Pathways of dehydroepiandrosterone formation in rat brain glia

Can longer lifespan be associated with gut microbiota involvement in lipid metabolism?

Biological aging is linked to altered body composition and reduced neuroactive steroid hormones like dehydroepiandrosterone sulfate (DHEAS), which can stimulate the GABA signaling pathway via gut microbiota. Our study examined the association of gut microbiota with lifespan in mice through comprehensive analysis of its composition and functional involvement in cholesterol sulfate, a precursor of DHEAS, metabolism. We used 16S rRNA and metagenomic sequencing, followed by metabolic pathway prediction and thin layer chromatography and MALDI-TOF cholesterol sulfate identification. Significant increases in bacteria such as Bacteroides, typical for long-lived and Odoribacter and Colidextribacter, specific for short-lived mice were detected. Furthermore, for males (Rikenella and Alloprevotella) and females (Lactobacillus and Bacteroides), specific bacterial groups emerged as predictors (AUC = 1), highlighting sex-specific patterns. Long-lived mice showed a strong correlation of Bacteroides (0.918) with lipid and steroid hormone metabolism, while a negative correlation of GABAergic synapse with body weight (-0.589). We found that several Bacteroides species harboring the sulfotransferase gene and gene cluster for sulfonate donor synthesis are involved in converting cholesterol to cholesterol sulfate, significantly higher in the feces of long-lived individuals. Overall, we suggest that increased involvement of gut bacteria, mainly Bacteroides spp., in cholesterol sulfate synthesis could ameliorate aging through lipid metabolism.

Advances in steroid research from the pioneering neurosteroid concept to metabolomics: New insights into pregnenolone function

Advances in neuroendocrinology have led to major discoveries since the 19th century, identifying adaptive loops for maintaining homeostasis. One of the most remarkable discoveries was the concept of neurosteroids, according to which the brain is not only a target but also a source of steroid production. The identification of new membrane steroid targets now underpins the neuromodulatory effects of neurosteroids such as pregnenolone, which is involved in functions mediated by the GPCR CB1 receptor. Structural analysis of steroids is a key feature of their interactions with the phospholipid membrane, receptors and resulting activity. Therefore, mass spectrometry-based methods have been developed to elucidate the metabolic pathways of steroids, the ultimate approach being metabolomics, which allows the identification of a large number of metabolites in a single sample. This approach should enable us to make progress in understanding the role of neurosteroids in the functioning of physiological and pathological processes.

Neuropeptidergic control of neurosteroids biosynthesis

Neurosteroids are steroids synthesized within the central nervous system either from cholesterol or by metabolic reactions of circulating steroid hormone precursors. It has been suggested that neurosteroids exert pleiotropic activities within the central nervous system, such as organization and activation of the central nervous system and behavioral regulation. It is also increasingly becoming clear that neuropeptides exert pleiotropic activities within the central nervous system, such as modulation of neuronal functions and regulation of behavior, besides traditional neuroendocrinological functions. It was hypothesized that some of the physiological functions of neuropeptides acting within the central nervous system may be through the regulation of neurosteroids biosynthesis. Various neuropeptides reviewed in this study possibly regulate neurosteroids biosynthesis by controlling the activities of enzymes that catalyze the production of neurosteroids. It is now required to thoroughly investigate the neuropeptidergic control mechanisms of neurosteroids biosynthesis to characterize the physiological significance of this new neuroendocrinological phenomenon.

New perspectives on the role of the neurosteroid pregnenolone as an endogenous regulator of type-1 cannabinoid receptor (CB1R) activity and function

Pregnenolone is a steroid with specific characteristics, being the first steroid to be synthesised from cholesterol at all sites of steroidogenesis, including the brain. For many years, pregnenolone was defined as an inactive precursor of all steroids because no specific target had been discovered. However, over the last decade, it has become a steroid of interest because it has been recognised as being a biomarker for brain-related disorders through the development of metabolomic approaches and advanced analytical methods. In addition, physiological roles for pregnenolone emerged when specific targets were discovered. In this review, we highlight the discovery of the selective interaction of pregnenolone with the type-1 cannabinoid receptor (CB1R). After describing the specific characteristic of CB1Rs, we discuss the newly discovered mechanisms of their regulation by pregnenolone. In particular, we describe the action of pregnenolone as a negative allosteric modulator and a specific signalling inhibitor of the CB1R. These particular characteristics of pregnenolone provide a great strategic opportunity for therapeutic development in CB1-related disorders. Finally, we outline new perspectives using innovative genetic tools for the discovery of original regulatory mechanisms of pregnenolone on CB1-related functions.

Neurosteroidogenic enzymes: CYP11A1 in the central nervous system

Neurosteroids, steroid hormones synthesized locally in the nervous system, have important neuromodulatory and neuroprotective effects in the central nervous system. Progress in neurosteroid research has led to the successful translation of allopregnanolone into an approved therapy for postpartum depression. However, there is insufficient evidence to support the assumption that steroidogenesis is exactly the same between the nervous system and the periphery. This review focuses on CYP11A1, the only enzyme currently known to catalyze the first reaction in steroidogenesis to produce pregnenolone, the precursor to all other steroids. Although CYP11A1 mRNA has been found in brain of many mammals, the presence of CYP11A1 protein has been difficult to detect, particularly in humans. Here, we highlight the discrepancies in the current evidence for CYP11A1 in the central nervous system and propose new directions for understanding neurosteroidogenesis, which will be crucial for developing neurosteroid-based therapies for the future.

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.

Expression and regulation of CYP17A1 and 3β-hydroxysteroid dehydrogenase in cells of the nervous system: Potential effects of vitamin D on brain steroidogenesis

Steroids are reported to have diverse functions in the nervous system. Enzymatic production of steroid hormones has been reported in different cell types, including astrocytes and neurons. However, the information on some of the steroidogenic enzymes involved is insufficient in many respects. Contradictory results have been reported concerning the relative importance of different cell types in the nervous system for expression of CYP17A1 and 3β-hydroxysteroid dehydrogenase (3β-HSD). 3β-HSD is important in all basic steroidogenic pathways and CYP17A1 is required to form sex hormones. In the current investigation we studied the expression of these enzymes in cultured primary rat astrocytes, in neuron-enriched cells from rat cerebral cortex and in human neuroblastoma SH-SY5Y cells, a cell line often used as an in vitro model of neuronal function and differentiation. As part of this study we also examined potential effects on CYP17A1 and 3β-HSD by vitamin D, a compound previously shown to have regulatory effects in steroid hormone-producing cells outside the brain. The results of our study indicate that astrocytes are a major site for expression of 3β-HSD whereas expression of CYP17A1 is found in both astrocytes and neurons. The current data suggest that neurons, contrary to some previous reports, are not involved in 3β-HSD reactions. Previous studies have shown that vitamin D can influence gene expression and hormone production by steroidogenic enzymes in some cells. We found that vitamin D suppressed CYP17A1-mediated activity by 20% in SH-SY5Ycells and astrocytes. Suppression of CYP17A1 mRNA levels was considerably stronger, about 50% in SH-SY5Y cells and 75% in astrocytes. In astrocytes 3β-HSD was also suppressed by vitamin D, about 20% at the enzyme activity level and 60% at the mRNA level. These data suggest that vitamin D-mediated regulation of CYP17A1 and 3β-HSD, particularly on the transcriptional level, may play a role in the nervous system.

Targeting translocator protein (18 kDa) (TSPO) dampens pro-inflammatory microglia reactivity in the retina and protects from degeneration

BACKGROUND

Reactive microglia are commonly seen in retinal degenerative diseases, and neurotoxic microglia responses can contribute to photoreceptor cell death. We and others have previously shown that translocator protein (18 kDa) (TSPO) is highly induced in retinal degenerations and that the selective TSPO ligand XBD173 (AC-5216, emapunil) exerts strong anti-inflammatory effects on microglia in vitro and ex vivo. Here, we investigated whether targeting TSPO with XBD173 has immuno-modulatory and neuroprotective functions in two mouse models of acute retinal degeneration using bright white light exposure.

METHODS

BALB/cJ and Cx3cr1(GFP/+) mice received intraperitoneal injections of 10 mg/kg XBD173 or vehicle for five consecutive days, starting 1 day prior to exposure to either 15,000 lux white light for 1 h or 50,000 lux focal light for 10 min, respectively. The effects of XBD173 treatment on microglia and Müller cell reactivity were analyzed by immuno-stainings of retinal sections and flat mounts, fluorescence-activated cell sorting (FACS) analysis, and mRNA expression of microglia markers using quantitative real-time PCR (qRT-PCR). Optical coherence tomography (OCT), terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) stainings, and morphometric analyses were used to quantify the extent of retinal degeneration and photoreceptor apoptosis.

RESULTS

Four days after the mice were challenged with bright white light, a large number of amoeboid-shaped alerted microglia appeared in the degenerating outer retina, which was nearly completely prevented by treatment with XBD173. This treatment also down-regulated the expression of TSPO protein in microglia but did not change the TSPO levels in the retinal pigment epithelium (RPE). RT-PCR analysis showed that the microglia/macrophage markers Cd68 and activated microglia/macrophage whey acidic protein (Amwap) as well as the pro-inflammatory genes Ccl2 and Il6 were reduced after XBD173 treatment. Light-induced degeneration of the outer retina was nearly fully blocked by XBD173 treatment. We further confirmed these findings in an independent mouse model of focal light damage. Retinas of animals receiving XBD173 therapy displayed significantly more ramified non-reactive microglia and more viable arrestin-positive cone photoreceptors than vehicle controls.

CONCLUSIONS

Targeting TSPO with XBD173 effectively counter-regulates microgliosis and ameliorates light-induced retinal damage, highlighting a new pharmacological concept for the treatment of retinal degenerations.

Expression of steroidogenesis-related genes in murine male germ cells

For decades, only few tissues and cell types were defined as steroidogenic, capable of de novo steroid synthesis from cholesterol. However, with the refinement of detection methods, several tissues have now been added to the list of steroidogenic tissues. Besides their critical role as long-range acting hormones, steroids are also playing more discreet roles as local mediators and signaling molecules within the tissues they are produced. In testis, steroidogenesis is carried out by the Leydig cells through a broad network of proteins, mediating cholesterol delivery to CYP11A1, the first cytochrome of the steroidogenic cascade, and the sequential action of enzymes insuring the production of active steroids, the main one being testosterone. The knowledge that male germ cells can be directly regulated by steroids and that they express several steroidogenesis-related proteins led us to hypothesize that germ cells could produce steroids, acting as autocrine, intracrine and juxtacrine modulators, as a way to insure synchronized progression within spermatogenic cycles, and preventing inappropriate cell behaviors between neighboring cells. Gene expression and protein analyses of mouse and rat germ cells from neonatal gonocytes to spermatozoa showed that most steroidogenesis-associated genes are expressed in germ cells, showing cell type-, spermatogenic cycle-, and age-specific expression profiles. Highly expressed genes included genes involved in steroidogenesis and other cell functions, such as Acbd1 and 3, Tspo and Vdac1-3, and genes involved in fatty acids metabolism or synthesis, including Hsb17b4 10 and 12, implying broader roles than steroid synthesis in germ cells. These results support the possibility of an additional level of regulation of spermatogenesis exerted between adjacent germ cells.

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.

DHEA effects on brain and behavior: insights from comparative studies of aggression

Historically, research on the neuroendocrinology of aggression has been dominated by the paradigm that the brain receives sex steroid hormones, such as testosterone (T), from the gonads, and then these gonadal hormones modulate behaviorally relevant neural circuits. While this paradigm has been extremely useful for advancing the field, recent studies reveal important alternatives. For example, most vertebrate species are seasonal breeders, and many species show aggression outside of the breeding season, when the gonads are regressed and circulating levels of gonadal steroids are relatively low. Studies in diverse avian and mammalian species suggest that adrenal dehydroepiandrosterone (DHEA), an androgen precursor and prohormone, is important for the expression of aggression when gonadal T synthesis is low. Circulating DHEA can be converted into active sex steroids within the brain. In addition, the brain can synthesize sex steroids de novo from cholesterol, thereby uncoupling brain steroid levels from circulating steroid levels. These alternative mechanisms to provide sex steroids to specific neural circuits may have evolved to avoid the costs of high circulating T levels during the non-breeding season. Physiological indicators of season (e.g., melatonin) may allow animals to switch from one neuroendocrine mechanism to another across the year. DHEA and neurosteroids are likely to be important for the control of multiple behaviors in many species, including humans. These studies yield fundamental insights into the regulation of DHEA secretion, the mechanisms by which DHEA affects behavior, and the brain regions and neural processes that are modulated by DHEA. It is clear that the brain is an important site of DHEA synthesis and action. This article is part of a Special Issue entitled 'Essential role of DHEA'.

Exogenous dehydroisoandrosterone sulfate reverses the dendritic changes of the central neurons in aging male rats

Sex hormones are known to help maintaining the cognitive ability in male and female rats. Hypogonadism results in the reduction of the dendritic spines of central neurons which is believed to undermine memory and cognition and cause fatigue and poor concentration. In our previous studies, we have reported age-related regression in dendrite arbors along with loss of dendritic spines in the primary somatosensory cortical neurons in female rats. Furthermore, castration caused a reduction of dendritic spines in adult male rats. In light of this, it was surmised that dendritic structures might change in normal aging male rats with advancing age. Recently, dehydroepiandrosterone sulfate (DHEAS) has been reported to have memory-enhancing properties in aged rodents. In this study, normal aging male rats, with a reduced plasma testosterone level of 75-80%, were used to explore the changes in behavioral performance of neuronal dendritic arbor and spine density. Aging rats performed poorer in spatial learning memory (Morris water maze). Concomitantly, these rats showed regressed dendritic arbors and spine loss on the primary somatosensory cortical and hippocampal CA1 pyramidal neurons. Exogenous DHEAS and testosterone treatment reversed the behavioral deficits and partially restored the spine loss of cortical neurons in aging male rats but had no effects on the dendritic arbor shrinkage of the affected neurons. It is concluded therefore that DHEAS, has the efficacy as testosterone, and that it can exert its effects on the central neuron level to effectively ameliorate aging symptoms.

Translocator protein (18 kDa) (TSPO) is expressed in reactive retinal microglia and modulates microglial inflammation and phagocytosis

BACKGROUND

The translocator protein (18 kDa) (TSPO) is a mitochondrial protein expressed on reactive glial cells and a biomarker for gliosis in the brain. TSPO ligands have been shown to reduce neuroinflammation in several mouse models of neurodegeneration. Here, we analyzed TSPO expression in mouse and human retinal microglia and studied the effects of the TSPO ligand XBD173 on microglial functions.

METHODS

TSPO protein analyses were performed in retinoschisin-deficient mouse retinas and human retinas. Lipopolysaccharide (LPS)-challenged BV-2 microglial cells were treated with XBD173 and TSPO shRNAs in vitro and pro-inflammatory markers were determined by qRT-PCR. The migration potential of microglia was determined with wound healing assays and the proliferation was studied with Fluorescence Activated Cell Sorting (FACS) analysis. Microglial neurotoxicity was estimated by nitrite measurement and quantification of caspase 3/7 levels in 661 W photoreceptors cultured in the presence of microglia-conditioned medium. The effects of XBD173 on filopodia formation and phagocytosis were analyzed in BV-2 cells and human induced pluripotent stem (iPS) cell-derived microglia (iPSdM). The morphology of microglia was quantified in mouse retinal explants treated with XBD173.

RESULTS

TSPO was strongly up-regulated in microglial cells of the dystrophic mouse retina and also co-localized with microglia in human retinas. Constitutive TSPO expression was high in the early postnatal Day 3 mouse retina and declined to low levels in the adult tissue. TSPO mRNA and protein were also strongly induced in LPS-challenged BV-2 microglia while the TSPO ligand XBD173 efficiently suppressed transcription of the pro-inflammatory marker genes chemokine (C-C motif) ligand 2 (CCL2), interleukin 6 (IL6) and inducible nitric oxide (NO)-synthase (iNOS). Moreover, treatment with XBD173 significantly reduced the migratory capacity and proliferation of microglia, their level of NO secretion and their neurotoxic activity on 661 W photoreceptor cells. Furthermore, XBD173 treatment of murine and human microglial cells promoted the formation of filopodia and increased their phagocytic capacity to ingest latex beads or photoreceptor debris. Finally, treatment with XBD173 reversed the amoeboid alerted phenotype of microglial cells in explanted organotypic mouse retinal cultures after challenge with LPS.

CONCLUSIONS

These findings suggest that TSPO is highly expressed in reactive retinal microglia and a promising target to control microglial reactivity during retinal degeneration.

Oxidative metabolism of dehydroepiandrosterone (DHEA) and biologically active oxygenated metabolites of DHEA and epiandrosterone (EpiA)--recent reports

Dehydroepiandrosterone (DHEA) is a multifunctional steroid with a broad range of biological effects in humans and animals. DHEA can be converted to multiple oxygenated metabolites in the brain and peripheral tissues. The mechanisms by which DHEA exerts its effects are not well understood. However, evidence that the effects of DHEA are mediated by its oxygenated metabolites has accumulated. This paper will review the panel of oxygenated DHEA metabolites (7, 16 and 17-hydroxylated derivatives) including a number of 5α-androstane derivatives, such as epiandrosterone (EpiA) metabolites. The most important aspects of the oxidative metabolism of DHEA in the liver, intestine and brain are described. Then, this article reviews the reported biological effects of oxygenated DHEA metabolites from recent findings with a specific focus on cancer, inflammatory and immune processes, osteoporosis, thermogenesis, adipogenesis, the cardiovascular system, the brain and the estrogen and androgen receptors.

Oxidative Stress-Mediated Brain Dehydroepiandrosterone (DHEA) Formation in Alzheimer's Disease Diagnosis

Neurosteroids are steroids made by brain cells independently of peripheral steroidogenic sources. The biosynthesis of most neurosteroids is mediated by proteins and enzymes similar to those identified in the steroidogenic pathway of adrenal and gonadal cells. Dehydroepiandrosterone (DHEA) is a major neurosteroid identified in the brain. Over the years we have reported that, unlike other neurosteroids, DHEA biosynthesis in rat, bovine, and human brain is mediated by an oxidative stress-mediated mechanism, independent of the cytochrome P450 17α-hydroxylase/17,20-lyase (CYP17A1) enzyme activity found in the periphery. This alternative pathway is induced by pro-oxidant agents, such as Fe(2+) and β-amyloid peptide. Neurosteroids are involved in many aspects of brain function, and as such, are involved in various neuropathologies, including Alzheimer's disease (AD). AD is a progressive, yet irreversible neurodegenerative disease for which there are limited means for ante-mortem diagnosis. Using brain tissue specimens from control and AD patients, we provided evidence that DHEA is formed in the AD brain by the oxidative stress-mediated metabolism of an unidentified precursor, thus depleting levels of the precursor in the blood stream. We tested for the presence of this DHEA precursor in human serum using a Fe(2+)-based reaction and determined the amounts of DHEA formed. Fe(2+) treatment of the serum resulted in a dramatic increase in DHEA levels in control patients, whereas only a moderate or no increase was observed in AD patients. The DHEA variation after oxidation correlated with the patients' cognitive and mental status. In this review, we present the cumulative evidence for oxidative stress as a natural regulator of DHEA formation and the use of this concept to develop a blood-based diagnostic tool for neurodegenerative diseases linked to oxidative stress, such as AD.

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.

Dehydroepiandrosterone formation is independent of cytochrome P450 17alpha-hydroxylase/17, 20 lyase activity in the mouse brain

Cytochrome P450 17alpha-hydroxylase/17, 20 lyase (CYP17) is a microsomal enzyme reported to have two distinct catalytic activities, 17alpha-hydroxylase and 17, 20 lyase, that are essential for the biosynthesis of peripheral androgens such as dehydroepiandrosterone (DHEA). Paradoxically, DHEA is present and plays a role in learning and memory in the adult rodent brain, while CYP17 activity and protein are undetectable. To determine if CYP17 is required for DHEA formation and function in the adult rodent brain, we generated CYP17 chimeric mice that had reduced circulating testosterone levels. There were no detectable differences in cognitive spatial learning between CYP17 chimeric and wild-type mice. In addition, while CYP17 mRNA levels were reduced in CYP17 chimeric compared to wild-type mouse brain, the levels of brain DHEA levels were comparable. To determine if adult brain DHEA is formed by an alternative Fe(2+)-dependent pathway, brain microsomes were isolated from wild-type and CYP17 chimeric mice and treated with FeSO(4). Fe(2+) caused comparable levels of DHEA production by both wild-type and CYP17 chimeric mouse brain microsomes; DHEA production was not reduced by a CYP17 inhibitor. Taken together these in vivo studies suggest that in the adult mouse brain DHEA is formed via a Fe(2+)-sensitive CYP17-independent pathway.

Translocator protein 18 kDa (TSPO): molecular sensor of brain injury and repair

For over 15 years, the peripheral benzodiazepine receptor (PBR), recently named translocator protein 18 kDa (TSPO) has been studied as a biomarker of reactive gliosis and inflammation associated with a variety of neuropathological conditions. Early studies documented that in the brain parenchyma, TSPO is exclusively localized in glial cells. Under normal physiological conditions, TSPO levels are low in the brain neuropil but they markedly increase at sites of brain injury and inflammation making it uniquely suited for assessing active gliosis. This research has generated significant efforts from multiple research groups throughout the world to apply TSPO as a marker of "active" brain pathology using in vivo imaging modalities such as Positron Emission Tomography (PET) in experimental animals and humans. Further, in the last few years, there has been an increased interest in understanding the molecular and cellular function(s) of TSPO in glial cells. The latest evidence suggests that TSPO may not only serve as a biomarker of active brain disease but also the use of TSPO-specific ligands may have therapeutic implications in brain injury and repair. This review presents an overview of the history and function of TSPO focusing on studies related to its use as a sensor of active brain disease in experimental animals and in human studies.

Immunohistochemical localization and biological activity of the steroidogenic enzyme cytochrome P450 17α-hydroxylase/C17, 20-lyase (P450C17) in the frog brain and pituitary

It is now clearly established that the brain has the capability of synthesizing various biologically active steroids including 17-hydroxypregnenolone (17OH-Delta(5)P), 17-hydroxyprogesterone (17OH-P), dehydroepiandrosterone (DHEA) and androstenedione (Delta(4)). However, the presence, distribution and activity of cytochrome P450 17alpha-hydroxylase/C17, 20-lyase (P450(C17)), a key enzyme required for the conversion of pregnenolone (Delta(5)P) and progesterone (P) into these steroids, are poorly documented. Here, we show that P450(C17)-like immunoreactivity is widely distributed in the frog brain and pituitary. Prominent populations of P450(C17)-containing cells were observed in a number nuclei of the telencephalon, diencephalon, mesencephalon and metencephalon, as well as in the pars distalis and pars intermedia of the pituitary. In the brain, P450(C17)-like immunoreactivity was almost exclusively located in neurons. In several hypothalamic nuclei, P450(C17)-positive cell bodies also contained 3beta-hydroxysteroid dehydrogenase-like immunoreactivity. Incubation of telencephalon, diencephalon, mesencephalon, metencephalon or pituitary explants with [(3)H]Delta(5)P resulted in the formation of several tritiated steroids including 17OH-Delta(5)P, 17OH-P, DHEA and Delta(4). De novo synthesis of C(21) 17-hydroxysteroids and C(19) ketosteroids was reduced in a concentration-dependent manner by ketoconazole, a P450(C17) inhibitor. This is the first detailed immunohistochemical mapping of P450(C17) in the brain and pituitary of any vertebrate. Altogether, the present data provide evidence that CNS neurons and pituitary cells can synthesize androgens.

Dehydroepiandrosterone sulphate sulphohydrolase [correction of sulphoydrolase] from human placenta microsomes--properties of the purified enzyme

A form of steroid sulphate sulphohydrolase (EC 3.1.6.2) hydrolysing the dehydroepiandrosterone sulphate (DHEAS-ase) was purified from human placenta microsomes. During the purification procedure the DHEAS-ase was separated from the oestrone sulphate sulphohydrolase (OS-ase). The purified DHEAS-ase revealed specific activity of 1520 nmolxmin-1xmgprotein-1 and exhibited optimal activity at pH 8.4. The Km value was established to be 3.3+/-0.07x10(-5) M. The pI value was around 8.7. The molecular weight estimated by gel filtration was 7.4 kDa. The purified DHEAS-ase was not sensitive to the common sulphohydrolase inhibitors, such as phosphate, sulphate and sulphide ions, but was inhibited by several phosphohydrolase inhibitors (ammonium molybdate, vanadium oxide(V), zinc acetate). Steroids effected inhibition or activation of the purified enzyme. The data concerning substances reacting with -SH groups suggest that in the physiological conditions DHEAS-ase is controlled by the redox status of the cell.

Imaging the peripheral benzodiazepine receptor response in central nervous system demyelination and remyelination

We used a rodent model of cuprizone-induced demyelination to examine the peripheral benzodiazepine receptor (PBR) response during remyelination. C57BL/6J mice were fed a 0.2% cuprizone-containing or control diet for 3 weeks and then removed to allow for remyelination. Quantitative autoradiography of 3H-(R)-PK11195 binding to PBR in the corpus callosum showed increased levels at 3 weeks of demyelination and gradually decreased as a function of remyelination. PBR levels were associated with the degree of remyelination and activation of microglia and astrocytes. However, the temporal pattern suggests that the PBR signal during the late stages of remyelination was primarily associated with astrocytes. We also used small-animal positron-emission tomography (PET) imaging to determine if this technique could be used to monitor PBR levels in the brain of living mice. The results indicate that 11C-(R)-PK11195 levels are significantly elevated in the mouse brain during cuprizone-induced demyelination and normalize at a time in which remyelination is complete. These findings support the notion that PBR is a sensitive marker for the visualization and quantification of brain injury and recovery. Further, the in vivo imaging of the PBR response is now possible in the living rodent brain.

Is brain dehydroepiandrosterone synthesis modulated by free radicals in mice?

Dehydroepiandrosterone (DHEA) is a neurosteroid synthesized de novo in the brain, in addition to the periphery, modulating some membrane, ion-gated channel neurotransmitter receptors. P450-17alpha-hydroxylase activity converting pregnenolone to DHEA, has not yet been identified in the brain of rodents. Studies in brain-derived primary cultures and cell lines, suggest a possible alternative pathway for DHEA synthesis involving oxygenated hydroxyperoxides. We investigated DHEA synthesis in the brains of castrated male mice before and after treatment with N-acetylcysteine amide (AD4) (a newly developed brain penetrating antioxidant). We found a significant increase in brain DHEA level 24 h after castration, which was totally blocked by AD4. This blockade of castration-induced increased brain DHEA synthesis, supports the assumption that this synthesis may also be affected by free radicals. This is the first in vivo study indicating the possible existence of an in-brain oxidative stress-related pathway leading to brain DHEA production.

Peripheral benzodiazepine receptor and its clinical targeting

Tumor cell targeted therapies, by induction or enhancement of apoptosis, constitute recent promising approaches achieving more specific anti-tumor efficacy. The peripheral benzodiazepine receptor (PBR), which belongs to the permeability transition pore (PTP), the central regulatory complex of apoptosis, is a potential target. A number of findings argue in favor of the development of PBR targeting approaches: (i) overexpression of PBR has been described in a large range of human cancers, (ii) PTP-mediated regulation of programmed cell death is an apoptotic-inducing factor-independent check-point that could be modulated by various conventional cancer therapies, and (iii) PBR ligation enhances apoptosis induction in many types of tumors and reverses Bcl-2 cytoprotective effects. Altogether, these observations support the use of PBR-directed drugs, particularly PBR ligands such as Ro5-4864, in the treatment of human cancers.

Novel lipoidal derivatives of pregnenolone and dehydroepiandrosterone and absence of their sulfated counterparts in rodent brain

A new sample preparation method coupled to GC-MS analysis was developed and validated for quantification of sulfate esters of pregnenolone (PREG-S) and dehydroepiandrosterone (DHEA-S) in rat brain. Using a solid-phase extraction recycling protocol, the results show that little or no PREG-S and DHEA-S (<1 pmol/g) is present in rat and mouse brain. These data are in agreement with studies in which steroid sulfates were analyzed without deconjugation. We suggest that the discrepancies between analyses with and without deconjugation are caused by internal contamination of brain extract fractions, supposed to contain steroid sulfates, by lipoidal forms of PREG and DHEA (L-PREG and L-DHEA, respectively). These derivatives can be acylated very efficiently with heptafluorobutyric anhydride and triethylamine, and their levels in rodent brain (approximately 1 nmol/g) are much higher than those of their unconjugated counterparts. They are distinct from fatty acid esters, and preliminary data do not favor structures such as sulfolipids or sterol peroxides. Noncovalent interactions between steroids and proteolipidic elements, such as lipoproteins, could account for some experimental data. Given their abundance in rodent brain, the structural characterization and biological functions of L-PREG and L-DHEA in the central nervous system merit considerable attention.

Steroid sulfatase (STS) expression in the human temporal lobe: enzyme activity, mRNA expression and immunohistochemistry study

Dehydroepiandrosterone (DHEA) and its sulfate (DHEAS) are suggested to be important neurosteroids. We investigated steroid sulfatase (STS) in human temporal lobe biopsies in the context of possible cerebral DHEA(S) de novo biosynthesis. Formation of DHEA(S) in mature human brain tissue has not yet been studied. 17 alpha-Hydroxylase/C17-20-lyase and hydroxysteroid sulfotransferase catalyze the formation of DHEA from pregnenolone and the subsequent sulfoconjugation, respectively. Neither their mRNA nor activity were detected, indicating that DHEA(S) are not produced within the human temporal lobe. Conversely, strong activity and mRNA expression of DHEAS desulfating STS was found, twice as high in cerebral neocortex than in subcortical white matter. Cerebral STS resembled the characteristics of the known placental enzyme. Immunohistochemistry revealed STS in adult cortical neurons as well as in fetal and adult Cajal-Retzius cells. Organic anion transporting proteins OATP-A, -B, -D, and -E showed high mRNA expression levels with distinct patterns in cerebral neocortex and subcortical white matter. Although it is not clear whether they are expressed at the blood-brain barrier and facilitate an influx rather than an efflux, they might well be involved in the transport of steroid sulfates from the blood. Therefore, we hypothesize that DHEAS and/or other sulfated 3beta-hydroxysteroids might enter the human temporal lobe from the circulation where they would be readily converted via neuronal STS activity.

Steroid hormones and neurosteroids in normal and pathological aging of the nervous system

Without medical progress, dementing diseases such as Alzheimer's disease will become one of the main causes of disability. Preventing or delaying them has thus become a real challenge for biomedical research. Steroids offer interesting therapeutical opportunities for promoting successful aging because of their pleiotropic effects in the nervous system: they regulate main neurotransmitter systems, promote the viability of neurons, play an important role in myelination and influence cognitive processes, in particular learning and memory. Preclinical research has provided evidence that the normally aging nervous system maintains some capacity for regeneration and that age-dependent changes in the nervous system and cognitive dysfunctions can be reversed to some extent by the administration of steroids. The aging nervous system also remains sensitive to the neuroprotective effects of steroids. In contrast to the large number of studies documenting beneficial effects of steroids on the nervous system in young and aged animals, the results from hormone replacement studies in the elderly are so far not conclusive. There is also little information concerning changes of steroid levels in the aging human brain. As steroids present in nervous tissues originate from the endocrine glands (steroid hormones) and from local synthesis (neurosteroids), changes in blood levels of steroids with age do not necessarily reflect changes in their brain levels. There is indeed strong evidence that neurosteroids are also synthesized in human brain and peripheral nerves. The development of a very sensitive and precise method for the analysis of steroids by gas chromatography/mass spectrometry (GC/MS) offers new possibilities for the study of neurosteroids. The concentrations of a range of neurosteroids have recently been measured in various brain regions of aged Alzheimer's disease patients and aged non-demented controls by GC/MS, providing reference values. In Alzheimer's patients, there was a general trend toward lower levels of neurosteroids in different brain regions, and neurosteroid levels were negatively correlated with two biochemical markers of Alzheimer's disease, the phosphorylated tau protein and the beta-amyloid peptides. The metabolism of dehydroepiandrosterone has also been analyzed for the first time in the aging brain from Alzheimer patients and non-demented controls. The conversion of dehydroepiandrosterone to Delta5-androstene-3beta,17beta-diol and to 7alpha-OH-dehydroepiandrosterone occurred in frontal cortex, hippocampus, amygdala, cerebellum and striatum of both Alzheimer's patients and controls. The formation of these metabolites within distinct brain regions negatively correlated with the density of beta-amyloid deposits.

Neurosteroids, GABAA receptors, and escalated aggressive behavior

Aggressive behavior can serve important adaptive functions in social species. However, if it exceeds the species-typical pattern, it may become maladaptive. Very high or escalated levels of aggressive behavior can be induced in laboratory rodents by pharmacological (alcohol-heightened aggression), environmental (social instigation), or behavioral (frustration-induced aggression) means. These various forms of escalated aggressive behavior may be useful in further elucidating the neurochemical control over aggression and violence. One neurochemical system most consistently linked with escalated aggression is the GABAergic system, in conjunction with other amines and peptides. Although direct stimulation of GABA receptors generally suppresses aggression, a number of studies have found that positive allosteric modulators of GABAA receptors can cause increases in aggressive behavior. For example, alcohol, benzodiazepines, and many neurosteroids are all positive modulators of the GABAA receptor and all can cause increased levels of aggressive behavior. These effects are dose-dependent and higher doses of these compounds generally shift from heightening aggressive behavior to being sedative and anti-aggressive. In addition, these modulators interact with each other and can have additive effects on the GABAA receptor and on behavior, including aggression. The GABAA receptor is a heteropentameric protein that can be constituted from various subunits. It has been shown that subunit composition can affect sensitivity of the receptor to some modulators and that subunit composition differentially affects the sedative vs anxiolytic actions of benzodiazepines. Initial studies targeting alpha subunits of the GABAA receptor point to their significant role in the aggression-heightening effects of alcohol, benzodiazepines, and neurosteroids.

Oxidative stress and kidney dysfunction due to ischemia/reperfusion in rat: attenuation by dehydroepiandrosterone

BACKGROUND

The pathogenesis of ischemia/reperfusion (I/R) involves generation of reactive oxygen and nitrogen species. This in vivo study investigates the effect of dehydroepiandrosterone (DHEA), a physiologic steroid with antioxidant properties, on oxidative balance and renal dysfunctions induced by monolateral I/R.

METHODS

Normal and DHEA-treated rats (4 mg/day x 21 days, orally) were subjected to monolateral renal I/R (30 minutes/6 hours). The oxidative state was determined by measuring hydrogen peroxide level and activities of glutathione-peroxidase, catalase, and superoxide dismutase. Tumor necrosis factor-alpha (TNF-alpha) and nitric oxide production and inducible nitric oxide synthase (iNOS) levels were also measured. Hydroxynonenal content was used to probe lipid peroxidation. Functional parameters determined were creatinine levels and Na/K-ATPase activity. Immunohistochemical and morphologic studies were also performed.

RESULTS

A markedly pro-oxidant state was evident in the kidney of rats subjected to I/R. Both hydrogen peroxide and reactive nitrogen species (nitric oxide and iNOS) increased, whereas antioxidants decreased. Oxidant species induce TNF-alpha increase, which, in turn, produces lipoperoxidative processes, as documented by the increased hydroxynonenal (HNE) level. As final result, impaired renal functionality, hydropic degeneration, and vacuolization of proximal convolute tubules were observed in kidneys of I/R rats. DHEA pretreatment improved the parameters considered.

CONCLUSION

I/R induces oxidative stress and consequently damages the proximal convolute renal tubules. Rats supplemented with DHEA and subjected to I/R had reduced pro-oxidant state, oxidative damage, and improved renal functionality, indicating an attenuation of oxidative injury and dysfunctions mediated by I/R.

Dehydroepiandrosterone (DHEA) and the aging brain: flipping a coin in the "fountain of youth"

The physiological role of dehydroepiandrosterone (DHEA) and its sulphated ester DHEA(S) has been studied for nearly 2 decades and still eludes final clarification. The major interest in DHEA derives from its unique pattern of activity. Its levels exhibit a dramatic age-related decline that supports significant involvement of DHEA(S) in the aging process. Particularly relevant to the aging process is the functional decline that involves memory and cognitive abilities. DHEA is derived mainly from synthesis in the adrenal glands and gonads. It can also be detected in the brain where it is derived from a synthesis that is independent from peripheral steroid sources. For this reason DHEA and other steroid molecules have been named "neurosteroids." Pharmacological studies on animals provided evidence that neurosteroids could be involved in learning and memory processes because they can display memory-enhancing properties in aged rodents. However, human studies have reported contradictory results that so far do not directly support the use of DHEA in aging-related conditions. As such, it is important to remember that plasma levels of DHEA(S) may not reflect levels in the central nervous system (CNS), due to intrinsic ability of the brain to produce neurosteroids. Thus, the importance of neurosteroids in the memory process and in age-related cognitive impairment should not be dismissed. Furthermore, the fact that the compound is sold in most countries as a health food supplement is hampering the rigorous scientific evaluation of its potential. We will describe the effect of neurosteroids, in particular DHEA, on neurochemical mechanism involved in memory and learning. We will focus on a novel effect on a signal transduction mechanism involving a classical "cognitive kinase" such as protein kinase C. The final objective is to provide additional tools to understand the physiological role and therapeutic potentials of neurosteroids in normal and/or pathological aging, such as Alzheimer's disease.