A1) is controlled by multiple promoters that give rise to different aromatase transcripts. Its regulation has been studied in cells from multiple origins, including placenta, bone, adipose tissue, and breast cancer. However, little is known about its regulation in cells from neural origin. We assessed whether vitamin D, dexamethasone, and the glucocorticoid receptor antagonist mifepristone regulate the aromatase gene in human glioma, neuroblastoma, and breast cancer cells. The results show that these compounds enhance the activity of different aromatase promoters in glioma cells, but not in neuroblastoma and breast cancer cells. Vitamin D increased the expression of I.3, I.7, and I.4 aromatase transcripts and induced de novo expression of the I.6 transcript; dexamethasone increased the expression of I.4, PII, and I.3 transcripts and mifepristone increased the expression of PII and I.3 aromatase transcripts. The cell specific regulation of CYP19A1 by vitamin D, dexamethasone, and mifepristone opens the possibility for cellular selective modulation of estrogen biosynthesis within the brain.

Forebrain steroid levels fluctuate rapidly during social interactions

Neurosteroids are powerful modulators of brain function and behavior, yet their dynamics within the brain have remained elusive. Using in vivo microdialysis in male zebra finches, we show that local estradiol levels increase rapidly within the forebrain during social interactions with females. Further, when males are exposed to other males’ song, local estradiol levels also increase and testosterone levels drop within a cortical/pallial auditory region that is analogous to mammalian auditory cortex. We also report that local estradiol and testosterone levels are differentially regulated in this same region by the conventional neurotransmitters glutamate and GABA, respectively. This study provides direct evidence that forebrain steroid levels are acutely and differentially regulated during social behavior, in a region-specific manner, and in a rapid time-course akin to that of traditional neuromodulators.

Rapid enhancement of two-step wiring plasticity by estrogen and NMDA receptor activity

Cortical information storage requires combined changes in connectivity and synaptic strength between neurons, but the signaling mechanisms underlying this two-step wiring plasticity are unknown. Because acute 17beta-estradiol (E2) modulates cortical memory, we examined its effects on spine morphogenesis, AMPA receptor trafficking, and GTPase signaling in cortical neurons. Acute E2 application resulted in a rapid, transient increase in spine density, accompanied by temporary formation of silent synapses through reduced surface GluR1. These rapid effects of E2 were dependent on a Rap/AF-6/ERK1/2 pathway. Intriguingly, NMDA receptor (NMDAR) activation after E2 treatment potentiated silent synapses and elevated spine density for as long as 24 h. Hence, we show that E2 transiently increases neuronal connectivity by inducing dynamic nascent spines that "sample" the surrounding neuropil and that subsequent NMDAR activity is sufficient to stabilize or "hold" E2-mediated effects. This work describes a form of two-step wiring plasticity relevant for cortical memory and identifies targets that may facilitate recovery from brain injuries.

Aromatase in the brain: not just for reproduction anymore

Aromatase, the enzyme that synthesises oestrogens from androgen precursors, is expressed in the brain, where it has been classically associated with the regulation of neuroendocrine events and behaviours linked with reproduction. Recent findings, however, have revealed new unexpected roles for brain aromatase, indicating that the enzyme regulates synaptic activity, synaptic plasticity, neurogenesis and the response of neural tissue to injury, and may contribute to control nonreproductive behaviours, mood and cognition. Therefore, the function of brain aromatase is not restricted to the regulation of reproduction as previously thought.

Aromatase distribution in the monkey temporal neocortex and hippocampus

Numerous studies have shown that neuronal plasticity in the hippocampus and neocortex is regulated by estrogen and that aromatase, the key enzyme for estrogen biosynthesis, is present in cerebral cortex. Although the expression pattern of aromatase mRNA has been described in the monkey brain, its precise cellular distribution has not been determined. In addition, the degree to which neuronal aromatase is affected by gonadal estrogen has not been investigated. In this study, we examined the immunohistochemical distribution of aromatase in young ovariectomized female rhesus monkeys with or without long-term cyclic estradiol treatment. Both experimental groups showed that aromatase is localized in a large population of CA1-3 pyramidal cells, in granule cells of the dentate gyrus and in some interneurons in which it was co-expressed with the calcium-binding proteins calbindin, calretinin, and parvalbumin. Moreover, numerous pyramidal cells were immunoreactive for aromatase in the neocortex, whereas only small subpopulations of neocortical interneurons were immunoreactive for aromatase. The widespread expression of the protein in a large neuronal population suggests that local intraneuroral estrogen synthesis may contribute to estrogen-induced synaptic plasticity in monkey hippocampus and neocortex of female rhesus monkeys. In addition, the apparent absence of obvious differences in aromatase distribution between the two experimental groups suggests that these localization patterns are not dependent on plasma estradiol levels.

Molecular characterization of the injury-induced aromatase transcript in the adult zebra finch brain

In the zebra finch (Taeniopygia guttata), the aromatase gene is transcribed from one of two promoters resulting in two transcripts constitutively expressed in brain or ovary. These transcripts differ only in Exon 1 which lies in the 5' un-translated region (UTR). An inducible form of aromatase is expressed following brain injury in glia. Towards characterizing this transcript, we (a) examined the up-regulation of amplicons within the aromatase transcript using quantitative PCR (qPCR), (b) performed 5' and 3' rapid amplification of cDNA ends (RACE) on injured brain RNA and (c) sequenced the injury-induced aromatase transcript. qPCR suggested that inducible aromatase may contain a novel 3'UTR. However, neither 3' nor 5' RACE revealed novel UTRs in the injured telencephalon. We then sequenced aromatase from injured entopallium, a region that lacks detectable constitutive aromatase. Inducible aromatase was identical in sequence to the known neural aromatase transcript. These data suggest that injury-induced aromatase differs from ovarian, but is indistinguishable from neuronal aromatase. We suggest that an injury-specific signal in glia may modulate aromatase transcription. Alternatively, injury-induced aromatase transcription may be silenced under constitutive conditions. To the best of our knowledge, this is the first report that documents the sequence of inducible aromatase in any vertebrate.

Interference of endocrine disrupting chemicals with aromatase CYP19 expression or activity, and consequences for reproduction of teleost fish

Many natural and synthetic compounds present in the environment exert a number of adverse effects on the exposed organisms, leading to endocrine disruption, for which they were termed endocrine disrupting chemicals (EDCs). A decrease in reproduction success is one of the most well-documented signs of endocrine disruption in fish. Estrogens are steroid hormones involved in the control of important reproduction-related processes, including sexual differentiation, maturation and a variety of others. Careful spatial and temporal balance of estrogens in the body is crucial for proper functioning. At the final step of estrogen biosynthesis, cytochrome P450 aromatase, encoded by the cyp19 gene, converts androgens into estrogens. Modulation of aromatase CYP19 expression and function can dramatically alter the rate of estrogen production, disturbing the local and systemic levels of estrogens. In the present review, the current progress in CYP19 characterization in teleost fish is summarized and the potential of several classes of EDCs to interfere with CYP19 expression and activity is discussed. Two cyp19 genes are present in most teleosts, cyp19a and cyp19b, primarily expressed in the ovary and brain, respectively. Both aromatase CYP19 isoforms are involved in the sexual differentiation and regulation of the reproductive cycle and male reproductive behavior in diverse teleost species. Alteration of aromatase CYP19 expression and/or activity, be it upregulation or downregulation, may lead to diverse disturbances of the above mentioned processes. Prediction of multiple transcriptional regulatory elements in the promoters of teleost cyp19 genes suggests the possibility for several EDC classes to affect cyp19 expression on the transcriptional level. These sites include cAMP responsive elements, a steroidogenic factor 1/adrenal 4 binding protein site, an estrogen-responsive element (ERE), half-EREs, dioxin-responsive elements, and elements related to diverse other nuclear receptors (peroxisome proliferator activated receptor, retinoid X receptor, retinoic acid receptor). Certain compounds including phytoestrogens, xenoestrogens, fungicides and organotins may modulate aromatase CYP19 activity on the post-transcriptional level. As is shown in this review, diverse EDCs may affect the expression and/or activity of aromatase cyp19 genes through a variety of mechanisms, many of which need further characterization in order to improve the prediction of risks posed by a contaminated environment to teleost fish population.

Synthesis of estrogens in progenitor cells of adult fish brain: evolutive novelty or exaggeration of a more general mechanism implicating estrogens in neurogenesis?

In contrast to other vertebrates, in which the adult brain shows limited adult neurogenesis, teleost fishes exhibit an unparalleled capacity to generate new neurons as adults, suggesting that their brains present a highly permissive environment for the maintenance and proliferation of adult progenitors. Here, we examine the hypothesis that one of the factors permitting establishment of this favourable environment is estradiol. Indeed, recent data showed that radial glial cells strongly expressed one of two aromatase duplicated genes. Aromatase is the estrogen-synthesizing enzyme and this observation is of great interest, given that radial glial cells are progenitor cells capable of generating new neurons. Given the well-documented roles of estrogens on cell fate, and notably on cell proliferation, these data suggest that estradiol could be involved in maintaining and/or activating these progenitors. Examination of recent data in birds and mammals suggests that the situation in fish could well be an exaggeration of a more general mechanism implicating estrogens in neurogenesis. Indeed, there is accumulating evidence that estrogens are involved in embryonic, adult or reparative neurogenesis in other vertebrates, notably in mammals.

Brain aromatase: roles in reproduction and neuroprotection

It is well established that aromatization constitutes an essential part of testosterone's signaling pathway in brain and that estrogen metabolites, often together with testosterone, organize and activate masculine neural circuits. This paper summarizes the current understanding regarding the distribution, regulation and function of brain aromatase in mammals. Data from our laboratory are presented that highlight the important function of aromatase in the regulation of androgen feedback sensitivity in non-human primates and the possible role that aromatase plays in determining the brain structure and sexual partner preferences of rams. In addition, new data is presented indicating that the capacity for aromatization in cortical astrocytes is associated with cell survival and may be important for neuroprotection. It is anticipated that a better appreciation of the physiological and pathophysiological functions of aromatase will lead to important clinical insights.

Evaluation of neuroactive steroid levels by liquid chromatography-tandem mass spectrometry in central and peripheral nervous system: effect of diabetes

The nervous system is a target for physiological and protective effects of neuroactive steroids. Consequently, the assessment of their levels in nervous structures under physiological and pathological conditions is a top priority. To this aim, identification and quantification of pregnenolone (PREG), progesterone (PROG), dihydroprogesterone (DHP), tetrahydroprogesterone (THP), testosterone (T), dihydrotestosterone (DHT), 5alpha-androstan-3alpha, 17beta-diol (3alpha-diol), 17alpha- and 17beta-estradiol (17alpha-E and 17beta-E) by liquid chromatography and tandem mass spectrometry (LC-MS/MS) has been set up. After validation, this method was applied to determine the levels of neuroactive steroids in central (i.e., cerebral cortex, cerebellum and spinal cord) and peripheral (i.e., brachial nerve) nervous system of control and diabetic rats. In controls only the brachial nerve had detectable levels of all these neuroactive steroids. In contrast, 17alpha-E in cerebellum, 17alpha-E, 17beta-E, DHP and THP in cerebral cortex, and 17alpha-E, 17beta-E and DHP in spinal cord were under the detection limit. Diabetes, induced by injection with streptozotocin, strongly affected the levels of some neuroactive steroids. In particular, the levels of PREG, PROG and T in cerebellum, of PROG, T and 3alpha-diol in cerebral cortex, of PROG, DHT and 3alpha-diol in spinal cord and of PREG, DHP, THP, T, DHT and 3alpha-diol in brachial nerve were significantly decreased. In conclusion, the data here reported demonstrate that the LC-MS/MS method allows the assessment of neuroactive steroids in the nervous system with high sensitivity and specificity and that diabetes strongly affects their levels, providing a further basis for new therapeutic tools based on neuroactive steroids aimed at counteracting diabetic neuropathy.

In search of neuroprotective therapies based on the mechanisms of estrogens

Although estradiol is a neuroprotective factor, estrogen therapy in older women increases the risk of adverse cognitive outcomes and poses additional peripheral risks, requiring careful use of estrogenic compounds as treatments for neurodegenerative conditions or neural injury. Potential alternatives to estrogen therapy to promote neuroprotection might include treatment with molecules that are able to interact with estrogen receptors, with alternative mechanisms of action, or with molecules that induce local estradiol synthesis in the brain, or a combination of all. However, before considering the broad clinical applications, more basic research is required to clarify the mechanisms of action and potential risks of some of these estrogen-based treatments.

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

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

Role of P450 aromatase in sex-specific astrocytic cell death

Female animals are protected from ischemic brain damage relative to age-matched males, in part through protection provided by endogenous estradiol. In brain, estradiol is produced from testosterone by cytochrome P450 aromatase (cyp 19), a steroid synthetic enzyme present in astrocytes. We tested the hypothesis that astrocytes derived from neonatal female rat brain are less susceptible than male cells to oxygen-glucose deprivation (OGD), and that this endogenous protection is associated with enhanced aromatase activity. Primary cultured cortical astrocytes were prepared from male and female rat pups separately and grown to confluence in estrogen-free media. Cell death in response to OGD, alone or in combination with hydrogen peroxide, lipopolysaccharides, interleukin-1beta, tissue necrosis factor-alpha, or nitric oxide (NO) donor diethylenetriamine/nitric oxide adduct (DETA/NO) was quantified as the ratio of propidium iodide to calcein AM-positive cells. Aromatase activity and cyp19 mRNA and protein levels were measured in cultures from each sex. Female astrocytes are more resistant to OGD and oxidant cell death induced by H(2)O(2) , but sustain greater cell death when inflammatory mediators are combined with OGD compared with OGD alone. Media transfer from female to male cells conferred protection against OGD-induced cell death. Aromatase activity and expression is greater in female than in male astrocytes. The aromatase inhibitor, Arimidex (100 nmol/L), abolishes sex differences in OGD-induced cell death, whereas treatment with 17beta-estradiol (10 nmol/L) protects cells of either sex. We conclude that astrocytes isolated from neonatal cortex exhibit marked sex differences in sensitivity to OGD, in part because of enhanced aromatization and estradiol formation in female cells.

Steroids and glial cell function

Hormonal and locally produced steroids act in the nervous system as neuroendocrine regulators, as trophic factors and as neuromodulators and have a major impact on neural development and function. Glial cells play a prominent role in the local production of steroids and in the mediation of steroid effects on neurons and other glial cells. In this review, we examine the role of glia in the synthesis and metabolism of steroids and the functional implications of glial steroidogenesis. We analyze the mechanisms of steroid signaling on glia, including the role of nuclear receptors and the mechanisms of membrane and cytoplasmic signaling mediated by changes in intracellular calcium levels and activation of signaling kinases. Effects of steroids on functional parameters of glia, such as proliferation, myelin formation, metabolism, cytoskeletal reorganization, and gliosis are also reviewed, as well as the implications of steroid actions on glia for the regulation of synaptic function and connectivity, the regulation of neuroendocrine events, and the response of neural tissue to injury.

Brain aromatase: new lessons from non-mammalian model systems

This review highlights recent studies of the anatomical and functional implications of brain aromatase (estrogen synthase) expression in two vertebrate lineages, teleost fishes and songbirds, that show remarkably high levels of adult brain aromatase activity, protein and gene expression compared to other vertebrate groups. Teleosts and birds have proven to be important neuroethological models for investigating how local estrogen synthesis leads to changes in neural phenotypes that translate into behavior. Region-specific patterns of aromatase expression, and thus estrogen synthesis, include the vocal and auditory circuits that figure prominently into the life history adaptations of vocalizing teleosts and songbirds. Thus, by targeting, for example, vocal motor circuits without inappropriate steroid exposure to other steroid-dependent circuits, such as those involved in either copulatory or spawning behaviors, the neuroendocrine system can achieve temporal and spatial specificity in its modulation of neural circuits that lead to the performance of any one behavior.